Narrow Results

Structural stability and energetics of nickel clusters, Ni_N (N =3-459), have been investigated by molecular-dynamics simulations. A size-dependent empirical model potential energy function has been used in the simulations. Stable structures of the microclusters with sizes N = 3-55 and clusters generated from fcc crystal structure with sizes N = 79-459 have been determined by molecular-dynamics simulations. It has been found that the five-fold symmetry appears on the surface of the spherical clusters. The average coordination number shows a size-dependent characteristic, on the other hand the average nearest-neighbor distance does not show a size-dependence.

The effect of radiation damage on copper clusters has been investigated by performing molecular-dynamics simulation using empirical potential energy function for interaction between copper atoms. The external radiation is modeled by giving extra kinetic energy in the range of 5–50 eV to initially chosen atom in the cluster. It has been found that the atom having extra kinetic energy dissociates independently from the amount of given energy in the studied range.

We have investigated systematically the energetics of arsenic terminated GaAs(001) surfaces. Available surface models proposed in the literature have been considered, and relaxation and surface energies of each model have been calculated using an empirical many-body potential energy function comprising two and three-body atomic interactions.

Melting and fragmentation behaviors of Ni₄₂₉ cluster have been studied with molecular-dynamics simulations using a size-dependent empirical model potential energy function. To monitor thermal behaviors of the cluster, we calculated some physical quantities such as average potential energy per atom, specific heat, radial atomic distribution, bond length distribution, average interatomic distance, nearest neighbor distance and average coordination number as a function of temperature. The roles of the surface and core atoms in the melting and fragmentation process of the cluster are also investigated by considering the surface and the bulk coordination numbers of the cluster.

We have simulated the gold deposition on arsenic and gallium terminated GaAs(001) surfaces at low and room temperatures. It has been found that gallium terminated surface is relatively less stable in comparison to the arsenic terminated surface. On the other hand, a single gold adatom on the surface has different characteristics than full coverage gold atoms on the surface; a single gold atom diffuses into the surface at room temperature. Simulations have been performed by considering classical molecular-dynamics technique using an empirical many-body potential energy function comprising two- and three-body atomic interactions.

An empirical many-body potential energy function has been developed to investigate the structural features and energetics of Zn_kCd_l (k+l=3, 4) microclusters. The most stable structures were found to be triangular for the three-atom clusters and tetrahedral for the four-atom clusters. The present results are in good agreement with available literature values.

Using an empirical potential energy function parametrized for each of the Ni, Cu, Pd, Pt, and Pb systems, minimum-energy structures of Ni_n, Cu_n, Pd_n, Pt_n, and Pb_n (n=3–13) microclusters have been determined by performing molecular-dynamics simulations. The structural and energetic features of the obtained microclusters have been investigated.

Brief information about nanoparticles and size dependency of their properties is given. Structural properties of copper nanoparticles, Cu_n (n = 50, 100, 150) have been investigated by a modified version of diffusion Monte Carlo method, using an empirical pair potential developed and parameterized for copper. Radial distribution of atoms and the coordination numbers are investigated by the optimum geometries obtained. It has been found that stable structures of copper nanoparticles considered have compact spherical shapes.

Si_n clusters in the size range n = 4–30 have been investigated using a combination of global structure optimization methods with DFT and ab initio calculations. One of the central aims is to provide explanations for the structural transition from prolate to spherical outer shapes at about n = 25, as observed in ion mobility measurements. Firstly, several existing empirical potentials for silicon and a newly generated variant of one of them were better adapted to small silicon clusters, by global optimization of their parameters. The best resulting empirical potentials were then employed in global cluster structure optimizations. The most promising structures from this stage were relaxed further at the DFT level with the hybrid B3LYP functional. For the resulting structures, single point energies have been calculated at the LMP2 level with a reasonable medium-sized basis set, cc-pVTZ. These DFT and LMP2 calculations were also carried out for the best structures proposed in the literature, including the most recent ones, to obtain the currently best and most complete overall picture of the structural preferences of silicon clusters. In agreement with recent findings, results obtained at the DFT level do support the shape transition from prolate to spherical structures, beginning with Si₂₆ (albeit not completely without problems). In stark contrast, at the LMP2 level, the dominance of spherical structures after the transition region could not be confirmed. Instead, just as below the transition region, prolate isomers are obtained as the lowest-energy structures for n ≤ 29. We conclude that higher (probably multireference) levels of theoretical treatments are needed before the puzzle of the silicon cluster shape transition at n = 25 can safely be considered as explained.