Uncovering the distinctive phase transition characteristics and thermochromic performance of VO₂ with different N-doping sites

Nitrogen is a rare dopant that can not only enter the interstitial sites (N-i) but also oxygen substitutional sites (N-s) of vanadium dioxide (VO2). The incorporation of the N atom has been demonstrated to modulate the phase transition behavior of VO2. However, a deep understanding of the effect of N doping sites on the phase transition and thermochromic properties of VO2 remains lacking. Here, we prepare N-doped VO2 with tunable doping sites. N-i demonstrates the ability to slightly increase phase transformation temperature (T-c), while greatly improving the visible light transmittance and near-infrared (NIR) modulation performance by 5 % and 44 %, respectively. Interestingly, the introduction of N-s reduces the T-c of N-doped VO2 with double doping sites (interstitial and O substitution sites, N-i,N-s), from 65.2 to 60.6 degrees C. More attractively, the thermal hysteresis width of the phase transition is reduced to near zero (0.2 degrees C). This is of great interest for the precise regulation of the VO2 phase transition. These distinctions are attributed to the differences in the effects of N-doping sites on the lattice and energy band structure of VO2. N-s injects hole carriers into the d(//) band raising the valence band maximum and thus narrowing the band gap. The point defect-induced nucleation mechanism supports a narrow hysteresis width in the combination effect of oxygen vacancies and N impurities.