Molecular dynamics simulations of surface diffusion and growth on silver and gold clusters

We report a systematic study of the diffusion of single adatoms and of the growth of fcc silver and gold clusters (Wulff polyhedra) by molecular dynamics simulations. Both metals have been modelled by many-body tight-binding potentials. The energy barriers for adatom diffusion on the cluster facets are calculated by the nudged elastic band method. Concerning single-adatom diffusion, we have studied the mechanisms connecting different facets [(111) and (100)] that play the main role during growth. We have found that the diffusion among different facets takes place by exchange processes in both metals; however, we have found that the mobility from a (111) to a (100) facet is much easier in gold than in silver. This has important consequences for the growth modes of the clusters. In fact, our growth simulations have shown that the transition from the Wulff shape to the octahedron is possible with gold even at low temperatures and quite fast deposition rates, whereas for silver much higher temperatures are required.