Excited-state interactions in monolayer MoS$_{2}$/graphene heterostructures

Recent progress in the formation of atomically thin 2-dimensional crystals by mechanical exfoliation and other synthetic techniques has led to the availability and study of various 2D materials other than graphene. Among them, molybdenum disulfide (MoS$_{2})$ has attracted particular attention. Although an indirect-gap material in the bulk, MoS$_{2}$ exhibits a direct gap in its monolayer form. Correspondingly, the material exhibits strong photoluminescence (PL), very sensitive to the environment. With the development of transfer techniques, it is now possible to create stacks of differing atomically thin materials. In this paper we apply this to investigate the influence of adjacent graphene layers on the PL of MoS$_{2}$ monolayers. Comparing the PL from MoS$_{2}$ on graphene with reference samples, we find that graphene induces strong quenching. Raman measurements of the graphene do not indicate the presence of any significant static charge transfer between layers. This suggests that the graphene layer provides efficient relaxation channels for the photoexcited MoS$_{2}$, rather than modifying its intrinsic properties. In this context, we discuss the relative contributions to PL quenching arising from excited-state charge and energy transfer processes.