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Article
Interfacial Interactions in van der Waals
Heterostructures of MoS2 and Graphene
Hai Li, Jiang-Bin Wu, Feirong Ran, Miao-Ling Lin, Xue-Lu Liu, Yanyuan Zhao,
Xin Lu, Qihua Xiong, Jun Zhang, wei Huang, Hua Zhang, and Ping-Heng Tan
ACS Nano, Just Accepted Manuscript • DOI: 10.1021/acsnano.7b07015 • Publication Date (Web): 25 Oct 2017
Downloaded from http://pubs.acs.org on October 25, 2017
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ACS Nano is published by the American Chemical Society. 1155 Sixteenth Street
N.W., Washington, DC 20036
Published by American Chemical Society. Copyright © American Chemical Society.
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Interfacial
Interactions
in
van
der
Waals
Heterostructures of MoS2 and Graphene
Hai Li,1,2,† Jiang-Bin Wu,3,† Feirong Ran,1 Miao-Ling Lin,3 Xue-Lu Liu,3 Yanyuan
Zhao,4 Xin Lu,4 Qihua Xiong,4,5 Jun Zhang,3,6 Wei Huang,1,7,8 Hua Zhang,2*
Ping-Heng Tan3,6*
1
Key Laboratory of Flexible Electronics (KLOFE) and Institute of Advanced Materials
(IAM), Jiangsu National Synergetic Innovation Center for Advanced Materials
(SICAM), Nanjing Tech University (NanjingTech), 30 South Puzhu Road, Nanjing
211816, P.R. China
2
Center for Programmable Materials, School of Materials Science and Engineering,
Nanyang Technological University, 50 Nanyang Avenue, Singapore 639798,
Singapore
3
State Key Laboratory of Superlattices and Microstructures, Institute
Semiconductors, Chinese Academy of Sciences, Beijing 100083, China
of
4
Division of Physics and Applied Physics, School of Physical and Mathematical
Sciences, Nanyang Technological University, 21 Nanyang Link, Singapore 637371,
Singapore
5
NOVITAS, Nanoelectronics Centre of Excellence, School of Electrical and Electronic
Engineering, Nanyang Technological University, Singapore 639798, Singapore
6
CAS Center of Excellence in Topological Quantum Computation, University of
Chinese Academy of Sciences, Beijing 100190, China
7
Key Laboratory for Organic Electronics and Information Displays & Institute of
Advanced Materials (IAM), SICAM, Nanjing University of Posts &
Telecommunications, 9 Wenyuan Road, Nanjing 210023, China.
8
Shaanxi Institute of Flexible Electronics (SIFE), Northwestern Polytechnical
University (NPU), 127 West Youyi Road, Xi'an 710072, China
†
These authors contributed equally to this work.
*To whom correspondence should be addressed. E-mail: hzhang@ntu.edu.sg;
phtan@semi.ac.cn
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Abstract
Interfacial coupling between neighboring layers of van der Waals heterostructures
(vdWHs), formed by vertically stacking more than two types of two-dimensional
materials (2DMs), greatly affects their physical properties and device performance.
Although high-resolution cross-sectional scanning tunneling electron microscopy can
directly image the atomically sharp interfaces in the vdWHs, the interfacial coupling
and lattice dynamics of vdWHs formed by two different types of 2DMs, such as
semimetal and semiconductor, are not clear so far. Here, we report the
ultralow-frequency Raman spectroscopy investigation on the interfacial interactions in
the vdWHs formed by graphene and MoS2 flakes. Due to the significant interfacial
coupling between MoS2 and graphene flakes, a series of layer breathing modes with
frequencies dependent on their layer numbers are observed in the vdWHs, which can
be described by the linear chain model. It is found that the interfacial interlayer
breathing force constant between MoS2 and graphene, (I) = 60 × 10 N/m , is
comparable with the interlayer breathing force constant of multi-layer MoS2 and
graphene. The results suggest that the interfacial layer-breathing couplings in the
vdWHs formed by MoS2 and graphene flakes are not sensitive to their stacking order
and twist angle between the two constituents. Our results demonstrate that the
interfacial interlayer coupling in vdWHs formed by two-dimensional semimetals and
semiconductors can lead to new lattice vibration modes, which not only can be used
to measure the interfacial interactions in vdWHs but also is beneficial to
fundamentally understand the properties of vdWHs for further engineering the
vdWHs-based electronic and photonic devices.
KEYWORDS: two-dimensional materials (2DMs), van der Waals heterostructures
(vdWHs), interfacial interaction, lattice dynamics, ultralow-frequency Raman
spectroscopy
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Vertical van der Waals heterostructures (vdWHs) are formed by vertically stacking
various two-dimensional materials (2DMs) by van der Waals (vdW) forces but
without any constraint of lattice matching and fabrication compatibility. vdWHs
offer promising properties and intriguing possibilities for controlling and
manipulating charge carriers, excitons, photons and phonons within these atomic
interfaces, facilitating the design of unique electronic and photonic devices.1-16
Normally, monolayer graphene and transition metal dichalcogenides (TMDs) are two
types of essential building blocks for vdWHs.
