Dark Exciton Formation and Relaxation Dynamics in Monolayer WSe2

Abstract

Transition metal dichalcogenides (TMDs) are promising 2D semiconductor in optoelectronics. Due to the reduced dielectric screening, strong bound excitons that are even stable at room temperature exist in these materials and have strong influence on optical properties. In tungsten-based TMDs (WSe 2 and WS 2 ), spin-forbidden dark excitons exist at lower energy than bright excitons. Such dark excitons provide relaxation channels of bright excitons and effect on the bright exciton photoluminescence (PL) properties such as temperature dependence [1] . In addition, spin-forbidden dark excitons have long valley lifetime, which are suitable for valleytronics [2] . Hence, understanding on dark exciton formation and relaxation dynamics in TMDs is one of the most important research topic for both fundamental exciton physics and applications. Recently it has been revealed that one of the spin-forbidden dark excitons have electric dipoles perpendicular to the crystal plane and emit in-plane propagating PL, which enables us to study on dark excitons easily without magneto-optical measurement system [3] , [4] .