The structures of AgCu clusters containing 40 atoms are investigated. The most promising structural families (fcc clusters, capped decahedra, and two types of capped polyicosahedra) are singled out by means of global optimization techniques within an atom-atom potential model. Then, representative clusters of each family are relaxed by means of density-functional methods. It is shown that, for a large majority of compositions, a complex interplay of geometric and electronic shell-closure effects stabilizes a specific polyicosahedral family, whose clusters are much lower in energy and present large HOMO-LUMO gaps. Within this family, geometric and quantum effects concur to favor magic structures associated with core-shell chemical ordering and high symmetry, so that these clusters are very promising from the point of view of their optical properties. Our results also suggest a natural growth pathway of AgCu clusters through high-stability polyicosahedral structures. Results for AuCu clusters of the same size are reported for comparison, showing that the interplay of the different effects is highly material specific.

The structure of metal clusters supported on a MgO(001) substrate is investigated by a computational approach, with the aim to locate stable structural motifs and possible transition sizes between different epitaxies. Metal-metal interactions are modeled by a second-moment approximation tight-binding potential, while metal-oxide interactions are modeled by an analytic function fitted to first-principles calculations. Global optimization techniques are used to search for the most stable structural motifs at small sizes (N < or = 200), while at larger sizes different structural motifs are compared at geometric magic numbers for clusters up to several thousand atoms. Metals studied are Ag, Au, Pd, and Pt. They are grouped according to their mismatch to the oxide substrate (lattice constant of the metal versus oxygen-oxygen distance on the surface). Ag and Au, which have a smaller mismatch with MgO, are studied in Paper I, while Pd and Pt, with a larger mismatch, are investigated in Paper II. For Ag the cube-on-cube (001) epitaxy is favored in the whole size range studied, while for Au a transition from the (001) to the (111) epitaxy is located at N=1200. The reliability of the model is discussed in the light of the available experimental data.

The control of the structure of oxide-supported metal nanoparticles is crucial in determining their properties and possible applications. Here, building principles are derived for predicting the epitaxies of metal nanoparticles on square-symmetry oxide surfaces. Unusual phases are found for an appropriate choice of the metal-oxide pair, where nanoparticles with hcp structure are stabilized for fcc metals such as Ni, Pd, and Pt, or for Co in a size range in which Co has typically nonhcp arrangements. These predictions are supported by a comparison with available experimental data on Ni/MgO(100) nanodots, and generalized to a whole class of metal-oxide systems of great potential interest, such as Pd and Pt on CaO, Ni on CoO, and Co on MgO. The atomistic features of the nanoparticles in turn suggest that these materials should possess peculiar properties; in particular, the facets exposed by the nanodots reveal adsorption sites with unusual geometry of possible effect on their catalytic properties, while the destabilization of stacking faults and the structural deformations observed for these particles are expected to influence their magnetic behavior.

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.

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The structure of metal clusters on MgO(001) is searched for by different computational methods. For sizes N < or = 200, a global optimization basin-hopping algorithm is employed, whereas for larger sizes the most significant structural motifs are compared at magic sizes. This paper is focused on Pt and Pd/MgO(001), which present a non-negligible mismatch between the nearest-neighbor distance in the metal and the oxygen-oxygen distance in the substrate. For both metals, a transition from the cube-on-cube (001) epitaxy to the (111) epitaxy is found. The results of our simulations are compared to experimental data, to results found for Au and Ag in the previous paper (paper I), and to predictions derived from the Wulff-Kaischew construction.

The structure of Pd clusters adsorbed on MgO(001) is determined by a combination of global-optimization methods using semiempirical potentials and density functional calculations. The transition to fcc clusters with (001) epitaxy is shown to take place in the size range 10

A three-dimensional kinetic Monte Carlo (KMC) model has been developed and used to simulate the microstructure and growth morphology of cubic transition metal nitride (TMN) thin films deposited by reactive magnetron sputtering. Results are presented for the case of stoichiometric TiN, chosen as a representative TMN prototype. The model is based on a NaCl-type rigid lattice and includes deposition and diffusion events for both N and Ti species. It is capable of reproducing voids and overhangs, as well as surface faceting. Simulations were carried out assuming a uniform flux of incoming particles approaching the surface at normal incidence. The ballistic deposition model is parametrized with an interaction parameter r 0 that mimics the capture distance at which incoming particles may stick on the surface, equivalently to a surface trapping mechanism. Two diffusion models are implemented, based on the different ways to compute the site-dependent activation energy for hopping atoms. The influence of temperature (300-500 K), deposition flux (0.1-100 monolayers/s), and interaction parameter r 0 (1.5-6.0Å) on the obtained growth morphology are presented. Microstructures ranging from highly porous, [001]-oriented straight columns with smooth top surface to rough columns emerging with different crystallographic facets are reproduced, depending on kinetic restrictions, deposited energy (seemingly captured by r 0 ), and shadowing effect. The development of facets is a direct consequence of the diffusion model which includes an intrinsic (minimum energy-based) diffusion anisotropy, although no crystallographic diffusion anisotropy was explicitly taken into account at this stage. The time-dependent morphological evolution is analyzed quantitatively to extract the growth exponent β and roughness exponent α, as indicators of kinetic roughening behavior. For dense TiN films, values of α ≈ 0.7 and β = 0.24 are obtained in good agreement with existing experimental data. At this stage a single lattice is considered but the KMC model will be extended further to address more complex mechanisms, such as anisotropic surface diffusion and grain boundary migration at the origin of the competitive columnar growth observed in polycrystalline TiN-based films.