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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