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Katarzyna Olkowska Pucko, Elena Blundo, Natalia Zawadzka, Salvatore Cianci, Diana Vaclavkova, Piotr Kapuściński, Dipankar Jana, Giorgio Pettinari, Marco Felici, Karol Nogajewski, Miroslav Bartoš, [Kenji Watanabe](https://orcid.org/0000-0003-3701-8119), [Takashi Taniguchi](https://orcid.org/0000-0002-1467-3105), Clement Faugeras, Marek Potemski, Adam Babiński, Antonio Polimeni, Maciej R Molas

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[Excitons and trions in WSSe monolayers](https://mdr.nims.go.jp/datasets/4ec2bb70-45b2-4c99-a8d9-87aad214bba4)

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Excitons and trions in WSSe monolayers2D MaterialsPAPER • OPEN ACCESSExcitons and trions in WSSe monolayersTo cite this article: Katarzyna Olkowska Pucko et al 2023 2D Mater. 10 015018 View the article online for updates and enhancements.You may also likeIntrinsic dipole-induced self-doping inJanus MXY-based (M = Mo, W; X = S; Y =Se, Te) p–n junctionsKun-Xing Xiao, Yuan Si, Ke Yang et al.-The mirror asymmetry induced nontrivialproperties of polar WSSe/MoSSeheterostructuresYuanyuan Wang, Wei Wei, Baibiao Huanget al.-Unidirectional Rashba spin splitting insingle layer WS2(1x)Se2x alloyJihene Zribi, Debora Pierucci, FedericoBisti et al.-This content was downloaded from IP address 144.213.253.16 on 25/12/2022 at 01:54https://doi.org/10.1088/2053-1583/aca915/article/10.1088/1361-6463/ac8601/article/10.1088/1361-6463/ac8601/article/10.1088/1361-6463/ac8601/article/10.1088/1361-648X/aaffb7/article/10.1088/1361-648X/aaffb7/article/10.1088/1361-648X/aaffb7/article/10.1088/1361-6528/aca0f6/article/10.1088/1361-6528/aca0f6/article/10.1088/1361-6528/aca0f6/article/10.1088/1361-6528/aca0f6/article/10.1088/1361-6528/aca0f62D Mater. 10 (2023) 015018 https://doi.org/10.1088/2053-1583/aca915OPEN ACCESSRECEIVED5 October 2022REVISED21 November 2022ACCEPTED FOR PUBLICATION6 December 2022PUBLISHED19 December 2022Original Content fromthis work may be usedunder the terms of theCreative CommonsAttribution 4.0 licence.Any further distributionof this work mustmaintain attribution tothe author(s) and the titleof the work, journalcitation and DOI.PAPERExcitons and trions in WSSe monolayersKatarzyna Olkowska Pucko1,∗, Elena Blundo2, Natalia Zawadzka1, Salvatore Cianci2,Diana Vaclavkova3, Piotr Kapuściński3, Dipankar Jana3, Giorgio Pettinari4, Marco Felici2,Karol Nogajewski1, Miroslav Bartoš5, Kenji Watanabe6, Takashi Taniguchi7, Clement Faugeras3,Marek Potemski1,3,8, Adam Babiński1, Antonio Polimeni2 and Maciej R Molas1,∗1 Institute of Experimental Physics, Faculty of Physics, University of Warsaw, 02-093 Warsaw, Poland2 Physics Department, Sapienza University of Rome, 00185 Rome, Italy3 Laboratoire National des Champs Magnétiques Intenses, CNRS-UGA-UPS-INSA-EMFL, 38042 Grenoble, France4 Institute for Photonics and Nanotechnologies, National Research Council (CNR-IFN), 00133 Rome, Italy5 Central European Institute of Technology, Brno University of Technology, Brno 61200, Czech Republic6 Research Center for Functional Materials, National Institute for Materials Science, Tsukuba 305-0044, Japan7 International Center for Materials Nanoarchitectonics, National Institute for Materials Science, Tsukuba 305-0044, Japan8 CENTERA Laboratories, Institute of High Pressure Physics, Polish Academy of Sciences, 01-142 Warsaw, Poland∗ Authors to whom any correspondence should be addressed.E-mail: katarzyna.olkowska-pucko@fuw.edu.pl andmaciej.molas@fuw.edu.plKeywords: excitons, transition metal dichalcogenides, photoluminescence, g-factor, alloy, dark trion, phonon replicaSupplementary material for this article is available onlineAbstractThe possibility of almost linear tuning of the band gap and of the electrical and optical propertiesin monolayers (MLs) of semiconducting transition metal dichalcogenide (S-TMD) alloysopens up the way to fabricate materials with on-demand characteristics. By making use ofphotoluminescence spectroscopy, we investigate optical properties of WSSe MLs with a S/Se ratioof 57/43 deposited on SiO2/Si substrate and encapsulated in hexagonal BN flakes. Similarly to the‘parent’ WS2 and WSe2 