Ab initiostructural predictions for ultrathin aluminum oxide films on metallic substrates

Abstract

On several metallic substrates, first-principles density-functional calculations of ${\mathrm{Al}}_2{\mathrm{O}}_3$ 5–7 Å films predict a structure, stabilized by interfacial electrostatics, which has no bulk counterpart. In two and three O-layer films, Al ions prefer distorted tetrahedral sites, over the normal octahedral sites. The film/substrate interface is found to consist of strongly chemisorbed oxygen and is the determining factor in geometry; these oxygens are only weakly perturbed by the presence of ${\mathrm{Al}}_2{\mathrm{O}}_3$ overlayers. In an experimentally relevant film (two O layers, as on NiAl), Al ions are nearly coplanar with surface O ions; thus observing “oxygen termination” does not indicate a polar surface. As a test case, we find Pt overlayers bind similarly as to $\alpha -{\mathrm{Al}}_2{\mathrm{O}}_3,$ suggesting these films may indeed be useful models for thicker materials.