Synthesis of Large-Scale Single-Crystalline

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Feb 11, 2018 - properties were evaluated by Raman and photoluminescence (PL) spectra. ... CVD has been successfully used for the synthesis of MoS2 film at ...
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Synthesis of Large-Scale Single-Crystalline Monolayer WS2 Using a Semi-Sealed Method Feifei Lan 1,2 ID , Ruixia Yang 1, *, Yongkuan Xu 2 , Shengya Qian 1 , Song Zhang 2 , Hongjuan Cheng 2 and Ying Zhang 2 1 2

*

Institute of electronic information and engineering, Hebei University of Technology, Tianjin 300401, China; [email protected] (F.L.); [email protected] (S.Q.) China Electronics Technology Group Corp 46th Research Institute, Tianjin 300220, China; [email protected] (Y.X.); [email protected] (S.Z.); [email protected] (H.C.); [email protected] (Y.Z.) Correspondence: [email protected]; Tel.: +86-159-2221-7509

Received: 17 December 2017; Accepted: 8 February 2018; Published: 11 February 2018

Abstract: As a two-dimensional semiconductor, WS2 has attracted great attention due to its rich physical properties and potential applications. However, it is still difficult to synthesize monolayer single-crystalline WS2 at larger scale. Here, we report the growth of large-scale triangular single-crystalline WS2 with a semi-sealed installation by chemical vapor deposition (CVD). Through this method, triangular single-crystalline WS2 with an average length of more than 300 µm was obtained. The largest one was about 405 µm in length. WS2 triangles with different sizes and thicknesses were analyzed by optical microscope and atomic force microscope (AFM). Their optical properties were evaluated by Raman and photoluminescence (PL) spectra. This report paves the way to fabricating large-scale single-crystalline monolayer WS2 , which is useful for the growth of high-quality WS2 and its potential applications in the future. Keywords: semi-sealed; CVD; WS2 ; Raman; AFM

1. Introduction Two-dimensional material such as transition mental dichalcogenides (TMDCs) and black phosphorus have attracted great interest for their unique physical properties [1–11], especially for TMDCs. In contrast to zero-bandgap graphene [12], TMDCs are two-dimensional semiconductor with available bandgap when in bulk [1,8]. With the reduction of the thickness, the bandgap transforms from an indirect to a direct one. Meanwhile, the bandgap can be adjusted through the sythesis of the alloy with different stoichiometry based on TMDCs. This intrinsic adjustable bandgap and flexibility allows them to be used in optoelectronic and nanoelectronic devices. There have been plentiful and inventive works focused on the synthesis methods [13–15], and their optical [16,17], electronic [18–20], and catalysis properties [21,22]. Among the TMDCs, MoS2 and WS2 are two perfect examples. Now, a great deal of research is focused on the study of MoS2 . In fact, WS2 is a more promising transition mental dichalcogenides for electronics [23], because of its superior mobility and its chemical robustness [24]. However, compared to MoS2 , the research of WS2 is a long way away from being enough, especially with regard to the synthesis of large-scale single-crystalline WS2 with monolayer. Mechanical exfoliation has been extensively used to obtain an atomically thin WS2 film for the research of its related properties, but the size of the film obtained by this method is too small to study the devices based on two-dimensional WS2 . Most recently, CVD has been successfully used for the synthesis of MoS2 film at large scale [25–29]. For this reason, CVD has also been considered an efficient method for the growth of WS2 . The growth process consists of the sulfidation of WO3 powders through S vapor. Although monolayer WS2 Nanomaterials 2018, 8, 100; doi:10.3390/nano8020100

