Applied Science and Convergence Technology 2024; 33(1): 18-22
Published online January 30, 2024
Copyright © The Korean Vacuum Society.
School of Materials Science and Engineering, Kyungpook National University, Daegu 41566, Republic of Korea
This is an Open Access article distributed under the terms of the Creative Commons Attribution Non-Commercial License (https://creativecommons.org/licenses/by-nc-nd/4.0/) which permits non-commercial use, distribution and reproduction in any medium without alteration, provided that the original work is properly cited.
A solution-based thermolysis route for MoS2 synthesis was systematically investigated to identify the key process parameters for achieving scalable and high-quality two-dimensional MoS2 with a uniform thickness. MoS2 was synthesized by spin coating a solution containing (NH4)2MoS4 as the precursor. The crystallinity was found to be closely related to the process temperature, while the thickness exhibited a stronger dependence on the precursor solution concentration than the other process variables. However, controlling the thickness of MoS2 is strongly dependent on the solvent properties, such as viscosity. The thickness uniformity and surface coverage could be enhanced by adding ethanolamine to a mixture of N-dimethylformamide and n-butylamine, which hindered the formation of MoS42− clusters in the precursor and facilitated the synthesis of uniform thin films. With an appropriate solvent, the MoS2 thickness can be controlled more precisely by varying the precursor concentration.
Keywords: MoS2, Thermolysis, Large scale, Uniformity
Since the discovery of graphene in 2004, there has been a surge in research focused on the properties of two-dimensional (2D) materials . Graphene, which has exceptional electronic and structural properties, has been used for applications in transistors [2–5], photovoltaics [6–10], and photodetectors . Its inherent 2D nature has sparked interest in its use in flexible electronic components [12–14]. However, the drawbacks of graphene are its stringent growth requirements, which are primarily constrained to specific substrates, and the challenging nature of the associated manufacturing processes [15–21]. Furthermore, the absence of a sizeable band gap in graphene limits its use in electronic devices.
To address these shortcomings, research focus has shifted considerably toward transition metal dichalcogenides as promising 2D materials. Among these, 2D MoS2 exhibits a large direct bandgap (approximately 1.9 eV) and strong photoluminescence properties, particularly at the monolayer level [22,23]. These attributes make MoS2 a versatile candidate for integration into field-effect transistors (FETs) [24–27] and sensor [28–32] applications. Furthermore, its potential spans a broad spectrum of optoelectronic devices, including photodetectors [33–36] and photovoltaics [37,38]. Additionally, single-crystalline MoS2 can spontaneously nucleate even on amorphous films, such as SiO2, allowing more seamless heterogeneous integration with other semiconductors. However, despite its potential application in diverse fields, achieving a uniform thickness of MoS2 over a large area remains challenging.
Conventionally, the most widely recognized method for 2D MoS2 synthesis involves the chemical vapor deposition of MoO3 and S precursors. However, this method faces challenges in terms of precise thickness control and large-area growth. Recent research has succeeded in achieving monolayer MoS2 growth on a scale of up to 4 in . Nevertheless, uniformity becomes a limiting factor as the growth area increases. Therefore, an innovative solution-based thermolysis synthesis method was developed using a Mo precursor . This approach facilitates precise control of MoS2 thickness through the manipulation of precursor [(NH4)2MoS4, (NH4)6Mo7O24·4H2O, Na2MoO4] concentration. This method allows precise control over the MoS2 thickness by adjusting the precursor concentrations, achieving large-scale growth of up to 6 in [41–46]. Furthermore, its solutionbased process characteristics allow its growth on flexible substrates, and 20-inch large-scale growth using roll-to-roll processes has been demonstrated . However, there is limited systematic research on the thickness uniformity and crystallinity of MoS2 during solutionbased thermolysis.
In this study, we investigated the process-dependent (i.e., precursor concentration, temperature, spin-coating speed, and choice of solvent) thickness uniformity and crystallinity of 2D MoS2 synthesized by solution-based thermolysis.
