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Article

Mechanism of Alkali Metal Compounds-Promoted Growth of Monolayer MoS: Eutectic Intermediates 2

Peng Wang, Jiayu Lei, Jiafan Qu, Shengyong Cao, Hu Jiang, Mengci He, Hongyan Shi, Xiudong Sun, Bo Gao, and Wenjun Liu Chem. Mater., Just Accepted Manuscript • DOI: 10.1021/acs.chemmater.8b04022 • Publication Date (Web): 10 Jan 2019 Downloaded from http://pubs.acs.org on January 11, 2019

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Chemistry of Materials

Mechanism of Alkali Metal Compounds-Promoted Growth of Monolayer MoS2: Eutectic Intermediates

Peng Wang,1 Jiayu Lei,1 Jiafan Qu,1 Shengyong Cao, Hu Jiang,1 Mengci He, Hongyan Shi,1,2 Xiudong Sun,1,2 Bo Gao, 1,2,* and Wenjun Liu3

1

Institute of Modern Optics, Department of Physics, Key Laboratory of Micro-Nano

Optoelectronic Information System, Ministry of Industry and Information Technology, Key Laboratory of Micro-Optics and Photonic Technology of Heilongjiang Province, Harbin Institute of Technology, Harbin 150001, China 2

Collaborative Innovation Center of Extreme Optics, Shanxi University, Taiyuan

030006, China 3

Department of Physics, Harbin Institute of Technology at Weihai, Weihai 264209,

China

-1-

ACS Paragon Plus Environment

Chemistry of Materials 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ABSTRACT: Monolayer MoS2, processing flexibility and remarkable physical properties derived from its direct bandgap feature, has been endowed to be one of the potential materials for practical applications such as integrated circuits and logic devices. Recently, a facile CVD method using alkali metal compounds as promoters attracted a lot of attention. Here, we systematically investigated the mechanism of alkali metal compounds-promoted growth of monolayer MoS2 by CVD and proposed a eutectic intermediates model. In the presence of alkali metal compounds, large monolayer MoS2 was obtained, regardless of the anions. However, non-alkali metal compounds did not promote the growth of monolayer MoS2. We proposed that the formation of eutectic intermediates, containing alkali metal molybdates and molybdenum oxides, played a crucial role in promoting the growth of monolayer MoS2. It is because the low melting point of eutectic intermediates could facilitate their mobility, favoring less nuclei and lateral growth. The proposal of eutectic intermediates model could not only contribute to growing ultra-large monolayer MoS2 and other 2D materials, but also inspire new ideas about growing 2D materials based on low melting point and high mobility of eutectic intermediates.

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INTRODUCTION

Two-dimensional (2D) transition metal dichalcogenides (TMDs), expected to be substitute materials for next generation electronics and optoelectronics, have attracted extensive interest owing to its atomic-scale thickness, flexibility and remarkable physical properties.1-7 As an early member of the TMDs family, molybdenum disulfide (MoS2) possesses many desirable properties.8, 9 In particular, the existence of direct bandgap in monolayer MoS2 suggests a great potential for practical applications such as integrated circuits and logic devices.10-12 Thus, developing reliable and scalable synthesis methods for monolayer MoS2 is highly desired. Considerable efforts have been devoted to synthesize MoS2 monolayers, including mechanical exfoliation,13,

14

liquid phase exfoliation,15-17 physical vapor deposition,18 and

chemical vapor deposition (CVD) approaches.19-21 Therein, CVD was thought to be the most promising method for synthesizing large scale, high quality monolayer MoS2 for electronics and optoelectronics industries.

