Volume 16 Number 29 7 August 2014 Pages 14973–15718

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Physical Chemistry Chemical Physics www.rsc.org/pccp

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PERSPECTIVE Jiang Pu, Lain-Jong Li and Taishi Takenobu Flexible and stretchable thin-film transistors based on molybdenum disulphide

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Flexible and stretchable thin-film transistors based on molybdenum disulphide Jiang Pu,*a Lain-Jong Li*b and Taishi Takenobu*ac

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The outstanding physical and chemical properties of two-dimensional materials, which include graphene and transition metal dichalcogenides, have allowed significant applications in next generation electronics. In particular, atomically thin molybdenum disulphide (MoS2) is attracting widespread attention because of its large bandgap, effective carrier mobility, and mechanical strength. In addition, recent developments in Received 13th December 2013, Accepted 27th February 2014 DOI: 10.1039/c3cp55270e

large-area high-quality sample preparation methods via chemical vapour deposition have enabled the use of MoS2 in novel functional applications, such as flexible and stretchable electronic devices. In this perspective, we focus on the current progress in generating MoS2-based flexible and stretchable thin-film transistors. The reported virtues and novelties of MoS2 provide significant advantages for future flexible

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and stretchable electronics.

1. Introduction Recently, two-dimensional (2D) materials, such as graphene and transition metal dichalcogenides (TMDCs), have attracted significant interest because of their unique electronic, optical and mechanical properties.1–4 One of the most promising 2D materials for transistor applications is molybdenum disulphide (MoS2), because of its large intrinsic bandgap.4–11 In particular, MoS2 has an indirect bandgap of 1.2 eV for bulk forms; additionally, MoS2 can be scaled down to atomically thin 2D forms and can be transformed to obtain a direct bandgap of 1.8 eV.9,10 As a result of this large bandgap, monolayer MoS2 transistors fabricated on SiO2 substrates have been created and exhibit excellent current on/off ratios (of the order of 108).5 In addition to these electronic features, the mechanical properties, including flexibility and stretchability, of atomically thin MoS2 are highly desirable. Within its 2D forms, the strong bonding between chalcogen and transition metal atoms results in in-plane mechanical strength values comparable to those of steel, providing significant possibilities for realising novel device applications, such as flexible or stretchable transistors.12–14 Although many investigations have been performed on mechanically exfoliated thin-layer MoS2, such MoS2 can only produce films measuring approximately 10 mm in size, which hampers the practical utility of MoS2 for application in large-area flexible and stretchable electronics. Accordingly, recent developments in scalable synthesis methods of chemical vapour deposition a

Applied Physics, Waseda University, Tokyo, Japan. E-mail: [email protected] Research Center for Applied Science, Academia Sinica, Taipei, Taiwan c Kagami Memorial Laboratory for Material Science and Technology, Waseda University, Tokyo 169-0051, Japan b

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(CVD) can be transferred to other arbitrary substrates, which opens up the possibility of using MoS2 in the field of flexible and stretchable electronics.15 The CVD approach can yield large-area MoS2 thin films and is applicable to flexible and stretchable device applications on bendable plastic and elastic rubber substrates.16,17 This perspective focuses on the current progress in generating MoS2 thin-film transistors (TFTs) for flexible and stretchable electronics. Firstly, film preparation approaches and the transistor applications of these films are introduced. Secondly, the demonstration of flexible MoS2 transistors and their advantages are highlighted. Finally, we discuss the device and materials strategies for the use of MoS2 in the field of flexible and stretchable electronics, and we provide perspective on the future direction of promising studies.

2. MoS2 synthesis and transistors 2.1

Synthesis of MoS2

The most important efforts under way involve establishing reliable methods for synthesising high-quality atomically thin MoS2, in order to investigate the fundamental electronic, optical and mechanical properties of atomically thin MoS2 as well as its possible applications. The current approaches for fabricating thin layers of MoS2 adopt two main methods. One is the top-down exfoliation of bulk materials (for example, by mechanical and liquid exfoliation), which can generate high-quality single crystalline thin flakes. Another is the direct bottom-up method of CVD for producing large-area uniform polycrystalline thin films. Here, we review the recent findings from studies using each method and describe the respective applications to transistor fabrication.

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2.2

Mechanical exfoliation

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The most widely adopted method for obtaining atomically thin flakes of MoS2 is mechanical cleavage using Scotch tape, which is the same technique developed for the production of graphene and can be applied to bulk crystals of MoS2.2,4–10 In this method, the layered structure of the material, which is composed of vertically stacked monolayer films held together by weak van der Waals interactions, can be easily peeled from the bulk crystal, resulting in few-layer 2D forms (Fig. 1a and b).2,5 The layer dependence of the optical and vibrational properties of this material has recently attracted attention because of the significant changes observed in the electronic structure.9–11,18–22 One signature change upon decreasing the layer thickness appears in the associated

Fig. 1 (a) A three-dimensional schematic of the crystal structure of layered MoS2. (b) An optical image of monolayer MoS2 exfoliated on a SiO2/Si substrate. (scale bar: 1 mm) (c) A schematic illustration of HfO2-top-gated monolayer MoS2 transistors. (d) The source–drain current (Ids) versus the top gate voltage (Vtg) curve for a monolayer device (c) measured at room temperature with the back gate grounded. The bias voltage ranges from 10 mV to 500 mV. (e) A schematic of unipolar and ambipolar carrier accumulation in an ionic liquid gated MoS2 transistor. Top: the unipolar accumulation mode. The drain voltage (VDS) is significantly smaller than the gate voltage (VGS). Bottom: the ambipolar accumulation mode. VDS is significantly larger than VGS. (f) The transfer curve of an ambipolar MoS2 thin flake transistor. The figures are reprinted with permission from: ref. 5, r 2011 Macmillan Publishers Ltd; ref. 2, r 2005 National Academy of Sciences, U.S.A.; ref. 43, r 2013 American Chemical Society.

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Raman spectra. Calculated and experimental studies have demonstrated a shift in the main two Raman peaks, the in-plane vibration of E12g and the out-of-plane A1g phonon mode, showing a decrease in the frequency of the E12g mode but an increase in that of the A1g mode.18–20 These peak shifts originate from interactions within the layered structure and the associated increase in the dielectric screening of long-range Coulombic interlayer interactions.19 Thus, the verification of Raman peak positions can be used to identify the layer thickness of MoS2. Another important layer dependent behaviour is the changes observed in the electronic band structure. The indirect bandgap of the bulk forms transitions into a direct bandgap when in the 2D form as a result of quantum confinement.9,10 Particularly in monolayer MoS2, with a thickness of approximately 0.8 nm, an intrinsic direct bandgap of 1.8 eV emerges at the K point and can be detected by strong photoluminescence and absorption spectra.9,10 As a result of this large direct bandgap, atomically thin MoS2 is a promising material for electronic and optoelectronic applications. The strong spin–orbit interaction that arises due to the presence of a transition metal in this material leads to energy splitting of the valence band.23–29 The combination of a symmetric band structure and strong spin–orbit coupling also enables valley polarisation, logic operations controlled with spin–valley coupling, and the possibility of valleytronics.27–29 One of the most promising electronic applications of atomically thin MoS2 is as a transistor fabricated for logic electronics because of the intrinsic bandgap of the material.5–8,30–40 As shown in Fig. 1c and d, Radisavljevic et al. first demonstrated monolayer MoS2 transistors using HfO2 as a high-k top-gate dielectric on SiO2 insulating substrates and achieved an extremely high current on/off ratio (approximately 1  108).5 Additionally, the back-gate-based multilayer MoS2 transistors exhibited high mobility values (4100 cm2 V 1 s 1) and sufficient current enhancement (on/off ratios of the order of 1  106).30 The excellent switching properties of these transistors quickly associated them with logic circuits.31,32 Recently, integrated circuits based on bilayer MoS2, including inverters, NAND gates, and ring oscillators, were successfully demonstrated.32 The oscillation frequency of a fabricated five-stage ring oscillator is as high as 1.6 MHz, which is an important step towards high-performance 2D nanoelectronics. Further advances in logic devices require CMOS circuits composed of n-type and p-type transistors. Although MoS2 transistors typically show only electron transport behaviour, ambipolar MoS2 transistors have been realised using ionic liquids as gate dielectrics.41–43 Fig. 1e shows a simple schematic of ionic liquid gated MoS2 transistors. When the gate voltage is applied, the transistor channel is formed by a single type of carrier (Fig. 1e, top). However, when the gate voltage is significantly smaller than the drain voltage (Fig. 1e, bottom), the effective gate voltage at the drain electrode is inversed, resulting in the accumulation of another type of carrier and inducing hole transport (Fig. 1f ). Furthermore, the large capacitance of the electric double layer is able to accumulate an order of magnitude higher carrier density than that of the conventional SiO2 insulator, resulting in the strong depletion of donor carriers and efficient carrier doping.44,45 Electrically induced

