Growth and synthesis of mono and few-layers transition metal dichalcogenides by vapour techniques: a review

Abstract

Nanosheet materials such as graphene, boron nitride and transition metal dichalcogenides have gathered a lot of interest in recent years thanks to their outstanding properties and promises for future technology, energy generation and post-CMOS device concepts. Amongst this class of materials transition metal dichalcogenides based on molybdenum, tungsten, sulfur and selenium gathered a lot attention because of their semiconducting properties and the possibility to be synthesized by bottom up techniques. Vapour phase processes such as chemical vapour deposition permit to produce high quality layers and to precisely control their thickness. In order to target industrial applications of transition metal dichalcogenides it is important to develop synthesis methods that allow to scale up wafer size, and eventually integrate them with other technologically important materials. This review will cover all the currently proposed methods for the bottom up synthesis of transition metal dichalcogenides from the vapour phase, with particular emphasis on the precursors available and on the most common semiconductor techniques like metal organic chemical vapour deposition and atomic layer epitaxy. A summary of the most common characterization technique is included and an overview of the growth issues that still limit the application of TMD is given.

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Introduction

The improvement of electronic devices, their ubiquitous employments in modern society and the opportunities offered by information technology are driven by the continuous scaling down of transistors, mostly based on silicon. Currently, the most advanced computer processors are based on silicon metal-oxide semiconductor field effect transistors (MOSFET), with gate lengths of 22 nm.¹ Further improvements of device performance will soon approach the limits posed, amongst others, by quantum mechanics effects, by heat dissipation and by shrinking of the gate dielectric. In order to meet the requirements to fulfill Moore's law, scientists are investigating new materials and new concepts that will permit to go beyond the boundaries of current designs. Silicon-germanium (SiGe) and germanium are some of the candidates to push up device performances, since they have higher electron mobility and allows for lower voltages and reduced power consumption, tunneling, and leakage.² Strain engineering and high-k gate dielectrics are studied in order to enhance the possibilities offered by standard materials.³ Nanostructures and low dimensional materials are also being considered as candidates for novel devices, since they would permit to adopt radical new design concepts.⁴

Amongst all the proposed solutions, graphene is the one that attracted most attentions.⁵ Its exceptional mechanical, electronic, optical and transport properties have promoted a lot of fundamental and applied research, resulting in a nobel prize assignment and in the development of new physics. Its carriers have mobility of 10⁶ cm² V⁻¹ s⁻¹ and are Dirac fermions with zero effective mass, making possible to study phenomena related to the quantum mechanics theory in a condensed-matter playground. Integrated circuits based on graphene transistor were demonstrated and high-frequency operation up to gigahertz was proven.⁶ Graphene functionalization permitted to realize novel chemical and biological sensors⁷ and is also being considered for hydrogen conversion and storage.⁸

Despite all these opportunities, graphene has several major drawbacks such as the lack of an electronic bandgap that makes certain kind of applications, for instance logic circuits, very difficult to develop: a graphene transistor cannot have the two conventional on and off states, it cannot be completely switched off, it has more energy leakage and produces more heat. Bandgap opening is possible through bandgap engineering (by nanostructuring, chemical functionalization or by applying a high electric field) but this step adds another level of complexity and degrades material properties. Graphene intrinsic defects and the presence of surface adatoms degrades the carrier mobility. Moreover it is chemically inert and functionalization, necessary to make it reactive to selected molecules, results in loss or degradation of several of its properties. These drawbacks fostered the research on alternative materials with similar structure and characteristics.

There is a wide variety of materials that can be thinned down to monolayer like graphene, exhibiting strong in-plane bonds and weak interplane interaction. The list includes transition metal dichalcogenides (TMD), silicene,⁹ germanene¹⁰ and boron nitride.¹¹ The properties of these materials depend on their dimension rather than their lateral size and, due to their novelty and excellent electronic properties, they are considered an exiting field for new research.

TMD are materials with chemical formula MX₂, where M is a transition metal of group 4–10 (Nb, Ta, Mo, W, etc.) and X a chalcogen such as S, Se, Te. They are studied since long time and, in bulk form, were grown since 1970.^12–15 The typical crystal structure (Fig. 1) consists in a stack of a transition metal layer between two chalcogen layers and it can be described as a van der Waals solid with strong (covalent) bonds in 2D plane, while the interaction between different planes is much weaker. Their symmetry is hexagonal or rhombohedral, with atoms arranged in octahedral or trigonal prismatic coordination.

Fig. 1 Typical lattice stacking of transition metal dichalcogenide.

TMD bulk material can be exfoliated with different methods such as scotch tape,¹⁶ solvents¹⁷ or lithium intercalation.¹⁸ More than 40 combinations of M and X were observed,¹⁹ with very different characteristics and a large range of electronic properties from metallic to semiconducting to superconducting. Bulk properties differ significantly from the ones of low dimensional, few layer stacks of TMD: for example the bandgap may change from indirect (bulk) to direct (monolayer), resulting in strong enhancement of optical absorption and emission.²⁰ When TMD are thinned down to monolayers, they have the same honeycomb structure of graphene and posses very similar properties, but the most important difference is that they exhibit a finite direct bandgap, from 1 to 2 eV, making 2D TMD ideal candidates for electronic devices, logic circuits, detectors, optoelectronic, photovoltaic, energy storage and catalytic applications.²¹

Novel properties are observed in monolayer TMD, such as valley polarization: conduction and valence bands of TMD monolayers present two inequivalent valleys at points K and K0, giving a new degree of freedom to carriers. New device concepts can be based on valleytronics, addressing the carriers by their valley index and permitting a control of information using circular polarized light.²² Moreover, spin–valley coupling may occur, offering a feature that may lead to completely new device design.²³

TMD electronic and semiconducting properties (Table 1) make them ideal candidates for the realization of digital electronic devices, FET and transistor, with the possibility to achieve high on/off current ratio.²⁴ Important milestones were achieved with TMD, such as the development of a top-gated MoS₂ FET with mobility of 60–70 cm² V⁻¹ s⁻¹ and on/off ratio of 10⁸ at room temperature,²⁵ inverters with gain between 2 and 16,^26,27 phototransistors and photosensors,^28,29 gas and biological sensors.³⁰ The trend is now to integrate different building blocks into a single device and to fabricate high performance CMOS based circuits.³¹

Table 1 Electronic properties of nanosheet materials (adapted from ref. 37)

Material	Type	Monolayer bandgap (eV)	Mobility (cm^2 V^−1 s^−1)
Graphene	Semi metallic	—	2 × 10^5
Silicene	Semi metallic	—	1 × 10^5 (theory)
Germanene	Semi metallic	—	1 × 10^5 (theory)
BN	Insulating	5.2
MoS2	Semiconducting	1.8	200 (Monolayer)
MoSe2	Semiconducting	1.5	50 (exfoliated)
MoTe2	Semiconducting	1.1	50 (bulk)
WS2	Semiconducting	1.9–2.1	100–200 (bulk)
WSe2	Semiconducting	1.7	100–500 (bulk)
WTe2	Semiconducting	1.1	20 (bulk)
TaS2	Metallic super conducting	—
TaSe2	Metallic super conducting	—
NbS2	Metallic super conducting	—
NbSe2	Metallic super conducting	—

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The continuous advance of personal portable devices, smartphones over all, is pushing researchers to develop flexible high performance electronics. Low dimensional materials such as graphene and TMD have high mechanical strength and can be deformed up to 10% before breaking and, thanks to the low dimensionality, they can be deposited or transferred on flexible substrates: this makes TMD ideal candidates for the next generation of portable devices and indeed the first demonstration of flexible devices with these materials already occurred.^32,33

