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Huije Ryu, Seong Chul Hong, Kangwon Kim, Yeonjoon Jung, Yangjin Lee, Kihyun Lee, Youngbum Kim, Hyunjun Kim, [Kenji Watanabe](https://orcid.org/0000-0003-3701-8119), [Takashi Taniguchi](https://orcid.org/0000-0002-1467-3105), Jeongyong Kim, Kwanpyo Kim, Hyeonsik Cheong, Gwan-Hyoung Lee

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[Optical grade transformation of monolayer transition metal dichalcogenides <i>via</i> encapsulation annealing](https://mdr.nims.go.jp/datasets/ffc0eded-ad62-4c77-a566-bad576accb13)

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Optical grade transformation of monolayer transition metal dichalcogenides via encapsulation annealingNanoscalePAPERCite this: Nanoscale, 2024, 16, 5836Received 28th December 2023,Accepted 26th February 2024DOI: 10.1039/d3nr06641jrsc.li/nanoscaleOptical grade transformation of monolayertransition metal dichalcogenides via encapsulationannealing†Huije Ryu,‡a Seong Chul Hong,‡a Kangwon Kim,b Yeonjoon Jung,a Yangjin Lee,c,dKihyun Lee,c,d Youngbum Kim,e Hyunjun Kim,a Kenji Watanabe, fTakashi Taniguchi,g Jeongyong Kim, e Kwanpyo Kim,c,d Hyeonsik Cheong b andGwan-Hyoung Lee *aMonolayer transition metal dichalcogenides (TMDs) have emerged as highly promising candidates for opto-electronic applications due to their direct band gap and strong light–matter interactions. However, exfoliatedTMDs have demonstrated optical characteristics that fall short of expectations, primarily because of significantdefects and associated doping in the synthesized TMD crystals. Here, we report the improvement of opticalproperties in monolayer TMDs of MoS2, MoSe2, WS2, and WSe2, by hBN-encapsulation annealing. MonolayerWSe2 showed 2000% enhanced photoluminescence quantum yield (PLQY) and 1000% increased lifetime afterencapsulation annealing at 1000 °C, which are attributed to dominant radiative recombination of excitonsthrough dedoping of monolayer TMDs. Furthermore, after encapsulation annealing, the transport character-istics of monolayer WS2 changed from n-type to ambipolar, along with an enhanced hole transport, whichalso support dedoping of annealed TMDs. This work provides an innovative approach to elevate the opticalgrade of monolayer TMDs, enabling the fabrication of high-performance optoelectronic devices.IntroductionSemiconducting TMDs have significant potential in opticalapplications owing to their large exciton binding energy, indir-ect-to-direct band transition, and strong light–matterinteraction.1–3 However, exfoliated or synthesized TMDs haveshown different optical properties because they are stronglyaffected by several factors, such as doping, strain, chemicalreaction, dielectric environment, and defects, that are alsocorrelated.4–12 For example, chalcogen vacancies cause n-typedoping and provide non-radiative recombination pathways in theTMDs.11 Numerous studies have demonstrated improvements inthe PL of monolayer TMDs through various approaches, includ-ing decreasing chalcogen vacancies, electrostatically doping, orapplying mechanical strain.4,7,13,14 However, the critical factorresponsible for the enhancement of PL remains elusive. Thisposes challenges in the fabrication of optical grade TMDs withoptical properties approaching the theoretical upper limit. Here,we report a significant enhancement of optical properties inmonolayer TMDs, including MoS2, MoSe2, WS2, and WSe2, byhBN-encapsulation annealing. Despite the 400% increase indefect density resulting from annealing hBN-encapsulated1L-WSe2 at 1000 °C, it astonishingly led to a remarkable 2000%increase in PL quantum yield (QY) and a 1000% extension in PLlifetime. Additionally, transport characteristics of the 1L-WS2transformed from n-type to ambipolar behavior along with anenhanced hole transport after encapsulation annealing. Ourobservation suggests that modulation of doping by hBN-encapsu-lation annealing plays a critical role in the optical grade of mono-layer TMDs.Results and discussionFor encapsulation annealing of monolayer TMDs, we fabri-cated stacks of hBN/TMDs/hBN using a pick-up