부산대학교 도서관

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, near-field 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 ($\mathrm{MoSe}_2$) 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 $\mathrm{MoSe}_2$ 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 heterostructure and the $\mathrm{MoSe}_2$ excitonic 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 from long-lived states. Hence, emission from the neutral band edge exciton largely dominates the photoluminescence spectrum of the $\mathrm{MoSe}_2$/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 $3.3 \pm 1$ and $4.4 \pm 1$ in the presence of mono- and bilayer graphene, respectively. Finally, while our bare $\mathrm{MoSe}_2$ on hBN exhibits negligible valley polarization at low temperature and under near-resonant excitation, we show that interfacing $\mathrm{MoSe}_2$ with graphene yields a single-line emitter with degrees of valley polarization and coherence up to ${\sim }$ 15 %.