Mechanical exfoliation, also known as micromechanical cleavage, is a straightforward technique that takes advantage of the weak bonding between layers, for the production of high-quality mono- to few-layer MoS₂ [58,59,60]. It consists of exfoliating thin films of 2D-MoS₂ from a bulk MoS₂ crystal by using a low surface tension tape to break the weak interlayer bonds in a similar way as for grapheme [61]. Additional exfoliation of the extracted films may be needed to obtain few- to monolayer MoS₂. Tapes could be attached to glass slides to achieve planar exfoliation and slow peeling. The obtained monolayers are usually transferred to an appropriate substrate for further analysis and testing.

The advantage of the mechanical exfoliation process lies in its simplicity that requires the sole use of a confocal microscope to localize the 2D-MoS₂ layers deposited on the substrate. Conveniently, this technique can produce high crystalline quality mono- to few layers with a lateral size up to few tens of micrometers, making them highly suitable for sensing applications. However, this approach suffers from a lack of a consistent control in producing the 2D monolayers as it is heavily user-dependent and does not permit the control of the size and/or thickness uniformity of the exfoliated 2D-MoS₂ layers [62]. Therefore, the mechanical exfoliation technique is not necessarily suitable for the production of 2D-MoS₂ layers intended for large-area and high-throughput applications.

Chemical exfoliation, on the other hand, appears as a promising approach to produce large quantities of mono- and few-layer MoS₂ nanosheets [60,63,64,65]. Eda et al. [54] reported a high yield of monolayer crystal synthesis using chemical exfoliation of bulk MoS₂ via Li intercalation. However, this approach may induce an alteration in the quality of the produced 2D-MoS₂. For instance, the chemically exfoliated MoS₂ layers can lose their semiconducting properties because of the structural changes resulting from the Li intercalation process. However, this fabrication route stands by its ease of processing, low production costs, and suitability for catalysis and/or sensing applications [66].

Chemical vapor deposition (CVD) is one of the most popular routes for large-scale, high-quality, and low-cost 2D-MoS₂ material production [49,67,68,69]. CVD is a bottom-up fabrication method at the equilibrium state, which enables the processing of layered 2D-MoS₂ with controlled morphology and good crystallinity while minimizing structural defects. The control of the CVD process is ensured by tuning the deposition parameters such as temperature, pressure, gas flow rate, precursor’s quantities, and substrate types. The 2D-MoS₂ synthesis via the CVD technique can be achieved by means of thermal vapor sulfurization (TVS), thermal vapor deposition (TVD), and thermal decomposition (TD). Deokar et al. [43] used TVS for high quality and vertically-aligned luminescent MoS₂ nanosheets. A similar process could be used to grow 2D-MoS₂ layers [36,70] by employing two sources, such as molybdenum thin film (below 20 nm) or molybdenum oxide (MoO₃) powder deposited on a SiO₂/Si substrate as a first precursor and the sulfur powder or gaseous sulfur source (H₂S, etc.) as the second precursor [49,67,68,69,71,72]. A typical CVD sulfurization process (Figure 2a) is usually performed in a tubular furnace reactor, where a continuous argon flow (typical flow rate 100 sccm) is used as a carrier gas to stream the evaporated sulfur into the Mo source materials.

One of the critical aspects to be controlled in such a CVD tubular reactor is the temperature gradient between the S powder and the substrate. In fact, while the S powder is at 150–200 °C, the substrate’s temperature—with or without Mo thin film—should be maintained in the 700–900 °C range to obtain the 2D-MoS₂ phase. This technique offers sufficient latitude to fairly control the thickness and the homogeneity of the grown 2D-MoS₂. The typical average lateral crystal size obtained by CVD is usually in the 10–30 nm range. Table 2 shows few examples of CVD-TVS grown MoS₂ nanostructures along with their associated processing conditions.

Table 2 shows the typical morphologies obtained for MoS₂, which seem to depend on the carrier gas and the type of the substrate used. The reaction time and the spatial position of the substrate strongly affect the number of resulting layers.

The TVD based MoS₂ growth (Figure 2b) involves the concomitant evaporation of both MoO₃ and S powders. This approach consists of a stepwise sulfurization of MoO₃ to form the MoS₂ phase. It has been shown that, by increasing the S vapor flux, the sulfurization proceeds through several phase changes before reaching the final product. First, MoO₃ is formed, then MoO₂ followed by MoOS₂, and finally MoS₂. This approach is very useful to obtain 2D MoS₂ layers with a lateral size of few tens of microns. The TVD growth conditions of MoS₂ under various conditions and with different characteristics are summarized in Table 3.