technique.15Fig. 1a shows an optical microscopic image and atomic force†Electronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d3nr06641j‡These authors contributed equally to this work.aDepartment of Materials Science and Engineering, Seoul National University, Seoul08826, Republic of Korea. E-mail: gwanlee@snu.ac.krbDepartment of Physics, Sogang University, Seoul 04107, Republic of KoreacDepartment of Physics, Yonsei University, Seoul 03722, Republic of KoreadCenter for Nanomedicine, Institute for Basic Science, Seoul 03722, Republic ofKoreaeDepartment of Energy Science, Sungkyunkwan University, Suwon 16419, Republic ofKoreafResearch Center for Electronic and Optical Materials, National Institute forMaterials Science, 1-1 Namiki, Tsukuba 305-0044, JapangResearch Center for Materials Nanoarchitectonics, National Institute for MaterialsScience, 1-1 Namiki, Tsukuba 305-0044, Japan5836 | Nanoscale, 2024, 16, 5836–5844 This journal is © The Royal Society of Chemistry 2024Open Access Article. Published on 26 February 2024. Downloaded on 6/10/2024 10:37:27 AM.  This article is licensed under a Creative Commons Attribution-NonCommercial 3.0 Unported Licence.View Article OnlineView Journal  | View Issuehttp://rsc.li/nanoscalehttp://orcid.org/0000-0003-3701-8119http://orcid.org/0000-0003-4679-0370http://orcid.org/0000-0002-2347-4044http://orcid.org/0000-0002-3028-867Xhttps://doi.org/10.1039/d3nr06641jhttps://doi.org/10.1039/d3nr06641jhttps://doi.org/10.1039/d3nr06641jhttp://crossmark.crossref.org/dialog/?doi=10.1039/d3nr06641j&domain=pdf&date_stamp=2024-03-12http://creativecommons.org/licenses/by-nc/3.0/http://creativecommons.org/licenses/by-nc/3.0/https://doi.org/10.1039/d3nr06641jhttps://pubs.rsc.org/en/journals/journal/NRhttps://pubs.rsc.org/en/journals/journal/NR?issueid=NR016011microscopy (AFM) topography image of the as-stacked hBN/1L-WSe2/hBN. After being annealed at 1000 °C in vacuum for1 hour, no significant change was observed as shown inFig. 1b. Furthermore, as indicated by thickness profiles (whitedashed lines) in Fig. 1a and b, there was no change in thethickness of the 1L-WSe2 after encapsulation annealing. Incontrast, the as-exfoliated 1L-WSe2 on a SiO2 substrate wasthermally degraded and finally removed at 1000 °C duringannealing (Fig. S1†), which is consistent with previousstudies.16,17 To examine the structural changes and dopingeffect due to encapsulation annealing, we measured theRaman spectra of hBN-encapsulated 1L-WSe2 before and afterannealing (Fig. 1c). After encapsulation annealing, no notice-able change was observed. To clearly identify the effect ofstrain and doping, we used 1L-MoS2 as an indicator used inprevious studies.18–20 Fig. 1d shows the Raman spectra of hBN-encapsulated 1L-MoS2 before and after annealing. The fullwidth at half maximum (FWHM) of both A1g and E12g peaksdecreased. Moreover, the encapsulation annealing processleads to a decrease in the intensity of the defect-related LAmode (∼227 cm−1) and an increase in the intensities of A1gand E12g peaks. These results are summarized in Fig. 1e: theFWHM values of A1g and E12g peaks and ratios of I(LA)/I(A1g)and I(LA)/I(E12g) decreased after encapsulation annealing. Theposition shifts of A1g and E12g peaks in Fig. S2 and S3† showthat the 1L-MoS2 becomes dedoped after encapsulationannealing.To study the effect of encapsulation annealing on theoptical properties of monolayer TMDs, we measured the PLspectra of 1L-WSe2 before and after encapsulation annealing(Fig. 2a). After encapsulation annealing, the PL intensity of1L-WSe2 exhibited a significant increase by a factor of 8.8. ThePL intensity map in Fig. 2b verifies that the PL was uniformlyenhanced over whole area of 1L-WSe2. To explore how anneal-ing temperature influences the PL intensity, we assessed thePL intensity of the annealed hBN/1L-WSe2/hBN and 1L-WSe2/SiO2 structures, as depicted in Fig. 2d. The annealing processwas conducted in increments of 100 °C, spanning a rangefrom Tanneal = 100 to 1000 °C. Up to 300 °C, the PL intensityfor the hBN/1L-WSe2/hBN remained almost unchanged. It wasonly after annealing at 400 °C