Distinct Substrate Effect on the Reversibility of the Metal-Insulator Transitions in Electrolyte-Gated VO2 Thin Films

Electrolyte gating on correlated VO2 thin films enables electrical control of the "bulk" electronic and structural phases over the electrostatic screening length. Although this unique functionality potentially provides novel electronic and optoelectronic device applications, there are intense discussions on the mechanism of the device operation both from electrostatic and electrochemical viewpoints. Here it is shown that the reversibility of the device operation strongly depends on substrates, suggesting that a governing mechanism might differ depending on substrates. Electrolyte gating on VO2 films grown on lattice-matched TiO2 substrates shows reversible gating effects, whereas that on hexagonal Al2O3 substrates become irreversible, although in both cases metallic states can be induced electrically. X-ray absorption spectroscopy measurements on irreversibly gated VO2/Al2O3 reveal permanent reduction of the valence state of vanadium upon gate-induced metallization, presumably originating from irreversible electrochemical doping under the presence of the extremely large electric field created at an electrolyte/ VO2 interface. Our findings suggest essential importance of the film quality for future fundamental researches as well as for practical device applications based on electrolyte-gated devices.