Nano Res
1
Bandgap-tunable lateral and vertical heterostructures
based on monolayer Mo1-xWxS2 alloys
Yu Kobayashi1, Shohei Mori1, Yutaka Maniwa1, and Yasumitsu Miyata1,2 ()
Nano Res., Just Accepted Manuscript • DOI 10.1007/s12274-015-0826-7
http://www.thenanoresearch.com on June 1, 2015
© Tsinghua University Press 2015
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Nano Research
DOI 10.1007/s12274-015-0826-7
TABLE OF CONTENTS (TOC)
Bandgap-tunable lateral and vertical
heterostructures based on monolayer
Mo1-xWxS2 alloys
Yu Kobayashi,1 Shohei Mori,1 Yutaka Maniwa,1
and Yasumitsu Miyata.*,1,2
1Tokyo Metropolitan University, Japan
2JST, PRESTO, Japan
Lateral and vertical heterostructures based on bandgap-tunable atomic layer
Mo1-xWxS2 alloys are grown by the sulfurization of patterned thin films of WO3
and MoO3.
Bandgap-tunable lateral and vertical heterostructures
based on monolayer Mo1-xWxS2 alloys
Yu Kobayashi,1 Shohei Mori,1 Yutaka Maniwa,1 and Yasumitsu Miyata1,2()
Received: day month year
Revised: day month year
Accepted: day month year
(automatically inserted by
the publisher)
© Tsinghua University Press
and Springer-Verlag Berlin
Heidelberg 2014
KEYWORDS
Transition metal
dichalcogenide,
Mo1-xWxS2, alloy,
heterostructure,
thin film sulfurization,
photoluminescence,
stability.
ABSTRACT
Fabricating heterostructures of two-dimensional semiconductors with specific
bandgaps is one of the most important aspects of realizing the full potential of
these materials in electronics and optoelectronics. Recently, several groups have
reported the direct growth of lateral and vertical heterostructures based on
typical semiconductor transition metal dichalcogenides (TMDCs) such as WSe2,
MoSe2, WS2 and MoS2 monolayers. Here, we demonstrate the single-step direct
growth of lateral and vertical heterostructures based on bandgap-tunable
atomic layer Mo1-xWxS2 alloys by the sulfurization of patterned thin films of
WO3 and MoO3. These patterned films are capable of generating a wide variety
of concentration gradients due to the diffusion of transition metals during the
crystal growth. Under high temperature condition, this leads to the formation
of monolayer crystals of Mo1-xWxS2 alloys with various compositions and
bandgaps depending on the growth positions on the substrates.
Heterostructures of these alloys are also obtained through the stepwise changes
in W/Mo ratios within a single domain during low temperature growth. The
stabilization of monolayer Mo1-xWxS2 alloys, which often degrade even at room
temperature, was accomplished by covering the alloys with other atomic layers.
The present findings demonstrate an efficient means of both studying and
optimizing the optical and electrical properties of TMDC-based
heterostructures to allow their use in future device applications.
1. Introduction
The fabrication of atomic-layer heterostructures is
one of the most important steps in fully realizing the
intrinsic properties of such structures and developing
novel device applications[1]. In initial studies,
vertically stacked heterostructures were primarily
prepared by mechanical exfoliation and multiple
transfers of atomic layers of materials such as
graphene, boron nitride and transition metal
dichalcogenides (TMDCs)[2-11]. In addition to such
vertical heterostructures, recent progress in growth
techniques such as chemical vapor deposition (CVD)
and physical vapor transport have allowed the direct
synthesis of lateral heterostructures based on
graphene/boron nitrides[12-19], two different types
Nano Research
DOI (automatically inserted by the publisher)
Address correspondence to Yasumitsu Miyata, [email protected]
Research Article
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2 Nano Res.
of TMDCs[20-23]. In particular, TMDC-based
heterostructures have attracted much attention due
to their semiconducting properties, which are
essential to the realization of novel functional
electronics and optoelectronics.