that a noticeable increase in PLintensity became evident. Between 400 °C and 600 °C, therewas a sharp increase in PL intensity, aligning with the temp-erature range where the trion/exciton intensity ratio andFWHM of A exciton peak rapidly decreases, as indicated in theinset of Fig. 2a and S4† respectively. Given that trions primar-ily follow a non-radiative pathway, the diminished trion/exciton ratio suggests that the radiative recombination of exci-tons dominates in our samples.4 However, the PL intensityshowed no further increase at higher temperature than 700 °C.In contrast, the 1L-WSe2/SiO2 showed a marked decline in PLintensity at 500 °C and no PL at higher temperature due tothermal degradation of WSe2 (see Fig. S5† for PL spectra of1L-WSe2/SiO2 before and after annealing at 1000 °C).Fig. 1 hBN-encapsulation annealing of 1L-TMDs. Optical microscopic images (left) and AFM topography images (right) of hBN-encapsulated1L-WSe2 (a) before and (b) after annealing at 1000 °C. Raman spectra of hBN-encapsulated (c) 1L-WSe2 and (d) 1L-MoS2 before (blue) and afterannealing (red). (e) FWHM of E12g and A1g peak (top) and intensity ratio of I(LA)/I(E12g) and I(LA)/I(A1g) (bottom) of hBN-encapsulated 1L-MoS2 before(blue) and after annealing (red).Nanoscale PaperThis journal is © The Royal Society of Chemistry 2024 Nanoscale, 2024, 16, 5836–5844 | 5837Open Access Article. Published on 26 February 2024. Downloaded on 6/10/2024 10:37:27 AM.  This article is licensed under a Creative Commons Attribution-NonCommercial 3.0 Unported Licence.View Article Onlinehttp://creativecommons.org/licenses/by-nc/3.0/http://creativecommons.org/licenses/by-nc/3.0/https://doi.org/10.1039/d3nr06641jAdditionally, we annealed monolayers of MoS2, MoSe2, andWS2 1000 °C for 1 h, all encapsulated with hBN. As shown inFig. 2d, it was verified that the enhancement in PL is not exclu-sive to WSe2. The PL intensity of MoS2, MoSe2, and WS2increased by a factor of 3.5, 4.2, and 9, respectively. The PLintensities of these four monolayer TMDs after encapsulationannealing are displayed in the inset of Fig. 2d.In Fig. 3, we investigated the optical properties of 1L-WSe2before and after encapsulation annealing. Fig. 3a and b showthe PL contour plots of the as-stacked and annealed 1L-WSe2as a function of temperature, respectively. Below 100 K, thereis no significant difference between PL intensity and peakpositions of two samples. However, the A exciton (XA) PL peakof annealed 1L-WSe2 is higher than that of the as-stacked oneFig. 2 PL enhancement of monolayer TMD by hBN-encapsulation annealing. (a) PL intensity spectra of hBN-encapsulated 1L-WSe2 before (blue)and after annealing (red). Inset: Annealing temperature (100–1000 °C) dependent PL intensity ratio of trion and exciton. (b) PL intensity map of hBN-encapsulated 1L-WSe2 before (top) and after annealing (bottom). (c) Annealing temperature dependent PL intensity of hBN/1L-WSe2/hBN (red) and1L-WSe2/SiO2 (blue) structures. (d) Bar plot indicating the PL enhancement factor for each 1L-MX2 (M = Mo, W/X = S, Se) after encapsulation anneal-ing. Inset: PL intensity of each monolayer TMD after encapsulation annealing.Fig. 3 Optical property measurement of hBN-encapsulated 1L-WSe2 before and after annealing. Contour plots of PL spectra of hBN-encapsulated1L-WSe2 as a function of temperature (10–180 K) (a) before and (b) after annealing. (c) A exciton intensity as a function of temperature of exfoliated1L-WSe2 (green), hBN-encapsulated 1L-WSe2 before (blue) and after annealing (red). (d) PL spectra (10 K) of hBN-encapsulated 1L-WSe2 before(blue) and after annealing (red). (e) Plot of QY as a function of the generation rate and (f ) normalized TRPL decay curves for hBN-encapsulated1L-WSe2 before (blue) and after annealing (red).Paper Nanoscale5838 | Nanoscale, 2024, 16, 5836–5844 This journal is © The Royal Society of Chemistry 2024Open Access Article. Published on 26 February 2024. Downloaded on 6/10/2024 10:37:27 AM.  