Influence of the charge compensation effect on the metal–insulator transition of Mg-W co-doped VO₂

Cation co-doping was proved an effective approach to reduce the metal–insulator transition (MIT) temperature (θ_c) of vanadium dioxide (VO₂) for realizing excellent optical switching properties near room temperature, but its mechanism is remaining a great challenge. Herein, the Mg-W co-doped VO₂ films are fabricated through hydrothermal growth. By changing the Mg content with maintaining 1 at% W doping level, it is observed that Mg dopants raise the θ_c of co-doped VO₂ at a rate of ∼ 22 ℃ per at% Mg as the Mg content is less than ∼ 1 at%, but loses efficacy more than 1 at% Mg. The resistance of the co-doped VO₂ films show the similar variation trend. The valence-band XPS spectra indicate that the Mg dopants counteract the VBE uplifting of VO₂ caused by W dopants, consequently enlarging the optical band gap of W-doped VO₂, which would partially explain the improvement of the visible light transmittance of W-doped VO₂ induced by Mg dopants. The results clearly evidence that the charge compensation takes effect in the co-doped VO₂, demonstrating the co-doping of high- and low-valent cations is unfavorable for effectively decreasing θ_c of VO₂, and reveal the electron doping plays a key role in reducing θ_c.