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Accepted Manuscript
Controlled growth of MoS2 nanopetals on the silicon nanowire array using the
chemical vapor deposition method
Shang-Min Chen, Yow-Jon Lin
PII:
DOI:
Reference:
S0022-0248(17)30635-8
https://doi.org/10.1016/j.jcrysgro.2017.10.028
CRYS 24348
To appear in:
Journal of Crystal Growth
Received Date:
Revised Date:
Accepted Date:
6 September 2017
8 October 2017
22 October 2017
Please cite this article as: S-M. Chen, Y-J. Lin, Controlled growth of MoS2 nanopetals on the silicon nanowire array
using the chemical vapor deposition method, Journal of Crystal Growth (2017), doi: https://doi.org/10.1016/
j.jcrysgro.2017.10.028
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Controlled growth of MoS2 nanopetals on the silicon nanowire array
using the chemical vapor deposition method
Shang-Min Chena, Yow-Jon Linb,*
a
Department of Physics, National Changhua University of Education, Changhua 500,
Taiwan
b
Institute of Photonics, National Changhua University of Education, Changhua 500,
Taiwan
*
Author
to
whom correspondence
should
rzr2390@yahoo.com.tw
1
be
addressed.
E-mail
address:
Abstract
In order to get a physical/chemical insight into the formation of nanoscale
semiconductor heterojunctions, MoS2 flakes are deposited on the silicon nanowire
(SiNW) array by chemical vapor deposition (CVD). In this study, H2O2 treatment
provides a favorable place where the formation of Si-O bonds on the SiNW surfaces
that play important roles (i.e., the nucleation centers, catalyst control centers or
“seeds”) can dominate the growth of MoS2 on the SiNWs. Using this configuration,
the effect of a change in the S/MoO3 mass ratio (MS/MMoO3) on the surface
morphology of MoS2 is studied. It is shown that an increase in the value of MS/MMoO3
leads to the increased nucleation rate, increasing the size of curved MoS 2 nanopetals.
This study provides valuable scientific information for directly CVD-grown
edge-oriented MoS2/SiNWs heterojunctions for various nanoscale applications,
including hydrogen evolution reaction and electronic and optoelectronic devices.
Keywords: A1. Crystal morphology; A3. Chemical vapor deposition processes; B1.
Nanomaterials; B2. Semiconducting silicon
2
1. Introduction
Transition
metal
dichalcogenides
(TMDs)
are
important
layered
semiconductors because of their unique physical and chemical properties. TMDs have
garnered considerable interest as the thinnest building blocks for next-generation
electronic and optoelectronic devices. Of these TMDs, MoS2 has been extensively
studied because it has good electrical and optical properties [1]. MoS2 has been
synthesized by chemical vapor deposition (CVD), lithium-based intercalation,
metal-organic chemical vapor deposition, and pulsed laser deposition but primarily on
insulating oxide substrates [2-18]. MoS2 grown on insulating substrates is limited to
simple device architectures and generally requires transfer to other substrates in order
to be integrated into heterostructure devices. The preparation process and the
equipment for these methods are very complex. On the other hand, a heterojunction
that is composed of MoS2 and Si has many potential applications in electronic and
optoelectronic devices. The integration of MoS2 on Si could lower the cost of
electronic and optoelectronic devices and multifunctional devices would be possible
[2]. Silicon nanowires (SiNWs) have been considered as building blocks for
nano-electronics due to their high surface to volume ratio and their unique quasi
one-dimensional electronic structure as well as their excellent electronic, photonic,
thermal, and mechanical properties and hence have been studied for various nanoscale
3
applications, including field-effect transistors, chemical/biological sensors, and
energy conversion devices [19-26]. Therefore, the SiNWs can provide an outstanding
platform to study the role of dimensionality and size effects in semiconductor devices.
This study represents a step forward toward the integration of MoS 2 into existing
SiNW technology for new generation electronic/optoelectronic nanoscale devices.
The entire process for MoS2/SiNWs heterojunctions that is demonstrated in this study
requires no transfer processing, which minimizes cost and leverages the existing Si
manufacturing infrastructure to maximize performance. To date, there have been no
reports of the fabrication and characterization of MoS 2 flakes that were directly
CVD-grown on the SiNWs. In addition, little is known about the specifics of the
nucleation and growth mechanisms occurring during the CVD process. In this study,
H2O2 treatment provides a favorable place where the formation of Si-O bonds on the
SiNW surfaces that play important roles can dominate the growth of MoS2 on the
SiNWs. To demonstrate growth mechanisms, the mass of sulfur powder is increased
and the effect of an increases in the value of the S/MoO3 mass ratio (MS/MMoO3) on
the morphology of MoS2 nanostructures is studied.
