Lipid monolayer structure and interactions in the presence of peptides and proteins

NASA Astrophysics Data System (ADS)

Freites, Juan Alfredo

Structural aspects of two simple model systems, protein-lipid monolayer and peptide-lipid monolayer, were studied by experimental and computer simulation techniques. In both cases, both the choice of system and the approach employed to studying it, were motivated by specific biological problems. The interaction of annexin A1 with monolayers of dipalmitoylphosphatidylcholine (DPPC) was studied by fluorescence microscopy as a function of lipid monolayer phase and pH. It was shown that the annexin A1-DPPC interaction depends strongly on both the domain structure and phase behavior of the DPPC monolayer, and only weakly on the subphase pH. Annexin A1 was found to be line-active, adsorbing preferentially at phase boundaries. Also, annexin A1 was found to form networks in the presence of a domain structure in the lipid monolayer. Molecular dynamics simulations were carried out on a model system composed of a surfactant protein B peptide, SP-B1--25, and a monolayer of hexadecanoic acid. A detailed structural characterization was performed as a function of the lipid monolayer specic area. It was found that the peptide remains inserted in the monolayer up to values of specific area corresponding to an untilted condensed phase of the pure hexadecanoic acid monolayer. The system remains stable by altering the conformational order of both the anionic lipid monolayer and the peptide secondary structure, and effectively constitutes a unique disordered lipid-peptide monolayer phase. Two elements appear to be key for the constitution of this phase: an electrostatic interaction between the cationic residues of the peptide with the anionic headgroups of the lipids, and an exclusion of the aromatic residues on the hydrophobic end of the peptide from the hydrophilic and aqueous regions of the system. A direct comparison between molecular dynamics simulations and laboratory experiments was performed for hexadecanoic acid monolayer systems. In order to simulate specific points on the surface pressure vs. area isotherm, an algorithm for the control of surface pressure was developed based on previous work by Martyna, Tobias and Klein. The algorithm was implemented and tested with the hexadecanoic acid monolayer system.

Morphological variation of stimuli-responsive polypeptide at air-water interface

NASA Astrophysics Data System (ADS)

Shin, Sungchul; Ahn, Sungmin; Cheng, Jie; Chang, Hyejin; Jung, Dae-Hong; Hyun, Jinho

2016-12-01

The morphological variation of stimuli-responsive polypeptide molecules at the air-water interface as a function of temperature and compression was described. The surface pressure-area (π-A) isotherms of an elastin-like polypeptide (ELP) monolayer were obtained under variable external conditions, and Langmuir-Blodgett (LB) monolayers were deposited onto a mica substrate for characterization. As the compression of the ELP monolayer increased, the surface pressure increased gradually, indicating that the ELP monolayer could be prepared with high stability at the air-water interface. The temperature in the subphase of the ELP monolayer was critical in the preparation of LB monolayers. The change in temperature induced a shift in the π-A isotherms as well as a change in ELP secondary structures. Surprisingly, the compression of the ELP monolayer influenced the ELP secondary structure due to the reduction in the phase transition temperature with decreasing temperature. The change in the ELP secondary structure formed at the air-water interface was investigated by surface-enhanced Raman scattering. Moreover, the morphology of the ELP monolayer was subsequently imaged using atomic force microscopy. The temperature responsive behavior resulted in changes in surface morphology from relatively flat structures to rugged labyrinth structures, which suggested conformational changes in the ELP monolayers.

Structural, electronic and magnetic properties of Au-based monolayer derivatives in honeycomb structure

DOE Office of Scientific and Technical Information (OSTI.GOV)

Kapoor, Pooja, E-mail: pupooja16@gmail.com; Sharma, Munish; Ahluwalia, P. K.

2016-05-23

We present electronic properties of atomic layer of Au, Au{sub 2}-N, Au{sub 2}-O and Au{sub 2}-F in graphene-like structure within the framework of density functional theory (DFT). The lattice constant of derived monolayers are found to be higher than the pristine Au monolayer. Au monolayer is metallic in nature with quantum ballistic conductance calculated as 4G{sub 0}. Similarly, Au{sub 2}-N and Au{sub 2}-F monolayers show 4G{sub 0} and 2G{sub 0} quantum conductance respectively while semiconducting nature with calculated band gap of 0.28 eV has been observed for Au{sub 2}-O monolayer. Most interestingly, half metalicity has been predicted for Au{sub 2}-Nmore » and Au{sub 2}-F monolayers. Our findings may have importance for the application of these monolayers in nanoelectronic and spintronics.« less

Simple theoretical determination of the thickness of an alkali monolayer adsorbed on a jellium surface

NASA Astrophysics Data System (ADS)

Wojciechowski, K. F.; Rogowska, J. M.; Bogdanów, H.

1991-05-01

The thickness of a jellium slab representing an alkali monolayer on a jellium surface has been calculated, using parametrized trial functions for the electron density profile at the surface fulfilling the Budd-Vannimenus theorem and the charge neutrality condition. Reasonable values of the thicknesses of potassium, sodium, rubidium and caesium monolayers are obtained, which, contrary to earlier assumptions, depend on the bulk electron density of the substrate.

Structural, electronic and photocatalytic properties of atomic defective BiI3 monolayers

NASA Astrophysics Data System (ADS)

Yan, Huang; Ziyu, Hu; Xu, Gong; Xiaohong, Shao

2018-01-01

The structural, electronic and photocatalytic properties of five vacancy-containing 2D BiI3 monolayers are investigated by the first-principle calculations. The electronic structures show that the five structures are stable and have comparable binding energies to that of the pristine BiI3 monolayer, and the defects can tune the band gaps. Optical spectra indicate that the five structures retain high absorption capacity for visible light. The spin-orbit coupling (SOC) effect is found to play an important role in the band edge of defective structures, and the VBi and VBi-I3 defective BiI3 monolayers can make absolute band edges straddle water redox potentials more easily.

Ion specific 2D to 3D structural modification of Langmuir monolayer at lower surface pressure

NASA Astrophysics Data System (ADS)

Das, Kaushik; Kundu, Sarathi

2017-05-01

2D to 3D structural transformation of stearic acid Langmuir monolayer in presence of Ca2+ and Zn2+ ions at lower surface pressure (≈25 mN/m) has been studied at lower (pH ≈ 6.8) and higher (pH ≈ 9.5) subphase pH. Generally, 2D to 3D structural transformation of monolayer occurs at higher surface pressure (>50 mN/m) after collapse point which can be identified from surface pressure (π) vs. specific molecular area (A) isotherms. In presence of Ca2+ ions and for both lower and higher subphase pH, stearic acid monolayer remains as 2D monolayer at that lower surface pressure as confirmed from the Atomic Force Microscopy (AFM) studies on the films deposited at π ≈ 25mN/m. However, in presence of Zn2+ at higher subphase pH, stearic acid monolayer shows 2D to 3D structural transformation where less covered bilayer-like structure forms on top of the monolayer as obtained from the AFM studies. Fourier transform infrared (FTIR) spectroscopy results reveal that formation of relatively more amount of bidentate bridging coordination of metal carboxylate headgroup may be the key reason of such 2D to 3D structural transformation for Zn2+.

Study of iridium silicide monolayers using density functional theory

NASA Astrophysics Data System (ADS)

Popis, Minh D.; Popis, Sylvester V.; Oncel, Nuri; Hoffmann, Mark R.; ćakır, Deniz

2018-02-01

In this study, we investigated physical and electronic properties of possible two-dimensional structures formed by Si (silicon) and Ir (iridium). To this end, different plausible structures were modeled by using density functional theory and the cohesive energies calculated for the geometry of optimized structures, with the lowest equilibrium lattice constants. Among several candidate structures, we identified three mechanically (via elastic constants and Young's modulus), dynamically (via phonon calculations), and thermodynamically stable iridium silicide monolayer structures. The lowest energy structure has a chemical formula of Ir2Si4 (called r-IrSi2), with a rectangular lattice (Pmmn space group). Its cohesive energy was calculated to be -0.248 eV (per IrSi2 unit) with respect to bulk Ir and bulk Si. The band structure indicates that the Ir2Si4 monolayer exhibits metallic properties. Other stable structures have hexagonal (P-3m1) and tetragonal (P4/nmm) cell structures with 0.12 and 0.20 eV/f.u. higher cohesive energies, respectively. Our calculations showed that Ir-Si monolayers are reactive. Although O2 molecules exothermically dissociate on the surface of the free-standing iridium silicide monolayers with large binding energies, H2O molecules bind to the monolayers with a rather weak interaction.

Janus Monolayer Transition-Metal Dichalcogenides.

PubMed

Zhang, Jing; Jia, Shuai; Kholmanov, Iskandar; Dong, Liang; Er, Dequan; Chen, Weibing; Guo, Hua; Jin, Zehua; Shenoy, Vivek B; Shi, Li; Lou, Jun

2017-08-22

The crystal configuration of sandwiched S-Mo-Se structure (Janus SMoSe) at the monolayer limit has been synthesized and carefully characterized in this work. By controlled sulfurization of monolayer MoSe 2 , the top layer of selenium atoms is substituted by sulfur atoms, while the bottom selenium layer remains intact. The structure of this material is systematically investigated by Raman, photoluminescence, transmission electron microscopy, and X-ray photoelectron spectroscopy and confirmed by time-of-flight secondary ion mass spectrometry. Density functional theory (DFT) calculations are performed to better understand the Raman vibration modes and electronic structures of the Janus SMoSe monolayer, which are found to correlate well with corresponding experimental results. Finally, high basal plane hydrogen evolution reaction activity is discovered for the Janus monolayer, and DFT calculation implies that the activity originates from the synergistic effect of the intrinsic defects and structural strain inherent in the Janus structure.

Janus Monolayer Transition-Metal Dichalcogenides

DOE PAGES

Zhang, Jing; Jia, Shuai; Kholmanov, Iskandar; ...

2017-08-03

In this work, the crystal configuration of sandwiched S–Mo–Se structure (Janus SMoSe) at the monolayer limit has been synthesized and carefully characterized. By controlled sulfurization of monolayer MoSe 2, the top layer of selenium atoms is substituted by sulfur atoms, while the bottom selenium layer remains intact. Furthermore, the structure of this material is systematically investigated by Raman, photoluminescence, transmission electron microscopy, and X-ray photoelectron spectroscopy and confirmed by time-of-flight secondary ion mass spectrometry. Density functional theory (DFT) calculations are performed to better understand the Raman vibration modes and electronic structures of the Janus SMoSe monolayer, which are found tomore » correlate well with corresponding experimental results. Finally, high basal plane hydrogen evolution reaction activity is discovered for the Janus monolayer, and DFT calculation implies that the activity originates from the synergistic effect of the intrinsic defects and structural strain inherent in the Janus structure.« less

Methylene blue adsorption on a DMPA lipid langmuir monolayer.

PubMed

Giner Casares, Juan José; Camacho, Luis; Martín-Romero, Maria Teresa; López Cascales, José Javier

2010-07-12

Adsorption of methylene blue (MB) onto a dimyristoylphosphatidic acid (DMPA) Langmuir air/water monolayer is studied by molecular dynamics (MD) simulations, UV reflection spectroscopy and surface potential measurements. The free-energy profile associated with MB transfer from water to the lipid monolayer shows two minima of -66 and -60 kJ mol(-1) for its solid and gas phase, respectively, corresponding to a spontaneous thermodynamic process. From the position of the free-energy minima, it is possible to predict the precise location of MB in the interior of the DMPA monolayer. Thus, MB is accommodated in the phosphoryl or carbonyl region of the DMPA Langmuir air/water interface, depending on the isomorphic state (solid or gas phase, respectively). Reorientation of MB, measured from the bulk solution to the interior of the lipid monolayer, passes from a random orientation in bulk solution to an orientation parallel to the surface of the lipid monolayer when MB is absorbed.

Nanoscale structure of the oil-water interface

DOE PAGES

Fukuto, M.; Ocko, B. M.; Bonthuis, D. J.; ...

2016-12-15

X-ray reflectivity (XR) and atomistic molecular dynamics (MD) simulations, carried out to determine the structure of the oil-water interface, provide new insight into the simplest liquid-liquid interface. For several oils (hexane, dodecane, and hexadecane) the XR shows very good agreement with a monotonic interface-normal electron density profile (EDP) broadened only by capillary waves. Similar agreement is also found for an EDP including a sub-Å thick electron depletion layer separating the oil and the water. As a result, the XR and MD derived depletions are much smaller than reported for the interface between solid-supported hydrophobic monolayers and water.

Unanticipated C=C bonds in covalent monolayers on silicon revealed by NEXAFS.

PubMed

Lee, Michael V; Lee, Jonathan R I; Brehmer, Daniel E; Linford, Matthew R; Willey, Trevor M

2010-02-02

Interfaces are crucial to material properties. In the case of covalent organic monolayers on silicon, molecular structure at the interface controls the self-assembly of the monolayers, which in turn influences the optical properties and electrical transport. These properties intrinsically affect their application in biology, tribology, optics, and electronics. We use near-edge X-ray absorption fine structure spectroscopy to show that the most basic covalent monolayers formed from 1-alkenes on silicon retain a double bond in one-fifth to two-fifths of the resultant molecules. Unsaturation in the predominantly saturated monolayers will perturb the regular order and affect the dependent properties. The presence of unsaturation in monolayers produced by two different methods also prompts the re-evaluation of other radical-based mechanisms for forming covalent monolayers on silicon.

Modes of Cell Death Induced by Photodynamic Therapy Using Zinc Phthalocyanine in Lung Cancer Cells Grown as a Monolayer and Three-Dimensional Multicellular Spheroids.

PubMed

Manoto, Sello L; Houreld, Nicolette; Hodgkinson, Natasha; Abrahamse, Heidi

2017-05-16

Photodynamic therapy (PDT) involves interaction of a photosensitizer, light, and molecular oxygen which produces singlet oxygen and subsequent tumour eradication. The development of second generation photosensitizers, such as phthalocyanines, has improved this technology. Customary monolayer cell culture techniques are, unfortunately, too simple to replicate treatment effects in vivo. Multicellular tumour spheroids may provide a better alternative since they mimic aspects of the human tumour environment. This study aimed to profile 84 genes involved in apoptosis following treatment with PDT on lung cancer cells (A549) grown in a monolayer versus three-dimensional multicellular tumour spheroids (250 and 500 μm). Gene expression profiling was performed 24 h post irradiation (680 nm; 5 J/cm²) with zinc sulfophthalocyanine (ZnPcS mix ) to determine the genes involved in apoptotic cell death. In the monolayer cells, eight pro-apoptotic genes were upregulated, and two were downregulated. In the multicellular tumour spheroids (250 µm) there was upregulation of only 1 gene while there was downregulation of 56 genes. Apoptosis in the monolayer cultured cells was induced via both the intrinsic and extrinsic apoptotic pathways. However, in the multicellular tumour spheroids (250 and 500 µm) the apoptotic pathway that was followed was not conclusive.

Assembling and compressing a semifluorinated alkane monolayer on a hydrophobic surface: Structural and dielectric properties

NASA Astrophysics Data System (ADS)

El Abed, Abdel I.; Ionov, Radoslav; Daoud, Mohamed; Abillon, Olivier

2004-11-01

We investigate the dynamic behavior upon lateral compression of a semifluorinated alkane F(CF2)8(CH2)18H (denoted F8H18 ), spread on the hydrophobic top of a suitable amphiphilic monolayer: namely, a natural α -helix alamethicin peptide (alam). We show, in particular, the formation of an asymmetric flat bilayer by compressing at the air-water interface a mixed Langmuir film made of F8H18 and alam. The particular chemical structure of F8H18 , the suitable structure of the underlying alam monolayer and its collapse properties, allow for a continuous compression of the upper F8H18 monolayer while the density of the lower alam monolayer remains constant. Combining grazing incidence x-ray reflectivity, surface potential, and atomic force microscopy data allow for the determination of the orientation and dielectric constant of the upper F8H18 monolayer.

Electronic structure in 1T-ZrS2 monolayer by strain

NASA Astrophysics Data System (ADS)

Xin, Qianqian; Zhao, Xu; Ma, Xu; Wu, Ninghua; Liu, Xiaomeng; Wei, Shuyi

2017-09-01

We report electronic structure of 1T-ZrS2 monolayer with biaxial strain from -10% to 15%, basing the first principles calculations. Our calculation results indicate that the band structure of ZrS2 monolayer was changed clearly. The location of conduction band minimum (CBM) and valence band maximum (VBM) changed with the variation of isotropic strain. At compressive strain, the location of CBM and VBM retains at M and Γ point, respectively. The band gap of ZrS2 monolayer decreases from 1.111 eV to 0 eV when compressive strain increases from 0% to -8%, which means that the ZrS2 monolayer turns to metal at -8% compressive strain. Under the tensile strain, the ZrS2 monolayer also retains be an indirect band gap semiconductor. The location of CBM moves from M to Γ point and the location of VBM moves along Γ-A-K-Γ direction. The band gap of ZrS2 monolayer firstly increases and then decreases and the biggest band gap is 1.577 eV at tensile strain 6%. We can see the compression strain is more effective than tensile strain in modulating band gap of 1T-ZrS2 monolayer.

Ultra-thin ZnSe: Anisotropic and flexible crystal structure

NASA Astrophysics Data System (ADS)

Bacaksiz, C.; Senger, R. T.; Sahin, H.

2017-07-01

By performing density functional theory-based calculations, we investigate the structural, electronic, and mechanical properties of the thinnest ever ZnSe crystal [11]. The vibrational spectrum analysis reveals that the monolayer ZnSe is dynamically stable and has flexible nature with its soft phonon modes. In addition, a direct electronic band gap is found at the gamma point for the monolayer structure of ZnSe. We also elucidate that the monolayer ZnSe has angle dependent in-plane elastic parameters. In particular, the in-plane stiffness values are found to be 2.07 and 6.89 N/m for the arm-chair and zig-zag directions, respectively. The angle dependency is also valid for the Poisson ratio of the monolayer ZnSe. More significantly, the in-plane stiffness of the monolayer ZnSe is the one-tenth of Young modulus of bulk zb-ZnSe which indicates that the monolayer ZnSe is a quite flexible single layer crystal. With its flexible nature and in-plane anisotropic mechanical properties, the monolayer ZnSe is a good candidate for nanoscale mechanical applications.

Ionization state and structure of l-1,2-dipalmitoylphosphatidylglycerol monolayers at the liquid/air interface.

PubMed

Maltseva, Elena; Shapovalov, Vladimir L; Möhwald, Helmuth; Brezesinski, Gerald

2006-01-19

Phosphatidylglycerols are components of biological membranes. The phase behavior of these phospholipids was extensively investigated. However, there is still no definite picture about the dependence of the ionization state and monolayer structure on subphase composition. The major problem of previous investigations is that none of the methods used allow obtaining the ionization degree directly. In the present work we apply techniques developed in the past decades for Langmuir monolayers: infrared reflection absorption spectroscopy (IRRAS) as well as X-ray diffraction and reflectivity techniques, which provide straightforward information about structure and ionization state of a L-1,2-dipalmitoylphosphatidylglycerol (DPPG) monolayer. The Gouy-Chapman model is applied to evaluate the intrinsic pKa. Therewith, the ionization degree can be determined even at low pH values. The experimental titration curves are in good agreement with theoretical curves based on the Gouy-Chapman model. The obtained instrinic pKa amounts to 1. The ionization degree of a DPPG monolayer is independent of the monovalent cation size. In contrast, the structure of a DPPG monolayer is strongly affected by the type of divalent cations.

The characterization of organic monolayers at gold surfaces using scanning tunneling microscopy and atomic force microscopy correlation with macrostructural properties

NASA Astrophysics Data System (ADS)

Alves, C. A.

1992-09-01

Monolayer films formed by self-assembly of organothiols at epitaxially grown Au(111) films at mica were examined in air using scanning tunneling (STM) and atomic force microscopies (AFM). n-Alkanethiolate monolayers exhibit a hexagonal packing arrangement with nearest-neighbor and next-nearest-neighbor spacings of 0.50 and 0.87 nm. This arrangement is consistent with (the square root of 3 x the square root of 3)R30 deg adlayer structure at Au(111). STM reveals the structure of the Au-bound sulfur, while AFM details the structure at the monolayer/air interface, revealing that the order at the Au-S interface is retained up to the monolayer/air interface. The investigation of the self-assembled (CF3CF2)7(CH2)2SH monolayer at Au(111) by AFM reveals a (2 x 2) adlayer structure, with nearest-neighbor and next-nearest-neighbor spacings of 0.58 plus or minus 0.02 nm and 1.0 plus or minus 0.02 nm, respectively. This is consistent with the larger van der Waals diameter of the fluorinated chain. Coverage of this fluorinated thiolate monolayer is (6.3 plus or minus 0.8) x 10(exp -10) mol/cm(sup 2), consistent with the expected 0.25 monolayer coverage of the (2 x 2) adlayer structure at Au(111). Infrared reflection spectroscopy also confirmed this. Upon prolonged exposure to air, the thiolate species is oxidized to elemental sulfur in the forms of cyclooctasulfur (cyclo-S8) and other allotropes. STM reveals square structures on aged thiolate monolayers. Dimensions of these squares (0.40-0.50 nm per side) are close to those of cyclo-S8. Electrochemical reductive desorption experiments also reveal a change in the surface species with time, with a second desorption wave.

Diamondoid monolayers as electron emitters

DOEpatents

Yang, Wanli [El Cerrito, CA; Fabbri, Jason D [San Francisco, CA; Melosh, Nicholas A [Menlo Park, CA; Hussain, Zahid [Orinda, CA; Shen, Zhi-Xun [Stanford, CA

2012-04-10

Provided are electron emitters based upon diamondoid monolayers, preferably self-assembled higher diamondoid monolayers. High intensity electron emission has been demonstrated employing such diamondoid monolayers, particularly when the monolayers are comprised of higher diamondoids. The application of such diamondoid monolayers can alter the band structure of substrates, as well as emit monochromatic electrons, and the high intensity electron emissions can also greatly improve the efficiency of field-effect electron emitters as applied to industrial and commercial applications.

Diamondoid monolayers as electron emitters

DOEpatents

Yang, Wanli; Fabbri, Jason D.; Melosh, Nicholas A.; Hussain, Zahid; Shen, Zhi-Xun

2013-10-29

Provided are electron emitters based upon diamondoid monolayers, preferably self-assembled higher diamondoid monolayers. High intensity electron emission has been demonstrated employing such diamondoid monolayers, particularly when the monolayers are comprised of higher diamondoids. The application of such diamondoid monolayers can alter the band structure of substrates, as well as emit monochromatic electrons, and the high intensity electron emissions can also greatly improve the efficiency of field-effect electron emitters as applied to industrial and commercial applications.

Fullerene-derivative PC61BM forms three types of phase-pure monolayer on the surface of Au(111)

NASA Astrophysics Data System (ADS)

Li, Wen-Jie; Du, Ying-Ying; Zhang, Han-Jie; Chen, Guang-Hua; Sheng, Chun-Qi; Wu, Rui; Wang, Jia-Ou; Qian, Hai-Jie; Ibrahim, Kurash; He, Pi-Mo; Li, Hong-Nian

2016-12-01

We have studied the packing structures of C60-derivative PC61BM on the surface of Au(111) in ultrahigh vacuum using scanning tunneling microscopy. The Au(111) has a triangle-like reconstructed surface, which results in some packing structures different from those reported for low coverages. PC61BM can form three types of phase-pure monolayer, namely, the compact straight molecular double-row monolayer, the hexagonal-packing monolayer and the glassy monolayer. The different types of monolayer form for different molecular densities and different annealing temperatures. In addition to the already known inter-molecular interactions (Van de Waals interaction and hydrogen bond), the steric effect of the phenyl-butyric-acid-methyl-ester side tail plays conspicuous role in the molecular self-assembly at high coverages. The steric effect makes it difficult to prepare a hexagonal-packing monolayer at room temperature and decides the instability of the hexagonal-packing monolayer prepared by thermal annealing.

GaAs monolayer: Excellent SHG responses and semi metallic to metallic transition modulated by vacancy effect

NASA Astrophysics Data System (ADS)

Rozahun, Ilmira; Bahti, Tohtiaji; He, Guijie; Ghupur, Yasenjan; Ablat, Abduleziz; Mamat, Mamatrishat

2018-05-01

Monolayer materials are considered as a promising candidate for novel applications due to their attractive magnetic, electronic and optical properties. Investigation on nonlinear optical (NLO) properties and effect of vacancy on monolayer materials are vital to property modulations of monolayers and extending their applications. In this work, with the aid of first-principles calculations, the crystal structure, electronic, magnetic, and optical properties of GaAs monolayers with the vacancy were investigated. The result shows gallium arsenic (GaAs) monolayer produces a strong second harmonic generation (SHG) response. Meanwhile, the vacancy strongly affects structural, electronic, magnetic and optical properties of GaAs monolayers. Furthermore, arsenic vacancy (VAs) brings semi metallic to metallic transition, while gallium vacancy (VGa) causes nonmagnetic to magnetic conversion. Our result reveals that GaAs monolayer possesses application potentials in Nano-amplifying modulator and Nano-optoelectronic devices, and may provide useful guidance in designing new generation of Nano-electronic devices.

Metal ion interaction with phosphorylated tyrosine analogue monolayers on gold.

PubMed

Petoral, Rodrigo M; Björefors, Fredrik; Uvdal, Kajsa

2006-11-23

Phosphorylated tyrosine analogue molecules (pTyr-PT) were assembled onto gold substrates, and the resulting monolayers were used for metal ion interaction studies. The monolayers were characterized by X-ray photoelectron spectroscopy (XPS), infrared reflection-absorption spectroscopy (IRAS), cyclic voltammetry (CV), and electrochemical impedance spectroscopy (EIS), both prior to and after exposure to metal ions. XPS verified the elemental composition of the molecular adsorbate and the presence of metal ions coordinated to the phosphate groups. Both the angle-dependent XPS and IRAS results were consistent with the change in the structural orientation of the pTyr-PT monolayer upon exposure to metal ions. The differential capacitance of the monolayers upon coordination of the metal ions was evaluated using EIS. These metal ions were found to significantly change the capacitance of the pTyr-PT monolayers in contrast to the nonphosphorylated tyrosine analogue (TPT). CV results showed reduced electrochemical blocking capabilities of the phosphorylated analogue monolayer when exposed to metal ions, supporting the change in the structure of the monolayer observed by XPS and IRAS. The largest change in the structure and interfacial capacitance was observed for aluminum ions, compared to calcium, magnesium, and chromium ions. This type of monolayer shows an excellent capability to coordinate metal ions and has a high potential for use as sensing layers in biochip applications to monitor the presence of metal ions.

SiP monolayers: New 2D structures of group IV-V compounds for visible-light photohydrolytic catalysts

NASA Astrophysics Data System (ADS)

Ma, Zhinan; Zhuang, Jibin; Zhang, Xu; Zhou, Zhen

2018-06-01

Because of graphene and phosphorene, two-dimensional (2D) layered materials of group IV and group V elements arouse great interest. However, group IV-V monolayers have not received due attention. In this work, three types of SiP monolayers were computationally designed to explore their electronic structure and optical properties. Computations confirm the stability of these monolayers, which are all indirect-bandgap semiconductors with bandgaps in the range 1.38-2.21 eV. The bandgaps straddle the redox potentials of water at pH = 0, indicating the potential of the monolayers for use as watersplitting photocatalysts. The computed optical properties demonstrate that certain monolayers of SiP 2D materials are absorbers of visible light and would serve as good candidates for optoelectronic devices.

Pressure-induced enhancement in the thermoelectric properties of monolayer and bilayer SnSe2

NASA Astrophysics Data System (ADS)

Zou, Daifeng; Yu, Chuanbin; Li, Yuhao; Ou, Yun; Gao, Yongyi

2018-03-01

The electronic structures of monolayer and bilayer SnSe2 under pressure were investigated by using first-principles calculations including van der Waals interactions. For monolayer SnSe2, the variation of electronic structure under pressure is controlled by pressure-dependent lattice parameters. For bilayer SnSe2, the changes in electronic structure under pressure are dominated by intralayer and interlayer atomic interactions. The n-type thermoelectric properties of monolayer and bilayer SnSe2 under pressure were calculated on the basis of the semi-classical Boltzmann transport theory. It was found that the electrical conductivity of monolayer and bilayer SnSe2 can be enhanced under pressure, and such dependence can be attributed to the pressure-induced changes of the Se-Sn antibonding states in conduction band. Finally, the doping dependence of power factors of n-type monolayer and bilayer SnSe2 at three different pressures were estimated, and the results unveiled that thermoelectric performance of n-type monolayer and bilayer SnSe2 can be improved by applying external pressure. This study benefits to understand the nature of the transport properties for monolayer and bilayer SnSe2 under pressure, and it offers valuable insight for designing high-performance thermoelectric few-layered SnSe2 through strain engineering induced by external pressure.

Pressure-induced enhancement in the thermoelectric properties of monolayer and bilayer SnSe2.

PubMed

Zou, Daifeng; Yu, Chuanbin; Li, Yuhao; Ou, Yun; Gao, Yongyi

2018-03-01

The electronic structures of monolayer and bilayer SnSe 2 under pressure were investigated by using first-principles calculations including van der Waals interactions. For monolayer SnSe 2 , the variation of electronic structure under pressure is controlled by pressure-dependent lattice parameters. For bilayer SnSe 2 , the changes in electronic structure under pressure are dominated by intralayer and interlayer atomic interactions. The n -type thermoelectric properties of monolayer and bilayer SnSe 2 under pressure were calculated on the basis of the semi-classical Boltzmann transport theory. It was found that the electrical conductivity of monolayer and bilayer SnSe 2 can be enhanced under pressure, and such dependence can be attributed to the pressure-induced changes of the Se-Sn antibonding states in conduction band. Finally, the doping dependence of power factors of n -type monolayer and bilayer SnSe 2 at three different pressures were estimated, and the results unveiled that thermoelectric performance of n -type monolayer and bilayer SnSe 2 can be improved by applying external pressure. This study benefits to understand the nature of the transport properties for monolayer and bilayer SnSe 2 under pressure, and it offers valuable insight for designing high-performance thermoelectric few-layered SnSe 2 through strain engineering induced by external pressure.

Theory-Guided Innovation of Noncarbon Two-Dimensional Nanomaterials

DTIC Science & Technology

2016-05-24

unique structures and exceptional properties, such as Be5C2 monolayers with quasi -planaer pentacoordinate carbon, FeB6 monolayers hypercoordinate...properties, such as Be5C2 monolayers with quasi -planaer pentacoordinate carbon, FeB6 monolayers hypercoordinate transition metal, semiconducting Group 15...theoretical and experimental studies, we have developed a convenient chemical approach to etch hexagonal boron nitride monolayers to achieve holes

Asymmetric or symmetric bilayer formation during oblique drop impact depends on rheological properties of saturated and unsaturated lipid monolayers.

PubMed

Vranceanu, Marcel; Terinte, Nicoleta; Nirschl, Hermann; Leneweit, Gero

2011-02-01

Bilayer structures are formed by approaching two liquid surfaces with phospholipid monolayers, which are brought into contact by oblique drop impact on a liquid surface. Asymmetric bilayers can be produced by the coupling of drop and target monolayers. In contrast, symmetric bilayers or multilayers are formed by collapse of the compressed target monolayer. We show that under all studied conditions bilayer/multilayer synthesis takes place. The experimental conditions for the synthesis of asymmetric or symmetric bilayers are described quantitatively in terms of the surface rheological (surface elasticity and dilational viscosity) and the hydrodynamical parameters (Weber number and impact angle). The composition and mechanical properties of the phospholipid monolayers strongly influences the patterns of drop impact and the bilayer/multilayer formation. Cholesterol stiffens unsaturated phospholipid monolayers and fluidifies saturated monolayers. All monolayers form asymmetric vesicle-like structures, which are stable in the aqueous medium. Additionally, unsaturated phospholipid monolayers without cholesterol form symmetric vesicles by folding parts of the target monolayer. Sufficient presence of cholesterol in unsaturated phospholipid monolayers inhibits the folding of the target monolayer and the subsequent formation of symmetric bilayers. The rheological properties of saturated and unsaturated phospholipid monolayers and their mixtures with cholesterol are discussed. Based on drop impact results it is shown that the state of a so far undefined region in the DPPC/cholesterol phase diagram is a fluid phase. Copyright © 2010 Elsevier Inc. All rights reserved.

Simulating the Surface Relief of Nanoaerosols Obtained via the Rapid Cooling of Droplets

NASA Astrophysics Data System (ADS)

Tovbin, Yu. K.; Zaitseva, E. S.; Rabinovich, A. B.

2018-03-01

An approach is formulated that theoretically describes the structure of a rough surface of small aerosol particles obtained from a liquid droplet upon its rapid cooling. The problem consists of two stages. In the first stage, a concentration profile of the droplet-vapor transition region is calculated. In the second stage, local fractions of vacant sites and their pairs are found on the basis of this profile, and the rough structure of a frozen droplet surface transitioning to the solid state is calculated. Model parameters are the temperature of the initial droplet and those of the lateral interaction between droplet atoms. Information on vacant sites inside the region of transition allows us to identify adsorption centers and estimate the monolayer capacity, compared to that of the total space of the region of transition. The approach is oriented toward calculating adsorption isotherms on real surfaces.

Multimicrometer Noncovalent Monolayer Domains on Layered Materials through Thermally Controlled Langmuir-Schaefer Conversion for Noncovalent 2D Functionalization.

PubMed

Hayes, Tyler R; Bang, Jae Jin; Davis, Tyson C; Peterson, Caroline F; McMillan, David G; Claridge, Shelley A

2017-10-18

As functionalized 2D materials are incorporated into hybrid materials, ensuring large-area structural control in noncovalently adsorbed films becomes increasingly important. Noncovalent functionalization avoids disrupting electronic structure in 2D materials; however, relatively weak molecular interactions in such monolayers typically reduce stability toward solution processing and other common material handling conditions. Here, we find that controlling substrate temperature during Langmuir-Schaefer conversion of a standing phase monolayer of diynoic amphiphiles on water to a horizontally oriented monolayer on a 2D substrate routinely produces multimicrometer domains, at least an order of magnitude larger than those typically achieved through drop-casting. Following polymerization, these highly ordered monolayers retain their structures during vigorous washing with solvents including water, ethanol, tetrahydrofuran, and toluene. These findings point to a convenient and broadly applicable strategy for noncovalent functionalization of 2D materials in applications that require large-area structural control, for instance, to minimize desorption at defects during subsequent solution processing.

Three-dimensional tissue assemblies: novel models for the study of Salmonella enterica serovar Typhimurium pathogenesis

NASA Technical Reports Server (NTRS)

Nickerson, C. A.; Goodwin, T. J.; Terlonge, J.; Ott, C. M.; Buchanan, K. L.; Uicker, W. C.; Emami, K.; LeBlanc, C. L.; Ramamurthy, R.; Clarke, M. S.;

Three-Dimensional Tissue Assemblies: Novel Models for the Study of Salmonella enterica Serovar Typhimurium Pathogenesis

PubMed Central

Nickerson, Cheryl A.; Goodwin, Thomas J.; Terlonge, Jacqueline; Ott, C. Mark; Buchanan, Kent L.; Uicker, William C.; Emami, Kamal; LeBlanc, Carly L.; Ramamurthy, Rajee; Clarke, Mark S.; Vanderburg, Charles R.; Hammond, Timothy; Pierson, Duane L.

2001-01-01

The lack of readily available experimental systems has limited knowledge pertaining to the development of Salmonella-induced gastroenteritis and diarrheal disease in humans. We used a novel low-shear stress cell culture system developed at the National Aeronautics and Space Administration in conjunction with cultivation of three-dimensional (3-D) aggregates of human intestinal tissue to study the infectivity of Salmonella enterica serovar Typhimurium for human intestinal epithelium. Immunohistochemical characterization and microscopic analysis of 3-D aggregates of the human intestinal epithelial cell line Int-407 revealed that the 3-D cells more accurately modeled human in vivo differentiated tissues than did conventional monolayer cultures of the same cells. Results from infectivity studies showed that Salmonella established infection of the 3-D cells in a much different manner than that observed for monolayers. Following the same time course of infection with Salmonella, 3-D Int-407 cells displayed minimal loss of structural integrity compared to that of Int-407 monolayers. Furthermore, Salmonella exhibited significantly lower abilities to adhere to, invade, and induce apoptosis of 3-D Int-407 cells than it did for infected Int-407 monolayers. Analysis of cytokine expression profiles of 3-D Int-407 cells and monolayers following infection with Salmonella revealed significant differences in expression of interleukin 1α (IL-1α), IL-1β, IL-6, IL-1Ra, and tumor necrosis factor alpha mRNAs between the two cultures. In addition, uninfected 3-D Int-407 cells constitutively expressed higher levels of transforming growth factor β1 mRNA and prostaglandin E2 than did uninfected Int-407 monolayers. By more accurately modeling many aspects of human in vivo tissues, the 3-D intestinal cell model generated in this study offers a novel approach for studying microbial infectivity from the perspective of the host-pathogen interaction. PMID:11598087

Surface profiles and modulation of ultra-thin perfluoropolyether lubricant in contact sliding

NASA Astrophysics Data System (ADS)

Sinha, S. K.; Kawaguchi, M.; Kato, T.

2004-08-01

Deformation in shear and associated tribological behaviours of ultra-thin lubricants are of significant importance for the lubrication of magnetic hard disks and for other applications such as micro-electromechanical systems, nano-fluidics and nanotechnology. This paper presents the characteristics of the perfluoropolyether ultra-thin lubricant, in terms of its surface profiles when subjected to a contact sliding test. The results indicate that for a several-monolayers thick (~4.0-4.5 nm) lubricant film, sliding produces a considerable amount of surface roughness due to peaks of lubricant that persist during sliding; however, it can flow back or return to a smooth profile after a lapse of time when the sliding is stopped. For a monolayer-thin (~1.4-1.57 nm) film, the lubricant flow is restricted, and the rough profile created due to sliding persists and almost becomes permanent on the wear track. During sliding, due to high shear stress, a characteristic feature of lubricant profile modulation is observed. This modulation, or waviness, is due to the accumulation of lubricant in piles or islands, giving certain amplitudes and frequencies, which themselves depend upon the percentage of lubricant molecules that are chemically bonded to the substrate and the lubricant thickness. The results indicate that ultra-thin lubricants (monolayer and thicker) behave more like a semi-solid (having some sliding characteristics similar to those of rubbers) than a liquid when subjected to a high shear rate during contact sliding.

Tuning the structure of thermosensitive gold nanoparticle monolayers.

PubMed

Rezende, Camila A; Shan, Jun; Lee, Lay-Theng; Zalczer, Gilbert; Tenhu, Heikki

2009-07-23

Gold nanoparticles grafted with poly(N-isopropylacrylamide) (PNIPAM) are rendered amphiphilic and thermosensitive. When spread on the surface of water, they form stable Langmuir monolayers that exhibit surface plasmon resonance. Using Langmuir balance and contrast-matched neutron reflectivity, the detailed structural properties of these nanocomposite monolayers are revealed. At low surface coverage, the gold nanoparticles are anchored to the interface by an adsorbed PNIPAM layer that forms a thin and compact pancake structure. Upon isothermal compression (T=20 degrees C), the adsorbed layer thickens with partial desorption of polymer chains to form brush structures. Two distinct polymer conformations thus coexist: an adsorbed conformation that assures stability of the monolayer, and brush structures that dangle in the subphase. An increase in temperature to 30 degrees C results in contractions of both adsorbed and brush layers with a concomitant decrease in interparticle distance, indicating vertical as well as lateral contractions of the graft polymer layer. The reversibility of this thermal response is also shown by the contraction-expansion of the polymer layers in heating-cooling cycles. The structure of the monolayer can thus be tuned by compression and reversibly by temperature. These compression and thermally induced conformational changes are discussed in relation to optical properties.

DNA-lipid complexes: stability of honeycomb-like and spaghetti-like structures.

PubMed Central

May, S; Ben-Shaul, A

1997-01-01

A molecular level theory is presented for the thermodynamic stability of two (similar) types of structural complexes formed by (either single strand or supercoiled) DNA and cationic liposomes, both involving a monolayer-coated DNA as the central structural unit. In the "spaghetti" complex the central unit is surrounded by another, oppositely curved, monolayer, thus forming a bilayer mantle. The "honeycomb" complex is a bundle of hexagonally packed DNA-monolayer units. The formation free energy of these complexes, starting from a planar cationic/neutral lipid bilayer and bare DNA, is expressed as a sum of electrostatic, bending, mixing, and (for the honeycomb) chain frustration contributions. The electrostatic free energy is calculated using the Poisson-Boltzmann equation. The bending energy of the mixed lipid layers is treated in the quadratic curvature approximation with composition-dependent bending rigidity and spontaneous curvature. Ideal lipid mixing is assumed within each lipid monolayer. We found that the most stable monolayer-coated DNA units are formed when the charged/neutral lipid composition corresponds (nearly) to charge neutralization; the optimal monolayer radius corresponds to close DNA-monolayer contact. These conclusions are also valid for the honeycomb complex, as the chain frustration energy is found to be negligible. Typically, the stabilization energies for these structures are on the order of 1 k(B)T/A of DNA length, reflecting mainly the balance between the electrostatic and bending energies. The spaghetti complexes are less stable due to the additional bending energy of the external monolayer. A thermodynamic analysis is presented for calculating the equilibrium lipid compositions when the complexes coexist with excess bilayer. PMID:9370436

Apparatus and method for intra-layer modulation of the material deposition and assist beam and the multilayer structure produced therefrom

NASA Technical Reports Server (NTRS)

Wadley, Hadyn N. G. (Inventor); Zhou, Xiaowang (Inventor); Quan, Junjie (Inventor)

2002-01-01

A method of producing a multilayer structure that has reduced interfacial roughness and interlayer mixing by using a physical-vapor deposition apparatus. In general the method includes forming a bottom layer having a first material wherein a first plurality of monolayers of the first material is deposited on an underlayer using a low incident adatom energy. Next, a second plurality of monolayers of the first material is deposited on top of the first plurality of monolayers of the first material using a high incident adatom energy. Thereafter, the method further includes forming a second layer having a second material wherein a first plurality of monolayers of the second material is deposited on the second plurality of monolayers of the first material using a low incident adatom energy. Next, a second plurality of monolayers of the second material is deposited on the first plurality of monolayers of the second material using a high incident adatom energy.

Exploring Ag(111) Substrate for Epitaxially Growing Monolayer Stanene: A First-Principles Study

PubMed Central

Gao, Junfeng; Zhang, Gang; Zhang, Yong-Wei

2016-01-01

Stanene, a two-dimensional topological insulator composed of Sn atoms in a hexagonal lattice, is a promising contender to Si in nanoelectronics. Currently it is still a significant challenge to achieve large-area, high-quality monolayer stanene. We explore the potential of Ag(111) surface as an ideal substrate for the epitaxial growth of monolayer stanene. Using first-principles calculations, we study the stability of the structure of stanene in different epitaxial relations with respect to Ag(111) surface, and also the diffusion behavior of Sn adatom on Ag(111) surface. Our study reveals that: (1) the hexagonal structure of stanene monolayer is well reserved on Ag(111) surface; (2) the height of epitaxial stanene monolayer is comparable to the step height of the substrate, enabling the growth to cross the surface step and achieve a large-area stanene; (3) the perfect lattice structure of free-standing stanene can be achieved once the epitaxial stanene monolayer is detached from Ag(111) surface; and finally (4) the diffusion barrier of Sn adatom on Ag(111) surface is found to be only 0.041 eV, allowing the epitaxial growth of stanene monolayer even at low temperatures. Our above revelations strongly suggest that Ag(111) surface is an ideal candidate for growing large-area, high-quality monolayer stanene. PMID:27373464

Surface Coverage and Structure of Mixed DNA/Alkylthiol Monolayers on Gold: Characterization by XPS, NEXAFS, and Fluorescence Intensity Measurements

PubMed Central

Lee, Chi-Ying; Gong, Ping; Harbers, Gregory M.; Grainger, David W.; Castner, David G.; Gamble, Lara J.

2006-01-01

Self-assembly of thiol-terminated single-stranded DNA (HS-ssDNA) on gold has served as an important model system for DNA immobilization at surfaces. Here, we report a detailed study of the surface composition and structure of mixed self-assembled DNA monolayers containing a short alkylthiol surface diluent [11-mercapto-1-undecanol (MCU)] on gold supports. These mixed DNA monolayers were studied with X-ray photoelectron spectroscopy (XPS), near-edge X-ray absorption fine structure spectroscopy (NEXAFS), and fluorescence intensity measurements. XPS results on sequentially adsorbed DNA/MCU monolayers on gold indicated that adsorbed MCU molecules first incorporate into the HS-ssDNA monolayer and, upon longer MCU exposures, displace adsorbed HS-ssDNA molecules from the surface. Thus, HS-ssDNA surface coverage steadily decreased with MCU exposure time. Polarization-dependent NEXAFS and fluorescence results both show changes in signals consistent with changes in DNA orientation after only 30 min of MCU exposure. NEXAFS polarization dependence (followed by monitoring the N 1s → π* transition) of the mixed DNA monolayers indicated that the DNA nucleotide base ring structures are oriented more parallel to the gold surface compared to DNA bases in pure HS-ssDNA monolayers. This indicates that HS-ssDNA oligomers reorient toward a more-upright position upon MCU incorporation. Fluorescence intensity results using end-labeled DNA probes on gold show little observable fluorescence on pure HS-ssDNA monolayers, likely due to substrate quenching effects between the fluorophore and the gold. MCU diluent incorporation into HS-ssDNA monolayers initially increases DNA fluorescence signal by densifying the chemisorbed monolayer, prompting an upright orientation of the DNA, and moving the terminal fluorophore away from the substrate. Immobilized DNA probe density and DNA target hybridization in these mixed DNA monolayers, as well as effects of MCU diluent on DNA hybridization in complex milieu (i.e., serum) were characterized by surface plasmon resonance (SPR) and 32P-radiometric assays and reported in a related study PMID:16689533

Topological insulator behavior of WS{sub 2} monolayer with square-octagon ring structure

DOE Office of Scientific and Technical Information (OSTI.GOV)

Kumar, Ashok, E-mail: ashok@cup.ac.in; Pandey, Ravindra; Ahluwalia, P. K.

We report electronic behavior of an allotrope of monolayer WS{sub 2} with a square octagon ring structure, refereed to as (so-WS{sub 2}) within state-of-the-art density functional theory (DFT) calculations. The WS{sub 2} monolayer shows semi-metallic characteristics with Dirac-cone like features around Γ. Unlike p-orbital’s Dirac-cone in graphene, the Dirac-cone in the so-WS{sub 2} monolayer originates from the d-electrons of the W atom in the lattice. Most interestingly, the spin-orbit interaction associated with d-electrons induce a finite band-gap that results into the metal-semiconductor transition and topological insulator-like behavior in the so-WS{sub 2} monolayer. These characteristics suggest the so-WS{sub 2} monolayer tomore » be a promising candidate for the next-generation electronic and spintronics devices.« less

X-ray study of the structure of phospholipid monolayers on the water surface

NASA Astrophysics Data System (ADS)

Asadchikov, V. E.; Tikhonov, A. M.; Volkov, Yu. O.; Roshchin, B. S.; Ermakov, Yu. A.; Rudakova, E. B.; D'yachkova, I. G.; Nuzhdin, A. D.

2017-10-01

The possibility of laboratory X-ray reflectometry study of the structure of dimyristoyl phosphatidylserine (DMPS) phospholipid monolayers on the water surface in various phase states has been demonstrated.

Two-dimensional MoS2 electromechanical actuators

NASA Astrophysics Data System (ADS)

Hung, Nguyen T.; Nugraha, Ahmad R. T.; Saito, Riichiro

2018-02-01

We investigate the electromechanical properties of two-dimensional MoS2 monolayers with 1H, 1T, and 1T‧ structures as a function of charge doping by using density functional theory. We find isotropic elastic moduli in the 1H and 1T structures, while the 1T‧ structure exhibits an anisotropic elastic modulus. Moreover, the 1T structure is shown to have a negative Poisson’s ratio, while Poisson’s ratios of the 1H and 1T‧ are positive. By charge doping, the monolayer MoS2 shows a reversible strain and work density per cycle ranging from  -0.68% to 2.67% and from 4.4 to 36.9 MJ m-3, respectively, making them suitable for applications in electromechanical actuators. We also examine the stress generated in the MoS2 monolayers and we find that 1T and 1T‧ MoS2 monolayers have relatively better performance than 1H MoS2 monolayer. We argue that such excellent electromechanical performance originate from the electrical conductivity of the metallic 1T and semimetallic 1T‧ structures and also from their high Young’s modulus of about 150-200 GPa.

DOE Office of Scientific and Technical Information (OSTI.GOV)

Das, Kaushik, E-mail: kaushikdas2089@gmail.com; Kundu, Sarathi

Long chain fatty acid molecules (e.g., stearic and behenic acids) form a monolayer on water surface in the presence of Ba{sup 2+} ions at low subphase pH (≈ 5.5) and remain as a monolayer before collapse generally occurs at higher surface pressure (π{sub c} > 50 mN/m). Monolayer formation is verified from the surface pressure vs. area per molecule (π-A) isotherms and also from the atomic force microscopy (AFM) analysis of the films deposited by single upstroke of hydrophilic Si (001) substrate through the monolayer covered water surface. At high subphase pH (≈ 9.5), barium stearate molecules form multilayer structuremore » at lower surface pressure which is verified from the π-A isotherms and AFM analysis of the film deposited at 25 mN/m. Such monolayer to multilayer structure formation or monolayer collapse at lower surface pressure is unusual as at this surface pressure generally fatty acid salt molecules form a monolayer on the water surface. Formation of bidentate chelate coordination in the metal containing headgroups is the reason for such monolayer to multilayer transition. However, for longer chain barium behenate molecules only monolayer structure is maintained at that high subphase pH (≈ 9.5) due to the presence of relatively more tail-tail hydrophobic interaction.« less

First-principles study on the structure and electronic property of gas molecules adsorption on Ge2Li2 monolayer

NASA Astrophysics Data System (ADS)

Hu, Yiwei; Long, Linbo; Mao, Yuliang; Zhong, Jianxin

2018-06-01

Using first-principles methods, we have studied the adsorption of gas molecules (CO2, CH4, H2S, H2 and NH3) on two dimensional Ge2Li2 monolayer. The adsorption geometries, adsorption energies, charge transfer, and band structures of above mentioned gas molecules adsorption on Ge2Li2 monolayer are analyzed. It is found that the adsorption of CO2 on Ge2Li2 monolayer is a kind of strong chemisorption, while other gas molecules such as CH4, H2S, H2 and NH3 are physisorption. The strong covalent binding is formed between the CO2 molecule and the nearest Ge atom in Ge2Li2 monolayer. This adsorption of CO2 molecule on Ge2Li2 monolayer leads to a direct energy gap of 0.304 eV. Other gas molecules exhibit mainly ionic binding to the nearest Li atoms in Ge2Li2 monolayer, which leads to indirect energy gap after adsorptions. Furthermore, it is found that the work function of Ge2Li2 monolayer is sensitive with the variation of adsorbents. Our results reveal that the Ge2Li2 monolayer can be used as a kind of nano device for gas molecules sensor.

Mechanical and electronic properties of Janus monolayer transition metal dichalcogenides

NASA Astrophysics Data System (ADS)

Shi, Wenwu; Wang, Zhiguo

2018-05-01

The mechanical and electronic properties of Janus monolayer transition metal dichalcogenides MXY (M  =  Ti, Zr, Hf, V, Nb, Ta, Cr, Mo, W; X/Y  =  S, Se, Te) were investigated using density functional theory. Results show that breaking the out-of-plane structural symmetry can be used to tune the electronic and mechanical behavior of monolayer transition metal dichalcogenides. The band gaps of monolayer WXY and MoXY are in the ranges of 0.16–1.91 and 0.94–1.69 eV, respectively. A semiconductor to metallic phase transition occurred in Janus monolayer MXY (M  =  Ti, Zr and Hf). The monolayers MXY (M  =  V, Nb, Ta and Cr) show metallic characteristics, which show no dependence on the structural symmetry breaking. The mechanical properties of MXY depended on the composition. Monolayer MXY (M  =  Mo, Ti, Zr, Hf and W) showed brittle characteristic, whereas monolayer CrXY and VXY are with ductile characteristic. The in-plane stiffness of pristine and Janus monolayer MXY are in the range between 22 and 158 N m‑1. The tunable electronic and mechanical properties of these 2D materials would advance the development of ultra-sensitive detectors, nanogenerators, low-power electronics, and energy harvesting and electromechanical systems.

Monolayer collapse regulating process of adsorption-desorption of palladium nanoparticles at fatty acid monolayers at the air-water interface.

PubMed

Goto, Thiago E; Lopez, Ricardo F; Iost, Rodrigo M; Crespilho, Frank N; Caseli, Luciano

2011-03-15

In this paper, we investigate the affinity of palladium nanoparticles, stabilized with glucose oxidase, for fatty acid monolayers at the air-water interface, exploiting the interaction between a planar system and spheroids coming from the aqueous subphase. A decrease of the monolayer collapse pressure in the second cycle of interface compression proved that the presence of the nanoparticles causes destabilization of the monolayer in a mechanism driven by the interpenetration of the enzyme into the bilayer/multilayer structure formed during collapse, which is not immediately reversible after monolayer expansion. Surface pressure and surface potential-area isotherms, as well as infrared spectroscopy [polarization modulation infrared reflection adsorption spectroscopy (PM-IRRAS)] and deposition onto solid plates as Langmuir-Blodgett (LB) films, were employed to construct a model in which the nanoparticle has a high affinity for the hydrophobic core of the structure formed after collapse, which provides a slow desorption rate from the interface after monolayer decompression. This may have important consequences on the interaction between the metallic particles and fatty acid monolayers, which implies the regulation of the multifunctional properties of the hybrid material.

Enhanced photoresponse of monolayer molybdenum disulfide (MoS2) based on microcavity structure

NASA Astrophysics Data System (ADS)

Lu, Yanan; Yang, Guofeng; Wang, Fuxue; Lu, Naiyan

2018-05-01

There is an increasing interest in using monolayer molybdenum disulfide (MoS2) for optoelectronic devices because of its inherent direct band gap characteristics. However, the weak absorption of monolayer MoS2 restricts its applications, novel concepts need to be developed to address the weakness. In this work, monolayer MoS2 monolithically integrates with plane microcavity structure, which is formed by the top and bottom chirped distributed Bragg reflector (DBR), is demonstrated to improve the absorption of MoS2. The optical absorption is 17-fold enhanced, reaching values over 70% at work wavelength. Moreover, the monolayer MoS2-based photodetector device with microcavity presents a significantly increased photoresponse, demonstrating its promising prospects in MoS2-based optoelectronic devices.

Tuning electrocatalytic activity of Pt monolayer shell by bimetallic Ir-M (M=Fe, Co, Ni or Cu) cores for the oxygen reduction reaction

DOE PAGES

Kuttiyiel, Kurian A.; Choi, YongMan; Sasaki, Kotaro; ...

2016-05-18

Here, platinum monolayer electrocatalyst are known to exhibit excellent oxygen reduction reaction (ORR) activity depending on the type of substrate used. Here we demonstrate a relationship between the ORR electrocatalytic activity and the surface electronic structure of Pt monolayer shell induced by various IrM bimetallic cores (M=Fe, Co, Ni or Cu). The relationship is rationalized by comparing density functional theory calculations and experimental results. For an efficient Pt monolayer electrocatalyst, the core should induce sufficient contraction to the Pt shell leading to a downshift of the d-band center with respect to the Fermi level. Depending on the structure of themore » IrM, relative to that of pure Ir, this interaction not only alters the electronic and geometric structure but also induces segregation effects. Combined these effects significantly enhance the ORR activities of the Pt monolayer shell on bimetallic Ir cores electrocatalysts.« less

Strain-mediated electronic properties of pristine and Mn-doped GaN monolayers

NASA Astrophysics Data System (ADS)

Sharma, Venus; Srivastava, Sunita

2018-04-01

Graphene-like two-dimensional (2D) monolayer structures GaN has gained enormous amount of interest due to high thermal stability and inherent energy band gap for practical applications. First principles calculations are performed to investigate the electronic structure and strain-mediated electronic properties of pristine and Mn-doped GaN monolayer. Binding energy of Mn dopant at various adsorption site is found to be nearly same indicating these sites to be equally favorable for adsorption of foreign atom. Depending on the adsorption site, GaN monolayer can act as p-type or n-type magnetic semiconductor. The tensile strength of both pristine and doped GaN monolayer (∼24 GPa) at ultimate tensile strain of 34% is comparable with the tensile strength of graphene. The in-plane biaxial strain modulate the energy band gap of both pristine and doped-monolayer from direct to indirect gap semiconductor and finally retendered theme into metal at critical value of applied strain. These characteristics make GaN monolayer to be potential candidate for the future applications in tunable optoelectronics.

Maxwell displacement current allows to study structural changes of gramicidin A in monolayers at the air-water interface.

PubMed

Vitovic, Pavol; Weis, Martin; Tomcík, Pavol; Cirák, Július; Hianik, Tibor

2007-05-01

We applied methods of measurement Maxwell displacement current (MDC) pressure-area isotherms and dipole potential for analysis of the properties of gramicidin A (gA) and mixed gA/DMPC monolayers at an air-water interface. The MDC method allowed us to observe the kinetics of formation of secondary structure of gA in monolayers at an air-water interface. We showed, that secondary structure starts to form at rather low area per molecule at which gA monolayers are in gaseous state. Changes of the MDC during compression can be attributed to the reorientation of dipole moments in a gA double helix at area 7 nm(2)/molecule, followed by the formation of intertwined double helix of gA. The properties of gA in mixed monolayers depend on the molar fraction of gA/DMPC. At higher molar fractions of gA (around 0.5) the shape of the changes of dipole moment of mixed monolayer was similar to that for pure gA. The analysis of excess free energy in a gel (18( ) degrees C) and in a liquid-crystalline phase (28( ) degrees C) allowed us to show influence of the monolayer structural state on the interaction between gA and the phospholipids. In a gel state and at the gA/DMPC molar ratio below 0.17 the aggregates of gA were formed, while above this molar ratio gA interacts favorably with DMPC. In contrast, for DMPC in a liquid-crystalline state aggregation of gA was observed for all molar fractions studied. The effect of formation ordered structures between gA and DMPC is more pronounced at low temperatures.

Multifunctional Binary Monolayers Ge xP y: Tunable Band Gap, Ferromagnetism, and Photocatalyst for Water Splitting.

PubMed

Li, Pengfei; Zhang, Wei; Li, Dongdong; Liang, Changhao; Zeng, Xiao Cheng

2018-06-04

The most stable structures of two-dimensional Ge x P y and Ge x As y monolayers with different stoichiometries (e.g., GeP, GeP 2 , and GeP 3 ) are explored systematically through the combination of the particle-swarm optimization technique and density functional theory optimization. For GeP 3 , we show that the newly predicted most stable C2/ m structure is 0.16 eV/atom lower in energy than the state-of-the-art P3̅m1 structure reported previously ( Nano Lett. 2017, 17, 1833). The computed electronic band structures suggest that all the stable and metastable monolayers of Ge x P y are semiconductors with highly tunable band gaps under the biaxial strain, allowing strain engineering of their band gaps within nearly the whole visible-light range. More interestingly, the hole doping can convert the C2/ m GeP 3 monolayer from nonmagnetic to ferromagnetic because of its unique valence band structure. For the GeP 2 monolayer, the predicted most stable Pmc2 1 structure is a (quasi) direct-gap semiconductor that possesses a high electron mobility of ∼800 cm 2 V -1 s -1 along the k a direction, which is much higher than that of MoS 2 (∼200 cm 2 V -1 s -1 ). More importantly, the Pmc2 1 GeP 2 monolayer not only can serve as an n-type channel material in field-effect transistors but also can be an effective catalyst for splitting water.

Well-ordered structure of methylene blue monolayers on Au(111) surface: electrochemical scanning tunneling microscopy studies.

PubMed

Song, Yonghai; Wang, Li

2009-02-01

Well-ordered structure of methylene blue (MB) monolayers on Au(111) surface has been successfully obtained by controlling the substrate potential. Electrochemical scanning tunneling microscopy (ECSTM) examined the monolayers of MB on Au(111) in 0.1 M HClO(4) and showed long-range ordered, interweaved arrays of MB with quadratic symmetry on the substrate in the potential range of double-layer charging. High-resolution ECSTM image further revealed the details of the MB monolayers structure of c(5 x 5 radical 3)rect and the flat-lying orientation of ad-molecules. The dependence of molecular organization on the substrate potential and the formation mechanism of well-ordered structure on Au(111) surface were investigated in detail. The obtained well-ordered structure at the interface between a metal and an aqueous electrolyte might possibly be used as high-density device for signal memory and templates for the advanced nanopatterning of surfaces. (c) 2008 Wiley-Liss, Inc.

A DMPA Langmuir monolayer study: from gas to solid phase. An atomistic description by molecular dynamics Simulation.

PubMed

Giner-Casares, J J; Camacho, L; Martín-Romero, M T; Cascales, J J López

2008-03-04

In this work, a DMPA Langmuir monolayer at the air/water interface was studied by molecular dynamics simulations. Thus, an atomistic picture of a Langmuir monolayer was drawn from its expanded gas phase to its final solid condensed one. In this sense, some properties of monolayers that were traditionally poorly or even not reproduced in computer simulations, such as lipid domain formation or pressure-area per lipid isotherm, were properly reproduced in this work. Thus, the physical laws that control the lipid domain formation in the gas phase and the structure of lipid monolayers from the gas to solid condensed phase were studied. Thanks to the atomistic information provided by the molecular dynamics simulations, we were able to add valuable information to the experimental description of these processes and to access experimental data related to the lipid monolayers in their expanded phase, which is difficult or inaccessible to study by experimental techniques. In this sense, properties such as lipids head hydration and lipid structure were studied.

Semi-metallic Be5C2 monolayer global minimum with quasi-planar pentacoordinate carbons and negative Poisson's ratio.

PubMed

Wang, Yu; Li, Feng; Li, Yafei; Chen, Zhongfang

2016-05-03

Designing new materials with novel topological properties and reduced dimensionality is always desirable for material innovation. Here we report the design of a two-dimensional material, namely Be5C2 monolayer on the basis of density functional theory computations. In Be5C2 monolayer, each carbon atom binds with five beryllium atoms in almost the same plane, forming a quasi-planar pentacoordinate carbon moiety. Be5C2 monolayer appears to have good stability as revealed by its moderate cohesive energy, positive phonon modes and high melting point. It is the lowest-energy structure with the Be5C2 stoichiometry in two-dimensional space and therefore holds some promise to be realized experimentally. Be5C2 monolayer is a gapless semiconductor with a Dirac-like point in the band structure and also has an unusual negative Poisson's ratio. If synthesized, Be5C2 monolayer may find applications in electronics and mechanics.

Monolayer PtSe₂, a New Semiconducting Transition-Metal-Dichalcogenide, Epitaxially Grown by Direct Selenization of Pt.

PubMed

Wang, Yeliang; Li, Linfei; Yao, Wei; Song, Shiru; Sun, J T; Pan, Jinbo; Ren, Xiao; Li, Chen; Okunishi, Eiji; Wang, Yu-Qi; Wang, Eryin; Shao, Yan; Zhang, Y Y; Yang, Hai-tao; Schwier, Eike F; Iwasawa, Hideaki; Shimada, Kenya; Taniguchi, Masaki; Cheng, Zhaohua; Zhou, Shuyun; Du, Shixuan; Pennycook, Stephen J; Pantelides, Sokrates T; Gao, Hong-Jun

2015-06-10

Single-layer transition-metal dichalcogenides (TMDs) receive significant attention due to their intriguing physical properties for both fundamental research and potential applications in electronics, optoelectronics, spintronics, catalysis, and so on. Here, we demonstrate the epitaxial growth of high-quality single-crystal, monolayer platinum diselenide (PtSe2), a new member of the layered TMDs family, by a single step of direct selenization of a Pt(111) substrate. A combination of atomic-resolution experimental characterizations and first-principle theoretic calculations reveals the atomic structure of the monolayer PtSe2/Pt(111). Angle-resolved photoemission spectroscopy measurements confirm for the first time the semiconducting electronic structure of monolayer PtSe2 (in contrast to its semimetallic bulk counterpart). The photocatalytic activity of monolayer PtSe2 film is evaluated by a methylene-blue photodegradation experiment, demonstrating its practical application as a promising photocatalyst. Moreover, circular polarization calculations predict that monolayer PtSe2 has also potential applications in valleytronics.

Unique Zigzag-Shaped Buckling Zn2C Monolayer with Strain-Tunable Band Gap and Negative Poisson Ratio.

PubMed

Meng, Lingbiao; Zhang, Yingjuan; Zhou, Minjie; Zhang, Jicheng; Zhou, Xiuwen; Ni, Shuang; Wu, Weidong

2018-02-19

Designing new materials with reduced dimensionality and distinguished properties has continuously attracted intense interest for materials innovation. Here we report a novel two-dimensional (2D) Zn 2 C monolayer nanomaterial with exceptional structure and properties by means of first-principles calculations. This new Zn 2 C monolayer is composed of quasi-tetrahedral tetracoordinate carbon and quasi-linear bicoordinate zinc, featuring a peculiar zigzag-shaped buckling configuration. The unique coordinate topology endows this natural 2D semiconducting monolayer with strongly strain tunable band gap and unusual negative Poisson ratios. The monolayer has good dynamic and thermal stabilities and is also the lowest-energy structure of 2D space indicated by the particle-swarm optimization (PSO) method, implying its synthetic feasibility. With these intriguing properties the material may find applications in nanoelectronics and micromechanics.

Biaxial tensile strain tuned up-and-down behavior on lattice thermal conductivity in β-AsP monolayer

NASA Astrophysics Data System (ADS)

Guo, San-Dong; Dong, Jun

2018-07-01

Various two-dimensional (2D) materials with a graphene-like buckled structure have emerged, and the β-phase AsP monolayer has been recently proposed to be thermodynamically stable from first-principles calculations. The studies of thermal transport are very useful for these 2D materials-based nano-electronics devices. Motivated by this, a comparative study of strain-dependent phonon transport of AsP monolayers is performed by solving the linearized phonon Boltzmann equation within the single-mode relaxation time approximation (RTA). It is found that the lattice thermal conductivity () of the AsP monolayer is very close to the one of As monolayer with a similar buckled structure, which is due to neutralization between the reduction of phonon lifetimes and group velocity enhancement from As to AsP monolayer. The corresponding room-temperature sheet thermal conductance of AsP monolayer is 152.5 . It is noted that the increasing tensile strain can harden a long wavelength out-of-plane (ZA) acoustic mode, and soften the in-plane longitudinal acoustic (LA) and transversal acoustic (TA) modes. Calculated results show that of AsP monolayer presents a nonmonotonic up-and-down behavior with increased strain. The unusual strain dependence is due to the competition among the reduction of phonon group velocities, improved phonon lifetimes of ZA mode and nonmonotonic up-and-down phonon lifetimes of TA/LA mode. It is found that acoustic branches dominate the in the considered strain range, and the contribution from ZA branch increases with increased strain, while it is opposite for TA/LA branch. By analyzing cumulative with respect to phonon mean free path, tensile strain can modulate effectively the size effects on in the AsP monolayer. Our work enriches the studies of thermal transports of 2D materials with graphene-like buckled structures, and strengthens the idea to engineer thermal transport properties by simple mechanical strain, and stimulates further experimental works to synthesize AsP monolayers.

Molecular dynamics simulation of the evolution of hydrophobic defects in one monolayer of a phosphatidylcholine bilayer: relevance for membrane fusion mechanisms.

PubMed Central

Tieleman, D Peter; Bentz, Joe

2002-01-01

The spontaneous formation of the phospholipid bilayer underlies the permeability barrier function of the biological membrane. Tears or defects that expose water to the acyl chains are spontaneously healed by lipid lateral diffusion. However, mechanical barriers, e.g., protein aggregates held in place, could sustain hydrophobic defects. Such defects have been postulated to occur in processes such as membrane fusion. This gives rise to a new question in bilayer structure: What do the lipids do in the absence of lipid lateral diffusion to minimize the free energy of a hydrophobic defect? As a first step to understand this rather fundamental question about bilayer structure, we performed molecular dynamic simulations of up to 10 ns of a planar bilayer from which lipids have been deleted randomly from one monolayer. In one set of simulations, approximately one-half of the lipids in the defect monolayer were restrained to form a mechanical barrier. In the second set, lipids were free to diffuse around. The question was simply whether the defects caused by removing a lipid would aggregate together, forming a large hydrophobic cavity, or whether the membrane would adjust in another way. When there are no mechanical barriers, the lipids in the defect monolayer simply spread out and thin with little effect on the other intact monolayer. In the presence of a mechanical barrier, the behavior of the lipids depends on the size of the defect. When 3 of 64 lipids are removed, the remaining lipids adjust the lower one-half of their chains, but the headgroup structure changes little and the intact monolayer is unaffected. When 6 to 12 lipids are removed, the defect monolayer thins, lipid disorder increases, and lipids from the intact monolayer move toward the defect monolayer. Whereas this is a highly simplified model of a fusion site, this engagement of the intact monolayer into the fusion defect is strikingly consistent with recent results for influenza hemagglutinin mediated fusion. PMID:12202375

Room Temperature Intrinsic Ferromagnetism in Epitaxial Manganese Selenide Films in the Monolayer Limit

NASA Astrophysics Data System (ADS)

O'Hara, Dante J.; Zhu, Tiancong; Trout, Amanda H.; Ahmed, Adam S.; Luo, Yunqiu Kelly; Lee, Choong Hee; Brenner, Mark R.; Rajan, Siddharth; Gupta, Jay A.; McComb, David W.; Kawakami, Roland K.

2018-05-01

Monolayer van der Waals (vdW) magnets provide an exciting opportunity for exploring two-dimensional (2D) magnetism for scientific and technological advances, but the intrinsic ferromagnetism has only been observed at low temperatures. Here, we report the observation of room temperature ferromagnetism in manganese selenide (MnSe$_x$) films grown by molecular beam epitaxy (MBE). Magnetic and structural characterization provides strong evidence that in the monolayer limit, the ferromagnetism originates from a vdW manganese diselenide (MnSe$_2$) monolayer, while for thicker films it could originate from a combination of vdW MnSe$_2$ and/or interfacial magnetism of $\\alpha$-MnSe(111). Magnetization measurements of monolayer MnSe$_x$ films on GaSe and SnSe$_2$ epilayers show ferromagnetic ordering with large saturation magnetization of ~ 4 Bohr magnetons per Mn, which is consistent with density functional theory calculations predicting ferromagnetism in monolayer 1T-MnSe$_2$. Growing MnSe$_x$ films on GaSe up to high thickness (~ 40 nm) produces $\\alpha$-MnSe(111), and an enhanced magnetic moment (~ 2x) compared to the monolayer MnSe$_x$ samples. Detailed structural characterization by scanning transmission electron microscopy (STEM), scanning tunneling microscopy (STM), and reflection high energy electron diffraction (RHEED) reveal an abrupt and clean interface between GaSe(0001) and $\\alpha$-MnSe(111). In particular, the structure measured by STEM is consistent with the presence of a MnSe$_2$ monolayer at the interface. These results hold promise for potential applications in energy efficient information storage and processing.

Surface chemistry of liquid metals

NASA Technical Reports Server (NTRS)

Mann, J. Adin, Jr.; Peebles, Henry; Peebles, Diamond; Rye, Robert; Yost, Fred

1993-01-01

The fundamental surface chemistry of the behavior of liquid metals spreading on a solid substrate is not at all well understood. Each of these questions involves knowing the details of the structure of interfaces and their dynamics. For example the structure of a monolayer of tin oxide on pure liquid tin is unknown. This is in contrast to the relatively large amount of data available on the structure of copper oxide monolayers on solid, pure copper. However, since liquid tin has a vapor pressure below 10(exp -10)torr for a reasonable temperature range above its melting point, it is possible to use the techniques of surface science to study the geometric, electronic and vibrational structures of these monolayers. In addition, certain techniques developed by surface chemists for the study of liquid systems can be applied to the ultra-high vacuum environment. In particular we have shown that light scattering spectroscopy can be used to study the surface tension tensor of these interfaces. The tin oxide layer in particular is very interesting in that the monolayer is rigid but admits of bending. Ellipsometric microscopy allows the visualization of monolayer thick films and show whether island formation occurs at various levels of dosing.

Liquid-Phase Exfoliation into Monolayered BiOBr Nanosheets for Photocatalytic Oxidation and Reduction

DOE Office of Scientific and Technical Information (OSTI.GOV)

Yu, Hongjian; Huang, Hongwei; Xu, Kang

2017-09-26

Monolayered photocatalytic materials have attracted huge research interests in terms of their large specific surface area and ample active sites. Sillén-structured layered BiOX (X = Cl, Br, I) casts great prospects owing to their strong photo-oxidation ability and high stability. Fabrication of monolayered BiOX by a facile, low-cost, and scalable approach is highly challenging and anticipated. Herein, we describe the large-scale preparation of monolayered BiOBr nanosheets with a thickness of ~0.85 nm via a readily achievable liquid-phase exfoliation strategy with assistance of formamide at ambient conditions. The as-obtained monolayered BiOBr nanosheets are allowed diverse superiorities, such as enhanced specific surfacemore » area, promoted band structure, and strengthened charge separation. Profiting from these benefits, the advanced BiOBr monolayers not only show excellent adsorption and photodegradation performance for treating contaminants, but also demonstrate a greatly promoted photocatalytic activity for CO2 reduction into CO and CH4. Additionally, monolayered BiOI nanosheets have also been obtained by the same synthetic approach. Our work offers a mild and general approach for preparation of monolayered BiOX, and may have huge potential to be extended to the synthesis of other single-layer two-dimensional materials.« less

THz emission of donor and acceptor doped GaAs/AlGaAs quantum well structures with inserted thin AlAs monolayer

NASA Astrophysics Data System (ADS)

van Dommelen, Paphavee; Daengngam, Chalongrat; Kalasuwan, Pruet

2018-04-01

In this paper, we explore THz range optical intersubband transition energies in a donor doped quantum well of a GaAs/AlGaAs system as a function of the insertion position of an AlAs monolayer in the GaAs quantum well. In simulated models, the optical transition energies between electron subband levels 1 and 2 were higher in the doped structure than in the undoped structure. This may be because the envelope wave function of the second electron subband strongly overlapped the envelope wave function of the first electron subband and influenced the optical intersubband transition between the two levels in the THz range. At different levels of bias voltage at the Schottky barrier on the donor doped structure, the electric field in the growth direction of the structure linearly increased the further away the AlAs monolayer was placed from the reference position. We also simulated the optical transition energies between acceptor energy levels of the acceptor doped structure as a function of the insertion position of the AlAs monolayer. The acceptor doped structure induced THz range emission whereas the undoped structure induced mid-IR emission.

Lithium halide monolayers: Structural, electronic and optical properties by first principles study

NASA Astrophysics Data System (ADS)

Safari, Mandana; Maskaneh, Pegah; Moghadam, Atousa Dashti; Jalilian, Jaafar

2016-09-01

Using first principle study, we investigate the structural, electronic and optical properties of lithium halide monolayers (LiF, LiCl, LiBr). In contrast to graphene and other graphene-like structures that form hexagonal rings in plane, these compounds can form and stabilize in cubic shape interestingly. The type of band structure in these insulators is identified as indirect type and ionic nature of their bonds are illustrated as well. The optical properties demonstrate extremely transparent feature for them as a result of wide band gap in the visible range; also their electron transitions are indicated for achieving a better vision on the absorption mechanism in these kinds of monolayers.

Theoretical Prediction of an Antimony-Silicon Monolayer (penta-Sb2Si): Band Gap Engineering by Strain Effect

NASA Astrophysics Data System (ADS)

Morshedi, Hosein; Naseri, Mosayeb; Hantehzadeh, Mohammad Reza; Elahi, Seyed Mohammad

2018-04-01

In this paper, using a first principles calculation, a two-dimensional structure of silicon-antimony named penta-Sb2Si is predicted. The structural, kinetic, and thermal stabilities of the predicted monolayer are confirmed by the cohesive energy calculation, phonon dispersion analysis, and first principles molecular dynamic simulation, respectively. The electronic properties investigation shows that the pentagonal Sb2Si monolayer is a semiconductor with an indirect band gap of about 1.53 eV (2.1 eV) from GGA-PBE (PBE0 hybrid functional) calculations which can be effectively engineered by employing external biaxial compressive and tensile strain. Furthermore, the optical characteristics calculation indicates that the predicted monolayer has considerable optical absorption and reflectivity in the ultraviolet region. The results suggest that a Sb2Si monolayer has very good potential applications in new nano-optoelectronic devices.

