BINDING-ENERGIES AND STABILITY OF CU-ADATOM CLUSTERS ON CU(100) AND CU(111)

The binding energies of two-dimensional Cu-adatom clusters on the (100) and (111) faces of a Cu crystal are calculated as a function of cluster size and shape by means of molecular statics calculations using the atom-embedding model of Finnis and Sinclair. A simple model is presented to calculate the binding energy of a large adatom cluster by adding the contributions to the binding energy of the individual atoms in the cluster. The contribution of an individual atom to the total binding energy of a cluster depends on the occupation of nearest neighbour (NN) as well as next-nearest neighbour (NNN) sites on the (100) surface. On the (111) surface only the occupation of NN sites was found to be important. At low temperatures, square and rectangular islands with an aspect ratio close to unity are the most stable configurations on the (100) surface, while on the (111) surface hexagonal islands, in which all atoms have at least three nearest neighbours in the island, are found to be the most stable. At higher temperatures, the corners of large adatom islands on Cu(100) are rounded due to entropic effects, as is shown by two-dimensional Ising model calculations with NN interactions according to the atom-embedding calculations.