Hcp-icosahedron structural transformation induced by the change in Ag concentration during the freezing of （CoAg）561 clusters

The synergy effect of alloy elements in bimetallic clusters can be used to tune the chemical and physical properties. Research on the influences of alloy concentration and distribution on the frozen structure of bimetallic clusters plays a key rolc in exploring new structural materials. In this paper, we study the influence of Ag concentration on the frozen structure of the （AgCo）561 cluster by using molecular dynamics simulation with a general embedded atom method. The results indicate that tt~e structure and chemical ordering of the （AgCo）561 cluster are strongly related to Ag concentration. Hcp-icosahedron structural transformation in the frozen （CoAg）561 cluster can be induced by changing Ag concentration. The chemical ordering also transforms to Janus-like Co Ag from core-shell Co-Ag.     

Molecular Dynamics Simulation of Icosahedral Transformations in Solid Cu-Co Clusters

We study the icosahedral transformations of solid Cu Co clusters with different initial configurations by using molecular dynamics with the embedded atom method. It is found that the formation of symmetric icosahedral cluster is strongly related to the atomic number and initial configuration. The transformation originates from the surface into the interior of the cluster and is a structural change which is rapid and diffusionless. The icosahedral clusters with any composition and configuration, such as core-shell or three-shell duster, can be prepared by the means of solid-solid phase transition in bimetallic clusters.     

GEOMETRIC AND ELECTRONIC STRUCTURES OF Ti8C12 CLUSTER

The electronic structure of Ti₈C₁₂ clusters with three possible geometric structures suggested in the literature is studied using the discrete-variational local-density-functional method. The results show that the ground states of the clusters are all degenerate, which means further Jahn-Teller distortions for the geometric structures of clusters. The results also indicate that the distorted dodecahedral Ti₈C₁₂ cluster, which is proposed by Guo et al. and optimized by the first principle calculations, is the most stable one among the clusters we considered and its electronic structure can explain the experimental observations. In this cluster, there is a strong pd bonding between Ti and C atoms, and the density of states at the Fermi energy is high.     

Structural, electronic and vibrational properties of indium oxide clusters

Geometric, electronic and vibrational properties of the most stable and energetically favourable configurations of indium oxide clusters In_mO_n (1≤m, n≤4) are investigated using density functional theory. The lowest energy geometries prefer the planar arrangement of the constituent atoms with a trend to maximize the number of ionic In-O bonds. Due to the charge transfer from In to O atoms, the electrostatic repulsion occurs between the atoms with the same kind of charge. The minimization of electrostatic repulsion and the maximization of In-O bond number compete between each other and determine the location of the isometric total energy. The most stable linear In-O-In-O structure of In₂O₂ cluster is attributed to the reduced electrostatic repulsive energy at the expense of In-O bond number, while the lowest energy rhombus-like structure of In₂O₃ cluster reflects the maximized number of In-O bonds. Furthermore, the vibrational frequencies of the lowest energy clusters are calculated and compared with the available experimental results. The energy gap and the charge density distribution for clusters with varying oxygen/indium ratio are also discussed.     

Hcp-icosahedron structural transformation induced by the change in Ag concentration during the freezing of （CoAg）₅₆₁ clusters

The synergy effect of alloy elements in bimetallic clusters can be used to tune the chemical and physical properties. Research on the influences of alloy concentration and distribution on the frozen structure of bimetallic clusters plays a key role in exploring new structural materials. In this paper, we study the influence of Ag concentration on the frozen structure of the (AgCo) 561 cluster by using molecular dynamics simulation with a general embedded atom method. The results indicate that the structure and chemical ordering of the (AgCo) 561 cluster are strongly related to Ag concentration. Hcp-icosahedron structural transformation in the frozen (CoAg) 561 cluster can be induced by changing Ag concentration. The chemical ordering also transforms to Janus-like Co–Ag from core-shell Co–Ag.     

Coalescence of heteroclusters Au767 and Ag767: a molecular-dynamics study

This paper studies the coalescence of heteroclusters Au767 and Ag767 by using molecular dynamics with the embedded atom method, where layer atomic energy is employed to describe the potential energy variation of per atom in different layers along radial direction. The results show that the coalescence is driven by releasing the atomic energy of the coalesced zone. The deformation, which is induced by substitutional and vacancy diffusion during the coalescence, makes the coalesced cluster disorder. If the summation of the thermal energy and the released atomic energy is large enough to keep the disorder state, the clusters form a metastable liquid droplet; otherwise, the clusters coalesce into a solid cluster when the coalesced cluster reaches the equilibrium state, and the coalesced cluster experiences liquid to solid ordering changes during the coalescence of a solid Au767 with a liquid Ag767 and a liquid Au767 with a liquid Ag767. The centre of figure of the cluster system is shifted during the coalescence process, and higher coalescence temperature causes larger shift degree.     

