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Sample records for alkane monolayers studied

  1. Nanoscale Trapping and Squeeze-Out of Confined Alkane Monolayers.

    PubMed

    Gosvami, N N; O'Shea, S J

    2015-12-01

    We present combined force curve and conduction atomic force microscopy (AFM) data for the linear alkanes CnH2n+2 (n = 10, 12, 14, 16) confined between a gold-coated AFM tip and a graphite surface. Solvation layering is observed in the force curves for all liquids, and conduction AFM is used to study in detail the removal of the confined (mono)layer closest to the graphite surface. The squeeze-out behavior of the monolayer can be very different depending upon the temperature. Below the monolayer melting transition temperatures the molecules are in an ordered state on the graphite surface, and fast and complete removal of the confined molecules is observed. However, above the melting transition temperature the molecules are in a disordered state, and even at large applied pressure a few liquid molecules are trapped within the tip-sample contact zone. These findings are similar to a previous study for branched alkanes [ Gosvami Phys. Rev. Lett. 2008, 100, 076101 ], but the observation for the linear alkane homologue series demonstrates clearly the dependence of the squeeze-out and trapping on the state of the confined material. PMID:26529283

  2. Structure and Phase Transitions of Monolayers of Intermediate-length n-alkanes on Graphite Studied by Neutron Diffraction and Molecular Dynamics Simulation

    SciTech Connect

    Taub, H.; Hansen, F.Y.; Diama, Amand; Matthies, Blake; Criswell, Leah; Mo, Haiding; Bai, M; Herwig, Kenneth W

    2009-01-01

    We present evidence from neutron diffraction measurements and molecular dynamics (MD) simulations of three different monolayer phases of the intermediate-length alkanes tetracosane (n-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.

  3. Ordered Self-assembled Alkane Monolayer on Graphite and Graphene Surface

    NASA Astrophysics Data System (ADS)

    Su, Yudan; Han, Huiling; Wang, Feng; Cai, Qun; Tian, Chuanshan; Shen, Y. R.

    2015-03-01

    The 2D self-assembly of long chain alkane molecule on graphite and graphene had been studied with phase-sensitive sum-frequency vibrational spectroscopy (PS-SFVS) and scanning tunneling microscopy (STM). The spectrum of Imχs(2) (ωIR) which directly characterizes the surface resonances, shows 10-cm-1 red-shift of the symmetric-stretch frequency of the CH2 groups pointing towards graphite (or graphene) surface indicating Van der Waals interaction in between. The Gibbs adsorption energy of polyethylene (PE, n ~ 140) on graphite from chloroform solution was determined to be -42kJ/mol per molecule or -0.6 kJ/mol per CH2 unit. This large adsorption energy drives the long alkane chain to form an ordered self-assembled monolayer on graphite (or graphene). The sum frequency spectra suggest the orientation of carbon skeleton plane of alkane is predominately perpendicular to the graphite/graphene surface. Our STM result also provides clear evidence for the proposed molecular adsorption model. These results explain the large amount residual of long chain alkane on polystyrene (PS) or poly(methyl methacrylate) (PMMA) transferred graphene, and facilitate a better way to fabricate cleaner large-size graphene.

  4. Adsorption Behaviors of Mixed Monolayers of n-Alkanes at the Liquid-Solid Interface.

    PubMed

    Hibino, Masahiro

    2016-05-17

    To understand the self-assembly of monolayers at the liquid-solid interface, a thermodynamic model, which describes the contributions of the molecular interactions, is essential. We present an adapted Zimm-Bragg model of the cooperativity transitions for determining the Gibbs free energy for self-assembly at the liquid-solid interface. Scanning tunneling microscopy was used to observe the monolayers formed on graphite from phenyloctane solutions of binary mixtures of n-hexacosane (C26H54) and n-tetratriacontane (C34H70). This revealed that the sharp transition in the monolayers from the full surface coverage of the long-chain alkane, which is adsorbed preferentially, to the full coverage of the short-chain alkane is a function of the mixture composition. The model allows for the estimation of the free-energy changes associated with the difference in the alkyl chain length and the interface between the two different alkane regions in the monolayers. It is also suitable for understanding more complex systems that exhibit intermolecular interactions. PMID:27124544

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

    SciTech Connect

    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(CF{sub 2}){sub 8}(CH{sub 2}){sub 18}H (denoted F{sub 8}H{sub 18}), spread on the hydrophobic top of a suitable amphiphilic monolayer: namely, a natural {alpha}-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 F{sub 8}H{sub 18} and alam. The particular chemical structure of F{sub 8}H{sub 18}, the suitable structure of the underlying alam monolayer and its collapse properties, allow for a continuous compression of the upper F{sub 8}H{sub 18} 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 F{sub 8}H{sub 18} monolayer.

  6. Fractal analysis methods for solid alkane monolayer domains at SiO2/air interfaces.

    PubMed

    Knüfing, Lydia; Schollmeyer, Hauke; Riegler, Hans; Mecke, Klaus

    2005-02-01

    A systematic evaluation of various fractal analysis methods is essential for studying morphologies of finite and noisy experimental patterns such as domains of long chain alkanes at SiO(2)/air interfaces. The derivation of trustworthy fractal dimensions crucially relies on the definition of confidence intervals for the assumed scaling range. We demonstrate that the determination of the intervals can be improved largely by comparing the scaling behavior of different morphological measures (area, boundary, curvature). We show that the combination of area and boundary data from coarse-grained structures obtained with the box-counting method reveals clear confidence limits and thus credible morphological data. This also holds for the Minkowski density method. It also reveals the confidence range. Its main drawback, the larger swing-in period at the lower cutoff compared to the box-counting method, is compensated by more details on the scaling behavior of area, boundary, and curvature. The sandbox method is less recommendable. It essentially delivers the same data as box-counting, but it is more susceptible to finite size effects at the lower cutoff. It is found that the domain morphology depends on the surface coverage of alkanes. The individual domains at low surface coverage have a fractal dimension of approximately 1.7, whereas at coverages well above 50% the scaling dimension is 2 with a large margin of uncertainty at approximately 50% coverage. This change in morphology is attributed to a crossover from a growth regime dominated by diffusion-limited aggregation of individual domains to a regime where the growth is increasingly affected by annealing and the interaction of solid growth fronts which approach each other and thus compete for the alkane supply. PMID:15667180

  7. Structural investigation of Langmuir and Langmuir-Blodgett monolayers of semifluorinated alkanes.

    PubMed

    Dynarowicz Łatka, Patrycja; Pérez-Morales, Marta; Muñoz, Eulogia; Broniatowski, Marcin; Martín-Romero, María T; Camacho, Luis

    2006-03-30

    The behavior of a semi-fluorinated alkane (C(10)F(21)C(19)H(39)) has been studied at the air-water interface by using surface pressure and surface potential-area isotherms as well as infrared spectroscopy for the Langmuir-Blodgett films. In addition, based on the quantum chemical PM3 semiempirical approach, the dimer structure was investigated, and the double helix was found to be the most stable conformation of the dimer. The obtained results allow us to imply that the phase transition observed in the course of the surface pressure/area isotherm is due to a conformational change originating from the double helix to a vertical, single helix configuration. PMID:16553421

  8. Biochemical studies on the metabolic activation of halogenated alkanes.

    PubMed Central

    Cheeseman, K H; Albano, E F; Tomasi, A; Slater, T F

    1985-01-01

    This paper reviews recent investigations by Slater and colleagues into the metabolic activation of halogenated alkanes in general and carbon tetrachloride in particular. It is becoming increasingly accepted that free radical intermediates are involved in the toxicity of many such compounds through mechanisms including lipid peroxidation, covalent binding, and cofactor depletion. Here we describe the experimental approaches that are used to establish that halogenated alkanes are metabolized in animal tissues to reactive free radicals. Electron spin resonance spectroscopy is used to identify free-radical products, often using spin-trapping compounds. The generation of specific free radicals by radiolytic methods is useful in the determination of the precise reactivity of radical intermediates postulated to be injurious to the cell. The enzymic mechanism of the production of such free radicals and their subsequent reactions with biological molecules is studied with specific metabolic inhibitors and free-radical scavengers. These combined techniques provide considerable insight into the process of metabolic activation of halogenated compounds. It is readily apparent, for instance, that the local oxygen concentration at the site of activation is of crucial importance to the subsequent reactions; the formation of peroxy radical derivatives from the primary free-radical product is shown to be of great significance in relation to carbon tetrachloride and may be of general importance. However, while these studies have provided much information on the biochemical mechanisms of halogenated alkane toxicity, it is clear that many problems remain to be solved. PMID:3007102

  9. A molecular dynamics study of the packing structures in monolayers of partially fluorinated amphiphiles

    NASA Astrophysics Data System (ADS)

    Shin, Seokmin; Collazo, Nancy; Rice, Stuart A.

    1992-01-01

    We report the results of molecular dynamics simulations of liquid-supported monolayers of three partially fluorinated amphiphile molecules, namely CF3(CF2)9CH2COOH, CF3(CF2)6CH2(CF2)3COOH, and CF3(CF2)6(CH2)4COOH. These studies were undertaken to provide information on the interplay between molecular flexibility and the packing structure in a monolayer so as to better interpret the results of recent experiments. The qualitative aspects of the predictions of the simulations are consistent with the recent experimental data for monolayers of CF3(CF2)9CH2COOH [S. W. Barton, A. Goudot, O. Boulassa, F. Rondelez, B. Lin, F. Novak, A. Acero, and S. A. Rice, J. Chem. Phys. 96, xxx (1992)]. In particular, the observed breakup of the homogeneous ordered monolayer into ordered islands with the same collective tilt of the molecules is correctly predicted, and the fact that the collective tilt angle is small is correctly predicted. However, the experimental and theoretical values of the tilt angles are not in quantitative agreement, which we attribute to the inadequacy of the atom-atom potentials used in the simulations. In general, for monolayers of CF3(CF2)9CH2COOH we find that the collective tilt angle predicted is a sensitive function of the area per molecule and is smaller than in monolayers of alkane alcohols and alkane acids. The results of the simulations of monolayers of other partially fluorinated species suggest that the difference in size between the fluorocarbon segments and the smaller head groups or flexible ``spacer'' CH2 segments can generate subtle changes in the packing structure of a monolayer and the relative stabilities of the untilted and tilted structures.

  10. Two-Step Freezing in Alkane Monolayers on Colloidal Silica Nanoparticles: From a Stretched-Liquid to an Interface-Frozen State.

    PubMed

    Gao, Xia; Huber, Patrick; Su, Yunlan; Zhao, Weiwei; Wang, Dujin

    2016-08-01

    The crystallization behavior of an archetypical soft/hard hybrid nanocomposite, that is, an n-octadecane C18/SiO2-nanoparticle composite, was investigated by a combination of differential scanning calorimetry (DSC) and variable-temperature solid-state (13)C nuclear magnetic resonance (VT solid-state (13)C NMR) as a function of silica nanoparticles loading. Two latent heat peaks prior to bulk freezing, observed for composites with high silica loading, indicate that a sizable fraction of C18 molecules involve two phase transitions unknown from the bulk C18. Combined with the NMR measurements as well as experiments on alkanes and alkanols at planar amorphous silica surfaces reported in the literature, this phase behavior can be attributed to a transition toward a 2D liquid-like monolayer and subsequently a disorder-to-order transition upon cooling. The second transition results in the formation of a interface-frozen monolayer of alkane molecules with their molecular long axis parallel to the nanoparticles' surface normal. Upon heating, the inverse phase sequence was observed, however, with a sizable thermal hysteresis in accord with the characteristics of the first-order phase transition. A thermodynamic model considering a balance of interfacial bonding, chain stretching elasticity, and entropic effects quantitatively accounts for the observed behavior. Complementary synchrotron-based wide-angle X-ray diffraction (WAXD) experiments allow us to document the strong influence of this peculiar interfacial freezing behavior on the surrounding alkane melts and in particular the nucleation of a rotator phase absent in the bulk C18. PMID:27386888

  11. Nonlinear optical techniques for surface studies. [Monolayers

    SciTech Connect

    Shen, Y.R.

    1981-09-01

    Recent effort in developing nonlinear optical techniques for surface studies is reviewed. Emphasis is on monolayer detection of adsorbed molecules on surfaces. It is shown that surface coherent antiStokes Raman scattering (CARS) with picosecond pulses has the sensitivity of detecting submonolayer of molecules. On the other hand, second harmonic or sum-frequency generation is also sensitive enough to detect molecular monolayers. Surface-enhanced nonlinear optical effects on some rough metal surfaces have been observed. This facilitates the detection of molecular monolayers on such surfaces, and makes the study of molecular adsorption at a liquid-metal interface feasible. Advantages and disadvantages of the nonlinear optical techniques for surface studies are discussed.

  12. Nonlinear optical studies of organic monolayers

    SciTech Connect

    Shen, Y.R.

    1988-02-01

    Second-order nonlinear optical effects are forbidden in a medium with inversion symmetry, but are necessarily allowed at a surface where the inversion summary is broken. They are often sufficiently strong so that a submonolayer perturbation of the surface can be readily detected. They can therefore be used as effective tools to study monolayers adsorbed at various interfaces. We discuss here a number of recent experiments in which optical second harmonic generation (SHG) and sum-frequency generation (SFG) are employed to probe and characterize organic monolayers. 15 refs., 5 figs.

  13. Kinetic study of asphaltene dissolution in amphiphile/alkane solutions

    SciTech Connect

    Permsukarome, P.; Chang, C.; Fogler, H.S.

    1997-09-01

    The kinetics of dissolution of pentane-insoluble solid asphaltene precipitates by amphiphile/alkane solutions were investigated using a differential reactor flow system. Two amphiphiles, dodecylbenzenesulfonic acid and nonylphenol, and five alkane solvents, ranging from hexane to hexadecane, were used. Results showed that the rate of asphaltene dissolution in amphiphile/alkane fluids could be approximated with a first-order kinetics with respect to the undissolved asphaltene mass in solution. The specific dissolution rate constant, k, varied with the concentration of amphiphiles, the type of alkane solvents, the temperature, and the fluid flow rate. The rate of asphaltene dissolution displayed a Langmuir-Hinshelwood kinetics with respect to the concentration of amphiphiles. Increasing the temperature of amphiphile/alkane fluids also enhanced the rate of asphaltene dissolution. The apparent activation energy for asphaltene dissolution was approximated to be 4--7 kcal/mol. The rate of asphaltene dissolution was also greater in amphiphile solutions containing lighter alkanes, such as hexane, with lower viscosities. These trends suggest that both surface reaction and mass transfer processes are important to the rate of asphaltene dissolution in amphiphile/alkane fluids.

  14. A nonequilibrium molecular dynamics study of the rheology of alkanes

    SciTech Connect

    Gupta, S.A.; Cui, S.T.; Cummings, P.T.; Cochran, H.D. |

    1996-05-01

    We examine the rheological properties of four different alkanes: n-decane, n-hexadecane, n-tetracosane, and squalane. Simulations of Couette flow are performed for a range of shear rates with 100 molecules in each case using a replicated data version of our code. Number of interaction sites ranges from 1000 to 3000. We have performed extremely long simulations required to obtain acceptable statistics at low shear rates. The alkanes show a transition from non-Newtonian to Newtonian behavior as the shear rate decreases to low values. 1 tab, 1 fig, 17 refs.

  15. STM studies of synthetic peptide monolayers

    SciTech Connect

    Bergeron, David J.; Clauss, Wilfried; Johnson, Alan T.; Pilloud, Denis L.; Leslie Dutton, P.

    1998-08-11

    We have used scanning probe microscopy to investigate self-assembled monolayers of chemically synthesized peptides. We find that the peptides form a dense uniform monolayer, above which is found a sparse additional layer. Using scanning tunneling microscopy, submolecular resolution can be obtained, revealing the alpha helices which constitute the peptide. The nature of the images is not significantly affected by the incorporation of redox cofactors (hemes) in the peptides.

  16. Magnetoluminescence study of WS2 monolayers

    NASA Astrophysics Data System (ADS)

    Scrace, T.; Tsai, Y.; Barman, B.; Schweidenback, L.; Petrou, A.; Kioseoglou, G.; Hawrylak, P.

    2014-03-01

    We have studied the photoluminescence (PL) spectra[2] from WS2 monolayers in the 5-150 K temperature range in magnetic fields up to 7 tesla applied along the normal to the sample plane. The luminescence was excited by a 488nm linearly polarized laser beam. The PL spectra have two features identified as the neutral (X) and negatively charged (X-) exciton. At zero magnetic field the X- feature has a large (as high as 30%), laser power-dependent circular polarization, in contrast to the small polarization of Xthat does not depend on laser power. The application of an external magnetic field has a profound effect on the circular polarization of the charged exciton. Its polarization increases by 10% at 7 tesla for any laser-power while its energy exhibits a small magnetic splitting (2meV at 7 tesla). On the other hand, the emitted circular polarization of the free exciton is not affected by the external magnetic field. This work has been supported by ONR.

  17. Stable isotopic studies of n-alkane metabolism by a sulfate-reducing bacterial enrichment culture.

    PubMed

    Davidova, Irene A; Gieg, Lisa M; Nanny, Mark; Kropp, Kevin G; Suflita, Joseph M

    2005-12-01

    Gas chromatography-mass spectrometry and nuclear magnetic resonance spectroscopy were used to study the metabolism of deuterated n-alkanes (C6 to C12) and 1-13C-labeled n-hexane by a highly enriched sulfate-reducing bacterial culture. All substrates were activated via fumarate addition to form the corresponding alkylsuccinic acid derivatives as transient metabolites. Formation of d14-hexylsuccinic acid in cell extracts from exogenously added, fully deuterated n-hexane confirmed that this reaction was the initial step in anaerobic alkane metabolism. Analysis of resting cell suspensions amended with 1-13C-labeled n-hexane confirmed that addition of the fumarate occurred at the C-2 carbon of the parent substrate. Subsequent metabolism of hexylsuccinic acid resulted in the formation of 4-methyloctanoic acid, and 3-hydroxy-4-methyloctanoic acid was tentatively identified. We also found that 13C nuclei from 1-13C-labeled n-hexane became incorporated into the succinyl portion of the initial metabolite in a manner that indicated that 13C-labeled fumarate was formed and recycled during alkane metabolism. Collectively, the findings obtained with a sulfate-reducing culture using isotopically labeled alkanes augment and support the previously proposed pathway (H. Wilkes, R. Rabus, T. Fischer, A. Armstroff, A. Behrends, and F. Widdel, Arch. Microbiol. 177:235-243, 2002) for metabolism of deuterated n-hexane by a denitrifying bacterium. PMID:16332800

  18. A sum-frequency generation spectroscopic study of the Gibbs analysis paradox: monolayer or sub-monolayer adsorption?

    PubMed

    Shahir, Afshin Asadzadeh; Nguyen, Khoi Tan; Nguyen, Anh V

    2016-03-23

    The Gibbs adsorption isotherm (GAI) has been considered as the foundation of surfactant adsorption studies for over a century; however, its application in determining the limiting surface excess has recently been intensively discussed, with contradictory experimental evidence either supporting or refuting the theory. The available arguments are based on monolayer adsorption models. In this paper, we experimentally and intellectually propose and validate the contribution of sub-monolayer adsorption to the GAI paradox. We utilize a powerful intrinsically surface-sensitive technique, vibrational sum-frequency generation spectroscopy (SFG), complementing with conventional tensiometric measurements to address these controversies both quantitatively and qualitatively. Our SFG results revealed that the precipitous decrease in surface tension directly corresponds to surface occupancy by adsorbates. In addition, the Gibbs analysis was successfully applied to the soluble monolayer of a surface-active alcohol to full saturation. However, the full saturation of the topmost monolayer does not necessarily mean that the surface adsorption was completed because the adsorption was observed to continuously occur in the sub-monolayer region soon after the topmost monolayer became saturated. Nonetheless, the Gibbs isotherm failed to account for the excess of alcohol adsorbed in this sub-monolayer region. This new concept of surface excess must therefore be treated thermodynamically. PMID:26661072

  19. Synthesis, conformational and theoretical studies of 1,n-di(2-formyl-4-phenylazophenoxy)alkanes

    NASA Astrophysics Data System (ADS)

    Balachander, R.; Manimekalai, A.

    2016-01-01

    1,n-di(2-Formyl-4-phenylazophenoxy)alkanes 1 and 2 and 1,3-di(2-formyl-4-phenylazophenoxymethyl)benzene 3 were synthesis and characterized by FT-IR, UV-Vis, 1H, 13C NMR and mass spectral studies. The stable conformations of 1-3 were predicted theoretically and selected geometrical parameters were derived from optimized structures. The molecular parameters of HOMO-LUMO energies, polarizability, hyperpolarizability, natural bond orbital (NBO), atom in molecule (AIM) analysis and molecular electrostatic potential (MEP) surfaces were determined by the density functional theory (DFT) method and analysed.

  20. Second harmonic generation study of liquid surface and amphilphile monolayer

    SciTech Connect

    Zhao, X.

    1992-12-31

    This thesis contains the following topics. In Chapter 2, we use temperature dependent SHC to study the surface structure of some neat liquids. It was found all the hydrogen bonding liquids have a stronger SHG dependence of temperature while for the non-hydrogen bonding liquids SH response is insensitive to temperature change. In Chapter 3, we discussed the general feature of the electric field induced second harmonic generation (EFISHG). We experimentally established the importance of the {chi}{sup (3)} mechanism at the air/water charged interface. In Chapter 4, we used SHG to study the p-nitrophenol (PNP) penetration into the insoluble monolayer of hexadecyl and pentadecyl acid. It was found that PNP preferentially adsorbs at the fatty acid monolayer. The population and orientation of PNP is strongly dependent on the fatty acid monolayer packing. In Chapter 5, SH signal fluctuation spectroscopy was used to study the surface self-diffusion within the monolayer gas-liquid co-existence region. Assuming the uniform distribution of the liquid phase cluster size, a diffusion model was developed to fit the experimental measured autocorrelation function. In Chapter 6, SH fluctuation spectroscopy was used to study the monolayer orientation spontaneous fluctuation. It was found that only the electron rich aromatic head groups exhibit the orientational fluctuation while the electron poor aromatic molecules do not have a fluctuation anomalies. Fluctuation time scale is strongly influenced by the hydrophobic chain length. A quantitative theory was presented based on Landau-Ginzburg theory to fit the time dependent orientation fluctuation. In Chapter 7, we focused on the acid-base equilibrium constant measurement at the air/water interface using both {chi}{sup (2)} and {chi}{sup (2)} method. We have measured pKa of aniline, phenol, amine, and acid at the interface. It turned out that for all these species, the neutral form is more stable at the interface.

  1. Added Alkane Allows Thermal Thinning of Supramolecular Columns by Forming Superlattice-An X-ray and Neutron Study.

    PubMed

    Yen, Ming-Huei; Chaiprapa, Jitrin; Zeng, Xiangbing; Liu, Yongsong; Cseh, Liliana; Mehl, Georg H; Ungar, Goran

    2016-05-11

    We report a columnar superlattice formed by blends of dendron-like Li 3,4,5-tris(n-alkoxy)benzoates with n-alkanes. Without the alkane, the wedge-shaped molecules form liquid crystal columns with 3 dendrons in a supramolecular disk. The same structure exists in the blend, but on heating one dendron is expelled from the disks in every third column and is replaced by the alkane. This superlattice of unequal columns is confirmed by complementary X-ray and neutron diffraction studies. Lateral thermal expansion of dendrons normally leads to the expulsion of excess molecules from the column, reducing the column diameter. However, in the already narrow columns of pure Li salt, expulsion of one of only three dendrons in a disk is not viable. The added alkane facilitates the expulsion, as it replaces the missing dendron. Replacing the alkane with a functional compound can potentially lead to active nanoarrays with relatively large periodicity by using only small molecules. PMID:27101731

  2. Line tension of alkane lenses on aqueous surfactant solutions at phase transitions of coexisting interfaces.

    PubMed

    Matsubara, Hiroki; Ushijima, Baku; Law, Bruce M; Takiue, Takanori; Aratono, Makoto

    2014-04-01

    Alkane droplets on aqueous solutions of surfactants exhibit a first-order wetting transition as the concentration of surfactant is increased. The low-concentration or "partial wetting" state corresponds to an oil lens in equilibrium with a two-dimensional dilute gas of oil and surfactant molecules. The high-concentration or "pseudo-partial wetting" state consists of an oil lens in equilibrium with a mixed monolayer of surfactant and oil. Depending on the combination of surfactant and oil, these mixed monolayers undergo a thermal phase transition upon cooling, either to a frozen mixed monolayer or to an unusual bilayer structure in which the upper leaflet is a solid layer of pure alkane with hexagonal packing and upright chains while the lower leaflet remains a disordered liquid-like mixed monolayer. Additionally, certain long-chain alkanes exhibit a surface freezing transition at the air-oil interface where the top monolayer of oil freezes above its melting point. In this review, we summarize our previous studies and discuss how these wetting and surface freezing transitions influence the line tension of oil lenses from both an experimental and theoretical perspective. PMID:24007861

  3. Thermodynamic aspects of cholesterol effect on properties of phospholipid monolayers: Langmuir and Langmuir-Blodgett monolayer study.

    PubMed

    Jurak, Małgorzata

    2013-04-01

    Cholesterol is an important component of lipid rafts in mammalian cell membranes. Studies of phospholipid monolayers containing cholesterol provide insight into the role of cholesterol in regulating the properties of animal cells, raft stability, and organization. In this contribution, a study of the characteristics of binary Langmuir monolayers consisting of phospholipids, 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC), 1,2-dioleoyl-sn-glycero-3-phosphocholine (DOPC), 2-oleoyl-1-palmitoyl-sn-glycero-3-phosphocholine (POPC), 1,2-dipalmitoyl-sn-glycero-3-phospho-rac-(1-glycerol) (DPPG), and cholesterol (Chol), was conducted on the basis of the surface pressure-area per molecule (π-A) isotherms. Analysis of the results obtained provided information on the mean molecular area, the excess Gibbs energy of mixing, and condensation in the monolayer. The mixed monolayers were also deposited onto the mica plates and investigated by the contact angle measurements of water, formamide, and diiodomethane. The contact angles allowed calculating surface free energy of the films from the van Oss et al. approach. It was found that cholesterol determines the molecular packing and ordering of the monolayers closely connected with the kind of phospholipid. This is reflected in the values of surface free energy of the model membranes. From the thermodynamic analysis of phospholipid/cholesterol/liquid interactions, one may draw conclusions about the most favorable composition (stoichiometry) of the binary film which is especially important in view of the lipid rafts formation. PMID:23470025

  4. Ab initio studies of excitations in monolayer black phosphorus

    NASA Astrophysics Data System (ADS)

    Frank, Tobias; Kurpas, Marcin; Gmitra, Martin; Derian, Rene; Stich, Ivan; Fabian, Jaroslav

    Monolayer black phosphorus, or phosphorene, represents an ideal system to study many-body electron-electron and electron-hole interactions due to its strong anisotropy driven 1d electronic nature. In particular, the size of the fundamental band gap value and excitonic binding energies remain unresolved given the different gap values of 1.6 to 2.4 eV obtained by many-body GW calculations. We present our contribution to this issue studying excitations in phosphorene employing quantum monte carlo (QMC) calculations. We show the evolution of finite size effects of the fundamental and optical gap, with respect to relatively large supercell sizes in the theoretical framework of diffusion monte carlo (DMC) explicitly including electronic correlations. Our studies point to a significant influence of electron correlation on the fundamental gap as well as to a strong anisotropic nature of the excitonic state. Furthermore we address the question of a multiconfigurational ground state in monolayer black phosphorus. This work is supported by the DFG GRK 1570, SFB 689, and European Union Seventh Framework Programme under Grant Agreement No. 604391 Graphene Flagship.

  5. Numerical and experimental studies of ethanol flames and autoignition theory for higher alkanes

    NASA Astrophysics Data System (ADS)

    Saxena, Priyank

    In order to enhance the fuel efficiency of an engine and to control pollutant formation, an improved understanding of the combustion chemistry of the fuels at a fundamental level is paramount. This knowledge can be gained by developing detailed reaction mechanisms of the fuels for various combustion processes and by studying combustion analytically employing reduced-chemistry descriptions. There is a need for small detailed reaction mechanisms for alkane and alcohol fuels with reduced uncertainties in their combustion chemistry that are computationally cheaper in multidimensional CFD calculations. Detailed mechanisms are the starting points in identifying reduced-chemistry descriptions of combustion processes to study problems analytically. This research includes numerical, experimental and analytical studies. The first part of the dissertation consists of numerical and experimental studies of ethanol flames. Although ethanol has gained popularity as a possible low-pollution source of renewable energy, significant uncertainties remain in its combustion chemistry. To begin to address ethanol combustion, first a relatively small detailed reaction mechanism, commonly known as the San Diego Mech, is developed for the combustion of hydrogen, carbon monoxide, formaldehyde, methane, methanol, ethane, ethylene, and acetylene, in air or oxygen-inert mixtures. This mechanism is tested for autoignition, premixed-flame burning velocities, and structures and extinction of diffusion flames and of partially premixed flames of many of these fuels. The reduction in uncertainties in the combustion chemistry can best be achieved by consistently updating a reaction mechanism with reaction rate data for the elementary steps based on newer studies in literature and by testing it against as many experimental conditions as available. The results of such a testing for abovementioned fuels are reported here along with the modifications of reaction-rate parameters of the most important

  6. Comparative study of alkali-vapour cells with alkane-, alkeneand 1-nonadecylbenzene-based antirelaxation wall coatings

    NASA Astrophysics Data System (ADS)

    Balabas, M. V.; Tretiak, O. Yu

    2013-12-01

    The dependence of both longitudinal and transverse relaxation times of ground-state magnetic polarisation in alkali atoms on the coating temperature is experimentally studied for the first time in a rubidium-vapour cell with 1-nonadecylbenzene antirelaxation coating of inner walls. The comparison of these times with the relaxation times in a caesium-vapour cell with alkane wall coatings is presented. It is found that within the studied temperature range (294 - 340K) the transverse relaxation time decreases with increasing temperature of alkene and 1-nonadecylbenzene coatings. For the alkane coating such a dependence was not explicitly found. The longitudinal relaxation time begins to decrease in all cases when passing a certain critical temperature of the coating material. It is found that the unsaturated radical structure of the coating material molecules strongly affects its antirelaxation properties.

  7. Comparative study of alkali-vapour cells with alkane-, alkeneand 1-nonadecylbenzene-based antirelaxation wall coatings

    SciTech Connect

    Balabas, M V; Tretiak, O Yu

    2013-12-31

    The dependence of both longitudinal and transverse relaxation times of ground-state magnetic polarisation in alkali atoms on the coating temperature is experimentally studied for the first time in a rubidium-vapour cell with 1-nonadecylbenzene antirelaxation coating of inner walls. The comparison of these times with the relaxation times in a caesium-vapour cell with alkane wall coatings is presented. It is found that within the studied temperature range (294 – 340K) the transverse relaxation time decreases with increasing temperature of alkene and 1-nonadecylbenzene coatings. For the alkane coating such a dependence was not explicitly found. The longitudinal relaxation time begins to decrease in all cases when passing a certain critical temperature of the coating material. It is found that the unsaturated radical structure of the coating material molecules strongly affects its antirelaxation properties. (optical pumping)

  8. Doping of rhenium disulfide monolayers: a systematic first principles study.

    PubMed

    Çakır, Deniz; Sahin, Hasan; Peeters, François M

    2014-08-21

    The absence of a direct-to-indirect band gap transition in ReS2 when going from the monolayer to bulk makes it special among the other semiconducting transition metal dichalcogenides. The functionalization of this promising layered material emerges as a necessity for the next generation technological applications. Here, the structural, electronic, and magnetic properties of substitutionally doped ReS2 monolayers at either the S or Re site were systematically studied by using first principles density functional calculations. We found that substitutional doping of ReS2 depends sensitively on the growth conditions of ReS2. Among the large number of non-metallic atoms, namely H, B, C, Se, Te, F, Br, Cl, As, P, and N, we identified the most promising candidates for n-type and p-type doping of ReS2. While Cl is an ideal candidate for n-type doping, P appears to be the most promising candidate for p-type doping of the ReS2 monolayer. We also investigated the doping of ReS2 with metal atoms, namely Mo, W, Ti, V, Cr, Co, Fe, Mn, Ni, Cu, Nb, Zn, Ru, Os and Pt. Mo, Nb, Ti, and V atoms are found to be easily incorporated in a single layer of ReS2 as substitutional impurities at the Re site for all growth conditions considered in this work. Tuning chemical potentials of dopant atoms energetically makes it possible to dope ReS2 with Fe, Co, Cr, Mn, W, Ru, and Os at the Re site. We observe a robust trend for the magnetic moments when substituting a Re atom with metal atoms such that depending on the electronic configuration of dopant atoms, the net magnetic moment of the doped ReS2 becomes either 0 or 1 μB. Among the metallic dopants, Mo is the best candidate for p-type doping of ReS2 owing to its favorable energetics and promising electronic properties. PMID:25001566

  9. Quantifying Dimer and Trimer Formation of Tri-n-butyl Phosphates in Different Alkane Diluents: FTIR Study.

    PubMed

    Vo, Quynh N; Unangst, Jaclynn L; Nguyen, Hung D; Nilsson, Mikael

    2016-07-21

    Tri-n-butyl phosphate (TBP), a representative of neutral organophosphorous metal-ion-extracting reagents, is an important ligand used in solvent extraction processes for the recovery of uranium and plutonium from spent nuclear fuel, as well as other non-nuclear applications. Ligand-ligand and organic solvent-ligand interactions play an important role in these processes. The self-association behavior of TBP in various alkane diluents of different chain lengths (8, 12, and 16 carbons) and a branched alkane (iso-octane) was investigated by Fourier transform infrared spectroscopic measurements. By careful deconvolution of the spectra into multiple peaks, our results indicate that TBP self-associates to form not only dimers, as previous studies showed, but also trimers in the practical concentration range. Using a mathematical fitting procedure, the dimerization and trimerization constants were determined. As expected, these equilibrium constants are dependent on the solvent used. As the alkane chain for linear hydrocarbon solvents becomes longer, dimerization decreases whereas trimerization increases. For the more branched hydrocarbon, we observe a significantly higher dimerization constant. These effects are most likely due to the intermolecular van der Waals interactions between the butyl tails of each TBP molecule and the diluent hydrocarbon chain as all solvents in this study are relatively nonpolar. PMID:27399338

  10. Semifluorinated Alkane Eye Drops for Treatment of Dry Eye Disease—A Prospective, Multicenter Noninterventional Study

    PubMed Central

    Scherer, Dieter; Krösser, Sonja; Beckert, Michael; Cursiefen, Claus; Kaercher, Thomas

    2015-01-01

    Abstract Purpose: Evaporation of the tear film is heavily discussed as one core reason for dry eye disease (DED). Subsequently, new artificial tear products are developed that specifically target this pathomechanism. Perfluorohexyloctane (F6H8, NovaTears®) from the family of semifluorinated alkanes is a novel substance that has been approved as a medical device, as a nonblurring wetting agent for the ocular surface. Methods: Thirty patients with hyperevaporative dry eye received F6H8 during a prospective, multicenter, observational 6-week study. Patients were advised to apply 1 drop 4 times daily in both eyes. Parameters assessed included best corrected visual acuity, intraocular pressure, Schirmer I test, tear fluid, tear film breakup time (TFBUT), corneal staining, meibum secretion, and Ocular Surface Disease Index (OSDI©). Results: From the 30 patients recruited, 25 completed the trial per protocol. Four patients discontinued F6H8 and 1 patient did not present for follow-up. F6H8 treatment led to significant reduction of corneal staining and significant increase of Schirmer I and TFBUT. In addition, OSDI score dropped significantly from a mean of 55 (±23.0) to 34 (±22.4). Visual acuity and ocular pressure did not change. Conclusions: This prospective observational study shows significant beneficial effects in patients suffering from evaporative DED, using F6H8 in all the relevant parameters tested. The decrease of the OSDI by a mean of 21 points was particularly remarkable and clearly exceeds minimal, clinical important differences for mild or moderate and severe disease. Overall, F6H8 (NovaTears) seems to be safe and effective in treating mild to moderate hyperevaporative DED. PMID:26296040

  11. Flow reactor studies of non-equilibrium plasma-assisted oxidation of n-alkanes.

    PubMed

    Tsolas, Nicholas; Lee, Jong Guen; Yetter, Richard A

    2015-08-13

    The oxidation of n-alkanes (C1-C7) has been studied with and without the effects of a nanosecond, non-equilibrium plasma discharge at 1 atm pressure from 420 to 1250 K. Experiments have been performed under nearly isothermal conditions in a flow reactor, where reactive mixtures are diluted in Ar to minimize temperature changes from chemical reactions. Sample extraction performed at the exit of the reactor captures product and intermediate species and stores them in a multi-position valve for subsequent identification and quantification using gas chromatography. By fixing the flow rate in the reactor and varying the temperature, reactivity maps for the oxidation of fuels are achieved. Considering all the fuels studied, fuel consumption under the effects of the plasma is shown to have been enhanced significantly, particularly for the low-temperature regime (T<800 K). In fact, multiple transitions in the rates of fuel consumption are observed depending on fuel with the emergence of a negative-temperature-coefficient regime. For all fuels, the temperature for the transition into the high-temperature chemistry is lowered as a consequence of the plasma being able to increase the rate of fuel consumption. Using a phenomenological interpretation of the intermediate species formed, it can be shown that the active particles produced from the plasma enhance alkyl radical formation at all temperatures and enable low-temperature chain branching for fuels C3 and greater. The significance of this result demonstrates that the plasma provides an opportunity for low-temperature chain branching to occur at reduced pressures, which is typically observed at elevated pressures in thermal induced systems. PMID:26170423

  12. Structural and Kinetic Studies of Novel Cytochrome P450 Small-Alkane Hydroxylases

    SciTech Connect

    Arnold, Frances H.