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superconductivity in MoS2 has also been realised.46,47 The demonstration of electron and hole transport properties will allow the construction of CMOS devices and the formation of p–n junctions for optoelectronics.43,48 Another important application of the MoS2 transistor is in optoelectronics, such as photodetectors and photovoltaic cells.49–53 The visible range of the direct bandgap offers effective light absorption and luminescence capabilities, which allow MoS2 to be used as a channel material for phototransistors. Lee and co-workers reported the thickness dependence of the photoresponse in various layers of MoS2, indicating that different wavelengths of light can be detected as a result of the thickness-tunable bandgap.51 For example, mono- and bilayer MoS2, which possess respective optical energy gaps of 1.8 eV and 1.6 eV, are useful for green light detection; meanwhile, trilayer MoS2 exhibits an energy gap of 1.4 eV and is suitable for red light. More recently, ultrasensitive monolayer MoS2 phototransistors with a broad spectral range were demonstrated.53 The maximum external photoresponsivity reached 880 A W 1, and the noise under dark current is lower than that of commercial state-of-the-art silicon avalanche photodiodes, indicating high sensitivity due to the direct bandgap and efficient carrier excitation. This result supports the understanding that monolayer MoS2 is a promising semiconducting material for use in imaging circuits, light sensing applications and photovoltaic cells. Furthermore, electroluminescence has also been observed with monolayer MoS2 in a transistor configuration, opening up the possibility for using this material in lightemitting devices, such as LEDs and diode lasers.54 The high surface-to-volume ratio of MoS2 provides natural opportunities in sensor applications.55–57 The first demonstration of a gas sensor based on mono- and few-layer MoS2 transistors was performed by Li et al., who developed a sensor that can detect the adsorption of NO gas.55 Because the MoS2 transistors show n-type doping behaviour, certain changes occur in charge transfer, doping level, and conductivity as a result of the exposure of such devices to NO gas, which is most likely p-donor. These sensing behaviours extend to chemical sensors for a wide range of analytes, exhibiting a strong response upon exposure to nerve gas.56 Additionally, thin-film humidity sensors have been reported, demonstrating a sensitive response to water vapour at room temperature and atmospheric pressure.57 Followed by the rapid progress in electronic and optoelectronic devices, transistors made from monolayer MoS2 will pave the way for new applications, such as thermopower generation and energy harvesting. A large value of the Seebeck coefficient in MoS2 was recently observed, and this value can be tuned using an electric field from 4  102 to 1  105 mV K 1.58 The reported value is 70–25 000 times larger than that of graphene, offering further potential for thermoelectric nanodevices. 2.3

Liquid exfoliation

Although the mechanical cleavage technique yields highquality single crystal samples that can be used to fabricate high-performance devices, from which the intrinsic physical phenomena can be investigated, growth methods able to yield

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Fig. 2 (a) A photograph of a dispersion of MoS2 in NMP. (b) An optical image of as-deposited MoS2 thin films on a PET substrate. (c) An AFM image of a deposited ultrathin film with an average thickness of 1.3 nm. The white line indicates a height profile taken at the position of the red line. (d) A schematic diagram showing the fabrication of MoS2 TFT arrays on PET substrates and a photograph of the flexible TFT sensor arrays. The figures are reprinted with permission from: ref. 60, r 2011 AAAS; ref. 66, r 2011 American Chemical Society; ref. 68, r 2012 Wiley.

significant quantities are required for additional applications, such as energy storage, composites and hybrids. In liquid exfoliation, the layered compounds are dispersed in commonly used organic solvents, such as N-methyl-2-pyrrolidone (NMP), as shown in Fig. 2a; these approaches can produce gram-sized quantities of various flakes or few-layer materials from their dispersions, such as MoS2, WS2, or boron nitride (BN).59–63 Another effective approach for the mass production of layered materials is the method of electrochemical lithium intercalation.64–66 The Li-intercalated materials can be exfoliated by ultrasonication, yielding few-layer or monolayer films. However, Li intercalation generally results in a significant loss of semiconducting properties with an emerging metallic phase. In response to this drawback, Eda et al. demonstrated the recovery of the semiconducting properties of the pristine materials by introducing mild annealing to the preparation process.66 The obtained chemically exfoliated MoS2 films exhibited specific direct band gap PL, supporting the presence of a low density of defects. Additionally, this solution process can be applied to the transfer of films to arbitrary substrates, including glass and flexible PET (Fig. 2b and c).66 In addition to these contributions, Zeng and co-workers developed high-yield, monolayer semiconducting nanosheets of MoS2 and WS2 based on a controllable lithiation process.64,65 Specifically, the obtained monolayer MoS2 was in excess of 90%, indicating the effectiveness of large-scale production.64 The solution processability of MoS2 has enabled

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various device applications, such as photoelectrochemical devices and flexible arrays.67,68 For example, as illustrated in Fig. 2d, flexible gas sensors fabricated by MoS2 TFTs have been realised and have achieved high sensitivity and excellent reproducibility.68 Reduced graphene oxides were patterned on PET as electrodes, and suspended monolayer MoS2 sheets were used as the active channel. The TFT sensor arrays can detect the common toxic gas NO2 and can survive 5000 bending cycles without any degradation of sensing performance. The use of solution-processed deposition through spin coating paves the way for simple, low-cost, and environmentally friendly approaches to the high-yield production of MoS2 thin films, as well as a wide range of potential applications.69 In contrast, in liquid exfoliation, the thickness is relatively difficult to control, which results in a low concentration of monolayer films. Furthermore, the reported methods easily yield few-layer or monolayer flakes, but the size of the flakes tends to be very small, that is, generally below a few micrometres; this technique cannot deliver layer-controlled, highly uniform and largearea thin films. Large-area monolayer MoS2 films are required for electronic or optoelectronic applications; thus, new synthetic methods are desired for sorting the thickness, obtaining scalability, and maintaining reproducibility as a key step towards the fabrication of practical devices. 2.4

