First-principles studies: Thiolated Au2Cr and Au6Cr clusters

The structural, energetic and magnetic properties of thiol-passivating Au₂Cr and Au₆Cr clusters are investigated by performing first-principles calculation based on density functional theory. We find that the adsorption of thiolate is energetically more favorable than thiol and that the thiolates favor “top” site adsorption on Cr atom in one methanethiolate adsorbed Au₂Cr cluster while they favor “bridge” site adsorption on top of the middle point of Cr–Au bonds in three methanethiolates adsorbed Au₂Cr cluster. In thiol-passivating Au₆Cr cluster, the thiol favor “top” site adsorption on top of atom Au while the thiolate favor “bridge” site adsorption on top of the middle point of Au–Au bonds. The energetics of the reactions indicates that these thiol-passivating Au₂Cr or Au₆Cr clusters can be used as hydrogen storage materials. There are large and positive spin populations on atom Cr. The spin populations of atoms Au, S and H are attributed to both the spin delocalization and the spin polarization mechanisms.     

Effect of severe plastic deformation on the electronic properties of the Cu<Subscript>72</Subscript>Au<Subscript>24</Subscript>Ag<Subscript>4</Subscript> alloy

The electrical, magnetic, and optical properties of the Cu₇₂Au₂₄Ag₄ ternary alloy in the ordered and disordered states have been studied by the method of severe torsional plastic deformation under pressure in Bridgman anvils. It has been shown that, as a result of this deformation, the residual electrical resistivity of the alloy increases by approximately 11% and the magnitude of the negative thermopower decreases. The high diamagnetic susceptibility of the alloy has been explained by a significant role of charge carriers with the effective mass considerably smaller than the free-electron mass. The behavior of the optical conductivity has been discussed with due regard for the results of energy-band calculations. The experimental data obtained for the Cu₇₂Au₂₄Ag₄ alloy have been compared with the results of similar studies of the Cu₃Au binary alloy.     

Structural features of liquid metallic glass former

The structural parameters of Au₇₅Si₂₅ alloy, pure Au, Al₈₈Si₁₂, Cu₈₇Sn₁₃, In₉₈Al₂ and Al₉₃Ni₇ alloys have been measured by X-ray diffractometer. It is found that there are subpeaks in the pair correlation functions in liquid Au₇₅Si₂₅ alloy. The addition of Si in liquid Au results in a decrease in both the correlation radius and the coordination number of the nearest atomic neighbors. The Au₇₅Si₂₅ alloy nearest atomic distance has a more stable dependence on temperature compared to In₉₈Al₂, Al₉₃Ni₇ and Cu₈₇Sn₁₃ alloys. The atomic density change of liquid Au₇₅Si₂₅ alloy is less dependent on temperature than the liquid Au and Al₈₈Si₁₂ alloy. The liquid metallic good glass former Au₇₅Si₂₅ alloy possesses a more stable liquid structure than that of poor glass formers, indicating the essential of the fragility of the superheated melts.     

Chemically shifted surface core-levels and surface segregation in EuAu and YbAu alloys

Chemically shifted surface core-level binding energies are observed for the rare-earth alloy component in Eu_1?xAu_x and Yb_1?xAu_x. Furthermore, these shifts are different from the chemical shifts of the bulk 4f levels. The surface core-levels are used to identify surface segregation of the lanthanide metal in the present alloys.     

All-electron relativistic calculation on Au5X (X = Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu and Zn) clusters

All-electron scalar relativistic calculations on Au₅X (X = Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu and Zn) clusters have been performed by using density functional theory with the generalized gradient approximation. Our calculation results indicate that all the lowest energy geometries of Au₅X clusters have planar structures; the doped X atoms prefer to occupy the fourfold coordination site. Except Au₅Fe, Au₅Co and Au₅Zn, for other clusters including pure Au₆ cluster, the HOMO are delocalized obviously with a contribution from all atoms in the cluster. On the contrary, the electron localization in Au₅Zn is very strong resulting in the least stability of this cluster. Au₅Cu cluster with six delocalized electrons being defined as magic number for two-dimensional system has the largest VIP and deepest HOMO energy level. With the substitution Au for X atoms, the metallicity of all Au₅X clusters is reinforced.     

