  The adsorption of Ag$_3$ and Ag$_4$ 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 silver 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
the similar bond enthalpies of Ag--Al and Ag--O bonds, and its importance for
nanoparticle catalysis is highlighted.
