1. Introduction

Molybdenum disulfide (MoS₂) is an inorganic compound of the transition metal dichalcogenides (TMDs) series, that has one atom of Molybdenum and two atoms of Sulfur. Dichalcogenides are chemical compounds consisting of a transition metal, like Molybdenum, and a chalcogen (element of group 16 in the periodic table) like sulfur (S) [1]. The physical, chemical, and electronic properties of this compound grabbed the attention of many researchers and were found promising materials to substitute previously used semiconductor and/or graphene devices. As the world is moving towards miniaturization, researchers were searching for a material to substitute semiconductor devices that seemed to reach an end when entering the nanoscale era [2]. While semiconductors devices based on Silicon were facing quantum and tunneling effects on a nanoscale level, MoS₂ showed favorable and promising electronic and quantum characteristics when going from bulk to two-dimensional (2D) structure [3].

MoS₂ seems to solve many problems facing previous devices [4,5,6,7,8], it has a large band gap (~1.8 eV) which changes from an indirect gap to a direct one in thin structures. This would permit downscaling electronic devices, rather than graphene which nearly has a zero-band gap [9,10]. It does not have surface dangling bonds and has high mobility even with high-κ dielectric materials. It is ideal for thin-film transistors, and its fabrication is simple which means large production yield and low cost [11,12]. The covalent bonds between Molybdenum and Sulfur and the Van der Waals bonds between its layers make it optimal for gas sensing. One of the major problems with Silicon devices was that related to the metal-semiconductor interface [13]. MoS₂ has less contact resistance and high performance. In other words, MoS₂ has potency to be used in 1 nm gate transistors with excellent on/off switching characteristics and high efficiency [14].

Silicon transistors fabrication faced some problems on the bulk scale that were overcome by new structures such as multi-gate transistors, but when going down to the nanoscale, the fabrication process seems to reach an end. The metal interconnection lines between transistors have high density and are very narrow, which cause an increase in resistance and capacitance between interconnect lines and high delays. Tunneling problems are more obvious with thin gate oxides and when trying to substitute silicon dioxide with other high-κ dielectric materials, more serious problems like thermal instability, channel mobility degradation, incompatibility with the interface aroused. All these issues lead to performance degradation, and high cost with a small yield. The high lithography resolution needed for small half-pitch (HP) ~20 nm, is not easily achieved and needs high techniques and tools in lithography [15,16]. On the other side, MoS₂ showed easier and simpler ways of synthesis and device fabrication [17]. It is easily prepared by spreading the molybdenum metal and sulfur and letting them self-propagate under a high temperature [18]. A MoS₂ field effect transistor was fabricated in [19] by immersing it in an electrolyte, the device showed lower contact resistance and a better gate control.

MoS₂ has a wide range of applications in different fields. Like Silicon and graphene, it has applications in biosensing, and optical sensors but the most important ones are those related to bio-applications like DNA, cancer, and Corona Virus detection [20,21,22,23]. While Silicon and graphene were still under study in their compatibility with human bodies, a study, in [24], showed that MoS₂ may be very effective in curing cancer and Alzheimer’s disease. It also showed that the compound has no biological interaction which makes it safe for injection in human bodies. Another study in [25] proposed a biocompatible device made of MoS₂ to restore some visual malfunctioning. The compound applications are not restricted to electronics, but it can serve as a lubricant [26] and is used in hydrogen evolution reactions. It is a suitable material for batteries’ electrodes [27]. Indeed, MoS₂ properties and structure made it promising for several electronic, sensing, microwave, and terahertz applications.

2. Structure and Properties

MoS₂ structures differ from 3D, 2D, one-dimensional (1D), or dot structures. Its characteristics and applications also change from one dimension to another, they can be semiconducting, metallic or superconducting. It exists in several layers and shapes. Its bulk (3D) structure can be tri-agonal (T), hexagonal (H), and Rhombohedral (R), where 2H MoS₂ means 2-layer hexagonal shape MoS₂. The three main structures are 1T, 2H and 3R, where the 1T phase coordinates in an octahedral structure, 2H and 3R in a trigonal prismatic structure [32,33] as shown in Figure 1. The lattice constants for each structure are shown in Table 1 [34]. The 1T structure is known to be metallic while the other two are semiconducting. The monolayer of hexagonal MoS₂ is also semiconducting. Both 2H and 3R are used as dry lubricants. Due to the nonlinear optical properties of 3R phase it is used in nonlinear optical mass sensing in quantum measurements and biomedicine [35]. As an example, for gas sensors, the different phase materials of MoS₂ can be interesting in obtaining high sensitivity and rapid desorption [36].

Figure 1. Different coordination and stacking sequences of the three MoS₂ structures 1T, 2H and 3R. Reproduced from [33]. Published by The Royal Society of Chemistry.

Table 1. Comparison between different MoS₂ structures.

1H MoS₂ is the most stable configuration and it is formed of one layer of Sulfur and one layer of Mo where S-Mo-S are attached through strong covalent bond like a sandwich, with a thickness of approximately 0.65 nm [28], The sandwiched S-Mo-S layers are attached through weak Wander Val forces [37]. The conductivity of nano MoS₂ depends on the temperature and thickness of the flakes, where conductivity increases with the temperature increase and decreases with increasing the thickness till it reaches the bulk structure [38]. The binding energies and photoluminescence properties are summarized in Table 2 [39].

Table 2. Comparison between Bulk and monolayer MoS₂.

