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Salvatore Cianci, Elena Blundo, Federico Tuzi, Giorgio Pettinari, Katarzyna Olkowska‐Pucko, Eirini Parmenopoulou, Djero B. L. Peeters, Antonio Miriametro, [Takashi Taniguchi](https://orcid.org/0000-0002-1467-3105), [Kenji Watanabe](https://orcid.org/0000-0003-3701-8119), Adam Babinski, Maciej R. Molas, Marco Felici, Antonio Polimeni

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[Spatially Controlled Single Photon Emitters in hBN‐Capped WS<sub>2</sub> Domes](https://mdr.nims.go.jp/datasets/2ec8a4f4-5e4d-4498-9a01-45fb3f169ab1)

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Spatially Controlled Single Photon Emitters in hBN‐Capped WS2 DomesRESEARCH ARTICLEwww.advopticalmat.deSpatially Controlled Single Photon Emitters in hBN-CappedWS2 DomesSalvatore Cianci, Elena Blundo, Federico Tuzi, Giorgio Pettinari,Katarzyna Olkowska-Pucko, Eirini Parmenopoulou, Djero B. L. Peeters,Antonio Miriametro, Takashi Taniguchi, Kenji Watanabe, Adam Babinski,Maciej R. Molas, Marco Felici,* and Antonio Polimeni*Monolayers (MLs) of transition-metal dichalcogenides host efficientsingle-photon emitters (SPEs) usually associated to the presence of nanoscalemechanical deformations or strain. Large-scale spatial control of strain wouldenhance the scalability of such SPEs and allow for their incorporation intophotonic structures. Here, the formation of regular arrays of strainedhydrogen-filled one-layer-thick micro-domes obtained by H-ion irradiation andlithography-based approaches is reported. Typically, the H2 liquefaction fortemperatures T<32 K causes the disappearance of the domes preventing theiruse as potential SPEs. Here, it is shown that the dome deflation can beovercome by hBN heterostructuring, that is by depositing thin hBN flakes onthe domes. This leads to the preservation of the dome structure at alltemperatures, as found by micro-Raman and micro-photoluminescence (μ-PL)studies. Eventually, spatially controlled hBN-capped WS2 domes show theappearance, at 5 K, of intense emission lines originating from localizedexcitons, which are shown to behave as quantum emitters here. Theelectronic properties of the emitters are addressed by time-resolved μ-PLyielding time decays of 1–10 ns, and by magneto-μ-PL measurements. Thelatter provide an exciton magnetic moment a factor of two larger than thevalue observed in planar strain-free MLs.S. Cianci, E. Blundo, F. Tuzi, E. Parmenopoulou, D. B. L. Peeters,A. Miriametro, M. Felici, A. PolimeniPhysics DepartmentSapienza University of RomeRome 00185, ItalyE-mail: marco.felici@uniroma1.it; antonio.polimeni@uniroma1.itG. PettinariInstitute for Photonics and Nanotechnologies (CNR-IFN)National Research CouncilRome 00133, ItalyThe ORCID identification number(s) for the author(s) of this articlecan be found under https://doi.org/10.1002/adom.202202953© 2023 The Authors. Advanced Optical Materials published byWiley-VCH GmbH. This is an open access article under the terms of theCreative Commons Attribution License, which permits use, distributionand reproduction in any medium, provided the original work is properlycited.DOI: 10.1002/adom.2022029531. IntroductionTransition metal dichalcogenides (TMDCs)in the monolayer (ML) form have beenextensively studied since the discovery ofthe direct nature of their bandgap,[1–5]making them ideal candidates for flex-ible opto-electronic devices.[6–8] Indeed,the 2D nature of these materials andtheir exceptional robustness to mechani-cal deformations[9–16] have opened an en-tire field of research dedicated to thestudy of the effects of lattice deformations(strain) on the material electronic and opti-cal properties.[17]Strain gradients in TMDC MLs havealso been related to the appearance ofsingle-photon emitters (SPEs) at cryogenictemperatures,[18–26] greatly increasing thepotential of these materials for quantumtechnologies. Indeed, layered materials areadvantageous from a production point ofview—since bulk crystals provide many ex-foliable MLs—and by virtue of the high ex-traction efficiency of the emitted photons,K. Olkowska-Pucko, A. Babinski, M. R. MolasInstitute of Experimental PhysicsFaculty of PhysicsUniversity of WarsawWarsaw 02-093, PolandD. B. L. PeetersDepartment of Applied Physics and Science EducationEindhoven University of TechnologyEindhoven 5600 MB, The NetherlandsT. TaniguchiInternational Center for Materials NanoarchitectonicsNational Institute for Materials Science1-1 Namiki, Tsukuba 305-0044, JapanK. WatanabeResearch Center for Functional MaterialsNational Institute for Materials Science1-1 Namiki, Tsukuba 305-0044, JapanAdv. Optical Mater. 2023, 11, 2202953 2202953 (1 of 10) © 2023 The Authors. Advanced Optical Materials published by Wiley-VCH GmbHhttp://www.advopticalmat.demailto:marco.felici@uniroma1.itmailto:antonio.polimeni@uniroma1.ithttps://doi.org/10.1002/adom.202202953http://creativecommons.org/licenses/by/4.0/http://crossmark.crossref.org/dialog/?doi=10.1002%2Fadom.202202953&domain=pdf&date_stamp=2023-04-08www.advancedsciencenews.com www.advopticalmat.dedue to the lack of internal reflection. Even though the link be-tween strain gradients and SPEs in TMDCs is yet to be fully un-derstood, in the last few years several different structures (e.g.,etched holes,[19] nanorods,[27,28] nanopillars,[29–31] nanowires,[32]nanostars,[33] etc.) have been used as stressors for TMDC MLs togenerate ordered arrays of SPEs. Moreover, by exploiting plas-monic effects[28,31,33,34] and optical cavities[35,36] in which these2D systems can be easily integrated, it is possible to enhancethe emitter brightness by increasing their radiative recombina-tion rate.In order to advance the field of quantum nanotechnologies,single-photon sources should be produced using manufactur-ing methods that are cheap, scalable, reproducible, and prefer-ably compatible with current photonic integration technologies.In particular, among the numerous ways by which a strainedTMDC ML can be obtained, for example, deposition on dissimilarsubstrates, bending, bulging, or indenting devices etc.,[17] low-energy hydrogen irradiation of bulk flakes has been proven tobe a reliable and efficient way to form strained, ML-thick, H2-filled bubbles, hereafter called domes, on the surface of the irra-diated flakes.