This article is licensed under a Creative Commons Attribution-NonCommercial 3.0 Unported Licence.View Article Onlinehttp://creativecommons.org/licenses/by-nc/3.0/http://creativecommons.org/licenses/by-nc/3.0/https://doi.org/10.1039/d3nr06641jabove 100 K. Fig. 3c shows the temperature-dependent Aexciton PL intensities of the two samples. As the temperaturedecreases, the PL intensity of both samples increases, thendecreases around 100 and 120 K for as-stacked and annealed1L-WSe2, respectively. This is in contrast to the previous reportthat the PL intensity continuously is reduced as the tempera-ture decreases due to the dominant population of dark exci-tons of WSe2, which is shown in Fig. 3c.21 The high qualityWSe2 synthesized by a flux method in ref. 21 shows the similartrend in the temperature-dependent measurement of PLbecause of the interplay between defect- and phonon-scatter-ing and the dark exciton state. It is noteworthy that, eventhough hBN-encapsulation enhances the PL of WSe2, addition-ally annealed WSe2 shows higher PL intensity in the tempera-ture range above 100 K. Fig. 3d shows the PL spectra of as-stacked and annealed 1L-WSe2 measured at 10 K. The individ-ual PL peaks correspond to the A exciton (P1), biexciton (P2),intervalley trion (P3), intravalley trion (P4), negatively chargedbiexciton (P5), and localized excitons (P6–P10), respectively asindicated in Fig. 3d.22–24 At 10 K, spin-forbidden dark excitonsare prevalent and A exciton PL peaks of both samples show nosignificant difference. However, after encapsulation annealing,the negatively charged biexciton (P5) is suppressed, while thebiexciton (P2) is enhanced. This phenomenon can also beattributed to a dedoping effect as a higher concentration ofbiexcitons is expected in neutralized WSe2 compared to nega-tively charged biexcitons. In the case of hBN-encapsulationannealed 1L-MoS2 with bright excitons (Fig. S6†),25 the PLintensity increases with decreasing temperature and A excitonPL peak becomes much higher after encapsulation annealingdue to the dedoping effect.Next, we measured the PL QY of as-stacked and annealed1L-WSe2 in Fig. 3e. The annealed 1L-WSe2 exhibited higher PLQY of 0.18% at a generation rate of 1017 cm−2 s−1 by 20 timesthan that of the as-stacked 1L-WSe2 (0.009%). The significantimprovement in PL QY can be ascribed to the prevalent for-mation of neutral excitons, which predominantly follow aradiative recombination path. We also verified that the PL QYof annealed 1L-MoS2 increases by a factor of 5 (Fig. S7a†). Toanalyze the exciton recombination dynamics of annealedWSe2, time-resolved photoluminescence (TRPL) measurementswere performed in Fig. 3f. The PL decay curves were fitted witha single exponential decay function, IðtÞ ¼ Ð t�1 IRFðt′Þe�t�t′τ dt,where IRF is a Gaussian instrument response function.26 Theannealed WSe2 exhibits a relatively prolonged lifetime of 1.3ns, surpassing that of the as-stacked one by an order of magni-Fig. 4 Electrical properties of hBN/1L-WS2/hBN (WS2 FET) device before and after annealing. (a) Schematic illustration and (b) optical microscopicimage of the fabricated WS2 device. Ids–Vg characteristics for WS2 FET (c) before and (d) after annealing. Ids–Vds characteristics for WS2 FET (e)before and (f ) after annealing as a function of gate voltage (Vg) ranging from 0 to 12 V for n-type transport and 0 to −8 V for p-type transport (inset).Nanoscale PaperThis journal is © The Royal Society of Chemistry 2024 Nanoscale, 2024, 16, 5836–5844 | 5839Open Access Article. Published on 26 February 2024. Downloaded on 6/10/2024 10:37:27 AM.  This article is licensed under a Creative Commons Attribution-NonCommercial 3.0 Unported Licence.View Article Onlinehttp://creativecommons.org/licenses/by-nc/3.0/http://creativecommons.org/licenses/by-nc/3.0/https://doi.org/10.1039/d3nr06641jtude. The recombination lifetime (τob) is determined by combi-nation of the radiative (τr) and nonradiative (τnr) lifetimes asper the following equation:1τob¼ 1τrþ 1τnr:Because the radiative lifetime is longer than nonradiativelifetime by an order of magnitude, the prolonged lifetime ofthe annealed WSe2 indicates that the radiative recombinationof neutral excitons is dominant and the formation of trionswith nonradiative recombination is suppressed due to thededoping effect.26,27 The annealed 1L-MoS2 also showed anincreased lifetime as shown in Fig. S7b.