Exciton Complexes and Spin/Valley Pumping in Doped Monolayer Semiconductors

We have investigated the exciton and optical properties in charge adjustable devices made of an exfoliated WSe2 monolayer embedded in high quality hBN crystals using a dry stamping technique [1].<br/>First, we show that knowledge of the selection rules associated to optical transitions of different exciton complexes, in particular the dark positive trion and its zone-edge phonon replica, allows us to measure the g factors of the K valley conduction and valence bands [2].<br/>Second, we observe that the photoluminescence intensity of the neutral bright exciton is strongest when the monolayer is hole-doped rather than charge neutral or electron-doped. On the basis of a theoretical analysis we demonstrate that this effect results from efficient exchange interactions between neutral exciton and charged particles. In particular, the exchange scattering with electrons renders bright exciton dark in monolayer WSe2 on accounts of the unique valley-spin configuration in this material [3].<br/>Third, we demonstrate very efficient spin-valley pumping of resident electrons in both WSe2 and WS2 monolayers using circularly polarized light [4]. We use the degree of circular polarization of the photoluminescence (PL) associated with negative trions as probes of the polarization of resident electrons : both the intervalley triplet trion and the intravalley singlet trion which consist in the binding of a photo-generated electron-hole pair with a resident electron from the opposite and same valley respectively. We show that circularly polarized excitation photo-generates electron-hole pairs in one valley and dynamically polarize resident electrons in the opposite valley. The spatial variation of the resident electron spin/valley polarization will also be discussed.<br/>[1] Cadiz et al, Phys. Rev. X 7, 021026 (2017).<br/>[2] C. Robert et al, Phys. Rev. Lett. 126, 067403 (2021)<br/>[3] M. Yang et al, Arxiv 2110.00887 (2021)<br/>[4] C. Robert et al, Nature Com. 12, 5455 (2021)