Band Gap-Tunable Molybdenum Sulﬁ de Selenide Monolayer Alloy · ©2014 Wiley-VCH Verlag GmbH &...

2589© 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim wileyonlinelibrary.com

Band Gap-Tunable Molybdenum Sulfi de 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 *

band gap engineering of TMD has become an important

topic. In early studies the TMD solid solutions both in the

metal (e.g., Mo x W 1−x S 2 ) and chalcogen (e.g., MoS 2x Se 2(1−x) )

sublattice forms have been realized by the direct vapor trans-

port growth, where the stoichiometric amounts of desired

powder elements were introduced into a quartz ampoule

for crystal growth. [ 17,18 ] Meanwhile, the growth of MoS 2 ,

WSe 2 and WS 2 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 MoS 2x Se 2(1−x) are ther-

modynamically stable at room temperature, [ 22 ] so that such

materials can be manufactured using chemical-vapor depo-

sition technique. It is therefore useful to know whether it is

possible to realize the synthesis of MoS 2x Se 2(1−x) monlayers

which exhibit intriguing electronic properties and tunable

optical band gaps. Very recently, the transition-metal dichal-

cogenide monolayer alloys (Mo 1–x W x S 2 ) 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 MoS 2 monolayer

fl akes prepared by CVD can be selenized in the presence of

selenium vapors to form MoS x Se y monolayers. The optical

band gap of the obtained MoS x Se y , ranging from 1.86 eV to

1.57 eV, is easily controllable by the selenization tempera-

ture. 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 MoS 2 monolayers were synthesized

based on our previous reports. [ 19 ] In brief, the triangular MoS 2

fl akes are formed by the vapor phase reaction of MoO 3 with

S powders, where the MoS 2 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 MoS 2

monolayer, we perform the selenization in a hot-wall furnace

at various temperatures. The scheme in Figure 1 a illustrates

the experimental set-up for the selenization process, where

the inlet gas (a mixture of Ar and H 2 ) carries the vaporized DOI: 10.1002/smll.201302893

2D Materials

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 SciencesAcademia Sinica, Taipei 10617 , TaiwanFax: (+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 ElectrophysicsNational Chiao-Tung University HsinChu 300 , Taiwan

S.-H. Su, Dr. J.-H. He Graduate Institute of Photonics and OptoelectronicsNational Taiwan University Taipei 106 , Taiwan E-mail: [email protected]

Transition metal dichalcogenide (TMD) monolayers have

attracted much attention recently since they exhibit mod-

erate 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 ] fl exible [ 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 effi ciency

and optical responsivity to different wavelengthes of light.

Recently, the exfoliated TMD monolayers including MoS 2 ,

MoSe 2 , and WS 2 , 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 effi ciency 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 pro-

posed as an approach to modulate their band gaps. [ 16 ] The

[+]S.-H.S. and Y.-T.H. contributed equally.

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http://doi.wiley.com/10.1002/smll.201302893

S.-H. Su et al.

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communications

© 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

selenium to the heated MoS 2 fl akes. It is noted that hydrogen

gas is necessary in the process to avoid the oxidation of MoS 2

by residual oxygen or unavoidable oxygen leaking from the

environment to the chamber. Figures 1 b and 1 c show the

optical micrographs for the as-synthesized MoS 2 and sele-

nized MoS 2 (at 800 °C) on sapphire substrates, respectively.

We note that there is no obvious change in size and shape of

the MoS 2 fl akes after selenization. Figures 1 d and 1 e display

the atomic force microscopy (AFM) images for the MoS 2

fl akes before and after selenization (800 °C). Note that the

change in thickness after selenization is within the measure-

ment errors.

To reveal the optical properties of the selenized MoS 2

fl akes, we perform the photoluminescence measurements

using microscopy- focused light (spot size: 0.7 µm). Figure 2 a

shows the optical micrographs of a triangular MoS 2 , 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 2 b is the photoluminescence

spectra collected for the samples before and after seleniza-

tion at different temperatures. The photoluminescence peak

position (∼668 nm) for the pristine MoS 2 does not vary with

the measurement sites from the corner to the edge. The

emission peak wavelength 667 nm for the MoS 2 selenized at

600 °C is still pretty similar to that of the pristine MoS 2 sam-

ples. 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

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Figure 1. (a) Schematic illustration of the experimental set-up for the selenization process, where the inlet gas (a mixture of Ar and H 2 ) carries the vaporized selenium to the heated MoS 2 fl akes. And optical micrographs for the (b) as-synthesized MoS 2 and (c) selenized MoS 2 (at 800 °C) on sapphire substrates. AFM images for the MoS 2 fl ake (d) before and (e) after selenization (at 800 °C)


2591www.small-journal.com© 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

900 °C respectively. The emission wavelength 790 nm for the

MoS 2 selenized at 900 °C is very close to the reported values

from 792 nm [ 26 ] to 800 nm [ 27 ] for exfoliated monolayer MoSe 2 .

Our experimental results suggest that the temperature is a

dominant parameter to control the optical properties of the

obtained MoS x Se y materials. The calculations by Hannu-

Pekka Komsa et al. [ 28 ] predict that the mixture of MoS 2 and

MoSe 2 should be energetically favored over the segregated

phases even at 300 K due to the fact that the entropic contri-

butions promote the mixing. [ 22 ] However, our reaction pro-

cess 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 MoS 2 . Taking the 900 °C selenization as an

example, the photoluminescence spectra in Figure 2 b 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 indi-

cates that the selenization is preferable at the location with

more edges or defects. The center part is relatively inert to

the selenization. Based on Figure 2 b, the largest band gap

energy difference across the 900 °C selenized sample is esti-

mated 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 (tempera-

ture) is dominating the structures and optical proerties of

selenized samples.

