Shape Evolution of Monolayer MoS2 Crystals Grown by ChemicalVapor DepositionShanshan Wang, Youmin Rong, Ye Fan, Merce Pacios, Harish Bhaskaran, Kuang He,and Jamie H. Warner*

Department of Materials, University of Oxford, Parks Road, Oxford OX1 3PH, United Kingdom

*S Supporting Information

ABSTRACT: Atmospheric-pressure chemical vapor deposi-tion (CVD) is used to grow monolayer MoS2 two-dimensionalcrystals at elevated temperatures on silicon substrates with a300 nm oxide layer. Our CVD reaction is hydrogen free, withthe sulfur precursor placed in a furnace separate from theMoO3 precursor to individually control their heating profilesand provide greater flexibility in the growth recipe. Weintentionally establish a sharp gradient of MoO3 precursorconcentration on the growth substrate to explore its sensitivityto the resultant MoS2 domain growth within a relativelyuniform temperature range. We find that the shape of MoS2domains is highly dependent upon the spatial location on thesilicon substrate, with variation from triangular to hexagonal geometries. The shape change of domains is attributed to localchanges in the Mo:S ratio of precursors (1:>2, 1:2, and 1:<2) and its influence on the kinetic growth dynamics of edges. Theseresults improve our understanding of the factors that influence the growth of MoS2 domains and their shape evolution.

I n recent years, two-dimensional (2D) semiconductingtransition metal dichalcogenides (TMDs), including MX2

(M = Mo, W; X = S, Se), have attracted a great deal of attentionbecause of their unique structure as well as remarkable physicaland chemical properties.1−3 As a member of the TMD family,molybdenum disulfide (MoS2) with a direct band gap of 1.8 eVfor a single molecular layer is complementary to the zero-bandgap graphene, with potential in catalysis, valleytronicapplications, nanoelectronics, and optoelectonic devices.2−7

Efforts have been made in developing reliable and flexibleapproaches for obtaining thin layer MoS2, including differenttypes of exfoliation,8−10 hydrothermal synthesis,11 and physicalvapor deposition.12,13 However, the lateral size of MoS2domains synthesized by the aforementioned methods is oftenrestricted to several micrometers, making the synthesis of alarge-area MoS2 thin layer challenging.Recently, chemical vapor deposition (CVD) has been one of

the most promising methods of producing large-area and high-quality MoS2 thin films. Several precursors have been used,such as Mo films,14 MoO3,

15−19 MoCl5,20 and (NH4)2MoS4.

21

Many kinds of domain shapes have been synthesized in theCVD method, such as triangles, hexagons, truncated triangles,three-point stars, and six-point stars.15,16 However, theunderstanding of the shape evolution of MoS2 domains is stilllimited. For graphene, great attention has been paid to thestudy of morphology control, because many of graphene’smechanical and electrical properties are highly dependent onhow differently shaped domains interconnect within thepolycrystalline film.22 Depending on their shape and size,

graphene films exhibit a wide variety of microstructurescharacterized by domain sizes, shapes, crystal orientations,and lattice defects.23 Thus, as an analogue of graphene, it isimportant to study the shape evolution of 2D MoS2 in CVD.In this work, we present an experimental method to study

the shape evolution of the CVD-grown MoS2 domain. Scanningelectron microscopy (SEM), Raman spectroscopy, photo-luminescence (PL), and atomic force microscopy (AFM)techniques are employed to comprehensively study the crystalstructure of MoS2 on the Si substrates with a 300 nm SiO2surface after growth. We explain the different shapes usingsimple principles of crystal growth and present a qualitativeshape transformation model with dependence on the Mo:Satom ratio.

