Open Access

Synthesis, properties and potential applications of two-dimensional transition metal dichalcogenides

Nano Convergence20152:17

DOI: 10.1186/s40580-015-0048-4

Received: 5 March 2015

Accepted: 16 March 2015

Published: 1 September 2015

Abstract

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.

Keywords

Two-dimensional materials Transition metal dichalcogenides Synthesis Electronic device Optoelectronic device Gas sensing device Energy storage device

1 Introduction

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 Review

2.1 Properties of TMDs

2.1.1 Composition

TMDs materials are comprised of a combination of two elements, a transition metal (M) of groups 4 ~ 10 of the transition metal series and a chalcogen (X) such as sulfur (S), selenium (Se), or tellurium (Te). Generally, TMDs materials containing group 4 ~ 7 transition elements have a layered structure, while those with group 8 ~ 10 transition metals have non-layered structures. Figure 1 shows the possible layered and non-layered structures of TMDs materials. Each layer has a thickness of 6 ~ 7 Å, which consists of a hexagonally packed layer of metal atoms combined by weak van der Waals forces [43]. The metal atoms provide four electrons to fill the bonding states of the TMDs such that the oxidation states of the metal and chalcogen atoms are +4 and −2, respectively. The bonding length of the M-X atom lies between 3.15 Å and 4.03 Å, depending on the size of the metal and chalcogen ions.
Figure 1

There are a number of possible layered structure TMDs materials consist of 16 transition metals and three chalcogen atoms. In the case of Co, Rh, Ir and Ni, only a few layered structures are observed, for example NiS2 forms an apyrite structure, however NiTe2 forms layered structure [43].

2.1.2 Mechanical properties

Castellanos-Gomez et al. measured the elastic properties of a suspended portion of MoS2 nanosheets containing 5 to 25 layers [16]. The measurement of its elastic mechanical properties was performed by using the AFM tip to apply a load in the center of the suspended region of the MoS2 nanosheets (Figure 2a and b). When the tip and sample are in contact, the elastic deformation of the nanosheets (δ), the deflection of the AFM cantilever (Δzc) and the displacement of the scanning piezotube of the AFM instrument (Δzpiezo) are related by the equation δ = Zpiezo- Zc. The applied force is related to the cantilever deflection as F = kc · Δzc, where kc is the spring constant of the cantilever (kc = 0.88 ± 0.20 N m−1). They showed that the Young’s modulus of the MoS2 nanosheets is exceptionally high, i.e. E = 0.33 ± 0.07 TPa [16]. Another experimental work also showed the mechanical properies of suspended MoS2 nanosheets. Bertolazzi et al. reported the measurement of the stiffness and breaking strength of single-layer MoS2 [17]. Exfoliated single and bilayer MoS2 nanosheets have been found to deform, and eventually become broken using AFM (Figures 2c and d). The measured in-plane stiffness of single-layer MoS2 is found to be 180 ± 60 Nm−1, and the Young's modulus of single-layer MoS2 is around 270 ± 100 GPa (Figure 2e). Breaking takes place at an effective strain of between 6 and 11% with an average breaking strength of 15 ± 3 Nm−1 (23 GPa). The strength of the strongest single-layer membranes is 11% of their Young's modulus, corresponding to the upper theoretical limit, which indicates that the material is highly crystalline and almost defect-free. This results show that single-layer MoS2 could be suitable for a variety of applications, such as reinforcing elements in composites and for the fabrication of flexible electronic devices [17].
Figure 2

Mechanical propertis of MoS 2 . (a) AFM topography image of a suspended 5 ~ 7 layer MoS2 flake on SiO2/Si substrate [16]. (b) Elastic constant versus t3R−2 measured for 26 layer MoS2. The inset graph shows the same graph on a linear scale [16]. (c) AFM topography image of suspended MoS2 membranes before (left) and after (right) the experiment [17]. (d) The height profile of the membrane highlighted in c shows that adheres to the sidewalls over a vertical distance of the order of 5 nm, resulting in a pretension of between 0.02 and 0.1 Nm−1 (left). Acquired force versus z-piezo extension curves for the suspended membrane and the substrate (right). (e) Examples of loading curves for single and bilayer MoS2 and the least-squares fit of the experimental indentation curves [17].

2.1.3 Electrical structure and optical property

The various electronic properties of TMDs arise from the filling of the non-bonding d bands from the group 4 to group 10 species. When the orbitals are partially occupied, the TMDs display metallic properties, whereas when they are fully occupied, they exhibit semiconducting ones. The influence of the chalcogen atoms on the electronic structure is minor compared with that of the metal atoms, however it is observed that the broadening of the d bands decreases the bandgap by increasing the atomic number of the chalcogen [43]. Table 1 summarize the electronic character of different layered TMDs. The bulk TMDs material has an indirect bandgap according to both the theoretical calculations and experimental results [8-15]. For example, in the case of MoS2, bulk MoS2 exhibits a negligible PL signal, however thinner MoS2 nanosheets shows pronounced emissions at ~670 and ~627 nm [11]. It also shown that the PL intensity of inversely dependent on the number of MoS2 layers. Especially, single-layer MoS2 shows very strong PL intensity [11,13].
Table 1

Electronic character of different layered TMDs [43]

Group

M

X

Properties

4

Ti, Hf, Zr

S, S, Te

Semiconducting (Eg = 0.2 ~ 2 eV)

5

V, Nb, Ta

S, Se Te

Narrow band metals or semimetals

6

Mo, W

S, Se, Te

Sulfides and Selenides are semiconducting. Telurides are semietallic.