Chemical vapour deposition

Following the breakthrough of the CVD approach for graphene films, the synthesis of large-area and uniform atomically thin layers of MoS2 is currently feasible.70,71 Some CVD methods have been developed to grow monolayer or few-layer MoS2 on insulating substrates.72–78 Lee and co-workers have demonstrated the synthesis of large-area monolayer MoS2 on Si/SiO2 substrates through the co-deposition of sulphur powders and MoO3 powders, as illustrated in Fig. 3a.73 The growth conditions are very sensitive to the surface treatment, where aromatic molecules, such as reduced graphene oxides (rGO), perylene-3,4,9,10-tetracarboxylic acid tetrapotassium salt (PTAS) and perylene-3,4,9,10-tetracarboxylic dianhydride (PTCDA), promote layer growth (Fig. 3a). As-grown MoS2 films remain typical n-type semiconductors, and bottom gate transistors show semiconducting properties with a high current on/off ratio (4108) and high mobility (410 cm2 V 1 s 1).79–82 Using the same chemical approach, highly crystalline islands that can scale up to domains of 120 mm in size were realised, and the resultant material possesses optical and electronic properties comparable or superior to that of exfoliated MoS2.74 Although co-deposition of different precursors can grow highquality single domain MoS2, full coverage of the whole surface of substrates is substantially difficult. The lateral dimension of the obtained MoS2 layer is typically less than a millimetre, which is not suitable for the fabrication of wafer-scale devices intended for large-area or flexible electronics. An alternative scalable growth strategy is illustrated in Fig. 3b and was reported by Liu and co-workers for the deposition of trilayer MoS2 thin layers.83 Highly crystalline and large-area MoS2 sheets were synthesised through the two-step high-temperature annealing of a thermally decomposed ammonium thiomolybdate

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Fig. 3 (a) Left: a schematic depiction of the experimental set-up used for the co-deposition of MoO3 and S powders through gas-phase reaction. The red circles represent the heating reaction chamber. Right: an optical image of MoS2 layers grown on a substrate treated with rGO solution. (b) A schematic illustration of the two-step annealing process for the synthesis of MoS2 thin layers on insulating substrates. The precursor (NH4)2MoS4 is dip-coated on sapphire and SiO2/Si substrates, followed by annealing in the presence of sulphur. The synthesised MoS2 films can be transferred onto the arbitrary substrates. (c) A schematic of a CVD-grown MoS2 TFT constructed with an ion gel as gate dielectric on a SiO2/Si substrate. (d) The transfer characteristics of the MoS2 TFTs. VD is the drain voltage, and VG is the gate voltage. (e) The temperature dependence of the drain current at a VG of 2.0 V (red), 1.2 V (blue) and 0.1 V (green). A metal-to-insulator transition is observed in a MoS2 TFT. The figures are reprinted with permission from: ref. 73, r 2012 Wiley; ref. 16 and 83, r 2012 American Chemical Society.

layer dip-coated on sapphire or SiO2/Si substrates. The addition of sulphur during the second annealing process improved the crystallinity of MoS2 so that various spectroscopic and microscopic properties could be characterised.83 Moreover, we constructed TFTs with CVD-grown polycrystalline MoS2 thin films and with the use of ion gels, i.e., the gelation of ionic liquids,84 as gate dielectrics.16 Owing to the large capacitance of ion gels (of the order of 10 mF cm 2),85–87 the electrical performance of the material exhibits a low drive voltage (o1 V), high electron mobility (412 cm2 V 1 s 1) and high on/off current ratio (105) on SiO2/Si substrates, as shown in Fig. 3c and d.16 The evaluation

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of the temperature dependence of conductivity is shown in Fig. 3e. The drain current is inversely proportional to the temperature in the on-state of TFTs, which reveals the significant band transport in polycrystalline films. The strong interaction of MoS2 domains may assist carrier transport across the domain boundaries, suggesting the capability of stable conduction under deformation. Although ion-gel dielectric layers are very suitable for MoS2 films, we should also comment on possible disadvantages of electrochemical gating transistors. In these TFTs, when we increased the gate voltage to more than 4 V, which is higher than the electrochemical window of ionic liquids, a significant gate current (4100 nA) was detected. This leakage current indicates a redox reaction between the ionic liquid and MoS2, which is a critical problem for future applications. To avoid or reduce these electrochemical reactions, ionic liquids with a wide electrochemical window are supposed to be necessary. In addition to these rich transport properties, the synthesised MoS2 thin layers can be easily transferred onto other arbitrary substrates, enabling transistor applications on unconventional substrates, such as thin plastic and rubber substrates.16,17 A simple and scalable sulphurisation approach was recently developed for the wafer-scale deposition of MoS2 thin films.88–90 Highly uniform films were obtained with the direct sulphurisation of MoO3 thin layers evaporated on 2-inch sapphire wafers and were transferable to SiO2/Si wafers after carrying out a PMMA-assisted solution procedure, as shown in Fig. 4. In addition to the application of these direct bottom-up methods on bare surfaces, MoS2 can also be synthesised on graphene surfaces, used as growth templates.91 The demonstration of MoS2–graphene heterostructures suggests that other hexagonally structured materials are able to serve as a growth substrate. The vertical heterostructured

Fig. 4 The wafer-scale deposition of MoS2 thin films obtained by the direct sulphurisation of MoO3 thin films. The pictures show the procedure used to transfer as-grown MoS2 thin films from sapphire to SiO2/Si wafers. The figures are reproduced with permission from: ref. 88, r 2012 The Royal Society of Chemistry.

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surface of the atomic films could potentially provide hybrid electronic applications and new optical phenomena, such as charge transfer, exciton generation and high-performance transistors, because of the absence of dangling bonds. As a result of these scalable growth techniques, CVD polycrystalline MoS2 thin films are suitable for practical use in large-area integrated circuits, flexible electronics and optoelectronic devices.16,17,92,93

3. Flexible MoS2 transistors Flexibility and transparency are two of the most desirable characteristics for flexible electronics. In this respect, semiconducting 2D layered materials are some of the best candidates. In particular, graphene is a widely explored 2D material, and many investigations have been performed on the use of graphene for flexible and transparent transistors.94–96 Although the extremely high mobility of graphene (of the order of 105 cm2 V 1 s 1 at room temperature) allows its use in high-frequency devices, the gap-less feature limits the application of graphene in logic electronics.97 Therefore, researchers have recently renewed their interest in the discovery of a new type of 2D analogue material with superior mechanical, electronic, and optical attributes. Atomically thin MoS2 is a particularly intriguing post-graphene candidate for use in flexible logic devices. The mechanical strength of monolayer MoS2 is 30 times higher than that of steel.12–14 Furthermore, the robustness of MoS2 allows it to endure deformation of up to 11% before breaking due to the stiff chemical bonds of the Mo–S networks.13 Ultrathin MoS2 possessing tunable bandgaps can also manage higher current amplification, and recently, significant effort has been devoted to the realisation of flexible transistors.16,68,98–101 The subnanometre thickness of the 2D forms affords the advantages of flexibility and transparency. The rigidity of the bulk forms causes the material to be fundamentally difficult to bend and results in cracking of the crystal structure; however, pliable atomic-scale thickness films can be mechanically flexed. With the advantages of these electronic and flexible properties, we have demonstrated highly flexible MoS2 TFTs on plastic substrates (Fig. 5a).16 The ion gels are adopted as gate dielectrics, and as a result, the fabricated material combines both flexibility and a large specific capacitance.85–87 The TFTs were fabricated on polyimide substrates (with a thickness of 12.5 mm) and were placed on a home-built bending apparatus to test their electrical performance under mechanical bending, as illustrated in Fig. 5b. Fig. 5c presents the transfer characteristics of a flexible MoS2 TFT when the device was bent to a curvature radius of 0.75 mm; the device also demonstrated clear recovery of the transport properties. Furthermore, a detailed evaluation of the curvature radius-dependence of the drain current and mobility is shown in Fig. 5d and indicates that the electrical characteristics exhibit no degradation when bent down to a curvature radius of 0.75 mm. Hence, this is one of the most bendable TFTs available today in the field of 2D nanosheet materials. The high flexibility is also supported by the flexible nature of the ion gel films and the thinner