■ RESULTS AND DISCUSSIONFigure 1a−c shows an illustration of the CVD setup for MoS2growth, along with the main steps of the growing method, andthe temperature curves for both precursors (MoO3 and Spowder), for the whole experimental procedure. Briefly, thesamples were grown by CVD with solid MoO3 and Sprecursors.15 In contrast to the previous work,15 we used twoseparate furnaces to provide accurate control of the temper-ature of MoO3 and S separately. Fifteen milligrams of MoO3(molybdenum trioxide) powder was put in a ceramic boat,

Received: July 14, 2014Revised: September 29, 2014

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where it is directly below the gap of the first and secondsubstrate. The boat has a greater length along the gas flowdirection, creating a growth condition with a wider change inthe MoO3 concentration gradient. This ceramic boat wasplaced in the center of the second furnace. Also, 80 mg of sulfurpowder was put in another ceramic boat and the boat placed inthe first furnace. Four pieces of SiO2 substrates were tightlyaligned on the boat, being face down to the MoO3 powder, andonly leaving one small gap between the boat head and the firstsubstrate for Ar gas and S vapor in and another small gapbetween the boat tail and the last substrate for gas out. Thedistance between the two ceramic boats is 18 cm. This longdistance is to ensure that the S vapor concentration gradient onthe Si substrates can be ignored, compared with the MoO3concentration gradient. We heated S for 15 min after increasingthe temperature of MoO3 to let MoO3 evaporate first. Thegrowth temperature was set to 700 °C. Figure 1d shows thedeposition pattern of pure MoO3 on the second Si substratewhen S is not introduced, revealing the MoO3 concentrationvariation on the Si substrate. This resulted in the depositionpattern after reacting with S (Figure 1e).Figure 2 presents a schematic illustration of how the second

growth substrate is spatially sectioned into six parts withcorresponding SEM images from those sections shown below.Toward the edge of the deposited material, as shown by thedashed yellow oval, isolated MoS2 islands with edge lengthsranging from 2 to 47.9 μm were observed. The islands mergeinto a continuous film toward the inner region of the depositedfilm. Surprisingly, we observe regular shape change along theflow direction. To clearly show the dependence of the shapechange on the distance from the MoO3 precursor, we dividedthe second Si substrate into six regions (1−6) along the gas

flow direction (Figure 2). The exact location of the MoO3powder precursor in the experiment was directly underneaththe left side of the second Si substrate. The MoS2 domainmorphology in each region is shown by the SEM images(Figure 2). It can be seen that, with the increase in the distancebetween the precursor and the growth location, MoS2 domainsexperience a regular morphology transformation as well as asize change. Section 1 is dominated by ∼6 μm truncatedtriangles. However, as the deposition location moves fartherfrom the MoO3 powder, the length of the truncated sidesbecomes shorter, and the morphology finally changes totriangular with sharp edges and smooth sides. Also, the domainis obviously enlarged and reaches a peak at ∼50 μm. Then thedomain shape gradually transforms back to truncated triangles.Within section 4, the truncated sides become longer, whichresults in a hexagonal shape in section 5, ∼10 mm from theMoO3 powder. In the last region in the downstream part of thesubstrate, the morphology of MoS2 is mainly ∼2−3 μm smalltriangles. In general, the morphology of MoS2 along the gasflow direction on the surface of the second Si substrateexperiences a transformation from medium-sized truncatedtriangles to large triangles, then back to medium-sizedtruncated triangles followed by decreasing sizes of hexagons,and finally very small triangles. The small dark regions on theflakes and films are secondary layers that form during CVDgrowth.To be sure that the domains imaged by SEM in Figure 2

were indeed monolayer MoS2, we characterized them usingRaman spectroscopy and AFM (Figure 3). Two characteristicRaman vibration modes can be seen in the spectra in Figure 3a,the E1

2g mode representing the in-plane vibration ofmolybdenum and sulfur atoms and the A1g mode related to

Figure 1. (a) Schematic illustration of the MoS2 CVD system. (b) Flowchart showing the main steps of the MoS2 growing method. (c) Temperatureprogramming process of MoO3 and S precursors. (d) Pattern of deposition of MoO3 on the Si chip without introducing S. (e) Pattern of depositionof MoS2 on the Si chip after growth. The side length of the Si ship is 2 cm.