7

Tc, Re

S, Se, Te

Small gap semiconductors.

10

Pd, Pt

S, Se, Te

Sulfides and Selenides are semiconducting. Telurides are metallic. PdTe2 is superconducting

Lee et al. first reported on the characterization of single and few layer MoS2nanosheets by Raman spectroscopy and AFM [14]. They clearly observed signal in-plane (E1 2g) and out-of-plane (A1g) modes for the MoS2 sample with number of layers varying from 1 to 6. The frequency of E1 2g decreases and that of A1g increases with increasing number of MoS2 layers. Figures 3a and b show the optical image of the thin MoS2 film on a SiO2/Si substrate and AFM height image, respectively, and Figures 3c and d shows the clear red shift of E1 2g and blue shift of A1g with increasing number of MoS2 layers. The reason for the opposite direction of the frequency shift is the partially Columbic interaction and possible stacking-induced charge of the intra-layer bonding [14]. Optical absorption is another important characteristic of TMDs materials and is related to the band structure of these semiconducting layered materials. Two main peaks can be observed for Si/SiO2-supported MoS2 at 1.85 eV (670 nm) and 1.98 eV (627 nm), respectively. The contrast of the MoS2 layer on a Si/SiO2 substrate is generally related to the reflective index and absorption contrast of the MoS2 and SiO2 layers. Benameur et al. reported that they were able to distinguish single-, bi-, and tri-layer MoS2 and WSe2 on a 90 or 270 nm SiO2 substrate by measuring the contrast under broadband green illumination [44]. Recently, Li et al. demonstrated a simple approach that can be used to identify single-layer to tri-layer MoS2nanosheets on a 300 nm SiO2 substrate by the normal optical microscopy imaging method with Image J software [15]. The grayscale image of the R channel shows distinct layers ranging from single-layer to tri-layers of MoS2nanohseets. Also, the intensity difference between the MoS2nanosheets and SiO2 can be used to distinguish the number of MoS2 layers. The direct identification method is a very simple and non-destructive technique for distinguishing the number of MoS2 layers and MoS2 based devices.
Figure 3

Optical properties of MoS 2 . (a) Optical micrograph of thin MoS2 films on SiO2/Si substrate. [14] (b) AFM height image taken for the 8 x 8 μm2 area indicated by dotted lines in (a) [14]. (c) Raman spectra of thin and bulk MoS2 films. The solid line for the 2 L spectrum is a double Voigt fit through the data [14]. (d) Frequencies of E1 2g and A1g Raman modes (left vertical axis) and their difference (right vertical axis) as a function of layer thickness [14].

2.2 Synthesis method of TMDs

2.2.1 Mechanical exfoliation method

In 2004, Novoselov et al. successfully produced various single-layer 2D crystals from bulk materials, such as graphite, BN, MoS2, NbSe2, and Bi2Sr2CaCu2Ox [1,2,45]. Figures 4a and b show the optical and AFM images of the atomically thin 2D material prepared by the mechanical exfoliation method. This method is typically adopted to prepare single-layer TMDs samples. The single crystal TMDs samples prepared by the mechanical exfoliation method are of good quality, and can be used for studying their basic properties [26,30,46] by optical microscopy, atomic force microscopy (AFM), scanning tunneling microscopy (STM), transmission electron microscopy (TEM) and so on. However, the size of the TMDs material prepared by the mechanical exfoliation method is quite small approximately on the tens of microns scale, posing a limitation to real device applications.
Figure 4

Mechanical and liquid exfoliation method. (a) Optical image of graphene, MoS2, NbSe2, and h-BN flakes prepared by mechanical exfoliation method. The red dotted squares represent the subsequent AFM scan areas [45]. (b) AFM friction images measured simultaneously by AFM from the indicated areas in the red dotted squares. S indicates SiO2 substrate [45]. (c) Photographs of dispersions of MoS2 (in NMP), WS2 (in NMP), and BN (in IPA) [59]. (d)-(f) Low resolution TEM images of BN, MoS2, and WS2 nanosheets prepared by liquid exfoliation method [59].

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 [47] 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 [53]. 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 [50]. 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 [54]. 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 [30]. 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 [59]. 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