†The effects of encapsulation annealing on the electrical pro-perties of monolayer TMDs are investigated in Fig. 4. We fabri-cated an hBN-encapsulated 1L-WS2 device with graphene elec-trodes as shown in the schematic of Fig. 4a. Fig. 4b shows anoptical microscopic image of a representative 1L-WS2 device.As shown in transport curves of Fig. 4c and d, the 1L-WS2device exhibited enhanced p-type transport after encapsulationannealing. This result also supports that the WS2 channel isdedoped as confirmed by the Raman spectroscopic measure-ments and PL analyses in Fig. S3† and Fig. 3. Despite dedop-ing induced by encapsulation annealing, the field effect mobi-lity for electrons increased from 4.94 to 5.14 cm2 V−1 s−1. Then-type transport properties of the 1L-WS2 remained consistent,whereas the p-type transport exhibited enhancement afterencapsulation annealing, as shown in the output curves ofFig. 4e and f. These findings indicate that the dedopinginduced by encapsulation annealing results in alterations inthe electrical properties, while still preserving high electricalperformance.To probe the structural changes of 1L-WSe2 during encap-sulation annealing, we employed scanning transmission elec-tron microscopy (STEM) in Fig. 5a–c. For the STEM character-ization, 1L-graphene was used as an encapsulation layer as thegraphene has a small background signal (see Methods fordetails of the TEM sample preparation).28–30 The high-angleannular dark field (HAADF) STEM images of Fig. 5a and bshow the atomic structures of as-stacked and annealed1L-WSe2. We calculated the defect densities of two samplesusing a deep learning process with 1000 simulated STEMimages (1024 × 1024 pixels) as we previously reported.31 Thedefect density increased from 0.33% to 1.38% (see the ESI† fordetails of the defect density calculation). This is contrary toprevious reports that defect density and PL intensity have aninverse correlation.32 In order to differentiate the types ofdefects in as-stacked and annealed WSe2, we plot the defectFig. 5 Structural analysis of hBN-encapsulated WSe2 before and after annealing. HAADF-STEM image of graphene-encapsulated 1L-WSe2 (a)before and (b) after annealing. (c) Spectra of number of defects per total atoms as a function of intensity of defect sites in 1L-WSe2 before (blue) andafter annealing (red). Inset: Normalized Spectra. XPS spectra of (d) W 4f, (e) Se 3d core levels of hBN-encapsulated 2L-WSe2 before and afterannealing.Paper Nanoscale5840 | Nanoscale, 2024, 16, 5836–5844 This journal is © The Royal Society of Chemistry 2024Open Access Article. Published on 26 February 2024. Downloaded on 6/10/2024 10:37:27 AM.  This article is licensed under a Creative Commons Attribution-NonCommercial 3.0 Unported Licence.View Article Onlinehttp://creativecommons.org/licenses/by-nc/3.0/http://creativecommons.org/licenses/by-nc/3.0/https://doi.org/10.1039/d3nr06641jdensity, i.e. counts, as a function of contrast intensity of STEMimages as shown in Fig. 5c. It is clear that the encapsulationannealed WSe2 has more defects with relatively high contrastintensity as shown in the inset of Fig. 5c. The vacant sites indefects with higher contrast intensity might be substituted byimpurities, such as oxygen, because it has been reported thatchalcogen vacancies are substituted by oxygen during posttreatments, such as thermal annealing and laser exposure,leading to reduction of vacancy-induced in-gap states belowthe conduction band.27,33–40To examine the compositions of as-stacked and annealedWSe2, X-ray photoelectron spectroscopy (XPS) was used. Notethat we used 2L-WSe2 for XPS measurements as the monolayergenerates low signals. Fig. 5d and e show the XPS spectra of W4f and Se 3d core levels before and after encapsulation anneal-ing, respectively. The three distinct features of the W 4f7/2, W4f5/2, and W 5p3/2 are observed at 32.3, 34.4, and 37.6 eV,respectively. The doublets observed at 54.6 and 55.5 eV corres-pond to the Se 3d5/2 and