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Hibiki Naito, Yasuyuki Makino, Wenjin Zhang, Tomoya Ogawa, Takahiko Endo, Takumi Sannomiya, Masahiko Kaneda, Kazuki Hashimoto, Hong En Lim, Yusuke Nakanishi, [Kenji Watanabe](https://orcid.org/0000-0003-3701-8119), [Takashi Taniguchi](https://orcid.org/0000-0002-1467-3105), Kazunari Matsuda, Yasumitsu Miyata

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[High-throughput dry transfer and excitonic properties of twisted bilayers based on CVD-grown transition metal dichalcogenides](https://mdr.nims.go.jp/datasets/08778f89-37fc-4755-bc1d-e51967ce3e32)

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High-throughput dry transfer and excitonic properties of twisted bilayers based on CVD-grown transition metal dichalcogenidesNanoscaleAdvancesPAPEROpen Access Article. Published on 04 September 2023. Downloaded on 10/22/2023 3:07:02 AM.  This article is licensed under a Creative Commons Attribution 3.0 Unported Licence.View Article OnlineView Journal  | View IssueHigh-throughpuaDepartment of Physics, Tokyo MetropolitaE-mail: ymiyata@tmu.ac.jp; wjzhang@tmu.bDepartment of Materials Science and EngYokohama 226-8503, JapancDepartment of Chemistry, Saitama UniversdResearch Center for Electronic and OpticJapaneResearch Center for Materials NanoarchitecfInstitute of Advanced Energy, Kyoto Univers† Electronic supplementary informahttps://doi.org/10.1039/d3na00371j‡ These authors have contributed equallyCite this:Nanoscale Adv., 2023, 5, 5115Received 30th May 2023Accepted 21st August 2023DOI: 10.1039/d3na00371jrsc.li/nanoscale-advances© 2023 The Author(s). Published byt dry transfer and excitonicproperties of twisted bilayers based on CVD-growntransition metal dichalcogenides†Hibiki Naito,‡a Yasuyuki Makino,‡a Wenjin Zhang, ‡*a Tomoya Ogawa,aTakahiko Endo,a Takumi Sannomiya,b Masahiko Kaneda,a Kazuki Hashimoto,aHong En Lim, c Yusuke Nakanishi, a Kenji Watanabe, d Takashi Taniguchi,eKazunari Matsudaf and Yasumitsu Miyata *avan der Waals (vdW) layered materials have attracted much attention because their physical properties can becontrolled by varying the twist angle and layer composition. However, such twisted vdW assemblies are oftenprepared using mechanically exfoliated monolayer flakes with unintended shapes through a time-consumingsearch for such materials. Here, we report the rapid and dry fabrication of twisted multilayers using chemicalvapor deposition (CVD) grown transition metal chalcogenide (TMDC) monolayers. By improving the adhesionof an acrylic resin stamp to the monolayers, the single crystals of various TMDCmonolayers with desired grainsize and density on a SiO2/Si substrate can be efficiently picked up. The present dry transfer processdemonstrates the one-step fabrication of more than 100 twisted bilayers and the sequential stacking ofa twisted 10-layer MoS2 single crystal. Furthermore, we also fabricated hBN-encapsulated TMDCmonolayers and various twisted bilayers including MoSe2/MoS2, MoSe2/WSe2, and MoSe2/WS2. Theinterlayer interaction and quality of dry-transferred, CVD-grown TMDCs were characterized by usingphotoluminescence (PL), cathodoluminescence (CL) spectroscopy, and cross-sectional electronmicroscopy. The prominent PL peaks of interlayer excitons can be observed for MoSe2/MoS2 and MoSe2/WSe2 with small twist angles at room temperature. We also found that the optical spectra were locallymodulated due to nanosized bubbles, which are formed by the presence of interface carbon impurities.The present findings indicate the widely applicable potential of the present method and enable an efficientsearch of the emergent optical and electrical properties of TMDC-based vdW heterostructures.Introductionvan der Waals (vdW) heterostructures of two-dimensional (2D)materials have recently been studied intensively because oftheir emergent physical properties and potentialapplications.1–7 In particular, stacked 2D materials with mis-aligned crystal orientation generate a long-range periodicpotential as visualized by a moiré pattern. Because this moirépattern depends on the twist angle between two layers as well asn University, Hachioji 