2. Experimental section
Four-inch 525 μm-thick n-Si (100) wafers with an electrical resistivity of
about 5 Ω cm (Guv Team International Co., Ltd.) were used in the experiment. The
4
n-Si substrates were ultrasonically cleaned for 10 min each in acetone, then in
methanol, then in de-ionized water and dried in nitrogen. The SiNW array was then
formed adopting a developed silver-induced wet-chemical-etching process in an
aqueous buffered HF and AgNO3 etching solution at 25 °C. The etching time was 25
min. The surface color of these wafers appeared black after removing the Ag
remnants by immersing them in the concentrated HNO3 solution for one hour. Some
of the as-grown SiNWs/n-Si samples were then dipped in a H2O2 solution at 60 oC for
10 min and dried in nitrogen (referred to as H2O2-treated SiNWs/n-Si samples). The
morphologies of the SiNWs were studied using field emission scanning electron
microscopy (FESEM). To verify the effect of H2O2 treatment on the surface
properties of the SiNWs, the Si 2p core levels were analyzed using X-ray
photoelectron spectroscopy (XPS). XPS measurements were performed using a
monochromatic Al Kα X-ray source. These were calibrated by using the C 1s peak as
a reference. Few-layer MoS2 was grown on the as-grown and H2O2-treated SiNWs
using the CVD method. This CVD setup included a three-temperature-zone furnace
and three quartz tubes into which sulfur, MoO3, and substrates were sequentially
placed in each temperature zone. A CVD method for growing MoS2 on the SiNWs
that allows independent control over all deposition parameters is described. The
growth process was performed in a CVD system, using MoO3 (15 mg) and sulfur (100
5
mg) powder as the precursor (referred to as group A). That is, the value of MS/MMoO3
is 6.7 during the CVD-grown process. The growth process was performed in a CVD
system, using MoO3 (15 mg) and sulfur (300 mg) powder as the precursor (referred to
as group B). That is, the value of MS/MMoO3 is 20.0 during the CVD-grown process.
The lengths of zones I (for S) and II (for MoO3) were both 3 cm, and the distance
between S and MoO3 was about 20 cm. The SiNWs/n-Si substrate (2×1 cm2) was
placed downwind along zone III. The temperature in zone I was 110 oC, which was
maintained for 900 s. The temperature in zone II was 110 oC, which was maintained
for 900 s. The temperature in zone III was 750 oC, which was maintained for 1000 s.
During the growth period, argon was used as the carrying gas at a flow rate of 130
SCCM (standard cubic centimeters per minute) and the vacuum pressure was
maintained at 89.3 Pa. Figure 1 shows a schematic illustration of the CVD system and
the locations of S and MoO3 precursor holders together with the substrate positions.
The structural properties of MoS2 flakes were determined using Raman spectroscopy.
A 532-nm laser was used for excitation. The morphologies of MoS2 flakes were
studied using FESEM. XPS was used to identify the chemical bonding state of the
MoS2 samples. The XPS core-level peaks are deconvolved into their various
components
using
an
interactive
least-squares
Gaussian–Lorentzian peaks were used in this analysis.
6
computer
program.
Mixed
3. Results and discussion
Figure 2 shows cross-sectional and plane FESEM images of the SiNWs/n-Si
sample. The SiNW length, as estimated from FESEM, was about 1.9 μm. Raman
spectroscopy has been employed to confirm the formation of MoS2 flakes. Figure 3
shows the Raman spectra for MoS2/H2O2-treated SiNWs/n-Si samples from groups A
and B, indicating that there were MoS2 flakes on the H2O2-treated SiNW array. For
comparison, the actual position of Raman bands and shifts of MoS 2 flakes are shown
in reference to the Si peak of 521 cm−1 [18]. The observed A1g and E2g1 bands in
MoS2 are inferred as the out-of-plane vibration of sulfur atoms along the c-axis and
in-plane vibration of S−Mo−S atoms along the basal plane, respectively [18]. No
Raman spectra that correspond to A1g and E2g1 bands were identified for the
CVD-grown MoS2/as-grown SiNWs/n-Si samples from groups A and B, which shows
that there were no MoS2 flakes on the as-grown SiNW array.