The band gap engineering of TMDCs is an
important challenge that must be addressed in order
to maximize the potential of these materials, and one
effective means of doing so is the use of alloying
materials. To date, several approaches have been
applied to the design of various TMDC-based alloys.
Chen et al. prepared monolayers of Mo1-xWxS2 via
mechanical exfoliation of single-crystal Mo1-xWxS2
grown by chemical vapor transport[24], while Gong
et al. demonstrated the single-step direct growth of
mono- and bilayers of MoS2(1-x)Se2x by CVD[25]. More
recently, similar alloys have been observed around
the heterojunction interfaces between the lateral
heterostructures of TMDCs[20-22].
Here we report the single-step direct growth of
lateral and vertical heterostructures based on
monolayers of bandgap-tunable Mo1-xWxS2 alloys
having a wide range of compositions. These
Mo1-xWxS2 monolayers and heterostructures were
prepared by sulfurization of two different spatially
separate thin films of transition metal oxides on a
substrate, as shown in Figs. 1 and 1S in the Electronic
Supplementary Material (ESM). As previously
observed[20, 22], the Mo atoms contribute to crystal
growth at an earlier stage than the W atoms,
resulting in the self-assembly of TMDC-based
heterostructures even in the case of a single-step
sulfurization process. Unlike these previous studies
which basically use powder-type precursors[20-22],
we found that the use of patterned films amplifies
the concentration gradient of the spreading transition
metal on the substrate during the crystal growth
process. As a result, both lateral and vertical
heterostructures can be obtained, composed of
Mo1-xWxS2 atomic layers with various compositions.
We also report the stabilization of these Mo1-xWxS2
alloys, which often degrade even at room
temperature, using atomic-layer passivation. The
present findings demonstrate an efficient means of
investigating and optimizing the optical and
electrical properties of TMDC-based heterostructures
for use in various device applications.
2. Results & Discussion
We initially assessed the monolayer Mo1-xWxS2
alloys with relatively uniform compositions within
single grains grown under high temperature
conditions. Figure 2a presents an optical microscopy
image of typical crystals of a monolayer Mo1-xWxS2
alloy grown at 900 °C on a SiO2/Si substrate. The
layer number was confirmed by the strong PL peak
energies, which have been reported in a previous
study of exfoliated monolayer Mo1-xWxS2 alloys[24].
Here it is evident that the crystals were
approximately 10 to 20 µm in size with triangular
morphologies, as was also observed in the case of
monolayers of MoS2 and WS2 (Fig. S2 (in the ESM)).
Figures 2b and c present the Raman and PL spectra
of five individual crystals at different positions on
the same substrate and the monolayers of pure WS2
and MoS2. It has been reported that the Raman
spectra of monolayer Mo1-xWxS2 alloys exhibit three
characteristic peaks in the region from 350 to 420 cm-1,
assigned to the WS2-like E’, MoS2-like E’, and A1’
modes[24,26]. The relative intensities and peak
positions of these peaks vary depending on the
Mo/W ratio in each crystal as reported previously[26].
The Mo/W ratios can also be determined from the
peak energies of the PL spectra, since the PL spectra
of exfoliated monolayer Mo1-xWxS2 alloys on SiO2/Si
substrates have been investigated in detail[24]. In the
present sample, the W composition x ranges from 0.9
to 0.2. As was observed in the case of exfoliated
samples, at high Mo concentrations (x = 0.2), the PL
peaks exhibit a red shift, while the peaks move to
higher energies with increases in the W concentration.
This redshift has been observed in the previous PL
study of exfoliated monolayer Mo1-xWxS2 alloys, and
is attributed to the different orbital compositions in
the lowest unoccupied molecular orbital (LUMO)
between MoS2 and WS2 monolayers[24]. As shown in
Fig. 2d, A1’ mode Raman frequencies also increase
with increasing the W composition, which can be
well reproduced by the fitting result for exfoliated
Mo1-xWxS2 monolayers[26]. This agreement evidences
to support the validity of composition evaluation
from PL and Raman spectra in the present study. The
wide variations in the Mo/W ratio seen here are
related to the distance between the growth position
of crystals and the different WO3 and MoO3 regions
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3 Nano Res.
in the patterned films. As an example, the Mo content
is enriched near the MoO3 film, whereas W-rich
alloys grow in the vicinity of the WO3 film.
