Two-dimensional semiconductors: recent progress and future perspectives

June 2, 2017 | Autor: Jinlian Hu | Categoria: Engineering, Materials Chemistry, CHEMICAL SCIENCES

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FEATURE ARTICLE

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Two-dimensional semiconductors: recent progress and future perspectives Xiufeng Song,a Jinlian Hub and Haibo Zeng*a Graphene with a sp2-honeycomb carbon lattice has drawn a large amount of attention due to its excellent properties and potential applications in many ﬁelds. Similar to the structure of graphene, two-dimensional semiconductors are its two-dimensional and isostructural counterparts based on the typical layerstructured semiconductors, such as boron nitride (h-BN) and transition metal dichalcogenides (e.g. MoS2 and WS2), whose layers are bound by weak van der Waals forces. Unlike the semi-metal features of graphene, the two-dimensional semiconductors are natural semiconductors with thicknesses on the atomic scale. When one of the dimensions is extremely reduced, the two-dimensional semiconductors exhibit some unique properties, such as a transition from indirect to direct semiconductor properties,

Received 25th November 2012 Accepted 28th January 2013

and hence have great potential for applications in electronics, energy storage, sensors, catalysis and composites, which arise both from the dimension-reduced eﬀect and from the modiﬁed electronic structure. In this feature article, recent developments in the synthesis, properties and applications of two-dimensional semiconductors are discussed. The reported virtues and novelties of two-dimensional

DOI: 10.1039/c3tc00710c

semiconductors are highlighted and the current problems in their developing process are clariﬁed, in

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addition to their challenges and future prospects.

1 a

State Key Laboratory of Mechanics and Control of Mechanical Structures, Key Laboratory for Intelligent Nano Materials and Devices of the Ministry of Education, and College of Material Science and Technology, Nanjing University of Aeronautics and Astronautics, Nanjing 210016, China. E-mail: [email protected]

b

School of Materials Science and Engineering, Anhui University of Technology, Ma-AnShan, Anhui, 243002, China

Xiufeng Song was born in Shangdong, China, in 1983. He received his PhD degree in Material Science from Nanjing University of Aeronautics and Astronautics in 2010. Now, he is a postdoctoral researcher in Prof. Zeng's group. His current research interests include twodimensional semiconductors (BN, MoS2, etc.) and inorganic luminescent materials for LED.

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Introduction

Since the early report of Geim and Novoselov et al. in 2004,1 graphene, a at monolayer of carbon atoms arranged in a twodimensional (2D) honeycomb lattice, has rapidly risen to be one of the hottest topics in materials science due to its fascinating properties and great potential for various applications. Owing to its zero-bandgap, super thin and at structure, graphene

Haibo Zeng is a professor and director of Optoelectronic Materials and Devices Research Center in Nanjing University of Aeronautics and Astronautics (NUAA). He received his PhD degree from the Chinese Academy of Sciences (CAS) in 2006, then worked in CAS, Universit¨ at Karlsruhe (Germany) and National Institute for Materials Science (Japan). In 2011, he joined NUAA and built a Nano-optoelectronics Group. He has published more than 80 peer-reviewed journal articles and the total number of citations is over 2000. The H-index of these publications is 30. The research in his current group focuses on two-dimensional semiconductors- and ZnO-based optoelectronics.

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Feature Article exhibits remarkable electronic, thermal, optical and mechanical properties, such as high mobility of charge carriers at room temperature (>200 000 cm2 V1 s1),2 superior thermal conductivity (5000 W m1 K1),3,4 high optical transmittance (97.7%),5,6 extremely high Young's modulus (1.1 TPa)7 and fracture strength (125 GPa),7 specic surface area (theoretically, 2630 m2 g1),8 high chemical stability, as well as excellent transport phenomena such as the quantum Hall eﬀect9,10 and ambipolar electric eld eﬀect.11 These intriguing properties endow graphene-based materials many applications, such as in ultra-strong lightweight components used in space shuttles to improve fuel eﬃciency, in transparent conductive electrodes for solar cells, various photoelectronic devices (e.g., liquid crystal displays, electric-eld-activated optical modulators, and electrical energy storage elements in batteries), and supercapacitors to enable renewable energy production at a large scale.8,12,13 As a promising nanomaterial, graphene can also be used in catalysis, sensors, biomedicine and in reinforced composites.8,14–25 The development of graphene will undoubtedly not only cause a revolution in understanding the fundamental properties of twodimensional structural materials with delocalized electrons, but will also bring us new truly transformative technologies which may dramatically change the future. In fact, such monolayer structures are not limited to carbon, similar to the case of inorganic fullerenes and nanotubes. Similar to graphene, two-dimensional semiconductors are its two-dimensional and isostructural counterparts based on the typical layer-structured compounds, such as h-BN and transition metal dichalcogenides, whose layers are bound by weak van der Waals forces. Two-dimensional semiconductors have also attracted researchers' great interest with the development of the graphene-related research. Theoretical and experimental work has shown that these two-dimensional semiconductors have exceptional properties, which will bring new breakthroughs in nanomaterials science.26–36 However, this eld is not well developed due to several experimental challenges and the lack of our understanding of their intrinsic properties. The most recent experimental achievements on two-dimensional semiconductors are quite encouraging. Novoselov et al.37 successfully isolated individual crystal planes from a large variety of strong layered materials including BN, several dichalcogenides (such as MoS2, NbS2) and complex oxides (Ba2Sr2CaCu2Ox), and showed that the resulting atomically thin 2D sheets are stable at room temperature, and in air, exhibit high crystal quality, and are continuous on a macroscopic scale. Since then, inspiring results were achieved in the synthesis of layered BN, MoS2, WS2 and other inorganic analogues of graphene, which provide a potent research platform for future fundamental studies of the basic properties and related application of 2D crystal materials.36–46 Atomic scale thickness endows two-dimensional semiconductors with peculiar and fascinating properties in contrast with those of their bulk parent compounds. As a typical example, bulk MoS2 has an indirect gap of 1.2 eV; however, monolayer MoS2 is a direct gap semiconductor with a bandgap of 1.8 eV due to quantum connement, which results in the enhancement of its photoluminescence.35,47 Theoretical and

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Journal of Materials Chemistry C experimental results indicate that BN nanoribbons (BNNRs) display semiconductor behavior, which makes them very interesting for applications in nanoelectronics and optoelectronics.26,34 Monolayer two-dimensional semiconductors exhibit stronger piezoelectric coupling than traditionally employed bulk forms, which enables 2D material applications in sensing, actuating, and new electronic components.28,29,48,49 In addition, there are also many novel properties, implying that new-fashioned applications should be further explored. Due to the extraordinary physical properties (such as high carrier mobility), graphene is considered as the most promising candidate to replace silicon in future electronic devices, especially in radio-frequency transistors. However, graphene is semimetallic with zero bandgap, which is not suitable to switch oﬀ devices. Bandgap engineering is thus required to open a bandgap of graphene, or a search for replacement of 2D layer materials with nature semiconducting properties for graphene is necessary. Monolayer MoS2 has a direct gap of 1.8 eV, while that of Si is 1.1 eV. The properties of a MoS2 monolayer based transistor are lower than those of silicon and graphene nanoribbons, and there still remain many challenges. However, with further progress, monolayer MoS2 could be a new competitor for graphene. For graphene based electronic technology, insulators are also required, especially as a high at dielectric layer. BN with a single (or a few) layer is a promising candidate with an atomically smooth surface leading to low scattering due to a low density of charge traps and adsorbed impurities. Compared with graphene, two-dimensional semiconductors are in an embryonic stage. Further works should be undertaken to explore their properties and expand applications. In this review, we focus on a recent progress in the eld of two-dimensional semiconductors, such as BN, dichalcogenide layers and other 2D semiconductors. Their synthesis, properties and applications are briey summarized. The advantages and disadvantages of two-dimensional crystals, current challenges, and future perspectives are also discussed.