[37] These domes feature radii ranging from tens ofnm to a few μm and their size and position can be controlledby the lithographic definition of H-opaque masks on the flakesurface, prior to the irradiation process, leading to the forma-tion of μm- (or even nm-) sized light emitters at room tempera-ture (RT).[37] However, when brought to cryogenic temperatures,the domes deflate due to the gas-to-liquid transition of molec-ular hydrogen, taking place at a critical temperature TC ≈ 32 K(the domes then reversibly re-inflate if the temperature is in-creased above TC).[37] Here, we show how to prevent the deflationof the domes via deposition (capping) of few-layer-thick hexago-nal boron nitride (hBN) on top of the domes. The striking preser-vation of the dome shape, and corresponding strain field, viahBN capping is investigated by means of μ-Raman spectroscopyperformed on MoS2 domes and of micro-photoluminescence (μ-PL) spectroscopy performed on an ordered array of WS2 domes.Importantly, this phenomenon is here exploited to create site-controlled SPEs, which appear at low temperature (≈5 K) inthe spectra of the hBN-capped WS2 domes. The single-photonpurity of these emitters was assessed by second-order auto-correlation measurements, and the SPE electronic propertieswere further investigated by time-resolved μ-PL and magneto-μ-PL measurements. These latter showed an exciton gyromag-netic factor much larger than that observed in strain-free WS2MLs providing an enhanced response of the exciton to externalmagnetic fields.2. Results and DiscussionIn this work, we will focus on MoS2 and WS2 crystals, eventhough the dome formation was also demonstrated for severalother TMDC materials[37] and for hBN.[38] Such a choice is mo-tivated by the experimental probes used to determine the domestrain, that is the particularly clear and intense Raman signal ofMoS2 and the high PL efficiency of WS2.WS2 emitters were observed in regular arrays of domesobtained by hydrogenating a WS2 bulk flake patterned witha lithographically-designed poly(methyl methacrylate) (PMMA)mask, whose openings (diameter, 3 μm) allow the space-controlled formation of hydrogen-filled domes. We point out thatPMMA self-cleans during the H-ion irradiation process, thusleaving the so-formed domes isolated and free from neighboringsmaller domes or debris, a process that facilitates the subsequentdeposition of hBN. The patterned hydrogen irradiation proce-dure was described in previous works[13,14,37] and more details areprovided in the Experimental Section. Figure 1a shows an atomicforce microscope (AFM) image of an ordered array of WS2 domescapped with few-layer-thick (≈10 nm) hBN on a specific regionmarked by a white dashed line. The AFM image shows an opti-mal adherence of the hBN flake to the domes underneath, as wellas the dome robustness against the capping process. The struc-tural effects of the capping of the TMDC domes can be readilyappreciated by looking at the optical microscope images of thesample at RT and 5 K shown in Figure 1b,c, respectively. The lat-ter clearly shows the dome deflation in the uncapped region onthe right top corner of the image, while the domes covered withhBN unexpectedly retain their shape with minimal differencesin size.In order to attain clues on this phenomenon and informa-tion on the variations of the dome strain with temperature, abare MoS2 dome (see AFM image in Figure 2a) was first stud-ied by μ-Raman measurements at different Ts (see Experimen-tal Section). The same study was then carried out after cappingthe dome with hBN (see AFM image in Figure 2b). The corre-sponding T-dependent μ-Raman spectra are shown in Figure 2c(bare) and Figure 2d (capped). The spectra referring to all tem-peratures can be found in Figure S1, Supporting Information.In both cases, the μ-Raman spectra were recorded on the cen-ter of the dome, where the tensile strain is maximum and fea-tures a biaxial character.[14,38,39] The μ-Raman spectra are charac-terized by the peaks associated to the in-plane (E2g, Raman shift< 400 cm−1) and out-of-plane (A1g, Raman shift >400 cm−1) nor-mal modes of oscillation. Two contributions are present for eachmode, one due to the bulk MoS2 flake underneath the dome andthe other associated to the ML-thick dome membrane. In the lat-ter case, the Raman mode is found at lower frequency with re-spect to the bulk one, due to the inherent tensile strain of thedomes.[39]An immediately recognizable difference between the two setsof data regards the variation with T of the relative intensity ofthe dome and bulk peaks. Specifically, the signal from the bareMoS2 dome exhibits a largely different intensity for different tem-peratures. This can be ascribed to the dome progressive defla-tion, which, in turn, affects the interference effect responsible forthe amplification of the Raman signal occurring when the domeheight matches about half the wavelength of the Raman signal(≈532/2 nm). Instead, the T-dependent spectra of the same domeafter being capped with hBN exhibit a remarkable stability of thedome to bulk relative intensity even at the lowest temperatureconsidered, offering evidence of the fact that the capped domeretains its shape, regardless of the hydrogen phase. Figure 2edisplays the results of the Lorentzian fits performed on each μ-Raman spectrum, plotting the shift of the E2g and A1g peaks ofthe dome as a function of temperature. The Raman shift valueswere fitted with the formula𝜔(T) = 𝜔0 + A[1 + 2∕(eℏ𝜔02kBT − 1)](1)Adv. Optical Mater. 