ISLAND MORPHOLOGY AND ADATOM SELF-DIFFUSION ON PT(111)

The results of a density-functional-theory study of the formation ener gies of (100)- and (111)-faceted steps on the Pt(111) surface, as well as of the barrier for diffusion of an adatom on the flat surface, are presented. The step formation energies are found to be in a ratio of 0.88 in favor of the (111)-faceted step, in excellent agreement with e xperiment; the equilibrium shape of islands should, therefore, clearly be nonhexagonal. The origin of the difference between the two steps i s discussed in terms of the release of stress at the surface through r elaxation. For the diffusion barrier, we also find relaxation to be im portant, leading to a 20% decrease of its energy. The value we obtain: 0.33 eV, however, remains higher than available experimental data; po ssible reasons for this discrepancy are discussed. We find the ratio o f step formation energies and the diffusion barrier to be the same whe ther using the local-density approximation or the generalized-gradient approximation for the exchange-and-correlation energy.