Bidisperse silica nanoparticles close-packed monolayer on silicon substrate by three step spin method

NASA Astrophysics Data System (ADS)

Khanna, Sakshum; Marathey, Priyanka; Utsav, Chaliawala, Harsh; Mukhopadhyay, Indrajit

2018-05-01

We present the studies on the structural properties of monolayer Bidisperse silica (SiO2) nanoparticles (BDS) on Silicon (Si-100) substrate using spin coating technique. The Bidisperse silica nanoparticle was synthesised by the modified sol-gel process. Nanoparticles on the substrate are generally assembled in non-close/close-packed monolayer (CPM) form. The CPM form is obtained by depositing the colloidal suspension onto the silicon substrate using complex techniques. Here we report an effective method for forming a monolayer of bidisperse silica nanoparticle by three step spin coating technique. The samples were prepared by mixing the monodisperse solutions of different particles size 40 and 100 nm diameters. The bidisperse silica nanoparticles were self-assembled on the silicon substrate forming a close-packed monolayer film. The scanning electron microscope images of bidisperse films provided in-depth film structure of the film. The maximum surface coverage obtained was around 70-80%.

Supramolecular domains in mixed peptide self-assembled monolayers on gold nanoparticles.

PubMed

Duchesne, Laurence; Wells, Geoff; Fernig, David G; Harris, Sarah A; Lévy, Raphaël

2008-09-01

Self-organization in mixed self-assembled monolayers of small molecules provides a route towards nanoparticles with complex molecular structures. Inspired by structural biology, a strategy based on chemical cross-linking is introduced to probe proximity between functional peptides embedded in a mixed self-assembled monolayer at the surface of a nanoparticle. The physical basis of the proximity measurement is a transition from intramolecular to intermolecular cross-linking as the functional peptides get closer. Experimental investigations of a binary peptide self-assembled monolayer show that this transition happens at an extremely low molar ratio of the functional versus matrix peptide. Molecular dynamics simulations of the peptide self-assembled monolayer are used to calculate the volume explored by the reactive groups. Comparison of the experimental results with a probabilistic model demonstrates that the peptides are not randomly distributed at the surface of the nanoparticle, but rather self-organize into supramolecular domains.

Colloidal membranes: The rich confluence of geometry and liquid crystals

NASA Astrophysics Data System (ADS)

Kaplan, Cihan Nadir

A simple and experimentally realizable model system of chiral symmetry breaking is liquid-crystalline monolayers of aligned, identical hard rods. In these materials, tuning the chirality at the molecular level affects the geometry at systems level, thereby inducing a myriad of morphological transitions. This thesis presents theoretical studies motivated by the rich phenomenology of these colloidal monolayers. High molecular chirality leads to assemblages of rods exhibiting macroscopic handedness. In the first part we consider one such geometry, twisted ribbons, which are minimal surfaces to a double helix. By employing a theoretical approach that combines liquid-crystalline order with the preferred shape, we focus on the phase transition from simple flat monolayers to these twisted structures. In these monolayers, regions of broken chiral symmetry nucleate at the interfaces, as in a chiral smectic A sample. The second part particularly focuses on the detailed structure and thermodynamic stability of two types of observed interfaces, the monolayer edge and domain walls in simple flat monolayers. Both the edge and "twist-walls" are quasi-one-dimensional bands of molecular twist deformations dictated by local chiral interactions and surface energy considerations. We develop a unified theory of these interfaces by utilizing the de Gennes framework accompanied by appropriate surface energy terms. The last part turns to colloidal "cookies", which form in mixtures of rods with opposite handedness. These elegant structures are essentially flat monolayers surrounded by an array of local, three dimensional cusp defects. We reveal the thermodynamic and structural characteristics of cookies. Furthermore, cookies provide us with a simple relation to determine the intrinsic curvature modulus of our model system, an important constant associated with topological properties of membranes. Our results may have impacts on a broader class of soft thin films.

Analysis of the induction of the myelin basic protein binding to the plasma membrane phospholipid monolayer

NASA Astrophysics Data System (ADS)

Zhang, Lei; Hao, Changchun; Feng, Ying; Gao, Feng; Lu, Xiaolong; Li, Junhua; Sun, Runguang

2016-09-01

Myelin basic protein (MBP) is an essential structure involved in the generation of central nervous system (CNS) myelin. Myelin shape has been described as liquid crystal structure of biological membrane. The interactions of MBP with monolayers of different lipid compositions are responsible for the multi-lamellar structure and stability of myelin. In this paper, we have designed MBP-incorporated model lipid monolayers and studied the phase behavior of MBP adsorbed on the plasma membrane at the air/water interface by thermodynamic method and atomic force microscopy (AFM). By analyzing the pressure-area (π-A) and pressure-time (π-T) isotherms, univariate linear regression equation was obtained. In addition, the elastic modulus, surface pressure increase, maximal insertion pressure, and synergy factor of monolayers were detected. These parameters can be used to modulate the monolayers binding of protein, and the results show that MBP has the strongest affinity for 1,2-dipalmitoyl-sn-glycero-3- phosphoserine (DPPS) monolayer, followed by DPPC/DPPS mixed and 1,2-dipalmitoyl-sn-glycero-3-phospho-choline (DPPC) monolayers via electrostatic and hydrophobic interactions. AFM images of DPPS and DPPC/DPPS mixed monolayers in the presence of MBP (5 nM) show a phase separation texture at the surface pressure of 20 mN/m and the incorporation of MBP put into the DPPC monolayers has exerted a significant effect on the domain structure. MBP is not an integral membrane protein but, due to its positive charge, interacts with the lipid head groups and stabilizes the membranes. The interaction between MBP and phospholipid membrane to determine the nervous system of the disease has a good biophysical significance and medical value. Project supported by the National Natural Science Foundation of China (Grant Nos. 21402114 and 11544009), the Natural Science Basic Research Plan in Shaanxi Province of China (Grant No. 2016JM2010), the Fundamental Research Funds for the Central Universities of China (Grant No. GK201604004), and the National University Science and Technology Innovation Project of China (Grant Nos. 201610718014 and cx16018).

Janus monolayers of transition metal dichalcogenides.

PubMed

Lu, Ang-Yu; Zhu, Hanyu; Xiao, Jun; Chuu, Chih-Piao; Han, Yimo; Chiu, Ming-Hui; Cheng, Chia-Chin; Yang, Chih-Wen; Wei, Kung-Hwa; Yang, Yiming; Wang, Yuan; Sokaras, Dimosthenis; Nordlund, Dennis; Yang, Peidong; Muller, David A; Chou, Mei-Yin; Zhang, Xiang; Li, Lain-Jong

2017-08-01

Structural symmetry-breaking plays a crucial role in determining the electronic band structures of two-dimensional materials. Tremendous efforts have been devoted to breaking the in-plane symmetry of graphene with electric fields on AB-stacked bilayers or stacked van der Waals heterostructures. In contrast, transition metal dichalcogenide monolayers are semiconductors with intrinsic in-plane asymmetry, leading to direct electronic bandgaps, distinctive optical properties and great potential in optoelectronics. Apart from their in-plane inversion asymmetry, an additional degree of freedom allowing spin manipulation can be induced by breaking the out-of-plane mirror symmetry with external electric fields or, as theoretically proposed, with an asymmetric out-of-plane structural configuration. Here, we report a synthetic strategy to grow Janus monolayers of transition metal dichalcogenides breaking the out-of-plane structural symmetry. In particular, based on a MoS 2 monolayer, we fully replace the top-layer S with Se atoms. We confirm the Janus structure of MoSSe directly by means of scanning transmission electron microscopy and energy-dependent X-ray photoelectron spectroscopy, and prove the existence of vertical dipoles by second harmonic generation and piezoresponse force microscopy measurements.

Selectively Plasmon-Enhanced Second-Harmonic Generation from Monolayer Tungsten Diselenide on Flexible Substrates.

PubMed

Wang, Zhuo; Dong, Zhaogang; Zhu, Hai; Jin, Lei; Chiu, Ming-Hui; Li, Lain-Jong; Xu, Qing-Hua; Eda, Goki; Maier, Stefan A; Wee, Andrew T S; Qiu, Cheng-Wei; Yang, Joel K W

2018-02-27

Monolayer two-dimensional transition-metal dichalcogenides (2D TMDCs) exhibit promising characteristics in miniaturized nonlinear optical frequency converters, due to their inversion asymmetry and large second-order nonlinear susceptibility. However, these materials usually have very short light interaction lengths with the pump laser because they are atomically thin, such that second-harmonic generation (SHG) is generally inefficient. In this paper, we fabricate a judiciously structured 150 nm-thick planar surface consisting of monolayer tungsten diselenide and sub-20 nm-wide gold trenches on flexible substrates, reporting ∼7000-fold SHG enhancement without peak broadening or background in the spectra as compared to WSe 2 on as-grown sapphire substrates. Our proof-of-concept experiment yields effective second-order nonlinear susceptibility of 2.1 × 10 4 pm/V. Three orders of magnitude enhancement is maintained with pump wavelength ranging from 800 to 900 nm, breaking the limitation of narrow pump wavelength range for cavity-enhanced SHG. In addition, SHG amplitude can be dynamically controlled via selective excitation of the lateral gap plasmon by rotating the laser polarization. Such a fully open, flat, and ultrathin profile enables a great variety of functional samples with high SHG from one patterned silicon substrate, favoring scalable production of nonlinear converters. The surface accessibility also enables integration with other optical components for information processing in an ultrathin and flexible form.

Nonreciprocal optical isolation via graphene based photonic crystals

NASA Astrophysics Data System (ADS)

Roshan Entezar, S.; Karimi Habil, M.

2018-03-01

The transmission properties of a one-dimensional photonic crystal containing graphene mono-layers are studied using the transfer matrix method. It is shown that the structure can be used as a polarization-selective nonreciprocal device which discriminates between the two circularly polarized waves with different handedness impinging in the same direction. This structure may be utilized in designing optical isolators for the circularly polarized waves due to the gyrotropic behavior of the graphene mono-layers under the perpendicularly applied external magnetic field. Moreover, the effect of an external magnetic field and the chemical potential of the graphene mono-layers on the band gap of the structure are investigated.

How Correlated is the FeSe /SrTiO3 System?

NASA Astrophysics Data System (ADS)

Mandal, Subhasish; Zhang, Peng; Ismail-Beigi, Sohrab; Haule, K.

2017-08-01

Recent observation of ˜10 times higher critical temperature in a FeSe monolayer compared with its bulk phase has drawn a great deal of attention because the electronic structure in the monolayer phase appears to be different than bulk FeSe. Using a combination of density functional theory and dynamical mean field theory, we find electronic correlations have important effects on the predicted atomic-scale geometry and the electronic structure of the monolayer FeSe on SrTiO3 . The electronic correlations are dominantly controlled by the Se-Fe-Se angle either in the bulk phase or the monolayer phase. But the angle sensitivity increases and the orbital differentiation decreases in the monolayer phase compared to the bulk phase. The correlations are more dependent on Hund's J than Hubbard U . The observed orbital selective incoherence to coherence crossover with temperature confirms the Hund's metallic nature of the monolayer FeSe. We also find electron doping by oxygen vacancies in SrTiO3 increases the correlation strength, especially in the dx y orbital by reducing the Se-Fe-Se angle.

X-Ray Reflectometry of DMPS Monolayers on a Water Substrate

NASA Astrophysics Data System (ADS)

Tikhonov, A. M.; Asadchikov, V. E.; Volkov, Yu. O.; Roshchin, B. S.; Ermakov, Yu. A.

2017-12-01

The molecular structure of dimyristoyl phosphatidylserine (DMPS) monolayers on a water substrate in different phase states has been investigated by X-ray reflectometry with a photon energy of 8 keV. According to the experimental data, the transition from a two-dimensional expanded liquid state to a solid gel state (liquid crystal) accompanied by the ordering of the hydrocarbon tails C14H27 of the DMPS molecule occurs in the monolayer as the surface pressure rises. The monolayer thickness is 20 ± 3 and 28 ± 2 Å in the liquid and solid phases, respectively, with the deflection angle of the molecular tail axis from the normal to the surface in the gel phase being 26° ± 8°. At least a twofold decrease in the degree of hydration of the polar lipid groups also occurs under two-dimensional monolayer compression. The reflectometry data have been analyzed using two approaches: under the assumption about the presence of two layers with different electron densities in the monolayer and without any assumptions about the transverse surface structure. Both approaches demonstrate satisfactory agreement between themselves in describing the experimental results.

InSe monolayer: synthesis, structure and ultra-high second-harmonic generation

NASA Astrophysics Data System (ADS)

Zhou, Jiadong; Shi, Jia; Zeng, Qingsheng; Chen, Yu; Niu, Lin; Liu, Fucai; Yu, Ting; Suenaga, Kazu; Liu, Xinfeng; Lin, Junhao; Liu, Zheng

2018-04-01

III–IV layered materials such as indium selenide have excellent photoelectronic properties. However, synthesis of materials in such group, especially with a controlled thickness down to monolayer, still remains challenging. Herein, we demonstrate the successful synthesis of monolayer InSe by physical vapor deposition (PVD) method. The high quality of the sample was confirmed by complementary characterization techniques such as Raman spectroscopy, atomic force microscopy (AFM) and high resolution annular dark field scanning transmission electron microscopy (ADF-STEM). We found the co-existence of different stacking sequence (β- and γ-InSe) in the same flake with a sharp grain boundary in few-layered InSe. Edge reconstruction is also observed in monolayer InSe, which has a distinct atomic structure from the bulk lattice. Moreover, we discovered that the second-harmonic generation (SHG) signal from monolayer InSe shows large optical second-order susceptibility that is 1–2 orders of magnitude higher than MoS2, and even 3 times of the largest value reported in monolayer GaSe. These results make atom-thin InSe a promising candidate for optoelectronic and photosensitive device applications.

Thermal stability and molecular ordering of organic semiconductor monolayers: effect of an anchor group.

PubMed

Jones, Andrew O F; Knauer, Philipp; Resel, Roland; Ringk, Andreas; Strohriegl, Peter; Werzer, Oliver; Sferrazza, Michele

2015-06-08

The thermal stability and molecular order in monolayers of two organic semiconductors, PBI-PA and PBI-alkyl, based on perylene derivatives with an identical molecular structure except for an anchor group for attachment to the substrate in PBI-PA, are reported. In situ X-ray reflectivity measurements are used to follow the stability of these monolayers in terms of order and thickness as temperature is increased. Films have thicknesses corresponding approximately to the length of one molecule; molecules stand upright on the substrate with a defined structure. PBI-PA monolayers have a high degree of order at room temperature and a stable film exists up to 250 °C, but decomposes rapidly above 300 °C. In contrast, stable physisorbed PBI-alkyl monolayers only exist up to 100 °C. Above the bulk melting point at 200 °C no more order exists. The results encourage using anchor groups in monolayers for various applications as it allows enhanced stability at the interface with the substrate. © 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

Monolayer PtSe 2 , a New Semiconducting Transition-Metal-Dichalcogenide, Epitaxially Grown by Direct Selenization of Pt

DOE PAGES

Wang, Yeliang; Li, Linfei; Yao, Wei; ...

2015-05-21

For single-layer transition-metal dichalcogenides (TMDs) receive significant attention due to their intriguing physical properties for both fundamental research and potential applications in electronics, optoelectronics, spintronics, catalysis, and so on. Here, we demonstrate the epitaxial growth of high-quality single-crystal, monolayer platinum diselenide (PtSe2), a new member of the layered TMDs family, by a single step of direct selenization of a Pt(111) substrate. We found that a combination of atomic-resolution experimental characterizations and first-principle theoretic calculations reveals the atomic structure of the monolayer PtSe2/Pt(111). Angle-resolved photoemission spectroscopy measurements confirm for the first time the semiconducting electronic structure of monolayer PtSe2 (in contrastmore » to its semimetallic bulk counterpart). The photocatalytic activity of monolayer PtSe2 film is evaluated by a methylene-blue photodegradation experiment, demonstrating its practical application as a promising photocatalyst. Moreover, circular polarization calculations predict that monolayer PtSe2 has also potential applications in valleytronics.« less

Strain induced chemical potential difference between monolayer graphene sheets.

PubMed

Zhang, Yupeng; Luo, Chengzhi; Li, Weiping; Pan, Chunxu

2013-04-07

Monolayer graphene sheets were deposited on a transparent and flexible polydimethylsiloxane (PDMS) substrate, and a tensile strain was loaded by stretching the substrate in one direction. It was found that an electric potential difference between stretched and static monolayer graphene sheets reached 8 mV when the strain was 5%. Theoretical calculations for the band structure and total energy revealed an alternative way to experimentally tune the band gap of monolayer graphene, and induce the generation of electricity.

Structural and electronic properties of monolayer group III monochalcogenides

NASA Astrophysics Data System (ADS)

Demirci, S.; Avazlı, N.; Durgun, E.; Cahangirov, S.

2017-03-01

We investigate the structural, mechanical, and electronic properties of the two-dimensional hexagonal structure of group III-VI binary monolayers, M X (M =B , Al, Ga, In and X =O , S, Se, Te) using first-principles calculations based on the density functional theory. The structural optimization calculations and phonon spectrum analysis indicate that all of the 16 possible binary compounds are thermally stable. In-plane stiffness values cover a range depending on the element types and can be as high as that of graphene, while the calculated bending rigidity is found to be an order of magnitude higher than that of graphene. The obtained electronic band structures show that M X monolayers are indirect band-gap semiconductors. The calculated band gaps span a wide optical spectrum from deep ultraviolet to near infrared. The electronic structure of oxides (M O ) is different from the rest because of the high electronegativity of oxygen atoms. The dispersions of the electronic band edges and the nature of bonding between atoms can also be correlated with electronegativities of constituent elements. The unique characteristics of group III-VI binary monolayers can be suitable for high-performance device applications in nanoelectronics and optics.

Strain engineering of atomic and electronic structures of few-monolayer-thick GaN

NASA Astrophysics Data System (ADS)

Kolobov, A. V.; Fons, P.; Saito, Y.; Tominaga, J.; Hyot, B.; André, B.

2017-07-01

Two-dimensional (2D) semiconductors possess the potential to ultimately minimize the size of devices and concomitantly drastically reduce the corresponding energy consumption. In addition, materials in their atomic-scale limit often possess properties different from their bulk counterparts paving the way to conceptually novel devices. While graphene and 2D transition-metal dichalcogenides remain the most studied materials, significant interest also exists in the fabrication of atomically thin structures from traditionally 3D semiconductors such as GaN. While in the monolayer limit GaN possesses a graphenelike structure and an indirect band gap, it was recently demonstrated that few-layer GaN acquires a Haeckelite structure in the direction of growth with an effectively direct gap. In this work, we demonstrate the possibility of strain engineering of the atomic and electronic structure of few-monolayer-thick GaN structures, which opens new avenues for their practical application in flexible nanoelectronics and nano-optoelectronics. Our simulations further suggest that due to the weak van der Waals-like interaction between a substrate and an overlayer, the use of a MoS2 substrate may be a promising route to fabricate few-monolayer Haeckelite GaN experimentally.

Miscibility of binary monolayers at the air-water interface and interaction of protein with immobilized monolayers by surface plasmon resonance technique.

PubMed

Wang, Yuchun; Du, Xuezhong

2006-07-04

The miscibility and stability of the binary monolayers of zwitterionic dipalmitoylphosphatidylcholine (DPPC) and cationic dioctadecyldimethylammonium bromide (DOMA) at the air-water interface and the interaction of ferritin with the immobilized monolayers have been studied in detail using surface pressure-area isotherms and surface plasmon resonance technique, respectively. The surface pressure-area isotherms indicated that the binary monolayers of DPPC and DOMA at the air-water interface were miscible and more stable than the monolayers of the two individual components. The surface plasmon resonance studies indicated that ferritin binding to the immobilized monolayers was primarily driven by the electrostatic interaction and that the amount of adsorbed protein at saturation was closely related not only to the number of positive charges in the monolayers but also to the pattern of positive charges at a given mole fraction of DOMA. The protein adsorption kinetics was determined by the properties of the monolayers (i.e., the protein-monolayer interaction) and the structure of preadsorbed protein molecules (i.e., the protein-protein interaction).

Measuring mechanodynamics in an unsupported epithelial monolayer grown at an air–water interface

PubMed Central

Gullekson, Corinne; Walker, Matthew; Harden, James L.; Pelling, Andrew E.

2017-01-01

Actomyosin contraction and relaxation in a monolayer is a fundamental biophysical process in development and homeostasis. Current methods used to characterize the mechanodynamics of monolayers often involve cells grown on solid supports such as glass or gels. The results of these studies are fundamentally influenced by these supporting structures. Here we describe a new method for measuring the mechanodynamics of epithelial monolayers by culturing cells at an air–liquid interface. These model monolayers are grown in the absence of any supporting structures, removing cell–substrate effects. This method’s potential was evaluated by observing and quantifying the generation and release of internal stresses upon actomyosin contraction (800 ± 100 Pa) and relaxation (600 ± 100 Pa) in response to chemical treatments. Although unsupported monolayers exhibited clear major and minor strain axes, they were not correlated with nuclear alignment as observed when the monolayers were grown on soft deformable gels. It was also observed that both gels and glass substrates led to the promotion of long-range cell nuclei alignment not seen in the hanging-drop model. This new approach provides us with a picture of basal actomyosin mechanodynamics in a simplified system, allowing us to infer how the presence of a substrate affects contractility and long-range multicellular organization and dynamics. PMID:28035043

Origin of the instability of octadecylamine Langmuir monolayer at low pH

DOE PAGES

Avazbaeva, Zaure; Sung, Woongmo; Lee, Jonggwan; ...

2015-11-30

In this paper, it has been reported that an octadecylamine (ODA) Langmuir monolayer becomes unstable at low pH values with no measurable surface pressure at around pH 3.5, suggesting significant dissolution of the ODA molecule into the subphase solution (Albrecht, Colloids Surf. A 2006, 284–285, 166–174). However, by lowering the pH further, ODA molecules reoccupy the surface, and a full monolayer is recovered at pH 2.5. Using surface sum-frequency spectroscopy and pressure–area isotherms, it is found that the recovered monolayer at very low pH has a larger area per molecule with many gauche defects in the ODA molecules as comparedmore » to that at high pH values. This structural change suggests that the reappearance of the monolayer is due to the adsorbed Cl– counterions to the protonated amine groups, leading to partial charge neutralization. This proposition is confirmed by intentionally adding monovalent salts (i.e., NaCl, NaBr, or NaI) to the subphase to recover the monolayer at pH 3.5, in which the detailed structure of the monolayer is confirmed by sum frequency spectra and the adsorbed anions by X-ray reflectivity.« less

Layer-by-layer evolution of structure, strain, and activity for the oxygen evolution reaction in graphene-templated Pt monolayers.

PubMed

Abdelhafiz, Ali; Vitale, Adam; Joiner, Corey; Vogel, Eric; Alamgir, Faisal M

2015-03-25

In this study, we explore the dimensional aspect of structure-driven surface properties of metal monolayers grown on a graphene/Au template. Here, surface limited redox replacement (SLRR) is used to provide precise layer-by-layer growth of Pt monolayers on graphene. We find that after a few iterations of SLRR, fully wetted 4-5 monolayer Pt films can be grown on graphene. Incorporating graphene at the Pt-Au interface modifies the growth mechanism, charge transfers, equilibrium interatomic distances, and associated strain of the synthesized Pt monolayers. We find that a single layer of sandwiched graphene is able to induce a 3.5% compressive strain on the Pt adlayer grown on it, and as a result, catalytic activity is increased due to a greater areal density of the Pt layers beyond face-centered-cubic close packing. At the same time, the sandwiched graphene does not obstruct vicinity effects of near-surface electron exchange between the substrate Au and adlayers Pt. X-ray photoelectron spectroscopy (XPS) and extended X-ray absorption fine structure (EXAFS) techniques are used to examine charge mediation across the Pt-graphene-Au junction and the local atomic arrangement as a function of the Pt adlayer dimension. Cyclic voltammetry (CV) and the oxygen reduction reaction (ORR) are used as probes to examine the electrochemically active area of Pt monolayers and catalyst activity, respectively. Results show that the inserted graphene monolayer results in increased activity for the Pt due to a graphene-induced compressive strain, as well as a higher resistance against loss of the catalytically active Pt surface.

Structural and electronic properties of multilayer graphene on monolayer hexagonal boron nitride/nickel (111) interface system: A van der Waals density functional study

NASA Astrophysics Data System (ADS)

Yelgel, Celal

2016-02-01

The structural and electronic properties of multilayer graphene adsorbed on monolayer hexagonal boron nitride (h-BN)/Ni(111) interface system are investigated using the density functional theory with a recently developed non-local van der Waals density functional (rvv10). The most energetically favourable configuration for a monolayer h-BN/Ni(111) interface is found to be N atom atop the Ni atoms and B atom in fcc site with the interlayer distance of 2.04 Å and adsorption energy of 302 meV/BN. Our results show that increasing graphene layers on a monolayer h-BN/Ni(111) interface leads to a weakening of the interfacial interaction between the monolayer h-BN and Ni(111) surface. The adsorption energy of graphene layers on the h-BN/Ni(111) interface is found to be in the range of the 50-120 meV/C atom as the vertical distance from h-BN to the bottommost graphene layers decreases. With the adsorption of a multilayer graphene on the monolayer h-BN/Ni(111) interface system, the band gap of 0.12 eV and 0.25 eV opening in monolayer graphene and bilayer graphene near the K point is found with an upward shifting of the Fermi level. However, a stacking-sensitive band gap is opened in trilayer graphene. We obtain the band gap of 0.35 eV close to the K point with forming a Mexican hat band structure for ABC-stacked trilayer graphene.

Strain Control of Exciton-Phonon Coupling in Atomically Thin Semiconductors.

PubMed

Niehues, Iris; Schmidt, Robert; Drüppel, Matthias; Marauhn, Philipp; Christiansen, Dominik; Selig, Malte; Berghäuser, Gunnar; Wigger, Daniel; Schneider, Robert; Braasch, Lisa; Koch, Rouven; Castellanos-Gomez, Andres; Kuhn, Tilmann; Knorr, Andreas; Malic, Ermin; Rohlfing, Michael; Michaelis de Vasconcellos, Steffen; Bratschitsch, Rudolf

2018-03-14

Semiconducting transition metal dichalcogenide (TMDC) monolayers have exceptional physical properties. They show bright photoluminescence due to their unique band structure and absorb more than 10% of the light at their excitonic resonances despite their atomic thickness. At room temperature, the width of the exciton transitions is governed by the exciton-phonon interaction leading to strongly asymmetric line shapes. TMDC monolayers are also extremely flexible, sustaining mechanical strain of about 10% without breaking. The excitonic properties strongly depend on strain. For example, exciton energies of TMDC monolayers significantly redshift under uniaxial tensile strain. Here, we demonstrate that the width and the asymmetric line shape of excitonic resonances in TMDC monolayers can be controlled with applied strain. We measure photoluminescence and absorption spectra of the A exciton in monolayer MoSe 2 , WSe 2 , WS 2 , and MoS 2 under uniaxial tensile strain. We find that the A exciton substantially narrows and becomes more symmetric for the selenium-based monolayer materials, while no change is observed for atomically thin WS 2 . For MoS 2 monolayers, the line width increases. These effects are due to a modified exciton-phonon coupling at increasing strain levels because of changes in the electronic band structure of the respective monolayer materials. This interpretation based on steady-state experiments is corroborated by time-resolved photoluminescence measurements. Our results demonstrate that moderate strain values on the order of only 1% are already sufficient to globally tune the exciton-phonon interaction in TMDC monolayers and hold the promise for controlling the coupling on the nanoscale.

Enhanced piezoelectric effect in Janus group-III chalcogenide monolayers

NASA Astrophysics Data System (ADS)

Guo, Yu; Zhou, Si; Bai, Yizhen; Zhao, Jijun

2017-04-01

Piezoelectricity is a unique material property that converts mechanical energy into electricity or vice versa. Starting from the group-III monochalcogenide monolayers, we design a series of derivative Janus structures for piezoelectric materials, including Ga2SSe, Ga2STe, Ga2SeTe, In2SSe, In2STe, In2SeTe, GaInS2, GaInSe2, and GaInTe2. Our first-principles calculations show that these Janus structures are thermodynamically and dynamically stable. They have a bandgap in the range of 0.89-2.03 eV, lower than those of the perfect monolayers, and Ga2STe, Ga2SeTe, In2STe, and In2SeTe monolayers are direct gap semiconductors. They possess piezoelectric coefficients up to 8.47 pm/V, over four times the maximum value obtained in perfect group-III monochalcogenide monolayers. Moreover, the broken mirror symmetry of these Janus structures induces out-of-plane dipolar polarization, yielding additional out-of-plane piezoelectric coefficients of 0.07-0.46 pm/V. The enhanced piezoelectric properties enable the development of these novel two-dimensional materials for piezoelectric sensors and nanogenerators.

Topology-Scaling Identification of Layered Solids and Stable Exfoliated 2D Materials.

PubMed

Ashton, Michael; Paul, Joshua; Sinnott, Susan B; Hennig, Richard G

2017-03-10

The Materials Project crystal structure database has been searched for materials possessing layered motifs in their crystal structures using a topology-scaling algorithm. The algorithm identifies and measures the sizes of bonded atomic clusters in a structure's unit cell, and determines their scaling with cell size. The search yielded 826 stable layered materials that are considered as candidates for the formation of two-dimensional monolayers via exfoliation. Density-functional theory was used to calculate the exfoliation energy of each material and 680 monolayers emerge with exfoliation energies below those of already-existent two-dimensional materials. The crystal structures of these two-dimensional materials provide templates for future theoretical searches of stable two-dimensional materials. The optimized structures and other calculated data for all 826 monolayers are provided at our database (https://materialsweb.org).

Ordered Fe(II)Ti(IV)O3 Mixed Monolayer Oxide on Rutile TiO2(011).

PubMed

Halpegamage, Sandamali; Ding, Pan; Gong, Xue-Qing; Batzill, Matthias

2015-08-25

Oxide monolayers supported or intermixed with an oxide support are potential nanocatalysts whose properties are determined by the interplay with the support. For fundamental studies of monolayer oxides on metal oxide supports, well-defined systems are needed, but so far, the synthesis of monolayer oxides with long-range order on single-crystal oxide surfaces is rare. Here, we show by a combination of scanning tunneling microscopy, photoemission spectroscopy, and density functional theory (DFT)-based computational analysis that the rutile TiO2(011) surface supports the formation of an ordered mixed FeTiO3 monolayer. Deposition of iron in a slightly oxidizing atmosphere (10(-8) Torr O2) and annealing to 300 °C results in a well-ordered surface structure with Fe in a 2+ charge state and Ti in a 4+ charge states. Low-energy ion scattering suggests that the cation surface composition is close to half Fe and half Ti. This surface is stable in ultrahigh vacuum to annealing temperatures of 300 °C before the iron is reduced. DFT simulations confirm that a surface structure with coverage of 50% FeO units is stable and forms an ordered structure. Although distinct from known bulk phases of the iron-titanium oxide systems, the FeTiO3 monolayer exhibits some resemblance to the ilmenite structure, which may suggest that a variety of different mixed oxide phases (of systems that exist in a bulk ilmenite phase) may be synthesized in this way on the rutile TiO2(011) substrate.

Femtosecond transient absorption dynamics of close-packed gold nanocrystal monolayer arrays*1

NASA Astrophysics Data System (ADS)

Eah, Sang-Kee; Jaeger, Heinrich M.; Scherer, Norbert F.; Lin, Xiao-Min; Wiederrecht, Gary P.

2004-03-01

Femtosecond transient absorption spectroscopy is used to investigate hot electron dynamics of close-packed 6 nm gold nanocrystal monolayers. Morphology changes of the monolayer caused by the laser pump pulse are monitored by transmission electron microscopy. At low pump power, the monolayer maintains its structural integrity. Hot electrons induced by the pump pulse decay through electron-phonon (e-ph) coupling inside the nanocrystals with a decay constant that is similar to the value for bulk films. At high pump power, irreversible particle aggregation and sintering occur in the nanocrystal monolayer, which cause damping and peak shifting of the transient bleach signal.

Electronic structure of monolayer 1T'-MoTe2 grown by molecular beam epitaxy

NASA Astrophysics Data System (ADS)

Tang, Shujie; Zhang, Chaofan; Jia, Chunjing; Ryu, Hyejin; Hwang, Choongyu; Hashimoto, Makoto; Lu, Donghui; Liu, Zhi; Devereaux, Thomas P.; Shen, Zhi-Xun; Mo, Sung-Kwan

2018-02-01

Monolayer transition metal dichalcogenides (TMDCs) in the 1T' structural phase have drawn a great deal of attention due to the prediction of quantum spin Hall insulator states. The band inversion and the concomitant changes in the band topology induced by the structural distortion from 1T to 1T' phases are well established. However, the bandgap opening due to the strong spin-orbit coupling (SOC) is only verified for 1T'-WTe2 recently and still debated for other TMDCs. Here we report a successful growth of high-quality monolayer 1T'-MoTe2 on a bilayer graphene substrate through molecular beam epitaxy. Using in situ angle-resolved photoemission spectroscopy (ARPES), we have investigated the low-energy electronic structure and Fermi surface topology. The SOC-induced breaking of the band degeneracy points between the valence and conduction bands is clearly observed by ARPES. However, the strength of SOC is found to be insufficient to open a bandgap, which makes monolayer 1T'-MoTe2 on bilayer graphene a semimetal.

Electronic structure of monolayer 1T'-MoTe 2 grown by molecular beam epitaxy

DOE PAGES

Tang, Shujie; Zhang, Chaofan; Jia, Chunjing; ...

2017-11-14

Monolayer transition metal dichalcogenides (TMDCs) in the 1T' structural phase have drawn a great deal of attention due to the prediction of quantum spin Hall insulator states. The band inversion and the concomitant changes in the band topology induced by the structural distortion from 1T to 1T' phases are well established. However, the bandgap opening due to the strong spin-orbit coupling (SOC) is only verified for 1T'-WTe 2 recently and still debated for other TMDCs. Here we report a successful growth of high-quality monolayer 1T'-MoTe 2 on a bilayer graphene substrate through molecular beam epitaxy. Using in situ angle-resolved photoemissionmore » spectroscopy (ARPES), we have investigated the low-energy electronic structure and Fermi surface topology. The SOC-induced breaking of the band degeneracy points between the valence and conduction bands is clearly observed by ARPES. However, the strength of SOC is found to be insufficient to open a bandgap, which makes monolayer 1T'-MoTe 2 on bilayer graphene a semimetal.« less

Peptide interfaces with graphene: an emerging intersection of analytical chemistry, theory, and materials.

PubMed

Russell, Shane R; Claridge, Shelley A

2016-04-01

Because noncovalent interface functionalization is frequently required in graphene-based devices, biomolecular self-assembly has begun to emerge as a route for controlling substrate electronic structure or binding specificity for soluble analytes. The remarkable diversity of structures that arise in biological self-assembly hints at the possibility of equally diverse and well-controlled surface chemistry at graphene interfaces. However, predicting and analyzing adsorbed monolayer structures at such interfaces raises substantial experimental and theoretical challenges. In contrast with the relatively well-developed monolayer chemistry and characterization methods applied at coinage metal surfaces, monolayers on graphene are both less robust and more structurally complex, levying more stringent requirements on characterization techniques. Theory presents opportunities to understand early binding events that lay the groundwork for full monolayer structure. However, predicting interactions between complex biomolecules, solvent, and substrate is necessitating a suite of new force fields and algorithms to assess likely binding configurations, solvent effects, and modulations to substrate electronic properties. This article briefly discusses emerging analytical and theoretical methods used to develop a rigorous chemical understanding of the self-assembly of peptide-graphene interfaces and prospects for future advances in the field.

The ferromagnetic monolayer Fe(110) on W(110)

NASA Astrophysics Data System (ADS)

Gradmann, U.; Liu, G.; Elmers, H. J.; Przybylski, M.

1990-07-01

Ferromagnetic order in the pseudomorphic monolayer Fe(110) on W(110) was analyzed experimentally using Conversion Electron Mössbauer Spectroscopy (CEMS) and Torsion Oscillation Magnetometry (TOM). The monolayer is thermodynamically stable, crystallizes to large monolayer patches at elevated temperatures and therefore forms an excellent approximation to the ideal monolayer structure. It is ferromagnetic below a Curie-temperature T c,mono, which is given by (282±3) K for the Ag-coated layer, (290±10) K for coating by Cu, Ag or Au and ≈210 K for the free monolayer. For the Ag-coated monolayer, ground state hyperfine field B hf (0)=(11.9±0.3) T and magnetic moment per atom μ=2.53 μB could be determined, in fair agreement with theoretical predictions. Unusual properties of the phase transition are detected by the combination of both experimental techniques. Strong magnetic anisotropies, which are essential for ferromagnetic order, are determined by CEMS.

Chain Stretching and Order-Disorder Transitions in Block Copolymer Monolayers and Multilayers

NASA Astrophysics Data System (ADS)

Kramer, Edward J.; Mishra, Vindhya; Stein, Gila E.; Sohn, Karen E.; Hur, Sumi; Fredrickson, Glenn H.; Cochran, Eric W.

2009-03-01

Both monolayers of block copolymer cylinders and spheres undergo order to disorder transitions (ODT) at temperatures well below those of the bulk. Monolayers of PS-b-P2VP cylinders undergo a ``nematic'' to ``isotropic'' transition at temperatures about 20 K below the bulk ODT while monolayers of PS-b-P2VP with P2VP spheres undergo a 2D crystal to hexatic transition at least 10 K below the bulk ODT. Bilayers of each structure disorder at temperatures well above that of the monolayers. While one is tempted to attribute all of the difference to the fact that ordered monolayers are quasi 2 dimensional while bilayers are not, an alternative explanation exists. In the cylinder monolayer the corona PS chains must stretch to fill a nearly square cross-section domain rather than a hexagonal one in the bulk, while the corona PS chains in a sphere monolayer must stretch to fill a hexagonal prism rather than an octahedron in the bulk. The more non-uniform stretching of the chains in the monolayer should increase its free energy and decrease its order-disorder temperature.

7A projection map of the S-layer protein sbpA obtained with trehalose-embedded monolayer crystals.

PubMed

Norville, Julie E; Kelly, Deborah F; Knight, Thomas F; Belcher, Angela M; Walz, Thomas

2007-12-01

Two-dimensional crystallization on lipid monolayers is a versatile tool to obtain structural information of proteins by electron microscopy. An inherent problem with this approach is to prepare samples in a way that preserves the crystalline order of the protein array and produces specimens that are sufficiently flat for high-resolution data collection at high tilt angles. As a test specimen to optimize the preparation of lipid monolayer crystals for electron microscopy imaging, we used the S-layer protein sbpA, a protein with potential for designing arrays of both biological and inorganic materials with engineered properties for a variety of nanotechnology applications. Sugar embedding is currently considered the best method to prepare two-dimensional crystals of membrane proteins reconstituted into lipid bilayers. We found that using a loop to transfer lipid monolayer crystals to an electron microscopy grid followed by embedding in trehalose and quick-freezing in liquid ethane also yielded the highest resolution images for sbpA lipid monolayer crystals. Using images of specimens prepared in this way we could calculate a projection map of sbpA at 7A resolution, one of the highest resolution projection structures obtained with lipid monolayer crystals to date.

DOE Office of Scientific and Technical Information (OSTI.GOV)

Avazbaeva, Zaure; Sung, Woongmo; Lee, Jonggwan

In this paper, it has been reported that an octadecylamine (ODA) Langmuir monolayer becomes unstable at low pH values with no measurable surface pressure at around pH 3.5, suggesting significant dissolution of the ODA molecule into the subphase solution (Albrecht, Colloids Surf. A 2006, 284–285, 166–174). However, by lowering the pH further, ODA molecules reoccupy the surface, and a full monolayer is recovered at pH 2.5. Using surface sum-frequency spectroscopy and pressure–area isotherms, it is found that the recovered monolayer at very low pH has a larger area per molecule with many gauche defects in the ODA molecules as comparedmore » to that at high pH values. This structural change suggests that the reappearance of the monolayer is due to the adsorbed Cl– counterions to the protonated amine groups, leading to partial charge neutralization. This proposition is confirmed by intentionally adding monovalent salts (i.e., NaCl, NaBr, or NaI) to the subphase to recover the monolayer at pH 3.5, in which the detailed structure of the monolayer is confirmed by sum frequency spectra and the adsorbed anions by X-ray reflectivity.« less

Positional ordering of hard adsorbate particles in tubular nanopores

NASA Astrophysics Data System (ADS)

Gurin, Péter; Varga, Szabolcs; Martínez-Ratón, Yuri; Velasco, Enrique

2018-05-01

The phase behavior and structural properties of a monolayer of hard particles is examined in such a confinement where the adsorbed particles are constrained to the surface of a narrow hard cylindrical pore. The diameter of the pore is chosen such that only first- and second-neighbor interactions occur between the hard particles. The transfer operator method of [Percus and Zhang, Mol. Phys. 69, 347 (1990), 10.1080/00268979000100241] is reformulated to obtain information about the structure of the monolayer. We have found that a true phase transition is not possible in the examined range of pore diameters. The monolayer of hard spheres undergoes a structural change from fluidlike order to a zigzaglike solid one with increasing surface density. The case of hard cylinders is different in the sense that a layering takes place continuously between a low-density one-row and a high-density two-row monolayer. Our results reveal a clear discrepancy with classical density functional theories, which do not distinguish smecticlike ordering in bulk from that in narrow periodic pores.

Cold denaturation induces inversion of dipole and spin transfer in chiral peptide monolayers

PubMed Central

Eckshtain-Levi, Meital; Capua, Eyal; Refaely-Abramson, Sivan; Sarkar, Soumyajit; Gavrilov, Yulian; Mathew, Shinto P.; Paltiel, Yossi; Levy, Yaakov; Kronik, Leeor; Naaman, Ron

2016-01-01

Chirality-induced spin selectivity is a recently-discovered effect, which results in spin selectivity for electrons transmitted through chiral peptide monolayers. Here, we use this spin selectivity to probe the organization of self-assembled α-helix peptide monolayers and examine the relation between structural and spin transfer phenomena. We show that the α-helix structure of oligopeptides based on alanine and aminoisobutyric acid is transformed to a more linear one upon cooling. This process is similar to the known cold denaturation in peptides, but here the self-assembled monolayer plays the role of the solvent. The structural change results in a flip in the direction of the electrical dipole moment of the adsorbed molecules. The dipole flip is accompanied by a concomitant change in the spin that is preferred in electron transfer through the molecules, observed via a new solid-state hybrid organic–inorganic device that is based on the Hall effect, but operates with no external magnetic field or magnetic material. PMID:26916536

Robust ferromagnetism in monolayer chromium nitride

PubMed Central

Zhang, Shunhong; Li, Yawei; Zhao, Tianshan; Wang, Qian

2014-01-01

Design and synthesis of two-dimensional (2D) materials with robust ferromagnetism and biocompatibility is highly desirable due to their potential applications in spintronics and biodevices. However, the hotly pursued 2D sheets including pristine graphene, monolayer BN, and layered transition metal dichalcogenides are nonmagnetic or weakly magnetic. Using biomimetic particle swarm optimization (PSO) technique combined with ab initio calculations we predict the existence of a 2D structure, a monolayer of rocksalt-structured CrN (100) surface, which is both ferromagnetic and biocompatible. Its dynamic, thermal and magnetic stabilities are confirmed by carrying out a variety of state-of-the-art theoretical calculations. Analyses of its band structure and density of states reveal that this material is half-metallic, and the origin of the ferromagnetism is due to p-d exchange interaction between the Cr and N atoms. We demonstrate that the displayed ferromagnetism is robust against thermal and mechanical perturbations. The corresponding Curie temperature is about 675 K which is higher than that of most previously studied 2D monolayers. PMID:24912562

Cold denaturation induces inversion of dipole and spin transfer in chiral peptide monolayers

NASA Astrophysics Data System (ADS)

Eckshtain-Levi, Meital; Capua, Eyal; Refaely-Abramson, Sivan; Sarkar, Soumyajit; Gavrilov, Yulian; Mathew, Shinto P.; Paltiel, Yossi; Levy, Yaakov; Kronik, Leeor; Naaman, Ron

2016-02-01

Chirality-induced spin selectivity is a recently-discovered effect, which results in spin selectivity for electrons transmitted through chiral peptide monolayers. Here, we use this spin selectivity to probe the organization of self-assembled α-helix peptide monolayers and examine the relation between structural and spin transfer phenomena. We show that the α-helix structure of oligopeptides based on alanine and aminoisobutyric acid is transformed to a more linear one upon cooling. This process is similar to the known cold denaturation in peptides, but here the self-assembled monolayer plays the role of the solvent. The structural change results in a flip in the direction of the electrical dipole moment of the adsorbed molecules. The dipole flip is accompanied by a concomitant change in the spin that is preferred in electron transfer through the molecules, observed via a new solid-state hybrid organic-inorganic device that is based on the Hall effect, but operates with no external magnetic field or magnetic material.

Strain engineering on electronic structure and carrier mobility in monolayer GeP3

NASA Astrophysics Data System (ADS)

Zeng, Bowen; Long, Mengqiu; Zhang, Xiaojiao; Dong, Yulan; Li, Mingjun; Yi, Yougen; Duan, Haiming

2018-06-01

Using density functional theory coupled with the Boltzmann transport equation with relaxation time approximation, we have studied the strain effect on the electronic structure and carrier mobility of two-dimensional monolayer GeP3. We find that the energies of valence band maximum and conduction band minimum are nearly linearly shifted with a biaxial strain in the range of  ‑4% to 6%, and the band structure experiences a remarkable transition from semiconductor to metal with the appropriate compression (‑5% strain). Under biaxial strain, the mobility of the electron and hole in monolayer GeP3 reduces and increases by more than one order of magnitude, respectively. It is suggested that it is possible to perform successive transitions from an n-type semiconductor (‑4% strain) to a good performance p-semiconductor (+6% strain) by applying strain in monolayer GeP3, which is potentially useful for flexible electronics and nanosized mechanical sensors.

Secondary electron imaging of monolayer materials inside a transmission electron microscope

DOE Office of Scientific and Technical Information (OSTI.GOV)

Cretu, Ovidiu, E-mail: cretu.ovidiu@nims.go.jp; Lin, Yung-Chang; Suenaga, Kazutomo

2015-08-10

A scanning transmission electron microscope equipped with a backscattered and secondary electron detector is shown capable to image graphene and hexagonal boron nitride monolayers. Secondary electron contrasts of the two lightest monolayer materials are clearly distinguished from the vacuum level. A signal difference between these two materials is attributed to electronic structure differences, which will influence the escape probabilities of the secondary electrons. Our results show that the secondary electron signal can be used to distinguish between the electronic structures of materials with atomic layer sensitivity, enhancing its applicability as a complementary signal in the analytical microscope.

MgF2 monolayer as an anti-reflecting material

NASA Astrophysics Data System (ADS)

Mahida, H. R.; Singh, Deobrat; Sonvane, Yogesh; Gupta, Sanjeev K.; Thakor, P. B.

2017-02-01

The single-layer atomic sheet of magnesium fluoride (MgF2) having 1H and 1T phase structure (hexagonal and tetragonal phase) has been calculated by density functional theory (DFT). Further, we have investigated the structural, electronic and optical properties such as frequency dependent dielectric function, absorption spectra, energy loss spectra, reflectivity, refractive index and optical conductivity of monolayer MgF2 for the direction of parallel and perpendicular electric field polarizations. Our results suggest that monolayer MgF2 provides promising applications in anti-reflection coatings, high-reflective systems and in opto-electronic materials.

Interfacial binding of divalent cations to calixarene-based Langmuir monolayers

DOE PAGES

Tulli, Ludovico G.; Wang, Wenjie; Lindemann, William R.; ...

2015-02-20

The interactions of Langmuir monolayers produced through the self-assembly of an amphiphilic p-carboxycalix[4]arene with a series of divalent, fourth-period transition metals, at the air-water interface, were investigated. Changes in the interfacial behavior of 1 in response to the presence of CuCl 2, CoCl 2, MnCl 2, and NiCl 2 were studied by means of Langmuir compression isotherms and Brewster angle microscopy (BAM). The measurements revealed that the self-assembly properties of 1 are significantly affected by Cu 2+ ions. The interactions of 1-based monolayers with Co 2+ and Cu 2+ ions were further investigated by means of synchrotron radiation-based X-ray reflectivitymore » (XRR), X-ray near-total-reflection fluorescence (XNTRF), and grazing incidence X-ray diffraction (GIXD). XNTRF and XRR analyses revealed that the monolayer of 1 binds more strongly to Cu 2+ than Co 2+ ions. In the presence of relatively high concentrations of Cu 2+ ions in the subphase (1.4 × 10 -3 M), XNTRF exhibited anomalous depth profile behavior and GIXD measurements showed considerably strong diffuse scattering. Furthermore, both measurements suggest the formation of Cu 2+ clusters contiguous to the monolayer of 1.« less

Langmuir monolayers composed of single and double tail sulfobetaine lipids.

PubMed

Hazell, Gavin; Gee, Anthony P; Arnold, Thomas; Edler, Karen J; Lewis, Simon E

2016-07-15

Owing to structural similarities between sulfobetaine lipids and phospholipids it should be possible to form stable Langmuir monolayers from long tail sulfobetaines. By modification of the density of lipid tail group (number of carbon chains) it should also be possible to modulate the two-dimensional phase behaviour of these lipids and thereby compare with that of equivalent phospholipids. Potentially this could enable the use of such lipids for the wide array of applications that currently use phospholipids. The benefit of using sulfobetaine lipids is that they can be synthesised by a one-step reaction from cheap and readily available starting materials and will degrade via different pathways than natural lipids. The molecular architecture of the lipid can be easily modified allowing the design of lipids for specific purposes. In addition the reversal of the charge within the sulfobetaine head group relative to the charge orientation in phospholipids may modify behaviour and thereby allow for novel uses of these surfactants. Stable Langmuir monolayers were formed composed of single and double tailed sulfobetaine lipids. Surface pressure-area isotherm, Brewster Angle Microscopy and X-ray and neutron reflectometry measurements were conducted to measure the two-dimensional phase behaviour and out-of-plane structure of the monolayers as a function of molecular area. Sulfobetaine lipids are able to form stable Langmuir monolayers with two dimensional phase behaviour analogous to that seen for the well-studied phospholipids. Changing the number of carbon tail groups on the lipid from one to two promotes the existence of a liquid condensed phase due to increased Van der Waals interactions between the tail groups. Thus the structure of the monolayers appears to be defined by the relative sizes of the head and tail groups in a predictable way. However, the presence of sub-phase ions has little effect on the monolayer structure, behaviour that is surprisingly different to that seen for phospholipids. Copyright © 2016 The Authors. Published by Elsevier Inc. All rights reserved.

Semi-Metallic Be5C2 Monolayer Global Minimum with Quasi-Planar Pentacoordinate Carbons and Negative Poissons Ratio (Open Access Publisher’s Version)

DTIC Science & Technology

2016-05-03

ARTICLE Received 25 Nov 2015 | Accepted 1 Apr 2016 | Published 3 May 2016 Semi-metallic Be5C2 monolayer global minimum with quasi -planar...forming a quasi -planar pentacoordinate carbon moiety. Be5C2 monolayer appears to have good stability as revealed by its moderate cohesive energy...some promise to be realized experimentally . Be5C2 monolayer is a gapless semiconductor with a Dirac-like point in the band structure and also has an

Monolayer coated gold nanoparticles for delivery applications

PubMed Central

Rana, Subinoy; Bajaj, Avinash; Mout, Rubul; Rotello, Vincent M.

2011-01-01

Gold nanoparticles (AuNPs) provide attractive vehicles for delivery of drugs, genetic materials, proteins, and small molecules. AuNPs feature low core toxicity coupled with the ability to parametrically control particle size and surface properties. In this review, we focus on engineering of the AuNP surface monolayer, highlighting recent advances in tuning monolayer structures for efficient delivery of drugs and biomolecules. This review covers two broad categories of particle functionalization, organic monolayers and biomolecule coatings, and discusses their applications in drug, DNA/RNA, protein and small molecule delivery. PMID:21925556

Phases and phase transition in insoluble and adsorbed monolayers of amide amphiphiles: Specific characteristics of the condensed phases.

PubMed

Vollhardt, D

2015-08-01

For understanding the role of amide containing amphiphiles in inherently complex biological processes, monolayers at the air-water interface are used as simple biomimetic model systems. The specific characteristics of the condensed phases and phase transition in insoluble and adsorbed monolayers of amide amphiphiles are surveyed to highlight the effect of the chemical structure of the amide amphiphiles on the interfacial interactions in model monolayers. The mesoscopic topography and/or two-dimensional lattice structures of selected amino acid amphiphiles, amphiphilic N-alkylaldonamide, amide amphiphiles with specific tailored headgroups, such as amide amphiphiles based on derivatized ethanolamine, e.g. acylethanolamines (NAEs) and N-,O-diacylethanolamines (DAEs) are presented. Special attention is devoted the dominance of N,O-diacylated ethanolamine in mixed amphiphilic acid amide monolayers. The evidence that a first order phase transition can occur in adsorption layers and that condensed phase domains of mesoscopic scale can be formed in adsorption layers was first obtained on the basis of the experimental characteristics of a tailored amide amphiphile. New thermodynamic and kinetic concepts for the theoretical description of the characteristics of amide amphiphile's monolayers were developed. In particular, the equation of state for Langmuir monolayers generalized for the case that one, two or more phase transitions occur, and the new theory for phase transition in adsorbed monolayers are experimentally confirmed at first by amide amphiphile monolayers. Despite the significant progress made towards the understanding the model systems, these model studies are still limited to transfer the gained knowledge to biological systems where the fundamental physical principles are operative in the same way. The study of biomimetic systems, as described in this review, is only a first step in this direction. Copyright © 2014 Elsevier B.V. All rights reserved.

Mechanism of charge transfer and its impacts on Fermi-level pinning for gas molecules adsorbed on monolayer WS2.

PubMed

Zhou, Changjie; Yang, Weihuang; Zhu, Huili

2015-06-07

Density functional theory calculations were performed to assess changes in the geometric and electronic structures of monolayer WS2 upon adsorption of various gas molecules (H2, O2, H2O, NH3, NO, NO2, and CO). The most stable configuration of the adsorbed molecules, the adsorption energy, and the degree of charge transfer between adsorbate and substrate were determined. All evaluated molecules were physisorbed on monolayer WS2 with a low degree of charge transfer and accept charge from the monolayer, except for NH3, which is a charge donor. Band structure calculations showed that the valence and conduction bands of monolayer WS2 are not significantly altered upon adsorption of H2, H2O, NH3, and CO, whereas the lowest unoccupied molecular orbitals of O2, NO, and NO2 are pinned around the Fermi-level when these molecules are adsorbed on monolayer WS2. The phenomenon of Fermi-level pinning was discussed in light of the traditional and orbital mixing charge transfer theories. The impacts of the charge transfer mechanism on Fermi-level pinning were confirmed for the gas molecules adsorbed on monolayer WS2. The proposed mechanism governing Fermi-level pinning is applicable to the systems of adsorbates on recently developed two-dimensional materials, such as graphene and transition metal dichalcogenides.

Isoelectronic tungsten doping in monolayer MoSe 2 for carrier type modulation

DOE PAGES

Li, Xufan; Lin, Ming -Wei; Basile, Leonardo; ...

2016-07-06

Doping and alloying are effective ways to engineer the band structure and modulate the optoelectronic functionality of monolayer transition metal dichalcogenides (TMDs). In this work, we explore the synthesis and electronic properties of monolayer Mo 1-xW xSe 2 (0 < x < 0.18) alloys with almost 100% alloying degree. The isoelectronic substitutional doping of tungsten for molybdenum in the monolayer MoSe 2 is shown to suppress its intrinsically n-type conduction behavior, with p-type conduction gradually emerging to become dominant with increasing W concentration in the alloys. Atomic resolution Z-contrast electron microscopy show that W is shown to substitute directly formore » Mo without the introduction of noticeable vacancy or interstitial defects, however with randomly-distributed W-rich regions ~2 nm in diameter. Scanning tunneling microscopy/spectroscopy measurements reveal that these W-rich regions exhibit a local band structure with the valence band maximum (VBM) closer to the Fermi level as compared with the Mo-rich regions in the monolayer Mo 1-xW xSe 2 crystal. These localized upshifts of the VBM in the local band structure appear responsible for the overall p-type behavior observed for the monolayer Mo 1-xW xSe 2 crystals. Stacked monolayers of n-type MoSe 2 and p-type Mo 1-xW xSe 2 were demonstrated to form atomically thin, vertically stacked p n homojunctions with gate-tunable characteristics, which appear useful for future optoelectronic applications. Lastly, these results indicate that alloying with isoelectronic dopant atoms appears to be an effective and advantageous alternate strategy to doping or alloying with electron donors or acceptors in two-dimensional TMDs.« less

Carbon Dioxide Electroreduction into Syngas Boosted by a Partially Delocalized Charge in Molybdenum Sulfide Selenide Alloy Monolayers.

PubMed

Xu, Jiaqi; Li, Xiaodong; Liu, Wei; Sun, Yongfu; Ju, Zhengyu; Yao, Tao; Wang, Chengming; Ju, Huanxin; Zhu, Junfa; Wei, Shiqiang; Xie, Yi

2017-07-24

Structural parameters of ternary transition-metal dichalcogenide (TMD) alloy usually obey Vegard law well, while interestingly it often exhibits boosted electrocatalytic performances relative to its two pristine binary TMDs. To unveil the underlying reasons, we propose an ideal model of ternary TMDs alloy monolayer. As a prototype, MoSeS alloy monolayers are successfully synthesized, in which X-ray absorption fine structure spectroscopy manifests their shortened Mo-S and lengthened Mo-Se bonds, helping to tailor the d-band electronic structure of Mo atoms. Density functional theory calculations illustrate an increased density of states near their conduction band edge, which ensures faster electron transfer confirmed by their lower work function and smaller charge-transfer resistance. Energy calculations show the off-center charge around Mo atoms not only benefits for stabilizing COOH* intermediate confirmed by its most negative formation energy, but also facilitates the rate-limiting CO desorption step verified by CO temperature programmed desorption and electro-stripping tests. As a result, MoSeS alloy monolayers attain the highest 45.2 % Faradaic efficiency for CO production, much larger than that of MoS 2 monolayers (16.6 %) and MoSe 2 monolayers (30.5 %) at -1.15 V vs. RHE. This work discloses how the partially delocalized charge in ternary TMDs alloys accelerates electrocatalytic performances at atomic level, opening new horizons for manipulating CO 2 electroreduction properties. © 2017 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.

Interactions of the Human Calcitonin Fragment 9–32 with Phospholipids: A Monolayer Study

PubMed Central

Wagner, Kerstin; Van Mau, Nicole; Boichot, Sylvie; Kajava, Andrey V.; Krauss, Ulrike; Le Grimellec, Christian; Beck-Sickinger, Annette; Heitz, Frédéric

2004-01-01

Human calcitonin and its C-terminal fragment 9–32 (hCT(9–32)) administered in a spray translocate into respiratory nasal epithelium with an effect similar to intravenous injection. hCT(9–32) is an efficient carrier to transfer the green fluorescent protein into excised bovine nasal mucosa. To understand the translocation of hCT(9–32) across plasma membranes, we investigated its interactions with phospholipids and its interfacial structure using model lipid monolayers. A combination of physicochemical methods was applied including surface tension measurements on adsorbed and spread monolayers at the air-water interface, Fourier transform infrared, circular dichroism, and atomic force microscopy on Langmuir-Blodgett monolayers. The results disclose that hCT(9–32) preferentially interacts with negatively charged phospholipids and does not insert spontaneously into lipid monolayers. This supports a nonreceptor-mediated endocytic internalization pathway as previously suggested. Structural studies revealed a random coil conformation of hCT(9–32) in solution, transforming to α-helices when the peptide is localized at lipid-free or lipid-containing air-water interfaces. Atomic force microscopy studies of monolayers of the peptide alone or mixed with dioleoylphosphatidylcholine revealed that hCT(9–32) forms filaments rolled into spirals. In contrast, when interacting with dioleoylphosphatidylglycerol, hCT(9–32) does not adopt filamentous structures. A molecular model and packing is proposed for the spiral-forming hCT(9–32). PMID:15240473

Observation of chain stretching in Langmuir diblock copolymer monolayers

DOE Office of Scientific and Technical Information (OSTI.GOV)

Factor, B.J.; Lee, L.; Kent, M.S.

1993-10-01

We report observations of chain stretching in diblock copolymer monolayers on the surface of a selective solvent. Using neutron reflectivity, we have studied the concentration profile of the submerged block over a large range of surface density [sigma] (chains per area) for two different molecular weights. The observed increase in the layer thickness is weaker than the [sigma][sup 1/3] prediction of mean-field and scaling theories for the limiting behavior, but is in agreement with recent numerical self-consistent-field calculations by Whitmore and Noolandi [Macromolecules 23, 3321 (1990)].

Chemical films and monolayers on the water surface and their interactions with ultraviolet radiation: a pilot investigation

NASA Astrophysics Data System (ADS)

Schouten, Peter; Lemckert, Charles; Turnbull, David; Parisi, Alfio; Downs, Nathan; Underhill, Ian; Turner, Geoff

2011-06-01

Over the past 50 years numerous types of chemical films and monolayers have been deployed on top of a wide variety of water reserves in an endeavour to reduce evaporation. To date very little knowledge has been assimilated on how these chemical films and monolayers, once applied to a water surface, influence the underwater UV light field and, in turn, the delicate ecosystems that exist in aquatic environments. This manuscript presents underwater UV exposure profiles weighted to the DNA damage action spectrum measured under an octadecanol/hexadecanol/lime chemical film mixture, a silicone-based chemical film and an octadecanol monolayer applied to the water surface. UV transmission and absorption properties were also evaluated for each of these chemical films and monolayers. From this it was found that when chemical films/monolayers are applied to surface water they can reduce the penetration of biologically effective UV into the water column by up to 85% at a depth as small as 1 cm. This could have a positive influence on the aquatic ecosystem, as harmful UV radiation may be prevented from reaching and consequently damaging a variety of life forms or it could have a negative effect by potentially stopping aquatic organisms from adapting to solar ultraviolet radiation over extended application intervals. Additionally, there is currently no readily applicable system or technique available to readily detect or visualize chemical films and monolayers on the water surface. To overcome this problem a new method of monolayer and chemical film visualization, using a UV camera system, is detailed and tested and its applicability for usage in both laboratory-based trials and real-world operations is evaluated.

Short communication: Inhibition of angiotensin 1-converting enzyme by peptides derived from variants of bovine β-casein upon apical exposure to a Caco-2 cell monolayer.

PubMed

Petrat-Melin, Bjørn; Le, Thao T; Møller, Hanne S; Larsen, Lotte B; Young, Jette F

2017-02-01

This study investigated the consequence of genetically contingent amino acid substitutions in bovine β-casein (CN) genetic variants A 1 , A 2 , B, and I on the structure and bioactive potential of peptides following in vitro digestion. The β-CN variants were digested in vitro using pepsin and pancreatin, and a peptide profile was obtained by liquid chromatography tandem mass spectrometry, revealing among others, the β-casomorphin precursor peptides VYPFPGPIHN and VYPFPGPIPN, derived from variant A 1 /B and from A 2 /I, respectively. These 2 peptides were synthesized and assessed for angiotensin 1-converting enzyme (ACE) inhibitory capacity before and after incubation with a monolayer of Caco-2 intestinal cells. The VYPFPGPIHN was a stronger ACE inhibitor than VYPFPGPIPN, with the concentration needed to reach half-maximal inhibition (IC 50 ) of 123 ± 14.2 μM versus 656 ± 7.6 μM. Exposure to a Caco-2 intestinal cell monolayer did not affect ACE inhibition by VYPFPGPIHN, but resulted in an almost 2-fold increase in inhibition by VYPFPGPIPN after incubation. Subsequent tandem mass spectrometric analysis identified the truncated peptide VYPFPGPIP, suggesting hydrolysis by a cell membrane associated peptidase. Thus, genetic variation in bovine β-CN results in the generation of peptides that differ in bioactivity, and are differently affected by intestinal brush border peptidases. Copyright © 2017 American Dairy Science Association. Published by Elsevier Inc. All rights reserved.

Surface properties and morphology of mixed POSS-DPPC monolayers at the air/water interface.

PubMed

Rojewska, Monika; Skrzypiec, Marta; Prochaska, Krystyna

2017-02-01

From the point of view of the possible medical applications of POSS (polyhedral oligomeric silsesquioxanes), it is crucial to analyse interactions occurring between POSS and model biological membrane at molecular level. Knowledge of the interaction between POSS and DPPC (1,2-dipalmitoyl-sn-glycero-3-phosphocholine) allows prediction of the impact of POSS contained in biomaterials or cosmetics on a living organism. In the study presented, the surface properties and morphology of Langmuir monolayers formed by mixtures of POSS and the phospholipid (DPPC) at the air/water surface are examined. We selected two POSS derivatives, with completely different chemical structure of substituents attached to the corner of the silicon open cage, which allowed the analysis of the impact of the character of organic moieties (strongly hydrophobic or clearly hydrophilic) on the order of POSS molecules and their tendency to form self-aggregates at the air/water surface. POSS derivatives significantly changed the profile of the π-A isotherms obtained for DPPC but in different ways. On the basis of the regular solution theory, the miscibility and stability of the two components in the monolayer were analysed in terms of compression modulus (C s -1 ), excess Gibbs free energy (ΔG exc ), activity coefficients (γ) and interaction parameter (ξ). The results obtained indicate the existence of two different interaction mechanisms between DPPC and POSS which depend on the chemical character of moieties present in POSS molecules. Copyright © 2016 Elsevier B.V. All rights reserved.

Optical and structural characterization of InAs/GaAs quantum wells

NASA Technical Reports Server (NTRS)

Ksendzov, A.; George, T.; Grunthaner, F. J.; Liu, J. K.; Rich, D. H.; Terhune, R. W.; Wilson, B. A.; Pollak, F. H.; Huang, Y.-S.

1991-01-01

Three InAs/GaAs single quantum wells of two-, three-, and four-monolayer thickness were characterized using optical and structural techniques. The results of high-resolution transmission electron (HRTEM) microscopy and optical studies which combine absorption, photoluminescence (PL), photoreflectance, and cathodoluminescence are presented. Using the polarization modulated absorptance technique, we observed two absorption features in our samples at 77 K. On the basis of their polarization properties and comparison with an envelope function calculation, these structures are assigned to transitions between the confined heavy-hole and confined and unconfined electron levels. Photoreflectance spectra of the three-monolayer sample in 77-300 K range show only the fundamental quantum well transition. The temperature dependence of this transition is approximately linear with a slope of 2.2 x 10 exp -4 eV/K, which is significantly lower than in both constituent materials. Comparison to the absorption data reveals that the PL spectra are affected by the carrier diffusion and therefore do not provide direct measure of the exciton density of states. The HRTEM images indicate that, while the interfaces of the two-monolayer sample are smooth and the well thickness is uniform, the four-monolayer sample has uneven interfaces and contains domains of two, three, and four monolayers.

First Principles Study of Electronic Band Structure and Structural Stability of Al2C Monolayer and Nanotubes

NASA Astrophysics Data System (ADS)

Pramchu, S.; Jaroenjittichai, A. P.; Laosiritaworn, Y.

2017-09-01

We used density functional theory (DFT) based on generalized gradient approximation (GGA) and hybrid functional (HSE06) to investigate band gap and structural stability of Al2C monolayer and nanotubes. From the results, both GGA and HSE06 band gaps of Al2C monolayer agree well with previously reported data. For the Al2C nanotubes, we found that their band gaps are more sensitive to the size and the chirality than that of the widely studied SiC2 nanotubes, indicating the Al2C nanotubes may have higher band gap tuning capabilities (with varying diameter size and chirality) compared with those of SiC2 nanotubes. We have also discovered a desirable direct band gap in the case of (n,0) nanotubes, although Al2C monolayer band gap is indirect. The calculated strain energy reveals that (n,0) nanotubes constructed by wrapping up Al2C monolayer consume less energy than (0,n) nanotubes. Thus, (n,0) nanotubes is easier to synthesize than (0,n) nanotubes. This discovery of direct band gap in (n,0) Al2C nanotubes and their adjustable band gap suggests them as promising sensitizer for enhancing power conversion efficiency of excitonic solar cells.

DOE Office of Scientific and Technical Information (OSTI.GOV)

Y Itoh; B Kim; R Gearba

C{sub 60} and C{sub 60}-ferrocene conjugated molecule bearing five carboxylic acids successfully anchor onto a silicon oxide surface as a monolayer through a simple method of simply dipping an amino-terminated surface into the solution of the C{sub 60} derivatives. The monolayer structure was characterized by UV-vis spectroscopy, X-ray reflectivity, X-ray photoelectron spectroscopy, and IR spectroscopy to reveal that the molecules are standing presenting its C{sub 60} spherical face at the surface. The electronic effect of the C{sub 60} monolayer and the ferrocene-functionalized C{sub 60} monolayer in OFET devices was investigated. When an n-type OFET was fabricated on the ferrocene functionalizedmore » monolayer, we see an enhancement in the mobility. When a p-type OFET was made the ferrocene-functionalized C{sub 60} monolayer showed a lowering of the carrier mobility.« less

Role of Thermal Process on Self-Assembled Structures of 4′-([2,2′:6′,2″-Terpyridin]-4′-Yl)-[1,1′-Biphenyl]-4-Carboxylic Acid on Au(III)

PubMed Central

Liu, Xiaoqing; Wang, Yongli; Song, Xin; Chen, Feng; Ouyang, Hongping; Zhang, Xueao; Cai, Yingxiang; Liu, Xiaoming; Wang, Li

2013-01-01

The role of dynamic processes on self-assembled structures of 4′-([2,2′:6′, 2″-terpyridin]-4′-yl)-[1,1′-biphenyl]-4-carboxylic acid (l) molecules on Au(III) has been studied by scanning tunneling microscopy. The as-deposited monolayer is closed-packed and periodic in a short-range due to dipole forces. A thermal annealing process at 110 degrees drives such disordered monolayer into ordered chain-like structures, determined by the combination of the dipole forces and hydrogen bonding. Further annealing at 130 degrees turns the whole monolayer into a bowknot-like structure in which hydrogen bonding plays the dominant role in the formation of assembled structures. Such dependence of an assembled structure on the process demonstrates that an assembled structure can be regulated and controlled not only by the molecular structure but also by the thermal process to form the assembled structure. PMID:23478440

Pt monolayer coating on complex network substrate with high catalytic activity for the hydrogen evolution reaction

PubMed Central

Li, Man; Ma, Qiang; Zi, Wei; Liu, Xiaojing; Zhu, Xuejie; Liu, Shengzhong (Frank)

2015-01-01

A deposition process has been developed to fabricate a complete-monolayer Pt coating on a large-surface-area three-dimensional (3D) Ni foam substrate using a buffer layer (Ag or Au) strategy. The quartz crystal microbalance, current density analysis, cyclic voltammetry integration, and X-ray photoelectron spectroscopy results show that the monolayer deposition process accomplishes full coverage on the substrate and the deposition can be controlled to a single atomic layer thickness. To our knowledge, this is the first report on a complete-monolayer Pt coating on a 3D bulk substrate with complex fine structures; all prior literature reported on submonolayer or incomplete-monolayer coating. A thin underlayer of Ag or Au is found to be necessary to cover a very reactive Ni substrate to ensure complete-monolayer Pt coverage; otherwise, only an incomplete monolayer is formed. Moreover, the Pt monolayer is found to work as well as a thick Pt film for catalytic reactions. This development may pave a way to fabricating a high-activity Pt catalyst with minimal Pt usage. PMID:26601247

Oxygen reduction kinetics on Pt monolayer shell highly affected by the structure of bimetallic AuNi cores

DOE PAGES

Chen, Guangyu; Kuttiyiel, Kurian A.; Su, Dong; ...

2016-07-12

Here, we describe pronounced effects of structural changes of the AuNi cores on the oxygen reduction reaction (ORR) activity of a Pt monolayer shell. The study of alloyed AuNi nanoparticles compared with AuNi core–shell structured nanoparticles revealed configurations having different electronic and electrochemical properties. Controlled alloying of Au with Ni was essential to tune the electronic properties of Au interacting with the Pt monolayer shell to achieve suitable adsorption of O 2 on Pt for expediting the ORR. The alloyed AuNi nanoparticles made the Pt shell more catalytically active for the ORR than the core–shell structured AuNi nanoparticles. The Ptmore » monolayer supported on the alloyed AuNi nanoparticles showed the Pt mass and specific activities as high as 1.52 A mg –1 and 1.18 mA cm –2, respectively, with almost no loss over 5 000 cycles of stability test. This high ORR activity is ascribed to the role of nonspecific steric configuration of Ni atoms changing the electronic properties of the alloy that affect the oxygen and water interaction with the Pt shell and facilitate increased ORR kinetics.« less

Rational Design of Multilayer Collagen Nanosheets with Compositional and Structural Control.

PubMed

Jiang, Tao; Vail, Owen A; Jiang, Zhigang; Zuo, Xiaobing; Conticello, Vincent P

2015-06-24

Two collagen-mimetic peptides, CP(+) and CP(-), are reported in which the sequences comprise a multiblock architecture having positively charged N-terminal (Pro-Arg-Gly)3 and negatively charged C-terminal (Glu-Hyp-Gly)3 triad extensions, respectively. CP(+) rapidly self-associates into positively charged nanosheets based on a monolayer structure. In contrast, CP(-) self-assembles to form negatively charged monolayer nanosheets at a much slower rate, which can be accelerated in the presence of calcium(II) ion. A 2:1 mixture of unassociated CP(-) peptide with preformed CP(+) nanosheets generates structurally defined triple-layer nanosheets in which two CP(-) monolayers have formed on the identical surfaces of the CP(+) nanosheet template. Experimental data from electrostatic force microscopy (EFM) image analysis, zeta potential measurements, and charged nanoparticle binding assays support a negative surface charge state for the triple-layer nanosheets, which is the reverse of the positive surface charge state observed for the CP(+) monolayer nanosheets. The electrostatic complementarity between the CP(+) and CP(-) triple helical cohesive ends at the layer interfaces promotes a (CP(-)/CP(+)/CP(-)) compositional gradient along the z-direction of the nanosheet. This structurally informed approach represents an attractive strategy for the fabrication of two-dimensional nanostructures with compositional control.

Electronic Devices with Strontium Barrier Film and Process for Making Same

DTIC Science & Technology

1998-08-20

structure of the barrier film on an atomic level where the barrier film is comprised of a plurality of contiguous monolayers, while FIG. 7B shows another...another embodiment where the barrier film is comprised of a plurality of contiguous monolayers in which different monolayers thereof are formed of...High Energy Electron 10 Diffraction (RHEED) diagnostic system directed toward the substrate 26. A diffusion barrier precursor compound effusion

Electronic Devices with Composite Atomic Barrier Film and Process for Making Same

DTIC Science & Technology

1998-08-20

structure of the barrier film on an atomic level where the barrier film is comprised of a plurality of contiguous monolayers, while FIG. 7B shows...another embodiment where the barrier film is comprised of a plurality of contiguous monolayers in which different monolayers thereof are formed of...High Energy Electron 10 Diffraction (RHEED) diagnostic system directed toward the substrate 26. A diffusion barrier precursor compound effusion

Characterization and reactivity of organic monolayers on gold and platinum surfaces

DOE Office of Scientific and Technical Information (OSTI.GOV)

Wu, Chien-Ching

1995-12-06

Purpose is to understand how the mobilization, dielectric, orientation, composition, coverage, and structure of self-assembled organic monolayers on metal surfaces affects the surface reactivities and properties of these films in order to facilitate the construction of desired films. Two model systems were used: tiols at Au and aromatic acids at Pt. Surface analysis methods, including contact angle, electrochemistry, ellipsometry, infrared reflection absorption spectroscopy (IRRAS), and x-ray photospectroscopy, were used to study the self-assembled organic monolayers on Au and Pt. IRRAS, contact angle, and electrochemistry were used to determine the surface pK a of phenylcarboxylic acids and pyridylcarboxylic acids monolayers onmore » Pt. These techniques were also used to determine the orientation of polymethylene chain axis and the carboxylic follow the structural evolution of the chains and end group of the thiolate monolayers during formation. IRRAS was also used to assess the carboxylic acid group in terms of its possible existence as the non-hydrogen-bonded species, the hydrogen-bonded dimeric group, and the hydrogen-bonded polymeric group. These different forms of the end group were also followed vs coverage, as well as the reactivity vs solution pH. IRRAS and contact angle were used to calculate the rate constant of the esterification of carboxylic acid-terminated monolayers on Au.« less

GeAs and SiAs monolayers: Novel 2D semiconductors with suitable band structures

NASA Astrophysics Data System (ADS)

Zhou, Liqin; Guo, Yu; Zhao, Jijun

2018-01-01

Two dimensional (2D) materials provide a versatile platform for nanoelectronics, optoelectronics and clean energy conversion. Based on first-principles calculations, we propose a novel kind of 2D materials - GeAs and SiAs monolayers and investigate their atomic structure, thermodynamic stability, and electronic properties. The calculations show that monolayer GeAs and SiAs sheets are energetically and dynamically stable. Their small interlayer cohesion energies (0.191 eV/atom for GeAs and 0.178 eV/atom for SiAs) suggest easy exfoliation from the bulk solids that exist in nature. As 2D semiconductors, GeAs and SiAs monolayers possess band gap of 2.06 eV and 2.50 eV from HSE06 calculations, respectively, while their band gap can be further engineered by the number of layers. The relatively small and anisotropic carrier effective masses imply fast electric transport in these 2D semiconductors. In particular, monolayer SiAs is a direct gap semiconductor and a potential photocatalyst for water splitting. These theoretical results shine light on utilization of monolayer or few-layer GeAs and SiAs materials for the next-generation 2D electronics and optoelectronics with high performance and satisfactory stability.

