Island formation and dynamics of gold clusters on amorphous carbon films

Samples of Au clusters deposited by laser ablation on an amorphous-carbon substrate are investigated. After a few months' storage at room temperature, the initially statistically distributed clusters are found to be collected in agglomerates consisting of larger clusters embedded in a Au film typically covering areas of size $25\ifmmode\times\else\texttimes\fi{}70\phantom{\rule{0.3em}{0ex}}{\mathrm{nm}}^{2}$. The Au film is determined to be probably 4 to 8 monolayers but at most $7\phantom{\rule{0.3em}{0ex}}\mathrm{nm}$ thick. Evidence is found that a number of clusters consisting of less than 50 atoms are pinned at intrusions of the substrate. These results were derived using high-resolution transmission electron microscopy and off-axis holography measurements to characterize the agglomerates as well as the substrate. Monte Carlo simulations were performed to model the film formation process. To this end, the substrate-Au interaction was determined using density functional calculations, while the $\mathrm{Au}\char21{}\mathrm{Au}$ interaction was modeled with effective many-body Gupta potentials. The film formation can be understood as diffusion and fusion of clusters of intermediate ($50lNl300$ atoms) size. Larger clusters are more stable at room temperature and remain adsorbed on the Au film.