Tailoring oxide properties: An impact on adsorption characteristics of molecules and metals

Both density functional theory calculations and numerous experimental studies demonstrate a variety of unique features in metal supported oxide films and transition metal doped simple oxides, which are markedly different from their unmodified counterparts. This review highlights, from the computational perspective, recent literature on the properties of the above mentioned surfaces and how they adsorb and activate different species, support metal aggregates, and even catalyse reactions. The adsorption of Au atoms and clusters on metal-supported MgO films are reviewed together with the cluster׳s theoretically predicted ability to activate and dissociate O₂ at the Au–MgO(100)/Ag(100) interface, as well as the impact of an interface vacancy to the binding of an Au atom. In contrast to a bulk MgO surface, an Au atom binds strongly on a metal-supported ultra-thin MgO film and becomes negatively charged. Similarly, Au clusters bind strongly on a supported MgO(100) film and are negatively charged favouring 2D planar structures. The adsorption of other metal atoms is briefly considered and compared to that of Au. Existing computational literature of adsorption and reactivity of simple molecules including O₂, CO, NO₂, and H₂O on mainly metal-supported MgO(100) films is discussed. Chemical reactions such as CO oxidation and O₂ dissociation are discussed on the bare thin MgO film and on selected Au clusters supported on MgO(100)/metal surfaces. The Au atoms at the perimeter of the cluster are responsible for catalytic activity and calculations predict that they facilitate dissociative adsorption of oxygen even at ambient conditions. The interaction of H₂O with a flat and stepped Ag-supported MgO film is summarized and compared to bulk MgO. The computational results highlight spontaneous dissociation on MgO steps. Furthermore, the impact of water coverage on adsorption and dissociation is addressed. The modifications, such as oxygen vacancies and dopants, at the oxide–metal interface and their effect on the adsorption characteristics of water and Au are summarized. Finally, more limited computational literature on transition metal (TM) doped CaO(100) and MgO(100) surfaces is presented. Again, Au is used as a probe species. Similar to metal-supported MgO films, Au binds more strongly than on undoped CaO(100) and becomes negatively charged. The discussion focuses on rationalization of Au adsorption with the help of Born–Haber cycle, which reveals that the so-called redox energy including the electron transfer from the dopant to the Au atom together with the simultaneous structural relaxation of lattice atoms is responsible for enhanced binding. In addition, adsorption energy dependence on the position and type of the dopant is summarized.     

Charging of metal adatoms on ultrathin oxide films: Au and Pd on FeO/Pt(111)

We present a combined experimental (STM/scanning tunneling spectroscopy) and theoretical (density functional theory) study on the deposition of Au and Pd metal atoms on FeO/Pt(111) ultrathin films. We show that while the Pd atoms are only slightly oxidized, the Au atoms form positive ions upon deposition, at variance to a charge transfer into the Au atoms as observed for MgO/Ag(100). The modulation of the adsorption properties within the surface Moiré cell and the charging induce the formation a self-assembled array of gold adatoms on FeO/Pt(111), whereas Pd atoms are randomly distributed.     

Bonding trends and dimensionality crossover of gold nanoclusters on metal-supported MgO thin films

Bonding of gold clusters, , 16, and 20, on MgO(100) and on thin MgO films supported on Mo(100) is investigated using first-principles density-functional theory. Enhanced adhesive bonding is found for clusters deposited on metal-supported MgO films of thickness up to about 1 nm, or 4 to 5 MgO layers, originating from electrostatic interaction between the underlying metal and metal-induced excess electronic charge accumulated at the cluster interface with the oxide film. The increased wetting propensity is accompanied by a dimensionality crossover from three-dimensional optimal cluster geometries on MgO(100) to energetically favored two-dimensional structures on the metal-supported films.     

