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

Just Accepted

This is a “Just Accepted” manuscript, which has been examined by the peer-review process and has been

accepted for publication. A “Just Accepted” manuscript is published online shortly after its acceptance,

which is prior to technical editing and formatting and author proofing. Tsinghua University Press (TUP)

provides “Just Accepted” as an optional and free service which allows authors to make their results available

to the research community as soon as possible after acceptance. After a manuscript has been technically

edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP

article. Please note that technical editing may introduce minor changes to the manuscript text and/or

graphics which may affect the content, and all legal disclaimers that apply to the journal pertain. In no event

shall TUP be held responsible for errors or consequences arising from the use of any information contained

in these “Just Accepted” manuscripts. To cite this manuscript please use its Digital Object Identifier (DOI® ),

which is identical for all formats of publication.

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, ymiyata@tmu.ac.jp

Research Article

| www.editorialmanager.com/nare/default.asp

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

www.theNanoResearch.com∣www.Springer.com/journal/12274 | Nano Research

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’

| www.editorialmanager.com/nare/default.asp

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,

| www.editorialmanager.com/nare/default.asp

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).

References

[1] Geim, A. K.; Grigorieva, I. V. Van der Waals

heterostructures. Nature 2013, 499, 419-425.

[2] Dean, C. R.; Young, A. F.; MericI; Lee, C.; Wang, L.;

Sorgenfrei, S.; Watanabe, K.; Taniguchi, T.; Kim, P.;

Shepard, K. L. et al. Boron nitride substrates for

high-quality graphene electronics. Nat. Nanotechnol. 2010,

5, 722-726.

[3] Dean, C.; Young, A. F.; Wang, L.; Meric, I.; Lee, G. H.;

Watanabe, K.; Taniguchi, T.; Shepard, K.; Kim, P.; Hone,

J. Graphene based heterostructures. Solid State Commun.

2012, 152, 1275-1282.

[4] Georgiou, T.; Jalil, R.; Belle, B. D.; Britnell, L.;

Gorbachev, R. V.; Morozov, S. V.; Kim, Y.-J.; Gholinia,

A.; Haigh, S. J.; Makarovsky, O. et al. Vertical field-effect

transistor based on graphene-WS2 heterostructures for

flexible and transparent electronics. Nat. Nanotechnol.

2013, 8, 100-103.

[5] Wang, Q. H.; Kalantar-Zadeh, K.; Kis, A.; Coleman, J. N.;

Strano, M. S. Electronics and optoelectronics of

two-dimensional transition metal dichalcogenides. Nat.

Nanotechnol. 2012, 7, 699-712.

[6] Lee, G.-H.; Yu, Y.-J.; Cui, X.; Petrone, N.; Lee, C.-H.;

Choi, M. S.; Lee, D.-Y.; Lee, C.; Yoo, W. J.; Watanabe, K.

et al. Flexible and transparent MoS2 field-effect transistors

on hexagonal boron nitride-graphene heterostructures.

ACS Nano 2013, 7, 7931-7936.

[7] Wang, L.; Meric, I.; Huang, P. Y.; Gao, Q.; Gao, Y.; Tran,

H.; Taniguchi, T.; Watanabe, K.; Campos, L. M.; Muller,

D. A. et al. One-dimensional electrical contact to a

two-dimensional material. Science 2013, 342, 614-617.

www.theNanoResearch.com∣www.Springer.com/journal/12274 | Nano Research

7 Nano Res.

[8] Gorbachev, R. V.; Song, J. C. W.; Yu, G. L.; Kretinin, A.

V.; Withers, F.; Cao, Y.; Mishchenko, A.; Grigorieva, I. V.;

Novoselov, K. S.; Levitov, L. S. et al. Detecting

topological currents in graphene superlattices. Science

2014, 346, 448-451.

[9] Roy, K.; Padmanabhan, M.; Goswami, S.; Sai, T. P.;

Ramalingam, G.; Raghavan, S.; Ghosh, A.

Graphene-MoS2 hybrid structures for multifunctional

photoresponsive memory devices. Nat. Nanotechnol. 2013,

8, 826-830.

[10] Hunt, B.; Sanchez-Yamagishi, J. D.; Young, A. F.;

Yankowitz, M.; LeRoy, B. J.; Watanabe, K.; Taniguchi, T.;

Moon, P.; Koshino, M.; Jarillo-Herrero, P. et al. Massive

dirac fermions and hofstadter butterfly in a van der Waals

heterostructure. Science 2013, 340, 1427-1430.

