Thermal-Strain-Enabled Enhanced Emission from UV Laser-Induced Defect Levels near the Surface of Multilayer MoS₂

Monolayer two-dimensional (2D) materials have been intensively studied, while research on multilayers is still in its infancy. Here, we induce defects inside bulk MoS2 through thermal annealing and near the surface of multilayer MoS2 using 375 nm laser irradiation and investigate their photoluminescence (PL) and fluorescence lifetime imaging. Enhanced emission is limited within a certain MoS2 thickness. The observed enhanced emission is evidenced by a threshold behavior in superlinear PL intensity increase, strong polarization effects, and increased lifetime of defect peak. The laser power threshold for enhanced emission is much smaller in defects near the surface than that inside the bulk of multilayer MoS2. The mechanical strain from a wrinkle in the sample further lowers the laser power threshold for enhanced emission. By exciting with a 639 nm laser that is close to the fundamental gap between the conduction band minimum and the valence band maximum, the lifetime of defect enhanced emission increased by 5 times. Furthermore, one of the competing indirect band gap emissions disappears, and the defect emission peak dominates the PL spectrum in the wrinkle area with a strain. The discovered principle can be applied to future studies on the integration of enhanced emission and single photon emitters|single photon emission involving selective depopulation of the conduction band of the host crystal to defect levels for quantum emitters.