Facile Preparation of Single MoS2 Atomic Crystals with Highly

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Facile preparation of single MoS2 atomic crystals with highly tunable photoluminescence by morphology and atomic structure Jie Li, Chenli Hu, Hao Wu, Zhixuan Liu, Shuai Cheng, Wenfeng Zhang, Haibo Shu, and Haixin Chang Cryst. Growth Des., Just Accepted Manuscript • DOI: 10.1021/acs.cgd.6b01330 • Publication Date (Web): 24 Oct 2016 Downloaded from http://pubs.acs.org on October 24, 2016

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Crystal Growth & Design

Facile preparation of single MoS2 atomic crystals with highly tunable photoluminescence by morphology and atomic structure Jie Li,1 Chenli Hu,2 Hao Wu,1 Zhixuan Liu,1 Shuai Cheng,1 Wenfeng Zhang,1 Haibo Shu,2 Haixin Chang1,* 1

Center for Joining and Electronic Packaging, State Key Laboratory of Material

Processing and Die & Mold Technology, School of Materials Science and Engineering, Huazhong University of Science and Technology (HUST), Wuhan 430074, China. 2 College of Optical and Electronic Technology, China Jiliang Univeristy, Hangzhou 310018, China.

Email: [email protected]

Abstract: To prepare atomically thin two-dimensional (2D) transition metal dichalcogenides

(TMDs)

single

atomic

crystals

with

highly

tunable

photoluminescence is of special importance in 2D photonics and optoelectronics. However, to control the photoluminescence (PL) in 2D TMDs atomic crystals is still a big challenge due to atomically thin 2D nature which induces far more complicated and variable PL behaviors than normal semiconductors. In this report, triangular and tetragonal MoS2 atomic crystals are prepared simultaneously on one single growth substrate for the first time in a single CVD process. Through manipulating the growth dynamics to reduce nucleation density and enhance lateral growth with mixture precursor and other growth parameters, both the morphology and PL of the MoS2 1

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atomic crystals can be efficiently modulated. The PL of the optimized triangular MoS2 atomic crystals can be over one to two orders higher than PL of other triangular morphology modulated by different growth dynamics. In addition, position-dependent PL and a gradient decrease in PL intensity from the center towards edge region are found in both triangular and tetragonal MoS2 atomic crystals, which show 1-2 orders of magnitude of decrease in the PL and imply the strong influences of atomic structure disorders and edges considering the high-resolution transmission electron microscopy (HRTEM) observations and first-principles calculations. These results indicate a facile way to tune the PL of TMDs by morphology and atomic structure.

Key words: MoS2, atomic crystals, photoluminescence, morphology

Introduction Since the discovery and comprehensive study of graphene,1-16 other atomically thin two-dimensional (2D) materials, such as layered transition metal dichalcogenides (TMDs) have also attracted great interest.17-27 Atomically-thin 2D TMDs crystals show unique physical and chemical properties, which make them prominent from their bulk counterparts and enables a variety of potential applications in many fields.28-41 For example, atomically thin semiconducting TMDs are promising channel materials for transistors due to their quantum well effect and relatively high charge mobility.42-44 Meanwhile, several TMDs (MoS2, WS2, MoSe2, WSe2) show the layer dependent electronic band structures thus undergo indirect-to-direct bandgap transition45-47, which provides great chances for 2D photonics and optoelectronics.48-50 2


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Photoluminescence (PL) is one of the most noteworthy properties for the atomically thin 2D TMDs, and has been extensively studied. Conventionally, the PL intensity are dependent on several factors such as strain,28 chemical doping,51, 52 ion itercalation53 and structure defects.54, 55 However, the PL of atomically thin 2D TMDs is far more complicated due to atomically thin 2D nature. For example, the non-uniformity of PL has been a great obstacle for various applications, while the origin of such non-uniform characteristics is still not clear to date.56,

57

In general, to prepare

atomically thin two-dimensional (2D) transition metal dichalcogenides (TMDs) single atomic crystals with highly tunable photoluminescence is of special importance in 2D optoelectronics and is still a big challenge. Recently, chemical vapor deposition (CVD) method has been successfully adopted to synthesize single crystalline 2D TMDs with large scale.20, 58 Concerning about the MoS2 atomic Crystals, triangular domain is the dominated morphology reported.59, 60 Meanwhile, tetragonal MoS2 with high crystallinity was also obtained through sulfurization of MoO2 microplates.61,