To apply TMDs materials to real devices, their large scale growth is essential. The chemical vapor deposition (CVD) method is the most effective way to achieve large-area growth. This method can be divided into two tyes, the sulfurization (or selenization) of metal thin films and vapor phase reaction of metal oxides with chalcogen precursor. Attempts to synthesis MoS2 layers by the simple sulfurization of Mo metal thin films have been reported. Zhan et al. report MoS2 film synthesis by thermal annealing in a sulfur atmosphere with a Mo thin film deposited on a SiO2/Si substrate [61]. The size and thickness of the pre-deposited Mo film determine the size and thickness of the MoS2 thin film, respectively. The direct sulfurization of the Mo metal thin film provides a quick and easy way to access atomically thin MoS2 layers on insulating substrates. However, it is challenging to deposit a uniform Mo film. Kong et al. also reported that vertically aligned MoS2 and molybdenum diselenide (MoSe2) layers can be produced by a rapid sulfurization/selenization process at 550°C [62]. Uniform TMDs edge-terminated films with densely packed, strip-like grains can be produced on various substrates including glassy carbon, quartz and oxidized silicon. However, the uncontrolled sulfur/selenium diffusion poses a limitation on the growth process. Further, the anisotropic structure of the TMDs layers makes it much faster for sulfur/selenium diffusion along the van der Waals gaps. Alternatively, attempts have also been made to prepare wafer-scale semiconducting MoS2 thin layers [63] by the direct sulfurization of an MoO3 thin layer (Figures 5a-c). To produce wafer-scale MoS2 thin films, an MoO3 thin layer with the desired thickness is prepared by thermal evaporation on a sapphire substrate. During the growth, an MoO3-coated sapphire substrate is initially reduced to MoO2 or other reduced Mo form in an H2/Ar environment at 500°C. The sample is then annealed in a sulfur-rich environment at 1000°C. The as-grown MoS2 thin film can be transferred to an arbitrary substrate for the fabrication of electronic devices [63]. A similar technique has also been adopted for the synthesis of large-area WS2 sheets with controllable thickness [64]. However, the synthesis of TMDs by the direct sulfurization (or selenization) of a metal oxide thin film has several limitations, such as the difficulty to control the thickness of the pre-deposited metal oxide or metal thin film, which affects the wafer-scale uniformity. To obtain high quality TMDs with the desired number of layers, the thickness of the metal oxide needs to be precisely controlled. Recently attempts have been made to improve the synthetic process by depositing metal oxide layers via atomic layer deposition (ALD) [65]. A atomically thin TMDs nanosheets with systematic thickness controllability and wafer-scale uniformity can be achieved using this method (Figures 5d and e). Song et al. demonstrated that the number of tungsten disulfide (WS2) layers can be controlled by tuning the number of cycles of ALD of tungsten trioxide (WO3) [65].
Figure 5

Sulfurization (or selenization) of metal (or metal oxide) thin film. (a) Schematic ilustration for the synthesis of MoS2 layers by MoO3 sulfurization. A layer of MoO3 (3.6 nm) was thermally evaporated on the sapphire substrate. The MoO3 was then converted to MoS2 by a two-step thermal process [63]. (b) MoS2 layer grown on a sapphire wafer [63]. (c) X-Ray photoemission spectroscopy (XPS) results for Mo and S binding energies of the MoO3 layer before and after sulfurization. Raman (lower left) and PL spectra (lower right) for the obtained MoS2 trilayer after MoO3 sulfurization [63]. (d) Synthesis procedure for the ALD-based WS2 nanosheets [65]. (e) OM images of the transferred WS2 nanosheet on the SiO2 substrate for the single-, bi-, and tetralayered thicknesses [65]. (f) AFM images and height profiles (inset) of the WS2 nanosheet transferred onto the SiO2 substrate for the single-, bi-, and tetralayered thicknesses [65]. (g) Raman spectra for the single- (red), bi- (blue), and tetralayer (black) WS2 nanosheets on SiO2 substrates [65]. (h) Relative Raman peak intensities (red) and peak distances (blue) of the E1 2g and A1g bands for the single-, bi-, and tetralayer WS2 nanosheets [65]. (i) PL spectra for the single- (red), bi- (blue), and tetralayer (black) WS2 nanosheets on SiO2 substrates [65]. (j) XPS measurements for the W4f (red) and S2p (blue) core levels of the single-layer WS2 nanosheet [65].

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 [66]. 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

The very first work on the large area growth of MoS2 atomic layers was reported by Li’s group based on the direct chemical vapor phase reaction of MoO3 and S powders [67,68]. During the MoS2 growth, MoO3 in the vapor phase undergoes a two-step reaction, the first of which involves the formation of MoO3-x that further reacts with the sulfur vapor to grow MoS2 layers. The growth of singlecrystalline MoS2 flakes directly on arbitrary substrates is quite possible by this method and, hence, it has been widely used for producing synthetic TMDs single-layers. The growth of MoS2 is very sensitive to the substrate treatment prior to the growth [67]. Facilitating the nucleation by seeding the substrate with graphene-like species has also been explored [69-71]. Figures 6a and b shows the growth setup and condition of MoS2 using MoS2 powder. Up to 400 μm2 single-layer MoS2 flakes with a triangular shape can be formed on SiO2, sapphire, and glass substrates [70].
Figure 6

Vaporization of metal oxide with chalcogen precursor. (a) Growth setup and conditions of single-layer MoS2 [70]. (b) Optical microscope images of MoS2 crystallites grown on sapphire, glass, and SiO2/Si substrates. The scale bar is 10 μm. AFM image with 3 μm scale bar. The inset plot is the height profile along the black line shown in the image, demonstrating its single-layer thickness [70]. (c) Schematic view of the related chemical reaction and experimental setup of the LPCVD system [77]. (d) SEM images of jagged edge WS2 flakes synthesized under pure Ar gas flow and mixed Ar and H2 gas flow. AFM characterization of the WS2 and optical image of bare sapphire and single-layer as-grown WS2 [77]. (e) Schematic of the controlled synthesis of single-layer MoSe2 via CVD. Typical optical images of single-layer triangles and continuous film. Small bilayer domains with a darker color can be observed in the lower left and right [78].