Nowadays most of the optoelectronic devices are made from III–V and III–N compounds. Their great success is due to the possibility to form ternary compounds such as In_xGa_1−xAs, In_xGa_1−xN, Al_xGa_1−xAs etc. with different composition. It is thus possible to fabricate lattice matched and mismatched heterostructures, to tune the bandgap, optical emission, absorption, electron properties, etc., and to realize complex devices such as LED, solar cells, photodetectors. TMD and 2D nanosheets such as graphene and boron nitride can be considered as a new class of materials that in principle would permit to obtain ternary compounds and stacked layers similar to the one realized for III–V compounds. Theoretical models predict that mix of TMD is thermodynamically stable³⁴ and the bandgap of a ternary alloy can be continuously tuned.^35,36 Stacks of exfoliated 2D material were combined to realize LED by van der Waals heterostructures.³²

Most of TMD devices developed so far were realized from mechanically exfoliated sheets. Although flakes quality is very high and excellent proof of concepts may be obtained in this way, the method pose critical limits on large scale production. In order to target TMD industrial applications it is mandatory to develop reliable and high-throughput deposition techniques on large area substrates, and compatibility with the current systems and processes could be a tremendous added value. Control of the precursors flow in the vapour phase through the use of gases or controlled evaporation of liquid precursors would permit to increase deposition reproducibility. Chemical vapour deposition (CVD) and atomic layer deposition (ALD) are nowadays the most used processes to realize almost all kind of electronic devices with the current used materials (Si, Ge, III–V, III–N, oxides, etc.): it would be highly desirable to develop TMD synthesis processes that share the same deposition equipments used for other compounds, since it could open the possibility to directly integrate TMD with other materials.

Not all TMD are technologically interesting, since most of them are not stable in standard conditions and reacts rapidly with atmosphere, losing their peculiar properties. The most promising, and the ones on which this review will focus, are MoS₂, MoSe₂, WS₂ and WSe₂, thanks to their peculiar electronic, optical and chemical properties and the possibility to synthesize them by bottom up techniques.

Several papers were already published on the most important properties, applications and synthesis of TMD^19,37–41 but a comprehensive review on the different vapour phase deposition techniques, in particular with focus on CVD and ALD has not been proposed, yet.

An analysis of the growth methods, processes and precursors for this novel class of 2D materials is extremely important in order to understand the best routes to obtain the highest quality material and the most feasible way to realize TMD nanosheets. For example, the controlled and reproducible synthesis of monolayer TMD with low defects and crystallinity, the deposition scalability to large area, the fabrication of TMD heterostructures and the possibility to integrate them with other materials are still open issues to be addressed. Providing technological solutions for the growth of TMD would pave way towards the realization of new applications and device based on these materials.

TMD characterization and properties measurements

TMD characterization, applications and device development have been deeply analyzed in dedicated reviews and papers, to which the reader is addressed for details.^{24,37,38,41–44} However, in order to give a complete overview of this class of materials, a brief summary of the most common characterization techniques will be provided in this section.

Raman spectroscopy and photoluminescence are considered the most useful and reliable techniques to identify the numbers of layer in both exfoliated and synthesized TMD. Optical interference and Atomic Force Microscopy (AFM) are also used to exactly determine the layer numbers but, although they can provide precise results, they have limited throughput and are more time consuming. In order to develop synthesis on large area substrates and to target mass production, fast and reliable characterization techniques are needed, and Raman and PL can successfully meet these requirements.

Raman spectroscopy is considered a powerful tool to determine the number of layers: the vibrational modes of TMD flakes are strongly influenced by the thickness, since any additional layer beyond the first interacts with the bottom layer and change energy, width and amplitude of the peaks.⁴⁵ Only two peaks are usually permitted in Raman backscattering geometry of few-layers TMD, namely E¹_2g and A_1g, related to the in-plane and out-of-plane vibrations of M and X atoms, respectively. As the layer number increase, the out of plane vibrations A_1g are dampened, leading to a blue shift. The E¹_2g peak, instead, undergoes a red shift due to the relaxation of in-plane phonon modes. Moreover, A_1g modes are sensitive to carrier density and doping, resulting in red shift and peak broadening, respectively.²⁴ Table 2 summarizes the expected Raman peak position of bulk and monolayer TMD.

Table 2 Raman peak of most common TMD in monolayer and bulk form

	Raman peak	Peak position (cm^−1)		Ref.
		1L	Bulk
MoS2	E^12g	384	383.5	45
	A1g	405	408.6
MoSe2	E^12g	287.2	286	45
	A1g	243	240.5–242.5
WS2	E^12g	356	355.8	46
	A1g	418	420
WSe2	E^12g	250	248	45 and 47
	A1g	250	250.2

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One of the most important characteristics of monolayer TMD is the strong enhancement of PL signal with respect to bulk emission.²⁰ As discussed in the previous section, bulk TMD are indirect bandgap materials: the indirect bandgap is due to a transition from the top of the Γ point valence band to the bottom of the conduction band, between Γ and K point. As the number of layers decrease down to one, the indirect bandgap energy increases and cross the direct gap, that remains in the same position. This is the reason for the strong enhancement of the emission signal, up to 10–20×, coupled to a peak blue shift (Table 3). Thanks to micro-PL mapping, different zones of a single flake can be analyzed in order to identify defects or the presence of additional layers.

Table 3 Main PL peaks of most common TMD in monolayer and bulk form

	PL peak position (eV)		Ref.
	1L	Bulk
MoS2	1.8	1.2	45
MoSe2	1.82	1.1	45
WS2	1.95	1.3	48
WSe2	1.65	1.2	45

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Methods for TMD nanosheets synthesis

Top down

Top-down methods for TMD nanosheets synthesis can produce high quality monolayer samples at low cost and are very convenient for fundamental research and for the realization of proof-of-concept devices.

There are basically two top down strategies adopted: mechanical exfoliation through Scotch tape and liquid methods, which can be subsequently divided in two categories: lithium intercalation and solvent-assisted sonication.

Since most TMD, and in particular MoS₂, MoSe₂,WS₂ and WSe₂, are available in bulk form, single layers can be exfoliated using the Scotch tape methods, similarly to what is done on graphene. Exfoliated flakes up to 10 μm wide and high crystallinity can be obtained and are usually identified using optical microscopy, considering the color contrast given by layers with different thicknesses on SiO₂ (Fig. 2).⁴⁹ However, despite these results, the scale up of Scotch tape method is extremely difficult and the throughput is very limited.

Fig. 2 Optical and atomic force microscopy images of dichalcogenide nanolayers deposited on 270 nm SiO₂ with corresponding contrast and height profiles of monolayers: (a)–(c) for MoS₂, (d)–(f) for WSe₂ and (g)–(i) for NbSe₂. Contrast and height profiles of monolayer flakes are taken across the black lines drawn on optical images, and red lines on AFM images. Measured thicknesses correspond well with interlayer distances in dichalcogenide crystals. Observed optical contrast is in the 25–30% range for MoS₂ and WSe₂ and is slightly lower that the values predicted in the model. In the case of NbSe₂ optical contrast is in the 5–10% range.⁴⁹ © IOP Publishing. Reproduced with permission. All rights reserved.

TMD can be exfoliated by ultrasonic treatment in appropriate solvents or by using surfactants (Fig. 3):¹⁷ sonication usually results in crystal flakes with dimension of a few hundreds nanometers^50,51. A more viable solution is to utilize lithium containing compounds that intercalate into atomical layers, followed by a reaction in water, that separates the sheets by producing H₂.^18,52 Although exfoliation in liquid permits to obtain high yield of monolayers dispersed in a solution that can be adopted for applications such as electrochemical energy storage, catalysis, sensing, in ink-jet printing or to prepare composites and hybrids by mixing different solutions with different materials, its main disadvantages are the small size of the obtained flakes, that can limit some applications where large area flakes or films are needed. The challenges to be addressed are the yield enhancement of single layers (most of the flakes are few-layers) and the control of the lateral dimensions of the exfoliated sheets.

