Vibrational and optical properties of vanadium dioxide
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Vanadium dioxide, VO2, is an interesting oxide which undergoes a metal-to-insulator phase transition (MIT) at T_MIT=68 ℃. When MIT happens, the material transforms from rutile structure into monoclinic lattice at 〖T<T〗_MIT. This doubles the size of unit cell. MIT brings significant changes in electrical and optical properties of VO2. Electrical conductivity increases 4-5 orders of magnitude when metallic phase is achieved. In this work we studied optical, electrical and structural properties of VO2 using Raman spectroscopy and spectroscopic ellipsometry. Raman study shows diverse behavior of phonon in VO2. This diversity arises from the native doping, substrate effect and finite-size effect. Increasing native free carrier concentration is found to decrease phase transition temperature with broader insulator-to-metal transition region. Blue shift of V—O related vibration is found and helped us to introduce temperature-carrier concentration phase-diagram for this material. Spectroscopic ellipsometry is applied to extract the optical properties of VO2 grown on sapphire with different orientations. Tensile strain in CR axis of VO2 is found to increase TMIT and energy gap of insulator VO2. Raman spectroscopy and atomic force microscopy are used to study the finite-size effect in VO2. Phase transition temperature is found to scale inversely with grain size in the film. A characteristic length of 300 nm is found and is associated with the electron correlation length just below the metallic phase.