MLs, we assign the WSSe MLs to the ML family with the dark groundexciton state. We find that, in addition to the neutral bright A exciton line, three observedemission lines are associated with negatively charged excitons. The application of in-plane andout-of-plane magnetic fields allows us to assign undeniably the bright and dark (spin- andmomentum-forbidden) negative trions as well as the phonon replica of the dark spin-forbiddencomplex. Furthermore, the existence of the single photon emitters in the WSSe ML is alsodemonstrated, thus prompting the opportunity to enlarge the wavelength range for potentialfuture quantum applications of S-TMDs.1. IntroductionSemiconducting transition metal dichalcogenides(S-TMDs) MX2, which crystallise in the 2H phase,include only five compounds, i.e. WS2, WSe2, MoS2,MoSe2, andMoTe2 [1].Monolayers (MLs) ofMX2 aredirect-band-gap semiconductors and can emit lightmore efficiently than their bulk counterparts havingindirect band gaps. The fundamental optical trans-ition in S-TMD MLs, the so-called A exciton, spansthe spectral range from 1.15 eV for the MoTe2 ML [2]up to∼2.1 eV for the WS2 ML [3]. One possibility ofadjusting the A-exciton energy is to change the layerthickness, but this is accompanied with a change inthe band gap character (from direct to indirect) withdetrimental effects on the radiative efficiency [2, 4–6].Yet another strategy is that of straining S-TMDs, thusshifting the exciton energy. However, strain also leadsto direct-to-indirect exciton transitions, affecting notonly the optical efficiency and carrier decay time[7], but also having huge effects on the exciton g-factor [8]. Another opportunity to adjust the energyof the optical band gap is by mixing different atoms,e.g. transition metals (Mo and W) or chalcogens (S,Se, and Te), which leads to the formation of ternarycompounds (alloys), such asMoSSe,WSSe,MoWSe2,etc. In such materials, it is possible to tune the opticalband gap by variation of the stoichiometry ratioof each element [9–12]. Using a variety of candid-ates, the band gap of ternary compounds can be© 2022 The Author(s). Published by IOP Publishing Ltdhttps://doi.org/10.1088/2053-1583/aca915https://crossmark.crossref.org/dialog/?doi=10.1088/2053-1583/aca915&domain=pdf&date_stamp=2022-12-19https://creativecommons.org/licenses/by/4.0/https://creativecommons.org/licenses/by/4.0/https://orcid.org/0000-0002-6036-7096https://orcid.org/0000-0003-0423-4798https://orcid.org/0000-0002-3282-9513https://orcid.org/0000-0003-4020-369Xhttps://orcid.org/0000-0003-3116-2224https://orcid.org/0000-0003-0241-0583https://orcid.org/0000-0003-0187-3770https://orcid.org/0000-0002-0977-2301https://orcid.org/0000-0001-8839-5032https://orcid.org/0000-0002-5923-0260https://orcid.org/0000-0003-3701-8119https://orcid.org/0000-0002-1467-3105https://orcid.org/0000-0002-9615-8739https://orcid.org/0000-0001-8881-6618https://orcid.org/0000-0002-5591-4825https://orcid.org/0000-0002-2017-4265https://orcid.org/0000-0002-5516-9415mailto:katarzyna.olkowska-pucko@fuw.edu.plmailto:maciej.molas@fuw.edu.plhttps://doi.org/10.1088/2053-1583/aca9152D Mater. 10 (2023) 015018 K O Pucko et alcontinuously tuned within a wide spectral range.This opens up new possibilities for absorbing andemitting light at desired wavelengths, which plays acrucial role in potential optoelectronic applications.The S-TMD MLs are known to be organised in twosubgroups, i.e. bright and darkish, due to the type ofthe ground exciton state (bright and dark, respect-ively) [13]. In bright MLs, the optical recombinationbetween spin-polarised subbands of the top valenceband (VB) and the bottom conduction band (CB) isoptically active (bright). In the case of darkish MLs,that transition is optically inactive (dark). MoSe2and MoTe2 MLs are bright, while MoS2, WS2, andWSe2 MLs are darkish (see [14–16]). The determin-ation of bright or darkish character of the excitonicground state of alloy MLs is still missing. However, inthe mean field approximation, it is expected that theelectronic and optical properties of alloy MLs (e.g.WSSe) are an average of those of the corresponding‘parent’ (e.g. WS2 and WSe2).In this work, we investigate the optical