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Scanning tunneling microscopy/spectroscopy on MoS2 embedded nanowire formed in CVDgrown Mo1− x W x S2 alloy

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2017 Jpn. J. Appl. Phys. 56 08LB06(http://iopscience.iop.org/1347-4065/56/8S1/08LB06)
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REGULAR PAPER

Japanese Journal of Applied Physics 56, 08LB06 (2017)https://doi.org/10.7567/JJAP.56.08LB06

Scanning tunneling microscopy/spectroscopy on MoS2 embedded nanowire formedin CVD-grown Mo1%xWxS2 alloy
Hiroyuki Mogi1, Yu Kobayashi2, Atsushi Taninaka1, Ryuji Sakurada1, Takahiro Takeuchi1, Shoji Yoshida1,
Osamu Takeuchi1, Yasumitsu Miyata2, and Hidemi Shigekawa1*
1Faculty of Pure and Applied Sciences, University of Tsukuba, Tsukuba, Ibaraki 305-8573, Japan
Department of Physics, Tokyo Metropolitan University, Hachioji, Tokyo 192-0397, Japan2*E-mail: hidemi@ims.tsukuba.ac.jp
Received February 17, 2017; revised May 7, 2017; accepted May 10, 2017; published online July 19, 2017
MoS2 embedded nanowires formed in a transition-metal dichalcogenide (TMDC) layered semiconductor of Mo1%xWxS2 alloy grown by chemical
vapor deposition (CVD) on graphite were observed for the ﬁrst time. Three nanowires radiated outward from the center of each triangular
Mo1%xWxS2 island to its three corners, suggesting that they were formed during the growth process. The bandgap energy in the wires was 2.38 eV,
0.03 eV narrower than the average bandgap energy in the region surrounding the nanowire. The observed results suggest the possibility ofdesigning embedded nanostructures in a TMDC by controlling the growth conditions, which should lead to further advances in TMDC materials forthe development of new types of devices. © 2017 The Japan Society of Applied Physics

1.

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Introduction

Monolayer transition-metal dichalcogenides (TMDCs), consisting of a transition-metal atomic layer (e.g., W or Mo)
sandwiched by two chalcogen atomic layers (S, Se, and Te),have been attracting considerable attention because of their
excellent electrical and optical characteristics for applications.1–9) They are chemically stable and their bandgap
energy varies in the visible-light range (for example, MoS2:
2.4 eV,10) WS2: 2.7 eV11)). To increase the applicability of
their characteristics, tunability of the bandgap energy is a key
factor and strongly desired. Recently, this requirement has
been realized by the growth of Mo1−xWxS2 and Mo1−xWxSe2
alloys, for which the bandgap energy has been conﬁrmed tobe tunable via their compositional ratios.12–15) The formationof heterostructures using these alloys has enabled tunableband alignment.16–18)
For the further development of these promising materials,
the fabrication of nanostructures is the next target. For III–V
and other compound semiconductors, metal organic chemical
vapor deposition (MOCVD) and molecular beam epitaxy(MBE) are well-established growth methods for producing
highly functional devices based on the fabrication ofquantum structures such as quantum wells, quantum wires,and quantum dots.19–21) To utilize such characteristics ofalloys in TMDCs, a deeper understanding of their growthmechanism is urgently required. Here we present the resultsof a scanning tunneling microscopy=spectroscopy (STM=
STS) study on the growth process of Mo1−xWxS2, in which
the formation of MoS2 embedded nanowires was observed
for the ﬁrst time.
2.

