Single- And narrow-line photoluminescence in a boron nitride-supported MoSe2/graphene heterostructure

Abstract

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 MoSe2layer 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 heterostructure and the MoSe2excitonic linewidth gets as narrow as 1.6 meV, approaching the homogeneous limit. The semi-metallic graphene layer neutralizes the 2D semiconductor and enables picosecond non-radiative energy transfer that quenches radiative recombination fromlong-lived states.Hence, emission from the neutral band edge exciton 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 MoSe2on hBN exhibits negligible valley polarization at low temperature and under near-resonant excitation, we show that interfacing MoSe2with graphene yields a single-line emitter with degrees of valley polarization and coherence up to ~ 15 %.