Enhancing exciton DOLP by scattering valley excitons in mono- and few-layer MoS₂

The generated exciton valley coherence on Linearly Polarized light excitation is affected by strong exchange interaction and (impurity, phonon) scattering in Transition Metal Dichalcogenides (TMDs). The final state is a mixed ensemble, and the detected Degree of Linear Polarization (DOLP) quantifies the residual coherence at the time of emission. We decouple the role of multiple factors degrading valley coherence in Monolayer (ML) and Bilayer (BL) MoS₂. We observe that impurity scattering degrades DOLP in clean MoS₂ with low defect emission but significantly enhances it when MoS₂ has huge defect emission. This is because of the Motional narrowing effect due to the dense charge fluctuating environment in excitons' vicinity in defected MoS₂. We artificially improve the DOLP by stacking MoS₂ on Graphene as (a) due to fast exciton transfer to Gr, the collection of long-lived decoherent excitons is reduced (Gr filtering effect) and (b) the exciton wavefunction overlap and hence the exchange interaction is reduced due to dielectric screening. The DOLP degrades with temperature (T) in a ML due to phonon scattering inside the light cone but shows a slight improvement in Bi- and Tri-layer due to transfer (, filtering) to the low energy indirect peak.