High-Temperature-Induced Intervalley Carrier Transfer in Two-Dimensional Semiconductors: WSe₂ versus WS₂

Abstract

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 WSe 2 is 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 WS 2 and WSe 2 with a theoretical analysis of their spectral behavior. Both direct and indirect exciton emissions in WSe 2 are relatively enhanced, which is different from the hightemperature exciton emission behavior of WS 2 in the same layer. The PL enhancement in WSe 2 could 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.