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Luis Enrique Parra López, Loïc Moczko, Joanna Wolff, Aditya Singh, Etienne Lorchat, Michelangelo Romeo, [Takashi Taniguchi](https://orcid.org/0000-0002-1467-3105), [Kenji Watanabe](https://orcid.org/0000-0003-3701-8119), Stéphane Berciaud

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[Single- and narrow-line photoluminescence in a boron nitride-supported MoSe<math>  <msub><mrow></mrow> <mn>2</mn> </msub></math>/graphene heterostructure](https://mdr.nims.go.jp/datasets/d452816a-2a90-4fdf-ade6-73fc9d192d02)

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Single- and narrow-line photoluminescence in a boron nitride-supported MoSe2/graphene heterostructureComptes RendusPhysiqueLuis Enrique Parra López, Loïc Moczko, Joanna Wolff, Aditya Singh,Etienne Lorchat, Michelangelo Romeo, Takashi Taniguchi, KenjiWatanabe and Stéphane BerciaudSingle- and narrow-line photoluminescence in a boronnitride-supported MoSe2/graphene heterostructureVolume 22, issue S4 (2021), p. 77-88<https://doi.org/10.5802/crphys.58>Part of the Special Issue: Recent advances in 2D material physicsGuest editors: Xavier Marie (INSA Toulouse, Université Toulouse III Paul Sabatier,CNRS, France) and Johann Coraux (Institut Néel, Université Grenoble Alpes,CNRS, France)© Académie des sciences, Paris and the authors, 2021.Some rights reserved.This article is licensed under theCreative Commons Attribution 4.0 International License.http://creativecommons.org/licenses/by/4.0/Les Comptes Rendus. Physique sont membres duCentre Mersenne pour l’édition scientifique ouvertewww.centre-mersenne.orghttps://doi.org/10.5802/crphys.58http://creativecommons.org/licenses/by/4.0/https://www.centre-mersenne.orghttps://www.centre-mersenne.orgComptes RendusPhysique2021, 22, n S4, p. 77-88https://doi.org/10.5802/crphys.58Recent advances in 2D material physics / Physique des matériauxbidimensionnelsSingle- and narrow-line photoluminescencein a boron nitride-supported MoSe2/grapheneheterostructureLuis Enrique Parra Lópeza, Loïc Moczkoa, JoannaWolffa, Aditya Singha, b,Etienne Lorchata, Michelangelo Romeoa, Takashi Taniguchic,Kenji Watanabed and Stéphane Berciaud ∗, a, ea Université de Strasbourg, CNRS, Institut de Physique et Chimie des Matériaux deStrasbourg (IPCMS), UMR 7504, F-67000 Strasbourg, Franceb Department of Physics, Indian Institute of Technology Delhi, 110016, New Delhi,Indiac International Center for Materials Nanoarchitectonics, National Institute forMaterials Science, 1-1 Namiki, Tsukuba 305-0044, Japand Research Center for Functional Materials, National Institute for Materials Science,1-1 Namiki, Tsukuba 305-0044, Japane Institut Universitaire de France, 1 rue Descartes, 75231 Paris cedex 05, FranceE-mails: luis.parralopez@ipcms.unistra.fr (L. E. Parra López),loic.moczko@ipcms.unistra.fr (L. Moczko), joanna.wolff@etu.unistra.fr (J. Wolff),phz168205@iitd.ac.in (A. Singh), etienne.lorchat@gmail.com (E. Lorchat),michelangelo.romeo@ipcms.unistra.fr (M. Romeo), taniguchi.takashi@nims.go.jp(T. Taniguchi), WATANABE.Kenji.AML@nims.go.jp (K. Watanabe),stephane.berciaud@ipcms.unistra.fr (S. Berciaud)Abstract. Heterostructures made from van der Waals (vdW) materials provide a template to investigate awealth 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 metaldichalcogenides (TMD) have recently attracted interest to design model 2D “donor–acceptor” systems andnew optoelectronic components. Here, using Raman scattering and photoluminescence spectroscopies, wereport a comprehensive characterization of a molybedenum diselenide (MoSe2) monolayer deposited ontohexagonal boron nitride (hBN) and capped by mono- and bilayer graphene. Along with the atomically flathBN susbstrate, a single graphene epilayer is sufficient to passivate the MoSe2 layer and provides a homoge-nous environment without the need for an extra capping layer. As a result, we do not observe photo-induceddoping in our heterostructure and the MoSe2 excitonic linewidth gets as narrow as 1.6 meV, approachingthe