Se 3d3/2.41 After encapsulation anneal-ing, the binding energies of W 4f and Se 3d core-levels blue-shifted, possibly indicating improved crystallinity. Based onour observed results, the origin of the dedoping effect inducedby encapsulation annealing can be ascribed to the substitutionof vacancies with oxygen and the enhanced crystallinity indefect-free areas.ConclusionsIn conclusion, we demonstrate a hBN-encapsulation annealingmethod to achieve the optical grade of monolayer TMDs. Afterannealing at 1000 °C for 1 h, we observed enhancement of PLintensity, QY, and lifetime in hBN-encapsulated 1L-TMD. ThehBN-encapsulated 1L-WS2 exhibited a transition in the trans-port characteristics from n-type to ambipolar behavior, whichcan be attributed to the dedoping of WS2 after encapsulationannealing. STEM measurements indicated an increase ofdefect density after encapsulation annealing. Our observationthat the n-type TMD undergoes dedoping despite increaseddefect density is an exceptional case. Our deep learning ana-lysis implies that the majority of the newly introduced defectsmight be impurities rather than selenium vacancies. Our workshows a novel way to transform monolayer TMDs into opticalgrade materials by the hBN-encapsulation annealing and hasgreat potential for various optoelectronic applications.MethodsChemical vapor deposition growth1L-MoS2 crystals were grown on silicon dioxide/silicon (SiO2/Si) substrates with SiO2 thickness of 285 nm by the CVDmethod within a 2-inch quartz tube, conducted under atmos-pheric pressure. To synthesize 1L-MoS2, a quartz boat contain-ing MoO3 powder (99.97%, Sigma-Aldrich) was positioned inthe center of a furnace, and a SiO2/Si substrate was suspendedface-down on top of the boat. Another quartz boat loaded withsulfur powder (Sigma-Aldrich) was located upstream withinthe quartz tube, approximately 21.5 centimeters away from thecenter of the furnace. The furnace temperature was graduallyincreased to 750 °C at a rate of 50 °C per minute and main-tained for 15 minutes, and then the furnace was naturallycooled to room temperature. Ar (300 sccm) was flowed for theentire growth process.Sample preparation2D flakes utilized in the fabrication process were mechanicallyexfoliated onto SiO2/Si substrates. The thickness of eachmaterial was individually verified using a combination ofAtomic Force Microscopy (AFM) and optical contrast. Toencapsulate monolayer TMD between hBN, the pick-up trans-fer technique was used with a polypropylene carbonate(bisphenol A carbonate, Sigma Aldrich) (PC)-coated poly(dimethyl siloxane) (PDMS) lens mounted on a slide glass topick-up and release.15 The PDMS/PC/slide glass assembly wasmanipulated using a 3-axis micromanipulator to control theposition of the contact area. By solely regulating the stagetemperature within the range of 80–130 °C, the top hBN waspicked up completely by the PC, ensuring there were no cracksor folds. Once the hBN had been separated from the sub-strates, monolayer TMD and the bottom hBN were picked-upby van der Waals forces from the top hBN to form hBN/1L-TMD/hBN ultrathin 2D heterostructures. It’s important tonote that monolayer TMD had to be fully encapsulated by hBNwithout any bubbles; otherwise, the monolayer TMD woulddegrade during the annealing process. After stacking, theheterostructure was transferred onto a clean SiO2/Si substrateby releasing the PC film from the PDMS lens at a temperatureexceeding 180 °C. Finally, to remove the PC film, the sampleswere immersed in chloroform for 30 minutes. Similar to hBN-encapsulated monolayer TMDs, graphene-encapsulated mono-layer TMDs for STEM imaging were also prepared using thedry-transfer method.29,30 To prevent degradation of hetero-structures during annealing, bi- or trilayer graphene was used.Nonetheless, removing graphene from the SiO2/Si substrateproves challenging due to the stronger adhesion of Gr/SiOx incomparison to hBN/SiOx.42 Therefore, hBN was employed asthe upper layer to facilitate the easy detachment of the gra-phene. Thus, hBN/Gr/TMDs/Gr heterostructures were fabri-cated initially. Next, XeF2 gas, which can