Sample preparation

Figure 1 shows a schematic illustration of the sample
preparation. Mo1−xWxS2 was formed on kish graphite(Covalent Materials) by high-temperature CVD.22) The
graphite was mechanically exfoliated onto a quartz substrateusing Nitto tape (SPV-224). The substrate was placed in aquartz tube (3 cm diameter, 100 cm length) with WO3 powder(Aldrich, 99% purity, 100 mg), MoO3 powder (Aldrich, 99%
purity, 0.2 mg), and sulfur ﬂakes (Aldrich, 99.99% purity,
2 g). The quartz tube was then ﬁlled with Ar gas at a ﬂow rate

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Fig. 1. (Color online) Schematic illustration of the growth process ofMo1−xWxS2 on graphite by CVD with an Ar gas ﬂow.

of 100 cm3=min. The temperature of the substrate and the
WO3 and MoO3 was gradually increased to the growthtemperature (1100 °C) over 60 min using an electrical
furnace. When the substrate temperature reached the set
value, the sulfur was heated at 200 °C for 15–30 min using
another electrical furnace to supply sulfur vapor to thesubstrate. After the growth, the quartz tube was immediately
cooled using an electric fan.
3.

Characterization of the sample

First, compositional analysis was carried out using Raman
spectroscopy (RS). Figure 2 shows secondary electron microscope (SEM; acceleration energy of 0.8 keV) images ofMo1−xWxS2 alloy samples formed on kish graphite and theRaman spectra obtained from a triangular island. The shapeof the islands depends on the MoþW : S ratio in the vapor
and the temperature,23) and equilateral triangular islands withthreefold symmetry of the lattice were grown under thepresent conditions. Multilayered islands were brightly imaged
and the dark area outside the islands corresponds to thegraphite substrate. As shown in Fig. 2(b), the island
comprises a small equilateral triangle surrounded by a slightly
larger one. As previously studied, the inner equilateraltriangular area is not a second layer but an area of Mo-rich
Mo1−xWxS2 alloy surrounded by an area of W-rich alloy
corresponding to the larger triangle, i.e., a heterostructure of
W-rich and Mo-rich Mo1−xWxS2 alloys is formed.14,16) As
indicated in Fig. 2(b), a rodlike nucleus existednear thecenter of each Mo-rich area.
Figures 2(c) and 2(d) show Raman spectra obtained for thepure MoS2 and WS2, and Mo1−xWxS2 alloy formed on

08LB06-1© 2017 The Japan Society of Applied Physics

Jpn. J. Appl. Phys. 56, 08LB06 (2017)

H. Mogi et al.

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Fig. 2. (Color online) (a) SEM image of Mo1−xWxS2 grown on graphite. (b) Magniﬁcation of a triangular Mo1−xWxS2 island in the square in (a). (c) Raman
spectra obtained for pure MoS2 and WS2 formed on graphite. (d) Raman spectrum obtained from Mo1−xWxS2 grown on graphite. From the shift of the Ramanpeaks in the spectrum from those for the pure MoS2 and WS2 in (c), the component ratio x was estimated to be 0.3 and 0.9 for Mo-rich and W-rich areas,
respectively.

graphite, respectively. The sample was excited by a laser
with a wavelength of 532 nm and spot size of 1 µm at room
temperature. The peaks for the pure MoS2 (405 cm−1) andWS2 (419 cm−1) are assigned to A1g in Fig. 2(c),22) and it
has been reported that the Raman spectra of monolayer
Mo1−xWxS2 alloys exhibit three characteristic peaks in theregion from 350 to 420 cm−1, which are assigned to the WS2like EA, MoS2-like EA, and A01 modes in order of increasing
frequency. The frequency of the A01 mode increases almost
linearly with increasing W concentration from the value for
the A01 mode of pure MoS2 to that for WS2.12,14,24) Therefore,
from the observed positions of A01 for the MoS2-like(409 cm−1) and WS2-like (419 cm−1) peaks, the component
ratio of x was estimated to be 0.3 (Mo-rich area) and 0.9(W-rich area), respectively, where the value of 0.3 is in good
agreement with that obtained in our previous paper.16)
4.

