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Wenjin Zhang, Zheng Liu, Hiroshi Nakajo, Soma Aoki, Haonan Wang, Yanlin Wang, Yanlin Gao, Mina Maruyama, Takuto Kawakami, Yasuyuki Makino, Masahiko Kaneda, Tongmin Chen, Kohei Aso, Tomoya Ogawa, Takahiko Endo, Yusuke Nakanishi, [Kenji Watanabe](https://orcid.org/0000-0003-3701-8119), [Takashi Taniguchi](https://orcid.org/0000-0002-1467-3105), Yoshifumi Oshima, Yukiko Yamada‐Takamura, Mikito Koshino, Susumu Okada, Kazunari Matsuda, Toshiaki Kato, Yasumitsu Miyata

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[Chemically Tailored Semiconductor Moiré Superlattices of Janus Heterobilayers](https://mdr.nims.go.jp/datasets/97d6f943-9257-4eaa-be54-1b781ade8410)

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Chemically Tailored Semiconductor Moiré Superlattices of Janus HeterobilayersChemically Tailored Semiconductor Moiré Superlattices ofJanus HeterobilayersWenjin Zhang,* Zheng Liu, Hiroshi Nakajo, Soma Aoki, Haonan Wang, Yanlin Wang,Yanlin Gao, Mina Maruyama, Takuto Kawakami, Yasuyuki Makino, Masahiko Kaneda,Tongmin Chen, Kohei Aso, Tomoya Ogawa, Takahiko Endo, Yusuke Nakanishi,Kenji Watanabe, Takashi Taniguchi, Yoshifumi Oshima, Yukiko Yamada-Takamura,Mikito Koshino, Susumu Okada, Kazunari Matsuda, Toshiaki Kato,*and Yasumitsu Miyata*1. IntroductionMoiré superlattices, which arise from a lat-tice mismatch or relative twist anglebetween two different layers, have recentlyemerged as an ideal platform for exploringexotic quantum phenomena and futureapplications of two-dimensional (2D) mate-rials. Nanoscale periodic potential in moirésuperlattices provides a new degree of free-dom for tuning and studying the flat bandeffect and correlated physics.[1–6] To date,moiré superlattices have been fabricatedusing a variety of 2D materials, includinggraphene, hexagonal boron nitride (hBN),and transition metal dichalcogenides(TMDCs). Among these, Janus monolayersof TMDCs (represented as MXY (M=Moand W; X, Y= S, Se, and Te)) provide anadditional degree of freedom in moirésuperlattices. Janus monolayers have twodifferent chalcogen atoms above and belowthe central metal atom, and because of thisasymmetric structure, they have a built-inJanus monolayers of transition metal dichalcogenides (TMDCs) are promisingbuilding blocks for moiré superlattices because of their built-in electric field andclean fabrication process. In particular, Janus TMDC monolayers can bechemically converted from conventional TMDC monolayers by atomic substi-tution, enabling the direct formation of lattice-mismatched heterobilayers fromTMDC bilayers. However, the moiré superlattices of Janus heterobilayers havenot been studied experimentally. Herein, this work reports the fabrication andcharacterization of semiconductor moiré superlattices in chemically tailoredJanus heterobilayers. The MoSSe/MoSe2 (or WSSe/WSe2) Janus heterobilayersare prepared by replacing the top layer Se atoms with S atoms in MoSe2 (or WSe2)bilayer using H2 plasma treatment. Scanning transmission electron microscopyreveals that an average moiré period of about 14 nm formed due to latticemismatch resulting from the chalcogen substitutions. The cryogenic photolu-minescence spectra show sharp, near-infrared emissions, which are attributed toexcitons trapped by moiré potentials based on comparison with theoreticalcalculations. The Janus-based heterostructures provide a long-period moirésystem with built-in potential even for nontwisted heterobilayers, allowing thefunctionalization of confined and correlated electron systems.W. Zhang, Y. Makino, M. Kaneda, T. Ogawa, T. Endo, Y. Nakanishi,Y. MiyataDepartment of PhysicsTokyo Metropolitan UniversityHachioji 192-0397, JapanE-mail: wjzhang@tmu.ac.jp; ymiyata@tmu.ac.jpZ. LiuInnovative Functional Materials Research InstituteNational Institute of Advanced Industrial Science and Technology (AIST)Nagoya 463-8560, JapanThe ORCID identification number(s) for the author(s) of this articlecan be found under https://doi.org/10.1002/sstr.202300514.