Najmaei et al. used MoO3 nanoribbons and sulfur as the reactants for MoS2 growth [72]. 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 [72]. 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 [73]. 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 [73] 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 [74] 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 [80]. 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 [77]. 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

Due to its excellent semiconducting properties with a direct bandgap of 1.83 eV, single-layer MoS2 nanosheets are seen as one of the most appropriate supplementing materials to graphene for the fabrication of low power electronic devices. It has been experimentally observed that back-gate transistors based on MoS2 nanosheets with SiO2 dielectric exhibit low carrier mobilities of less than 10 cm2 V−1 s−1 [2,6], however, this value can be significantly increased by using a buffer layer of dielectric materials such as HfO2. Thus, the carrier mobility of MoS2 based devices can be increased to over 200 cm2 V−1 s−1, along with a large on/off ratio of 1 x 108 and ultralow standby power dissipation at room temperature [6]. The performance limit of MoS2 transistors with HfO2 as the dielectric [20] has been theoretically studied by Yoon et al., who showed that an On/Off current ratio exceeding 1010 can be achieved. Furthermore, in Figures 7a and b, Pu et al. showed the potential of MoS2 based FETs by fabricating the electric bi-layer transistors (EDLTs, FETs gated by ionic liquids) [21,22]. Through the accumulation of carriers, this technique can produce FETs whose performance is up to 2 orders of magnitude higher than that of conventional FETs (e.g. bottom-gate FETs with 300 nm SiO2 dielectric). Also, using multi-layer MoS2 instead of single-layer MoS2 offers additional advantages. First of all, the density of states of the multi-layer MoS2 is three times that of the single-layer MoS2 leading to a considerably higher drive current in the ballistic limit. In addition, multiple conducting channels can also be created by the field effect in multi-layer MoS2 leading to an increase in the current drive of the device. Radisvljevic et al. experimentally demonstrated that a simple integrated circuit consisting of two MoS2 transistors can be used for amplification as well as performing basic logic operations, as shown in Figures 7c and d [24]. The feasibility of incorporating MoS2 transistors into complex circuits was further demonstrated by Wang et al. [25] by integrating 12 transistors side-by-side based on a single sheet of bi-layer MoS2, for various logic operations, including a NAND (Negated AND or NOT AND) gate, astatic random access memory, and a five-stage ring oscillator. Thus, it is expected that semiconducting MoS2 nanosheets can be used as potential building blocks for next generation integrated circuits and various nano-electronic devices.
Figure 7

Electronic device applications. (a) A thin film MoS2 EDLT constructed with an ion gel on a rigid substrate [22]. (b) Transfer and output characteristics of the MoS2 EDLT. VD is the drain voltage, and VG is the gate voltage. Specific capacitance and phase angle of the ion-gel/MoS2 interface capacitor as a function of the applied frequency [22]. (c) Schematic ilustration for single-layer MoS2 FET device [24]. (d) Drain-source current Ids through the MoS2 single-layer transistor measured as a function of the top gate voltage Vtg (upper graph). Drain-source current Ids as a function of drain-source voltage for different values of Vtg (lower graph) [22].

2.3.2 Optoelectronic devices

The thickness dependent band gap of MoS2 nanosheets makes them a potential material for optoelectronic devices. Recently, single-layer MoS2 based phototransistors were demonstrated, which exhibit an on/off ratio of ~103 and a carrier mobility of 0.11 cm2 V−1 s−1 can be used for switching them on and off within ca. 50 ms [26], which is higher than that of single-layer graphene based devices (tens of picosecond) [81]. Also, the photoresponsivity of the MoS2 phototransistors (7.5 mA W−1) is much higher than that of graphene-based devices (1 mA W−1) [26], due to the zero bandgap, fast carrier transport and short photocarrier lifetime in the pristine graphene, which leads to the fast recombination of photogenerated carriers. Further, Lee et al. fabricated single-, bi- and triple-layer MoS2 phototransistors in the top-gate configuration, consisting of a transparent 50 nm thick Al2O3 dielectric to boost the carrier mobility, e.g. up to 80 cm2 V−1 s−1 for the single-layer device [28]. The phototransistors based on single-layer (bandgap of 1.82 eV) and bi-layer (bandgap of 1.65 eV) MoS2 are promising for greenlight detection, while the triple-layer MoS2 phototransistor with a bandgap of 1.35 eV is suitable for the detection of red light. Additionally, MoS2 nanosheets can be utilized in the fabrication of light emitting diodes (LEDs) (Figure 8). Frey et al. utilized a thin film of chemically exfoliated MoS2 sheets as the anode for polymer LEDs (PLEDs) [29]. It was found that the MoS2 sheets acted as hole injectors due to their high work function and the wide-gap MoO3 layers acted as an electron-blocking layer in the LED based on the MoO3/MoS2 hybrid structure, which improved the carrier balance in the device and led to significantly enhanced light emitting performance and efficiency.
Figure 8

Optoelectronic device applications. (a) Optical image of the single-layer MoS2 based top-gate transistor [28]. (b) Raman spectra of single-, bi-, and triple-layer MoS2. The inset image shows the atomic displacements of the two Raman-active modes: E1 2g and A1g [28]. (c) The photoinduced transfer curves of respective top-gate transistors with single-, bi-, and ttriple-layer MoS2 under monochromatic red, green, and UV light [28]. (d) The schematic band diagrams of ITO (gate)/Al2O3 (dielectric)/single (1 L)-, bi (2 L)-, triple (3 L)-layer MoS2 (n-channel) under the light (Elight = ) illustrate the photoelectric effects for the band gap measurements [28]. (e) The photon energy-dependent ΔQeff plots indicate the approximate optical energy gaps to be 1.35, 1.65, and 1.82 eV for the triple-, bi-, and single-layer MoS2 nanosheets respectively [28].