selectively etch onlyhBN remaining on the graphene, was treated on the hetero-structures to remove the top hBN.28 For the encapsulationannealing of monolayer TMD, the sandwiched 1L-TMD wasannealed in a vacuum of 10−4 Torr. The furnace temperaturewas ramped up to the target annealing temperature for 3 hand maintained for 1 h. Then, the furnace was naturallycooled to room temperature.Raman and PL spectroscopyRoom-temperature Raman and PL intensity map images andspectra were obtained using a Raman spectroscopy instrumentequipped with a 532 nm laser and a spot size of ∼1 μmNanoscale PaperThis journal is © The Royal Society of Chemistry 2024 Nanoscale, 2024, 16, 5836–5844 | 5841Open Access Article. Published on 26 February 2024. Downloaded on 6/10/2024 10:37:27 AM.  This article is licensed under a Creative Commons Attribution-NonCommercial 3.0 Unported Licence.View Article Onlinehttp://creativecommons.org/licenses/by-nc/3.0/http://creativecommons.org/licenses/by-nc/3.0/https://doi.org/10.1039/d3nr06641j(JASCO). To minimize any potential damage to the samplecaused by laser exposure, a power of <5 mW was used for anacquisition time of 60 s. Given that the laser spot size wasapproximately 1 μm, the mapping area was scanned with apoint-to-point distance of 1 μm. Low-temperature micro-photo-luminescence (micro-PL) measurements were carried outusing a diode-pumped solid-state laser with wavelength of532 nm (2.33 eV) and power of ∼5 μW. We used a 40× objectivelens (numerical aperture 0.6) to focus the laser on the sampleswith a spot of diameter ∼1 μm and to collect PL signals fromsamples. A substrate with hBN-encapsulated 1L-TMD sampleswas loaded into an optical cryostat (Oxford MicrostatHe2) andcooled to 10 K with liquid helium. PL signals emitted by thesamples were spectrally resolved using a Horiba TRIAX 320spectrometer (300 grooves per mm) and were detected with athermoelectrically cooled charge-coupled device (CCD)camera. Raman and PL spectra were obtained at the sameposition before and after annealing.Atomic force microscopyAFM (Park Systems, NX10) in the contact mode was used toobserve the surface morphology, and thickness of hBN-encap-sulated 1L-WSe2.Quantum yield measurementTo estimate the QY of 1L-WSe2 and MoS2, we encapsulated amechanically exfoliated 1L-WSe2 flake and CVD-grown1L-MoS2 with hBN and transferred it onto a SiO2 substrate. Asa reference sample, we utilized a thin poly(methyl methacry-late) (PMMA) film that contained dispersed rhodamine 6G(R6G).43–45 We acquired photoluminescence (PL) spectra froma 300 nm-thick PMMA film with a concentration of 10−4 M ofR6G and from a bare quartz substrate. These spectra weremeasured utilizing an integrating sphere under a 514 nm laserexcitation. We estimated the QY of the reference sample fromthe emission and absorption of the R6G. Next, we conductedphotoluminescence (PL) intensity and absorption measure-ments on the hBN-encapsulated 1L-WSe2 and MoS2 using aconfocal microscope under identical measurement conditions.By comparing the PL intensity and absorption of 1L-WSe2 andMoS2 with those of the reference sample, we were able to esti-mate the quantum yield (QY) of 1L-WSe2 and MoS2.Time resolved-PL measurementFor TRPL measurements, we employed the same microscope,which was equipped with a pulsed excitation at 488 nm(BDL-488, Becker & Hickl GmbH), with a pulse width of 70 psand a repetition rate of 80 MHz, along with a high-speedhybrid detector (HPM-100-40, Becker & Hickl GmbH) and atime-correlated single-photon counting module (TCSPC,Becker & Hickl GmbH). All measurements were conducted atroom temperature.Device fabrication and electrical measurementElectron beam lithography was utilized to develop metal padpatterns and establish via contacts for graphene electrodesencapsulated by hBN. The van der Waals heterostructure wasetched by exposing the pre-patterned structure to XeF2.28 Thetop hBN was etched away, and the embedded graphene electro-des stopped the etching process. Metals of Cr/Pd/Au (1 nm/30 nm/40 nm) were then deposited on the exposed grapheneelectrodes using an e-beam evaporator. Subsequently, a