Giant reversible, facet-dependent, structural changes in a correlated-electron insulator induced by ionic liquid gating

The use of electric fields to alter the conductivity of correlated electron oxides is a powerful tool to probe their fundamental nature as well as for the possibility of developing novel electronic devices. Vanadium dioxide (VO ₂) is an archetypical correlated electron system that displays a temperature-controlled insulating to metal phase transition near room temperature. Recently, ionic liquid gating, which allows for very high electric fields, has been shown to induce a metallic state to low temperatures in the insulating phase of epitaxially grown thin films of VO ₂. Surprisingly, the entire film becomes electrically conducting. Here, we show, from in situ synchrotron X-ray diffraction and absorption experiments, that the whole film undergoes giant, structural changes on gating in which the lattice expands by up to ∼3% near room temperature, in contrast to the 10 times smaller (∼0.3%) contraction when the system is thermally metallized. Remarkably, these structural changes are fully reversible on reverse gating. Moreover, we find these structural changes and the concomitant metallization are highly dependent on the VO ₂ crystal facet, which we relate to the ease of electric-field–induced motion of oxygen ions along chains of edge-sharing VO ₆ octahedra that exist along the (rutile) c axis. Significance We report a remarkable reversible change in structure of vanadium dioxide films when gated with an ionic liquid. We show that the film expands by more than 3% in the out-of-plane direction when gated to the metallic state. This giant structural change is not only more than 10 times larger than the one at the thermally controlled insulator-to-metal transition measured in the same films, but is in the opposite direction—an expansion rather than a contraction. These results are very important to the field of ionic liquid gating, which has largely ignored the possibility that the high electric fields created on gating at the liquid–oxide interface can result in significant structural changes rather than a purely electrostatic phenomenon.