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Terunobu Nakanishi, Shoji Yoshida, Kota Murase, Osamu Takeuchi, [Takashi Taniguchi](https://orcid.org/0000-0002-1467-3105), [Kenji Watanabe](https://orcid.org/0000-0003-3701-8119), Hidemi Shigekawa, Yu Kobayashi, Yasumitsu Miyata, Hisanori Shinohara, [Ryo Kitaura](https://orcid.org/0000-0001-8108-109X)

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[The Atomic and Electronic Structure of 0° and 60° Grain Boundaries in MoS2](https://mdr.nims.go.jp/datasets/736f1e41-391c-40ea-b21a-c3e011d196ef)

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The Atomic and Electronic Structure of 0˚ and 60˚ Grain Boundaries in MoS2ORIGINAL RESEARCHpublished: 17 April 2019doi: 10.3389/fphy.2019.00059Frontiers in Physics | www.frontiersin.org 1 April 2019 | Volume 7 | Article 59Edited by:Sabina Botti,Italian National Agency for NewTechnologies, Energy and SustainableEconomic Development (ENEA), ItalyReviewed by:Fausto Sirotti,École Polytechnique, FranceNiklas Nilius,University of Oldenburg, Germany*Correspondence:Ryo Kitaurar.kitaura@nagoya-u.jpSpecialty section:This article was submitted toCondensed Matter Physics,a section of the journalFrontiers in PhysicsReceived: 25 January 2019Accepted: 27 March 2019Published: 17 April 2019Citation:Nakanishi T, Yoshida S, Murase K,Takeuchi O, Taniguchi T, Watanabe K,Shigekawa H, Kobayashi Y, Miyata Y,Shinohara H and Kitaura R (2019) TheAtomic and Electronic Structure of 0◦and 60◦ Grain Boundaries in MoS2.Front. Phys. 7:59.doi: 10.3389/fphy.2019.00059The Atomic and Electronic Structureof 0◦ and 60◦ Grain Boundaries inMoS2Terunobu Nakanishi 1, Shoji Yoshida 2, Kota Murase 2, Osamu Takeuchi 2,Takashi Taniguchi 3, Kenji Watanabe 3, Hidemi Shigekawa 2, Yu Kobayashi 4,Yasumitsu Miyata 4, Hisanori Shinohara 1 and Ryo Kitaura 1*1Department of Chemistry, Nagoya University, Nagoya, Japan, 2 Research Center for Functional Materials, National Institutefor Materials Science, Tsukuba, Japan, 3 Faculty of Pure and Applied Sciences, University of Tsukuba, Tsukuba, Japan,4Department of Physics, Tokyo Metropolitan University, Hachioji, JapanWe have investigated atomic and electronic structure of grain boundaries in monolayerMoS2, where relative angles between two different grains are 0 and 60 degree. The grainboundaries with specific relative angle have been formed with chemical vapor deposition(CVD) growth on graphite and hexagonal boron nitride flakes; van der Waals interlayerinteraction between MoS2 and the flakes restricts the relative angle between two differentgrains of MoS2. Through scanning tunneling microscopy (STM) and spectroscopymeasurements, we have found that the perfectly stitched structure between two differentgrains of MoS2 was realized in the case of the 0 degree grain boundary. We alsofound that even with the perfectly stitched structure, valence band maximum (VBM) andconduction band minimum (CBM) shows significant blue shift, which probably arise fromlattice strain at the boundary.Keywords: grain boundaries, transition metal dichalchogenides, scanning tunneling microscopy, boundary states,chemical vapor deposition (CVD)INTRODUCTIONA post-graphene material, transition metal dichalcogenide (TMD), has recently attracted a greatdeal of attention. TMDs have a long research history, but research on properties of monolayerTMDs, three-atom-thick atomic layers, has only recently been started [1–3]. One of the mostdistinct in TMDs from graphene is that TMDs can have sizable bandgap (∼2 eV), leading toelectronic and optoelectronic applications of TMD atomic layers [4]. In fact, various TMD-baseddevices, including high-performance FET devices, light-emitting transistors, and photodetectors,have actually been demonstrated [5–7]. In conjunction with the flexibility arising from the ultrathinstructure, flexible electronic and optoelectronic devices can also be made [8, 9]. In addition,monolayer TMDs in 2H form can have valley-degree-of-freedom, which may lead to future novelelectronic devices based on valleytronics [10, 11].For future applications of TMDs for electronic and optoelectronic devices, wafer-scalemonolayer TMDs grown by chemical vapor deposition (CVD) are indispensable [12, 13].Top-down approaches, such as mechanical exfoliation, are not compatible with wafer-scalemonolayer TMDs, and a bottom-up approach is required for that purpose [14]. Crystal growthby CVD is a bottom-up approach to obtain thin films, having been successfully applied to growvarious atomic