Coarsening of mass-selected Au clusters on amorphous carbon at room temperature

Abstract

Low-energy cluster beam deposition was used to deposit mass-selected Au n clusters ( n = 4, 6, 13 and 20) on amorphous carbon (a-C) substrates. The resulting samples were stored at room temperature under ambient conditions for time periods up to 32 months to analyze the coarsening behaviour of the clusters. Cluster-size distributions were measured in regular time intervals by transmission electron microscopy (TEM). The TEM experiments show a significant increase of the average cluster size with time analogous to classical surface Ostwald ripening (OR). The coarsening of Au clusters can be well described by steady-state diffusion-limited kinetics. The derived surface mass-transport diffusion coefficients at room temperature range between 1.1 and 3.8·10 −25 m 2 s −1 for our samples. A detailed analysis of D s ′ values suggests that, the rate of the surface OR for mass-selected Au n clusters increases with the cluster size in the sequence: Au 4 ≈ Au 6 < Au 13 < Au 20 for the investigated range of Au clusters. Given that the initial, on-surface cluster-size distributions are nominally monodisperse, classical OR with cluster coarsening based only on the Gibbs–Thomson effect cannot explain the observed coarsening. The activation of the coarsening process is rationalized by initial variations of the cluster sizes due to the deposition process itself and/or the interaction of the clusters with the substrate. Moreover, the presence of initial deposited Au clusters as different isomers with slightly different chemical potential on the substrate, may also initiate the coarsening by surface OR. Furthermore, we find that the coarsening is most pronounced for the paucidispersed sample with Au m (10 ⩽ m ⩽ 20) clusters. A possible explanation of this behaviour is the presence of an initial distribution of different cluster sizes directly after deposition.