62

But there is still no report on

synthesizing triangular and tetragonal MoS2 crystals simultaneously in a single one-step CVD process. In this report, triangular and tetragonal MoS2 atomic crystals were simultaneously prepared on one single growth substrate for the first time in a single CVD process. Through manipulating the growth dynamics to reduce nucleation density and enhance lateral growth with mixture precursor and other growth parameters, both the triangular and tetragonal morphology and PL of the MoS2 atomic 3



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crystals can be efficiently modulated. The PL of the optimized triangular MoS2 atomic crystals can be over one to two orders higher than PL of other triangular morphology modulated by different growth dynamics. In addition, we further studied PL in different positions of individual atomic crystals with different morphology. Position-dependent PL and a gradient decrease in PL intensity from the center towards edge region are found in both triangular and tetragonal MoS2 atomic crystals, which show 1-2 orders of magnitude of decrease in the PL and imply the strong influences of atomic structure disorders and edges considering the high-resolution transmission electron microscopy (HRTEM) observations and first-principles calculations. These results indicate a facile way to tune the PL of TMDs by morphology and atomic structure disorders.

Experimental section Growth of single MoS2 atomic crystals: We have used a home-made two-zone CVD furnace to grow MoS2 atomic crystals. Sulfur powder and MoO3 powder was used as the source for synthesizing MoS2, various mole ratio of MnO2 (x=M MnO2 : M MoO3 from 2:1 to 0:1) plays an important role in controlling morphology and quality of as-grown MoS2. Prior to growth, substrates (Si with 300 nm SiO2) were cleaned in acetone and ethanol, followed by 2 h in conventional piranha solution and washed with DI water three times. MoO3 and MnO2 with different fractions (x=0, 0.25, 0.5, 1, 1.5, 2) were mixed and well-grounded in a mortar. The mixture was placed in a ceramic boat with a width of 1 cm. A substrate (1x2 cm) was placed on top of the 4


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powders, perpendicular to the boat. The region faced down on upper side of the powders was referred to as part B, the other region beyond the boat width as part A. Then the boat was loaded at the center of the second hot-zone and the sulfur was placed upstream 19-20 cm from the mixture. The furnace was pumped down to 10-2 torr, refilled with high purity nitrogen, and repeated twice. After purging the system, the second zone was heated up to 650℃ in 35 min, and kept this temperature for 20 min. When the temperature of second zone was reached to 500℃,The temperature of the first zone where loaded sulfur can be reached to 180℃ due to the heating influence from the second zone. Then the furnace was opened and cooled down to the room temperature naturally. During the growth, 50 sccm of nitrogen was used as the carrier gas and the pressure was kept at 40 torr.

Theoretical calculations: All density-functional theory (DFT) calculations are performed using the projector augmented wave (PAW) method63 with the generalized-gradient approximation (GGA) functional64 as implemented in the VASP code.65, 66 In order to describe accurately interlayer van der Waals interactions (vdWs) of bilayer MoS2, the PBE functional with the vdWs correction (DFT-D2)67 has been used. The kinetic energy cutoff for the plane-wave expansion was set to 400 eV. The MoS2 nanoclusters are created on the basis of supercell models of MoS2 bilayer and monolayer, as shown in Figure 7. For the structural optimizations, the convergence criteria of electronic and ionic iterations are 10-3 eV and 10-2 eV/Å, respectively. Characterizations: The morphology of the as-grown MoS2 was observed by 5



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optical microscopy and field emission scanning electron microscopy (FSEM, JSM-7600F). We measured the thickness of the samples by atomic force microscopy (AFM, SPM9700). Raman, PL and PL mapping (LabRAM HR800) were carried out with a 100x objective under ambient condition. The wavelength of laser is 532 nm. X-ray photoelectron spectroscopy (XPS, AXIS-ULTRA DLD-600W) was used to characterize the atomic binding energy. C1s spectra were used to calibrate the binding energy. Field emission transmission electron microscopy (FETEM, Tecnai G2 F30) was applied to analyze to atomic structures of atomic crystals. The TEM samples were made by a typical wet-transfer method.

