Ultrafast Laser Crystallization Dynamics of Thermochromic VO2 Thin Films by Simple Moisture-Assisted Sol-Gel

Vanadium dioxide (VO2) is a versatile thermochromic material exploited for multiple applications, including smart windows, gas sensing, radiative cooling, and metamaterials. Traditional methods for crystallizing solution-based VO2 thin films are usually challenging, requiring temperatures above 400 °C and strict process control to avoid unwanted oxides. Here, we adopt a synergic approach exploiting the versatility of sol-gel reactions and the ultrafast processing of pulsed laser annealing (PLA). The latter selectively heats the film with a local temperature increase, without involving the substrate over which the film is deposited. The resulting process is simple, green, and straightforward, avoiding carcinogenic vanadium precursors and not requiring a strict control of the processing atmospheres. We have found that similar values of thermochromic efficiency can be obtained within a broad working window, performing every step in air. Taking advantage of environmental moisture, the drying temperature before PLA is reduced to 100 °C, enabling a process extension to polymer substrates. VO2 (M1) crystallization is reached within only a few seconds in air, without the need for a protected atmosphere and with comparable results to furnace annealing. By combining optical spectroscopy, scanning/transmission electron microscopy, atomic force microscopy with grazing incidence X-ray diffraction, and Rutherford backscattering spectrometry, the sol-gel transition from amorphous to crystalline under PLA is investigated. The first crystallites are formed within 5 laser pulses and the sol-gel laser conversion is proposed to feature two distinct mechanisms, an initial partial photothermal ablation followed by solid-state diffusion. Simulations of the temperature distribution during single laser pulses confirm the experimental results.