layers, such as graphene, hexagonal boron nitrides (hBN), and TMDs [15–20]. Intypical CVD growth of TMDs, solid sources such as metal oxides and elemental sulfur are used, andhttps://www.frontiersin.org/journals/physicshttps://www.frontiersin.org/journals/physics#editorial-boardhttps://www.frontiersin.org/journals/physics#editorial-boardhttps://www.frontiersin.org/journals/physics#editorial-boardhttps://www.frontiersin.org/journals/physics#editorial-boardhttps://doi.org/10.3389/fphy.2019.00059http://crossmark.crossref.org/dialog/?doi=10.3389/fphy.2019.00059&domain=pdf&date_stamp=2019-04-17https://www.frontiersin.org/journals/physicshttps://www.frontiersin.orghttps://www.frontiersin.org/journals/physics#articleshttps://creativecommons.org/licenses/by/4.0/mailto:r.kitaura@nagoya-u.jphttps://doi.org/10.3389/fphy.2019.00059https://www.frontiersin.org/articles/10.3389/fphy.2019.00059/fullhttp://loop.frontiersin.org/people/246712/overviewhttp://loop.frontiersin.org/people/677252/overviewhttp://loop.frontiersin.org/people/388462/overviewNakanishi et al. Grain Boundaries in MoS2monolayer TMDs film with a lateral size of millimeters havebeen reported [19, 21]. Recently, the growth of TMDs by metal-organic CVD (MOCVD) with volatile liquid sources has beensuccessfully demonstrated, and MOCVD is a promising methodto realize wafer-scale TMDs that are compatible with deviceapplications [22–24].In CVD-grown large-area TMDs, grain boundaries (GBs) areinevitably formed, which can significantly alter the electronic andoptical properties of TMDs [25–28]. During the CVD growthof TMDs, nuclei form at the beginning of the CVD process,growing to form a large-area continuous sheet of TMDwith GBs.Because the orientation of nuclei is normally random, a widevariety of GB structures can be formed. For example, a GBwith 7-5 and 8-4-4membered rings forms in a CVD-grownMoS2, wheremidgap boundary states appear [25, 29–31]. The existence of GB-induced midgap states significantly affects electronic transportacross the boundary, leading to reduction of carrier mobility viaadditional carrier scattering at the GB [25]. Therefore, control ofGB structure by controlling grain orientation and understandingthe boundary-oriented electronic structure provide a basis for therealization of future TMD-based devices.In this work, we have focused on orientation-limited growthof a TMD and investigation of localized boundary states usingscanning tunneling microscopy (STM) and scanning tunnelingspectroscopy (STS); the STS is a powerful tool to investigatedomain boundaries [32, 33]. The key for the successful controlof crystal orientation in CVD growth of TMDs is the interactionbetween TMDs and the substrates used in CVD processes.In conventional CVD growth of TMDs, SiO2/Si substrateswith amorphous surfaces are used, leading to random crystalorientations of grown TMDs. In contrast, substrates withcrystalline structures can limit the crystal orientation of grownTMDs through TMD-substrate interactions [20, 29, 34, 35]. Forthe control of crystal orientation, we used hBN and graphiteas substrates for CVD growth of TMDs. The atomically flatsurfaces with three-fold (hBN) and six-fold rotation (graphite)symmetries successfully limited crystal orientations of grownTMD flakes; only two different orientations were observed. GBsbetween MoS2 flakes with different orientations (relative angleof 60◦) shows boundary states localized at specific location nearthe Fermi level. On the other hand, a GB between MoS2 flakeswith the same orientation shows a perfectly-stitched structurewithout any defects in scanned areas in STM images. We alsofound that both the conduction band minimum (CBM) and thevalence band maximum (VBM) shift to the higher energy side atthe GB even with a perfectly-stitched structure. This means thatthe GB state does not arise from defects but from strain at the GB,and strain formed at the growth process cannot be released evenwith the low friction coefficient between MoS2 and graphite.METHODOLOGYWe grew monolayer MoS2 on hBN and graphite (Kish graphite,type C, Covalent Materials) flakes exfoliated on quartz substrateswith a multi-furnace CVD apparatus. We prepared hBN andgraphite flakes by the mechanical exfoliation method withadhesive tape (Scotch tape, 3M). As precursors for growth ofMoS2, we used molybdenum trioxide (Sigma-Aldrich, 99.5%purity) and sulfur powder (Sigma-Aldrich, 99.98% purity).Furnace temperatures at the locations where molybdenumtrioxide and elemental sulfur were placed were set