Results and Discussion To modulate the growth dynamics, the well-grounded mixture of MoO3 and MnO2 was used as precursor for Mo sources. Various amounts of MnO2 in the mixture were applied to modulate the growth of MoS2 atomic crystals in a typical CVD process. More details are described in Methods and the Supporting Information. Figure S1 illuminates the schematic diagram of experimental setup. We separate the growth substrate into two parts. Part B is the region right above the mixture sources in the ceramic boat. The position next to Part B is named as Part A, which is outside the ceramic boat. The morphology of the grown MoS2 atomic crystals in Part A was found to be almost triangular and the morphology in Part B would be dominantly tetragonal. Figure 1a-1f show the optical microscopy (OM) images of as-grown MoS2 from Part A with six different mole ratios of MnO2 to MoO3 (x= MnO2/ MoO3). 6


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When the fraction x=0, 0.25, triangular MoS2 are too small to be found clearly. It is noteworthy that, with the increasing of the amount of MnO2, triangular MoS2 crystals are much larger in size. Among the samples, the largest domain size about 18-20 µm appears at the x=1, while the size would decrease to several micron, if the mole ratio is as high as x=1.5 or higher (more details were displayed in Table S1). For all the triangular atomic crystals distinguished by optical microscopy, curve edges are always observed. This is consistent with the previous reports.59,

68

For Part B,

tetragonal atomic crystal is dominated in this region. For example, Figure 1g-1i show optical images taken from part B from a sample grown with x=1. The tetragonal MoS2 crystals with an 1-2 µm domain size are found as discussed more with the Raman and XPS spectra later. Raman, Photoluminescence measurements and atomic force microscopy were used to characterize the MoS2 atomic crystals. Figure 2a shows the Raman spectra of the MoS2 atomic crystals with different triangular morphology grown under six different MnO2 content in the mixture precursors. They all exhibit two predominating peaks at around 384.5 cm-1 and 404.5 cm-1, corresponding to E2g1 (in-plane vibration) and A1g (out-of-plane) modes.69 For single-layer and few-layer MoS2, the Raman peak difference between E2g1 and A1g can be used to estimate the layer number, smaller peak difference thinner atomic crystals. Note that the peak difference between E2g1 and A1g for MoS2 grown with more MnO2 in precursor is 19.2 cm-1 smaller than 22.2 cm-1 for that grown without or with few MnO2, indicating vertical growth would like to be suppressed with certain quantity of MnO2. The A1g/ E2g1 distance of 19.2 cm-1 7



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agrees well with the bilayer MoS2 and is consistent with ～1.62 nm thickness (Fig. 2b).69 For the PL spectra, considering the difference of the intensity of center and edge rigions, the PL displayed in Fig. 2 were all measured at the center area. The A and B direct exciton transitions induced by spin-orbit coupling at the K point of the Brillouin zone are detected around ～625 nm and 675 nm. However the peak around 770-900 nm arises from the indirect transition is not observed due to the susceptibility under ambient conditions.46 The MoS2 atomic crystals from mole ratio of 1:1 for MoO3 and MnO2 in the mixture precursors have the highest PL intensity which is one or two orders of magnitude higher than that without MnO2. The large difference of the PL for triangular MoS2 atomic crystals grown with and without MnO2 results from different growth dynamics. The triangular MoS2 atomic crystals without or with few MnO2 are much smaller and usually have a thicker thickness of 3.2 nm (3-4 layers) than that with large quantity of MnO2. The fast growth and high nucleation density without MnO2 lead to smaller and thicker atomic crystals via vertical growth rather than lateral growth of larger, thinner triangular MoS2 atomic crystals grown with more MnO2. This is in good agreement with the Raman analysis of the larger distance between E2g1 and A1g peaks for MoS2 atomic crystals grown without MnO2. MoS2 flakes of tetragonal shape in part B were also investigated by Raman spectra. Raman peaks except the two characteristic peaks of A1g and E2g1 for MoS2 can be mainly assigned to MoO2 (Fig. 2d), which have been confirmed by XRD, Raman and TEM tests (Fig S3, S4 in supporting information). We can assign the the peak at near 130 cm-1 to MoOx due to the diffusion of MoOx.70, 71 So the observed tetragonal 8