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the out-of-plane vibration of sulfur atoms.24 The frequencydifference between these two modes depends on the number oflayers of MoS2. Here, the fitting results show that these twomodes are located at 384.7 and 405.0 cm−1, respectively, givinga frequency difference Δk of 20.4 cm−1. This matches well withthe frequency difference of CVD-grown monolayer MoS2 inprevious work.16,18,19 The full width at half-maximum (fwhm)of the E1

2g peak is 3.8 cm−1, which is close to that of theexfoliated monolayer MoS2, 3.7 cm−1, suggesting goodcrystalline quality of the CVD-synthesized monolayer do-mains.18 We also performed Raman mapping on a largetriangular MoS2 domain by plotting the 2D spatial variation ofthe magnitude of the frequency difference between the A1g andE2g peaks to show the thickness uniformity of a large area(Figure 3b). Most of the domain has a Δk of <20.5 cm−1,confirming a homogeneous monolayer. Interestingly, anobvious increase in the frequency difference can be observedat the edge of the triangular domain. Panels c and d of Figure 3give a typical AFM measurement, showing the thickness of thedomain is ∼0.6 nm. This layer thickness is not only in the rangeof a single-layer MoS2 film on the bare substrates (0.6−0.9nm)25 but also consistent with the 0.6−0.7 nm value typicallyquoted for exfoliated monolayer films, indicating that the MoS2

monolayer is of high quality without the presence of large scaleabsorbents or other interactions between the film and the oxidesubstrate surface.26

We characterized the photoluminescence from differentlyshaped MoS2 domains. Figure 4a shows the typical PL spectrafrom a synthesized MoS2 domain. All three domains showed asimilar spectral profile in terms of peak position and fwhm. Astrong PL signal is located at 675 nm, which can be correlatedto the A1 excitation of MoS2, and another broad peak is locatedat ∼625 nm, which is known as the resonance of B1excitation.27 2D images of the PL intensity for different domainshapes have also been measured by stepping a focusedexcitation laser (532 nm) across the sample and integratingthe PL signal from each point. Panels b−d of Figure 4 showcorresponding PL intensity maps for a large triangular, mediumtruncated triangular, and a small hexagonal MoS2 domain,respectively, suggesting high crystallinity and uniformity of theMoS2 monolayer. Additionally, for all kinds of shapes, there is aclearly visible increase in PL intensity at the center of domainsand no edge enhancement.Here, we establish a qualitative model to explain the change

in the morphology of MoS2 along the gas flow direction. Themain reason for this shape change phenomenon is the change

Figure 2. Schematic illustration showing the spatial sectioning of the growth substrate into sections 1−6 and the corresponding SEM images fromthe edge of those sections. The dashed yellow oval indicates the typical region within a section examined by SEM, where individual domains can befound. Each section of sections 1−5 has a width of 2−3 mm, and section 6 has a width of ∼5 mm.

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in the Mo:S atom ratio along the Si substrate surface. Beforethe explanation, it is worthwhile to note that all theaforementioned characterized monolayer MoS2 domains weregrown in the last 10 min of the main reaction session, which ismarked by the green bracket in Figure 1c. During this period,the S precursor has been in the heat preservation stage at 150°C for 5 min. Associated with considering the long distancebetween S and MoO3, the S vapor gradient on one Si substrateduring the growth of monolayer MoS2 can be negligible. On theother hand, because the distance between the substrate and theMoO3 is very small, there will be an obvious concentrationgradient on the surface of substrates.On the basis of the principles of crystal growth,28 the shape

of a crystal is determined by the growing rate of different crystalfaces. The slowest growing faces become the largest, and therapidly growing faces either become smaller or disappearaltogether. The growing rate of faces is determined by thesurface free energy, and in 2D crystals, this corresponds to theedge free energy. Not surprisingly, the low-energy faces tend tobe those that grow slowly. For monolayer MoS2, its final shapewill be related to the growing rate of different edgeterminations. The most commonly observed edge structuresare Mo zigzag (Mo-zz) terminations and S zigzag (S-zz)terminations, which are supposed to be the most energeticallystable structures.15 These two kinds of terminations are bothzigzag edges; however, for S-zz edges, the S atoms are exposedto the outside and each S atom has only two bonds with two

