Preparing molybdenum disulphide by vapour deposition

Chemical vapour deposition (CVD)

Chemical vapour deposition is a simple, efficient way to produce 2D films because of its wide tunability in growth parameters, including temperature, airflow, pressure, time, heating rate and reactant ratio. ³⁶ In this technique, the growth substrate is usually insulating SiO₂/Si, mica or sapphire.^37–39 As grown MoS₂ by CVD have different properties than that produced by mechanical exfoliation.^40,41 Plechinger et al. directly compared large area MoS₂ films grown by CVD and MoS₂ flakes prepared by mechanical exfoliation from a mineral bulk crystal. ⁴² Raman spectroscopy and photoluminescence measurements showed that the two structures had different phonon mode positions and exciton emission positions.

Following the breakthrough of using CVD to produce MoS₂ films, some CVD methods have been developed to grow monolayer or few layer MoS₂ on insulating substrates with different solid precursors under high temperatures.^17,43 Table 1 summarises the reactants used in CVD of Mo and Mo based compounds.

OPEN IN VIEWER

Table 1

Reactants of MoS₂ preparation by chemical vapour deposition

Source of molybdenum	Source of sulphur
Mo	S
Mo(CO)6	H2S
Mo	H2S
MoO3	S
MoCl5/MoF6	H2S
MoCl5	S
Mo(S-t-Bu)	Mo(S-t-Bu)
MoS2	MoS2

One of the most popular vapour deposition methods involves reacting Mo with S precursor gas evaporated from a solid sulphur source. This approach can facilitate large area synthesis of MoS₂ films over a length scale of a few inches. Zhan et al. have directly prepared monolayer and few layered MoS₂ on substrates by predepositing an Mo film in sulphur (113°C) using CVD. ⁸ Figure 1a shows the typical procedure: first, a thin layer of Mo was predeposited on SiO₂/Si by e-beam evaporation, then S was introduced and reacted with Mo at 750°C for 10 min, using N₂ as a protective atmosphere, which forms a very thin film of MoS₂. The size and thickness of the MoS₂ layer depend on the size of the substrate and the thickness of the predeposited Mo. Laskar et al. showed for the first time that CVD on epitaxial substrates can generate large area, oriented crystalline films of unprecedented high quality by sulphurising e-beam evaporated Mo films. ⁴⁴ The growth temperature and choice of substrate critically impacted the quality of the grown film. They sulphurised samples at 500, 700, 900 and 1100°C for 12 h, and their results suggest that the films grown at higher growth temperatures had much higher structural quality than those grown at lower temperatures. Lee et al. developed a large area synthesis method for preparing high quality MoS₂ films using H₂S gas and a Mo metal source (Fig. 1b). ⁴⁵ First, they deposited Mo metal on SiO₂/Si or a quartz wafer by e-beam evaporation and then placed the samples in a tube furnace, which they heated to 750°C under rough vacuum. After preannealing the Mo, they injected a reaction mixture of H₂S/H₂/Ar to synthesise MoS₂ over 15 min; during synthesis, the chamber pressure was 3·1 × 10^− 1 torr.

OPEN IN VIEWER

1

Variety of preparation methods of MoS₂ film. a model of introducing S to Mo thin film predeposited on SiO₂ substrate, CVD system used to prepare MoS₂ samples and SiO₂/Si substrate with few layers of peeled MoS₂. ⁸ b schematic of growth and transfer system used to prepare MoS₂ films; CVD can be used to synthesise large area MoS₂ films on SiO₂/Si wafer or quartz substrate. This film can be transferred to arbitrary substrate. ⁴⁵ c schematic of experimental set-up, and AFM image of monolayer MoS₂ film on SiO₂/Si substrate. ⁴⁹ d schematic of promoter and growth substrates, showing various regions (A-P) divided according to distance from promoter substrate to growth substrate, along with corresponding optical images of MoS₂ flakes in those regions. ⁵² e schematic of synthesising MoS₂ layers by MoO₃ sulphurisation. First, MoO₃ layer is thermally evaporated on sapphire substrate, then MoO₃ is converted to MoS₂ by two-step thermal process. ⁵⁴ f Growth set-up, conditions and SEM of triangular MoS₂ monolayer ⁵⁷