[11] Yu, Y.; Hu, S.; Su, L.; Huang, L.; Liu, Y.; Jin, Z.; Purezky,

A. A.; Geohegan, D. B.; Kim, K. W.; Zhang, Y. et al.

Equally efficient interlayer exciton relaxation and

improved absorption in epitaxial and nonepitaxial

MoS2/WS2 heterostructures. Nano Lett. 2015, 15, 486-491.

[12] Levendorf, M. P.; Kim, C.-J.; Brown, L.; Huang, P. Y.;

Havener, R. W.; Muller, D. A.; Park, J. Graphene and

boron nitride lateral heterostructures for atomically thin

circuitry. Nature 2012, 488, 627-632.

[13] Sutter, P.; Cortes, R.; Lahiri, J.; Sutter, E. Interface

formation in monolayer graphene-boron nitride

heterostructures. Nano Lett. 2012, 12, 4869-4874.

[14] Miyata, Y.; Maeda, E.; Kamon, K.; Kitaura, R.; Sasaki, Y.;

Suzuki, S.; Shinohara, H. Fabrication and characterization

of graphene/hexagonal boron nitride hybrid sheets. Appl.

Phys. Express 2012, 5, 085102.

[15] Liu, Z.; Ma, L.; Shi, G.; Zhou, W.; Gong, Y.; Lei, S.;

Yang, X.; Zhang, J.; Yu, J.; Hackenberg, K. P. et al.

In-plane heterostructures of graphene and hexagonal boron

nitride with controlled domain sizes. Nat. Nanotechnol.

2013, 8, 119-124.

[16] Liu, L.; Park, J.; Siegel, D. A.; McCarty, K. F.; Clark, K.

W.; Deng, W.; Basile, L.; Idrobo, J. C.; Li, A.-P.; Gu, G.

Heteroepitaxial growth of two-dimensional hexagonal

boron nitride templated by graphene edges. Science 2014,

343, 163-167.

[17] Han, G. H.; Rodríguez-Manzo, J. A.; Lee, C.-W.; Kybert,

N. J.; Lerner, M. B.; Qi, Z. J.; Dattoli, E. N.; Rappe, A. M.;

Drndic, M.; Johnson, A. T. C. Continuous growth of

hexagonal graphene and boron nitride in-plane

heterostructures by atmospheric pressure chemical vapor

deposition. ACS Nano 2013, 7, 10129-10138.

[18] Havener, R. W.; Kim, C.-J.; Brown, L.; Kevek, J. W.;

Sleppy, J. D.; McEuen, P. L.; Park, J. Hyperspectral

imaging of structure and composition in atomically thin

heterostructures. Nano Lett. 2013, 13, 3942-3946.

[19] Gao, Y.; Zhang, Y.; Chen, P.; Li, Y.; Liu, M.; Gao, T.; Ma,

D.; Chen, Y.; Cheng, Z.; Qiu, X. et al. Toward single-layer

uniform hexagonal boron nitride-graphene patchworks

with zigzag linking edges. Nano Lett. 2013, 13,

3439-3443.

[20] Gong, Y.; Lin, J.; Wang, X.; Shi, G.; Lei, S.; Lin, Z.; Zou,

X.; Ye, G.; Vajtai, R.; Yakobson, B. I. et al. Vertical and

in-plane heterostructures from WS2/MoS2 monolayers. Nat.

Mater. 2014, 13, 1135-1142.

[21] Duan, X.; Wang, C.; Shaw, J. C.; Cheng, R.; Chen, Y.; Li,

H.; Wu, X.; Tang, Y.; Zhang, Q.; Pan, A. et al. Lateral

epitaxial growth of two-dimensional layered

semiconductor heterojunctions. Nat. Nanotechnol. 2014, 9,

1024-1030.

[22] Huang, C.; Wu, S.; Sanchez, A. M.; Peters, J. J. P.;

Beanland, R.; Ross, J. S.; Rivera, P.; Yao, W.; Cobden, D.

H.; Xu, X. Lateral heterojunctions within monolayer

MoSe2/WSe2 semiconductors. Nat. Mater. 2014, 13,

1096-1101.

[23] Zhang, X.-Q.; Lin, C.-H.; Tseng, Y.-W.; Huang, K.-H.;

Lee, Y.-H. Synthesis of lateral heterostructures of

semiconducting atomic layers. Nano Lett. 2015, 15,

410-415.

[24] Chen, Y.; Xi, J.; Dumcenco, D. O.; Liu, Z.; Suenaga, K.;

Wang, D.; Shuai, Z.; Huang, Y.-S.; Xie, L. Tunable band

gap photoluminescence from atomically thin

transition-metal dichalcogenide alloys. ACS Nano 2013, 7,

4610-4616.

[25] Gong, Y.; Liu, Z.; Lupini, A. R.; Shi, G.; Lin, J.; Najmaei,

S.; Lin, Z.; Elías, A. L.; Berkdemir, A.; You, G. et al.

