Recent Advances in Spin-coating Precursor Mediated Chemical Vapor Deposition of Two-Dimensional Transition Metal Dichalcogenides

Abstract

Two-dimensional (2D) transition metal dichalcogenides (TMDs) have garnered widespread interest in the scientific community and industry for their exceptional physical and chemistry properties, and great potential for applications in diverse fields including (opto)electronics, electrocatalysis, and energy storage. Chemical vapor deposition (CVD) is one of the most compelling growth methods for the scalable growth of high-quality 2D TMDs. However, the conventional CVD process for synthesis of 2D TMDs still encounters significant challenges, primarily attributed to the high melting point of precursor powders, and achieving a uniform distribution of precursor atmosphere on the substrate to obtain controllable smaple domains is difficult. The spin-coating precursor mediated chemical vapor deposition (SCVD) strategy provides refinement over traditional methods by eliminating the use of solid precursors and ensuring a more clean and uniform distribution of the growth material on the substrate. Additionally, the SCVD process allows fine-tuning of material thickness and purity by manipulating solution composition, concentration, and the spin coating process. This Review presents a comprehensive summary of recent advances in controllable growth of 2D TMDs with a SCVD strategy. First, a series of various liquid precursors, additives, source supply methods, and substrate engineering strategies for preparing atomically thin TMDs by SCVD are introduced. Then, 2D TMDs heterostructures and novel doped TMDs fabricated through the SCVD method are discussed. Finally, the current challenges and perspectives to synthesize 2D TMDs using SCVD are discussed.

1. Introduction

The discovery of graphene¹ marked a significant milestone in two-dimensional (2D) materials research. Since then, other 2D materials, including transition metal dichalcogenides (TMDs), with the formula MX₂ (M: transition metal atom; X: chalcogen atom), have garnered considerable interest in various fields such as electronics, optoelectronics, magnetics, and catalysis research due to their ultrathin thickness, unique crystal structures, excellent physical and chemical properties.²⁻¹⁰ It is widely recognized that material preparation is a fundamental prerequisite for meeting the increasing demands of various applications. Therefore, the exploration of diverse synthesis methods for materials is crucial. Conventional mechanical exfoliation methods prove to be inadequate in fulfilling the demands for the precise construction of large-area thin films and heterostructures of 2D TMDs, rendering them unsuitable for practical applications.

Chemical vapor deposition (CVD) method stands out as a preferred method for the synthesis of 2D TMDs and their heterostructures due to relatively good controllability, high quality, and high yield of samples. However, controlling the gas phase distribution and reaction rate of the gas phase during the traditional CVD growth process presents a challenge. The nonuniform distribution of the precursor concentration on the substrate will directly affect the nucleation sites, resulting in undesirable nucleation and poor control of the nucleation density. Research indicates that incorporating additives into the CVD process can lower the melting point of solid precursors and enhance source delivery. For instance, the incorporation of salt additives with metal oxides has been utilized to form intermediate volatile compounds, which can reduce the reaction temperature and promote the reaction.^11,12 However, this method introduces unwanted and unavoidable byproducts or impurities, leading to a notable reduction in the quality of ultrathin TMDs, which affects their stability and results in poor electrical and optical performance.^13,14

To address this issue, an updated CVD synthesis technique termed spin-coating-mediated precursor chemical vapor deposition (SCVD) is gradually coming into view for the growth of TMDs and has made rapid progress in recent years. In SCVD, one or multiple suitable precursors are dissolved in solvents to form uniform mixed solutions serving as metal-containing sources. Subsequently, the precursor solutions are spin-coated onto the pretreated substrates, reacting with chalcogen to produce 2D TMDs. And the thickness of the synthesized 2D TMDs can be regulated by adjusting the concentration of solution and spin coating speed. As the synthesis process involves heating, the solutes gradually transform into a molten liquid state, facilitating more complete reactions with chalcogen feedings. Unlike the conventional CVD growth method where the positioning of substrates with precursors is crucial, SCVD method enables direct reaction and nucleation on the substrate, which eliminates the issues related to position-dependent substrates with precursors and reduces the formation of undesirable byproducts and additional contaminants during growth. It enables precise control of the precursor atmosphere, ensuring a clean and uniform distribution of the resulting sample on the substrate, thus achieving controllability and repeatability of material preparation. It is worth noting that SCVD technology is particularly advantageous for synthesizing large-area single-layer or multilayer TMD thin films. For example, the liquid precursor (NH₄)₆H₂W₁₂O₄₀·x(H₂O) (AMT) was reported to be deposited on Au to synthesize WS₂ monolayers with domain sizes up to ∼420 μm.¹⁵ The wafer-scale MoS₂ layers were obtained through thermolysis of a spin-coated (NH₄)₂MoS₄ solution film.¹⁶ Furthermore, by regulating the tunable doping concentrations of various dopants in mixed solutions, the properties and functionalities of 2D TMDs can be engineered. Overall, TMDs grown by SCVD have the advantages of precise controllability of composition, thickness, and other key parameters.

Herein, we present the recent advancements in controllable SCVD growth of 2D TMDs. The first part focuses on the exploration of liquid precursors of SCVD, and introduces the different kinds of liquid precursors, additives, and new source supply methods, such as bubble method, dissolution–precipitation method, and monomer feeding method. The second part discusses the substrate engineering strategies including substrate choice, surface treatment, and patterning. It is worth noting that substrate engineering plays a crucial role in controlling the nucleation and growth of TMD films. The third part mainly introduces 2D TMDs heterostructures and the novel doped TMDs in the SCVD. Finally, we provide insights into the challenges and potential opportunities for fabricating and commercializing 2D TMDs using SCVD in the future.

2. SCVD Growth of 2D TMDs

Nowadays, the SCVD technique has demonstrated its superiority in synthesizing 2D TMDs with excellent reproducibility and homogeneity under mild conditions. In this section, our primary focus is on the liquid precursor categories, additives, and new source supply method in SCVD. And the implementation of effective substrate pretreatment strategies can significantly increase the size of the synthesized material, potentially extending it to the centimeter level even to the wafer-level films.

2.1. Liquid Precursors for SCVD

2.1.1. Importance of Liquid Precursors in Determining TMDs Quality

Utilizing liquid precursors in the SCVD method effectively overcomes the challenge of unstable nucleation that arises from the difficulty in controlling the evaporation of solid precursors in the CVD method. The selection of liquid-phase precursor is a critical factor in the success of SCVD. In addition, a lot of progress has been made in the growth of various 2D TMDs in monolayer and few-layer forms using SCVD.

It is worth summarizing that the precursors generally should (i) have a melting point lower or close to the growth temperature, (ii) be enabled to form stable melts in the reaction, and (iii) be soluble in the target solvent and become stable compounds in solutions. The first two conditions promote the growth of the target material, while the last condition facilitates the formation of a homogeneous solution mixture. In the literature, precursors such as (NH₄)₂MoS₄,¹⁷ Na₂MoO₄,¹⁸ Na₂WO₄,¹⁹ NaReO₄,²⁰ and C₄H₄NNbO₉²¹ have been used for the SCVD growth of 2D TMDs. The possible chemical reactions in SCVD growth of 2D TMDs are illustrated in Table 1.

Table 1. Possible Chemical Reactions in SCVD Growth of 2D TMDs.

