Enhancement of direct and indirect exciton emissions in few-layer WSe2 at high temperatures

Abstract

We present a temperature-dependent and time-resolved photo-luminescence (PL) study for few-layer tungsten diselenide (WSe2) that is exfoliated from the bulk crystals. The PL intensities of direct and indirect exciton emissions in monolayer and bilayer WSe2 decrease while temperature increases. However, abnormal enhancement of PL emission from direct and indirect excitons in ≥3-layer WSe2 is observed at high temperatures, ranging from 300 K to 400 K. A crossover of Λ → Γ and Κ → Γ indirect transitions occurs in few-layer WSe2 at elevated temperature and this phenomenon is verified by comparing the temperature-dependent shift of indirect exciton PL peak and ab initio band structure calculations. The enhancement of high-temperature PL from direct and indirect excitons can be attributed to the rapid intervalley transfer of thermally activated carriers from Λ point to K point and the crossover of Λ → Γ and Κ → Γ indirect transitions. The distinct behavior of temperature dependence of the PL intensities at a high temperature between bilayer and trilayer WSe2 reflects their band structure discrepancy. This work provides a thorough understanding of the origin of the observed indirect optical transition in few-layer WSe2 and suggests a possible means of improving the luminescence efficiency of multilayer WSe2.