Recently,
the maximum
photon-to-electron conversion external quantum efficiency of 12% was achieved in
the MoS2/WSe2 vdWHs.17 The photoluminescence (PL) peak with a large
Stokes-like shift was also observed in the MoS2/WSe2 hetero-bilayer due to the
electronic interaction.18 Graphene is not suitable for channel material in transistor
due to the lack of energy gap. TMDs are not suitable for high-speed electronic devices
due to their low mobility. However, the TMD/graphene vdWHs have been used for
various high-performance devices, such as field-effect tunneling transistors, logic
transistors, photovoltaic and memory devices,2, 4, 7, 19-23 by taking advantages of the
high mobility of graphene and the natural band gap of TMDs. In these devices,
graphene is used as an electrode and also introduces a barrier. Until now, monolayer
and multilayer graphene sheets have been widely used as electrodes in vdWH-based
devices to achieve higher performance compared to devices with directly deposited
metal contacts.2, 19-20, 23 As known, these metal contacts can introduce defects into the
2DMs in devices, resulting in Fermi-level pinning, a finite Schottky barrier and large
excessive contact resistance in the two-dimensional semiconductor (2DS)/metal
heterostructures. Alternatively, by integrating graphene and ultrathin 2DS flakes
with a proper process to form 2DS/graphene vdWHs, an atomically sharp and
non-damaging interface can be obtained to minimize defects and prevent the
Fermi-level pinning, leading to a much lower Schottky barrier height between
graphene and 2DS. These advantages guarantee the realization of excellent
performances of the vdWHs-based devices. The van der Waals coupling at the
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interface of vdWHs is mainly dependent on the distance, twist angle and stacking
order of disparate 2DMs.24-27 Therefore, it is very important to investigate the
interface
coupling
in
vdWHs
for
their
future
high-performance
device
applications.18, 25, 28-29
Raman spectroscopy is widely used to monitor the crystal quality, polytypism,
doping, defects, strain, disorder, chemical modification and relative orientation of
2DM flakes.30-34 Each Raman peak is characterized by the peak position (Pos), full
width at half maximum (FWHM) and intensity (I). The in-plane vibration modes of
2DMs are very sensitive to their doping level, disorder and chemical modifications,
but they are not sensitive for probing the interface coupling. Recently, the
ultralow-frequency (ULF, < 50 cm-1) Raman spectroscopy has been proven to be a
versatile tool for probing the interface coupling, stacking order and layer number of
2DM flakes.31-33, 35-40 In this work, by using Raman spectroscope, we systematically
study the interfacial interactions in MoS2/graphene vdWHs formed by stacking
mechanically exfoliated single- and few-layer MoS2 and graphene flakes. Although
the high-frequency Raman modes above 100 cm-1 have been used as fingerprints of
van der Waals interactions in MoS2/graphene vdWHs,27 we found that this
assignment is inconvincible since they are also sensitive to the strain and doping.
Here, we demonstrate that the peak position of layer-breathing (LB) mode in the
ULF range, i.e., Pos(LB), is very sensitive to the layer number of both MoS2 and
graphene flakes, proving the existence of the interfacial LB coupling and good
quality of the interface of graphene on MoS2 (graphene/MoS2/substrate, referred to
as graphene/MoS2) and MoS2 on graphene (MoS2/graphene/substrate, referred to as
MoS2/graphene). Our improved linear chain model (LCM) indicates that the LB
force
constant
of
MoS2/graphene
vdWHs
at
the
interface
is
αLB(MoS2/graphene)=60×1017 N/m2, which is comparable with the LB force constant
of multi-layer MoS2 and graphene flakes.
Results and Discussion
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We denote the m-layer MoS2 (mLM) deposited on the n-layer graphene (nLG) as
mLM/nLG, and the nLG deposited on the mLM as nLG/mLM, so the total layer
number of the vdWH is N = m + n. The shear (S) and LB modes exist in multilayer
flakes. The shear mode in the ULF range was previously referred to as the C mode in
(twisted) multilayer graphenes (MLG)31-32,
38
because it provided a direct
measurement of the interlayer coupling in MLG.38 Here, as for a general notation for
interlayer vibration modes in multilayer flakes and vdWHs, we denote the shear
mode as the S mode in the ULF range. For a given n-layer graphene or MoS2 flake,
there are n-1 S or LB modes (n > 1), which are denoted as Snn-i and LBnn-i (i = 1, 2,
..., n-1), respectively, where, Sn1 and LBn1 are the S and LB modes of pristine
n-layer graphene or MoS2 flakes with the highest frequencies below 130 cm-1,
respectively.31-32,
39
The m and n numbers of nLG and mLM constituents in
nLG/mLM vdWHs can be directly confirmed by the corresponding Pos(S) and/or
Pos(LB)31-32, 38-39 and/or the corresponding optical contrast measurement of prinstine
mLM and nLG before transferring.41-43
The 2LM/1LG vdWHs were first investigated. Figure 1a schematically shows the
preparation of MoS2/graphene vdWHs (see Figure 1b for 2LM/1LG). The optical
microscopy image of as-transferred MoS2/graphene vdWHs with layer numbers is
shown in Figure 1c. The as-transferred MoS2/graphene vdWHs were annealed at
different time (5, 30 and 60 min) in Ar environment at 300 ∘C to improve the
interfacial contact.44 The exfoliated pristine 2LM and 1LG flakes close to the
as-transferred 2LM/1LG vdWHs (denoted as as-transferred 2LM and 1LG) are used
as reference in this study. Figure 1d shows the Raman spectra of the 2LM/1LG
vdWHs and the reference samples in the S, LB, A1g and E
peak spectral ranges.
