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Nanostructured molybdenum dichalcogenides: a review

With increasing interest in transition metal dichalcogenides (TMDs), a great amount of literature has come into existence covering a broad range of preparation methods as well as their applications in diverse fields. Often dichalcogenides include sulphides, selenides and tellurides; however, they diﬀer in various ways depending upon their crystal structures, methods of synthesis and application requirements. But each TMD is exceptional from the modern technological viewpoint. This review article focuses in detail on the synthesis, properties and applications of molybdenum dichalcogenides [MoX 2 (X = S, Se, Te)] because of the desired expectations of the scientific community for specific types of chalcogenides. The majority of researchers have focused their research on nanocrystalline molybdenum disulphide; however, the present article also discusses selenides and tellurides as they also attract significant interest due to vast new avenues in advanced applications. Through this comprehensive review, we aim to provide a complete description of molybdenum dichalcogenides in one place which will serve as an important source of information for the large number of researchers in the field.


Introduction
Dichalcogenides of transition metals popularly termed as TMDs have captured significant attention due to their layered structure like graphene, accompanied by semiconducting properties. These layered materials are thermally robust, have high dielectric constants and show decent photocatalytic activity owing to a wide variety of chemical structures and compositions. TMDs are a class of compounds consisting of transition metals and chalcogens (S, Se, Te). [1][2][3][4] The prominent members of this family are molybdenum dichalcogenides (MoX 2 , X = S, Se, Te). Molybdenum is naturally abundant and is the smallest atom of the transition metals to form layered dichalcogenides. These materials offer bandgap tunability and have significant electron mobility in the planes. There is a general understanding that, because of discrete energy level formation, the quantum confinement effect is observed in nanometer-sized semiconducting materials; therefore, they display improved and unique optical, electrical, and structural properties when compared with their bulk counterparts. For example, nano-sized MoS 2 is reported to show enhanced performance as a solid lubricant in comparison to micrometer-sized MoS 2 . Therefore, extensive efforts have been made by researchers globally to synthesise MoX 2 in the nanometer regime. Depending upon the synthesis methods, these materials have been obtained in various dimensions (D) and with different morphologies like 0D (nanoplatelets), 1D (nanorods and nanowires), 2D (nanosheets and nanoribbons), and 3D (flowers and snowflakes). The most studied form among these is the 2D form owing to its similarity to graphene. A monolayer of graphene is only one atom thick, while the MoX 2 monolayer consists of molybdenum sandwiched between the layers of chalcogen atoms. The common synthesis methods include exfoliation, chemical vapour deposition, hydrothermal and solvothermal approaches. Because of their multi-dimensional structural properties, MoX 2 have applications in advanced technologies such as in hydrogen evolution reactions, [5][6][7] light-emitting diodes, 8 solar cells, 9 Li/Na-ion batteries, 10 supercapacitors, 11 field-effect transistors, 12,13 photo-detectors, 14 bio-sensors, [15][16][17] solid lubricants, dehydrosulfurization, 10 photocatalysis, 18 desalination of water 19,20 and the biomedical field. 21 Therefore, studying these materials allows us to go beyond graphene and explore new properties of 2D inorganic materials. The current review discusses the exciting properties of MoX 2 with a focus on their synthesis and application in energy devices. semiconducting properties, while in 1T the central atom is octahedrally coordinated, and it is metallic in nature. In a trigonal prismatic structure, the atoms' packing is AbA type, while in an octahedral structure it is AbC type (capital letters denote chalcogen atoms and the lowercase letter denotes a metal atom). 22 The 1T phase shows better activity in hydrogen evolution and energy storage than the 2H phase. 23 The Earth's crust contains MoS 2 in the form of molybdenite. It exists in three polymorphic phases -1T, 2H, and 3R belonging to point groups D 6d , D 6h and C 3v , respectively. Here, the numerals 1, 2 and 3 indicate the numbers of layers and T, H, and R denote tetragonal, hexagonal, and rhombohedral structures, respectively. The crystal structure for all the three phases is depicted in Fig. 1. 24 Naturally, MoS 2 occurs in the 2H phase. Mo coordinates octahedrally in the 1T phase and exhibits paramagnetic behaviour. It transforms into a more stable form, i.e., 2H-MoS 2 , upon aging or heating e.g. when Guo et al. 25 annealed 1T 0 -MoS 2 to 300 1C in an Ar atmosphere, it transformed to 2H-MoS 2 with stability up to B1200 1C, thereafter it is converted to Mo 2 S 3 . 3R-MoS 2 has Mo in trigonal prismatic coordination and transforms to its 2H-prototype upon heating. 10 Guo et al. 25 reported a phase change by intercalation with alkali metal ions. This transformation is attributed to the change in the electronic structure from d 2 to d 3 , which leads to destabilisation in the trigonal prismatic 2H phase and conversion to the octahedral 1T phase. The S-Mo-S layer-to-layer distance is about 6 Å.
Similarly, MoSe 2 is also found in the Earth's crust as a scarce mineral drysdallite. 2H-MoSe 2 with the hP 6 space group has cell parameters of a = 3.283 Å and c = 12.918 Å. The electron mobility is higher in 2D MoSe 2 than that in 2D MoS 2 . Bond lengths of 2.528 Å for Mo-Se and about 3.293 Å for Se-Se make them better candidates for various applications. MoSe 2 monolayers are thicker than MoS 2 because of the large atomic radius of Se atoms. 26 Fig. 2 represents the crystal structure of MoSe 2 which is analogous to that of MoS 2 . 27 MoTe 2 crystallises in two phases, i.e., 2H and 1T 0 , whose crystal structure is presented in Fig. 3. 28 The 2H phase belongs to the hP6, P6 3 /mmc space group with unit cell sizes of a = 3.519 Å and c = 13.946 Å. The 1T phase belongs to the mP12, P2 2 /m space group with unit cell constants of a = 6.33 Å, b = 3.469 Å, and c = 13.86 Å. The unit cell sizes change with temperature due to thermal expansion as the energy gap between these phases is very low (40 meV). 29 It is reported that the 2H-phase is slightly less stable, i.e., 815 1C and beyond that it changes to the more stable phase of 1T 0 which is stable even beyond 900 1C. 30 Due to the weak bonding between Mo and Te, its synthesis in one pure phase is highly challenging, thus limiting its exploration and applications. Therefore, it is desired to conduct research in materials like MoTe 2 with rapid pace. The role of chemists therefore becomes more demanding to propose simpler methods for the synthesis of pure phase MoTe 2 .

Electronic properties
On moving from bulk to nano-scale, MoX 2 give rise to diversified properties due to their transformation from indirect to direct bandgap semiconductors. It is well known that upon particle size reduction the bandgap increases in semiconductor materials and MoX 2 are no exception to that. The bandgap values of these materials in bulk and the nano range are summarised in Table 1. The bandgap for bulk MoS 2 is about 1.2 eV which can reach up to 1.8 eV for nano-MoS 2 and thus IR-active MoS 2 can be converted to visible active, which makes it a potential photocatalyst. Similarly, MoSe 2 has been reported to attain a bandgap of 1.5 eV in the nano-regime, thus extending its scope in optoelectronic applications. Likewise, bulk 2H-MoTe 2 has an indirect bandgap of 0.93 eV, whereas a direct bandgap of 1.1 eV is reported for monolayer 2H-MoTe 2 . 31 This value is very close to that of Si, and hence it finds numerous applications in photonics. As the bandgap of MoTe 2 lies in the infrared region, it can be used as an infrared detector. It is also reported that MoTe 2 can exhibit a higher phonon mobility than MoS 2 at room temperature. 32 The bandgap in MoX 2 decreases gradually from 1.8 to 1.1 eV as the size of the chalcogen increases. Therefore, it also gives an opportunity to tune the bandgap using different concentrations of chalcogens. [36][37][38] In MoX 2 , Mo is in the +4 oxidation state having a 4d 2 valence electronic configuration. In a trigonal prismatic structure, the electrons are filled in the d z 2 orbital and the transition from this orbital to the d x 2 -y 2 and d xy orbitals makes it semiconducting in nature, while in octahedral coordination, the electrons are filled in the t 2g orbital (as unpaired in d xy and d yz ) which corresponds to the valence band in the electronic structure of the 1T phase (Fig. 4). 22 This partially occupied valence band in the 1T phase is responsible for its semi-metallic nature. 39 As the d z 2 orbital has a slightly lower energy than the t 2g orbital, the overall energy of electrons is lower for the 2H phase, making it thermodynamically more stable than the 1T phase. Additionally, the lone pairs of chalcogens terminate the edges of layers, giving them stability against reaction with environmental species. 22 Synthesis Several approaches are documented for the synthesis of nano-MoX 2 . As the MoX 2 layers are linked together through van der Waals forces, these forces can easily be dealt with to obtain few/ monolayers of these materials conveniently from the bulk forms. Some notable approaches for the synthesis of nano-MoX 2 practiced by researchers are hydrothermal, solvothermal, thermal decomposition, ultrasonication, chemical vapour deposition, microwave, mechanical exfoliation and chemical exfoliation.
These techniques result in a wide diversity of morphologies. The commonly used molybdenum precursors are molybdenum trioxide, ammonium heptamolybdate (AHM), and sodium molybdate and the synthesis methods are discussed one by one in the following sub-sections.