© 2024 The Authors. Small Structures published by Wiley-VCH GmbH.This is an open access article under the terms of the CreativeCommons Attribution License, which permits use, distribution andreproduction in any medium, provided the original work is properly cited.DOI: 10.1002/sstr.202300514H. Nakajo, S. Aoki, T. KatoGraduate School of EngineeringTohoku UniversitySendai 980-8579, JapanE-mail: kato12@tohoku.ac.jpH. Nakajo, S. Aoki, T. KatoAdvanced Institute for Materials Research (AIMR)Tohoku UniversitySendai 980-8577, JapanH. NakajoKOKUSAI ELECTRIC CORP.Toyama 939-2393, JapanH. Wang, Y. Wang, K. MatsudaInstitute of Advanced EnergyKyoto UniversityKyoto 611-0011, JapanRESEARCH ARTICLEwww.small-structures.comSmall Struct. 2024, 5, 2300514 2300514 (1 of 8) © 2024 The Authors. Small Structures published by Wiley-VCH GmbHmailto:wjzhang@tmu.ac.jpmailto:ymiyata@tmu.ac.jphttps://doi.org/10.1002/sstr.202300514http://creativecommons.org/licenses/by/4.0/http://creativecommons.org/licenses/by/4.0/mailto:kato12@tohoku.ac.jphttp://www.small-structures.comout-of-plane electric field.[7] Recent theoretical studies have pre-dicted that this asymmetric structure leads to various properties,such as large Rashba spin-orbit coupling,[8–15] piezoelectricity,[16]nonlinear optical response,[17,18] and long-lived charge transferexcitons.[10,11,19–25] Furthermore, the excitons in Janus heterobi-layer moiré superlattices have been calculated to realize a high-temperature Bose–Einstein condensation state as a result of thebuilt-in electric field that enhances the exciton lifetime and therepulsive interactions between trapped excitons.[23]In addition to these theoretical works, recent advances insurface-chalcogen exchange techniques have experimentallyrealized Janus monolayers, such as MoSSe and WSSe, fromthe corresponding monolayer TMDCs.[26–34] This techniquehas been applied to the direct preparation of Janus heterobi-layers, including WSSe/WSe2 from bilayer WSe2.[35–37] TheWSSe/MoSSe heterobilayer was also obtained by the transferprocess.[36,37] These advances allow us to access their excitonicproperties. Previous reports demonstrated ultrafast chargetransfer and enhanced interlayer coupling effects in Janusheterobilayers.[35,37,38] However, the moiré superlattices ofJanus heterobilayers are still unknown experimentally. Notably,the MSSe monolayers have an intermediate lattice constantbetween MS2 and MSe2.[7,26] This small lattice mismatch enablesthe formation of long-period moiré superlattices even from non-twisted bilayers in Janus heterobilayers. Such a long moiréperiod compared to conventional nontwisted heterobilayerscould be useful for further tuning the electron correlation inmoiré superlattices. In addition, the direct preparation ofJanus heterobilayers provides a method to fabricate scalablemoiré superlattices from MX2 bilayers with thermodynamicallystable configurations such as Bernal stacking.Here, we present the fabrication and characterization of moirésuperlattices in Janus heterobilayers of MoSSe/MoSe2 andWSSe/WSe2. Janus heterobilayers were prepared from bilayerMoSe2 (or WSe2) using plasma functionalization and were char-acterized by Raman and photoluminescence (PL) measurements.Nanoscale moiré structures were visualized using electronmicroscopy observations. Furthermore, their excitonic propertieswere investigated using cryogenic PL spectroscopy together withtheoretical calculations.2. Results and Discussion2.1. Preparation and Structure Analysis of Janus HeterobilayerWe first synthesized bilayer MSe2 (M=Mo and W) on SiO2/Sisubstrates using chemical vapor deposition (CVD). Then, the topsurface Se atoms were substituted by S atoms using H2 plasmatreatment (Figure 1a). We note that the S substitution was alsoconfirmed by high-resolution elemental mapping in our previousstudy.[39] As a result, the transition metal atoms (M) were cova-lently bonded to the top S and underlying Se atoms within the toplayer, resulting in the formation of MSSe/MSe2 heterobilayers.Owing to the smaller lattice constant of the upper Janus mono-layers, the present process leads to the formation of moiré super-lattices from nontwisted bilayer MSe2, as shown in Figure 1b.Figure 1c shows an optical microscopy image of a grain com-posed of monolayer and bilayer regions after the plasma treat-ment. The optical image shows stacked triangular grains ofdifferent sizes. The same orientation of the triangles suggeststhe formation of 3R-like stacking.