Au Slab Research Articles

Strain hardening and toughening in metal/molecular nanolayer/metal nanosandwiches

Introducing molecular nanolayers (MNLs) is attractive for enhancing the stability of, and inducing unusual properties at, inorganic thin film interfaces. Although organic molecules anchored to inorganic surfaces have been studied extensively, property enhancement mechanisms underpinned by molecular assemblies at inorganic thin film interfaces are yet to be revealed and understood. Here, ab initio molecular dynamics simulations of tensile strain of Au/MNL/Au thin film nanosandwich models provide insights into molecularly induced strain hardening and toughening. Au/MNL/Au nanosandwiches support up to ≈30% higher stresses and exhibit up to ≈140% higher toughness than pure Au slab models. Both hardening and toughening are governed by molecular length and terminal chemistry in the MNL. Strong Au/MNL interface bonding and greater molecular length promote defect creation in Au, which results in strain hardening. Accommodation of increasing post-hardening strains in the MNL mitigates the stress increase in the Au slabs, delays interface fracture, and contributes to toughening. Remarkably, toughening correlates with equilibrium interface strain, which could be used as a proxy for efficiently identifying promising inorganic/MNL combinations that provide toughening. Our findings are important for the discovery and design of inorganic–organic interfaces, nanomaterials, and composites.

Facilitating H migration on graphene by adsorbing on Au

H spillover is an important phenomenon for hydrogen transport control. Our first-principles calculations show that H migration on graphene can happen via a chemisorbed or physisorbed state. In the former process, the midpoint is the transition state; there the charge state of H is H+ and the activation barrier is 1.0–1.2 eV. Oxidation of the H-on-graphene system by artificially removing the number of electrons lowers the activation barrier because H is positively charged at the initial and end states, thereby decreasing the cost of moving charge from H to graphene during the migration. Adsorbing graphene on a Au slab decreases the activation barrier to 0.8 eV because electrons spontaneously transfer from the H-and-graphene system to Au. Oxidation of graphene by adsorbing on a high work function metal with appropriate bonding strength is therefore proposed as a strategy to promote H spillover.

Effect of substrate charge density on the adsorption of intrinsically disordered protein amyloid β40: a molecular dynamics study.

The inhibitory effect of negatively charged gold nanoparticles (AuNPs) on amyloidogenic protein fibrillation has been established from experiments and computer simulations. Here, we investigate the effect of the charge density (σ) of gold (Au) surfaces on the adsorption of the intrinsically disordered amyloid β40 (Aβ40) monomer using molecular dynamics (MD) simulations. On the basis of the binding free energy, some key residues (ARG5, LYS16, LYS28, LEU17-ALA21, ILE31-VAL38) were found to be responsible for preventing the β-sheet formation, which is known to be a precursor for fibrillation. Until a critical charge density (σc) of -0.167 e nm-2, the key residues remained adsorbed on the Au slab. A saturation in the number of condensed counterions (Na+) on Aβ40 was also observed at σc. Beyond σc, the condensation of Na+ occurs only on the Au slab, leading to competition between positively charged key residues and condensed ions. This competition was found to be responsible for the lack of adsorption of the key residues, leading to β-sheet formation for σ > -0.167 e nm-2. This study suggests that if the key residues are not adsorbed, then β-sheet formation is observed, which can then lead to the development of proto-fibrils and subsequently fibrillation. Therefore the surface should have an optimal charge density to be an effective inhibitor of fibrillation.

Solid-state bonding behavior between surface-nanostructured Cu and Au: a molecular dynamics simulation

In recent years, solid-state bonding has attracted attention for various electronic packaging applications as an alternative to conventional solders. Surface-nanostructured materials enable solid-state bonding without complex surface modifications and operate at a low bonding temperature and pressure. Therefore, in this study, molecular dynamics simulations were conducted to investigate the solid-state bonding behavior between surface-nanostructured Cu and Au, with a focus on diffusion phenomena. A periodic ligament-cavity nanostructured Cu (NS-Cu) model was prepared at the bonding interface between Cu and Au slabs. The simulation results indicated that the larger the specific surface area of NS-Cu, the faster the densification at the bonding interface. Atomic displacement analysis showed that rapid densification occurred via the displacement of Cu and Au atoms in the vicinity of NS-Cu. The preferential diffusion of atoms along NS-Cu cavities contributed to this phenomenon. At this stage of densification, the diffusion coefficients were higher than the surface diffusion coefficients estimated based on literature, which indicates that this behavior is specific to surface-nanostructured materials. The highly disordered atomic arrangement at the bonding interface enabled significant atomic diffusion. Therefore, this study confirmed that the use of surface-nanostructured materials would contribute to a promising bonding technology for application in electronics.

