Tuning Phase Transition and Thermochromic Properties of Vanadium Dioxide Thin Films via Cobalt Doping

Vanadium dioxide (VO₂) featuring a distinct thermally triggered phase transition is regarded as the most attractive thermochromic material for smart window applications. However, the high transition temperature (∼67 °C) and moderate luminous transmittance (<50%) of the pristine VO₂ circumvent room temperature applications. In this work, epitaxial cobalt-doped VO₂ thin films were fabricated to tailor the electric and optical properties on a c-plane sapphire substrate. At the highest doping concentration of 10%, the transition temperature of VO₂ is reduced to 44 °C, accompanied by a high luminous transmittance of 79% for single-element Co-doped VO₂. The roles of cobalt doping and detailed band variation are fully explained experimentally and by modeling (DFT calculation), respectively. Furthermore, the dramatically increased carrier concentration in cobalt-doped VO₂ underscores the promising future of cobalt-doped VO₂ unveiled by temperature-dependent Hall effect measurement.