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2D Materials
Band Gap-Tunable Molybdenum Sulﬁde Selenide
Monolayer Alloy
Sheng-Han Su, Yu-Te Hsu, Yung-Huang Chang, Ming-Hui Chiu, Chang-Lung Hsu,
Wei-Ting Hsu, Wen-Hao Chang, Jr-Hau He,* and Lain-Jong Li*
Transition metal dichalcogenide (TMD) monolayers have
attracted much attention recently since they exhibit moderate carrier mobility values,[1,2] good bendability and direct
band gaps,[3,4] which may allow them to serve for low-power
electronics,[1,5,6] flexible[7] and optoeletronic devices.[8–10]
In the application of optoelectonics such as photodetectors,
photovoltaic cells and light-emitting diodes, the optical band
gap of the semiconductor TMD may determine the efficiency
and optical responsivity to different wavelengthes of light.
Recently, the exfoliated TMD monolayers including MoS2,
MoSe2, and WS2, which can absorb up to 5−10% incident
sunlight in a thickness of less than 1 nm, have been shown to
achieve 1 order of magnitude higher sunlight absorption than
the most commonly used absorbers in solar cells GaAs and
Si.[9] This strongly suggests that the TMD materials hold great
promise for the device applications in nanoscale. To realize
the high efficiency solar cells or other optoelectronic devices
based on the TMD monolayers, it is crucially important to
develop a strategy to tune the optical band gap of the TMD
monolayers. Strain engineering has been proposed to modify
the optical band gap of the monolayer TMDs.[11–15] Moreover,
the stacking of various TMD monolayers has also been proposed as an approach to modulate their band gaps.[16] The
S.-H. Su,[+] Y.-T. Hsu,[+] Dr. Y.-H. Chang, M.-H. Chiu,
C.-L. Hsu, Dr. L.-J. Li
Institute of Atomic and Molecular Sciences
Academia Sinica, Taipei 10617, Taiwan
Fax: (+886) 223668264
E-mail: [email protected]
Dr. L.-J. Li
Department of Medical Research
China Medical University Hospital
Taichung, Taiwan
W.-T. Hsu, Dr. W.-H. Chang
Department of Electrophysics
National Chiao-Tung University
HsinChu 300, Taiwan
S.-H. Su, Dr. J.-H. He
Graduate Institute of Photonics and Optoelectronics
National Taiwan University
Taipei 106, Taiwan
E-mail: [email protected]
[+]S.-H.S. and Y.-T.H. contributed equally.
DOI: 10.1002/smll.201302893
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band gap engineering of TMD has become an important
topic. In early studies the TMD solid solutions both in the
metal (e.g., MoxW1−xS2) and chalcogen (e.g., MoS2xSe2(1−x))
sublattice forms have been realized by the direct vapor transport growth, where the stoichiometric amounts of desired
powder elements were introduced into a quartz ampoule
for crystal growth.[17,18] Meanwhile, the growth of MoS2,
WSe2 and WS2 monolayers has been reported recently by
using sulfurization or selenization of transition metal oxides
with chemical vapor deposition (CVD) techniques.[19–21] The
density-functinoal theory (DFT) calculations show that the
single layers of mixed TMDs, such as MoS2xSe2(1−x) are thermodynamically stable at room temperature,[22] so that such
materials can be manufactured using chemical-vapor deposition technique. It is therefore useful to know whether it is
possible to realize the synthesis of MoS2xSe2(1−x) monlayers
which exhibit intriguing electronic properties and tunable
optical band gaps. Very recently, the transition-metal dichalcogenide monolayer alloys (Mo1–xWxS2) have been obtained
by mechanical cleaving from their bulk crystals,[23] where
the band gap emission ranges from 1.82 eV to 1.99 eV. Note
that the mechanical cleavage is valuable for fundamental
research; however, a simple and scalable method to obtain
TMD monolayers with controllable optical energy gaps is
still urgently needed.
In this contribution, we report that the MoS2 monolayer
flakes prepared by CVD can be selenized in the presence of
selenium vapors to form MoSxSey monolayers. The optical
band gap of the obtained MoSxSey, ranging from 1.86 eV to
1.57 eV, is easily controllable by the selenization temperature. It is key demonstration for controlling electronic and
optoelectronic structures of TMD monolayers using a simple
method, where pproach is straightforward and applicable to
the band gap engineering for other TMD monolayers.
