Abstract
Heterostructures made from van der Waals materials provide a template to investigate proximity effects at atomically sharp heterointerfaces. In particular, near-field charge and energy transfer in heterostructures made from semiconducting transition metal dichalcogenides (TMD) have attracted interest to design model 2D "donor-acceptor" systems and new optoelectronic components. Here, using of Raman scattering and photoluminescence spectroscopies, we report a comprehensive characterization of a molybedenum diselenide (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 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 MoSe$_2$ excitonic linewidth gets as narrow as 1.6~meV, hence 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 MoSe$_2$/graphene heterostructure. Since this exciton has a picosecond radiative lifetime at low temperature, comparable with the energy 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 MoSe$_2$ on hBN exhibits negligible valley polarization at low temperature and under near-resonant excitation, we show that interfacing MoSe$_2$ with graphene yields a single-line emitter with degrees of valley polarization and coherence up to $\sim 15\,\%$.
Highlights
Semiconducting transition metal dichalcogenides (TMDs) are layered crystals that exhibit a unique set of optical and electronic properties
In this paper, using a combination of PL and Raman spectroscopies, we investigate an hBNsupported MoSe2 monolayer capped by a graphene flake containing monolayer (1LG) and bilayer domains (2LG)
We show that this minimal van der Waals (vdW) assembly benefits both from the capping properties of hexagonal boron nitride (hBN) and from the emission filtering effect of graphene, leading to the absence of photoinduced doping combined with single-line intrinsic excitonic emission with linewidths as low as 1.6 meV at 14 K
Summary
Semiconducting transition metal dichalcogenides (TMDs) are layered crystals that exhibit a unique set of optical and electronic properties. Since the Dirac point of graphene lies within the bandgap of Mo- and W-based TMDs, the graphene layer acts as a broadband acceptor of electrons and holes for moderate doping levels below ∼1013 cm−2 [23,24,25], resulting in a complete neutralization of the TMD in the dark through static charge transfer [26] The combination of these two filtering effects produces single-line PL spectra arising from the radiative recombination of neutral excitons [22]. In this paper, using a combination of PL and Raman spectroscopies, we investigate an hBNsupported MoSe2 monolayer capped by a graphene flake containing monolayer (1LG) and bilayer domains (2LG) We show that this minimal vdW assembly benefits both from the capping properties of hBN and from the emission filtering effect of graphene, leading to the absence of photoinduced doping combined with single-line intrinsic excitonic emission with linewidths as low as 1.6 meV at 14 K. These results are promising considering the small valley contrasts reported in MoSe2 samples [36, 37]
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