to 1,029Kand 473K, respectively, and the growth of MoS2 was carriedout at 1,373K for 20min under Ar flow with a flow rateof 200 sccm. Atomic force microscope (AFM) observationswere performed by the Veeco AFM system (Dimension3100SPM, Nanoscope IV) operated at a scanning rate of1Hz. We measured photoluminescence (PL) spectra by amicrospectroscopy system with a confocal microscope (JobinYvon HR-800, Horiba) with an excitation laser wavelength of488 nm. For PL imaging, an LED light source (Mightex GCS-6500-15) was used to illuminate samples, and PL intensity(λ > 600 nm) was imaged with CCD (Princeton InstrumentsPIXIS-1024BR-eXelon).We formed electrical contacts to samplesfor STM/STS measurements by deposition of gold thougha shadow mask or patterning conductive silver paste. Aftermaking the electrical contact, samples were introduced to anultrahigh vacuum (UHV) environment and degassed at 473K.The STM/STS measurements were conducted using a scanningtunneling microscope (Omicron LT–STM) in constant currentmode operated at 90K with an electrochemically etched W tipcoated with PtIr (UNISOKU Co., Ltd.). A numerical derivativewas used to acquire dI/dV curves, and WSxM software was usedto process the STM images [36].RESULTS AND DISCUSSIONPL imaging and spectroscopy have clearly shown that the qualityof the present samples is high. Figure S1 shows a typical PLimage of MoS2/hBN and typical PL spectrum of MoS2/hBNand MoS2/graphite. As clearly seen, the PL image shows brightand uniform contrast, which clearly demonstrates high qualityof samples we use. The observed FWHM values of PL spectraare 35∼45 meV, which are much smaller than those of samplesexfoliated onto SiO2 substrates [37, 38]. These PL spectra clearlydemonstrate that quality of our sample is high.The crystal orientations of MoS2 grown on hBN andgraphite are limited to two orientations due to the van derWaals interactions between MoS2 and hBN. Figure 1A showsan AFM image of monolayer MoS2 crystals grown on ahBN flake. As clearly seen, all crystals possess a hexagonalshape with long and short facets, and their orientations arelimited to only two different ones, where 60◦ rotation ofone orientation matches the other orientation. The observedlong and short facets in the crystals correspond to chalcogenand metal zigzag edges; the relationship between crystalshape and crystallographic orientation was investigated withtransmission electron microscopy and electron diffraction(additional information is given in Figure S2). Figure 1B showsstructural models of hexagonalMoS2 flakes with the two differentorientations. The limited orientations of MoS2 flakes are alsoobserved in MoS2 flakes grown on graphite substrates. Thisclearly demonstrates that the orientation-dependent potentialFrontiers in Physics | www.frontiersin.org 2 April 2019 | Volume 7 | Article 59https://www.frontiersin.org/journals/physicshttps://www.frontiersin.orghttps://www.frontiersin.org/journals/physics#articlesNakanishi et al. Grain Boundaries in MoS2FIGURE 1 | (A) An AFM image of monolayer MoS2 grown on h BN. Long and short edges of MoS2 flakes are marked by green and blue dotted line, respectively.(B) Structural model of the grown MoS2 with relative angle of 60◦. (C,D) An AFM image of MoS2 before and after the oxidation. The relative angle between the twograins in this case is 0◦. (E,F) An AFM image of MoS2 before and after the oxidation. The relative angle between the two grains in this case is 60◦. White linearcontrasts in Figures 1A,C,F are wrinkles in hBN flakes.arising from crystalline substrates is crucial to limiting the crystalorientation of grown MoS2.Because the crystal orientation of MoS2 on hBN is limited,the resulting structure of the GBs should also be limited: GBsbetween grains with the same orientation (GB-0◦) and grainswith 60◦ mutual orientation (GB-60◦). To investigate if defectsexist at such GBs, we investigated the reactivity for an oxidationreaction. MoS2 flakes with GBs were heated at 573K under aflow of dry air. It has been shown that defects are sensitiveto oxidation and reactions under the conditions above lead tothe formation of oxides. Because oxidation from MoS2 to thecorresponding oxides heightens the pristine structure, position-sensitive detection of oxidation of MoS2 can easily be donethrough AFM height images. Figures 1C–F are AFM images ofpristine (oxidized) MoS2 flakes that have GB-0◦ and GB-60◦,respectively. As clearly seen in Figures 1D,F, oxidation at Mozigzag edges (shorter edges) is faster than