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crystals on the substrate actually consist of MoS2 on the top with ultrathin MoO2 underneath the few-layer MoS2 layers. Figure 2e is the AFM image for these MoS2 crystals, and the MoS2 usually have more numbers of layers than bilayer triangular MoS2 from Part A, e.g, 5-7 layers MoS2 in Figure S5. The dominant total thickness of MoS2 and MoO2 ranges from several nanometers to tens of nanometers. Note that the tetragonal MoS2 crystals obtained here have much lower PL intensity than that of triangular ones due to thicker MoS2 in tetragonal MoS2 crystals with more layers (Fig. 2f). In order to further investigate the controllable morphology by using additive well-mixed MnO2 in the precursor, SEM characterizations for samples have been done (Fig. 3). Figure 3a,3b indicate that there are very small (a few tens or hundreds of nanometers) triangular MoS2 atomic crystals which can not be recognized under optical microscope. As shown in Figure 1a-1f and Figure 3a-3c, the addition of MnO2 in mixture precursor facilitate the larger MoS2 atomic crystal growth by controlling the nucleation density. Meanwhile, Figure 3d-3f is taken from Part B location prepared with precursors of different MnO2 ratios. If there is no MnO2 in the precursor, the resulting crystals in Part B would be “crowded” and vertically grown on the substrate (Fig. 3d), and no obvious atomically-thin 2D crystal morphology are observed, which is in contrast with that with using MnO2 (Fig. 3f). So the presence of MnO2 can inhibit the nucleation to decrease the nucleation density and promote the lateral 2D growth. We measured the XRD spectra of the remains of sources with 1:1 molar ratio after the growth reaction (shown in Fig. S2). The remains were MoMnO4 9



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which indicate that the MnO2 in the mixture of MoO3 and MnO2 can regulate the evaporation speed of MoO3 by forming a new alloy MoMnO4 with MnO2. Such result indicates that using an assistant precursor source (e.g., MnO2) with the capability to form alloy (e.g., MoMnO4) with main sources (e.g., MoO3) may be one general route to tune the growth dynamics of TMD atomic crystals. Note that no large size triangular or tetragonal atomic crystals form when little pure MoO3 is applied as sources, indicating a better control by mixture than just reducing the reacting sources. So according to the above characterizations (OM iamges, Raman spectra, SEM ) and related analysis, we provide a schematic for the growth process in Figure 3g, illustrating the growth dynamics of triangular and tetragonal MoS2. For triangular MoS2 atomic crystals, the addition of MnO2 can decrease the nucleation density and promote the lateral 2D growth which is also confirmed by the statistical analysis of thickness and lateral and lateral dimension (Table S1). The optimum condition of lateral size and thickness can be reached with proper ratio of MnO2/MoO3. For the tetragonal MoS2 atomic crystals, abundant MoO3 is firstly reduced to MoO2 and deposited on the substrate just over the precursor boat, and then tetragonal crystals are the result of sulfurization of MoO2 by the diffusion of sulfur atoms. X-ray photoelectron spectra (XPS) has always been used to analyze the chemical bonding of two dimensional materials. Figure 4 displays the detailed XPS scans for Mo, S and O binding energies for the as-grown MoS2 from the optimized condition. The XPS spectra in Figure 4a-4c for triangular atomic crystals grown at the part A demonstrate that binding energy for Mo4+ 3d5/2, Mo4+ 3d3/2 and S 2s locate at 229.8, 10


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232.9 eV and 226.9 eV, respectively.