2023, 11, 2202953 2202953 (2 of 10) © 2023 The Authors. Advanced Optical Materials published by Wiley-VCH GmbH 21951071, 2023, 12, Downloaded from https://onlinelibrary.wiley.com/doi/10.1002/adom.202202953 by Cochrane Japan, Wiley Online Library on [23/06/2023]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons Licensehttp://www.advancedsciencenews.comhttp://www.advopticalmat.dewww.advancedsciencenews.com www.advopticalmat.deFigure 1. a) AFM image of an ordered array of WS2 domes partially capped with a thin hBN flake (≈10 nm). The white dashed line marks the borderbetween the uncapped or bare, area (top-right) and the remaining area capped with hBN. b) Optical microscope image of the same area of panel (a),taken at RT. c) Same as (b) but with the sample at 5 K. The domes in the capped area kept their shape, while those which were not capped with hBNclearly deflated. The lower quality of the image at T = 5 K is caused by the presence of the cryostat window.Figure 2. a) 3D AFM image of a bare MoS2 dome. b) 3D AFM image of the same dome of panel (a) after the capping with hBN. Both images weretaken at RT. c) Stacked normalized μ-Raman spectra at different temperatures of the bare MoS2 dome. The peaks associated to the E2g (in-plane) andA1g (out-of-plane) modes of oscillation are visible for both the bulk material (indicated by a dashed arrow) and the dome membrane (indicated by asolid arrow). d) Stacked normalized μ-Raman spectra at different temperatures of the same MoS2 dome of panel (c) after the capping with hBN. A laserexcitation wavelength equal to 532 nm and laser power equal to 30 μW were employed for the spectra of panels (c,d). e) Temperature dependence ofthe E2g (squares) and A1g (circles) modes of the dome membrane before (olive, full) and after (red, empty) the hBN capping. The gray shaded areahighlights the T interval for which hydrogen condenses (<32 K). The evolution in temperature is fitted (solid lines) via Equation (1). For the bare dome,the fit was performed for T = 150 − 300 K, where a well defined trend could be observed (the dashed lines are an extrapolation at lower temperatures).f) Values of the biaxial strain versus T for the bare (olive full squares) and capped (red empty squares) MoS2 dome obtained from Equation (2). In thecase of the capped dome the strain is kept constant even at the temperatures for which the hydrogen is no longer in the gas phase (gray shaded area).The lines are guides to the eye.where only the lower-order three-phonon processes of the anhar-monic Klemens model are taken into account via the parameterA.[40,41] The frequency trends of the dome peaks show a strongdifference between the bare and capped configurations, with thelatter featuring regular temperature shifts, even in the range oftemperatures for which the hydrogen gas condenses (indicated inthe plots by a shaded gray area). Instead, the bare dome is charac-terized by a much more erratic change in the frequency positionof its Raman peaks, reflecting the major structural changes it un-dergoes at lower temperatures. For these reasons, only the peaksAdv. Optical Mater. 2023, 11, 2202953 2202953 (3 of 10) © 2023 The Authors. Advanced Optical Materials published by Wiley-VCH GmbH 21951071, 2023, 12, Downloaded from https://onlinelibrary.wiley.com/doi/10.1002/adom.202202953 by Cochrane Japan, Wiley Online Library on [23/06/2023]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons Licensehttp://www.advancedsciencenews.comhttp://www.advopticalmat.dewww.advancedsciencenews.com www.advopticalmat.dein the temperature range 150–300 K were fitted by Equation (1)with the resulting fit being indicated by an olive solid line. Theline obtained from the fit is extended, for lower temperatures,by an olive dashed line, which highlights the stark contrast be-tween the expected evolution in temperature and the actual mea-sured frequencies. The Raman mode frequency of the referenceunstrained ML, 𝜔ML(T), can be obtained from the measured bulkmode by just adding (subtracting) a small (1.5 cm−1), rigid shiftto the E2g (A1g) peak.[39] Therefore, from the frequency position ofthe Raman peaks associated to the bulk and to the dome modes(the full set of data is shown in Supporting Figure S1, SupportingInformation), the information on the biaxial strain 𝜖 at differenttemperatures can be extracted quantitatively by applying the fol-lowing formula:𝜀(T) = 12×⎛⎜⎜⎝𝜔MLE2g(T) − 𝜔domeE2g(T)𝜔MLE2g(T)⎞⎟⎟⎠1𝛾E2g(2)where 𝜔MLE2g(T) is the Raman shift of the in-plane mode of a strain-free MoS2 ML at temperature T, 𝛾E2gis the Grüneisen parameterequal to 0.84 ± 0.11[39] (assumed constant at all temperatures[42])and the total biaxial strain is divided by two to obtain the strainexerted on just one direction (i.e., the radial or the circumfer-ential one that are equivalent at the top of the dome[39]). Thestrain values obtained with such a procedure are shown in Fig-ure 2f. The evolution of the strain characterizing the bare dome(denoted by olive full squares) with decreasing T indicates thatthe dome membrane becomes progressively less stretched dueto the diminishing pressure of the hydrogen gas inside. Whenthe gas eventually condenses, for temperatures falling in the grayshaded area, the dome collapses onto the bulk crystal, so that thecorresponding μ-Raman signal disappears at T = 5 K (see thebottom-most spectrum in Figure 2c). In the case of the cappedMoS2 dome, instead, the values of strain show virtually no evolu-tion for all temperatures, as displayed by the red empty squares.The same strain evolution in temperature was observed for sev-eral other bare and capped MoS2 domes, see Figure S2, Support-ing Information. This provides further confirmation that MoS2domes can be rigidly and systematically kept in shape by the hBNlayer deposited atop. Even though the exact mechanism by whichthe hBN thin flake causes such a phenomenon is still not fully un-derstood, we envisage that the hBN high rigidity may play an im-portant role. Additionally, it is noteworthy that at RT the cappingprocedure reduces the membrane strain from 𝜖 = 2% to about1.7% (see Figure 2f). This indicates a likely elastic energy transferfrom the dome membrane to the capping hBN layer, which possi-bly provides the energy necessary to maintain the dome structureregardless of the inner hydrogen phase state. It is worth mention-ing that hydrogen-filled strained domes made of hBN are alsoknown to deflate at low Ts.