Grain wall boundaries in centimeter-scale continuous monolayer WS2 film grown by chemical vapor deposition.

PubMed

Jia, Zhiyan; Hu, Wentao; Xiang, Jianyong; Wen, Fusheng; Nie, Anmin; Mu, Congpu; Zhao, Zhisheng; Xu, Bo; Tian, Yongjun; Liu, Zhongyuan

2018-06-22

Centimeter-scale continuous monolayer WS 2 film with large tensile strain has been successfully grown on oxidized silicon substrate by chemical vapor deposition, in which monolayer grains can be more than 200 μm in size. Monolayer WS 2 grains are observed to merge together via not only traditional grain boundaries but also non-traditional ones, which are named as grain walls (GWs) due to their nanometer-scale widths. The GWs are revealed to consist of two or three layers. Though not a monolayer, the GWs exhibit significantly enhanced fluorescence and photoluminescence. This enhancement may be attributed to abundant structural defects such as stacking faults and partial dislocations in the GWs, which are clearly observable in atomically resolved high resolution transmission electron microscopy and scanning transmission electron microscopy images. Moreover, GW-based phototransistor is found to deliver higher photocurrent than that based on monolayer film. These features of GWs provide a clue to microstructure engineering of monolayer WS 2 for specific applications in (opto)electronics.

First principles study of the electronic properties and band gap modulation of two-dimensional phosphorene monolayer: Effect of strain engineering

NASA Astrophysics Data System (ADS)

Phuc, Huynh V.; Hieu, Nguyen N.; Ilyasov, Victor V.; Phuong, Le T. T.; Nguyen, Chuong V.

2018-06-01

The effect of strain on the structural and electronic properties of monolayer phosphorene is studied by using first-principle calculations based on the density functional theory. The intra- and inter-bond length and bond angle for monolayer phosphorene is also evaluated. The intra- and inter-bond length and the bond angle for phosphorene show an opposite tendency under different directions of the applied strain. At the equilibrium state, monolayer phosphorene is a semiconductor with a direct band gap at the Γ-point of 0.91 eV. A direct-indirect band gap transition is found in monolayer phosphorene when both the compression and tensile strain are simultaneously applied along both zigzag and armchair directions. Under the applied compression strain, a semiconductor-metal transition for monolayer phosphorene is observed at -13% and -10% along armchair and zigzag direction, respectively. The direct-indirect and phase transition will largely constrain application of monolayer phosphorene to electronic and optical devices.

Grain wall boundaries in centimeter-scale continuous monolayer WS2 film grown by chemical vapor deposition

NASA Astrophysics Data System (ADS)

Jia, Zhiyan; Hu, Wentao; Xiang, Jianyong; Wen, Fusheng; Nie, Anmin; Mu, Congpu; Zhao, Zhisheng; Xu, Bo; Tian, Yongjun; Liu, Zhongyuan

2018-06-01

Centimeter-scale continuous monolayer WS2 film with large tensile strain has been successfully grown on oxidized silicon substrate by chemical vapor deposition, in which monolayer grains can be more than 200 μm in size. Monolayer WS2 grains are observed to merge together via not only traditional grain boundaries but also non-traditional ones, which are named as grain walls (GWs) due to their nanometer-scale widths. The GWs are revealed to consist of two or three layers. Though not a monolayer, the GWs exhibit significantly enhanced fluorescence and photoluminescence. This enhancement may be attributed to abundant structural defects such as stacking faults and partial dislocations in the GWs, which are clearly observable in atomically resolved high resolution transmission electron microscopy and scanning transmission electron microscopy images. Moreover, GW-based phototransistor is found to deliver higher photocurrent than that based on monolayer film. These features of GWs provide a clue to microstructure engineering of monolayer WS2 for specific applications in (opto)electronics.

The stability of aluminium oxide monolayer and its interface with two-dimensional materials

NASA Astrophysics Data System (ADS)

Song, Ting Ting; Yang, Ming; Chai, Jian Wei; Callsen, Martin; Zhou, Jun; Yang, Tong; Zhang, Zheng; Pan, Ji Sheng; Chi, Dong Zhi; Feng, Yuan Ping; Wang, Shi Jie

2016-07-01

The miniaturization of future electronic devices requires the knowledge of interfacial properties between two-dimensional channel materials and high-κ dielectrics in the limit of one atomic layer thickness. In this report, by combining particle-swarm optimization method with first-principles calculations, we present a detailed study of structural, electronic, mechanical, and dielectric properties of Al2O3 monolayer. We predict that planar Al2O3 monolayer is globally stable with a direct band gap of 5.99 eV and thermal stability up to 1100 K. The stability of this high-κ oxide monolayer can be enhanced by substrates such as graphene, for which the interfacial interaction is found to be weak. The band offsets between the Al2O3 monolayer and graphene are large enough for electronic applications. Our results not only predict a stable high-κ oxide monolayer, but also improve the understanding of interfacial properties between a high-κ dielectric monolayer and two-dimensional material.

Geometric and electronic structures of monolayer hexagonal boron nitride with multi-vacancy

NASA Astrophysics Data System (ADS)

Kim, Do-Hyun; Kim, Hag-Soo; Song, Min Woo; Lee, Seunghyun; Lee, Sang Yun

2017-05-01

Hexagonal boron nitride (h-BN) is an electrical insulator with a large band gap of 5 eV and a good thermal conductor of which melting point reaches about 3000 °C. Due to these properties, much attention was given to the thermal stability rather than the electrical properties of h-BN experimentally and theoretically. In this study, we report calculations that the electronic structure of monolayer h-BN can be influenced by the presence of a vacancy defect which leads to a geometric deformation in the hexagonal lattice structure. The vacancy was varied from mono- to tri-vacancy in a supercell, and different defective structures under the same vacancy density were considered in the case of an odd number of vacancies. Consequently, all cases of vacancy defects resulted in a geometric distortion in monolayer h-BN, and new energy states were created between valence and conduction band with the Fermi level shift. Notably, B atoms around vacancies attracted one another while repulsion happened between N atoms around vacancies, irrespective of vacancy density. The calculation of formation energy revealed that multi-vacancy including more B-vacancies has much lower formation energy than vacancies with more N-vacancies. This work suggests that multi-vacancy created in monolayer h-BN will have more B-vacancies and that the presence of multi-vacancy can make monolayer h-BN electrically conductive by the new energy states and the Fermi level shift.

Geometric and electronic structures of monolayer hexagonal boron nitride with multi-vacancy.

PubMed

Kim, Do-Hyun; Kim, Hag-Soo; Song, Min Woo; Lee, Seunghyun; Lee, Sang Yun

2017-01-01

Hexagonal boron nitride (h-BN) is an electrical insulator with a large band gap of 5 eV and a good thermal conductor of which melting point reaches about 3000 °C. Due to these properties, much attention was given to the thermal stability rather than the electrical properties of h-BN experimentally and theoretically. In this study, we report calculations that the electronic structure of monolayer h-BN can be influenced by the presence of a vacancy defect which leads to a geometric deformation in the hexagonal lattice structure. The vacancy was varied from mono- to tri-vacancy in a supercell, and different defective structures under the same vacancy density were considered in the case of an odd number of vacancies. Consequently, all cases of vacancy defects resulted in a geometric distortion in monolayer h-BN, and new energy states were created between valence and conduction band with the Fermi level shift. Notably, B atoms around vacancies attracted one another while repulsion happened between N atoms around vacancies, irrespective of vacancy density. The calculation of formation energy revealed that multi-vacancy including more B-vacancies has much lower formation energy than vacancies with more N-vacancies. This work suggests that multi-vacancy created in monolayer h-BN will have more B-vacancies and that the presence of multi-vacancy can make monolayer h-BN electrically conductive by the new energy states and the Fermi level shift.

Ferroelasticity and domain physics in two-dimensional transition metal dichalcogenide monolayers.

PubMed

Li, Wenbin; Li, Ju

2016-02-24

Monolayers of transition metal dichalcogenides can exist in several structural polymorphs, including 2H, 1T and 1T'. The low-symmetry 1T' phase has three orientation variants, resulting from the three equivalent directions of Peierls distortion in the parental 1T phase. Using first-principles calculations, we predict that mechanical strain can switch the relative thermodynamic stability between the orientation variants of the 1T' phase. We find that such strain-induced variant switching only requires a few percent elastic strain, which is eminently achievable experimentally with transition metal dichalcogenide monolayers. Calculations indicate that the transformation barrier associated with such variant switching is small (<0.2 eV per chemical formula unit), suggesting that strain-induced variant switching can happen under laboratory conditions. Monolayers of transition metal dichalcogenides with 1T' structure therefore have the potential to be ferroelastic and shape memory materials with interesting domain physics.

Giant piezoelectricity of monolayer group IV monochalcogenides

NASA Astrophysics Data System (ADS)

Fei, Ruixiang; Li, Wenbin; Li, Ju; Yang, Li

We predict enormous, anisotropic piezoelectric effects in intrinsic monolayer group IV monochalcogenides (MX, M =Sn or Ge, X =Se or S), including SnSe, SnS, GeSe, and GeS. Using first-principle simulations based on the modern theory of polarization, we find that their piezoelectric coefficients are about one to two orders of magnitude larger than those of other 2D materials, such as MoS2 and GaSe, and bulk quartz and AlN which are widely used in industry. This enhancement is a result of the unique ``puckered'' C2v symmetry and electronic structure of monolayer group IV monochalcogenides. Given the achieved experimental advances in the fabrication of monolayers, their flexible character, and ability to withstand enormous strain, these 2D structures with giant piezoelectric effects may be promising for a broad range of applications such as nano-sized sensors, piezotronics, and energy harvesting in portable electronic devices.

First-principles simulation on thermoelectric propertiesof transition metal dichalcogenide monolayers

NASA Astrophysics Data System (ADS)

Nakamura, Koichi

2018-06-01

Thermoelectric properties of transition metal dichalcogenide (TMDC) monolayer models, such as Seebeck coefficient and lattice heat capacity, were simulated on the basis of first-principles calculations. The calculated Seebeck coefficients are appropriate for the thermoelectric element of all the TMDC monolayer models introduced in this study. In the MoX2/WX2 (X = S, Se, and Te) heterojunction structure, carrier electrons and holes are respectively distributed in the MoX2 and WX2 regions by adopting a common Fermi energy for both electronic structures. In particular, in the X = Te case, the practical carrier concentration with a large Seebeck coefficient can be evaluated without doping. The lattice heat capacities and their temperature dependence tendencies can be classified on the basis of the minimum frequencies of the optical modes. The quotient of the lattice thermal conductivity over the phonon relaxation time gives the temperature-independent specific values according to the kind of TMDC monolayer.

Ferroelasticity and domain physics in two-dimensional transition metal dichalcogenide monolayers

DOE PAGES

Li, Wenbin; Li, Ju

2016-02-24

Monolayers of transition metal dichalcogenides can exist in several structural polymorphs, including 2H, 1T and 1T'. The low-symmetry 1T' phase has three orientation variants, resulting from the three equivalent directions of Peierls distortion in the parental 1T phase. Using first-principles calculations, we predict that mechanical strain can switch the relative thermodynamic stability between the orientation variants of the 1T' phase. We find that such strain-induced variant switching only requires a few percent elastic strain, which is eminently achievable experimentally with transition metal dichalcogenide monolayers. Calculations indicate that the transformation barrier associated with such variant switching is small (<0.2 eV permore » chemical formula unit), suggesting that strain-induced variant switching can happen under laboratory conditions. Furthermore, monolayers of transition metal dichalcogenides with 1T' structure therefore have the potential to be ferroelastic and shape memory materials with interesting domain physics.« less

Mechanical properties, structure, bioadhesion, and biocompatibility of pectin hydrogels.

PubMed

Markov, Pavel A; Krachkovsky, Nikita S; Durnev, Eugene A; Martinson, Ekaterina A; Litvinets, Sergey G; Popov, Sergey V

2017-09-01

The surface structure, biocompatibility, textural, and adhesive properties of calcium hydrogels derived from 1, 2, and 4% solutions of apple pectin were examined in this study. An increase in the pectin concentration in hydrogels was shown to improve their stability toward elastic and plastic deformation. The elasticity of pectin hydrogels, measured as Young's modulus, ranged from 6 to 100 kPa. The mechanical properties of the pectin hydrogels were shown to correspond to those of soft tissues. The characterization of surface roughness in terms of the roughness profile (Ra) and the root-mean-square deviation of the roughness profile (Rq) indicated an increased roughness profile for hydrogels depending on their pectin concentration. The adhesion of AU2% and AU4% hydrogels to the serosa abdominal wall, liver, and colon was higher than that of the AU1% hydrogel. The adhesion of macrophages and the non-specific adsorption of blood plasma proteins were found to increase as the pectin concentration in the hydrogels increased. The rate of degradation of all hydrogels was higher in phosphate buffered saline (PBS) than that in DMEM and a fibroblast cell monolayer. The pectin hydrogel was also found to have a low cytotoxicity. © 2017 Wiley Periodicals, Inc. J Biomed Mater Res Part A: 105A: 2572-2581, 2017. © 2017 Wiley Periodicals, Inc.

Features of the complexation of octadecane-2,4-dione and lanthanide ions in Langmuir monolayers

NASA Astrophysics Data System (ADS)

Sokolov, M. E.; Repina, I. N.; Raitman, O. A.; Kolokolov, F. A.; Panyushkin, V. T.

2016-05-01

Monolayers of octadecane-2,4-dione on the surfaces of EuCl3 and TbCl3 solutions in the concentration range of 1 × 10-4 to 5 × 10-3 M at pH 5.8 are studied. It is found that the limiting area of octadecane-2,4-dione molecule in a monolayer dependence on Eu3+ and Tb3+ concentration is of extreme nature. The formation of complex compounds in the ligand monolayer is postulated, and structures are proposed for these compounds at different concentrations of metal ions.

Lattice distortion of square iron nitride monolayers induced by changing symmetry of substrate

NASA Astrophysics Data System (ADS)

Hattori, Takuma; Iimori, Takushi; Miyamachi, Toshio; Komori, Fumio

2018-04-01

Rectangular iron nitride monatomic layers are fabricated on the threefold symmetric Cu(111) substrate by taking advantage of the stability of the square nitride film. Two different ordered structures are observed on the Cu(111) substrate by scanning tunneling microscopy after annealing at 510 K and 580 K. Their chemical composition and lattice symmetry are investigated by x-ray photoelectron spectroscopy and low energy electron diffraction. The monolayer film prepared at 580 K is a distorted Fe2N monolayer analogous to a ferromagnetic square Fe2N monolayer with a clock reconstruction on the Cu(001) substrate. The lattice deformation of the square Fe2N monolayer is induced by using Cu(111) with threefold symmetry.

Interdigitation between Triglycerides and Lipids Modulates Surface Properties of Lipid Droplets.

PubMed

Bacle, Amélie; Gautier, Romain; Jackson, Catherine L; Fuchs, Patrick F J; Vanni, Stefano

2017-04-11

Intracellular lipid droplets (LDs) are the main cellular site of metabolic energy storage. Their structure is unique inside the cell, with a core of esterified fatty acids and sterols, mainly triglycerides and sterol esters, surrounded by a single monolayer of phospholipids. Numerous peripheral proteins, including several that were previously associated with intracellular compartments surrounded by a lipid bilayer, have been recently shown to target the surface of LDs, but how they are able to selectively target this organelle remains largely unknown. Here, we use atomistic and coarse-grained molecular dynamics simulations to investigate the molecular properties of the LD surface and to characterize how it differs from that of a lipid bilayer. Our data suggest that although several surface properties are remarkably similar between the two structures, key differences originate from the interdigitation between surface phospholipids and core neutral lipids that occurs in LDs. This property is extremely sensitive to membrane undulations, unlike in lipid bilayers, and it strongly affects both lipid-packing defects and the lateral pressure profile. We observed a marked change in overall surface properties for surface tensions >10 mN/m, indicative of a bimodal behavior. Our simulations provide a comprehensive molecular characterization of the unique surface properties of LDs and suggest how the molecular properties of the surface lipid monolayer can be modulated by the underlying neutral lipids. Copyright © 2017 Biophysical Society. Published by Elsevier Inc. All rights reserved.

Electronic, Vibrational and Thermoelectric Properties of Two-Dimensional Materials

NASA Astrophysics Data System (ADS)

Wickramaratne, Darshana

The discovery of graphene's unique electronic and thermal properties has motivated the search for new two-dimensional materials. Examples of these materials include the layered two-dimensional transition metal dichalcogenides (TMDC) and metal mono-chalcogenides. The properties of the TMDCs (eg. MoS 2, WS2, TaS2, TaSe2) and the metal mono-chalcogenides (eg. GaSe, InSe, SnS) are diverse - ranging from semiconducting, semi-metallic and metallic. Many of these materials exhibit strongly correlated phenomena and exotic collective states such as exciton condensates, charge density waves, Lifshitz transitions and superconductivity. These properties change as the film thickness is reduced down to a few monolayers. We use first-principles simulations to discuss changes in the electronic and the vibrational properties of these materials as the film thickness evolves from a single atomic monolayer to the bulk limit. In the semiconducting TMDCs (MoS2, MoSe2, WS2 and WSe2) and monochalcogenides (GaS, GaSe, InS and InSe) we show confining these materials to their monolayer limit introduces large band degeneracies or non-parabolic features in the electronic structure. These changes in the electronic structure results in increases in the density of states and the number of conducting modes. Our first-principles simulations combined with a Landauer approach show these changes can lead to large enhancements up to an order of magnitude in the thermoelectric performance of these materials when compared to their bulk structure. Few monolayers of the TMDCs can be misoriented with respect to each other due to the weak van-der-Waals (vdW) force at the interface of two monolayers. Misorientation of the bilayer semiconducting TMDCs increases the interlayer van-der-Waals gap distance, reduces the interlayer coupling and leads to an increase in the magnitude of the indirect bandgap by up to 100 meV compared to the registered bilayer. In the semi-metallic and metallic TMDC compounds (TiSe2, TaS 2, TaSe2) a phase transition to a charge density wave (CDW) ground state occurs at a temperature that is unique to each material. Confining these materials to a single monolayer or few-monolayers can increase or decrease their CDW transition temperature and change the magnitude of the CDW energy gap. We show the low energy Raman modes observed in 1T-TaSe2 and 1T-TaS2 in their CDW ground state can emerge from zone folded phonons due to the reconstruction of the lattice in the bulk and monolayer structures. In 1T-TiSe2 the driving mechanism of the CDW is excitonic condensation. We show the excitonic gap of the monolayer and bilayer structures can increase by up to a factor of 3 compared to the excitonic gap of the bulk structure.

Theoretical discovery of stable structures of group III-V monolayers: The materials for semiconductor devices

DOE Office of Scientific and Technical Information (OSTI.GOV)

Suzuki, Tatsuo, E-mail: dr.tatsuosuzuki@gmail.com

Group III-V compounds are very important as the materials of semiconductor devices. Stable structures of the monolayers of group III-V binary compounds have been discovered by using first-principles calculations. The primitive unit cell of the discovered structures is a rectangle, which includes four group-III atoms and four group-V atoms. A group-III atom and its three nearest-neighbor group-V atoms are placed on the same plane; however, these connections are not the sp{sup 2} hybridization. The bond angles around the group-V atoms are less than the bond angle of sp{sup 3} hybridization. The discovered structure of GaP is an indirect transition semiconductor,more » while the discovered structures of GaAs, InP, and InAs are direct transition semiconductors. Therefore, the discovered structures of these compounds have the potential of the materials for semiconductor devices, for example, water splitting photocatalysts. The discovered structures may become the most stable structures of monolayers which consist of other materials.« less

Atomically thin transition metal layers: Atomic layer stabilization and metal-semiconductor transition

NASA Astrophysics Data System (ADS)

Hwang, Jeongwoon; Oh, Young Jun; Kim, Jiyoung; Sung, Myung Mo; Cho, Kyeongjae

2018-04-01

We have performed first-principle calculations to explore the possibility of synthesizing atomically thin transition metal (TM) layers. Buckled structures as well as planar structures of elemental 2D TM layers result in significantly higher formation energies compared with sp-bonded elemental 2D materials with similar structures, such as silicene and phosphorene. It is shown that the TM layers can be stabilized by surface passivation with HS, C6H5S2, or O, and O passivation is most effective. The surface oxygen passivation can improve stability leading to thermodynamically stable TM monolayers except Au, which is the most non-reactive metal element. Such stabilized TM monolayers also show an electronic structure transition from metallic state of free-standing TM layer to semiconducting O-passivated Mo and W monolayers with band gaps of 0.20-1.38 eV.

Zwitterionic lipid assemblies: Molecular dynamics studies of monolayers, bilayers, and vesicles using a new coarse grain force field

PubMed Central

Shinoda, Wataru; DeVane, Russell; Klein, Michael L.

2010-01-01

A new coarse-grained (CG) intermolecular force field is presented for a series of zwitterionic lipids. The model is an extension of our previous work on nonionic surfactants and is designed to reproduce experimental surface/interfacial properties as well as distribution functions from all-atom molecular dynamics (MD) simulations. Using simple functional forms, the force field parameters are optimized for multiple lipid molecules, simultaneously. The resulting CG lipid bilayers have reasonable molecular areas, chain order parameters, and elastic properties. The computed surface pressure vs. area (π-A) curve for a DPPC monolayer demonstrates a significant improvement over the previous CG models. The DPPC monolayer has a longer persistence length than a PEG lipid monolayer, exhibiting a long-lived curved monolayer surface under negative tension. The bud ejected from an oversaturated DPPC monolayer has a large bicelle-like structure, which is different from the micellar bud formed from an oversaturated PEG lipid monolayer. We have successfully observed vesicle formation during CG-MD simulations, starting from an aggregate of DMPC molecules. Depending on the aggregate size, the lipid assembly spontaneously transforms into a closed vesicle or a bicelle. None of the various intermediate structures between these extremes seem to be stable. An attempt to observe fusion of two vesicles through the application of an external adhesion force was not successful. The present CG force field also supports stable multi-lamellar DMPC vesicles. PMID:20438090

UV-induced reaction kinetics of dilinoleoylphosphatidylethanolamine monolayers.

PubMed Central

Viitala, T; Peltonen, J

1999-01-01

The UV-induced reactivity of dilinoleoylphosphatidylethanolamine (DLiPE) Langmuir and Langmuir-Blodgett films has been studied by in situ measurements of the changes in the mean molecular area, UV-vis and Fourier transform infrared spectroscopy, and atomic force microscopy (AFM). Optimum orientation and packing density of the DLiPE molecules in the monolayer were achieved by adding uranyl acetate to the subphase. A first-order reaction kinetic model was successfully fitted to the experimental reaction kinetics data obtained at a surface pressure of 30 mN/m. Topographical studies of LB films by AFM were performed on bilayer structures as a function of subphase composition and UV irradiation time. The orientational effect of the uranyl ions on the monolayer molecules was observed as an enhanced homogeneity of the freshly prepared monomeric LB films. However, the long-term stability of these films proved to be bad; clear reorganization and loss of a true monolayer structure were evidenced by the AFM images. This instability was inhibited for the UV-irradiated films, indicating that the UV irradiation gave rise to a cross-linked structure. PMID:10233096

Simulations of molecular self-assembled monolayers on surfaces: packing structures, formation processes and functions tuned by intermolecular and interfacial interactions.

PubMed

Wen, Jin; Li, Wei; Chen, Shuang; Ma, Jing

2016-08-17

Surfaces modified with a functional molecular monolayer are essential for the fabrication of nano-scale electronics or machines with novel physical, chemical, and/or biological properties. Theoretical simulation based on advanced quantum chemical and classical models is at present a necessary tool in the development, design, and understanding of the interfacial nanostructure. The nanoscale surface morphology, growth processes, and functions are controlled by not only the electronic structures (molecular energy levels, dipole moments, polarizabilities, and optical properties) of building units but also the subtle balance between intermolecular and interfacial interactions. The switchable surfaces are also constructed by introducing stimuli-responsive units like azobenzene derivatives. To bridge the gap between experiments and theoretical models, opportunities and challenges for future development of modelling of ferroelectricity, entropy, and chemical reactions of surface-supported monolayers are also addressed. Theoretical simulations will allow us to obtain important and detailed information about the structure and dynamics of monolayer modified interfaces, which will guide the rational design and optimization of dynamic interfaces to meet challenges of controlling optical, electrical, and biological functions.

Penta-SiC5 monolayer: A novel quasi-planar indirect semiconductor with a tunable wide band gap

NASA Astrophysics Data System (ADS)

Naseri, Mosayeb

2018-03-01

In this paper, by using of the first principles calculations in the framework of the density functional theory, we systematically investigated the structure, stability, electronic and optical properties of a novel two-dimensional pentagonal monolayer semiconductors namely penta-SiC5 monolayer. Comparing elemental silicon, diamond, and previously reported 2D carbon allotropes, our calculation shows that the predicted penta-SiC5 monolayer has a metastable nature. The calculated results indicate that the predicted monolayer is an indirect semiconductor with a wide band gap of about 2.82 eV by using Heyd-Scuseria-Ernzerhof (HSE06) hybrid functional level of theory which can be effectively tuned by external biaxial strains. The obtained exceptional electronic properties suggest penta-SiC5 monolayer as promising candidates for application in new electronic devices in nano scale.

A pentacene monolayer trapped between graphene and a substrate.

PubMed

Zhang, Qicheng; Peng, Boyu; Chan, Paddy Kwok Leung; Luo, Zhengtang

2015-09-21

A self-assembled pentacene monolayer can be fabricated between the solid-solid interface of few-layered graphene (FLG) and the mica substrate, through a diffusion-spreading method. By utilizing a transfer method that allows us to sandwich pentacene between graphene and mica, followed by controlled annealing, we enabled the diffused pentacene to be trapped in the interfaces and led to the formation of a stable monolayer. We found that the formation of a monolayer is kinetically favored by using a 2D Ising lattice gas model for pentacene trapped between the graphene-substrate interfaces. This kinetic Monte Carlo simulation results indicate that, due to the graphene substrate enclosure, the spreading of the first layer proceeds faster than the second layer, as the kinetics favors the filling of voids by molecules from the second layer. This graphene assisted monolayer assembly method provides a new avenue for the fabrication of two-dimensional monolayer structures.

Monolayer Contact Doping of Silicon Surfaces and Nanowires Using Organophosphorus Compounds

PubMed Central

Hazut, Ori; Agarwala, Arunava; Subramani, Thangavel; Waichman, Sharon; Yerushalmi, Roie

2013-01-01

Monolayer Contact Doping (MLCD) is a simple method for doping of surfaces and nanostructures1. MLCD results in the formation of highly controlled, ultra shallow and sharp doping profiles at the nanometer scale. In MLCD process the dopant source is a monolayer containing dopant atoms. In this article a detailed procedure for surface doping of silicon substrate as well as silicon nanowires is demonstrated. Phosphorus dopant source was formed using tetraethyl methylenediphosphonate monolayer on a silicon substrate. This monolayer containing substrate was brought to contact with a pristine intrinsic silicon target substrate and annealed while in contact. Sheet resistance of the target substrate was measured using 4 point probe. Intrinsic silicon nanowires were synthesized by chemical vapor deposition (CVD) process using a vapor-liquid-solid (VLS) mechanism; gold nanoparticles were used as catalyst for nanowire growth. The nanowires were suspended in ethanol by mild sonication. This suspension was used to dropcast the nanowires on silicon substrate with a silicon nitride dielectric top layer. These nanowires were doped with phosphorus in similar manner as used for the intrinsic silicon wafer. Standard photolithography process was used to fabricate metal electrodes for the formation of nanowire based field effect transistor (NW-FET). The electrical properties of a representative nanowire device were measured by a semiconductor device analyzer and a probe station. PMID:24326774

Contrasting suspended covers reveal the impact of an artificial monolayer on heat transfer processes at the interfacial boundary layer.

PubMed

Pittaway, P; Martínez-Alvarez, V; Hancock, N

2015-01-01

The highly variable performance of artificial monolayers in reducing evaporation from water storages has been attributed to wind speed and wave turbulence. Other factors operating at the interfacial boundary layer have seldom been considered. In this paper, two physical shade covers differing in porosity and reflectivity were suspended over 10 m diameter water tanks to attenuate wind and wave turbulence. The monolayer octadecanol was applied to one of the covered tanks, and micrometeorological conditions above and below the covers were monitored to characterise diurnal variation in the energy balance. A high downward (air-to-water) convective heat flux developed under the black cover during the day, whereas diurnal variation in the heat flux under the more reflective, wind-permeable white cover was much less. Hourly air and water temperature profiles under the covers over 3 days when forced convection was minimal (low wind speed) were selected for analysis. Monolayer application reduced temperature gain in surface water under a downward convective heat flux, and conversely reduced temperature loss under an upward convective heat flux. This 'dual property' may explain why repeat application of an artificial monolayer to retard evaporative loss (reducing latent heat loss) does not inevitably increase water temperature.

The structure of [MnIII6 CrIII]3+ single-molecule magnets deposited in submono-layers and monolayers on surfaces studied by means of molecular resolved atomic force microscopy (AFM) and Kelvin Probe Force Microscopy in UHV

NASA Astrophysics Data System (ADS)

Heinzmann, U.; Gryzia, A.; Volkmann, T.; Brechling, A.; Hoeke, V.; Glaser, T.

2014-04-01

Single molecule magnets (SMM) deposited in submonolayers and monolayers have been analyzed with respect to their structures by means of non-contact AFM (topographic as well as damping mode) and Kelvin Probe Force Microscopy with molecular resolution.

Electronic Devices with Barium Barrier Film and Process for Making Same

DTIC Science & Technology

1998-08-20

structure of the barrier film on an atomic level 15 where the barrier .film is comprised of a plurality of contiguous monolayers, while FIG. 7B...yet another embodiment where the barrier film is comprised of a plurality of 20 contiguous monolayers in which different monolayers thereof are...barrier precursor compound effusion cell, for example a barium fluoride, strontium fluoride or the like effusion cell, is provided at 32, and has a

Electronic, magnetic and optical properties of B, C, N and F doped MgO monolayer

NASA Astrophysics Data System (ADS)

Moghadam, A. Dashti; Maskane, P.; Esfandiari, S.

2018-06-01

MgO as one of the alkaline earth oxides has various applications in industry. In this work, we aim to investigate the electronic, optical and magnetic properties of MgO monolayers. Furthermore, monolayer structures with substituted B, N, C and F atoms instead of O atom are studied. These results indicate that MgO layer has possessed potential application in optoelectronic and spintronic nano-devices.

Deducing 2D Crystal Structure at the Solid/Liquid Interface with Atomic Resolution by Combined STM and SFG Study

NASA Astrophysics Data System (ADS)

McClelland, Arthur; Ahn, Seokhoon; Matzger, Adam J.; Chen, Zhan

2009-03-01

Supplemented by computed models, Scanning Tunneling Microscopy (STM) can provide detailed structure of 2D crystals formed at the liquid/solid interface with atomic resolution. However, some structural information such as functional group orientations in such 2D crystals needs to be tested experimentally to ensure the accuracy of the deduced structures. Due to the limited sensitivity, many other experimental techniques such as Raman and infrared spectroscopy have not been allowed to provide such structural information of 2D crystals. Here we showed that Sum Frequency Generation Vibrational Spectroscopy (SFG) can measure average orientation of functional groups in such 2D crystals, or physisorbed monolayers, providing key experimental data to aid in the modeling and interpretation of the STM images. The usefulness of combining these two techniques is demonstrated with a phthalate diesters monolayer formed at the 1-phenyloctane/ highly oriented pyrolytic graphite (HOPG) interface. The spatial orientation of the ester C=O of the monolayer was successfully determined using SFG.

Solar cells based on particulate structure of active layer: Investigation of light absorption by an ordered system of spherical submicron silicon particles

NASA Astrophysics Data System (ADS)

Miskevich, Alexander A.; Loiko, Valery A.

2015-12-01

Enhancement of the performance of photovoltaic cells through increasing light absorption due to optimization of an active layer is considered. The optimization consists in creation of particulate structure of active layer. The ordered monolayers and multilayers of submicron crystalline silicon (c-Si) spherical particles are examined. The quasicrystalline approximation (QCA) and the transfer matrix method (TMM) are used to calculate light absorption in the wavelength range from 0.28 μm to 1.12 μm. The integrated over the terrestial solar spectral irradiance "Global tilt" ASTM G173-03 absorption coefficient is calculated. In the wavelength range of small absorption index of c-Si (0.8-1.12 μm) the integral absorption coefficient of monolayer can be more than 20 times higher than the one of the plane-parallel plate of the equivalent volume of material. In the overall considered range (0.28-1.12 μm) the enhancement factor up to ~1.45 for individual monolayer is observed. Maximum value of the spectral absorption coefficient approaches unity for multilayers consisting of large amount of sparse monolayers of small particles. Multilayers with variable concentration and size of particles in the monolayer sequences are considered. Absorption increasing by such gradient multilayers as compared to the non-gradient ones is illustrated. The considered structures are promising for creation of high efficiency thin-film solar cells.

Composition and structure of spontaneously adsorbed monolayers of n-perfluorocarboxylic acids on silver

NASA Astrophysics Data System (ADS)

Chau, Lai-Kwan; Porter, Marc D.

1990-03-01

Monolayer films of n-perfluorocarboxylic acids (CF 3(CF 2) nCOOH, where n = 0-2, 5-8) have been formed by spontaneous adsorption at silver. Infrared reflection spectroscopy, optical ellipsometry, and contact angle measurements indicate that these films exhibit low surface free energies, that the carboxylic acid group is symmetrically bound at the silver substrate as a carboxylate bridging ligand, and that the structure is composed of tilted (≈ 40° from the surface normal) perfluorocarbon chains and small structural defects.

Nonlinear vibrational spectroscopy of surfactants at liquid interfaces

NASA Astrophysics Data System (ADS)

Miranda, Paulo Barbeitas

Surfactants are widely used to modify physical and chemical properties of interfaces. They play an important role in many technological problems. Surfactant monolayers are also of great scientific interest because they are two-dimensional systems that may exhibit a very rich phase transition behavior and can also be considered as a model system for biological interfaces. In this Thesis, we use a second-order nonlinear optical technique (Sum-Frequency Generation - SFG) to obtain vibrational spectra of surfactant monolayers at liquid/vapor and solid/liquid interfaces. The technique has several advantages: it is intrinsically surface-specific, can be applied to buried interfaces, has submonolayer sensitivity and is remarkably sensitive to the conformational order of surfactant monolayers. The first part of the Thesis is concerned with surfactant monolayers at the air/water interface (Langmuir films). Surface crystallization of an alcohol Langmuir film and of liquid alkanes are studied and their phase transition behaviors are found to be of different nature, although driven by similar intermolecular interactions. The effect of crystalline order of Langmuir monolayers on the interfacial water structure is also investigated. It is shown that water forms a well-ordered hydrogen-bonded network underneath an alcohol monolayer, in contrast to a fatty acid monolayer which induces a more disordered structure. In the latter case, ionization of the monolayer becomes more significant with increase of the water pH value, leading to an electric-field-induced ordering of interfacial water molecules. We also show that the orientation and conformation of fairly complicated molecules in a Langmuir monolayer can be completely mapped out using a combination of SFG and second harmonic generation (SHG). For a quantitative analysis of molecular orientation at an interface, local-field corrections must be included. The second part is a study of self-assembled surfactant monolayers at the solid/liquid interface. It is shown that the conformation of a monolayer adsorbed onto a solid substrate and immersed in a liquid is highly dependent on the monolayer surface density and on the nature of intermolecular interactions in the liquid. Fully packed monolayers are well ordered in any environment due to strong surfactant-surfactant interactions and limited liquid penetration into the monolayer. In contrast, loosely packed monolayers are very sensitive to the liquid environment. Non-polar liquids cause a mild increase in the surfactant conformational disorder. Polar liquids induce more disorder and hydrogen-bonding liquids produce highly disordered conformations due to the hydrophobic effect. When immersed in alkanes, under certain conditions the surfactant chains may become highly ordered due to their interaction with the liquid molecules (chain-chain interaction). In the case of long-chain alcohols, competition between the hydrophobic effect and chain-chain interaction is observed.

Limiting Size of Monolayer Graphene Flakes Grown on Silicon Carbide or via Chemical Vapor Deposition on Different Substrates

NASA Astrophysics Data System (ADS)

Alekseev, N. I.

2018-05-01

The maximum size of homogeneous monolayer graphene flakes that form during the high-temperature evaporation of silicon from a surface of SiC or during graphene synthesis via chemical vapor deposition is estimated, based on the theoretical calculations developed in this work. Conditions conducive to the fragmentation of a monolayer graphene sheet to form discrete fragments or terrace-type structures in which excess energy due to dangling bonds at the edges is compensated for by the lack of internal stress are indentified and described. The results from calculations for the sizes of graphene structures are compared with experimental findings for the most successful graphene syntheses reported in the literature.

Indirect absorption spectroscopy using quantum cascade lasers: mid-infrared refractometry and photothermal spectroscopy.

PubMed

Pfeifer, Marcel; Ruf, Alexander; Fischer, Peer

2013-11-04

We record vibrational spectra with two indirect schemes that depend on the real part of the index of refraction: mid-infrared refractometry and photothermal spectroscopy. In the former, a quantum cascade laser (QCL) spot is imaged to determine the angles of total internal reflection, which yields the absorption line via a beam profile analysis. In the photothermal measurements, a tunable QCL excites vibrational resonances of a molecular monolayer, which heats the surrounding medium and changes its refractive index. This is observed with a probe laser in the visible. Sub-monolayer sensitivities are demonstrated.

The structure and dynamics of Nano Particles encapsulated by the SDS monolayer collapse at the water/TCE interface

PubMed Central

Shi, Wenxiong

2016-01-01

The super-saturated surfactant monolayer collapses with the nanoparticles (NPs) at the water/trichloroethylene (TCE) interface are investigated using molecular dynamics (MD) simulations. The results show that sodium alkyl sulfate (SDS) monolayer collapse is initiated by buckling and followed primarily by budding and the bud encapsulating the NPs and oil molecules. The developed bud detaches from the monolayer into a water phase and forms the swollen micelle emulsion with NPs and oil molecules. We investigate the wavelength of the initial budding and the theoretical description of the budding process. The wavelength of the monolayer increases with bending modulus. The energy barrier of the budding can be easily overcome by thermal fluctuation energy, which indicates that budding process proceeds rapidly. PMID:27853312

The structure and dynamics of Nano Particles encapsulated by the SDS monolayer collapse at the water/TCE interface

NASA Astrophysics Data System (ADS)

Shi, Wenxiong

2016-11-01

The super-saturated surfactant monolayer collapses with the nanoparticles (NPs) at the water/trichloroethylene (TCE) interface are investigated using molecular dynamics (MD) simulations. The results show that sodium alkyl sulfate (SDS) monolayer collapse is initiated by buckling and followed primarily by budding and the bud encapsulating the NPs and oil molecules. The developed bud detaches from the monolayer into a water phase and forms the swollen micelle emulsion with NPs and oil molecules. We investigate the wavelength of the initial budding and the theoretical description of the budding process. The wavelength of the monolayer increases with bending modulus. The energy barrier of the budding can be easily overcome by thermal fluctuation energy, which indicates that budding process proceeds rapidly.

Self-organization of a wedge-shaped surfactant in monolayers and multilayers.

PubMed

Cain, Nicholas; Van Bogaert, Josh; Gin, Douglas L; Hammond, Scott R; Schwartz, Daniel K

2007-01-16

The self-organization behavior of a wedge-shaped surfactant, disodium-3,4,5-tris(dodecyloxy)phenylmethylphosphonate, was studied in Langmuir monolayers (at the air-water interface), Langmuir-Blodgett (LB) monolayers and multilayers, and films adsorbed spontaneously from isooctane solution onto a mica substrate (self-assembled films). This compound forms an inverted hexagonal lyotropic liquid crystal phase in the bulk and in thick adsorbed films. Surface pressure isotherm and Brewster angle microscope (BAM) studies of Langmuir monolayers revealed three phases: gas (G), liquid expanded (LE), and liquid condensed (LC). The surface pressure-temperature phase diagram was determined in detail; a triple point was found at approximately 10 degrees C. Atomic force microscope (AFM) images of LB monolayers transferred from various regions of the phase diagram were consistent with the BAM images and indicated that the LE regions are approximately 0.5 nm thinner than the LC regions. AFM images were also obtained of self-assembled films after various adsorption times. For short adsorption times, when monolayer self-assembly was incomplete, the film topography indicated the coexistence of two distinct monolayer phases. The height difference between these two phases was again 0.5 nm, suggesting a correspondence with the LE/LC coexistence observed in the Langmuir monolayers. For longer immersion times, adsorbed multilayers assembled into highly organized periodic arrays of inverse cylindrical micelles. Similar periodic structures, with the same repeat distance of 4.5 nm, were also observed in three-layer LB films. However, the regions of organized periodic structure were much smaller and more poorly correlated in the LB multilayers than in the films adsorbed from solution. Collectively, these observations indicate a high degree of similarity between the molecular organization in Langmuir layers/LB films and adsorbed self-assembled films. In both cases, monolayers progress through an LE phase, into LE/LC coexistence, and finally into LC phase as surface density increases. Following the deposition of an additional bilayer, the film reorganizes to form an array of inverted cylindrical micelles.

I Situ Structural Study of Underpotential Deposition and Electrocatalysis on GOLD(111) Electrodes

NASA Astrophysics Data System (ADS)

Chen, Chun-Hsien

This thesis work has studied systems of Bi, Pb, Ag, and Hg underpotential deposition (UPD) on Au(111) electrodes. The application of the atomic force microscope (AFM), the scanning tunneling microscope (STM), and the surface x-ray scattering (SXS) to these UPD studies has provided in situ measurements from which we investigate factors that determine UPD surface structures and correlate these structures with surface reactivity. For all the UPD systems in this thesis work, atomic level features of the electrode surface have been revealed. In the case of Pb UPD, Pb starts to deposit by forming islands which exhibit a hexagonal close packed structure of Pb adatoms, while, in the other systems, the UPD adatoms form open lattices. In the Bi and Pb studies, we correlate the activities of the modified surface toward electroreduction of H_2O_2 with the adlattice structures. A heterobimetallic bridge model for H_2O_2 on the surface could explain the enhanced reactivity. The full monolayers of Bi and Hg, rhombohedral metals, form rectangular lattice structures on the hexagonal Au(111) surfaces. The partial charge retention on the Bi and Hg adatom opens the adlayer structure when the coverage is less than a full monolayer. The structure of the first submonolayers of Ag UPD is electrolyte-dependent. The electrode surface exhibits 3 x 3 and 4 x 4 overlayer structures in solutions containing sulfate and nitrate, respectively. In perchloric acid another open structure is observed and a close-packed monolayer is formed in acetic acid. The different monolayer structures give rise to packing densities which correlate with electrolyte size. This implies that the anions participate in reducing metal ions.

Dielectric function, critical points, and Rydberg exciton series of WSe2 monolayer.

PubMed

Diware, M S; Ganorkar, S P; Park, K; Chegal, W; Cho, H M; Cho, Y J; Kim, Y D; Kim, H

2018-06-13

The complex dielectric function ([Formula: see text]) of WSe 2 monolayer grown by atomic layer deposition is investigated using spectroscopic ellipsometry. Band structure parameters are obtained by standard line-shape analysis of the second-energy-derivative of [Formula: see text] spectra. The fundamental band gap is observed at 2.26 eV, corresponds to transition between valence band (VB) maximum at the K point and conduction band (CB) minimum at Q point in the Brillouin zone (BZ). Two strong so-called A and B excitonic peaks in [Formula: see text] spectra originate from vertical transitions from spin-orbit split (0.43 eV) VB to CB at K point of the BZ. Binding energies of A and B exactions are 0.71 and 0.28 eV, respectively. Well resolved five excited excitons states has been detected within the spectral region between A and B. Energy profile of the Rydberg series shows significant deviation from the hydrogenic behavior, discussed in connection with the 2D hydrogen model. Results presented here will improve our understanding about the optical response of 2D materials and will help to design better optoelectronic applications and validate theoretical considerations.

Dielectric function, critical points, and Rydberg exciton series of WSe2 monolayer

NASA Astrophysics Data System (ADS)

Diware, M. S.; Ganorkar, S. P.; Park, K.; Chegal, W.; Cho, H. M.; Cho, Y. J.; Kim, Y. D.; Kim, H.

2018-06-01

The complex dielectric function () of WSe2 monolayer grown by atomic layer deposition is investigated using spectroscopic ellipsometry. Band structure parameters are obtained by standard line-shape analysis of the second-energy-derivative of spectra. The fundamental band gap is observed at 2.26 eV, corresponds to transition between valence band (VB) maximum at the K point and conduction band (CB) minimum at Q point in the Brillouin zone (BZ). Two strong so-called A and B excitonic peaks in spectra originate from vertical transitions from spin–orbit split (0.43 eV) VB to CB at K point of the BZ. Binding energies of A and B exactions are 0.71 and 0.28 eV, respectively. Well resolved five excited excitons states has been detected within the spectral region between A and B. Energy profile of the Rydberg series shows significant deviation from the hydrogenic behavior, discussed in connection with the 2D hydrogen model. Results presented here will improve our understanding about the optical response of 2D materials and will help to design better optoelectronic applications and validate theoretical considerations.

Full membrane spanning self-assembled monolayers as model systems for UHV-based studies of cell-penetrating peptides

DOE Office of Scientific and Technical Information (OSTI.GOV)

Franz, Johannes; Graham, Daniel J.; Schmüser, Lars

2015-03-01

Biophysical studies of the interaction of peptides with model membranes provide a simple yet effective approach to understand the transport of peptides and peptide based drug carriers across the cell membrane. Therein, the authors discuss the use of self-assembled monolayers fabricated from the full membrane-spanning thiol (FMST) 3-((14-((4'-((5-methyl-1-phenyl-35-(phytanyl)oxy-6,9,12,15,18,21,24,27,30,33,37-undecaoxa-2,3-dithiahenpentacontan-51-yl)oxy)-[1,1'-biphenyl]-4-yl)oxy)tetradecyl)oxy)-2-(phytanyl)oxy glycerol for ultrahigh vacuum (UHV) based experiments. UHV-based methods such as electron spectroscopy and mass spectrometry can provide important information about how peptides bind and interact with membranes, especially with the hydrophobic core of a lipid bilayer. Moreover, near-edge x-ray absorption fine structure spectra and x-ray photoelectron spectroscopy (XPS) data showed thatmore » FMST forms UHV-stable and ordered films on gold. XPS and time of flight secondary ion mass spectrometry depth profiles indicated that a proline-rich amphipathic cell-penetrating peptide, known as sweet arrow peptide is located at the outer perimeter of the model membrane.« less

Effect of ionization on the behavior of n-eicosanephosphonic acid monolayers at the air/water interface. Experimental determinations and molecular dynamics simulations.

PubMed

Schulz, Erica P; Piñeiro, Ángel; Miñones, José; Miñones Trillo, José; Frechero, Marisa A; Pieroni, Olga; Schulz, Pablo C

2015-03-03

Monolayers of n-eicosanephosphonic acid, EPA, were studied using a Langmuir balance and a Brewster angle microscope at different subphase pH values to change the charge of the polar headgroups (Zav) from 0 to -2. Molecular dynamics simulations (MDS) results for |Zav| = 0, 1, and 2 were compared with the experimental ones. EPA monolayers behave as mixtures of mutually miscible species (C20H41-PO3H2, C20H41-PO3H(-), and C20H41-PO3(2-), depending on the subphase pH). The order and compactness of the monolayers decrease when increasing |Zav|, while go from strongly interconnected by phosphonic-phosphonic hydrogen bonds (|Zav| = 0-0.03) through an equilibrium between the total cohesive energy and the electrostatic repulsion between the charged polar groups (0.03 < |Zav| < 1.6) to an entirely ionic monolayer (|Zav| ≈ 2). MDS reveal for |Zav| = 0 that the chains form spiralled nearly rounded structures induced by the hydrogen-bonded network. When |Zav| ≈ 1 fingering domains were identified. When Z ≈ 2, the headgroups are more disordered and distanced, not only in the xy plane but also in the z direction, forming a rough layer and responding to compression with a large plateau in the isotherm. The monolayers collapse behavior is consistent with the structures and domains founds in the different ionization states and their consequent in-plane rigidity: there is a transition from a solid-like response at low pH subphases to a fluid-like response at high pH subphases. The film area in the close-packed state increases relatively slow when the polar headgroups are able to form hydrogen bonds but increases to near twice that this value when |Zav| ≈ 2. Other nanoscopic properties of monolayers were also determined by MDS. The computational results confirm the experimental findings and offer a nanoscopic perspective on the structure and interactions in the phosphonate monolayers.

Molecular beam studies of the growth and kinetics of self-assembled monolayers

NASA Astrophysics Data System (ADS)

Schwartz, Peter Vincent

Low energy helium diffraction, a quantitative structural characterization tool, has been used to measure the growth kinetics of self-assembled monolayers (SAMs). Special attention was given to the growth of decanethiol monolayers deposited from a molecular beam onto the (111) face of gold single crystals especially at the initial stages of growth. The influence of changing impingement rate, substrate temperature, and annealing treatments was investigated. We also studied the structure and dynamics of physisorbed adlayers on top of the monolayers and structural variations in monolayers caused by changes in chemical composition such as the addition of phenyl groups, and hydroxyl groups. Experimental work involved renovations to the existing diffractometer. The apparatus was improved with respect to its signal to noise ratio; efficiency in sample preparation and data collection; and the reproducibility of obtaining clean crystal surfaces. The renovations greatly extended the range of experiments of which the diffraction machine is capable. The growth of n-decanethiol SAMs by gas deposition was identified as a multi-stage process where the initial "lying down" layer grows on the bare gold surface with a near unity sticking coefficient, while the subsequent, "standing-up" phase grows with a sticking coefficient of about 10sp{-3}. The ordering and chemisorption of a single "lying down" layer of decanethiol was investigated by annealing a single layer physisorbed on a 130 K Au(111) surface to incrementally higher temperatures. The molecules first align themselves with the underlying gold substrate, then orient themselves in the "head to head" two molecule unit mesh, then chemisorb at still higher temperatures. Overlayers of long chain molecules grown on top of monolayers on Au(111) are found to be more ordered than the underlying monolayers themselves. The energy of adsorption to the organic surface is found to be very close to that of the bulk value, even for a gold-adlayer separation distance of about 4 A. Debye-Waller experiments were done to measure the stiffness of monolayers of different chain lengths, coverages and functional groups as well as overlayers.

Toward Exploring the Structure of Monolayer to Few-layer TaS2 by Efficient Ultrasound-free Exfoliation

NASA Astrophysics Data System (ADS)

Hu, Yiwei; Hao, Qiaoyan; Zhu, Baichuan; Li, Biao; Gao, Zhan; Wang, Yan; Tang, Kaibin

2018-01-01

Tantalum disulfide nanosheets have attracted great interest due to its electronic properties and device applications. Traditional solution-ased ultrasonic process is limited by ultrasound which may cause the disintegration into submicron-sized flake. Here, an efficient multi-step intercalation and ultrasound-free process has been successfully used to exfoliate 1T-TaS2. The obtained TaS2 nanosheets reveal an average thickness of 3 nm and several micrometers in size. The formation of few-layer TaS2 nanosheets as well as monolayer TaS2 sheets is further confirmed by atomic force microscopy images. The few-layer TaS2 nanosheets remain the 1T structure, whereas monolayer TaS2 sheets show lattice distortion and may adopt the 1H-like structure with trigonal prism coordination.

Organic memory device with self-assembly monolayered aptamer conjugated nanoparticles

NASA Astrophysics Data System (ADS)

Oh, Sewook; Kim, Minkeun; Kim, Yejin; Jung, Hunsang; Yoon, Tae-Sik; Choi, Young-Jin; Jung Kang, Chi; Moon, Myeong-Ju; Jeong, Yong-Yeon; Park, In-Kyu; Ho Lee, Hyun

2013-08-01

An organic memory structure using monolayered aptamer conjugated gold nanoparticles (Au NPs) as charge storage nodes was demonstrated. Metal-pentacene-insulator-semiconductor device was adopted for the non-volatile memory effect through self assembly monolayer of A10-aptamer conjugated Au NPs, which was formed on functionalized insulator surface with prostate-specific membrane antigen protein. The capacitance versus voltage (C-V) curves obtained for the monolayered Au NPs capacitor exhibited substantial flat-band voltage shift (ΔVFB) or memory window of 3.76 V under (+/-)7 V voltage sweep. The memory device format can be potentially expanded to a highly specific capacitive sensor for the aptamer-specific biomolecule detection.

Transmission X-ray scattering as a probe for complex liquid-surface structures

DOE PAGES

Fukuto, Masafumi; Yang, Lin; Nykypanchuk, Dmytro; ...

2016-01-28

The need for functional materials calls for increasing complexity in self-assembly systems. As a result, the ability to probe both local structure and heterogeneities, such as phase-coexistence and domain morphologies, has become increasingly important to controlling self-assembly processes, including those at liquid surfaces. The traditional X-ray scattering methods for liquid surfaces, such as specular reflectivity and grazing-incidence diffraction, are not well suited to spatially resolving lateral heterogeneities due to large illuminated footprint. A possible alternative approach is to use scanning transmission X-ray scattering to simultaneously probe local intermolecular structures and heterogeneous domain morphologies on liquid surfaces. To test the feasibilitymore » of this approach, transmission small- and wide-angle X-ray scattering (TSAXS/TWAXS) studies of Langmuir films formed on water meniscus against a vertically immersed hydrophilic Si substrate were recently carried out. First-order diffraction rings were observed in TSAXS patterns from a monolayer of hexagonally packed gold nanoparticles and in TWAXS patterns from a monolayer of fluorinated fatty acids, both as a Langmuir monolayer on water meniscus and as a Langmuir–Blodgett monolayer on the substrate. The patterns taken at multiple spots have been analyzed to extract the shape of the meniscus surface and the ordered-monolayer coverage as a function of spot position. These results, together with continual improvement in the brightness and spot size of X-ray beams available at synchrotron facilities, support the possibility of using scanning-probe TSAXS/TWAXS to characterize heterogeneous structures at liquid surfaces.« less

Electronic Devices with Cesium Barrier Film and Process for Making Same

DTIC Science & Technology

1998-08-20

interfacial structure of the barrier film on an atomic level where the barrier film is comprised of a plurality of contiguous monolayers, while FIG. 7B shows...another 20 embodiment where the barrier film is comprised of a plurality of contiguous monolayers in which different monolayers thereof are formed...compound effusion cell, for example a barium fluoride, strontium fluoride or the like effusion cell, is provided at 32, and has a shutter 33. A

Different Interfacial Behaviors of Peptides Chemically Immobilized on Surfaces with Different Linker Lengths and via Different Termini

DTIC Science & Technology

2014-02-20

spectroscopy was applied to investigate such structures of peptides immobilized on self-assembled monolayers (SAMs). Here cysteine-modified antimicrobial ...modified antimicrobial peptide cecropin P1 (CP1) was chemically immobilized onto SAM with a maleimide terminal group. Two important characteristics...applied to investigate such structures of peptides immobilized on self-assembled monolayers (SAMs). Here cysteine-modified antimicrobial peptide cecropin

Introducing double polar heads to highly fluorescent Thiazoles: Influence on supramolecular structures and photonic properties.

PubMed

Kaufmann, M; Hupfer, M L; Sachse, T; Herrmann-Westendorf, F; Weiß, D; Dietzek, B; Beckert, R; Presselt, M

2018-04-30

Supramolecular structures determine properties of optoelectronically active materials and can be tailored via the Langmuir-Blodgett (LB) technique. Interactions between dyes can cause high crystallinities of Langmuir monolayers, thus rendering retaining their integrity during the LB-deposition challenging. However, increasing degrees of freedom exclusively at the polar anchoring moieties of dyes might improve processability without perturbing the dye's optoelectronic properties nor the function-determining crystallinity of the layer. (Amphiphilic) thiazole dyes without, with a mono-polar, and with a double-polar anchor were synthesized, whereas the two constituting polar moieties of the latter derivate are separated by a flexible alkyl chain. The supramolecular structures and crystallinities of Langmuir and LB monolayers were characterized by means of LB isotherms, atomic force microscopy and polarization-resolved fluorescence spectroscopy. As compared to the mono-polar reference the introduction of a flexible double-polar head did not deteriorate UV-vis absorption, emission or electrochemical properties of the thiazole but significantly extended the range of constant compressibility modulus, thus indicating improved processability of the Langmuir monolayers. Indeed, AFM studies revealed that the integrity of the monolayers could be retained during LB-deposition. Additionally, also the underlying supramolecular structure of the chromophore moieties is largely identical to those obtained from the mono-polar reference thiazoles. Copyright © 2018. Published by Elsevier Inc.

Indium-chlorine and gallium-chlorine tetrasubstituted phthalocyanines in a bulk system, Langmuir monolayers and Langmuir-Blodgett nanolayers--spectroscopic investigations.

PubMed

Bursa, B; Wróbel, D; Biadasz, A; Kędzierski, K; Lewandowska, K; Graja, A; Szybowicz, M; Durmuş, M

2014-07-15

The paper deals with spectroscopic characterization of metallic phthalocyanines (Pc's) (indium and gallium) complexed with chlorine and substituted with four benzyloxyphenoxy peripheral groups in bulk systems, 2D Langmuir monolayers and Langmuir-Blodgett nanolayers. An influence of the molecular structure of dyes (the presence of metal and of substitutes attached to the phthalocyanine macroring) on the in situ measurements of light absorption is reported. Molecular arrangement of the phthalocyanine molecular skeleton in the Langmuir monolayers on water substrate and in the Langmuir-Blodgett nanolayers is evaluated. A comparison of the light absorption spectra of the phthalocyanine monolayers with the spectra of the dyes in solution supports the existence of dye aggregates in the monolayer. It was shown that the type of dye aggregates (oblique and H types) depends markedly on the dye molecular structures. The NIR-IR, IR reflection-absorption and Raman spectra are also monitored for Langmuir-Blodgett nanolayers in non-polarized and polarized light. It was shown that the dye molecules in the Langmuir-Blodgett layers are oriented nearly vertically with respect to a gold substrate. Copyright © 2014 Elsevier B.V. All rights reserved.

Protecting the properties of monolayer MoS 2 on silicon based substrates with an atomically thin buffer

DOE PAGES

Man, Michael K. L.; Deckoff-Jones, Skylar; Winchester, Andrew; ...

2016-02-12

Semiconducting 2D materials, like transition metal dichalcogenides (TMDs), have gained much attention for their potential in opto-electronic devices, valleytronic schemes, and semi-conducting to metallic phase engineering. However, like graphene and other atomically thin materials, they lose key properties when placed on a substrate like silicon, including quenching of photoluminescence, distorted crystalline structure, and rough surface morphology. The ability to protect these properties of monolayer TMDs, such as molybdenum disulfide (MoS 2), on standard Si-based substrates, will enable their use in opto-electronic devices and scientific investigations. Here we show that an atomically thin buffer layer of hexagonal-boron nitride (hBN) protects themore » range of key opto-electronic, structural, and morphological properties of monolayer MoS 2 on Si-based substrates. The hBN buffer restores sharp diffraction patterns, improves monolayer flatness by nearly two-orders of magnitude, and causes over an order of magnitude enhancement in photoluminescence, compared to bare Si and SiO 2 substrates. Lastly, our demonstration provides a way of integrating MoS 2 and other 2D monolayers onto standard Si-substrates, thus furthering their technological applications and scientific investigations.« less

Large-area snow-like MoSe2 monolayers: synthesis, growth mechanism, and efficient electrocatalyst application.

PubMed

Huang, Jingwen; Liu, Huiqiang; Jin, Bo; Liu, Min; Zhang, Qingchun; Luo, Liqiong; Chu, Shijin; Chu, Sheng; Peng, Rufang

2017-07-07

This study explores the large-area synthesis of controllable morphology, uniform, and high-quality monolayer. MoSe 2 is essential for its potential application in optoelectronics, photocatalysis, and renewable energy sources. In this study, we successfully synthesized snow-like MoSe 2 monolayers using a simple chemical vapor deposition method. Results reveal that snow-like MoSe 2 is a single crystal with a hexagonal structure, a thickness of ∼0.9 nm, and a lateral dimension of up to 20 μm. The peak position of the photoluminescence spectra is ∼1.52 eV corresponding to MoSe 2 monolayer. The growth mechanism of the snow-like MoSe 2 monolayer was investigated and comprised a four-step process during growth. Finally, we demonstrate that the snow-like MoSe 2 monolayers are ideal electrocatalysts for hydrogen evolution reactions (HERs), reflected by a low Tafel slope of ∼68 mV/decade. Compared with the triangular-shaped MoSe 2 monolayer, the hexangular snow-like shape with plentiful edges is superior for perfect electrocatalysts for HERs or transmission devices of optoelectronic signals.

Ice Nucleation Efficiency of Hydroxylated Organic Surfaces Is Controlled by Their Structural Fluctuations and Mismatch to Ice.

PubMed

Qiu, Yuqing; Odendahl, Nathan; Hudait, Arpa; Mason, Ryan; Bertram, Allan K; Paesani, Francesco; DeMott, Paul J; Molinero, Valeria

2017-03-01

Heterogeneous nucleation of ice induced by organic materials is of fundamental importance for climate, biology, and industry. Among organic ice-nucleating surfaces, monolayers of long chain alcohols are particularly effective, while monolayers of fatty acids are significantly less so. As these monolayers expose to water hydroxyl groups with an order that resembles the one in the basal plane of ice, it was proposed that lattice matching between ice and the surface controls their ice-nucleating efficiency. Organic monolayers are soft materials and display significant fluctuations. It has been conjectured that these fluctuations assist in the nucleation of ice. Here we use molecular dynamic simulations and laboratory experiments to investigate the relationship between the structure and fluctuations of hydroxylated organic surfaces and the temperature at which they nucleate ice. We find that these surfaces order interfacial water to form domains with ice-like order that are the birthplace of ice. Both mismatch and fluctuations decrease the size of the preordered domains and monotonously decrease the ice freezing temperature. The simulations indicate that fluctuations depress the freezing efficiency of monolayers of alcohols or acids to half the value predicted from lattice mismatch alone. The model captures the experimental trend in freezing efficiencies as a function of chain length and predicts that alcohols have higher freezing efficiency than acids of the same chain length. These trends are mostly controlled by the modulation of the structural mismatch to ice. We use classical nucleation theory to show that the freezing efficiencies of the monolayers are directly related to their free energy of binding to ice. This study provides a general framework to relate the equilibrium thermodynamics of ice binding to a surface and the nonequilibrium ice freezing temperature and suggests that these could be predicted from the structure of interfacial water.

Structure and phase transitions of monolayers of intermediate-length n-alkanes on graphite studied by neutron diffraction and molecular dynamics simulation

NASA Astrophysics Data System (ADS)

Diama, A.; Matthies, B.; Herwig, K. W.; Hansen, F. Y.; Criswell, L.; Mo, H.; Bai, M.; Taub, H.

2009-08-01

We present evidence from neutron diffraction measurements and molecular dynamics (MD) simulations of three different monolayer phases of the intermediate-length alkanes tetracosane (n-C24H50 denoted as C24) and dotriacontane (n-C32H66 denoted as C32) adsorbed on a graphite basal-plane surface. Our measurements indicate that the two monolayer films differ principally in the transition temperatures between phases. At the lowest temperatures, both C24 and C32 form a crystalline monolayer phase with a rectangular-centered (RC) structure. The two sublattices of the RC structure each consists of parallel rows of molecules in their all-trans conformation aligned with their long axis parallel to the surface and forming so-called lamellas of width approximately equal to the all-trans length of the molecule. The RC structure is uniaxially commensurate with the graphite surface in its [110] direction such that the distance between molecular rows in a lamella is 4.26 Å=√3 ag, where ag=2.46 Å is the lattice constant of the graphite basal plane. Molecules in adjacent rows of a lamella alternate in orientation between the carbon skeletal plane being parallel and perpendicular to the graphite surface. Upon heating, the crystalline monolayers transform to a "smectic" phase in which the inter-row spacing within a lamella expands by ˜10% and the molecules are predominantly oriented with the carbon skeletal plane parallel to the graphite surface. In the smectic phase, the MD simulations show evidence of broadening of the lamella boundaries as a result of molecules diffusing parallel to their long axis. At still higher temperatures, they indicate that the introduction of gauche defects into the alkane chains drives a melting transition to a monolayer fluid phase as reported previously.

Structure and phase transitions of monolayers of intermediate-length n-alkanes on graphite studied by neutron diffraction and molecular dynamics simulation.

PubMed

Diama, A; Matthies, B; Herwig, K W; Hansen, F Y; Criswell, L; Mo, H; Bai, M; Taub, H

2009-08-28

We present evidence from neutron diffraction measurements and molecular dynamics (MD) simulations of three different monolayer phases of the intermediate-length alkanes tetracosane (n-C(24)H(50) denoted as C24) and dotriacontane (n-C(32)H(66) denoted as C32) adsorbed on a graphite basal-plane surface. Our measurements indicate that the two monolayer films differ principally in the transition temperatures between phases. At the lowest temperatures, both C24 and C32 form a crystalline monolayer phase with a rectangular-centered (RC) structure. The two sublattices of the RC structure each consists of parallel rows of molecules in their all-trans conformation aligned with their long axis parallel to the surface and forming so-called lamellas of width approximately equal to the all-trans length of the molecule. The RC structure is uniaxially commensurate with the graphite surface in its [110] direction such that the distance between molecular rows in a lamella is 4.26 A=sqrt[3a(g)], where a(g)=2.46 A is the lattice constant of the graphite basal plane. Molecules in adjacent rows of a lamella alternate in orientation between the carbon skeletal plane being parallel and perpendicular to the graphite surface. Upon heating, the crystalline monolayers transform to a "smectic" phase in which the inter-row spacing within a lamella expands by approximately 10% and the molecules are predominantly oriented with the carbon skeletal plane parallel to the graphite surface. In the smectic phase, the MD simulations show evidence of broadening of the lamella boundaries as a result of molecules diffusing parallel to their long axis. At still higher temperatures, they indicate that the introduction of gauche defects into the alkane chains drives a melting transition to a monolayer fluid phase as reported previously.

Selectivity and Sensitivity of Ultrathin Monolayer Electrodes

NASA Astrophysics Data System (ADS)

Cheng, Quan

The objective of this work is to build a molecular architecture on the electrode surface with a well-defined morphology and desirable electrochemical characteristics. The goal is accomplished by means of self-assembly of thioctic acid, a sulfur-terminated organic molecule with a short alkyl chain and a hydrophilic carboxylic headgroup, on a gold electrode. Characterization of the monolayer structure and the electrochemical response of the monolayer electrodes is performed by means of capacitance measurements and voltammetry. Investigation of the capacitance of the self-assembled monolayers provides insight into the macroscopic permeability of the films and reveals that penetration of solvent/ions into the thioctic acid monolayer film occurs extensively. Voltammetric results demonstrate that permselectivity of the monolayer electrode can be obtained as a result of the induced electrostatic interactions between the monolayer interface and the electroactive species. Measurement of the voltammetric response of the redox probes at the monolayers as a function of the electrolyte concentration and composition is used to qualitatively analyze the effect of electrolyte on response. A model describing the role of the interfacial charge in the electrochemical response of the monolayers as a function of the solution composition and surface smoothness is proposed. A strategy is developed to further explore the applications of the monolayer electrodes to control the electrochemical response of the biological molecules such as catecholamines. The ability to control the surface hydrophobicity of the monolayer electrodes through coadsorption of thioctic acid and hexanethiol, to display different electrochemical properties towards biological molecules is tested. The optimum conditions for detection of the biological molecules on the monolayer electrodes are discussed. In order to pursue selective analysis in microenvironments, the thioctic acid monolayer formed on the ultramicroelectrodes (UME) is investigated, demonstrating high permselectivity and high sensitivity of the monolayer modified UMEs. Because of the more effective mass transport to the UMEs, effects of electrolyte on the monolayer response can be characterized facilely. Amperometric pH sensing on the thioctic acid UMEs using a redox mediator is discussed. Finally, the thioctic acid monolayer microelectrode is applied to investigate direct electrochemistry of a redox protein, cytochrome c. A sketch for developing a biosensor via mediation effects using the monolayer assembly is proposed.

Interfacial engineering of microstructured materials

NASA Astrophysics Data System (ADS)

Poda, Aimee

The tribological behavior of octadecyltrichlorosilane self assembled monolayers (OTS-SAMs) has been successfully exploited to reduce energy losses and to produce adequate adhesion barrier properties on many MEMS surfaces. Unfortunately, performance discrepancies are reported in the literature between films produced on smooth surfaces as compared to typical MEMS surfaces maintaining topographical roughness. Rational explanations in terms of reproducibility issues, production considerations, and the scale of measurement technique have been introduced to account for some of the variation. The tribological phenomena at the micro-scale are complicated by the fact that rather than inertial effects, the forces associated with the surface become dominant factors influencing the mechanical behavior of contacting components. In MEMS, real mechanical contacts typically consist of a few nanometer scale asperities. Furthermore, various surface topographies exist for MEMS device fabrication and their corresponding asperity profiles can vary drastically based on the production process. This dissertation presents research focusing on the influence of topographical asperities on OTS film properties of relevance for efficient tribological improvement. A fundamental approach has been taken to carefully examine the factors that contribute to high quality film formation, specifically formation temperature and the role of interfacial water layer associated with the sample surface. As evidenced on smooth surfaces, the characteristics for successful tribological performance of OTS films are strongly dependent on the lateral packing density and molecular orientation of the monolayer. Limited information is available on how monolayers associate on topographical asperities and whether these topographical asperities influence the interfacial reactivity of MEMS surfaces. A silica film produced from a low temperature, vapor-phase hydrolysis of tetrachlorosilane with a tunable topography is introduced and leveraged as a novel investigative platform for advanced analytical investigations often restricted to use on smooth surfaces. This tunable surface allows intellectual insight into the nature of surface properties associated with silica surfaces, the uptake of interfacial water and the subsequent influence of surface morphology on OTS film formation. FTIR analysis was utilized for an examination of interfacial properties on both smooth Si(100) surfaces and on the tunable MVD topography in combination with an investigation of OTS film formation mechanism. A dilute etchant technique is developed to provide topographic contrast for AFM imaging to allow direct examination of film packing characteristics in relation to surface asperities. A relationship between monolayer adsorption characteristics and topographical asperities with observed variations in monolayer order resultant from surface roughness has been elucidated. Results show that the packing structure of OTS monolayers is dependent on the local asperity curvature which is qualitatively different from that observed on flat surfaces. In addition, a difference in surface reactivity is observed as a result of different surface topographies with thicker silica layers maintaining a thicker interfacial water layer resulting in a higher coverage of OTS monolayers at similar reaction times and conditions. This work shows changes in surface reactivity as a consequence of different morphological surface characteristics and preparation procedures. Additional research is presented on a new class of SAM, namely octadecylphoshonic acid and its monolayer formation mechanism and properties are compared to conventional OTS monolayers. This monolayer is translated to investigative probes based on Aluminum oxide specifically tailored for a tribological comparison across multi-scale friction regimes.

Giant piezoelectricity of monolayer group IV monochalcogenides: SnSe, SnS, GeSe, and GeS

NASA Astrophysics Data System (ADS)

Fei, Ruixiang; Li, Wenbin; Li, Ju; Yang, Li

2015-10-01

We predict enormous, anisotropic piezoelectric effects in intrinsic monolayer group IV monochalcogenides (MX, M=Sn or Ge, X=Se or S), including SnSe, SnS, GeSe, and GeS. Using first-principle simulations based on the modern theory of polarization, we find that their piezoelectric coefficients are about one to two orders of magnitude larger than those of other 2D materials, such as MoS2 and GaSe, and bulk quartz and AlN which are widely used in industry. This enhancement is a result of the unique "puckered" C2v symmetry and electronic structure of monolayer group IV monochalcogenides. Given the achieved experimental advances in the fabrication of monolayers, their flexible character, and ability to withstand enormous strain, these 2D structures with giant piezoelectric effects may be promising for a broad range of applications such as nano-sized sensors, piezotronics, and energy harvesting in portable electronic devices.

Giant fluctuations and structural effects in a flocking epithelium

NASA Astrophysics Data System (ADS)

Giavazzi, Fabio; Malinverno, Chiara; Corallino, Salvatore; Ginelli, Francesco; Scita, Giorgio; Cerbino, Roberto

2017-09-01

Epithelial cells cultured in a monolayer are very motile in isolation but reach a near-jammed state when mitotic division increases their number above a critical threshold. We have recently shown that a monolayer can be reawakened by over-expression of a single protein, RAB5A, a master regulator of endocytosis. This reawakening of motility was explained in terms of a flocking transition that promotes the emergence of a large-scale collective migratory pattern. Here we focus on the impact of this reawakening on the structural properties of the monolayer. We find that the unjammed monolayer is characterised by a fluidisation at the single cell level, and by enhanced non-equilibrium large-scale number fluctuations at a larger length scale. Also, with the help of numerical simulations, we trace back the origin of these fluctuations to the self-propelled active nature of the constituents, and to the existence of a local alignment mechanism, leading to the spontaneous breaking of the orientational symmetry.

AFM investigation of effect of absorbed water layer structure on growth mechanism of octadecyltrichlorosilane self-assembled monolayer on oxidized silicon

DOE Office of Scientific and Technical Information (OSTI.GOV)

Li, Shaowei; Zheng, Yanjun, E-mail: zhengyj@cup.edu.cn; Chen, Changfeng

2016-06-28

The growth mechanism of an octadecyltrichlorosilane (OTS) self-assembled monolayer on a silicon oxide surface at various relative humidities has been investigated. Atomic force microscopy images show that excess water may actually hinder the nucleation and growth of OTS islands. A moderate amount of water is favorable for the nucleation and growth of OTS islands in the initial stage; however, the completion of the monolayer is very slow in the final stage. The growth of OTS islands on a low-water-content surface maintains a relatively constant speed and requires the least amount of time. The mobility of water molecules is thought tomore » play an important role in the OTS monolayers, and a low-mobility water layer provides a steady condition for OTS monolayer growth.« less

Patterning monolayer graphene with zigzag edges on hexagonal boron nitride by anisotropic etching

DOE Office of Scientific and Technical Information (OSTI.GOV)

Wang, Guole; Wu, Shuang; Zhang, Tingting

2016-08-01

Graphene nanostructures are potential building blocks for nanoelectronic and spintronic devices. However, the production of monolayer graphene nanostructures with well-defined zigzag edges remains a challenge. In this paper, we report the patterning of monolayer graphene nanostructures with zigzag edges on hexagonal boron nitride (h-BN) substrates by an anisotropic etching technique. We found that hydrogen plasma etching of monolayer graphene on h-BN is highly anisotropic due to the inert and ultra-flat nature of the h-BN surface, resulting in zigzag edge formation. The as-fabricated zigzag-edged monolayer graphene nanoribbons (Z-GNRs) with widths below 30 nm show high carrier mobility and width-dependent energy gaps atmore » liquid helium temperature. These high quality Z-GNRs are thus ideal structures for exploring their valleytronic or spintronic properties.« less

AFM investigation of effect of absorbed water layer structure on growth mechanism of octadecyltrichlorosilane self-assembled monolayer on oxidized silicon

NASA Astrophysics Data System (ADS)

Li, Shaowei; Zheng, Yanjun; Chen, Changfeng

2016-06-01

The growth mechanism of an octadecyltrichlorosilane (OTS) self-assembled monolayer on a silicon oxide surface at various relative humidities has been investigated. Atomic force microscopy images show that excess water may actually hinder the nucleation and growth of OTS islands. A moderate amount of water is favorable for the nucleation and growth of OTS islands in the initial stage; however, the completion of the monolayer is very slow in the final stage. The growth of OTS islands on a low-water-content surface maintains a relatively constant speed and requires the least amount of time. The mobility of water molecules is thought to play an important role in the OTS monolayers, and a low-mobility water layer provides a steady condition for OTS monolayer growth.

