Tuning the work function of ultrathin oxide films on metals by adsorption of alkali atoms

We report a theoretical investigation of the adsorption of alkali metal atoms deposited on ultrathin oxide films. The properties of Li, Na, and K atoms adsorbed on SiO2/Mo(112) and of K on MgO/Ag(100) and TiO2/Pt(111) have been analyzed with particular attention to the induced changes in the work function of the system, Phi. On the nonreducible SiO2 and MgO oxide films there is a net transfer of the outer ns electron of the alkali atom to the metal substrate conduction band; the resulting surface dipole substantially lowers Phi. The change in Phi depends (a) on the adsorption site (above the oxide film or at the interface) and (b) on the alkali metal coverage. Deposition of K on reducible TiO2 oxide films results in adsorbed K+ ions and in the formation of Ti3+ ions. No charge transfer to the metal substrate is observed but also in this case the surface dipole resulting from the K-TiO2 charge transfer has the effect to considerably reduce the work function of the system. (C) 2008 American Institute of Physics.