Defect-induced Burstein-Moss shift in reduced V2O5 nanostructures

The main effects of oxygen vacancy defects on the electronic and optical properties of ${\mathrm{V}}_{2}{\mathrm{O}}_{5}$ nanowires were studied through in situ Raman, photoluminescence, absorption, and photoemission spectroscopy. Both thermal reduction and electrochemical reduction via lithium insertion leads to the creation of oxygen vacancy defects in the crystal that gives rise to new electronic midgap defect states at energy 0.75 eV below the conduction band edge. The defect formation results in delocalization and injection of excess electrons into the conduction band, as opposed to localized electron injection as previously suggested. Contrary to what is seen in most oxides, the presence of vacancy defects leads to band filling and an increase in the optical band gap of ${\mathrm{V}}_{2}{\mathrm{O}}_{5}$ from 1.95 to 2.45 eV, which is attributed to the Burstein-Moss effect. Other observed changes in the optical properties are correlated to the changes in the electronic structure of the oxide as a result of defect formation. Further, in situ Raman measurements during the electrochemical reduction at room temperature show that the oxygen atom that is most readily reduced is the threefold coordinated oxygen (O3).