The nature of Ag(111)/TiO2 rutile and anatase interfaces, of interest for the design of memristors, was studied by means of density functional theory calculations using various computational approaches. Interfaces were considered where the lattice mismatch of the oxide crystalline phase and the metal electrode did not result in excessive strain. The bonding at the interface was very weak, and the charge transfer was negligible for stoichiometric oxides. The formation of O vacancies had a lower cost at the interface with Ag than on the bare titania surface and results in stronger adhesion between the Ag electrode and the reduced TiO2-x oxide. The diffusion of Ag and O atoms or ions across the interface was a thermodynamically unfavourable process which could occur only at high temperatures or under the effect of an external electric field. Once Ag atoms were incorporated into the bulk of TiO2 they could be stabilized in an interstitial site (more favorable) or a position substitutional to Ti. In both cases Ag was ionized and transfers the valence electron to the host crystal with formation of Ti3+ states. The Ag atoms remain positively charged, even when extended Ag chains were formed (nanofilaments). For an Ag filament inside TiO2 to exhibit conductive behavior, a higher density of Ag atoms was required, but this was hardly possible in the regular bulk crystalline lattice of TiO2 without inducing a structural breakdown.

Density Functional Theory Study of TiO2/Ag Interfaces and their Role in Memristor Devices. Prada, S., Rosa, M., Giordano, L., Di Valentin, C., Pacchioni, G.: Physical Review B, 2011, 83[24], 245314