The S 2p spectrum for part A exhibits two

peaks at about 163.7 and 162.6 eV which are assigned to the doublet, S 2p1/2 and S 2p3/2 for S2-. Importantly, only 533.1 eV for Si-O in O1s spectra is observed and there is no Mo-O bonding at the part A (Fig. 4c). Figure 4d-4f shows the XPS spectrum for tetragonal atomic crystals grown at part B. The peaks at 229.5 and 232.6 eV are attributed to the doublet of Mo4+ 3d5/2 and Mo4+ 3d3/2 core levels in MoS2, respectively. Obviously, XPS spectra of O 1s from Part B can be de-convoluted to 533.1 eV for Si-O and 529.7 eV for Mo-O (Fig. 4f), in agreement with the MoO2 Raman peaks in Raman spectra for tetragonal MoS2 atomic crystals at Part B (Fig. 3d) and the TEM observation (Fig. S4). We note that the XPS characterization of Mn 2p for all the samples show that there is no Mn signal detected (Fig. S5 in Supporting Information). So the doping effect of MnO2 is negligible, and MnO2 mostly only modulates the growth of MoS2 in our CVD-growth by reducing the reacting quantity of MoO3 through forming MoMnO4 alloy with MoO3 (Fig. S2). It is consistent with the XRD measurements of the MoMnO4 remains of mixed precursors after growth (Fig. S2). More importantly, position-dependent photoluminescence is observed both in single triangular and tetragonal atomic crystals. The position-dependent PL in a single MoS2 atomic crystal is displayed in Figure 5. In contrast to previous reports,56, 57 the central area has the highest PL and the PL decreases significantly in the regions between center and edge and much more quenched at the edges in triangular MoS2 atomic crystals (Fig. 5a). Similar situations are observed in PL of tetragonal crystals (Fig. 5b). Take the triangular MoS2 as example, PL intensity in the center of triangular 11



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MoS2 atomic crystals can be over 2 times higher than that at the regions between center and edge and about one order of magnitude higher than that at the edges. For tetragonal MoS2, the PL in the center is over 3 times higher than that at the regions between center and edge and is two orders of magnitude higher than that at the edges. High-resolution PL mapping was further applied to characterize the position dependent PL behaviors. Figure 5c and 5d show the PL mapping of the triangular and tetragonal MoS2 atomic crystals, typically integrated PL intensity from 630 to 730 nm. Obviously, integrated PL intensity of central area is higher than that at edges in both kinds of atomic crystals. Importantly, the integrated PL intensity shows a gradient decrease from the center to the edge. Outside the central regions, the PL intensity gradients occupy a narrow equal-intensity-PL ribbon with a width from a few tens to several hundreds of nanometers around the center. This is consistent with our observations of single-point PL spectra measurements, and implies a possibility to modulate the PL in a single TMD atomic crystal at nanometer scale. Note that the PL changes at the different positions are comparable with that induced by morphology. To investigate the profound origin of the PL differences, we first exclude the effect of different thickness within the triangular singal crystal range (Fig. S6). We find that the thickness of

triangular MoS2 is uniform from the center to edge. High

resolution TEM (HRTEM) has been done to reveal the atomic structures of MoS2 atomic crystals. Usually, the atomic structure, such as defections, disorder has great influence on physical properties. According to the above results, we investigate the atomic structures of center and edge areas in triangular MoS2 atomic crystals, 12


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respectively (Fig. 6). Figure 6a and 6b illustrate the HRTEM image of central area for a triangular MoS2 atomic crystal, where highly ordered atomic structures are found, while less ordered atomic structures are observed at the areas close to the edge (Fig. 6d, 6e). FFT pattern in center areas (inset in Fig. 6a) displays hexagonally arranged spots with a six-fold symmetry. However, the FFT pattern of areas close to edge has an irregular, slightly distorted six-fold symmetry, consistent with the emergence of disorder of atomic structure as shown in the enlarged images from selected regions (Fig. 6d).72, 73 The disorder of the atomic structures is further confirmed by the bright field intensity analysis for the HRTEM images. The intensity profile along the lines in Fig. 6b for center areas show a much ordered patterns than that in Fig. 6d from edge areas. Such disorders at the edge areas of the triangular MoS2 crystals will influence the PL properties especially for the PL intensity. Similar disorders are also found at the edge areas in the tetragonal MoS2 atomic crystals (Supporting Information Fig. S4). The FFT pattern in tetragonal MoS2 atomic crystals (inset in Fig. S4a) shows an irregular, clearly distorted six-fold symmetry, implying the atomic disorders observed (Fig. S4b). To further understand the dramatic reduction of PL intensity at the edges of MoS2 atomic crystals as compared to their center regions, we calculated the electronic structures of triangular MoS2

bilayer and monolayer nanoclusters using

first-principles calculations (Fig. 7). The density of states (DOS) distributions indicate that the central basal plane of both triangular MoS2 bilayer and monolayer nanoclusters are semiconducting with opened band gap while their edges indicate a 13



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typical metallic character without band gap, in good agreement with previous scanning tunnel microscopy (STM) observations.74 This is another possible reason why the PL intensity has a rapid drop from the center region to edges of triangular MoS2 atomic crystals. For the tetragonal MoS2 crystals, similar transition is expected for sharing similar electronic properties at their central basal planes and edges. Therefore both the disorder and the edge states may finally result in the PL properties observed in TMD atomic crystals.