homogeneous limit. The semi-metallic graphene layer neutralizes the 2D semiconductor and enables pi-cosecond 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∗Corresponding author.ISSN (electronic) : 1878-1535 https://comptes-rendus.academie-sciences.fr/physique/https://doi.org/10.5802/crphys.58https://orcid.org/0000-0002-5753-3671mailto:luis.parralopez@ipcms.unistra.frmailto:loic.moczko@ipcms.unistra.frmailto:joanna.wolff@etu.unistra.frmailto:phz168205@iitd.ac.inmailto:etienne.lorchat@gmail.commailto:michelangelo.romeo@ipcms.unistra.frmailto:taniguchi.takashi@nims.go.jpmailto:WATANABE.Kenji.AML@nims.go.jpmailto:stephane.berciaud@ipcms.unistra.frhttps://comptes-rendus.academie-sciences.fr/physique/78 Luis Enrique Parra López et al.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 quenchedby a factor of 3.3± 1 and 4.4± 1 in the presence of mono- and bilayer graphene, respectively. Finally, whileour bare MoSe2 on hBN exhibits negligible valley polarization at low temperature and under near-resonantexcitation, we show that interfacing MoSe2 with graphene yields a single-line emitter with degrees of valleypolarization and coherence up to ∼ 15 %.Keywords. van der Waals heterostructures, Transition metal dichalcogenides, Graphene, Energy transfer,Excitons, Optoelectronics, Valleytronics.Available online 18th March 20211. IntroductionSemiconducting transition metal dichalcogenides (TMDs) are layered crystals that exhibit aunique set of optical and electronic properties. In particular, TMDs undergo an indirect-to-directbandgap transition when thinned down to the monolayer limit [1,2]. Enhanced Coulomb interac-tions combined with reduced dielectric screening in two dimensions endow mono and few-layerTMDs with room-temperature stable excitonic manifolds [3, 4]. In addition, TMD monolayerspossess a valley pseudo-spin degree of freedom that can be manipulated using schemes inspiredby decades of developments in optically-controlled spin dynamics [5, 6]. Moreover, the relativelyeasy isolation of TMD monolayers layers and their stacking with partner layered materials in vander Waals (vdW) heterostructures offer opportunities to tune their properties and discover newelectronic and optical phenomena [7], as well as new proximity effects [8, 9] at atomically-sharpheterointerfaces. These fundamental properties can potentially be harnessed for an emerginggeneration of optoelectronic [7, 10] and valleytronic systems [11, 12].A pivotal example are TMD/graphene vdW heterostructures. Indeed, combining the semi-metallic and optically transparent character of graphene [13, 14] with the unique optical prop-erties of TMDs [3] yields a platform for light-emitting systems which, as opposed to bare TMDmonolayers, retain a high degree of valley polarization and, importantly, of valley coherence upto room temperature [15]. The reason is that there is an efficient non-radiative transfer of pho-toexcited carriers and excitons from the TMD to graphene [16–19]. This coupling shortens theexcitonic lifetime down to the picosecond range and may thus significantly quench photolumi-nescence (PL), in particular at room temperature [16–18], where the effective excitonic lifetimeof a bare TMD monolayer can exceed one nanosecond [17]. An interesting situation arises at lowtemperature, where the radiative lifetime of the neutral exciton [20, 21] is of the same order asthe exciton transfer time towards graphene [22]. In this case, neutral exciton emission is mini-mally quenched. In contrast, non-radiative transfer remains sufficiently fast to massively quenchemission from all the other long-lived excitonic species (including charged excitons, spin-darkexcitons, localized excitons, exciton-phonon replica, . . . [3]). Moreover, since the Dirac point ofgraphene lies within the bandgap of Mo- and W-based TMDs, the graphene layer acts as a broad-band acceptor of electrons and holes for moderate doping levels below ∼1013 cm−2 [23–25], re-sulting 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 theradiative