    2012-02-27

    The goals of this project are to investigate (1) the kinetics and stabilities of engineered cytochrome P450 (P450) small alkane hydroxylases and their evolutionary intermediates, (2) the structural basis for catalytic proficiency on small alkanes of these engineered P450s, and (3) the changes in redox control resulting from protein engineering. To reach these goals, we have established new methods for determining the kinetics and stabilities of multicomponent P450s such as CYP153A6. Using these, we were able to determine that CYP153A6 is proficient for hydroxylation of alkanes as small as ethane, an activity that has never been observed previously in any natural P450. To elucidate the structures of the engineered P450s, we obtained x-ray diffraction data for two variants in the P450PMO (propane monooxygenase) lineage and a preliminary structure for the most evolved variant. This structure shows changes in the substrate binding regions of the enzyme and a reduction in active site volume that are consistent with the observed changes in substrate specificity from fatty acids in the native enzyme to small alkanes in P450PMO. We also constructed semi-rational designed libraries mutating only residues in the enzyme active site that in one round of mutagenesis and screening produced variants that achieved nearly half of the activity of the most evolved enzymes of the P450PMO lineage. Finally, we found that changes in redox properties of the laboratory-evolved P450 alkane hydroxylases did not reflect the improvement in their electron transfer efficiency. The heme redox potential remained constant throughout evolution, while activity increased and coupling efficiency improved from 10% to 90%. The lack of correlation between heme redox potential and enzyme activity and coupling efficiency led us to search for other enzyme properties that could be better predictors for activity towards small alkanes, specifically methane. We investigated the oxidation potential of the radical

  13. Raman study of local ordering processes of solid n-alkanes

    NASA Astrophysics Data System (ADS)

    Hacura, A.; Zimnicka, B.; Wrzalik, R.

    2016-02-01

    The microphase separation of n-alkanes with different chain length was investigated by Raman spectroscopy for binary mixture rapidly quenched from the melt. The process was observed as a function of time. The first several minutes after solidification were crucial for the demixing process. For a few weeks old sample the orientational order parameters and were calculated based on the analysis of polarized spectra recorded in the area of the formed domains. The measured values are significantly greater than zero (from 0.17 to 0.32), which indicates the mutual parallel arrangement of the molecules in the domains composed of n-alkanes of the same chain length.

  14. Scanning tunneling microscopy studies of mixed self-assembled monolayers

    NASA Astrophysics Data System (ADS)

    Raigoza, Annette Fernandez

    This thesis examines the formation of multicomponent self-assembled mono-layers (SAMs) on the Au(111) surface using scanning tunneling microscopy. Two methods, sequential adsorption and coadsorption, are used to create these mixed SAMs. In the sequential adsorption experiments, a clean Au(111)-on-mica sub-strate is exposed to the first molecular species and then this adsorbate-covered sample is exposed to the second molecular species. Alternately, in the coadsorption experiments, a gold surface is exposed to both adsorbates simultaneously. Exposing a coronene- or dithiocarbamate-covered surface to excess thiol in the vapor phase results in a drastic restructuring of the initial surface. This is primarily driven by the kinetics of the octanethiol monolayer formation process, but the extent to which this happens is dependent on the molecule-molecule and molecule-surface interactions of the adsorbate due to the initial coverage and order of the monolayer. An octanethiolate monolayer is also substantially modified when immersed in a solution containing dithiocarbamate (DTC). Defects in the octanethiol monolayer are prime sites for molecular exchange. A surplus of DTC in the solution drives substitution that can lead to the complete removal of thiol from the surface. When a Au(111) surface is exposed to solutions containing both octanethiol and dithiocarbamate (DTC), both molecular species compete for available ad- sorption sites. At equal octanethiol-to-DTC ratios, molecular exchange hinders octanethiol monolayer formation. Higher octanethiol concentration in solution results in the incorporation of thiol into the resulting monolayer, with a strong dependence on the chain length of the DTC molecules.

  15. First principles study of metal contacts to monolayer black phosphorous

    SciTech Connect

    Chanana, Anuja; Mahapatra, Santanu

    2014-11-28

    Atomically thin layered black phosphorous (BP) has recently appeared as an alternative to the transitional metal dichalcogenides for future channel material in a metal-oxide-semiconductor transistor due to its lower carrier effective mass. Investigation of the electronic property of source/drain contact involving metal and two-dimensional material is essential as it impacts the transistor performance. In this paper, we perform a systematic and rigorous study to evaluate the Ohmic nature of the side-contact formed by the monolayer BP (mBP) and metals (gold, titanium, and palladium), which are commonly used in experiments. Employing the Density Functional Theory, we analyse the potential barrier, charge transfer and atomic orbital overlap at the metal-mBP interface in an optimized structure to understand how efficiently carriers could be injected from metal contact to the mBP channel. Our analysis shows that gold forms a Schottky contact with a higher tunnel barrier at the interface in comparison to the titanium and palladium. mBP contact with palladium is found to be purely Ohmic, where as titanium contact demonstrates an intermediate behaviour.

  16. 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

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

    NASA Astrophysics Data System (ADS)

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

    2016-07-01

    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.

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

    PubMed

    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

  19. Drug-Membrane Interactions Studied in Phospholipid Monolayers Adsorbed on Non-porous Alkylated Microspheres

    PubMed Central

    LUKACOVA, VIERA; PENG, MING; FANUCCI, GAIL; TANDLICH, ROMAN; HINDERLITER, ANNE; MAITY, BIKASH; MANIVANNAN, ETHIRAJAN; COOK, GREGORY R.; BALAZ, STEFAN

    2008-01-01

    Characterization of interactions with phospholipids is an integral part of the in vitro profiling of drug candidates because of the roles the interactions play in tissue accumulation and passive diffusion. Currently used test systems may inadequately emulate the bilayer core solvation properties (immobilized artificial membranes - IAM), suffer from potentially slow transport of some chemicals (liposomes in free or immobilized forms), and require a tedious separation (if used for free liposomes). Here we introduce a well-defined system overcoming these drawbacks: nonporous octadecylsilica particles coated with a self-assembled phospholipid monolayer. The coating mimics the structure of the headgroup region, as well as the thickness and properties of the hydrocarbon core more closely than IAM. The monolayer has a similar transition temperature pattern as the corresponding bilayer. The particles can be separated by filtration or a mild centrifugation. The partitioning equilibria of 81 tested chemicals were dissected into the headgroup and core contributions, the latter using the alkane/water partition coefficients. The deconvolution allowed a successful prediction of the bilayer/water partition coefficients with the standard deviation of 0.26 log units. The plate-friendly assay is suitable for high-throughput profiling of drug candidates without sacrificing the quality of analysis or details of the drug-phospholipid interactions. PMID:17218665

  20. Interactions of gas molecules with monolayer MoSe2: A first principle study

    NASA Astrophysics Data System (ADS)

    Sharma, Munish; Jamdagni, Pooja; Kumar, Ashok; Ahluwalia, P. K.

    2016-05-01

    We present a first principle study of interaction of toxic gas molecules (NO, NO2 and SO2) with monolayer MoSe2. The predicted order of sensitivity of gas molecule is NO2 > SO2 > NO. Adsorbed molecules strongly influence the electronic behaviour of monolayer MoSe2 by inducing impurity levels in the vicinity of Fermi energy. NO and SO2 is found to induce p-type doping effect while semiconductor to metallic transitions occur on NO2 adsorption. Our findings may guide the experimentalist for fabricating sensor devices based on MoSe2 monolayer.

  1. Atomic force microscopy studies of domain structures in phase-separated monolayers

    NASA Astrophysics Data System (ADS)

    Xiao, Shou-Jun; Wu, Hai-Ming; Yang, Xiao-Min; Wei, Yu; Tai, Zi-Hou; Sun, Xing-Zhong

    1994-10-01

    Domain structures were studied with atomic force microscopy (AFM) in binary phase-separated monolayer films composed of 5, 10, 15-triphenyl-20-(4-dl-α-phenylalanylamindo) phenyl porphyrin (TPPP) and one of a series of fatty acids which are arachidic acid (AA), palmitic acid (PA), and lauric acid (LA). The liquid-condensed (LC) domain structures of AA and PA were observed in their corresponding mixed monolayers. However, instead of the fatty acid domain, a liquid-expanded (LE) domain structure of TPPP appears in the mixed monolayer of LA/TPPP.

  2. [Molecular topology study of gas chromatographic retention indices of alkane series].

    PubMed

    Nie, Changming; Dai, Yimin; Wen, Songnian; Li, Zhonghai

    2005-01-01

    The gas chromatographic retention indices can be used to qualify some organic compounds. A new topological index based on distance matrix and branch vertex of the atoms in a molecule is proposed by defining equilibrium electronegativity of atoms in the molecule and coloring atoms in the molecular graph with equilibrium electronegativity, which appears unique to the molecular structures and has excellent structural selectivity. The multivariate linear equations of gas chromatographic retention indices are as follows: I(Squalane) = 23.97842N1 - 3.86562N2 + 0.787379N3 + 42.33061, R = 0.9922, n = 70, S = 13.70405, F = 1396.601; I(SE-30) = 23.83937N1 - 3.5687N2 + 0.939876N3 + 22.11952, R = 0.9919, n = 37, S = 11.96088, F = 668.8781; where the N1, N2 and N3 are a group of topological indices; n, R, S and F are sample number, regression coefficient, residual standard deviation and F-statistic value, respectively. The calculated results by the formulae indicate that the average relative deviations between calculated values and experimental data of gas chromatographic retention indices of alkane series on both squalane (column temperature 50 degrees C) and SE-30 (column temperature 80 degrees C) were all 1.31% and the errors were within experimental deviations. The equations can express well the change rule of the relative gas chromatographic retention indices of alkane series. PMID:15881357

  3. Liquid-liquid interfaces of semifluorinated alkane diblock copolymers with water, alkanes, and perfluorinated alkanes.

    SciTech Connect

    Perahia, Dvora, Dr.; Pierce, Flint; Tsige, Mesfin; Grest, Gary Stephen, Dr.

    2008-08-01

    The liquid-liquid interface between semifluorinated alkane diblock copolymers of the form F3C(CF2)n-1-(CH2)m-1CH3 and water, protonated alkanes, and perfluorinated alkanes are studied by fully atomistic molecular dynamics simulations. A modified version of the OPLS-AA (Optimized Parameter for Liquid Simulation All-Atom) force field of Jorgensen et al. has been used to study the interfacial behavior of semifluorinated diblocks. Aqueous interfaces are found to be sharp, with correspondingly large values of the interfacial tension. Due to the reduced hydrophobicity of the protonated block compared to the fluorinated block, hydrogen enhancement is observed at the interface. Water dipoles in the interfacial region are found to be oriented nearly parallel to the liquid-liquid interface. A number of protonated alkanes and perfluorinated alkanes are found to be mutually miscible with the semifluorinated diblocks. For these liquids, interdiffusion follows the expected Fickian behavior, and concentration-dependent diffusivities are determined.

  4. Synchrotron X-Ray Studies of Monolayers at Air - Interfaces and on Solid Substrates.

    NASA Astrophysics Data System (ADS)

    Shih, Mingchih

    We have used X-ray diffraction to study the structures of Langmuir monolayers. We find that both heneicosanoic acid and heneicosanol monolayers show the same untilted structures at high monolayer pressure as do lamellar paraffins; in other words, the high-pressure untilted structures are determined by chain packing and not by the head group. At lower pressures, however, the structures, lattice spacings, and tilt angles and tilt directions all appear to be determined by competition between head group and tail group interactions. For example, we find that the effect of increasing subphase pH on heneicosanoic acid monolayers is to reduce the in -plane spacings at zero pressure, and thus reduce the tilt angles and move the phase transitions to lower pressure. Again, while acid monolayers show phases with both nearest-neighbor and next-nearest-neighbor tilts, alcohol monolayers show next-nearest-neighbor tilts only. Finally, we have studied the correlation between structures of phases on water and structures of films transferred from these phases to solid substrates; we find that the structures are not preserved, although there are small "memory effects".

  5. Comparative Study on Single-Molecule Junctions of Alkane- and Benzene-Based Molecules with Carboxylic Acid/Aldehyde as the Anchoring Groups.

    PubMed

    Chen, Fang; Peng, Lin-Lu; Hong, Ze-Wen; Mao, Jin-Chuan; Zheng, Ju-Fang; Shao, Yong; Niu, Zhen-Jiang; Zhou, Xiao-Shun

    2016-12-01

    We have measured the alkane and benzene-based molecules with aldehyde and carboxylic acid as anchoring groups by using the electrochemical jump-to-contact scanning tunneling microscopy break junction (ECSTM-BJ) approach. The results show that molecule with benzene backbone has better peak shape and intensity than those with alkane backbone. Typically, high junction formation probability for same anchoring group (aldehyde and carboxylic acid) with benzene backbone is found, which contributes to the stronger attractive interaction between Cu and molecules with benzene backbone. The present work shows the import role of backbone in junction, which can guide the design molecule to form effective junction for studying molecular electronics. PMID:27566686

  6. Mechanistic studies on the gas-phase dehydrogenation of alkanes at cyclometalated platinum complexes.

    PubMed

    Butschke, Burkhard; Schwarz, Helmut

    2012-10-29

    In the ion/molecule reactions of the cyclometalated platinum complexes [Pt(L-H)](+) (L=2,2'-bipyridine (bipy), 2-phenylpyridine (phpy), and 7,8-benzoquinoline (bq)) with linear and branched alkanes C(n)H(2n+2) (n=2-4), the main reaction channels correspond to the eliminations of dihydrogen and the respective alkenes in varying ratios. For all three couples [Pt(L-H)](+)/C(2)H(6), loss of C(2)H(4) dominates clearly over H(2) elimination; however, the mechanisms significantly differs for the reactions of the "rollover"-cyclometalated bipy complex and the classically cyclometalated phpy and bq complexes. While double hydrogen-atom transfer from C(2)H(6) to [Pt(bipy-H)](+), followed by ring rotation, gives rise to the formation of [Pt(H)(bipy)](+), for the phpy and bq complexes [Pt(L-H)](+), the cyclometalated motif is conserved; rather, according to DFT calculations, formation of [Pt(L-H)(H(2))](+) as the ionic product accounts for C(2)H(4) liberation. In the latter process, [Pt(L-H)(H(2))(C(2)H(4))](+) (that carries H(2) trans to the nitrogen atom of the heterocyclic ligand) serves, according to DFT calculation, as a precursor from which, due to the electronic peculiarities of the cyclometalated ligand, C(2)H(4) rather than H(2) is ejected. For both product-ion types, [Pt(H)(bipy)](+) and [Pt(L-H)(H(2))](+) (L=phpy, bq), H(2) loss to close a catalytic dehydrogenation cycle is feasible. In the reactions of [Pt(bipy-H)](+) with the higher alkanes C(n)H(2n+2) (n=3, 4), H(2) elimination dominates over alkene formation; most probably, this observation is a consequence of the generation of allyl complexes, such as [Pt(C(3)H(5))(bipy)](+). In the reactions of [Pt(L-H)](+) (L=phpy, bq) with propane and n-butane, the losses of the alkenes and dihydrogen are of comparable intensities. While in the reactions of "rollover"-cyclometalated [Pt(bipy-H)](+) with C(n)H(2n+2) (n=2-4) less than 15 % of the generated product ions are formed by C-C bond-cleavage processes, this value is

  7. Improved Alkane Production in Nitrogen-Fixing and Halotolerant Cyanobacteria via Abiotic Stresses and Genetic Manipulation of Alkane Synthetic Genes.

    PubMed

    Kageyama, Hakuto; Waditee-Sirisattha, Rungaroon; Sirisattha, Sophon; Tanaka, Yoshito; Mahakhant, Aparat; Takabe, Teruhiro

    2015-07-01

    Cyanobacteria possess the unique capacity to produce alkane. In this study, effects of nitrogen deficiency and salt stress on biosynthesis of alkanes were investigated in three kinds of cyanobacteria. Intracellular alkane accumulation was increased in nitrogen-fixing cyanobacterium Anabaena sp. PCC7120, but decreased in non-diazotrophic cyanobacterium Synechococcus elongatus PCC7942 and constant in a halotolerant cyanobacterium Aphanothece halophytica under nitrogen-deficient condition. We also found that salt stress increased alkane accumulation in Anabaena sp. PCC7120 and A. halophytica. The expression levels of two alkane synthetic genes were not upregulated significantly under nitrogen deficiency or salt stress in Anabaena sp. PCC7120. The transformant Anabaena sp. PCC7120 cells with additional alkane synthetic gene set from A. halophytica increased intracellular alkane accumulation level compared to control cells. These results provide a prospect to improve bioproduction of alkanes in nitrogen-fixing halotolerant cyanobacteria via abiotic stresses and genetic engineering. PMID:25971893

  8. Carbon Isotopes of Alkanes in Hydrothermal Abiotic Organic Synthesis Processes at High Temperatures and Pressures: An Experimental Study

    NASA Technical Reports Server (NTRS)

    Fu, Qi; Socki, Richard A.; Niles, Paul B.

    2010-01-01

    Observation of methane in the Martian atmosphere has been reported by different detection techniques [1-4]. With more evidence showing extensive water-rock interaction in Martian history [5-7], abiotic formation by Fischer-Tropsch Type (FTT) synthesis during serpentization reactions may be one possible process responsible for methane generation on Mars [8, 9]. While the experimental studies performed to date leave little doubt that chemical reactions exist for the abiotic synthesis of organic compounds by mineral surface-catalyzed reactions [10-12], little is known about the reaction pathways by which CO2 and/or CO are reduced under hydrothermal conditions. Carbon and hydrogen isotope measurements of alkanes have been used as an effective tool to constrain the origin and reaction pathways of hydrocarbon formation. Alkanes generated by thermal breakdown of high molecular weight organic compounds have carbon and hydrogen isotopic signatures completely distinct from those formed abiotically [13-15]. Recent experimental studies, however, showed that different abiogenic hydrocarbon formation processes (e.g., polymerization vs. depolymerization) may have different carbon and hydrogen isotopic patterns [16]. Results from previous experiments studying decomposition of higher molecular weight organic compounds (lignite) also suggested that pressure could be a crucial factor affecting fractionation of carbon isotopes [17]. Under high pressure conditions, no experimental data are available describing fractionation of carbon isotope during mineral catalyzed FTT synthesis. Thus, hydrothermal experiments present an excellent opportunity to provide the requisite carbon isotope data. Such data can also be used to identify reaction pathways of abiotic organic synthesis under experimental conditions.

  9. In situ detection of anaerobic alkane metabolites in subsurface environments

    PubMed Central

    Agrawal, Akhil; Gieg, Lisa M.

    2013-01-01

    Alkanes comprise a substantial fraction of crude oil and refined fuels. As such, they are prevalent within deep subsurface fossil fuel deposits and in shallow subsurface environments such as aquifers that are contaminated with hydrocarbons. These environments are typically anaerobic, and host diverse microbial communities that can potentially use alkanes as substrates. Anaerobic alkane biodegradation has been reported to occur under nitrate-reducing, sulfate-reducing, and methanogenic conditions. Elucidating the pathways of anaerobic alkane metabolism has been of interest in order to understand how microbes can be used to remediate contaminated sites. Alkane activation primarily occurs by addition to fumarate, yielding alkylsuccinates, unique anaerobic metabolites that can be used to indicate in situ anaerobic alkane metabolism. These metabolites have been detected in hydrocarbon-contaminated shallow aquifers, offering strong evidence for intrinsic anaerobic bioremediation. Recently, studies have also revealed that alkylsuccinates are present in oil and coal seam production waters, indicating that anaerobic microbial communities can utilize alkanes in these deeper subsurface environments. In many crude oil reservoirs, the in situ anaerobic metabolism of hydrocarbons such as alkanes may be contributing to modern-day detrimental effects such as oilfield souring, or may lead to more beneficial technologies such as enhanced energy recovery from mature oilfields. In this review, we briefly describe the key metabolic pathways for anaerobic alkane (including n-alkanes, isoalkanes, and cyclic alkanes) metabolism and highlight several field reports wherein alkylsuccinates have provided evidence for anaerobic in situ alkane metabolism in shallow and deep subsurface environments. PMID:23761789

  10. Brewster Angle Microscopy Study of Model Stratum Corneum Lipid Monolayers at the Air-Water Interface

    NASA Astrophysics Data System (ADS)

    Adams, Ellen; Champagne, Alex; William, Joseph; Allen, Heather

    2012-04-01

    As the first and last barrier in the body, the stratum corneum (SC) is essential to life. Understanding the interactions and organization of lipids within the SC provides insight into essential physiological processes, including water loss prevention and the adsorption of substances from the environment. Langmuir monolayers have long been used to study complex systems, such as biological membranes and marine aerosols, due to their ability to shed light on intermolecular interactions. In this study, lipid mixtures with varying cholesterol and cerebroside ratios were investigated at the air/water interface. Surface tension measurements along with Brewster angle microscopy (BAM) images were used to examine the lipid phase transitions. Results indicate that cholesterol and cerebrosides form miscible monolayers, exhibiting ideal behavior. BAM images of a singular, uniform collapse phase also suggest formation of a miscible monolayer.

  11. A molecular dynamics study of the effect of thermal boundary conductance on thermal transport of ideal crystal of n-alkanes with different number of carbon atoms

    NASA Astrophysics Data System (ADS)

    Rastgarkafshgarkolaei, Rouzbeh; Zeng, Yi; Khodadadi, J. M.

    2016-05-01

    Phase change materials such as n-alkanes that exhibit desirable characteristics such as high latent heat, chemical stability, and negligible supercooling are widely used in thermal energy storage applications. However, n-alkanes have the drawback of low thermal conductivity values. The low thermal conductivity of n-alkanes is linked to formation of randomly oriented nano-domains of molecules in their solid structure that is responsible for excessive phonon scattering at the grain boundaries. Thus, understanding the thermal boundary conductance at the grain boundaries can be crucial for improving the effectiveness of thermal storage systems. The concept of the ideal crystal is proposed in this paper, which describes a simplified model such that all the nano-domains of long-chain n-alkanes are artificially aligned perfectly in one direction. In order to study thermal transport of the ideal crystal of long-chain n-alkanes, four (4) systems (C20H42, C24H50, C26H54, and C30H62) are investigated by the molecular dynamics simulations. Thermal boundary conductance between the layers of ideal crystals is determined using both non-equilibrium molecular dynamics (NEMD) and equilibrium molecular dynamics (EMD) simulations. Both NEMD and EMD simulations exhibit no significant change in thermal conductance with the molecular length. However, the values obtained from the EMD simulations are less than the values from NEMD simulations with the ratio being nearly three (3) in most cases. This difference is due to the nature of EMD simulations where all the phonons are assumed to be in equilibrium at the interface. Thermal conductivity of the n-alkanes in three structures including liquid, solid, and ideal crystal is investigated utilizing NEMD simulations. Our results exhibit a very slight rise in thermal conductivity values as the number of carbon atoms of the chain increases. The key understanding is that thermal transport can be significantly altered by how the molecules and the

  12. The study of the formation of monolayers of quantum dots at different temperatures

    NASA Astrophysics Data System (ADS)

    Gorbachev, Ilya A.; Goryacheva, Irina Y.; Brezesinski, Gerald; Gluhovskoy, Evgeny G.

    2016-04-01

    The process of formation of Langmuir monolayers of quantum dots at the different subphase temperatures was studied by means of compression isotherm, Brewster angle microscopy and transmission electron microscopy. The increasing of the maximum surface pressure from 32 to 44 mN/m takes place with decreasing the temperature from 34 to 11°C. This is due to a decrease in the rate of dissolution of surfactant molecules in water. The increasing of a filling degree of monolayer by the quantum dots and increasing of it uniformity in thickness takes place in this temperature range. The area of bilayer and multilayer film of quantum dots decreasing and the area of quantum dots monolayer is increasing. This change explained by the difference in the phase condition of oleic acid molecules, which stabilized quantum dots.

  13. Ultra-high vacuum scanning tunneling microscopy and theoretical studies of 1-halohexane monolayers on graphite

    PubMed Central

    Müller, Thomas; Werblowsky, Tova L.; Florio, Gina M.; Berne, Bruce J.; Flynn, George W.

    2005-01-01

    A simple model system for the 2D self-assembly of functionalized organic molecules on surfaces was examined in a concerted experimental and theoretical effort. Monolayers of 1-halohexanes were formed through vapor deposition onto graphite surfaces in ultrahigh vacuum. Low-temperature scanning tunneling microscopy allowed the molecular conformation, orientation, and monolayer crystallographic parameters to be determined. Essentially identical noncommensurate monolayer structures were found for all 1-halohexanes, with differences in image contrast ascribed mainly to electronic factors. Energy minimizations and molecular dynamics simulations reproduced structural parameters of 1-bromohexane monolayers quantitatively. An analysis of interactions driving the self-assembly process revealed the crucial role played by small but anisotropic electrostatic forces associated with the halogen substituent. While alkyl chain dispersion interactions drive the formation of a close-packed adsorbate monolayer, electrostatic headgroup forces are found to compete successfully in the control of both the angle between lamella and backbone axes and the angle between surface and backbone planes. This competition is consistent with energetic tradeoffs apparent in adsorption energies measured in earlier temperature-programmed desorption studies. In accordance with the higher degree of disorder observed in scanning tunneling microscopy images of 1-fluorohexane, theoretical simulations show that electrostatic forces associated with the fluorine substituent are sufficiently strong to upset the delicate balance of interactions required for the formation of an ordered monolayer. The detailed dissection of the driving forces for self-assembly of these simple model systems is expected to aid in the understanding of the more complex self-assembly processes taking place in the presence of solvent. PMID:15758073

  14. Ab initio study of ZrO2 monolayers epitaxial on Si

    NASA Astrophysics Data System (ADS)

    Dogan, Mehmet; Kumah, Divine; Ahn, Charles; Walker, Frederick; Ismail-Beigi, Sohrab

    Growing thin films of crystalline metal oxides on semiconductors has been of much scientific interest because of their applications in electronic devices. One research goal is to achieve ferroelectricity in a crystalline and thin oxide film that is epitaxial on a semiconductor. This would enable the realization of non-volatile field-effect transistors where the state is encoded in the polarization direction of the oxide. We study oxides that are not ferroelectric in the bulk but become ferroelectric as an ultra-thin film on a semiconductor. Recent advances in epitaxial growth methods permit fabrication of such systems. We use density functional theory to study the interface between ZrO2 monolayers and Si (001). These monolayers have multiple metastable states. We present an analysis of these configurations and energy barriers between them. We consider the likely experimental situation where different configurations coexist to form a multi-domain system, and investigate domain dynamics. Furthermore, we demonstrate that the ZrO2 monolayers can be used as a buffer layer to induce ferroelectricity in perovskite oxides such as SrTiO3 on Si. We also show that these monolayers modify the transport properties of Si which would allow for the desired device applications. This work is supported by the National Science Foundation through Grant MRSEC NSF DMR-1119826.

  15. Formaldehyde molecule adsorption on the doped monolayer MoS2: A first-principles study

    NASA Astrophysics Data System (ADS)

    Ma, Dongwei; Ju, Weiwei; Li, Tingxian; Yang, Gui; He, Chaozheng; Ma, Benyuan; Tang, Yanan; Lu, Zhansheng; Yang, Zongxian

    2016-05-01

    Based on first-principles calculations, formaldehyde (H2CO) adsorption on the pristine monolayer MoS2 and that doped with Cl, P, or Si was theoretically studied to explore the potential of the MoS2 sheets as H2CO gas sensors. It is found that under Mo-rich conditions it is viable for Cl to be filled into the S vacancies acting as n-type dopant and for P and Si acting as p-type dopants. The results on the H2CO adsorption on the pristine and the Cl-doped monolayer MoS2 indicate that both are insensitive to H2CO. In contrast, H2CO exhibits strong adsorption on the P or Si-doped monolayer MoS2. And there are large electron transfer from the P or Si-doped monolayer MoS2 to the H2CO and obvious change in the electronic densities of states of both systems induced by the H2CO adsorption. These suggest that P and Si can be appropriate dopants filled into MoS2 sheets for detecting H2CO molecule.

  16. Self-assembled monolayers for studying enzyme immobilization and ion recognition

    NASA Astrophysics Data System (ADS)

    Kang, Jie

    This thesis explores the use of self-assembled monolayers on gold for studying enzyme immobilization and ion recognition. Chapter 1 serves as a general introduction to biosensing, self-assembled monolayers, protein immobilization, and surface characterization techniques. Chapter 2 through Chapter 5 describe the immobilization of a redox enzyme, glucose oxidase, to a variety of functional self-assembled monolayers by either noncovalent adsorption or covalent attachment. The characteristics of different immobilization methods are investigated, and the activity of the immobilized enzyme is assessed electrochemically. Chapter 2 presents detailed procedures for measuring glucose oxidase activity by an electrochemical technique---cyclic voltammetry. Chapter 3 describes the adsorption of glucose oxidase to hydrophobic and hydrophilic self-assembled monolayers (SAMs). Significant glucose oxidase adsorption to hydrophobic, methyl-terminated SAMs was observed, while long chain, hydrophilic SAMs terminated by hydroxyl and carboxyl groups resist enzyme adsorption. Chapter 4 examines the covalent attachment of glucose oxidase to N-hydroxysuccinimide ester (NHS ester)-terminated self-assembled monolayers. The reactivity of the surface NHS ester group is found to increase as its coverage is lowered. This observation is explained by the steric effect. Chapter 5 reports the electrostatic adsorption of glucose oxidase to self-assembled monolayers of cystamine. The adsorbed enzyme shows superior activity to enzyme immobilized by other means. The rate constants of surface enzyme catalysis are determined and compared with those of the enzyme in solution. Chapter 6 is concerned with iron (III) recognition by a self-assembled monolayer terminated with a siderophore group, desferrioxamine (H3DFO). We first demonstrate that the iron coverage of the ferrioxamine (FeDFO)-terminated SAM can be successfully assayed by cyclic voltammetry. We then present results for iron (III) binding to the H3

  17. Study of polystyrene-poly(ethylene oxide) diblock copolymer monolayers as barriers to protein adsorption

    NASA Astrophysics Data System (ADS)

    Jogikalmath, Gangadhar

    Protein adsorption resistant surfaces find use in many biomedical applications, such as catheters, dialysis devices and biosensors that involve blood contacting surfaces. To ensure long-term functioning of a device in an environment containing protein, there is a need to produce homogeneous surfaces that are resistant to protein adsorption. A polymer brush covered surface, produced by either physical adsorption or chemical grafting of hydrophilic polymers to surfaces, is one of the approaches used in creating such surfaces. High grafting densities needed to make an effective barrier are usually not realized in chemical grafting/adsorption from solution, due to self-exclusion of surface grafted molecules. In this dissertation polymer brush surfaces formed by chemically grafted PEO molecules and transferred monolayers of PS-b-PEO diblock copolymers are investigated using atomic force microscopy (AFM), surface plasmon resonance (SPR) and surface pressure measurement techniques. An AFM adhesion mapping technique was used to evaluate the surface heterogeneity of chemically modified PEO and transferred diblock copolymer monolayer surfaces. The behavior of PS-b-PEO molecules at the air-water interface was studied using Langmuir trough. The stability of transferred diblock copolymer monolayers was investigated using AFM. Using SPR, protein adsorption to the diblock copolymer layers was investigated as a function of protein size (using HSA and ferritin) as a function of grafting density of PEO in the monolayer. It was seen that a lower density of the PS-b-PEO monolayer was sufficient to prevent ferritin adsorption (larger protein) while a higher density brush layer was required to achieve complete prevention of HSA adsorption to the surface. The effect of mobility of the polymer brush layer on protein adsorption prevention was analyzed using SPR and surface pressure measurements. It was seen that the copolymer monolayer (at the air-buffer interface) rearranged itself to

  18. Manipulating the Assembly of Spray-Deposited Nanocolloids: In Situ Study and Monolayer Film Preparation.

    PubMed

    Zhang, Peng; Santoro, Gonzalo; Yu, Shun; Vayalil, Sarathlal K; Bommel, Sebastian; Roth, Stephan V

    2016-05-01

    Fabrication of nanoparticle arrays on a substrate is one of the most concerned aspects for manipulating assembly of nanoparticles and preparing functional nanocomposites. Here, we studied in situ the assembly kinetics of polystyrene nanocolloids by using grazing incidence small-angle X-ray scattering. The structure formation of the nanoparticle film is monitored during air-brush spraying, which provides a rapid and scalable preparation. By optimizing the substrate temperature, the dispersion of the nanocolloids can be tailored to prepare monolayer film. The success of the monolayer preparations is attributed to the fast solvent evaporation which inhibits the aggregation of the nanocolloids. The present study may open a new avenue for the manufacture-friendly preparation of well-dispersed nanoparticle thin films. PMID:27070283

  19. Conformation, orientation and interaction in molecular monolayers: A surface second harmonic and sum frequency generation study

    SciTech Connect

    Superfine, R.; Huang, J.Y.; Shen, Y.R.

    1988-12-01

    We have used sum frequency generation (SFG) to study the order in a silane monolayer before and after the deposition of a coadsorbed liquid crystal monolayer. We observe an increase in the order of the chain of the silane molecule induced by the interpenetration of the liquid crystal molecules. By using second harmonic generation (SHG) and SFG, we have studied the orientation and conformation of the liquid crystal molecule on clean and silane coated glass surfaces. On both surfaces, the biphenyl group is tilted by 70{degree} with the alkyl chain end pointing away from the surface. The shift in the C-H stretch frequencies in the coadsorbed system indicates a significant interaction between molecules. 9 refs., 3 figs.

  20. Metathesis of alkanes and related reactions.

    PubMed

    Basset, Jean-Marie; Copéret, Christophe; Soulivong, Daravong; Taoufik, Mostafa; Cazat, Jean Thivolle

    2010-02-16

    The transformation of alkanes remains a difficult challenge because of the relative inertness of the C-H and C-C bonds. The rewards for asserting synthetic control over unfunctionalized, saturated hydrocarbons are considerable, however, because converting short alkanes into longer chain analogues is usually a value-adding process. Alkane metathesis is a novel catalytic and direct transformation of two molecules of a given alkane into its lower and higher homologues; moreover, the process proceeds at relatively low temperature (ambient conditions or higher). It was discovered through the use of a silica-supported tantalum hydride, ([triple bond]SiO)(2)TaH, a multifunctional catalyst with a single site of action. This reaction completes the story of the metathesis reactions discovered over the past 40 years: olefin metathesis, alkyne metathesis, and ene-yne cyclizations. In this Account, we examine the fundamental mechanistic aspects of alkane metathesis as well as the novel reactions that have been derived from its study. The silica-supported tantalum hydride catalyst was developed as the result of systematic and meticulous studies of the interaction between oxide supports and organometallic complexes, a field of study denoted surface organometallic chemistry (SOMC). A careful examination of this surface-supported tantalum hydride led to the later discovery of alumina-supported tungsten hydride, W(H)(3)/Al(2)O(3), which proved to be an even better catalyst for alkane metathesis. Supported tantalum and tungsten hydrides are highly unsaturated, electron-deficient species that are very reactive toward the C-H and C-C bonds of alkanes. They show a great versatility in various other reactions, such as cross-metathesis between methane and alkanes, cross-metathesis between toluene and ethane, or even methane nonoxidative coupling. Moreover, tungsten hydride exhibits a specific ability in the transformation of isobutane into 2,3-dimethylbutane as well as in the metathesis

  1. Boron nitride as a substrate for H2 monolayer studies

    NASA Astrophysics Data System (ADS)

    Evans, M. D.; Patel, N.; Sullivan, N. S.

    1992-11-01

    We report measurements of the adsorption isotherms of helium and methane on boron nitride. The suitability of using BN as a substrate for studying the two-dimensional, orientational ordering of quantum quadrupoles on a triangular lattice is also discussed.

  2. Study for fabrication, evaluation, and testing of monolayer woven type materials for space suit insulation

    NASA Technical Reports Server (NTRS)

    Merrick, E. B.

    1979-01-01

    An alternative space suit insulation concept using a monolayer woven pile material is discussed. The material reduces cost and improves the durability of the overgarment, while providing protection similar to that provided by multilayer insulation (MLI). Twelve samples of different configurations were fabricated and tested for compressibility and thermal conductivity as a function of compression loading. Two samples which showed good results in the initial tests were further tested for thermal conductivity with respect to ambient pressure and temperature. Results of these tests were similar to results of the MLI tests, indicating the potential of the monolayer fabric to replace the present MLI. A seaming study illustrated that the fabric can be sewn in a structurally sound seam with minimal heat loss. It is recommended that a prototype thermal meteroid garment be fabricated.

  3. Wedge energy bands of monolayer black phosphorus: a first-principles study

    NASA Astrophysics Data System (ADS)

    Park, Minwoo; Bae, Hyeonhu; Lee, Seunghan; Yang, Li; Lee, Hoonkyung

    2016-08-01

    On the basis of first-principles calculations, we present intriguing electronic properties of halogen-striped functionalized monolayer black phosphorus. The halogen-striped monolayer black phosphorus is found to have a wedge energy band with the energy-momentum relation of E\\propto {{p}y} when the stripe–stripe distance is smaller than ~40 Å. Our tight-binding study shows that the wedge energy band occurs when 2-atom basis 1D lattices are periodically repeated aligned with each other in a 2D lattice. We also discuss the possible applications of this wedge energy band in electron supercollimation with high mobility or severely anisotropic electronic transport, which can be used for the development of optics-like nano-electronics.

  4. Wedge energy bands of monolayer black phosphorus: a first-principles study.

    PubMed

    Park, Minwoo; Bae, Hyeonhu; Lee, Seunghan; Yang, Li; Lee, Hoonkyung

    2016-08-01

    On the basis of first-principles calculations, we present intriguing electronic properties of halogen-striped functionalized monolayer black phosphorus. The halogen-striped monolayer black phosphorus is found to have a wedge energy band with the energy-momentum relation of [Formula: see text] when the stripe-stripe distance is smaller than ~40 Å. Our tight-binding study shows that the wedge energy band occurs when 2-atom basis 1D lattices are periodically repeated aligned with each other in a 2D lattice. We also discuss the possible applications of this wedge energy band in electron supercollimation with high mobility or severely anisotropic electronic transport, which can be used for the development of optics-like nano-electronics. PMID:27299467

  5. Ab initio study of magnetism in nonmagnetic metal substituted monolayer MoS2

    NASA Astrophysics Data System (ADS)

    Hu, Ai-Ming; Wang, Ling-ling; Meng, Bo; Xiao, Wen-Zhi

    2015-10-01

    Based on density functional theory, the electronic structures and magnetic properties have been studied in MoS2 monolayer via substitutional doping of nonmagnetic elements (IA, IIA, and IIIA elements). The magnetic moment of those doped systems origins form the interplay between the crystal-field of MoS2 matrix and localized Mo 4d states. On the whole, these doped MoS2 monolayers exhibit a half-metal→spin gapless (or narrow gap) semiconductor→ferromagnetic semiconductor transition as the dopants change from IA to IIIA groups. Electron and hole doping by a potential gate can realize a transition from ferromagnetic semiconductor to half metal. In important, the spin-polarization direction is switchable depending on the doped carrier's type.