[40,41] Notably, 2H- and 3R-likestacking were randomly observed in the present CVD-grownsamples (Figure S1, Supporting Information). Following theplasma treatment, the conversion from MoSe2 to MoSSe wasconfirmed from room-temperature PL/Raman intensity mapsand spectra (Figure 1d–h and S2, Supporting Information).Figure 1d,e shows the A10 mode Raman intensity maps ofMoSSe at 290 cm�1 and MoSe2 at 240 cm�1. The A10 mode at290 cm�1 in Figure 1g is consistent with that of MoSSe in theprevious study.[42] The Raman signal of MoSSe can be seenthroughout the region of the triangular grain, while that ofMoSe2 is only observed in the bilayer region. This indicates thatthe surface Se atoms were uniformly substituted by S atoms, andin the bilayer region, the bottom MoSe2 was protected from thesubstitution reaction. As shown in Figure 1g, in the bilayerregion, the A 01 mode of MoSSe at 290 cm�1 had comparableintensity to the A 01 mode of MoSe2 at 240 cm�1, whereas onlythe A 01 mode at 240 cm�1 was observed in the pristine bilayerMoSe2. For the PL intensity map (Figure 1f ), the bilayer regionshows weak PL intensity at 1.69 eV compared to the monolayerregion. The PL spectra of the MoSSe/MoSe2 showed two typicalpeaks corresponding to MoSSe (1.65 eV) and MoSe2 (1.47 eV),which differed from that of the bilayer MoSe2 before the plasmatreatment (Figure 1h). The PL peak at 1.65 eV was also consistentwith the reported spectra of monolayer MoSSe.[26] Similarchanges in Raman and PL spectra were observed in the mono-layer region (Figure S2, Supporting Information). We note thatthese Raman and PL peaks of MoSSe differ from those of theintermediate partially substituted Janus structure and metallicstate of MoSH[28,43,44] and random alloyed MoS2xSe2(1�x) mono-layer, which shows multiple peaks including both MoS2 andMoSe2 derived Raman modes.[45,46] The present plasma treat-ment was also applied to fabricate the WSSe/WSe2 samples(Figure S3,and S4, Supporting Information).Y. Gao, M. Maruyama, S. OkadaDepartment of PhysicsGraduate School of Pure and Applied SciencesUniversity of TsukubaTsukuba 305-8571, JapanT. Kawakami, M. KoshinoDepartment of PhysicsOsaka UniversityToyonaka, Osaka 560-0043, JapanT. Chen, K. Aso, Y. Oshima, Y. Yamada-TakamuraSchool of Materials ScienceJapan Advanced Institute of Science and Technology (JAIST)Ishikawa 923-1292, JapanK. WatanabeResearch Center for Electronic and Optical MaterialsNIMSTsukuba 305-0044, JapanT. TaniguchiResearch Center for Materials NanoarchitectonicsNIMSTsukuba 305-0044, Japanwww.advancedsciencenews.com www.small-structures.comSmall Struct. 2024, 5, 2300514 2300514 (2 of 8) © 2024 The Authors. Small Structures published by Wiley-VCH GmbH 26884062, 2024, 5, Downloaded from https://onlinelibrary.wiley.com/doi/10.1002/sstr.202300514 by Cochrane Japan, Wiley Online Library on [08/05/2024]. 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.small-structures.com2.2. Atomic Structure of Janus MoSSe/MoSe2 HeterobilayerThe atomic arrangement of Janus heterobilayers was investigatedusing scanning transmission electron microscopy (STEM).Figure 2a shows the high-angle annular dark-field (HAADF)STEM image of MoSSe/MoSe2. Figure 2b shows the enlargedimages of the four regions labeled i–iv in Figure 2a. Images iand iv can be assigned to Rhh stacking, whereas the images iiand iii correspond to RXh and RMh stacking, respectively(Figure 2d). The Rhh stacking shows simple honeycomb lattices,which are displayed in Figure 2a as lighter contrasts. In thisregion, the distance of the moiré pattern, aM, is about 16 nm.Figure 2e shows the fast Fourier transform (FFT) patternobtained from Figure 2a, and presents the sixfold symmetryof the spots. Notably, when each spot was magnified, two distinctpeaks were observed (Figure 2e). The inner and outer spots wereassigned to the FFT patterns of the honeycomb lattices of MoSe2and MoSSe with the same orientation, respectively. From thepeak distances, the lattice mismatch between the MoSSe andMoSe2 monolayers was estimated to be 2.1% in this region.The lattice constant of MoSSe can be estimated to be0.321 nm by using that of MoSe2 (0.328 nm). This lattice mis-match, δ, corresponds to a moiré period of 15.4 nm, based onthe relation of aM ≈ aMoSSeδ .