Pd, Co and Co-Pd clusters on the ordered alumina film on NiAl(1Â 1Â 0): Contact angle, surface structure and composition

Pd clusters deposited at room temperature grow in two different modes. At the reflection domain boundaries the clusters grow in their thermodynamically favorable shape. The clusters do not have a single crystallographic orientation and their shape can be approximated by a truncated sphere with a high contact angle of about 110°, especially at very low coverages (below 0.05 ML). At the antiphase domain boundaries, the Pd clusters grow in (1 1 1) orientation and on some of them small (1 1 1) facets appear at their tops already at low coverages. For higher coverages of Pd, the majority of Pd clusters are rather flat with a large Pd(1 1 1) facet on top. The clusters' shape at the antiphase domain boundaries differs from the thermodynamically favorable one, due to kinetic limitations, especially at higher coverages. Annealing the Pd clusters to 300 °C leads to re-structuring of these Pd clusters. They transform into higher and more rounded clusters and a thin disordered alumina film is formed on top of the clusters. When Pd is deposited at 300 °C, about 16% of the Pd clusters have a steep slope and rounded tops. The rest of the Pd forms lower clusters, goes subsurface and is covered by a disordered alumina film. When Co and Pd are deposited sequentially, Pd covers the Co clusters forming a shell. The resulting mixed clusters are still truncated spheres with a lowered contact angle. For deposition in the reverse order (first Pd and then Co) we found that Co forms an alloy with Pd already at room temperature.