# Fileset

[s41699-023-00438-5.pdf](https://mdr.nims.go.jp/filesets/fd2e8e4f-566d-46f9-b860-27bab82db73a/download)

## Creator

M. Zinkiewicz, M. Grzeszczyk, T. Kazimierczuk, M. Bartos, K. Nogajewski, W. Pacuski, [K. Watanabe](https://orcid.org/0000-0003-3701-8119), [T. Taniguchi](https://orcid.org/0000-0002-1467-3105), A. Wysmołek, P. Kossacki, M. Potemski, A. Babiński, M. R. Molas

## Rights

[Creative Commons BY Attribution 4.0 International](https://creativecommons.org/licenses/by/4.0/)

## Other metadata

[Raman scattering excitation in monolayers of semiconducting transition metal dichalcogenides](https://mdr.nims.go.jp/datasets/7b2b69ae-d222-4c9e-9c1c-6fed97dc8b62)

## Fulltext

Raman scattering excitation in monolayers of semiconducting transition metal dichalcogenidesARTICLE OPENRaman scattering excitation in monolayers of semiconductingtransition metal dichalcogenidesM. Zinkiewicz 1✉, M. Grzeszczyk1, T. Kazimierczuk1, M. Bartos2, K. Nogajewski1, W. Pacuski 1, K. Watanabe 3, T. Taniguchi 4,A. Wysmołek 1, P. Kossacki 1, M. Potemski1,5,6, A. Babiński1 and M. R. Molas 1✉Raman scattering excitation (RSE) is an experimental technique in which the spectrum is made up by sweeping the excitationenergy when the detection energy is fixed. We study the low-temperature (T= 5 K) RSE spectra measured on four high qualitymonolayers (ML) of semiconducting transition metal dichalcogenides (S-TMDs), i.e. MoS2, MoSe2, WS2, and WSe2, encapsulated inhexagonal BN. The outgoing resonant conditions of Raman scattering reveal an extraordinary intensity enhancement of thephonon modes, which results in extremely rich RSE spectra. The obtained spectra are composed not only of Raman-active peaks, i.e.in-plane E0 and out-of-plane A01, but the appearance of 1st, 2nd, and higher-order phonon modes is recognized. The intensityprofiles of the A01 modes in the investigated MLs resemble the emissions due to neutral excitons measured in the corresponding PLspectra for the outgoing type of resonant Raman scattering conditions. Furthermore, for the WSe2 ML, the A01 mode was observedwhen the incoming light was in resonance with the neutral exciton line. The strength of the exciton-phonon coupling (EPC) inS-TMD MLs strongly depends on the type of their ground excitonic state, i.e. bright or dark, resulting in different shapes of the RSEspectra. Our results demonstrate that RSE spectroscopy is a powerful technique for studying EPC in S-TMD MLs.npj 2D Materials and Applications             (2024) 8:2 ; https://doi.org/10.1038/s41699-023-00438-5INTRODUCTIONThe electron-phonon coupling is, in addition to the Coulombinteraction, one of the fundamental interactions betweenquasiparticles in solids1. It plays an important role in a variety ofphysical phenomena, in particular, low-energy electronic excita-tions can be strongly modified by coupling to lattice vibrations,which influences e.g. their transport2 and thermodynamic3properties.Semiconducting transition metal dichalcogenides (S-TMDs)based on molybdenum and tungsten, i.e. MoS2, MoSe2, MoTe2,WS2, and WSe2, are the most well-known representatives of vander Waals (vdW) materials4,5. Their most distinguished hallmark isthe transition from indirect- to direct-band gap, when thinneddown from a bulk to a monolayer (ML)6–9. Due to the very strongabsorption and direct energy band gap in the ML limit, in recentyears this class of materials has become of great interest fromboth research4,10–12 and development point of view13,14. Thephotoluminescence (PL) signal of S-TMDs is caused mainly byexcitonic effects, even at room temperature, due to the largeexcitonic binding energy at the level of hundreds of meV15–17. Itarises from the reduced dimensionality of the material and limiteddielectric screening of the environment18. Constant progress insample preparation, particularly the encapsulation of MLs in thinlayers of hexagonal BN (hBN)11, leads to narrowing of theobserved emission lines to the limit of a few meV, which opensthe possibility of studying a variety of individual excitoniccomplexes associated with both bright and dark states19–37.Raman scattering excitation (RSE) experiment performed onS-TMD MLs was proposed a few years ago in ref. 38 as a powerfultechnique to investigate the interaction between differentexcitonic complexes and phonons, i.e. exciton-phonon coupling(EPC). This approach is analogous to the PL excitation (PLE)method, in which the detected spectra are measured as a functionof the excitation energy, while the detection energy/window isfixed. In the RSE experiment, the detection window can be set tocover the emission of different excitonic complexes, while theexcitation energy is tuned in the energy region only slightly above(a few dozen of meV) these excitonic resonances. This results inthe outgoing resonant conditions of Raman scattering (RS)39. Theanalysis of the outgoing phonon modes crossing the excitonicemission allows one to reveal the details of the exciton-phononinteraction in a given material. The RSE spectroscopy might beseen as the extension of the resonant Raman scattering method(RRS), which was extensively used to study the characteristics ofphonon modes in S-TMDs40–46. However, a typical RRS experimentis carried out with a few selected excitation points42–44 offeringonly limited access to investigate the resonant changes in theshape/intensities of the phonon modes. This restriction is liftedwith the RSE measurements performed when practically con-tinuously tuning the excitation energy38,47,48. RSE spectra werepreviously reported for bare MLs of MoSe247 and WS238 exfoliatedon Si/SiO2 substrates, as well as for the MoSe2 ML encapsulated inhBN flakes48. In the case of the MoSe2 MLs47,48, the RSE spectrumwas dominated by several phonon replicas of the LA mode. Incontrast, for the WS2 ML38, a very rich Raman spectrum waspresented with numerous phonon modes originating from theedge of the Brillouin zone (BZ) as well as multiphonon modes.Knowing that S-TMD MLs are organised into two subgroups, i.e.bright and darkish, due to the type of the ground exciton state(bright and dark, respectively)28,32,34,49, their low-temperature1Institute of Experimental Physics, Faculty of Physics, University of Warsaw, 02-093 Warsaw, Poland. 