As previously reported,45-47 the mode assignments of bulk TMDs are used to assign
the corresponding modes in monolayer and few-layer MoS2. Figure 1e shows the
Raman spectra of the 2LM/1LG vdWHs and the reference samples in the G and 2D
peak spectral ranges.
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Pos(G) and Pos(2D) of 1LG are highly sensitive to their doping level.48 The doping,
both n- and p-doping, can make the G peak blue-shift, and the doping level (Fermi
level) is positively correlated to Pos(G).49 Furthermore, p-doping and n-doping can
result in the blue-shift and red-shift of Pos(2D), respectively.48-49 For the
as-transferred 1LG, the Pos(G) and Pos(2D) exhibit blue-shifts of 14 and 15 cm-1,
respectively (Figure 1e). The FWHM of the G mode, FWHM(G), is 10.4 cm-1, about
1 cm-1 narrower than that of pristine 1LG, which also results from the doping effect.
When 2LM and 1LG were combined to form as-transferred 2LM/1LG vdWHs,
besides the doping induced by the moisture and impurity during the transfer process,
the charge transfer between two constituents of 2LM/1LG vdWHs can also induce
the additional doping. Previous theoretical calculation demonstrated that 1LG loses
electrons and becomes p-doping during the formation of MoS2/Graphene vdWHs.50
However, if the p-doping level of 1LG is too high, additional electrons can also be
transferred from MoS2 to 1LG,51 which makes Pos(G) of MoS2/Graphene vdWHs
lower than that of 1LG. As shown in Figure 1e, Pos(G) of as-transferred 2LM/1LG
vdWHs is almost equal to that of as-transferred 1LG. This indicates that the charge
transfer between 2LM and 1LG can be ignored for Pos(G) of as-transferred
2LM/1LG vdWHs due to the weak interface coupling in as-transferred
MoS2/graphene vdWHs.27
During the fabrication of two-dimensional heterostructures, adsorbates, such as
water, hydrocarbons, etc., are easily adsorbed on the surfaces of 2DMs, resulting in
the doping of 2DMs and weakening the van der Waals coupling at the interface of
vdWHs.27, 52 Normally, annealing is used to remove the moisture and impurities and
thus increase the interfacial quality of vdWHs,44 i.e., less doping of constituents,
enhancement of the interfacial coupling of heterostructures, and change the peak
profile, intensity and position of Raman modes of two constituents. Indeed, as shown
in Figure 1e, with increasing the annealing time from 5 to 60 min, Pos(G) of
annealed 2LM/1LG vdWHs monotonously decreases in frequency from 1597 cm-1 to
1587 cm-1. The interfacial coupling between 2LM and 1LG should be a weak vdW
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interaction and has small effect on Pos(G) and Pos(2D). This red-shift of the G mode
should result from the less doping after removal of moisture and impurity absorbed on
1LG surfaces by annealing. Moreover, I(2D)/I(G) of 2LM/1LG vdWHs also
increases with increased annealing time, indicating the decreased doping level in
1LG.49, 53 However, the annealing effect on Pos(2D) in 2LM/1LG vdWHs is quite
different from that of Pos(G). After being annealed for 30 min, Pos(2D) of
2LM/1LG vdWHs shows a blue-shift of 8 cm-1 compared to that of as-transferred
2LM/1LG. While with increasing the annealing time to 60 min, the Pos(2D) of
2LM/1LG vdWHs shows a red-shift of 3 cm-1 compared to that of as-transferred
2LM/1LG vdWHs. In addition, the lineshape is also dependent on the annealing
time. As-transferred 2LM/1LG vdWHs show asymmetrical 2D peak. While 2D peak
in annealed 2LM/1LG vdWHs becomes from asymmetrical to symmetrical with
annealing time increased from 5 min to 60 min. Aforementioned results indicate
annealing is necessary to increase the quality of interfacial contact.
As known, Pos(E
) and Pos(A1g) of TMD flakes are also sensitive to their doping
level.33 The n-doping and p-doping can make the E
and A1g modes red-shift and
blue-shift,54 respectively. However, such effect on the E
mode is much weaker
than that on the A1g mode.33,
55-58
Compared to the pristine 2LM, Pos(E
) and
Pos(A1g) in as-transferred 2LM exhibit shifts of +0.2 and +0.4 cm-1, respectively,
indicating its p-doping by moisture and impurity induced in the transfer process.
However, Pos(E
) and Pos(A1g) of as-transferred 2LM/1LG exhibit shift of −0.2
and +1.3 cm-1 relative to those of pristine 2LM, respectively. Besides the
doping-induced shift, the interfacial coupling in MoS2/graphene vdWHs can also
affect Pos(A1g). As previously reported, Pos(A1g) of 1LM and 2LM can blue-shift
∼2 cm-1 after the formation of MoS2/graphene vdWHs,27 which is comparable or
larger to the shift by doping. This makes it difficult to evaluate the interface quality
of MoS2/graphene vdWHs by the shift of Pos(E ) and Pos(A1g). Indeed, in our
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work, there is no discipline for the shift of Pos(E
) and Pos(A1g) with the annealing
time.