2.3.3 Gas sensing devices

In recent years, chemical, biological and gas sensors utilizing FET-device structures have become very popular. The change in resistance of the FET channel upon the adsorption of target molecules allows for their detection [32]. Recently, mechanically cleaved single- and multi-layer MoS2 nanosheets based FET devices have been employed for NO detection [30]. It was observed that the FET-based sensors fabricated using bilayer, trilayer, and quadrilayer MoS2 nanosheets exhibited high sensitivity for NO with a detection limit of less than 1 ppm (Figures 9a and b). However FET-based sensing using a single-layer device presented an unstable response. Further, a flexible gas sensor using MoS2-rGO hybrid structure was fabricated on a polyethylene terephthalate (PET) substrate for the detection of NO2 (Figures 9c and d) [33]. In addition, the functionalization of the MoS2 thin film with Pt nanoparticles (NPs) led to an increase in the sensing sensitivity by a factors of 3, with a detection limit of ~2 ppb for NO2. Also, a systematic study on the electrochemical sensing behavior of solution processable MoS2 nanosheets was reported [34] in which the single-layer MoS2 nanosheets attached to a (3-aminopropyl)-triethoxysilane (APTES)-functionalized glass carbon electrode exhibited a fast electron transfer rate in the [Fe(CN)6]3-/4-and [Ru(NH3)6]2+/3+redox systems.
Figure 9

Gas sensing devices application. (a) Optical microscope image of an FET device based on 2 L MoS2 [30]. (b) Real-time current response after exposure of the 2 L MoS2 FET to increasing concentrations of NO. Inset: A typical adsorption and desorption process of NO on the 2 L MoS2 FET [30]. (c) Schematic illustration of the fabrication process of MoS2 TFT array on PET substrate and a photograph of the TFT sensor array [33]. (d) Detection of 1.2 ppm NO2 using MoS2 TFT sensors on PET with different thicknesses of MoS2 thin film [33].

2.3.4 Energy storage devices

The large-scale production of single- and multi-layer MoS2 nanosheets using exfoliation techniques can enable their wide spread application for energy storage devices such as batteries. Recently, MoS2 nanosheets prepared by chemical lithiation and exfoliation in a stacking structure were fabricated for use as electrodes for lithium ion batteries (LIBs) and compared with electrodes made from the bulk MoS2 (Figures 10a and b) [37]. The stacked MoS2 nanostructure showed much better cycling stability than the bulk MoS2, retaining a high capacity of 750 mA h g−1 even after 50 cycles. The stability and Li-storage capacity of MoS2 based energy storing devices can be improved to a greater extent by adding polymer molecules such as poly(ethylene oxide)(PEO) to the Li-intercalation solution [38], as the presence of PEO can increase in the interlayer spacing of the MoS2 nanosheets. Much improved Li-storage capacity and cycling stability are observed in the case of electrodes made from the MoS2–PEO nanocomposites. This improvement in performance is attributed to the fact that large amounts of lithium ions can be accommodated over the PEO. Recently, few-layer MoS2 and reduced graphene oxide (rGO) hybrids were used for the fabrication of electrodes that exhibited a specific capacity of ~ 1100 mA h g−1 at a current of 100 mA g−1 with excellent cycling stability.
Figure 10

Energy storage device application. (a) TEM images of raw and restacked MoS2 [37]. (b) Electrochemical properties of raw and restacked MoS2: cyclic voltammograms of raw and restacked MoS2 at a scanning rate of 0.2 mV s−1 [37]. (c) Microstructure of MoS2/G (1:2) composite: SEM image, TEM image, HRTEM image, and electron diffraction pattern [40]. (d) First three charge and discharge curves of the samples after annealing in H2/N2 at 800°C for 2 h at a current density of 100 mA/g in a half-cell composed of MoS2/G and Li: MoS2, MoS2/G (1:1), MoS2/G (1:2),(d) MoS2/G (1:4) [40].

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 [40] 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.

3 Conclusion

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.

Declarations

Acknowledgements

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.