Hydrothermal method
The hydrothermal method is a technique used to carry out chemical reactions at high temperatures and pressures. A properly sealed vessel, generally a Teflon lined stainless-steel autoclave, is used. The precursors and chemical reagents are dissolved in water and kept in an autoclave at high temperatures for long durations. Hydrothermal synthesis of molybdenum dichalcogenides has been extensively reported. Li et al. 40 successfully synthesized MoS 2 nanowires via a hydrothermal method. Prabhakar et al. 41 synthesised MoS 2 nanoflowers and nanosheets of less than 100 nm diameter and 10-50 nm thickness by the reaction of AHM with thioacetamide and thiourea, respectively, and using citric acid as a surfactant. The sulphur source was added to a moderately acidic aqueous solution of AHM and citric acid and then kept in an autoclave for about 10 h at 160 1C to collect a black powder. Similarly, by use of an autoclave, Tian et al. 42 reported a green protocol for synthesis of MoS 2 fine crystals using Na 2 MoO 4 and potassium thiocyanate (KSCN) as precursors sealed in a stainless-steel autoclave at 260 1C for 24 h. They reported silvery dark crystalline nanosheets with a size of 0.3 mm from transmission electron microscopy (TEM) images. The suggested reaction is represented by eqn (1) Nagaraju et al. 43 prepared bundles of MoS 2 nanofibres via two different routes using AHM as the molybdenum precursor Pourabbas et al. 44 compared the formation of nano-ribbons of nano-MoS 2 via two different methods where, in the first case, MoO 3 was mixed with Na 2 S in water and autoclaved for 12 h at 300 1C to isolate large poly-disperse particles with a size in the range of 150-200 nm composed of nanoribbons of 2-3 nm thickness and about 100 nm length. However, a better size domain was obtained when the authors dispersed AHM and thiourea in water with 1-octanol and sodium lauryl sulphate as surfactants, sealed them in an autoclave at 180 1C for 5 h and obtained particles in the size range of 20-30 nm in the form of ribbons with uniform distribution. Synthesis of relatively small particles of 3R-MoS 2 with a mean diameter of about 5.5 nm was reported by Manuja et al. 45 by autoclaving the aq. mixture of Na 2 MoO 4 , polyethylene glycol (PEG) and thiourea at 200 1C for 11 h. Hou et al. 46 synthesised spherical and flower-like MoS 2 nanoparticles by modifying the additive used such as CTAC, PEG and ethyl alcohol in the reaction of sodium molybdate and thioacetamide. Sodium molybdate and thioacetamide were taken in the stoichiometric ratio, and then the additive was added with some amount of water. They further acidified the mixture to set the pH to 1 and the mixture was then sealed in an autoclave and heated in a muffle furnace for 6 h at 220 1C. The course of the reaction is represented by eqn (4)- (9). The authors found that spherical nanoparticles were obtained using CTAC and PEG as additives with little difference in their sizes, whereas flower-like MoS 2 NPs were obtained using ethanol.
Similarly, there is an extensive use of the hydrothermal method for the synthesis of nanocrystalline MoSe 2 . 47-51 Common selenium precursors often employed are Se and Na 2 SeO 3 . Several researchers described excellent use of the hydrothermal method to obtain MoSe 2 , e.g. the mixture of Na 2 MoO 4 , hydrazine hydrate/ L-ascorbic acid and Se in distilled water can be processed in an autoclave for several hours at 200 1C to obtain flower-like morphology made up of very thin nanosheets, 14,52 as supported by high resolution transmission electron microscopy (HRTEM) images (Fig. 5).
The typical XRD and UV absorption spectra of MoSe 2 formed by this method are shown in Fig. 6. 14 The XRD results supported that the 2H phase was formed and the absorption was observed at B 450 nm and B650 nm, indicating that the thus-formed compound is a useful photo-sensor in the range of 400-700 nm. Modifications and alterations are often performed by different groups to enhance the product yield as well as the properties for making them suitable for a variety of applications. In one such modification, a pH-based synthesis has been explored by Mittal et al. 53 to study its impact on the size and morphology of MoSe 2 . Nanosheets and nano-urchins that contained several nanospheres were obtained at pH 5 and pH 8, respectively, via a three-step mechanism. In the first step, hydrazine hydrate reduced selenium to give selenium ions as per eqn (10). When this solution was added to a solution of sodium molybdate in water, excess hydrazine hydrate reduced Mo 6+ ions in Na 2 MoO 4 to Mo 4+ ions (eqn (11)). Finally, the product was formed as per eqn (12) where the reaction between molybdenum and selenium ions took place in an autoclave. In similar efforts, Xiao et al. 54 used sodium molybdate and selenium powder with various amounts of basic reducing agent such as NaBH 4 to synthesize MoSe 2 in mixed 1T and 2H phases where it has been described that by increasing the amount of NaBH 4 , the number of stacked layers can be decreased. Likewise, MoSe 2 nanorods and spherical particles were synthesized by Ghritlahre et al. 55 using a basic aq. solution of sodium molybdate or AHM as the molybdenum precursor and Se powder with a small amount of hydrazine hydrate (HH) in an autoclave at 200 1C.
Often it is realized that the source of selenium is a tricky choice and it is crucial for the preparation of the product of desired morphology and physical properties. Thus, researchers constantly explore a variety of such combinations and look for newer, unexplored and more appropriate selenium precursors to suit the metal precursors for establishing the appropriate kinetics. Inorganic selenium precursors may offer more versatility in surface properties when compared with selenium metal as a source. Setayeshgar et al. 56 were the first to use sodium selenite as the selenium source for the synthesis of nanocrystalline MoSe 2 . This eliminated the requirement of any reducing agent like hydrazine hydrate as Na 2 SeO 3 readily dissociates in water to give Se 2À ions. They proposed that the reaction of MoCl 5 /NaOH/ethylene glycol and Na 2 SeO 3 can yield bi-phase 1T, 3.5-to-6 nm thick 2H-MoSe 2 nanosheets of 5 to 8 layers via autoclave heating at 200 1C for 18 h. The fine tuning of the size and morphology was reported by varying the ratio of reagents and precursors. The suggested chemical reactions in this process are represented by eqn (13) and (14). 2MoCl 5 + 8NaOH -2Mo(OH) 4 + 8NaCl + Cl 2 (13) 2Na 2 SeO 3 + Mo(OH) 4 + NaCl + H 2 O -MoSe 2 + 5NaOH + HCl + 3O 2 (14) It is noticed that MoCl 5 was reduced to Mo 4+ in the first stage, followed by in situ release of Se 2À from Na 2 SeO 3 . In another study, Liu et al. 57 used potassium selenocyanate as the selenium source with AHM in the water/ethylene glycol (EG) mixture and synthesized MoSe 2 microspheres consisting of MoSe 2 nanosheets. According to them, potassium selenocyanate dissociates to give H 2 Se in the reaction as per eqn (15) which reacts with Mo 4+ ions produced in the mixture to give MoSe 2 .
With set methodologies for sulphides and selenides, counterpart tellurides have not been much explored but still there are a few reports on its hydrothermal synthesis, e.g. Li and co-workers 58 synthesized MoTe 2 nanosheets having thickness in the range of 20-80 nm. Te powder was mixed with water in the presence of NaBH 4 , followed by addition of sodium molybdate with continuous stirring and heating. The mixture was then transferred to an autoclave and maintained at 200 (203) were seen in the XRD spectrum of 2H-MoTe 2 . Although the method offers easy processing and a green alternative, products of uncertain/mixed morphology are obtained.

Solvothermal method
Solvothermal and hydrothermal methods are analogues, except that the organic solvents are used instead of water for the synthesis. The precursor and solvent mixture are sealed in a vessel and the reaction is carried out at a temperature more than the boiling point of the solvent. The Mo-chalcogenide synthesis reactions require a very high temperature/energy as the precursors like MoO 3 and chalcogen sources often do not break/dissociate easily at normal temperatures. In such a situation solvothermal methods for synthesis of nanomaterials are considered very handy to obtain uniform sized particles. Zhan et al. 59 synthesised uniform thin platelets of MoS 2 (100 nm thickness) from MoO 3 and elemental sulphur using hydrazine in pyridine in an autoclave that was maintained at 300 1C for 12 h. They proposed that, firstly, MoO 3 may be reduced to MoO 2 by hydrazine hydrate and, on the addition of sulphur powder, O-atoms are replaced by S-atoms to form MoS 2 nanoparticles (NPs). The reaction methodology is depicted in Fig. 7.
Similarly, Li et al. 60 mentioned the synthesis of free MoS 2 in a single step where (NH 4 ) 2 MoS 4 and N 2 H 4 ÁH 2 O were mixed in DMF and heated in a Teflon-lined autoclave at 200 1C for 10 h. They, initially, synthesized MoS 2 particles on graphene and elaborated that even without graphene free standing MoS 2 can be prepared from this method. Such an approach was found to be useful for catalytic applications. The formation of a colloidal solution of MoS 2 nanosheets was reported by Zhu et al. 61 from micro MoS 2 powder dispersed in n-hexane and reacted with n-butyllithium (n-BuLi) in an autoclave for 5 h at 100 1C. The MoS 2 nanosheets were obtained via repetitive washing of the intermediate compound Li x MoS 2 , followed by its hydrolysis by sonication for 1 h in water. Similarly, flower like nanoparticles composed of ultrathin nanosheets of MoS 2 have been reported by use of the sodium molybdate, thiourea and hydroxylamine hydrochloride mixture (1 : 2 : 2 molar ratio) in an autoclave heated for 24 h at 190 1C, followed by annealing for 5 h at 500 1C. 62 The authors observed that a lower temperature and reaction time leads to formation of MoO 2 impurities. Several researchers have also reported synthesis of nano-MoSe 2 by the solvothermal method using an autoclave. The synthesis method is very much similar to that of MoS 2 where only the sulphur source is replaced by a selenium source. The most commonly used precursors are MoO 3 and Se powder with some reducing agents like N 2 H 4 ÁH 2 O. 63 Jiang et al. 64 suggested that the thickness of MoSe 2 nanosheets can be decreased with increasing carbon chains in alcohol when performing the synthesis using various solvents like ethanol, butanol, hexanol and octanol. They also commented on the morphology with respect to the reaction temperature. The particles formed at 120 1C were spherical in shape with no clear evidence of nanosheets. When the temperature was increased, nanoflowers composed of thin nanosheets were observed; with further increase in temperature, these nanosheets became thicker.
Despite the popularity of the solvothermal method, it has limitations for high melting solids which have poor or no solubility. Therefore, compounds like MoTe 2 have not been popularly synthesized by this method unlike by the hydrothermal method where synthesis of MoTe 2 nanosheets has been possible with ease via in situ reduction of Te powder by NaBH 4 which is useful in generating Te 2À ions. Similarly, the reaction medium in the solvothermal method may have its utility for MoTe 2 formation. It is challenging, however, to monitor the pressure during the synthesis and the nature of solvents versus the duration/pressure may limit its operational understanding.