that at S zigzag edges(longer edges) [39, 40]. We also found that GB-60◦ is oxidized asedges are oxidized, whereas GB-0◦ essentially retains its pristinestructure. This means that GB-0◦ does not have defects that aresensitive to oxidation reactions, indicating that, unlike GB-60◦,GB-0◦ has a well-stitched structure.To investigate the structure and local electronic structure ofGBs, we performed STM/STS measurements around the GBs.For this purpose, we use MoS2 grown on graphite, where thesame orientation-limited growth of MoS2 occurs. Figure 2A isa STM image of a MoS2 grown on graphite, where positions ofGB-0◦ are highlighted by arrows; we confirmed the monolayerstructure by a line profile analysis at the edge (Figure S3). As canbe seen, the GB-0◦ image is slightly darker than its peripheralplace, which indicates that GB-0◦ has different local densityof states from its peripheral place. For detailed investigationsof structure and electronic structure, atomic-resolution STMobservation of the GB-0◦ was carried out. Figure 2B is a STMimage of the GB-0◦ at high magnification, showing the triangulararray of sulfur atoms as bright spots. Based on a close inspectionof the STM image, the misorientation angle between the twodomains is almost zero (Figures S4, S5). GB-0◦ is imaged asslightly darker than its peripheral place at the middle of the STMtopographic image, and we observed no defects at the GB-0◦;neither vacancies nor insertion of atomic rows are seen. Thewell-stitched structure of GB-0◦ revealed by STM observationis consistent with its observed low reactivity toward oxidationreactions. It should be noted that a translational mismatch shouldexist at the boundary even with orientation matching betweentwo grains. This result, however, clearly demonstrates that GB-0◦has a stitched structure without defects, indicating that most ofthe translational mismatch can be relaxed through sliding on thegraphite plane. The ultraflat surfaces of graphite and MoS2 maylead to ultralow friction between them, which should facilitate thesliding [41–43].As demonstrated by darker contrasts in the STM image, eventhough GB-0◦ has a well-stitched structure, the local electronicstructure at GB-0◦ is different from that of its peripheral places.To see the differences in the electronic structures, we carried outSTS and dI/dV mapping to visualize the local density of states.Figure 2C shows a dI/dV map across the GB-0◦, which is locatedat a lateral position of around 6 nm in Figure 2C. As clearly seenin the figure, both CBM and VBM show upward shifts at theGB-0◦. Figure 2D is a STS spectrum at GB-0◦, showing that theupward shift at VBM (0.8 eV) is larger than that at CBM (0.4 eV).This results in a reduction of the bandgap at the GB-0◦ fromFrontiers in Physics | www.frontiersin.org 3 April 2019 | Volume 7 | Article 59https://www.frontiersin.org/journals/physicshttps://www.frontiersin.orghttps://www.frontiersin.org/journals/physics#articlesNakanishi et al. Grain Boundaries in MoS2FIGURE 2 | (A) An STM image of MoS2 grown on graphite. The blue arrows indicate the position of GB. (B) A magnified STM image around the GB-0◦. The GB-0◦can be imaged as a dark linear contrast, which are indicated by the arrow. (C) dI/dV mapping across the GB-0◦. (D) STS spectra measured at a place out of the GBand on the GB.2.2 to 1.8 eV; the observed bandgap of pristine monolayer MoS2(2.2 eV) is consistent with the value reported previously: 2.15–2.4eV [44–46].Because GB-0◦ has a stitched structure, this upward shiftcannot be explained by formation of defects-mediated midgapstates and can probably be explained by the local strainat the GB-0◦. The bandgap of monolayer MoS2 is verysensitive to strain, and strain causes bandgap narrowing throughupward/downward shift of VBM/CBM [46]. The observedmodulation of bands, however, is upward shift in both CB andVB, which probably originates from accumulation of electrons atthe boundary. This discrepancy can be understood if piezoelectriccharge is taken into account [47]. As monolayer MoS2 has a non-centrosymmetric structure, the local strain can induce chargeaccumulation at the GB-0◦, leading to the observed upwardshift of CBM and VBM. One important implication is that asmall strain, which probably arises from residual translationalmismatch even after the sliding-based relaxation, remains atGB-0◦, where the local electronic structure is strongly altered.To investigate the degree of strain at GB-0◦, we performeddetailed image analyses with the high-resolution STM