Raman and PL intensity maps and spectra of a
monolayer Mo1-xWxS2 alloy were obtained to
investigate the compositional variation within a
single crystal (Fig. 3). The crystal has triangle shape
as shown in the optical image (Fig. 3a), which is also
confirmed from the intensity maps of PL (Fig. 3b)
and WS2–like E’ Raman mode (Fig. 3c). The
intensities in these maps are observed to increase
with increasing distance from the center of the crystal.
It should be noted that the MoS2-like E’ peak
generates the different Raman intensity map from
that of WS2–like E’ peak (Figs. 3c and d). These
variations in intensity correspond to changes in the
Mo/W ratio within the crystal. As the distance from
the center is increased, the PL peaks continuously
shift from 1.91 to 1.95 eV (Fig. 3e) and the relative
intensities of the WS2–like E’ Raman mode increase
compared to those of MoS2-like E’ mode (Fig. 3f). We
note that there is no abrupt shift of the PL peak
energies as shown in Fig. S3 (in the ESM). These
results indicate that the proportion of W increases
slightly, from 0.8 to 0.9, as the crystal grows, meaning
that the relative rates at which Mo and W atoms are
supplied changes during the single-step sulfurization
process. This compositional variation also modulates
the PL intensity within the single crystal probably
due to the changes in resonance condition and/or PL
efficiency. However, it should be noted that these
crystals exhibit relatively small spatial variations in
the Mo/W ratio compared to crystals obtained under
the low temperature growth conditions described
below.
The formation of lateral and vertical
heterostructures based on Mo1-xWxS2 alloys was
subsequently assessed under low temperature
growth conditions. Following reduced temperature
growth at 750 °C, the atomic-layer Mo1-xWxS2 crystals
had more symmetrical shapes with smooth edges, as
shown in the optical microscopy image in Figs. 4a
and b. The resulting grains were approximately 10
µm in size and described near-regular triangles.
Figures 4d to i present the Raman and PL intensity
maps of this same crystal shown in Fig. 4b. In Figs.
4d to f, cyan and red indicate the intensities of the
WS2 and MoS2 E’ peaks, respectively. It can be seen
that the MoS2 peak is primarily observed in the inner
part of crystal, whereas the WS2 peak is dominant
around the periphery of the inner MoS2 region.
Almost the same image was generated from PL
intensity maps of different energy ranges (Figs. 4g to
i). The proportion of W in the inner and outer regions
were estimated to be 0 and 0.9, respectively, from the
Raman and PL spectra (Figs. 4j and k). This result
clearly indicates the formation of a lateral
heterostructure based on monolayer Mo1-xWxS2 alloys
with high spatial variations in the Mo/W ratio as
illustrated in Fig. 4c.
The unique aspects of the present sample include
multistep compositional variation. The PL spectra
obtained from various locations within the crystal
exhibit three major peaks at 1.83, 1.88 and 1.94 eV
(Fig. 4k), indicating that there are three main alloy
compositions (x = 0, 0.7 and 0.9) within the single
grain. This suggests that the present lateral
heterostructure consists of two heterojunctions. It can
be concluded that this lateral heterostructure grows
as the result of two-step changes in the rates at which
W and Mo atoms are supplied during the single-step
sulfurization process. Even though the mechanism of
two-step change is still unclear, the situation implies
that there are two types of Mo sources with different
lifetime until deactivation during sulfurization.
Possible candidates are MoO3 and MoO2 because the
reduction of MoO3 to MoO2 could occur during the
temperature rise under Ar atmosphere before
sulfurization.