Fig. 3 Schematic description of the main liquid exfoliation mechanisms. (A) Ion intercalation. (B) Ion exchange. (C) Sonication-assisted exfoliation. From V. Nicolosi, M. Chhowalla, M. G. Kanatzidis, M. S. Strano, and J. N. Coleman, “Liquid Exfoliation of Layered Materials,” Science, 340, no. 6139, 1226419, 2013. Reprinted with permission from AAAS.

Since this review will mainly focus on gas phase methods, the reader is addressed to other reviews for details about the above cited top down methods. For example MoS₂, MoSe₂, WS₂, NbSe₂, WSe₂, VSe₂ and MoTe₂ obtained by mechanical exfoliation are reported in ref. 53–57; a comprehensive list of solvents used for the liquid exfoliation is cited in ref. 50 while lithium intercalation is discussed in ref. 58 and 59.

Bottom up

In order to integrate TMD in existing electronics and to design new kind of devices it is mandatory to obtain a good control in terms of number of layers (one layer as opposed to few-layers), crystal quality (absence of defects, grain boundaries), lateral dimension of sheets and scalability to large area substrates. Top down methods, even if permit to produce very high quality material, fail to fulfill most of these requirements. For this reason several techniques were developed with a bottom up approach, starting from TMD constituent elements to controllably synthesize nanosheets over a given substrate.

Most of the proposed processes utilize one or two of the TMD constituent elements (metal and sulfur/selenium) in vapour form, transporting the selected species over the substrate with a carrier gas. This permit, in principle, to achieve a good control over the gas composition and, through flowdynamic optimizations obtained with chamber/tube design, to deliver the precursors in a homogeneous way over all the substrate area: large area scalability, uniformity and thickness control are routinely achieved for thin films deposited by CVD and can potentially be obtained also for monolayer TMD.

CVD is a widely used technique in material science and it is commonly employed to deposit a large class of semiconductors, metals, oxides, etc. on a variety of substrates. Only recently it was applied for the synthesis of low dimensional materials, such as graphene and TMD. Li et al.⁶⁰ were the first to demonstrate large scale deposition of graphene utilizing methane on copper foil and later a variety of papers followed on TMD and boron nitride nanosheets.

Depending on the particular setup, the gas flow can be carefully controlled through the chamber design and a good flowdynamic optimization can, in principle, guarantee an efficient transport of species, good homogeneity, easily scale up to large area and excellent reproducibility. For these reasons CVD is one of the most promising techniques to synthesize monolayers TMD on large area with controlled thickness, good crystallographic quality and excellent electronic properties.

ALD is a modified CVD technique in which the two precursors are delivered separately in the growth chamber. It was invented by T. Suntola in 1974 (ref. 61) and was originally used to deposit ZnS film for electroluminescent devices. ALD is nowadays very common for oxides growth and for conformal deposition on nanostructures, patterned substrates, high aspect ratio structures such as trenches or pillars, etc., and it is largely used for the deposition of high-k oxides.

A single ALD growth cycle consists in: injection of precursor 1, purge with inert gas, injection of precursor 2, purge with inert gas. Excess reactants and reaction by-products are evacuated during purge cycles and the surface reactions during precursors supply are self-limited: if the substrate surface is sufficiently reactive to the precursor and if the precursor does not bond with itself, one ALD growth cycle should in principle deposit a single monolayer of the selected specie: uniform films with excellent conformality can be thus deposited even onto complex-shaped large area substrates. For this reason, ALD should be considered as one of the most promising technique for the deposition of TMD, because theoretically it would permit to tailor the number of deposited layers by defining the number of cycles. Despite its potential, ALD is still not so widespread for TMD, mainly because of a lack of suitable chemistry.⁶² Nevertheless, several groups used ALD also for the deposition of metals or metal oxides of controlled thickness to be further sulfurized or selenized by other techniques.

Before discussing the details of the different synthesis procedures, it is to be noted that most of these techniques share common precursors: it is thus very important to understand which kind of chemicals are available for TMD CVD and their physical properties. Beside the most commonly used S or Se powders or H₂S gas for chalcogens, there are also less common chemicals worth to be mentioned. Also, M or MO_x may be deposited on a substrate for further selenization/sulfurization: knowledge on how to control their deposition is often mandatory to obtain monolayer TMD.

The following section will exhaustively discuss the precursors used for TMD synthesis by CVD. To the best of author's knowledge this summary was never proposed before. The main focus will be on S, Se, Mo and W because MoS₂, MoSe₂, WS₂ and WSe₂ are considered to be the most technologically relevant compounds for the realization of devices.

Precursors

Sulfur

Sulfur powders. Sulfur powders are the most used precursor to deliver S to the substrate. S powders are available with high purity (up to 99 998%), can be easily handled and are non-toxic: for this reason they are preferred to H₂S. The material safety data sheet (MSDS) of S reports skin irritation as the only hazard, requires to avoid contact with skin and eyes, formation of dust and aerosols and indicates to provide appropriate exhaust ventilation in places where dust is formed.

S powder melts at 115 °C and evaporates to gaseous sulfur, with vapour pressure reported in Fig. 4: knowing the value of this parameter permits, in principle, to get a better control of the gas phase and to deliver a controlled quantity of precursor towards the substrate. However, the quantity of vapourized sulphur depends also on the mass of powder put in the crucible: for this reason the use of powders as main reagents, even if it is very common due to its simplicity, may pose some problems for reproducibility and control of the deposited layers.

Fig. 4 Sulfur vapour pressure vs. temperature (adapted from ref. 63).

H₂S. Hydrogen sulfide is a colorless gas with characteristic odor of rotten eggs; it is heavier than air, very poisonous, corrosive, flammable, and explosive. It is used as a source for S in the deposition of TMD but its handling requires particular attentions and safety measures (Table 4). MSDS of this substance describes the consequences of exposure to eye, skin and inhalation and should be carefully readed. Flammable range of H₂S is between 4.3% and 46% in air and poses an immediate fire hazard when mixed with air. Distant ignition and flashback are possible and flames or high temperature exposure of the cylinder can cause an explosion. Although its odor is very pungent, it quickly deadens the sense of smell, so the victims may be unaware of its presence until it is too late.

Table 4 Sulfur exposure limits

	Limit (ppm)
Short-term exposure limit (STEL)	15
Immediately dangerous to life and health (IDLH)	100

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For all these reasons, special care should be taken to design a reactor with H₂S line⁶⁴ and it is not so commonly used for the synthesis of TMD but, despite safety drawbacks, several research groups utilize it because the delivery of a gas to the growth chamber is much more controllable than evaporation of solid sources, ensuring higher reproducibility of the process.

Thanks to its fast decomposition at high temperature, H₂S should be more efficient that elemental S in the growth of TMD, and should help in improving uniformity.⁶⁵ Partial pressure of H₂S in the gas phase during TMD growth is usually in the range 10⁻¹ to 10⁻².^66,67 H₂S may also act as reducing agent for metal oxides.

Other sulfur precursors. A few less common S precursors are reported in literature. For example dimethyl disulfide (CH₃SSCH₃, DMDS) was used with Mo(CO)₆ to grow by ALD MoS₂ on SiO₂ (ref. 62) and 1,2-ethanedithiol (HS(CH₂)₂SH), 2-methylpropanethiol (HSC(CH₃)₃) were used for the CVD of WS₂ thin films on glass.⁶⁸ The sources were heated to 63 °C and 55 °C, respectively. In the cited paper thick nanocrystalline film were deposited: no reports about mono or few-layer of WS₂ were found in literature using these precursors.

Selenium

Selenium powders. Selenium powders are the most common precursor used for the selenization of metals or metal. They are easily handled and are available in pellets (<5 mm particle size) or powders with purity up to 99 999%. On the contrary to S, Se powders must be properly handled because Se is toxic by inhalation and if swallowed. It has severe danger of cumulative effects if the operator is exposed for long time and it must be correctly disposed because it may cause long-term adverse effects in the aquatic environment. Danger of acute Se exposure are described in the MSDS and safety handling precautions must be adopted.