The structure of graphene oxide membranes in liquid water, ethanol and water-ethanol mixtures

NASA Astrophysics Data System (ADS)

Talyzin, Alexandr V.; Hausmaninger, Tomas; You, Shujie; Szabó, Tamás

2013-12-01

The structure of graphene oxide (GO) membranes was studied in situ in liquid solvents using synchrotron radiation X-ray diffraction in a broad temperature interval. GO membranes are hydrated by water similarly to precursor graphite oxide powders but intercalation of alcohols is strongly hindered, which explains why the GO membranes are permeated by water and not by ethanol. Insertion of ethanol into the membrane structure is limited to only one monolayer in the whole studied temperature range, in contrast to precursor graphite oxide powders, which are intercalated with up to two ethanol monolayers (Brodie) and four ethanol monolayers (Hummers). As a result, GO membranes demonstrate the absence of ``negative thermal expansion'' and phase transitions connected to insertion/de-insertion of alcohols upon temperature variations reported earlier for graphite oxide powders. Therefore, GO membranes are a distinct type of material with unique solvation properties compared to parent graphite oxides even if they are composed of the same graphene oxide flakes.The structure of graphene oxide (GO) membranes was studied in situ in liquid solvents using synchrotron radiation X-ray diffraction in a broad temperature interval. GO membranes are hydrated by water similarly to precursor graphite oxide powders but intercalation of alcohols is strongly hindered, which explains why the GO membranes are permeated by water and not by ethanol. Insertion of ethanol into the membrane structure is limited to only one monolayer in the whole studied temperature range, in contrast to precursor graphite oxide powders, which are intercalated with up to two ethanol monolayers (Brodie) and four ethanol monolayers (Hummers). As a result, GO membranes demonstrate the absence of ``negative thermal expansion'' and phase transitions connected to insertion/de-insertion of alcohols upon temperature variations reported earlier for graphite oxide powders. Therefore, GO membranes are a distinct type of material with unique solvation properties compared to parent graphite oxides even if they are composed of the same graphene oxide flakes. Electronic supplementary information (ESI) available. See DOI: 10.1039/c3nr04631a

Monolayer optical memory cells based on artificial trap-mediated charge storage and release

NASA Astrophysics Data System (ADS)

Lee, Juwon; Pak, Sangyeon; Lee, Young-Woo; Cho, Yuljae; Hong, John; Giraud, Paul; Shin, Hyeon Suk; Morris, Stephen M.; Sohn, Jung Inn; Cha, Seungnam; Kim, Jong Min

2017-03-01

Monolayer transition metal dichalcogenides are considered to be promising candidates for flexible and transparent optoelectronics applications due to their direct bandgap and strong light-matter interactions. Although several monolayer-based photodetectors have been demonstrated, single-layered optical memory devices suitable for high-quality image sensing have received little attention. Here we report a concept for monolayer MoS2 optoelectronic memory devices using artificially-structured charge trap layers through the functionalization of the monolayer/dielectric interfaces, leading to localized electronic states that serve as a basis for electrically-induced charge trapping and optically-mediated charge release. Our devices exhibit excellent photo-responsive memory characteristics with a large linear dynamic range of ~4,700 (73.4 dB) coupled with a low OFF-state current (<4 pA), and a long storage lifetime of over 104 s. In addition, the multi-level detection of up to 8 optical states is successfully demonstrated. These results represent a significant step toward the development of future monolayer optoelectronic memory devices.

Structural, mechanical and tribocorrosion behaviour in artificial seawater of CrN/AlN nano-multilayer coatings on F690 steel substrates

NASA Astrophysics Data System (ADS)

Ma, Fuliang; Li, Jinlong; Zeng, Zhixiang; Gao, Yimin

2018-01-01

The CrN monolayer and CrN/AlN nano-multilayer coating were successfully fabricated by reactive magnetron sputtering on F690 steel. The results show that CrN monolayer exhibits a face centered cubic crystalline structure with (111) preferred orientation and CrN/AlN nano-multilayer coating has a (200) preferred orientation. This design of the nano-multilayer can interrupt the continuous growth of columnar crystals making the coating denser. The CrN/AlN nano-multilayer coating has a better wear resistance and corrosion resistance compared with the CrN monolayer coating. The tribocorrosion tests reveal that the evolution of potential and current density of F690 steel and CrN monolayer or CrN/AlN nano-multilayer coating see an opposite trend under the simultaneous action of wear and corrosion, which is attributed to that F690 steel is a non-passive material and PVD coatings is a passive material. The nano-multilayer structure has a good ;Pore Sealing Effect;, and the corrosive solution is difficult to pass through the coating to corrode the substrate.

Intravascular hemolysis induced by phospholipases A2 from the venom of the Eastern coral snake, Micrurus fulvius: Functional profiles of hemolytic and non-hemolytic isoforms.

PubMed

Fernández, María Laura; Quartino, Pablo Yunes; Arce-Bejarano, Ruth; Fernández, Julián; Camacho, Luis F; Gutiérrez, José María; Kuemmel, Daniel; Fidelio, Gerardo; Lomonte, Bruno

2018-04-01

A unique feature of the venom of Micrurus fulvius (Eastern coral snake) is its ability to induce severe intravascular hemolysis in particular species, such as dogs or mice. This effect was previously shown to be induced by distinct phospholipase A 2 (PLA 2 ) isoforms which cause direct hemolysis in vitro, an uncommon finding for such enzymes. The functional profiles of PLA 2 -17, a direct hemolytic enzyme, and PLA 2 -12, a co-existing venom isoform lacking such effect, were compared. The enzymes differed not only in their ability to cause intravascular hemolysis: PLA 2 -17 additionally displayed lethal, myotoxic, and anticoagulant actions, whereas PLA 2 -12 lacked these effects. PLA 2 -12 was much more active in hydrolyzing a monodisperse synthetic substrate than PLA 2 -17, but the catalytic activity of latter was notably higher on a micellar substrate, or towards pure phospholipid artificial monolayers under controlled lateral pressures. Interestingly, PLA 2 -17 could hydrolyze substrate at a pressure of 20 mN m -1 , in contrast to PLA 2 -12 or the non-toxic pancreatic PLA 2 . This suggests important differences in the monolayer penetrating power, which could be related to differences in toxicity. Comparative examination of primary structures and predicted three-dimensional folding of PLA 2 -12 and PLA 2 -17, revealed that differences concentrate in their N-terminal and central regions, leading to variations of the surface properties at the membrane interacting interface. PLA 2 -17 presents a less basic interfacial surface than PLA 2 -12, but more bulky aromatic residues, which could be associated to its higher membrane-penetrating strength. Altogether, these structural and functional comparative observations suggest that the ability of PLA 2 s to penetrate substrate interfaces could be a major determinant of toxicity, perhaps more important than protein surface charge. Copyright © 2017 Elsevier B.V. All rights reserved.

Structural and electronic properties of the V-V compounds isoelectronic to GaN and isostructural to gray arsenic

NASA Astrophysics Data System (ADS)

Yang, Zhao; Han, Dan; Chen, Guohong; Chen, Shiyou

2018-03-01

The III-V binary compound semiconductors such as GaN, GaP, InN and InP have extensive applications in various optoelectronic, microwave and power-electronic devices. Using first-principles calculation, we systematically studied the structural and electronic properties of the V-V binary compounds (BiN, BiP, SbN and SbP) that are isoelectronic to GaN, GaP, InN and InP if Bi and Sb are in the +3 valence state. Interestingly, we found that the ground-state structures of BiP, SbN and SbP have the R-3m symmetry and are isostructural to the layered structure of gray arsenic, whereas BiN prefers a different ground-state structure with the C2 symmetry. Electronic structure calculations showed that the bulk BiN is a narrow bandgap semiconductor for its bandgap is about 0.2 eV. In contrast, BiP, SbN and SbP are metallic. The layered ground-state structure of the V-V binary compounds motivates us to study the electronic properties of their few-layer structures. As the structure becomes monolayer, their bandgaps increase significantly and are all in the range from about 1 eV to 1.7 eV, which are comparative to the bandgap of the monolayer gray arsenic. The monolayer BiP, SbN and SbP have indirect bandgaps, and they show a semiconductor-metal transition as the number of layers increase. Interestingly, the monolayer BiP has the largest splitting (350 meV) of the CBM valley, and thus may have potential application in novel spintronics and valleytronics devices.

Electronic and Optical Properties of Twisted Bilayer Graphene

NASA Astrophysics Data System (ADS)

Huang, Shengqiang

The ability to isolate single atomic layers of van der Waals materials has led to renewed interest in the electronic and optical properties of these materials as they can be fundamentally different at the monolayer limit. Moreover, these 2D crystals can be assembled together layer by layer, with controllable sequence and orientation, to form artificial materials that exhibit new features that are not found in monolayers nor bulk. Twisted bilayer graphene is one such prototype system formed by two monolayer graphene layers placed on top of each other with a twist angle between their lattices, whose electronic band structure depends on the twist angle. This thesis presents the efforts to explore the electronic and optical properties of twisted bilayer graphene by Raman spectroscopy and scanning tunneling microscopy measurements. We first synthesize twisted bilayer graphene with various twist angles via chemical vapor deposition. Using a combination of scanning tunneling microscopy and Raman spectroscopy, the twist angles are determined. The strength of the Raman G peak is sensitive to the electronic band structure of twisted bilayer graphene and therefore we use this peak to monitor changes upon doping. Our results demonstrate the ability to modify the electronic and optical properties of twisted bilayer graphene with doping. We also fabricate twisted bilayer graphene by controllable stacking of two graphene monolayers with a dry transfer technique. For twist angles smaller than one degree, many body interactions play an important role. It requires eight electrons per moire unit cell to fill up each band instead of four electrons in the case of a larger twist angle. For twist angles smaller than 0.4 degree, a network of domain walls separating AB and BA stacking regions forms, which are predicted to host topologically protected helical states. Using scanning tunneling microscopy and spectroscopy, these states are confirmed to appear on the domain walls when inversion symmetry is broken with an external electric field. We observe a double-line profile of these states on the domain walls, only occurring when the AB and BA regions are gaped. These states give rise to channels that could transport charge in a dissipationless manner making twisted bilayer graphene a promising platform to realize controllable topological networks for future applications.

Process for Making a Semiconductor Device with Barrier Film Formation Using a Metal Halide and Products Thereof

DTIC Science & Technology

1998-08-20

structure of the barrier film on an atomic level where the barrier film is comprised of a plurality of contiguous monolayers, while FIG. 7B shows...another embodiment where the barrier film is comprised of a plurality of i contiguous monolayers in which different monolayers thereof are formed... effusion cell, for example a barium fluoride, strontium fluoride or the like effusion cell, is provided at 32, and has a shutter 33. A 15 shutter 35

Growth Mechanism of Transition Metal Dichalcogenide Monolayers: The Role of Self-Seeding Fullerene Nuclei.

PubMed

Cain, Jeffrey D; Shi, Fengyuan; Wu, Jinsong; Dravid, Vinayak P

2016-05-24

Due to their unique optoelectronic properties and potential for next generation devices, monolayer transition metal dichalcogenides (TMDs) have attracted a great deal of interest since the first observation of monolayer MoS2 a few years ago. While initially isolated in monolayer form by mechanical exfoliation, the field has evolved to more sophisticated methods capable of direct growth of large-area monolayer TMDs. Chemical vapor deposition (CVD) is the technique used most prominently throughout the literature and is based on the sulfurization of transition metal oxide precursors. CVD-grown monolayers exhibit excellent quality, and this process is widely used in studies ranging from the fundamental to the applied. However, little is known about the specifics of the nucleation and growth mechanisms occurring during the CVD process. In this study, we have investigated the nucleation centers or "seeds" from which monolayer TMDs typically grow. This was accomplished using aberration-corrected scanning transmission electron microscopy to analyze the structure and composition of the nuclei present in CVD-grown MoS2-MoSe2 alloys. We find that monolayer growth proceeds from nominally oxi-chalcogenide nanoparticles which act as heterogeneous nucleation sites for monolayer growth. The oxi-chalcogenide nanoparticles are typically encased in a fullerene-like shell made of the TMD. Using this information, we propose a step-by-step nucleation and growth mechanism for monolayer TMDs. Understanding this mechanism may pave the way for precise control over the synthesis of 2D materials, heterostructures, and related complexes.

Network structure control of binary mixed langmuir monolayers of homo-PS and PS-b-P2VP.

PubMed

Wen, Gangyao

2010-03-25

Our recent work showed there existed a composition window for mixed Langmuir monolayers of homopolystyrene (h-PS) and a symmetric diblock copolymer polystyrene-block-poly(2-vinylpyridine) (PS-b-P2VP) to form necklace-network structures at the air/water interface. In order to study further the possible mechanism and control the network structure (i.e., surface coverage and nanoaggregate diameter), effects of spreading solution concentration and volume, subphase temperature, and transfer pressure on the network structure were studied by the Langmuir monolayer technique and tapping mode atomic force microscopy. With the increase of transfer pressure, there existed a novel nonlinear behavior for the nanoaggregate diameter first to increase, then to decrease, and finally to increase again, while the surface coverage tended to increase step by step. Moreover, with the elevation of temperature, chain motion between the adjoining nanoaggregates tended to be improved and thus the nanoaggregate diameter tended to be more uniform.

Direct measurement of the thickness-dependent electronic band structure of MoS2 using angle-resolved photoemission spectroscopy.

PubMed

Jin, Wencan; Yeh, Po-Chun; Zaki, Nader; Zhang, Datong; Sadowski, Jerzy T; Al-Mahboob, Abdullah; van der Zande, Arend M; Chenet, Daniel A; Dadap, Jerry I; Herman, Irving P; Sutter, Peter; Hone, James; Osgood, Richard M

2013-09-06

We report on the evolution of the thickness-dependent electronic band structure of the two-dimensional layered-dichalcogenide molybdenum disulfide (MoS2). Micrometer-scale angle-resolved photoemission spectroscopy of mechanically exfoliated and chemical-vapor-deposition-grown crystals provides direct evidence for the shifting of the valence band maximum from Γ to K, for the case of MoS2 having more than one layer, to the case of single-layer MoS2, as predicted by density functional theory. This evolution of the electronic structure from bulk to few-layer to monolayer MoS2 had earlier been predicted to arise from quantum confinement. Furthermore, one of the consequences of this progression in the electronic structure is the dramatic increase in the hole effective mass, in going from bulk to monolayer MoS2 at its Brillouin zone center, which is known as the cause for the decreased carrier mobility of the monolayer form compared to that of bulk MoS2.

In Situ Structural Studies of the Underpotential Deposition of Copper onto an Iodine Covered Platinum Surface Using X-Ray Standing Waves

DTIC Science & Technology

1991-01-01

electrocrystallization, catalysis, and surface chemistry. In this process, submonolayer to monolayer(s) amounts of a metal can be electrodeposited on a foreign...mechanisms involving nucleation and growth processes. Although electrochemical methods are invaluable in controlling and measuring thermodynamic...obtain direct atomic structural information about metal deposits on an iodine covered Pt(IIl) surface . They found that electrodeposition occurred in a

Lead bromide-based layered perovskite Langmuir-Blodgett films having π-conjugated molecules as organic layer prepared by using squeezed out technique

NASA Astrophysics Data System (ADS)

Era, Masanao; Shironita, Yu; Soda, Koichi

2018-03-01

Using the squeezed out technique, we successfully prepared PbBr-based layered perovskite Langmuir-Blodgett (LB) films, which have π-conjugated materials as an organic layer (i.e., a phenylenevinylene oligomer, a dithienylethene derivative, and a π-conjugated polyfluorene derivative). The mixed monolayers of π-conjugated materials and octadecylammonium bromide were spread on an aqueous subphase containing saturated PbBr2. During pressing, octadecylammonium molecules were squeezed from the mixed monolayer, and the squeezed ammonium molecules formed the PbBr-based layered perovskite structure at the air-aqueous subphase interface. The monolayers with the PbBr-based layered perovskite structure could be deposited on fused quartz substrates by the LB technique. In addition to the preparation procedure, the structural and optical properties of the layered perovskite LB films and their formation mechanism are reported in this paper.

Ordering of Air-Oxidized Decanethiols on Au(111).

PubMed

Sotthewes, Kai; Kap, Özlem; Wu, Hairong; Thompson, Damien; Huskens, Jurriaan; Zandvliet, Harold J W

2018-04-19

Self-assembled monolayers (SAMs) of alkanethiols on gold are a commonly used platform for nanotechnology owing to their ease of preparation and high surface coverage. Unfortunately, the gold-sulfur bond is oxidized at ambient conditions which alters the stability and structure of the monolayer. We show using scanning tunneling microscopy and X-ray photoelectron spectroscopy that decanethiolate molecules oxidize into decanesulfonates that organize into a hitherto unknown striped phase. Air-exposed SAMs oxidize, as can be determined by a shift of the S 2p peak and the appearance of O 1s photoelectrons as part of the decanethiol monolayer transforms into a lamellae-like decanesulfonate structure when exposed to air. The herringbone structure of the Au(111) surface is preserved, indicating that the interaction between the molecules and the surface is rather weak as these findings are substantiated by density functional theory calculations.

FRET study of G-quadruplex forming fluorescent oligonucleotide probes at the lipid monolayer interface.

PubMed

Swiatkowska, Angelika; Kosman, Joanna; Juskowiak, Bernard

2016-01-05

Spectral properties and G-quadruplex folding ability of fluorescent oligonucleotide probes at the cationic dioctadecyldimethylammonium bromide (DODAB) monolayer interface are reported. Two oligonucleotides, a 19-mer bearing thrombin binding aptamer sequence and a 21-mer with human telomeric sequence, were end-labeled with fluorescent groups (FAM and TAMRA) to give FRET probes F19T and F21T, respectively. The probes exhibited abilities to fold into a quadruplex structure and to bind metal cations (Na(+) and K(+)). Fluorescence spectra of G-quadruplex FRET probes at the monolayer interface are reported for the first time. Investigations included film balance measurements (π-A isotherms) and fluorescence spectra recording using a fiber optic accessory interfaced with a spectrofluorimeter. The effect of the presence of DODAB monolayer, metal cations and the surface pressure of monolayer on spectral behavior of FRET probes were examined. Adsorption of probe at the cationic monolayer interface resulted in the FRET signal enhancement even in the absence of metal cations. Variation in the monolayer surface pressure exerted rather modest effect on the spectral properties of probes. The fluorescence energy transfer efficiency of monolayer adsorbed probes increased significantly in the presence of sodium or potassium ion in subphase, which indicated that the probes retained their cation binding properties when adsorbed at the monolayer interface. Copyright © 2015 Elsevier B.V. All rights reserved.

Calculating the Maximum Density of the Surface Packing of Ions in Ionic Liquids

NASA Astrophysics Data System (ADS)

Kislenko, S. A.; Moroz, Yu. O.; Karu, K.; Ivaništšev, V. B.; Fedorov, M. V.

2018-05-01

The maximum density of monolayer packing on a graphene surface is calculated by means of molecular dynamics (MD) for ions of characteristic size and symmetry: 1-butyl-3-methylimidazolium [BMIM]+, tetrabutylammonium [TBA]+, tetrafluoroborate [BF4]-, dicyanamide [DCA]-, and bis(trifluoromethane) sulfonimide [TFSI]-. The characteristic orientations of ions in a closely packed monolayer are found. It is shown that the formation of a closely packed monolayer is possible for [DCA]- and [BF4]- anions only at surface charges that exceed the limit of the electrochemical stability of the corresponding ionic liquids. For the [TBA]+ cation, a monolayer structure can be observed at the charge of nearly 30 μC/cm2 attainable in electrochemical experiment.

On the interaction of the highly charged peptides casocidins with biomimetic membranes.

PubMed

Becucci, Lucia; Aloisi, Giovanni; Scaloni, Andrea; Caira, Simonetta; Guidelli, Rolando

2018-04-19

Casocidin I and II (CI and CII) are structurally related antimicrobial peptides made of 39 and 31 amino acids, respectively, which derive from natural proteolytic processing of α s2 -casein and adopt an ordered α-helical structure in biomimetic membranes. Their putative membrane-permeabilizing activity was investigated at Hg-supported self-assembled monolayers (SAMs) and at tethered bilayer lipid membranes (tBLMs); the latter consisted of a monolayer of 2,3,di-O-phytanyl-sn-glycerol-1-tetraethylene-glycol-d,l-α lipoic acid ester thiolipid (DPTL), with a dioleoylphosphatidylcholine (DOPC) or dioleoylphosphatidylserine (DOPS) monolayer on top of it. Interaction of CI and CII with these biomimetic membranes was studied by four electrochemical techniques at pH 3, 5.4 and 6.8. Peptide incorporation in tBLMs was attempted via scans of electrochemical impedance spectra. Experiments demonstrated that CI and CII penetrate SAMs as well as the distal DOPC monolayer of DPTL/DOPC tBLMs, but not the proximal phytanyl monolayer, with the only exception of CII at pH 5.4. Conversely, CII permeabilized DPTL/DOPS tBLMs to a moderate extent at all investigated pH values by forming holes across the membrane, but not ion channels. Structural distribution of charged residues seemed to prevent CII from having a hydrophobic side of the α-helix capable of stabilizing a regular ion channel in the lipid matrix. Copyright © 2018 Elsevier B.V. All rights reserved.

Omeprazole decreases magnesium transport across Caco-2 monolayers

PubMed Central

Thongon, Narongrit; Krishnamra, Nateetip

2011-01-01

AIM: To elucidate the effect and underlying mechanisms of omeprazole action on Mg2+ transport across the intestinal epithelium. METHODS: Caco-2 monolayers were cultured in various dose omeprazole-containing media for 14 or 21 d before being inserted into a modified Ussing chamber apparatus to investigate the bi-directional Mg2+ transport and electrical parameters. Paracellular permeability of the monolayer was also observed by the dilution potential technique and a cation permeability study. An Arrhenius plot was performed to elucidate the activation energy of passive Mg2+ transport across the Caco-2 monolayers. RESULTS: Both apical to basolateral and basolateral to apical passive Mg2+ fluxes of omeprazole-treated epithelium were decreased in a dose- and time-dependent manner. Omeprazole also decreased the paracellular cation selectivity and changed the paracellular selective permeability profile of Caco-2 epithelium to Li+, Na+, K+, Rb+, and Cs+ from series VII to series VI of the Eisenman sequence. The Arrhenius plot revealed the higher activation energy for passive Mg2+ transport in omeprazole-treated epithelium than that of control epithelium, indicating that omeprazole affected the paracellular channel of Caco-2 epithelium in such a way that Mg2+ movement was impeded. CONCLUSION: Omeprazole decreased paracellular cation permeability and increased the activation energy for passive Mg2+ transport of Caco-2 monolayers that led to the suppression of passive Mg2+ absorption. PMID:21472124

Omeprazole decreases magnesium transport across Caco-2 monolayers.

PubMed

Thongon, Narongrit; Krishnamra, Nateetip

2011-03-28

To elucidate the effect and underlying mechanisms of omeprazole action on Mg(2+) transport across the intestinal epithelium. Caco-2 monolayers were cultured in various dose omeprazole-containing media for 14 or 21 d before being inserted into a modified Ussing chamber apparatus to investigate the bi-directional Mg(2+) transport and electrical parameters. Paracellular permeability of the monolayer was also observed by the dilution potential technique and a cation permeability study. An Arrhenius plot was performed to elucidate the activation energy of passive Mg(2+) transport across the Caco-2 monolayers. Both apical to basolateral and basolateral to apical passive Mg(2+) fluxes of omeprazole-treated epithelium were decreased in a dose- and time-dependent manner. Omeprazole also decreased the paracellular cation selectivity and changed the paracellular selective permeability profile of Caco-2 epithelium to Li(+), Na(+), K(+), Rb(+), and Cs(+) from series VII to series VI of the Eisenman sequence. The Arrhenius plot revealed the higher activation energy for passive Mg(2+) transport in omeprazole-treated epithelium than that of control epithelium, indicating that omeprazole affected the paracellular channel of Caco-2 epithelium in such a way that Mg(2+) movement was impeded. Omeprazole decreased paracellular cation permeability and increased the activation energy for passive Mg(2+) transport of Caco-2 monolayers that led to the suppression of passive Mg(2+) absorption.

Large-area synthesis of high-quality monolayer 1T’-WTe2 flakes

NASA Astrophysics Data System (ADS)

Naylor, Carl H.; Parkin, William M.; Gao, Zhaoli; Kang, Hojin; Noyan, Mehmet; Wexler, Robert B.; Tan, Liang Z.; Kim, Youngkuk; Kehayias, Christopher E.; Streller, Frank; Zhou, Yu Ren; Carpick, Robert; Luo, Zhengtang; Park, Yung Woo; Rappe, Andrew M.; Drndić, Marija; Kikkawa, James M.; Johnson, A. T. Charlie

2017-06-01

Large-area growth of monolayer films of the transition metal dichalcogenides is of the utmost importance in this rapidly advancing research area. The mechanical exfoliation method offers high quality monolayer material but it is a problematic approach when applied to materials that are not air stable. One important example is 1T’-WTe2, which in multilayer form is reported to possess a large non saturating magnetoresistance, pressure induced superconductivity, and a weak antilocalization effect, but electrical data for the monolayer is yet to be reported due to its rapid degradation in air. Here we report a reliable and reproducible large-area growth process for obtaining many monolayer 1T’-WTe2 flakes. We confirmed the composition and structure of monolayer 1T’-WTe2 flakes using x-ray photoelectron spectroscopy, energy-dispersive x-ray spectroscopy, atomic force microscopy, Raman spectroscopy and aberration corrected transmission electron microscopy. We studied the time dependent degradation of monolayer 1T’-WTe2 under ambient conditions, and we used first-principles calculations to identify reaction with oxygen as the degradation mechanism. Finally we investigated the electrical properties of monolayer 1T’-WTe2 and found metallic conduction at low temperature along with a weak antilocalization effect that is evidence for strong spin-orbit coupling.

Understanding the structural, electrical, and optical properties of monolayer h-phase RuO2 nanosheets: a combined experimental and computational study

NASA Astrophysics Data System (ADS)

Ko, Dong-Su; Lee, Woo-Jin; Sul, Soohwan; Jung, Changhoon; Yun, Dong-Jin; Kim, Hee-Goo; Son, Won-Joon; Chung, Jae Gwan; Jung, Doh Won; Kim, Se Yun; Kim, Jeongmin; Lee, Wooyoung; Kwak, Chan; Shin, Jai Kwang; Kim, Jung-Hwa; Roh, Jong Wook

2018-04-01

The structural, electrical, and optical properties of monolayer ruthenium oxide (RuO2) nanosheets (NSs) fabricated by chemical exfoliation of a layered three-dimensional form of K-intercalated RuO2 are studied systematically via experimental and computational methods. Monolayer RuO2 NS is identified as having a distorted h-MX2 structure. This is the first observation of a RuO2 NS structure that is unlike the t-MX2 structure of the RuO2 layers in the parent material and does not have hexagonal symmetry. The distorted h-MX2 RuO2 NSs are shown to have optical transparency superior to that of graphene, thereby predicting the feasibility of applying RuO2 NSs to flexible transparent electrodes. In addition, it is demonstrated that the semiconducting band structures of RuO2 NSs can be manipulated to be semi-metallic by adjusting the crystal structure, which is related to band-gap engineering. This finding indicates that RuO2 NSs can be used in a variety of applications, such as flexible transparent electrodes, atomic-layer devices, and optoelectronic devices.

Spectroscopic study of fluorescent probes based on G-quadruplex oligonucleotides labeled with ethynylpyrenyldeoxyuridine.

PubMed

Switalska, Angelika; Kierzek, Ryszard; Dembska, Anna; Juskowiak, Bernard

2017-12-01

The design, synthesis, and spectral properties of four pyrene labeled oligonucleotide probes with G-quadruplex structure (Tel22-Tpy, Tel22-Upy, Tel22-6Upy, Tel22-18Upy) based on the 22-mer human telomeric sequence (Tel22) have been reported. Pyrene labels in the form of ethynylpyrenyldeoxyuridine have been inserted efficiently into oligodeoxynucleotides probes using phosphoramidite chemistry. The probes exhibited abilities to fold into G-quadruplex structures and to bind metal cations (Na + and K + ). Folding properties of probes and their spectral behavior were examined by recording the UV-vis, fluorescence, and CD spectra as well as by analyzing melting profiles. Fluorescence characteristics and G-quadruplex folding of probes were also studied at the interface of cationic dioctadecyldimethylammonium bromide (DODAB) monolayer. Investigations included film balance measurements (π-A isotherms) and fluorescence spectra recording using a fiber optic accessory interfaced with a spectrofluorimeter. Copyright © 2017 Elsevier B.V. All rights reserved.

Coadsorption of Human Milk Lactoferrin into the Dipalmitoylglycerolphosphatidylcholine Phospholipid Monolayer Spread at the Air/Water Interface

PubMed Central

Miano, Fausto; Zhao, Xiubo; Lu, Jian R.; Penfold, Jeff

2007-01-01

The coadsorption of human milk lactoferrin into a spread monolayer of dipalmitoylglycerol phosphatidylcholine (DPPC) at the air/water interface has been studied by neutron reflection. The system is a good model of the preocular tear film outer interface, which was the motivation for the study. The association of the protein with the surface was indicated by an increase of the surface pressure exerted by the DPPC monolayer. The extent of lactoferrin coadsorption was found to decrease with increasing surface pressure in the lipid monolayer, a trend consistent with the observation reported for other proteins, such as lysozyme and β-lactoglobulin. The neutron reflectivity measurements were subsequently carried out at the three surface pressures of 8, 15, and 35 mN/m to examine the structure and composition of lactoferrin coadsorbed at the interface. Whereas the DPPC monolayer effectively prevented lactoferrin insertion at the high surface pressure, a measurable amount of lactoferrin was found at the air/water interface at the two lower surface pressures. At 15 mN/m it was difficult to identify the distribution of lactoferrin with respect to the DPPC monolayer, due to its relatively low adsorbed amount and much broader distribution. At the lowest surface pressure of 8 mN/m, the lactoferrin coadsorption was found to increase with time over the first few hours. After 5 h the distribution of the lactoferrin layer became similar to, though quantitatively lower than, that adsorbed in the absence of the DPPC monolayer. It is characterized by a top dense sublayer of 15 Å with a bottom diffuse sublayer of 60 Å, indicating structural unfolding induced by surface adsorption under these conditions. PMID:17114223

On the inclusion of alkanes into the monolayer of aliphatic alcohols at the water/alkane vapor interface: a quantum chemical approach.

PubMed

Vysotsky, Yuri B; Fomina, Elena S; Belyaeva, Elena A; Fainerman, Valentin B; Vollhardt, Dieter

2013-02-14

In the framework of the quantum chemical semiempirical PM3 method thermodynamic and structural parameters of the formation and clusterization of aliphatic alcohols C(n)H(2n+1)OH (n(OH) = 8-16) at 298 K at the water/alkane vapor C(n)H(2n+2), (n(CH(3)) = 6-16) interface were calculated. The dependencies of enthalpy, entropy and Gibbs' energy of clusterization per one monomer molecule of 2D films on the alkyl chain length of corresponding alcohols and alkanes, the molar fraction of alkanes in the monolayers and the immersion degree of alcohol molecules into the water phase were shown to be linear or stepwise. The threshold of spontaneous clusterization of aliphatic alcohols at the water/alkane vapor interface was 10-11 carbon atoms at 298 K which is in line with experimental data at the air/water interface. It is shown that the presence of alkane vapor does not influence the process of alcohol monolayer formation. The structure of these monolayers is analogous to those obtained at the air/water interface in agreement with experimental data. The inclusion of alkane molecules into the amphiphilic monolayer at the water/alkane vapor interface is possible for amphiphiles with the spontaneous clusterization threshold at the air/water interface (n(s)(0)) of at least 16 methylene units in the alkyl chain, and it does not depend on the molar fraction of alkanes in the corresponding monolayer. The inclusion of alkanes from the vapor phase into the amphiphilic monolayer also requires that the difference between the alkyl chain lengths of alcohols and alkanes is not larger than n(s)(0) - 15 and n(s)(0) - 14 for the 2D film 1 and 2D film 2, respectively.

Proteomic profiling of halloysite clay nanotube exposure in intestinal cell co-culture

PubMed Central

Lai, Xianyin; Agarwal, Mangilal; Lvov, Yuri M.; Pachpande, Chetan; Varahramyan, Kody; Witzmann, Frank A.

2013-01-01

Halloysite is aluminosilicate clay with a hollow tubular structure with nanoscale internal and external diameters. Assessment of halloysite biocompatibility has gained importance in view of its potential application in oral drug delivery. To investigate the effect of halloysite nanotubes on an in vitro model of the large intestine, Caco-2/HT29-MTX cells in monolayer co-culture were exposed to nanotubes for toxicity tests and proteomic analysis. Results indicate that halloysite exhibits a high degree of biocompatibility characterized by an absence of cytotoxicity, in spite of elevated pro-inflammatory cytokine release. Exposure-specific changes in expression were observed among 4081 proteins analyzed. Bioinformatic analysis of differentially expressed protein profiles suggest that halloysite stimulates processes related to cell growth and proliferation, subtle responses to cell infection, irritation and injury, enhanced antioxidant capability, and an overall adaptive response to exposure. These potentially relevant functional effects warrant further investigation in in vivo models and suggest that chronic or bolus occupational exposure to halloysite nanotubes may have unintended outcomes. PMID:23606564

Isostructural solid-solid phase transition in monolayers of soft core-shell particles at fluid interfaces: structure and mechanics.

PubMed

Rey, Marcel; Fernández-Rodríguez, Miguel Ángel; Steinacher, Mathias; Scheidegger, Laura; Geisel, Karen; Richtering, Walter; Squires, Todd M; Isa, Lucio

2016-04-21

We have studied the complete two-dimensional phase diagram of a core-shell microgel-laden fluid interface by synchronizing its compression with the deposition of the interfacial monolayer. Applying a new protocol, different positions on the substrate correspond to different values of the monolayer surface pressure and specific area. Analyzing the microstructure of the deposited monolayers, we discovered an isostructural solid-solid phase transition between two crystalline phases with the same hexagonal symmetry, but with two different lattice constants. The two phases corresponded to shell-shell and core-core inter-particle contacts, respectively; with increasing surface pressure the former mechanically failed enabling the particle cores to come into contact. In the phase-transition region, clusters of particles in core-core contacts nucleate, melting the surrounding shell-shell crystal, until the whole monolayer moves into the second phase. We furthermore measured the interfacial rheology of the monolayers as a function of the surface pressure using an interfacial microdisk rheometer. The interfaces always showed a strong elastic response, with a dip in the shear elastic modulus in correspondence with the melting of the shell-shell phase, followed by a steep increase upon the formation of a percolating network of the core-core contacts. These results demonstrate that the core-shell nature of the particles leads to a rich mechanical and structural behavior that can be externally tuned by compressing the interface, indicating new routes for applications, e.g. in surface patterning or emulsion stabilization.

Disorder-derived, strong tunneling attenuation in bis-phosphonate monolayers.

PubMed

Pathak, Anshuma; Bora, Achyut; Liao, Kung-Ching; Schmolke, Hannah; Jung, Antje; Klages, Claus-Peter; Schwartz, Jeffrey; Tornow, Marc

2016-03-09

Monolayers of alkyl bisphosphonic acids (bisPAs) of various carbon chain lengths (C4, C8, C10, C12) were grown on aluminum oxide (AlO(x)) surfaces from solution. The structural and electrical properties of these self-assembled monolayers (SAMs) were compared with those of alkyl monophosphonic acids (monoPAs). Through contact angle (CA) and Kelvin-probe (KP) measurements, ellipsometry, and infrared (IR) and x-ray photoelectron (XPS) spectroscopies, it was found that bisPAs form monolayers that are relatively disordered compared to their monoPA analogs. Current-voltage (J-V) measurements made with a hanging Hg drop top contact show tunneling to be the prevailing transport mechanism. However, while the monoPAs have an observed decay constant within the typical range for dense monolayers, β(mono)  =  0.85  ±  0.03 per carbon atom, a surprisingly high value, β(bis) =  1.40  ±  0.05 per carbon atom, was measured for the bisPAs. We attribute this to a strong contribution of 'through-space' tunneling, which derives from conformational disorder in the monolayer due to strong interactions of the distal phosphonic acid groups; they likely form a hydrogen-bonding network that largely determines the molecular layer structure. Since bisPA SAMs attenuate tunnel currents more effectively than do the corresponding monoPA SAMs, they may find future application as gate dielectric modification in organic thin film devices.

Disorder-derived, strong tunneling attenuation in bis-phosphonate monolayers

NASA Astrophysics Data System (ADS)

Pathak, Anshuma; Bora, Achyut; Liao, Kung-Ching; Schmolke, Hannah; Jung, Antje; Klages, Claus-Peter; Schwartz, Jeffrey; Tornow, Marc

2016-03-01

Monolayers of alkyl bisphosphonic acids (bisPAs) of various carbon chain lengths (C4, C8, C10, C12) were grown on aluminum oxide (AlO x ) surfaces from solution. The structural and electrical properties of these self-assembled monolayers (SAMs) were compared with those of alkyl monophosphonic acids (monoPAs). Through contact angle (CA) and Kelvin-probe (KP) measurements, ellipsometry, and infrared (IR) and x-ray photoelectron (XPS) spectroscopies, it was found that bisPAs form monolayers that are relatively disordered compared to their monoPA analogs. Current-voltage (J-V) measurements made with a hanging Hg drop top contact show tunneling to be the prevailing transport mechanism. However, while the monoPAs have an observed decay constant within the typical range for dense monolayers, β mono  =  0.85  ±  0.03 per carbon atom, a surprisingly high value, β bis  =  1.40  ±  0.05 per carbon atom, was measured for the bisPAs. We attribute this to a strong contribution of ‘through-space’ tunneling, which derives from conformational disorder in the monolayer due to strong interactions of the distal phosphonic acid groups; they likely form a hydrogen-bonding network that largely determines the molecular layer structure. Since bisPA SAMs attenuate tunnel currents more effectively than do the corresponding monoPA SAMs, they may find future application as gate dielectric modification in organic thin film devices.

Domain size polydispersity effects on the structural and dynamical properties in lipid monolayers with phase coexistence

NASA Astrophysics Data System (ADS)

Rufeil-Fiori, Elena; Banchio, Adolfo J.

Lipid monolayers with phase coexistence are a frequently used model for lipid membranes. In these systems, domains of the liquid-condensed phase always present size polydispersity. However, very few theoretical works consider size distribution effects on the monolayer properties. Because of the difference in surface densities, domains have excess dipolar density with respect to the surrounding liquid expanded phase, originating a dipolar inter-domain interaction. This interaction depends on the domain area, and hence the presence of a domain size distribution is associated with interaction polydispersity. Inter-domain interactions are fundamental to understanding the structure and dynamics of the monolayer. For this reason, it is expected that polydispersity significantly alters monolayer properties. By means of Brownian dynamics simulations, we study the radial distribution function (RDF), the average mean square displacement and the average time-dependent self-diffusion coefficient, D(t), of lipid monolayers with normal distributed size domains. It was found that polydispersity strongly affects the value of the interaction strength obtained, which is greatly underestimated if polydispersity is not considered. However, within a certain range of parameters, the RDF obtained from a polydisperse model can be well approximated by that of a monodisperse model, suitably fitting the interaction strength, even for 40% polydispersities. For small interaction strengths or small polydispersities, the polydisperse systems obtained from fitting the experimental RDF have an average mean square displacement and D(t) in good agreement with that of the monodisperse system.

Structural and shear characteristics of adsorbed sodium caseinate and monoglyceride mixed monolayers at the air-water interface.

PubMed

Rodríguez Patino, Juan M; Cejudo Fernández, Marta; Carrera Sánchez, Cecilio; Rodríguez Niño, Ma Rosario

2007-09-01

The structural and shear characteristics of mixed monolayers formed by an adsorbed Na-caseinate film and a spread monoglyceride (monopalmitin or monoolein) on the previously adsorbed protein film have been analyzed. Measurements of the surface pressure (pi)-area (A) isotherm and surface shear viscosity (eta(s)) were obtained at 20 degrees C and at pH 7 in a modified Wilhelmy-type film balance. The structural and shear characteristics of the mixed films depend on the surface pressure and on the composition of the mixed film. At surface pressures lower than the equilibrium surface pressure of Na-caseinate (at pipi(e)(CS) have important repercussions on the shear characteristics of the mixed films.

Effects of structure on the interactions between five natural antimicrobial compounds and phospholipids of bacterial cell membrane on model monolayers

USDA-ARS?s Scientific Manuscript database

Monolayers composed of bacterial phospholipids were used as model membranes to study interactions of naturally occurring phenolic compounds 2,5-dihydroxybenzaldehyde, 2-hydroxy-5-methoxybenzaldehyde and the plant essential oil compounds carvacrol, cinnamaldehyde, and geraniol, previously found to be...

Integration of Ganglioside GT1b Receptor into DPPE and DPPC Phospholipid Monolayers: An X-Ray Reflectivity and Grazing-Incidence Diffraction Study

PubMed Central

Miller, C. E.; Busath, D. D.; Strongin, B.; Majewski, J.

2008-01-01

Using synchrotron grazing-incidence x-ray diffraction (GIXD) and reflectivity, the in-plane and out-of-plane structures of mixed-ganglioside GT1b-phospholipid monolayers were investigated at the air-liquid interface and compared with monolayers of the pure components. The receptor GT1b is involved in the binding of lectins and toxins, including botulinum neurotoxin, to cell membranes. Monolayers composed of 20 mol % ganglioside GT1b, the phospholipid dipalmitoyl phosphatidylethanolamine (DPPE), and the phospholipid dipalmitoyl phosphatidylcholine (DPPC) were studied in the gel phase at 23°C and at surface pressures of 20 and 40 mN/m, and at pH 7.4 and 5. Under these conditions, the two components did not phase-separate, and no evidence of domain formation was observed. The x-ray scattering measurements revealed that GT1b was intercalated within the host DPPE/DPPC monolayers, and slightly expanded DPPE but condensed the DPPC matrix. The oligosaccharide headgroups extended normally from the monolayer surfaces into the subphase. This study demonstrated that these monolayers can serve as platforms for investigating toxin membrane binding and penetration. PMID:18599631

Pseudorotational epitaxy of self-assembled octadecyltrichlorosilane monolayers on sapphire (0001)

DOE PAGES

Steinrück, H. -G.; Magerl, A.; Deutsch, M.; ...

2014-10-06

The structure of octadecyltrichlorosilane self-assembled monolayers (SAMs) on sapphire (0001) was studied by Å-resolution surface-specific x-ray scattering methods. The monolayer was found to consist of three sublayers where the outermost layer corresponds to vertically oriented, closely packed alkyl tails. Laterally, the monolayer is hexagonally packed and exhibits pseudorotational epitaxy to the sapphire, manifested by a broad scattering peak at zero relative azimuthal rotation, with long powderlike tails. The lattice mismatch of ~1% – 3% to the sapphire’s and the different length scale introduced by the lateral Si-O-Si bonding prohibit positional epitaxy. However, the substrate induces an intriguing increase in themore » crystalline coherence length of the SAM’s powderlike crystallites when rotationally aligned with the sapphire’s lattice. As a result, the increase correlates well with the rotational dependence of the separation of corresponding substrate-monolayer lattice sites.« less

Study of zinc-induced changes in lymphocyte membranes using atomic force microscopy, luminescence, and light scattering methods

NASA Astrophysics Data System (ADS)

Filimonenko, D. S.; Khairullina, A. Ya.; Yasinskii, V. M.; Kozlova, N. M.; Zubritskaja, G. P.; Slobozhanina, E. I.

2011-07-01

Changes in the surface structure of lymphocyte membranes exposed to various concentrations of zinc ions are studied. It is found by atomic force microscopy that increasing the concentration of zinc ions leads to a reduction in the correlation length of the autocorrelation function of the roughness profile of a lymphocyte compared to control samples; this may indicate the existence of fine structure in the membrane surface. Fluorescence markers are used to observe a reduction in the microviscosity of the lipids in the outer monolayer of the lipid bilayer after lymphocytes are exposed to Zn ions, as well as the exposure of phosphatidylserine on the surface membrane, and the oxidation of HS-groups of membrane proteins. Calculations of the absorption coefficients of lymphocytes modified with zinc reveal the existence of absorption bands owing to the formation of metal-protein complexes and zinc oxide nanoparticles. These results indicate significant changes in the structural and functional state of lymphocyte membranes exposed to zinc ions.

Effect of Structure and Disorder on the Charge Transport in Defined Self-Assembled Monolayers of Organic Semiconductors.

PubMed

Schmaltz, Thomas; Gothe, Bastian; Krause, Andreas; Leitherer, Susanne; Steinrück, Hans-Georg; Thoss, Michael; Clark, Timothy; Halik, Marcus

2017-09-26

Self-assembled monolayer field-effect transistors (SAMFETs) are not only a promising type of organic electronic device but also allow detailed analyses of structure-property correlations. The influence of the morphology on the charge transport is particularly pronounced, due to the confined monolayer of 2D-π-stacked organic semiconductor molecules. The morphology, in turn, is governed by relatively weak van-der-Waals interactions and is thus prone to dynamic structural fluctuations. Accordingly, combining electronic and physical characterization and time-averaged X-ray analyses with the dynamic information available at atomic resolution from simulations allows us to characterize self-assembled monolayer (SAM) based devices in great detail. For this purpose, we have constructed transistors based on SAMs of two molecules that consist of the organic p-type semiconductor benzothieno[3,2-b][1]benzothiophene (BTBT), linked to a C 11 or C 12 alkylphosphonic acid. Both molecules form ordered SAMs; however, our experiments show that the size of the crystalline domains and the charge-transport properties vary considerably in the two systems. These findings were confirmed by molecular dynamics (MD) simulations and semiempirical molecular-orbital electronic-structure calculations, performed on snapshots from the MD simulations at different times, revealing, in atomistic detail, how the charge transport in organic semiconductors is influenced and limited by dynamic disorder.

Electronic Structure of a Self-Assembled Monolayer with Two Surface Anchors: 6-Mercaptopurine on Au(111).

PubMed

Fernández, Cynthia C; Pensa, Evangelina; Carro, Pilar; Salvarezza, Roberto; Williams, Federico J

2018-05-22

The electronic structure of aromatic and aliphatic thiols on Au(111) has been extensively studied in relation to possible applications in molecular electronics. In this work, the effect on the electronic structure of an additional anchor to the S-Au bond using 6-mercaptopurine as a model system has been investigated. Results from X-ray photoelectron spectroscopy, near-edge X-ray absorption fine structure spectroscopy, and density functional theory (DFT) confirm that this molecule adsorbs on Au(111) with S-Au and iminic N-Au bonds. Combined ultraviolet photoelectron spectroscopy and DFT data reveal that formation of the 6MP self-assembled monolayer generates a molecular dipole perpendicular to the surface, with negative charges residing at the metal/monolayer interface and positive charges at the monolayer/vacuum interface, which lowers the substrate work function. Scanning tunneling microscopy shows two surface molecular domains: a well-ordered rectangular lattice where molecules are tilted on average 30° with respect to the substrate and aligned 6MP islands where molecules are standing upright. Finally, we found a new electronic state located at -1.7 eV with respect to the Fermi level that corresponds to a localized π molecular state, while the state corresponding to the N-Au bond is hybridized with Au d electrons and stabilized at much lower energies (-3 eV).

Cascaded exciton energy transfer in a monolayer semiconductor lateral heterostructure assisted by surface plasmon polariton.

PubMed

Shi, Jinwei; Lin, Meng-Hsien; Chen, I-Tung; Mohammadi Estakhri, Nasim; Zhang, Xin-Quan; Wang, Yanrong; Chen, Hung-Ying; Chen, Chun-An; Shih, Chih-Kang; Alù, Andrea; Li, Xiaoqin; Lee, Yi-Hsien; Gwo, Shangjr

2017-06-26

Atomically thin lateral heterostructures based on transition metal dichalcogenides have recently been demonstrated. In monolayer transition metal dichalcogenides, exciton energy transfer is typically limited to a short range (~1 μm), and additional losses may be incurred at the interfacial regions of a lateral heterostructure. To overcome these challenges, here we experimentally implement a planar metal-oxide-semiconductor structure by placing a WS 2 /MoS 2 monolayer heterostructure on top of an Al 2 O 3 -capped Ag single-crystalline plate. We find that the exciton energy transfer range can be extended to tens of microns in the hybrid structure mediated by an exciton-surface plasmon polariton-exciton conversion mechanism, allowing cascaded exciton energy transfer from one transition metal dichalcogenides region supporting high-energy exciton resonance to a different transition metal dichalcogenides region in the lateral heterostructure with low-energy exciton resonance. The realized planar hybrid structure combines two-dimensional light-emitting materials with planar plasmonic waveguides and offers great potential for developing integrated photonic and plasmonic devices.Exciton energy transfer in monolayer transition metal dichalcogenides is limited to short distances. Here, Shi et al. fabricate a planar metal-oxide-semiconductor structure and show that exciton energy transfer can be extended to tens of microns, mediated by an exciton-surface-plasmon-polariton-exciton conversion mechanism.

Combined Diffraction and Density Functional Theory Calculations of Halogen-Bonded Cocrystal Monolayers

PubMed Central

2013-01-01

This work describes the combined use of synchrotron X-ray diffraction and density functional theory (DFT) calculations to understand the cocrystal formation or phase separation in 2D monolayers capable of halogen bonding. The solid monolayer structure of 1,4-diiodobenzene (DIB) has been determined by X-ray synchrotron diffraction. The mixing behavior of DIB with 4,4′-bipyridyl (BPY) has also been studied and interestingly is found to phase-separate rather than form a cocrystal, as observed in the bulk. DFT calculations are used to establish the underlying origin of this interesting behavior. The DFT calculations are demonstrated to agree well with the recently proposed monolayer structure for the cocrystal of BPY and 1,4-diiodotetrafluorobenzene (DITFB) (the perfluorinated analogue of DIB), where halogen bonding has also been identified by diffraction. Here we have calculated an estimate of the halogen bond strength by DFT calculations for the DITFB/BPY cocrystal monolayer, which is found to be ∼20 kJ/mol. Computationally, we find that the nonfluorinated DIB and BPY are not expected to form a halogen-bonded cocrystal in a 2D layer; for this pair of species, phase separation of the components is calculated to be lower energy, in good agreement with the diffraction results. PMID:24215390

Combined diffraction and density functional theory calculations of halogen-bonded cocrystal monolayers.

PubMed

Sacchi, Marco; Brewer, Adam Y; Jenkins, Stephen J; Parker, Julia E; Friščić, Tomislav; Clarke, Stuart M

2013-12-03

This work describes the combined use of synchrotron X-ray diffraction and density functional theory (DFT) calculations to understand the cocrystal formation or phase separation in 2D monolayers capable of halogen bonding. The solid monolayer structure of 1,4-diiodobenzene (DIB) has been determined by X-ray synchrotron diffraction. The mixing behavior of DIB with 4,4'-bipyridyl (BPY) has also been studied and interestingly is found to phase-separate rather than form a cocrystal, as observed in the bulk. DFT calculations are used to establish the underlying origin of this interesting behavior. The DFT calculations are demonstrated to agree well with the recently proposed monolayer structure for the cocrystal of BPY and 1,4-diiodotetrafluorobenzene (DITFB) (the perfluorinated analogue of DIB), where halogen bonding has also been identified by diffraction. Here we have calculated an estimate of the halogen bond strength by DFT calculations for the DITFB/BPY cocrystal monolayer, which is found to be ∼20 kJ/mol. Computationally, we find that the nonfluorinated DIB and BPY are not expected to form a halogen-bonded cocrystal in a 2D layer; for this pair of species, phase separation of the components is calculated to be lower energy, in good agreement with the diffraction results.

The behavior of the adsorption of cytochrome C on lipid monolayers: A study by the Langmuir-Blodgett technique and theoretical analysis.

PubMed

Li, Junhua; Sun, Runguang; Hao, Changchun; He, Guangxiao; Zhang, Lei; Wang, Juan

2015-10-01

Cytochrome c (Cyt c) is an essential component of the inner mitochondrial respiratory chain because of its function of transferring electrons. The feature is closely related to the interaction between Cyt c and membrane lipids. We used Langmuir-Blodgett monolayer technique combined with AFM to study the interaction of Cyt c with lipid monolayers at air-buffer interface. In our work, by comparing the mixed Cyt c-anionic (DPPS) and Cyt c-zwitterionic (DPPC/DPPE) monolayers, the adsorption capacity of Cyt c on lipid monolayers is DPPS>DPPE>DPPC, which is attributed to their different headgroup structures. π-A isothermal data show that Cyt c (v=2.5 μL) molecules are at maximum adsorption quantity on lipid monolayer. Moreover, Cyt c molecules would form aggregations and drag some lipids with them into subphase if the protein exceeds the maximum adsorption quantity. π-T curve indicates that it takes more time for Cyt c molecular conformation to rearrange on DPPE monolayer than on DPPC. The compressibility study reveals that the adsorption or intermolecular aggregation of Cyt c molecules on lipid monolayer will change the membrane fluidization. In order to quantitatively estimate Cyt c molecular adsorption properties on lipid monolayers, we fit the experimental isotherm with a simple surface state equation. A theoretical model is also introduced to analyze the liquid expanded (LE) to liquid condensed (LC) phase transition of DPPC monolayer. The results of theoretical analysis are in good agreement with the experiment. Copyright © 2015 Elsevier B.V. All rights reserved.

Inter-layer and intra-layer heat transfer in bilayer/monolayer graphene van der Waals heterostructure: Is there a Kapitza resistance analogous?

NASA Astrophysics Data System (ADS)

Rajabpour, Ali; Fan, Zheyong; Vaez Allaei, S. Mehdi

2018-06-01

Van der Waals heterostructures have exhibited interesting physical properties. In this paper, heat transfer in hybrid coplanar bilayer/monolayer (BL-ML) graphene, as a model layered van der Waals heterostructure, was studied using non-equilibrium molecular dynamics (MD) simulations. The temperature profile and inter- and intra-layer heat fluxes of the BL-ML graphene indicated that, there is no fully developed thermal equilibrium between layers and the drop in the average temperature profile at the step-like BL-ML interface is not attributable to the effect of Kapitza resistance. By increasing the length of the system up to 1 μm in the studied MD simulations, the thermally non-equilibrium region was reduced to a small area near the step-like interface. All MD results were compared to a continuum model and a good match was observed between the two approaches. Our results provide a useful understanding of heat transfer in nano- and micro-scale layered materials and van der Waals heterostructures.

Structure-Property Relationship of Phenylene-Based Self-Assembled-Monolayers for Record Low Work Function of Indium Tin Oxide.

PubMed

Benneckendorf, Frank S; Hillebrandt, Sabina; Ullrich, Florian; Rohnacher, Valentina; Hietzschold, Sebastian; Jänsch, Daniel; Freudenberg, Jan; Beck, Sebastian; Mankel, Eric; Jaegermann, Wolfram; Pucci, Annemarie; Bunz, Uwe H F; Müllen, Klaus

2018-06-20

Studying the structure-property relations of tailored dipolar phenyl and biphenylphosphonic acids we report self-assembled monolayers with a significant decrease of the work function (WF) of indium-tin oxide (ITO) electrodes. While the strengths of the dipoles are varied through the different molecular lengths and the introduction of electron-withdrawing fluorine atoms, the surface energy is kept constant through the electron-donating N,N dimethylamine head groups. The self-assembled monolayer formation and its modification of the electrodes are investigated via infrared reflection absorption spectroscopy, contact angle measurements, and photoelectron spectroscopy. The WF decrease of ITO correlates with increasing molecular dipoles. The lowest ever recorded WF of 3.7 eV is achieved with the fluorinated biphenylphosphonic acid.

Monolayer Colloidal Crystals by Modified Air-Water Interface Self-Assembly Approach

PubMed Central

Ye, Xin; Huang, Jin; Zeng, Yong; Sun, Lai-Xi; Geng, Feng; Liu, Hong-Jie; Wang, Feng-Rui; Jiang, Xiao-Dong; Wu, Wei-Dong; Zheng, Wan-Guo

2017-01-01

Hexagonally ordered arrays of polystyrene (PS) microspheres were prepared by a modified air-water self-assembly method. A detailed analysis of the air-water interface self-assembly process was conducted. Several parameters affect the quality of the monolayer colloidal crystals, i.e., the colloidal microsphere concentration on the latex, the surfactant concentration, the polystyrene microsphere diameter, the microsphere polydispersity, and the degree of sphericity of polystyrene microspheres. An abrupt change in surface tension was used to improve the quality of the monolayer colloidal crystal. Three typical microstructures, i.e., a cone, a pillar, and a binary structure were prepared by reactive-ion etching using a high-quality colloidal crystal mask. This study provides insight into the production of microsphere templates with flexible structures for large-area patterned materials. PMID:28946664

Structural phase transition in monolayer MoTe2 driven by electrostatic doping

NASA Astrophysics Data System (ADS)

Wang, Ying; Xiao, Jun; Zhu, Hanyu; Li, Yao; Alsaid, Yousif; Fong, King Yan; Zhou, Yao; Wang, Siqi; Shi, Wu; Wang, Yuan; Zettl, Alex; Reed, Evan J.; Zhang, Xiang

2017-10-01

Monolayers of transition-metal dichalcogenides (TMDs) exhibit numerous crystal phases with distinct structures, symmetries and physical properties. Exploring the physics of transitions between these different structural phases in two dimensions may provide a means of switching material properties, with implications for potential applications. Structural phase transitions in TMDs have so far been induced by thermal or chemical means; purely electrostatic control over crystal phases through electrostatic doping was recently proposed as a theoretical possibility, but has not yet been realized. Here we report the experimental demonstration of an electrostatic-doping-driven phase transition between the hexagonal and monoclinic phases of monolayer molybdenum ditelluride (MoTe2). We find that the phase transition shows a hysteretic loop in Raman spectra, and can be reversed by increasing or decreasing the gate voltage. We also combine second-harmonic generation spectroscopy with polarization-resolved Raman spectroscopy to show that the induced monoclinic phase preserves the crystal orientation of the original hexagonal phase. Moreover, this structural phase transition occurs simultaneously across the whole sample. This electrostatic-doping control of structural phase transition opens up new possibilities for developing phase-change devices based on atomically thin membranes.

Synthesis and enhanced electrochemical catalytic performance of monolayer WS2(1-x) Se2x with a tunable band gap.

PubMed

Fu, Qi; Yang, Lei; Wang, Wenhui; Han, Ali; Huang, Jian; Du, Pingwu; Fan, Zhiyong; Zhang, Jingyu; Xiang, Bin

2015-08-26

The first realization of a tunable band-gap in monolayer WS2(1-x) Se2x is demonstrated. The tuning of the bandgap exhibits a strong dependence of S and Se content, as proven by PL spectroscopy. Because of its remarkable electronic structure, monolayer WS2(1-x) Se2x exhibits novel electrochemical catalytic activity and offers long-term electrocatalytic stability for the hydrogen evolution reaction. © 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

Ferromagnetism induced by point defect in Janus monolayer MoSSe regulated by strain engineering

NASA Astrophysics Data System (ADS)

Meng, Ming; Li, Tinghui; Li, Shaofeng; Liu, Kuili

2018-03-01

The formation and regulation of magnetism dependent on introduced defects in the Janus MoSSe monolayer has attracted much attention because of its potential application in spintronics. Here, we present a theoretical study of defect formation in the MoSSe monolayer and its introduced magnetism under external strain. The tensile deformation induced by external strain not only leads to decreases in defect formation energy, but also enhances magnetic characteristics. However, as compressed deformation increases, the magnetism in the structure induced by Se or S defects remains unchanged because this microstructural deformation adequately spin polarizes unpaired electrons of neighboring Mo atoms. Our results suggest the use of point defect and strain engineering in the Janus MoSSe monolayer for spintronics applications.

Nanomechanical resonators based on group IV element monolayers

NASA Astrophysics Data System (ADS)

He, Ji-Dong; Sun, Jia-Sheng; Jiang, Jin-Wu

2018-04-01

We perform molecular dynamics simulations to investigate the energy dissipation of the resonant oscillation for the group IV monolayers of puckered configuration, in which the oscillation is driven with different actuation velocities. We find that, in the moderate actuation velocity regime, the nonlinear coupling between the resonant oscillation mode and other high-frequency modes will lead to the non-resonant motion of the system. For the larger actuation velocity, the effective strain generated during the resonant oscillating causes a structural transition from the puckered configuration into the planar configuration, which is a characteristic energy dissipation mechanism for the resonant oscillation of these group IV puckered monolayers. Our findings shed light on mechanical applications of the group IV monolayers in the nanomechanical resonator field.

Tuning the Electronic Properties, Effective Mass and Carrier Mobility of MoS2 Monolayer by Strain Engineering: First-Principle Calculations

NASA Astrophysics Data System (ADS)

Phuc, Huynh V.; Hieu, Nguyen N.; Hoi, Bui D.; Hieu, Nguyen V.; Thu, Tran V.; Hung, Nguyen M.; Ilyasov, Victor V.; Poklonski, Nikolai A.; Nguyen, Chuong V.

2018-01-01

In this paper, we studied the electronic properties, effective masses, and carrier mobility of monolayer MoS_2 using density functional theory calculations. The carrier mobility was considered by means of ab initio calculations using the Boltzmann transport equation coupled with deformation potential theory. The effects of mechanical biaxial strain on the electronic properties, effective mass, and carrier mobility of monolayer MoS_2 were also investigated. It is demonstrated that the electronic properties, such as band structure and density of state, of monolayer MoS_2 are very sensitive to biaxial strain, leading to a direct-indirect transition in semiconductor monolayer MoS_2. Moreover, we found that the carrier mobility and effective mass can be enhanced significantly by biaxial strain and by lowering temperature. The electron mobility increases over 12 times with a biaxial strain of 10%, while the carrier mobility gradually decreases with increasing temperature. These results are very useful for the future nanotechnology, and they make monolayer MoS_2 a promising candidate for application in nanoelectronic and optoelectronic devices.

Electrical Tuning of Valley-Polarized Circular Photogalvanic Current in a Monolayer Transition Metal Dichalcogenide

NASA Astrophysics Data System (ADS)

Liu, Lei; Lenferink, Erik J.; Stanev, Teodor K.; Stern, Nathaniel P.; Wei, Guohua

In a monolayer transition metal dichalcogenide that lacks structural inversion symmetry, the valley contrasting properties, particularly the magnetic moment and Berry curvature, offer the possibility to create a population imbalance between the two valleys simply with an external optical field. With the circular photogalvanic effect, the generation of the spin-valley-coupled photocurrent has been demonstrated in chalcogenides. Continuously tuning the valley-polarized current so far has remained largely unexplored in monolayer devices. Here we show the voltage-tunable photocurrent polarization can be achieved in monolayer MoS2 where electric field facilitates the disassociation of excitons and the carrier drift. Gating that modulates the contact barrier and carrier density can switch the monolayer photocurrent polarization on and off with a large valley-polarized current on-off ratio greater than 103. The efficient electrical tuning of valley-polarized photocurrent opens new possibilities for exploiting polarized currents in monolayer semiconductor devices. This work is supported by the National Science Foundation MRSEC program (DMR-1121262) and the U.S. Department of Energy (BES DE-SC0012130). N.P.S. is an Alfred P. Sloan Research Fellow.

Two-dimensional pentagonal CrX (X = S, Se or Te) monolayers: antiferromagnetic semiconductors for spintronics and photocatalysts.

PubMed

Chen, Wenzhou; Kawazoe, Yoshiyuki; Shi, Xingqiang; Pan, Hui

2018-06-25

Two dimensional (2D) materials with hexagonal building blocks have received tremendous interest in recent years and show promise as nanoscale devices for versatile applications. Herein, we propose a new family of 2D pentagonal CrX (X = S, Se or Te) monolayers (penta-CrX) for applications in electronics, spintronics and photocatalysis. We find that the 2D penta-CrX monolayers are thermally, structurally and mechanically stable. The penta-CrX monolayers are antiferromagnetic and semiconducting. We show that the magnetism is attributed to the super-exchange induced by the ionic interactions between the Cr and X atoms and can be enhanced upon applying tension. We further show that the penta-CrS and penta-CrSe monolayers show good redox potentials versus a normal hydrogen electrode, and their band gaps are comparable to the energy of a photon in the visible light region, indicating their capability of maximal utilization of solar energy for water splitting. With intrinsic semiconducting and controllable magnetic properties, the proposed penta-CrX monolayers may hold promise as flexible spintronics and photocatalysts.

Electronic properties of B and Al doped graphane: A hybrid density functional study

NASA Astrophysics Data System (ADS)

Mapasha, R. E.; Igumbor, E.; Andriambelaza, N. F.; Chetty, N.

2018-04-01

Using a hybrid density functional theory approach parametrized by Heyd, Scuseria and Ernzerhof (HSE06 hybrid functional), we study the energetics, structural and electronic properties of a graphane monolayer substitutionally doped with the B (BCH) and Al (AlCH) atoms. The BCH defect can be integrated within a graphane monolayer at a relative low formation energy, without major structural distortions and symmetry breaking. The AlCH defect relaxes outward of the monolayer and breaks the symmetry. The density of states plots indicate that BCH doped graphane monolayer is a wide band gap semiconductor, whereas the AlCH defect introduces the spin dependent mid gap states at the vicinity of the Fermi level, revealing a metallic character with the pronounced magnetic features. We further examine the response of the Al dependent spin states on the multiple charge states doping. We find that the defect formation energy, structural and electronic properties can be altered via charge state modulation. The +1 charge doping opens an energy band gap of 1.75 eV. This value corresponds to the wavelength in the visible spectrum, suggesting an ideal material for solar cell absorbers. Our study fine tunes the graphane band gap through the foreign atom doping as well as via defect charge state modulation.

Thermal transport properties of bulk and monolayer MoS2: an ab-initio approach

NASA Astrophysics Data System (ADS)

Bano, Amreen; Khare, Preeti; Gaur, N. K.

2017-05-01

The transport properties of semiconductors are key to the performance of many solid-state devices (transistors, data storage, thermoelectric cooling and power generation devices, etc). In recent years simulation tools based on first-principles calculations have been greatly improved, being able to obtain the fundamental ground-state properties of materials accurately. The quasi harmonic thermal properties of bulk and monolayer of MoS2 has been computed with ab initio periodic simulations based of density functional theory (DFT). The temperature dependence of bulk modulus, specific heat, thermal expansion and gruneisen parameter have been calculated in our work within the temperature range of 0K to 900K with projected augmented wave (PAW) method using generalized gradient approximation (GGA). Our results show that the optimized lattice parameters are in good agreement with the earlier reported works and also for thermoelastic parameter, i.e. isothermal bulk modulus (B) at 0K indicates that monolayer MoS2 (48.5 GPa)is more compressible than the bulk structure (159.23 GPa). The thermal expansion of monolayer structure is slightly less than the bulk. Similarly, other parameters like heat capacity and gruneisen parameter shows different nature which is due to the confinement of 3 dimensional structure to 2 dimension (2D) for improving its transport characteristics.

Characterization of buried metal-molecule-metal junctions using Fourier transform infrared microspectroscopy

NASA Astrophysics Data System (ADS)

Babayco, Christopher B.; Land, Donald P.; Parikh, Atul N.; Kiehl, Richard A.

2014-09-01

We have devised an infrared spectromicroscopy based experimental configuration to enable structural characterization of buried molecular junctions. Our design utilizes a small mercury drop at the focal point of an infrared microscope to act as a mirror in studying metal-molecule-metal (MmM) junctions. An organic molecular monolayer is formed either directly on the mercury drop or on a thin, infrared (IR) semi-transparent layer of Au deposited onto an IR transparent, undoped silicon substrate. Following the formation of the monolayer, films on either metal can be examined independently using specular reflection spectroscopy. Furthermore, by bringing together the two monolayers, a buried molecular bilayer within the MmM junction can be characterized. Independent examination of each half of the junction prior to junction formation also allows probing any structural and/or conformational changes that occur as a result of forming the bilayer. Because our approach allows assembling and disassembling microscopic junctions by forming and withdrawing Hg drops onto the monolayer covered metal, spatial mapping of junctions can be performed simply by translating the location of the derivatized silicon wafer. Finally, the applicability of this technique for the longer-term studies of changes in molecular structure in the presence of electrical bias is discussed.

Layered graphene-mica substrates induce melting of DNA origami

NASA Astrophysics Data System (ADS)

Green, Nathaniel S.; Pham, Phi H. Q.; Crow, Daniel T.; Burke, Peter J.; Norton, Michael L.

2018-04-01

Monolayer graphene supported on mica substrates induce melting of cross-shaped DNA origami. This behavior can be contrasted with the case of origami on graphene on graphite, where an expansion or partially re-organized structure is observed. On mica, only well-formed structures are observed. Comparison of the morphological differences observed for these probes after adsorption on these substrates provides insights into the sensitivity of DNA based nanostructures to the properties of the graphene monolayer, as modified by its substrate.

Self-assembled monolayers improve protein distribution on holey carbon cryo-EM supports

PubMed Central

Meyerson, Joel R.; Rao, Prashant; Kumar, Janesh; Chittori, Sagar; Banerjee, Soojay; Pierson, Jason; Mayer, Mark L.; Subramaniam, Sriram

2014-01-01

Poor partitioning of macromolecules into the holes of holey carbon support grids frequently limits structural determination by single particle cryo-electron microscopy (cryo-EM). Here, we present a method to deposit, on gold-coated carbon grids, a self-assembled monolayer whose surface properties can be controlled by chemical modification. We demonstrate the utility of this approach to drive partitioning of ionotropic glutamate receptors into the holes, thereby enabling 3D structural analysis using cryo-EM methods. PMID:25403871

Orientation and Interaction of Oblique Cylindrical Inclusions Embedded in a Lipid Monolayer: A Theoretical Model for Viral Fusion Peptides

PubMed Central

Kozlovsky, Yonathan; Zimmerberg, Joshua; Kozlov, Michael M.

2004-01-01

We consider the elastic behavior of flat lipid monolayer embedding cylindrical inclusions oriented obliquely with respect to the monolayer plane. An oblique inclusion models a fusion peptide, a part of a specialized protein capable of inducing merger of biological membranes in the course of fundamental cellular processes. Although the crucial importance of the fusion peptides for membrane merger is well established, the molecular mechanism of their action remains unknown. This analysis is aimed at revealing mechanical deformations and stresses of lipid monolayers induced by the fusion peptides, which, potentially, can destabilize the monolayer structure and enhance membrane fusion. We calculate the deformation of a monolayer embedding a single oblique inclusion and subject to a lateral tension. We analyze the membrane-mediated interactions between two inclusions, taking into account bending of the monolayer and tilt of the hydrocarbon chains with respect to the surface normal. In contrast to a straightforward prediction that the oblique inclusions should induce tilt of the lipid chains, our analysis shows that the monolayer accommodates the oblique inclusion solely by bending. We find that the interaction between two inclusions varies nonmonotonically with the interinclusion distance and decays at large separations as square of the distance, similar to the electrostatic interaction between two electric dipoles in two dimensions. This long-range interaction is predicted to dominate the other interactions previously considered in the literature. PMID:15298906

Manipulation of local optical properties and structures in molybdenum-disulfide monolayers using electric field-assisted near-field techniques.

PubMed

Nozaki, Junji; Fukumura, Musashi; Aoki, Takaaki; Maniwa, Yutaka; Yomogida, Yohei; Yanagi, Kazuhiro

2017-04-05

Remarkable optical properties, such as quantum light emission and large optical nonlinearity, have been observed in peculiar local sites of transition metal dichalcogenide monolayers, and the ability to tune such properties is of great importance for their optoelectronic applications. For that purpose, it is crucial to elucidate and tune their local optical properties simultaneously. Here, we develop an electric field-assisted near-field technique. Using this technique we can clarify and tune the local optical properties simultaneously with a spatial resolution of approximately 100 nm due to the electric field from the cantilever. The photoluminescence at local sites in molybdenum-disulfide (MoS 2 ) monolayers is reversibly modulated, and the inhomogeneity of the charge neutral points and quantum yields is suggested. We successfully etch MoS 2 crystals and fabricate nanoribbons using near-field techniques in combination with an electric field. This study creates a way to tune the local optical properties and to freely design the structural shapes of atomic monolayers using near-field optics.

Structural and electric properties of two semifluorinated alkane monolayers compressed on top of a controlled hydrophobic monolayer substrate

NASA Astrophysics Data System (ADS)

El Abed, Abdel-Illah; Ionov, Radoslav; Goldmann, Michel

2007-10-01

We investigate the dynamic behavior upon lateral compression of two mixed films made with one of the two semifluorinated alkanes F(CF2)8(CH2)18H and F(CF2)10(CH2)10H and the natural α -helix alamethicin peptide. Surface pressure, surface potential versus molecular area isotherms, and grazing-incidence x-ray diffraction were applied to characterize this system. We show that both mixed films demix vertically to form two asymmetric flat bilayers where the lower layer is made of alamethicin and the upper layer is made of semifluorinated molecules. The structure matching of the semifluorinated alkanes (where the hydrophilic group is missing) with a suitable organization of the underlying alamethicin monolayer allows for a continuous compression of the upper semifluorinated layers while the density of the lower alamethicin monolayer remains constant. Comparing data of the two studied mixed films enables us to evaluate the effect of chain length on the in-plane organization of the molecules and on the electric properties of the upper layers.

N-Functionalized MXenes: ultrahigh carrier mobility and multifunctional properties.

PubMed

Shao, Yangfan; Zhang, Fang; Shi, Xingqiang; Pan, Hui

2017-11-01

Two dimensional (2D) nanomaterials have demonstrated huge potential in wide applications from nanodevices to energy harvesting/storage. In this work, we propose a new class of 2D monolayers, nitrogen-functionalized MXenes (Nb 2 CN 2 and Ta 2 CN 2 ), based on density-functional theory (DFT). We find that these monolayers are direct semiconductors with near linear energy dispersions at the Γ point. M 2 CN 2 monolayers have significant small effective mass and show an ultra-high mobility of up to 10 6 cm 2 V -1 s -1 . We show that the electronic structures of the M 2 CN 2 monolayers can be easily controlled by biaxial and uniaxial strains. Importantly, the carrier mobility and direct band gap can be dramatically increased within a certain range of strain. A direct-indirect band gap transition can be triggered and the band gap can be tuned under strain. The tunable electronic properties are attributed to the structural changes and charge redistribution under stain. Our findings demonstrate that N-functionalized MXenes are promising materials for nanodevices with high speed and low power.

Intrinsic Ferroelasticity and/or Multiferroicity in Two-Dimensional Phosphorene and Phosphorene Analogues.

PubMed

Wu, Menghao; Zeng, Xiao Cheng

2016-05-11

Phosphorene and phosphorene analogues such as SnS and SnSe monolayers are promising nanoelectronic materials with desired bandgap, high carrier mobility, and anisotropic structures. Here, we show first-principles calculation evidence that these monolayers are potentially the long-sought two-dimensional (2D) materials that can combine electronic transistor characteristic with nonvolatile memory readable/writeable capability at ambient condition. Specifically, phosphorene is predicted to be a 2D intrinsic ferroelastic material with ultrahigh reversible strain, whereas SnS, SnSe, GeS, and GeSe monolayers are multiferroic with coupled ferroelectricity and ferroelasticity. Moreover, their low-switching barriers render room-temperature nonvolatile memory accessible, and their notable structural anisotropy enables ferroelastic or ferroelectric switching readily readable via electrical, thermal, optical, mechanical, or even spintronic detection upon the swapping of the zigzag and armchair direction. In addition, it is predicted that the GeS and GeSe monolayers as well as bulk SnS and SnSe can maintain their ferroelasticity and ferroelectricity (anti-ferroelectricity) beyond the room temperature, suggesting high potential for practical device application.