Effect of Au nanosandwiching on the structural, optical and dielectric properties of the as grown and annealed InSe thin films

In the current work, the structural, optical and dielectric properties of the InSe/Au/InSe nanosandwiched structures are investigated by means of X-ray diffraction and UV–visible light spectrophotometry techniques. The insertion of a 20 and 100nm thick Au metal slabs between two InSe layers did not alter the amorphous nature of the as grown InSe films but decreased the energy band gap and the free carrier density. It also increased; the absorption ratio and the values of dielectric constant by ~ 3 times. The insertion of 100nm Au layers as a nanosandwich enhanced the drift mobility (31.3cm2/Vs) and plasmon frequency (1.53GHz) of the InSe films. On the other hand, upon annealing, a metal induced crystallization process is observed for the InSe/Au (100nm)/InSe sandwiches. Particularly, while the samples sandwiched with a layer of 20nm thickness hardly revealed hexagonal γ−In2Se3 when annealed at 300°C, those sandwiched with 100nm Au slab, displayed well crystalline phase of hexagonal γ−In2Se3 at annealing temperature of 200°C. The further annealing at 300°C, forced the appearing of the orthorhombic In4Se3 phase. Optically, the annealing of the InSe/Au(100nm)/InSe at 200°C improved the absorption ratio by ~ 9 times and decreased the energy band gap. The nanosandwiching technique of InSe seems to be promising for the engineering of the optical properties of the InSe photovoltaic material.

Resonance modulation and optical force of nanostructures with Au slabs array

Abstract We demonstrate a three dimensional nanoparticle trapping approach aided by the surface plasmon resonance and polarization modulation effect of nanostructure with Au slabs array and Si substrate. The numerical simulations indicate that the polarization direction of incident light relative to long side of slabs can modulate plasmon resonance effect. The plasmon resonance effect is strongest when the polarization is parallel to the long side of slabs, in contrast, it is disappear with the polarization perpendicular to the long side of slabs. The novel method is presented in the paper that the electric amplitude of resonance spots can be enhanced by increasing the slabs number. The simulation results show that the resonance electric amplitude with four slabs can be raised 18-fold than that with one slab. It is very beneficial for more effectively operating small particle. Furthermore, the absorption spectra are broadened and absorption ratio is raised with increasing the slabs number until full spectra are involved in the case of four slabs. It may be utilized for getting stronger optic gradient and efficient solar E-O conversion.

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

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.

Strain effects on the structural, magnetic, and thermodynamic properties of the Au(001)/Fe(001) interface from first principles

The structural, magnetic, and thermodynamic properties of the Au(001)/Fe(001) interface are investigated as a function of the in-plane strain using density functional theory calculations for two different Au slab thicknesses: 2 and 8 monolayers. The structural and magnetic properties are analyzed by studying the interlayer distance in the direction perpendicular to the interface and the atomic magnetic moments of Fe atoms, as a function of the in-plane strain. The structural study evidences both the bulk elastic and surface and interface contributions. The atomic magnetic moments of Fe atoms are essentially dependent on their local environment (number and distance of the Fe first neighbors). Thermodynamic properties of the interface are investigated through the calculation of the interface energy and interface stress. These thermodynamic quantities are subsequently used in a simple model to evaluate the strain state of an ideal spherical symmetric Fe@Au core-shell nanoparticle. The surface elastic effects are found to be significant for nanoparticles of diameter smaller than $\ensuremath{\sim}20$ nm and predominant for diameters smaller than $\ensuremath{\sim}2.3$ nm. Interface elastic effects are weaker than surface elastic effects but can not be neglected for very small nanoparticles ($\ensuremath{\lesssim}1.9$ nm) or for thin shells.

Beneficial Effect of Cu on a Cu-Modified Au Catalytic Surface for CO Oxidation Reaction: A DFT Study

Density functional theory calculations within the generalized gradient approximations were employed to examine the molecular oxygen adsorption, decomposition, and CO oxidation upon a number of Cu modified Au surfaces cleavaged in (100) orientation. Amount of Cu was varied on the Au slab to optimize the model, which serves as the best one to investigate the synergic effect between Au and Cu for CO oxidation process. The adsorption energy of O2 varies between −0.61 and −2.20 eV depending on the surface composition. O–O bond activation up to 1.86 Å has been observed for the most favorable configuration. CO adsorbs preferably on top configuration of the Cu–Au catalytic surface with an Eads of −1.12 eV. The atomic O adsorption is strongly site specific, 4-fold hollow being the most favorable. A coincident activation barrier of 0.60 eV has been found employing constraint minimization and cNEB method for CO2 formation. Initially, a stable O–CO complex is formed, which leads to CO2 formation without the expenditure of any significant energy. Both the O–CO complex formation and its conversion to CO2 are thermodynamically favorable processes. The process has been repeated on a pure Cu(100) surface to distinguish the role of Au. CO on a bridge yields the highest adsorption energy −0.97 eV, being slightly less stable on other locations. O2 has strong adsorption on this surface as well, but bond activation and Eads are comparatively less. The minimum energy path follows a similar route to CO2 as on a Au–Cu surface but with higher activation energy and no exothermicity. CO2 created desorbs by surmounting a small barrier through an exothermic process. Comparison suggests that the Cu-modified Au surface is superior and more active than pure Cu toward CO oxidation. Analogous to the Au–Cu bimetallic structure model, a Ag–Cu slab consisting of a top monolayer of Cu and three layers beneath Ag was considered for comparison. A similar pronounced effect regarding O2 adsorption and an easy CO oxidation reaction was revealed. The results suggest that Cu bearing Au and Ag surfaces appear to be good candidates for low-temperature CO oxidation.