responseof MLs of WSSe with a S/Se ratio of 57/43 encap-sulated in hexagonal BN (hBN) flakes, by means ofphotoluminescence (PL) spectroscopy. TheWSSeMLis ascribed to the family of MLs with a dark groundexciton state, like its ‘parent’ WS2 and WSe2 MLs.Our optical spectroscopy investigations furthermorereveal additional emission lines at energies lower thanthat of the bright excitons. By applying magneticfields perpendicular and parallel to the layer plane, weare able to determine that these lines arise from brightand dark (spin- and momentum-forbidden) negat-ively charged excitons and to identify a phonon rep-lica of the dark spin-forbidden charged exciton com-plex.We finally demonstrateWSSeMLs as a potentialplatform for quantum communication as these MLs,similar to WSe2 or WS2 [1, 17–19] host centres emit-ting photons one-by-one, as shown by a clear anti-bunching at zero delay in our auto-correlation meas-urements of their emission statistics.2. ResultsLow-temperature (T= 5K) PL spectra of WSe2,WSSe, and WS2 MLs deposited on SiO2/Si substrates(top) and encapsulated in hBN flakes (bottom) areshown in figure 1. The relative concentration of S andSe atoms in the WSSe crystal, from which MLs wereexfoliated was determined by means of energy dis-persive x-ray (EDX) analysis. The alloy was found tobe homogeneous over the whole investigated crystal(∼half-mm in size) with a S/Se ratio ≈ 57/43, seesupplementary information (SI) for details. First, wefocus on the analysis of the PL spectra measured onMLs deposited on a SiO2/Si substrate, see figure 1(a).The corresponding PL spectra of the WS2 and WSe2MLs display characteristic emission lines, labelled X,associated with the recombination of neutral brightA excitons comprising carriers from the K± pointsof the Brillouin zone (BZ) [1]. The determined Xenergies for the WS2 and WSe2 MLs are equal toabout 2.090 eV and 1.745 eV, respectively. By ana-logy, we ascribe the emission line apparent at about1.909 eV in the WSSe ML to the A exciton. Its energyis very close to 1.942 eV, as expected for the alloyML from the linear dependence of the X energy withthe S/Se relative concentration. As can be seen infigure 1(b), the encapsulation of MLs in hBN red-shifts the X energies. This results from the reduc-tion of both the band gap energies and the bindingenergies of excitons in the hBN-encapsulated MLs,as compared to MLs deposited on a Si/SiO2 sub-strate. This reduction is due to the different dielec-tric environments of the MLs (hBN encapsulationversus vacuum and Si/SiO2 substrate) [20]. Note thata series of low-energy emission lines can be observedin figure 1. Their attribution for WS2 and WSe2 MLswas discussed in the literature [3, 4, 6, 21], while thecorresponding analysis for the WSSe ML was miss-ing and will be presented in the following. The mostwell-known effect of the hBN encapsulation is a sig-nificant reduction in the inhomogeneous broaden-ing of the spectral lines, leading to linewidths thatapproach the radiative decay limit [22]. To study thehBN influence in our case, we fitted the X lines,seen in figure 1, with Lorentzian functions. Theirextracted linewidths, i.e. full width at half maximum,decrease substantially from 13meV (16meV) for theWSe2 (WS2) MLs deposited on a Si/SiO2 substrateto a few meV for MLs encapsulated in hBN flakes(3.5meV for WSe2 and 5.2meV for WS2). Interest-ingly, the linewidth reduction for theWSSeML (from23meV for Si/SiO2 to 18meV for encapsulated ML)is not as large. The main difference may come fromthe intrinsic quality of the WSSe alloy crystal com-pared with the pure WS2 and WSe2 crystals. Whilethe S/Se composition in our WSSe crystal is relat-ively homogeneous at themacroscale, local variationsof the chalcogen concentration are expected at thenanoscale. The alloy disorder as the main source ofintrinsic scattering strongly limits the exciton lifetimein alloy MLs, leading to a broadening of the lines.Consequently, it is expected that hBN-encapsulationis not able to yield to a sizeable reduction of theX linewidth.The assignment of the lower-in-energy features,labelled T, TI and TDE′′(Γ) in figure 2(a), requires amore detailed analysis. The T line can be ascribedto the negatively charged exciton (negative trion)according to the following evidence: (a) the energyseparation between the X and T lines is of 32meV,which is very close to the corresponding separationbetween the neutral exciton and the negative trionreported in theWSe2 (32meV) [21, 23] andWS2 MLs(34meV) [16, 20]; (b) the T emission line can beobserved up to room temperature (see SI for details),22D Mater. 