Visualizing Point Defects in Transition-Metal Dichalcogenides Using Optical Microscopy

While transmission electron microscopy and scanning tunneling microscopy reveal atomic structures of point defect and grain boundary in monolayer transitionmetal dichalcogenides (TMDs), information on point defect distribution in macroscale is still not available. Herein, we visualize the point defect distribution of monolayer TMDs using dark-field optical microscopy. This was realized by anchoring silver nanoparticles on defect sites of MoS₂ under light illumination. The optical images clearly revealed that the point defect distribution varies with light power and exposure time. The number of silver nanoparticles increased initially and reached a plateau in response to light power or exposure time. The size of silver nanoparticles was a few hundred nanometers in the plateau region as observed using optical microscopy. The measured defect density in macroscale was ∼2 × 10¹⁰ cm^-2, slightly lower than the observed value (4 × 10¹¹ cm^-2) from scanning tunneling microscopy.