The CVD-grown MoS2 monolayers were synthesized
based on our previous reports.[19] In brief, the triangular MoS2
flakes are formed by the vapor phase reaction of MoO3 with
S powders, where the MoS2 monolayers with a lateral size up
to tens micron can be obtained and which growth method
has been adopted by many other groups.[24,25] To modulate
the electronic structures and optical band gaps of the MoS2
monolayer, we perform the selenization in a hot-wall furnace
at various temperatures. The scheme in Figure 1a illustrates
the experimental set-up for the selenization process, where
the inlet gas (a mixture of Ar and H2) carries the vaporized
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S.-H. Su et al.
Figure 1. (a) Schematic illustration of the experimental set-up for the selenization process, where the inlet gas (a mixture of Ar and H2) carries the
vaporized selenium to the heated MoS2 flakes. And optical micrographs for the (b) as-synthesized MoS2 and (c) selenized MoS2 (at 800 °C) on
sapphire substrates. AFM images for the MoS2 flake (d) before and (e) after selenization (at 800 °C)
selenium to the heated MoS2 flakes. It is noted that hydrogen
gas is necessary in the process to avoid the oxidation of MoS2
by residual oxygen or unavoidable oxygen leaking from the
environment to the chamber. Figures 1b and 1c show the
optical micrographs for the as-synthesized MoS2 and selenized MoS2 (at 800 °C) on sapphire substrates, respectively.
We note that there is no obvious change in size and shape of
the MoS2 flakes after selenization. Figures 1d and 1e display
the atomic force microscopy (AFM) images for the MoS2
flakes before and after selenization (800 °C). Note that the
change in thickness after selenization is within the measurement errors.
To reveal the optical properties of the selenized MoS2
flakes, we perform the photoluminescence measurements
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using microscopy- focused light (spot size: 0.7 µm). Figure 2a
shows the optical micrographs of a triangular MoS2, where
the circles indicated with the colors from purple to black
represent the measurement sites from the corner though
the center to the edge. Figure 2b is the photoluminescence
spectra collected for the samples before and after selenization at different temperatures. The photoluminescence peak
position (∼668 nm) for the pristine MoS2 does not vary with
the measurement sites from the corner to the edge. The
emission peak wavelength 667 nm for the MoS2 selenized at
600 °C is still pretty similar to that of the pristine MoS2 samples. The peak wavelength for the sample selenized at 700 °C
is at 726 nm and the wavelength continues to increase to
768 nm and 790 nm for the samples selenized at 800 °C and
© 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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Band Gap-Tunable Molybdenum Sulfide Selenide Monolayer Alloy
Figure 2. (a) Optical micrograph of a triangular MoS2, where the circles with the colors from purple to black represent the measurement sites
from the corner though the center to the edge (b) Photoluminescence spectra collected for the samples before and after selenization at different
temperatures. The peak at 694 nm was caused by the sapphire substrate and the sharp spikes at around 775 and 807 nm were system noises.
900 °C respectively. The emission wavelength 790 nm for the
MoS2 selenized at 900 °C is very close to the reported values
from 792 nm[26] to 800 nm[27] for exfoliated monolayer MoSe2.
Our experimental results suggest that the temperature is a
dominant parameter to control the optical properties of the
obtained MoSxSey materials. The calculations by HannuPekka Komsa et al.[28] predict that the mixture of MoS2 and
MoSe2 should be energetically favored over the segregated
phases even at 300 K due to the fact that the entropic contributions promote the mixing.[22] However, our reaction process involves the Mo-S bond breaking, where enough thermal
energy should be provided to overcome the reaction barrier
and then enable the replacement of S with Se.
It is also informative to examine the homogeneity of the
selenization of MoS2. Taking the 900 °C selenization as an
example, the photoluminescence spectra in Figure 2b show
that the emission wavelength at the corner site is 10 nm
longer than that obtained at the center site of the sample.
The longer emission wavelength at the corner site indicates that the selenization is preferable at the location with
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more edges or defects. The center part is relatively inert to
the selenization. Based on Figure 2b, the largest band gap
energy difference across the 900 °C selenized sample is estimated to be at most 17 meV (eg. The gap energy difference
between the center and the corner sites). These results in
Figure 2 suggest that thermodynamic parameter (temperature) is dominating the structures and optical proerties of
selenized samples.
