Modulation of Structure and Optical Property of Nitrogen-Incorporated VO 2 (M1) Thin Films by Polyvinyl Pyrrolidone.

VO 2 , as a promising material for smart windows, has attracted much attention, and researchers have been continuously striving to optimize the performance of VO 2 -based materials. Herein, nitrogen-incorporated VO 2 (M1) thin films, using a polyvinylpyrrolidone (PVP)-assisted sol-gel method followed by heat treatment in NH 3 atmosphere, were synthesized, which exhibited a good solar modulation efficiency ( ΔT sol ) of 4.99% and modulation efficiency of 37.6% at 2000 nm ( ΔT 2000 nm ), while their visible integrated transmittance ( T lum ) ranged from 52.19% to 56.79% after the phase transition. The crystallization, microstructure, and thickness of the film could be regulated by varying PVP concentrations. XPS results showed that, in addition to the NH 3 atmosphere-N doped into VO 2 lattice, the pyrrolidone-N introduced N-containing groups with N-N, N-O, or N-H bonds into the vicinity of the surface or void of the film in the form of molecular adsorption or atom (N, O, and H) filling. According to the Tauc plot, the estimated bandgap of N-incorporated VO 2 thin films related to metal-to-insulator transition ( E g 1 ) was 0.16-0.26 eV, while that associated with the visible transparency ( E g 2 ) was 1.31-1.45 eV. The calculated E g 1 and E g 2 from the first-principles theory were 0.1-0.5 eV and 1.4-1.6 eV, respectively. The Tauc plot estimation and theoretical calculations suggested that the combined effect of N-doping and N-adsorption with the extra atom (H, N, and O) decreased the critical temperature ( τ c ) due to the reduction in E g 1 .