▎ 摘　　要

Heterostructures made from van der Waals (vdW) materials provide a template to investigate a wealth of proximity effects at atomically sharp two-dimensional (2D) heterointerfaces. in particular, nearfield charge and energy transfer in vdW heterostructures made from semiconducting transition metal dichalcogenides (TMD) have recently attracted interest to design model 2D "donor-acceptor" systems and new optoelectronic components. Here, using Raman scattering and photoluminescence spectroscopies. we report a comprehensive characterization of a molybedenum diselenide (MoSe2) monolayer deposited Onto hexagonal boron nitride (hBN) and capped by mono- and bilayer graphene. Along with the atomically flat hBN susbstrate, a single graphene epilayer is sufficient to passivate the MoSe2 layer and provides a homogenous environment without the need for an extra capping layer. As a result, we do not observe photo-induced doping in our heterostnicture and the MoSe2 excitonic linewidth gets as narrow as 1.6 rrieV, approaching the homogeneous limit. The semi-metallic graphene layer neutralizes the 2D semiconductor and enables picosecond non-radiative energy transfer that quenches radiative recombination from long-lived states. Hence, emission from the neutral band edge CM largely dominates the photoluminescence spectrum of the MoSe2 /graphene heterostructure. Since this exciton has a picosecond radiative lifetime at low temperature, comparable with the non-radiative transfer time, its low-temperature photoluminescence is only quenched by a factor of 3.3 +/- 1 and 4.4 +/- 1 in the presence of mono- and bilayer graphene, respectively. Finally, while our bare MoSe2 on hBN exhibits negligible valley polarization at low temperature and under near-resonant excitation, we show that interfacing MoSe2 with graphene yields a single-line emitter with degrees of valley polarization and coherence up to similar to 15 %.