In comparison to the results obtained by CVD-TVS summarized in Table 2, TVD exhibits high-yield fabrication of 2D-MoS₂ monolayers generally exhibiting a triangular flakes shape. Besides, one can notice the two possible configurations of the substrate of interest in TVD face-up and face-down compared to CVD-TVS [75,76,77,78,79].

Moreover, the TD-based CVD method presents an alternative approach to produce highly crystalline MoS₂ thin layers with superior electrical properties on insulating substrates [34]. Typically, the TD-CVD is based on the high-temperature annealing of a thermally decomposed ammonium thiomolybdate layer (NH₄)₂MoS₄ in the presence of S, as illustrated in Figure 2c. It is worth noting that the excess in sulfur introduces changes in the shape, size, and morphology of fabricated MoS₂. It also leads to a p-type MoS₂ semiconductor by increasing the electrons deficiency. In contrast, the presence of sulfur vacancies in MoS₂ was reported to have a direct impact on the catalytic properties of MoS₂, suggesting a carriers’ mobility alteration [80]

Besides, the addition of S during the high-temperature annealing drastically enhances the crystallinity of MoS₂. Relatively, centimeter-sized MoS₂ crystals could be formed on Al₂O₃ substrates compared to SiO₂ ones [35]. The fully covered Al₂O₃ substrate with an epitaxial monolayer of MoS₂ was achieved at 930 °C. The MoS₂ crystals nucleate in a single domain to pursue by domain-to-domain stitching process occurring during annealing at 1000 °C mediated by the oxygen flow. The difference in the self-limited monolayer growth observed between the SiO₂ and Al₂O₃ substrates is related to the absorption energy barrier on MoS₂ [37]. In particular, the growth of MoS₂ on Al₂O₃ obeys the surface-limited epitaxial growth mode, which is not the case for the SiO₂ due to lattice mismatch. Moreover, the patterning of the as-grown MoS₂ layers has been reported by means of the polydimethylsiloxane (PDMS) stamps and the reuse of the substrate after transferring the MoS₂ layers [35]. Recently, the epitaxial growth of centimeter wafer-scale single-crystal MoS₂ monolayers on vicinal Au (111) thin films were also obtained at a processing temperature of 720 °C, by melting and re-solidifying commercial Au foils [36]. This allows overcoming the evolution of antiparallel domains and twin boundaries, leading to the formation of polycrystalline films. It has been proposed that the step edge of Au (111) induced the unidirectional nucleation, growth, and subsequent merging of MoS₂ monolayer domains into single-crystalline films.

The atomic layer deposition (ALD) technique is known to produce high-quality thin films even at low temperatures, typically between 150 and 350 °C. Since ALD is an atom stepwise growth process, where the reactants are alternately injected into the growth area, it allows the purging of excess species and by-products after each reaction. As a result, high-quality films are obtained by sequential surface reactions. A schematic representation of the ALD synthesis of 2D-MoS₂ can be found elsewhere [81].

Despite the challenges related to its synthesis conditions, ALD makes it possible to deposit crystalline MoS₂ thin films at a relatively low temperature (<350 °C) followed by annealing. For instance, L.K. Tan et al. [82] reported the possibility to use ALD for the synthesis of highly crystallized MoS₂ films on sapphire substrates at 300 °C. They prepared MoS₂ films by alternating exposure of the substrate to Mo(V) chlorides (MoCl₅) and hydrogen disulfide (H₂S) vapors. Similarly, Mattinen et al. [83] proposed the use of a Mo based precursor, namely Mo(thd)₃ (thd = 2,2,6,6 tetramethylheptane 3,5-dionato), with H₂S as a sulfur source. They have been able to achieve a self-limiting growth and a linear film thickness control (with a very low growth rate of ≈0.025 Å per cycle). While the crystallinity of these MoS₂ films was found to be particularly good (taking into account that the deposition was done at a low temperature), their surface was rather rough, consisting of flake-like grains with a size of ≈10–30 nm. One of the advantages of this process is the possibility to deposit layered MoS₂ films on various substrates. Table 4 summarizes the main processing conditions used by different groups along with the achieved MoS₂ film thicknesses.

The ALD appears as a potentially interesting technique for the production of high-quality MoS₂ ultrathin films at relatively low temperatures and with the ability to achieve excellent step coverage onto different substrates. However, the very low throughput of the ALD might hinder its scalability and competitiveness in comparison with other physical and/or chemical deposition methods.