Brooks⁶⁹ studied the vapour pressure of solid selenium with quartz Boudon gages, reported in Fig. 5.

Fig. 5 Selenium vapour pressure (adapted from ref. 69).

H₂Se. Hydrogen selenide is a colorless, flammable gas under standard conditions, heavier than air. It is reported to be the most toxic selenium compound and it must be treated and handled with extreme attention, because it may explode in contact with water, it is a flammable gas and may cause flash fire. Due to its high toxicity (Table 5) particular care should be taken to handle the exhaust gases from the growth chamber. Even at extremely low concentrations H₂Se has a very irritating smell, resembling decayed horseradish, and smells of rotten eggs at higher concentrations. Concentrations of 0.3 ppm are readily detected by the nose, but there is no noticeable irritant effect at that level. Concentrations of 1.5 ppm or higher are strongly irritating to the eyes and nose.

Table 5 H₂Se exposure limits

	Limit (ppm)
8 hour period	0.05
Immediately dangerous to life and health (IDLH)	100

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However, like H₂S, the odor in concentrations below 1 ppm rapidly disappears because of olfactory fatigue.

MSDS of this compound reports that it is deadly poison by inhalation and a very poisonous irritant to skin, eyes, and mucous membranes. It may also cause central nervous system effects. It has been found that repeated exposures to concentrations of 0.3 ppm are fatal to experimental animals by causing a pneumonitis and injury to liver and spleen.

Due to its high toxicity and the ready availability of safer alternatives, H₂Se is usually not used for the synthesis of TMD, even if it can react with W to form WSe₂ according to the reaction (1):⁷⁰

Bozheyev et al.⁷¹ have used H₂Se as a selenization agent to prepare WSe₂ thin films 600 nm thick, starting from selenium-rich WSe_2−x deposited by reactive magnetron sputtering but, at the best of author knowledge, no other paper was found employing H₂Se for nanosheet TMD synthesis.

Other Se precursors

Beside Se powders several groups are investigating the use of alternative Se precursors. As discussed above, it not straightforward to control Se supply by the use of powders and reproducibility could be an issue. Liquid Se precursors kept at a definite temperature permit to get a precise control of the vapour pressure and to deliver a well-defined amount of reagent towards the growth chamber by the use of a carrier gas.

The synthesis of WSe₂ was carried out via metal organic chemical vapour deposition (MOCVD) using tungsten hexacarbonyl W(CO)₆ and dimethyl selenide (CH₃)₂Se as the W and Se sources.⁷² Dimethyl selenium is a stable colourless liquid with an unpleasant odour, which hydrolyzes in water and reacts violently with acids and strong oxidizing agents. Its boiling point is 56–58 °C so it can be conveniently used in a MOCVD system. Its vapour pressure was determined by Karlson et al.⁷³ and reported in Fig. 6.

Fig. 6 Vapour pressure of DMSe and DMDSe (adapted from ref. 73).

Film quality and carbon contamination depends strongly on (CH₃)₂Se purity: 99.99% pure (CH₃)₂Se performs better than 99% pure precursor.⁷²

Diethyl selenide is another metal organic compound that was used to grow WSe₂ thin films⁷⁴ but not yet used for the synthesis of nanosheet TMD. It was heated to 60 °C and delivered to the growth chamber by N₂. Its vapour pressure was studied by Tanaka et al.⁷⁵ It should be noted that chemical reaction of WSe₂ and diethyl selenide could produce H₂Se, so waste gas must be properly handled.

Di-tert-butylselenide and diethylselenide were used by Boscher et al. to deposit MoSe₂ films using MoCl₅ as the other precursor⁷⁴ and VSe₂ using VCl₄ and VOCl₃.⁷⁶ In these works the focus was on thin film and not on monolayers: no other paper was found, on the best of author knowledge, about the use of these precursors for 2D TMD.

Mo

Depending on the growth method, Mo can be delivered to the growth chamber starting from metal oxide powders, from vapourized liquids or can be directly deposited in form on thin metallic or metal-oxide layer on the substrate. The use of powders poses the same problems already discussed for S and Se: reproducibility and large scale homogeneity could be an issue. The direct deposition of Mo onto the substrate is in principle the most controllable method, but a precise control of Mo thickness must be achieved. It is often reported that the synthesis of mono or few-layers MoS₂ or MoSe₂ depends on the thickness and homogeneity of the initial Mo deposition. Several techniques are available and will be discussed in the following paragraphs to assess this problem, and the same applies to W, too. Thermal decomposition of thiosalts is another route to coat the substrate with a controlled layer of Mo.

Evaporation of liquids, solids or metal-organic Mo compounds such as MoCl₅, MoF₆, Mo(CO)₆ could be one of the most straightforward methods to integrate Mo into a standard CVD system and to obtain good reproducibility and homogeneity over a large area.

MoO_x powders

In its pure form, molybdenum is a silver-grey metal with melting point of 2623 °C. It starts to weakly oxidate at 300 °C and bulk oxidation starts at temperatures above 600 °C, resulting in molybdenum trioxide.

MoO₃ is produced either by heating MoS₂ or by acidification of aqueous solutions of sodium molybdate with perchloric acid according to the reaction (2):⁷⁷

MoO₃ powders are available with purity up to 99.98%. The vapour pressure of MoO₃ is reported in Fig. 7.

Fig. 7 Vapour pressure of MoO₃, adapted from ref. 78.

Mo and MoO_x deposited onto a substrate. In order to achieve a better control over the number of deposited layer and to scale up to large area substrates one possible route is to start from a controlled Mo or MoO_x layer deposited onto a substrate and then convert the metal to MoS₂ or MoSe₂ through a controlled sulfurization or selenization. Since the final number of layer of the TMD is controlled by the number of layers initially deposited on the substrate, it is extremely important to get a precise control over the metal thickness and a good homogeneity over the substrate.

Several techniques are used to deposit a thin Mo film onto a variety of substrates (Si, SiO₂, sapphire, Cr, Au, etc.), the most common being e-beam evaporation and sputtering. The thickness of the deposited layer is usually monitored through a quartz microbalance.

An alternative starting layer is MoO₃ deposited by evaporation,⁷⁹ because this oxide has a lower evaporation temperature with respect to Mo and it is more easily sulfurized/selenized. In order to increase reliability and control over the number of Mo layers, several groups used ALD with different kind of precursors: this is in principle the best technique control the thickness. There are different precursors available for CVD deposition of either Mo or MoO₃ layers, which will be discussed in the following paragraphs.

Mo(CO)₆. Molybdenum hexacarbonyl‎ (Mo(CO)₆) can be used with water and ozone to deposit thin MoO₃ films through ALD. This precursor is solid at room temperature and very volatile, with a melting point of 150 °C and boiling point of 156 °C. It has a vapour pressure of about 0.10–0.15 mbar at room temperature, so it is a convenient precursor for CVD.

Diskus et al.⁸⁰ used Mo(CO)₆ to deposit MoO₃ layers on single crystalline Si(111) and soda lime glass. Mo(CO)₆ was kept at room temperature and delivered to the growth chamber with the use of N₂. An onset decomposition of 163 °C was determined for Mo(CO)₆. After 1000 cycles the film thickness was between 35 and 75 nm: fine control over layer thickness can thus in principle be obtained.

MoCl₅. Molybdenum(V) chloride (MoCl₅) is a volatile solid that melts at 194 °C and boils at 268 °C, available with purity up to 99.99%. Its vapour pressure is 1.75 torr at 25 °C and 131 torr at 250 °C. The compound is very air-sensitive and care should be taken to load the powder into the growth chamber.⁸¹ It is toxic by ingestion, an irritant to skin, and may react with water to produce corrosive hydrochloric acid and toxic fumes.

