Excitons or not excitons: Contrasting fates of photo-excitations in monolayer transition metal dichalcogenides

Photo-excitation of atomically thin transition metal dichalcogenide (TMD) monolayers is commonly assumed to result in excitons with large binding energies. Here, we apply time-resolved THz photoconductivity spectroscopy to mechanically exfoliated, large-area monolayer WS₂ and MoSe₂ at excitation densities well below the exciton-Mott transition. We observe contrasting photoconductivity responses, depending on defect density. For monolayers with moderate defect densities, a positive THz conductivity emerges just after photoexcitation under both above-gap and resonant excitation conditions, with ≥ 20% of photogenerated excitons immediately dissociating into charge carriers, likely due to a trap-mediated process. In contrast, in monolayers with low defect density, a negative THz conductivity is observed following photo-excitation; this is consistent with the binding of photo-generated excitons with free carriers from intrinsic doping to form heavier exciton-carrier complexes commonly known as trions. These contrasting fates of photo-excitation in monolayer TMDs reveal the complexity of exciton-carrier physics at the 2D limit.