Size and shape evolution of faceted bicrystal nanoparticles of gold on sapphire

We produced an array of Au nanoparticles on the basal plane of sapphire via solid-state dewetting of a thin film, and followed the size and shape evolution of individual particles during a 950 °C anneal in air. The particles exhibited {1 0 0} and {1 1 1} facets and shapes that tend to be far from those predicted based upon equilibrium considerations. Most of the single-crystal particles exhibited remarkable size and shape stability up to the longest annealing time, 65 h. The bicrystal particles rapidly transformed into single crystals after the shortest annealing time, 1 h. This transformation was accompanied by an apparent rotation of one of the grains and its alignment along its immobile counterpart. We interpreted both phenomena in terms of very slow mass transport along the singular {1 1 1} and {1 0 0} facets. In the case of bicrystal particles, the fast migration and escape of the boundary from the particle changed the crystallography of the facets in the grain swept by the boundary from a crystallographically singular orientation to a non-special orientation. Such surfaces exhibit high diffusivity and, hence, rapid shape evolution via curvature-driven surface diffusion. A quantitative model of this rotation was proposed and showed to confirm our qualitative description. Based on these findings, we conclude that grain-boundary migration, rather than surface diffusivity, controls the kinetics of mid- and late-term agglomeration of thin polycrystalline films of Au.