2

h-BN or ‘white graphene’

2.1

Structure

Hexagonal boron nitride (h-BN) is isostructural to graphite, while monolayer h-BN is a structural analogue of graphene and has been referred to as ‘white graphene’. h-BN is comprised of alternating boron and nitrogen atoms replacing carbon in a honeycomb arrangement. sp2-bonded h-BN shows strong covalent bonds within the plane and weak bonds with van der Waals forces between diﬀerent planes. The crystal structure of h-BN is hexagonal, with the space group P63/mmc (no. 194), the ˚ c ¼ 6.6612 A, ˚ a¼b¼ lattice constants being a ¼ b ¼ 2.5040 A, 90 , g ¼ 120 . The crystalline structure of h-BN with one, two and three-dimensions are shown in Fig. 1. Although the crystal structures of h-BN and graphite are similar, the electronic properties are very diﬀerent: while graphite is a semimetal (zero-gap), h-BN is a good insulator (or a wide band-gap semiconductor) with a direct energy gap of 5.97 eV.50 Thus, monolayer and few-layer h-BN sheets possess enormous potential in comparable or complementary

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Journal of Materials Chemistry C

Fig. 1

One, two and three-dimensional crystalline structures of h-BN.

electronic and composite applications. With a low dielectric constant, large thermal conductivity and high mechanical strength, h-BN can serve as a dielectric layer for electronic devices.51,52 Due to its large energy gap, h-BN can be used as a promising deep ultraviolet (DUV) emitter.50,53,54 h-BN could also be utilized as a solid-state lubricant or a ller material in composites.55–59 Its outstanding properties and potential applications have put h-BN at the forefront of current nanoscience research.

2.2

Properties

Theoretical investigations on the properties of monolayer h-BN have just begun to emerge. Similar to graphene, single- or fewlayered h-BN exhibits special properties diﬀerent from those of bulk h-BN. Due to atomic scale thickness, the edge structure and vacancy defects play important roles in the character of monolayer h-BN. Zigzag and armchair edge and vacancy defects have been observed by high-resolution electronic microscopy.60,61 Theoretically, two-dimensional BN white graphene shows electronic, optical and magnetic properties which arise from its edge structure and defects.26,34,62–67 For example, irrespective of their width and chirality, perfect BNNRs display semiconductor behaviors;67 bare zigzag BNNRs (zBNNRs)27,64 and H-terminated BNNRs67 are magnetic and non-magnetic, respectively. And, zBNNRs with two-hydrogen-terminated edges become ferromagnetic metals.63 Edge modication with F, Cl, OH, and NO2 groups can change the bandgap of BNNRs,68 though such decorated BNNRs still keep the semiconducting character. An extra transversal electric eld can control the bandgap of bare zBNNRs to undergo metallic-semiconducting-half-metallic transition.27 Moreover, the BNNRs with only the B edge terminated by hydrogen present half-metallic characteristics.26,64 These properties make 2D h-BN promising for potential application in electronics and optoelectronics.26,34,63–65,69 Electronic property. Due to its zigzag sharp edges and vacancies, BN white graphene becomes a semiconductor and its conductivity is enhanced. Zeng et al.34 investigated the electrical transport of BN nanoribbons (BNNRs) with

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Feature Article

Fig. 2 Transformation of electrical transport. (a) TEM image of a ribbon connected to the counter gold electrode (upper black part) and tungsten STM tip (bottom black part); the insets present the experimental conﬁguration, and the scale bar is 20 nm. (b) Typical I–V curves of several BN nanoribbons. (c) I–V curve of a BN nanoribbon under an increased voltage until its structural degradation induced by Joule overheating. (d) Typical ln I vs. V plot of a BN nanoribbon. Reprinted with permission from ref. 34. Copyright 2010, American Chemical Society.

N-terminated zigzag edges, as shown in Fig. 2. The results illustrated that compared to insulating BN nanotubes, BNNRs are p-type semiconductors with a much higher conductivity. The current is seen in Fig. 2c to be 2 mA under a 9 V voltage and 15 mA under a 18 V voltage. Broken voltage, conductance and carrier mobility are 20 V, 104 S m1 and 58.8 cm2 V1 s1, respectively.34 With H,62 F,70 O66 and S66 functionalized zigzag edges of h-BN white graphene, 2D BN exhibited semiconductor–half-metal–metal transition, which makes BN white graphene a novel material for application in the novel integrated functional nanodevices. Treating h-BN membranes with a hydrogen plasma, the bandgap was shown to be tuned from 5.6 eV to 4.25 eV, while the resistance decreased by 38%, which can be explained by hydrogenation leading to a smaller bandgap.71 Similar experiments were also carried out on oxygen plasma-treated graphene.72 Both works imply that plasma-treated doping can be an eﬀective way to modify the band structure of BN nanosheets (BNNSs). By in situ electron beam irradiation, C substitution for B and N atoms in BNNSs was achieved and BNNSs could be transformed from insulators to conductors with a resistance as low as 10 kU.73 Wang et al.74 also obtained Cx–BN nanosheets, which are semiconductors with a controlled resistivity varying in a large range, from 102 to 101 U m1, depending on the C content. Wide bandgap and low conductivity become obstacles for applying h-BN in electronic and optoelectronic devices. In order to introduce 2D BN in electronic devices as a semiconductor, the conductivity of the latter needs to be improved. Besides utilizing an extra transversal electric eld27,75–77 and planar strain,78 doping and surface modication are the most eﬀective routes to improve the transport property of BN white graphene.73,79–82 However, 2D h-BN is a one or several atomic layer material; it is still a challenge to achieve eﬀective doping in 2D

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Feature Article h-BN for scientists. It has been of great demand to explore an easy and controllable way to modify the electronic structure of BN white graphene. Optical properties. Optical property is another important parameter for BN white graphene, especially in optoelectronic devices. 2D h-BN layers present a very high transparency (transmittance over 99%) in the wavelength range of 250–900 nm, and show a sharp absorption peak below 250 nm. The calculated optical bandgap of monolayer BN is 6.07 eV.83 For few-layered BN, the same parameter was reported to be of 5.92 eV,84 while for bulk h-BN, it is about 5.2 eV.85 These results indicate that the bandgap becomes smaller with increasing number of h-BN layers, which is associated with layer–layer interactions leading to the dispersion of electronic bands and reduction of the bandgap.85 2D h-BN shows an ultraviolet luminescence band at 215 nm, which opens up a new research eld for applications in deep-UV light-emitting and laser devices.50,86,87 The cathodoluminescence (CL) spectrum of BN whiskers shows distinct emission bands centered at 330, 461, and 541 nm with an emission line centered at about 212 nm.88 This deep UV emission at 212 nm could be attributed to a band-to-band optical transition. Yu et al.89 prepared vertically aligned BN nanosheets and found that the CL spectra of the BNNSs exhibited a broad emission band centered at 265 nm (4.68 eV) in the range of 200–400 nm. This emission band is ascribed to the near-band-edge emission of highly crystallized BN layers. At the same time, for the granular lms of h-BN, the emission band is in the range of 260–520 nm, being centered around 360 nm, which was attributed to defects due to poor crystallinity.89 However, to our knowledge, there are no reports about cathodoluminescence and photoluminescence of monolayer or few-layer h-BN lms. This becomes a challenge in this eld to explore the optical properties of BN white graphene, which will help to expand the potential applications of 2D layer materials, towards, e.g., deep UV LEDs and lasers. Mechanical properties. Graphene is known to exhibit excellent mechanical properties. The mechanical properties of BN nanotubes (BNNTs), having a similar structure, might be comparable to those of CNTs. The reported Young's modulus and fracture strength of graphene are 1.1 TPa and 125 GPa, suggesting that graphene is a promising structural material.7 In theoretical works, layered h-BN was reported to possess a Young's modulus ranging from 0.198 to 0.328 TPa,90 which is soer than graphene.91 The bulk modulus of BN is 160 Pa m, whereas that of graphene is 202 Pa m. Li et al.92 tested the bending moduli of chemically exfoliated BNNSs, and found that the bending moduli increase with decreasing sheet thickness. While for BN sheets with a thickness smaller than 50 nm, the bending modulus is close to the reported bulk BN values (31.2 GPa). For thicker BNNSs (above 100 nm), a smaller bending modulus was found, which nally approaches 18 GPa. Song et al.93 investigated mechanical properties of layered h-BN synthesized by chemical vapor deposition (CVD). The elastic constant of an h-BN lm with a thickness of 1–2 nm is in the range of 220–510 N m1. With a typical fracture test, the lms break at deections of about 70 nm and forces of about This journal is ª The Royal Society of Chemistry 2013

Journal of Materials Chemistry C 221 nN. With increasing thickness of the h-BN lm, the elastic constant increases. Theoretically, due to the eﬀect of vacancy defects, the elastic constant values decrease linearly with the concentration of vacancies. The test modulus value of 2D h-BN layers is lower than the calculated value. This might be attributed to the layer distribution of stacking faults resulting from the growth by CVD.93 Other properties. BN white graphene possesses an ultimately high surface area due to its atomic-scale thin layer. The high value of Brunauer–Emmett–Teller (BET) surface area is up to 927 m2 g1 with 1–4 layers of BN.91 The surface area of BN increases progressively with a decrease in the number of layers. And with the high surface area, BN white graphene exhibits high CO2 adsorption, but negligible H2 adsorption. Yu et al.89 found that the vertically aligned BNNSs are superhydrophobic with contact angles over 150 due to their nanoscale rough surfaces. Recent studies by Bando's group94 also reported a superhydrophobic behavior of BNNSs, pointing the excellent chemical inertness of the material. Even strongly acidic and basic conditions did not aﬀect the water-repellent properties of the BNNT lms. The superhydrophobic properties suggest that BNNSs can be excellent candidates for self-cleaning applications. There are also many other physical and chemical properties of BN white graphene, such as thermal transport property95,96 and piezoelectric eﬀect,29,48,49,97 which are not detected experimentally. It does not indicate that they are not important and can be ignored; rather, it is mainly attributed to the studies and understanding on the essences and characteristics of BN white graphene being still at the starting stage and there is still a lack of an eﬀective approach for synthesis and characterization of BN white graphene.