1–50 mg of MoCl₅ are generally used as powders for the growth of MoS₂ along with S powders^82,83 but MoCl₅ is also used for the ALD of MoS₂ with H₂S as the other precursor.⁸⁴ For the ALD process, in order to have a suitable vapour pressure, MoCl₅ source was kept at 120 °C.

Other Mo precursors. Molybdenum(VI) fluoride (MoF₆) is colourless solid, with a melting point of 17.5 °C and a boiling point of 34 °C. It is highly unstable toward hydrolysis and reacts violently with water, with the release of toxic HF and molybdenum oxides.

It was used as precursor for ALD of atomic Mo layers with Si₂H₆ as the other reactant.⁸⁵ It was also used to deposit thin films of MoS₂ by CVD using H₂S as the other precursor^86,87 but no reports about the direct synthesis of low dimension TMD were found in literature.

Bis(tert-butylimido)-bis(diethylamido)molybdenum[(tBuN)₂Mo(NEt₂)₂] and bis(tert-butylimido)-bis(di-isopropylamido)molybdenum [(tBuN)₂Mo(NiPr₂)₂ were used for ALD of molybdenum nitrides⁸⁸ but no evidence of a suitable chemistry was found for sulfide/selenide compounds.

Thiosalts. Ammonium tetrathiomolybdate (NH₄)₂MoS₄ is a compound that contains both Mo and S, prepared from molybdate solutions [MoO₄]²⁻ with hydrogen sulfide in the presence of ammonia.⁸⁹ It is solid at room temperature, available with purity up to 99.99%. It is usually added to polar organic solvents such as dimethylformamide (DMF): the solution is spinned onto the substrate and used as starting point to form MoS₂ by sulfurization.⁹⁰ The drawback of this precursor is the difficulties to obtain a uniform (NH₄)₂MoS₄ film: the thickness of the deposited layer controls the number of MoS₂ layers that will be produced by the subsequent annealing and the scale up to large area is very difficult. Moreover, the presence of C residues from the solvents may reduce the incorporation of S, thus leading to defective or polycrystalline layers.

W

W is usually deposited in form of a thin layer on a substrate, used in form of oxide powders or delivered trough vaporization of solids or liquid to the gas chamber. All the argumentations discussed above for Mo remain valid for W.

W and WO_x deposited onto a substrate. A thin WO₃ layer can be deposited onto a substrate (SiO₂,sapphire, graphene/SiC) by thermal evaporation or sputtering.^91,92 ALD of W or WO₃ layers is reported with WF₆ (ref. 93–95) or W(CO)₆.⁹⁶ Although few works were found about the sulfurization/selenization of W or WO₃ layers deposited by ALD, the layer control, homogeneity and scalability offered by this technique could significantly help to further develop WS₂ and WSe₂ synthesis.

WO₃ powders. WO₃ is available in powders with purity up to 99 995%. Its melting point is 1473 °C and its vapour pressure is reported in Fig. 8. WO₃ are obtained from W ores by reactions with alkalis.

Fig. 8 WO₃ vapour pressure, adapted from ref. 97.

Other W precursors.
WOCl₄, WOCl₆, WCl₆. Tungsten hexacarbonyl (W(CO)₆), tungsten(VI) chloride (WCl₆) and Tungsten oxychloride (WOCl₄) were used as precursors for WS₂ CVD synthesis along with S sources (CH₃)₂Se, 1,2-ethanedithiol [HS(CH₂)₂SH] and 2-methylpropanethiol [HSC(CH₃)₃]. The W precursors were kept in stainless steel bubblers at temperatures of 165, 245 and 176 °C, respectively.^68,72

WCl₆, kept at 100 °C in a stainless steel bubbler, was used for CVD of WS₂ with H₂S.⁹⁸

Novel sources such as WH₂(iPrCp)₂ and WH₂ (EtCp)₂ were used for CVD synthesis of W containing films, including WO₃ and WC_x and WNC_x. They have the advantage to be oxygen free and to have a low onset temperature of 350–400 °C.^67,99 However, no reports about the use of these precursors for the direct synthesis of WS₂ or WSe₂ were found.

Precursors for other elements

MoS₂, MoSe₂, WS₂ and WSe₂ are the most widely synthesized compounds but there are also a few literature reports dealing with other TMD bottom up synthesis. For example Boscher et al.⁷⁶ tried to synthesise VSe₂ by atmospheric pressure CVD using vanadium tetrachloride (VCl₄), vanadium oxytrichloride (VOCl₃) and butyl selenide but no significant results were obtained.

MoTe₂ and ReS₂ were synthesized using respectively Te pellets with Mo powders¹⁰⁰ and Re and S powders.¹⁰¹

Methods

Table 6 summarizes the combinations of different metals and chalcogen precursors used in literature for the bottom up synthesis of TMD.

Table 6 Combinations of different precursors proposed for TMD synthesis

Chalcogen		Metal
		Metal deposited on substrate			Metal in vapour phase
		M	MOx	Thiosalt	MOx	Other
Powder	S	X	X	X	X	X
	Se	X	X		X
Gas	H2S		X			X
Other						X

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The methods currently used can be summarized in two big categories:

• synthesis of TMD from gaseous precursors in the vapour phase, supplied from powder, liquids or directly from H₂S.

• sulfurization or selenization of the metal deposited, in different forms, onto a substrate (thin metal or metal-oxide layer, thiosalt layer).

The most commonly adopted setups used for these methods are sketched in Fig. 9. In some cases standard CVD/ALD equipment is also used, in particular when liquid or gaseous precursor are adopted. In this case the use of mass flow controllers permits to control the gas phase composition more precisely. In the following paragraph the various methods will be discussed and analyzed.

Fig. 9 Schematics of the most common methods used to deposit TMD from vapour phase. (a) Metal (M) and Chalcogen (X) powders. (b) Metal or metal oxides deposited on substrate and chalcogen powders. (c) Metal or metal oxides deposited on substrate and chalcogen supplied as gaseous precursors. (d) Metal and chalcogen compounds supplied by gaseous precursors.

Synthesis of TMD with both precursors supplied from the vapour phase

Metal and chalcogen powders. Due to the simplicity of the setup, of the availability and ease of handling of the precursors, a lot of research groups still synthesize TMD using vapour phase reactions between MO_x and chalcogen powders: even if high quality flakes were obtained and several proof-of-concept devices were realized starting from materials deposited with these techniques, control of layers number, scalability to large area and reproducibility are still an issue.

For the growth of MoS₂ or WS₂ usually 0.5–1 grams of S powders are placed in a crucible 7–15 cm away from the substrate, heated independently at temperatures ranging from 100 to 200 °C and delivered, in form of vapours, to the substrate. Sulfur vapour pressure at this temperature corresponds to about 10⁻² -1 torr. Because sulfur evaporates very fast at high temperature, some authors report the use of two different crucibles, placed at different temperatures and distances from the substrate:¹⁰² the nearest crucible starts to supply S by sublimation at lower temperatures, while the second one supplies S via evaporation before the entire consumption of the first source at higher temperatures occurs. This design assures the continuous supply of reagent during the whole process.

Se powders (0, 5–1 g) are also usually placed upstream in the growth tube (10–20 cm away from the substrate) and Se vapours are transported to the substrate by a carrier gas. The powders are typically heated at 250–300 °C, corresponding to a vapour pressure in the range 10⁻² to 10⁻¹ torr, according to Fig. 4.