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Fig. 5 (a) A schematic depiction of a MoS2 TFT fabricated on a flexible plastic substrate. (b) Left: an optical image of a MoS2 TFT on a 12.5 mmthick polyimide substrate. Right: a schematic illustration of the bending measurements. (c) The transfer characteristics of the MoS2 TFT. The red, black dotted, and blue dotted lines correspond to the transfer curve for a curvature radius of 0.75 mm and to the transfer curves before and after the bending experiments. The inset shows an optical image of the bending measurement when the device is set to a curvature radius of 0.75 mm. (d) The dependence of the drain current at a gate voltage of 1.5 V (red) and the carrier mobility on the curvature radius. The carrier mobility is normalised by the results without bending (blue). The inset shows optical images of the MoS2 TFT rolled to a curvature radius of 0.75 mm. The figures are reprinted with permission from: ref. 16, r 2012 American Chemical Society.

polyimide substrates, which reduce the effective applied strains on the MoS2 films to values below 1%. Yoon and co-workers soon followed with the realisation of flexible and transparent MoS2 transistors that can withstand tension and compression.98 Fig. 6a illustrates the procedure used for the fabrication of flexible and transparent transistors based on MoS2 and graphene. Because patterned graphene was used as the electrode, the fabricated devices showed good transparency with an optical transmittance of 74% and a high stability against tension and compression (Fig. 6b). Although the materials in these reports achieve high flexibility, the measured field effect mobility on the flexible substrates is less than 5 cm2 V 1 s 1; however, further improvements and optimisation of the device performance are required. To enhance the transistor characteristics, Chang et al. presented highperformance flexible MoS2 thin flake transistors with high-k dielectrics.99 The strong local screening effect and suppression of Coulomb scattering increased the mobility up to 30 cm2 V 1 s 1; the on/off ratio was in excess of 107, and the subthreshold swing was reduced to 82 mV dec 1. Additionally, the strategy used to transfer the MoS2 transistors has already been established. This technique can construct atomic layer deposited (ALD) dielectricbased TFTs on any arbitrary substrate, which enables a schematic of device fabrication that is both easy and reliable.100 More recently, Lee and co-workers demonstrated novel heterostructural,

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Fig. 6 (a) A schematic diagram of the fabrication and transfer process used for multilayer MoS2 TFTs on a plastic substrate. (b) A photograph of the highly flexible and transparent MoS2 TFTs on a PET substrate. The inset shows an illustration of the device structure. (c) An optical micrograph of the flexible heterostructured MoS2–hBN–graphene (MBG) device. (d) A photograph of the MBG device on the PEN substrate, showing its flexibility and transparency. The figures are reprinted with permission from: ref. 98, r 2013 Wiley; ref. 101, r 2013 American Chemical Society.

flexible MoS2 transistors.101 The flexible and transparent MoS2 devices were stacked on hexagonal BN (hBN)–graphene heterostructures, as shown in Fig. 6c. Compared to the conventional solid dielectrics, such as SiO2, hBN is beneficial for carrier transport in association with atomically flat surfaces that are free of charge traps. When hBN was utilised as the gate dielectric and graphene was used for the electrodes, these heterostructural MoS2 transistors recorded the highest mobility values (45 cm2 V 1 s 1) on flexible plastic substrates. The results indicate that 2D-material-based heterostructure devices are promising for high-performance flexible and transparent electronics. Furthermore, these heterostructured devices open up a route for novel device applications. For example, vertically stacked graphene–MoS2 hybrid structures recently enabled vertically integrated CMOS inverters,102 memory devices,103,104 and photodetection devices.105,106 The development of multiheterostructures of 2D materials will lead to important roles for these materials in novel functional devices in future flexible and transparent electronics.

4. Stretchable MoS2 transistors Beyond flexible electronics, future electronics aim for ubiquitous and ambient electronic systems: for example, wearable computers and electronic skins. One of the key steps towards the realisation of these ultimate goals is the development of transistors, circuits, and sensors that afford arbitrary form factors.107–115 In particular, transistors that can mechanically flex and stretch are the most essential component of flexible and stretchable electronics. The major challenge in this field is

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the requirement for semiconducting materials that can remain robust despite mechanical strain. However, conventional semiconductors, including organic and inorganic materials, lack robustness and stretchability; as a result, the strains experienced by these materials must remain below 1% to avoid inducing fracture and cracking. Therefore, recent studies towards the realisation of stretchable devices have explored highly unique approaches. These studies have focused on the design of systems that can release the substantial strain of semiconductors.114,115 For example, one common method in the fabrication of stretchable organic devices is the construction of isolated rigid areas on an elastic substrate.107–109 These rigid islands are undeformable; thus, the transistors fabricated on these protected areas can avoid deformation when the whole substrate is stretched. Another effective approach is the engineering of wavy forms in device components.110–112 Accordiontype structural configurations can relax against applied tension, allowing induced strains to be applied to semiconductors within the limit of 1%. Following these strategies, graphene has been involved in highly simple and desirable geometries for stretchable electrodes and transistors.70,116,117 Owing to its atomic thickness and very large Young’s modulus,118 graphene can intentionally be formed into ripples or accordion structures on a film surface during the process of transferring a graphene film.70,116,117 Ripple relaxation allows the graphene films to stretch to values in excess of 20%,70,116 while graphene transistors fabricated on rubber substrates can operate at a mechanical strain of 5%.117 The beneficial properties of atomically thin 2D films suggest that these are advantageous candidates for use in stretchable electronics. In principle, the same strategy can be applied to MoS2 films, which are more suitable for transistor applications because of the existing bandgap. The development of CVD synthesis and the transfer technique for MoS2 thin films has enabled the fabrication of MoS2 transistors on elastic substrates.83,88 Although atomically thin MoS2 is a likely material for use in stretchable electronics, further investigation of the intrinsic effect of strain within the MoS2 crystal structure on the electronic properties is essential for device applications. Recently, some groups reported changes in the electronic properties of monolayer and bilayer MoS2 based on first-principle calculations.119–123 These studies predicted that the bandgap of MoS2 decreases with increasing strain and that a semiconductor–metal transition can be induced at a strain of 10%. To probe these theoretical predictions, He et al. and Conley et al. measured the PL and absorption spectrum via strain in monolayer MoS2.124,125 These two reports investigated the influence of uniaxial tensile strain in the range from 0% to 2%. Consequently, a strain-induced redshift of the band gap was observed, which is in agreement with the first-principle calculations. Furthermore, phonon softening of the doubly degenerate E12g mode followed by energy splitting into two modes with strain in excess of 1% breaks the original lattice symmetry.125–127 These demonstrations of electronic structure modulation support the possibility of strain engineering and the tuning of optical and electronic properties, with the potential to result in novel physical phenomena, pressure sensors,

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Fig. 7 (a) An optical image of the channel region of a MoS2 TFT mounted on PDMS substrates. (b) A differential AFM image, recorded on the surface of MoS2 films transferred onto PDMS. (c) The topography (left) and height profile (right) of the ripple structure on the MoS2 film surfaces. The figures are reprinted with permission from: ref. 17, r 2013 AIP Publishing LLC.

