Depth-dependent giant lattice expansion of up to 5% in ionic liquid-gated 90 nm thick VO_2 (001 )/Al_2O_3 (1010) films

摘要

We have previously shown that ionic liquid gating suppresses the metal to insulator transition (MIT) from near room temperature to low temperatures in epitaxial VO_2 films grown on TiO_2(001) and (101), and Al_2O_3 (1010), for thicknesses up to ～40nm. We also showed that for 10nm VO_2 films grown on TiO_2 (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 VO_2 films, grown on Al_2O_3 (1010), 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.