Pd, Co and Co-Pd clusters on the ordered alumina film on NiAl(110): Contact angle, surface structure and composition

We have investigated the structure and morphology of Co and Pd clusters grown at room temperature on an alumina film on NiAl(110) by scanning tunneling microscopy, low energy ion scattering and Auger electron spectroscopy. We have also studied the clusters after annealing to 300 degrees C and Pd clusters deposited at 300 degrees C. Mixed Co-Pd clusters obtained by sequential deposition at room temperature were also studied. Pure Co deposited at room temperature forms a single type of clusters, most or all of them with close-packed planes parallel to the oxide surface. Their shape can be approximated by truncated spheres with a high contact angle of 115-125 degrees. These clusters are stable upon annealing up to 300 degrees C. 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 degrees, especially at very low coverages (below 0.05 ML). At the antiphase domain boundaries. the Pd clusters grow in (1 11) orientation and on some of them small (111) facets appear at their tops already at low coveraizes. For higher coverages of Pd, the majority of Pd clusters are rather flat with a large Pd(111) 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 degrees 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 degrees 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. (c) 2007 Elsevier B.V. All rights reserved.