Shape-Dependent Defect Structures of Monolayer MoS2 Crystals

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Article

Shape-Dependent Defect Structures of Monolayer MoS Crystals Grown by Chemical Vapor Deposition 2

Guozhu Zhang, Jinwei Wang, Zefei Wu, Run Shi, Wenkai Ouyang, Abbas Amini, Bananakere Nanjegowda Chandrashekar, Ning Wang, and Chun Cheng ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b13777 • Publication Date (Web): 08 Dec 2016 Downloaded from http://pubs.acs.org on December 11, 2016

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ACS Applied Materials & Interfaces

Shape-Dependent Defect Structures of Monolayer MoS2 Crystals Grown by Chemical Vapor Deposition

Guozhu Zhang1, Jinwei Wang1, Zefei Wu2, Run Shi1, Wenkai Ouyang1, Abbas Amini3, Bananakere Nanjegowda Chandrashekar1, Ning Wang2, Chun Cheng1,*

1

Department of Materials Science and Engineering and Shenzhen Key Laboratory of

Nanoimprint Technology, South University of Science and Technology, Shenzhen 518055, P. R. China 2

Department of Physics and Center for 1D/2D Quantum Materials, the Hong Kong

University of Science and Technology, Clear Water Bay, Hong Kong, P. R. China 3

Center for Infrastructure Engineering, Western Sydney University, Kingswood,

NSW 2751, Australia

* Corresponding author:

E-mail: [email protected] (Chun Cheng)

ACS Paragon Plus Environment


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Abstract: Monolayer MoS2 crystals with tailored morphologies have been shown to exhibit shape-dependent properties and thus have potential applications in building nano-devices. However, a deep understanding of the relationship between the shape and defect structures in monolayer MoS2 is yet elusive. Monolayer MoS2 crystals in polygonal shapes, including triangle, tetragon, pentagon and hexagon, are grown using the chemical vapor deposition technique. Compared with other shapes, the hexagon MoS2 crystal contains more electron-donor defects that are mainly due to sulfur vacancies. In the triangular shapes, the defects are mainly distributed at the vertices of the shapes while they are located at the center of hexagonal shapes. Based on the Coulomb interaction of exciton and trion, quantitative calculations demonstrate a high electron density (~1012/cm2) and high Fermi level (EC-EF = 15meV) for hexagonal shape at room temperature, compared to triangular shapes (~1011/cm2, EC-EF = ~30meV). These findings verify that a much higher number of donor-like sulfur vacancies are formed in hexagonal MoS2 shapes. This property allows more electrons or trions to localize in such sites through the physical/chemical adsorption of O2/H2O, which results in a strong enhancement of the light emission efficiency in the hexagonal crystal. The findings provide a better understanding of the formation of shape-dependent defect structures of monolayer MoS2 crystals, and are useful in applications for fabricating nano-electronic and optoelectronic devices through defect engineering.

Keywords: Transition-metal chalcogenides, chemical vapor deposition, monolayer MoS2, excitons, trions, shape, defect structures


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INTRODUCTION Following the discovery and successful applications of graphene, new two-dimensional (2D) materials have been discovered on the basis of single layer transition-metal dichalcogenides (TMDs) with unique physical and chemical properties and significant potentials in nanoelectronics, photonics, optoelectronic etc.1-7 Among the TMDs family, a single atomic layer molybdenum disulfide (MoS2) has a direct bandgap of 1.8 eV which is complementary to the zero bandgap of graphene. MoS2 has gained significant attention for specific applications in catalysis,8-10 sensors,11-14 valleytronic devices,15-17 solar cells,18 and optoelectonic devices.19-20 Thus, there is considerable effort recently to develop high-quality and continuous single-layered MoS2 on a large scale. To date, the chemical vapor deposition (CVD) method has been considered as a promising technique for the growth of large-area and high-quality MoS2 thin films.21-25 Also, single-layered MoS2 sheets with different morphologies, such as triangles,26-30 three-point stars,21 butterfly-shape,31-33 and hexagons31-32 have been synthesized using the CVD method. The morphology of 2D materials, which correlates with the crystallinity, crystal orientation, edge structure and lattice defect, plays a vital role in the mechanical and electrical properties of crystalline materials.34-36 Despite recent attempts to advance the physical and chemical properties of MoS2 crystals, there has not been a systematic study on the shape-dependent defect structures of single-layered MoS2 with different morphologies. In particular, defect