Since the synthesis of TMD occurs between 650 and 800 °C, careful furnace temperature profile control/measurement or additional heaters outside the main growth chamber are usually adopted to control the vaporization of S or Se powders at lower temperatures. Synthesis at both atmospheric pressure and low pressure (1–500 torr) is reported.⁴⁰

MoO₃ and WO₃ powders (about 0.5–1 g) for metal precursor supply are usually placed in a crucible in the high temperature zone of the growth furnace. Mo is usually placed near the substrate or directly underneath it while W, having a lower vapour pressure, is heated farther from the substrate to temperatures up to 1070 °C, some hundreds degrees higher than the temperature used to synthesize WS₂.^103,104

MoO₃ and WO₃ powders are initially reduced to volatile suboxides MoO_3−x or WO_3−x by S vapours and are then transported to the substrate, where they further react with S vapour to form MoS₂ or WS₂.^104,105

The proposed reaction mechanism for Mo is:

with the formation of MoO₂ as an intermediate phase when x = 1.

MoS₂ and WS₂ are usually grown in inert N₂ or Ar atmosphere, because S is a strong reducing agent and it is sufficient to reduce MoO₃ or WO₃ to suboxides. However, Se chemical reactivity is much lower than S one and a suitable reducer agent is required for the suboxide formation. For this reason the synthesis of MoSe₂ and WSe₂ from oxide powders necessarily requires the addition of H₂ to the gas phase: without H₂ the growth of MoSe₂ or WSe₂ does not occur.²⁷ Even the addition of some percent of H₂ to the gas phase is reported to be sufficient to produce MoO_3−x or WO_3−x and it may also have a role in the nucleation of flakes on the substrate.¹⁰⁶

Thermodynamic calculations have confirmed the importance of H₂ for the selenization of Mo and W, according to the reaction:¹⁰⁷

WO₃ + 3Se + H₂ → WSe₂ + H₂O + SeO₂ (4)

The reaction between metal and chalcogen powders may lead to the formation of nanoparticles or nanorods, as opposed to monolayer nucleation. In order to reduce the presence of unwanted structures and to promote large area growth of high quality and continuous monolayer TMD, pretreatment of the substrate surface with aromatic molecules such as reduced graphene oxide, perylene-3,4,9,10-tetracarboxylic acid tetrapotassium salt (PTAS), perylene-3,4,9,10-tetracarboxylic dianhydride (PTCDA) was proposed. These substances were diluted in water and a droplet was spun on the substrate surface, followed by drying at 50 °C.¹⁰⁵

A more comprehensive study about the role of seeding promoters for 2D growth of MoS₂ was proposed by Ling et al.,¹⁰⁸ by using different aromatic and inorganic molecules on the substrate. They verified that the use of PTAS as seeding promoter permits to obtain large area, continuous and high quality MoS₂ monolayers already at 650 °C, while only nanoparticles were achieved in the same conditions without PTAS. An optimum seed concentration of different aromatic organic molecules (Table 7) can also promote the growth of monolayer MoS₂, while the investigated inorganic particles were not effective. Several promoters may be thermally evaporated on the substrate, permitting to achieve a better control over TMD nucleation, even permitting to pattern the substrate, with respect to the use of aqueous solutions.

Table 7 Seeding promoters used in MoS₂ growth (adapted from ref. 108, reader is addressed to the reference for explanation of promoters abbreviations)

	Seeding promoter	Temperature	Thickness/quality
Organic	PTAS	>600	Monolayer/excellent
	F16CuPc	>430	Monolayer/excellent
	PTCDA	>450	Monolayer/good
	CuPc	>430	Monolayer/good
	DBP	350–450	Monolayer/good
	CV	205	Monolayer/good
	NAA	200	Monolayer/good
	Spiro-TPD	>280	Monolayer/fair
	TCTA	>410	Monolayer + multilayer/fair
	BCP	240/300	Monolayer + multilayer/fair
	TPBI	>350	Monolayer + multilayer/poor
	Spiro-2-NPB	>390	Monolayer + multilayer/poor
	Ir(ppy)3	>300	Monolayer + multilayer/poor
Inorganic	Al2O3		Nothing/bad
	HfO2		Nothing/bad
	5 Å Au		Particles/bad
	Silicon		Nothing/bad

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In order to control the nucleation of 2D TMD without the use of seeding layers or precursor, an appropriate cleaning and storage of the substrate may be required. Zande et al.¹⁰⁹ grew monolayer MoS₂ on SiO₂/Si by cleaning the substrate in acetone, isopropanol H₂SO₄/H₂O₂ (3 : 1) and 5 min of O₂ plasma in ultrahigh pure N₂. Higher monolayer yield was obtained by using fresh precursors and ultraclean substrates, with minimized air contamination during storage: the use of uncleaned substrates and/or old precursors reduced significantly yield and reproducibility.

Another proposed powder for the growth of MoS₂ is MoCl₅,¹¹⁰ placed in the hot zone of the furnace and several cm away from the substrate, with 1 g of sulfur powder used as sulfurizing agent. The number of MoS₂ layers was found dependent on the weight of MoCl₅ placed in the crucible.

Bulk MoTe₂ was synthesized using Mo powder and Te pellets¹⁰⁰ at a temperature of 1150 °C. A long growth (1.5 weeks) and a controlled cooling permitted to obtain crystals that were later exfoliated by scotch tape and transferred to SiO₂/Si wafers. Single crystals of ReS₂ were grown with Br₂ as a transport agent¹⁰¹ at 1060–1100 °C. The bulk crystal was exfoliated to bilayers for further analysis.

By this method vertical and in-plane heterostructures from WS₂/MoS₂ monolayers were obtained:¹¹¹ MoO₃ powders were placed in front of the wafer, while a mix of W and Te were deposited on it for the growth of WS₂. The addition of Te promotes W melting during the growth. S powders were put in a zone at lower temperature. MoS₂ and WS₂ were grown sequentially thanks to the difference in their nucleation and growth rates, and the formation of Mo_xW_1−xS₂ was avoided by a precise control of the reaction temperature: vertically stacked bilayers are preferred at 850 °C, while in-plane lateral heterojunctions appear at 650 °C.

Metal and chalcogen compounds supplied directly in gaseous phase. In order to achieve a better control of chalcogen supply, a possible route would be the use of H₂S or other S/Se precursors. Nevertheless, no works in literature were found using metal powders and alternative S/Se precursors. For example one may argue that the supply of H₂S may, in principle, offer a better control over the S partial pressure, since the growth and nucleation is sensitive to precursors concentrations and gradients.

H₂S should decompose in S₂ at the typical MoS₂ or WS₂ deposition temperatures, even if the cracking efficiency is low (1–10%).¹¹²

Also precursors such as dimethyl disulfide, dimethyl selenide, etc. were never considered for reaction with metal powders, at the best of author's knowledge.

A few works exist, however, about the CVD or ALD deposition of TMD using combination of “other” metal and chalcogen precursors described in the corresponding paragraphs. These precursors have high vapour pressure and are stored in liquid or solid forms in canister or bubblers. They are delivered to the growth chamber using a carrier gas and a standard CVD deposition system could be used. As discussed previously, the advantages of this approach are the availability of a well-understood and developed deposition system, the precise control of precursor supply and homogeneity achieved in the growth chamber due to a optimized flowdynamic design, despite the more complex experimental setup needed.

Ideally, the most convenient method for the deposition of monolayer TMD should be ALD, with intrinsic control of layer number and surface reactions. Despite these advantages, very few groups explored the possibilities offered by this technique, yet. A comprehensive study about ALD of metal sulfide was presented by Dasgupta et al.¹¹³ but it was not focussed on transition metals. However, it describes in details several reaction mechanisms involving the reactions of sulfide, observing that ligand-exchange mechanism of surface thiols (–SH) in sulfide ALD should play the same role as surface hydroxyls (–OH) in oxide processes and identifying several challenges, such as ion exchange, air reactivity, and H₂S toxicity, to be addressed in order to incorporate sulfide processes into device manufacturing.