Thermal method
The generally adopted methods are mostly based on the autoclave requirement which requires special skills to operate and, hence, is highly challenging. It is always desirable that simpler methods be developed for enhancement of the applicability of materials with ease. In this regard, direct reactions in standard laboratory glassware are generally favourable and preferred. The thermal method, therefore, being the most conventional, brings its own advantages. By this method, researchers can monitor the reaction changes physically as well as spectroscopically at desired time intervals during the course of reactions. The precursors in such methods are solvent-friendly in terms of dispersion/solubility. Some of the popular reagents are metal complexes, molecular precursors and/or chalcogen sources which react at normal pressure and at a temperature below 300 1C. There are a number of reports where soft solution methods have been practiced. For example, Duphil et al. 65 synthesised MoS 2 by thermal decomposition of Mo(CO) 6 . The sulphur powder was dissolved in p-xylene by heating, followed by the addition of Mo(CO) 6 at room temperature and refluxing at 140 1C for several hours. The reaction setup is shown in Fig. 8. Raman spectroscopy was extensively and effectively applied for monitoring the reaction by way of disappearance of the CO peak as the formation of a black precipitate starts. It was opined that Mo(CO) 6 decomposed to give elemental Mo, followed by its reaction with the S present to give a black precipitate of MoS 2 of 10-30 nm size albeit not without aggregation. The overall reaction is represented by eqn (16): Park et al. 66 reported preparation of MoS 2 via generation of a colloidal solution of Mo NPs, followed by its reaction with sulphur via a thermal method. In this method, Mo(CO) 6 and S were reacted in the presence of oleic acid to control the particle size. The reaction consists of two steps -(1) formation of polycrystalline Mo/MoO x nanoflakes and (2) reaction with sulphur to produce MoS 2 . By this method, highly crystalline and spherical MoS 2 NPs of about B2 nm size were obtained. The schematic illustration is shown in Fig. 9.
It is well documented and understood by this time that chemical methods can produce high quality nanostructured materials and the challenges associated with their synthesis can be addressed by chemists via skill manipulation. Often temperatures less than 300 1C are appropriate to operate in a Schlenk system in an advanced chemistry laboratory and the solvents and precursors can be degassed prior to or during an in situ stage. Sun et al. 67 and Wang et al. 68 dissolved Se powder in 1-octadecene (ODE) by heating and stirring it. The mixture of sodium molybdate, oleic acid (OLA) and octylamine was heated in an Ar atmosphere to obtain a red-brown solution. Later, they added ODE-Se solution to it and heated at B300 1C for about half an hour. The SEM/EDAX and TEM analyses of the black   73 by increasing the amount of surfactant during their experiment involving the use of MoO 2 (acac) 2 in OAm, followed by heating with Se-precursor (prepared by stirring Se powder, OAm and 1-butanethiol) under an inert atmosphere. It was described that the reaction occurs via reduction of Se to Se 2À ions. In line with MoSe 2 , thermal methods are also effectively useful for synthesis of its tellurium counterpart. Thus, Sun et al. 74 were able to synthesize 1T 0 -MoTe 2 via a low-temperature thermal route by using TOPTe prepared by dissolution of Te powder in trioctylphosphine (TOP) and OAm by heating to obtain a yellow solution. Subsequent injection of hexamethyldisilazane along with MoCl 5 solution in OLA led to formation of a nanocrystalline product within half an hour at 300 1C. OLA was used as the complexing agent and OAm as the reducing reagent and HDMS was required for high crystallinity. 1T 0 -MoTe 2 nanoflowers were formed directly at this temperature instead of the more preferred and stable 2H polymorph. The authors suggested that this could be due to the low energy barrier for the 1T 0 phase by the organic ligands. Qiu et al. 75 carried out a reaction between Mo(CO) 6 and Te powder in diphenylmethane by heating it for 24 h in a N 2 atmosphere. It was then annealed at 650 1C for 4 h to obtain MoTe 2 in a stoichiometric ratio. They observed that excess of Te was present in the sample before annealing. According to them, this was probably due to faster sublimation of Mo(CO) 6 and deposition of MoTe 2 particles over Te powder. SEM and TEM images indicated mixed morphologies in the form of nanorods and nanosheets. Liu et al. 76 synthesized nanosheets of 1T 0 -MoTe 2 with lateral dimensions of 250-350 nm and about 4 nm thickness. Te was first dissolved in TOP, then injected into Mo(CO) 6 dispersed in OAm and heated at more than 300 1C for 2 h. When the same experiment was repeated using MoCl 5 , spherical nanostructures with smaller dimensions composed of defect rich nanosheets were observed. This states that different precursors can have different effects on the product morphology.