A 2 × 2 cm2 c-plane sapphire substrate was ultrasonicated for 5 min in acetone, isopropyl alcohol, and deionized water. Subsequently, ultraviolet (UV)-ozone treatment (20 mW/cm2) was performed for 30 min to improve the hydrophilicity of the sapphire surface before spin coating. The precursor solutions were prepared using (NH4)2MoS4 (Sigma Aldrich, 99.97 %) with N, N-Dimethylformamide (DMF), a DMF + n-butylamine mixture, and a DMF + n-butylamine + ethanolamine mixture at concentrations of 0.024, 0.012, and 0.0048 M.
The solution was sonicated for 30 min before spin coating, which was performed at speeds of 2,000 and 4,000 rpm, followed by baking at 60 °C for 15 min. Subsequently, a MoS2 thin film was synthesized via a two-step annealing process in a tube furnace. During the first annealing process, the film was held in an N2/H2 gas atmosphere at a pressure below 1 Torr, and the temperature was maintained at 300 °C for 30 min to remove solvent residue. The second annealing process occurred in an N2 atmosphere at various temperatures (700, 800, 900, and 1,000 °C) for 30 min. In the second annealing step, additional S powder was heated to provide subplementary sulfur, preventing the decomposition of MoS2 during the process and preventing MoS2 oxidation. A schematic of the process flow is shown in Fig. 1.
Raman spectra of the synthesized 2D MoS2 samples were obtained by dm500i (Dongwoo Optron) Raman spectrometer using a 532-nm laser line and a spot size of 1 μm. The surface morphology of the MoS2 samples was analyzed by field emission scanning electron microscopy (FE-SEM) using a JSM-6701F (JEOL) and by atomic force microscopy (AFM) using an NX20 (Park Systems) instrument.
The crucial factors to consider in the solution-based synthesis of 2D MoS2 are the uniformity and crystallinity of the layer. Choosing a solvent with high polarity that effectively dissolves the precursor (NH4)2MoS4 is essential. Among the candidate solvents, such as DMF, dimethyl sulfoxide, and ethylene glycol, selecting a solvent with a low viscosity is crucial for uniform coating. Therefore, DMF with a viscosity of 0.92 mPa·s was chosen. In terms of the process, a one-step annealing method for growing MoS2 directly at elevated temperatures is not suitable for obtaining uniform stoichiometric MoS2 because of the possibility of decomposition of MoS2 in a strongly reducing environment . Hence, we adopted a two-step annealing process . In the first step, the spin-coated sample was annealed at 300 °C under the N2/H2 ambient. The gas was introduced for 30 min, while maintaining a pressure of less than 1 Torr. This step led to the decomposition of (NH4)2MoS4 into MoS3. The first annealing step ensured that any residue from this step did not interfere with the subsequent MoS2 growth in the second annealing step. The process temperature was set at 300 °C to minimize decomposition of MoS2 in the presence of H2 . In the second annealing step, the process temperature was increased to 700–1,000 °C, and the atmosphere inside the reactor was adjusted to N2 gas. During the second annealing, vaporized sulfur was introduced using N2 as the carrier gas, facilitating a 30 min synthesis of MoS2. A sulfur-rich environment was created to promote the formation of stoichiometric MoS2 .
The thickness and crystallinity of the MoS2 were analyzed using Raman spectroscopy, specifically by examining the spacing and full width at half-maximum (FWHM) of the E12g and A1g peaks. Figure 2 shows the peak spacing in the range of 23.12–25.31 cm−1 (four to five layers) , regardless of the concentration of the solution and speed at which the solution is coated. Surprisingly, the Raman data indicate that an increase in the annealing temperature led to an increase in the MoS2 thickness. The observed decrease in FWHM from 700 (7.44 cm−1) to 1,000 °C (5.47 cm−1) suggested an improvement in MoS2 crystallinity. The diffusion rate of sulfur (S) through the layers and parallel diffusion along the van der Waals bonding plane in MoS2 varies with annealing temperature. Therefore, the increase in the thickness with increasing annealing temperature can be attributed to the diffusion of S through the layers, provided that the precursor concentration is sufficiently high. However, as discussed below, the overall thickness uniformity could only be enhanced with a proper solvent, even with 900 and 1,000 °C annealing [50,51].