Electron energy loss spectroscopy of the vibration modes of water on Ag(100) and Ag(115) surfaces and comparison to Au(100), Au(111) and Au(115)

Motivated by rather similar behavior of the Helmholtz capacitances of stepped Au(11n) and Ag(11n) electrodes we have extended a previous study on the vibration spectrum of water adsorbed at low temperatures on stepped gold surfaces to Ag(100), Ag(115) and Au(111) surfaces. On Ag(100) surfaces, the spectra show the presence of the typical H-bonded network of water molecules. The rather weak intensity, the absence of non-hydrogen bonded hydrogen atoms, the similarity to the infrared spectrum of ice crystallites, and the increase in the angular spread of the elastic peak are indicative of adsorption in form of three-dimensional clusters. This is stark contrast to Au(100) and Au(111) where the spectra match to a model involving stacks of water bilayers. The low coverage spectra on Ag(115) resemble the results on Au(115): A considerable fraction of the H-atoms remains in the non-H-bonded state and spectral features of water adsorbed at step-sites are identified. The first layer of water on Ag(115) surfaces should therefore have a similar structure as recently proposed in a theoretical study concerning water on Au(115). For larger doses, the experimental results on Ag(115) suggests the formation of three-dimensional clusters. This is contrary to Au(115) where the layered structure with a constant fraction of non-hydrogen-bonded H-atoms persists at higher doses.     

M2As-和M2Br+(M=Cu,Ag,Au)的赝势从头算研究

使用赝势在HF和MP2水平上计算了M2As-和M2Br+(M=Cu,Ag,Au)的平均几何和振动频率。计算的结果表明,这些化合物呈弯曲结构(C2v)。在MP2水平上研究了电子相关对这些化合物的几何结构的修正,键角减少10°到20°。在MP2,MP3,MP4,CCSD,和CCSD(T)水平上还较详细地研究了电子相关对Au2As-的几何结构的修正。通过对这些化合物稳定性的研究,预言了Au2As-的成键可能性。     

Microstructure of Ag/BaTiO3 films grown on MgO(100) substrate under external electric field

By applying an electric field parallel to the substrate surface, highly ordered Ag nanoparticles were homogeneously embedded in BaTiO₃ (BTO) films grown on a MgO(100) substrate. Transmission electron microscopy studies show that the BTO film is [100]-oriented. The general crystallographic orientation relationships between the composite film and MgO substrate are [100]_BTO//[100]_Ag//[100]_MgO and (010)_BTO//(010)_Ag//(010)_MgO. However, in films grown without an external electric field, Ag particles grow with random orientations and the BTO matrix is polycrystalline. Thus, electric fields are thought to meliorate the quality of the films by changing the growth orientation. In addition, the BTO matrix and Ag particles were found to be multiply twinned from studies on cross-sectional specimens. The contribution is discussed of Ag particles with ordered growth orientation and the large amount of BTO microtwins with higher dielectric constant to the improved optical properties of the as-prepared composite films.     

Cu、Ag和Au掺杂AlN的电磁性质的第一性原理研究

采用基于密度泛函理论（DFT）的平面波超软赝势法,研究了Cu、Ag、Au掺杂AlN的晶格常数、磁矩、能带结构和态密度。电子结构表明,Cu、Ag、Au的掺杂使在带隙中引入了由杂质原子的d态与近邻N原子的2p态杂化而成的杂质带,都为p型掺杂,增强了体系的导电性。Cu掺杂AlN具有半金属铁磁性,半金属能隙为0.442eV,理论上可实现100%的自旋载流子注入;Ag掺杂AlN具有很弱的半金属铁磁性;而Au掺杂AlN不具有半金属铁磁性。因此,与Ag、Au相比,Cu更适合用来制作AlN基稀磁半导体。     

Charging effects on the vibrational properties of Au and Au2 on MgO(100)

Vibrational spectra of charge-neutral and charged Au and Au₂ on MgO(100) were investigated using ab-initio density functional perturbation theory. The calculated vibrational spectra showed vibrational features associated with the charge states of Au and Au₂ on MgO(100). Further analyses of surface in-plane and normal phonon modes of Au and Au₂ on MgO(100) were performed to extract vibrational features involving the Au modes. These features provide important information for experimentally explaining the charge states of Au and Au₂ on MgO(100).     

