ORIGINAL RESEARCH
published: 17 April 2019
doi: 10.3389/fphy.2019.00059

The Atomic and Electronic Structureof 0◦ and 60◦ Grain Boundaries in
MoS2
Terunobu 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*
Department 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,
4
Department of Physics, Tokyo Metropolitan University, Hachioji, Japan

1

Edited by:
Sabina Botti,
Italian National Agency for New
Technologies, Energy and Sustainable
Economic Development (ENEA), Italy
Reviewed by:
Fausto Sirotti,
École Polytechnique, France
Niklas Nilius,
University of Oldenburg, Germany*Correspondence:
Ryo Kitaura
r.kitaura@nagoya-u.jp
Specialty section:
This article was submitted toCondensed Matter Physics,
a section of the journal
Frontiers in Physics
Received: 25 January 2019
Accepted: 27 March 2019
Published: 17 April 2019
Citation:
Nakanishi T, Yoshida S, Murase K,
Takeuchi O, Taniguchi T, Watanabe K,
Shigekawa H, Kobayashi Y, Miyata Y,
Shinohara H and Kitaura R (2019) The
Atomic and Electronic Structure of 0◦
and 60◦ Grain Boundaries in MoS2 .
Front. Phys. 7:59.
doi: 10.3389/fphy.2019.00059

Frontiers in Physics | www.frontiersin.org

We have investigated atomic and electronic structure of grain boundaries in monolayer
MoS2 , 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 different
grains of MoS2 . Through scanning tunneling microscopy (STM) and spectroscopy
measurements, we have found that the perfectly stitched structure between two different
grains of MoS2 was realized in the case of the 0 degree grain boundary. We also
found 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)

INTRODUCTION
A post-graphene material, transition metal dichalcogenide (TMD), has recently attracted a great
deal of attention. TMDs have a long research history, but research on properties of monolayer
TMDs, 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-based
devices, including high-performance FET devices, light-emitting transistors, and photodetectors,
have actually been demonstrated [5–7]. In conjunction with the flexibility arising from the ultrathin
structure, 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 novel
electronic 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 growth
by 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]. In
typical CVD growth of TMDs, solid sources such as metal oxides and elemental sulfur are used, and1April 2019 | Volume 7 | Article 59

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adhesive tape (Scotch tape, 3M). As precursors for growth of
MoS2 , we used molybdenum trioxide (Sigma-Aldrich, 99.5%
purity) and sulfur powder (Sigma-Aldrich, 99.98% purity).
Furnace temperatures at the locations where molybdenum
trioxide and elemental sulfur were placed were set to 1,029 K
and 473 K, respectively, and the growth of MoS2 was carriedout at 1,373 K for 20 min under Ar flow with a flow rateof 200 sccm. Atomic force microscope (AFM) observations
were performed by the Veeco AFM system (Dimension
3100SPM, Nanoscope IV) operated at a scanning rate of
1 Hz. We measured photoluminescence (PL) spectra by amicrospectroscopy system with a confocal microscope (Jobin
Yvon HR-800, Horiba) with an excitation laser wavelength of
488 nm. For PL imaging, an LED light source (Mightex GCS6500-15) was used to illuminate samples, and PL intensity(λ > 600 nm) was imaged with CCD (Princeton Instruments
PIXIS-1024BR-eXelon). We formed electrical contacts to samples
for STM/STS measurements by deposition of gold thougha shadow mask or patterning conductive silver paste. Aftermaking the electrical contact, samples were introduced to an
ultrahigh vacuum (UHV) environment and degassed at 473 K.
The STM/STS measurements were conducted using a scanningtunneling microscope (Omicron LT–STM) in constant current
mode operated at 90 K with an electrochemically etched W tip
coated with PtIr (UNISOKU Co., Ltd.). A numerical derivative
was used to acquire dI/dV curves, and WSxM software was used
to process the STM images [36].

