Clusters obtained by sputter deposition of cobalt atoms on alumina

We report on aggregation of Co on a smooth amorphous alumina surface, during sputter deposition of Co atoms at room temperature. The atomic structure of the Co grains appears to be size dependent! hep in the larger bean-shaped clusters (greater than 3nm), and often fee in the smaller, almost spherical clusters. The deposits show relatively narrow distributions of the cluster sizes and of the intercluster distances, which results in a noticeable local order and explains how charge blocking (the so-called Coulomb blockade) could be observed in large sets of such particles by Schelp et al. (1997, Phys. Rev. B, 56, R5747). Electrical percolation occurs at a nominal thickness of about 2 nm.