1 Introduction

In the past few years, two-dimensional (2D) materials have attracted a great deal of attention due to their unique physical and chemical properties associated with their ultrathin structure^[1-6]. Unlike gapless graphene^[7-8], transition metal dichalcogenides (TMDs) served as layered semiconductors have been wildly studied and employed in many applications, such as electrical^[9-10], optical^[11-12], catalytic^[13-14], and photovoltaic devices^[15-16].

Molybdenum disulfide (MoS₂), as a prototypical member of TMDs family, is a layered semiconductor with a bandgap in the range of 1.2 eV to 1.8 eV^[17]. Upon reducing the dimension of MoS₂ from a three-dimensional bulk into a monolayer, the band gap converts from an indirect to a direct one (Fig. 1(a))^[18], making them useful for novel electronic devices as building blocks. In addition, monolayer MoS₂ consists of two planes of S atoms separated by an intermediate plane of Mo atoms (Fig. 1(b), (c)), which exhibits an outstanding on/off ratio current up to 10⁸, and high carrier mobilities of 200 cm²/(V·s)^[19]. Furthermore, its satisfactory flexibility and excellent mechanical strength derived from strong covalent bonding also ensure that this material is an excellent candidate for future practical applications. Therefore, it is highly desirable to develop reliable and scalable synthetic methods for large-domain, single-crystal monolayer MoS₂.

At present, monolayer and few-layer MoS₂ have been prepared by both "top-down"^[20-26] and "bottom-up"^[27-38]synthetic strategies (Table 1). The top-down strategies include mechanical cleavage, mechanical force-assisted liquid exfoliation, ion intercalation-assisted liquid exfoliation, and so on. Just like graphene^[39-40], 2D MoS₂ synthesized by mechanical cleavage is of good quality, however the size of it is pretty small (usually a few micrometers), furthermore, their shape and thickness are not controllable. Using liquid exfoliation to synthesize layered MoS₂ makes large-scale production achievable, while its application in electronic devices is hindered by the low quality.

The bottom-up strategies, that is, the direct preparation of 2D materials via chemical reactions from different precursors at certain conditions. The CVD technique is a common bottom-up strategy utilized to synthesize atomically thin nanomaterials on special substrates. Typically, the substrate is put inside of the furnace chamber, and then the gas-phase precursors will react above the substrate to generate ultrathin flakes or films at high temperature. Up to now, it has been successful in growing high-quality single- and few-layered MoS₂ on various substrates, including SiO₂/Si, sapphire, SrTiO₃ and Au. Compared to other bottom-up strategies, the CVD method is most promising in terms of scalability, simple operation and low cost.

However, it is well known that there are so many parameters we should take into account in CVD process like temperature, pressure, flow rate of carrier gas, and so on, which makes it hard for us to control the process of growth. Although many papers and reviews have summarized the synthetic methods of MoS₂ and its applications, a systematic review that expounds how these various important factors influence the process of growth is lacking. This work aims to discuss in detail how some parameters such as substrates, kinds of carrier gases and species of Mo precursors affect the quality of the obtained MoS₂ by CVD (Fig. 2), which can be also applied to the synthesis of other TMDs.

2 Influence Factors 2.1 Substrates

It is well-known that the CVD routes typically involve a chemical reaction of the precursors, followed by a vapor-solid phase transformation giving rise to the growth of ultrathin MoS₂ flakes or films on target substrates^[41]. Since different substrates possess different surface structures, various substrates like SiO₂/Si, mica, sapphire and Au foil may give diverse growth environments with a wide tunability of domain shape, orientation, as well as size for monolayer MoS₂.

2.1.1 SiO₂/Si

Owing to its well compatibility with Si-based electronics, insulating SiO₂/Si is the most prevalent substrate for the preparation of MoS₂ by CVD, that means we can fabricate some electron devices like field effect transistor (FET) directly without any transfer process which may have unfavorable effects on the electrical properties of MoS₂.

Typically, Najmaei et al.^[29] achieved the controlled CVD synthesis of MoS₂ atomic layers on SiO₂/Si substrate (Fig. 3(a)). They demonstrated that the key point for the synthesis of large-area and high-quality MoS₂ films was the uniform nucleation on SiO₂ substrates. The Raman results (Fig. 3(c), (d)) shown that MoS₂ domains tended to nucleate in the places near to the substrates′ edges, scratches, or rough areas. Based on this phenomenon, they uniformly patterned the substrates with many rectangular SiO₂ pillars, directly to create some step edges for nucleation of MoS₂ in CVD process (Fig. 3(b)). The as-grown MoS₂ films were predominantly monolayer, while a small fraction of which consisted of two or more layers. Because of the isotropy of the amorphous SiO₂ surface, the resulting MoS₂ films were made up of randomly oriented domains. Finally, they fabricated FET devices to estimate the electrical property of this material by patterning this film via lithography and reactive ion etching. Mobility measurements shown an average of 4.3 ± 0.8 cm²/(V·s), on/off ratios could reach 6×10⁶ (Fig. 3(e), (f)).

2.1.2 Mica

Other than SiO₂/Si substrate, other insulating materials such as mica, sapphire, SrTiO₃ and LaAlO₃ were also used for synthesizing the monolayer MoS₂ by CVD. Noteworthily, their atomically smooth crystalline surfaces, as well as different interfacial energetics, and possible lattice-match to MoS₂, could also lead to additional controllable orientation and morphology of MoS₂ by van der Waals (vdW) epitaxy.