Mo atoms (the saturated S in the MoS2 should have threebonds with three different Mo atoms), while for Mo-zz edges,the Mo atoms are exposed to the outside and each has onlyfour bonds with S atoms (the saturated Mo in the MoS2 shouldhave six bonds with six different S atoms). This structuraldifference gives them different levels of chemical activity underdifferent Mo:S ratio conditions, which may impact the growingrate and finally influence the domain shape. Generally, we candiscuss this problem by separating the Mo:S ratio into threeconditions: >1:2, 1:2, <1:2.We assume all shapes of domains start growing from a

hexagonal nucleus with three sides of Mo-zz terminations andanother three sides of S-zz terminations (in fact, this hypothesisis not necessary, just for simplifying the explanations). Underthe first condition, S-zz terminations grow faster than the Mo-zz terminations, because in Mo sufficient atmosphere, S-zzterminations with unsaturated S atoms exposed to the outsideare more energetically unstable than the Mo-zz terminationsand have higher probability of meeting and bonding with freeMo atoms. In Figure 5 under the first condition, it can be seenthat if S-zz terminations grow faster than the Mo-zzterminations, the domain shape will change from a hexagonto a triangle with three sides of Mo-zz terminations. Under thesecond condition, where the Mo:S ratio corresponds to thestoichiometric ratio of MoS2, the termination stability and theprobability for two types of terminations meeting correspond-ing free atoms are similar, which results in similar growing rates.

Figure 3. (a) Raman spectrum and (b) Raman map of the MoS2 domain, plotting the spatial variation of the magnitude of the frequency differencebetween A1g and E2g. Laser excitation of 532 nm was used. (c and d) AFM image and height profile, respectively, for MoS2 domains taken across thedotted white line in panel c.

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In this case, the final shape of the domains will be hexagon.Under the third condition with a S rich atmosphere, theanalysis is similar to that of the first condition. The domain

shape will also transform to triangular but having three sides ofS-zz terminations. To be more detailed, the exact ratio betweenMo and S atoms on the substrate will influence the energetic

Figure 4. (a) Photoluminescence (PL) spectrum of a synthesized MoS2 domains. (b−d) 2D images of the PL intensity of triangular, truncatedtriangular, and hexagonal MoS2 domains, respectively. The excitation wavelength is 532 nm.

Figure 5. Schematic illustration of the relationship between the Mo:S atom ratio and the domain shape. The ball-and-stick models in the central partshow the top view microstructure of the monolayer MoS2 crystal in different shapes, while the ball models on the left show two kinds of MoS2termination structures. The schematic diagram on the right illustrates the domain shape changing procedure depending on the growing rates of twodifferent terminations.

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stability of Mo-zz and S-zz terminations, causing the growthrate difference. If the Mo:S ratio is ≫1:2, the rate of growth ofS-zz terminations will increase much faster than that of Mo-zz,making crystal domains transform to a triangular shape in avery short period of time. However, if the Mo:S ratio is slightlyabove 1:2, the difference in the rate of growth between twotypes of terminations will be small, resulting in the formation oftruncated triangles within the same period of growing time.

There are several reasons to support the hypothesisdescribed above. The first is that, in previous work,15,16,29

different shapes of MoS2 domains were synthesized byadjusting the ratio between MoO3 and S precursor amounts.The relationship between the domain shape and the Mo:S ratiofits the hypothesis described above well. Lee et al. used 0.03 gof MoO3 and 0.01 g of S (Mo:S ratio of 1:1.5 to >1:2) toproduce monolayer triangular MoS2 domains.

29 Najmaei et al.

Figure 6. SEM images of the MoS2 shape changing process at 750 °C on the third Si chip. The observation direction is along the red arrow shown inFigure S1 of the Supporting Information.

Figure 7. (a−e) SEM images indicating the MoS2 shape evolution along the red arrow direction in Figure S1 of the Supporting Information at 650°C on the second Si chip. (f) Schematic illustration of three-point star formation.

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synthesized a triangular monolayer MoS2 crystal in S sufficientatmosphere, and when they decreased the amount of the Sprecursor, hexagonal and truncated triangles were observed.16