recombination of neutral excitons [22].Beyond these general considerations, the photophysics of 2D materials and a fortiori ofTMD/graphene heterostructures subtly depends on extrinsic factors and it is therefore neces-sary to carefully control their local chemical and dielectric environment. For instance, inves-tigations in high vacuum prevents physisorption of water and organic molecules. Besides, itis known that conventional transparent susbtrates such as SiO2 can host trapped charges andC. R. Physique — 2021, 22, n S4, 77-88Luis Enrique Parra López et al. 79favor photoinduced doping [17, 27–29]. A workaround is to use a more inert material such ashexagonal boron nitride (hBN). Indeed, hBN has proven to be an invaluable vdW material eversince its introduction as a dielectric substrate to reveal the intrinsic electron transport propertiesof graphene [30]. Its atomically flat nature and optical transparency provides a smooth, homo-geneous environment that preserves the intrinsic optical features of TMDs. A direct manifesta-tion is the narrowing of the emission lines in hBN-capped samples [31, 32], with linewidths near1 meV that approach the homogeneous limit at cryogenic temperatures [21, 33–35].In this paper, using a combination of PL and Raman spectroscopies, we investigate an hBN-supported MoSe2 monolayer capped by a graphene flake containing monolayer (1LG) and bi-layer domains (2LG). We show that this minimal vdW assembly benefits both from the cappingproperties of hBN and from the emission filtering effect of graphene, leading to the absence ofphotoinduced doping combined with single-line intrinsic excitonic emission with linewidths aslow as 1.6 meV at 14 K. In addition, we measure finite degrees of valley polarization and, impor-tantly, of valley coherence up to ∼15% at cryogenic temperatures. These results are promisingconsidering the particularly small valley contrasts reported in MoSe2 samples [36, 37].2. Optical characterization at room temperatureFigure 1(a) shows an optical image of our hBN-supported MoSe2/graphene sample depositedonto a Si/SiO2 substrate. The side view of the hBN/MoSe2 and hBN/MoSe2/1LG are sketched inFigure 1(b) and typical room temperature PL spectra these two systems are shown in Figure 1(c).The main PL peak slightly below 1.6 eV arises from the recombination of the lowest lying opticallyactive MoSe2 exciton (X0, also known as the A-exciton) possibly with a minor contributioncoming from the trion (charged exciton, X∗) [38]. The photophysics of this heterostructure isdriven by the efficient energy transfer from the thermalized excitonic population towards thegraphene layer, leading to strong PL quenching by about two orders of magnitude (see scalingfactor in Figure 1(c)). As an indirect consequence of the picosecond lifetime of the band-edgeexciton, we observe a broad, high energy shoulder on the MoSe2/1LG PL spectrum that isassigned to hot luminescence from excited excitonic states, including Rydberg-like states fromfrom the A-exciton manifold as well as the B-exciton (located near 190 meV above the band-edgeA-exciton) [17]. No sizeable hot PL signal is detected in the linear regime in the hBN-supportedMoSe2 reference.We now investigate the possibility of photoinduced charge transfer (i.e., photodoping) inthe hBN/MoSe2/1LG region of our sample. As indicated above, band offsets between TMD andgraphene allow electron and hole transfer from the TMD to graphene. Besides this intrinsic phe-nomenon, the presence of surface traps and molecular adsorbates (either on the substrate or onthe 2D material) [27, 28], as well as chalcogen vacancies and other local defects in TMDs [39]can facilitate charge transfer and alter exciton dynamics. Under light illumination above theTMD bandgap, a net photoinduced charge transfer to graphene has been observed on SiO2-supported samples on timescales that are orders of magnitude longer than the TMD excitoniclifetime, hence with no sizeable effect on the PL intensity [17, 40, 41]. This photodoping leadsto a stationary