Thermodynamic balance of perylene self-assembly on Ag(110)

NASA Astrophysics Data System (ADS)

Bobrov, Kirill; Kalashnyk, Nataliya; Guillemot, Laurent

2016-10-01

We present a room temperature STM study of perylene adsorption on Ag(110) at the monolayer coverage regime. We found that structure and symmetry of the perylene monolayer are settled by thermodynamic balance of the three factors: (i) the ability of perylene molecules to recognize specific adsorption sites on the (110) lattice, (ii) the intermolecular interaction, and (iii) the accommodation of thermal motion of the molecules. The moderate strength of the site recognition and the intermolecular interaction, of the same order of magnitude as kT ˜ 25 meV, represents a key feature of the thermodynamic balance. It bestows to this system the unique quality to form the quasi-liquid monolayer of epitaxial as well as self-assembling character. The perylene monolayer accommodates the short-range motion of the molecules instead of quenching it. It precludes the formation of possible solid nuclei and maintains common registry of the included molecules. The surface registry of the quasi-liquid phase is provided by locking of a structure-related fraction of the perylene molecules into specific adsorption sites of the (110) lattice favorable in terms of intermolecular interaction.

Characterization of BaTiO3 nanocubes assembled into highly ordered monolayers using micro- and nano-Raman spectroscopy

NASA Astrophysics Data System (ADS)

Itasaka, Hiroki; Mimura, Ken-ichi; Nishi, Masayuki; Kato, Kazumi

2018-05-01

We investigated the influence of heat treatment on the crystallographic structure and ferroelectric phase transition behavior of barium titanate (BaTiO3, BT) nanocubes assembled into highly ordered monolayers, using tip-enhanced Raman spectroscopy (TERS), temperature-dependent micro-Raman spectroscopy, and scanning transmission electron microscopy (STEM). TER spectra from individual BT nanocubes with the size of about 20 nm were obtained with a side-illumination optical setup, and revealed that heat treatment enhances their tetragonality. The result of temperature-dependent micro-Raman spectroscopy showed that the ferroelectric phase transition behavior of the monolayers becomes similar to that of bulk BT through heat treatment in spite of their thickness. STEM observation for the cross-section of the heated BT nanocube monolayer showed that amorphous layers exist at the interface between BT nanocubes in face-to-face contact. These results indicate that the tetragonal crystal structure of BT nanocubes is stabilized by heat treatment and the formation of the interfacial amorphous layer during heat treatment may be a key to this phenomenon.

Size Effects in Nanoscale Structural Phenomena

NASA Astrophysics Data System (ADS)

McElhinny, Kyle Matthew

The creation of nanostructures offers the opportunity to modify and tune properties in ways inaccessible in bulk materials. A key component in this development is the introduction of size effects which reduce the physical size, dimensionality, and increase the contribution of surface effects. The size effects strongly modify the structural dynamics in nanoscale systems and leads to changes in the vibrational, electrical, and optical properties. An increased level of understanding and control of nanoscale structural dynamics will enable more precise control over nanomaterial transport properties. My work has shown that 1D spatial confinement through the creation of semiconducting nanomembranes modifies the phonon population and dispersion. X ray thermal diffuse scattering distributions show an excess in intensity for nanomembranes less than 100 nm in thickness, for phonon modes with wavevectors spanning the entire Brillouin zone. This excess intensity indicates the development of new low energy phonon modes or the softening of elastic constants. Furthermore, an additional anisotropy in the phonon dispersion is observed with a symmetry matching the direction of spatial confinement. This work has also extended x ray thermal diffuse scattering for use in studying nanomaterials. In electro- and photoactive monolayers a structural reconfiguration can be produced by external optical stimuli. I have developed an electro and photoactive molecular monolayers on oxide surfaces. Using x ray reflectivity, I have evaluated the organization and reconfiguration of molecular monolayers deposited by Langmuir Blodgett technique. I have designed and probed the reconfiguration of optically reconfigurable monolayers of azobenzene donor molecules on semiconducting surfaces. These monolayers reconfigure through a cooperative switching process leading to the development of large isomeric domains. This work represents an advancement in the interpretation of x ray reflectivity from molecular monolayers and inhomogeneous surfaces. The growth 2D materials depends on the interactions between the substrate and the 2D material. I have studied the competition between kinetics and surface energetics which lead to a faceted Ge surface during the growth of Graphene nanoribbons. As part of this work, I have developed new methodologies for interpreting x ray reflectivity patterns from surfaces with multiple reflections. A systematic analysis of the temperature dependence of the faceting process indicates that the process is thermodynamically dominated at high temperatures.

The interfacial-organized monolayer water film (MWF) induced ``two-step'' aggregation of nanographene: both in stacking and sliding assembly pathways

NASA Astrophysics Data System (ADS)

Lv, Wenping; Wu, Ren'an

2013-03-01

A computational investigation was carried out to understand the aggregation of nanoscale graphene with two typical pathways of stacking assembly and sliding assembly in water. The interfacial-organized monolayer water film (MWF) induced ``two-step'' aggregation of nanographene in both stacking and sliding assembly pathways was reported for the first time. By means of potential mean forces (PMFs) calculation, no energy barrier was observed during the sliding assembly of two graphene nanosheets, while the PMF profiles could be impacted by the contact forms of nanographene and the MWF within the interplate of two graphene nanosheets. To explore the potential physical basis of the ``hindering role'' of self-organized interfacial water, the dynamical and structural properties as well as the status of hydrogen bonds (H-bonds) for interfacial water were investigated. We found that the compact, ordered structure and abundant H-bonds of the MWF could be taken as the fundamental aspects of the ``hindering role'' of interfacial water for the hydrophobic assembly of nanographene. These findings are displaying a potential to further understand the hydrophobic assembly which mostly dominate the behaviors of nanomaterials, proteins etc. in aqueous solutions.A computational investigation was carried out to understand the aggregation of nanoscale graphene with two typical pathways of stacking assembly and sliding assembly in water. The interfacial-organized monolayer water film (MWF) induced ``two-step'' aggregation of nanographene in both stacking and sliding assembly pathways was reported for the first time. By means of potential mean forces (PMFs) calculation, no energy barrier was observed during the sliding assembly of two graphene nanosheets, while the PMF profiles could be impacted by the contact forms of nanographene and the MWF within the interplate of two graphene nanosheets. To explore the potential physical basis of the ``hindering role'' of self-organized interfacial water, the dynamical and structural properties as well as the status of hydrogen bonds (H-bonds) for interfacial water were investigated. We found that the compact, ordered structure and abundant H-bonds of the MWF could be taken as the fundamental aspects of the ``hindering role'' of interfacial water for the hydrophobic assembly of nanographene. These findings are displaying a potential to further understand the hydrophobic assembly which mostly dominate the behaviors of nanomaterials, proteins etc. in aqueous solutions. Electronic supplementary information (ESI) available: The evolution of interaction energy for two graphene nanosheets assembly in stacking (a) and sliding (b) pathway was plotted in Fig. S1. The time evolution of three dimension distance for stacking assembly of two graphene nanosheets with the edge-edge orientation of 45° was plotted in Fig. S2. The initial orientations of graphene nanosheets in three simulations (edge-edge distance in x-direction (dx) was 0.3 nm, but in z-direction (dz) was 0.0 nm, 0.4 nm and 0.7 nm, respectively) were shown in Fig. S3. The snapshots of the evolution of hydration shells during the sliding assembly of nanographene were shown in Fig. S4, with the separation of two graphene nanosheets in z-direction is (a) 0 nm and (b) 0.7 nm, respectively. The process of two graphene nanosheets assembly in stacking pathway was shown in Movie S1 as video. The process of two graphene nanosheets (with a separation of 0.7 nm in normal direction) assembly in sliding pathway was shown in Movie S2 as video. The dynamical evolution of interfacial water during the sliding assembly of nanographene was shown in Movie S3 as video. The process of extruding the monolayer water film (MWF) out of the interplate of two graphene nanosheets was shown in Movie S4 as video. Movie S5 displays that the graphene-water-graphene sandwiched structure was successfully maintained during a 10 ns MD simulation. See DOI: 10.1039/c3nr33447c

Electrochemical Properties of Organosilane Self Assembled Monolayers on Aluminum 2024

NASA Technical Reports Server (NTRS)

Hintze, Paul E.; Calle, Luz Marina

2004-01-01

Self assembled monolayers are commonly used to modify surfaces. Within the last 15 years, self assembled monolayers have been investigated as a way to protect from corrosion[1,2] or biofouling.[3] In this study, self assembled monolayers of decitriethoxysilane (C10H21Si(OC2H5)3) and octadecyltriethoxysilane (C18H37Si(OC2H5)3) were formed on aluminum 2024-T3. The modified surfaces and bare Al 2024 were characterized by dynamic water contact angle measurements, x-ray photoelectron spectroscopy (XIPS) and infrared spectroscopy. Electrochemical impedance spectroscopy (EIS) in 0.5 M NaCl was used to characterize the monolayers and evaluate their corrosion protection properties. The advancing water contact angle and infrared measurements show that the mono layers form a surface where the hydrocarbon chains are packed and oriented away from the surface, consistent with what is found in similar systems. The contact angle hysteresis measured in these systems is relatively large, perhaps indicating that the hydrocarbon chains are not as well packed as monolayers formed on other substrates. The results of the EIS measurements were modeled using a Randle's circuit modified by changing the capacitor to a constant phase element. The constant phase element values were found to characterize the monolayer. The capacitance of the monolayer modified surface starts lower than the bare Al 2024, but approaches values similar to the bare Al 2024 within 24 hours as the monolayer is degraded. The n values found for bare Al 2024 quickly approach the value of a true capacitor and are greater than 0.9 within hours after the start of exposure. For the monolayer modified structure, n can stay lower than 0.9 for a longer period of time. In fact, n for the monolayer modified surfaces is different from the bare surface even after the capacitance values have converged. This indicates that the deviation from ideal capacitance is the most sensitive indicator of the presence of the monolayer.

Carbon phosphide monolayers with superior carrier mobility

NASA Astrophysics Data System (ADS)

Wang, Gaoxue; Pandey, Ravindra; Karna, Shashi P.

2016-04-01

Two dimensional (2D) materials with a finite band gap and high carrier mobility are sought after materials from both fundamental and technological perspectives. In this paper, we present the results based on the particle swarm optimization method and density functional theory which predict three geometrically different phases of the carbon phosphide (CP) monolayer consisting of sp2 hybridized C atoms and sp3 hybridized P atoms in hexagonal networks. Two of the phases, referred to as α-CP and β-CP with puckered or buckled surfaces are semiconducting with highly anisotropic electronic and mechanical properties. More remarkably, they have the lightest electrons and holes among the known 2D semiconductors, yielding superior carrier mobility. The γ-CP has a distorted hexagonal network and exhibits a semi-metallic behavior with Dirac cones. These theoretical findings suggest that the binary CP monolayer is a yet unexplored 2D material holding great promise for applications in high-performance electronics and optoelectronics.Two dimensional (2D) materials with a finite band gap and high carrier mobility are sought after materials from both fundamental and technological perspectives. In this paper, we present the results based on the particle swarm optimization method and density functional theory which predict three geometrically different phases of the carbon phosphide (CP) monolayer consisting of sp2 hybridized C atoms and sp3 hybridized P atoms in hexagonal networks. Two of the phases, referred to as α-CP and β-CP with puckered or buckled surfaces are semiconducting with highly anisotropic electronic and mechanical properties. More remarkably, they have the lightest electrons and holes among the known 2D semiconductors, yielding superior carrier mobility. The γ-CP has a distorted hexagonal network and exhibits a semi-metallic behavior with Dirac cones. These theoretical findings suggest that the binary CP monolayer is a yet unexplored 2D material holding great promise for applications in high-performance electronics and optoelectronics. Electronic supplementary information (ESI) available: Fig. S1 cohesive energy and structure of the CP monolayer with various stoichiometric compositions obtained using CALYPSO, Fig. S2 history of CALYPSO steps and structure of the CP monolayer, Fig. S3 phonon dispersion with DFT-D2 functional, Fig. S4 band structure for β-CP using the DFT-PBE and DFT-D2 functional forms, Fig. S5 strain energy curves, Fig. S6 projected band structure for α-CP, Fig. S7 projected band structure for β-CP, Fig. S8 projected band structure for γ-CP, Fig. S9 band structures obtained with the GGA-PBE and HSE06 functional; Table S1 lattice parameters with the DFT-D2 functional form; Video S1 AIMD simulation of α-CP at 300 K, Video S2 AIMD simulation of β-CP at 300 K, Video S3 AIMD simulation of γ-CP at 300 K. See DOI: 10.1039/c6nr00498a

Au Nanoparticle Sub-Monolayers Sandwiched between Sol-Gel Oxide Thin Films

PubMed Central

Della Gaspera, Enrico; Menin, Enrico; Sada, Cinzia

2018-01-01

Sub-monolayers of monodisperse Au colloids with different surface coverage have been embedded in between two different metal oxide thin films, combining sol-gel depositions and proper substrates functionalization processes. The synthetized films were TiO2, ZnO, and NiO. X-ray diffraction shows the crystallinity of all the oxides and verifies the nominal surface coverage of Au colloids. The surface plasmon resonance (SPR) of the metal nanoparticles is affected by both bottom and top oxides: in fact, the SPR peak of Au that is sandwiched between two different oxides is centered between the SPR frequencies of Au sub-monolayers covered with only one oxide, suggesting that Au colloids effectively lay in between the two oxide layers. The desired organization of Au nanoparticles and the morphological structure of the prepared multi-layered structures has been confirmed by Rutherford backscattering spectrometry (RBS), Secondary Ion Mass Spectrometry (SIMS), and Scanning Electron Microscopy (SEM) analyses that show a high quality sandwich structure. The multi-layered structures have been also tested as optical gas sensors. PMID:29538338

Lattice thermal conductivity of monolayer AsP from first-principles molecular dynamics.

PubMed

Sun, Yajing; Shuai, Zhigang; Wang, Dong

2018-05-23

Few-layered arsenic-phosphorus alloys, AsxP(1-x), with a puckered structure have been recently synthesized and demonstrated with fully tunable band gaps and optical properties. It is predicted that the carrier mobility of monolayer AsP compounds is even higher than that of black phosphorene (b-P). The anisotropic and orthogonal electrical and thermal transport properties of the puckered group VA elements make them intriguing materials for thermoelectric applications. Herein, we investigated the thermal transport properties of AsP based on first-principles molecular dynamics and the Boltzmann transport equation. We reveal that monolayer AsP with three different chemical structures possesses thermal conductivities lower than b-P, but with increased anisotropy. Further, these structures behave profoundly different on heat conduction. This can be attributed to the distinct low-frequency optical modes associated with their bonding nature. Our results highlight the impact of atomic arrangement on the thermal conductivity of AsP, and the structure-property relationship established may guide the fabrication of thermoelectric materials via the engineered alloying method.

Surface structure of bulk 2H-MoS2(0001) and exfoliated suspended monolayer MoS2: A selected area low energy electron diffraction study

NASA Astrophysics Data System (ADS)

Dai, Zhongwei; Jin, Wencan; Grady, Maxwell; Sadowski, Jerzy T.; Dadap, Jerry I.; Osgood, Richard M.; Pohl, Karsten

2017-06-01

We have used selected area low energy electron diffraction intensity-voltage (μLEED-IV) analysis to investigate the surface structure of crystalline 2H molybdenum disulfide (MoS2) and mechanically exfoliated and suspended monolayer MoS2. Our results show that the surface structure of bulk 2H-MoS2 is distinct from its bulk and that it has a slightly smaller surface relaxation at 320 K than previously reported at 95 K. We concluded that suspended monolayer MoS2 shows a large interlayer relaxation compared to the MoS2 sandwich layer terminating the bulk surface. The Debye temperature of MoS2 was concluded to be about 600 K, which agrees with a previous theoretical study. Our work has shown that the dynamical μLEED-IV analysis performed with a low energy electron microscope (LEEM) is a powerful technique for determination of the local atomic structures of currently extensively studied two-dimensional (2-D) materials.

Surface structure of bulk 2H-MoS 2 (0001) and exfoliated suspended monolayer MoS 2 : A selected area low energy electron diffraction study

DOE PAGES

Dai, Zhongwei; Jin, Wencan; Grady, Maxwell; ...

2017-02-10

Here, we used selected area low energy electron diffraction intensity-voltage (μLEED-IV) analysis to investigate the surface structure of crystalline 2H molybdenum disulfide (MoS 2) and mechanically exfoliated and suspended monolayer MoS 2. Our results show that the surface structure of bulk 2H-MoS 2 is distinct from its bulk and that it has a slightly smaller surface relaxation at 320 K than previously reported at 95 K. We concluded that suspended monolayer MoS 2 shows a large interlayer relaxation compared to the MoS 2 sandwich layer terminating the bulk surface. The Debye temperature of MoS 2 was concluded to be aboutmore » 600 K, which agrees with a previous theoretical study. Our work has shown that the dynamical μLEED-IV analysis performed with a low energy electron microscope (LEEM) is a powerful technique for determination of the local atomic structures of currently extensively studied two-dimensional (2-D) materials.« less

Remarkable effects of disorder on superconductivity of single atomic layers of lead on silicon

NASA Astrophysics Data System (ADS)

Brun, Christophe

2015-03-01

It is well known that conventional superconductivity is very robust against non-magnetic disorder. Nevertheless for thin and ultrathin films the structural properties play a major role in determining the superconducting properties, through a subtle interplay between disorder and Coulomb interactions. Unexpectedly, in 2010 superconductivity was discovered in single atomic layers of lead and indium grown on silicon substrate using scanning tunneling spectroscopy and confirmed later on by macroscopic transport measurements. Such well-controlled and tunable crystalline monolayers are ideal systems for studying the influence of various kinds of structural defects on the superconducting properties at the atomic and mesoscopic scale. In particular, Pb monolayers offer the opportunity of probing new effects of disorder because not only superconductivity is 2D but also the electronic wave functions are 2D. Our study of two Pb monolayers of different crystal structures by very-low temperature STM (300 mK) under magnetic field reveals unexpected results involving new spatial spectroscopic variations. Our results show that although the sheet resistance of the Pb monolayers is much below the resistance quantum, strong non-BCS corrections appear leading to peak heights fluctuations in the dI/dV tunneling spectra at a spatial scale much smaller than the superconducting coherence length. Furthermore, strong local evidence of the signature of Rashba effect on the superconductivity of the Pb/Si(111) monolayer is revealed through filling of in gap states and local spatial variations of this filling. Finally the nature of vortices in a monolayer is found to be very sensitive to the properties of step edges areas. This work was supported by University Pierre et Marie Curie UPMC `Emergence' project, French ANR Project `ElectroVortex,' ANR-QuDec and Templeton Foundation (40381), ARO (W911NF-13-1-0431) and CNRS PICS funds. Partial funding by US-DOE Grant DE-AC02-07CH1.

Characterization of poly(2-hydroxyethyl methacrylate) (PHEMA) contact lens using the Langmuir monolayer technique.

PubMed

Nogueira, N; Conde, O; Miñones, M; Trillo, J M; Miñones, J

2012-11-01

The behavior of poly(2-hydroxyethyl methacrylate) (PHEMA) polymer monolayer spread on water was studied under various experimental conditions. The influence of subphase pH and temperature, compression speed, elapsed time from the deposit of the monolayer and the recording of the surface pressure-area (π-A) isotherms, as well as the number of polymer molecules deposited at the air/water surface (surface concentration) was studied. The obtained results show that PHEMA exhibits a very stable monolayer given that it is unaffected by modifications in the majority of these variables. Only the elapsed time between the spreading of the monolayer and the beginning of compression causes a small change in the π-A isotherms that consists in an increase in the area occupied by the film. This is attributed to the greater unfolding with time of the polymer's monomers at the air/water interface. The plateau that appears on π-A curves of the PHEMA monolayer is attributed to the reorientation of their hydroxyethyl polar groups through their C-O-C bonds, as well as to the reorientation of the ethylene (CH(2)) groups that link the monomers, which provokes a folding of the polymer's chains causing an accordion configuration. The existence of this structure is confirmed by the presence of numerous noise peaks in the relative thickness versus time curve corresponding to this region. In the same fashion, the images observed from Brewster angle microscopy (BAM) reveal the existence of light-dark "bands" relative to the different regions of this particular structure. Copyright © 2012 Elsevier Inc. All rights reserved.

Tripod self-assembled monolayer on Au(111) prepared by reaction of hydroxyl-terminated alkylthiols with SiCl4.

PubMed

Ichimura, Andrew S; Lew, Wanda; Allara, David L

2008-03-18

Infrared reflection spectroscopy (IRS), single wavelength ellipsometry, and density functional theory were used to elucidate the structure of a molecular tripod self-assembled monolayer (SAM) on polycrystalline gold{111} substrates. The tripod SAM was formed by the reaction of SiCl4 with a densely packed monolayer of 2-mercaptoethanol, 6-mercaptohexanol, and 16-mercaptohexadecanol under inert atmosphere. After reaction with SiCl4, IRS spectra show an intense absorption at approximately 1112 cm(-1) that is attributed to Si-O-C asymmetric stretching vibration of a molecular tripod structure. Harmonic vibrational frequencies computed at the B3LYP/6-311+g** level of theory for the mercaptoethanol tripod SAM closely match the experimental IRS spectra, giving further support for the tripod structure. When rinsed with methanol or water, the Si-Cl-terminated SAM becomes capped with Si-OMe or Si-OH. The silanol-terminated tripod SAM is expected to find use in the preparation of thin zeolite and silica films on gold substrates.

Quantum spin Hall state in monolayer 1T '-WTe 2

DOE PAGES

Tang, Shujie; Zhang, Chaofan; Wong, Dillon; ...

2017-06-26

A quantum spin Hall (QSH) insulator is a novel two-dimensional quantum state of matter that features quantized Hall conductance in the absence of a magnetic field, resulting from topologically protected dissipationless edge states that bridge the energy gap opened by band inversion and strong spin–orbit coupling. By investigating the electronic structure of epitaxially grown monolayer 1T '-WTe 2 using angle-resolved photoemission (ARPES) and first-principles calculations, we observe clear signatures of topological band inversion and bandgap opening, which are the hallmarks of a QSH state. Scanning tunnelling microscopy measurements further confirm the correct crystal structure and the existence of a bulkmore » bandgap, and provide evidence for a modified electronic structure near the edge that is consistent with the expectations for a QSH insulator. Our results establish monolayer 1T '-WTe 2 as a new class of QSH insulator with large band gap in a robust two-dimensional materials family of transition metal dichalcogenides (TMDCs).« less

Quantum spin Hall state in monolayer 1T '-WTe 2

DOE PAGES

Tang, Shujie; Zhang, Chaofan; Wong, Dillon; ...

2017-06-26

A quantum spin Hall (QSH) insulator is a novel two-dimensional quantum state of matter that features quantized Hall conductance in the absence of a magnetic field, resulting from topologically protected dissipationless edge states that bridge the energy gap opened by band inversion and strong spin–orbit coupling. By investigating the electronic structure of epitaxially grown monolayer 1T '-WTe 2 using angle-resolved photoemission (ARPES) and first-principles calculations, we observe clear signatures of topological band inversion and bandgap opening, which are the hallmarks of a QSH state. Scanning tunnelling microscopy measurements further confirm the correct crystal structure and the existence of a bulkmore » bandgap, and provide evidence for a modified electronic structure near the edge that is consistent with the expectations for a QSH insulator. Finally, our results establish monolayer 1T '-WTe 2 as a new class of QSH insulator with large band gap in a robust two-dimensional materials family of transition metal dichalcogenides (TMDCs).« less

Polycyclic aromatic hydrocarbons in model bacterial membranes - Langmuir monolayer studies.

PubMed

Broniatowski, Marcin; Binczycka, Martyna; Wójcik, Aneta; Flasiński, Michał; Wydro, Paweł

2017-12-01

High molecular weight polycyclic aromatic hydrocarbons (HMW-PAHs) are persistent organic pollutants which due to their limited biodegradability accumulate in soils where their increased presence can lead to the impoverishment of the decomposer organisms. As very hydrophobic PAHs easily penetrate cellular membranes of soil bacteria and can be incorporated therein, changing the membrane fluidity and other functions which in consequence can lead to the death of the organism. The structure and size of PAH molecule can be crucial for its membrane activity; however the correlation between PAH structure and its interaction with phospholipids have not been investigated so far. In our studies we applied phospholipid Langmuir monolayers as model bacterial membranes and investigated how the incorporation of six structurally different PAH molecules change the membrane texture and physical properties. In our studies we registered surface pressure and surface potential isotherms upon the monolayer compression, visualized the monolayer texture with the application of Brewster angle microscopy and searched the ordering of the film-forming molecules with molecular resolution with the application of grazing incidence X-ray diffraction (GIXD) method. It turned out that the phospholipid-PAH interactions are strictly structure dependent. Four and five-ring PAHs of the angular or cluster geometry can be incorporated into the model membranes changing profoundly their textures and fluidity; whereas linear or large cluster PAHs cannot be incorporated and separate from the lipid matrix. The observed phenomena were explained based on structural similarities of the applied PAHs with membrane steroids and hopanoids. Copyright © 2017. Published by Elsevier B.V.

Anionic poly(p-phenylenevinylene)/layered double hydroxide ordered ultrathin films with multiple quantum well structure: a combined experimental and theoretical study.

PubMed

Yan, Dongpeng; Lu, Jun; Ma, Jing; Wei, Min; Wang, Xinrui; Evans, David G; Duan, Xue

2010-05-18

The sulfonated phenylenevinylene polyanion derivate (APPV) and exfoliated Mg-Al-layered double hydroxide (LDH) monolayers were alternatively assembled into ordered ultrathin films (UTFs) employing a layer-by-layer method, which shows uniform yellow luminescence. UV-vis absorption and fluorescence spectroscopy present a stepwise and regular growth of the UTFs upon increasing deposited cycles. X-ray diffraction, atomic force microscopy, and scanning electron microscopy demonstrate that the UTFs are orderly periodical layered structure with a thickness of 3.3-3.5 nm per bilayer. The APPV/LDH UTFs exhibit well-defined polarized photoemission characteristic with the maximum luminescence anisotropy of approximately 0.3. Moreover, the UTF exhibit longer fluorescence lifetime (3-3.85-fold) and higher photostability than the drop-casting APPV film under UV irradiation, suggesting that the existence of a LDH monolayer enhances the optical performance of the APPV polyanion. A combination study of electrochemistry and periodic density functional theory was used to investigate the electronic structure of the APPV/LDH system, illustrating that the APPV/LDH UTF is a kind of organic-inorganic hybrid multiple quantum well (MQW) structure with a low band energy of 1.7-1.8 eV, where the valence electrons of APPV can be confined into the energy wells formed by the LDH monolayers effectively. Therefore, this work not only gives a feasible method for fabricating a luminescence ultrathin film but also provides a detailed understanding of the geometric and electronic structures of photoactive polyanions confined between the LDH monolayers.

Vacancy-Induced Formation and Growth of Inversion Domains in Transition-Metal Dichalcogenide Monolayer

DOE Office of Scientific and Technical Information (OSTI.GOV)

Lin, Junhao; Pantelides, Sokrates T.; Zhou, Wu

2015-04-23

Sixty degree grain boundaries in semiconducting transition-metal dichalcogenide (TMDC) monolayers have been shown to act as conductive channels that have profound influence on both the transport properties and exciton behavior of the monolayers. We show that annealing TMDC monolayers at high temperature induces the formation of large-scale inversion domains surrounded by such 60° grain boundaries. To study the formation mechanism of such inversion domains, we use the electron beam in a scanning transmission electron microscope to activate the dynamic process within pristine TMDC monolayers. Moreover, the electron beam acts to generate chalcogen vacancies in TMDC monolayers and provide energy formore » them to undergo structural evolution. We directly visualize the nucleation and growth of such inversion domains and their 60° grain boundaries atom-by-atom within a MoSe 2 monolayer and explore their formation mechanism. Combined with density functional theory, we conclude that the nucleation of the inversion domains and migration of their 60° grain boundaries are driven by the collective evolution of Se vacancies and subsequent displacement of Mo atoms, where such a dynamical process reduces the vacancy-induced lattice shrinkage and stabilizes the system. Our results can help to understand the performance of such materials under severe conditions (e.g., high temperature).« less

Defects and oxidation of group-III monochalcogenide monolayers

NASA Astrophysics Data System (ADS)

Guo, Yu; Zhou, Si; Bai, Yizhen; Zhao, Jijun

2017-09-01

Among various two-dimensional (2D) materials, monolayer group-III monochalcogenides (GaS, GaSe, InS, and InSe) stand out owing to their potential applications in microelectronics and optoelectronics. Devices made of these novel 2D materials are sensitive to environmental gases, especially O2 molecules. To address this critical issue, here we systematically investigate the oxidization behaviors of perfect and defective group-III monochalcogenide monolayers by first-principles calculations. The perfect monolayers show superior oxidation resistance with large barriers of 3.02-3.20 eV for the dissociation and chemisorption of O2 molecules. In contrast, the defective monolayers with single chalcogen vacancy are vulnerable to O2, showing small barriers of only 0.26-0.36 eV for the chemisorption of an O2 molecule. Interestingly, filling an O2 molecule to the chalcogen vacancy of group-III monochalcogenide monolayers could preserve the electronic band structure of the perfect system—the bandgaps are almost intact and the carrier effective masses are only moderately disturbed. On the other hand, the defective monolayers with single vacancies of group-III atoms carry local magnetic moments of 1-2 μB. These results help experimental design and synthesis of group-III monochalcogenides based 2D devices with high performance and stability.

On the buckling of hexagonal boron nitride nanoribbons via structural mechanics

NASA Astrophysics Data System (ADS)

Giannopoulos, Georgios I.

2018-03-01

Monolayer hexagonal boron nitride nanoribbons have similar crystal structure as graphene nanoribbons, have excellent mechanical, thermal insulating and dielectric properties and additionally present chemical stability. These allotropes of boron nitride can be used in novel applications, in which graphene is not compatible, to achieve remarkable performance. The purpose of the present work is to provide theoretical estimations regarding the buckling response of hexagonal boron nitride monolayer under compressive axial loadings. For this reason, a structural mechanics method is formulated which employs the exact equilibrium atomistic structure of the specific two-dimensional nanomaterial. In order to represent the interatomic interactions appearing between boron and nitrogen atoms, the Dreiding potential model is adopted which is realized by the use of three-dimensional, two-noded, spring-like finite elements of appropriate stiffness matrices. The critical compressive loads that cause the buckling of hexagonal boron nitride nanoribbons are computed with respect to their size and chirality while some indicative buckled shapes of them are illustrated. Important conclusions arise regarding the effect of the size and chirality on the structural stability of the hexagonal boron nitride monolayers. An analytical buckling formula, which provides good fitting of the numerical outcome, is proposed.

Tinene: a two-dimensional Dirac material with a 72 meV band gap.

PubMed

Cai, Bo; Zhang, Shengli; Hu, Ziyu; Hu, Yonghong; Zou, Yousheng; Zeng, Haibo

2015-05-21

Dirac materials have attracted great interest for both fundamental research and electronic devices due to their unique band structures, but the usual near zero bandgap of graphene results in a poor on-off ratio in the corresponding transistors. Here, we report on tinene, monolayer gray tin, as a new two-dimensional material with both Dirac characteristics and a remarkable 72 meV bandgap based on density functional theory calculations. Compared with silicene and germanene, tinene has a similar hexagonal honeycomb monolayer structure, but it has an obviously larger buckling height (∼0.70 Å). Interestingly, such a moderate buckling structure results in phonon dispersion without appreciable imaginary modes, indicating the strong dynamic stability of tinene. Significantly, a distinct transformation is discovered from the band structure that six Dirac cones would appear at high symmetry K points in the first Brillouin zone when gray tin is thinned from the bulk to monolayer, but a bandgap as large as 72 meV is still preserved. Considering the recent successful realization of silicene and germanene with a similar structure, the predicted stable tinene with Dirac characteristics and a suitable bandgap is a possibility for the "more than Moore" materials and devices.

Structure and magnetic properties of mono- and bi-layer graphene films on ultraprecision figured 4H-SiC(0001) surfaces.

PubMed

Hattori, Azusa N; Okamoto, Takeshi; Sadakuni, Shun; Murata, Junji; Oi, Hideo; Arima, Kenta; Sano, Yasuhisa; Hattori, Ken; Daimon, Hiroshi; Endo, Katsuyoshi; Yamauchi, Kazuto

2011-04-01

Monolayer and bilayer graphene films with a few hundred nm domain size were grown on ultraprecision figured 4H-SiC(0001) on-axis and 8 degrees -off surfaces by annealing in ultra-high vacuum. Using X-ray photoelectron spectroscopy (XPS), atomic force microscopy, reflection high-energy electron diffraction, low-energy electron diffraction (LEED), Raman spectroscopy, and scanning tunneling microscopy, we investigated the structure, number of graphene layers, and chemical bonding of the graphene surfaces. Moreover, the magnetic property of the monolayer graphene was studied using in-situ surface magneto-optic Kerr effect at 40 K. LEED spots intensity distribution and XPS spectra for monolayer and bilayer graphene films could become an obvious and accurate fingerprint for the determination of graphene film thickness on SiC surface.

Dynamic instabilities in strongly correlated VSe2 monolayers and bilayers

NASA Astrophysics Data System (ADS)

Esters, Marco; Hennig, Richard G.; Johnson, David C.

2017-12-01

With the emergence of graphene and other two-dimensional (2D) materials, transition-metal dichalcogenides have been investigated intensely as potential 2D materials using experimental and theoretical methods. VSe2 is an especially interesting material since its bulk modification exhibits a charge-density wave (CDW), the CDW is retained even for few-layer nanosheets, and monolayers of VSe2 are predicted to be ferromagnetic. In this work, we show that electron correlation has a profound effect on the magnetic properties and dynamic stability of VSe2 monolayers and bilayers. Including a Hubbard-U term in the density-functional-theory calculations strongly affects the magnetocrystalline anisotropy in the 1 T -VSe2 structure while leaving the 2 H -polytype virtually unchanged. This demonstrates the importance of electronic correlations for the electrical and magnetic properties of 1 T -VSe2 . The Hubbard-U term changes the dynamic stability and the presence of imaginary modes of ferromagnetic 1 T -VSe2 while affecting only the amplitudes in the nonmagnetic phase. The Fermi surface of nonmagnetic 1 T -VSe2 allows for nesting along the CDW vector, but it plays no role in ferromagnetic 1 T -VSe2 . Following the eigenvectors of the soft modes in nonmagnetic 1 T -VSe2 monolayers yields a CDW structure with a 4 ×4 supercell and Peierls-type distortion in the atomic positions and electronic structure. The magnetic order indicates the potential for spin-density-wave structures.

Optical nonlinearities of excitons in monolayer MoS2

NASA Astrophysics Data System (ADS)

Soh, Daniel B. S.; Rogers, Christopher; Gray, Dodd J.; Chatterjee, Eric; Mabuchi, Hideo

2018-04-01

We calculate linear and nonlinear optical susceptibilities arising from the excitonic states of monolayer MoS2 for in-plane light polarizations, using second-quantized bound and unbound exciton operators. Optical selection rules are critical for obtaining the susceptibilities. We derive the valley-chirality rule for the second-order harmonic generation in monolayer MoS2 and find that the third-order harmonic process is efficient only for linearly polarized input light while the third-order two-photon process (optical Kerr effect) is efficient for circularly polarized light using a higher order exciton state. The absence of linear absorption due to the band gap and the unusually strong two-photon third-order nonlinearity make the monolayer MoS2 excitonic structure a promising resource for coherent nonlinear photonics.

Substrate interactions with suspended and supported monolayer MoS 2: Angle-resolved photoemission spectroscopy

DOE PAGES

Jin, Wencan; Yeh, Po -Chun; Zaki, Nader; ...

2015-03-17

We report the directly measured electronic structure of exfoliated monolayer molybdenum disulfide (MoS₂) using micrometer-scale angle-resolved photoemission spectroscopy. Measurements of both suspended and supported monolayer MoS₂ elucidate the effects of interaction with a substrate. Thus, a suggested relaxation of the in-plane lattice constant is found for both suspended and supported monolayer MoS₂ crystals. For suspended MoS₂, a careful investigation of the measured uppermost valence band gives an effective mass at Γ¯ and Κ¯ of 2.00m₀ and 0.43m₀, respectively. We also measure an increase in the band linewidth from the midpoint of Γ¯Κ¯ to the vicinity of Κ¯ and briefly discussmore » its possible origin.« less

Synthesis, Characterization, and Properties of the Two-Dimensional Chalcogenides: Monolayers, Alloys, and Heterostructures

NASA Astrophysics Data System (ADS)

Cain, Jeffrey D.

Inspired by the triumphs of graphene, and motivated by its limitations, the science and engineering community is rapidly exploring the landscape of other layered materials in their atomically-thin forms. Dominating this landscape are the layered chalcogenides; diverse in chemistry, crystal structure, and properties, there are well over 100 primary members of this material family. Driven by quantum confinement, single layers (or few, in some cases) of these materials exhibit electronic, optical, and mechanical properties that diverge dramatically from their bulk counterparts. While initially isolated in monolayer form via mechanical exfoliation, the field of two-dimensional (2D) materials is being forced evolve to more scalable and reliable methods. Focusing on the chalcogenides (e.g. MoS2, Bi 2Se3, etc.), this dissertation introduces and mechanistically examines multiple novel synthetic approaches for the direct growth of monolayers, heterostructures, and alloys with the desired quality, reproducibility and generality. The first methods described in this thesis are physical vapor transport (PVT) and evaporative thinning (ET): a facile, top-down synthesis approach for creating ultrathin specimens of layered materials down to the two-dimensional limit. Evaporative thinning, applied in this study to the fabrication of A2X3 (Bi2Se3 and Sb2Te3) monolayers, is based on the controlled evaporation of material from initially thick specimens until the 2D limit is reached. The resultant flakes are characterized with a suite of imaging and spectroscopic techniques and the mechanism of ET is investigated via in-situ heating within a transmission electron microscope. Additionally, the basic transport properties of the resultant flakes are probed. The growth of ultrathin GeSe flakes is explored using PVT and the material's basic structure, properties, and stability are addressed. Second, oxide precursor based chemical vapor deposition (CVD) is presented for the direct growth of monolayer transition metal dichalcogenides materials (TMDs), MX2. Specifically, synthetic protocols for monolayers of the semiconducting TMDs, MoS2, WS2, and MoSe2, are presented and the layers are completely characterized with a full suite of techniques including scanning/transmission electron microscopy, scanning probe microscopy, and photo-spectroscopy (photoluminescence, Raman scattering) among others. Furthermore, the nucleation and growth processes in CVD grown TMD monolayers is investigated using aberration-corrected scanning transmission electron microscopy and a step-by-step nucleation and growth mechanism for monolayer TMDs is elucidated. Third, the use of monolayers as building blocks for more complex structures is explored and a predictive approach for the synthesis of van der Waals heterostructures and monolayer alloys is designed. This is accomplished using an intellectual framework analogous to the Time-Temperature-Transformation (TTT) diagrams that are ubiquitous in traditional materials science and metallurgy. These are called Time-Temperature-Architecture (TTA) diagrams, wherein the term "architecture" defines the specific conformation of the 2D components. These diagrams provide process guidance to synthesize a library of heterostructures with a range of morphologies and monolayer alloys within both the transition metal sulfide and selenide systems. Lastly, some next steps for future research are outlined. An appendix of summaries of collaborative studies of the optical and electronic properties of CVD grown monolayers TMDs is appended.

Nonequilibrium 2-Hydroxyoctadecanoic Acid Monolayers: Effect of Electrolytes

DOE Office of Scientific and Technical Information (OSTI.GOV)

Lendrum, Conrad D.; Ingham, Bridget; Lin, Binhua

2012-02-06

2-Hydroxyacids display complex monolayer phase behavior due to the additional hydrogen bonding afforded by the presence of the second hydroxy group. The placement of this group at the position {alpha} to the carboxylic acid functionality also introduces the possibility of chelation, a utility important in crystallization including biomineralization. Biomineralization, like many biological processes, is inherently a nonequilibrium process. The nonequilibrium monolayer phase behavior of 2-hydroxyoctadecanoic acid was investigated on each of pure water, calcium chloride, sodium bicarbonate and calcium carbonate crystallizing subphases as a precursor study to a model calcium carbonate biomineralizing system, each at a pH of {approx}6. Themore » role of the bicarbonate co-ion in manipulating the monolayer structure was determined by comparison with monolayer phase behavior on a sodium chloride subphase. Monolayer phase behavior was probed using surface pressure/area isotherms, surface potential, Brewster angle microscopy, and synchrotron-based grazing incidence X-ray diffraction and X-ray reflectivity. Complex phase behavior was observed for all but the sodium chloride subphase with hydrogen bonding, electrostatic and steric effects defining the symmetry of the monolayer. On a pure water subphase hydrogen bonding dominates with three phases coexisting at low pressures. Introduction of calcium ions into the aqueous subphase ensures strong cation binding to the surfactant head groups through chelation. The monolayer becomes very unstable in the presence of bicarbonate ions within the subphase due to short-range hydrogen bonding interactions between the monolayer and bicarbonate ions facilitated by the sodium cation enhancing surfactant solubility. The combined effects of electrostatics and hydrogen bonding are observed on the calcium carbonate crystallizing subphase.« less

Interaction of 3‧,4‧,6‧-trimyristoyl-uridine derivative as potential anticancer drug with phospholipids of tumorigenic and non-tumorigenic cells

NASA Astrophysics Data System (ADS)

Salis, Luiz Fernando Grosso; Jaroque, Guilherme Nuñez; Escobar, Jhon Fernando Berrío; Giordani, Cristiano; Martinez, Alejandro Martinez; Fernández, Diana Margarita Márquez; Castelli, Francesco; Sarpietro, Maria Grazia; Caseli, Luciano

2017-12-01

Investigating the mechanism of action of drugs whose pharmaceutical activity is associated with cell membranes is fundamental to comprehending the biochemical and biophysical processes that occur on membrane surfaces. In this work, we investigated the interaction of an ester-type derivative of uridine, 3‧,4‧,6‧-trimyristoyl uridine, with models for cell membranes formed by lipid monolayers at the air-water interface. For that, selected lipids have been chosen in order to mimic tumorigenic and non-tumorigenic cells. For mixed monolayers with 2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC) or 1,2-dihexadecanoyl-sn-glycero-3-phospho-L-serine (DPPS), the surface pressure-area isotherms exhibited a noticeable shift to lower areas in relation to the areas predicted for ideal mixtures, indicating a condensation of the monolayer structure. Changes in the viscoelastic properties of the interfacial film could be inferred by analyzing the compressibility modulus of the monolayer. Structural and morphological changes were also evidenced by using vibrational spectroscopy and Brewster angle microscopy, respectively, with distinctive effects on DPPC and DPPS. As conclusion we can state that the lipid composition of the monolayer modulates the interaction with this lipophilic drug, which may have important implications in understanding how this drug acts on specific sites of the cellular membrane.

Structural Phase Transformation in Strained Monolayer MoWSe2 Alloy.

PubMed

Apte, Amey; Kochat, Vidya; Rajak, Pankaj; Krishnamoorthy, Aravind; Manimunda, Praveena; Hachtel, Jordan A; Idrobo, Juan Carlos; Syed Amanulla, Syed Asif; Vashishta, Priya; Nakano, Aiichiro; Kalia, Rajiv K; Tiwary, Chandra Sekhar; Ajayan, Pulickel M

2018-04-24

Two-dimensional (2D) materials exhibit different mechanical properties from their bulk counterparts owing to their monolayer atomic thickness. Here, we have examined the mechanical behavior of 2D molybdenum tungsten diselenide (MoWSe 2 ) precipitation alloy grown using chemical vapor deposition and composed of numerous nanoscopic MoSe 2 and WSe 2 regions. Applying a bending strain blue-shifted the MoSe 2 and WSe 2 A 1g Raman modes with the stress concentrated near the precipitate interfaces predominantly affecting the WSe 2 modes. In situ local Raman measurements suggested that the crack propagated primarily thorough MoSe 2 -rich regions in the monolayer alloy. Molecular dynamics (MD) simulations were performed to study crack propagation in an MoSe 2 monolayer containing nanoscopic WSe 2 regions akin to the experiment. Raman spectra calculated from MD trajectories of crack propagation confirmed the emergence of intermediate peaks in the strained monolayer alloy, mirroring experimental results. The simulations revealed that the stress buildup around the crack tip caused an irreversible structural transformation from the 2H to 1T phase both in the MoSe 2 matrix and WSe 2 patches. This was corroborated by high-angle annular dark-field images. Crack branching and subsequent healing of a crack branch were also observed in WSe 2 , indicating the increased toughness and crack propagation resistance of the alloyed 2D MoWSe 2 over the unalloyed counterparts.

MURI Center for Materials Chemistry in the Space Environment

DTIC Science & Technology

2006-11-30

ionic species in relevant reaction environments, surface photochemistry expertise, synchrotron-based measurement and irradiation, synthesis of structural...and Ne+ ions with dodecanethiolate and semifluorinated dodecanethiolate self-assembled monolayers (SAM), polyhedral oligomeric silsesquioxane (POSS...POSS/Kapton models as gas phase species, and with alkane thiol self assembled monolayers on gold surfaces, and with liquid squalane. We have also

Water penetration/accommodation and phase behaviour of the neutral Langmuir monolayer at the air/water interface probed with sum frequency generation vibrational spectroscopy (SFG-VS).

PubMed

Zhang, Zhen; Zheng, De-Sheng; Guo, Yuan; Wang, Hong-Fei

2009-02-14

A strong and broad hydrogen bonded O-H band around 3520 cm(-1) is observed in the insoluble monolayer of the neutral liquid crystal molecules of 4''-n-pentyl-4-p-cyanobiphenyl (5CB) and 4''-n-octyl-4-p-cyanobiphenyl (8CB) throughout the whole surface density range, but not in the 4-pentyl-4'-cyanoterphenyl (5CT) monolayer, at the air/water interface. This novel spectral feature suggests the existence of an oriented water cluster species which has penetrated or accommodated into the Langmuir monolayer of the 8CB and 5CB molecules. This finding provided a molecular level mechanism for the stark difference in the phase behaviour between the CB and CT insoluble Langmuir monolayers at the air/water interface. It also calls for attention to the details of the specific water-surface interaction in mediating the structure and the phase behaviour of the molecular assemblies at the heterogeneous aqueous interfaces.

Exciton band structure in layered MoSe2: from a monolayer to the bulk limit.

PubMed

Arora, Ashish; Nogajewski, Karol; Molas, Maciej; Koperski, Maciej; Potemski, Marek

2015-12-28

We present the micro-photoluminescence (μPL) and micro-reflectance contrast (μRC) spectroscopy studies on thin films of MoSe(2) with layer thicknesses ranging from a monolayer (1L) up to 5L. The thickness dependent evolution of the ground and excited state excitonic transitions taking place at various points of the Brillouin zone is determined. Temperature activated energy shifts and linewidth broadenings of the excitonic resonances in 1L, 2L and 3L flakes are accounted for by using standard formalisms previously developed for semiconductors. A peculiar shape of the optical response of the ground state (A) exciton in monolayer MoSe(2) is tentatively attributed to the appearance of a Fano-type resonance. Rather trivial and clearly decaying PL spectra of monolayer MoSe(2) with temperature confirm that the ground state exciton in this material is optically bright in contrast to a dark exciton ground state in monolayer WSe(2).

Adsorption of gas molecules on Cu impurities embedded monolayer MoS2: A first- principles study

NASA Astrophysics Data System (ADS)

Zhao, B.; Li, C. Y.; Liu, L. L.; Zhou, B.; Zhang, Q. K.; Chen, Z. Q.; Tang, Z.

2016-09-01

Adsorption of small gas molecules (O2, NO, NO2 and NH3) on transition-metal Cu atom embedded monolayer MoS2 was investigated by first-principles calculations based on the density-functional theory (DFT). The embedded Cu atom is strongly constrained on the sulfur vacancy of monolayer MoS2 with a high diffusion barrier. The stable adsorption geometry, charge transfer and electronic structures of these gas molecules on monolayer MoS2 embedded with transition-metal Cu atom are discussed in detail. It is found that the monolayer MoS2 with embedded Cu atom can effectively capture these gas molecules with high adsorption energy. The NH3 molecule acts as electron donor after adsorption, which is different from the other gas molecules (O2, NO, and NO2). The results suggest that MoS2-Cu system may be promising for future applications in gas molecules sensing and catalysis, which is similar to those of the transition-metal embedded graphene.

Exciton band structure in layered MoSe2: from a monolayer to the bulk limit

NASA Astrophysics Data System (ADS)

Arora, Ashish; Nogajewski, Karol; Molas, Maciej; Koperski, Maciej; Potemski, Marek

2015-12-01

We present the micro-photoluminescence (μPL) and micro-reflectance contrast (μRC) spectroscopy studies on thin films of MoSe2 with layer thicknesses ranging from a monolayer (1L) up to 5L. The thickness dependent evolution of the ground and excited state excitonic transitions taking place at various points of the Brillouin zone is determined. Temperature activated energy shifts and linewidth broadenings of the excitonic resonances in 1L, 2L and 3L flakes are accounted for by using standard formalisms previously developed for semiconductors. A peculiar shape of the optical response of the ground state (A) exciton in monolayer MoSe2 is tentatively attributed to the appearance of a Fano-type resonance. Rather trivial and clearly decaying PL spectra of monolayer MoSe2 with temperature confirm that the ground state exciton in this material is optically bright in contrast to a dark exciton ground state in monolayer WSe2.

Conformal and highly luminescent monolayers of Alq3 prepared by gas-phase molecular layer deposition.

PubMed

Räupke, André; Albrecht, Fabian; Maibach, Julia; Behrendt, Andreas; Polywka, Andreas; Heiderhoff, Ralf; Helzel, Jonatan; Rabe, Torsten; Johannes, Hans-Hermann; Kowalsky, Wolfgang; Mankel, Eric; Mayer, Thomas; Görrn, Patrick; Riedl, Thomas

2014-01-22

The gas-phase molecular layer deposition (MLD) of conformal and highly luminescent monolayers of tris(8-hydroxyquinolinato)aluminum (Alq3) is reported. The controlled formation of Alq3 monolayers is achieved for the first time by functionalization of the substrate with amino groups, which serve as initial docking sites for trimethyl aluminum (TMA) molecules binding datively to the amine. Thereby, upon exposure to 8-hydroxyquinoline (8-HQ), the self-limiting formation of highly luminescent Alq3 monolayers is afforded. The growth process and monolayer formation were studied and verified by in situ quartz crystal monitoring, optical emission and absorption spectroscopy, and X-ray photoelectron spectroscopy. The nature of the MLD process provides an avenue to coat arbitrarily shaped 3D surfaces and porous structures with high surface areas, as demonstrated in this work for silica aerogels. The concept presented here paves the way to highly sensitive luminescent sensors and dye-sensitized metal oxides for future applications (e.g., in photocatalysis and solar cells).

A Self-Limiting Electro-Ablation Technique for the Top-Down Synthesis of Large-Area Monolayer Flakes of 2D Materials

DOE Office of Scientific and Technical Information (OSTI.GOV)

Das, Saptarshi; Bera, Mrinal K.; Tong, Sheng

2016-06-21

We report the discovery of an electrochemical process that converts two dimensional layered materials of arbitrary thicknesses into monolayers. The lateral dimensions of the monolayers obtained by the process within a few seconds time at room temperature were as large as 0.5 mm. The temporal and spatial dynamics of this physical phenomenon, studied on MoS2 flakes using ex-situ AFM imaging, Raman mapping, and photoluminescence measurements trace the origin of monolayer formation to a substrate-assisted self-limiting electrochemical ablation process. Electronic structure and atomistic calculations point to the interplay between three essential factors in the process: (1) strong covalent interaction of monolayermore » MoS2 with the substrate; (2) electric-field induced differences in Gibbs free energy of exfoliation; (3) dispersion of MoS2 in aqueous solution of hydrogen peroxide. This process was successful in obtaining monolayers of other 2D transition metal dichalcogenides, like WS2 and MoTe2 as well.« less

Intrinsic exciton-state mixing and nonlinear optical properties in transition metal dichalcogenide monolayers

NASA Astrophysics Data System (ADS)

Glazov, M. M.; Golub, L. E.; Wang, G.; Marie, X.; Amand, T.; Urbaszek, B.

2017-01-01

Optical properties of transition metal dichalcogenides monolayers are controlled by Wannier-Mott excitons forming a series of 1 s ,2 s ,2 p ,... hydrogen-like states. We develop the theory of the excited excitonic states energy spectrum fine structure. We predict that p - and s -shell excitons are mixed due to the specific D3 h point symmetry of the transition metal dichalcogenide monolayers. Hence, both s - and p -shell excitons are active in both single- and two-photon processes, providing an efficient mechanism of second harmonic generation. The corresponding contribution to the nonlinear susceptibility is calculated.

The Structure of Self-Assembled Monolayers of Alkylsiloxanes on Silicon: A Comparison of Results from Ellipsometry and Low-Angle X-Ray Reflectivity

DTIC Science & Technology

1989-05-01

Thicknesses measured by the two methods differ by 2.2 (rlms) for alkyl chains of 10 - 18 carbon atoms and have a maximum difference of 4.2 e Ellipsometry...the approximate area projected by each alkyl group in the plane of the monolayer is 0,, 1’ ) # . Preliminary studies indicate that the use of this...projected by each alkyl group in the plane of the monolayer is - 21 ± 3 A2 . Preliminary studies indicate that this technique can be used to follow the

I Situ Surface X-Ray Diffraction Studies of Electrochemically Deposited Monolayers

NASA Astrophysics Data System (ADS)

Yee, Dennis

1995-01-01

In situ x-ray diffraction has been used to determine the detailed atomic structure of electrochemically deposited lead, thallium, and bismuth monolayers on the silver (111) electrode surface. A review of our previously published lead and thallium monolayer results and the first in situ surface x-ray crystallographic study of the bismuth monolayer structure is presented. The crystallographic analysis of the bismuth Bragg rod intensities and the interference between the bismuth Bragg rod and silver crystal truncation rod scattering were used to determine the detailed atomic structure of the bismuth on silver (111) system at the liquid-solid interface. Our previous in situ x-ray diffraction studies showed that the bismuth monolayer lattice is rectangular and uniaxially incommensurate with the underlying hexagonal silver surface. A crystallographic analysis of the measured structure factor magnitudes reveals that the monolayer forms chains of atoms on the silver surface, similar to the bulk Bi(110)_{rh} plane, with a near neighbor distance of 3.12 +/- 0.01 A and a bond angle of 93 +/- 1^circ, consistent with the bulk Bi(110) _{rh} plane values. The crystallographic refinement also shows that the bismuth monolayer atoms are anisotropically disordered with a rms disorder of 0.25 +/- 0.03 A in the incommensurate direction and 0.09 +/- 0.03 A rms in the commnensurate direction. The interference between the Bi(20) Bragg rod and the Ag(10L)_ {h} crystal truncation rod scattering reveals that one set of bismuth atoms is registered near the bridge sites of the silver (111) surface while another set is registered near the 3-fold hollow sites. In addition, the Bi-Ag d-spacing (3.1 +/- 0.1 A) is found to be consistent with the bulk bismuth near neighbor distance. The bismuth z-direction rms disorder (1.01 +/- 0.08 A) is found to be dominated by the roughness of the underlying silver (sigma_{Ag} = 0.9 +/- 0.1 A rms). Using the estimated bismuth-bismuth spring constant of 1.41 +/- 0.07 eV/A^2 from our measured bismuth two-dimensional compressibility, two simple models are used to try and understand the origin of the anisotropic disorder. A simple two-dimensional isotropic thermal fluctuation model shows that thermal fluctuations are not large enough to account for all of the measured excess disorder in the incommensurate direction. A simple one-dimensional Frenkel-Kontorova model shows that the substrate-induced disorder can account for the anisotropic disorder, assuming a substrate sinusoidal potential strength of 0.35 +/- 0.02 eV.

Molecular dynamics simulations of structural transformation of perfluorooctane sulfonate (PFOS) at water/rutile interfaces.

PubMed

He, Guangzhi; Zhang, Meiyi; Zhou, Qin; Pan, Gang

2015-09-01

Concentration and salinity conditions are the dominant environmental factors affecting the behavior of perfluorinated compounds (PFCs) on the surfaces of a variety of solid matrices (suspended particles, sediments, and natural minerals). However, the mechanism has not yet been examined at molecular scales. Here, the structural transformation of perfluorooctane sulfonate (PFOS) at water/rutile interfaces induced by changes of the concentration level of PFOS and salt condition was investigated using molecular dynamics (MD) simulations. At low and intermediate concentrations all PFOS molecules directly interacted with the rutile (110) surface mainly by the sulfonate headgroups through electrostatic attraction, yielding a typical monolayer structure. As the concentration of PFOS increased, the molecules aggregated in a complex multi-layered structure, where an irregular assembling configuration was adsorbed on the monolayer structure by the van der Waals interactions between the perfluoroalkyl chains. When adding CaCl2 to the system, the multi-layered structure changed to a monolayer again, indicating that the addition of CaCl2 enhanced the critical concentration value to yield PFOS multilayer assemblies. The divalent Ca(2+) substituted for monovalent K(+) as the bridging counterion in PFOS adsorption. MD simulation may trigger wide applications in study of perfluorinated compounds (PFCs) from atomic/molecular scale. Copyright © 2015 Elsevier Ltd. All rights reserved.

A class of monolayer metal halogenides MX{sub 2}: Electronic structures and band alignments

DOE Office of Scientific and Technical Information (OSTI.GOV)

Lu, Feng; Wang, Weichao; Luo, Xiaoguang

2016-03-28

With systematic first principles calculations, a class of monolayer metal halogenides MX{sub 2} (M = Mg, Ca, Zn, Cd, Ge, Pb; M = Cl, Br, I) has been proposed. Our study indicates that these monolayer materials are semiconductors with the band gaps ranging from 2.03 eV of ZnI{sub 2} to 6.08 eV of MgCl{sub 2}. Overall, the band gap increases with the increase of the electronegativity of the X atom or the atomic number of the metal M. Meanwhile, the band gaps of monolayer MgX{sub 2} (X = Cl, Br) are direct while those of other monolayers are indirect. Based on the band edge curvatures, the derived electronmore » (m{sub e}) and hole (m{sub h}) effective masses of MX{sub 2} monolayers are close to their corresponding bulk values except that the m{sub e} of CdI{sub 2} is three times larger and the m{sub h} for PbI{sub 2} is twice larger. Finally, the band alignments of all the studied MX{sub 2} monolayers are provided using the vacuum level as energy reference. These theoretical results may not only introduce the monolayer metal halogenides family MX{sub 2} into the emerging two-dimensional materials, but also provide insights into the applications of MX{sub 2} in future electronic, visible and ultraviolet optoelectronic devices.« less

Comparison of electronic structure between monolayer silicenes on Ag (111)

NASA Astrophysics Data System (ADS)

Chun-Liang, Lin; Ryuichi, Arafune; Maki, Kawai; Noriaki, Takagi

2015-08-01

The electronic structures of monolayer silicenes (4 × 4 and ) grown on Ag (111) surface are studied by scanning tunneling spectroscopy (STS) and density functional theory (DFT) calculations. While both phases have similar electronic structures around the Fermi level, significant differences are observed in the higher energy unoccupied states. The DFT calculations show that the contributions of Si 3pz orbitals to the unoccupied states are different because of their different buckled configurations. Project supported by the Ministry of Education, Culture, Sports, Science and Technology (MEXT) through Grants-in-Aid for Scientific Research (Grant Nos. 24241040 and 25110008) and the World Premier International Research Center Initiative (WPI), MEXT, Japan.

One-pot synthesis of powder-form β-Ni(OH)2 monolayer nanosheets with high electrochemical performance

NASA Astrophysics Data System (ADS)

Wang, Minmin; Ren, Wanzhong; Zhao, Yunan; Liu, Yan; Cui, Hongtao

2013-08-01

In this work, β-Ni(OH)2 monolayer nanosheets, which had been thought to be unachievable, were successfully prepared for the first time by a one-pot strategy using epoxide as precipitation agent and sodium dodecyl sulfate (SDS) as surfactant. The characterization results indicate that the formation of monolayer morphology depends on the mediation of SDS molecules. The XRD patterns demonstrate the loose and defective packing of Ni(OH)2 layers in the SDS intercalated samples. The disappearing of vibration band of free hydroxyl groups in the FTIR spectra suggests the interlayer separation resulted by SDS. The TEM and AFM images further confirm the formation of monolayer nanosheets. It is proposed that the in situ modification of the secondary growth unit of β-Ni(OH)2 by SDS allows its two-dimensional anisotropic growth through steric hindrance of SDS molecules. In addition, this effect allows isolation of β-Ni(OH)2 from solvent with keeping of monolayer nanosheet state in dry powder. The electrochemical measurement results indicate that β-Ni(OH)2 monolayer nanosheets own much higher urea electrolysis performance than their corresponding multilayer structure.

High resolution Talbot self-imaging applied to structural characterization of self-assembled monolayers of microspheres.

PubMed

Garcia-Sucerquia, J; Alvarez-Palacio, D C; Kreuzer, H J

2008-09-10

We report the observation of the Talbot self-imaging effect in high resolution digital in-line holographic microscopy (DIHM) and its application to structural characterization of periodic samples. Holograms of self-assembled monolayers of micron-sized polystyrene spheres are reconstructed at different image planes. The point-source method of DIHM and the consequent high lateral resolution allows the true image (object) plane to be identified. The Talbot effect is then exploited to improve the evaluation of the pitch of the assembly and to examine defects in its periodicity.

DOE Office of Scientific and Technical Information (OSTI.GOV)

Novikova, N. N., E-mail: nn-novikova07@yandex.ru; Kovalchuk, M. V.; Yakunin, S. N.

The processes of structural rearrangement in a model membrane, i.e., an arachic acid monolayer formed on a colloidal solution of cerium dioxide or magnetite, are studied in situ in real time by the methods of X-ray standing waves and 2D diffraction. It is shown that the character of the interaction of nanoparticles with the monolayer is determined by their nature and sizes and depends on the conditions of nanoparticle synthesis. In particular, the structure formation in the monolayer–particle system is greatly affected by the stabilizer (citric acid), which is introduced into the colloidal solution during synthesis.

Structure, transport and photoconductance of PbS quantum dot monolayers functionalized with a copper phthalocyanine derivative.

PubMed

André, A; Theurer, C; Lauth, J; Maiti, S; Hodas, M; Samadi Khoshkhoo, M; Kinge, S; Meixner, A J; Schreiber, F; Siebbeles, L D A; Braun, K; Scheele, M

2017-01-31

We simultaneously surface-functionalize PbS nanocrystals with Cu 4,4',4'',4'''-tetraaminophthalocyanine and assemble this hybrid material into macroscopic monolayers. Electron microscopy and X-ray scattering reveal a granular mesocrystalline structure with strong coherence between the atomic lattice and the superlattice of nanocrystals within each domain. Terahertz spectroscopy and field-effect transistor measurements indicate efficient coupling of holes throughout the hybrid thin film, in conjunction with a pronounced photoresponse. We demonstrate the potential of this material for optoelectronic applications by fabricating a light-effect transistor.

Coherent X-ray Scattering from Liquid-Air Interfaces

NASA Astrophysics Data System (ADS)

Shpyrko, Oleg

Advances in synchrotron x-ray scattering techniques allow studies of structure and dynamics of liquid surfaces with unprecedented resolution. I will review x-ray scattering measurements of thermally excited capillary fluctuations in liquids, thin polymer liquid films and polymer surfaces in confined geometry. X-ray Diffuse scattering profile due to Debye-Waller like roughening of the surface allows to probe the distribution of capillary fluctuations over a wide range of length scales, while using X-ray Photon Correlation Spectroscopy (XPCS) one is able to directly couple to nanoscale dynamics of these surface fluctuations, over a wide range of temporal and spacial scales. I will also discuss recent XPCS measurements of lateral diffusion dynamics in Langmuir monolayers assembled at the liquid-air interface. This research was supported by NSF CAREER Grant 0956131.

Complex band structures of transition metal dichalcogenide monolayers with spin-orbit coupling effects

NASA Astrophysics Data System (ADS)

Szczęśniak, Dominik; Ennaoui, Ahmed; Ahzi, Saïd

2016-09-01

Recently, the transition metal dichalcogenides have attracted renewed attention due to the potential use of their low-dimensional forms in both nano- and opto-electronics. In such applications, the electronic and transport properties of monolayer transition metal dichalcogenides play a pivotal role. The present paper provides a new insight into these essential properties by studying the complex band structures of popular transition metal dichalcogenide monolayers (MX 2, where M  =  Mo, W; X  =  S, Se, Te) while including spin-orbit coupling effects. The conducted symmetry-based tight-binding calculations show that the analytical continuation from the real band structures to the complex momentum space leads to nonlinear generalized eigenvalue problems. Herein an efficient method for solving such a class of nonlinear problems is presented and yields a complete set of physically relevant eigenvalues. Solutions obtained by this method are characterized and classified into propagating and evanescent states, where the latter states manifest not only monotonic but also oscillatory decay character. It is observed that some of the oscillatory evanescent states create characteristic complex loops at the direct band gap of MX 2 monolayers, where electrons can directly tunnel between the band gap edges. To describe these tunneling currents, decay behavior of electronic states in the forbidden energy region is elucidated and their importance within the ballistic transport regime is briefly discussed.

Diverse carrier mobility of monolayer BNC_x: A combined density functional theory and Boltzmann transport theory study.

PubMed

Wu, Tao; Deng, Kaiming; Deng, Wei-Qiao; Lu, Ruifeng

2017-09-19

BNCX monolayer as a kind of two-dimensional material has numerous chemical atomic ratios and arrangements with different electronic structures. Via calculations on the basis of density functional theory and Boltzmann transport theory under deformation potential approximation, the band structures and carrier mobilities of BNCX (x=1,2,3,4) nanosheets are systematically investigated. The calculated results show that BNC2-1 is a material with very small band gap (0.02 eV) among all the structures while other BNCX monolayers are semiconductors with band gap ranging from 0.51 to 1.32 eV. The carrier mobility of BNCX varies considerably from tens to millions of cm2 V-1 s-1. For BNC2-1, the hole mobility and electron mobility along both x and y directions can reach 105 orders of magnitude, which is similar to the carrier mobility of graphene. Besides, all studied BNCX monolayers obviously have anisotropic hole mobility and electron mobility. In particular, for semiconductor BNC4, its hole mobility along y direction and electron mobility along x direction unexpectedly reach 106 orders of magnitude, even higher than that of graphene. Our findings suggest that BNCX layered materials with proper ratio and arrangement of carbon atoms will possess desirable charge transport properties, exhibiting potential applications in nanoelectronic devices. © 2017 IOP Publishing Ltd.

Hydrogen-assisted post-growth substitution of tellurium into molybdenum disulfide monolayers with tunable compositions

NASA Astrophysics Data System (ADS)

Yin, Guoli; Zhu, Dancheng; Lv, Danhui; Hashemi, Arsalan; Fei, Zhen; Lin, Fang; Krasheninnikov, Arkady V.; Zhang, Ze; Komsa, Hannu-Pekka; Jin, Chuanhong

2018-04-01

Herein we report the successful doping of tellurium (Te) into molybdenum disulfide (MoS2) monolayers to form MoS2x Te2(1-x) alloy with variable compositions via a hydrogen-assisted post-growth chemical vapor deposition process. It is confirmed that H2 plays an indispensable role in the Te substitution into as-grown MoS2 monolayers. Atomic-resolution transmission electron microscopy allows us to determine the lattice sites and the concentration of introduced Te atoms. At a relatively low concentration, tellurium is only substituted in the sulfur sublattice to form monolayer MoS2(1-x)Te2x alloy, while with increasing Te concentration (up to ˜27.6% achieved in this study), local regions with enriched tellurium, large structural distortions, and obvious sulfur deficiency are observed. Statistical analysis of the Te distribution indicates the random substitution. Density functional theory calculations are used to investigate the stability of the alloy structures and their electronic properties. Comparison with experimental results indicate that the samples are unstrained and the Te atoms are predominantly substituted in the top S sublattice. Importantly, such ultimately thin Janus structure of MoS2(1-x)Te2x exhibits properties that are distinct from their constituents. We believe our results will inspire further exploration of the versatile properties of asymmetric 2D TMD alloys.

Adsorption of GST-PI3Kgamma at the air-buffer interface and at substrate and nonsubstrate phospholipid monolayers.

PubMed

Hermelink, Antje; Kirsch, Cornelia; Klinger, Reinhard; Reiter, Gerald; Brezesinski, Gerald

2009-02-01

The recruitment of phosphoinositide 3-kinase gamma (PI3Kgamma) to the cell membrane is a crucial requirement for the initiation of inflammation cascades by second-messenger production. In addition to identifying other regulation pathways, it has been found that PI3Kgamma is able to bind phospholipids directly. In this study, the adsorption behavior of glutathione S-transferase (GST)-PI3Kgamma to nonsubstrate model phospholipids, as well as to commercially available substrate inositol phospholipids (phosphoinositides), was investigated by use of infrared reflection-absorption spectroscopy (IRRAS). The nonsubstrate phospholipid monolayers also yielded important information about structural requirements for protein adsorption. The enzyme did not interact with condensed zwitterionic or anionic monolayers; however, it could penetrate into uncompressed fluid monolayers. Compression to values above its equilibrium pressure led to a squeezing out and desorption of the protein. Protein affinity for the monolayer surface increased considerably when the lipid had an anionic headgroup and contained an arachidonoyl fatty acyl chain in sn-2 position. Similar results on a much higher level were observed with substrate phosphoinositides. No structural response of GST-PI3Kgamma to lipid interaction was detected by IRRAS. On the other hand, protein adsorption caused a condensing effect in phosphoinositide monolayers. In addition, the protein reduced the charge density at the interface probably by shifting the pK values of the phosphate groups attached to the inositol headgroups. Because of their strongly polar headgroups, an interaction of the inositides with the water molecules of the subphase can be expected. This interaction is disturbed by protein adsorption, causing the ionization state of the phosphates to change.

Adsorption of GST-PI3Kγ at the Air-Buffer Interface and at Substrate and Nonsubstrate Phospholipid Monolayers

PubMed Central

Hermelink, Antje; Kirsch, Cornelia; Klinger, Reinhard; Reiter, Gerald; Brezesinski, Gerald

2009-01-01

The recruitment of phosphoinositide 3-kinase γ (PI3Kγ) to the cell membrane is a crucial requirement for the initiation of inflammation cascades by second-messenger production. In addition to identifying other regulation pathways, it has been found that PI3Kγ is able to bind phospholipids directly. In this study, the adsorption behavior of glutathione S-transferase (GST)-PI3Kγ to nonsubstrate model phospholipids, as well as to commercially available substrate inositol phospholipids (phosphoinositides), was investigated by use of infrared reflection-absorption spectroscopy (IRRAS). The nonsubstrate phospholipid monolayers also yielded important information about structural requirements for protein adsorption. The enzyme did not interact with condensed zwitterionic or anionic monolayers; however, it could penetrate into uncompressed fluid monolayers. Compression to values above its equilibrium pressure led to a squeezing out and desorption of the protein. Protein affinity for the monolayer surface increased considerably when the lipid had an anionic headgroup and contained an arachidonoyl fatty acyl chain in sn-2 position. Similar results on a much higher level were observed with substrate phosphoinositides. No structural response of GST-PI3Kγ to lipid interaction was detected by IRRAS. On the other hand, protein adsorption caused a condensing effect in phosphoinositide monolayers. In addition, the protein reduced the charge density at the interface probably by shifting the pK values of the phosphate groups attached to the inositol headgroups. Because of their strongly polar headgroups, an interaction of the inositides with the water molecules of the subphase can be expected. This interaction is disturbed by protein adsorption, causing the ionization state of the phosphates to change. PMID:19186139

Characterization of Surface-Active Biofilm Protein BslA in Self-Assembling Langmuir Monolayer at the Air-Water Interface.

PubMed

Liu, Wei; Li, Shanghao; Wang, Zhuguang; Yan, Elsa C Y; Leblanc, Roger M

2017-08-01

Biofilm is an extracellular matrix of bacteria and serves as a protective shield of bacterial communities. It is crucial for microbial growth and one of the leading causes of human chronic infections as well. However, the structures and molecular mechanism of biofilm formation remain largely unknown. Here, we examined a protein, BslA, expressed in the biofilms of Bacillus subtilis. We characterized the Langmuir monolayers of BslA at the air/water interface. Using techniques in surface chemistry and spectroscopy, we found that BslA forms a stable and robust Langmuir monolayer at the air/water interface. Our results show that the BslA Langmuir monolayer underwent two-stage elasticity in the solid state phase upon mechanical compression: one is possibly due to the intermolecular interaction and the other is likely due to both the intermolecular compulsion and the intramolecular distortion. The Langmuir monolayer of BslA shows abrupt changes in rigidities and elasticities at ∼25 mN/m. This surface pressure is close to the one at which BlsA saturates the air/water interface as a self-assembled film without mechanical compression, corresponding to a mean molecular area of ∼700 Å 2 per molecule. Based on the results of surface UV-visible spectroscopy and infrared reflective-absorption spectroscopy, we propose that the BslA Langmuir monolayer carries intermolecular elasticity before ∼25 mN/m and both intermolecular and intramolecular elasticity after ∼25 mN/m. These results provide valuable insights into the understanding of biofilm-associated protein under high mechanical force, shedding light on further investigation of biofilm structure and functionalities.

Domain size polydispersity effects on the structural and dynamical properties in lipid monolayers with phase coexistence.

PubMed

Rufeil-Fiori, Elena; Banchio, Adolfo J

2018-03-07

In lipid monolayers with phase coexistence, domains of the liquid-condensed phase always present size polydispersity. However, very few theoretical works consider size distribution effects on the monolayer properties. Because of the difference in surface densities, domains have excess dipolar density with respect to the surrounding liquid expanded phase, originating a dipolar inter-domain interaction. This interaction depends on the domain area, and hence the presence of a domain size distribution is associated with interaction polydispersity. Inter-domain interactions are fundamental to understanding the structure and dynamics of the monolayer. For this reason, it is expected that polydispersity significantly alters monolayer properties. By means of Brownian dynamics simulations, we study the radial distribution function (RDF), the average mean square displacement and the average time-dependent self-diffusion coefficient, D(t), of lipid monolayers with normally distributed size domains. For this purpose, we vary the relevant system parameters, polydispersity and interaction strength, within a range of experimental interest. We also analyze the consequences of using a monodisperse model to determine the interaction strength from an experimental RDF. We find that polydispersity strongly affects the value of the interaction strength, which is greatly underestimated if polydispersity is not considered. However, within a certain range of parameters, the RDF obtained from a polydisperse model can be well approximated by that of a monodisperse model, by suitably fitting the interaction strength, even for 40% polydispersities. For small interaction strengths or small polydispersities, the polydisperse systems obtained from fitting the experimental RDF have an average mean square displacement and D(t) in good agreement with that of the monodisperse system.

Structure of a monolayer of molecular rotors on aqueous subphase from grazing-incidence X-ray diffraction.

PubMed

Kaleta, Jiří; Wen, Jin; Magnera, Thomas F; Dron, Paul I; Zhu, Chenhui; Michl, Josef

2018-03-23

In situ grazing-incidence X-ray scattering shows that a monolayer of artificial rod-shaped dipolar molecular rotors produced on the surface of an aqueous subphase in a Langmuir trough has a structure conducive to a 2D ferroelectric phase. The axes of the rotors stand an average of 0.83 nm apart in a triangular grid, perpendicular to the surface within experimental error. They carry 2,3-dichlorophenylene rotators near rod centers, between two decks of interlocked triptycenes installed axially on the rotor axle. The analysis is based first on simultaneous fitting of observed Bragg rods and second on fitting the reflectivity curve with only three adjustable parameters and the calculated rotor electron density, which also revealed the presence of about seven molecules of water near each rotator. Dependent on preparation conditions, a minor and variable amount of a different crystal phase may also be present in the monolayer.

Characterisation of the membrane affinity of an isoniazide peptide conjugate by tensiometry, atomic force microscopy and sum-frequency vibrational spectroscopy, using a phospholipid Langmuir monolayer model.

