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Synthesis, properties and potential applications of two-dimensional transition metal dichalcogenides
© Han et al.; licensee Springer. 2015
Received: 5 March 2015
Accepted: 16 March 2015
Published: 1 September 2015
In recent years, 2-dimensional (2D) materials such as graphene and h-BN have been spotlighted, because of their unique properties and high potential applicability. Among these 2D materials, transition metal dichalcogenides (TMDs) have attracted a lot of attention due to their unusual electrical, optical, and mechanical properties. Also, TMDs have virtually unlimited potential in various fields, including electronic, optoelectronic, sensing, and energy storage applications. For these various applications, there are many methods for sample preparation, such as the mechanical, liquid exfoliation and chemical vapor deposition techniques. In this review, we introduce the properties, preparation methods and various applications of TMDs materials.
In the last few years, a great deal of attention has been dedicated to layered two-dimensional (2D) materials, such as graphene and hexagonal boron nitride (h-BN), owing to their potential applications in various fields [1-5]. The great potential of graphene has stimulated a lot of interest in the exploration of other layered 2D nanomaterials, which can complement the requirements associated with graphene. It is well known that graphene generally exhibits semi-metallic properties, and, therefore, semiconducting and insulating 2D layered nanomaterials having structural properties akin to graphene are needed in order to integrate them into nano-electronic devices for different applications. Recently, layered transition metal dichalcogenides (TMDs), such as MoS2, WS2, MoSe2 and WSe2, have been identified as semiconducting 2D layered materials. TMDs have received significant attention because they exhibit unique electrical [6,7], optical [8-15], and mechanical [16,17] properties. Layered 2D nanostructures with atomic scale thicknesses may exhibit peculiar and fascinating properties in contrast with those of their bulk parent compounds. Both the experimental and theoretical results have shown that 2D semiconductors have exceptional properties that can result in novel and important breakthroughs in the field of nanomaterials and nanodevices.
Because of these attractive properties, there are many potential applications of TMDs materials, such as electronic devices [6,18-25], optoelectronic devices [26-29], gas sensing [18,30-34] and energy storage devices [35-42]. In this review, we introduce the properties, preparation methods and various applications of TMDs materials.
2.1 Properties of TMDs
2.1.2 Mechanical properties
2.1.3 Electrical structure and optical property
Electronic character of different layered TMDs 
Ti, Hf, Zr
S, S, Te
Semiconducting (Eg = 0.2 ~ 2 eV)
V, Nb, Ta
S, Se Te
Narrow band metals or semimetals
S, Se, Te
Sulfides and Selenides are semiconducting. Telurides are semietallic.
S, Se, Te
Small gap semiconductors.
S, Se, Te
Sulfides and Selenides are semiconducting. Telurides are metallic. PdTe2 is superconducting
2.2 Synthesis method of TMDs
2.2.1 Mechanical exfoliation method
2.2.2 Liquid exfoliation method
To exploit the extraordinary potential of these layered materials, large quantities of TMDs nanosheets are required. To obtain large amounts of single- or few-layer TMDs nanosheets, a solution processing strategy would be more appropriate. The first report on the liquid phase exfoliation of sheets of clay materials in the early 1960s  has inspired many studies into methods of exfoliating sheets of TMDs [48-52].
Seo et al. reported an interesting sulfidation-induced shape transformation process for the fabricationof 2D WS2 sheets from 1D W18O49 nanorods . The resulting single sheets of 2D WS2 can further assemble together via van der Waals interactions to form nanosheets containing a number of layers.
However, the lateral dimension of the WS2 sheets is restricted by the size of the rods. Due to their layered structures, TMDs bulk materials can be intercalated by various kinds of intercalates such as organic molecules, transition metal halides and lithium ions . The resulting intercalated compounds can be exfoliated to single and few-layer 2D TMDs nanosheets by ultrasonication [54-58]. For example, Ramakrishna Matte et al. reported the insertion of MoS2 and WS2 with lithium by using n-butyllithium in hexaneas the intercalation agent, and subsequent exfoliation in water with ultrasonication to yield single-layer materials . However, this method is time-consuming and the degree of lithium insertion is not controllable, which limits it feasibility. Zheng et al. developed a controllable electrochemical lithiation method to produce high-yield, single-layer TMDs nanosheets . By incorporating the layered TMDs bulk materials, such as MoS2, WS2, TiS2, and ZrS2, as the cathode in an electrochemical cell, the lithium intercalation in these materials can be monitored and finely controlled during the discharge process. The obtained intercalated compounds can be ultrasonicated and exfoliated in water or ethanol to achieve high-grade TMDs single-layer materials in large amounts.
In another work, Coleman et al. developed an effective and reliable liquid exfoliation technique to produce 2D nanosheets, including single-layers . After the dispersion and ultrasonication of each inorganic starting material in about 30 common solvents with varying surface tensions and adsoption properties, it was demonstrated that the best solvents have a surface tension close to 40 mJ m−2 by using optical absorption spectroscopy. Based on their theoretical investigation, Coleman et al. proposed that successful solvents are those that minimize the energy of exfoliation. For example, N-methyl-pyrrolidone (NMP) and isopropanol (IPA) are very promising solvents for exfoliating various layeredcompounds. The TEM images in Figures 4c and d show that very thin sheets of MoS2 and WS2, are produced with lateral sizes ranging from 50 to 1000 nm. These images and associated Fourier transforms illustrate that no substantial deviation from the hexagonal symmetry of these materials is observed, unlike the MoS2 and WS2 nanosheets exfoliated by lithium intercalation [27,60].
