Depth-dependent giant lattice expansion of up to 5% in ionic liquid-gated 90 nm thick VO₂ (001)/Al₂O₃ (10 1 ¯ 0) films

Abstract

We have previously shown that ionic liquid gating suppresses the metal to insulator transition (MIT) from near room temperature to low temperatures in epitaxial VO2 films grown on TiO2(001) and (101), and Al2O3 (10 1 ¯ 0), for thicknesses up to ∼40 nm. We also showed that for 10 nm VO2 films grown on TiO2 (001), the suppression of the MIT is preceded by a structural phase transition in which the films expand nearly uniformly out-of-plane by as much as 3%. Here, we report that the MIT of much thicker VO2 films, grown on Al2O3 (10 1 ¯ 0), even as thick as 90 nm, is suppressed throughout the thickness of the film by ionic liquid gating, and that these films also show a significant lattice expansion. This lattice expansion is non-uniform with a near surface region that is expanded by up to 5%, with the bulk of the film exhibiting a depth-dependent expansion that decreases from the top to the bottom of the film.