[2,47] Using the obtained latticeFigure 1. Fabrication and optical characterization of MoSSe/MoSe2 heterobilayer. a) Schematic of the experimental setup and the conversion processfrom bilayer (2 L) MoSe2 to heterobilayer MoSSe/MoSe2 using H2 plasma treatment. b) Side and top views of bilayer MoSe2 and Janus heterobilayerMoSSe/MoSe2. The periodic moiré superlattice is visualized by changing the lattice constant of the top layer. The blue-dashed shape indicates a moiréunit cell. c) Typical optical microscope image of the heterobilayer MoSSe/MoSe2 prepared on a SiO2/Si substrate. Raman intensity map of A1’ mode ofd) MoSSe at 290 cm�1 and e) MoSe2 at 240 cm�1. f ) PL intensity map of MoSSe (1.69 eV). g) Raman and h) PL spectra of the bilayer MoSe2 andheterobilayer MoSSe/MoSe2.Figure 2. Atomic structure of the moiré superlattice in the Janus heterobilayer MoSSe/MoSe2. a) HAADF-STEM image of MoSSe/MoSe2. b) Enlarged andc) simulated images of the selected regions from (a), and the scale bar is 0.25 nm. d) The schematics show the top and side views with a periodic stackingwww.advancedsciencenews.com www.small-structures.comSmall Struct. 2024, 5, 2300514 2300514 (3 of 8) © 2024 The Authors. Small Structures published by Wiley-VCH GmbH 26884062, 2024, 5, Downloaded from https://onlinelibrary.wiley.com/doi/10.1002/sstr.202300514 by Cochrane Japan, Wiley Online Library on [08/05/2024]. 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.small-structures.comparameters, we simulated the HAADF-STEM images. As shownin Figure 2b,c, the experimental images are in good agreementwith the simulated ones of the structure model in Figure 2d.Figure 3a shows the wide-area annular bright-field (ABF)STEM image of the MoSSe/MoSe2 heterobilayer. Here, we showthe ABF STEM image due to its high contrast, and the contrast ofthe moiré patterns between the ABF and HAADF–STEM imageswas reversed (Figure S5, Supporting Information). The blackdots correspond to the moiré patterns. The distance betweentwo adjacent black dots ranges from 12 nm to 17 nm, and theaverage moiré period was 13.9� 0.5 nm. The lattice mismatchand the MoSSe lattice constant were estimated to be around2.3% and 0.320 nm using the equation, aM ≈ aMoSSeδ , respectively.These values are close to those obtained from the local region, asshown in Figure 2, and were consistent with the lattice constantof previous experimental and theoretical works[13,26,33] Thisresult also supports uniform conversion of the top layers bythe present plasma treatment. We note that the inhomogeneityof moiré patterns can be explained by the introduction ofin-plane tensile strain into the MoSSe during the conversionfrom MoSe2 (Figure S5, Supporting Information). We alsoobserved similar moiré structures in the WSSe/WSe2 Janusheterobilayer (Figure S6, Supporting Information). Notably,due to their small lattice mismatch, the MoSSe/MoSe2 andWSSe/WSe2 heterobilayers have longer moiré period comparedto other nontwisted (or nearzero twist angle) heterobilayers, suchas MoTe2/MoS2, WS2/WSe2, GaSe/MoSe2, and WSe2/MoTe2(Figure 3b).[48–51]2.3. Low-Temperature PL Spectra of Janus WSSe/WSe2HeterobilayerWith the emergence of moiré structure in the present Janus het-erobilayers, the nanoscale periodic moiré potentials could locallytrap the excitons at low temperatures. For low-temperature PLmeasurement, the Janus heterobilayers were encapsulated withhBN flakes to obtain high optical quality (Figure S7, SupportingInformation). In this work, we focused on the WSSe/WSe2because relatively bright PL was measured compared to theMoSSe/MoSe2 in the present study. Figure 4a shows the PLspectra of the hBN-encapsulated WSSe monolayer and WSSe/WSe2 heterobilayer at 8 K. The monolayer WSSe exhibited abroad peak between 1.6 and 1.8 eV. This broad peak resultedfrom a mixed contribution from defect-derived localized exci-tons, charged excitons, and neutral excitons.