and tunable photonic devices. Meanwhile, stable operation without the degradation or transformation of the electronic properties under strain is a major requirement for stretchable devices. Therefore, we fabricated accordion structure CVDgrown MoS2 thin films directly on PDMS substrates in analogy to graphene.17 Fig. 7 shows an atomic force microscope image of the MoS2 films created with ripples through a transfer process from as-grown sapphire substrates to swellable PDMS substrates. These ripples likely assist the MoS2 films in terms of increasing their expandability due to the strain relaxation allowed against mechanical stretching. Based on the material and device strategies employed for stretchable electronics, as shown in Fig. 8a, we built MoS2 TFTs on PDMS using elastic ion gel gate dielectrics. Fig. 8b presents the transfer curves under four different tensile conditions and shows that the variation in the drain current was very small during stretching for values as great as 4%. Additionally, the mobility, on current, and off current of these transistors were approximately constant, even up to values of 5% strain, which demonstrates the stretchability of CVD-grown MoS2 TFTs (Fig. 8c and d). Although a significant mobility decrease was observed at strain values in excess of 5%, as shown in Fig. 8d, the strain relaxation in the domain boundaries may be the origin of this degradation, as suggested in the case of graphene films.117 These results provide the potential for stretchable electronics based on MoS2 films. We anticipate that further improvements can be made to obtain superb stretchability, such as the application of pre-strain to PDMS substrates before performing the film transfer process, which would result in larger elastic ripples. Furthermore, the same strategy and process can also be applied to other atomically thin 2D TMDCs, such as MoSe2,128 WS2,129 and WSe2.130 The material variety of TMDCs offers various transistor polarities, including p-type, n-type, and ambipolar transport, which will enable new electronic and optical functionalities as a result of combining these materials.

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Moreover, a variety of 2D TMDCs have inspired growing interest in this material with respect to the exploration of new complementary functional devices. The significant potential of 2D TMDCs provides a wide range of possible electronic and optoelectronic applications for flexible and stretchable electronics.

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Acknowledgements T.T. was partially supported by a Waseda University Grant (2011A-501), the Funding Program for the Next Generation of World-Leading Researchers and Grants-in-Aid from MEXT (26107533 ‘‘Science of Atomic Layers’’). L.-J. Li acknowledges the support of the Academia Sinica (IAMS and Nano program) and the National Science Council Taiwan (NSC-99-2112-M-001021-MY3 and 99-2738-M-001-001).

Notes and references

Fig. 8 (a) A schematic illustration and optical images of the MoS2 TFTs under uniaxial stretching. (b) The transfer characteristics of the MoS2 TFTs. The red, orange, blue and green lines correspond to the transfer curves for strains of 0, 3 and 4% and after stretching, respectively. (c) Top: the strain dependence of the drain current as a function of a reference voltage, VR, of 1.6 V (red) and 0.3 V (blue). Bottom: the strain dependence of the on/off ratio (black). (d) Top: the electron mobility at various strains (red). Bottom: the specific capacitance of the ion-gel/MoS2 interface at 15 Hz (bottom, black) at various strains. The mobility is normalised by the results obtained in the absence of an applied tension. The blue square in the top panel corresponds to the normalised mobility after stretching at a 5% strain. The figures are reprinted with permission from: ref. 17, r 2013 AIP Publishing LLC.

The outstanding properties of 2D TMDCs provide significant potential for future flexible and stretchable electronic devices.

5. Conclusions and future outlook We have highlighted recent studies on transistors constructed from novel two-dimensional MoS2 materials for use in flexible and stretchable electronics. The rapid progress of high-quality sample preparation methods, which range from mechanical exfoliation to scalable CVD, have allowed the fabrication of high-performance MoS2 transistors and have expanded the applications for device fabrication on unconventional substrates. Although MoS2 transistors built on plastic and rubber substrates have exhibited superior flexibility and stretchability, further improvements in terms of the large-area integration, device characteristics such as mobility and switching speed, and operation durability are required. Improving air-stability of device performance is also crucial for practical applications. In addition, printing and patterning techniques of highly crystalline uniform films will offer the possibility of low-temperature, low-cost and environmentally friendly fabrication processes.

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1 K. S. Novoselov, A. K. Geim, S. V. Morozov, D. Jiang, Y. Zhang, S. V. Dubonos, I. V. Grigorieva and A. A. Firsov, Science, 2004, 306, 666–669. 2 K. S. Novoselov, D. Jiang, F. Schedin, T. J. Booth, V. V. Khotkevich, S. V. Morozov and A. K. Geim, Proc. Natl. Acad. Sci. U. S. A., 2005, 102, 10451–10453. 3 V. Podzorov, M. E. Gershenson, Ch. Kloc, R. Zeis and E. Bucher, Appl. Phys. Lett., 2004, 84, 3301. 4 Q. H. Wang, K. Kalantar-Zadeh, A. Kis, J. N. Coleman and M. S. Strano, Nat. Nanotechnol., 2012, 7, 699–712. 5 B. Radisavljevic, A. Radenovic, J. Brivio, V. Giacometti and A. Kis, Nat. Nanotechnol., 2011, 6, 147–150. 6 Y. Yoon, K. Ganapathi and S. Salahuddin, Nano Lett., 2011, 11, 3768–3773. 7 S. Ghatak, A. N. Pal and A. Ghosh, ACS Nano, 2011, 5, 7707–7712. 8 H. Liu and P. D. Ye, IEEE Electron Device Lett., 2012, 33, 546–548. 9 A. Splendiani, L. Sun, Y. Zhang, T. Li, J. Kim, C.-Y. Chim, G. Galli and F. Wang, Nano Lett., 2010, 10, 1271–1275. 10 K. F. Mak, C. Lee, J. Hone, J. Shan and T. F. Heinz, Phys. Rev. Lett., 2010, 105, 136805. 11 A. Ramasubramaniam, D. Naveh and E. Towe, Phys. Rev. B: Condens. Matter Mater. Phys., 2011, 84, 205325. 12 J. Brivio, D. T. L. Alexander and A. Kis, Nano Lett., 2011, 11, 5148–5153. 13 S. Bertolazzi, J. Brivio and A. Kis, ACS Nano, 2011, 5, 9703–9709. 14 A. Castellanos-Gomez, M. Poot, G. A. Steele, H. S. J. van der Zant, N. Agrait and G. Rubio-Bollinger, Adv. Mater., 2012, 24, 772–775. 15 M. Chhowalla, H. S. Shin, G. Eda, L.-J. Li, K. P. Loh and H. Zhang, Nat. Chem., 2013, 5, 263–275. 16 J. Pu, Y. Yomogida, K.-K. Liu, L.-J. Li, Y. Iwasa and T. Takenobu, Nano Lett., 2012, 12, 4013–4017. 17 J. Pu, Y. Zhang, Y. Wada, J. T.-W. Wang, L.-J. Li, Y. Iwasa and T. Takenobu, Appl. Phys. Lett., 2013, 103, 023505.

Phys. Chem. Chem. Phys., 2014, 16, 14996--15006 | 15003

View Article Online

Published on 27 February 2014. Downloaded on 14/07/2014 05:46:52.