Authors’ Affiliations

(1)
SKKU Advanced Institute of Nanotechnology (SAINT), Center for Human Interface Nanotechnology (HINT), Sungkyunkwan University (SKKU)
(2)
School of Advanced Materials Science and Engineering, Sungkyunkwan University (SKKU)

References

  1. KS Novoselov, AK Geim, SV Morozov, D Jiang, Y Zhang, SV Dubonos, IV Grigorieva, AA Firsov, Science 306, 666 (2004)View ArticleGoogle Scholar
  2. KS Novoselov, D Jiang, F Schedin, TJ Booth, VV Khotkevich, SV Morozov, AK Geim, Proc. Natl. Acad. Sci. U. S. A. 102, 10451 (2005)View ArticleGoogle Scholar
  3. L Song, L Ci, H Lu, PB Sorokin, C Jin, J Ni, AG Kvashnin, DG Kvashnin, J Lou, BI Yakobson, PM Ajayan, Nano Lett. 10, 3209 (2010)View ArticleGoogle Scholar
  4. Y Shi, C Hamsen, X Jia, KK Kim, A Reina, M Hofmann, AL Hsu, K Zhang, H Li, ZY Juang, MS Dresselhaus, LJ Li, J Kong, Nano Lett. 10, 4134 (2010)View ArticleGoogle Scholar
  5. KH Lee, HJ Shin, JY Lee, IY Lee, GH Kim, JY Choi, SW Kim, Nano Lett. 12, 714 (2012)View ArticleGoogle Scholar
  6. B Radisavljevic, A Radenovic, J Brivio, V Giacometti, A Kis, Nat. Nanotechnol. 6, 147 (2011)View ArticleGoogle Scholar
  7. O Lopez-Sanchez, D Lembke, M Kayci, A Radenovic, A Kis, Nat. Nanotechnol. 8, 497 (2013)View ArticleGoogle Scholar
  8. H Zeng, J Dai, W Yao, D Xiao, X Cui, Nat. Nanotechnol. 7, 490 (2012)View ArticleGoogle Scholar
  9. W Zhang, C-P Chuu, J-K Huang, C-H Chen, M-L Tsai, Y-H Chang, C-T Liang, Y-Z Chen, Y-L Chueh, J-H He, M-Y Chou, L-J Li, Sci. Rep. 4, 3826 (2014)Google Scholar
  10. C Cong, J Shang, X Wu, B Cao, N Peimyoo, C Qiu, L Sun, T Yu, Adv. Opt. Mater. 2, 131 (2014)View ArticleGoogle Scholar
  11. A Splendiani, L Sun, YB Zhang, TS Li, J Kim, CY Chim, G Galli, F Wang, Nano Lett. 10, 1271 (2010)View ArticleGoogle Scholar
  12. T Li, G Galli, J. Phys. Chem. C 111, 16192 (2007)View ArticleGoogle Scholar
  13. KF Mak, C Lee, J Hone, J Shan, TF Heinz, Phys. Rev. Lett. 105, 136805 (2010)View ArticleGoogle Scholar
  14. C Lee, H Yan, LE Brus, TF Heinz, J Hone, S Ryu, ACS Nano 4, 2695 (2010)View ArticleGoogle Scholar
  15. H Li, G Lu, ZY Yin, QY He, Q Zhang, H Zhang, Small 8, 682 (2012)View ArticleGoogle Scholar
  16. A Castellanos-Gomez, M Poot, GA Steele, HSJ van der Zant, N Agraït, G Rubio-Bollinger, Adv. Mater. 24, 772 (2012)View ArticleGoogle Scholar
  17. S Bertolazzi, J Brivio, A Kis, ACS Nano 5, 9703 (2011)View ArticleGoogle Scholar
  18. X Huang, Z Yin, S Wu, X Qi, Q He, Q Zhang, Q Yan, F Boey, H Zhang, Small 7, 1876 (2011)View ArticleGoogle Scholar
  19. Y Zhang, T-T Tang, C Girit, Z Hao, MC Martin, A Zettl, MF Crommie, YR Shen, F Wang, Nature 459, 820 (2009)View ArticleGoogle Scholar
  20. Y Yoon, K Ganapathi, S Salahuddin, Nano Lett. 11, 3768 (2011)View ArticleGoogle Scholar
  21. YJ Zhang, JT Ye, Y Matsuhashi, Y Iwasa, Nano Lett. 12, 1136 (2012)View ArticleGoogle Scholar
  22. J Pu, Y Yomogida, K-K Liu, L-J Li, Y Iwasa, T Takenobu, Nano Lett. 12, 4013 (2012)View ArticleGoogle Scholar
  23. S Kim, A Konar, W-S Hwang, JH Lee, J Lee, J Yang, C Jung, H Kim, J-B Yoo, J-Y Choi, YW Jin, SY Lee, D Jena, W Choi, K Kim, Nat. Commun. 3, 1011 (2012)View ArticleGoogle Scholar
  24. B Radisavljevic, MB Whitwick, A Kis, ACS Nano 5, 9934 (2011)View ArticleGoogle Scholar
  25. H Wang, L Yu, Y-H Lee, Y Shi, A Hsu, ML Chin, L-J Li, M Dubey, J Kong, T Palacios, Nano Lett. 12, 4674 (2012)View ArticleGoogle Scholar
  26. ZY Yin, H Li, L Jiang, YM Shi, YH Sun, G Lu, Q Zhang, XD Chen, H Zhang, ACS Nano 6, 74 (2012)View ArticleGoogle Scholar