2.3

Fabrication

Even though the layered h-BN has the above-mentioned particular properties, a prerequisite for the studies of the basic properties of the material is the synthesis of high-quality BN white graphene with controlled size, layers, morphology and edges. Thus developing the eﬀective strategies for controllable and scalable production of BN white graphene is of great and crucial importance for its applications in diﬀerent elds. Up to now, many methods have been designed to obtain high-quality BNNSs, such as mechanical exfoliation, chemical exfoliation, unzipping BN nanotubes, chemical vapor deposition, and thermal decomposition. Details of experimental studies on the synthesis of BN white graphene are summarized in Table 1. Mechanical exfoliation. Mechanical exfoliation was rst utilized to prepare graphene from graphite by Novoselov et al.1 Now, this method is widely used to prepare high-quality graphene samples. This technique is quite simple, but it provides highly crystalline samples to study the fundamental properties of graphene such as ballistic transport, carrier mobility, thermal conductivity and so on.1–4,15,18,21 Pacile et al.98 obtained a similar structure of layered BN by mechanical exfoliation. The thinnest region of BN is 3.5 nm thick or roughly of ten layers. The surface roughness of the thinnest region is 0.14 nm. With further

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Table 1

Feature Article

Summary of recent synthetic methods to prepare 2D h-BN

Method

Structure

Mechanical exfoliation Plasma etching of BNNTs

Nanosheets Nanoribbons

Potassium-intercalation–splitting of BNNTs Liquid exfoliation Thermal decomposition

Nanoribbons Nanosheets Nanomeshs

Chemical synthesis Low pressure CVD

Nanosheets Monolayer lms

Atmospheric pressure CVD

Few layer lms

Environment

BN nanotubes partially embedded in a polymer lm 106 Torr, 300 C for 72 h in quartz tube In various organic solvents 3 107 mbar, 800 C on metal surfaces 900 C in a nitrogen atmosphere 350 Torr, 1000 C for 1 h on a metal surface in a quartz tube 700 C for 1 h on a metal surface in a quartz tube

peeling, single layered BN could be obtained. Utilizing this method, some interesting work was carried out on layered h-BN.99,100 With electron diﬀraction and plasmon electron energy loss spectroscopy, single and double layers were observed.101 However, it is hard to realize large-area fabrication. The layers, morphology and edges are still not controllable, and the yield is extremely low. So, this technology is expected to have limited relevance in commercial high-end applications. Liquid phase exfoliation. Liquid phase chemical exfoliation of graphite is a viable route to obtain a high volume production of graphene.102–104 Unlike mechanical exfoliation, this method employs chemicals to intercalate bulk graphite and produces a graphene–solvent interaction, which can overcome the van der Waals forces between graphene layers.102,103 Ultrasonic energy is oen used to drive the exfoliation in charged liquid media. Han et al.105 were the rst to report on the liquid-phase exfoliation of layered BN via sonicating BN powder in a 1,2-dichloroethane solution of poly(m-phenylenevinylene-co-2,5-dictoxy-p-phenylenevinylene), and obtained single, double, and triple layered BN nanosheets. Thereaer, N,N-dimethylformamide (DMF),106 methanesulfonic acid,107 and 1,2-dichloroethane108 were successfully used to realize the chemical exfoliation of h-BN. Lin et al.109 found that water was an eﬀective solvent for the exfoliation and dispersion of h-BN under sonication conditions, and “clean” aqueous dispersions of h-BN nanosheets were formed. Recently, Coleman et al.42 pointed out the best 20 solvents for chemical exfoliation of h-BN according to the dispersion of exfoliated BN in liquids. They proposed that this exfoliation technique could be applied not only to transition metal dichalcogenides, graphene, BN, and Bi2Te3, but also to transition metal oxides and other layered compounds. Before the exfoliation of h-BN, it was rst functionalized by lipophilic and hydrophilic amine molecules, which could be homogeneously dispersed in organic solvents and/or water. Lin et al.110 used octadecylamine (ODA) and an amine-terminated polyethylene glycol (PEG) as the Lewis bases in the functionalization and exfoliation of h-BN. Reactive nitrene radicals can also be used to realize a covalent chemical functionalization of h-BN nanosheets.111 Recently, Nazarov et al.112 indicated h-BN nanosheets

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Yield

Width

Ref.

Low Medium

Large than 100 mm 15–500 nm

35,95 and 96 32

Low

40 nm

97

High Very low

39 and 98 99

High Low

Less than 100 nm Nano-to microscopiclength scale Several hundred nm Centimeter-scale

Low

Several centimetres

81

88 80

functionalized via a treatment with inorganic reagents, such as hydrazine, H2O2, HNO3/H2SO4, and oleum. On functionalization, the dispersibility of h-BN nanosheets is enhanced and chemical compatibility is also facilitated within polymer matrices. This would increase the yield of h-BN nanosheets and expand the application of such nanosheets in polymer matrices. Unzipping BN nanotubes. BN nanotubes can be formed by rolling up 2D BN sheets; therefore, it would seem natural to unroll them to obtain layered BN. Zeng et al.34 unzipped multiwalled BN nanotubes to BNNSs by delicate plasma etching. The BN nanotubes were rst deposited on a Si substrate, and then covered with polymethyl methacrylate (PMMA), forming a PMMA–nanotube lm. This lm was peeled oﬀ, turned over and then exposed to Ar plasma. Aer removing PMMA in an acetone vapor, BN nanoribbons (BNNRs) were formed. The sketch of BNNR fabrication by plasma etching is shown in Fig. 3. Single-, bi-, and multi-layer BN nanoribbons with zigzag edges were produced and a p-type doped semiconductor was formed.34 Erickson et al.113 synthesized BNNRs through the potassiumintercalation-induced longitudinal splitting of boron nitride nanotubes. Narrow (about 20 nm), several sheet (typically 2–10) and high crystallinity BNNRs with uniform widths were obtained. This method can provide long (1 mm) BNNRs with low defects in the ribbon plane and along the ribbon edges. During the unzipping of a BN nanotube, BNNTs oen form with all their walls being mostly of armchair or zigzag orientation, yielding ribbons with edges dened by the tube chirality. Theoretically, the electronic, optical, and magnetic properties of BNNRs arise from various edge structures and terminations.27,48,64,69,96,97,114 It is worthwhile to consider the possibility of

Fig. 3 Sketch of a PMMA–BN ﬁlm fabrication for plasma etching. Reprinted with permission from ref. 34. Copyright 2010, American Chemical Society.

