Synthesis Methods of Two-Dimensional MoS₂: A Brief Review

Abstract

Molybdenum disulfide (MoS₂) is one of the most important two-dimensional materials after graphene. Monolayer MoS₂ has a direct bandgap (1.9 eV) and is potentially suitable for post-silicon electronics. Among all atomically thin semiconductors, MoS₂’s synthesis techniques are more developed. Here, we review the recent developments in the synthesis of hexagonal MoS₂, where they are categorized into top-down and bottom-up approaches. Micromechanical exfoliation is convenient for beginners and basic research. Liquid phase exfoliation and solutions for chemical processes are cheap and suitable for large-scale production; yielding materials mostly in powders with different shapes, sizes and layer numbers. MoS₂ films on a substrate targeting high-end nanoelectronic applications can be produced by chemical vapor deposition, compatible with the semiconductor industry. Usually, metal catalysts are unnecessary. Unlike graphene, the transfer of atomic layers is omitted. We especially emphasize the recent advances in metalorganic chemical vapor deposition and atomic layer deposition, where gaseous precursors are used. These processes grow MoS₂ with the smallest building-blocks, naturally promising higher quality and controllability. Most likely, this will be an important direction in the field. Nevertheless, today none of those methods reproducibly produces MoS₂ with competitive quality. There is a long way to go for MoS₂ in real-life electronic device applications.

1. Introduction

Recently, graphene has received considerable attention in scientific communities. It is recognized as a new wonder material (e.g., record carrier mobility, thermal conductivity, mechanical strength and flexibility), and applications in nanocomposites, paints, transparent electrodes, heat spreaders, etc. are being explored [1]. However, one should not forget that the original motivation for studying graphene was in its field effect [2], with a hope that it might play a key role in nanoelectronics, rather than serving as a passive element such as electrodes [3]. Nevertheless, at this stage, due to the difficulties in opening an energy bandgap in graphene, it is not clear whether graphene will become a key material in electronics at all. Therefore, the search for other two-dimensional (2D) materials (materials with one or few atomic layers) with an inherent energy bandgap that is suitable to make transistors is essential. Black phosphorus [4], transition metal dichalcogenide (TMD) [5], gallium nitride [6], etc. are extensively explored. For any type of application, the material production is always the first experimental step. Among these 2D semiconductors, molybdenum disulfide (MoS₂) is relatively more mature in terms of its synthesis technologies. Hence, in this paper, we provide a short literature survey of recent progresses in the synthesis of MoS₂. The paper focuses only on the production techniques rather than serving as a complete review on MoS₂-related physics, materials and electronics. Also, we put special emphasis on the recent achievements in metalorganic chemical vapor deposition (MOCVD) and atomic layer deposition (ALD), where gaseous precursors are used to grow MoS₂. It has a high potential for producing MoS₂ with better quality and controllability for nanoelectronic device applications.

2. Synthesis Methods of MoS₂

MoS₂ has several polymorphs, where the 2H MoS₂ has a layered hexagonal structure with lattice parameters of a = 0.316 nm and c = 1.229 nm [7], yielding an inter-layer spacing of 0.615 nm [8], as schematically shown in Figure 1. Bulk 2H MoS₂ is an indirect bandgap semiconductor (~1.29 eV). With decreasing thickness, the bandgap increases. When it comes to monolayer MoS₂, it becomes a direct bandgap material, and the energy gap can be as large as ~1.90 eV [9], suitable for fabricating most electronic devices. The synthesis methods of MoS₂ can be divided into top-down and bottom-up approaches. We hereby describe them in turn.