Deuterium cluster jet produced at moderate backing pressures

A deuterium cluster jet produced in the supersonic expansion into vacuum of deuterium gas at liquid nitrogen temperature and moderate backing pressures are studied by Rayleigh scattering techniques. The experimental results show that deuterium clusters can be created at moderate gas backing pressures ranging from 8 to 23 bar, and a maximum average cluster size of 350 atoms per cluster is estimated. The temporal evolution of the cluster jet generated at the backing pressure of 20 bar demonstrates a two-plateau structure. The possible mechanism responsible for this structure is discussed. The former plateau with higher average atom and cluster densities is more suitable for the general laser-cluster interaction experiments.     

Diffusion of Pt dimers on a Wulff polyhedral surface

The surface diffusion of Pt dimers on Wulff polyhedral clusters with 586-12934 atoms was studied by the embedded atom method. The minimum energy diffusion path and the corresponding energy barrier for dimer diffusion on cluster surfaces were determined through a combination of the quenched Molecular Dynamics and the Nudged Elastic Band method. It was found that the diffusion of a dimer across the step-edge by dissociation and consecutive single-atom exchange with the edge atoms, rather than diffusion over t...     

Geometries,stabilities, and electronic properties analysis in In_nNi~((0,±1)) clusters: Molecular modeling and DFT calculations

Density functional theory(DFT) with the B3 LYP method and the SDD basis set is selected to investigate In_nNi,In_nNi~-, and In_nNi~+ (n = 1–14) clusters. For neutral and charged systems, several isomers and different multiplicities are studied with the aim to confirm the most stable structures. The structural evolution of neutral, cationic, and anionic In_nNi clusters, which favors the three-dimensional structures for n = 3–14. The main configurations of the In_nNi isomers are not affected by adding or removing an electron, the order of their stabilities is also nearly not affected. The obtained binding energy exhibits that the Ni-doped In_(13) cluster is the most stable species of all different sized clusters. The calculated fragmentation energy and the second-order energy difference as a function of the cluster size exhibit a pronounced even–odd alternation phenomenon. The electronic properties including energy gap(E_g), adiabatic electron affinity(AEA), vertical electron detachment energy(VDE), adiabatic ionization potential energy(AIP), and vertical ionization potential energy(VIP) are studied. The total magnetic moments show that the different magnetic moments depend on the number of the In atoms for charged In_nNi. Additionally, the natural population analysis of In_nNi~((0,±1)clusters is also discussed.     

Na decorated B6 cluster and its hydrogen storage properties

The structures and hydrogen storage properties of sodium atoms decorated B6 clusters are investigated by the B3LYP method with a 6-311＋G （d, p） basis set. For NamB6 （m = 1-3） clusters, Na atoms are always inclined to separate far enough from each other and not cluster together on a B6 cluster surface so that each Na atom has sufficient space to bind hydrogen molecules. The hydrogen storage gravimetric density of a two Na atoms decorated B6 cluster is 17.91 wt% with an adsorption energy per H2 molecule （AAE/H2） of 0.6851 kcal.mo1^-1. The appropriate AAE/H2 and preferable gravimetric density of the two Na atoms decorated B6 cluster complex indicate that it is feasible for hydrogen storage application in ambient conditions.     

Atomic scale models of $\mathsf{Co{-}Ag}$ mixed nanoclusters at thermodynamic equilibrium

The configurations at thermodynamic equilibrium of Co_xAg_201-x nanoparticles are explored for 0 < x < 201 by means of Metropolis Monte Carlo simulations with a semi-empirical embedded atom potential at temperatures from 100 K to 1000 K. Remarkable configurations are predicted in this temperature range. As a consequence of a competition between strain and Co binding at low temperature, for x < 20, Co is distributed just below the cluster surface layer into groups of no more than 5 atoms, favouring well-defined positions, and the cluster central area is avoided. To increase the temperature favours the clustering of these small groups. Their dissolution is predicted at temperatures higher than the melting temperature of the cluster. For x > 50, Co regroups at the centre of the cluster and intersects {111}-facets when Ag atoms are not numerous enough to form an entire surface shell. At these stoechiometries, temperature is not sufficient to mix Ag and Co, even above the melting point. At still smaller Ag concentrations, the Ag atoms are distributed at lowest coordination sites, along the edges of the cluster, avoiding the cluster facets as well as inner sites. At intermediate stoechiometries (20 < x < 50), either oblate Co groups below the surface or a compact group at the centre of the cluster are possible.Received: 1 October 2003, Published online: 27 January 2004PACS: 61.46. + w Nanoscale materials: clusters, nanoparticles, nanotubes, and nanocrystals - 61.43.Bn Structural modeling: serial-addition models, computer simulation     

Interface formation by low energy deposition of core-shell Ag-Co nanoclusters on Ag(100)

Molecular Dynamics is used to study the formation of an interface between a single crystalline Ag matrix and core-shell AgCo nanoclusters deposited with energies ranging from 0.25 eV to 1.5 eV per atom. As a consequence of the slowing down, clusters deform, may become epitaxial with the substrate and keep their core-shell structure. The consequences of the cluster-surface interaction are studied in detail for a realistic size and energy range and the accumulation of clusters is modelled. It is found that the interface formed is no more than a few atomic layers thick and that both the cobalt cluster cores and the silver shells display limited epitaxy with the substrate. The effect is not much energy dependent and is larger for the Ag shells that for the Co cores.     

