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Composition-Modulated Two-Dimensional
Semiconductor Lateral Heterostructures via
Layer-Selected Atomic Substitution
Honglai Li,
†
Xueping Wu,
†
Hongjun Liu,
†
Biyuan Zheng,
†
Qinglin Zhang,
†
Xiaoli Zhu,
†
Zheng Wei,
§
Xiujuan Zhuang,
†
Hong Zhou,
†
Wenxin Tang,
§
Xiangfeng Duan,
‡
and Anlian Pan*
,†
†
Key Laboratory for Micro-Nano Physics and Technology of Hunan Province, School of Physics and Electronic Science, and State
Key Laboratory of Chemo/Biosensing and Chemometrics, Hunan University, Changsha, Hunan 410082, P. R. China
‡
Department of Chemistry and Biochemistry, University of California, Los Angeles, California 90095, United States
§
College of Materials Science and Engineering, Chongqing University, Chongqing 400030, P. R. China
*
SSupporting Information
ABSTRACT: Composition-controlled growth of two-di-
mensional layered semiconductor heterostructures is
crucially important for their applications in multifunctional
integrated photonics and optoelectronics devices. Here, we
report the realization of composition completely modulated
layered semiconductor MoS2−MoS2(1−x)Se2x(0 < x<1)
lateral heterostructures via the controlled layer-selected
atomic substitution of pregrown stacking MoS2, with a
bilayer located at the center of a monolayer. Through
controlling the reaction time, S at the monolayer MoS2at the peripheral area can be selectively substituted by Se atoms at
different levels, while the bilayer region at the center retains the original composition. Microstructure characterizations
demonstrated the formation of lateral heterostructures with a sharp interface, with the composition at the monolayer area
gradually modulated from MoS2to MoSe2and having high-quality crystallization at both the monolayer and the bilayer
areas. Photoluminescence and Raman mapping studies exhibit the tunable optical properties only at the monolayer region
of the as-grown heterostructures, which further demonstrates the realization of high-quality composition/bandgap
modulated lateral heterostructures. This work offers an interesting and easy route for the development of high-quality
layered semiconductor heterostructures for potential broad applications in integrated nanoelectronic and optoelectronic
devices.
KEYWORDS: layered semiconductor, transition-metal dichalcogenides, lateral heterostructures, tunable compositions,
atomic substitution
Two dimensional (2D) atomic crystal materials,
1−3
especially transition-metal dichalcogenides
(TMDs),
4−11
have attracted considerable interest
recently due to their atomically thin geometry structure and
unique electronic and optical properties for potential
applications in integrated optoelectronic devices and sys-
tems.
12−22
Semiconductor heterostructures with spatially
modulated bandgaps and sharp composition interfaces are
important for high-performance device applications.
23−25
In the
past several years, controlled growth of TMD atomic crystal
semiconductor heterostructures has received more and more
attention.
26−36
For example, Gong et al. have shown the growth
of high-quality vertical and in-plane WS2−MoS2heterostruc-
tures with light emissions broadly tuned in both vertical and
lateral directions.
26
Duan et al. have reported the lateral growth
of MoS2−MoSe2and WS2−WSe2heterostructures, based on
which atomic p−n diodes and inverters have been achieved.
27
Li et al. have reported the two-step epitaxial growth of the
lateral WSe2−MoS2heterojunction with an atomically sharp
interface, where the edge of WSe2induces the epitaxial MoS2
growth despite a large lattice mismatch.
28
The optoelectronic
properties of semiconductor heterostructures are directly
related to the energy band diagram at their interfaces.
37
Controlled growth of atomic crystal semiconductor hetero-
structures with band gap engineered interfaces is particularly
important for their further broad applications. However, to the
best of our knowledge, high quality 2D semiconductor
heterostructures with continuously modulated composition or
band gap have never been reported.
Received: November 10, 2016
Accepted: December 19, 2016
Published: December 19, 2016
Article
www.acsnano.org
© 2016 American Chemical Society 961 DOI: 10.1021/acsnano.6b07580
ACS Nano 2017, 11, 961−967
It was reported that the pregrown TMD monolayer
nanosheets can be easily selenized and sulfurized with a simple
annealing approach, which provides a simple method for the
synthesis of composition modulated 2D layered semiconductor
alloys.
