Carbon film deposition on SnO{sub 2}/Si(111) using DC unbalanced magnetron sputtering

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

Aji, A. S.; Darma, Y.

In this paper, carbon deposition on SnO{sub 2} layer using DC unbalanced magnetron-sputtering technique at low temperature has been systematically studied. Sputtering process were carried out at pressure of 4.6×10{sup −2} Torr by keeping the substrate temperature at 300 °C. SnO{sub 2} were growth on silicon (111) substrate using thermal evaporation and continuing with dry oxidation of Sn at 225 °C. Thermal evaporation for high purity Sn was conducted by maintain the current source as high as 40 ampere. The quality of SnO{sub 2} on Si(111) and the characteristic of carbon thin film on SnO{sub 2} were analized by meanmore » XRD, FTIR and Raman spectra. XRD analysis shows that SnO{sub 2} film is growth uniformly on Si(111). FTIR and Raman spectra confirm the formation of thin film carbon on SnO{sub 2}. Additionally, thermal annealing for some sample series have been performed to study their structural stability. The change of atomic structure due to thermal annealing were analized by Raman and XRD spectra.« less

STM imaging ortho- and para-fluorothiophenol self-assembled monolayers on Au(111).

PubMed

Jiang, Peng; Deng, Ke; Fichou, Denis; Xie, Si-Shen; Nion, Aymeric; Wang, Chen

2009-05-05

Self-assembled monolayers (SAMs) of para- and ortho-fluorothiophenol (p- and o-FTP) spontaneously formed on Au(111) substrate have been contrasted through investigation by a scanning tunneling microscope (STM) at room temperature. High-resolution STM imaging reveals that p-FTP adopts a 6 x radical3R30 degrees molecule arrangement containing six molecules. Two different kinds of p-FTP molecule dimer line structures have been formed on Au(111) by intermolecular pi-pi stacking along 112 substrate directions, besides a single p-FTP molecule line. In contrast, o-FTP molecules self-assemble into a much looser wave-like SAM, which can be described as a 5 x 3 radical3R30 degrees structure containing two molecules. Periodic density functional theory (DFT) calculations for the two systems suggest that these kinds of FTP molecules preferentially take the asymmetrical positions between 3-fold face-centered cubic (fcc) hollow and bridge sites on Au(111), tilting from the substrate surface. Theoretical simulation gives apparent average tilted angles of 58 degrees and 68 degrees for p-FTP and o-FTP with respect to the surface normal, respectively. This simulation shows that o-FTP is more inclined to lie down toward the Au(111) surface compared to p-FTP. The difference between p-FTP and o-FTP SAM structures can be qualitatively understood in terms of the variation of intermolecular dipole-dipole orientation. This suggests that, besides well-known Au-S and pi-pi interactions, electrostatic interactions including dipole-dipole, quadrupole-quadrupole, and dipole-quadrupole interactions might also play an important role in influencing the SAM structures formed by aromatic thiols with a permanent dipole moment.

Energy-resolved attosecond interferometric photoemission from Ag(111) and Au(111) surfaces

NASA Astrophysics Data System (ADS)

Ambrosio, M. J.; Thumm, U.

2018-04-01

Photoelectron emission from solid surfaces induced by attosecond pulse trains into the electric field of delayed phase-coherent infrared (IR) pulses allows the surface-specific observation of energy-resolved electronic phase accumulations and photoemission delays. We quantum-mechanically modeled interferometric photoemission spectra from the (111) surfaces of Au and Ag, including background contributions from secondary electrons and direct emission by the IR pulse, and adjusted parameters of our model to energy-resolved photoelectron spectra recently measured at a synchrotron light source by Roth et al. [J. Electron Spectrosc. 224, 84 (2018), 10.1016/j.elspec.2017.05.008]. Our calculated spectra and photoelectron phase shifts are in fair agreement with the experimental data of Locher et al. [Optica 2, 405 (2015), 10.1364/OPTICA.2.000405]. Our model's not reproducing the measured energy-dependent oscillations of the Ag(111) photoemission phases may be interpreted as evidence for subtle band-structure effects on the final-state photoelectron-surface interaction not accounted for in our simulation.

Interaction of Au with thin ZrO2 films: influence of ZrO2 morphology on the adsorption and thermal stability of Au nanoparticles.

PubMed

Pan, Yonghe; Gao, Yan; Kong, Dandan; Wang, Guodong; Hou, Jianbo; Hu, Shanwei; Pan, Haibin; Zhu, Junfa

2012-04-10

The model catalysts of ZrO(2)-supported Au nanoparticles have been prepared by deposition of Au atoms onto the surfaces of thin ZrO(2) films with different morphologies. The adsorption and thermal stability of Au nanoparticles on thin ZrO(2) films have been investigated using synchrotron radiation photoemission spectroscopy (SRPES) and X-ray photoelectron spectroscopy (XPS). The thin ZrO(2) films were prepared by two different methods, giving rise to different morphologies. The first method utilized wet chemical impregnation to synthesize the thin ZrO(2) film through the procedure of first spin-coating a zirconium ethoxide (Zr(OC(2)H(5))(4)) precursor onto a SiO(2)/Si(100) substrate at room temperature followed by calcination at 773 K for 12 h. Scanning electron microscopy (SEM) investigations indicate that highly porous "sponge-like nanostructures" were obtained in this case. The second method was epitaxial growth of a ZrO(2)(111) film through vacuum evaporation of Zr metal onto Pt(111) in 1 × 10(-6) Torr of oxygen at 550 K followed by annealing at 1000 K. The structural analysis with low energy electron diffraction (LEED) of this film exhibits good long-range ordering. It has been found that Au forms smaller particles on the porous ZrO(2) film as compared to those on the ordered ZrO(2)(111) film at a given coverage. Thermal annealing experiments demonstrate that Au particles are more thermally stable on the porous ZrO(2) surface than on the ZrO(2)(111) surface, although on both surfaces, Au particles experience significant sintering at elevated temperatures. In addition, by annealing the surfaces to 1100 K, Au particles desorb completely from ZrO(2)(111) but not from porous ZrO(2). The enhanced thermal stability for Au on porous ZrO(2) can be attributed to the stronger interaction of the adsorbed Au with the defects and the hindered migration or coalescence resulting from the porous structures. © 2012 American Chemical Society

Formation of Surface and Quantum-Well States in Ultra Thin Pt Films on the Au(111) Surface

PubMed Central

Silkin, Igor V.; Koroteev, Yury M.; Echenique, Pedro M.; Chulkov, Evgueni V.

2017-01-01

The electronic structure of the Pt/Au(111) heterostructures with a number of Pt monolayers n ranging from one to three is studied in the density-functional-theory framework. The calculations demonstrate that the deposition of the Pt atomic thin films on gold substrate results in strong modifications of the electronic structure at the surface. In particular, the Au(111) s-p-type Shockley surface state becomes completely unoccupied at deposition of any number of Pt monolayers. The Pt adlayer generates numerous quantum-well states in various energy gaps of Au(111) with strong spatial confinement at the surface. As a result, strong enhancement in the local density of state at the surface Pt atomic layer in comparison with clean Pt surface is obtained. The excess in the density of states has maximal magnitude in the case of one monolayer Pt adlayer and gradually reduces with increasing number of Pt atomic layers. The spin–orbit coupling produces strong modification of the energy dispersion of the electronic states generated by the Pt adlayer and gives rise to certain quantum states with a characteristic Dirac-cone shape. PMID:29232833

Production of Au clusters by plasma gas condensation and their incorporation in oxide matrixes by sputtering

NASA Astrophysics Data System (ADS)

Figueiredo, N. M.; Serra, R.; Manninen, N. K.; Cavaleiro, A.

2018-05-01

Gold clusters were produced by plasma gas condensation method and studied in great detail for the first time. The influence of argon flow, discharge power applied to the Au target and aggregation chamber length on the size distribution and deposition rate of Au clusters was evaluated. Au clusters with sizes between 5 and 65 nm were deposited with varying deposition rates and size dispersion curves. Nanocomposite Au-TiO2 and Au-Al2O3 coatings were then deposited by alternating sputtering. These coatings were hydrophobic and showed strong colorations due to the surface plasmon resonance effect. By simulating the optical properties of the nanocomposites it was possible to identify each individual contribution to the overall surface plasmon resonance signal. These coatings show great potential to be used as high performance localized surface plasmon resonance sensors or as robust self-cleaning decorative protective layers. The hybrid method used for depositing the nanocomposites offers several advantages over co-sputtering or thermal evaporation processes, since a broader range of particle sizes can be obtained (up to tens of nanometers) without the application of any thermal annealing treatments and the properties of clusters and matrix can be controlled separately.

Structural and electronic properties of graphene nanoflakes on Au(111) and Ag(111)

PubMed Central

Tesch, Julia; Leicht, Philipp; Blumenschein, Felix; Gragnaniello, Luca; Fonin, Mikhail; Marsoner Steinkasserer, Lukas Eugen; Paulus, Beate; Voloshina, Elena; Dedkov, Yuriy

2016-01-01

We investigate the electronic properties of graphene nanoflakes on Ag(111) and Au(111) surfaces by means of scanning tunneling microscopy and spectroscopy as well as density functional theory calculations. Quasiparticle interference mapping allows for the clear distinction of substrate-derived contributions in scattering and those originating from graphene nanoflakes. Our analysis shows that the parabolic dispersion of Au(111) and Ag(111) surface states remains unchanged with the band minimum shifted to higher energies for the regions of the metal surface covered by graphene, reflecting a rather weak interaction between graphene and the metal surface. The analysis of graphene-related scattering on single nanoflakes yields a linear dispersion relation E(k), with a slight p-doping for graphene/Au(111) and a larger n-doping for graphene/Ag(111). The obtained experimental data (doping level, band dispersions around EF, and Fermi velocity) are very well reproduced within DFT-D2/D3 approaches, which provide a detailed insight into the site-specific interaction between graphene and the underlying substrate. PMID:27002297

Sputtering-growth of seeded Au nanoparticles for nanogap-assisted surface-enhanced Raman scattering (SERS) biosensing

NASA Astrophysics Data System (ADS)

Fu, Chit Yaw; U. S., Dinish; Rautela, Shashi; Goh, Douglas Wenda; Olivo, Malini

2011-12-01

Gold-coated array patterned with tightly-packed nanospheres was developed as a substrate base for constructing SERSenriched nanogaps with Au-nanoparticles (GNPs). Using 1,2-ethanedithiol as a linker, Au-NPs (=17-40nm) were anchored covalently on the sphere-array. Thin Au layer was sputtered on the substrate to mask the citrate coating of GNPs that could demote the sensing mechanism. The negatively-charged GNP surface warrants the colloidal stability, but the resulting repulsive force keeps the immobilized NPs apart by about 40nm. The attained gap size is inadequately narrow to sustain any intense enhancement owing to the near-field nature of SERS. Minimal amount of NaCl was then added to slightly perturb the colloidal stability by reducing their surface charge. Notably, the interparticle-gap reduces at increasing amount of salt, giving rise to increased packing density of GNPs. The SERS enhancement is also found to exponentially increase at decreasing gap size. Nevertheless, the minimum gap achieved is limited to merely 7nm. Excessive addition of salt would eventually induce complete aggregation of particles, forming clustered NPs on the array. A simple sputtering-growth approach is therefore proposed to further minimize the interparticle gap by enlarging the seeded NPs based on mild sputtering. The SEM images confirm that the gap below 7nm is achievable. With advent of the colloidal chemistry, the combined salt-induced aggregation and sputtering-growth techniques can be applied to engineer interparticle gap that is crucial to realize an ultrasensitive SERS biosensor. The proposed two-step preparation can be potentially adopted to fabricate the SERS-enriched nanogaps on the microfluidics platform.

Enhancing the reactivity of gold: Nanostructured Au(111) adsorbs CO

DOE PAGES

Hoffmann, F. M.; Hrbek, J.; Ma, S.; ...

2015-12-02

Low-coordinated sites are surface defects whose presence can transform a surface of inert or noble metal such as Au into an active catalyst. We prepared gold surfaces modified by pits, starting with a well-ordered Au(111) surface; we then used microscopy (STM) for their structural characterization and CO spectroscopy (IRAS and NEXAFS) for probing reactivity of surface defects. In contrast to the Au(111) surface CO adsorbs readily on the pitted surfaces bonding to low-coordinated sites identified as step atoms forming {111} and {100} microfacets. Finally, pitted nanostructured surfaces can serve as interesting and easily prepared models of catalytic surfaces with definedmore » defects that offer an attractive alternative to vicinal surfaces or nanoparticles commonly employed in catalysis science.« less

Self-organization of a self-assembled supramolecular rectangle, square, and three-dimensional cage on Au111 surfaces.

PubMed

Yuan, Qun-Hui; Wan, Li-Jun; Jude, Hershel; Stang, Peter J

2005-11-23

The structure and conformation of three self-assembled supramolecular species, a rectangle, a square, and a three-dimensional cage, on Au111 surfaces were investigated by scanning tunneling microscopy. These supramolecular assemblies adsorb on Au111 surfaces and self-organize to form highly ordered adlayers with distinct conformations that are consistent with their chemical structures. The faces of the supramolecular rectangle and square lie flat on the surface, preserving their rectangle and square conformations, respectively. The three-dimensional cage also forms well-ordered adlayers on the gold surface, forming regular molecular rows of assemblies. When the rectangle and cage were mixed together, the assemblies separated into individual domains, and no mixed adlayers were observed. These results provide direct evidence of the noncrystalline solid-state structures of these assemblies and information about how they self-organize on Au111 surfaces, which is of importance in the potential manufacturing of functional nanostructures and devices.

Atomic and molecular adsorption on Au(111)

DOE PAGES

Santiago-Rodriguez, Yohaselly; Herron, Jeffrey A.; Curet-Arana, Maria C.; ...

2014-05-02

Periodic self-consistent density functional theory (DFT-GGA) calculations were used to study the adsorption of several atomic species, molecular species and molecular fragments on the Au(111) surface with a coverage of 1/4 monolayer (ML). Binding geometries, binding energies, and diffusion barriers were calculated for 27 species. Furthermore, we calculated the surface deformation energy associated with the binding events. The binding strength for all the analyzed species can be ordered as follows: NH 3 < NO < CO < CH 3 < HCO < NH 2 < COOH < OH < HCOO < CNH 2 < H < N < NH O < F < S < C < CH. Although the atomic species preferred to bind at the three-fold fcc site, no tendency was observed in site preference for the molecular species and fragments. The intramolecular and adsorbate-surface vibrational frequencies were calculated for all the adsorbates on their most energetically stable adsorption site. Most of the theoretical binding energies and frequencies agreed with experimental values reported in the literature. In general, the values obtained with the PW91 functional are more accurate than RPBE in reproducing these experimental binding energies. The energies of the adsorbed species were used to calculate the thermochemical potential energy surfaces for decomposition of CO, NO, N 2, NH 3 and CH 4, oxidation of CO, and hydrogenation of CO, CO 2 and NO, giving insight into the thermochemistry of these reactions on gold nanoparticles. As a result, these potential energy surfaces demonstrated that: the decomposition of species is not energetically favorable on Au(111); the desorption of NH 3, NO and CO are more favorable than their decomposition; the oxidation of CO and hydrogenation of CO and NO on Au(111) to form HCO and HNO, respectively, are also thermodynamically favorable.« less

On the nature of the {SO2-4}/{Ag(111) } and {SO2-4}/{Au(111) } surface bonding

NASA Astrophysics Data System (ADS)

Patrito, E. M.; Olivera, P. Paredes; Sellers, Harrell

1997-05-01

The nature of sulfate-Ag(111) and sulfate-Au(111) surface bonding has been investigated at the SCF + MP2 level of theory. Convergence of binding energy with cluster size is investigated and, unlike neutral adsorbates, large clusters are required in order to obtain reliable binding energies. In the most stable adsorption mode, sulfate binds to the surface via three oxygen atoms (C 3v symmetry) with a binding energy of 159.3 kcal/mol on Ag(111) and 143.9 kcal/mol on Au(111). The geometry of adsorbed sulfate was optimized at the SCF level. While the bond length between sulfur and the oxygens coordinated to the surface increases, the sulfur-uncoordinated oxygen bond length decreases. This weakening and strengthening of the bonds, respectively, is consistent with bond order conservation in adsorbates on metal surfaces. Although a charge transfer of 0.4 electrons towards the metal is observed, the adsorbate remains very much sulfate-like. The molecular orbital analysis indicates that there is also some charge back-donation towards unoccupied orbitals of sulfate. This results in an increased electron density around sulfur as revealed in the electron density difference maps. Analysis of the Laplacian of the charge density of free sulfate provides a suitable framework to understand the nature of the different charge transfer processes and allows us to establish some similarities with the CO- and SO 2-metal bondings.

Adsorption of squaraine molecules to Au(111) and Ag(001) surfaces

NASA Astrophysics Data System (ADS)

Luft, Maike; Groß, Boris; Schulz, Matthias; Lützen, Arne; Schiek, Manuela; Nilius, Niklas

2018-02-01

The adsorption of anilino squaraines, an important chromophore for the use in organic solar cells, to Ag(001) and Au(111) has been studied with scanning tunneling microscopy. Self-assembly into square building blocks with eight molecules per unit cell is revealed on the Ag surface, while no ordering effects occur on gold. The squaraine-silver interaction is mediated by the carbonyl and hydroxyl oxygens located in the center of the molecule. The intermolecular coupling, on the other hand, is governed by hydrogen bonds formed between the terminal isobutyl groups and oxygen species of adjacent molecules. The latter gets maximized by rotating the molecules by a few degrees against a perfect square alignment. A similar molecular pattern does not form on Au(111) due to symmetry mismatch. Moreover, the high electronegativity of gold reduces the directing effect of oxygen-metal bonds that trigger the ordering process on silver. As a consequence, only frustrated three-fold symmetric units that do not expand into an ordered molecular network are present on the gold surface.

Influence of Au and TiO2 structures on hydrogen dissociation over TiO2/Au(100)

NASA Astrophysics Data System (ADS)

Nakamura, I.; Mantoku, H.; Furukawa, T.; Takahashi, A.; Fujitani, T.

2012-11-01

We performed H2-D2 exchange reactions over TiOx/Au(100) and compared the observed reaction kinetics with those reported for TiOx/Au(111) in order to clarify the influence of the Au and TiO2 structures on dissociation of H2 molecules. Low energy electron diffraction observations showed that the TiO2 produced on Au(100) was disordered, in contrast to the comparatively ordered TiO2 structure formed on Au(111). The activation energies and the turnover frequencies for HD formation over TiO2/Au(100) agreed well with those for TiO2/Au(111), clearly indicating that the hydrogen dissociation sites created over TiO2/Au(100) were the perimeter interface between stoichiometric TiO2 and Au, as was previously concluded for TiO2/Au(111). We concluded that the creation of active sites for hydrogen dissociation was independent of the Au and TiO2 structures consisting perimeter interface, and that local bonds that formed between Au and O atoms of stoichiometric TiO2 were essential for the creation of active sites.

Experimental and DFT studies of gold nanoparticles supported on MgO(111) nano-sheets and their catalytic activity.

PubMed

Li, Zhi; Ciobanu, Cristian V; Hu, Juncheng; Palomares-Báez, Juan-Pedro; Rodríguez-López, José-Luis; Richards, Ryan

2011-02-21

A wet chemical preparation of MgO with the (111) facet as the primary surface has recently been reported and with alternating layers of oxygen anions and magnesium cations, this material shows unique chemical and physical properties. The potential to utilize the MgO(111) surface for the immobilization of metal particles is intriguing because the surface itself offers a very different environment for the metal particle with an all oxygen interface, as opposed to the typical (100) facet that possesses alternating oxygen anion and magnesium cation sites on the surface. Gold nanoparticles have demonstrated a broad range of interesting catalytic properties, but are often susceptible to aggregation at high temperatures and are very sensitive to substrate effects. Here, we investigate gold-supported on MgO(111) nanosheets as a catalyst system for the aerobic oxidation of benzyl alcohol. Gold nanoparticles deposited on MgO(111) show an increased level of activity in the solvent-free benzyl alcohol aerobic oxidation as compared to gold nanoparticles deposited on a typical MgO aerogel. TEM studies reveal that the gold nanoparticles have a hemispherical shape while sitting on the main surface of MgO(111) nanosheets, with a large Au-MgO interface. Given that the gold nanoparticles deposited on the two types of MgO have similar size, and that the two types of unmodified MgO show almost the same activities in the blank reaction, we infer that the high activity of Au/MgO(111) is due to the properties of the (111) support and/or those of the gold-support interface. To understand the binding of Au on low-index MgO surfaces and the charge distribution at the surface of the support, we have performed density functional theory (DFT) calculations on all low-index MgO substrates (with and without gold), using a model Au(10) cluster. Due to similar lattice constants of Au(111) and MgO(111) planes, the Au cluster retains its structural integrity and binds strongly on MgO(111) with either

The (111) Surface of NaAu 2. Structure, Composition, and Stability

DOE PAGES

Kwolek, Emma J.; Widmer, Roland; Gröning, Oliver; ...

2014-12-17

The (111) surface of single-crystal NaAu 2 is a model for catalytically active, powdered NaAu 2. We prepare and characterize this surface with a broad suite of techniques. Preparation in ultrahigh vacuum consists of the traditional approach of ion bombardment (to remove impurities) and thermal annealing (to restore surface order). Both of these steps cause loss of sodium (Na), and repeated treatments eventually trigger conversion of the surface and near-surface regions to crystalline gold. The bulk has a limited ability to repopulate the surface Na. Under conditions where Na depletion is minimized, electron diffraction patterns are consistent with the bulk-terminatedmore » structure, and scanning tunneling microscopy reveals mesa-like features with lateral dimensions of a few tens of nanometers. The tops of the mesas do not possess fine structure characteristic of a periodic lattice, suggesting that the surface layer is disordered under the conditions of these experiments.« less

Collision-spike sputtering of Au nanoparticles

DOE PAGES

Sandoval, Luis; Urbassek, Herbert M.

2015-08-06

Ion irradiation of nanoparticles leads to enhanced sputter yields if the nanoparticle size is of the order of the ion penetration depth. While this feature is reasonably well understood for collision-cascade sputtering, we explore it in the regime of collision-spike sputtering using molecular-dynamics simulation. For the particular case of 200-keV Xe bombardment of Au particles, we show that collision spikes lead to abundant sputtering with an average yield of 397 ± 121 atoms compared to only 116 ± 48 atoms for a bulk Au target. Only around 31 % of the impact energy remains in the nanoparticles after impact; themore » remainder is transported away by the transmitted projectile and the ejecta. As a result, the sputter yield of supported nanoparticles is estimated to be around 80 % of that of free nanoparticles due to the suppression of forward sputtering.« less

Mesoscopic self-organization of a self-assembled supramolecular rectangle on highly oriented pyrolytic graphite and Au(111) surfaces.

PubMed

Gong, Jian-Ru; Wan, Li-Jun; Yuan, Qun-Hui; Bai, Chun-Li; Jude, Hershel; Stang, Peter J

2005-01-25

A self-assembled supramolecular metallacyclic rectangle was investigated with scanning tunneling microscopy on highly oriented pyrolytic graphite and Au(111) surfaces. The rectangles spontaneously adsorb on both surfaces and self-organize into well ordered adlayers. On highly oriented pyrolytic graphite, the long edge of the rectangle stands on the surface, forming a 2D molecular network. In contrast, the face of the rectangle lays flat on the Au(111) surface, forming linear chains. The structures and intramolecular features obtained through high-resolution scanning tunneling microscopy imaging are discussed.

Influence of Substrate Bonding and Surface Morphology on Dynamic Organic Layer Growth: Perylenetetracarboxylic Dianhydride on Au(111).

PubMed

Schmidt, Thomas; Marchetto, Helder; Groh, Ullrich; Fink, Rainer H; Freund, Hans-Joachim; Umbach, Eberhard

2018-05-15

We investigated the dynamics of the initial growth of the first epitaxial layers of perylenetetracarboxylic dianhydride (PTCDA) on the Au(111) surface with high lateral resolution using the aberration-corrected spectro-microscope SMART. With this instrument, we could simultaneously study the different adsorption behaviors and layer growth on various surface areas consisting of either a distribution of flat (111) terraces, separated by single atomic steps ("ideal surface"), or on areas with a high density of step bunches and defects ("realistic surface"). The combined use of photoemission electron microscopy, low-energy electron microscopy, and μ-spot X-ray absorption provided a wealth of new information, showing that the growth of the archetype molecule PTCDA not only has similarities but also has significant differences when comparing Au(111) and Ag(111) substrate surfaces. For instance, under otherwise identical preparation conditions, we observed different growth mechanisms on different surface regions, depending on the density of step bunches. In addition, we studied the spatially resolved desorption behavior which also depends on the substrate morphology.

Snapshots of crystal growth: Nanoclusters of organic conductors on Au(111) surfaces

NASA Astrophysics Data System (ADS)

Schott, J. H.; Ward, M. D.

1994-06-01

Mono- and multilayer crystalline nanoclusters of tetra-hiafulvalene-tetracyanoquinodimethane ((TTF) (TCNO)), a low-dimensional organic conductor in the bulk form, can be formed readily on Au(111) surfaces by vapor phase sublimation under ambient conditions. Scanning tunneling microscopy of monolayer (TTF)(TCNQ) films reveals a two-dimensional density of states (DOS) that is consistent with the arrangement of TTF and TCNO molecules in the ac face of bulk (TTF)(TCNO), in which the molecular planes are nearly parallel to the Au(111) substrate. In contrast, clusters with thicknesses corresponding to two or three molecular layers exhibit a transformation to a highly anisotropic DOS that can be attributed to interlayer molecular overlap in segregated TTF and TCNQ molecular chains along the c-axis, which can be described as 'molecular wires'. The orientation of the crystalline (TTF)(TCNO) clusters is preserved throughout the crystal growth sequence, leading to meso- and macroscopic (TTF)(TCNO) needles that are oriented perpendicular to the Au(111) substrate. These studies provide visualization of crystal growth from the initial stages of nucleation to macroscopic crystals, and a revealing example of the changes in electronic structure that occur during the evolution of molecular (TTF)(TCNQ) nuclei into a bulk crystalline phase.

Heteroepitaxial growth of Pt and Au thin films on MgO single crystals by bias-assisted sputtering

DOE PAGES

Tolstova, Yulia; Omelchenko, Stefan T.; Shing, Amanda M.; ...

2016-03-17

The crystallographic orientation of a metal affects its surface energy and structure, and has profound implications for surface chemical reactions and interface engineering, which are important in areas ranging from optoelectronic device fabrication to catalysis. However, it can be very difficult and expensive to manufacture, orient, and cut single crystal metals along different crystallographic orientations, especially in the case of precious metals. One approach is to grow thin metal films epitaxially on dielectric substrates. In this work, we report on growth of Pt and Au films on MgO single crystal substrates of (100) and (110) surface orientation for use asmore » epitaxial templates for thin film photovoltaic devices. We develop bias-assisted sputtering for deposition of oriented Pt and Au films with sub-nanometer roughness. We show that biasing the substrate decreases the substrate temperature necessary to achieve epitaxial orientation, with temperature reduction from 600 to 350 °C for Au, and from 750 to 550 °C for Pt, without use of transition metal seed layers. Additionally, this temperature can be further reduced by reducing the growth rate. Biased deposition with varying substrate bias power and working pressure also enables control of the film morphology and surface roughness.« less

Solvent effects on the adsorption and self-organization of Mn12 on Au(111).

PubMed

Pineider, Francesco; Mannini, Matteo; Sessoli, Roberta; Caneschi, Andrea; Barreca, Davide; Armelao, Lidia; Cornia, Andrea; Tondello, Eugenio; Gatteschi, Dante

2007-11-06

A sulfur-containing single molecule magnet, [Mn12O12(O2CC6H4SCH3)16(H2O)4], was assembled from solution on a Au(111) surface affording both submonolayer and monolayer coverages. The adsorbate morphology and the degree of coverage were inspected by scanning tunneling microscopy (STM), while X-ray photoelectron spectroscopy (XPS) allowed the determination of the chemical nature of the adsorbate on a qualitative and quantitative basis. The properties of the adsorbates were found to be strongly dependent on the solvent used to dissolve the magnetic complex. In particular, systems prepared from tetrahydrofuran solutions gave arrays of isolated and partially ordered clusters on the gold substrate, while samples prepared from dichloromethane exhibited a homogeneous monolayer coverage of the whole Au(111) surface. These findings are relevant to the optimization of magnetic addressing of single molecule magnets on surfaces.

Evolution of magnetism of Cr nanoclusters on a Au(111) surface

NASA Astrophysics Data System (ADS)

Gotsis, Harry; Kioussis, Nicholas; Papaconstantopoulos, Dimitri

2004-03-01

Advances in low-temperature scanning tunneling microscopy under ultrahigh vacuum have provided new opportunities for investigating the magnetic structures of nanoclusters adsorbed on surfaces. Recent STM studies of Cr trimers on the Au(111) surface suggest a switching between two distinct electronic states. We have carried out ab initio electronic structure calculations to investigate the structural, electronic and magnetic properties of isolated Cr atoms, Cr dimers and trimers in different geometry. We will present results for the evolution of magnetic behavior including noncollinear magnetism and provide insight in the connection between magnetism and geometry.

Identifying Few-Molecule Water Clusters with High Precision on Au(111) Surface.

PubMed

Dong, Anning; Yan, Lei; Sun, Lihuan; Yan, Shichao; Shan, Xinyan; Guo, Yang; Meng, Sheng; Lu, Xinghua

2018-06-01

Revealing the nature of a hydrogen-bond network in water structures is one of the imperative objectives of science. With the use of a low-temperature scanning tunneling microscope, water clusters on a Au(111) surface were directly imaged with molecular resolution by a functionalized tip. The internal structures of the water clusters as well as the geometry variations with the increase of size were identified. In contrast to a buckled water hexamer predicted by previous theoretical calculations, our results present deterministic evidence for a flat configuration of water hexamers on Au(111), corroborated by density functional theory calculations with properly implemented van der Waals corrections. The consistency between the experimental observations and improved theoretical calculations not only renders the internal structures of absorbed water clusters unambiguously, but also directly manifests the crucial role of van der Waals interactions in constructing water-solid interfaces.

Electronic sputtering of vitreous SiO2: Experimental and modeling results

NASA Astrophysics Data System (ADS)

Toulemonde, M.; Assmann, W.; Trautmann, C.

2016-07-01

The irradiation of solids with swift heavy ions leads to pronounced surface and bulk effects controlled by the electronic energy loss of the projectiles. In contrast to the formation of ion tracks in bulk materials, the concomitant emission of atoms from the surface is much less investigated. Sputtering experiments with different ions (58Ni, 127I and 197Au) at energies around 1.2 MeV/u were performed on vitreous SiO2 (a-SiO2) in order to quantify the emission rates and compare them with data for crystalline SiO2 quartz. Stoichiometry of the sputtering process was verified by monitoring the thickness decreases of a thin SiO2 film deposited on a Si substrate. Angular distributions of the emitted atoms were measured by collecting sputtered atoms on arc-shaped Cu catcher foils. Subsequent analysis of the number of Si atoms deposited on the catcher foils was quantified by elastic recoil detection analysis providing differential as well as total sputtering yields. Compared to existing data for crystalline SiO2, the total sputtering yields for vitreous SiO2 are by a factor of about five larger. Differences in the sputtering rate and track formation characteristics between amorphous and crystalline SiO2 are discussed within the frame of the inelastic thermal spike model.

Stability of M 3S 3 complexes on fcc M(111) surfaces: M = Au, Ag, Cu, and Ni

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

Liu, Da-Jiang; Lee, Jiyoung; Windus, Theresa L.

Density Functional Theory is utilized to assess the stability of metal (M)-sulfur (S) complexes adsorbed on fcc M(111) surfaces, specifically considering S-decorated planar M trimers, M 3S 3. Scanning Tunneling Microscopy studies have identified structures proposed to be Ni 3S 3 on Ni(111), and Au 3S 3 on Au(111). In addition, Cu 3S 3 on Cu(111) has been suggested to facilitate enhanced Cu surface mass transport. Our analysis considers M 3S 3 complexes for M = Au, Ag, Cu, and Ni, assessing key measures of stability on surfaces, and also comparing behavior with trends in gas-phase stability. These surface andmore » gas-phase analyses are systematically related within the framework of Hess’s law, which allows elucidation of various contributions to the overall energetics. In all cases, the adsorbed complex is stable relative to its separated constituents adsorbed on the terrace. However, only for Ag does one find a negative energy of formation from excess S on terraces and M extracted from kink sites along step edges, implying spontaneous complex formation for this pathway. We interpret various experimental observations in the context of our results for energetics.« less

Stability of M 3S 3 complexes on fcc M(111) surfaces: M = Au, Ag, Cu, and Ni

DOE PAGES

Liu, Da-Jiang; Lee, Jiyoung; Windus, Theresa L.; ...

2018-02-08

Density Functional Theory is utilized to assess the stability of metal (M)-sulfur (S) complexes adsorbed on fcc M(111) surfaces, specifically considering S-decorated planar M trimers, M 3S 3. Scanning Tunneling Microscopy studies have identified structures proposed to be Ni 3S 3 on Ni(111), and Au 3S 3 on Au(111). In addition, Cu 3S 3 on Cu(111) has been suggested to facilitate enhanced Cu surface mass transport. Our analysis considers M 3S 3 complexes for M = Au, Ag, Cu, and Ni, assessing key measures of stability on surfaces, and also comparing behavior with trends in gas-phase stability. These surface andmore » gas-phase analyses are systematically related within the framework of Hess’s law, which allows elucidation of various contributions to the overall energetics. In all cases, the adsorbed complex is stable relative to its separated constituents adsorbed on the terrace. However, only for Ag does one find a negative energy of formation from excess S on terraces and M extracted from kink sites along step edges, implying spontaneous complex formation for this pathway. We interpret various experimental observations in the context of our results for energetics.« less

A first principles study on the electronic origins of silver segregation at the Ag-Au (111) surface

NASA Astrophysics Data System (ADS)

Hoppe, Sandra; Müller, Stefan

2017-12-01

The special electronic structure of gold gives rise to many interesting phenomena, such as its color. The surface segregation of the silver-gold system has been the subject of numerous experimental and theoretical studies, yielding conflicting results ranging from strong Ag surface enrichment to Au surface segregation. Via a combined approach of density functional theory (DFT) and statistical physics, we have analyzed the segregation at the Ag-Au (111) surface with different Ag bulk concentrations. Interestingly, we observe a moderate Au surface segregation, which is due to a charge transfer from the less electronegative Ag to Au. Canonical Monte Carlo simulations suggest that the calculated concentration profile with a Au-enriched surface layer remains stable up to higher temperatures. However, the presence of adsorbed oxygen reverses the segregation behavior and leads to strong Ag enrichment of the surface layer.

Ultrahigh vacuum dc magnetron sputter-deposition of epitaxial Pd(111)/Al2O3(0001) thin films.

PubMed

Aleman, Angel; Li, Chao; Zaid, Hicham; Kindlund, Hanna; Fankhauser, Joshua; Prikhodko, Sergey V; Goorsky, Mark S; Kodambaka, Suneel

2018-05-01

Pd(111) thin films, ∼245 nm thick, are deposited on Al 2 O 3 (0001) substrates at ≈0.5 T m , where T m is the Pd melting point, by ultrahigh vacuum dc magnetron sputtering of Pd target in pure Ar discharges. Auger electron spectra and low-energy electron diffraction patterns acquired in situ from the as-deposited samples reveal that the surfaces are compositionally pure 111-oriented Pd. Double-axis x-ray diffraction (XRD) ω-2θ scans show only the set of Pd 111 peaks from the film. In triple-axis high-resolution XRD, the full width at half maximum intensity Γ ω of the Pd 111 ω-rocking curve is 630 arc sec. XRD 111 pole figure obtained from the sample revealed six peaks 60°-apart at a tilt angles corresponding to Pd 111 reflections. XRD ϕ scans show six 60°-rotated 111 peaks of Pd at the same ϕ angles for 11[Formula: see text]3 of Al 2 O 3 based on which the epitaxial crystallographic relationships between the film and the substrate are determined as [Formula: see text]ǁ[Formula: see text] with two in-plane orientations of [Formula: see text]ǁ[Formula: see text] and [Formula: see text]ǁ[Formula: see text]. Using triple axis symmetric and asymmetric reciprocal space maps, interplanar spacings of out-of-plane (111) and in-plane (11[Formula: see text]) are found to be 0.2242 ± 0.0003 and 0.1591 ± 0.0003 nm, respectively. These values are 0.18% lower than 0.2246 nm for (111) and the same, within the measurement uncertainties, as 0.1588 nm for (11[Formula: see text]) calculated from the bulk Pd lattice parameter, suggesting a small out-of-plane compressive strain and an in-plane tensile strain related to the thermal strain upon cooling the sample from the deposition temperature to room temperature. High-resolution cross-sectional transmission electron microscopy coupled with energy dispersive x-ray spectra obtained from the Pd(111)/Al 2 O 3 (0001) samples indicate that the Pd-Al 2 O 3 interfaces are essentially atomically abrupt and

In situ STM imaging of the structures of pentacene molecules adsorbed on Au(111).

PubMed

Pong, Ifan; Yau, Shuehlin; Huang, Peng-Yi; Chen, Ming-Chou; Hu, Tarng-Shiang; Yang, Yawchia; Lee, Yuh-Lang

2009-09-01

In situ scanning tunneling microscope (STM) was used to examine the spatial structures of pentacene molecules adsorbed onto a Au(111) single-crystal electrode from a benzene dosing solution containing 16-400 microM pentacene. Molecular-resolution STM imaging conducted in 0.1 M HClO(4) revealed highly ordered pentacene structures of ( radical31 x radical31)R8.9 degrees , (3 x 10), ( radical31 x 10), and ( radical7 x 2 radical7)R19.1 degrees adsorbed on the reconstructed Au(111) electrode dosed with different pentacene solutions. These pentacene structures and the reconstructed Au(111) substrate were stable between 0.2 and 0.8 V [vs reversible hydrogen electrode, RHE]. Increasing the potential to E > 0.8 V lifted the reconstructed Au(111) surface and disrupted the ordered pentacene adlattices simultaneously. Ordered pentacene structures could be restored by applying potentials negative enough to reinforce the reconstructed Au(111). At potentials negative of 0.2 V, the adsorption of protons became increasingly important to displace adsorbed pentacene admolecules. Although the reconstructed Au(111) structure was not essential to produce ordered pentacene adlayers, it seemed to help the adsorption of pentacene molecules in a long-range ordered pattern. At room temperature (25 degrees C), approximately 100 pentacene molecules seen in STM images could rotate and align themselves to a neighboring domain in 10 s, suggesting that pentacene admolecules could be mobile on Au(111) under the STM imaging conditions of -150 mV in bias voltage and 1 nA in feedback current.

Intermolecular interactions and substrate effects for an adamantane monolayer on a Au(111) surface

NASA Astrophysics Data System (ADS)

Sakai, Yuki; Nguyen, Giang D.; Capaz, Rodrigo B.; Coh, Sinisa; Pechenezhskiy, Ivan V.; Hong, Xiaoping; Wang, Feng; Crommie, Michael F.; Saito, Susumu; Louie, Steven G.; Cohen, Marvin L.

2013-12-01

We study theoretically and experimentally the infrared (IR) spectrum of an adamantane monolayer on a Au(111) surface. Using a STM-based IR spectroscopy technique (IRSTM) we are able to measure both the nanoscale structure of an adamantane monolayer on Au(111) as well as its infrared spectrum, while DFT-based ab initio calculations allow us to interpret the microscopic vibrational dynamics revealed by our measurements. We find that the IR spectrum of an adamantane monolayer on Au(111) is substantially modified with respect to the gas-phase IR spectrum. The first modification is caused by the adamantane-adamantane interaction due to monolayer packing, and it reduces the IR intensity of the 2912 cm-1 peak (gas phase) by a factor of 3.5. The second modification originates from the adamantane-gold interaction, and it increases the IR intensity of the 2938 cm-1 peak (gas phase) by a factor of 2.6 and reduces its frequency by 276 cm-1. We expect that the techniques described here can be used for an independent estimate of substrate effects and intermolecular interactions in other diamondoid molecules and for other metallic substrates.

Quantum tunneling in real space: Tautomerization of single porphycene molecules on the (111) surface of Cu, Ag, and Au.

PubMed

Kumagai, Takashi; Ladenthin, Janina N; Litman, Yair; Rossi, Mariana; Grill, Leonhard; Gawinkowski, Sylwester; Waluk, Jacek; Persson, Mats

2018-03-14

Tautomerization in single porphycene molecules is investigated on Cu(111), Ag(111), and Au(111) surfaces by a combination of low-temperature scanning tunneling microscopy (STM) experiments and density functional theory (DFT) calculations. It is revealed that the trans configuration is the thermodynamically stable form of porphycene on Cu(111) and Ag(111), whereas the cis configuration occurs as a meta-stable form. The trans → cis or cis → trans conversion on Cu(111) can be induced in an unidirectional fashion by injecting tunneling electrons from the STM tip or heating the surface, respectively. We find that the cis ↔ cis tautomerization on Cu(111) occurs spontaneously via tunneling, verified by the negligible temperature dependence of the tautomerization rate below ∼23 K. Van der Waals corrected DFT calculations are used to characterize the adsorption structures of porphycene and to map the potential energy surface of the tautomerization on Cu(111). The calculated barriers are too high to be thermally overcome at cryogenic temperatures used in the experiment and zero-point energy corrections do not change this picture, leaving tunneling as the most likely mechanism. On Ag(111), the reversible trans ↔ cis conversion occurs spontaneously at 5 K and the cis ↔ cis tautomerization rate is much higher than on Cu(111), indicating a significantly smaller tautomerization barrier on Ag(111) due to the weaker interaction between porphycene and the surface compared to Cu(111). Additionally, the STM experiments and DFT calculations reveal that tautomerization on Cu(111) and Ag(111) occurs with migration of porphycene along the surface; thus, the translational motion couples with the tautomerization coordinate. On the other hand, the trans and cis configurations are not discernible in the STM image and no tautomerization is observed for porphycene on Au(111). The weak interaction of porphycene with Au(111) is closest to the gas-phase limit and therefore the absence

Quantum tunneling in real space: Tautomerization of single porphycene molecules on the (111) surface of Cu, Ag, and Au

NASA Astrophysics Data System (ADS)

Kumagai, Takashi; Ladenthin, Janina N.; Litman, Yair; Rossi, Mariana; Grill, Leonhard; Gawinkowski, Sylwester; Waluk, Jacek; Persson, Mats

2018-03-01

Tautomerization in single porphycene molecules is investigated on Cu(111), Ag(111), and Au(111) surfaces by a combination of low-temperature scanning tunneling microscopy (STM) experiments and density functional theory (DFT) calculations. It is revealed that the trans configuration is the thermodynamically stable form of porphycene on Cu(111) and Ag(111), whereas the cis configuration occurs as a meta-stable form. The trans → cis or cis → trans conversion on Cu(111) can be induced in an unidirectional fashion by injecting tunneling electrons from the STM tip or heating the surface, respectively. We find that the cis ↔ cis tautomerization on Cu(111) occurs spontaneously via tunneling, verified by the negligible temperature dependence of the tautomerization rate below ˜23 K. Van der Waals corrected DFT calculations are used to characterize the adsorption structures of porphycene and to map the potential energy surface of the tautomerization on Cu(111). The calculated barriers are too high to be thermally overcome at cryogenic temperatures used in the experiment and zero-point energy corrections do not change this picture, leaving tunneling as the most likely mechanism. On Ag(111), the reversible trans ↔ cis conversion occurs spontaneously at 5 K and the cis ↔ cis tautomerization rate is much higher than on Cu(111), indicating a significantly smaller tautomerization barrier on Ag(111) due to the weaker interaction between porphycene and the surface compared to Cu(111). Additionally, the STM experiments and DFT calculations reveal that tautomerization on Cu(111) and Ag(111) occurs with migration of porphycene along the surface; thus, the translational motion couples with the tautomerization coordinate. On the other hand, the trans and cis configurations are not discernible in the STM image and no tautomerization is observed for porphycene on Au(111). The weak interaction of porphycene with Au(111) is closest to the gas-phase limit and therefore the absence

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

PubMed

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

2018-05-22

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

Spin switch in iron phthalocyanine on Au(111) surface by hydrogen adsorption

NASA Astrophysics Data System (ADS)

Wang, Yu; Li, Xiaoguang; Zheng, Xiao; Yang, Jinlong

2017-10-01

The manipulation of spin states at the molecular scale is of fundamental importance for the development of molecular spintronic devices. One of the feasible approaches for the modification of a molecular spin state is through the adsorption of certain specific atoms or molecules including H, NO, CO, NH3, and O2. In this paper, we demonstrate that the local spin state of an individual iron phthalocyanine (FePc) molecule adsorbed on an Au(111) surface exhibits controllable switching by hydrogen adsorption, as evidenced by using first-principles calculations based on density functional theory. Our theoretical calculations indicate that different numbers of hydrogen adsorbed at the pyridinic N sites of the FePc molecule largely modify the structural and electronic properties of the FePc/Au(111) composite by forming extra N-H bonds. In particular, the adsorption of one or up to three hydrogen atoms induces a redistribution of charge (spin) density within the FePc molecule, and hence a switching to a low spin state (S = 1/2) from an intermediate spin state (S = 1) is achieved, while the adsorption of four hydrogen atoms distorts the molecular conformation by increasing Fe-N bond lengths in FePc and thus breaks the ligand field exerted on the Fe 3d orbitals via stronger hybridization with the substrate, leading to an opposite switching to a high-spin state (S = 2). These findings obtained from the theoretical simulations could be useful for experimental manipulation or design of single-molecule spintronic devices.

Exploration of new methods for growing Ag films on Au(111) studied by ARPES

NASA Astrophysics Data System (ADS)

Luh, Dah-An; Cheng, Cheng-Maw; Tsai, Chi-Ting; Tsuei, Ku-Ding

2007-03-01

Ag/Au(111) thin films have attracted lots of interests as a model system in the past decades. Ag and Au are lattice-matched, and thin Ag films of very high quality are expected to grow on Au(111). However, the intermixing between Ag and Au at elevated temperatures has been a major concern during the growth of Ag films on the Au(111) surface. In many previous studies, Ag was deposited on the Au(111) surface at near room temperature to avoid the intermixing problem. Investigating the results from these studies, the Ag films on Au(111) grown by this recipe still show clear thickness variation. This thickness variation may result from Ag-Au intermixing or film roughening during the process of room temperature deposition. We are revisiting this classical model system with new growth methods. Our goal is to find growth methods that will stop the intermixing between Ag and Au and reduce the variation in the thickness of Ag films. Preliminary results from our study will be presented in this poster.

Comparing Ullmann Coupling on Noble Metal Surfaces: On-Surface Polymerization of 1,3,6,8-Tetrabromopyrene on Cu(111) and Au(111).

PubMed

Pham, Tuan Anh; Song, Fei; Nguyen, Manh-Thuong; Li, Zheshen; Studener, Florian; Stöhr, Meike

2016-04-18

The on-surface polymerization of 1,3,6,8-tetrabromopyrene (Br4 Py) on Cu(111) and Au(111) surfaces under ultrahigh vacuum conditions was investigated by a combination of scanning tunneling microscopy (STM), X-ray photoelectron spectroscopy (XPS) and density functional theory (DFT) calculations. Deposition of Br4 Py on Cu(111) held at 300 K resulted in a spontaneous debromination reaction, generating the formation of a branched coordination polymer network stabilized by C-Cu-C bonds. After annealing at 473 K, the C-Cu-C bonds were converted to covalent C-C bonds, leading to the formation of a covalently linked molecular network of short oligomers. In contrast, highly ordered self-assembled two-dimensional (2D) patterns stabilized by both Br-Br halogen and Br-H hydrogen bonds were observed upon deposition of Br4 Py on Au(111) held at 300 K. Subsequent annealing of the sample at 473 K led to a dissociation of the C-Br bonds and the formation of disordered metal-coordinated molecular networks. Further annealing at 573 K resulted in the formation of covalently linked disordered networks. Importantly, we found that the chosen substrate not only plays an important role as catalyst for the Ullmann reaction, but also influences the formation of different types of intermolecular bonds and thus, determines the final polymer network morphology. DFT calculations further support our experimental findings obtained by STM and XPS and add complementary information on the reaction pathway of Br4 Py on the different substrates. © 2016 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

Shell-isolated nanoparticle-enhanced Raman spectroscopy study of the adsorption behaviour of DNA bases on Au(111) electrode surfaces.

