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High-quality VO2 nanoparticle (NP) films were prepared by a facile self-template synthesis method, including sputtering of V films and postannealing, and were identified as a parabolic nanocone array, namely, the moth-eye nanostructure. Compared to a VO2 flat thin film with a refractive index of ∼3 at a 550 nm wavelength, the fabricated subwavelength VO2 NP films with the graded and lower effective refractive index (less than 2) exhibited ultrahigh luminous transmittance (T lum) due to the decreased reflectance differences at air/VO2 and VO2/glass interfaces. Meanwhile, subwavelength rutile VO2 NPs provided the localized surface plasmon resonance excited at ∼1250 nm wavelength to enhance the solar modulation (ΔT sol). Using simulations, the optimal NP size and porosity of the NP film were designed to obtain the optimum balance between ΔT sol and T lum, which were confirmed by experimental results. The single-sided VO2 NP film with an ultrahigh T lum of 93.3% exhibited a transparent vision with a ΔT sol of 6.1%, and the double-sided VO2 NP film provided an excellent ΔT sol of 14%, while the T lum remained at ∼70%. The outstanding performances of the VO2 NP films exceeded the reported results for porous films, multilayers, and nanostructures. Thus, we believe that the VO2 NP film is a promising nanostructure to lead VO2-based coatings to an era for smart window applications, including the building glazing and windshields of vehicles.

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Nanophotonics has been extensively researched in the past decades owing to the rapid development of the precision nanofabrication techniques, such as focused-ion beam milling and electron-beam lithography. However, it was limited by some disadvantages, such as high cost and low productivity. Therefore, a facile and economics method, nanosphere lithography (NSL) [1], for fabricating multiple nanostructures has been reported. Polystyrene nanospheres, with diameters of several hundred nanometers, were aligned into a monolayer. By using close-packed polystyrene nanospheres array as a mask, periodic metal bowties can be fabricated after metal evaporation and nanospheres lift-off.

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