2Central European Institute of Technology, Brno University ofTechnology, Brno 61200, Czech Republic. 3Research Center for Electronic and Optical Materials, National Institute for Materials Science, 1-1 Namiki, Tsukuba 305-0044, Japan.4Research Center for Materials Nanoarchitectonics, National Institute for Materials Science, 1-1 Namiki, Tsukuba 305-0044, Japan. 5Laboratoire National des Champs MagnétiquesIntenses, CNRS-UGA-UPS-INSA-EMFL, 38042 Grenoble, France. 6CENTERA Laboratories, Institute of High Pressure Physics, Polish Academy of Sciences, 01-142 Warsaw, Poland.✉email: malgorzata.zinkiewicz@fuw.edu.pl; maciej.molas@fuw.edu.plwww.nature.com/npj2dmaterialsPublished in partnership with FCT NOVA with the support of E-MRS1234567890():,;http://crossmark.crossref.org/dialog/?doi=10.1038/s41699-023-00438-5&domain=pdfhttp://crossmark.crossref.org/dialog/?doi=10.1038/s41699-023-00438-5&domain=pdfhttp://crossmark.crossref.org/dialog/?doi=10.1038/s41699-023-00438-5&domain=pdfhttp://crossmark.crossref.org/dialog/?doi=10.1038/s41699-023-00438-5&domain=pdfhttp://orcid.org/0000-0002-7472-5501http://orcid.org/0000-0002-7472-5501http://orcid.org/0000-0002-7472-5501http://orcid.org/0000-0002-7472-5501http://orcid.org/0000-0002-7472-5501http://orcid.org/0000-0001-8329-5278http://orcid.org/0000-0001-8329-5278http://orcid.org/0000-0001-8329-5278http://orcid.org/0000-0001-8329-5278http://orcid.org/0000-0001-8329-5278http://orcid.org/0000-0003-3701-8119http://orcid.org/0000-0003-3701-8119http://orcid.org/0000-0003-3701-8119http://orcid.org/0000-0003-3701-8119http://orcid.org/0000-0003-3701-8119http://orcid.org/0000-0002-1467-3105http://orcid.org/0000-0002-1467-3105http://orcid.org/0000-0002-1467-3105http://orcid.org/0000-0002-1467-3105http://orcid.org/0000-0002-1467-3105http://orcid.org/0000-0002-8302-2189http://orcid.org/0000-0002-8302-2189http://orcid.org/0000-0002-8302-2189http://orcid.org/0000-0002-8302-2189http://orcid.org/0000-0002-8302-2189http://orcid.org/0000-0002-7558-1044http://orcid.org/0000-0002-7558-1044http://orcid.org/0000-0002-7558-1044http://orcid.org/0000-0002-7558-1044http://orcid.org/0000-0002-7558-1044http://orcid.org/0000-0002-5516-9415http://orcid.org/0000-0002-5516-9415http://orcid.org/0000-0002-5516-9415http://orcid.org/0000-0002-5516-9415http://orcid.org/0000-0002-5516-9415https://doi.org/10.1038/s41699-023-00438-5mailto:malgorzata.zinkiewicz@fuw.edu.plmailto:maciej.molas@fuw.edu.plwww.nature.com/npj2dmaterials(T ~ 4.2–20 K) PL spectra display completely different complexity.The high quality of the MLs embedded between the hBN flakes,accompanied by the division of the MLs into two subgroups, hasmotivated us to conduct a comprehensive study devoted to theEPC in such S-TMD MLs with the aid of the RSE technique.In this work, we use the RSE technique to investigate theexciton-phonon interaction in four high-quality samples con-sisting of the MoS2, MoSe2, WS2, and WSe2 MLs encapsulated inhBN flakes. We observe the intensity enhancements of Ramanmodes, while their emission energies match the emission energyof the corresponding neutral exciton (X). The measured RSEspectra are composed of many peaks that can be attributed notonly from the BZ centre (Γ points) but also from other points inthe BZ (e.g. M points), which are followed by lines arising frommultiphonon processes. In the case of outgoing resonanceconditions with the X emission, the intensity profiles of the A01modes in the studied MLs resemble the emission lines of thecorresponding X emission. The A01 modes’ enhancements areextraordinarily strong for the WS2 and WSe2 MLs, but thecorresponding increases of the A01 peaks in the MoS2 and MoSe2MLs are significantly smaller. Instead, for the Mo-based MLs, themodes involving LA phonons from the edge of the BZ, such as2LA, 3LA, and so on, are greatly enhanced. Moreover, weobserve that the A01 intensity is also significantly enhanced whenthe excitation energy is in the vicinity of the X emission, whichleads to the incoming resonance condition. Our experimentsdemonstrate that the landscape of the exciton-phonon interac-tion, as traced with our RSE experiments, appears to bequalitatively different in two distinct subgroups, of bright anddarkish, S-TMD monolayers. The specific alignment of single-particle energy bands and the related characteristic scatteringprocesses are speculated to account for the observeddifferences.RESULTSRaman scattering excitation spectraFigure 1 presents false colour maps of the low-temperature(T= 5 K) emission intensities collected for the MoS2, MoSe2, WS2and WSe2 MLs encapsulated in hBN flakes. The vertical position ofa given map has been shifted to reflect the relative energy of theneutral exciton (X) emission. The assignment of the X line asarising from the neutral A exciton is straightforward andconsistent with many other studies on these MLs11,17,28,34,36. Wefocus on the X lines, but we are aware that the low-temperaturePL spectra of the investigated MLs are composed of severaladditional emission lines (see, for example, Fig. 5), particularly dueto charged excitons, dark complexes, and the correspondingphonon replicas22,23,29,31,36,50–54.It can be seen that the line shape of the detected signalsignificantly depends on the excitation energy. Several parallelnarrow lines superimposed on neutral exciton emissions areclearly observed in Fig. 1. These sharp lines follow the tunedexcitation energy, which points out the Raman scattering as theirorigin and are examined in detail in the following. Moreover, forthe MoS2 and WS2 MLs, the X emissions are strongly enhancedwith decreasing