Charge versus energy transfer in atomically-thin graphene-transition metal dichalcogenide van der Waals heterostructures

TL;DR

This study investigates charge and energy transfer mechanisms in atomically thin graphene/TMD heterostructures, revealing that energy transfer dominates over charge transfer in influencing optoelectronic interactions.

Contribution

It introduces new experimental insights into interlayer coupling, demonstrating energy transfer as the primary mechanism in graphene/TMD heterostructures.

Findings

Photoluminescence is quenched by over two orders of magnitude.

Room temperature MoSe₂ exciton lifetime is shortened to about 1 ps.

Energy transfer dominates over charge transfer in interlayer coupling.

Abstract

Van der Waals heterostuctures, made from stacks of two-dimensional materials, exhibit unique light-matter interactions and are promising for novel optoelectronic devices. The performance of such devices is governed by near-field coupling through, e.g., interlayer charge and/or energy transfer. New concepts and experimental methodologies are needed to properly describe two-dimensional heterointerfaces. Here, we report on interlayer charge and energy transfer in atomically thin metal (graphene)/semiconductor (transition metal dichalcogenide (TMD, here MoSe$_2$)) heterostructures using a combination of photoluminescence and Raman scattering spectroscopies. The photoluminescence intensity in graphene/MoSe$_2$ is quenched by more than two orders of magnitude and rises linearly with the photon flux, demonstrating a drastically shortened ($\sim 1~\tr{ps}$) room temperature MoSe$_2$ exciton lifetime. Key complementary insights are provided from analysis of the graphene and MoSe$_2$ Raman modes, which reveals net photoinduced electron transfer from MoSe$_2$ to graphene and hole accumulation in MoSe$_2$. Remarkably, the steady state Fermi energy of graphene saturates at $290\pm 15~\tr{meV}$ above the Dirac point. This behavior is observed both in ambient air and in vacuum and is discussed in terms of band offsets and environmental effects. In this saturation regime, balanced photoinduced flows of electrons and holes may transfer to graphene, a mechanism that effectively leads to energy transfer. Using a broad range of photon fluxes and diverse environmental conditions, we find that the presence of net photoinduced charge transfer has no measurable impact on the near-unity photoluminescence quenching efficiency in graphene/MoSe$_2$. This absence of correlation strongly suggests that energy transfer to graphene is the dominant interlayer coupling mechanism between atomically-thin TMDs and graphene.

Full text

I Introduction

Charge and energy transfer (CT, ET) play a prominent role in atomic, molecular and nanoscale systems. On the one hand, Förtser-type energy transfer Förster (1948), mediated by relatively long-range (up to several nm) near-field dipole-dipole coupling is an essential step in photosynthesis van Grondelle et al. (1994) and is now engineered in a variety of light-harvesting devices and distance sensors Guzelturk and Demir (2016); Govorov et al. (2016). Charge transfer, on the other hand is a much shorter range process ($\sim 1~{}{\textrm{nm}}$) that plays a key role in a number of molecular and solid-state systems and is at the origin of the operation of photodetectors and solar cells May and Kühn (2008); Sze and Ng (2006). In the limit of orbital overlap between donor and acceptor systems, electron exchange, resulting in no net charge transfer and also known as Dexter-type energy transfer Dexter (1953), may occur. The efficiencies of CT and ET depend very sensitively on the donor-acceptor distance, on the energy levels (or bands) offsets, and on the local dielectric and electrostatic environment. CT and ET processes may have beneficial or detrimental impact on the performance of optoelectronic devices and therefore deserve fundamental investigations.

In this context, two-dimensional materials (2DM, such as graphene, boron nitride, transition metal dichalcogenides (TMDs), black phosphorus,…) provide an extraordinary toolkit to investigate novel regimes of CT/ET. Indeed the very diverse and complementary physical properties of 2DM can be tailored and controlled at the single-layer level, but also combined and possibly enhanced within so-called van der Waals heterostructures (vdWHs) Geim and Grigorieva (2013); Novoselov et al. (2016); Mak and Shan (2016). VdWHs provide a new paradigm of clean, ultra smooth two-dimensional heterointerfaces Haigh et al. (2012). Since their van der Waals gap is only of a few \angstrom, band bending and depletion regions cannot develop in vdWH. As a result, well-established concepts borrowed from the physics of bulk or low-dimensional heterojunctions Sze and Ng (2006) must be adapted with great care when describing the optoelectronic response of vdWHs. In addition, the ultimate proximity between the atomically thin building blocks that compose a vdWH potentially allows ultra efficient CT and/or ET.

Among the vast library of 2DM, graphene Castro Neto et al. (2009) and atomically-thin semiconducting TMDs (with formula MX2, with M = Mo, W and X = S, Se, Te) Mak et al. (2010); Splendiani et al. (2010); Mak and Shan (2016) have attracted particular interest for optoelectronic applications Britnell et al. (2013); Yu et al. (2013); Roy et al. (2013); Zhang et al. (2014); De Fazio et al. (2016); Massicotte et al. (2016); McCreary et al. (2014); Shim et al. (2014); Pierucci et al. (2016); He et al. (2014a); Henck et al. (2016). Indeed, graphene (Gr) may act as a highly tunable transparent electrode, endowed with exceptional physical properties Koppens et al. (2014); Mak et al. (2012); Tielrooij et al. (2015), while monolayer TMDs are direct bandgap semiconductors with unusually strong light-matter interactions and excitonic effects Wang et al. (2017); Xia et al. (2014); Mak and Shan (2016), as well as unique spin, valley and optoelectronic properties Xia et al. (2014); Mak and Shan (2016); Schaibley et al. (2016). Photodetectors based on graphene and TMDs display high photoresponsivity and photogain Britnell et al. (2013); Yu et al. (2013); Roy et al. (2013); Zhang et al. (2014); De Fazio et al. (2016), down to picosecond timescales Massicotte et al. (2016). The photophysics of Gr/TMD vdWHs is governed by near-field interlayer CT and/or ET (ICT, IET). In the related and most studied case of TMD/TMD heterojunctions with type II band alignment, sub-picosecond ICT and subsequent interlayer exciton formation is thought to be the dominant coupling mechanism Ceballos et al. (2014); Hong et al. (2014); Fang et al. (2014); Lee et al. (2014); Rivera et al. (2015); Schaibley et al. (2016). However, recent photoluminescence excitation spectroscopy studies in MoSe2/WS2 vdWH have suggested that IET may be at least as efficient as ICT Kozawa et al. (2016).

