Electronic vs optical bandgap of stoichiometric V2O5: Effects of surface transfer doping and electron accumulation layer

The experimental and theoretical bandgap reported for stoichiometric V2O5, a layered semiconductor of great technological importance, spans a wide range of 1.7–4.8 eV. Using combined photoemission, absorption, and photoluminescence measurements, we show that the fundamental electronic gap of V2O5 is 1.85 eV, which is in close agreement with the value of 1.8 eV predicted by the first density functional theory studies, but is lower than the value of 2.2–2.8 eV obtained from optical absorption and photoemission studies. It is shown that this difference between the fundamental and optical gaps is due to the presence of a surface electron accumulation layer, which results in a Burstein–Moss shift of the Fermi level well into the conduction band. The underlying cause of degenerate electron doping is due to “surface transfer doping” by ambient water molecules as a result of the high electron affinity of the semiconductor.