192-0397, Japan.ac.jpineering, Tokyo Institute of Technology,ity, Saitama 338-8570, Japanal Materials, NIMS, Tsukuba 305-0044,tonics, NIMS, Tsukuba 305-0044, Japanity, Kyoto, 611-0011, Japantion (ESI) available. See DOI:to this work.the Royal Society of Chemistrythe constituent materials, many studies have focused on thetwist-angle-dependent properties of various 2D materials, suchas graphene, hexagonal boron nitride (hBN), and transitionmetal dichalcogenides (TMDCs). Such twisted vdW systemsshow various properties including superconductivity,8 ferro-electricity,9,10 and moiré-related excitonic states.11–14 It is note-worthy that this progress has been driven by the development ofsophisticated transfer techniques for 2D materials.Many studies have reported ways of improving the transfertechnique of 2D materials with controlled twist angles andvarious components. The transfer was oen conducted by usingsolution assisted or wet etching processes of substrates.15–17 Incontrast, dry transfer techniques were also developed to preparevdW heterostructures with a clean 2D–2D interface.18–21 Eventhough much progress has been made in the dry transfertechnique, the search for thin and sufficiently large akes of 2Dmaterials is still very time-consuming. This is because akes of2D materials are usually exfoliated from a bulk layered crystalonto a substrate as a mixture of various layer numbers andshapes. To overcome the limitations posed by this method,current research employs two major approaches. One approachNanoscale Adv., 2023, 5, 5115–5121 | 5115http://crossmark.crossref.org/dialog/?doi=10.1039/d3na00371j&domain=pdf&date_stamp=2023-09-09http://orcid.org/0000-0002-3803-4770http://orcid.org/0000-0003-0347-8897http://orcid.org/0000-0001-8782-9556http://orcid.org/0000-0003-3701-8119http://orcid.org/0000-0002-9733-5119https://doi.org/10.1039/d3na00371jhttp://creativecommons.org/licenses/by/3.0/http://creativecommons.org/licenses/by/3.0/https://doi.org/10.1039/d3na00371jhttps://pubs.rsc.org/en/journals/journal/NAhttps://pubs.rsc.org/en/journals/journal/NA?issueid=NA005018Fig. 1 Twisted multilayers fabricated from CVD-grown TMDCmonolayers. (a) Schematic of CVD growth of monolayer TMDC singlecrystals and assemblies into vdW twistedmultilayers. Optical images ofCVD-grown (b) WS2 and (c) WSe2 monolayer grains, and (d) stackedbilayers of WS2 and WSe2. The scale bars are 50 mm. (e) Optical imagesof 10-layer twisted MoS2 in each fabrication step. The scale bars are 10mm. Arrows indicate newly stacked monolayer MoS2 single crystals ineach fabrication step.Nanoscale Advances PaperOpen Access Article. Published on 04 September 2023. Downloaded on 10/22/2023 3:07:02 AM.  This article is licensed under a Creative Commons Attribution 3.0 Unported Licence.View Article Onlineis the use of a robotic search system, which can dramaticallyreduce the time needed for a person to explore monolayers withsufficient ake size.22 The other is the preparation of 2D mate-rials by direct vapor-phase growth rather than by mechanicalexfoliation.23–27 In particular, recent advances in chemical vapordeposition (CVD) growth enable the preparation of large-areapolycrystalline lms and single crystals of TMDC monolayerson a wafer-scale substrate.28–31 In addition, triangular singlecrystals of TMDCs enable easy determination of the crystalorientation, allowing easy control of twist angles. Indeed, theuse of such CVD-grown polycrystalline TMDC lms and singlecrystals has recently enabled fast, continuous fabrication ofvdW heterostructures.23–27 For example, Mannix et al. demon-strated the fabrication of solid materials consisting of 80 layersof MoS2 and twist-controlled 4-layer WS2, respectively, by usinga robotic assembly with a multi-component polymer stamp.26Despite these great advances, there have been only a few reportson dry transfer and its effect on the optical properties of CVD-grown TMDCs.26,32 This is presumably due to the technicaldifficulties encountered in the growth and dry transfer of CVD-grown TMDC single crystals. One of the major technical diffi-culties is the efficient peeling of CVD-grown monolayers fromgrowth