Metal-insulator transition tuned by valence variation of Nb dopants in Nb-doped VO₂ films

High-valence cation doping was proved to be an effective method in reducing the high metal–insulator transition (MIT) temperature (θ_c). However, the mechanism of cation-dopants reducing θ_c is not quite clear and the reported data about the θ_c reduction induced by Nb dopants are dispersive. In this work, we prepared Nb-doped VO₂ films by hydrothermal process and investigated the effects of Nb dopants on the MIT behavior. Valence variation of Nb dopants was observed in the Nb-doped VO₂. The Nb⁵⁺ dopants elevate the valence band top to higher level than the Nb⁴⁺-containing sample. Correspondingly, the Nb⁵⁺ doping reduces θ_c in a higher rate of ∼21 ℃/at%Nb than the Nb⁴⁺-containing sample which has the decline rate of ∼18 ℃/at%Nb. The results imply Nb⁵⁺ dopants contributing excess electrons modify the band structure and consequently promoting the MIT transition. In addition, the Nb doping realizes the θ_c reduction almost without detriment to the optical properties of VO₂, suggesting the promising applications in smart window and other optical switching devices.