Graphene as an integrated optical filter for two-dimensional semiconductors
Physicists from the University of Strasbourg and CNRS (France), in collaboration with INSA Toulouse (France) and NIMS (Japan) have demonstrated 2D luminescent systems, emitting one single, bright and spectrally narrow line that can readily be implemented in nanoscale opto-electronic devices.
Since the discovery of graphene in 2004 and its celebration by the Nobel Prize in Physics in 2010, the vast family of two-dimensional (2D) materials has kept fascinating the scientific community and offers many prospects for the realisation of transistors, lasers, photodetectors and sensors with reduced dimensions.
2D materials are crystalline sheets with a thickness of only one or a few atoms, whose physical properties, both unique and complementary, differ drastically from those of a stack of several sheets. By coupling several sheets with distinct structures (a 2D stack known as a van der Waals heterostructure), it becomes possible to observe a multitude of unexpected physical phenomena and to control them within innovative devices.
Transition metal dichalcogenides (TMD, e.g., MoS2, WS2) are among the most popular 2D materials. These 2D semiconductors interact strongly with light and emit an intense photoluminescence signal. As such, they are at the heart of photonic and optoelectronic devices based on 2D materials. In practice, these device-oriented developments come up against two obstacles: First, the light emission spectra of TMD contain several "peaks", corresponding to physical phenomena that are particularly challenging to decipher. In addition, it is difficult to make good electrical contacts on TMDs.
By coupling a TMD monolayer to a graphene monolayer, our team at the University of Strasbourg (with PhD students Etienne Lorchat and Luis Parra López), in close collaboration with our colleagues at INSA Toulouse and NIMS (Japan), has demonstrated that graphene acts as a narrow and selective optical filter, capable of radically "cleaning" the emission spectrum of TMD, whose emission becomes exclusively intrinsic . These results take on an additional "dimension" if one remembers that graphene is a quasi-transparent electrode with excellent electronic mobility. The TMD/graphene heterostructures are then emerging as 2D opto-electronic building blocks, which can be exploited to produce electrically-driven light-emitting devices with an emission rate approaching one terahertz (1012 Hz) and with an efficiency (number of photons emitted divided by the number of excited states formed) exceeding 50 %.
In practice, we have fabricated van der Waals heterostructures from bulk crystals of TMD and graphite, using micro manipulation and transfer of selected monolayers (see Figure 1). Then, we studied the optically induced light emission (photoluminescence, PL) of these samples at variable temperature in an optical cryostat. Under laser illumination, the electrons in the TMD are excited, leading to the formation of excitons, i.e., quasi-particles that are analogous to the hydrogen atom in condensed matter systems. Several types of excitons can coexist and the dynamics associated with their formation and recombination is very complex. In the presence of graphene, two important phenomena occur:
1) The TMD, which usually exhibits unintentional doping by resident charge carriers, is completely neutralized by graphene and the light emission features due to the recombination of charged excitons vanish.
2) The presence of graphene gives rise to a very fast non-radiative exciton transfer (in a few picoseconds) that bypasses the radiative recombination of all excitonic species endowed with sufficiently long lifetimes. This is where the specificity of TMD comes into play: the radiative lifetime of their intrinsic neutral excitons is exceptionally short (about 2 ps) compared to other excitonic states, and more broadly compared to all excitons in other conventional semiconductors. Thus, neutral intrinsic excitons continue to emit light efficiently, while the emission from all other excitons is drastically inhibited due to non-radiative transfer to graphene.
Figure 1 – Photoluminescence spectra of a Tungsten diselenide (WS2) monolayer in the absence (top) or presence (bottom) of a graphene monolayer (Gr). Here, the sample is encapsulated by thin layers of hexagonal boron nitride (BN, see crystal structures in the two panels). Measurements were carried out at 15 K.
All in all, graphene acts as an atomically-thin integrated filter that is naturally tuned to the intrinsic excitonic line of the TMD, as clearly shown in Figure 1. This important result is supported by a detailed microscopic study of the exciton dynamics in these heterostructures, which sheds new light on the microscopic exciton transfer mechanisms taking place at atomically sharp 2D heterointerfaces.
While the uncovered filtering effect operates very efficiently up to room temperature , it is accompanied by a decrease in luminescence intensity when the temperature exceeds 100 K (a temperature already higher than that of liquid nitrogen) and this question will have to be addressed in view of opto-electronic applications. Nevertheless, this study also shows that, protected by graphene, a TMD is exceptionally "photostable" and sustains larger exciton densities than a “bare” TMD monolayer. It is therefore possible to engineer TMD/graphene heterostructures as bright as bare TMD monolayers even at room temperature.
Such heterostructures also exhibit high degrees of valley polarization and coherence, as we recently observed  and therefore hold promise for applications in chiral optics and opto-valleytronics .
 Filtering the photoluminescence spectra of atomically thin semiconductors with graphene. E. Lorchat, L. E. Parra López, C. Robert, D. Lagarde, G. Froehlicher, T. Taniguchi, K. Watanabe, X. Marie, S. Berciaud. Nature Nanotechnology, (2020) doi: 10.1038/s41565-020-0644-2
 Charge versus energy transfer in atomically-thin graphene-transition metal dichalcogenide van der Waals heterostructures. G. Froehlicher, E. Lorchat, S. Berciaud. Phys. Rev. X. 8, 011007 (2018)
 Room-temperature valley polarization and coherence in transition metal dichalcogenide-graphene van der Waals Heterostructures. E. Lorchat, S. Azzini, T. Chervy, T. Taniguchi, K. Watanabe, T. W Ebbesen, C. Genet, S. Berciaud. ACS Photonics 5, 5047 (2018)
 Room Temperature Chiral Coupling of Valley Excitons with Spin-Momentum Locked Surface Plasmons. T. Chervy, S. Azzini, E. Lorchat, S. Wang, Y. Gorodetski, J. A. Hutchison, S. Berciaud, T. W. Ebbesen, C. Genet. ACS Photonics 5, 1281 (2018)