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hasgrowth been synthesized a size of hundreds microns by CVD [30–32], the uniformity and the of WS2. The with growth process consists ofofthe sulfidation of WO 3 powders through S vapor. repeatability is really poor. The main reasons for these results are the high melting point of WO3 Although monolayer WS2 has been synthesized with a size of hundreds of microns by CVD [30–32], powders, and theand factthe thatrepeatability the growth is process is veryThe sensitive to the sulfidation rate.are In this paper, the uniformity really poor. main reasons for these results the high point of WOof 3 powders, and thetriangular fact that WS the 2growth process is very sensitive to the wemelting report the synthesis single-crystalline film with large size through a semi-sealed sulfidation In this paper, we report single-crystalline WSWO 2 film with a CVD method.rate. A semi-sealed quartz boat the wassynthesis used to of enhance the partialtriangular pressure of 3 . With large size through a semi-sealed CVD method. A semi-sealed quartz boat was used to enhance the higher partial pressure of WO3 , the WS2 monolayer film with the grain size of more than 400 µm partial pressure WO3. the With a higher of WO3WS , the WS 2 related monolayer film with the was obtained. Thisofpaves way to the partial growthpressure of monolayer and TMDCs with large 2 grain size of more than 400 µm was obtained. This paves the way to the growth of monolayer WS2 grain size. and related TMDCs with large grain size. 2. Growth Process 2. Growth Process Triangular WS2 monolayer films were grown by CVD in a horizontal furnace. High-purity Ar 2 monolayer films were grown by CVD in a horizontal furnace. High-purity Ar was theTriangular carrier gasWS with a flow rate of 100 sccm, 3 mg WO3 powders were placed into a small quartz was the carrier gas with a flowSrate of 100 sccm, WO3 powders were placed into a small quartz boat as W source, high-purity powders were 3Smg source, and Al2 O 3 was chosen as the substrate, boat as W source, high-purity S powders were S source, and Al2O3 was chosen as the substrate, face-down above the WO3 powders. Compared to the lower growth temperature of MoS2 , the growth face-down above the WO3 powders. Compared to the lower growth temperature of MoS2, the temperature of WS2 was as high as 1050 ◦ C, with a pressure of 10 mbar. growth temperature of WS2 was as high as 1050 °C, with a pressure of 10 mbar. 3. Results and Discussion 3. Results and Discussion Similar to the growth of MoS2 , the synthesis of WS2 is very sensitive to the sulfidation rate; too fast Similar to the growth of MoS2, the synthesis of WS2 is very sensitive to the sulfidation rate; too or too slow are both detrimental to the growth of large-scale WS2 film. An effective way to solve this fast or too slow are both detrimental to the growth of large-scale WS2 film. An effective way to solve problem is to control the evaporation rate of the S source. In order to control the temperature and this problem is to control the evaporation rate of the S source. In order to control the temperature evaporation rate of the S source, S powders were placed into an independent stainless-steel cylinder and evaporation rate of the S source, S powders were placed into an independent stainless-steel outcylinder of the furnace with a heating and abelt thermocouple to control temperature. The integral out of the furnace with belt a heating and a thermocouple to the control the temperature. The structure is shown in Figure 1a. Through this system, we successfully obtained triangular monolayer integral structure is shown in Figure 1a. Through this system, we successfully obtained triangular WSmonolayer edge length thelength triangles was about 150 as 150 shown 2 film; the WS 2 film; theof edge of the triangles wasµm, about µm,in asFigure shown1b. in Figure 1b.

Figure 1. (a) Schematic diagram of CVD growth system; (b) optical image of WS2. Figure 1. (a) Schematic diagram of CVD growth system; (b) optical image of WS2 .

