Charging of metal atoms on ultrathin MgO/Mo(100) films

The chemical activity of supported metal nanoclusters is enhanced by electronic charging induced by the interaction with surface defects. We use density functional theory plane wave calculations to show that charging of metal atoms with high electron affinity like Au is possible also in the absence of defects by atom deposition on ultrathin MgO films (1 to 3 layers) grown on Mo(100). The Au 6s level falls below the Fermi level of Mo, leading to electron transfer from Mo to Au by direct tunneling through the insulating MgO film. The effect is not observed for Pd, whose 5s empty level falls just above the Fermi level of Mo, or for thicker MgO films.