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tailoring in single layer TMDs has become a long-term goal, which is largely necessitated by the development of modern functional devices.10, 37 According to recent studies, single layer MoS2 is a natively n-type semiconductor due to the predominant sulfur vacancy.38-41 In single-layered MoS2, the sulfur vacancy is an electron-donating point defect which serves as an electron localization center. This property affects the MoS2’s charge transport,40 electron-phonon interaction,42 and charge transfer among the interfaces.43-44 In contrast to the plane zone, the edge sites of the MoS2 crystal have a much higher chemical catalytic activity and gas sensing property due to its higher number of defect sites and edge dangling bonds.45-46 It is expected that monolayer MoS2 with controlled defect structures and defect distribution would exhibit unique and functional electronic properties, as compared to its bulk material. Therefore, a comprehensive understanding through measuring the defect structures and distribution in CVD-grown monolayer MoS2 crystals has become a necessary. In this paper, with a comprehensive photoluminescence (PL) and Raman study, we report on the defect structures and distribution in CVD-grown monolayer MoS2 crystals with different morphologies. The hexagon MoS2 crystals feature significantly enhanced PL emission, about three times higher than those from triangular, tetragonal and pentagonal shapes. PL mapping studies indicate that more defects are present in the vertices of triangular MoS2 while they are gradually shifted to the center in hexagonal MoS2 shapes. Based on the Coulomb interaction of the exciton with trion,47 quantitative calculations demonstrate that hexagonal MoS2 crystal has a high electron


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density (~1012/cm2) and a high Fermi level (EC-EF =15 meV) at room temperature, compared with other shapes of MoS2 (~1011/cm2, EC-EF = ~30meV). This implies that more donor-like sulfur vacancies are formed in the hexagonal MoS2 crystals.



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RESULTS AND DISCUSSION Figure 1(a) schematically shows the CVD setup for the growth of monolayer MoS2. More details regarding the MoS2 growth are given in the Experimental Section. Figure 1(b) shows a typical optical microscopic image of the as-grown MoS2 crystals on the SiO2/Si substrate. It can be seen that the synthesized MoS2 sheets have regular polygon shapes, mainly triangle, tetragon, pentagon and hexagon. The edge lengths of most of the isolated sheets range between 15 and 30 µm. Sub figures 1(c)-(f) show typical optical images of single ultrathin MoS2 crystals taking various shapes from simple triangle to multi-edged hexagon. The bright dots located at the center of the MoS2 layers (Figure 1(b)) indicate the original nucleation sites for the crystals.30 Similar to graphene and h-BN, MoS2 has a hexagonal lattice structure with a six-fold lattice symmetry.31, 48 The bright-field TEM image and diffraction pattern of triangular growth nuclei (Supporting Information Figure S1) show the single crystal structure of triangular MoS2 shape in Figure 1(c). The MoS2 flakes in tetragonal, pentagonal and hexagonal shapes display approximately the same angle spacing as the ones for triangular shapes. In fact, a 60° or multiple of 60° (120° or 180°) angular spacing between major angles is retained for the tetragon, pentagon and hexagon structures which indicate a common six-fold symmetry. Nevertheless, the lattice of monolayer MoS2 contains two sub-lattices, including two layers of S atoms sandwiching a layer of Mo atom. This leads to a reduction of six-fold symmetry in the hexagonal lattice and the formation of a three-fold symmetry.31 Such a symmetric MoS2 crystal with tetragonal, pentagonal and hexagonal shapes is formed as a result