The effect of gold on platinum oxidation in homogeneous Au-Pt electrocatalysts

Oxidation of Au-Pt thin films was carried out in ambient air at room temperature and characterized by X-ray photoelectron spectroscopy. The homogeneous films were prepared by RF co-sputtering with concentrations varying from Au₉Pt₉₁ to Au₈₉Pt₁₁ and compared to pure Pt and Au thin films. Spectral deconvolution of the Au 4f and Pt 4f core levels revealed linear peak shifts for both the Au-Au and Pt-Pt bonding components as a function of alloy mixture and metallic component peak asymmetry that remained constant for all alloy stoichiometries. The predominant oxidation products were PtO and PtO₂ and were characterized by stable core level binding energies for all films. A gradual decline in the Pt-O_x products and corresponding levels of elemental oxygen was observed with increasing Au content but was similar in proportion to the metallic Pt components. Based on these results, variations in Pt oxide phases and/or concentration do not appear to contribute to enhanced electrocatalytic activity for oxygen reduction observed for the intermediate alloy stoichiometries.     

Probing the structural,electronic and magnetic properties of small Au4M (M = Sc–Zn) clusters

Density-functional method PW91 has been selected to investigate the structural, electronic and magnetic properties of Au₄M (M =Sc–Zn) clusters. Geometry optimisations show that the M atoms in the ground-state Au₄M clusters favour the most highly coordinated position. The ground-state Au₄M clusters possess a solid structure for M = Sc and Ti and a planar structure for M = V–Zn. The characteristic frequency of the doped clusters is much greater than that of pure gold cluster. The relative stability and chemical activity are analysed by means of the averaged binding energy and highest occupied molecular orbital and lowest unoccupied molecular orbital energy gap for the lowest energy Au₄M clusters. It is found that the dopant atoms can enhance the thermal stability of the host cluster except for Zn atom. The Au₄Ti, Au₄Mn and Au₄Zn clusters have relatively higher chemical stability. The vertical detachment energy, electron affinity and photoelectron spectrum are calculated and simulated theoretically for all the ground-state structures. The magnetism calculations reveal that the total magnetic moment of Au₄M cluster is mainly localised on the M atom and vary from 0 to 5 μ_B by substituting an Au atom in Au₅ cluster with different transition-metal atoms.     

Au/In bilayers studied with 111In deposited at the interface

Au/In bilayer samples of different stoichiometries were studied using the time-differential perturbed angular correlation technique. The ¹¹¹In probe atoms were deposited at the gold/indium interface to follow the formation of the intermetallic compounds at an early stage. Au-In phases of different composition (AuIn₂, AuIn, Au₇In₃, Au₃In) were created at the interfaces upon annealing. In addition a metastable Au₇In₃(m) phase, observed only after quenching in the bulk Au_0.7In_0.3 alloy, was found. This revised version was published online in August 2006 with corrections to the Cover Date.     

Insights into the structures,stabilities, electronic and magnetic properties of X2Aun (X = La,Y, and Sc; n = 1–9) clusters: comparison with pure gold clusters

A systematic study of the X₂Au_n (X = La, Y, Sc; n = 1–9) clusters are performed by using the density functional theory at TPSS level. The structures, stabilities, electronic, and magnetic properties are investigated in comparison with pure gold clusters. The results show that the transition points of the doped clusters from two-dimensional to three-dimensional structure are obviously earlier than gold clusters. The impurity X atoms tend to occupy the most highly coordinated position and form the largest probable number of bonds with gold atoms. In addition, the impurity atoms can strongly enhance the stabilities of gold clusters. It indicates that the impurity atoms dramatically affect the geometries and stabilities of the Au_n clusters. The highest occupied molecular orbital–lowest occupied molecular orbital gap, vertical ionisation potential, and chemical hardness show that the X₂Au₆ clusters have higher stabilities than the others. In La₂Au_1–9, Y₂Au_1–7, and Sc₂Au_1–4 clusters, the charges transfer from X atoms to the Au_n frames. The total magnetic moments of X₂Au_n clusters exist distinctly odd–even alternation behaviours except for La₂Au₄ and Sc₂Au₄ clusters.     

Features of the ordered phase formation in Au4V alloy

The kinetics of ordering in Au₄V alloy has been studied by X-ray diffraction. The features of ordering, coexisting with the ordered phase decomposition, have been investigated. Coexistence of these processes leads to a nonmonotonic change in the volume fraction of the ordered phase. Possible models of realization of ordering and decomposition processes are considered.     