The structure of 55-atom Cu–Au bimetallic clusters: Monte Carlo study

We have investigated segregation phenomena in Cu–Au bimetallic clusters with decahedral structures at 100 K and 300 K, based on the second-moment approximation of the tight-binding (TB-SMA) potentials by using Monte Carlo method. The simulation results indicate that there are three regions (split, three-shell onion-like and core-shell region) at 100 K and two regions (split and core-shell) at 300 K with the structure of decahedral clusters, as the chemical potential difference Δμ changes. It is found that the structure of decahedral clusters undergoes a division into smaller clusters in the split region. In the core-shell structure, Au atoms are enriched in surface and Cu atoms occupy the core of the clusters because of the different surface energy of Cu and Au. The Au atoms are enriched in the surface shell, and the Cu atoms are in the middle shell, while a single Au atom is located in the center to form the three-shell onion-like structure. The structure and binding energy of smaller clusters after splitting are also discussed. The Au atoms generally lie on the surface of the smaller clusters after splitting.     

Structure and luminescence of Ca2Si5N8：Eu^2＋ phosphor for warm white light-emitting diodes

We have synthesized Ca 2 Si 5 N 8:Eu 2+ phosphor through a solid-state reaction and investigated its structural and luminescent properties.Our Rietveld refinement of the crystal structure of Ca 1.9 Eu 0.1 Si 5 N 8 reveals that Eu atoms substituting for Ca atoms occupy two crystallographic positions.Between 10 K and 300 K,Ca 2 Si 5 N 8:Eu 2+ phosphor shows a broad red emission band centred at ～1.97 eV-2.01 eV.The gravity centre of the excitation band is located at 3.0 eV-3.31 eV.The centroid shift of the 5d levels of Eu 2+ is determined to be ～1.17 eV,and the red-shift of the lowest absorption band to be ～ 0.54 eV due to the crystal field splitting.We have analysed the temperature dependence of PL by using a configuration coordinate model.The Huang-Rhys parameter S=6.0,the phonon energy ν=52 meV,and the Stokes shift S=0.57 eV are obtained.The emission intensity maximum occurring at ～200 K can be explained by a trapping effect.Both photoluminescence (PL) emission intensity and decay time decrease with temperature increasing beyond 200 K due to the non-radiative process.     

Molecular-dynamics investigation of structural transformations of a Cu201 cluster in its melting process

We perform molecular-dynamics calculations to investigate the structural transformation of a copper cluster containing 201 atoms in its melting process within the framework of the embedded-atom method (EAM). Concerning melting, the obtained results reveal that its structural changes are different from those of larger-size clusters containing several hundreds or more atoms and smaller-size clusters containing tens of atoms. The melting process of this Cu₂₀₁ cluster involves three stages, firstly some atoms in inner regions of this cluster move into outer regions accompanying the structural transformation of the local atom packing, followed by the continuous interchange of atomic positions, and finally this cluster is wholly disordered. During the temperature increase, the structural changes of different regions determined by atom density profiles result in apparent increases in internal energy. By decomposing peaks of pair distribution functions (PDFs) according to the pair analysis (PA) technique, the local structural patterns are identified for the melting of this cluster.     

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.     

Strong influence of defects on the electronic structure of Pt adatoms and clusters on graphite

We study the specific impact of defects such as step edges at the graphite surface on the electronic configuration of adsorbed Pt atoms and Pt₈ clusters. Surface diffusion is strongly reduced by depositing Pt and Pt₈ into a thin rare gas layer. In this configuration a very narrow adatom Pt 4f spectrum is found at an exceptionally small binding energy, similar to Pt surfaces. Both, adatom and cluster spectra are strongly shifted towards higher binding energy when allowed to diffuse towards defects like step edges. The strong shifts are indicative of a chemical reaction at the step edges and are conjectured to be part of the particle size dependent binding energy shifts typically observed for transition metal clusters grown on the surface of graphite.     

强激光场中原子团簇的电离和光辐射

通过数值求解一维含时薛定谔方程,研究了原子与原子围簇的电离、电子的时间演化和光辐射现象。结果表明,原子团簇比原子更易于电离,原子团簇中电子的时间演化和原子中电子的时间演化不同,光辐射频率也要高很多。