10 (2023) 015018 K O Pucko et alFigure 1. Low-temperature (T= 5K) PL spectra of WSe2, WS2, and WSSe MLs (a) deposited directly onto a SiO2/Si substrate and(b) encapsulated in hBN flakes. The spectra were measured under excitation energy of 2.41 eV and power of 50µW.Figure 2. (a) Power dependence of low-temperature (T= 5K) PL spectra measured on a WSSe monolayer with 2.41 eV laser lightexcitation. (b) The intensity evolution of the emission features with excitation power. The dashed black line indicates the lineardependence as a guide to the eye. (c) The low-temperature PL spectrum of a single photon emitter, denoted as LX, measured on aWSSe ML. Note that the presented spectrum was measured on a specific region of a WSSe ML different from the ML displayed inpanel (a). (d) The photon autocorrelation histogram recorded for the LX line.32D Mater. 10 (2023) 015018 K O Pucko et alwhich is typical of the negative trion lines inW-basedMLs [4, 13, 20]; (c) the sign of free carriers can berevealed indirectly due to the apparent double struc-ture of the T lines at the highest magnetic fields,as discussed later (see figure 5(a)), which is a sig-nature of negatively charged excitons in WSe2 andWS2 MLs [3, 20, 21, 24]. The assignment of the TIand TDE′′(Γ) lines to the correspondingly intervalleymomentum-forbidden negative dark trion and thephonon replica of the intravalley spin-forbidden darktrion, requires the use of external magnetic fieldsapplied in different configurations with respect to theML plane (parallel or perpendicular), which is dis-cussed below. To confirm the origin of the aforemen-tioned emission lines, we investigated their intensityevolution as a function of the excitation power. Thelow-temperature PL spectra of a WSSe ML encap-sulated in hBN flakes measured with four differ-ent excitation powers are presented in figure 2(a),while the obtained power dependences of the stud-ied emission lines are shown in figure 2(b). Notethat the power evolution of the TDE′′(Γ) line couldnot be resolved for an excitation power greater than300µW. The integrated intensities of all investigatedlines are characterised by an almost linear evolutionwith the excitation power. To analyse quantitativelythe power evolutions of the line intensities, we fit-ted the corresponding dependences using the follow-ing formula I∝ Pβ , where I is integrated intensity oflines, P is the excitation power, and β is a power coef-ficient. The extracted values for the slopes of these lin-ear evolutions range from about 1 for the X, T, andTDE′′(Γ) lines to 1.2 for the TI, which is expected inthe case of excitonic complexes composed of a singleelectron–hole (e–h) pair (in contrast to biexcitons,whose intensities increase quadratically with excita-tion power) [25].It is important to mention that in addition tothe free exciton and trion lines, a zoo of states res-ulting in a broad band below 1.8 eV can also beobserved in figure 2(a). These emission lines fea-ture a sublinear behaviour with power and are thusattributed to defect-related emissions. Interestingly,at some specific locations where the ML is wrinkledor blistered (and thus strained [26, 27]) due to thetrapping of contaminants or close to the edges, isol-ated narrow lines show up from the broad defect-related band, like those in figure 2(c). These linescan be attributed to localised excitons (LXs), sim-ilar to what was reported for WSe2 [1, 19]. To verifytheir quantum nature, we performed autocorrela-tion measurements on the LX line, see figure 2(c).As shown in figure 2(d), a clear antibunching beha-viour is observed, resulting in a second-order auto-correlation function g(0) = 0.388± 0.081, thus prov-ing the single-photon emitter nature of the LXs. Ourresult confirms the promise of WSSe for quantumapplications