So far, various strategies have been proposed for the growth of MoS2 using CVD, such as annealing (NH4)2MoS4,22, 23 sulfurizing MoO3,19 Mo20 films and MoCl5,24 etc. among which sulfurization of molybdenum oxide is mostly used.19, 25, 26 Briefly, the sulfur vapor carried by an inert gas is introduced to react with vaporized molybdenum oxide at elevated temperature. However, the inhomogeneous diffusion and reaction of vaporized molybdenum oxide and sulfur might result in significant thickness variations of as-grown layers.27, 28 Meantime, dense small flakes tend to form for lack -3-



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of control over the nucleation density. To control the nucleation, many attempts, such as adjusting the carrier gas flow,26, 29, 30 growth temperature,31, 32 and the distance between precursor sources,33-38 have been made. Notably, a space-confined vapor-phase deposition method, forming micro-reactor between precursors and substrates for less nucleation, was reported.39,

40

Nevertheless, the delicate layout

would hamper large-scale production and industrial application. Recently, a promoter-assistant

approach,

loading

perylene-3,4,9,10-tetracarboxylic

acid

tetrapotassium salt (PTAS)19, 27, 41 on the substrate or mixing alkali meta salts42-46 with precursor, was developed and showed remarkable tolerance of growth parameters for high quality monolayer MoS2. Ling27 held that the presence of PTAS increased the surface adhesive force of MoS2, resulting in the layered growth. Furthermore, PTAS might offer a heterogeneous nucleation site for the formation of MoS2 nuclei, which needed less energy than homogeneous nucleation.41 In NaCl-assisted CVD growth, Song42 attributed the catalytic effect of NaCl on the synthesis of MoS2 to the formation of a Na2Sx chain, which reacted with MoCl5 precursor and decreased the energy necessary for the final product. Wang44 considered the growth mechanism of NaCl-assisted process as the formation of intermediate NaxMoOy, which provide nucleation sites on the substrate and enhance the solid-vapor reaction. Zhou46 believed that the formation of volatile MoO2Cl2 by fusion of MoO3 with NaCl played an important role. In this aspect, the gas-gas phase reactions among metal oxychlorides, sulfur vapor and hydrogen were thought to be much faster than the solid-gas phase reactions between metal oxides and sulfur vapor. Kim43 proposed that intermediate -4-


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sodium oxides were formed during CVD growth when using mixed solution containing ammonium heptamolybdate (AHM) as precursor and NaOH as promoter. The intermediate sodium oxides further reacted with the oxide substrate to form Na2SiO3 at high temperatures improving the wettability of Mo greatly. By synthesizing large uniform monolayer MoS2 on soda-lime glass substrate, Yang45 supposed that the adsorption of Na at the flake edges reduced the energy barriers to increase the growth rate. Quite recently, highly crystalline MoS2 nanoribbons were grown on NaCl substrate or on a pre-grown monolayer MoS2 film assisted with NaCl, which was attributed to the formation of molten Na-Mo-O droplets and the anisotropic crawling due to the different interfacial free energy between droplet/ribbon and droplet/substrate.47

In this work, we systematically investigated the mechanism of alkali metal compounds in promoting the CVD growth of monolayer MoS2 and proposed a new understanding into the role of alkali metal cations. We chose MoO3, which could convert into MoO3-x during evaporation,48, 49 as molybdenum precursor and various alkali metal salts or hydroxides as promoter. In the presence of alkali metal compounds, large monolayer MoS2 was obtained, regardless of the anions. However, non-alkali metal compounds did not promote the growth of monolayer MoS2. Raman spectroscopy, electron diffraction spectroscopy (EDS) and thermogravimetric analysis (TGA) measurements showed that eutectic intermediates, containing alkali metal molybdates and molybdenum oxides, were formed in the absence of sulfur during growth. These eutectic intermediates could transform to monolayer MoS2 in -5-



subsequent sulfurization. We proposed that the low melting point of eutectic intermediates facilitates their mobility, favoring less nuclei and lateral growth for large monolayer MoS2.

EXPERIMENTAL SECTION

Preparation of promoter-coated substrate. 0.1M promoter solution was prepared by dissolving a certain amount of salts in 20 ml deionized water. A droplet of 10 μL promoter solution was dropped on a hydrophilic silicon substrate with 300 nm thick oxide, followed by spin-coating at 500 rpm for 6 s and 2000 rpm for 30 s. Afterwards, the substrate was baked at 80℃ for 2 min.