The proposed precursors for the ALD or CVD of MoS₂ are MoCl₅, Mo(CO)₆, H₂S and dimethyl disulfide (DMDS, (CH₃)₂S₂), while WSe₂ was grown using W(CO)₆, WCl₅ and dimethyl selenium (DMSe, (CH₃)₂Se).

Physisorbed Mo(CO)₆ undergoes to decarbonylation processes, with the formation of chemisorbed subcarbonyls (Mo(CO)_n n ≤ 5). For this reason Mo(CO)₆ was chosen as a good Mo precursor for the first-half reaction of ALD for MoS₂ synthesis.¹¹⁴ It was used in combination with both H₂S¹¹⁵ and DMDS.⁶² It is vapourized from room temperature up to 130 °C and delivered into the reactor using N₂ as carrier gas. Doses of Mo(CO)₆ and dimethyl disulfide were 1.8 × 10⁻⁶ and 4.0 × 10⁻⁴ mol s⁻¹, respectively, while Mo : H₂S : (Ar or H₂) ratio were maintained at 1 : 10 : 100 or 1 : 10 : 1000. Self-limiting process for the ALD processes was verified, and the total number of layers deposited was always found linear with the number of cycles. Generally, growth rates al low as 10 cycles per monolayer were observed, permitting in principle an high degree of control.

ALD chemistry for Mo(CO)₆ and DMDS was proposed as:⁶²

where – denotes surface adsorbed species.

The deposition was obtained at temperatures as low as 100–140 °C but the flakes were amorphous, even if local S–Mo–S order with an atomic arrangement of 2H-MoS₂ was observed by Raman analysis. A subsequent annealing at 900 °C for 5 min in Ar atmosphere permitted to recrystallize the domains and obtain better Raman signals.⁶² Thermodynamic models were developed to understand Mo(CO)₆ and H₂S gas phase and deposition chemistry, in order to identify a process windows for CVD deposition of MoS₂ (ref. 66) (Fig. 10).

Fig. 10 (a) Calculated stability windows for various condensed phases at P = 20 torr. (b) Raman spectra (with exfoliated bulk MoS₂ as reference) of multilayer MoS₂ films deposited in argon ambient on sapphire over a wide range of T, with P_tot = 20 torr and Mo(CO)₆ : H₂S = 1 : 10 with indices in bracket showing FWHM of corresponding E¹_2g and A_1g peaks. (c) Contour plot showing the variation in the mole percentage of MoS₂ in the MoS₂-C film at different temperatures and pressures for the composition represented by the dotted line in (a). Reproduced from⁶⁶ with permission of The Royal Society of Chemistry.

MoS₂ deposition with characteristic monolayer Raman peaks was obtained only at temperature higher than 350 °C with an optimal setting of 850 °C. The growth parameters were extensively studied and optimal deposition pressure of 500 mbar, gas phase ratio Mo : H₂S of 1 : 10 and H₂ partial pressure were identified. C contamination was found in the deposited layer, with level depending on total pressure and H₂ content, but was minimized by using temperature of 850 °C. Supersaturation was found to be the most critical parameter to control the number of monolayer. With the precursors concentration used in the paper, a MoS₂ monolayer is obtained in about 1 minute.

MoCl₅ was used in combination with H₂S for ALD of MoS₂. It was kept at temperature between 105 and 120 °C to get a sufficient vapour pressure, with N₂ as carrier gas.^84,115 Deposition temperature was set at 350 °C, 400 °C or 800 °C, with evidence of monolayer formation even in the lowest range. Smooth, uniform, continuous MoS₂ films with remarkable Raman peaks, no visible grain boundaries and strong photoluminescence signal were obtained. However, Tan et al. report that a post-annealing process at 500–800 °C in S vapour permitted to reconstruct the as-grown layers and to improve the crystallinity, as observed by the enhancement of Raman peaks. With the use of MoCl₅ and H₂S the number of deposited layers was reported as linear with the number of cycles, with at least 10 cycles needed to obtain a single monolayer.

A similar process was proposed for WS₂ using WCl₅ and H₂S.⁹⁸ Large area nanosheets were found after 25 min growth at 700 °C in Ar atmosphere. The domains initially nucleated with small size (100 nm), and later merged to form a continuous film with a layer-by-layer process.

Growth of WSe₂ using W(CO)₆ and DMSe was described by Eichfeld et al.⁷² The deposition was controlled by optimizing growth parameters such as temperature, pressure, Se : W ratio, and substrate type. Optimal temperature (900 °C), pressure (700 torr) and Se : W ratio (up to 20 000) settings permitted to obtain size domains up to 8 μm. Amongst the different substrates used for the experiments, epitaxial and CVD graphene gave the highest nucleation density of monolayer WSe₂ domains. The lowest nucleation density was obtained on boron nitride, with preference for vertical (3D) growth of WSe₂ versus lateral (2D) growth. Sapphire provided the largest triangular WSe₂ domains, suggesting that sticking coefficient of adatoms on its surface is higher, as well as their surface mobility. Precursor purity have a significant impact on the film quality, and carbon contamination was reduced by using high purity DMSe. Also, the presence of H₂ promoted WSe₂ growth and permitted to obtain lower C contamination.

Thin WS₂ films were deposited on glass by CVD⁶⁸ but the thickness was about 400 nm, so no direct informations about 2D TMD can be obtained from this work. However, this demonstrate that WS₂ can be deposited using precursors such as W(CO)₆, WCl₆, WOCl₄, 1,2-ethanedithiol [HS(CH₂)₂SH] and 2-methylpropanethiol (HSC(CH₃)₃) at temperature ranging from 300 to 600 °C and atmospheric pressure. Growth rates up to 400 nm min⁻¹ at the proposed growth conditions were obtained, with evidence of Volmer–Weber 3D nucleation and WS₂ domains up to 200 nm in size. No evidence of C or Cl contamination was found in the films.

Synthesis of TMD from metal or metal oxides deposited on substrate

One of the most convenient methods to obtain monolayer TMD is, in principle, to deposit a thin metal layer over the substrate, control its thickness in the range 1–5 nm, and then sulfurize it to obtain MoS₂ or WS₂. Drawback of this process is the difficulty to get precise metal thickness and homogeneity over large area. The interaction of sulfur with Mo or W films has been studied extensively, both experimentally and theoretically in ref. 116–119. Sulfur or MoS₂ powders were used as sulfurizing agent, although also H₂S could be used because its decomposition on Mo metal surface is thermodynamically favoured.¹¹⁸ The temperatures required for the synthesis of TMD with this method are usually in the range of 750–900 °C.

Zhan et al.¹²⁰ sulfurized a Mo metal film with S powders, forming single and double MoS₂ layer. They found that the thickness of TMD layer is directly related to the thickness of the initial Mo deposited layer. Ma et al.¹²¹ observed that the nucleation of MoS₂ on metal substrate is controlled by S supersaturation, which must be low enough in order to avoid the formation of polycrystalline and defective layer. The use of MoS₂ powder instead of elemental S permitted to reduce the S partial pressure in the vapour phase. This process generally occurs in nitrogen atmosphere, at temperature of about 750 °C.

Vertically aligned MoS₂ and MoSe₂ layers were obtained with S or Se powders and an evaporated Mo layer on glassy carbon, quartz, and SiO₂/Si substrates.¹²² The rapid sulfurization/selenization processes were performed in Ar at 550 °C: diffusion mechanisms of S and Se into the metal promote the formation of MoS₂/MoSe₂ layers aligned perpendicular to the substrate. Chemical conversion in TMD occurs much faster than the diffusion of S and Se gas, which is then the process limiting factor.

Sulfurization of a thin W or Mo layer deposited on MgO was described by Orofeo et al.¹²³ W film was converted to WS₂ via chemical reactions at 800 °C using sulfur powders. No aggregation of the metal into smaller island was observed, due to the fact that the surface energy and mobility of the metal atoms are not high enough at this growth temperature. The characteristics of final TMD flakes were determined by the initial thickness of the metal layer.

