Excitonic Effect Drives Ultrafast Transition in Two-Dimensional Transition Metal Dichalcogenides.

Two-dimensional (2D) transition metal dichalcogenides (TMDCs) are ideal platforms for exploring excitonic physics because of the tightly bound excitons. In this work, we observed the onset of band-edge exciton formation in monolayer MoS 2 (WS 2 ) and bilayer MoS 2 -WS 2 by measuring the transient optical response upon excitation with ultrashort laser pulses. In addition to wavelength dependence on excitation under nonresonant excitation, we found that the onset of band-edge exciton formation in monolayer MoS 2 (WS 2 ) pumped in the exciton state is significantly faster than that with pumping in the nonexciton state, which could be attributed to the effective transition between exciton states induced by the excitonic effect. Besides, the onset of band-edge exciton formation in van der Waals heterostructures is similar to that for monolayer TMDCs regardless of charge transfer at the interface. Our work contributes to a better understanding of exciton dynamics in 2D TMDCs, providing a solid basis of the rational design of the 2D optoelectronic applications based on TMDCs.