Metallic monoclinic phase in VO₂ induced by electrochemical gating: In situ Raman study

We report in situ Raman scattering studies of electrochemically top gated VO₂ 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 Å in VO₆ octahedra hardens with increasing gate voltage and the B_g(3) mode near 654 cm^-1 softens. This shows that the distortion of the VO₆ octahedra in the monoclinic phase decreases with gating. The time-dependent Raman data at fixed gate voltages of 1 V (for 50 minutes, showing enhancement of conductivity by a factor of 50) and 2 V (for 130 minutes, 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 due to the diffusion-controlled oxygen vacancy formation due to the applied electric field.