PubMed

Wen, Bao-Ying; Jin, Xi; Li, Yue; Wang, Ya-Hao; Li, Chao-Yu; Liang, Miao-Miao; Panneerselvam, Rajapandiyan; Xu, Qing-Chi; Wu, De-Yin; Yang, Zhi-Lin; Li, Jian-Feng; Tian, Zhong-Qun

2016-06-21

For the first time, we used the electrochemical shell-isolated nanoparticle-enhanced Raman spectroscopy (EC-SHINERS) technique to in situ characterize the adsorption behaviour of four DNA bases (adenine, guanine, thymine, and cytosine) on atomically flat Au(111) electrode surfaces. The spectroscopic results of the various molecules reveal similar features, such as the adsorption-induced reconstruction of the Au(111) surface and the drastic Raman intensity reduction of the ring breathing modes after the lifting reconstruction. As a preliminary study of the photo-induced charge transfer (PICT) mechanism, the in situ spectroscopic results obtained on single crystal surfaces are excellently illustrated with electrochemical data.

Interplay between Self-Assembled Structures and Energy Level Alignment of Benzenediamine on Au(111) Surfaces

NASA Astrophysics Data System (ADS)

Li, Guo; Neaton, Jeffrey

2015-03-01

Using van der Waals-corrected density functional theory (DFT) calculations, we study the adsorption of benzene-diamine (BDA) molecules on Au(111) surfaces. We find that at low surface coverage, the adsorbed molecules prefer to stay isolated from each other in a monomer phase, due to the inter-molecular dipole-dipole repulsions. However, when the coverage rises above a critical value of 0.9nm-2, the adsorbed molecules aggregate into linear structures via hydrogen bonding between amine groups, consistent with recent experiments [Haxton, Zhou, Tamblyn, et al, Phys. Rev. Lett. 111, 265701 (2013)]. Moreover, we find that these linear structures at high density considerably reduces the Au work function (relative to a monomer phase). Due to reduced surface polarization effects, we estimate that the resonance energy of the highest occupied molecular orbital of the adsorbed BDA molecule relative to the Au Fermi level is significantly lower than the monomer phase by more than 0.5 eV, consistent with the experimental measurements [DellAngela, Kladnik, and Cossaro, et al., Nano Lett. 10, 2470 (2010)]. This work supported by DOE (the JCAP under Award Number DE-SC000499 and the Molecular Foundry of LBNL), and computational resources provided by NERSC.

Detection of organic vapors on sputtered and annealed thin Au films

NASA Astrophysics Data System (ADS)

Kvitek, O.; Kopacek, V.; Reznickova, A.; Svorcik, V.

2018-03-01

Unique optical properties of metal nanostructures enable construction of new types of chemical sensors. Nanostructures composed of Au on glass substrate were prepared by annealing of 2-20 nm thick sputtered Au films at 300 °C for 1 h. The annealing leads to transformation of the as sputtered continuous Au layers to a nanoisland structure. The forming nanostructure shows a strong, well defined surface plasmon resonance absorption band in UV-Vis spectrum, which is useful for construction of a chemical sensor. The samples were used to detect vapors of acetone and water in an experimental testing apparatus. The achieved signal-to-noise ratio was 583 and 386 for acetone and water vapors, respectively on the nanostructure prepared from 4 nm thick Au layer. The nanostructured sensitive layers, however, showed poor signal stability; therefore a polymer overlayer was introduced to protect it. The employed polystyrene film prepared by spin-coating improved sensitivity and selectivity of the sensor, while the dynamic properties of the sensing influenced only slightly.

Nanoscale thin film growth of Au on Si(111)-7 × 7 surface by pulsed laser deposition method

NASA Astrophysics Data System (ADS)

Yokotani, Atsushi; Kameyama, Akihiro; Nakayoshi, Kohei; Matsunaga, Yuta

2017-03-01

To obtain important information for fabricating atomic-scale Au thin films that are used for biosensors, we have observed the morphology of Au particles adsorbed on a Si(111)-7 × 7 surface, which is supposed to be the initial stage of Au atomistic thin film formation. Au particles were adsorbed on the clean Si surface using a PLD method, and the adsorbed particles were observed using a scanning tunneling microscope. As the number of laser shots was increased in the PLD method, the size of the adsorbed particle became larger. The larger particles seemed to form clusters, which are aggregations of particles in which each particle is distinguished, so we call this type of cluster a film-shaped cluster. In this work, we have mainly analyzed this type of cluster. As a result the film-shaped clusters were found to have a structure of nearly monoatomic layers. The particles in the clusters were gathered closely in roughly a 3-fold structure with an inter particle distance of 0.864 nm. We propose a model for the cluster structure by modifying Au(111) face so that each observed particle consists of three Au atoms.

Tunable surface plasmon resonance frequency of Au-Ag bimetallic asymmetric structure thin films in the UV and IR region

NASA Astrophysics Data System (ADS)

Hong, Ruijin; Ji, Jialin; Tao, Chunxian; Zhang, Dawei

2016-10-01

Au/ZnO/Ag sandwich structure films were fabricated by DC magnetron sputter at room temperature. The tunability of the surface plasmon resonance wavelength was realized by varying the thickness of ZnO thin film. The effects of ZnO layer on the optical properties of Au/ZnO/Au thin films were investigated by optical absorption and Raman scattering measurements. It has been found that both the surface plasmon resonance frequency and SERS can be controlled by adjusting the thickness of ZnO layer due to the coupling of metal and semiconductor.

Silica-sandwiched Au nanoparticle arrays by a soft PE-CVD/RF sputtering approach

NASA Astrophysics Data System (ADS)

Barreca, Davide; Gasparotto, Alberto; Maccato, Chiara; Tondello, Eugenio

2008-06-01

This work is focused on the development of an innovative synthetic route to SiO2-sandwiched Au nanoparticle arrays. The adopted strategy consists of: (i) the radio frequency sputtering of gold on thermally oxidized Si(100) and silica substrates from Ar plasmas; (ii) the plasma enhanced chemical vapor deposition of a SiO2 overlayer using tetramethoxysilane as precursor from Ar-O2 plasmas. A common feature of both preparative stages is the use of very soft processing conditions at temperatures close to room temperature, in order to tailor the Au nanoparticle morphology and to preserve it upon SiO2 coverage. In situ monitoring of gold deposition was accomplished by means of laser reflection interferometry. Valuable information on the system morphology before and after SiO2 coverage was provided by field emission-scanning electron microscopy for samples with different Au content. Additional important information on the system chemical composition, structure and optical response was gained by the combined use of x-ray photoelectron spectroscopy, glancing incidence x-ray diffraction and UV-visible absorption spectroscopy. The results obtained highlight the formation of high-purity SiO2/Au/SiO2-sandwiched stacks, in which the gold content and distribution, as well as the nanoparticle morphology, could be tailored by the sole variation of the sputtering time, without any further ex situ treatment.

HF treatment effect for carbon deposition on silicon (111) by DC sputtering technique

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

Aji, A. S., E-mail: aji.ravazes70@gmail.com; Darma, Y., E-mail: aji.ravazes70@gmail.com

Surface modifications of Si (111) substrate by HF solution for thin film carbon deposition have been systematically studied. Thin film carbon on Si (111) has been deposited using DC Unbalanced Magnetron Sputtering with carbon pellet doped by 5% Fe as the target. EDAX characterization confirmed that the carbon fraction on Si substrate much higher by dipping a clean Si substrate by HF solution before sputtering process in comparison with carbon fraction on Si substrate just after conventional RCA. Moreover, SEM and AFM images show the uniform thin film carbon on Si with HF treatment, in contrast to the Si withoutmore » HF solution treatment. These experimental results suggest that HF treatment of Si surface provide Si-H bonds on top Si surface that useful to enhance the carbon deposition during sputtering process. Furthermore, we investigate the thermal stability of thin film carbon on Si by thermal annealing process up to 900 °C. Atomic arrangements during annealing process were characterized by Raman spectroscopy. Raman spectra indicate that thin film carbon on Si is remaining unchanged until 600 °C and carbon atoms start to diffuse toward Si substrate after annealing at 900 °C.« less

Identification of the Structure Model of the Si(111)-(5×2)-Au Surface

NASA Astrophysics Data System (ADS)

Shirasawa, Tetsuroh; Voegeli, Wolfgang; Nojima, Takehiro; Iwasawa, Yusaku; Yamaguchi, Yudai; Takahashi, Toshio

2014-10-01

The atomic structure of the Si(111)-(5×2)-Au surface, a periodic gold chain on the silicon surface, has been a long-debated issue in surface science. The recent three candidates, the so-called Erwin-Barke-Himpsel (EBH) model [S. C. Erwin, I. Barke, and F. J. Himpsel, Phys. Rev. B 80, 155409 (2009)], the Abukawa-Nishigaya (AN) model [T. Abukawa and Y. Nishigaya, Phys. Rev. Lett. 110, 036102 (2013)], and the Kwon-Kang (KK) model [S. G. Kwon and M. H. Kang, Phys. Rev. Lett. 113, 086101 (2014)] that has one additional Au atom than the EBH model are tested by surface x-ray diffraction data. A two-dimensional Patterson map constructed from the in-plane diffraction intensities rejects the AN model and prefers the KK model over the EBH model. On the basis of the arrangement of Au obtained from the Patterson map, all the reconstructed Si atoms, such as the so-called honeycomb chain structure, are directly imaged out by utilizing a holographic method. The KK model reproduces out-of-plane diffraction data as well.

Coverage dependent molecular assembly of anthraquinone on Au(111)

NASA Astrophysics Data System (ADS)

DeLoach, Andrew S.; Conrad, Brad R.; Einstein, T. L.; Dougherty, Daniel B.

2017-11-01

A scanning tunneling microscopy study of anthraquinone (AQ) on the Au(111) surface shows that the molecules self-assemble into several structures depending on the local surface coverage. At high coverages, a close-packed saturated monolayer is observed, while at low coverages, mobile surface molecules coexist with stable chiral hexamer clusters. At intermediate coverages, a disordered 2D porous network interlinking close-packed islands is observed in contrast to the giant honeycomb networks observed for the same molecule on Cu(111). This difference verifies the predicted extreme sensitivity [J. Wyrick et al., Nano Lett. 11, 2944 (2011)] of the pore network to small changes in the surface electronic structure. Quantitative analysis of the 2D pore network reveals that the areas of the vacancy islands are distributed log-normally. Log-normal distributions are typically associated with the product of random variables (multiplicative noise), and we propose that the distribution of pore sizes for AQ on Au(111) originates from random linear rate constants for molecules to either desorb from the surface or detach from the region of a nucleated pore.

Coverage dependent molecular assembly of anthraquinone on Au(111).

PubMed

DeLoach, Andrew S; Conrad, Brad R; Einstein, T L; Dougherty, Daniel B

2017-11-14

A scanning tunneling microscopy study of anthraquinone (AQ) on the Au(111) surface shows that the molecules self-assemble into several structures depending on the local surface coverage. At high coverages, a close-packed saturated monolayer is observed, while at low coverages, mobile surface molecules coexist with stable chiral hexamer clusters. At intermediate coverages, a disordered 2D porous network interlinking close-packed islands is observed in contrast to the giant honeycomb networks observed for the same molecule on Cu(111). This difference verifies the predicted extreme sensitivity [J. Wyrick et al., Nano Lett. 11, 2944 (2011)] of the pore network to small changes in the surface electronic structure. Quantitative analysis of the 2D pore network reveals that the areas of the vacancy islands are distributed log-normally. Log-normal distributions are typically associated with the product of random variables (multiplicative noise), and we propose that the distribution of pore sizes for AQ on Au(111) originates from random linear rate constants for molecules to either desorb from the surface or detach from the region of a nucleated pore.

Tunable optical properties of plasmonic Au/Al2O3 nanocomposite thin films analyzed by spectroscopic ellipsometry accounting surface characteristics.

PubMed

Jaiswal, Jyoti; Mourya, Satyendra; Malik, Gaurav; Chandra, Ramesh

2018-05-01

In the present work, we have fabricated plasmonic gold/alumina nanocomposite (Au/Al 2 O 3 NC) thin films on a glass substrate at room temperature by RF magnetron co-sputtering. The influence of the film thickness (∼10-40  nm) on the optical and other physical properties of the samples was investigated and correlated with the structural and compositional properties. The X-ray diffractometer measurement revealed the formation of Au nanoparticles with average crystallite size (5-9.2 nm) embedded in an amorphous Al 2 O 3 matrix. The energy-dispersive X ray and X-ray photoelectron spectroscopy results confirmed the formation of Au/Al 2 O 3 NC quantitatively and qualitatively and it was observed that atomic% of Au increased by increasing thickness. The optical constants of the plasmonic Au/Al 2 O 3 NC thin films were examined by variable angle spectroscopic ellipsometry in the wide spectral range of 246-1688 nm, accounting the surface characteristics in the optical stack model, and the obtained results are expected to be unique. Additionally, a thickness-dependent blueshift (631-590 nm) of surface plasmon resonance peak was observed in the absorption spectra. These findings of the plasmonic Au/Al 2 O 3 NC films may allow the design and fabrication of small, compact, and efficient devices for optoelectronic and photonic applications.

Stability of surface and subsurface hydrogen on and in Au/Ni near-surface alloys

NASA Astrophysics Data System (ADS)

Celik, Fuat E.; Mavrikakis, Manos

2015-10-01

Periodic, self-consistent DFT-GGA (PW91) calculations were used to study the interaction of hydrogen atoms with the (111) surfaces of substitutional near-surface alloys (NSAs) of Au and Ni with different surface layer compositions and different arrangements of Au atoms in the surface layer. The effect of hydrogen adsorption on the surface and in the first and second subsurface layers of the NSAs was studied. Increasing the Au content in the surface layer weakens hydrogen binding on the surface, but strengthens subsurface binding, suggesting that the distribution of surface and subsurface hydrogen will be different than that on pure Ni(111). While the metal composition of the surface layer has an effect on the binding energy of hydrogen on NSA surfaces, the local composition of the binding site has a stronger effect. For example, fcc hollow sites consisting of three Ni atoms bind H nearly as strongly as on Ni(111), and fcc sites consisting of three Au atoms bind H nearly as weakly as on Au(111). Sites with one or two Au atoms show intermediate binding energies. The preference of hydrogen for three-fold Ni hollow sites alters the relative stabilities of different surface metal atom arrangements, and may provide a driving force for adsorbate-induced surface rearrangement.

Dewetting process of Au films on SiO2 nanowires: Activation energy evaluation

NASA Astrophysics Data System (ADS)

Ruffino, F.; Grimaldi, M. G.

2015-05-01

SiO2 nanowires gain scientific and technological interest in application fields ranging from nano-electronics, optics and photonics to bio-sensing. Furthermore, the SiO2 nanowires chemical and physical properties, and so their performances in devices, can be enhanced if decorated by metal nanoparticles (such Au) due to local plasmonic effects. In the present paper, we propose a simple, low-cost and high-throughput three-steps methodology for the mass-production of Au nanoparticles coated SiO2 nanowires. It is based on (1) production of the SiO2 nanowires on Si surface by solid state reaction of an Au film with the Si substrate at high temperature; (2) sputtering deposition of Au on the SiO2 nanowires to obtain the nanowires coated by an Au film; and (3) furnace annealing processes to induce the Au film dewetting on the SiO2 nanowires surface. Using scanning electron microscopy analyses, we followed the change of the Au nanoparticles mean versus the annealing time extracting values for the characteristic activation energy of the dewetting process of the Au film on the SiO2 nanowires surface. Such a study can allow the tuning of the nanowires/nanoparticles sizes for desired technological applications.

Extraordinary epitaxial alignment of graphene islands on Au(111)

NASA Astrophysics Data System (ADS)

Wofford, Joseph M.; Starodub, Elena; Walter, Andrew L.; Nie, Shu; Bostwick, Aaron; Bartelt, Norman C.; Thürmer, Konrad; Rotenberg, Eli; McCarty, Kevin F.; Dubon, Oscar D.

2012-05-01

Pristine, single-crystalline graphene displays a unique collection of remarkable electronic properties that arise from its two-dimensional, honeycomb structure. Using in situ low-energy electron microscopy, we show that when deposited on the (111) surface of Au carbon forms such a structure. The resulting monolayer, epitaxial film is formed by the coalescence of dendritic graphene islands that nucleate at a high density. Over 95% of these islands can be identically aligned with respect to each other and to the Au substrate. Remarkably, the dominant island orientation is not the better lattice-matched 30° rotated orientation but instead one in which the graphene [01] and Au [011] in-plane directions are parallel. The epitaxial graphene film is only weakly coupled to the Au surface, which maintains its reconstruction under the slightly p-type doped graphene. The linear electronic dispersion characteristic of free-standing graphene is retained regardless of orientation. That a weakly interacting, non-lattice matched substrate is able to lock graphene into a particular orientation is surprising. This ability, however, makes Au(111) a promising substrate for the growth of single crystalline graphene films.

Stability of surface and subsurface hydrogen on and in Au/Ni near-surface alloys

DOE PAGES

Celik, Fuat E.; Mavrikakis, Manos

2015-01-12

Periodic, self-consistent DFT-GGA (PW91) calculations were used to study the interaction of hydrogen atoms with the (111) surfaces of substitutional near-surface alloys (NSAs) of Au and Ni with different surface layer compositions and different arrangements of Au atoms in the surface layer. The effect of hydrogen adsorption on the surface and in the first and second subsurface layers of the NSAs was studied. Increasing the Au content in the surface layer weakens hydrogen binding on the surface, but strengthens subsurface binding, suggesting that the distribution of surface and subsurface hydrogen will be different than that on pure Ni(111). While themore » metal composition of the surface layer has an effect on the binding energy of hydrogen on NSA surfaces, the local composition of the binding site has a stronger effect. For example, fcc hollow sites consisting of three Ni atoms bind H nearly as strongly as on Ni(111), and fcc sites consisting of three Au atoms bind H nearly as weakly as on Au(111). Sites with one or two Au atoms show intermediate binding energies. The preference of hydrogen for three-fold Ni hollow sites alters the relative stabilities of different surface metal atom arrangements, and may provide a driving force for adsorbate-induced surface rearrangement.« less

Stability of Surface and Subsurface Hydrogen on and in Au/Ni Near-Surface Alloys

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

Celik, Fuat E.; Mavrikakis, Manos

2015-10-01

Periodic, self-consistent DFT-GGA (PW91) calculations were used to study the interaction of hydrogen atoms with the (111) surfaces of substitutional near-surface alloys (NSAs) of Au and Ni with different surface layer compositions and different arrangements of Au atoms in the surface layer. The effect of hydrogen adsorption on the surface and in the first and second subsurface layers of the NSAs was studied. Increasing the Au content in the surface layer weakens hydrogen binding on the surface, but strengthens subsurface binding, suggesting that the distribution of surface and subsurface hydrogen will be different than that on pure Ni(111). While themore » metal composition of the surface layer has an effect on the binding energy of hydrogen on NSA surfaces, the local composition of the binding site has a stronger effect. For example, fcc hollow sites consisting of three Ni atoms bind H nearly as strongly as on Ni(111), and fcc sites consisting of three Au atoms bind H nearly as weakly as on Au(111). Sites with one or two Au atoms show intermediate binding energies. The preference of hydrogen for three-fold Ni hollow sites alters the relative stabilities of different surface metal atom arrangements, and may provide a driving force for adsorbate-induced surface rearrangement.« less

Ab Initio calculation on magnetism of monatomic Fe nanowire on Au (111) surface

NASA Astrophysics Data System (ADS)

Yasui, Takashi; Nawate, Masahiko

2010-01-01

The magnetic anisotropy of the one-dimensional monatomic Fe wire on the Au (111) texture has been theoretically analyzed using Wien2k flamework. The model simulates experimentally observed ferromagnetic Fe monatomic wire self-organized along the terrace edge of the Au (788) plane, which exhibits the magnetizaiton perpendicular both the wire and Au plane. In the case of the model consisting the one-dimensional Fe wire placed on the Au (111) plane with the Au lattice cite, no significant anisotropy is resulted by the calculation. On the other hand, the model where the Fe wire is formed along the Au terrace like step indicates the anisotropy of which easy direction is along the wire, resulting in differenct direction from the experiment. When we introduce the disorder in the Fe wire array, the easy direction changes. As for the model that the every other Fe atoms are slightly closer to the Au step (approx 0.0091 nm) the easy direction turns to be perpendicular to the wire and parallel to the Au plane, that is, the dislocation direction. The disorder in the Fe wire seems to play significant roll in the anisotropy.

The Preparation of Au@TiO2 Yolk-Shell Nanostructure and its Applications for Degradation and Detection of Methylene Blue

NASA Astrophysics Data System (ADS)

Wan, Gengping; Peng, Xiange; Zeng, Min; Yu, Lei; Wang, Kan; Li, Xinyue; Wang, Guizhen

2017-09-01

This paper reports the synthesis of a new type of Au@TiO2 yolk-shell nanostructures by integrating ion sputtering method with atomic layer deposition (ALD) technique and its applications as visible light-driven photocatalyst and surface-enhanced Raman spectroscopy (SERS) substrate. Both the size and amount of gold nanoparticles confined in TiO2 nanotubes could be facilely controlled via properly adjusting the sputtering time. The unique structure and morphology of the resulting Au@TiO2 samples were investigated by using various spectroscopic and microscopic techniques in detail. It is found that all tested samples can absorb visible light with a maximum absorption at localized surface plasmon resonance (LSPR) wavelengths (550-590 nm) which are determined by the size of gold nanoparticles. The Au@TiO2 yolk-shell composites were used as the photocatalyst for the degradation of methylene blue (MB). As compared with pure TiO2 nanotubes, Au@TiO2 composites exhibit improved photocatalytic properties towards the degradation of MB. The SERS effect of Au@TiO2 yolk-shell composites was also performed to investigate the detection sensitivity of MB.

The Preparation of Au@TiO2 Yolk-Shell Nanostructure and its Applications for Degradation and Detection of Methylene Blue.

PubMed

Wan, Gengping; Peng, Xiange; Zeng, Min; Yu, Lei; Wang, Kan; Li, Xinyue; Wang, Guizhen

2017-09-18

This paper reports the synthesis of a new type of Au@TiO 2 yolk-shell nanostructures by integrating ion sputtering method with atomic layer deposition (ALD) technique and its applications as visible light-driven photocatalyst and surface-enhanced Raman spectroscopy (SERS) substrate. Both the size and amount of gold nanoparticles confined in TiO 2 nanotubes could be facilely controlled via properly adjusting the sputtering time. The unique structure and morphology of the resulting Au@TiO 2 samples were investigated by using various spectroscopic and microscopic techniques in detail. It is found that all tested samples can absorb visible light with a maximum absorption at localized surface plasmon resonance (LSPR) wavelengths (550-590 nm) which are determined by the size of gold nanoparticles. The Au@TiO 2 yolk-shell composites were used as the photocatalyst for the degradation of methylene blue (MB). As compared with pure TiO 2 nanotubes, Au@TiO 2 composites exhibit improved photocatalytic properties towards the degradation of MB. The SERS effect of Au@TiO 2 yolk-shell composites was also performed to investigate the detection sensitivity of MB.

Template growth of Au, Ni and Ni–Au nanoclusters on hexagonal boron nitride/Rh(111): a combined STM, TPD and AES study

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

Wu, Fanglue; Huang, Dali; Yue, Yuan

In this study, the template growth of Au, Ni, and Ni–Au bimetallic nanoclusters on hexagonal boron nitride/Rh(111), i.e. h-BN/Rh(111), was investigated via scanning tunneling microscopy (STM), temperature programmed-desorption (TPD), and Auger electron spectroscopy (AES). STM study shows that template growth of Au clusters on h-BN/Rh(111) forms mainly well-dispersed monolayer clusters. In contrast, Ni forms large multilayer clusters showing a relatively high diffusivity on h-BN/Rh(111) substrate. Ni–Au bimetallic clusters are effectively formed first by Au deposition followed by Ni deposition, with the Au clusters functioning as nucleation sites for the subsequently deposited Ni. Further structural analysis was carried out via TPDmore » and AES. The resulting TPD and AES data show the surface composition and charge transfer between Au and Ni of the bimetallic clusters. These results suggest that the h-BN/Rh(111) substrate represents a unique candidate for supporting Ni–Au bimetallic clusters in further catalytic reactions.« less

Template growth of Au, Ni and Ni–Au nanoclusters on hexagonal boron nitride/Rh(111): a combined STM, TPD and AES study

DOE PAGES

Wu, Fanglue; Huang, Dali; Yue, Yuan; ...

2017-09-12

In this study, the template growth of Au, Ni, and Ni–Au bimetallic nanoclusters on hexagonal boron nitride/Rh(111), i.e. h-BN/Rh(111), was investigated via scanning tunneling microscopy (STM), temperature programmed-desorption (TPD), and Auger electron spectroscopy (AES). STM study shows that template growth of Au clusters on h-BN/Rh(111) forms mainly well-dispersed monolayer clusters. In contrast, Ni forms large multilayer clusters showing a relatively high diffusivity on h-BN/Rh(111) substrate. Ni–Au bimetallic clusters are effectively formed first by Au deposition followed by Ni deposition, with the Au clusters functioning as nucleation sites for the subsequently deposited Ni. Further structural analysis was carried out via TPDmore » and AES. The resulting TPD and AES data show the surface composition and charge transfer between Au and Ni of the bimetallic clusters. These results suggest that the h-BN/Rh(111) substrate represents a unique candidate for supporting Ni–Au bimetallic clusters in further catalytic reactions.« less

Epitaxial growth of thermally stable cobalt films on Au(111)

NASA Astrophysics Data System (ADS)

Haag, N.; Laux, M.; Stöckl, J.; Kollamana, J.; Seidel, J.; Großmann, N.; Fetzer, R.; Kelly, L. L.; Wei, Z.; Stadtmüller, B.; Cinchetti, M.; Aeschlimann, M.

2016-10-01

Ferromagnetic thin films play a fundamental role in spintronic applications as a source for spin polarized carriers and in fundamental studies as ferromagnetic substrates. However, it is challenging to produce such metallic films with high structural quality and chemical purity on single crystalline substrates since the diffusion barrier across the metal-metal interface is usually smaller than the thermal activation energy necessary for smooth surface morphologies. Here, we introduce epitaxial thin Co films grown on an Au(111) single crystal surface as a thermally stable ferromagnetic thin film. Our structural investigations reveal an identical growth of thin Co/Au(111) films compared to Co bulk single crystals with large monoatomic Co terraces with an average width of 500 Å, formed after thermal annealing at 575 K. Combining our results from photoemission and Auger electron spectroscopy, we provide evidence that no significant diffusion of Au into the near surface region of the Co film takes place for this temperature and that no Au capping layer is formed on top of Co films. Furthermore, we show that the electronic valence band is dominated by a strong spectral contribution from a Co 3d band and a Co derived surface resonance in the minority band. Both states lead to an overall negative spin polarization at the Fermi energy.

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

PubMed

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

2018-04-19

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

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

NASA Astrophysics Data System (ADS)

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

2016-12-01

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

Annealing temperature effect on self-assembled Au droplets on Si (111).

PubMed

Sui, Mao; Li, Ming-Yu; Kim, Eun-Soo; Lee, Jihoon

2013-12-13

We investigate the effect of annealing temperature on self-assembled Au droplets on Si (111). The annealing temperature is systematically varied while fixing other growth parameters such as deposition amount and annealing duration clearly to observe the annealing temperature effect. Self-assembled Au droplets are fabricated by annealing from 50°C to 850°C with 2-nm Au deposition for 30 s. With increased annealing temperatures, Au droplets show gradually increased height and diameter while the density of droplets progressively decreases. Self-assembled Au droplets with fine uniformity can be fabricated between 550°C and 800°C. While Au droplets become much larger with increased deposition amount, the extended annealing duration only mildly affects droplet size and density. The results are systematically analyzed with cross-sectional line profiles, Fourier filter transform power spectra, height histogram, surface area ratio, and size and density plots. This study can provide an aid point for the fabrication of nanowires on Si (111).

Confinement of surface state electrons in self-organized Co islands on Au(111)

NASA Astrophysics Data System (ADS)

Schouteden, Koen; Lijnen, Erwin; Janssens, Ewald; Ceulemans, Arnout; Chibotaru, Liviu F.; Lievens, Peter; Van Haesendonck, Chris

2008-04-01

We report on detailed low temperature scanning tunneling spectroscopy measurements performed on nanoscale Co islands on Au(111) films. At low coverages, Co islands self-organize in arrays of mono- and bilayer nanoscale structures that often have an hexagonal shape. The process of self-organization is induced by the Au(111) 'herringbone' reconstruction. By means of mapping of the local density of states with lock-in detection, electron standing wave patterns are resolved on top of the atomically flat Co islands. The surface state electrons are observed to be strongly confined laterally inside the Co nanosized islands, with their wavefunctions reflecting the symmetry of the islands. To complement the experimental work, particle-in-a-box calculations were performed. The calculations are based on a newly developed variational method that can be applied to '2D boxes' of arbitrary polygonal shape. The experimental patterns are found to fit nicely to the calculated wavefunctions for a box having a symmetry corresponding to the experimental island symmetry. The small size of the Co islands under study (down to 7.7 nm2) is observed to induce a strong discretization of the energy levels, with very large energy separations between the eigenstates up to several 100 meV. The observed standing wave patterns are identified either as individual eigenstates or as a 'mixture' of two or more energetically close-lying eigenstates of the cobalt island. Additionally, the Co surface state appears not to be limited to mono- and bilayer islands, but this state remains observable for multilayered islands up to five monolayers of Co.

Correction: Shell-isolated nanoparticle-enhanced Raman spectroscopy study of the adsorption behaviour of DNA bases on Au(111) electrode surfaces.

PubMed

Wen, Bao-Ying; Jin, Xi; Li, Yue; Wang, Ya-Hao; Li, Chao-Yu; Liang, Miao-Miao; Panneerselvam, Rajapandiyan; Xu, Qing-Chi; Wu, De-Yin; Yang, Zhi-Lin; Li, Jian-Feng; Tian, Zhong-Qun

2016-06-21

Correction for 'Shell-isolated nanoparticle-enhanced Raman spectroscopy study of the adsorption behaviour of DNA bases on Au(111) electrode surfaces' by Bao-Ying Wen et al., Analyst, 2016, DOI: 10.1039/c6an00180g.

Metallic Properties of the Si(111) - 5 × 2 - Au Surface from Infrared Plasmon Polaritons and Ab Initio Theory.

PubMed

Hötzel, Fabian; Seino, Kaori; Huck, Christian; Skibbe, Olaf; Bechstedt, Friedhelm; Pucci, Annemarie

2015-06-10

The metal-atom chains on the Si(111) - 5 × 2 - Au surface represent an exceedingly interesting system for the understanding of one-dimensional electrical interconnects. While other metal-atom chain structures on silicon suffer from metal-to-insulator transitions, Si(111) - 5 × 2 - Au stays metallic at least down to 20 K as we have proven by the anisotropic absorption from localized plasmon polaritons in the infrared. A quantitative analysis of the infrared plasmonic signal done here for the first time yields valuable band structure information in agreement with the theoretically derived data. The experimental and theoretical results are consistently explained in the framework of the atomic geometry, electronic structure, and IR spectra of the recent Kwon-Kang model.

Understanding the interfacial behavior of lysozyme on Au (111) surfaces with multiscale simulations

NASA Astrophysics Data System (ADS)

Samieegohar, Mohammadreza; Ma, Heng; Sha, Feng; Jahan Sajib, Md Symon; Guerrero-García, G. Iván; Wei, Tao

2017-02-01

The understanding of the adsorption and interfacial behavior of proteins is crucial to the development of novel biosensors and biomaterials. By using bottom-up atomistic multiscale simulations, we study here the adsorption of lysozyme on Au(111) surfaces in an aqueous environment. Atomistic simulations are used to calculate the inhomogeneous polarization of the gold surface, which is induced by the protein adsorption, and by the presence of an interfacial layer of water molecules and monovalent salts. The corresponding potential of mean force between the protein and the gold surface including polarization effects is used in Langevin Dynamics simulations to study the time dependent behavior of proteins at finite concentration. These simulations display a rapid adsorption and formation of a first-layer of proteins at the interface. Proteins are initially adsorbed directly on the gold surface due to the strong protein-surface attractive interaction. A subsequent interfacial weak aggregation of proteins leading to multilayer build-up is also observed at long times.

Monoatomic and dimer Mn adsorption on the Au(111) surface from first principles

NASA Astrophysics Data System (ADS)

Muñoz, Francisco; Romero, Aldo H.; Mejía-López, Jose; Morán-López, J. L.

2011-05-01

A theoretical study based on the density functional theory of the adsorption of Mn monomers and dimers on a Au-(111) surface is presented. As necessary preliminary steps, the bulk and clean surface electronic structure are calculated, which agree well with previous reports. Then, the electronic structure of the Mn adatom, chemisorbed on four different surface geometries, is analyzed. It is found that the most stable geometry is when the Mn atom is chemisorbed on threefold coordinated sites. Using this geometry for a single adatom a second Mn atom is chemisorbed and the most stable dimer geometrical structure is calculated. The lowest-energy configuration corresponds to the molecule lying parallel to the surface, adsorbed on two topological equivalent threefold coordinated sites. It is also found that the lowest-energy magnetic configuration corresponds to the antiferromagnetic arrangement with individual magnetic moments of 4.64μB. Finally, it is concluded that the dimer is not stable and should fragment at the surface.

Superstructures at Te/Au(111) interface evolving upon increasing Te coverage

NASA Astrophysics Data System (ADS)

Guan, Jiaqi; Huang, Xiaochun; Xu, Xiaofeng; Zhang, Shuyuan; Jia, Xun; Zhu, Xuetao; Wang, Weihua; Guo, Jiandong

2018-03-01

By in-situ low temperature scanning tunneling microscopy, we systematically investigated the superstructure evolution at Te/Au(111) interface upon increasing Te coverage. Te atoms form one-dimensional √{ 3} R30∘ chains at ∼0.10 monolayer (ML) coverage. Two two-dimensional chiral superstructures, (√{ 111} ×√{ 111}) R 4 .7∘ and (3√{ 21} × 3√{ 21}) R 10 .9∘ , are selectively formed with the Te coverage below and above 1/3 ML, respectively. The two chiral superstructures can be converted to each other reversibly by adding Te atoms or moderately annealing. A honeycomb-like superstructure, decorated with adatoms that are distributed in quasi-one-dimensional chains, is observed by further increasing the Te coverage to 4/9 ML. At the Te/Au(111) interface, an interfacial state at -0.65 eV to -0.55 eV below the Fermi level is also resolved by scanning tunneling spectroscopy. The formation of these Te-induced high-order superstructures is accompanied by relaxation of gold atoms in the surface layer, indicating a strong Te-Au interaction. Our work demonstrates a reliable method to fabricate Te nanostructures on noble metals in a controlled way.

Spin-flip transitions and departure from the Rashba model in the Au(111) surface

NASA Astrophysics Data System (ADS)

Ibañez-Azpiroz, Julen; Bergara, Aitor; Sherman, E. Ya.; Eiguren, Asier

2013-09-01

We present a detailed analysis of the spin-flip excitations induced by a periodic time-dependent electric field in the Rashba prototype Au(111) noble metal surface. Our calculations incorporate the full spinor structure of the spin-split surface states and employ a Wannier-based scheme for the spin-flip matrix elements. We find that the spin-flip excitations associated with the surface states exhibit an strong dependence on the electron momentum magnitude, a feature that is absent in the standard Rashba model [E. I. Rashba, Sov. Phys. Solid State 2, 1109 (1960)]. Furthermore, we demonstrate that the maximum of the calculated spin-flip absorption rate is about twice the model prediction. These results show that, although the Rashba model accurately describes the spectrum and spin polarization, it does not fully account for the dynamical properties of the surface states.

The role of charge transfer in the oxidation state change of Ce atoms in the TM13-CeO2(111) systems (TM = Pd, Ag, Pt, Au): a DFT + U investigation.

PubMed

Tereshchuk, Polina; Freire, Rafael L H; Ungureanu, Crina G; Seminovski, Yohanna; Kiejna, Adam; Da Silva, Juarez L F

2015-05-28

Despite extensive studies of transition metal (TM) clusters supported on ceria (CeO2), fundamental issues such as the role of the TM atoms in the change in the oxidation state of Ce atoms are still not well understood. In this work, we report a theoretical investigation based on static and ab initio molecular dynamics density functional theory calculations of the interaction of 13-atom TM clusters (TM = Pd, Ag, Pt, Au) with the unreduced CeO2(111) surface represented by a large surface unit cell and employing Hubbard corrections for the strong on-site Coulomb correlation in the Ce f-electrons. We found that the TM13 clusters form pyramidal-like structures on CeO2(111) in the lowest energy configurations with the following stacking sequence, TM/TM4/TM8/CeO2(111), while TM13 adopts two-dimensional structures at high energy structures. TM13 induces a change in the oxidation state of few Ce atoms (3 of 16) located in the topmost Ce layer from Ce(IV) (itinerant Ce f-states) to Ce(III) (localized Ce f-states). There is a charge flow from the TM atoms to the CeO2(111) surface, which can be explained by the electronegativity difference between the TM (Pd, Ag, Pt, Au) and O atoms, however, the charge is not uniformly distributed on the topmost O layer due to the pressure induced by the TM13 clusters on the underlying O ions, which yields a decrease in the ionic charge of the O ions located below the cluster and an increase in the remaining O ions. Due to the charge flow mainly from the TM8-layer to the topmost O-layer, the charge cannot flow from the Ce(IV) atoms to the O atoms with the same magnitude as in the clean CeO2(111) surface. Consequently, the effective cationic charge decreases mainly for the Ce atoms that have a bond with the O atoms not located below the cluster, and hence, those Ce atoms change their oxidation state from IV to III. This increases the size of the Ce(III) compared with the Ce(IV) cations, which builds-in a strain within the topmost Ce layer, and

Threshold voltage tuning in AlGaN/GaN HFETs with p-type Cu2O gate synthesized by magnetron reactive sputtering

NASA Astrophysics Data System (ADS)

Wang, Lei; Li, Liuan; Xie, Tian; Wang, Xinzhi; Liu, Xinke; Ao, Jin-Ping

2018-04-01

In present study, copper oxide films were prepared at different sputtering powers (10-100 W) using magnetron reactive sputtering. The crystalline structure, surface morphologies, composition, and optical band gap of the as-grown films are dependent on sputtering power. As the sputtering power decreasing from 100 to 10 W, the composition of films changed from CuO to quasi Cu2O domination. Moreover, when the sputtering power is 10 W, a relative high hole carrier density and high-surface-quality quasi Cu2O thin film can be achieved. AlGaN/GaN HFETs were fabricated with the optimized p-type quasi Cu2O film as gate electrode, the threshold voltage of the device shows a 0.55 V positive shift, meanwhile, a lower gate leakage current, a higher ON/OFF drain current ratio of ∼108, a higher electron mobility (1465 cm2/Vs), and a lower subthreshold slope of 74 mV/dec are also achieved, compared with the typical Ni/Au-gated HFETs. Therefore, Cu2O have a great potential to develop high performance p-type gate AlGaN/GaN HFETs.

Effect of dispersion on surface interactions of cobalt(II) octaethylporphyrin monolayer on Au(111) and HOPG(0001) substrates: a comparative first principles study.

PubMed

Chilukuri, Bhaskar; Mazur, Ursula; Hipps, K 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.

Aging behavior of near atmospheric N2 ambient sputtered/patterned Au IR absorber thin films

NASA Astrophysics Data System (ADS)

Gaur, Surender P.; Kothari, Prateek; Rangra, Kamaljit; Kumar, Dinesh

2018-03-01

Near atmospheric N2 ambient sputtered Au thin films exhibit significant spectral absorptivity over medium to long wave infrared radiations. Thin films were found adequately robust for micropatterning using conventional photolithography and metal lift off processes. Since long term spectral absorptivity is major practical concern for Au blacks, this paper reports on aging behavior of near atmospheric Ar and Ar + N2 (1:1) ambient sputtered infrared absorber Au thin films. Comparative analysis on electrical, morphological and spectral absorption behavior of twenty-five weeks room temperature/vacuum aged Au infrared absorber thin films is performed. The Ar and Ar + N2 ambient sputtered Au thing films have shown anticipated consistency in their physical, electrical and spectral properties regardless the long term aging in this work.

Chemical modifications of Au/SiO2 template substrates for patterned biofunctional surfaces.

PubMed

Briand, Elisabeth; Humblot, Vincent; Landoulsi, Jessem; Petronis, Sarunas; Pradier, Claire-Marie; Kasemo, Bengt; Svedhem, Sofia

2011-01-18

The aim of this work was to create patterned surfaces for localized and specific biochemical recognition. For this purpose, we have developed a protocol for orthogonal and material-selective surface modifications of microfabricated patterned surfaces composed of SiO(2) areas (100 μm diameter) surrounded by Au. The SiO(2) spots were chemically modified by a sequence of reactions (silanization using an amine-terminated silane (APTES), followed by amine coupling of a biotin analogue and biospecific recognition) to achieve efficient immobilization of streptavidin in a functional form. The surrounding Au was rendered inert to protein adsorption by modification by HS(CH(2))(10)CONH(CH(2))(2)(OCH(2)CH(2))(7)OH (thiol-OEG). The surface modification protocol was developed by testing separately homogeneous SiO(2) and Au surfaces, to obtain the two following results: (i) SiO(2) surfaces which allowed the grafting of streptavidin, and subsequent immobilization of biotinylated antibodies, and (ii) Au surfaces showing almost no affinity for the same streptavidin and antibody solutions. The surface interactions were monitored by quartz crystal microbalance with dissipation monitoring (QCM-D), and chemical analyses were performed by polarization modulation-reflexion absorption infrared spectroscopy (PM-RAIRS) and X-ray photoelectron spectroscopy (XPS) to assess the validity of the initial orthogonal assembly of APTES and thiol-OEG. Eventually, microscopy imaging of the modified Au/SiO(2) patterned substrates validated the specific binding of streptavidin on the SiO(2)/APTES areas, as well as the subsequent binding of biotinylated anti-rIgG and further detection of fluorescent rIgG on the functionalized SiO(2) areas. These results demonstrate a successful protocol for the preparation of patterned biofunctional surfaces, based on microfabricated Au/SiO(2) templates and supported by careful surface analysis. The strong immobilization of the biomolecules resulting from the described

Effect of nickel diffusion and oxygen behavior on heterojunction Schottky diodes of Au/NiO/ZnO with a NiO interlayer prepared by radio-frequency magnetron sputtering

NASA Astrophysics Data System (ADS)

Hwang, Jun-Dar; Chen, Hsin-Yu; Chen, Yu-Huang; Ho, Ting-Hsiu

2018-07-01

The rectifying characteristic of Au/ZnO Schottky diodes (SDs) was remarkably improved by introducing a NiO layer in-between the Au and ZnO layers. Compared with the Au/ZnO SDs, the introduction of the NiO layer significantly enhanced the rectification ratio from 1.38 to 1300, and reduced the ideality factor from 5.78 to 2.14. The NiO and ZnO layers were deposited on an indium-tin-oxide/glass substrate by radio-frequency magnetron sputtering. Secondary ion mass spectroscopy showed that Ni atoms diffused from NiO to ZnO, leading to a graded distribution of Ni in ZnO. X-ray diffraction demonstrated that the diffusion of Ni atoms increased the grain size and electron concentration of ZnO. X-ray photoelectron spectroscopy showed that the interstitial oxygen (Oi) atoms in NiO and ZnO compensated the oxygen vacancies (OV) at the NiO/ZnO interface; the amount of OV was significantly reduced, while Oi vanished at the interface. The band diagram revealed a potential drop in the bulk ZnO, owing to the graded distribution of Ni in ZnO, which accelerated the carriers, collected by the outer circuit. The carriers at the NiO/ZnO interface easily crossed over the barrier height, instead of being recombined by OV, owing to the lower amount of OV at the interface.

Effect of nickel diffusion and oxygen behavior on heterojunction Schottky diodes of Au/NiO/ZnO with a NiO interlayer prepared by radio-frequency magnetron sputtering.

PubMed

Hwang, Jun-Dar; Chen, Hsin-Yu; Chen, Yu-Hung; Ho, Ting-Hsiu

2018-05-03

The rectifying characteristic of Au/ZnO Schottky diodes (SDs) was remarkably improved by introducing a NiO layer in-between the Au and ZnO layers. Compared with the Au/ZnO SDs, the introduction of the NiO layer significantly enhanced the rectification ratio from 1.38 to 1,300, and reduced the ideality factor from 5.78 to 2.14. The NiO and ZnO layers were deposited on an indium-tin-oxide/glass substrate by radio-frequency magnetron sputtering. Secondary ion mass spectroscopy showed that Ni atoms diffused from NiO to ZnO, leading to a graded distribution of Ni in ZnO. X-ray diffraction demonstrated that the diffusion of Ni atoms increased the grain size and electron concentration of ZnO. X-ray photoelectron spectroscopy showed that the interstitial oxygen (Oi) atoms in NiO and ZnO compensated the oxygen vacancies (OV) at the NiO/ZnO interface; the amount of OV was significantly reduced, while Oi vanished at the interface. The band diagram revealed a potential drop in the bulk ZnO, owing to the graded distribution of Ni in ZnO, which accelerated the carriers, collected by the outer circuit. The carriers at the NiO/ZnO interface easily crossed over the barrier height, instead of being recombined by OV, owing to the lower amount of OV at the interface. © 2018 IOP Publishing Ltd.

Epitaxial growth of (111)-oriented BaTiO3/SrTiO3 perovskite superlattices on Pt(111)/Ti/Al2O3(0001) substrates

NASA Astrophysics Data System (ADS)

Panomsuwan, Gasidit; Takai, Osamu; Saito, Nagahiro

2013-09-01

Symmetric BaTiO3/SrTiO3 (BTO/STO) superlattices (SLs) were epitaxially grown on Pt(111)/Ti/Al2O3(0001) substrates with various modulation periods (Λ = 4.8 - 48 nm) using double ion beam sputter deposition. The BTO/STO SLs exhibit high (111) orientation with two in-plane orientation variants related by a 180° rotation along the [111]Pt axis. The BTO layer is under an in-plane compressive state, whereas the STO layer is under an in-plane tensile state due to the effect of lattice mismatch. A remarkable enhancement of dielectric constant is observed for the SL with relatively small modulation period, which is attributed to both the interlayer biaxial strain effect and the Maxwell-Wagner effect.