Chemical vapour deposition
Solid-state reactions mostly use MoO 3 , sulphur, hydrogen, H 2 S, etc. as precursors for the synthesis of nano-MoS 2 utilizing a high temperature tube furnace. [77][78][79] The temperature required to carry out solid-state reactions is very high to realize high yields and homogeneous quality, and to have precise control over layer numbers. CVD grown layers are prone to chalcogenide deficiency due to the volatile nature of chalcogens. Balendhran et al. 80 carried out a solid-state reaction between MoO 3 and S powder in a tube furnace which they referred to as the thermal evaporation technique using a quartz substrate in an argon environment. The reaction methodology for the same is shown in Fig. 10. It was demonstrated that layered MoS 2 deposition can be achieved via choosing the right combination of temperatures and durations. Highly crystalline and ordered layered structures were reported at an annealing temperature of 830 1C for 3 h. The suggested reaction mechanism is represented by eqn (17) and (18): On the same line, Jeon and co-workers 81 synthesised monolayered MoS 2 with a controlled size of nanoparticles in the range of 5-60 nm by regulating the flow of hydrogen and argon. Here, they used a dual temperature zone furnace to carry out the reaction where sulphur powder was placed in the first zone and MoO 3 powder and the silicon substrate were placed next to each other in the second zone. Instead of one heating zone, two were preferred as sulphur is highly volatile than MoO 3 , and to cause their evaporation at the same time, it is required to heat them at different temperatures. Otherwise, the product would be S-deficient as reported by several researchers where the reaction was carried out in a single zone. The setup of the reaction is shown in Fig. 11. The first zone was heated at 120 1C and the second zone at 800 1C for 20 minutes with the ratio of Ar/H 2 well-regulated as required. The authors observed that the size of the nanoparticles can be reduced with increasing the H 2 flow rate. It is also worth mentioning that a typical MoS 2 stoichiometry was obtained when no hydrogen was used which the authors suggested could be due to the possible reduction of S, leading to substantial loss of sulphur during sulfurization of Mo. When MoS 2 is formed, the edges are covered by S dimers. As the flow of H 2 increases, it acts as the reducing agent and reacts with these surface-attached dimers to form H 2 S. Therefore, instead of pure MoS 2 , partially reduced MoO 3-x S is deposited. The proposed reaction is represented by eqn (19) and (20): Reacting MoO 3 nanobelts with sulphur powder, Li et al. 82 synthesised MoS 2 fullerenes. The reaction was performed in a tube furnace in an inert atmosphere of argon gas at a temperature higher than 800 1C.
Based on TEM analysis, the authors stated that nanotubes of MoS 2 as well as fullerene-like nanoparticles were formed by rolling of the layered structures. It is also evident from this article that the morphology of products is dependent on the morphology of the reactants. When the authors carried out the same reaction with micro MoO 3 , nanoparticles of MoS 2 were formed instead of nanotubes. In an attempt to identify whether the formation of MoS 2 nanotubes from the reaction of MoO 3 with H 2 S gas takes place through the MoS 3 intermediate, 83 Nath et al. 84 Kong et al. 7 presented synthesis of thin MoS 2 and MoSe 2 layers vertically aligned on a substrate so as to achieve maximum catalytic activity. They obtained samples with an excellent arrangement of the layers as shown in the HRTEM images (Fig. 12). Recently, the use of KCl to obtain mixed phase MoS 2 heterostructures was also reported. 85 Lee et al. 86 prepared thin MoS 2 layers of B0.72 nm thickness and more than 150 nm in lateral dimensions over an RGO coated Si/SiO 2 substrate via chemical vapour deposition taking MoO 3 and sulphur powder as precursors. The authors observed two prominent peaks at 627 and 677 nm in their PL spectra due to A 1 and B 1 direct excitonic transitions. Wypych et al. 87 carried out the reaction of potassium molybdate and H 2 S by heating it for almost 24 h in the temperature range of 300-400 1C, followed by reduction in a H 2 /N 2 atmosphere at a much elevated temperature (850 1C) for 3 days to form KMoS 2 . After hydration and oxidation of KMoS 2 , 1T 0 MoS 2 was formed. The process took a very long time, and the product too contained some impurities. Modifying this process, Yu et al. 23 were successful to synthesise pure 1T 0 -MoS 2 crystals. After grinding K 2 MoO 4 and S powder together, the mixture was annealed for 1.5 h at 450 1C under a H 2 /Ar flow. Excess S was added to the obtained powder and again annealed for 10 h at 850 1C with increased H 2 flow, followed by soaking and washing with DI water. Later, it was soaked for a few hours in I 2 /acetonitrile solution and dried after washing thoroughly with DI water to achieve 1T 0 -MoS 2 crystals. Replacing S with Se in this process, 1T 0 -MoSe 2 was prepared. Annealing 1T 0 -MoS 2 and 1T 0 -MoSe 2 at 200 1C produced 2H-MoS 2 and 2H-MoSe 2 , respectively.
The higher negative reduction potential of Se poses greater challenges in the synthesis of MoSe 2 when compared with MoS 2 . 88 Most reports on the synthesis of MoSe 2 layers use the CVD method in a very similar manner to the synthesis of MoS 2 . Often selenization of MoO 3 with Se is a popular methodology to generate MoSe 2 . It was demonstrated that Ar and N 2 are good carrier gases to maintain an inert atmosphere with some percentage of H 2 for facilitating reduction. 89  containing Se powder was maintained at B300 1C to avoid faster vaporization of Se. Using the same method, Huang et al. 5 synthesised snow-like single crystal MoSe 2 with a hexagonal structure by use of Se and MoO 3 and placing a Si/SiO 2 substrate inverted over Se in a three-zone tube furnace. The reaction was carried out in an Ar/H 2 atmosphere, heating Se at 520 1C and MoO 3 at 820 1C for half an hour. Depending upon the gas flow rate, large triangular monolayers of lateral dimensions close to 20 mm and snow-like monolayers of 0.9 nm thickness were formed. The PL peak was observed at 814 nm corresponding to a 1.52 eV bandgap. The 2H-phase was confirmed from the SAED pattern and the d-spacing calculated matched [110] and [100] crystal planes.
Hanson et al. 93 transformed MoSe 2 from 2H to 1T 0 via exposure to n-BuLi. They first synthesized 2H-MoSe 2 monolayers by the above-described CVD method by heating MoO 3 and Se powder at 800 1C. The chips were treated with n-BuLi solution in hexane for 12 h in an Ar atmosphere. Various characterization studies supported the successful formation of the 1T 0 -MoSe 2 monolayer. The PL spectra (Fig. 13) gave a bandgap value of B1.5 eV for 2H-MoSe 2 , while no photoluminescence was observed for 1T 0 -MoSe 2 . Taking pointers from the synthesis of MoS 2 by reduction of the corresponding trisulphide, 84 Nath et al. 94 were successful in synthesizing MoSe 2 nanotubes in a similar manner. In addition, MoSe 2 nanotubes via decomposition of (NH 4 ) 2 MoSe 4 in a reducing H 2 atmosphere were also successfully synthesized. The reaction involved is represented by eqn (23). The morphology of the products was analysed through TEM (Fig. 14).
(NH 4 ) 2 MoSe 4 + H 2 -MoSe 2 + 2H 2 Se + 2NH 3 (23) Vikraman et al. 95 used a chemical bath deposition method to deposit MoSe 2 on the substrate and post-annealing at 450 1C for an hour. The bath solution in which the substrate was dipped contained AHM, SeO 2 , N 2 H 4 , and NH 3 solution with a certain amount of HCl. They represented the overall reaction by eqn (24). The authors observed that with an increase in the deposition time, the thickness of the layers increased as expected. It was also noticed that the layers align vertically till the deposition time reached 15 minutes, and beyond that there was a random alignment of layers.
Gas phase reactions to synthesise MoTe 2 are not easily achievable as a consequence of a lower bonding energy between Mo and Te or the requirement of high temperatures and longer durations to react. Moreover, at higher temperatures, instead of forming MoTe 2 vapours, it loses Te as vapours leading to Te deficiency in the prepared MoTe 2 samples. The MoTe 2 layers being prone to oxidation are often coated with a protective layer of materials such as aluminium, polymethylmethacrylate (PMMA), silica, etc. Therefore, the synthesis of few-layer MoTe 2 is highly challenging. The melting point of Te is about 450 1C. A large number of articles have reported the use of the CVD method for MoTe 2 synthesis. 28,96-98 Huang et al. 31 used the physical vapour deposition technique to produce 3L 2H-MoTe 2 . MoTe 2 was deposited on a silica substrate from a MoTe 2 target for half a minute at 400 1C in an inert environment using a DC magnetron sputtering system. Then, the sample was annealed for several hours at 650 1C in a N 2 atmosphere after coating it with a protective layer of silica, in an attempt to avoid decomposition of MoTe 2 . The 1T 0 phase transformed to the 2H phase post-annealing and the cross-sectional HRTEM image of MoTe 2 showed a three-layer stacked 2H-MoS 2 structure with 2.1 nm thickness.
Zhou et al. 99 and Hussain et al. 100 synthesised MoTe 2 via tellurization of MoO 3 and Mo films. Mo film was deposited on a SiO 2 substrate and converted to MoO 3 via oxidation in air. It was then placed over Te powder in a CVD chamber in a reducing environment of Ar/H 2 at 700 1C. Large nanosheets of B30 mm lateral size were observed. The authors also mentioned that when a Mo film was used instead of the MoO 3 film, the 1T 0 phase was obtained. In place of depositing the MoO 3 film directly from MoO 3 powder, the authors oxidised a Mo film to form MoO 3 because MoO 3 has a tendency to lose oxygen during evaporation. Similarly, in another experiment MoO 3 , MoCl 5 and Te were taken in a 1 : 1 : 1 weight ratio and heated in a tube furnace at 780 1C for nearly 15 minutes, resulting in formation of 1T'-MoTe 2 monolayers. 101 Shirpay et al. 102 deposited nanosheets of 2H-MoTe 2 via the chemical vapour transport method  using MoO 3 and TeO 2 . Kim and co-workers 103 were able to synthesize ultrathin MoTe 2 mixed phase films with a thickness of about 1-2 nm at a lower temperature using the Mo(CO) 6 complex and dibutyl telluride as the precursors. Pradhan et al. 104 synthesized bi-layered MoTe 2 crystals on a silica substrate using iodine as the transport agent. As starting materials, they utilized Mo and Te powders, which required a very high temperature (B1100 1C) and a very long duration (1.5 weeks) to react. Yang et al. 105 controlled the MoTe 2 nanosheet phases by optimizing the flow of the carrier gas employed and the tellurization temperature. The synthesis was carried out in a twozone CVD chamber using MoO 3 and Te to obtain thin films with thickness around 4 nm. The four stages observed were the 2H phase dominant coexistence of both phases, pure 2H phase, 1T 0 phase dominant coexistence of both phases and phase pure 1T 0 -MoTe 2 . They observed that when the flow rate of N 2 was increased from 1 sccm (standard cubic centimeters per minute) to 40 sccm, the 1T 0 dominant phase was transformed to the 2H dominant phase with phase pure 2H-MoTe 2 at 3 sccm and 1T 0 -MoTe 2 at approximately 90 sccm. The dependence of phase on the synthesis temperature was also studied and the authors suggested that achieving 100% pure 2H-MoTe 2 was highly unapproachable, while a pure 1T 0 phase can be easily obtained with a high rate of tellurization. In a similar attempt to utilize MoTe 2 as a phase change material, Empante et al. 106 obtained 1T, 1T 0 and 2H phases by varying the cooling rate of the furnace after the reaction. They carried out a reaction between MoO 3 and Te in a CVD chamber for about 30 min at 650 1C for the 2H phase and at 680 1C for the other two phases. 1T, 1T 0 and 2H phases were obtained when the chamber was opened after the temperature reached below 450 1C, 350 1C and 100 1C, respectively. This suggested that the 2H-phase is favoured at lower temperatures and an increase in temperature maximises the probability of 1T-MoTe 2 formation. In CVD, the film quality, i.e., the dimensions and the stoichiometry, is dependent on the amount of solid precursors, the distance at which they are placed, the substrate and its position, growth temperature and duration. The cooling rates of the sample also determine the phase of MoX 2 compounds.

Microwave method
Microwave radiation causes direct and targeted heating of the sample with a minimum dissipation of energy in the surrounding. The use of microwave transparent glassware minimises the loss of energy to heat the sample container. Under microwave irradiation, solvents are superheated. As a result of this overheating, metal ions may reduce to a zerovalent state. Metal particles strongly interact with microwave irradiation, yielding very favourable reduction conditions. Microwave energy is more intense as compared to thermal heating and also there is targeted heating of the reaction mixture, thereby avoiding the energy wastage in heating a higher quantity of solvent and the reaction vessel which is generally the case in thermal heating. Therefore, microwave reactions are more efficient and less time-consuming. Qureshi et al. 107 synthesised biocompatible and non-cytotoxic MoS 2 by a microwave-assisted solvothermal method. AHM and elemental sulfur were taken in a 1 : 1 ratio in water, followed by addition of hydrazine monohydrate, for initial reaction activation. The reaction mixture was then irradiated with microwave radiation for 10 min at 270 W and the black precipitate suggested the formation of MoS 2 . The authors observed the morphology of NPs to be elongated rod-shaped from TEM images, and the SEM images show stacked plate-like structures. The reaction schematic is shown in Fig. 15.
The polyol method is a potentially useful approach for the synthesis of metal NPs. Polyol serves as a reducing agent as well as a surface protecting agent. Harpeness et al. 108 utilised this method to synthesise MoSe 2 nanorods using a household microwave oven at 900 W, switched on for 21 s and off for 9 s to avoid overheating of the solvent and microwaved Mo(CO) 6 and Se mixture in ethylene glycol under an inert atmosphere. The black powder so obtained was further annealed at 600 1C for 5 h to obtain MoSe 2 with no impurities. The crystallite size obtained was within the 11-16 nm range consisting of nanorods 45-55 nm in length and diameter varying from 2.5 to 6 nm. The method is very much efficient and simple but still there are not many reports available on this. There are no reports on the synthesis of nanostructured MoTe 2 via this method. Therefore, the use of a household microwave oven is yet to be explored. It is expected to make the process simpler and more convenient.

Sono-chemical method
Uzcanga et al. 109 carried out the preparation of MoS 2 sonochemically using AHM and thiolacetic acid (CH 3 COSH) in water by completely dissolving them and the reaction mixture thus generated was treated with sonication for 1 h at a high ultrasound intensity (20 kHz) and the black precipitate was annealed at 400 1C in a H 2 /H 2 S atmosphere for 4 hours. During sonication, water decomposes to produce hydrogen radicals which then produce H 2 S gas on attacking thiolacetic acid acting as a sulfiding agent. Xu et al. 110 synthesised MoS 2 quantum dots (QDs) and nanosheets via a simple sonication method where MoS 2 in DMF was sonicated to exfoliate MoS 2 . Following that, it was heated at 140 1C with stirring for several hours and then centrifuged to collect MoS 2 QDs from the supernatant and QD and nanosheet mixture via centrifugation. The overall synthetic approach is depicted in Fig. 16.
Kristl et al. 111 synthesised nanocrystalline MoSe 2 via a simple sono-chemical approach. The Mo(CO) 6 and Se mixture in decalin was prepared in a sono-chemical reaction vessel. This slurry was then irradiated with high-intensity ultrasound for several hours under N 2 . The reaction temperature was maintained at 0 1C using a cooling bath. Amorphous MoSe 2 , thus formed, was then annealed at 330 1C for 12 h to obtain nanocrystalline MoSe 2 spherical particles of 15-60 nm size analysed through TEM ( Fig. 17(a)). The authors here commented on the very crucial role of temperature. The temperature of the reaction can reach up to 80 1C under these conditions and the rate of sono-chemical reactions decreases with an increase in temperature. When they kept the temperature of the water bath at 20 1C, there was an excess of Se in MoSe 2 compared to the stoichiometric ratio. They found it to be because of unreacted Mo(CO) 6 that sublimed and deposited on the walls before it could react with Se.
Yuwen et al. 112 synthesized MoSe 2 nanodots (NDs) from bulk MoSe 2 by ultrasonication using the triblock copolymer F-127, which gets absorbed on the surface of MoSe 2 particles and thereby offers water dispersibility. First, F-127 was dissolved in water to obtain a clear solution, followed by addition of MoSe 2 and stirring for some time. This solution was then sonicated for 4 h in an ice bath with a 2 s on/off cycle at a high power. The authors adopted gradient centrifugation to separate MoSe 2 NDs from larger particles. 2H-MoSe 2 NDs of size in the range of 2-3 nm and thickness of up to three layers were obtained after 3-4 cycles of centrifugation. In another report, MoSe 2 nanodots were retrieved from bulk-MoSe 2 via ultrasonication, conjugated with folic acid (FA) and stabilized by protein molecules. The corresponding XRD spectrum is shown in Fig. 17(b). Two peaks were observed at 700 and 800 nm in the absorbance spectra indicating a bandgap of 1.5-1.7 eV. 113 In the synthesis of MoTe 2 , Mo(CO) 6 was used as the precursor. The suspension of Mo(CO) 6 and tellurium powder was sonicated for 4 h in an Ar atmosphere, followed by drying and calcination. First, they observed formation of a mixture of nanosheets and nanotubes which upon calcination transformed to nanotubes due to rolling of nanosheets. The nanotubes of 20-50 nm diameter and of length 100-500 nm were observed in the HRTEM images (Fig. 17c) with a d-spacing of 0.345 nm for the (004) plane of monoclinic MoTe 2 . 114 The XRD also supported the formation of monoclinic MoTe 2 .

