Water-Assisted Synthesis of Molybdenum Disulfide Film with Single Organic Liquid Precursor

We report on the synthesis of large-area molybdenum disulfide (MoS2) film on an insulating substrate by means of chemical vapor deposition. A single mixture of molybdenum hexacarbonyl (Mo(CO)6) and dimethyl disulfide (C2H6S2) was utilized as an organic liquid precursor for the synthesis of MoS2 film. Carbon impurities stemming from the dissociation of the organic precursor are effectively removed by water oxidation, and hydrogen gas, which is a by-product of the oxidation of carbon impurities, inhibits the formation of molybdenum oxides. The use of a liquid precursor assisted with water oxidation ensures high reproducibility and full-coverage of MoS2 film for large area, which is not typically achieved with solid precursors such as molybdenum oxide and sulfur powder. We believe that our approach will advance the synthesis of transition metal dichalcogenides.

coverage uniformities of MoS 2 film cannot be ensured. Unlike solid-phase precursors, a gas phase precursor has an advantage in that it offers controllability 28 . Hydrogen sulfide can be utilized as sulfur precursor, but it is very toxic, and special care is therefore necessary 28 . Unfortunately, to the best of our knowledge, a gas phase precursor for molybdenum has not yet been reported. On the other hand, a liquid phase precursor is also an alternative. Metal organic chemical vapor deposition (MOCVD) with a bubbler system is widely used to grow III-nitride materials such as GaN and AlN using liquid precursors 29,30 . Recently, a combination of diethyl sulfide ((C 2 H 5 ) 2 S, liquid phase) and molybdenum hexacarbonyl (Mo(CO) 6 , solid phase) was used to grow a monolayer MoS 2 film 31 . However, the growth time for the complete monolayer MoS 2 film took around a day, so it is still necessary to investigate new types of precursors.
Herein, we report on the use of a single organic liquid precursor in the synthesis of large-area MoS 2 film. The single liquid precursor is prepared by the dissolution of molybdenum hexacarbonyl in dimethyl disulfide ((CH 3 ) 2 S 2 ). The coverage of MoS 2 film is controlled by adjusting the growth time, resulting in the formation of full-coverage MoS 2 films within 15 minutes. Carbon impurities stemming from the dissociation of organic precursors are effectively removed by water oxidation, as confirmed via Raman spectroscopy and photoluminescence (PL) measurements. Furthermore, the detailed growth mechanism is discussed.

Results and Discussion
The bubbler system was equipped in CVD as shown in Fig. 1(a), to synthesize the MoS 2 film. 0.04 M of Mo(CO) 6 powder was dissolved in (CH 3 ) 2 S 2 , and the precursor solution was further analyzed via liquid chromatography-mass spectrometry (see Figure S1 in Supplementary Information). The presence of Mo and S chemicals was confirmed as Mo ions, C 2 H 8 OS, Mo(CO) 2 , Mo(CO) 5 , C 5 HMoO 6 , and C 6 HMoO 7 . Unfortunately, dimethyl disulfide was not detected due to the detection limit, but we assume that dimethyl disulfide should also be present in the precursor solution. To remove the carbon impurities, a separate water bubbler is installed. Argon is used as a carrier, and the adhesion between the precursor and the SiO 2 /Si substrate is increased by coating perylene-3,4,9,10-tetracarboxylic acid tetrapotassium salt (PTAS) on the SiO 2 /Si substrate 20 . Figure 1(b) shows photographs of bare SiO 2 /Si, as-grown MoS 2 film, and transferred MoS 2 film on SiO 2 /Si substrate. The color of SiO 2 /Si substrate changed violet to blue-green after growth. Furthermore, the MoS 2 film was transferred on the SiO 2 /Si substrate using the conventional poly(methyl methacrylate) (PMMA) method, and it exhibit similar color as that of as-grown MoS 2 film, indicating that the MoS 2 film was well transferred on the target substrate. Figure 1(c) shows AFM image of the transferred MoS 2 film. The characteristic wrinkles of MoS 2 are clearly visible. The inset in Fig. 1(c) shows the height profile along the white-dotted line in Fig. 1(c), and the thickness of MoS 2 film is of around 0.85 nm, which is similar to the thickness of monolayer (ML) MoS 2 (0.615 nm) 32 . It is worth noting