substrates. In general, CVD-grown monolayers areknown to adhere strongly to growth substrates compared toexfoliated akes. In addition, the adhesion of polymers used forthe dry transfer is highly dependent on the process tempera-tures, which are not optimized for most polymers. Comparedwith the solution or chemical assisted transfer of CVD grownTMDCs,33–36 the dry transfer should keep the intrinsic propertiesof as grown TMDCs because the solution processes inducephysical or chemical property modulations of TMDCs.37–39 Toexpand the versatility of this growth-assisted approach, it ishighly desirable to demonstrate the dry transfer and investigateits effects on physical properties.In this study, we report the rapid and dry fabrication oftwisted multilayers of CVD-grown TMDCs using a simple acrylicresin stamp. We introduced the melting and solidicationprocess of an acrylic resin stamp in contact with the sample toimprove the adhesion of the stamp. This improvement allowsus to efficiently pick up the single crystals of various TMDCmonolayers with desired grain size and density from the SiO2surface, and to perform high-throughput and continuous drytransfer. The present process demonstrates the one-step fabri-cation of more than 100 twisted bilayers, and the creation of 10layers of MoS2 single crystals with different crystal orientations.Furthermore, the hBN encapsulated monolayers and hetero-bilayers (MoSe2/MoS2, MoSe2/WSe2, and MoSe2/WS2) werecreated and their twist angle dependent properties were char-acterized by photoluminescence (PL). We have also investigatedthe local optical properties of monolayer MoSe2 by cath-odoluminescence (CL) spectroscopy. The bubbles wereobserved by cross sectional analysis.Results and discussionFirst, we demonstrate a high-throughput and continuous drytransfer method for creating vdW heterostructures based on5116 | Nanoscale Adv., 2023, 5, 5115–5121a large number of monolayer TMDC single crystals. A majoradvantage of the CVD process is the controllability of the sizeand density of single-crystal grains. The small size of the grainsensures the simultaneous transfer of a large number ofsamples, as shown in Fig. 1a. Fig. 1b and c show the opticalimages of a large-area WS2 single crystal with a size of 170 mm(Fig. 1b) and high-density, small-area WSe2 single crystals witha size of around 10 mm (Fig. 1c). Regarding the edge structure,our previous studies have revealed that metal-terminated zigzagedges are formed along the triangular grain edges under thepresent growth conditions.40,41 It should be noted that the exactedge congurations including the edge reconstruction and thestep edge cannot be identied simply from the grain shapealone.42–44 These individual crystals are oriented in randomdirections owing to the non-epitaxial growth on the amorphousSiO2 surface. First, the large WS2 crystal was lied with a stamp.This large WS2 crystal was then used to li smaller WSe2 crys-tals. Finally, WSe2/WS2 bilayers were transferred onto a newSiO2/Si substrate. This single stacking process yields more than100 twisted bilayers of WS2 and WSe2 with different crystalorientations (Fig. 1d). The process was completed in about 1–2hours. The present dry transfer process enables the continuousstacking of a twisted 10-layer MoS2 single crystal (Fig. 1e). Thefabrication of such a large number of vdW heterostructurescould be useful for exploring the physical properties dependingon the twist angle and layer composition.The present dry transfer has been conducted by using anacrylic resin stamp prepared on glass slides, as reported previ-ously.22 The stamp was rst contacted with the TMDC sampleson a hotplate with a motorized xyz stage (Fig. 2a). The sample© 2023 The Author(s). Published by the Royal Society of Chemistryhttp://creativecommons.org/licenses/by/3.0/http://creativecommons.org/licenses/by/3.0/https://doi.org/10.1039/d3na00371jFig. 2 Dry transfer process with the acrylic resin stamp. (a) Schematicof the transfer system used in the present study. (b) Optical images ofMoS2 grains in contact with the stamp (top) at room temperature and(bottom) during heating. (c) Schematic of the