In addition to the monolayer lateral
heterostructures, we also identified the formation of
vertically stacked, lateral heterostructures on the
same substrate grown at 750 °C (Fig. 5). As shown in
the optical image (Fig. 5a), the resulting crystal was
approximately 20 µm in size and had a six-pointed
star morphology. Figure 5b displays a structural
model of this crystal, estimated as described below.
As shown in Fig. 5c, the Raman spectra of this crystal
changes depending on the position as observed in the
above lateral heterostructure. A notable point is that
the WS2-like E’ mode Raman intensity map exhibits
an intense spot at the center of the star, three highly
intense, symmetrical lines shaped like a regular
triangle and a six-pointed star (Fig. 5d). This
indicates that there are three major regions with high
concentration of WS2 within the crystal. MoS2-like E’
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4 Nano Res.
Raman signals were primarily generated only within
the outer WS2 region, and are seen to overlap with
the inner WS2 portion (Figs. 5e and f). Atomic force
microscopy (AFM) analysis supports that the crystal
had triangular second and center third layers within
the inner WS2 region (Fig. S4 (in the ESM)).
This layered structure was also examined using PL
spectra (Fig. 5g) and intensity maps (Figs. 5h to j). As
shown in Fig. 5g, the PL peak energies were shifted
from 1.85 to 1.96 eV depending on the position
assessed. For comparison purposes, two PL intensity
maps were constructed from the peak areas in the
ranges of 1.94 to 1.99 eV for W-rich region (Fig. 5h)
and 1.80 to 1.89 eV for Mo-rich region (Fig. 5i), and
their combined image is presented in Fig. 5j. These
figures demonstrate that strong luminescence was
observed only for the outer region of inner
W-enriched triangle lines in Fig. 5d. In contrast, the
inner region showed a remarkable reduction in the
PL intensity of the Mo1-xWxS2 alloy. This reduction
can be explained as the result of the stacking-induced
transition from the direct to indirect bandgaps of the
TMDC[27-29]. Within this stacked region, the
associated wide-range PL spectra displayed
additional peaks likely derived from interlayer
excitons (Fig. S5 (in the ESM)), meaning that the top
and bottom layers were able to strongly interact with
one another even in the vertical heterostructures of
an alloy-based TMDC. The crystal was therefore
composed of stacked layers consisting of at least two
different lateral heterostructures, as shown in Fig. 5b.
It is worth noting the variations in the PL around
the interfaces in the heterostructure. Figure S6a
presents the PL spectra around the interface
indicated by the orange line in Fig. 5a. The peak
energy is seen to gradually shift from 1.87 to 1.96 eV.
As shown in the peak plot (Fig. S6b (in the ESM)), the
stepwise transition occurs within a span of one
micrometer, which is close to the diffraction limit
defined by the laser wavelength employed (532 nm).
This spatial peak shift also supports our contention,
discussed above, that the composition of the
Mo1-xWxS2 alloy rapidly changes over short distances.
The stability of atomic-layer Mo1-xWxS2 alloys
obtained by thin-film sulfurization was subsequently
assessed. As shown in the optical, Raman, and PL
images (Figs. 6a to d), the sample was found to have
partially degraded following room temperature
storage in air for 45 days after its initial synthesis.
The region that underwent the most pronounced
degradation corresponds to the outer monolayer, for
which both the PL and Raman peaks are seen to
decrease, as indicated by the bottom panels in Figs.
6e and f. In contrast, almost no changes were
observed in the Raman and PL spectra and intensity
maps of the vertical heterostructure containing the
triangular second layer. These results indicate the
low structural stability of Mo1-xWxS2 monolayers
grown on SiO2/Si substrates and the increased
stability of Mo1-xWxS2 alloys via atomic-layer
stacking.