monolayer TMDs film with a lateral size of millimeters havebeen reported [19, 21]. Recently, the growth of TMDs by metalorganic CVD (MOCVD) with volatile liquid sources has beensuccessfully demonstrated, and MOCVD is a promising method
to realize wafer-scale TMDs that are compatible with device
applications [22–24].
In CVD-grown large-area TMDs, grain boundaries (GBs) are
inevitably 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 TMD with GBs.
Because the orientation of nuclei is normally random, a wide
variety of GB structures can be formed. For example, a GB with 75 and 8-4-4 membered rings forms in a CVD-grown MoS2 , where
midgap boundary states appear [25, 29–31]. The existence of GBinduced midgap states significantly affects electronic transport
across the boundary, leading to reduction of carrier mobility via
additional carrier scattering at the GB [25]. Therefore, control of
GB structure by controlling grain orientation and understanding
the 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 using
scanning tunneling microscopy (STM) and scanning tunnelingspectroscopy (STS); the STS is a powerful tool to investigate
domain boundaries [32, 33]. The key for the successful controlof crystal orientation in CVD growth of TMDs is the interaction
between TMDs and the substrates used in CVD processes.
In conventional CVD growth of TMDs, SiO2 /Si substrates
with amorphous surfaces are used, leading to random crystal
orientations of grown TMDs. In contrast, substrates withcrystalline structures can limit the crystal orientation of grown
TMDs through TMD-substrate interactions [20, 29, 34, 35]. For
the control of crystal orientation, we used hBN and graphite
as substrates for CVD growth of TMDs. The atomically flat
surfaces with three-fold (hBN) and six-fold rotation (graphite)
symmetries successfully limited crystal orientations of grown
TMD flakes; only two different orientations were observed. GBs
between MoS2 flakes with different orientations (relative angleof 60◦ ) shows boundary states localized at specific location near
the Fermi level. On the other hand, a GB between MoS2 flakes
with the same orientation shows a perfectly-stitched structure
without any defects in scanned areas in STM images. We also
found that both the conduction band minimum (CBM) and thevalence band maximum (VBM) shift to the higher energy side at
the GB even with a perfectly-stitched structure. This means that
the 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.

RESULTS AND DISCUSSION
PL imaging and spectroscopy have clearly shown that the qualityof the present samples is high. Figure S1 shows a typical PL
image of MoS2 /hBN and typical PL spectrum of MoS2 /hBN
and 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 spectra
are 35∼45 meV, which are much smaller than those of samples
exfoliated onto SiO2 substrates [37, 38]. These PL spectra clearly
demonstrate 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 der
Waals 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 hexagonal
shape with long and short facets, and their orientations arelimited to only two different ones, where 60◦ rotation of
one orientation matches the other orientation. The observed
long and short facets in the crystals correspond to chalcogenand metal zigzag edges; the relationship between crystal
shape and crystallographic orientation was investigated withtransmission electron microscopy and electron diffraction(additional information is given in Figure S2). Figure 1B shows
structural models of hexagonal MoS2 flakes with the two different
orientations. The limited orientations of MoS2 flakes are also
observed in MoS2 flakes grown on graphite substrates. This
clearly demonstrates that the orientation-dependent potential

METHODOLOGY
We grew monolayer MoS2 on hBN and graphite (Kish graphite,
type C, Covalent Materials) flakes exfoliated on quartz substrates
with a multi-furnace CVD apparatus. We prepared hBN andgraphite flakes by the mechanical exfoliation method with

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FIGURE 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 two
grains 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 linear
contrasts in Figures 1A,C,F are wrinkles in hBN flakes.

of states from its peripheral place. For detailed investigationsof structure and electronic structure, atomic-resolution STM
observation of the GB-0◦ was carried out. Figure 2B is a STM
image 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 STM
topographic image, and we observed no defects at the GB-0◦ ;neither vacancies nor insertion of atomic rows are seen. The
well-stitched structure of GB-0◦ revealed by STM observationis consistent with its observed low reactivity toward oxidation
reactions. It should be noted that a translational mismatch should
exist at the boundary even with orientation matching between
two grains. This result, however, clearly demonstrates that GB-0◦
has a stitched structure without defects, indicating that most of
the 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 electronic
structure at GB-0◦ is different from that of its peripheral places.
To see the differences in the electronic structures, we carried out
STS 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 the
GB-0◦ . Figure 2D is a STS spectrum at GB-0◦ , showing that the
upward 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◦ fromarising from crystalline substrates is crucial to limiting the crystal
orientation of grown MoS2 .
Because the crystal orientation of MoS2 on hBN is limited,
the resulting structure of the GBs should also be limited: GBs
between grains with the same orientation (GB-0◦ ) and grains
with 60◦ mutual orientation (GB-60◦ ). To investigate if defects
exist at such GBs, we investigated the reactivity for an oxidation
reaction. MoS2 flakes with GBs were heated at 573 K 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, positionsensitive detection of oxidation of MoS2 can easily be done
through 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 Mo
zigzag 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 pristine
structure. This means that GB-0◦ does not have defects that are
sensitive 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 of
GB-0◦ are highlighted by arrows; we confirmed the monolayer
structure 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 density

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FIGURE 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 GB
and on the GB.