imageshown in Figure 2B. Figure S4 shows a contrast-enhanced STMimage after applying high-pass filter to filter out the low-frequency noise. It is clear that there are no atomic defects atGB-0◦. A line profile along the yellow line clearly demonstratesthat location of bright spots in the STM image align periodicallywithout noticeable distortion. In addition, we performed fastFourier transform (FTT) analysis on the STM image shownin Figure 2B. As shown in Figure S5, a FFT image at GB-0◦shows spots with 6-fold symmetry, which is consistent witha triangular lattice of the sulfur array. The 6-fold symmetricpattern in the FFT image of GB-0◦ is almost identical to a FFTimage at a corresponding peripheral place; line profiles alongthe green arrows in the FFT image at GB-0◦ and the peripheralplace also coincide well. This means that the difference in latticeconstants at the GB-0◦ and its peripheral place is less than theexperimental resolution (2%). As discussed above, the observeddifference in bandgap at GB-0◦ and peripheral places is 0.4 eV.Even though we assume that the difference in bandgap originatesonly from lattice strain, the strain should be comparable to theexperimental resolution, and it is difficult to image the straindirectly [44]. These analyses mean that small distortion lessthan the experimental resolution can remain at the GB-0◦, andsignificant bandgap modulation can occur even in the case ofGB-0◦. This suggests that it is important to grow large singlecrystal of TMDs without any boundaries for future applicationwith high-mobility TMD films.In the case of GB-60◦, structural defects exist and the localelectronic structure is strongly modified. Figure 3A is a STMimage of MoS2 on graphite near the GB-60◦. The vertical linearcontrast at the middle of the image corresponds to an impurityattached at GB-60◦, where strong binding sites for impuritiesFrontiers in Physics | www.frontiersin.org 4 April 2019 | Volume 7 | Article 59https://www.frontiersin.org/journals/physicshttps://www.frontiersin.orghttps://www.frontiersin.org/journals/physics#articlesNakanishi et al. Grain Boundaries in MoS2FIGURE 3 | (A) An STM image of MoS2 grown on graphite with a grain boundary of 60◦. (B) A magnified STM image around the GB-60◦. (C) dI/dV mapping acrossthe GB-60◦ measured along the blue line in (B). (D–F) STS spectral image of the GB-60◦ measured with different bias voltage of 0.827, −0.780, and −1.539V,respectively.should exist. Figure 3B and Figure S6 are magnified STM imagesof clean GB-60◦, where the atomic structure can be seen. Basedon close investigation of the STM image, we found that theangle between GB-60◦ and the zigzag edge of MoS2 is about20◦. Figure 3C shows the STS spectral mapping along the blueline in Figure 3B. As clearly seen in Figure 3C, the electronicstructure is significantly modulated at GB-60◦, where both CBMand VBM upshift to reduce the bandgap from 2.3 to 1.9 eV. Toinvestigate the spatial distribution of the boundary state at GB-60◦, we performed dI/dVmapping at three different bias voltagesof 0.83, −0.78, and −1.54V. Figures 3D–F show the observeddI/dV mappings at bias voltages of 0.83, −0.78, and −1.54V,respectively. As clearly seen, the boundary state strongly localizesat GB-60◦. In addition, the boundary state corresponding to abias voltage of −0.78V shows a dotted distribution rather thana linear uniform distribution, and this means that the boundarystate originates from a specific defect site existing at the GB-60◦.CONCLUSIONIn this paper, electronic properties and defect densities intwo types of GBs in MoS2 grown by the CVD process wereinvestigated. The orientations of MoS2 grown on hBN andgraphite by the CVD process are limited to two directions andthe misorientation angles of the two flakes are 0◦ and 60◦. It isconfirmed that two grains are stitched completely in the GB-0◦,but have an upshift of band structure due to the local stress andcharge accumulation. In the GB-60◦, the structure of GB is clearlyimaged by STM/STS without the disturbance of adsorbates onGB. The band structure in GB-60◦ upshifts and localized statesappear. In the case of two grains of MoS2 stitched at the sameangle, the electronic structure of GB-0◦ is modified due to localstress and carrier accumulation. It will be a challenge to makea MoS2 sheet without modulation of the electronic state orthe structure.AUTHOR CONTRIBUTIONSTN prepared MoS2 samples. TN, KM, and SY performedSTM/STS measurements. YK and YM performed opticalcharacterizations of MoS2 