In contrast, fundamental studies of IET and ICT remain scarce in Gr/TMD vdWH. Photoinduced ICT has been observed in Gr/MoS2 photodetectors Zhang et al. (2014). Recent transient absorption studies have evidenced fast interlayer coupling in Gr/WS2 vdWHs and tentatively assigned it to photoinduced ICT He et al. (2014a). Yet, such studies were mostly performed under ambient conditions and the share of environmental effects needs to be assessed. Importantly, in Ref. Massicotte et al., 2016, the internal quantum efficiency of Gr/TMD photodetectors degrades when the active TMD layer is thinned down to the monolayer limit, possibly due to efficient IET to graphene. Overall, IET has been surprisingly overlooked in vdWH, whereas related studies in hybrid heterostructures composed of nanoscale emitters (molecules, quantum dots, quantum wells,…) interfaced with carbon nanotubes Roquelet et al. (2010), graphene Chen et al. (2010); Gaudreau et al. (2013); Tisler et al. (2013); Federspiel et al. (2015), TMDs Prins et al. (2014); Raja et al. (2016) have consistently demonstrated highly efficient Förster-type ET.

Unraveling the relative efficiencies of ICT and IET in vdWH is a timely challenge for optoelectronics. For this purpose, optical spectroscopy offer minimally invasive and spatially-resolved tools. First exciton dynamics and interlayer coupling can be probed with great sensitivity using micro-photoluminescence (PL) spectroscopy Mak and Shan (2016); Schaibley et al. (2016). Second, micro-Raman scattering spectroscopy allows quantitative measurements of doping and charge transfer as it has been demonstrated in graphene Yan et al. (2007); Pisana et al. (2007); Das et al. (2008); Froehlicher and Berciaud (2015); Ryu et al. (2010) and in MoS2 Chakraborty et al. (2012); Miller et al. (2015), but not yet in vdWHs.

In this paper, using an original combination of PL and Raman spectroscopies, we are able to disentangle contributions from ICT and IET in model vdWHs made of single-layer graphene stacked onto single-layer molybdenum diselenide (MoSe2) (hereafter denoted Gr/MoSe2) in the absence of any externally applied electric field. While highly efficient exciton-exciton annihilation and subsequent saturation of the PL intensity is – as expected – observed in bare MoSe2 as the incident photon flux increases, the PL in Gr/MoSe2 is massively quenched and its intensity rises linearly with the photon flux, demonstrating a drastically shortened room-temperature exciton lifetime in MoSe2. Key complementary insights are provided from an comprehensive analysis of the graphene and MoSe2 Raman modes, which reveals net photoinduced electron transfer from MoSe2 to graphene and hole accumulation in MoSe2. Remarkably, the steady state Fermi energy of graphene saturates at $290\pm 15~{}{\textrm{meV}}$ above the Dirac point. In this saturation regime, balanced flows of electrons and holes transfer to graphene, resulting in no net photoinduced charge transfer. This reproducible behavior is observed both in ambient air and in vacuum and is discussed in terms of intrinsic factors (i.e., band offsets) and extrinsic effects associated with native doping and charge trapping. Using a broad range of photon fluxes and diverse environmental conditions, we find that the existence of net photoinduced charge transfer has no measurable impact on the near-unity photoluminescence quenching efficiency in graphene/MoSe2. This absence of correlation strongly suggests that energy transfer to graphene (either in the form of Dexter or Förster processes) is the dominant interlayer coupling mechanism between atomically-thin TMDs and graphene. Our results provide a better understanding of the atomically thin two-dimensional metal-semiconductor (i.e., Schottky) junction, an ubiquitous building block in emerging optoelectronic devices, and will serve as a guide to engineer charge carrier and exciton transport in two-dimensional materials.

II Characterization of the Gr/MoSe2 heterostructure

Figure 1(a) shows an optical image of a Gr/MoSe2 vdWH (Sample S1) deposited onto a Si/SiO2 substrate. From AFM measurements (see Supplemental Material SMn , Fig. S1), we can distinguish a region of the heterostucture (highlighted with a white dashed contour in Fig. 1(a)), where the two layers are well coupled, as evidenced by the small surface roughness Novoselov et al. (2016) and the small height difference of approximately $0.65~{}{\textrm{nm}}$ between the surface of MoSe2 and Gr (see Fig. 1(b)). Outside this region, the interface shows sub-micrometer size “pockets” and an average step of $\sim 2-3~{}{\textrm{nm}}$ (see Fig. 1(b)) between MoSe2 and Gr. Hereafter, the former and the latter are referred to coupled and decoupled Gr/MoSe2, respectively (see Fig1(c) and Supplemental Material SMn , Fig. S1).

Typical photoluminescence (PL) and Raman spectra from three different points of the sample are shown in Fig. 1(d) and Fig. 1(e), respectively. Unless otherwise noted, the samples were optically excited in the continuous wave regime using a single longitudinal mode, linearly polarized laser at a photon energy of $2.33~{}{\textrm{eV}}$ well above the optical bandgap of MoSe2.

Figure. 1(f-k) displays the hyperspectral maps of (f) the MoSe2 PL intensity, (g-j) the frequencies ($\omega_{\textrm{G,2D}}$) and full-widths at half maximum (FWHM, $\Gamma_{\textrm{G,2D}}$) of the Raman G- and 2D-mode features Ferrari and Basko (2013), and (k) of the ratio of their integrated intensities ($I_{\textrm{2D}}/I_{{\textrm{G}}}$). Note that no defect-induced D-mode feature Ferrari and Basko (2013) (expected around $1350~{}{\textrm{cm}}^{-1}$) emerges from the background showing the very good quality of our sample. All hyperspectral maps allow to distinctively identify the coupled and decoupled Gr/MoSe2 regions and confirm the trends observed on selected points.

The PL spectra in Fig. 1(d) are characteristic of single-layer MoSe2 with the A and B excitons Li et al. (2014) near 1.57 eV and 1.75 eV, respectively. Remarkably, the MoSe2 PL intensity is $\sim 300$ times smaller on coupled Gr/MoSe2 than on MoSe2/SiO2, while it is only reduced by a modest factor of $\sim 2$ on decoupled Gr/MoSe2 (Fig. 1(d,f). Such massive PL quenching, also observed for other Gr/TMD vdWHs He et al. (2014a); Massicotte et al. (2016) demonstrates strong interlayer coupling and suggest a much reduced exciton lifetime.