Digital and analog memory devices based on 2D layered MPS_3 (M= Mn,Co, Ni) materials

We demonstrate digital and analog devices with an Ag/MPS_3/Au structure based on layered MPS_3(M=Mn,Co,Ni)2 D materials.All devices show the bipolar behavior of resistive switching.In addition,Ag/MnPS3/Au and Ag/NiPS_3/Au devices show synaptic characteristics of potentiation and depression.The digital and analog characteristics of resistance states enable Ag/MPS_3/Au devices to work as both binary memory and artificial synapse devices.The Ag/MPS_3/Au memory devices are promising for applications of flexible eye-like and brain-like systems on a chip when they are integrated with photodetectors and FETs composed of full MPS_3 materials.     

New surface structure analysis of Ag-Si phase by quantitative auger electron spectroscopy method

Ag or Au was deposited on a clean Si substrate at room temperature. These systems, Ag/Si and Au/Si, were annealed at various temperatures or various heating times. Due to the annealing, Ag or Au diffused into Si and/or Si diffused into the metal. The changes of the surface composition are analyzed by a quantitative Auger Electron Spectroscopy (AES) method which is newly developed as a non-destructive method. In the case of Ag/Si, Ag migrated into the Si substrate and/or Si diffused into Ag. Then, Ag-Si solid solution was produced. After the annealing, the Ag/Si system is changed into Ag/(Ag-Si)/Si of the three-phase structure. In the case of Au/Si (Au film thickness < 15 Å), the Au film thickness became thinner by annealing. The Au/Si system always keeps the Au/Si phase after annealing, while there was no Au-Si solution area. The difference between the Ag/(Ag-Si)/Si and the Au/Si structure is attributed to the reason that Au diffuses more quickly than Ag into the Si substrate. AES results after annealing cannot be explained by the model of the formation of the three-dimensional island structure which is commonly referenced.     

Bonding in Cu,Ag, and Au clusters: relativistic effects,trends, and surprises

Electronic structure and bonding in anionic coinage metal clusters are investigated via density-functional calculations, focusing on an extensive set of isomers of Cu(-)(7), Ag(-)(7), and Au(-)(7). While the ground states of Cu(-)(7) and Ag(-)(7) are three dimensional (3D), that of Au(-)(7) is planar, separated from the optimal 3D isomer by 0.5 eV. The simulated thermally weighted photoabsorption spectrum of Au(-)(7) is dominated by planar structures, and it agrees well with the measured one. The propensity of Au(-)(N) clusters to favor planar structures (with N as large as 13) is correlated with strong hybridization of the atomic 5d and 6s orbitals due to relativistic effects.     

A facile strategy to synthesize bimetallic Au/Ag nanocomposite film by layer-by-layer assembly technique

A facile strategy has been developed for the preparation of bimetallic gold–silver (Au–Ag) nanocomposite films by alternating absorption of poly-(ethyleneimine)–silver ions and Au onto substrates and subsequent reduction of the silver ions. The composition, micro-structure and properties of the {PEI–Ag/Au}_n nanocomposite films were characterized by ultraviolet visible spectroscopy (UV–vis), transmisson electron microscopy (TEM), field emission scanning electron microscopy (FESEM), X-ray photoelectron spectroscopy (XPS), surface enhanced Raman scattering (SERS) and cyclic voltammetry (CV). The UV–vis characteristic absorbances of {PEI–Ag/Au}_n nanocomposite thin film increase almost linear with the number of bilayers, which indicates a process of uniform assembling. Appearance of a double plasmon bands in the visible region and the lack of apparent core–shell structures in the TEM images confirm the formation of bimetallic Au–Ag nanoparticles. The result of XPS also demonstrates the existence of Ag and Au nanoparticles in the nanocomposite films. TEM and FESEM images show that these Ag and Au nanoparticles in the films possess sphere structure with the size of 20–25 nm. The resulting {PEI–Ag/Au}_n films inherit the properties from both the metal Ag and Au, which exhibits a unique performance in SERS and electrocatalytic activities to the oxidation of dopamine. As a result, the {PEI–Ag/Au}_n films are more attractive compared to {PEI–Ag/PSS}_n and {PEI/Au}_n films.     

