Bonding of transition metal atoms on the Al(100) surface from density functional calculations I. Anomalous work function change as relativistic effect

For the series Ni, Pd, Pt, Cu, Ag, and Au, we have studied the interaction of transition metal (TM) atoms with the (100) surface of Al by means of cluster models and density functional calculations. Adsorption of K has also been considered for comparison. This study aims at analyzing the inter-metallic bond. In particular, we have correlated the adsorbate-induced changes in the surface dipole with the measured change in the work function of the bimetallic system. We find that, differently from K which is characterized by a charge transfer interaction, TM atoms form largely covalent bonds accompanied by only small charge displacements which, in turn, induce small surface dipoles. The sign of these dipoles, hence the expected change in the work function, is different for Pt and Au; this "anomalous" behavior of Pt and Au relative to the other members of the TM series is clearly due to intra-atomic electronic relativistic effects.