Photoluminescence properties of WSe2 monolayer and bilayer nanosheets

We systematically studied the low-temperature (12 K) photoluminescence (PL) spectroscopies of WSe2 nanosheets prepared by vapor deposition technique. It is found that PL intensity monotonically decreases with the increasing of WSe2 nanosheet layers. In particular, the PL intensity dramatically decreased when the thickness of WSe2 films changed from monolayer to bilayer, which indicated that there is a direct-to-indirect transition in the band gap of WSe2 nanosheets. Then we focused on the variable temperature photoluminescence of the bilayer structure. When the temperature increases from 12 K to 300 K, the temperature-dependent evolution of the direct transition energy (peak A) is approximately consistent with the common formula obeyed by bulk semiconductors, while the indirect transition energy (peak I) can only be described by a linear relationship with temperature. This indicates that peak A and peak I have different transition characteristics and these two transition characteristics coexist in bilayer WSe2 simultaneously.