Self-doped nanocolumnar vanadium oxides thin films for highly selective NO₂ gas sensing at low temperature

Vanadium dioxide (VO₂) as strong correlated oxide has been intensively studied due to the unique properties accompanied by metal-insulator transition (MIT) near room temperature. Their extrinsic electrical and optical transformations on MIT have been greatly attracted in many potential applications. In this study, we propose to apply an intrinsic electrical property of VO₂ in metallic phase as doping agent of chemiresistive sensor. Self-doped nanocoulmnar vanadium oxides (SNVO) thin films are simply fabricated by glancing angle deposition without any template and chemical additives. The nanocolumnar VO₂ thin films were deposited by e-beam evaporator, and then the SNVO with residual VO₂ in V₂O₅ nanocolums were synthesized by an air atmosphere annealing, which are composed of dominant oxidized V₂O₅ which plays a sensing role on the surface and the residual VO₂ with self-doping effect inside nanocolumns. Especially at low temperature (150 °C), SNVO sensor shows an ultra-high performance (R_gas/R_air > 100) to NO₂ (5 ppm) gas. It is supposed that the residual VO₂ which shows metallic properties above Mott transition temperature (∼67 °C) acts as an intrinsic electron donor by pseudo-free electron, which helps adsorption of oxygen ions on the surface and extends the depletion region of few nanometers of narrow necks between nanocolumns.