STM/STS analysis

Figure 3(a) shows an STM image of a Mo1−xWxS2 heterostructure formed on graphite. A Mo-rich triangular structure
is surrounded by a W-rich area. As shown by the crosssection in Fig. 3(b), the heights of the islands are ∼0.6 and∼0.8 nm for the W-rich and Mo-rich areas, respectively,
which are in good agreement with the height of a monolayerof Mo1−xWxS2 (0.6 nm). The observed diﬀerence of 0.2 nm
for the Mo-rich area originates from the higher local densityof states (LDOS) in this area. As shown in the previous
paper, we counted Mo and W atoms using high-resolution
STM images of the same sample.16) The average Mo : W
ratios were 1 : 9 for a W-rich area and 7 : 3 for a Mo-richarea in good agreement with the results obtained by Raman
measurement in this work.
As shown in Figs. 3(c) and 3(d), three wires with nanoscale
width radiated outward from the nucleus in the triangularisland to its three corners, as schematically shown in the inset
of Fig. 3(d) and in Fig. 3(e). These characteristics wereobserved for all the islands. Figure 4(a) shows an STM imageof a nanowire including its surrounding area. As describedabove, the average Mo : W ratio in the Mo-rich area was 7 : 3
according to the result of Raman measurement (averaged overan area of ∼1 µm2). As shown in Fig. 4(a), however, there arenanoscale domains with diﬀerent brightnesses in the areasurrounding the nanowire, which correspond to Mo1−xWxS2
areas with diﬀerent values of x, namely, the alloy was not
uniformly formed at the nanoscale. Figure 4(b) shows a highresolution image of a nanowire, in which each atom in the
nanowire is clearly imaged. Since Mo and W atoms arerespectively imaged as bright and dark areas in Mo1−xWxS2
alloy,16) the central area in the nanowire, where all atoms areimaged as bright areas, is considered to be pure MoS2. As
shown in our previous paper, the electronic structure of a
Mo-rich=W-rich area is strongly inﬂuenced by the Mo=W
composition in the surrounding areas,16) which is considered
to produce the variation in the brightness in the nanowire. In
fact, the wide part of the nanowire, which is surrounded by alarger Mo-rich area, is observed to be brighter.
These nanowires are considered to show the growthtrajectory of the corner of the island. Namely, the Mo-richarea is grown from both ends of the nucleus. Since thegrowth nucleus has a rod shape, the starting point of
nanowire growth is not located at the center of the Mo-richarea. However, since a metal-terminated edge (Mo or W) is
stable, the island becomes an equilateral triangle, which is the
structure of TMDC schematically shown in Fig. 5.23,25)
Figure 4(c) shows spectra obtained from above the central
area of a nanowire (red) and those averaged over the areasurrounding the nanowire in the Mo-rich area (blue). These
spectra were obtained by averaging 500 spectra in each area.

08LB06-2© 2017 The Japan Society of Applied Physics

Jpn. J. Appl. Phys. 56, 08LB06 (2017)

H. Mogi et al.

(a)

(c)

(d)

(b)

(e)

Fig. 3. (Color online) (a) STM image of a Mo1−xWxS2 heterostructure formed on a graphite in a Mo-rich triangle area surrounded by a W-rich area (sample
bias Vs = 1.4 V, setpoint tunnel current It = 80 pA). (b) Cross section obtained along the line in (a). (c) and (d) magniﬁcations of the center and a corner of the
Mo-rich triangular island in (a), respectively (Vs = 1.6 V, It = 200 pA, 80 K). The inset in (d) shows a schematic view of the structure. Red arrows mark thepositions of nanowires. The nucleus is enclosed within a blue dashed ellipse. (e) Composition-emphasized schematic model of the same area as in (a). Red lines
represent the nanowires shown in (c) and (d). The white arrow corresponds to that shown in (c).

