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

•N-doped VO2(M1) thin films have been synthesized by annealing in NH3 atmosphere.•The phase purity, microstructure and optical property of VO2 thin film can be regulated by NH3 concentration.•First-principles calculations have been carried out to study the mechanism of N-doping on energy band structures of VO2(M1).•The energy band gaps of VO2(M1) are tuned by substitution N-doping or interstitial N-doping.