One can expect that the degradation of monolayer
Mo1-xWxS2 alloys derives from the reaction of the
TMDC alloys with oxygen and/or water in air[30]. To
test this hypothesis, monolayer Mo1-xWxS2 alloy
samples were prepared under the high temperature
condition and covered with a monolayer graphene
film acting as a gas barrier layer. Figures 6g and h
show the degradation over time of non-covered and
graphene-covered Mo1-xWxS2 monolayers with
different composition ratios. In addition, the
degradation over time was followed by assessing the
Raman intensities (Figs. S7 and S8 (in the ESM)). In
the case of the non-covered samples, almost no
decreases in intensity were observed for the
monolayers for which x was 0.3 and 0.9, while the
intensities of those layers for which x was 0.5 to 0.8
decreased gradually, dropping to half their original
levels after 30 to 50 days. These data indicate that the
composition ratio is a major factor to determine the
degradation rates for the alloy monolayers. This
composition-dependent reactivity probably derives
from the local changes in the density of defects such
as S vacancy, lattice strains, or electronic states. In
contrast, the graphene-covered samples exhibited
less change in their Raman intensities even under the
same environmental conditions, demonstrating that
the passivation layer was able to protect the
monolayer TMDC alloys from reactive species in air.
One may think that the unstable Mo1-xWxS2
monolayers have much defect densities. To prove this
more, we have measured low-temperature PL spectra.
In previous studies, additional PL peaks of
defect-derived bound excitons appear at 0.2 eV lower
than the PL peak of neutral excitons[31]. However, no
PL peaks of bound excitons are observed for the
www.theNanoResearch.com∣www.Springer.com/journal/12274 | Nano Research
5 Nano Res.
present Mo1-xWxS2 (x=0.7) monolayers alloy samples
at 80 K (Fig. S9 (in the ESM)). This result suggests
that the present samples have relatively low defect
densities.
Finally, we note that the room-temperature
degradation has never been reported for the
exfoliated Mo1-xWxS2 monolayers so far. This strongly
suggests that the present Mo1-xWxS2 monolayers
directly grown on SiO2/Si substrates become more
reactive than those prepared by mechanical
exfoliation from bulk samples. One possible reason
could be the lattice strain because the chemical
reactivity of graphene is also enhanced by
mechanical strain[32]. Furthermore, the lattice strain
can be induced during the cooling process after the
growth by the mismatch of thermal expansion
coefficient between Mo1-xWxS2 monolayers and
SiO2/Si substrates, as observed previously for WS2
monolayers on a SiO2/Si substrate[33]. Actually, both
WS2 and MoS2 monolayers grown on SiO2/Si
substrates show a lower E’ phonon energy compared
with the samples grown on sapphire substrates,
while the A1’-Raman peaks have no change between
SiO2/Si and sapphire substrates (Fig. S10 (in the
ESM)). This is consistent with the substrate-induced
lattice strain as previously observed. These would be
improved by using proper substrates for the growth
of TMDCs with less lattice strain.
3. Conclusion
We have demonstrated the single-step direct
growth of lateral and vertical heterostructures based
on bandgap-tunable atomic layer Mo1-xWxS2 alloys.
Unlike the previous approaches for heterostructure
synthesis using powder-type precursors, the present
sulfurization process of patterned films can produce
a large variety of concentration gradients from the
diffusion of transition metals during crystal growth.
At high temperatures (900 °C), this process results in
the growth of monolayer crystals of Mo1-xWxS2 alloys
with various compositions depending on the growth
positions on the substrates. In contrast, low
temperature growth at 750 °C generates lateral and
vertical heterostructures of these alloys through the
stepwise changes in W/Mo ratios within a single
domain. Furthermore, it is possible to stabilize
monolayer Mo1-xWxS2 alloys, which often degrade
even at room temperature, by covering them with
other atomic layers. The present findings
demonstrate a useful process for both the study and
optimization of the optical and electrical properties
of TMDC-based heterostructures so as to allow their
use in various device applications. In particular, the
bandgap-tunable lateral heterostructures also pave
the way for the precise control of electronic structure
of one-dimensional interface. Such designed interface
will be a fascinating system for realizing
one-dimensional electron gas which can be regarded
as an ultrathin conducting wire. The realization and
applications of these interface-related phenomena
would be one of the most interesting challenges in
the field of two-dimensional materials.