As shown in Fig. 1(e),(g-k), interlayer coupling also dramatically affects the Raman response of graphene. Indeed, on coupled Gr/MoSe2, the G-mode feature upshifts, gets narrower, and the $I_{\textrm{2D}}/I_{{\textrm{G}}}$ ratio decreases (Fig. 1(k)) with respect to reference measurements on the neighboring pristine graphene deposited on SiO2 (Gr/SiO2) and decoupled Gr/MoSe2 regions (Fig. 1(e,k)). These observations are robust evidence of an increased charge carrier concentration in graphene Das et al. (2008); Froehlicher and Berciaud (2015). Surprisingly, we observe an upshift of the 2D-mode frequency on coupled Gr/MoSe2 (Fig. 1(i)), which is too high to be solely induced by doping or strain Lee et al. (2012); Froehlicher and Berciaud (2015); Metten et al. (2014), and seems qualitatively similar to previous reports on graphene deposited on thick boron nitride terraces Ahn et al. (2013); Forster et al. (2013) and monolayer MoS2 grown on graphene McCreary et al. (2014). Possible origins for this upshift are discussed in the Supplemental Material SMn (Fig. S15).

In the following we quantitatively investigate exciton dynamics (Sec. III) and interlayer charge transfer (Sec. IV) .

III Exciton dynamics in Gr/MoSe2

Although PL quenching has been reported in previous studies of Gr/TMD heterostructures (see Figure 1 in Ref. He et al., 2014b and Supplementary Figure 6 in Ref. Massicotte et al. (2016)), quantitative analysis of PL quenching and its interpretation in terms of IET and ICT have not been reported thus far.

Figure 2(a,b) displays the normalized PL spectra of MoSe2 recorded on MoSe2/SiO2, decoupled and coupled Gr/MoSe2 at low and high incident photon flux $\Phi_{{\textrm{ph}}}$. The A exciton PL feature of coupled Gr/MoSe2/SiO2 is marginally redshifted (by $\approx 10~{}{\textrm{meV}}$) with respect to that of air/MoSe2/SiO2, irrespective of $\Phi_{{\textrm{ph}}}$. Considering the drastically different dielectric environments, such a surprisingly small reduction of the optical bandgap is assigned to the near-perfect compensation of the reductions of electronic bandgap and exciton binding energy in graphene-capped MoSe2Ugeda et al. (2014); Stier et al. (2016); Raja et al. (2017). The lineshapes of the A exciton features are quite similar, except for a small but reproducible narrowing of the A-exciton linewidth in coupled Gr/MoSe2. Similar narrowing has recently been observed in TMD layers fully encapsulated in boron nitride Cadiz et al. (2017); Ajayi et al. (2017) and likely results from a reduction of inhomogeneous broadening and pure dephasing in graphene-capped TMD samples.

The integrated PL intensities of the A and B exciton features (denoted $I_{\textrm{PL}}^{\textrm{A,B}}$) normalized by $\Phi_{\textrm{ph}}$, are plotted as a function of $\Phi_{\textrm{ph}}$ in Figure 2(c) and (d), respectively. For MoSe2/SiO2 and decoupled Gr/MoSe2, $I_{\textrm{PL}}^{{\textrm{A}}}/\Phi_{{\textrm{ph}}}$ drops abruptly as $\Phi_{{\textrm{ph}}}$ augments due to highly efficient exciton-exciton annihilation (EEA), as previously evidenced in TMD monolayers Kumar et al. (2014); Mouri et al. (2014). In the case of coupled Gr/MoSe2, $I_{\textrm{PL}}^{{\textrm{A}}}/\Phi_{{\textrm{ph}}}$ remains constant, within experimental accuracy, up to $\Phi_{{\textrm{ph}}}\sim 10^{24}~{}{\textrm{cm}}^{-2}~{}{\textrm{s}}^{-1}$. As a result, while $I_{\textrm{PL}}^{{\textrm{A}}}$ is about 300 times weaker on coupled Gr/MoSe2 than on bare MoSe2 at $\Phi_{{\textrm{ph}}}=3\times 10^{20}~{}{\textrm{cm}}^{-2}~{}{\textrm{s}}^{-1}$, this quenching factor reduces down to $\sim 3$ at $\Phi_{{\textrm{ph}}}=6\times 10^{23}~{}{\textrm{cm}}^{-2}~{}{\textrm{s}}^{-1}$. Strong PL quenching together with the linear scaling of $I_{\textrm{PL}}^{{\textrm{A}}}$ with $\Phi_{{\textrm{ph}}}$ demonstrate that interlayer coupling between graphene and MoSe2 opens up non-radiative decay channel for A excitons, that dramatically reduces of the A exciton lifetime and is sufficiently fast to bypass EEA. Very similar PL quenching and exciton dynamics have been observed in other Gr/MoSe2/SiO2 samples (see Supplemental Material SMn , Fig. S11-S13) as well as in Gr/WS2/SiO2 (see Supplemental Material SMn , Fig. S14) and Gr/WSe2/SiO2 (data not shown). The shortening of the A exciton lifetime is further substantiated by the analysis of the hot luminescence from the B exciton (note that our samples are photoexcited at 2.33 eV, i.e., well-above the B exciton in MoSe2). In bare MoSe2 and decoupled Gr/MoSe2, $I_{\textrm{PL}}^{{\textrm{A}}}\gg I_{\textrm{PL}}^{{\textrm{B}}}$, whereas $I_{\textrm{PL}}^{\textrm{B}}\sim I_{\textrm{PL}}^{{\textrm{A}}}$ in coupled Gr/MoSe2. Interestingly, $I_{\textrm{PL}}^{{\textrm{B}}}$ is very similar in the three cases and scales linearly with $\Phi_{\textrm{ph}}$ (see Fig. 2(d)). These observations suggest (i) that interlayer coupling does not significantly affect exciton formation and exciton decay until a population of A excitons is formed, and (ii) that the A exciton lifetime in Gr/MoSe2 is not appreciably longer than the ${\textrm{B}}\rightarrow{\textrm{A}}$ decay time. The latter is typically in the subpicosecond range Shi et al. (2013) in atomically thin TMDs, and provides a lower bound for the A exciton lifetime in Gr/MoSe2. Additional insights are provided by time-resolved photoluminescence measurements recorded in ambient conditions (see Fig. 2(e)). Bare MoSe2 and decoupled Gr/MoSe2 display non-monoexponential decays Robert et al. (2016) with average exciton lifetime of $\sim 1~{}{\textrm{ns}}$. As anticipated, the PL decay of Gr/MoSe2 is too fast to be resolved using our experimental apparatus, confirming that the A exciton lifetime is significantly shorter that our time-resolution of $\sim 20~{}{\textrm{ps}}$. Using the estimated decay time of bare MoSe2 and a typical quenching factor of $\sim 300$ (i.e., a quenching efficiency of $\sim 99.7\%$) in the low fluence limit, we can reckon a conservative upper bound of a few ps for the exciton lifetime in coupled Gr/MoSe2.

IV Interlayer charge transfer

IV.1 Net photoinduced electron transfer to graphene

The fast MoSe2 exciton decay in Gr/MoSe2 heterostructures may arise from a combination of ICT and IET processes. In this section, we introduce an original Raman-based readout of the steady state charge carrier density in both materials.

