Tailoring the Visible Spectral Response of Phase-Change VO ₂ Sol–Gel Coatings with Large Resistivity Contrast Using Nanoimprinted Metasurfaces

VO₂ phase change material (PCM) was used to elaborate optical thin films and metasurfaces from sol-gel processing with a performant switching ability in the visible spectra. The initial formulation was developed from the VO(acac)₂ precursor, reacting in a typical hydroalcoholic medium, and was deposited through liquid deposition before treatment at high temperature under vacuum. The crystalline switch between the monoclinic (M1) and tetragonal (rutile) has been well put in evidence around 53 °C, confirming the efficient metal-insulator transition, characteristics of the VO₂ PCM. We also highlight the major role of crystallite preferential orientation (texture) in the coating to achieve an efficient optical switching. This effect was attributed to thermodynamic and kinetic aspects governing the nucleation growth and intrinsic high birefringence of the M1 phase. Under optimal conditions, the VO₂ PCM film with preferential M1(011) orientation exhibited an electrical resistance switching over 2 orders of magnitude and an optical contrast above 1 refractive index unit in the near-infrared region. This optimal material was then deposited as a conformal layer above a dielectric SiO₂ metasurface composed of submicronic pillars organized into periodical square arrays between 400 and 880 nm pitches. Upon thermal modulation, the composite dielectric/PCM metasurfaces exhibit an efficient reflective spectral switching (Δλ_r up to 70 nm) located in the visible. Finally, the optimal material optical contrast as well as high stability and reproducibility upon switching were investigated through in situ simultaneous X-ray diffraction and sheet resistance analysis. The established synthesis-structure-property relationships of the proposed method provide a foundation for developing advanced optoelectronic devices and dynamically tunable photonic systems in the visible, compatible with a rapid and simple sol-gel-based synthetic approach.