[38,43] This proves the unique effectplayed by the capping process, during which the hBN flake adaptsto the sample morphology to minimize the total energy of the sys-tem. In turn, the deflation of the dome underneath would imply astrain transfer to the hBN capping layer and thus an energy cost.The cost of keeping the dome in-shape is likely lower, leading tothis peculiar effect. We also verified that the same structural ef-fect is obtained by capping with hBN flakes hBN domes insteadof TMDC domes.Figure 3. Stacked normalized μ-PL spectra taken at different temperatureson a WS2 dome, before (olive dashed line) and after (red solid line) thecapping with few-layer-thick hBN. Excitation wavelength equal to 532 nmand laser power equal to 44 μW. The free neutral exciton is indicated asX, while recombination from charged excitons (trions) is indicated withT. At T = 290 K, the X (and T) band of the capped dome is about 90meV higher in energy than that of the bare dome indicating a sizable ten-sile strain reduction ensuing the capping. At lower temperatures, for thebare dome several bands contribute to the emission spectra likely origi-nating from morphological defects associated to the progressively deflat-ing membrane. On the contrary, the hBN-capped dome maintains a well-defined lineshape at all temperatures (the T band label is omitted for claritypurposes). The inset shows a 3D AFM map of the dome capped with hBN,taken at RT.The effects of hBN capping were also investigated by perform-ing μ-PL measurements (see Experimental Section) both priorto and after the capping procedure on the ordered array of WS2domes displayed in Figure 1. For these domes, we also performedRaman studies analogous to those performed on MoS2 domes,see Figure S3, Supporting Information. Usually, the PL emis-sion spectra of TMDC MLs are characterized by the presenceof the band gap exciton peak,[44] whose recombination energyis highly sensitive to the amount of strain experienced by theML.[17,45] For our system, the redshift of the exciton emission isintimately related to the evolution of the strain tensor across thedome surface.[37,46] In particular, the biaxial strain values that canbe attained on top of the bare WS2 domes are close to those nec-essary (𝜖 ≃ 2.5%) to observe the direct-to-indirect band gap tran-sition, as reported previously.[46,47]Figure 3 illustrates the effects of hBN capping on the emis-sion properties of a WS2 dome belonging to the array shown inFigure 1. The spectra were recorded on the top of the dome bothbefore (olive dashed lines) and after (red solid lines) the hBN cap-ping. At 290 K, the first clear difference between the bare andcapped configuration is a reduction in strain following the cap-ping procedure, as demonstrated by the blueshift of the neutralexciton peak (labeled X) in the capped dome (the lower-energyAdv. Optical Mater. 2023, 11, 2202953 2202953 (4 of 10) © 2023 The Authors. Advanced Optical Materials published by Wiley-VCH GmbH 21951071, 2023, 12, Downloaded from https://onlinelibrary.wiley.com/doi/10.1002/adom.202202953 by Cochrane Japan, Wiley Online Library on [23/06/2023]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons Licensehttp://www.advancedsciencenews.comhttp://www.advopticalmat.dewww.advancedsciencenews.com www.advopticalmat.debands, labeled T, can be attributed in both cases to charged exci-ton species[45,48]). This finding is in accordance with the μ-Ramanresults of the MoS2 domes (see Figure 2 and related discussion).However, in the patterned WS2 dome, the ≃90 meV blueshift ofthe exciton energy at 290 K indicates a remarkable biaxial strainreduction by about 1% after the hBN capping.[46] This points toan optimal adherence of the hBN layer over the dome surfacethanks to the lithographic isolation of the patterned WS2 domesthat avoids the formation of satellite smaller domes around them(as demonstrated by the AFM images of an uncapped region ofthe sample shown in Figure S4, Supporting Information). Thisisolation does not occur in the randomly formed MoS2 domesdiscussed previously (see AFM images in Figure 2a,b), where asmaller strain reduction was indeed found. When cooling thesample from RT, the changes in the bare dome morphology canbe seen in the evolution of the dome emission spectrum, char-acterized by a broad lineshape with several contributing bandsand by an exciton peak showing an anomalously large “thermal”blueshift of 250 meV. This value clearly contrasts the regularblueshift observed for the exciton of the capped dome (85 meV)when T decreases. Indeed, at 5.5 K the bare dome is completelydeflated, and the peak visible at ≈2.1 eV comes from the WS2 MLlaying on the WS2 bulk, being decoupled from this latter by thelikely presence of condensed hydrogen. Lower-energy recombi-nation bands ought to be ascribed to the morphologically disor-dered structure (e.g., wrinkles) of the deflated ML. As a matter offact, the progressive decrease of tensile strain when the H2 pres-sure inside the dome decreases induces a blueshift of the excitonenergy that adds to the usual band gap increase with decreasingT. For decreasing T, the regular blueshift of the X peak of thecapped dome (lower-energy bands are ascribable to charged ex-citon species) leads thus to a peculiar trend in which the cappeddome exciton, compared to the bare one, is at higher energiesfor high temperatures and lower energies for low temperatures.A more detailed temperature study for this capped WS2 domecan be found in the Figure S5, Supporting Information. In thesame figure, it is also shown that the evolution in temperature ofthe X energy of the capped dome overlaps that of an unstrainedWS2 ML.The hBN capping procedure—along with the high spatial con-trol over the dome formation site—prompts the opportunity forthe creation of light sources in TMDC materials. Indeed, one ofthe most striking features of the capping procedure is the appear-ance, in the μ-PL spectra of domes brought to low temperature,of intense, relatively narrow (full-width at half-maximum = 1–5meV) and spectrally isolated lines, whose quantum nature will beshown next.Previous studies already reported the appearance of SPEs inWSe2 and WSSe MLs at low temperature. In many cases, theirorigin was ascribed to strain gradients, which are provided ei-ther by the wrinkling caused by the deposition procedure[25,49]or by nanostructured stressors, such as nanopillars, nanorodsetc.