MoS₂ exists in different 2D structure like nanosheets, and nanoribbons or 1D structures as nanowires and nanotubes, or 0D structure as quantum dots and nanoplatelets. The thickness of 2D nanoribbons was found to be of 1 to 3 layers of MoS₂, while the thickness of 1D nanowires (NW) can have lengths from 14 to 30 nm and a width of 0.6 nm approximately [40]. The structure of 1D nanoplatelets and their properties were investigated in [41]. The nanoplatelets are 12–30 nm with one-unit cell width. They have very high catalytic activity for hydrodesulphurization. The quantum dots range from 2 to 10 nm in size. They have higher band gap than nanosheets, and stronger bonds between Mo atoms than monolayers. The change in band gap of MoS₂ from one dimension to another, changes the photoluminescence characteristics and thus has different optical properties according to its dimension. Additionally, monolayers or other low dimension forms are also easy to be implemented in optical nanostructures to enhance the photoluminescence intensities and emission rates through light-matter interactions [42,43,44]. This is a strong motivation for MoS₂ to be included in optical applications [29].

3. Synthesis

There are different techniques used to obtain material layers Figure 2, and each one of them results in different quantities, shapes, and sizes. Mainly the approaches used in synthesizing TMDs nanostructures, are the top-down approach and the bottom-up approach [55,56]. The first approach depends on etching crystal planes from a substrate that has the crystals laid over it, while in the second approach, the crystals are stacked over the substrate. Exfoliation is one of the top-bottom techniques for obtaining MoS₂ layers. The weak Van der Waal forces between layers of TMDs paved the way in front of different exfoliation synthesizing techniques [57]. Mechanical exfoliation is done using a sticky tape which is rubbed out and shifted on a substrate having MoS₂ flakes over it. The method gives low yield and is good for lab use. Exfoliation can also be done in the liquid phase by adding a chemical compound and stirring, bubbling, or grinding. This method is simple and cheap but has low quality. The low yield in liquid exfoliation was avoided using carbon aerogel composites in [58]. The synthesizing is fast and completed in 30 minutes. It also avoids pyrophoric materials that are typically used in liquid exfoliation and increases the electrical conductivity and porosity of MoS₂. Sonication is one of the techniques that showed simple synthesis when used with liquid exfoliation [59,60]. It disposes of the use of hazardous materials used in liquid exfoliation. It is based on ultrasonic waves emitted from a probe in shape of bubbles that peels MoS₂ layers when they burst. The challenge in using sonication assisted techniques is that it produces relatively small area MoS₂ nanosheets that limits its use in practical applications. Generally Top down techniques are said to have low controllability, and scalability and high cost [55]. The yield is increased to >90% when using ultrasound sonication with supercritical carbon in [61] with an intercalating solvent N-methyl-2-pyrrolidone (NMP). The method is fast, simple and scalable. Sputtering is used to prepare layers of MoS₂ to be used as lubricants, the layers have a low friction coefficient, but these frictional properties can be changed under humidity, especially for thin films of MoS₂.

Figure 2. Different MoS₂ synthesis techniques [27].

Physical layer deposition (PVD) is one of the bottom-up techniques that includes ion implantation like molecular beam epitaxy (MBE) [62]. The method can be applied only to thin layers of MoS₂ and the resulting grain sizes are variable [63]. Chemical vapor deposition (CVD) is applied to thin and thick layers, where Mo is laid over a substrate and Sulfur vapor passes over it. This method has good quality, but low yield. The atomic layer deposition (ALD) method is used to fabricate thick and thin films. The method is considered efficient and the layers have fewer impurities that can be used in different applications, including electronics and sensors. MoS₂ layers can be synthesized with the help of chemical solutions, using hydrothermal and solvothermal reactions wherein both cases Mo and S react in an aqueous solution above the boiling point and in a nonaqueous solution at high temperature, respectively. The size and shape of the layers can be controlled where we can get powder and thin films of MoS₂ by this method. It is considered cheap and scalable [64].

In [65], MoS₂ is synthesized using a liquid organic precursor on an insulating substrate using CVD. The used method is more reproducible and is used to obtain larger areas of MoS₂ layers than those obtained with methods using powder Molybdenum oxide and sulfur powder. Another method in [66] used thermal evaporation and ALD, where it used metalloporphyrin as a promoter layer. The method allowed to manipulate the carrier density and conductivity of MoS₂ according to the thickness of the metalloporphyrin layer used. It is used to produce MoS₂ nanosheets on a large scale. In [18], MoS₂ is synthesized using self-propagating, under high temperature, where Mo nanopowders and elementary Sulfur are used. The mixture is put into cylinders and then under pressure. The main resulting structure is 2H MoS₂, but there are other phases like rhombohedral MoS¬₂ and Mo₂S₃. Thermal sulfidation is another method like CVD that uses Sulfur gas as a precursor. It uses a Mo [67] or Mo-oxide [68] film deposited on a substrate, where evaporated sulfur passes over it under certain temperature. The method is known to reduce the effect of gas flows that occurs in CVD, and results in self-aligned patterns of MoS₂. The sulfidation of two different oxides of Mo: MoO₃, and MoO₂ discussed in [68], showed more stable MoS₂ monolayer films, produced from MoO₂. The films were integrated with bottom-gate transistors and they showed on/off ratio of 10³–10⁴ and electron mobility of 10⁻⁴ cm²/V·s. The PL spectrum of the synthesized monolayers has an exciton peak at 1.89 eV.

Another approach to avoid the drawbacks of exfoliation and intercalation or liquid exfoliation like low electrical performance (low mobility of 0.3-0.4 square centimeters per volt per second and low on/off ratios ~10–100) is using electrochemical intercalation [69]. The method involves quaternary ammonium molecules into 2D crystals, with mild sonication and exfoliation techniques. The technique gives high performance MoS₂ nanosheets with 10 square centimeters per volt per second mobility, and on/off ratios of 10⁶. Table 3 summarizes some synthesis techniques that are already known until now.