Electronic properties of rutile TiO2 ultrathin films: Odd-even oscillations with the number of layers

The dependence of the surface properties of thin rutile TiO2(110) films on the number of layers in two-dimensional slab models has been investigated with first-principles methods based on density-functional theory. A pronounced oscillation of the interlayer distances, surface and adhesion energies, and electronic structure as a function of the number of layers has been found. The effect can be explained by surface-induced hybridization of Ti 3d and O 2p orbitals among the layers. This leads to reinforced interactions between the first and second layers and to weaker bonding and longer distances between the second and third layers of the films. By removing the 3d orbitals from the Ti basis set we found that TiO2 behaves almost exactly as the isostructural SnO2(110) surface. The results are of importance for the properties of epitaxial ultrathin films grown on metal substrates as well as for single crystal surfaces.