Fluorophlogopite mica (KMg₃AlSi₃O₁₀F₂) is regarded as an ideal vdW epitaxy substrate due to its surface inertness, atomic flatness, as well as the ordered hexagonal lattice structure. These unique features endow mica as a superior material for epitaxially growing layered materials which possess the same crystal symmetry. Particularly, the lattice symmetry of mica is in agreement with that of MoS₂ and there is small lattice mismatch (~2.7%) between mica and MoS₂, which could be used for growing monolayer MoS₂ on mica via the vdW epitaxy.

Using low-pressure chemical vapor deposition (LPCVD), Ji et al.^[42] successfully synthesized MoS₂ monolayer on a mica substrate (Fig. 4(a)). As shown in SEM (Fig. 4(b)), the initial grown MoS₂ were triangular flakes, exhibiting almost the same size and orientation, as labelled by the dashed yellow rhombus. With the increasing of the growth time, the MoS₂ domain enlarged in size to generate larger flakes and even centimeter-scale, monolayer MoS₂ films (Fig. 4(c), (d)). Another interesting finding was that the MoS₂ domains on mica preferred to merge together via in-plane stitching, rather than overlap with each other. Thus, this growth system was identified to be dominated by a vdW epitaxial mechanism.

2.1.3 Sapphire

Additionally, sapphire (0001) is also an ideal substrate frequently used for the epitaxial growth of a large amount of crystals due to its atomically flat surface and possible lattice-matching with the target materials. In addition, it must be mentioned that monolayer MoS₂ grown on sapphire may have higher quality than those on SiO₂/Si substrates owing to the large-scale thickness uniformity and low content of oxygen doping.

Similarly, Ji et al.^[43] controllably synthesized high-quality monolayer MoS₂ flakes and films on sapphire (0001) substrates by using LPCVD (Fig. 5(a)). With growth time increasing, triangular MoS₂ domains of different orientation grew up gradually and began to coalesce with each other (Fig. 5(b)), finally resulting a uniform thickness, high crystal quality but polycrystalline monolayer MoS₂ (Fig. 5(c)). This phenomenon revealed that perfect epitaxial characteristics of MoS₂ on sapphire were almost totally absent. Based on density functional theory (DFT), this growth behavior was thought to result from the small differences of the interfacial binding energy between sapphire and MoS₂ at diverse relative rotations.

In order to decrease the grain boundaries which were unfavorable for the mechanical and electrical performance of the MoS₂, Dumcenco et al.^[44] used EPI-ready grade sapphire substrates to regulate the crystal orientation of MoS₂ domains during CVD process, thus these highly aligned domains could merge into a uniform layer with a decreased amount of grain boundaries. The key to better aligning of individual MoS₂ flakes was preparing atomically smooth terraces on the surface of sapphire (Fig. 5(d)) by annealing it at 1 000 ℃ for 1 h before the growth process. A careful orientation examination disclosed that the most of single-crystal MoS₂ flakes (91.5% displayed in Fig. 5(e)) were well aligned with edge orientations as multiples of 60° (Fig. 5(f)). Only a small part of the MoS₂ flakes (6%) possessed an angle of ±30°, while the rest (2.5%) displayed other orientations. Undoubtedly, such highly aligned MoS₂ domains sharply decreased the number of tilted grain boundaries.

2.1.4 SrTiO₃ and LaAlO₃

Recently, some novel single-crystal substrates were also reported to be capable of mediating the diffusion pathways of precursor species, which provided additional chances for us to engineer the morphology of MoS₂.

For example, Zhang et al.^[45] reported a controllable preparation of dendritic monolayer MoS₂ on SrTiO₃ (STO (001)) substrate using LPCVD. The obtained MoS₂ flakes (Fig. 6(a)) or films (Fig. 6(c)) possessed the unique feature of strictly monolayer, coverage adjustable from sub-monolayer to complete monolayer (Fig. 6(b)), and flake shape tunable from fractal to compact by changing the conditions of growth. This phenomenon could be explained by a diffusion-limited aggregation mechanism. The unique dendritic structure of MoS₂ with abundant edges which were considered to be the active sites for hydrogen evolution reaction (HER) made it a promising catalyst for practical applications.

Afterwards, Li et al.^[46]also controllably synthesized the dendritic MoS₂ (Fig. 6(d)) on a novel single-crystal LaAlO₃ (LAO (100)) substrate through LPCVD. It was found that the LAO substrates commonly twinned and generated a triangular surface corrugation (Fig. 6(e), (f)), which could be ascribed to a phase transformation between the high-temperature cubic prototype phase and rhombohedral lattice below critical temperature. Therefore, the domain wall would divide the corrugation into two parts, isolating the steps from one part to another. Thus, there were steps on the one side, however the other side of the domain wall owned more smooth surface, even if both two parts were (100) lattice planes. The atomic steps on one side supplied plenty of nucleation sites, giving rise to lots of nuclei located along the steps, which leaded to the growth of fractal MoS₂ flakes with a mass of active edge sites, exhibiting a promising HER performance.

