Optical properties and formation mechanism of M1-phase VO2 thin films annealed in a closed NH3 atmosphere

Abstract Monoclinic (M1) VO 2 film has potential applications in thermochromic modulators owing to the particular optical performance caused by phase transition. However, its controllable preparation is very difficult due to the multivalence of V element. Here, we studied a sol-gel process followed by annealing in a closed vaccum furnace containing NH 3 atmosphere, and discussed the formation mechanism of M1-phase VO 2 films. Thermochromic M1-phase VO 2 thin films can be fabricated under the base pressure of 600–2000 Pa with the annealing temperature at 470–590 °C for more than 0.5 h, and the optimal condition is at 1000–2000 Pa and 500–530 °C for 0.5 h, by synthetically considering their luminous transmittance ( T lum ), solar modulation efficiency ( ΔT sol ), and modulation efficiency at 2000 nm ( ΔT 2000nm ). The M1-phase VO 2 thin film generally exhibits the optical properties with T lum  ≈ 39–41%, ΔT sol  ≈ 7.7% and ΔT 2000nm  ≈ 50%. A higher base pressure introduces more oxygen to remove polymers in precursors, enhancing the transparency of films. Improving the temperature or the duration promotes the growth of VO 2 crystallite with limits. However, inappropriate base pressure, temperature or duration will reduce or oxidize the VO 2 films, resulting in the inferior thermochromic properties. According to thermal and XPS analysis, NH 3 can act as a reductant to prevent oxidation of VO 2 films during the annealing process. This convenient heat treatment method to synthesize M1-VO 2 films may provide a useful and practical technique for the production of VO 2 -based smart window and optical devices.