Identification of parameters through which surface chemistry determines the lifetimes of hot electrons in small Au nanoparticles.

This paper describes measurements of the dynamics of hot electron cooling in photoexcited gold nanoparticles (Au NPs) with diameters of ∼3.5 nm, and passivated with either a hexadecylamine or hexadecanethiolate adlayer, using ultrafast transient absorption spectroscopy. Fits of these dynamics with temperature-dependent Mie theory reveal that both the electronic heat capacity and the electron-phonon coupling constant are larger for the thiolated NPs than for the aminated NPs, by 40% and 30%, respectively. Density functional theory calculations on ligand-functionalized Au slabs show that the increase in these quantities is due to an increased electronic density of states near the Fermi level upon ligand exchange from amines to thiolates. The lifetime of hot electrons, which have thermalized from the initial plasmon excitation, increases with increasing electronic heat capacity, but decreases with increasing electron-phonon coupling, so the effects of changing surface chemistry on these two quantities partially cancel to yield a hot electron lifetime of thiolated NPs that is only 20% longer than that of aminated NPs. This analysis also reveals that incorporation of a temperature-dependent electron-phonon coupling constant is necessary to adequately fit the dynamics of electron cooling.

Structural and electronic properties of the Au(001)/Fe(001) interface from density functional theory calculations

A density functional theory (DFT) investigation of the structural and electronic properties of the Au(001)/Fe(001) interface, as a function of the number of Au layers deposited on the Fe substrate (from 1 to 11 Au monolayers), is presented. The elastic effects on the interface properties due to the lattice mismatch between Fe and Au, calculated by DFT using the generalized gradient approximation, are also evaluated. At the interface, the interlayer distances in the Fe substrate as well as in the Au slab expand. The Fe atoms of the interface exhibit an enhanced magnetic moment and the Au atoms of the interface bear a nonzero (but very low) magnetic moment. The calculated interface energy favors the formation of core-shell Fe@Au nanoparticles, where Au(001) is in epitaxy at 45${}^{\ensuremath{\circ}}$ on (001) facets of a Fe nanocube. Finally, the analysis of the electronic properties shows that the work of adhesion of the interface is maximum for a coating of Fe with 2 Au monolayers, which can be explained by a strong overlap between the electronic densities of the Fe interface atoms with those of the Au surface atoms.

Coverage and charge dependent adsorption of butanethiol on the Au(1 1 1) surface: A density functional theory study

A theoretical study based on periodic DFT calculations of the structure, the surface bonding, and the energetics of butanethiols adsorbed on the Au(1 1 1) surface is reported. Several sites and coverage have been considered, and neutral and charged metal surfaces have been simulated. Whatever the coverage is, the preferred site is a hollow-bridge type-site in which sulfur atom simultaneously binds to two gold atoms. Thiol adsorption parameters are sensitive to the coverage, especially the adsorption energy that shows a clear response to the number of thiols adsorbed, and the thiol-surface interaction decreases when the coverage grows as the lateral repulsion between the alkyl tails weaken the strength of the Au-S bond. The thiol-surface interaction parameters are also sensitive to the charge of the metal. Also, we found that the adsorption becomes more favorable when the metal surface is negatively charged, and less favorable on positive surfaces. Finally, in order to analyze dynamical effects, we performed a molecular dynamics (MD) simulation considering a system with ethanethiol on an Au slab pre-covered by water. The MD simulation shows that the proton transfer occurs within a few femtoseconds and that prior to the transfer itself, the sulfur atom binds to a gold surface atom. This Au atom is clearly pulled out of the surface, what could be interpreted as the onset of the islands and cluster formation already observed in the low coverage regime.