The S and LB modes are proved to be sensitive to the interlayer coupling of
2DMs38-39 and also the interfacial coupling of two-dimensional heterostructures.31-32,
35-37, 59
Figure 1d shows the S and LB modes in pristine and as-transferred 2LM,
referred to as S21 and LB21, respectively. Similar S and LB modes are also observed
in the as-transferred 2LM/1LG vdWHs and those annealed. The Pos(S) in
as-transferred 2LM/1LG vdWHs is identical to the Pos(S21) in pristine 2LM. Indeed,
the polarized Raman measurements (Figure S1) on the annealed 2LM/1LG vdWHs
show that the S mode is present in both parallel (XX) and cross (XY) polarizations
while the LB mode vanishes in the XY configuration, analogy to the polarization
behaviors of the S21 and LB21 modes in pristine 2LM.33 The lineshape and peak
position of the S mode of 2LM/1LG vdWHs are found to be dependent on the
annealing time. A proper annealing of 30 min can make the Pos(S) in annealed
2LM/1LG vdWHs identical to the Pos(S21) in as-transferred one. Shorter or longer
annealing time will make the S mode weaker in intensity and red-shift in peak
position. The E
and A1g modes in 2LM/1LG vdWHs annealed for 30 min are also
identical to those in as-transferred one, further indicating 30 min annealing time is
optimal.
In contrast to the S mode, the LB mode in 2LM/1LG vdWHs exhibited different
behavior under annealing. Compared to pristine 2LM, the LB mode in as-transferred
2LM/1LG vdWHs exhibits a red-shift in frequency with a broad profile, indicating
that the transfer process significantly changes the LB coupling due to the presence of
moisture and impurity. With increasing the annealing time, Pos(LB) monotonously
decreases in frequency. For 2LM/1LG vdWHs annealed for 30 min, although its
Pos(S) is identical to those of as-transferred one and pristine 2LM, its Pos(LB)
red-shifts about 1.4 and 2.7 cm−1, respectively, relative to Pos(LB) of as-transferred
one and Pos(LB21) of pristine 2LM. If the annealing time for 2LM/1LG vdWHs was
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increased to 60 min, the S and LB modes became weaker in intensity and red-shifted
0.5 and 1.6 cm−1 relative to that with annealing for 30 min, respectively.
Simultaneously, its I(E
) and I(A1g) became weaker than that with annealing for 30
min, suggesting that longer annealing time makes the quality of 2LM/1LG vdWHs
worse. The Pos(LB), Pos(G) and (G)/(2D) of this 2LM/1LG vdWHs show
homogeneous Raman mapping image (Figure S2), indicating uniform interfacial
coupling of this 2LM/1LG vdWHs and uniform doping of the 1LG constituent. The
good heterointerfaces of the vdWHs can be further confirmed by the atomic force
microscope (AFM) image of the samples after proper annealing. A 2LM flake was
transferred onto a 2LG flakes to form a 2LM/2LG vdWH, as shown by its optical
image in Figure 2a. Figure 2b shows that the height of the as-transferred 2LM flake
on the 2LG flake was measured as 2.3nm by AFM. This value is much larger than
the expected thickness of 2LM, which results from the defects or organic molecular
absorbed between the heterointerface. However, after annealing at 300 oC for 30
minutes, as depicted in Figure 2c, the height of the 2LM flake on the 2LG flake was
decreased to 1.3 nm, which is almost identical to the thickness of pristine 2LM. Two
LB modes were observed in the annealed 2LM/2LG, in contrast to one LB21 mode
observed in pristine 2LM flake, as demonstrated in Figure 2d. This indicates that the
good heterointerface of 2LM/2LG generates new interlayer LB modes, different
from those only present in 2LM or 2LG constituents. This annealing condition was
further used in other MoS2/graphene vdWHs.
In order to understand the interface S and LB coupling of MoS2/graphene vdWHs,
1-5L MoS2 (referred to as mLM, m=1-5) flakes were transferred onto 1LG followed
by annealing for 30 min in Ar at 300 oC to form the mLM/1LG vdWHs. Their
Raman spectra are shown in Figure 3a. For a comparison, the Raman spectra of the
corresponding pristine mLMs are plotted by the gray-dashed curves. Pos(A1g)
difference between mLM/1LG vdWHs and the corresponding mLM decreases with
increased m, so does the E
mode. This indicates that the perturbation of 1LG to
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the mLMs constituent becomes smaller and smaller with increasing m. This
perturbation makes Pos(A1g) of the mLM constituent blue-shift and Pos( E
)
red-shift relative to those of the pristine mLM. The S mode between 1LM and 1LG
is not observed in the Raman spectrum of 1LM/1LG vdWHs in ULF range, which is
in good agreement with previous results on twisted bilayer graphene (t2LG).31-32, 59
The S modes of mLM/1LG vdWHs and the Sm1 modes of mLMs (m > 1) are
observed, and Pos(S) of mLM/1LG vdWHs is almost identical to Pos(Sm1) of mLM
for each m (m > 1), which indicates that the observed S mode in mLM/nLG vdWHs
(m > 1) is mainly localized in mLM constituent. The weaken interfacial shear
coupling had also been observed in other mLM/nLG vdWHs, as discussed later.
Thereafter, the Sm1 mode of the mLM constituent is used to denote the observed S
mode in mLM/nLG vdWHs.
