Clusters obtained by sputter deposition of cobalt atoms on alumina

We report on aggregation of Co on a smooth amorphous alumina surface, durin g sputter deposition of Co atoms at room temperature. The atomic structure of the Co grains appears to be size dependent: hcp in the larger bean-shape d clusters (greater than 3 nm), and often fee in the smaller, almost spheri cal clusters. The deposits show relatively narrow distributions of the clus ter sizes and of the intercluster distances, which results in a noticeable local order and explains how charge blocking (the so-called Coulomb blockad e) 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 thic kness of about 2 nm.