Growth of Au clusters on amorphous Al₂O₃: Are small clusters more mobile than atoms?

We study the growth by sputtering of Au clusters on amorphous Al₂O₃ using transmission electron microscopy (TEM). The weak decrease of the cluster density with temperature is a clear sign that the defects of the substrate strongly influence the growth. We observe two phenomena which cannot be explained in a classical model of nucleation on defects: (i) there is some missing material in TEM micrographs that can re-appear after annealing at room temperature, and (ii) there is a delay in the nucleation of clusters. Moreover, in the case of clusters visible by TEM, we show by comparing the observed growth with kinetic Monte-Carlo simulations that clusters detach from defects above a critical size. We propose that a similar behavior explains the blockade and re-appearance of the invisible material, and the delay in nucleation: a large density of clusters nucleate on traps that release them at a size below TEM resolution (≃1 nm in these experiments).