  6. 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.

  7. Superacid-promoted ionization of alkanes without carbonium ion formation: a density functional theory study.

    PubMed

    Dinér, Peter

    2012-10-11

    The carbonium ion has been suggested to be the intermediate in superacid-promoted reactions (SbF(5)-HF) such as hydrogen-deuterium exchange and in the electrophilic C-H cleavage into hydrogen and the carbenium ion. In this study, the superacid-promoted C-H cleavage into hydrogen and the carbenium ion was studied using density functional theory (B3LYP and M062X) and ab initio methods (MP2 and CCSD). The calculations suggest that the superacid-promoted C-H cleavage proceeds via a concerted transition state leading to hydrogen (H(2)) and the carbenium ion without the formation of the elusive carbonium ion. The reactivity for the superacid-promoted C-H cleavage decreases upon going from isobutane (tertiary) > propane (secondary) > isobutane (primary) > propane (primary) > ethane > methane. PMID:22998332

  8. A SERS study of the molecular structure of alkanethiol monolayers on Ag nanocubes in the presence of aqueous glucose

    NASA Astrophysics Data System (ADS)

    Rycenga, Matthew; McLellan, Joseph M.; Xia, Younan

    2008-09-01

    We report progress towards the surface-enhanced Raman scattering (SERS) characterization of self-assembled monolayers (SAMs) on uniform Ag nanocubes. This study quantifies changes in the SAMs induced by the presence of aqueous glucose. The SAMs were prepared from dodecanethiol and they were representative of highly ordered monolayers as indicated by SERS analysis. We examined the SAMs' response to glucose and observed conformational changes in the alkanethiolate SAMs. Analysis of the trans and gauche bands as well as the C-H stretching modes of the SAMs suggests that the analyte-SAM interactions were superficial and there was no penetration for the glucose molecules into the monolayers.

  9. A SERS study of the molecular structure of alkanethiol monolayers on Ag nanocubes in the presence of aqueous glucose

    PubMed Central

    Rycenga, Matthew; McLellan, Joseph M.; Xia, Younan

    2008-01-01

    We report progress towards the surface-enhanced Raman scattering (SERS) characterization of self-assembled monolayers (SAMs) on uniform Ag nanocubes. This study quantifies changes in the SAMs induced by the presence of aqueous glucose. The SAMs were prepared from dodecanethiol and they were representative of highly ordered monolayers as indicated by SERS analysis. We examined the SAMs response to glucose and observed conformational changes in the alkanethiolate SAMs. Analysis of the trans and gauche bands as well as the C-H stretching modes of the SAMs suggest that the analyte-SAM interactions were superficial and there was no penetration for the glucose molecules into the monolayers. PMID:20160847

  10. Formation, characterization, and stability of methaneselenolate monolayers on Au(111): an electrochemical high-resolution photoemission spectroscopy and DFT study.

    PubMed

    Cometto, F P; Calderón, C A; Morán, M; Ruano, G; Ascolani, H; Zampieri, G; Paredes-Olivera, P; Patrito, E M

    2014-04-01

    We investigated the mechanism of formation and stability of self-assembled monolayers (SAMs) of methaneselenolate on Au(111) prepared by the immersion method in ethanolic solutions of dimethyl diselenide (DMDSe). The adsorbed species were characterized by electrochemical measurements and high-resolution photoelectron spectroscopy (HR-XPS). The importance of the headgroup on formation mechanism and the stability of the SAMs was addressed by comparatively studying methaneselenolate (MSe) and methanethiolate (MT) monolayers. Density Functional Theory (DFT) calculations were performed to identify the elementary reaction steps in the mechanisms of formation and decomposition of the monolayers. Reductive desorption and HR-XPS measurements indicated that a MSe monolayer is formed at short immersion times by the cleavage of the Se-Se bond of DMDSe. However, the monolayer decomposes at long immersion times at room temperature, as evidenced by the appearance of atomic Se on the surface. The decomposition is more pronounced for MSe than for MT monolayers. The MSe monolayer stability can be greatly improved by two modifications in the preparation method: immersion at low temperatures (-20 °C) and the addition of a reducing agent to the forming solution. PMID:24645647

  11. Couette flows of a granular monolayer: An experimental study

    SciTech Connect

    Elliott, K.E.; Ahmadi, G.; Kvasnak, W.

    1995-03-01

    An experimental study concerning rapid flows of granular materials in a two dimensional planar granular Couette flow apparatus is performed. The device is capable of generating particulate flows in grain-inertia regime at different shearing rates and solid volume fractions. Multi-color spherical glass particles are sheared across an annular test-section for several wall angular velocities. A video recorder is used to record the motion of particles, and consecutive images are stored and analyzed by an image processing technique for evaluating individual grain velocities. Experimental data for the mean velocity, the root mean-square fluctuation velocity components and the solid volume fraction profile are obtained. The resulting mean velocity profiles have a roughly linear variation for the range of solid volume fractions and shear rates studied. The solid volume fraction profiles exhibit nonuniform variations with the highest concentration occuring near the center of the shearing cell. The RMS-fluctuation velocities are roughly constant, with the streamwise fluctuation being somewhat larger than the cross-stream direction. The experimentally measured flow properties are in reasonable agreement with the earlier theoretical and simulation results.

  12. Revealing fibrinogen monolayer conformations at different pHs: electrokinetic and colloid deposition studies.

    PubMed

    Nattich-Rak, Małgorzata; Adamczyk, Zbigniew; Wasilewska, Monika; Sadowska, Marta

    2015-07-01

    Adsorption mechanism of human fibrinogen on mica at different pHs is studied using the streaming potential and colloid deposition measurements. The fibrinogen monolayers are produced by a controlled adsorption under diffusion transport at pH of 3.5 and 7.4. Initially, the electrokinetic properties of these monolayers and their stability for various ionic strength are determined. It is shown that at pH 3.5 fibrinogen adsorbs irreversibly on mica for ionic strength range of 4×10(-4) to 0.15 M. At pH 7.4, a partial desorption is observed for ionic strength below 10(-2) M. This is attributed to the desorption of the end-on oriented molecules whereas the side-on adsorbed molecules remain irreversibly bound at all ionic strengths. The orientation of molecules and monolayer structure is evaluated by the colloid deposition measurements involving negatively charged polystyrene latex microspheres, 820 nm in diameter. An anomalous deposition of negative latex particles on substrates exhibiting a negative zeta potential is observed. At pH 3.5 measurable deposition of latex is observed even at low ionic strength where the approach distance of latex particles exceeded 70 nm. At pH 7.4 this critical distance is 23 nm. This confirms that fibrinogen monolayers formed at both pHs are characterized by the presence of the side-on and end-on oriented molecules that prevail at higher coverage range. It is also shown that positive charge is located at the end parts of the αA chains of the adsorbed fibrinogen molecules. Therefore, it is concluded that the colloid deposition method is an efficient tool for revealing protein adsorption mechanisms at solid/electrolyte interfaces. PMID:25453169

  13. Interaction of a nonspecific wheat lipid transfer protein with phospholipid monolayers imaged by fluorescence microscopy and studied by infrared spectroscopy.

    PubMed Central

    Subirade, M; Salesse, C; Marion, D; Pézolet, M

    1995-01-01

    The interaction of a nonspecific wheat lipid transfer protein (LTP) with phospholipids has been studied using the monolayer technique as a simplified model of biological membranes. The molecular organization of the LTP-phospholipid monolayer has been determined by using polarized attenuated total internal reflectance infrared spectroscopy, and detailed information on the microstructure of the mixed films has been investigated by using epifluorescence microscopy. The results show that the incorporation of wheat LTP within the lipid monolayers is surface-pressure dependent. When LTP is injected into the subphase under a dipalmytoylphosphatidylglycerol monolayer at low surface pressure (< 20 mN/m), insertion of the protein within the lipid monolayer leads to an expansion of dipalmytoylphosphatidylglycerol surface area. This incorporation leads to a decrease in the conformational order of the lipid acyl chains and results in an increase in the size of the solid lipid domains, suggesting that LTP penetrates both expanded and solid domains. By contrast, when the protein is injected under the lipid at high surface pressure (> or = 20 mN/m) the presence of LTP leads neither to an increase of molecular area nor to a change of the lipid order, even though some protein molecules are bound to the surface of the monolayer, which leads to an increase of the exposure of the lipid ester groups to the aqueous environment. On the other hand, the conformation of LTP, as well as the orientation of alpha-helices, is surface-pressure dependent. At low surface pressure, the alpha-helices inserted into the monolayers are rather parallel to the monolayer plane. In contrast, at high surface pressure, the alpha-helices bound to the surface of the monolayers are neither parallel nor perpendicular to the interface but in an oblique orientation. Images FIGURE 5 FIGURE 6 FIGURE 9 PMID:8519997

  14. Reaction pathway for alkane dehydrocyclization

    SciTech Connect

    Shi, Buchang; Davis, B.H.

    1996-08-01

    Naphtha reforming to produce high octane gasoline is an important process. Many reaction mechanisms are involved in this process. For example, the study of the fundamentals of this process led to the concept of bi- or poly-functional catalysis. The results of this study provide additional mechanistic information about the dehydrocyclization of an n-alkane to produce aromatics. The reaction coordinate diagram advanced to account for the observation of irreversible adsorption should be modified to account for the present results. 32 refs., 1 fig.

  15. Nonlinear Optical Studies of Self-Assembled Monolayers (SAM) Silica-SAM-Water Interface Probed With Second Harmonic Generation

    NASA Astrophysics Data System (ADS)

    James, Kenneth

    2010-03-01

    Second harmonic generation (SHG) is a successful and widely used technique for the study of surfaces and surface phenomena. We present a novel technique using second harmonic generation from oriented water molecules in the Gouy-Chapman diffuse layer at the alkylsiloxane and biomolecular self assembled monolayer (SAM) interface with water to measure distance between the solid surface and the average location of the oriented water in the diffuse layer. Distances of one nanometer can be distinguished. This in situ probe is applicable for organic adsorbates which in general will push the diffuse layer away from the solid surface. The organic layer thickness can be used to obtain the adsorption fraction. From this and an understanding of the likely chemistry, the orientation of the molecules can be inferred. We have demonstrated this technique on three molecular systems: hydrophobic self assembled monolayers of methoxysilane molecules of varying hydrocarbon chain length, self assembled monolayers of streptavidin glycoproteins and the combined streptavidin-biotinylated antibody monolayer.

  16. Nonlinear optical studies of self-assembled monolayers (SAM) silica-SAM-water interface probed with second harmonic generation

    NASA Astrophysics Data System (ADS)

    James, Kenneth Edward

    Second harmonic generation (SHG) is a successful and widely used technique for the study of surfaces and surface phenomena. We present a novel technique using second harmonic generation from oriented water molecules in the Gouy-Chapman diffuse layer at the alkylsiloxane and biomolecular self assembled monolayer (SAM) interface with water to measure distance between the solid surface and the average location of the oriented water in the diffuse layer. This distance is manifest in the SHG angular dependence profile from the relative phases of the second harmonic light generated at the diffuse layer and at the solid surface. Distances of one nanometer can be distinguished. Values for the diffuse layer potential, diffuse layer decay length, magnitude and phase of the nonlinear susceptibility were obtained. This in situ probe is universally applicable for organic adsorbates which in general will push the diffuse layer away from the solid surface. The organic layer thickness can be used to obtain the adsorption fraction. From this and an understanding of the likely chemistry, the orientation of the molecules can be inferred. We have demonstrated this technique on three molecular systems: hydrophobic self assembled monolayers of methoxysilane molecules of varying hydrocarbon chain length, self assembled monolayers of streptavidin glycoproteins and the combined streptavidin-biotinylated antibody monolayer. In the methoxysilane monolayers a relationship between hydrophobicity and molecular orientation was observed. The thickness of the streptavidin monolayer was determined to be 5.6 nm. This is strikingly close to the length of the of the streptavidin molecule which implies a close packed monolayer of streptavidin molecules. The average height of the antibodies was determined to be 10.9 nm or about two thirds the height of an antibody molecule. This too confirms a monolayer and allows for good approximation of surface coverage. This method does nothing to disturb or alter

  17. Ultrafast electron diffraction optimized for studying structural dynamics in thin films and monolayers.

    PubMed

    Badali, D S; Gengler, R Y N; Miller, R J D

    2016-05-01

    A compact electron source specifically designed for time-resolved diffraction studies of free-standing thin films and monolayers is presented here. The sensitivity to thin samples is achieved by extending the established technique of ultrafast electron diffraction to the "medium" energy regime (1-10 kV). An extremely compact design, in combination with low bunch charges, allows for high quality diffraction in a lensless geometry. The measured and simulated characteristics of the experimental system reveal sub-picosecond temporal resolution, while demonstrating the ability to produce high quality diffraction patterns from atomically thin samples. PMID:27226978

  18. Ultrafast electron diffraction optimized for studying structural dynamics in thin films and monolayers

    PubMed Central

    Badali, D. S.; Gengler, R. Y. N.; Miller, R. J. D.

    2016-01-01

    A compact electron source specifically designed for time-resolved diffraction studies of free-standing thin films and monolayers is presented here. The sensitivity to thin samples is achieved by extending the established technique of ultrafast electron diffraction to the “medium” energy regime (1–10 kV). An extremely compact design, in combination with low bunch charges, allows for high quality diffraction in a lensless geometry. The measured and simulated characteristics of the experimental system reveal sub-picosecond temporal resolution, while demonstrating the ability to produce high quality diffraction patterns from atomically thin samples. PMID:27226978

  19. ARPES Studies on the substrate effect on monolayer FeSe

    NASA Astrophysics Data System (ADS)

    Rebec, Slavko; Jia, Tao; Lee, James; Li, Wei; Zhang, Chaofan; Moore, Robert; Shen, Z. X.

    For 2D films, interface interactions can play a critical role in determining the prevailing physics of the system. In the case of FeSe on SrTiO3, reducing the FeSe thickness to 1 monolayer (ML) from bulk leads to a significantly increased superconducting transition temperature (Tc). To fully utilize and maximize this approach to increasing Tc in FeSe and potentially apply it to other superconducting materials, the role which the substrate plays in this system must be understood. Here we present recent in-situ angle-resolved photo emission studies of the substrate effect on MBE grown 1 ML FeSe films.

  20. Products of Chemistry: Alkanes: Abundant, Pervasive, Important, and Essential.

    ERIC Educational Resources Information Center

    Seymour, Raymond B.

    1989-01-01

    Discusses the history and commercialization of alkanes. Examines the nomenclature and uses of alkanes. Studies polymerization and several types of polyethylenes: low-density, high-density, low-molecular-weight, cross-linked, linear low-density, and ultrahigh-molecular-weight. Includes a glossary of hydrocarbon terms. (MVL)

  1. STM study of morphology and electron transport features in cytochrome c and nanocluster molecule monolayers.

    PubMed

    Khomutov, G B; Belovolova, L V; Gubin, S P; Khanin, V V; Obydenov, A Yu; Sergeev-Cherenkov, A N; Soldatov, E S; Trifonov, A S

    2002-01-01

    The morphology and electron tunneling through single cytochrome c and nanocluster Pt(5)(CO)(7)[P(C(6)H(5))](4) molecules organized as monolayer Langmuir-Blodgett (LB) films on graphite substrate have been studied experimentally using scanning tunneling microscopy (STM) and spectroscopy techniques with sub-nanometer spatial resolution in a double barrier tunnel junction configuration STM tip-monomolecular film-conducting substrate at ambient conditions. STM images of the films revealed globular structures with characteristic diameters (approximately 3.5 nm for the protein molecule and approximately 1.2 nm for the nanocluster). The spectroscopic study by recording the tunneling current-bias voltage (I-V) curves revealed tunneling I-V characteristics with features as steps of different width and heights that are dependent on the STM tip position over the molecule in the monolayer, giving evidence for sequential discrete electron-tunneling effects with the combination of the single electron Coulomb-charging energy and the electronic energy level separation (molecular spectrum) in such immobilized metalloprotein and nanocluster structures that can be of interest for the development of bioelectronic and hybrid functional nanosystems. PMID:11786369

  2. Perforated monolayers

    SciTech Connect

    Regen, S.L.

    1992-12-01

    Goal of this research program is to create ultrathin organic membranes that possess uniform and adjustable pores ( < 7[angstrom] diameter). Such membranes are expected to possess high permeation selectivity (permselectivity) and high permeability, and to provide the basis for energy-efficient methods of molecular separation. Work carried out has demonstrated feasibility of using perforated monolayer''-based composites as molecular sieve membranes. Specifically, composite membranes derived from Langmuir-Blodgett multilayers of the calix[6]arene-based surfactant shown below plus poly[l-(trimethylsilyl)-l-propyne] (PTMSP) were found to exhibit sieving behavior towards He, N[sub 2] and SF[sub 6]. Results of derivative studies that have also been completed are also described in this report.

  3. 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

    2008-03-01

    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. PMID:18225932

  4. Study of dithiol monolayer as the interface for controlled deposition of gold nanoparticles

    SciTech Connect

    Cichomski, M.; Tomaszewska, E.; Kosla, K.; Kozlowski, W.; Grobelny, J.

    2011-03-15

    Self-assembled monolayer of dithiol molecules, deposited on polycrystalline Au (111), prepared at room atmosphere, was studied using scanning tunneling microscopy (STM) and X-ray photoelectron spectroscopy (XPS). Dithiols were used as interface, which chemically bonds to the deposited gold nanoparticles through strong covalent bonds. The size and size distribution of the deposited nanoparticles were measured using dynamic light scattering (DLS), scanning electron microscopy (SEM) and atomic force microscopy (AFM). The AFM results showed that nanoparticles are immobilized and stable during scanning procedure and do not contaminate the AFM tip. The size of monodisperse nanoparticles obtained from the DLS measurements is slightly higher than that obtained from the AFM and SEM measurements. This is due to the fact that the DLS measures the hydrodynamic radius, dependent on the protective chemical layer on nanoparticles. - Research Highlights: {yields} Dithiols molecules create chemically bounded layers on a Au (111) surface. {yields} Gold nanoparticles can be chemically bounded to a self-assembled monolayer. {yields} Nanoparticles are stable during AFM probe interactions.

  5. The insertion of human apolipoprotein H into phospholipid membranes: a monolayer study.

    PubMed Central

    Wang, S X; Cai, G P; Sui, S F

    1998-01-01

    Apolipoprotein H (ApoH) is a plasma glycoprotein isolated from human serum. The interactions of ApoH with lipid membrane were reported to be essential for its physiological and pathogenic roles. In this paper we studied the ability of ApoH to insert into phospholipid membranes using the monolayer approach. The results show that ApoH is surface active and can insert into the lipid monolayers. The insertion ability of ApoH is stronger when a higher content of negatively charged lipids is present in the membrane. The acidic-pH and low-ionic-strength conditions will also enhance ApoH insertion, but these factors may not have much influence on the final insertion ability of ApoH, suggesting that, in the mechanism of ApoH insertion, not only electrostatic forces, but also hydrophobic interactions, are evidently involved. Modification by heat inactivation and reduction/alkylation does not change the critical insertion pressure (pic) of ApoH, suggesting a stable domain, maybe a linear sequence motif, but not the native three-dimensional structure of ApoH, is responsible for its insertion. The extent to which insertion of ApoH into phospholipid membranes may facilitate the 'immune cleaning' of plasma liposomes is discussed. PMID:9761718

  6. First principles kinetic Monte Carlo study on the growth patterns of WSe2 monolayer

    NASA Astrophysics Data System (ADS)

    Nie, Yifan; Liang, Chaoping; Zhang, Kehao; Zhao, Rui; Eichfeld, Sarah M.; Cha, Pil-Ryung; Colombo, Luigi; Robinson, Joshua A.; Wallace, Robert M.; Cho, Kyeongjae

    2016-06-01

    The control of domain morphology and defect level of synthesized transition metal dichalcogenides (TMDs) is of crucial importance for their device applications. However, current TMDs synthesis by chemical vapor deposition and molecular beam epitaxy is in an early stage of development, where much of the understanding of the process-property relationships is highly empirical. In this work, we use a kinetic Monte Carlo coupled with first principles calculations to study one specific case of the deposition of monolayer WSe2 on graphene, which can be expanded to the entire TMD family. Monolayer WSe2 domains are investigated as a function of incident flux, temperature and precursor ratio. The quality of the grown WSe2 domains is analyzed by the stoichiometry and defect density. A phase diagram of domain morphology is developed in the space of flux and the precursor stoichiometry, in which the triangular compact, fractal and dendritic domains are identified. The phase diagram has inspired a new synthesis strategy for large TMD domains with improved quality.

  7. The 5f localization/delocalization in square and hexagonal americium monolayers: a FP-LAPW electronic structure study

    NASA Astrophysics Data System (ADS)

    Gao, D.; Ray, A. K.

    2006-04-01

    The electronic and geometrical properties of bulk americium and square and hexagonal americium monolayers have been studied with the full-potential linearized augmented plane wave (FP-LAPW) method. The effects of several common approximations are examined: (1) non-spin polarization (NSP) vs. spin polarization (SP); (2) scalar-relativity (no spin-orbit coupling (NSO)) vs. full-relativity (i.e., with spin-orbit (SO) coupling included); (3) local-density approximation (LDA) vs. generalized-gradient approximation (GGA). Our results indicate that both spin polarization and spin orbit coupling play important roles in determining the geometrical and electronic properties of americium bulk and monolayers. A compression of both americium square and hexagonal monolayers compared to the americium bulk is also observed. In general, the LDA is found to underestimate the equilibrium lattice constant and give a larger total energy compared to the GGA calculations. While spin orbit coupling shows a similar effect on both square and hexagonal monolayer calculations regardless of the model, GGA versus LDA, an unusual spin polarization effect on both square and hexagonal monolayers is found in the LDA results as compared with the GGA results. The 5f delocalization transition of americium is employed to explain our observed unusual spin polarization effect. In addition, our results at the LDA level of theory indicate a possible 5f delocalization could happen in the americium surface within the same Am II (fcc crystal structure) phase, unlike the usually reported americium 5f delocalization which is associated with crystal structure change. The similarities and dissimilarities between the properties of an Am monolayer and a Pu monolayer are discussed in detail.

  8. Radical formation in the [MeReO3]-catalyzed aqueous peroxidative oxidation of alkanes: a theoretical mechanistic study.

    PubMed

    Kuznetsov, Maxim L; Pombeiro, Armando J L

    2009-01-01

    Plausible mechanisms of radical formation in the catalytic system [MeReO(3)]/H(2)O(2)/H(2)O-CH(3)CN for the oxidation of alkanes to alcohols and ketones, via radical pathways, are investigated extensively at the density functional theory level. The most favorable route is based on the monoperoxo complex [MeReO(2)(O(2))(H(2)O)] and includes the formation of an H(2)O(2) adduct, water-assisted H-transfer from H(2)O(2) to the peroxo ligand, and generation of HOO(*). The thus formed reduced Re(VI) complex [MeReO(2)(OOH)(H(2)O)] reacts with H(2)O(2), resulting, upon water-assisted H-transfer and O-OH bond homolysis, in the regeneration of the oxo-Re(VII) catalyst and formation of the HO(*) radical that reacts further with the alkane. Water plays a crucial role by (i) stabilizing transition states for the proton migrations and providing easy intramolecular H-transfers in the absence of any N,O-ligands and (ii) saturating the Re coordination sphere what leads to a decrease of the activation barrier for the formation of HOO(*). The activation energy of the radical formation calculated for [MeReO(3)] (17.7 kcal/mol) is compatible with that determined experimentally [Shul'pin et al. J. Chem. Soc., Perkin Trans. 2 2001, 1351 .] for oxo-V-based catalytic systems (17 +/- 2 kcal/mol), and the overall type of mechanism proposed for such V catalysts is also effective for [MeReO(3)]. PMID:19049432

  9. Study of lnter-Molecular Dynamics within Alkylsiloxane Self-Assembled Monolayer and Elastomer Systems

    NASA Astrophysics Data System (ADS)

    Roman, Michael

    In this work, molecular motion, and in particular, glassy relaxations are studied in two novel experimental systems. Both experimental systems offer a significant degree of control over molecule-molecule, or group-group (where group refers to a portion of a molecule), interactions by controlling density and the type of inter-molecular interaction. Both systems have rigid elements that decrease the tendency of bulk materials to spontaneously change their density with temperature. Thus, density can be maintained and controlled and the effect of density and temperature can be (at least in part) de-convolved. The goal of this work is to experimentally observe the transition from simple, local relaxations to glassy dynamics as density is increased and to understand how this transition differs as the inter-molecular interactions are altered. In both approaches, the system is fabricated from individual parts where the nature, spacing, and particular arrangement of the parts can be controlled and the resultant changes in molecular motion can be observed. Building up a custom system from parts enables fundamental investigation into the glass transition (as discussed above) and also makes possible the development of materials that have engineered responses as a function of temperature. As a short-hand, we refer to the two systems as the monolayer or SAM (short for Self-Assembled Monolayer) and elastomer approaches. In Chapters 4-7 we discuss results from the monolayer approach. Chapter 8 summarizes results from the elastomer approach. In particular, Chapter 4 introduces you to dielectric spectroscopy and briefly summarizes the previous work by former students in the Clarke group which identified the local and glass relaxations in silane monolayers of substituted alkyl chains as analogous to the local and glassy relaxations in polymeric systems containing phase segregated alkyl chains, and similar to the local and glass modes in poly(ethylene). The remainder of Chapter 4

  10. Do n-alkane biomarkers in soils/sediments reflect the δ²H isotopic composition of precipitation? A case study from Mt. Kilimanjaro and implications for paleoaltimetry and paleoclimate research.

    PubMed

    Zech, Michael; Zech, Roland; Rozanski, Kazimierz; Gleixner, Gerd; Zech, Wolfgang

    2015-01-01

    During the last decade compound-specific deuterium ((2)H) analysis of plant leaf wax-derived n-alkanes has become a promising and popular tool in paleoclimate research. This is based on the widely accepted assumption that n-alkanes in soils and sediments generally reflect δ(2)H of precipitation (δ(2)H(prec)). Recently, several authors suggested that δ(2)H of n-alkanes (δ(2)H(n-alkanes)) can also be used as a proxy in paleoaltimetry studies. Here, we present results from a δ(2)H transect study (∼1500 to 4000 m above sea level [a.s.l.]) carried out on precipitation and soil samples taken from the humid southern slopes of Mt. Kilimanjaro. Contrary to earlier suggestions, a distinct altitude effect in δ(2)H(prec) is present above ∼2000 m a.s.l., that is, δ(2)H(prec) values become more negative with increasing altitude. The compound-specific δ(2)H values of nC27 and nC29 do not confirm this altitudinal trend, but rather become more positive both in the O-layers (organic layers) and the Ah-horizons (mineral topsoils). Although our δ(2)H(n-alkane) results are in agreement with previously published results from the southern slopes of Mt. Kilimanjaro [Peterse F, van der Meer M, Schouten S, Jia G, Ossebaar J, Blokker J, Sinninghe Damsté J. Assessment of soil n-alkane δD and branched tetraether membrane lipid distributions as tools for paleoelevation reconstruction. Biogeosciences. 2009;6:2799-2807], a re-interpretation is required given that the δ(2)H(n-alkane) results do not reflect the δ(2)H(prec) results. The theoretical framework for this re-interpretation is based on the evaporative isotopic enrichment of leaf water associated with the transpiration process. Modelling results show that relative humidity, decreasing considerably along the southern slopes of Mt. Kilimanjaro (from 78% in ∼2000 m a.s.l. to 51% in 4000 m a.s.l.), strongly controls δ(2)H(leaf water). The modelled (2)H leaf water enrichment along the altitudinal transect matches well the

  11. Case studies on the formation of chalcogenide self-assembled monolayers on surfaces and dissociative processes.

    PubMed

    Tong, Yongfeng; Jiang, Tingming; Bendounan, Azzedine; Harish, Makri Nimbegondi Kotresh; Giglia, Angelo; Kubsky, Stefan; Sirotti, Fausto; Pasquali, Luca; Sampath, Srinivasan; Esaulov, Vladimir A

    2016-01-01

    This report examines the assembly of chalcogenide organic molecules on various surfaces, focusing on cases when chemisorption is accompanied by carbon-chalcogen atom-bond scission. In the case of alkane and benzyl chalcogenides, this induces formation of a chalcogenized interface layer. This process can occur during the initial stages of adsorption and then, after passivation of the surface, molecular adsorption can proceed. The characteristics of the chalcogenized interface layer can be significantly different from the metal layer and can affect various properties such as electron conduction. For chalcogenophenes, the carbon-chalcogen atom-bond breaking can lead to opening of the ring and adsorption of an alkene chalcogenide. Such a disruption of the π-electron system affects charge transport along the chains. Awareness about these effects is of importance from the point of view of molecular electronics. We discuss some recent studies based on X-ray photoelectron spectroscopy that shed light on these aspects for a series of such organic molecules. PMID:26977383

  12. Case studies on the formation of chalcogenide self-assembled monolayers on surfaces and dissociative processes

    PubMed Central

    Bendounan, Azzedine; Harish, Makri Nimbegondi Kotresh; Giglia, Angelo; Kubsky, Stefan; Sirotti, Fausto; Pasquali, Luca; Sampath, Srinivasan

    2016-01-01

    Summary This report examines the assembly of chalcogenide organic molecules on various surfaces, focusing on cases when chemisorption is accompanied by carbon–chalcogen atom-bond scission. In the case of alkane and benzyl chalcogenides, this induces formation of a chalcogenized interface layer. This process can occur during the initial stages of adsorption and then, after passivation of the surface, molecular adsorption can proceed. The characteristics of the chalcogenized interface layer can be significantly different from the metal layer and can affect various properties such as electron conduction. For chalcogenophenes, the carbon–chalcogen atom-bond breaking can lead to opening of the ring and adsorption of an alkene chalcogenide. Such a disruption of the π-electron system affects charge transport along the chains. Awareness about these effects is of importance from the point of view of molecular electronics. We discuss some recent studies based on X-ray photoelectron spectroscopy that shed light on these aspects for a series of such organic molecules. PMID:26977383

  13. Adsorption of linear alkanes on Cu(111): Temperature and chain-length dependence of the softened vibrational mode

    NASA Astrophysics Data System (ADS)

    Fosser, Kari A.; Kang, Joo H.; Nuzzo, Ralph G.; Wöll, Christof

    2007-05-01

    The vibrational spectra of linear alkanes, with lengths ranging from n-propane to n-octane, were examined on a copper surface by reflection-absorption infrared spectroscopy. The appearance and frequency of the "soft mode," a feature routinely seen in studies of saturated hydrocarbons adsorbed on metals, were examined and compared between the different adsorbates. The frequency of the mode was found to be dependent on both the number of methylene units of each alkane as well as specific aspects of the order of the monolayer phase. Studies of monolayer coverages at different temperatures provide insights into the nature of the two-dimensional (2D) melting transitions of these adlayer structures, ones that can be inferred from observed shifts in the soft vibrational modes appearing in the C-H stretching region of the infrared spectrum. These studies support recently reported hypotheses as to the origins of such soft modes: the metal-hydrogen interactions that mediate them and the dynamics that underlay their pronounced temperature dependencies. The present data strongly support a model for the 2D to one-dimensional order-order phase transition arising via a continuous rather than discrete first-order process.

  14. Structure and growth of stearate monolayers on calcite: First results of an in situ X-ray reflectivity study

    SciTech Connect

    Fenter, P.; Sturchio, N.C.

    1999-10-01

    The adsorption of organic molecules at mineral-fluid interfaces has a profound influence upon geochemical reaction and transport processes, yet little is known about the in situ structures or properties of organic layers at mineral-fluid interfaces. The authors describe an X-ray reflectivity study of stearate monolayers adsorbed at the calcite surface from methanolic solutions. Using these measurements the authors are able to determine important aspects of the in situ structure, bonding, adsorption, and growth mechanisms of stearate monolayers. The experimental approach demonstrated here can be applied widely in studying the interaction of organic molecules with mineral surfaces in aqueous systems.

  15. A vibrational spectroscopy study of the orientational ordering in CH 3 Cl monolayers physisorbed on graphite

    NASA Astrophysics Data System (ADS)

    Nalezinski, R.; Bradshaw, A. M.; Knorr, K.

    1997-12-01

    Methylchloride physisorbed on highly ordered pyrolytic graphite (HOPG) has been investigated by infrared reflection-absorption spectroscopy (IRAS). The results confirm the change in orientation of the molecules from flat to inclined between the two 2D crystalline monolayer phases and the up-down staggering in the inclined phase as suggested by previous diffraction studies. At lower coverages the molecules are found to be oriented perpendicular to the substrate, in disagreement with calculations for single, isolated molecules physisorbed on a smooth graphite surface. Measurements of the transient growth behaviour show that this latter state is long-lived and gives rise to complex growth laws. The results show that IRAS is a valuable tool for the study of physisorbed molecular layers.

  16. Reflectance spectroscopy of organic compounds: 1. Alkanes

    USGS Publications Warehouse

    Clark, R.N.; Curchin, J.M.; Hoefen, T.M.; Swayze, G.A.

    2009-01-01

    Reflectance spectra of the organic compounds comprising the alkane series are presented from the ultraviolet to midinfrared, 0.35 to 15.5 /??m. Alkanes are hydrocarbon molecules containing only single carbon-carbon bonds, and are found naturally on the Earth and in the atmospheres of the giant planets and Saturn's moon, Titan. This paper presents the spectral properties of the alkanes as the first in a series of papers to build a spectral database of organic compounds for use in remote sensing studies. Applications range from mapping the environment on the Earth, to the search for organic molecules and life in the solar system and throughout the. universe. We show that the spectral reflectance properties of organic compounds are rich, with major diagnostic spectral features throughout the spectral range studied. Little to no spectral change was observed as a function of temperature and only small shifts and changes in the width of absorption bands were observed between liquids and solids, making remote detection of spectral properties throughout the solar system simpler. Some high molecular weight organic compounds contain single-bonded carbon chains and have spectra similar to alkanes even ' when they fall into other families. Small spectral differences are often present allowing discrimination among some compounds, further illustrating the need to catalog spectral properties for accurate remote sensing identification with spectroscopy.

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

    NASA Astrophysics Data System (ADS)

    Swiatkowska, Angelika; Kosman, Joanna; Juskowiak, Bernard

    2016-01-01

    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.

  18. Microscopy and spectroscopy studies of (silicon(m)germanium(n))(p) superlattices and alkylthiol monolayers

    NASA Astrophysics Data System (ADS)

    Zhang, Chun

    1999-12-01

    The main ingredients of this thesis are divided into two parts. In the first part, we have investigated the second-order nonlinear optical susceptibility of short- period (Si mGen) p strained layer superlattices (SLS). The measured c(2) 311 for (Si5Ge5)3 sample is 1.0 × 10-7 esu at 1.17 eV (1064 nm) and 7.2 × 10 -7 esu at 1.55 eV (800 nm). In addition to the Si-Ge bond contribution from Si/Ge interfaces, we attribute an inhomogeneous strain due to lattice mismatch in the superlattice layers to be the main source of the SH responses we observed. Theoretical calculation of strain induced c(2) gives the value comparable to the one experimentally determined. This strain model can also explain the larger SH responses from superlattices samples containing stacking fault defects. We finally show that a multistructure of alternative strained superlattices and Si buffer layers can coherently generate SH, hence make such material potentially useful in practice. In the second part, we have studied the frictional property for alkyl-thiol monolayers CH3(CH2)nSH (n = 6, 9, 17) self-assembled on Au(111) with different molecular configurations under annealing. From scanning tunneling microscopy (STM) studies, it is known that these molecules form ordered (3× 3) R30° structure after self- assembly at room temperature. With annealing at sufficient high temperatures for an extended time period, the ordering of the films would first degrade and then form another order stripe-phase as partial desorption of the molecules takes place. Such thermally induced irreversible structural changes are further confirmed by using X-ray photoelectron spectroscopy (XPS) and Fourier transform inferred spectroscopy (FTIR). Our friction results on these films measured by frictional force microscope (FFM) with a Si3N4 tip show that friction increases as the monolayer ordering deteriorates and then reaches to a saturation level of ~4 times larger for the

  19. Isolating the non-polar contributions to the intermolecular potential for water-alkane interactions

    NASA Astrophysics Data System (ADS)

    Ballal, Deepti; Venkataraman, Pradeep; Fouad, Wael A.; Cox, Kenneth R.; Chapman, Walter G.

    2014-08-01

    Intermolecular potential models for water and alkanes describe pure component properties fairly well, but fail to reproduce properties of water-alkane mixtures. Understanding interactions between water and non-polar molecules like alkanes is important not only for the hydrocarbon industry but has implications to biological processes as well. Although non-polar solutes in water have been widely studied, much less work has focused on water in non-polar solvents. In this study we calculate the solubility of water in different alkanes (methane to dodecane) at ambient conditions where the water content in alkanes is very low so that the non-polar water-alkane interactions determine solubility. Only the alkane-rich phase is simulated since the fugacity of water in the water rich phase is calculated from an accurate equation of state. Using the SPC/E model for water and TraPPE model for alkanes along with Lorentz-Berthelot mixing rules for the cross parameters produces a water solubility that is an order of magnitude lower than the experimental value. It is found that an effective water Lennard-Jones energy ɛW/k = 220 K is required to match the experimental water solubility in TraPPE alkanes. This number is much higher than used in most simulation water models (SPC/E—ɛW/k = 78.2 K). It is surprising that the interaction energy obtained here is also higher than the water-alkane interaction energy predicted by studies on solubility of alkanes in water. The reason for this high water-alkane interaction energy is not completely understood. Some factors that might contribute to the large interaction energy, such as polarizability of alkanes, octupole moment of methane, and clustering of water at low concentrations in alkanes, are examined. It is found that, though important, these factors do not completely explain the anomalously strong attraction between alkanes and water observed experimentally.