[27,29,30,50] Even though the exact origin of these SPEs has notbeen fully clarified, yet, it is strongly suggested that they couldstem from the interplay between dark states, strain gradients, anddefect-related levels.[51–53] In particular, darkish materials such asWSe2 and WS2 are characterized by opposite spins in the bottomconduction band (CB) and in the top valence band (VB), result-ing in a dark ground state. Indeed, strain lowers the CBs and,when it reaches a suitable value, may bring the lowest energy CBto become resonant with defect states, leading to a hybridizationbetween the two.[51] While dark strain-localized exciton states re-main dark, hybridization with a point defect breaks the valley se-lectivity and leads to efficient light emission.In our system the strain gradient is naturally provided by thedome morphology with an ensuing strain increase from the edgeto the top of the dome, where the strain is the largest.[37,39] Basedon a theoretical ground, such a strain increases from the edgetoward the summit should lead to the formation of localizedstates toward the dome apex, due to strain-induced quantumconfinement.[54] However, in our system, by approaching the topof the dome, the exciton recombination character changes fromdirect to indirect, thus leading to a less intense emission.[46] Asa matter of fact, we observe that the emitters appear almost ex-clusively at the edge of slightly asymmetric domes, as depictedin the optical images in the insets of Figure 4a,f. The imagesshow the point of the domes excited by the laser spot from whichthe corresponding spectra displayed in the main panel originate.Our result is the first experimental investigation of micron-sizedWS2 domes, whose exciton character changes from direct to in-direct from the edge toward the center. Some previous studies in-vestigated instead TMDC nanodomes by near-field PL measure-ments at room temperature.[55] In that case, the presence of lo-calized, deeply bound exciton states only in the dome peripherywas reported.[55] Those nanodomes were created by depositionof a TMD ML on an hBN substrate (and ensuing encapsulationof contaminants), and the observation of SPEs at the dome pe-riphery was explained in terms of atomic-scale wrinkling takingplace at the edge of the nanodomes, thus causing strain maximato be at the edges and not at the center, as theoretically supportedby the findings of ref. [56]. The system simulated in that theo-retical work also differs remarkably from ours, since it focuseson the study of a MoS2 ML deposited over a MoS2 ML. Indeed,the theoretical framework described in that work[56] and used toexplain the near-field experiments[55] cannot be applied to ourdomes, which are created by a bottom-up approach (hydrogen-ion irradiation) resulting in well-clamped edges (with no wrin-kling) and in a strain increase from the edges toward the center.We rather believe that the more pronounced direct character ofthe exciton at the edge of the domes along with the suitable strainextent act as the driving force leading to the appearance of theemitters therein.The spectrum of Figure 4a features three distinct lines (labeledwith the letters 𝛼, 𝛽, and 𝛾), whose intensity for increasing laserpower is shown in Figure 4b. For each line, the emission inten-sity displays the saturating behavior usually associated to discretedefect states[19] and described by the equationI = Isat[P∕(P + PN)](3)where Isat is the saturation intensity and PN is the laser power atwhich the intensity is half of Isat.Time-resolved μ-PL measurements (see Experimental Sec-tion), performed by exciting the sample with a supercontinuumlaser spectrally filtered at 532 nm, were taken for all these emit-ters in order to extract their decay time. In Figure 4c the PL de-cay curves of the emitters 𝛼, 𝛽, and 𝛾 are fitted with an exponen-tial function, yielding time decay time values of (2.690 ± 0.029),Adv. Optical Mater. 2023, 11, 2202953 2202953 (5 of 10) © 2023 The Authors. Advanced Optical Materials published by Wiley-VCH GmbH 21951071, 2023, 12, Downloaded from https://onlinelibrary.wiley.com/doi/10.1002/adom.202202953 by Cochrane Japan, Wiley Online Library on [23/06/2023]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons Licensehttp://www.advancedsciencenews.comhttp://www.advopticalmat.dewww.advancedsciencenews.com www.advopticalmat.deFigure 4. a) Narrow lines (labeled 𝛼, 𝛽, and 𝛾) in the μ-PL spectrum of a capped dome at 6.8 K. The excitation wavelength is 532 nm and the laser poweris 200 nW. The right inset is an optical image, taken at 6.8 K, showing the dome’s area excited by the laser. The white dashed line highlights the domeasymmetric profile. Scale bar 2 μm. b) Saturation of the PL intensity (I) for increasing laser power (P) of all the three lines featured in panel (a), fittedwith Equation (3). c) Time-decay curves of the three emitters of panel (a) fitted with an exponential decay function. The decay times extracted from thefits are equal to (2.690 ± 0.029), (6.58 ± 0.20), and (3.045 ± 0.042) ns for the emitters 𝛼 (green), 𝛽 (red), and 𝛾 (blue), respectively. d,e) Second-orderautocorrelation function g(2)(𝜏) measured for emitter 𝛼 (panel d) and 𝛽 (panel e) whose values at zero-time delay are equal to 0.248 ± 0.046 and 0.151± 0.033. f) μ-PL spectrum of a different WS2 dome capped with hBN at a temperature of 6.8 K. Laser power, 20 μW. The spectrally isolated emitter islabeled 𝛿. The inset on the right shows an optical image of the dome and its area excited by the laser at 6.8 K. The white dashed line highlights thedome’s asymmetric profile. Scalebar, 2 μm. g) Second-order autocorrelation function g(2)(𝜏) measured for emitter 𝛿, whose g(2)(0) value is equal to0.183 ± 0.069. h) Time-decay curve of emitter 𝛿 fitted with an exponential decay function. The extracted decay time of (4.148 ± 0.055) ns.(6.58± 0.20), and (3.045± 0.042) ns, respectively. All the obtaineddecay times are in accordance with those measured for local-ized states in WSe2 MLs.