Briefly, although SiO₂/Si is the prevailing substrate for synthesizing high-quality layered MoS₂ currently by CVD, a lot of recent literature demonstrates that the amorphous surface of SiO₂ is not effective in catching the MoO_3-x species and controlling the crystallographic orientation for growing large-area, thickness uniform and single-crystal MoS₂ films. On the country, insulating single-crystal substrates are more reasonable for the growth of layered MoS₂, creating more opportunities for us to engineer the domain orientation, micro-morphology, merging behavior and growth mechanism.

2.1.5 Au foil

Recently, synthesizing layered MoS₂ on Au foil emerges as a fascinating growth system owing to its direct application in HER without a complex transfer process which will lead some quality degradation, on the other hand, directly growing MoS₂ on Au substrates is convenient for the further characterizations by scanning tunneling microscopy and spectroscopy (STM/STS), which can reveal some fundamental characteristices of the monolayer MoS₂.

For example, Shi et al.^[47] developed the controllable synthesis of domain size tunable monolayer MoS₂ flakes (Fig. 7(a), (b)) on Au foils by LPCVD. Interestingly, on the basis of Raman spectra of MoS₂ flakes grown under different source-substrate distance (D_ss) and temperature, they concluded that the MoS₂ flakes grown on Au foil were strictly monolayer regardless of growth temperature and D_ss, which indicated that the growth traits of MoS₂ on Au foil resulted from the strong interaction between Au foil and MoS₂. This phenomenon was in great contrast with that of MoS₂ on some insulating substrates like SiO₂/Si and sapphire, having some possibility to form few-layer MoS₂.

Another unanticipated finding was that the crystallography of Au substrate had a dramatic impact on the growth process^[48], where MoS₂ domains were more likely to generate on Au (110) and Au (100) facets than Au (111) at high temperatures (Fig. 7(c), (d)).Obviously, Au foil has many advantages when it comes to synthesize high-quality monolayer MoS₂, however high cost of which may hinder its future application. Thus, hunting for other stable metals or alloys is also vital for lowering the costs of production and achieving their practical applications.

2.1.6 Polyimide

Besides insulating and metal substrates, polymers can also be utilized as target substrates for the synthesis of large-domain and high-quality monolayer MoS₂. Directly growing 2D materials on flexible materials provides a good idea to combine the scalable synthesis technique with the target flexible substrates, taking a crucial step toward practical fabrication of flexible functional films. However, the high-temperature synthetic condition (>600 ℃) for MoS₂ by CVD are not suitable for polymers, which usually have relatively low thermal stability^[49].

What's exciting is that Gong et al.^[50] directly grew monolayer single-crystal MoS₂ flakes on polyimide (PI) substrates by a simple CVD method using ammonium molybdate ((NH₄)₂MoO₄) as a Mo precursor rather than traditionally used molybdenum trioxide (MoO₃) at 450 ℃(Fig. 8(a), (b)), at which temperature PI was still stable and flexible (Fig. 8(c), (d)). Moreover, the MoO₃ derived from the decomposition of (NH₄)₂MoO₄ was more active than the commercial MoO₃ powder and the gaseous products accompanied with the decomposition process could bring out the MoO₃ molecules, making the reaction kinetically more favored.

As discussed above, we can be clearly aware that the substrates play a significant role in the synthesis and application of atomically thin MoS₂. Notably, various substrates possess diverse surface structures and interfacial binding strengths, that means during growth, the interaction between the gaseous precursors and the substrates is different, which leads to different densities of MoS₂ nuclei at the initial period of the growth. In addition, distinct crystalline states of the substrates determine the orientation of MoS₂ domains, if the surface of the substrate is amorphous, the MoS₂ domains may display randomly on the substrates, however, while the substrate possesses atomically smooth crystalline surface and small lattice mismatch with MoS₂, the epitaxial domains may align at one direction. Upon different domains merging together, there will be no grain boundaries generated, which would be detrimental to the electrical and mechanical performance of the MoS₂. From the perspective of application, how to choose a proper growth substrate is also important. Even though amorphous surface of SiO₂ is not effective in capturing the MoO_3-x species and controlling the crystallographic orientation, SiO₂/Si is still the prevailing substrate for synthesizing layered MoS₂ currently by CVD owing to its high compatibility with Si-based electronics. Moreover, if we want to apply it in the area of electrocatalysis, we would better choose some substrates with high conductivity like Au or carbon paper. However, if we want to combine 2D materials with flexible films, polymers substrates should be our choices.

Up to now, so many kinds of substrates have been utilized in the growth of MoS₂ such as SiO₂/Si, mica, sapphire, STO, Au foils and PI. Briefly, every substrate has its own advantages and disadvantages, different substrates may give diverse growth mechanism with a wide tunability of domain shape, orientation and size for monolayer MoS₂, and the final decision of the substrate may depend on the further application of the obtained MoS₂ flakes or films.

2.2 Pretreatment of the Substrates

It is generally accepted that the formation process of MoS₂ domains mainly contains two steps: nucleation and growth. Achieving a proper concentration of nuclei is very vital to synthesize high quality layered MoS₂. Pretreating the surface of substrates before growth is one of the novel strategies to create some nucleation sites artificially or change the state of the surface of the substrates for enhancing the interaction between the precursors and the substrates, which will promote the nucleation of MoS₂.