	Reactions	References
MoS2	(NH4)2MoO4 + 4S + 2KOH + H2O → MoO3 + 3H2S + K2SO4 + 2NH3	(22)
	2MoO3 + 7S → 2MoS2 + 3SO2
	(NH4)6Mo7O24·4H2O → 6NH3 + 7H2O + 7MoO3	(23)
	2MoO3 + 7S → 2MoS2 + 3SO2
MoSe2	2(NH4)2MoO4 → 2MoO3 + 4NH3 + 2H2O	(24)
	2MoO3 + 2KI → MoO2I2 + K2MoO4
	MoO2I2 + 4H2 + 3Se → MoSe2 + 2HI + 2H2O + H2Se
	Na2MoO4 + 2Na2SeO3 + 3Se + 10H2 → MoSe2 + 3Na2Se + 10H2O	(20)
WS2	(NH4)10W12O41 → 10NH3 + 5H2O + 12WO3	(25)
	2WO3 + 7S → 2WS2 + 3SO2
MoTe2	Na2MoO4 + 3Te + 3H2O → TeO2 + 2H2Te + MoO3 + 2NaOH	(26)
	MoO3 + 2TeO2 + 7H2 → MoTe2 + 7H2O
WTe2	Na2WO4 + 2Na2TeO3 + 3Te + 10H2 → WTe2 + 3Na2Te + 10H2O	(20)

In 2012, (NH₄)₂MoS₄ solution was dip-coated onto an insulating substrate and subsequently decomposed through high-temperature annealing, resulting in the production large-area MoS₂ thin layers (Figure 1a).¹⁷ However, the dip-coating method frequently results in nonuniform distribution of precursor on the substrate, leading to the formation of multidomains with different layer numbers. To tackle this challenge, Liu et al. utilized a Na₂WO₄ solution as a precursor for spin coating and successfully synthesized monolayer WS₂ crystals after annealing the precursor.¹⁸Figure 1b displays the OM images of a WS₂ crystal with and without annealing. The precursor annealing process resulted in the redistribution of the molten liquid precursor, effectively preventing aggregation and promoting monolayer growth. Park et al. utilized mixed solutions of C₄H₄NNbO₉·xH₂O with different concentrations to construct the different morphologies of metallic NbSe₂ from hexagonal, triangular, windmill-like, to herringbone-like (Figure 1c).²¹ The concentration of the Nb precursor gradually increases and surpasses that of the Se precursor. As a result, Se-terminated facets grow faster than Nb-terminated facets under Nb-rich conditions. Monolayer WTe₂ and MoTe₂ have also been successfully synthesized using (NH₄)₆H₂W₁₂O₄₀·xH₂O and (NH₄)₆Mo₇O₂₄·4H₂O as precursors, respectively.^27,28 Various precursors with the formula A_xMO_y (where A is an alkali metal or ammonium and M is a transition metal) have been employed in the growth of TMDs, such as Na₂MoO₄,¹⁹ Na₂WO₄¹⁹ and (NH₄)₂MoO₄.²⁹

Figure 1.

Synthesis of TMDs with SCVD. (a) Schematic illustration of decomposing the dip-coating (NH₄)₂MoS₄ to produce multilayer MoS₂. Reproduced from ref (17). Copyright 2012 American Chemical Society. (b) OM image of WS₂ crystal before and after annealing using Na₂WO₄ solution as a precursor; scale bar: 20 μm. Reproduced from ref (18). Copyright 2020 American Chemical Society. (c) Change in the morphology of NbSe₂ at various NaOH/C₄H₄NNbO₉·xH₂O ratios. Reproduced from ref (21). Copyright 2020 American Chemical Society. (d) Melting points of several precursors and synthetic strategy of MoSe₂ with mixed-salt precursors. (e) OM image of MoSe₂ monolayer grown with or without adding Na₂SeO₃. The inset shows a typical MoSe₂ nanoflake in an area of 10 × 10 μm². Reproduced from ref (20). Copyright 2021 American Chemical Society.

On the other hand, the synthesis of high-quality 2D TMDs is also inseparable from the precise feeding of chalcogen. Similarly, liquid precursors can also be used as a source of chalcogen supply, such as Na₂SeO₃ and Na₂TeO₃. Taniguchi et al. proposed various groups of mixed precursors to synthesis transition metal selenides and tellurides. Figure 1d shows the melting points of several precursors. The 2D MoSe₂, ReSe₂, and WTe₂ were successfully produced with Na₂MoO₄–Na₂SeO_3, NaReO₄–Na₂SeO₃, and Na₂WO₄–Na₂TeO₃ as precursors, respectively.²⁰ The OM images of the MoSe₂ monolayer grown with or without the addition of Na₂SeO₃ are shown in Figure 1e. Despite the SCVD technique being a universal option for growing large groups of 2D TMDs, the limited suitable precursors degrade the versatility of this strategy. Notably, along with 2D TMDs, a group of transition metal carbides (TMNxs) including MoNx, WNx, CrNx, VNx, and transition metal carbides (TMCs) like MoC₂ have been fabricated via SCVD technique,^30,31 which suggests the potential diversification of the 2D material family. The more detailed information regarding synthesized 2D TMDs by SCVD is illustrated in Table 2.

Table 2. Summary of SCVD Synthesis of 2D TMDs.

