Nanoscale Control of Oxygen Defects and Metal-Insulator Transition in Epitaxial Vanadium Dioxides.

Strongly correlated vanadium dioxide (VO2) is one of the most promising materials that exhibits a temperature-driven, metal-insulator transition (MIT) near room temperature. The ability to manipulate the MIT at nanoscale offers both insight into understanding the energetics of phase transition and a promising potential for nanoelectronic devices. In this work, we study nanoscale electrochemical modifications of the MIT in epitaxial VO2 thin films using a combined approach with scanning probe microscopy (SPM) and theoretical calculations. We find that applying electric voltages of different polarity through an SPM tip locally changes the contact potential difference and conductivity on the surface of VO2 by modulating the oxygen stoichiometry. We observed nearly 2 orders of magnitude change in resistance between positive and negative biased-tip written areas of the film, demonstrating the electric field modulated MIT behavior at the nanoscale. Density functional theory calculations, benchmarked against more accurate many-body quantum Monte Carlo calculations, provide information on the formation energetics of oxygen defects that can be further manipulated by strain. This study highlights the crucial role of oxygen vacancies in controlling the MIT in epitaxial VO2 thin films, useful for developing advanced electronic and iontronic devices.