Figure 4(a) shows the Si 2p core-level spectra for SiNWs/n-Si samples with
and without H2O2 treatment. The peak positioned at 99.4 eV is attributed to Si-Si
bonds and the peak positioned at 103.3 eV is attributed to Si–O bonds [27-29]. Note
that the Si-O bonds were formed at the H2O2-treated SiNW surfaces. The ratio of the
103.3 eV-peak intensity to the 99.4 eV-peak intensity (ISi-O/ISi-Si) was calculated. The
ISi-O/ISi-Si ratio for the as-grown SiNWs/n-Si and 10 min-H2O2-treated SiNWs/n-Si
7
samples was, respectively, calculated to be 0 and 1.05. The increased ISi-O/ISi-Si ratio
may lead to an increase in the thickness of the SiOx layer [27,29]. Malik at al. [30]
reported that an ultra-thin (1.5–1.7 nm thick) SiOx layer was grown on the Si surface
by immersing the wafer in a hot hydrogen peroxide solution for 10 min. It is
suggested that few-layer MoS2 were directly deposited on the H2O2-treated SiNWs by
the CVD method because an ultra-thin SiOx film forms on the SiNW surfaces. This
observation clearly shows the importance of Si-O bonds on the SiNW surfaces for the
CVD growth process. H2O2 treatment provides a favorable place where the formation
of Si-O bonds that play important roles (i.e., the nucleation centers, catalyst control
centers or “seeds”) can dominate the growth of MoS2 flakes on the SiNWs. The
growth of MoS2 has been commonly observed on bare SiO2 surfaces [3,7]. Figure 4(b)
shows the Mo 3d and S 2s XPS spectra for MoS2/H2O2–treated SiNWs samples from
groups A and B. From the XPS spectra (Fig. 4b), the peaks at 233 and 229.7 eV
represent the Mo 3d3/2 and Mo 3d5/2 for the MoS2 sample from groups A or B,
respectively, and the peak at 226.6 eV corresponds to S 2s for MoS2 samples from
groups A or B. These binding energies are in good agreement with the reported values
for MoS2 samples [31].
The multilayer flakes show characteristic A1g and E2g1 Raman modes for
MoS2/H2O2-treated SiNWs/n-Si samples from group A, which are respectively located
8
at around 403.7 cm-1 and 378.2 cm−1. The multilayer flakes show characteristic A1g
and E2g1 Raman modes for MoS2/H2O2-treated SiNWs/n-Si samples from group B,
which are respectively located at around 403.9 cm-1 and 377.2 cm−1. The frequency
difference between the A1g and E2g1 peaks was about 25.5 (26.7) cm−1 for
MoS2/H2O2-treated SiNWs/n-Si samples from group A (group B). The frequency
difference between the A1g and E2g1 peaks indicated that multilayers of MoS2 were
formed in this study [18,31,32]. Raman spectroscopy has been widely utilized as a
useful tool for probing the intriguing physical properties such as the lattice strain and
the number of layers [31]. Considerable variation in the Raman shift is observed for
MoS2/H2O2-treated SiNWs/n-Si samples from groups A and B. It is obvious that the
A1g peak of MoS2 from group B shows a negligible blue-shift of 0.2 cm−1, whereas the
E2g1 peak of MoS2 from group B shows a significant red-shift of 1.0 cm−1, compared
to those of MoS2 from group A. Because the E2g1 in-plane vibration mode is highly
sensitive to the built-in tensile strain of two-dimensional MoS2 [31], the red-shift of
the E2g1 peak position can be attributed to the effect of tensile strain that results from
an induced high nucleation rate by an increase in the value of MS/MMoO3 [18]. Using
the Gruneisen parameter of the E2g1 mode for MoS2 and the initial wavenumber of
383.8 cm-1, we can calculate the biaxial lattice strain in the MoS 2 layer from groups A
or B [31,33,34]. The biaxial tensile strain on MoS2 directly deposited on the SiNW
9
array from group A (group B) can be estimated as 1.1 (1.3) %, which is greater than
the previously reported strain values [31,33,34]. We consider the possibility of a
nonuniform strain distribution within the illuminated area for MoS2 that was deposited
on the SiNWs, which would lead to a larger range of Raman shifts for the E2g1 peak
given its larger shift rate.
The ratio of E2g1 to A1g peak intensities (IE2g/IA1g) calculated from the Raman
results can be used to distinguish between basal- and edge-oriented MoS2 [35,36]. The
value of IE2g/IA1g of MoS2 observed from group A (group B) is 0.40, (0.34), which is
attributed to the existence of both horizontally and vertically aligned MoS2 layers in
the nanopetals and consistent with that of edge-oriented MoS2 nanopetals [35]. With
the increased value of MS/MMoO3, mass transport in both horizontal and vertical
direction is enhanced, leading to a change in the size of curved nanopetals. Özden et
al. [37] suggested that local changes of the S to Mo vapor ratio in the growth zone is a
key factor for the change of shape and size and of the resulting MoS2 formations. In
addition, the edge-oriented MoS2 nanopetals deliver excellent hydrogen evolution
reaction (HER) activity with enhanced kinetics and long-term cycling stability [36,38].