Fermi level shift in graphene and complementary fingerprints of charge transferin the TMD monolayer [17, 29]. The Fermi level of graphene ultimately saturates as the photonflux increases [17, 29]. Although such extrinsic effects could be of practical use for photodetec-tion [40, 41], they may hamper access to the intrinsic photophysics of the heterostructure. In oursample, the bottom hBN flake provides a flat substrate that passivates the MoSe2/graphene het-erostructure and could minimize extrinsic photodoping. Quantitative insights into photodopingcan be obtained using micro-Raman spectroscopy [42] of the graphene flake.C. R. Physique — 2021, 22, n S4, 77-8880 Luis Enrique Parra López et al.Figure 1. (a) Optical image of a hBN-supported MoSe2/graphene heterostructure de-posited onto an Si/SiO2 substrate. Three regions are contoured and correspond tohBN/MoSe2 (blue), hBN/MoSe22/1LG (orange) and hBN/MoSe2/2LG (red). (b) Sketch ofthe heterostructure showing the bare MoSe2 (top) and the MoSe2/1LG (bottom). (c) RoomTemperature photoluminescence spectra recorded on bare MoSe2 (top) and MoSe2/1LG(bottom). The full-width at half maximum ΓX0 of the main excitonic line is indicated. Bothspectra are normalized to a common value and the scaling factor between the two spec-tra allowing an estimation of the room temperature PL quenching factor is indicated in thelower panel. The laser wavelength is 532 nm and the laser intensity used in this experimentis near 1 µW/µm2.Indeed, the presence of charge carriers in 1LG leads to well-documented changes in the fre-quency (ωG), full-width at half maximum (FWHM, ΓG) and intensity of its one-phonon G-mode(near 1582 cm−1) due to the non-adiabatic renormalization of the Kohn anomaly at the Γ point.Under moderate doping below 1013 cm−2, this effect leads to an electron–hole symmetric up-shift of ωG accompanied by a reduction of ΓG arising from the suppression of Landau damp-ing [43–45]. Similar, albeit less prominent effects are also observable in 2LG [46]. Contributionsfrom doping can be disentangled from other perturbations, in particular due to built-in or ap-plied strain, by inspecting the correlation between ωG and the frequency ω2D of the 2D mode(near 2690 cm−1, involving a pair of near-zone edge phonons with opposite momenta [42]). Theslope ∂ω2D/∂ωG is near 2.2 under biaxial strain, directly reflecting the values of the Grüneisenparameters 2D- and G-mode phonons [47–49] but is significantly smaller than 1 in the case ofhole or electron doping (Figure 2(a)) because the 2D-mode phonons have momenta significantlyaway from the Kohn anomaly at the zone edge (K and K′ points) [45, 50].Figure 2(a) shows the correlation between ωG and ω2D extracted from a Raman map of themonolayer MoSe2/1LG area of our sample (see Figure 1(a)). Clearly, ω2D and ωG follow a linearcorrelation with a slope near 2.2 (Figure 2(a)), suggesting native compressive strain (i.e., stiffeningof the Raman modes compared to the undoped/unstrained reference indicated in Figure 2(a))and ruling out sizable spatially inhomogeneous residual doping. The native strain level can be aslarge as 0.05% (if one assumes biaxial strain [49]) and the existence of a strain gradient likelyarises from the stacking process. Figure 2(b) shows the evolution of the Raman spectra as afunction of the incident laser intensity from 10 µW/µm2 up to 1 mW/µm2. We observe that ωGdoes not change appreciably and instead remains at 1582.2 ± 0.5 cm−1. Similarly, ΓG remainsconstant around ≈14 ± 1 cm−1 as it can be seen in Figure 2(c). These values are typical forquasi-neutral graphene with a residual charge density of at most a few 1011 cm−2 [45]. Hence,our observations strongly support the hypothesis that the hBN flake shields the heterostructureC. R. Physique — 2021, 22, n S4, 77-88Luis Enrique Parra López et al. 81Figure 2. (a) Correlation between ω2D and ωG extracted from a Raman map of the regionboxed in Figure 1(a). The dashed, short-dashed and solid gray lines indicate the correlationsexpected under biaxial strain, hole and electron doping, respectively. These lines crossat (1582, 2686) cm−1, a point that corresponds to undoped graphene with a minimalamount of built-in compressive strain below 10−4. Our measurements line up on the strainline, demonstrating the existence of compressive strain gradient on the graphene layer.