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Fig. 4. (Color online) (a) Wide-range STM image of a MoS2 wire formed in a Mo-rich area (Vs = 0.7 V, It = 100 pA). (b) High-resolution image of a MoS2
wire and the cross section along the blue line in the image (Vs = 1.5 V, It = 300 pA, 80 K). (c) Spectra obtained from above a MoS2 wire and the surroundingarea (Mo-rich area). A spectrum obtained over a W-rich area is also shown for comparison. (d) Enlargement of part of (c) showing the conduction band edge.
The horizontal dashed line indicates the threshold where the eﬀective signal is higher than the noise level.

For comparison, a spectrum obtained in the W-rich area isalso shown (green), for which the bandgap energy was
2.73 eV and close to the value for pure WS2 (2.74 eV). Since
the bandgap energy markedly changes with the shift of the
conduction band edge in the Mo-rich area,16) a magniﬁcationof the conduction band edge is shown in a logarithmic plot
in Fig. 4(d) to show the shift clearly. The bandgap energy inthe wires was about 2.38 eV, close to the value for pure MoS2

(2.4 eV) as expected, and 0.03 eV less than the bandgap
energy averaged over the region surrounding the nanowire.
Here, the conduction band edge ECBM and the valence bandedge EVBM were determined using the bias voltages at which
the signal became higher than the noise level. The diﬀerencein the bandgap is less than that obtained in our previous
paper, 0.075 eV, because measurement was carried out in Mo
and W areas in Mo1−xWxS2 in the previous paper. To obtain a

08LB06-3© 2017 The Japan Society of Applied Physics

Jpn. J. Appl. Phys. 56, 08LB06 (2017)(a)

H. Mogi et al.

(b)

(c)

(d)

(c)

(b)

(a)

Fig. 5. (Color online) (a–c) Three structures used for simulation. (d) Relative change in the total energy.

larger shift in the bandgap energy to make the nanowire more
applicable, a larger value of x such as 0.9 is required.26)
5.

Simulation

To determine the mechanism of the nanowire growth process
in more detail, a simple simulation was carried out.
Figures 5(a)–5(c) show the schematic structures used forthe simulation. A pair of Mo atoms in a triangular MoS2
structure were replaced by two W atoms, as indicated by two
red circles in each diagram. The simulation was carried out
using Atomistix ToolKit with the Perdew–Burke–Ernzerhof(PBE) exchange interaction potential for the generalized
gradient approximation (GGA).27) A double-ζ base was used
for all atoms and the cutoﬀ energy was set to 100 Ry. The
relative diﬀerence in the total energy with the change in theposition of W atoms is shown in Fig. 5(d). The fact that the
total energy decreases as the structure changes from (a) to (c)suggests the preferential adsorption of W atoms at the edges
away from the corner of the triangular MoS2 crystal. The
distance moved by W atoms is considered to depend on thegrowth conditions, such as the temperature and the composition of the Mo=W vapor. This could lead to the formationof a MoS2 wire with a certain width (about 3 nm in thisexperiment) during the growth of Mo1−xWxS2 alloy. Moredetailed simulations using molecular dynamics, for example,
with a more realistic size are necessary to obtain further
understanding of the formation of a MoS2 wire.
6.

Summary

MoS2 embedded nanowires formed in a transition-metal
dichalcogenide (TMDC) layered semiconductor ofMo1−xWxS2 alloy during its growth by chemical vapor
deposition (CVD) on graphite were observed for the ﬁrst
time. The bandgap energy in the wires was about 2.38 eV,
0.03 eV narrower than the average bandgap energy in theregion surrounding the nanowire. The observed results
suggest the possibility of designing nanostructures in a
TMDC by controlling the growth conditions, which should
lead to the development of new types of devices.
Acknowledgements

This work was supported by Japan Society for the Promotionof Science in the form of Grants-in-Aid for ScientiﬁcResearch (15H05734 and 16H00918) and CREST, Japan
Science and Technology Agency.

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08LB06-4© 2017 The Japan Society of Applied Physics