In the course of conducting this research, we found it difficult to obtain triangles with larger In the this research, we found difficult of to WO obtain triangles with size. size. Thecourse reason of forconducting this phenomenon is the lower vaporitpressure 3. As we know, thelarger melting Thepoint reason for this phenomenon the lower vapor pressure ofmakes WO3 . itAs we know, the melting point of of WO 3 is as high as 1300 is °C; such a high melting point difficult to enhance the partial ◦ C; such a high melting point makes it difficult to enhance the partial pressure WO as 31300 pressure of WO vapor. A lower pressure of WO3 vapor will result in a shortage of the W source on 3 is as high of WO lower pressure will result in a shortage sourcetoon the surface the surface substrate. So of weWO have to enhance the partial pressureof ofthe WOW 3 vapor enlarge the 3 vapor.ofAthe 3 vapor sizesubstrate. of WS2 film. mosttoefficient way enhance the partial pressure WO3 is to of the So The we have enhance theto partial pressure of WO to enlarge thereduce size ofthe WS2 3 vapor of pressure of the furnace during the growth of WS2.pressure A low pressure thethe melting pointofofthe film. The most efficient way to enhance the partial of WO3can is tolower reduce pressure WO3 to increase partialofpressure WO 3. However, this the condition, the transport speed of the furnace during thethe growth WS2 . A of low pressure can in lower melting point of WO 3 to increase S vapor will also increase. This will increase the sulfidation rate. As we know, a high sulfidation the partial pressure of WO3 . However, in this condition, the transport speed of the S vapor will also rate is This adverse the migration and diffusion the atoms and molecules theissurface thethe increase. willfor increase the sulfidation rate. Asofwe know, a high sulfidationon rate adverseoffor substrate. This will make it difficult for the acquisition of single-crystal 2 film with large size. So migration and diffusion of the atoms and molecules on the surface of theWS substrate. This will make it we need to find an efficient way to increase the partial pressure of WO 3 and keep the transport of S difficult for the acquisition of single-crystal WS2 film with large size. So we need to find an efficient vapor under a low speed. way to increase the partial pressure of WO3 and keep the transport of S vapor under a low speed. In this paper, a semi-sealed quartz boat was used to enhance the partial pressure of WO3 vapor. In this paper, a semi-sealed quartz boat was used to enhance the partial pressure of WO3 vapor. The small quartz with WO3 powders and the substrate were put into a semi-sealed quartz boat, and The small quartz with WO3 powders and the substrate were put into a semi-sealed quartz boat, and the the substrate was placed downstream of the W source to reduce the nucleation centers at the substrate was placed downstream of the W source to reduce the nucleation centers at the beginning of beginning of the growth to enlarge the size of the single-crystalline triangles. The distance between the growth to enlarge the size of the single-crystalline triangles. The distance between the W source

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and the was 3–5 cm,REVIEW as shown Figure 2a. During growth, WO3 vapor was limited in3such Nanomaterials 2018, 8, xthe FOR PEER of 7 a the W substrate source and substrate was 3–5incm, as shown in Figure 2a. During growth, WO 3 vapor was semi-sealed quartz boat. The partial pressure of the WO vapor can be greatly enhanced relative to limited in such a semi-sealed quartz boat. The partial3 pressure of the WO3 vapor can be greatlythe the W source and thefurnace. substrate was 3–5 cm, shown in Figure 2a. During 3 vapor was pressure of the whole Meanwhile theaspressure the furnace canpressure begrowth, keptof atWO a higher pressure enhanced relative to the pressure of the whole furnace.ofMeanwhile the the furnace can limited in such a semi-sealed quartz boat. The partial pressure of the WO 3 vapor can be greatly to be reduce the transport speed of the S vapor. With this method, the length of the largest triangular WS2 kept at a higher pressure to reduce the transport speed of the S vapor. With this method, the enhanced relative to the pressure of in the whole2b. furnace. Meanwhile the pressure of the furnace can increased to about 405 µm, as shown Figure length of the largest triangular WS2 increased to about 405 µm, as shown in Figure 2b. be kept at a higher pressure to reduce the transport speed of the S vapor. With this method, the length of the largest triangular WS2 increased to about 405 µm, as shown in Figure 2b.

