Demystifying the Modulation Effects of the Graphene Transport Layers on the Interlayer Charge Transfer Mechanism and Broadband Optical Properties of MoS2/Graphene/WSe2 van der Waals Heterojunctions

2D materials and heterojunctions have extraordinary potential in the field of next-generation integrated photodetectors. Recently, inserting graphene (Gra) into van der Waals heterojunctions as transport layers has been proven to be an effective method for improving the responsivity and response speed of photodetectors. However, how did the physical mechanisms caused by the insertion (interlayer coupling and electron transfer) regulate its optical properties are unclear. Meanwhile, whether the insertion will modulate the broadband optical properties of the heterojunction should also be carefully verified. In this work, the broadband (1.25-6.50 eV) electronic band structures and exciton properties of MoS2/WSe2 and MoS2/Gra/WSe2 were studied using spectroscopic ellipsometry, meanwhile Raman and PL spectroscopy were used to assist in analysis. We found that graphene insertion not only promote the spontaneous charge transfer from WSe2 to MoS2, but also effectively inject electrons into the MoS2 layer, which benefits the interlayer charge separation and net charge accumulation in MoS2. Moreover, the almost stable CP energies near the band edge represent that graphene insertion does not change the electronic band structures of MoS2/WSe2. Furthermore, due to the increasing effective dielectric screening induced by graphene insertion, the exciton binding energies redshift and the exciton transition energy blueshift consequently.