Nanostructured MoS₂ can be fabricated via both top-down and bottom-up approaches. In the case of the top-down method, the commercially available bulk crystal of MoS₂ is physically downsized into MoS₂ nanomaterials (Figure 1) [29,40,41], while in the bottom-up approach, MoS₂ nanomaterials are synthesized via chemical reaction with small molecules using chemical vapor deposition (CVD) and hydrothermal or solvothermal methods, etc. [42,43,44]. Single layers, multilayers, nanoparticles, and quantum dots of MoS₂ have also been reported [45,46,47,48]. Continued efforts have been reported for the fabrication of MoS₂ nanomaterials via the top-down and bottom-up strategies [16,17,18,19,28,29,30,31,40,41,42,43,44].

Due to the layered structure and van der Waals interactions, MoS₂ nanosheets can be easily prepared through the exfoliation method. Mechanical, chemical, electrochemical, and liquid-phase exfoliation processes have been reported for the synthesis of MoS₂ nanosheets [39,40,41,42,43,44,45,46,47,48,49,50,51,52]. For example, in the mechanical exfoliation technique, the suitable MoS₂ flakes are peeled off from the bulk crystal of MoS₂ using adhesive tape and shifted onto a specific substrate [46,53]. When the scotch tape is detached, some parts of MoS₂ remain on the substrate. As result, single- or few-layer MoS₂ nanosheets with random shapes and sizes are obtained. The 2D materials prepared by the exfoliation method have good quality and allow to study the pristine properties of materials and device performance. However, during this process, the thickness and size of the MoS₂ are difficult to control, and the resulting materials are inappropriate for large-scale production and scaled-up applications [53,54]. Li et al., mechanically exfoliated single- and multilayer MoS₂ nanosheets from SiO₂/Si with the adhesive tape method [41]. The flakes of MoS₂ were mechanically stripped on Si/SiO₂ substrate. The obtained single-layer and multilayer MoS₂ materials were characterized using a bright-field optical microscope and an atomic force microscope (AFM). From the AFM measurements, the height of a single MoS₂ sheet was found to be 0.8 nm, while the thickness of two, three, and four layers of MoS₂ nanosheets was 1.5, 2.1, and 2.9 nm, respectively (Figure 2). The MoS₂ nanosheet monolayers showed an enhanced optical performance, especially single-layer MoS₂ nanosheets. It was observed that the van der Waals interactions between MoS₂ to SiO₂ were much weaker. For this purpose, gold can be used as a substrate to exfoliate the MoS₂ nanosheets due to its strong affinity for sulfur. It can exfoliate the MoS₂ monolayer from the bulk because of the strong van der Waals interactions between Au and MoS₂ layers [55,56,57]. Huang et al. prepared large-area MoS₂ nanosheets using a Au-assisted exfoliation strategy [50]. In a typical synthesis, a Au thin layer was deposited on a Ti or Cr adhesion-covered substrate. To develop good contact between a MoS₂ bulk crystal on tape and a Au-covered substrate, it should be passed under high pressure. The monolayer sheets with a large area were collected from the surface of the Au after peeling off the tape.

In the top-down approaches, single- and multilayer MoS₂ nanosheets are prepared, which have been used to study some fundamental properties of MoS₂ nanosheets.

The CVD technique has a long history and is commonly used for the synthesis of high-quality semiconductor materials. In a typical CVD process of MoS₂ nanosheets, the Mo sources are solid precursors of Mo or MoO₃ powder, and the S sources are H₂S gas or solid S powder [58,59,60,61]. The solid MoO₃ and vaporized S react with each other in a low-pressure chamber, forming nuclei for the growth of MoS₂ [58]. Then, MoS₂ slowly grows and enlarges its size on the substrates under carrier gas flow. The temperatures at which MoS₂ grows during the CVD process are usually between 700 and 1000 °C, with a metal catalyst such as Au [61]. Plasma-enhanced CVD requires a low temperature (150–300 °C) for the growth of MoS₂ nanosheets, and MoS₂ can even be directly deposited on the plastic substrate [62]. Recently, metal organic CVD has been reported for the synthesis of MoS₂ nanosheets [63,64], where organometallic precursors were used as starting materials.

Advanced technology such as molecular beam epitaxy (MBE) can be used to prepare single-crystal semiconductor thin films. However, its applications are limited to the synthesis of 2D materials [65]. Ordinary physical vapor deposition is rarely reported for 2D materials. A MoS₂–Ti composite was prepared by direct current magnetron sputtering, using Ti and MoS₂ materials [66]. In this process, the MoS₂ was amorphous.

Solution-based processes are commonly used to synthesize MoS₂ nanosheets. Hydrothermal and solvothermal methods are the most interesting for the preparation of MoS₂ nanosheets [67,68]. In these methods, the Mo source is commonly a molybdate, such as Na₂MoO₄ or (NH₄)₆Mo₇O₂₄, and the S source is thiourea and thioacetamide and L-cysteine [69,70,71,72,73]. The molybdate reacts with the S or S compound in a stainless steel autoclave. The physicochemical reaction takes place at high temperatures (160–200 °C) and pressure for at least a few hours. In the solvothermal method, organic solvents such as 1-methyl-2-pyrrolidinone, N,N-dimethylformamide, and polyethylene glycol-600 are used to proceed with the reaction, while in the hydrothermal method, water is used as a solvent. The MoS₂ powders obtained from these methods have different sizes and shapes. The sizes and shapes of the products can be adjusted by altering the experimental conditions. To improve the crystalline quality of MoS₂, the products are usually post-annealed at high temperature.

The MoS₂ nanomaterials prepared through different bottom-up approaches have various sizes, shapes, morphologies, and thicknesses and can be used for many applications.