CVD growth of MoS2 flakes. A firebrick crucible filled with 0.02 g sulfur powder was placed upstream in the quartz tube outside the tube furnace. 0.001 g of MoO3 powder was evenly spread at the bottom of an alumina crucible, which was loaded in the center of the furnace. Two substrates, one with promoter and the other without promoter, were placed face-down on the crucible containing MoO3 powder side by side. In a typical growth, the whole CVD system was purged with 300 sccm Ar for 4 min. Then, 60 sccm of Ar was introduced into the system as carrier gas during MoS2 growth. The furnace was heated from room temperature to 680 °C in 20 min, and kept for 12 min. One minute before reaching 680 °C, we pushed the firebrick crucible with a magnet into the tube furnace at specific location. Temperature programming process is shown in Figure S1 (See Supporting Information). After growth, the system was quickly cooled down to room temperature with 105 sccm of Ar flow. -6-


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Characterization of MoS2 flakes. We used AFM (NT-MDT NEXT) for thickness, optical microscope (OLYMPUS BX51) for morphology, micro-Raman spectroscope (NT-MDT NTEGRA Spectra) for components analysis and uniformity. The laser excitation wavelength for Raman measurement is 532 nm and the spectral resolution is 0.9 cm-1. High-resolution TEM (HRTEM, JEOL 2100) and selected area electron diffraction (SAED) were conducted to evaluate the crystallinity of the sample. SEM and EDS (Hitachi SU8010) was utilized to characterize the morphology and elements, respectively. X-ray photoelectron spectroscopy (XPS, ThermoFisher, ESCALAB 250Xi) analyses were employed to determine the chemical compositions and valence states of as-grown MoS2 associated with promoters.

RESULTS AND DISCUSSION

Figure 1a shows an illustration of the CVD setup for MoS2 growth in a horizontal tube furnace with 1-inch inner diameter. In order to avoid the uneven diffusion of the precursor in traditional point-to-face layout, we used a facile face-to-face configuration for precursor feeding. For comparison, we put the other bare substrate without alkali metal compounds side by side as control substrate.

Figure 1b and 1c shows the optical image of the MoS2 flakes with and without NaNO3 as promoter. It can be seen in Figure 1b that on the substrate with NaNO3 there are large triangular MoS2 flakes with edge length in tens of microns. AFM measurement shows the thickness of 0.65 nm, indicting monolayer (See Figure S2 in Supporting Information). However, on the substrate without NaNO3 shown in Figure 1c there are -7-



dense small triangular MoS2 flakes with edge length in couples of microns. It can be speculated that the existence of NaNO3 decreased the nucleation density of MoS2 and facilitated the lateral growth. The MoS2 flakes were also characterized by micro-Raman spectroscopy using an excitation laser with a wavelength of 532 nm 1 (See Figure S3 in Supporting Information). The peaks of E2g and A1g mode are

located at 382.33 cm-1 and 401.69 cm-1 for MoS2 monolayer, with a difference of 19.36 cm-1, which is another indication of monolayer. For small MoS2 flakes, the peaks of the two modes are located at 382.29 cm-1 and 402.26 cm-1 with a difference of 19.97 cm-1. To assess the uniformity and crystallinity of the as-grown MoS2, Raman mapping, PL imaging and HRTEM imaging were performed on a single MoS2 flake, respectively. Figure 1d shows the optical image, Raman intensity mapping 1 images of E2g and A1g peaks and PL images for a single MoS2 flake. The spatial

homogeneity of the Raman and PL images highlights a single crystal with good uniformity of the MoS2 flake. From the HRTEM image shown in Figure 1e, the defect-free crystal structure of the MoS2 monolayer indicates high crystallinity of the sample. The corresponding hexagonal symmetrical SAED pattern (inset of Figure 1e) also confirmed that the MoS2 sample is indeed a single crystal.