Methods
Atomic layers of Mo1-xWxS2 alloys were grown
using an improved method based on thin film
sulfurization that has been reported previously
[34-36]. Firstly, thin films of WO3 (Aldrich, 99%
purity) and MoO3 (Aldrich, 99.5 % purity) were
deposited in a grid-like pattern by employing a
shadow mask with slits (slit width of 0.1 mm) on
SiO2/Si (100 nm or 285 nm SiO2) substrates, as shown
in Fig. 1S (in the ESM). Typical film thicknesses were
approximately 10 and 1 nm for WO3 and MoO3,
respectively. During subsequent evaporation by
resistive or electron beam heating, the shadow mask
was held approximately 1 mm above the substrate to
produce gradients of film thickness. The substrate
was then placed in a quartz tube (3 cm in diameter,
100 cm in length) together with sulfur flakes (Aldrich,
99.99 % purity, 2 g). The substrate was located
approximately 30 cm downstream from the sulfur so
as to heat the two materials separately. The quartz
tube was next filled with Ar gas at a flow rate of 200
cm3/min and the substrate was gradually heated to
the sulfurization temperature (750 to 900 °C) over 45
min using an electric furnace. When the substrate
temperature obtained the set point, the sulfur flake
was heated to 190 °C for 30 min using a second
furnace, so as to supply sulfur vapor to the substrate.
Following the sulfurization, the tube was
immediately cooled using an electric fan. For
reference, monolayers of pure WS2 and MoS2 were
grown only from WO3 or MoO3 films, respectively,
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6 Nano Res.
under the same conditions. Typical growth
temperatures were 900 °C for WS2 and 775 °C for
MoS2.
To obtain passivation by atomic layer films,
monolayer Mo1-xWxS2 alloys on SiO2/Si substrates
were covered by CVD-grown monolayer graphene
films. The graphene were grown by atmospheric
pressure chemical vapor deposition on commercial
Cu foil (thickness of 20 µm, 99.9%, Nilaco), as
reported previously[37]. In this process, the Cu foil
was placed in a quartz tube and the tube was filled
with Ar/H2 (3%) gas at a flow rate of 400 cm3/min and
subsequently annealed at 1075 °C for 1 hour to
remove surface oxide. The growth was performed at
1075 °C for 2 h under a mixture of Ar/H2 (3%)/CH4
(0.0125%) at a flow rate of 200 cm3/min. The sample
was floated on a 1 M Fe(NO3)2 aqueous solution
during etching of the Cu foil, as described in the
previous report[38]. After dissolving the Cu, the
Mo1-xWxS2 atomic layers on the SiO2/Si substrate were
placed face-down onto the floating graphene. The
sample was rinsed with pure water several times and
then dried under a N2 flow.
Optical images were recorded with an optical
microscope (Nicon, ECLIPSE-LV100D). Raman and
PL spectra were acquired with a micro-Raman
spectroscope (Renishaw, inVia) with an excitation
laser operating at 532 nm at room temperature.
Acknowledgements
This work was supported by a Grant-in-Aid for
Young Scientist (A) (No. 15H05412) and for Scientific
Research on Innovative Areas (No. 26107530) from
the Ministry of Education, Culture, Sports, Science
and Technology (MEXT), Japan, and by the Izumi
Science and Technology Foundation.
Electronic Supplementary Material: Supplementary
material (Photographic images of substrates with
patterned oxide thin films before and after the
sulfurization process, optical microscopy images, PL
and Raman spectra of monolayers of WS2 and MoS2,
PL peak energies measured along the dotted line in
Fig. 3b, AFM images and wide-range PL spectra of
Mo1-xWxS2 heterostructures, PL spectra and peak
energies measured along the orange line in Fig. 5a,
variations in the Raman spectra of non-covered and
graphene-covered Mo1-xWxS2 alloys, Raman spectra
of WS2 and MoS2 on sapphire and SiO2/Si substrate,
and PL spectra of pure WS2 and Mo1-xWxS2 at 300 K
and 80 K.) is available in the online version of this
article at http://dx.doi.org/10.1007/s12274-***-****-*
(automatically inserted by the publisher).
