Spontaneous formation of dipolar metal nanoclusters.

The adsorption of three- and four-atom Ag and Pd clusters on the alpha-Al(2)O(3) (0001) surface is explored with density functional theory. Within each adsorbed cluster, two different cluster-surface interactions are present. We find that clusters simultaneously form both ionic bonds with surface oxygen and intermetallic bonds with surface aluminum. The simultaneous formation of disparate electronic structure motifs within a single metal nanoparticle is termed a "dipolar nanocluster". This coexistence is ascribed to a balance of geometric constraints and metal electronic structure, and its importance for nanoparticle catalysis is highlighted.