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Feature Article functionalizing the edges of these BNNRs. By unzipping BN nanotubes, zigzag or armchair terminations are obtained. For graphene, nanocutting of graphene is another promising method for graphene nanoribbon (GNR) synthesis. For BN, not only potassium but also other metal particles are expected to be utilized to achieve highly reactive BNNR edges. Thermal decomposition. Over ten years, thermal decomposition has been used to prepare monolayer h-BN on metals by weak physisorption.115–117 By thermal decomposition of borazine (HBNH)3, a highly regular nanomesh of h-BN with a 3 nanometer periodicity and a 2 nanometer hole size was formed on a Rh(111) surface.118,119 Hole formation is likely driven by the lattice mismatch of the lm and the rhodium substrate. Hereaer, Cr(110),120 Fe(100),121 Ru(1000),122 Ir(111),123 Ni(111)124,125 and Pd(110)126 are used as substrates for the growth of h-BN. Depending on the substrate, there are two adsorption geometries: borazine bound with the molecule ring either perpendicular (Pt(111)) or parallel to the substrate (Au(111) and Ru(0001)).123,127 At low temperatures borazine molecules adsorb intact on a metal surface with its molecule ring parallel to the substrate surface. Molecular dissociation occurs near room temperature, with dehydrogenation accompanied also by partial cracking of the benzene-like ring, a process which precedes the formation of the BN layer.123 Due to a weakly physisorbed metal–insulator interface, this h-BN lm growth was self-limiting at one monolayer. The sticking coeﬃcient of the precursor molecule becomes exceedingly small.118 The growth of the multilayer turns out to be diﬃcult. For this method, most of the researchers mainly focus on the characteristics of the morphology, structure and properties of the layered h-BN lms, but little on practical application. Chemical vapor deposition (CVD). CVD growth is perhaps the most promising technique for large-scale production of single or few-layer h-BN lms. CVD oﬀers a relatively controllable means to produce monolayer h-BN lms with large area and high quality, which are of great importance for practical applications. Thus more and more research has been focused on CVD deposited h-BN recently, especially on the controllable CVD growth of few-layer h-BN. This technique has also proven to be very eﬃcient for producing h-BN lms using various chemical precursors such as BF3/NH3,128 BCl3/NH3,129 B2H6/ NH3,130 B/NH3 (ref. 131) as well as single precursors such as borazine (B3N3H6),132 trichloroborazine (B3N3H3Cl3),133,134 or hexachloroborazine (B3N3Cl6)135 with the B/N ratio of 1 : 1. A remarkable progress on CVD-produced BN white graphene has been achieved in the last several years. Shi et al.84 reported a fewlayer h-BN thin lm grown on Ni in an ambient pressure chemical vapor deposition (APCVD) system with borazine (B3N3H6). The obtained h-BN lms showed the thickness in the range of 5 to 50 nm and a lateral size of about 20 mm, which was only limited by the size of the Ni single-crystal grains. At the same time, Song et al.93 reported on the large area synthesis of h-BN lms consisting of two to ve atomic layers, using CVD with the solid state boron source ammonia borane (NH3-BH3). As shown in Fig. 4, the obtained BN lms had continuous large areas up to several square centimeters with a thickness of about 1.3 nm. More recently, Kim et al.83 obtained a large area

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Journal of Materials Chemistry C

Fig. 4 Topography of ultrathin hexagonal BN ﬁlms. (a) Photograph of a largearea h-BN ﬁlm on a Si substrate. (b) SEM image shows an h-BN ﬁlm. (c and d) AFM image and line-scan proﬁle indicate that the h-BN ﬁlm has a uniform thickness of 1 nm. Reprinted with permission from ref. 93. Copyright 2010, American Chemical Society.

monolayer h-BN lm on Cu with a low-pressure CVD (LPCVD). Chatterjee et al.136 also achieved a monolayer h-BN lm on Cu and Ni substrates by CVD with solid state decaborane (B10H14). A triangular-shaped h-BN was found, which is possibly due to the diﬀerent chemical potentials of B and N species and more energetically favored nitrogen-terminated edges.133,137,138 Recently, M¨ uller and Sachdev et al. synthesized h-BN via a CVD of borazine (B3N3H6) on Ag(111),139 Ru(111)138 and Cr(110).120 With surface controlled chemical reactions, they obtained high-quality BN white graphene by a three-step boration–oxidation–nitration process.140 Aer the oxidation of BN on a metal surface, oxygen containing compounds in the B–N–O– H-system were formed, which may act as surfactants to deliver mobile species responsible for the BN-layer formation. So, trimethylborate B(OM)3 (with M ¼ CH3) can be introduced as a boron source to prepare layered BN.141,142 Aer nitration by NH3, highly ordered crystalline monolayer h-BN lms form. With a two-step boration–nitration process, there are two drawbacks in the preparation of hexagonal BN layers. First, not all boron from the B–Rh species transform into BN-like species; some boron still remains in the Rh crystal. Second, the structural quality of the BN lm cannot compete with that of a lm prepared via the three-step process.140,141 This new growth mechanism is expected to provide a promising synthetic route to the preparation of high-quality monolayer BN lms. Poly- or mono-crystalline Ni and Cu are oen utilized as substrates for the growth of monolayer and few layer h-BN by CVD, which is explained by their catalytic activity.93 Hence, the quality of the metal surface plays a key role in controlling the formation of continuous layers on the substrate and the number of layers of h-BN formed. The substrate surface morphology aﬀects the nucleation of the h-BN growth. h-BN mostly nucleates along the metal rolling lines, which is due to the gas ow eﬀect and more impurity sites on the metal surface along the rolling lines.83 Lee et al.143 studied the inuence of the Cu morphology on the high-quality growth of h-BN nanosheets and found that the heterostructure (c-BN/BN soot) can be decreased by introducing a at Cu surface with large grain boundaries and low surface J. Mater. Chem. C, 2013, 1, 2952–2969 | 2957

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roughness. The crystal orientation of the substrate is another parameter aﬀecting the quality of layered h-BN lms, which is due to the lattice mismatch leading to a tensile stress. There is a lattice mismatch of about 0.4% between Ni(111) and h-BN. For Cu(111) and h-BN, it is of +2%.83 Lee et al.144 found that the growth of h-BN layers strongly depends on the underlying Ni crystal orientation. The growth rate of h-BN was larger on Ni(100)or Ni(100)-oriented crystal surfaces, while that on Ni(111)- or Ni(111)-oriented surfaces is not detected. The kinetics rather than thermodynamics were concluded to play a primary role in the CVD growth of h-BN on Ni.144 2.4

Applications

Fillers for composites. BN white graphene is known to have remarkable mechanical properties90–93 and thermal conductivity,95,96 which make it advantageous as novel nanollers in composite materials aiming at mechanical reinforcement and high thermal conductivity in a matrix.56 Bulk h-BN145 and BN nanotubes146 have been tested as llers in lled polybenzoxazine and PMMA to improve their mechanical and thermal properties. Recently, Zhi et al.106 reported on BNNSenhanced PMMA. Aer embedding BNNSs, the transmission of light at a wavelength longer than 600 nm is up to 91%, which is quite close to that of the blank PMMA (92%). The elastic modulus of PMMA also increased from 1.74 to 2.13 GPa (22% increase), and the strength also increased by 11%. This result reveals that an applied mechanical load can eﬀectively be transferred to BNNSs due to profound interfacial interactions. Wang et al.107 prepared a poly[2,20 -(p-oxydiphenylene)-5,50 bibenzimidazole] (OPBI)/BNNS composite, and addition of 4 wt % of BNNS increased the Young's modulus up to 4.2 GPa, i.e. much higher than that of 4 wt% of BN particles added (Fig. 5). With 2 wt% of BNNSs, the tensile stress increased to 174 MPa, which is 14% higher than that of OPBI. The enhancement of modulus and strength is believed to be the result of the good dispersion state of the BNNSs and the eﬀective stress transfer due to the interactions, such as p–p interactions, between the polymer chains and BNNSs. Due to a large surface area, the interface between the BNNS and polymer can oﬀer more

Fig. 5 Typical stress–strain curves of OPBI and the composites (a) and comparison of the Young's modulus (b), tensile stress and (c) ultimate strain of OPBI and the composites. Reprinted with permission from ref. 107. Copyright 2011, Royal Society of Chemistry.

