Abstract
Molybdenum disulfide (MoS2) is an actively pursuing material of the 2D family due to its semiconducting characteristics, making it a potential candidate for nano and optoelectronics application. MoS2 growth from molybdenum and sulphur precursors by chemical vapor depositions (CVD) is used widely, but molybdates’ conversion into MoS2 via CVD is overlooked previously. Direct growth of MoS2 on the desired pattern not only reduces the interfacial defects but also reduces the complexities in device fabrication. In this work, we combine the wet synthesis and chemical vapor deposition method where sodium molybdate and L-cysteine are used to make a solution. With the dip coating, the mixture is coated on the substrates, and then, chemical vapor deposition is used to convert the chemicals into MoS2. Raman spectroscopy revealed the presence of oxysulphides (peaks number value) other than and , where heat treatment was performed in the presence of Ar gas flow only. On the other hand, the films reducing in the presence of sulphur and argon gas promote only and peaks of MoS2, which confirms complete transformation. XRD diffraction showed a very small change in the diffraction peaks and value of strain, whereas SEM imaging showed the flakes formation for MoS2 samples which were heated in the presence of sulphur. X-ray photoelectron spectroscopy is also performed for the chemical composition and to understand the valence state of Mo, S, and O and other species.
1. Introduction
Transition metal dichalcogenides (TMDs) garner attention due to their potential in practical applications [1–6]. The physical and electronic properties of these 2D layered materials gave them credit for an exceptional class of materials. Photonics [4, 5], energy storage [7], sensing [8], electronic and optoelectronic [9, 10], lubrication and catalysis [11–14], and many other devices of TMDs showed remarkable results. Different chemical compositions of TMDs exhibit a semiconducting, semimetal, and superconducting behavior. Van der Waals forces stack covalently bound MoS2 (S-Mo-S) in binary MX2-layered materials [15]. In bulk form, it has an indirect band gap of 1.2 eV, which increases as the number of layers decreases due to the quantum confinement effect, reaching direct 1.9 eV band gap for the MoS2 monolayer [16]. It is an indirect band gap semiconductor with a band gap energy of 1.2 eV in bulk form. Moreover, it exhibits mobility up to 200 cm2/Vs [9], melting temperature greater than 1000°C [17], and room temperature stability for a longer duration [18], making it an ideal material for efficient device applications.
Various top-down approaches such as mechanical (scotch tape) and chemical (sonication) exfoliations have been used to synthesize MoS2 [19–21]. Molecular defects, flake size, and control on layers number are not good in top-down approaches, which promote the low performance of electronic devices. To overcome these problems, various bottom-up approaches such as physical vapor deposition [22, 23], chemical vapor deposition [24], hydrothermal technique [25], thiosalts thermal decomposition [26], and solvothermalization [27] are used for the direct growth of MoS2 layers on the desired substrate. These bottom-up methods used a variety of molybdenum salts. A few of them contain sulphur, and few are without sulphur content and require extra sulphur content during growth, mostly called sulphurization. In the bottom-up approaches, chemical vapor deposition is widely used for MoS2 growth on inert substrates. Mostly, in CVD approach, molybdenum trioxide (MoO3) is reacted with sulphur compounds in the gas phases and resides on the desired substrate, but the real understanding of chemical phenomena and optimization to obtain high-quality MoS2 is complicated in it. In sulphuration, sulphur powder and H2S gas usually have been used as a sulphur source, and their appropriate amount snub the formation oxysulphides. In the present work, both sources of sulphur have been utilized. Sulphur powder is added in the furnace to react with the coated sodium molybdate and L-cystein solution, while H2S gas is formed as a byproduct of the reaction during heating which promotes formation of MoS2. Raman spectroscopy confirms the formation of monolayer MoS2 and oxysulphides peaks for the samples where sulphur source was not added in the chamber. XRD also showed extra peaks for the samples without sulphur content and a higher value of strain compared to the sample formed in the presence of sulphur.
2. Materials and Methods
In this approach, we prepared MoS2 via the solution of L-cysteine (C6H12N2O4S2) and sodium molybdate dihydrate (H4MoNa2O6) in DI water. Sodium molybdate is used as the precursor of molybdenum, while L-cystine is a source of sulphur in the growth of MoS2. 2 g of L-cystein, as shown in Figure 1, and 1 g of sodium molybdate was mixed into 50 ml DI water under continuous sonication for 30 minutes. After sonication, a homogenous mixture is obtained, as shown in Figure 1 (c).