In contrast to the observation of the LB mode in t2LG, twisted bilayer MoS2,
MoS2/WSe2 and MoSe2/MoS2 heterobilayers,31,
35-37, 59
the LB mode cannot be
observed in 1LM/1LG vdWHs. This is reasonable because TMD and graphene
flakes are two quite different systems and the electron-phonon coupling (EPC) of the
LB modes in graphene flakes are very weak.31 Similar to the case of 2LM/1LG
vdWHs, the LB mode had been observed in mLM/1LG vdWHs (m=3, 4, 5). In some
vdWHs, like 3LM/1LG and 5LM/1LG, the LB peaks are overlapped with the S
peaks. To get the intrinsic LB modes in mLM/1LG vdWHs, the Raman spectrum
under the XY polarization is subtracted from that under XX polarization and is
shown in Figure 3b for each mLM/1LG vdWHs and also each pristine mLM. The
LBmm−1 mode of each pristine mLM (m > 1) is observed and shown in Figure 3b by
gray dashed curve. The corresponding LB modes of mLM/1LGs (m > 1) are also
clearly observed. The LB mode of 3LM/1LG vdWHs is observed at 26.8 cm−1,
which is 1.4 cm−1 lower than Pos(LB32) of the pristine 3LM. Such red-shift of the LB
mode in mLM/1LG vdWHs relative to Pos(LBmm−1) of pristine mLMs decreases with
increasing m of mLM, from 2.7 cm−1 for 2LM/1LG to 0.5 cm−1 for 5LM/1LG
vdWHs. There are m-1 LB modes in mLM, in which the LBmm-1 mode can be
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observed with the most intense intensity.39
Pos(LBmm-1) becomes smaller with
increasing m of mLM. The formation of mLM/1LG vdWHs leads to m+1 monolayer
constituents and m LB modes. Similar to the case in mLM, the LBm+1m mode should
be observed in mLM/1LG vdWHs as confirmed later, therefore, its frequency will be
smaller than Pos(LBmm-1) of mLM. Because the mass per unit area of 1LG is about
1/4 of that of 1LM, this redshift of the LB mode in mLM/1LG vdWHs is smaller
than that of the LBmm-1 mode of mLM to the LBm+1m mode of (m+1)LM. Moreover,
similar to mLM again, with increasing m of mLM, the relative shift of Pos(LB)
becomes smaller. Frequency differences of Pos(E
), Pos(A1g), Pos(S) and Pos(LB)
between mLM/1LG vdWHs and pristine mLMs are summarized in Figure 3c. It
shows that the LB modes are more sensitive to the interfacial coupling of mLM/1LG
(m > 1) than the E
and A1g mode, although Pos(LB) is only about 1/10 of Pos(E
)
and Pos(A1g).
Before systematically revealing interfacial LB coupling of mLM/nLG vdWHs, we
first investigate 2LM/nLG vdWHs, i.e., 2LM flakes were transferred on top of 1LG
to 6LG and 8LG flakes, respectively. Three branches of the LB modes are detected
in 2LM/nLG vdWHs, whose frequencies red-shift with increasing n of nLG, as
indicated by the dashed lines. The experimental data are summarized in Figure 4b. It
demonstrates that the interfacial coupling between 2LM and nLG constituents is not
so weak that both the two constituents contribute to the observed LB modes.
Therefore, the vdWHs must be considered as an overall system to model the LB
vibration in 2LM/nLG vdWHs, similar to the case of twisted multilayer
graphenes.31-32 Pos(LB) of 2LM/nLG vdWHs can be analyzed by an LCM, in which
each MoS2 or and graphene layer is treated as a ball with corresponding mass per
unit area. The frequencies ω (in cm−1) and displacement patterns of the LB modes in
mLM/nLG vdWHs can be calculated by solving linear homogeneous equations
follows:31
=
! "
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where ui is the phonon eigenvector of the ith mode with frequency ωi, M is the
diagonal mass matrix of mLM/nLG vdWHs in which Mii is the mass per unit area of
each rigid layer, c=3.0×1010cm s−1 is the speed of light, and D is the LB force
constant matrix. In multi-layer MoS2, the layer-number dependent Pos(LB) can be
fitted well by the LCM only considering the nearest force constant of (M) = 84 ×
10 N/m .30 However, Pos(LB) of tMLG and nLG (n > 2) must be described by the
nearest LB force constant (G) of 106.5×1017 N/m2 along with the additional
second nearest LB force constant ' (G) of 9.5×1017 N/m2.32 For the case of
2LM/nLG vdWHs, we assume that the interfacial LB force constant between 2LM
and nLG constituents is a constant (I). The LB modes observed in Figure 4a
indicate that (I) is comparable to (M) and (G), so the LB mode can be
denoted by the total layer number (N) of 2LM/nLG vdWHs, i.e., N = n + 2, similar
to the case of the LB modes in tMLGs.32 There are n+1 LB modes in 2LM/nLG
vdWHs although no more than two LB modes are clearly observed in 2LM/nLG
vdWHs, as shown in Figure 4a. It is found that only one parameter of
(I)=60×1017 N/m2 can be used to well reproduce the experimental data by LCM,
and the theoretical evolution of the LB modes in 2LM/nLG vdWHs with n of nLG is
demonstrated in Figure 4b. The results show that the three branches in Figure 4a can
be denoted as LBNN−1, LBNN−2 and LBNN−3, respectively. The n-dependent frequency
of the n-1 LB modes in nLG is depicted in Figure 4b as gray circles linked by
gray-dashed lines. Because the fitted (I) is smaller than (M) but larger than
(G)/2, the n-dependent Pos(LB) evolution of the LB modes in 2LM/nLG vdWHs
are quite different from that in nLG, which clearly demonstrates how the interfacial
LB coupling can modify the interlayer lattice dynamics of vdWHs from their
constituents.