2.2.1 Seeding aromatic molecules on surfaces

For the first time, Lee et al.^[51] obtained highly crystalline and large-area MoS₂ layers on amorphous SiO₂/Si substrate using graphene-like molecules like reduced graphene oxide (rGO), perylene-3, 4, 9, 10-tetracarboxylic acid tetrapotassium salt (PTAS) and perylene-3, 4, 9, 10-tetracarboxylic dianhydride (PTCDA) as the seeds for growing. However, the authors did not get the strict monolayer MoS₂ and further discussed the effect of the graphene-like molecules in detail. Lately, Kong et al.^[52] synthesized high-quality uniform single crystalline structure MoS₂ monolayers using APCVD with the seeding of PTAS (Fig. 9(a)), which was stable at high temperature compared to other aromatic molecules (Fig. 9(b)). In addition, PTAS had a high solubility in water, making the PTAS molecules distributed uniform on the surface of substrates. After seeding the substrate with PTAS, monolayer MoS₂ flakes (Fig. 9(c)) can grow successfully on the substrates. As a comparison, they can only observe MoS₂ particles on the bare substrate without the seeds. Thus, the existence of PTAS was able to adsorb the precursor molecules and promote the initiation of heterogeneous nucleation, significantly enabling layer growth. As a conclusion, they disclosed that seeding the substrates with an appropriate aromatic molecule having a high thermal stability and better solubility in solvent was crucial for the synthesis of high-quality MoS₂ monolayers.

To further explore this concept, they systematically investigated the effect of the seeding promoter in preparing monolayer MoS₂^[53]. Owing to different distance between the promoter substrate and various regions of growth substrate, the different growth regions might have disparate concentration of seeds (Fig. 9(d)). By analyzing the growth results of different growth regions (Fig. 9(e)), they indicated that a proper concentration of the promoters was of great importance to facilitate the nucleation of MoS₂ on the SiO₂/Si substrate. Furthermore, except for the already found seeding promoters like PTAS, twelve kinds of other aromatic molecules such as copper phthalocyanine (CuPc), PTCDA, copper (Ⅱ) 1, 2, 3, 4, 8, 9, 10, 11, 15, 16, 17, 18, 22, 23, 24, 25-hexadecafluoro-29H, 31H-phthalocyanine (F₁₆CuPc) were further used as promoters for the growth of MoS₂, and many of them were discovered to be effective. However, when some inorganic nanoparticles were utilized as promoters, they found no enhancement for the synthesis of monolayer MoS₂. Moreover, since PTAS was a salt and had a high solubility in water, it can only work normally on hydrophilic substrates. However, the above newfound aromatic molecules can be uniformly and easily deposited on various substrates by thermal evaporation, hence promoting the synthesis of MoS₂ on various hydrophobic substrates, such as graphene, gold, and h-BN. It is anticipated that these studies will provide more understanding on the mechanism of seed-enhanced growth.

2.2.2 Plasma treatment

Pretreating the substrate with plasma is a fresh approach to facilitate uniform nucleation and layer growth of MoS₂. Jeon et al.^[54] reported that large-area, varied numbers of MoS₂ layers can be obtained by pretreating SiO₂/Si substrates with the oxygen plasma (Fig. 10(a)). The XPS characterization shown that the chemical structure of the SiO₂/Si surface had changed after oxygen plasma treatment. More specifically, with the time of treatment increasing, the Si 2p and O 1s peaks shifted to higher binding energies, corresponding to Si-(O or OH)₄ bonding, which resulted in a higher surface reactivity and a better lattice matching with hexagonal MoS₂. Compared to the original SiO₂/Si substrate, the surface of those pretreated with oxygen plasma became more hydrophilic, thus decreasing the surface energy and facilitating uniform nucleation and layer growth of MoS₂. Another interesting finding was that they can controllably synthesize monolayer, bilayer and trilayer MoS₂ films by simply adjusting the duration (90 s, 120 s and 300 s) of the plasma treatment on the SiO₂/Si, which can be proved by AFM characterization (Fig. 10(b), (c), (d)). This interesting phenomenon indicated that the growth process of these layer-controlled MoS₂ film was self-limiting, which can be explained by the interaction between MoS₂ and substrates which changed due to the plasma treatment time.

2.2.3 Pre-deposited Au film

Atomic Mo film can serve as a Mo precursor to synthesize atomic MoS₂ layers, and the thickness of the MoS₂ layer is only dependent on the thickness of pre-deposited Mo film. However, the preparation of large-area atomically thin Mo film on the substrate is a difficult problem to overcome at the present. Notably, Song et al.^[55] have developed a fancy method to prepare atomically thin MoS₂ layers in expected sizes and patterns via Au-assisted CVD. Firstly, they pre-deposited 50 nm Au thin film on a Si substrate (Fig. 11(a)), and then it would react with [Mo(CO)₆] to form the Mo-Au alloy on the surface of Au film, which was able to act as an excellent Mo reservoir. When the H₂S introduced into the chamber at 300 ℃ afterwards, the surface Mo-Au alloy would react with it and the buried Mo atom diffused out, then forming atomically thin MoS₂ layers (Fig. 11(e)). Therefore, the formation of the surface Mo-Au alloy was supposed to be the key role in realization of uniform MoS₂ atomic layers (Fig. 11(b), (c), (d)). In this sense, we can achieve patterned growth of MoS₂ atomic layers simply by designing the pattern of the pre-deposited Au film. This was the first time to present the growth of atomically thin MoS₂ in a desired pattern without any pretreatment.

