Laser-Induced Crystallisation Offers a Quicker Route to Smart Windows

Synchrotron studies show laser annealing is a simpler route to thermochromic VO2 thin films

Smart windows change their properties in response to external factors, with glazing that can switch between transparent and opaque depending on temperature, light levels or an applied voltage. They can be used for privacy and visual effects or to improve energy efficiency. Smart windows using thermochromic materials, for example, can change to block infrared transmission as temperatures rise, remaining transparent to visible light. The thermochromic properties of vanadium dioxide (VO2) offer great potential for energy-saving smart windows. However, depositing VO2 films and coatings through sputtering, chemical or physical vapour deposition can be time-consuming and requires complex and expensive equipment. Solution-based methods are a simpler option, but usually require using a furnace to heat the materials above around 400°C to achieve the necessary crystalline structure, limiting the materials that can be used as a substrate. In work recently published in Applied Surface Science, an international team of researchers crystallised VO2 thin film using laser annealing, substantially reducing the annealing time and crystallisation temperature. Their results showed that the thermochromic properties were comparable with those of furnace-treated samples and that pulsed laser annealing of VO2 could be exploited for a range of applications, including smart windows, metamaterials and flexible electronics.

“Vanadium dioxide is a thermochromic material,” said lead author Maria Basso, a PhD candidate at the University of University of Padova in Italy.

We can tune the material so that at a given temperature – the transition temperature – it continues to transmit visible light but starts to reflect the near-IR. By depositing this phase-change material as a thin film on windows, we can create smart windows as a passive way to improve the energy efficiency of a building. Since they don’t need an active energy input, they’re a low-energy way of keeping a room cooler. In this research, we crystallised the material in an unconventional way. Using a laser allowed us to induce the crystallisation locally, without using a furnace, which could be extended to substrates that are sensitive to temperature, e.g. plastics.

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