  20. Magnetism from 2p states in K-doped ZnO monolayer: A density functional study

    NASA Astrophysics Data System (ADS)

    Fang, D. Q.; Zhang, Y.; Zhang, S. L.

    2016-05-01

    Using density-functional–based methods, we have studied 2p-based magnetic moments and magnetic coupling in potassium (K)-doped ZnO monolayer. We find that the substitution of a K atom at a Zn site in a ZnO monolayer induces a magnetic moment of 1.0~μB per cell mainly originating from the O-2p states and has much lower formation energy than a magnetic Zn vacancy. A half-metallic electronic property and long-range ferromagnetic coupling between the magnetic moments are obtained based on the generalized gradient approximation (GGA) calculations, which is explained by a double-exchange–like mechanism. Moreover, with stronger correlation correction on 2p states, the structure of the substitutional K impurity undergoes a Jahn-Teller–like distortion. Incorporating magnetism into a two-dimensional ZnO monolayer will promote its application in nanodevices.

  1. Cholesterol in condensed and fluid phosphatidylcholine monolayers studied by epifluorescence microscopy.

    PubMed Central

    Worthman, L A; Nag, K; Davis, P J; Keough, K M

    1997-01-01

    Epifluorescence microscopy was used to investigate the effect of cholesterol on monolayers of dipalmitoylphosphatidylcholine (DPPC) and 1 -palmitoyl-2-oleoyl phosphatidylcholine (POPC) at 21 +/- 2 degrees C using 1 mol% 1-palmitoyl-2-[12-[(7-nitro-2-1, 3-benzoxadizole-4-yl)amino]dodecanoyl]phosphatidylcholine (NBD-PC) as a fluorophore. Up to 30 mol% cholesterol in DPPC monolayers decreased the amounts of probe-excluded liquid-condensed (LC) phase at all surface pressures (pi), but did not effect the monolayers of POPC, which remained in the liquid-expanded (LE) phase at all pi. At low pi (2-5 mN/m), 10 mol% or more cholesterol in DPPC induced a lateral phase separation into dark probe-excluded and light probe-rich regions. In POPC monolayers, phase separation was observed at low pi when > or =40 mol% or more cholesterol was present. The lateral phase separation observed with increased cholesterol concentrations in these lipid monolayers may be a result of the segregation of cholesterol-rich domains in ordered fluid phases that preferentially exclude the fluorescent probe. With increasing pi, monolayers could be transformed from a heterogeneous dark and light appearance into a homogeneous fluorescent phase, in a manner that was dependent on pi and cholesterol content. The packing density of the acyl chains may be a determinant in the interaction of cholesterol with phosphatidylcholine (PC), because the transformations in monolayer surface texture were observed in phospholipid (PL)/sterol mixtures having similar molecular areas. At high pi (41 mN/m), elongated crystal-like structures were observed in monolayers containing 80-100 mol% cholesterol, and these structures grew in size when the monolayers were compressed after collapse. This observation could be associated with the segregation and crystallization of cholesterol after monolayer collapse. Images FIGURE 3 FIGURE 4 PMID:9168032

  2. Dynamics of Functionalized Surface Molecular Monolayers Studied with Ultrafast Infrared Vibrational Spectroscopy

    PubMed Central

    Rosenfeld, Daniel E.; Nishida, Jun; Yan, Chang; Gengeliczki, Zsolt; Smith, Brian J.; Fayer, Michael D.

    2012-01-01

    The structural dynamics of thin films consisting of tricarbonyl (1,10-phenanthroline)rhenium chloride (RePhen(CO)3Cl) linked to an alkyl silane monolayer through a triazole linker synthesized on silica-on-calcium-fluoride substrates are investigated using ultrafast infrared (IR) techniques. Ultrafast 2D IR vibrational echo experiments and polarization selective heterodyne detected transient grating (HDTG) measurements, as well as polarization dependent FT-IR and AFM experiments are employed to study the samples. The vibrational echo experiments measure spectral diffusion, while the HDTG experiments measure the vibrational excited state population relaxation and investigate the vibrational transition dipole orientational anisotropy decay. To investigate the anticipated impact of vibrational excitation transfer, which can be caused by the high concentration of RePhen(CO)3Cl in the monolayer, a concentration dependence of the spectral diffusion is measured. To generate a range of concentrations, mixed monolayers consisting of both hydrogen terminated and triazole/RePhen(CO)3Cl terminated alkyl silanes are synthesized. It is found that the measured rate of spectral diffusion is independent of concentration, with all samples showing spectral diffusion of 37 ± 6 ps. To definitively test for vibrational excitation transfer, polarization selective HDTG experiments are conducted. Excitation transfer will cause anisotropy decay. Polarization resolved heterodyne detected transient grating spectroscopy is sensitive to anisotropy decay (depolarization) caused by excitation transfer and molecular reorientation. The HDTG experiments show no evidence of anisotropy decay on the appropriate time scale, demonstrating the absence of excitation transfer the RePhen(CO)3Cl. Therefore the influence of excitation transfer on spectral diffusion is inconsequential in these samples, and the vibrational echo measurements of spectral diffusion report solely on structural dynamics. A small

  3. A new tribological experimental setup to study confined and sheared monolayers

    NASA Astrophysics Data System (ADS)

    Fu, L.; Favier, D.; Charitat, T.; Gauthier, C.; Rubin, A.

    2016-03-01

    We have developed an original experimental setup, coupling tribology, and velocimetry experiments together with a direct visualization of the contact. The significant interest of the setup is to measure simultaneously the apparent friction coefficient and the velocity of confined layers down to molecular scale. The major challenge of this experimental coupling is to catch information on a nanometer-thick sheared zone confined between a rigid spherical indenter of millimetric radius sliding on a flat surface at constant speed. In order to demonstrate the accuracy of this setup to investigate nanometer-scale sliding layers, we studied a model lipid monolayer deposited on glass slides. It shows that our experimental setup will, therefore, help to highlight the hydrodynamic of such sheared confined layers in lubrication, biolubrication, or friction on solid polymer.

  4. A new tribological experimental setup to study confined and sheared monolayers.

    PubMed

    Fu, L; Favier, D; Charitat, T; Gauthier, C; Rubin, A

    2016-03-01

    We have developed an original experimental setup, coupling tribology, and velocimetry experiments together with a direct visualization of the contact. The significant interest of the setup is to measure simultaneously the apparent friction coefficient and the velocity of confined layers down to molecular scale. The major challenge of this experimental coupling is to catch information on a nanometer-thick sheared zone confined between a rigid spherical indenter of millimetric radius sliding on a flat surface at constant speed. In order to demonstrate the accuracy of this setup to investigate nanometer-scale sliding layers, we studied a model lipid monolayer deposited on glass slides. It shows that our experimental setup will, therefore, help to highlight the hydrodynamic of such sheared confined layers in lubrication, biolubrication, or friction on solid polymer. PMID:27036787

  5. Density functional study of CaN monolayer on Si(001)

    NASA Astrophysics Data System (ADS)

    Saati asr, Maryam; Zahedifar, Maedeh; Hashemifar, S. Javad; Akbarzadeh, Hadi

    2016-01-01

    In this work, the first-principles computations are performed to study the structural and magnetic properties of CaN/Si(001) interface. Bulk CaN in the zinc-blende (ZB) structure is argued to be an ionic magnetic compound with a total spin moment of 1 μB per formula unit, originated from the p electrons of N ions. Various interface configurations of a ZB CaN monolayer on Si (001) surface are investigated and the lowest energy and the highest spin polarized interfaces are extracted. Then the minimum energy path between the lowest energy and the highest spin polarized interfaces are calculated by using the nudged elastic band method and it is argued that both these systems are unstable toward a nonmagnetic interface with a rock-salt arrangement of Ca and N atoms.

  6. Protein interactions with self-assembled monolayers presenting multimodal ligands: a surface plasmon resonance study.

    PubMed

    Vutukuru, Srinavya; Bethi, Sridhar R; Kane, Ravi S

    2006-11-21

    This paper describes the use of surface plasmon resonance (SPR) spectroscopy and self-assembled monolayers (SAMs) to understand the characteristics of surfaces that promote the adsorption of proteins at high ionic strengths (high-salt conditions). We synthesized SAMs presenting different multimodal ligands and determined the influence of surface composition, solution composition, and the nature of the protein on the extent of protein adsorption onto the SAMs. Our results confirm that hydrophobic interactions can contribute significantly to protein adsorption under high-salt conditions. In particular, the extent of protein adsorption under high-salt conditions increased with increasing surface hydrophobicity. The extent of protein adsorption was also influenced by the solution composition and decreased with an increase in the chaotropicity of the anion. The combination of SPR and SAMs is well-suited for studying the interaction of proteins with complex surfaces of relevance to chromatography. PMID:17107014

  7. Transepithelial transport of aliphatic carboxylic acids studied in Madin Darby canine kidney (MDCK) cell monolayers

    SciTech Connect

    Cho, M.J.; Adson, A.; Kezdy, F.J. )

    1990-04-01

    Transport of 14C-labeled acetic, propionic (PA), butyric, valeric, heptanoic (HA), and octanoic (OA) acids across the Madin Darby canine kidney (MDCK) epithelial cell monolayer grown on a porous polycarbonate membrane was studied in Hanks' balanced salt solution (HBSS) at 37{degrees}C in both apical-to-basolateral and basolateral-to-apical directions. At micromolar concentrations of solutes, metabolic decomposition was significant as evidenced by (14C)CO2 production during the OA transport. The apparent permeability (Pe) indicates that as lipophilicity increases, diffusion across the unstirred boundary layer becomes rate limiting. In support of this notion, transport of OA and HA was enhanced by agitation, showed an activation energy of 3.7 kcal/mol for OA, and resulted in identical Pe values for both transport directions. Analysis of Pe changes with varying alkyl chain length resulted in a delta G of -0.68 +/- 0.09 kcal/mol for -CH2-group transfer from an aqueous phase to the MDCK cells. When the intercellular tight junctions were opened by the divalent chelator EGTA in Ca2+/Mg2(+)-free HBSS, transport of the fluid-phase marker Lucifer yellow greatly increased because of paracellular leakage. PA transport also showed a significant increase, but OA transport was independent of EGTA. Although albumin also undergoes paracellular transport in the presence of EGTA and OA binds strongly to albumin, OA transport in EGTA solution was unchanged by albumin. These observations indicate that transmembrane transport is the major mechanism for lipophilic substances. The present study, together with earlier work on the transport of polar substances, shows that the MDCK cell monolayer is an excellent model of the transepithelial transport barrier.

  8. 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. PMID:26071844

  9. Kinetic studies on the interactions between glycolipid biosurfactant assembled monolayers and various classes of immunoglobulins using surface plasmon resonance.

    PubMed

    Ito, Seya; Imura, Tomohiro; Fukuoka, Tokuma; Morita, Tomotake; Sakai, Hideki; Abe, Masahiko; Kitamoto, Dai

    2007-08-01

    Kinetic studies on the interactions between self-assembled monolayers of mannosylerythritol lipids (MELs), which are glycolipid biosurfactants abundantly produced by microorganisms, and various classes of immunoglobulins including human IgG, IgA, and IgM were performed using surface plasmon resonance (SPR). The effect of the MEL structure on the binding behavior of HIgG was examined. Assembled monolayers of MEL-A having two acetyl groups on the headgroup gave a high affinity (K(d)=1.7x10(-6)M) toward HIgG, while those of MEL-B or MEL-C having only one acetyl group at C-6' or C-4' position gave little affinity. Our kinetic analysis revealed that the binding manner of HIgG, HIgA (K(d)=2.4x10(-7)M), and HIgM (K(d)=2.2x10(-7)M) to the assembled monolayers of MEL-A is not the monovalent mode but the bivalent mode, and both the first and second rate association constants (k(a1), k(a2)) increase with an increase in the number of antibody binding sites, while those for dissociation (k(d1), k(d2)) changed little. Moreover, we succeeded in directly observing great amounts of HIgG, HIgA, and HIgM bound to MEL-A monolayers using atomic force microscopy (AFM). Finally, we found that MEL-A assembled monolayer binds toward various IgG derived from mouse, pig, rabbit, horse, goat, rat, and bovine as well as human IgG (HIgG), and the only exception was sheep IgG. These results clearly demonstrate that MEL-A assembled monolayers would be useful as noble affinity ligand system for various immunoglobulins. PMID:17428643

  10. Electron diffraction studies of molecular ordering and orientation in phospholipid monolayer domains.

    PubMed Central

    Hui, S W; Yu, H

    1993-01-01

    The molecular order and orientation of phase separated domains in monolayers of DP(Me)PE and DP(Me)2PE were determined by electron diffraction. Dark and bright fluorescent domains at the air-water interface were observed by fluorescence microscopy. The monolayers were transferred to Formvar coated electron microscope grids for electron diffraction studies. The positions of domains on the marker grids were recorded in fluorescence micrographs, which were used as guide maps to locate these domains in the electron microscope. Selected area electron diffraction patterns were obtained from predetermined areas within and outside the dark domains. Sharp hexagonal diffraction patterns were recorded from dark domains, and diffuse diffraction rings from bright areas in between dark domains. The diffraction results indicated that the dark domains and bright areas were comprised of lipid molecules in solid and fluid states, respectively. The orientation of diffraction patterns from adjacent locations within a dark domains changed gradually, indicating a continuous bending of the molecular packing lattice vector within these domains. Orientation directors in U-shaped DP(Me)2PE domains followed the turn of the arm; no vortex nor branching was indicated by electron diffraction. Directors branching from the "stem" of highly invaginated DP(Me)PE domains usually occurred at twinning angles of n pi/3 from the stem director, which would minimize packing defects in the development of thinner branches. Electron diffraction from local areas of individual domains proved that dark fluorescent domains were solid ones, and that pseudo-long range order existed in these solid domains. Images FIGURE 1 FIGURE 2 FIGURE 3 FIGURE 4 FIGURE 5 FIGURE 6 FIGURE 7 FIGURE 8 PMID:8431539

  11. Computational Study of Metal Contacts to Monolayer Transition-Metal Dichalcogenide Semiconductors

    NASA Astrophysics Data System (ADS)

    Kang, Jiahao; Liu, Wei; Sarkar, Deblina; Jena, Debdeep; Banerjee, Kaustav

    2014-07-01

    Among various 2D materials, monolayer transition-metal dichalcogenide (mTMD) semiconductors with intrinsic band gaps (1-2 eV) are considered promising candidates for channel materials in next-generation transistors. Low-resistance metal contacts to mTMDs are crucial because currently they limit mTMD device performances. Hence, a comprehensive understanding of the atomistic nature of metal contacts to these 2D crystals is a fundamental challenge, which is not adequately addressed at present. In this paper, we report a systematic study of metal-mTMD contacts with different geometries (top contacts and edge contacts) by ab initio density-functional theory calculations, integrated with Mulliken population analysis and a semiempirical van der Waals dispersion potential model (which is critical for 2D materials and not well treated before). Particularly, In, Ti, Au, and Pd, contacts to monolayer MoS2 and WSe2 as well as Mo-MoS2 and W-WSe2 contacts are evaluated and categorized, based on their tunnel barriers, Schottky barriers, and orbital overlaps. Moreover, going beyond Schottky theory, new physics in such contact interfaces is revealed, such as the metallization of mTMDs and abnormal Fermi level pinning. Among the top contacts to MoS2, Ti and Mo show great potential to form favorable top contacts, which are both n-type contacts, while for top contacts to WSe2, W or Pd exhibits the most advantages as an n- or p-type contact, respectively. Moreover, we find that edge contacts can be highly advantageous compared to top contacts in terms of electron injection efficiency. Our formalism and the results provide guidelines that would be invaluable for designing novel 2D semiconductor devices.

  12. STM study of monolayer MoS2 synthesized by Chemical Vapor Deposition

    NASA Astrophysics Data System (ADS)

    Mills, Adam; Chen, Chuanhui; Yu, Yifei; Cao, Linyui; Tao, Changgang

    2014-03-01

    Monolayer molybdenum disulfide (MoS2) , an atomically thin transition-metal dichalcogenide semiconductor with a direct band gap, as opposed to an indirect band gap in bulk MoS2, has recently captured a lot of research interest for its distinctive optical and electronic properties, and potential applications such as field effect transistors, optoelectronic devices and chemical sensors. Using scanning tunneling microscopy, we have investigated monolayer MoS2 synthesized by chemical vapor deposition. The structural and electronic properties of monolayer MoS2 grown on glassy carbon and other substrates will be presented. We will also discuss our preliminary scanning tunneling spectroscopy measurements on these samples.

  13. Long-chain alkane production by the yeast Saccharomyces cerevisiae.

    PubMed

    Buijs, Nicolaas A; Zhou, Yongjin J; Siewers, Verena; Nielsen, Jens

    2015-06-01

    In the past decade industrial-scale production of renewable transportation biofuels has been developed as an alternative to fossil fuels, with ethanol as the most prominent biofuel and yeast as the production organism of choice. However, ethanol is a less efficient substitute fuel for heavy-duty and maritime transportation as well as aviation due to its low energy density. Therefore, new types of biofuels, such as alkanes, are being developed that can be used as drop-in fuels and can substitute gasoline, diesel, and kerosene. Here, we describe for the first time the heterologous biosynthesis of long-chain alkanes by the yeast Saccharomyces cerevisiae. We show that elimination of the hexadecenal dehydrogenase Hfd1 and expression of a redox system are essential for alkane biosynthesis in yeast. Deletion of HFD1 together with expression of an alkane biosynthesis pathway resulted in the production of the alkanes tridecane, pentadecane, and heptadecane. Our study provides a proof of principle for producing long-chain alkanes in the industrial workhorse S. cerevisiae, which was so far limited to bacteria. We anticipate that these findings will be a key factor for further yeast engineering to enable industrial production of alkane based drop-in biofuels, which can allow the biofuel industry to diversify beyond bioethanol. PMID:25545362

  14. Abnormal carbon and hydrogen isotopes of alkane gases from the Qingshen gas field, Songliao Basin, China, suggesting abiogenic alkanes?

    NASA Astrophysics Data System (ADS)

    Liu, Quanyou; Dai, Jinxing; Jin, Zhijun; Li, Jian; Wu, Xiaoqi; Meng, Qingqiang; Yang, Chun; Zhou, Qinghua; Feng, Zihui; Zhu, Dongya

    2016-01-01

    It is great debate that the alkane gases of abiogenic origin would constitute a major portion of the commercial accumulation of the Qingshen gas field, Songliao Basin, China. In this study, abiogenic gases characterized by heavy δ13C1 values, reversal of the usual carbon isotopic trend of C1-C5 alkanes, very narrow variation in δ2HC1 values, and low CH4/3He ratios associated with high R/Ra values (>1) were identified. The hydrocarbon gas in the Qingshen gas field is a mixture of thermogenic alkanes derived from Cretaceous mudstone (type I kerogen) or Jurassic coal (type III kerogen) and abiogenic alkanes (mainly CH4) from mantle degassing. A quantitative estimation of abiogenic alkanes contribution to the Qingshen gas field is made based on a δ13C1 vs. δ13C2 plot: about 30-40% of alkane gases in the Qingshen gas field, along with its helium, are estimated to be derived from the mantle via magmatic activity. Particularly, the abiogenic formation of CH4 generated from the reduction of CO2 by hydrothermal activity may contribute. Our study suggests that abiogenic alkane gases in certain geological settings could be more widespread than previously thought, and may accumulate into economic reservoirs.

  15. Alkane metathesis by tandem alkane-dehydrogenation-olefin-metathesis catalysis and related chemistry.

    PubMed

    Haibach, Michael C; Kundu, Sabuj; Brookhart, Maurice; Goldman, Alan S

    2012-06-19

    stable solid metal oxides as the olefin-metathesis catalysts. Both the pincer complexes and the alkylidene complexes have been supported on alumina via adsorption through basic para-substituents. This process does not significantly affect catalyst activity, and in some cases it increases both the catalyst lifetime and the compatibility of the co-catalysts. These molecular catalysts are the first systems that effect alkane metathesis with molecular-weight selectivity, particularly for the conversion of C(n)n-alkanes to C(2n-2)n-alkanes plus ethane. This molecular-weight selectivity offers a critical advantage over the few previously reported alkane metathesis systems. We have studied the factors that determine molecular-weight selectivity in depth, including the isomerization of the olefinic intermediates and the regioselectivity of the pincer-iridium catalyst for dehydrogenation at the terminal position of the n-alkane. Our continuing work centers on the development of co-catalysts with improved interoperability, particularly olefin-metathesis catalysts that are more robust at high temperature and dehydrogenation catalysts that are more active at low temperature. We are also designing dehydrogenation catalysts based on metals other than iridium. Our ongoing mechanistic studies are focused on the apparently complex combination of factors that determine molecular-weight selectivity. PMID:22584036

  16. Cholesterol Depletion from a Ceramide/Cholesterol Mixed Monolayer: A Brewster Angle Microscope Study

    NASA Astrophysics Data System (ADS)

    Mandal, Pritam; Noutsi, Pakiza; Chaieb, Sahraoui

    2016-06-01

    Cholesterol is crucial to the mechanical properties of cell membranes that are important to cells’ behavior. Its depletion from the cell membranes could be dramatic. Among cyclodextrins (CDs), methyl beta cyclodextrin (MβCD) is the most efficient to deplete cholesterol (Chol) from biomembranes. Here, we focus on the depletion of cholesterol from a C16 ceramide/cholesterol (C16-Cer/Chol) mixed monolayer using MβCD. While the removal of cholesterol by MβCD depends on the cholesterol concentration in most mixed lipid monolayers, it does not depend very much on the concentration of cholesterol in C16-Cer/Chol monolayers. The surface pressure decay during depletion were described by a stretched exponential that suggested that the cholesterol molecules are unable to diffuse laterally and behave like static traps for the MβCD molecules. Cholesterol depletion causes morphology changes of domains but these disrupted monolayers domains seem to reform even when cholesterol level was low.

  17. Studies on the interactions between parabens and lipid membrane components in monolayers at the air/aqueous solution interface.

    PubMed

    Flasiński, Michał; Gawryś, Maciej; Broniatowski, Marcin; Wydro, Paweł

    2016-04-01

    The interactions between parabens (PBs) and lipid components of mammalian and bacterial cell membranes were investigated in model systems of Langmuir monolayers. Me-, Et-, Pr- and Bu-paraben studied in this paper are frequently applied as cosmetics and food preservatives, since they possess broad antimicrobial activity. The mode of PB action is connected with their incorporation into the membrane of bacterial organisms, however; it is not known what is the role of the respective lipid species in this mechanism. This problem is crucial to understand the differences in paraben activity toward individual microorganisms and to shed the light onto the problem of PB cytotoxicity reported in studies on mammalian cells. In this paper, the mentioned aspects were investigated with application of the Langmuir monolayer technique complemented with BAM and GIXD. Our experiments revealed that the influence of PBs depends on their chemical structure, solution concentration and on the class of lipid. The strongest modification of the monolayer characteristics, leading to its collapse at low surface pressure, occurred in the presence of BuPB, having the largest chain. PBs interact preferentially with the monolayers possessing low degree of condensation, whereas for LC state, the effect was weaker and observed only as modification of the 2D unit cells. In the model systems, PBs interact with phospholipids characteristic for mammalian membranes (phosphatidylcholine) stronger than with bacterial (phosphatidylglycerol and cardiolipin). This strong influence of parabens on the model systems composed of animal lipids may explain cytotoxic activity of these preservatives. PMID:26777770

  18. A combined vibrational sum frequency generation spectroscopy and atomic force microscopy study of sphingomyelin-cholesterol monolayers.

    PubMed

    Weeraman, Champika; Chen, Maohui; Moffatt, Douglas J; Lausten, Rune; Stolow, Albert; Johnston, Linda J

    2012-09-11

    A combination of vibrational sum frequency generation spectroscopy and atomic force microscopy is used to study the changes in morphology and conformational order in monolayers prepared from three natural sphingomyelin (SM) mixtures as a function of surface pressure and cholesterol concentration. The most homogeneous SM gave monolayers with well-ordered acyl chains and few gauche defects with relatively small effects of either increasing surface pressure or cholesterol addition. Heterogeneous SM mixtures with a mixture of acyl chain lengths or with significant fractions of unsaturated acyl chains had much larger contributions from gauche defects at low surface pressure and gave increasingly well-ordered monolayers as the surface pressure increased. They also showed substantial increases in lipid chain order after cholesterol addition. Overall, these results are consistent with the strong hydrogen bonding capacity of SM leading to well-ordered monolayers over a range of surface pressures. The changes in acyl chain order for natural SMs as a function of cholesterol are relevant to formation of sphingolipid-cholesterol enriched domains in cell membranes. PMID:22889131

  19. Structural and thermodynamic behavior of alkane chains at the liquid/vapor interface

    NASA Astrophysics Data System (ADS)

    Hernandez, David Alejandro; Domínguez, Hector

    2013-04-01

    Computer simulations for several alkane fluids were carried out to study thermodynamics and structural behavior of the molecules at the liquid-vapor interface. Three different models were used to simulate the fluids, one of them was proposed in this work and we obtained a slightly better agreement than the other models with experimental data. The fluid structure at the interface was analyzed at temperatures close to the melting point using the new model and it was found that molecules at the free surface present more order than those at the bulk liquid phase. By calculating the order of the hydrocarbon chains a strong structure of molecules was observed at the interface than those in bulk, moreover, some of those molecules at the interface were aligned perpendicular to the interface. Previous simulations report stronger structures at the interface by the formation of a monolayer of alkane chains, however, those simulations started at very low temperatures and they did not reproduce thermodynamic properties such as the interfacial tension correctly. The model proposed in the present work not only presents good agreement with surface tension data but also shows evidence that the fluid structured as experiments indicated at temperatures close to the melting temperature.

  20. First-principle studies of electronic structure and magnetic excitations in FeSe monolayer

    NASA Astrophysics Data System (ADS)

    Bazhirov, Timur; Cohen, Marvin L.

    2013-03-01

    Recent experimental advances made it possible to study single-layered superconducting systems of iron-based compounds. The results show evidence of significant enhancement of superconducting properties compared to the bulk case. We use first-principle pseudopotential density functional theory techniques and the local spin-density approximation to study the electronic properties of an FeSe monolayer in different spin configurations. The results show that the experimental shape of the Fermi surface is best described by a checkerboard antiferromagnetic (AFM) spin arrangement. To explore the underlying pairing mechanism, we study the evolution of the non-magnetic to the AFM-ordered structures under constrained magnetization, and we estimate the electronic coupling to magnetic excitations involving transfer and increase of iron magnetic moments and compare it to the electron-phonon coupling. Finally, we simulate the substrate-induced interaction by using uniform charge doping and show that the latter can lead to an increase in the density of states at the Fermi level and possibly produce higher superconducting transition temperatures. This work was supported by NSF grant No. DMR10-1006184 and U.S. DOE under Contract No. DE-AC02-05CH11231. Computational resources have been provided by DOE at Lawrence Berkeley National Laboratory's NERSC facility

  1. A systematic study of atmospheric pressure chemical vapor deposition growth of large-area monolayer graphene†

    PubMed Central

    Liu, Lixin; Zhou, Hailong; Cheng, Rui; Chen, Yu; Lin, Yung-Chen; Qu, Yongquan; Bai, Jingwei; Ivanov, Ivan A.; Liu, Gang

    2012-01-01

    Graphene has attracted considerable interest as a potential material for future electronics. Although mechanical peel is known to produce high quality graphene flakes, practical applications require continuous graphene layers over a large area. The catalyst-assisted chemical vapor deposition (CVD) is a promising synthetic method to deliver wafer-sized graphene. Here we present a systematic study on the nucleation and growth of crystallized graphene domains in an atmospheric pressure chemical vapor deposition (APCVD) process. Parametric studies show that the mean size of the graphene domains increases with increasing growth temperature and CH4 partial pressure, while the density of domains decreases with increasing growth temperature and is independent of the CH4 partial pressure. Our studies show that nucleation of graphene domains on copper substrate is highly dependent on the initial annealing temperature. A two-step synthetic process with higher initial annealing temperature but lower growth temperature is developed to reduce domain density and achieve high quality full-surface coverage of monolayer graphene films. Electrical transport measurements demonstrate that the resulting graphene exhibits a high carrier mobility of up to 3000 cm2 V−1 s−1 at room temperature. PMID:25414547

  2. The rheological properties of beta amyloid Langmuir monolayers: Comparative studies with melittin peptide.

    PubMed

    Caruso, Benjamín; Ambroggio, Ernesto E; Wilke, Natalia; Fidelio, Gerardo Daniel

    2016-10-01

    We determined the rheological properties of β-amyloid Langmuir films at the air/water interface, a peptide whose interfacial structure is extended β-sheet, and compared them with those of films composed of Melittin (Mel), which adopts an α-helical conformation at neutral pH. To determine the dilatational and shear moduli we evaluated the response of pure peptide monolayers to an oscillatory anisotropic compressive work. Additionally, a micro-rheological characterization was performed by tracking the diffusion of micrometer sized latex beads onto the interface. This technique allowed us the detection of different rheological behaviour between monolayers presenting a low shear response. Monolayers of the β-sheet structure-adopting peptides, such as β-amyloid peptides, exhibited a marked shear (elastic) modulus even at low surface pressures. In contrast, Mel monolayers exhibited negligible shear modulus and the micro-rheological shear response was markedly lower than that observed for either Aβ1-40 or Aβ1-42 amyloid peptides. When Mel monolayers were formed at the interface of an aqueous solution at pH 11, we observed an increase in both the lateral stability and film viscosity as detected by a slower diffusion of the latex beads, in keeping with an increase in β-sheet structure at this high pH (verified by ATR and FT-IR measurements). We suggest that the interactions responsible for the marked response upon shear observed for β-amyloid peptide monolayers are the hydrogen bonds of the β-sheet structure that can form an infinite planar network at the interface. Conversely, α-helical Mel peptide lack of these inter-molecular interactions and, therefore the shear contribution was negligible. We propose that the secondary structure is important for modulating the rheological behavior of short peptide monolayers regardless of the mass density or surface charge at the surface. PMID:27318963

  3. Incremental oxidation of the surface of monolayer and bilayer graphene: A computational study

    NASA Astrophysics Data System (ADS)

    Lawson, Daniel B.; Beregszaszy, Erich J.

    2015-04-01

    We report the binding energies for a monolayer and bilayer graphene sheet coated with up to 24 oxygen atoms added sequentially to one surface of a monolayer and bilayer. Our graphene/graphite system consists of an arrangement of 3×5 phenyl rings or 48 carbon atoms in the monolayer and 96 carbon atoms in the bilayer. Geometries were energy optimized using the RM1 semiempirical method employing Periodic Boundary Conditions (PBC) followed by single point PBE and HSE06, all with the 6-31g* basis and PBC. Results indicate that the first O atom bound to pure graphene has a binding energy 2.16 eV on the monolayer and 2.14 eV on the bilayer. As O atoms are added the binding energy increases to 2.61 eV when the surface coverage on the monolayer reaches 45.8% or 11 O atoms on the unit cell, and for the bilayer this maximum occurs with 45.8% coverage or 11 O atoms on the top of the bilayer. The binding energy then gradually declines to 2.41 eV for the monolayer and to 1.93 eV for the bilayer with 100% coverage or 24 O atoms covering the top of the surface. Oxygen atoms added in close proximity to one another have a greater binding energy than O atoms added with larger separations relative to the unit cell. The difference in O binding energy between the monolayer system and the bilayer system is on average 0.02 eV less for the bilayer, the second, and as the number of O atoms are increased, the binding energy between the graphene layers falls to zero after 45.8% coverage or with 11 O atoms.

  4. A comparison study of oxygen reduction on the supported Pt, Pd, Au monolayer on WC(0001)

    NASA Astrophysics Data System (ADS)

    Zhang, Xilin; Lu, Zhansheng; Yang, Zongxian

    2016-07-01

    Using the first principles methods, the geometric and electronic structures of the metal monolayers supported on WC(0001) surfaces (MML/WC(0001) (M = Pt, Pd, and Au)) and their catalytic activity toward the oxygen reduction reaction (ORR) were comparatively studied. Both the kinetics and the density of states results demonstrated that the direct dissociation of O2 on all three MML/WC(0001) surfaces are almost impossible. Yet the barriers of the formation and dissociation of OOH on AuML/WC(0001) are much smaller than those on the PtML/WC(0001) and the PdML/WC(0001) surfaces, implying that the AuML/WC(0001) exhibits the highest catalytic activity for ORR via a combination of 2e- hydrogenation of O2 and 4e- dissociation of OOH. The rate-limiting step barrier of 0.83 eV for the hydrogenation of OH forming H2O is also comparable to that on the traditional Pt-based catalysts. The deactivation mechanism of PtML/WC(0001) and the performance of PdML/WC(0001) for ORR were identified. The present study is conductive to designing new efficient catalyst without using of the precious Pt for efficiently promoting ORR.

  5. Design of BAs-AlN monolayered honeycomb heterojunction structures: A first-principles study

    NASA Astrophysics Data System (ADS)

    Camacho-Mojica, Dulce C.; López-Urías, Florentino

    2016-04-01

    BAs and AlN are semiconductor materials with an indirect and direct gap respectively in the bulk phase. Recently, electronic calculations have demonstrated that a single-layer or few layers of BAs and AlN exhibit a graphite-like structure with interesting electronic properties. In this work, infinite sheets single-layer heterojunction structures based on alternated strips with honeycomb BAs and AlN layers are investigated using first-principles density functional theory calculations. Optimized geometries, density of states, band-gaps, formation energies, and wave functions are studied for different strip widths joined along zigzag and armchair edges. Results in optimized heterojunction geometries revealed that BAs narrow strips exhibit a corrugation effect due to a lattice mismatch. It was found that zigzag heterojunctions are more energetically favored than armchair heterojunctions. Furthermore, the formation energy presents a maximum at the point where the heterojunction becomes a planar structure. Electronic charge density results yielded a more ionic behavior in Alsbnd N bonds than the Bsbnd As bonds in accordance with monolayer results. It was observed that the conduction band minimum for both heterojunctions exhibit confined states located mainly at the entire AlN strips whereas the valence band maximum exhibits confined states located mainly at BAs strips. We expect that the present investigation will motivate more experimental and theoretical studies on new layered materials made of III-V semiconductors.

  6. Adsorption of alkanes on stoichiometric and oxygen-rich RuO2(110).

    PubMed

    Li, Tao; Kim, Minkyu; Rai, Rahul; Liang, Zhu; Asthagiri, Aravind; Weaver, Jason F

    2016-08-10

    We investigated the molecular adsorption of methane, ethane, propane and n-butane on stoichiometric and oxygen-rich RuO2(110) surfaces using temperature-programmed desorption (TPD) and dispersion-corrected density functional theory (DFT-D3) calculations. We find that each alkane adsorbs strongly on the coordinatively-unsaturated Ru (Rucus) atoms of s-RuO2(110), with desorption from this state producing a well-defined TPD peak at low alkane coverage. As the coverage increases, we find that alkanes first form a compressed layer on the Rucus atoms and subsequently adsorb on the bridging O atoms of the surface until the monolayer saturates. DFT-D3 calculations predict that methane preferentially adsorbs on top of a Rucus atom and that the C2 to C4 alkanes preferentially adopt bidentate configurations in which each molecule aligns parallel to the Rucus atom row and datively bonds to neighboring Rucus atoms. DFT-D3 predicts binding energies that agree quantitatively with our experimental estimates for alkane σ-complexes on RuO2(110). We find that oxygen atoms adsorbed on top of Rucus atoms (Oot atoms) stabilize the adsorbed alkane complexes that bind in a given configuration, while also blocking the sites needed for σ-complex formation. This site blocking causes the coverage of the most stable, bidentate alkane complexes to decrease sharply with increasing Oot coverage. Concurrently, we find that a new peak develops in the C2 to C4 alkane TPD spectra with increasing Oot coverage, and that the desorption yield in this TPD feature passes through a maximum at Oot coverages between ∼50% and 60%. We present evidence that the new TPD peak arises from C2 to C4 alkanes that adsorb in upright, monodentate configurations on stranded Rucus sites located within the Oot layer. PMID:27477390

  7. Mixed alkanethiol monolayers on submicrometric gold patterns: a controlled platform for studying cell-ligand interactions.

    PubMed

    Fishler, Rami; Artzy-Schnirman, Arbel; Peer, Elad; Wolchinsky, Ron; Brener, Reuven; Waks, Tova; Eshhar, Zelig; Reiter, Yoram; Sivan, Uri

    2012-09-12

    Nanoscale organization of surface ligands often has a critical effect on cell-surface interactions. We have developed an experimental system that allows a high degree of control over the 2-D spatial distribution of ligands. As a proof of concept, we used the developed system to study how T-cell activation is independently affected by antigen density and antigen amount per cell. Arrays of submicrometer gold islands at varying surface coverage were defined on silicon by electron beam lithography (EBL). The gold islands were functionalized with alkanethiol self-assembled monolayers (SAMs) containing a small antigen, 2,4,6-trinotrophenyl (TNP), at various densities. Genetically engineered T-cell hybridomas expressing TNP-specific chimeric T-cell antigen receptor (CAR) were cultured on the SAMs, and their activation was assessed by IL-2 secretion and CD69 expression. It was found that, at constant antigen density, activation increased monotonically with the amount of antigen, while at constant antigen amount activation was maximal at an intermediate antigen density, whose value was independent of the amount of antigen. PMID:22900991

  8. Comparative study of decyl thiocyanate and decanethiol self-assembled monolayers on gold substrates

    NASA Astrophysics Data System (ADS)

    Dreesen, L.; Volcke, C.; Sartenaer, Y.; Peremans, A.; Thiry, P. A.; Humbert, C.; Grugier, J.; Marchand-Brynaert, J.