Directly growing 2D semiconductor materials on SiO₂/Si substrate is of great important to its potential application in the field of electron devices. However, as mentioned above, the interaction between the amorphous surface of SiO₂/Si substrate and gaseous Mo precursors is pretty weak, resulting the initial nucleation process being rather difficult. In order to solve this problem, researchers try to pretreat the substrate before the growth process, specifically including seeding the substrate with some aromatic molecule like PTAS to adsorb the precursor molecules and promote the initiation of heterogeneous nucleation, pretreating the substrate with plasma to decrease the surface energy and facilitate uniform nucleation, depositing Au thin film on the SiO₂/Si substrate to form Mo-Au alloy which could act as an excellent Mo reservoir during the growth, and so on. In general, artificially modifying the SiO₂/Si substrate and activating the inert state of the surface are the key points to enhance the interaction between the SiO₂/Si substrate and the Mo precursors, thus promoting the initial nucleation of MoS₂.

2.3 Kinds of Carrier Gases

As we all know, the carrier gases play an important role in the growth of MoS₂, whose effects can be divided into two parts. Apparently, the flow rates of the carrier gases determine the amount of the precursors brought to the substrates per unit time, which will influence the growth kinetics of layered MoS₂. Furthermore, the recipe of the carrier gases is also vital for the growth of MoS₂, which is usually proceeded under inert gas like high purity argon or nitrogen. However, if some reactive gas like O₂ or H₂ is included, the growth process will be greatly influenced^[56-58].

2.3.1 O₂

Recently, Chen et al.^[56]developed a novel oxygen-assisted chemical vapor deposition (OACVD) strategy to synthesize high-quality monolayer MoS₂. Fig. 12(a)-(c) shown the optical images of MoS₂ grown on sapphire substrates with the O₂ flow rate ranging from 0 to 2 sccm. According to the AFM image in the inset of Fig. 12(a), a great many MoS₂ particles were observed on both the MoS₂ and substrate without O₂. By contrast, a clean monolayer MoS₂ film was obtained with just 1 sccm O₂ brought into the chamber (Fig. 12(b)), which was also consistent with the observation from the AFM image of Fig. 12(b). With increasing the O₂ flow rate to 2 sccm, the length of triangular MoS₂ flakes could reach about 350 μm (Fig. 12(c)). It was interesting to find that the maximum domain size of the obtained MoS₂ decreased gradually with the O₂ flow rate increased from 3 to 5 sccm (Fig. 12(d)-(f)). To gain a better understanding of this phenomenon, the authors plotted the average length (l) of the MoS₂ flakes grown under 2 sccm O₂ as a function of growth duration (Fig. 12(g)). Typically, after growing for 10, 20, and 30 min, maximum domain size of MoS₂ could reach 125, 275, and 350 μm, respectively. However, when further increasing the growth time, the as-grown domains would be etched little by little. Finally, the as-grown MoS₂ domains would completely disappear after 60 min of growth.

Their findings suggest that a small amount of O₂ could decrease nucleation density of MoS₂ by smearing out some unsteady nuclei and chemically oxidizing its edges, resulting in the Mo and S atoms becoming MoO₃ and SO₂ gas to decompose. Hence, they concluded that the process of growth was accompanied by the etching effect of O₂. To gain more insight, the relationship between the growth, etching rates of MoS₂ and growth duration was illuminated in Fig. 12(h). The blue curves indicated that, in the initial period, the growth rate was almost stable, and then it gradually decreased as the time went on. While the rate of etching was supposed to be fixed, it just depended on the flow rate of O₂. Hence, only when the growth rate was higher than that of etching, the growth was proceeding. Upon the increasing of the growth duration further, the rate of etching would be higher than the growth, resulting the MoS₂ domains shrinking rapidly.

Using this OACVD method, this group recently synthesized wafer-scale highly aligned uniform and continuous monolayer MoS₂ films by epitaxial growth on sapphire wafer^[57]. They illustrated that the formation of MoS₂ on sapphire substrate coincides with the classic surface-limited growth mechanism. More specifically, the energy barrier of absorption on MoS₂ flakes was larger than that on a sapphire substrate. In this case, the arriving precursors would quickly migrate to the edge of MoS₂ forming large film or be etched by O₂. Except for the etching effect, the existence of O₂ could prevent the MoO₃ from sulfidation and guarantee its incessant evaporation in the growth process. During the experiment, they did not find any obvious change of MoO₃ before and after the growth, as shown in Fig. 13(b) and (d). As a comparison, the MoO₃ would react with S vapor to form MoS₂ and MoO₂ (Fig. 13(a), (c)) without the introduction of O₂. It was commonly known that the melting point of MoS₂ was higher than that of MoO₃, thus the evaporation would stop at that temperature when the MoO₃ was sulfured, which was a typical poisoning effect we avoided by introducing small amounts of O₂. With the assistance of O₂, the MoO₃ was protected and could evaporate continuously enabling us to effectively synthesize very large domain sizes and even obtain wafer scale uniform MoS₂ films (Fig. 13(e)).