samples. TT and KW synthesized hBNcrystals. RK designed experiments andwrote the paper. OT,HidS,and HisS discussed the results and reviewed the manuscript.FUNDINGThis work was supported by JSPS KAKENHI Grant numbersJP16H06331, JP16H03825, JP16H00963, JP15K13283,JP25107002, and JST CREST Grant Number JPMJCR16F3.KW and TT acknowledge support from the Elemental StrategyInitiative conducted by the MEXT, Japan and the CREST(JPMJCR15F3), JST.SUPPLEMENTARY MATERIALThe Supplementary Material for this article can be foundonline at: https://www.frontiersin.org/articles/10.3389/fphy.2019.00059/full#supplementary-materialFrontiers in Physics | www.frontiersin.org 5 April 2019 | Volume 7 | Article 59https://www.frontiersin.org/articles/10.3389/fphy.2019.00059/full#supplementary-materialhttps://www.frontiersin.org/journals/physicshttps://www.frontiersin.orghttps://www.frontiersin.org/journals/physics#articlesNakanishi et al. Grain Boundaries in MoS2REFERENCES1. Joensen P, Frindt RF, Morrison SR. Single-layer MoS2. Mater Res Bull.(1986) 21:457–61.2. Radisavljevic B, Radenovic A, Brivio J, Giacometti V, Kis A. Single-layerMoS2 transistors. Nat Nanotechnol. (2011) 6:147–50. doi: 10.1038/nnano.2010.2793. Mak KF, Lee C, Hone J, Shan J, Heinz TF. Atomically thin MoS2:a new direct-gap semiconductor. Phys Rev Lett. (2010) 105:136805.doi: 10.1103/PhysRevLett.105.1368054. Liu GB, Xiao D, Yao YG, Xu XD, Yao W. Electronic structures and theoreticalmodelling of two-dimensional group-VIB transition metal dichalcogenides.Chem Soc Rev. (2015) 44:2643–63. doi: 10.1039/C4CS00301B5. Pu J, Takenobu T. Monolayer transition metal dichalcogenides as lightsources. Adv Mater. (2018) 30:1707627. doi: 10.1002/adma.2017076276. Wang QH, Kalantar-Zadeh K, Kis A, Coleman JN, Strano MS.Electronics and optoelectronics of two-dimensional transition metaldichalcogenides. Nat Nanotechnol. (2012) 7:699–712. doi: 10.1038/nnano.2012.1937. Jariwala D, Sangwan VK, Lauhon LJ, Marks TJ, Hersam MC. Emergingdevice applications for semiconducting two-dimensional transition metaldichalcogenides. Acs Nano. (2014) 8:1102–20. doi: 10.1021/nn500064s8. Lee GH, Yu YJ, Cui X, Petrone N, Lee CH, Choi MS, et al.Flexible and transparent MoS2 field-effect transistors on hexagonalboron nitride-graphene heterostructures. Acs Nano. (2013) 7:7931–6.doi: 10.1021/nn402954e9. Pu J, Yomogida Y, Liu KK, Li LJ, Iwasa Y, Takenobu T. Highly flexible MoS2thin-film transistors with Ion gel dielectrics. Nano Lett. (2012) 12:4013–7.doi: 10.1021/nl301335q10. Xiao D, Liu GB, Feng WX, Xu XD, Yao W. Coupled spin and valley physicsin monolayers of MoS2 and other group-VI dichalcogenides. Phys Rev Lett.(2012) 108:196802. doi: 10.1103/PhysRevLett.108.19680211. Schaibley JR, Yu HY, Clark G, Rivera P, Ross JS, Seyler KL, et al.Valleytronics in 2D materials. Nat Rev Mater. (2016) 1:16055.doi: 10.1038/natrevmats.2016.5512. Kitaura R, Miyata Y, Xiang R, Hone J, Kong J, Ruoff RS, et al. Chemicalvapor deposition growth of graphene and related materials. J Phys Soc Jpn.(2015) 84:121013. doi: 10.7566/JPSJ.84.12101313. Shi YM, Li HN, Li LJ. Recent advances in controlled synthesis oftwo-dimensional transition metal dichalcogenides via vapour depositiontechniques. Chem Soc Rev. (2015) 44:2744–56. doi: 10.1039/C4CS00256C14. Novoselov KS, Jiang D, Schedin F, Booth TJ, Khotkevich VV, Morozov SV,et al. Two-dimensional atomic crystals. Proc Natl Acad Sci USA. (2005)102:10451–3. doi: 10.1073/pnas.050284810215. Lee YH, Zhang XQ, ZhangWJ, Chang MT, Lin CT, Chang KD, et al. Synthesisof large-area MoS2 atomic layers with chemical vapor deposition. Adv Mater.(2012) 24:2320–5. doi: 10.1002/adma.20110479816. Reina A, Jia XT, Ho J, Nezich D, Son HB, Bulovic V, et al. Large area, few-layergraphene films on arbitrary substrates by chemical vapor deposition. NanoLett. (2009) 9:30–5. doi: 10.1021/nl801827v17. Sinha S, Takabayashi Y, Shinohara H, Kitaura R. Simple fabrication ofair-stable black phosphorus heterostructures with large-area hBN sheetsgrown by chemical vapor deposition method. 2d Mater. (2016) 3:035010.doi: 10.1088/2053-1583/3/3/03501018. Okada M, Miyauchi Y, Matsuda K, Taniguchi T, Watanabe K, Shinohara H,et al. Observation of biexcitonic emission at extremely low power density intungsten disulfide atomic layers grown on hexagonal boron nitride. Sci Rep.