Ultrafast formation and dynamics of interlayer exciton in a large-area CVD-grown WS₂/WSe₂ heterostructure

A WS₂/WSe₂ heterostructure is constructed by stacking a WS₂ monolayer on the top of WSe₂ monolayer fabricated with chemical vapor deposition (CVD) method. Ultrafast transient spectroscopy is used to demonstrate the ultrafast charge transfer and interlayer exciton dynamics in the heterostructure. When the WS₂/WSe₂ heterostructure was photoexcitated at 617 nm (2.01 eV) to excite the A-exciton transition of WS₂, an ultrafast photobleaching was observed around the WSe₂ A-exciton transition at 749 nm. The bleaching signal lasts several nanoseconds, which is much longer than the A-exciton lifetime in both the WS₂ and WSe₂ monolayer film. Moreover, by selectively photoexciting the A-exciton of WSe₂ at 749 nm in the heterostructure, an ultrafast photobleaching occurs around the WS₂ A-exciton transition, the recovery of the bleaching shows a single exponential relaxation with typical time constant of ~1.8 ps. The very fast relaxation in the heterostructure probing around 620 nm is indicative that rich defect states exist below the conduction band in WS₂, which can efficiently trap these electrons transferred from the WSe₂ upon photoexcitation. Our spectroscopic results reveal that our CVD-grown WS₂/WSe₂ bilayer film has a type II heterostructure in nature at room temperature. With photoexcitation, electrons and holes can be separately confined in the WS₂ and WSe₂ layer, respectively; as a result, interlayer excitons are formed.