PubMed

Hill, Katalin; Pénzes, Csanád Botond; Schnöller, Donát; Horváti, Kata; Bosze, Szilvia; Hudecz, Ferenc; Keszthelyi, Tamás; Kiss, Eva

2010-10-07

Tensiometry, sum-frequency vibrational spectroscopy, and atomic force microscopy were employed to assess the cell penetration ability of a peptide conjugate of the antituberculotic agent isoniazide. Isoniazide was conjugated to peptide (91)SEFAYGSFVRTVSLPV(106), a functional T-cell epitope of the immunodominant 16 kDa protein of Mycobacterium tuberculosis. As a simple but versatile model of the cell membrane a phospholipid Langmuir monolayer at the liquid/air interface was used. Changes induced in the structure of the phospholipid monolayer by injection of the peptide conjugate into the subphase were followed by tensiometry and sum-frequency vibrational spectroscopy. The drug penetrated lipid films were transferred to a solid support by the Langmuir-Blodgett technique, and their structures were characterized by atomic force microscopy. Peptide conjugation was found to strongly enhance the cell penetration ability of isoniazide.

Fracture Characteristics of Monolayer CVD-Graphene

PubMed Central

Hwangbo, Yun; Lee, Choong-Kwang; Kim, Sang-Min; Kim, Jae-Hyun; Kim, Kwang-Seop; Jang, Bongkyun; Lee, Hak-Joo; Lee, Seoung-Ki; Kim, Seong-Su; Ahn, Jong-Hyun; Lee, Seung-Mo

2014-01-01

We have observed and analyzed the fracture characteristics of the monolayer CVD-graphene using pressure bulge testing setup. The monolayer CVD-graphene has appeared to undergo environmentally assisted subcritical crack growth in room condition, i.e. stress corrosion cracking arising from the adsorption of water vapor on the graphene and the subsequent chemical reactions. The crack propagation in graphene has appeared to be able to be reasonably tamed by adjusting applied humidity and stress. The fracture toughness, describing the ability of a material containing inherent flaws to resist catastrophic failure, of the CVD-graphene has turned out to be exceptionally high, as compared to other carbon based 3D materials. These results imply that the CVD-graphene could be an ideal candidate as a structural material notwithstanding environmental susceptibility. In addition, the measurements reported here suggest that specific non-continuum fracture behaviors occurring in 2D monoatomic structures can be macroscopically well visualized and characterized. PMID:24657996

DOE Office of Scientific and Technical Information (OSTI.GOV)

Han, Bong-Gyoon; Watson, Zoe; Kang, Hannah

We describe a rapid and convenient method of growing streptavidin (SA) monolayer crystals directly on holey-carbon EM grids. As expected, these SA monolayer crystals retain their biotin-binding function and crystalline order through a cycle of embedding in trehalose and, later, its removal. This fact allows one to prepare, and store for later use, EM grids on which SA monolayer crystals serve as an affinity substrate for preparing specimens of biological macromolecules. In addition, we report that coating the lipid-tail side of trehalose-embedded monolayer crystals with evaporated carbon appears to improve the consistency with which well-ordered, single crystals are observed tomore » span over entire, 2 μm holes of the support films. Randomly biotinylated 70S ribosomes are used as a test specimen to show that these support films can be used to obtain a high-resolution cryo-EM structure« less

Molecular orientation of copper phthalocyanine thin films on different monolayers of fullerene on SiO{sub 2} or highly oriented pyrolytic graphite

DOE Office of Scientific and Technical Information (OSTI.GOV)

Wang, Chenggong; Wang, Congcong; Liu, Xiaoliang

2015-03-23

The interface electronic structures of copper phthalocyanine (CuPc) have been studied using ultraviolet photoemission spectroscopy as different monolayers of C{sub 60} were inserted between CuPc and a SiO{sub 2} or highly ordered pyrolytic graphite (HOPG) substrate. The results show that CuPc has standing up configuration with one monolayer of C{sub 60} insertion on SiO{sub 2} while lying down on HOPG, indicating that the insertion layer propagates the CuPc-substrate interaction. Meanwhile, CuPc on more than one monolayers of C{sub 60} on different substrates show that the substrate orientation effect quickly vanished. Our study elucidates intriguing molecular interactions that manipulate molecular orientationmore » and donor-acceptor energy level alignment.« less

A new Dirac cone material: a graphene-like Be3C2 monolayer.

PubMed

Wang, Bing; Yuan, Shijun; Li, Yunhai; Shi, Li; Wang, Jinlan

2017-05-04

Two-dimensional (2D) materials with Dirac cones exhibit rich physics and many intriguing properties, but the search for new 2D Dirac materials is still a current hotspot. Using the global particle-swarm optimization method and density functional theory, we predict a new stable graphene-like 2D Dirac material: a Be 3 C 2 monolayer with a hexagonal honeycomb structure. The Dirac point occurs exactly at the Fermi level and arises from the merging of the hybridized p z bands of Be and C atoms. Most interestingly, this monolayer exhibits a high Fermi velocity in the same order of graphene. Moreover, the Dirac cone is very robust and retains even included spin-orbit coupling or external strain. These outstanding properties render the Be 3 C 2 monolayer a promising 2D material for special electronics applications.

Electronic and magnetic properties of SnS2 monolayer doped with non-magnetic elements

NASA Astrophysics Data System (ADS)

Xiao, Wen-Zhi; Xiao, Gang; Rong, Qing-Yan; Wang, Ling-Ling

2018-05-01

We performed a systematic study of the electronic structures and magnetic properties of SnS2 monolayer doped with non-magnetic elements in groups IA, IIA and IIIA based on the first-principles methods. The doped systems exhibit half-metallic and metallic natures depending on the doping elements. The formation of magnetic moment is attributable to the cooperative effect of the Hund's rule coupling and hole concentration. The spin polarization can be stabilized and enhanced through confining the delocalized impurity states by biaxial tensile strain in hole-doped SnS2 monolayer. Both the double-exchange and p-p exchange mechanisms are simultaneously responsible for the ferromagnetic ground state in those hole-doped materials. Our results demonstrate that spin polarization can be induced and controlled in SnS2 monolayers by non-magnetic doping and tensile strain.

Characterization of 10,12-pentacosadiynoic acid Langmuir–Blodgett monolayers and their use in metal–insulator–metal tunnel devices

PubMed Central

Sharma, Saumya; Khawaja, Mohamad; Ram, Manoj K; Goswami, D Yogi

2014-01-01

Summary The characterization of Langmuir–Blodgett thin films of 10,12-pentacosadiynoic acid (PDA) and their use in metal–insulator–metal (MIM) devices were studied. The Langmuir monolayer behavior of the PDA film was studied at the air/water interface using surface tension–area isotherms of polymeric and monomeric PDA. Langmuir–Blodgett (LB, vertical deposition) and Langmuir–Schaefer (LS, horizontal deposition) techniques were used to deposit the PDA film on various substrates (glass, quartz, silicon, and nickel-coated film on glass). The electrochemical, electrical and optical properties of the LB and LS PDA films were studied using cyclic voltammetry, current–voltage characteristics (I–V), and UV–vis and FTIR spectroscopies. Atomic force microscopy measurements were performed in order to analyze the surface morphology and roughness of the films. A MIM tunnel diode was fabricated using a PDA monolayer assembly as the insulating barrier, which was sandwiched between two nickel layers. The precise control of the thickness of the insulating monolayers proved critical for electron tunneling to take place in the MIM structure. The current–voltage characteristics of the MIM diode revealed tunneling behavior in the fabricated Ni–PDA LB film–Ni structures. PMID:25551052

Characterization of 10,12-pentacosadiynoic acid Langmuir-Blodgett monolayers and their use in metal-insulator-metal tunnel devices.

PubMed

Sharma, Saumya; Khawaja, Mohamad; Ram, Manoj K; Goswami, D Yogi; Stefanakos, Elias

2014-01-01

The characterization of Langmuir-Blodgett thin films of 10,12-pentacosadiynoic acid (PDA) and their use in metal-insulator-metal (MIM) devices were studied. The Langmuir monolayer behavior of the PDA film was studied at the air/water interface using surface tension-area isotherms of polymeric and monomeric PDA. Langmuir-Blodgett (LB, vertical deposition) and Langmuir-Schaefer (LS, horizontal deposition) techniques were used to deposit the PDA film on various substrates (glass, quartz, silicon, and nickel-coated film on glass). The electrochemical, electrical and optical properties of the LB and LS PDA films were studied using cyclic voltammetry, current-voltage characteristics (I-V), and UV-vis and FTIR spectroscopies. Atomic force microscopy measurements were performed in order to analyze the surface morphology and roughness of the films. A MIM tunnel diode was fabricated using a PDA monolayer assembly as the insulating barrier, which was sandwiched between two nickel layers. The precise control of the thickness of the insulating monolayers proved critical for electron tunneling to take place in the MIM structure. The current-voltage characteristics of the MIM diode revealed tunneling behavior in the fabricated Ni-PDA LB film-Ni structures.

On atomic structure of Ge huts growing on the Ge/Si(001) wetting layer

NASA Astrophysics Data System (ADS)

Arapkina, Larisa V.; Yuryev, Vladimir A.

2013-09-01

Structural models of growing Ge hut clusters—pyramids and wedges—are proposed on the basis of data of recent STM investigations of nucleation and growth of Ge huts on the Si(001) surface in the process of molecular beam epitaxy. It is shown that extension of a hut base along ⟨110⟩ directions goes non-uniformly during the cluster growth regardless of its shape. Growing pyramids, starting from the second monolayer, pass through cyclic formation of slightly asymmetrical and symmetrical clusters, with symmetrical ones appearing after addition of every fourth monolayer. We suppose that pyramids of symmetrical configurations composed by 2, 6, 10, etc., monolayers over the wetting layer are more stable than asymmetrical ones. This might explain less stability of pyramids in comparison with wedges in dense arrays forming at low temperatures of Ge deposition. Possible nucleation processes of pyramids and wedges on wetting layer patches from identical embryos composed by 8 dimers through formation of 1 monolayer high 16-dimer nuclei different only in their symmetry is discussed. Schematics of these processes are presented. It is concluded from precise STM measurements that top layers of wetting layer patches are relaxed when huts nucleate on them.

Excitonic Properties of Chemically Synthesized 2D Organic-Inorganic Hybrid Perovskite Nanosheets.

PubMed

Zhang, Qi; Chu, Leiqiang; Zhou, Feng; Ji, Wei; Eda, Goki

2018-05-01

2D organic-inorganic hybrid perovskites (OIHPs) represent a unique class of materials with a natural quantum-well structure and quasi-2D electronic properties. Here, a versatile direct solution-based synthesis of mono- and few-layer OIHP nanosheets and a systematic study of their electronic structure as a function of the number of monolayers by photoluminescence and absorption spectroscopy are reported. The monolayers of various OIHPs are found to exhibit high electronic quality as evidenced by high quantum yield and negligible Stokes shift. It is shown that the ground exciton peak blueshifts by ≈40 meV when the layer thickness reduces from bulk to monolayer. It is also shown that the exciton binding energy remains effectively unchanged for (C 6 H 5 (CH 2 ) 2 NH 3 ) 2 PbI 4 with the number of layers. Similar trends are observed for (C 4 H 9 NH 3 ) 2 PbI 4 in contrast to the previous report. Further, the photoluminescence lifetime is found to decrease with the number of monolayers, indicating the dominant role of surface trap states in nonradiative recombination of the electron-hole pairs. © 2018 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

Insights about α-tocopherol and Trolox interaction with phosphatidylcholine monolayers under peroxidation conditions through Brewster angle microscopy.

PubMed

Castro, Carla M; Pinheiro, Marina; Lúcio, Marlene; Giner-Casares, Juan J; Camacho, Luis; Lima, José L F C; Reis, Salette; Segundo, Marcela A

2013-11-01

Membranes are major targets to oxidative damage, particularly due to lipid oxidation, which has been associated to aging. The role, efficacy and membrane interaction of antioxidants is still unclear, requiring further understanding of molecular interaction. Hence, the objective of this work was to evaluate the interaction between antioxidants (α-tocopherol and its aqueous soluble analog Trolox) and the monolayer formed by phosphatidylcholine molecules at air/liquid interface upon peroxidation conditions, promoted by peroxyl radicals from thermal decomposition of 2,2'-azobis(2-methylpropionamidine) (AAPH). The interaction with three different monolayers, containing (i) 1,2-dihexadecanoyl-sn-glycero-3-phosphocholine (DPPC), (ii) DDPC+α-linolenic acid, or (iii) egg yolk l-α-phosphatidylcholine (EPC), was ascertain by surface pressure (π)-molecular area (A) isotherms and by monitoring monolayer features through Brewster angle microscopy (BAM). The interaction of antioxidants with DPPC monolayers was confirmed by modifications on DPPC domain shape for α-tocopherol and through the maintenance of typical multilobed domain shape during an extended surface pressure interval for Trolox. Under peroxidation conditions, BAM images showed a clear interaction between components of AAPH subphase with the monolayer through changes on DPPC domain shape and appearance of white dots, located mainly at the frontier between the condensed and expanded liquid phases. White branched structures were also observed whenever both α-linolenic acid and α-tocopherol were present, indicating the segregation of these components within the monolayer, which is highly significant in biological systems. For EPC monolayers, no information from BAM was obtained but π-A isotherms confirmed the existence of the same interactions observed within the other two monolayers. Copyright © 2013 Elsevier B.V. All rights reserved.

The Electronic Properties of O-Doped Pure and Sulfur Vacancy-Defect Monolayer WS₂: A First-Principles Study.

PubMed

Wang, Weidong; Bai, Liwen; Yang, Chenguang; Fan, Kangqi; Xie, Yong; Li, Minglin

2018-01-31

Based on the density functional theory (DFT), the electronic properties of O-doped pure and sulfur vacancy-defect monolayer WS₂ are investigated by using the first-principles method. For the O-doped pure monolayer WS₂, four sizes (2 × 2 × 1, 3 × 3 × 1, 4 × 4 × 1 and 5 × 5 × 1) of supercell are discussed to probe the effects of O doping concentration on the electronic structure. For the 2 × 2 × 1 supercell with 12.5% O doping concentration, the band gap of O-doped pure WS₂ is reduced by 8.9% displaying an indirect band gap. The band gaps in 3 × 3 × 1 and 4 × 4 × 1 supercells are both opened to some extent, respectively, for 5.55% and 3.13% O doping concentrations, while the band gap in 5 × 5 × 1 supercell with 2.0% O doping concentration is quite close to that of the pure monolayer WS₂. Then, two typical point defects, including sulfur single-vacancy (V S ) and sulfur divacancy (V 2S ), are introduced to probe the influences of O doping on the electronic properties of WS₂ monolayers. The observations from DFT calculations show that O doping can broaden the band gap of monolayer WS₂ with V S defect to a certain degree, but weaken the band gap of monolayer WS₂ with V 2S defect. Doping O element into either pure or sulfur vacancy-defect monolayer WS₂ cannot change their band gaps significantly, however, it still can be regarded as a potential method to slightly tune the electronic properties of monolayer WS₂.

Pulmonary surfactant protein C containing lipid films at the air-water interface as a model for the surface of lung alveoli.

PubMed

Post, A; Nahmen, A V; Schmitt, M; Ruths, J; Riegler, H; Sieber, M; Galla, H J

1995-01-01

The pulmonary surfactant lines as a complex monolayer of lipids and proteins the alveolar epithelial surface. The monolayer dynamically adapts the surface tension of this interface to the varying surface areas during inhalation and exhalation. Its presence in the alveoli is thus a prerequisite for a proper lung function. The lipid moiety represents about 90% of the surfactant and contains mainly dipalmitoylphosphatidylcholine (DPPC) and phosphatidylglycerol (PG). The surfactant proteins involved in the surface tension adaption are called SP-A, SP-B and SP-C. The aim of the present investigation is to analyse the properties of monolayer films made from pure SP-C and from mixtures of DPPC, DPPG and SP-C in order to mimic the surfactant monolayer with minimal compositional requirement. Pressure-area diagrams were taken. Ellipsometric measurements at the air-water interface of a Langmuir film balance allowed measurement of the changes in monolayer thickness upon compression. Isotherms of pure SP-C monolayers exhibit a plateau between 22 and 25 mN/m. A further plateau is reached at higher compression. Structures of the monolayer formed during compression are reversible during expansion. Together with ellipsometric data which show a stepwise increase in film thickness (coverage) during compression, we conclude that pure SP-C films rearrange reversibly into multilayers of homogenous thickness. Lipid monolayers collapse locally and irreversibly if films are compressed to approximately 0.4 nm2/molecule. In contrast, mixed DPPG/SP-C monolayers with less than 5 mol% protein collapse in a controlled and reversible way. The pressure-area diagrams exhibit a plateau at 20 mN/m, indicating partial demixing of SP-C and DPPG.(ABSTRACT TRUNCATED AT 250 WORDS)

Cytogenetic landscape of paired neurospheres and traditional monolayer cultures in pediatric malignant brain tumors.

PubMed

Zhao, Xiumei; Zhao, Yi-Jue; Lin, Qi; Yu, Litian; Liu, Zhigang; Lindsay, Holly; Kogiso, Mari; Rao, Pulivarthi; Li, Xiao-Nan; Lu, Xinyan

2015-07-01

New therapeutic targets are needed to eliminate cancer stem cells (CSCs). We hypothesize that direct comparison of paired CSCs and nonstem tumor cells (NSTCs) will facilitate identification of primary "driver" chromosomal aberrations that can serve as diagnostic markers and/or therapeutic targets. We applied spectral karyotyping and G-banding to matched pairs of neurospheres (CSC-enriched cultures) and fetal bovine serum-based monolayer cultures (enriched with NSTCs) from 16 patient-derived orthotopic xenograft mouse models, including 9 medulloblastomas (MBs) and 7 high-grade gliomas (HGGs), followed by direct comparison of their numerical and structural abnormalities. Chromosomal aberrations were detected in neurospheres of all 16 models, and 82.0% numerical and 82.4% structural abnormalities were maintained in their matching monolayer cultures. Among the shared abnormalities, recurrent clonal changes were identified including gain of chromosomes 18 and 7 and loss of chromosome 10/10q (5/16 models), isochromosome 17q in 2 MBs, and a new breakpoint of 13q14 in 3 HGGs. Chromothripsis-like evidence was also observed in 3 HGG pairs. Additionally, we noted 20 numerical and 15 structural aberrations that were lost from the neurospheres and found 26 numerical and 23 structural aberrations that were only present in the NSTCs. Compared with MBs, the neurosphere karyotypes of HGG were more complex, with fewer chromosomal aberrations preserved in their matching NSTCs. Self-renewing CSCs in MBs and pediatric HGGs harbor recurrent numerical and structural aberrations that were maintained in the matching monolayer cultures. These primary chromosomal changes may represent new markers for anti-CSC therapies. © The Author(s) 2014. Published by Oxford University Press on behalf of the Society for Neuro-Oncology. All rights reserved. For permissions, please e-mail: journals.permissions@oup.com.

Effect of Na+ and Ca2+ ions on a lipid Langmuir monolayer: an atomistic description by molecular dynamics simulations.

PubMed

Giner Casares, Juan José; Camacho, Luis; Martín-Romero, Maria Teresa; López Cascales, José Javier

2008-12-01

Studying the effect of alkali and alkaline-earth metal cations on Langmuir monolayers is relevant from biophysical and nanotechnological points of view. In this work, the effect of Na(+) and Ca(2+) on a model of an anionic Langmuir lipid monolayer of dimyristoylphosphatidate (DMPA(-)) is studied by molecular dynamics simulations. The influence of the type of cation on lipid structure, lipid-lipid interactions, and lipid ordering is analyzed in terms of electrostatic interactions. It is found that for a lipid monolayer in its solid phase, the effect of the cations on the properties of the lipid monolayer can be neglected. The influence of the cations is enhanced for the lipid monolayer in its gas phase, where sodium ions show a high degree of dehydration compared with calcium ions. This loss of hydration shell is partly compensated by the formation of lipid-ion-lipid bridges. This difference is ascribed to the higher charge-to-radius ratio q/r for Ca(2+), which makes ion dehydration less favorable compared to Na(+). Owing to the different dehydration behavior of sodium and calcium ions, diminished lipid-lipid coordination, lipid-ion coordination, and lipid ordering are observed for Ca(2+) compared to Na(+). Furthermore, for both gas and solid phases of the lipid Langmuir monolayers, lipid conformation and ion dehydration across the lipid/water interface are studied.

A systems biology approach to defining regulatory mechanisms for cartilage and tendon cell phenotypes.

PubMed

Mueller, A J; Tew, S R; Vasieva, O; Clegg, P D; Canty-Laird, E G

2016-09-27

Phenotypic plasticity of adult somatic cells has provided emerging avenues for the development of regenerative therapeutics. In musculoskeletal biology the mechanistic regulatory networks of genes governing the phenotypic plasticity of cartilage and tendon cells has not been considered systematically. Additionally, a lack of strategies to effectively reproduce in vitro functional models of cartilage and tendon is retarding progress in this field. De- and redifferentiation represent phenotypic transitions that may contribute to loss of function in ageing musculoskeletal tissues. Applying a systems biology network analysis approach to global gene expression profiles derived from common in vitro culture systems (monolayer and three-dimensional cultures) this study demonstrates common regulatory mechanisms governing de- and redifferentiation transitions in cartilage and tendon cells. Furthermore, evidence of convergence of gene expression profiles during monolayer expansion of cartilage and tendon cells, and the expression of key developmental markers, challenges the physiological relevance of this culture system. The study also suggests that oxidative stress and PI3K signalling pathways are key modulators of in vitro phenotypes for cells of musculoskeletal origin.

Electrical transport across grain boundaries in graphene monolayers on SiC(0 0 0 \\bar{1} )

NASA Astrophysics Data System (ADS)

Zhou, Xiaodong; Ji, Shuai-Hua; Chockalingam, S. P.; Hannon, J. B.; Tromp, R. M.; Heinz, T. F.; Pasupathy, A. N.; Ross, F. M.

2018-07-01

We measure the role of structural defects, including grain boundaries and step edges, in determining the electrical transport characteristics of polycrystalline graphene monolayers synthesized on C-face SiC(0 0 0 ) by thermal decomposition. A combination of multi-probe scanning tunneling microscopy/potentiometry and low-energy electron microscopy allows the transport properties of individual grain boundaries to be correlated with their misorientation and atomic-level structure, without any device fabrication. We find that different types of grain boundary show dramatically different transport properties, and that boundaries can change structure and resistivity along their length. Boundary regions made up of dislocation superlattices separated by continuous graphene exhibit relatively low resistivity which is comparable to the resistivity of the graphene sheet itself. Other grain boundaries display trench structures with a resistivity 1–2 orders of magnitude greater and sufficient to dominate transport through the polycrystalline sheet. We also measure the transport properties of step edges and monolayer-bilayer boundaries on C-face graphene and compare them to Si-face graphene. Such measurements offer a guideline for optimizing graphene growth on SiC to improve its electronic properties.

A scanning tunneling microscope study on an ordered mixed monolayer of bis(4,5-dihydronaphtho[1,2-d])-tetrathiafulvalene and n-tetradecane on highly oriented pyrolytic graphite.

PubMed

Zhao, Miao; Jiang, Peng; Deng, Ke; Jiang, Chao

2010-11-01

Tetrathiafulvalene (TTF) and its derivatives (TTFs) have been successfully used as building blocks to form charge transfer salts and organic semiconductors because of their special structures and rich electron nature. We report the formation of ordered mixed binary-component monolayer consisting of Bis(4,5-dihydronaphtho[1,2-d])tetrathiafulvalene (DH-TTF) and n-tetradecane (n-C14H30) molecules on highly oriented pyrolytic graphite (HOPG) surface. Scanning tunneling microscope (STM) imaging reveals that the two different kinds of molecules can spontaneously form ordered periodic phase separation structures on the substrate, in which ordered DH-TTF double- (or single-) lamella structures are periodically tuned by ordered n-C14H30 double- (or single-) lamella structures. Furthermore, scanning tunneling spectrum (STS) measurements by addressing the individual DH-TTF and n-C14H30 molecules in the ordered monolayer show that the two different kinds of molecules exhibit completely different I(V) characters on the HOPG substrate. The modulated arrangement of the TTF derivative by insulating molecules opens a possible route to construct organic conducting molecule ribbons for potential application in nanodevices.

Fabrication of sub-diffraction-limit molecular structures by scanning near-field photolithography

NASA Astrophysics Data System (ADS)

Ducker, Robert E.; Montague, Matthew T.; Sun, Shuqing; Leggett, Graham J.

2007-09-01

Using a scanning near-field optical microscope coupled to a UV laser, an approach we term scanning near-field photolithography (SNP), structures as small as 9 nm (ca. λ/30) may be fabricated in self-assembled monolayers of alkanethiols on gold surfaces. Selective exposure of the adsorbate molecules in the near field leads to photoconversion of the alkylthiolate to a weakly bound alkylsulfonate which may be displaced readily be a contrasting thiol, leading to a chemical pattern, or used as a resist for the selective etching of the underlying metal. A novel ultra-mild etch for gold is reported, and used to etch structures as small as 9 nm. Photopatterning of oligo(ethylene glycol) (OEG) terminated selfassembled monolayers facilitates the fabrication of biomolecular nanostructures. Selective removal of the protein-resistant OEG terminated adsorbates created regions that may be functionalized with a second thiol and derivatized with a biomolecule. Finally, the application of SNP to nanopatterning on oxide surfaces is demonstrated. Selective exposure of monolayers of phosphonic acids adsorbed onto aluminum oxide leads to cleavage of the P-C bond and desorption of the adsorbate molecule. Subsequent etching, using aqueous based, yields structures as small as 100 nm.

Shape Transitions and Lattice Structuring of Ceramide-Enriched Domains Generated by Sphingomyelinase in Lipid Monolayers

PubMed Central

Härtel, Steffen; Fanani, María Laura; Maggio, Bruno

2005-01-01

Sphingomyelinases (SMases) hydrolyze the membrane constituent sphingomyelin (SM) to phosphocholine and ceramide (Cer). Growing evidence supports that SMase-induced SM→Cer conversion leads to the formation of lateral Cer-enriched domains which drive structural reorganization in lipid membranes. We previously provided visual evidence in real-time for the formation of Cer-enriched domains in SM monolayers through the action of the neutral Bacillus cereus SMase. In this work, we disclose a succession of discrete morphologic transitions and lateral organization of Cer-enriched domains that underlay the SMase-generated surface topography. We further reveal how these structural parameters couple to the generation of two-dimensional electrostatic fields, based upon the specific orientation of the lipid dipole moments in the Cer-enriched domains. Advanced image processing routines in combination with time-resolved epifluorescence microscopy on Langmuir monolayers revealed: 1), spontaneous nucleation and circular growth of Cer-enriched domains after injection of SMase into the subphase of the SM monolayer; 2), domain-intrinsic discrete transitions from circular to periodically undulating shapes followed by a second transition toward increasingly branched morphologies; 3), lateral superstructure organization into predominantly hexagonal domain lattices; 4), formation of super-superstructures by the hexagonal lattices; and 5), rotationally and laterally coupled domain movement before domain border contact. All patterns proved to be specific for the SMase-driven system since they could not be observed with Cer-enriched domains generated by defined mixtures of SM/Cer in enzyme-free monolayers at the same surface pressure (Π = 10 mN/m). Following the theories of lateral shape transitions, dipolar electrostatic interactions of lipid domains, and direct determinations of the monolayer dipole potential, our data show that SMase induces a domain-specific packing and orientation of the molecular dipole moments perpendicular to the air/water interface. In consequence, protein-driven generation of specific out-of-equilibrium states, an accepted concept for maintenance of transmembrane lipid asymmetry, must also be considered on the lateral level. Lateral enzyme-specific out-of-equilibrium organization of lipid domains represents a new level of signal transduction from local (nm) to long-range (μm) scales. The cross-talk between lateral domain structures and dipolar electrostatic fields adds new perspectives to the mechanisms of SMase-mediated signal transduction in biological membranes. PMID:15489298

Monolayer atomic crystal molecular superlattices.

PubMed

Wang, Chen; He, Qiyuan; Halim, Udayabagya; Liu, Yuanyue; Zhu, Enbo; Lin, Zhaoyang; Xiao, Hai; Duan, Xidong; Feng, Ziying; Cheng, Rui; Weiss, Nathan O; Ye, Guojun; Huang, Yun-Chiao; Wu, Hao; Cheng, Hung-Chieh; Shakir, Imran; Liao, Lei; Chen, Xianhui; Goddard, William A; Huang, Yu; Duan, Xiangfeng

2018-03-07

Artificial superlattices, based on van der Waals heterostructures of two-dimensional atomic crystals such as graphene or molybdenum disulfide, offer technological opportunities beyond the reach of existing materials. Typical strategies for creating such artificial superlattices rely on arduous layer-by-layer exfoliation and restacking, with limited yield and reproducibility. The bottom-up approach of using chemical-vapour deposition produces high-quality heterostructures but becomes increasingly difficult for high-order superlattices. The intercalation of selected two-dimensional atomic crystals with alkali metal ions offers an alternative way to superlattice structures, but these usually have poor stability and seriously altered electronic properties. Here we report an electrochemical molecular intercalation approach to a new class of stable superlattices in which monolayer atomic crystals alternate with molecular layers. Using black phosphorus as a model system, we show that intercalation with cetyl-trimethylammonium bromide produces monolayer phosphorene molecular superlattices in which the interlayer distance is more than double that in black phosphorus, effectively isolating the phosphorene monolayers. Electrical transport studies of transistors fabricated from the monolayer phosphorene molecular superlattice show an on/off current ratio exceeding 10 7 , along with excellent mobility and superior stability. We further show that several different two-dimensional atomic crystals, such as molybdenum disulfide and tungsten diselenide, can be intercalated with quaternary ammonium molecules of varying sizes and symmetries to produce a broad class of superlattices with tailored molecular structures, interlayer distances, phase compositions, electronic and optical properties. These studies define a versatile material platform for fundamental studies and potential technological applications.

Monolayer atomic crystal molecular superlattices

NASA Astrophysics Data System (ADS)

Wang, Chen; He, Qiyuan; Halim, Udayabagya; Liu, Yuanyue; Zhu, Enbo; Lin, Zhaoyang; Xiao, Hai; Duan, Xidong; Feng, Ziying; Cheng, Rui; Weiss, Nathan O.; Ye, Guojun; Huang, Yun-Chiao; Wu, Hao; Cheng, Hung-Chieh; Shakir, Imran; Liao, Lei; Chen, Xianhui; Goddard, William A., III; Huang, Yu; Duan, Xiangfeng

2018-03-01

Artificial superlattices, based on van der Waals heterostructures of two-dimensional atomic crystals such as graphene or molybdenum disulfide, offer technological opportunities beyond the reach of existing materials. Typical strategies for creating such artificial superlattices rely on arduous layer-by-layer exfoliation and restacking, with limited yield and reproducibility. The bottom-up approach of using chemical-vapour deposition produces high-quality heterostructures but becomes increasingly difficult for high-order superlattices. The intercalation of selected two-dimensional atomic crystals with alkali metal ions offers an alternative way to superlattice structures, but these usually have poor stability and seriously altered electronic properties. Here we report an electrochemical molecular intercalation approach to a new class of stable superlattices in which monolayer atomic crystals alternate with molecular layers. Using black phosphorus as a model system, we show that intercalation with cetyl-trimethylammonium bromide produces monolayer phosphorene molecular superlattices in which the interlayer distance is more than double that in black phosphorus, effectively isolating the phosphorene monolayers. Electrical transport studies of transistors fabricated from the monolayer phosphorene molecular superlattice show an on/off current ratio exceeding 107, along with excellent mobility and superior stability. We further show that several different two-dimensional atomic crystals, such as molybdenum disulfide and tungsten diselenide, can be intercalated with quaternary ammonium molecules of varying sizes and symmetries to produce a broad class of superlattices with tailored molecular structures, interlayer distances, phase compositions, electronic and optical properties. These studies define a versatile material platform for fundamental studies and potential technological applications.

Wetting and interfacial properties of water nanodroplets in contact with graphene and monolayer boron-nitride sheets.

PubMed

Li, Hui; Zeng, Xiao Cheng

2012-03-27

Born-Oppenheim quantum molecular dynamics (QMD) simulations are performed to investigate wetting, diffusive, and interfacial properties of water nanodroplets in contact with a graphene sheet or a monolayer boron-nitride (BN) sheet. Contact angles of the water nanodroplets on the two sheets are computed for the first time using QMD simulations. Structural and dynamic properties of the water droplets near the graphene or BN sheet are also studied to gain insights into the interfacial interaction between the water droplet and the substrate. QMD simulation results are compared with those from previous classic MD simulations and with the experimental measurements. The QMD simulations show that the graphene sheet yields a contact angle of 87°, while the monolayer BN sheet gives rise to a contact angle of 86°. Hence, like graphene, the monolayer BN sheet is also weakly hydrophobic, even though the BN bonds entail a large local dipole moment. QMD simulations also show that the interfacial water can induce net positive charges on the contacting surface of the graphene and monolayer BN sheets, and such charge induction may affect electronic structure of the contacting graphene in view that graphene is a semimetal. Contact angles of nanodroplets of water in a supercooled state on the graphene are also computed. It is found that under the supercooled condition, water nanodroplets exhibit an appreciably larger contact angle than under the ambient condition. © 2012 American Chemical Society

Surface-enhanced Raman spectroscopic and surface plasmon resonance in situ study of self-assembly of 4-mercaptobenzoic acid on gold surface

NASA Astrophysics Data System (ADS)

Thi, Minh Do; Volka, Karel

2010-07-01

A feasibility study has been undertaken to assess the suitability of a commercially available SERS substrate for monitoring of self-assembling deposition process. Monolayer self-assembly of 4-mercaptobenzoic acid on SERS active substrate Klarite™ from absolute and acidified ethanol was studied and compared with deposition on SPR substrate from absolute ethanol. Changes in integral intensity of the phenyl bands at 1587 and 1076 cm -1 and ethanol band at 1451 cm -1 allow to follow structural changes in the monolayer. Stability of the monolayer assembled from acidified ethanol in contrast to the pure ethanol was demonstrated.

Organization of Amphiphilic Molecular Disks with Branched Hydrophilic Tails and Hexa-peri-hexabenzocoronene Core

NASA Astrophysics Data System (ADS)

Lee, Myongsoo; Kim, Jung-Woo; Yoo, Yong-Sik; Peleshanko, Sergey; Larson, Kirsten; Vaknin, David; Markutsya, Sergei; Tsukruk, Vladimir V.

2002-03-01

Amphiphilic branched discotics consisting of the aromatic core and oligoethers as the branched peripheral chains have been characterized in bulk and monolayer states. The discotics based on di-branched oligoether side chains have been observed to self-organize into an ordered hexagonal columnar structure within liquid crystalline (LC) phases. The tetrabranched molecule showed only an isotropic liquid. The LC molecules with di-branched tails have been observed to form stable monolayers on the water surface in contrast to the tetra-branched tails. We suggest a crab-like molecular conformation and cluster-segregated monolayers with six-fold symmetry of face-on packing on a solid surface.

Experimental and Theoretical Structural Investigation of AuPt Nanoparticles Synthesized using a Direct Electrochemical Method

DOE Office of Scientific and Technical Information (OSTI.GOV)

Lapp, Aliya S.; Duan, Zhiyao; Marcella, Nicholas

In this report we examine the structure of bimetallic nanomaterials prepared by an electrochemical approach known as hydride-terminated (HT) electrodeposition. It has been shown previously that this method can lead to deposition of a single Pt monolayer on bulk-phase Au surfaces. Specifically, under appropriate electrochemical conditions and using a solution containing PtCl 4 2-, a monolayer of Pt atoms electrodeposits onto bulk-phase Au immediately followed by a monolayer of H atoms. The H-atom capping layer prevents deposition of Pt multilayers. We applied this method to ~1.6 nm Au nanoparticles (AuNPs) immobilized on an inert electrode surface. In contrast to themore » well-defined, segregated Au/Pt structure of the bulk-phase surface, we observe that HT electrodeposition leads to the formation of AuPt quasi-random alloy NPs rather than the core@shell structure anticipated from earlier reports relating to deposition onto bulk phases. The results provide a good example of how the phase behavior of macro materials does not always translate to the nano world. A key component of this study was the structure determination of the AuPt NPs, which required a combination of electrochemical methods, electron microscopy, X-ray absorption spectroscopy, and theory (DFT and MD).« less

SAM-based Cell Transfer to Photopatterned Hydrogels for Microengineering Vascular-Like Structures

PubMed Central

Sadr, Nasser; Zhu, Mojun; Osaki, Tatsuya; Kakegawa, Takahiro; Yang, Yunzhi; Moretti, Matteo; Fukuda, Junji; Khademhosseini, Ali

2011-01-01

A major challenge in tissue engineering is to reproduce the native 3D microvascular architecture fundamental for in vivo functions. Current approaches still lack a network of perfusable vessels with native 3D structural organization. Here we present a new method combining self-assembled monolayer (SAM)-based cell transfer and gelatin methacrylate hydrogel photopatterning techniques for microengineering vascular structures. Human umbilical vein cell (HUVEC) transfer from oligopeptide SAM-coated surfaces to the hydrogel revealed two SAM desorption mechanisms: photoinduced and electrochemically triggered. The former, occurs concomitantly to hydrogel photocrosslinking, and resulted in efficient (>97%) monolayer transfer. The latter, prompted by additional potential application, preserved cell morphology and maintained high transfer efficiency of VE-cadherin positive monolayers over longer culture periods. This approach was also applied to transfer HUVECs to 3D geometrically defined vascular-like structures in hydrogels, which were then maintained in perfusion culture for 15 days. As a step toward more complex constructs, a cell-laden hydrogel layer was photopatterned around the endothelialized channel to mimic the vascular smooth muscle structure of distal arterioles. This study shows that the coupling of the SAM-based cell transfer and hydrogel photocrosslinking could potentially open up new avenues in engineering more complex, vascularized tissue constructs for regenerative medicine and tissue engineering applications. PMID:21802723

Experimental and Theoretical Structural Investigation of AuPt Nanoparticles Synthesized using a Direct Electrochemical Method

DOE PAGES

Lapp, Aliya S.; Duan, Zhiyao; Marcella, Nicholas; ...

2018-06-01

In this report we examine the structure of bimetallic nanomaterials prepared by an electrochemical approach known as hydride-terminated (HT) electrodeposition. It has been shown previously that this method can lead to deposition of a single Pt monolayer on bulk-phase Au surfaces. Specifically, under appropriate electrochemical conditions and using a solution containing PtCl 4 2-, a monolayer of Pt atoms electrodeposits onto bulk-phase Au immediately followed by a monolayer of H atoms. The H-atom capping layer prevents deposition of Pt multilayers. We applied this method to ~1.6 nm Au nanoparticles (AuNPs) immobilized on an inert electrode surface. In contrast to themore » well-defined, segregated Au/Pt structure of the bulk-phase surface, we observe that HT electrodeposition leads to the formation of AuPt quasi-random alloy NPs rather than the core@shell structure anticipated from earlier reports relating to deposition onto bulk phases. The results provide a good example of how the phase behavior of macro materials does not always translate to the nano world. A key component of this study was the structure determination of the AuPt NPs, which required a combination of electrochemical methods, electron microscopy, X-ray absorption spectroscopy, and theory (DFT and MD).« less

Experimental and Theoretical Structural Investigation of AuPt Nanoparticles Synthesized Using a Direct Electrochemical Method.

PubMed

Lapp, Aliya S; Duan, Zhiyao; Marcella, Nicholas; Luo, Long; Genc, Arda; Ringnalda, Jan; Frenkel, Anatoly I; Henkelman, Graeme; Crooks, Richard M

2018-05-11

In this report, we examine the structure of bimetallic nanomaterials prepared by an electrochemical approach known as hydride-terminated (HT) electrodeposition. It has been shown previously that this method can lead to deposition of a single Pt monolayer on bulk-phase Au surfaces. Specifically, under appropriate electrochemical conditions and using a solution containing PtCl 4 2- , a monolayer of Pt atoms electrodeposits onto bulk-phase Au immediately followed by a monolayer of H atoms. The H atom capping layer prevents deposition of Pt multilayers. We applied this method to ∼1.6 nm Au nanoparticles (AuNPs) immobilized on an inert electrode surface. In contrast to the well-defined, segregated Au/Pt structure of the bulk-phase surface, we observe that HT electrodeposition leads to the formation of AuPt quasi-random alloy NPs rather than the core@shell structure anticipated from earlier reports relating to deposition onto bulk phases. The results provide a good example of how the phase behavior of macro materials does not always translate to the nano world. A key component of this study was the structure determination of the AuPt NPs, which required a combination of electrochemical methods, electron microscopy, X-ray absorption spectroscopy, and theory (DFT and MD).

Proteomic profiling of halloysite clay nanotube exposure in intestinal cell co-culture.

PubMed

Lai, Xianyin; Agarwal, Mangilal; Lvov, Yuri M; Pachpande, Chetan; Varahramyan, Kody; Witzmann, Frank A

2013-11-01

Halloysite is aluminosilicate clay with a hollow tubular structure with nanoscale internal and external diameters. Assessment of halloysite biocompatibility has gained importance in view of its potential application in oral drug delivery. To investigate the effect of halloysite nanotubes on an in vitro model of the large intestine, Caco-2/HT29-MTX cells in monolayer co-culture were exposed to nanotubes for toxicity tests and proteomic analysis. Results indicate that halloysite exhibits a high degree of biocompatibility characterized by an absence of cytotoxicity, in spite of elevated pro-inflammatory cytokine release. Exposure-specific changes in expression were observed among 4081 proteins analyzed. Bioinformatic analysis of differentially expressed protein profiles suggest that halloysite stimulates processes related to cell growth and proliferation, subtle responses to cell infection, irritation and injury, enhanced antioxidant capability, and an overall adaptive response to exposure. These potentially relevant functional effects warrant further investigation in in vivo models and suggest that chronic or bolus occupational exposure to halloysite nanotubes may have unintended outcomes. Copyright © 2013 John Wiley & Sons, Ltd.

Direct observation of impact propagation and absorption in dense colloidal monolayers

NASA Astrophysics Data System (ADS)

Buttinoni, Ivo; Cha, Jinwoong; Lin, Wei-Hsun; Job, Stéphane; Daraio, Chiara; Isa, Lucio

2017-11-01

Dense colloidal suspensions can propagate and absorb large mechanical stresses, including impacts and shocks. The wave transport stems from the delicate interplay between the spatial arrangement of the structural units and solvent-mediated effects. For dynamic microscopic systems, elastic deformations of the colloids are usually disregarded due to the damping imposed by the surrounding fluid. Here, we study the propagation of localized mechanical pulses in aqueous monolayers of micron-sized particles of controlled microstructure. We generate extreme localized deformation rates by exciting a target particle via pulsed-laser ablation. In crystalline monolayers, stress propagation fronts take place, where fast-moving particles (V approximately a few meters per second) are aligned along the symmetry axes of the lattice. Conversely, more viscous solvents and disordered structures lead to faster and isotropic energy absorption. Our results demonstrate the accessibility of a regime where elastic collisions also become relevant for suspensions of microscopic particles, behaving as “billiard balls” in a liquid, in analogy with regular packings of macroscopic spheres. We furthermore quantify the scattering of an impact as a function of the local structural disorder.

Complexation and phase evolution at dimethylformamide-Ag(111) interfaces

DOE Office of Scientific and Technical Information (OSTI.GOV)

Song, Wentao; Leung, Kevin; Shao, Qian

The interaction of solvent molecules with metallic surfaces impacts many interfacial chemical processes. We investigate the chemical and structure evolution that follows adsorption of the polar solvent dimethylformamide (DMF) on Ag(111). An Ag(DMF) 2 coordination complex forms spontaneously by DMF etching of Ag(111), yielding mixed films of the complexes and DMF. Utilizing ultrahigh vacuum scanning tunneling microscopy (UHV-STM), in combination with X-ray photoelectron spectroscopy (XPS) and density functional theory (DFT) computations, we map monolayer phases from the 2-D gas regime, consisting of a binary mixture of DMF and Ag(DMF) 2, through the saturation monolayer limit, in which these two chemicalmore » species phase separate into ordered islands. Structural models for the near-square DMF phase and the chain-like Ag(DMF) 2 phase are presented and supported by DFT computation. Interface evolution is summarized in a surface pressure-composition phase diagram, which allows structure prediction over arbitrary experimental conditions. In conclusion, this work reveals new surface coordination chemistry for an important electrolyte-electrode system, and illustrates how surface pressure can be used to tune monolayer phases.« less

Complexation and phase evolution at dimethylformamide-Ag(111) interfaces

DOE PAGES

Song, Wentao; Leung, Kevin; Shao, Qian; ...

2016-09-15

The interaction of solvent molecules with metallic surfaces impacts many interfacial chemical processes. We investigate the chemical and structure evolution that follows adsorption of the polar solvent dimethylformamide (DMF) on Ag(111). An Ag(DMF) 2 coordination complex forms spontaneously by DMF etching of Ag(111), yielding mixed films of the complexes and DMF. Utilizing ultrahigh vacuum scanning tunneling microscopy (UHV-STM), in combination with X-ray photoelectron spectroscopy (XPS) and density functional theory (DFT) computations, we map monolayer phases from the 2-D gas regime, consisting of a binary mixture of DMF and Ag(DMF) 2, through the saturation monolayer limit, in which these two chemicalmore » species phase separate into ordered islands. Structural models for the near-square DMF phase and the chain-like Ag(DMF) 2 phase are presented and supported by DFT computation. Interface evolution is summarized in a surface pressure-composition phase diagram, which allows structure prediction over arbitrary experimental conditions. In conclusion, this work reveals new surface coordination chemistry for an important electrolyte-electrode system, and illustrates how surface pressure can be used to tune monolayer phases.« less

Direct observation of impact propagation and absorption in dense colloidal monolayers

PubMed Central

Cha, Jinwoong; Lin, Wei-Hsun; Job, Stéphane; Daraio, Chiara

2017-01-01

Dense colloidal suspensions can propagate and absorb large mechanical stresses, including impacts and shocks. The wave transport stems from the delicate interplay between the spatial arrangement of the structural units and solvent-mediated effects. For dynamic microscopic systems, elastic deformations of the colloids are usually disregarded due to the damping imposed by the surrounding fluid. Here, we study the propagation of localized mechanical pulses in aqueous monolayers of micron-sized particles of controlled microstructure. We generate extreme localized deformation rates by exciting a target particle via pulsed-laser ablation. In crystalline monolayers, stress propagation fronts take place, where fast-moving particles (V approximately a few meters per second) are aligned along the symmetry axes of the lattice. Conversely, more viscous solvents and disordered structures lead to faster and isotropic energy absorption. Our results demonstrate the accessibility of a regime where elastic collisions also become relevant for suspensions of microscopic particles, behaving as “billiard balls” in a liquid, in analogy with regular packings of macroscopic spheres. We furthermore quantify the scattering of an impact as a function of the local structural disorder. PMID:29087329

Structural and configurational properties of nanoconfined monolayer ice from first principles

PubMed Central

Corsetti, Fabiano; Matthews, Paul; Artacho, Emilio

2016-01-01

Understanding the structural tendencies of nanoconfined water is of great interest for nanoscience and biology, where nano/micro-sized objects may be separated by very few layers of water. Here we investigate the properties of ice confined to a quasi-2D monolayer by a featureless, chemically neutral potential, in order to characterize its intrinsic behaviour. We use density-functional theory simulations with a non-local van der Waals density functional. An ab initio random structure search reveals all the energetically competitive monolayer configurations to belong to only two of the previously-identified families, characterized by a square or honeycomb hydrogen-bonding network, respectively. We discuss the modified ice rules needed for each network, and propose a simple point dipole 2D lattice model that successfully explains the energetics of the square configurations. All identified stable phases for both networks are found to be non-polar (but with a topologically non-trivial texture for the square) and, hence, non-ferroelectric, in contrast to previous predictions from a five-site empirical force-field model. Our results are in good agreement with very recently reported experimental observations. PMID:26728125

Structural and configurational properties of nanoconfined monolayer ice from first principles

NASA Astrophysics Data System (ADS)

Corsetti, Fabiano; Matthews, Paul; Artacho, Emilio

2016-01-01

Understanding the structural tendencies of nanoconfined water is of great interest for nanoscience and biology, where nano/micro-sized objects may be separated by very few layers of water. Here we investigate the properties of ice confined to a quasi-2D monolayer by a featureless, chemically neutral potential, in order to characterize its intrinsic behaviour. We use density-functional theory simulations with a non-local van der Waals density functional. An ab initio random structure search reveals all the energetically competitive monolayer configurations to belong to only two of the previously-identified families, characterized by a square or honeycomb hydrogen-bonding network, respectively. We discuss the modified ice rules needed for each network, and propose a simple point dipole 2D lattice model that successfully explains the energetics of the square configurations. All identified stable phases for both networks are found to be non-polar (but with a topologically non-trivial texture for the square) and, hence, non-ferroelectric, in contrast to previous predictions from a five-site empirical force-field model. Our results are in good agreement with very recently reported experimental observations.

Structural distortions in monolayers of binary semiconductors

NASA Astrophysics Data System (ADS)

Kumari, Poonam; Debnath, Saikat; Mahadevan, Priya

2018-01-01

We examine the structural properties of free-standing II-VI and III-V semiconductors at the monolayer limit within first principle density functional theory calculations. A nonpolar buckled structure was found to be favored over a polar buckled structure. While an obvious reason for this may be traced to the contribution from dipole-dipole interactions present in the polar structure which would destabilize it with respect to the nonpolar structure, Coulomb interactions between electrons on the cations and anions are found to be the reason for the nonpolar structure to be favoured. A route to tune the Coulomb interaction between the electrons on the cations and anions is through biaxial tensile strain. This allows for a planar graphitic phase in CdS to be stabilized at just 2% tensile strain. Strain also shifts the valence band maximum from the Γ point to the K point, opening up opportunities for exploring spin-valley physics in these materials.

Approaching perfect absorption of monolayer molybdenum disulfide at visible wavelengths using critical coupling.

PubMed

Jiang, Xiaoyun; Wang, Tao; Xiao, Shuyuan; Yan, Xicheng; Cheng, Le; Zhong, Qingfang

2018-08-17

A simple perfect absorption structure is proposed to achieve the high efficiency light absorption of monolayer molybdenum disulfide (MoS 2 ) by the critical coupling mechanism of guided resonances. The results of numerical simulation and theoretical analysis show that the light absorption in this atomically thin layer can be as high as 98.3% at the visible wavelengths, which is over 12 times more than that of a bare monolayer MoS 2 . In addition, the operating wavelength can be tuned flexibly by adjusting the radius of the air hole and the thickness of the dielectric layers, which is of great practical significance to improve the efficiency and selectivity of the absorption in monolayer MoS 2 . The novel idea of using critical coupling to enhance the light-MoS 2 interaction can be also adopted in other atomically thin materials. The meaningful improvement and tunability of the absorption in monolayer MoS 2 provides a good prospect for the realization of high-performance MoS 2 -based optoelectronic applications, such as photodetection and photoluminescence.

An effective approach to synthesize monolayer tungsten disulphide crystals using tungsten halide precursor

NASA Astrophysics Data System (ADS)

Thangaraja, Amutha; Shinde, Sachin M.; Kalita, Golap; Tanemura, Masaki

2016-02-01

The synthesis of large-area monolayer tungsten disulphide (WS2) single crystal is critical for realistic application in electronic and optical devices. Here, we demonstrate an effective approach to synthesize monolayer WS2 crystals using tungsten hexachloride (WCl6) as a solid precursor in atmospheric chemical vapor deposition process. In this technique, 0.05M solution of WCl6 in ethanol was drop-casted on SiO2/Si substrate to create an even distribution of the precursor, which was reduced and sulfurized at 750 °C in Ar atmosphere. We observed growth of triangular, star-shaped, as well as dendritic WS2 crystals on the substrate. The crystal geometry evolves with the shape and size of the nuclei as observed from the dendritic structures. These results show that controlling the initial nucleation and growth process, large WS2 single crystalline monolayer can be grown using the WCl6 precursor. Our finding shows an easier and effective approach to grow WS2 monolayer using tungsten halide solution-casting, rather than evaporating the precursor for gas phase reaction.

Self-Assembly of Large Gold Nanoparticles for Surface-Enhanced Raman Spectroscopy

DOE Office of Scientific and Technical Information (OSTI.GOV)

Yang, Guang; Nanda, Jagjit; Wang, Boya

Performance of portable technologies from mobile phones to electric vehicles is currently limited by the energy density and lifetime of lithium batteries. Expanding the limits of battery technology requires in situ detection of trace components at electrode–electrolyte interphases. Surface-enhance Raman spectroscopy could satisfy this need if a robust and reproducible substrate were available. Gold nanoparticles (Au NPs) larger than 20 nm diameter are expected to greatly enhance Raman intensity if they can be assembled into ordered monolayers. A three-phase self-assembly method is presented that successfully results in ordered Au NP monolayers for particle diameters ranging from 13 to 90 nm.more » The monolayer structure and Raman enhancement factors (EFs) are reported for a model analyte, rhodamine, as well as the best performing polymer electrolyte salt, lithium bis(trifluoromethane)sulfonimide. Experimental EFs for the most part correlate with predictions based on monolayer geometry and with numerical simulations that identify local electromagnetic field enhancements. Lastly, the EFs for the best performing Au NP monolayer are between 10 6 and 10 8 and give quantitative signal response when analyte concentration is changed.« less

Mechanical control of the electro-optical properties of monolayer and bilayer BC3 by applying the in-plane biaxial strain

NASA Astrophysics Data System (ADS)

Behzad, Somayeh

2017-11-01

Recently, a new two-dimensional (2D) material, the 2D BC3 crystal, has been synthesized. Here, the mechanical control of the electro-optical properties of monolayer and bilayer BC3 by applying the biaxial strain is investigated. The electronic structure calculations showed that the strain-free monolayer and bilayer BC3 are indirect band-gap semiconductors with band gap of 0.62 and 0.29 eV, respectively, where the conduction band minimum (CBM) is at the M point whereas the valence band maximum (VBM) is at the Γ point. The doubly degenerated bands in the monolayer BC3 are splitted in the bilayer BC3 due to the interlayer interactions. Both monolayer and bilayer BC3 remain indirect gap semiconductor under biaxial tensile strain and their band gaps increases with strain. On the other hand, by increasing the magnitude of tensile strain, the optical spectra shift to the lower energies and the static dielectric constant increases. These findings suggest the potential of strain-engineered 2D BC3 in electronic and optoelectronic device applications.

Self-Assembly of Large Gold Nanoparticles for Surface-Enhanced Raman Spectroscopy.

PubMed

Yang, Guang; Nanda, Jagjit; Wang, Boya; Chen, Gang; Hallinan, Daniel T

2017-04-19

Performance of portable technologies from mobile phones to electric vehicles is currently limited by the energy density and lifetime of lithium batteries. Expanding the limits of battery technology requires in situ detection of trace components at electrode-electrolyte interphases. Surface-enhance Raman spectroscopy could satisfy this need if a robust and reproducible substrate were available. Gold nanoparticles (Au NPs) larger than 20 nm diameter are expected to greatly enhance Raman intensity if they can be assembled into ordered monolayers. A three-phase self-assembly method is presented that successfully results in ordered Au NP monolayers for particle diameters ranging from 13 to 90 nm. The monolayer structure and Raman enhancement factors (EFs) are reported for a model analyte, rhodamine, as well as the best performing polymer electrolyte salt, lithium bis(trifluoromethane)sulfonimide. Experimental EFs for the most part correlate with predictions based on monolayer geometry and with numerical simulations that identify local electromagnetic field enhancements. The EFs for the best performing Au NP monolayer are between 10 6 and 10 8 and give quantitative signal response when analyte concentration is changed.

Electrically tunable magnetic configuration on vacancy-doped GaSe monolayer

NASA Astrophysics Data System (ADS)

Tang, Weiqing; Ke, Congming; Fu, Mingming; Wu, Yaping; Zhang, Chunmiao; Lin, Wei; Lu, Shiqiang; Wu, Zhiming; Yang, Weihuang; Kang, Junyong

2018-03-01

Group-IIIA metal-monochalcogenides with the enticing properties have attracted tremendous attention across various scientific disciplines. With the aim to satisfy the multiple demands of device applications, here we report a design framework on GaSe monolayer in an effort to tune the electronic and magnetic properties through a dual modulation of vacancy doping and electric field. A half-metallicity with a 100% spin polarization is generated in a Ga vacancy doped GaSe monolayer due to the nonbonding 4p electronic orbital of the surrounding Se atoms. The stability of magnetic moment is found to be determined by the direction of applied electric field. A switchable magnetic configuration in Ga vacancy doped GaSe monolayer is achieved under a critical electric field of 0.6 V/Å. Electric field induces redistribution of the electronic states. Finally, charge transfers are found to be responsible for the controllable magnetic structure in this system. The magnetic modulation on GaSe monolayer in this work offers some references for the design and fabrication of tunable two-dimensional spintronic device.

Self-Assembly of Large Gold Nanoparticles for Surface-Enhanced Raman Spectroscopy

DOE PAGES

Yang, Guang; Nanda, Jagjit; Wang, Boya; ...

2017-04-04

Performance of portable technologies from mobile phones to electric vehicles is currently limited by the energy density and lifetime of lithium batteries. Expanding the limits of battery technology requires in situ detection of trace components at electrode–electrolyte interphases. Surface-enhance Raman spectroscopy could satisfy this need if a robust and reproducible substrate were available. Gold nanoparticles (Au NPs) larger than 20 nm diameter are expected to greatly enhance Raman intensity if they can be assembled into ordered monolayers. A three-phase self-assembly method is presented that successfully results in ordered Au NP monolayers for particle diameters ranging from 13 to 90 nm.more » The monolayer structure and Raman enhancement factors (EFs) are reported for a model analyte, rhodamine, as well as the best performing polymer electrolyte salt, lithium bis(trifluoromethane)sulfonimide. Experimental EFs for the most part correlate with predictions based on monolayer geometry and with numerical simulations that identify local electromagnetic field enhancements. Lastly, the EFs for the best performing Au NP monolayer are between 10 6 and 10 8 and give quantitative signal response when analyte concentration is changed.« less

X-Ray Synchrotron and Neutron Reflectivity Studies of = Polymer-Modified Lipid Monolayers on Water

NASA Astrophysics Data System (ADS)

Smith, G. S.; Majewski, J.; Kuhl, T.; Israelachvili, J.; Kjaer, K.; Gerstenberg, M. C.; Als-Nielsen, J.

1997-03-01

We studied monolayers (at air-water interface) composed of mixtures of distearoyl phosphatidyl ethanolamine (DSPE) mixed with 1.3, 4.5 and 9% of the same lipid but modified by polyethylene glycol chains (PEG) covalently linked to its head group. The GID data yielded three reflections leading to a hexagonal unit cell a_H=4.7Åarea per lipid molecule = 38.3Åindependent of PEG concentration. The in-plane coherence lengths decreased from 360Åfor the pure lipid to 230Åfor 9.0% DSPE-PEG. The FWHM(q_z) of each of the Bragg rods increased with PEG-lipid concentration suggesting decreasing of the lengths of the coherently diffracting part of the hydrocarbon chains. Reflectivities show that both the density and the extension of the polymer segments increase with DSPE-PEG concentration and can be well modeled with a parabolic density profile. Our data indicates that the bulky hydrophilic polymer disrupts the lipid monolayer. This is attributed to an increase in lipid protrusions and a relaxation of the lateral force between PEG portions by staggering of the lipid headgroups.

Penta-P2X (X=C, Si) monolayers as wide-bandgap semiconductors: A first principles prediction

NASA Astrophysics Data System (ADS)

Naseri, Mosayeb; Lin, Shiru; Jalilian, Jaafar; Gu, Jinxing; Chen, Zhongfang

2018-06-01

By means of density functional theory computations, we predicted two novel two-dimensional (2D) nanomaterials, namely P2X (X=C, Si) monolayers with pentagonal configurations. Their structures, stabilities, intrinsic electronic, and optical properties as well as the effect of external strain to the electronic properties have been systematically examined. Our computations showed that these P2C and P2Si monolayers have rather high thermodynamic, kinetic, and thermal stabilities, and are indirect semiconductors with wide bandgaps (2.76 eV and 2.69 eV, respectively) which can be tuned by an external strain. These monolayers exhibit high absorptions in the UV region, but behave as almost transparent layers for visible light in the electromagnetic spectrum. Their high stabilities and exceptional electronic and optical properties suggest them as promising candidates for future applications in UV-light shielding and antireflection layers in solar cells.

Nanoparticle-Based Receptors Mimic Protein-Ligand Recognition.

PubMed

Riccardi, Laura; Gabrielli, Luca; Sun, Xiaohuan; De Biasi, Federico; Rastrelli, Federico; Mancin, Fabrizio; De Vivo, Marco

2017-07-13

The self-assembly of a monolayer of ligands on the surface of noble-metal nanoparticles dictates the fundamental nanoparticle's behavior and its functionality. In this combined computational-experimental study, we analyze the structure, organization, and dynamics of functionalized coating thiols in monolayer-protected gold nanoparticles (AuNPs). We explain how functionalized coating thiols self-organize through a delicate and somehow counterintuitive balance of interactions within the monolayer itself and with the solvent. We further describe how the nature and plasticity of these interactions modulate nanoparticle-based chemosensing. Importantly, we found that self-organization of coating thiols can induce the formation of binding pockets in AuNPs. These transient cavities can accommodate small molecules, mimicking protein-ligand recognition, which could explain the selectivity and sensitivity observed for different organic analytes in NMR chemosensing experiments. Thus, our findings advocate for the rational design of tailored coating groups to form specific recognition binding sites on monolayer-protected AuNPs.

Marrying Excitons and Plasmons in Monolayer Transition-Metal Dichalcogenides

NASA Astrophysics Data System (ADS)

Van Tuan, Dinh; Scharf, Benedikt; Žutić, Igor; Dery, Hanan

2017-10-01

Just as photons are the quanta of light, plasmons are the quanta of orchestrated charge-density oscillations in conducting media. Plasmon phenomena in normal metals, superconductors, and doped semiconductors are often driven by long-wavelength Coulomb interactions. However, in crystals whose Fermi surface is comprised of disconnected pockets in the Brillouin zone, collective electron excitations can also attain a shortwave component when electrons transition between these pockets. In this work, we show that the band structure of monolayer transition-metal dichalcogenides gives rise to an intriguing mechanism through which shortwave plasmons are paired up with excitons. The coupling elucidates the origin for the optical sideband that is observed repeatedly in monolayers of WSe2 and WS2 but not understood. The theory makes it clear why exciton-plasmon coupling has the right conditions to manifest itself distinctly only in the optical spectra of electron-doped tungsten-based monolayers.

Determination of low levels of cadmium ions by the under potential deposition on a self-assembled monolayer on gold electrode.

PubMed

Noyhouzer, Tomer; Mandler, Daniel

2011-01-17

The electrochemical determination of low levels of Cd using a self-assembled monolayer (SAM) modified Au electrode is reported. Determination was based on the stripping of Cd, which was deposited by under potential deposition (UPD). A series of short alkanethiol SAMs bearing different end groups, i.e., sulfonate, carboxylate and ammonium, were examined. Lowest level of detection (ca. 50 ngL(-1)) was achieved with a 3-mercaptopropionic acid (MPA) monolayer using subtractive anodic square wave voltammetry (SASV). Additional surface methods, namely, reductive desorption and X-ray photoelectron spectroscopy, were applied to determine the interfacial structure of the electrodeposited Cd on the modified electrodes. We conclude that the deposited Cd forms a monoatomic layer, which bridges between the gold surface and the alkanethiol monolayer associating with both the gold and the sulfur atoms. Copyright © 2010 Elsevier B.V. All rights reserved.

Introducing Overlapping Grain Boundaries in Chemical Vapor Deposited Hexagonal Boron Nitride Monolayer Films

PubMed Central

2017-01-01

We demonstrate the growth of overlapping grain boundaries in continuous, polycrystalline hexagonal boron nitride (h-BN) monolayer films via scalable catalytic chemical vapor deposition. Unlike the commonly reported atomically stitched grain boundaries, these overlapping grain boundaries do not consist of defect lines within the monolayer films but are composed of self-sealing bilayer regions of limited width. We characterize this overlapping h-BN grain boundary structure in detail by complementary (scanning) transmission electron microscopy techniques and propose a catalytic growth mechanism linked to the subsurface/bulk of the process catalyst and its boron and nitrogen solubilities. Our data suggest that the overlapping grain boundaries are comparatively resilient against deleterious pinhole formation associated with grain boundary defect lines and thus may reduce detrimental breakdown effects when polycrystalline h-BN monolayer films are used as ultrathin dielectrics, barrier layers, or separation membranes. PMID:28410557

Transition metal doped (X = V, Cr) CdS monolayer: A DFT study

NASA Astrophysics Data System (ADS)

Deb, Jyotirmoy; Paul, Debolina; Sarkar, Utpal

2018-05-01

In this work based on density functional theory approach with generalized gradient approximation we have investigated the effect doping and co-doping of transition metal atoms in CdS monolayer sheet. On the basis cohesive energy, we have determined the stability of all the transition metal doped systems. CdS monolayer is of nonmagnetic character but the insertion of transition metal atoms introduces the spontaneous spin polarization which results in a significant value of magnetic moment. The band structure analysis reveals that three different types of conducting nature such as spin-select-half-semiconductor, half metallic and metallic nature with total spin polarization has also been observed. The versatile conducting nature of the transition metal doped CdS monolayer predicts the possibility of using these systems in spintronics mainly as a spin filter and also to form metal-semiconductor interface etc. at nanoscale level.

Accelerating oxygen reduction on Pt monolayer via substrate compression

NASA Astrophysics Data System (ADS)

Zhang, Xilin; Chen, Yue; Yang, Zongxian; Lu, Zhansheng

2017-11-01

Many methods have been proposed to accelerate the oxygen reduction and save the dosage of Pt. Here, we report a promising way in fulfilling these purposes by applying substrate strain on the supported Pt monolayer. The compressive strain would modify the geometric and electronic structures of tungsten carbide (WC) substrate, changing the interaction nature between substrate and Pt monolayer and resulting in a downward shift of the d-band center of surface Pt atoms. The activity of Pt monolayer on the compressed WC is further evaluated from the kinetics of the dissociation and protonation of O2. The dissociation barrier of O2 is increased and the hydrogenation barrier of O atom is decreased, indicating that the recovery of the catalytically active sites is accelerated and the deactivation by oxygen poison is alleviated. The present study provides an effective way in tuning the activity of Pt-based catalysts by applying the substrate strain.

ON THE FREEZING AND IDENTIFICATION OF LIPID MONOLAYER 2-D ARRAYS FOR CRYOELECTRON MICROSCOPY

PubMed Central

Taylor, Dianne W.; Kelly, Deborah F.; Cheng, Anchi; Taylor, Kenneth A.

2008-01-01

Lipid monolayers provide a convenient vehicle for the crystallization of biological macromolecules for 3-D electron microscopy. Although numerous examples of 3-D images from 2-D protein arrays have been described from negatively stained specimens, only six structures have been done from frozen hydrated specimens. We describe here a method that makes high quality frozen-hydrated specimens of lipid monolayer arrays for cryoelectron microscopy. The method uses holey carbon films with patterned holes for monolayer recovery, blotting and plunge freezing to produce thin aqueous films which cover >90% of the available grid area. With this method, even specimens with relatively infrequent crystals can be screened using automated data collection techniques. Though developed for microscopic examination of 2-D arrays, the method may have wider application to the preparation of single particle specimens for 3-D image reconstruction. PMID:17561414

Long shelf-life streptavidin support-films suitable for electron microscopy of biological macromolecules

DOE PAGES

Han, Bong-Gyoon; Watson, Zoe; Kang, Hannah; ...

2016-06-15

We describe a rapid and convenient method of growing streptavidin (SA) monolayer crystals directly on holey-carbon EM grids. As expected, these SA monolayer crystals retain their biotin-binding function and crystalline order through a cycle of embedding in trehalose and, later, its removal. This fact allows one to prepare, and store for later use, EM grids on which SA monolayer crystals serve as an affinity substrate for preparing specimens of biological macromolecules. In addition, we report that coating the lipid-tail side of trehalose-embedded monolayer crystals with evaporated carbon appears to improve the consistency with which well-ordered, single crystals are observed tomore » span over entire, 2 μm holes of the support films. Randomly biotinylated 70S ribosomes are used as a test specimen to show that these support films can be used to obtain a high-resolution cryo-EM structure« less

Plasmon-Polaritons in Island Metal Films: A Solution in the Framework of the Coherent Potential Approximation

NASA Astrophysics Data System (ADS)

Maksimenko, V. V.; Zagaynov, V. A.; Semina, P. N.; Zheltova, A. V.; Maslenkova, E. V.; Smolyanskiy, A. S.

2018-05-01

The photon propagator describing the interaction of light with a monolayer of metal particles (island film) is calculated in the coherent potential approximation. It is shown that the shift in the frequency peak of a dipole surface plasmon for a monolayer particle relative to the analogous frequency of the plasma resonance for an isolated particle is not the only manifestation of the influence of neighboring particles. Neighboring particles also produce a bimodal structure in the spectrum line of the plasmon resonance. The possibility of fine structure in the plasmon resonance spectrum lines is predicted.

Adsorption of halogens on metal surfaces

NASA Astrophysics Data System (ADS)

Andryushechkin, B. V.; Pavlova, T. V.; Eltsov, K. N.

2018-06-01

This paper presents a review of the experimental and theoretical investigations of halogen interaction with metal surfaces. The emphasis was placed on the recent measurements performed with a scanning tunneling microscope in combination with density functional theory calculations. The surface structures formed on metal surface after halogen interaction are classified into three groups: chemisorbed monolayer, surface halide, bulk-like halide. Formation of monolayer structures is described in terms of surface phase transitions. Surface halide phases are considered to be intermediates between chemisorbed halogen and bulk halide. The modern theoretical approaches in studying the dynamics of metal halogenation reactions are also presented.

Dynamical simulation of electron transfer processes in self-assembled monolayers at metal surfaces using a density matrix approach.

PubMed

Prucker, V; Bockstedte, M; Thoss, M; Coto, P B

2018-03-28

A single-particle density matrix approach is introduced to simulate the dynamics of heterogeneous electron transfer (ET) processes at interfaces. The characterization of the systems is based on a model Hamiltonian parametrized by electronic structure calculations and a partitioning method. The method is applied to investigate ET in a series of nitrile-substituted (poly)(p-phenylene)thiolate self-assembled monolayers adsorbed at the Au(111) surface. The results show a significant dependence of the ET on the orbital symmetry of the donor state and on the molecular and electronic structure of the spacer.

Gold nanoparticles protected by mixed hydrogenated/fluorinated monolayers: controlling and exploring the surface features

NASA Astrophysics Data System (ADS)

Şologan, Maria; Gentilini, Cristina; Bidoggia, Silvia; Boccalon, Mariangela; Pace, Alice; Pengo, Paolo; Pasquato, Lucia

2018-06-01

Harnessing the reciprocal phobicity of hydrogenated and fluorinated thiolates proved to be a valuable strategy in preparing gold nanoparticles displaying mixed monolayers with a well-defined and pre-determined morphology. Our studies display that the organisation of the fluorinated ligands in phase-separated domains takes place even when these represent a small fraction of the ligands grafted on the gold surface. Using simple model ligands and by combining 19F NMR or ESR spectroscopies, and multiscale molecular simulations, we could demonstrate how the monolayer morphology responds in a predictable manner to structural differences between the thiolates. This enables a straightforward preparation of gold nanoparticles with monolayers displaying stripe-like, Janus, patchy, and random morphologies. Additionally, solubility properties may be tuned as function of the nature of the ligands and of the monolayer morphology obtaining gold nanoparticles soluble in organic solvents or in aqueous solutions. Most importantly, this rich diversity can be achieved not by resorting to ad hoc developed fabrication techniques, but rather relying on the spontaneous self-sorting of the ligands upon assembly on the nanoparticle surface. Besides enabling control over the monolayer morphology, fluorinated ligands endow the nanoparticles with several properties that can be exploited in the development of novel materials with applications, for instance in drug delivery and diagnostic imaging.

Formation of protein molecular imprints within Langmuir monolayers: A quartz crystal microbalance study

PubMed Central

Turner, Nicholas W.; Wright, Bryon E.; Hlady, Vladimir; Britt, David W.

2008-01-01

Protein imprinting leading to enhanced rebinding of ferritin to ternary lipid monolayers is demonstrated using a quartz crystal microbalance. Monolayers consisting of cationic dioctadecyldimethylammonium bromide, non-ionic methyl stearate, and poly(ethylene glycol) bearing phospholipids were imprinted with ferritin at the air/water interface of a Langmuir-Blodgett trough and transferred hydrated to hydrophobic substrates for study. This immobilization was shown by fluorescence correlation spectroscopy to significantly hinder any further diffusion of lipids, while rebinding studies demonstrated up to a six-fold increase in ferritin adsorption to imprinted versus control monolayers. A diminished rebinding of ferritin to its imprint was observed through pH reduction to below the protein isoelectric point, demonstrating the electrostatic nature of the interaction. Rebinding to films where imprint pockets remained occupied by the template protein was also minimal. Studies with a smaller acidic protein revealed the importance of the steric influence of poly(ethylene glycol) in forming the protein binding pockets, as albumin-imprinted monolayers showed low binding of ferritin, while ferritin-imprinted monolayers readily accommodated albumin. The controllable structure-function relationship and limitations of this system are discussed with respect to the application of protein imprinting in sensor development as well as fundamental studies of proteins at dynamic interfaces. PMID:17204279

Stability and electronic properties of Gex(BN)y monolayers

NASA Astrophysics Data System (ADS)

Freitas, A.; Machado, L. D.; Tromer, R. M.; Bezerra, C. G.; Azevedo, S.

2017-10-01

In this work, we employ ab initio simulations to propose a new class of monolayers with stoichiometry Gex(BN)y . These monolayers belong to a family of 2D materials combining B, N and group IV atoms, such as BxCyNz and SixByNz . We calculated the formation energy for ten atomic arrangements, and found that it increases when the number of Bsbnd Ge and Nsbnd Ge bonds increases, and decreases when the number of Bsbnd N and Gesbnd Ge bonds increases. We found that the lowest energy monolayer presented a Ge2 BN stoichiometry, and maximized the number of Bsbnd N and Gesbnd Ge bonds. This structure also presented mixed sp2 and sp3 bonds and out-of-plane buckling. Moreover, it remained stable in our ab initio molecular dynamics simulations carried out at T = 300 K. The calculated electronic properties revealed that Gex(BN)y monolayers might present conductor or semiconductor behavior, with band gaps ranging from 0.0 to 0.74 eV, depending on atomic arrangement. Tunable values of band gap can be useful in applications. In optoelectronics, for instance, this property might be employed to control absorbed light wavelengths. Our calculations add a new class of monolayers to the increasing library of 2D materials.

Cation effects on phosphatidic acid monolayers at various pH conditions.

PubMed

Zhang, Ting; Cathcart, Matthew G; Vidalis, Andrew S; Allen, Heather C

2016-10-01

The impact of pH and cations on phase behavior, stability, and surface morphology for dipalmitoylphosphatidic acid (DPPA) monolayers was investigated. At pH<10, DPPA monolayers on water are predominantly populated by neutral species and display the highest packing density. Cations are found to expand and stabilize the monolayer in the following order of increasing magnitude at pH 5.6: Na + >K + ∼Mg 2+ >Ca 2+ . Additionally, cation complexation is tied to the pH and protonation state of DPPA, which are the primary factors controlling the monolayer surface behavior. The binding affinity of cations to the headgroup and thus deprotonation capability of the cation, ranked in the order of Ca 2+ >Mg 2+ >Na + >K + , is found to be well explained by the law of matching water affinities. Nucleation of surface 3D lipid structures is observed from Ca 2+ , Mg 2+ , and Na + , but not from K + , consistent with the lowest binding affinity of K + . Unraveling cation and pH effects on DPPA monolayers is useful in further understanding the surface properties of complex systems such as organic-coated marine aerosols where organic films are directly influenced by the pH and ionic composition of the underlying aqueous phase. Copyright © 2016 Elsevier Ireland Ltd. All rights reserved.

Diverse carrier mobility of monolayer BNC x : a combined density functional theory and Boltzmann transport theory study.