Another CVD method involved using MoO₃ in place of the Mo metal source. Sulphurising MoO₃ has been comprehensively studied; it is the main MoS₂ synthesis technique, as MoO₃ has low melting and evaporation temperatures. ⁴⁶ Typically, MoO₃ is partially reduced by sulphur vapour to form volatile MoO_3-x species, which flow downstream in a carrier gas, adsorb onto the substrate, diffuse on the surface and react with sulphur to rearrange into MoS₂ layers (equation (1))^47,48 (1) Lee et al. produced MoS₂ film with a thick of ∼0·72 nm in 2012. They placed a separate ceramic boat with S powder (0·8 g) next to the boat with MoO₃ powder (0·4 g), with the SiO₂/Si substrate mounted face down on top of the boat containing MoO₃ powder, the reaction chamber was heated to 650°C in nitrogen ⁴⁹ (Fig. 1c). Cheng and Park synthesised monolayer MoS₂ on a SiO₂/Si substrate in a hot wall furnace. High purity MoO₃ and S powders were placed in the furnace, and the reaction chamber was heated to 700°C in flowing Ar. Then, they kept the chamber at 700°C for 15 min before cooling.^50,51

The growth conditions of MoS₂ are very sensitive to surface treatment. Layer growth can be promoted with aromatic molecules, such as graphene, reduced graphene oxides, perylene-3,4,9,10-tetracarboxylic acid tetrapotassium salt and perylene-3,4,9,10-tetracarboxylic dianhydride, as well as with F₁₆CuPc and CuPc.^49,52,53 Lee et al. used various aromatic molecules as seeding promoters to produce large area, highly crystalline, uniform MoS₂ monolayers with CVD (Fig. 1d). Growing MoS₂ also depends on seed concentration. For example, Fig. 1d shows a promoter substrate covered with condensed perylene-3,4,9,10-tetracarboxylic acid tetrapotassium salt seeding promoters. The growth substrate was divided into various regions (A–P), which shows the different flake sizes and surface coverage varied in interesting ways. From region I onward, the size of monolayer flake decreases gradually, but the density of the flake appears to be increased.

Lin et al. deposited MoO₃ on a c-face sapphire substrate using a thermal evaporator. ⁵⁴ First, they placed an MoO₃ film coated on a c-face sapphire substrate in the cold zone of a quartz tube flowing with Ar/H₂. When the centre of the furnace reached 500°C, they moved the substrate to the hot zone for the first anneal. After 1 h, they moved the sample to the cold zone and placed a ceramic boat of S powder in the cold zone. After the hot zone reached 1000°C, they moved the samples and the boat to the centre of the furnace for a second anneal for 30 min with Ar flowing at 70 sccm (Fig. 1e). The chemical equations for the two-step reaction are proposed in equations (2) and (3) (2) (3) Amani et al. also fabricated monolayer MoS₂ films using MoO₃ and sulphur powder. ⁵⁵ They first dispersed MoO₃ nanoribbons on an auxiliary silicon substrate by a hytrothermal process; the temperature of sulphur powder placed upstream of furnace is ∼600°C, and the deposition temperature and time is 850°C and 10–15 min under a constant flow of nitrogen respectively. Wang et al. synthesised MoS₂ flakes by sulphurisation of MoO₂ microcrystals. ⁵⁶ First, MoO₂ was nucleated on substrates using MoO₃ powder and sulphur vapour at 650–850°C. Second, the MoO₂ microplates were annealed in sulphur vapour at 850–950°C for 0·5–6 h in nitrogen. Last, the MoS₂ was separated from as made MoS₂/MoO₂ plates. The MoS₂ flakes showed high crystallinity with size of ∼10 μm.

Wu et al. reported a new, straightforward method for obtaining monolayer MoS₂ with high optical quality, ⁵⁷ using MoS₂ powder as the source of both Mo and S. The powders were placed at the centre of a horizontal quartz tube furnace, maintaining the carrier gas flow and setting the pressure to ∼20 torr. The furnace temperature was increased to ∼900°C, holding it for 15–20 min (Fig. 1f ). In this way, MoS₂ can be grown on various insulating substrates, which should facilitate device fabrication without need for a transfer process.