Figure 2. (a) Semi-sealed equipment schematic diagram; (b) optical microscope of 405µm monolayer Figure 2. (a) Semi-sealed equipment schematic diagram; (b) optical microscope of 405µm monolayer WS2 film. WSFigure 2 film.2. (a) Semi-sealed equipment schematic diagram; (b) optical microscope of 405µm monolayer WS2 film. Figure 3 shows the optical microscopy images of triangular WS2 films. Most of the films are Figure 3 shows optical microscopy triangular WS of nucleation the films are monolayer, the size the of the triangles enlarged images to more of than 300 µm on each side, Most and the 2 films. Figure 3 shows the optical microscopy images of triangular WS2 films. Most of the films are monolayer, the size of the triangles enlarged more thanimages, 300 µm oncan each andorientation the nucleation density reduced obviously. Furthermore, fromtothe optical we seeside, that the of monolayer, the size of the triangles enlarged to more than 300 µm on each side, and the nucleation density reduced obviously. Furthermore, optical images, we can see thatepitaxial the orientation the triangles was not complete disorder. from Manythe of the triangles present a slightly growth of density reduced obviously. Furthermore, from the optical images, we can see that the orientation of mechanism. Thisnot maybe resultsdisorder. from theMany high growth temperature of 1050 °C andepitaxial an annealing the triangles was complete of the triangles present a slightly growth the triangles was not complete disorder. Many of the triangles present a◦ slightly epitaxial growth process of the sapphire before sulfidation. According to the research of the Kis group [33], annealing mechanism. This maybe results from the high growth temperature of 1050 C and an annealing process mechanism. This maybe results from the high growth temperature of 1050 °C and an annealing sapphirebefore is helpful for the growth of WS triangles with the Kis same orientation. This results ofof thethe sapphire sulfidation. According to 2the research of the group [33], annealing of the process of the sapphire before sulfidation. According to the research of the Kis group [33], annealing from theisenhanced Van der Waalsofforce. Additionally, the annealing of the sapphire is helpful for sapphire helpful for the growth WS triangles with the same orientation. This results from 2 of WS2 triangles with the same orientation. This resultsthe of the sapphire is helpful for the growth the reduction of nucleation density because of the clean surface of the substrate. These results enhanced der Waals Additionally, the annealing the sapphire is sapphire helpful for reduction from theVan enhanced Vanforce. der Waals force. Additionally, theofannealing of the is the helpful for a new method for the growth of surface continuous single-crystal WS2 monolayers. of provide nucleation density because of the clean of the substrate. These results provide a new method the reduction of nucleation density because of the clean surface of the substrate. These results forprovide the growth ofmethod continuous single-crystal WS2 monolayers. a new for the growth of continuous single-crystal WS2 monolayers.

Figure 3. Optical images of the triangles at different locations with different magnifications: (a–c) are the images with a magnification of 10×; (d–f) are the images with a magnification of 20×. Figure 3. Optical images of the triangles at different locations with different magnifications: (a–c) are Figure 3. Optical images of the triangles at different locations with different magnifications: (a–c) are the images with a magnification of 10×; are the images magnification of 20×. we found an; (d–f) interesting thing,with as ashown in Figure the grain theDuring imagesthe withresearch, a magnification of 10× (d–f) are the images with a magnification of 4a. 20×On .

boundary of two connecting triangles with great difference in torsion angles, the film is multilayer. During the research, we found an interesting thing, as shown in Figure 4a. On the grain However, the multilayer film is only concentrated on the grain boundary. This can also be seen During of thetwo research, we found an interesting as shown in Figure 4a.the Onfilm the is grain boundary boundary connecting triangles with great thing, difference in torsion angles, multilayer. through AFM, Figure 4b,c shows the 2D and 3D image of the film. Through the image, we can see the multilayer only difference concentrated on the grain boundary. can also be seen of However, two connecting trianglesfilm withisgreat in torsion angles, the film This is multilayer. However, that the film is multilayer on the grain boundary. Through the research, we find that this through AFM, Figure 4b,cconcentrated shows the 2Don and 3D image of the film. Through the image, we can see the multilayer film is only the grain boundary. This can also be seen through AFM, phenomenon can be observed only on the two connecting triangles with great difference in torsion that 4b,c the film is the multilayer on the grain boundary. Through the research, weseefind that this is Figure shows 2D and 3D image of the film. Through the image, we can that the film angles. For those triangles that do not connect with each other, or that connect with each other but phenomenon can be observed only on thethe two connecting triangles withphenomenon great difference torsion multilayer on the grain boundary. we find that this can in be observed with a small difference in torsionThrough angles, this research, growth does not appear. This maybe results from the angles. For those triangles that do not connect with each other, or that connect with each other but only on the two connecting triangles with great difference in torsion angles. For those triangles great difference of torsion angles between two connecting triangles. With a great difference inthat a small difference torsion angles, this growth doesother not appear. This maybe results from the dowith not connect with eachin other, or that connect with each but with a small difference in torsion great difference of torsion angles between two connecting triangles. With a great difference in

