AVS 50th International Symposium
    Surface Science Monday Sessions
       Session SS3-MoM

Paper SS3-MoM9
Mobility of Nanostructures on the Surface of a Desorbing Solid: Friction at the Nanoscale

Monday, November 3, 2003, 11:00 am, Room 328

Session: Surface Diffusion and Wetting
Presenter: V.N. Antonov, University of Illinois at Urbana-Champaign
Authors: V.N. Antonov, University of Illinois at Urbana-Champaign
J.S. Palmer, University of Illinois at Urbana-Champaign
A.S. Bhatti, University of Illinois at Urbana-Champaign
J.H. Weaver, University of Illinois at Urbana-Champaign
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Physical vapor deposition of Au (or other atoms) on rare gas solids leads to spontaneous formation of clusters. The thermal desorption of the buffer causes the clusters to move and aggregate into larger structures, a process known as buffer-layer-assisted growth (BLAG) and desorption assisted coalescence. Our results demonstrate that the initial nucleation density is independent of the buffer thickness. We have studied the extent of aggregation and the size distribution of Au nanostructures as a function of the buffer composition (Xe, Kr, or Ar) and thickness. In the limit of large Au nanostructures (>20 nm), 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. A model for BLAG is proposed, based on the concept that nanostructure growth kinetics is controlled by competition between the rate of diffusion and the rate of buffer depletion. From this model, the effective activation energy for Au cluster diffusion is found to be within a few times the single atom binding energy on the surface. For small Au nanostructures (<10 nm), the diffusivity varies strongly, and even increases with average size in a limited size range. This effect was stronger on Kr than on Xe buffer layers. It is attributed to a competition between the rate of release of energy by cluster-cluster coalescence and its rate of dissipation through the buffer. These results demonstrate that coalescence-driven diffusion enhancement is an important phenomenon that needs to be considered in processes where nanostructure self-assembly is involved.