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.     

Ag-Cu双金属团簇中Ag原子偏析行为的 分子动力学研究

采用了恒温分子动力学方法系统模拟研究了不同尺寸不同组分的Ag-Cu双金属团簇的退火过程。分析低温退火结构可得团簇中Ag原子的偏析行为：在Ag原子所占比率较少时,Ag原子全部占据在团簇表面;随着Ag原子数的增多,直到Ag和Cu的比率接近时,绝大多数Ag原子仍占据在团簇表面;这与实验观测Ag-Cu混合团簇中Ag原子的偏析行为完全一致。通过细致研究Ag-Cu双金属团簇中Ag原子的偏析行为与团簇尺寸、组分和温度的关系发现：当Ag原子所占比重明显少于Cu原子时,在各不同尺寸下,Ag原子偏析温度点均高于熔点,即在熔点以上一定温度范围内仍会出现Ag原子的偏析现象;当Ag原子所占比重明显多于Cu原子时,在各不同尺寸下,体系偏析温度点均低于团簇熔点;对于较大尺寸团簇,在Ag原子所占比重接近于Cu原子时,体系偏析温度点与团簇熔点相同。     

直接置换法制备包覆型纳米铜-银双金属粉末

以AgNO3为主盐,采用直接置换法初步制备了包覆型纳米铜-银双金属粉末,分析了工艺条件对包覆效果的影响,并用透射电子显微镜、X射线能量色散谱仪和X射线衍射仪进行了表征。实验结果表明：纳米铜-银双金属粉末为包覆型结构,平均粒径约70 nm,分散性较好,表面银的原子分数达到74.28%。在洗涤过程中加入一定量的保护剂,有效解决了纳米铜粉的氧化问题。     

直接置换法制备包覆型纳米铜-银双金属粉末

 以AgNO₃为主盐,采用直接置换法初步制备了包覆型纳米铜-银双金属粉末,分析了工艺条件对包覆效果的影响,并用透射电子显微镜、X射线能量色散谱仪和X射线衍射仪进行了表征。实验结果表明：纳米铜-银双金属粉末为包覆型结构,平均粒径约70 nm,分散性较好,表面银的原子分数达到74.28%。在洗涤过程中加入一定量的保护剂,有效解决了纳米铜粉的氧化问题。     

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.     

Stability of small Ni-Ti bimetallic clusters studied by density functional theory

The low-energy structures and the electronic and the magnetic properties of small NiNiNi$_{m}$Ti$_{n}$ clusters, lowest-energy structure, electronic and magnetic propertiesProject supported by the National Natural Science Foundation of China (Grant No.~10874039), and the Natural Science Foundation of Hebei Province of China (Grant Nos.~A2009000246 and 2009000243).3120A, 3640B, 2110K7/3/2009 12:00:00 AMThe low-energy structures and the electronic and the magnetic properties of small NiNiNi$_{m}$Ti$_{n}$ clusters, lowest-energy structure, electronic and magnetic propertiesProject supported by the National Natural Science Foundation of China (Grant No.~10874039), and the Natural Science Foundation of Hebei Province of China (Grant Nos.~A2009000246 and 2009000243).3120A, 3640B, 2110K7/3/2009 12:00:00 AMThe low-energy structures and the electronic and the magnetic properties of small Ni$_{n}$Ti$_{n}$ ($n=1$--$6$) and Ni$_{m}$Ti$_{n}$ ($1 \le n \le 4$, $1 \le m \le 4$, $n \ne m$) clusters are investigated by performing all-electron calculations based on density functional theory. Ground states and several isomers near the ground states are determined for these clusters. The results indicate that the growth of small Ni$_{m}$Ti$_{n}$ clusters prefers to form rich Ti--Ni and Ti--Ti bonds. When the percentage of titanium atoms is significantly greater than that of nickel atoms, the nickel atoms are most frequently found above the surface; in contrast, the titanium atoms prefer the bridging sites. A M\"{u}lliken spin population analysis indicates that the total spin of titanium-nickel clusters is not always zero.http://cpb.iphy.ac.cn/CN/10.1088/1674-1056/19/4/043102https://cpb.iphy.ac.cn/CN/article/downloadArticleFile.do?attachType=PDF&id=111644NimTin;clusters;lowest-energy;structure;electronic;and;magnetic;propertiesThe low-energy structures and the electronic and the magnetic properties of small Nin Tin(n = 1-6) and Ni m Ti n(1 ≤ n ≤ 4,1 ≤ m ≤ 4,n ≠ m) clusters are investigated by performing all-electron calculations based on density functional theory.Ground states and several isomers near the ground states are determined for these clusters.The results indicate that the growth of small Ni m Ti n clusters prefers to form rich Ti-Ni and Ti-Ti bonds.When the percentage of titanium atoms is significantly greater than that of nickel atoms,the nickel atoms are most frequently found above the surface;in contrast,the titanium atoms prefer the bridging sites.A Mu¨lliken spin population analysis indicates that the total spin of titanium-nickel clusters is not always zero.     