[18–20,28–30,33,34,57] To determine whetherthe studied lines originate from quantum emitters, the second-order autocorrelation function g(2)(𝜏) was measured for line 𝛼(Figure 4d) and 𝛽 (Figure 4e). The samples were excited with acontinuous-wave laser at 532 nm and the emission was filteredby a monochromator in order to select a specific emitter (see de-tails on our Hanbury–Brown and Twiss setup in the Experimen-tal Section). The autocorrelation function was not measured forline 𝛾 due to its lower intensity and to the presence of nearbysmaller-intensity emissions that would have hindered the sourcepurity. All the measured emitters display the characteristic anti-bunching dip at zero time delay, with values of g(2)(0) equal to0.248 ± 0.046 and 0.151 ± 0.033 for the 𝛼 and 𝛽 emitters, re-spectively. The spectrum of another capped WS2 dome is shownin Figure 4f, and it is characterized by the presence of a single,isolated emitter (labeled 𝛿) that is observed by exciting the domeclose to its edge. Figure 4g shows the second-order autocorrela-tion function of this line, whose behavior as an SPE is confirmedby a value of g(2)(0) equal to 0.183 ± 0.069. The correspondingtime-resolved μ-PL measurement is displayed in Figure 4h, andit shows a mono-exponential decay curve, whose fitting returnsa decay time of (4.148 ± 0.055) ns. We point out that while mostof the research interest on TMDC-based SPEs has been focusedon WSe2, WS2 has been the object of much fewer studies.[24,29,58]Our work presents, to our knowledge, the first non-electrical gen-eration of WS2 SPEs, with the highest purity to date and the firstmeasurement of the emitter decay time. We exclude that theseemitters may come from the hBN capping layer since, i) emittersin hBN are typically created via electron irradiation, plasma treat-ments, and/or high-T annealings;[36,59,60] ii) emitters in hBN aretypically broader, are distributed over a larger energy range andpersist up to RT (unlike ours);[59,60] iii) we do not observe SPEs inthe capped regions around the WS2 domes, nor in hBN-cappedMoS2 domes, nor in hBN domes.SPEs in TMDC MLs can be also relevant for valley- and spin-tronics applications. To this regard, the exciton gyromagnetic fac-tor, g, is an important parameter determining the response of thecarrier states to magnetic fields. It also represents an insightfulquantity for understanding the properties of the band structurelevels involved in the exciton recombination. Therefore, the emit-Adv. Optical Mater. 2023, 11, 2202953 2202953 (6 of 10) © 2023 The Authors. Advanced Optical Materials published by Wiley-VCH GmbH 21951071, 2023, 12, Downloaded from https://onlinelibrary.wiley.com/doi/10.1002/adom.202202953 by Cochrane Japan, Wiley Online Library on [23/06/2023]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons Licensehttp://www.advancedsciencenews.comhttp://www.advopticalmat.dewww.advancedsciencenews.com www.advopticalmat.deFigure 5. a) Helicity-resolved normalized μ-magneto-PL spectra of the SPElabeled 𝛼 in Figure 4a. In order to compensate for slight changes in theemission energy caused by a drift of the sample during the field sweep,the spectra are shifted on the x-axis with respect to the mean value in en-ergy of the two polarizations. A splitting of a less intense line is also visibleat higher energy. b) Zeeman splitting ΔE of the line of panel (a) as a func-tion of the magnetic field. The linear fit, following Equation (4), gives avalue of the exciton g-factor equal to −8.034 ± 0.054. c) In pink, we showthe histogram the g-factors obtained by performing fits analogous to thatof panel (b) on 18 narrow lines (see Figure S6, Supporting Information).The histogram is characterized by a mean value of g = −8.20 ± 0.51. Inpurple, we show the histogram of the g-factors of the free exciton of un-strained WS2 MLs. The histogram was built from the values available inthe literature[47,62–69] giving a mean value of g = −4.00 ± 0.28. The in-set shows the correspondence between the energy of the SPE and theirg-factor, showing no apparent correlation.ters discussed above, as well as several others, were studied inthe presence of a magnetic field (B) parallel to the luminescencewavevector (Faraday configuration) up to 12 T, at a temperature of10 K (see Experimental Section). After being excited by a 515 nmlaser, the light emitted from the sample is analyzed in terms of 𝜎+and 𝜎− polarizations. Figure 5a shows the magneto-μ-PL spectraof line 𝛼 displayed in Figure 4a. The intensity factors next to eachspectrum show the increasing degree of circular polarization ofthe emission with increasing B, reaching a value of 69% at 8 T,which is consistent with the one obtained for similar emitters inWSe2.[61] By fitting the spectra of the two circular polarizationswith a Lorentzian function we can extract the Zeeman splittingenergy of the exciton ΔE(B) = E(𝜎+) − E(𝜎−), and from the rela-tionΔE = g𝜇BB (4)(where 𝜇B is the Bohr magneton) we can obtain the value of theexciton g-factor (Figure 5b). Emitters 𝛼 and 𝛽 (for this latter, referto Figure S6(l), Supporting Information) have values of g equalto −8.034 ± 0.054 and −8.176 ± 0.025, respectively. The valuesof g related to several other narrow lines (see linear fits on theZeeman splittings in Figure S6, Supporting Information) werecollected in the pink histogram of Figure 5c, giving a mean valueof g = −8.20 ± 0.51. The additional high-energy peak visible inthe spectra of Figure 5a was not included in this histogram sinceit is the only outlier of the distribution, with a g equal to −5.39 ±0.13. This anomalous value of g requires further studies, since itcould be linked to an excitonic species different from that of theother emitters.Due to the lack of preceding results on the exciton g-factor ofWS2 SPEs, we compare our results with those of free excitons instrain-free WS2 MLs reported in the literature;[47,62–69] see the pur-ple histogram in Figure 5c. A comparison between the two his-tograms shows that the absolute value of g for our SPEs is abouttwice that of the free exciton of unstrained WS2 MLs. We also no-tice that a similar finding was observed for analogous emitters instrained WSe2 MLs.