Fig. 3 shows the evolution of $\omega_{\textrm{G,2D}}$, $\Gamma_{\textrm{G,2D}}$ and $I_{\textrm{2D}}/I_{{\textrm{G}}}$ measured in sample S1 as a function of $\Phi_{\textrm{ph}}$, in ambient air. The corresponding spectra are shown in the Supplemental Material SMn (Fig. S2). First, for Gr/SiO2 and decoupled Gr/MoSe2, $\omega_{\textrm{G}}\approx 1583~{}{\textrm{cm}}^{-1}$, $\Gamma_{\textrm{G}}\approx 16~{}{\textrm{cm}}^{-1}$, $\omega_{\textrm{2D}}\approx 2674~{}{\textrm{cm}}^{-1}$ and $I_{\textrm{2D}}/I_{\textrm{G}}\approx 6.5$ do not show any appreciable variation as $\Phi_{\textrm{ph}}$ augments. These values correspond to very weakly doped graphene ($\left|n_{{\textrm{G}}r}\right|\sim 10^{11}~{}{\textrm{cm}}^{-2}$ or $\left|E^{{\textrm{G}}r}_{\textrm{F}}\right|\lesssim 100~{}{\textrm{meV}}$) Das et al. (2008); Berciaud et al. (2009); Froehlicher and Berciaud (2015). In addition, the absence of measurable phonon softening at high $\Phi_{\textrm{ph}}$, indicates that the laser-induced temperature rise remains below $\sim 100~{}{\textrm{K}}$ Balandin (2011).

Second, for coupled Gr/MoSe2, $\omega_{{\textrm{G}}}$ distinctly rises as $\Phi_{\textrm{ph}}$ increases, whereas $\Gamma_{{\textrm{G}}}$ decreases (see Figs. 3(a)-(c)). Additionally, $I_{\textrm{2D}}/I_{\textrm{G}}$ (Figs. 3(e)) drops significantly. These spectroscopic features provide strong evidence for net photoinduced ICT from MoSe2 to graphene Das et al. (2008); Froehlicher and Berciaud (2015). We can now identify the sign of the net transferred charge flow using the correlation between $\omega_{\textrm{2D}}$ and $\omega_{{\textrm{G}}}$ in Fig. 3(f) as in Ref. Lee et al., 2012; Froehlicher and Berciaud, 2015. As $\Phi_{\textrm{ph}}$ increases, the data for Gr/SiO2 and decoupled Gr/MoSe2 show no clear correlations. In contrast, on coupled Gr/MoSe2 $\omega_{\textrm{2D}}$ and $\omega_{{\textrm{G}}}$ display a linear correlation with a slope of $\approx 0.11$, a value that clearly points towards photoinduced electron doping in graphene Froehlicher and Berciaud (2015).

Using well-established theoretical modelling of electron-phonon coupling in doped graphene Ando (2006); Pisana et al. (2007), we quantitatively determine the Fermi energy of graphene relative to the Dirac point $E^{\textrm{Gr}}_{\textrm{F}}$ or equivalently its doping level $n_{\textrm{Gr}}$. The values of $E^{\textrm{Gr}}_{\textrm{F}}$ and $n_{\textrm{Gr}}$ extracted from a global fitting procedure (see Ref. Froehlicher and Berciaud, 2015) are plotted in Fig. 3(f). As further discussed in Sec. V, $E^{\textrm{Gr}}_{\textrm{F}}$ ($n_{\textrm{Gr}}$) saturates as $\Phi_{{\textrm{ph}}}$ increases and reaches up to $\approx 280~{}{\textrm{meV}}$ ($\approx 5\times 10^{12}~{}{\textrm{cm}}^{-2}$).

IV.2 Hole accumulation in MoSe2

Net electron transfer to graphene naturally implies hole accumulation in MoSe2. Depending on the initial doping of MoSe2, photoinduced hole accumulation in MoSe2 should allow or impede the formation of charged excitons (trions). However, at room temperature, trions in MoSe$2$ are not stable enough Ross et al. (2013) to allow the observation of trion emission in our PL spectra. However, as in Sec. IV.1, we can seek for fingerprints of ICT in the high-resolution Raman response of MoSe2.

Figure 4(a) shows the MoSe2 Raman spectra from MoSe2/SiO2, decoupled and coupled Gr/MoSe2. In addition to several higher-order resonant features involving finite momentum phonons, one can identify the two Raman-active one-phonon modes in monolayer MoSe2 with $A^{\prime}_{1}$ symmetry (near $242~{}{\textrm{cm}}^{-1}$) and $E^{\prime}$ symmetry (near $289~{}{\textrm{cm}}^{-1})$ Soubelet et al. (2016); Zhang et al. (2015). The faint $E^{\prime}$ mode-feature is slightly downshifted on coupled Gr/MoSe2, as compared to MoSe2/SiO2. The prominent $A^{\prime}_{1}$-mode feature is much similar for MoSe2/SiO2 and decoupled Gr/MoSe2, but distinctively blueshifts (by $\approx 0.5~{}{\textrm{cm}}^{-1}$) and gets narrower (by $\approx 20\%$) for coupled Gr/MoSe2 (see Fig. 4(b)).

As in the case of graphene, changes in the Raman spectra can tentatively be assigned to doping, with possible spurious contributions from native strain, laser-induced heating, as well as van der Waals coupling Zhou et al. (2014) and surface effects Luo et al. (2013); Froehlicher et al. (2015)111The observed upshift may in part stem from van der Waals coupling between the graphene and MoSe2monolayers Zhang et al. (2015); Zhou et al. (2014); Luo et al. (2013); Froehlicher et al. (2015) (similarly to the case of TMD bilayers), as well as from surface effects Luo et al. (2013); Froehlicher et al. (2015), i.e. in the present case, slightly larger force constants between Mo and Se atoms in Gr/MoSe2/SiO2 than in air/MoSe2/SiO2. However both kinds of effects would not lead to the significant narrowing of the $A^{\prime}_{1}$ feature observed in Gr/MoSe2/SiO2 and cannot account for the differential effects shown in Fig. 4c.. Interestingly, recent Raman studies in MoS2 monolayers have demonstrated that the $A^{\prime}_{1}$-mode phonon undergoes modest doping-induced phonon renormalization, namely a downshift and a broadening for increasing electron concentration whereas the $E^{\prime}$-mode phonon is largely insensitive to doping Chakraborty et al. (2012); Miller et al. (2015). Conversely, also in MoS2, it was shown that under tensile (resp. compressive) strain the $E^{\prime}$-mode feature undergoes much larger shifts than the $A^{\prime}_{1}$-mode feature Conley et al. (2013); Zhang et al. (2015). The $A^{\prime}_{1}$ and $E^{\prime}$ phonons may thus be used as probes of ICT and strain, respectively. Based on these considerations, the minute $E^{\prime}$ phonon softening observed irrespective of $\Phi_{\textrm{ph}}$ in Gr/MoSe2 relative to MoSe2/SiO2 suggests a slightly larger native tensile strain on Gr/MoSe2, that has no impact whatsoever on ICT (see Supplemental Material SMn , Fig. S4-S8). More importantly, the upshifted and narrower $A^{\prime}_{1}$-mode feature consistently observed up to $\Phi_{\textrm{ph}}\approx 6\times 10^{23}~{}{\textrm{cm}}^{-2}~{}{\textrm{s}}^{-1}$ in coupled Gr/MoSe2 indicates a lower electron density in MoSe2 than in decoupled Gr/MoSe2 and MoSe2/SiO2.