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Feature Article polymer chains, and the polymer chains at the matrix–ller interface may degrade more slowly. So the interaction plays an important role in composites. Therefore the BNNS surface must be functionalized to enhance the interface eﬀect. Octadecylamine (ODA) and hyperbranched aromatic polyamide (HBP) can be utilized to functionalize BNNS.147 The chemical reaction of HBP functionalized BN with an epoxy matrix forms a strong interface. Intermediate interface interaction leads to a moderate improvement of thermal properties. Substrates for graphene electronics. Graphene has been considered as a promising material in electronics, photonics, plasmonics, and optoelectronic devices.16 The electronic quality of a graphene device strongly depends on the electronic interaction between the graphene layer and the substrate, which limits the excellent charge transport properties of graphene. The most commonly used substrates, silicon oxide (SiO2) or high-k gate dielectrics, limit the mobility of charge carriers in devices by introducing scattering from charged surface states, remote impurities, remote surface optical phonons, and substrate roughness.148 One promising dielectric for potential integration with graphene is BN white graphene, because it has an atomically smooth surface which is relatively free of dangling bonds and charge traps.148–153 It also has a lattice constant similar to that of graphite with a lattice mismatch of approximately 1.7%. h-BN has large optical phonon modes and a large electrical bandgap. Dean et al.154 utilized exfoliated h-BN as the graphene substrate instead of SiO2. The charge carrier mobility concentration was up to 60 000 cm2 V1 s1, i.e. three times larger than that on SiO2, and rS z 71 U, which is similar to values obtained on SiO2. This result was ascribed to a substantially reduced charge impurity contribution on BN substrates, which reveals the eﬀect of short-range scattering at comparatively lower densities.154 Gannett et al.51 achieved a mobility as high as 37 000 cm2 V1 s1, an order of magnitude higher than that commonly reported for CVD graphene and better than those of most exfoliated graphene samples. Zomer et al.100 introduced a new transfer technique which yields an atomically at graphene on BN with almost no bubbles or wrinkles. They obtained the mobilities of graphene on h-BN as high as 120 000 cm2 V1 s1 at room temperature and up to 275 000 cm2 V1 s1 at 4.2 K. During the transfer process, impurities were introduced into the interface at the h-BN/graphene, which aﬀected carrier mobility concentration.148 It is of great demand to achieve high-quality and large-area layered hBN as a dielectric layer to satisfy the requirements for highperformance graphene electronic devices. Graphene–BN hybrid devices. Hexagonal BN is a wide-gap insulator with a crystalline structure similar to that of graphene. The lattice-mismatch between h-BN and graphene is only 1.7%, which lets them match with each other to form a bilayer superlattice. In 1997, Suenaga et al.155 obtained hybrid C–BN–C sandwich geometry nanotubes, which can be used in nanostructured electronic devices. For a 2D crystal, layered h-BN can be used as a barrier layer for electron tunnelling between two graphene layers.156 In a recent theoretical study, the bipolar eld-eﬀect transistor utilizing graphene heterostructures with layered BN exhibited high room-temperature switching ratios,

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Feature Article and showed a potential application for high-frequency operation and large-scale integration.157 Britnell et al.158 experimentally realized the electron tunnel current of graphene by ultrathin BN. They fabricated the graphene/BN/graphene and graphite/BN/graphite devices, as shown in Fig. 6. The I–V curves of the devices were linear at a zero bias but had an exponential dependence on V at higher biases. The zero-bias resistance of the device scaled exponentially with the BN barrier thickness from monolayer to 4 atomic layers, as seen in Fig. 6. They concluded that a high quality barrier material of h-BN has a great potential in novel electron tunneling devices and for investigating strongly coupled and narrowly separated electrodes of diﬀerent compositions.158 Recently, Levendorf et al.159 controlled the fabrication of lateral heterostructures of graphene/h-BN via a patterned regrowth technique. The resulting lms formed mechanically continuous sheets across the heterojunctions, which can be used in atomically thin integrated circuitry. h-BN could also be used to open a small bandgap in graphene.160,161 Theoretical studies suggested that graphene deposited on a h-BN substrate will introduce a gap of 53 meV,149 but recent experimental studies found no such bandgap.51,100,154 The stacked h-BN/graphene/h-BN sandwich structure shows the bandgap of 0.16 eV. The bandgap could be enhanced up to 0.34 eV by introducing a strong vertical electric eld.162 Ramasubramaniam et al.163 also investigated the bandgap tuning of bilayer graphene between h-BN sheets, by applying an external electric eld. They found that the gap is continuously tunable from 0 to 0.2 eV, being robust to stacking disorder. However, it is very diﬃcult to realize the h-BN/graphene/h-BN sandwich structures. Gao et al.164 prepared randomly stacked layers of graphene and h-BN through a chemical exfoliation technique and mixing approach. The bandgap of a hybrid of the h-BN/graphene was tuned from 1.19 eV to 5.25 eV varying with the concentration of h-

Journal of Materials Chemistry C BN and graphene. Recently, Liu et al.165 reported that h-BN can be directly grown on graphene by a CVD process to form graphene/ h-BN stacks. Ding et al.166 realized high-quality graphene layers grown on h-BN akes by CVD without any metal catalysts. They are expected to be obtained as h-BN/graphene/h-BN sandwiches or h-BN/graphene superlattices, which could provide a stage to explore the properties of the h-BN/graphene structure with a tunable and sizable bandgap. Other applications. h-BN has a high thermal conductivity, temperature stability, acid–base resistance, and appropriate chemical inertness. Furthermore, h-BN is hydrophobic, thus preventing moisture condensation on its surface. Wu et al.167 rst employed BN as the support for metal catalysts. The activity of metal/BN is superior to that of the metal/g-Al2O3 system, and opens new perspectives in supports for catalysis.168 With a higher surface area of BN, the activity of the BN-supported metal catalysts is stronger. The surface area of BNNSs is as high as 927 m2 g1 (for 1–4 layers), which makes them a promising support to metal catalysts.91 An ultraviolet absorption band at 215 nm makes the broad bandgap h-BN the material of choice for DUV optoelectronic devices. However, h-BN is an excellent electrical insulator; the conductivity of BN needs to be improved for applications in optoelectronic devices. Dahal et al.169 realized p-type Mg doped h-BN epilayers. The properties of h-BN:Mg with Mg acceptor level and p-type resistivity superior to AlN:Mg make h-BN an excellent candidate for applications in deep ultraviolet optoelectronics. However, great eﬀorts are to be made to take this direction for few layered h-BN.

3

With the development of graphene, transition metal dichalcogenides (TMDs) with the common structural formula MeX2 (Me ¼ Mo, W, Ti, etc. and X ¼ S, Se, Te) have drawn much attention due to their analogous structure with graphene. The 2D TMDs are expected to have electronic properties varying from metals to wide-gap semiconductors with excellent mechanical characteristics. The monolayer TMD materials have already shown a good potential in nanoelectronic and photonic applications.35,45,47,170–174 3.1

Fig. 6 Structure and character of graphene/BN/graphene devices. (a) Schematic view of a graphene/BN/graphene structure. (b) SEM micrograph of Au/BN/Au and graphene/BN/graphene devices. (c) Characteristic I–V curves for graphite/ BN/graphite devices with diﬀerent thicknesses of the BN insulating layer. The inset shows a typical I–V curve where a breakdown in the BN is observed at +3 V; the thickness of the ﬂake is 4 layers of BN (1.3 nm). The dotted line indicates the continuation of the exponential dependence. Reprinted with permission from ref. 158. Copyright 2012, American Chemical Society.

This journal is ª The Royal Society of Chemistry 2013

Dichalcogenide layers

Structure

The layered MeX2 have two crystal structures; one is specied as 2H-MX2 with D6h point group symmetry and the layered structures formed by the stacking of weakly interacting two-dimensional (2D) MX2 layers. The other one is known as the 1T structure with D3d point-group symmetry and is common to several MX2 compounds. The atomic structures of 2H-MX2 and 1T-MX2 are shown in Fig. 7(a) and (b). The two structures can be marked as honeycomb (H) and centered honeycomb (T) structures. They can be considered as a positively charged 2D hexagonal lattice of M atoms sandwiched between two hexagonal lattices of negatively charged X atoms. In the two structures, each M atom has six nearest X atoms, and each X atom has three nearest M atoms forming p–d hybridized ionic M–X bonds.

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Fig. 8 Fig. 7 Atomic structure of 2H-MX2 (a) and 1T-MX2 (b). (c) Summary of the results of our stability analysis comprising 44 diﬀerent MX2 compounds that can form stable, 2D single-layer H and/or T structures. Transition-metal atoms indicated by M are divided into 3d, 4d and 5d groups. MX2 compounds shaded light gray form neither a stable H nor T structure. Reprinted with permission from ref. 41. Copyright 2012, American Chemical Society.

According to extensive stability analysis using rst-principles calculations, Ataca et al.41 summarized the crystal structure of dichalcogenides, as shown in Fig. 7(c). It is easy to distinguish the structure and electronic properties of these dichalcogenides. MoX2 and WX2 (X ¼ S, Se, and Te) have semiconducting properties and possess a bandgap suitable for device applications, which makes them potential candidates as the complement or substitute to graphene. The structures of 2H-MoS2 chalcogenides are shown in Fig. 8 with one, two and three dimensions. 3.2

MoS2 with one, two and three-dimensional crystalline structures.

Fig. 9 The simpliﬁed band structure of bulk MoS2, showing the lowest conduction band c1 and the highest split valence bands v1 and v2. A and B are the 0 direct-gap transitions, and I is the indirect-gap transition. Eg is the indirect gap for the bulk, and Eg is the direct gap for the monolayer. Reprinted with permission from ref. 35. Copyright 2010, American Physics Society.

