TiO2—a prototypical memristive material

Abstract

Redox-based memristive switching has been observed in many binary transition metaloxides and related compounds. Since, on the one hand, many recent reports utilizeTiO2 for their studies of the memristive phenomenon and, on the other hand, there isa long history of the electronic structure and the crystallographic structure ofTiO2 under the impact of reduction and oxidation processes, we selected this materialas a prototypical material to provide deeper insight into the mechanismsbehind memristive switching. In part I, we briefly outline the results of thehistorical and recent studies of electroforming and resistive switching ofTiO2-based cells. We describe the (tiny) stoichiometrical range forTiO2 − x as a homogeneous compound, the aggregation of point defects (oxygen vacancies) intoextended defects, and the formation of the various Magnéli phases. Furthermore, we discussthe driving forces for these solid-state reactions from the thermodynamical point of view.In part II, we provide new experimental details about the hierarchical transformation ofTiO2 single crystals into Magnéli phases, and vice versa, under the influence of chemical,electrical and thermal gradients, on the basis of the macroscopic and nanoscopicmeasurements. Those include thermogravimetry, high-temperature x-ray diffraction (XRD),high-temperature conductivity measurements, as well as low-energy electron diffraction(LEED), x-ray photoelectron spectroscopy (XPS), and LC-AFM (atomic force microscopeequipped with a conducting tip) studies. Conclusions are drawn concerning the relevantparameters that need to be controlled in order to tailor the memristive properties.