lift-offprocess was carried out by immersing the samples in acetone.Electrical measurements of the devices were then conducted atroom temperature under ambient conditions, employing aparameter analyzer, specifically the Keithley 2400.Transmission electron microscopyTransmission electron microscopy samples were preparedusing a poly(methyl methacrylate)-based wet-transfer method.Samples on poly(methyl methacrylate) film were transferred onSi3N4 nanofilm TEM grids (TEM Windows, SN100-A10Q33B).The PMMA film was removed by placing samples in acetonefor 12 h. A double-Cs-aberration-corrected JEOL ARM-200F wasused for HAADF-STEM images at an operating voltage of 80 kVwith a 23 mrad convergence angle and collection semi anglesfrom 68 to 280 mrad.Model building and trainingWe generated a ResUNet deep learning model with 5 residualblocks and gradual increased filters from 32 to 128 in the con-volutional layers to evaluate the number and type of defects inWSe2. We used 1000 simulated STEM images (1024 × 1024pixels) of WSe2 as training and validation data. The loss func-tion (categorical cross entropy) decreased under 0.03.31 Pixelswith defects were extracted by the model, and we averaged thepixel’s intensity out to compare annealed and as-stackedsample. We confirmed that the local intensity of the annealedSe vacancies was higher than that of untreated Se vacancies,which may represent oxygen adsorbed on Se vacancy sitesduring annealing.X-ray photoelectron spectroscopyXPS samples were prepared with a top hBN layer approximately3 nm thick to ensure a distinct signal. The XPS spectra of Wand Se are obtained with Al Kα radiation using a 10 μm beamspot size (Versaprobe III).Author contributionsH. R. and G.-H. L. designed and conceived theproject. K. W. K. and H. C. performed PL measurements at lowtemperature. Y. J. synthesized TMDs by CVD. Y. L. and K. P. K.performed HAADF-STEM imaging. K. L. and K. P. K. per-formed STEM image analysis based on deep learning. Y. K.and J. K. performed QY and TRPL measurements. H. K. per-formed XPS measurements. K. W. and T. T. supplied boronnitride crystals. H. R., S. C. H., and G.-H. L. collectively ana-lyzed the data and wrote the paper. All authors read and con-tributed to the manuscript.Paper Nanoscale5842 | Nanoscale, 2024, 16, 5836–5844 This journal is © The Royal Society of Chemistry 2024Open Access Article. Published on 26 February 2024. Downloaded on 6/10/2024 10:37:27 AM.  This article is licensed under a Creative Commons Attribution-NonCommercial 3.0 Unported Licence.View Article Onlinehttp://creativecommons.org/licenses/by-nc/3.0/http://creativecommons.org/licenses/by-nc/3.0/https://doi.org/10.1039/d3nr06641jConflicts of interestThere are no conflicts to declare.AcknowledgementsThis research was supported by the National ResearchFoundation of Korea (NRF) funded by the Ministry of Scienceand ICT (NRF-2021R1A2C301431613, NRF-2019R1A2C3006189,and NRF-2017R1A5A1014862 (SRC program: vdWMRC center))and the Creative-Pioneering Researchers Program throughSeoul National University (SNU). K. W. and T. T. acknowledgesupport from the JSPS KAKENHI (Grant Numbers 21H05233and 23H02052) and World Premier International ResearchCenter Initiative (WPI), MEXT, Japan. G.-H. L. acknowledgesthe support from the Research Institute of Advanced Materials(RIAM), Institute of Engineering Research (IER), Institute ofApplied Physics (IAP), and Inter-University SemiconductorResearch Center (ISRC) at the Seoul National University.References1 A. Raja, A. Chaves, J. Yu, G. Arefe, H. M. Hill, A. F. Rigosi,T. C. Berkelbach, P. Nagler, C. Schuller, T. Korn,C. Nuckolls, J. Hone, L. E. Brus, T. F. Heinz,D. R. Reichman and A. Chernikov, Nat. Commun., 2017, 8,15251.2 Q. H. Wang, K. Kalantar-Zadeh, A. Kis, J. N. Coleman andM. S. Strano, Nat. Nanotechnol., 2012, 7, 699–712.3 X. Liu, T. Galfsky, Z. Sun, F. Xia, E.-c. Lin, Y.-H. Lee,S. Kéna-Cohen and V. M. Menon, Nat. Photonics, 2015, 9,30–34.4 D.-H. Lien, S. Z. Uddin, M. Yeh, M. Amani, H. Kim,J. W. Ager, E. Yablonovitch and A. Javey, Science, 2019, 364,468–471.5 E. 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