excitation energies towards the respective Xenergies. The observed enhancement of the X intensity can bedescribed in terms of extremely efficient formation of neutralexcitons at larger k-vectors due to the near-resonant excitation.Note that the laser line used in experiments is narrow enough toinvestigate individual narrow peaks, but the excitation range ofmeasurements is limited by the spectral range of the lasers.Moreover, the dye laser applied for MoS2 measurements has alimited tuning accuracy. This leads to the observation of thenarrow lines only at specific excitation and detection energies, incontrast to almost continuous linear evolutions present for otherMLs, see Fig. 1. As is seen in the figure, the intensities of theRaman modes strongly depend on the material. For Mo-basedMLs, i.e. MoS2 and MoS2, the intensity enhancement of different1.930 1.935 1.940 1.9451.961.982.002.022.042.06Excitationenergy(eV)(a) (d)(c)(b)A'11.630 1.635 1.640 1.6451.661.681.701.721.741.76Detection energy (eV)highlow2.045 2.050 2.055 2.0602.082.102.122.142.16A'1A'1A'11.715 1.720 1.725 1.7301.741.761.781.801.821.84A'1EnergydistancefromXemission(meV)MoS2MoSe2 WS2WSe220406080100120Fig. 1 Raman scattering excitation. False-colour maps of optical response of the a MoS2, b MoSe2, c WS2, and d WSe2 MLs encapsulated inhBN measured at low temperature (T= 5 K) under excitation of tuneable lasers (excitation power ~ 150 μW). The maps are vertically alignedwith respect to the energy distance from the energy of the neutral exciton (X) emission in these materials, which is marked on the red right-hand energy scales. The colour scales have been normalised to the maximum intensity. The detection energies of the maps are centredaround the X emission. The phonon modes are visible as several narrow parallel resonances passing diagonally across the maps. For the MoS2ML, white arrows point out lines which are investigated in the following.M. Zinkiewicz et al.2npj 2D Materials and Applications (2024)     2 Published in partnership with FCT NOVA with the support of E-MRS1234567890():,;phonon modes is similar and relatively small compared to the Xintensity. In contrast, extraordinary increases in the selectedRaman peaks accompanied by a large number of smaller modesare observed in the spectra measured on the WS2 and WSe2 MLs.In order to investigate in detail the apparent phonon modes inthe aforementioned optical responses of the MLs, the RSE spectraare plotted in Fig. 2. The RSE spectra were obtained by fixing thedetection energy at the emission energy of the X line, while theexcitation energy was being tuned. Note that the horizontalenergy scale in the figure corresponds to the relative distancebetween the excitation and X energies, and is given in cm−1,which is a typical unit for RS experiments and represents the so-called Raman shift. As can be seen in the inset of Fig. 2, theintensity of the A01 modes in the W-based MLs is extraordinarilyhigh and exceeds the X emission several times (compare with Fig.1c and d). In contrast, the phonon modes related to thelongitudinal acoustic branch, i.e. 2LA, 3LA, …, are enhanced forthe Mo-based MLs, while the corresponding intensities of the A01modes are much smaller. All of the presented RSE spectra are veryrich, consisting of many phonon modes. This confirms theresonant excitation conditions of RS, since the correspondingnon-resonant Raman spectra are composed of only two Raman-active modes in the backscattering geometry of the experiment,i.e. A01 and E055, whereas RS spectra become especially rich underresonant excitation conditions40. Using the previous resultspresented in the literature38,40,43,56–58, we have identified almostall observed peaks, marked in Fig. 2 with vertical black and greyarrows. Their energies and assignments to the respective phononsare summarised in the Supporting Information (SI). Besides thetwo Raman-active modes from the Γ point of the BZ, the majorityof the observed peaks were ascribed to phonons from the Mpoints, which are located at the edge of the hexagonal BZ of theMLs in the middle between the K+ and K− points. For example, wehave identified peaks related to combinations of all threebranches of acoustic phonons (e.g. LA, ZA, TA) as well as theirhigher orders (e.g. 2LA, 3LA,…); see the SI for details. Theirobservation can be associated with the resonant excitation of RS,which results in the appearance of Raman inactive momentum-conserving combinations of acoustic modes from the edge of theBZ40,59,60.We have also investigated the RSE spectrum measured on theMoSe2 ML grown on hBN flake using the molecular beam epitaxy(MBE) technique61. The ML was also covered with a thin hBN flakeusing mechanical exfoliation and dry transfer technique. Thespectra measured for epitaxial layers exhibit peaks at energiessimilar to those in the case of exfoliated layers, but the relativeintensity of various peaks differs. Particularly pronounced forepitaxial layers are LA replicas, which are observed from 2nd to12th order, see SI for details.Enhancement profiles of A01 modes due to outgoing resonanceThe resonant enhancement of the phonon modes presentedabove originates from the so-called outgoing resonance, whichrequires that the resonant excitation energy must be equal to thesum of the phonon and exciton energies58. To verify thishypothesis we compare the PL emissions of the X lines and theintegrated intensities of the A01 peaks in Fig. 3. The choice of A01modes is motivated by their substantial intensities compared tothe remaining phonon modes, see Fig. 2. Note that we alsoFig. 