Conclusions In conclusion, we have prepared triangular and tetragonal MoS2 atomic crystals simultaneously on one substrate in a single CVD process. The PL intensity of the atomic crystals can be well tuned in over 2 orders of magnitude by modulating the growth dynamics and the morphology. Furthermore, the PL in a single triangular and tetragonal MoS2 atomic crystals show a gradient decrease from center region to edge with equal-intensity PL distributions occupying a width from a few tens to hundreds of nanometers. The PL difference in a single MoS2 atomic crystal at different positions may result from the atomic structure disorder and edge states, implying an efficient route to tune the PL in TMD atomic crystals.

Supporting information available: Figures S1-S6, Table S1.

Acknowledgements 14


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This work is supported by the Natural Science Foundation of China (No. 51402118, 51502101), National Basic Research Program of China (No. 2015CB258400), National Key Research and Development Program of China (No. 2016YFB0700700) and start-up funding from HUST. References (1) Novoselov, K. S.; Geim, A. K.; Morozov, S. V.; Jiang, D.; Katsnelson, M. I.; Grigorieva, I. V.; Dubonos, S. V.; Firsov, A. A. Nature 2005, 438, 197-200. (2) Chen, Z.; Ren, W.; Gao, L.; Liu, B.; Pei, S.; Cheng, H.-M. Nat. Mater. 2011, 10, 424-428. (3) Bao, Q.; Zhang, H.; Wang, Y.; Ni, Z.; Yan, Y.; Shen, Z. X.; Loh, K. P.; Tang, D. Y. Adv. Funct. Mater. 2009, 19, 3077-3083. (4) Li, X.; Cai, W.; An, J.; Kim, S.; Nah, J.; Yang, D.; Piner, R.; Velamakanni, A.; Jung, I.; Tutuc, E.; Banerjee, S. K.; Colombo, L.; Ruoff, R. S. Science 2009, 324, 1312-1314. (5) Gao, L.; Ren, W.; Xu, H.; Jin, L.; Wang, Z.; Ma, T.; Ma, L. P.; Zhang, Z.; Fu, Q.; Peng, L. M.; Bao, X.; Cheng, H. M. Nat. Commun. 2012, 3, 699. (6) Ling, X.; Xie, L.; Fang, Y.; Xu, H.; Zhang, H.; Kong, J.; Dresselhaus, M. S.; Zhang, J.; Liu, Z. Nano Lett. 2010, 10, 553-561. (7) Chang, H.; Sun, Z.; Yuan, Q.; Ding, F.; Tao, X.; Yan, F.; Zheng, Z. Adv. Mater. 2010, 22, 4872-6. (8) Chang, H.; Wu, H. Energy. Environ. Sci. 2013, 6, 3483. (9) Chang, H.; Wu, H. Adv. Funct. Mater. 2013, 23, 1984-1997. (10) Zhu, J.; Yang, D.; Yin, Z.; Yan, Q.; Zhang, H. Small 2014, 10, 3480-3498. (11) Cao, X.; Yin, Z.; Zhang, H. Energ. Environ. Sci. 2014, 7, 1850-1865. (12) Ito, Y.; Zhang, W.; Li, J.; Chang, H.; Liu, P.; Fujita, T.; Tan, Y.; Yan, F.; Chen, M. Adv. Funct. Mater. 2016, 26, 1271-1277. (13) Yin, Z.; Zhu, J.; He, Q.; Cao, X.; Tan, C.; Chen, H.; Yan, Q.; Zhang, H. Adv. Energy Mater. 2014, 4, 1300574. (14) Zhang, H.; Lv, X.; Li, Y.; Wang, Y.; Li, J. ACS Nano 2010, 4, 380-386. (15) Tang, L.; Feng, H.; Cheng, J.; Li, J. Chem. Commun. 2010, 46, 5882-5884. (16) Chen, D.; Feng, H.; Li, J. Chem. Rev. 2012, 112, 6027-6053. (17) Huang, X.; Zeng, Z.; Zhang, H. Chem. Soc. Rev. 2013, 42, 1934-46. (18) Xu, M. S.; Liang, T.; Shi, M. M.; Chen, H. Z. Chem. Rev. 2013, 113, 3766-3798. (19) Rao, C. N. R.; Matte, H. S. S. R.; Maitra, U. Angew. Chem. Int. Ed. 2013, 52, 13162-13185. (20) Shi, Y.; Li, H.; Li, L. J. Chem. Soc. Rev. 2015, 44, 2744-56. (21) Butler, S. Z.; Hollen, S. M.; Cao, L.; Cui, Y.; Gupta, J. A.; Gutiérrez, H. R.; Heinz, T. F.; Hong, S. S.; Huang, J.; Ismach, A. F.; Johnston-Halperin, E.; Kuno, M.; Plashnitsa, V. V.; Robinson, R. D.; Ruoff, R. S.; Salahuddin, S.; Shan, J.; Shi, L.; Spencer, M. G.; Terrones, M.; Windl, W.; Goldberger, J. E. ACS Nano 2013, 7, 15



