The shapes of islands of chemisorbed atoms as a probe of long-range interadatom interactions

Abstract

When ordered superlattices of adatoms are produced by attractive electronic indirect interactions, islands form at low coverages. At low temperatures, the shapes of these domains should bear the mark of these interactions. (For example, at zero temperature a c(2 × 2) island on a square lattice should be square if the second neighbor attraction is at least roughly three times the third neighbor attraction.) First, asymptotic and explicit evaluations of the energies of different configurations are presented. Then some thermal disordering effects are included in a quasicontinuous way. Polygonal-like shapes should be observable at low temperatures, below (possibly much below) 100K. Complications due to three-adatom interactions can arise in model calculations. Island shapes should be observable using LEED; the adlayer-induced spots should exhibit star-like patterns. Problems such as diffusion and heterogeneity limit the size of islands, making this a problem of metastability. The (111) faces of fcc crystals most likely are the best substrate on which to seek polygonal (i.e., hexagonal) shapes. In any case these ideas provide another aspect by which to compare Monte Carlo simulations with experimental observations.