    2006-09-01

    In a recent paper Ciszek et al. [J.W. Ciszek, M.P. Stewart, J.M. Tour, J. Am. Chem. Soc. 126 (2004) 13172] showed that organic thiocyanates may be an interesting alternative to the use of thiols for thiolate assemblies. We use scanning tunnelling microscopy (STM), infrared reflection absorption and sum-frequency generation spectroscopies (IRRAS and SFG) in order to study the adsorption properties of decyl thiocyanates (DTCN) and compare them to the decanethiol (DT) ones. Firstly, IRRAS measurements show that DTCN molecules form self-assembled monolayers (SAMs) on gold via a thiolate link with the metallic substrate. Secondly, the DTCN SAM on gold is less ordered than the DT one as highlighted by SFG spectroscopy. Indeed, the intensities of the methyl vibration modes vanish while the methylene ones increase when DTCN molecules are adsorbed on the substrate instead of DT. We explain the differences in SAMs quality on the basis of STM measurements which reveal differences in molecular order and packing.

  9. Migrations of pentagon-heptagon defects in hexagonal boron nitride monolayer: the first-principles study.

    PubMed

    Wang, J; Li, S N; Liu, J B

    2015-04-16

    The first-principles calculations are employed to study the migrations of pentagon-heptagon (5-7) defects in hexagonal boron nitride monolayer (h-BN). A type of grain boundaries, consisted of 5-7 defects, is constructed on the basis of experimental observations. With the absorption of a pair of atoms, one 5-7 defect in the grain boundary migrates apart by one unit cell and afterward migrates again through the bond rotation. It is also found that the two migrations could be replaced by one single step when the pair of absorbed atoms is located at another specific site in the same heptagon. Energy barriers and reaction paths for the migrations of 5-7 defects in h-BN by the bond rotation are theoretically investigated by the standard nudged elastic band method and the generalized solid-state nudged elastic band method. To elucidate the difference between the bond rotation process of the 5-7 defects with N-N bonds and those with B-B bonds, a couple of typical 21.7° grain boundaries with either N-N or B-B bonds are investigated. It is shown that the energy barrier of the migration of defects with N-N bonds is lower than that with B-B bonds in this type of grain boundaries. PMID:25811102

  10. Theoretical study of polyiodide formation and stability on monolayer and bilayer graphene.

    PubMed

    Tristant, Damien; Puech, Pascal; Gerber, Iann C

    2015-11-28

    The presence of polyiodide complexes have been reported several times when carbon-based materials were doped by iodine molecules, but their formation mechanism remains unclear. By using first-principles calculations that include nonlocal correlation effects by means of a van der Waals density functional approach, we propose that the formation of triiodide (I3(-)) and pentaiodide (I5(-)) is due to a large density of iodine molecules (I2) in interaction with a carbonaceous substrate. As soon as the concentration of surface iodine reaches a threshold value of 12.5% for a graphene monolayer and 6.25% for a bilayer, these complexes spontaneously appear. The corresponding structural and energetic aspects, electronic structures and vibrational frequencies support this statement. An upshift of the Dirac point from the Fermi level with values of 0.45 and 0.52 eV is observed for adsorbed complexes on graphene and intercalated complexes between two layers, respectively. For doped-graphene, it corresponds to a graphene hole density of around 1.1 × 10(13) cm(-2), in quantitative agreement with experiments. Additionally, we have studied the thermal stability at room temperature of these adsorbed ions on graphene by means of ab initio molecular dynamics, which also shows successful p-doping with polyiodide complexes. PMID:26497888

  11. Study on transport properties of silicene monolayer under external field using NEGF method

    SciTech Connect

    Syaputra, Marhamni Wella, Sasfan Arman; Wungu, Triati Dewi Kencana; Purqon, Acep; Suprijadi

    2015-09-30

    We investigate the current-voltage (I-V) characteristics of a pristine monolayer silicene using non-equilibrium Green function (NEGF) method combining with density functional theory (DFT). This method succeeded in showing the relationship of I and V on silicene corresponding to the electronic characteristics such as density of states. The external field perpendicular to the silicene monolayer affects in increasing of the current. Under 0.2 eV external field, the current reaches the maximum peak at Vb = 0.3 eV with the increase is about 60% from what it is in zero external field.

  12. Study on transport properties of silicene monolayer under external field using NEGF method

    NASA Astrophysics Data System (ADS)

    Syaputra, Marhamni; Wella, Sasfan Arman; Wungu, Triati Dewi Kencana; Purqon, Acep; Suprijadi

    2015-09-01

    We investigate the current-voltage (I-V) characteristics of a pristine monolayer silicene using non-equilibrium Green function (NEGF) method combining with density functional theory (DFT). This method succeeded in showing the relationship of I and V on silicene corresponding to the electronic characteristics such as density of states. The external field perpendicular to the silicene monolayer affects in increasing of the current. Under 0.2 eV external field, the current reaches the maximum peak at Vb = 0.3 eV with the increase is about 60% from what it is in zero external field.

  13. Interaction between Lipid Monolayers and Poloxamer 188: An X-Ray Reflectivity and Diffraction Study

    PubMed Central

    Wu, Guohui; Majewski, Jaroslaw; Ege, Canay; Kjaer, Kristian; Weygand, Markus Jan; Lee, Ka Yee C.

    2005-01-01

    The mechanism by which poloxamer 188 (P188) seals a damaged cell membrane is examined using the lipid monolayer as a model system. X-ray reflectivity and grazing-incidence x-ray diffraction results show that at low nominal lipid density, P188, by physically occupying the available area and phase separating from the lipids, forces the lipid molecules to pack tightly and restore the barrier function of the membrane. Upon compression to bilayer equivalent pressure, P188 is squeezed out from the lipid monolayer, allowing a graceful exit of P188 when the membrane integrity is restored. PMID:16100276

  14. Surface and liquid-crystalline properties of FmHnFm triblock semifluorinated n-alkanes.

    PubMed

    Chachaj-Brekiesz, Anna; Górska, Natalia; Osiecka, Natalia; Makyła-Juzak, Katarzyna; Dynarowicz-Łątka, Patrycja

    2016-05-01

    A series of triblock semifluorinated n-alkanes of the general formula: F(CF2)m(CH2)n(CF2)mF, (in short FmHnFm), where m=10, 12, and n=6, 8, and 12 have been synthesized and employed for liquid crystalline studies and Langmuir monolayer characterization. Differential scanning calorimetry (DSC) measurements together with texture observation with polarizing microscope (POM) revealed the presence of liquid crystalline smectic phases for all the investigated homologs. The behavior of the studied molecules spread at the free water surface has also been investigated. Our results show for the first time that these unusual film-forming materials, which are completely hydrophobic in nature and do not possess any polar group in their structure, are surface active and form insoluble (Langmuir) monolayers at the air/water interface. Due to the fact that these molecules are chemically inert and, similar to the semifluorinated diblocks, are not toxic, they may be destined for biomedical uses as gas carriers and contrast agents, as well as in drug delivery systems. PMID:26952494

  15. Cholesterol Depletion from a Ceramide/Cholesterol Mixed Monolayer: A Brewster Angle Microscope Study

    PubMed Central

    Mandal, Pritam; Noutsi, Pakiza; Chaieb, Sahraoui

    2016-01-01

    Cholesterol is crucial to the mechanical properties of cell membranes that are important to cells’ behavior. Its depletion from the cell membranes could be dramatic. Among cyclodextrins (CDs), methyl beta cyclodextrin (MβCD) is the most efficient to deplete cholesterol (Chol) from biomembranes. Here, we focus on the depletion of cholesterol from a C16 ceramide/cholesterol (C16-Cer/Chol) mixed monolayer using MβCD. While the removal of cholesterol by MβCD depends on the cholesterol concentration in most mixed lipid monolayers, it does not depend very much on the concentration of cholesterol in C16-Cer/Chol monolayers. The surface pressure decay during depletion were described by a stretched exponential that suggested that the cholesterol molecules are unable to diffuse laterally and behave like static traps for the MβCD molecules. Cholesterol depletion causes morphology changes of domains but these disrupted monolayers domains seem to reform even when cholesterol level was low. PMID:27245215

  16. Influence of Surface Morphology on the Shear-Induced Wear of Alkylsilane Monolayers: Molecular Dynamics Study.

    PubMed

    Summers, Andrew Z; Iacovella, Christopher R; Billingsley, Matthew R; Arnold, Steven T; Cummings, Peter T; McCabe, Clare

    2016-03-15

    Chemisorbed alkylsilane monolayer coatings have been shown to possess favorable lubrication properties; however, film degradation prevents the widespread use of these materials as lubricants in micro- and nanoelectromechanical systems (MEMS/NEMS). In this work, molecular dynamics (MD) simulations are used to provide insight into the conditions that promote the degradation and wear of these materials. This is achieved through removal of interfacial chain-substrate bonds during shear and the examination of the mobility of the resulting free, unbound chains. Specific focus is given to the effects of surface morphology, which has been shown previously to strongly influence frictional forces in monolayer systems. In-plane order of chain attachments is shown to lead to pressure-induced orientational ordering of monolayers, promoting film stability. This behavior is lost as nonideality is introduced into the substrate and chain patterning on the surface becomes disordered. The presence of surface roughness is found to reduce film stability, with localization of wear observed for chain attachment sites nearest the interface of contact. The influence of substrate nonideality on monolayer degradation is shown to diminish as chain length is increased. PMID:26885941

  17. An atomic force microcopy study of the mechanical and electricalproperties of monolayer films of molecules with aromatic end groups

    SciTech Connect

    Fang, Liang; Park, J.Y.; Ma, H.; Jen, A.K.-Y.; Salmeron, M.

    2007-09-06

    The effect of intermolecular {pi}-{pi} stacking on the electrical and mechanical properties of monolayer films molecules containing aromatic groups was studied using atomic force microscopy. Two types of aromatic molecules, (4-mercaptophenyl) anthrylacetylene (MPAA) and (4-mercaptophenyl)-phenylacetylene (MPPA) were used as model systems with different {pi}-{pi} stacking strength. Monolayer films of these molecules on Au(111) surfaces exhibited conductivities differing by more than one order of magnitude, MPAA being the most conductive and MPPA the least conductive. The response to compressive loads by the AFM tip was also found to be very different for both molecules. In MPAA films distinct molecular conductivity changes are observed upon mechanical perturbation. This effect however was not observed on the MPPA film, where intermolecular {pi}-{pi} interactions are likely weaker.

  18. Dynamics and kinetics of monolayer CH4 on MgO(001) studied by helium-atom scattering

    NASA Astrophysics Data System (ADS)

    Jung, David R.; Cui, Jinhe; Frankl, Daniel R.

    1991-05-01

    The structure, vibrational excitations, and adsorption and desorption kinetics of monolayer CH4 on MgO have been investigated using several techniques of helium scattering. Structural information is presented in the form of high-order diffraction-peak intensities. A vibrational excitation of 7.5 meV measured by time-of-flight methods shows no dispersion. This excitation energy is used in an analysis of the Debye-Waller effect for the [00] and [1¯0] beams. Studies of adsorption and desorption rates exploiting the He-methane diffuse-scattering cross section indicate an island-growth mode and allow determination of the desorption activation energy. Differences between the low-coverage adsorption rates for adsorption on fresh versus previously exposed surfaces suggest that higher-binding-energy sites are present after the desorption of a methane monolayer.

  19. Studies of molecular monolayers at air-liquid interfaces by second harmonic generation: question of orientational phase transition

    SciTech Connect

    Rasing, T.; Shen, Y.R.; Kim, M.W.; Grubb, S.; Bock, J.

    1985-06-01

    Insoluble molecular monolayers at gas-liquid interfaces provide an insight to the understanding of surfactants, wetting, microemulsions and membrane structures and offer a possibility to study the rich world of 2-dimensional phase transitions. In the interpretation of the observed properties of these systems various assumptions about the molecular orientation are often made, but so far few clear experimental data exist. In this paper we will show how optical second harmonic generation (SHG) can be used to measure the molecular orientation of monolayers of surfactant molecules at water-air interfaces. By simultaneously measuring the surface pressure versus surface molecular area we can show for the first time that the observed liquid condensed-liquid expanded transition is an orientational phase transition. 7 refs., 4 figs.

  20. Solar photothermochemical alkane reverse combustion.

    PubMed

    Chanmanee, Wilaiwan; Islam, Mohammad Fakrul; Dennis, Brian H; MacDonnell, Frederick M

    2016-03-01

    A one-step, gas-phase photothermocatalytic process for the synthesis of hydrocarbons, including liquid alkanes, aromatics, and oxygenates, with carbon numbers (Cn) up to C13, from CO2 and water is demonstrated in a flow photoreactor operating at elevated temperatures (180-200 °C) and pressures (1-6 bar) using a 5% cobalt on TiO2 catalyst and under UV irradiation. A parametric study of temperature, pressure, and partial pressure ratio revealed that temperatures in excess of 160 °C are needed to obtain the higher Cn products in quantity and that the product distribution shifts toward higher Cn products with increasing pressure. In the best run so far, over 13% by mass of the products were C5+ hydrocarbons and some of these, i.e., octane, are drop-in replacements for existing liquid hydrocarbons fuels. Dioxygen was detected in yields ranging between 64% and 150%. In principle, this tandem photochemical-thermochemical process, fitted with a photocatalyst better matched to the solar spectrum, could provide a cheap and direct method to produce liquid hydrocarbons from CO2 and water via a solar process which uses concentrated sunlight for both photochemical excitation to generate high-energy intermediates and heat to drive important thermochemical carbon-chain-forming reactions. PMID:26903631

  1. Solar photothermochemical alkane reverse combustion

    PubMed Central

    Chanmanee, Wilaiwan; Islam, Mohammad Fakrul; Dennis, Brian H.; MacDonnell, Frederick M.

    2016-01-01

    A one-step, gas-phase photothermocatalytic process for the synthesis of hydrocarbons, including liquid alkanes, aromatics, and oxygenates, with carbon numbers (Cn) up to C13, from CO2 and water is demonstrated in a flow photoreactor operating at elevated temperatures (180–200 °C) and pressures (1–6 bar) using a 5% cobalt on TiO2 catalyst and under UV irradiation. A parametric study of temperature, pressure, and partial pressure ratio revealed that temperatures in excess of 160 °C are needed to obtain the higher Cn products in quantity and that the product distribution shifts toward higher Cn products with increasing pressure. In the best run so far, over 13% by mass of the products were C5+ hydrocarbons and some of these, i.e., octane, are drop-in replacements for existing liquid hydrocarbons fuels. Dioxygen was detected in yields ranging between 64% and 150%. In principle, this tandem photochemical–thermochemical process, fitted with a photocatalyst better matched to the solar spectrum, could provide a cheap and direct method to produce liquid hydrocarbons from CO2 and water via a solar process which uses concentrated sunlight for both photochemical excitation to generate high-energy intermediates and heat to drive important thermochemical carbon-chain-forming reactions. PMID:26903631

  2. Interactions of a Tetrazine Derivative with Biomembrane Constituents: A Langmuir Monolayer Study.

    PubMed

    Nakahara, Hiromichi; Hagimori, Masayori; Mukai, Takahiro; Shibata, Osamu

    2016-07-01

    Tetrazine (Tz) is expected to be used for bioimaging and as an analytical reagent. It is known to react very fast with trans-cyclooctene under water in organic chemistry. Here, to understand the interaction between Tz and biomembrane constituents, we first investigated the interfacial behavior of a newly synthesized Tz derivative comprising a C18-saturated hydrocarbon chain (rTz-C18) using a Langmuir monolayer spread at the air-water interface. Surface pressure (π)-molecular area (A) and surface potential (ΔV)-A isotherms were measured for monolayers of rTz-C18 and biomembrane constituents such as dipalmitoylphosphatidylcholine (DPPC), dipalmitoylphosphatidylglycerol (DPPG), dipalmitoyl phosphatidylethanolamine (DPPE), palmitoyl sphingomyelin (PSM), and cholesterol (Ch). The lateral interaction between rTz-C18 and the lipids was thermodynamically elucidated from the excess Gibbs free energy of mixing and two-dimensional phase diagram. The binary monolayers except for the Ch system indicated high miscibility or affinity. In particular, rTz-C18 was found to interact more strongly with DPPE, which is a major constituent of the inner surface of cell membranes. The phase behavior and morphology upon monolayer compression were investigated by using Brewster angle microscopy (BAM), fluorescence microscopy (FM), and atomic force microscopy (AFM). The BAM and FM images of the DPPC/rTz-C18, DPPG/rTz-C18, and PSM/rTz-C18 systems exhibited a coexistence state of two different liquid-condensed domains derived mainly from monolayers of phospholipids and phospholipids-rTz-C18. From these morphological observations, it is worthy to note that rTz-C18 is possible to interact with a limited amount of the lipids except for DPPE. PMID:27280946

  3. Mechanism of Pb Adsorption to Fatty Acid Langmuir Monolayers Studied by X-ray Absorption Fine Structure Spectroscopy

    SciTech Connect

    Boyanov, M.I.; Kmetko, J.; Shibata, T.; Datta, A.; Dutta, P.; Bunker, B.A.

    2010-09-30

    The local atomic environment of lead (Pb) adsorbed to a CH{sub 3}(CH{sub 2}){sub 19}COOH Langmuir monolayer was investigated in situ using grazing-incidence X-ray absorption fine structure (GI-XAFS) spectroscopy at the Pb L{sub III} edge. Measurements were performed at pH 6.5 of the 10{sup -5} M PbCl{sub 2} solution subphase, a condition under which grazing incidence diffraction (GID) revealed a large-area commensurate superstructure underneath the close-packed organic monolayer. The XAFS results indicate covalent binding of the Pb cations to the carboxyl headgroups, and the observed Pb-Pb coordination suggests that the metal is adsorbed as a hydrolysis polymer, rather than as individual Pb{sup 2+} ions. The data are consistent with a bidentate chelating mechanism and a one Pb atom to one carboxyl headgroup binding stoichiometry. We discuss how this adsorption model can explain the peculiarities observed with Pb in previous metal-Langmuir monolayer studies. A systematic study of lead perchlorate and lead acetate aqueous solutions is presented and used in the analysis. XAFS multiple scattering effects from alignment of the Pb-C-C atoms in the lead acetate solutions are reported.

  4. The electronic structure, mechanical flexibility and carrier mobility of black arsenic-phosphorus monolayers: a first principles study.

    PubMed

    Sun, Jie; Lin, Na; Ren, Hao; Tang, Cheng; Yang, Letao; Zhao, Xian

    2016-04-14

    New artificial layered semiconductors - black arsenic-phosphorus (b-AsP) - which have tunable band gaps owing to good tunability of the chemical compositions have been synthesized in a recent experiment. In the present work, first principles calculations are performed to systematically study the structure, and mechanical, electrical, and transport properties of b-AsP monolayers. The mechanical analysis demonstrates that the exfoliation of single-layer b-AsP systems from the bulk form is more difficult compared with that of pure black phosphorus (BP). In addition, the breaking strain of the b-AsP monolayer is comparable with other widely studied two dimensional materials, indicating their excellent mechanical flexibility and good potential for flexible device applications. Besides, the electronic structures of b-AsP system monolayers are not sensitive to their specific compositions, which however, can be flexibly modulated by the strain effect. The predicted carrier mobilities of b-AsP systems are directionally anisotropic, similar to pure BP. However, the degradation of their carrier mobilities may become a practical limitation in real electronic device applications. PMID:27003857

  5. Studying and controlling order within nanoparticle monolayers fabricated through electrophoretic deposition

    NASA Astrophysics Data System (ADS)

    Krejci, Alexander J.

    Langmuir Blodgett films can be used to create very thin NP films. Templated substrates in combination with spin coating have been used to order blockcopolymers; this could be adapted for NP arrays as well. Some of these techniques can be applied for forming ordered arrays of NPs in two-dimensions, creating nanoparticle monolayers (NPMs), the focus of this work. NPMs are attractive for many applications in devices such as magnetic storage, solar cells, and biosensors. One particularly attractive feature of NPMs is the high surface area to volume ratio of the films. For example, through collaboration, we are investigating PL properties of two monolayers, composed of two different types of NPs, stacked on top of one another. Although challenging, there now are a variety of techniques for the fabrication of NPMs. This dissertation introduces a new process by which one can fabricate monolayers, electrophoretic deposition (EPD). Literature exists on using EPD to fabricate NPMs, but this literature is very limited. One such study deposited films of Au NPs on carbon films and another Pt NPs on carbon films. To the best of our knowledge, only NPMs of metallic NPs on carbon have been fabricated. Of the EPD studies in which NPMs have been fabricated, the technique has not been investigated in depth or has not been generalized for deposition of many types of materials. If NPM formation via EPD could be generalized, the NPMs could be industrially attractive as EPD has many industrially advantageous properties. For instance, EPD is highly versatile in multiple ways: many types of particles can be deposited, the size of the electrodes can be varied over many orders of magnitude, and a large variety of solvents can be used to suspend NPs. For example, our group has deposited materials of different shapes including tubes, sheets, and spheres; different materials such as polymers, metals, semiconductors, and magnetic materials; and on a variety of substrates including steel, silicon

  6. Study of structural order in porphyrin-fullerene dyad ZnDHD6ee monolayers by electron diffraction and atomic force microscopy

    SciTech Connect

    D'yakova, Yu. A.; Suvorova, E. I.; Orekhov, Andrei S.; Orekhov, Anton S.; Alekseev, A. S.; Gainutdinov, R. V.; Klechkovskaya, V. V. Tereschenko, E. Yu.; Tkachenko, N. V.; Lemmetyinen, H.; Feigin, L. A.; Kovalchuk, M. V.

    2013-11-15

    The structure of porphyrin-fullerene dyad ZnDHD6ee monolayers formed on the surface of aqueous subphase in a Langmuir trough and transferred onto solid substrates has been studied. The data obtained are interpreted using simulation of the structure of isolated molecules and their packing in monolayer and modeling of diffraction patterns from molecular aggregates having different sizes and degrees of order. Experiments on the formation of condensed ZnDHD6ee monolayers are described. The structure of these monolayers on a water surface is analyzed using {pi}-A isotherms. The structure of the monolayers transferred onto solid substrates is investigated by electron diffraction and atomic force microscopy. The unit-cell parameters of two-dimensional domains, which are characteristic of molecular packing in monolayers and deposited films, are determined. Domains are found to be organized into a texture (the molecular axes are oriented by the [001] direction perpendicular to the substrate). The monolayers contain a limited number of small 3D domains.

  7. A Quantitative Study of Tethered Chains in Various Solution Conditions Using Langmuir Diblock Copolymer Monolayers

    SciTech Connect

    Kent, Michael S.

    1999-08-13

    This article summarizes our investigations of tethered chain systems using Langmuir monolayer of polydimethysiloxane-poly styrene (PDMS-PS) diblock copolymers on organic liquids. In this system, the PDMS block adsorbs to the air surface while the PS block dangles into the subphase liquid. The air surface can be made either repulsive or attractive for the tethered PS chain segments by choosing a subphase liquid which has a surface tension lower or greater than that of PS, respectively. The segment profile of the PS block is determined by neutron reflection as a function of the surface density, the molecular weights of the PS and PDMS blocks, and the solution conditions. We cover the range of reduced surface density (SIGMA) characteristic of the large body of data in the literature for systems of chains tethered onto solid surfaces from dilute solution in good or theta solvent conditions (SIGMA < 12). We emphasize quantitative comparisons with analytical profile forms and scaling predictions. We find that the strong-stretching limit invoked in analytical SCF and scaling theories is not valid over this Z range. On the other hand, over a large portion of this range (SIGMA < 5) tethered layers are well described by a renormalization group theory addressing weakly interacting or noninteracting chains. Simultaneous with the study of the profile form, the free energy of the chains is examined through the surface tension. A strong increase in the surface pressure is observed with increasing surface density which determines the maximum surface density which can be achieved. This apparently nonequilibrium effect is attributed to steric interactions and limited lateral interpenetration. This effect may explain several outstanding discrepancies regarding the adsorption of end-functionalized chains and diblock copolymers onto solid surfaces.

  8. Markedly different adsorption behaviors of gas molecules on defective monolayer MoS2: a first-principles study.

    PubMed

    Li, Hongxing; Huang, Min; Cao, Gengyu

    2016-06-01

    Sulfur vacancy (SV) is one of the most typical defects in two-dimensional monolayer MoS2, leading to reactive sites. We presented a systematic study of the adsorption behaviors of gas molecules, CO2, N2, H2O, CO, NH3, NO, O2, H2 and NO2, on monolayer MoS2 with single SV by first-principles calculations. It was found that CO2, N2 and H2O molecules physisorbed at the proximity of single SV. Our adsorption energy calculations and charge transfer analysis showed that the interactions between CO2, N2 and H2O molecules and defective MoS2 are stronger than the cases of CO2, N2 and H2O molecules adsorbed on pristine MoS2, respectively. The defective MoS2 based gas sensors may be more sensitive to CO2, N2 and H2O molecules than pristine MoS2 based ones. CO, NO, O2 and NH3 molecules were found to chemisorb at the S vacancy site and thus modify the electronic properties of defective monolayer MoS2. Magnetism was induced upon adsorption of NO molecules and the defective states induced by S vacancy can be completely removed upon adsorption of O2 molecules, which may provide some helpful information for designing new MoS2 based nanoelectronic devices in future. The H2 and NO2 molecules were found to dissociate at S vacancy. The dissociation of NO2 molecules resulted in O atoms located at the S vacancy site and NO molecules physisorbed on O-doped MoS2. The calculated results showed that NO2 molecules can help heal the S vacancy of the MoS2 monolayer. PMID:27198064

  9. Abundance and diversity of n-alkane-degrading bacteria in a forest soil co-contaminated with hydrocarbons and metals: a molecular study on alkB homologous genes.

    PubMed

    Pérez-de-Mora, Alfredo; Engel, Marion; Schloter, Michael

    2011-11-01

    Unraveling functional genes related to biodegradation of organic compounds has profoundly improved our understanding of biological remediation processes, yet the ecology of such genes is only poorly understood. We used a culture-independent approach to assess the abundance and diversity of bacteria catalyzing the degradation of n-alkanes with a chain length between C(5) and C(16) at a forest site co-contaminated with mineral oil hydrocarbons and metals for nearly 60 years. The alkB gene coding for a rubredoxin-dependent alkane monooxygenase enzyme involved in the initial activation step of aerobic aliphatic hydrocarbon metabolism was used as biomarker. Within the area of study, four different zones were evaluated: one highly contaminated, two intermediately contaminated, and a noncontaminated zone. Contaminant concentrations, hydrocarbon profiles, and soil microbial respiration and biomass were studied. Abundance of n-alkane-degrading bacteria was quantified via real-time PCR of alkB, whereas genetic diversity was examined using molecular fingerprints (T-RFLP) and clone libraries. Along the contamination plume, hydrocarbon profiles and increased respiration rates suggested on-going natural attenuation at the site. Gene copy numbers of alkB were similar in contaminated and control areas. However, T-RFLP-based fingerprints suggested lower diversity and evenness of the n-alkane-degrading bacterial community in the highly contaminated zone compared to the other areas; both diversity and evenness were negatively correlated with metal and hydrocarbon concentrations. Phylogenetic analysis of alkB denoted a shift of the hydrocarbon-degrading bacterial community from Gram-positive bacteria in the control zone (most similar to Mycobacterium and Nocardia types) to Gram-negative genotypes in the contaminated zones (Acinetobacter and alkB sequences with little similarity to those of known bacteria). Our results underscore a qualitative rather than a quantitative response of

  10. The Interface between Gd and Monolayer MoS2: A First-Principles Study

    PubMed Central

    Zhang, Xuejing; Mi, Wenbo; Wang, Xiaocha; Cheng, Yingchun; Schwingenschlögl, Udo

    2014-01-01

    We analyze the electronic structure of interfaces between two-, four- and six-layer Gd(0001) and monolayer MoS2 by first-principles calculations. Strong chemical bonds shift the Fermi energy of MoS2 upwards into the conduction band. At the surface and interface the Gd f states shift to lower energy and new surface/interface Gd d states appear at the Fermi energy, which are strongly hybridized with the Mo 4d states and thus lead to a high spin-polarization (ferromagnetically ordered Mo magnetic moments of 0.15 μB). Gd therefore is an interesting candidate for spin injection into monolayer MoS2. PMID:25482498

  11. The work function of sub-monolayer cesium-covered gold: A photoelectronspectroscopy study

    SciTech Connect

    LaRue, J.L.; White, J.D.; Nahler, N.H.; Liu, Z.; Sun, Y.; Pianetta, P.A.; Auerbach, D.J.; Wodtke, A.M.; /SLAC, SSRL /UC, Santa Barbara, Chem. Dept.

    2008-06-13

    Using visible and X-ray photoelectron spectroscopy we measured the work function of a Au(111) surface at a well-defined sub-monolayer coverage of Cs. For a Cs coverage producing a photoemission maximum with a He-Ne laser, the work function is 1.61 {+-} 0.08 eV consistent with previous assumptions used to analyze vibrationally promoted electron emission. A discussion of possible Cs layer structures is also presented.

  12. Theoretical study of the interaction of electron donor and acceptor molecules with monolayer WS2

    NASA Astrophysics Data System (ADS)

    Zhou, C. J.; Yang, W. H.; Wu, Y. P.; Lin, W.; Zhu, H. L.

    2015-07-01

    With the aim of understanding recent experimental data concerning molecular doping in WS2-based FET gas sensors, we have investigated the interaction of NH3 and H2O molecules with monolayer WS2, by means of first-principles calculations. The structural relaxations and total energy calculations are performed to determine the preferential binding configurations and it is found that both NH3 and H2O molecules are physisorbed on monolayer WS2. The Bader analysis combined with the plane-averaged differential charge density results indicate that NH3 acts as the electron donor, while H2O acts as the electron acceptor, leading to n- and p-type doping of WS2, respectively. The charge transfer mechanism is discussed in light of the mixing of the molecular highest occupied molecular orbital and lowest unoccupied molecular orbital with the underlying WS2 orbitals. In addition, the modification of the work function is found to be almost linearly dependent on the total charge transfer. The modification of the work function and the carrier concentration can be obtained by tuning the molecule coverages, without destroying the band structure of monolayer WS2. The electrical sensitivities to the gas adsorption make WS2 a gas sensor that promises wide-ranging applications.

  13. First-principles study on magnetism of Ru monolayer under an external electric field

    NASA Astrophysics Data System (ADS)

    Kitaoka, Yukie; Imamura, Hiroshi

    Electric field control of magnetic properties such as magnetic moment and magnetic anisotropy has been attracted. For the 4 d TM films, on the other hand, it was recently reported that the ferromagnetism Pd thin-film is induced by application of an external electric field otherwise Pd thin-film shows paramagnetic. However, little attention has been paid to the magnetism of other 4 d TMs. Here, we investigate the magnetism of the free-standing Ru monolayer and that on MgO(001) substrate under an external electric field by using first-principles FLAPW method. We found that the free-standing Ru monolayer is ferromagnet with magnetic moment of 1.50 ¥muB /atom. The MA energy is 3.45 meV/atom, indicating perpendicular MA, at zero electric field (E=0) and increases up to 3.84 meV/atom by application of E=1 (V/¥AA). The Ru monolayer on MgO(001) substrate is also ferromagnet with magnetic moment of 0.89 ¥muB /atom. The MA energy is 1.49 meV/atom, indicating perpendicular MA, at E=0 and decreases to 1.33 meV/atom by application of E=1 (V/¥AA).

  14. Variation in n-Alkane Distributions of Modern Plants: Questioning Applications of n-Alkanes in Chemotaxonomy and Paleoecology

    NASA Astrophysics Data System (ADS)

    Bush, R. T.; McInerney, F. A.

    2010-12-01

    Long chain n-alkanes (n-C21 to n-C37) are synthesized as part of the epicuticular leaf wax of terrestrial plants and are among the most recognizable and widely used plant biomarkers. n-Alkane distributions have been utilized in previous studies on modern plant chemotaxonomy, testing whether taxa can be identified based on characteristic n-alkane profiles. Dominant n-alkanes (e.g. n-C27 or n-C31) have also been ascribed to major plant groups (e.g. trees or grasses respectively) and have been used in paleoecology studies to reconstruct fluctuations in plant functional types. However, many of these studies have been based on relatively few modern plant data; with the wealth of modern n-alkane studies, a more comprehensive analysis of n-alkanes in modern plants is now possible and can inform the usefulness of n-alkane distributions as paleoecological indicators. The work presented here is a combination of measurements made using plant leaves collected from the Chicago Botanic Garden and a compilation of published literature data from six continents. We categorized plants by type: angiosperms, gymnosperms, woody plants, forbs, grasses, ferns and pteridophytes, and mosses. We then quantified n-alkane distribution parameters such as carbon preference index (CPI), average chain length (ACL), and dispersion (a measure of the spread of the profile over multiple chain lengths) and used these to compare plant groups. Among all plants, one of the emergent correlations is a decrease in dispersion with increasing CPI. Within and among plant groups, n-alkane distributions show a very large range of variation, and the results show little or no correspondence between broad plant groups and a single dominant n-alkane or a ratio of n-alkanes. These findings are true both when data from six continents are combined and when plants from a given region are compared (North America). We also compared the n-alkane distributions of woody angiosperms, woody gymnosperms, and grasses with one

  15. a Study of the Materials Chemistry of Monolayer Oxides on Compound Semiconductors

    NASA Astrophysics Data System (ADS)

    Lu, Zhong

    In this dissertation, a study of deep-ultraviolet -light-enhanced (4.1 < hnu < 5.1 eV) oxygen reaction on GaAs from submonolayer to several monolayers coverage is presented. The reaction is nonthermal and does not involve gas-phase excitation or dissociation of O_2. Our experiments show a distinct wavelength and coverage dependence for the photoenhancement. The results indicate that a mechanism based on photoemission of electrons into the growing oxide film is most in accord with the experimental observations. The surface chemistry of GaAs-oxide removal and the passivation mechanism with Electron Cyclotron Resonance (ECR) hydrogen plasma has been investigated. It is found that As-oxide is efficiently removed at room temperature, and heating expedites the removal of Ga-oxide. Band bending, which correlates with the surface state density changes during ECR hydrogen-plasma oxide reduction, is also observed. This type of change in band bending could well be responsible for the hydrogen plasma passivation effect on many GaAs based devices. ECR oxidation at room temperature forms a stoichiometric oxide layer which is primarily composed of As_2 O_5 and Ga_2 O_3. We also studied the thermal reaction of As _2O_5 with GaAs at temperatures below 550^circC. A solid-state interface reaction of 4GaAs + 3As_2 O_5 to 2Ga _2O_3 + 3As _2O_3 + 4As, which includes the usual native oxide thermal reaction: 2GaAs + As_2O_3 to Ga_2O_3 + 4As, as well as a decomposition reaction, As _2O_5 to As _2O_3 + O _2, is responsible for the thermal reaction in this temperature range. A similar ECR-H oxide removal on GaSb surfaces shows that Sb-oxide removal occurs at room temperature, while Ga-oxide removal occurs at a temperature of ~250^circC. In addition, we have found that subsequent exposure to N _2 plasma leaves a thin nitride layer which prevents degradation of the H-cleaned surface and passivates the surface. We have applied this technique to the processing of an AlGaSb PIN photodiode. Our

  16. Gas-Phase Reactions of Atomic Gold Cations with Linear Alkanes (C2-C9).

    PubMed

    Zhang, Ting; Li, Zi-Yu; Zhang, Mei-Qi; He, Sheng-Gui

    2016-06-30

    To develop proper ionization methods for alkanes, the reactivity of bare or ligated transition metal ions toward alkanes has attracted increasing interests. In this study, the reactions of the gold cations with linear alkanes from ethane up to nonane (CnH2n+2, n = 2-9) under mild conditions have been characterized by mass spectrometry and density functional theory calculations. When reacting with Au(+), small alkanes (n = 2-6) were confirmed to follow specific reaction channels of dehydrogenation for ethane and hydride transfer for others to generate product ions characteristic of the original alkanes, which indicates that Au(+) can act as a reagent ion to ionize alkanes from ethane to n-hexane. Strong dependence of the chain length of alkanes was observed for the rate constants and reaction efficiencies. Extensive fragmentation took place for larger alkanes (n > 6). Theoretical results show that the fragmentation induced by the hydride transfer occurs after the release of AuH. Moreover, the fragmentation of n-heptane was successfully avoided when the reaction took place in a high-pressure reactor. This implies that Au(+) is a potential reagent ion to ionize linear and even the branched alkanes. PMID:27266670

  17. Conformation of liquid N-alkanes.

    PubMed Central

    Goodsaid-Zalduondo, F; Engelman, D M

    1981-01-01

    The conformations of liquid n-alkanes have been studied using neutron scattering techniques to better understand the conformational forces present in membrane lipid interiors. We have studied hydrocarbon chains having lengths comparable to those found for esterified membrane lipid fatty acids, and find that the steric constraints of packing in the liquid state do not change the conformational distributions of hydrocarbon chains from those imposed by the intrachain forces present in the gas phase. It follows that the central region of membranes containing lipids in the disordered state should contain hydrocarbon chain conformations determined primarily by intrachain forces. PMID:7272453

  18. High-Yield Excited Triplet States in Pentacene Self-Assembled Monolayers on Gold Nanoparticles through Singlet Exciton Fission.

    PubMed

    Kato, Daiki; Sakai, Hayato; Tkachenko, Nikolai V; Hasobe, Taku

    2016-04-18

    One of the major drawbacks of organic-dye-modified self-assembled monolayers on metal nanoparticles when employed for efficient use of light energy is the fact that singlet excited states on dye molecules can be easily deactivated by means of energy transfer to the metal surface. In this study, a series of 6,13-bis(triisopropylsilylethynyl)pentacene-alkanethiolate monolayer protected gold nanoparticles with different particle sizes and alkane chain lengths were successfully synthesized and were employed for the efficient generation of excited triplet states of the pentacene derivatives by singlet fission. Time-resolved transient absorption measurements revealed the formation of excited triplet states in high yield (172±26 %) by suppressing energy transfer to the gold surface. PMID:26997657

  19. Atomic force microscopy study of the adsorption of protein molecules on transferred Langmuir monolayer

    SciTech Connect

    Gainutdinov, R. V. Tolstikhina, A. L.; Stepina, N. D.; Novikova, N. N.; Yur'eva, E. A.; Khripunov, A. K.

    2010-09-15

    Ordered protein films have been obtained by the adsorption of protein molecules on a Langmuir monolayer, which had previously formed on a silicon substrate, using the Langmuir-Blodgett and molecular self-organization methods. A mixture of cholesterol with dipalmitoylphosphatidylcholine (DPPC) and a polymer-cellulose acetopivalinate-were used as immobilization materials. Protein molecules (catalase and alkaline phosphatase) immobilized on solid substrates have been investigated by atomic force micros-copy. It was shown that the developed combined technique provides a deposition of homogeneous ultrathin protein films with a high degree of filling.