Enhanced Oxygen Activation over Supported Bimetallic Au−Ni Catalysts

New bimetallic Ni−Au supported nanoparticle catalysts were prepared by using dendrimer templated nanoparticles. Amine-terminated generation 5 polyamidoamine (PAMAM) dendrimers were anchored to a commercial silica with a siloxane linked anhydride. The dendrimer was then alkylated and used to template Ni−Au nanoparticles, which were subsequently extracted into organic solution as thiol monolayer protected clusters (MPCs). Transmission electron microscopy (TEM) and energy dispersive spectroscopy (EDS) indicated bimetallic nanoparticles of about 2 nm in size. Nanoparticles were deposited onto P-25 TiO2, and the capping thiol ligands were removed under flowing H2. DRIFTS infrared spectra of adsorbed CO showed only Au on the catalyst surface; no bands attributable to Ni or NiO were observed. Density functional theory (DFT) calculations showed that Au is substantially more stable than Ni on the surface of model slabs. DFT calculations also indicated that the incorporation of Ni into Au slabs results in stronger ...

Grating couplers for broadside input and output coupling of long-range surface plasmons

Metal gratings for in-coupling a Gaussian beam incident from broadside to the long-range surface plasmon polariton (LRSPP) propagating in one direction along a membrane-supported Au slab bounded by air or water are proposed and modeled by the finite-difference time-domain method. Grating couplers for out-coupling the propagating LRSPP into free radiation directed along broadside are also investigated. Short grating designs consisting of a small number of Au bumps yield 15% to 20% in-coupling efficiencies, and about 60% out-coupling efficiencies. LRSPP back-reflections along the membrane waveguide caused by the out-coupling grating are also calculated and discussed.

Broadside Excitation of Long-Range Surface Plasmons via Grating Coupling

A metal grating for coupling a Gaussian beam incident from broadside to the long-range surface plasmon-polariton (LRSPP) propagating along a membrane-supported Au slab is proposed and modelled by the finite-difference time-domain method. A short grating design consisting of a small number of Au bumps yields a 19.5% coupling efficiency at ¿ <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0</sub> = 1310 nm into the LRSPP propagating in a single direction when the grating is excited off-center near one of its edges. The grating coupler is easy to fabricate and it facilitates experimental alignments.

Optical second-harmonic spectroscopy of Au(887) and Au(443) surfaces

In order to investigate the electronic states of step sites on Au surfaces, we have observed the reflected optical second-harmonic (SH) intensity from Au(887) and Au(443) surfaces in ultrahigh vacuum with a normal incident excitation light beam, as a function of the photon energy and the incident and SH light polarizations. The second-order surface nonlinear susceptibility elements observed in this measurement configuration were ${\ensuremath{\chi}}_{xxx}^{(2)}$, ${\ensuremath{\chi}}_{xyy}^{(2)}$, and ${\ensuremath{\chi}}_{yxy}^{(2)}$, where $x$ and $y$ are defined as the $[11\overline{2}]$ and $[\overline{1}10]$ directions, respectively. The step edges lie in the $y$ direction. The ratios of the nonlinear susceptibility elements $|{\ensuremath{\chi}}_{xyy}^{(2)}|/|{\ensuremath{\chi}}_{xxx}^{(2)}|$ and $|{\ensuremath{\chi}}_{yxy}^{(2)}|/|{\ensuremath{\chi}}_{xxx}^{(2)}|$ were different on the two surfaces in the photon energy range from $2\ensuremath{\hbar}\ensuremath{\omega}=2.5$ to 3.3 eV. The deviation of the SH response from that of an ideal $3m$ symmetric Au(111) surface was found to be larger for the Au(443) surface than for the Au(887) surface. This deviation is attributed to the atomic steps created by the miscut of the samples. In order to analyze the observed SH spectra, we calculated the electronic states of a Au(554) slab using a density-functional theory. We found that the low-energy onset of the SH intensity caused by the steps can be qualitatively interpreted by referring to the calculated partial density of the $d$-electronic states of the step and terrace atoms on the Au slab.

Heating a slab of finite thickness with a pulsed laser in view of the theory of linear systems

The problem of heating a slab of finite thickness induced by surface absorption of a pulsed laser is solved using the two-dimensional Laplace integral transform technique. In the solution procedure the cases of constant and temperature-dependent absorption coefficient of the front surface considering different cooling conditions at the front and rear surfaces are taken into account. Mathematical expressions for the temperature distribution within the slab, and the temperature of its front and rear surfaces as a function of the time are obtained for all cases considered. The time required to initiate melting is found only for the case of temperature-dependent surface absorption and negligible cooling at both surfaces. The linear system theoretical treatment showed that the slab behaves like a linear system only for the case of temperature-independent absorption coefficient, where it is found to be invariant and its transfer function is defined. To illustrate the physical validity of the results obtained, computations are carried out on Al and Au slabs during and after irradiation with a measured time-dependent laser pulse profile.