In the earlier time, Mo(CO)₆ and H₂S can be used to produce MoS₂ in the temperature range of 175 to 300°C.^58,59 After a short while, researchers found that the MoS₂ films can be deposited by reaction of molybdenum halides with H₂S. ⁶⁰ Lee et al. reported that the MoS₂ CVD film was fabricated by reaction of H₂S with MoF₆ at 1 kPa and at temperature as low as 703 K. ⁶¹ Endlera et al. obtained MoS₂ lubricant coatings film using a gas mixture of MoCl₅, H₂S and Ar. ⁶² In 2014, Yu et al. synthesised MoS₂ atomic films by placing MoCl₅ and S powders in a furnace at a growth temperature of 850°C, flowrate of 50 sccm and pressure of ∼2 torr. ⁶³ Cheon et al. deposited MoS₂ from the metal organic precursor Mo(S-t-Bu)₄. ⁶⁴ The MoS₂ film was deposited using a Pyrex high vacuum apparatus containing substrate temperatures of 110–350°C and a pressure of 10^− 4 torr, and the film thickness depended on the deposition time and the total precursor flux. However, in recent years, few have used MoCl₅ and Mo(S-t-Bu)₄ as Mo sources.

There are many types of CVD, including low pressure CVD, which is the most common type. ⁶⁵ In this method, the reaction chamber in the furnace should be at low pressure; this makes the reaction more downy, which is better than depositing on a single sample. Other methods include metal organic CVD and plasma activated CVD,^66–68 which can produce films and nanoparticles. However, these methods are not usually used to produce MoS₂ films.

Physical vapour deposition (PVD)

An early application of MoS₂ was lubrication, as surface coating can effectively improve the durability of materials used in aggressive environments.^69,70 Methods useful for depositing coatings include PVD and magnetron sputtering.^33,71

Recently, some researchers have tried to prepare MoS₂ films by PVD. ⁷² Most recent papers have reported monolayer MoS₂ exhibiting triangular morphology. However, the triangular morphology appeared as early as 2000 in samples produced by PVD. ⁷³ Later, Lauritsen et al. obtained the same morphology of triangular MoS₂. Initially, they deposited Mo by PVD onto Au(111) in H₂S at 10^− 6 mbar at 400 K. The Au substrate acted as a nanotemplate. They then annealed the substrate at 673 K for 15 min in H₂S. The cluster morphology of the triangular MoS₂ that they produced depended strongly on the size of substrate. ⁷⁴

Depositing metal contacts may also substantially affect the structure and electronic properties of MoS₂.^75,76 For example, Gong et al. deposited three different metals (Pd, Au and Ag) on monolayer MoS₂. ⁷⁷ They placed MoS₂ powder source downstream, far from the oven centre in a cooler zone, and increased the furnace temperature to 900°C, keeping the growth temperature to ∼650°C. For experimental details, refer to Ref. 54. They heated the metals before deposition for 5 min, and then e-beam evaporated the metals on the MoS₂ samples at 2 × 10^− 6 torr. Their Raman spectra showed that Pd wet the MoS₂, forming a uniform, intimate contact, while the Au and Ag adsorbed inhomogeneously.

Composite

Currently, graphene is one of the most studied compounding materials for MoS₂.^81,82 Yu et al. produced high performance graphene/MoS₂ heterostructure device, using the MoS₂ as the transistor channel and graphene as the contact electrodes and circuit interconnects (Fig. 2a). ⁸³ They deposited 20 nm aluminium oxide (Al₂O₃) on the CVD-MoS₂ by atomic layer deposition. Large area single layer graphene was then grown on Cu foils at 1035°C by CVD and transferred onto a sample in a vacuum probe station at 3 × 10^− 6 torr. The result shows that this contact between graphene and MoS₂ was ohmic, and the paper is the first demonstration of logic circuits using CVD grown 2D heterostructures. The output characteristics of their inverter, a low voltage of − 4 V represents logic state 0 and a voltage close to 0 V represents logic state 1.

