NUCLEATION AND GROWTH OF METALLIC SUBMONOLAYERS ON COMPACT METAL-SURFACES

By use of the molecular-dynamics method we study the high-temperature nonequilibrium and equilibrium properties of a metallic overlayer on a compact metal surface. The system, is modeled by the embedded-atom method, and consists of a Pt slab with a large ideal (111) surface (cross section of 930 atoms) on which Ag atoms are randomly distributed with coverages THETA=0.10, 0.25, 0.30. We simulate the short-time (tau approximately 1 ns) process by which the adatoms form dynamically stable aggregates in local equilibrium with a dilute two-dimensional vapor. We analyze the equilibrium properties of the adlayer as a function of T and THETA. We compute the cluster size distribution, the adatom radial-distribution function, the diffusion coefficient, and we characterize the cluster shape by the ratio of its principal moments of inertia. At all THETA and T the Ag islands grow pseudomorphically on Pt(111). At high temperatures (T approximately 1000 K) the clusters are fluidlike. There is no evidence of peculiar stability at the sizes corresponding to the filling of two-dimensional close-packing shells (''magic numbers,'' N=7, 10, etc.). At intermediate temperatures (T=600-800 K) the size distribution is shifted toward larger aggregates and, at low coverage, starts to display stability peaks at the magic numbers. The larger clusters develop a solid-like core surrounded by a fluidlike boundary. At the lowest temperature of our study (T=400 K) the clusters display a high degree of local order. The size distribution function is restricted to small N's by the slowing down of the cluster growth beyond the nucleation stage. At THETA=0. 10 the stability peaks at N = 7 and 10 are apparent. We discuss the influence of a step on the behavior of the system.