Diffusion and coalescence of bilayer surface-alloy islands of Pd(0 0 1)-p(2×2)-p4g–Al

We have studied thermally induced coarsening process in bilayer islands of ordered Al–Pd surface alloy on Pd(0 0 1). The island size distribution was deduced from scanning tunneling microscopy images recorded after annealing the surface at 500–700 K. Normalized size distribution function after annealing at 500 K, at which temperature the bilayer surface alloy is formed, shows a sharp peak at u∼0.2, where u denotes the island size normalized to the average size, with a long tail extending to u∼4. Upon annealing at higher temperatures, the peak shifts to larger u and gets broader. After annealing at 700 K, at which temperature the decomposition of surface alloy sets in, the normalized distribution function resembles closely the universal scaling function predicted for coarsening due to diffusion and coalescence of two-dimensional islands. The data is not consistent with the Ostwald ripening.