Nucleation and growth of order in Cu(3)Au (111) films

NASA Astrophysics Data System (ADS)

Bonham, Scott William

The present work epitaxial investigated two types of ordering phenomena using films of Cusb3Au, the order-disorder phase transition on the (111) crystal surface, and preferential selection of one of two possible stacking domains. Cusb3Au has long been a model system for studying order-disorder phase transition. Bulk material exhibits a discontinuous transition while the surfaces exhibit continuos transitions and the long-range order parameter S is proportional to (Tsb{c}-T)sp{beta}, where Tsb{c} is the critical temperature. The transition of the (111) surface is studied with qualitative reflection high-energy electron diffraction (RHEED), which is sensitive to only the first few atomic layers. This work significantly improves on an earlier study through both improved data collection and more comprehensive data analysis. The measured value of beta =0.50± 0.02 agrees with both the earlier measurements and with predictions of mean field theory. In addition, data on surface defects during the transition and on the kinetics of ordering are presented. During epitaxial growth of (111) face-centered cubic crystal films, such as disordered Cusb3Au, there are two possible ways that successive layers can be laid down, leading to two types of stacking domains. However, a small vicinal miscut (0.5sp° {-}1sp° ) of the crystal surface introduces step edges that change nucleation preferences of the domains, resulting in one being preferred over the other by ratios up to 700:1. Fifteen samples were measured and this preference has been found to depend systematically and strongly on the magnitude and direction of the sample miscut. A qualitative RHEED study confirms that a preference for one of the stacking senses is present after deposition of a few monlolayers of Cusb3Au. The observed behavior of the film can be explained by a model in which Cu and Au atoms minimize their number of Nb nearest neighbors when growing over the Nb step edges. This represents both a discovery of a

Ohmic contacts to p-GaN Using Au/Ni-Mg-O Metallization

NASA Astrophysics Data System (ADS)

Liday, Jozef; Vogrinčič, Peter; Hotový, Ivan; Bonanni, Alberta; Sitter, Helmut; Lalinský, Tibor; Vanko, Gabriel; Řeháček, Vlastimil; Breza, Juraj; Ecke, Gernot

2010-11-01

Electrical characteristics and elemental depth profiles of ohmic contacts to p-GaN using Au/Ni-Mg-Ox metallization have been investigated. The objective was to examine the possibilities of increasing the charge carrier concentration in the surface region of GaN by adding Mg, thus of a p-type dopant into the Au/NiOx metallization structure. For this purpose, a Ni-Mg-Ox layer with a low concentration of Mg was deposited on p-GaN by dc reactive magnetron sputtering. The top Au layer was deposited in a similar way. The fabricated contact structures were annealed in N2. When the Ni-Mg layer in the Au/Ni-Mg-Ox/p-GaN structure was deposited in an atmosphere with a low concentration of oxygen (0.2 at%), the structure exhibited a low resistance ohmic nature. The contact resistance was lower than in the case of a Au/Ni-Ox/p-GaN structure without the Mg dopant in the metallic layer. An increase in the concentration of oxygen in the working atmosphere resulted in higher values of the contact resistance of the Au/Ni-Mg-Ox/p-GaN structure. In our opinion the ohmic nature of the contact structure is related to the existence of a metal/p-NiO/p-GaN scheme. The measured values of the contact resistance in the Au/Ni-Mg-Ox/p-GaN structure in comparison with the Au/Ni-Ox/p-GaN structure are caused by an increased charge carrier concentration in the surface region of p-GaN, which is a consequence of Mg diffusion from the Ni-Mg-Ox layer.

Investigation of trimethylacetic acid adsorption on stoichiometric and oxygen-deficient CeO 2 (111) surfaces

DOE PAGES

Sanghavi, Shail; Wang, Weina; Nandasiri, Manjula I.; ...

2016-05-12

We studied the interactions between the carboxylate anchoring group from trimethylacetic acid (TMAA) and CeO 2(111) surfaces as a function of oxygen stoichiometry using in situ X-ray photoelectron spectroscopy (XPS). The stoichiometric CeO 2(111) surface was obtained by annealing the thin film under 2.0 × 10 –5 Torr of oxygen at ~550 °C for 30 min. In order to reduce the CeO 2(111) surface, the thin film was annealed under ~5.0 × 10 –10 Torr vacuum conditions at 550 °C, 650 °C, 750 °C and 850 °C for 30 min to progressively increase the oxygen defect concentration on the surface.more » The saturated TMAA coverage on the CeO 2(111) surface determined from XPS elemental composition is found to increase with increasing oxygen defect concentration. This is attributed to the increase of under-coordinated cerium sites on the surface with the increase in the oxygen defect concentrations. Furthermore, XPS results were in agreement with periodic density functional theory (DFT) calculations and indicate a stronger binding between the carboxylate group from TMAA and the oxygen deficient CeO 2–δ(111) surface through dissociative adsorption.« less

In and Si adatoms on Si(111)5×2-Au : Scanning tunneling microscopy and first-principles density functional calculations

NASA Astrophysics Data System (ADS)

Stępniak, A.; Nita, P.; Krawiec, M.; Jałochowski, M.

2009-09-01

Structural properties of monatomic indium chains on Si(111)5×2-Au surface are investigated by scanning tunneling microscopy (STM) and first-principles density functional calculations (DFT). The STM topography data show that submonolayer coverage of indium leads to a well-ordered chain structure with the same periodicity as the Si adatoms form on Si(111)5×2-Au surface. Bias-dependent STM topography and spectroscopy reveal two different mechanisms of In-atoms adsorption on the surface: bonding to Si adatoms and substitution for Si atoms in the adatom positions. Those mechanisms are further corroborated by DFT calculations. The obtained structural model of In-modified Si(111)5×2-Au surface remains in good agreement with the experimental data.

Interaction of amino acids with the Au(111) surface: adsorption free energies from molecular dynamics simulations.

PubMed

Hoefling, Martin; Iori, Francesco; Corni, Stefano; Gottschalk, Kay-Eberhard

2010-06-01

Interactions of proteins with inorganic surfaces are of high importance in biological events and in modern biotechnological applications. Therefore, peptides have been engineered to recognize inorganic surfaces with high specificity. However, the underlying interactions are still not well understood. Here, we investigated the adsorption of amino acids as protein building blocks onto a Au(111) surface. In particular, using molecular dynamics simulations, we calculated the potential of mean force between all the 20 amino acids and the gold surface. We found a strong dependence of the binding affinities on the chemical character of the amino acids. Additionally, the interaction free energy is correlated with the propensity of amino acids to form beta-sheets, hinting at design principles for gold binding peptides and induction of beta-sheet formation near surfaces.

A periodic mixed gaussians-plane waves DFT study on simple thiols on Au(111): adsorbate species, surface reconstruction, and thiols functionalization.

PubMed

Rajaraman, Gopalan; Caneschi, Andrea; Gatteschi, Dante; Totti, Federico

2011-03-07

Here we present DFT calculations based on a periodic mixed gaussians/plane waves approach to study the energetics, structure, bonding of SAMs of simple thiols on Au(111). Several open issues such as structure, bonding and the nature of adsorbate are taken into account. We started with methyl thiols (MeSH) on Au(111) to establish the nature of the adsorbate. We have considered several structural models embracing the reconstructed surface scenario along with the MeS˙-Au(ad)-MeS˙ type motif put forward in recent years. Our calculations suggest a clear preference for the homolytic cleavage of the S-H bond leading to a stable MeS˙ on a gold surface. In agreement with the recent literature studies, the reconstructed models of the MeS˙ species are found to be energetically preferred over unreconstructed models. Besides, our calculations reveal that the model with 1:2 Au(ad)/thiols ratio, i.e. MeS˙-Au(ad)-MeS˙, is energetically preferred compared to the clean and 1:1 ratio models, in agreement with the experimental and theoretical evidences. We have also performed Molecular Orbital/Natural Bond Orbital, MO/NBO, analysis to understand the electronic structure and bonding in different structural motifs and many useful insights have been gained. Finally, the studies have then been extended to alkyl thiols of the RSR' (R, R' = Me, Et and Ph) type and here our calculations again reveal a preference for the RS˙ type species adsorption for clean as well as for reconstructed 1:2 Au(ad)/thiols ratio models.

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

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

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

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 tomore » 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.« less

Energy Level Alignment at the Interface between Linear-Structured Benzenediamine Molecules and Au(111) Surface

NASA Astrophysics Data System (ADS)

Li, Guo; Rangel, Tonatiuh; Liu, Zhenfei; Cooper, Valentino; Neaton, Jeffrey

Using density functional theory with model self-energy corrections, we calculate the adsorption energetics and geometry, and the energy level alignment of benzenediamine (BDA) molecules adsorbed on Au(111) surfaces. Our calculations show that linear structures of BDA, stabilized via hydrogen bonds between amine groups, are energetically more favorable than monomeric phases. Moreover, our self-energy-corrected calculations of energy level alignment show that the highest occupied molecular orbital energy of the BDA linear structure is deeper relative to the Fermi level relative to the isolated monomer and agrees well with the values measured with photoemission spectroscopy. This work supported by DOE.

HCl dissociating on a rigid Au(111) surface: A six-dimensional quantum mechanical study on a new potential energy surface based on the RPBE functional.

PubMed

Liu, Tianhui; Fu, Bina; Zhang, Dong H

2017-04-28

The dissociative chemisorption of HCl on the Au(111) surface has recently been an interesting and important subject, regarding the discrepancy between the theoretical dissociation probabilities and the experimental sticking probabilities. We here constructed an accurate full-dimensional (six-dimensional (6D)) potential energy surface (PES) based on the density functional theory (DFT) with the revised Perdew-Burke-Ernzerhof (RPBE) functional, and performed 6D quantum mechanical (QM) calculations for HCl dissociating on a rigid Au(111) surface. The effects of vibrational excitations, rotational orientations, and site-averaging approximation on the present RPBE PES are investigated. Due to the much higher barrier height obtained on the RPBE PES than on the PW91 PES, the agreement between the present theoretical and experimental results is greatly improved. In particular, at the very low kinetic energy, the QM-RPBE dissociation probability agrees well with the experimental data. However, the computed QM-RPBE reaction probabilities are still markedly different from the experimental values at most of the energy regions. In addition, the QM-RPBE results achieve good agreement with the recent ab initio molecular dynamics calculations based on the RPBE functional at high kinetic energies.

Dewetting induced Au-Ge composite nanodot evolution in SiO2

NASA Astrophysics Data System (ADS)

Datta, D. P.; Chettah, A.; Siva, V.; Kanjilal, D.; Sahoo, P. K.

2018-01-01

A composite nanostructure comprising of Au and Ge gradually evolves on SiO2 surface when a bilayer of Au and Ge is irradiated by medium keV Xe-ion beam. The morphology progresses through different stages from nucleating patches to extended islands and finally a Au-Ge composite nanodot array develops on the insulator surface. While ion energy and fluence are found to determine dimensions of the nanostructures, existence of a characteristic lateral length scale is also detected at every stage of evolution. Through morphological and compositional analysis, the observed evolution is understood as an effect of ion beam induced dewetting of Au top layer. Numerical estimation based on the unified thermal spike model using the present experimental condition demonstrates formation of molten zones around the ion track due to nuclear and electronic energy deposition in the target. Dewetting results from mass flow onto the surface driven by local melting along the ion track and combines with sputter erosion of the bilayer film to lead to composite nanodot evolution. The generality of the ion induced processes provides possible route towards metal-semiconductor hybrid nanostructure synthesis on insulator surface.

Photoluminescence from Au nanoparticles embedded in Au:oxide composite films

NASA Astrophysics Data System (ADS)

Liao, Hongbo; Wen, Weijia; Wong, George K.

2006-12-01

Au:oxide composite multilayer films with Au nanoparticles sandwiched by oxide layers (such as SiO2, ZnO, and TiO2) were prepared in a magnetron sputtering system. Their photoluminescence (PL) spectra were investigated by employing a micro-Raman system in which an Argon laser with a wavelength of 514 nm was used as the pumping light. Distinct PL peaks located at a wavelength range between 590 and 680 nm were observed in most of our samples, with Au particle size varying from several to hundreds of nanometers. It was found that the surface plasmon resonance (SPR) in these composites exerted a strong influence on the position of the PL peaks but had little effect on the PL intensity.

Facile synthesis of Au-ZnO plasmonic nanohybrids for highly efficient photocatalytic degradation of methylene blue

NASA Astrophysics Data System (ADS)

Kuriakose, Sini; Sahu, Kavita; Khan, Saif A.; Tripathi, A.; Avasthi, D. K.; Mohapatra, Satyabrata

2017-02-01

Au-ZnO plasmonic nanohybrids were synthesized by a facile two step process. In the first step, nanostructured ZnO thin films were prepared by carbothermal evaporation followed by thermal annealing in oxygen atmosphere. Deposition of ultrathin Au films onto the nanostructured ZnO thin films by sputtering combined with thermal annealing resulted in the formation of Au-ZnO plasmonic nanohybrid thin films. The structural, optical, plasmonic and photocatalytic properties of the Au-ZnO nanohybrid thin films were studied. XRD studies on the Au-ZnO hybrid thin films revealed the presence of Au and ZnO nanostructures. UV-visible absorption studies showed two peaks corresponding to the excitonic absorption of ZnO nanostructures in the UV region and the surface plasmon resonance (SPR) absorption of Au nanoparticles in the visible region. The Au-ZnO nanohybrid thin films annealed at 400 °C showed enhanced photocatalytic activity as compared to nanostructrured ZnO thin films towards sun light driven photocatalytic degradation of methylene blue (MB) dye in water. The observed enhanced photocatalytic activity of Au-ZnO plasmonic nanohybrids is attributed to the efficient suppression of the recombination of photogenerated charge carriers in ZnO due to the strong electron scavenging action of Au nanoparticles combined with the improved sun light utilization capability of Au-ZnO nanohybrids coming from the plasmonic response of Au nanoparticles decorating ZnO nanostructures.

Au/Cr Sputter Coating for the Protection of Alumina During Sliding at High Temperatures

NASA Technical Reports Server (NTRS)

Benoy, Patricia A.; Dellacorte, Christopher

1995-01-01

A sputter deposited bilayer coating of gold and chromium was investigated as a potential solid lubricant to protect alumina substrates in applications involving sliding at high temperature. The proposed lubricant was tested in a pin-on-disk tribometer with coated alumina disks sliding against uncoated alumina pins. Three test parameters; temperature, load, and sliding velocity were varied over a wide range in order to determine the performance envelope on the gold/chromium (Au/Cr) solid lubricant film. The tribo-tests were run in an air atmosphere at temperatures of 25 to 1000 C, under loads of 4.9 to 49.0 N and at sliding velocities from 1 to 15 m/sec. Post test analyses included surface profilometry, wear factor determination and scanning electron microscopy/energy dispersive spectroscopy (SEM/EDS) examination of worn surfaces. Compared to unlubricated Al2O3 sliding, the use of the Au/Cr film reduced friction by 30 to 50 percent and wear by one to two orders of magnitude. Increases in test temperature resulted in lower friction and the Au/Cr film continued to provide low friction, about 0.3, even at 1000 C. Pin wear factors and friction were largely unaffected by increasing loads up to 29.4 N. Sliding velocity had essentially no effect on friction, however, increased velocity reduced coating life (total sliding distance). Based upon these research results, the Au/Cr film is a promising lubricant for moderately loaded, low speed applications operating at temperatures as high as 1000 C.

Formation, Migration, and Reactivity of Au CO Complexes on Gold Surfaces

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

Wang, Jun; McEntee, Monica; Tang, Wenjie

2016-01-12

Here, we report experimental as well as theoretical evidence that suggests Au CO complex formation upon the exposure of CO to active sites (step edges and threading dislocations) on a Au(111) surface. Room-temperature scanning tunneling microscopy (STM), X-ray photoelectron spectroscopy, transmission infrared spectroscopy, and density functional theory calculations point to Au CO complex formation and migration. Room-temperature STM of the Au(111) surface at CO pressures in the range from 10^ 8 to 10^ 4 Torr (dosage up to 10^6 langmuir) indicates Au atom extraction from dislocation sites of the herringbone reconstruction, mobile Au CO complex formation and diffusion, and Aumore » adatom cluster formation on both elbows and step edges on the Au surface. The formation and mobility of the Au CO complex result from the reduced Au Au bonding at elbows and step edges leading to stronger Au CO bonding and to the formation of a more positively charged CO (CO +) on Au. These studies indicate that the mobile Au CO complex is involved in the Au nanoparticle formation and reactivity, and that the positive charge on CO increases due to the stronger adsorption of CO at Au sites with lower coordination numbers.« less

Double layer effects in electrocatalysis: The oxygen reduction reaction and ethanol oxidation reaction on Au(111), Pt(111) and Ir(111) in alkaline media containing Na and Li cations

DOE PAGES

Lopes, Pietro P.; Strmcnik, Dusan; Jirkovsky, Jakub S.; ...

2015-09-28

Oxygen reduction and ethanol oxidation reactions were studied on Au(111), Pt(111) and Ir(111) in alkaline solutions containing sodium and/or lithium cations. By keeping the same (111) surface orientation and exploring oxophilicity trends and non-covalent interactions between OH ad and alkali metal cations (AMC n+), we were able to gain deep insights into the multiple roles that OH ad plays in these important electrocatalytic reactions. Cyclic voltammetry experiments revealed that OH ad formation initiates at distinct electrode potentials, governed by the oxophilicity of the specific metal surface, with further OH ad adlayer stabilization by non-covalent alkali-cation interactions and affecting the formationmore » of a “true oxide” layer at higher electrode potentials. Although OH ad is a simple spectator for the ORR, it promotes the ethanol oxidation reaction (EOR) at lower potentials and act as spectator at high OHad coverages. By changing the alkali metal cation at the interface (Li +) on more oxophilic surfaces, it was possible to promote the EOR even more, relative to Na +, without changing the product distribution for the reaction. This cation effect suggests that OH ad—Li +(H 2O) x clusters can stabilize the ethoxide adlayer, thus improving the EOR activity. Finally, our results indicate the importance of the entire electrochemical interface in determining the electrocatalytic activity during reaction.« less

From porphyrins to pyrphyrins: adsorption study and metalation of a molecular catalyst on Au(111)

NASA Astrophysics Data System (ADS)

Mette, Gerson; Sutter, Denys; Gurdal, Yeliz; Schnidrig, Stephan; Probst, Benjamin; Iannuzzi, Marcella; Hutter, Jürg; Alberto, Roger; Osterwalder, Jürg

2016-04-01

The molecular ligand pyrphyrin, a tetradentate bipyridine based macrocycle, represents an interesting but widely unexplored class of molecules. It resembles the well-known porphyrin, but consists of pyridyl subunits instead of pyrroles. Metal complexes based on pyrphyrin ligands have recently shown promise as water reduction catalysts in homogeneous photochemical water splitting reactions. In this study, the adsorption and metalation of pyrphyrin on a single crystalline Au(111) surface is investigated in an ultrahigh vacuum by means of scanning tunneling microscopy, low-energy electron diffraction, X-ray photoelectron spectroscopy and density functional theory. Pyrphyrin coverages of approximately one monolayer and less are obtained by sublimation of the molecules on the substrate kept at room temperature. The molecules self-assemble in two distinct phases of long-range molecular ordering depending on the surface coverage. The deposition of cobalt metal and subsequent annealing lead to the formation of Co-ligated pyrphyrin molecules accompanied by a pronounced change of the molecular self-assembly. Electronic structure calculations taking the herringbone reconstruction of Au(111) into account show that the molecules are physisorbed, but preferred adsorption sites are identified where Co and the N atoms of the two terminal cyano groups are optimally coordinated to the surface Au atoms. An intermediate state of the metalation reaction is observed and the reaction steps for the Co metalation of pyrphyrin molecules on Au(111) are established in a joint experimental and computational effort.The molecular ligand pyrphyrin, a tetradentate bipyridine based macrocycle, represents an interesting but widely unexplored class of molecules. It resembles the well-known porphyrin, but consists of pyridyl subunits instead of pyrroles. Metal complexes based on pyrphyrin ligands have recently shown promise as water reduction catalysts in homogeneous photochemical water splitting

Effect of core-shell structure on optical properties of Au-Cu2O nanoparticles

NASA Astrophysics Data System (ADS)

Sai, Cong Doanh; Ngac, An Bang

2018-03-01

Solid Au-Cu2O core-shell nanoparticles were synthesized using gold nanoparticles of 16.6 nm in size as the core. The core-shell structure of the synthesized particles was confirmed and characterized by TEM and HRTEM images. Due to their similar crystal structure, the (111) planes of Cu2O are nucleated and grown epitaxially on the {111} facets of Au nanoparticles with the lattice mismatch of about 4.3% resulting in a polycrystallized Cu2O shell covering the Au nanocore. Due to the quantum confinement effect, the band gap energy Eg of the synthesized Cu2O shells is blue-shifted from 2.35 to 2.70 eV as the shell thickness decreases from of 24.6±3.6 to 9.0±1.7 nm. The localized SPR (Surface Plasmon Resonance) peak of the Au nanocore undergoes a large red shift of the order of a hundred of nm due to both the high refractive index and the increase of the thickness of Cu2O shell. Theoretical models within the Drude framework significantly underestimate the experimental data and predict a wrong rate of change of the SPR peak position with respect to the shell thickness.

Scanning tunneling microscopy study of graphene on Au(111): Growth mechanisms and substrate interactions

NASA Astrophysics Data System (ADS)

Nie, Shu; Bartelt, Norman C.; Wofford, Joseph M.; Dubon, Oscar D.; McCarty, Kevin F.; Thürmer, Konrad

2012-05-01

We use scanning tunneling microscopy to study the structure of graphene islands on Au(111) grown by deposition of elemental carbon at 950 °C. Consistent with low-energy electron microscopic observations, we find that the graphene islands have dendritic shapes. The islands tend to cover depressed regions of the Au surface, suggesting that Au is displaced as the graphene grows. If small tunneling currents are used, it is possible to image simultaneously the graphene/Au moiré and the Au herringbone reconstruction, which forms underneath the graphene on cooling from the growth temperature. The delicate herringbone structure and its periodicity remain unchanged from the bare Au surface. Using a Frenkel-Kontorova model, we deduce that this striking observation is consistent with an attraction between graphene and Au of less than 13 meV per C atom. Raman spectroscopy supports this weak interaction. However, at the tunneling currents necessary for atomic-resolution imaging of graphene, the Au reconstruction is altered, implying influential tip-sample interactions and a mobile Au surface beneath the graphene.

DFT calculations for Au adsorption onto a reduced TiO2 (110) surface with the coexistence of Cl

NASA Astrophysics Data System (ADS)

Tada, Kohei; Sakata, Kohei; Yamada, Satoru; Okazaki, Kazuyuki; Kitagawa, Yasutaka; Kawakami, Takashi; Yamanaka, Shusuke; Okumura, Mitsutaka

2014-02-01

Residual chlorines, which originate from HAuCl4, enhance the aggregation of gold (Au) nanoparticles and clusters, preventing the generation of highly active supported Au catalysts. However, the detailed mechanism of residual-chlorine-promoted aggregation of Au is unknown. Herein to investigate this mechanism, density functional theory (DFT) calculations of Au and Cl adsorption onto a reduced rutile TiO2 (110) surface were performed using a generalised gradient approximation Perdew, Burke, and Ernzerhof formula (GGA-PBE) functional and plane-wave basis. Although both Au and Cl atoms prefer to mono-absorb onto oxygen defect sites, Cl atoms have a stronger absorption onto a reduced TiO2 (110) surface, abbreviated as rTiO2 (110) in the following, than Au atoms. Additionally, co-adsorption of a Cl atom and a Au atom or Au nanorod onto a rTiO2 surface was investigated; Cl adsorption onto an oxygen defect site weakens the interaction between a Au atom or Au nanorod and rTiO2 (110) surface. The calculation results suggest that the depletion of interaction between Au and rTiO2 surface is due to strong interaction between Cl atoms at oxygen defect sites and neighbouring bridging oxygen (OB) atoms.

Crystal-face-selective adsorption of Au nanoparticles onto polycrystalline diamond surfaces.

PubMed

Kondo, Takeshi; Aoshima, Shinsuke; Hirata, Kousuke; Honda, Kensuke; Einaga, Yasuaki; Fujishima, Akira; Kawai, Takeshi

2008-07-15

Crystal-face-selective adsorption of Au nanoparticles (AuNPs) was achieved on polycrystalline boron-doped diamond (BDD) surface via the self-assembly method combined with a UV/ozone treatment. To the best of our knowledge, this is the first report of crystal-face-selective adsorption on an inorganic solid surface. Hydrogen-plasma-treated BDD samples and those followed by UV/ozone treatment for 2 min or longer showed almost no adsorption of AuNP after immersion in the AuNP solution prepared by the citrate reduction method. However, the samples treated by UV/ozone for 10 s showed AuNP adsorption on their (111) facets selectively after the immersion. Moreover, the sample treated with UV/ozone for 40-60 s showed AuNP adsorption on the whole surface. These results indicate that the AuNP adsorption behavior can be controlled by UV/ozone treatment time. This phenomenon was highly reproducible and was applied to a two-step adsorption method, where AuNPs from different batches were adsorbed on the (111) and (100) surface in this order. Our findings may be of great value for the fabrication of advanced nanoparticle-based functional materials via bottom-up approaches with simple macroscale procedures.

Surface acidity scales: Experimental measurements of Brønsted acidities on anatase TiO2 and comparison with coinage metal surfaces

NASA Astrophysics Data System (ADS)

Silbaugh, Trent L.; Boaventura, Jaime S.; Barteau, Mark A.

2016-08-01

The first quantitative surface acidity scale for Brønsted acids on a solid surface is presented through the use of titration-displacement and equilibrium experiments on anatase TiO2. Surface acidities of species on TiO2 correlated with gas phase acidities, as was previously observed in qualitative studies of Brønsted acid displacement on Ag(110), Cu(110) and Au(111). A 90% compression of the surface acidity scale relative to the gas phase was observed due to compensation from the covalent component of the conjugate base - surface bond. Adsorbed conjugate bases need not be completely anionic for correlations with gas phase acidities to hold. Positive and negative substituent effects, such as substituted fluorine and hydrocarbon sidechain dispersion interactions with the surface, may modify the surface acidity scale, in agreement with previous experimental and theoretical work on Au(111).

Halo-substituted azobenzenes adsorbed at Ag(111) and Au(111) interfaces: Structures and optical properties

NASA Astrophysics Data System (ADS)

Hughes, Zak E.; Baev, Alexander; Prasad, Paras N.; Walsh, Tiffany R.

2017-05-01

The adsorption of azobenzene (AB), ortho fluoro-azobenzene (FAB) and ortho chlor-azobenzol (ClAB), in both the cis and trans isomers, at the Au(111) and Ag(111) surfaces is investigated using plane-wave density functional calculations with the revPBE-vdW-DF functional. The resulting adsorption energies and internal structures of AB adsorbed to both metal surfaces are in broad agreement with available experimental data. In the gas phase, FAB and ClAB feature a significant reduction in the energy difference between the two isomeric states, compared with AB. This relative reduction in the energy difference is still significant for the adsorbed form of FAB but is only weakly apparent for ClAB. The absorption spectra of the molecules have also been calculated, with the halogen substituents generating significant changes in the gas phase, but only a modest difference for the adsorbed molecules.

Thin films of Ag–Au nanoparticles dispersed in TiO2: influence of composition and microstructure on the LSPR and SERS responses

NASA Astrophysics Data System (ADS)

Borges, Joel; Ferreira, Catarina G.; Fernandes, João P. C.; Rodrigues, Marco S.; Proença, Manuela; Apreutesei, Mihai; Alves, Eduardo; Barradas, Nuno P.; Moura, Cacilda; Vaz, Filipe

2018-05-01

Thin films containing monometallic (Ag,Au) and bimetallic (Ag–Au) noble nanoparticles were dispersed in TiO2, using reactive magnetron sputtering and post-deposition thermal annealing. The influence of metal concentration and thermal annealing in the (micro)structural evolution of the films was studied, and its correlation with the localized surface plasmon resonance (LSPR) and surface enhanced Raman spectroscopy (SERS) behaviours was evaluated. The Ag/TiO2 films presented columnar to granular microstructures, developing Ag clusters at the surface for higher annealing temperatures. In some cases, the films presented dendrite-type fractal geometry, which led to an almost flat broadband optical response. The Au/TiO2 system revealed denser microstructures, with Au nanoparticles dispersed in the matrix, whose size increased with annealing temperature. This microstructure led to the appearance of LSPR bands, although some Au segregation to the surface hindered this effect for higher concentrations. The structural results of the Ag–Au/TiO2 system suggested the formation of bimetallic Ag–Au nanoparticles, which presence was supported by the appearance of a single narrow LSPR band. In addition, the Raman spectra of Rhodamine-6G demonstrated the viability of these systems for SERS applications, with some indication that the Ag/TiO2 system might be preferential, contrasting to the notorious behaviour of the bimetallic system in terms of LSPR response.

Thermal and ion-induced surface reactions of 1,1-difluoroethylene on Si(111)7 x 7 and vitreous SiO2.

PubMed

He, Zhenhua; Leung, K T

2005-08-11

Thermal and ion-induced reactions of 1,1-difluoroethylene (1,1-C2H2F2 or iso-DFE) on Si(111)7 x 7 and vitreous SiO2 surfaces have been investigated by vibrational electron energy loss spectroscopy and thermal desorption spectrometry. Like ethylene, iso-DFE predominantly chemisorbs via a [2 + 2] cycloaddition mechanism onto the 7 x 7 surface as a di-sigma-bonded difluoroethane-1,2-diyl adstructure, which undergoes H abstraction and defluorination, producing hydrocarbon fragments and SiF(x) (x = 1-3) upon annealing to >700 K. Ion irradiation of Si(111)7 x 7 in iso-DFE at 50 eV impact energy appears to substantially enhance the production of hydrocarbon fragments and SiF(x)(), leading to stronger SiF4 desorption products over an extended temperature range (400-900 K). The observed SiC and SiF(x) produced on the 7 x 7 surface by ion irradiation in iso-DFE are found to be similar to those obtained by ion irradiation in the fluoromethane homologues, CF4 and CH2F2. The production of higher relative concentrations for the larger SiF(x) and C2-containing fragments is evidently favored on the 7 x 7 surface. On a vitreous SiO2 surface, ion irradiation in iso-DFE, unlike that in CF4 and CH2F2, appears to produce less SiF(x) than that on the 7 x 7 surface, which indicates that surface O does not interact strongly with the C2-containing fragments. The presence or absence of a C=C bond and the relative F-to-C ratio of the sputtering gas could therefore produce important effects on the resulting surface products obtained by low-energy ion irradiation.

Room temperature deposition of gold onto the diffuse and sharp diffraction spot Si(111)-( 3 × 3) R30° Au surfaces

NASA Astrophysics Data System (ADS)

Plass, Richard; Marks, Laurence D.

1996-06-01

Room temperature gold depositions onto Si(111)-( 3 × 3) R30° Au surfaces with diffuse and sharp diffraction spots [Surf. Sci. 242 (1991) 73] (diffuse and sharp 3 × 3 Au hereafter) under UHV conditions has been monitored using transmission electron diffraction (TED). Both systems display an increase in surface structure diffraction spot intensities up to the completion of 1.0 monolayer (ML) after which the surface beams display an exponential decrease in intensity with coverage. The exponential decay rate decreases after roughly 1.33 ML. These results can be attributed to gold initially diffusing to and filling 3 × 3 Au gold trimer sites in vacancy type surface domain walls [Surf. Sci. 342 (1995) 233], then filling one of three possible sites on the 3 × 3 Au structure with essentially no surface diffusion, disrupting nearby gold trimers. Gold deposition onto the diffuse type structure caused the formation and expansion of satellite arcs around the strongest 3 × 3 beams similar to those seen by others [Surf. Sci. 242 (1991) 73; Jpn. J. Appl. Phys. 16 (1977) 891; J. Vac. Sci. Technol. A 10 (1992) 3486] at elevated temperatures while the sharp structure displayed only a modest shoulder formation near the strongest 3 × 3 beams.

Initial Reduction of CO2 on Pd-, Ru-, and Cu-Doped CeO2(111) Surfaces: Effects of Surface Modification on Catalytic Activity and Selectivity.

PubMed

Guo, Chen; Wei, Shuxian; Zhou, Sainan; Zhang, Tian; Wang, Zhaojie; Ng, Siu-Pang; Lu, Xiaoqing; Wu, Chi-Man Lawrence; Guo, Wenyue

2017-08-09

Surface modification by metal doping is an effective treatment technique for improving surface properties for CO 2 reduction. Herein, the effects of doped Pd, Ru, and Cu on the adsorption, activation, and reduction selectivity of CO 2 on CeO 2 (111) were investigated by periodic density functional theory. The doped metals distorted the configuration of a perfect CeO 2 (111) by weakening the adjacent Ce-O bond strength, and Pd doping was beneficial for generating a highly active O vacancy. The analyses of adsorption energy, charge density difference, and density of states confirmed that the doped metals were conducive for enhancing CO 2 adsorption, especially for Cu/CeO 2 (111). The initial reductive dissociation CO 2 → CO* + O* on metal-doped CeO 2 (111) followed the sequence of Cu- > perfect > Pd- > Ru-doped CeO 2 (111); the reductive hydrogenation CO 2 + H → COOH* followed the sequence of Cu- > perfect > Ru- > Pd-doped CeO 2 (111), in which the most competitive route on Cu/CeO 2 (111) was exothermic by 0.52 eV with an energy barrier of 0.16 eV; the reductive hydrogenation CO 2 + H → HCOO* followed the sequence of Ru- > perfect > Pd-doped CeO 2 (111). Energy barrier decomposition analyses were performed to identify the governing factors of bond activation and scission along the initial CO 2 reduction routes. Results of this study provided deep insights into the effect of surface modification on the initial reduction mechanisms of CO 2 on metal-doped CeO 2 (111) surfaces.

Water–gas shift reaction over gold nanoparticles dispersed on nanostructured CeO x–TiO 2(110) surfaces: Effects of high ceria coverage

DOE PAGES

Grinter, D. C.; Park, J. B.; Agnoli, S.; ...

2016-08-01

We used scanning tunnelling microscopy to study the morphology of an overlayer of ceria in contact with a TiO 2(110) substrate. Two types of domains were observed after ceria deposition. An ordered ceria film covered half of the surface and high-resolution imaging suggested a near-c(6 × 2) relationship to the underlying TiO 2(110)-(1 × 1). For the other half of the surface, it comprised CeO x nanoparticles and reconstructed TiOx supported on TiO 2(110)-(1 × 1). Exposure to a small amount of gold resulted in the formation of isolated gold atoms and small clusters on the ordered ceria film andmore » TiO 2(110)-(1 × 1) areas, which exhibited significant sintering at 500 K and showed strong interaction between the sintered gold clusters and the domain boundaries of the ceria film. The Au/CeO x/TiO 2(110) model system proved to be a good catalyst for the water–gas shift (WGS) exhibiting much higher turnover frequencies (TOFs) than Cu(111) and Pt(111) benchmarks, or the individual Au/TiO 2(110) and Au/CeO 2(111) systems. Finally, for Au/CeO x/TiO 2(110) catalysts, there was a decrease in catalytic activity with increasing ceria coverage that correlates with a reduction in the concentration of Ce 3 + formed during WGS reaction conditions.« less

Water–gas shift reaction over gold nanoparticles dispersed on nanostructured CeO x–TiO 2(110) surfaces: Effects of high ceria coverage

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

Grinter, D. C.; Park, J. B.; Agnoli, S.

We used scanning tunnelling microscopy to study the morphology of an overlayer of ceria in contact with a TiO 2(110) substrate. Two types of domains were observed after ceria deposition. An ordered ceria film covered half of the surface and high-resolution imaging suggested a near-c(6 × 2) relationship to the underlying TiO 2(110)-(1 × 1). For the other half of the surface, it comprised CeO x nanoparticles and reconstructed TiOx supported on TiO 2(110)-(1 × 1). Exposure to a small amount of gold resulted in the formation of isolated gold atoms and small clusters on the ordered ceria film andmore » TiO 2(110)-(1 × 1) areas, which exhibited significant sintering at 500 K and showed strong interaction between the sintered gold clusters and the domain boundaries of the ceria film. The Au/CeO x/TiO 2(110) model system proved to be a good catalyst for the water–gas shift (WGS) exhibiting much higher turnover frequencies (TOFs) than Cu(111) and Pt(111) benchmarks, or the individual Au/TiO 2(110) and Au/CeO 2(111) systems. Finally, for Au/CeO x/TiO 2(110) catalysts, there was a decrease in catalytic activity with increasing ceria coverage that correlates with a reduction in the concentration of Ce 3 + formed during WGS reaction conditions.« less

Coverage Dependent Assembly of Anthraquinone on Au(111)

NASA Astrophysics Data System (ADS)

Conrad, Brad; Deloach, Andrew; Einstein, Theodore; Dougherty, Daniel

A study of adsorbate-adsorbate and surface state mediated interactions of anthraquinone (AnQ) on Au(111) is presented. We utilize scanning tunneling microscopy (STM) to characterize the coverage dependence of AnQ structure formation. Ordered structures are observed up to a single monolayer (ML) and are found to be strongly dependent on molecular surface density. While the complete ML forms a well-ordered close-packed layer, for a narrow range of sub-ML coverages irregular close-packed islands are observed to coexist with a disordered pore network linking neighboring islands. This network displays a characteristic pore size and at lower coverages, the soliton walls of the herringbone reconstruction are shown to promote formation of distinct pore nanostructures. We will discuss these nanostructure formations in the context of surface mediated and more direct adsorbate interactions.

CO adsorption on the “29” Cu xO/Cu(111) surface: An integrated DFT, STM, and TPD study

DOE PAGES

Hensley, Alyssa J. R.; Therrien, Andrew J.; Zhang, Renqin; ...

2016-10-04

The elucidation of an accurate atomistic model of surface structures is crucial for the design and understanding of effective catalysts, a process requiring a close collaboration between experimental observations and theoretical models. Any developed surface theoretical model must agree with experimental results for the surface when both clean and adsorbate covered. Here, we present a detailed study of the adsorption of CO on the “29” Cu xO/ Cu(111) surface, which is important in the understanding of ubiquitous Cubased catalysis. This study uses scanning tunneling microscopy, temperatureprogrammed desorption, and density functional theory to analyze CO adsorption on the “29” Cu xO/Cu(111)more » surface. From the experimental scanning tunneling microscopy images, CO was found to form six different ordered structures on the “29” Cu xO/Cu(111) surface depending on the surface CO coverage. By modeling the adsorption of CO on our atomistic model of the “29” Cu xO/Cu(111) surface at different coverages, we were able to match the experimentally observed CO ordered structures to specific combinations of sites on the “29” Cu xO/Cu(111) surface. Lastly, the high degree of agreement seen here between experiment and theory for the adsorption of CO on the “29” Cu xO/Cu(111) surface at various CO coverages provides further support that our atomistic model of the “29” Cu xO/Cu(111) surface is experimentally accurate.« less

Phosphine Functionalization GaAs(111)A Surfaces

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

Traub, M.; Biteen, J; Michalak, D

Phosphorus-functionalized GaAs surfaces have been prepared by exposure of Cl-terminated GaAs(111)A surfaces to triethylphosphine (PEt3) or trichlorophosphine (PCl3), or by the direct functionalization of the native-oxide terminated GaAs(111)A surface with PCl3. The presence of phosphorus on each functionalized surface was confirmed by X-ray photoelectron spectroscopy. High-resolution, soft X-ray photoelectron spectroscopy was used to evaluate the As and Ga 3d regions of such surfaces. On PEt3 treated surfaces, the Ga 3d spectra exhibited a bulk Ga peak as well as peaks that were shifted to 0.35, 0.92 and 1.86 eV higher binding energy. These peaks were assigned to residual Cl-terminated Gamore » surface sites, surficial Ga2O and surficial Ga2O3, respectively. For PCl3-treated surfaces, the Ga 3d spectra displayed peaks ascribable to bulk Ga(As), Ga2O, and Ga2O3, as well as a peak shifted 0.30 eV to higher binding energy relative to the bulk signal. A peak corresponding to Ga(OH)3, observed on the Cl-terminated surface, was absent from all of the phosphine-functionalized surfaces. After reaction of the Cl-terminated GaAs(111)A surface with PCl3 or PEt3, the As 3d spectral region was free of As oxides and As0. Although native oxide-terminated GaAs surfaces were free of As oxides after reaction with PCl3, such surfaces contained detectable amounts of As0. Photoluminescence measurements indicted that phosphine-functionalized surfaces prepared from Cl-terminated GaAs(111)A surfaces had better electrical properties than the native-oxide capped GaAs(111)A surface, while the native-oxide covered surface treated with PCl3 showed no enhancement in PL intensity.« less

Elementary surface chemistry during CuO/Al nanolaminate-thermite synthesis: copper and oxygen deposition on aluminum (111) surfaces.

PubMed

Lanthony, Cloé; Guiltat, Mathilde; Ducéré, Jean Marie; Verdier, Agnes; Hémeryck, Anne; Djafari-Rouhani, Mehdi; Rossi, Carole; Chabal, Yves J; Estève, Alain

2014-09-10

The surface chemistry associated with the synthesis of energetic nanolaminates controls the formation of the critical interfacial layers that dominate the performances of nanothermites. For instance, the interaction of Al with CuO films or CuO with Al films needs to be understood to optimize Al/CuO nanolaminates. To that end, the chemical mechanisms occurring during early stages of molecular CuO adsorption onto crystalline Al(111) surfaces are investigated using density functional theory (DFT) calculations, leading to the systematic determination of their reaction enthalpies and associated activation energies. We show that CuO undergoes dissociative chemisorption on Al(111) surfaces, whereby the Cu and O atoms tend to separate from each other. Both Cu and O atoms form islands with different properties. Copper islanding fosters Cu insertion (via surface site exchange mechanism) into the subsurface, while oxygen islands remain stable at the surface. Above a critical local oxygen coverage, aluminum atoms are extracted from the Al surface, leading to oxygen-aluminum intermixing and the formation of aluminum oxide (γ-alumina). For Cu and O co-deposition, copper promotes oxygen-aluminum interaction by oxygen segregation and separates the resulting oxide from the Al substrate by insertion into Al and stabilization below the oxide front, preventing full mixing of Al, Cu, and O species.

Structural paradox in submonolayer chlorine coverage on Au(111)

NASA Astrophysics Data System (ADS)

Zheltov, V. V.; Cherkez, V. V.; Andryushechkin, B. V.; Zhidomirov, G. M.; Kierren, B.; Fagot-Revurat, Y.; Malterre, D.; Eltsov, K. N.

2014-05-01

In this work, we present a combined low-temperature scanning tunneling microscopy (STM) and density functional theory (DFT) study of chlorine adsorption on Au(111) at low coverages. Our STM study of Cl/Au(111) system has shown that at submonolayer coverages (θ < 0.1 ML) chlorine atoms form chainlike structures with abnormally short distances of 3.8 Å between them. Our DFT calculations have shown that chlorine atoms can interact with each other through distortion of the substrate and this indirect elastic interaction is strong enough to affect their arrangement in the chainlike structures.

Compression-Induced Conformation and Orientation Changes in an n-Alkane Monolayer on a Au(111) Surface.

PubMed

Endo, Osamu; Nakamura, Masashi; Amemiya, Kenta; Ozaki, Hiroyuki

2017-04-25

The influence of the preparation method and adsorbed amount of n-tetratetracontane (n-C 44 H 90 ) on its orientation in a monolayer on the Au(111) surface is studied by near carbon K-edge X-ray absorption fine structure spectroscopy (C K-NEXAFS), scanning tunneling microscopy (STM) under ultrahigh vacuum, and infrared reflection-absorption spectroscopy (IRAS) at the electrochemical interface in sulfuric acid solution. The n-C 44 H 90 molecules form self-assembled lamellar structures with the chain axis parallel to the surface, as observed by STM. For small amounts adsorbed, the carbon plane is parallel to the surface (flat-on orientation). An increase in the adsorbed amount by ∼10-20% induces compression of the lamellar structure either along the lamellar axis or alkyl chain axis. The compressed molecular arrangement is observed by STM, and induced conformation and orientation changes are confirmed by in situ IRAS and C K-NEXAFS.

Fabrication and characterization of copper oxide (CuO)–gold (Au)–titania (TiO{sub 2}) and copper oxide (CuO)–gold (Au)–indium tin oxide (ITO) nanowire heterostructures

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

Chopra, Nitin, E-mail: nchopra@eng.ua.edu; Department of Biological Sciences, The University of Alabama, Tuscaloosa, AL 35487; Shi, Wenwu

2014-10-15

Nanoscale heterostructures composed of standing copper oxide nanowires decorated with Au nanoparticles and shells of titania and indium tin oxide were fabricated. The fabrication process involved surfactant-free and wet-chemical nucleation of gold nanoparticles on copper oxide nanowires followed by a line-of-sight sputtering of titania or indium tin oxide. The heterostructures were characterized using high resolution electron microscopy, diffraction, and energy dispersive spectroscopy. The interfaces, morphologies, crystallinity, phases, and chemical compositions were analyzed. The process of direct nucleation of gold nanoparticles on copper oxide nanoparticles resulted in low energy interface with aligned lattice for both the components. Coatings of polycrystalline titaniamore » or amorphous indium tin oxide were deposited on standing copper oxide nanowire–gold nanoparticle heterostructures. Self-shadowing effect due to standing nanowire heterostructures was observed for line-of-sight sputter deposition of titania or indium tin oxide coatings. Finally, the heterostructures were studied using Raman spectroscopy and ultraviolet–visible spectroscopy, including band gap energy analysis. Tailing in the band gap energy at longer wavelengths (or lower energies) was observed for the nanowire heterostructures. - Highlights: • Heterostructures comprised of CuO nanowires coated with Au nanoparticles. • Au nanoparticles exhibited nearly flat and low energy interface with nanowire. • Heterostructures were further sputter-coated with oxide shell of TiO{sub 2} or ITO. • The process resulted in coating of polycrystalline TiO{sub 2} and amorphous ITO shell.« less

Interface electronic structures of reversible double-docking self-assembled monolayers on an Au(111) surface

PubMed Central

Zhang, Tian; Ma, Zhongyun; Wang, Linjun; Xi, Jinyang; Shuai, Zhigang

2014-01-01

Double-docking self-assembled monolayers (DDSAMs), namely self-assembled monolayers (SAMs) formed by molecules possessing two docking groups, provide great flexibility to tune the work function of metal electrodes and the tunnelling barrier between metal electrodes and the SAMs, and thus offer promising applications in both organic and molecular electronics. Based on the dispersion-corrected density functional theory (DFT) in comparison with conventional DFT, we carry out a systematic investigation on the dual configurations of a series of DDSAMs on an Au(111) surface. Through analysing the interface electronic structures, we obtain the relationship between single molecular properties and the SAM-induced work-function modification as well as the level alignment between the metal Fermi level and molecular frontier states. The two possible conformations of one type of DDSAM on a metal surface reveal a strong difference in the work-function modification and the electron/hole tunnelling barriers. Fermi-level pinning is found to be a key factor to understand the interface electronic properties. PMID:24615153

Study of p-diaminobenzene Adsorption on Au(111) by Scanning Tunneling Microscopy

NASA Astrophysics Data System (ADS)

Zhou, Hui; Hu, Zonghai; Eom, Daejin; Rim, Kwang; Liu, Li; Flynn, George; Venkataraman, Latha; Morgante, Alberto; Heinz, Tony

2008-03-01

From the well-defined conductivity obtained for various individual diamino-substituted molecules spanning two gold contacts, as well as from theoretical analysis [1], researchers have suggested that amines adsorb preferentially to coordinatively unsaturated surface Au atoms through the N lone pair. To understand the nature of the amine binding, we have applied ultrahigh vacuum scanning tunneling microscope (STM) to investigate the adsorption of p-diaminobenzene molecules on the reconstructed Au(111) surface. The STM topography images (taken at 4 K) show that the molecules adsorb preferentially to step edges, corresponding to sites of reduced Au atom coordination. The adsorbed molecules are found to display a distinctive orientation along the step edges. The two-lobe topographic structure of each molecule seen by STM is compatible with the previously calculated charge density of the HOMO level. [1] L. Venkataraman at el., Nano Lett. 7, 502 (2007).

Ag@Au concave cuboctahedra: A unique probe for monitoring Au-catalyzed reduction and oxidation reactions by surface-enhanced Raman spectroscopy

DOE PAGES

Zhang, Jiawei; Winget, Sarah A.; Wu, Yiren; ...