Mechanical exfoliation
The process to peel off the atomically thin layer of a compound from its parent bulk crystals by micromechanical cleavage is called mechanical exfoliation. As MoX 2 are layered materials held together by weak forces, atomically thin layers can easily be peeled using Scotch tape. These cleaved thin layers on the Scotch tape are normally placed on a substrate and can be transferred via rubbing carefully. The monolayer and multilayer nanosheets can thus be isolated once the Scotch tape is peeled off. Silicon oxide with a thickness of 200-300 mm is generally the substrate of choice. 12,[115][116][117] The method requires mild conditions, but the yield is very low. Additionally, there is little control over the number of layers being produced. In an attempt to resolve this, Dicamillo et al. 118 used a rheometer as an automated tool for mechanical exfoliation and obtained few-layer flakes of MoS 2 and MoTe 2 . This automation offers reproducibility and adds to the efficiency of this technique. Here, a Nitto Blue tape was used instead of the regular Scotch tape as the amount of residue left on the substrate was less. The only disadvantage of this procedure is a low yield; however, researchers prefer this method as it produces MoS 2 layers of few micrometers thickness. Wafer anodic bonding was another modified method that promised a higher yield where one side of Pyrex glass was made anodic by placing pristine MoS 2 and the other side was made cathodic. It was then subjected to a voltage of 200-1500 V at a temperature of 130-200 1C and an adhesive tape was used to mechanically peel off the flakes and transfer the thin film on the substrate. 119 Tongay et al. 120 exfoliated monolayer MoS 2 and MoSe 2 on a 90 nm SiO 2 substrate and studied the effect of layer thickness and temperature on the PL intensity. In bulk, the bandgap of MoSe 2 is 1.1 eV, and therefore weak PL emission is expected. Splendiani et al. 117 also observed increased photoluminescence with a lower number of layers (emission peaks at 627 and 677 nm) which were absent in bulk MoS 2 . The increased PL emission in the  single layer compared to multi-layer one was attributed to the indirect-to-direct bandgap crossover due to quantum confinement in the perpendicular direction. The PL intensity generally decreases for semiconductors with an increase in temperature as non-radiative processes are enhanced. 120 Similar observations were observed for monolayer and few-layer MoS 2 and fewlayer MoSe 2 but contradictory results were observed for singlelayer MoSe 2 ( Fig. 18(a)). 27 Several authors have reported the synthesis of MoSe 2 via mechanical exfoliation. 26,121,122 Similarly, for preparation of nanocrystalline MoTe 2 , Ruppert et al., 32 Feng et al. 123 and several other researchers also adopted the same method. [124][125][126][127][128][129][130][131][132][133] In Fig. 18(b), the PL spectra of the MoS 2 and MoSe 2 monolayers synthesized by mechanical exfoliation are shown. Strong PL emission was observed at 1.56 eV and 1.85 eV for MoSe 2 and MoS 2 , respectively. 134 Yu et al. 23 also reported the PL spectroscopy results of 1T 0 -MoS 2 and 2H-MoS 2 obtained by thermal annealing of 1T 0 -MoS 2 ( Fig. 19(a)). 1T 0 -MoS 2 did not show PL emission; however, 2H-MoS 2 has shown strong photoluminescence at about 670 nm which is probably due to interlayer interactions and spin-orbit coupling. Li et al. 135 in their review article have discussed the relation of the PL bands to the number of layers. As the photoluminescence falls in the visible region, this property can be utilised for various applications such as photocatalysis, photonic devices, etc. The foremost shortcoming of this method is the production of small and arbitrary shaped flakes distributed over a surface, thus forbidding their use in large-scale applications.

Chemical exfoliation
Chemical exfoliation overcomes the low-yield problem of mechanical exfoliation. It includes two approaches involving ion intercalation and solvent exfoliation. The ion intercalation method includes the insertion of small ions between the gaps of layers increasing the space between the layers. Then, these intercalated layers are reacted with water or some solvent like ethanol or methanol. Hydrogen gas is evolved during hydrolysis of ions and separates layers in single sheets suspended in the solvent. Mostly Li ions are used for intercalation due to their small size. 136 The amount of lithium intercalated decides the phase purity of the material. Lin et al. 137 used n-BuLi (n-butyl lithium) for exfoliation of layers from bulk MoS 2 . n-BuLi in hexane was added to MoS 2 and sealed in an autoclave which was heated at 80 1C for 4 h. The dried Li x MoS 2 thus formed was hydrolyzed and sonicated for some time to obtain a colloidal suspension of MoS 2 layers. In another report, ultrathin 1T 0 -MoS 2 nanosheets were exfoliated from micro-MoS 2 using Li 2 S. A mixture of MoS 2 , Li 2 S and Mo powders was taken in a quartz tube, sealed and heated to 800 1C for 10 h. The thus-obtained LiMoS 2 powder was shaken in water for 12 h, which resulted in a colloidal solution of MoS 2 nanosheets, and the 1T 0 phase was confirmed by STEM ( Fig. 19(b)). 25 Although the low-yield problem of the mechanical exfoliation method gets resolved to some extent, the control over size and thickness gets reduced. Additionally, expensive reagents are required, and the problem of Li-ion toxicity exists. Impurities are easily introduced as the Li x MoX 2 compounds are highly air sensitive, thereby requiring extensive cleaning which hampers the yield and may even cause structural destruction.

Pulsed laser ablation
The pulsed laser ablation (PLA) method is extensively used for the preparation of NPs. In this process, a laser beam is used as the primary excitation source for the vaporisation of the metal precursor from the surface. A high energy pulsed laser beam hitting the target material generates high temperatures which vaporize the substance quickly. The wavelength and duration of the pulse (Hz), and laser ablation time are chosen depending upon the material to be synthesised. Han

Raman spectroscopy
Raman spectroscopy is a reliable and non-destructive method to study lattice vibrations and phonon modes in materials. The Raman study becomes crucial when dealing with layered compounds. It is a very definitive tool to determine the number of layers in the synthesized samples. As MoX 2 compounds exist in different phases, semiconducting as well as metallic, Raman spectroscopy is also the most convenient tool to identify these phases. Almost every report on synthesis of MoX 2 compounds has mentioned the use of Raman spectroscopy for characterization. Kong et al. 7 analysed the as-synthesized MoS 2 using Raman spectroscopy where they observed two peaks for the E 1 2g and A 1 g modes at approximately 380 cm À1 and 420 cm À1 , respectively. Late et al. 12 synthesized monolayer MoS 2 whose Raman spectrum was then compared with that of bulk MoS 2 . Fig. 20(a) shows the out-of-plane A 1 g and in-plane E 1 2g mode position in the Raman analysis of monolayer MoS 2 and compares with the peak positions in the bulk form. Byrley et al. 140 photochemically induced a 2H to 1T phase transition in MoS 2 and monitored by Raman spectroscopy. The appearance of additional peaks at 330 and 370 cm À1 confirms the 1T phase and a distance of B20 cm À1 between E 1 2g and A 1 g peaks indicated a MoS 2 monolayer. Baledharan et al. 80 reported that as the number of layers decreases, the intensity of the Raman peaks decreases. It has been reported that the distance between the out-of-plane A 1 g or B 1 2g and in-plane E 1 2g peaks is indicative of the number of layers and decreases when the thickness of the films increases. 27 For example, a gap of almost B18 cm À1 indicates monolayer MoS 2 . 86 Guo et al. 25 very qualitatively represented the phase transformation in MoS 2 through the Raman spectra of primitive (1T 0 -MoS 2 ), mixed phase and 2H-MoS 2 ( Fig. 20(b)). Peaks at 156, 228, 283, 330, 403 cm À1 were obtained for 1T 0 -MoS 2 . The non-appearance of any peak at 383 cm À1 indicates the absence of the 2H counterpart.
The doublet in the range of 380-420 cm À1 is characteristic of 2H-MoS 2 . 141 Although most of the reports support that blue and red shifts are observed in the A 1 g and E 1 2g peaks, resulting in a decreased peak distance, 142 there are a few studies which report divergent behaviour. For instance, Zhu et al. 143 observed Raman frequencies at 379.6 and 401.8 cm À1 for 4 nm thickness, blue-shifted from the pristine MoS 2 . The reason provided by them was the distortion in the structure due to intercalating ion induced strain. Thanh et al. 144 reported a blue shift in the Raman peaks (Fig. 20(c)) when MoS 2 layers of 1.5 to 3.5 nm thickness were produced via N 2 quenched exfoliation. The peak distance however decreased, consistent with other reports. Yang et al. 145 observed blue and red shifts in both the Raman peaks upon inducing strain in the plane and out of the plane, respectively.
2H-MoSe 2 shows 12 modes of vibration, of which A 1 g , E 1 g , E 1 2g , and E 1 2g are Raman active. As shown in Fig. 20(d), the out-ofplane A 1 g is the more prominent peak at about 240 cm À1 and the peak at 280 cm À1 is reported to be due to the E 1 2g in-plane mode of vibration. 27 A peak at around 350 cm À1 due to the interlayer interaction is also considered as a characteristic peak which is absent in monolayer MoSe 2 . 134 Likewise, Raman spectroscopy is also often used to identify various phases of MoTe 2 where the 2H-phase exhibit peaks at 119, 171, 233 and 289 cm À1 corresponding to in-plane E 1g , outof-plane A 1g , in-plane E 1 2g and out-of-plane B 1 2g modes, of which the peak at 233 cm À1 was the most prominent. 99,106 In another report, the peaks at 171.4 (A 1g ), 234.7 (E 1 2g ), and 290.0 cm À1 (B 1 2g ) corresponded to the major Raman vibrational modes of 2H-MoTe 2 . 29 It is also reported that the thickness of the MoTe 2 layers can be determined by the intensity ratio of the B 1 2g and E 1 2g peaks, e.g. a decrease in the ratio means an increase in the number of layers. In the case of the 1T 0 -phase of MoTe 2 , it has been reported that the chalcogenide layer is slightly offset compared to other phases, resulting in a distorted structure which gives rise to a higher number of Raman peaks compared to the 2H-phase. Empante et al. 106 observed Raman peaks at 80, 85, 102, 112, 126 and 162 cm À1 for the 1T 0 -phase and some additional peaks at 189 and 257 cm À1 were also reported by Zhou et al. 99 Generally, the broad peaks at 155 and 242 cm À1 can be related to the 1T-phase of MoTe 2 . 106 Few-layer MoTe 2 shows also a characteristic out-of-plane B 1 2g peak which is normally absent in monolayer and bulk MoTe 2 . Also, the peak at about 255 cm À1 for multilayer MoTe 2 can be