2.3.2 H₂

As the same with O₂, H₂ also has a double action in the growth of MoS₂. Shi et al.^[58] successfully prepared monolayer MoS₂ flakes on Au substrates and systematically explored the effect of H₂ via LPCVD. SEM images in Fig. 14(a) and (b) shown that when the H₂ flow rate changing from 0 to 3 sccm, the length of MoS₂ flakes would increase from 7.8 to 20.9 μm. With the flow rate of H₂ increasing further to 5 and 6 sccm, the average length could reach about 47.8 and 55.8 μm (Fig. 14(c), (d)), respectively. However, while further increasing the H₂ flow rate to 8 and 10 sccm, the edge lengths of grown MoS₂ domains began to decrease to 13.5 and 7.3 μm (Fig. 14(e), (f)).

In order to shed light on the interesting experimental results, the role of H₂ was playing in the CVD process should be analyzed in detail. On one hand, H₂ could provide a reductive atmosphere which might be favorable for the efficient sulfurization of MoO₃ and the fast formation of MoS₂ at low H₂ flow rate (the involved chemical reaction was shown in Eq.(Ⅰ) of Fig. 14(g)). In addition, a small amount of H₂ was also able to reduce the nucleation density of MoS₂ by removing some impurities on Au substrates since the impurities could serve as nucleation sites for the growth of MoS₂. On the other hand, as the flow rate of H₂ becoming higher, MoO₃ source would be reduced to Mo metals and this might directly prevent the Mo precursor from suppling and then stop the subsequent growth of MoS₂. Furthermore, a high flow rate of H₂ would also etch the existing MoS₂ domains, as shown in Eq.(Ⅱ) of Fig. 14(g). From the XPS results (Fig. 14(h)), we observed that the Mo metal (etching product) can form Mo-Au alloys with the Au films at high temperature. Therefore, the reduced MoO₃ precursor supply and etching effect would work together to decrease domain size of MoS₂ at high H₂ flow rate. In this sense, we can achieve large-size MoS₂ flakes on Au foils by simply controlling an appropriate flow rate of H₂.

Based on the above experiment details, we can infer that the growth results are very sensitive to the ambient atmospheres. Only a small change about the amount of reactive gas like O₂ or H₂, results in the quality of the obtained MoS₂ having a big difference. Generally, these active gases often have a double action to the growth of MoS₂, even though their specific effect are different. When the amount of reactive gas is small, it will help to synthesize large-size MoS₂ flakes or films via reducing the nucleation density of MoS₂. Upon increasing the amount of reactive gas, it will etch the existing MoS₂ domains predominantly, resulting a small-size MoS₂ domain. Bearing this in mind, we can easily achieve high-quality MoS₂ monolayer with a controllable size by adjusting the amount of reactive gas or reaction time.

2.4 Species of Mo Precursors

Hitherto, there are so many different kinds of Mo precursors employed for the growth of atomically thin MoS₂ including Mo film, MoO₂, MoO₃, MoCl₅, (NH₄)₂MoS₄, Mo(CO)₆, and so on. Since different Mo precursors have various physical and chemical properties, the concrete parameters used in the experiments may differ greatly. In addition, the thickness and size of MoS₂ flakes or films obtained by diverse Mo precursors may also vary from each other.

2.4.1 Mo film

Generally, Mo foil might be one of the earliest Mo precursors used to synthesize atomically thin MoS₂^{[28, 59-61]}. Zhan et al.^[28] demonstrated a direct synthesis of single- and few layered MoS₂ by APCVD using a pre-deposition of Mo film as Mo precursor on SiO₂ substrates followed by sulphurization. Specifically, a thin Mo layer (approximately 1-5 nm) was pre-deposited on SiO₂ substrate by an e-beam evaporator firstly, and then sulfur vapor was brought in and reacted with Mo film to form atomically thin MoS₂ film at 750 ℃. It was interesting to find that the thickness and size of the MoS₂ layer were only dependent on the thickness of pre-deposited Mo film and the size of the SiO₂ substrate, indicating that the synthetic process was controllable and scalable, that is, films of any size and thickness can be prepared with good uniformity, which made it a potential candidate to meet the demands from future applications. However, they cannot get strict monolayer and single-crystal MoS₂ via this method of directly sulfurating the thin Mo film.

Recently, Li et al.^[61] developed a facile and environmentally friendly strategy to synthesize monolayer and few-layer MoS₂ single crystals by a simple one-step rapid sulfurization of Mo film. In this growth system, they designed a special reactor with a confined space to suppress the sublimation of sulfur and increase the probability of nucleation for synthesizing MoS₂ under an sufficient sulfur atmosphere (Fig. 15(a)). For Reactor 1, the Mo film coated with S plate was placed in the gap between two bare SiO₂/Si substrates. Fig. 15(b) shown the optical images of MoS₂ flakes obtained by Reactor 1. More importantly, most of the as-grown MoS₂ flakes were strictly monolayers, which can be proved by AFM characterization. As for Reactor 2, a single S plate was sandwiched between two SiO₂/Si substrates coated with Mo films. Fig. 15(c) displayed the optical images of monolayer and few-layer MoS₂ flakes synthesized by Reactor 2, the thickness of which can be confirmed easily by optical contrast. Therefore, simply by changing the reactor design, they can controllably obtain monolayer and few-layer MoS₂ flakes. In order to illuminate the important effects of the space-confined design, as a comparison, the top SiO₂/Si substrate in Reactor 1 was removed and then annealed under the same conditions. While the experiment results shown that no monolayer MoS₂ single crystals were obtained, which can be explained by the rapid sulfur evaporation and the reduced probability of nucleation for synthesizing MoS₂ single crystals. As far as I known, this is the first time to achieve monolayer MoS₂ single crystals by direct sulfurization of Mo foil. In addition, only very small amounts of precursors (Mo~3 μg, S~50 mg) were needed in this synthesis, far less than those used in conventional CVD approaches.

