Metallic monoclinic phase in VO

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 Ag(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 Bg(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 Ag(7) and Bg(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