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of the cyclic twinning mechanism which is commonly found in other inorganic and geological crystal growths.49 As shown in Figure 2(a)-(d), the layer structure of as-grown MoS2 was further confirmed using the atomic force microscope (AFM) technique. It is found that the surface of the MoS2 samples was not perfectly clean and some bright dots were distributed within and at the edge of the polygon areas. These bright dots came from the PTAS, which served as a seeding promoter for the growth of MoS2.28 In spite of this, the AFM characterization with a line-scan profile shows as-grown MoS2 crystals with a thickness of ∼0.8 nm, confirming it as the sole layer of MoS2 (0.69 nm).50 It is worth to note that the majority of the separated sheets were uniform monolayers, except for small bilayers or multilayer patches. As effective methods for the characterization of crystal quality and bandgap of 2D materials, Raman and photoluminescence spectroscopic examinations were carried out on the as-grown MoS2 samples. Figure 2(e) shows the Raman spectra of the MoS2 1 samples, where the distance between the in-plane E2g mode and out-of-plane A1g

mode is about 20.3 cm-1. This indicates the formation of predominately monolayer MoS2, which is consistent with the previous reports.11,

30

Additionally, PL

characterizations were performed at the center of different shape samples (Figure 2(f)) to identify the layer number of MoS2 films. All these samples have a prominent PL peak at about 1.82-1.87 eV (peak A) and a weak peak at ~1.98eV (peak B). These two resonance peaks are the direct exciton transitions and their energy difference is originated from the spin-orbital splitting of the valence band (inset Figure 2(f)).51-52



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The strong emissions at ~1.85 eV indicate the monolayer nature and the direct bandgap of the as-grown MoS2 crystals. Significantly, the strong PL emission intensity of the hexagon MoS2 is three times higher than those of the triangular, tetragonal and pentagonal samples (Figure 2(f)). This may indicate much more sulfur vacancy in the hexagon MoS2 crystal.43 Sulfur vacancy, a type of electron-donating defect, acts as an adsorption site on the MoS2 surface and is very sensitive to the O2 and H2O in the atmosphere.43-44 O2/H2O molecules (electron/acceptor) in the atmosphere can be physically/chemically adsorbed onto and strongly localized at such defect sites (p-type doping).44 The binding of free electrons or trions (one hole combined with two electrons) with such defects form localized excitons, which are very stable and can even prevent non-radiative recombination. At the same time, the resonance peaks A of the triangular, tetragonal and pentagonal MoS2 are located at ~1.82 eV. Such peak blue shifts to ~1.87 eV suggest that more p-type doping, mainly from O2 adsorption, occurs in hexagonal MoS2.43-44 In other words, hexagonal MoS2 can accommodate more sulfur vacancies for the physical adsorption of O2/H2O molecules. To quantitatively analyze the crystal defects in the samples with different shapes, an analysis of the PL spectra at different locations of MoS2 shapes were performed at room temperature. Figure 3 gives the normalized PL intensity (peak A) distribution of the as-grown samples. The bottom of each panel in this figure shows the PL measurement location and the upper part displays the contour map of PL intensity. The maximum emission of a triangular shape in PL mapping occurs close to the


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vertices (Figure 3(a)), similar to that reported by Gutiérrez et al.53 This enhanced PL emission at the vertices has been ascribed to the formation of lattice defect or edge dangling bond. When the crystal shape transforms from triangle to tetragon (Figure 3(b)) and pentagon (Figure 3(c)), no maxima in PL intensity are observed in each vertex of the uniform polygons. However, an unexpected result is observed in the hexagon shape in Figure 3(d), where the maximum PL intensity is at the center of the sample, in contrast to the intensity distribution of the triangular shape. The above findings necessitated a systematic study of the PL emission intensity profiles to obtain a clear scenario of defect structures in different shapes of MoS2. Optical excitation often generates electron-hole pairs in 2D materials and then forms stable exciton states due to their extremely large Coulomb interaction. In particular, in MoS2, the formed excitons have the capacity of interacting with the charge carriers, giving rise to the formation of the many-body bound state mainly as trions (charged excitons).54-57 Because of the Coulomb interaction, the formed trions (negative center) are prone to be localized in the donor-like defect (positive center). Thus, a trion-related study may be considered as an effective mean to explore the defect nature in single layer MoS2. To do this, the contributions of exciton ( A0 ) and the trion ( A− ) in PL peak A is analyzed here (Figure 4(a)).54-55 In triangular and hexagonal shapes, peak A consists of three peaks which are the radiative combination peaks of A− trion (∼1.79 eV/triangle, ∼1.84 eV/hexagon), A0 exciton (∼1.84 eV/triangle,