The solution is dropped on the cleaned SiO2/Si substrates (3 drops on each substrate using a micrometer pipette). After drop coating (Figure 1 (e)), the substrates were dried on a hot plate at 100°C for 15 minutes (Figure 1 (f)). After drying, the samples are transferred to the CVD chamber, where the substrate was kept in the tube centre.
Argon (Ar) gas is used as a biproduct carrying gas maintained at a flow rate of 60 sccm. The samples are heated at 680°C for 1 hour in the presence-absence of 0.08 gm of sulphur (Figure 2). During the heating process, H2S gas is being released and react with the precursors. As a result, the MoS2 layer is formed on the substrates. The chemical reaction that occurs during the heating is
Sodium molybdate decomposes, H2S reacts with different species such as Mo by-products and converts it into MoS2 on substrate, and further reaction with sulphur powder placed at the edge of the furnace zone occurs to avoid oxysulphides. The furnace used is calibrated, and temperature profile was obtained. The temperature mentioned in the centre of furnace was 680°C, and at the start of heating zone where sulphur was kept, it was measured as 120°C. After 1 hour, the CVD furnace was stopped, and the samples cool down in the presence of Ar gas. The substrates after chemical deposition showed a colour zone present on the substrate, which indicates MoS2 growth, which is further confirmed by different characterization techniques.
Various characterization tools are employed to understand the complexities of the thin films. The Shimazdu X-ray diffractometer is used for structural information of MoS2 thin films.
Horiba, Jobin Yvon iHR 550 Raman spectroscopy setup is used to know the optical signatures, JEOL JSM scanning electron microscopy is used for surface morphology, and VG ESCALAB 220I-XL X-ray photoelectron spectroscopy is used to get information about chemical bonding of MoS2 films.
3. Results and Discussion
Raman spectra of MoS2 samples with and without a sulphur source are shown in Figure 3. MoS2 exhibits two feature modes and ; the characteristic peaks of MoS2 ( and ) look to be due to Mo and S atoms vibrating out-of-plane and in-plane, respectively. In our samples, the Raman spectra revealed a high strength of these two modes. Identification of the number of layers is possible by using the frequency difference () between the two peaks/modes. The thickness information of MoS2 layers is correlated to frequency changes between and [19]. As shown in Figure 3, the and peaks for MoS2 without sulphur were 383.5 and 408.7 cm−1, respectively. Moreover, the peaks for MoS2 with sulphur were 383.2 and 408.4 cm−1. A 25.2 cm−1 difference in frequency between the two peaks implies multilayer growth of MoS2. Although certain peaks indicating oxysulphides are present in the samples, and Raman modes have the same location and intensity ratios, indicating that the quality of both CVD materials is comparable.
In addition, several experiments are performed at different concentrations of sulphur, and the presence of oxysulphides is observed for lower concentration of sulphur such as 0.01 gm. Figure 4 shows the Raman spectrum of MoS2 films sulphurated in the presence of 0.01 gm of sulphur. The presence of oxysulphides showed that the sulphur content is not enough to complete the reaction which results in peaks arising due to stretching and bending of Mo bonds with oxygen atoms. The ambient parameters are very important in the growth of 2D materials. For the applications of MoS2, the presence of oxysulphides is not very encouraged, and sulphurisation is recommendable to remove sulphur-oxygen compounds arising due to shortage of sulphur.
CVD is a versatile technique for the growth of 2D materials [24, 28]. In Figure 5(a), the surface morphology of MoS2 grown sample in the presence of sulphur clearly showed flakes-like structures arranged on top of each other. These flakes are multilayers that are arranged randomly on the substrate. During the conversion of MoS2 layers, excessive sulphur content helps MoS2 to aggregate. Whereas, MoS2 grown without sulphur showed a rough sheet but not in the form of aggregated wide flakes (Figure 5(b)).
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XRD pattern of MoS2 matched with the Xpert high score database (JCPDS #00-037-1492) card. In Figure 6(a), MoS2 in the presence of sulphur showed an intense peak of (002) at 14.8o, whereas the intensity of MoS2 diffraction peak that appeared at (101) is less resembling the exact pattern of MoS2. For the XRD of MoS2 without sulphur content, the intensity of the (101) peaks enhance hints toward more defects due to oxygen or oxysulphide information. Multiple diffraction peaks are obtained in MoS2 samples, which showed polycrystalline nature.
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Lattice strain was computed using Williamson–Hall (W-H) analysis and X-ray peak broadening analysis, assuming peak widths as a function of 2θ [29], where θ is Bragg’s diffraction angle. The strain produced in powders as a result of crystal defects and deformations is estimated using lattice strain equation:where β is the full-width half maximum (FWHM) of diffraction peak (in radian).