PubMed

Wu, Tao; Deng, Kaiming; Deng, Weiqiao; Lu, Ruifeng

2017-10-19

BNC x monolayer as a kind of two-dimensional material has numerous chemical atomic ratios and arrangements with different electronic structures. Via calculations on the basis of density functional theory and Boltzmann transport theory under deformation potential approximation, the band structures and carrier mobilities of BNC x (x  =  1,2,3,4) nanosheets are systematically investigated. The calculated results show that BNC 2 -1 is a material with very small band gap (0.02 eV) among all the structures while other BNC x monolayers are semiconductors with band gap ranging from 0.51 eV to 1.32 eV. The carrier mobility of BNC x varies considerably from tens to millions of cm 2 V -1 s -1 . For BNC 2 -1, the hole mobility and electron mobility along both x and y directions can reach 10 5 orders of magnitude, which is similar to the carrier mobility of graphene. Besides, all studied BNC x monolayers obviously have anisotropic hole mobility and electron mobility. In particular, for semiconductor BNC 4 , its hole mobility along the y direction and electron mobility along the x direction unexpectedly reach 10 6 orders of magnitude, even higher than that of graphene. Our findings suggest that BNC x layered materials with the proper ratio and arrangement of carbon atoms will possess desirable charge transport properties, exhibiting potential applications in nanoelectronic devices.

Diverse carrier mobility of monolayer BNC x : a combined density functional theory and Boltzmann transport theory study

NASA Astrophysics Data System (ADS)

Wu, Tao; Deng, Kaiming; Deng, Weiqiao; Lu, Ruifeng

2017-11-01

BNC x monolayer as a kind of two-dimensional material has numerous chemical atomic ratios and arrangements with different electronic structures. Via calculations on the basis of density functional theory and Boltzmann transport theory under deformation potential approximation, the band structures and carrier mobilities of BNC x (x  =  1,2,3,4) nanosheets are systematically investigated. The calculated results show that BNC2-1 is a material with very small band gap (0.02 eV) among all the structures while other BNC x monolayers are semiconductors with band gap ranging from 0.51 eV to 1.32 eV. The carrier mobility of BNC x varies considerably from tens to millions of cm2 V-1 s-1. For BNC2-1, the hole mobility and electron mobility along both x and y directions can reach 105 orders of magnitude, which is similar to the carrier mobility of graphene. Besides, all studied BNC x monolayers obviously have anisotropic hole mobility and electron mobility. In particular, for semiconductor BNC4, its hole mobility along the y direction and electron mobility along the x direction unexpectedly reach 106 orders of magnitude, even higher than that of graphene. Our findings suggest that BNC x layered materials with the proper ratio and arrangement of carbon atoms will possess desirable charge transport properties, exhibiting potential applications in nanoelectronic devices.

Strain-engineering of Janus SiC monolayer functionalized with H and F atoms

NASA Astrophysics Data System (ADS)

Drissi, L. B.; Sadki, K.; Kourra, M.-H.; Bousmina, M.

2018-05-01

Based on ab initio density functional theory calculations, the structural, electronic, mechanical, acoustic, thermodynamic, and piezoelectric properties of (F,H) Janus SiC monolayers are studied. The new set of derivatives shows buckled structures and different band gap values. Under strain, the buckling changes and the structures pass from semiconducting to metallic. The elastic limits and the metastable regions are determined. The Young's modulus and Poisson ratio reveal stronger behavior for the modified conformers with respect to graphene. The values of the Debye temperature make the new materials suitable for thermal application. Moreover, all the conformers show in-plane and out-of-plane piezoelectric responses comparable with known two-dimensional materials. If engineered, such piezoelectric Janus structures may be promising materials for various nanoelectromechanical applications.

Versatile mechanical properties of novel g-SiC x monolayers from graphene to silicene: a first-principles study.

PubMed

Lu, X K; Xin, T Y; Zhang, Q; Xu, Q; Wei, T H; Wang, Y X

2018-08-03

Recently, a series of graphene-like binary monolayers (g-SiC x ), where Si partly substitutes the C positions in graphene, have been obtained by tailoring the band gaps of graphene and silicene that have made them a promising material for application in opto-electronic devices. Subsequently, evaluating the mechanical properties of g-SiC x has assumed great importance for engineering applications. In this study, we quantified the in-plane mechanical properties of g-SiC x (x = 7, 5, 3, 2 and 1) monolayers (also including graphene and silicene) based on density function theory. It was found that the mechanical parameters of g-SiC x , such as the ideal strength, Young's modulus, shear modulus, Poisson's ratio, as well as fracture toughness, are overall related to the ratio of Si-C to C-C bonds, which varies with Si concentration. However, for g-SiC 7 and g-SiC 3 , the mechanical properties seem to depend on the structure because in g-SiC 7 , the C-C bond strength is severely weakened by abnormal stretching, and in g-SiC 3 , conjugation structure is formed. The microscopic failure of g-SiC x exhibits diverse styles depending on the more complex structural deformation modes introduced by Si substitution. We elaborated the structure-properties relationship of g-SiC x during the failure process, and in particular, found that the structural transformation of g-SiC 3 and g-SiC is due to the singular symmetry of their structure. Due to the homogeneous phase, all the g-SiC x investigated in this study preserve rigorous isotropic Young's moduli and Poisson's ratios. With versatile mechanical performances, the family of g-SiC x may facilitate the design of advanced two-dimensional materials to meet the needs for practical mechanical engineering applications. The results offer a fundamental understanding of the mechanical behaviors of g-SiC x monolayers.

Nonlinear Optical Spectroscopy of Two-Dimensional Materials

NASA Astrophysics Data System (ADS)

Cui, Qiannan

Nonlinear optical properties of two-dimensional (2D) materials, such as transition metal dichalcogenides (TMDs), graphene, black phosphorus, and so on, play a key role of understanding nanoscale light-matter interactions, as well as developing nanophotonics applications from solar cells to quantum computation. With ultrafast lasers, we experimentally study nonlinear optical properties of 2D materials. Employing transient absorption microscopy, we study several members of 2D materials, such as WSe2, TiS3 and ReS2. The dynamical saturable absorption process of 2D excitons is spatiotemporally resolved. Intrinsic parameters of these 2D materials, such as exciton lifetime, exciton diffusion coefficient, and exciton mobility, are effectively measured. Especially, in-plane anisotropy of transient absorption and diffusive transport is observed for 2D excitons in monolayer ReS2, demonstrating the in-plane degree of freedom. Furthermore, with quantum interference and control nanoscopy, we all-optically inject, detect and manipulate nanoscale ballistic charge currents in a ReS2 thin film. By tuning the phase difference between one photon absorption and two photon absorption transition paths, sub-picosecond timescale of ballistic currents is coherently controlled for the first time in TMDs. In addition, the spatial resolution is two-order of magnitude smaller than optical diffraction limit. The second-order optical nonlinearity of 2D monolayers is resolved by second harmonic generation (SHG) microscopy. We measure the second-order susceptibility of monolayer MoS 2. The angular dependence of SHG in monolayer MoS2 shows strong symmetry dependence on its crystal lattice structure. Hence, second harmonic generation microscopy can serve as a powerful tool to noninvasively determine the crystalline directions of 2D monolayers. The real and imaginary parts of third-order optical nonlinearity of 2D monolayers are resolved by third harmonic generation (THG) microscopy and two-photon transient absorption microscopy, respectively. With third harmonic generation microscopy, we observe strong and anisotropic THG in monolayer and multilayer ReS2. Comparing with 2D materials with hexagonal lattice, such as MoS2, the third-order susceptibility is higher by one order of magnitude in ReS2 with a distorted 1T structure. The in-plane anisotropy of THG is attributed to the lattice distortion in ReS2 after comparing with a symmetry analysis. With two-photon transient absorption microscopy, we observe a giant two-photon absorption coefficient of monolayer WS2.

Quantification of Malignant Breast Cancer Cell MDA-MB-231 Transmigration across Brain and Lung Microvascular Endothelium

PubMed Central

Fan, Jie; Fu, Bingmei M.

2015-01-01

Tumor cell extravasation through the endothelial barrier forming the microvessel wall is a crucial step during tumor metastasis. However, where, how and how fast tumor cells transmigrate through endothelial barriers remain unclear. Using an in vitro transwell model, we performed a transmigration assay of malignant breast tumor cells (MDA-MB-231) through brain and lung microvascular endothelial monolayers under control and pathological conditions. The locations and rates of tumor cell transmigration as well as the changes in the structural components (integrity) of endothelial monolayers were quantified by confocal microscopy. Endothelial monolayer permeability to albumin Palbumin was also quantified under the same conditions. We found that about 98% of transmigration occurred at the joints of endothelial cells instead of cell bodies; tumor cell adhesion and transmigration degraded endothelial surface glycocalyx and disrupted endothelial junction proteins, consequently increased Palbumin; more tumor cells adhered to and transmigrated through the endothelial monolayer with higher Palbumin; Palbumin and tumor transmigration were increased by vascular endothelial growth factor (VEGF), a representative of cytokines, and lipopolysaccharides (LPS), a typical systemic inflammatory factor, but reduced by adenosine 3′, 5′-cyclic monophosphate (cAMP). These results suggest that reinforcing endothelial structural integrity is an effective approach for inhibiting tumor extravasation. PMID:26603751

The electronic and transport properties of monolayer transition metal dichalcogenides: a complex band structure analysis

NASA Astrophysics Data System (ADS)

Szczesniak, Dominik

Recently, monolayer transition metal dichalcogenides have attracted much attention due to their potential use in both nano- and opto-electronics. In such applications, the electronic and transport properties of group-VIB transition metal dichalcogenides (MX2 , where M=Mo, W; X=S, Se, Te) are particularly important. Herein, new insight into these properties is presented by studying the complex band structures (CBS's) of MX2 monolayers while accounting for spin-orbit coupling effects. By using the symmetry-based tight-binding model a nonlinear generalized eigenvalue problem for CBS's is obtained. An efficient method for solving such class of problems is presented and gives a complete set of physically relevant solutions. Next, these solutions are characterized and classified into propagating and evanescent states, where the latter states present not only monotonic but also oscillatory decay character. It is observed that some of the oscillatory evanescent states create characteristic complex loops at the direct band gaps, which describe the tunneling currents in the MX2 materials. The importance of CBS's and tunneling currents is demonstrated by the analysis of the quantum transport across MX2 monolayers within phase field matching theory. Present work has been prepared within the Qatar Energy and Environment Research Institute (QEERI) grand challenge ATHLOC project (Project No. QEERI- GC-3008).

Spectroscopic and electrochemical characterization of interfacial biomimetic assemblies on electrochemically generated gold oxide surfaces.

PubMed

Krysiński, Paweł; Blanchard, G J

2005-04-01

We report on the formation of a gold oxide layer and the reaction of this oxide with an acid chloride to form a stable, relatively complete monolayer bound through an ester-like bond to the gold oxide surface. We have used cyclic voltammetry, FTIR and optical ellipsometry to characterize this novel monolayer structure. The exposed functional groups of this monolayer can participate in subsequent surface reactions, opening the door to the use of oxide-based surface attachment chemistry on metallic substrates. This chemistry will allow for the formation of films tailored to contain both hydrophobic and hydrophilic regions, stacked at predetermined distances from the substrate that may serve as biomembrane mimetic assemblies.

Organosilicon derivatives of BTBT for monolayer organic field effect transistors

NASA Astrophysics Data System (ADS)

Agina, Elena V.; Polinskaya, Marina S.; Trul, Askold A.; Chekusova, Viktoria P.; Sizov, Alexey S.; Borshchev, Oleg V.; Ponomarenko, Sergey A.

2017-08-01

Synthesis of novel organosilicon derivatives of [1]benzothieno[3,2-b][1]-benzothiophene (BTBT) linked though flexible aliphatic spacers to a disiloxane anchor group is reported. They were successfully used in monolayer OFETs with the charge carrier mobilities up to 0.02 cm2 /Vs, threshold voltage close to 0 V and On/Off ratio up to 10,000. Influence of the chemical structure of the molecules synthesized on the morphology, molecular 2D ordering in the monolayers and their semiconducting properties is considered. The effect of different methods of the ultrathin semiconducting layer preparation, such as Langmuir-Blodgett, Langmuir-Schaefer, spin coating or doctor blade, on the OFET performance is discussed.

Scanning tunneling microscope observation of the phosphatidylserine domains in the phosphatidylcholine monolayer.

PubMed

Matsunaga, Soichiro; Yamada, Taro; Kobayashi, Toshihide; Kawai, Maki

2015-05-19

A mixed monolayer of 1,2-dihexanoyl-sn-glycero-3-phospho-l-serine (DHPS) and 1,2-dihexanoyl-sn-glycero-3-phosphocholine (DHPC) on an 1-octanethiol-modified gold substrate was visualized on the nanometer scale using in situ scanning tunneling microscopy (STM) in aqueous solution. DHPS clusters were evident as spotty domains. STM enabled us to distinguish DHPS molecules from DHPC molecules depending on their electronic structures. The signal of the DHPS domains was abolished by neutralization with Ca(2+). The addition of the PS + Ca(2+)-binding protein of annexin V to the Ca(2+)-treated monolayer gave a number of spots corresponding to a single annexin V molecule.

First-principle study of single TM atoms X (X=Fe, Ru or Os) doped monolayer WS2 systems

NASA Astrophysics Data System (ADS)

Zhu, Yuan-Yan; Zhang, Jian-Min

2018-05-01

We report the structural, magnetic and electronic properties of the pristine and single TM atoms X (X = Fe, Ru or Os) doped monolayer WS2 systems based on first-principle calculations. The results show that the W-S bond shows a stronger covalent bond, but the covalency is obviously weakened after the substitution of W atom with single X atoms, especially for Ru (4d75s1) with the easily lost electronic configuration. The smaller total energies of the doped systems reveal that the spin-polarized states are energetically favorable than the non-spin-polarized states, and the smallest total energy of -373.918 eV shows the spin-polarized state of the Os doped monolayer WS2 system is most stable among three doped systems. In addition, although the pristine monolayer WS2 system is a nonmagnetic-semiconductor with a direct band gap of 1.813 eV, single TM atoms Fe and Ru doped monolayer WS2 systems transfer to magnetic-HM with the total moments Mtot of 1.993 and 1.962 μB , while single TM atom Os doped monolayer WS2 systems changes to magnetic-metal with the total moments Mtot of 1.569 μB . Moreover, the impurity states with a positive spin splitting energies of 0.543, 0.276 and 0.1999 eV near the Fermi level EF are mainly contributed by X-dxy and X-dx2-y2 states hybridized with its nearest-neighbor atom W-dz2 states for Fe, Ru and Os doped monolayer WS2 system, respectively. Finally, we hope that the present study on monolayer WS2 will provide a useful theoretical guideline for exploring low-dimensional spintronic materials in future experiments.

Sucrose esters as biocompatible surfactants for penetration enhancement: An insight into the mechanism of penetration enhancement studied using stratumcorneum model lipids and Langmuir monolayers.

PubMed

Todosijević, Marija N; Brezesinski, Gerald; Savić, Snežana D; Neubert, Reinhard H H

2017-03-01

Up to now, the molecular mechanism of the penetration enhancing effect of sucrose esters (SEs) on stratumcorneum (SC) has not been explained in details. In this study, variety of surface sensitive techniques, including surface pressure-area (π-A) isotherms, infrared reflection-absorption spectroscopy (IRRAS), and Brewster angle microscopy (BAM), have been used to investigate interactions between SEs and SC intercellular lipids. A monolayer of the mixture of ceramide AS C18:18, stearic acid and cholesterol in the molar ratio of 1:1:0.7 on an aqueous subphase is a good model to mimic a single layer of intercellular SC lipids. The π-A isotherms of mixed monolayers and parameters derived from the curves demonstrated the interaction between nonionic surfactants such as SEs and SC lipids. With increasing SE concentration, the resultant monolayer films became more fluid and better compressible. IRRAS measurements showed that SEs disordered the acyl chains of SC lipids, and the BAM images demonstrated the modification of the domain structures in SC monolayers. Longer chain-SE has a stronger disordering effect and is better miscible with ceramides in comparison to SE with a shorter hydrophobic part. In conclusion, this study demonstrates the disordering effect of SEs on the biomimetic SC model, pointing out that small changes in the structure of surfactant may have a strong influence on a penetration enhancement of lipophilic drugs through intercellular lipids of skin. Copyright © 2016 Elsevier B.V. All rights reserved.

Gene expression analysis of growth factor receptors in human chondrocytes in monolayer and 3D pellet cultures

PubMed Central

Witt, Anika; Salamon, Achim; Boy, Diana; Hansmann, Doris; Büttner, Andreas; Wree, Andreas; Bader, Rainer; Jonitz-Heincke, Anika

2017-01-01

The main goal of cartilage repair is to create functional tissue by enhancing the in vitro conditions to more physiological in vivo conditions. Chondrogenic growth factors play an important role in influencing cartilage homeostasis. Insulin-like growth factor (IGF)-1 and transforming growth factor (TGF)-β1 affect the expression of collagen type II (Col2) and glycosaminoglycans (GAGs) and, therefore, the targeted use of growth factors could make chondrogenic redifferentiation more efficient. In the present study, human chondrocytes were postmortally isolated from healthy articular cartilage and cultivated as monolayer or 3D pellet cultures either under normoxia or hypoxia and stimulated with IGF-1 and/or TGF-β1 to compare the impact of the different growth factors. The mRNA levels of the specific receptors (IGF1R, TGFBR1, TGFBR2) were analyzed at different time points. Moreover, gene expression rates of collagen type 1 and 2 in pellet cultures were observed over a period of 5 weeks. Additionally, hyaline-like Col2 protein and sulphated GAG (sGAG) levels were quantified. Stimulation with IGF-1 resulted in an enhanced expression of IGF1R and TGFBR2 whereas TGF-β1 stimulated TGFBR1 in the monolayer and pellet cultures. In monolayer, the differences reached levels of significance. This effect was more pronounced under hypoxic culture conditions. In pellet cultures, increased amounts of Col2 protein and sGAGs after incubation with TGF-β1 and/or IGF-1 were validated. In summary, constructing a gene expression profile regarding mRNA levels of specific growth factor receptors in monolayer cultures could be helpful for a targeted application of growth factors in cartilage tissue engineering. PMID:28534942

Laser exposure induced alteration of WS2 monolayers in the presence of ambient moisture

NASA Astrophysics Data System (ADS)

Atkin, P.; Lau, D. W. M.; Zhang, Q.; Zheng, C.; Berean, K. J.; Field, M. R.; Ou, J. Z.; Cole, I. S.; Daeneke, T.; Kalantar-Zadeh, K.

2018-01-01

Photoluminescence (PL) emergence in monolayer transition metal dichalcogenides (TMDs) such as WS2, has been one of the key attractions of such materials. However, there have been many observational contradictions in PL measurements presented in the past literature. This work addresses such issues. Firstly, the observational changes of the flakes’ PL patterns under exposure to various intensities of radiant exposure via laser sources are presented. These experiments show that these changes are a function of radiant exposure. Interestingly, it is observed that PL loss is accompanied by a change of the profile height for WS2 monolayers. In order to explore the fundamental mechanism for PL and height variations, laser irradiation was applied to monolayer WS2 flakes with varying radiant exposure to obtain PL maps, under the absence and presence of oxygen, H2O and nitrogen molecules in the atmosphere. It was seen that, after relatively high radiant exposure (>15 mJ µm-2), the PL pattern loss occurs only in the presence of atmospheric H2O molecules (45% humidity) and is also accompanied by an increase in height. Compositional analysis determined that this height increase was due to the substitution of surface S atoms with sulphate groups. This discovery represents an important step forward in understanding the necessary precautions when investigating optical properties of 2D TMDs in atmospheric conditions, and highlights the need for precise evaluation of the thresholds for radiant exposure at which specific reactions begin to occur. This knowledge is crucial for efficient and effective control of ambient operating conditions for optical characterisation of monolayer WS2 and TMDs in general.

DFT study of structural and electronic properties of MoS2(1-x)Se2x alloy (x = 0.25)

NASA Astrophysics Data System (ADS)

Gusakova, Julia; Gusakov, Vasilii; Tay, Beng Kang

2018-04-01

First-principles calculations have been performed to study the structural features of the monolayer MoS2(1-x)Se2x (x = 0.25) alloy and its electronic properties. We studied the effects of the relative positions of Se atoms in a real monolayer alloy. It was demonstrated that the distribution of the Se atoms between the top and bottom chalcogen planes was most energetically favorable. For a more probable distribution of Se atoms, a MoS2(1-x)Se2x (x = 0.25) monolayer alloy is a direct semiconductor with a fundamental band gap equal to 2.35 eV (calculated with the GVJ-2e method). We also evaluated the optical band gap of the alloy at 77 K (1.86 eV) and at room temperature (1.80 eV), which was in good agreement with the experimentally measured band gap of 1.79 eV.

High-efficient light absorption of monolayer graphene via cylindrical dielectric arrays and the sensing application

NASA Astrophysics Data System (ADS)

Zhou, Peng; Zheng, Gaige

2018-04-01

The efficiency of graphene-based optoelectronic devices is typically limited by the poor absolute absorption of light. A hybrid structure of monolayer graphene with cylindrical titanium dioxide (TiO2) array and aluminum oxide (Al2O3) spacer layer on aluminum (Al) substrate has been proposed to enhance the absorption for two-dimensional (2D) materials. By combining dielectric array with metal substrate, the structure achieves multiple absorption peaks with near unity absorbance at near-infrared wavelengths due to the resonant effect of dielectric array. Completed monolayer graphene is utilized in the design without any demand of manufacture process to form the periodic patterns. Further analysis indicates that the near-field enhancement induced by surface modes gives rise to the high absorption. This favorable field enhancement and tunability of absorption not only open up new approaches to accelerate the light-graphene interaction, but also show great potential for practical applications in high-performance optoelectronic devices, such as modulators and sensors.

Atomic defects in monolayer titanium carbide (Ti 3C 2T x) MXene

DOE PAGES

Sang, Xiahan; Xie, Yu; Lin, Ming -Wei; ...

2016-09-06

Here, the 2D transition metal carbides or nitrides, or MXenes, are emerging as a group of materials showing great promise in lithium ion batteries and supercapacitors. Until now, characterization and properties of single-layer MXenes have been scarcely reported. Here, using scanning transmission electron microscopy, we determined the atomic structure of freestanding monolayer Ti 3C 2T x flakes prepared via the minimally intensive layer delamination method and characterized different point defects that are prevalent in the monolayer flakes. We determine that the Ti vacancy concentration can be controlled by the etchant concentration during preparation. Density function theory-based calculations confirm the defectmore » structures and predict that the defects can influence the surface morphology and termination groups, but do not strongly influence the metallic conductivity. Using devices fabricated from single- and few-layer Ti 3C 2T x MXene flakes, the effect of the number of layers in the flake on conductivity has been demonstrated.« less

Defect states of complexes involving a vacancy on the boron site in boronitrene

NASA Astrophysics Data System (ADS)

Ngwenya, T. B.; Ukpong, A. M.; Chetty, N.

2011-12-01

First principles calculations have been performed to investigate the ground state properties of freestanding monolayer hexagonal boronitrene (h-BN). We have considered monolayers that contain native point defects and their complexes, which form when the point defects bind with the boron vacancy on the nearest-neighbor position. The changes in the electronic structure are analyzed to show the extent of localization of the defect-induced midgap states. The variations in formation energies suggest that defective h-BN monolayers that contain carbon substitutional impurities are the most stable structures, irrespective of the changes in growth conditions. The high energies of formation of the boron vacancy complexes suggest that they are less stable, and their creation by ion bombardment would require high-energy ions compared to point defects. Using the relative positions of the derived midgap levels for the double vacancy complex, it is shown that the quasi-donor-acceptor pair interpretation of optical transitions is consistent with stimulated transitions between electron and hole states in boronitrene.

Interface electronic structures of reversible double-docking self-assembled monolayers on an Au(111) surface

PubMed Central

Zhang, Tian; Ma, Zhongyun; Wang, Linjun; Xi, Jinyang; Shuai, Zhigang

2014-01-01

Double-docking self-assembled monolayers (DDSAMs), namely self-assembled monolayers (SAMs) formed by molecules possessing two docking groups, provide great flexibility to tune the work function of metal electrodes and the tunnelling barrier between metal electrodes and the SAMs, and thus offer promising applications in both organic and molecular electronics. Based on the dispersion-corrected density functional theory (DFT) in comparison with conventional DFT, we carry out a systematic investigation on the dual configurations of a series of DDSAMs on an Au(111) surface. Through analysing the interface electronic structures, we obtain the relationship between single molecular properties and the SAM-induced work-function modification as well as the level alignment between the metal Fermi level and molecular frontier states. The two possible conformations of one type of DDSAM on a metal surface reveal a strong difference in the work-function modification and the electron/hole tunnelling barriers. Fermi-level pinning is found to be a key factor to understand the interface electronic properties. PMID:24615153

Wedge-shaped potential and Airy-function electron localization in oxide superlattices.

PubMed

Popovic, Z S; Satpathy, S

2005-05-06

Oxide superlattices and microstructures hold the promise for creating a new class of devices with unprecedented functionalities. Density-functional studies of the recently fabricated, lattice-matched perovskite titanates (SrTiO3)n/(LaTiO3)m reveal a classic wedge-shaped potential well for the monolayer (m = 1) structure, originating from the Coulomb potential of a two-dimensional charged La sheet. The potential in turn confines the electrons in the Airy-function-localized states. Magnetism is suppressed for the monolayer structure, while in structures with a thicker LaTiO3 part, bulk antiferromagnetism is recovered, with a narrow transition region separating the magnetic LaTiO3 and the nonmagnetic SrTiO3.

Chemical and Bandgap Engineering in Monolayer Hexagonal Boron Nitride

PubMed Central

Ba, Kun; Jiang, Wei; Cheng, Jingxin; Bao, Jingxian; Xuan, Ningning; Sun, Yangye; Liu, Bing; Xie, Aozhen; Wu, Shiwei; Sun, Zhengzong

2017-01-01

Monolayer hexagonal boron nitride (h-BN) possesses a wide bandgap of ~6 eV. Trimming down the bandgap is technically attractive, yet poses remarkable challenges in chemistry. One strategy is to topological reform the h-BN’s hexagonal structure, which involves defects or grain boundaries (GBs) engineering in the basal plane. The other way is to invite foreign atoms, such as carbon, to forge bizarre hybrid structures like hetero-junctions or semiconducting h-BNC materials. Here we successfully developed a general chemical method to synthesize these different h-BN derivatives, showcasing how the chemical structure can be manipulated with or without a graphene precursor, and the bandgap be tuned to ~2 eV, only one third of the pristine one’s. PMID:28367992

Spin splitting in band structures of BiTeX (X=Cl, Br, I) monolayers

NASA Astrophysics Data System (ADS)

Hvazdouski, D. C.; Baranava, M. S.; Stempitsky, V. R.

2018-04-01

In systems with breaking of inversion symmetry a perpendicular electric field arises that interacts with the conduction electrons. It may give rise to electron state splitting even without influence of external magnetic field due to the spin-orbital interaction (SOI). Such a removal of the spin degeneracy is called the Rashba effect. Nanostructure with the Rashba effect can be part of a spin transistor. Spin degeneracy can be realized in a channel from a material of this type without additive of magnetic ions. Lack of additive increases the charge carrier mobility and reliability of the device. Ab initio simulations of BiTeX (X=Cl, Br, I) monolayers have been carried out using VASP wherein implemented DFT method. The study of this structures is of interest because such sort of structures can be used their as spin-orbitronics materials. The crystal parameters of BiTeCl, BiTeBr, BiTeI have been determined by the ionic relaxation and static calculations. It is necessary to note that splitting of energy bands occurs in case of SOI included. The values of the Rashba coefficient aR (in the range from 6.25 to 10.00 eV·Å) have high magnitudes for spintronics materials. Band structure of monolayers structures have ideal Rashba electron gas, i.e. there no other energy states near to Fermi level except Rashba states.

SAM-based cell transfer to photopatterned hydrogels for microengineering vascular-like structures.

PubMed

Sadr, Nasser; Zhu, Mojun; Osaki, Tatsuya; Kakegawa, Takahiro; Yang, Yunzhi; Moretti, Matteo; Fukuda, Junji; Khademhosseini, Ali

2011-10-01

A major challenge in tissue engineering is to reproduce the native 3D microvascular architecture fundamental for in vivo functions. Current approaches still lack a network of perfusable vessels with native 3D structural organization. Here we present a new method combining self-assembled monolayer (SAM)-based cell transfer and gelatin methacrylate hydrogel photopatterning techniques for microengineering vascular structures. Human umbilical vein cell (HUVEC) transfer from oligopeptide SAM-coated surfaces to the hydrogel revealed two SAM desorption mechanisms: photoinduced and electrochemically triggered. The former, occurs concomitantly to hydrogel photocrosslinking, and resulted in efficient (>97%) monolayer transfer. The latter, prompted by additional potential application, preserved cell morphology and maintained high transfer efficiency of VE-cadherin positive monolayers over longer culture periods. This approach was also applied to transfer HUVECs to 3D geometrically defined vascular-like structures in hydrogels, which were then maintained in perfusion culture for 15 days. As a step toward more complex constructs, a cell-laden hydrogel layer was photopatterned around the endothelialized channel to mimic the vascular smooth muscle structure of distal arterioles. This study shows that the coupling of the SAM-based cell transfer and hydrogel photocrosslinking could potentially open up new avenues in engineering more complex, vascularized tissue constructs for regenerative medicine and tissue engineering applications. Copyright © 2011 Elsevier Ltd. All rights reserved.

First principles study of structural, vibrational and electronic properties of graphene-like MX 2 (M=Mo, Nb, W, Ta; X=S, Se, Te) monolayers

NASA Astrophysics Data System (ADS)

Ding, Yi; Wang, Yanli; Ni, Jun; Shi, Lin; Shi, Siqi; Tang, Weihua

2011-05-01

Using first principles calculations, we investigate the structural, vibrational and electronic structures of the monolayer graphene-like transition-metal dichalcogenide (MX 2) sheets. We find the lattice parameters and stabilities of the MX 2 sheets are mainly determined by the chalcogen atoms, while the electronic properties depend on the metal atoms. The NbS 2 and TaS 2 sheets have comparable energetic stabilities to the synthesized MoS 2 and WS 2 ones. The molybdenum and tungsten dichalcogenide (MoX 2 and WX 2) sheets have similar lattice parameters, vibrational modes, and electronic structures. These analogies also exist between the niobium and tantalum dichalcogenide (NbX 2 and TaX 2) sheets. However, the NbX 2 and TaX 2 sheets are metals, while the MoX 2 and WX 2 ones are semiconductors with direct-band gaps. When the Nb and Ta atoms are doped into the MoS 2 and WS 2 sheets, a semiconductor-to-metal transition occurs. Comparing to the bulk compounds, these monolayer sheets have similar structural parameters and properties, but their vibrational and electronic properties are varied and have special characteristics. Our results suggest that the graphene-like MX 2 sheets have potential applications in nano-electronics and nano-devices.

Characterization of a Biomimetic Polymeric-Lipid Bilayer by Phase Sensitive Neutron Reflectivity

NASA Astrophysics Data System (ADS)

Perez-Salas, Ursula A.; Krueger, Susan; Majkrzak, Charles F.; Berk, Norman F.; Faucher, Keith M.; Chaikof, Elliot L.

2003-03-01

Lipid membranes, the boundaries for cellular and intracellular structures, regulate many crucial biological processes. Planar supported mimics of cell membranes are of great interest as model systems for the study of membrane structure/function phenomena in fundamental biophysics research. We studied a supported biomedically relevant membrane-mimetic system composed of a polyelectrolyte cushion, a terpolymer and a self-assembled phospholipid monolayer and obtained a detailed profile characterization of the system by neutron reflectometry. The water-swellable hydrophilic polyelectrolyte acts as a support for the biomembrane, not unlike the cytoskeletal support found in actual mammalian cell membranes. The "cushion" polymers are fixed to the flat, hard surface by having the polymer interact with it electrostatically. The terpolymer has the following desirable features: it tethers to the polyelectrolyte layer and it creates a hydrophilic and a hydrophobic region. Unilamellar phospholipid vesicle fusion on to the hydrophobic region of the terpolymer creates the hybrid tethered membrane. For added stability to external force fields (such as shear flow), the phospholipid monolayer is then polymerized in situ, effectively anchoring the lipid layer to the hydrophobic region of the terpolymer. Neutron reflectivity measurements were done on the polyelectrolyte layer, the polyelectrolyte layer plus terpolymer and the polylectrolyte layer plus terpolymer plus phospholipid. The layers were studied dry and hydrated and under 95α D_2O and 50% \\ 50% α H_2O \\ α D_2O) on the polyelectrolyte layer plus terpolymer and the polylectrolyte layer plus terpolymer plus phospholipid the distribution of water in the layers was obtained. The results will be correlated to impedance measurements flourescence measurements and infrared spectroscopy measurements made on equivalent samples.

Multilayered phantoms with tunable optical properties for a better understanding of light/tissue interactions

NASA Astrophysics Data System (ADS)

Roig, Blandine; Koenig, Anne; Perraut, François; Piot, Olivier; Vignoud, Séverine; Lavaud, Jonathan; Manfait, Michel; Dinten, Jean-Marc

2015-03-01

Light/tissue interactions, like diffuse reflectance, endogenous fluorescence and Raman scattering, are a powerful means for providing skin diagnosis. Instrument calibration is an important step. We thus developed multilayered phantoms for calibration of optical systems. These phantoms mimic the optical properties of biological tissues such as skin. Our final objective is to better understand light/tissue interactions especially in the case of confocal Raman spectroscopy. The phantom preparation procedure is described, including the employed method to obtain a stratified object. PDMS was chosen as the bulk material. TiO2 was used as light scattering agent. Dye and ink were adopted to mimic, respectively, oxy-hemoglobin and melanin absorption spectra. By varying the amount of the incorporated components, we created a material with tunable optical properties. Monolayer and multilayered phantoms were designed to allow several characterization methods. Among them, we can name: X-ray tomography for structural information; Diffuse Reflectance Spectroscopy (DRS) with a homemade fibered bundle system for optical characterization; and Raman depth profiling with a commercial confocal Raman microscope for structural information and for our final objective. For each technique, the obtained results are presented and correlated when possible. A few words are said on our final objective. Raman depth profiles of the multilayered phantoms are distorted by elastic scattering. The signal attenuation through each single layer is directly dependent on its own scattering property. Therefore, determining the optical properties, obtained here with DRS, is crucial to properly correct Raman depth profiles. Thus, it would be permitted to consider quantitative studies on skin for drug permeation follow-up or hydration assessment, for instance.

Simulation studies on structural and thermal properties of alkane thiol capped gold nanoparticles.

PubMed

Devi, J Meena

2017-06-01

The structural and thermal properties of the passivated gold nanoparticles were explored employing molecular dynamics simulation for the different surface coverage densities of the self-assembled monolayer (SAM) of alkane thiol. The structural properties of the monolayer protected gold nanoparticles such us overall shape, organization and conformation of the capping alkane thiol chains were found to be influenced by the capping density. The structural order of the thiol capped gold nanoparticles enhances with the increase in the surface coverage density. The specific heat capacity of the alkane thiol capped gold nanoparticles was found to increase linearly with the thiol coverage density. This may be attributed to the enhancement in the lattice vibrational energy. The present simulation results suggest, that the structural and thermal properties of the alkane thiol capped gold nanoparticles may be modified by the suitable selection of the SAM coverage density. Copyright © 2017 Elsevier Inc. All rights reserved.

Evolution of the Valley Position in Bulk Transition-Metal Chalcogenides and Their Monolayer Limit.

PubMed

Yuan, Hongtao; Liu, Zhongkai; Xu, Gang; Zhou, Bo; Wu, Sanfeng; Dumcenco, Dumitru; Yan, Kai; Zhang, Yi; Mo, Sung-Kwan; Dudin, Pavel; Kandyba, Victor; Yablonskikh, Mikhail; Barinov, Alexei; Shen, Zhixun; Zhang, Shoucheng; Huang, Yingsheng; Xu, Xiaodong; Hussain, Zahid; Hwang, Harold Y; Cui, Yi; Chen, Yulin

2016-08-10

Layered transition metal chalcogenides with large spin orbit coupling have recently sparked much interest due to their potential applications for electronic, optoelectronic, spintronics, and valleytronics. However, most current understanding of the electronic structure near band valleys in momentum space is based on either theoretical investigations or optical measurements, leaving the detailed band structure elusive. For example, the exact position of the conduction band valley of bulk MoS2 remains controversial. Here, using angle-resolved photoemission spectroscopy with submicron spatial resolution (micro-ARPES), we systematically imaged the conduction/valence band structure evolution across representative chalcogenides MoS2, WS2, and WSe2, as well as the thickness dependent electronic structure from bulk to the monolayer limit. These results establish a solid basis to understand the underlying valley physics of these materials, and also provide a link between chalcogenide electronic band structure and their physical properties for potential valleytronics applications.

Evolution of the Valley Position in Bulk Transition-Metal Chalcogenides and Their Monolayer Limit

DOE PAGES

Yuan, Hongtao; Liu, Zhongkai; Xu, Gang; ...

2016-07-12

Valley physics based on layered transition metal chalcogenides have recently sparked much interest due to their potential spintronics and valleytronics applications. However, most current understanding of the electronic structure near band valleys in momentum space is based on either theoretical investigations or optical measurements, leaving the detailed band structure elusive. For example, the exact position of the conduction band valley of bulk MoS 2 remains controversial. Here, using angle-resolved photoemission spectroscopy with sub-micron spatial resolution (micro- ARPES), we systematically imaged the conduction/valence band structure evolution across representative chalcogenides MoS 2, WS 2 and WSe 2, as well as the thicknessmore » dependent electronic structure from bulk to the monolayer limit. These results establish a solid basis to understand the underlying valley physics of these materials, and also provide a link between chalcogenide electronic band structure and their physical properties for potential valleytronics applications.« less

Amyloid-beta-sheet formation at the air-water interface.

PubMed Central

Schladitz, C; Vieira, E P; Hermel, H; Möhwald, H

1999-01-01

An amyloid(1-40) solution rich in coil, turn, and alpha-helix, but poor in beta-sheet, develops monolayers with a high beta-sheet content when spread at the air-water interface. These monolayers are resistant to repeated compression-dilatation cycles and interaction with trifluoroethanol. The secondary structure motifs were detected by circular dichroism (CD) in solution and with infrared reflection-absorption spectroscopy (IRRAS) at the interface. Hydrophobic influences are discussed for the structure conversion in an effort to understand the completely unknown reason for the natural change of the normal prion protein cellular (PrP(C)) into the abnormal prion protein scrapie (PrP(Sc)). PMID:10585952

Conduction quantization in monolayer MoS2

NASA Astrophysics Data System (ADS)

Li, T. S.

2016-10-01

We study the ballistic conduction of a monolayer MoS2 subject to a spatially modulated magnetic field by using the Landauer-Buttiker formalism. The band structure depends sensitively on the field strength, and its change has profound influence on the electron conduction. The conductance is found to demonstrate multi-step behavior due to the discrete number of conduction channels. The sharp peak and rectangular structures of the conductance are stretched out as temperature increases, due to the thermal broadening of the derivative of the Fermi-Dirac distribution function. Finally, quantum behavior in the conductance of MoS2 can be observed at temperatures below 10 K.

Strain-induced enhancement of thermoelectric performance of TiS2 monolayer based on first-principles phonon and electron band structures

NASA Astrophysics Data System (ADS)

Li, Guanpeng; Yao, Kailun; Gao, Guoying

2018-01-01

Using first-principle calculations combined with Boltzmann transport theory, we investigate the biaxial strain effect on the electronic and phonon thermal transport properties of a 1 T (CdI2-type) structural TiS2 monolayer, a recent experimental two-dimensional (2D) material. It is found that the electronic band structure can be effectively modulated and that the band gap experiences an indirect-direct-indirect transition with increasing tensile strain. The band convergence induced by the tensile strain increases the Seebeck coefficient and the power factor, while the lattice thermal conductivity is decreased under the tensile strain due to the decreasing group velocity and the increasing scattering chances between the acoustic phonon modes and the optical phonon modes, which together greatly increase the thermoelectric performance. The figure of merit can reach 0.95 (0.82) at an 8 percent tensile strain for the p-type (n-type) doping, which is much larger than that without strain. The present work suggests that the TiS2 monolayer is a good candidate for 2D thermoelectric materials, and that biaxial strain is a powerful tool with which to enhance thermoelectric performance.

Strain-induced enhancement of thermoelectric performance of TiS2 monolayer based on first-principles phonon and electron band structures.

PubMed

Li, Guanpeng; Yao, Kailun; Gao, Guoying

2018-01-05

Using first-principle calculations combined with Boltzmann transport theory, we investigate the biaxial strain effect on the electronic and phonon thermal transport properties of a 1 T (CdI 2 -type) structural TiS 2 monolayer, a recent experimental two-dimensional (2D) material. It is found that the electronic band structure can be effectively modulated and that the band gap experiences an indirect-direct-indirect transition with increasing tensile strain. The band convergence induced by the tensile strain increases the Seebeck coefficient and the power factor, while the lattice thermal conductivity is decreased under the tensile strain due to the decreasing group velocity and the increasing scattering chances between the acoustic phonon modes and the optical phonon modes, which together greatly increase the thermoelectric performance. The figure of merit can reach 0.95 (0.82) at an 8 percent tensile strain for the p-type (n-type) doping, which is much larger than that without strain. The present work suggests that the TiS 2 monolayer is a good candidate for 2D thermoelectric materials, and that biaxial strain is a powerful tool with which to enhance thermoelectric performance.

Synchrotron X-Ray Diffraction Study of Structure and Growth of Adsorbed Layers

NASA Astrophysics Data System (ADS)

Dai, Pengcheng

Synchrotron x-ray diffraction and scanning-tunneling -microscopy (STM) experiments reveal a new commensurate monolayer structure of 10CB (decylcyanobiphenyl) molecules adsorbed on the (0001) graphite surface. Our results are consistent with two generic structures for nCB monolayers on surfaces of hexagonal symmetry. The monolayer d spacing of the new phase inferred by STM is 10% layer than that obtained by x-ray diffraction on the same sample. We suggest that part of this discrepancy results from a systematic error introduced in calibration of the STM length scale against the graphite substrate. For multilayer nCB films, we find that a polycrystalline structure is formed and most of the adsorbed molecules are aligned with their long axis perpendicular to the graphite surface. Synchrotron x-ray scattering has been used to investigate the structure and growth of xenon physisorbed on the Ag(111) surface using a specially designed ultra -high vacuum (UHV) chamber. For growth under quasi-equilibrium conditions, the bulk Xe-Xe spacing is reached at monolayer completion and solid films of thickness >= 220 A are observed in which an 'ABC' stacking sequence predominates. Under kinetic growth conditions, intensity oscillations at the Xe anti-Bragg position of the specular rod are observed as a function of time, indicating layer -by-layer growth. Analysis of the specular reflectivity at different coverages yields the fractional layer occupancies and the spacing between the Ag(111) surface and first Xe layer. We have conducted a series of low-energy electron diffraction (LEED) 'kinetic isotherm' experiments on both xenon and hexane rm(C_6H_{14 }) films adsorbed on the Ag(111) surface. Our preliminary results show that under the pressure and temperature range accessible to the experiments, all of the Xe kinetic isotherms fall on a universal curve which is concave upward. However, the hexane kinetic isotherms have a qualitatively different shape (S-like) at the higher temperatures while being similar to Xe at low temperatures. From these experiments, we determine that the growth of xenon from submonolayer to 0.9 monolayer is 'zero-order'. However, the growth of hexane is more complicated. It follows the 'first-order' at low temperatures, and changes to S-like shape at high temperatures which we do not yet understand.

Surface modification of GC and HOPG with diazonium, amine, azide, and olefin derivatives.

PubMed

Tanaka, Mutsuo; Sawaguchi, Takahiro; Sato, Yukari; Yoshioka, Kyoko; Niwa, Osamu

2011-01-04

Surface modification of glassy carbon (GC) and highly oriented pyrolytic graphite (HOPG) was carried out with diazonium, amine, azide, and olefin derivatives bearing ferrocene as an electroactive moiety. Features of the modified surfaces were evaluated by surface concentrations of immobilized molecule, blocking effect of the modified surface against redox reaction, and surface observation using cyclic voltammetry and electrochemical scanning tunneling microscope (EC-STM). The measurement of surface concentrations of immobilized molecule revealed the following three aspects: (i) Diazonium and olefin derivatives could modify substrates with the dense-monolayer concentration. (ii) The surface concentration of immobilized amine derivative did not reach to the dense-monolayer concentration reflecting their low reactivity. (iii) The surface modification with the dense-monolayer concentration was also possible with azide derivative, but the modified surface contained some oligomers produced by the photoreaction of azides. Besides, the blocking effect against redox reaction was observed for GC modified with diazonium derivative and for HOPG modified with diazonium and azide derivatives, suggesting fabrication of a densely modified surface. Finally, the surface observation for HOPG modified with diazonium derivative by EC-STM showed a typical monolayer structure, in which the ferrocene moieties were packed densely at random. On the basis of those results, it was demonstrated that surface modification of carbon substrates with diazonium could afford a dense monolayer similar to the self-assembled monolayer (SAM) formation.

Intermolecular interactions and substrate effects for an adamantane monolayer on a Au(111) surface

NASA Astrophysics Data System (ADS)

Sakai, Yuki; Nguyen, Giang D.; Capaz, Rodrigo B.; Coh, Sinisa; Pechenezhskiy, Ivan V.; Hong, Xiaoping; Wang, Feng; Crommie, Michael F.; Saito, Susumu; Louie, Steven G.; Cohen, Marvin L.

2013-12-01

We study theoretically and experimentally the infrared (IR) spectrum of an adamantane monolayer on a Au(111) surface. Using a STM-based IR spectroscopy technique (IRSTM) we are able to measure both the nanoscale structure of an adamantane monolayer on Au(111) as well as its infrared spectrum, while DFT-based ab initio calculations allow us to interpret the microscopic vibrational dynamics revealed by our measurements. We find that the IR spectrum of an adamantane monolayer on Au(111) is substantially modified with respect to the gas-phase IR spectrum. The first modification is caused by the adamantane-adamantane interaction due to monolayer packing, and it reduces the IR intensity of the 2912 cm-1 peak (gas phase) by a factor of 3.5. The second modification originates from the adamantane-gold interaction, and it increases the IR intensity of the 2938 cm-1 peak (gas phase) by a factor of 2.6 and reduces its frequency by 276 cm-1. We expect that the techniques described here can be used for an independent estimate of substrate effects and intermolecular interactions in other diamondoid molecules and for other metallic substrates.

An effective approach to synthesize monolayer tungsten disulphide crystals using tungsten halide precursor

DOE Office of Scientific and Technical Information (OSTI.GOV)

Thangaraja, Amutha; Shinde, Sachin M.; Kalita, Golap, E-mail: kalita.golap@nitech.ac.jp

2016-02-01

The synthesis of large-area monolayer tungsten disulphide (WS{sub 2}) single crystal is critical for realistic application in electronic and optical devices. Here, we demonstrate an effective approach to synthesize monolayer WS{sub 2} crystals using tungsten hexachloride (WCl{sub 6}) as a solid precursor in atmospheric chemical vapor deposition process. In this technique, 0.05M solution of WCl{sub 6} in ethanol was drop-casted on SiO{sub 2}/Si substrate to create an even distribution of the precursor, which was reduced and sulfurized at 750 °C in Ar atmosphere. We observed growth of triangular, star-shaped, as well as dendritic WS{sub 2} crystals on the substrate. The crystalmore » geometry evolves with the shape and size of the nuclei as observed from the dendritic structures. These results show that controlling the initial nucleation and growth process, large WS{sub 2} single crystalline monolayer can be grown using the WCl{sub 6} precursor. Our finding shows an easier and effective approach to grow WS{sub 2} monolayer using tungsten halide solution-casting, rather than evaporating the precursor for gas phase reaction.« less

Superior ionic and electronic properties of ReN2 monolayers for Na-ion battery electrodes.

PubMed

Zhang, Shi-Hao; Liu, Bang-Gui

2018-08-10

Excellent monolayer electrode materials can be used to design high-performance alkali-metal-ion batteries. Here, we propose two-dimensional ReN 2 monolayers as superior sodium-ion battery materials. Our total energy optimization results in a buckled tetragonal structure for the ReN 2 monolayer, and our phonon spectrum and elastic moduli prove that it is dynamically and mechanically stable. Further investigations show that it is metallic and still keeps its metallic feature after the adsorption of Na or K atoms, and the adsorption of Na (or K) atoms changes the lattice parameters by 3.2% (or 3.8%) at most. Its maximum capacity reaches 751 mA h g -1 for Na-ion batteries or 250 mA h g -1 for K-ion batteries, and the diffusion barrier is only 0.027 eV for the Na atom or 0.127 eV for the K atom. The small lattice changes, high storage capacity, metallic feature, and extremely low ion diffusion barriers make the ReN 2 monolayers a superior electrode material for Na-ion rechargeable batteries with ultrafast charging/discharging processes.

Electrostatic effects in the collapse transition of phospholiquid monolayer

NASA Astrophysics Data System (ADS)

Nguyen, Toan T.; Gopal, Ajaykumar; Lee, Ka Yee C.; Witten, Thomas A.

2004-03-01

We study the collapse transition of fluidic phospholipid surfactant monolayers under lateral compression. DMPC, DPPC or POPG surfactants and their binary mixtures are used. Various collapsed structures (circular discs, cylinderical tubes and pearls-on-a-string) were observed during the transition. We show that electrostatics plays an important role in the formation of these structures. By changing the composition of charged surfactant (POGP) or the screening condition of the solution, one can change the dominant collapsed structure from discs to tubes to pearls in the order of increasing the strength of electrostatic interactions, in accordance with theoretical estimates. We also study a complimentary electrostatic effect due charge relaxation in the transitions between these structures. It is shown that free energy gained from relaxations of charge molecule is small and can be neglected when considering electrostatics of these systems.

Dirac Fermions in Borophene

NASA Astrophysics Data System (ADS)

Feng, Baojie; Sugino, Osamu; Liu, Ro-Ya; Zhang, Jin; Yukawa, Ryu; Kawamura, Mitsuaki; Iimori, Takushi; Kim, Howon; Hasegawa, Yukio; Li, Hui; Chen, Lan; Wu, Kehui; Kumigashira, Hiroshi; Komori, Fumio; Chiang, Tai-Chang; Meng, Sheng; Matsuda, Iwao

2017-03-01

Honeycomb structures of group IV elements can host massless Dirac fermions with nontrivial Berry phases. Their potential for electronic applications has attracted great interest and spurred a broad search for new Dirac materials especially in monolayer structures. We present a detailed investigation of the β12 sheet, which is a borophene structure that can form spontaneously on a Ag(111) surface. Our tight-binding analysis revealed that the lattice of the β12 sheet could be decomposed into two triangular sublattices in a way similar to that for a honeycomb lattice, thereby hosting Dirac cones. Furthermore, each Dirac cone could be split by introducing periodic perturbations representing overlayer-substrate interactions. These unusual electronic structures were confirmed by angle-resolved photoemission spectroscopy and validated by first-principles calculations. Our results suggest monolayer boron as a new platform for realizing novel high-speed low-dissipation devices.

Epitaxial MoS2/GaN structures to enable vertical 2D/3D semiconductor heterostructure devices

NASA Astrophysics Data System (ADS)

Ruzmetov, D.; Zhang, K.; Stan, G.; Kalanyan, B.; Eichfeld, S.; Burke, R.; Shah, P.; O'Regan, T.; Crowne, F.; Birdwell, A. G.; Robinson, J.; Davydov, A.; Ivanov, T.

MoS2/GaN structures are investigated as a building block for vertical 2D/3D semiconductor heterostructure devices that utilize a 3D substrate (GaN) as an active component of the semiconductor device without the need of mechanical transfer of the 2D layer. Our CVD-grown monolayer MoS2 has been shown to be epitaxially aligned to the GaN lattice which is a pre-requisite for high quality 2D/3D interfaces desired for efficient vertical transport and large area growth. The MoS2 coverage is nearly 50 % including isolated triangles and monolayer islands. The GaN template is a double-layer grown by MOCVD on sapphire and allows for measurement of transport perpendicular to the 2D layer. Photoluminescence, Raman, XPS, Kelvin force probe microscopy, and SEM analysis identified high quality monolayer MoS2. The MoS2/GaN structures electrically conduct in the out-of-plane direction and across the van der Waals gap, as measured with conducting AFM (CAFM). The CAFM current maps and I-V characteristics are analyzed to estimate the MoS2/GaN contact resistivity to be less than 4 Ω-cm2 and current spreading in the MoS2 monolayer to be approx. 1 μm in diameter. Epitaxial MoS2/GaN heterostructures present a promising platform for the design of energy-efficient, high-speed vertical devices incorporating 2D layered materials with 3D semiconductors.

Metal adsorption on monolayer blue phosphorene: A first principles study

NASA Astrophysics Data System (ADS)

Khan, Imran; Son, Jicheol; Hong, Jisang

2018-01-01

We investigated the electronic structure, adsorption energies, magnetic properties, dipole moment and work function of metal adatoms (Mg, Cr, Mo, Pd, Pt, and Au) adsorption on a blue phosphorene monolayer. For Mg, Pt and Au metals, the most stable state was found in hollow site while for Cr, Mo and Pd metals we found an adsorption in valley site. We suggest that the Pd and Pt atoms prefer 2D growth mode while the Mg, Cr, Mo and Au atoms prefer 3D island growth mode on monolayer phosphorene. The electronic band structures and magnetic properties were dependent on the doping site and dopant materials. For instance, the semiconducting features were preserved in Mg, Pd, Pt, and Au doped systems. However, the Cr and Mo doped systems displayed half-metallic band structures. The total magnetic moment of 4.05, 2.0 and 0.77 μB /impurity atom were obtained in Cr, Mo and Au doped systems whereas the Mg, Pd and Pt doped systems remained nonmagnetic. We also investigated the magnetic interaction between two transition metal impurities. We observed ferromagnetic coupling between two transition metal impurities in Cr and Mo doped systems while the Au doped system displayed almost degenerated magnetic state. For Mg, Cr, and Mo adsorptions, we found relatively large values of dipole moments compared to those in the Pd, Pt and Au adsorptions. This resulted in a significant suppression of the work function in Mg, Cr and Mo adsorptions. Overall, adsorption can tune the physical and magnetic properties of phosphorene monolayer.

Deuterium Labeling Strategies for Creating Contrast in Structure-Function Studies of Model Bacterial Outer Membranes Using Neutron Reflectometry.

PubMed

Le Brun, Anton P; Clifton, Luke A; Holt, Stephen A; Holden, Peter J; Lakey, Jeremy H

2016-01-01

Studying the outer membrane of Gram-negative bacteria is challenging due to the complex nature of its structure. Therefore, simplified models are required to undertake structure-function studies of processes that occur at the outer membrane/fluid interface. Model membranes can be created by immobilizing bilayers to solid supports such as gold or silicon surfaces, or as monolayers on a liquid support where the surface pressure and fluidity of the lipids can be controlled. Both model systems are amenable to having their structure probed by neutron reflectometry, a technique that provides a one-dimensional depth profile through a membrane detailing its thickness and composition. One of the strengths of neutron scattering is the ability to use contrast matching, allowing molecules containing hydrogen and those enriched with deuterium to be highlighted or matched out against the bulk isotopic composition of the solvent. Lipopolysaccharides, a major component of the outer membrane, can be isolated for incorporation into model membranes. Here, we describe the deuteration of lipopolysaccharides from rough strains of Escherichia coli for incorporation into model outer membranes, and how the use of deuterated materials enhances structural analysis of model membranes by neutron reflectometry. © 2016 Elsevier Inc. All rights reserved.

Nearly-free-electron system of monolayer Na on the surface of single-crystal HfSe 2

DOE PAGES

Eknapakul, T.; Fongkaew, I.; Siriroj, S.; ...

2016-11-15

Here, the electronic structure of a single Na monolayer on the surface of single-crystal HfSe 2 is investigated using angle-resolved photoemission spectroscopy. We find that this system exhibits an almost perfect "nearly-free-electron" behavior with an extracted effective mass of ~1m e, in contrast to heavier masses found previously for alkali-metal monolayers on other substrates. Our density-functional-theory calculations indicate that this is due to the large lattice constant, causing both exchange and correlation interactions to be suppressed, and to the weak hybridization between the overlayer and the substrate. This is therefore an ideal model system for understanding the properties of two-dimensionalmore » materials.« less

Photoswitching in azobenzene self-assembled monolayers capped on zinc oxide: nanodots vs nanorods.

PubMed

Shah, Syed Mujtaba; Martini, Cyril; Ackermann, Jörg; Fages, Frédéric

2012-02-01

We report the synthesis and spectroscopic characterization of nanohybrid structures consisting of an azobenzene compound grafted on the surface of zinc oxide nanoparticles. Characteristic bathochromic shifts indicate that the azobenzene photochromic molecules self-assemble onto the surface of the nanocrystals. The extent of packing is dependent on the shape of the nanoparticle. ZnO nanorods, with flat facets, enable a tighter organization of the molecules in the self-assembled monolayer than in the case of nanodots that display a more curvated shape. Consistently, the efficiency of photochromic switching of the self-assembled monolayer on ZnO nanoparticles is also shown to be strongly affected by nanoparticle shape. Copyright © 2011 Elsevier Inc. All rights reserved.

Stability enhancement and electronic tunability of two-dimensional SbIV compounds via surface functionalization

NASA Astrophysics Data System (ADS)

Zhou, Wenhan; Guo, Shiying; Liu, Xuhai; Cai, Bo; Song, Xiufeng; Zhu, Zhen; Zhang, Shengli

2018-01-01

We propose a family of hydrogenated- and halogenated-SbIV (SbIVX-2) materials that simultaneously have two-dimensional (2D) structures, high stability and appealing electronic properties. Based on first-principles total-energy and vibrational-spectra calculations, SbIVX-2 monolayers are found both thermally and dynamically stable. Varying IV and X elements can rationally tune the electronic properties of SbIVX-2 monolayers, effectively modulating the band gap from 0 to 3.42 eV. Regarding such superior stability and broad band-gap range, SbIVX-2 monolayers are expected to be synthesized in experiments and taken as promising candidates for low-dimensional electronic and optoelectronic devices, such as blue-to-ultraviolet light-emitting diodes (LED) and photodetectors.

Multispectral selective near-perfect light absorption by graphene monolayer using aperiodic multilayer microstructures

NASA Astrophysics Data System (ADS)

Zand, Iman; Dalir, Hamed; Chen, Ray T.; Dowling, Jonathan P.

2018-03-01

We investigate one-dimensional aperiodic multilayer microstructures in order to achieve near-total absorptions at preselected wavelengths in a graphene monolayer. The proposed structures are designed using a genetic optimization algorithm coupled to a transfer matrix code. Coupled-mode-theory analysis, consistent with transfer matrix method results, indicates the existence of a critical coupling in the graphene monolayer for perfect absorptions. Our findings show that the near-total-absorption peaks are highly tunable and can be controlled simultaneously or independently in a wide range of wavelengths in the near-infrared and visible ranges. The proposed approach is metal-free, does not require surface texturing or patterning, and can be also applied for other two-dimensional materials.

Transition from overlayer growth to alloying growth of Ga on Si(111)-alpha-(sqrt[3]xsqrt[3])-Au.

PubMed

Yamanaka, T; Ino, S

2002-11-04

Atomic depth distribution and growth modes of Ga on an Si(111)-alpha-(sqrt[3]xsqrt[3])-Au surface at room temperature were studied after each monolayer deposition of Ga via reflection high-energy electron diffraction and characteristic x-ray spectroscopy measurements as functions of glancing angle theta(g) of the incident electron beam. One monolayer of Ga grew on the Au layer, and the sqrt[3]xsqrt[3] periodicity was conserved below the Ga overlayer. Above a critical Ga coverage of about one monolayer, this growth mode drastically changed; i.e., Au atoms dissociated from the sqrt[3]xsqrt[3] structure and Ga grew into islands of Ga-Au alloy.

Large enhancement of second harmonic generation from transition-metal dichalcogenide monolayer on grating near bound states in the continuum.

PubMed

Wang, Tiecheng; Zhang, Shihao

2018-01-08

Second harmonic generation from the two-layer structure where a transition-metal dichalcogenide monolayer is put on a one-dimensional grating has been studied. This grating supports bound states in the continuum which have no leakage lying within the continuum of radiation modes, we can enhance the second harmonic generation from the transition-metal dichalcogenide monolayer by more than four orders of magnitude based on the critical field enhancement near the bound states in the continuum. In order to complete this calculation, the scattering matrix theory has been extended to include the nonlinear effect and the scattering matrix of a two-dimensional material including nonlinear terms; furthermore, two methods to observe the bound states in the continuum are considered, where one is tuning the thickness of the grating and the other is changing the incident angle of the electromagnetic wave. We have also discussed various modulation of the second harmonic generation enhancement by adjusting the azimuthal angle of the transition-metal dichalcogenide monolayer.

Polarization-modulated FTIR spectroscopy of lipid/gramicidin monolayers at the air/water interface.

PubMed Central

Ulrich, W P; Vogel, H

1999-01-01

Monolayers of gramicidin A, pure and in mixtures with dimyristoylphosphatidylcholine (DMPC), were studied in situ at the air/H2O and air/D2O interfaces by polarization-modulated infrared reflection absorption spectroscopy (PM-IRRAS). Simulations of the entire set of amide I absorption modes were also performed, using complete parameter sets for different conformations based on published normal mode calculations. The structure of gramicidin A in the DMPC monolayer could clearly be assigned to a beta6.3 helix. Quantitative analysis of the amide I bands revealed that film pressures of up to 25-30 mN/m the helix tilt angle from the vertical in the pure gramicidin A layer exceeded 60 degrees. A marked dependence of the peptide orientation on the applied surface pressure was observed for the mixed lipid-peptide monolayers. At low pressure the helix lay flat on the surface, whereas at high pressures the helix was oriented almost parallel to the surface normal. PMID:10049344

Thiol-ene immobilisation of carbohydrates onto glass slides as a simple alternative to gold-thiol monolayers, amines or lipid binding.

PubMed

Biggs, Caroline I; Edmondson, Steve; Gibson, Matthew I

2015-01-01

Carbohydrate arrays are a vital tool in studying infection, probing the mechanisms of bacterial, viral and toxin adhesion and the development of new treatments, by mimicking the structure of the glycocalyx. Current methods rely on the formation of monolayers of carbohydrates that have been chemically modified with a linker to enable interaction with a functionalised surface. This includes amines, biotin, lipids or thiols. Thiol-addition to gold to form self-assembled monolayers is perhaps the simplest method for immobilisation as thiolated glycans are readily accessible from reducing carbohydrates in a single step, but are limited to gold surfaces. Here we have developed a quick and versatile methodology which enables the use of thiolated carbohydrates to be immobilised as monolayers directly onto acrylate-functional glass slides via a 'thiol-ene'/Michael-type reaction. By combining the ease of thiol chemistry with glass slides, which are compatible with microarray scanners this offers a cost effective, but also useful method to assemble arrays.

Electrochemistry of redox-active self-assembled monolayers

PubMed Central

Eckermann, Amanda L.; Feld, Daniel J.; Shaw, Justine A.; Meade, Thomas J.

2010-01-01

Redox-active self-assembled monolayers (SAMs) provide an excellent platform for investigating electron transfer kinetics. Using a well-defined bridge, a redox center can be positioned at a fixed distance from the electrode and electron transfer kinetics probed using a variety of electrochemical techniques. Cyclic voltammetry, AC voltammetry, electrochemical impedance spectroscopy, and chronoamperometry are most commonly used to determine the rate of electron transfer of redox-activated SAMs. A variety of redox species have been attached to SAMs, and include transition metal complexes (e.g., ferrocene, ruthenium pentaammine, osmium bisbipyridine, metal clusters) and organic molecules (e.g., galvinol, C60). SAMs offer an ideal environment to study the outer-sphere interactions of redox species. The composition and integrity of the monolayer and the electrode material influence the electron transfer kinetics and can be investigated using electrochemical methods. Theoretical models have been developed for investigating SAM structure. This review discusses methods and monolayer compositions for electrochemical measurements of redox-active SAMs. PMID:20563297

Strain and structure heterogeneity in MoS 2 atomic layers grown by chemical vapour deposition

DOE PAGES

Liu, Zheng; Amani, Matin; Najmaei, Sina; ...

2014-11-18

Monolayer molybdenum disulfide (MoS 2) has attracted tremendous attention due to its promising applications in high-performance field-effect transistors, phototransistors, spintronic devices, and nonlinear optics. The enhanced photoluminescence effect in monolayer MoS 2 was discovered and, as a strong tool, was employed for strain and defect analysis in MoS 2. Recently, large-size monolayer MoS 2 has been produced by chemical vapor deposition but has not yet been fully explored. Here we systematically characterize chemical vapor deposition grown MoS 2 by PL spectroscopy and mapping, and demonstrate non-uniform strain in single-crystalline monolayer MoS 2 and strain-induced band gap engineering. We also evaluatemore » the effective strain transferred from polymer substrates to MoS 2 by three-dimensional finite element analysis. In addition, our work demonstrates that PL mapping can be used as a non-contact approach for quick identification of grain boundaries in MoS 2.« less

First-principles studies of Te line-ordered alloys in a MoS2 monolayer

NASA Astrophysics Data System (ADS)

Andriambelaza, N. F.; Mapasha, R. E.; Chetty, N.

2018-04-01

The thermodynamic stability, structural and electronic properties of Te line-ordered alloys are investigated using density functional theory (DFT) methods. Thirty four possible Te line-ordered alloy configurations are found in a 5×5 supercell of a MoS2 monolayer. The calculated formation energies show that the Te line-ordered alloy configurations are thermodynamically stable at 0 K and agree very well with the random alloys. The lowest energy configurations at each concentration correspond to the configuration where the Te atom rows are far apart from each other (avoiding clustering) within the supercell. The variation of the lattice constant at different concentrations obey Vegard's law. The Te line-ordered alloys fine tune the band gap of a MoS2 monolayer although deviating from linearity behavior. Our results suggest that the Te line-ordered alloys can be an effective way to modulate the band gap of a MoS2 monolayer for nanoelectronic, optoelectronic and nanophotonic applications.

Atomic Scale Study on Growth and Heteroepitaxy of ZnO Monolayer on Graphene.

PubMed

Hong, Hyo-Ki; Jo, Junhyeon; Hwang, Daeyeon; Lee, Jongyeong; Kim, Na Yeon; Son, Seungwoo; Kim, Jung Hwa; Jin, Mi-Jin; Jun, Young Chul; Erni, Rolf; Kwak, Sang Kyu; Yoo, Jung-Woo; Lee, Zonghoon

2017-01-11

Atomically thin semiconducting oxide on graphene carries a unique combination of wide band gap, high charge carrier mobility, and optical transparency, which can be widely applied for optoelectronics. However, study on the epitaxial formation and properties of oxide monolayer on graphene remains unexplored due to hydrophobic graphene surface and limits of conventional bulk deposition technique. Here, we report atomic scale study of heteroepitaxial growth and relationship of a single-atom-thick ZnO layer on graphene using atomic layer deposition. We demonstrate atom-by-atom growth of zinc and oxygen at the preferential zigzag edge of a ZnO monolayer on graphene through in situ observation. We experimentally determine that the thinnest ZnO monolayer has a wide band gap (up to 4.0 eV), due to quantum confinement and graphene-like structure, and high optical transparency. This study can lead to a new class of atomically thin two-dimensional heterostructures of semiconducting oxides formed by highly controlled epitaxial growth.

Resolving the chemical nature of nanodesigned silica surface obtained via a bottom-up approach.

PubMed

Rahma, Hakim; Buffeteau, Thierry; Belin, Colette; Le Bourdon, Gwenaëlle; Degueil, Marie; Bennetau, Bernard; Vellutini, Luc; Heuzé, Karine

2013-08-14

The covalent grafting on silica surfaces of a functional dendritic organosilane coupling agent inserted, in a long alkyl chain monolayer, is described. In this paper, we show that depending on experimental parameters, particularly the solvent, it is possible to obtain a nanodesigned surface via a bottom-up approach. Thus, we succeed in the formation of both homogeneous dense monolayer and a heterogeneous dense monolayer, the latter being characterized by a nanosized volcano-type pattern (4-6 nm of height, 100 nm of width, and around 3 volcanos/μm(2)) randomly distributed over the surface. The dendritic attribute of the grafted silylated coupling agent affords enough anchoring sites to immobilize covalently functional gold nanoparticles (GNPs), coated with amino PEG polymer to resolve the chemical nature of the surfaces and especially the volcano type nanopattern structures of the heterogeneous monolayer. Thus, the versatile surface chemistry developed herein is particularly challenging as the nanodesign is straightforward achieved in a bottom-up approach without any specific lithography device.

C4N3H monolayer: A two-dimensional organic Dirac material with high Fermi velocity

NASA Astrophysics Data System (ADS)

Pan, Hongzhe; Zhang, Hongyu; Sun, Yuanyuan; Li, Jianfu; Du, Youwei; Tang, Nujiang

2017-11-01

Searching for two-dimensional (2D) organic Dirac materials, which have more adaptable practical applications compared with inorganic ones, is of great significance and has been ongoing. However, only two such materials with low Fermi velocity have been discovered so far. Herein, we report the design of an organic monolayer with C4N3H stoichiometry that possesses fascinating structure and good stability in its free-standing state. More importantly, we demonstrate that this monolayer is a semimetal with anisotropic Dirac cones and very high Fermi velocity. This Fermi velocity is roughly one order of magnitude larger than the largest velocity ever reported in 2D organic Dirac materials, and it is comparable to that in graphene. The Dirac states in this monolayer arise from the extended π -electron conjugation system formed by the overlapping 2 pz orbitals of carbon and nitrogen atoms. Our finding paves the way to a search for more 2D organic Dirac materials with high Fermi velocity.

Room-temperature Tamm-plasmon exciton-polaritons with a WSe2 monolayer

PubMed Central

Lundt, Nils; Klembt, Sebastian; Cherotchenko, Evgeniia; Betzold, Simon; Iff, Oliver; Nalitov, Anton V.; Klaas, Martin; Dietrich, Christof P.; Kavokin, Alexey V.; Höfling, Sven; Schneider, Christian

2016-01-01

Solid-state cavity quantum electrodynamics is a rapidly advancing field, which explores the frontiers of light–matter coupling. Metal-based approaches are of particular interest in this field, as they carry the potential to squeeze optical modes to spaces significantly below the diffraction limit. Transition metal dichalcogenides are ideally suited as the active material in cavity quantum electrodynamics, as they interact strongly with light at the ultimate monolayer limit. Here, we implement a Tamm-plasmon-polariton structure and study the coupling to a monolayer of WSe2, hosting highly stable excitons. Exciton-polariton formation at room temperature is manifested in the characteristic energy–momentum dispersion relation studied in photoluminescence, featuring an anti-crossing between the exciton and photon modes with a Rabi-splitting of 23.5 meV. Creating polaritonic quasiparticles in monolithic, compact architectures with atomic monolayers under ambient conditions is a crucial step towards the exploration of nonlinearities, macroscopic coherence and advanced spinor physics with novel, low-mass bosons. PMID:27796288

Fine structure and lifetime of dark excitons in transition metal dichalcogenide monolayers

NASA Astrophysics Data System (ADS)

Robert, C.; Amand, T.; Cadiz, F.; Lagarde, D.; Courtade, E.; Manca, M.; Taniguchi, T.; Watanabe, K.; Urbaszek, B.; Marie, X.

2017-10-01

The intricate interplay between optically dark and bright excitons governs the light-matter interaction in transition metal dichalcogenide monolayers. We have performed a detailed investigation of the "spin-forbidden" dark excitons in WSe2 monolayers by optical spectroscopy in an out-of-plane magnetic field Bz. In agreement with the theoretical predictions deduced from group theory analysis, magnetophotoluminescence experiments reveal a zero-field splitting δ =0.6 ±0.1 meV between two dark exciton states. The low-energy state is strictly dipole forbidden (perfectly dark) at Bz=0 , while the upper state is partially coupled to light with z polarization ("gray" exciton). The first determination of the dark neutral exciton lifetime τD in a transition metal dichalcogenide monolayer is obtained by time-resolved photoluminescence. We measure τD˜110 ±10 ps for the gray exciton state, i.e., two orders of magnitude longer than the radiative lifetime of the bright neutral exciton at T =12 K .

Grafting cavitands on the Si(100) surface.

PubMed

Condorelli, Guglielmo G; Motta, Alessandro; Favazza, Maria; Fragalà, Ignazio L; Busi, Marco; Menozzi, Edoardo; Dalcanale, Enrico; Cristofolini, Luigi

2006-12-19

Cavitand molecules having double bond terminated alkyl chains and different bridging groups at the upper rim have been grafted on H-terminated Si(100) surface via photochemical hydrosilylation of the double bonds. Pure and mixed monolayers have been obtained from mesitylene solutions of either pure cavitand or cavitand/1-octene mixtures. Angle resolved high-resolution X-ray photoelectron spectroscopy has been used as the main tool for the monolayer characterization. The cavitand decorated surface consists of Si-C bonded layers with the upper rim at the top of the layer. Grafting of pure cavitands leads to not-well-packed layers, which are not able to efficiently passivate the Si(100) surface. By contrast, monolayers obtained from cavitand/1-octene mixtures consist of well-packed layers since they prevent silicon oxidation after aging. AFM measurements showed that these monolayers have a structured topography, with objects protruding from the Si(100) surface with average heights compatible with the expected ones for cavitand molecules.

Active microrheology and simultaneous visualization of sheared phospholipid monolayers

PubMed Central

Choi, S.Q.; Steltenkamp, S.; Zasadzinski, J.A.; Squires, T.M.

2011-01-01

Two-dimensional films of surface-active agents—from phospholipids and proteins to nanoparticles and colloids—stabilize fluid interfaces, which are essential to the science, technology and engineering of everyday life. The 2D nature of interfaces present unique challenges and opportunities: coupling between the 2D films and the bulk fluids complicates the measurement of surface dynamic properties, but allows the interfacial microstructure to be directly visualized during deformation. Here we present a novel technique that combines active microrheology with fluorescence microscopy to visualize fluid interfaces as they deform under applied stress, allowing structure and rheology to be correlated on the micron-scale in monolayer films. We show that even simple, single-component lipid monolayers can exhibit viscoelasticity, history dependence, a yield stress and hours-long time scales for elastic recoil and aging. Simultaneous visualization of the monolayer under stress shows that the rich dynamical response results from the cooperative dynamics and deformation of liquid-crystalline domains and their boundaries. PMID:21587229

Established breast cancer stem cell markers do not correlate with in vivo tumorigenicity of tumor-initiating cells

PubMed Central

LEHMANN, CHRISTIAN; JOBS, GABRIELE; THOMAS, MARKUS; BURTSCHER, HELMUT; KUBBIES, MANFRED

2012-01-01

The tumor-initiating capacity of primary human breast cancer cells is maintained in vitro by culturing these cells as spheres/aggregates. Inoculation of small cell numbers derived from these non-adherent cultures leads to rapid xenograft tumor formation in mice. Accordingly, injection of more differentiated monolayer cells derived from spheres results in significantly decelerated tumor growth. For our study, two breast cancer cell lines were generated from primary tumors and cultured as mammospheres or as their adherent counterparts. We examined the in vivo tumorigenicity of these cells by injecting serial dilutions into immunodeficient mice. Inoculation of 106 cells per mouse led to rapid tumor formation, irrespective of cell line or culture conditions. However, after injection of only 103 cells, solely sphere cells were highly tumorigenic. In vitro, we investigated differentiation markers, established breast CSC markers and conducted mRNA profiling. Cytokeratin 5 and 18 were increased in both monolayer cell types, indicating a more differentiated phenotype. All cell lines were CD24−/CD44+ and did not express CD133, CD326 or E-cadherin. ALDH1 activity was not detectable in any cell line. A verapamil-sensitive Hoechst side population was present in sphere cells, but there was no correlation with tumorigenicity in vivo. mRNA profiling did not reveal upregulation of relevant transcription factors. In vitro cell cycle kinetics and in vivo tumor doubling times displayed no difference between sphere and monolayer cultures. Our data indicate that intrinsic genetic and functional markers investigated are not indicative of the in vivo tumori-genicity of putative breast tumor-initiating cells. PMID:23042145

Film growth and structure design in the barium oxide-strontium oxide-titanium dioxide system

NASA Astrophysics Data System (ADS)

Fisher, Patrick J.