However, on the three regions of the sample, the frequency and FWHM of the $A^{\prime}_{1}$-mode feature downshifts and increases linearly as $\Phi_{\textrm{ph}}$ augments, respectively. Such trends counter-intuitively suggest photoinduced electron doping in MoSe2. We tentatively assign the observed evolution of the $A^{\prime}_{1}$-mode feature to slight laser-induced temperature increase (estimated below $100~{}{\textrm{K}}$ Late et al. (2014) at $\Phi_{\textrm{ph}}\approx 6\times 10^{23}~{}{\textrm{cm}}^{-2}~{}{\textrm{s}}^{-1}$), possibly combined with related photogating effects involving the presence of molecular adsorbates and trapped charges both acting as electron acceptors and laser-assisted desorption of the latter Miller et al. (2015). Remarkably, as shown in Fig. 4(c), the difference between the $A^{\prime}_{1}$ frequencies (FWHM) measured on coupled Gr/MoSe2 and decoupled Gr/MoSe2 or MoSe2/SiO2 monotonically increases (decreases) as $\Phi_{\textrm{ph}}$ augments. These observations correspond to a net photoinduced hole doping for MoSe2 in coupled Gr/MoSe2, relative to decoupled Gr/MoSe2 and MoSe2/SiO2, consistently with the net photoinduced electron transfer from MoSe2 to graphene demonstrated in Fig. 3.

V Environmental effects

The charge density and exciton dynamics in 2D materials are known to be influenced by environmental effects, in particular by molecular adsorbates and the underlying substrate Ryu et al. (2010); Miller et al. (2015); Tongay et al. (2013); Cadiz et al. (2016). To determine the generality of the results presented above, we compare in Fig. 5(a,b) the evolution of $E^{\textrm{Gr}}_{\textrm{F}}$ with increasing $\Phi_{\textrm{ph}}$ recorded in ambient air and under high vacuum ($\lesssim 10^{-4}~{}{\textrm{mbar}}$) for a set of five samples, wherein strong PL quenching has been observed (see Supplemental Material SMn , Fig. S11). Remarkably, in ambient air, all samples display (i) different initial doping at low $\Phi_{\textrm{ph}}$, (ii) distinct sub-linear rises of $E^{\textrm{Gr}}_{\textrm{F}}$ with increasing $\Phi_{\textrm{ph}}$ and (iii) similar saturation at $E^{\textrm{Gr}}_{\textrm{F}}$ around $290\pm 15~{}{\textrm{meV}}$ (i.e., $n_{\textrm{Gr}}\approx(5\pm 0.5)\times 10^{12}~{}{\textrm{cm}}^{-2}$). Interestingly, under vacuum, we systematically observe a transient regime with a photoinduced rise of $E^{\textrm{Gr}}_{\textrm{F}}$ (at fixed $\Phi_{{\textrm{ph}}}$) towards a saturation value that is attained on a rather long timescale (typically several minutes, depending on $\Phi_{{\textrm{ph}}}$, see Fig. 5(b) and Supplemental Material SMn , Fig. S10). Once $E^{\textrm{Gr}}_{\textrm{F}}$ has reached its saturation value, it becomes completely independent on $\Phi_{{\textrm{ph}}}$ (see Fig. 5(b)).

The distinct charge transfer dynamics observed under ambient conditions and in vacuum shed light on the role of molecular adsorbates at the surface of the vdWH. In vacuum, a significant fraction of the molecular adsorbates are removed, including through laser-assisted desorption. These adsorbates are efficient electron traps Ryu et al. (2010); Miller et al. (2015); Tongay et al. (2013), acting against the net photoinduced electron transfer from MoSe2 to graphene. In the absence of molecular adsorbates, the electrons that are transferred from MoSe2 to graphene remain on graphene as long as the laser illumination is on (see Sec. VI). Such extrinsic effects impact the optoelectronic response of 2DM and vdWH - most often examined under ambient conditions - and therefore provide an impetus for further studies under controlled atmospheres Ryu et al. (2010), using different substrates, stacking sequences and encapsulating materials Raja et al. (2017); Ajayi et al. (2017); Cadiz et al. (2017). Along this line, we have studied (see Fig.5(b) and Supplemental Material SMn , Fig. S11) a MoSe2/Gr/SiO2 vdWH. Remarkably, the results obtained on this inverted heterostructure are very similar to those obtained in Gr/MoSe2/SiO2 vdWHs.

Finally, we have compared the PL in Gr/MoSe2/SiO2 and MoSe2/Gr/SiO2 in ambient air and under vacuum conditions. While the PL of bare MoSe2 is -as previously reported Tongay et al. (2013)- quenched under vacuum, the PL intensity and lineshape measured as a function of $\Phi_{{\textrm{ph}}}$ in ambient air and under vacuum in Gr/MoSe2 are not appreciably different (See Fig. 6 and Supplemental Material SMn , Fig. S12).

VI Discussion

The complementary results reported in Sec. III-V now make it possible to address a set of open questions of high relevance for fundamental photophysics and optoelectronic applications. What are the microscopic mechanisms responsible for net electron transfer and its saturation (Sec. VI.1)? What is the impact of excitonic effects on interlayer coupling (Sec. 7)? What are the relative contributions of ICT and IET to the massive photoluminescence quenching analyzed in Fig. 2 (Sec. VI.3)?

VI.1 Charge transfer mechanism

The clear saturation of the net photoinduced ICT in Gr/TMD heterostructures shown in Fig. 5 had not been reported thus far and we shall first discuss the underlying microscopic ICT mechanisms. Since the Dirac point of graphene is located between the valence band maximum and the conduction band minimum of MoSe2 Yu et al. (2009); Liang et al. (2013); Wilson et al. (2017), the tunneling of photoexcited electrons and holes to graphene can be envisioned as long as energy and momentum are conserved and that $E^{\textrm{Gr}}_{\textrm{F}}$ lies sufficiently below (above) the conduction band minimum (valence band maximum) of MoSe2. Electron and hole transfer to graphene are sketched in Fig. 7(a,b). To account for our experimental findings, we propose the following scenario.

