  Vanadium (IV) oxide is one of the most promising materials for thermochromic
films due to its unique, reversible crystal phase transition from monoclinic
(M1) to rutile (R) at its critical temperature (T$_c$) which corresponds to a
change in optical properties: above T$_c$, VO$_2$ films exhibit a decreased
transmittance for wavelengths of light in the near-infrared region. However, a
high transmittance modulation often sacrifices luminous transmittance which is
necessary for commercial and residential applications of this technology. In
this study, we explore the potential for synthesis of VO$_2$ films in a matrix
of metal oxide nanocrystals, using In$_2$O$_3$, TiO$_2$, and ZnO as diluents.
We seek to optimize the annealing conditions to yield desirable optical
properties. Although the films diluted with TiO$_2$ and ZnO failed to show
transmittance modulation, those diluted with In$_2$O$_3$ exhibited strong
thermochromism. Our investigation introduces a novel window film consisting of
a 0.93 metal ionic molar ratio VO$_2$-In$_2$O$_3$ nanocrystalline matrix,
demonstrating a significant increase in luminous transmittance without any
measurable impact on thermochromic character. Furthermore, solution-processing
mitigates costs, allowing this film to be synthesized 4x-7x cheaper than
industry standards. This study represents a crucial development in film
chemistry and paves the way for further application of VO$_2$ nanocomposite
films in chromogenic fenestration.
