Density functional theory study of TiO(2)/Ag interfaces and their role in memristor devices

The nature of Ag(111)/TiO${}_{2}$ rutile and anatase interfaces, of interest for the design of memristors, has been studied by means of density functional theory calculations using various computational approaches. We have considered interfaces where the lattice mismatch of the oxide crystalline phase and the metal electrode does not result in excessive strain. The bonding at the interface is very weak, and the charge transfer is negligible for stoichiometric oxides. The formation of O vacancies has a lower cost at the interface with Ag than on the bare titiania surface and results in stronger adhesion between the Ag electrode and the reduced TiO${}_{2\ensuremath{-}x}$ oxide. The diffusion of Ag and O atoms or ions across the interface is a thermodynamically unfavorable process which can occur only at high temperatures or under the effect of an external electric field. Once Ag atoms are incorporated in the bulk of TiO${}_{2}$ they can be stabilized in an interstitial site (more favorable) or a position substitutional to Ti. In both cases Ag is ionized and transfers the valence electron to the host crystal with formation of Ti${}^{3+}$ states. The Ag atoms remain positively charged, even when extended Ag chains are formed (nanofilaments). For an Ag filament inside TiO${}_{2}$ to exhibit conductive behavior, a higher density of Ag atoms is required, but this is hardly possible in the regular bulk crystalline lattice of TiO${}_{2}$ without inducing a structural breakdown.