Structure of cobalt cluster films obtained by sputter deposition on alumina

We report on the structure of cobalt cluster films obtained by aggregation of metal atoms sputter-deposited on amorphous alumina. The cluster layers w ere encapsulated with a second amorphous alumina layer, also made by sputte ring in the same chamber. Electron energy loss spectroscopy (EELS) indicate s that encapsulation with sputtered alumina keeps the clusters free from co balt oxide. Transmission electron microscopy (TEM) exhibits relatively narr ow distributions of the cluster sizes and of the inter-cluster distances, w hich results in a noticeable local order. Size distributions appear quasi-G aussian, but TEM misses an important number of small particles. Extended X ray absorption fine structure (EXAFS) data shows that the actual average si zes are smaller, and Monte Carlo simulations suggest that the actual distri butions could be bimodal, with a secondary peak in the small-size range.