  27. GL Frey, KJ Reynolds, RH Friend, H Cohen, Y Feldman, J. Am. Chem. Soc. 125, 5998 (2003)View ArticleGoogle Scholar
  28. HS Lee, S-W Min, Y-G Chang, MK Park, T Nam, H Kim, JH Kim, S Ryu, S Im, Nano Lett. 12, 3695 (2012)View ArticleGoogle Scholar
  29. C Zhong, C Duan, F Huang, H Wu, Y Cao, Chem. Mater. 23, 326 (2010)View ArticleGoogle Scholar
  30. H Li, ZY Yin, QY He, X Huang, G Lu, DWH Fam, AIY Tok, Q Zhang, H Zhang, Small 8, 63 (2012)View ArticleGoogle Scholar
  31. ZY Zeng, ZY Yin, X Huang, H Li, QY He, G Lu, F Boey, H Zhang Angew, Chem Int. Ed 50, 11093 (2011)View ArticleGoogle Scholar
  32. Q He, S Wu, Z Yin, H Zhang, Chem. Sci. 3, 1764 (2012)View ArticleGoogle Scholar
  33. Q He, Z Zeng, Z Yin, H Li, S Wu, X Huang, H Zhang, Small 8, 2994 (2012)View ArticleGoogle Scholar
  34. S Wu, Z Zeng, Q He, Z Wang, SJ Wang, Y Du, Z Yin, X Sun, W Chen, H Zhang, Small 8, 2264 (2012)View ArticleGoogle Scholar
  35. MR Palacin, Chem. Soc. Rev. 38, 2565 (2009)View ArticleGoogle Scholar
  36. MS Whittingham, Chem. Rev. 104, 4271 (2004)View ArticleGoogle Scholar
  37. G Du, Z Guo, S Wang, R Zeng, Z Chen, H Liu, Chem. Commun. 46, 1106 (2010)View ArticleGoogle Scholar
  38. J Xiao, D Choi, L Cosimbescu, P Koech, J Liu, JP Lemmon, Chem. Mater. 22, 4522 (2010)View ArticleGoogle Scholar
  39. X Huang, X Qi, F Boey, H Zhang, Chem. Soc. Rev. 41, 666 (2012)View ArticleGoogle Scholar
  40. K Chang, WX Chen, ACS Nano 5, 4720 (2011)View ArticleGoogle Scholar
  41. BE Conway, Electrochemical Supercapacitors: Scientific Fundamentals and Technological Applications (Kulwer Academic/Plenum Press, New York, 1999)View ArticleGoogle Scholar
  42. JM Soon, KP Loh, Electrochem. Solid-State Lett. 10, A250 (2007)View ArticleGoogle Scholar
  43. M Chhowalla, HS Shin, G Eda, LJ Li, KP Loh, H Zhang, Nat. Chem. 5, 263 (2013)View ArticleGoogle Scholar
  44. MM Benameur, B Radisavljevic, JS He´ron, S Sahoo, H Berger, A Kis, Nanotechnology 22, 125706 (2011)View ArticleGoogle Scholar
  45. C Lee, QY Li, W Kalb, XZ Liu, H Berger, RW Carpick, J Hone, Science 328, 76 (2010)View ArticleGoogle Scholar
  46. S Ghatak, AN Pal, A Ghosh, ACS Nano 5, 7707 (2011)View ArticleGoogle Scholar
  47. GF Walker, Nature 187, 312 (1960)View ArticleGoogle Scholar
  48. J Feng, X Sun, CZ Wu, LL Peng, CW Lin, SL Hu, JL Yang, Y Xie, J. Am. Chem. Soc. 133, 17832 (2011)View ArticleGoogle Scholar
  49. J Feng, LL Peng, CZ Wu, X Su, SL Hu, CW Lin, J Dai, JL Yang, Y Xie, Adv. Mater. 24, 1969 (2012)View ArticleGoogle Scholar
  50. MB Dines, J. Chem. Educ. 51, 211 (1974)View ArticleGoogle Scholar
  51. T Sasaki, M Watanabe, H Hashizume, H Yamada, H Nakazawa, J. Am. Chem. Soc. 118, 8329 (1996)View ArticleGoogle Scholar
  52. D Golberg, Nat. Nanotechnol. 6, 200 (2011)View ArticleGoogle Scholar
  53. JW Seo, YW Jun, SW Park, H Nah, T Moon, B Park, JG Kim, YJ Kim, J Cheon, Angew. Chem. Int. Ed. 46, 8828 (2007)View ArticleGoogle Scholar
  54. HSS Ramakrishna Matte, A Gomathi, AK Manna, DJ Late, R Datta, SK Pati, CNR Rao, Angew. Chem. Int. Ed. 122, 4153 (2010)View ArticleGoogle Scholar
  55. P Joensen, RF Frindt, SR Morrison, Mater. Res. Bull. 21, 457 (1986)View ArticleGoogle Scholar
  56. BK Miremadi, SR Morrison, J. Appl. Phys. 63, 4970 (1988)View ArticleGoogle Scholar
  57. D Yang, RF Frindt, J. Phys. Chem. Solids 57, 1113 (1996)View ArticleGoogle Scholar
  58. G Eda, H Yamaguchi, D Voiry, T Fujita, MW Chen, M Chhowalla, Nano Lett. 11, 5111 (2011)View ArticleGoogle Scholar
