Nanostructure diffusion and aggregation on desorbing rare-gas solids: Slip on an incommensurate lattice

Physical vapor deposition of a wide range of materials on rare-gas solids leads to spontaneous cluster formation. Desorption of the rare-gas buffer causes the clusters to aggregate, a process known as buffer-layer-assisted growth. We have studied the extent of aggregation and the size distribution of Au nanostructures as a function of the buffer composition (Xe, Kr, and Ar) and thickness, using transmission electron microscopy to image them after buffer desorption and delivery to amorphous carbon substrates. For small compact Au nanostructures (less than similar to5 nm mean radius, less than or equal to3x10(4) atoms), the diffusivity varies strongly with size and even increases with average size in a limited range. This enhanced diffusion phenomenon is attributed to self-heating during coalescence. It is most important for small particles and is more evident on Kr than on Xe because of weaker interface coupling. In the limit of large ramified Au nanostructures (exceeding similar to10 nm mean radius, greater than or equal to2x10(5) atoms), the diffusivity scales as the inverse of the contact area, in agreement with molecular dynamics simulations of fast slip diffusion of nanocrystals on incommensurate surfaces. Motion is driven by phonons of the cluster and substrate, and is controlled by friction between a cluster facet and the buffer surface. A simple model is proposed that explains the observed exponential dependence of cluster size on buffer thickness. In this model, the growth kinetics are controlled by competition between the rate of cluster diffusion and the rate of buffer depletion.