(b) Typical Raman spectra taken with increasing, color coded, incident laser intensity.(c) Correlation between ΓG and ωG under increasing incident laser intensity (color codedsymbols, as in (b)). The solid gray line is the theoretically predicted correlation expected inthe presence of doping using the model in Ref. [43]. All measurements were performed inambient conditions under laser excitation at 532 nm.from substrate-induced electron redistribution and photo-doping, yielding a pristine system toinvestigate exciton dynamics.3. Low temperature hyperspectral PL mappingFigure 3(a) shows typical low temperature (14 K) PL spectra recorded on selected spots of oursample indicated in the PL intensity map shown in Figure 3(b). An hyperspectral PL mappingstudy is shown in Figure 3(c)–(g). On the MoSe2/hBN area, the PL spectrum is dominated bytwo main peaks located at 1.645 ± 0.002 eV and 1.616 ± 0.003 eV (Figure 3(c, d)). These peakshave Voigt profiles and originate from the recombination of the neutral exciton (X0) and the trion(X∗), respectively [38]. In contrast, in the 1LG and 2LG-capped MoSe2 regions, the PL spectradisplay single, narrower quasi-Lorentzian emission lines (Figure 3(c, e)). As shown in Figure 3(a,c, d), these peaks are located at 1.636± 0.002 eV for hBN/MoSe2/1LG and 1.633± 0.003 eV forhBN/MoSe2/2LG, respectively. These values are slightly redshifted by 9 and 12 meV with respectto the bare MoSe2 reference, respectively, due to dielectric screening [17, 51].Considering the simplicity of the PL-spectrum of bright TMDs such as MoSe2 [3,38], the single-line character of the MoSe2/graphene PL spectra is essentially a consequence of the completeneutralization of the MoSe2 monolayer. As confirmed by Figure 3(a, f), the integrated PL intensitynear the expected location of the X∗ feature is vanishingly small compared to that of X0 line (IX0 )over all the 1LG and 2LG-capped MoSe2 area.The observed PL quenching, however, is a consequence of non-radiative energy transfer tographene, stemming predominantly from hot excitons (either finite momentum X0 excitons re-siding outside the light cone or excitons occupying higher-order optically active states), with asmaller contribution from cold X0 excitons in the light cone [22]. As a result, the selected PL spec-tra in Figure 3(a) and total PL intensity map (Itot, Figure 3(b)) reveal sizeable reduction of Itoton the MoSe2/1LG and MoSe2/2LG regions. However, if we consider the quenching factor of theC. R. Physique — 2021, 22, n S4, 77-8882 Luis Enrique Parra López et al.Figure 3. Low-temperature photoluminescence of the MoSe2/graphene van der Waalsheterostructure shown in Figure 1(a). (a) From top to bottom, selected PL spectra taken inhBN-supported MoSe2 (blue), MoSe2/1LG (orange) and MoSe2/2LG (red) at spots indicatedin the PL intensity map shown in (b). Dashed red lines in (a) are Lorentzian fits to thedata (filled circles). The integrated intensity of the X0 line is indicated in each case. Blue,orange and red contours in (b), (also in d–g) indicate the MoSe2, 1LG and 2LG flakes,respectively. (c) Correlation between ΓX0 and EX0 extracted from the hyperspectral maps ofthe (d) neutral exciton emission energy EX0 , (e) full-width at half maximum ΓX0 , extractedfrom Voigt (bare MoSe2) and Lorentzian (MoSe2/1LG and MoSe2/2LG) fits. The averagevalues in each zone are shown with their respective standard deviations following the colorcode in (a, b). (f) Map of the trion-to-exciton integrated intensity ratio IX∗/IX0 . (g) Map ofthe PL quenching factor of the X0 line. This factor, denoted QX0 , is defined relative to thespatially averaged value of IX0 over the MoSe2 flake shown on the lower part of the map.The data in