2016-01-26

In this paper, we report a facile synthesis of Ag@Au concave cuboctahedra by titrating aqueous HAuCl4 into a suspension of Ag cuboctahedra in the presence of ascorbic acid (AA), NaOH, and poly(vinylpyrrolidone) (PVP) at room temperature. Initially, the Au atoms derived from the reduction of Au 3+ by AA are conformally deposited on the entire surface of a Ag cuboctahedron. Upon the formation of a complete Au shell, however, the subsequently formed Au atoms are preferentially deposited onto the Au{100} facets, resulting in the formation of a Ag@Au cuboctahedron with concave structures at the sites of {111} facets. The concavemore » cuboctahedra embrace excellent SERS activity that is more than 70-fold stronger than that of the original Ag cuboctahedra at an excitation wavelength of 785 nm. The concave cuboctahedra also exhibit remarkable stability in the presence of an oxidant such as H 2O 2 because of the protection by a complete Au shell. These two unique attributes enable in-situ SERS monitoring of the reduction of 4-nitrothiophenol (4-NTP) to 4-aminothiophenol (4-ATP) by NaBH4 through a 4,4'-dimercaptoazobenzene ( trans-DMAB) intermediate and the subsequent oxidation of 4-ATP back to trans-DMAB upon the introduction of H 2O 2.« less

Ag@Au concave cuboctahedra: A unique probe for monitoring Au-catalyzed reduction and oxidation reactions by surface-enhanced Raman spectroscopy

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

Zhang, Jiawei; Winget, Sarah A.; Wu, Yiren

In this paper, we report a facile synthesis of Ag@Au concave cuboctahedra by titrating aqueous HAuCl4 into a suspension of Ag cuboctahedra in the presence of ascorbic acid (AA), NaOH, and poly(vinylpyrrolidone) (PVP) at room temperature. Initially, the Au atoms derived from the reduction of Au 3+ by AA are conformally deposited on the entire surface of a Ag cuboctahedron. Upon the formation of a complete Au shell, however, the subsequently formed Au atoms are preferentially deposited onto the Au{100} facets, resulting in the formation of a Ag@Au cuboctahedron with concave structures at the sites of {111} facets. The concavemore » cuboctahedra embrace excellent SERS activity that is more than 70-fold stronger than that of the original Ag cuboctahedra at an excitation wavelength of 785 nm. The concave cuboctahedra also exhibit remarkable stability in the presence of an oxidant such as H 2O 2 because of the protection by a complete Au shell. These two unique attributes enable in-situ SERS monitoring of the reduction of 4-nitrothiophenol (4-NTP) to 4-aminothiophenol (4-ATP) by NaBH4 through a 4,4'-dimercaptoazobenzene ( trans-DMAB) intermediate and the subsequent oxidation of 4-ATP back to trans-DMAB upon the introduction of H 2O 2.« less

Au-nanoparticles grafted on plasma treated PE

NASA Astrophysics Data System (ADS)

Švorčík, V.; Chaloupka, A.; Řezanka, P.; Slepička, P.; Kolská, Z.; Kasálková, N.; Hubáček, T.; Siegel, J.

2010-03-01

Polyethylene (PE) surface was treated with Ar plasma. Activated surface was grafted from methanol solution of 1,2-ethanedithiol. Then the sample was immersed into freshly prepared colloid solution of Au-nanoparticles. Finally Au layer was sputtered on the samples. Properties of the modified PE were studied using various methods: AFM, EPR, RBS and nanoindentation. It was shown that the plasma treatment results in degradation of polymer chain (AFM) and creation of free radicals by EPR. After grafting with dithiol, the concentration of free radicals declines. The presence of Au and S in the surface layer after the coating with Au-nanoparticles was proved by RBS. Plasma treatment changes PE surface morphology and increases surface roughness, too. Another significant change in surface morphology and roughness was observed after deposition of Au-nanoparticles. Nanoindentation measurements show that the grafting with Au-nanoparticles increases adhesion of subsequently sputtered Au layer.

Six-dimensional quantum dynamics study for the dissociative adsorption of HCl on Au(111) surface

NASA Astrophysics Data System (ADS)

Liu, Tianhui; Fu, Bina; Zhang, Dong H.

2013-11-01

The six-dimensional quantum dynamics calculations for the dissociative chemisorption of HCl on Au(111) are carried out using the time-dependent wave-packet approach, based on an accurate PES which was recently developed by neural network fitting to density functional theory energy points. The influence of vibrational excitation and rotational orientation of HCl on the reactivity is investigated by calculating the exact six-dimensional dissociation probabilities, as well as the four-dimensional fixed-site dissociation probabilities. The vibrational excitation of HCl enhances the reactivity and the helicopter orientation yields higher dissociation probability than the cartwheel orientation. A new interesting site-averaged effect is found for the title molecule-surface system that one can essentially reproduce the six-dimensional dissociation probability by averaging the four-dimensional dissociation probabilities over 25 fixed sites.

Silicide surface phases on gold

NASA Technical Reports Server (NTRS)

Green, A. K.; Bauer, E.

1981-01-01

The crystalline silicide layers formed on (111) and (100) surfaces of Au films on various Si single-crystal substrates are studied by LEED and AES in conjunction with sputter-depth profiling as a function of annealing temperature. On the (111) surface, three basic silicide structures are obtained corresponding to layers of various thicknesses as obtained by different preparation conditions. The (100) surface shows only two different structures. None of the structures is compatible with the various bulk silicide structures deduced from X-ray diffraction. Using LEED as a criterion for the presence or absence of silicide on the surface, smaller layer thicknesses are obtained than reported previously on the basis of AES studies.

Theoretical Study of Trimethylacetic Acid Adsorption on CeO 2 (111) Surface

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

Wang, Weina; Thevuthasan, S.; Wang, Wenliang

We investigated trimethylacetic acid (TMAA) adsorption on stoichiometric and oxygen-deficient CeO 2(111) surfaces using density functional theory that accounts for the on-site Coulomb interaction via a Hubbard term (DFT+U) and long-range dispersion correction. Both the molecular state and dissociative state (TMAA → TMA– + H +) were identified on stoichiometric and oxygen-deficient CeO 2(111) surfaces. For the stoichiometric surface, two thermodynamically favorable configurations with adsorption energies of the order of -30 kcal/mol are identified; one is a molecule adsorption state, and the other one is a dissociative state. For the oxygen-deficient surface, dissociative states are more favorable than molecular states.more » Moreover, the most favorable configuration is the dissociative adsorption of TMAA with the adsorption energy of the order of -77 kcal/mol. The dissociated TMA moiety takes the position of oxygen vacancy, forming three Ce–O bonds. The signature vibrational frequencies for these thermodynamically stable structures are reported as well as their electronic structures. The effects of long-range dispersion interactions are found to be negligible for geometries but important for adsorption energies.« less

Theoretical Study of Trimethylacetic Acid Adsorption on CeO 2 (111) Surface

DOE PAGES

Wang, Weina; Thevuthasan, S.; Wang, Wenliang; ...

2016-01-11

We investigated trimethylacetic acid (TMAA) adsorption on stoichiometric and oxygen-deficient CeO 2(111) surfaces using density functional theory that accounts for the on-site Coulomb interaction via a Hubbard term (DFT+U) and long-range dispersion correction. Both the molecular state and dissociative state (TMAA → TMA– + H +) were identified on stoichiometric and oxygen-deficient CeO 2(111) surfaces. For the stoichiometric surface, two thermodynamically favorable configurations with adsorption energies of the order of -30 kcal/mol are identified; one is a molecule adsorption state, and the other one is a dissociative state. For the oxygen-deficient surface, dissociative states are more favorable than molecular states.more » Moreover, the most favorable configuration is the dissociative adsorption of TMAA with the adsorption energy of the order of -77 kcal/mol. The dissociated TMA moiety takes the position of oxygen vacancy, forming three Ce–O bonds. The signature vibrational frequencies for these thermodynamically stable structures are reported as well as their electronic structures. The effects of long-range dispersion interactions are found to be negligible for geometries but important for adsorption energies.« less

Characterization of one-dimensional molecular chains of 4,4'-biphenyl diisocyanide on Au(111) by scanning tunneling microscopy

DOE PAGES

Zhou, Jing; Li, Yan; Zahl, Percy; ...

2015-03-14

The morphology and electronic structure of vapor deposited 4,4'-biphenyldiisocyanide (BPDI) on a Au(111) surface were investigated using variable-temperature scanning tunneling microscopy (STM). When deposited at room temperature, BPDI molecules form one-dimensional molecular chains similar to that recently observed for the structurally related 1,4-phenyl diisocyanide (PDI). Compared to PDI, the longer periodicity for the BPDI molecular chains is consistent with the addition of a second phenyl ring and supports a structural model in which the BPDI molecules lie parallel to the surface and interconnected by Au-adatoms. The molecular chains are mostly aligned along the [110] direction of the Au(111) substrate, butmore » exhibit frequent changes in angle that are consistent with directions between fcc and hcp three-fold hollow sites. Dispersion-corrected density functional theory calculations for one-dimensional chains of BPDI molecules bound end-to-end via their isocyanide groups to Au-adatoms reproduce the observed periodicity of the chains and show that this morphology is energetically favored over upright binding with one free –NC group. The spatially resolved conductance (dI/dV) map for BPDI on Au(111) exhibits a feature centered at -0.67 eV below the Fermi level which are delocalized along the chain with maxima at the Au-adatom and biphenyl positions. This occupied resonant feature is close to that previously observed for the PDI in both photoemission and conductance measurements and is attributed to an occupied interfacial state resulting from BPDI-Au interactions« less

Clarification of the interaction between Au atoms and the anatase TiO2 (112) surface using density functional theory

NASA Astrophysics Data System (ADS)

Tada, Kohei; Koga, Hiroaki; Okumura, Mitsutaka; Tanaka, Shingo

2018-04-01

A model (112) surface slab of anatase TiO2 (112) was optimized, and the adsorption of Au atoms onto the (112) surface was investigated by first-principles calculations based on DFT (density functional theory) with the generalized gradient approximation (GGA). Furthermore, the results were compared with those of Au/anatase TiO2 (101) system. The (112) surface has a ridge and a groove (zig-zag structure). The Au atoms were strongly adsorbed in the grooves but became unstable as they climbed toward the ridges, and the promotion of electrons in the 5d orbitals to the 6s and 6p orbitals in the absorbed Au atom occurred. At the Au/anatase TiO2 interface, the Au-Ti4+ coordinate bond in the (112) system is stronger than that in the (101) system because the promotion of electrons is greater in the former interaction than the latter. The results suggest that Au/anatase TiO2 catalysts with a higher dispersion of Au nanoparticles could be prepared when the (112) surface is preferentially exposed.

Water adsorption on the Fe3O4(111) surface: dissociation and network formation.

PubMed

Zaki, Eman; Mirabella, Francesca; Ivars-Barceló, Francisco; Seifert, Jan; Carey, Spencer; Shaikhutdinov, Shamil; Freund, Hans-Joachim; Li, Xiaoke; Paier, Joachim; Sauer, Joachim

2018-06-13

We monitored adsorption of water on a well-defined Fe3O4(111) film surface at different temperatures as a function of coverage using infrared reflection-absorption spectroscopy, temperature programmed desorption, and single crystal adsorption calorimetry. Additionally, density functional theory was employed using a Fe3O4(111)-(2 × 2) slab model to generate 15 energy minimum structures for various coverages. Corresponding vibrational properties of the adsorbed water species were also computed. The results show that water molecules readily dissociate on regular surface Fetet1-O ion pairs to form "monomers", i.e., terminal Fe-OH and surface OH groups. Further water molecules adsorb on the hydroxyl covered surface non-dissociatively and form "dimers" and larger oligomers, which ultimately assemble into an ordered (2 × 2) hydrogen-bonded network structure with increasing coverage prior to the formation of a solid water film.

Unraveling the oxygen vacancy structures at the reduced Ce O2(111 ) surface

NASA Astrophysics Data System (ADS)

Han, Zhong-Kang; Yang, Yi-Zhou; Zhu, Beien; Ganduglia-Pirovano, M. Verónica; Gao, Yi

2018-03-01

Oxygen vacancies at ceria (Ce O2 ) surfaces play an essential role in catalytic applications. However, during the past decade, the near-surface vacancy structures at Ce O2(111 ) have been questioned due to the contradictory results from experiments and theoretical simulations. Whether surface vacancies agglomerate, and which is the most stable vacancy structure for varying vacancy concentration and temperature, are being heatedly debated. By combining density functional theory calculations and Monte Carlo simulations, we proposed a unified model to explain all conflicting experimental observations and theoretical results. We find a novel trimeric vacancy structure which is more stable than any other one previously reported, which perfectly reproduces the characteristics of the double linear surface oxygen vacancy clusters observed by STM. Monte Carlo simulations show that at low temperature and low vacancy concentrations, vacancies prefer subsurface sites with a local (2 × 2) ordering, whereas mostly linear surface vacancy clusters do form with increased temperature and degree of reduction. These results well explain the disputes about the stable vacancy structure and surface vacancy clustering at Ce O2(111 ) , and provide a foundation for the understanding of the redox and catalytic chemistry of metal oxides.

ZIF-8 derived hexagonal-like α-Fe2O3/ZnO/Au nanoplates with tunable surface heterostructures for superior ethanol gas-sensing performance

NASA Astrophysics Data System (ADS)

Chen, Ying; Li, Hui; Ma, Qian; Che, Quande; Wang, Junpeng; Wang, Gang; Yang, Ping

2018-05-01

A series of hexagonal-like α-Fe2O3/ZnO/Au nanoplate heterostructures with tunable morphologies and superior ethanol gas-sensing performance were successfully synthesized via the facile multi-step reaction processes. Hexagonal-like α-Fe2O3 nanoplates with uniform size around 150 nm are employed as new sensor substrates for loading the well-distributed ZnO and Au nanoparticles with adjustable size distribution on the different surfaces. Brunauer-EmmeQ-Teller (BET) surface areas of α-Fe2O3 and α-Fe2O3/ZnO samples are evaluated to be 37.94 and 61.27 m2/g, respectively, while α-Fe2O3/ZnO/Au composites present the highest value of 79.08 m2/g. These α-Fe2O3-based functional materials can exhibit outstanding sensing properties to ethanol. When the ethanol concentration is 100 ppm, the response value of α-Fe2O3/ZnO/Au composites can reach up to 170, which is 14.6 and 80.3 times higher than that of α-Fe2O3/ZnO and pure α-Fe2O3, respectively. The recycling stability and long-time effectiveness can be availably maintained within 30 days, as well as the response and recovery times are shortened to 4 and 5 s, respectively. Significantly, the response value of α-Fe2O3/ZnO/Au composite is still up to 63 at an operating temperature of 280 °C even though the ethanol concentration decreases to 10 ppm. The enhanced gas sensing mechanism would be focused on the synergistic effects of phase compositions, surface heterogeneous structures, large specific surface area, and the selective depositions of Au nanoparticles in α-Fe2O3/ZnO/Au sensors. The synergistic effect of different surface heterostructures referring to α-Fe2O3/Au and α-Fe2O3/ZnO/Au and their novel electron transport processes on the surfaces are first investigated and discussed in details. It is expected that hexagonal-like α-Fe2O3/ZnO/Au nanoplate heterostructures with excellent sensing performance can be the promising highly-sensitive materials in the actual application for monitoring and detecting ethanol.

The effect of sputter temperature on vacancy island behavior on Ni(111) measured by photoemission of adsorbed xenon

NASA Astrophysics Data System (ADS)

Malafsky, Geoffrey P.

1994-04-01

The temperature dependence of vacancy coalescence on an ion bombarded Ni(111) surface is measured by photoemission of adsorbed xenon (PAX). The Ni(111) crystal is sputtered by a low fluence (0.06 ML incident ions) Ar + ion beam with incident kinetic energies of 500-3000 eV. The Xe coverage decreases rapidly with increasing temperature between 88 and 375 K with little additional change from 375 to 775 K. The PAX spectra are acquired with a Xe chamber pressure of 8 × 10 -10 Torr and at a temperature of 88 K. Under these conditions, the Xe is selectively adsorbed at defect sites which would make the Xe coverage proportional to the surface defect density on simple defect structures but the large size of the Xe atom relative to the Ni atom prevents the direct relationship of Xe coverage to the defect density when complex and varying defect structures are present. The decrease in Xe coverage is not attributed to the loss of defect sites by adatom-vacancy recombination but the changing vacancy island shape and size with temperature which alters the ratio of adsorbed Xe atoms to surface vacancy sites. This ratio decreases with increasing temperature as the vacancy islands progress from small and irregularly shaped islands to larger and hexagonally shaped islands. This transition is seen in Monte Carlo simulations of the kinetically driven atomic diffusion on the sputtered surface.

Six-dimensional quantum dynamics study for the dissociative adsorption of HCl on Au(111) surface

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

Liu, Tianhui; Fu, Bina; Zhang, Dong H., E-mail: zhangdh@dicp.ac.cn

The six-dimensional quantum dynamics calculations for the dissociative chemisorption of HCl on Au(111) are carried out using the time-dependent wave-packet approach, based on an accurate PES which was recently developed by neural network fitting to density functional theory energy points. The influence of vibrational excitation and rotational orientation of HCl on the reactivity is investigated by calculating the exact six-dimensional dissociation probabilities, as well as the four-dimensional fixed-site dissociation probabilities. The vibrational excitation of HCl enhances the reactivity and the helicopter orientation yields higher dissociation probability than the cartwheel orientation. A new interesting site-averaged effect is found for the titlemore » molecule-surface system that one can essentially reproduce the six-dimensional dissociation probability by averaging the four-dimensional dissociation probabilities over 25 fixed sites.« less

Elementary surface processes during reactive magnetron sputtering of chromium

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

Monje, Sascha; Corbella, Carles, E-mail: carles.corbella@rub.de; Keudell, Achim von

2015-10-07

The elementary surface processes occurring on chromium targets exposed to reactive plasmas have been mimicked in beam experiments by using quantified fluxes of Ar ions (400–800 eV) and oxygen atoms and molecules. For this, quartz crystal microbalances were previously coated with Cr thin films by means of high-power pulsed magnetron sputtering. The measured growth and etching rates were fitted by flux balance equations, which provided sputter yields of around 0.05 for the compound phase and a sticking coefficient of O{sub 2} of 0.38 on the bare Cr surface. Further fitted parameters were the oxygen implantation efficiency and the density of oxidationmore » sites at the surface. The increase in site density with a factor 4 at early phases of reactive sputtering is identified as a relevant mechanism of Cr oxidation. This ion-enhanced oxygen uptake can be attributed to Cr surface roughening and knock-on implantation of oxygen atoms deeper into the target. This work, besides providing fundamental data to control oxidation state of Cr targets, shows that the extended Berg's model constitutes a robust set of rate equations suitable to describe reactive magnetron sputtering of metals.« less

Vibrational properties of the Au-(√{3 }×√{3 } )/Si(111) surface reconstruction

NASA Astrophysics Data System (ADS)

Halbig, B.; Liebhaber, M.; Bass, U.; Geurts, J.; Speiser, E.; Räthel, J.; Chandola, S.; Esser, N.; Krenz, M.; Neufeld, S.; Schmidt, W. G.; Sanna, S.

2018-01-01

The vibrational properties of the Au-induced (√{3 }×√{3 })R 30∘ reconstruction of the Si(111) surface are investigated by polarized surface Raman spectroscopy and density-functional theory. The Raman measurements are performed in situ at room temperature as well as 20 K, and they reveal the presence of vibrational eigenmodes in the spectral range from 20 to 450 cm-1. In particular, two peaks of E symmetry at 75 and 183 cm-1 dominate the spectra. No substantial difference between room- and low-temperature spectra is observed, suggesting that the system does not undergo a phase transition down to 20 K. First-principles calculations are performed based on the structural models discussed in the literature. The thermodynamically stable conjugate honeycomb-chained-trimer model (CHCT) [Surf. Sci. 275, L691 (1992), 10.1016/0039-6028(92)90785-5] leads to phonon eigenvalues compatible with the experimental observations in the investigated spectral range. On the basis of the phonon eigenfrequencies, symmetries, and Raman intensities, we assign the measured spectral features to the calculated phonon modes. The good agreement between measured and calculated modes provides a strong argument in favor of the CHCT model.

In Situ Observations of UV-Induced Restructuring of Self-Assembled Porphyrin Monolayer on Liquid/Au(111) Interface at Molecular Level.

PubMed

Kim, Yongman; Doh, Won Hui; Kim, Jeongjin; Park, Jeong Young

2018-05-29

Porphyrin-derived molecules have received much attention for use in solar energy conversion devices, such as artificial leaves and dye-sensitized solar cells. Because of their technological importance, a molecular-level understanding of the mechanism for supramolecular structure formation in a liquid, as well as their stability under ultraviolet (UV) irradiation, is important. Here, we observed the self-assembled structure of free-base, copper(II), and nickel(II) octaethylporphyrin formed on Au(111) in a dodecane solution using scanning tunneling microscopy (STM). As evident in the STM images, the self-assembled monolayers (SAMs) of these three porphyrins on the Au(111) surface showed hexagonal close-packed structures when in dodecane solution. Under UV irradiation (λ = 365 nm), the porphyrin molecules in the SAM or the dodecane solution move extensively and form new porphyrin clusters on the Au sites that have a high degree of freedom. Consequently, the Au(111) surface was covered with disordered porphyrin clusters. However, we found that the porphyrin molecules decomposed under UV irradiation at 254 nm. Molecular-scale observation of the morphological evolution of the porphyrin SAM under UV irradiation can provide a fundamental understanding of the degradation processes of porphyrin-based energy conversion devices.

An efficient Monte Carlo algorithm for the fast equilibration and atomistic simulation of alkanethiol self-assembled monolayers on a Au(111) substrate.

PubMed

Alexiadis, Orestis; Daoulas, Kostas Ch; Mavrantzas, Vlasis G

2008-01-31

A new Monte Carlo algorithm is presented for the simulation of atomistically detailed alkanethiol self-assembled monolayers (R-SH) on a Au(111) surface. Built on a set of simpler but also more complex (sometimes nonphysical) moves, the new algorithm is capable of efficiently driving all alkanethiol molecules to the Au(111) surface, thereby leading to full surface coverage, irrespective of the initial setup of the system. This circumvents a significant limitation of previous methods in which the simulations typically started from optimally packed structures on the substrate close to thermal equilibrium. Further, by considering an extended ensemble of configurations each one of which corresponds to a different value of the sulfur-sulfur repulsive core potential, sigmass, and by allowing for configurations to swap between systems characterized by different sigmass values, the new algorithm can adequately simulate model R-SH/Au(111) systems for values of sigmass ranging from 4.25 A corresponding to the Hautman-Klein molecular model (J. Chem. Phys. 1989, 91, 4994; 1990, 93, 7483) to 4.97 A corresponding to the Siepmann-McDonald model (Langmuir 1993, 9, 2351), and practically any chain length. Detailed results are presented quantifying the efficiency and robustness of the new method. Representative simulation data for the dependence of the structural and conformational properties of the formed monolayer on the details of the employed molecular model are reported and discussed; an investigation of the variation of molecular organization and ordering on the Au(111) substrate for three CH3-(CH2)n-SH/Au(111) systems with n=9, 15, and 21 is also included.

Ab initio molecular dynamics calculations on scattering of hyperthermal H atoms from Cu(111) and Au(111).

PubMed

Kroes, Geert-Jan; Pavanello, Michele; Blanco-Rey, María; Alducin, Maite; Auerbach, Daniel J

2014-08-07

Energy loss from the translational motion of an atom or molecule impinging on a metal surface to the surface may determine whether the incident particle can trap on the surface, and whether it has enough energy left to react with another molecule present at the surface. Although this is relevant to heterogeneous catalysis, the relative extent to which energy loss of hot atoms takes place to phonons or electron-hole pair (ehp) excitation, and its dependence on the system's parameters, remain largely unknown. We address these questions for two systems that present an extreme case of the mass ratio of the incident atom to the surface atom, i.e., H + Cu(111) and H + Au(111), by presenting adiabatic ab initio molecular dynamics (AIMD) predictions of the energy loss and angular distributions for an incidence energy of 5 eV. The results are compared to the results of AIMDEFp calculations modeling energy loss to ehp excitation using an electronic friction ("EF") model applied to the AIMD trajectories, so that the energy loss to the electrons is calculated "post" ("p") the computation of the AIMD trajectory. The AIMD calculations predict average energy losses of 0.38 eV for Cu(111) and 0.13-0.14 eV for Au(111) for H-atoms that scatter from these surfaces without penetrating the surface. These energies closely correspond with energy losses predicted with Baule models, which is suggestive of structure scattering. The predicted adiabatic integral energy loss spectra (integrated over all final scattering angles) all display a lowest energy peak at an energy corresponding to approximately 80% of the average adiabatic energy loss for non-penetrative scattering. In the adiabatic limit, this suggests a way of determining the approximate average energy loss of non-penetratively scattered H-atoms from the integral energy loss spectrum of all scattered H-atoms. The AIMDEFp calculations predict that in each case the lowest energy loss peak should show additional energy loss in the

Ab initio molecular dynamics calculations on scattering of hyperthermal H atoms from Cu(111) and Au(111)

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

Kroes, Geert-Jan, E-mail: g.j.kroes@chem.leidenuniv.nl; Pavanello, Michele; Blanco-Rey, María

2014-08-07

Energy loss from the translational motion of an atom or molecule impinging on a metal surface to the surface may determine whether the incident particle can trap on the surface, and whether it has enough energy left to react with another molecule present at the surface. Although this is relevant to heterogeneous catalysis, the relative extent to which energy loss of hot atoms takes place to phonons or electron-hole pair (ehp) excitation, and its dependence on the system's parameters, remain largely unknown. We address these questions for two systems that present an extreme case of the mass ratio of themore » incident atom to the surface atom, i.e., H + Cu(111) and H + Au(111), by presenting adiabatic ab initio molecular dynamics (AIMD) predictions of the energy loss and angular distributions for an incidence energy of 5 eV. The results are compared to the results of AIMDEFp calculations modeling energy loss to ehp excitation using an electronic friction (“EF”) model applied to the AIMD trajectories, so that the energy loss to the electrons is calculated “post” (“p”) the computation of the AIMD trajectory. The AIMD calculations predict average energy losses of 0.38 eV for Cu(111) and 0.13-0.14 eV for Au(111) for H-atoms that scatter from these surfaces without penetrating the surface. These energies closely correspond with energy losses predicted with Baule models, which is suggestive of structure scattering. The predicted adiabatic integral energy loss spectra (integrated over all final scattering angles) all display a lowest energy peak at an energy corresponding to approximately 80% of the average adiabatic energy loss for non-penetrative scattering. In the adiabatic limit, this suggests a way of determining the approximate average energy loss of non-penetratively scattered H-atoms from the integral energy loss spectrum of all scattered H-atoms. The AIMDEFp calculations predict that in each case the lowest energy loss peak should show additional

Enhanced photoelectrocatalytic performance of α-Fe2O3 thin films by surface plasmon resonance of Au nanoparticles coupled with surface passivation by atom layer deposition of Al2O3.

PubMed

Liu, Yuting; Xu, Zhen; Yin, Min; Fan, Haowen; Cheng, Weijie; Lu, Linfeng; Song, Ye; Ma, Jing; Zhu, Xufei

2015-12-01

The short lifetime of photogenerated charge carriers of hematite (α-Fe2O3) thin films strongly hindered the PEC performances. Herein, α-Fe2O3 thin films with surface nanowire were synthesized by electrodeposition and post annealing method for photoelectrocatalytic (PEC) water splitting. The thickness of the α-Fe2O3 films can be precisely controlled by adjusting the duration of the electrodeposition. The Au nanoparticles (NPs) and Al2O3 shell by atom layer deposition were further introduced to modify the photoelectrodes. Different constructions were made with different deposition orders of Au and Al2O3 on Fe2O3 films. The Fe2O3-Au-Al2O3 construction shows the best PEC performance with 1.78 times enhancement by localized surface plasmon resonance (LSPR) of NPs in conjunction with surface passivation of Al2O3 shells. Numerical simulation was carried out to investigate the promotion mechanisms. The high PEC performance for Fe2O3-Au-Al2O3 construction electrode could be attributed to the Al2O3 intensified LSPR, effective surface passivation by Al2O3 coating, and the efficient charge transfer due to the Fe2O3-Au Schottky junctions.

Six-dimensional quantum dynamics study for the dissociative adsorption of DCl on Au(111) surface

NASA Astrophysics Data System (ADS)

Liu, Tianhui; Fu, Bina; Zhang, Dong H.

2014-04-01

We carried out six-dimensional quantum dynamics calculations for the dissociative adsorption of deuterium chloride (DCl) on Au(111) surface using the initial state-selected time-dependent wave packet approach. The four-dimensional dissociation probabilities are also obtained with the center of mass of DCl fixed at various sites. These calculations were all performed based on an accurate potential energy surface recently constructed by neural network fitting to density function theory energy points. The origin of the extremely small dissociation probability for DCl/HCl (v = 0, j = 0) fixed at the top site compared to other fixed sites is elucidated in this study. The influence of vibrational excitation and rotational orientation of DCl on the reactivity was investigated by calculating six-dimensional dissociation probabilities. The vibrational excitation of DCl enhances the reactivity substantially and the helicopter orientation yields higher dissociation probability than the cartwheel orientation. The site-averaged dissociation probability over 25 fixed sites obtained from four-dimensional quantum dynamics calculations can accurately reproduce the six-dimensional dissociation probability.

Six-dimensional quantum dynamics study for the dissociative adsorption of DCl on Au(111) surface

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

Liu, Tianhui; Fu, Bina, E-mail: bina@dicp.ac.cn, E-mail: zhangdh@dicp.ac.cn; Zhang, Dong H., E-mail: bina@dicp.ac.cn, E-mail: zhangdh@dicp.ac.cn

We carried out six-dimensional quantum dynamics calculations for the dissociative adsorption of deuterium chloride (DCl) on Au(111) surface using the initial state-selected time-dependent wave packet approach. The four-dimensional dissociation probabilities are also obtained with the center of mass of DCl fixed at various sites. These calculations were all performed based on an accurate potential energy surface recently constructed by neural network fitting to density function theory energy points. The origin of the extremely small dissociation probability for DCl/HCl (v = 0, j = 0) fixed at the top site compared to other fixed sites is elucidated in this study. The influence of vibrational excitationmore » and rotational orientation of DCl on the reactivity was investigated by calculating six-dimensional dissociation probabilities. The vibrational excitation of DCl enhances the reactivity substantially and the helicopter orientation yields higher dissociation probability than the cartwheel orientation. The site-averaged dissociation probability over 25 fixed sites obtained from four-dimensional quantum dynamics calculations can accurately reproduce the six-dimensional dissociation probability.« less

Self-organization of S adatoms on Au(111): √3R30° rows at low coverage

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

Walen, Holly, E-mail: hwalen@iastate.edu; Liu, Da-Jiang; Oh, Junepyo

Using scanning tunneling microscopy, we observe an adlayer structure that is dominated by short rows of S atoms, on unreconstructed regions of a Au(111) surface. This structure forms upon adsorption of low S coverage (less than 0.1 monolayer) on a fully reconstructed clean surface at 300 K, then cooling to 5 K for observation. The rows adopt one of three orientations that are rotated by 30° from the close-packed directions of the Au(111) substrate, and adjacent S atoms in the rows are separated by √3 times the surface lattice constant, a. Monte Carlo simulations are performed on lattice-gas models, derivedmore » using a limited cluster expansion based on density functional theory energetics. Models which include long-range pairwise interactions (extending to 5a), plus selected trio interactions, successfully reproduce the linear rows of S atoms at reasonable temperatures.« less

Self-organization of S adatoms on Au(111): √3R30° rows at low coverage.

PubMed

Walen, Holly; Liu, Da-Jiang; Oh, Junepyo; Lim, Hyunseob; Evans, J W; Kim, Yousoo; Thiel, P A

2015-07-07

Using scanning tunneling microscopy, we observe an adlayer structure that is dominated by short rows of S atoms, on unreconstructed regions of a Au(111) surface. This structure forms upon adsorption of low S coverage (less than 0.1 monolayer) on a fully reconstructed clean surface at 300 K, then cooling to 5 K for observation. The rows adopt one of three orientations that are rotated by 30° from the close-packed directions of the Au(111) substrate, and adjacent S atoms in the rows are separated by √3 times the surface lattice constant, a. Monte Carlo simulations are performed on lattice-gas models, derived using a limited cluster expansion based on density functional theory energetics. Models which include long-range pairwise interactions (extending to 5a), plus selected trio interactions, successfully reproduce the linear rows of S atoms at reasonable temperatures.

Surface enhanced Raman scattering activity of dual-functional Fe3O4/Au composites

NASA Astrophysics Data System (ADS)

Wang, Li-Ping; Huang, Yu-Bin; Lai, Ying-Huang

2018-03-01

There is a high demand for multifunctional materials that can integrate sample collection and sensing. In this study, magnetic Fe3O4 clusters were fabricated using a simple solvent-thermal method. The effect of the reductant (sodium citrate, SC) on the structure and morphology of Fe3O4 was examined by the variation in the reagent amount. The resulting Fe3O4 clusters were functionalized with 3-aminopropyltriethoxysilane (APTES) to anchor Au nanoparticles to its surface. The fabricated composites were characterized by X-ray diffraction (XRD), transmission electron microscopy (TEM), and a superconducting quantum interference device (SQUID) magnetometer. Dual-functional Fe3O4/Au clusters were obtained, effectively combining magnetic and plasmonic optical properties. The magnetic Fe3O4 cluster cores permitted the adsorption of the probe molecules, while sample concentration and collection were carried out under an external magnetic field. In addition, 4-nitrothiophenol (4-NTP) was chosen as the probe molecule to examine the analyte concentration ability and surface-enhanced Raman scattering (SERS) activity of the Fe3O4/Au composites. The results indicated that the Fe3O4/Au clusters exhibit a prominent SERS effect. The best 4-NTP detection limit obtained was 1 × 10-8 M, with a corresponding SERS analytical enhancement factor (AEF) exceeding 2 × 105.

Molecular layers of ZnPc and FePc on Au(111) surface: Charge transfer and chemical interaction

NASA Astrophysics Data System (ADS)

Ahmadi, Sareh; Shariati, M. Nina; Yu, Shun; Göthelid, Mats

2012-08-01

We have studied zinc phthalocyanine (ZnPc) and iron phthalocyanine (FePc) thick films and monolayers on Au(111) using photoelectron spectroscopy and x-ray absorption spectroscopy. Both molecules are adsorbed flat on the surface at monolayer. ZnPc keeps this orientation in all investigated coverages, whereas FePc molecules stand up in the thick film. The stronger inter-molecular interaction of FePc molecules leads to change of orientation, as well as higher conductivity in FePc layer in comparison with ZnPc, which is reflected in thickness-dependent differences in core-level shifts. Work function changes indicate that both molecules donate charge to Au; through the π-system. However, the Fe3d derived lowest unoccupied molecular orbital receives charge from the substrate when forming an interface state at the Fermi level. Thus, the central atom plays an important role in mediating the charge, but the charge transfer as a whole is a balance between the two different charge transfer channels; π-system and the central atom.

Scanning Tunneling Microscopy Study of Carbon Tetrachloride Adsorption and Degradation on a Natural a-Fe2O3(0001) Surface in Ultrahigh Vacuum

NASA Astrophysics Data System (ADS)

Taeg Rim, Kwang; Fitts, Jeffrey; Adib, Kaveh; Camillone, Nicholas, III; Schlosser, Peter; Osgood, Richard, Jr.; Flynn, George; Joyce, Stephen

2001-03-01

Scanning tunneling microscopy and low energy electron diffraction have been used to study a natural a-Fe2O3(0001) surface and the adsorption and degradation of carbon tetrachloride on the reduced Fe3O4(111) terminated surface. A natural a-Fe2O3 (0001) surface was prepared by repeated cycles of Ar+ ion sputtering and annealing in vacuum or in O2 at 850 K. STM images and a LEED pattern indicate that an Fe3O4(111) terminated surface and a bi-phase can be formed depending on annealing conditions. The Fe3O4(111) terminated surface was dosed with CCl4 at room temperature, and flashed up to 590 K and 850 K. STM images show adsorbates on the surface at room temperature and the degradation products of CCl4 are isolated on the surface as the flashing temperature increases up to 850 K. Results from a companion temperature programmed desorption investigation are used in conjunction with the STM images to propose site specific reactions of CCl4 on the Fe3O4(111) terminated surface.

Effect of bottom electrode on dielectric property of sputtered-(Ba,Sr)TiO{sub 3} films

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

Ito, Shinichi; Yamada, Tomoaki; Takahashi, Kenji

2009-03-15

(Ba{sub 0.5}Sr{sub 0.5})TiO{sub 3} (BST) films were deposited on (111)Pt/TiO{sub 2}/SiO{sub 2}/Al{sub 2}O{sub 3} substrates by rf sputtering. By inserting a thin layer of SrRuO{sub 3} in between BST film and (111)Pt electrode, the BST films grew fully (111)-oriented without any other orientations. In addition, it enables us to reduce the growth temperature of BST films while keeping the dielectric constant and tunability as high as those of BST films directly deposited on Pt at higher temperatures. The dielectric loss of the films on SrRuO{sub 3}-top substrates was comparable to that on Pt-top substrates for the same level of dielectricmore » constant. The results suggest that the SrRuO{sub 3} thin layer on (111)Pt electrode is an effective approach to growing highly crystalline BST films with (111) orientation at lower deposition temperatures.« less

Frictional and morphological properties of Au-MoS2 films sputtered from a compact target

NASA Technical Reports Server (NTRS)

Spalvins, T.

1984-01-01

AuMoS2 films 0.02 to 1.2 microns thick were sputtered from target compacted from 5 wt % Au + 95 wt % MoS2, to investigate the frictional and morphological film growth characteristics. The gold dispersion effects in MoS2 films are of interest to increase the densitification and strengthening of the film structure. Three microstructural growth stages were identified on the nano-micro-macrostructural level. During sliding both sputtered Au-MoS2 and MoS2 films have a tendency to break within the columner region. The remaining or effective film, about 0.2 microns thick, performs the lubrication. The Au-MoS2 films displayed a lower friction coefficient with a high degree of frictional stability and less wear debris generation as compared to pure MoS2 films. The more favorable frictional characteristics of the Au-MoS2 films are attributed to the effective film thickness and the high density packed columner zone which has a reduced effect on the fragmentation of the tapered crystallites during fracture.

DFT studies of elemental mercury oxidation mechanism by gaseous advanced oxidation method: Co-interaction with H2O2 on Fe3O4 (111) surface

NASA Astrophysics Data System (ADS)

Zhou, Changsong; Song, Zijian; Zhang, Zhiyue; Yang, Hongmin; Wang, Ben; Yu, Jie; Sun, Lushi

2017-12-01

Density functional theory calculations have been carried out for H2O2 and Hg0 co-interaction on Fe3O4 (111) surface. On the Fetet1-terminated Fe3O4 (111) surface, the most favored configurations are H2O2 decomposition and produce two OH groups, which have strong interaction with Hg atom to form an OHsbnd Hgsbnd OH intermediate. The adsorbed OHsbnd Hgsbnd OH is stable and hardly detaches from the catalyst surface due to the highly endothermic process. A large amount of electron transfer has been found from Hg to the produced OH groups and has little irreversible effect on the Fe3O4 (111) surface. On the Feoct2-terminated Fe3O4 (111) surface, the Feoct2 site is more active than Fetet1 site. H2O2 decomposition and Hg0 oxidation processes are more likely to occur due to that the Feoct2 site both contains Fe2+ and Fe3+ cations. The calculations reveal that Hg0 oxidation by the OH radical produced from H2O2 is energetically favored. Additionally, Hg0 and H2O2 co-interaction mechanism on the Fe3O4 (111) interface has been investigated on the basis of partial local density of state calculation.

Density functional theory study on the metal-support interaction between a Au9 cluster and an anatase TiO2(001) surface.

PubMed

Jiang, Zong-You; Zhao, Zong-Yan

2017-08-23

Noble metals supported on TiO 2 surfaces have shown extraordinary photocatalytic properties in many important processes such as hydrogenation, water splitting, degradation of hazards, and so on. Using density functional theory calculations, this work has systematically investigated the microstructure and electronic structure of three different Au 9 isomers loaded on anatase TiO 2 (001) surface. The calculated results show that the interaction between the Au 9 cluster and the TiO 2 support is closely related to the adsorption site and the stability of the Au 9 cluster in the gas phase. The adsorption energy of the 2D configuration is larger than that of the 3D configuration of the Au 9 cluster, owing to the stronger interactions between more adsorption sites. The stable adsorption site for Au 9 clusters deposited on the anatase TiO 2 (001) surface tends to be the O 2c -O 2c hollow site. The presentation of the MIGS of the Au 9 cluster, the disappearance of surface states of the TiO 2 (001) surface, and the shifting of the Fermi level from the top of the valence band to the bottom of the conduction band suggest strong interactions between the Au 9 clusters and the TiO 2 (001) surface. Importantly, the electron transfer from the Au 9 clusters to the TiO 2 support occurs mainly through Au-O 2c interactions, which are mainly localized at the contact layer of the Au 9 clusters. These conclusions are useful to understand various physical and chemical properties of noble metal clusters loaded onto an oxide surface, and helpful to design novel metal/semiconductor functional composite materials and devices.

Visualizing Electric Fields at Au(111) Step Edges via Tip-Enhanced Raman Scattering

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

None, None

Tip-enhanced Raman scattering (TERS) can be used to image plasmon-enhanced local electric fields on the nanoscale. This is illustrated through ambient TERS measurements recorded using silver atomic force microscope tips coated with 4-mercaptobenzonitrile molecules and used to image step edges on an Au(111) surface. The observed 2D TERS images uniquely map electric fields localized at Au(111) step edges following 671-nm excitation. We establish that our measurements are not only sensitive to spatial variations in the enhanced electric fields but also to their vector components. We also experimentally demonstrate that (i) few nanometer precision is attainable in TERS nanoscopy using corrugatedmore » tips with nominally radii on the order of 100-200 nm, and (ii) TERS signals do not necessarily exhibit the expected E4 dependence. Overall, we illustrate the concept of electric field imaging via TERS and establish the connections between our observations and conventional TERS chemical imaging measurements.« less

Bifunctional Ag@SiO 2 /Au Nanoparticles for Probing Sequential Catalytic Reactions by Surface-Enhanced Raman Spectroscopy

DOE PAGES

Wu, Yiren; Su, Dong; Qin, Dong

2017-02-22

Here, we report the synthesis of bifunctional Ag@SiO 2/Au nanoparticles with an “islands in the sea” configuration by titrating HAuCl 4 solution into an aqueous suspension of Ag@SiO 2 core–shell nanocubes in the presence of NaOH, ascorbic acid, and poly(vinyl pyrrolidone) at pH 11.9. The NaOH plays an essential role in generating small pores in the SiO 2 shell in situ, followed by the epitaxial deposition of Au from the Ag surface through the pores, leading to the formation of Au islands (6–12 nm in size) immersed in a SiO 2 sea. Furthermore, by controlling the amount of HAuCl 4more » titrated into the reaction system, the Au islands can be made to pass through and protrude from the SiO 2 shell, embracing catalytic activity toward the reduction of 4-nitrophenol to 4-aminophenol by NaBH4. And while the Ag in the core provides a strong surface-enhanced Raman scattering activity, the SiO 2 sea helps maintain the Au component as compact, isolated, and stabilized islands. The Ag@SiO 2/Au nanoparticles can serve as a bifunctional probe to monitor the stepwise Au-catalyzed reduction of 4-nitrothiophenol to 4-aminothiophenol by NaBH 4 and Ag-catalyzed oxidation of 4-aminothiophenol to trans-4,4'-dimercaptoazobenzene by the O 2 from air in the same reaction system.« less

Bifunctional Ag@SiO 2 /Au Nanoparticles for Probing Sequential Catalytic Reactions by Surface-Enhanced Raman Spectroscopy

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

Wu, Yiren; Su, Dong; Qin, Dong

Here, we report the synthesis of bifunctional Ag@SiO 2/Au nanoparticles with an “islands in the sea” configuration by titrating HAuCl 4 solution into an aqueous suspension of Ag@SiO 2 core–shell nanocubes in the presence of NaOH, ascorbic acid, and poly(vinyl pyrrolidone) at pH 11.9. The NaOH plays an essential role in generating small pores in the SiO 2 shell in situ, followed by the epitaxial deposition of Au from the Ag surface through the pores, leading to the formation of Au islands (6–12 nm in size) immersed in a SiO 2 sea. Furthermore, by controlling the amount of HAuCl 4more » titrated into the reaction system, the Au islands can be made to pass through and protrude from the SiO 2 shell, embracing catalytic activity toward the reduction of 4-nitrophenol to 4-aminophenol by NaBH4. And while the Ag in the core provides a strong surface-enhanced Raman scattering activity, the SiO 2 sea helps maintain the Au component as compact, isolated, and stabilized islands. The Ag@SiO 2/Au nanoparticles can serve as a bifunctional probe to monitor the stepwise Au-catalyzed reduction of 4-nitrothiophenol to 4-aminothiophenol by NaBH 4 and Ag-catalyzed oxidation of 4-aminothiophenol to trans-4,4'-dimercaptoazobenzene by the O 2 from air in the same reaction system.« less

SERS study of surface plasmon resonance induced carrier movement in Au@Cu2O core-shell nanoparticles

NASA Astrophysics Data System (ADS)

Chen, Lei; Zhang, Fan; Deng, Xin-Yu; Xue, Xiangxin; Wang, Li; Sun, Yantao; Feng, Jing-Dong; Zhang, Yongjun; Wang, Yaxin; Jung, Young Mee

2018-01-01

A plasmon induced carrier movement enhanced mechanism of surface-enhanced Raman scattering (SERS) was investigated using a charge-transfer (CT) enhancement mechanism. Here, we designed a strategy to study SERS in Au@Cu2O nanoshell nanoparticles with different shell thicknesses. Among the plasmonically coupled nanostructures, Au spheres with Cu2O shells have been of special interest due to their ultrastrong electromagnetic fields and controllable carrier transfer properties, which are useful for SERS. Au@Cu2O nanoshell nanoparticles (NPs) with shell thicknesses of 48-56 nm are synthesized that exhibit high SERS activity. This high activity originates from plasmonic-induced carrier transfer from Au@Cu2O to 4-mercaptobenzoic acid (MBA). The CT transition from the valence band (VB) of Cu2O to the second excited π-π* transition of MBA, and is of b2 electronic symmetry, which was enhanced significantly. The Herzberg-Teller selection rules were employed to predict the observed enhanced b2 symmetry modes. The system constructed in this study combines the long-range electromagnetic effect of Au NPs, localized surface plasmon resonance (LSPR) of the Au@Cu2O nanoshell, and the CT contribution to assist in understanding the SERS mechanism based on LSPR-induced carrier movement in metal/semiconductor nanocomposites.

The effect of SiO2, Pt, and Pt /Au templates on the microstructure and permittivity of BaxSr1-xTiO3 films

NASA Astrophysics Data System (ADS)

Rundqvist, Pär; Liljenfors, Tomas; Vorobiev, Andrei; Olsson, Eva; Gevorgian, Spartak

2006-12-01

Ba0.25Sr0.75TiO3 (BSTO) and SrTiO3 (STO) ferroelectric thin films were grown on templates of SiO2/Si, Pt /TiO2/SiO2/Si, and Pt /Au/Pt/TiO2/SiO2/Si using pulsed laser deposition. The microstructure and surface morphology of the multilayer stacks were studied using x-ray diffraction, atomic force microscopy, and transmission electron microscopy. The microstructural analysis shows that the ferroelectric films are polycrystalline textured with a columnar structure where the grain size is 50-100nm. The BSTO films deposited at 800°C on an amorphous SiO2/Si template reveal a textured structure with a dominant (110) orientation, which is explained by a dominant growth of BSTO (110) grains due to the lower surface energy of the (110) phase. The STO and BSTO films deposited at 650°C on the Pt /TiO2/SiO2/Si and Pt /Au/Pt/TiO2/SiO2/Si templates, respectively, reveal a structure with a dominant (111) orientation, which is explained by the dominant growth of BSTO (STO) (111) grains imposed by the underlying Pt (111) texture. In all cases the ferroelectric films are subject to compressive in-plane strain which is different for different grain orientations. Strain modified permittivities of ferroelectric films grown on different templates are calculated from first principles for different orientations and compared with measured results. The correlations between grain orientations, grain sizes, grain boundaries, strain, and dielectric permittivity of ferroelectric films on different templates are discussed.