TMDs	Source materials	Phase	Morphology	Thickness	Temperature (°C)	Substrate	Carrier gas (sccm)	Growth time [min]
MoS2	(NH4)2MoS4 + S32	2H	Nanosheet	1.5–2 nm	700	SiO2/Si	300Ar	14
	Na2MoO4 + NaOH + S33	2H	Nanosheet	2.07 nm	800	Sapphire		2
	(NH4)2MoO4 + KOH + S22	2H	Film	0.7 nm	750	Sapphire/SiO2/Si	30Ar	10
	MoO3 + NH4OH + S34	2H	Nanosheet	Monolayer	760–880	Glass	150Ar	5
	(NH4)6Mo7O24·4H2O + S35	2H	Film	0.6–2.5 nm	800	SiO2/Si	100Ar	10
	Na2MoO4·2H2O + C12H25SH36	2H	Nanosheet	Monolayer	850	SiO2/Si	10Ar	60
	(NH4)2MoO4 + KI + S37	2H	Film	1 nm	720	SiO2/Si	30Ar	3–5
	(NH4)2MoS417	2H	Film	2 nm	1000	Sapphire, SiO2/Si	80%Ar/20%H2	30
	(NH4)2MoO4 + S29	2H	Nanosheet	0.78 nm	750	Sapphire	Ar	20
	(NH4)2MoS438	2H	Film	2–30 nm	700	SiO2/Si	96%Ar/4% H2	60
	Na2MoO4·2H2O + (C2H5)2S39	2H	Film	0.7 nm	850	Sapphire, SiO2/Si	350Ar/10 H2	20
	Na2MoO4 + Mo(CO)6 + CH3SSCH340	2H	Film	0.9 nm	850	SiO2/Si	350Ar/15H2	20
	MoO3 + NH4OH + S41	2H	Film	0.8 nm	800	SiO2/Si	150Ar	5
	Na2MoO4 + S19	2H	Nanosheet	Monolayer	730–750	soda lime glass slide	80Ar	10–20
	Na2MoO4 + ZnS42	2H	Nanosheet	Monolayer	780	Sapphire/SiO2/Si	100Ar	10–60
	(NH4)2MoO4 + KI + S24	2H	Film	Monolayer	800	Sapphire	Ar	10
MoSe2	(NH4)2MoO4 + KI + Se24	2H	Film	Monolayer	800	sapphire	Ar/H2	10
	Na2MoO4 + Se19	2H	Nanosheet	Monolayer	750–800	soda lime glass slide	80Ar/8 H2	10–20
	Na2MoO4 + ZnSe42	2H	Nanosheet	Monolayer	800	Sapphire/SiO2/Si	100Ar	10–60
	Na2MoO4 + Na2SeO320	2H	Nanosheet	Monolayer	775–850	SiO2/Si	190Ar/10H2	5–10
WS2	(NH4)10H2(W2O7)6 + S32	2H	Nanosheet	1.5–2 nm	700	SiO2/Si	300Ar	14
	Na2WO4·H2O + N2H4 + (CH3)2S243	2H	Film	0.65 nm	850	SiO2/Si	350Ar	30
	Na2WO4 + S19	2H	Nanosheet	Monolayer	730–780	soda lime glass slide	80Ar/4H2	10–20
	Na2WO4 + ZnS42	2H	Nanosheet	Monolayer	930	Sapphire/SiO2/Si	100Ar	10–60
	(NH4)2WO4 + KI + S24	2H	Film	Monolayer	850	sapphire	Ar/H2	10
WSe2	Na2WO4 + ZnSe42	2H	Nanosheet	Monolayer	820	Sapphire/SiO2/Si	100Ar	10–60
	(NH4)2WO4 + KI + Se24	2H	Film	Monolayer	850	sapphire	Ar/H2	10
MoTe2	Na2MoO4 + Te19	1T’	Nanosheet		730–750	soda lime glass slide	120Ar/20H2;	10–20
	(NH4)6Mo7O24·4H2O + C24H39NaO5 + Te27	1T’	Nanosheet	0.8 nm	700/500	SiO2/Si	400N2/25H2;	5
	Na2MoO4 + ZnTe42	2H	Nanosheet	Monolayer	750–800	Sapphire/SiO2/Si	100Ar	10–60
	Na2MoO4 + ZnTe42	1T’	Nanosheet	Monolayer	800	Sapphire/SiO2/Si	100Ar	10–60
	(NH4)6Mo7O24 + NaOH + Te44	1T’	Nanosheet	0.82 ± 0.04 nm	730	SiO2/Si	100Ar/3H2	15
WTe2	Na2WO4 + ZnTe42	1T’	Nanosheet	Monolayer	780	Sapphire/SiO2/Si	100Ar	10–60
	(NH4)6H2W12O40 + Te28	1T’	Nanosheet	0.8 nm	650	SiO2/Si	N2/H2	6
	Na2WO4 + Na2TeO320	1T’	Nanosheet	2.1 nm	775–825	sapphire	95Ar/5H2	5
NbSe2	C4H4NNbO9·xH2O + NaOH + Se21	2H	Nanosheet	0.7 nm	800	SiO2/Si	600N2/15H2	20
ReSe2	NaReO4 + Na2SeO320	1T’	Nanosheet		750	sapphire	190Ar/10H2	5–10

2.1.2. Overview of Additives Used in SCVD Growth

Many research results indicate that the addition of additives can significantly improve the interaction between the precursor vapor and substrate during the growth of materials.^11,45−47 Here, the additives used during SCVD growth are divided into inorganic and organic compounds for further discussion and analysis.

In recent years, researchers have discovered that inorganic additives play multifaceted roles in SCVD growth, including growth promotion, acting as reducing agents, and acting as solvents. Notably, additives such as KI and NaOH have been widely studied.

For example, Kong et al. reported that through introducing KI into the (NH₄)₂MoO₄ solution, the domain size of synthesized MoS₂ monolayers could reach up to approximately 0.62 mm from vertex to vertex (Figure 2a),³⁷ which demonstrates the effectiveness of KI as an additive in promoting the growth of large-area monolayer MoS₂. Kim et al. reported the synthesis of large-area monolayer MoSe₂ with good uniformity on 3 cm × 3 cm sapphire substrate via KI promoter assisted as shown in Figure 2b,c.²⁴ Further research indicates that KI not only facilitates the formation of reactive transition metal oxyhalide, reducing the energy barrier of chalcogenization, but also promotes the growth along the in-plane direction. The addition of KI is also suitable for other TMDs families, including MoS₂, WS₂, and WSe₂. Similarly, centimeter scale Nb_1–xW_xSe₂ monolayers have been reported with the assistance of promoter KI, allowing for tunable composition to achieve continuously adjustable bandgaps.⁴⁸ In another study, Zhao et al. added NaCl salt into (NH₄)₆Mo₇O₂₄ (AHM)) solution to achieve rapid growth of large-area monolayer MoSe₂ films.⁴⁹ Nguyen et al. found that the presence of NaNO₃ suppressed nucleation and subsequently increased crystal size significantly. Figure 2d,e shows OM images of MoS₂ nanoflakes with and without NaNO₃ catalyst.⁵⁰ As illustrated in Figure 2f, NaNO₃ reacts with MoO_3–x to form a metal oxynitrate [MoO_3–x[NO₃]_y], which has a melting point lower than that of the metal oxide, reducing the overall activation energy. The formation of the metal oxynitrate affected the reaction kinetics and expedited the reaction equilibrium, resulting in an increase in the density and size of the MoS₂ monolayers. However, the corresponding PL intensity is significantly reduced due to the presence of excess residues (Figure 2g).

Figure 2.

Synthesis of MoS₂ using inorganic additives. (a) OM image of MoS₂ crystal with introducing KI into the (NH₄)₂MoO₄ solution precursor, inset is the atomic force microscopy with height of 1 nm. Reproduced from ref (37). Copyright 2021 American Association for the Advancement of Science (AAAS). (b,c) Schematic illustration of mixing (NH₄)₂MoO₄/KI liquid precursors (b) to synthesize centimeter-scale monolayer MoSe₂ film (c). Reproduced from ref (24). Copyright 2021 American Chemical Society. (d,e) OM image of synthesized MoS₂ monolayers without and with the addition of NaNO₃ catalyst. Scale bars: 20 μm. (f) The reaction mechanism of synthesis of MoS₂ monolayers with the addition of NaNO₃. (g) Photoluminescence spectra of MoS₂ obtained by mechanical exfoliation and grown by SCVD. Reproduced from ref (50). Copyright 2022 American Chemical Society.

Besides, NaOH(KOH) is also commonly used as an additive because it could improve the uniform distribution of precursors on the substrate, which is important especially in growing centimeter-scale uniform monolayer TMDs films.^33,51 Zhu et al. brought KOH (or NaOH) to the liquid-phase precursor to introduce −OH groups.²² As illustrated in Figure 3a, −OH groups in solution aimed at attaching to the (001) surface of MoS₂ to form an S–Mo–S-OH bilayer structure during the growth process, which leads to preferential growth of MoS₂ layers in the lateral direction instead of in the vertical direction, further promoting the generation of large area monolayer MoS₂. The OM and AFM images in Figure 3b verified the uniform atomically flat MoS₂ film with a surface roughness of about 0.12 nm. Figure 3c shows representative uniform MoS₂ monolayers grown with 100% coverage on 1 × 1 and 3 × 3 cm² sapphire. Moreover, the test of devices fabricated from the obtained MoS₂ films suggested that the −OH layer on the monolayer MoS₂ surface could effectively suppress oxidation of MoS₂ in air, resulting in the highest carrier mobility up to 30 cm² V^–1 s^–1 in air. Hu et a reported a method for preparing nonfilter light storage transistors using a hydrophilic monolayer molybdenum disulfide film with strong charge trapping capability.⁵² The hydroxyl groups covalently bonded with MoS₂ possess strong charge trapping capabilities, which can control the capture and release of electrons by laser and electric operation, thus achieving stable image memory functions, and promoting the development of artificial visual systems in the future. Furthermore, 1 × 1 cm² MoTe₂ monolayer can be grown using NaOH as additive (Figure 3d).²⁶ The spin-coated precursors, which can be dehydrated into immobilized particles during the reaction, were consumed continually to provide a source for the growth of the monolayer. The small roughness of 0.19 nm calculated from the AFM image in Figure 3e indicates the smoothness of the fabricated monolayer. The time-of-flight secondary ion mass spectrometry (ToF-SIMS) data in Figure 3f showed that −OH groups could be adsorbed on the surface of the MoTe₂ layer, which can inhibit the growth of MoTe₂ in the vertical direction. Ammonium salts have also been proved to be a clean and nondestructive additives for the synthesis of MoS₂.⁵³ Overall, the novel additives in the SCVD technique provide a new opportunity for the growth of inch-sized monolayer TMDs films, moving us closer to the realization of inch-scale functional nanomaterials.