transfer processincluding (i) the contact of the stamp with the sample, (ii) heating tomelt the stamp, (iii) cooling to solidify the stamp, and (iv) lifting thestamp to pick up TMDC monolayers. Optical images of CVD-growngrains of MoS2 (d) before and (e) after stamp lifting. The scale bars are200 mm. White dotted lines indicate the areas in contact with thestamp.Fig. 3 Evaluation of interlayer interaction for twisted bilayer MoS2. (a)Optical images of four representative twisted bilayers with differenttwist angles for CVD-grown MoS2. Scale bars are 5 mm. (b) Room-temperature PL spectra and (c) PL peak positions for direct and indirectoptical transitions for 16 twisted bilayer MoS2 with different twistangles. Black line shows the PL peak of the A exciton of MoS2, whereasthe red line indicates the PL peaks derived from an indirect gap.Paper Nanoscale AdvancesOpen Access Article. Published on 04 September 2023. Downloaded on 10/22/2023 3:07:02 AM.  This article is licensed under a Creative Commons Attribution 3.0 Unported Licence.View Article Onlineand stamp were then heated with a hotplate, which soenedand spread the acrylic resin around 100 °C (Fig. 2b). To li theCVD monolayers, the stamp and sample were heated at 160 °C,and then cooled to 50 °C in the present study. Aer cooling, thestamp was gradually peeled from the substrate (Fig. 2c).Notably, the introduction of melting and solidicationprocesses of the stamp improves the adhesion of the stamp tothe monolayer akes. This improvement enables us to pick upefficiently the single crystals of various TMDC monolayers fromthe SiO2 surface. Fig. 2d and e show the typical optical images ofmonolayer MoS2 grown on a SiO2/Si substrate before and aerthe contact and peeling of stamps, respectively. This process ledto the liing of almost all grains in contact with the stamp.Without cooling, the soened stamp tends to remain on thesubstrate, and liing the TMDCs from the substrates is difficult.The liing yield also depends on other factors such as growthsubstrates. For example, TMDCs grown on single crystalsapphire substrates can hardly be lied directly by the stampprobably due to the strong adhesion between TMDCs and singlecrystal sapphire. Further studies are required to understand theinteraction between TMDCs and growth substrates in thefuture. In the following, we will focus on the PL properties ofdry-transferred, CVD-grown TMDCs.The interlayer interactions of such vdW heterostructureswere then investigated by PL spectroscopy for twisted MoS2© 2023 The Author(s). Published by the Royal Society of Chemistrybilayers. Fig. 3a shows the representative optical images of fourtwisted MoS2 bilayers with different twist angles. The PL spectraof 16 different twisted MoS2 bilayers are shown in Fig. 3b. Thesesamples show two prominent PL peaks at 1.8 eV and 1.6 eV,which are due, respectively, to the direct transition associatedwith the free excitons (A exciton) and the indirect transition.45The A exciton is almost independent of the twist angle, whileindirect transitions show a low energy shi at twist angles of 0°and 60° (Fig. 3b and c). These trends are consistent withprevious studies and can be explained by the difference in theinterlayer distance.46 We further investigated the surface qualityof these transferred TMDC homobilayers using atomic forcemicroscopy (AFM) as shown in Fig. S1.† The height of the toptransferred layer (0.8–0.9 nm) is comparable with that of thebottom layer (0.8–0.9 nm), which indicates a well couplingbetween the two layers. These results indicate a sufficientinterlayer interaction in the present vdW heterostructures ob-tained by dry transfer of CVD-grown TMDC monolayers.To improve sample quality, TMDC heterobilayers wereencapsulated into hBN by using the present dry transferprocess. PL measurements reveal that the hBN encapsulationprocess releases inhomogeneous lattice strain for monolayerTMDCs grown on the SiO2 surface (Fig. S2†). Fig. 4a and b showthe optical and PL images of the hBN-encapsulated MoS2/MoSe2heterostructure on an SiO2/Si substrate, respectively. In Fig. 4b,bright PL can be observed from small triangles of monolayerMoSe2 