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

Excitation power and energy dependent photoluminescence (PL) and transient absorption spectroscopy (TAS) studies are carried out on chemical vapour deposition grown monolayer-MoS₂films 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 theK/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 theK/K' valleys. Dissimilarity in the rates of relaxation of the photo-excited electrons and the holes to the bottom of theK/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 theK/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, which could be crucial in designing valleytronic devices based on trions.