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.
Previous studies have shown that contact with air causes a thin oxidised layer to form on VO2, and that humidity can then induce a gradual transition to V2O5 that would impact the thermochromic nature of the materials. On Diamond's I09 beamline, the research team used hard X-ray photoelectron spectroscopy (HAXPES) to probe the oxidation state of the material beneath the surface. By comparing the synchrotron results with those gained from laboratory based SXPS and bulk XRD studies, the researchers confirmed that this undesirable oxidation is only superficial.
This was my first visit to a synchrotron and it was really interesting to see how things work, how intense it can be and how much data you can get in a small amount of time. The experiments we did at Diamond, combined with other techniques, demonstrated that laser annealing is a viable process for crystallising thin films of VO2. We identified the range in which we can play with the parameters to induce crystallisation and we investigated the influence of different substrates.
Maria Basso
A solution-based production system for VO2 thin films is cheaper, avoids the need for a vacuum, and is scalable for industrial production. Using lasers to induce crystallisation is much quicker and more energy efficient, paving the way for smart windows to become more cost-effective and more readily available. Laser annealing is a promising alternative to conventional thermal treatments and could be exploited for other VO2 applications, including flexible electronics and metamaterials.
Maria's group in Italy and our group at UCL are both investigating metal oxides and their functional properties, and this project is part of a larger effort around improving metal oxides for specific applications. Smart windows are a very specific application, but it's really crucial to understand the relationship between the structure of a material, the chemistry and the electronic structure, and for that experiments such as the one we did at Diamond using X-ray Photoelectron Spectroscopy are crucial.
Dr Anna Regoutz from University College London who led the team that conducted the Diamond experiments.
To find out more about the I09 beamline or discuss potential applications, please contact Principal Beamline Scientist Tien-Lin Lee: tien-lin.lee@diamond.ac.uk.
Basso M. et al. Rapid laser-induced low temperature crystallization of thermochromic VO2 sol-gel thin films. Applied Surface Science 631 (2023): 157507. DOI:10.1016/j.apsusc.2023.157507.
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