around the larger grain of monolayer MoS2. Further-more, the dark small triangles within the monolayer MoS2single crystal correspond to MoS2/MoSe2 heterobilayers withvarious twist angles. The weak PL signal in the stacked area ismainly from the charge and energy transfer induced PLquenching of A excitons of MoS2 and MoSe2. Fig. 4c shows thePL spectra of these twisted heterobilayers measured at roomNanoscale Adv., 2023, 5, 5115–5121 | 5117http://creativecommons.org/licenses/by/3.0/http://creativecommons.org/licenses/by/3.0/https://doi.org/10.1039/d3na00371jFig. 4 PL properties of MoS2/MoSe2 heterobilayers with various twistangles. (a) Optical images of hBN encapsulated MoS2/MoSe2 hetero-bilayers. (b) PL intensity image (smaller white triangles are 1L MoSe2and larger triangle is 1L MoS2, and the dark triangles within MoS2 aretwisted area). Scale bars are 10 mm. (c) Room-temperature PL spectraof the twisted area with various twist angles. The dashed line indicatesthe trend of the interlayer exciton peak. (d) PL peak positions of theintralayer exciton from MoS2 and MoSe2, and the interlayer excitonfrom the MoSe2/MoS2 heterobilayer with different twist angles. (e)Fitting results of the PL spectra of the hBN encapsulated MoSe2/MoS2twisted bilayers at 3°, 28°, and 60° twist angles.Fig. 5 Cathodoluminescence (CL) analysis of hBN-encapsulatedmonolayer MoSe2. (a) STEM image of hBN-encapsulated monolayerMoSe2 suspended on a TEM grid. (b) CL map at 1.57 eV of the samearea as (a). Scale bars in (a) and (b) are 1 mm. (c) CL spectra recorded atthe positions indicated by P1, P2, P3, and P4 in (a). The inset showsa schematic of the CL measurement under electron beam excitation.Nanoscale Advances PaperOpen Access Article. Published on 04 September 2023. Downloaded on 10/22/2023 3:07:02 AM.  This article is licensed under a Creative Commons Attribution 3.0 Unported Licence.View Article Onlinetemperature. Here, clear peaks derived from interlayer excitonswere observed at 1.35 eV for the heterobilayers with smaller (orlarger) twist angles of 3, 5 and 60°. These peaks can be assignedto interlayer excitons as reported in a previous study.47 Incontrast, such peaks can be hardly detected for the otherintermediate twist angles. This indicates a crystal orientationinduced variation in interlayer coupling strength.48,49 The PLpeak energies are summarized in Fig. 4d. Furthermore, the peakshapes of A excitons at lower (or higher) twist angles becomeasymmetric and broadened compared with those at interme-diate twist angles. The spectral decomposition results show thatthere is new peak at lower (or higher) twist angles (Fig. 4e),which probably comes from the atomic reconstruction withinthe moiré unit cell.14 Other hBN encapsulated heterobilayers ofMoSe2/WSe2 and MoSe2/WS2 were also investigated (Fig. S3–S5†). Further investigation including low temperaturemeasurements will be required to fully understand the twistangle dependent excitonic properties of TMDC-basedheterobilayers.Finally, the quality and challenges of dry transfer samplesare discussed. In the present samples, the dot-like reduction ofPL intensities was frequently observed in the PL intensity map5118 | Nanoscale Adv., 2023, 5, 5115–5121(Fig. S6†). This corresponds to the positions of bubbles formedin the hBN encapsulated monolayer MoS2. The peak energiesshow slight variations within a few tens of meV at differentpositions. This is derived from the bandgap modulation due toinhomogeneous lattice strain, which was induced by contami-nation and folding introduced during the present dry transferprocess. To obtain local optical properties at higher spatialresolution, a cathodoluminescence (CL) experiment was carriedout using a scanning transmission microscope (STEM) equip-ped with a parabolic mirror and a spectrometer.50 In the presentsystem, an electron beam can be focused down to 1 nm scales.Fig. 5a shows the STEM darkeld (DF) image of hBN encapsu-lated MoSe2 on a SiN TEM grid. In the DF image, bubbles andcracks can be clearly identied within the MoSe2 grain. The CLmap of the same area at 1.57 eV (Fig. 5b) shows weaker signalswithin the bubbles or