Mechanical Control of Metal-Insulator Transition in Epitaxial Vanadium Dioxides

Strongly correlated vanadium dioxide (VO$_{2})$ is one of the most promising materials exhibiting a temperature-driven metal-insulator transition (MIT) in the vicinity of room temperature. Thus, establishing control over the MIT of VO$_{2}$ by means of external stimuli, such as strain, temperature, and electric field, is an important task to realize VO$_{2}$-based advanced electronic devices. Herein, we explore the mechanical control of the conductivity and hence the MIT in epitaxial VO$_{2}$ thin films using electronic transport and scanning probe microscopy approaches. We found that the mechanical pressure could modulate VO$_{2}$ film's conductivity. This result is attributed to the piezochemical effect as oxygen stoichiometry is strongly influenced by the applied mechanical pressure. This piezochemical coupling is further realized by observed shifts in electrochemical potential of oxygen vacancies as a function of applied pressure and electric field. We relate our results to calculations of oxygen vacancy formation energy as function of pressure. Such strain-induced changes in electrochemical potential and conductance of VO$_{2}$ films indicate the local pressure-induced MIT, and can be explained based on the flexoelectric effect and/or Vegard strain effect. Our findings provide the basis for piezochemical control of MIT in VO$_{2}$ thin films.~