Using DMF, additional analysis was conducted on the synthesis and thickness control of MoS2 by changing the first annealing atmosphere from N2/H2 to H2. The first annealing was conducted at 300 °C in an H2 gas atmosphere, while the second annealing remained consistent with the previous process, using only N2 gas at 900 °C. For this analysis, Raman spectra were collected from various regions of the sample, as shown in Fig. 3(a). Possibly due to a stronger reducing environment created by the H2-only atmosphere, thickness of MoS2 could be reduced to two to three layers, as suggested by the Raman peak spacing of 19.67 cm−1 in Fig. 3(a). The Raman spectra obtained from five different regions of the sample showed almost identical profiles with similar peak spacings. However, the AFM analysis [Fig. 3(b)] revealed that a triangular MoS2 domain was formed, and the MoS2 film was not continuous. The formation of the MoS2 domains was attributed to the use of DMF, as discussed below.
Surface coverage and thickness uniformity strongly depend on the viscosity of the solvent. DMF has the lowest viscosity among the solvents tested in this study; however, the SEM images in Fig. 4 suggest that the solution with only DMF shows incomplete surface coverage due to the aggregation of the MoS2 precursor [indicated by the white circles in Fig. 4(a)], regardless of the precursor concentration. This is likely the reason why the DMF-based process showed no concentrationdependent thickness control (Fig. 2) and poor surface coverage [Fig. 3(b)]. Additionally, pinholes were observed in the synthesized MoS2, indicating nonuniform growth. This was further confirmed by Raman spectroscopy, which was conducted at five different points on the sample, as shown in Fig. 5. For the samples processed at a low rpm, peaks were observed only near the center of the sample, with no MoS2 peaks detected in the side regions. Although this trend was less pronounced in the samples processed at high rpm, there were still points on the sample where MoS2 peaks were not detected. This was attributed to the formation of ionic thiomolybdate (
Another solvent containing an amine group was added to DMF to enhance the coating uniformity. Initially, a solution was prepared using DMF and n-butylamine with a viscosity of 0.5 Pa·s in a 1:1 ratio. Despite improving the wettability of the sapphire substrate surface through UV-ozone treatment before spin coating, the solution did not coat the substrate surface because of its lower viscosity and adhesion compared to the original solution, regardless of the concentration. To address this issue, ethanolamine, a higher viscosity amine group solvent with a viscosity of 24.1 mPa·s, was added in the subsequent step . The two-step annealing process was performed using with a 0.024-M and 0.0048-M solutions. With a 0.024-M solution, the peak spacing was approximately 25.31 cm−1, indicating a 5-layer thick MoS2 film. Raman spectra were obtained at five different points across the sample, as shown in Fig. 6(a). When the precursor concentration of the solution was lowered to 0.0048 M, the peak spacing became approximately 19.67 cm−1, indicating MoS2 with two to three layers. With the thinnest MoS2 sample synthesized at 900 °C shown in Fig. 6(b), the Raman spectra were taken from 12 different points across the sample with an average spacing of 19.6 cm−1 and a standard deviation of 0.417. This result confirms the successful synthesis of uniformly thick MoS2. Figures 3 and 4 suggest that the control parameters, such as concentration and annealing temperature, had minimal impact on the domain size without a uniform distribution of precursors. Compared with the MoS2 films shown in Figs. 3(b) and 4, these films are continuous. A top-view SEM image in Fig. 6(c) taken from the 900 °C sample shows no distinguishable domains. The AFM image in Fig. 6(d) also suggests the absence of microscale domains. The root-meansquare roughness of the surface is 0.34 nm.
In this study, solution-based thermolysis of 2D MoS2 was investigated by exploring the impact of process variables (i.e., coating speed, solvent viscosity, solution concentration, annealing ambient temperature, and annealing temperature) on the crystallinity, thickness uniformity, thickness controllability, and surface coverage of 2D MoS2. First, the coating speed had no impact on the thickness control. Raman analysis showed that the crystallinity of MoS2 improved as the process temperature increased. Although there were variations in the growth thickness of the 2D MoS2 with varying temperatures in the second annealing step, the impact was considered marginal. Although DMF has a low viscosity, aggregation of the precursors leads to poor surface coverage and a lack of thickness control by the precursor concentration. With addition of an amine group solvent to prevent the formation of ionic thiomolybdate (
This research was subported by the BK21 Four project funded by the Ministry of Education, Korea (2120231314753) and the Basic Research Laboratory program funded by the Ministry of Science and ICT (RS-2023-00222070).
The authors declare no conflicts of interest.