cracks compared to the at areas. Asshown in Fig. 5c, the CL spectra at different positions (P1–4)show the variation of the CL intensity and peak energy even forrelatively at regions such as P2, P3, and P4. This indicates thedifficulty of obtaining strain-free TMDC samples even aer thehBN encapsulation. In future, this issue needs to be addressedthrough improvements in the transfer and post-transferprocesses.It is noted that the bubble area around P1 shows a quench-ing of CL, suggesting energy and/or charge transfer to impuri-ties in the bubbles. The CL quenching may also result from thedifference in surrounding hBN layers due to the bubbleformation. To identify the impurities in bubbles formed in thepresent process, the cross-sectional structure was observed bySTEM. Fig. 6a shows the STEM image of hBN-encapsulatedmonolayer MoS2. This sample was prepared by dry transferunder vacuum (10−3 Pa) to reduce the contamination of waterand oxygen in air for elemental analysis. It is noted that the© 2023 The Author(s). Published by the Royal Society of Chemistryhttp://creativecommons.org/licenses/by/3.0/http://creativecommons.org/licenses/by/3.0/https://doi.org/10.1039/d3na00371jFig. 6 Cross-sectional structural analysis of hBN-encapsulated MoS2stacked in a vacuum. (a) Cross-sectional STEM image with bubbles. (b)EDS mapping of the selected area (dotted rectangle) in (a). (c) Lineelemental analysis along the red line in (a). Light red and green regionsare MoS2 and the bubble area, respectively.Paper Nanoscale AdvancesOpen Access Article. Published on 04 September 2023. Downloaded on 10/22/2023 3:07:02 AM.  This article is licensed under a Creative Commons Attribution 3.0 Unported Licence.View Article Onlinebubbles still can be seen as shown in Fig. 6a. Energy dispersivespectroscopy (EDS) mapping shows that carbon is the mainelement in the bubble area (Fig. 6b and c). In the image, carbonimpurities exist between monolayer MoS2 and bottom hBN.Considering the hBN exfoliation by adhesive tape, the origin ofthese carbon impurities is probably from organic materials onthe adhesive tape. These impurities may be removed by a post-exfoliation annealing process. Even though further efforts areneeded to overcome these disadvantages, it is noteworthy thatthe present ndings provide a clue to improve the samplequality of vdW heterostructures prepared by the dry transfer ofCVD-grown TMDCs.ConclusionsWe have investigated the dry transfer process that employssimple acrylic resin stamps and characterized the opticalproperties of transferred CVD-grown monolayer TMDCs. Thepresent study demonstrated a simple and rapid way of fabri-cating a variety of TMDC-based twisted multilayers. To improvethe dry transfer process, we introduced the melting and solid-ication process of the acrylic resin stamp. This improvementallows us to efficiently pick up the single crystals of CVD-grownTMDC monolayers from the SiO2 surface. It should be notedthat the present transfer is not a fully automated system,although motorized stages were used in our setup. A fullyautomated system is very powerful to search for monolayerakes in exfoliated samples, because searching for monolayerakes with the desired size is themost time-consuming process.Importantly, we can skip the search process by using CVD-grown TMDC samples, where the size and density of themonolayers can be controlled by the growth conditions and thecrystal orientation can be easily identied. This allows us toperform high throughput and continuous dry transfer. Becauseof the simplicity of the process and setup, we believe that thisapproach is not only scalable but also easy to use for manyresearchers. PL spectroscopy revealed a sufficient interlayerinteraction in the twisted MoS2 bilayers obtained by the presentdry transfer. Furthermore, the good interlayer coupling was alsosupported by the observation of room temperature interlayer© 2023 The Author(s). Published by the Royal Society of Chemistryexcitons and their twist angle sensitive properties for hBNencapsulated heterobilayers with various twist angles. The localbandgap modulation and quenching were investigated by CLimaging