Experimental demonstration of two distinct pathways of trion generation in monolayer MoS2

TL;DR

This study reveals two distinct pathways for trion formation in monolayer MoS2, involving direct creation in valleys via photon absorption and transformation from excitons due to carrier relaxation, supported by spectroscopy and modeling.

Contribution

It provides the first experimental demonstration of two separate trion generation pathways in monolayer MoS2, validated by a rate equation model.

Findings

Direct trion creation in K/K' valleys via photon absorption

Transformation of excitons into trions due to carrier relaxation

Coexistence of both trion generation pathways confirmed

Abstract

Excitation power and energy dependent photoluminescence (PL) and transient absorption spectroscopy (TAS) studies are carried out on chemical vapour deposition (CVD) grown 1L-MoS2 films to understand the process of trion formation. The study shows that the excitation with sufficiently low photon energy results in the creation of trions directly in the K/K' valleys through photon absorption followed by phonon scattering events. On the other hand, excitation energy sufficiently larger than the band-gap can generate the carriers away from the K/K' valleys. Dissimilarity in the rates of relaxation of the photo-excited electrons and the holes to the bottom of the K/K' valleys results in the transformation of the excitons residing there into trions. Our TAS study clearly demonstrates a temporary increase of the trion population in the K/K' valleys. Moreover, excitation intensity dependent PL spectroscopy performed under above-band-gap excitation, also suggests the coexistence of both the pathways of trion generation in this material. This conclusion is further validated by a rate equation model. Our findings provide valuable insight into the formation of trions in monolayer transition metal dichalcogenides (TMDC), which could be crucial in designing valleytronic devices based on trions.