Figure 3.

Synthesis of TMDs films using inorganic additives. (a) Schematic mechanism of MoS₂ growth process with −OH or not. (b) OM image of a continuous MoS₂ film grown on sapphire. The inset is an AFM image showing the atomically flat surface. (c) Photographs of 1 × 1 and 3 × 3 cm² MoS₂ monolayer films on sapphire. Reproduced from ref (22). Copyright 2019 American Chemical Society. (d) Photograph of 1 cm × 1 cm bare sapphire, as-grown MoTe₂ monolayer on sapphire, and transferred MoTe₂ monolayer on SiO₂/Si substrate. (e) Typical AFM image of monolayer MoTe₂ with small roughness (Ra = 0.19 nm). (f) ToF-SIMS depth profile of the MoTe₂–OH sample. Reproduced from ref (26). Copyright 2021 American Chemical Society.

Inorganic additives in the SCVD process for TMDs are multifaceted, acting not just as growth promoters but also as reducing and solubilizing agents. For example, Kim et al. incorporated hydrazine (N₂H₄) as a strong reducing agent into an aqueous Na₂WO₄ solution to prereduce Na₂WO₄ and facilitate the synthesis of WS₂.⁴³ The introduction of hydrazine into the reaction provides regulation over the synthesis, yielding a notable enhancement in the quality of the WS₂ product. The reduction of sodium tungstate by hydrazine generates enough volatile tungsten suboxides to directly participate in the growth of WS₂, avoiding the etching caused by H₂ reduction, thus ensuring the attainment of full coverage of the WS₂ film. Another method of forming a precursor solution is dissolving the oxide in an alkaline solvent.^34,41 Lee et al. reported the synthesis of MoS₂ monolayer with a size of up to 500 μm using the preprocessed solution composed of MoO₃ and NH₄OH.⁴¹ However, the oxides dissolved in alkaline solvent as precursors are somewhat more complex in nature. Clearly, the role of inorganic additives during the SCVD growth of TMDs is versatile and extends beyond a singular function, underscoring their critical importance in tailoring and refining the quality and dimensions of the resulting materials.

Furthermore, organic additives such as dimethylformamide (DMF),^54,55 ethylene glycol,⁵⁶ and n-methylpyrollidone (NMP)¹⁶ could be also used as effective agents for dissolving poorly water-soluble precursors. Theses precursors and the organic solvents can chemically bond to the precursor to form a stable solute in the mixture solution. Kim et al. utilized (NH₄)₂MoS₄-DMF solution with additional amine- and amino alcohol-based solvents (n-butylamine and 2-aminoethanol) to synthesize 2 in. wafer-scale MoS₂ thin films (Figure 4a).⁵⁴ As shown in Figure 4b–d, the spin-coated precursor film exhibits dewetted regions and pinholes when only DMF was used as solvent. After adding 2-aminoethanol to DMF, thiomolybdate clusters formed with a hydrogen bond with a hydroxide group, which enhanced the solubility of (NH₄)₂MoS₄. However, microsize defects still existed in this case. With the further addition of n-butylamine again, a defect-free spin-coating precursor film was finally obtained, and the 2 in. wafer-scale MoS₂ thin films were obtained in the following thermal process. Ozkan et al. used ethylenediaminetetraacetic acid (EDTA) and dimethyl sulfoxide (DMSO) as a solvent that dissolved (NH₄)₂MoS₄ to fabricate MoS₂ films (Figure 4e).⁵⁷Figure 4f illustrates the mechanism for the synthesis of MoS₂ crystals through this strategy. Tetrathiomolybdate coordinates with EDTA, where the presence of both coordinated and uncoordinated carboxylic groups could enhance the interaction with the silanol-functionalized silica surface. Therefore, with EDTA and DMSO, the precursor solution can have superior wettability on the hydroxylated surface. Besides, Jeong et al. designed a precursor-polymer complex film consisting of anhydrous ammonium tetrathiopolybdate (ATM) precursor and linear polyimide (L-PEI) (Figure 4g,h),³⁸ which facilitates uniform coating of precursor on substrate over large areas and LPEI fully decomposes at 400 °C without residual carbon. From the above examples, it is evident that the utilization of organic additives can enhance the interaction of the precursors in the solution and enhance the wettability of the precursor on the substrate, thereby contributing to the synthesis of wafer-level TMDs film. However, while some organic additives can be decomposed without leaving residue, most of them still remain as contaminants and potentially compromise the quality of the crystal.

Figure 4.

Synthesis of MoS₂ films using organic additives. (a) Optical image of a 2 in.-sized MoS₂ wafer thin film from SCVD using (NH₄)₂MoS₄-DMF solution with additional amine- and amino alcohol-based solvents (n-butylamine and 2-aminoethanol). (b–d) AFM images of the different spin-coated precursor films with additive of (b) only DMF, (c) DMF:2-aminoethanol, and (d) DMF:n-butylamine:2-aminoethanol. Reproduced from ref (54). Copyright 2015 Royal Society of Chemistry. (e) Schematic of wet behavior of solution of (NH₄)₂MoS₄-EDTA-DMSO. (f) Mechanism for the synthesis of MoS₂ crystal. Reproduced from ref (57). Copyright 2017 Springer Nature. (g) Schematic illustration of the formation of MoS₂ thin film from the anhydrous ammonium tetrathiopolybdate (ATM) precursor and linear polyimide (L-PEI) complex film. (h) Image of a 6 in.-sized MoS₂ thin film synthesized through the method illustrate in (g). Reproduced from ref (38). Copyright 2017 American Chemical Society.

In summary, the strategic incorporation of the appropriate additives into precursor solutions in the SCVD process is remarkably beneficial. These additives can lower the melting point of the precursor, increase the vapor pressure, enable the formation of intermediates, reduce the activation energy, and increase the wettability of the precursor solution on substrate. Collectively, these effects synergistically contribute to the efficient synthesis of 2D TMDs.