[28,34,52] By contrast,only the WSSe/WSe2 heterobilayer showed multiple sharp peakswith line widths of 3–6meV between 1.3 and 1.7 eV. These sharppeaks have never been observed for the other samples, includingmonolayer WSSe, monolayer and bilayer WSe2. From this com-parison, these multiple, sharp PL peaks could be attributed to0 5 10 1505101520MoSSe/MoSe(WSSe/WSe ) WSe /MoTeWS /WSeMoS /MoTeGaSe/MoSe)mn(doirepéri oMLattice mismatch(b)(a)Figure 3. Wide-field moiré patterns of the heterobilayer MoSSe/MoSe2. a) ABF-STEM image of the MoSSe/MoSe2 heterobilayer on the TEM grid. Scalebar is 20 nm. b) Relationship between the moiré period and the lattice mismatch for five nontwisted (or nearzero twist angle) heterobilayers includingMoSSe/MoSe2 (WSSe/WSe2) in the present work, MoTe2/MoS2, WS2/WSe2, GaSe/MoSe2, and WSe2/MoTe2 nontwisted heterobilayers.[48–51].1.50 1.55 1.60 1.6501 198 W cm20 W cm10 W cm4 W cm)s/c(ytisnet niLPPhoton energy (eV)10 100110100).u.a(ytis netn iLPLaser power (W cm )1.519 eV1.575 eV1.645 eV1.664 eV1.675 eV1.2 1.4 1.6 1.801).u.a(ytisnetniLPPhoton energy (eV)WSSe/WSe1L WSSe2L WSe1L WSe(a) (b)(c)Figure 4. Moiré excitons in the Janus heterobilayer WSSe/WSe2. a) PLspectra of hBN-encapsulated (red) heterobilayer WSSe/WSe2, (pink)monolayer (1 L) Janus WSSe, (blue) bilayer (2 L), and (light blue) 1 LWSe2 measured at 8 K. b) Excitation power-dependent PL spectra andc) PL peak intensities at 1.519, 1.575, 1.645, 1.664, and 1.675 eV of theheterobilayer WSSe/WSe2. A linear power dependence is observed forthe peaks at 1.664 and 1.675 eV, whereas the other three peaks show asaturating behavior above 20W cm�2.www.advancedsciencenews.com www.small-structures.comSmall Struct. 2024, 5, 2300514 2300514 (4 of 8) © 2024 The Authors. Small Structures published by Wiley-VCH GmbH 26884062, 2024, 5, Downloaded from https://onlinelibrary.wiley.com/doi/10.1002/sstr.202300514 by Cochrane Japan, Wiley Online Library on [08/05/2024]. 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.small-structures.comexcitons trapped in the moiré potential rather than defects or lat-tice strain.[53,54]We found that these multiple PL peaks have different excita-tion power dependence, as shown in Figure 4b. Figure 4c showsthe PL intensities of different peaks plotted as a function of exci-tation laser power. For the three peaks below 1.645 eV, the PLintensity tends to show a saturation behavior at a relatively lowexcitation power density of 20W cm�2. Conversely, a linear powerdependence was observed for the peaks at 1.664 and 1.675 eV.The saturation behavior is probably due to the longer lifetimeof interlayer excitons than that of intralayer excitons. Owing tothe staggered band alignment of WSSe/WSe2 (shown later), inter-layer excitons were expected to be formed by efficient interlayercharge transfer. Because of their long lifetimes, the interlayerexcitons could easily occupy the individual moiré potentials at rel-atively low exciton power and show saturation behavior.Besides the exciton types mentioned above, it should be notedthat the multiple peaks are due to the inhomogeneity of moirépotential. Indeed, the PL spectra showed significant variations indifferent regions (Figure S8, Supporting Information). The inho-mogeneous moiré patterns were observed in the STEM images(Figure 3a) and also in the previous reports.[55,56]. Such inhomo-geneity is probably derived from introducing unintentionallattice strain during the plasma treatment and/or the hBNencapsulation. Further improvement of the fabrication processis also necessary to suppress the strain effect on Janusheterobilayers.2.4. Electronic Band Structure of WSSe/WSe2 MoiréSuperlatticeTo confirm the staggered band alignment, the band structures ofmonolayer WSSe and WSe2 and their heterobilayer wereobtained using first-principles calculations. The valence andconduction band edges of monolayer WSSe were lower thanthose of monolayer WSe2 (Figure 5a). This suggests the forma-tion of staggered band alignment for the heterobilayer asreported previously.