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 an Au film typically covering areas of size 25$\times$70 nm$^2$. The Au film is determined to be probably 4 to 8 monolayers but at most 7 nm thick. Evidence is found that a number of clusters consisting of less then 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 (GGA) while the Au-Au interaction was modeled with effective many body Gupta potentials. The film formation can be understood as diffusion and fusion of clusters of intermediate ($50< N < 300$ atoms) size. Larger clusters are more stable at room temperature and remain adsorbed on the Au film.