Radiation-induced direct bandgap transition in few-layer MoS2

Abstract

We report photoluminescence (PL) spectroscopy of air-suspended and substrate-supported molybdenum disulfide (MoS2) taken before and after exposure to proton radiation. For 2-, 3-, and 4-layer MoS2, the radiation causes a substantial (>10×) suppression of the indirect bandgap emission, likely due to a radiation-induced decoupling of the layers. For all samples measured (including the monolayer), we see the emergence of a defect-induced shoulder peak at around 1.7 eV, which is redshifted from the main direct bandgap emission at 1.85 eV. Here, defects induced by the radiation trap the excitons and cause them to be redshifted from the main direct band emission. After annealing, the defect-induced sideband disappears, but the indirect band emission remains suppressed, indicating a permanent transition into a direct bandgap material. While suspended 2-, 3-, and 4-layer MoS2 show no change in the intensity of the direct band emission after radiation exposure, substrate-supported MoS2 exhibits an approximately 2-fold increase in the direct bandgap emission after irradiation. Suspended monolayer MoS2 shows a 2–3× drop in PL intensity; however, substrate-supported monolayer MoS2 shows a 2-fold increase in the direct band emission.