Metallic monoclinic phase in VO$_2$ induced by electrochemical gating: in-situ Raman study

TL;DR

This study uses in-situ Raman spectroscopy to show that electrochemical gating induces a metallic state in VO$_2$ while maintaining its monoclinic structure, likely due to oxygen vacancy formation.

Contribution

It reveals that electrochemical gating induces metallicity in VO$_2$ without structural phase change, highlighting oxygen vacancies as the key mechanism.

Findings

VO$_2$ becomes metallic at 2.6 V gating while remaining monoclinic.

Raman modes indicate decreased distortion of VO$_6$ octahedra with gating.

Conductance increases are linked to oxygen vacancy diffusion.

Abstract

We report in-situ Raman scattering studies of electrochemically top gated VO$_2$ thin film to address metal-insulator transition (MIT) under gating. The room temperature monoclinic insulating phase goes to metallic state at a gate voltage of 2.6 V. However, the number of Raman modes do not change with electrolyte gating showing that the metallic phase is still monoclinic. The high frequency Raman mode A$_g$(7) near 616 cm$^{-1}$ ascribed to V-O vibration of bond length 2.06 \AA~ in VO$_6$ octahedra hardens with increasing gate voltage and the B$_g$(3) mode near 654 cm$^{-1}$ softens. This shows that the distortion of the VO$_6$ octahedra in the monoclinic phase decreases with gating. The time dependent Raman data at fixed gate voltages of 1 V (for 50 minute, showing enhancement of conductivity by a factor of 50) and 2 V (for 130 minute, showing further increase in conductivity by a factor of 5) show similar changes in high frequency Raman modes A$_g$(7) and B$_g$(3) as observed in gating. This slow change in conductance together with Raman frequency changes show that the governing mechanism for metalization is more likely to the diffusion controlled oxygen vacancy formation due to the applied electric field.