2.1.3. Recent Advances in Developing the Source Supply Method

Not only will the metal or chalcogen source itself affect the material preparation but also the source supply method can affect the growth process. In recent years, new developed source supply methods such as bubble method, dissolution–precipitation method and monomer feeding method have introduced advanced capabilities in SCVD growth.^19,36,42,58 The supply of CH₃SSCH₃ and C₁₂H₂₅SH precursor have been precisely controlled by bubble method during SCVD growth.^36,40 As Figure 5a shows, Liu et al. utilized bubbling suppling C₁₂H₂₅SH as a continuous sulfur sources to react with Na₂MoO₄ during the synthesis of 2D MoS₂.³⁶ The synthesized MoS₂ crystals possess the lower density of sulfur vacancies (0.32 nm^–2) and the quality of that can even be close to that of exfoliated MoS₂. As simulated in Figure 5b, this is because thiol molecules could be chemically absorbed into the S vacancy to repair these defects in MoS₂. It offers a new idea to repair defects and decrease vacancies density in TMDs. Liu et al. reported the growth of uniform monolayer TMDs and their monolayer alloys, such as MoSe₂, WS₂, MoTe₂, and Mo_xW_1–xS₂, using the dissolution–precipitation (DP) method (Figure 5c–e).¹⁹ The embedded metal source melted and diffused through the molten glass to its surface and reacted with chalcogen sources to attain TMDs. The soda lime glass acts as both a spin-coated substrate and metal source transferred layer, which achieves a uniform feed of the metal source while eliminating unwanted gas-phase reaction via independent diffusion path of the metal and chalcogen sources. On account of the inhibition of secondary nucleation by the DP method, the synthesized MoS₂ monolayers appear cleaner. Another strategy is to introduce an active chalcogen monomer supply.⁴² As shown in Figure 5f (left panel), Zuo et al. put forward a stacked sandwiched model of ZnS, Na₂MoO₄ coated silica fiber fabric, mica spacer, and the substrate. When heated to growth temperature, the released S monomers from the ZnS and vaporized Na₂MoO₄ passed through the fiber fabric to form monolayer MoS₂ on the substrate (Figure 5f, right panel). Figure 5g shows the OM image of as-grown MoS₂. The density of sulfur vacancy defects in MoS₂ were measured as ∼2 × 10¹² cm^–2 from STEM characterizations, which is among the lowest value of as-grown monolayer TMDs in previous literatures. Subsequently, a series of TMDs and alloys were synthesized in the same way by changing the transition metal sources (e.g., Na₂MoO₄ and Na₂WO₄) and chalcogenide plates (e.g., ZnS, ZnSe, and ZnTe). Moreover, Liu et al. produced a new bright spot in nonlinear optical fibers.⁵⁹

Figure 5.

TMDs synthesized through new resource supply. (a) Schematic image of the bubbling setup and OM images of grown MoS₂ monolayers through the bubble method. (b) Simulation of in situ repairing process by thiol molecules. Reproduced from ref (36). Copyright 2020 John Wiley and Sons. (c) Illustration of the DP growth process. (d) Detail of DP growth during nucleation. (e) OM images of synthesized MoSe₂, MoTe₂, and WS₂ nanosheets through the DP method, respectively. Reproduced from ref (19). Copyright 2021 Oxford University Press. (f) Schematic model of sulfur (S) monomer supply for the growth of MoS₂. (g) OM image of the as-grown MoS₂ monolayers through monomer feeding method. Reproduced from ref (42). Copyright 2022 Springer Nature.

The development of new source supply methods overcomes the uncontrollable nature of vaporization of solid metal and chalcogen precursors and therefore ensures the constant delivery of precursor vapors to the substrate surface. This steady delivery is essential for achieving uniformity and high quality in the synthesis of materials.

2.2. Substrate Engineering Strategies in SCVD Growth

The substrate plays a crucial role in the adsorption of source materials, promoting nucleation, and stimulating the epitaxial growth of nucleus during synthesis of 2D TMDs.⁶⁰ In the SCVD technique, various substrate engineering strategies have been developed to control nucleation, enhance growth, and improve the film quality. These strategies include careful selection of the substrate material, surface treatment, and substrate patterning.

2.2.1. Substrate Selection

Commonly used substrates in the SCVD technique include SiO₂/Si, sapphire, glass, and quartz. Given the requirements of SCVD growth, SiO₂/Si and sapphire stand out as the preferred substrate materials, meeting the specific requirements of growth of SCVD. The use of SiO₂/Si as a substrate for large-scale SCVD growth facilitates subsequent nanopatterning and integration and is highly compatible with metal-oxide-semiconductor (CMOS) technology. However, due to the amorphous surface of the SiO₂/Si substrate, the TMDs films grown on the SiO₂/Si are mostly polycrystalline with randomly oriented grains. The C-plane sapphire substrate, characterized by lattice symmetry that matches well with TMDs, proves to be more conducive for the growth of wafer-scale single crystal TMDs. Park group regulated chalcogen vapor pressure to guide MoS₂ seeds to grow along the preferred orientation of the lattice structure of the sapphire substrate, resulting in the well-aligned MoS₂ crystals (Figure 6a).²⁹ As shown in Figure 6b–d, the chalcogen atoms are inclined to low potential energy sites guided by the sapphire lattice structure, enabling the alignment of TMDs under an orientation of 0°/60°. Numerous studies⁶¹⁻⁶³ have demonstrated the crucial influence of substrates on the crystal orientation. It is expected to further investigate the impact of substrates on the orientation of TMDs in the SCVD technique.

Figure 6.

Well-aligned MoS₂ crystals on sapphire substrate. (a) Schematic illustration of controlling the chalcogen vapor pressure to grow MoS₂ on a sapphire substrate. (b–d) OM images (b,c) and the orientation histogram (d) of as-grown MoS₂ under gradient and quasi-static distributions of chalcogen vapor pressure. Reproduced from ref (29). Copyright 2021 American Chemical Society.

2.2.2. Substrate Pretreatment Strategy

To ensure uniform deposition of precursors, substrates are typically pretreated using simple methods such as O₂ plasma,^18,42,64 piranha solution,^17,65 alkaline solution^39,40 and UV–O₃⁶⁶⁻⁶⁸ to enhance hydrophilicity. These treatments improve the surface properties of the substrate and promote the adsorption of source materials, resulting in improved nucleation and growth of 2D TMDs.

For example, Figure 7 presents the OM of water contact angles on the SiO₂ surface and corresponding schematic illustration of the mechanism before and after O₂ plasma treatment.⁶⁴ After the O₂ plasma treatment, the SiO₂ surface becomes more hydrophilic. Oxygen plasma removes organic contaminants and breaks siloxane group (Si–O–Si) through chemical reactions with oxygen radicals, thus promoting the formation of Si–OH. That is, SiO₂ surface treated by oxygen plasma could form hydrogen bond with water molecules.⁶⁹ In particular, the water contact angles of the treated SiO₂ surface are 11.4°, which is lower than the water contact angle of the untreated SiO₂ surface of 45.2°, indicating an enhanced surface energy of the substrate. It is noteworthy that O₂ plasma treatment can not only eliminate natural contaminants on the surface of SiO₂, but also increase the surface energy, thereby improving the hydrophilicity. Moreover, Sodium cholate and iodixanol have been reported to function as a surfactant for increasing the wettability and viscosity of the aqueous precursor on the SiO₂/Si substrate.^21,70

Figure 7.

Optical images of the water contact angle on the SiO₂ surface and corresponding schematic illustrations before and after O₂ plasma treatment, respectively. Reproduced from ref (64). Copyright 2021 American Chemical Society.

