Epitaxial growth of TiO2 films with the rutile(110) structure on Ag(100)

Ultrathin films of TiO2 were grown on Ag(100) by evaporation of titanium in the presence of O2 at a pressure in the 10-4 Pa range and annealing at 770 K. The composition of the deposited films was monitored by XPS and LEIS. The morphology at the nanometric scale of the TiO2 films and their crystallographic structure were investigated by means of STM, LEED and XPD. Above the monolayer coverage (at which the oxide film has a lepidocrocite-like structure), STM images show the formation of multilayer islands with a distribution of heights. XPD results indicate that these oxide islands have the rutile (110) structure and are epitaxially oriented with the sides of the oxide unit cell parallel to those of the substrate unit cell. The results of the DFT calculations justify the 3D growth of rutile (110) on Ag(100). The calculated strain energy required to match the metal substrate can explain the incommensurate growth of the overlayer in the direction of the long side of the oxide unit cell. The results of the calculations indicate that a commensurate growth of rutile (110) may be possible along the short side of the oxide unit cell, taking into account the relatively small strain energy to fit the lattice parameter of the substrate. The DFT calculations predict a considerable increase of the work function upon deposition of titania films on Ag(100), which can be attributed to a charge transfer from the metal to the 3d Ti empty states.