PCCP

18 C. Lee, H. Yan, L. E. Brus, T. F. Heinz, J. Hone and S. Ryu, ACS Nano, 2010, 4, 2695–2700. 19 A. Molina-Sanchez and L. Wirtz, Phys. Rev. B: Condens. Matter Mater. Phys., 2011, 84, 155413. 20 H. Li, Q. Zhang, C. C. R. Yap, B. K. Tay, T. H. T. Edwin, A. Olivier and D. Baillargeat, Adv. Funct. Mater., 2012, 22, 1385–1390. 21 B. Chakraborty, A. Bera, D. V. S. Muthu, S. Bhowmick, U. V. Waghmare and A. K. Sood, Phys. Rev. B: Condens. Matter Mater. Phys., 2012, 85, 161403. 22 S.-L. Li, K. Wakabayashi, Y. Xu, S. Nakaharai, K. Komatsu, W.-W. Li, Y.-F. Lin, A. Aparecido-Ferreira and K. Tsukagoshi, Nano Lett., 2013, 13, 3546–3552. 23 Z. Y. Zhu, Y. C. Cheng and U. Schwingenschlogl, Phys. Rev. B: Condens. Matter Mater. Phys., 2011, 84, 153402. 24 D. Xiao, G.-B. Liu, W. Feng, X. Xu and W. Yao, Phys. Rev. Lett., 2012, 108, 196802. 25 A. Molina-Sanchez, D. Sangalli, K. Hummer, A. Marini and L. Wirtz, Phys. Rev. B: Condens. Matter Mater. Phys., 2013, 88, 045412. 26 A. Kormanyos, V. Zolyomi, N. D. Drummond, P. Rakyta, G. Burkard and V. I. Fal’ko, Phys. Rev. B: Condens. Matter Mater. Phys., 2013, 88, 045416. 27 T. Cao, G. Wang, W. Han, H. Ye, C. Zhu, J. Shi, Q. Niu, P. Tan, E. Wang, B. Liu and J. Feng, Nat. Commun., 2012, 3, 887. 28 H. Zeng, J. Dai, W. Yao, D. Xiao and X. Cui, Nat. Nanotechnol., 2012, 7, 490–493. 29 K. F. Mak, K. He, J. Shan and T. F. Heinz, Nat. Nanotechnol., 2012, 7, 494–498. 30 S. Kim, A. Konar, W.-S. Hwang, J. H. Lee, J. Lee, J. Yang, C. Jung, H. Kim, J.-B. Yoo, J.-Y. Choi, Y. W. Jin, S. Y. Lee, D. Jena, W. Choi and K. Kim, Nat. Commun., 2012, 3, 1011. 31 B. Radisavljevic, M. B. Whitwick and A. Kis, ACS Nano, 2011, 5, 9934–9938. 32 H. Wang, L. Yu, Y.-H. Lee, Y. Shi, A. Hsu, M. L. Chin, L.-J. Li, M. Dubey, J. Kong and T. Palacios, Nano Lett., 2012, 12, 4674–4680. 33 H. Qin, L. Pan, Z. Yao, J. Li and Y. Shi, Appl. Phys. Lett., 2012, 100, 123104. 34 H. Liu, A. T. Neal and P. D. Ye, ACS Nano, 2012, 6, 8563–8569. 35 S. Das, H.-Y. Chen, A. V. Penumatcha and J. Appenzeller, Nano Lett., 2013, 13, 100–105. 36 M.-W. Lin, L. Liu, Q. Lan, X. Tan, K. S. Dhindsa, P. Zeng, V. M. Naik, M. M.-C. Cheng and Z. Zhou, J. Phys. D: Appl. Phys., 2012, 45, 345102. 37 H. Nam, S. Wi, H. Rokni, M. Chen, G. Priessnitz, W. Lu and X. Liang, ACS Nano, 2013, 7, 5870–5881. 38 D. Jariwala, V. K. Sangwan, D. J. Late, J. E. Johns, V. P. Dravid, T. J. Marks, L. J. Lauhon and M. C. Hersam, Appl. Phys.Lett., 2013, 102, 173107. 39 B. Radisavljevic and A. Kis, Nat. Mater., 2013, 12, 815–820. 40 B. W. H. Baugher, H. O. H. Churchill, Y. Yang and P. Jarillo-Herrero, Nano Lett., 2013, 13, 4212–4216.

15004 | Phys. Chem. Chem. Phys., 2014, 16, 14996--15006

Perspective

41 Y. Zhang, J. Ye, Y. Matsuhashi and Y. Iwasa, Nano Lett., 2012, 12, 1136–1140. 42 M. M. Perera, M.-W. Lin, H.-J. Chuang, B. P. Chamlagain, C. Wang, X. Tan, M. M.-C. Cheng, D. Tomanek and Z. Zhou, ACS Nano, 2013, 7, 4449–4458. 43 Y. J. Zhang, J. T. Ye, Y. Yomogida, T. Takenobu and Y. Iwasa, Nano Lett., 2013, 13, 3023–3028. 44 H. Shimotani, H. Asanuma, J. Takeya and Y. Iwasa, Appl. Phys. Lett., 2006, 89, 203501. 45 J. T. Ye, S. Inoue, K. Kobayashi, Y. Kasahara, H. T. Yuan, H. Shimotani and Y. Iwasa, Nat. Mater., 2010, 9, 125–128. 46 K. Taniguchi, A. Matsumoto, H. Shimotani and H. Takagi, Appl. Phys. Lett., 2012, 101, 042603. 47 J. T. Ye, Y. J. Zhang, R. Akashi, M. S. Bahramy, R. Arita and Y. Iwasa, Science, 2012, 338, 1193–1196. 48 W. Bao, X. Cai, D. Kim, K. Sridhara and M. S. Fuhrer, Appl. Phys. Lett., 2013, 102, 042104. 49 Z. Yin, H. Li, H. Li, L. Jiang, Y. Shi, Y. Sun, G. Lu, Q. Zhang, X. Chen and H. Zhang, ACS Nano, 2012, 6, 74–80. 50 W. Choi, M. Y. Cho, A. Konar, J. H. Lee, G.-B. Cha, S. C. Hong, S. Kim, J. Kim, D. Jena, J. Joo and S. Kim, Adv. Mater., 2012, 24, 5832–5836. 51 H. S. Lee, S.-W. Min, Y.-G. Chang, M. K. Park, T. Nam, H. Kim, J. H. Kim, S. Ryu and S. Im, Nano Lett., 2012, 12, 3695–3700. 52 M. Fontana, T. Deppe, A. K. Boyd, M. Rinzan, A. Y. Liu, M. Paranjape and P. Barbara, Sci. Rep., 2013, 3, 1634. 53 O. Lopez-Sanchez, D. Lembke, M. Kayci, A. Radenovic and A. Kis, Nat. Nanotechnol., 2013, 8, 497–501. 54 R. S. Sundaram, M. Engel, A. Lombardo, R. Krupke, A. C. Ferrari, Ph. Avouris and M. Steiner, Nano Lett., 2013, 13, 1416–1421. 55 H. Li, Z. Yin, Q. He, H. Li, X. Huang, G. Lu, D. W. H. Fam, A. I. Y. Tok, Q. Zhang and H. Zhang, Small, 2012, 8, 63–67. 56 F. K. Perkins, A. L. Friedman, E. Cobas, P. M. Campbell, G. G. Jernigan and B. T. Jonker, Nano Lett., 2013, 13, 668–673. 57 D. J. Late, Y.-K. Huang, B. Liu, J. Acharya, S. N. Shirodkar, J. Luo, A. Yan, D. Charles, U. V. Waghmare, V. P. Dravid and C. N. R. Rao, ACS Nano, 2013, 7, 4879–4891. 58 M. Buscema, M. Barkelid, V. Zwiller, H. S. J. van der Zant, G. A. Steele and A. Castellanos-Gomez, Nano Lett., 2013, 13, 358–363. 59 H. S. S. R. Matte, A. Gomathi, A. K. Manna, D. J. Late, R. Datta, S. K. Pati and C. N. R. Rao, Angew. Chem., Int. Ed., 2010, 49, 4059–4062. 60 J. N. Coleman, M. Lotya, A. O’Neill, S. D. Bergin, P. J. King, U. Khan, K. Young, A. Gaucher, S. De, R. J. Smith, I. V. Shvets, S. K. Arora, G. Stanton, H.-Y. Kim, K. Lee, G. T. Kim, G. S. Duesberg, T. Hallam, J. J. Boland, J. J. Wang, J. F. Donegan, J. C. Grunlan, G. Moriarty, A. Shmeliov, R. J. Nicholls, J. M. Perkins, E. M. Grieveson, K. Theuwissen, D. W. McComb, P. D. Nellist and V. Nicolosi, Science, 2011, 331, 568–571. 61 K. Lee, H.-Y. Kim, M. Lotya, J. N. Coleman, G.-T. Kim and G. S. Duesberg, Adv. Mater., 2011, 23, 4178–4182.