2.2.3 Sulfurization (or selenization) of metal (or metal oxide) thin film
Wang et al. reported a method capable of producing highly crystalline MoS2 flakes with a controlled number of layers by using MoO2 microcrystals as the template . In this method, MoO2 nanoplates are synthesized by the thermal evaporation of MoO3 powder in a sulfur environment at 650–850°C. The surface of the MoO2 plates is further sulfurized to MoS2 at a higher temperature (850–950°C) at a later stage. The surface sulfurization of the crystalline MoO2 micro-plates produces a top MoS2 layer with a high degree of crystallinity However, the MoS2 growth is still determined by the crystal size of the MoO2 flakes, where the MoS2 single crystal obtained is randomly distributed as an isolated island.
2.2.4 Vaporization of metal oxide with chalcogen precursor
Najmaei et al. used MoO3 nanoribbons and sulfur as the reactants for MoS2 growth . The triangular-shaped MoS2 crystals are observed to be nucleated and formed on the step edges. Using substrate patterning by lithography processes, the nucleation of the MoS2 layers can be controlled. The observed catalytic process along the edges is due to the significant reduction in the nucleation energy barrier of MoS2 at the step edges as compared with the flat surface . Further experiments revealed that small triangular MoS2 domains are preferentially nucleated at the step edges and then continue to grow and form boundaries with other domains. Their coalescence finally results in the formation of a continuous MoS2 film . It is also found that the coalescence of the grains leads to the formation of chemically bounded grain boundaries or the simple joining together of the grains by growing on top of each other, without forming any chemical bonds. Van der Zande et al. reported a refined route for ultra-large MoS2 single crystal growth with solid MoO3 and S precursors  The resulting highly crystalline islands of single-layer MoS2 can be up to 120 mm in lateral size. Neither seeding molecules nor step-edges were used to promote the nucleation of MoS2. Large MoS2 crystalline islands with average sizes between 20 and 100 mm were obtained by using ultraclean substrates and fresh precursors. According to report, the yield can be significantly decreased if dirty substrates or old precursors are used.
The direct sulfurization/selenation of various metal oxides or metal chlorides  has been widely applied by many research groups to produce TMDs layers such as MoS2 [74,75], WS2 [64,76,77], MoSe2 [78,79] and WSe2 . Among the numerous reports on the synthesis of sulfides and selenides, one should notice that for the synthesis of selenide, H2 gas is commonly introduced as an additional reducing agent along with Se to further reduce the metal oxides and assist in the selenization reaction [77,80]. Furthermore, Zhang et al. revealed that using H2 as the minor carrier gas can also tailor the shape of single-layer WS2 from jagged to straight edge triangles under low pressure chemical vapor deposition . In Figure 6d, Wang et al. demonstrated the CVD growth of uniform MoSe2 single-layer under ambient pressure, resulting in large area single crystalline flakes with a size of 135 μm. They used MoO3 powder and Se pellets with Ar/H2 carrier gas. The grown MoSe2 has a direct band gap of 1.48 eV, average mobility of 50 cm2V−1 s−1 and on/off ratio of 106.
2.3 Applications of TMDs
2.3.1 Electronic devices
2.3.2 Optoelectronic devices
2.3.3 Gas sensing devices
2.3.4 Energy storage devices
The MoS2 nanosheets can be used for the fabrication of supercapacitors or bi-layer capacitors, as they possess a large interlayer space, as well as a large specific surface area, that can be used for ion intercalation, and exhibit several stable oxidation states from Mo2+ to Mo6+. Recently, a thin film (~100 nm) of CVD-grown edge-oriented MoS2 was employed as a supercapacitor electrode  and the as-fabricated supercapacitor exhibited dual functionality, i.e. it exhibits both bi-layer and faradaic capacitance, and can function even at alternating current frequencies of ~100 Hz.
In summary, we discussed the various properties of TMDs, including their electrical, mechanical, and optical ones. Because of their unique properties, TMDs can be applied in various fields, such as electronics, optoelectronics, sensing and energy storage applications. As a typical example, bulk MoS2 has an indirect band gap of 1.2 eV, whereas single-layer MoS2 is a direct band gap semiconductor with a band gap of 1.8 eV due to the quantum confinement effect, which results in the enhancement of its photoluminescence. Further, it is experimentally found that the single-layer MoS2 based FETs exhibit a remarkably high on/off ratio of ~100 at room temperature. In addition, single-layer MoS2 transistors have been shown to exhibit much better photoresponsivity than graphene based ones. Moreover, gas sensors based on few layer MoS2 nanosheets have shown high sensitivity for NO detection. Also, we discussed the various sample preparation methods. The TMDs samples prepared by mechanical and liquid exfoliation methods have good quality, but their size is small. The CVD approach is suitable for wafer scale fabrication and real device applications.
This work was financially supported by Basic Science Research Program (2012R1A2A1A01002787, 2009–0083540) and the Center for Advanced Soft-Electronics as Global Frontier Project (2013M3A6A5073177) through the National Research Foundation (NRF) of Korea Grant funded by the Ministry of Science, ICT & Future Planning.
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