Another study claims that Mo-zz triangles have sharper,straighter edges than S-zz triangles.15 This morphologicaldifference can be clearly seen for truncated triangles in section 4of Figure 2b, where three short sides are much rougher than theother three long sides. On the basis of the SEM characterizationresults, these truncated triangles grow between large trianglesand small hexagonal domains. On the basis of the modeldescribed above, it should grow at a Mo:S ratio of >1:2,meaning that the S-zz terminations grow faster than the Mo-zzterminations, so that S-zz terminations should be shorter thanthe Mo-zz one. The greater roughness of shorter sides supportsits S-zz edge structure and corresponds to the analysis well. Thethird reason is that, this regular shape changing phenomenonnot only exists along the gas flow direction but also is observedin the direction vertical to the gas flow direction on thesubstrate (Figure S2 of the Supporting Information), indicatingthe change in the Mo:S ratio is the most important reason forthe shape change. Despite the shape change, the crystal sizealso experiences a regular change. This may relate to theconcentration gradient of the gas phase MoO3 along the gasflow direction, which impacts the average growing rate of theMoS2 crystals.The effect of MoO3 precursor temperature on the MoS2

shape change has also been investigated, and we found that thismorphology transformation phenomenon widely exists. How-ever, as the temperature influences the evaporation amount ofMoO3, leading to a different MoO3 concentration gradient in

the gas phase, the location of the deposition showing thiscrystal shape changing process slightly moves. Figure 6 showsthe MoS2 shape transformation along the Ar gas flow directionat 750 °C on the third Si chip. The crystal morphologyevolution is similar to that at 700 °C, indicating that thisphenomenon commonly exists in the MoS2 growth process.When the MoO3 temperature was decreased to 650 °C,

except the aforementioned shape changing trend, the domainshape ended up with three-point stars instead of triangles(Figure 7a−e). This is possibly because the decreasingtemperature reduces the evaporation amount of MoO3,resulting in a Mo:S ratio even lower than that under 700 °C,which makes the difference in growing rate between Mo-zzterminations and S-zz terminations larger. The growth of S-zzedges cannot catch up with that of Mo-zz edges, so that S-zzedge is able to form only a curved side instead of a straight one(Figure 7f), which looks like a three-point star. This shape isvery easy to transform into several-point stars, because thecurved crystal edge is not perfectly S-zz structure anymore.During the slow growing process of the curved side, when adefect emerges with a Mo-zz structure on it, the Mo-zzstructure will grow with high speed and form a sharp pointwithin a short period of time. This is why many several-pointstars can be observed. Panels a−c of Figure 7 show twodifferent sized domains have formed, large 10 μm domains andsmaller submicrometer domains.We also investigated the effect of Ar gas flow rate on MoS2

crystal growth. Figure 8 shows the SEM images of MoS2domain morphologies in different areas marked by red spotsin the central schematic diagram on the first Si chip, with an

Figure 8. SEM images showing the MoS2 shape evolution under an Ar flow rate of 100 sccm on the first Si chip. Each SEM image with a number onthe top right corner corresponds to the location having the same number on the schematic illustration in the center.

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increase in Ar gas flow rate from 10 to 100 sccm. In general, theloss of stability during crystal growth can be clearly observed,because the smoothness of the domain sides is severelyreduced, and the crystal shape becomes dendritic-like in someareas. The high flow rate may promote the mass transferprocess, which contributes to the increase in the crystal growthrate. In this case, instability may occur as atoms do not haveenough time to move into the right lattice locations, wherecrystal domains could have the lowest surface free energy, andthe probability of defect formation increases. Therefore, underthe high-flow rate condition, the MoS2 crystals are more likelyto grow under “kinetic” conditions rather than thermodynamicones, which is typical for colloidal nanocrystals with highprecursor feedstock. However, the regular shape domains withsmooth sides and sharp edges can still be seen in somelocations, as shown in section 3 in Figure 8. This is because thedirection from section 1 to 3 is perpendicular to the Ar gas flowdirection and there is a decrease in the concentration of MoO3on the substrate surface, as the distance from the MoO3 powderprecursor to each spot increases.The decrease in precursor concentration has an effect of

slowing the crystal growth rate, which can balance the positiveinfluence from the high flow rate, thus leading to stable crystalgrowth under thermodynamic control. Furthermore, in thedirection parallel to the gas flow, the MoO3 concentration onthe first substrate decreases as the sections of investigationmove farther from the MoO3 powder. This is why in section 8in Figure 8, crystal domains transform back into more regularshapes compared to those before it. However, as the MoO3concentration in the central area of the substrate is too high, weare not able to achieve isolated monolayer domains there,making the direct comparison of crystal shape along the gasflow direction not feasible. From these studies, we found thatthe increase in Ar flow rate could turn the control of crystalgrowth from the thermodynamic to kinetic regime, resulting inthe formation of dendritic morphologies, which are notfavorable for the production of high-quality 2D crystal.However, with a decrease in the precursor concentration, anew balance can be achieved to stabilize the crystal growth andshift the growing conditions to the thermodynamic control.In conclusion, we observed the regular morphology evolution

