Evolution of Insulator-Metal Phase Transitions in Epitaxial Tungsten Oxide Films during Electrolyte-Gating

TL;DR

This study investigates how electrostatic and electrochemical gating processes induce phase transitions in epitaxial tungsten oxide films, revealing distinct mechanisms and structural changes during the insulator-metal transition.

Contribution

It provides a direct comparison of electrostatic versus electrochemical gating effects on phase transitions in WO₃ films using in situ x-ray diffraction and resistance measurements.

Findings

Electrostatic gating with ionic liquid induces charge accumulation and metallic state.

Electrochemical intercalation with polymer electrolyte causes uniform ion insertion and phase change.

Different gating methods lead to distinct lattice deformation behaviors and transition dynamics.

Abstract

An interface between an oxide and an electrolyte gives rise to various processes as exemplified by electrostatic charge accumulation/depletion and electrochemical reactions such as intercalation/decalation under electric field. Here we directly compare typical device operations of those in electric double layer transistor geometry by adopting ${A}$-site vacant perovskite WO$_3$ epitaxial thin films as a channel material and two different electrolytes as gating agent. $\textit{In situ}$ measurements of x-ray diffraction and channel resistance performed during the gating revealed that in both the cases WO$_3$ thin film reaches a new metallic state through multiple phase transitions, accompanied by the change in out-of-plane lattice constant. Electrons are electrostatically accumulated from the interface side with an ionic liquid, while alkaline metal ions are more uniformly intercalated into the film with a polymer electrolyte. We systematically demonstrate this difference in the electrostatic and electrochemical processes, by comparing doped carrier density, lattice deformation behavior, and time constant of the phase transitions.