First-principles density-functional calculations are used to study metal adsorption (Li, K, Y, Nb, Ru, Pd, Pt, Cu, Ag, Au, and Al at $\frac{1}{3}$–4 monolayer coverages) atop 5 Å ${\mathrm{Al}}_2{\mathrm{O}}_3$ films on Al(111). The oxide-metal bond is ionic at low coverages but, with interesting exceptions, caused by polarization at high coverages where the overlayer is metallic. Binding trends are explained in terms of simple concepts. Increasing overlayer thickness can cause the adsorbate-oxide interface structure to change, and while some metals wet, most do not.

• Received 16 October 1998

DOI:https://doi.org/10.1103/PhysRevLett.82.4050