38
In this work, we find that the selenylation of layered
MoS2is highly dependent on the layer number, with selenium
substitution temperature of the monolayer greatly decreased
more than that of the bilayer and multilayer. Based on this
finding, we realized the composition completely modulated
MoS2−MoS2(1−x)Se2x(0 < x< 1) lateral heterostructures via
the controlled layer-selected atomic substitution of pregrown
stacking MoS2nanosheets composed of a bilayer located at the
center of a monolayer. Scanning transmission electron
microscopy (STEM), photoluminescence (PL), and Raman
scattering measurements demonstrate the realization of the
atomic layered lateral heterostructures. The achieved hetero-
structures display high-quality crystallization and a very sharp
interface, with the composition at the monolayer area
completely modulated from MoS2to MoSe2, accompanying
the continuously tuned PL from 668 to 760 nm. These
composition-modulated 2D semiconductor lateral heterostruc-
tures may find potential applications in integrated nano-
electronics and nanophotonics.
RESULTS
Figure 1a shows the real-color optical image of a MoS2
nanosheet, and the inset gives the corresponding atomic force
microscopy (AFM) image, revealing the monolayer nature of
the sheet. Parts b and c of Figure 1 give the PL spectra of this
sample excited with a 488 nm argon ion laser before and after
annealing under Se vapor for 1, 3, and 5 min at a temperature
below 730 and 740 °C, respectively, which shows that the PL
spectra for the annealing time below 730 °C remain the same as
that before annealing (0 min), while the peak wavelength is
gradually red-shifted with increasing annealing time at 740 °C,
from ∼670 nm (before annealing, pure MoS2) to 714 nm. The
results indicate that the monolayer sample is chemically stable
at a relatively low temperature below 730 °C, while selenium
atomic substitution can take place in this sample at 740 °C, and
the substitution rate is increased with elevation of the annealing
time. By contrast, similar experiments were conducted for
bilayer MoS2nanosheets. Figure 1d gives the optical image of a
typical bilayer MoS2with the corresponding AFM image
(inset), and parts e and f of Figure 1 give the corresponding PL
before and after annealing in the Se atmosphere, with the same
annealing times as those conducted for the monolayer sample.
Figure 1. Contrastive characters of Se substitution in monolayer and bilayer MoS2nanosheets, respectively. (a, d) Optical images of
monolayer and bilayer MoS2nanosheets after substitution, respectively. Insets: corresponding AFM images with section analysis along the
black lines. Annealing temperature-related PL spectra of the monolayer (b, c) and bilayer (e, f) before and after annealing for 1, 3, and 5 min.
(g and h) Annealing temperature-dependent bandgap values and compositions of the two sheets after annealing for 5 min.
Figure 2. Schematic diagram of the preparation of the lateral heterostructured MoS2−MoS2(1‑x)Se2xnanosheet by the Se substitution in a
designed stacking MoS2nanosheet.
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DOI: 10.1021/acsnano.6b07580
ACS Nano 2017, 11, 961−967
962
It is interesting to find that the bilayer sample is always
chemically stable when the annealing temperature is below 800
°C and the selenium substitution begins to take place at 810 °C
since the peak wavelength of the PL spectra is red-shifted with
increasing annealing time. Figures 1g gives the annealing
temperature dependent PL peak energy of both the monolayer
and the bilayer samples after annealing for 5 min, respectively,
and Figure 1h shows the corresponding annealing temperature-
dependent Se molar fraction xconverted from the PL spectra
using Vegard’s law,
15
which more clearly demonstrates that the
Se substitution temperature of the bilayer MoS2(810 °C) is
much higher than that of the monolayer MoS2(740 °C). For
both samples above their respective substitution temperature,
the substitution rate is gradually increased with increasing
annealing temperature.
