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

• Published in: Journal of alloys and compounds Vol. 706; pp. 289 - 296
• Main Authors: Wan, Meinan, Liu, Baoshun, Wang, Shuo, Hu, Lingting, He, Youjia, Tao, Haizheng, Zhao, Xiujian
• Format: Journal Article
• Language: English
• Published: Lausanne Elsevier B.V 05.06.2017; Elsevier BV
• Subjects: Ammonia; Annealing furnaces; Annealing temperature; Base pressure; Chemical compounds; Heat conductivity; Heat treatment; Modulation; Modulators; NH3 atmosphere; Optical properties; Oxidation; Phase transitions; Sol-gel processes; Thermochromic property; Thin films; Vanadium oxides; VO2 thin film; X ray photoelectron spectroscopy; NH3 atmosphere; Annealing temperature; Base pressure; VO2 thin film; Thermochromic property
• ISSN: 0925-8388; 1873-4669
• DOI: 10.1016/j.jallcom.2017.02.151

Monoclinic (M1) VO2 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 NH3 atmosphere, and discussed the formation mechanism of M1-phase VO2 films. Thermochromic M1-phase VO2 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 (Tlum), solar modulation efficiency (ΔTsol), and modulation efficiency at 2000 nm (ΔT2000nm). The M1-phase VO2 thin film generally exhibits the optical properties with Tlum ≈ 39–41%, ΔTsol ≈ 7.7% and ΔT2000nm ≈ 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 VO2 crystallite with limits. However, inappropriate base pressure, temperature or duration will reduce or oxidize the VO2 films, resulting in the inferior thermochromic properties. According to thermal and XPS analysis, NH3 can act as a reductant to prevent oxidation of VO2 films during the annealing process. This convenient heat treatment method to synthesize M1-VO2 films may provide a useful and practical technique for the production of VO2-based smart window and optical devices. [Display omitted] •A new method to fabricate M1-VO2 film by annealing in a closed NH3 atmosphere was studied in detail.•This annealing approach has good repeatability.•Annealing base pressure, temperature and duration are the crucial parameters to synthesize pure M1-VO2 thin film.•NH3 can act as a reductant to inhibite the over-oxidization of VO2.