[37] We found that the staggered band align-ment of WSSe/WSe2 was resilient to variations in lattice constant(Figure S9, Supporting Information). The present calculationsalso showed a potential difference of 0.7 V between the vacuumon the S- and Se-sides (Figure S9, Supporting Information),which was comparable to previously reported results of variousJanus monolayer TMDCs.[9,15,23,57] Together with the confine-ment effect of the moiré potential, the staggered band alignmentcould lead to the formation of long-lived, interlayer excitons inthe WSSe/WSe2 heterobilayer. Evaluation of the intrinsic excitonlifetime would require highly clean and carrier-tunable devices ofJanus heterobilayers, which is a future challenge.The above first-principles calculation ignored the effect of themoiré pattern caused by a tiny difference in the lattice constantsof WSSe and WSe2. To see this, we calculated the band structureof WSSe/WSe2 moiré superlattice using a tight binding model(See S10, Supporting Information). Here, we take the ratio in lat-tice constants betweenWSSe andWSe2 to be 35:36, which approx-imates the ratio obtained by first-principle calculation (Figure 5a).Due to the large periods of the moiré superlattice, the electronicbands are folded in the mini-Brillouin zone and divided into thesubband structure (Figure 5b). As shown in Figure 5c, the densityof states exhibits multiple sharp peaks, which is attributed to theemergence of the subband formation. The spatial distribution ofthe density of states shows the long-range periodic patterns(Figure 5d). Particularly, electronic states at VBM exhibit an ener-getically isolated narrow band and are confined to the centralregion of moiré pattern where chalcogen and transition metalatoms are vertically aligned (as in the top panel of Figure 2b–d).The isolated narrow band and localized density of states supportthe observation of sharp PL peaks due to the formation of moirépotentials that can trap the intra- and interlayer excitons.-8-6-4-20)Ve(ygrenEK M aaa0a a0––––––––––a3.94.04.15.75.85.9K μ0aa0γ 0.0 2.5 5.0(a) (b) (c) (d)(e)WSSe WSe2 Density of statesEnergy (eV)WSSeWSe2Figure 5. Electronic structures of Janus monolayer WSSe, WSe2, and moiré superlattice of WSSe/WSe2. a) Electronic band structures of monolayer(orange) WSSe and (gray) WSe2 under optimized lattice constants of 3.191 and 3.28 Å calculated by first-principles calculation. b–d) Electronic structuresof moiré superlattice of WSSe/WSe2 calculated by a tight binding model. b) Band structure along the κ–γ–μ points of the mini-Brillouin zone (inset). Colorcode indicates the probability amplitudes of the wave function in WSSe and WSe2 layers. c) Density of states with sharp peak structure associated with thesubband formation. Local density of states at d) E=�4.12 eV and e)�5.68 eV, respectively. aM is the lattice constant of moiré superlattice. Here, we takesuperlattice including 36� 36WSSe units and 35� 35WSe2.www.advancedsciencenews.com www.small-structures.comSmall Struct. 2024, 5, 2300514 2300514 (5 of 8) © 2024 The Authors. Small Structures published by Wiley-VCH GmbH 26884062, 2024, 5, Downloaded from https://onlinelibrary.wiley.com/doi/10.1002/sstr.202300514 by Cochrane Japan, Wiley Online Library on [08/05/2024]. 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.small-structures.com3. ConclusionIn summary, we have demonstrated the fabrication and struc-ture/optical characterizations of moiré superlattices based onMoSSe/MoSe2 and WSSe/WSe2 Janus heterobilayers. These het-erobilayers were prepared by plasma functionalization frombilayer TMDCs. The STEM observations visualized the moirésuperlattices with an average lattice constant of about 14 nm.The moiré excitons were studied using cryogenic PL measure-ments together with the theoretical calculations. The staggeredband alignment was confirmed by the first-principles calcula-tions, and the formation of moiré potential was also supportedby the tight-binding calculations.The present study provides a novel way to prepare the moirésuperlattices using a simple