Simulation of the interaction of bipartite bimetallic clusters with low-energy argon clusters

Molecular-dynamics simulation of the evolution of bipartite bimetallic clusters consisting of 390 atoms during bombardment by Ar_n (n = 1, 2, 13) clusters with initial energies from 1 eV to 1.4 keV is performed. Binary Cu–Au and Ni–Al clusters consisting of equal atomic fractions of corresponding elements were used as a target. As a result of simulation, the temperatures, changes in the potential energy, sputtering yields, and intensities of collision-stimulated displacement of atoms through the interface of monometallic parts of binary clusters, depending on the size and energy of incident particle, are obtained.

     

Structural properties and reactivity of bimetallic silver-gold clusters

Bimetallic silver-gold clusters are well suited to study changes in metallic versus ionic properties involving charge transfer as a function of the size and the composition. We present structures, ionization potentials (IP) and vertical detachment energies (VDE) for neutral and charged bimetallic Au_nAg_m ( 2(n + m)5) clusters obtained from density functional level of theory. In the stable structures of these clusters Au atoms assume positions which favor charge transfer from Ag atoms. In clusters with equal numbers of hetero atoms (n = m = 1- 4) heteronuclear bonding is preferred to homonuclear bonding, giving rise to large values of ionization potentials. For larger clusters (n=m=5, 10) stable structures do not favor neither hetero bonding nor segregation into the single components, although they exhibit more metallic than ionic features. This remains valid also for Au₈Ag₁₂ cluster characterized by strong charge transfer to gold subunit. The influence of doping of pure gold clusters with silver atoms on VDE and IP values is discussed in context of their reactivity towards O₂ and CO molecules. As a starting point we consider reactivity towards CO and O₂ molecules on the example of AgAu^- dimer. The results show that the catalytic cycle can be fullfilled.     

Density functional calculations on small bimetallic magnetic clusters

Structural and magnetic properties of small bimetallic clusters like Co_MRh_N, NiNa_N, and NiCu_N are determined in the framework of a generalized gradient-corrected approximation to density functional theory. The role of magnetism on the most stable structure and on the energy differences among the low-lying isomers is quantified by comparing magnetic and non-magnetic solutions of the Kohn-Sham equations. The correlation between structure, chemical order, and environment-dependent magnetic properties is discussed.     

Thermodynamic properties of AuyAgx bimetallic clusters through the evolutive ensemble

The caloric and specific heat curves for the bimetallic nanoclusters Au_7-xAg_x (x=0,3,4,7) are obtained through a statistical determination of the configurational density of states in the evolutive ensemble obtained with a genetic algorithm. The effect of the value of x (the relative concentrations) on the thermodynamics is studied. Three peaks are observed in the specific heat curves for all values of x. This is interpreted as being due to melting, and fragmentation of the cluster into first two, and then into 3 or more parts. A fourth pre-melting peak is observed for Au₄Ag₃ and is attributed to a new phenomena related to the breaking of the degeneracy of the permutational isomers. The melting transition for the bimetallic clusters is significantly wider than that for the pure clusters. The boiling transition displays a larger specific heat for the bimetallic clusters.     

Size and composition dependence of the frozen structures in Co-based bimetallic clusters

This Letter studies the size-dependent freezing of Co, Co-Ni, and Co-Cu clusters by using molecular dynamics with embedded atom method. Size effect occurs in these three types of clusters. The clusters with large sizes always freeze to form their bulk-like structures. However, the frozen structures for small sizes are generally related to their compositions. The icosahedral clusters are formed for Co clusters (for ?3.2 nm diameter) and also for Co-Ni clusters but at a larger size range (for ?4.08 nm). Upon the Co-Cu clusters, decahedral structure is obtained for small size (for 2.47 nm). The released energy induced the structural transformation plays a key role in the frozen structures. These results indicate that the preformed clusters with special structures can be tuned by controlling their compositions and sizes.     

Plasma oscillations in oxide free Al clusters

Plasma oscillations in oxide free Al clusters with radii of the order 10 Å are excited by 50 keV electrons. Only surface plasma oscillations are observed. Results are in excellent agreement with the hydrodynamic theory put forward by Fujimoto and Komaki. This theory is for the first time quantitatively confirmed.     

Molecular dynamics simulation of bipartite bimetallic clusters under low-energy argon ion bombardment

The evolution of bipartite bimetallic atomic clusters within 5 ps under bombardment with monoenergetic argon ions at the initial energy ranging from 1 eV to 1.4 keV has been simulated by the classical molecular dynamics method with a target obtained from Ni?Al and Cu?Au clusters consisting of 78 and 390 atoms, equally divided between the corresponding monometallic parts, the simulated pairs of which have different heats of intermixing. The changes in the potential energy and temperature, the sputtering yields, and the intensity of the ion-stimulated movement of atoms at the interface of the monometallic parts of clusters of both sizes have been determined as functions of the energy of the bombardment.     