Initial oxidation stages on FeCr(100) and FeCr(110) surfaces

Simultaneous LEED and AES are used to follow early stages of oxidation of monocrystalline FeCr(100) and (110) between 700 and 900 K in the oxygen pressure range 10^?9–10^?6 Torr. A chromium-rich oxide region at the alloy/oxide interface is observed, which exhibits different surface structures on oxidized FeCr(100) and FeCr(110). The chromium concentration in this initially formed oxide film is found to be enhanced by low oxygen pressures or high temperatures. During further oxidation different behaviours are observed on FeCr(100) and FeCr(110), which are explained by assuming different ion permeabilities through the initial chromium rich oxide regions on the two surface planes. On FeCr(110) surfaces oxidation is initiated on chromium enriched (100) facets at 800 K or below. At 900 K a film consisting of rhombohedral Cr₂O₃ or (Fe, Cr)₂O₃ is epitaxially growing with its (001) plane parallel to the alloy (110) face. On FeCr(100) surfaces the chromium rich oxide region next to the substrate is of fcc type. As soon as the diffusion of iron from the alloy to the gas/oxide interface is observable, a spinel type oxide is formed and connected with the location of iron in tetrahedral lattice sites. Closer to the fcc lattice the spinel oxide consists of FeCr₂O₄ or a solid solution of FeCr₂O₄ and Fe₃O₄ whereas next to the gas phase the oxide is pure Fe₃O₄.     

Initial oxidation kinetics and energetics of Cu0.5Au0.5 (0 0 1) film investigated by in situ ultrahigh vacuum transmission electron microscopy

The initial oxidation behavior of Cu_0.5Au_0.5 (0 0 1) thin film was investigated by in situ ultrahigh vacuum transmission electron microscopy to model nano-oxidation of alloys with one active component and one noble component. The formation of irregular-shaped octahedron Cu₂O islands with cube-on-cube crystallographic orientation to the substrate film was observed at all temperature studied. The energetics of Cu₂O nucleation for Cu and Cu_0.5Au_0.5 oxidation was compared. Cu_0.5Au_0.5 oxidation has lower nucleation activation energy due to the reduced mismatch strain between Cu₂O and Cu_0.5Au_0.5 films. On the other hand, the reaction kinetics for Cu_0.5Au_0.5 alloy oxidation is slower due to the higher diffusion activation energy of Cu.     

合金催化制备氧化锌纳米双晶薄片及其结构表征 (cited: 1)

借助气相输运凝结方法,采用Ag_xAu_1-x合金作为催化剂,在较低的650 oC下成功制备出氧化锌纳米双晶薄片. 高分辨结构表征证实该薄片中存在{10-13}孪晶界面. 系列地对比实验表明高的锌蒸汽浓度和Ag_xAu_1-x合金催化作用是孪晶成核的必要条件. 还发现衬底上制备出的氧化锌纳米孪晶薄片的密度可以通过合金催化剂中的银金成分比例加以调节.     

Internal field distribution around spin-glas freezing temperature in AuFe-alloys

We demonstrate an application of PAC to the study of magnetic ordering in random alloys. Using¹¹¹Cd as a probe the spin-glass freezing inAuFe-alloys of different concentrations could be observed. In the Au₈₂Fe₁₈ alloy no indication of ferromagnetism above the freezing temperature was found, due to the fact that the¹¹¹In probe nuclei are located in a Au rich environment.     

Structural stability and electronic properties of Au5Hn (n=1–10) clusters

A systematic study on the geometrical structures, electronic and magnetic properties of Au₅H _n (n=1–10) clusters has been performed by using the all-electron scalar relativistic density functional theory with generalized gradient approximation at the PW91 level. It is found that all Au₅H _n clusters prefer to keep the planar structures like pure Au₅ cluster, the Au₅ structures in Au₅H₄, Au₅H₅ and Au₅H₆ clusters are distorted obviously. The adsorption of a number of hydrogen atoms enhances the stability of Au₅ cluster and all Au₅H _n clusters are more stable than pure Au₅ cluster energetically. The odd-even alteration of magnetic moment is observed in Au₅H _n clusters and may be served as the material with tunable code capacity of “0” and “1” by adsorbing odd or even number of H atoms. It seems that the most favorable adsorption between Au₅ cluster and a number of hydrogen atoms takes place in the case that the odd number of hydrogen atoms is adsorbed onto Au₅ cluster and becomes Au₅H _n cluster with even number of valence electrons.     