∼1.87 eV/hexagon), and B exciton (∼1.94 eV/triangle, ∼1.99 eV/hexagon). The band diagrams of these three quasiparticles are schematically depicted in Figure 4(b). As



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discussed, such a blue-shift of the exciton and trion peaks, compared with triangular MoS2, may be a result of the variation in band structure induced by different defect structures and concentrations. In addition, it is observed that the intensity of the A− trion is almost comparable to the A0 exciton in the triangular MoS2 crystal, whereas the intensity of the A− trion is relatively lower than that of the A0 exciton in hexagon crystal. This implies that the common radiative recombination of the exciton of hexagon MoS2 crystals is stronger than that of triangular shapes. To study the relation between the band structure and samples’ morphology, temperature-dependent photoluminescence examinations were performed from 93-293K on the as-grown MoS2 samples. The original temperature-dependent PL spectra and the peak position of A excitation are provided in Figure 5 and Supporting Information (Figure S2). It is seen that the integrated intensity of peak A decreases with the temperature rises (Figure 5(a) and 5(b)). The gradual drop in quantum efficiency occurs owing to the thermally activated non-radiative recombination.58 Meanwhile, the A and B peaks are broadened and shifted to the lower energy zone as the temperature rises. The PL peak energy of direct-bandgap semiconductors is called the optical bandgap. A standard temperature-dependent bandgap ( E g ) of a semiconductor has been introduced with the following equation:59

  hω E g (T ) = Eg (0) − S hω coth( ) − 1 2k BT  


(1)

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where Eg(0) represents the optical bandgap of the semiconductor at 0 K, S is the dimensionless coupling constant and hω

stands for the average phonon energy.

Here, the calculated optical bandgap is based on the temperature-dependent variation of peak A, as depicted in Figures 5(a) and (b). Figure 5(c) shows that the experimental results coincide with those of the calculated ones. The feature parameters, including

Eg (0) , S and hω , are listed in Table 1. The extracted parameters Eg (0) are consistent with those reported by others.58 The parameters S and hω

have a

corresponding relation with the maximum value in hexagonal MoS2 shape (34 meV). Here, the average phonon energy belongs to the emission photon from the recombination of an electron-hole pair, whose energy is higher than the photon emitted from the recombination of trion (emitting a photon and leaving one free electron). Specific analyses are proposed in Supporting Information Note S1. A peak fitting method was utilized to analyze the trions derivation with temperature (Figure 4(a)); the results are given in Supporting Information (Figure S3-S6). According to the populations of excitons and trions derived from the steady-state solutions, the intensity ratio of the A - trion and A0 exciton can be expressed as:55

I(A- ) γ A- N A = I ( A0 ) γ A0 N A0

(2)

The parameters γ A - and γ A0 express the radiative decay rate of the exciton and trion, respectively. N A0 and N A- are the equilibrium densities of respectively exciton A0 and trion A− . The radiative recombination of the excitons and trions influences the electron density of the MoS2 systems, thus, in terms of the electron



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density Boltzmann distribution function, the temperature-dependent

A − / A0

intensity ratio is given by: I(A- ) E − ( EC − E F ) = M (k BT )1 / 2 exp( T ) 0 k BT I(A )

M=

(3)

γ A m A (2πme )1 / 2 ⋅ 4hmA γA -

0

−

(4)