2.4.2 MoO₃

In a general way, the sulphurization of MoO₃ has been one of the most prevalent approaches for the synthesis of atomically thin MoS₂. So far, majority of growing MoS₂ monolayers through CVD processes involved the utilization of MoO₃ powder as a Mo precursor. Lee et al.^[51]firstly demonstrated that MoO₃ is an appropriate precursor for controllable synthesis of MoS₂ thin films. They systematically investigated the nucleation and growth of layered MoS₂ in CVD process assisted by some graphene-like species, the effect of which has been discussed above. In this sense, we can directly obtain large-size MoS₂ monolayer on amorphous SiO₂ substrate without using highly crystalline substrates or rigorous ultrahigh vacuum. During this synthesis of MoS₂, the furnace was heated to 650 ℃ in a N₂ environment, at which temperature MoO₃ powder would be reduced by the sulfur to produce volatile suboxide MoO_3-x. These compounds would diffuse quickly to the substrate and react further with sulfur vapor to form highly crystalline MoS₂ films, whose size can be up to several millimeters. Furthermore, some characterizations such as Raman, PL, AFM all suggested that the obtained layered MoS₂ films contained monolayer, bilayer and few-layer.

Inspired by using the thin Mo film as Mo precursor to synthesize atomic-layered MoS₂, Lin et al.^[62]presented that wafer-scale MoS₂ atomic thin layers can be achieved by a simple and scalable two-step annealing process to transform MoO₃ layers into MoS₂ layers. Firstly, a thin layer of MoO₃ (approximately 3.6 nm) was pre-deposited on a bare sapphire substrate via thermal evaporation, and then the coated MoO₃ was reduced to MoO₂ or other suboxides at 500 ℃ for 1 h in Ar/H₂ atmosphere during the first annealing process. In the second thermal process, the sample was annealed at 600 Torr and 1 000 ℃ for 30 min in a sulfur-rich environment. Further HRTEM, Raman, PL and AFM characterizations shown that these obtained films were polycrystalline and uniform few-layer. In addition, these MoS₂ films were thickness controllable and can be easily transferred to other substrates, making these films favorable for flexible electronic and optoelectronic devices.

Instead of using solid MoO₃ powder as a Mo precursor, Lee et al.^[63]developed a novel and simple way for using a low concentration of MoO₃ solution as Mo precursor to lead a relatively low supersaturation state of the vapor reactants, resulting in a reduced nucleation density, which was vital for growth of large domain size MoS₂. Adopting this proposal, a thin precursor film containing a small amount of MoO₃ (≈0.01 mg) was prepared on SiO₂/Si as a source substrate (Fig. 16(a), (b)), which can decrease nucleation density to 32 nuclei mm^-2 over the whole substrate surface at a high temperature of 800 ℃. Due to the relative low nucleation density and proper growth rate, a highly crystalline and centimeter-scale MoS₂ monolayer can be grown directly on the target substrate, and a large size (500 μm) MoS₂ single crystals can be observed in the edge of the substrate (Fig. 16(c), (d), (e)). In addition to this, they discovered that the monolayer MoS₂ possessed an outstanding optical property with a relative small full-width-half-maximum (FWHM) of 48 meV (Fig. 16(f)), which was comparable to or even better than that of exfoliated single-crystal MoS₂ monolayers.

2.4.3 MoO₂

As an another kind of molybdenum oxide, MoO₂ can also be applied to prepare highly crystalline MoS₂. For example, Wang et al.^[64] reported a fresh method for synthesis of rhombohedral MoS₂ flakes via sulfurizing of MoO₂ nanoplates (Fig. 17(a)). In the first step, evaporated MoO₃ reacted with S at 650-850 ℃ to form the MoO₂ nanoplates on SiO₂/Si substrate, and then the MoO₂ templates were secondary annealed at 850-950 ℃ for 0.5-6 h in a sulfur atmosphere. During this process, varied numbers of MoO₂ layers can be sulfurized to MoS₂ depending on the annealing time (Fig. 17(b)). As shown in Raman mapping (Fig. 17(c), (d), (e)), the obtained MoS₂ flakes were remarkably uniform and possess a domain size of ~10 μm. Because of high quality of the MoS₂ flakes, the carrier mobilities and on/off current ratio of back-gated FETs fabricated by the obtained MoS₂ was comparable to that of prepared by mechanical cleavage. This novel strategy paves a new way for synthesis of highly crystalline MoS₂ with controlled number of layers, making this material a promising material for various future applications.

2.4.4 MoCl₅

As we discussed in Section 2.3.1, MoO₃ powders will be poisoned during the process of sulfurization and cannot evaporate incessantly to effectively grow large domain MoS₂. Furthermore, toxic and corrosive SO₂ gas will be generated during the sulfurization of MoO₃. Compared with MoO₃, MoCl₅ has a lower sublimation temperature, thus it cannot be sulfurized and poisoned before migrating to the high temperature growth zone.

