A multi-modal study on the metal-to-insulator transition and optoelectronic properties of laser-textured W-VO2 thin films

Abstract

Vanadium dioxide (VO2) has emerged as one of the most attractive candidates for thermochromic coatings in smart windows, due to its ability to reversibly regulate solar infrared transmission through a metal-to-insulator transition (MIT). W-doped VO2 (M1) thin films were synthesized by a polymer-assisted sol-gel route and deposited onto borosilicate glass by dip-coating. Optimization of the thermal treatment and doping with 1 wt% of W6+ yielded a pronounced reduction of the phase transition temperature from 68 °C to 37.5 °C, together with a well-defined thermochromic response in the UV–Vis–NIR range. Moreover, a comprehensive optical characterization was performed by spectrophotometry and spectroscopic ellipsometry, combined with the study of nanomechanical, textural and electrical properties of the thin films through diverse Atomic Force Microscopy (AFM) modes. The combination of these techniques has revealed distinct evolutions of the optical and electronic properties across the MIT, thereby providing further insight on its nature. Furthermore, novel line-based ultrashort-pulse laser texturing patterns were developed to selectively remove portions of the film while allowing more control over the laser ablated area. Results indicate that these laser treatments can increase luminous transmittance in thin films by 30 % while preserving their solar and IR modulation, thus enhancing the applicability of W-VO2 thin films in smart window technologies. The incorporation of a textured sample into a proof of concept thermochromic system produced a 50 % reduction in system heating after IR radiation, compared with an equivalent plain-glass system, thus demonstrating the practical thermal-management benefits of these W-doped VO2 textured samples.