0

The parameter T refers to temperature, k B is Boltzmann constant, ET is the trion binding energy, and (EC-EF) is the position of Fermi level. More information on the temperature-dependent mass action model is provided in Supporting Information S2. Finally, after taking logarithms of equation (3), a linear relation of

ln{[I ( A− ) /I ( A0 )] ⋅T −1 / 2} versus 1 / T is obtained: ln[

As

shown

in

I(A- ) −1 / 2 E − ( EC − EF ) ⋅T ] = ln[ M ( k B )1 / 2 ] + T 0 k BT I(A ) Figure

6,

the

well-fitted

experimental

(5)

results

of

ln{[I ( A− ) /I ( A0 )] ⋅T −1/ 2} versus 1000 / T with Eq. (5) were obtained based on the peak fitting results (Figure S3-S6). The fitting parameters of Fermi level ( EC − EF ) 293 K and electron concentration ne / 293 K are listed in Table 1. The Fermi level obtained here (~15-35 meV) is much lower than the reported values of others (~200 meV)60. This due to the fact that CVD grown MoS2 crystals often contain more defects, typically as the n-type donor-like sulfur vacancy, than those created by mechanical exfoliation61. This shifts the Fermi level to the conduction band.62-63 Also, the electron density calculated from this model is around 1011 to 1012 cm-2, which is in good agreement with the values reported by others.1, 42, 54-55


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Subsequently, a semi-quantitative analysis is conducted to determine the density of sulfur vacancy among these different shapes of MoS2 crystals. According to the semiconductor physics theory, the Fermi level (EF) of the semiconductor is directly correlated with the donor density as illustrated in following

E F = EC + k B T ln(

ND − NA ) NC

(6)

where N D and N A are the densities of the donor-like and acceptor-like defect. N C is the effective density of states in the conduction band. For the n-type MoS2 crystal, it contains more donor-like sulfur vacancies ( N D ) and thus the N A could be ignored. Since the Fermi level of the hexagon MoS2 is more close to the conduction band as compared to other shapes. Thus, based on equation (6), the concentrations of sulfur vacancy in the center of the hexagonal MoS2 crystals are higher than those in triangular, tetragonal and pentagonal shapes. Generally speaking, increase of the defect density in the semiconductor would result in a photoluminescence quenching owing to such sites serve as centers for the non-radiative recombination.64 However, it can be seen that the hexagonal MoS2 present a high light emission efficiency as compared with other shapes (Figure 2f). Considering the PL and Raman tests condition (in the atmosphere environment), it is suggested that more surface adsorbed O2/H2O encountered on the hexagonal MoS2 surface due to they can localize free electrons or trions as well as prevent non-radiative recombination in such defects.43-44 In view if this, it is further confirmed that the hexagonal MoS2 crystal possesses a



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higher density of sulfur vacancy than those in triangle, tetragonal and pentagonal shapes. This result may be ascribed to the symmetric growth of the MoS2 crystal under the cycle twinning mechanism in the hexagonal, pentagonal and tetragonal ones.53 With regard to the single layer MoS2 grown through the CVD method, the triangle shape often shows single crystal (Supporting Information S1), and its edge and vertex contain more defect sites and adsorbates due to the edge dangling bonds.45-46 Whereas, the other polygonal shapes of MoS2 crystals usually involve a twin crystal growth. Especially for the highly symmetric hexagonal MoS2, it contains several rotationally symmetric mirror twins, forming a cyclic twin.31, 53 Such hexagonal structure has been observed under dark-field TEM in the former research and contains a common line defect in MoS2 crystals: mirror twin boundaries.31 This defect structure often displays an 8-4-4 ring motif, that is, the mirror twin boundaries periodically recurred from 4and 8- membered rings.31 Due to the coordination change in the 8-4-4 defects, the mirror boundaries are molybdenum rich, which can be seen as n-type doping. In other words, many sulfur vacancies are formed around the mirror boundaries in the hexagonal MoS2 crystal. While for the tetragonal and pentagonal ones, they are unable to strictly grow through the mirror twin mechanism. In this way, another common line defect, 5-7 ring motif (periodically recurred from 5- and 7- membered rings), may form in the grain boundary.30 Conversely, such structure of line defect is sulfur rich, which would p-dope the boundary. From the PL mapping tests in Figure 3,


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we can also see that the intensity distributions in the tetragonal and pentagonal are not uniform as compared with the hexagonal one. This also indicates that different line defects are formed in the grain boundary of the tetragonal and pentagonal MoS2. Therefore, the electron density difference between the tetragonal, pentagonal MoS2 and triangular crystal is very limited.



