Influence of a substrate on ultrafast interfacial charge transfer and dynamical interlayer excitons in monolayer WSe2/graphene heterostructures

Efficient interfacial light–electric interconversion in van der Waals heterostructures is critical for their optoelectronic applications. Using time-resolved terahertz spectroscopy and transient absorption spectroscopy, the charge transfer and the dynamical interlayer excitons were investigated in the heterostructures comprising monolayer WSe2 and monolayer graphene with varying stacking order on a sapphire substrate. Herein, a more comprehensive understanding of ultrafast charge transfer and exciton dynamics in two-dimensional heterostructures is shown. Owing to the effective electric field induced by the sapphire substrate, the WSe2/graphene heterostructure exhibits positive terahertz photoconductivity after photoexcitation, while negative terahertz photoconductivity is observed in the graphene/WSe2 heterostructure. The transient absorption spectra indicate that the exciton lifetimes also exhibit a considerable difference, where WSe2/graphene exhibits the longest exciton lifetime, followed by monolayer WSe2, while graphene/WSe2 exhibits the shortest lifetime. These observations provide a new idea for using van der Waals heterostructures in electronic and photonic devices.