of CVD-grown MoS2 domains along the gas flow direction onone substrate. The microstructure and properties of the MoS2crystals were measured by SEM, Raman spectra, AFM, and PLspectra, confirming that the MoS2 film was a uniform, singlelayer with high crystallinity. A possible explanation for shapeevolution was based on the principles of crystal growth. Inaddition, the effects of MoO3 precursor temperature and Ar gasflow rate on MoS2 crystal shape were also investigated. It isanticipated that these studies will allow the shape-controllablesynthesis of MoS2 and improve the ability to discover of moreshape-dependent properties.

■ EXPERIMENTAL METHODSThe results presented in this paper were reproduced more than fivetimes, and the phenomenon of the domain shape change in the sameplace on the chip along the flow direction always existed. We usedCVD to grow MoS2 with two furnaces to control the temperature ofMoO3 and S separately, and a ceramic boat to create a wider change inMoO3 concentration on the substrates. Growth substrates were Si witha 285 nm layer of SiO2. Substrates were cleaned in acetone for 30 minand then 2-propanol for 15 min, followed by O2 plasma for 5 min.After being cleaned, four substrates were tightly aligned and placedface down above a ceramic boat containing 15 mg of molybdenum(VI)

oxide (MoO3) powder (≥99.5%, Sigma-Aldrich). The exact location ofthe MoO3 powder is directly below the tiny gap between the first andsecond substrate. The ceramic boat covered with four substrates wasloaded into a 1 in. quartz tube together with another boat containing80 mg of sulfur powder (≥99.5%, Sigma-Aldrich). The boat containingS is 18 cm from the boat of MoO3 and was at the upstream of the tube.They were then put into two different furnaces (furnace 1 having Sand furnace 2 having MoO3). The CVD growth occurred atatmospheric pressure while ultra-high-purity argon was flowing. TheCVD system was first flushed with 500 sccm of Ar gas for 1.5 h whenthe temperatures of the first and second furnaces were set to 30 and150 °C, respectively. Then the second furnace was heated at a rate of15 °C/min to 760 °C under a flow rate of 10 sccm of Ar, held at thesetting temperature for 30 min, and then slowly cooled at a coolingrate of −8 °C/min followed by a fast cooling under 500 sccm of Ar.The temperature programming for the first furnace having S was asfollows: temperature held at 30 °C until the second furnace was heatedfor 15 min, reaching a temperature of 375 °C, and then increased witha ramping rate of 3 °C/min to 150 °C and held for 30 min followed byrapid cooling.

Characterization. The crystal structures of the resulting productswere analyzed with a scanning electron microscope (Hitachi-4300)under an accelerating voltage of 3.0 kV. The thickness and surfacetopology were measured by an atomic force microscope (AsylumResearch MFP-3D). Typical scans were conducted in AC mode with asilicon AC160TS cantilever (Olympus, spring constant of ∼42 N/mand resonant frequency of ∼300 kHz). Raman spectroscopy andphotoluminescence were conducted using a JY Horiba LabRAMARAMIS imaging confocal Raman microscope under an excitationwavelength of 532 nm.

■ ASSOCIATED CONTENT*S Supporting InformationShape evolution along the direction vertical to the Ar gas flow,demonstrating the S concentration is uniform, and PL andRaman spectra of MoS2 in the same plot. This material isavailable free of charge via the Internet at http://pubs.acs.org.

■ AUTHOR INFORMATIONCorresponding Author*E-mail: jamie.warner@materials.ox.ac.uk.NotesThe authors declare no competing financial interest.

■ ACKNOWLEDGMENTSJ.H.W. thanks the Royal Society for support. S.W. acknowl-edges financial support from the China Scholarship Counciland thanks S. Zhou for discussions.

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