Band gap engineering and layer-by-layer mapping of

| www.editorialmanager.com/nare/default.asp

8 Nano Res.

selenium-doped molybdenum disulfide. Nano Lett. 2014,

14, 442-449.

[26] Chen, Y.; Dumcenco, D. O.; Zhu, Y.; Zhang, X.; Mao, N.;

Feng, Q.; Zhang, M.; Zhang, J.; Tan, P.-H.; Huang, Y.-S.

et al. Composition-dependent raman modes of Mo1-xWxS2

monolayer alloys. Nanoscale 2013, 6, 2833-2839.

[27] Zhao, W.; Ghorannevis, Z.; Chu, L.; Toh, M.; Kloc, C.;

Tan, P.-H.; Eda, G. Evolution of electronic structure in

atomically thin sheets of WS2 and WSe2. ACS Nano 2012,

7, 791-797.

[28] Mak, K. F.; Lee, C.; Hone, J.; Shan, J.; Heinz, T. F.

Atomically thin MoS2 : A new direct-gap semiconductor.

Phys. Rev. Lett. 2010, 105, 136805.

[29] Splendiani, A.; Sun, L.; Zhang, Y.; Li, T.; Kim, J.; Chim,

C.-Y.; Galli, G.; Wang, F. Emerging photoluminescence in

monolayer MoS2. Nano Lett. 2010, 10, 1271-1275.

[30] Zhou, H.; Yu, F.; Liu, Y.; Zou, X.; Cong, C.; Qiu, C.; Yu,

T.; Yan, Z.; Shen, X.; Sun, L. et al. Thickness-dependent

patterning of MoS2 sheets with well-oriented triangular

pits by heating in air. Nano Res. 2013, 6, 703-711.

[31] Tongay, S.; Suh, J.; Ataca, C.; Fan, W.; Luce, A.; Kang, J.

S.; Liu, J.; Ko, C.; Raghunathanan, R.; Zhou, J. et al.

Defects activated photoluminescence in two-dimensional

semiconductors: Interplay between bound, charged, and

free excitons. Sci. Rep. 2013, 3, 2657.

[32] Bissett, M. A.; Konabe, S.; Okada, S.; Tsuji, M.; Ago, H.

Enhanced chemical reactivity of graphene induced by

mechanical strain. ACS Nano 2013, 7, 10335-10343.

[33] Kobayashi, Y.; Sasaki, S.; Mori, S.; Hibino, H.; Liu, Z.;

Watanabe, K.; Taniguchi, T.; Suenaga, K.; Maniwa, Y.;

Miyata, Y. Growth and optical properties of high-quality

monolayer WS2 on graphite. ACS Nano 2015, 9,

4056-4063.

[34] Orofeo, C. M.; Suzuki, S.; Sekine, Y.; Hibino, H. Scalable

synthesis of layer-controlled WS2 and MoS2 sheets by

sulfurization of thin metal films. Appl. Phys. Lett. 2014,

105, 083112.

[35] Gutiérrez, H. R.; Perea-López, N.; Elías, A. L.; Berkdemir,

A.; Wang, B.; Lv, R.; López-Urías, F.; Crespi, V. H.;

Terrones, H.; Terrones, M. Extraordinary

room-temperature photoluminescence in triangular WS2

monolayers. Nano Lett. 2012, 13, 3447-3454.

[36] Chhowalla, M.; Shin, H. S.; Eda, G.; Li, L.-J.; Loh, K. P.;

Zhang, H. The chemistry of two-dimensional layered

transition metal dichalcogenide nanosheets. Nat. Chem.

2013, 5, 263-275.

[37] Yu, Q.; Jauregui, L. A.; Wu, W.; Colby, R.; Tian, J.; Su, Z.;

Cao, H.; Liu, Z.; Pandey, D.; Wei, D. et al. Control and

characterization of individual grains and grain boundaries

in graphene grown by chemical vapour deposition. Nat.

Mater. 2011, 10, 443-449.

[38] Ren, Y.; Zhu, C.; Cai, W.; Li, H.; Hao, Y.; Wu, Y.; Chen, S.;

Wu, Q.; Piner, R. D.; Ruoff, R. S. An improved method for

transferring ggarphene grown by chemical vapor

deposition. Nano 2012, 7, 1150001.

www.theNanoResearch.com∣www.Springer.com/journal/12274 | Nano Research

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.

| www.editorialmanager.com/nare/default.asp

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.

www.theNanoResearch.com∣www.Springer.com/journal/12274 | Nano Research

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.

| www.editorialmanager.com/nare/default.asp

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.

www.theNanoResearch.com∣www.Springer.com/journal/12274 | Nano Research

Nano Res.