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Heating zone

(a) Gas out

Substrate MoO3 S

(b)

Ar

(c)

50 μm

(d)

A1g

E12g

PL

50 μm

(e)

0.5 nm

Figure 1. (a) A schematic illustration of CVD setup for MoS2 growth. Typical optical images of MoS2 flakes grown (b) with and (c) without NaNO3 as promoter. (d) Optical image of a single MoS2 flake grown with NaNO3, and the corresponding Raman intensity mapping and PL images. (e) HRTEM image representing the defect-free hexagonal structure of the monolayer. Inset: the corresponding SAED image confirming the hexagonal structure.

In the growth using PTAS and other aromatic molecules as promoters, PTAS containing alkali metal cation and organic anion worked best.50 It is proposed that the aromatic molecules had two roles: decreasing the surface energy for lateral growth -9-



and offering low-energy sites for less nucleation.27, 41 In the growth using alkali metal compounds as promoters, some proposed that the alkali metal cations played a role by forming Na2Sx42 or Na2SiO3,43 or molten Na-Mo-O droplets,47 while others thought that chloride ion fused with MoO3 forming MoO2Cl2.46 Those intermediate products decreased the energy barrier for lateral growth. In order to explore which components of the promoters facilitated the growth of monolayer MoS2 in previous and present work, we used more common Na-, K- and Li-based alkali metal salts and hydroxides as promoters to do the growth, including NaOH, NaCl, Na2SO4, CH3COONa, Na3C6H5O7, EDTA-4Na, KOH, KH2PO4, KBr, KI, KCl, LiCl. Figure 2 shows the optical images of the grown MoS2 flakes with the above promoters. Similar to NaNO3, triangular monolayer MoS2 were obtained with all the promoters (See Figure S4 and Table S1 in Supporting Information for Raman data), indicating it is alkali metal cations not anion components that played a decisive role in facilitating the monolayer MoS2 growth. Images with lower magnifications of the as-grown MoS2 with alkali metal compounds indicate that MoS2 flakes uniformly distributed in a large area on the substrates (see Figure S5 in Supporting Information), while small MoS2 flakes preferred to grow on control substrates without alkali metal compounds. We also used soluble non-alkali metal salts as promoters to do the growth, including AgF, CuCl2, MgCl2, ZnCl2, CaCl2, BaCl2, PbI2 and AlCl3. However, inhomogeneous and irregular MoS2 were obtained (See Figure S6 in Supporting Information), which indicates that non-alkali metal cations could not facilitate monolayer MoS2 growth. Furthermore, it can be inferred that the anion components have no effects (either good or bad) in -10-


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facilitating the growth.

NaOH

Na2SO4

NaCl

EDTA-4Na

CH3COONa

C6H5Na3O7

KH2PO4

KI

KCl

KBr

KOH

LiCl

Figure 2. Optical images of as-grown MoS2 using various common alkali metal salts and hydroxides as promoters. Large triangular monolayer MoS2 were obtained with all promoters. Scale bars: 50 μm

In order to figure out what role the alkali metal cations played in the growth, we carried out a two-step growth experiment on NaNO3-coated SiO2/Si substrate. In the first step, the growth was done in the absence of sulfur. Figure 3a shows the optical image of the SiO2/Si surface. It can be seen that many particles with diameters ranging from 1 to 3 μm were randomly distributed with average spacing around 10 μm. The SEM image in the inset of Figure 3a shows that the particle has a circular shape, indicating a condensation process. EDS and Raman measurements on the condensed particles were performed to determine the chemical compositions. As shown in Figure 3c, EDS peaks corresponding to elements Na and Mo were observed in the particles. Furthermore, the EDS images of Na, Mo and N in Figure 3d-3f -11-



