High-Temperature-Induced Intervalley Carrier Transfer in Two-Dimensional Semiconductors: WSe2versus WS2

Thin layer transition metal dichalcogenides (TMDs) have shown great potential in the field of electronics and optoelectronics devices due to their unique electronic and optical properties. Multilayer WSe2is an indirect bandgap semiconductor and generally optically inactive. To improve the optical properties of multilayer TMDs, heating as a simple and effective method was widely chosen. Herein, we analyze high-temperature photoluminescence (PL) enhancement results on excitons in three-layer and four-layer WS2and WSe2with a theoretical analysis of their spectral behavior. Both direct and indirect exciton emissions in WSe2are relatively enhanced, which is different from the high-temperature exciton emission behavior of WS2in the same layer. The PL enhancement in WSe2could be attributed to the transfer of thermally activated electrons from the Λ valley to K valley involving K → K and K → Γ transition at high temperature, where the conduction band extreme is located at the Λ valley. We quantitatively describe this enhancement phenomenon and demonstrate that the observed spectral behavior reflects the competition between intervalley carrier transfer and high-temperature PL quenching. An excellent agreement between calculated and measured intensities is obtained. This research provides a deeper understanding of the thermally induced intervalley carrier transfer model in multilayer TMDs.