2 RSE spectra. Raman scattering excitation (RSE) spectradetected at the energies of the X line measured on the MoS2,MoSe2, WS2, and WSe2 MLs encapsulated in hBN. The vertical scalehas been adjusted to make the low-intensity peaks visible, while theinset shows the RSE spectra with the most pronounced peaks. Notethat the RSE spectrum of the MoSe2 ML was multiplied by a factor of2.5 for clarity. The black arrows denote the labelled peaks in theFigure, while the grey arrows point to the identified peaks with theirassignment presented in Supplementary Table 1 in the SI. The A01peaks are marked with their energy in cm−1.2.050 2.055 2.060015304560Energy (eV)05101520WS2 WSe21.720 1.725 1.73002040608005101520IntegratedA' 1intensity(arb.u.)(b)(a) (c) (d)1.636 1.641 1.6460246810MoSe20.00.10.20.30.41.935 1.940 1.945020406080100PLcounts(x102)MoS20.00.10.20.30.4Fig. 3 PL emission versus RSE response. Comparison of (orange points) PL emissions of the X lines and (blue points) integrated intensities ofthe A01 peaks measured on the a MoS2, b MoSe2, c WS2, and d WSe2 MLs encapsulated in hBN. Solid black curves represent the correspondingfitting using Lorentzian function.M. Zinkiewicz et al.3Published in partnership with FCT NOVA with the support of E-MRS npj 2D Materials and Applications (2024)     2 analyse some other selected modes, e.g. 2ZA, 2LA, etc., in the SI,which show a similar behaviour as the one found for A01. As can beseen in Fig. 3, the data were fitted using Lorentzian functions,which nicely reproduce the profiles shown for both the X line andthe A01 peak. Furthermore, the parameters of the Lorentzianprofiles fitted for the X lines and the A01 peaks are summarised inTable 1. Except for the results obtained for the MoS2 ML, theextracted profiles for the A01 peaks, described by their energies andlinewidths, follow the shape of the X emission line. For the MoS2ML, the observed small discrepancy in the energies of the X andA01 peaks of about 1 meV may be explained in terms of the smallenergy resolution of the laser used in this case. Nevertheless, theresults obtained confirm that the enhancements of the A01 peaks inthe S-TMD MLs are due to the outgoing resonance.Outgoing versus incoming resonanceFigure 4a and b shows correspondingly a schematic illustration ofthe incoming and outgoing excitation resonance with theexcitonic transition (X). The resonance type depends on theresonant conditions between the incident or scattered light withthe X energy, while the energy difference between the excitationand the emission amounts to the phonon energy. The resultspresented so far are associated with the outgoing X resonance,which occurs when the scattered-photon energy is equal to thatof the optical X transition. In contrast, the incoming X resonancetakes place when the incident photon energy equals the energy ofan optical X transition.To study the possibility of achieving the incoming resonantconditions in S-TMD MLs, we measured the optical response of theWSe2 ML in both the outgoing and incoming resonanceconditions, see Fig. 5. The low-temperature PL spectrum of theWSe2 ML encapsulated in hBN flakes, presented in panel (a) of thefigure, displays several emission lines with a characteristic patternsimilar to that previously reported in several works on WSe2 MLsembedded in between hBN flakes19–37. Panel (b) and (c) displaycorrespondingly the optical response of the WSe2 ML in theoutgoing and incoming resonances with the neutral excitonemission. As for the outgoing conditions, the observed results areanalogous to those shown in Fig. 1, the incoming resonance leadsto the two prominent effects. The first one, seen as a significantenhancement of all the emission lines, is associated with theresonant excitation to the X transition. Due to these conditions,the shape of the PL spectrum is strongly modified by an enormousincrease in the emission intensity of the negatively chargedbiexciton (XX−). The outstanding XX− increment can be explainedin terms of the considerable n-type doping of the studied WSe2ML, which results in a large reservoir of negative dark trions21–23,36.Due to the fact that a negatively charged biexciton is formed by anegative dark trion and a neutral bright exciton (X)21–23,36, theresonant excitation of the X complex should cause the creation ofa large population of XX−, as is seen in Fig. 5c. The second effect isassociated with the observation of several narrow peaks, parallelto the excitation laser, which are superimposed on the enhancedemission lines below the X peak. These peaks can be ascribed tophonon modes, whose intensities are enlarged due to theincoming resonance. As in the outgoing case, the increase inthe intensity depends on the phonon symmetry, resulting in themost pronounced A01 peak.Alike in Fig. 3, we analyse the intensity profiles of the A01 modemeasured on the WSe2 ML under the incoming and outgoingresonance conditions of the Raman scattering with the X line, seeFig. 6. It can be seen that the A01 evolutions can be nicelydescribed by Lorentzian functions with similar linewidths, whiletheir intensities differ considerably. The enhancement of the A01intensity is about 3 times larger in the outgoing resonance ascompared to the incoming one, which can be because of severaldifferences, such as: the strength of the exciton-phonon couplingin these two regimes, involvement of other excitonic states (e.g.dark trion or dark exciton) and of emission/absorption subpro-cesses, etc. For a given phonon mode, the energy separationbetween the outgoing and incoming resonances with a particulartransition should be equal to this phonon energy. The obtainedenergy separation for the A01 mode presented in Fig. 6 is of about30 meV. This value is equivalent of 242 cm−1, which is very closeto the A01 wavenumber from Fig. 2 (~251 cm−1).DISCUSSIONIn the literature on the subject, a physical picture has been so fardrawn that the exciton-phonon coupling in S-TMD MLs can beunderstood considering the symmetries of phonon modes withrespect to the symmetries of orbitals associated with the involvedtransitions (excitons)42. For example, it has been established thatthe A01 mode is enhanced when the excitation laser is inresonance with A and B excitons in S-TMDs, while the E0 intensityis increased under