Synthesis Methods of Two-Dimensional MoS 2 : A Brief Review

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.


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 (MoS 2 ) 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 MoS 2 .The paper focuses only on the production techniques rather than serving as a complete review on MoS 2 -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 MoS 2 .It has a high potential for producing MoS 2 with better quality and controllability for nanoelectronic device applications.

Synthesis Methods of MoS 2
MoS 2 has several polymorphs, where the 2H MoS 2 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 MoS 2 is an indirect bandgap semiconductor (~1.29 eV).With decreasing thickness, the bandgap increases.When it comes to monolayer MoS 2 , 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 MoS 2 can be divided into top-down and bottom-up approaches.We hereby describe them in turn.
Crystals 2017, 7,198 2 of 10 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.

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.

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 µ m) 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

Micromechanical Exfoliation
Similar to the micromechanical exfoliation of graphene (a vivid lab demonstration can be found in [10]), MoS 2 flakes can be produced on a substrate using sticky tapes.The starting material is bulk MoS 2 , 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 MoS 2 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 MoS 2 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 SiO 2 , 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 MoS 2 flakes can be exfoliated by slightly modifying the technique.Magda et al. obtained mono MoS 2 layers with lateral size of several hundreds of microns by improving the MoS 2 -substrate adhesion, as shown in Figure 2 [15].The mechanism is that sulfur atoms can bind more strongly to gold than to SiO 2 .The drawback is that such as-exfoliated MoS 2 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 MoS 2 synthesized by this method is limited to fundamental study at lab scale.
Crystals 2017, 7,198 3 of 10 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.

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
Liquid phase exfoliation also starts from bulk MoS 2 , 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 MoS 2 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 MoS 2 layers and enlarge the interlayer spacing, easing the following exfoliation by mechanical treatment (e.g., sonication).Fan et al. developed a process to exfoliate MoS 2 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 MoS 2 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 MoS 2 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.

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.

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 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, MoS 2 thin films are prepared by Walia et al. [30][31][32], where optical and electrical properties can be studied.

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 MoS 2 -Ti composite by vacuum sputtering, using Ti and MoS 2 as the targets [34].However, the MoS 2 is an amorphous structure in this case.

Solution Chemical Process
There are various methods for producing MoS 2 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 MoS 2 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 (NH 4 ) 2 MoS 4 (ammonium tetrathiomolybdate), or something similar.(NH 4 ) 2 MoS 4 decomposes to form MoO 3 at 120-360 • C, which can be further converted into MoS 2 .In some cases, electrochemical deposition [42][43][44] and photo-assisted deposition are used [45].For instance, Li et al. fabricated MoS 2 nanowires and nanoribbons using MoO x as the precursor [43].The MoO x precursor is then exposed to H 2 S at high temperature (500-800 • C) to synthesize MoS 2 .Depending on the temperature for processing, both 2H and 3R phase of MoS 2 can be synthesized.The MoS 2 nanoribbons can be 50 µm in length and are aligned in parallel arrays which are preferable for device fabrication, as shown in Figure 4. 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 MoOx as the precursor [43].The MoOx 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 µ m in length and are aligned in parallel arrays which are preferable for device fabrication, as shown in Figure 4.

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
( 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.

