Changes in VO₂ band structure induced by charge localization and surface segregation

Vanadium vacancies introduce acceptor doping with hole localization, while oxygen vacancies cause electron localization and donor doping. As deposition temperature increases, donor concentration stays constant, whereas acceptor concentration significantly increases, leading to enhanced (011) lattice-plane compression and surface segregation. Localized charges result in shifts of O 1s and V⁴⁺ 2p core levels toward higher binding energies, and O 2p and V⁴⁺ 3d valence bands toward the Fermi level, but e_g ^π bands lifting and a1g bands splitting energies are both insensitive to charge localization. Particularly, band-gap energy decreases with increasing V-V pair distance, and is significantly reduced by band tailing.