Since MoO₃ and WO₃, thanks to a lower melting point, can be more easily deposited by means of evaporation or sputtering than the metal counterparts, they were chosen as alternative starting layer for sulfurization or selenization. Process for DMS synthesis is similar to the previous one: S, Se powders or H₂S are used to sulfurize/selenize the coated substrate.

MoS₂ synthesis from MoO₃ is usually a two step process: a first annealing in S-rich inert atmosphere is performed at 500–600 °C, in order to reduce the oxide to MoO_3−x. A second stage follows, with MoS₂ formation at higher temperature (850–1000 °C):

The synthesis was also performed in mixed Ar–H₂ atmosphere.⁷⁹ The reactions were proposed as follows:

Lin et al. also pointed out that the direct sulfurization at high temperature, without the suboxide formation at lower temperature, resulted in MoS₂ films with electrical carrier mobility one magnitude lower.

Since the initial layer must be reduced to a suboxide, selenization of the MoO₃ layer requires the addition of H₂ to the gas phase, as discussed for the synthesis of MoSe₂ from powder. In this case the process may be similar to the one described in formula (7) with Se in the place of S for the second half of the reaction.

Sulfurization of WO₃ layer by sulfur powders is performed at temperatures between 750 and 900 °C in argon atmosphere.^48,124 In this case the two step process to produce WO_3−x suboxides was not necessary.

Thiosalts–thiomolybdates. Ammonium thiomolybdates (NH₄)₂MoS₄ can be reduced to MoS₃ and, subsequently, to MoS₂ after annealing in N₂ atmosphere.¹²⁵ A thin layer of (NH₄)₂MoS₄ is usually deposited or spin coated over different substrates (SiO₂, quartz, etc.) and a two step process is used to synthesis MoS₂. A detailed description of substrate preparation with (NH₄)₂MoS₄ is reported in ref. 90. High temperature annealing of (NH₄)₂MoS₄ is likely to result in rapid oxidation, and many reactions are involved in the conversion of (NH₄)₂MoS₄ to MoS₂, resulting in irreproducibility and polycrystalline material with many layers. For this reason, 2 step processes were developed, with an initial annealing at 500 °C and a second heating to 1000 °C in S vapours: the addition of S (through the use of powders) was found to be essential to produce crystalline MoS₂ flakes.

MoS₂ formation is proposed as follows:

Hydrogen may also play a role in the initial MoS₃ conversion through the reactions:^126,127

H₂ inhibits the etching during film growth and promotes the desulfurization reaction by decreasing the S/Mo atomic ratio and oxidation rate of the MoS_x films.

High quality MoS₂ flakes were obtained by this method, although the drawback is the difficulty to control the initial (NH₄)₂MoS₄ coating of the substrate.

Growth mechanisms

The exact growth processes of TMD are still not fully understood. Several papers propose simple chemical reactions and kinetic mechanisms to explain the synthesis of mono and multi layers, corroborated by theoretical calculations based on density functional theory.^{41,79,107,128}

As discussed in the previous section, the basic chemical framework is the reduction and further sulfurization/selenization of the metal oxide, with H₂ acting as an additional reducing agent when selenium is involved. H₂ may directly reduce MO_x or it can react with S to produce H₂S: the exact reaction pathway is still under investigation but a combination of the two processes may indeed occur. The reactions involved in the growth of TMD were summarized in eqn (3)–(9).

Both homogenous and heterogeneous reactions may be involved in the growth, and two possible mechanisms were identified:

(a) MO_3−x species are adsorbed on the substrate and further react with S/Se to form MS₂/MSe₂

(b) MO_3−x and chalcogen react heterogeneously in the gas phase, resulting in MX₂ clusters that are adsorbed on the substrate and then grow laterally or form a bulk depending on the growth conditions. In this case surface mobility plays a crucial role in the growth.

These two mechanisms are in competition and it is difficult to control them accurately, eventually separating their contribute. Temperature controls reaction kinetics, adsorption, desorption, mobility of cluster on the surface and thus it is considered the main parameter affecting these growth mechanisms, although partial pressure of the precursors plays also a role. Considering the phase diagram of the Mo–O–S system,⁴¹ a suitable S partial pressure is needed in order to get a proper reducing atmosphere. If S concentration in the gas phase is either too high or too low the conversion of MoO₃ to suboxides may be strongly inhibited or enhanced, leading to suppression of MoS₂ growth or formation of 3D clusters. For this reason the addition of H₂ to the carrier gas may change chemical equilibrium and help the synthesis process.

Given their molecular structure and chemical bonds, TMD are usually observed in triangular form but complex shapes, resulting from assembling of smaller triangles, may be also observed.^104,129

Atoms on the flake basal plane have very weak chemical reactivity, while the flake edges have higher free energy and are thus preferred nucleation sites. For this reason, under the favoured growth conditions, the flakes tend to expand laterally, forming monolayer structures.

Through direct in situ observation by TEM on single and multilayer MoS₂ structures during growth, obtained from MoO₃ on MgAl₂O₄ substrates sulfurized by H₂S, Hansen et al.¹³⁰ confirmed that step sites either in the substrate or in existing flakes can behave as nucleation centers. An existing MoS₂ flake is stick on the substrate and expands as MoO_x is converted to MoO_xS_y by sulfurization and migrate on the surface to reach energetically favoured sites on flakes edges. Formation of multilayer or bulk MoS₂ is explained by local higher concentration of MoO_xS_y on the substrate, that leads to an increased transfer of Mo to the MoS₂ basal plane. The overall growth mechanism leading to 2D layered structures is attributed to van der Waals epitaxy, which relaxes the lattice matching conditions of the epitaxial layer and permits the synthesis of TMD on a wide variety of substrates.^91,130,131 The mechanisms described by Hansen et al. for MoS₂ could apply for other TMD such as WS₂, MoSe₂, WSe₂ etc., because of similarities between structures and chemical bonds between these materials.

Superficial energy has an important role in nucleation, migration and chemical reactivity of TMD. This is the reason why the use of surfactants such as graphene oxides or other compounds, described previously, has a great effect in the synthesis of TMD. The presence of additional nucleation sites and the lower surface energy promoted by the pretreatment is able to enhance and promote the growth of monolayer flakes.¹³²

In order to get a full understanding of TMD growth processes, thermodynamic calculations, kinetic models and eventually density functional theory framework should be considered. The complexity of the system poses several hurdles to the growth process. Even if synthesis methods are conceptually simple, the biggest issues that delays the development of TMD devices are the careful control of precursor concentration and growth chamber temperature gradients, eventually over a large area substrate. Growth of high quality material requires to control thickness, purity and defects. The defects (point defects, vacancies, impurities) are also controlled by the growth conditions (temperature, precursor vapor pressure).

Conclusions

Transition metal dichalcogenides are the most promising class of materials to develop novel device concepts and to scale down transistors, in order to continue to fulfill Moore's Law. Contrary to graphene, they have a finite bandgap in the range 1.4–1.9 eV and most of them shows semiconducting behaviour. Despite the promises given by their physical characteristics, a lot of open issues still exist to obtain reliable, reproducible and large scale growth of TMD, with particular emphasis of heterostructures, doping and ternary compounds. In this review the important topic of bottom up synthesis of TMD has been analyzed and discussed. Description and analysis of the different available precursors for TMD synthesis and on different growth methods from the vapour phase were provided, with particular emphasis to metal-organic chemical vapour deposition and atomic layer deposition. These growth techniques are the most widespread in the semiconductor industry and are nowadays used in the mass production of a vast class of devices. Integration of TMD with other materials by using these methods would open the possibility to integrate them with current manufacture technology.

Acknowledgements

The author is grateful to Dr Giovanni Attolini for reading the manuscript and for useful suggestions and discussions.

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