Structural Investigation of Palladium Clusters on γ-AlO₃(111)/NiAl(110) with Transmission Electron Microscopy

For Pd clusters on γ-Al2O3 epitaxially grown on NiAl(110) the lattice constant has been determined as a function of the cluster size by evaluating patterns induced by double diffraction involving the NiAl substrate and the cluster lattice. Similar to results reported previously for platinum and tantalum clusters on the same substrate (Surf. Sci. 1997, 391, 27; Surf. Sci. 1998, 413, 192), we observe a reduction of the distances within the lattice with decreasing cluster size. The highest observed reduction is 5% for clusters with a diameter of about 12 Å. Within the limits of the experimental errors the reduction of the lattice distances is isotropic. Although the samples have been exposed to air prior to investigation, the clusters are only weakly oxidized as concluded from XPS data.