Photothermal effects from Au-Cu2O core-shell nanocubes, octahedra, and nanobars with broad near-infrared absorption tunability

NASA Astrophysics Data System (ADS)

Wang, Hsiang-Ju; Yang, Kung-Hsun; Hsu, Shih-Chen; Huang, Michael H.

2015-12-01

Other than the display of purely optical phenomenon, the recently-discovered facet-dependent optical properties of metal-Cu2O nanocrystals have become useful by illuminating Au-Cu2O nanocubes and octahedra having a surface plasmon resonance (SPR) absorption band in the near-infrared (NIR) region from octahedral Au cores with 808 nm light for heat generation. After 5 min of light irradiation, a solution of Au-Cu2O nanocubes can reach 65 °C with their Au SPR band matching the illuminating light wavelength. Photothermal efficiency has been found to be facet-dependent. In addition, short gold nanorods were employed to synthesize {100}-bound rectangular Au-Cu2O nanobars with a tunable longitudinal Au SPR absorption band covering a broad NIR range from ~1050 to 1400 nm. Because the Au SPR bands can become fixed with relatively thin Cu2O shells of less than 15 nm, ultrasmall nanobars having a size of 61 nm directly red-shift the Au SPR band to 1047 nm. And 73 nm nanobars can give a Au SPR band at 1390 nm. Truncated nanobars exposing {100}, {110}, and {111} facets give a very blue-shifted Au SPR band. The nanobars also exhibit photothermal activity when illuminated by 1064 nm light. These small Au-Cu2O nanocrystals represent the simplest nanostructure design to absorb light covering the entire NIR wavelengths.Other than the display of purely optical phenomenon, the recently-discovered facet-dependent optical properties of metal-Cu2O nanocrystals have become useful by illuminating Au-Cu2O nanocubes and octahedra having a surface plasmon resonance (SPR) absorption band in the near-infrared (NIR) region from octahedral Au cores with 808 nm light for heat generation. After 5 min of light irradiation, a solution of Au-Cu2O nanocubes can reach 65 °C with their Au SPR band matching the illuminating light wavelength. Photothermal efficiency has been found to be facet-dependent. In addition, short gold nanorods were employed to synthesize {100}-bound rectangular Au-Cu2O nanobars

The Effects of the Location of Au Additives on Combustion-generated SnO2 Nanopowders for CO Gas Sensing

PubMed Central

Bakrania, Smitesh D.; Wooldridge, Margaret S.

2010-01-01

The current work presents the results of an experimental study of the effects of the location of gold additives on the performance of combustion-generated tin dioxide (SnO2) nanopowders in solid state gas sensors. The time response and sensor response to 500 ppm carbon monoxide is reported for a range of gold additive/SnO2 film architectures including the use of colloidal, sputtered, and combustion-generated Au additives. The opportunities afforded by combustion synthesis to affect the SnO2/additive morphology are demonstrated. The best sensor performance in terms of sensor response (S) and time response (τ) was observed when the Au additives were restricted to the outermost layer of the gas-sensing film. Further improvement was observed in the sensor response and time response when the Au additives were dispersed throughout the outermost layer of the film, where S = 11.3 and τ = 51 s, as opposed to Au localized at the surface, where S = 6.1 and τ = 60 s. PMID:22163586

Kondo effect in single cobalt phthalocyanine molecules adsorbed on Au(111) monoatomic steps

NASA Astrophysics Data System (ADS)

Zhao, Aidi; Hu, Zhenpeng; Wang, Bing; Xiao, Xudong; Yang, Jinlong; Hou, J. G.

2008-06-01

The Kondo effect in single dehydrogenated cobalt phthalocyanine (CoPc) molecules adsorbed on Au(111) monoatomic steps was studied with a low temperature scanning tunneling microscope. The CoPc molecules adsorbed on Au(111) monoatomic steps show two typical configurations, which can be dehydrogenated to reveal Kondo effect. Moreover, the Kondo temperatures (TK) measured for different molecules vary in a large range from ~150 to ~550 K, increasing monotonically with decreasing Co-Au distance. A simple model consisting of a single Co 3dz2 orbital and a Au 6s orbital is considered and gives a qualitative explanation to the dependence. The large variation of TK is attributed to the variation of the interaction between the magnetic-active cobalt ion and the Au substrate resulted from different Co-Au distances.

An accurate full-dimensional potential energy surface for H-Au(111): Importance of nonadiabatic electronic excitation in energy transfer and adsorption.

PubMed

Janke, Svenja M; Auerbach, Daniel J; Wodtke, Alec M; Kandratsenka, Alexander

2015-09-28

We have constructed a potential energy surface (PES) for H-atoms interacting with fcc Au(111) based on fitting the analytic form of the energy from Effective Medium Theory (EMT) to ab initio energy values calculated with density functional theory. The fit used input from configurations of the H-Au system with Au atoms at their lattice positions as well as configurations with the Au atoms displaced from their lattice positions. It reproduces the energy, in full dimension, not only for the configurations used as input but also for a large number of additional configurations derived from ab initio molecular dynamics (AIMD) trajectories at finite temperature. Adiabatic molecular dynamics simulations on this PES reproduce the energy loss behavior of AIMD. EMT also provides expressions for the embedding electron density, which enabled us to develop a self-consistent approach to simulate nonadiabatic electron-hole pair excitation and their effect on the motion of the incident H-atoms. For H atoms with an energy of 2.7 eV colliding with Au, electron-hole pair excitation is by far the most important energy loss pathway, giving an average energy loss ≈3 times that of the adiabatic case. This increased energy loss enhances the probability of the H-atom remaining on or in the Au slab by a factor of 2. The most likely outcome for H-atoms that are not scattered also depends prodigiously on the energy transfer mechanism; for the nonadiabatic case, more than 50% of the H-atoms which do not scatter are adsorbed on the surface, while for the adiabatic case more than 50% pass entirely through the 4 layer simulation slab.

Physisorption of three amine terminated molecules (TMBDA, BDA, TFBDA) on the Au(111) Surface: The Role of van der Waals Interaction

NASA Astrophysics Data System (ADS)

Aminpour, Maral; Le, Duy; Rahman, Talat S.

2012-02-01

Recently, the electronic properties and alignment of tetramethyl-1,4-benzenediamine (TMBDA), 1,4-benzenediamine (BDA) and tetrafluro-1,4-benzenediamine (TFBDA) molecules were studied experimentally. Discrepancies were found for both the binding energy and the molecule tilt angle with respect to the surface, when results were compared with density functional theory calculations [1]. We have included the effect of vdW interactions both between the molecules and the Au(111) surface and find binding energies which are in very good agreement with experiments. We also find that at low coverages each of these molecules would adsorb almost parallel to the surface. N-Au bond lengths and charge redistribution on adsorption of the molecules are also analyzed. Our calculations are based on DFT using vdW-DF exchange correlation functionals. For BDA (since we are aware of experimental data), we show that for higher coverage, inclusion of intermolecular van der Waals interaction leads to tilting of the molecules with respect to the surface and formation of line structures. Our results demonstrate the central role played by intermolecular interaction in pattern formation on this surface.[4pt] [1] M. Dell'Angela et al, Nano Lett. 2010, 10, 2470; M. Kamenetska et al, J. Phys. Chem. C, 2011, 115, 12625

Atomic force microscopy investigation of the kinetic growth mechanisms of sputtered nanostructured Au film on mica: towards a nanoscale morphology control

PubMed Central

2011-01-01

The study of surface morphology of Au deposited on mica is crucial for the fabrication of flat Au films for applications in biological, electronic, and optical devices. The understanding of the growth mechanisms of Au on mica allows to tune the process parameters to obtain ultra-flat film as suitable platform for anchoring self-assembling monolayers, molecules, nanotubes, and nanoparticles. Furthermore, atomically flat Au substrates are ideal for imaging adsorbate layers using scanning probe microscopy techniques. The control of these mechanisms is a prerequisite for control of the film nano- and micro-structure to obtain materials with desired morphological properties. We report on an atomic force microscopy (AFM) study of the morphology evolution of Au film deposited on mica by room-temperature sputtering as a function of subsequent annealing processes. Starting from an Au continuous film on the mica substrate, the AFM technique allowed us to observe nucleation and growth of Au clusters when annealing process is performed in the 573-773 K temperature range and 900-3600 s time range. The evolution of the clusters size was quantified allowing us to evaluate the growth exponent 〈z〉 = 1.88 ± 0.06. Furthermore, we observed that the late stage of cluster growth is accompanied by the formation of circular depletion zones around the largest clusters. From the quantification of the evolution of the size of these zones, the Au surface diffusion coefficient was evaluated in D(T) = [(7.42 × 10−13) ± (5.94 × 10−14) m2/s]exp(−(0.33±0.04) eVkT). These quantitative data and their correlation with existing theoretical models elucidate the kinetic growth mechanisms of the sputtered Au on mica. As a consequence we acquired a methodology to control the morphological characteristics of the Au film simply controlling the annealing temperature and time. PMID:24576328

Self-organization of S adatoms on Au(111): √3R30° rows at low coverage

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

Walen, Holly; Liu, Da -Jiang; Oh, Junepyo

Using scanning tunneling microscopy, we observe an adlayer structure that is dominated by short rows of S atoms, on unreconstructed regions of a Au(111) surface. This structure forms upon adsorption of low S coverage (less than 0.1 monolayer) on a fully reconstructed cleansurface at 300 K, then cooling to 5 K for observation. The rows adopt one of three orientations that are rotated by 30° from the close-packed directions of the Au(111) substrate, and adjacent S atoms in the rows are separated by √3 times the surface lattice constant, a. Monte Carlo simulations are performed on lattice-gas models, we derivedmore » using a limited cluster expansion based on density functional theory energetics. Furthermore, models which include long-range pairwise interactions (extending to 5a), plus selected trio interactions, successfully reproduce the linear rows of S atoms at reasonable temperatures.« less

Self-organization of S adatoms on Au(111): √3R30° rows at low coverage

DOE PAGES

Walen, Holly; Liu, Da -Jiang; Oh, Junepyo; ...

2015-07-06

Using scanning tunneling microscopy, we observe an adlayer structure that is dominated by short rows of S atoms, on unreconstructed regions of a Au(111) surface. This structure forms upon adsorption of low S coverage (less than 0.1 monolayer) on a fully reconstructed cleansurface at 300 K, then cooling to 5 K for observation. The rows adopt one of three orientations that are rotated by 30° from the close-packed directions of the Au(111) substrate, and adjacent S atoms in the rows are separated by √3 times the surface lattice constant, a. Monte Carlo simulations are performed on lattice-gas models, we derivedmore » using a limited cluster expansion based on density functional theory energetics. Furthermore, models which include long-range pairwise interactions (extending to 5a), plus selected trio interactions, successfully reproduce the linear rows of S atoms at reasonable temperatures.« less

The role of van der Waals interaction in the tilted binding of amine molecules to the Au(111) surface

NASA Astrophysics Data System (ADS)

Le, Duy; Aminpour, Maral; Kiejna, Adam; Rahman, Talat S.

2012-06-01

We present the results of ab initio electronic structure calculations for the adsorption characteristics of three amine molecules on Au(111), which show that the inclusion of van der Waals interactions between the isolated molecule and the surface leads in general to good agreement with experimental data on the binding energies. Each molecule, however, adsorbs with a small tilt angle (between -5 and 9°). For the specific case of 1,4-diaminobenzene (BDA) our calculations reproduce the larger tilt angle (close to 24°) measured by photoemission experiments, when intermolecular (van der Waals) interactions (for about 8% coverage) are included. These results point not only to the important contribution of van der Waals interactions to molecule-surface binding energy, but also that of intermolecular interactions, often considered secondary to that between the molecule and the surface, in determining the adsorption geometry and pattern formation.

Direct Observation of Charge Transfer at a MgO(111) Surface

NASA Astrophysics Data System (ADS)

Subramanian, A.; Marks, L. D.; Warschkow, O.; Ellis, D. E.

2004-01-01

Transmission electron diffraction (TED) combined with direct methods have been used to study the √(3)×√(3)R30° reconstruction on the polar (111) surface of MgO and refine the valence charge distribution. The surface is nonstoichiometric and is terminated by a single magnesium atom. A charge-compensating electron hole is localized in the next oxygen layer and there is a nominal charge transfer from the oxygen atoms to the top magnesium atom. The partial charges that we obtain for the surface atoms are in reasonable agreement with empirical bond-valence estimations.

Rotational homogeneity in graphene grown on Au(111)

NASA Astrophysics Data System (ADS)

Wofford, Joseph; Starodub, Elena; Walter, Andrew; Nie, Shu; Bostwick, Aaron; Bartelt, Norman; Thürmer, Konrad; Rotenberg, Eli; McCarty, Kevin; Dubon, Oscar

2012-02-01

The set of properties offered by the (111) surface of gold makes it intriguing as a platform on which to study the fundamental processes that underpin graphene growth on metals. Among these are the low carbon solubility and an interaction strength with graphene that is predicted to be smaller than most transition metals. We have investigated this synthesis process using low-energy electron microscopy and diffraction to monitor the sample surface in real time, and found that the resulting graphene film possesses a remarkable degree of rotational homogeneity. The dominant orientation of the graphene is aligned with the Au lattice, with a small minority rotated by 30 degrees. The origins of this in-plane structuring are puzzling because angularly resolved photo-emission spectroscopy and scanning tunneling microscopy experiments both suggest only a relatively small interaction between the two materials. Finally, the implications of these findings for the growth of high structural-quality graphene films are discussed.

Scanning tunneling microscopy studies of pulse deposition of dinuclear organometallic molecules on Au(111)

NASA Astrophysics Data System (ADS)

Guo, Song; Alex Kandel, S.

2008-01-01

Ultrahigh-vacuum scanning tunneling microscopy (STM) was used to study trans-[Cl(dppe)2Ru(CC)6Ru(dppe)2Cl] [abbreviated as Ru2, diphenylphosphinoethane (dppe)] on Au(111). This large organometallic molecule was pulse deposited onto the Au(111) surface under ultrahigh-vacuum (UHV) conditions. UHV STM studies on the prepared sample were carried out at room temperature and 77K in order to probe molecular adsorption and to characterize the surface produced by the pulse deposition process. Isolated Ru2 molecules were successfully imaged by STM at room temperature; however, STM images were degraded by mobile toluene solvent molecules that remain on the surface after the deposition. Cooling the sample to 77K allows the solvent molecules to be observed directly using STM, and under these conditions, toluene forms organized striped domains with regular domain boundaries and a lattice characterized by 5.3 and 2.7Å intermolecular distances. When methylene chloride is used as the solvent, it forms analogous domains on the surface at 77K. Mild annealing under vacuum causes most toluene molecules to desorb from the surface; however, this annealing process may lead to thermal degradation of Ru2 molecules. Although pulse deposition is an effective way to deposit molecules on surfaces, the presence of solvent on the surface after pulse deposition is unavoidable without thermal annealing, and this annealing may cause undesired chemical changes in the adsorbates under study. Preparation of samples using pulse deposition must take into account the characteristics of sample molecules, solvent, and surfaces.

Nanosecond laser switching of surface wettability and epitaxial integration of c-axis ZnO thin films with Si(111) substrates.

PubMed

Molaei, R; Bayati, M R; Alipour, H M; Estrich, N A; Narayan, J

2014-01-08

We have achieved integration of polar ZnO[0001] epitaxial thin films with Si(111) substrates where cubic yttria-stabilized zirconia (c-YSZ) was used as a template on a Si(111) substrate. Using XRD (θ-2θ and φ scans) and HRTEM techniques, the epitaxial relationship between the ZnO and the c-YSZ layers was shown to be [0001]ZnO || [111]YSZ and [21¯1¯0]ZnO || [1¯01](c-YSZ), where the [21¯1¯0] direction lies in the (0001) plane, and the [1¯01] direction lies in the (111) plane. Similar studies on the c-YSZ/Si interface revealed epitaxy as (111)YSZ || (111)Si and in-plane (110)YSZ || (110)Si. HRTEM micrographs revealed atomically sharp and crystallographically continuous interfaces. The ZnO epilayers were subsequently laser annealed by a single pulse of a nanosecond excimer KrF laser. It was shown that the hydrophobic behavior of the pristine sample became hydrophilic after laser treatment. XPS was employed to study the effect of laser treatment on surface stoichiometry of the ZnO epilayers. The results revealed the formation of oxygen vacancies, which are envisaged to control the observed hydrophilic behavior. Our AFM studies showed surface smoothing due to the coupling of the high energy laser beam with the surface. The importance of integration of c-axis ZnO with Si(111) substrates is emphasized using the paradigm of domain matching epitaxy on the c-YSZ[111] buffer platform along with their out-of-plane orientation, which leads to improvement of the performance of the solid-state devices. The observed ultrafast response and switching in photochemical characteristics provide new opportunities for application of ZnO in smart catalysts, sensors, membranes, DNA self-assembly and multifunctional devices.

Tuning the Kondo effect in thin Au films by depositing a thin layer of Au on molecular spin-dopants.

PubMed

Ataç, D; Gang, T; Yilmaz, M D; Bose, S K; Lenferink, A T M; Otto, C; de Jong, M P; Huskens, J; van der Wiel, W G

2013-09-20

We report on the tuning of the Kondo effect in thin Au films containing a monolayer of cobalt(II) terpyridine complexes by altering the ligand structure around the Co(2+) ions by depositing a thin Au capping layer on top of the monolayer on Au by magnetron sputtering (more energetic) and e-beam evaporation (softer). We show that the Kondo effect is slightly enhanced with respect to that of the uncapped film when the cap is deposited by evaporation, and significantly enhanced when magnetron sputtering is used. The Kondo temperature (TK) increases from 3 to 4.2/6.2 K for the evaporated/sputtered caps. X-ray absorption spectroscopy and surface-enhanced Raman spectroscopy investigation showed that the organic ligands remain intact upon Au e-beam evaporation; however, sputtering inflicts significant change in the Co(2+) electronic environment. The location of the monolayer-on the surface or embedded in the film-has a small effect. However, the damage of Co-N bonds induced by sputtering has a drastic effect on the increase of the impurity-electron interaction. This opens up the way for tuning of the magnetic impurity states, e.g. spin quantum number, binding energy with respect to the host Fermi energy, and overlap via the ligand structure around the ions.

The Improvement of Ion Plated Ag and Au Film Adherence to Si3N4 and SiC Surfaces for Increased Tribological Performance

NASA Technical Reports Server (NTRS)

Spalvins, Talivaldis

1998-01-01

A modified dc-diode plating system, utilizing a metallic screen cage as a cathode and referred as SCREEN CAGE ION PLATING (SCIP), is used to deposit Ag and Au lubricating films on Si3N4 and SiC surfaces. When deposition is performed in Ar or N2, glow discharge, the surface displays poor adhesive strength (less than 5 MPa). A dramatic increase in adhesive strength (less than 80 MPa) is achieved when plating is performed in a reactive 50% 02 + 50% Ar glow discharge. The excited/ionized oxygen species (O2(+)/O(+) in the glow discharge contribute to the oxidation of the Si3N4 or SiC surfaces as determined by X-ray Photoelectron Spectroscopy (XTS) depth profiling. The reactively sputter-oxidized S3N4 or SiC surfaces and the activated-oxidized-metastable Ag or Au species formed in the plasma cooperatively contribute to the increased adherence. As a result, the linear thermal expansion coefficient mismatch at the interface is reduced. These lubricating Ag and Au films under sliding conditions reduce the friction coefficient by a factor of 2-1/2 to 4.

STM study of C60F18 high dipole moment molecules on Au(111)

NASA Astrophysics Data System (ADS)

Bairagi, K.; Bellec, A.; Chumakov, R. G.; Menshikov, K. A.; Lagoute, J.; Chacon, C.; Girard, Y.; Rousset, S.; Repain, V.; Lebedev, A. M.; Sukhanov, L. P.; Svechnikov, N. Yu.; Stankevich, V. G.

2015-11-01

Scanning tunneling microscopy and spectroscopy studies of C60F18 molecules deposited on Au(111) are reported and compared to C60 molecules both at liquid helium temperature and room temperature (RT). Whereas adsorption and electronic properties of C60F18 single molecules were studied at low temperature (LT), self-assemblies were investigated at RT. In both cases, the fluorine atoms of the C60F18 molecules are pointed towards the surface. Individual C60F18 molecules on Au(111) have a HOMO-LUMO gap of 2.9 eV. The self-assembled islands exhibit a close-packed hexagonal lattice with amorphous borders. The comparison with C60 molecules clearly demonstrates the influence of the C60F18 electric dipole moment (EDM) on the electronic properties of single molecules and on the thermodynamics of self-assembled islands. Besides, the apparent height value of a separate molecule increases in a self-assembly environment as a result of a depolarization phenomenon.

Spontaneous desorption and phase transitions of self-assembled alkanethiol and alicyclic thiol monolayers chemisorbed on Au(111) in ultrahigh vacuum at room temperature.

PubMed

Ito, Eisuke; Kang, Hungu; Lee, Dongjin; Park, Joon B; Hara, Masahiko; Noh, Jaegeun

2013-03-15

Scanning tunneling microscopy (STM) and X-ray photoelectron spectroscopy (XPS) were used to examine the surface structure and adsorption conditions of hexanethiol (HT) and cyclohexanethiol (CHT) self-assembled monolayers (SAMs) on Au(111) as a function of storage period in ultrahigh vacuum (UHV) conditions of 3×10(-7) Pa at room temperature (RT). STM imaging revealed that after storage for 7 days, HT SAMs underwent phase transitions from c(4×2) phase to low coverage 4×√3 phase. This transition is due to a structural rearrangement of hexanethiolates that results from the spontaneous desorption of chemisorbed HT molecules on Au(111) surface. XPS measurements showed approximately 28% reduction in sulfur coverage, which indicates desorption of hexanethiolates from the surfaces. Contrary to HT SAMs, the structural order of CHT SAMs with (5×2√3)R35° phase completely disappeared after storage for 3 or 7 days. XPS results show desorption of more than 80% of the cyclohexanethiolates, even after storage for 3 days. We found that spontaneous desorption of CHT molecules on Au(111) in UHV at RT occurred quickly, whereas spontaneous desorption of HT molecules was much slower. Thermal desorption spectroscopy (TDS) results suggest CHT SAMs in UHV at RT can desorb more efficiently than HT SAMs due to formation of thiol desorption fragments that result from chemical reactions between surface hydrogen atoms and thiolates on Au(111) surfaces. This study clearly demonstrated that organic thiols chemisorbed on gold surfaces are desorbed spontaneously in UHV at RT and van der Waals interactions play an important role in determining the structural stability of thiolate SAMs in UHV. Copyright © 2012 Elsevier Inc. All rights reserved.

Thickness and surface roughness study of co-sputtered nanostructured alumina/tungsten (Al2O3/W) thin films

NASA Astrophysics Data System (ADS)

Naveen, A.; Krishnamurthy, L.; Shridhar, T. N.

2018-04-01

Tungsten (W) and Alumina (Al2O3) thin films have been developed using co-sputtering technique on SS304, Copper (Cu) and Glass slides using Direct Current magnetron sputtering (DC) and Radio Frequency (RF) magnetron sputtering methods respectively. Central Composite Design (CCD) method approach has been adopted to determine the number of experimental plans for deposition and DC power, RF power and Argon gas flow rate have been input parameters, each at 5 levels for development of thin films. In this research paper, study has been carried out determine the optimized condition of deposition parameters for thickness and surface roughness of the thin films. Thickness and average Surface roughness in terms of nanometer (nm) have been characterized by thickness profilometer and atomic force microscopy respectively. The maximum and minimum average thickness observed to be 445 nm and 130 respectively. The optimum deposition condition for W/Al2O3 thin film growth was determined to be at 1000 watts of DC power and 800 watts of RF power, 20 minutes of deposition time, and almost 300 Standard Cubic Centimeter(SCCM) of Argon gas flow. It was observed that average roughness difference found to be less than one nanometer on SS substrate and one nanometer on copper approximately.

Secondary ion mass spectroscopy study of Au trapping and migration in the Au-irradiated YBa2Cu3O7 - delta film

NASA Astrophysics Data System (ADS)

Li, Yupu; Kilner, J. A.; Liu, J. R.; Chu, W. K.; Wagner, G. A.; Somekh, R. E.

1996-05-01

The range data and migration of Au in YBa2Cu3O7-δ film were studied with implanted 197Au (1.5 MeV 5×1015 Au+/cm2) as a tracer. The film was a c-axis oriented film, ˜750 nm thick, deposited by high-pressure planar dc sputtering on <100> LaAlO3. Analysis by secondary ion mass spectroscopy shows that the as-implanted Au concentration distribution is essentially Gaussian-like and the depth (R̂p) of maximum Au concentration (˜1.2 wt %) is 201 nm. The projected range (R¯p) and (R̂p) are found to be in very good agreement with the simulated data by TRIM-95, whereas the measured ``straggle'' (ΔRp*) is about 20% larger than that by TRIM-95 simulation. It has also been found that the implanted 197Au starts to migrate within the film at a temperature between 650 and 700 °C, which is much higher than that for the implanted 2H (˜175 °C) and the implanted 18O (between 250 and 300 °C) in c-oriented YBa2Cu3O7-δ films.

Self-assembled growth of Au islands on a Mo(110) surface.

PubMed

Wawro, A; Sobańska, M; Petroutchik, A; Baczewski, L T; Pankowski, P

2010-08-20

The self-assembled growth of epitaxial Au(111) islands on a Mo(110) buffer layer has been investigated as a function of growth temperature and amount of deposited material by reflection high energy electron diffraction and atomic force microscopy. At the growth temperature of 385 degrees C the dendrite-shaped islands coexist with the compact ones. The uniform islands formed at 500 degrees C adopt mostly a shape of truncated pyramids with a well developed (111) top plane and {111} and {100} side facets. As the growth temperature reaches 800 degrees C the Au islands take less regular shapes due to occurrence of coalescence. The averaged area and height of the islands increase with the deposition temperature and the amount of deposited material. The surface density of the islands decreases with increasing temperature. The epitaxial relations at the interface between the Au islands and the Mo buffer determined from the angular dependence of the electron diffraction pattern favour the Nishiyama-Wassermann growth mode. Factors responsible for the island-like growth and possible mechanisms of diffusion are discussed in details.

Optimization of nanocomposite Au/TiO2 thin films towards LSPR optical-sensing

NASA Astrophysics Data System (ADS)

Rodrigues, M. S.; Costa, D.; Domingues, R. P.; Apreutesei, M.; Pedrosa, P.; Martin, N.; Correlo, V. M.; Reis, R. L.; Alves, E.; Barradas, N. P.; Sampaio, P.; Borges, J.; Vaz, F.

2018-04-01

Nanomaterials based on Localized Surface Plasmon Resonance (LSPR) phenomena are revealing to be an important solution for several applications, namely those of optical biosensing. The main reasons are mostly related to their high sensitivity, with label-free detection, and to the simplified optical systems that can be implemented. For the present work, the optical sensing capabilities were tailored by optimizing LSPR absorption bands of nanocomposite Au/TiO2 thin films. These were grown by reactive DC magnetron sputtering. The main deposition parameters changed were the number of Au pellets placed in the Ti target, the deposition time, and DC current applied to the Ti-Au target. Furthermore, the Au NPs clustering, a key feature to have biosensing responses, was induced by several post-deposition in-air annealing treatments at different temperatures, and investigated via SEM analysis. Results showed that the Au/TiO2 thin films with a relatively low thickness (∼100 nm), revealing concentrations of Au close to 13 at.%, and annealed at temperatures above 600 °C, had the most well-defined LSPR absorption band and thus, the most promising characteristics to be explored as optical sensors. The NPs formation studies revealed an incomplete aggregation at 300 and 500 ⁰C and well-defined spheroidal NPs for higher temperatures. Plasma treatment with Ar led to a gradual blue-shift of the LSPR absorption band, which demonstrates the sensitivity of the films to changes in the dielectric environment surrounding the NPs (essential for optical sensing applications) and the exposure of the Au nanoparticles (crucial for a higher sensitivity).

Synthesis of Epitaxial Single-Layer MoS2 on Au(111).

PubMed

Grønborg, Signe S; Ulstrup, Søren; Bianchi, Marco; Dendzik, Maciej; Sanders, Charlotte E; Lauritsen, Jeppe V; Hofmann, Philip; Miwa, Jill A

2015-09-08

We present a method for synthesizing large area epitaxial single-layer MoS2 on the Au(111) surface in ultrahigh vacuum. Using scanning tunneling microscopy and low energy electron diffraction, the evolution of the growth is followed from nanoscale single-layer MoS2 islands to a continuous MoS2 layer. An exceptionally good control over the MoS2 coverage is maintained using an approach based on cycles of Mo evaporation and sulfurization to first nucleate the MoS2 nanoislands and then gradually increase their size. During this growth process the native herringbone reconstruction of Au(111) is lifted as shown by low energy electron diffraction measurements. Within the MoS2 islands, we identify domains rotated by 60° that lead to atomically sharp line defects at domain boundaries. As the MoS2 coverage approaches the limit of a complete single layer, the formation of bilayer MoS2 islands is initiated. Angle-resolved photoemission spectroscopy measurements of both single and bilayer MoS2 samples show a dramatic change in their band structure around the center of the Brillouin zone. Brief exposure to air after removing the MoS2 layer from vacuum is not found to affect its quality.

Surface Plasmon Enhanced Photocatalysis of Au/Pt-decorated TiO2 Nanopillar Arrays

NASA Astrophysics Data System (ADS)

Shuang, Shuang; Lv, Ruitao; Xie, Zheng; Zhang, Zhengjun

2016-05-01

The low quantum yields and lack of visible light utilization hinder the practical application of TiO2 in high-performance photocatalysis. Herein, we present a design of TiO2 nanopillar arrays (NPAs) decorated with both Au and Pt nanoparticles (NPs) directly synthesized through successive ion layer adsorption and reaction (SILAR) at room temperature. Au/Pt NPs with sizes of ~4 nm are well-dispersed on the TiO2 NPAs as evidenced by electron microscopic analyses. The present design of Au/Pt co-decoration on the TiO2 NPAs shows much higher visible and ultraviolet (UV) light absorption response, which leads to remarkably enhanced photocatalytic activities on both the dye degradation and photoelectrochemical (PEC) performance. Its photocatalytic reaction efficiency is 21 and 13 times higher than that of pure TiO2 sample under UV-vis and visible light, respectively. This great enhancement can be attributed to the synergy of electron-sink function of Pt and surface plasmon resonance (SPR) of Au NPs, which significantly improves charge separation of photoexcited TiO2. Our studies demonstrate that through rational design of composite nanostructures one can harvest visible light through the SPR effect to enhance the photocatalytic activities initiated by UV-light, and thus realize more effectively utilization of the whole solar spectrum for energy conversion.

Morphology of size-selected Ptn clusters on CeO2(111)

NASA Astrophysics Data System (ADS)

Shahed, Syed Mohammad Fakruddin; Beniya, Atsushi; Hirata, Hirohito; Watanabe, Yoshihide

2018-03-01

Supported Pt catalysts and ceria are well known for their application in automotive exhaust catalysts. Size-selected Pt clusters supported on a CeO2(111) surface exhibit distinct physical and chemical properties. We investigated the morphology of the size-selected Ptn (n = 5-13) clusters on a CeO2(111) surface using scanning tunneling microscopy at room temperature. Ptn clusters prefer a two-dimensional morphology for n = 5 and a three-dimensional (3D) morphology for n ≥ 6. We further observed the preference for a 3D tri-layer structure when n ≥ 10. For each cluster size, we quantitatively estimated the relative fraction of the clusters for each type of morphology. Size-dependent morphology of the Ptn clusters on the CeO2(111) surface was attributed to the Pt-Pt interaction in the cluster and the Pt-O interaction between the cluster and CeO2(111) surface. The results obtained herein provide a clear understanding of the size-dependent morphology of the Ptn clusters on a CeO2(111) surface.

Morphology of size-selected Ptn clusters on CeO2(111).

PubMed

Shahed, Syed Mohammad Fakruddin; Beniya, Atsushi; Hirata, Hirohito; Watanabe, Yoshihide

2018-03-21

Supported Pt catalysts and ceria are well known for their application in automotive exhaust catalysts. Size-selected Pt clusters supported on a CeO 2 (111) surface exhibit distinct physical and chemical properties. We investigated the morphology of the size-selected Pt n (n = 5-13) clusters on a CeO 2 (111) surface using scanning tunneling microscopy at room temperature. Pt n clusters prefer a two-dimensional morphology for n = 5 and a three-dimensional (3D) morphology for n ≥ 6. We further observed the preference for a 3D tri-layer structure when n ≥ 10. For each cluster size, we quantitatively estimated the relative fraction of the clusters for each type of morphology. Size-dependent morphology of the Pt n clusters on the CeO 2 (111) surface was attributed to the Pt-Pt interaction in the cluster and the Pt-O interaction between the cluster and CeO 2 (111) surface. The results obtained herein provide a clear understanding of the size-dependent morphology of the Pt n clusters on a CeO 2 (111) surface.

Method of sputter etching a surface

DOEpatents

Henager, Jr., Charles H.

1984-01-01

The surface of a target is textured by co-sputter etching the target surface with a seed material adjacent thereto, while the target surface is maintained at a pre-selected temperature. By pre-selecting the temperature of the surface while sputter etching, it is possible to predetermine the reflectance properties of the etched surface. The surface may be textured to absorb sunlight efficiently and have minimal emittance in the infrared region so as to be well-suited for use as a solar absorber for photothermal energy conversion.

Influence of Cu, Au and Ag on structural and surface properties of bioactive coatings based on titanium.

PubMed

Wojcieszak, D; Mazur, M; Kalisz, M; Grobelny, M

2017-02-01

In this work influence of copper, silver and gold additives on structural and surface properties of biologically active thin films based on titanium have been described. Coatings were prepared by magnetron sputtering method. During each process metallic discs (targets) - Ti and the additive (Cu, Ag or Au) were co-sputtered in argon atmosphere. Structural investigation of as-deposited coatings was performed with the aid of XRD and SEM/EDS method. It was found that all prepared thin films were homogenous. Addition of Cu, Ag and Au resulted in nanocrystalline structure. Moreover, influence of these additives on hardness and antibacterial activity of titanium coatings was also studied. Ti-Cu, Ti-Ag and Ti-Au films had lower hardness as-compared to Ti. According to AAS results the difference of their activity was related to the ion migration process. It was found that Ti-Ag and Ti-Au coatings had biocidal effect related to direct contact of their surface with microorganisms. In the case of Ti-Cu antimicrobial activity had direct and indirect nature due to efficient ion migration process from the film surface to the surrounding environment. Functional features of coatings such as wettability and corrosion resistance were also examined and included in the comprehensive analysis. Copyright © 2016 Elsevier B.V. All rights reserved.

Role of copper/vanadium on the optoelectronic properties of reactive RF magnetron sputtered NiO thin films

NASA Astrophysics Data System (ADS)

Panneerselvam, Vengatesh; Chinnakutti, Karthik Kumar; Thankaraj Salammal, Shyju; Soman, Ajith Kumar; Parasuraman, Kuppusami; Vishwakarma, Vinita; Kanagasabai, Viswanathan

2018-04-01

In this study, pristine nickel oxide (NiO), copper-doped NiO (Cu-NiO) and vanadium-doped NiO (V-NiO) thin films were deposited using reactive RF magnetron co-sputtering as a function of dopant sputtering power. Cu (0-8 at%) and V (0-1 at%) were doped into the NiO lattice by varying the sputtering power of Cu and V in the range of 5-15 W. The effect of dopant concentration on optoelectronic behavior is investigated by UV-Vis-NIR spectrophotometer and Hall measurements. XRD analysis showed that the preferred orientation of the cubic phase for undoped NiO changes from (200) to (111) plane when the sputtering parameters are varied. The observed changes in the lattice parameters and bonding states of the doped NiO indicate the substitution of Ni ions by monovalent Cu and trivalent V ions. The optical bandgap of pristine NiO, Cu-NiO, and V-NiO was found to be 3.6, 3.45, and 3.05 eV, respectively, with decreased transmittance and resistivity. Further analysis using SEM and AFM described the morphological behavior of doped NiO thin films and Raman spectroscopy indicated the structural changes on doping. These findings would be helpful in fabricating solid-state solar cells using doped NiO as efficient hole transporting material.

Molecular dynamics simulations with electronic stopping can reproduce experimental sputtering yields of metals impacted by large cluster ions

NASA Astrophysics Data System (ADS)

Tian, Jiting; Zhou, Wei; Feng, Qijie; Zheng, Jian

2018-03-01

An unsolved problem in research of sputtering from metals induced by energetic large cluster ions is that molecular dynamics (MD) simulations often produce sputtering yields much higher than experimental results. Different from the previous simulations considering only elastic atomic interactions (nuclear stopping), here we incorporate inelastic electrons-atoms interactions (electronic stopping, ES) into MD simulations using a friction model. In this way we have simulated continuous 45° impacts of 10-20 keV C60 on a Ag(111) surface, and found that the calculated sputtering yields can be very close to the experimental results when the model parameter is appropriately assigned. Conversely, when we ignore the effect of ES, the yields are much higher, just like the previous studies. We further expand our research to the sputtering of Au induced by continuous keV C60 or Ar100 bombardments, and obtain quite similar results. Our study indicates that the gap between the experimental and the simulated sputtering yields is probably induced by the ignorance of ES in the simulations, and that a careful treatment of this issue is important for simulations of cluster-ion-induced sputtering, especially for those aiming to compare with experiments.

Sputtering growth of Y3Fe5O12/Pt bilayers and spin transfer at Y3Fe5O12/Pt interfaces

NASA Astrophysics Data System (ADS)

Chang, Houchen; Liu, Tao; Reifsnyder Hickey, Danielle; Janantha, P. A. Praveen; Mkhoyan, K. Andre; Wu, Mingzhong

2017-12-01

For the majority of previous work on Y3Fe5O12 (YIG)/normal metal (NM) bi-layered structures, the YIG layers were grown on Gd3Ga5O12 first and were then capped by an NM layer. This work demonstrates the sputtering growth of a Pt/YIG structure where the Pt layer was grown first and the YIG layer was then deposited on the top. The YIG layer shows well-oriented (111) texture, a surface roughness of 0.15 nm, and an effective Gilbert damping constant less than 4.7 × 10-4, and the YIG/Pt interface allows for efficient spin transfers. This demonstration indicates the feasibility of fabricating high-quality NM/YIG/NM tri-layered structures for new physics studies.

Positronic probe of vacancy defects on surfaces of Au nanoparticles embedded in MgO

NASA Astrophysics Data System (ADS)

Xu, Jun; Moxom, J.; Somieski, B.; White, C. W.; Mills, A. P., Jr.; Suzuki, R.; Ishibashi, S.

2001-09-01

Clusters of four atomic vacancies were found in Au nanoparticle-embedded MgO by positron lifetime spectroscopy [Phys. Rev. Lett. 83, 4586 (1999)]. These clusters were also suggested to locate at the surface of Au nanoparticles by one-detector measurements of Doppler broadening of annihilation radiation. In this work we provide evidence, using two-detector coincidence experiments of Doppler broadening (2D-DBAR), to clarify that these vacancy clusters reside on the surfaces of Au nanoparticles. This work also demonstrates a method for identifying defects at nanomaterials interfaces: a combination of both positron lifetime spectroscopy, which tells the type of the defects, and 2D-DBAR measurements, which reveals chemical environment of the defects.

Method of sputter etching a surface

DOEpatents

Henager, C.H. Jr.

1984-02-14

The surface of a target is textured by co-sputter etching the target surface with a seed material adjacent thereto, while the target surface is maintained at a pre-selected temperature. By pre-selecting the temperature of the surface while sputter etching, it is possible to predetermine the reflectance properties of the etched surface. The surface may be textured to absorb sunlight efficiently and have minimal emittance in the infrared region so as to be well-suited for use as a solar absorber for photothermal energy conversion. 4 figs.

The surface plasmon-induced hot carrier effect on the catalytic activity of CO oxidation on a Cu2O/hexoctahedral Au inverse catalyst.

PubMed

Lee, Si Woo; Hong, Jong Wook; Lee, Hyunhwa; Wi, Dae Han; Kim, Sun Mi; Han, Sang Woo; Park, Jeong Young

2018-06-14

The intrinsic correlation between an enhancement of catalytic activity and the flow of hot electrons generated at metal-oxide interfaces suggests an intriguing way to control catalytic reactions and is a significant subject in heterogeneous catalysis. Here, we show surface plasmon-induced catalytic enhancement by the peculiar nanocatalyst design of hexoctahedral (HOH) Au nanocrystals (NCs) with Cu2O clusters. We found that this inverse catalyst comprising a reactive oxide for the catalytic portion and a metal as the source of electrons by localized surface plasmon resonance (localized SPR) exhibits a change in catalytic activity by direct hot electron transfer or plasmon-induced resonance energy transfer (PIRET) when exposed to light. We prepared two types of inverse catalysts, Cu2O at the vertex sites of HOH Au NCs (Cu2O/Au vertex site) and a HOH Au NC-Cu2O core-shell structure (HOH Au@Cu2O), to test the structural effect on surface plasmons. Under broadband light illumination, the Cu2O/Au vertex site catalyst showed 30-90% higher catalytic activity and the HOH Au@Cu2O catalyst showed 10-30% higher catalytic activity than when in the dark. Embedding thin SiO2 layers between the HOH Au NCs and the Cu2O verified that the dominant mechanism for the catalytic enhancement is direct hot electron transfer from the HOH Au to the Cu2O. Finite-difference time domain calculations show that a much stronger electric field was formed on the vertex sites after growing the Cu2O on the HOH Au NCs. These results imply that the catalytic activity is enhanced when hot electrons, created from photon absorption on the HOH Au metal and amplified by the presence of surface plasmons, are transferred to the reactive Cu2O.

Transition from overlayer growth to alloying growth of Ga on Si(111)-alpha-(sqrt[3]xsqrt[3])-Au.

PubMed

Yamanaka, T; Ino, S

2002-11-04

Atomic depth distribution and growth modes of Ga on an Si(111)-alpha-(sqrt[3]xsqrt[3])-Au surface at room temperature were studied after each monolayer deposition of Ga via reflection high-energy electron diffraction and characteristic x-ray spectroscopy measurements as functions of glancing angle theta(g) of the incident electron beam. One monolayer of Ga grew on the Au layer, and the sqrt[3]xsqrt[3] periodicity was conserved below the Ga overlayer. Above a critical Ga coverage of about one monolayer, this growth mode drastically changed; i.e., Au atoms dissociated from the sqrt[3]xsqrt[3] structure and Ga grew into islands of Ga-Au alloy.

Stable Au–C bonds to the substrate for fullerene-based nanostructures

PubMed Central

Chutora, Taras; Redondo, Jesús; de la Torre, Bruno; Švec, Martin

2017-01-01

We report on the formation of fullerene-derived nanostructures on Au(111) at room temperature and under UHV conditions. After low-energy ion sputtering of fullerene films deposited on Au(111), bright spots appear at the herringbone corner sites when measured using a scanning tunneling microscope. These features are stable at room temperature against diffusion on the surface. We carry out DFT calculations of fullerene molecules having one missing carbon atom to simulate the vacancies in the molecules resulting from the sputtering process. These modified fullerenes have an adsorption energy on the Au(111) surface that is 1.6 eV higher than that of C60 molecules. This increased binding energy arises from the saturation by the Au surface of the bonds around the molecular vacancy defect. We therefore interpret the observed features as adsorbed fullerene-derived molecules with C vacancies. This provides a pathway for the formation of fullerene-based nanostructures on Au at room temperature. PMID:28685108

Enhanced interface perpendicular magnetic anisotropy in electrodeposited Co/Au(111) layers

NASA Astrophysics Data System (ADS)

Cagnon, L.; Devolder, T.; Cortes, R.; Morrone, A.; Schmidt, J. E.; Chappert, C.; Allongue, P.

2001-03-01

This work investigates the structure and interface perpendicular magnetic anisotropy (PMA) of electrodeposited Cu/Co/Au(111) sandwiches with variable Co thickness [2-20 monolayers (ML's)]. In optimum deposition conditions, polar magneto-optical Kerr effect measurements show that the axis of easy magnetization is perpendicular to the layers for thicknesses below ca. 7.2 ML's. This value is among the best ever reported for the Cu/Co/Au(111) structure. While extended x-ray-absorption fine structure indicates that layers are hcp, in situ STM imaging suggests that magnetoelastic effects contribute significantly to PMA. The correlation observed between the strength of PMA and film structure is discussed in details.

Analysis of surface sputtering on a quantum statistical basis

NASA Technical Reports Server (NTRS)

Wilhelm, H. E.

1975-01-01

Surface sputtering is explained theoretically by means of a 3-body sputtering mechanism involving the ion and two surface atoms of the solid. By means of quantum-statistical mechanics, a formula for the sputtering ratio S(E) is derived from first principles. The theoretical sputtering rate S(E) was found experimentally to be proportional to the square of the difference between incident ion energy and the threshold energy for sputtering of surface atoms at low ion energies. Extrapolation of the theoretical sputtering formula to larger ion energies indicates that S(E) reaches a saturation value and finally decreases at high ion energies. The theoretical sputtering ratios S(E) for wolfram, tantalum, and molybdenum are compared with the corresponding experimental sputtering curves in the low energy region from threshold sputtering energy to 120 eV above the respective threshold energy. Theory and experiment are shown to be in good agreement.

Island morphology statistics and growth mechanism for oxidation of the Al(111) surface with thermal O2 and NO

NASA Astrophysics Data System (ADS)

Sexton, J. Z.; Kummel, A. C.

2004-10-01

Scanning tunneling microscopy (STM) was employed to study the mechanism for the oxidation of Al(111) with thermal O2 and NO in the 20%-40% monolayer coverage regime. Experiments show that the islands formed upon exposure to thermal O2 and NO have dramatically different shapes, which are ultimately dictated by the dynamics of the gas surface interaction. The circumference-to-area ratio and other island morphology statistics are used to quantify the average difference in the two island types. Ultrahigh-vacuum STM was employed to make the following observations: (1) Oxygen islands on the Al(111) surface, formed upon exposure to thermal oxygen, are elongated and noncompact. (2) Mixed O/N islands on the Al(111) surface, formed upon exposure to thermal nitric oxide (NO), are round and compact. (3) STM movies acquired during thermal O2 exposure indicate that a complex mechanism involving chemisorption initiated rearrangement of preexisting oxygen islands leads to the asymmetric and elongated island shapes. The overall mechanism for the oxidation of the Al(111) surface can be summarized in three regimes. Low coverage is dominated by widely isolated small oxygen features (<3 O atoms) where normal dissociative chemisorption and oxygen abstraction mechanisms are present. At 20%-40% monolayer coverage, additional oxygen chemisorption induces rearrangement of preexisting islands to form free-energy minimum island shapes. At greater than ˜40% monolayer coverage, the apparent surface oxygen coverage asymptotes corresponding to the conversion of the 2D islands to 3D Al2O3 surface crystallites. The rearrangement of oxygen islands on the surface to form the observed islands indicates that there is a short-range oxygen-oxygen attractive potential and a long-range oxygen-oxygen repulsive potential.