Density functional study on structural and electronic properties of bimetallic gold--yttrium clusters: comparison with pure gold and yttrium clusters

Employing first-principles methods, based on the density functional theory, this paper investigates the ground state geometric and electronic properties of pure gold clusters, pure yttrium clusters and gold clusters doped each with one yttrium atom. It is shown that the average bond lengths in the Augold--yttrium bimetallic cluster, density functional theory, HOMO--LUMO gap, equilibrium structureProject supported by the Education Committee of Chongqing (Grant No KJ051105) and the National Natural Science Foundation of China (Grant No 10276028).3640, 3640B, 3120A, 3130J9/1/2007 12:00:00 AMEmploying first-principles methods, based on the density functional theory, this paper investigates the ground state geometric and electronic properties of pure gold clusters, pure yttrium clusters and gold clusters doped each with one yttrium atom. It is shown that the average bond lengths in the Augold--yttrium bimetallic cluster, density functional theory, HOMO--LUMO gap, equilibrium structureProject supported by the Education Committee of Chongqing (Grant No KJ051105) and the National Natural Science Foundation of China (Grant No 10276028).3640, 3640B, 3120A, 3130J9/1/2007 12:00:00 AMEmploying first-principles methods, based on the density functional theory, this paper investigates the ground state geometric and electronic properties of pure gold clusters, pure yttrium clusters and gold clusters doped each with one yttrium atom. It is shown that the average bond lengths in the Au$_{n - 1}$Y($n \le $9) bimetallic clusters are shorter than those in the corresponding pure gold and yttrium clusters. The most stable isomers of the yttrium-doped gold clusters tend to equally delocalize valence s, p and d electrons of the constituent atoms over the entire structure. The Y atom has maximum number of neighbouring Au atom, which tends to be energetically favourable in the lowest-energy equilibrium structures, because the Au--Y bond is stronger than the Au-Au bond. The three-dimensional isomers of Au$_{n - 1}$Y structures are found in an early appearance starting at $n$=5 (Au$_{4}$Y). Calculated vertical ionization potential and electron affinities as a function of the cluster size show odd-even oscillatory behaviour, and resemble pure gold clusters. However, one of the most striking feature of pure yttrium clusters is the absence of odd-even alternation, in agreement with mass spectrometric observations. The HOMO--LUMO gap of Au$_{3}$Y is the biggest in all the doped Au$_{n - 1}$Y($n \le $9) bimetallic clusters.     

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.     

Fe-Ni混合团簇基态结构的遗传算法研究

采用半经验的Gupta多体势结合遗传算法对不同组分的FenNim(N=n+m,N=13,38)混合团簇的基态结构和性质进行了模拟研究.结果显示:总原子数为13的混合团簇基态几何构型是基于单质团簇的基态二十面体结构,并且随着Fe原子数的增加表现出Fe原子首先占据中心位置的规律性;对总原子数为38的混合团簇,在轻混合(类掺杂)情形(n≤15,34≤n≤38)下基态几何构型为类似于纯单质团簇基态的截角八面体结构,而在重混合(n=16～33)时基态几何结构表现为不同于单质团簇的类截角二十面体构型;分析二级差分能表明Fe1Ni12、Fe7Ni31及Fe14Ni24具有相对高的稳定性,我们提出了基于有效键数的简化模型以解释此幻数结构序列.