(2017) 7:322. doi: 10.1038/s41598-017-00068-019. Kobayashi Y, Sasaki S, Mori S, Hibino H, Liu Z, Watanabe K,et al. Growth and optical properties of high-quality monolayer WS2on graphite. Acs Nano. (2015) 9:4056–63. doi: 10.1021/acsnano.5b0010320. Okada M, Sawazaki T, Watanabe K, Taniguch T, Hibino H, Shinohara H, et al.Direct chemical vapor deposition growth of WS2 atomic layers on hexagonalboron nitride. Acs Nano. (2014) 8:8273–7. doi: 10.1021/nn503093k21. Zhan YJ, Liu Z, Najmaei S, Ajayan PM, Lou J. Large-area vapor-phase growthand characterization of MoS2 atomic layers on a SiO2 substrate. Small. (2012)8:966–71. doi: 10.1002/smll.20110265422. Eichfeld SM, Hossain L, Lin YC, Piasecki AF, Kupp B, Birdwell AG, et al.Highly scalable, atomically thin WSe2 grown via metal-organic chemicalvapor deposition. Acs Nano. (2015) 9:2080–7. doi: 10.1021/nn507328623. Kim H, Ovchinnikov D, Deiana D, Unuchek D, Kis A. Suppressingnucleation in metal-organic chemical vapor deposition of MoS2monolayers by alkali metal halides. Nano Lett. (2017) 17:5056–63.doi: 10.1021/acs.nanolett.7b0231124. Kang K, Xie SE, Huang LJ, Han YM, Huang PY, Mak KF, et al.High-mobility three-atom-thick semiconducting films with wafer-scalehomogeneity. Nature. (2015) 520:656–60. doi: 10.1038/nature1441725. van der Zande M, Huang PY, Chenet DA, Berkelbach TC, You YM,Lee GH, et al. Grains and grain boundaries in highly crystallinemonolayer molybdenum disulphide. Nat Mater. (2013) 12:554–61.doi: 10.1038/nmat363326. Ji HG, Lin YC, Nagashio K, Maruyama M, Solis-Fernandez P, AjiAS, et al. Hydrogen-assisted epitaxial growth of monolayer tungstendisulfide and seamless grain stitching. Chem Mater. (2018) 30:403–11.doi: 10.1021/acs.chemmater.7b0414927. Elibol K, Susi T, O’Brien M, Bayer BC, Pennycook TJ, McEvoy N, et al.Grain boundary-mediated nanopores in molybdenum disulfide grown bychemical vapor deposition. Nanoscale. (2017) 9:1591–8. doi: 10.1039/C6NR08958E28. Karvonen L, Saynatjoki A, Huttunen MJ, Autere A, AmirsolaimaniB, Li S, et al. Rapid visualization of grain boundaries in monolayerMoS2 by multiphoton microscopy. Nat Commun. (2017) 8:15714.doi: 10.1038/ncomms1571429. Yu H, Yang ZZ, Du LJ, Zhang J, Shi JN, Chen W, et al. Precisely alignedmonolayer MoS2 epitaxially grown on h-BN basal plane. Small. (2017)13:1603005. doi: 10.1002/smll.20160300530. Najmaei S, Liu Z, Zhou W, Zou XL, Shi G, Lei SD, et al. Vapour phase growthand grain boundary structure of molybdenum disulphide atomic layers. NatMater. (2013) 12:754–9. doi: 10.1038/nmat367331. Zhou W, Zou XL, Najmaei S, Liu Z, Shi YM, Kong J, et al. Intrinsic structuraldefects in monolayer molybdenum disulfide. Nano Lett. (2013) 13:2615–22.doi: 10.1021/nl400747932. Nilius N, Kulawik M, Rust HP, Freund HJ. Defect-induced gap statesin Al2O3 thin films on NiAl(110). Phys Rev B. (2004) 69:1214011.doi: 10.1103/PhysRevB.69.12140133. Schmid M, Shishkin M, Kresse G, Napetschnig E, Varga P, Kulawik M, et al.Oxygen-deficient line defects in an ultrathin aluminum oxide film. Phys RevLett. (2006) 97:046101. doi: 10.1103/PhysRevLett.97.04610134. Hotta T, Tokuda T, Zhao S, Watanabe K, Taniguchi T, Shinohara H, et al.Molecular beam epitaxy growth of monolayer niobium diselenide flakes. ApplPhys Lett. (2016) 109:133101. doi: 10.1063/1.496317835. Ago H, Endo H, Solis-Fernandez P, Takizawa R, Ohta Y, Fujita Y, et al.Controlled van der Waals epitaxy of mono layer MoS2 triangular domainson graphene. Acs Appl Mater Inter. (2015) 7:5265–73. doi: 10.1021/am508569m36. Horcas I, Fernandez R, Gomez-Rodriguez JM, Colchero J, Gomez-HerreroJ, Baro AM. WSXM: a software for scanning probe microscopy and atool for nanotechnology. Rev Sci Instrum. (2007) 78:013705. doi: 10.1063/1.243241037. Zafar A, Nan HY, Zafar Z, Wu ZT, Jiang J, You YM, et al. Probing theintrinsic optical quality of CVD grown MoS2. Nano Res. (2017) 10:1608–17.doi: 10.1007/s12274-016-1319-z38. Kaplan D, Gong Y, Mills K, Swaminathan V, Ajayan PM, Shirodkar S,et al. Excitation intensity dependence of photoluminescence frommonolayersof MoS2 and WS2/MoS2 heterostructures. 2d Mater. (2016) 3:015005.doi: 10.1088/2053-1583/3/1/01500539. Yamamoto M, Dutta S, Aikawa S, Nakaharai S, Wakabayashi K, Fuhrer MS,et al. Self-limiting layer-by-layer oxidation of atomically thinWSe2.Nano Lett.