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demonstrate that elements Na and Mo existed only in the particles, while there is no element N on the surface. Figure 3b shows there are multiple Raman peaks located at 157 cm-1, 196 cm-1, 216 cm-1, 282 cm-1, 662 cm-1 corresponding Raman modes from MoO3, 363 cm-1 for MoO2,51 244 cm-1, 290 cm-1, 956 cm-1 and 816 cm-1 from Na2MoO452 and Na2Mo2O7,53 respectively. From the EDS and Raman results, it can be known that MoO3, with the help of NaNO3, formed multiple intermediates, which further aggregated into eutectic particles. In the second step, the substrate with randomly distributed eutectic particles was loaded back to the CVD setup for subsequent sulfurization without additional molybdenum source. Figure 3g and 3h shows the optical images of the same area before and after sulfurization, respectively. It can be seen that the randomly distributed eutectic particles were transformed into triangular monolayer MoS2 flakes. Using the impurities marked by red circles as references, it is apparent that the location of the formed MoS2 flakes does not match the randomly distributed eutectic particles, and the MoS2 flakes are much fewer than the eutectic particles. We can infer that the condensed eutectic particles remelted at high

temperature

in

the

sulfurization

process,

and

some

intermediate

molecules/clusters gradually left the melted eutectic particle and migrated on SiO2/Si surface. These molecules/clusters were sulfurized as nuclei, and laterally merged to form small MoS2 flakes. Ultimately, large MoS2 flakes were formed by continuous attachment of more sulfurized molecules/clusters.

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(a) 1 μm

MoO3 MoO2 ● SiO2 ★ Na2 MoO4 ☆ Na2 Mo2 O 7 ■

●

(b) Intensity (a.u.)

◆

★

■ ☆★ ■ ■ ■☆ ◆

(c)

☆ ★

■

10 μm

(d)

400

600

800

Raman shift (cm-1)

1000

(e)

(f)

Na ka 1_2

Mo La 1

(g)

Energy (keV)

(h)

N ka 1_2

(i)

100

80% MoO3 20% NaNO3

95 90

0.8 0.6 0.4

85 0.2 80

50 μm

50 μm

0.0 75

0

200

400

600

800

Deriv. Weight (%/°C)

200

Weight (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


1000

Temperature (°C)

Figure 3. Eutectic particles synthesized on SiO2/Si substrate in the absence of sulfur and subsequent sulfurization. (a) Typical optical images of Eutectic particles formed by fusion of MoO3-x and Na2MoO4 (Na2Mo2O7). Inset: SEM image of one eutectic particle. (b) Raman and (c) EDS spectra of eutectic particles. (d-f) are EDS images of Na, Mo and N elements corresponding to the red rectangular area of the inset in (a). Optical images of the same area (g) before and (h) after sulfurization. (i) Thermogravimetric-derivate thermogravimetric (TG-DTG) curves of MoO3-NaNO3 mixture (80%: 20%) in Ar with a temperature ramping rate of 10℃/min.

To explore the eutectic intermediates at high temperature, we conducted TGA (TA Instruments, SDT Q600) measurement of MoO3-NaNO3 mixture and pure MoO3, -13-



respectively. As shown in Figure 3i, the TGA result of MoO3-NaNO3 mixture (80%: 20%) displayed a prominent weight loss at ~400°C due to the decomposable feature of NaNO3. The decomposed products would form eutectic with molybdenum oxides at ~500°C. For pure MoO3, the weight loss occurred at ~800°C. (See Figure S7 in Supporting Information) These results indicate that NaNO3 chemically reacts with MoO3 to form eutectic intermediates at temperatures well below the melting point of pure MoO3.

The two-step growth experiment using KCl as promoter was also performed (See Figure S8 in Supporting Information). After the first step, same as NaNO3, randomly distributed particles composed of K2MoO4 and MoO3-x could be seen. Besides, no chlorine peak was observed from EDS, suggesting the high volatile metal oxychlorides (MoOxCly)46, 54 is not an intermediate product in the condensed particles. After sulfurization in the second step, uniform monolayer MoS2 flakes were obtained after sulfurizing the condensed particles in the CVD setup. However, when we used ZnCl2 and Cu(NO3)2 as promoter to do the two-step growth experiment (See Figure S9 in Supporting Information), irregular islands composed of small particles were observed after the first step, and no Raman peaks of molybdates were detected. After sulfurization in the second step, no MoS2 flakes were obtained. Notably, the optical images after sulfurization match pretty well with those before sulfurization, indicating no migration in the sulfurization process, which probably accounts for the failing growth of monolayer MoS2.

