THE MOBILITY OF PT ATOMS AND SMALL PT CLUSTERS ON PT(111) AND ITS IMPLICATIONS FOR THE EARLY STAGES OF EPITAXIAL-GROWTH

We use the effective medium approximation to calculate the activation energy for the site-to-site motion of Pt atoms and Pt clusters with two to seven atoms, on the Pt(111) surface. We find that clusters are highly mobile and will move as a whole rather than by breaking into fragments that reunite in a different place. The results are used to examine the properties of the islands formed in the earliest stages of Pt deposition on the surface. We show that island formation at temperatures below 500 K occurs through the formation of stable clusters. There is no critical nucleus and the density of the supersaturated gas of single atoms on the surface is very close to zero; the islands do not originate through density fluctuations. We also rationalize the existence of three distinct regimes in the temperature dependence of island sizes at low coverage. They are due to absence of atom ''evaporation'' from islands onto the surface, at low temperature; to evaporation som open shell islands but not from the closed shell ones, at intermediate temperature; and to evaporation from all islands, at higher temperature.