Rapid Detection of Tetrodotoxin Using Surface-Enhanced Raman Spectroscopy and Fe3O4/SiO2/Au Gold/Magnetic Nanoparticles

NASA Astrophysics Data System (ADS)

Neng, Jing; Wang, Xujun; Jia, Kan; Sun, Peilong

2018-03-01

Fe3O4 nanoparticles were first modified with tetraethoxylsilane to form Fe3O4/SiO2 nanoparticles, followed by the addition of 3-aminopropyltriethoxysilane and 3-thiolpropyltriethoxysilane to introduce -NH2 and -SH groups to the surface of Fe3O4/SiO2 nanoparticles. Gold nanoparticles were further assembled on the surface of Fe3O4/SiO2 via the electrostatic adsorption of -NH2 and the Au-S bond to produce stable core-shell Fe3O4/SiO2/Au gold/magnetic nanoparticles. These Fe3O4/SiO2/Au gold/magnetic nanoparticles were characterized by a variety of techniques such as transmission electron microscopy (TEM) and energy dispersive X-ray spectroscopy (EDX), and afterwards conjugated with tetrodotoxin antibodies (Ab) and used as a Raman active substrate (Fe3O4/SiO2/Au-Ab) with Rhodamine B (RhB)-labeled tetrodotoxin antibody as a Raman reporter (Ab-RhB). Upon mixing these reagents with tetrodotoxin (TTX), a sandwich complex [Fe3O4/SiO2/Au-Ab···TTX···Ab-RhB] was generated due to the specific antibody-antigen interactions. The immunocomplex was subsequently separated by an externally applied magnetic source and concentrated into a pellet point, where the laser interrogation of the pellet produced a strong signal characteristic of RhB. The logarithmic intensity of the signal was found to be proportional to the concentration of TTX with a limit of detection of 0.01 μg/mL and a detection linearity range of 0.01-0.5 μg/mL. The established method eliminates the complicated procedures of traditional centrifuging, column separation, and incubation and achieves a rapid detection of tetrodotoxin with improved detection sensitivity.

Solar Wind sputtering from the surface of Comet Churyumov-Gerasimenko

NASA Astrophysics Data System (ADS)

Wurz, Peter; Rubin, Martin; Altwegg, Kathrin; Balsiger, Hans; Gasc, Sébastien; Galli, André; Jäckel, Annette; Le Roy, Lena; Calmonte, Ursina; Tzou, Chia-Yu; Mall, Urs; Korth, Axel; Fiethe, Björn; De Keyser, Johan; Berthelier, Jean-Jacques; Rème, Henri; Gombosi, Tamas; Fuselier, Steven

2015-04-01

While the European Space Agency's Rosetta spacecraft is orbiting close to the comet 67P/Churyumov-Gerasimenko (67P/C-G) we performed continuous measurements of the chemical inventory of its coma with the the Rosetta Orbiter Spectrometer for Ion and Neutral Analysis (ROSINA) instrument suite. ROSINA consists of two mass spectrometers, the Double Focusing Mass Spectrometer (DFMS) and the Reflectron-type Time-Of-Flight (RTOF), as well as the COmet Pressure Sensor (COPS). Most of the observed species in the coma are volatile material that are released from the comet's surface by sublimation, for example H2O, CO, CO2 and many others. The number densities in the coma of these species show temporary variation compatible with the solar illumination (diurnal cycle), with seasonal variation (summer and winter hemispheres), and with compositional heterogeneity of the surface. We can trace back the measurements of the observed species to the cometary surface to create maps of their probable origin on the surface. In addition to the volatile material we detected atoms of Na, K, Si, S, and some more, which cannot or only partially be set free via sublimation. Again, we project these measurements down onto the surface of the comet. These maps for the sputtered atoms differ significantly from the maps for volatile species, like the water map, in some cases they are almost the opposite. Our present understanding is that these atoms are the result of solar wind sputtering of refractory material, i.e., of dust located on the cometary surface. Since the release of material from the surface by sputtering is almost stoichiometric we can infer the chemical composition of major elements for the areas affected by sputtering and the average mineralogy of these locations can be derived. The sputter signal will disappear with the comet getting closer to the Sun and becomes more active. Once the coma is dense enough, the solar wind will be absorbed by the gas layer above the surface and will

Wettability of Y2O3: A Relative Analysis of Thermally Oxidized, Reactively Sputtered and Template Assisted Nanostructured Coatings

PubMed Central

Barshilia, Harish C.; Chaudhary, Archana; Kumar, Praveen; Manikandanath, Natarajan T.

2012-01-01

The wettability of reactively sputtered Y2O3, thermally oxidized Y-Y2O3 and Cd-CdO template assisted Y2O3 coatings has been studied. The wettability of as-deposited Y2O3 coatings was determined by contact angle measurements. The water contact angles for reactively sputtered, thermally oxidized and template assisted Y2O3 nanostructured coatings were 99°, 117° and 155°, respectively. The average surface roughness values of reactively sputtered, thermally oxidized and template assisted Y2O3 coatings were determined by using atomic force microscopy and the corresponding values were 3, 11 and 180 nm, respectively. The low contact angle of the sputter deposited Y2O3 and thermally oxidized Y-Y2O3 coatings is attributed to a densely packed nano-grain like microstructure without any void space, leading to low surface roughness. A water droplet on such surfaces is mostly in contact with a solid surface relative to a void space, leading to a hydrophobic surface (low contact angle). Surface roughness is a crucial factor for the fabrication of a superhydrophobic surface. For Y2O3 coatings, the surface roughness was improved by depositing a thin film of Y2O3 on the Cd-CdO template (average roughness = 178 nm), which resulted in a contact angle greater than 150°. The work of adhesion of water was very high for the reactively sputtered Y2O3 (54 mJ/m2) and thermally oxidized Y-Y2O3 coatings (43 mJ/m2) compared to the Cd-CdO template assisted Y2O3 coating (7 mJ/m2). PMID:28348296

Seed-induced growth of flower-like Au-Ni-ZnO metal-semiconductor hybrid nanocrystals for photocatalytic applications.

PubMed

Chen, Yuanzhi; Zeng, Deqian; Cortie, Michael B; Dowd, Annette; Guo, Huizhang; Wang, Junbao; Peng, Dong-Liang

2015-03-25

The combination of metal and semiconductor components in nanoscale to form a hybrid nanocrystal provides an important approach for achieving advanced functional materials with special optical, magnetic and photocatalytic functionalities. Here, a facile solution method is reported for the synthesis of Au-Ni-ZnO metal-semiconductor hybrid nanocrystals with a flower-like morphology and multifunctional properties. This synthetic strategy uses noble and magnetic metal Au@Ni nanocrystal seeds formed in situ to induce the heteroepitaxial growth of semiconducting ZnO nanopyramids onto the surface of metal cores. Evidence of epitaxial growth of ZnO{0001} facets on Ni {111} facets is observed on the heterojunction, even though there is a large lattice mismatch between the semiconducting and magnetic components. Adjustment of the amount of Au and Ni precursors can control the size and composition of the metal core, and consequently modify the surface plasmon resonance (SPR) and magnetic properties. Room-temperature superparamagnetic properties can be achieved by tuning the size of Ni core. The as-prepared Au-Ni-ZnO nanocrystals are strongly photocatalytic and can be separated and re-cycled by virtue of their magnetic properties. The simultaneous combination of plasmonic, semiconducting and magnetic components within a single hybrid nanocrystal furnishes it multifunctionalities that may find wide potential applications. © 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

Adlayer structure of octa-alkoxy-substituted copper(II) phthalocyanine on Au(111) by electrochemical scanning tunneling microscopy.

PubMed

Wang, Li; Ou-Yang, Liangyue; Yau, Shueh-Lin

2008-01-01

Electrochemical scanning tunneling microscopy (ECSTM) has been used to examine the adlayer of octa-alkoxy-substituted copper(II) phthalocyanines (CuPc(OC(8)H(17))(8)) on Au(111) in 0.1 M HClO(4), where the molecular adlayer was prepared by spontaneous adsorption from a benzene solution containing this molecule. Topography STM scans revealed long-range ordered, interweaved arrays of CuPc(OC(8)H(17))(8) with coexistent rectangular and hexagonal symmetries. High-quality STM molecular resolution yielded the internal molecular structure and the orientation of CuPc(OC(8)H(17))(8) admolecules. These STM results could shed insight into the method of generating ordered molecular assemblies of phthalocyanine molecules with long-chained substitutes on metal surface. 2007 Wiley-Liss, Inc

Visualizing the optical field strengths in Au/dielectric nanostructures and its correlation to SERS enhancements

NASA Astrophysics Data System (ADS)

Rajesh, Y.; Sangani, L. D. Varma; Shaik, Ummar Pasha; Gaur, Anshu; Mohiddon, Md Ahamad; Krishna, M. Ghanashyam

2017-05-01

The role of dielectric surrounding over the Au nanostructure for surface plasmon resonance (SPR) behavior is investigated by scanning near field optical microscopy (SNOM). The observed optical field strengths are correlated with the surface enhanced Raman scattering (SERS) enhancement recorded for R6G molecule. Discontinuous nanostructured Au thin films are deposited by RF magnatron sputtering at very low rate on to three different dielectric substrates, ZnO, TiO2 and SiO2. These three Au/dielectric nanostructures are investigated using SNOM by illuminating it in near field and collecting in transmission far field configuration. The observed optical near field images of the three different nanostructures are discussed by taking their dielectric constant into the account. The SERS enhancements are correlated with the optical field strengths derived from the near field optical imaging.

Titration of submonolayer Au growth on Si(111)

NASA Astrophysics Data System (ADS)

Kautz, J.; Copel, M. W.; Gordon, M. S.; Tromp, R. M.; van der Molen, S. J.

2014-01-01

We study and analyze the growth of submonolayers of Au on Si(111) by a complementary set of surface techniques. Specifically, we focus on the 5×2 and the α√3 ×√3 structures. We determine the gold coverage of these structures as a function of temperature by means of low energy electron diffraction (LEED) and low energy electron microscopy (LEEM). These results are independently calibrated by ex-situ ion scattering experiments. This allows us to present a phase diagram for this system. Remarkably, for all temperatures considered (820-1040 K), we find a coverage for the 5×2 phase that is significantly (≈10%) higher than the value of 0.6 monolayers which is assumed in the latest structural models. Therefore, a further refinement of the present picture of the quasi-one-dimensional 5×2 structure is required.

High Activity of Au/K/TiO 2(110) for CO Oxidation: Alkali-Metal-Enhanced Dispersion of Au and Bonding of CO

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

Rodriguez, Jose A.; Grinter, David C.; Ramirez, Pedro J.

In this paper, images from scanning tunneling microscopy show high mobility for potassium (K) on an oxidized TiO 2(110) surface. At low coverages, the alkali metal occupies mainly terrace sites of the o-TiO 2(110) system. The results of X-ray photoelectron spectroscopy indicate that K is fully ionized. The electron transferred from K to the titania affects the reactivity of this oxide, favoring the dispersion of Au particles on the terraces of the o-TiO 2(110) surface. When small coverages of K and Au are present on the o-TiO 2(110) system, only a few K–Au pairs are formed and the alkali metalmore » affects Au chemisorption mainly through the oxide interactions. Addition of K to Au/o-TiO 2(110) enhances the reactivity of the system, opening new reaction paths for the adsorption and oxidation of carbon monoxide. CO can undergo disproportionation (2CO → C ads + CO 2,ads) on K/o-TiO 2(110) and Au/K/o-TiO 2(110) surfaces. The Au–KO x interface binds CO much better than plain Au–TiO 2, increasing the surface coverage of CO and facilitating its oxidation. Kinetic tests show that K promotes CO oxidation on Au/TiO 2. Finally, turnover frequencies of 2.1 and 10.8 molecules (Au site) -1 s –1 were calculated for oxidation of CO on Au/o-TiO 2(110) and Au/K/o-TiO 2(110) catalysts, respectively.« less

High Activity of Au/K/TiO 2(110) for CO Oxidation: Alkali-Metal-Enhanced Dispersion of Au and Bonding of CO

DOE PAGES

Rodriguez, Jose A.; Grinter, David C.; Ramirez, Pedro J.; ...

2018-02-14

In this paper, images from scanning tunneling microscopy show high mobility for potassium (K) on an oxidized TiO 2(110) surface. At low coverages, the alkali metal occupies mainly terrace sites of the o-TiO 2(110) system. The results of X-ray photoelectron spectroscopy indicate that K is fully ionized. The electron transferred from K to the titania affects the reactivity of this oxide, favoring the dispersion of Au particles on the terraces of the o-TiO 2(110) surface. When small coverages of K and Au are present on the o-TiO 2(110) system, only a few K–Au pairs are formed and the alkali metalmore » affects Au chemisorption mainly through the oxide interactions. Addition of K to Au/o-TiO 2(110) enhances the reactivity of the system, opening new reaction paths for the adsorption and oxidation of carbon monoxide. CO can undergo disproportionation (2CO → C ads + CO 2,ads) on K/o-TiO 2(110) and Au/K/o-TiO 2(110) surfaces. The Au–KO x interface binds CO much better than plain Au–TiO 2, increasing the surface coverage of CO and facilitating its oxidation. Kinetic tests show that K promotes CO oxidation on Au/TiO 2. Finally, turnover frequencies of 2.1 and 10.8 molecules (Au site) -1 s –1 were calculated for oxidation of CO on Au/o-TiO 2(110) and Au/K/o-TiO 2(110) catalysts, respectively.« less

Influence of Under-layer Morphology on Structural and Magnetic Properties of Sputtered Co81Pd19 Films

NASA Astrophysics Data System (ADS)

Ponchaiya, Pairin; Rattanasakulthong, Watcharee

2017-09-01

Sputtered Co81Pd19 films with thickness of about 60 nm were deposited on various under-layers (Co, Ni, Cr and Al) and on glass substrate. Structural, morphological and magnetic properties of Co81Pd19 films were investigated. All of prepared Co81Pd19 film showed CoPd-FCC phase in (111) direction on CoO-FCC (111), NiO-FCC (200), Cr-BCC (200) and (201) and AlO-FCC (200) phases of Co, Ni, Cr and Al under-layer, respectively. AFM images revealed that the film on Cr under-layers and glass substrate exhibited the maximum roughness with the highest grain size and the minimum roughness with the continuous grain size, respectively. Both parallel and perpendicular maximum coercive field were found in the film on glass under-layer and the film on Co-under-layer film showed the highest saturation magnetization from both in-plane and out-of-plane measurements. These results confirmed that the structural and magnetic properties of sputtered Co81Pd19 films were affected by under-layer surface roughness and morphology by the virtue of particle size and distribution on the under-layer film surface.

Characterization of Gold-Sputtered Zinc Oxide Nanorods-a Potential Hybrid Material.

PubMed

Perumal, Veeradasan; Hashim, Uda; Gopinath, Subash C B; Rajintra Prasad, Haarindraprasad; Wei-Wen, Liu; Balakrishnan, S R; Vijayakumar, Thivina; Rahim, Ruslinda Abdul

2016-12-01

Generation of hybrid nanostructures has been attested as a promising approach to develop high-performance sensing substrates. Herein, hybrid zinc oxide (ZnO) nanorod dopants with different gold (Au) thicknesses were grown on silicon wafer and studied for their impact on physical, optical and electrical characteristics. Structural patterns displayed that ZnO crystal lattice is in preferred c-axis orientation and proved the higher purities. Observations under field emission scanning electron microscopy revealed the coverage of ZnO nanorods by Au-spots having diameters in the average ranges of 5-10 nm, as determined under transmission electron microscopy. Impedance spectroscopic analysis of Au-sputtered ZnO nanorods was carried out in the frequency range of 1 to 100 MHz with applied AC amplitude of 1 V RMS. The obtained results showed significant changes in the electrical properties (conductance and dielectric constant) with nanostructures. A clear demonstration with 30-nm thickness of Au-sputtering was apparent to be ideal for downstream applications, due to the lowest variation in resistance value of grain boundary, which has dynamic and superior characteristics.

Sputtering of rough surfaces: a 3D simulation study

NASA Astrophysics Data System (ADS)

von Toussaint, U.; Mutzke, A.; Manhard, A.

2017-12-01

The lifetime of plasma-facing components is critical for future magnetic confinement fusion power plants. A key process limiting the lifetime of the first-wall is sputtering by energetic ions. To provide a consistent modeling of the sputtering process of realistic geometries, the SDTrimSP-code has been extended to enable the processing of analytic as well as measured arbitrary 3D surface morphologies. The code has been applied to study the effect of varying the impact angle of ions on rough surfaces on the sputter yield as well as the influence of the aspect ratio of surface structures on the 2D distribution of the local sputtering yields. Depending on the surface morphologies reductions of the effective sputter yields to less than 25% have been observed in the simulation results.

Performance analyses of Schottky diodes with Au/Pd contacts on n-ZnO thin films as UV detectors

NASA Astrophysics Data System (ADS)

Varma, Tarun; Periasamy, C.; Boolchandani, Dharmendar

2017-12-01

In this paper, we report fabrication and performance analyses of UV detectors based on ZnO thin film Schottky diodes with Au and Pd contacts. RF magnetron sputtering technique has been used to deposit the nano-crystalline ZnO thin film, at room temperature. Characterization techniques such as XRD, AFM and SEM provided valuable information related to the micro-structural & optical properties of the thin film. The results show that the prepared thin film has good crystalline orientation and minimal surface roughness, with an optical bandgap of 3.1 eV. I-V and C-V characteristics were evaluated that indicate non-linear behaviour of the diodes with rectification ratios (IF/IR) of 19 and 427, at ± 4 V, for Au/ZnO and Pd/ZnO Schottky diodes, respectively. The fabricated Schottky diodes when exposed to a UV light of 365 nm wavelength, at an applied bias of -2 V, exhibited responsivity of 10.16 and 22.7 A/W, for Au and Pd Schottky contacts, respectively. The Pd based Schottky photo-detectors were found to exhibit better performance with superior values of detectivity and photoconductive gain of 1.95 × 1010 cm Hz0.5/W & 77.18, over those obtained for the Au based detectors which were observed to be 1.23 × 1010 cm Hz0.5/W & 34.5, respectively.

Construction of single-crystalline supramolecular networks of perchlorinated hexa-peri-hexabenzocoronene on Au(111)

NASA Astrophysics Data System (ADS)

Zhang, Yi; Zhang, Yanfang; Li, Geng; Lu, Jianchen; Lin, Xiao; Tan, Yuanzhi; Feng, Xinliang; Du, Shixuan; Müllen, Klaus; Gao, Hong-Jun

2015-03-01

The self-assembly of the perchlorinated hexa-peri-hexabenzocoronene (PCHBC) molecules on Au(111) has been studied by a low temperature scanning tunneling microscopy (STM) combining with density functional theory based first principle calculations. Highly ordered supramolecular networks with single domains limited by the terraces are formed on Au(111) substrate. High resolution images of the PCHBC molecules, confirmed by first principle simulations, are obtained. It reveals the close-packed arrangement of the PCHBC molecules on Au(111). The calculated charge distribution of PCHBC molecules shows the existence of attractive halogen-halogen interaction between neighboring molecules. Compared with the disordered adsorption of hexa-peri-hexabenzocoronene on Au(111), we conclude that the formation of attractive ClCl interactions between neighbors is the key factor to form the highly ordered, close-packed networks. Due to the steric hindrance resulted from the peripheral chlorine atoms, the PCHBC molecule is contorted and forms the doubly concave conformation, which is different from the hexa-peri-hexabenzocoronene with a planar structure. By using this supramolecular network as a template, we deposited C60 molecules on it at room temperature with low coverage. The STM images taken at low temperature show that the C60 molecules are mono-dispersed on the networks and adsorb on top of the PCHBC molecules, forming a typical concave-convex host-guest system.

Selective adsorption of toluene-3,4-dithiol on Si(553)-Au surfaces

NASA Astrophysics Data System (ADS)

Suchkova, Svetlana; Hogan, Conor; Bechstedt, Friedhelm; Speiser, Eugen; Esser, Norbert

2018-01-01

The adsorption of small organic molecules onto vicinal Au-stabilized Si(111) surfaces is shown to be a versatile route towards controlled growth of ordered organic-metal hybrid one-dimensional nanostructures. Density functional theory is used to investigate the site-specific adsorption of toluene-3,4-dithiol (TDT) molecules onto the clean Si(553)-Au surface and onto a co-doped surface whose steps are passivated by hydrogen. We find that the most reactive sites involve bonding to silicon at the step edge or on the terraces, while gold sites are relatively unfavored. H passivation and TDT adsorption both induce a controlled charge redistribution within the surface layer, causing the surface metallicity, electronic structure, and chemical reactivity of individual adsorption sites to be substantially altered.

Infrared spectroscopy of molecular submonolayers on surfaces by infrared scanning tunneling microscopy: tetramantane on Au111.

PubMed

Pechenezhskiy, Ivan V; Hong, Xiaoping; Nguyen, Giang D; Dahl, Jeremy E P; Carlson, Robert M K; Wang, Feng; Crommie, Michael F

2013-09-20

We have developed a new scanning-tunneling-microscopy-based spectroscopy technique to characterize infrared (IR) absorption of submonolayers of molecules on conducting crystals. The technique employs a scanning tunneling microscope as a precise detector to measure the expansion of a molecule-decorated crystal that is irradiated by IR light from a tunable laser source. Using this technique, we obtain the IR absorption spectra of [121]tetramantane and [123]tetramantane on Au(111). Significant differences between the IR spectra for these two isomers show the power of this new technique to differentiate chemical structures even when single-molecule-resolved scanning tunneling microscopy (STM) images look quite similar. Furthermore, the new technique was found to yield significantly better spectral resolution than STM-based inelastic electron tunneling spectroscopy, and to allow determination of optical absorption cross sections. Compared to IR spectroscopy of bulk tetramantane powders, infrared scanning tunneling microscopy (IRSTM) spectra reveal narrower and blueshifted vibrational peaks for an ordered tetramantane adlayer. Differences between bulk and surface tetramantane vibrational spectra are explained via molecule-molecule interactions.

CO adsorption on small Au{sub n} (n = 1–4) structures supported on hematite. II. Adsorption on the O-rich termination of α-Fe{sub 2}O{sub 3}(0001) surface

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

Pabisiak, Tomasz; Kiejna, Adam, E-mail: kiejna@ifd.uni.wroc.pl; Winiarski, Maciej J.

2016-01-28

The adsorption of small Au{sub n} (n = 1–4) nanostructures on oxygen terminated α-Fe{sub 2}O{sub 3}(0001) surface was investigated using density functional theory in the generalized gradient approximation of Perdew-Burke-Ernzerhof (PBE) form with Hubbard correction U, accounting for strong electron correlations (PBE+U). The structural, energetic, and electronic properties were examined for two classes of the adsorbed Au{sub n} nanostructures with vertical and flattened configurations. Similarly to the Fe-terminated α-Fe{sub 2}O{sub 3}(0001) surface considered in Part I, the flattened configurations were found energetically more favored than vertical ones. The binding of Au{sub n} to the O-terminated surface is much stronger thanmore » to the Fe-termination. The adsorption bonding energy of Au{sub n} and the work function of the Au{sub n}/α-Fe{sub 2}O{sub 3}(0001) systems decrease with the increased number of Au atoms in a structure. All of the adsorbed Au{sub n} structures are positively charged. The bonding of CO molecules to the Au{sub n} structures is distinctly stronger than on the Fe-terminated surface; however, it is weaker than the binding to the bare O-terminated surface. The CO molecule binds to the Au{sub n}/α-Fe{sub 2}O{sub 3}(0001) system through a peripheral Au atom partly detached from the Au{sub n} structure. The results of this work indicate that the most energetically favored sites for adsorption of a CO molecule on the Au{sub n}/α-Fe{sub 2}O{sub 3}(0001) systems are atoms in the Au{sup 0.5+} oxidation state.« less

Water dissociative adsorption on NiO(111): Energetics and structure of the hydroxylated surface

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

Zhao, Wei; Bajdich, Michal; Carey, Spencer

The energetics of the reactions of water with metal oxide surfaces are of tremendous interest for catalysis, electrocatalysis, and geochemistry, yet the energy for the dissociative adsorption of water was only previously measured on one well-defined oxide surface, iron oxide. In the present paper, the enthalpy of the dissociative adsorption of water is measured on NiO(111)-2 × 2 at 300 K using single-crystal adsorption calorimetry. The differential heat of dissociative adsorption decreases with coverage from 170 to 117 kJ/mol in the first 0.25 ML of coverage. Water adsorbs molecularly on top of that, with a heat of ~92 kJ/mol. Densitymore » functional theory (DFT) calculations reproduce the measured energies well (all within 17 kJ/mol) and provide insight into the atomic-level structure of the surfaces studied experimentally. They show that the oxygen-terminated O-octo(2 × 2) structure is the most stable NiO(111)-2 × 2 termination and gives reaction energies with water that are more consistent with the calorimetry results than the metal-terminated surface. They show that water adsorbs dissociatively on this (2 × 2)-O-octo surface to produce a hydroxyl-covered surface with a heat of adsorption of 171 ± 5 kJ/mol in the low-coverage limit (very close to 170 kJ/mol experimentally) and an integral heat that decreases by 14 kJ/mol up to saturation (compared to ~30 kJ/mol experimentally). As a result, sensitivity of this reaction’s energy to choice of DFT method is tested using a variety of different exchange correlation functionals, including HSE06, and found to be quite weak.« less

Water dissociative adsorption on NiO(111): Energetics and structure of the hydroxylated surface

DOE PAGES

Zhao, Wei; Bajdich, Michal; Carey, Spencer; ...

2016-09-19

The energetics of the reactions of water with metal oxide surfaces are of tremendous interest for catalysis, electrocatalysis, and geochemistry, yet the energy for the dissociative adsorption of water was only previously measured on one well-defined oxide surface, iron oxide. In the present paper, the enthalpy of the dissociative adsorption of water is measured on NiO(111)-2 × 2 at 300 K using single-crystal adsorption calorimetry. The differential heat of dissociative adsorption decreases with coverage from 170 to 117 kJ/mol in the first 0.25 ML of coverage. Water adsorbs molecularly on top of that, with a heat of ~92 kJ/mol. Densitymore » functional theory (DFT) calculations reproduce the measured energies well (all within 17 kJ/mol) and provide insight into the atomic-level structure of the surfaces studied experimentally. They show that the oxygen-terminated O-octo(2 × 2) structure is the most stable NiO(111)-2 × 2 termination and gives reaction energies with water that are more consistent with the calorimetry results than the metal-terminated surface. They show that water adsorbs dissociatively on this (2 × 2)-O-octo surface to produce a hydroxyl-covered surface with a heat of adsorption of 171 ± 5 kJ/mol in the low-coverage limit (very close to 170 kJ/mol experimentally) and an integral heat that decreases by 14 kJ/mol up to saturation (compared to ~30 kJ/mol experimentally). As a result, sensitivity of this reaction’s energy to choice of DFT method is tested using a variety of different exchange correlation functionals, including HSE06, and found to be quite weak.« less

Plasmonic enhancement of visible-light water splitting with Au-TiO2 composite aerogels

NASA Astrophysics Data System (ADS)

Desario, Paul A.; Pietron, Jeremy J.; Devantier, Devyn E.; Brintlinger, Todd H.; Stroud, Rhonda M.; Rolison, Debra R.

2013-08-01

We demonstrate plasmonic enhancement of visible-light-driven splitting of water at three-dimensionally (3D) networked gold-titania (Au-TiO2) aerogels. The sol-gel-derived ultraporous composite nanoarchitecture, which contains 1 to 8.5 wt% Au nanoparticles and titania in the anatase form, retains the high surface area and mesoporosity of unmodified TiO2 aerogels and maintains stable dispersion of the ~5 nm Au guests. A broad surface plasmon resonance (SPR) feature centered at ~550 nm is present for the Au-TiO2 aerogels, but not Au-free TiO2 aerogels, and spans a wide range of the visible spectrum. Gold-derived SPR in Au-TiO2 aerogels cast as films on transparent electrodes drives photoelectrochemical oxidation of aqueous hydroxide and extends the photocatalytic activity of TiO2 from the ultraviolet region to visible wavelengths exceeding 700 nm. Films of Au-TiO2 aerogels in which Au nanoparticles are deposited on pre-formed TiO2 aerogels by a deposition-precipitation method (DP Au/TiO2) also photoelectrochemically oxidize aqueous hydroxide, but less efficiently than 3D Au-TiO2, despite having an essentially identical Au nanoparticle weight fraction and size distribution. For example, 3D Au-TiO2 containing 1 wt% Au is as active as DP Au/TiO2 with 4 wt% Au. The higher photocatalytic activity of 3D Au-TiO2 derives only in part from its ability to retain the surface area and porosity of unmodified TiO2 aerogel. The magnitude of improvement indicates that in the 3D arrangement either a more accessible photoelectrochemical reaction interphase (three-phase boundary) exists or more efficient conversion of excited surface plasmons into charge carriers occurs, thereby amplifying reactivity over DP Au/TiO2. The difference in photocatalytic efficiency between the two forms of Au-TiO2 demonstrates the importance of defining the structure of Au||TiO2 interfaces within catalytic Au-TiO2 nanoarchitectures.We demonstrate plasmonic enhancement of visible-light-driven splitting of

Single-layer ZnS supported on Au(111): A combined XPS, LEED, STM and DFT study

DOE PAGES

Deng, Xingyi; Sorescu, Dan C.; Lee, Junseok

2016-12-31

Single-layer of ZnS, consisting of one atomic layer of ZnS(111) plane, has been grown on Au(111) and characterized using X-ray photoelectron spectroscopy (XPS), low energy electron diffraction (LEED) and scanning tunneling microscopy (STM). While the LEED measurement indicates a coincidence structure of ZnS-(3×3)/Au(111)-(4×4), high resolution STM images reveal hexagonal unit cells of 6.7×6.7 Å 2 and 11.6×11.6 Å 2, corresponding to √3 and 3 times the unit cell of the ideal zincblende ZnS-(1×1), respectively, depending on the tunneling conditions. Calculations based on density functional theory (DFT) indicate a significantly reconstructed non-planar structure of ZnS single-layer on Au(111) with 2/3 ofmore » the S anions being located nearly in the plane of the Zn cations and the rest 1/3 of the S anions protruding above the Zn plane. In conclusion, the calculated STM image shows similar characteristics to those of the experimental STM image. Additionally, the DFT calculations reveal the different bonding nature of the S anions in ZnS single-layer supported on Au(111).« less

Epitaxial ZnO gate dielectrics deposited by RF sputter for AlGaN/GaN metal-oxide-semiconductor high-electron-mobility transistors

NASA Astrophysics Data System (ADS)

Yoon, Seonno; Lee, Seungmin; Kim, Hyun-Seop; Cha, Ho-Young; Lee, Hi-Deok; Oh, Jungwoo

2018-01-01

Radio frequency (RF)-sputtered ZnO gate dielectrics for AlGaN/GaN metal-oxide-semiconductor high-electron-mobility transistors (MOS-HEMTs) were investigated with varying O2/Ar ratios. The ZnO deposited with a low oxygen content of 4.5% showed a high dielectric constant and low interface trap density due to the compensation of oxygen vacancies during the sputtering process. The good capacitance-voltage characteristics of ZnO-on-AlGaN/GaN capacitors resulted from the high crystallinity of oxide at the interface, as investigated by x-ray diffraction and high-resolution transmission electron microscopy. The MOS-HEMTs demonstrated comparable output electrical characteristics with conventional Ni/Au HEMTs but a lower gate leakage current. At a gate voltage of -20 V, the typical gate leakage current for a MOS-HEMT with a gate length of 6 μm and width of 100 μm was found to be as low as 8.2 × 10-7 mA mm-1, which was three orders lower than that of the Ni/Au Schottky gate HEMT. The reduction of the gate leakage current improved the on/off current ratio by three orders of magnitude. These results indicate that RF-sputtered ZnO with a low O2/Ar ratio is a good gate dielectric for high-performance AlGaN/GaN MOS-HEMTs.

Ferroelectricity emerging in strained (111)-textured ZrO{sub 2} thin films

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

Fan, Zhen, E-mail: a0082709@u.nus.edu, E-mail: msecj@nus.edu.sg; Deng, Jinyu; Liu, Ziyan

2016-01-04

(Anti-)ferroelectricity in complementary metal-oxide-semiconductor (CMOS)-compatible binary oxides have attracted considerable research interest recently. Here, we show that by using substrate-induced strain, the orthorhombic phase and the desired ferroelectricity could be achieved in ZrO{sub 2} thin films. Our theoretical analyses suggest that the strain imposed on the ZrO{sub 2} (111) film by the TiN/MgO (001) substrate would energetically favor the tetragonal (t) and orthorhombic (o) phases over the monoclinic (m) phase of ZrO{sub 2}, and the compressive strain along certain 〈11-2〉 directions may further stabilize the o-phase. Experimentally ZrO{sub 2} thin films are sputtered onto the MgO (001) substrates buffered bymore » epitaxial TiN layers. ZrO{sub 2} thin films exhibit t- and o-phases, which are highly (111)-textured and strained, as evidenced by X-ray diffraction and transmission electron microscopy. Both polarization-electric field (P-E) loops and corresponding current responses to voltage stimulations measured with appropriate applied fields reveal the ferroelectric sub-loop behavior of the ZrO{sub 2} films at certain thicknesses, confirming that the ferroelectric o-phase has been developed in the strained (111)-textured ZrO{sub 2} films. However, further increasing the applied field leads to the disappearance of ferroelectric hysteresis, the possible reasons of which are discussed.« less

Solar-Wind Protons and Heavy Ions Sputtering of Lunar Surface Materials

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

Barghouty, N.; Meyer, Fred W; Harris, Peter R

2011-01-01

Lunar surface materials are exposed to {approx}1 keV/amu solar-wind protons and heavy ions on almost continuous basis. As the lunar surface consists of mostly oxides, these materials suffer, in principle, both kinetic and potential sputtering due to the actions of the solar-wind ions. Sputtering is an important mechanism affecting the composition of both the lunar surface and its tenuous exosphere. While the contribution of kinetic sputtering to the changes in the composition of the surface layer of these oxides is well understood and modeled, the role and implications of potential sputtering remain unclear. As new potential-sputtering data from multi-charged ionsmore » impacting lunar regolith simulants are becoming available from Oak Ridge National Laboratory's MIRF, we examine the role and possible implications of potential sputtering of Lunar KREEP soil. Using a non-equilibrium model we demonstrate that solar-wind heavy ions induced sputtering is critical in establishing the timescale of the overall solar-wind sputtering process of the lunar surface. We also show that potential sputtering leads to a more pronounced and significant differentiation between depleted and enriched surface elements. We briefly discuss the impacts of enhanced sputtering on the composition of the regolith and the exosphere, as well as of solar-wind sputtering as a source of hydrogen and water on the moon.« less

Sputtered deposited nanocrystalline ZnO films: A correlation between electrical, optical and microstructural properties

NASA Astrophysics Data System (ADS)

Lee, J.; Gao, W.; Li, Z.; Hodgson, M.; Metson, J.; Gong, H.; Pal, U.

2005-05-01

Zinc oxide thin films were prepared by dc (direct current) and rf (radio frequency) magnetron sputtering on glass substrates. ZnO films produced by dc sputtering have a high resistance, while the films produced using rf sputtering are significantly more conductive. While the conductive films have a compact nodular surface morphology, the resistive films have a relatively porous surface with columnar structures in cross section. Compared to the dc sputtered films, rf sputtered films have a microstructure with smaller d spacing, lower internal stress, higher band gap energy and higher density. Dependence of conductivity on the deposition technique and the resulting d spacing , stress, density, band gap, film thickness and Al doping are discussed. Correlations between the electrical conductivity, microstructural parameters and optical properties of the films have been made.

The Au(111)/IL interfacial nanostructure in the presence of precursors and its influence on the electrodeposition process.

PubMed

Borisenko, Natalia; Lahiri, Abhishek; Pulletikurthi, Giridhar; Cui, Tong; Carstens, Timo; Zahlbach, Janine; Atkin, Rob; Endres, Frank

2018-01-01

Ionic liquids have attracted significant interest as electrolytes for the electrodeposition of metals and semiconductors, but the details of the deposition processes are not yet well understood. In this paper, we give an overview of how the addition of various precursors (TaF 5 , SiCl 4 , and GaCl 3 ) affects the solid/IL interfacial structure. In situ Atomic Force Microscopy (AFM) and vibrational spectroscopy have been employed to study the changes of the Au(111)/IL interface and in the electrolytes, respectively. Ionic liquids with the 1-butyl-1-methylpyrrolidinium ([Py 1,4 ] + ) cation and bis(trifluoromethylsulfonyl)amide ([TFSA] - ), trifluoromethylsulfonate ([TfO] - ) and tris(pentafluoroethyl)trifluorophosphate ([FAP] - ) as anions were chosen for this purpose. In situ AFM force-distance measurements reveal that both the anion of the IL and the solutes (TaF 5 or GaCl 3 ) influence the Electrical Double Layer (EDL) structure of the Au(111)/IL interface, which can affect the deposition process of Ta and the morphology of the Ga electrodeposits, respectively. Furthermore, the concentration of the precursor can significantly alter the Au(111)/[Py 1,4 ][FAP]-SiCl 4 interfacial structure wherein the presence of 0.25 M SiCl 4 a double layer structure forms that facilitates Si deposition. This study may provide some critical insights into the structure of the electrode/IL interface for specific applications.

Controlling the stereochemistry and regularity of butanethiol self-assembled monolayers on au(111).

PubMed

Yan, Jiawei; Ouyang, Runhai; Jensen, Palle S; Ascic, Erhad; Tanner, David; Mao, Bingwei; Zhang, Jingdong; Tang, Chunguang; Hush, Noel S; Ulstrup, Jens; Reimers, Jeffrey R

2014-12-10

The rich stereochemistry of the self-assembled monolayers (SAMs) of four butanethiols on Au(111) is described, the SAMs containing up to 12 individual C, S, or Au chiral centers per surface unit cell. This is facilitated by synthesis of enantiomerically pure 2-butanethiol (the smallest unsubstituted chiral alkanethiol), followed by in situ scanning tunneling microscopy (STM) imaging combined with density functional theory molecular dynamics STM image simulations. Even though butanethiol SAMs manifest strong headgroup interactions, steric interactions are shown to dominate SAM structure and chirality. Indeed, steric interactions are shown to dictate the nature of the headgroup itself, whether it takes on the adatom-bound motif RS(•)Au(0)S(•)R or involves direct binding of RS(•) to face-centered-cubic or hexagonal-close-packed sites. Binding as RS(•) produces large, organizationally chiral domains even when R is achiral, while adatom binding leads to rectangular plane groups that suppress long-range expression of chirality. Binding as RS(•) also inhibits the pitting intrinsically associated with adatom binding, desirably producing more regularly structured SAMs.

Photoluminescent Au-Ge composite nanodots formation on SiO2 surface by ion induced dewetting

NASA Astrophysics Data System (ADS)

Datta, D. P.; Siva, V.; Singh, A.; Kanjilal, D.; Sahoo, P. K.

2017-09-01

Medium energy ion irradiation on a bilayer of Au and Ge on SiO2 is observed to result in gradual morphological evolution from an interconnected network to a nanodot array on the insulator surface. Structural and compositional analyses reveal composite nature of the nanodots, comprising of both Au and Ge. The growing nanostructures are found to be photoluminescent at room temperature where the emission intensity and wavelengths vary with morphology. The growth of such nanostructures can be understood in terms of dewetting of the metal layer under ion irradiation due to ion-induced melting along the ion tracks. The visible PL emission is found to be related with evolution of the Au-Ge nanodots. The study indicates a route towards single step synthesis of metal-semiconductor nanodots on insulator surface.

Effect of surface plasmon resonance on the photocatalytic activity of Au/TiO2 under UV/visible illumination.

PubMed

Tseng, Yao-Hsuan; Chang, I-Guo; Tai, Yian; Wu, Kung-Wei

2012-01-01

In this study, gold-loaded titanium dioxide was prepared by an impregnation method to investigate the effect of surface plasmon resonance (SPR) on photoactivity. The deposited gold nanoparticles (NPs) absorb visible light because of SPR. The effects of both the gold content and the TiO2 size of Au/TiO2 on SPR and the photocatalytic efficiency were investigated. The morphology, crystal structure, light absorption, emission from the recombination of a photoexcited electron and hole, and the degree of aggregation were investigated using transmission electron microscopy (TEM), X-ray diffraction (XRD), UV-visible-diffuse reflectance spectra (UV-VIS-DRS), photoluminescence (PL) spectroscopy, and turbidimetry, respectively. Photocatalytic activity was evaluated by the decolorization of methyl orange solution over modified titania under UV and UV/GLED (green light emitting diode) illumination. Au/TiO2 NPs exhibited an absorption peak (530-570 nm) because of SPR. The results of our photocatalytic experiments indicated that the UV-inducedly photocatalytic reaction rate was improved by simultaneously using UV and green light illumination; this corresponds to the adsorption region of SPR. Au/TiO2 could use the enhanced electric field amplitude on the surface of the Au particle in the spectral vicinity of its plasmon resonance and thus improve the photoactivity. Experimental results show that the synergistic effect between UV and green light for the improvement of photoactivity increases with increasing the SPR absorption, which in turn is affected by the Au content and TiO2 size.

Growth mechanism, surface and optical properties of ZnO nanostructures deposited on various Au-seeded thickness obtained by mist-atomization

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

Afaah, A. N., E-mail: afaahabdullah@yahoo.com; Aadila, A., E-mail: aadilaazizali@gmail.com; Asib, N. A. M., E-mail: amierahasib@yahoo.com

2016-07-06

In this paper, growth mechanisms of ZnO nanostructures on non-seeded glass, 6 nm and 12 nm Au seed layer obtained by mist-atomization was proposed. ZnO films were successfully deposited on glass substrate with different thickness of Au seed layer i.e. 6 nm and 12 nm. The surface and optical properties of the prepared samples were investigated using Field emission scanning electron microscopy (FESEM) and photoluminescence (PL). FESEM micrograph show that ZnO nanostructure deposited on 6 nm Au seed layer has uniform formation and well distributed. From PL spectroscopy, the UV emission shows that ZnO deposited on 6 nm Au seedmore » layer has the more intense UV intensity which proved that high crystal quality of nanostructured ZnO deposited on 6 nm Au seed layer.« less

Low temperature rf sputtering deposition of (Ba, Sr) TiO3 thin film with crystallization enhancement by rf power supplied to the substrate

NASA Astrophysics Data System (ADS)

Yoshimaru, Masaki; Takehiro, Shinobu; Abe, Kazuhide; Onoda, Hiroshi

2005-05-01

The (Ba, Sr) TiO3 thin film deposited by radio frequency (rf) sputtering requires a high deposition temperature near 500 °C to realize a high relative dielectric constant over of 300. For example, the film deposited at 330 °C contains an amorphous phase and shows a low relative dielectric constant of less than 100. We found that rf power supplied not only to the (Ba, Sr) TiO3 sputtering target, but also to the substrate during the initial step of film deposition, enhanced the crystallization of the (Ba, Sr) TiO3 film drastically and realized a high dielectric constant of the film even at low deposition temperatures near 300 °C. The 50-nm-thick film with only a 10 nm initial layer deposited with the substrate rf biasing is crystallized completely and shows a high relative dielectric constant of 380 at the deposition temperature of 330 °C. The (Ba, Sr) TiO3 film deposited at higher temperatures (upwards of 400 °C) shows <110> preferred orientation, while the film deposited at 330 °C with the 10 nm initial layer shows a <111> preferred orientation on a <001>-oriented ruthenium electrode. The unit cell of (Ba, Sr) TiO3 (111) plane is similar to that of ruthenium (001) plane. We conclude that the rf power supplied to the substrate causes ion bombardments on the (Ba, Sr) TiO3 film surface, which assists the quasiepitaxial growth of (Ba, Sr) TiO3 film on the ruthenium electrode at low temperatures of less than 400 °C.

Effect of dissolved LiCl on the ionic liquid-Au(111) interface: an in situ STM study

NASA Astrophysics Data System (ADS)

Borisenko, Natalia; Atkin, Rob; Lahiri, Abhishek; Zein El Abedin, Sherif; Endres, Frank

2014-07-01

The structure of the electrolyte/electrode interface plays a significant role in electrochemical processes. To date, most studies are focusing on understanding the interfacial structure in pure ionic liquids. In this paper in situ scanning tunnelling microscopy (STM) has been employed to elucidate the structure of the charged Au(111)-ionic liquid (1-butyl-1-methylpyrrolidinium tris(pentafluoroethyl)trifluorophosphate, [Py1,4]FAP) interface in the presence of 0.1 M LiCl. The addition of the Li salt to the ionic liquid has a strong influence on the interfacial structure. In the first STM scan in situ measurements reveal that Au(111) undergoes the (22 \\times \\surd 3) ‘herringbone’ reconstruction in a certain potential regime, and there is strong evidence that the gold surface dissolves at negative electrode potentials in [Py1,4]FAP containing LiCl. Bulk deposition of Li is obtained at -2.9 V in the second STM scan.

Secondary ion formation during electronic and nuclear sputtering of germanium

NASA Astrophysics Data System (ADS)

Breuer, L.; Ernst, P.; Herder, M.; Meinerzhagen, F.; Bender, M.; Severin, D.; Wucher, A.

2018-06-01

Using a time-of-flight mass spectrometer attached to the UNILAC beamline located at the GSI Helmholtz Centre for Heavy Ion Research, we investigate the formation of secondary ions sputtered from a germanium surface under irradiation by swift heavy ions (SHI) such as 5 MeV/u Au by simultaneously recording the mass spectra of the ejected secondary ions and their neutral counterparts. In these experiments, the sputtered neutral material is post-ionized via single photon absorption from a pulsed, intensive VUV laser. After post-ionization, the instrument cannot distinguish between secondary ions and post-ionized neutrals, so that both signals can be directly compared in order to investigate the ionization probability of different sputtered species. In order to facilitate an in-situ comparison with typical nuclear sputtering conditions, the system is also equipped with a conventional rare gas ion source delivering a 5 keV argon ion beam. For a dynamically sputter cleaned surface, it is found that the ionization probability of Ge atoms and Gen clusters ejected under electronic sputtering conditions is by more than an order of magnitude higher than that measured for keV sputtered particles. In addition, the mass spectra obtained under SHI irradiation show prominent signals of GenOm clusters, which are predominantly detected as positive or negative secondary ions. From the m-distribution for a given Ge nuclearity n, one can deduce that the sputtered material must originate from a germanium oxide matrix with approximate GeO stoichiometry, probably due to residual native oxide patches even at the dynamically cleaned surface. The results clearly demonstrate a fundamental difference between the ejection and ionization mechanisms in both cases, which is interpreted in terms of corresponding model calculations.

Modeling Solar-Wind Heavy-Ions' Potential Sputtering of Lunar KREEP Surface

NASA Technical Reports Server (NTRS)

Barghouty, A. F.; Meyer, F. W.; Harris, R. P.; Adams, J. H., Jr.

2012-01-01

Recent laboratory data suggest that potential sputtering may be an important weathering mechanism that can affect the composition of both the lunar surface and its tenuous exosphere; its role and implications, however, remain unclear. Using a relatively simple kinetic model, we will demonstrate that solar-wind heavy ions induced sputtering of KREEP surfaces is critical in establishing the timescale of the overall solar-wind sputtering process of the lunar surface. We will also also show that potential sputtering leads to a more pronounced and significant differentiation between depleted and enriched surface elements. We briefly discuss the impacts of enhanced sputtering on the composition of the regolith and the exosphere, as well as of solar-wind sputtering as a source of hydrogen and water on the moon.

Biocompatibility and Surface Properties of TiO2 Thin Films Deposited by DC Magnetron Sputtering

PubMed Central

López-Huerta, Francisco; Cervantes, Blanca; González, Octavio; Hernández-Torres, Julián; García-González, Leandro; Vega, Rosario; Herrera-May, Agustín L.; Soto, Enrique

2014-01-01

We present the study of the biocompatibility and surface properties of titanium dioxide (TiO2) thin films deposited by direct current magnetron sputtering. These films are deposited on a quartz substrate at room temperature and annealed with different temperatures (100, 300, 500, 800 and 1100 °C). The biocompatibility of the TiO2 thin films is analyzed using primary cultures of dorsal root ganglion (DRG) of Wistar rats, whose neurons are incubated on the TiO2 thin films and on a control substrate during 18 to 24 h. These neurons are activated by electrical stimuli and its ionic currents and action potential activity recorded. Through X-ray diffraction (XRD), the surface of TiO2 thin films showed a good quality, homogeneity and roughness. The XRD results showed the anatase to rutile phase transition in TiO2 thin films at temperatures between 500 and 1100 °C. This phase had a grain size from 15 to 38 nm, which allowed a suitable structural and crystal phase stability of the TiO2 thin films for low and high temperature. The biocompatibility experiments of these films indicated that they were appropriated for culture of living neurons which displayed normal electrical behavior. PMID:28788667

Growth and characterization of MnAu2 films

NASA Astrophysics Data System (ADS)

Cheng, S. F.; Bussmann, K. M.