that WS 2 was successfully synthesized using another single liquid precursor that had been prepared by the dissolving of W(CO) 6 in (CH 3 ) 2 S 2 , instead of Mo(CO) 6 (see Figure S2 in Supplementary Information). A time evolution experiment was carried out to understand the growth behavior. Figure 1 It is expected that carbon impurities will be present on the growth substrate since the precursor contains carbon. Therefore, the presence of carbon was confirmed via Raman spectroscopy. Figure 2(a,b) show the Raman spectra of MoS 2 for different growth conditions: without water, with 1 sccm water, and with 10 sccm water. The peaks of E 1 2g (382 cm −1 ) and A 1g (404 cm −1 ) originated from in-plane and out-of-plane phonon vibrations, respectively, are clearly seen for the sample grown without water supply ( Fig. 2(a)). As expected, the peaks of the defect-related D-band near 1330 cm −1 and the graphite related G-band near 1580 cm −1 are detected in the Raman spectra, indicating that carbon impurities exist. We further confirmed the presence of amorphous carbon (a-C) on whole regions of the growth substrate via Raman mapping technique (see Figure S3 in Supplementary Information). Previous work showed that the a-C can be effectively eliminated with water, realizing the super growth of carbon nanotube forest 34 . Therefore, the water was introduced in this work as a weak oxidizer. The carbon atoms are eliminated by the following chemical reaction 34 .
It is reported that the Gibbs free energy of Reaction 1 is changes from plus to minus at ~670 °C. This indicates that for Reaction 1 to be spontaneous, a minimum temperature of ~670 °C is required 35 . At 650 °C growth, the carbon impurities were not effectively removed (see Figure S4 in Supplementary Information). Reaction 1 shows that carbon monoxide and hydrogen molecule are evolved. With 1 sccm of water, the peaks of the D-band and G-band slightly decreased ( Fig. 2(b)) whereas the peak intensities for E 1 2g and A 1g increased ( Fig. 2(a)). Eventually, the a-C is completely removed when 10 sccm water supply is used. We further confirmed the entire removal of a-C via Raman mapping (see Figure S5 in Supplementary Information). Typically, ML MoS 2 exhibits a strong PL intensity due to the direct band transition nature 7 . Figure 2(c) shows the PL spectra of MoS 2 with/ without the water supply. While the PL intensity is very weak without a water supply, it gradually increases as a function of the flow rate of water. To clarify the change in the PL intensity, each PL spectrum was fitted with a Lorentzian curve, as shown in Fig. 2(d-f). In the absence of water, the intensity of the neutral exciton (A°) peak near 1.895 eV is very weak, whereas multiexciton (A − ) near 1.852 eV and neutral exciton (B) near 1.988 eV are observed to be stronger 36 . However, those peaks are not strong when compared to those of water supply. As the water supply increases, the intensity of the A° exciton peak becomes more intense compared to those of other peaks, indicating that the overall PL intensity has increased. The change of PL intensity, such as PL quenching without water supply and the increase of A° exciton intensity with water supply might be related to the presence of a-C and molybdenum oxide. In the presence of a-C, the excited electron might be transferred to the conduction band of the conductive a-C, leading to PL quenching 37 . On the other hand, in the absence of a-C, a strong PL intensity is clearly observed. The presence of molybdenum oxides will be discussed later on, but the increase of the neutral exciton A° with the water supply might also be attributed to the undoping effect on MoS 2 by the removal of the molybdenum oxide [38][39][40] . As a consequence, a-C is effectively removed via water oxidation. It is noted that the water supply promotes the generation of radicals by hydrolysis and hydrogenolysis reactions of precursors, expecting that small MoS 2 flakes are grown 31 . Even though the flux of radicals increases in the presence of water, the amount of radicals which participate in the growth of MoS 2 at high temperature decreases due to the higher desorption rate 41 . Therefore, a fair quality of MoS 2 film within ~15 min is achieved.