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Eutectic intermediates model

Based on the above results and discussions, we proposed eutectic intermediates model for promoting monolayer MoS2 growth by alkali metal compounds, which is schematically depicted in Figure 4.

Figure 4. Eutectic intermediates model for promoting monolayer MoS2 growth by alkali metal compounds. (a) Promoter uniformly distributed on the substrate. (b) Two routes arise from elevating the furnace temperature. Route 1: thermally stable alkali metal compounds directly react with vaporized MoO3-x; Route 2: thermally unstable alkali metal compounds would decompose for subsequent reaction (c) Eutectic intermediates formed by MoO3-x and MxMoOy. (d) Sulfurization of eutectic intermediate for triangular monolayer MoS2

For thermally stable alkali metal compounds, such as NaCl, KCl, KI, KBr, etc., they would directly react with vaporized MoO3-x forming alkali metal molybdates43, 44, 46 based on Eq. 1, which fuse with excess vaporized MoO3-x forming eutectic intermediates (Route 1 in Figure 4).

Ma X + MoO3-x → M2 MoO4 + M2 Mo2O7 + byproduct  -15-


(1)


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Where M refers to alkali metal elements (Na, K and Li), while X indicates the anions. The gaseous byproducts could be MoOxCly,46 H2O,43 etc., depending on the anions. For thermally unstable alkali metal compounds at high temperature, such as NaNO3, EDTA-4Na, CH3COONa, C6H5Na3O7, etc., they would decompose when elevating the furnace temperature (Route 2 in Figure 4). Subsequently, the decomposed products fuse with excess vaporized MoO3-x, forming eutectic intermediates containing alkali metal molybdates and MoO3-x in similar way to Route 1. Due to the low melting point (See Figure Table S2 in Supporting Information), eutectic intermediates are at melting state at growth temperature, and have high mobility on substrate. So they tend to collide with each other and joint together, accompanying by reaction with sulfur vapor based on Eqs. 2 and 3.43, 44

2M 2 MoO4 + 7S → 2MoS2 + 2M 2O + 3SO2

(2)

M 2 MoO4 + 5S → MoS2 + M 2S + 2SO2

(3)

Both reactions have extremely large negative Gibbs free energy (ΔG), meaning thermodynamically favorable (See Figure Table S2 in Supporting Information). The collision and jointing of eutectic intermediates lead to less nuclei, which accounts for the mismatch of eutectic particles and MoS2 flakes (shown in Figure 3g and 3h). Besides, due to high mobility, eutectic intermediates are more likely to attach to edges of small MoS2 flakes, favoring lateral growth for large monolayer MoS2.

Given that the melting points of eutectic intermediates are around 500℃, we did the -16-


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growth using NaNO3 as promoter at 500℃ and 480℃ (See Figure S10 in Supporting Information), which were rarely thought to be a suitable growth temperature. However, we still got triangular MoS2 flakes, but dense, small and thick compared to those at 680℃. Meanwhile, the MoS2 flakes grown at 480℃ are denser, smaller and thicker than those grown at 500℃. These results also revealed that the low melting point of eutectic intermediates and resultant high mobility played a crucial role in alkali metal compounds-promoted growth of monolayer MoS2.

For non-alkali metal compounds, containing Zn, Cu, etc., the melting points of eutectic intermediates are higher than growth temperature, resulting in low mobility (See Figure Table S2 in Supporting Information). In such cases, there are more nuclei and hence dense flakes, along with smaller size and more layers. We also tried doing the growth at higher temperatures, such as 800℃, but a large amount of white fibrous silicon sulfide with pungent smell was formed due to the SiO2/Si substrate (See Figure S11 in Supporting Information).