without noticeable distortion. In addition, we performed fast
Fourier 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 FFT
image at a corresponding peripheral place; line profiles along
the green arrows in the FFT image at GB-0◦ and the peripheralplace also coincide well. This means that the difference in lattice
constants at the GB-0◦ and its peripheral place is less than theexperimental resolution (2%). As discussed above, the observed
difference in bandgap at GB-0◦ and peripheral places is 0.4 eV.
Even though we assume that the difference in bandgap originates
only from lattice strain, the strain should be comparable to theexperimental resolution, and it is difficult to image the strain
directly [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 of
GB-0◦ . This suggests that it is important to grow large single
crystal of TMDs without any boundaries for future application
with high-mobility TMD films.
In the case of GB-60◦ , structural defects exist and the local
electronic structure is strongly modified. Figure 3A is a STM
image of MoS2 on graphite near the GB-60◦ . The vertical linear
contrast at the middle of the image corresponds to an impurity
attached at GB-60◦ , where strong binding sites for impurities

2.2 to 1.8 eV; the observed bandgap of pristine monolayer MoS2
(2.2 eV) is consistent with the value reported previously: 2.15–2.4
eV [44–46].
Because GB-0◦ has a stitched structure, this upward shiftcannot be explained by formation of defects-mediated midgap
states and can probably be explained by the local strain
at the GB-0◦ . The bandgap of monolayer MoS2 is very
sensitive to strain, and strain causes bandgap narrowing throughupward/downward shift of VBM/CBM [46]. The observed
modulation of bands, however, is upward shift in both CB and
VB, which probably originates from accumulation of electrons at
the boundary. This discrepancy can be understood if piezoelectriccharge is taken into account [47]. As monolayer MoS2 has a noncentrosymmetric structure, the local strain can induce charge
accumulation at the GB-0◦ , leading to the observed upward
shift of CBM and VBM. One important implication is that asmall strain, which probably arises from residual translational
mismatch even after the sliding-based relaxation, remains at
GB-0◦ , where the local electronic structure is strongly altered.
To investigate the degree of strain at GB-0◦ , we performed
detailed image analyses with the high-resolution STM image
shown in Figure 2B. Figure S4 shows a contrast-enhanced STM
image after applying high-pass filter to filter out the lowfrequency noise. It is clear that there are no atomic defects at
GB-0◦ . A line profile along the yellow line clearly demonstrates
that location of bright spots in the STM image align periodically

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FIGURE 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 −1.539 V,
respectively.

GB. The band structure in GB-60◦ upshifts and localized states
appear. In the case of two grains of MoS2 stitched at the sameangle, the electronic structure of GB-0◦ is modified due to local
stress and carrier accumulation. It will be a challenge to makea MoS2 sheet without modulation of the electronic state or
the structure.

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 about
20◦ . Figure 3C shows the STS spectral mapping along the blueline in Figure 3B. As clearly seen in Figure 3C, the electronic
structure is significantly modulated at GB-60◦ , where both CBM
and VBM upshift to reduce the bandgap from 2.3 to 1.9 eV. To
investigate the spatial distribution of the boundary state at GB60◦ , we performed dI/dV mapping at three different bias voltagesof 0.83, −0.78, and −1.54 V. Figures 3D–F show the observed
dI/dV mappings at bias voltages of 0.83, −0.78, and −1.54 V,
respectively. As clearly seen, the boundary state strongly localizes
at GB-60◦ . In addition, the boundary state corresponding to abias voltage of −0.78 V shows a dotted distribution rather thana linear uniform distribution, and this means that the boundary
state originates from a specific defect site existing at the GB-60◦ .

AUTHOR CONTRIBUTIONS
TN prepared MoS2 samples. TN, KM, and SY performed
STM/STS measurements. YK and YM performed opticalcharacterizations of MoS2 samples. TT and KW synthesized hBN
crystals. RK designed experiments and wrote the paper. OT, HidS,
and HisS discussed the results and reviewed the manuscript.

FUNDING
This work was supported by JSPS KAKENHI Grant numbers
JP16H06331,
JP16H03825,
JP16H00963,
JP15K13283,
JP25107002, and JST CREST Grant Number JPMJCR16F3.
KW and TT acknowledge support from the Elemental Strategy
Initiative conducted by the MEXT, Japan and the CREST(JPMJCR15F3), JST.

CONCLUSION
In this paper, electronic properties and defect densities in
two types of GBs in MoS2 grown by the CVD process were
investigated. 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 is
confirmed 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 clearly
imaged by STM/STS without the disturbance of adsorbates on

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SUPPLEMENTARY MATERIALThe Supplementary Material for this article can be foundonline at: https://www.frontiersin.org/articles/10.3389/fphy.
2019.00059/full#supplementary-material

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Conflict of Interest Statement: The authors declare that the research was
conducted in the absence of any commercial or financial relationships that could
be 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 article
distributed 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 original
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