The schematic diagram of the LCM for 2LM/3LG vdWH is shown in Figure 4c as
an
example,
where
2LM/3LG
was
considered
as
MoS2-MoS2-graphene-graphene-graphene. To clearly show how the nLG constituent
involves into the LB vibration of the whole vdWH, Figure 4d depicts the calculated
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normal mode displacements and mode frequencies of the observed LB modes for
2LM/1LG, 2LM/2LG and 2LM/3LG vdWHs. From the calculated normal mode
displacements, it is evident that the nLG constituents completely contribute to the
LB modes in 2LM/nLG vdWHs. The atomic mass per unit area of 1LM is m1LM
=3.0×10−7g/cm2, while that of 1LG, m1LG=0.76×10−7g/cm2, is only about 1/4 of
m1LM. The light m1LG results in that the graphene layer has the similar atomic
displacement to its nearest MoS2 layer in 2LM/1LG vdWHs, as indicated in Figure
4d. However, if more graphene layers are included in the MoS2/graphene vdWHs,
the large total mass of graphene layers can be comparable to m1LM, and the graphene
layers cannot be regarded as a perturbation to normal mode displacement of the LB
mode of 2LM anymore. For example, 2LG can be treated as an independent part in
normal mode displacements of the LB modes of 2LM/2LG vdWHs. This makes that
normal mode displacements of the LB42 and LB43 modes of 2LM/2LG vdWHs are
very similar to those of the LB31 and LB32 modes of pristine 3LM, respectively, as
indicated by red arrows in Figure 4d. For the LB53 mode in 2LM/3LG vdWHs,
normal mode displacement between the top graphene layer and the bottom two
graphene layers is out-of-phase, while the top graphene layer vibrates in-phase
together with its nearest MoS2 layer. Therefore, Figure 4 clearly demonstrates how
nLGs are coupled with 2LM by interfacial coupling between the two constituents to
generate the LB modes of total layers of vdWHs.
In contrast to the n-dependent Pos(LB) in 2LM/nLG vdWHs, as shown in Figure 4a,
the observed Pos(S) of all the 2LM/nLG vdWHs are almost identical to Pos(S21) of
pristine 2LM, which indicates the weak interfacial shear coupling between graphene
and MoS2 layers in the vdWHs. The largest variation of Pos(S) observed in all the
2LM/nLG vdWHs is only 0.6cm-1. There exist the lattice mismatch between
graphene and MoS2 layers. The local alignment of two interfacial layers can
significantly alters the interlayer stacking and coupling, leading to notable frequency
and intensity changes of low-frequency modes, as reported in twisted bilayer MoS2
and MoSe2.35-37 The disappearance of the shear mode is also observed in unaligned
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regions of twisted bilayers of MoSe2 with the mismatched registry between atoms in
two monolayers, which had been confirmed by the theoretical analysis.37 The
mismatched lattices in twisted bilayer MoS2 also weaken the interlayer shear
coupling.35 In the twisted multilayer graphene, the first principle calculation shows
that the shear restoring forces are nearly canceled at the twisted interface, resulting
in a much weaker shear coupling than in AB-stacked interfaces.33 This is confirmed
by the corresponding experiments.31 It was found that the interfacial shear coupling
is only 20% of the bulk value, which results in that the observed S mode in twisted
multilayer graphene is localized in each constituent.31 The n-dependent Pos(S) in
//
2LM/nLG vdWHs are also calculated for different interfacial shear coupling (),
//
//
and the cases that () is equal to 0, 0.2 and 0.4 of the shear coupling (() of
nLG are depicted in Figure S3. The calculated results show that a similar S mode of
//
//
mLM is observed in 2LM/nLG vdWHs even ()=0.4 ((). By comparing the
//
theoretical and experimental Pos(S21) variation in 2LM/nLG vdWHs, () is less
//
than 0.2 (() . Similar to the case in twisted multilayer graphene,31-32 the
mismatched lattices between graphene and MoS2 layers may cause the local
interfacial shar coupling either attractive or repulsive, which make the total shear
coupling weakened. Further theoretical calculation is necessary to clarify the
experimental results. As shown in Figure S3, 2LM/nLG exhibits serial branches of
the nLG-like S modes because of the weak interfacial shear coupling. However, the
observed Sn1 modes38 of nLG (n > 1) constituents are not be observed in 2LM/nLG,
which may be overlapped by other Raman modes due to the weak intensity.38-39
3-6LM flakes were also transferred on top of nLG flakes. Figure 5a shows Raman
spectra of 4LM/nLG (n=1, 2, 3, 5, 7, 9) vdWHs along with pristine 4LM in the S and
LB peaks spectral range. Two branches of the LB modes, LBNN−1 and LBNN−2, are
also observed in this series of 4LM/nLG vdWHs and the interfacial coupling ( (I))
of 60×1017 N/m2 can also be enough to understand the observed Pos(LBNN−1) and
Pos(LBNN−2). As shown in Figure 5a, the LBNN−1 mode of 4LM/1LG vdWHs exhibits