and calls for a deeper investigation of theLX properties and of the role played by strain, whichwill be the object of future studies.From the analysis presented so far, we can con-clude that the low-temperature PL spectra of theWSSeML encapsulated in hBN flakes comprise emis-sion lines due to excitonic complexes composed of ane–h pair. Figure 3 shows possible spin configurationsfor the bright and dark excitons formed in the vicin-ity of the A exciton. The neutral bright exciton in theK± valley is composed of an electron from the higher-lying level of the CB and a hole from the top levelof the VB from the same K± point, see figure 3(a).The negatively charged exciton (negative trion) is athree-particle complex composed of an e–h pair andan excess electron. There are four negative trions indarkish MLs based on tungsten (W) atoms (WS2 orWSe2) [3, 21, 23], i.e. two bright and two dark states.These states can be formed at the K+ and K− pointsdue to the location of a hole, leading to two possibleconfigurations of a given complex. Note that only asingle configuration of a specific complex is shown inpanels (b), (c), and (d) of figure 3. Due to the spinconservation rule for S-TMDMLs, the bright (optic-ally active) negative trion can be found in both theintravalley singlet (TS), involving two electrons fromthe same valley, and the intervalley triplet (TT), com-prising two electrons from different valleys [20]. Fordark (optically inactive) negative trions, the corres-ponding electrons are located in different valleys andcharacterised by antiparallel alignment of their spins.This configuration leads to two complexes, depend-ing on the state of electron associated with the recom-bination process: intravalley spin-forbidden (TD) andintervalleymomentum-forbidden (TI), which cannotrecombine optically due to spin andmomentum con-servation, respectively [3, 16, 28]. One of the pos-sibilities to fulfil the spin and momentum conserva-tion rules during optical recombination of dark trionsis phonon emission from the Γ and K points of theBZ, respectively [3, 21, 23, 29]. Figure 3(d) presentsthe schematic illustration of a possible recombina-tion pathway of dark negative trions involving opticalphonon emission from the Γ point of the BZ, TSE′′(Γ).To verify the assignment of the T, TI, and TDE′′(Γ) lines,we measured PL spectra as functions of in-plane andout-of-plane magnetic fields up to 30 T, see figures 4and 5.To investigate the effect of the in-plane mag-netic field (B∥) on the emission of the WSSe ML,we measured the evolution of the low-temperature(T= 4.2 K) PL spectra under fields up to B∥ = 30 T.Figure 4(a) presents the PL spectra at selectedmagnetic fields, B∥ = 0, 10, 20, and 30 T. Withincreasing B∥, the intensity of the TI line increasessignificantly, while the intensities of the otheremission lines, seen in the PL spectra, remain almost42D Mater. 10 (2023) 015018 K O Pucko et alFigure 3. Schematic illustration of possible spin configurations for (a) the neutral exciton (X), (b) the bright singlet and tripletnegative trions (TS and TT, respectively), (c) the dark intravalley and intervalley negative trions (TD and TI, respectively), and(d) the dark trion emission assisted by the optical phonon E′′from Γ point of the BZ (TDE′′(Γ)) formed in the vicinity of opticalband gap of WSSe ML. Note that, except for the X complex, we draw only complexes for which a hole is located at the K+ point ofthe BZ.Figure 4. (a) Low-temperature (T= 4.2 K) PL spectra of a WSSe monolayer encapsulated in hBN flakes measured at selectedmagnetic fields applied in the ML’s plane. The PL spectra are normalised to the intensity of the X line. (b) The correspondingmagnetic-field dependence of the relative intensity of the dark trion line to the neutral exciton line, ITI/D /IX. The solid black curverepresents a quadratic fit according to the equation described in the text. The spectra were measured under excitation energy of2.41 eV and power of 50µW.unchanged. The energy separation between the Xand TI lines of approximately 57meV is very similarto the reported energy separations between the brightneutral exciton and the spin-forbidden negative trionin both WSe2 (54–60meV) [21, 21, 23, 30] and WS2(57meV) [3]. Consequently, the brightening of theemission line in the in-plane field, labelled TD, isascribed to the intravalley spin-forbidden dark negat-ive trion. To support further our attribution, we ana-lyse the magnetic field evolution of the TD/I intensity.The B∥ evolution of the intensity of the dark trionTD is expected to be quadratic I= αB2∥ [3, 13, 31].Figure 4(b) displays the B∥ dependence of the relativeintensities of the TI/D and X lines in magnetic fieldsup to 30 T. Note that the division by the X intens-ity allows us to eliminate the variation of the signalintensity during measurements, e.g. the measuredsignal in the magnetic field setup is affected by theFaraday effect. The extracted data are accompaniedby quadratic fits with the formula I= I0 +αB2∥, whereI0 corresponds to the TI intensity at zero field. As canbe appreciated in figure 4(b), the obtained data canbe described well by the proposed formula with the αparameter equal to 9.1× 10−3T−2.The last part of this work is devoted to theeffects of out-of-plane magnetic fields (B⊥) on theproperties of the studied lines. Figure 5(a) demon-strates the helicity-resolved PL spectra measured52D Mater. 10 (2023) 015018 K O Pucko et alFigure 5. (a) Helicity-resolved PL spectra of an hBN-encapsulated WSSe monolayer at T= 4.2 K measured at selected values ofthe applied out-of-plane magnetic field. The red (blue) colour corresponds to the σ+ (σ−) polarised spectra. The spectra arevertically shifted for clarity. (b) Energy difference between the two circularly polarised split components of the X, T, TI, andTDE′′(Γ)transitions as a function of out-of-plane magnetic field. The solid lines represent fits according to the equation describedin the text. The spectra were measured under excitation energy of 2.41 eV and power of 50µW.on the hBN-encapsulated WSSe ML in magneticfields up to 30 T oriented perpendicularly to the MLplane. Due to both the large binding energies of freeexcitons and their huge reduced masses in S-TMDMLs [32–34], applying the out-of-plane magneticfield results mostly in the exciton Zeeman effect [35](the diamagnetic shift of the ground excitonic statescan hardly be seen in magnetic fields as high as70 T [32, 33]), which manifests itself as a splitting ofthe two counter-circularly-polarised components ofa given transition (σ±). As can be seen in figure 5(a),all the observed emission lines split into two σ± com-ponents, but the magnitude of the Zeeman split-tings differs, providing us with a tool to distinguishbetween the different complexes. Note that a doublestructure of the σ+ component of the T line canbe appreciated at 30 T with the determined energyseparation between the TS and TT components ofabout 6meV (see SI for details), which is in verygood agreement with the previously reported cor-responding energy splitting of the spin-singlet andspin-triplet negative trions in the WS0.6Se1.4 [36],WSe2 (6–7meV) [37–39], and WS2 (7–8meV) [20,40] MLs. The energy separation of the σ± compon-ents in the B⊥ field, ∆E(B⊥) = Eσ+ − Eσ− can beexpressed as ∆E(B⊥) = gµBB⊥, where g denotes theeffective g-factor of the considered excitonic com-plex and µB is the Bohr magneton. The magneticfield evolutions of ∆E with linear fits to the exper-imental data for the X, T, TI, and TDE′′(Γ) lines areshown in figure 5(b). The extracted g-factors for boththe X and T lines are about −3.7. Simultaneously,the g-factors found for the TI and TDE′′(Γ) lines aremuch larger and correspond to approximately −13.8and−7.9. The TDE′′(Γ) assignment to the phonon rep-lica of the TD line is due to its g-factor of about−8, which is a characteristic value of spin-forbiddentransitions (see [3, 23, 29] for details). As this typeof transition requires spin-flip process of an electronin the CB (see figure 3(d)), it can be only achievedby a chiral phonon of a specific symmetry, that is E′′from the Γ point of the BZ. [3, 23, 29] Moreover,the attribution of the TDE′′(Γ) to the emission of theE′′(Γ) phonon allows us to determine its energyto be about 28meV. Note that the E′′(Γ) phononcan not be observed in a typical experimental back-scattering geometry [41, 42], which requires