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tuned and a blue shift for the monolayer may be realized. 160 Table 3 summarises the Raman peak values of MoX 2 from various reports. However, these values may vary from report to report as the peak positions are phase and thickness dependent. Fig. 20(d) shows a representation of the effect of layer thickness on the Raman peaks of MoS 2 and MoSe 2 . 27 It is also well known that the Raman peak values are dependent upon the wavelength of laser light used. 122 Yang et al. 134 studied the variations in the Raman frequencies of MoS 2 and MoSe 2 with temperature and pressure. Fig. 21(a) shows that the peaks are blue-shifted on increasing pressure. However, a red shift was observed by the authors when the temperature was increased.
Inter-atomic interactions are enhanced when the pressure is increased causing compression of the out-of-plane mode leading to broadening of the A 1g peak in comparison to the E 1 2g peak. Similarly, with an increase in temperature, the kinetic energy increases. Therefore, inter-atomic interactions are diminished, and a red shift was observed.

X-Ray photoelectron spectroscopy (XPS)
XPS is another important tool to obtain useful information about the bonding, structure/phases and morphology of these compounds. The CVD method generally involves the use of MoO 3 as the Mo precursor, and the extent of reaction completion can be estimated from the XPS spectra. Molybdenum is present as Mo +6 in MoO 3 and shows binding energies of 232.5 and 235.9 eV for 3d 5/2 and 3d 3/2 , respectively, while these values are much lower in respective chalcogenides. 5 The typical binding energies of various elements in MoX 2 , e.g. Mo, S, Se and Te, are listed in Table 4. Fig. 21(b) shows the binding energy  position and shift in peaks on transforming the phase. The binding energies are slightly higher for the 2H-phase than for 1T 0 -MoS 2 . The peaks for Mo +4 for MoS 2 in the 2H phase appear at around 230 and 233 eV which shift to a lower energy transformation of the semiconducting phase to the metallic phase. 25 Byrley et al. 140 observed similar phenomena where the broadening in the peaks indicated a mixture of both the phases with peaks at 230 and 233 eV shifted to lower energies. Wang et al. 165 reported a peak shift in Mo(3d 5/2 ) and S(2p) in MoS 2 nanoparticles by +1.6 eV and À2.2 eV, respectively, in comparison to elemental Mo and S because of partial electron transfer from the Mo metal to the chalcogen owing to the greater electronegativity of S than Mo. As a result, Mo gains a partial positive charge which is reflected by the increase in binding energy and sulfur gains a negative charge, hence a decrease in binding energy. 43

X-Ray diffraction (XRD) technique
XRD patterns give a clear indication of the phase of MoX 2 compounds with measurement of the cluster size and also provides information on lattice spacings. The XRD peak positions for 2H-MoS 2 are reported to be around 2y B141, 32.71, 33.51, and 58.31 prominently due to the [002], [100], [101] and [110] planes, respectively. 46,60,80,86 The peak at 2y = 141 is the most intense one in MoS 2 and can give information about the phase and the number of layers as it varies marginally even with slight changes in the structure and morphology. It is reported that the absence of a peak at 2y = 14.41 is a typical characteristic of monolayer MoS 2 . 43 Similarly, the peak at about 14.81, typical for 1T 0 -MoS 2 , may shift to lower angles (B14.41) when transformed to the 2H phase ( Fig. 22(a)). 23 Other Mo-chalcogenides show similar behaviour, e.g. an almost 0.31 variation has been shown in the case of 1T 0 and 2H-MoSe 2 for the peak at B13.91 ( Fig. 22(b)). 23 Atomic force microscopy (AFM) AFM is a conclusive tool to estimate the number of layers/ thickness of the deposited materials. Dividing the height of the as-synthesized layers by the thickness of one layer gives a good estimation of the number of layers present in the as-synthesized material. 110,161 Zhou et al. 99 found the height of the as-prepared MoTe 2 through AFM to be approximately 3.1 nm. The interlayer

Applications
Mo-chalcogenide nanostructured materials offer great scope in modern technological advancement. They are suitable for a variety of electronic and optoelectronic applications due to tailoring of their band gap and nature of the structure, e.g. simple spherical nanoparticles to layered structures can alter the properties, leading to possibilities of new applications. Not only pure MoX 2 nanostructured materials but their hybrid counterparts add tremendous value in converting traditional applications to modern requirements. In search of hybrid materials, researchers tried to combine nanoparticles with graphene for betterment of electrical and thermal properties. Indeed, it has been reported that graphene, having excellent conductivity when combined with MoX 2 , may get further enriched, opening new avenues in the field of catalysis. In addition, MoX 2 can tackle the no bandgap problem of graphene. Therefore, the fabrication of hybrid structures achieving a synergistic effect is the latest research trend. Several reports and reviews were published discussing various applications of Mo-chalcogenides and a few of them are referred in this article. 168,169 Mo-dichalcogenides have been proposed as potential materials for energy applications such as thermoelectrics, 170 Li/Na-ion batteries, 10 supercapacitors, 11 etc. Due to the high ionic conductivity and surface area, they are potential materials for electrical double layers in supercapacitors. 171 Hydrothermally synthesized MoSe 2 has shown a specific capacitance of 133 F g À1 at a current density of 2 A g À1 in a full solid-state supercapacitor, thereby opening avenues for flexible supercapacitors. 172 Table 2 presents some of the important applications of MoX 2 compounds prepared by various methods. The following sections discuss some selected applications of layered molybdenum dichalcogenide compounds in their intrinsic forms and integrated with other materials wherever seems appropriate.