Low-energy dynamics of the two-dimensional S=1/2 Heisenberg antiferromagnet on percolating clusters

We investigate the quantum dynamics of site diluted S=1/2 Heisenberg antiferromagnetic clusters at the 2D percolation threshold. We use Lanczos diagonalization to calculate the lowest excitation gap Delta and, to reach larger sizes, use quantum Monte Carlo simulations to study an upper bound for Delta obtained from sum rules involving the staggered structure factor and susceptibility. Scaling the gap distribution with the cluster length L, Delta approximately L(-), we obtain a dynamic exponent z approximately 2D(f), where D(f)=91/48 is the fractal dimensionality of the percolating cluster. This is in contrast with previous expectations of z=D(f). We argue that the low-energy excitations are due to weakly coupled effective moments formed due to local imbalance in sublattice occupation.     

Isovector quadrupole resonance observed in the 60Ni(13C,13N)60Co reaction at E/A=100 MeV

The charge-exchange reaction 60Ni(13C,13N)60Co at E/A=100 MeV has been studied to locate isovector (deltaT=1) non-spin-flip (deltaS=0) giant resonances. Besides the giant dipole resonance at E(x)=8.7 MeV, another resonance has been observed at E(x)=20 MeV with a width of 9 MeV. Distorted-wave Born approximation analysis on the angular distribution clearly indicated the L=2 multipolarity, attributing the E(x)=20 MeV state to the giant isovector quadrupole resonance.     

F+ laser performance and interaction of Cu,Ag and Au at the reduced oxygen coordination of MgO surface: first principles calculations

F⁺ laser performance and interaction of the title group IB transition metals at the reduced oxygen coordination of MgO surface were investigated using the TD and DFT methods of ab initio molecular electronic structure calculations. The considered ion clusters were embedded in simulated Coulomb fields that closely approximate the Madelung fields of the host surfaces and the nearest neighbor ions to the F⁺ site were allowed to relax to equilibrium in each case. The F⁺ laser performance fades quickly as the reduced oxygen coordination decreases from 5 to 4 to 3. The relaxed excited states (RESs) of the defect containing surfaces are compact and deep below the conduction bands of the perfect MgO surface. The probability of orientational destruction of the center in laser experiment is expected to follow the order flat>corner>edge. The excited state at the edge has higher energy than that at the flat or at the corner. F⁺ is easily formed at the lower oxygen coordination and the disappearance of anisotropy and 2p splitting observed in absorption of F⁺ at the surface follow the order corner>flat>edge. The Glasner–Tompkins relation is generalized to include the F⁺ bands at the reduced oxygen coordination of a metal oxide surface. As far as the adsorbate–substrate interactions are concerned, the F⁺ center enhances the adsorptivity of Ag, Cu and Au by ca. 1.91–3.33 eV and changes the nature of adsorption from physical adsorption to chemical adsorption. The adsorption energies follow the order Cu>Au>Ag and are explainable in terms of electrostatic potential curves, energy gaps and spin pairing. Cu and Ag act as electron donors while Au acts as electron acceptor and the MgO surface cannot be made semiconducting by F⁺ imperfection.     