that theelectric field of the incident wave is perpendicularto the ML plane. Moreover, due to the identifica-tion of the phonon replica involving the E′′phononfrom the Γ point of the BZ, its energy was determ-ined to be about 28meV. If we consider the corres-ponding E′′(Γ) energies in WS2 (36meV) [3] andWSe2 (21meV) [30] MLs, the determined value isvery close to the estimate of 30meV obtained bymaking the assumption of a linear dependence ofthe phonon energy on the S/Se relative concentra-tion in the ML alloys. Consequently, as reported inthe literature, excitonic complexes in S-TMD MLscan be arranged into three groups due to their Zee-man splitting magnitude [3, 21, 23, 35]: (a) g-factorsaround−4 for bright transitions (X and T:−3.7); (b)spin-forbidden dark transitions are described by theg-factor equal to around −8 (TDE′′(Γ): −7.9); and (c)values of g-factors of about−14 are characteristic formomentum-forbidden dark transitions (TI:−13.8).62D Mater. 10 (2023) 015018 K O Pucko et al3. SummaryTo conclude, we presented a PL-based investigation ofthe optical response of MLs of WSSe with a S/Se ratioof 57/43 deposited on Si/SiO2 substare and encapsu-lated in hBN flakes. We found that the WSSe ML wascharacterised by a dark ground exciton state, like its‘parent’ WS2 and WSe2 MLs. Moreover, the existenceof single photon emitters in the WSSe ML was alsodemonstrated. Through the application of in-planeand out-of-planemagnetic fields, the fourmain emis-sion lines apparent in the low-temperature PL spec-tra were ascribed to the neutral bright exciton, thebright and dark (spin- and momentum-forbidden)negatively charged excitons, and the phonon replicaof the dark spin-forbidden trion. Our results showthat alloyed S-TMD MLs can represent a powerfulmaterial platform towiden the spectral range of oper-ation of 2D crystals for optoelectronics and quantumtechnology applications.4. MethodsMLs of WSSe were mechanically exfoliated from thebulk crystals grown by the Flux zone method andpurchased from 2D semiconductors. The MLs wereexfoliated with a scotch tape onto polydimethylsilox-ane (PDMS). Relatively thick hBN flakes were exfo-liated mechanically onto SiO2/Si substrates (300 nmthick SiO2 on top of a 500µm thick Si). The WSSeMLs were then deterministically transferred fromthe PDMS to the hBN flake. Thin hBN flakes wereexfoliated onto PDMS and deposited on the WSSeML to encapsulate it. After each deposition step, thesample was annealed in high vacuum (10−6 mbar) at∼120◦C for several hours to induce the coalescence ofthe contaminants, thus improving the adhesion of thesample. For the WSSe MLs on SiO2/Si substrates, theMLs were first isolated onto PDMS and then depos-ited on the substrate. The investigated MLs of WSe2and WS2 ML encapsulated in hBN flakes were fab-ricated in an analogue way by mechanical exfoliationbased on PDMS, but the whole structure was depos-ited on a bare Si substrate. The WSe2 and WS2 MLson SiO2/Si substrates were fabricated by simple exfo-liation of the bulk crystals onto the substrates.The WSSe alloy was examined by EDX analysisusing a ZEISS-Sigma300 scanning electron micro-scope equipped with an Oxford Instruments X-Act100mm energy-dispersive spectrometer. Data wereacquired with an acceleration voltage of 28 kV andanalysed by INCA software. The spatial resolutionwas in the 10–15µm range.The PL experiments at zero magnetic field wereperformed using a λ=514.5 nm (2.41 eV) continu-ous wave (CW) laser diode. The studied sampleswere placed on a cold finger in a continuous flowcryostat mounted on x–y motorised positioners. Theexcitation light was focused bymeans of a 100× long-working-distance objective with a 0.55 numericalaperture producing a spot of about 1µm diameter.The signal was collected via the same microscopeobjective, sent through a 0.75m monochromator,and then detected using a liquid nitrogen cooledcharge-coupled device (CCD) camera.Low-temperature micro-magneto-PL experi-ments were performed in the Voigt and Faraday geo-metries, i.e. magnetic field orientated parallel andperpendicular with respect to ML’s plane, respect-ively. Measurements (spatial resolution∼1µm) werecarried out with the aid of a resistive