2.1. Top-down Approaches

2.1.1. Micromechanical Exfoliation

Similar to the micromechanical exfoliation of graphene (a vivid lab demonstration can be found in [10]), MoS₂ flakes can be produced on a substrate using sticky tapes. The starting material is bulk MoS₂, where some parts are peeled off with the tape and pressed into the substrate. Upon the release of the tape, some parts stay with the substrate rather than the tape due to the van der Waals force to the substrate. Repeating the process may produce flakes of MoS₂ with random shapes, sizes and numbers of layers. This method offers 2D materials of the highest quality, enabling to study the pristine properties and ultimate device performances. For example, Kis et al. have used micro-exfoliation to produce MoS₂ monolayers that are suitable for ultrasensitive photodetectors [11], analogue [12] and digital circuits [13], and observation of interesting physics such as mobility engineering and a metal–insulator transition [14]. However, it was found that the van der Waals adhesion of many TMDs to SiO₂, a typical substrate, is much weaker than that of graphene. Therefore, the produced flakes are very small (typically <10 µm) in their lateral size [15]. These results notwithstanding, large area MoS₂ flakes can be exfoliated by slightly modifying the technique. Magda et al. obtained mono MoS₂ layers with lateral size of several hundreds of microns by improving the MoS₂-substrate adhesion, as shown in Figure 2 [15]. The mechanism is that sulfur atoms can bind more strongly to gold than to SiO₂. The drawback is that such as-exfoliated MoS₂ flakes have to be transferred to another new substrate, since in most applications the material is used on insulating substrates. In addition, like their graphene counterpart, the tape-assisted micromechanical exfoliation method offers very low yield, and it’s not possible to scale up for large volume production, therefore MoS₂ synthesized by this method is limited to fundamental study at lab scale.

2.1.2. Liquid Phase Exfoliation

Liquid phase exfoliation also starts from bulk MoS₂, producing flakes with random shapes, sizes and number of layers. The quantities, however, are much larger, and the quality lower. Roughly, there are 2 routes to exfoliate MoS₂ in solution. The first is exfoliation by mechanical means such as sonication, shearing, stirring, grinding and bubbling [16,17,18,19,20,21]. The essence is purely physical, although some chemistry may still be involved [22,23,24]. For example, surfactants such as sodium deoxycholate (SDC) bile salt [22] and chitosan [23] may be added to the solution to prevent the exfoliated flakes from recombination. Electrolysis is employed to generate bubbles in some cases [24]. It is known that fine bubbles can squeeze into material interfaces and exfoliate them [25]. The yield of this method is dramatically improved from tape-assisted exfoliation, but the efficiency is still low for industrial applications. Therefore, a second route by atomic intercalation via solution chemistry [26] or electrochemistry [27] has been proposed. Lithium is typically used to intercalate between the MoS₂ layers and enlarge the interlayer spacing, easing the following exfoliation by mechanical treatment (e.g., sonication). Fan et al. developed a process to exfoliate MoS₂ nanosheets very efficiently using sonication assisted Li intercalation. They found that the complete Li intercalation with butylithium can be effected within 1.5 hours [28]. An example of the MoS₂ flakes synthesized by this method is shown in Figure 3. However, this method results in the loss of semiconducting properties due to the structural changes during Li intercalation. It is reported that above an annealing temperature of 300 °C, these MoS₂ flakes can be somewhat recovered [26].

Liquid phase exfoliation is a simple technology with low cost, producing large quantity 2D nanosheets with relatively high quality. Therefore, it is regarded as the most suitable method for the large-scale industrial production of one- to few-layer 2D material flakes at low expense [29]. It is worth noting that liquid phase exfoliation can produce not only wet suspensions of flakes or dry powders, but macroscopically continuous thin films as well [30,31,32]. On substrates, through drop casting of the suspension and drying, MoS₂ thin films are prepared by Walia et al. [30,31,32], where optical and electrical properties can be studied.

2.2. Bottom-up Approaches

2.2.1. Physical Vapor Deposition

Although molecular beam epitaxy (MBE) is an advanced technology for growing single crystal semiconductor thin films, its application in producing 2D materials needs further development, and the grain size of the 2D material is not as high as expected [33]. In fact, even ordinary physical vapor deposition is rare. There is a report on growing MoS₂-Ti composite by vacuum sputtering, using Ti and MoS₂ as the targets [34]. However, the MoS₂ is an amorphous structure in this case.