2.2.3. Substrate Patterning

Precise patterned growth is crucial for the integration of functional materials into circuit devices but is difficult for the TMDs monolayers grown using typical CVD method due to the uncontrollable random nucleation seeds. While there have been reports of successful growth of patterned TMDs using predesigned Au, Cr, and Pt–Ti seeds, the complete removal of residual seeds remains exceedingly difficult, thereby limiting further research in this area.⁷¹⁻⁷³ People utilize micropipette tip, photolithography, and inkjet printing to pattern the substrate in the SCVD technique, enabling seed-free synthetic patterned TMDs. As is shown in Figure 8a, the strip patterned MoS₂ layers were obtained by dragging a precursor solution droplet via micropipette tip.⁷⁴ The droplet, dragged with variable velocities, induces an asymmetric shape as it moves across the substrate, resulting in spatial material deposition. It is well-known that lithography is an available patterning method.^51,64,75Figure 8b shows arrays of MoS₂ nanoflakes grown from patterned Na₂MoO₄ squares. After arrays of square windows was fabricated on SiO₂/Si substrate by photolithography, the molybdenum-containing seed material was spin-coated with Na₂MoO₄ aqueous solution.⁷⁵ The seeds accumulated at the edges of the patterned squares to form MoS₂ nanoflakes. However, it is obviously observed that the arrays of MoS₂ are not regular. Another mask-free approach seems simpler compared with photolithography. It is utilized to deposit arrays of patterns on preselected locations via the direct write patterning (DWP) technique.³²Figure 8c shows the representative images of grown patterned MoS₂ and WS₂ nanoflakes with a domain size of 8–10 μm and a thickness of average 2 nm. As for inkjet printing, it is supposed to be a desirable patterning way. The corresponding printed patterns in centimeter size like the word “JNU” were displayed in Figure 8d. When printed aqueous precursors along parallel direction began to melt, the inkjet-printed small nucleation centers appeared moving, coalescing, and merging with each other under maintaining the initial printing patterns.²³ However, further research is still needed to explore how to achieve high-precision control of precursors, enabling the preparation of single crystal patterns with high quality and industrial prospects.

Figure 8.

Synthesis of patterned TMDs via patterned substrate. (a) Schematic illustration and OM image of patterned MoS₂ layers by dragging a precursor solution droplet via micropipette tip. Reproduced from ref (74). Copyright 2021 Springer Nature. (b) Optical image of arrays of MoS₂ nanoflakes grown from patterned Na₂MoO₄ squares. Reproduced from ref (75). Copyright 2019 Royal Society of Chemistry. (c) OM and the corresponding atomic force microscope (AFM) images of patterned MoS₂ (upper) and WS₂ (bottom) nanoflakes via the DWP. Reproduced from ref (32). Copyright 2019 IOP Publishing Ltd. (d) Schematic model and OM images of monolayer “JNU” MoS₂ based on patterned aqueous precursors (upper) and the OM images of as-grown TMDs with different growth temperature along the printed aqueous precursors direction on the transparent glass substrate (bottom) via inkjet printing. Reproduced from ref (23). Copyright 2021 John Wiley and Sons.

Currently, the growth dynamics of 2D TMDs on substrates, particularly nucleation and growth processes aiming at wafer scale production of 2D TMDs, still need being explored. With the appropriate selection of substrates, modification of surface properties, or the utilization of prepatterned substrates, it is expected to gain more control over the nucleation and growth of 2D TMDs, ultimately enhancing film quality and achieving desired material properties.

3. Novel Structure of TMDs Obtained by SCVD

3.1. TMDs-Based Heterostructures

Combining different atomically thin transition metal dichalcogenides (TMDs) into vertically stacked or seamlessly connected planar structures can form van der Waals heterostructures (vdWHs) or lateral heterostructures. These heterostructures have significantly expanded the range of available material systems and provided a reliable platform for exploring novel physical and chemical properties, greatly broadening the application prospects of 2D TMDs. In recent developments, the SCVD growth method has been successfully applied in producing TMD heterostructures with large crystal sizes and clean interfaces.

Several studies have been reported on the successful synthesis of high-quality MoS₂–WS₂ lateral heterostructures using the SCVD method.⁷⁶⁻⁷⁸ Kim et al. reported the preparation of various lateral heterostructures and alloys (WS₂–MoS₂, MoS₂-alloy, alloy-WS₂) (Figure 9a,b) by precisely adjusting growth parameters including the subsequent growth temperature, and the relative supersaturation level of MoO₃ and WO₃ precursors.⁷⁶ The MoO₃ and WO₃ precursor solutions were prepared by dissolving MoO₃ and WO₃ powders in NH₄OH. Following regular spin- coating treatment, the substrates containing MoO₃ film and WO₃ particles were positioned at the bottom and top of the crucible in the furnace, respectively (Figure 9a). Four types of as-grown monolayer lateral structures can be obtained: MoS₂/WS₂ heterostructures (type A), MoS₂/alloy heterostructures (type B), alloy/WS₂ heterostructures (type C), and alloy monolayer structures (type D) (Figure 9b). At low growth temperatures (∼680 °C), due to the relatively high vaporization temperature of WO₃, there is almost no W precursor arrive the substrate coated with MoO₃, resulting in the formation of a single MoS₂ monolayer inside (referred to as type A and B). Upon heating to ∼840 °C, both MoO₃ and WO₃ simultaneously vaporize and react with gaseous sulfur, leading to the formation of alloy-type monolayers in the core (referred to as types C and D). Then at the second growth step, at a low growth temperature of about 800 °C, WO₃ vaporize while MoO₃ do not, MoS₂/WS₂ lateral heterostructure with a clean interface (referred to as type A) can be synthesized. At higher growth temperatures of about 950 °C for the second growth step, alloy often appears at the interface of lateral heterostructure (referred to as type B and D) because MoO₃ violently vaporizes at high temperatures and coexists with the WO₃ precursor in the gas phase. Until MoO₃ is completely consumed, WS₂ will grow from the lateral side of the alloy monolayer, and a type C heterostructure can be formed. Figure 9c−e shows the OM images and the corresponding Raman mappings of the well-crystallized in-plane heterostructure consisting of MoS₂ monolayer inside and WS₂ outside. In another example, it is reported by Xu et al. that through H₂-triggered reaction, MoS₂–WS₂ lateral heterostructures with an ultraclean and defect-free narrow interface were synthesized.⁷⁷Figure 9f schematically shows the two stages of the growth process. On the first stage, the MoS₂ monolayers were grown at 800 °C under pure Ar flow. Once exhausting Mo precursor completely, the H₂ was then introduced to trigger second growth of WS₂ at 900 °C. In fact, the sequential growth of MoS₂ and WS₂ is due to the different reaction conditions for the Mo-based precursor and the W-based precursor. Without H₂, only MoS₂ can grow on substrate. Under the presence of H₂, conversion of WO₃ to low-volatile W-based precursors happened and WS₂ can grow from the side of MoS₂. The typical OM image of the lateral MoS₂–WS₂ is shown in Figure 9f. The associated photoluminescence (PL) intensity mappings at 620 and 662 nm verified well the spatial distribution of MoS₂ and WS₂ domains (Figure 9g,h).

Figure 9.

Synthesis of MoS₂–WS₂ lateral heterostructure with the SCVD strategy. (a) Growth process diagram of MoS₂–WS₂ lateral heterostructures and alloys. (b) Summary table of conditions for each monolayer structure. (c-e) OM image of a large sized typical MoS₂–WS₂ lateral heterostructure and the corresponding Raman mapping images of the E′ mode of MoS₂ and the A′1 mode of WS₂. Scale bar: 50 μm. Reproduced from ref (76). Copyright 2019 American Chemical Society. (f) Schematic diagram of MoS₂–WS₂ lateral heterostructure synthesized with hydrogen-triggered reaction. (g–i) OM image of MoS₂–WS₂ lateral heterostructure on SiO₂/Si substrate and the corresponding PL intensity mapping at 620 and 662 nm. Scale bar: 5 μm. Reproduced from ref (77). Copyright 2021 American Chemical Society.