Figure 3 shows the Raman spectra for the MoS2 flakes
before and after selenization at different temperatures. It is
clearly seen that the characteristic peaks of MoS2 including
E12g at 385.6 cm−1 and A1g at 405.8 cm−1[19] are observed
for both the pristine and 600 °C treated samples, indicating
that the selenization at 600 °C does not obviously change
the structure of MoS2. The MoS2 flakes after selenization
at 700 °C exhibit several unidentified peaks at 225.1 and
267.1 cm−1, which are likely attributed to the vibration from
the partially selenized Mo-S structures and worth further
investigations in the future. When the selenization temperature is increased to 800 °C, the observed Raman features, at
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Figure 4. Two optical absorption spectra for seleized samples at 600 °C
and 900 °C, respectively
Figure 3. Raman spectra for the MoS2 flakes before and after
selenization at different temperatures.
287.1 cm−1 and 239 cm−1, assigned to E12g and A1g modes of
MoSe2 respectively,[26] suggest the formation of Mo-Se binds.
It is noted that the weak shoulder and unassiged peak at
around 249.8 cm−1 normally appears when the MoSe2 quality
is not perfect. The MoS2 flake selenized at 900 °C displays a
single and sharp A1g peak at 242.2 cm−1 and a pronounced
E12g peak at 289 cm−1. Meanwhile, no shoulder peak at
249.8 cm−1 is observed and no MoS2 characteristic peaks are
observed, which further confirms the success of selenization.
The Raman specta taken at several representative sites of the
MoS2 triangle flake after selenization at 900 °C are shown in
the supporting Figure S1. It is observed that the peak position distrbution is reasonably uniform across the triangle
flake. Most importantly, no MoS2 characteristic Raman peaks
are found for the whole sample area, further suggesting that
the selenization process is homogeneous. To further confirm
the homogeneity of the selenized sample at 900 °C, we show
the optical absorption spectrum for the sample in Figure 4.
Two distinct absorption peaks at approximately 790.6 nm and
684.3 nm, identified as A and B excitonic absorption, are
observed and these peak positions are consistent with those
for exfoliated MoSe.2[26] The absorption spectrum for the
pristine MoS2 sample is also shown in Figure 4 for comparison. By considering the photoluminescence, absorption
spectra and Raman features, it is suggested that the 900 °C
selenized sample are close to that of the reported MoSe2
monolayer.[26]
To understand the differences between the MoS2 flakes
selenized at various temperatures, X-ray photoemission spectroscopy (XPS) was adopted to characterize the chemical
bonding structures. Figure 5 displays the detailed XPS scans
for the Mo, S and Se binding energies for the as-grown MoS2
and those after selenization, where the magnitude of each
profile was normalized for easier comparison. The as-grown
MoS2 exhibits two characteristic peaks at 232.5 and 229.3 eV,
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attributed to the Mo 3d3/2 and Mo 3d5/2 binding energies for
Mo4+.[29] The peaks, corresponding to the S 2p1/2 and S 2p3/2
orbital of divalent sulfide ions (S2−) are observed at 163.3
and 162.1 eV.[30] A doublet peak (232.4 cm−1, 235.6 cm−1)
attributed to the MoO3 is also observed. When the sample
is selenized at 600 °C, a weak doublet (55.3 cm−1, 54.5 cm−1)
assigned to Se 3d3/2 and Se 3d5/2 binding energy appears in
addition to the above mentioned XPS peaks for as-grown
MoS2 flakes. With the increasing selenization temperature,
the doublets peaks (Se 3d3/2, Se 3d5/2) and (Se 3p3/2, Se 3p5/2)
become more prominent. Meanwhile, the S 2p1/2 and S 2p3/2
binding energies become less pronounced. For the sample
selenized at 900 °C, only a Mo doublet and two Se doublets
are observed, confirming the selenization of MoS2.