this figure were recorded at 14 K under continuous wave laser illumination at633 nm with a laser intensity near 100 µW/µm2.neutral exciton line X0 only (denoted QX0 , see Figure 3(g)), we observe a moderate QX0 = 3.3±1 onMoSe2/1LG and a slightly larger value of 4.4±1 in the MoSe2/2LG, in qualitative agreement withprevious studies of energy transfer from individual emitters to mono and few-layer graphene [52].The reduced quenching efficiency compared to the massive quenching observed at room tem-perature stems from the picosecond X0 radiative lifetime at low temperatures [20, 21], which iscomparable to the energy transfer time to graphene (see Ref. [22] for details).Our spatially-resolved PL study also reveals narrow X0 lines with spatially averaged FWHM(ΓX0 ) of 4± 2 meV and 3± 1 meV, on the MoSe2/1LG and MoSe2/2LG areas, respectively (Fig-ure 3(c, e)). Interestingly, ΓX0 gets as narrow as 1.6 meV in selected locations (see Figure 3(e)), avalue that, even if it exceeds the homogeneous limit (0.3 meV for an estimated X0 lifetime of 2 ps)C. R. Physique — 2021, 22, n S4, 77-88Luis Enrique Parra López et al. 83Figure 4. Photoluminescence spectra of hBN/MoSe2/2LG recorded under low and highlaser intensity, near 1 µW/µm2 and 1 mW/µm2 in red and grey, respectively. Both spectrawere recorded under cw excitation at 633 nm with the same integration time. The lowintensity spectrum has been multiplied by a factor of 1000 for comparison. A fit of the datareveals linewidths (FWHM) of ΓX0 = 1.8 and 2.6 meV at low and high intensity, respectivelyand an integrated intensity ratio of the exciton line very near 1000, warranting that thesample remains in the linear regime under intense cw excitation. The inset shows the lowenergy side of the spectra and evidences a faint feature assigned to a photoinduced trionX∗ (see Ref. [22] for details). The data were recorded under cw excitation at 633 nm.suggests that our simple sample architecture warrants reduced inhomogeneous broadening anddephasing, with values that are close to state of the art hBN-encapsulated MoSe2 samples [21,35].Figure 3(c) presents the correlation between ΓX0 and EX0 in each region of the sample, extractedfrom the maps in Figure 3(d, e). As expected, the presence of a second graphene layer improvescapping and further screens the MoSe2 excitons.Let us note that although our sample is spatially homogeneous over domains as large as50 µm2, we observe more scattered EX0 , larger ΓX0 and residual trion emission (Figure 3(d–g))near the boundaries of the 1LG and 2LG domains and also near localized micro-sized spots of oursample. During the stacking process, organic residues tend to segregate and form bubbles whichlocally decouple the TMD and graphene layers leading to complex spectra that reflect spatialinhomogeneities.To close this section, let us comment on photoinduced doping at low-temperature. As indi-cated in Ref. [22], TMD/graphene heterostructures are extremely photostable, in stark contrastwith bare TMD monolayers. By illuminating our hBN/MoSe2/2LG sample at high laser inten-sity (∼1 mW/µm2), a dim photo-induced trion feature with a reduced binding energy of 23 meVemerges, with an integrated intensity nearly three orders of magnitude smaller than that of theX0 line (Figure 4). These results indicate that the MoSe2 layer coupled to graphene remains es-sentially neutral under intense cw illumination, and that one can safely neglect photo-induceddoping, including at cryogenic temperatures.C. R. Physique — 2021, 22, n S4, 77-8884 Luis Enrique Parra López et al.Figure 5. Low-temperature polarization-resolved photoluminescence spectra in hBN-supported MoSe2 (a, b), and MoSe2/1LG (c, d) and MoSe2/2LG (e, f). Measurements weretaken at 9 K and under optical excitation at 60 meV above the X0 line with both circularly(σ±) and linearly (x, y ,) polarized laser excitation. Spectra in solid lines (resp. thin solid lines+ symbols) correspond to cross-polarized (resp. co-polarized) incoming and emitted pho-tons. The associated degrees of linear