(2015) 15:2067–73. doi: 10.1021/nl504975340. Lauritsen JV, Kibsgaard J, Helveg S, Topsoe H, Clausen BS, Laegsgaard E,et al. Size-dependent structure of MoS2 nanocrystals.Nat Nanotechnol. (2007)2:53–8. doi: 10.1038/nnano.2006.17141. Kobayashi Y, Taniguchi T, Watanabe K, Maniwa Y, Miyata Y. Slidable atomiclayers in van derWaals heterostructures. Appl Phys Express. (2017) 10:045201.doi: 10.7567/APEX.10.045201Frontiers in Physics | www.frontiersin.org 6 April 2019 | Volume 7 | Article 59https://doi.org/10.1038/nnano.2010.279https://doi.org/10.1103/PhysRevLett.105.136805https://doi.org/10.1039/C4CS00301Bhttps://doi.org/10.1002/adma.201707627https://doi.org/10.1038/nnano.2012.193https://doi.org/10.1021/nn500064shttps://doi.org/10.1021/nn402954ehttps://doi.org/10.1021/nl301335qhttps://doi.org/10.1103/PhysRevLett.108.196802https://doi.org/10.1038/natrevmats.2016.55https://doi.org/10.7566/JPSJ.84.121013https://doi.org/10.1039/C4CS00256Chttps://doi.org/10.1073/pnas.0502848102https://doi.org/10.1002/adma.201104798https://doi.org/10.1021/nl801827vhttps://doi.org/10.1088/2053-1583/3/3/035010https://doi.org/10.1038/s41598-017-00068-0https://doi.org/10.1021/acsnano.5b00103https://doi.org/10.1021/nn503093khttps://doi.org/10.1002/smll.201102654https://doi.org/10.1021/nn5073286https://doi.org/10.1021/acs.nanolett.7b02311https://doi.org/10.1038/nature14417https://doi.org/10.1038/nmat3633https://doi.org/10.1021/acs.chemmater.7b04149https://doi.org/10.1039/C6NR08958Ehttps://doi.org/10.1038/ncomms15714https://doi.org/10.1002/smll.201603005https://doi.org/10.1038/nmat3673https://doi.org/10.1021/nl4007479https://doi.org/10.1103/PhysRevB.69.121401https://doi.org/10.1103/PhysRevLett.97.046101https://doi.org/10.1063/1.4963178https://doi.org/10.1021/am508569mhttps://doi.org/10.1063/1.2432410https://doi.org/10.1007/s12274-016-1319-zhttps://doi.org/10.1088/2053-1583/3/1/015005https://doi.org/10.1021/nl5049753https://doi.org/10.1038/nnano.2006.171https://doi.org/10.7567/APEX.10.045201https://www.frontiersin.org/journals/physicshttps://www.frontiersin.orghttps://www.frontiersin.org/journals/physics#articlesNakanishi et al. Grain Boundaries in MoS242. Mandelli D, Leven I, Hod O, Urbakh M. Sliding friction ofgraphene/hexagonal -boron nitride heterojunctions: a route to robustsuperlubricity. Sci Rep UK. (2017) 7:10851. doi: 10.1038/s41598-017-10522-843. Wang LF, Zhou X, Ma TB, Liu DM, Gao L, Li X, et al. Superlubricity of agraphene/MoS2 heterostructure: a combined experimental and DFT study.Nanoscale. (2017) 9:10846–53. doi: 10.1039/C7NR01451A44. Li H, Contryman AW, Qian XF, Ardakani SM, Gong YJ, Wang XL, et al.Optoelectronic crystal of artificial atoms in strain-textured molybdenumdisulphide. Nat Commun. (2015) 6:7381. doi: 10.1038/ncomms838145. Liu XL, Balla I, Bergeron H, Hersam MC. Point defects andgrain boundaries in rotationally commensurate MoS2 on epitaxialgraphene. J Phys Chem C. (2016) 120:20798–805. doi: 10.1021/acs.jpcc.6b0207346. Huang YL, Chen YF, Zhang WJ, Quek SY, Chen CH, Li LJ, et al. Bandgaptunability at single-layer molybdenum disulphide grain boundaries. NatCommun. (2015) 6:6298. doi: 10.1038/ncomms729847. Kobayashi Y, Yoshida S, Sakurada R, Takashima K, Yamamoto T,Saito T, et al. Modulation of electrical potential and conductivity inan atomic-layer semiconductor heterojunction. Sci Rep. (2016) 6:31223.doi: 10.1038/srep31223Conflict of Interest Statement: The authors declare that the research wasconducted in the absence of any commercial or financial relationships that couldbe construed as a potential conflict of interest.Copyright © 2019 Nakanishi, Yoshida, Murase, Takeuchi, Taniguchi, Watanabe,Shigekawa, Kobayashi, Miyata, Shinohara and Kitaura. This is an open-access articledistributed under the terms of the Creative Commons Attribution License (CC BY).The use, distribution or reproduction in other forums is permitted, provided theoriginal author(s) and the copyright owner(s) are credited and that the originalpublication in this journal is cited, in accordance with accepted academic practice.No use, distribution or reproduction is permitted which does not comply with theseterms.Frontiers in Physics | www.frontiersin.org 7 April 2019 | Volume 7 | Article 59https://doi.org/10.1038/s41598-017-10522-8https://doi.org/10.1039/C7NR01451Ahttps://doi.org/10.1038/ncomms8381https://doi.org/10.1021/acs.jpcc.6b02073https://doi.org/10.1038/ncomms7298https://doi.org/10.1038/srep31223http://creativecommons.org/licenses/by/4.0/http://creativecommons.org/licenses/by/4.0/http://creativecommons.org/licenses/by/4.0/http://creativecommons.org/licenses/by/4.0/http://creativecommons.org/licenses/by/4.0/https://www.frontiersin.org/journals/physicshttps://www.frontiersin.orghttps://www.frontiersin.org/journals/physics#articles The Atomic and Electronic Structure of 0 and 60 Grain Boundaries in MoS2 Introduction Methodology Results and Discussion Conclusion Author Contributions Funding Supplementary Material References