Catalysis
Electrocatalysis. The world is pursuing renewable and clean sources of energy. Molecular hydrogen is by far the cleanest fuel. Given the right catalyst, the hydrogen evolution reaction (HER) is the most efficient and simplest electrode reaction to produce hydrogen. Production of hydrogen via splitting of water through the HER is a 2e À electrochemical reaction and is believed to be an excellent way to produce renewable hydrogen fuel. In the HER, the H + ions from the solution combine with electrons on the surface of the catalyst to produce H 2 gas. Therefore, adsorption and binding of hydrogen becomes the most essential step. This step is quantized in the form of Gibbs free energy of adsorption (DG H ). The lower the DG H value, the higher the binding efficiency. The layered nature of MoX 2 compounds makes them highly suitable for catalysis due to the presence of dangling bonds and exposed edges, 7,173 whereas basal planes are catalytically inert. Therefore, nanosized MoX 2 are more active due to increased accessible edges as compared to their bulk counterparts due to the increased surface area.
HER devices use acidic electrolytes, and hence MoS 2 and MoSe 2 act as acid-resistant catalysts. 174 Several reports have reported the remarkable capability of MoX 2 compounds in the HER. In HER electrocatalysis, the HER catalyst should minimize the overpotential and should have a low Tafel slope (intrinsic properties of the catalyst) and consequently increase the process efficiency. It is therefore warranted that researchers should undertake advanced research to develop highly active HER catalysts including materials that are highly abundant and inexpensive. The HER on a commercial Pt surface gives a Tafel slope of B 30 mV dec À1 , while commercial MoS 2 shows a value of B115 mV dec À1 . MoS 2 has the advantages of being resistant to sulphur poisoning and highly reactive. Xu et al. 110 explored MoS 2 QDs and nanosheets for HER performance. The QDs gave an overpotential of about 350 mV, whereas the mixture gave an overpotential of 120 mV which was due to the high number of active edges in interspersed QDs and nanosheets and obtained a Tafel slope of B69 mV dec À1 . There was a negligible change in performance even after the 2000th cycle. The main focus is to maximize exposed sites to obtain a higher efficiency. For this motive, MoS 2 has been supported on various materials like Au, graphene, CdS, ZnS, carbon fibres, SiO 2 , etc. MoS 2 /RGO hybrids synthesized by Yanguang Li and co-workers 60 gave a very low overpotential and a small Tafel slope. They obtained a Tafel slope of B41 mV dec À1 from MoS 2 /RGO hybrids. This catalyst was reported to be highly durable and reproducible even after 1000 cycles. The progress in MoS 2 based HER performance was thoroughly reviewed by He et al. 175 On the other hand, MoSe 2 show HER performance with a much lower Tafel slope and a lower overpotential than MoS 2 as MoSe 2 provides a higher coverage area for hydrogen adsorption. 52 Guo et al. 176 fabricated a nanocomposite of MoSe 2 /RGO and found that the electrocatalytic activity was well comparable to that of the commercial Pt/C material. The role of electrical conductivity is very pivotal for HER performance. The electrical conductivity of 1T-MoX 2 is much higher than that of 2H-MoX 2 . Using 1T-MoX 2 provides more active sites and a greater electrical conductivity leading to an enhancement in HER activity. [177][178][179] Zhang et al. 180 reported core-shell type 1T@2H-MoSe 2 nanosheets via a simple hydrothermal method with excellent HER performance having a Tafel slope of 41 mV dec À1 equal to that reported for MoS 2 and an overpotential of 20 mV. 60 Huang et al. 181 synthesized MoSe 2 /SnO 2 composites where MoSe 2 nanosheets were decorated on SnO 2 nanotubes. This open structure made the edges fully accessible leading to high HER activity with a Tafel slope of 51 mV dec À1 . These reports suggest that a secondary material is required in order to expose the edges and make full utilisation of these MoX 2 compounds. Similarly, Singh et al. 182 reported the improved performance of the MoSe 2 /SnS nanocomposite for water splitting.
When it comes to tellurium-based Mo nanostructures, 2H-MoTe 2 was found to be catalytically inert; however, 1T-MoTe 2 was found to be a competent electrocatalyst for the HER with a moderate current density of À10 mA cm À2 at about 360 mV in sulfuric acid. 183,184 Likewise, Qiao et al. 185 reported liquid exfoliated MoTe 2 nanosheets for electrocatalytic activity showing a Tafel slope of 118.9 mV dec À1 with an overpotential of 309 mV. As the HER activity is extensively dependent on the exposed edges, a large portion of the layered surface, i.e., the basal plane, remains useless. By activating the basal planes of 1T 0 -MoTe 2 , McGlynn et al. 28 reported an improvement of overpotential in MoTe 2 from 320 to 178 mV with the Tafel slope increasing to 116 mV dec À1 from 68 mV dec À1 . This value was obtained by applying a reverse bias to the MoTe 2 electrodes which introduced extra electrons leading to activation of basal planes resulting in superior HER activity. There is truly little research carried out for HER applications of MoTe 2 and it may be possible to further enhance its applicability by combining it with other materials. In view of this, Donglin Lu and co-workers 186 synthesized 1T 0 -MoTe 2 on a carbon cloth substrate via the CVD process. The activity of the material was evident from the Tafel slope value of 127 mV dec À1 and the overpotential value of À230.7 mV. The inter-combination of MoX 2 may further enhance their activity, but no reports on that matter can be found. Hence, this area can be looked upon to expand their application and gain more insights into their properties.
Photocatalysis. MoS 2 , being chemically inert, easily available, and having low toxicity, good thermal stability and high absorption capacity, has been considered as a useful heterogenous photocatalyst for dye degradation and water decontamination through advanced oxidation processes (Fig. 24). The small bandgap of TMDs, especially those of MoS 2 and MoSe 2 , allows solar energy to be harvested in the visible region for photocatalytic degradation of organic pollutants. The narrow bandgap of nano-MoSe 2 (B1.55 eV) is highly resistant to photocorrosion. The photocatalytic activity of MoSe 2 is limited by virtue of a higher charge carrier recombination rate, poor visible light adsorption and a low electrical conductivity. These drawbacks can be overcome by the synergistic effect of other nanomaterials. Many reports mention the use of TMD composites with other materials like ZnO, TiO 2 , GO, and rGO as photocatalysts. 187 The review article by Wu et al. 188 gives a general comparison of the photocatalytic efficiency of MoS 2 in combination with such materials. Ritika et al. 18 observed more than 95% degradation of 10 ppm methylene blue (MB) solution in 20 minutes using a meager quantity (0.15 g l À1 ) of the MoS 2 -ZnO catalyst. However, the degradation percentage was found to be pH dependent with maximum degradation observed at pH 11. According to them, the combination of MoS 2 and ZnO has reduced the recombination rate of charge carriers due to fast charge transfer in the conduction bands of MoS 2 and ZnO leading to a high rate of reaction. Wu et al. 189 synthesized MoSe 2 composites with reduced graphene oxide (rGO) so as to minimize the recombination rate and increase the charge transfer. The hydrothermal method resulted in formation of 2-3 layer thick MoSe 2 nanosheets vertically aligned on the surface of rGO. These nanocomposites were able to remove more than 90% of MB and RhB effluents. Haipengchu et al. 50 reported up to 90% reduction of Cr(VI) in visible light in 2 h with MoSe 2 /TiO 2 nanocomposites. They also compared the results with those of individual MoSe 2 and TiO 2 which resulted in a much lower reduction of the same up to 60% and 5% only. The maximum efficiency was obtained when the ratio of MoSe 2 and TiO 2 was 2 : 1 and further increase in the amount of TiO 2 decreased the reduction efficiency. Mittal et al. 190 studied the photocatalytic activity of MoSe 2 and polyaniline (PANI) nanocomposites. These composites were effective against both cationic and anionic dyes due to the presence of both +ve and Àve charges on the surface and thus 80% degradation was observed for Congo red (CR) and Rhodamine B (RhB) by a MoSe 2 /PANI composite with a 2 : 1 ratio. Huang et al. 191 were even successful in degrading nitro aromatic compound contaminants present in water with the help of the as-synthesized MoSe 2 microspheres composed of MoSe 2 nanosheets. However, in the case of Mo-tellurides, out of two phases of MoTe 2 , only the 2H phase has fairly good photoresponsivity to visible and infrared light. 192

Water desalination and decontamination
Shortage of clean potable water is the major problem faced by most of the population. It is realized by the researchers that, without disturbing natural hydrological cycles, seawater desalination technologies are the best alternative to the problem of water shortage. Driven by their huge ecological benefits, they are gaining a lot of research interest. MoS 2 nanosheets are highly effective in decontaminating water by absorbing heavy ions. The monolayer nanosheets have a high absorption capacity due to a high surface area and excellent affinity towards many transition metal ions (heavy metals) such as Hg 2+ , Ag + , Pb 2+ , Zn 2+ , and Cd 2+ (soft Lewis acids) due to the presence of S 2À ions (soft Lewis base) on the exposed edges. The order of selectivity is Hg 2+ 4 Pb 2+ 4 Zn 2+ 4 Cd 2+ according to the order of their softness. These negatively charged ions are also helpful in dispersing these nanomaterials into water. The H + or other intercalating ions such as Li + are present in contaminated water as counterions with a negative charge on the surface. These ions are replaced by the contaminant metal ions and make complexes with sulfur ions, leading to decontamination of water. Another factor in the adsorption of these ions on the nanosheets is that the ions in the water are pulled and hold together by the surface ions through electrostatic force of attraction. Thus, the whole process is believed to be based on the interaction at the surface as well as the inner layer complex formation. 193 Therefore, the best adsorbent for decontamination would be the one with a higher sulphur content, large surface area and high pore volume having enough size to accommodate the ions. Kankeu et al. 194 recently synthesized MWCNT/MoS 2 hybrids loaded with mercaptopropyl trimethoxysilane to create a sulfur rich surface and then tested them for removing mercury ions from water. A high absorption capacity of 0.9661 mg g À1 was achieved, and the study also revealed that the decontamination takes place through the chemisorption phenomenon. Ai et al. 195 reported the absorption capacity of the hydrothermally prepared MoS 2 nanosheets as high as 2563 mg g À1 and excellent mercury selectivity. The concentration of mercury ions was drastically reduced from 10 000 ppb to 0.3 ppb by using 10 mg of the adsorbent for 100 mL solution. MoS 2 adsorbents are also highly efficient in absorbing cationic/alkaline dyes like MB, malachite green (MG), and RhB 196 through van der Waals forces and electrostatic forces of attraction. Massey et al. 197 in their study found that 0.1 g of the synthesized MoS 2 nanosheets were able to adsorb 90% of MB within 10 minutes from its 4100 ppm solution and has shown good recyclability. Similarly, an 80% removal efficiency was observed in the case of MG and RhB within an hour. However, acidic dyes like CR and fuchsin acid have shown very low adsorption. In contrast to most studies, fungus like MoS 2 nanosheets, hydrothermally prepared by Song et al.,198 were selective towards Congo red instead of MB or RhB. MoS 2 nanosheets with a contact angle greater than 801 (against water) exhibit hydrophobicity and hence can be helpful in removal of oils from water. 199 These nanostructures having high adsorption properties combined with photocatalytic properties serve as productive and cost-efficient materials for water purification. A review article discussing specifically the adsorption of heavy metal ions in water on MoS 2 has been recently published. It thoroughly explains the mechanism involved, the factors affecting the adsorption process and its efficiency compared to other adsorbents. 200 In their study, Hieranian et al. 201 applied membrane technology for the desalination of water. As it is obvious that the thinner the membrane, the greater the water flux through it, single layer MoS 2 nanosheets with nanopores were applied, achieving a desalination capacity up to 80%.

Sensors
The use of nanocrystalline MoX 2 as a sensing material is widely being studied. The most reported MoX 2 based sensors are field effect transistor (FET) type. The advantage of MoS 2 in FETs has been thoroughly reviewed by Singh et al. 202 The general device structure of a FET with the example of MoS 2 as the sensing material is depicted in Fig. 25. 203 The strength of MoX 2 transistors lies in a large bandgap, which offers a high on/off ratio, and the layered nature provides excellent electrostatic integrity. Therefore, they can overcome the limitations in graphene-based devices due to the absence of bandgap despite having an excellent mobility of up to 2 Â 10 5 cm 2 V À1 s À1 . 204 They may not be ideal candidates for high performance applications but they can be conveniently utilized for applications having low-power requirements. MoS 2 -based FET sensors have exhibited exceptional charge carrier mobilities and high on/off ratios with great sensitivity in the detection of biomolecules, heavy metal ions, and toxic gases. 193 The following sub-sections discuss the use of molybdenum dichalcogenides in gas sensors, photo-detectors and bio-sensors.