postgrowth plasma treatment atroom temperature. This process enables the rapid and scalablefabrication of moiré superlattices with a clean interface fromnontwisted bilayers and multilayers, which can be obtained byconventional CVD process. In terms of structural features, asmall lattice mismatch of the Janus heterobilayers allows the for-mation of long-period moiré superlattices exceeding 10 nm innontwisted heterobilayers. Such long-period potentials have usu-ally been difficult to create in conventional heterobilayer TMDCsdue to large lattice mismatches. It is noted that twisted “homo-bilayers” allow the formation of longer-period moiré structures,but cannot avoid large inhomogeneities due to small angularmisalignments. In addition to the longer-period moiré struc-tures, the built-in potential of Janus monolayers offers the pos-sibility to modulate a wide variety of quantum phenomena,including noncentrosymmetric superconductivity, Majorana fer-mions, topological phases, and Bose–Einstein condensation ofexcitons.[23,58]4. Experimental SectionCVD Growth of TMDCs: MoSe2 and WSe2 used in this study were grownon Si substrates with a SiO2 thickness of 285 nm using the CVD methodwith two electric furnaces. For WSe2, the substrate was set in a quartz tubewith WO3 powder (300mg) and Se beads (3–4 g). N2 gas with a constantflow rate of 450 sccm was used to fill the quartz tube under atmosphericpressure. The WO3 powder was heated to 950 °C using the downstreamelectric furnace. When the furnace reached the target temperature, Se washeated at 380 °C for 2 min under H2/N2 (H2= 0.7%) gas with a total flowrate of 450 sccm using the upstream electric furnace. Finally, the entiresystem was cooled using a fan. MoSe2 was prepared using MoO2 powder(80–120mg) and heated at 870–880 °C under a mixed gas of N2(200–250 sccm) and H2 (1 sccm). The Se was heated at 420 °C for 2 min.Plasma Treatment: Janus heterobilayers were prepared by the plasma-assisted surface chemical substitution from bilayer MoSe2 and WSe2, asshown in Figure 1a.[36,39] The CVD-grown TMDC samples (MoSe2 andWSe2) were placed (z= 300–350mm) in a quartz tube (diameter20mm and length= 600mm), where z= 0mm is the end of quartz tubein the upstream side. The quartz tube was pumped down to base pressure(≈few Pa) with a rotary pump. Then, 99.99% pure H2 was supplied(≈20 sccm) and the tube pressure was kept at 38 Pa. S powder was placedin the upstream. A copper coil (diameter= 50mm) was placed around thedownstream (z= 400–500mm) region of the quartz tube as an antennafor inductively coupled plasma generation. S powder was placed in theupstream (z= 200–250mm). The radio frequency power for the coppercoil was set to 15–30W. The reaction time was adjusted between30 and 60min. All treatments were carried out under room temperatureconditions.hBN Encapsulation: The hBN-encapsulated samples were preparedusing a typical polymer-assisted lifting and peeling process using acrylicresin stamps, as reported previously.[59] Thin hBN flakes were mechani-cally exfoliated onto the SiO2/Si substrates from bulk crystals.[60] Thedetailed processes are presented in Figure S11, Supporting Information.Optical Measurements: Room temperature PL spectroscopy wasconducted using a Renishaw inVia spectrometer equipped with a 532 nmexcitation light. Cryogenic PL was performed using a lab-made opticalsetup with a cryostat under vacuum conditions (<10�4 Pa). A 635 nm con-tinuous-wave semiconductor laser was used as the excitation source. Thelaser was focused using a 50� objective lens. The PL signals were collectedusing the same objective lens and finally detected using a cooled charge-coupled device through a spectrometer.STEM Observations and Analyses: For the STEM observations, HAADF-and ABF-STEM images and STEM-EDS mapping were collected at roomtemperature using a JEM-ARM200F (Cold FEG) equipped with a CEOSASCOR corrector and a 100mm2 SDD detector operated at 120 kV.The simulation of HAADF-STEM images was performed using Tempassimulation package, with parameters similar to the experimental ones.Computational Methods: The theoretical calculations, as shown inFigure 5a, were conducted using the STATE program package,[61,62] basedon density functional theory.