2017-01-01

MnAu2 films ranging from 60 to 200 nm thickness are deposited by co-sputtering from elemental targets. X-ray diffraction confirmed these films to be nearly single phase with tetragonal lattice parameters of a=0.336 nm and c=0.872 nm that compare well to the bulk values of a=0.336 nm and c=0.876 nm. The density of the films is analyzed using x-ray reflectivity to be 14.95 g/cm3 and within experimental error of previously determined value of 15.00 g/cm3. The films grown on c-plane sapphire, (100)MgO and (100)MgF2 are randomly oriented polycrystalline, while the films grown on a-plane sapphire, (111)MgO and (111)Si/(0001)AlN showed that the (110) plane is parallel to the film plane and there are three sets of domains in equal amount differing by 60° in-plane rotation. Magnetic order is found to become paramagnetic near 360 K which is in close proximity to the bulk value. There are deviations in the slope of hysteresis loops observed at 10 K around 10 kOe that indicate complex magnetic switching.

Nanoporous Au structures by dealloying Au/Ag thermal- or laser-dewetted bilayers on surfaces

NASA Astrophysics Data System (ADS)

Ruffino, F.; Torrisi, V.; Grillo, R.; Cacciato, G.; Zimbone, M.; Piccitto, G.; Grimaldi, M. G.

2017-03-01

Nanoporous Au attracts great technological interest and it is a promising candidate for optical and electrochemical sensors. In addition to nanoporous Au leafs and films, recently, interest was focused on nanoporous Au micro- and nano-structures on surfaces. In this work we report on the study of the characteristics of nanoporous Au structures produced on surfaces. We developed the following procedures to fabricate the nanoporous Au structures: we deposited thin Au/Ag bilayers on SiO2 or FTO (fluorine-doped tin oxide) substrates with thickness xAu and xAg of the Au and Ag layers; we induced the alloying and dewetting processes of the bilayers by furnace annealing processes of the bilayers deposited on SiO2 and by laser irradiations of the bilayers deposited on FTO; the alloying and dewetting processes result in the formation of AuxAgy alloy sub-micron particles being x and y tunable by xAu and xAg. These particles are dealloyed in HNO3 solution to remove the Ag atoms. We obtain, so, nanoporous sub-micron Au particles on the substrates. Analyzing the characteristics of these particles we find that: a) the size and shape of the particles depend on the nature of the dewetting process (solid-state dewetting on SiO2, molten-state dewetting on FTO); b) the porosity fraction of the particles depends on how the alloying process is reached: about 32% of porosity for the particles fabricated by the furnace annealing at 900 °C, about 45% of porosity for the particles fabricated by the laser irradiation at 0.5 J/cm2, in both cases independently on the Ag concentration in the alloy; c) After the dealloying process the mean volume of the Au particles shrinks of about 39%; d) After an annealing at 400 °C the nanoporous Au particles reprise their initial volume while the porosity fraction is reduced. Arguments to justify these behaviors are presented.

[Study on anti-coagulant property of radio frequency sputtering nano-sized TiO2 thin films].

PubMed

Tang, Xiaoshan; Li, Da

2010-12-01

Nano-TiO2 thin films were prepared by Radio frequency (RF) sputtering on pyrolytic carbon substrates. The influences of sputtering power on the structure and the surface morphology of TiO2 thin films were investigated by X-ray diffraction (XRD), and by scanning electron microscopy (SEM). The results show that the TiO2 films change to anatase through the optimum of sputtering power. The mean diameter of nano-particle is about 30 nm. The anti-coagulant property of TiO2 thin films was observed through platelet adhesion in vitro. The result of experiment reveals the amount of thrombus on the TiO2 thin films being much less than that on the pyrolytic carbon. It also indicates that the RF sputtering Nano-sized TiO2 thin films will be a new kind of promising materials applied to artificial heart valve and endovascular stent.

Enhanced middle-infrared light transmission through Au/SiO(x)N(y)/Au aperture arrays.

PubMed

Xiao, Gongli; Yao, Xiang; Ji, Xinming; Zhou, Jia; Bao, Zongming; Huang, Yiping

2011-12-01

The enhanced middle-infrared light transmission through Au/SiO(x)N(y)/Au aperture arrays by changing the refractive index and the thickness of a dielectric layer was studied experimentally. The results indicated that the transmission spectra was highly dependent on the refractive index and the thickness of SiO(x)N(y). We found that the transmission peaks redshifted regularly along with the refractive index from 1.6 to 1.8, owing to the role of surface plasmon polaritons (SPP) coupling in the Au/SiO(x)N(y)/Au cascaded metallic structure. Simultaneously, a higher transmission efficiency and narrower transmission peak was obtained in Au/SiO2.1N0.3/Au cascaded metallic structure with small refractive index (1.6) than in Au/SiO0.6N1/Au cascaded metallic structure with large refractive index (1.8). When the thickness of SiO(x)N(y) changes from 0.2 to 0.4 microm, the shape of transmission spectra exhibits a large change. It was found that a higher transmission efficiency and narrower transmission peak was obtained in Au/SiO(x)N(y)/Au cascaded metallic structure with a thin dielectric film (0.2 microm), with the increase of SiO(x)N(y) film's thickness, the transmission peak gradually widened and disappeared finally. This effect is useful in applications of biochemical sensing and tunable integrated plasmonic devices in the middle-infrared region.

Solar Wind Sputtering of Lunar Surface Materials: Role and Some Possible Implications of Potential Sputtering

NASA Technical Reports Server (NTRS)

Barghouty, A. F.; Adams, J. H., Jr.; Meyer, F.; Reinhold, c.

2010-01-01

Solar-wind induced sputtering of the lunar surface includes, in principle, both kinetic and potential sputtering. The role of the latter mechanism, however, in many focused studies has not been properly ascertained due partly to lack of data but can also be attributed to the assertion that the contribution of solar-wind heavy ions to the total sputtering is quite low due to their low number density compared to solar-wind protons. Limited laboratory measurements show marked enhancements in the sputter yields of slow-moving, highly-charged ions impacting oxides. Lunar surface sputtering yields are important as they affect, e.g., estimates of the compositional changes in the lunar surface, its erosion rate, as well as its contribution to the exosphere as well as estimates of hydrogen and water contents. Since the typical range of solar-wind ions at 1 keV/amu is comparable to the thickness of the amorphous rim found on lunar soil grains, i.e. few 10s nm, lunar simulant samples JSC-1A AGGL are specifically enhanced to have such rims in addition to the other known characteristics of the actual lunar soil particles. However, most, if not all laboratory studies of potential sputtering were carried out in single crystal targets, quite different from the rim s amorphous structure. The effect of this structural difference on the extent of potential sputtering has not, to our knowledge, been investigated to date.

High-temperature stability of Au/Pd/Cu and Au/Pd(P)/Cu surface finishes

NASA Astrophysics Data System (ADS)

Ho, C. E.; Hsieh, W. Z.; Lee, P. T.; Huang, Y. H.; Kuo, T. T.

2018-03-01

Thermal reliability of Au/Pd/Cu and Au/Pd(4-6 wt.% P)/Cu trilayers in the isothermal annealing at 180 °C were investigated by X-ray photoelectron spectroscopy (XPS), time-of-flight secondary ion mass spectrometry (TOF-SIMS), and transmission electron microscopy (TEM). The pure Pd film possessed a nanocrystalline structure with numerous grain boundaries, thereby facilitating the interdiffusion between Au and Cu. Out-diffusion of Cu through Pd and Au grain boundaries yielded a significant amount of Cu oxides (CuO and Cu2O) over the Au surface and gave rise to void formation in the Cu film. By contrast, the Pd(P) film was amorphous and served as a good diffusion barrier against Cu diffusion. The results of this study indicated that amorphous Pd(P) possessed better oxidation resistance and thermal reliability than crystalline Pd.

Facing-target mid-frequency magnetron reactive sputtered hafnium oxide film: Morphology and electrical properties

NASA Astrophysics Data System (ADS)

Zhang, Yu; Xu, Jun; Wang, You-Nian; Choi, Chi Kyu; Zhou, Da-Yu

2016-03-01

Amorphous hafnium dioxide (HfO2) film was prepared on Si (100) by facing-target mid-frequency reactive magnetron sputtering under different oxygen/argon gas ratio at room temperature with high purity Hf target. 3D surface profiler results showed that the deposition rates of HfO2 thin film under different O2/Ar gas ratio remain unchanged, indicating that the facing target midfrequency magnetron sputtering system provides effective approach to eliminate target poisoning phenomenon which is generally occurred in reactive sputtering procedure. X-ray photoelectron spectroscopy (XPS) and Fourier transform infrared spectroscopy (FTIR) demonstrated that the gradual reduction of oxygen vacancy concentration and the densification of deposited film structure with the increase of oxygen/argon (O2/Ar) gas flow ratio. Atomic force microscopy (AFM) analysis suggested that the surface of the as-deposited HfO2 thin film tends to be smoother, the root-meansquare roughness (RMS) reduced from 0.876 nm to 0.333 nm while O2/Ar gas flow ratio increased from 1/4 to 1/1. Current-Voltage measurements of MOS capacitor based on Au/HfO2/Si structure indicated that the leakage current density of HfO2 thin films decreased by increasing of oxygen partial pressure, which resulted in the variations of pore size and oxygen vacancy concentration in deposited thin films. Based on the above characterization results the leakage current mechanism for all samples was discussed systematically.

Electronic sputtering of LiF, CaF2, LaF3 and UF4 with 197 MeV Au ions. Is the stoichiometry of atom emission preserved?

NASA Astrophysics Data System (ADS)

Toulemonde, M.; Assmann, W.; Muller, D.; Trautmann, C.

2017-09-01

Sputtering experiments with swift heavy ions in the electronic energy loss regime were performed by using the catcher technique in combination with elastic recoil detection analysis. Four different fluoride targets, LiF, CaF2, LaF3 and UF4 were irradiated in the electronic energy loss regime using 197 MeV Au ions. The angular distribution of particles sputtered from the surface of freshly cleaved LiF and CaF2 single crystals is composed of a broad cosine distribution superimposed by a jet-like peak that appears perpendicular to the surface independent of the angle of beam incidence. For LiF, the particle emission in the entire angular distribution (jet plus broad cosine component) is stoichiometric, whereas for CaF2 the ratio of the sputtered F to Ca particles is at large angles by a factor of two smaller than the stoichiometry of the crystal. For single crystalline LaF3 no jet component is observed and the angular distribution is non-stoichiometric with the number of sputtered F particles being slightly larger than the number of sputtered La particles. In the case of UF4, the target was polycrystalline and had a much rougher surface compared to cleaved crystals. This destroys the appearance of a possible jet component leading to a broad angular distribution. The ratio of sputtered U atoms compared to F atoms is in the order of 1-2, i.e. the number of collected particles on the catcher is also non-stoichiometric. Such unlike behavior of particles sputtered from different fluoride crystals creates new questions.

Nanostructured zinc oxide thin film for application to surface plasmon resonance based cholesterol biosensor

NASA Astrophysics Data System (ADS)

Kaur, Gurpreet; Tomar, Monika; Gupta, Vinay

2015-11-01

ZnO thin film was deposited on gold coated glass prism by RF sputtering technique in glancing angle deposition (GLAD) configuration. The structural, morphological and optical properties of the deposited film were investigated using X-ray diffraction (XRD), Atomic Force Microscopy (AFM) and Fourier Transform Infrared (FTIR) Spectroscopy. ZnO coated Au prisms (ZnO/Au/prism) were used to excite surface plasmons in Kretschmann configuration at the Au- ZnO interface on a laboratory assembled Surface Plasmon Resonance (SPR) measurement setup. Cholesterol oxidase (ChOx) enzyme was immobilized on the ZnO/Au/prism structure by physical adsorption technique. Polydimethylsiloxane (PDMS) microchannels were fabricated over ChOx/ZnO/Au/prism system and various concentrations of cholesterol were passed over the sensor surface. The concentration of cholesterol was varied from 0.12 to 10.23 mM and the SPR reflectance curves were recorded in both static as well as dynamic modes demonstrating a high sensitivity of 0.36° mM-1.

The effect of metal surface passivation on the Au-InP interaction

NASA Technical Reports Server (NTRS)

Fatemi, Navid S.; Weizer, Victor G.

1989-01-01

The effect of SiO2 encapsulation on reaction rates in the Au-InP system was studied. Scanning electron microscopy and x-ray photoelectron spectroscopy were used to investigate surface and/or interface morphologies and in-depth compositional profiles. It was found that the rate of dissolution of InP into Au and subsequent phase transformations are largely dependent on the condition of the free surface of the metalization. SiO2 capping of Au is reported for the first time to suppress the Au-InP reaction rate. The Au-InP interaction is shown to be quite similar to the Au-GaAs interaction despite differences in behavior of the group-V elements.

Bi-functional Au/FeS (Au/Co3O4) composite for in situ SERS monitoring and degradation of organic pollutants

NASA Astrophysics Data System (ADS)

Ma, Shuzhen; Cai, Qian; Lu, Kailing; Liao, Fan; Shao, Mingwang

2016-01-01

The bi-functional Au/FeS (Au/Co3O4) composite was fabricated by in situ reducing Au nanoparticles onto the surface of FeS (Co3O4). The as-prepared FeS possessed a multi-structure composed of plenty of nanoplates, which were coated by Au nanoparticles with an average size of 47.5 nm. While the Co3O4 showed a thin hexagonal sheet containing Au nanoparticles on its surface with an average size of 79.0 nm. Both the as-prepared Au/FeS and Au/Co3O4 composites exhibited excellent SERS performance, capable of enhancing the Raman signals of R6G molecules with the enhancement factor up to 1.81 × 106 and 7.60 × 104, respectively. Moreover, Au/FeS (Au/Co3O4) composite also has been verified to have intrinsic peroxidase-like activity, which could decompose H2O2 into hydroxyl radicals and then degrade organic pollutants into small molecules. Therefore, SERS can be used to real-time and in situ monitoring the degradation process of R6G molecules, employing the Au/FeS (Au/Co3O4) composite both as SERS substrate and catalyst.

Au/ZnO nanoarchitectures with Au as both supporter and antenna of visible-light

NASA Astrophysics Data System (ADS)

Liu, Tianyu; Chen, Wei; Hua, Yuxiang; Liu, Xiaoheng

2017-01-01

In this paper, we fabricate Au/ZnO nanostructure with smaller ZnO nanoparticles loaded onto bigger gold nanoparticles via combining seed-mediated method and sol-gel method. The obtained Au/ZnO nanocomposites exhibit excellent properties in photocatalysis process like methyl orange (MO) degradation and oxidative conversion of methanol into formaldehyde under visible light irradiation. The enhanced properties were ascribed to the surface plasmon resonance (SPR) effect of Au nanoparticles, which could contribute to the separation of photo-excited electrons and holes and facilitate the process of absorbing visible light. This paper contributes to the emergence of multi-functional nanocomposites with possible applications in visible-light driven photocatalysts and makes the Au/ZnO photocatalyst an exceptional choice for practical applications such as environmental purification of organic pollutants in aqueous solution and the synthesis of fine chemicals and intermediates.

Atomic structure of (111) SrTiO3/Pt interfaces

NASA Astrophysics Data System (ADS)

Schmidt, Steffen; Klenov, Dmitri O.; Keane, Sean P.; Lu, Jiwei; Mates, Thomas E.; Stemmer, Susanne

2006-03-01

Atomic resolution high-angle annular dark field (HAADF) imaging in scanning transmission electron microscopy was used to investigate the interface atomic structure of epitaxial, (111) oriented SrTiO3 films on epitaxial Pt electrodes grown on (0001) sapphire. The cube-on-cube orientation relationship of SrTiO3 on Pt was promoted by the use of a Ti adhesion layer underneath the Pt electrode. While a Ti-rich Pt surface was observed before SrTiO3 growth, HAADF images showed an atomically abrupt SrTiO3/Pt interface with no interfacial layers. The SrTiO3 films contained two twin variants that were related by a 180° rotation about the ⟨111⟩ surface normal. HAADF images showed two different interface atomic arrangements for the two twins. The role of Ti in promoting (111) epitaxy and the implications for the dielectric properties are discussed.

Sandwiched ZnO@Au@CdS nanorod arrays with enhanced visible-light-driven photocatalytical performance

NASA Astrophysics Data System (ADS)

Ren, Shoutian; Wang, Yingying; Fan, Guanghua; Gao, Renxi; Liu, Wenjun

2017-11-01

The development of high-performance photocatalysts is central to efforts focused on taking advantage of solar energy to overcome environmental and energy crises. Integrating different functional materials artfully into nanostructures can deliver more efficient photocatalytic activity. Here, sandwiched ZnO@Au@CdS nanorod films were synthesized via successive ZnO nanorod electrodeposition, Au sputtering and CdS electrodeposition. The as-synthesized composites were characterized by UV-vis spectrophotometer, x-ray diffractometer, scanning and transmission electron microscopy. Their photocatalytic activity was assessed by degrading Rhodamine B solution under visible light irradiation. ZnO@Au@CdS exhibited better photocatalytic performance than ZnO@CdS throughout the visible light region, and the corresponding enhancement factor of Au nanoparticles was measured as a function of CdS loading amount, and it could reach 190% with CdS deposition for 1 min. The normalized rate constant could reach 0.387 h-1 for ZnO@Au@CdS-1min, which was equivalent to or better than results in reference photocatalysts. The enhancement mechanism of Au nanoparticles was estimated by comparing the monochromatic photocatalytic action spectra with the absorption spectrum of ZnO@Au@CdS, and it was mainly determined by incident photon energy. With selective excitation of Au nanoparticles by incident photons, the excited hot electrons in Au NPs are transferred to the conduction band of ZnO to boost photocatalytic reaction. With selective excitation of CdS, the enhanced interband absorption of CdS and relay station effect of Au nanoparticles should be responsible for the enhanced photocatalytic performance. Our work not only opens the door to the design of efficient supported photocatalysts, but also helps to understand the enhancement mechanism of LSPR effect on the photoelectric conversion of semiconductors.

Electrochemical and PM-IRRAS Studies of the Effect of Cholesterol on the Structure of a DMPC Bilayer Supported at an Au (111) Electrode Surface, Part 1: Properties of the Acyl Chains

PubMed Central

Bin, Xiaomin; Horswell, Sarah L.; Lipkowski, Jacek

2005-01-01

Charge density measurements and polarization modulation infrared reflection absorption spectroscopy were employed to investigate the spreading of small unilamellar vesicles of a dimyristoylphosphatidylcholine (DMPC)/cholesterol (7:3 molar ratio) mixture onto an Au (111) electrode surface. The electrochemical experiments demonstrated that vesicles fuse and spread onto the Au (111) electrode surface, forming a bilayer, at rational potentials −0.4 V < (E − Epzc) < 0.4 V or field strength <6×107 V m−1. Polarization modulation infrared reflection absorption spectroscopy experiments provided information concerning the conformation and orientation of the acyl chains of DMPC molecules. Deuterated DMPC was used to subtract the contribution of C-H stretching bands of cholesterol and of the polar head region of DMPC from spectra in the C-H stretching region. The absorption spectra of the C-H stretch bands in the acyl chains were determined in this way. The properties of the DMPC/cholesterol bilayer have been compared with the properties of a pure DMPC bilayer. The presence of 30% cholesterol gives a thicker and more fluid bilayer characterized by a lower capacity and lower tilt angle of the acyl chains. PMID:15849259

Fabrication of Au- and Ag–SiO{sub 2} inverse opals having both localized surface plasmon resonance and Bragg diffraction

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

Erola, Markus O.A.; Philip, Anish; Ahmed, Tanzir

The inverse opal films of SiO{sub 2} containing metal nanoparticles can have both the localized surface plasmon resonance (LSPR) of metal nanoparticles and the Bragg diffraction of inverse opal crystals of SiO{sub 2}, which are very useful properties for applications, such as tunable photonic structures, catalysts and sensors. However, effective processes for fabrication of these films from colloidal particles have rarely been reported. In our study, two methods for preparation of inverse opal films of SiO{sub 2} with three different crystal sizes and containing gold or silver nanoparticles (NPs) via self-assembly using electrostatic interactions and capillary forces are reported. Themore » Bragg diffraction of inverse opal films of SiO{sub 2} in the presence and absence of the template was measured and predicted on the basis of with UV–vis spectroscopy and scanning electron microscopy. The preparation methods used provided good-quality inverse opal SiO{sub 2} films containing highly dispersed, plasmonic AuNPs or AgNPs and having both Bragg diffractions and LSPRs. - Graphical abstract: For syntheses of SiO{sub 2} inverse opals containing Au/Ag nanoparticles two approaches and three template sizes were employed. Self-assembly of template molecules and metal nanoparticles occurred using electrostatic interactions and capillary forces. Both the Bragg diffraction of the photonic crystal and the localized surface plasmon resonance of Au/Ag nanoparticles were detected. - Highlights: • Fabrication methods of silica inverse opals containing metal nanoparticles studied. • Three template sizes used to produce SiO{sub 2} inverse opals with Au/Ag nanoparticles. • PS templates with Au nanoparticles adsorbed used in formation of inverse opals. • Ag particles infiltrated in inverse opals with capillary and electrostatic forces. • Bragg diffractions of IOs and surface plasmon resonances of nanoparticles observed.« less

Electronic, magnetic and optical properties of C- and N-doped CeO2 bulk and (111) surface from first-principles

NASA Astrophysics Data System (ADS)

Dai, Shuhua; Zhou, Wei; Liu, Yanyu; Lu, Yi-Lin; Sun, Lili; Wu, Ping

2018-05-01

The electronic, magnetic and optical properties of C- and N-doped CeO2 bulk and (111) surface have been systematically investigated by first-principles calculations. The results show that the spin splitting occurs when doping atoms replace the anion sites in the CeO2 matrix, causing a local magnetic moment of 2.00, and 1.00 μB for C- and N-doping, respectively. The strong hybridization between dopants 2p and O 2p triplet-states around the Fermi level gives rise to the half-metallic character for doped bulk systems, while substitution onto the surface eliminates the degeneration of dopants 2p orbitals, which results in anisotropic spin atmosphere. Especially, owing to the low formation energy and available RTFM, N-doped CeO2 would be easily realized in the experiment and should also be wonderful candidate materials for oxide spintronics. In addition, compared with N-doped CeO2, the calculated optical properties reveal that C-doped CeO2 (111) surface is able to enhance the absorption of the visible light.

Azobenzene versus 3,3',5,5'-tetra-tert-butyl-azobenzene (TBA) at Au(111): characterizing the role of spacer groups.

PubMed

McNellis, Erik R; Bronner, Christopher; Meyer, Jörg; Weinelt, Martin; Tegeder, Petra; Reuter, Karsten

2010-06-28

We present large-scale density-functional theory (DFT) calculations and temperature programmed desorption measurements to characterize the structural, energetic and vibrational properties of the functionalized molecular switch 3,3',5,5'-tetra-tert-butyl-azobenzene (TBA) adsorbed at Au(111). Particular emphasis is placed on exploring the accuracy of the semi-empirical dispersion correction approach to semi-local DFT (DFT-D) in accounting for the substantial van der Waals component in the surface bonding. In line with previous findings for benzene and pure azobenzene at coinage metal surfaces, DFT-D significantly overbinds the molecule, but seems to yield an accurate adsorption geometry as far as can be judged from the experimental data. Comparing the trans adsorption geometry of TBA and azobenzene at Au(111) reveals a remarkable insensitivity of the structural and vibrational properties of the -N[double bond, length as m-dash]N- moiety. This questions the established view of the role of the bulky tert-butyl-spacer groups for the switching of TBA in terms of a mere geometric decoupling of the photochemically active diazo-bridge from the gold substrate.

Simple model of surface roughness for binary collision sputtering simulations

NASA Astrophysics Data System (ADS)

Lindsey, Sloan J.; Hobler, Gerhard; Maciążek, Dawid; Postawa, Zbigniew

2017-02-01

It has been shown that surface roughness can strongly influence the sputtering yield - especially at glancing incidence angles where the inclusion of surface roughness leads to an increase in sputtering yields. In this work, we propose a simple one-parameter model (the "density gradient model") which imitates surface roughness effects. In the model, the target's atomic density is assumed to vary linearly between the actual material density and zero. The layer width is the sole model parameter. The model has been implemented in the binary collision simulator IMSIL and has been evaluated against various geometric surface models for 5 keV Ga ions impinging an amorphous Si target. To aid the construction of a realistic rough surface topography, we have performed MD simulations of sequential 5 keV Ga impacts on an initially crystalline Si target. We show that our new model effectively reproduces the sputtering yield, with only minor variations in the energy and angular distributions of sputtered particles. The success of the density gradient model is attributed to a reduction of the reflection coefficient - leading to increased sputtering yields, similar in effect to surface roughness.

Negative differential resistance by molecular resonant tunneling between neutral tribenzosubporphine anchored to a Au(111) surface and tribenzosubporphine cation adsorbed on to a tungsten tip.

PubMed

Majima, Yutaka; Ogawa, Daisuke; Iwamoto, Masachika; Azuma, Yasuo; Tsurumaki, Eiji; Osuka, Atsuhiro

2013-09-25

Tribenzosubporphyrins are boron(III)-chelated triangular bowl-shaped ring-contracted porphyrins that possess a 14π-aromatic circuit. Their flat molecular shapes and discrete molecular orbital diagrams make them ideal for observation by scanning tunneling microscopy (STM). Expanding their applications toward single molecule-based devices requires a fundamental knowledge of single molecular conductance between tribenzosubporphines and the STM metal tip. We utilized a tungsten (W) STM tip to investigate the electronic properties of B-(5-mercaptopentoxy)tribenzosubporphine 1 at the single molecular level. B-(5-mercaptopentoxy)-tribenzosubporphine 1 was anchored to the Au(111) surface via reaction with 1-heptanethiol linkers that were preorganized as a self-assembled monolayer (C7S SAM) on the Au(111) substrate. This arrangement ensured that 1 was electronically decoupled from the metal surface. Differential conductance (dI/dV - V) measurements with the bare W tip exhibited a broad gap region of low conductance and three distinct responses at 2.4,-1.3, and -2.1 V. Bias-voltage-dependent STM imaging of 1 at 65 K displayed a triangle shape at -2.1 < V < -1.3 V and a circle shape at V < -2.1 V, reflecting its HOMO and HOMO-1, respectively. In addition, different conductance behaviors were reproducibly observed, which has been ascribed to the adsorption of a tribenzosubporphine-cation on the W tip. When using a W tip doped with preadsorbed tribenzosubporphine-cation, negative differential resistance (NDR) phenomena were clearly observed in a reproducible manner with a peak-to-valley ratio of 2.6, a value confirmed by spatial mapping conductance measurements. Collectively, the observed NDR phenomena have been attributed to effective molecular resonant tunneling between a neutral tribenzosubporphine anchored to the metal surface and a tribenzosubporphine cation adsorbed on a W tip.

Surface electrochemistry of CO on reconstructed gold single crystal surfaces studied by infrared reflection absorption spectroscopy and rotating disk electrode.

PubMed

Blizanac, Berislav B; Arenz, Matthias; Ross, Philip N; Marković, Nenad M

2004-08-18

The electrooxidation of CO has been studied on reconstructed gold single-crystal surfaces by a combination of electrochemical (EC) and infrared reflection absorption spectroscopy (IRAS) measurements. Emphasis is placed on relating the vibrational properties of the CO adlayer to the voltammetric and other macroscopic electrochemical responses, including rotating disk electrode measurements of the catalytic activity. The IRAS data show that the C-O stretching frequencies are strongly dependent on the surface orientation and can be observed in the range 1940-1990 cm(-1) for the 3-fold bridging, 2005-2070 cm(-1) for the 2-fold bridging, and 2115-2140 for the terminal position. The most complex CO spectra are found for the Au(110)-(1 x 2) surface, i.e., a band near 1965 cm(-1), with the second, weaker band shifted positively by about 45 cm(-1) and, finally, a weak band near 2115 cm(-1). While the C-O stretching frequencies for a CO adlayer adsorbed on Au(111)-(1 x 23) show nu(CO) bands at 2029-2069 cm(-1) and at 1944-1986 cm(-1), on the Au(100)-"hex" surface a single CO band is observed at 2004-2029 cm(-1). In the "argon-purged" solution, the terminal nu(CO) band on Au(110)-(1 x 2) and the 3-fold bridging band on the Au(111)-(1 x 23) disappear entirely. The IRAS/EC data show that the kinetics of CO oxidation are structure sensitive; i.e., the onset of CO oxidation increases in the order Au(110)-(1 x 2) > or = Au(100)-"hex" > Au(111)-(1 x 23). Possible explanations for the structure sensitivity are discussed.

Chemical waves in the O2 + H2 reaction on a Rh(111) surface alloyed with nickel. II. Photoelectron spectroscopy and microscopy

NASA Astrophysics Data System (ADS)

Smolinsky, Tim; Homann, Mathias; von Boehn, Bernhard; Gregoratti, Luca; Amati, Matteo; Al-Hada, Mohamed; Sezen, Hikmet; Imbihl, Ronald

2018-04-01

Chemical waves in the H2 + O2 reaction on a Rh(111) surface alloyed with Ni [ΘNi < 1.5 monolayers (ML)] have been investigated in the 10-7 and 10-6 mbar range at T = 773 K using scanning photoelectron microscopy and x-ray photoelectron spectroscopy as in situ methods. The local intensity variations of the O 1s and the Ni 2p signal display an anticorrelated behavior. The coincidence of a high oxygen signal with a low Ni 2p intensity, which seemingly contradicts the chemical attraction between O and Ni, has been explained with a phase separation of the oxygen covered Rh(111)/Ni surface into a 3D-Ni oxide and into a Ni poor metallic phase. Macroscopic NiO islands (≈1 μm size) formed under reaction conditions have been identified as 2D-Ni oxide. Titration experiments of the oxygen covered Rh(111)/Ni surface with H2 demonstrated that the reactivity of oxygen is decreased by an order of magnitude through the addition of 0.6 ML Ni. An excitation mechanism is proposed in which the periodic formation and reduction of NiO modulate the catalytic activity.

Surface-structure sensitivity of CeO 2 nanocrystals in photocatalysis and enhancing the reactivity with nanogold

DOE PAGES

Lei, Wanying; Zhang, Tingting; Gu, Lin; ...

2015-06-19

Structure–function correlations are a central theme in heterogeneous (photo)catalysis. In this research, using aberration-corrected scanning transmission electron microscopy (STEM), the atomic surface structures of well-defined one-dimensional (1D) CeO 2 nanorods (NRs) and 3D nanocubes (NCs) are directly visualized at subangstrom resolution. CeO 2 NCs predominantly expose the {100} facet, with {110} and {111} as minor cutoff facets at the respective edges and corners. Notably, the outermost surface layer of the {100} facet is nearly O-terminated. Neither surface relaxations nor reconstructions on {100} are observed, indicating unusual polarity compensation, which is primarily mediated by near-surface oxygen vacancies. The surface of CeOmore » 2 NRs is highly stepped, with the enclosed {110} facet exposing Ce cations and O anions on terraces. On the basis of STEM profile-view imaging and electronic structure analysis, the photoreactivity of CeO2 nanocrystals toward aqueous methyl orange degradation under UV is revealed to be surface-structure-sensitive, following the order: {110} >> {100}. The underlying surface-structure sensitivity can be attributed to the variation in low-coordinate surface cerium cations between {110} and {100} facets. To further enhance light absorption, Au nanoparticles (NPs) are deposited on CeO 2 NRs to form Au/CeO 2 plasmonic nanocomposites, which dramatically promotes the photoreactivity that is Au particle size- and excitation light wavelength-dependent. The mechanisms responsible for the enhancement of photocatalytic activity are discussed, highlighting the crucial role of photoexcited charge carrier transfer.« less

A Closer Look at Solar Wind Sputtering of Lunar Surface Materials

NASA Technical Reports Server (NTRS)

Barghouty, A. F.; Adams, J. H., Jr.; Meyer, F.; Mansur, L.; Reinhold, C.

2008-01-01

Solar-wind induced potential sputtering of the lunar surface may be a more efficient erosive mechanism than the "standard" kinetic (or physical) sputtering. This is partly based on new but limited laboratory measurements which show marked enhancements in the sputter yields of slow-moving, highly-charged ions impacting oxides. The enhancements seen in the laboratory can be orders of magnitude for some surfaces and highly charged incident ions, but seem to depend very sensitively on the properties of the impacted surface in addition to the fluence, energy and charge of the impacting ion. For oxides, potential sputtering yields are markedly enhanced and sputtered species, especially hydrogen and light ions, show marked dependence on both charge and dose.

Enhanced catalyst activity by decorating of Au on Ag@Cu2O nanoshell

NASA Astrophysics Data System (ADS)

Chen, Lei; Liu, Maomao; Zhao, Yue; Kou, Qiangwei; Wang, Yaxin; Liu, Yang; Zhang, Yongjun; Yang, Jinghai; Jung, Young Mee

2018-03-01

We successfully synthesized Au-decorated Ag@Cu2O heterostructures via a simple galvanic replacement method. As the Au precursor concentration increased, the density of the Au nanoparticles (NPs) on the Ag@Cu2O surface increased, which changed the catalytic activity of the Ag@Cu2O-Au structure. The combination of Au, Ag, and Cu2O exhibited excellent catalytic properties, which can further effect on the catalyst activity of the Ag@Cu2O-Au structure. In addition, the proposed Ag@Cu2O-Au nanocomposite was used to transform the organic, toxic pollutant, 4-nitrophenol (4-NP), into its nontoxic and medicinally important amino derivative via a catalytic reduction to optimize the material performance. The proposed Au-decorated Ag@Cu2O exhibited excellent catalytic activity, and the catalytic reduction time greatly decreased (5 min). Thus, three novel properties of Ag@Cu2O-Au, i.e., charge redistribution and transfer, adsorption, and catalytic reduction of organic pollutants, were ascertained for water remediation. The proposed catalytic properties have potential applications for photocatalysis and localized surface plasmon resonance (LSPR)- and peroxidase-like catalysis.

Structural and optical properties of gold-incorporated diamond-like carbon thin films deposited by RF magnetron sputtering

NASA Astrophysics Data System (ADS)

Majeed, Shahbaz; Siraj, K.; Naseem, S.; Khan, Muhammad F.; Irshad, M.; Faiz, H.; Mahmood, A.

2017-07-01

Pure and gold-doped diamond-like carbon (Au-DLC) thin films are deposited at room temperature by using RF magnetron sputtering in an argon gas-filled chamber with a constant flow rate of 100 sccm and sputtering time of 30 min for all DLC thin films. Single-crystal silicon (1 0 0) substrates are used for the deposition of pristine and Au-DLC thin films. Graphite (99.99%) and gold (99.99%) are used as co-sputtering targets in the sputtering chamber. The optical properties and structure of Au-DLC thin films are studied with the variation of gold concentration from 1%-5%. Raman spectroscopy, atomic force microscopy (AFM), Vickers hardness measurement (VHM), and spectroscopic ellipsometry are used to analyze these thin films. Raman spectroscopy indicates increased graphitic behavior and reduction in the internal stresses of Au-DLC thin films as the function of increasing gold doping. AFM is used for surface topography, which shows that spherical-like particles are formed on the surface, which agglomerate and form larger clusters on the surface by increasing the gold content. Spectroscopy ellipsometry analysis elucidates that the refractive index and extinction coefficient are inversely related and the optical bandgap energy is decreased with increasing gold content. VHM shows that gold doping reduces the hardness of thin films, which is attributed to the increase in sp2-hybridization.

Fabrication of Porous Ag/TiO2/Au Coatings with Excellent Multipactor Suppression

PubMed Central

Wu, Duoduo; Ma, Jianzhong; Bao, Yan; Cui, Wanzhao; Hu, Tiancun; Yang, Jing; Bai, Yuanrui

2017-01-01

Porous Ag/TiO2/Au coatings with excellent multipactor suppression were prepared by fabrication of porous Ag surface through two-step wet chemical etching, synthesis of TiO2 coatings by electroless-plating-like solution deposition and deposition of Au coatings via electroless plating. Porous structure of Ag surface, TiO2 coatings on porous Ag surface and Au coatings on porous Ag/TiO2 surface were verified by field-emission scanning electron microscopy, the composition and crystal type of TiO2 coatings was characterized by X-ray photoelectron spectroscopy and X-ray diffraction. Secondary electron yield (SEY) measurement was used to monitor the SEY coefficient of the porous Ag coatings and Ag/TiO2/Au coatings. The as-obtained porous Ag coatings were proved exhibiting low SEY below 1.2, and the process was highly reproducible. In addition, the porous Ag/TiO2/Au coatings showed excellent multipactor suppression with the SEY 1.23 and good environmental stability. It is worth mentioning that the whole preparation process is simple and feasible, which would provide a promising application in RF devices. PMID:28281546

Surface Chemistry of Trimethylaluminum on Pd(111) and Pt(111)

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

Gharachorlou, Amir; Detwiler, Michael D.; Mayr, Lukas

The behavior of trimethylaluminum (TMA) was investigated on the surfaces of Pt(111) and Pd(111) single crystals. TMA was found to dissociatively adsorb on both surfaces between 300–473 K. Surfaces species observed by high-resolution electron energy loss spectroscopy (HREELS) and X-ray photoelectron spectroscopy (XPS) after TMA adsorption at 300 K included Al-CH3 and CHx,ads (x = 1, 2, or 3) on Pt(111), and ethylidyne (CCH3), CHx,ads (x = 1, 2, or 3), and metallic Al on Pd(111). Density functional theory (DFT) calculations predicted methylaluminum (MA, Al-CH3) to be the most kinetically favorable TMA decomposition product on (111) terraces of both surfaces,more » however, HREELS signatures for Al-CH3 were detected only on Pt(111), whereas ethylidyne was observed on Pd(111). XPS demonstrated higher amounts of carbonaceous species on Pt(111) than on Pd(111). DFT calculations showed that further dissociation of MA to metallic aluminum and methyl groups to be more kinetically favorable on step sites of both metals. In our proposed reaction mechanism, MA migrates to and dissociates at Pd(111) steps at 300 K forming adsorbed methyl groups and metallic Al. Some methyl groups dehydrogenate and recombine forming ethylidyne. Metallic Al or ejected Pd atoms from steps diffuse across Pd(111) terraces until coalescing into irregularly shaped islands on terraces or steps, as observed by scanning tunneling microscopy (STM). Upon heating above 300 K, the Pd–Al alloy diffuses into the Pd bulk. On Pt(111), a high coverage of carbon-containing species following TMA adsorption at 300 K prevented MA diffusion and dissociation at steps, as evidenced by isolated clusters of MA in STM images. Heating above 300 K resulted in MA dissociation, but no Pt–Al alloy formation was observed. We conclude that the differing abilities of Pd and Pt to hydrogenate carbonaceous species plays a key role in MA dissociation and alloy formation, and therefore, the adsorption and dissociation

First-principles theoretical investigation of monoatomic and dimer Mn adsorption on noble metal (111) surfaces

NASA Astrophysics Data System (ADS)

Muñoz, Francisco; Romero, Aldo H.; Mejía-López, Jose; Morán-López, J. L.

2012-03-01

A theoretical investigation of the adsorption of Mn single atoms and dimers on the (111) surface of Cu, Ag, and Au, within the framework of the density functional theory, is presented. First, the bulk and the clean (111) surface electronic structures are calculated, with results that agree well with previous reports. To understand the adatom-substrate interaction, also the electronic characteristics of the free Mn dimer are determined. Then, the electronic structure of the Mn adatom, chemisorbed on four different surface geometries, is analyzed for the three noble metals. It is found that the most stable geometry, in all three cases, Cu, Ag, and Au, occurs when the Mn atom is chemisorbed on threefold coordinated sites. For the dimer, the lowest-energy configuration corresponds to the molecule lying parallel to the surface. In the three noble metals, the geometry corresponds to both atoms chemisorbed in threefold coordinated sites, but with different local symmetry. It is also found that the magnetic configuration with the lowest energy corresponds to the antiferromagnetic arrangement of Mn atoms, with individual magnetic moments close to 5μB. The ferromagnetic and antiferromagnetic solutions, in the case of a Ag substrate, are close in energy. It is also found that in this case the Mn2 molecule is chemisorbed with very similar energy on various geometries. To study the dynamical motion of the dimer components, we calculated the potential energy barriers for the Mn motion in the various surfaces. In contrast to Cu and Au, this leads to the conclusion that on Ag the Mn dimer moves relatively freely.

High rate DC-reactive sputter deposition of Y 2O 3 film on the textured metal substrate for the superconducting coated conductor

NASA Astrophysics Data System (ADS)

Kim, Ho-Sup; Park, Chan; Ko, Rock-Kil; Shi, Dongqui; Chung, Jun-Ki; Ha, Hong-Soo; Park, Yu-Mi; Song, Kyu-Jeong; Youm, Do-Jun

2005-10-01

Y2O3 film was directly deposited on Ni-3at%W substrate by DC reactive sputtering. DC reactive sputtering was carried out using metallic Y target and water vapor for oxidizing the elements of metallic target on the substrate. The detailed conditions of DC reactive sputtering for depositions of Y2O3 films were investigated. The window of water vapor for proper growth of Y2O3 films was determined by sufficient oxidations of the Y2O3 films and the non-oxidation of the target surface, which was required for high rate sputtering. The window turned out to be fairly wide in the chamber used. As the sputtering power was raised, the deposition rate increased without narrowing the window. The fabricated Y2O3 films showed good texture qualities and surface morphologies. The YBCO film deposited directly on the Y2O3 buffered Ni-3at%W substrate showed Tc, Ic (77 K, self field), and Jc (77 K, self field) of 89 K, 64 A/cm and 1.1 MA/cm2, respectively.

Effects of substrate on the structure and orientation of ZnO thin film grown by rf-magnetron sputtering

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

Liu, H. F.; Chua, S. J.; Hu, G. X.

2007-10-15

X-ray diffractions, Nomarski microscopy, scanning electron microscopy, and photoluminescence have been used to study the effects of substrate on the structure and orientation of ZnO thin films grown by rf-magnetron sputtering. GaAs(001), GaAs(111), Al{sub 2}O{sub 3}(0002) (c-plane), and Al{sub 2}O{sub 3}(1102) (r-plane) wafers have been selected as substrates in this study. X-ray diffractions reveal that the ZnO film grown on GaAs(001) substrate is purely textured with a high c-axis orientation while that grown on GaAs(111) substrate is a single ZnO(0002) crystal; a polycrystalline structure with a large-single-crystal area of ZnO(0002) is obtained on a c-plane Al{sub 2}O{sub 3} substrate whilemore » a ZnO(1120) single crystal is formed on an r-plane Al{sub 2}O{sub 3} substrate. There is absence of significant difference between the photoluminescence spectra collected from ZnO/GaAs(001), ZnO/GaAs(111), and ZnO/Al{sub 2}O{sub 3}(0002), while the photoluminescence from ZnO/Al{sub 2}O{sub 3}(1102) shows a reduced intensity together with an increased linewidth, which is, likely, due to the increased incorporation of native defects during the growth of ZnO(1120)« less

Decomposition mechanism of formic acid on Cu (111) surface: A theoretical study

NASA Astrophysics Data System (ADS)

Jiang, Zhao; Qin, Pei; Fang, Tao

2017-02-01

The study of formic acid decomposition on transition metal surfaces is important to obtain useful information for vapor phase catalysis involving HCOOH and for the development of direct formic acid fuel cells. In this study, periodic density functional theory calculations have been employed to investigate the dissociation pathways of HCOOH on Cu (111) surface. About adsorption, it is found that the adsorption of HCOO, COOH, HCO, CO, OH and H on Cu (111) are considered chemisorption, whereas HCOOH, CO2, H2O and H2 have the weak interaction with Cu (111) surface. Furthermore, the minimum energy pathways are analyzed for the decomposition of HCOOH to CO2 and CO through the scission of Hsbnd O, Csbnd H and Csbnd O bonds. It is found that HCOOH, HCOO and COOH prefer to dissociate in the related reactions rather than desorb. For the decomposition, it is indicated that HCO and COOH are the main dissociated intermediates of trans-HCOOH, CO2 is the main dissociated intermediates of bidentate-HCOO, and CO is the main dissociated product of cis-COOH. The co-adsorbed H atom is beneficial for the formation of CO2 from cis-COOH. Besides, it is found that the most favorable path for HCOOH decomposition on Cu (111) surface is HCOOH-HCO-CO (Path 5), where the step of CO formation from HCO dehydrogenation is considered to be the rate-determining step. The results also show that CO is preferentially formed as the dominant product of HCOOH on Cu (111) surface.

Effect of Homo-buffer Layers on the Properties of Sputtering Deposited Ga2O3 Films

NASA Astrophysics Data System (ADS)

Huang, Jian; Li, Bing; Ma, Yuncheng; Tang, Ke; Huang, Haofei; Hu, Yan; Zou, Tianyu; Wang, Linjun

2018-05-01

β- Ga2O3 films were grown by radio-frequency magnetron sputtering method. The influence of Ga2O3 buffer layers and annealing treatment on the structural, optical, morphological and electrical properties of Ga2O3 films was studied. The results revealed an improvement of crystalline quality and transmittance of annealed β- Ga2O3 films prepared with homo-buffer layers. Ga2O3 film UV photodetectors were fabricated with a new B and Ga co-doped ZnO films (BGZO)/Au interdigitated electrode. A good ohmic contact was formed between the film and the electrode. For the detector based on Ga2O3 films with buffer layers, a higher value of photo response and faster response times was obtained.

The structure, energetics, and nature of the chemical bonding of phenylthiol adsorbed on the Au(111) surface: implications for density-functional calculations of molecular-electronic conduction.

PubMed

Bilić, Ante; Reimers, Jeffrey R; Hush, Noel S

2005-03-01

The adsorption of phenylthiol on the Au(111) surface is modeled using Perdew and Wang density-functional calculations. Both direct molecular physisorption and dissociative chemisorption via S-H bond cleavage are considered as well as dimerization to form disulfides. For the major observed product, the chemisorbed thiol, an extensive potential-energy surface is produced as a function of both the azimuthal orientation of the adsorbate and the linear translation of the adsorbate through the key fcc, hcp, bridge, and top binding sites. Key structures are characterized, the lowest-energy one being a broad minimum of tilted orientation ranging from the bridge structure halfway towards the fcc one. The vertically oriented threefold binding sites, often assumed to dominate molecular electronics measurements, are identified as transition states at low coverage but become favored in dense monolayers. A similar surface is also produced for chemisorption of phenylthiol on Ag(111); this displays significant qualitative differences, consistent with the qualitatively different observed structures for thiol chemisorption on Ag and Au. Full contours of the minimum potential energy as a function of sulfur translation over the crystal face are described, from which the barrier to diffusion is deduced to be 5.8 kcal mol(-1), indicating that the potential-energy surface has low corrugation. The calculated bond lengths, adsorbate charge and spin density, and the density of electronic states all indicate that, at all sulfur locations, the adsorbate can be regarded as a thiyl species that forms a net single covalent bond to the surface of strength 31 kcal mol(-1). No detectable thiolate character is predicted, however, contrary to experimental results for alkyl thiols that indicate up to 20%-30% thiolate involvement. This effect is attributed to the asymptotic-potential error of all modern density functionals that becomes manifest through a 3-4 eV error in the lineup of the adsorbate and

Strain evolution of each type of grains in poly-crystalline (Ba,Sr)TiO3 thin films grown by sputtering

PubMed Central

Park, Woo Young; Park, Min Hyuk; Lee, Jong Ho; Yoon, Jung Ho; Han, Jeong Hwan; Choi, Jung-Hae; Hwang, Cheol Seong

2012-01-01

The strain states of [111]-, [110]-, and [002]-oriented grains in poly-crystalline sputtered (Ba,Sr)TiO3 thin films on highly [111]-oriented Pt electrode/Si substrates were carefully examined by X-ray diffraction techniques. Remarkably, [002]-oriented grains respond more while [110]- and [111]-oriented grains do less than the theoretically estimated responses, which is understandable from the arrangement of the TiO6 octahedra with respect to the stress direction. Furthermore, such mechanical responses are completely independent of the degree of crystallization and film thickness. The transition growth temperature between the positive and negative strains was also different depending on the grain orientation. The unstrained lattice parameter for each type of grain was different suggesting that the oxygen vacancy concentration for each type of grain is different, too. The results reveal that polycrystalline (Ba,Sr)TiO3 thin films are not an aggregation of differently oriented grains which simply follow the mechanical behavior of single crystal with different orientations. PMID:23230505

120 MeV Ag ion induced effects in Au/HfO2/Si MOSCAPs

NASA Astrophysics Data System (ADS)

Manikanthababu, N.; Prajna, K.; Pathak, A. P.; Rao, S. V. S. Nageswara

2018-05-01

HfO2/Si thinfilms were deposited by RF sputtering technique. 120 MeV Ag ion irradiation has been used to study the electrical properties of Au/HfO2/Si MOSCAPs. SHI (120 MeV Ag) induced annealing, defects creation and intermixing effects on the electrical properties of these systems have been studied. Here, we have observed that the high electronic excitation can cause a significant reduction of leakage currents in these MOSCAP devices. Various quantum mechanical tunneling phenomenon has been observed from the I-V characteristics.