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Figures and captions

Figure 1 Morphology control of MoS2 atomic crystals. (a-f) Optical images of MoS2 crystal morphology with different ratios between MoO3 and MnO2 at Part A of substrates (the region outside ceramic boat). The ratio of MnO2 to MoO3 is a) 0:1; b) 0.25:1; c) 0.5:1; d) 1:1; e) 1.5:1; f) 2:1. (g-i) Tetragonal morphology with various thickness grown on part B of substrates (the region just above the powders in ceramic boat) at the 1:1 ratio of MnO2 to MoO3.

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Figure 2 (a-c) Raman spectra, AFM image and PL of triangular MoS2 atomic crystals with different ratios of MnO2 to MoO3. (d-f) Raman, AFM and PL characterizations of tetragonal atomic crystals. Inset in b, d: AFM height profiles for atomic crystals, and inset in f is the enlargement of PL of tetragonal atomic crystals. Note PL spectra presented here were taken from center region of MoS2 samples.

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Figure 3 SEM characterizations obtained from two parts of MoS2 samples. (a-c), Part A on substrate; (d-f), Part B on substrate. The ratios of of MnO2 to MoO3 are 0: 1 for a and d, 0.5:1 for b and e, and 1:1 for c and f. (g) Schematic for the growth process.

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Figure 4 XPS analysis of the as-grown MoS2 atomic crystals. a-c are obtained from part A and d-f from part B on SiO2/Si substrates.

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Figure 5 a,b) Position-dependent PL spectra from the center towards the edge Positions are indicated by color points in atomic crystals in insets, center (red), middle (green), and edge (blue). c,d) PL mapping of as-grown MoS2 triangular (c) and tetragonal (d) atomic crystals according to integrated characteristic photon emission intensities from wavelength 630 to 730 nm.

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Figure 6 a,d) HRTEM bright field images of the center and edge region in the triangular MoS2 atomic crystals, respectively. Insets in a and d are FFT patterns of the selected regions in HRTEM images. b,e) enlarged images from a selected rectangular regions of a,d). c,f) Bright field intensity profile along the lines selected in b,e), respectively.

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Figure 7 The Optimized atomic structures and density of states (DOS) of monolayer (a) and bilayer (b) MoS2 atomic crystals at basal plane and edges. The left at the panels of (a) and (b) is atomic model and the right DOS at the basal plane and edges for monolayer and bilayer MoS2 atomic crystals, respectively.

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"For Table of Contents Use Only"

Triangular and tetragonal MoS2 atomic crystals are prepared simultaneously on one single growth substrate for the first time in a single CVD process. The photoluminescence (PL) of the MoS2 atomic crystals can be efficiently modulated by both morphology and atomic structures and position-dependent PL with a gradient decrease in PL intensity from the center towards edge region are found.

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Facile Preparation of Single MoS2 Atomic Crystals with Highly

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