and spectroscopy. Importantly, nanoscale bandgapvariation was observed even for at regions in the hBN-encapsulated MoS2 monolayer. Cross-sectional STEM analysissuggests that the PL/CL quenching is derived from the carboncontamination in bubbles. These ndings provide a basis forfabricating high-quality vdW heterostructures. This is alsoimportant for investigating the emergent electrical and opticalproperties of these vdW heterostructures, and for future deviceapplications in electronics and optoelectronics.Experimental methodSample preparationTMDC monolayers, including MoS2, WS2, WSe2, and MoSe2,were grown on SiO2/Si (SiO2 thickness: 285 nm) substrates byCVD, as reported previously.40,51,52 For large-area WS2 growth,the substrate was placed in a quartz tube with WO3 powder (300mg) and sulfur akes (2 g). Aer lling the quartz tube with N2gas at atmospheric pressure (and a constant ow rate of 600sccm), the temperature of the WO3 powder was gradually raisedto 1050 °C using an electric furnace, to supply W precursors tothe downstream substrate. Once the target temperature wasreached, the sulfur was heated at 180 °C for 10 min witha second electric furnace. Then, the entire system was imme-diately cooled using an electric fan. The same growth condi-tions were employed for MoS2, MoSe2, and WSe2. MoS2 wasgrown with MoO2 (100–250 mg) at 850–1000 °C under N2 gas(200–300 sccm) for 5–10 min. The growth temperatures werechanged to obtain single crystals of different sizes and densi-ties. WSe2 was grown with Se beads (2 g) instead of sulfur akes,and the Se was heated at 385 °C for 2 min under H2 (3%)/N2 gasat a ow rate of 300 sccm. Similarly, MoSe2 was grown withMoO2 powder (100mg) and Se beads (2 g) at 820 °C with amixedgas of 400 sccm N2 and 1.2 sccm H2, and the Se was heated at420 °C for 2 min. Salt-assisted CVD was also employed toprepare monolayer MoS2 and WSe2 single crystals for hBNencapsulation.53,54 Thin akes of MoS2 and hBN were preparedon SiO2/Si substrates by mechanical exfoliation of bulk MoS2(SPI supplies) and bulk hBN,55 respectively.Transfer processThe transfer of TMDC samples was performed through thepolymer-assisted liing and peeling process using acrylic resinstamps of the type reported in a previous study.22 First, 1.8 mg ofacrylic resin (Elvacite 2552C, Mitsubishi Chemical America) wasdissolved in anisole (1.4 mL, Tokyo Chemical Industry Co., Ltd).A droplet of the solution was deposited on a glass slide and thendried on a hotplate at 185 °C for 30 min. This generates a dome-shaped stamp of acrylic resin with a size of 1 mm. The glassslide and the SiO2/Si substrate were xed in a lab-made transfersystem with xyz stages, a hot plate, and an optical microscope(Fig. 2a). To li the TMDC samples from the substrates, thestamp was brought into contact with the samples at roomNanoscale Adv., 2023, 5, 5115–5121 | 5119http://creativecommons.org/licenses/by/3.0/http://creativecommons.org/licenses/by/3.0/https://doi.org/10.1039/d3na00371jNanoscale Advances PaperOpen Access Article. Published on 04 September 2023. Downloaded on 10/22/2023 3:07:02 AM.  This article is licensed under a Creative Commons Attribution 3.0 Unported Licence.View Article Onlinetemperature. Each sample was then heated at 160 °C for 10 minon a hotplate. Aer the heating step, the hotplate was cooled to50 °C using an electric fan. The stamp was then gradually liedfrom the substrate with a motorized stage. To peel the samplefrom the stamp, the stamp was placed on a target substrate atroom temperature and melted at 185 °C. Finally, the acrylicresin on the substrate was washed away with chloroform. ForhBN encapsulation, the exfoliated akes of hBN were lied bythe same process, and used as a stamp to li the TMDCmonolayers. The liing of the TMDC monolayers is enabled bythe strong interlayer vdW interaction of the atomically at,clean surface of hBN. Finally, hBN/TMDC was placed onanother hBN ake for encapsulation.CharacterizationOptical images were recorded with an optical microscope(Nikon, ECLIPSE-LV100D). PL spectra were recorded by usinga micro-spectrometer (Renishaw, inVia) with an excitation laseroperating at 532 nm. Low-temperature