The band structure of coupled Gr/MoSe2 can be, in first approximation, considered as the superposition of the bands of the different materials Kośmider and Fernández-Rossier (2013); Pierucci et al. (2016) separated by a subnanometer “van der Waal gap”. The relative position of the band structure is determined by the offsets between the Dirac point of graphene and the valence (conduction) band maximum (minimum) of MoSe2. In the dark, without loss of generality we may assume that both graphene and MoSe2 are quasi-neutral. When visible light is shined onto Gr/MoSe2, electron-hole pairs and excitons are mainly created in MoSe2 since the latter absorbs significantly more than graphene Mak et al. (2012); Li et al. (2014). At this point, given the very close electron and hole effective masses in MoSe2 Kormányos et al. (2015), the rates of photoinduced electron and hole transfer from MoSe2 to graphene will chiefly depend on the the wavefunction overlap, the density of states in graphene and the energy difference between the band extrema in MoSe2 and $E^{\textrm{Gr}}_{\textrm{F}}$.

Assuming the Dirac point lies closer to the valence band maximum than to the conduction band minimum Yu et al. (2009); Liang et al. (2013), the photoinduced electron current to graphene should exceed the hole current immediately after sample illumination, consistently with our experimental findings. Due to the small density of states of graphene near its Dirac point Castro Neto et al. (2009), the net electron transfer to graphene induces a sizeable rise of $E^{\textrm{Gr}}_{\textrm{F}}$ above the Dirac point. Thus, as $E^{\textrm{Gr}}_{\textrm{F}}$ moves away from the valence band maximum in MoSe2, the hole current to $n$-doped graphene increases significantly. The vanishing of the net electron transfer to graphene then results from the cancellation of the photoinduced electron (Fig. 7(a)) and hole (Fig. 7(b)) currents. In vacuum and in the absence of adsorbates, this saturation is reached in the steady state at any $\Phi_{\textrm{ph}}$. In ambient air, electrons may escape from graphene (in Gr/MoSe2/SiO2) or MoSe2 (in MoSe2/Gr/SiO2) resulting (at intermediate $\Phi_{\textrm{ph}}<10^{23}~{}{\textrm{cm}}^{-2}~{}{\textrm{s}}^{-1}$) in a steady state $E^{\textrm{Gr}}_{\textrm{F}}$ below the $\Phi_{\textrm{ph}}$-independent saturation value observed in vacuum (see Fig. 5(b)) 222As a result, the relative magnitudes of the electron and hole flows, and the resulting steady state $E^{\textrm{Gr}}_{\textrm{F}}$ are not exclusively determined by $\Phi_{\textrm{ph}}$ (compare data in Fig. 5 and see Supplemental Material SMn , Fig. S13)..

The very similar saturation values of $E^{\textrm{Gr}}_{\textrm{F}}$ uncovered in several Gr/MoSe2 samples both in ambient air and in vacuum (see Fig. 5) suggest an limit set by the intrinsic band offsets between graphene and MoSe2, as well as the electron and hole tunnelling efficiencies. The latter be affected by extrinsic materials properties, such as the presence of band tails states as well as other traps and defects Furchi et al. (2014). Systematic studies using other TMDs with distinct band offsets relative to graphene, and controlled amounts of impurities and/or defects will help determining the shares of extrinsic and intrinsic effects in the net charge transfer saturation. Nevertheless, our work is a step towards optical determination of band offsets in van der Waals heterostructures. Confronted to electron transport measurements Kim et al. (2015), or angle-resolved photoemission spectroscopy Pierucci et al. (2016); Wilson et al. (2017), our Raman-based approach may unveil the impact of strong bandgap renormalization and exciton binding energy on the optoelectronic properties of TMDs and related vdWHs.

VI.2 Impact of excitonic effects

Indeed, although PL measurements (see Fig. 2) make it clear that excitons are formed in MoSe2, the impact of excitonic effects on interlayer coupling and more generally on the optoelectronic response of vdWH remains elusive.

Upon optical excitation well-beyond the optical bandgap (as it is the case in Fig. 1-6), free electron-hole pairs and tightly bound excitons can be formed in Gr/MoSe2 Raja et al. (2017); Ugeda et al. (2014). Despite exciton formation being highly efficient and occurring on sub-picosecond to a few picosecond timescales Robert et al. (2016); Steinleitner et al. (2017), our PL measurements have revealed equally short band-edge (A) exciton lifetimes in Gr/MoSe2 (see Fig 2). Therefore the observed interlayer coupling processes may certainly involve band-edge excitons but may also imply direct hot carrier and/or higher-order exciton transfer to graphene. To assess the contribution of out of equilibrium effects, we have combined PL and Raman measurements in ambient air and in vacuum on a Gr/WS2/SiO2 vdWH at two different laser photon energies near the B exciton (2.33 eV) and slightly below the A exciton (1.96 eV). In the latter case only A excitons can be formed by means of an upconversion process Jones et al. (2016); Chervy et al. (2017) (see Supplemental Material SMn , Fig. S14). For both incoming photon energies, we observe strong PL quenching as well as photoinduced doping, very similar to the observations discussed in Fig. 2-6 for Gr/MoSe2 vdWHs. These observations indicate that ICT and IET processes in GR/TMD vdWH mainly involve band-edge TMD excitons 333This conclusion is consistent with the fact that momentum conservation can be more easily fulfilled for an exciton than for a free charge carrier. Indeed, an exciton in the TMD can decay by transferring an electron or hole to a finite momentum state graphene leaving the other carrier in the TMD with the excess momentum (see Fig. 7(a,b)), whereas at room temperature, a free charge carrier near the band-edges would need extra momentum provided by defect or phonon scattering.. This result illustrates the unusually strong excitonic effects in TMDs, which must be taken into consideration when adapting free-carrier optoelectronic models to the case of vdWH-based devices.

VI.3 Charge vs energy transfer

Finally, we address the competition between ICT and IET. Let us first recall that Raman measurements probe the steady state charge carrier densities in our samples and do not make it possible to extract electron and hole transfer rates. Figure 6 summarizes our findings by confronting the dependence of $E^{\textrm{Gr}}_{\textrm{F}}$ and $I^{\textrm{A}}_{\textrm{PL}}$ on $\Phi_{{\textrm{ph}}}$ in sample S2. The key implications of our combined PL and Raman study are that the short exciton lifetime in Gr/MoSe2 is (i) independent on $\Phi_{{\textrm{ph}}}$ (over nearly four orders of magnitude), (ii) unaffected by the environmental conditions (air vs vacuum), and, crucially by (iii) the presence (in air, at low $\Phi_{{\textrm{ph}}}$) or absence (in vacuum at any $\Phi_{{\textrm{ph}}}$, or in air at high $\Phi_{{\textrm{ph}}}$) of net photoinduced ICT (here, electron transfer from MoSe2 to graphene, see also Supplemental Material SMn , Fig. S13).

