Positive and Negative Photoconductivity in Monolayer MoS₂ as a Function of Physisorbed Oxygen

We investigate the effect of molecular oxygen on the photoconductivity of monolayer MoS 2 via broad-band time-resolved terahertz spectroscopy. We observe that the photoconductivity of monolayer MoS 2 transitions from negative to positive when the environment of MoS 2 changes from vacuum to atmospheric pressure. We argue that this transition from negative to positive photoconductivity results from physically adsorbed oxygen depleting excess electrons from the n-type MoS 2 . We attribute the negative photoconductivity to negative trion formation, in which photoinduced excitons capture excess electrons from MoS 2 . We attribute the positive photoconductivity to negative trion formation as well; however, in this case, photoinduced excitons capture photoinduced defect electrons rather than excess electrons, which have been immobilized by physisorbed oxygen. These results should prove useful to those who look to make sensors and other types of devices out of monolayer MoS 2 as physisorbed gases, particularly oxygen, can dramatically affect the conductivity of the monolayer.