OPEN IN VIEWER

2

Compositing, decorating and doping of MoS₂ film: a schematic of important steps to fabricate large scale electronics from graphene/MoS₂ heterostructures, using MoS₂ as channel and graphene as contact electrodes and interconnects; ⁸³ b SEM image of MoS₂/3DGN composite (arrow indicates microscale MoS₂ flakes); ⁸⁴ c selenised MoS₂ (at 800°C) on sapphire substrates; ⁸⁸ d schematic of Nb doping in CVD grown MoS₂ ⁹⁴

As a different example of composite films, Zhang et al. produced MoS₂ coated 3D graphene networks (3DGNs) (Fig. 2b). ⁸⁴ After synthesising the 3DGNs by CVD, they prepared an MoS₂ precursor solution by mixing (NH₄)₂MoS₄ in dimethylformamide. Then, they placed the 3DGNs into the MoS₂ precursor solution for 5 min, before transferring it to the CVD chamber. The CVD chamber was pumped down to 10 mtorr, and the MoS₂ precursor coated 3DGNs were annealed at 50°C for 1 h. Then, the temperature was increased to 400°C in an Ar/H₂ protective atmosphere. After the temperature stabilised, the MoS₂ precursor coated 3DGNs was annealed at 400°C for 1 h. Then, the furnace was cooled naturally to room temperature. The MoS₂/3DGN composite exhibited good cycling performance when used as an anode for lithium ion batteries. It exhibited reversible capacities of 877 and 665 mA h g^− 1 during the fiftieth cycle at current densities of 100 and 500 mA g^− 1 respectively.

Decorating

To improve the performance of hydrodesulphurisation in petroleum, researchers have attempted to deposit Co on MoS₂.^85,86 For example, Okamoto deposited Co(CO)₃NO onto MoS₂ catalysts supported on SiO₂. They presulphided MoS₂/SiO₂ in a 10%H₂S/H₂ stream at 400°C using Co(CO)₃NO. ⁸⁷ After exposing the MoS₂/SiO₂ to the Co(CO)₃NO vapour for 5 min, they evacuated the sample for 10 min at room temperature, and then performed a second sulphidation under the same conditions as the first.

MoS₂ is very interesting because its band gap changes from indirect in the bulk crystal to direct in single layers. Li et al. reported that the CVD grown MoS₂ monolayer flakes can be selenised in the presence of selenium vapour on sapphire substrates. ⁸⁸ First, they placed as grown triangular monolayer MoS₂ at the centre of a quartz tube furnace. Then, they heated Se powder, placed close to the open end of the furnace at the upstream position, to 270°C and various other temperatures using a separate heating belt for 4 h. As the selenisation temperature changed, the band gap varied from 1·86 to 1·57 eV (Fig. 2c).

Shi et al. decorated the edges of monolayer MoS₂ with Au nanoparticles using a controllable wet method. ⁸⁹ Using MoO₃ and S as precursors for CVD, they produced high quality crystalline MoS₂, which they directly immersed in 5 mM AuCl₃ for 20 s. They formed MoS₂ transistors with an ionic gel film, which works as a dielectric layer. Decorating the MoS₂ with Au NPs decoration changed the threshold gate voltage from 0·47 to 1·29 V and shifted the Fermi level in MoS₂ layers down. Decorating the device increased the on/off ratio by ∼50 times.

Doping

Dopants are impurities that enhance electrical conductivity and photocurrent gain. ⁹⁰ Currently, research on doped MoS₂ is very limited. Rhenium doped MoS₂ single crystals have been grown using chemical vapour transport with Br₂ as the transport agent. ⁹¹ In this study, pure Mo, Re and S were sealed in a quartz ampoule containing Br₂; at a temperature gradient of approximately 960–930°C, the productions are MoS₂ doped with 0·5–1% rhenium.

Doped niobium was reported several decades ago,^92,93 but only large crystals were investigated. In 2014, Laskar et al. reported a CVD method for niobium doped MoS₂. ⁹⁴ Using e-beam evaporation, they produced Mo (2·5 nm)/Nb/Mo (2·5 nm) on sapphire, which they sulphurised in a CVD chamber at 900°C for 10 min (Fig. 2d ). They found that Nb can act as an efficient acceptor in MoS₂ leading to a high hole density and relatively high mobility: at room temperature, a hole concentration of 4 × 10²⁰ cm^− 3 and hole mobility of 8·5 cm² V^− 1 s^− 1.