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appearance of the grain boundary will result in the disorder of the growth. This disorder growth will make a mismatching stitching between two different triangles, resulting in an overlapping growth angles, this growth does not appear. This maybe results from the great difference of torsion angles of two triangular grains with different orientations. However, we need some more tests to prove it. between two connecting triangles. With a great torsion angles, a grain boundary will Additionally, we found an interesting thingdifference during thein AFM testing. The film appears to have Nanomaterials 2018, 8, x FOR PEER REVIEW 4 of 7 appear on the interface oftotwo The appearance oftesting. the grain boundary will obviously fallen off due thedifferent scraping triangles. by the probe during the AFM After the falling offresult of in the thefilm, disorder ofclearly the growth. This disorder growth willin make a mismatching stitching between we can see the outline of the film, as shown Figure 4d. From the image, we can see torsion angles, a grain boundary will appear on the interface of two different triangles. The two different triangles, resulting in an overlapping growth of two triangular grains with different that the growth of the film begins at the centre of the triangles. The growth may be a symmetrical appearance of the grain boundary will result in the disorder of the growth. This disorder growth will orientations. However, weand need more to prove it. growth along the centre thesome diagonal oftests the triangles. make a mismatching stitching between two different triangles, resulting in an overlapping growth of two triangular grains with different orientations. However, we need some more tests to prove it. Additionally, we found an interesting thing during the AFM testing. The film appears to have obviously fallen off due to the scraping by the probe during the AFM testing. After the falling off of the film, we can clearly see the outline of the film, as shown in Figure 4d. From the image, we can see that the growth of the film begins at the centre of the triangles. The growth may be a symmetrical growth along the centre and the diagonal of the triangles. Figure 4. (a) optical image on the grain boundary; (b) 2D image of the grain boundary by atomic Figure 4. (a) optical image on the grain boundary; (b) 2D image of the grain boundary by atomic force force microscope (AFM); (c) 3D image of the grain boundary by AFM; (d) outline of the film by AFM. microscope (AFM); (c) 3D image of the grain boundary by AFM; (d) outline of the film by AFM.