Properties

These days, the TMD related research mainly focuses on MoS2 and WS2 layers with semiconducting properties. Owing to strong surface eﬀects, the properties of the TMDs vary drastically with the number of layers in a sheet. The electron energy loss spectra of TMDs shi toward low energy (long wavelengths) with decreasing number of layers.175,176 The bandgap increases from multilayers to a single layer, and MoX2 and WX2 transfer indirect-bandgap to direct-bandgap semiconductors.35,177,178 These special properties, especially as a direct bandgap semiconductor, make monolayer MoX2 and WX2 complements or substitutes of graphene in optoelectronic and energy harvesting applications. Electrical properties. The bulk MoS2 crystal is an indirectgap semiconductor with a bandgap of 1.29 eV. The monolayer of MoS2 is a direct gap semiconductor in which the lowest energy interband transition occurs at the K point of the Brillouin zone, as shown in Fig. 9. The bandgap of a monolayer of MoS2 is 1.9 eV.35 The few-layered MoS2 sheets can be considered as identical layers with only nearest-neighbor interlayer interactions; the 2D electronic structure can be regarded as a subset of the bulk electronic states with quantized out-of-plane momenta

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kt, i.e., momenta lying in planes perpendicular to the G–A or K–H directions.35 In monolayer MoS2, the allowed plane passes directly through the H and A points. The c1 minimum and the v1 maximum of the resulting 2D bands both occur at the H point (Fig. 9), which indicates that monolayer MoS2 is a directgap material. In the bulk material, with increasing thickness, cuts with kt approaching the G–K line are formed. Then the v1 maximum and the c1 minimum occur at the G point and along the G–K direction, respectively, which makes few-layer MoS2 samples become indirect-gap semiconductors.35 Novoselov et al.37 rst reported on the mobility of layered MoS2 being between 0.5 and 3 cm2 V1 s1 with a carrier density in the order of 1012 cm2. The mobility of MoS2 increases with the number of layers (1L: 0.03; 2L: 0.07; 3L: 0.17; 4L: 0.22 cm2 V1 s1).179 Using HfO2 with a high dielectric constant as the dielectric layer, the mobility of monolayer MoS2 can be increased to 200 cm2 V1 s1.172 The in-plane resistivity is in the range from 0.3 104 to 4 104 U ,1 for exfoliated MoS2.170 The resistivity of the CVD-prepared MoS2 layer is from 1.46 104 to 2.84 104 U ,1, i.e. about two orders of magnitude This journal is ª The Royal Society of Chemistry 2013

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Feature Article

Journal of Materials Chemistry C

higher than CVD-prepared graphene (125 U ,1).180 With excellent electrical properties layered MoS2 becomes a promising candidate for tunable nanoelectronics. Photoluminescence property. Due to the quantum connement eﬀects, the electronic structure and optical properties of the MoS2 nanostructure, such as nanoplates181 and nanotubes,182 are diﬀerent from those of the bulk form. Splendiani et al.177 explored the photoluminescence of monolayer MoS2, and revealed that the luminescence spectrum shows two strong emission bands centered at 627 nm and 677 nm. The photoluminescence intensity was the strongest for monolayer MoS2, which is due to an indirect to direct bandgap transition in the delectron system. Eda et al.47 systemically studied the optical properties of layered MoS2, as shown in Fig. 10(a) and (b). The absorption spectrum of MoS2 showed two absorption bands centered at 600 and 700 nm. The absorption of MoS2 is dominated by two prominent transitions which arise from the directgap transitions between the maxima of the split valence band (v1 and v2) and the minimum of the conduction band (c1), both of which are located at the K point of the Brillouin zone (Fig. 9). The splitting arises from the combined eﬀect of interlayer coupling and spin–orbit coupling.35 With the reduction of the thickness of MoS2 lms, the absorption intensities decrease. The emission spectrum consists of one major peak at 660 nm and one minor peak at 610 nm. The emission intensity gradually decreases with increasing lm thickness, which is due to the weak interlayer coupling between the restacked MoS2 sheets.35,183 Similar results were also reported for mechanically exfoliated MoS2,35 where the PL quantum yield dropped steadily

with increasing thickness N ¼ 1–6 (Fig. 10(c) and (d)). The PL spectrum of monolayer MoS2 is centered at 1.90 eV, which is attributed to a direct-gap luminescence. Few-layer samples display multiple emission peaks. The peak coinciding with that of the monolayer emission peak shis to the red and broadens slightly with increasing thickness. Another broad feature systematically shis to lower energies, approaching the indirect-gap energy of 1.29 eV, and becomes less prominent with increasing thickness.35 A slight red shi in both the absorption resonance and photoluminescence energy with increasing lm thickness was found.35,47 The small shi (20 meV) is consistent with the fact that the direct gap is only slightly sensitive to the quantum connement eﬀect due to the large electron and hole mass around the K point. The diﬀerent gap is only 20 meV between mono- and bi-layer MoS2.35 The direct gap of about 1.8 eV occurs between c1 and v1 at the K point of the Brillouin zone (transition A in Fig. 9). Whereas, the maximum v1 and minimum c1 are located at the G point and along the G–K direction forming an indirect gap of 1.29 eV.35 The photoluminescence intensity of layered MoS2 can be enhanced by Ag@SiO2 core/shell composites due to a metal-enhanced uorescence.184 These features of MoS2 make it attractive for novel electronic and optoelectronic devices such as solar cells, and light-emitting diodes. Elastic properties. Theoretical studies reported the in-plane stiﬀness and Poisson's ratio calculated in the harmonic elastic strain range to be 123 N m1 and 0.25, respectively, indicating that monolayer MoS2 is much soer than graphene.185 Bertolazzi et al.186 found that the in-plane elastic modulus of monolayer MoS2 is 180 60 N m1, corresponding to an eﬀective Young's modulus of 270 100 GPa, while for bilayer MoS2 the values are 260 70 N m1, 200 600 GPa, respectively. The membranes were broken when the averages of maximum stress values for monolayer and bilayer MoS2 are 15 3 and 28 8 N m1, respectively (corresponding to 22 4 GPa for a monolayer and 21 6 GPa for a bilayer). Gomez et al.187 studied the elastic properties of freely suspended MoS2 nanosheets, with thicknesses ranging from 5 to 25 layers. The Young's modulus and pre-tension were reported to be E ¼ 0.33 0.07 TPa and T ¼ 0.13 0.10 N m1, i.e. higher than those reported by Bertolazzi et al.186 The Young's modulus of bulk MoS2 is 0.24 TPa. This discrepancy is attributed to the presence of stacking faults. The low pre-tension and high elasticity of MoS2 make it suitable for application as a reinforcing element in composites and applications in exible semiconductor materials.

Fig. 10 Absorption (a) and photoluminescence (b) spectra of MoS2 thin ﬁlms with average thicknesses ranging from 1.3 to 7.6 nm. Insets of (a) and (b) show energy of the A exciton peak as a function of average ﬁlm thickness. The peak energies were extracted from the absorption and photoluminescence spectra in the main panel, respectively. (c) Normalized PL spectra by the intensity of peak A of thin layers of MoS2 for N ¼ 1–6. Feature I for N ¼ 4–6 is magniﬁed and the spectra are displaced for clarity. (d) Bandgap energy of thin layers of MoS2, inferred from the energy of the PL feature I for N ¼ 2–6 and from the energy of the PL peak A for N ¼ 1. The dashed line represents the (indirect) bandgap energy of bulk MoS2. Reprinted with permission from ref. 47 (a and b) and 35 (c and d). Copyright 2011, American Chemical Society (a and b) and 2010, American Physics Society (c and d).

