Changes in the Photoluminescence of Monolayer and Bilayer Molybdenum Disulfide during Laser Irradiation.

Various postsynthesis processes for transition metal dichalcogenides have been attempted to control the layer number and defect concentration, on which electrical and optical properties strongly depend. In this work, we monitored changes in the photoluminescence (PL) of molybdenum disulfide (MoS) until laser irradiation generated defects on the sample flake and completely etched it away. Higher laser power was required to etch bilayer MoS compared to monolayer MoS. When the laser power was 270 μW with a full width at half-maximum of 1.8 μm on bilayer MoS, the change in PL intensity over time showed a double maximum during laser irradiation due to a layer-by-layer etching of the flake. When the laser power was increased to 405 μW, however, both layers of bilayer MoS were etched all at once, which resulted in a single maximum in the change of PL intensity over time, as in the case of monolayer MoS. The dependence of the etching pattern for bilayer MoS on laser power was also reflected in position changes of both exciton and trion PL peaks. The subtle changes in the PL spectra of MoS as a result of laser irradiation found here are discussed in terms of PL quantum efficiency, conversion between trions and excitons, mean interatomic spacing, and the screening of Coulomb interaction.