The selenization process takes effect at the temperature
higher than 600 °C and the evolution of the optical band gap
suggests the gradual conversion of MoS2 to MoSxSey and then
MoSe2 with the increasing temperature. Figure 5 also reveals
that some Mo-O bonds exhibit in the as-grown and 600 °C
selenized MoS2 and the oxygen-species are not detectable
after selenization with a higher temperature, which is likely
due to that the hydrogen gas takes the effect. To get an idea
of the temperature effect, we estimate the percentage of selenization, a ratio between Mo-Se and (Mo-Se+Mo-S), using
the obtained XPS spectra. The Mo-Se percentage for the asgrwon MoS2 and those selenized at 600 °C, 700 °C, 800 °C
and 900 °C is 0%, 14.5%, 73.8%, 95% and 100% respectively.
The observed trend strongly agrees the conversion of Mo-S
to MoSe bonds. Most importantly, the conversion is governed
by the selenization temperature, indicating that the process is
thermodynamically controlled.
In summary, we report that the CVD-grown MoS2
monolayer flakes can be selenized in the presence of selenium
vapors. The optical band gap, ranging from 1.86 eV (667 nm)
to 1.57 eV (790 nm), is controllable by the selenization temperature. XPS analysis suggests the gradual conversion of
MoS2 to MoSxSey and then MoSe2 with the increasing selenization temperature. This approach, replacing one chalcogen
© 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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Band Gap-Tunable Molybdenum Sulfide Selenide Monolayer Alloy
Figure 5. XPS scans for the Mo, S and Se binding energies for the as-grown MoS2 and those after selenization, where the magnitude of each profile
was normalized for easier comparison.
by another in a gas phase, is promising in modulating the
optical and electronic properties of other TMD monolayers.
Experimental Section
Synthesis of Monolayer MoS2: Triangular MoS2 single crystals were synthesized by the modified processes of our previous
work.[19] In brief, c-plane sapphire (0001) substrates [Tera Xtal
Technology Corp.] were first cleaned in a piranha solution [H2SO4/
H2O2 (70:30)] at 100 °C for 1 h. Substrates were placed in the
center of a 4 inch tubular furnace on a quartz holder. The MoO3
powders (0.6 g; Sigma-Aldrich, 99.5%) in an Al2O3 crucible were
placed next to the sapphire substrates and S (Sigma-Aldrich,
99.5%) powders were placed close to the furnace open-end at
the upstream position, where the schematic illustration of the
growth system was described elsewhere.[31] The furnace was first
heated to 150 °C at 10 °C/min rate with 70 sccm Ar at 10 torr and
annealed for 20 minutes, then ramped to 650 °C at 25 °C/min rate
and kept for 20 minutes. Sulfur was heated separately by heating
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belt to 170 °C when the furnace reached 400 °C. After growth, furnace was slowly cooled to room temperature.
Selenization of MoS2: The as-grown monolayer MoS2 single
crystal flakes were selenized in a hot-wall furnace at 600 °C,
700 °C, 800 °C, and 900 °C, respectively. Briefly, as-grown MoS2
monolayers on sapphire were at the center of the furnace in the
quartz tube. The Selenium powders were placed close to the furnace open-end at the upstream position. The furnace was heated
to 600 °C, 700 °C, 800 °C, and 900 °C at 30 °C/min rate and kept
for 4 h, respectively. The Selenium powders was heated to 270 °C
using a separate heating belt. After selenization, the furnace was
slowly cooled to room temperature.
Characterization: Photoluminescence spectra were excited by
green light laser with 532 nm wavelength and 0.9 N.A. of objective (spot size: 0.7 µm). Raman spectra were collected in a NT-MDT
confocal Raman microscopic system (laser wavelength 473 nm and
laser spot size ∼0.5 µm). The Si peak at 520 cm−1 was used as reference for wavenumber calibration. The AFM images were performed
in a Veeco Dimension-Icon system. The transmittance spectra of
the MoS2 flakes were obtained using a JASCO-V-670 UV-vis spec-
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trophotometer. Chemical configurations were determined by X-ray
photoelectron spectroscope (XPS, Phi V5000). XPS measurements
were performed with an Mg Kα X-ray source on the samples. The
energy calibrations were made against the C 1s peak to eliminate
the charging of the sample during analysis.
Supporting Information
Supporting Information is available from the Wiley Online Library
or from the author.
Acknowledgements
This research was mainly supported by Academia Sinica (IAMS and
Nano program) and National Science Council Taiwan (NSC-99–
2112-M-001–021-MY3 and 99–2738-M-001–001).
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© 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Received: September 6, 2013
Published online: March 7, 2014
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