[63,64] The exchange-correlation potentialenergy between electrons is approximated using the generalized gradientapproximation with a Perdew–Burke–Ernzerhof functional.[65] The weakdispersive interaction between the WSe2 and WSSe layers was treatedusing vdW-DF2 with the C09 exchange-correlation functional.[66,67]Ultrasoft pseudopotentials were adopted to describe the interactionbetween valence electrons and ions.[68] The valence wave functions anddeficit charge density were expanded in terms of plane-wave basis setswith cutoff energies of 25 and 225 Ry, respectively. Brillouin-zone integra-tion was carried out with 9� 9 k-meshes. The lattice parameters and inter-nal atomic coordinates were fully optimized until the force acting on theatoms was <1.33� 10�3 hartree bohr�1. To exclude the unphysical dipoleinteraction with the periodic images normal to the sheet, we used theeffective screening medium method.[69,70] For the Janus heterobilayer cal-culation, the interlayer distance between WSSe and WSe2 was 3.18 Å(Figure S9, Supporting Information).Supporting InformationSupporting Information is available from the Wiley Online Library or fromthe author.AcknowledgementsThe authors acknowledge T. Tanaka and S. Furusawa for providingvaluable advice and help on HAADF-STEM image simulation. This workwas financially supported by the Japan Science and TechnologyAgency (JST) CREST program (JPMJCR20T3), the JST FORESTProgram (JPMJFR213X), the JST PRESTO program (JPMJPR23H5),Kakenhi Grants-in-Aid (JP19H00664, JP20H00354, JP20H05664, JP20K14415,JP20H01840, JP20H00127, JP21H05232, JP21H05233, JP21H05234,JP21H05235, JP21H05236, JP22H00280, JP22H00283, JP22K18986,JP22H04957, JP22H05441, JP22KJ2561, JP23K13635, JP23H02052, andJP23H01807) from the Japan Society for the Promotion of Science(JSPS), Sumitomo Foundation Fiscal 2021 Grant for Basic ScienceResearch Projects, Yazaki Memorial Foundation for Science andTechnology, Mitsubishi Foundation, Murata Science FoundationResearch Grant, and ZE Research Program, IAE (ZE2023B-05).Conflict of InterestThe authors declare no conflict of interest.www.advancedsciencenews.com www.small-structures.comSmall Struct. 2024, 5, 2300514 2300514 (6 of 8) © 2024 The Authors. Small Structures published by Wiley-VCH GmbH 26884062, 2024, 5, Downloaded from https://onlinelibrary.wiley.com/doi/10.1002/sstr.202300514 by Cochrane Japan, Wiley Online Library on [08/05/2024]. 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.small-structures.comData Availability StatementThe data that support the findings of this study are available from thecorresponding author upon reasonable request.Keywordsheterobilayers, Janus transition metal dichalcogenides, moiré excitons,moiré superlattices, plasma treatmentReceived: November 26, 2023Revised: January 31, 2024Published online: March 1, 2024[1] L. Balents, C. R. Dean, D. K. Efetov, A. F. Young, Nat. Phys. 2020, 16,725.[2] K. F. Mak, J. Shan, Nat. Nanotechnol. 2022, 17, 686.[3] D. Huang, J. Choi, C.-K. Shih, X. Li, Nat. 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Maruyama, K. Nagashio, S. Okada, ACS Appl. Electron. Mater.2020, 2, 1352.www.advancedsciencenews.com www.small-structures.comSmall Struct. 2024, 5, 2300514 2300514 (8 of 8) © 2024 The Authors. Small Structures published by Wiley-VCH GmbH 26884062, 2024, 5, Downloaded from https://onlinelibrary.wiley.com/doi/10.1002/sstr.202300514 by Cochrane Japan, Wiley Online Library on [08/05/2024]. 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 Licensehttps://state-doc.readthedocs.io/en/latest/http://www.advancedsciencenews.comhttp://www.small-structures.com Chemically Tailored Semiconductor Moiré Superlattices of Janus Heterobilayers 1. Introduction 2. Results and Discussion 2.1. Preparation and Structure Analysis of Janus Heterobilayer 2.2. Atomic Structure of Janus MoSSe/MoSe2 Heterobilayer 2.3. Low-Temperature PL Spectra of Janus WSSe/WSe2 Heterobilayer 2.4. Electronic Band Structure of WSSe/WSe2 Moiré Superlattice 3. Conclusion 4. Experimental Section