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a red-shift of 0.9 cm−1 relative to LB41 of pristine 4LM. Pos(LBNN−1) gradually
red-shifts to 10.0 cm−1 in 4LM/9LG vdWHs. Meanwhile, the LBNN−2 peak was
obviously observed in 4LM/3LG vdWHs resulting from the interfacial coupling. The
LBNN−1 and LBNN−2 modes are also observed in other series of MoS2/graphene
vdWHs, such as 3LM/nLG and 5LM/nLG vdWHs (Figure S4). Moreover, no
notable shift (< 0.2cm−1) of the shear mode (e.g., Sm1 of mLM/nLG vdWHs) is
observed in MoS2/graphene vdWHs. Figure 5b-c show Pos(LBNN−1) and Pos(LBNN−2)
of mLM/nLG vdWHs as a function of n of nLGs, respectively. The force constant
(60×1017 N/m2) of the interfacial coupling can be used to well fit all the observed
Pos(LBNN−1) and Pos(LBNN−2). It means that the strength of the interlayer LB
coupling at the interface of mLM/nLG vdWHs is quite uniform. The Raman spectra
of different 2LM/1LG vdWHs are measured (Figure S5) and all the Pos(LB32) are
identical to each other, indicating the uniform interfacial coupling in mLM/nLG
vdWHs again. Beyond the LBNN−1 and LBNN−2 branches, there exist other branches in
mLM/nLG vdWHs. All the n-dependent frequencies of the LB modes in mLM/nLGs
(m=2, 3, 4, 5) are included in Figure S6.
The MoS2/graphene vdWHs of mLM/nLG have been discussed above in detail. nLG
flakes were also transferred on the top of mLM flakes to form nLG/mLM vdWHs.
Figure 6a shows Raman spectra of 2LM, 2LM/3LG and 3LG/2LM vdWHs, while
Figure 6b show Raman spectra of 3LM, 3LM/7LG and 7LG/3LM vdWHs in the S
and LB peaks spectral range, respectively. It is found that the interfacial couplings in
the vdWHs formed by mLM and nLG flakes are not sensitive to their stacking order.
For an example, the LB modes observed in 3LG/2LM and 7LG/3LM vdWHs exhibit
almost identical peak position to those of 2LM/3LG and 3LM/7LG vdWHs,
respectively. In MoSe2/MoS2 heterobilayers, Pos(LB) is slightly dependent on the
twist angle (θt) between the top and bottom TMD flakes,36 and the average value is
found to decrease from 33.5 to 30.7 cm−1 as θt increases from 0o to 60o, indicating
the interface coupling becomes slightly weaker as θt increases.36 This effect had also
been observed in twisted bilayer MoS2 and MoSe2.35, 37 However, in the case of
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MoS2/graphene vdWHs, the results show that Pos(LBNN−1) and Pos(LBNN−2) in all
the nLG/mLM and nLG/mLM vdWHs can be well estimated by one parameter of the
interfacial coupling of (I) and are not sensitive to the stacking order. This implies
that the interfacial coupling in mLM/nLG vdWHs is also not sensitive to θt between
mLM and nLG constituents. Similar results had been observed in tMLGs.32
Conclusions
In summary, MoS2/graphene vdWHs are prepared and subsequently characterized by
ultralow-frequency Raman spectroscopy. The charge transferring between the
graphene and MoS2 are revealed by the G and 2D mode of graphene. It is found that
the new features of low-frequency LB modes in MoS2/graphene and graphene/MoS2
vdWHs are dependent on the thickness of graphene and MoS2 flakes. The LB modes
are much more sensitive to the interfacial coupling than the high frequency E
and
A1g modes. Importantly, the frequencies of these series of LB modes can be well
fitted by the improved LCM with the same force constant, indicating the uniform
and significant interfacial coupling in the all vdWHs. Unlike the LB modes, the S
mode in mLM/nLG is localized in the mLM constituent due to the weak interfacial
shear coupling between MoS2 and graphene adjacent layers. We believe that the
study on the interfacial interactions in vdWHs is fundamentally important, which
could be useful in practical device applications.
Methods/Experiments
Fabrication of van der Waals heterostructures
Briefly, the graphene or MoS2 flakes were first deposited on 90 nm SiO2/Si
substrates by mechanical exfoliation method. The top flake, such as graphene or
MoS2, was then transferred onto the pre-deposited flake by our recently reported
clean transfer method.60 The stacked vdWHs were annealed in Ar atmosphere at 300
o
C to enhance the interfacial interactions.
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Raman characterization
Ultralow-frequency Raman spectroscopy was carried out at room temperature using
a micro-Raman spectrometer (Horiba JY HR800) equipped with a liquid nitrogen
cooled charge-coupled device. The excitation wavelength is 488 nm from an Ar+
laser. A power of ∼ 0.2 mW is used to avoid sample heating. The laser plasma lines
are removed using a BragGrate bandpass filter (OptiGrate Corp), as these would
appear in the same spectral range as the modes of interest. The Rayleigh line is
suppressed using three BragGrate notch filters (OptiGrate Corp) with an optical
density 4 and a spectral bandwidth ∼ 5-10 cm−1.