resonanant conditions matching the energy ofC excitons42,43. Our results, shown in Fig. 2, demonstrate asubstantial difference in the intensity and complexity of the RSEspectra measured on the Mo- and W-based MLs. While the A01modes are extraordinarily enhanced for the WS2 and WSe2 MLsleading to the extremely rich RSE spectra (up to 16 identifiedphonon modes in the WS2 ML), the RSE spectra of the MoS2 andMoSe2 MLs are dominated by the modes involving acousticphonons, e.g. 2LA, 3LA, …. In our opinion, this difference can beunderstood in terms of the excitonic states in the vicinity of theirband gaps. Although S-TMD MLs share a very similar bandstructure, i.e. they are direct band-gap semiconductors with theminima (maxima) of the conduction (valence) band located at theK+ and K− points of the BZ4,5, the arrangement of the opticallyactive transitions in the vicinity of their band gaps is different. Astrong spin-orbit coupling results in spin-split and spin-polarisedsubbands in both the valence band (VB) and the conduction band(CB). Consequently, MLs of S-TMD are organised into twosubgroups, i.e. bright and darkish, due to the type of the groundexciton state (bright and dark, respectively)49. In bright MLs, theTable 1. Summary of the obtained fitting parameters of the Xemission lines and A01 profiles using Lorentzian functions, shown inFig. 3.MoS2 MoSe2 WS2 WSe2X xc (eV) 1.940 1.641 2.057 1.727X w (meV) 8.3 4.9 5.7 5.5A01 xc (eV) 1.939 1.642 2.057 1.725A01 w (meV) 4.6 4.8 4.5 5.3xc and w represent fitted energy position and the full width at halfmaximum (FWHM, linewidth), respectively.noitaticxeexcitationemissionemissionphononphononincoming X resonance outgoing X resonanceX X(a) (b)Fig. 4 Resonant Raman scattering. Schematic illustration of thea incoming and b outgoing excitation resonance with the excitonictransition, denoted as X.M. Zinkiewicz et al.4npj 2D Materials and Applications (2024)     2 Published in partnership with FCT NOVA with the support of E-MRSexcitonic recombination between the top VB and the bottom CBis optically active (bright), while the opposite happens darkishMLs. Nowadays, it is well established that for MLs encapsulated inhBN flakes, MoSe2 is bright, while MoS2, WS2, and WSe2 MLs aredarkish (see refs. 12,28,32,34). It means that the observed differencein the intensity of the phonon modes in the RSE spectra, shown inFig. 2, may coincide with a particular alignment of bright and darkstates in S-TMD MLs.The substantial enhancement of the LA phonons from the Mpoints in the MoSe2 has been explained by the efficient phonon-assisted scattering occurring between the bright X exciton and thedark indirect exciton (IX) formed by an electron and a hole locatedat the Q and K points, respectively (see ref. 47 for details). Recently,similar description of the EPC, which occurs between the acousticphonons from the M point and the continuum dark exciton statesrelated to the optically forbidden transition at K and Q valleys, wasdemonstrated in thin layers of WS262. This confirms that for theacoustic phonons there is no significant difference between theMoSe2 (bright) and WS2 (darkish) MLs. Simultaneously, the smallincrease of the A01(Γ) intensity in the MoS2 ML suggests that thestrength of the EPC is not remarkable. In contrast, theextraordinarily high intensity of the Raman-active A01(Γ) phononmodes (does not require any additional scattering betweendifferent points in the BZ) in the W-based MLs resulting in thevery rich Raman spectra can not be described using the sameapproach, see Fig. 2. Note that the similar difference of one orderof magnitude in increment of the A01 peaks in outgoing conditionswere reported for the MoSe2 and WS2 MLs44, which corroborateour results.Particularly, similar results obtained for the WS2 and WSe2 MLsmay suggest the involvement of the ground dark excitons in theefficient EPC in these materials. In the following, we speculate onthe possible mechanisms, which may affect the EPC in thesedarkish materials. As the ground exciton state in W-based MLs isdark, the intensity of the bright exciton emission might becontrolled by concurrent processes of the radiative recombination(emission of photons) and the relaxation process to the darkexciton and dark trion (depending on the doping level of the ML).It is known that the relative intensity of the bright and darkexcitons in WSe2 is a function of temperature63 with a significantquenching of the X emission at low temperature. Consequently,large reservoirs of dark excitons and dark trions in W-based MLsare formed at low temperature12,28,34, while the analogous oneswill be absent in bright MLs (ground exciton state is bright). Theselong-lived reservoirs may increase the probability of intravalleyscattering between bright excitons and dark complexes, facilitat-ing the emission of phonons from Γ points in the WS2 and WSe2MLs. It suggests that the strength of the EPC in S-TMD MLs is notonly associated with the symmetries of both phonons andelectronic bands42, but may be affected by other phenomena,such as the relative order of excitonic states in the vicinity of directtransitions in K points of their BZ. The most complex are theresults obtained for the MoS2 ML, which are very similar to theones of the MoSe2 ML, i.e. the LA phonons from the M points aresignificantly enhanced. It is known that the MoS2 MLs encapsu-lated in hBN flakes exhibit dual character. The theoreticallypredicted structure of the CB and VB yields optical activity of theenergetically lowest transition, while including the excitoniceffects leads to the dark ground excitonic state32,50. This mayindicate that the EPC in the MoS2 ML is more associated with itselectronic band structure than with the excitonic one. Theaforementioned phenomena, which may be responsible for theEPC in S-TMD MLs, requires solid theoretical analysis, which isbeyond the scope of our experimental work.Summarising, we have presented