[18–21,23,49,70,71] No correlation is observed be-tween the energy of the emitters and the value of their g-factor,as it can be seen from the inset of Figure 5c.In the case of WSe2 emitters, it was proposed that this large gvalue is a consequence of the emitter radiative transition involv-ing an electron in a defect state and a hole in the VB. While thehybridized orbitals dx2−y2 ± idxy (lz =±2h) constituting the VB areresponsible for the intrinsic valley exciton gyromagnetic factor of−4 (the CB dz2 orbitals do no contribute having lz = 0), the defectstate adds to the orbital component of g increasing its absolutevalue.[70] Alternatively, the large gyromagnetic factor reported inthe histogram of Figure 5c would be compatible with the valuefound for dark excitonic species (both neutral and charged) inWS2,[62] WSe2,[72] and WSSe[50] MLs. In our case, the presence ofstrain and ensuing microscopic disorder could favor dark statesstate as the origin of the observed emission lines and new exper-iments are planned to address this point.3. ConclusionsBy creation of hBN/dome heterostructures, we succeeded in pre-venting the deflation of H2-filled, highly strained TMDC micro-domes below the H2 liquefaction temperature (≈32 K). Opti-cal microscopy, μ-Raman, and μ-PL measurements provided evi-dence of such phenomenon. The strained configuration, kept inplace by the hBN layer, is crucial for the appearance of intensenarrow lines in the emission spectra of spatially controlled WS2domes formed in the openings of a lithographically defined maskand brought to 5 K. Second-order autocorrelation measurementsconfirmed these lines as originating from SPEs. Magneto-μ-PLmeasurements on the SPEs provided a localized exciton g-factorwith a value of about −8. These results confirm the feasibility ofour method for the generation of ordered arrays of SPEs in WS2.Adv. Optical Mater. 2023, 11, 2202953 2202953 (7 of 10) © 2023 The Authors. Advanced Optical Materials published by Wiley-VCH GmbH 21951071, 2023, 12, Downloaded from https://onlinelibrary.wiley.com/doi/10.1002/adom.202202953 by Cochrane Japan, Wiley Online Library on [23/06/2023]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons Licensehttp://www.advancedsciencenews.comhttp://www.advopticalmat.dewww.advancedsciencenews.com www.advopticalmat.deNotably, this method does not require any etching procedure onthe substrate in order to create stressors and the TMDC ML is alsoprotected from possible sources of contamination since it is natu-rally lifted up from the bulk crystal by the hydrogen gas pressure.In addition, both our results and several recent reports[27–31] seemto suggest that the formation of TMDC-based SPEs is associatedwith the presence of sharp strain gradients, such as those present,for example, in proximity of the dome edges. Since the formationof triangular and elliptical domes in predetermined patterns wasalready achieved,[73] the freedom offered by our nanofabricationmethod represents an invaluable asset for finding the dome opti-mal shape and dimensions. Improvements on the emitter inten-sity and lifetime (≈1–10 ns as determined here by time-resolvedμ-PL) can also be explored by deterministically integrating thecapped domes with photonic and plasmonic cavities.[28,35,74] Theinvestigation of the coupling of our SPEs with these cavities willbe crucially important for unleashing the full potential of TMDC-based SPEs for quantum technologies.4. Experimental SectionSample Preparation: The TMDC flakes were exfoliated from bulk crys-tals (purchased from 2D semiconductors) via the scotch-tape methodand were then deposited on SiO2/Si substrates. In the case of the WS2flakes the sample was covered with a litographically-patterned poly(methylmethacrylate) (PMMA) mask, whose openings allow the formation of or-dered arrays of hydrogen-filled domes. The patterned hydrogen irradia-tion procedure was described in detail in previous works.[13,14,37] The Sisubstrate on which the flakes were deposited was mounted in a vacuumchamber, connected to an electrical ground and heated to a temperatureof ≈150 °C. A Kaufman source irradiated the sample with low-energy (≈20eV) hydrogen ions, which penetrated into the top-most layers of the crys-tal. In the case of the samples on which the H-opaque mask had beendeposited, only the regions that had been left exposed were interested bythis phenomenon. The accumulation of molecular hydrogen beneath thefirst crystal layer led to the formation of spherical domes with footprint de-pending on the diameter of the mask aperture. In previous works the poly-mer used for the H-opaque masks was hydrogen silesquioxane (HSQ),which needed to be removed by chemical etching after the hydrogenationtreatment.[37] Instead, the PMMA mask used in this work was etched awayby the hydrogen ion beam itself, a self-cleaning process, which does notaffect the position and sizes of the formed domes. To cap the domes withfew-layer hBN, bulk hBN crystals were also exfoliated with the scotch-tapemethod and few-nm-thick flakes were isolated on PDMS. These flakes werethen positioned over the domes via the dry viscoelastic method describedin ref. [75]. A wide range of hBN flakes were tested to cap the domes. Forrelatively thick flakes (>20 nm), it is generally not possible to achieve agood adherence with the domes due to the flake rigidity and consequentcapping of air during the deposition process. Good results are generallyachieved for thin flakes (<20 nm), provided that the deposition process isperformed slowly enough. The domes were observed to remain in shapeat low T irrespective of the hBN thickness.Atomic Force Microscopy Measurements: AFM measurements wereperformed using a Veeco Digital Instruments Dimension D3100 micro-scope equipped with a Nanoscope IIIa controller, employing TappingMode monolithic silicon probes with a nominal tip curvature radius of 5–10 nm and a force constant of 40 N m−1. All the scans were performed atroom temperature and at ambient conditions. All the data were analyzedwith the Gwyddion software.