Figure 3 shows the Raman spectra for the MoS 2 fl akes

before and after selenization at different temperatures. It is

clearly seen that the characteristic peaks of MoS 2 including

E 1 2g at 385.6 cm −1 and A 1g 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 MoS 2 . The MoS 2 fl akes after selenization

at 700 °C exhibit several unidentifi ed 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 tempera-

ture is increased to 800 °C, the observed Raman features, at

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Figure 2. (a) Optical micrograph of a triangular MoS 2 , 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.

S.-H. Su et al.


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287.1 cm −1 and 239 cm −1 , assigned to E 1 2g and A 1g modes of

MoSe 2 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 MoSe 2 quality

is not perfect. The MoS 2 fl ake selenized at 900 °C displays a

single and sharp A 1g peak at 242.2 cm −1 and a pronounced

E 1 2g peak at 289 cm −1 . Meanwhile, no shoulder peak at

249.8 cm −1 is observed and no MoS 2 characteristic peaks are

observed, which further confi rms the success of selenization.

The Raman specta taken at several representative sites of the

MoS 2 triangle fl ake after selenization at 900 °C are shown in

the supporting Figure S1. It is observed that the peak posi-

tion distrbution is reasonably uniform across the triangle

fl ake. Most importantly, no MoS 2 characteristic Raman peaks

are found for the whole sample area, further suggesting that

the selenization process is homogeneous. To further confi rm

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, identifi ed 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 MoS 2 sample is also shown in Figure 4 for com-

parison. 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 MoSe 2

monolayer. [ 26 ]

To understand the differences between the MoS 2 fl akes

selenized at various temperatures, X-ray photoemission spec-

troscopy (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 MoS 2

and those after selenization, where the magnitude of each

profi le was normalized for easier comparison. The as-grown

MoS 2 exhibits two characteristic peaks at 232.5 and 229.3 eV,

attributed to the Mo 3d 3/2 and Mo 3d 5/2 binding energies for

Mo 4+ . [ 29 ] The peaks, corresponding to the S 2p 1/2 and S 2p 3/2

orbital of divalent sulfi de ions (S 2− ) are observed at 163.3

and 162.1 eV. [ 30 ] A doublet peak (232.4 cm −1 , 235.6 cm −1 )

attributed to the MoO 3 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 3d 3/2 and Se 3d 5/2 binding energy appears in

addition to the above mentioned XPS peaks for as-grown

MoS 2 fl akes. With the increasing selenization temperature,

the doublets peaks (Se 3d 3/2 , Se 3d 5/2 ) and (Se 3p 3/2 , Se 3p 5/2 )

become more prominent. Meanwhile, the S 2p 1/2 and S 2p 3/2

binding energies become less pronounced. For the sample

selenized at 900 °C, only a Mo doublet and two Se doublets

are observed, confi rming the selenization of MoS 2 .

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 MoS 2 to MoS x Se y and then

MoSe 2 with the increasing temperature. Figure 5 also reveals

that some Mo-O bonds exhibit in the as-grown and 600 °C

selenized MoS 2 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 sele-

nization, a ratio between Mo-Se and (Mo-Se+Mo-S), using

the obtained XPS spectra. The Mo-Se percentage for the as-

grwon MoS 2 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 MoS 2

monolayer fl akes 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 tem-

perature. XPS analysis suggests the gradual conversion of

MoS 2 to MoS x Se y and then MoSe 2 with the increasing sele-

nization temperature. This approach, replacing one chalcogen

small 2014, 10, No. 13, 2589–2594

Figure 3. Raman spectra for the MoS 2 fl akes before and after selenization at different temperatures.

Figure 4. Two optical absorption spectra for seleized samples at 600 °C and 900 °C, respectively


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by another in a gas phase, is promising in modulating the

optical and electronic properties of other TMD monolayers.

Experimental Section

Synthesis of Monolayer MoS 2 : Triangular MoS 2 single crys-tals were synthesized by the modifi ed processes of our previous work. [ 19 ] In brief, c-plane sapphire (0001) substrates [Tera Xtal Technology Corp.] were fi rst cleaned in a piranha solution [H 2 SO 4 /H 2 O 2 (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 MoO 3 powders (0.6 g; Sigma-Aldrich, 99.5%) in an Al 2 O 3 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 fi rst 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

belt to 170 °C when the furnace reached 400 °C. After growth, fur-nace was slowly cooled to room temperature.

Selenization of MoS 2 : The as-grown monolayer MoS 2 single crystal fl akes were selenized in a hot-wall furnace at 600 °C, 700 °C, 800 °C, and 900 °C, respectively. Briefl y, as-grown MoS 2 monolayers on sapphire were at the center of the furnace in the quartz tube. The Selenium powders were placed close to the fur-nace 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 objec-tive (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 refer-ence for wavenumber calibration. The AFM images were performed in a Veeco Dimension-Icon system. The transmittance spectra of the MoS 2 fl akes were obtained using a JASCO-V-670 UV-vis spec-

Figure 5. XPS scans for the Mo, S and Se binding energies for the as-grown MoS 2 and those after selenization, where the magnitude of each profi le was normalized for easier comparison.

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trophotometer. Chemical confi gurations 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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Received: September 6, 2013 Published online: March 7, 2014

Band Gap-Tunable Molybdenum Sulﬁ de Selenide Monolayer Alloy · ©2014 Wiley-VCH Verlag GmbH &...

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