the investigation of theexciton-phonon interaction in four high-quality samples consistingof MoS2, MoSe2, WS2, and WSe2 MLs encapsulated in hBN flakes.By sweeping the excitation energy for a fixed value of thedetection energy, we have observed an astonishing amplificationof phonon-modes’ intensity, while the detection energy was inresonance with neutral exciton emission. Due to this phenom-enon, the measured RSE spectra are composed of many phononmodes originating not only from the BZ centre (Γ points) but alsofrom others (e.g. M points), which are followed by lines due tomultiphonon processes. For the outgoing X resonance, we havefound that the intensity profiles of the A01 modes in the studiedMLs resemble the emission lines of the corresponding X emission.The A01 enhancements are extraordinarily strong for the WS2 and1.69 1.70 1.71 1.72 1.7302468Emission energy (eV)IntegratedA' 1intensity(arb.u.) ~30 meV 242 cm-1x3Fig. 6 RSE profiles of the A01 mode. Intensity profiles of the A01mode measured on the WSe2 ML encapsulated in hBN flakes. Blueand orange circles correspond to the incoming and outgoingresonance conditions of the Raman scattering with the X line.1.65 1.66 1.67 1.68 1.69 1.70 1.71 1.72 1.731.721.731.741.751.761.77Detection energy (eV)A'1(a)(c)Excitationenergy(eV)A'1(b)highlowPLint.(arb.u.)XDE"(�)XX-XD XITS TTXXXWSe2TDlaserFig. 5 Outgoing versus incoming resonant conditions. a Low-temperature (T= 5 K) PL spectrum of the WSe2 ML encapsulated inhBN flakes measured using excitation energy of 2.4 eV and power of150 μW. The lines' assignments are as follows: X—neutral exciton; XX—neutral biexciton; TS and TT—singlet (intravalley) and triplet(intervalley) negatively charged excitons, respectively; XI and XD—momentum- and spin-forbidden neutral dark excitons, respectively;XX−—negatively charged biexciton; TD—negatively charged darkexciton (dark trion); XDE00ðΓÞ—phonon replica of the XD complex.b, c False-colour maps of low-temperature (T= 5 K) optical responseof the WSe2 MLs encapsulated in hBN under excitation of tuneable-energy lasers (excitation power ~ 10 μW) measured in the vicinity ofthe outgoing and incoming resonance conditions of Ramanscattering with the X line, respectively. The colour scales in panels(b) and (c) have been normalised to the maximum intensity. Thevertical red dashed lines denote the edge of the used long-passfilters.M. Zinkiewicz et al.5Published in partnership with FCT NOVA with the support of E-MRS npj 2D Materials and Applications (2024)     2 WSe2 MLs, but an analogous effect in the MoS2 and MoSe2 MLs aresignificantly smaller. For the Mo-based MLs, the modes involvingacoustic phonons, such as 2LA and 3LA, are significantlyenhanced. Moreover, we have observed that the A01 intensity isalso greatly strengthened since the excitation energy is in thevicinity of the X emission, leading to the incoming resonancecondition. We have proposed that the difference in the obtainedRSE measured on S-TMD MLs at low temperature can beunderstood in terms of the division of MLs into bright and darkishsubgroups. These results shine new light on the exciton-phononinteraction in the MLs of S-TMDs, pointing out that not only thesymmetries of phonons and excitons play an important role in thisprocess, but also the type of their ground excitonic states.METHODSSample preparationThe studied samples were composed of four S-TMD MLs, i.e. MoS2,MoSe2, WS2 and WSe2, encapsulated in hBN flakes and supportedby a bare Si substrate. The structures were obtained by two-stagepolydimethylsiloxane (PDMS)-based64 mechanical exfoliation ofbulk crystals of S-TMDs and hBN. A bottom layer of hBN inheterostructures was created in the course of non-deterministicexfoliation to achieve the highest quality. The assembly ofheterostructures was realised via successive dry transfers of aML and capping hBN flake from PDMS stamps onto the bottomhBN layer.Experimental setupOptical measurements were performed at low temperature(T= 5 K) using typical setups for the PL and PLE experiments.The investigated sample was placed on a cold finger in acontinuous-flow cryostat mounted on x–y manual positioners. Thenon-resonant PL measurements were carried out using 514.5 nm(2.41 eV) and 532 nm (2.33 eV) radiations from the continuouswave Ar+ and Nd:YAG lasers, respectively. To study the opticalresponse of a ML as a function of excitation energy, i.e. to measurePLE spectra, two types of tuneable lasers were used: dye lasersbased on Rhodamine 6G and DCM, and a Ti:Sapphire laser. Theexcitation light was focused by means of a 50 × long-workingdistance objective that produced a spot of about 1 μm diameter.The signal was collected via the same microscope objective, sentthrough a monochromator, and then detected by a charge-coupled device (CCD) camera. Measurements of the low-energypart of the RSE spectra, i.e. from around 15 meV from the laserline, were carried out using ultra steep edge long-pass filtersmounted in front of the spectrometer.DATA AVAILABILITYThe data that support the findings of this work are available from the correspondingauthors upon reasonable request.Received: 17 August 2023; Accepted: 11 December 2023;REFERENCES1. Vogl, P. in Physics of Nonlinear Transport in Semiconductors (NATO Science SeriesB) Vol. 52 (Springer New York, 1980).2. Huewe, F. et al. Energy exchange between phononic and electronic subsystemsgoverning the nonlinear conduction in DCNQI2 Cu. Phys. Rev. B 92, 155107 (2015).3. Zhou, J. et al. Direct observation of large electron-phonon interaction effect onphonon heat transport. Nat. Commun. 11, 6040 (2020).4. Koperski, M. et al. Optical properties of atomically thin transition metal dichal-cogenides: observations and puzzles. Nanophotonics 6, 1289 (2017).5. Wang, G. et al. Colloquium: Excitons in atomically thin transition metal dichal-cogenides. Rev. Mod. Phys. 90, 021001 (2018).6. Mak, K. F., He, K., Shan, J. & Heinz, T. F. Control of valley polarization in monolayerMoS2 by optical helicity. Nat. Nanotechnol. 