  20. Experimental Study of the Temperature Dependence of Substrate Coverage in Ionic Self-Assembled Monolayers

    NASA Astrophysics Data System (ADS)

    Simpson, Brian; Abudayyeh, Mohammad; Ali, Md; Hamrick, Alena; Mazilu, Dan; Mazilu, Irina

    2014-03-01

    We investigate the temperature dependence of the surface coverage of thin films consisting of silica nanoparticles deposited on glass substrates via the ISAM (ionically self-assembled monolayers) technique. Variables such as the concentration and pH of the silica colloidal suspension and polyelectrolyte solution, dipping time, and particle size among others are controlled, and the thin films are deposited on substrates under a range of temperature values. The samples are analyzed using scanning electron microscopy. The surface coverage is estimated by employing a pixel-count comparison of grayscale values in the SEM micrographs and compared to analytical results obtained using a cooperative sequential adsorption model. Presentor.

  1. Lateral Heterostructures of Monolayer Transition Metal Dichalcogenides: a First-principles Study

    NASA Astrophysics Data System (ADS)

    Wu, Meng; Cao, Ting; Louie, Steven G.

    Using first-principles calculations, we investigate the electronic structure and optical properties of lateral heterostructures consisting of different monolayer transition metal dichalcogenides (TMDs). We find that the spin-orbital coupling effect plays an important role in modifying the ground-state electronic structure and excited-state properties such as optical responses. The anisotropy of optical absorption is investigated including local-field effects. This work was supported by NSF Grant No. DMR15-1508412, the U.S. DOE under Contract No. DE-AC02-05CH11231. Computational resources have been provided by DOE at Lawrence Berkeley National Laboratory's NERSC facility.

  2. Alkane inducible proteins in Geobacillus thermoleovorans B23

    PubMed Central

    2009-01-01

    Background Initial step of β-oxidation is catalyzed by acyl-CoA dehydrogenase in prokaryotes and mitochondria, while acyl-CoA oxidase primarily functions in the peroxisomes of eukaryotes. Oxidase reaction accompanies emission of toxic by-product reactive oxygen molecules including superoxide anion, and superoxide dismutase and catalase activities are essential to detoxify them in the peroxisomes. Although there is an argument about whether primitive life was born and evolved under high temperature conditions, thermophilic archaea apparently share living systems with both bacteria and eukaryotes. We hypothesized that alkane degradation pathways in thermophilic microorganisms could be premature and useful to understand their evolution. Results An extremely thermophilic and alkane degrading Geobacillus thermoleovorans B23 was previously isolated from a deep subsurface oil reservoir in Japan. In the present study, we identified novel membrane proteins (P16, P21) and superoxide dismutase (P24) whose production levels were significantly increased upon alkane degradation. Unlike other bacteria acyl-CoA oxidase and catalase activities were also increased in strain B23 by addition of alkane. Conclusion We first suggested that peroxisomal β-oxidation system exists in bacteria. This eukaryotic-type alkane degradation pathway in thermophilic bacterial cells might be a vestige of primitive living cell systems that had evolved into eukaryotes. PMID:19320977

  3. First-principles study of the contractive reconstruction of gold and silver monolayers on gold, silver and aluminum

    SciTech Connect

    Takeuchi, Noboru.

    1990-11-16

    Using first-principles calculations in conjunction with modeling techniques, the author has investigated the structures of Au and Ag monolayers on a number of metal surfaces. Au(100) has a c(26 {times} 68) surface unit cell and the reconstruction has been interpreted as the top layer transforming to a contracted hexagonal-close-packed layer, superimposed on the square lattice of the underlying substrate atoms. Similar reconstructions have been observed on the 5d fcc metals Ir and Pt, but not in the 4d Rh, Pd, and Ag. The author studied the energetics of a monolayer of Au and Ag using first-principles calculations. The author found that it is energetically favorable for both Au and Ag to transform from a square to hexagonal arrangement and to contract to a higher surface density, but Au gains substantially more energy than Ag. This is true both for a monolayer in isolation as well as on top of a jellium surface. The author also calculated the mismatch energy (energy loss when the top layer loses registry with the substrate) for Au and Ag, and found that Ag has a slightly higher mismatch energy. The first-principles results thus offer a strong indication that Au(100) can reconstruct but Ag will not. The reconstruction is further studied with a 2 dimensional Frenkel-Kontorowa model, with parameters extracted from the total energy calculations. The author found that it is indeed energetically favorable for the top layer of Au(100), but not for Ag, to transform to a hexagonal-close-packed structure and contract. 85 refs., 34 figs., 8 tabs.

  4. From monomer to monolayer: a global optimisation study of (ZnO)n nanoclusters on the Ag surface.

    PubMed

    Demiroglu, Ilker; Woodley, Scott M; Sokol, Alexey A; Bromley, Stefan T

    2014-12-21

    We employ global optimisation to investigate how oxide nanoclusters of increasing size can best adapt their structure to lower the system energy when interacting with a realistic extended metal support. Specifically, we focus on the (ZnO)@Ag(111) system where experiment has shown that the infinite Ag(111)-supported ZnO monolayer limit corresponds to an epitaxially 7 : 8 matched graphene-like (Zn(3)O(3))-based hexagonal sheet. Using a two-stage search method based on classical interatomic potentials and then on more accurate density functional theory, we report global minina candidate structures for Ag-supported (ZnO)n cluster with sizes ranging from n = 1-24. Comparison with the respective global minina structure of free space (ZnO)n clusters reveals that the surface interaction plays a decisive role in determining the lowest energy Ag-supported (ZnO)n cluster structures. Whereas free space (ZnO)n clusters tend to adopt cage-like bubble structures as they grow larger, Ag-supported (ZnO)n clusters of increasing size become progressively more like planar cuts from the infinite graphene-like ZnO single monolayer. This energetic favourability for planar hexagonal Ag-supported clusters over their 3D counterparts can be partly rationalised by the ZnO-Ag(111) epitaxial matching and the increased number of close interactions with the Ag surface. Detailed analysis shows that this tendency can also be attributed to the capacity of 2D clusters to distort to improve their interaction with the Ag surface relative to more rigid 3D bubble cluster isomers. For the larger sized clusters we find that the adsorption energies and most stable structural types appear to be rather converged confirming that our study makes a bridge between the Ag-supported ZnO monomer and the infinite Ag-supported ZnO monolayer. PMID:25354937

  5. Tethered Chains in Poor Solvent Conditions: An Experimental Study Involving Langmuir Diblock Copolymer Monolayers

    SciTech Connect

    Kent, M.S.; Lee, L.T.; Majewski, J.; Satija, S.; Smith, G.S.

    1998-10-13

    We have employed Langmuir monolayer of highly asymmetric polydimethylsiloxane- polystyrene (PDMS-PS) diblock copolymers on dioctyl phthalate (DOP) at temperatures ranging from 22 "C to -35 `C as a model system for tethered chains in poor solvent conditions. The thicknesses of the tethered PS layers extending into the DOP subphase, measured by neutron reflection, decrease with decreasing temperature (T) over this entire r~ge. However, the v~iation with T becomes weak below -20 "C. At the ]owest T, the layer thicknesses are contracted 55 % -75 `% of their values at the theta condition (T8 = 22 "C), but are still quite swollen compared to the fully collapsed, nonsolvent limit. The contraction of the layer with decreasing T is determined as a function of surface density and molecular weight. These data are compared to universal scaling forms. The PS segments are depleted from the air surface over the entire T range, the thickness of the depletion layer increasing slightly with decreasing T. The free energy of the surface layer is probed by surface tension measurements. Negative surface pressures are observed at low coverages for both PDMS-PS and PDMS monolayer, indicating metastability toward lateral phase separation. Evidence for a trruisition from a dispersed phase to a condensed phase with decreasing T was observed in the reflectivity at very low PDMS-PS coverage.

  6. Studies of low temperature photoluminescence spectra and excitonic valley polarization in monolayer MoTe2

    NASA Astrophysics Data System (ADS)

    Koirala, Sandhaya; Mouri, Shinichiro; Miyauchi, Yuhei; Matsuda, Kazunari; Kyoto University Team

    Recently, atomically thin layered transition-metal dichalcogenide (TMDs) in the form MX2 (M = Mo, W, X = S, Se, Te) have attracted much interest from the viewpoints of their fundamental physics and potential applications. The characteristic optical features of semiconducting TMDs arise from excitons confined in their atomically thin layers. Molybdenum ditelluride MoTe2 has attracted emerging research interest because of optical gap energy (lowest exciton transition) of 1.09 eV, and large spin-orbit coupling of 250 meV. Temperature-dependent photoluminescence (PL) and polarization-resolved PL measurement were performed for mechanically exfoliated monolayer MoTe2 from 4.4 to 300 K. At a low temperature, the PL spectra from MoTe2 show two sharp peaks for excitons and charged excitons (trions). The systematic temperature-dependent PL measurements revel that the homogeneous linewidth of the exciton peak broadens linearly as the temperature increased due to exciton-acoustic-phonon interactions. From polarization-resolved PL measurements, the valley polarization of above 40 % in the exciton state has been observed at low temperatures. In this meeting, we will discuss about exciton dephasing and valley polarization in monolayer MoTe2.

  7. CO oxidation catalyzed by silicon carbide (SiC) monolayer: A theoretical study.

    PubMed

    Wang, Nan; Tian, Yu; Zhao, Jingxiang; Jin, Peng

    2016-05-01

    Developing metal-free catalysts for CO oxidation has been a key scientific issue in solving the growing environmental problems caused by CO emission. In this work, the potential of the silicon carbide (SiC) monolayer as a metal-free catalyst for CO oxidation was systematically explored by means of density functional theory (DFT) computations. Our results revealed that CO oxidation reaction can easily proceed on SiC nanosheet, and a three-step mechanism was proposed: (1) the coadsorption of CO and O2 molecules, followed by (2) the formation of the first CO2 molecule, and (3) the recovery of catalyst by a second CO molecule. The last step is the rate-determining one of the whole catalytic reaction with the highest barrier of 0.65eV. Remarkably, larger curvature is found to have a negative effect on the catalytic performance of SiC nanosheet for CO oxidation. Therefore, our results suggested that flat SiC monolayer is a promising metal-free catalyst for CO oxidation. PMID:27135172

  8. Studies of intestinal permeability of 36 flavonoids using Caco-2 cell monolayer model.

    PubMed

    Tian, Xiao-Juan; Yang, Xiu-Wei; Yang, Xiaoda; Wang, Kui

    2009-02-01

    To investigate the structure-permeability relationship of dietary/nutriceutic flavonoids, the transepithelial transport and cellular uptake of 36 flavonoids (including flavones, flavonols, dihydroflavones, dihydroflavonols, isoflavones, chalcones, flavanes, flavanols, methylated and glycosidic derivatives) were investigated using the Caco-2 cell monolayer. The apparent permeability coefficients (P(app)) of the flavonoids were calculated from bilateral transport assays in the Transwell system with flavonoid determination using a high performance liquid chromatography (HPLC) coupled with a UV detector. The most flavonoids exhibited concentration-independent P(app) values and a ratio of 0.5-1.8 for P(app AB to BL)/P(app BL to AB), suggesting passive diffusion pathways. However, certain flavonoids e.g. morin and some flavonoid glycosides may involve the efflux mechanisms. For isoflavones, flavones, and dihydroflavones, the oil/water partition coefficients (additionally modified by the number and position of the three hydroxyl groups) was the key determinant for Caco-2 cell permeation. However, the permeability of flavonols is more complex with their structure possibly related to their high rate of cell accumulation. Overall, the parental skeleton structure, the number and position of free hydroxyl groups, accumulation and efflux in Caco-2 cell play the key roles in the transport of flavonoids across Caco-2 cell monolayer. PMID:18848870

  9. Catalytic conversion of light alkanes

    SciTech Connect

    Lyons, J.E.

    1992-06-30

    The second Quarterly Report of 1992 on the Catalytic Conversion of Light Alkanes reviews the work done between April 1, 1992 and June 31, 1992 on the Cooperative Agreement. The mission of this work is to devise a new catalyst which can be used in a simple economic process to convert the light alkanes in natural gas to oxygenate products that can either be used as clean-burning, high octane liquid fuels, as fuel components or as precursors to liquid hydrocarbon uwspomdon fuel. During the past quarter we have continued to design, prepare, characterize and test novel catalysts for the mild selective reaction of light hydrocarbons with air or oxygen to produce alcohols directly. These catalysts are designed to form active metal oxo (MO) species and to be uniquely active for the homolytic cleavage of the carbon-hydrogen bonds in light alkanes producing intermediates which can form alcohols. We continue to investigate three molecular environments for the active catalytic species that we are trying to generate: electron-deficient macrocycles (PHASE I), polyoxometallates (PHASE II), and regular oxidic lattices including zeolites and related structures as well as other molecular surface structures having metal oxo groups (PHASE I).

  10. SPALEED Studies of the Growth of Zero to Mono-layer Graphene on SiC(0001)

    NASA Astrophysics Data System (ADS)

    Hupalo, M.; Hershberger, M. T.; Hattab, H.; McDougall, D. C.; Horn von Hoegen, M.; Tringides, M. C.

    The growth of graphene on SiC was studied in detail with SPA LEED to understand the transition from zero to monolayer graphene with increasing temperature starting at 1200°C. Both the changing diffraction spots with annealing and their line shapes are studied in detail until a fully completed monolayer is obtained with only 6x6 spots remaining. In particular we focus on two strong features not investigated previously: (i) superstructures spots at n/13 locations present between the specular and the graphene spots. These spots are possibly related to different coincidence lattices before graphene locks into its final 6x6 orientation. (ii) The presence of a very broad background intensity covering ~60% of the BZ both around the specular and graphene spots whose origin is still unknown. Detailed studies of the dependence of this background component on energy and comparison between the graphene and specular spots suggest that the origin is not due to the standard variation with electron energy, i.e. a g(s) curve caused by the topography. Throughout the literature this broad background has been seen in graphene grown in different types of substrates. We comment on possible reasons for the origin of the background. Ames Laboratory is operated by the US-DOE under Contract No. DE-AC02-07CH11358.

  11. Development and application of thin-layer spectroelectrochemical techniques for the study of organosulfur monolayers adsorbed at gold

    SciTech Connect

    Simmons, N.

    1997-10-08

    A main research interest is the characterization of monolayers formed by the spontaneous adsorption of organosulfur compounds at gold. This dissertation describes the development and application of long optical pathlength thin-layer spectroelectrochemistry in an attempt to address key issues regarding the reactivity of surface-immobilized molecules. The first section of this introductory chapter briefly describes the general approach to the preparation and characterization of these films. The last section provides an overview of the main principles and advantages of thin-layer spectroelectrochemistry for studying surface-adsorbed species. The body of this dissertation is divided into four chapters. Chapter 2 consists of a paper describing the design, construction, and characterization of a cuvette-based LOPTLC. Chapter 3 is a paper which examines the reductive desorption process using thin-layer spectroelectrochemistry to monitor and identify the desorption product. Chapter 4 is a paper describing the characterization of monolayers functionalized with a catechol terminal group which serves as a redox transformable coordination site for metal ion binding. Chapter 5 discusses the application of thin-layer spectroelectrochemistry to acid-base reactivity studies of surface-immobilized molecules. The final section provides some general conclusions and a prospectus for future studies. These chapters have been processed separately for inclusion on the data base. This report contains the introduction, references, and general conclusions. 78 refs.

  12. Transporter engineering for improved tolerance against alkane biofuels in Saccharomyces cerevisiae

    PubMed Central

    2013-01-01

    Background Hydrocarbon alkanes, components of major fossil fuels, are considered as next-generation biofuels because their biological production has recently been shown to be possible. However, high-yield alkane production requires robust host cells that are tolerant against alkanes, which exhibit cytotoxicity. In this study, we aimed to improve alkane tolerance in Saccharomyces cerevisiae, a key industrial microbial host, by harnessing heterologous transporters that potentially pump out alkanes. Results To this end, we attempted to exploit ABC transporters in Yarrowia lipolytica based on the observation that it utilizes alkanes as a carbon source. We confirmed the increased transcription of ABC2 and ABC3 transporters upon exposure to a range of alkanes in Y. lipolytica. We then showed that the heterologous expression of ABC2 and ABC3 transporters significantly increased tolerance against decane and undecane in S. cerevisiae through maintaining lower intracellular alkane level. In particular, ABC2 transporter increased the tolerance limit of S. cerevisiae about 80-fold against decane. Furthermore, through site-directed mutagenesis for glutamate (E988 for ABC2, and E989 for ABC3) and histidine (H1020 for ABC2, and H1021 for ABC3), we provided the evidence that glutamate was essential for the activity of ABC2 and ABC3 transporters, with ATP most likely to be hydrolyzed by a catalytic carboxylate mechanism. Conclusions Here, we demonstrated that transporter engineering through expression of heterologous efflux pumps led to significantly improved tolerance against alkane biofuels in S. cerevisiae. We believe that our results laid the groundwork for developing robust alkane-producing yeast cells through transporter engineering, which will greatly aid in next-generation alkane biofuel production and recovery. PMID:23402697

  13. Indentation and self-healing mechanisms of a self-assembled monolayer--a combined experimental and modeling study.

    PubMed

    Meltzer, Christian; Paul, Jonas; Dietrich, Hanno; Jäger, Christof M; Clark, Timothy; Zahn, Dirk; Braunschweig, Björn; Peukert, Wolfgang

    2014-07-30

    A combination of in situ vibrational sum-frequency generation (SFG) spectroscopy and molecular-dynamics (MD) simulations has allowed us to study the effects of indentation of self-assembled octadecylphosphonic acid (ODPA) monolayers on α-Al2O3(0001). Stress-induced changes in the vibrational signatures of C-H stretching vibrations in SFG spectra and the results of MD simulations provide clear evidence for an increase in gauche-defect density in the monolayer as a response to indentation. A stress-dependent analysis indicates that the defect density reaches saturation at approximately 155 MPa. After stress is released, the MD simulations show an almost instantaneous healing of pressure-induced defects in good agreement with experimental results. The lateral extent of the contact areas was studied with colocalized SFG spectroscopy and compared to theoretical predictions for pressure gradients from Hertzian contact theory. SFG experiments reveal a gradual increase in gauche-defect density with pressure before saturation close to the contact center. Furthermore, our MD simulations show a spatial anisotropy of pressure-induced effects within ODPA domains: molecules tilted in the direction of the pressure gradient increase in tilt angle while those on the opposite side form gauche-defects. PMID:25054614

  14. a Heat Capacity Study of Para-Hydrogen Monolayers on Graphite.

    NASA Astrophysics Data System (ADS)

    Motteler, Frederick Clark

    The heat capacity of monolayer p-H(,2) adsorbed on graphite foam has been measured over the 0.0631(ANGSTROM)(' -2) to 0.0891(ANGSTROM)('-2) density range and the 2 to 25(DEGREES)K temperature range. Specific heat peaks are observed over almost the entire density range and are similar to those observed for ('4)He and ('3)He monolayers on graphite for similar densities. Low density data (0.0631(ANGSTROM)('-2) to 0.0727(ANGSTROM)(' -2)) contains a heat capacity peak attributed to the commensurate order-disorder transition. At commensurate density. 0.0637(ANGSTROM)(' -2), the peak attains a maximum height of C/Nk(,b) = 11.7 at 21.4(DEGREES)K and may be characterized by the critical exponent (alpha) = 0.33. For temperatures below the order -disorder transition, the commensurate monolayer heat capacity is characterized by Einstein like behavior with (theta)(,E) = 53.4(DEGREES)K. High density data (0.0787(ANGSTROM)('-2) to 0.0815(ANGSTROM)(' -2)) contains a heat capacity peak attributed to the disordering of a close packed triangular incommensurate solid. The incommensurate solid is characterized by its Debye like heat capacity behavior. The density region between the commensurate structure and incommensurate solid (0.0707(ANGSTROM)('-2) to 0.772(ANGSTROM)(' -2)) contains two heat capacity peaks. At about 20(DEGREES)K there is broad rounded remnant of the order-disorder peak. At lower temperatures (6 to 9(DEGREES)K) there is a small, but very sharp heat capacity peak. The peak attains a maximum height of C/Nk(,b) = 0.89 at a density of 0.0727(ANGSTROM)(' -2) and a temperature of 9.54(DEGREES)K. The remnant of the order-disorder transition seen in p-H(,2) data, together with re-examination of ('4)He data indicates that a shoulder-like remnant intersects the melting line. The melting peak of ('4)He undergoes significant change at the intersection point. The low temperature intermediate density region is consistent with a striped phase of domain walls. The sharp low

  15. Alkane biohydroxylation: Interests, constraints and future developments.

    PubMed

    Soussan, Laurence; Pen, Nakry; Belleville, Marie-Pierre; Marcano, José Sanchez; Paolucci-Jeanjean, Delphine

    2016-03-20

    Alkanes constitute one of the vastest reserves of raw materials for the production of fine chemicals. This paper focuses on recent advances in alkane biohydroxylation, i.e. the bioactivation of alkanes into their corresponding alcohols. Enzyme and whole-cell biocatalysts have been reviewed. Process considerations to implement such biocatalysts in bioreactors at large scale by coupling the bioconversion with cofactor regeneration and product removal are also discussed. PMID:26853477

  16. Phase Transitions in Dipalmitoylphosphatidylcholine Monolayers.

    PubMed

    Zuo, Yi Y; Chen, Rimei; Wang, Xianju; Yang, Jinlong; Policova, Zdenka; Neumann, A Wilhelm

    2016-08-23

    A self-assembled phospholipid monolayer at an air-water interface is a well-defined model system for studying surface thermodynamics, membrane biophysics, thin-film materials, and colloidal soft matter. Here we report a study of two-dimensional phase transitions in the dipalmitoylphosphatidylcholine (DPPC) monolayer at the air-water interface using a newly developed methodology called constrained drop surfactometry (CDS). CDS is superior to the classical Langmuir balance in its capacity for rigorous temperature control and leak-proof environments, thus making it an ideal alternative to the Langmuir balance for studying lipid polymorphism. In addition, we have developed a novel Langmuir-Blodgett (LB) transfer technique that allows the direct transfer of lipid monolayers from the droplet surface under well-controlled conditions. This LB transfer technique permits the direct visualization of phase coexistence in the DPPC monolayer. With these technological advances, we found that the two-dimensional phase behavior of the DPPC monolayer is analogous to the three-dimensional phase transition of a pure substance. This study has implications in the fundamental understanding of surface thermodynamics as well as applications such as self-assembled monolayers and pulmonary surfactant biophysics. PMID:27479299

  17. Bandgap engineering of monolayer MoS2 under strain: A DFT study

    NASA Astrophysics Data System (ADS)

    Li, Can; Fan, Bowen; Li, Weiyi; Wen, Luowei; Liu, Yan; Wang, Tao; Sheng, Kuang; Yin, You

    2015-06-01

    In this paper, density functional theory calculations are used to investigate the monolayer MoS2 in terms of the strain by analyzing the structure parameters: the bandgap, the density of states (DOS) and the Milliken charges. The calculations indicate that an increasing external stain tends to depress the ripple structure with a shorter S-S interlayer spacing and to enlarge the length of the Mo-S bond. Tensile strain dramatically alteres the bandgap; however, compressive strain almost does not. The change in the bandgap is explained by an analysis of the DOS, the partial density of states (PDOS), the structure parameters and the Mulliken charge distribution. The effects of strain on the Mulliken charge and the length of the Mo-S band cause bandgap differences under tensile and compressive strain.

  18. Oxidation of a potassium monolayer on Ru(001) studied with photoemission, NEXAFS and vibrational EELS

    NASA Astrophysics Data System (ADS)

    Hoffmann, F. M.; Weisel, M.; Eberhardt, W.; Fu, Zugen

    1990-08-01

    The interaction of oxygen with a monolayer of potassium on a Ru(001) surface has been investigated with photoemission and NEXAFS. O 1s core-level data exhibit for low exposures of oxygen a single peak at 531.8 eV. This indicates together with the earlier observation of KO bond formation by EELS that a single K xO y species with equivalent oxygen atoms is formed. NEXAFS data indicate a partially filled O π* orbital and a strongly shifted σ-resonance. Photoemission, vibrational and Auger data suggest an ionic species which is close to potassium Superoxide with the OO bond oriented parallel to the surface.

  19. First-principle study of hydrogenation on monolayer MoS2

    NASA Astrophysics Data System (ADS)

    Xu, Yong; Li, Yin; Chen, Xi; Zhang, Chunfang; Zhang, Ru; Lu, Pengfei

    2016-07-01

    The structural and electronic properties of hydrogenation on 1H-MoS2 and 1T-MoS2 have been systematically explored by using density functional theory (DFT) calculations. Our calculated results indicate an energetically favorable chemical interaction between H and MoS2 monolayer for H adsorption when increasing concentration of H atoms. For 1H-MoS2, single H atom adsorption creates midgap approaching the fermi level which increases the n-type carrier concentration effectively. As a consequence, its electrical conductivity is expected to increase significantly. For 1T-MoS2, H atoms adsorption can lead to the opening of a direct gap of 0.13eV compared to the metallic pristine 1T-MoS2.

  20. Neutron Reflection Study of Bovine β-Casein Adsorbed on OTS Self- Assembled Monolayers

    NASA Astrophysics Data System (ADS)

    Fragneto, Giovanna; Thomas, Robert K.; Rennie, Adrian R.; Penfold, Jeffrey

    1995-02-01

    Specular neutron reflection has been used to determine the structure and composition of bovine β-casein adsorbed on a solid surface from an aqueous phosphate-buffered solution at pH 7. The protein was adsorbed on a hydrophobic monolayer self-assembled from deuterated octadecyltrichlorosilane solution on a silicon (111) surface. A two-layer structure formed consisting of one dense layer of thickness 23 ± 1 angstroms and a surface coverage of 1.9 milligrams per square meter adjacent to the surface and an external layer protruding into the solution of thickness 35 ± 1 angstroms and 12 percent protein volume fraction. The structure of the (β-casein) layer is explained in terms of the charge distribution in the protein.

  1. Domain shapes in lipid monolayers studied as polar cholesteric liquid crystals

    NASA Astrophysics Data System (ADS)

    Iwamoto, M.; Liu, F.; Ou-Yang, Z.-C.

    2010-07-01

    Both bulk and boundary orientations, and boundary shape equations for tilted lipid domain are derived in analogy with a polar cholesteric liquid crystal. It shows that in a two-dimensional (2D) system the 3D spontaneous splay and chiral elastic energies, the s0 and q0 terms of Frank energy, can be regarded as an orientation-dependent line tension, and the domain formation is the equilibrium between the line tension, the surface pressure, the orientational stress, and the dipole-dipole interaction. An obvious and analytic shape solution for pinned boundary orientation for maximum boundary tension has been found and the diverse domain shapes observed in lipid monolayers in the past two decades, such as star, boojum, cardioid, ellipse, bola, and clover-leaf shapes, are dramatically well described by the solution.

  2. The fracture behaviors of monolayer phosphorene with grain boundaries under tension: a molecular dynamics study.

    PubMed

    Guo, Yangyang; Qiao, Chong; Wang, Aihua; Zhang, Jinping; Wang, Songyou; Su, Wan-Sheng; Jia, Yu

    2016-07-27

    The fracture behaviors of monolayer phosphorene (MP) with and without a grain boundary (GB) have been explored by molecular dynamics (MD) simulations. Firstly, in the case of perfect MP, fracture mostly happens on the bond in the zigzag direction when suffering random loading. With the existence of a GB, the crack propagates perpendicular to the GB in different ways under parallel tension to the GB, whereas it propagates along the GB under perpendicular tension to the GB. Then, we found that both the fracture strength and strain decrease with increasing temperature making fracture more likely at relatively high temperatures. Finally, we also found that, similar to graphene, the effect of strain rate on both the fracture strength and strain is not significant, demonstrating that MP is a typical brittle 2D material. Overall, our findings present a useful insight into utilizing phosphorene for mechanical design in electronic devices. PMID:27405397

  3. Electronic and vibrational properties of graphene monolayers with iron adatoms: A density functional theory study

    NASA Astrophysics Data System (ADS)

    Dimakis, Nicholas; Navarro, Nestor E.; Velazquez, Julian; Salgado, Andres

    2015-04-01

    Periodic density functional calculations on graphene monolayers with and without an iron adatom have been used to elucidate iron-graphene adsorption and its effects on graphene electronic and vibrational properties. Density-of-states calculations and charge density contour plots reveal charge transfer from the iron s orbitals to the d orbitals, in agreement with past reports. Adsorbed iron atoms covalently bind to the graphene substrate, verified by the strong hybridization of iron d-states with the graphene bands in the energy region just below the Fermi level. This adsorption is weak and compared to the well-analyzed CO adsorption on Pt: It is indicated by its small adsorption energy and the minimal change of the substrate geometry due to the presence of the iron adatoms. Graphene vibrational spectra are analyzed though a systematic variation of the graphene supercell size. The shifts of graphene most prominent infrared active vibrational modes due to iron adsorption are explored using normal mode eigenvectors.

  4. Adsorption and dissociation of O2 on MoSe2 and MoTe2 monolayers: ab initio study

    NASA Astrophysics Data System (ADS)

    Zhu, X. F.; Wang, L.; Chen, L. F.

    2014-07-01

    Adsorption and dissociation of O2 molecule on the MoSe2 and MoTe2 monolayers are studied by using density functional theory (DFT) within the generalized gradient approximation (GGA) and a supercell approach. The physisorbed O2 molecule on MoSe2 and MoTe2 with a magnetic moment (MM) close to that for an isolated O2 molecule has small adsorption energy and long distance from the surface. The dissociative adsorption of configuration R5(R6) is the most stable adsorption site, whereas the chemisorption of O2 is unfavorable at all adsorption sites. The dissociative adsorption of configuration R4 induces dramatic changes of electronic structures and localized spin polarization both for monolayer MoSe2 and MoTe2. The analysis of electronic density of states (DOSs) shows that the contribution of spin polarization is mainly from the hybridization between O-p, Se(Te)-p and Mo-d orbitals.

  5. Binding of Na+ and K+ to the Headgroup of Palmitic Acid Monolayers Studied by Vibrational Sum Frequency Generation Spectroscopy

    NASA Astrophysics Data System (ADS)

    Huang, Zishuai; Allen, Heather C.

    2012-06-01

    Alkali cations are critical in biological systems due to their electrical interaction with cell membranes. While Na+ and K+ share similar chemical and physical properties, they can exhibit differences when interacting with biological membranes. These phenomena may be modeled using a Langmuir monolayer of surfactant on alkali chloride solutions. Vibrational sum frequency generation (VSFG) spectroscopy is an interface specific technique that is widely employed to study molecular organization at surfaces and interfaces. VSFG spectroscopy was used to probe the CO2- vibrational mode for the carboxylic acid headgroup of palmitic acid (PA) spread on the surface of NaCl and KCl solutions in the vibrational region between 1400 and 1500 cm-1. The ability of Na+ and K+ to bind with the carboxylic headgroup of PA is revealed by observing peak positions (˜1410 cm-1 and ˜1470 cm-1) and relative intensity for the CO2- peaks. These results are compared and discussed with perspective toward elucidating interfacial PA headgroup organization. The time evolution for the PA CO2- peaks is also monitored after monolayer spreading via VSFG and these results are presented as well.

  6. Langmuir monolayers and Differential Scanning Calorimetry for the study of the interactions between camptothecin drugs and biomembrane models.

    PubMed

    Casadó, Ana; Giuffrida, M Chiara; Sagristá, M Lluïsa; Castelli, Francesco; Pujol, Montserrat; Alsina, M Asunción; Mora, Margarita

    2016-02-01

    CPT-11 and SN-38 are camptothecins with strong antitumor activity. Nevertheless, their severe side effects and the chemical instability of their lactone ring have questioned the usual forms for its administration and have focused the current research on the development of new suitable pharmaceutical formulations. This work presents a biophysical study of the interfacial interactions of CPT-11 and SN-38 with membrane mimetic models by using monolayer techniques and Differential Scanning Calorimetry. The aim is to get new insights for the understanding of the bilayer mechanics after drug incorporation and to optimize the design of drug delivery systems based on the formation of stable bilayer structures. Moreover, from our knowledge, the molecular interactions between camptothecins and phospholipids have not been investigated in detail, despite their importance in the context of drug action. The results show that neither CPT-11 nor SN-38 disturbs the structure of the complex liposome bilayers, despite their different solubility, that CPT-11, positively charged in its piperidine group, interacts electrostatically with DOPS, making stable the incorporation of a high percentage of CPT-11 into liposomes and that SN-38 establishes weak repulsive interactions with lipid molecules that modify the compressibility of the bilayer without affecting significantly neither the lipid collapse pressure nor the miscibility pattern of drug-lipid mixed monolayers. The suitability of a binary and a ternary lipid mixture for encapsulating SN-38 and CPT-11, respectively, has been demonstrated. PMID:26656185

  7. Formation and dissolution processes of the 6-thioguanine (6TG) self-assembled monolayer. A kinetic study.

    PubMed

    Madueño, Rafael; Pineda, Teresa; Sevilla, José Manuel; Blázquez, Manuel

    2005-02-01

    This is a report on the kinetics of the destruction and formation processes of the 6-thioguanine self-assembled monolayer (6TG SAM) on a mercury electrode from acid solutions by chronoamperometry. The destruction of the 6TG SAM that has been previously formed under open circuit potential conditions is carried out by stepping the potential from an initial value where the chemisorbed layer is stable up to potentials where the molecules are no longer chemisorbed. The destruction of the SAM has been described by a model that involves three types of contributions: (i) a Langmuir-type adsorption process, (ii) a 2D nucleation mechanism followed by a growth controlled by surface diffusion, and (iii) a 2D nucleation mechanism followed by a growth at a constant rate. The nonlinear fit of the experimental transients by using this procedure allows the quantitative determination of the individual contributions to the overall process. The kinetics of the formation process is studied under electrochemical conditions. The chronoamperometric experiment allows us to monitor the early stages of 6TG SAM formation. The implications of the physisorbed state at low potentials in the type of monolayer formation and destruction processes as well as the influence of temperature are also discussed. PMID:16851120

  8. First-principles study of nanometer-sharp domain walls in ferromagnetic Fe monolayers under in-plane strain.

    PubMed

    Shimada, T; Okuno, J; Ishii, Y; Kitamura, T

    2012-03-01

    We investigated a nanometer-sharp magnetic domain wall (DW) structure in a free-standing Fe(110) monolayer and studied the crucial role of in-plane strain using fully unconstrained noncollinear ab initio spin-density-functional theory calculations within the generalized gradient approximation. The DW width is calculated to be 0.86 nm. A precise vector-field description of the magnetization density revealed that a noncollinear character in the DW was spatially confined between atoms, whereas a collinear and high magnetization density was localized around each atom. In the rapid rotation of magnetic moments in the DW, we found an electron rearrangement from the d(zx) and d(x(2)-y(2)) states to the d(xy), d(yz) and d(z(2)) states due to a shift of band structures. Applied tensile and compressive in-plane strains both bring about narrower DWs in the monolayer except when the strain is small. The strain dependence of the DW width is discussed in terms of both exchange interaction and magnetocrystalline anisotropy. PMID:22322862

  9. Tethered chains in poor solvent conditions: An experimental study involving Langmuir diblock copolymer monolayers

    SciTech Connect

    Kent, M.S.; Majewski, J.; Smith, G.S.; Lee, L.T.; Satija, S.

    1999-02-01

    We have employed Langmuir monolayers of highly asymmetric polydimethylsiloxane-polystyrene (PDMS-PS) diblock copolymers on dioctyl phthalate (DOP) at temperatures ranging from 22 to {minus}35thinsp{degree}C as a model system for tethered chains in poor solvent conditions. The thicknesses of the tethered PS layers extending into the DOP subphase, measured by neutron reflection, decrease with decreasing temperature ({ital T}) over this entire range. However, the variation with {ital T} becomes weak below {minus}20thinsp{degree}C. At the lowest {ital T}, the layer thicknesses are 55{percent}{endash}75{percent} of the values at the theta condition (T{sub {theta}}=22thinsp{degree}C). The contraction of the layer with decreasing {ital T} is determined as a function of surface density and molecular weight, and these data are compared to universal scaling forms. The PS segments are depleted from the near surface region over the entire {ital T} range, with the thickness of the depletion layer increasing slightly with decreasing {ital T}. The free energy of the surface layer is probed by surface tension measurements. With decreasing {ital T}, negative surface pressures are observed at low coverages for both PDMS-PS and PDMS monolayers, indicating metastability toward lateral phase separation. Evidence for a transition from a dispersed phase to a condensed phase with decreasing {ital T} was observed in the reflectivity for very low PDMS-PS coverage. At high coverage where the submerged blocks are strongly interacting at 22thinsp{degree}C, only a modest decrease in surface pressure is observed over the experimental range of {ital T} despite the strong contraction. This latter result is discussed in terms of the relative contributions of enthalpic and entropic effects to the surface pressure. {copyright} {ital 1999 American Institute of Physics.}

  10. Ordered chlorinated monolayer silicene structures

    NASA Astrophysics Data System (ADS)

    Li, Wenbin; Sheng, Shaoxiang; Chen, Jian; Cheng, Peng; Chen, Lan; Wu, Kehui

    2016-04-01

    We report on a systematic experimental study on the chlorination of monatomic silicene layer on Ag(111) by scanning tunneling microscopy. Monolayer silicene on Ag(111) can form 4×4, (√13×√13)R ± 13.9°, and (2√3×2√3)R30° reconstructions due to their different buckling configurations. We found that at low dosage, Cl atoms attach to the upper buckled Si atoms without changing the buckling configuration of the silicene monolayer. However, at high coverage, the global buckling configuration will be significantly changed, resulting in new ordered structures. Interestingly, all monolayer silicene structures, regardless of their initial reconstructions, tend to form a local silicene 1×1 structure at the saturation coverage. The mechanism for chlorination of monolayer silicene is explained.

  11. Chemoresponsive Monolayer Transistors

    SciTech Connect

    Guo,X.; Myers, M.; Xiao, S.; Lefenfeld, M.; Steiner, R.; Tulevski, G.; Tang, J.; Baumert, J.; Leibfarth, F.; et al.