In Eq. 1, anion-derived byproducts could be generated. In Eqs. 2-3, cation-derived residues could be left. To elucidate whether the byproducts and residues would affect the properties of the MoS2 monolayers, XPS was used to analyze the chemical bonding of the sample. Figure 5 displays the XPS data for MoS2 monolayers using NaNO3 (blue) and KCl (red) as promoters. As shown in Figure 5a, the Mo 3d shows two peaks at 229.3 and 232.5 eV, attributed to the doublet Mo 3d5/2 and Mo 3d3/2, respectively. The peaks at 226.0 eV corresponds to the S 2s. All these results are -17-



consistent with the reported values for MoS2 crystal.22, 33, 55 Note that there is no peaks at the Cl 2p region for MoS2 monolayers using KCl as promoter, indicating gaseous byproducts MoOxCly generated through Eq. 1 were exhausted with the carrier gas.46 Meantime, no peaks at the Na 1s and K 2p regions existed for MoS2 monolayers using NaNO3 and KCl as promoter, respectively, indicating the residues produced through Eqs. 2-3 are too few to be detectable. Therefore, we can know the chemical bonding in MoS2 monolayers was not affected when using alkali metal compounds to facilitate the growth.

Mo 3d5/2

(a)

(b)

S 2s

235

230

Na 1s

Intensity (a.u.)

Intensity (a.u.)

Mo 3d3/2

1076

225

Binding engery (eV)

(c)

1072

1068

1064

Binding energy (eV)

(d)

Cl 2p

Intensity (a.u.)

K 2p

Intensity (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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304

300

296

292

210

205

200

195

190

Binding engery (eV)

Binding engery (eV)

Figure 5. (a) XPS data of Mo 3d and S 2s regions for MoS2 monolayers using NaNO3 (blue) and KCl (red) as promoters. (b) XPS data of Na 1s region for MoS2 monolayers using NaNO3 as promoter. XPS data of (c) K 2p and (d) Cl 2p regions for MoS2 monolayers using KCl as promoter. -18-


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CONCLUSIONS

In this work, we systematically investigated the promoting mechanism of alkali metal compounds in growing monolayer MoS2 by CVD and proposed a new understanding into the role of alkali metal cations. In the presence of alkali metal compounds, large monolayer MoS2 was obtained, regardless of the anions. However, non-alkali metal compounds did not promote the growth of monolayer MoS2. Raman and EDS measurements showed that eutectic intermediates, containing alkali metal molybdates and molybdenum oxides, were formed in the absence of sulfur during growth. We believed that the low melting point of eutectic intermediates facilitates their mobility, favoring less nuclei and lateral growth for large monolayer MoS2. The proposal of eutectic intermediates model could not only contribute to growing ultra-large monolayer MoS2 and other 2D materials, but also inspire new ideas about growing 2D materials based on low melting point and hence high mobility of eutectic intermediates.

ASSOCIATED CONTENT

Supporting Information

The Supporting Information is available free of charge on the ACS Publications website at http://pubs.acs.org.

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Temperature programming process used for CVD growth, AFM image of monolayer MoS2, Raman spectra of as-grown MoS2 flakes when using alkali metal compounds as promoters and non-alkali metal compounds as promoters, low-magnification optical images of MoS2 flakes with and without alkali metal compounds, TGA data of pure MoO3, two-step growth using KCl, ZnCl2 and Cu(NO3)2 as promoters, low-temperature growth when using alkali metal compounds as promoters, more details of thermodynamic data for eutectic products and the Gibbs free energy for MoS2 growth, high-temperature growth when using non-alkali metal compounds as promoters.

AUTHOR INFORMATION

Corresponding Author *E-mail: [email protected]

Author Contributions B. G. initiated the idea. P. W., J. L., J. Q. and S. C. did the growth experiments and Raman measurements. H. J. did the AFM measurements. M. H. did the SEM and EDS measurements. All authors analyzed and interpreted data. P. W. and B. G. wrote the manuscript. B. G. supervised the project.

Notes The authors declare no competing financial interest.

ACKNOWLEDGMENT -20-


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This work was financially supported by the National Natural Science Foundation of China (Nos. 21473046 and 21203046).

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TOC graph Eutectic

Monolayer MoS2

intermediates

50 μm

50 μm

MoS2

Eutectic intermediates

S

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Eutectic Intermediates - ACS Publications - American Chemical Society

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