Sintering of two-dimensional nanoclusters in metal(100) homoepitaxial systems: Deviations from predictions of Mullins continuum theory

We present a comparison of the predictions of atomistic and continuum models for the sintering of pairs of near-square two-dimensional nanoclusters adsorbed on the ͑100͒ surface in fcc metal homoepitaxial systems. Mass transport underlying these processes is dominated by periphery diffusion ͑PD͒ of adatoms along the edge of the clusters. A Mullins-type continuum model for cluster evolution incorporates anisotropy in the step edge stiffness ͑reflecting the energetics and adsorption site lattice structure in the atomistic model͒, and can also account for anisotropy in the step edge mobility ͑reflecting details of the kinetics͒. In such continuum treatments , the characteristic time ␶ eq for relaxation of clusters with linear size of order L satisfies ␶ eq ϳL 4. Deviations may generally be expected for small sizes L or low temperatures T. However, for the relaxation of dumbbell-shaped clusters ͑formed by corner-to-corner coalescence of square clusters͒, atomistic simulations for PD with no kink rounding barrier ͑␦ϭ0͒ reveal that ␶ eq ϳL 4 always applies. In contrast, atomistic simulations with a large kink rounding barrier ͑␦Ͼ0͒ reveal distinct scaling with ␶ eq ϳL 3 , for low T or small L, thus providing an effective way to test for ␦Ͼ0. For the relaxation of faceted rectangular clusters ͑formed by side-to-side coalescence of square clusters͒, atomistic simulations for PD with ␦ϭ0 reveal that ␶ eq ϳL 2 , for low T or small L. This is consistent with a recent proposal by Combe and Larralde. For large ␦Ͼ0, ␶ eq has an even weaker dependence on L. We elucidate scaling behavior and the effective activation barrier for relaxation in terms of the individual atomistic PD processes and their barriers.