  Metal-insulator transition materials such as NbO2 have generated much
excitement in recent years for their potential applications in computing and
sensing. NbO2 has generated considerable debate over the nature of the phase
transition, and the values for the band gap/band widths in the insulating
phase. We present a combined theoretical and experimental study of the band gap
and electronic structure of the insulating phase of NbO2. We carry out
ab-initio density functional theory plus U calculations, directly determining U
and J parameters for both the Nb 4d and O 2p subspaces through the recently
introduced minimum-tracking linear response method. We find a fundamental bulk
band gap of 0.80 eV for the full DFT+U+J theory. We also perform calculations
and measurements for a (100) oriented thin film. Scanning tunnelling
spectroscopy measurements show that the surface band gap varies from 0.75 eV to
1.35 eV due to an excess of oxygen in and near the surface region of the film.
Slab calculations indicate metallicity localised at the surface region caused
by an energy level shift consistent with a reduction in Coulomb repulsion. We
demonstrate that this effect in combination with the simple, low cost DFT+U+J
method can account for the band widths and p-d gap observed in X-ray
photoelectron spectroscopy experiments. Overall, our results indicate the
possible presence of a 2D anisotropic metallic layer at the (100) surface of
NbO2.
