Reduced Absorption Due to Defect-Localized Interlayer Excitons in Transition-Metal Dichalcogenide–Graphene Heterostructures

Abstract

Associating atomic vacancies to excited-state transport phenomena in two-dimensional semiconductors demands a detailed understanding of the exciton transitions involved. We study the effect of such defects on the electronic and optical properties of WS 2 −graphene and MoS 2 −graphene van der Waals heterobilayers, employing many-body perturbation theory. We find that chalcogen defects and the graphene interface radically alter the optical properties of the transition-metal dichalcogenide in the heterobilayer, due to a combination of dielectric screening and the manybody nature of defect-induced intralayer and interlayer optical transitions. By analyzing the intrinsic radiative rates of the subgap excitonic features, we show that while defects introduce low-lying optical transitions, resulting in excitons with non-negligible oscillator strength, they decrease the optical response of the pristine-like transition-metal dichalcogenide intralayer excitons. Our findings relate excitonic features with interface design for defect engineering in photovoltaic and transport applications.

Photovoltage and Photocurrent Absorption Spectra of Sulfur Vacancies Locally Patterned in Monolayer MoS2

“…Since the Fermi energy can be assumed to be only a few meV below c D2 (cf. Figure d and refs − ), we tentatively suggest that the thermal energy k B T bath , with k B being the Boltzmann constant, plays the following role. First, at room temperature, the exciton dissociation is more efficient than at cryogenic temperatures, and photogenerated holes are likely to be subject to lateral transport within MoS 2 , instead of tunneling across the hBN barrier to the b-graphene.…”

“…Figure d sketches the band alignment of the vertical tunneling circuit as derived from literature values, with the spatially localized defect conduction band states c D1 and c D2, the defect valence band state v D within MoS 2 , and the Fermi level E F being slightly below c D1 and c D2. ,,, In turn, for a sub-bandgap excitation, optical transitions can occur between v D and c D2/ c D1 (gray vertical arrow) as well as between the valence band maximum (VBM) of MoS 2 and c D2/ c D1. Since we measure a negative sign of the generated photocurrent at zero bias (cf.…”