Regarding the molybdenum oxide, we found unresolved particles from SEM and AFM images in the sample surface when water was not supplied, as shown in Fig. 3(a,b). Furthermore, it should be emphasized that full-coverage MoS 2 film was not achieved even with growth time exceeding 2 hours with the presence of several particles. To elucidate the chemical composition of those particles, the samples were further analyzed via X-ray photoelectron microscopy (XPS) and transmission electron microscopy (TEM). Figure 3(c,d) show the XPS spectra of Mo 3d and S 2p core levels with/without water supply. Typically, bulk MoS 2 shows three characteristic peaks for Mo 4+ 3d 3/2 (~232 eV), Mo 4+ 3d 5/2 (~229 eV), and S 2s (~226.13 eV) in the Mo 3d core level spectra and two peaks for S 2p 1/2 and S 2p 3/2 in the S 2p core level spectra 42 . Meanwhile the XPS spectra for the S 2p core level is similar regardless of the water supply, and the Mo 3d core level spectra changed due to the presence of water. When water was not supplied, four distinct peaks could be observed, and one peak near 235.2 eV disappeared gradually as the water supply increased (Fig. 3(c)). Gaussian curve fitting was used to assign the additional peaks in Mo 3d core level spectra to the Mo 6+ 3d 3/2 and Mo 6+ 3d 5/2 of molybdenum oxide 43 . It is not currently clear why molybdenum oxide formed, but those were eliminated when water was supplied. Hydrogen gases released in Reaction 1, reduces the molybdenum oxide as shown in Reaction 2, resulting in the generation of the molybdenum suboxide and water 44 . The generated suboxide participates in the formation of MoS 2 via Reaction 3 20 . The removal of molybdenum oxide was further confirmed via TEM analysis. Figure 4 shows TEM images of MoS 2 with/without water supply. Without water supply, molybdenum oxide particles are observed on the MoS 2 film (Fig. 4(a,b)). In Fig. 4(b), the inset shows the fast Fourier transformation (FFT) of the white-dashed box. The parallelogram-shaped dots are assigned to the (131), (−111), (004), and (034) planes of MoO 3 , according to a previous reports 45,46 . Figure 4(c) shows a high resolution TEM image of molybdenum oxide particles, and the parallelogram-shaped lattice structure is clearly identified. The value for d-spacing for the (−111) and (131) planes were obtained as 0.263 and 0.255 nm, which is in good agreement with the previous results obtained for molybdenum trioxide (MoO 3 ) 45, 46 . Therefore, the particle is deduced to be MoO 3 with an orthorhombic structure. With the water supply, such particles were not observed on the MoS 2 film, as shown in Fig. 4(d,e). The hexagonal-shaped FFT pattern in the inset of Fig. 4(e) is obtained from the white-dashed box in Fig. 4(e). The hexagonal dots are assigned to the (10-10) and (11)(12)(13)(14)(15)(16)(17)(18)(19)(20) planes of MoS 2 , according to a previous report 47 . The high-resolution TEM image in Fig. 4(f) shows the apparently hexagonal structure of MoS 2 . The d-spacings for the (10-10) and (11)(12)(13)(14)(15)(16)(17)(18)(19)(20) planes are 0.27 nm and 0.16 nm, respectively. Those values match well with previously reported values 20 , indicating that the MoS 2 film has been successfully synthesized. Figure 5 illustrates the growth mechanism for MoS 2 with/without water supply. In the absence of water, various kinds of molybdenum oxides and a-Cs are easily deposited on the growth substrate during growth, resulting in the formation of MoO 3 particles and a-Cs. These unwanted impurities limit the evolution of continuous the MoS 2 film (Fig. 5(a)). In contrast, the water supply during growth can lead to effective removal of a-C via Reaction 1. In addition, the hydrogen gases stemming from Reaction 1 plays an important role in removing the impurities and enhancing the lateral growth of the MoS 2 . Firstly, molybdenum oxide is reduced to molybdenum suboxide by the hydrogen, releasing water and carbon monoxide via Reaction 1. Secondly, the reduced molybdenum suboxides participate in the lateral growth of the MoS 2 film via Reaction 3, resulting in full coverage of the MoS 2 film without any other impurities (Fig. 5(b)). As a consequence, MoS 2 films can be synthesized with water oxidation of a-C and reduction of molybdenum oxide impurities.

