Multiscale Investigation of Oxygen Vacancies in TiO2 Anatase and Their Role in Memristor’s Behavior

The structure, energetics, and transport properties of TiO2 anatase with different densities of oxygen vacancies are studied by computer simulations using a variety of techniques, ranging from first-principles to Monte Carlo methods, to span different time scales. This work is motivated by the recent development of memristive electronic devices, usually made of metal oxides in which arrays of defects control the resistance switching mechanism. Anatase, in particular, emerged as one of the most promising candidates for memristor design. However, the microscopic behavior of these multivacancy systems is not yet entirely understood. In this regard, electronic and transport properties of TiO2 anatase containing neutral and charged oxygen vacancies are investigated within density functional theory (DFT) by adding a Hubbard-like term to the generalized gradient approximation of the electron density (GGA+U). Calculated observables are the formation energy of oxygen defects, the cohesion energy of multivacancy systems, and the energy profiles of oxygen diffusion pathways, computed through the nudged-elastic band (NEB) approach. Furthermore, a kinetic Monte Carlo model (KMC) of the conductive channel formation in bulk anatase, based on the corresponding diffusion rates, is discussed. Finally, to demonstrate the relation between energetically stable structures and the conductive phase of memristors, we study electron transport within a tight-binding approximation to DFT, using the nonequilibrium Green’s function (NEGF) formalism.