Inspired by the great difference of the Se substitution
temperature from the monolayer and the bilayer MoS2,
composition-tuned lateral MoS2−MoS2(1−x)Se2xheterostruc-
tures can, in principle, be obtained by layer-selected Se
substitution of stacking MoS2nanosheets with the monolayer
at the peripheral and the bilayer at the center, as shown
schematically in Figure 2. When the stacking MoS2nanosheets
are exposed in the Se atmosphere at an appropriate
temperature, the Se atoms can only react with the peripheral
monolayer MoS2and substitute their S atoms, keeping the
composition of the central bilayer MoS2unchanged, thus
resulting in the formation of MoS2−MoS2(1−x)Se2xlateral
heterostructures. The composition or substitution rate in the
monolayer region of the heterostructures can be controlled by
the substitution time and realize the tunability of the
composition or energy band diagram in these achieved
heterostructures.
The stacking MoS2nanosheets were synthesized through a
traditional chemical vapor deposition (CVD) route with a 300
nm SiO2/Siwaferasthesubstrate(seetheMaterials
Preparation section for details). As shown in Figure 3a (optical
image), the obtained stacking MoS2nanosheets have a well-
defined triangular shape, with a small triangle located at the
center of a large triangle. The thickness-dependent contrast can
distinguish between the monolayer and bilayer regions of the
sheet. After the pregrown stacking nanosheets were annealed in
Se atmosphere for different times, the microstructure and
composition characterizations of the obtained samples were
conducted with transmission electron microscopy (TEM)
combined with energy-dispersive X-ray spectroscopy (EDX).
Figure 3b shows the TEM image of a representative stacking
nanosheet after atomic substitution at 750 °C for 1 min, which
keeps a good appearance characteristic as that of before
annealing. Figure 3c plots the EDX spectroscopy spectra
collected from two positions in the two regions, respectively
(dots 1 and 2 in Figure 3b), which reveal that position 1
(peripheral monolayer region) is composed of considerable Se,
S, and Mo elements (the detected Cu element originates from
the copper grid), while position 2 (central bilayer region)
mainly consists of S and Mo elements with negligible Se
elements detected. The elemental analyses indicate that the
sheet is a lateral heterostructure with monolayer MoS2(1−x)Se2x
alloy at the peripheral region and bilayer MoS2at the central
region. EDX line scan profiles of the elemental distribution
along the black line in Figure 3b clearly show the opposite
Figure 3. (a) Optical image of the successfully grown stacking MoS2nanosheets. (b) Typical TEM image of the obtained nanosheet after Se-
substitution at 750 °C for 1 min and (c) the corresponding TEM−EDX profiles recorded at two positions of different thickness (1, 2) in the
sheet. (d) EDX line scan profiles for the different detected elements: S and Se, respectively, across the interface of the stacking MoS2after
substitution. (e) The HRTEM image taken from the interfacial regions (scale bars, 2 nm). Insets of (e): the SAED patterns taken from the
monolayer and bilayer positions of the nanosheet after substitution. (f) HAADF−STEM image taken across the interfacial regions between
the monolayer and the bilayer after substitution at 750 °C for 3 min (scale bar: 2 nm).
ACS Nano Article
DOI: 10.1021/acsnano.6b07580
ACS Nano 2017, 11, 961−967
963
modulation of elements Se and S (Figure. 3d). The Se content
is decreased while the S content is increased across the interface
region from the monolayer to the bilayer, which further
demonstrates the lateral heterostructure feature of the stacking
sheet after atomic substitution. Figure 3e gives the correspond-
ing high-resolution TEM (HRTEM) image across the interface
of the nanosheet, which demonstrates that the structure is
highly crystallized, with the measured lattice plane spacings of
2.72 and 2.70 Å at the monolayer and bilayer regions,
respectively, in agreement with the (100) plane spacing of
the composition tunable sheets. The insets of Figure 3e are the
selected area electron diffraction (SAED) patterns of the sheet
at the two regions. The obvious diffraction intensity contrast
verifies the layer number difference of the stacking nanosheet.
Both patterns show clearly defined single sets of diffraction
spots, which further demonstrate the high quality of the
substituted nanosheets.