Acknowledgements
This work was supported by the National Key Research and Development Program
of China (Grant No. 2016YFA0301200 and 2017YFB1002900); and MOE under
AcRF Tier 2 (ARC 19/15, No. MOE2014-T2-2-093; MOE2015-T2-2-057;
MOE2016-T2-2-103; MOE2017-T2-1-162) and AcRF Tier 1 (2016-T1-001-147;
2016-T1-002-051;
2017-T1-001-150)
and
NTU
under
Start-Up
Grant
(M4081296.070.500000) in Singapore. It was also supported by the Joint Research
Fund for Overseas Chinese, Hong Kong and Macao Scholars (Grant No. 51528201)
and by NSFC (Grant No. 21571101, 11434010, 11474277, 11574305 and 51527901),
the Natural Science Foundation of Jiangsu Province in China (Grant No.
BK20161543), and the Natural Science Foundation of the Jiangsu Higher Education
Institutions of China (Grant No. 15KJB430016). J. Z. acknowledges the support
from National Young 1000 Talent Plan of China and LU JIAXI International team
program supported by the K.C. Wong Education Foundation and CAS. P.T. and J.Z.
acknowledges the support from the Key Research Program of the Chinese Academy
of Sciences (Grant No. XDPB0602, XDPB08-2), and Beijing municipal Science and
technology commission.
Corresponding Author
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∗E-mail: hzhang@ntu.edu.sg; phtan@semi.ac.cn
Conflict of Interest: The authors declare no competing financial interest.
Supporting Information Available: Experimental and calculated low-frequency
Raman spectra of MoS2/graphene and graphene/MoS2 heterostructures. This material
is available free of charge via the Internet at http://pubs.acs.org.
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and Few-Layer MoS2 Sheets. Small 2012, 8, 682-686.
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Figure 1: (a) Schematic illustration of preparation of MoS2/graphene vdWHs. (b)
Schematic illustration of 2LM/1LG vdWHs. (c) Optical image of MoS2/graphene
vdWHs formed by transferring 2LM and 3LM on 1LG. The 1LG zone is marked by
the white dash line. (d) Stokes/anti-Stokes Raman spectra of the pristine 2LM,
as-transferred 2LM and 2LM/1LG vdWHs, and 2LM/1LG vdWHs annealed for 5,
30 and 60 min in the S and LB peak spectral range and the corresponding Stokes
Raman Spectra in the E2g1 and A1g peak spectral range. (e) Raman spectra of the
pristine 1LG, as-transferred 1LG and 2LM/1LG vdWHs, and 2LM/1LG vdWHs
annealed for 5, 30 and 60 min in the G and 2D peak spectral range. Vertical
dash-dotted lines are used for the guide of eyes.
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Figure 2: (a) Optical image of a vdWH stacked by a 2LM flake on a 2LG flake. AFM
images of (b) as-transferred and (c) annealed 2LM/2LG, in which the height profile
of the 2LM on the 2LG is shown for each image. (d) The S and LB modes of pristine
2LM and 2LM/2LG annealed at 300 oC for 30 minutes.
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Figure 3: (a) Stokes Raman spectra of mLM/1LG (m=1-5) in the S and LB, and E
and A1g peaks spectral ranges, along with those of pristine mLM used as reference in
gray dashed curves. The vertical dashed lines are the guides to eyes. (b) Raman spectra
of the LB-like modes in mLM/1LG (m=1-5) along with those of pristine mLM used as
reference in gray dashed curves. (c) Frequency differences of S, LB, A1g and E
modes between the mLM/1LG and the corresponding pristine mLM.
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Figure 4: (a) Stokes/anti-Stokes Raman spectra of 2LM/nLG in the S and LB peak
spectral range. The black dash lines are used for the guide of eyes. (b) Theoretical
(Theo.) evolution of the LB modes in 2LM/nLG vdWHs with layer number of nLG
based on the LCM, and the corresponding theoretical data of nLG and experimental
(Exp.) data in 2LM/nLG vdWHs. (c) Schematic diagram of a linear chain model
(LCM) for LB modes in 2LM/3LG, in which the next nearest LB coupling in the
3LG constituents is considered. (d) Normal mode displacements of the observed LB
modes in 2LM/1LG, 2LM/2LG and 2LM/3LG. The corresponding calculated
(Theo.) frequencies are indicated.
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Figure 5: (a) Stokes/anti-Stokes Raman spectra of 4LM/nLG in the S and LB peak
spectral range. The black dash lines are used for the guide of eyes. The S41 and S43
modes of the 4LM constituent are labeled. (b) Pos(LBNN−1) as a function of layer
number (n) of the nLG constituent. (c) Pos(LBNN−2) as a function of layer number (n)
of the nLG constituent. The solid lines show the theoretical trends of Pos(LBNN−1)
and Pos(LBNN−2) on n based on the linear chain model.
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Figure 6: (a) Stokes/anti-Stokes Raman spectra of 2LM, 2LM/3LG and 3LG/2LM in
the S and LB peak spectral range. (b) Stokes/anti-Stokes Raman spectra of 3LM,
3LM/7LG and 7LG/3LM in the S and LB peak spectral range. The dashed lines are
the guide of eyes.
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