Conclusions
We have synthesized MoS 2 films using a single liquid precursor made by dissolving Mo(CO) 6 in (CH 3 ) 2 S 2 . The liquid precursor consists of Mo ions, Mo compounds and sulfur compounds. Unlike with a solid precursor, full-coverage MoS 2 can be obtained simply by increasing the growth time. We found the introduction of water to be important in removing unwanted impurities, such a-C and molybdenum oxides from the MoS 2 film. Carbon was removed by water oxidation in Reaction 1: C (s) + H 2 O (g) = CO (g) + H 2 (g), and the hydrogen gas stemming from Reaction 1 helps effectively remove the molybdenum oxide. As a consequence, an impurity-free MoS 2 film was grown with the assistance of water. Our approach does not only open the use of organic liquid precursors in the synthesis of MoS 2 but also advances the synthesis of other s-TMdCs.

Methods
Substrate preparation. 20 mm × 20 mm Si substrate with a 300 nm thick SiO 2 layer was kept in Piranha solution to remove the organic residues and produce a hydrophilic surface. After rinsing and drying the substrate, 0.01 wt% PTAS solution was coated on the substrate as a seeding promoter via spin-coating. The substrate was placed on the center of a 5 cm × 5 cm quartz plate.
Preparation of the single liquid precursor. 0.5 g of molybdenum hexacarbonyl (Mo(CO) 6 , >99.9%, Sigma Aldrich) powder was dissolved in 50 mL of dimethyl disulfide (CH 3 SSCH 3 , >99%, Sigma Aldrich). The mixture was kept in a quartz bubbler. To prevent the agglomeration, the solution was stirred with a magnetic bar on a home-made stirring system. Growth of the MoS 2 film. A furnace and a 2-inch quartz tube were connected and equipped with high-purity (99.999%) argon, hydrogen gas, and two bubblers (liquid precursor and water). The growth substrate was loaded at the center of the quartz tube. Prior to growth, the system was purged using argon at 500 sccm for 10 minutes. The quartz tube was rapidly heated up to 750 °C for 8 min in a preheated furnace. During growth, the temperature was maintained with 5 sccm of precursor and 10 sccm of water flow for 15 minutes. After growth, the quartz tube was rapidly cooled down to room temperature by taking the quartz out of the furnace. 350 sccm of argon flow rate at atmospheric pressure was maintained throughout the entire growth process.
Characterization. The liquid precursor mixture was analyzed via liquid chromatography mass spectrometry (XEVO TQ-S, Waters). The surface morphologies of MoS 2 were characterized via optical microscopy (Nikon LV-IM, Nikon), scanning electron microscopy (JSM-7100F, JEOL), and atomic force microscopy (N8-NEOS, Bruker). To identify phonon vibration and photoluminescence of sample, a micro-Raman system (XperRam100, Nanobase) was used with a 532-nm laser. The laser power was kept at 0.1 mW to avoid damaging the sample. The chemical composition of MoS 2 was analyzed via X-ray photoelectron spectroscopy (K-alpha, Thermo fisher scientific), and the atomic structures of MoS 2 and the molybdenum oxide particles were characterized by transmission electron microscopy (Tecnai, FEI). The acceleration voltage was 200 kV during the TEM measurement. Prior to TEM measurement, the MoS 2 were transferred on a TEM grid using conventional PMMA transfer 48 .