Using MoCl₅ as Mo precursor, Yu et al.^[65]developed a controllable strategy to prepare high-quality monolayer and few-layer MoS₂ films with great uniformity. Typically, the MoCl₅ and sulfur vapor can react to form MoS₂ species at elevated temperature, which might deposit onto the receiving substrates to produce MoS₂ films subsequently. It should be pointed out that the number of the obtained MoS₂ layers can be accurately modulated through changing the dosage of MoCl₅ precursor used in experiments (Table 2). In addition, they had also successfully synthesized uniform MoS₂ films on diverse substrates including SiO₂/Si, sapphire and graphite. It was important to highlight that this synthetic method can be scaled up for the applications of electronics and optoelectronics.

2.4.5 (NH₄)₂MoS₄

Pyrolysis of the precursor containing Mo and S is also a good idea to synthesize layered MoS₂. As a typical example, Liu et al.^[66] developed a two-step decomposition of (NH₄)₂MoS₄ to synthesize highly crystalline MoS₂ thin film on insulating substrates (Fig. 18(a)). After the first thermal process under 500 ℃ for 1 h, the crystal structure of the obtained MoS₂ was not very good, and then the sample was placed into the furnace chamber for annealing to improve the quality of MoS₂ in an atmosphere of sulfur vapor. Notably, the introduction of sulfur during the second annealing observably improved the quality of MoS₂ film (Fig. 18(b), (c), (d)). Furthermore, these MoS₂ thin films on the growth substrate can be transferred with ease to other substrates. It was noteworthy that the bottom gate FET made by these MoS₂ thin layers displayed n-type behaviors with carrier mobility up to 6 cm²/(V·s) and the on/off current ratio ~10⁵, which was comparable with the FET devices fabricated by the mechanically exfoliated MoS₂.

2.4.6 Mo(CO)₆

Beyond solid and liquid Mo precursor, Mo(CO)₆ can be a special gaseous Mo precursor for the controllable growth of MoS₂. Recently, Kumar et al.^[67] demonstrated a simple CVD strategy using Mo(CO)₆ and H₂S as precursors (Fig. 19(a)), which could achieve synthesis of uniform n-layered (n = 1, 2, 3, … >6) MoS₂ films (Fig. 19(d)) on a variety of substrates, including SiO₂/Si, sapphire and fused quartz. In this growth system, the low boiling point of Mo(CO)₆ (~156 ℃) and H₂S made these ideal precursors for the growth of MoS₂ by CVD. It should be pointed out that all precursors (Mo(CO)₆ and H₂S) were introduced into the reaction chamber in the form of vapor phase, in which case we can easily control the concentration of the precursors and then reduce nucleation to form large domain and high quality MoS₂. The layered MoS₂ grown on the substrates has been researched by AFM (Fig. 19(c)) and Raman (Fig. 19(e)) to confirm the number of layers, their crystallinity and uniformity. In addition, through SEM characterization (Fig. 19(b)), they found that the edge length of monolayer single-crystalline MoS₂ flakes grown under relatively low pressure can reach as large as 25 μm. Because of the Mo vacancy, the as-prepared MoS₂ films were p-type semiconductors, whose field effect and Hall hole mobilities can up to 2.4 cm²/ (V·s) and 44 cm²/(V·s), respectively.

According to the above discussion, it is very clear to see that the species of Mo precursors are extremely vital to the growth of high quality MoS₂ since various Mo precursors have different sublimation temperatures, that means, during the growth process, the vapor concentration of Mo species above the substrates is diverse, which will determine the nucleation density of MoS₂. Generally, achieving a relatively low nucleation density is favorable to decrease the grain boundaries and improve the quality of the obtained MoS₂. Moreover, compared to solid and liquid Mo precursors, selecting a gaseous precursor like Mo(CO)₆ will make the synthesis process more controllable, for which is introduced into the reaction chamber in the form of vapor phase, thus we can easily control the concentration of the precursor and then reduce nucleation to form high quality MoS₂ films. In addition, during the growth of layered MoS₂, the Mo precursors should evaporate continuously to feed Mo source in the gas phase for achieving large size single-crystal MoS₂ domains and to avoid being poisoned.

3 Conclusion and Perspectives

At present, atomically thin MoS₂ has been achieved by various "top-down" and "bottom up" strategies, and applied in many fields, such as electrical, optical, catalysis and photovoltaic devices. Notably, atomically thin MoS₂ layer exhibits great potential of being fabricated into electronics due to its semi-conductive property, which is one of the primary advantages of MoS₂ over graphene, since the zero bandgap makes graphene not suitable for the fabrication of logic circuits. Thus, MoS₂-based electronics show great potential for future applications. In addition, stacking other kinds of 2D materials with MoS₂ for the research of new properties or phenomena has also attracted a lot of interest. Thus, developing a simple, controllable and scalable method to synthesize high quality MoS₂ is of great importance for future applications. As we all know, the CVD method is a preponderant bottom-up strategy used to prepare atomically thin nanomaterials in terms of scalability, simple operation and low cost, which are suitable for electronic and optoelectronic devices. Bearing this in mind, we provided a systematical overview about the preparation of single or few layered MoS₂ by CVD in recent years and highlighted that we can achieve high-quality single or few layered MoS₂ by engineering the main experiment parameters like growth substrates, carrier gases and Mo precursors for future various applications.