7, 494 (2012).7. Arora, A. et al. Excitonic resonances in thin films of WSe2: from monolayer to bulkmaterial. Nanoscale 7, 10421 (2015).8. Arora, A., Nogajewski, K., Molas, M., Koperski, M. & Potemski, M. Exciton bandstructure in layered MoSe2: from a monolayer to the bulk limit. Nanoscale 7,20769–20775 (2015).9. Molas, M. R. et al. The optical response of monolayer, few-layer and bulk tungstendisulfide. Nanoscale 9, 13128–13141 (2017).10. Mak, K. F., Lee, C., Hone, J., Shan, J. & Heinz, T. F. Atomically thin MoS2: a newdirect-gap semiconductor. Phys. Rev. Lett. 105, 136805 (2010).11. Cadiz, F. et al. Excitonic linewidth approaching the homogeneous limit in MoS2-based van der Waals heterostructures. Phys. Rev. X 7, 021026 (2017).12. Robert, C. et al. Fine structure and lifetime of dark excitons in transition metaldichalcogenide monolayers. Phys. Rev. B 96, 155423 (2017).13. Duan, X., Wang, C., Pan, A., Yu, R. & Duan, X. Two-dimensional transition metaldichalcogenides as atomically thin semiconductors: opportunities and chal-lenges. Chem. Soc. Rev. 44, 8859–8876 (2015).14. Ye, Y. et al. Electrical generation and control of the valley carriers in amonolayer transition metal dichalcogenide. Nat. Nanotechnol. 11, 598–602(2016).15. Stier, A. V., Wilson, N. P., Clark, G., Xu, X. & Crooker, S. A. Probing the influence ofdielectric environment on excitons in monolayer WSe2: insight from high mag-netic fields. Nano Lett. 16, 7054–7060 (2016).16. Goryca, M. et al. Revealing exciton masses and dielectric properties of monolayersemiconductors with high magnetic fields. Nat. Commun. 10, 4172 (2019).17. Molas, M. R. et al. Energy spectrum of two-dimensional excitons in a nonuniformdielectric medium. Phys. Rev. Lett. 123, 136801 (2019).18. Zhu, B., Chen, X. & Cui, X. Exciton binding energy of monolayer WS2. Sci. Rep. 5,9218 (2015).19. Courtade, E. et al. Charged excitons in monolayer WSe2: experiment and theory.Phys. Rev. B 96, 085302 (2017).20. Li, Z. et al. Revealing the biexciton and trion-exciton complexes in BN encapsu-lated WSe2. Nat. Commun. 9, 3719 (2018).21. Chen, S.-Y., Goldstein, T., Taniguchi, T., Watanabe, K. & Yan, J. Coulomb-boundfour- and five-particle intervalley states in an atomically-thin semiconductor. Nat.Commun. 9, 3717 (2018).22. Barbone, M. et al. Charge-tuneable biexciton complexes in monolayer WSe2. Nat.Commun. 9, 3721 (2018).23. Paur, M. et al. Electroluminescence from multi-particle exciton complexes intransition metal dichalcogenide semiconductors. Nat. Commun. 10, 1709 (2019).24. Liu, E. et al. Gate tunable dark trions in monolayer WSe2. Phys. Rev. Lett. 123,027401 (2019).25. Li, Z. et al. Direct observation of gate-tunable dark trions in monolayer WSe2.Nano Lett. 19, 6886 (2019).26. Li, Z. et al. Emerging photoluminescence from the dark-exciton phonon replica inmonolayer WSe2. Nat. Commun. 10, 2469 (2019).27. Li, Z. et al. Momentum-dark intervalley exciton in monolayer tungsten diselenidebrightened via chiral phonon. ACS Nano 13, 14107 (2019).28. Molas, M. R. et al. Probing and manipulating valley coherence of dark excitons inmonolayer WSe2. Phys. Rev. Lett. 123, 096803 (2019).29. Liu, E. et al. Valley-selective chiral phonon replicas of dark excitons and trions inmonolayer WSe2. Physical Review Research 1, 032007 (2019).30. Liu, E. et al. Multipath optical recombination of intervalley dark excitons andtrions in monolayer WSe2. Phys. Rev. Lett. 124, 196802 (2020).31. He, M. et al. Valley phonons and exciton complexes in a monolayer semi-conductor. Nat. Commun. 11, 618 (2020).32. Robert, C. et al. Measurement of the spin-forbidden dark excitons in MoS2 andMoSe2 monolayers. Nat. Commun. 11, 4037 (2020).33. Lu, Z. et al. Magnetic field mixing and splitting of bright and dark excitons inmonolayer MoSe2. 2D Mater. 7, 015017 (2019).34. Zinkiewicz, M. et al. Neutral and charged dark excitons in monolayer WS2.Nanoscale 12, 18153–18159 (2020).35. Robert, C. et al. Measurement of conduction and valence bands g-factors in atransition metal dichalcogenide monolayer. Phys. Rev. Lett. 126, 067403 (2021).36. Zinkiewicz, M. et al. Excitonic complexes in n-doped WS2 monolayer. Nano Lett.21, 2519–2525 (2021).37. Zinkiewicz, M. et al. The effect of dielectric environment on the brightening ofneutral and charged dark excitons in WSe2 monolayer. Appl. Phys. Lett. 120,163101 (2022).38. Molas, M. R., Nogajewski, K., Potemski, M. & Babiński, A. Raman scattering exci-tation spectroscopy of monolayer WS2. Sci. Rep. 7, 5036 (2017).M. Zinkiewicz et al.6npj 2D Materials and Applications (2024)     2 Published in partnership with FCT NOVA with the support of E-MRS39. Feng, Z. C., Perkowitz, S., Wrobel, J. M. & Dubowski, J. J. Outgoing multiphononresonant raman scattering and luminescence near the E0+ Δ0 gap in epitaxialCdTe films. Phys. Rev. B 39, 12997–13000 (1989).40. Gołasa, K. et al. Multiphonon resonant Raman scattering in MoS2. Appl. Phys. Lett.104, 092106 (2014).41. Livneh, T. & Sterer, E. Resonant Raman scattering at exciton states tuned bypressure and temperature in 2h-MoS2. Phys. Rev. B 81, 195209 (2010).42. Carvalho, B. R., Malard, L. M., Alves, J. M., Fantini, C. & Pimenta, M. A. Symmetry-dependent exciton-phonon coupling in 2d and bulk MoS2 observed by reso-nance Raman scattering. Phys. Rev. Lett. 114, 136403 (2015).43. Soubelet, P., Bruchhausen, A. E., Fainstein, A., Nogajewski, K. & Faugeras, C.Resonance effects in the Raman scattering of monolayer and few-layer MoSe2.Phys. Rev. B 93, 155407 (2016).44. McDonnell, L. P., Viner, J. J. S., Rivera, P., Xu, X. & Smith, D. C. Observation ofintravalley phonon scattering of 2s excitons in MoSe2 and WSe2 monolayers. 2DMater. 7, 045008 (2020).45. Kumar, D., Singh, B., Kumar, R., Kumar, M. & Kumar, P. Davydov splitting, reso-nance effect and phonon dynamics in chemical vapor deposition grown layeredMoS2. Nanotechnology 32, 285705 (2021).46. Klein, J. et al. Electrical control of orbital and vibrational interlayer coupling in bi-and trilayer 2H-MoS2. Phys. Rev. Mater. 