    2006-01-01

    This work details a method to make efficacious field-effect transistors from monolayers of polycyclic aromatic hydrocarbons that are able to sense and respond to their chemical environment. The molecules used in this study are functionalized so that they assemble laterally into columns and attach themselves to the silicon oxide surface of a silicon wafer. To measure the electrical properties of these monolayers, we use ultrasmall point contacts that are separated by only a few nanometers as the source and drain electrodes. These contacts are formed through an oxidative cutting of an individual metallic single-walled carbon nanotube that is held between macroscopic metal leads. The molecules assemble in the gap and form transistors with large current modulation and high gate efficiency. Because these devices are formed from an individual stack of molecules, their electrical properties change significantly when exposed to electron-deficient molecules such as tetracyanoquinodimethane (TCNQ), forming the basis for new types of environmental and molecular sensors.

  12. Photoluminescence study of the substitution of Cd by Zn during the growth by atomic layer epitaxy of alternate CdSe and ZnSe monolayers

    SciTech Connect

    Hernández-Calderón, I.; Salcedo-Reyes, J. C.

    2014-05-15

    We present a study of the substitution of Cd atoms by Zn atoms during the growth of alternate ZnSe and CdSe compound monolayers (ML) by atomic layer epitaxy (ALE) as a function of substrate temperature. Samples contained two quantum wells (QWs), each one made of alternate CdSe and ZnSe monolayers with total thickness of 12 ML but different growth parameters. The QWs were studied by low temperature photoluminescence (PL) spectroscopy. We show that the Cd content of underlying CdSe layers is affected by the exposure of the quantum well film to the Zn flux during the growth of ZnSe monolayers. The amount of Cd of the quantum well film decreases with higher exposures to the Zn flux. A brief discussion about the difficulties to grow the Zn{sub 0.5}Cd{sub 0.5}Se ordered alloy (CuAu-I type) by ALE is presented.

  13. First-principles study of B, C, N and F doped graphene-like MgO monolayer

    NASA Astrophysics Data System (ADS)

    Wu, Ping; Huang, Min; Cheng, Wenjing; Tang, Fuling

    2016-07-01

    Based on the first-principles calculations, we have investigated the stable geometries, electronic and magnetic properties of the graphene-like MgO monolayer with O atom substituted by B, C, N, and F atoms. The formation energy decreases in the order of B>C>N>F, which may be influenced by the different electronegativities. The band gaps of p-type doped MgO monolayers are tunable due to the emergence of impurity states within the band gap, while F-doped MgO monolayer realizes the transition from semiconductor to metal. The results show that p-type doped MgO monolayer exhibit magnetic behaviors due to polarizations of dopants and surrounding Mg or O atoms near the dopants, while no magnetism is observed in the case of F doped MgO monolayer. These results are potentially useful for spintronic applications and the development of magnetic nanostructures.

  14. X-ray Scattering Studies of Long-Chain Alkanol Monolayers at the Water-Hexane Interface

    SciTech Connect

    Schlossman, Mark L.; Tikhonov, Aleksey M.

    2006-01-17

    X-ray reflectivity and interfacial tension measurements demonstrate that long-chain alkanol monolayers at the water-hexane interface exhibit a well defined chain disorder and partial hexane mixing into the monolayer, in contrast to alkanol monolayers at the water-vapor interface that consist of close-packed rigid rod molecules. At the water-hexane interface triacontanol molecules form a condensed phase with progressive disordering of the chain from the -CH{sub 2}OH to the -CH{sub 3} group. At this interface the density in the head-group region is 10 to 15% greater than bulk water, an effect not seen for the ordered monolayer at the water-vapor interface. Monolayers of shorter length alkanols (consisting of 20, 22, and 24 carbons) and variations with temperature are also discussed.

  15. Comparative Study of Electroless Copper Film on Different Self-Assembled Monolayers Modified ABS Substrate

    PubMed Central

    Xu, Jiushuai; Fan, Ruibin; Wang, Jiaolong; Jia, Mengke; Xiong, Xuanrui; Wang, Fang

    2014-01-01

    Copper films were grown on (3-Mercaptopropyl)trimethoxysilane (MPTMS), (3-Aminopropyl)triethoxysilane (APTES) and 6-(3-(triethoxysilyl)propylamino)-1,3,5- triazine-2,4-dithiol monosodium (TES) self-assembled monolayers (SAMs) modified acrylonitrile-butadiene-styrene (ABS) substrate via electroless copper plating. The copper films were examined using scanning electron microscopy (SEM) and X-ray diffraction (XRD). Their individual deposition rate and contact angle were also investigated to compare the properties of SAMs and electroless copper films. The results indicated that the formation of copper nuclei on the TES-SAMs modified ABS substrate was faster than those on the MPTMS-SAMs and APTES-SAMs modified ABS substrate. SEM images revealed that the copper film on TES-SAM modified ABS substrate was smooth and uniform, and the density of copper nuclei was much higher. Compared with that of TES-SAMs modified resin, the coverage of copper nuclei on MPTMS and APTES modified ABS substrate was very limited and the copper particle size was too big. The adhesion property test demonstrated that all the SAMs enhanced the interfacial interaction between copper plating and ABS substrate. XRD analysis showed that the copper film deposited on SAM-modified ABS substrate had a structure with Cu(111) preferred orientation, and the copper film deposited on TES-SAMs modified ABS substrate is better than that deposited on MPTMS-SAMs or APTES-SAMs modified ABS resins in electromigrtion resistance. PMID:24739812

  16. Lipophilization of ascorbic acid: a monolayer study and biological and antileishmanial activities.

    PubMed

    Kharrat, Nadia; Aissa, Imen; Sghaier, Manel; Bouaziz, Mohamed; Sellami, Mohamed; Laouini, Dhafer; Gargouri, Youssef

    2014-09-17

    Ascorbyl lipophilic derivatives (Asc-C2 to Asc-C(18:1)) were synthesized in a good yield using lipase from Staphylococcus xylosus produced in our laboratory and immobilized onto silica aerogel. Results showed that esterification had little effect on radical-scavenging capacity of purified ascorbyl esters using DPPH assay in ethanol. However, long chain fatty acid esters displayed higher protection of target lipids from oxidation. Moreover, compared to ascorbic acid, synthesized derivatives exhibited an antibacterial effect. Furthermore, ascorbyl derivatives were evaluated, for the first time, for their antileishmanial effects against visceral (Leishmania infantum) and cutaneous parasites (Leishmania major). Among all the tested compounds, only Asc-C10, Asc-C12, and Asc-C(18:1) exhibited antileishmanial activities. The interaction of ascorbyl esters with a phospholipid monolayer showed that only medium and unsaturated long chain (Asc-C10 to Asc-C(18:1)) derivative esters were found to interact efficiently with mimetic membrane of leishmania. These properties would make ascorbyl derivatives good candidates to be used in cosmetic and pharmaceutical lipophilic formulations. PMID:25148258

  17. Monolayer MoS2 on HOPG Studied by Scanning Tunneling Microscopy / Spectroscopy

    NASA Astrophysics Data System (ADS)

    Lu, Chun-I.; Butler, C.; Chu, Y.-H.; Yang, H.-H.; Wei, C.-M.; Li, L.-J.; Lin, M.-T.; department of Physics, National Taiwan University Team; institute of Atomic; Molecular Sciences, Academia Sinica Team

    Chemical Vapor Deposition (CVD) is a promising way to prepare 2D material such as graphene and MoS2 for μm-scale. In this report, we deposit monolayer MoS2 by CVD method on HOPG to create the heterojunction. We observe that, the alignment of triangle MoS2 islands shows the tendency that they have some preferred directions from AFM morphology. From STM atomic resolution images, the moiré superstructures analysis could summarize that the MoS2 lattice tends to have a small angle with graphite's lattice. On the other hand, we also take the tunneling spectra from the different moiré domains and the moiré hills, moiré volleys of the single moiré domain. The results reveal the extraordinary states, which appear in the band gap range of MoS2. We consider these states are the consequence of hybridized of two layers and be detected from the interlayer space. C.-I Lu et al., Appl. Phys. Lett. 106, 181904 (2015).

  18. Molecular dynamics study of solubilization of immiscible solutes by a micelle: Free energy of transfer of alkanes from water to the micelle core by thermodynamic integration method.

    PubMed

    Fujimoto, K; Yoshii, N; Okazaki, S

    2010-08-21

    Free energy of transfer, DeltaG(w-->m), from water phase to a sodium dodecyl sulfate (SDS) micelle core has been calculated for a series of hydrophobic solutes originally immiscible with water by thermodynamic integration method combined with molecular dynamics calculations. The calculated free energy of transfer is in good correspondence to the experiment as well as the theoretical free energy of transfer. The calculated DeltaG(w-->m)'s are all negative, implying that the alkane molecules are more stable in the micelle than in the water phase. It decreases almost linearly as a function of the number of carbon atoms of the alkanes longer than methane with a decrement of 3.3 kJ mol(-1) per one methylene group. The calculated free energy of transfer indicates that, for example, at the micelle concentration of 50 CMC (critical micelle concentration), about only 1 of 6 micelles or 1 of 32 000 micelles does not contain a solute methane or n-octane molecule, respectively. PMID:20726656

  19. Molecular dynamics study of solubilization of immiscible solutes by a micelle: Free energy of transfer of alkanes from water to the micelle core by thermodynamic integration method

    NASA Astrophysics Data System (ADS)

    Fujimoto, K.; Yoshii, N.; Okazaki, S.

    2010-08-01

    Free energy of transfer, ΔGw→m, from water phase to a sodium dodecyl sulfate (SDS) micelle core has been calculated for a series of hydrophobic solutes originally immiscible with water by thermodynamic integration method combined with molecular dynamics calculations. The calculated free energy of transfer is in good correspondence to the experiment as well as the theoretical free energy of transfer. The calculated ΔGw→m's are all negative, implying that the alkane molecules are more stable in the micelle than in the water phase. It decreases almost linearly as a function of the number of carbon atoms of the alkanes longer than methane with a decrement of 3.3 kJ mol-1 per one methylene group. The calculated free energy of transfer indicates that, for example, at the micelle concentration of 50 CMC (critical micelle concentration), about only 1 of 6 micelles or 1 of 32 000 micelles does not contain a solute methane or n-octane molecule, respectively.

  20. Comparing the accuracy of high-dimensional neural network potentials and the systematic molecular fragmentation method: A benchmark study for all-trans alkanes

    NASA Astrophysics Data System (ADS)

    Gastegger, Michael; Kauffmann, Clemens; Behler, Jörg; Marquetand, Philipp

    2016-05-01

    Many approaches, which have been developed to express the potential energy of large systems, exploit the locality of the atomic interactions. A prominent example is the fragmentation methods in which the quantum chemical calculations are carried out for overlapping small fragments of a given molecule that are then combined in a second step to yield the system's total energy. Here we compare the accuracy of the systematic molecular fragmentation approach with the performance of high-dimensional neural network (HDNN) potentials introduced by Behler and Parrinello. HDNN potentials are similar in spirit to the fragmentation approach in that the total energy is constructed as a sum of environment-dependent atomic energies, which are derived indirectly from electronic structure calculations. As a benchmark set, we use all-trans alkanes containing up to eleven carbon atoms at the coupled cluster level of theory. These molecules have been chosen because they allow to extrapolate reliable reference energies for very long chains, enabling an assessment of the energies obtained by both methods for alkanes including up to 10 000 carbon atoms. We find that both methods predict high-quality energies with the HDNN potentials yielding smaller errors with respect to the coupled cluster reference.

  1. Comparing the accuracy of high-dimensional neural network potentials and the systematic molecular fragmentation method: A benchmark study for all-trans alkanes.

    PubMed

    Gastegger, Michael; Kauffmann, Clemens; Behler, Jörg; Marquetand, Philipp

    2016-05-21

    Many approaches, which have been developed to express the potential energy of large systems, exploit the locality of the atomic interactions. A prominent example is the fragmentation methods in which the quantum chemical calculations are carried out for overlapping small fragments of a given molecule that are then combined in a second step to yield the system's total energy. Here we compare the accuracy of the systematic molecular fragmentation approach with the performance of high-dimensional neural network (HDNN) potentials introduced by Behler and Parrinello. HDNN potentials are similar in spirit to the fragmentation approach in that the total energy is constructed as a sum of environment-dependent atomic energies, which are derived indirectly from electronic structure calculations. As a benchmark set, we use all-trans alkanes containing up to eleven carbon atoms at the coupled cluster level of theory. These molecules have been chosen because they allow to extrapolate reliable reference energies for very long chains, enabling an assessment of the energies obtained by both methods for alkanes including up to 10 000 carbon atoms. We find that both methods predict high-quality energies with the HDNN potentials yielding smaller errors with respect to the coupled cluster reference. PMID:27208939

  2. Alkane-Based Urethane Potting Compounds

    NASA Technical Reports Server (NTRS)

    Morris, D. E.

    1986-01-01

    New low viscosity urethanes easily mixed, molded, and outgassed. Alkane-based urethanes resist hydrolysis and oxidation and have excellent dielectric properties. Low-viscosity alkane-based urethane prepolymer prepared by one-step reaction of either isophorone diisocyanate or methyl-bis (4-cyclohexyl isocyanate) with hydrogenated, hydroxy-terminated polybutadiene (HTPBD).

  3. Cholesterol strongly affects the organization of lipid monolayers studied as models of the milk fat globule membrane: Condensing effect and change in the lipid domain morphology.

    PubMed

    Murthy, Appala Venkata Ramana; Guyomarc'h, Fanny; Paboeuf, Gilles; Vié, Véronique; Lopez, Christelle

    2015-10-01

    The biological membrane that surrounds the milk fat globules exhibits phase separation of polar lipids that is poorly known. The objective of this study was to investigate the role played by cholesterol in the organization of monolayers prepared as models of the milk fat globule membrane (MFGM). Differential scanning calorimetry and X-ray diffraction experiments allowed characterization of the gel to liquid crystalline phase transition temperature of lipids, Tm ~35°C, in vesicles prepared with a MFGM lipid extract. For temperature below Tm, atomic force microscopy revealed phase separation of lipids at 30 mN·m(-1) in Langmuir-Blodgett monolayers of the MFGM lipid extract. The high Tm lipids form liquid condensed (LC) domains that protrude by about 1.5 nm from the continuous liquid expanded (LE) phase. Cholesterol was added to the MFGM extract up to 30% of polar lipids (cholesterol/milk sphingomyelin (MSM) molar ratio of 50/50). Compression isotherms evidenced the condensing effect of the cholesterol onto the MFGM lipid monolayers. Topography of the monolayers showed a decrease in the area of the LC domains and in the height difference H between the LC domains and the continuous LE phase, as the cholesterol content increased in the MFGM lipid monolayers. These results were interpreted in terms of nucleation effects of cholesterol and decrease of the line tension between LC domains and LE phase in the MFGM lipid monolayers. This study revealed the major structural role of cholesterol in the MFGM that could be involved in biological functions of this interface (e.g. mechanisms of milk fat globule digestion). PMID:26087463

  4. Monte Carlo Study of the Semimetal-Insulator Phase Transition in Monolayer Graphene with a Realistic Interelectron Interaction Potential

    NASA Astrophysics Data System (ADS)

    Ulybyshev, M. V.; Buividovich, P. V.; Katsnelson, M. I.; Polikarpov, M. I.

    2013-08-01

    We report on the results of the first-principles numerical study of spontaneous breaking of chiral (sublattice) symmetry in suspended monolayer graphene due to electrostatic interaction, which takes into account the screening of Coulomb potential by electrons on σ orbitals. In contrast to the results of previous numerical simulations with unscreened potential, we find that suspended graphene is in the conducting phase with unbroken chiral symmetry. This finding is in agreement with recent experimental results by the Manchester group [D. C. Elias et al., Nat. Phys. 7, 701 (2011); A. S. Mayorov et al., Nano Lett. 12, 4629 (2012)]. Further, by artificially increasing the interaction strength, we demonstrate that suspended graphene is quite close to the phase transition associated with spontaneous chiral symmetry breaking, which suggests that fluctuations of chirality and nonperturbative effects might still be quite important.

  5. Scanning Tunneling Microscopy Study of Molecular Structure: Controlled Monolayer Formation on Graphite at the Liquid-solid Interface

    NASA Astrophysics Data System (ADS)

    Su, C.; Kannappan, K.; Chin, V. Nora; Avila-Bront, L.; Jayaraman, S.; Turro, N. J.; Flynn, G. W.

    2006-03-01

    The self-assembly of heptadecanoic acid 1 and racemic 2-bromoheptadecanoic acid 2 mixtures on the basal plane of a graphite surface has been studied using scanning tunneling microscopy at the liquid-solid interface. The domain structure varies as a function of the ratio of coadsorbed molecules. At lower concentration of acid 2, heptadecanoic acid controls the surface structure by forming a template with fixed lamellar axis-molecular axis angle and domains with alternating R- and S-enantiomer molecular rows. Increasing the concentration of acid 2 leads to the segregation of chiral domains. The inter-correlation between heptadecanoic acid and 2-bromoheptadecanoic acid determines the 2D chiral configuration in the mixed monolayer. A model based on energetically favorable molecular conformations is proposed and will be discussed.

  6. Self-assembled monolayers of methylselenolate on the Au(111) surface: A combined STM and DFT study

    NASA Astrophysics Data System (ADS)

    El-Kareh, L.; Mehring, P.; Caciuc, V.; Atodiresei, N.; Beimborn, A.; Blügel, S.; Westphal, C.

    2014-01-01

    In this study scanning tunneling microscopy (STM) and density functional theory (DFT) were used to investigate the structural formation of methylselenolate (CH3Se) self-assembled monolayers (SAMs) on the Au(111) surface. SAMs were prepared by two different methods, from solution and by exposing the Au(111) surface to gaseous dimethyldiselenide (DMDSe). For methylselenolate (MSe) on the Au(111) surface, our STM measurements revealed the presence of (4 × √3) and (3 × 2√3) rectangular striped phases. These structures were verified by DFT calculations. For both phases, the DFT calculations clearly found a bridge adsorption geometry for MSe on Au(111). Furthermore, they provide information about the electronic structure of the MSe-SAMs.

  7. Azide functional monolayers grafted to a germanium surface: model substrates for ATR-IR studies of interfacial click reactions.

    PubMed

    Zhang, Shuo; Koberstein, Jeffrey T

    2012-01-10

    High-quality azide-functional substrates are prepared by a low temperature reaction of 11-bromoundecyltrichlorosilane with UV-ozone-treated germanium ATR-IR plates followed by nucleophilic substitution of the terminal bromine by addition of sodium azide. The resulting monolayer films are characterized by atomic force microscopy (AFM), contact angle analysis, X-ray photoelectron spectroscopy (XPS), attenuated total reflectance infrared spectroscopy (ATR-IR), and ellipsometry. XPS and ellipsometric thickness data correspond well to the results of molecular model calculations confirming the formation of a densely packed azide-functional monolayer. These azide-functional substrates enable interfacial "click" reactions with complementary alkyne-functional molecules to be studied in situ by ATR-IR. To illustrate their potential utility for kinetic studies we show that, in the presence of copper(I) catalyst, the azide-modified surfaces react rapidly and quantitatively with 5-chloro-pentyne to form triazoles via a 1,3-dipolar cycloaddition reaction. Time-resolved ATR-IR measurements indicate that the interfacial click reaction is initially first order in azide concentration as expected from the reaction mechanism, with a rate constant of 0.034 min(-1), and then transitions to apparent second order dependence, with a rate constant of 0.017 min(-1)/(chains/nm(2)), when the surface azide and triazole concentrations become similar, as predicted by Oyama et al. The reaction achieves an ultimate conversion of 50% consistent with the limit expected due to steric hindrance of the 5-chloro-pentyne reactant at the surface. PMID:22081885

  8. Structure and shear response of lipid monolayers

    SciTech Connect

    Dutta, P.; Ketterson, J.B.

    1990-02-01

    Organic monolayers and multilayers are both scientifically fascinating and technologically promising; they are, however, both complex systems and relatively inaccessible to experimental probes. In this Progress Report, we describe our X-ray diffraction studies, which have given us substantial new information about the structures and phase transitions in monolayers on the surface of water; our use of these monolayers as a unique probe of the dynamics of wetting and spreading; and our studies of monolayer mechanical properties using a simple but effective technique available to anyone using the Wilhelmy method to measure surface tension.

  9. 1,6-hexanedithiol monolayers on Au(111): A multitechnique structural study

    SciTech Connect

    Leung, T.Y.B.; Gerstenberg, M.C.; Lavrich, D.J.; Scoles, G.; Schreiber, F.; Poirier, G.E.

    2000-01-25

    Monolayers of 1,6-hexanedithiol [HS(CH{sub 2}){sub 6}SH] deposited on Au(111) from the gas phase were characterized by scanning tunneling microscopy (STM), grazing incidence X-ray diffraction (GIXD), and low-energy atom diffraction (LEAD). Molecular resolution STM images suggest that the molecules lie prone in a striped arrangement with an inter-row spacing of 5 {angstrom}. For the films prepared at an elevated temperature, two uniaxial incommensurate phases were found by GIXD. With respect to the surface substrate net, the diffraction patterns of both phases can be described by rectangular (p x {radical}3) nets, where p is 4.24 {+-} 0.01 and 4.30 {+-} 0.01. These values of p correspond to spacings of 12.23 {+-} 0.04 and 12.40 {+-} 0.02 {angstrom} along the nearest-neighbor (NN) direction of the substrate, whereas the spacing along the next-nearest-neighbor direction is 5 {angstrom} in both cases. The LEAD patterns can be described by a 3 x 1 superlattice with respect to the mesh observed by GIXD. Lattice nonuniformity and angular broadening along the NN direction were observed by GIXD. The structure of the striped phases is consistent with the molecules being fully extended and flat on the surface with their molecular C-C-C plane parallel to the surface. Using different growth protocols, including liquid-phase deposition, the order of the striped phases was observed to change considerably; however, no evidence of nucleation of other ordered phases was found. Even if denser phases exist, the striped phases may act as effective kinetic traps preventing the transition to other denser phases. The results of both varied growth conditions and performed annealing experiments can be explained by the strong molecule/substrate interaction in the striped phases, which is a consequence of the strong, but not site-specific, interaction of both sulfur atoms with the gold surface.

  10. Semifluorinated thiols in Langmuir monolayers - a study by nonlinear and linear vibrational spectroscopies.

    PubMed

    Volpati, Diogo; Chachaj-Brekiesz, Anna; Souza, Adriano L; Rimoli, Caio Vaz; Miranda, Paulo B; Oliveira, Osvaldo N; Dynarowicz-Łątka, Patrycja

    2015-12-15

    A series of semifluorinated thiols of the general formula CmF2m+1CnH2nSH (abbr. FmHnSH) have been synthesized and characterized in Langmuir monolayers with surface pressure-area isotherms, complemented with polarization-modulated reflection absorption spectroscopy (PM-IRRAS) and sum-frequency generation (SFG) techniques. A comparative analysis was performed for compounds having the same length of fluorinated segment (F10) and variable length of the hydrogenated part (H6, H10, H12), and having identical hydrogenated segment (H12) connected to a fluorinated moiety of different lengths (F6, F8, F10). For the sake of comparison, an alkanethiol (H18SH) was also examined, and F10H10COOH and F10H10OH molecules were used for helping the assignment of SFG spectra of CH stretches. SFG was applied to investigate the hydrocarbon chain and the terminal CF3 group, while PM-IRRAS was used to probe CF2 groups. The number of gauche defects in the hydrocarbon chain increased with the increasing length of the molecule, either by elongation of the hydrogenated or perfluorinated part. SFG measurements recorded at three polarization combinations (ppp, ssp, sps) enabled us to estimate the tilt angle of the terminal CF3 group in semifluorinated thiol molecules as ranging from 35° to 45°, which is consistent with nearly vertical fluorinated segments. Upon increasing the surface pressure, the fluorinated segment gets slightly more upright, but the hydrocarbon chain tilt increases while keeping the same average number of gauche defects. The extent of disorder in the hydrogenated segment may be controlled by varying the size of the fluorinated segment, and this could be exploited for designing functionalized surfaces with insertion of other molecules in the defect region. PMID:26364075

  11. A Raft-Associated Species of Phosphatidylethanolamine Interacts with Cholesterol Comparably to Sphingomyelin. A Langmuir-Blodgett Monolayer Study

    PubMed Central

    Grzybek, Michal; Kubiak, Jakub; Łach, Agnieszka; Przybyło, Magdalena; Sikorski, Aleksander F.

    2009-01-01

    Background Specific interactions between sphingomyelin (SM) and cholesterol (Ch) are commonly believed to play a key role in the formation of rafts in the biological membranes. A weakness of this model is the implication that these microdomains are confined to the outer bilayer leaflet. The cytoplasmic leaflet, which contains the bulk of phosphatidylethanolamine (PE), phosphatidylserine (PS) and phosphatidylinositol (PI), is thought also to harbour half of the membrane cholesterol. Moreover, SLPE (1-stearoyl-2-linoleoyl-sn-glycero-3-phosphatidyl-ethanolamine) has recently been shown to be enriched in isolated detergent-resistant membranes (DRM), and this enrichment was independent of the method of isolation of DRM. Methodology/Principal Findings Here we present quantitative evidence coming from Langmuir-Blodgett monolayer experiments that SLPE forms complex with Ch similar to that between SM and Ch. The energies of these interactions as calculated form the monolayer studies are highly negative. FRAP analysis showed that NBD-Ch recovery was similar in liposomes composed of DOPC/Ch SM or SLPE but not DPPE, providing further evidence that SLPE may form an lo phase in the presence of high Ch concentration. Experiments on the solubility of DOPC liposomes containing DPPE/Ch (1∶1), SM/Ch (1∶1) or SLPE/Ch (1∶1) showed the presence of Triton X-100 insoluble floating fraction (TIFF) in the case of SM/Ch or SLPE/Ch but not in DPPE/Ch containing liposomes. Quantitative determination of particular lipid species in the TIFF fraction confirms the conclusion that SLPE (or similar PE species) could be an important constituent of the inner leaflet raft. Conclusion Such interactions suggest a possible existence of inner-leaflet nanoscale assemblies composed of cholesterol complexes with SLPE or similar unsaturated PE species. PMID:19330037

  12. PM-IRRAS Studies of DMPC Bilayers Supported on Au(111) Electrodes Modified with Hydrophilic Monolayers of Thioglucose.

    PubMed

    Matyszewska, Dorota; Bilewicz, Renata; Su, ZhangFei; Abbasi, Fatemah; Leitch, J Jay; Lipkowski, Jacek

    2016-02-23

    A phospholipid bilayer composed of 1,2-dimyristoyl-d54-sn-glycero-3-phosphocholine (d54-DMPC) was deposited onto the Au(111) electrode modified with a self-assembled monolayer of 1-thio-β-d-glucose (β-Tg) via the Langmuir-Blodgett and Langmuir-Schaefer (LB-LS) techniques. Polarization modulation infrared reflection absorption spectroscopy (PM-IRRAS) measurements were used to characterize structural and orientational changes in this model biological membrane on a hydrophilic surface modified gold electrode. The results of the spectroscopic measurements showed that the tilt angle of acyl chains obtained for deuterated DMPC bilayers supported on the β-Tg-modified gold is significantly lower than that reported previously for DMPC bilayers deposited directly on Au(111) electrodes. Moreover, tilt angles of ∼18° were obtained for d54-DMPC bilayers on β-Tg self-assembled monolayers (SAMs) at positive potentials, which are similar to the values calculated for h-DMPC deposited on bare gold in the desorbed state and to those observed for a stack of hydrated DMPC bilayers. This data confirms that the β-thioglucose SAM promotes the formation of a water cushion that separates the phospholipid bilayer from the metal surface. As a result, the DMPC polar heads are not in direct contact with the electrode and can adopt a zigzag configuration, which strengthens the chain-chain interactions and allows for an overall decrease in the tilt of the acyl chains. These novel supported model membranes may be especially useful in studies pertaining to the incorporation of peptides and proteins into phospholipid bilayers. PMID:26829620

  13. Orientation and Mg Incorporation of Calcite Grown on Functionalized Self-Assembled Monolayers: A Synchrotron X-ray Study

    SciTech Connect

    Kwak,S.; DiMasi, E.; Han, Y.; Aizenberg, J.; Kuzmenko, I.

    2005-01-01

    Calcite crystals were nucleated from MgCl2/CaCl2 solutions onto functionalized self-assembled monolayers adsorbed onto E-beam evaporated Au films. Synchrotron X-ray scattering studies of the crystals reveal new information about preferred orientation and Mg incorporation. The Au [111] axis is distributed within 2.6 degrees of the film surface normal, but the oriented crystals may be tilted up to 6 degrees away from this axis. For low Mg{sup 2+} content, SO{sub 3}--functionalized films nucleated primarily near the (106) calcite face, odd-chain-length carboxylic acid terminated alkanethiol films nucleated near the (012) face, and even-chain-length carboxylic acid terminated alkanethiol films nucleated near the (113) face. [Mg{sup 2+}]/[Ca{sup 2+}] concentration ratios (n) of 2 and greater defeated this preferred orientation and created a powder texture. Diffraction patterns within the layer plane from the coarse calcite powders indicated a shift to higher 2 accompanied by peak broadening with increasing n. For 0.5 < n < 3.5, a double set of calcite peaks is observed, showing that two distinct Mg calcite phases form: one of comparatively lower Mg content, derived from the templated crystals, and a Mg-rich phase derived from amorphous precursor particles. According to the refinement of lattice parameters, Mg incorporation of up to 18 mol % occurs for n = 4, independent of film functionality. We discuss the differences between the differently functionalized monolayers and also introduce the hypothesis that two separate routes to Mg calcite formation occur in this system.

  14. Diffusion of squalene in n-alkanes and squalane.

    PubMed

    Kowert, Bruce A; Watson, Michael B; Dang, Nhan C

    2014-02-27

    Squalene, an intermediate in the biosynthesis of cholesterol, has a 24-carbon backbone with six methyl groups and six isolated double bonds. Capillary flow techniques have been used to determine its translational diffusion constant, D, at room temperature in squalane, n-C16, and three n-C8-squalane mixtures. The D values have a weaker dependence on viscosity, η, than predicted by the Stokes-Einstein relation, D = kBT/(6πηr). A fit to the modified relation, D/T = ASE/η(p), gives p = 0.820 ± 0.028; p = 1 for the Stokes-Einstein limit. The translational motion of squalene appears to be much like that of n-alkane solutes with comparable chain lengths; their D values show similar deviations from the Stokes-Einstein model. The n-alkane with the same carbon chain length as squalene, n-C24, has a near-equal p value of 0.844 ± 0.018 in n-alkane solvents. The values of the hydrodynamic radius, r, for n-C24, squalene, and other n-alkane solutes decrease as the viscosity increases and have a common dependence on the van der Waals volumes of the solute and solvent. The possibility of studying squalene in lipid droplets and membranes is discussed. PMID:24528091

  15. Enrichment and Characterization of a Psychrotolerant Consortium Degrading Crude Oil Alkanes Under Methanogenic Conditions.

    PubMed

    Ding, Chen; Ma, Tingting; Hu, Anyi; Dai, Lirong; He, Qiao; Cheng, Lei; Zhang, Hui

    2015-08-01

    Anaerobic alkane degradation via methanogenesis has been intensively studied under mesophilic and thermophilic conditions. While there is a paucity of information on the ability and composition of anaerobic alkane-degrading microbial communities under low temperature conditions. In this study, we investigated the ability of consortium Y15, enriched from Shengli oilfield, to degrade hydrocarbons under different temperature conditions (5-35 °C). The consortium could use hexadecane over a low temperature range (15-30 °C). No growth was detected below 10 °C and above 35 °C, indicating the presence of cold-tolerant species capable of alkane degradation. The preferential degradation of short chain n-alkanes from crude oil was observed by this consortium. The structure and dynamics of the microbial communities were examined using terminal restriction fragment length polymorphism (T-RFLP) fingerprinting and Sanger sequencing of 16S rRNA genes. The core archaeal communities were mainly composed of aceticlastic Methanosaeta spp. Syntrophaceae-related microorganisms were always detected during consecutive transfers and dominated the bacterial communities, sharing 94-96 % sequence similarity with Smithella propionica strain LYP(T). Phylogenetic analysis of Syntrophaceae-related clones in diverse methanogenic alkane-degrading cultures revealed that most of them were clustered into three sublineages. Syntrophaceae clones retrieved from this study were mainly clustered into sublineage I, which may represent psychrotolerant, syntrophic alkane degraders. These results indicate the wide geographic distribution and ecological function of syntrophic alkane degraders. PMID:25783218

  16. Semiconductor-to-metal phase transition in monolayer ZrS{sub 2}: GGA+U study

    SciTech Connect

    Kumar, Ashok Tankeshwar, K.; He, Haiying; Pandey, Ravindra; Ahluwalia, P. K.

    2015-06-24

    We report structural and electronic properties of ZrS{sub 2} monolayer within density functional theory (DFT) by inclusion of Hubbard on-site Coulomb and exchange interactions. The importance of on-site interactions for both ZrS{sub 2} bulk and monolayer has been highlighted that significantly improves the electronic band-gap. It is demonstrated that mechanical strain induces structural phase transition that results in semiconductor-to-metal transition in monolayer ZrS{sub 2}. This phenomenon has important implications in technological applications such as flexible, low power and transparent electronic devices.

  17. Effect of alkane chain length and counterion on the freezing transition of cationic surfactant adsorbed film at alkane mixture - water interfaces.

    PubMed

    Tokiwa, Yuhei; Sakamoto, Hiroyasu; Takiue, Takanori; Aratono, Makoto; Matsubara, Hiroki

    2015-05-21

    Penetration of alkane molecules into the adsorbed film gives rise to a surface freezing transition of cationic surfactant at the alkane-water interface. To examine the effect of the alkane chain length and counterion on the surface freezing, we employed interfacial tensiometry and ellipsometry to study the interface of cetyltrimethylammonium bromide and cetyltrimethylammonium chloride aqueous solutions against dodecane, tetradecane, hexadecane, and their mixtures. Applying theoretical equations to the experimental results obtained, we found that the alkane molecules that have the same chain length as the surfactant adsorb preferentially into the surface freezing film. Furthermore, we demonstrated that the freezing transition temperature of cationic surfactant adsorbed film was independent of the kind of counterion. PMID:25932500

  18. From monomer to monolayer: a global optimisation study of (ZnO)n nanoclusters on the Ag surface

    NASA Astrophysics Data System (ADS)

    Demiroglu, Ilker; Woodley, Scott M.; Sokol, Alexey A.; Bromley, Stefan T.

    2014-11-01

    We employ global optimisation to investigate how oxide nanoclusters of increasing size can best adapt their structure to lower the system energy when interacting with a realistic extended metal support. Specifically, we focus on the (ZnO)@Ag(111) system where experiment has shown that the infinite Ag(111)-supported ZnO monolayer limit corresponds to an epitaxially 7 : 8 matched graphene-like (Zn3O3)-based hexagonal sheet. Using a two-stage search method based on classical interatomic potentials and then on more accurate density functional theory, we report global minina candidate structures for Ag-supported (ZnO)n cluster with sizes ranging from n = 1-24. Comparison with the respective global minina structure of free space (ZnO)n clusters reveals that the surface interaction plays a decisive role in determining the lowest energy Ag-supported (ZnO)n cluster structures. Whereas free space (ZnO)n clusters tend to adopt cage-like bubble structures as they grow larger, Ag-supported (ZnO)n clusters of increasing size become progressively more like planar cuts from the infinite graphene-like ZnO single monolayer. This energetic favourability for planar hexagonal Ag-supported clusters over their 3D counterparts can be partly rationalised by the ZnO-Ag(111) epitaxial matching and the increased number of close interactions with the Ag surface. Detailed analysis shows that this tendency can also be attributed to the capacity of 2D clusters to distort to improve their interaction with the Ag surface relative to more rigid 3D bubble cluster isomers. For the larger sized clusters we find that the adsorption energies and most stable structural types appear to be rather converged confirming that our study makes a bridge between the Ag-supported ZnO monomer and the infinite Ag-supported ZnO monolayer.We employ global optimisation to investigate how oxide nanoclusters of increasing size can best adapt their structure to lower the system energy when interacting with a realistic

  19. Using the BacMam Baculovirus System to Study Expression and Function of Recombinant Efflux Drug Transporters in Polarized Epithelial Cell Monolayers

    PubMed Central

    Fung, King Leung; Kapoor, Khyati; Pixley, Jessica N.; Talbert, Darrell J.; Kwit, Alexandra D.T.; Ambudkar, Suresh V.

    2016-01-01

    The ATP-binding cassette (ABC) transporter superfamily includes several membrane-bound proteins that are critical to drug pharmacokinetics and disposition. Pharmacologic evaluation of these proteins in vitro remains a challenge. In this study, human ABC transporters were expressed in polarized epithelial cell monolayers transduced using the BacMam baculovirus gene transfer system. The purpose of the study was to evaluate the efficacy of BacMam baculovirus to transduce cells grown in monolayers. In a porcine kidney cell line, LLC-PK1 cells, baculoviral transduction is successful only via the apical side of a polarized monolayer. We observed that recombinant ABC transporters were expressed on the cell surface with post-translational modification. Furthermore, sodium butyrate played a critical role in recombinant protein expression, and preincubation in the presence of tunicamycin or thapsigargin enhanced protein expression. Cells overexpressing human P-glycoprotein (P-gp) showed vectorial basolateral-to-apical transport of [3H]-paclitaxel, which could be reversed by the inhibitor tariquidar. Similarly, coexpression of human P-gp and ABCG2 in LLC-PK1 cells resulted in higher transport of mitoxantrone, which is a substrate for both transporters, than in either P-gp– or ABCG2-expressing cells alone. Taken together, our results indicate that a high level of expression of efflux transporters in a polarized cell monolayer is technically feasible with the BacMam baculovirus system PMID:26622052

  20. Hematite nanoparticle monolayers on mica electrokinetic characteristics.

    PubMed

    Morga, Maria; Adamczyk, Zbigniew; Oćwieja, Magdalena

    2012-11-15

    Electrokinetic properties of α-Fe(2)O(3) (hematite) nanoparticle monolayers on mica were thoroughly characterized using the streaming potential method. Hematite suspensions were obtained by acidic hydrolysis of ferric chloride. The average size of particles (hydrodynamic diameter), determined by dynamic light scattering (DLS) and AFM, was 22 nm (pH=5.5, I=10(-2)M). The hematite monolayers on mica were produced under diffusion-controlled transport from the suspensions of various bulk concentration. The monolayer coverage, quantitatively determined by AFM and SEM, was regulated within broad limits by adjusting the nanoparticle deposition time. This allowed one to uniquely express zeta potential of hematite monolayers, determined by the streaming potential measurements, in terms of the particle coverage. Such dependencies, obtained for various pH, were successfully interpreted in terms of the three-dimensional electrokinetic model. A universal calibrating graph was produced enabling one to determine hematite monolayer coverage from the measured value of the streaming potential. The influence of the ionic strength, varied between 10(-4) and 10(-2)M, on the zeta potential of hematite monolayers was also studied. Additionally, the stability of monolayers (desorption kinetics) was determined under in situ conditions using the streaming potential method. Our experimental data prove that it is feasible to produce uniform and stable hematite particle monolayers of well-controlled coverage. Such monolayers may find practical applications as universal substrates for protein immobilization (biosensors) and in electrocatalytic applications. PMID:22921408

  1. [Distribution Characteristics and Source Apportionment of n-Alkanes in Water from Yellow River in Henan Section].

