Filtering the photoluminescence spectra of atomically thin semiconductors with graphene

Abstract

Atomically thin semiconductors made from transition metal dichalcogenides (TMDs) are model systems for investigations of strong light–matter interactions and applications in nanophotonics, optoelectronics and valleytronics. However, the photoluminescence spectra of TMD monolayers display a large number of features that are particularly challenging to decipher. On a practical level, monochromatic TMD-based emitters would be beneficial for low-dimensional devices, but this challenge is yet to be resolved. Here, we show that graphene, directly stacked onto TMD monolayers, enables single and narrow-line photoluminescence arising solely from TMD neutral excitons. This filtering effect stems from complete neutralization of the TMD by graphene, combined with selective non-radiative transfer of long-lived excitonic species to graphene. Our approach is applied to four tungsten- and molybdenum-based TMDs and establishes TMD/graphene heterostructures as a unique set of optoelectronic building blocks that are suitable for electroluminescent systems emitting visible and near-infrared photons at near THz rate with linewidths approaching the homogeneous limit.

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Fig. 1: Graphene as a versatile emission filter.
Fig. 2: Neutralizing an atomically thin semiconductor with graphene.
Fig. 3: Laser power-dependent PL in BN-capped TMD/graphene at 4 K.
Fig. 4: Low-temperature exciton dynamics.
Fig. 5: Evidence for hot exciton transfer to graphene.

Data availability

The data that support the plots within this paper and other findings of this study are available from the corresponding author upon reasonable request.

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Acknowledgements

We thank D. Basko, T. Galvani, L. Wirtz, G. Schull, S. Azzini, T. Chervy, C. Genet, M.A. Semina and M.M. Glazov for fruitful discussions. We are grateful to H. Majjad and M. Rastei for help with AFM measurements, to M. Romeo, F. Chevrier, M. Acosta, A. Boulard and the StNano clean room staff for technical support. We acknowledge financial support from the Agence Nationale de la Recherche (under grants H2DH ANR-15-CE24-0016, 2D-POEM ANR-18-ERC1-0009, D-vdW-Spin, VallEx and MagicValley), from LabEx NIE (grant no. ANR-11-LABX-0058-NIE) and from EUR NanoX (grant no. VWspin and MILO).

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Contributions

S.B. conceived and led the project, with C.R., D.L. and X.M. supervising the time-resolved PL measurements. E.L. and L.E.P.L. fabricated the samples. E.L., L.E.P.L., C.R., D.L. and S.B. carried out the measurements. E.L., L.E.P.L. and S.B. analysed the data, with input from G.F., C.R., D.L. and X.M. T.T. and K.W. provided high-quality hexagonal BN crystals. S.B. wrote the manuscript, with input from X.M., C.R., E.L. and L.E.P.L.

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Correspondence to Stéphane Berciaud.

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Extended data

Extended Data Fig. 1 Approaching the homogeneous limit in BN/MoSe2/graphene.

Photoluminescence (PL) spectra of a MoSe2 monolayer directly deposited on hexagonal boron nitride (BN) (top, blue line), partly covered by a single layer of graphene (1LG middle, orange) and by a bilayer of graphene (2LG, bottom, dark red). The X0 exciton energy, full-width at half-maximum (FWHM, denoted \({\Gamma }_{{{\rm{X}}}^{0}}\)) and integrated PL intensity (I\({}_{{{\rm{X}}}^{0}}\), in arbitrary units) are indicated. The spectra were recorded at T=15 K with laser photon energy of 2.33 eV. We observe nearly identical \({\Gamma }_{{{\rm{X}}}^{0}}\), as small as 1.9 meV and 2.0 meV in the 1LG/MoSe2/BN and 2LG/MoSe2/BN, respectively. We find that I\({}_{{{\rm{X}}}^{0}}\) is only quenched by a factor 2.2 (resp. 3.0) in 1LG/MoSe2/BN (resp. 2LG/MoSe2/BN) with respect to the vacuum/MoSe2/BN reference. These results demonstrate that minimal X0 PL quenching and PL linewidths approaching the homogeneous limit can be achieved in MoSe2/graphene heterostructures without the need for an extra BN top layer. This figure appears as supplementary Fig. 2 in the supplementary information file.

Extended Data Fig. 2 Trion-free photoluminescence spectra at room temperature.

PL spectra of BN-capped TMD/graphene heterostructures compared to those of a nearby BN-capped TMD reference, all recorded in ambient air in the linear regime under continuous-wave laser excitation at 2.33 eV. The X0 PL lines are symmetric in the TMD/graphene heterostructures whereas they exhibit a lower-energy shoulder arising predominantly from trions (X) in the TMD references. The scaling factors allow estimating the large room temperature PL quenching factors that strongly contrast with the low X0 PL quenching factors observed at cryogenic temperatures (see main text and supplementary Table 1). The X0 lines are slightly redshifted in TMD/graphene, as discussed in the main text and supplementary Section 4. The red lines are multi-Lorentzian fits to the data, with their different components shown with grey dashed lines. Hot luminescence from excited excitonic states (for example, X\({}_{2{\rm{s}}}^{0}\) and B excitons) is clearly visible in MoS2/graphene and MoSe2/graphene. This figure appears as supplementary Fig. 7 in the supplementary information file.

Extended Data Fig. 3 Photostability and neutrality under high photon flux at room temperature.

(a) Laser power-dependent photoluminescence spectra of a BN-capped WS2/graphene heterostructure compared to a nearby BN-capped WS2 reference, recorded in ambient air using continuous-wave laser excitation at 2.33 eV. The spectra are shown on a semilogarithmic scale and are normalised by the incoming photon flux (Φph) and the integration time. Φph is color-coded with a gradient ranging from dark red (low Φph ~ 100 nW/μm2 or equivalently ~ 3 × 1019cm2s−1) to yellow (high Φph ~ 1mW∕μm2 or equivalently ~ 3 × 1023cm2s−1). PL saturation due to exciton–exciton annihilation is clearly visible in WS2, whereas a quasi linear scaling is observed in WS2/graphene. The PL spectra in WS2 remain quasi symmetric even under high Φph, while the PL spectra from the TMD reference exhibit a lower-energy shoulder, assigned to trion (X) emission. The latter grows significantly as Φph increases and ultimately overcomes the X0 line, as shown in (b) on the selected spectra recorded at Φph ~ 2 × 1023cm2s−1 and plotted on a linear scale. This figure appears as supplementary Fig. 8 in the supplementary information file.

Supplementary information

Supplementary Information

Supplementary Figs. 1–14, Tables 1 and 2, Sections 1–9 and refs. 1–18.

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Lorchat, E., López, L.E.P., Robert, C. et al. Filtering the photoluminescence spectra of atomically thin semiconductors with graphene. Nat. Nanotechnol. 15, 283–288 (2020). https://doi.org/10.1038/s41565-020-0644-2

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