Structural phase control and thermochromic modulation of VO2 thin films by post thermal annealing

Abstract

Monoclinic M1 to rutile structural phase transition (SPT) in thermochromic VO2 is accompanied by the insulator to metal transition and transition from an infrared (IR) transparent to IR opaque phase. Fine control over the phase stabilization and the functional properties without additional doping can offer the ability to tailor VO2 thin films for their respective applications. In this work, post deposition thermal annealing was used to control the phase stabilization and the modulation in thermochromic performance of polycrystalline VO2 thin films. Monoclinic M1 to rutile SPT in VO2 thin films was tracked by in-situ temperature dependent synchrotron X-ray diffraction measurements and an increase in the SPT temperature was observed with increasing annealing temperature. Intermediate monoclinic M2 phase emerges near the SPT temperature at higher annealing temperature. Temperature dependent X-ray absorption spectroscopy (XAS) measurements revealed the higher electron correlation effect among the V 3d parallel to states across the phase transition in VO2 thin film having the intermediate phase. Spectral weight of V 3d(parallel to) states was found to increase with the annealing temperature. Lower annealing temperature drives the IR switching temperature towards room temperature while the higher annealing temperature improves the IR switching performance of VO2 thin films.