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View Article Online

Published on 27 February 2014. Downloaded on 14/07/2014 05:46:52.

Perspective

62 K.-G. Zhou, N.-N. Mao, H.-X. Wang, Y. Peng and H.-L. Zhang, Angew. Chem., Int. Ed., 2011, 50, 10839–10842. 63 V. Nicolosi, M. Chhowalla, M. G. Kanatzidis, M. S. Strano and J. N. Coleman, Science, 2013, 340, 1226419. 64 Z. Zeng, Z. Yin, X. Huang, H. Li, Q. He, G. Lu, F. Boey and H. Zhang, Angew. Chem., Int. Ed., 2011, 50, 11093–11097. 65 Z. Zeng, T. Sun, J. Zhu, X. Huang, Z. Yin, G. Lu, Z. Fan, Q. Yan, H. H. Hng and H. Zhang, Angew. Chem., Int. Ed., 2012, 51, 9052–9056. 66 G. Eda, H. Yamaguchi, D. Voiry, T. Fujita, M. Chen and M. Chhowalla, Nano Lett., 2011, 11, 5111–5116. 67 L. A. King, W. Zhao, M. Chhowalla, D. J. Riley and G. Eda, J. Mater. Chem. A, 2013, 1, 8935–8941. 68 Q. He, Z. Zeng, Z. Yin, H. Li, S. Wu, X. Huang and H. Zhang, Small, 2012, 8, 2994–2999. 69 M. A. Ibrahem, T.-W. Lan, J. K. Huang, Y.-Y. Chen, K.-H. Wei, L.-J. Li and C. W. Chu, RSC Adv., 2013, 3, 13193–13202. 70 K. S. Kim, Y. Zhao, H. Jang, S. Y. Lee, J. M. Kim, K. S. Kim, J.-H. Ahn, P. Kim, J.-Y. Choi and B. H. Hong, Nature, 2009, 457, 706–710. 71 X. Li, W. Cai, J. An, S. Kim, J. Nah, D. Yang, R. Piner, A. Velamakanni, I. Jung, E. Tutuc, S. K. Banerjee, L. Colombo and R. S. Ruoff, Science, 2009, 324, 1312–1314. 72 Y. Zhan, Z. Liu, S. Najmaei, P. M. Ajayan and J. Lou, Small, 2012, 8, 966–971. 73 Y.-H. Lee, X.-Q. Zhang, W. Zhang, M.-T. Chang, C.-T. Lin, K.-D. Chang, Y.-C. Yu, J. T.-W. Wang, C.-S. Chang, L.-J. Li and T.-W. Lin, Adv. Mater., 2012, 24, 2320–2325. 74 A. M. van der Zande, P. Y. Huang, D. A. Chenet, T. C. Berkelbach, Y. You, G.-H. Lee, T. F. Heinz, D. R. Reichman, D. A. Muller and J. C. Hone, Nat. Mater., 2013, 12, 554–561. 75 S. Balendhran, J. Z. Ou, M. Bhaskaran, S. Sriram, S. Ippolito, Z. Vasic, E. Kats, S. Bhargava, S. Zhuiykov and K. Kalantar-zadeh, Nanoscale, 2012, 4, 461–466. 76 Q. Ji, Y. Zhang, T. Gao, Y. Zhang, D. Ma, M. Liu, Y. Chen, X. Qiao, P.-H. Tan, M. Kan, J. Feng, Q. Sun and Z. Liu, Nano Lett., 2013, 13, 3870–3877. 77 Y. Yu, C. Li, Y. Liu, L. Su, Y. Zhang and L. Cao, Sci. Rep., 2013, 3, 1866. 78 X. Wang, H. Feng, Y. Wu and L. Jiao, J. Am. Chem. Soc., 2013, 135, 5304–5307. 79 W. Wu, D. De, S.-C. Chang, Y. Wang, H. Peng, J. Bao and S.-S. Pei, Appl. Phys. Lett., 2013, 102, 142106. 80 M. Amani, M. L. Chin, A. G. Birdwell, T. P. O’Regan, S. Najmaei, Z. Liu, P. M. Ajayan, J. Lou and M. Dubey, Appl. Phys. Lett., 2013, 102, 193107. 81 H. Liu, M. Si, S. Najmaei, A. T. Neal, Y. Du, P. M. Ajayan, J. Lou and P. D. Ye, Nano Lett., 2013, 13, 2640–2646. 82 W. Zhu, T. Low, Y.-H. Lee, H. Wang, D. B. Farmer, J. Kong, F. Xia and P. Avouris, Nat. Commun., 2013, 5, 3087. 83 K.-K. Liu, W. Zhang, Y.-H. Lee, Y.-C. Lin, M.-T. Chang, C.-Y. Su, C.-S. Chang, H. Li, Y. Shi, H. Zhang, C.-S. Lai and L.-J. Li, Nano Lett., 2012, 12, 1538–1544. 84 Md. A. B. H. Susan, T. Kaneko, A. Noda and M. Watanabe, J. Am. Chem. Soc., 2005, 127, 4976–4983.