6, 024002 (2022).47. Chow, C. M. et al. Phonon-assisted oscillatory exciton dynamics in monolayerMoSe2. npj 2D Mater. Appl. 1, 33 (2017).48. Shree, S. et al. Observation of exciton-phonon coupling in MoSe2. Phys. Rev. B 98,035302 (2018).49. Molas, M. R. et al. Brightening of dark excitons in monolayers of semiconductingtransition metal dichalcogenides. 2D Mater. 4, 021003 (2017).50. Grzeszczyk, M. et al. Exposing the trion’s fine structure by controlling the carrierconcentration in hBN-encapsulated MoS2. Nanoscale 13, 18726–18733 (2021).51. Klein, J. et al. Trions in mos2 are quantum superpositions of intra- and intervalleyspin states. Phys. Rev. B 105, L041302 (2022).52. Park, S. et al. Efficient valley polarization of charged excitons and resident carriersin molybdenum disulfide monolayers by optical pumping. Commun. Phys. 5, 73(2022).53. Smoleński, T. et al. Interaction-induced Shubnikov–de Haas oscillations in opticalconductivity of monolayer MoSe2. Phys. Rev. Lett. 123, 097403 (2019).54. Rodek, A. et al. Controlled coherent-coupling and dynamics of exciton complexesin a MoSe2 monolayer. 2D Mater. 10, 025027 (2023).55. Zhang, X. et al. Phonon and Raman scattering of two-dimensional transitionmetal dichalcogenides from monolayer, multilayer to bulk material. Chem. Soc.Rev. 44, 2757–2785 (2015).56. Shi, W. et al. Raman and photoluminescence spectra of two-dimensional nano-crystallites of monolayer WS2. 2D Mater. 3, 025016 (2016).57. Carvalho, B. R. et al. Intervalley scattering by acoustic phonons in two-dimensional MoS2 revealed by double-resonance Raman spectroscopy. Nat.Commun. 8, 14670 (2017).58. Zinkiewicz, M. et al. Emission excitation spectroscopy in WS2 monolayer encap-sulated in hexagonal BN. Acta Phys. Pol. A 136, 624–627 (2019).59. Molas, M. R. et al. Tuning carrier concentration in a superacid treated MoS2monolayer. Sci. Rep. 9, 1989 (2019).60. Bhatnagar, M. et al. Temperature induced modulation of resonant Raman scat-tering in bilayer 2H-MoS2. Sci. Rep. 12, 14169 (2022).61. Pacuski, W. et al. Narrow excitonic lines and large-scale homogeneity oftransition-metal dichalcogenide monolayers grown by molecular beam epitaxyon hexagonal boron nitride. Nano Lett. 20, 3058–3066 (2020).62. Tan, Q.-H. et al. Quantum interference between dark-excitons and zone-edgedacoustic phonons in few-layer WS2. Nat. Commun. 14, 88 (2023).63. Zhang, X.-X., You, Y., Zhao, S. Y. F. & Heinz, T. F. Experimental evidence for darkexcitons in monolayer WSe2. Phys. Rev. Lett. 115, 257403 (2015).64. Castellanos-Gomez, A. et al. Deterministic transfer of two-dimensional materialsby all-dry viscoelastic stamping. 2D Mater. 1, 011002 (2014).ACKNOWLEDGEMENTSThe work was supported by the National Science Centre, Poland (grants no. 2017/27/B/ST3/00205, 2018/31/B/ST3/02111, and 2021/41/B/ST3/04183), EU Graphene Flag-ship Project, and the CNRS via IRP “2DM” project. M.B. acknowledges support fromthe Grant Agency of the Czech Republic under grant no. 23-05578S. K.W. and T.T.acknowledge support from the JSPS KAKENHI (Grant Numbers 21H05233 and23H02052) and World Premier International Research Center Initiative (WPI), MEXT,Japan. M.P. acknowledges the support from the Foundation for Polish Science (MAB/2018/9 Grant within the IRA Program financed by EU within SG OP Program).AUTHOR CONTRIBUTIONSM.Z., M.G., T.K., A.W., P.K., M.P., A.B. and M.R.M. performed the experiments. K.N. andM.B. fabricated the samples with exfoliated monolayers. W.P. grew the sample withan epitaxial MoSe2 monolayer K.W. and T.T. grew the hBN crystals. M.R.M. initiatedand supervised the project. M.Z. and M.R.M. wrote the manuscript with inputs fromall co-authors.COMPETING INTERESTSThe authors declare no competing interests.ADDITIONAL INFORMATIONSupplementary information The online version contains supplementary materialavailable at https://doi.org/10.1038/s41699-023-00438-5.Correspondence and requests for materials should be addressed to M. Zinkiewicz orM. R. Molas.Reprints and permission information is available at http://www.nature.com/reprintsPublisher’s note Springer Nature remains neutral with regard to jurisdictional claimsin published maps and institutional affiliations.Open Access This article is licensed under a Creative CommonsAttribution 4.0 International License, which permits use, sharing,adaptation, distribution and reproduction in anymedium or format, as long as you giveappropriate credit to the original author(s) and the source, provide a link to the CreativeCommons license, and indicate if changes were made. The images or other third partymaterial in this article are included in the article’s Creative Commons license, unlessindicated otherwise in a credit line to the material. If material is not included in thearticle’s Creative Commons license and your intended use is not permitted by statutoryregulation or exceeds the permitted use, you will need to obtain permission directlyfrom the copyright holder. To view a copy of this license, visit http://creativecommons.org/licenses/by/4.0/.© The Author(s) 2024M. Zinkiewicz et al.7Published in partnership with FCT NOVA with the support of E-MRS npj 2D Materials and Applications (2024)     2 https://doi.org/10.1038/s41699-023-00438-5http://www.nature.com/reprintshttp://www.nature.com/reprintshttp://creativecommons.org/licenses/by/4.0/http://creativecommons.org/licenses/by/4.0/ Raman scattering excitation in monolayers of semiconducting transition metal dichalcogenides Introduction Results Raman scattering excitation spectra Enhancement profiles of A1^  1′ modes due to outgoing resonance Outgoing versus incoming resonance Discussion Methods Sample preparation Experimental�setup DATA AVAILABILITY References Acknowledgements Author contributions Competing interests ADDITIONAL INFORMATION