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).