Modulation of Metal–Insulator Transition in VO2 by Electrolyte Gating‐Induced Protonation

In this study, we investigated the effect of electrolyte gating on epitaxial VO 2 fi lms grown on MgF 2 substrates, unlike the TiO 2 and Al 2 O 3 substrates that were used in previous studies. [ 11,13,19–21 ] Like TiO 2 , MgF 2 has a rutile structure, but the lattice mismatch between VO 2 and MgF 2 is larger than that between VO 2 and TiO 2 . As a result, epitaxial VO 2 fi lms on MgF 2 substrates consist of grains with an average diameter of 50 nm, [ 22 ] so that there are many grain boundaries in the fi lms. We found that electrolyte gating induces proton migration into the VO 2 fi lms, whereas it induces no apparent change in oxygen content. The electrolyte gating-induced protonation caused reversible and nonvolatile changes in the electronic states of VO 2 . In addition, electrolyte gating of W-substituted VO 2 fi lms revealed that protonation induces a combined effect, involving of carrier doping and lattice deformation, on the suppression of the MIT. We deposited epitaxial VO 2 thin fi lms onto MgF 2 (001) substrates by pulsed laser deposition [ 23 ] and we fabricated VO 2 based EDL transistors (EDLTs) by standard optical lithography techniques. The fi lm thickness was set at 40 nm. Figure 1 a shows the temperature dependence of the sheet resistance ( R S ) at various values of the gate voltage ( V G ) for an EDLT. Gating was carried out at 360 K and V G was increased monotonically in 0.1 V steps. The pristine state ( V G = 0 V) exhibited a MIT at around 320 K, accompanied by a thermal hysteresis, characteristic of a fi rst-order phase transition. The value of R S in the low-temperature insulating phase gradually decreased with increasing V G , and an almost temperature-independent R S was observed at a V G of 0.8 V. We refer to this state as the gated metallic state. Note that the R S above the MIT temperature ( T MI ) increased with increasing V G . These behaviors are quite similar to those previously reported for EDLTs consisting of VO 2 fi lms on TiO 2 substrates. [ 19 ]