Due to the high surface to volume silicon ratio and unique quasi one-dimensional
electronic structure, SiNW based devices have properties that can outperform their
traditional counterparts in many ways. The combination of the SiNWs with MoS2
10
flakes appears to be a promising way to realize application in HER by increasing the
conductivity. The concept of heterojunctions of MoS2/SiNWs opens a promising
direction for applications in HER.
In order to verify the effect of a change in the value of MS/MMoO3 on the
surface morphology of MoS2, the value of MS/MMoO3 was varied from 6.7 to 20.0.
Figure 3 shows that the intensity of the E2g1 (A1g) peak increases with increasing the
value of MS/MMoO3. This is because of a high nucleation rate induced by an increase
in the value of MS/MMoO3. Kumar and Viswanath [18] reported sulfur evaporation rate
controlled, screw dislocation driven CVD growth of MoS 2 flakes with a spiral
structure that provides enhanced active sites due to increased edge length by
approximately 5 fold. Figure 5 shows cross-sectional and plane FESEM images of the
MoS2/H2O2-treated SiNWs/n-Si sample from groups A and B. It is seen the shape of
flake, indicating that MoS2 nanopetals were formed on the H2O2-treated SiNWs. The
size of MoS2 nanopetals from group B is larger than that of MoS2 nanopetals from
group A. It is suggested that an increase in the value of MS/MMoO3 lead to the
increased sulfur evaporation rate, inducing a high nucleation rate. The developed
strategy of the increased value of MS/MMoO3 to induce a high nucleation rate promotes
the achievement of large area MoS2 flakes. Such sulfur evaporation rate controlled
growth is important for surface engineering of edge-oriented MoS2 nanopetals to
11
harvest the benefits of active edge sites for applications in catalyst and electronic and
optoelectronic devices.
4. Conclusions
We have demonstrated a CVD method for the growth MoS2 flakes on the
as-grown and H2O2-treated SiNWs. A full understanding of the specifics of the
nucleation and growth mechanisms is necessary. The CVD growth of MoS2 flakes on
the H2O2-treated SiNWs is observed. However, there were no MoS2 flakes on the
as-grown SiNWs. H2O2 treatment provides a favorable place where the formation of
Si-O bonds on the SiNW surfaces that play important roles (i.e., the nucleation centers,
catalyst control centers or “seeds”) can dominate the growth of MoS2 nanopetals on
the SiNWs. In order to confirm the specifics of the nucleation and to demonstrate
growth mechanisms, the effect of a change in the value of MS/MMoO3 on the surface
morphology of MoS2 is studied. An increase in the value of MS/MMoO3 leads to the
increased nucleation rate, increasing the size of curved edge-oriented MoS2
nanopetals. This understanding can provide guidance for the development of
nanoscale heterojunctions applied in the electronic and optoelectronic devices and
HER.
12
Acknowledgment
The authors acknowledge the support of the Ministry of Science and Technology,
Taiwan (Contract No. 106-2112-M-018-001-MY3) in the form of grants.
13
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19
Figure Captions
Fig. 1. Illustration of the CVD reaction for MoS2 growth.
Fig. 2. (a) Plane and (b) cross-sectional FESEM images of the SiNWs/n-Si sample.
Fig. 3. Raman spectra for MoS2/H2O2-treated SiNWs/n-Si samples from groups (a) A
and (b) B.
Fig. 4. (a) Si 2p core-level spectra for SiNWs/n-Si samples without and with H2O2
treatment and (b) Mo 3d and S 2s XPS spectra for MoS2/H2O2-treated
SiNWs/n-Si samples from groups A and B.
Fig. 5. (a) Plane and (b) cross-sectional FESEM images of the MoS2/SiNWs/n-Si
sample from group A and (c) plane and (d) cross-sectional FESEM images of
the MoS2/SiNWs/n-Si sample from group B.
20
Figure-1
Fig.1
Figure-2
(a)
Fig.2
Figure-3
Fig.3
Figure-4
)
Fig-4
Figure-5
(a)
Fig-5
Highlights
 MoS2 is deposited on the silicon nanowires (SiNWs) by chemical vapor
deposition.
 H2O2 treatment leads to the formation of Si-O bonds on the SiNW surfaces
 Si-O bonds played important roles can dominate the growth of MoS2 on the
SiNWs.
 The effect of a change in the S/MoO3 mass ratio on the size of MoS2 is studied.
 A nucleation rate for MoS2 growth increases with increasing the S/MoO3 mass
ratio.
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