  59. JN Coleman, M Lotya, A O’Neill, SD Bergin, PJ King, U Khan, K Young, A Gaucher, S De, RJ Smith, IV Shvets, SK Arora, G Stanton, HY Kim, K Lee, GT Kim, GS Duesberg, T Hallam, JJ Boland, JJ Wang, JF Donegan, JC Grunlan, G Moriarty, A Shmeliov, RJ Nicholls, JM Perkins, EM Grieveson, K Theuwissen, DW McComb, PD Nellist et al., Science 331, 568 (2011)View ArticleGoogle Scholar
  60. RA Gordon, D Yang, ED Crozier, DT Jiang, RF Frindt, Phys. Rev. B 65, 125407 (2002)View ArticleGoogle Scholar
  61. Y Zhan, Z Liu, S Najmaei, PM Ajayan, J Lou, Small 8, 966 (2012)View ArticleGoogle Scholar
  62. D Kong, H Wang, JJ Cha, M Pasta, KJ Koski, J Yao, Y Cui, Nano Lett. 13, 1341 (2013)View ArticleGoogle Scholar
  63. Y-C Lin, W Zhang, J-K Huang, K-K Liu, Y-H Lee, C-T Liang, C-W Chu, L-J Li, Nanoscale 4, 6637 (2012)View ArticleGoogle Scholar
  64. AL Elı´as, N Perea-Lo´pez, A Castro-Beltra´n, A Berkdemir, R Lv, S Feng, AD Long, T Hayashi, YA Kim, M Endo, HR Gutie´rrez, NR Pradhan, L Balicas, TE Mallouk, F Lo´pez Urı´as, H Terrones, M Terrones, ACS Nano 7, 5235 (2013)View ArticleGoogle Scholar
  65. J-G Song, J Park, W Lee, T Choi, H Jung, CW Lee, S-H Hwang, JM Myoung, J-H Jung, S-H Kim, C Lansalot-Matras, H Kim, ACS Nano 7, 11333 (2013)View ArticleGoogle Scholar
  66. X Wang, H Feng, Y Wu, L Jiao, J. Am. Chem. Soc. 135, 5304 (2013)View ArticleGoogle Scholar
  67. Y-H Lee, X-Q Zhang, W Zhang, M-T Chang, C-T Lin, K-D Chang, Y-C Yu, JT-W Wang, C-S Chang, L-J Li, T-W Lin, Adv. Mater. 24, 2320 (2012)View ArticleGoogle Scholar
  68. Y-H Lee, L Yu, H Wang, W Fang, X Ling, Y Shi, C-T Lin, J-K Huang, M-T Chang, C-S Chang, M Dresselhaus, T Palacios, L-J Li, J Kong, Nano Lett. 13, 1852 (2013)Google Scholar
  69. C Mai, A Barrette, Y Yu, YG Semenov, KW Kim, L Cao, K Gundogdu, Nano Lett. 14, 202 (2013)View ArticleGoogle Scholar
  70. S Wu, C Huang, G Aivazian, JS Ross, DH Cobden, X Xu, ACS Nano 7, 2768 (2013)View ArticleGoogle Scholar
  71. A Castellanos-Gomez, R Rolda´n, E Cappelluti, M Buscema, F Guinea, HSJ Van Der Zant, GA Steele, Nano Lett. 13, 5361 (2013)View ArticleGoogle Scholar
  72. S Najmaei, Z Liu, W Zhou, X Zou, G Shi, S Lei, BI Yakobson, J-C Idrobo, PM Ajayan, J Lou, Nat. Mater. 12, 754 (2013)View ArticleGoogle Scholar
  73. AM van der Zande, PY Huang, DA Chenet, TC Berkelbach, Y You, G-H Lee, TF Heinz, DR Reichman, DA Muller, JC Hone, Nat. Mater. 12, 554 (2013)View ArticleGoogle Scholar
  74. Y Yu, C Li, Y Liu, L Su, Y Zhang, L Cao, Sci. Rep. 3, 1866 (2013)Google Scholar
  75. H Schmidt, S Wang, L Chu, M Toh, R Kumar, W Zhao, AH Castro Neto, J Martin, S Adam, B O¨zyilmaz, G Eda, Nano Lett. 14, 1909 (2014)View ArticleGoogle Scholar
  76. HR Gutie´rrez, N Perea Lo´pez, AL Elı´as, A Berkdemir, B Wang, R Lv, F Lo´pez Urı´as, VH Crespi, H Terrones, M Terrones, Nano Lett. 13, 3447 (2012)View ArticleGoogle Scholar
  77. Y Zhang, Y Zhang, Q Ji, J Ju, H Yuan, J Shi, T Gao, D Ma, M Liu, Y Chen, X Song, HY Hwang, Y Cui, Z Liu, ACS Nano 7, 8964 (2013)Google Scholar
  78. X Wang, Y Gong, G Shi, WL Chow, K Keyshar, G Ye, R Vajtai, J Lou, Z Liu, E Ringe, BK Tay, PM Ajayan, ACS Nano 8, 5125 (2014)View ArticleGoogle Scholar
  79. J Shaw, H Zhou, Y Chen, N Weiss, Y Liu, Y Huang, X Duan, Nano Res. 7, 1 (2014)View ArticleGoogle Scholar
  80. J-K Huang, J Pu, C-L Hsu, M-H Chiu, Z-Y Juang, Y-H Chang, W-H Chang, Y Iwasa, T Takenobu, L-J Li, ACS Nano 8, 923 (2013)View ArticleGoogle Scholar
  81. F Xia, T Mueller, Y-M Lin, P Avouris, Nat. Nanotechnol. 4, 839 (2009)View ArticleGoogle Scholar

Copyright

© Han et al.; licensee Springer. 2015

This is an open access article distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/4.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly credited.