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CONCLUSION An in-depth study on the shape-dependent defect structure of single layered MoS2 crystals is presented. For the first time, monolayer MoS2 crystals in polygonal shapes, including triangle, tetragon, pentagon and hexagon, are grown using the CVD method. Experimental characterizations and quantitative analyses show that the electron density and Fermi level increased from the triangle MoS2 crystal (~1011/cm2, EC-EF = ~30meV) to the hexagon one (~1012/cm2, EC-EF = ~15meV), which demonstrates that more donor-like defects, sulfur vacancy, are formed in hexagonal MoS2 shapes. In addition, because of the cyclic twinning mechanism, the defects distributions are gradually transferred from the vertices and edges (triangular shape) to the center (hexagonal shape). The findings indicate that hexagon MoS2 shapes play an important role in the chemical catalysis process or gas sensing due to their higher distribution of active sites in the whole basal plane when compared with other MoS2 shapes. This study sheds light on the relationship between the defect structure of single layer MoS2 crystals and their plane shapes, which may be useful for fabricating nano-electronic and optoelectronic devices through defect engineering.


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EXPERIMENTAL METHODS MoS2 growth procedure. Monolayer MoS2 crystals were fabricated by the CVD method with similar process as ref. 28. Si substrates with a 300-nm oxide layer were cleaned for 10 minutes with piranha solution (H2SO4/H2O2 (2:1)), acetone, isopropanol and deionized water. Prior to growth, four drops (0.5 µL) of perylene-3,4,9,10-tetracarboxylic acid tetrapotassium salt (PTAS) solution (2 µM) were placed at each corner of the substrates and dried in the air. Then, the substrates were put in a 1-inch CVD furnace face-down above a crucible containing 1.3 mg of MoO3 (≥ 99.5% Sigma Aldrich#100932642) located in the middle. Another crucible containing 1.4 mg of sulfur (≥ 99.5% Sigma Aldrich #101144903) was place in the upstream area 22~25cm away from the MoO3. No extra heating equipment is required for sulfur evaporation. CVD growth took place under the atmospheric pressure with a flowing ultrahigh-purity Argon (Ar) gas. Before heating, the tube was pumped out to 2×10-2 torr vacuum and then purged with Ar gas three times. The Ar gas flow during the process was 18 sccm. After 30 minutes of Ar purging, the furnace temperature was gradually increased from room temperature (23oC) to 620oC at a rate of 15oC/min, and subsequently maintained at that temperature for 10 minutes. As the furnace temperature was naturally cooled to 600oC, it was opened for rapid cooling to room temperature.

AFM, Raman/PL, and TEM Measurements. The prepared samples were firstly characterized using an optical microscopy (Olympus BX51). The surface morphology was examined with an atomic force microscope (AFM) (Asylum Research,



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MFP-3D-Stand Alone). Raman spectra were obtained using a Raman spectroscope (Horiba, LabRAM HR-800) with a laser excitation wavelength of 532 nm. Temperature-dependent PL spectra were obtained by loading the temperature controlled stages (Linkam, THMS600) onto the Raman spectroscope system and measuring at a laser excitation wavelength of 325 nm. The Si peak at 520 cm−1 was used for calibration in the experiments. The Transmission Electron Microscope (TEM) samples were prepared using a lacey-carbon Cu grid to scratch the MoS2 sample after transferring to the PMMA solution. TEM imaging and diffraction were conducted using a TEM (FEI Tecnai F30) adjusted to 300 kV.