3.3

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Synthesis

Similar to h-BN, dichalcogenides are also mechanically exfoliated from a piece of natural crystalline sample, e.g., with a Scotch tape. Novoselov et al.37 prepared MoS2 and NbSe2 thin layers by mechanical exfoliation. And mono-, bi-, tri-, and quadrilayer MoS2 akes can be obtained.185 The intrinsic properties of MoS2, such as mobility, Raman scattering and photoluminescence of monolayers, are studied.35,37,87,172,178,188,189 The transformation of MoS2 from an indirect gap (as multilayer) to direct gap (as monolayer) semiconductor was also found.35

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However, this method limits its application in a commercially viable device due to low-yield. Chemical exfoliation. The solution-based exfoliation of layered materials is a promising route for producing 2D crystals in a large scale to realize their unique properties in practical applications.190,191 Lithium intercalation and exfoliation were rst used to prepare layered MoS2,192 as well as WS2.193,194 Intercalation and exfoliation of MoS2 were realized in two steps. First, commercial raw MoS2 was soaked in 1.5 equivalents of butyllithium (n-BuLi, 1.6 M in hexane) and kept in argon atmosphere for a week. Then the LiMoS2 product was exfoliated in water into single layers through the redox reaction. The reactions are described as follows: MoS2 + n-BuLi / LiMoS2 + 1/2C8H18 LiMoS2 + H2O / (MoS2)exfoliated

layers

+ LiOH + 1/2H2

(1) (2)

Gordon et al.195 obtained single layers of the transition-metal dichalcogenides WS2, MoS2, and MoSe2 by this method. The exfoliated MoS2 sheets were found to be 300–800 nm in lateral dimensions and exhibited a typical thickness of 1–1.2 nm.47 Annealing is benecial in preventing the phase transformation, which can improve structural uniformity and reduce local strains. Aer annealing at temperatures above 200 C, MoS2 was predominantly in the 2H phase, and the phase fraction reached 0.95 at 300 C. The intensities of absorption and photoluminescence were also improved due to the reduction of the metallic 1T phase component.47 Smith et al.196 and Zhou et al.197 utilized sonication-assisted exfoliation of two-dimensional semiconductors in water and in ethanol/water, respectively. The process is quick, easy, and insensitive to ambient conditions. While the degree of exfoliation is inferior to ion-exfoliated dispersions, this is more than compensated by the versatility of the procedure.196 During exfoliation, polymers can be added for steric stabilization due to the adsorption of polymer chains onto the surface of nanosheets in a solvent environment.198 Coleman et al.42 and Cunningham et al.190 initially sonicated commercial MoS2 and WS2 powders in a number of solvents with varying surface tensions and showed the best 20 solvents for each material. This method can be introduced to BN and other TMDs in a number of solvents. Likewise, all the layered materials such as transition metal oxides are also expected to be exfoliated via this technique.42,190 Hydrothermal synthesis. Hydrothermal and solvothermal methods have been successfully used to synthesize transition metal dichalcogenide nanostructures.199–202 Single-layer TMDs can also be synthesized by these methods.203 Tian et al.204 obtained molybdenum disulde nanotubes and nanorods by a hydrothermal reaction with MoO3 and KSCN at a low temperature. Rao et al.38,39 expanded it to layered MoS2 and WS2. They achieved ve or fewer layers of MoS2 and WS2. The interlayers of MoS2 and WS2 are 0.68 nm and 0.9 nm thick, respectively, which are slightly larger than the interplanar distance for the (002) plane of bulk MoS2 and WS2.38 During reaction, a part of the S

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atoms in SCN changed into SO42 and reduced Mo(VI) to Mo(IV), while the other part of S contributed to form MoS2 with Mo. At the same time, CO32 was also formed, and the alkaline gas released during the reaction revealed the existence of NH3.204 Thus, the possible reaction for the synthesis of MoS2 could be presented as follows:204 4MoO3 + 9SCN + 14H2O / 4MoS2 + SO42 + NH4+ + 4CO32 + 8NH3 (g) + 5CO2 (g) (3)

Chemical vapor deposition. Large-area synthesis of few layer MoS2 is of great demand for the preparation of high-performance devices. CVD can provide large-area, high-quality and well-controlled layered MoS2 thin lms. Zhan et al.180 utilized sulfur vapor reacting with Mo to synthesize layered MoS2. Sulfur was introduced and reacted with a thin lm of Mo on a substrate at 750 C forming two- and three-layered MoS2 lms. Shi et al.205 synthesized MoS2 thin lms on graphene substrates by CVD. (NH4)2MoS4 was rst dissolved in N,N-dimethylformamide (DMF), and by bubbling the organic solution with argon (Ar) gas, the DMF/(NH4)2MoS4 vapors were carried into the growth chamber where it was adsorbed onto the surface of the graphene substrate. The authors procured thicker MoS2 akes with typical thicknesses ranging from 10 to 50 nm. Utilizing (NH4)2MoS4, Liu et al.206 reported that a high temperature annealing of a thermally decomposed ammonium thiomolybdate layer in the presence of sulfur could produce largearea MoS2 thin layers. The lms were uniformly at with a thickness around 2 nm. The eﬀective eld-eﬀect mobility for the MoS2 could be up to 6 cm2 V1 s1, which is comparable with the previously reported data (0.1–10 cm2 V1 s1) for mechanically exfoliated MoS2.37 3.4

Application

Due to their excellent properties, TMDs can be applied in many elds. The excellent mechanical properties of TMDs also suggest their prospective use as llers to reinforce plastics and in exible semiconducting applications.186,187 TMDs are promising materials as electrodes in lithium-ion batteries and in supercapacitors196,207 and as catalysts for dehydrosulfurization.204,208 TMD thin lms can also be used in electronic and optoelectronic devices.172,173,209–222 Radisavljevic et al.172 rst fabricated a single-layer MoS2 based transistor with a HfO2 gate dielectric, as shown in Fig. 11. The single-layer MoS2 mobility is at least 200 cm2 V1 s1, i.e. similar to that of graphene nanoribbons, and this transistor shows a room-temperature current on/oﬀ ratio of 1 108 and ultralow standby power dissipation. Zhang et al.210 fabricated an electric double layer transistor with a thin ake of MoS2. They achieved a high on/oﬀ ratio (>200) for FET operation with both electron and hole carriers, as well as a high hole mobility of up to 86 cm2 V1 s1, i.e. twice the value of electron mobility. The density of the accumulated carriers reached 1 1014 cm2, which is one order of magnitude larger than that of conventional FETs with solid dielectrics.37 Late et al.211 investigated the hysteresis in singlelayer MoS2 FETs. They observed hysteretic and transient

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Fig. 11 (a) 3D schematic view of a MoS2 monolayer transistor. (b) Roomtemperature transfer characteristic for the FET with a 10 mV applied bias voltage Vds. Backgate voltage Vbg is applied to the substrate and the top gate is disconnected. Inset: Ids–Vds curve acquired for Vbg values of 0, 1 and 5 V. Reprinted with permission from ref. 172, copyright 2011, Nature Publishing Group.

behaviors in conductance characteristics, which are due to absorption of moisture and high photosensitivity of MoS2. By uniformly encapsulating the as-made devices with an amorphous Si3N4 coating, nearly “hysteresis-free” FETs with enhanced mobility could be obtained. Bulk WSe2 FETs exhibit an intrinsic hole mobility of up to 500 cm2 V1 s1 and a poor Ion/Ioﬀ ratio of less than 10 at room temperature.223 However, for monolayerWSe2 based FETs, the eﬀective hole mobility is up to 250 cm2 V1 s1, subthreshold swing is 60 mV dec1, and Ion/Ioﬀ reaches over 106 at room temperature.224 Layered WS2 based FETs exhibit ambipolar behavior and a high (105) on/oﬀ current ratio.225 It can also be used as an ionic liquid-gated transistor due to its virtually perfect electrostatic coupling between the ionic liquid gate and the transistor channel.219 These results promote layered transition metal dichalcogenide semiconductors as a new competitor to graphene, as well as to traditional semiconductors in a variety of applications, such as FETs. For monolayer MoS2, the photoconductivity increases about 3 orders of magnitude compared with that of bilayer MoS2, which is due to the onset of optical absorption from the direct band edge.35 The controllability of the bandgap may also be used to optimize the application of MoS2 in phototransistors.173,213 Phototransistors fabricated with a single-layer MoS2 exhibit n-type semiconducting properties. Photocurrent is generated only below the wavelength of 670 nm, indicating that the photon energy must be greater than the energy gap (1.83 eV) of single-layer MoS2, which is 676 nm in wavelength. Photocurrent generation and annihilation can be switched within 50 ms. The photoresponsivity from the single-layer MoS2 phototransistor is up to 7.5 mA W1 under illumination with a low optical power (Plight, 80 mW) and a medium gate voltage (Vg, 50 V).173 Importantly, the obtained photoresponsivity from the single-layer MoS2 was better than that of a single-layer graphene based FET (1 mA W1).17 Lee et al.226 reported top-gate phototransistors based on a few-layered MoS2 nanosheet. The phototransistor with a triple-layer exhibited improved photodetection capabilities for red light, while those with single- and double-layers were more useful for green light detection, which is due to signicant energy bandgap.226 With excellent optoelectronic properties (photogeneration and photoresponse in a broad range from UV to near-IR) the MoS2 layer can be applied in optical sensors as a potential alternative to the conventional Si and GaAs based photodetectors.