39
The atomic arrangement of the lateral
heterostructure is clearly resolved in high angle annular dark
field (HAADF)−STEM imaging. The HAADF−STEM image
in Figure 3f shows the interfacial regions between the
monolayer and the bilayer (brighter region) after annealing at
750 °C for 3 min. The bright spots (indicated by the green
arrow) in the monolayer region corresponding to Mo atoms are
nearly uniformly distributed, while the contents of S2 atoms
(indicated by the yellow arrow) and S + Se atoms (indicated by
the red arrow) in this region show obvious contrast, which
agrees with the previous report in the monolayer MoS2(1−x)Se2x
alloy.
40
However, the spots in the bilayer region are of the same
brightness, which is in agreement with the atomic structure of
bilayer MoS2,
41
and no Se atom was detected. The above
results clearly demonstrate the realization of high-quality
MoSSe (monolayer)−MoS2(bilayer) lateral heterostructures
with an atomic-level sharpened interface.
PL spectra were used to characterize the composition-
dependent optical modulation of these lateral heterostructures.
Figure 4a is the real-color image of a stacking sheet after
substitution, and Figure 4b shows the annealing time
dependent PL spectra at the monolayer and bilayer regions
of the stacking sheet, respectively. The black spectra were
collected from the monolayer region substituted at a temper-
ature of 750 °C for 0, 1, 3, 5 min, respectively, while the red
spectra were collected from the bilayer region. It can be seen
that all of the spectra collected from the monolayer region
(black) reveal single emission bands, with the peak wavelength
gradually red-shifted when the substituted time is increased. In
contrast, the spectra from the bilayer region (red) almost keep
the same peak position over time. The highly distinct PL peaks
for the central region and peripheral region demonstrate that
the composition-tuned layered semiconductor lateral hetero-
structures are successfully achieved. Figure 4c shows the
annealing time dependent PL peak energy (trilateral) and the
correspondingly induced Se molar fraction x(square) of the
monolayer and bilayer regions after the annealing, respectively,
which further demonstrates that the S in the monolayer region
can be gradually substituted by Se at 750 °C, while the bilayer
region is very stable at this temperature. In addition, the Se
molar fraction xinduced from the PL observation is highly
consistent with those from the direct EDX analysis (Table S1),
further supporting the finding of the different atomic
substitution behaviors in the two regions with different layers.
PL mapping studies can further reveal the optical modulation
within the triangular nanosheet. Taking the substituted time of
5 min into account, Figure 4d,e gives the wavelength-selected
PL emission mapping of the examined nanosheet in the spectral
regions of 675−705 and 745−775 nm, respectively. Obviously,
the short wavelength region (675−705 nm) is located at the
center of the nanosheet, while the long wavelength region
(745−775 nm) is located at the periphery of sheet. A PL
mapping image composed of the short wavelength region and
Figure 4. (a) Optical image of a typically stacking MoS2nanosheet after substitution. (b) Annealing time-related PL spectra of the stacking
lateral heterostructures. (c) Annealing time-related bandgap values and compositions of the composition modulated heterostructures. (d−f)
Wavelength-selected PL mapping of a stacking nanosheet substituted for 5 min in the spectral regions of 675−705 nm, 745−775 nm, and the
combination, respectively.
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ACS Nano 2017, 11, 961−967
964
the long one (Figure 4f) shows a seamless lateral integration,
further demonstrating the feature of the lateral heterostructure.
The formation of composition-tuned lateral heterostructures
can further be confirmed by the composition-dependent
vibration modes observed from the micro-Raman measure-
ments. Figure 5a plots the normalized annealing time-
dependent Raman spectra of the peripheral monolayer region
shown in Figure 4a. The results show that the intensities of Se−
Mo related modes are gradually enhanced, while the intensities
of S−Mo related modes are gradually attenuated. Meanwhile,
all of the vibration modes are increasingly shifted to the low
frequency with the substituted time. All of the above
observations show good agreement with the continuously
composition-tuned MoS2(1−x)Se2xalloy. On the other hand, the
annealing time-dependent Raman spectra of the central bilayer
region are shown in Figure 5b. Similar to the PL spectra, the
Raman spectra here are constant over time. The obvious
difference of Raman spectra collected from the two regions
after various substitution times reveals that the composition of
the lateral heterostructure is tuned gradually. Parts c−eof
Figure 4 give the frequency-selected Raman mapping of the
examined nanosheet at 260 cm−1, 403 cm−1,andthe
combination, respectively. Similar to those of PL mapping
studies, the low frequency (260 cm−1) is located at the center of
the nanosheet, while the high frequency (403 cm−1) is located
at the periphery of sheet, and a seamless lateral integration is
also demonstrated. The above results further confirm the
formation of MoS2−MoS2(1−x)Se2xlateral heterostructures.
