Effects of electronic correlation, physical structure, and surface termination on the electronic structure of V ₂O ₃ nanowires

We report on a density functional theory (DFT) study of the electronic structure of vanadium sesquioxide (V ₂O ₃) in both bulk and nanowire form. In particular, our study focuses on the role of spin polarization and electronic correlations, as computed within the local (spin) density approximation (L(S)DA) and the LDA+U formalism. As expected for a mean-field approach such as DFT, our optimized bulk V ₂O ₃ structure is shown to be metallic in nature, while an adequate choice of the Hubbard U parameter (U = 4 eV) is enough to open the band gap, making the system insulating. However, this formalism predicts a nonmagnetic insulator, as opposed to the experimentally observed antiferromagnetic structure, to be the ground state. The electronic structure of the V ₂O ₃ nanowire system is more complex, and it strongly depends on the surface termination of the structures. Our results show that non-spin-polarized LDA calculations of 001-grown nanowires are metallic in nature. However, LSDA predicts that some surface terminations are half-metals, with a large band gap opening for one of the spins. When LSDA+U was used to study the nanowire model with a closed-shell oxygen surface termination, we observe insulating behavior with no net magnetic moment, with a 104 meV band gap. This is consistent with the experimentally observed gap recently reported in the literature for similar wires. To experimentally address the surface structure of these nanowires, we perform surface specific nano-Auger electron spectroscopy on as-synthesized V ₂O ₃ nanowires. Our experimental results show a higher O:V peak ratio (1.93:1) than expected for pure V ₂O ₃, thereby suggesting higher oxygen content at the surface of the nanowires. From our results, we conclude that oxygen termination is likely the termination for our as-synthesized V ₂O ₃ nanowires.