    PubMed

    Feng, Jing-lan; Xi, Nan-nan; Zhang, Fei; Liu, Shu-hui; Sun, Jian-hui

    2016-03-15

    To investigate the distributions and possible sources of n-alkanes in water and suspended particulate matter from Yellow River in Henan section, 26 water and suspended particulate matter samples were collected in August 2010 and 22 n-alkanes (C₁₄-C₃₆) were quantitatively determined by gas chromatography-mass spectrometer (GC-MS). Potential sources of n-alkanes were analyzed using different characteristic parameters. The results indicated that total concentrations of 22 n-alkanes were 521-5,843 ng · L⁻¹ with a mean concentration of 1,409 ng · L⁻¹, while the total amounts of n-alkanes in the suspended particulate matter were 463-11,142 ng · L⁻¹ with a mean value of 1,951 ng · L⁻¹. The composition profiles of n-alkanes in water showed unimodal distribution with a peak at C₂₅ in water. However, the composition characteristics of n-alkanes in SPM were of bimodal type, but still with the advantage of high carbon hydrocarbons peak at C₂₅. Results of characteristic parameters including CPI, TAR, OEP and % WaxCn showed that n-alkanes in the studied area were derived mainly from combustion of fossil fuel, while terrestrial higher plant played a role in the existence of n-alkanes in water and suspended particulate matter from Yellow River in Henan section. PMID:27337879

  2. MIR and NIR group spectra of n-alkanes and 1-chloroalkanes

    NASA Astrophysics Data System (ADS)

    Kwaśniewicz, Michał; Czarnecki, Mirosław A.

    2015-05-01

    Numerous attempts were undertaken to resolve the absorption originating from different parts of alkanes. The separation of the contributions from the terminal and midchain methylene units was observed only in the spectra of solid alkanes at low temperatures. On the other hand, for liquid alkanes this effect was not reported as yet. In this study, ATR-IR, Raman and NIR spectra of eight n-alkanes and seven 1-chloroalkanes in the liquid phase were measured from 1000 to 12,000 cm-1. The spectra were analyzed by using two-dimensional (2D) correlation approach and chemometrics methods. It was shown that in 2D asynchronous contour plots, constructed from the spectra of n-alkanes and 1-chloroalkanes, the methylene band was resolved into two components. These two components were assigned to the terminal and midchain methylene groups. For the first time, the contributions from these two molecular fragments were resolved in the spectra of liquid n-alkanes and 1-chloroalkanes. MCR-ALS resolved these spectra into two components that were assigned to the ethyl and midchain methylene groups. These components represent the group spectra that can be used for assignment, spectral analysis and prediction of unknown spectra. The spectral prediction based on the group spectra provides very good results for n-alkanes, especially in the first and second overtone regions.

  3. 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

  4. Effect of n-alkanes on asphaltene structuring in petroleum oils.

    PubMed

    Stachowiak, Christian; Viguié, Jean-Romain; Grolier, Jean-Pierre E; Rogalski, Marek

    2005-05-24

    The interactions between asphaltenes and short- to medium-chain n-alkanes were studied using titration microcalorimetry and inverse chromatography. The exothermic heat effects observed upon mixing of asphaltenes and n-alkanes were interpreted in terms of assembling of the two types of compounds into mixed structures. We show that the energy of the interactions between n-alkanes and the asphaltene hydrocarbon chains is close to the energy of the interactions between the asphaltene chains. We propose that the latter interactions are responsible for the formation of the asphaltene aggregates and are the driving force of the aggregate assembly into higher structures. PMID:15896019

  5. Adsorption and dissociation kinetics of alkanes on CaO(100)

    NASA Astrophysics Data System (ADS)

    Chakradhar, A.; Liu, Y.; Schmidt, J.; Kadossov, E.; Burghaus, U.

    2011-08-01

    The adsorption kinetics of ethane, butane, pentane, and hexane on CaO(100) have been studied by multi-mass thermal desorption (TDS) spectroscopy. The sample cleanliness was checked by Auger electron spectroscopy. A molecular and dissociative adsorption pathway was evident for the alkanes, except for ethane, which does not undergo bond activation. Two TDS peaks appeared when recording the parent mass, which are assigned to different adsorption sites/configurations of the molecularly adsorbed alkanes. Bond activation leads to desorption of hydrogen and several alkane fragments assigned to methane and ethylene formation. Only one TDS feature is seen in this case. Formation of carbon residuals was absent.

  6. Surface plasmon Raman scattering studies of liquid crystal anchoring on liquid-crystal-based self-assembled monolayers.

    PubMed

    Critchley, Kevin; Cheadle, Edward M; Zhang, Hao-Li; Baldwin, Kurt J; Liu, Quanying; Cheng, Yaling; Fukushima, Hitoshi; Tamaki, Takashi; Batchelder, David N; Bushby, Richard J; Evans, Stephen D

    2009-11-26

    We studied the anchoring of 6CB on a series of self-assembled monolayers (SAMs) with a functional group that mimics that of the nematic liquid crystal (LC). The SAMs were first characterized by wetting, Fourier-transform infrared spectroscopy, and surface potential measurements. We found that, in two of these SAMs, the end group dipoles were oriented close to the normal of the surface and that these promoted homeotropic anchoring. In the case of the other SAM, the dipole was oriented parallel to the surface, and planar anchoring was obtained. Raman scattering by adsorbates on thin metal films is enhanced by the electromagnetic fields of surface plasmon polaritons (SPPs). Despite the inherent polarization of SPPs, there have been few reports in which SPP Raman scattering has been used to study molecular orientation. We have developed optical instrumentation to provide efficient excitation and collection of SPP Raman scattered light using attenuated total reflection geometry. The Kretschmann prism coupling configuration was used to excite SPPs on thin (500 A) gold films with adsorbed SAMs of alkanethiols in contact with thin films (50 microm) of the nematic liquid crystal 4'-hexylbiphenyl-4-carbonitrile (6CB, Merck). The anchoring and orientational wetting of the LC 6CB at the interface with omega-functionalized SAMs was studied using this arrangement. In agreement with the results of previous studies, a high-energy surface (-COOH) was found to promote planar anchoring, whereas a low-energy surface (-CF(3)) was found to induce homeotropic anchoring. PMID:19921953

  7. Molecular simulation studies of nanoscale friction between phosphorylcholine self-assembled monolayer surfaces: Correlation between surface hydration and friction

    NASA Astrophysics Data System (ADS)

    He, Yi; Chen, Shengfu; Hower, Jason C.; Bernards, Matthew T.; Jiang, Shaoyi

    2007-08-01

    We performed all-atom molecular dynamics simulations to study the friction between surfaces covered with two phosphorylcholine self-assembled monolayers (PC-SAM) under shear. PC-SAM surfaces with a √7×√7R19° lattice structure and a parallel arrangement of the head groups were used as model zwitterionic surfaces. They provide a full representation of the zwitterionic nature of phospholipid surfaces, which are believed to play an important role in the lubrication of biological joints such as knees and hips. The surfaces were immersed in aqueous solutions and kept in contact with two regions of bulk water. Sodium chloride and potassium chloride solutions at various concentrations were employed to study the effects of the presence of ions on friction. The results show a strong relationship between surface hydration and friction. Higher ionic concentrations or ions with shorter Debye lengths cause a larger disruption to the hydration around the zwitterionic surfaces, leading to larger friction forces. In addition, the results show that under nanoscale confinement, the friction coefficients of PC-SAM surfaces in pure water are directly proportional to both shear velocity and surface separation distance. These results are comparable to previously published experimental studies.

  8. Supported organoiridium catalysts for alkane dehydrogenation

    DOEpatents

    Baker, R. Thomas; Sattelberger, Alfred P.; Li, Hongbo

    2013-09-03

    Solid supported organoiridium catalysts, a process for preparing such solid supported organoiridium catalysts, and the use of such solid supported organoiridium catalysts in dehydrogenation reactions of alkanes is provided. The catalysts can be easily recovered and recycled.

  9. EPR studies on the organization of self-assembled spin-labeled organic monolayers adsorbed on GaAs.

    PubMed

    Ruthstein, Sharon; Artzi, Reit; Goldfarb, Daniella; Naaman, Ron

    2005-02-01

    Characterizing the structure and dynamic properties of a single monolayer is a challenge due to the minute amount of material that is probed. Here, EPR spectroscopy is used for investigating the spatial and temporal organization of self-assembled monolayers of 5- and 16-doxyl stearic acid (5 DSA and 16 DSA, respectively) adsorbed on a GaAs substrate. The results are complemented with FTIR and ellipsometery measurements, which provide the evidence for the formation of monolayers. Moreover, a comparison with the FTIR spectrum of a monolayer of stearic acid shows that the monolayers of the spin labeled molecules are less packed due to the hindrance introduced by the labeling group. The EPR spectra provide a new insight on the ordering in the layer and more interestingly, it reveals the time dependence of the organization. For 5DSA, with the spin-label group situated close to the substrate, the EPR spectrum immediately after adsorption is poorly resolved and dominated by the spin-exchange interaction between neighboring molecules. As time increases (up to 1 week) the resolution of the 14N hyperfine coupling increases, revealing a better organized monolayer where the molecules are more homogenously spaced. Moreover, the spectrum of the layer, after reaching equilibrium, shows that there is no motional freedom near the GaAs surface. Orientation dependence measurements on the equilibrated sample show the presence of a preferred orientation of the molecules, although with a wide distribution. The spectrum of the 16DSA monolayer, where the nitroxide spin label is situated at the end of the chain, far from the surface, also showed a poorly resolved spectrum at short times, but unlike 5DSA, it did not exhibit any time dependence. Through EPR line-shape simulations and by comparison with FTIR results, the differences between 5DSA and 16DSA were attributed to difference in coverage caused by the bulky spin label near the surface in the case of 5DSA. PMID:19785139

  10. The uptake of tritium-labelled carnitine by monolayer cultures of human fetal muscle and its potential as a label in cytotoxicity studies

    PubMed Central

    Cambridge, Geraldine; Stern, C. M. M.

    1981-01-01

    As a novel approach to the investigation of immune responses directed against muscle antigens in inflammatory muscle disease, the use of tritium-labelled carnitine as a selective marker for myotubes in monolayer cultures was investigated. Tritium-labelled carnitine was incubated either with monolayer cultures of human fetal muscle (which contain fibroblasts and myotubes) or with syngeneic monolayer cultures of human fetal fibroblasts. The rate of uptake and loss of tritium-labelled carnitine by muscle cultures was compared with that shown by fibroblast cultures; uptake being five times greater for muscle. Values for Km and Vmax were derived for both tissues in culture, the ratio Km/Vmax being 3·1 for muscle cultures and 0·46 for fibroblast cultures, indicating the presence of the active transport system for carnitine in the myotube membrane. Freeze-dried radioautographs of muscle monolayers, previously incubated with tritium-labelled carnitine, were made and confirmed the specific intra-tubular localization of the label. Fetal muscle monolayers, previously incubated with tritium-labelled carnitine, were used as targets in long-term cytotoxicity experiments into lymphocyte-mediated myotoxicity. Peripheral blood lymphocytes from patients with inflammatory muscle disease were shown to be myotoxic, but lymphocytes from normal individuals or those with non-inflammatory muscle disease were not. This system is likely to prove much more sensitive than those methods employing chromium-51-labelled cultures. Carnitine-based measures of myotoxicity closely followed the clinical activity of the disease in sequential studies carried out on one patient and the test shows considerable potential as a means of assessing myotube killing by lymphocytes on a per-cell basis. ImagesFig. 3 PMID:7261477

  11. Anaerobic oxidation of short-chain alkanes in hydrothermal sediments: potential influences on sulfur cycling and microbial diversity

    SciTech Connect

    Adams, MM; Hoarfrost, AL; Bose, A; Joye, SB; Girguis, PR

    2013-05-14

    Short-chain alkanes play a substantial role in carbon and sulfur cycling at hydrocarbon-rich environments globally, yet few studies have examined the metabolism of ethane (C-2), propane (C-3), and butane (C-4) in anoxic sediments in contrast to methane (C-1). In hydrothermal vent systems, short-chain alkanes are formed over relatively short geological time scales via thermogenic processes and often exist at high concentrations. The sediment-covered hydrothermal vent systems at Middle Valley (MV Juan de Fuca Ridge) are an ideal site for investigating the anaerobic oxidation of C-1-C-4 alkanes, given the elevated temperatures and dissolved hydrocarbon species characteristic of these metalliferous sediments. We examined whether MV microbial communities oxidized C-1-C-4 alkanes under mesophilic to thermophilic sulfate-reducing conditions. Here we present data from discrete temperature (25, 55, and 75 degrees C) anaerobic batch reactor incubations of MV sediments supplemented with individual alkanes. Co-registered alkane consumption and sulfate reduction (SR) measurements provide clear evidence for C-1-C-4 alkane oxidation linked to SR over time and across temperatures. In these anaerobic batch reactor sediments, 16S ribosomal RNA pyrosequencing revealed that Deltaproteobacteria, particularly a novel sulfate-reducing lineage, were the likely phylotypes mediating the oxidation of C-2-C-4 alkanes. Maximum C-1-C-4 alkane oxidation rates occurred at 55 degrees C, which reflects the mid-core sediment temperature profile and corroborates previous studies of rate maxima for the anaerobic oxidation of methane (AOM). Of the alkanes investigated, C-3 was oxidized at the highest rate over time, then C-4, C-2, and C-1, respectively. The implications of these results are discussed with respect to the potential competition between the anaerobic oxidation of C-2-C(4)alkanes with AOM for available oxidants and the influence on the fate of C-1 derived from these hydrothermal systems.

  12. Oxidation Products of Semi-volatile Alkanes by Hydroxyl Radicals

    NASA Astrophysics Data System (ADS)

    Zhang, H.; Worton, D. R.; Nah, T.; Goldstein, A. H.; Wilson, K. R.

    2013-12-01

    Alkanes are ubiquitous in the atmosphere and are important components that influence atmospheric chemistry. Semi-volatile alkanes are partitioned between the gas- and the particle-phases and can be readily oxidized in both phases. Previous studies have demonstrated that reaction rates and the products of OH oxidation are very different for organic compounds in the gas- and particle phases. In the present study, n-octadecane (C18H38), n-eicosane (C20H42), n-docosane (C22H46), n-tricosane (C24H50), and n-pentadecylcyclohexane (C21H42) were chosen as model compounds for semi-volatile alkanes to examine their OH-initiated oxidation reactions in a flow tube reactor. OH exposure was varied in the experiments, equivalent to oxidation of up to one week in the atmosphere. Oxidation products were collected on filters and analyzed using two-dimensional gas chromatography coupled to a high-resolution time-of-flight electron impact ionization and vacuum ultraviolet photoionization mass spectrometer. Most of the oxygenated higher molecular weight isomers were separated and quantified. Our results suggest that aerosol samples formed in the n-octadecane experiment were more oxidized than the other model compounds (i.e., functionalization products with three oxygen atoms per molecule compared to two oxygen atoms per molecule) at similar OH exposures and aerosol mass loadings. This is likely due to the concentration of n-octadecane in the gas phase where oxidation is more rapid. We find that the first-generation gas-phase oxidation products quickly partition to the particle phase after which higher-generation oxidation likely occurs in the particle phase. Interestingly, functionalized carbonyl isomers for the normal alkanes were only observed on the 4 carbon positions closest to the molecule end in all cases, which is in contrast to structure-reactivity relationship (SRR) predictions for gas-phase reactions. For n-octadecane, the concentrations of first-generation functionalization

  13. Chemoresponsive monolayer transistors

    PubMed Central

    Guo, Xuefeng; Myers, Matthew; Xiao, Shengxiong; Lefenfeld, Michael; Steiner, Rachel; Tulevski, George S.; Tang, Jinyao; Baumert, Julian; Leibfarth, Frank; Yardley, James T.; Steigerwald, Michael L.; Kim, Philip; Nuckolls, Colin

    2006-01-01

    This work details a method to make efficacious field-effect transistors from monolayers of polycyclic aromatic hydrocarbons that are able to sense and respond to their chemical environment. The molecules used in this study are functionalized so that they assemble laterally into columns and attach themselves to the silicon oxide surface of a silicon wafer. To measure the electrical properties of these monolayers, we use ultrasmall point contacts that are separated by only a few nanometers as the source and drain electrodes. These contacts are formed through an oxidative cutting of an individual metallic single-walled carbon nanotube that is held between macroscopic metal leads. The molecules assemble in the gap and form transistors with large current modulation and high gate efficiency. Because these devices are formed from an individual stack of molecules, their electrical properties change significantly when exposed to electron-deficient molecules such as tetracyanoquinodimethane (TCNQ), forming the basis for new types of environmental and molecular sensors. PMID:16855049

  14. Quantum chemical analysis of thermodynamics of 2D cluster formation of alkanes at the water/vapor interface in the presence of aliphatic alcohols.

    PubMed

    Vysotsky, Yu B; Kartashynska, E S; Belyaeva, E A; Fainerman, V B; Vollhardt, D; Miller, R

    2015-11-21

    Using the quantum chemical semi-empirical PM3 method it is shown that aliphatic alcohols favor the spontaneous clusterization of vaporous alkanes at the water surface due to the change of adsorption from the barrier to non-barrier mechanism. A theoretical model of the non-barrier mechanism for monolayer formation is developed. In the framework of this model alcohols (or any other surfactants) act as 'floats', which interact with alkane molecules of the vapor phase using their hydrophobic part, whereas the hydrophilic part is immersed into the water phase. This results in a significant increase of contact effectiveness of alkanes with the interface during the adsorption and film formation. The obtained results are in good agreement with the existing experimental data. To test the model the thermodynamic and structural parameters of formation and clusterization are calculated for vaporous alkanes C(n)H(2n+2) (n(CH3) = 6-16) at the water surface in the presence of aliphatic alcohols C(n)H(2n+1)OH (n(OH) = 8-16) at 298 K. It is shown that the values of clusterization enthalpy, entropy and Gibbs' energy per one monomer of the cluster depend on the chain lengths of corresponding alcohols and alkanes, the alcohol molar fraction in the monolayers formed, and the shift of the alkane molecules with respect to the alcohol molecules Δn. Two possible competitive structures of mixed 2D film alkane-alcohol are considered: 2D films 1 with single alcohol molecules enclosed by alkane molecules (the alcohols do not form domains) and 2D films 2 that contain alcohol domains enclosed by alkane molecules. The formation of the alkane films of the first type is nearly independent of the surfactant type present at the interface, but depends on their molar fraction in the monolayer formed and the chain length of the compounds participating in the clusterization, whereas for the formation of the films of the second type the interaction between the hydrophilic parts of the surfactant is

  15. Tight-binding model study of substrate induced pseudo-spin polarization and magnetism in mono-layer graphene

    NASA Astrophysics Data System (ADS)

    Sahu, Sivabrata; Rout, G. C.

    2016-06-01

    We present here a tight-binding model study of generation of magnetism and pseudo-spin polarization in monolayer graphene arising due to substrate, impurity and Coulomb correlation effects. The model Hamiltonian contains the first-, second- and third-nearest-neighbor hopping integrals for π electrons of graphene besides substrate induced gap, impurity interactions and Coulomb correlation of electrons. The Hubbard type Coulomb interactions present in both the sub-lattices A and B are treated within the mean-field approximation. The electronic Green's functions are calculated by using Zubarev's technique and hence the electron occupancies of both sub-lattices are calculated for up and down spins separately. These four temperature dependent occupancies are calculated numerically and self-consistently. Then we have calculated the temperature dependent pseudo-spin polarization, ferromagnetic and anti-ferromagnetic magnetizations. We observe that there exists pseudo-spin polarization for lower Coulomb energy, u < 2.2t1 and pseudo-spin polarization is enhanced with substrate induced gap and impurity effect. For larger Coulomb energy u > 2.5t1, there exists pseudo-spin polarization (p); while ferromagnetic (m) and antiferromagnetic (pm) magnetizations exhibit oscillatory behavior. With increase of the substrate induced gap, the ferromagnetic and antiferromagnetic transition temperatures are enhanced with increase of the substrate induced gap; while polarization (p) is enhanced in magnitude only.

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

    SciTech Connect

    Franz, Johannes; Graham, Daniel J.; Baio, Joe E.; Lelle, Marco; Peneva, Kalina; Müllen, Klaus; Castner, David G.; Weidner, Tobias

    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 that 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.

  17. Nanodimentional Aggregates In Organic Monolayers Studied With Atomic Force Microscopy (AFM) And Fluorescence Lifetime Imaging Microscopy (FLIM)

    NASA Astrophysics Data System (ADS)

    Ivanov, George R.; Burov, Julian

    2007-04-01

    Organic monolayers from a fluorescently labeled phospholipid (DPPE-NBD) were deposited on solid supports under special conditions that form stable nanometer wide bilayers cylinders that protrude from the monolayer. This molecule was frequently used in sensor applications due to its sensitivity to environment changes. The proposed configuration should provide both fast response times (ultra thin film) and increased sensitivity (greatly increased surface area). AFM can clearly distinguish between the different phases. The height difference between the solid-expanded and the liquid-expanded phase was measured to be 1.4 nm while the bilayer thickness was 5.6 nm. The solid domains show a 20 % decrease in fluorescence lifetime in comparison to the monolayer as measured by FLIM. This difference in lifetimes is explained in the model of fluorescence self quenching in the solid phase due to the molecules being closer to each other.

  18. Molecular ordering and phase transitions in alkanol monolayers at the water-hexane interface.

    PubMed

    Tikhonov, Aleksey M; Pingali, Sai Venkatesh; Schlossman, Mark L

    2004-06-22

    The interface between bulk water and bulk hexane solutions of n-alkanols (H(CH(2))(m)OH, where m=20, 22, 24, or 30) is studied with x-ray reflectivity, x-ray off-specular diffuse scattering, and interfacial tension measurements. The alkanols adsorb to the interface to form a monolayer. The highest density, lowest temperature monolayers contain alkanol molecules with progressive disordering of the chain from the -CH(2)OH to the -CH(3) group. In the terminal half of the chain that includes the -CH(3) group the chain density is similar to that observed in bulk liquid alkanes just above their freezing temperature. The density in the alkanol headgroup region is 10% greater than either bulk water or the ordered headgroup region found in alkanol monolayers at the water-vapor interface. We conjecture that this higher density is a result of water penetration into the headgroup region of the disordered monolayer. A ratio of 1:3 water to alkanol molecules is consistent with our data. We also place an upper limit of one hexane to five or six alkanol molecules mixed into the alkyl chain region of the monolayer. In contrast, H(CH(2))(30)OH at the water-vapor interface forms a close-packed, ordered phase of nearly rigid rods. Interfacial tension measurements as a function of temperature reveal a phase transition at the water-hexane interface with a significant change in interfacial excess entropy. This transition is between a low temperature interface that is nearly fully covered with alkanols to a higher temperature interface with a much lower density of alkanols. The transition for the shorter alkanols appears to be first order whereas the transition for the longer alkanols appears to be weakly first order or second order. The x-ray data are consistent with the presence of monolayer domains at the interface and determine the domain coverage (fraction of interface covered by alkanol domains) as a function of temperature. This temperature dependence is consistent with a

  19. Photocatalytic acceptorless alkane dehydrogenation: scope, mechanism, and conquering deactivation with carbon dioxide.

    PubMed

    Chowdhury, Abhishek Dutta; Julis, Jennifer; Grabow, Kathleen; Hannebauer, Bernd; Bentrup, Ursula; Adam, Martin; Franke, Robert; Jackstell, Ralf; Beller, Matthias

    2015-01-01

    Alkane dehydrogenation is of special interest for basic science but also offers interesting opportunities for industry. The existing dehydrogenation methodologies make use of heterogeneous catalysts, which suffer from harsh reaction conditions and a lack of selectivity, whereas homogeneous methodologies rely mostly on unsolicited waste generation from hydrogen acceptors. Conversely, acceptorless photochemical alkane dehydrogenation in the presence of trans-Rh(PMe3 )2 (CO)Cl can be regarded as a more benign and atom efficient alternative. However, this methodology suffers from catalyst deactivation over time. Herein, we provide a detailed investigation of the trans-Rh(PMe3 )2 (CO)Cl-photocatalyzed alkane dehydrogenation using spectroscopic and theoretical investigations. These studies inspired us to utilize CO2 to prevent catalyst deactivation, which leads eventually to improved catalyst turnover numbers in the dehydrogenation of alkanes that include liquid organic hydrogen carriers. PMID:25346450

  20. Structural insights into diversity and n-alkane biodegradation mechanisms of alkane hydroxylases

    PubMed Central

    Ji, Yurui; Mao, Guannan; Wang, Yingying; Bartlam, Mark

    2013-01-01

    Environmental microbes utilize four degradation pathways for the oxidation of n-alkanes. Although the enzymes degrading n-alkanes in different microbes may vary, enzymes functioning in the first step in the aerobic degradation of alkanes all belong to the alkane hydroxylases. Alkane hydroxylases are a class of enzymes that insert oxygen atoms derived from molecular oxygen into different sites of the alkane terminus (or termini) depending on the type of enzymes. In this review, we summarize the different types of alkane hydroxylases, their degrading steps, and compare typical enzymes from various classes with regard to their three-dimensional structures, in order to provide insights into how the enzymes mediate their different roles in the degradation of n-alkanes and what determines their different substrate ranges. Through the above analyzes, the degrading mechanisms of enzymes can be elucidated and molecular biological methods can be utilized to expand their catalytic roles in the petrochemical industry or in bioremediation of oil-contaminated environments. PMID:23519435

  1. Site isolation in vanadium phosphorus oxide alkane oxidation

    SciTech Connect

    Thompson, M R; Ebner, J R

    1991-06-01

    Single crystal X-ray diffraction studies of vanadyl pyrophosphate indicate that at least two polytypical structures exists for this active and selective alkane oxidation catalyst. The crystal structures of these materials differ with respect to the symmetry and direction of columns of vanadyl groups within the unit cell. Single crystals of vanadyl pyrophosphate have been generated at extreme temperatures not often experienced by microcrystalline catalysts. The crystallography of the system suggests that other crystalline modifications or disordered phases might also exist. Zeroth-order models of crystal surface termination of vanadyl pyrophosphate have been constructed which conceptually illustrate the ability of vanadyl pyrophosphate to accommodate varying amounts of surface phosphorus parallel to (1,0,0), (0,1,0) and (0,2,4). Pyrophosphate termination of surfaces parallel to (1,0,0) likely results in the isolation of clusters of reactive centers and limits overoxidation of the alkane substrate. 23 refs., 6 figs.

  2. Graphic model for calculating the entropy of С11Н24 alkanes with allowance for multiple non-valence interactions through three atoms along the chain of a molecule

    NASA Astrophysics Data System (ADS)

    Nilov, D. Yu.; Smolyakov, V. M.

    2016-08-01

    A fourteen-constant graphic scheme is proposed for evaluating the thermodynamic properties of branched paraffin hydrocarbons. Absolute entropy S f, 298 gas of 159 alkanes, of which 157 alkanes have yet to be studied experimentally, are calculated using 105 experimental data S f, 298 K, gas for alkanes CН4-С32Н66.

  3. 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.

  4. Theoretical study of the stereodynamics of CO collisions with CH3- and CF3-terminated alkanethiolate self-assembled monolayers.

    PubMed

    Alexander, William A; Morris, John R; Troya, Diego

    2009-04-23

    We present a classical-trajectory study of CO collisions with regular (CH3-terminated) and omega-fluorinated (CF3-terminated) alkanethiol self-assembled monolayers (SAMs) with a focus on analyzing the stereodynamics properties of the collision. The CO molecule is scattered with incident angles of either 30 degrees or 60 degrees with respect to the surface normal and with 60 kJ x mol(-1) collision energy, and we analyze final translational and rotational energy, mechanism of the collisions, and orientation and alignment of the rotational angular momentum. Analysis of the alignment of the final rotational angular momentum in collisions involving initially rotationally cold CO indicates a slight preference for "cartwheel" and "corkscrew" rotational motions. In contrast, collisions of initially excited CO slightly favor "helicopter" motion of the recoiling molecule. Moreover, studies of final orientation reveal that, while cartwheel "topspin" motion is favored for collisions in which initially cold CO becomes rotationally excited, no preferred handedness is observed when CO leaves the surfaces with "helicopter" motion. Analysis of trajectories involving initially rotationally excited CO in which the initial rotational angular momentum is aligned and/or oriented shows a non-negligible effect of the initial rotational motion on the dynamics of energy transfer. For instance, CO approaching the SAMs with helicopter motion retains a larger fraction of its initial rotation than molecules colliding with cartwheel-type motions. Conservation of the alignment and orientation of the initial rotational angular momentum vector is also enhanced with helicopter motion relative to cartwheel or random motions. The calculated trends in the stereodynamic properties for the two SAMs indicate that the CH3-SAM is effectively more corrugated than the CF3-SAM. PMID:19206227

  5. Boron nitride as a substrate for H{sub 2} monolayer studies

    SciTech Connect

    Evans, M.D.; Patel, N.; Sullivan, N.S.

    1992-11-01

    The authors report measurements of the adsorption isotherms of helium and methane on boron nitride. The suitability of using BN as a substrate for studying the two-dimensional, orientational ordering of quantum quadrupoles on a triangular lattice is also discussed. 6 refs., 3 figs.

  6. A New In Vitro Model to Study Cellular Responses after Thermomechanical Damage in Monolayer Cultures

    PubMed Central

    Hettler, Alice; Werner, Simon; Eick, Stefan; Laufer, Stefan; Weise, Frank

    2013-01-01

    Although electrosurgical instruments are widely used in surgery to cut tissue layers or to achieve hemostasis by coagulation (electrocautery), only little information is available concerning the inflammatory or immune response towards the debris generated. Given the elevated local temperatures required for successful electrocautery, the remaining debris is likely to contain a plethora of compounds entirely novel to the intracorporal setting. A very common in vitro method to study cell migration after mechanical damage is the scratch assay, however, there is no established model for thermomechanical damage to characterise cellular reactions. In this study, we established a new in vitro model to investigate exposure to high temperature in a carefully controlled cell culture system. Heatable thermostat-controlled aluminium stamps were developed to induce local damage in primary human umbilical vein endothelial cells (HUVEC). The thermomechanical damage invoked is reproducibly locally confined, therefore allowing studies, under the same experimental conditions, of cells affected to various degrees as well as of unaffected cells. We show that the unaffected cells surrounding the thermomechanical damage zone are able to migrate into the damaged area, resulting in a complete closure of the ‘wound’ within 48 h. Initial studies have shown that there are significant morphological and biological differences in endothelial cells after thermomechanical damage compared to the mechanical damage inflicted by using the unheated stamp as a control. Accordingly, after thermomechanical damage, cell death as well as cell protection programs were activated. Mononuclear cells adhered in the area adjacent to thermomechanical damage, but not to the zone of mechanical damage. Therefore, our model can help to understand the differences in wound healing during the early phase of regeneration after thermomechanical vs. mechanical damage. Furthermore, this model lends itself to study the

  7. A new in vitro model to study cellular responses after thermomechanical damage in monolayer cultures.

    PubMed

    Hettler, Alice; Werner, Simon; Eick, Stefan; Laufer, Stefan; Weise, Frank

    2013-01-01

    Although electrosurgical instruments are widely used in surgery to cut tissue layers or to achieve hemostasis by coagulation (electrocautery), only little information is available concerning the inflammatory or immune response towards the debris generated. Given the elevated local temperatures required for successful electrocautery, the remaining debris is likely to contain a plethora of compounds entirely novel to the intracorporal setting. A very common in vitro method to study cell migration after mechanical damage is the scratch assay, however, there is no established model for thermomechanical damage to characterise cellular reactions. In this study, we established a new in vitro model to investigate exposure to high temperature in a carefully controlled cell culture system. Heatable thermostat-controlled aluminium stamps were developed to induce local damage in primary human umbilical vein endothelial cells (HUVEC). The thermomechanical damage invoked is reproducibly locally confined, therefore allowing studies, under the same experimental conditions, of cells affected to various degrees as well as of unaffected cells. We show that the unaffected cells surrounding the thermomechanical damage zone are able to migrate into the damaged area, resulting in a complete closure of the 'wound' within 48 h. Initial studies have shown that there are significant morphological and biological differences in endothelial cells after thermomechanical damage compared to the mechanical damage inflicted by using the unheated stamp as a control. Accordingly, after thermomechanical damage, cell death as well as cell protection programs were activated. Mononuclear cells adhered in the area adjacent to thermomechanical damage, but not to the zone of mechanical damage. Therefore, our model can help to understand the differences in wound healing during the early phase of regeneration after thermomechanical vs. mechanical damage. Furthermore, this model lends itself to study the

  8. Effect of Dispersion on Surface Interactions of Cobalt(II) Octaethylporphyrin Monolayer on Au(111) and HOPG(0001) Substrates: a Comparative First Principles Study

    SciTech Connect

    Chilukuri, Bhaskar; Mazur, Ursula; Hipps, Kerry W.

    2014-07-21

    A density functional theory study of a cobalt(II) octaethylporphyrin (CoOEP) monolayer on Au(111) and HOPG(0001) surfaces was performed under periodic boundary conditions. Calculations with and without dispersion corrections are performed and the effect of van der Waals forces on the interface properties is analyzed. Calculations have determined that the CoOEP molecule tends to bind at the 3-fold and the 6-fold center sites on Au(111) and HOPG(0001), respectively. Geometric optimizations at the center binding sites have indicated that the porphyrin molecules (in the monolayer) lie flat on both substrates. Calculations also reveal that the CoOEP monolayer binds slightly more strongly to Au(111) than to HOPG(0001). Charge density difference plots disclose that charge is redistributed mostly around the porphyrin plane and the first layer of the substrates. Dispersion interactions cause a larger substrate to molecule charge pushback on Au(111) than on HOPG. CoOEP adsorption tends to lower the work functions of either substrate, qualitatively agreeing with the experimental photoelectron spectroscopic data. Comparison of the density of states (DOS) of the isolated CoOEP molecule with that on gold and HOPG substrates showed significant band shifts around the Fermi energy due to intermolecular orbital hybridization. Simulated STM images were plotted with the Tersoff–Hamann approach using the local density of states, which also agree with the experimental results. This study elucidates the role of dispersion for better describing porphyrin–substrate interactions. A DFT based overview of geometric, adsorption and electronic properties of a porphyrin monolayer on conductive surfaces is presented.

  9. A Langmuir monolayer study of the action of phospholipase A2 on model phospholipid and mixed phospholipid-GM1 ganglioside membranes.

    PubMed

    Schulte, Wiebke; Orlof, Monika; Brand, Izabella; Korchowiec, Beata; Rogalska, Ewa

    2014-04-01

    Polarization-modulation infrared reflection-absorption spectroscopy, surface pressure measurements and thermodynamic analysis were used to study enzymatic hydrolysis of lipid monolayers at the air/water interface. The Ca(2+)-requiring pork pancreatic phospholipase A2 was used as a catalyst. The substrates were pure 1,2-dilauroyl-sn-glycero-3-phosphocholine or mixed 1,2-dilauroyl-sn-glycero-3-phosphocholine - monosialotetrahexosylganglioside Langmuir films. The physicochemical properties of the monolayers were established with the aim of a correlation with enzyme activity. The infrared spectra were acquired upon the advancement of the catalysis; the latter was studied at a controlled surface pressure and area of the film. Changes of the intensity and frequency of different infrared signals characteristic for the two lipids were correlated with modification of the properties of the monolayer due to hydrolysis. The amide I signal characteristic for peptides permitted detecting the enzyme adsorbed at the interface. The thermodynamic and infrared results indicate that monosialotetrahexosylganglioside increases H-bonding of the lipid polar heads in the films. This effect, which may be responsible for the low activity of phospholipase A2 in the mixed films, could be used for developing enzyme-resistant lipid systems. PMID:24524938

  10. Crumpling deformation regimes of monolayer graphene on substrate: a molecular mechanics study.

    PubMed

    Al-Mulla, Talal; Qin, Zhao; Buehler, Markus J

    2015-09-01

    Experiments and simulations demonstrating reversible and repeatable crumpling of graphene warrant a detailed understanding of the underlying mechanisms of graphene crumple formation, especially for design of tailored nanostructures. To systematically study the formation of crumples in graphene, we use a simple molecular dynamics model, and perform a series of simulations to characterize the finite number of deformation regimes of graphene on substrate after compression. We formulate a quantitative measure of predicting these deformations based on observed results of the simulations and distinguish graphene crumpling considered in this study from others. In our study, graphene is placed on a model substrate while controlling and varying the interfacial energy between graphene and substrate and the substrate roughness through a set of particles embedded in the substrate. We find that a critical value of interfacial adhesion energy marks a transition point that separates two deformation regimes of graphene on substrate under uniaxial compression. The interface between graphene and substrate plays a major role in the formation of crumples, and we show that the choice of substrate can help in designing desired topologies in graphene. PMID:26252422