Communication: Energy transfer and reaction dynamics for DCl scattering on Au(111): An ab initio molecular dynamics study.

PubMed

Kolb, Brian; Guo, Hua

2016-07-07

Scattering and dissociative chemisorption of DCl on Au(111) are investigated using ab initio molecular dynamics with a slab model, in which the top two layers of Au are mobile. Substantial kinetic energy loss in the scattered DCl is found, but the amount of energy transfer is notably smaller than that observed in the experiment. On the other hand, the dissociative chemisorption probability reproduces the experimental trend with respect to the initial kinetic energy, but is about one order of magnitude larger than the reported initial sticking probability. While the theory-experiment agreement is significantly improved from the previous rigid surface model, the remaining discrepancies are still substantial, calling for further scrutiny in both theory and experiment.

Alkylation of Silicon(111) surfaces

NASA Astrophysics Data System (ADS)

Rivillon, S.; Chabal, Y. J.

2006-03-01

Methylation of chlorine-terminated silicon (111) (Si-Cl) is investigated by Infra Red Absorption Spectroscopy (IRAS). Starting from an atomically flat H-terminated Si(111), the surface is first chlorinated by a gas phase process, then methylated using a Grignard reagent. Methyl groups completely replace Cl, and are oriented normal to the surface. The surface remains atomically flat with no evidence of etching.

Surface Demixing in a AuSn Liquid Alloy

NASA Astrophysics Data System (ADS)

Balagurusamy, Venkat; Streitel, Reinhard; Shpyrko, Oleg; Pershan, Peter; Ocko, Ben; Deutsch, Moshe

2006-03-01

We present results of X-ray reflectivity studies of the eutectic AuSn alloy liquid-vapor interface. The analysis shows that in common with the BiSn eutectic, there is surface demixing that extends to more than one monolayer. This is in contrast to a common presumption that the Gibbs adsorption predicts complete demixing only in the surface monolayer. The composition profiles can be explained by surface segregation theory for attractive interaction between Sn and Au atoms, similar to BiIn [1] and BiSn [2]. [1] E. DiMasi, H. Tostmann, O. G. Shpyrko, P. Huber, B. M. Ocko, P. S. Pershan, M. Deutsch, and L. E. Berman, Phys. Rev. Lett. 86, 1538 (2001) [2] O. G. Shpyrko, A. Y. Grigoriev, R. Streitel, D. Pontoni, P. S. Pershan, M. Deutsch, and B. M. Ocko, Phys. Rev. Lett. 95, 106103 (2005) *Present address: Center for Nanoscale Materials, ANL

Photoinduced Glycerol Oxidation over Plasmonic Au and AuM (M = Pt, Pd and Bi) Nanoparticle-Decorated TiO2 Photocatalysts

PubMed Central

Jedsukontorn, Trin; Saito, Nagahiro; Hunsom, Mali

2018-01-01

In this study, sol-immobilization was used to prepare gold nanoparticle (Au NP)-decorated titanium dioxide (TiO2) photocatalysts at different Au weight % (wt. %) loading (Aux/TiO2, where x is the Au wt. %) and Au–M NP-decorated TiO2 photocatalysts (Au3M3/TiO2), where M is bismuth (Bi), platinum (Pt) or palladium (Pd) at 3 wt. %. The Aux/TiO2 photocatalysts exhibited a stronger visible light absorption than the parent TiO2 due to the localized surface plasmon resonance effect. Increasing the Au content from 1 wt. % to 7 wt. % led to increased visible light absorption due to the increasing presence of defective structures that were capable of enhancing the photocatalytic activity of the as-prepared catalyst. The addition of Pt and Pd coupled with the Au3/TiO2 to form Au3M3/TiO2 improved the photocatalytic activity of the Au3/TiO2 photocatalyst by maximizing their light-absorption property. The Au3/TiO2, Au3Pt3/TiO2 and Au3Pd3/TiO2 photocatalysts promoted the formation of glyceraldehyde from glycerol as the principle product, while Au3Bi3/TiO2 facilitated glycolaldehyde formation as the major product. Among all the prepared photocatalysts, Au3Pd3/TiO2 exhibited the highest photocatalytic activity with a 98.75% glycerol conversion at 24 h of reaction time. PMID:29690645

Characterization and methanol electrooxidation studies of Pt(111)/Os surfaces prepared by spontaneous deposition.

PubMed

Johnston, Christina M; Strbac, Svetlana; Lewera, Adam; Sibert, Eric; Wieckowski, Andrzej

2006-09-12

Catalytic activity of the Pt(111)/Os surface toward methanol electrooxidation was optimized by exploring a wide range of Os coverage. Various methods of surface analyses were used, including electroanalytical, STM, and XPS methods. The Pt(111) surface was decorated with nanosized Os islands by spontaneous deposition, and the Os coverage was controlled by changing the exposure time to the Os-containing electrolyte. The structure of Os deposits on Pt(111) was characterized and quantified by in situ STM and stripping voltammetry. We found that the optimal Os surface coverage of Pt(111) for methanol electrooxidation was 0.7 +/- 0.1 ML, close to 1.0 +/- 0.1 Os packing density. Apparently, the high osmium coverage Pt(111)/Os surface provides more of the necessary oxygen-containing species (e.g., Os-OH) for effective methanol electrooxidation than the Pt(111)/Os surfaces with lower Os coverage (vs e.g., Ru-OH). Supporting evidence for this conjecture comes from the CO electrooxidation data, which show that the onset potential for CO stripping is lowered from 0.53 to 0.45 V when the Os coverage is increased from 0.2 to 0.7 ML. However, the activity of Pt(111)/Os for methanol electrooxidation decreases when the Os coverage is higher than 0.7 +/- 0.1 ML, indicating that Pt sites uncovered by Os are necessary for sustaining significant methanol oxidation rates. Furthermore, osmium is inactive for methanol electrooxidation when the platinum substrate is absent: Os deposits on Au(111), a bulk Os ingot, and thick films of electrodeposited Os on Pt(111), all compare poorly to Pt(111)/Os. We conclude that a bifunctional mechanism applies to the methanol electrooxidation similarly to Pt(111)/Ru, although with fewer available Pt sites. Finally, the potential window for methanol electrooxidation on Pt(111)/Os was observed to shift positively versus Pt(111)/Ru. Because of the difference in the Os and Ru oxophilicity under electrochemical conditions, the Os deposit provides fewer

Sputtering of ices in the outer solar system

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

Johnson, R.E.

1996-01-01

Exploration of the outer solar system has led to studies in a new area of physics: electronically induced sputtering of low-temperature, condensed-gas solids (ices). Many of the icy bodies in the outer solar system were found to be bombarded by relatively intense fluxes of ions and electrons, causing both changes in their optical reflectance and ejection (sputtering) of molecules from their surfaces. The small cohesive energies of the condensed-gas solids afford relatively large sputtering rates from the electronic excitations produced in the solid by fast ions and electrons. Such sputtering produces an ambient gas about an icy body, often themore » source of the local plasma. This colloquium outlines the physics of the sputtering of ices and its relevance to several outer-solar-system phenomena: the sputter-produced plasma trapped in Saturn{close_quote}s magnetosphere; the O{sub 2} atmosphere on Europa; and optical absorption features such as SO{sub 2} in the surface of Europa and O{sub 2} and, possibly, O{sub 3} in the surface of Ganymede. {copyright} {ital 1996 The American Physical Society.}« less

Gold surfaces and nanoparticles are protected by Au(0)-thiyl species and are destroyed when Au(I)-thiolates form.

PubMed

Reimers, Jeffrey R; Ford, Michael J; Halder, Arnab; Ulstrup, Jens; Hush, Noel S

2016-03-15

The synthetic chemistry and spectroscopy of sulfur-protected gold surfaces and nanoparticles is analyzed, indicating that the electronic structure of the interface is Au(0)-thiyl, with Au(I)-thiolates identified as high-energy excited surface states. Density-functional theory indicates that it is the noble character of gold and nanoparticle surfaces that destabilizes Au(I)-thiolates. Bonding results from large van der Waals forces, influenced by covalent bonding induced through s-d hybridization and charge polarization effects that perturbatively mix in some Au(I)-thiolate character. A simple method for quantifying these contributions is presented, revealing that a driving force for nanoparticle growth is nobleization, minimizing Au(I)-thiolate involvement. Predictions that Brust-Schiffrin reactions involve thiolate anion intermediates are verified spectroscopically, establishing a key feature needed to understand nanoparticle growth. Mixing of preprepared Au(I) and thiolate reactants always produces Au(I)-thiolate thin films or compounds rather than monolayers. Smooth links to O, Se, Te, C, and N linker chemistry are established.

Reaction mechanism of WGS and PROX reactions catalyzed by Pt/oxide catalysts revealed by an FeO(111)/Pt(111) inverse model catalyst.

PubMed

Xu, Lingshun; Wu, Zongfang; Jin, Yuekang; Ma, Yunsheng; Huang, Weixin

2013-08-07

We have employed XPS and TDS to study the adsorption and surface reactions of H2O, CO and HCOOH on an FeO(111)/Pt(111) inverse model catalyst. The FeO(111)-Pt(111) interface of the FeO(111)/Pt(111) inverse model catalyst exposes coordination-unsaturated Fe(II) cations (Fe(II)CUS) and the Fe(II)CUS cations are capable of modifying the reactivity of neighbouring Pt sites. Water facilely dissociates on the Fe(II)CUS cations at the FeO(111)-Pt(111) interface to form hydroxyls that react to form both water and H2 upon heating. Hydroxyls on the Fe(II)CUS cations can react with CO(a) on the neighbouring Pt(111) sites to produce CO2 at low temperatures. Hydroxyls act as the co-catalyst in the CO oxidation by hydroxyls to CO2 (PROX reaction), while they act as one of the reactants in the CO oxidation by hydroxyls to CO2 and H2 (WGS reaction), and the recombinative reaction of hydroxyls to produce H2 is the rate-limiting step in the WGS reaction. A comparison of reaction behaviors between the interfacial CO(a) + OH reaction and the formate decomposition reaction suggest that formate is the likely surface intermediate of the CO(a) + OH reaction. These results provide some solid experimental evidence for the associative reaction mechanism of WGS and PROX reactions catalyzed by Pt/oxide catalysts.

Selected Growth of Cubic and Hexagonal GaN Epitaxial Films on Polar MgO(111)

NASA Astrophysics Data System (ADS)

Lazarov, V. K.; Zimmerman, J.; Cheung, S. H.; Li, L.; Weinert, M.; Gajdardziska-Josifovska, M.

2005-06-01

Selected molecular beam epitaxy of zinc blende (111) or wurtzite (0001) GaN films on polar MgO(111) is achieved depending on whether N or Ga is deposited first. The cubic stacking is enabled by nitrogen-induced polar surface stabilization, which yields a metallic MgO(111)-(1×1)-ON surface. High-resolution transmission electron microscopy and density functional theory studies indicate that the atomically abrupt semiconducting GaN(111)/MgO(111) interface has a Mg-O-N-Ga stacking, where the N atom is bonded to O at a top site. This specific atomic arrangement at the interface allows the cubic stacking to more effectively screen the substrate and film electric dipole moment than the hexagonal stacking, thus stabilizing the zinc blende phase even though the wurtzite phase is the ground state in the bulk.

Operation of the GaSb p-channel metal-oxide-semiconductor field-effect transistors fabricated on (111)A surfaces

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

Nishi, K., E-mail: nishi@mosfet.t.u-tokyo.ac.jp; Takenaka, M.; Takagi, S.

2014-12-08

We demonstrate the operation of GaSb p-channel metal-oxide-semiconductor field-effect transistors (p-MOSFETs) on (111)A surfaces with Al{sub 2}O{sub 3} gate dielectrics formed by atomic-layer deposition at 150 °C. The p-MOSFETs on (111)A surfaces exhibit higher drain current and lower subthreshold swing than those on (100) surfaces. We find that the interface-state density (D{sub it}) values at the Al{sub 2}O{sub 3}/GaSb MOS interfaces on the (111)A surfaces are lower than those on the (100) surfaces, which can lead to performance enhancement of the GaSb p-MOSFETs on (111)A surfaces. The mobility of the GaSb p-MOSFETs on (111)A surfaces is 80% higher than that onmore » (100) surfaces.« less

Comparison of the surfaces and interfaces formed for sputter and electroless deposited gold contacts on CdZnTe

NASA Astrophysics Data System (ADS)

Bell, Steven J.; Baker, Mark A.; Duarte, Diana D.; Schneider, Andreas; Seller, Paul; Sellin, Paul J.; Veale, Matthew C.; Wilson, Matthew D.

2018-01-01

Cadmium zinc telluride (CdZnTe) is a leading sensor material for spectroscopic X/γ-ray imaging in the fields of homeland security, medical imaging, industrial analysis and astrophysics. The metal-semiconductor interface formed during contact deposition is of fundamental importance to the spectroscopic performance of the detector and is primarily determined by the deposition method. A multi-technique analysis of the metal-semiconductor interface formed by sputter and electroless deposition of gold onto (111) aligned CdZnTe is presented. Focused ion beam (FIB) cross section imaging, X-ray photoelectron spectroscopy (XPS) depth profiling and current-voltage (IV) analysis have been applied to determine the structural, chemical and electronic properties of the gold contacts. In a novel approach, principal component analysis has been employed on the XPS depth profiles to extract detailed chemical state information from different depths within the profile. It was found that electroless deposition forms a complicated, graded interface comprised of tellurium oxide, gold/gold telluride particulates, and cadmium chloride. This compared with a sharp transition from surface gold to bulk CdZnTe observed for the interface formed by sputter deposition. The electronic (IV) response for the detector with electroless deposited contacts was symmetric, but was asymmetric for the detector with sputtered gold contacts. This is due to the electroless deposition degrading the difference between the Cd- and Te-faces of the CdZnTe (111) crystal, whilst these differences are maintained for the sputter deposited gold contacts. This work represents an important step in the optimisation of the metal-semiconductor interface which currently is a limiting factor in the development of high resolution CdZnTe detectors.

In situ Investigation of Methane Dry Reforming on M-CeO 2(111) {M= Co, Ni, Cu} Surfaces: Metal-Support Interactions and the activation of C-H bonds at Low Temperature

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

Rodriguez, Jose A.; Liu, Zongyuan; Lustemberg, Pablo

Studies with a series of M-CeO 2(111) {M= Co, Ni, Cu} surfaces indicate that metal-oxide interactions can play a very important role for the activation of methane and its reforming with CO 2 at relatively low temperatures (600-700 K). Among the systems examined, Co-CeO 2(111) exhibits the best performance and Cu-CeO 2(111) has negligible activity. Experiments using ambient pressure XPS indicate that methane dissociates on Co-CeO2(111), at temperatures as low as 300 K, generating CH x and CO x species on the catalyst surface. The results of density-functional calculations show a reduction in the methane activation barrier from 1.07 eVmore » on Co(0001) to 0.87 eV on Co 2+/CeO 2(111), and to only 0.05 eV on Co 0/CeO 2-x(111). At 700 K, under methane dry reforming conditions, CO 2 dissociates on the oxide surface and a catalytic cycle is established without coke deposition. In conclusion, a significant part of the CH x formed on the Co 0/CeO 2-x (111) catalyst recombines to yield ethane or ethylene.« less

In situ Investigation of Methane Dry Reforming on M-CeO 2(111) {M= Co, Ni, Cu} Surfaces: Metal-Support Interactions and the activation of C-H bonds at Low Temperature

DOE PAGES

Rodriguez, Jose A.; Liu, Zongyuan; Lustemberg, Pablo; ...

2017-08-16

Studies with a series of M-CeO 2(111) {M= Co, Ni, Cu} surfaces indicate that metal-oxide interactions can play a very important role for the activation of methane and its reforming with CO 2 at relatively low temperatures (600-700 K). Among the systems examined, Co-CeO 2(111) exhibits the best performance and Cu-CeO 2(111) has negligible activity. Experiments using ambient pressure XPS indicate that methane dissociates on Co-CeO2(111), at temperatures as low as 300 K, generating CH x and CO x species on the catalyst surface. The results of density-functional calculations show a reduction in the methane activation barrier from 1.07 eVmore » on Co(0001) to 0.87 eV on Co 2+/CeO 2(111), and to only 0.05 eV on Co 0/CeO 2-x(111). At 700 K, under methane dry reforming conditions, CO 2 dissociates on the oxide surface and a catalytic cycle is established without coke deposition. In conclusion, a significant part of the CH x formed on the Co 0/CeO 2-x (111) catalyst recombines to yield ethane or ethylene.« less

Polymorphism and metal-induced structural transformation in 5,5'-bis(4-pyridyl)(2,2'-bispyrimidine) adlayers on Au(111).

PubMed

Hötger, Diana; Carro, Pilar; Gutzler, Rico; Wurster, Benjamin; Chandrasekar, Rajadurai; Klyatskaya, Svetlana; Ruben, Mario; Salvarezza, Roberto C; Kern, Klaus; Grumelli, Doris

2018-05-31

Metal-organic coordination networks self-assembled on surfaces have emerged as functional low-dimensional architectures with potential applications ranging from the fabrication of functional nanodevices to electrocatalysis. Among them, bis-pyridyl-bispyrimidine (PBP) and Fe-PBP on noble metal surfaces appear as interesting systems in revealing the details of the molecular self-assembly and the effect of metal incorporation on the organic network arrangement. Herein, we report a combined STM, XPS, and DFT study revealing polymorphism in bis-pyridyl-bispyrimidine adsorbed adlayers on the reconstructed Au(111) surface. The polymorphic structures are converted by the addition of Fe adatoms into one unique Fe-PBP surface structure. DFT calculations show that while all PBP phases exhibit a similar thermodynamic stability, metal incorporation selects the PBP structure that maximizes the number of metal-N close contacts. Charge transfer from the Fe adatoms to the Au substrate and N-Fe interactions stabilize the Fe-PBP adlayer. The increased thermodynamic stability of the metal-stabilized structure leads to its sole expression on the surface.

Vibrational energy transfer near a dissociative adsorption transition state: State-to-state study of HCl collisions at Au(111).

PubMed

Geweke, Jan; Shirhatti, Pranav R; Rahinov, Igor; Bartels, Christof; Wodtke, Alec M

2016-08-07

In this work we seek to examine the nature of collisional energy transfer between HCl and Au(111) for nonreactive scattering events that sample geometries near the transition state for dissociative adsorption by varying both the vibrational and translational energy of the incident HCl molecules in the range near the dissociation barrier. Specifically, we report absolute vibrational excitation probabilities for HCl(v = 0 → 1) and HCl(v = 1 → 2) scattering from clean Au(111) as a function of surface temperature and incidence translational energy. The HCl(v = 2 → 3) channel could not be observed-presumably due to the onset of dissociation. The excitation probabilities can be decomposed into adiabatic and nonadiabatic contributions. We find that both contributions strongly increase with incidence vibrational state by a factor of 24 and 9, respectively. This suggests that V-T as well as V-EHP coupling can be enhanced near the transition state for dissociative adsorption at a metal surface. We also show that previously reported HCl(v = 0 → 1) excitation probabilities [Q. Ran et al., Phys. Rev. Lett. 98, 237601 (2007)]-50 times smaller than those reported here-were influenced by erroneous assignment of spectroscopic lines used in the data analysis.

Systematic Control of Self-Assembled Au Nanoparticles and Nanostructures Through the Variation of Deposition Amount, Annealing Duration, and Temperature on Si (111).

PubMed

Li, Ming-Yu; Sui, Mao; Pandey, Puran; Zhang, Quanzhen; Kim, Eun-Soo; Lee, Jihoon

2015-12-01

The size, density, and configurations of Au nanoparticles (NPs) can play important roles in controlling the electron mobility, light absorption, and localized surface plasmon resonance, and further in the Au NP-assisted nanostructure fabrications. In this study, we present a systematical investigation on the evolution of Au NPs and nanostructures on Si (111) by controlling the deposition amount (DA), annealing temperature (AT), and dwelling time (DT). Under an identical growth condition, the morphologies of Au NPs and nanostructures drastically evolve when the DA is only slightly varied, based on the Volmer-Weber and coalescence models: i.e. I: mini NPs, II: mid-sized round dome-shaped Au NPs, III: large Au NPs, and IV: coalesced nanostructures. With the AT control, three distinctive ranges are observed: i.e., NP nucleation, Au NPs maturation and melting. The gradual dimensional expansion of Au NPs is always compensated with the density reduction, which is explained with the thermodynamic theory. The DT effect is relatively minor on Au NPs, a sharp contrast to other metallic NPs, which is discussed based on the Ostwald-ripening.

Conductive atomic force microscopy study of the photoexcitation effect on resistive switching in ZrO2(Y) films with Au nanoparticles

NASA Astrophysics Data System (ADS)

Novikov, A. S.; Filatov, D. O.; Antonov, D. A.; Antonov, I. N.; Shenina, M. E.; Gorshkov, O. N.

2018-03-01

We report on the experimental observation of the effect of optical excitation on resistive switching in ultrathin ZrO2(Y) films with single-layered arrays of Au nanoparticles. The samples were prepared by depositing nanometer-thick Au films sandwiched between two ZrO2(Y) layers by magnetron sputtering followed by annealing. Resistive switching was studied by conductive atomic force microscopy by measuring cyclic current-voltage curves of a probe-to-sample contact. The contact area was illuminated by radiation of a semiconductor laser diode with the wavelength corresponding to the plasmon resonance in an Au nanoparticle array. The enhancement of the hysteresis in cyclic current-voltage curves due to bipolar resistive switching under illumination was observed. The effect was attributed to heating of Au nanoparticles due to plasmonic optical absorption and a plasmon resonance, which enhances internal photoemission of electrons from the Fermi level in Au nanoparticles into the conduction band of ZrO2(Y). Both factors promote resistive switching in a ZrO2(Y) matrix.

UV-Enhanced Ethanol Sensing Properties of RF Magnetron-Sputtered ZnO Film.

PubMed

Huang, Jinyu; Du, Yu; Wang, Quan; Zhang, Hao; Geng, Youfu; Li, Xuejin; Tian, Xiaoqing

2017-12-26

ZnO film was deposited by the magnetron sputtering method. The thickness of ZnO film is approximately 2 μm. The influence of UV light illumination on C₂H₅OH sensing properties of ZnO film was investigated. Gas sensing results revealed that the UV-illuminated ZnO film displays excellent C₂H₅OH characteristics in terms of high sensitivity, excellent selectivity, rapid response/recovery, and low detection limit down to 0.1 ppm. The excellent sensing performance of the sensor with UV activation could be attributed to the photocatalytic oxidation of ethanol on the surface of the ZnO film, the planar film structure with high utilizing efficiency of UV light, high electron mobility, and a good surface/volume ratio of of ZnO film with a relatively rough and porous surface.

Complete Au@ZnO core-shell nanoparticles with enhanced plasmonic absorption enabling significantly improved photocatalysis

NASA Astrophysics Data System (ADS)

Sun, Yiqiang; Sun, Yugang; Zhang, Tao; Chen, Guozhu; Zhang, Fengshou; Liu, Dilong; Cai, Weiping; Li, Yue; Yang, Xianfeng; Li, Cuncheng

2016-05-01

Nanostructured ZnO exhibits high chemical stability and unique optical properties, representing a promising candidate among photocatalysts in the field of environmental remediation and solar energy conversion. However, ZnO only absorbs the UV light, which accounts for less than 5% of total solar irradiation, significantly limiting its applications. In this article, we report a facile and efficient approach to overcome the poor wettability between ZnO and Au by carefully modulating the surface charge density on Au nanoparticles (NPs), enabling rapid synthesis of Au@ZnO core-shell NPs at room temperature. The resulting Au@ZnO core-shell NPs exhibit a significantly enhanced plasmonic absorption in the visible range due to the Au NP cores. They also show a significantly improved photocatalytic performance in comparison with their single-component counterparts, i.e., the Au NPs and ZnO NPs. Moreover, the high catalytic activity of the as-synthesized Au@ZnO core-shell NPs can be maintained even after many cycles of photocatalytic reaction. Our results shed light on the fact that the Au@ZnO core-shell NPs represent a promising class of candidates for applications in plasmonics, surface-enhanced spectroscopy, light harvest devices, solar energy conversion, and degradation of organic pollutants.Nanostructured ZnO exhibits high chemical stability and unique optical properties, representing a promising candidate among photocatalysts in the field of environmental remediation and solar energy conversion. However, ZnO only absorbs the UV light, which accounts for less than 5% of total solar irradiation, significantly limiting its applications. In this article, we report a facile and efficient approach to overcome the poor wettability between ZnO and Au by carefully modulating the surface charge density on Au nanoparticles (NPs), enabling rapid synthesis of Au@ZnO core-shell NPs at room temperature. The resulting Au@ZnO core-shell NPs exhibit a significantly enhanced plasmonic

Interaction of cytidine 5'-monophosphate with Au(111): an in situ infrared spectroscopic study.

PubMed

Doneux, Thomas; Fojt, Lukás

2009-07-13

The interaction of cytidine 5'-monophosphate (CMP) with gold surfaces is studied by means of in situ infrared spectroscopy and cyclic voltammetry at the Au(111)|aqueous solution interface. Similar to other nucleic acid components, cytidine 5'-monophosphate is chemisorbed on the surface at positive potentials, and the amount of adsorbed CMP increases with the potential. Subtractively normalized interfacial Fourier-transform infrared spectroscopy (SNIFTIRS) is used to identify the adsorbed and desorbed species. Upon electrochemical desorption, the molecules released in solution are unprotonated on the N3 atom. Striking similarities are found between the spectrum of adsorbed CMP and the solution spectrum of protonated CMP. The origin of such similarities is discussed. The results strongly suggest that chemisorption occurs through the N3 atom of the pyrimidine ring. A comparison is drawn with cytidine, whose electrochemical and spectroscopic behaviors are also investigated.

The structure, surface topography and mechanical properties of Si-C-N films fabricated by RF and DC magnetron sputtering

NASA Astrophysics Data System (ADS)

Shi, Zhifeng; Wang, Yingjun; Du, Chang; Huang, Nan; Wang, Lin; Ning, Chengyun

2011-12-01

Silicon carbon nitride thin films were deposited on Co-Cr alloy under varying deposition conditions such as sputtering power and the partial pressure ratio of N2 to Ar by radio frequency and direct current magnetron sputtering techniques. The chemical bonding configurations, surface topography and hardness were characterized by means of X-ray photoelectron spectroscopy, atomic force microscopy and nano-indentation technique. The sputtering power exhibited important influence on the film composition, chemical bonding configurations and surface topography, the electro-negativity had primary effects on chemical bonding configurations at low sputtering power. A progressive densification of the film microstructure occurring with the carbon fraction was increased. The films prepared by RF magnetron sputtering, the relative content of the Si-N bond in the films increased with the sputtering power increased, and Si-C and Si-Si were easily detachable, and C-O, N-N and N-O on the film volatile by ion bombardment which takes place very frequently during the film formation process. With the increase of sputtering power, the films became smoother and with finer particle growth. The hardness varied between 6 GPa and 11.23 GPa depending on the partial pressure ratio of N2 to Ar. The tribological characterization of Co-Cr alloy with Si-C-N coating sliding against UHMWPE counter-surface in fetal bovine serum, shows that the wear resistance of the Si-C-N coated Co-Cr alloy/UHMWPE sliding pair show much favourable improvement over that of uncoated Co-Cr alloy/UHMWPE sliding pair. This study is important for the development of advanced coatings with tailored mechanical and tribological properties.

Nanopatterning of optical surfaces during low-energy ion beam sputtering

NASA Astrophysics Data System (ADS)

Liao, Wenlin; Dai, Yifan; Xie, Xuhui

2014-06-01

Ion beam figuring (IBF) provides a highly deterministic method for high-precision optical surface fabrication, whereas ion-induced microscopic morphology evolution would occur on surfaces. Consequently, the fabrication specification for surface smoothness must be seriously considered during the IBF process. In this work, low-energy ion nanopatterning of our frequently used optical material surfaces is investigated to discuss the manufacturability of an ultrasmooth surface. The research results indicate that ion beam sputtering (IBS) can directly smooth some amorphous or amorphizable material surfaces, such as fused silica, Si, and ULE under appropriate processing conditions. However, for IBS of a Zerodur surface, preferential sputtering together with curvature-dependent sputtering overcome ion-induced smoothing mechanisms, leading to the granular nanopatterns' formation and the coarsening of the surface. Furthermore, the material property difference at microscopic scales and the continuous impurity incorporation would affect the ion beam smoothing of optical surfaces. Overall, IBS can be used as a promising technique for ultrasmooth surface fabrication, which strongly depends on processing conditions and material characters.

Highly efficient and porous TiO2-coated Ag@Fe3O4@C-Au microspheres for degradation of organic pollutants

NASA Astrophysics Data System (ADS)

Shen, Mao; Chen, Suqing; Jia, Wenping; Fan, Guodong; Jin, Yanxian; Liang, Huading

2016-12-01

In this paper, we reported a novel hierarchical porous Ag@Fe3O4@C-Au@TiO2 core@shell microspheres with a highly photocatalytic activity and magnetically separable properties. The synthesis method is included of a Fe3O4 magnetic embedded Ag core (Ag@Fe3O4), an interlayer of carbon modified by PEI to form sufficient amounts of amine functional groups (Ag@Fe3O4@C-PEI), the grafting of Au nanoparticles on the surface of Ag@Fe3O4@C-PEI (Ag@Fe3O4@C-Au), and an ordered porous TiO2 structured shell. As an example of the applications, the photocatalytic activities of the samples were investigated by the reduction of Rhodamine B (RhB) under visible-light irradiation. The results show that the porous Ag@Fe3O4@C-Au@TiO2 core@shell microspheres display higher adsorption and photocatalytic activities compared to the pure porous TiO2 and Ag@Fe3O4@C@TiO2 microspheres, which are attributed to the local surface plasmon resonance (LSPR) by the Ag and Au nanoparticles and the high specific surface area.

A facile approach for reducing the working voltage of Au/TiO2/Au nanostructured memristors by enhancing the local electric field

NASA Astrophysics Data System (ADS)

Arab Bafrani, Hamidreza; Ebrahimi, Mahdi; Bagheri Shouraki, Saeed; Moshfegh, Alireza Z.

2018-01-01

Memristor devices have attracted tremendous interest due to different applications ranging from nonvolatile data storage to neuromorphic computing units. Exploring the role of surface roughness of the bottom electrode (BE)/active layer interface provides useful guidelines for the optimization of the memristor switching performance. This study focuses on the effect of surface roughness of the BE electrode on the switching characteristics of Au/TiO2/Au three-layer memristor devices. An optimized wet-etching treatment condition was found to modify the surface roughness of the Au BE where the measurement results indicate that the roughness of the Au BE is affected by both duration time and solution concentrations of the wet-etching process. Then we fabricated arrays of TiO2-based nanostructured memristors sandwiched between two sets of cross-bar Au electrode lines (junction area 900 μm2). The results revealed a reduction in the working voltages in current-voltage characteristic of the device performance when increasing the surface roughness at the Au(BE)/TiO2 active layer interface. The set voltage of the device (Vset) significantly decreased from 2.26-1.93 V when we increased the interface roughness from 4.2-13.1 nm. The present work provides information for better understanding the switching mechanism of titanium-dioxide-based devices, and it can be inferred that enhancing the roughness of the Au BE/TiO2 active layer interface leads to a localized non-uniform electric field distribution that plays a vital role in reducing the energy consumption of the device.

Well-Ordered In Adatoms at the In 2 O 3 ( 111 ) Surface Created by Fe Deposition

DOE PAGES

Wagner, Margareta; Lackner, Peter; Seiler, Steffen; ...

2016-11-11

Metal deposition on oxide surfaces usually results in adatoms, clusters, or islands of the deposited material, where defects in the surface often act as nucleation centers. An alternate configuration is reported. Afterwards the vapor deposition of Fe on the In 2O 3(111) surface at room temperature, ordered adatoms are observed with scanning tunneling microscopy (STM). These are identical to the In adatoms that form when the sample is reduced by heating in ultrahigh vacuum. Our density functional theory (DFT) calculations confirm that Fe interchanges with In in the topmost layer, pushing the excess In atoms to the surface where theymore » arrange as a well-ordered adatom array.« less

Emergence of chirality in hexagonally packed monolayers of hexapentyloxytriphenylene on Au(111): a joint experimental and theoretical study.

PubMed

Sleczkowski, Piotr; Katsonis, Nathalie; Kapitanchuk, Oleksiy; Marchenko, Alexandr; Mathevet, Fabrice; Croset, Bernard; Lacaze, Emmanuelle

2014-11-11

We investigate the expression of chirality in a monolayer formed spontaneously by 2,3,6,7,10,11-pentyloxytriphenylene (H5T) on Au(111). We resolve its interface morphology by combining scanning tunneling microscopy (STM) with theoretical calculations of intermolecular and interfacial interaction potentials. We observe two commensurate structures. While both of them belong to a hexagonal space group, analogical to the triangular symmetry of the molecule and the hexagonal symmetry of the substrate surface, they surprisingly reveal a 2D chiral character. The corresponding breaking of symmetry arises for two reasons. First it is due to the establishment of a large molecular density on the substrate, which leads to a rotation of the molecules with respect to the molecular network crystallographic axes to avoid steric repulsion between neighboring alkoxy chains. Second it is due to the molecule-substrate interactions, leading to commensurable large crystallographic cells associated with the large size of the molecule. As a consequence, molecular networks disoriented with respect to the high symmetry directions of the substrate are induced. The high simplicity of the intermolecular and molecule-substrate van der Waals interactions leading to these observations suggests a generic character for this kind of symmetry breaking. We demonstrate that, for similar molecular densities, only two kinds of molecular networks are stabilized by the molecule-substrate interactions. The most stable network favors the interfacial interactions between terminal alkoxy tails and Au(111). The metastable one favors a specific orientation of the triphenylene core with its symmetry axes collinear to the Au⟨110⟩. This specific orientation of the triphenylene cores with respect to Au(111) appears associated with an energy advantage larger by at least 0.26 eV with respect to the disoriented core.

Structural Changes in Self-Catalyzed Adsorption of Carbon Monoxide on 1,4-Phenylene Diisocyanide Modified Au(111)

DOE PAGES

Kestell, John; Boscoboinik, J. Anibal; Cheng, Lanxia; ...

2015-07-23

The self-accelerated adsorption of CO on 1,4-phenylene diisocyanide (PDI)-derived oligomers on Au(111) is explored by reflection–absorption infrared spectroscopy and scanning tunneling microscopy. PDI incorporates gold adatoms from the Au(111) surface to form one-dimensional —(Au–PDI) n— chains that can also connect between gold nanoparticles on mica to form a conductive pathway between them. CO adsorption occurs in two stages; it first adsorbs adjacent to the oligomers that move to optimize CO adsorption. Further CO exposure induces PDI decoordination to form Au–PDI adatom complexes thereby causing the conductivity of a PDI-linked gold nanoparticle array on mica to decrease to act as amore » chemically drive molecular switch. This simple system enables the adsorption process to be explored in detail. DFT calculations reveal that both the —(Au–PDI) n— oligomer chain and the Au–PDI adatom complex are stabilized by coadsorbed CO. A kinetic “foot-in-the-door” model is proposed in which fluctuations in PDI coordination allow CO to diffuse into the gap between gold adatoms to prevent the PDI from reattaching, thereby allowing additional CO to adsorb, to provide kinetic model for allosteric CO adsorption on PDI-covered gold.« less

Ion irradiation synthesis of Ag-Au bimetallic nanospheroids in SiO2 glass substrate with tunable surface plasmon resonance frequency

NASA Astrophysics Data System (ADS)

Meng, Xuan; Shibayama, Tamaki; Yu, Ruixuan; Takayanagi, Shinya; Watanabe, Seiichi

2013-08-01

Ag-Au bimetallic nanospheroids with tunable localized surface plasmon resonance (LSPR) were synthesized by 100 keV Ar-ion irradiation of 30 nm Ag-Au bimetallic films deposited on SiO2 glass substrates. A shift of the LSPR peaks toward shorter wavelengths was observed up to an irradiation fluence of 1.0 × 1017 cm-2, and then shifted toward the longer wavelength because of the increase of fragment volume under ion irradiation. Further control of LSPR frequency over a wider range was realized by modifying the chemical components. The resulting LSPR frequencies lie between that of the pure components, and an approximate linear shift of the LSPR toward the longer wavelength with the Au concentration was achieved, which is in good agreement with the theoretical calculations based on Gans theory. In addition, the surface morphology and compositions were examined with a scanning electron microscope equipped with an energy dispersive spectrometer, and microstructural characterizations were performed using a transmission electron microscope. The formation of isolated photosensitive Ag-Au nanospheroids with a FCC structure partially embedded in the SiO2 substrate was confirmed, which has a potential application in solid-state devices.

Functional nicotinic acetylcholine receptor reconstitution in Au(111)-supported thiolipid monolayers

NASA Astrophysics Data System (ADS)

Pissinis, Diego E.; Diaz, Carolina; Maza, Eliana; Bonini, Ida C.; Barrantes, Francisco J.; Salvarezza, Roberto C.; Schilardi, Patricia L.

2015-09-01

The insertion and function of the muscle-type nicotinic acetylcholine receptor (nAChR) in Au(111)-supported thiolipid self-assembled monolayers have been studied by atomic force microscopy (AFM), surface plasmon resonance (SPR), and electrochemical techniques. It was possible for the first time to resolve the supramolecular arrangement of the protein spontaneously inserted in a thiolipid monolayer in an aqueous solution. Geometric supramolecular arrays of nAChRs were observed, most commonly in a triangular form compatible with three nAChR dimers of ~20 nm each. Addition of the full agonist carbamoylcholine activated and opened the nAChR ion channel, as revealed by the increase in capacitance relative to that of the nAChR-thiolipid system under basal conditions. Thus, the self-assembled system appears to be a viable biomimetic model to measure ionic conductance mediated by ion-gated ion channels under different experimental conditions, with potential applications in biotechnology and pharmacology.

Two-dimensional networks of brominated Y-shaped molecules on Au(111)

NASA Astrophysics Data System (ADS)

Jeon, Un Seung; Chang, Min Hui; Jang, Won-Jun; Lee, Soon-Hyung; Han, Seungwu; Kahng, Se-Jong

2018-02-01

In the design of supramolecular structures, Y-shaped molecules are useful to expand the structures in three different directions. The supramolecular structures of Y-shaped molecules with three halogen-ligands on surfaces have been extensively studied, but much less are done for those with six halogen-ligands. Here, we report on the intermolecular interactions of a Y-shaped molecule, 1,3,5-Tris(3,5-dibromophenyl)benzene, with six Br-ligands studied using scanning tunneling microscopy (STM). Honeycomb-like structures were observed on Au(111), and could be explained with chiral triple-nodes made of three Br···Br halogen bonds. Molecular models were proposed based on STM images and reproduced with density-functional theory calculations. Although the molecule has six Br-ligands, only three of them form Br···Br halogen bonds because of geometrical restrictions. Our study shows that halogenated Y-shaped molecules will be useful components for building supramolecular structures.

Growth, microstructure and electrical properties of sputter-deposited hafnium oxide (HfO2) thin films grown using HfO2 ceramic target

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

Aguirre, B.; Vemuri, R. S.; Zubia, David

2011-01-01

Hafnium oxide (HfO₂) thin films have been made by radio-frequency (rf) magnetron-sputtering onto Si(100) substrates under varying growth temperature (Ts). HfO₂ ceramic target has been employed for sputtering while varying the Ts from room temperature to 500⁰C during deposition. The effect of Ts on the growth and microstructure of deposited HfO₂ films has been studied using grazing incidence x-ray diffraction (GIXRD), X-ray photoelectron spectroscopy (XPS), and high-resolution scanning electron microscopy (HR-SEM) coupled with energy dispersive x-ray spectrometry (EDS). The results indicate that the effect of Ts is significant on the growth, surface and interface structure, morphology and chemical composition ofmore » the HfO₂ films. Structural characterization indicates that the HfO₂ films grown at Ts<200 ⁰C are amorphous while films grown at Ts>200 ⁰C are nanocrystalline. An amorphous-to-crystalline transition occurs at Ts=200 ⁰C. Nanocrystalline HfO₂ films crystallized in a monoclinic structure with a (-111) orientation. XPS measurements indicated the high surface-chemical quality and stoichiometric nature of the grown HfO₂ films. An interface layer (IL) formation occurs due to reaction at the HfO₂-Si interface for HfO₂ films deposited at Ts>200 ⁰C. The thickness of IL increases with increasing Ts. XPS and EDS at the HfO₂-Si cross-section indicate the IL is a (Hf, Si)-O compound. The electrical characterization using capacitance-voltage measurements indicate that the dielectric constant decreases from 25 to 16 with increasing Ts.« less

Lignin-derived oxygenate reforming on a bimetallic surface: The reaction of benzaldehyde on Zn/Pt(111)

NASA Astrophysics Data System (ADS)

Shi, Daming; Vohs, John M.

2016-08-01

Temperature programmed desorption (TPD) and high-resolution electron energy loss spectroscopy (HREELS) were used to characterize the adsorption and reaction of benzaldehyde (C6H5CHO) on hydrogen-covered Pt(111) and Zn-modified Pt(111) surfaces. Benzaldehyde was found to interact with Pt(111) via both the phenyl ring and carbonyl of the aldehyde group. This bonding configuration facilitates unselective decomposition of the benzaldehyde to produce CO, H2, and small hydrocarbon fragments at relatively low temperatures. On the other hand, benzaldehyde was found to bond to Zn-decorated Pt(111) surface exclusively via the carbonyl group in an η2(C, O) configuration, with the phenyl ring tilted away from the surface. This configuration weakens Csbnd O bond in the carbonyl facilitating its cleavage and helps prevent hydrogenation of the phenyl ring.

Hydrogenation of CO 2 to methanol on CeO x/Cu(111) and ZnO/Cu(111) catalysts: Role of the metal-oxide interface and importance of Ce 3+ sites

DOE PAGES

Senanayake, Sanjaya D.; Ramirez, Pedro J.; Waluyo, Iradwikanari; ...

2016-01-06

The role of the interface between a metal and oxide (CeO x–Cu and ZnO–Cu) is critical to the production of methanol through the hydrogenation of CO 2 (CO 2 + 3H 2 → CH 3OH + H 2O). The deposition of nanoparticles of CeO x or ZnO on Cu(111), θ oxi < 0.3 monolayer, produces highly active catalysts for methanol synthesis. The catalytic activity of these systems increases in the sequence: Cu(111) < ZnO/Cu(111) < CeO x/Cu(111). The apparent activation energy for the CO 2 → CH 3OH conversion decreases from 25 kcal/mol on Cu(111) to 16 kcal/mol on ZnO/Cu(111)more » and 13 kcal/mol on CeO x/Cu(111). The surface chemistry of the highly active CeO x–Cu(111) interface was investigated using ambient pressure X-ray photoemission spectroscopy (AP-XPS) and infrared reflection absorption spectroscopy (AP-IRRAS). Both techniques point to the formation of formates (HCOO –) and carboxylates (CO 2 δ–) during the reaction. Our results show an active state of the catalyst rich in Ce 3+ sites which stabilize a CO 2 δ– species that is an essential intermediate for the production of methanol. Furthermore, the inverse oxide/metal configuration favors strong metal–oxide interactions and makes possible reaction channels not seen in conventional metal/oxide catalysts.« less

AZO nanorods thin films by sputtering method

NASA Astrophysics Data System (ADS)

Rosli, A. B.; Shariffudin, S. S.; Awang, Z.; Herman, S. H.

2018-05-01

Al-doped zinc oxide (AZO) nanorods thin film were deposited on Au catalyst using RF sputtering at 300 °C. The 15 nm thickness Au catalyst were deposited on glass substrates by sputtering method followed by annealing for 15 min at 500 °C to form Au nanostructures on the glass substrate. The AZO thin films were then deposited on Au catalyst at different RF power ranging from 50 - 200 W. The morphology of AZO was characterized using Field Emission Scanning Electron Microscopy while X-ray Diffraction was used to examine crystallinity of AZO thin films. From this work, the AZO nanorods was found grow at 200 W RF power.

Study the target effect on the structural, surface and optical properties of TiO2 thin film fabricated by RF sputtering method

NASA Astrophysics Data System (ADS)

Vyas, Sumit; Tiwary, Rohit; Shubham, Kumar; Chakrabarti, P.

2015-04-01

The effect of target (Ti metal target and TiO2 target) on Titanium Dioxide (TiO2) thin films grown on ITO coated glass substrate by RF magnetron sputtering has been investigated. A comparative study of both the films was done in respect of crystalline structure, surface morphology and optical properties by using X-ray diffractometer (XRD), Atomic Force Microscopy (AFM) studies and ellipsometric measurements. The XRD results confirmed the crystalline structure and indicated that the deposited films have the intensities of anatase phase. The surface morphology and roughness values indicated that the film using Ti metal target has a smoother surface and densely packed with grains as compared to films obtained using TiO2 target. A high transmission in the visible region, and direct band gap of 3.67 eV and 3.75 eV for films derived by using Ti metal and TiO2 target respectively and indirect bandgap of 3.39 eV for the films derived from both the targets (Ti metal and TiO2 target) were observed by the ellipsometric measurements.

Influence of microstructure and chemical composition of sputter deposited TiO2 thin films on in vitro bioactivity.

PubMed

Lilja, Mirjam; Genvad, Axel; Astrand, Maria; Strømme, Maria; Enqvist, Håkan

2011-12-01

Functionalisation of biomedical implants via surface modifications for tailored tissue response is a growing field of research. Crystalline TiO(2) has been proven to be a bone bioactive, non-resorbable material. In contact with body fluids a hydroxyapaptite (HA) layer forms on its surface facilitating the bone contact. Thus, the path of improving biomedical implants via deposition of crystalline TiO(2) on the surface is interesting to follow. In this study we have evaluated the influence of microstructure and chemical composition of sputter deposited titanium oxide thin films on the in vitro bioactivity. We find that both substrate bias, topography and the flow ratio of the gases used during sputtering affect the HA layer formed on the films after immersion in simulated body fluid at 37°C. A random distribution of anatase and rutile crystals, formed at negative substrate bias and low Ar to O(2) gas flow ratios, are shown to favor the growth of flat HA crystal structures whereas higher flow ratios and positive substrate bias induced growth of more spherical HA structures. These findings should provide valuable information when optimizing the bioactivity of titanium oxide coatings as well as for tailoring process parameters for sputtered-based production of bioactive titanium oxide implant surfaces.