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Metal-insulator transition materials such as NbO2 have generated muchexcitement in recent years for their potential applications in computing andsensing. NbO2 has generated considerable debate over the nature of the phasetransition, and the values for the band gap/band widths in the insulatingphase. We present a combined theoretical and experimental study of the band gapand electronic structure of the insulating phase of NbO2. We carry outab-initio density functional theory plus U calculations, directly determining Uand J parameters for both the Nb 4d and O 2p subspaces through the recentlyintroduced minimum-tracking linear response method. We find a fundamental bulkband gap of 0.80 eV for the full DFT+U+J theory. We also perform calculationsand measurements for a (100) oriented thin film. Scanning tunnellingspectroscopy measurements show that the surface band gap varies from 0.75 eV to1.35 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 causedby an energy level shift consistent with a reduction in Coulomb repulsion. Wedemonstrate that this effect in combination with the simple, low cost DFT+U+Jmethod can account for the band widths and p-d gap observed in X-rayphotoelectron spectroscopy experiments. Overall, our results indicate thepossible presence of a 2D anisotropic metallic layer at the (100) surface ofNbO2.

Reconciling the theoretical and experimental electronic structure of NbO2