Palladium Nanocrystals onAl2O3: Structure and Adhesion Energy

We have investigated the structure and morphology of nanosized palladium clusters supported by a thin ${\mathrm{Al}}_2{\mathrm{O}}_3$ film on NiAl(110) using scanning tunneling microscopy. Well-ordered clusters with a diameter above $\approx 40Å$ were imaged with atomic resolution, allowing a crystallographic identification of the cluster facets. A new method to obtain quantitative information on the work of adhesion (adhesion energy) of metal clusters deposited on oxides is introduced. For Pd on ${\mathrm{Al}}_2{\mathrm{O}}_3$ we obtain a value of $W_{\mathrm{adh}}=2.8\mathrm{}\pm 0.2\mathrm{J}/{\mathrm{m}}^2$. This result is at variance with values recently derived on the basis of ab initio density-functional theory.