This journal is ª The Royal Society of Chemistry 2013

Journal of Materials Chemistry C With better adsorption of gas molecules and lower electrical noise and lower detection limits, graphene has been applied in high performance electronic sensors.14,15,18,19 As a semiconducting analogue of graphene, MoS2 is also expected to be a potential candidate for sensing applications. The single- and multilayer MoS2 FETs were used to detect the adsorption of NO. The single-layer MoS2 FETs show a rapid and dramatic response upon exposure to NO, but its current is unstable. The FET sensors based on bi-, tri-, and quadri-layer MoS2 exhibit both stable and sensitive responses down to a detection limit of 0.8 ppm of NO.179 Flexible thin-lm transistors (TFT) with layered MoS2 can be used as high-performance, easily-operable and robust gas sensors for the NO2 detection, which is superior to the reduced graphene oxide-based device.227 The sensitivity can be improved 3 times by, e.g., a functionalization of a MoS2 thin lm with Pt nanoparticles, although the response and recovery time is relatively long (>30 min) compared to the rGO TFT. However, this disadvantage can be ignored when it is used as an electronic sensor at a higher temperature.

4

Other two-dimensional semiconductors

4.1

Hybridized BCN

Engineering graphene bandgaps to improve its semiconducting properties for fabricating high-performance graphene devices has attracted great interest. Graphene with C atoms substituted by B and N atoms exhibits p-type and n-type semiconducting electronic properties.228–230 Hybridized BxCyNz with varying contents of B, C and N shows various semiconducting hexagonal layered structures, such as BCN, BC2N, BC4N, BC3, and C3N4. A hybrid atomic monolayer h-BNC is an interesting analogue of graphene that would enable the tailoring of physical properties of graphene. Raidongia et al.231 reported a new analogue of graphene, BC1.2N, prepared by the reaction of activated charcoal with a mixture of boric acid and urea at 900 C. The obtained BCN with two to three layers exhibited a larger electrical resistivity than graphene. BCN presented a surface area of 2911 m2 g1 with a high propensity for adsorbing CO2. Ci et al.232 successfully synthesized uniform and continuous h-BNC lms by a thermal catalytic CVD method using methane and ammonia borane. The atomic ratio of B, C and N could be tuned by controlling the experimental parameters. The h-BNC lms mainly consist of two or three layers. h-BCN lms show two absorption edges, indicating two possible domain structures. The optical bandgap was reduced from 5.69 eV to 3.85 eV by increasing the C concentration to 84 at.%. The second absorption edge corresponds to optical bandgaps of 1.62 eV and 1.51 eV, which are from h-BN-doped graphene domains. Electrical conductivity of h-BNC ribbon increases with increasing carbon concentration. The electron and hole mobilities of the h-BNC (40 at.% of C) devices are in the range of 5–20 cm2 V1 s1.232 Two-dimensional eld-eﬀect transistors were fabricated using hybrid hBCN–graphene heterostructures.233 At a supply voltage of 0.6 V, the FETs exhibited an Ion/Ioﬀ ratio larger than 104, an intrinsic delay time of about 0.1 ps, and a power-delay-product close to 0.1 nJ m1.

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Journal of Materials Chemistry C Recently, Niu et al.234 synthesized graphene-like carbon nitride nanosheets by thermal oxidation etching of bulk g-C3N4. The 2D g-C3N4 nanosheets exhibited a high specic surface area of 306 m2 g1 and a small sheet thickness of about 2 nm. The bandgap of the nanosheets was 2.97 eV, i.e. 0.2 eV larger than that of bulk C3N4, which is attributed to the well-known quantum connement eﬀect. As for the electronic properties of these nanosheets, their electron transport ability along the inplane direction was improved and the lifetime of charge carriers was prolonged. Compared with bulk g-C3N4, the photocatalytic activities of the nanosheets were remarkably enhanced in terms of _OH radical generation and photocatalytic hydrogen evolution.234 4.2

Other chalcogenides

GaSe has a hexagonal layered structure with vertically stacked Se–Ga–Ga–Se sheets, which is diﬀerent from the S–Mo–S structure of transition metal chalcogenides. Between the sheets, there are only weak van der Waals interactions with some ionic or Coulomb contributions. GaSe is a p-type semiconductor with an indirect bandgap of 2.11 eV. The bandgap is only 25 meV lower than the direct bandgap, which leads to easy electron transfers through this gap with a small energy. The peak of photoluminescence shows a slight blueshi of 20 meV with a decrease in the number of layers, similar to that of few layered MoS2.35 This blueshi is possibly associated with the modication of a bandgap structure caused by the thickness decrease of layered GaSe. Layered GaSe-based photodetectors showed a fast response of 0.02 s, high responsivity of 2.8 A W1 and high external quantum eﬃciency of 1367% at 254 nm.235 The 2D nanostructure of GaSe can be eﬀectively used in high performance nanoscale photodetectors. Bismuth telluride (Bi2Te3) is a hexagonal layered material comprised of ve atom thick covalently bonded stacks of Te–Bi– Te–Bi–Te within each layer with the sheets held together by the van der Waals forces.236 Like MoS2, Bi2Te3 can be prepared using intercalation chemistry followed by exfoliation to form thin layers, which is due to the weak van der Waals forces.236–238 Layered bismuth telluride is expected to be produced by liquid exfoliation with sonication.42,196

5

Summary and future prospects

In this article, we have briey reviewed the recent progress in the eld of two-dimensional semiconductors including h-BN, transition metal dichalcogenides, and so on. We focused on the synthesis, properties and applications of the 2D layered materials with emphasis on their special properties and applications with their dimension changing from 3D to 2D. Along with graphene, the other two-dimensional semiconductors have received renewed interest in fundamental research and an expansion of application because of their potential to function in next-generation electronics, optoelectronics, sensors, and so on. As a representative of 2D layered materials, graphene has attracted enormous interest in both fundamental studies and potential applications. However, graphene is a zero-gap

2964 | J. Mater. Chem. C, 2013, 1, 2952–2969

Feature Article half-metal, which limits its application in semiconductor devices. Transition metal dichalcogenides have electronic properties varying from metals to wide-gap semiconductors, while h-BN is an insulator. These two-dimensional semiconductors can be considered as the substitutes and complements of graphene in modern electronic and optoelectronic devices. Until now, various synthetic routes for two-dimensional semiconductors have been realized. However, the quality and yield of the products are still far away from the demands of practical application. The synthesis of two-dimensional semiconductors with controlled sizes, shapes, edges, and layers is yet a challenge in expanding the fundamental properties and potential applications. (1) Large-area, high-quality and layer control of two-dimensional semiconductors are crucial to be employed in highperformance devices. CVD is an eﬃcient method to achieve largescale and uniform atomically thin lms. Eﬀects of domains and boundaries should be breached, which limits the use of twodimensional semiconductors. The micro-growth and control mechanisms are still unclear and need to be further developed. (2) High-yield is another key for the requirements of practical applications. Liquid-phase exfoliation can be used to obtain bulk nanosheets, but the shape is not uniform with a random structure, which hampers practical applications. (3) The edge structures, such as zigzag and armchair edges, are of great importance in the properties of the layered materials. The atomic smooth edges must be controlled to tailor the properties of two-dimensional semiconductors for further applications. (4) Functionalization with doping or surface engineering still needs a reasonable and reliable technical route to achieve property adjustment and application expanding. The lack of functional diversity limits the new property investigations and applications of 2D materials. Therefore, controllable synthesis of two-dimensional semiconductors is of great signicance and challenge, and needs to be further explored. A variety of physical and chemical properties of the twodimensional semiconductors have been investigated and developed for electronic applications. The novel properties caused by the 2D atomic layered structure need to be further explored. We expect new and unprecedented functionalities in two-dimensional semiconductors, such as, theoretically, piezoelectric property with an electro-mechanical coupling is attributed to a non-centrosymmetric crystal structure (point group symmetry D3h). With large-surface area, the 2D atomic layer materials can be used as support for catalysts. Their surface modication has a great inuence on their properties. By doping, their electronic structure can be further tuned, which will well introduce interesting characteristics, especially in electronic and optical properties. On the whole, many challenges related to adjustable detailed properties need to be further addressed before practical applications. The twodimensional semiconductors reviewed here have already become a new class of 2D crystals. There are many new scientic problems which need to be overcome. In this new eld, the 2D layered materials provide the challenge along with opportunity to scientists.

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Acknowledgements This work was supported by the Natural Science Foundation of China (no. 61222403), the Jiangsu Introduction Program of High-Level Innovative and Pioneering Talents, Jiangsu Planned Projects for Postdoctoral Research Funds (no. 1101139C), and the Fundamental Research Funds for the Central Universities (no. 4015- 56XIA12004 and 4015-56YAH11047).

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Two-dimensional semiconductors: recent progress and future perspectives

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