This journal is © the Owner Societies 2014

PCCP

85 J. Lee, M. J. Panzer, Y. He, T. P. Lodge and C. D. Frisbie, J. Am. Chem. Soc., 2007, 129, 4532–4533. 86 J. H. Cho, J. Lee, Y. Xia, B. Kim, Y. He, M. J. Renn, T. P. Lodge and C. D. Frisbie, Nat. Mater., 2008, 7, 900–906. 87 Y. Yomogida, J. Pu, H. Shimotani, S. Ono, S. Hotta, Y. Iwasa and T. Takenobu, Adv. Mater., 2012, 24, 4392–4397. 88 Y.-C. Lin, W. Zhang, J.-K. Huang, K.-K. Liu, Y.-H. Lee, C.-T. Liang, C.-W. Chu and L.-J. Li, Nanoscale, 2012, 4, 6637–6641. 89 Y. Lee, J. Lee, H. Bark, I.-K. Oh, G. H. Ryu, Z. Lee, H. Kim, J. H. Cho, J.-H. Ahn and C. Lee, Nanoscale, 2014, 6, 2821–2826. 90 J. Yang, D. Voiry, S. J. Ahn, D. Kang, A. Y. Kim, M. Chhowalla and H. S. Shin, Angew. Chem., Int. Ed., 2013, 52, 1–5. 91 Y. Shi, W. Zhou, A.-Y. Lu, W. Fang, Y.-H. Lee, A. L. Hsu, S. M. Kim, K. K. Kim, H. Y. Yang, L.-J. Li, J.-C. Idrobo and J. Kong, Nano Lett., 2012, 12, 2784–2791. 92 D.-S. Tsai, K.-K. Liu, D.-H. Lien, M.-L. Tsai, C.-F. Kang, C.-A. Lin, L.-J. Li and J.-H. He, ACS Nano, 2013, 7, 3905–3911. 93 W. Zhang, J.-K. Huang, C.-H. Chen, Y.-H. Chang, Y.-J. Cheng and L.-J. Li, Adv. Mater., 2013, 25, 3456–3461. 94 Y. Lee, S. Bae, H. Jang, S. Jang, S.-E. Zhu, S. H. Sim, Y. I. Song, B. H. Hong and J.-H. Ahn, Nano Lett., 2010, 10, 490–493. 95 B. J. Kim, H. Jang, S.-K. Lee, B. H. Hong, J.-H. Ahn and J. H. Cho, Nano Lett., 2010, 10, 3464–3466. 96 S.-K. Lee, H. Y. Jang, S. Jang, E. Choi, B. H. Hong, J. Lee, S. Park and J.-H. Ahn, Nano Lett., 2012, 12, 3472–3776. 97 A. S. Mayorov, R. V. Gorbachev, S. V. Morozov, L. Britnell, R. Jalil, L. A. Ponomarenko, P. Blake, K. S. Novoselov, K. Watanabe, T. Taniguchi and A. K. Geim, Nano Lett., 2011, 11, 2396–2399. 98 J. Yoon, W. Park, G.-Y. Bae, Y. Kim, H. S. Jang, Y. Hyun, S. K. Lim, Y. H. Kahng, W.-K. Hong, B. H. Lee and H. C. Ko, Small, 2013, 9, 3295–3300. 99 H.-Y. Chang, S. Yang, J. Lee, L. Tao, W.-S. Hwang, D. Jena, N. Lu and D. Akinwande, ACS Nano, 2013, 7, 5446–5452. 100 G. A. Salvatore, N. Munzenrieder, C. Barraud, L. Petti, C. Zysset, L. Buthe, K. Ensslin and G. Troster, ACS Nano, 2013, 10, 8809–8815. 101 G.-H. Lee, Y.-J. Yu, X. Cui, N. Petrone, C.-H. Lee, M. S. Choi, D.-Y. Lee, C. Lee, W. J. Yoo, K. Watanabe, T. Taniguchi, C. Nuckolls, P. Kim and J. Hone, ACS Nano, 2013, 7, 7931–7936. 102 W. J. Yu, Z. Li, H. Zhou, Y. Chen, Y. Wang, Y. Huang and X. Duan, Nat. Mater., 2013, 12, 246–252. 103 S. Bertolazzi, D. Krasnozhon and A. Kis, ACS Nano, 2013, 7, 3246–3252. 104 K. Roy, M. Padmanabhan, S. Goswami, T. P. Sai, G. Ramalingam, S. Raghavan and A. Ghosh, Nat. Nanotechnol., 2013, 8, 826–830. 105 L. Britnell, R. M. Ribeiro, A. Eckmann, R. Jalil, B. D. Belle, A. Mishchenko, Y.-J. Kim, R. V. Gorbachev, T. Georgiou, S. V. Morozov, A. N. Grigorenko, A. K. Geim, C. Casiraghi, A. H. C. Neto and K. S. Novoselov, Science, 2013, 340, 1311–1314.

Phys. Chem. Chem. Phys., 2014, 16, 14996--15006 | 15005

View Article Online

Published on 27 February 2014. Downloaded on 14/07/2014 05:46:52.

PCCP

106 W. J. Yu, Y. Lin, H. Zhou, A. Yin, Z. Li, Y. Huang and X. Duan, Nat. Nanotechnol., 2013, 8, 952–958. 107 T. Someya, T. Sekitani, S. Iba, Y. Kato, H. Kawaguchi and T. Sakurai, Proc. Natl. Acad. Sci. U. S. A., 2004, 101, 9966–9970. 108 T. Sekitani, Y. Noguchi, K. Hata, T. Fukushima, T. Aida and T. Someya, Science, 2008, 321, 1468–1472. 109 I. M. Graz, D. P. J. Cotton, A. Robinson and S. P. Lacour, Appl. Phys. Lett., 2011, 98, 124101. 110 D.-Y. Khang, H. Jiang, Y. Huang and J. A. Rogers, Science, 2006, 311, 208–212. 111 D.-H. Kim, J.-H. Ahn, W. M. Choi, H.-S. Kim, T.-H. Kim, J. Song, Y. Y. Huang, Z. Liu, C. Lu and J. A. Rogers, Science, 2008, 320, 507–511. 112 K. Park, D.-K. Lee, B.-S. Kim, H. Jeon, N.-E. Lee, D. Whang, H.-J. Lee, Y. J. Kim and J.-H. Ahn, Adv. Funct. Mater., 2010, 20, 3577. 113 D. J. Lipomi, M. Vosgueritchian, B. C.-K. Tee, S. L. Hellstrom, J. A. Lee, C. H. Fox and Z. Bao, Nat. Nanotechnol., 2011, 6, 788–792. 114 J. A. Rogers, T. Someya and Y. Huang, Science, 2010, 327, 1603–1607. 115 T. Sekitani and T. Someya, Adv. Mater., 2010, 22, 2228–2246. 116 Y. Wang, R. Yang, Z. Shi, L. Zhang, D. Shi, E. Wang and G. Zhang, ACS Nano, 2011, 5, 3645–3650. 117 S.-K. Lee, B. J. Kim, H. Jang, S. C. Yoon, C. Lee, B. H. Hong, J. A. Rogers, J.-H. Cho and J. H. Ahn, Nano Lett., 2011, 11, 4642–4646. 118 C. Lee, X. Wei, J. W. Kysar and J. Hone, Science, 2008, 321, 385–388.

15006 | Phys. Chem. Chem. Phys., 2014, 16, 14996--15006

Perspective

119 E. Scalise, M. Houssa, G. Pourtois, V. Afanas’ev and A. Stesmans, Nano Res., 2012, 5, 43–48. 120 Q. Yue, J. Kang, Z. Shao, X. Zhang, S. Chang, G. Wang, S. Qin and J. Li, Phys. Lett. A, 2012, 376, 1166–1170. 121 T. Li, Phys. Rev. B: Condens. Matter Mater. Phys., 2012, 85, 235407. 122 S. Bhattacharyya and A. K. Singh, Phys. Rev. B: Condens. Matter Mater. Phys., 2012, 86, 075454. 123 P. Lu, X. Wu, W. Guo and X. C. Zeng, Phys. Chem. Chem. Phys., 2012, 14, 13035–13040. 124 K. He, C. Poole, K. F. Mak and J. Shan, Nano Lett., 2013, 13, 2931–2936. 125 H. J. Conley, B. Wang, J. I. Ziegler, R. F. Haglund, Jr., S. T. Pantelides and K. I. Bolotin, Nano Lett., 2013, 13, 3626–3630. 126 C. Rice, R. J. Young, R. Zan, U. Bangert, D. Wolverson, T. Georgiou, R. Jalil and K. S. Novoselov, Phys. Rev. B: Condens. Matter Mater. Phys., 2013, 87, 081307. 127 Y. Wang, C. Cong, C. Qin and T. Yu, Small, 2013, 9, 2857–2861. 128 J. C. Shaw, H. Zhou, Y. Chen, N. O. Weiss, Y. Liu, Y. Huang and X. Duan, Nano Res., 2014, DOI: 10.1007/s12274-0140417-z. 129 Y. Zhang, Y. Zhang, Q. Ji, J. Ju, H. Yuan, J. Shi, T. Gao, D. Ma, M. Liu, Y. Chen, X. Song, H. Y. Hwang, Y. Cui and Z. Liu, ACS Nano, 2013, 7, 8963–8971. 130 J. K. Huang, J. Pu, C. L. Hsu, M. H. Chiu, Z. Y. Juang, Y. H. Chang, W. H. Chang, Y. Iwasa, T. Takenobu and L. J. Li, ACS Nano, 2014, 8, 923–930.

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Flexible and stretchable thin-film transistors based on molybdenum disulphide.

The outstanding physical and chemical properties of two-dimensional materials, which include graphene and transition metal dichalcogenides, have allow...
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