Observation of reduced phase transition temperature in N-doped thermochromic film of monoclinic VO2

Research on monoclinic (M1) phase of VO 2 has attracted a great of interest for smart coating applications due to its exceptional thermochromic property. Herein, we report the results using a novel approach to synthesize N-doped VO 2 (M1) thin films with high purity by heat treatment in NH 3 atmosphere. The N dopant in the film can be regulated by varying NH 3 concentration during the annealing process. We find that the N atoms are located at the interstitial sites or substitute oxygen atoms, and the V-N bonds in the VO 2 thin films increase with NH 3 concentration. The metal to insulator transition (MIT) temperature ( τ c,h ) of the VO 2 thin film is effectively reduced from 80.0 to 62.9   °C, while the solar modulation efficiency (Δ T sol ) and the modulation efficiency at 2000   nm (Δ T 2000nm ) are 7.36% and 55.6% respectively. The band gap of N-doped VO 2 thin films related to MIT ( E g 1 ) is estimated to be as low as 0.18–0.25   eV whereas the band gap associated with the visible transparency ( E g 2 ) is about 1.50–1.58   eV. Based on the highly accurate first-principles calculations, the E g 1 of VO 2 (M1) is reduced after substituted or interstitial N-doping, while the E g 2 alters with the mode of N-doping, which is excellent agreement with experimental measurement.