Our data demonstrate that albeit electrons and holes may transfer to graphene, ICT processes alone (even in the case of balanced electron and hole transfer) cannot be responsible for the massive PL quenching and linear rise of the PL intensity vs $\Phi_{{\textrm{ph}}}$. Instead, IET – either in the form of electron exchange (i.e., Dexter-type IET, Fig. 7(c)) or dipole-dipole interaction (i.e., Förster-type IET, Fig. 7(d) – provides a highly efficient relaxation pathway for excitons in Gr/TMD heterostructures.

Consequently, the $\sim 1~{}\rm ps$ exciton lifetime deduced from PL measurements (see Sec. III) can be considered as a fair estimation of the energy transfer timescale from a TMD monolayer to a graphene monolayer placed in its immediate vicinity. Since interlayer coupling is highly sensitive to minute changes (at the \angstrom level) in the distance between 2D layers as well as to the distribution of TMD excitons in energy-momentum space Federspiel et al. (2015); Robert et al. (2016), a timely theoretical and experimental challenge is to unravel the distance and temperature dependence of the energy transfer rate and quantify contributions stemming from short range (Dexter) and longer range Förster mechanisms.

Let us add the following comments in order to tentatively pinpoint the microscopic energy transfer mechanism. First, although balanced ICT and Dexter-type IET follow a priori two distinct microscopic mechanisms (see Fig. 7(a-c)), both processes imply charge tunnelling (i.e., wavefunction overlap) and result in a similar final state where the energy of an exciton population is transferred to graphene. Interestingly, it was recently predicted in porphyrin/graphene hybrids that Dexter ET is largely inefficient compared to Förster ET even at sub-nanometer distances Malic et al. (2014). In the case of Gr/TMD vdWHs, the large in-plane dipoles in monolayer TMDs Schuller et al. (2013) should further favor Förster energy transfer to graphene. Along this line, the exciton lifetime measured in decoupled Gr/MoSe2/SiO2 (see Fig. 2(e)) is of the same order of magnitude yet appreciably shorter than in MoSe2/SiO2, an effect that may tentatively be assigned to long-range Förster energy transfer 444Let us note that the PL data in Fig. 1(f) and Fig. 2(e) have been recorded on freshly made sample S1, before the data in Fig. 2(c)-(d). Aging of the air-exposed MoSe2 layer in S1 is likely responsible for the fact that in Fig. 2(c)-(d), the PL intensity in MoSe2/SiO2 is slightly smaller than in decoupled Gr/MoSe2/SiO2, wherein graphene acts as an efficient passivating layer..

VII Conclusion and outlook

We have exploited complementary insights from micro-Raman and photoluminescence spectroscopies to disentangle contributions from interlayer charge and energy transfer in graphene/TMD heterostructures and establish the key role of energy transfer. These general findings advance our fundamental understanding of light-matter interactions at atomically-thin heterointerfaces and have far reaching consequences for applications.

Indeed the Gr/TMD system is a ubiquitous building block in emerging optoelectronic nanodevices. Having established that edge TMD excitons transfer to graphene with near-unity efficiency, a key challenge is now to separate the electron-hole pairs formed in graphene Brenneis et al. (2015) and enhance photoconductivity and/or photocurrent generation before these charge carriers release their energy into heat on a sub-picosecond timescaleJohannsen et al. (2013); Gierz et al. (2013).

The competition between interlayer charge and energy transfer is also a matter of active debate in related systems, e.g. in TMD/TMD type II heterojunctions Hong et al. (2014); Ceballos et al. (2014); Fang et al. (2014); Lee et al. (2014); Rivera et al. (2015); Kozawa et al. (2016), that are also of high relevance for optoelectronics Mak and Shan (2016) and valleytronics Schaibley et al. (2016). We have shown that fingerprints of interlayer charge transfer are encoded in the Raman response of TMD monolayers. Combining Raman measurements and photoluminescence spectroscopy of intra- and inter-layer excitons should provide decisive insights into exciton dynamics in these atomically-thin semiconductor heterostructures.

More broadly, van der Waals heterostructures are also emerging as a platform to explore many-body effects and new regimes of strong- and/or chiral light-matter interactions. Further developments in these emerging areas will benefit from the present insights into interlayer charge and energy transfer.

Appendix A Experimental details

Gr/MoSe2 vdWHs were prepared onto Si wafers covered with a 90 nm thick SiO2 epilayer using a viscoelactic transfer technique Castellanos-Gomez et al. (2014). The Gr/MoSe2 vdWHs were first characterized by atomic force microscopy (AFM) and then by micro-PL and micro-Raman measurements. As-prepared samples (such as sample $S_{1}$ discussed above, see Fig. 1(a)) as well as annealed samples (1 hour at 150${}^{\circ}{\textrm{C}}$ and 2 hours at 200${}^{\circ}{\textrm{C}}$ in high vacuum) such as sample S3 were studied. Although more “pockets” (see Supplemental Material SMn , Fig. S1) are present on as-prepared samples, we could observe, both in annealed and as-prepared samples, extended ($>25~{}\mu{\textrm{m}}^{2}$) coupled Gr/MoSe2 domains with smooth and uniform interfaces due to a self-cleaning process Haigh et al. (2012).

