Influence of atmospheric gases on the electrical properties of PbSe quantum-dot films

We report the effects of N₂ and O₂ on the electrical properties of PbSe quantum-dot (QD) films treated with 1,2-ethanedithiol (EDT) by measuring the changes in the current-voltage characteristics of QD field-effect transistors (FETs). EDT-treated PbSe QD films at a base pressure of ∼10^-5 Torr exhibit ambipolar transport. Exposing these films to N₂ shifts the transfer characteristics toward negative gate-voltage values and increases the electron mobility. These changes could be reversed entirely by removing the N₂ gas over the FET and returning to base pressure. Oxygen exposure shifts the transfer characteristics in the opposite direction toward positive gate-voltage values. Moreover, oxygen exposure reduces charge mobility but increases film conductivity. For exposures up to ∼10⁸ langmuir, these O₂-induced changes could be reversed completely by removing the O₂ gas over the sample and returning to base pressure. However, after ∼10¹⁰ langmuir of O₂ exposure, the changes are irreversible. The QD films then permanently become p-type and the decrease in charge mobility remains even after returning to base pressure.