Structure evolution during the cooling and coalesced cooling processes of Cu-Co bimetallic clusters

Constant-temperature molecular dynamics with general EAM was employed to study the structure evolutions during the cooling and coalesced cooling processes of Cu-Co bimetallic clusters. It shows that the desired particle morphologies and structures can be obtained by controlling the composition and distribution of hetero atoms during synthesis process.     

Diffusion of palladium clusters on magnesium oxide

The diffusion of small palladium clusters on MgO(100) is theoretically investigated. It is found that small clusters can diffuse even faster than isolated adatoms by a variety of mechanisms (some of which are novel), such as dimer rotation, trimer walking, tetramer rolling, and sliding. The consequences of the diffusion of small clusters on the growth of Pd aggregates on MgO(100) are investigated, and it is shown that fast mobility of clusters larger than a single atom is essential to bring the theoretical results into agreement with the outcome of molecular beam epitaxy experiments.     

Reducible and non-reducible defect clusters in tin-doped indium oxide

Density functional theory calculations are used to estimate the energy of interstitial oxygen (O_i) released from tin-doped indium oxide (ITO). The currently accepted explanation of defect clusters’ irreducibility is based on different arrangements of doping atoms around O_i. In the present contribution we demonstrate that this concept has only a limited domain of applicability and explains the relative stability of different defect clusters with the same and fixed Sn:O_i ratio. To describe practically the important case of ITO treatment under strong reduction conditions another limiting case of varying Sn:O_i ratio is considered. It is found that in this particular case local coordination of doping atoms around O_i plays only a minor role. The relative stability of the oxidized defect clusters has caused a noticeable change in the electronic part of the defect formation energy, i.e. the chemical potential of the conduction electrons determines the equilibrium concentration of the interstitial oxygen atoms.     

Structure and stability of small zinc oxide clusters

The structure and stability of small neutral and positively charged zinc oxide (ZnO) _n clusters (n = 2−9) have been studied within the density functional theory. For n ≤ 7, the most stable clusters are shown to be flat rings; for n = 8, 9, the clusters are mainly three-dimensional cage structures. The energies and main channels of fragmentation of the clusters have been determined. It has been found that the fragmentation of the charged clusters with n > 6 occurs predominantly with formation of a (ZnO)₄⁺ ion, which explains the available mass spectrometric data on ionization of the zinc oxide clusters by electron impact.     

Formation and photodecomposition of cationic titanium oxide clusters

In this paper we report on the titanium oxide cluster cations Ti _x O _y ⁺, generated by laser ablation of a titanium target in the region of the nozzle expansion of oxygen. The mass distribution of the clusters produced is recorded with a time-of-flight mass spectrometer. Three different series, namely TiO(TiO₂) _n ⁺ , TiO(TiO₂) _n O₂⁺, and (TiO₂) _n ⁺, appear in the spectra. Two different ablation wavelengths (infrared at 1064 nm and ultraviolet at 308 nm) are used to generate the titanium oxide clusters. At the shorter wavelength the maximum size of the clusters formed decreases. The interaction of the UV photons with the Ti _x O _y ⁺ clusters is further investigated in a separate two-laser arrangement with an IR laser for ablation and after some mm downstream with an UV system for the cluster beam irradiation. These studies indicate that the intensity of the T _x O _y ⁺ clusters with x≥4, y≥7 is strongly influenced by the absorption of UV photons. This is attributed mainly to dissociation into smaller ones.     

Decay rate constants of sputtered vanadium oxide clusters

Results from studying the emission and fragmentation of V _n O _m ^± clusters sputtered from a vanadium surface by Xe⁺ ions at O₂ pressures of P = 4–5 × 10^?3 Pa are presented. The average decay rate constants of V _n O _m ^± clusters for major fragmentation channels are determined. It is shown that the decay rate constants for clusters of similar stoichiometry do not depend on their charge states.     

Pdn Pbm(n+m≤5)混合团簇的结构与光谱性质

在相对论有效原子实势(RECP)近似下,用密度泛函中的B3LYP/LANL2DZ方法,对纯Pbn(n=2-5)团簇、PdnPbm(n+m≤5)混合团簇的各种可能几何构型进行全优化计算,得到它们的基态结构和光谱性质;从结构和振动光谱两个方面分析了其形成规律;最后计算了团簇的能级分布和最高占据轨道(HOMO)与最低空轨道(LUMO)之间的能级间隙(HLG),分析了团簇的化学活性.