Gas sensors
Gaseous toxicity detection is an essential requirement for human and environmental health. The timely monitoring and understanding of effluents from industries and the environment is a burning issue for living organisms. In this regard, the development of 2D sensors which can work at room temperature has become important. Several reports endorsed nano-MoS 2 for gas-sensing applications due to the ample surface area and abundant surface-active sites. Yoon et al. 203 reported a 0.5 to 320 cm 2 V À1 s À1 variation in field effect mobility values with an on/off 410 10 order of magnitude when sensing was performed using a thin MoS 2 FET. MoS 2 -based FET devices are useful in the detection of various gases (e.g., H 2 , NH 3 , and NO 2 ). 13,205 As the 2D configuration allows better adsorption of gas molecules, most reports use MoS 2 nanosheets for gas detection. 206 Li et al. 207 detected NO gas up to the conc. of 0.8 ppm through a mechanically exfoliated MoS 2 nanosheet based FET and also observed that multilayer MoS 2 was more efficient than the monolayer analogue. It is reported that the resistance increases due to the absorption of electronwithdrawing NO on MoS 2 . 208 Late et al. 26 reported a room temperature MoSe 2 sensor for the detection of NH 3 up to a level of 50 ppm. The detection of hazardous NO 2 and NH 3 gases up to a level of 1 ppm was reported using a MoSe 2 based FET sensor 209 and up to a level of 20 ppb for exfoliated MoS 2 . 210 It is often realized that MoX 2 nanomaterials offer high sensitivity, but the recovery time is longer when compared to metal oxides. 211,212 Photo-detectors Atomically thin MoS 2 shows excellent sensing towards chemicals and gaseous molecules and is also a very good photosensor owing to its sensitivity to the external environment and illumination conditions. Late et al. 12 synthesized monolayer MoS 2 FETs and studied the origin of hysteresis in these devices. They stated that the main cause of hysteresis was the absorption of water molecules which induce trap states in the material. The authors also mentioned that with MoS 2 being highly photosensitive, white light illumination can significantly contribute to hysteresis. It was thereby suggested that the controlled hysterical behaviour of FETs can be utilized in humidity sensors and the photosensitive behaviour can be utilised in photo-detectors. 213 Li et al. 116 fabricated phototransistors using 1L MoS 2 and SiO 2 as dielectric layers which have shown a photo-responsivity of 7.5 mA W À1 at an applied voltage of nearly 50 V. Similarly, phototransistors fabricated from 1L to 3L MoS 2 exhibited a photo-responsivity of nearly 100 mA W À1 at an applied voltage of up to 10 V. The report has also shown that 1L and 2L MoS 2 detected green light and 3L nanosheets were responsive to red light. 214 Lopez-Sanchez et al. 215 fabricated 1L-MoS 2 phototransistors and reported an excellent photoresponsivity of 880 A W À1 . Hang et al. 121 fabricated phototransistors based on 3L MoSe 2 , demonstrating an on/off ratio of B10 5 and a carrier mobility of 1.79 cm 2 V À1 s À1 . The humidity sensors developed from MoSe 2 nanoflowers showed a sensitivity of more than 70% with response and recovery times of 53 seconds and 13 seconds, respectively. 14 Selamneni et al. 216 fabricated a near infrared (NIR) photodetector incorporating MoSe 2 nanoflowers onto cellulose paper and reported a photoresponsivity of 9.73 mA W À1 when illuminated by 780 nm light and the expected improvement with an enhancement in light intensity. The fabricated device was flexible and robust in nature, promising its future in nanoelectronics. Having a narrower bandgap than nano-MoS 2 and MoSe 2 , nano-MoTe 2 is a favourable candidate for NIR photodetectors. The use of thin layered MoTe 2 is very prominent in FETs. 129,217 Few-layer MoTe 2 has the least direct bandgap in the MoX 2 family and therefore can be operated in the visible to infrared energy range. Giri et al. 218 reported 3L MoTe 2 photodetectors showing photoresponsivities over the range of 300 to 1100 nm with a maximum photoresponsivity of B53.1 mA W À1 at 870 nm and a minimum value of 30 mA W À1 at 1100 nm. By introducing an electron tunnelling mechanism, interestingly, Yin et al. 124 reported an outstanding photoresponsivity of 2560 A W À1 and an electron mobility of 25.2 cm 2 V À1 s À1 using few-layered MoTe 2 as the channel in a transistor.

Bio-sensors
Owing to their excellent electrochemical and luminescence properties, MoS 2 -based nanomaterials have been endorsed as excellent biosensing probes for the analyte detection over a wide range. 219 Due to their luminescence quenching properties and multidimensional structures, such chalcogenides can cover a wide band of applications. 193 Indeed, there are several reports that document successful utilization of MoS 2 nanosheets in the detection of biomolecules and metal ion contaminants. 220,221 MoS 2 nanostructures are highly biocompatible up to a conc. of 100 ppm, 222 and thus it facilitates the monitoring of Ag + ions in E. coli cells. 220 Other species such as proteins 223,224 and H 2 O 2 225 have also been sensed using MoS 2 nanosheets. A strong photoluminescence is observed in nanocrystalline MoS 2 which is absent in the bulk form. This property renders it useful in bio-imaging probes. Additionally, TMDs have a superior ionic conductivity and electron mobility to the respective oxides. 226 Sha et al. 16 demonstrated detection of uric acid via fabrication of a flexible, highly sensitive and low-cost electrochemical sensor based on MoS 2 nanosheets. The review by Sha et al. 17 encompasses the biosensing applications of MoS 2 , emphasizing its potential in biomedical and environmental detection in terms of sensitivity and selectivity.

Photovoltaics
Because of their high electron mobility and radiation absorption properties, MoX 2 compounds have been widely explored for solar cells. It is therefore expected and has also been demonstrated that MoS 2 can overtake Si as the absorber layer because of its better absorption efficiency. It can be used as the charge transport layer as well as the buffer layer in solar cells.
The recently published review article by Nawz et al. 227  cells (DSSCs) use toxic ruthenium-based dyes which degrade at high temperatures and use expensive Pt electrodes. MoS 2 , however, is highly stable and absorbs in the visible range and therefore has been widely applied; for example, Ortiz-Quelis et al. 9 studied MoS 2 as a photoanodic material in dye-sensitized solar cells, resulting in an increased photocurrent. A 6% conversion efficiency was reported using the MoS 2 counter electrode. 228 MoSe 2 and MoTe 2 and their composites can be good alternatives to Pt electrodes due to their good electrocatalytic activity. 229 Bi et al. 230 reported a conversion efficiency comparable to that of Pt electrodes in DSSCs using nitrogen doped graphene templated MoSe 2 hollow spheres. Hussain et al. 100 demonstrated a conversion efficiency of 7.25% using MoTe 2 as the counter electrode as compared to an 8.15% efficiency obtained using the commercial Pt electrode. An enhancement in the conversion efficiency, i.e., 8.07%, was reported using MoSe 2 /MoTe 2 . 231 It was also shown that, with an increase in film thickness, the efficiency decreased. MoS 2 has also been used in organic solar cells (OSCs) as it has been reported to be capable of absorbing 5-10% more incident light as compared to GaAs and Si. 232 Similarly, using MoSe 2 QDs in OSCs, Lian et al. 233 achieved a high conversion efficiency of 17%. MoSe 2 has also marked its role in copper indium gallium selenide (CIGS) solar cells where the selenization process results in formation of a MoSe 2 interlayer between the Mo layer (back contact) and the absorber layer. This thin MoSe 2 layer has been reported to reduce the Schottky barrier height. 234 The deposition techniques, doping concentration and thickness greatly influence the performance of CIGS solar cells. 235

Biology
Mo dichalcogenides have recently become useful materials for cancer cell detection and understanding their interaction provides useful information for biologists. The advancement in nanotechnology and bioscience is thus desired to undertake such newer materials for better diagnostics as well as sensing and bio-imaging technologies. There are, however, only a few reports on this topic. It is often desired that spherical particles of semiconductors, especially those with light emitting properties or generally photonic properties, can be exploited for biological applications as their optical behaviour will possibly alter to provide useful information. Appropriately, Xu et al. 110 tested the cytotoxicity of MoS 2 quantum dots (QDs) prepared by a simple sonochemical method against HeLa and HEK293T cells and found them to be less toxic, indicating that such particles are safer to cells for obtaining useful information. These QDs also show excellent fluorescence in the biological environment, permeable in the cell membrane but not in the nuclei promising non-toxic and bio-compatible probes for in vitro imaging. When compared with other TMD materials which stay accumulated in the organs, PEG-coated MoS 2 nanosheets have shown faster degradation and removal from the body within a month. 236 MoS 2 oxidation products readily undergo degradation and release soluble Mo-compounds and hence are responsible for the biocompatible nature of MoS 2 . 237

Conclusions
This survey has revealed that nanostructured molybdenum dichalcogenides are a hot area of research currently. Research in this area is still in its infancy and there is a lot more to touch upon in terms of convenient synthesis methods, insights into the structure and morphology, and also their potential in various energy storage and conversion applications. In an increasingly competitive material fabrication market, ecofriendly and reusable materials are the core needs of advanced technology at a reasonable cost. The research output of MoX 2 type compounds could lead to the development of better lubricants and tribological, model, advanced, multifunctional and smart materials. This trend will foster cutting-edge tech innovations in the materials space and provide invaluable insights into various aspects of material fabrication. In this article, synthesis of nanocrystalline MoX 2 (X = S, Se, Te) by various methods such as hydrothermal, solvothermal, thermal, chemical vapour deposition, microwave, sonochemical, mechanical and chemical exfoliation, and pulsed laser ablation routes was discussed thoroughly. Analysis and characterization are the key requirements for a synthetic chemist, and therefore a special focus was given to the various techniques that are used in the analysis of these structurally unique semiconductors. It is expected that this review will give a good overview of the state of the art to entry level researchers in this field and the quest for simpler synthesis methods for the synthesis of this type of compound on the laboratory scale will continue. Hence, unique types of morphologies will emerge which may give rise to unique properties. The article will encourage researchers to look beyond MoS 2 and MoSe 2 and also look into the potential of MoTe 2 which is a unique material as it can also be grown in both 2H and 1T 0 forms with a possibility of tuning the phases as desired for a variety of promising yet challenging electronic applications. Polymorphism control of MoTe 2 is scientifically interesting and requires more comprehensive investigations. An overview of the past progress and recent research revealed that there is a spectrum of interest stretching from the considerably basic to the very applied. It has been revealed in this article that TMDs are excellent materials for a large number of applications but their potential can be further tuned and enhanced by integrating them with other inorganic or organic semiconducting materials. Thus, hybrid semiconductors with MoX 2 can augment their usefulness in a variety of applications such as gas sensors, FETs, water-desalination and decontamination, tailor-made electrocatalysis and photocatalysis, and biological applications remain to be studied.