Optical properties were charactered by Raman and PL spectra. Figure 5a presents the Raman Additionally, we with found an interesting thing during the AFM testing. The film appears have peak of the triangles different thicknesses. With the increasing of the number of layers, thetoVan obviously fallen off due to the scraping by the probe during the AFM testing. After the falling off der Waals force suppresses atom vibration, resulting in higher force constants, so the blueshift of A1g of thecorresponds film, we cantoclearly see the outline of the film, as shown inthe Figure 4d. From the image, wewhen can see the predicted stiffening [34,35]. However, E peak exhibits redshifts Figure 4. (a) optical image on the grain boundary; (b) triangles. 2D image ofThe the growth grain boundary bya atomic that the growth of the film begins at the centre of the may be symmetrical increasing the number of WS2 layers. This suggests that long-range Coulombic interlayer interaction force microscope (AFM); (c) 3D image of the grain boundary by AFM; (d) outline of the film by AFM. growth the centre the diagonal thestacking triangles. or thealong changing of the and structure based onofthe of different layers plays a major role [4,35]. Optical properties were charactered by Raman and PL spectra. Figure 5a the presents theinRaman The peak frequency and the ratio of / are summarized in Table 1. With increase the Optical properties were charactered by Raman and PL spectra. Figure 5a presents the Raman peak of the triangles with different thicknesses. With the increasing of the number of layers, the Van number of WS 2 layers, the different ratios of thicknesses. / decreased 4.5 to 0.8, an number obvious of change. can peak of the triangles with With thefrom increasing of the layers,This the Van derbe Waals force suppresses vibration, resulting higher force constants, soblueshift the blueshift der Waals force suppresses atom vibration, resulting inin higher force constants, so the of A1g of used as an effective way atom to identify the WS 2 films with different thicknesses. 1 A1gcorresponds corresponds to the predicted stiffening [34,35]. However, the E peak exhibits redshifts when to the predicted stiffening [34,35]. However, the E 2g peak exhibits redshifts when increasing the number of WS layers. This suggests that long-range Coulombic interlayer interaction 2 2 layers. This suggests that long-range Coulombic interlayer interaction increasing the number of WS or or thethe changing ofdifferent differentlayers layersplays plays a major role [4,35]. changingofofthe thestructure structurebased based on on the the stacking stacking of a major role [4,35]. The peak frequency inTable Table1.1.With Withthe theincrease increase The peak frequencyand andthe theratio ratioof of IE1 /I / A1g are are summarized summarized in in in thethe 2g

number of of WSWS the ratios number 2 layers, the ratiosofofIE1 /I/A1g decreased decreasedfrom from4.5 4.5toto0.8, 0.8,an anobvious obviouschange. change.This This can can be 2 layers, 2g used as anas effective way way to identify the WS films with different thicknesses. be used an effective to identify the 2WS 2 films with different thicknesses.

Figure 5. (a) Raman spectra with different WS2 thicknesses; (b) frequency change of A1g and E with different thicknesses; (c) / ratio with different layers. Table 1. Summary of the peak frequency for A1g and E

and the intensity ratio of the two peaks as a

function of thickness with the excitation wavelength 514 nm. λExc

Phonon Modes 1 Layer 2 Layer 3 Layer Multilayer −1) E (cm 356.17 354.23 352.67 352.75 Figure 5. (a) Raman spectra with different WS2 thicknesses; (b) frequency change of A 1g and1 E Figure 5. (a) Raman spectra with different WS2 thicknesses; (b) frequency change of A1g and E2g with −1) A 1g (cm 419.64 421.03 421.25 422.12 514different nm with thicknesses; (c) / ratio with different layers. different thicknesses; (c) IE1 /IA1g ratio with different layers. /2g 4.5 1.6 1.04 0.8 Table 1. Summary of the peak frequency for A1g and E and the intensity ratio of the two peaks as a Table 1. Summary of the peak frequency for A1g and E12g and the intensity ratio of the two peaks as a function6a of thickness with excitation wavelength 514 nm. as the Raman spectra. The PL spectra Figure shows the PLthe spectra at the same position function of thickness with the excitation wavelength 514 nm. display an indirect to direct bandgap from multilayer to monolayer. With the decreasing of λExc Phonon Modes 1 Layer 2 Layer 3 Layer Multilayer thickness, the intensity peaks increases dramatically. The PL intensity is extremely weak in −1) λExc Phonon Modes 1356.17 Layer 2 Layer 3 Layer Multilayer Eof PL (cm 354.23 352.67 352.75 −1 (cm−1 )) 419.64 421.03 421.25 422.12 514 nm EA12g1g(cm 356.17 354.23 352.67 352.75 / −1 ) 514 nm 4.5 1.6 1.04 0.8 419.64 421.03 421.25 422.12 A1g (cm IE1 /IA1g 2g

4.5

1.6

1.04

0.8

Figure 6a shows the PL spectra at the same position as the Raman spectra. The PL spectra display an indirect to direct bandgap from multilayer to monolayer. With the decreasing of thickness, the intensity of PL peaks increases dramatically. The PL intensity is extremely weak in