2.2.2. Solution Chemical Process

There are various methods for producing MoS₂ by solution chemistry. Hydrothermal synthesis [35] and solvothermal synthesis [36] typically use molybdate to react with sulfide or just sulfur in a stainless steel autoclave, where a series of physicochemical reactions take place under relatively high temperature (e.g., 200 °C) and high pressure for several hours or longer. The resultant is MoS₂ powders of different shapes. The size of individual particles can be adjusted to some extent. Very frequently, the powders are high-temperature post-annealed, to improve their crystalline quality and purity. The only difference between hydrothermal and solvothermal synthesis is that the precursor solution in the latter case is usually not aqueous. Other solution chemical processes start at almost room temperature and atmospheric pressure, where post-annealing is often used anyway [37,38,39,40,41]. The products can be either a powder or thin film, depending on the preparation details. The most commonly-used precursor is (NH₄)₂MoS₄ (ammonium tetrathiomolybdate), or something similar. (NH₄)₂MoS₄ decomposes to form MoO₃ at 120–360 °C, which can be further converted into MoS₂. In some cases, electrochemical deposition [42,43,44] and photo-assisted deposition are used [45]. For instance, Li et al. fabricated MoS₂ nanowires and nanoribbons using MoO_x as the precursor [43]. The MoO_x precursor is then exposed to H₂S at high temperature (500–800 °C) to synthesize MoS₂. Depending on the temperature for processing, both 2H and 3R phase of MoS₂ can be synthesized. The MoS₂ nanoribbons can be 50 µm in length and are aligned in parallel arrays which are preferable for device fabrication, as shown in Figure 4.

2.2.3. Chemical Vapor Deposition (CVD)

Among all the synthesis methods, CVD is the most compatible to existing semiconductor technology. Typically, it forms the film on a substrate, where chemical reactions are involved in the growth mechanism. The large-area growth of highly uniform MoS₂ will enable the batch fabrication of atomically thin devices and circuitry. Currently, most works start with solid state precursors of molybdenum, such as MoO₃ [46,47] or Mo [48]. If the Mo precursors are not vaporized in the process, often the technique is also called sulfurization [46,47,48]. Typical S precursors are H₂S gas [49] or vaporized S [46,47,48,50]. It is argued that using MoCl₅ precursors are easier to achieve monolayer MoS₂ over large area [50]. Earlier, the MoS₂ could be noncontinuous [47], which is not a big problem nowadays. The usual growth temperatures are 700–1000 °C, and the inclusion of a metal catalyst such as Au is favorable for the film quality [49]. Typical reactions taking place in the CVD are one of these:

MoO₃ + 2H₂S + H₂ = MoS₂ + 3H₂O

(1)

8MoO₃ + 24H₂S = 8MoS₂ + 24H₂O+S₈

(2)

16MoO₃ + 7S₈ = 16MoS₂ + 24SO₂

(3)

With plasma-enhanced chemical vapor deposition (PECVD), the growth temperature of MoS2 can be lowered to 150–300 °C, making it possible to directly deposit MoS₂ even on plastic substrates [51].

Metal-organic CVD is a special case of CVD where organometallic precursors are used. It is used in the modern semiconductor industry to produce single crystal expitaxial films. Recently, a few reports have appeared on the application of this method in the growth of MoS₂ thin films [49,52,53,54]. Kang et al. [52] have grown MoS₂ and WS₂ on 4-inch oxidized silicon wafers by MOCVD. The MoS₂ is grown with gaseous precursors of Mo(CO)₆ (boiling point 156 °C) and (C₂H₅)₂S. At 550 °C, monolayer MoS₂ with full coverage of the substrate (typically SiO₂/Si and fused silica) is deposited for t₀~26 h. Figure 5a shows scanning electron microscopy (SEM) images of the samples, demonstrating the effect of adding H₂ in the gas mixture. It is argued that H₂ enhances the decomposition of (C₂H₅)₂S (increasing nucleation due to hydrogenolysis) and etches MoS₂ (preventing intergrain connection) [52]. It seems that a low H₂ rate is beneficial for growing continuous monolayer MoS₂. Nevertheless, H₂ is necessary for removing the carbonaceous species generated during the MOCVD. Therefore, the H₂ flow should be optimized. To have successful growth, dehydrating the growth environment is also necessary. Figure 5b indicates that the growth mode is layer-by-layer under this specific condition. When t < t₀ there is almost no nucleation of the second MoS₂ layer, which takes place mainly at the grain boundaries of the first layer when t > t₀. Figure 5c shows a top gated (V_TG) MoS₂ transistor with a field-effect mobility of 29 cm² V⁻¹ s⁻¹ and transconductance 2 µSµm, determined from the transfer properties. The current saturation occurs at relatively low V_SD (see the inset). These features compete well with the-state-of-the-art MoS₂ transistors. Figure 5d–f demonstrates the optical absorption, Raman and photoluminescence spectra measured from the MOCVD grown MoS₂ (and WS₂) films, respectively, where the features agree well with the characteristics of exfoliated monolayer counterparts (denoted by diamonds).