CONCLUSIONS
In summary, lateral composition-tuned atomic-layered hetero-
structures have been successfully prepared through an effective
control of the layer-dependent atomic substitution process.
Both microstructure and spectral characterizations demonstrate
that the achieved nanosheets after substitution are lateral
heterostructures, with the composition at the peripheral
monolayer region being continuously tuned to the ternary
alloy while the composition at the central bilayer region
remains in its original form. The lateral heterostructures with
tunable compositions can give composition-related optical
modulations with the PL peak positions broadly tunable at the
periphery while fixed at the center. These composition-tuned
lateral heterostructures could find significant applications in 2D
fundamental physical research and the construction of
functional electronic and photoelectric devices.
METHODS
Materials Preparation. The stacking MoS2nanosheets were
synthesized through a CVD route on the 300 nm SiO2/Si substrate,
with sulfur and MoO3powders as the source materials.
15
Before
heating, an Ar gas flow was introduced into the system to eliminate the
air, and then the furnace was rapidly heated to 830 °C while the
pressure inside the system was kept at 200−300 Torr. After 10 min of
growth, the furnace was naturally cooled to room temperature. The
substitution reaction was also took place through a common CVD
route. A boat with Se powder was placed upstream, and another boat
covered with Si/SiO2wafers adhering pregrown MoS2nanosheet was
placed at the heating zone of a quartz tube. Ar mixed with 5% H2gas
was first introduced into the system at a fast rate (120 sccm, 30 min)
to purge the oxygen from the chamber before the furnace was heated.
The temperature in the center of the furnace was then rapidly heated
to the reaction temperature, with the region of Se powder at 260 °C,
keeping the pressure inside the tube at about 75 Torr. Then the
furnace was naturally cooled to room temperature.
TEM and Optical Characterizations. The microstructure was
characterized by AFM (Bruker Multimode 8), TEM (Tecai F20,
voltage: 300 kV) equipped with an EDS detector, and HAADF-STEM
(FEI Titan G2, 60−300 keV). Before the survey of the TEM results,
the nanosheets were transferred onto grid of copper using a PMMA
(Mw= 950 K, 4 wt %, AR-P 679.04, Allresist)-mediated nanotransfer
method (speed: 3000 rpm, 1 min).
15
The PL and Raman
measurements were performed with the confocal μ-PL system
(WITec, alpha-300). A 488 nm argon ion laser (power: about 30
mW, spot size: 1−2μm) was used to characterize the structural and
optical modulation of the sheet.
ASSOCIATED CONTENT
*
SSupporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acsnano.6b07580.
Schematic for the experiment setup and the analysis of
the atomic substitution rate (PDF)
AUTHOR INFORMATION
Corresponding Author
*E-mail: anlian.pan@hnu.edu.cn.
ORCID
Xiangfeng Duan: 0000-0002-4321-6288
Anlian Pan: 0000-0003-3335-3067
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
We are grateful to the NSF of China (Nos. 51525202,
61574054, 61505051, and 61474040), the Hunan province
science and technology plan (Nos. 2014FJ2001 and
2014TT1004), and the Aid program for Science and
Technology Innovative Research Team in Higher Educational
Institutions of Hunan Province.
Figure 5. (a, b) Annealing time-related Raman spectra collected
from the monolayer and bilayer regions of the stacking lateral
heterostructure, respectively. (c−e) Frequency-selected Raman
mapping of a stacking nanosheet substituted for 5 min at 403
cm−1, 260 cm−1, and the combination, respectively.
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DOI: 10.1021/acsnano.6b07580
ACS Nano 2017, 11, 961−967
965
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