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ASSOCIATED CONTENT Supporting Information. TEM images and the diffraction pattern of single-crystal triangle nuclei (Figure S1), temperature-dependent PL spectra (Figure S2) as well as their

Lorentz

peak

fitting

results

(Figure

S3-S6),

analysis

of

the

temperature-dependent PL spectra (Note S1 and S2) are presented. This material is available free of charge via the Internet at http://pubs.acs.org.



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AUTHOR INFORMATION Corresponding Author: Prof. Chun Cheng, E-mail address: [email protected] Notes: The authors declare no competing ﬁnancial interest.


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ACKNOWLEDGMENTS This work was supported by Guangdong-Hong Kong joint innovation project（Grant No. 2016A050503012), the National Natural Science Foundation of China (Grant No. 51406075 and 51402147), Key Research and Development Project funding from the Ministry of Science and Technology (Grant No. 2016YFA0202400), the Guangdong Natural

Science

Funds

for

Distinguished

Young

Scholars

(Grant

No.

2015A030306044), Training Program for Outstanding Young Teachers at Higher Education Institutions of Guangdong Province (Grant No. YQ2015151), Foundation of Shenzhen Science and Technology Innovation Committee (Grant No. JCYJ20150331101823695) and the Shenzhen Peacock Team Plan (Grant No. KQTD2015033110182370). The authors are also grateful to the Analytical and Characterization Center of South University of Science and Technology of China (SUSTC).



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Table: Table 1. Numerically simulated feature parameters of the monolayer MoS2 samples fitted from the temperature-dependent PL spectra. MoS2

Eg(0)

Shape

eV

Triangle

1.86

Tetragon

1.86

〈ħω〉

ET/(293K)

(EC-EF)/293K

ne/293K

meV

meV

meV

1012 cm-2

1.9

34

45

35

0.73

1.8

26

44

35

0.73

S

Pentagon

1.87

2.3

32

45

32

0.83

Hexagon

1.91

2.6

46

30

15

1.62


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Figures:

Figure 1. (a) Schematic of the MoS2 CVD synthesis process. (b) Optical microscopy image of the as-grown large-grain MoS2 on a SiO2 (300 nm)/Si substrate. The image contrast is increased for visibility; magenta is the bare substrate and violet represents monolayer MoS2. (c) - (f) Optical images of monolayer MoS2 with shape morphologies of (c) triangle, (d) tetragon, (e) pentagon and (f) hexagon. Scale bars: 20 µm.



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Figure 2. AFM images of MoS2 of (a) triangular, (b) tetragonal, (c) pentagonal and (d) hexagonal shapes. The inset is the height profile plotted along a black line. The step height or the thickness of the MoS2 is around 0.8 nm, indicating a monolayer of MoS2. (e) Typical Raman and (f) photoluminescence spectra of as-grown MoS2 of different shapes. The inset of (f) shows the normalized PL spectra of monolayer MoS2.


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Figure 3. Normalized PL intensity (A excitons) distribution of (a) triangular, (b) tetragonal, (c) pentagonal and (d) hexagonal MoS2 shapes. Scale bars: 10 µm.



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Figure 4. (a) Peak analysis of the PL spectra of the triangular and hexagonal monolayer MoS2. The A peaks in the PL spectra were fitted by assuming two peaks with Lorentzian functions, corresponding to the trion (A−) and the exciton (A0) peaks. The peak B is the exciton emission due to the spin-orbital splitting of the valence band. (b) Schematics of the exciton-related radiative transition at the K-point in the Brillioun zone. EE and ET represent the binding energies of the A0 exciton and A− trion.


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Figure 5. Temperature-dependent PL spectra of (a) triangular and (b) hexagonal monolayer MoS2 which range from 93-293K. The wavelength of the excitation laser is 325 nm. (c) A peak position versus temperature with linear fitting (green dash lines).



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Figure 6. Temperature-dependent A−/A0 intensity ratio with different MoS2 shapes with linear fitting (green dash line).


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Graphical Abstract


Shape-Dependent Defect Structures of Monolayer MoS2 Crystals

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