Au1-xNix合金(x=0.30-0.42)低温电阻率极小值的机理探讨

为了解释Au_1-xNi_x合金(x=0.30—0.42)低温出现电阻率极小值的实验结果,本文提出一个低浓度自旋集团顺磁态合金的模型,得到自旋集团孤立近似下和自旋集团耦合作用下的电阻率公式,并和晶格散射的贡献(由Au₈₀Ni₂₀合金或Au-Cu合金的ρ_i(T)实验数据代替)联合起来,得到ρ(T)的计算曲线,和实验结果符合得很好。当Tmin,电阻率随温度增高而下降,主要是自旋集团孤立近似下的Kondo效应引起的。自旋集团之间的RKKY耦合作用对电阻率的贡献在低温时大,随着温度增高按1/T规律迅速减小,所以ρ(T)-ρ_i(T)实验值在相当宽温度范围出现logT关系。随着温度增高,晶格散射对电阻率的贡献将变得重要,当T=T_min,电阻率出现极小值。 关键词：     

Structural evolution and properties of small-size thiol-protected gold nanoclusters

Ligand-protected gold clusters are widely used in biosensors and catalysis. Understanding the structural evolution of these kinds of nanoclusters is important for experimental synthesis. Herein, based on the particle swarm optimisation algorithm and density functional theory method, we use [Au₁(SH)₂]_n, [Au₂(SH)₃]_n, [Au₃(SH)₄]_n (n?=?1–3) as basic units to research the structural evolution relationships from building blocks to the final whole structures. Results show that there is a ‘line-ring-core’ structural evolution pattern in the growth process of the nanoclusters. The core structures of the ligand-protected gold clusters consist of Au₃, Au₄, Au₆ and Au₇ atoms. The electronics and optics analysis reflects that stability and optical properties gradually enhance with increase in size. These results can be used to understand the initial growth stage and design new ligand-protected nanoclusters.     

Density functional investigation on the structures and properties of Li atom doped Au20 cluster

Structures and properties of an Au₂₀ cluster doped with two Li atoms, Au₁₈Li₂, have been investigated using relativistic density functional theory within the framework of the zeroth-order regular approximation. Various initial structures have been generated and employed for geometry optimization followed by vibration analysis to check the stability of the final optimized structures. We have calculated various properties like binding energy, ionization potential, electron affinity and the HOMO–LUMO gap of these structures. It has been found that two dopant Li atoms favour occupying two different surface positions of the pyramidal Au₂₀ cluster. The binding energy of the surface-doped Au₁₈Li₂ cluster is 1.017?eV higher than that of the pure Au₂₀ cluster and the HOMO–LUMO gap (1.742?eV) is as high as a pure Au₂₀ cluster (1.786?eV). Interestingly, we observe that the HOMO–LUMO gap as well as the binding energy can be increased beyond those of the Au₁₈Li₂ cluster with the help of further Li atom doping. In fact, a doped tetrahedral Au₁₆Li₄ cluster, where all the dopants are at the surface sites, possesses a very high HOMO–LUMO gap of 2.117?eV. Geometric and energetic parameters indicate that the Au₁₆Li₄ cluster might be considered as a possible ‘superatom’ in the design of novel cluster-assembled materials.     

Geometrical,electronic, and magnetic properties of Au n V (n = 1–8) clusters: A density functional study

The geometrical, electronic, and magnetic properties of small Au _n V (n?=?1–8) clusters have been investigated using density functional theory at the PW91 level. An extensive structural search indicates that the V atom in low-energy Au _n V isomers tends to occupy the most highly coordinated position and the ground-state configuration of Au _n V clusters favors a planar structure. The substitution of a V atom for an Au atom in the Au _{n +1} cluster transforms the structure of the host cluster. Maximum peaks are observed for the ground-state Au _n V clusters at n?=?2 and 4 for the size dependence of the second-order energy differences, implying that the Au₂V and Au₄V clusters possess relatively higher stability. The energy gap of the Au₃V cluster is the largest of all the clusters. This may be ascribed to its highly symmetrical geometry and closed eight-electron shell. For ground-state clusters with the same spin multiplicity, as the clusters size increases, the vertical ionization potential decreases and the electron affinity increases. Magnetism calculations for the most stable Au _n V clusters demonstrate that the V atom enhances the magnetic moment of the host clusters and carries most of the total magnetic moment.     

On the interstitial and substitutional distribution of Au dissolved in solid Pb

It has been proposed that Au, when dissolved in solid Pb, exists as isolated atoms (Au₁) and small clusters of two (Au₂) and four (Au₄) atoms. In this paper three sets of data are analyzed: (1) the volume change of Pb(Au) accompanying Au precipitation at room temperature, (2) the difference in lattice parameter between Pb and Pb(Au), and (3_) the equilibrium distribution of Au in Pb in a centrifugal field. On the basis of this analysis it is concluded that Au₁ is predominantly substitutional and Au in the quenched-in Au₄ state is either mostly substitutional (each Au atom occupying a former Pb site) or vacancies are created during cluster formation and survive the quench.