Pulsed laser deposition (PLD) has emerged as one of the most promising physical vapor deposition (PVD) techniques for the deposition of MoS₂ thin films. The PLD approach consists of shining a focused high-power laser beam onto the surface of a solid target to be ablated and deposited as a film on a substrate. PLD is a non-equilibrium process that leads to the absorption of very-short (15–20 ns) and highly-energetic laser pulses by the target and to the formation of a directive plasma plume. The laser-ablated species that form the plasma plume condense onto the substrate, leading to the growth of a thin film. The PLD is well known for its large process latitude, high-flexibility, and excellent process controllability. For instance, by controlling the number of laser ablation pulses and/or the background gas pressure, nanoparticles, and/or films with thicknesses varying from few nm to few microns can be synthesized. Figure 3 shows a schematic representation of a PLD system.

Among the advantages and the unique features of the PLD method, we can cite: (i) its ability to achieve a congruent transfer to the films when a multi-element target is used [91]; (ii) its highest instantaneous deposition rate along with the highly-energetic aspect of the ablated species (~10 times higher than in sputtering) enables the growth of metastable phases and/or crystalline phases even at room temperature; and (iii) its process latitude, which makes it easy to control almost independently each of the deposition parameters (laser intensity, number of laser ablation pulses, background gas pressure, and substrate temperature), and hence the properties of the deposited materials [92,93,94]. While the early studies on the PLD of MoS₂ date back to the 1990s [95,96,97,98,99,100], it is only recently that important advancements have been made in PLD synthesis of 2D-MoS₂ films onto various substrates opening thereby the way to their use for different optoelectronic applications. In 2014, PLD was successfully used to grow one to several layers of MoS₂ onto different metal, semiconducting, and sapphire substrates [101,102]. Siegel et al. [103] were the first to report, in 2015, the growth of MoS₂ films (from 1 to a few 10s of monolayers thick) on centimeter-sized areas. Other attempts were made to deposit ultrathin (≤3 nm) films of nearly-stoichiometric amorphous MoS₂ onto irregular surfaces such as silicon and tungsten tips and to study their field electron emission (FEE) properties [95]. The authors stated that the addition of the MoS₂ coating is beneficial to the FEE process since lower electric fields were required to extract an electron current density of 10 μA/cm² (namely, 2.8 V/μm for MoS₂-coated Si and ~5.5 V/μm for MoS₂-coated W tips). More recently, PLD has been used to fabricate high-quality MoS₂ films (monolayer to few layers) and integrated them into functional ultraviolet (UV) photodetectors [104]. The developed photodetectors were found to exhibit a very low dark current (~10 × 10⁻¹⁰ A), low operating voltage (2 V), and good response time (32 ms). Their performance surpassed that previously reported for 2D-MoS₂ synthesized by other routes [105,106,107,108,109]. Indeed, under UV irradiation, their detectivity, photoresponse (I_on/I_off ratio), and responsivity were found to be as high as 1.81 × 10¹⁴ Jones, 1.37 × 10⁵, and 3 × 10⁴ A/W, respectively. Table 5 summarizes most of the papers reported so far on the PLD of MoS₂ films. More specifically, it compares the main PLD growth conditions of 2D-MoS₂ films along with the obtained crystallographic phase and some of the reported optoelectronic properties.

In addition to the main fabrication methods presented above, other PVD techniques have been used to deposit 2D-MoS₂ films. Among these methods, magnetron sputtering has been used to deposit both MoS₂ and WS₂ films onto polydimethylsiloxane (PDMS) polymer substrates [37,127,128,129,130] with controllable defect densities. The PDMS substrate was chosen to fabricate flexible devices based on 2D-semiconducting materials. Interestingly, very smooth MoS₂ surfaces, with a roughness of less than 2 nm, were achieved by casting the polymer on a polished silicon wafer. It has also been shown that it is possible to induce subsequent crystallization of MoS₂ by exposing it to a pulsed 532 nm laser [127].

Finally, the use of any of the above-discussed techniques to fabricate 2D-MoS₂ films is mostly dictated by the availability of the equipment, expertise, and requirements of targeted application. In a general context, the physical-chemical and optoelectronic properties of the final MoS₂ films will be determined to select the appropriate synthesis route. Nevertheless, the level of complexity, throughput, and fabrication costs have to be considered to choose the appropriate synthesis technique particularly when a technology has to be adopted. Table 6 provides a general comparison of the preparation techniques of MoS₂ described in this review by listing their main advantages and limitations.