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9 Nano Res.
FIGURES.
Figure 1. Schematic illustration of the growth of Mo1-xWxS2 alloys and Mo1-xWxS2–based heterostructures via the sulfurization of
patterned WO3 and MoO3 thin films.
Figure 2. (a) Optical microscopy image of typical crystals of monolayer Mo1-xWxS2 alloys grown at 900 °C. (b) Raman and (c) PL
spectra of five different crystals on the same substrate and of monolayers of pure MoS2 and WS2. Raman and PL spectra were
recorded at the same points for each crystal and the W composition, x, in Mo1-xWxS2 is indicated for each point. The spectra of
monolayers of pure MoS2 and WS2 are included at the top (x=0) and bottom (x=1) of this graph, respectively, for reference purposes.
The Raman and PL intensities are normalized by the maximum intensities. (d) A1’ mode Raman frequencies plotted as a function of
the W composition x. The solid circles and line are the present experimental data and the fitting result for exfoliated samples [26],
respectively.
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10 Nano Res.
Figure 3. (a) Optical microscopy image, (b) PL intensity map for 1.77 to 2.07 eV, and Raman intensity maps of (c) WS2-like and (d)
MoS2-like E’ modes obtained for a monolayer Mo1-xWxS2 alloy grown at 900 °C. (e) PL and (f) Raman spectra measured at the seven
points from top to bottom on the dotted line in (b). The Raman and PL intensities are normalized by the maximum intensities.
Figure 4. (a) Low and (b) high magnification optical microscope images, (c) structural model, (d) WS2-like and (e) MoS2-like E’
Raman intensity maps, (f) combined Raman intensity map of (d) and (e), PL intensity maps for (g) 1.91 to 1.98 eV and (h) 1.77 to
1.86 eV, (g) combined PL intensity maps of (g) and (f) of the lateral heterostructure of a monolayer Mo1-xWxS2 alloy grown at 750 °C.
(j) Raman and (k) PL spectra at different points marked by solid circles in (f) and (i), respectively. The order of the spectra (from top
to bottom) corresponds to the positions of the circles in (f) and (i). The Raman and PL intensities are normalized by the maximum
intensities.
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11 Nano Res.
Figure 5. (a) Optical microscopy image and (b) structural model of the lateral and stacked heterostructure based on monolayer
Mo1-xWxS2 alloys grown at 750 °C. (Note: for the structural model in (b), cyan and red colors correspond to high populations of W
and Mo atoms, respectively.) (c) Raman spectra acquired at the points marked by solid circles in (f). (d) WS2-like and (e) MoS2-like
E’ Raman intensity maps, (f) combined Raman intensity map of (d) and (e). (g) PL spectra acquired at the points marked by solid
circles in (j). PL intensity maps from (h) 1.92 to 1.99 eV and (i) 1.80 to 1.88 eV, (j) combined PL intensity map of (h) and (i). The
order of the spectra (from top to bottom) corresponds to the positions of the circles in (f) and (j). The Raman and PL intensities are
normalized by the maximum intensities.
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12 Nano Res.
Figure 6. (a) Optical microscopy image, (b) combined PL intensity map from 1.84 to 1.91 eV and 1.92 to 1.99 eV, (c) WS2-like and
(d) MoS2-like E’ Raman intensity maps after the degradation of the crystal shown in Figure 5. (e) Raman and (f) PL spectra acquired
at the points indicated in (c) and (d), respectively, before (black) and after (red) the degradation. The order of the spectra (from top to
bottom) corresponds to the positions of the circles in (b), (c) and (e). The optical, Raman and PL images were obtained 7, 45 and 45
days following initial synthesis of the structure. Normalized Raman intensities of WS2–like E’ or A1’ modes plotted as a function of
post-growth elapsed time for (g) non-covered and (h) graphene-covered Mo1-xWxS2 monolayers with different composition ratios.
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Nano Res.