At this stage, the material quality in [53] is still too low to be competitive. The large-area deposition of monolayer MoS₂ as well as other TMDs with spatial homogeneity and high performance is still challenging. MoS₂ films today are polycrystalline with the grain size typically ranging from 1 to 10 micrometers. This notwithstanding, the MOCVD synthesis of 2D semiconductors indeed opens a pathway towards future exciting nanoelectronic materials. The most important reason for this is that, in MOCVD, ultrapure gaseous precursors are injected into the reactor and finely dosed to deposit a very thin layer of atoms onto the substrate. Even if a solid or liquid precursor is used, it will be vaporized in situ in the chamber [53]. Unlike solid and liquid precursors, here the building-blocks for growing the film are much finer, naturally leading to better quality and controllability. The heated metalorganic precursor molecules decompose via pyrolysis, leaving the atoms on the substrate. The atoms bond to the surface, and an ultrathin MoS₂ layer is grown.

Based on this argument, one can easily think of another promising synthesis method: ALD [55], which can be seen as a subclass of CVD. In ALD, the film is grown by exposing the substrate surface to alternating gas precursors. In contrast to ordinary CVD, the precursors are never simultaneously present in the chamber. Instead, they are introduced as sequential and non-overlapping pulses, separated by purges in between. In each pulse, molecules react with the surface in a self-limiting way. Hence, it is possible to grow materials with high-thickness precision (layer by layer). Indeed, very recently, people have used ALD to grow MoS₂, such as in [56]. Using MoCl₅ (vaporized) and H₂S precursors, monolayer to few-layer MoS₂ on large SiO₂/Si or quartz substrates is obtained [56]. In the photos of two 150 mm quartz wafers before and after the ALD, the substrate changes to dark yellow uniformly (~2 layers of MoS₂) [56]. It is found that photoresist residues on the surface lead to more nucleation sites for MoS₂, but this mechanism remains to be confirmed. Figure 6a plots Raman signals for 50 cycle ALD MoS₂, where the E¹_2g and A_1g characteristic peaks are sharper and higher for 475 °C deposition temperature, indicating its higher material quality. After the ALD, some samples are annealed in a sulfur environment (H₂S or S atmosphere). Figure 6b shows that after sulfur annealing at increasingly high temperature, the characteristic peaks get narrower, stronger and more symmetric. After annealing at 920 °C, the full width at half maximum of the E¹_2g peak is close to that of exfoliated MoS₂ monolayer (4.2 cm⁻¹ vs. 3.7 cm⁻¹). By analyzing the separation Δ between E¹_2g and A_1g peaks, one can tentatively determine the number of layers in MoS₂ films [56]. In Figure 6c, the sample grown at 475 °C and annealed at 920 °C seems to be 1–2 monolayers. Future electronic measurements are needed to further examine the film quality.

3. Conclusions

In this paper, we have reviewed the synthesis methods of MoS₂ in terms of top-down and bottom-up approaches. Micromechanical exfoliation offers the highest material quality but is limited by the yield so it is mainly for basic research, whereas liquid phase exfoliation and solution chemical process are of low cost, with decent material quality and suitable for large-scale industrial production. MoS₂ thin films aiming for high-end electronic applications are produced by CVD. In this review, we have put a special emphasis on the MOCVD and ALD methods, because they are more sophisticated technology with gas phase precursors and will most likely be the dominant techniques in producing MoS₂ films oriented towards real electronic device applications in the future. In most cases the MoS₂ films are transfer-free, which is vital for industrial applications that require a high process controllability and reproducibility [57]. At the moment, production of MoS₂ as a high quality electronic material is still challenging. This notwithstanding, most of the reviewed processes can be extended to other 2D materials (with some modification), and with future technological advances, the successful production of advanced 2D nanoelectronic materials can be envisioned.