Spinodal decomposition introduces strain-enhanced thermochromism in polycrystalline V_1-TiO₂ thin films

Processes of self-organization play a key role in the development of innovative functional nanocomposites, allowing, in particular, the transformation of metastable solid solutions into multilayers by activating spinodal decomposition instead of layer-by-layer film growth. We report the formation of strained layered (V,Ti)O₂ nanocomposites in thin polycrystalline films using a spinodal decomposition. Already during the growth of V_0.65Ti_0.35O₂ films, spinodal decomposition was detected while producing atomic-scale disordered V- and Ti-rich phases. Post-growth annealing enhances compositional modulation, arranges the local atomic structures of the phases, and yields periodically layered nanostructures that resemble superlattices. The coherent interfacing of the V- and Ti-rich layers results in the compression of the V-rich phase along the c-axis of the rutile structure and enables strain-enhanced thermochromism. The latter is characterized by a simultaneous decrease in the temperature and width of the metal-insulator transition in the V-rich phase. Our results provide proof-of-concept for an alternative strategy to develop VO₂-based thermochromic coatings by introducing strain-enhanced thermochromism into polycrystalline thin films.