This thesis describes the growth and characterization of thin films in the SrO-BaO-TiO2 system. The films are grown by molecular beam cpitaxy (MBE) and pulsed laser deposition (PLD) on ceramic substrates, and characterized using X-ray diffraction (XRD), atomic force microscopy (AFM), reflection-high energy electron diffraction (RHEED), and transmission electron microscopy (TEM). Films are grown with varied global and initial local stoichiometries, with the goal of determining the stability of specific cation organizations. Simple oxides, TiO2 (anatase) and SrO (rock salt) were grown on oxide substrates using MBE. Growth conditions, including substrate material, substrate temperature, O3 flux, and metal flux, are varied in each case. It is observed that the growth morphology of anatase is highly dependent on the ozone flux, with fluxes of 1.00 sccm and greater resulting in flat anatase surfaces. Increased roughness at higher substrate was determined to be a result of rutile inclusions. Growth oscillations are observed in the RHEED intensity for both TiO2 and SrO in overlapping regions of growth space, indicating 2D growth modes. Varied shuttering sequences were used during MBE growth of perovskites: globally non-stoichiometric films, as well as locally non-stoichiometric but globally stoichiometric perovskite. Films were grown within a (SrO) m(TiO2)n framework, where growth cycles involved m monolayers of SrO followed by n monolayers of TiO2. XRD results indicate that Ruddlesden-Popper defects, that is, rock salt double layers, enable incorporation of all levels of Sr excess, whereas excess Ti is observed to incorporate into the perovskite structure only at extreme excesses. A series of films with m equal to n were grown; that is, multiple monolayers of SrO deposited followed by multiple monolayers of TiO2. These initially locally non-stoichiometric arrangements interreact to form highly crystalline perovskite, even with layer thicknesses of up to 33 monolayers. The Ba0.6Sr0.4TiO3 films were characterized for their microwave dielectric properties, and were found to have high dielectric constants (epsilonr ˜1300 in each case, implying high tunabilities) but high tan delta values as well. The mechanisms by which the perovskite structure incorporates cation excesses is discussed, and it is argued that two probable mechanisms, one involving plane-sharing of Ti and Sr cations and the other involving rock salt multilayers, also enable the observed transport necessary for multilayer reaction. Working under the argument that these mechanisms involve low-energy architectures, a novel homologous series of phases based on rock salt multilayers is grown using monotayer control: the SrmTiO2+ m series, with each TiO2 monolayer followed by m SrO monolayers (m = 1-5). The phases in this series were characterized structurally, and an in-plane contraction was observed between the m = 2 and m = 3 phases, which is argued to be a relaxation of the SrO monolayers. Considering Ti-excess organizations, the BaTi2O5 structure is grown and observed to nucleate over a narrow window of growth conditions and substrates. LaAlO 3(100) promotes the nucleation of anatasc and ejection of perovskite; SrTiO3(100) promotes the nucleation of perovskite and ejection of TiO2; importantly, MgO(100) promotes the nucleation (010)-oriented BaTi2O5 growing with multiple domains. A BaTi2 O5 buffer layer was then used to promote the inclusion of Sr into (Ba,SOTi205 epilayers. Sr incorporation into a perovskite-related structure was observed to occur over the full range of (Ba,Sr)Ti2O 5 compositions.

Collective cell migration without proliferation: density determines cell velocity and wave velocity

NASA Astrophysics Data System (ADS)

Tlili, Sham; Gauquelin, Estelle; Li, Brigitte; Cardoso, Olivier; Ladoux, Benoît; Delanoë-Ayari, Hélène; Graner, François

2018-05-01

Collective cell migration contributes to embryogenesis, wound healing and tumour metastasis. Cell monolayer migration experiments help in understanding what determines the movement of cells far from the leading edge. Inhibiting cell proliferation limits cell density increase and prevents jamming; we observe long-duration migration and quantify space-time characteristics of the velocity profile over large length scales and time scales. Velocity waves propagate backwards and their frequency depends only on cell density at the moving front. Both cell average velocity and wave velocity increase linearly with the cell effective radius regardless of the distance to the front. Inhibiting lamellipodia decreases cell velocity while waves either disappear or have a lower frequency. Our model combines conservation laws, monolayer mechanical properties and a phenomenological coupling between strain and polarity: advancing cells pull on their followers, which then become polarized. With reasonable values of parameters, this model agrees with several of our experimental observations. Together, our experiments and model disantangle the respective contributions of active velocity and of proliferation in monolayer migration, explain how cells maintain their polarity far from the moving front, and highlight the importance of strain-polarity coupling and density in long-range information propagation.

Manifestly covariant classical correlation dynamics I. General theory

NASA Astrophysics Data System (ADS)

Lin, Shiru; Wang, Yanchao; Chen, Zhongfang

2018-06-01

By means of density functional theory (DFT) computations and particle-swarm optimization (PSO) structure searches, we herein predict five low-lying energy structures of two-dimensional (2D) aluminum monoxide (AlO) nanosheets. Their high cohesive energy, absence of imaginary phonon dispersion, and good thermal stability make them feasible targets for experimental realization. These monolayers exhibit diverse structural topologies, for instance, PmA- and Pmm-AlO possess buckled four- and six-membered AlO rings, whereas P62-, PmB-, and P6 m-AlO have pores of varied sizes. Interestingly, the most energetically preferred monolayers, PmA- and Pmm-AlO, feature wide band gaps (2.45 and 5.13 eV, respectively), which are promising for green and blue light-emitting devices (LEDs) and photodetectors.

Boltzmann transport properties of ultra thin-layer of h-CX monolayers

NASA Astrophysics Data System (ADS)

Kansara, Shivam; Gupta, Sanjeev K.; Sonvane, Yogesh

2018-04-01

Structural, electronic and thermoelectric properties of monolayer h-CX (X= Al, As, B, Bi, Ga, In, P, N, Sb and Tl) have been computed using density functional theory (DFT). The structural, electronic band structure, phonon dispersion curves and thermoelectric properties have been investigated. h-CGa and h-CTl show the periodically lattice vibrations and h-CB and h-CIn show small imaginary ZA frequencies. Thermoelectric properties are obtained using BoltzTrap code with the constant relaxation time (τ) approximation such as electronic, thermal and electrical conductivity calculated for various temperatures. The results indicate that h-CGa, h-CIn, h-CTl and h-CAl have direct band gaps with minimum electronic thermal and electrical conductivity while h-CB and h-CN show the high electronic thermal and electrical conductivity with highest cohesive energy.

Computational characterization of lightweight multilayer MXene Li-ion battery anodes

NASA Astrophysics Data System (ADS)

Ashton, Michael; Hennig, Richard G.; Sinnott, Susan B.

2016-01-01

MXenes, a class of two-dimensional transition metal carbides and nitrides, have shown promise experimentally and computationally for use in energy storage applications. In particular, the most lightweight members of the monolayer MXene family (M = Sc, Ti, V, or Cr) are predicted to have gravimetric capacities above 400 mAh/g, higher than graphite. Additionally, intercalation of ions into multilayer MXenes can be accomplished at low voltages, and low diffusion barriers exist for Li diffusing across monolayer MXenes. However, large discrepancies have been observed between the calculated and experimental reversible capacities of MXenes. Here, dispersion-corrected density functional theory calculations are employed to predict reversible capacities and other battery-related properties for six of the most promising members of the MXene family (O-functionalized Ti- and V-based carbide MXenes) as bilayer structures. The calculated reversible capacities of the V2CO2 and Ti2CO2 bilayers agree more closely with experiment than do previous calculations for monolayers. Additionally, the minimum energy paths and corresponding energy barriers along the in-plane [1000] and [0100] directions for Li travelling between neighboring MXene layers are determined. V4C3O2 exhibits the lowest diffusion barrier of the compositions considered, at 0.42 eV, but its reversible capacity (148 mAh/g) is dragged down by its heavy formula unit. Conversely, the V2CO2 MXene shows good reversible capacity (276 mAh/g), but a high diffusion barrier (0.82 eV). We show that the diffusion barriers of all bilayer structures are significantly higher than those calculated for the corresponding monolayers, advocating the use of dispersed monolayer MXenes instead of multilayers in high performance anodes.

Computational studies at the density functional theory (DFT) level about the surface functionalization of hexagonal monolayers by chitosan monomer

NASA Astrophysics Data System (ADS)

Ebrahimi, Javad; Ahangari, Morteza Ghorbanzadeh; Jahanshahi, Mohsen

2018-05-01

Theoretical investigations based on density functional theory have been carried out to understand the underlying interactions between the chitosan monomer and several types of hexagonal monolayers consisting of pristine and defected graphene and boron-nitride nanosheets. Based on the obtained results, it was found that the type of the interaction for all the systems is of non-covalent nature and the chitosan monomer physically interacts with the surface of mentioned nanostructures. The interaction strength was evaluated by calculating the adsorption energies for the considered systems and it was found that the adsorption of chitosan monomer accompanies by the release of about -0.67 and -0.66 eV energy for pristine graphene and h-BN monolayer, respectively. The role of structural defect has also been considered by embedding a Stone-Wales defect within the structure of mentioned monolayers and it was found that the introduced defect enhances the interactions between the chitosan monomer and nanostructures. The role of dispersion interactions has also been taken into account and it was found that these long-range interactions play the dominating role in the attachment of chitosan monomer onto the graphene sheet, while having strong contribution together with the electrostatic interactions for the stabilization of chitosan onto the surface of h-BN monolayer. For all the cases, the adsorption of chitosan monomer did not change the inherent electronic properties of the nanostructures based on the results of charge transfer analysis and energy gap calculations. The findings of the present work would be very useful in future investigations to explore the potential applications of these hybrid materials in materials science and bio-related fields.

Influence of calcium on ceramide-1-phosphate monolayers

PubMed Central

Brezesinski, Gerald; Hill, Alexandra; Gericke, Arne

2016-01-01

Summary Ceramide-1-phosphate (C1P) plays an important role in several biological processes, being identified as a key regulator of many protein functions. For instance, it acts as a mediator of inflammatory responses. The mediation of the inflammation process happens due to the interaction of C1P with the C2 domain of cPLA2α, an effector protein that needs the presence of submicromolar concentrations of calcium ions. The aim of this study was to determine the phase behaviour and structural properties of C1P in the presence and absence of millimolar quantities of calcium in a well-defined pH environment. For that purpose, we used monomolecular films of C1P at the soft air/liquid interface with calcium ions in the subphase. The pH was varied to change the protonation degree of the C1P head group. We used surface pressure versus molecular area isotherms coupled with other monolayer techniques as Brewster angle microscopy (BAM), infrared reflection–absorption spectroscopy (IRRAS) and grazing incidence X-ray diffraction (GIXD). The isotherms indicate that C1P monolayers are in a condensed state in the presence of calcium ions, regardless of the pH. At higher pH without calcium ions, the monolayer is in a liquid-expanded state due to repulsion between the negatively charged phosphate groups of the C1P molecules. When divalent calcium ions are added, they are able to bridge the highly charged phosphate groups, enhancing the regular arrangement of the head groups. Similar solidification of the monolayer structure can be seen in the presence of a 150 times larger concentration of monovalent sodium ions. Therefore, calcium ions have clearly a strong affinity for the phosphomonoester of C1P. PMID:26977381

Macroscopic monolayer of plasmon coupled gold nanoparticles on mirror for fluorescence enhancement

NASA Astrophysics Data System (ADS)

Kaydashev, V. E.; Zolotukhin, P.; Belanova, A.; Anokhin, A. S.; Zharinov, V. S.; Kaidashev, E. M.

2018-04-01

We study an ability of a large quasi-homogeneous monolayer of Au plasmon coupled nanoparticles separated from continuous Au film by polymer spacer to enhance a fluorescence of adsorbed molecular species. A fluorescence response of Methylene Blue molecules is studied as a function of polymer film thickness. A change of plasmonic properties of a system, its ability to enhance a fluorescence and the possible heating of a structure upon light absorption are discussed.

Turning things downside up: Adsorbate induced water flipping on Pt(111)

DOE Office of Scientific and Technical Information (OSTI.GOV)

Kimmel, Greg A., E-mail: gregory.kimmel@pnnl.gov, E-mail: bruce.kay@pnnl.gov; Zubkov, Tykhon; Smith, R. Scott

2014-11-14

We have examined the adsorption of the weakly bound species N{sub 2}, O{sub 2}, CO, and Kr on the (√(37)×√(37))R25.3{sup ∘} water monolayer on Pt(111) using a combination of molecular beam dosing, infrared reflection absorption spectroscopy, and temperature programmed desorption. In contrast to multilayer crystalline ice, the adsorbate-free water monolayer is characterized by a lack of dangling OH bonds protruding into the vacuum (H-up). Instead, the non-hydrogen-bonded OH groups are oriented downward (H-down) to maximize their interaction with the underlying Pt(111) substrate. Adsorption of Kr and O{sub 2} have little effect on the structure and vibrational spectrum of the “√(37)”more » water monolayer while adsorption of both N{sub 2}, and CO are effective in “flipping” H-down water molecules into an H-up configuration. This “flipping” occurs readily upon adsorption at temperatures as low as 20 K and the water monolayer transforms back to the H-down, “√(37)” structure upon adsorbate desorption above 35 K, indicating small energy differences and barriers between the H-down and H-up configurations. The results suggest that converting water in the first layer from H-down to H-up is mediated by the electrostatic interactions between the water and the adsorbates.« less

Molecular chirality and domain shapes in lipid monolayers on aqueous surfaces

NASA Astrophysics Data System (ADS)

Krüger, Peter; Lösche, Mathias

2000-11-01

The shapes of domain boundaries in the mesoscopic phase separation of phospholipids in aqueous surface monolayers are analyzed with particular attention to the influence of molecular chirality. We have calculated equilibrium shapes of such boundaries, and show that the concept of spontaneous curvature-derived from an effective pair potential between the chiral molecules-yields an adequate description of the contribution of chirality to the total energy of the system. For enantiomeric dipalmitoylphosphatidylcholine in pure monolayers, and in mixtures with impurities that adsorb preferentially at the (one-dimensional) boundary line between the isotropic and anisotropic fluid phases, such as cyanobiphenyl (5CB), a total energy term that includes line tension, electrostatic dipole-dipole interaction, and spontaneous curvature is sufficient to describe the shapes of well-separated domain boundaries in full detail. As soon as interdomain distances fall below the domain sizes upon compression of a monolayer, fluctuations take over in determining its detailed structural morphology. Using Minkowski measures for the well-studied dimyristoyl phosphatidic acid (DMPA)/cholesterol system, we show that calculations accounting for line tension, electrostatic repulsion, and molecular chirality yield boundary shapes that are of the same topology as the experimentally observed structures. At a fixed molecular area in the phase coexistence region, the DMPA/cholesterol system undergoes an exponential decay of the line tension λ with decreasing subphase temperature T.

Electronic, Magnetic and Optical Properties of 2D Metal Nanolayers: A DFT Study

NASA Astrophysics Data System (ADS)

Bhuyan, Prabal Dev; Gupta, Sanjeev K.; Singh, Deobrat; Sonvane, Yogesh; Gajjar, P. N.

2018-03-01

In the recent work, we have investigated the structural, electronic, magnetic and optical properties of graphene-like hexagonal monolayers and multilayers (up to five layers) of 3d-transition metals Fe, Co and Ni based on spin-polarized density functional theory. Here, we have taken two types of pattern namely AA-stacking and AB-stacking for the calculations. The binding energy calculations show that the AA-type configuration is energetically more stable. The calculated binding energies of Fe, Co and Ni-bilayer monolayer are - 3.24, - 2.53 and - 1.94 eV, respectively. The electronic band structures show metallic behavior for all the systems and each configurations of Fe, Co and Ni-atoms. While, the quantum ballistic conductances of these metallic systems are found to be higher for pentalayer than other layered systems. The density of states confirms the ferromagnetic behavior of monolayers and multilayers of Fe and Co having negative spin polarizations. We have also calculated frequency dependent complex dielectric function, electronic energy loss spectrum and reflectance spectrum of monolayer to pentalayer metallic systems. The ferromagnetic material shows different permittivity tensor (ɛ), which is due to high spin magnetic moment for n-layered Fe and Co two-dimensional (2D) nanolayers. The theoretical investigation suggests that the electronic, magnetic and optical properties of 3d-transition metal nanolayers offers great promise for their use in spintronics nanodevices and magneto-optical nanodevices applications.

Monolayer nanoparticle-covered liquid marbles derived from a sol-gel coating

NASA Astrophysics Data System (ADS)

Li, Xiaoguang; Wang, Yiqi; Huang, Junchao; Yang, Yao; Wang, Renxian; Geng, Xingguo; Zang, Duyang

2017-12-01

A sol-gel coating consisting of hydrophobic SiO2 nanoparticles (NPs) was used to produce monolayer NP-covered (mNPc) liquid marbles. The simplest approach was rolling a droplet on this coating, and an identifiable signet allowed determination of the coverage ratio of the resulting liquid marble. Alternatively, the particles were squeezed onto a droplet surface with two such coatings, generating surface buckling from interfacial NP jamming, and then a liquid marble was produced via a jamming-relief process in which water was added into the buckled droplet. This process revealed an ˜7% reduction in particle distance after interfacial jamming. The mNPc liquid marbles obtained by the two methods were transparent with smooth profiles, as naked droplets, and could be advantageously used in fundamental and applied researches for their unique functions.

Characterization of TEM Moiré Patterns Originating from Two Monolayer Graphenes Grown on the Front and Back Sides of a Copper Substrate by CVD Method

NASA Astrophysics Data System (ADS)

Yamazaki, Kenji; Maehara, Yosuke; Gohara, Kazutoshi

2018-06-01

The number of layers affects the electronic properties of graphene owing to its unique band structure, called the Dirac corn. Raman spectroscopy is a key diagnostic tool for identifying the number of graphene layers and for determining their physical properties. Here, we observed moiré structures in transmission electron microscopy (TEM) observations; these are signature patterns in multilayer, although Raman spectra showed the typical intensity of the 2D/G peak in the monolayer. We also performed a multi-slice TEM image simulation to compare the 3D atomic structures of the two graphene membranes with experimental TEM images. We found that the experimental moiré image was constructed with a 9-12 Å interlayer distance between graphene membranes. This structure was constructed by transferring CVD-grown graphene films that formed on both sides of the Cu substrate at once.

Self-organization processes and topological defects in nanolayers in a nematic liquid crystal

DOE Office of Scientific and Technical Information (OSTI.GOV)

Chuvyrov, A. N.; Girfanova, F. M.; Mal'tsev, I. S.

Atomic force microscopy is used to study the self-organization processes that occur during the formation of topological defects in nanomolecular layers in a nematic liquid crystal with the homeotropic orientation of its molecules with respect to the substrate. In this case, a smectic monolayer with a thickness of one molecule length (about 2.2 nm) forms on the substrate, and a nanomolecular layer of a nematic liquid crystal forms above this monolayer. In such virtually two-dimensional layers, numerous different nanoclusters, namely, hut structures, pyramids, raft structures with symmetry C{sub nm} (where n = 2, 4, 5, 6, 7, ?, {infinity}), cones,more » and nanopools, form [1]. They have a regular shape close to the geometry of solid crystals. Modulated linear structures and topological point defects appear spontaneously in the nanopools and raft structures.« less

Growth and stability of Langmuir-Blodgett films on OH-, H-, or Br-terminated Si(001)

NASA Astrophysics Data System (ADS)

Bal, J. K.; Kundu, S.; Hazra, S.

2010-01-01

Growth of Langmuir-Blodgett (LB) films of nickel arachidate (NiA) on differently terminated (OH-, H-, or Br-terminated) Si(001) substrates and their structural evolution with time have been investigated by x-ray reflectivity technique and complemented by atomic force microscopy. Stable and strongly attached asymmetric monolayer (AML) of NiA is found to grow on freshly prepared oxide-covered Si substrate while unstable and weakly attached symmetric monolayer (SML) of NiA grows on H-terminated Si substrate, corresponding to stable hydrophilic and unstable hydrophobic natures of the substrates, respectively. The structure of LB film on Br-terminated Si substrate, however, shows intermediate behavior, namely, both AML and SML are present on the substrate, indicative of coexisting (hydrophilic and hydrophobic) nature of this terminated surface. Such coexisting nature of the substrate shows unusual growth behavior of LB films: (i) hydrophilic and hydrophobic attachments of NiA molecules in single up stroke of deposition and (ii) growth of few ring-shaped large-heights islands in subsequent deposition. These probably occur due to the presence of substrate-induced perturbation in the Langmuir monolayer and release of initially accumulated strain in the film structures near hydrophilic/hydrophobic interface, respectively, and provide the possibility to grow desired structures (AML or SML) of LB films by passivation-selective surface engineering.

Direct measurements of multiple adhesive alignments and unbinding trajectories between cadherin extracellular domains.

PubMed Central

Sivasankar, S; Gumbiner, B; Leckband, D

2001-01-01

Direct measurements of the interactions between antiparallel, oriented monolayers of the complete extracellular region of C-cadherin demonstrate that, rather than binding in a single unique orientation, the cadherins adhere in three distinct alignments. The strongest adhesion is observed when the opposing extracellular fragments are completely interdigitated. A second adhesive alignment forms when the interdigitated proteins separate by 70 +/- 10 A. A third complex forms at a bilayer separation commensurate with the approximate overlap of cadherin extracellular domains 1 and 2 (CEC1-2). The locations of the energy minima are independent of both the surface density of bound cadherin and the stiffness of the force transducer. Using surface element integration, we show that two flat surfaces that interact through an oscillatory potential will exhibit discrete minima at the same locations in the force profile measured between hemicylinders covered with identical materials. The measured interaction profiles, therefore, reflect the relative separations at which the antiparallel proteins adhere, and are unaffected by the curvature of the underlying substrate. The successive formation and rupture of multiple protein contacts during detachment can explain the observed sluggish unbinding of cadherin monolayers. Velocity-distance profiles, obtained by quantitative video analysis of the unbinding trajectory, exhibit three velocity regimes, the transitions between which coincide with the positions of the adhesive minima. These findings suggest that cadherins undergo multiple stage unbinding, which may function to impede adhesive failure under force. PMID:11259289

Vibrational Studies of Saccharide-Induced Lipid Film Reorganization at Aqueous/Air Interfaces

DOE PAGES

Link, Katie A.; Hsieh, Chia -Yun; Tuladhar, Aashish; ...

2018-02-09

Vibrational sum frequency generation (VSFG) and surface tension experiments were used to examine the effects of aqueous phase soluble saccharides on the structure and organization of insoluble lipid monolayers adsorbed to aqueous-air interfaces. Changes in dipalmitoylphosphocholine (DPPC) chain structure as a function of aqueous phase saccharide concentration and pH are reported. Complementary differential scanning calorimetry (DSC) measurements performed on solutions containing soluble saccharides and DPPC vesicles measured the effects of the saccharides on the lipid membrane phase behavior. Here, data show that the saccharides glucosamine and glucuronic acid induce a higher degree of organization in compressed DPPC monolayers regardless ofmore » the saccharide’s charge.« less

Water-soluble Au13 clusters protected by binary thiolates: Structural accommodation and the use for chemosensing

NASA Astrophysics Data System (ADS)

Ding, Weihua; Huang, Chuanqi; Guan, Lingmei; Liu, Xianhu; Luo, Zhixun; Li, Weixue

2017-05-01

Here we report a successful synthesis of water-soluble 13-atoms gold clusters under the monolayer protection of binary thiolates, glutathione and penicillamine, under a molecular formula of Au13(SG)5(PA)7. This monolayer-protected cluster (MPC) finds decent stability and is demonstrated to possess an icosahedral geometry pertaining to structural accommodation in contrast to a planar bare Au13 of local minima energy. Natural bond orbital (NBO) analysis depicts the interaction patterns between gold and the ligands, enlightening to understand the origin of enhanced stability of the Au13 MPCs. Further, the water-soluble Au13 MPCs are found to be a decent candidate for chemosensing and bioimaging.

Vibrational Studies of Saccharide-Induced Lipid Film Reorganization at Aqueous/Air Interfaces

DOE Office of Scientific and Technical Information (OSTI.GOV)

Link, Katie A.; Hsieh, Chia -Yun; Tuladhar, Aashish

Vibrational sum frequency generation (VSFG) and surface tension experiments were used to examine the effects of aqueous phase soluble saccharides on the structure and organization of insoluble lipid monolayers adsorbed to aqueous-air interfaces. Changes in dipalmitoylphosphocholine (DPPC) chain structure as a function of aqueous phase saccharide concentration and pH are reported. Complementary differential scanning calorimetry (DSC) measurements performed on solutions containing soluble saccharides and DPPC vesicles measured the effects of the saccharides on the lipid membrane phase behavior. Here, data show that the saccharides glucosamine and glucuronic acid induce a higher degree of organization in compressed DPPC monolayers regardless ofmore » the saccharide’s charge.« less

Heterogeneous Amyloid β-Sheet Polymorphs Identified on Hydrogen Bond Promoting Surfaces Using 2D SFG Spectroscopy.

PubMed

Ho, Jia-Jung; Ghosh, Ayanjeet; Zhang, Tianqi O; Zanni, Martin T

2018-02-08

Two-dimensional sum-frequency generation spectroscopy (2D SFG) is used to study the structures of the pentapeptide FGAIL on hydrogen bond promoting surfaces. FGAIL is the most amyloidogenic portion of the human islet amyloid polypeptide (hIAPP or amylin). In the presence of a pure gold surface, FGAIL does not form ordered structures. When the gold is coated with a self-assembled monolayer of mercaptobenzoic acid (MBA), 2D SFG spectra reveal features associated with β-sheets. Also observed are cross peaks between the FGAIL peptides and the carboxylic acid groups of the MBA monolayer, indicating that the peptides are in close contact with the surface headgroups. In the second set of samples, FGAIL peptides chemically ligated to the MBA monolayer also exhibited β-sheet features but with a much simpler spectrum. From simulations of the experiments, we conclude that the hydrogen bond promoting surface catalyzes the formation of both parallel and antiparallel β-sheet structures with several different orientations. When ligated, parallel sheets with only a single orientation are the primary structure. Thus, this hydrogen bond promoting surface creates a heterogeneous distribution of polymorph structures, consistent with a concentration effect that allows nucleation of many different amyloid seeding structures. A single well-defined seed favors one polymorph over the others, showing that the concentrating influence of a membrane can be counterbalanced by factors that favor directed fiber growth. These experiments lay the foundation for the measurement and interpretation of β-sheet structures with heterodyne-detected 2D SFG spectroscopy. The results of this model system suggest that a heterogeneous distribution of polymorphs found in nature are an indication of nonselective amyloid aggregation whereas a narrow distribution of polymorph structures is consistent with a specific protein or lipid interaction that directs fiber growth.

The Scope of Direct Alkylation of Gold Surface with Solutions of C1-C4 n-Alkylstannanes.

PubMed

Kaletová, Eva; Kohutová, Anna; Hajduch, Jan; Kaleta, Jiří; Bastl, Zdeněk; Pospíšil, Lubomír; Stibor, Ivan; Magnera, Thomas F; Michl, Josef

2015-09-23

Treatment of cleaned gold surfaces with dilute tetrahydrofuran or chloroform solutions of tetraalkylstannanes (alkyl = methyl, ethyl, n-propyl, n-butyl) or di-n-butylmethylstannyl tosylate under ambient conditions causes a self-limited growth of disordered monolayers consisting of alkyls and tin oxide. Extensive use of deuterium labeling showed that the alkyls originate from the stannane and not from ambient impurities, and that trialkylstannyl groups are absent in the monolayers, contrary to previous proposals. Methyl groups attached to the Sn atom are not transferred to the surface. Ethyl groups are transferred slowly, and propyl and butyl rapidly. In all cases, tin oxide is codeposited in submonolayer amounts. The monolayers were characterized by ellipsometry, contact angle goniometry, polarization modulated IR reflection absorption spectroscopy, X-ray photoelectron spectroscopy, and electrochemical impedance spectroscopy with ferrocyanide/ferricyanide, which revealed a very low charge-transfer resistance. The thermal stability of the monolayers and their resistance to solvents are comparable with those of an n-octadecanethiol monolayer. A preliminary examination of the kinetics of monolayer deposition from a THF solution of tetra-n-butylstannane revealed an approximately half-order dependence on the bulk solution concentration of the stannane, hinting that more than one alkyl can be transferred from a single stannane molecule. A detailed structure of the attachment of the alkyl groups is not known, and it is proposed that it involves direct single or multiple bonding of one or more C atoms to one or more Au atoms.

A distinct correlation between the vibrational and thermal transport properties of group VA monolayer crystals.

PubMed

Kocabaş, Tuğbey; Çakır, Deniz; Gülseren, Oğuz; Ay, Feridun; Kosku Perkgöz, Nihan; Sevik, Cem

2018-04-26

The investigation of thermal transport properties of novel two-dimensional materials is crucially important in order to assess their potential to be used in future technological applications, such as thermoelectric power generation. In this respect, the lattice thermal transport properties of the monolayer structures of group VA elements (P, As, Sb, Bi, PAs, PSb, PBi, AsSb, AsBi, SbBi, P3As1, P3Sb1, P1As3, and As3Sb1) with a black phosphorus like puckered structure were systematically investigated by first-principles calculations and an iterative solution of the phonon Boltzmann transport equation. Phosphorene was found to have the highest lattice thermal conductivity, κ, due to its low average atomic mass and strong interatomic bonding character. As a matter of course, anisotropic κ was obtained for all the considered materials, owing to anisotropy in frequency values and phonon group velocities calculated for these structures. However, the determined linear correlation between the anisotropy in the κ values of P, As, and Sb is significant. The results corresponding to the studied compound structures clearly point out that thermal (electronic) conductivity of pristine monolayers might be suppressed (improved) by alloying them with the same group elements. For instance, the room temperature κ of PBi along the armchair direction was predicted to be as low as 1.5 W m-1 K-1, whereas that of P was predicted to be 21 W m-1 K-1. In spite of the apparent differences in structural and vibrational properties, we peculiarly revealed an intriguing correlation between the κ values of all the considered materials as κ = c1 + c2/m2, in particular along the zigzag direction. Furthermore, our calculations on compound structures clearly showed that the thermoelectric potential of these materials can be improved by suppressing their thermal properties. The presence of ultra-low κ values and high electrical conductivity (especially along the armchair direction) makes this class of monolayers promising candidates for thermoelectric applications.

IMMUNOBLOT ANALYSIS OF PROTEINS ASSOCIATED WITH SELF-ASSEMBLED MONOLAYER SURFACES OF DEFINED CHEMISTRIES

PubMed Central

Cornelius, Rena M.; Shankar, Sucharita P.; Brash, John L.; Babensee, Julia E.

2011-01-01

Intact and fragmented proteins, eluted from self assembled monolayer (SAM) surfaces of alkanethiols of different chemistries (-CH3, -OH, -COOH, -NH2 ), following exposure to human plasma (HP) or human serum (HS), were examined using sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE) and immunoblotting techniques. The SAM surfaces were incubated for 1 hour with 10% (v/v) sterile-filtered heat-inactivated (h.i.) HS or 1% (v/v) sterile-filtered h.i. HP preparations [both in phosphate buffered saline (PBS)]. Adsorbed proteins were eluted using 10% SDS/2.3% dithioerythritol for characterization of protein profiles. The type of incubating medium may be an important determinant of adsorbed protein profiles, since some variations were observed in eluates from filtered versus control unfiltered h.i. 10% HS or 1% HP. Albumin and apolipoprotein A1 were consistently detected in both filtered h.i 10% HS and 1% HP eluates from all SAM surfaces and from control tissue culture-treated polystyrene (TCPS). Interestingly, Factor H and Factor I, antithrombin, prothrombin, high molecular weight kininogen (HMWK) and IgG were present in eluates from OH, COOH and NH2 SAM surfaces and in eluates from TCPS, but not in eluates from CH3 SAM surfaces, following exposure to filtered h.i. 10% HS. These results suggest that CH3 SAM surfaces were the least pro-inflammatory of all SAM surfaces. Overall, similar trends were observed in the profiles of proteins eluted from surfaces exposed to filtered 10% HS or 1% HP. However the unique profiles of adsorbed proteins on different SAM surface chemistries may be related to their differential interactions with cells, including immune/inflammatory cells. PMID:21509932

Phase behavior of stratum corneum lipids in mixed Langmuir-Blodgett monolayers.

PubMed Central

ten Grotenhuis, E; Demel, R A; Ponec, M; Boer, D R; van Miltenburg, J C; Bouwstra, J A

1996-01-01

The lipids found in the bilayers of the stratum corneum fulfill the vital barrier role of mammalian bodies. The main classes of lipids found in stratum corneum are ceramides, cholesterol, and free fatty acids. For an investigation of their phase behavior, mixed Langmuir-Blodgett monolayers of these lipids were prepared. Atomic force microscopy was used to investigate the structure of the monolayers as a function of the monolayer composition. Three different types of ceramide were used: ceramide extracted from pigskin, a commercially available ceramide with several fatty acid chain lengths, and two synthetic ceramides that have only one fatty acid chain length. In pigskin ceramide-cholesterol mixed monolayers phase separation was observed. This phase separation was also found for the commercially available type III Sigma ceramide-cholesterol mixed monolayers with molar ratios ranging from 1:0.1 to 1:1. These monolayers separated into two phases, one composed of the long fatty acid chain fraction of Sigma ceramide III and the other of the short fatty acid chain fraction of Sigma ceramide III mixed with cholesterol. Mixtures with a higher cholesterol content consisted of only one phase. These observations were confirmed by the results obtained with synthetic ceramides, which have only one fatty acid chain length. The synthetic ceramide with a palmitic acid (16:0) chain mixed with cholesterol, and the synthetic ceramide with a lignoceric acid (24:0) chain did not. Free fatty acids showed a preference to mix with one of these phases, depending on their fatty acid chain lengths. The results of this investigation suggest that the model system used in this study is in good agreement with those of other studies concerning the phase behavior of the stratum corneum lipids. By varying the composition of the monolayers one can study the role of each lipid class in detail. Images FIGURE 2 FIGURE 3 FIGURE 4 FIGURE 5 FIGURE 6 PMID:8874014

Effects of external magnetic field and out-of-plane strain on magneto-optical Kerr spectra in CrI3 monolayer.

PubMed

Guo, Guanxing; Bi, Gang; Cai, Chunfeng; Wu, Huizhen

2018-07-18

Magnetic semiconductors based on two-dimensional (2D) crystals have attracted attention owing to their intrinsic ferromagnetism and have potential for spintronic devices. Here, full-potential linearized augmented plane wave plus local orbitals method is used to explore the structural, electronic, magnetic, and magneto-optical properties of CrI 3 monolayer. Our first-principles calculations show that CrI 3 monolayer is a ferromagnetic indirect semiconductor with spin-up and spin-down band gaps of 1.23 and 1.90 eV, respectively, and a magnetic moment of 2.93 [Formula: see text] per Cr atom. Based on the macroscopic linear response theory, we systematically study the influences of external magnetic field and out-of-plane strain on the magneto-optical Kerr effect spectra in CrI 3 monolayer. The Kerr rotation of CrI 3 monolayer at 1.96 eV photon energy is [Formula: see text], which is consistent with the recent experiments. We find that the Kerr rotation reaches its maximum when the external magnetic field is perpendicular to CrI 3 plane, while it is almost zero on turning the magnetic field in the plane. This result as well as the sizable magnetocrystalline anisotropy energy (MAE) of 0.79 meV verifies that CrI 3 monolayer has a strong magnetic anisotropy with an out-of-plane easy axis. Further, applying out-of-plane compressive and tensile strain upon CrI 3 monolayer, we observe a redshift of the Kerr rotation spectra with the increase of the strain and the peak values of the Kerr rotation increase correspondingly. The rich electronic and magnetic properties, especially the magneto-optical spectra, render CrI 3 monolayer a promising 2D magnetic material for applications from sensing to data storage.

Commensurability condition and hierarchy of fillings for FQHE in higher Landau levels in conventional 2DEG systems and in graphene—monolayer and bilayer

NASA Astrophysics Data System (ADS)

Jacak, Janusz; Jacak, Lucjan

2016-01-01

The structure of the filling rate hierarchy referred to as the fractional quantum Hall effect is studied in higher Landau levels using the commensurability condition. The hierarchy of fillings that are derived in this manner is consistent with the experimental observations of the first three Landau levels in conventional semiconductor Hall systems. The relative poverty of the fractional structure in higher Landau levels compared with the lowest Landau level is explained using commensurability topological arguments. The commensurability criterion for correlated states for higher Landau levels (with n≥slant 1) including the paired states at half fillings of the spin-subbands of these levels is formulated. The commensurability condition is applied to determine the hierarchy of the fractional fillings of Landau levels in the monolayer and bilayer graphene. Good agreement with current experimental observations of fractional quantum Hall effect in the graphene monolayer and bilayer is achieved. The presence of even denominator rates in the hierarchy for fractional quantum Hall effect in the bilayer graphene is also explained.

Ground state degeneracy, energy barriers, and molecular dynamics evidence for two-dimensional disorder in black phosphorus and monochalcogenide monolayers at finite temperature

NASA Astrophysics Data System (ADS)

Mehboudi, Mehrshad; Barraza-Lopez, Salvador; Dorio, Alex M.; Zhu, Wenjuan; van der Zande, Arend; Churchill, Hugh O. H.; Pacheco-Sanjuan, Alejandro A.; Harriss, Edmund O.; Kumar, Pradeep

Mono-layers of black phosphorus and other two dimensional materials such as mono-layers of SiSe, GeS, GeSe, GeTe, Sns, SnSe, and SnTe with a similar crystalline structure have a four-fold degenerate ground state that leads to two-dimensional disorder at finite temperature. Disorder happens when neighboring atoms gently re-accommodate bonds beyond a critical temperature. In this talk, the effect of atomic numbers on the transition temperature will be discussed. In addition Car-Parinello molecular dynamics calculations at temperatures 30, 300 and 1000 K were performed on supercells containing more than five hundred atoms and the results from these calculations confirm the transition onto a two-dimensional disordered structure past the critical temperature, which is close to room temperature for many of these compounds. References: M. Mehboudi, A.M. Dorio, W. Zhu, A. van der Zande, H.O.H. Churchill, A.A. Pacheco Sanjuan, E.O.H. Harris, P. Kumar, and S. Barraza-Lopez. arXiv:1510.09153.

Linear and Star Poly(ionic liquid) Assemblies: Surface Monolayers and Multilayers.

PubMed

Erwin, Andrew J; Xu, Weinan; He, Hongkun; Matyjaszewski, Krzysztof; Tsukruk, Vladimir V

2017-04-04

The surface morphology and organization of poly(ionic liquid)s (PILs), poly[1-(4-vinylbenzyl)-3-butylimidazolium bis(trifluoromethylsulfonyl)imide] are explored in conjunction with their molecular architecture, adsorption conditions, and postassembly treatments. The formation of stable PIL Langmuir and Langmuir-Blodgett (LB) monolayers at the air-water and air-solid interfaces is demonstrated. The hydrophobic bis(trifluoromethylsulfonyl)imide (Tf 2 N - ) is shown to be a critical agent governing the assembly morphology, as observed in the reversible condensation of LB monolayers into dense nanodroplets. The PIL is then incorporated as an unconventional polyelectrolyte component in the layer-by-layer (LbL) films of hydrophobic character. We demonstrate that the interplay of capillary forces, macromolecular mobility, and structural relaxation of the polymer chains influence the dewetting mechanisms in the PIL multilayers, thereby enabling access to a diverse set of highly textured, porous, and interconnected network morphologies for PIL LbL films that would otherwise be absent in conventional LbL films. Their compartmentalized internal structure is relevant to molecular separation membranes, ultrathin hydrophobic coatings, targeted cargo delivery, and highly conductive films.

Neutrally Charged Gas/Liquid Interface by a Catanionic Langmuir Monolayer

DOE Office of Scientific and Technical Information (OSTI.GOV)

Vaknin, David; Bu, Wei

Surface-sensitive synchrotron X-ray scattering and spectroscopic experiments were performed to explore the characteristics of Langmuir monolayers of oppositely charged mixed amphiphiles. A premixed (molar 1:1 stearic acid/stearylamine) solution was spread as a monolayer at the gas/liquid interface on pure water and on mono- and divalent salt solutions, revealing that the negatively charged carboxyl groups and positively charged amine groups are miscible into one another and tend to bond together to form a nearly neutral surface. Similar control experiments on pure stearic acid (SA) and stearylamine (ST) were also conducted for comparison. Due to the strong bonding, hexagonal structures in smallmore » domains with acyl-chains normal to the liquid surface are formed at zero surface pressures, that is, at molecular areas much larger than those of the densely packed acyl chains. In-plane X-ray diffraction indicates that the catanionic surface is highly ordered and modifies the structure of the water surface and thus can serve as a model system for interactions of an amino acid template with solutes.« less

Structure and Order of Phosphonic Acid-Based Self-Assembled Monolayers on Si(100)

PubMed Central

Dubey, Manish; Weidner, Tobias; Gamble, Lara J.; Castner, David G.

2010-01-01

Organophosphonic acid self-assembled monolayers (SAMs) on oxide surfaces have recently seen increased use in electrical and biological sensor applications. The reliability and reproducibility of these sensors require good molecular organization in these SAMs. In this regard, packing, order and alignment in the SAMs is important, as it influences the electron transport measurements. In this study, we examine the order of hydroxyl- and methyl- terminated phosphonate films deposited onto silicon oxide surfaces by the tethering by aggregation and growth method using complementary, state-of-art surface characterization tools. Near edge x-ray absorption fine structure (NEXAFS) spectroscopy and in situ sum frequency generation (SFG) spectroscopy are used to study the order of the phosphonate SAMs in vacuum and under aqueous conditions, respectively. X-ray photoelectron spectroscopy and time of flight secondary ion mass spectrometry results show that these samples form chemically intact monolayer phosphonate films. NEXAFS and SFG spectroscopy showed that molecular order exists in the octadecylphosphonic acid and 11-hydroxyundecylphosphonic acid SAMs. The chain tilt angles in these SAMs were approximately 37° and 45°, respectively. PMID:20735054

Covalent attachment of TAT peptides and thiolated alkyl molecules on GaAs surfaces.

PubMed

Cho, Youngnam; Ivanisevic, Albena

2005-07-07

Four TAT peptide fragments were used to functionalize GaAs surfaces by adsorption from solution. In addition, two well-studied alkylthiols, mercaptohexadecanoic acid (MHA) and 1-octadecanethiol (ODT) were utilized as references to understand the structure of the TAT peptide monolayer on GaAs. The different sequences of TAT peptides were employed in recognition experiments where a synthetic RNA sequence was tested to verify the specific interaction with the TAT peptide. The modified GaAs surfaces were characterized by atomic force microscopy (AFM), X-ray photoelectron spectroscopy (XPS), and Fourier transform infrared reflection absorption spectroscopy (FT-IRRAS). AFM studies were used to compare the surface roughness before and after functionalization. XPS allowed us to characterize the chemical composition of the GaAs surface and conclude that the monolayers composed of different sequences of peptides have similar surface chemistries. Finally, FT-IRRAS experiments enabled us to deduce that the TAT peptide monolayers have a fairly ordered and densely packed alkyl chain structure. The recognition experiments showed preferred interaction of the RNA sequence toward peptides with high arginine content.

Quaterrylene molecules on Ag(111): self-assembly behavior and voltage pulse induced trimer formation.

PubMed

He, Yangyong; Cai, Zeying; Shao, Jian; Xu, Li; She, Limin; Zheng, Yue; Zhong, Dingyong

2018-05-03

The self-assembly behavior of quaterrylene (QR) molecules on Ag(111) surfaces has been investigated by scanning tunneling microscopy (STM) and density functional theory (DFT) calculations. It is found that the QR molecules are highly mobile on the Ag(111) surface at 78 K. No ordered assembled structure is formed on the surface with a sub-monolayer coverage up to 0.8 monolayer due to the intermolecular repulsive interactions, whereas ordered molecular structures are observed at one monolayer coverage. According to our DFT calculations, charge transfer occurs between the substrate and the adsorbed QR molecule. As a result, out-of-plane dipoles appear at the interface, which are ascribed to the repulsive dipole-dipole interactions between the QR molecules. Furthermore, due to the planar geometry, the QR molecules exhibit relatively low diffusion barriers on Ag(111). By applying a voltage pulse between the tunneling gap, immobilization and aggregation of QR molecules take place, resulting in the formation of a triangle-shaped trimer. Our work demonstrates the ability of manipulating intermolecular repulsive and attractive interactions at the single molecular level.

Unusual phonon behavior and ultra-low thermal conductance of monolayer InSe.

PubMed

Zhou, Hangbo; Cai, Yongqing; Zhang, Gang; Zhang, Yong-Wei

2017-12-21

Monolayer indium selenide (InSe) possesses numerous fascinating properties, such as high electron mobility, quantum Hall effect and anomalous optical response. However, its phonon properties, thermal transport properties and the origin of its structural stability remain unexplored. Using first-principles calculations, we show that the atoms in InSe are highly polarized and such polarization causes strong long-range dipole-dipole interaction (DDI). For acoustic modes, DDI is essential for maintaining its structural stability. For optical modes, DDI causes a significant frequency shift of its out-of-phase vibrations. Surprisingly, we observed that there were two isolated frequency regimes, which were completely separated from other frequency regimes with large frequency gaps. Within each frequency regime, only a single phonon mode exists. We further reveal that InSe possesses the lowest thermal conductance among the known two-dimensional materials due to the low cut-off frequency, low phonon group velocities and the presence of large frequency gaps. These unique behaviors of monolayer InSe can enable the fabrication of novel devices, such as thermoelectric module, single-mode phonon channel and phononic laser.

Quantification of stromal vascular cell mechanics with a linear cell monolayer rheometer

DOE Office of Scientific and Technical Information (OSTI.GOV)

Elkins, Claire M., E-mail: cma9@stanford.edu; Fuller, Gerald G.; Shen, Wen-Jun

2015-01-15

Over the past few decades researchers have developed a variety of methods for measuring the mechanical properties of whole cells, including traction force microscopy, atomic force microscopy (AFM), and single-cell tensile testing. Though each of these techniques provides insight into cell mechanics, most also involve some nonideal conditions for acquiring live cell data, such as probing only one portion of a cell at a time, or placing the cell in a nonrepresentative geometry during testing. In the present work, we describe the development of a linear cell monolayer rheometer (LCMR) and its application to measure the mechanics of a live,more » confluent monolayer of stromal vascular cells. In the LCMR, a monolayer of cells is contacted on both top and bottom by two collagen-coated plates and allowed to adhere. The top plate then shears the monolayer by stepping forward to induce a predetermined step strain, while a force transducer attached to the top plate collects stress information. The stress and strain data are then used to determine the maximum relaxation modulus recorded after step-strain, G{sub r}{sup 0}, referred to as the zero-time relaxation modulus of the cell monolayer. The present study validates the ability of the LCMR to quantify cell mechanics by measuring the change in G{sub r}{sup 0} of a confluent cell monolayer upon the selective inhibition of three major cytoskeletal components (actin microfilaments, vimentin intermediate filaments, and microtubules). The LCMR results indicate that both actin- and vimentin-deficient cells had ∼50% lower G{sub r}{sup 0} values than wild-type, whereas tubulin deficiency resulted in ∼100% higher G{sub r}{sup 0} values. These findings constitute the first use of a cell monolayer rheometer to quantitatively distinguish the roles of different cytoskeletal elements in maintaining cell stiffness and structure. Significantly, they are consistent with results obtained using single-cell mechanical testing methods, suggesting that the rheology-based LCMR technique may be a useful tool for rapid analysis of cell mechanics by shearing an entire cell monolayer.« less

Graphene field-effect devices

NASA Astrophysics Data System (ADS)

Echtermeyer, T. J.; Lemme, M. C.; Bolten, J.; Baus, M.; Ramsteiner, M.; Kurz, H.

2007-09-01

In this article, graphene is investigated with respect to its electronic properties when introduced into field effect devices (FED). With the exception of manual graphene deposition, conventional top-down CMOS-compatible processes are applied. Few and monolayer graphene sheets are characterized by scanning electron microscopy, atomic force microscopy and Raman spectroscopy. The electrical properties of monolayer graphene sandwiched between two silicon dioxide films are studied. Carrier mobilities in graphene pseudo-MOS structures are compared to those obtained from double-gated Graphene-FEDs and silicon metal-oxide-semiconductor field-effect-transistors (MOSFETs).

Femtosecond dynamics of monolayer MoS2-Ag nanoparticles hybrid probed at 532 nm

NASA Astrophysics Data System (ADS)

Xu, Xuefeng; Shi, Ying; Liu, Xiaochun; Sun, Mengtao

2018-01-01

In this communication, plasmon-exciton couplings of monolayer MoS2/Ag nanoparticles (NPs) hybrids with different sizes are investigated, using transient absorption spectra. Ultrafast dynamics of coupling interactions inside these hybrid structures are carefully examined at 532 nm, which can well interpret the apllication of plasmon-exciton coupling for the co-driven chemical reactions excited at 532 nm. Our experimental results can promote the deeper understanding on the physical mechanism of plasmon-excition interaction, and applications in different fields.

Evidence for graphite-like hexagonal AlN nanosheets epitaxially grown on single crystal Ag(111)

DOE Office of Scientific and Technical Information (OSTI.GOV)

Tsipas, P.; Kassavetis, S.; Tsoutsou, D.

Ultrathin (sub-monolayer to 12 monolayers) AlN nanosheets are grown epitaxially by plasma assisted molecular beam epitaxy on Ag(111) single crystals. Electron diffraction and scanning tunneling microscopy provide evidence that AlN on Ag adopts a graphite-like hexagonal structure with a larger lattice constant compared to bulk-like wurtzite AlN. This claim is further supported by ultraviolet photoelectron spectroscopy indicating a reduced energy bandgap as expected for hexagonal AlN.

Photo-modulation of the spin Hall conductivity of mono-layer transition metal dichalcogenides

DOE Office of Scientific and Technical Information (OSTI.GOV)

Sengupta, Parijat; Bellotti, Enrico

2016-05-23

We report on a possible optical tuning of the spin Hall conductivity in mono-layer transition metal dichalcogenides. Light beams of frequencies much higher than the energy scale of the system (the off-resonant condition) do not excite electrons but rearrange the band structure. The rearrangement is quantitatively established using the Floquet formalism. For such a system of mono-layer transition metal dichalcogenides, the spin Hall conductivity (calculated with the Kubo expression in presence of disorder) exhibits a drop at higher frequencies and lower intensities. Finally, we compare the spin Hall conductivity of the higher spin-orbit coupled WSe{sub 2} to MoS{sub 2}; themore » spin Hall conductivity of WSe{sub 2} was found to be larger.« less

X-ray Study of the Electric Double Layer at the n-Hexane/Nanocolloidal Silica Interface

DOE Office of Scientific and Technical Information (OSTI.GOV)

Tikhonov,A.

The spatial structure of the transition region between an insulator and an electrolyte solution was studied with x-ray scattering. The electron-density profile across the n-hexane/silica sol interface (solutions with 5, 7, and 12 nm colloidal particles) agrees with the theory of the electrical double layer and shows separation of positive and negative charges. The interface consists of three layers, i.e., a compact layer of Na{sup +}, a loose monolayer of nanocolloidal particles as part of a thick diffuse layer, and a low-density layer sandwiched between them. Its structure is described by a model in which the potential gradient at themore » interface reflects the difference in the potentials of 'image forces' between the cationic Na{sup +} and anionic nanoparticles and the specific adsorption of surface charge. The density of water in the large electric field ({approx}10{sup 9}-10{sup 10} V/m) of the transition region and the layering of silica in the diffuse layer is discussed.« less

Structure and dynamics of water near the interface with oligo(ethylene oxide) self-assembled monolayers

NASA Astrophysics Data System (ADS)

Ismail, Ahmed E.; Grest, Gary S.; Stevens, Mark J.

2007-03-01

Oligo(ethylene oxide) self-assembled monolayers (OEO SAM's) deposited on Au are the prototypical materials used to study protein resistance. Recently, protein resistance has been shown to vary as a function of surface coverage and to be maximal at about two-thirds coverage, not complete coverage. We use molecular dynamics simulations to study the nature of the interface between water and the OEO SAM for a range of SAM coverages. As SAM coverage decreases, the amount of water within the OEO monolayer increases monotonically; however, the penetration depth of the water shows a maximum near the experimentally-found maximal coverage. As the water content increases, the SAM-water mixture becomes harder to distinguish from bulk water. Since the oxygen atoms of OEO are hydrogen bond acceptors, a hydrogen bond network forms within the SAM-water mixture. The water molecules diffuse freely within the monolayer and exchange with the bulk water. Because the monolayer becomes increasingly like bulk water as the coverage decreases, proteins stay in their bulk soluble conformation and do not adsorb. Sandia is a multiprogram laboratory operated by Sandia Corporation, a Lockheed Martin Company, for the United States Department of Energy under Contract No. DE-AC04-94AL85000.

Novel 2D RuPt core-edge nanocluster catalyst for CO electro-oxidation

NASA Astrophysics Data System (ADS)

Grabow, Lars C.; Yuan, Qiuyi; Doan, Hieu A.; Brankovic, Stanko R.

2015-10-01

A single layer, bi-metallic RuPt catalyst on Au(111) is synthesized using surface limited red-ox replacement of underpotentially deposited Cu and Pb monolayers though a two-step process. The resulting 2D RuPt monolayer nanoclusters have a unique core-edge structure with a Ru core and Pt at the edge along the perimeter. The activity of this catalyst is evaluated using CO monolayer oxidation as the probe reaction. Cyclic voltammetry demonstrates that the 2D RuPt core-edge catalyst morphology is significantly more active than either Pt or Ru monolayer catalysts. Density functional theory calculations in combination with infra-red spectroscopy data point towards oscillating variations (ripples) in the adsorption energy landscape along the radial direction of the Ru core as the origin of the observed behavior. Both, CO and OH experience a thermodynamic driving force for surface migration towards the Ru-Pt interface, where they adsorb most strongly and react rapidly. We propose that the complex interplay between epitaxial strain, ligand and finite size effects is responsible for the formation of the rippled RuPt monolayer cluster, which provides optimal conditions for a quasi-ideal bi-functional mechanism for CO oxidation, in which CO is adsorbed mainly on Pt, and Ru provides OH to the active Pt-Ru interface.

Vertical uniformity of cells and nuclei in epithelial monolayers.

PubMed

Neelam, Srujana; Hayes, Peter Robert; Zhang, Qiao; Dickinson, Richard B; Lele, Tanmay P

2016-01-22

Morphological variability in cytoskeletal organization, organelle position and cell boundaries is a common feature of cultured cells. Remarkable uniformity and reproducibility in structure can be accomplished by providing cells with defined geometric cues. Cells in tissues can also self-organize in the absence of directing extracellular cues; however the mechanical principles for such self-organization are not understood. We report that unlike horizontal shapes, the vertical shapes of the cell and nucleus in the z-dimension are uniform in cells in cultured monolayers compared to isolated cells. Apical surfaces of cells and their nuclei in monolayers were flat and heights were uniform. In contrast, isolated cells, or cells with disrupted cell-cell adhesions had nuclei with curved apical surfaces and variable heights. Isolated cells cultured within micron-sized square wells displayed flat cell and nuclear shapes similar to cells in monolayers. Local disruption of nuclear-cytoskeletal linkages resulted in spatial variation in vertical uniformity. These results suggest that competition between cell-cell pulling forces that expand and shorten the vertical cell cross-section, thereby widening and flattening the nucleus, and the resistance of the nucleus to further flattening results in uniform cell and nuclear cross-sections. Our results reveal the mechanical principles of self-organized vertical uniformity in cell monolayers.

Improvement of Biological Indicators by Uniformly Distributing Bacillus subtilis Spores in Monolayers To Evaluate Enhanced Spore Decontamination Technologies

PubMed Central

Raguse, Marina; Fiebrandt, Marcel; Stapelmann, Katharina; Madela, Kazimierz; Laue, Michael; Lackmann, Jan-Wilm; Thwaite, Joanne E.; Setlow, Peter; Awakowicz, Peter

2016-01-01

Novel decontamination technologies, including cold low-pressure plasma and blue light (400 nm), are promising alternatives to conventional surface decontamination methods. However, the standardization of the assessment of such sterilization processes remains to be accomplished. Bacterial endospores of the genera Bacillus and Geobacillus are frequently used as biological indicators (BIs) of sterility. Ensuring standardized and reproducible BIs for reliable testing procedures is a significant problem in industrial settings. In this study, an electrically driven spray deposition device was developed, allowing fast, reproducible, and homogeneous preparation of Bacillus subtilis 168 spore monolayers on glass surfaces. A detailed description of the structural design as well as the operating principle of the spraying device is given. The reproducible formation of spore monolayers of up to 5 × 107 spores per sample was verified by scanning electron microscopy. Surface inactivation studies revealed that monolayered spores were inactivated by UV-C (254 nm), low-pressure argon plasma (500 W, 10 Pa, 100 standard cubic cm per min), and blue light (400 nm) significantly faster than multilayered spores were. We have thus succeeded in the uniform preparation of reproducible, highly concentrated spore monolayers with the potential to generate BIs for a variety of nonpenetrating surface decontamination techniques. PMID:26801572

Using the electrochemical dimension to build water/Ru(0001) phase diagram

NASA Astrophysics Data System (ADS)

Lespes, Nicolas; Filhol, Jean-Sébastien

2015-01-01

The water monolayer/Ru(0001) electrochemical phase diagram as a function of surface potential and temperature is built using a DFT approach. The monolayer structure with temperature is extracted following the zero-charge line in good agreement with experiments. Below 140 K, a mix of oppositely charged hydroxyl/water and hydride/water domains is found stable; above 140 K, water molecules desorb from the hydride phase leading to a mixture of oppositely charged surface hydride and hydroxyl/water phases; above 280 K, all the residual adsorbed water desorbs. For undissociated water, a Chain structure is found stable and desorbs above 150 K. The observed nano-sized domains are suggested to be the balance between hydroxyl/hydride repulsion that tends to create two well separated domains and opposite charging that tends to favor a domain mix. An isotopic effect is computed to reduce by a factor of 160 the kinetic rate of D2O dissociation (compared to H2O) and is linked to the reduction of the ZPE in the transition state caused by a proton transport chain. Water monolayer/Ru(0001) has a specific reactivity and its organization is highly sensitive to the surface potential suggesting that under electrochemical conditions, the potential is not only tuning directly the chemical reactivity but also indirectly through the solvent structure.

Estimated phase transition and melting temperature of APTES self-assembled monolayer using surface-enhanced anti-stokes and stokes Raman scattering

NASA Astrophysics Data System (ADS)

Sun, Yingying; Yanagisawa, Masahiro; Kunimoto, Masahiro; Nakamura, Masatoshi; Homma, Takayuki

2016-02-01

A structure's temperature can be determined from the Raman spectrum using the frequency and the ratio of the intensities of the anti-Stokes and Stokes signals (the Ias/Is ratio). In this study, we apply this approach and an equation relating the temperature, Raman frequency, and Ias/Is ratio to in-situ estimation of the phase change point of a (3-aminopropyl)triethoxysilane self-assembled monolayer (APTES SAM). Ag nanoparticles were deposited on APTES to enhance the Raman signals. A time-resolved measurement mode was used to monitor the variation in the Raman spectra in situ. Moreover, the structural change in APTES SAM (from ordered to disordered structure) under heating was discussed in detail, and the phase change point (around 118 °C) was calculated.

Enhanced doping effect on tuning structural phases of monolayer antimony

NASA Astrophysics Data System (ADS)

Wang, Jizhang; Yang, Teng; Zhang, Zhidong; Yang, Li

2018-05-01

Doping is capable to control the atomistic structure, electronic structure, and even to dynamically realize a semiconductor-metal transition in two-dimensional (2D) transition metal dichalcogenides (TMDs). However, the high critical doping density (˜1014 electron/cm2), compound nature, and relatively low carrier mobility of TMDs limits broader applications. Using first-principles calculations, we predict that, via a small transition potential, a substantially lower hole doping density (˜6 × 1012 hole/cm2) can switch the ground-state structure of monolayer antimony from the hexagonal β-phase, a 2D semiconductor with excellent transport performance and air stability but an indirect bandgap, to the orthorhombic α phase with a direct bandgap and potentially better carrier mobility. We further show that this structural engineering can be achieved by the established electrostatic doping, surface functional adsorption, or directly using graphene substrate. This gives hope to dynamically tuning and large-scale production of 2D single-element semiconductors that simultaneously exhibit remarkable transport and optical performance.

Albumin-coated SPIONs: an experimental and theoretical evaluation of protein conformation, binding affinity and competition with serum proteins

NASA Astrophysics Data System (ADS)

Yu, Siming; Perálvarez-Marín, Alex; Minelli, Caterina; Faraudo, Jordi; Roig, Anna; Laromaine, Anna

2016-07-01

The variety of nanoparticles (NPs) used in biological applications is increasing and the study of their interaction with biological media is becoming more important. Proteins are commonly the first biomolecules that NPs encounter when they interact with biological systems either in vitro or in vivo. Among NPs, super-paramagnetic iron oxide nanoparticles (SPIONs) show great promise for medicine. In this work, we study in detail the formation, composition, and structure of a monolayer of bovine serum albumin (BSA) on SPIONs. We determine, both by molecular simulations and experimentally, that ten molecules of BSA form a monolayer around the outside of the SPIONs and their binding strength to the SPIONs is about 3.5 × 10-4 M, ten times higher than the adsorption of fetal bovine serum (FBS) on the same SPIONs. We elucidate a strong electrostatic interaction between BSA and the SPIONs, although the secondary structure of the protein is not affected. We present data that supports the strong binding of the BSA monolayer on SPIONs and the properties of the BSA layer as a protein-resistant coating. We believe that a complete understanding of the behavior and morphology of BSA-SPIONs and how the protein interacts with SPIONs is crucial for improving NP surface design and expanding the potential applications of SPIONs in nanomedicine.The variety of nanoparticles (NPs) used in biological applications is increasing and the study of their interaction with biological media is becoming more important. Proteins are commonly the first biomolecules that NPs encounter when they interact with biological systems either in vitro or in vivo. Among NPs, super-paramagnetic iron oxide nanoparticles (SPIONs) show great promise for medicine. In this work, we study in detail the formation, composition, and structure of a monolayer of bovine serum albumin (BSA) on SPIONs. We determine, both by molecular simulations and experimentally, that ten molecules of BSA form a monolayer around the outside of the SPIONs and their binding strength to the SPIONs is about 3.5 × 10-4 M, ten times higher than the adsorption of fetal bovine serum (FBS) on the same SPIONs. We elucidate a strong electrostatic interaction between BSA and the SPIONs, although the secondary structure of the protein is not affected. We present data that supports the strong binding of the BSA monolayer on SPIONs and the properties of the BSA layer as a protein-resistant coating. We believe that a complete understanding of the behavior and morphology of BSA-SPIONs and how the protein interacts with SPIONs is crucial for improving NP surface design and expanding the potential applications of SPIONs in nanomedicine. Electronic supplementary information (ESI) available. See DOI: 10.1039/c6nr01732k

Structure and property relations of macromolecular self-assemblies at interfaces

NASA Astrophysics Data System (ADS)

Yang, Zhihao

Hydrophilic polymer chains, poly(ethylene glycol) (PEG), are attached to glass surfaces by silylation of the silanol groups on glass surfaces with the omega-(methoxyl terminated PEG) trimethoxysilanes. These tethered polymer chains resemble the self-assembled monolayers (SAMs) of PEG, which exhibit excellent biocompatibility and provide a model system for studying the interactions of proteins with polymer surfaces. The low molecular weight PEGs tend to extend, forming a brush-like monolayer, whereas the longer polymer chains tend to interpenetrate each other, forming a mushroom-like PEG monolayer at the interface. Interactions between a plasma protein, bovine serum albumin, and the PEG-SAMs are investigated in terms of protein adsorption and diffusion on the surfaces by the technique of fluorescence recovery after photobleaching (FRAP). The diffusion and aggregation behaviors of the protein on the two monolayers are found to be quite different despite the similarities in adsorption and desorption behaviors. The results are analyzed with a hypothesis of the hydrated surface dynamics. A method of covalently bonding phospholipid molecules to silica substrates followed by loading with free phospholipids is demonstrated to form well organized and stable phospholipid self-assembled monolayers. Surfaces of such SAMs structurally mimic the aqueous sides of phospholipid bilayer membranes. The dynamics of phospholipids and an adsorbed protein, lipase, in the SAMs are probed with FRAP, in terms of lateral diffusion of both phospholipids and protein molecules. The esterase activity of lipase on the SAM surfaces is confirmed by the hydrolysis reaction of a substrate, umbelliferone stearate, showing such lipid SAMs posess biomembrane functionality in terms of interfacial activation of the membranous enzymes. Dynamics of polyethylene oxide and polypropylene oxide tri-block copolymers, PEO-PPO-PEO and PPO-PEO-PPO, at the air/water interface upon thermal stimulation is studied by surface light scattering, in terms of the dynamic surface tension changes in response to a temperature jump. The characteristic of the surface tension relaxation is found to be highly related to the molecular structure and concentration of the copolymers at the interface.

The Elementary Mass Action Rate Constants of P-gp Transport for a Confluent Monolayer of MDCKII-hMDR1 Cells

PubMed Central

Tran, Thuy Thanh; Mittal, Aditya; Aldinger, Tanya; Polli, Joseph W.; Ayrton, Andrew; Ellens, Harma; Bentz, Joe

2005-01-01

The human multi-drug resistance membrane transporter, P-glycoprotein, or P-gp, has been extensively studied due to its importance to human health and disease. Thus far, the kinetic analysis of P-gp transport has been limited to steady-state Michaelis-Menten approaches or to compartmental models, neither of which can prove molecular mechanisms. Determination of the elementary kinetic rate constants of transport will be essential to understanding how P-gp works. The experimental system we use is a confluent monolayer of MDCKII-hMDR1 cells that overexpress P-gp. It is a physiologically relevant model system, and transport is measured without biochemical manipulations of P-gp. The Michaelis-Menten mass action reaction is used to model P-gp transport. Without imposing the steady-state assumptions, this reaction depends upon several parameters that must be simultaneously fitted. An exhaustive fitting of transport data to find all possible parameter vectors that best fit the data was accomplished with a reasonable computation time using a hierarchical algorithm. For three P-gp substrates (amprenavir, loperamide, and quinidine), we have successfully fitted the elementary rate constants, i.e., drug association to P-gp from the apical membrane inner monolayer, drug dissociation back into the apical membrane inner monolayer, and drug efflux from P-gp into the apical chamber, as well as the density of efflux active P-gp. All three drugs had overlapping ranges for the efflux active P-gp, which was a benchmark for the validity of the fitting process. One novel finding was that the association to P-gp appears to be rate-limited solely by drug lateral diffusion within the inner monolayer of the plasma membrane for all three drugs. This would be expected if P-gp structure were open to the lipids of the apical membrane inner monolayer, as has been suggested by recent structural studies. The fitted kinetic parameters show how P-gp efflux of a wide range of xenobiotics has been maximized. PMID:15501934

Synthesis, characterization, and relative stabilities of self-assembled monolayers on gold generated from bidentate n-alkyl xanthic acids.

PubMed

Moore, H Justin; Colorado, Ramon; Lee, Han Ju; Jamison, Andrew C; Lee, T Randall

2013-08-27

A series of self-assembled monolayers (SAMs) on gold were generated by the adsorption of n-alkyl xanthic acids (NAXAs) having the general formula CH3(CH2)nOCS2H (n = 12-15). The structural features of these SAMs were characterized by optical ellipsometry, contact angle goniometry, polarization modulation infrared reflection absorption spectroscopy (PM-IRRAS), and X-ray photoelectron spectroscopy (XPS). This series of xanthate SAMs were compared to SAMs generated from the corresponding n-alkanethiols and aliphatic dithiocarboxylic acids (ADTCAs). The collected data indicate that the NAXAs generate densely packed and well-ordered monolayers. The contact angles of hexadecane on the xanthate monolayers exhibited a large "odd-even" effect similar to that produced by the ADTCA SAMs. The relative stability of these bidentate xanthate SAMs was evaluated by monitoring the changes in ellipsometric thicknesses and wettability as a function of time under various conditions. The results demonstrate that SAMs formed from NAXAs are much less stable than analogous n-alkanethiolate and ADTCA SAMs.

Bovine insulin-phosphatidylcholine mixed Langmuir monolayers: behavior at the air-water interface.

PubMed

Pérez-López, S; Blanco-Vila, N M; Vila-Romeu, N

2011-08-04

The behavior of the binary mixed Langmuir monolayers of bovine insulin (INS) and phosphatidylcholine (PC) spread at the air-water interface was investigated under various subphase conditions. Pure and mixed monolayers were spread on water, on NaOH and phosphate-buffered solutions of pH 7.4, and on Zn(2+)-containing solutions. Miscibility and interactions between the components were studied on the basis of the analysis of the surface pressure (π)-mean molecular area (A) isotherms, surface compression modulus (C(s)(-1))-π curves, and plots of A versus mole fraction of INS (X(INS)). Our results indicate that intermolecular interactions between INS and PC depend on both the monolayer state and the structural characteristics of INS at the interface, which are strongly influenced by the subphase pH and salt content. Brewster angle microscopy (BAM) was applied to investigate the peptide aggregation pattern at the air-water interface in the presence of the studied lipid under any experimental condition investigated. The influence of the lipid on the INS behavior at the interface strongly depends on the subphase conditions.

Optical spectroscopy of excited exciton states in MoS2 monolayers in van der Waals heterostructures

NASA Astrophysics Data System (ADS)

Robert, C.; Semina, M. A.; Cadiz, F.; Manca, M.; Courtade, E.; Taniguchi, T.; Watanabe, K.; Cai, H.; Tongay, S.; Lassagne, B.; Renucci, P.; Amand, T.; Marie, X.; Glazov, M. M.; Urbaszek, B.

2018-01-01

The optical properties of MoS2 monolayers are dominated by excitons, but for spectrally broad optical transitions in monolayers exfoliated directly onto SiO2 substrates detailed information on excited exciton states is inaccessible. Encapsulation in hexagonal boron nitride (hBN) allows approaching the homogenous exciton linewidth, but interferences in the van der Waals heterostructures make direct comparison between transitions in optical spectra with different oscillator strength more challenging. Here we reveal in reflectivity and in photoluminescence excitation spectroscopy the presence of excited states of the A exciton in MoS2 monolayers encapsulated in hBN layers of calibrated thickness, allowing us to extrapolate an exciton binding energy of ≈220 meV. We theoretically reproduce the energy separations and oscillator strengths measured in reflectivity by combining the exciton resonances calculated for a screened two-dimensional Coulomb potential with transfer matrix calculations of the reflectivity for the van der Waals structure. Our analysis shows a very different evolution of the exciton oscillator strength with principal quantum number for the screened Coulomb potential as compared to the ideal two-dimensional hydrogen model.

Multiple Dirac cones and topological magnetism in honeycomb-monolayer transition metal trichalcogenides

NASA Astrophysics Data System (ADS)

Sugita, Yusuke; Miyake, Takashi; Motome, Yukitoshi

2018-01-01

The discovery of monolayer graphene has initiated two fertile fields in condensed matter physics: Dirac semimetals and atomically thin layered materials. When these trends meet again in transition metal compounds, which possess spin and orbital degrees of freedom and strong electron correlations, more exotic phenomena are expected to emerge in the cross section of topological states of matter and Mott physics. Here, we show by using ab initio calculations that a monolayer form of transition metal trichalcogenides (TMTs), which has a honeycomb network of 4 d and 5 d transition metal cations, may exhibit multiple Dirac cones in the electronic structure of the half-filled eg orbitals. The Dirac cones are gapped by the spin-orbit coupling under the trigonal lattice distortion and, hence, can be tuned by tensile strain. Furthermore, we show that electron correlations and carrier doping turn the multiple Dirac semimetal into a topological ferromagnet with high Chern number. Our findings indicate that the honeycomb-monolayer TMTs provide a good playground for correlated Dirac electrons and topologically nontrivial magnetism.

Intestinal absorption of hawthorn flavonoids--in vitro, in situ and in vivo correlations.

PubMed

Zuo, Zhong; Zhang, Li; Zhou, Limin; Chang, Qi; Chow, Moses

2006-11-25

Our previous studies identified hyperoside (HP), isoquercitrin (IQ) and epicatechin (EC) to be the major active flavonoid components of the hawthorn phenolic extract from hawthorn fruits demonstrating inhibitory effect on in vitro Cu(+2)-mediated low density lipoproteins oxidation. Among these three hawthorn flavonoids, EC was the only one detectable in plasma after the oral administration of hawthorn phenolic extract to rats. The present study aims to investigate the intestinal absorption mechanisms of these three hawthorn flavonoids by in vitro Caco-2 monolayer model, rat in situ intestinal perfusion model and in vivo pharmacokinetics studies in rats. In addition, in order to investigate the effect of the co-occurring components in hawthorn phenolic extract on the intestinal absorption of these three major hawthorn flavonoids, intestinal absorption transport profiles of HP, IQ and EC in forms of individual pure compound, mixture of pure compounds and hawthorn phenolic extract were studied and compared. The observations from in vitro Caco-2 monolayer model and in situ intestinal perfusion model indicated that all three studied hawthorn flavonoids have quite limited permeabilities. EC and IQ demonstrated more extensive metabolism in the rat in situ intestinal perfusion model and in vivo study than in Caco-2 monolayer model. Moreover, results from the Caco-2 monolayer model, rat in situ intestinal perfusion model as well as the in vivo pharmacokinetics studies in rats consistently showed that the co-occurring components in hawthorn phenolic extract might not have significant effect on the intestinal absorption of the three major hawthorn flavonoids studied.

Two- and three-dimensional growth of Bi on i -Al-Pd-Mn studied using medium-energy ion scattering

NASA Astrophysics Data System (ADS)

Noakes, T. C. Q.; Bailey, P.; McConville, C. F.; Draxler, M.; Walker, M.; Brown, M. G.; Hentz, A.; Woodruff, D. P.; Lograsso, T. A.; Ross, A. R.; Smerdon, J. A.; Leung, L.; McGrath, R.

2010-11-01

Recent work on the growth of thin metal films on quasicrystalline substrates has indicated the formation of so-called “magic height” islands with multiples of 4 atomic layers (AL) arising as a result of quantum size effects, which lead to enhanced stability. Here the results of a study are reported of Bi deposition on i -Al-Pd-Mn using medium-energy ion scattering to characterize the island thickness and the structural arrangement of Bi atoms within the islands. In addition, data were taken from annealed surfaces after Bi cluster desorption to leave a single aperiodic monolayer of Bi at the surface. Scattered-ion energy spectra from the Bi islands are consistent with a single Bi monolayer covered with mainly 4 AL islands for both 1.8 and 3.2 monolayer equivalent coverages but with some occupation of 2 and 8 Al islands as well. The angular dependence of the scattered-ion intensity (“blocking curve”) from Bi has been compared with simulations for various models of both rhombohedral Bi and a distorted “black-phosphorus”-like structure. The data demonstrate bilayer formation within the Bi islands. In the case of the aperiodic Bi monolayer, the blocking curves from substrate scattering are found to be inconsistent with two high-symmetry sites on the quasicrystalline surface that theory indicates are energetically favorable but do not exclude the formation of pentagonal arrangements of Bi atoms as seen in other recent experimental work.

Ultrafast Interlayer Electron Transfer in Incommensurate Transition Metal Dichalcogenide Homobilayers.

PubMed

Li, Yuanyuan; Cui, Qiannan; Ceballos, Frank; Lane, Samuel D; Qi, Zeming; Zhao, Hui

2017-11-08

Two-dimensional materials, such as graphene, transition metal dichalcogenides, and phosphorene, can be used to construct van der Waals multilayer structures. This approach has shown potentials to produce new materials that combine novel properties of the participating individual layers. One key requirement for effectively harnessing emergent properties of these materials is electronic connection of the involved atomic layers through efficient interlayer charge or energy transfer. Recently, ultrafast charge transfer on a time scale shorter than 100 fs has been observed in several van der Waals bilayer heterostructures formed by two different materials. However, information on the transfer between two atomic layers of the same type is rare. Because these homobilayers are essential elements in constructing multilayer structures with desired optoelectronic properties, efficient interlayer transfer is highly desired. Here we show that electron transfer between two monolayers of MoSe 2 occurs on a picosecond time scale. Even faster transfer was observed in homobilayers of WS 2 and WSe 2 . The samples were fabricated by manually stacking two exfoliated monolayer flakes. By adding a graphene layer as a fast carrier recombination channel for one of the two monolayers, the transfer of the photoexcited carriers from the populated to the drained monolayers was time-resolved by femtosecond transient absorption measurements. The observed efficient interlayer carrier transfer indicates that such homobilayers can be used in van der Waals multilayers to enhance their optical absorption without significantly compromising the interlayer transport performance. Our results also provide valuable information for understanding interlayer charge transfer in heterostructures.