Photochemical Synthesis and Properties of Colloidal Copper,Silver and Gold Adsorbed on Quartz

Original methods for the photochemical production of stable copper, silver and gold colloids in the form of films on quartz, and dispersion in liquids were devised. It is shown that photochemical synthesis of colloidal metals is a difficult multiphase process, and includes the formation of low-valence forms of Cu(I), Au(I) and nonmetal clusters, colloidal particles and their agglomerates. Cluster stabilization and further growth to colloidal particles are achieved by adsorption onto the solid surface (quartz) or by increasing the viscosity of photolyte. In the absence of these methods of stabilization, the processes of intermediate reoxidation to Cu(II) and Au(III) and agglomeration of Ag and Au colloids proceed in a photolyte. Adsorption and the rate of cluster growth on a quartz surface are speeded up by the action of monochromatic UV light. Experimental models of the mechanism of colloidal formation are suggested. The dependence of the growth rate and the properties of the colloids on conditions of the photochemical procedure (energy and light intensity, concentration of initial complex) has been established.     

A molecular dynamics study of the structural change differences between Au225 and Au369 clusters on MgO surfaces at low temperature

The differences in structural change between Au 225 and Au 369 clusters with their(111) facets supported on MgO(100) surfaces at 5 K are studied by using molecular-dynamics simulations with the atomic interchange potentials of the Au/MgO interface.The parameters are obtained from the ab initio energies using the Chen-Mo¨bius inversion method.Analyses of the pair distribution functions show that the two Au clusters use different deformation processes to adjust the distances between the interface atoms,owing to the misfit between the atom distances among the clusters and the substrates.The local structural changes are identified by atomic density profiles.     

Growth of polymer–metal nanocomposites by pulsed laser deposition

Complex polymer–metal nanocomposites have a wide range of applications, e.g. as flexible displays and packaging materials. Pulsed laser deposition was applied to form nanostructured materials consisting of metal clusters (Ag, Au, Pd and Cu) embedded in a polymer (polycarbonate, PC) matrix. The size and amount of the metal clusters are controlled by the number of laser pulses hitting the respective targets. For Cu and Pd, smaller clusters and higher cluster densities are obtained as in the cases of Ag and Au due to a stronger reactivity with the polymers and thus a lower diffusivity. Implantation effects, differences in metal diffusivity and reactivity on the polymer surfaces, and the coalescence properties are discussed with respect to the observed microstructures on PC and compared to the metal growth on poly (methyl methacrylate), PMMA.     

Computational investigation of CO adsorbed on Au<Subscript>x</Subscript>, Ag<Subscript>x</Subscript> and (AuAg)<Subscript>x</Subscript> nanoclusters (x = 1 − 5, 147) and monometallic Au and Ag low-energy surfaces

Density functional theory calculations have been performed to investigate the use of CO as a probe molecule for the determination of the structure and composition of Au, Ag and AuAg nanoparticles. For very small nanoclusters (x = 1 ? 5), the CO vibrational frequencies can be directly correlated to CO adsorption strength, whereas larger 147-atom nanoparticles show a strong energetic preference for CO adsorption at a vertex position but the highest wavenumbers are for the bridge positions. We also studied CO adsorption on Au and Ag (100) and (111) surfaces, for a 1 monolayer coverage, which proves to be energetically favourable on atop only and bridge positions for Au (100) and atop for Ag (100); vibrational frequencies of the CO molecules red-shift to lower wavenumbers as a result of increased metal coordination. We conclude that CO vibrational frequencies cannot be solely relied upon in order to obtain accurate compositional analysis, but we do propose that elemental rearrangement in the core@shell nanoclusters, from Ag@Au (or Au@Ag) to an alloy, would result in a shift in the CO vibrational frequencies that indicate changes in the surface composition.     

Atomic exchange mean field study of intermixing of Au on Ag(110)

A mean field method to study heteroepitaxial thin film growth is applied to growth, intermixing and surface reconstructions of Au on Ag(110). The results are in accordance with experimentally observed “burrowing” at submonolayer coverages and growth of elongated, (1 × 3) reconstructed, Au clusters at higher coverages. At coverages of a few monolayers the surface between the clusters has a high concentration of Ag, and ordered rows of Au are formed just beneath the surface.