Lattice strain for the characteristic peak of (002) of MoS2 with sulphur (Figure 6(a)) was calculated and found that it is displaced by lattice strain of 0.001, and for the other peak (101), it is displaced by 0.003. While (Figure 6(b)), XRD of MoS2 without sulphur, lattice strain for the characteristic peak of (002) is calculated and found that it is displaced by lattice strain of 0.005, and for the other peak (101), it is displaced by 0.002. The lattice strain is higher for MoS2 samples without sulphur content, which hints toward a higher defect state.
Because of the surface charge, there is a noticeable change in the C 1 s binding energy for MoS2-generated samples in the C 1 s spectrum. Using the adventitious carbon C 1 s singlet (284.6 eV) as a reference, the binding energies of the elements were calibrated, as shown in Figure 7(a). For simplicity, we are presenting only the XPS spectra for the MoS2 sample, which is grown in the presence of sulphur. The core peak spectra for Mo, S, and O were analyzed to see the interaction of these elements with each other. The core peaks of Mo and S both exhibited their doublets, with 3d3/2 and Mo 3d5/2 correlating to Mo 3d at 231.6 and 228.3 eV, respectively, and S2p peaks as 2p3/2 and 2p1/2 at 161.2 and 162.3 eV for S, respectively, as shown in Figures 7(b) and 7(c) [30]. The other peak that appeared at higher binding energy around 168.8 eV represents sulphate and carbon bonding. The binding energies of Mo and S represent the Mo + 4 and S-2 valence states. The occurrence of Mo and S doublets is a key feature seen in MoS2 structures [31, 32], and their presence in our experimental finding supports the development of MoS2 films. In addition to Mo +4 valence, Mo + 6 is also obtained in the sample, and Mo + 6 valence is enveloped by the broad peak of Mo 3d3/2 and Mo 3d5/2 (Figure 7(c)), which indicate that all the Mo-O bonds are not broken, and the transitions from the higher valence to lower valence of Mo maybe a temperature-dependent effect as observed by Salazar et al. [33]. In addition, the observed binding energy values for Mo bonds in our samples are also reported as the interaction of Mo with other elements [34]. Moreover, the presence of sulphur in the environment also facilitates the breaking of bonds between oxygen and molybdenum. Two main peaks were identified in the O 1s spectra of MoS2 grown sample at 531.6 and 535.4.0 eV, and the peak at 535.4 eV suggests that Mo species and other oxidized compounds had complex oxygen bonding [26]. Deconvolution of the smaller doublets of Mo 3d enveloped in a broad peak at 226.4 eV also supports intermediate states of MoS2, which occur owing to nonstoichiometric MoS2 [35], as shown in the Raman spectra of samples. O 1 s peak is supposed to appear at 530 eV; but in our case, it is enveloped by the broad peak that appeared at 531.6 eV, as shown in Figure 7(d). The presence of other radicals on the surface is the cause of this broadening in the O 1 s spectrum. Sodium peak is also observed in the samples which showed some Na residues coming from precursor of sodium molybdate, as shown in Figure 7(e). Figure 7(f) shows the atomic percentage of all the elements present in our samples.
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4. Conclusions
The present work is based on the combination of wet chemistry and the CVD growth method to synthesize MoS2. Direct conversion of MoS2 from liquid precursor under Ar flow at high temperature enables the conversion of L-cysteine and sodium molybdate solution-coated sample to convert into MoS2. The effect of heating in the sulphur and nonsulphur environment is analyzed with the help of Raman and XRD analysis. Raman spectrum showed the presence of extra peaks of oxysulphides for the sample, which has no sulphur content. XRD peaks showed a slightly higher strain value for the samples with no sulphur than the samples with sulphur. SEM showed the formation of multilayer flakes for the MoS2 samples formed in the presence of sulphur. Furthermore, XPS provides extensive information on the presence of various components in the samples. We successfully formed MoS2 thin films by combining the wet chemistry and CVD method, which is confirmed by employing various characterizations. This method showed sulphurisation as an effective step to obtain good quality MoS2 which would help design sensors where direct deposition of MoS2 is required. However, few more experiments are still required to explore it further.
Data Availability
The data used to support the findings of this study are included within the article and are available from the corresponding author upon request.
Conflicts of Interest
The authors declare that they have no conflicts of interest.
Acknowledgments
The authors gratefully acknowledge Qassim University, represented by the Deanship of Scientific Research, on the financial support for this research under the number 10275-qec-2020-1-3-I during the academic year 1441 AH/2020 AD.