Changing the synthesis conditions to realize the sequential growth of varied materials is an important idea for synthesizing high quality vertical heterostructure with the SCVD method. Zhu et al. realized the optional growth of large WS₂/MoS₂ vertical heterostructures via hydroxide-assisted nucleation.⁷⁹ The agents KOH, (NH₄)₂MoO₄, and Na₂WO₄ were dissolved together in deionized water to obtain an aqueous solution containing −OH groups. Subsequently, the solution was spin-coated onto a clean sapphire substrate and then went through a three-stage growth process with sulfur atmospheres, resulting in vertical growth of WS₂/MoS₂. The specific reaction process and mechanism are described in Figure 10a. During the first stage, a large MoS₂–OH bilayer was obtained in an Ar + S atmosphere of about 700 °C. Figure 10b shows the OM image of a MoS₂–OH triangular domain with a length of about 600 μm. In the second stage, as the growth temperature was raised to 780 °C in the S + Ar/H₂ mixed atmosphere, the growth of WS₂ was activated, promoting the lateral epitaxy of WS₂ on the MoS₂ edges under the protection of the −OH groups protection. Figure 10c presents the as-obtained WS₂–MoS₂ lateral heterostructures with a large lateral size of 700 μm. Finally, in the third stage, as the temperature continued to rise to 850 °C, the immediate decomposition of WS₂ and the OH– layer occurred, leading to the vertical nucleation of WS₂ on the MoS₂ surface and the formation of WS₂/MoS₂ vertical heterostructures (Figure 10d). Here, the numerous OH^– ions derived from KOH in the precursor solution are crucial for the selective growth of the desired heterostructure. Besides, the NbSe₂/W_xNb_1–xSe₂ vertical heterostructures has been successfully synthesized with (NH₄)₆(H₂W₁₂O₄₂)·4H₂O and C₄H₄NNbO₉·xH₂O as W and Nb precursors.⁸⁰ As depicted in Figure 10e, at first the 2D W_xNb_1–xSe₂ domains formed; and when the reaction time increased, the Nb precursor continued to evaporate to preferentially nucleate at the edge of the as grown W_xNb_1–xSe₂ to form the NbSe₂/W_xNb_1–xSe₂ vertical heterostructures. Figure 10f shows the corresponding coverage of NbSe₂ on the W_xNb_1–xSe₂ monolayer with growth time.

Figure 10.

Synthesis of vertical heterostructures with SCVD strategy. (a) Schematic of the structural evolution of WS₂/MoS₂ heterostructures. (b–d) OM images of typical triangular MoS₂–OH and WS₂/MoS₂ lateral and vertical heterostructures. Reproduced from ref (79). Copyright 2020 American Chemical Society. (e) The Optical images and growth mechanism of NbSe₂/W_xNb_1–xSe₂ vdWHs (Nb-60%). (f) Coverage of NbSe₂ on the W_xNb_1–xSe₂ monolayer with growth time. Reproduced from ref (80). Copyright 2021 American Chemical Society.

Overall, 2D TMDs heterostructures provide a versatile platform for the integration of different materials with specific properties. To further broaden the applications of these materials, continued efforts are necessary to utilize SCVD technology to develop more extensive and various heterostructures.

3.2. Transition Metal Substitution of TMDs in SCVD

Compared with traditional CVD, the SCVD technique offers great flexibility in obtaining controllable transition metal (TM) doped TMDs. The utilization of mixed precursor solutions enables the tunability of doping concentrations for various dopants in different host materials, which resulting in tunable electrical,^{65,68,80−82} optoelectrical,^83,84 catalytic^85,86 and magnetic⁸⁷⁻⁸⁹ properties of TMDs beyond their intrinsic characteristics.

Recently, high-quality and tunable Re- and V-doped 2D TMDs⁶⁸ are synthesized using liquid precursor solutions (NaReO₄, NaVO₃, Na₂MoO₄, and Na₂WO₄) with controlled dopants molar ratio, as shown in Figure 11a. Figure 11b shows the typical FET transfer curves of V-doped WS₂ and WSe₂ monolayers. The V-doped WSe₂-contacted WSe₂ FETs has the high on-state current and on/off ratio than that of Au/Pd-contacted WSe₂ FETs in the Figure 11c, which indicated the SCVD grown V-doped WSe₂ monolayers could be as promising low-resistance electrode material for the p-type WSe₂ FETs. Such doping strategy is obvious convenient than doping through gas phase dopant, such as synthesis of Re-doped WSe₂ with precise control of the partial pressure of Re₂(CO)₁₀.⁸¹ Similarly, Fan et al. used SCVD method to regulate the electrical properties of V-doped WSe₂ with different V doping concentration and thus realizing various band arrangements.⁸²Figure 11d exhibits schematic for preparation of V-doped WSe₂ monolayers. The threshold voltage of the V-doped WSe₂ monolayer shifted to a positive value and showed obvious p-type transport characteristics (Figure 11e), and the hole mobility and carrier density increased with the increase of V doping concentration (Figure 11f). Additionally, Vu et al. precisely manipulated the ratio of Nb/(W + Nb) in the solution to control the hole carrier concentration and the corresponding contact resistance R_C value.⁸⁰

Figure 11.

Synthesis of transition meatal doped TMDs. (a) Schematic illustration of preparing different ratios of mixed salt solutions for the fabrication of Re- and V-doped 2D TMDs. (b) Typical transfer curves of V-doped WS₂ and WSe₂ monolayers. (c) Transfer characteristics of WSe₂–FETs with three kinds of contacts including Au, Pd, and V-doped WSe₂. Reproduced from ref (68). Copyright 2021 John Wiley and Sons. (d) Schematic for preparation of V-doped WSe₂. (e,f) On/off ratio and threshold voltage (e), as well as the field-effect hole mobility and intrinsic hole carrier concentration (f), vary as a function of V-doping concentration. Reproduced from ref (82). Copyright 2020 John Wiley and Sons. (g,h) HAADF-STEM images of undoped (g) and doped (h) monolayer WS₂ crystals. Scale bars are 2 nm. (i,j) Transfer characteristics of undoped (i) and doped (j) monolayer WS₂ crystals. Reproduced from ref (65). Copyright 2019 American Chemical Society.

In another example, different proportions of Na₂WO₄·2H₂O and Nb(HC₂O₄)₅·yH₂O mixed solutions were spin-coated on the SiO₂/Si to obtain Nb-doped WS₂ nanosheets with different Nb concentrations.⁶⁵ The STEM images reveal that 10.3% Nb-doped WS₂ exhibited darker spots at the W atom position compared to the undoped WS₂ (Figure 11g,h), confirming the random in situ atomic substitution of Nb doping. Furthermore, the FETs based on 10.3% Nb-doped WS₂ displayed clear indications of hole doping, while the undoped WS₂ FETs exhibited n-type behavior (Figure 11 i,j), providing evidence that the Nb dopant acts as a p-type dopant. Additionally, V-doped MoS₂ and Nb-doped WS₂ nanosheets displayed p-type conductivity,^65,90 whereas Re-doped MoS₂ presented n-type behavior.⁶⁸ In addition to the tunable electrical properties, there have also been reports of changes in optical properties. Zhang et al. continuously increased the proportion of Nb components in the Nb_1-XW_XSe₂ alloy to realize the continuous shift of PL peak from 760 to 845 nm.⁴⁸ The Nb atoms act as electron acceptors in the WSe₂, resulting in a change of the energy band gap. Eda et al. observed a well-defined impurity-induced emission in Re-doped WS₂ monolayers as neutral donors.⁸³ Gao et al. realized a fascinating up-converted luminescence on the Yb–Er codoped bilayer WSe₂ excited by 980 nm laser.⁸⁴ A series of experimental reports on as-grown doped-TMDs using the SCVD technique is summarized in Table 3.