magnetic coilproducing fields up to 30 T using a free-beam-opticsarrangement. The sample was placed on top of ax–y–z piezo-stage kept at T= 4.2 K and was excitedusing a CW laser diode with 515 nm wavelength(2.41 eV photon energy). The emitted light was dis-persed with a monochromator of a 0.5m focal lengthand detectedwith aCCDcamera. The combination ofa quarter-wave plate and a linear polariser was used toanalyse the circular polarisation of signals (the meas-urements were performed with a fixed circular polar-isation, whereas reversing the direction of magneticfield yielded the information corresponding to theother polarisation component due to time-reversalsymmetry).Autocorrelation measurements were performedin a Hanbury–Brown–Twiss setup by using afrequency-double Nd:YAG CW laser emitting atλ= 532.2 nm.Data availability statementThe data that support the findings of this study areavailable upon reasonable request from the authors.AcknowledgmentThe work has been supported by the National ScienceCentre, Poland (Grants No. 2017/27/B/ST3/00205and 2018/31/B/ST3/02111), EU Graphene FlagshipProject, and the CNRS via IRP ‘2DM’ project. Weacknowledge the support of the LNCMI-CNRS,member of the European Magnetic Field Laboratory(EMFL). The Polish participation in EMFL is suppor-ted by the DIR/WK/2018/07 Grant from Polish Min-istry of Education and Science. We acknowledge sup-port by the European Union’s Horizon 2020 researchand innovation programme through the ISABEL Pro-ject (No. 871106). E B acknowledges support from LaSapienza through the grant Avvio alla Ricerca 2021(Grant No. AR12117A8A090764). This project wasfunded within the QuantERA II Programme that hasreceived funding from the European Union’s Hori-zon 2020 research and innovation programme underGrant Agreement No. 101017733, and with fundingorganisationsMinistero dell’Universitá e della Ricerca72D Mater. 10 (2023) 015018 K O Pucko et al(MUR) and Consiglio Nazionale delle Ricerche(CNR). M B acknowledge support by the ESF underthe project CZ.02.2.69/0.0/0.0/20_079/0017 436. C Facknowledges support from Graskop project ANR-19-CE09-0026. K W and T T acknowledge sup-port from JSPS KAKENHI (Grants No. 19H05790,20H00354 and 21H05233).Author ContributionsKO-P, E B,NZ, S C,DV, PK,D J, G P,MF, C F,MP, AP, A B, andMRMperformed the experiments. E B, KN, andM B fabricated the samples KW and T T grewthe hBN crystals. K O-P, E B, A P, and M R M wrotethe manuscript with inputs from the all co-authors.Conflicts of interestThere are no conflicts to declare.ORCID iDsKatarzyna Olkowska Puckohttps://orcid.org/0000-0002-6036-7096Elena Blundo https://orcid.org/0000-0003-0423-4798Natalia Zawadzka https://orcid.org/0000-0002-3282-9513Salvatore Cianci https://orcid.org/0000-0003-4020-369XDiana Vaclavkova https://orcid.org/0000-0003-3116-2224Piotr Kapuściński https://orcid.org/0000-0003-0241-0583Giorgio Pettinari https://orcid.org/0000-0003-0187-3770Marco Felici https://orcid.org/0000-0002-0977-2301Karol Nogajewski https://orcid.org/0000-0001-8839-5032Miroslav Bartoš https://orcid.org/0000-0002-5923-0260Kenji Watanabe https://orcid.org/0000-0003-3701-8119Takashi Taniguchi https://orcid.org/0000-0002-1467-3105Clement Faugeras https://orcid.org/0000-0002-9615-8739Marek Potemski https://orcid.org/0000-0001-8881-6618Adam Babiński https://orcid.org/0000-0002-5591-4825Antonio Polimeni https://orcid.org/0000-0002-2017-4265Maciej R Molas https://orcid.org/0000-0002-5516-9415References[1] Koperski M, Molas M R, Arora A, Nogajewski K,Slobodeniuk A O, Faugeras C and Potemski M 2017Nanophotonics 6 1289[2] Lezama I G, Arora A, Ubaldini A, Barreteau C, Giannini E,Potemski M and Morpurgo A F 2015 Nano Lett. 15 2336–42[3] Zinkiewicz M et al 2021 Nano Lett. 21 2519–25[4] Arora A, Koperski M, Nogajewski K, Marcus J, Faugeras Cand Potemski M 2015 Nanoscale 7 10421[5] Arora A, Nogajewski K, Molas M, Koperski M andPotemski M 2015 Nanoscale 7 20769[6] Molas M R, Nogajewski K, Slobodeniuk A O,Binder J, Bartos M and Potemski M 2017 Nanoscale9 13128[7] Blundo E, Felici M, Yildirim T, Pettinari G, Tedeschi D,Miriametro A, Liu B, Ma W, Lu Y and Polimeni A 2020 Phys.Rev. 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