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 MoS2 via broad-band time-resolved terahertz spectroscopy. We observe that the photoconductivity of monolayer MoS2 transitions from negative to positive when the environment of MoS2 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 MoS2. We attribute the negative photoconductivity to negative trion formation, in which photoinduced excitons capture excess electrons from MoS2. 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 MoS2 as physisorbed gases, particularly oxygen, can dramatically affect the conductivity of the monolayer.