polarization (ρ, corresponding to the degree of valleypolarization, in a, c, e) and circular polarization (γ, corresponding to the degree of valleycoherence, in b, d, f) are indicated. The laser intensity is near 50 µW/µm2 and we have ver-ified that all regions of the sample were excited in the linear regime. The spectra are nor-malized for clarity and the quenching factors of the X0 line are QX0 = 1.8±0.1 and 2.0±0.1,respectively.4. Valley polarization and valley coherenceWe finally address the valley contrasting properties of our sample. TMD/graphene heterostruc-tures have recently been introduced as chiral emitters with large degrees of valley polarization (ρ)and coherence (γ) up to room-temperature. Indeed, picosecond energy transfer between TMDand graphene allows the excitonic population to recombine before undergoing intervalley scat-tering and dephasing processes [15,53]. In addition, graphene, as a capping material, reduces dis-order, spatial inhomogeneities and dephasing, which explains the large room temperature valueof γ≈ 20% recently reported in hBN-encapsulated WS2/1LG heterostructures [15]. Although im-proved valley contrasts come at the expense of strong PL quenching at room temperature, we ex-pect, in keeping with the discussion above, that sizeable values of ρ and γ combined with largeX0 emission yields can be achieved at lower temperatures.Figure 5 shows polarization-resolved PL spectra recorded using a circular and a linear basis,as well as the associated degrees of circular and linear polarization, which are directly yielding ρand γ, respectively. For the X0 emission line, we get ρ ≈ 0%, 4±4% and 13±3% and γ≈ 0%, 10±5%and 15±4% for hBN-supported MoSe2, MoSe2/1LG and MoSe2/2LG, respectively.Near-zero values of ρ have been reported in MoSe2 monolayers [36]. Since the low tempera-ture X0 lifetimes in bare MoSe2 and in MoSe2/graphene heterostructures are similar [21, 22], theobservation of modest values of ρ in MoSe2/graphene is not surprising. Noteworthy, γ signifi-C. R. Physique — 2021, 22, n S4, 77-88Luis Enrique Parra López et al. 85cantly exceeds ρ in MoSe2/1LG and MoSe2/2LG, suggesting significantly reduced dephasing inencapsulated samples, as recently observed in MoS2 monolayers fully encapsulated in hBN [31]and in hBN-capped WS2/1LG [15]. Here, our measurements are performed at 60 meV above theX0 line. We anticipate larger values of ρ and γ as we excite MoSe2/graphene samples closer to theX0 resonance. In addition, one may also optimize the sample geometry as in Refs [21,35] in orderto minimize the radiative lifetime of the MoSe2 monolayer, and hence optimize ρ and γ.5. Conclusion and perspectivesWe have demonstrated that hBN-supported TMD/graphene heterostructures offer an excellenttemplate to study electron redistribution and excitonic energy transfer in two dimensions, withdonor–acceptor distances in the sub-nanometer range. The absence of photodoping in hBN-supported samples as compared to SiO2-supported systems adds on to the merits of hBN as anideal dielectrics for van der Waals assembly. Our study also establishes that bright TMD emissionwith linewidths approaching the homogenous limit can be achieved over large areas using onlyone epilayer of graphene, hence without the need of an hBN capping layer. In this manner, onedoes not only benefit from the emission filtering properties of graphene but also from its assetsas a capping van der Waals material.In spite of recent advances [17–19, 22, 54] the microscopic details of the transfer mechanismare still under investigation. Focusing for instance on energy transfer, Dexter- and Förster-typemechanisms depend sensitively on the distance between the layers and on the local environ-ment. Investigations of the transfer efficiency while finely tuning the TMD-graphene distanceusing hBN spacers and as a function of the number of graphene layers should bring decisive in-sights into near-field coupling in van der Waals materials.Our work also holds promise for device-oriented research. Indeed, the absence