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 MoS 2 will enable the batch fabrication of atomically thin devices and circuitry.Currently, most works start with solid state precursors of molybdenum, such as MoO 3 [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 H 2 S gas [49] or vaporized S [46][47][48]50].It is argued that using MoCl 5 precursors are easier to achieve monolayer MoS 2 over large area [50].Earlier, the MoS 2 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: (1) 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 MoS 2 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 MoS 2 thin films [49,[52][53][54].Kang et al. [52] have grown MoS 2 and WS 2 on 4-inch oxidized silicon wafers by MOCVD.The MoS 2 is grown with gaseous precursors of Mo(CO) 6 (boiling point 156 • C) and (C 2 H 5 ) 2 S. At 550 • C, monolayer MoS 2 with full coverage of the substrate (typically SiO 2 /Si and fused silica) is deposited for t 0 ~26 h. Figure 5a shows scanning electron microscopy (SEM) images of the samples, demonstrating the effect of adding H 2 in the gas mixture.It is argued that H 2 enhances the decomposition of (C 2 H 5 ) 2 S (increasing nucleation due to hydrogenolysis) and etches MoS 2 (preventing intergrain connection) [52].It seems that a low H 2 rate is beneficial for growing continuous monolayer MoS 2 .Nevertheless, H 2 is necessary for removing the carbonaceous species generated during the MOCVD.Therefore, the H 2 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 < t 0 there is almost no nucleation of the second MoS 2 layer, which takes place mainly at the grain boundaries of the first layer when t > t 0 .Figure 5c shows a top gated (V TG ) MoS 2 transistor with a field-effect mobility of 29 cm 2 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 MoS 2 transistors.Figure 5d-f demonstrates the optical absorption, Raman and photoluminescence spectra measured from the MOCVD grown MoS 2 (and WS 2 ) films, respectively, where the features agree well with the characteristics of exfoliated monolayer counterparts (denoted by diamonds).
Crystals 2017, 7, 198 6 of 10 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 (VTG) MoS2 transistor with a field-effect mobility of 29 cm 2 V −1 s −1 and transconductance 2 µSµm, determined from the transfer properties.The current saturation occurs at relatively low VSD (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 At this stage, the material quality in [53] is still too low to be competitive.The large-area deposition of monolayer MoS 2 as well as other TMDs with spatial homogeneity and high performance is still challenging.MoS 2 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 MoS 2 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 MoS 2 , such as in [56].Using MoCl 5 (vaporized) and H 2 S precursors, monolayer to few-layer MoS 2 on large SiO 2 /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 2 ) [56].It is found that photoresist residues on the surface lead to more nucleation sites for MoS 2 , but this mechanism remains to be confirmed.Figure 6a plots Raman signals for 50 cycle ALD MoS 2 , where the E 1 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 (H 2 S 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 E 1 2g peak is close to that of exfoliated MoS 2 monolayer (4.2 cm −1 vs. 3.7 cm −1 ).By analyzing the separation ∆ between E 1 2g and A 1g peaks, one can tentatively determine the number of layers in MoS 2 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.
Crystals 2017, 7, 198 7 of 10 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 E 1 2g and A1g 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 E 1 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 E 1 2g and A1g 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.

Conclusions
In this paper, we have reviewed the synthesis methods of MoS 2 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.MoS 2 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 MoS 2 films oriented towards real electronic device applications in the future.In most cases the MoS 2 films are transfer-free, which is vital for industrial applications that require a high process controllability and reproducibility [57].At the moment, production of MoS 2 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.

Figure 5 .
Figure 5. (a) Grain size evolution of monolayer MoS2 as a function of H2 flow.From left to right, 5, 20 and 200 sccm H2.(b) Optical images of MoS2 films as a function of growth time.t0 is the optimal time for full monolayer coverage (~26 h).(c) Transfer and output properties of the field-effect transistor fabricated in the MOCVD grown MoS2 with top gate configuration.σ□ denotes sheet conductance.Scale bar: 10 µm.(d)-(f) Optical absorption, Raman and normalized photoluminescence spectra of as-grown monolayer MoS2 (red) and WS2 (orange) films.The peak positions in (d)-(f) are consistent with exfoliated flakes (denoted by diamonds) [52].

Figure 5 .
Figure 5. (a) Grain size evolution of monolayer MoS 2 as a function of H 2 flow.From left to right, 5, 20 and 200 sccm H 2 .(b) Optical images of MoS 2 films as a function of growth time.t 0 is the optimal time for full monolayer coverage (~26 h).(c) Transfer and output properties of the field-effect transistor fabricated in the MOCVD grown MoS 2 with top gate configuration.σ denotes sheet conductance.Scale bar: 10 µm.(d-f) Optical absorption, Raman and normalized photoluminescence spectra of as-grown monolayer MoS 2 (red) and WS 2 (orange) films.The peak positions in (d-f) are consistent with exfoliated flakes (denoted by diamonds) [52].

Figure 6 .
Figure 6.(a) Raman spectroscopy measurements of ALD MoS2 at 375 or 475 °C , where the latter shows higher quality.(b) Raman spectra of 475 °C deposited MoS2 films (some are post annealed in sulfur environment), where 920 °C annealing leads to the best quality.The curve for bulk MoS2 is for reference.(c) E 1 2g to A1g peak separation Δ plotted against the cycle number in ALD.Δ~19 cm −1 tentatively indicates MoS2 monolayer, whereas bulk (>5 layers) material has Δ~25 cm −1 [56].

Figure 6 .
Figure 6.(a) Raman spectroscopy measurements of ALD MoS 2 at 375 or 475 • C, where the latter shows higher quality.(b) Raman spectra of 475 • C deposited MoS 2 films (some are post annealed in sulfur environment), where 920 • C annealing leads to the best quality.The curve for bulk MoS 2 is for reference.(c) E 1 2g to A 1g peak separation ∆ plotted against the cycle number in ALD.∆~19 cm −1 tentatively indicates MoS 2 monolayer, whereas bulk (>5 layers) material has ∆~25 cm −1 [56].