μ-Raman and μ-PL Measurements: μ-Raman spectroscopy on theMoS2 domes was performed by placing the sample on a x–y piezoelec-tric stage in a closed-cycle He cryostat with variable temperature in or-der to monitor the evolution of the dome strain, as the hydrogen gas de-creases in volume and eventually condenses for temperatures below 32K.[37] The excitation laser was provided by a single frequency Nd:YVO4lasers (DPSS series by Lasos) emitting at 532 nm. A 100× objective withNA = 0.75 was employed to excite and collect the light in a backscatteringconfiguration. The laser light was filtered out by a very sharp long-pass Ra-zor edge filter (Semrock). The Raman signal was spectrally dispersed by a75 cm focal length ACTON SP750 monochromator equipped with a 1200grooves mm−1 grating and detected by a back-illuminated N2-cooled SiCCD camera (100BRX by Princeton Instruments). The μ-Raman spectralresolution was 0.7 cm−1. The ordered arrays of WS2 domes were studiedby means of μ-PL spectroscopy, using the same setup just described for theμ-Raman experiments, with the exception of a 20 cm focal length Isoplane160 monochromator, equipped with 150 and 300 grooves mm−1 gratings.A supercontinuum pulsed laser, with 50 ps pulse width and a maximumrepetition rate of 77.8 MHz, was used as a source for time-resolved μ-PLmeasurements, from which the emitter decay times were evaluated. Thelaser emission was filtered by an acoustic-optic tunable filter tuned at 532nm. The signal was time analyzed by a Si avalanche photodiode (APD)with nominal 50 ps temporal resolution.Second-Order Autocorrelation Measurements: The assessment of theemitter single-photon purity was performed with a Hanbury Brown-Twiss setup, which measured the second-order autocorrelation function(g(2)(𝜏)) of the emitted light. The measurements were performed undercontinuous-wave excitation via a 532 nm laser. The PL signal coming fromthe sample was spectrally filtered with a 20 cm focal length Isoplane 160monochromator, equipped with a 600 grooves mm−1 grating. The filteredsignal was then divided by a 50/50 beam splitter and collected by two Siavalanche photodiodes with 250 ps temporal resolution. The coincidencecounts were calculated by the PicoHarp 300 time-correlated single-photoncounting module with a maximum resolution of 4 ps.μ-PL Measurements Under Magnetic Field: Magneto-μ-PL measure-ments were performed at low temperature (<10 K) in a superconductingmagnet reaching up to 12 T. x–y–z piezoelectric stages were used to excitethe sample and collect the signal from the desired point of the sample.A 515-nm-laser and a 100× microscope objective with NA = 0.82 wereused. The same objective was used to collect the luminescence. The cir-cular polarization of the luminescence was analyzed using a quarter-waveplate and a Wollaston prism steering the components of opposite linearpolarization (and thus of opposite elicity) to different lines of the liquid-nitrogen-cooled Si-CCD (pylon by Princeton Instruments) employed. Inthis manner, opposite circular polarization components could be acquiredsimultaneously. A monochromator with 0.75 m focal length and a 600grooves mm−1 grating was used to disperse the PL signal. The field wasdirected perpendicular to the sample surface (i.e., parallel to the emittedphoton wavevector, Faraday configuration).Supporting InformationSupporting Information is available from the Wiley Online Library or fromthe author.AcknowledgementsThe authors thank Marzia Cuccu for preliminary work and AndreasStier for useful discussions. This project was funded within the Quan-tERA II Programme that has received funding from the EuropeanUnion’s Horizon 2020 research and innovation programme under GrantAgreement No 101017733, and with funding organisations Ministerodell’Universitá e della Ricerca (MUR) and Consiglio Nazionale delleRicerche (CNR). A.P. and M.F. acknowledge financial support from thePNRR MUR project PE0000023-NQSTI. The authors also acknowledgesupport by the European Union’s Horizon 2020 research and innova-tion programme through the ISABEL project (No. 871106). E.B. acknowl-edges support from La Sapienza through the grant Avvio alla Ricerca 2020(grant no. AR120172B94E733F) and Avvio alla Ricerca 2021 (grant no.AR12117A8A090764). E.B. acknowledges the support from the Nano Let-ters Seed Grant 2022 by the American Chemical Society. K.W. and T.T. ac-knowledge support from the JSPS KAKENHI (Grant Numbers 19H05790Adv. Optical Mater. 2023, 11, 2202953 2202953 (8 of 10) © 2023 The Authors. Advanced Optical Materials published by Wiley-VCH GmbH 21951071, 2023, 12, Downloaded from https://onlinelibrary.wiley.com/doi/10.1002/adom.202202953 by Cochrane Japan, Wiley Online Library on [23/06/2023]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons Licensehttp://www.advancedsciencenews.comhttp://www.advopticalmat.dewww.advancedsciencenews.com www.advopticalmat.deand 20H00354). The authors acknowledge support from the National Sci-ence Centre, Poland, through Grants No. 2018/31/B/ST3/02111 (K.O.-P.and M.R.M.) and No. 2017/27/B/ST3/00205 (A.B.). M.F. acknowledgesSapienza Progetti H2020-Collaborativi (no. PH120172B8A67DD1).Open Access Funding provided by Universita degli Studi di Roma LaSapienza within the CRUI-CARE Agreement.Conflict of InterestThe authors declare no conflict of interest.Data Availability StatementThe data that support the findings of this study are available from the cor-responding author upon reasonable request.Keywordsheterostructures, single photon emitters, strain, two-dimensional materi-alsReceived: December 10, 2022Revised: February 22, 2023Published online: April 8, 2023[1] K. F. Mak, C. Lee, J. Hone, J. Shan, T. F. Heinz, Phys. Rev. Lett. 2010,105, 136805.[2] A. Splendiani, L. Sun, Y. Zhang, T. Li, J. Kim, C.-Y. Chim, G. Galli, F.Wang, Nano Lett. 2010, 10, 1271.[3] P. Tonndorf, R. Schmidt, P. Böttger, X. Zhang, J. Börner, A. Liebig, M.Albrecht, C. Kloc, O. Gordan, D. R. T. Zahn, S. M.de Vasconcellos, R.Bratschitsch, Opt. Express 2013, 21, 4908.[4] H. Yuan, Z. Liu, G. Xu, B. Zhou, S. Wu, D. Dumcenco, K. Yan, Y. Zhang,S.-K. Mo, P. Dudin, V. Kandyba, M. 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