Exciton Transitions in Monolayer WS2 Activated by Swift Heavy Ion Irradiation

Abstract

The emergence of monolayer transition metal dichalcogenides (TMDCs) has provided a favorable platform for exploring a broad range of novel optoelectronic device applications and quantum phenomena due to their remarkable physical and chemical characteristics. Understanding the transition and motion of excitons in TMDCs is of fundamental interest for their optoelectronic devices. Here, we demonstrate the exciton transition in monolayer WS2 that can be fine-tuned by swift heavy ion (SHI) irradiation. The dependence of trion and exciton emissions on SHI irradiation in the as-transferred and as-grown monolayer WS2 was investigated by micro photoluminescence (PL) spectra where different PL responses were monitored. The trion-to-exciton transition in the as-transferred WS2 monolayer was ascribed to SHI irradiation that resulted in charge localization and pre-existing defect annealing, whereas the exciton-to-trion transition in the as-grown WS2 monolayer was attributed to the competition of charge localization and charge transfer caused by SHI irradiation. Femtosecond transient absorption measurements were performed to study the exciton relaxation dynamics in SHI-irradiated WS2, and it was shown that defect-assisted recombination dominates the exciton relaxation process for SHI-irradiated WS2. These results open up new opportunities to manipulate and tune the optical properties of two-dimensional TMDCs, which are of special importance for the development of optoelectronic and valleytronic devices.