PL and Raman studies were carried out in ambient air and in high vacuum, in a backscattering geometry, using a home-built confocal microscope. Unless otherwise noted, the samples were optically excited using a single longitudinal mode, linearly polarized, $2.33~{}{\textrm{eV}}$ ($532~{}{\textrm{nm}}$) continuous wave laser. The collected light was dispersed onto a charged-coupled device (CCD) array by a single (500 nm in focal length) monochromator equipped with a 150 (resp. 900 for graphene, 2400 for MoSe2) grooves/mm grating for PL (resp. Raman) measurements. Spectral resolutions of $1.4~{}{\textrm{cm}}^{-1}$ and $0.6~{}{\textrm{cm}}^{-1}$ were obtained for Raman measurements with the 900 and 2400 grooves/mm grating, respectively. The sample holder was mounted onto a x-y-z piezoelectric stage, allowing hyperspectral imaging. Time-resolved PL measurements were performed on the same setup using a pulsed supercontinuum laser, with a repetition rate tunable from $1.95~{}{\textrm{MHz}}$ up to $78~{}{\textrm{MHz}}$. The unpolarized output of the supercontinuum laser at $1.96~{}{\textrm{eV}}$ ($633~{}{\textrm{nm}}$) was selected using an acousto-optic tunable filter. PL decays were obtained using an avalanche photodiode coupled to a time-tagged, time-correlated single photon counting board. We have employed a very broad range of photon fluxes resulting in exciton densities below and beyond the values achieved in earlier works He et al. (2014b); Massicotte et al. (2016); Zhang et al. (2014). The incident photon flux $\Phi_{\textrm{ph}}$ is obtained by measuring the laser power and the area of the laser spot. For instance, with a measured optical power of $1~{}\mu{\textrm{W}}$ at the objective at 2.33 eV, we obtain a photon flux of $2.2\times 10^{21}~{}{\textrm{cm}}^{-2}~{}{\textrm{s}}^{-1}$ using a 100x objective with a numerical aperture of 0.9. The Raman peaks are fit using Lorentzian and modified Lorentzian Froehlicher and Berciaud (2015); Metten et al. (2014) profiles for the G- and 2D-mode features, respectively, and Voigt profiles (with a fixed Gaussian width taking into account our spectral resolution) for MoSe2 features. Therefore in Fig. 4(b), the linewidth $\Gamma_{A^{\prime}_{1}}$ has to be understood as the Lorentian FWHM extracted from a Voigt fit.

Acknowledgements.

We thank D.M. Basko, C. Robert, D. Lagarde, X. Marie, A. Ouerghi, G. Schull and C. Genet for fruitful discussions. We are grateful to J. Chrétien for his early contribution to experimental measurements, to H. Majjad and M. Rastei for help with AFM measurements, and to the StNano clean room staff and M. Romeo for technical assistance. We acknowledge financial support from the Agence Nationale de la Recherche (under grant H2DH ANR-15-CE24-0016) and from the LabEx NIE (under Grant ANR-11-LABX-0058-NIE within the Investissement d’Avenir program ANR-10-IDEX-0002-02). S.B. is a member of Institut Universitaire de France (IUF).

Appendix S1 Additional Results on Sample S1

S1.1 Atomic force microscopy

S1.2 Graphene Raman spectra for increasing $\Phi_{\textrm{ph}}$

S1.3 MoSe2 Raman spectra for increasing $\Phi_{\textrm{ph}}$

S1.3.1 $A^{\prime}_{1}$ mode

S1.3.2 $E^{\prime}$ mode

S1.4 Spatially-resolved Raman studies

S1.4.1 Two-dimensional maps

S1.4.2 Line scans at various incident photon fluxes

Appendix S2 Additional results obtained on other samples

S2.1 Interlayer charge transfer in a Gr/MoSe2 heterostructure with an initially hole-doped graphene layer

S2.2 Laser-assisted desorption of molecular adsorbates under high vacuum

S2.3 Photoluminescence quenching on various samples

S2.4 Exciton dynamics in ambient air and in vacuum

S2.5 Comparison between photoinduced doping and exciton dynamics

Figure S13 shows PL and Raman measurements recorded in ambient air on Sample $S_{3}$ (see also Fig. S9) along a forward sweep followed by a backward sweep of $\Phi_{\rm ph}$. As opposed to most samples studied in this work, the graphene layer is $p$-doped at low $\Phi_{\rm ph}$ and we clearly see that $E_{\rm F}^{\rm Gr}$ (extracted following the procedure described in the text) has a hysteretic behavior that we attribute to laser-assisted adsorption of electron trapping molecules, such as water or molecular oxygen Mitoma et al. (2013). Remarkably, the (linear) evolution of the PL intensity is non-hysterietic, and thus largely independent on the equilibrium value of $E_{\rm F}^{\rm Gr}$ obtained at a given $\Phi_{\rm ph}$. These results further confirm that the ICT processes are likely not solely responsible for the massive PL quenching in Gr/MoSe2, and that molecular adsorbates to affect the charge transfer dynamics.

S2.6 Measurements under quasi-resonant optical excitation

Appendix S3 Discussion on the frequency of the 2D-mode feature

In this section, we briefly comment on the rigid upshift of the 2D-mode frequency observed in coupled Gr-MoSe2. Figs. 1(i) and 2(d) in the main text, and Figs. S2-S5 reveal a rigid upshift of $\approx 15~{}{\textrm{cm}}^{-1}$ in coupled Gr/MoSe2 as compared to Gr/SiO2 and decoupled Gr/MoSe2. This upshift cannot be explained by a change of doping Froehlicher and Berciaud (2015). The 2D mode shows more sensitivity to mechanical strain than to doping. However, an upshift of the 2D-mode frequency of around $15~{}{\textrm{cm}}^{-1}$ caused by strain would also lead to a G-mode upshift of around $7~{}{\textrm{cm}}^{-1}$ Lee et al. (2012); Metten et al. (2014), irrespective of $\Phi_{\rm ph}$. Such a shift is clearly not observed in all the figures cited above. Interestingly, a similar upshift of the 2D-mode feature has been observed in graphene deposited of thick boron nitride (BN) flakes Ahn et al. (2013); Forster et al. (2013). For Gr/BN, the 2D-mode upshift has been tentatively explained by dielectric screening due to the thick BN substrate, which reduces the electron-phonon coupling at the $K$ and $K^{\prime}$ points. It is not obvious that a similar explanation could hold for Gr/single-layer TMD because of the atomic thickness of the TMD. Since the 2D-mode feature interweaves the electron and phonon dispersions Maultzsch et al. (2004); Basko (2008); Venezuela et al. (2011); Ferrari and Basko (2013); Berciaud et al. (2013), another possible explanation could be the modification of the graphene band structure due to van der Waals coupling to MoSe2. However, in the case of MoS2/SLG, it has been calculated that the effects of the interaction on graphene band structure at $\Gamma$, $K$ and $K^{\prime}$ can be neglected Komsa and Krasheninnikov (2013); Pierucci et al. (2016). This intriguing observation of significant 2D-mode stiffening in vdWH will need further theoretical investigations to be fully understood.

Appendix S4 Discussion on optical interference effects

Optical interferences are known to affect the PL and Raman scattering response of 2DM deposited on layered substrates such as Si/SiO2 Yoon et al. (2009); Li et al. (2012); Buscema et al. (2014); Froehlicher et al. (2016). Here, we calculated a PL enhancement of only $5~{}\%$ for air/MoSe2/SiO2/Si as compared to air/Gr/MoSe2/SiO2/Si. This value is much too low to explain the observed PL quenching. We also calculated that for Gr/MoSe2, optical interference effects lead to a negligible enhancement of $I_{\textrm{2D}}/I_{\textrm{G}}$ by about $4~{}\%$ as compared to the case of Gr/SiO2.