PL was obtained byusing a lab-made optical setup with a cryostat under vacuumconditions (<10−4 Pa). A continuous-wave (cw) semiconductorlaser operating at 635 nm was used as the excitation source forthe PL measurements. The laser was focused using a 50×objective lens. The PL signals were collected using the sameobjective lens and detected with a cooled charge-coupled device(CCD) through a spectrometer. Cathodoluminescencemeasurement was carried using a modied STEM (JEM-2100F,JEOL, Japan) at an acceleration voltage of 80 kV. Analuminum parabolic mirror was used to collimate the CLemission from the sample. The signal was nally recordedusing a spectrometer with a CCD camera.Author contributionsH. N., Y. Ma., W. Z., M. K., K. H., T. O., and T. E. prepared TMDCsamples and performed the characterization. T. S. conductedCLmeasurement. K. W. and T. T. prepared bulk crystals of hBN.H. N. conducted low-temperature PL measurements with K. M.Y. Mi. developed the concept and supervised the project. H. N.,Y. Ma, W. Z., H. E. L., Y. N., and Y. Mi. prepared the gures andwrote the paper. All authors discussed the results and com-mented on the manuscript.Conflicts of interestThere are no conicts to declare.AcknowledgementsThis work was nancially supported by the Japan Science andTechnology Agency (JST) FOREST program (JPMJFR213X,JPMJFR213J), Kakenhi Grants-in-Aid (JP19K22142, JP20H00354,JP20H05664, JP21H05232, JP21H05233, JP21H05234,JP21H05235, JP22H00280, JP22H00283, JP22K18986,JP22H04957, JP22KJ2561, JP23K13635, JP23K04530 andJP23H02052) from the Japan Society for the Promotion of5120 | Nanoscale Adv., 2023, 5, 5115–5121Science (JSPS), ZE Research Program, IAE (ZE2023B-05) fromKyoto University, and the Murata Science Foundation.References1 Y. Liu, N. O. Weiss, X. Duan, H.-C. Cheng, Y. Huang andX. Duan, Nat. Rev. Mater., 2016, 1, 16042.2 J. C. W. Song and N. M. Gabor, Nat. Nanotechnol., 2018, 13,986–993.3 A. K. Geim and I. V. Grigorieva, Nature, 2013, 499, 419–425.4 Z. Hu, Q. Liu, S.-L. Chou and S.-X. Dou, Cell Rep. Phys. Sci.,2021, 2, 100286.5 H. Ago, S. Okada, Y. 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See DOI: https://doi.org/10.1039/d3na00371j High-throughput dry transfer and excitonic properties of twisted bilayers based on CVD-grown transition metal dichalcogenidesElectronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d3na00371j High-throughput dry transfer and excitonic properties of twisted bilayers based on CVD-grown transition metal dichalcogenidesElectronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d3na00371j High-throughput dry transfer and excitonic properties of twisted bilayers based on CVD-grown transition metal dichalcogenidesElectronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d3na00371j High-throughput dry transfer and excitonic properties of twisted bilayers based on CVD-grown transition metal dichalcogenidesElectronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d3na00371j High-throughput dry transfer and excitonic properties of twisted bilayers based on CVD-grown transition metal dichalcogenidesElectronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d3na00371j High-throughput dry transfer and excitonic properties of twisted bilayers based on CVD-grown transition metal dichalcogenidesElectronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d3na00371j High-throughput dry transfer and excitonic properties of twisted bilayers based on CVD-grown transition metal dichalcogenidesElectronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d3na00371j High-throughput dry transfer and excitonic properties of twisted bilayers based on CVD-grown transition metal dichalcogenidesElectronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d3na00371j High-throughput dry transfer and excitonic properties of twisted bilayers based on CVD-grown transition metal dichalcogenidesElectronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d3na00371j High-throughput dry transfer and excitonic properties of twisted bilayers based on CVD-grown transition metal dichalcogenidesElectronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d3na00371j