Nanoparticle diffusion on desorbing solids: The role of elementary excitations in buffer-layer-assisted growth

Abstract

Physical vapor deposition onto rare gas buffer layers leads to the spontaneous formation of clusters. During the thermal desorption of the buffer, these clusters diffuse and aggregate into larger structures, a process known as buffer-layer-assisted growth and desorption assisted coalescence. We studied the effect of buffer thickness and the rate of buffer desorption on the extent of this aggregation for Ag, Au, Cu, Pd, Co, and Ni particles on a solid Xe surface. On the basis of these experiments, results from Monte Carlo simulations and the existing theoretical models for cluster--cluster aggregation, we report for the first time the Arrhenius parameters for nanoparticle slip-diffusion. The effective activation energies range from 0.12 for small Ag clusters (few hundred atoms) to $0.60\phantom{\rule{0.3em}{0ex}}\text{eV}$ for ramified Ni islands (millions of atoms), and the giant pre-exponential factors were found to differ by many orders of magnitude. Significantly, the pre-exponential factors follow a Meyer--Neldel-type dependence on the corresponding effective activation energy, with a characteristic Meyer--Neldel energy of $6.9\phantom{\rule{0.3em}{0ex}}\text{meV}$. This energy is associated with the phononic excitations in solid Xe that are responsible for nanostructure mobility. This dependence should be a characteristic feature of nanoparticle diffusion.