Anomalous Light-Induced Charging in MoS₂ Monolayers with Cracks

Monolayer MoS₂ devices with Au electrodes were fabricated on SiO₂/Si substrates with 50 nm high SiO₂ nanopillar (NP) array patterns. In the NP patterns, many cracks were found in the MoS₂ flakes, which were generated by the NP-induced mechanical strain during the wet transfer process. The cracks broke a few tens of micrometer MoS₂ flakes, producing micrometer-sized flakes. Some of the small MoS₂ flakes were suspended over the NPs, and others were not. The suspended flakes were highly strained, but the nonsuspended flakes were unstrained. Light-induced charging behaviors at the MoS₂ flakes on the NPs were distinct from those on flat SiO₂. More interestingly, positive and negative charging of an identical flake could be observed during repeated light on-and-off cycles. The strain-induced potential gradient in the MoS₂ flakes on the NPs could cause exciton dissociation and charge migration under illumination, giving rise to light-induced charging. The polarity and amount of charges could be determined by the strain states and initial net charges of a specific flake and its neighboring flakes.