of a top hBNlayer allows us to directly contact the graphene electrode for photodetectors and light-emittingdevices, as well as for local investigations, e.g., using scanning tunnelling microscopy [55].Conversely, TMDs have recently been shown to outperform hBN as a capping material, yieldingrecord-high room temperature electron mobility in graphene [56].Finally, a challenging yet appealing perspective would consist in exploiting low-energy surfaceplasmon polaritons in graphene to tailor light-matter interactions in near-field coupled two-dimensional semiconductors [57], including in quantum wells made from the latter [58].6. Experimental detailsOur van der Waals heterstructure was prepared using standard micromechanical exfoliation anddry transfer methods, as in Ref. [59]. The hBN flake was mechanically exfoliated on top of thesubstrate. A MoSe2 monolayer and a single- and bilayer graphene flake were mechanically ex-foliated from bulk crystals and then stacked on top of the hBN flake. The number of layers ofthe flakes were verified using room-temperature Raman and PL spectroscopy. PL and Ramanscattering measurements in Figures 1 and 2, respectively, were performed in ambient air using a532 nm diode pumped solid state laser and a home-built scanning confocal setup. Low temper-ature PL studies were performed in a continuous-flow liquid helium cryostat mounted onto ourRaman/PL microscope. All PL measurements in Figure 3 were recorded in the linear regime undercw excitation at 633 nm, i.e., slightly above the B exciton in MoSe2 [3, 17]. The low-temperaturepolarization-resolved measurements in Figure 5 were performed under near-resonant optical ex-citation 60 meV above the neutral exciton (X0) line using a tunable supercontinuum laser. Thedegree of valley polarization ρ = (Iσ+σ+ − Iσ+σ− )/(Iσ+σ+ + Iσ+σ− ) is measured in a circular ba-sis and the degree of valley coherence γ = (Ixx − Ix y )/(Ixx + Ix y ) is measured in a linear basis.C. R. Physique — 2021, 22, n S4, 77-8886 Luis Enrique Parra López et al.In these expressions, Ii j indicates the PL intensity detected in the “i ” polarization state whenupon excitation under “ j ” polarized light (i , j =σ± or x/y). A Linear polarizer and an achromaticquarter-waveplate were used to prepare the polarization state of the incoming beam. The emit-ted photons propagate through the same quarter-wave plate and are analyzed using a Wollastonprism, which permits to record simultaneously the co- and cross polarized PL signals. The valuesof ρ and γ were determined from fits of the X0 lines.AcknowledgementsThe authors thank X. Marie, C. Robert, D. Lagarde, S. Azzini, T. Chervy, C. Genet and G. Schull forfruitful discussions. We are grateful to Aditi Moghe and to the StNano clean room staff for techni-cal support. We acknowledge financial support from the Agence Nationale de la Recherche (un-der grants 2D-POEM ANR-18-ERC1-0009) and from the LabEx NIE (ANR-11-LABX-0058-NIE, un-der grant SPE2D). AS and SB acknowledge support for the Indo-French Centre for the Promotionof Advanced Research (CEFIPRA). KW and TT acknowledge support from the Elemental Strat-egy Initiative conducted by the MEXT, Japan, Grant Number JPMXP0112101001, JSPS KAKENHIGrant Number JP20H00354 and the CREST (JPMJCR15F3), JST.References[1] K. F. Mak, C. Lee, J. Hone, J. Shan, T. F. Heinz, “Atomically thin MoS2: a new direct-gap semiconductor”, Phys. Rev.Lett. 105 (2010), no. 13, article no. 136805.[2] A. Splendiani, L. Sun, Y. Zhang, T. Li, J. Kim, C.-Y. Chim, G. Galli, F. Wang, “Emerging photoluminescence inmonolayer MoS2”, Nano Lett. 10 (2010), no. 4, p. 1271-1275.[3] G. Wang, A. Chernikov, M. M. Glazov, T. F. Heinz, X. Marie, T. 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Physique — 2021, 22, n S4, 77-88https://arxiv.org/abs/1909.09523 1. Introduction 2. Optical characterization at room temperature 3. Low temperature hyperspectral PL mapping 4. Valley polarization and valley coherence 5. Conclusion and perspectives 6. Experimental details Acknowledgements References