Effects of thickness of sandwiched layer on the structure and optical properties of thermochromic multi-layered thin films

Abstract

Vanadium oxide (VO2) is the archetype parent material for thermochromic smart window and smart insulation for thermal control in buildings. VO2 shows a metal-insulator transition (MIT) at a critical temperature ( cT ) ~68 °C accompanied by greater changes in the crystal structure, the optical transparency and electrical properties in several orders of magnitudes. The characteristic phase transition temperature of VO2 can be tailored to room temperature by modifying the synthesis growth through doping, substitution and multi-layer deposition. VO2 thin films were synthesized from the multi-layered thin films with excellent modulation and optical switching. VO2 thin films were deposited by electron beam evaporator and further analysed by Auger electron spectroscopy (AES), secondary ion mass spectroscopy (SIMS), XRay diffraction (XRD), Raman Spectroscopy, Atomic force microscopy (AFM), scanning electron microscopy (SEM), and UV-Vis-NIR spectroscopy. SEM measurements showed the existence of nanostructured VO2 films with numerous irregular shapes and corresponded with AFM morphology, with surface average roughness between 10 and 40 nm. XRD and AES confirmed the prevalent form of polycrystalline VO2 (M) thin films. Optical transmittance measurements showed the phase transition temperature ( c T ) between 30 and 50 oC, which reveals a reduction of the bulk c T of VO2. These multi-layered thin film results demonstrated a promising approach to engineering VO2 with good structural and optical performance for potential integration in smart window technology based on their achieved transmittance-modulation.