Table 3. Summary of Common Substitutionally Doped TMDs by the SCVD Technique.

Dopant	Host	Precursor	Dopant type	Density (%)	References
P	WSe2	(NH4)6H2W12O40·4H2O + P2O5 + Se	p		(91)
V	MoS2	Na2MoO4 + NaVO4 + S	p	2.0∼6.0	(90)
	MoS2	(NH4)6Mo7O24·4H2O + NaVO3 + S	p	0.8–7.6	(92)
	MoS2	Na2MoO4 + NaVO3 + S	p	2.9	(90)
	WS2	Na2WO4 + NaVO3 + S		4.7
	WSe2	Na2WO4 + NaVO3 + Se	p	2.7
	WS2	(NH4)6H2W12O40·xH2O + VO[SO4] + S	p	0.4–12	(87)
	WSe2	(NH4)6H2W12O40 + VO[SO4] + Se		0.5∼8	(88)
	WSe2	(NH4)6H2W12O40·xH2O + NH4VO3 + Se	p	0.1∼10	(89)
	WSe2	NH4H2W12O4 + NH4VO3 + Se	p	1.0∼10	(82)
Cr	MoS2	Na2MoO4 + Na2CrO3 + S			(90)
Fe	MoS2	Na2MoO4 + FeCl3 + S			(90)
	MoSe2	Na2MoO4 + FeCl3·6H2O + Se	n	0.93–6.10	(93)
	WS2	(NH4)6H2W12O40 + FeCl3·6H2O + S		0.7–2.8	(70)
Nb	WSe2	(NH4)6H2W12O40·xH2O + C4H4NNbO9·xH2O + Se	p	0.9∼9.1	(21)
	WS2	Na2WO4·2H2O + Nb(HC2O4)5·yH2O + S	p		(65)
Re	WS2	Na2WO4·2H2O + NaReO4 + S	n	1	(83)
	WS2	Na2WO4 + NaReO4 + S			(68)
	MoS2	Na2MoO4 + NaReO4 + S	n	2.1
	WSe2	Na2WO4 + NaReO4 + Se
	MoS2	(NH4)6Mo7O24 + NaReO4 + S	n	0.3	(70)
Co	MoSe2	Na2MoO4 + CoCl2·6H2O + Se			(93)
Ni	MoSe2	Na2MoO4 + NiCl2 + Se			(93)
Yb/Er	WSe2	WO3 + NaCl + Yb2O3/Er2O3			(84)

Apart from altering the electrical properties of 2D TMDs, the incorporation of suitable doping atoms into the host can also change the magnetic properties of these materials. Both theoretical and experimental reseach have indicated that doping with transition metal elements such as Cr, Mn, Fe, and Co can enable the realization of ferromagnetism in inherently nonferromagnetic TMDs.⁹⁴⁻⁹⁹ The SCVD technique has been proven to be an effective and straightforward method for the controlled synthesis of dilute magnetic semiconductors (DMSs). V doping is one of the common magnetic doping.^87,89 Zhang et al. prepared V-doped WS₂ monolayers with V concentration ranges from 0 to 12% to obtain room-temperature ferromagnetic order.⁸⁷Figure 12a shows a schematic of the synthetic process of V-doped WS₂ monolayers. Figure 12b–d depicts the relationship between ferromagnetism and the V doping concentration at room temperature. It can be seen that, with the increase of V doping concentration, the ferromagnetic intensity of V-doped WS₂ increases. Specifically, when the concentration of V doping in WS₂ reaches 2%, the coercivity and saturation magnetization of the material reach their maximum values, which is also applied in the WSe₂ host. Yun et al. reported V-doped WSe₂ monolayer induces long-range ferromagnetism at room temperature. The OM image of V-doped WSe₂ monolayers is shown in Figure 12e. Figure 12f,g shows topography and MFM phase images of pristine WSe₂ at room temperature.⁸⁹ With the increasing V concentration from 0.5 to 8 atom % WSe₂, the magnetization gradually realized the enhancement. Figure 12h,i exhibits the existence of the ferromagnetic order from the microscopic scale of magnetic force microscopy (MFM). And the ferromagnetic enhancement of the V-doped WSe₂ monolayer has been achieved by adjusting the complex Se vacancies structure through post heat-treatment.¹⁰⁰ In addition to V atoms, Fe, Co and Ni has been successfully doped into MoSe₂ using the SCVD technique to modulate the magnetism.⁹³

Figure 12.

Synthesis of transition meatal doped TMDs. (a) Schematic of synthetic process of V-doped WS₂ monolayers. (b–d) Magnetization versus field loops of V-doped WS₂ monolayers at 0, 2, and 8 atom % at 300 K. Reproduced from ref (87). Copyright 2020 John Wiley and Sons. (e) Schematic of fabricated V-doped WSe₂ monolayers. The inset shows an optical image of SCVD-grown V-doped WSe₂ monolayer. Scale bar, 50 μm. (f,g) Topography and MFM phase images of pristine WSe₂ at room temperature, respectively. (h,i) Topography and MFM phase images of 0.1% V-doped WSe₂ at room temperature, respectively. Reproduced from ref (89). Copyright 2020 John Wiley and Sons.

4. Conclusions and Perspectives

In summary, we discussed recent advancements in the growth of 2D TMDs using the SCVD technique. A variety of liquid precursors, additives, and new source supply methods for the synthesis of larger area and higher quality monolayer TMDs, even wafer-level homogeneous films, are reviewed. Substrate engineering strategies, including selection of the substrate, surface treatment, and substrate patterning, are studied to precisely control the nucleation and growth of TMD films. In addition, we provide an overview of the latest progress in the SCVD growth of 2D TMDs heterostructures and 2D doped TMDs. Despite these advancements, challenges still exist in the SCVD technique, and it is important to address these challenges to explore new synthesis method of 2D TMD with wafer scale area, high crystal quality, high reproductivity, and low cost:

• (I)The SCVD technique offers numerous opportunities for synthesizing atomically thin and high-quality 2D TMDs. Further research should focus on finding more novel liquid precursors or utilizing additives which could increase the solubility of precursors to expand the materials library. Additionally, exploring the growth of ternary or multielement 2D materials by mixing multiple precursors could be a promising direction.
• (II)In view of the fact that monolayer MoS₂ and WS₂ single crystal films have been successfully achieved on single-crystal Au substrates and well-designed sapphire substrates, it seems to be a feasible approach to explore new or pretreated growth substrates that have good contact with precursor solutions, and then seamlessly grow and link unidirectional domains into wafer-level single crystal films by inhibiting nucleation rate and promoting edge-induced growth.
• (III)The development of patterned growth of TMDs using substrate patterning in the SCVD technique has shown many superiorities. However, there is still a need to realize large-area with well-controllable size and morphology arrays for the application in integrated circuits. Further advancements in patterning techniques will be crucial for enabling the practical implementation of TMD-based devices.
• (IV)More in-depth studies on doped/alloyed 2D TMDs are necessary. Controlling the doping concentrations and defect density of doped TMDs is crucial as it can effectively regulate the electrical and optical properties of TMDs. Further research efforts should be dedicated to understanding and optimizing the doping processes to unlock the full potential of doped TMDs in various applications.

Author Contributions

^‡ D.S. and Y.J. contributed equally.

The authors declare no competing financial interest.