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at the same position as the Raman spectra. The PL spectra 5 of 7 display an indirect to direct bandgap from multilayer to monolayer. With the decreasing of thickness, multilayer, with an indirect bandgap The semiconductor. increase of PL the intensityconsistent of PL peaks increases dramatically. PL intensity Meanwhile, is extremely the weak in multilayer, intensity increase of direct interband transition withthe theincrease decreasing thickness. The consistentimplies with anthe indirect bandgap semiconductor. Meanwhile, of PLofintensity implies peak movedof todirect shorter wavelength with the of thickness, which indicates an increase in the increase interband transition withdecreasing the decreasing of thickness. The peak moved to shorter the bandgap, and reaches its maximum at the monolayer, which is about 2.0 eV. In order to wavelength with the decreasing of thickness, which indicates an increase in the bandgap, and reaches investigate theat differences of the photoluminecence between the edgethe and the other areas, its maximum the monolayer, which is about 2.0 properties eV. In order to investigate differences of the as well as the grain boundaries, we choose a typical position on the surface of the film to perform PL photoluminecence properties between the edge and the other areas, as well as the grain boundaries, line scanning. Figureposition 6b shows ofthe thefilm PL line scanning. to Figure PL we choose a typical on the the position surface of to perform PL According line scanning. Figure6c, 6bthe shows peak did notofchanges obviously at different areas. This 6c, result a high quality with a good the position the PL line scanning. According to Figure the indicates PL peak did not changes obviously at uniformity of the film. different areas. This result indicates a high quality with a good uniformity of the film.

Figure Figure 6. 6.(a) (a)Photoluminescence Photoluminescence (PL) (PL) spectra spectra of of WS WS22with withdifferent different thickness; thickness; (b) (b) optical optical image image of of WS WS22 and and the the red red line line is is position position for for PL PL line line scanning; scanning; (c) (c) PL PL line line scanning scanning image. image.

4. 4. Conclusions Conclusions In In conclusion, conclusion, we we grew grew monolayer monolayer single-crystalline single-crystalline WS WS22 triangles triangles with with large large size size using using aa semi-sealed CVD method. The largest triangle was about 405 µm in length. Many of the triangles semi-sealed CVD method. The largest triangle was about 405 µm in length. Many of the triangles present present aa slightly slightly epitaxial epitaxial growth growth mechanism. mechanism. Raman Ramanspectra spectrashow show that that most most of of the the triangles triangles are are monolayer. PL spectra spectra indicate indicatethe thegood gooduniformity uniformityand and high quality triangles. method monolayer. PL high quality of of thethe triangles. ThisThis method can can be used for the growth of large-scale single-crystalline 2 film. film. be used for the growth of large-scale single-crystalline WS WS 2

Acknowledgments: Acknowledgments: This This work work is is financially financially supported supported by by the the National National Natural Natural Science Science of of Foundation Foundation of of China China (Grant (Grant No. No. 61774054). 61774054). The Theauthors authors thank thank for for Raman Raman Characterization Characterization and and Photoluminescence Photoluminescence measurement measurement of of Electronic Material Material Research Research Institute Institute of of Tianjin, Tianjin, China. China. Electronic Author Contributions: Contributions: Feifei Feifei Lan Lan and and Ruixia Ruixia Yang Yang conceived conceived and and designed designed the the experiments; experiments; Shengya Shengya Qian Qian Author performed the experiments; Yongkuan Xu and Hongjuan Cheng analyzed the data; Song Zhang ang Ying Zhang performed experiments; Yongkuan Xu and Hongjuan Cheng analyzed the data; Song Zhang ang Ying contributedthe reagents/materials/analysis tools; Feifei Lan wrote the paper. Zhang contributed reagents/materials/analysis tools; Feifei Lan wrote the paper. Conflicts of Interest: The authors declare no conflicts of interest. Conflicts of Interest: The authors declare no conflicts of interest.

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