Two-dimensional inorganic analogues of graphene: transition metal dichalcogenides.

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Two-dimensional inorganic analogues of graphene: transition metal dichalcogenides

Review

Manoj K. Jana and C. N. R. Rao

Cite this article: Jana MK, Rao CNR. 2016 Two-dimensional inorganic analogues of graphene: transition metal dichalcogenides. Phil. Trans. R. Soc. A 374: 20150318. http://dx.doi.org/10.1098/rsta.2015.0318

New Chemistry Unit, International Centre for Materials Science and Sheikh Saqr Laboratory, Jawaharlal Nehru Centre for Advanced Scientific Research (JNCASR), Bangalore 560 064, India

Accepted: 10 December 2015 One contribution of 12 to a theme issue ‘Fullerenes: past, present and future, celebrating the 30th anniversary of Buckminster Fullerene’. Subject Areas: nanotechnology, inorganic chemistry Keywords: transition metal dichalcogenides, two-dimensional layered materials, graphene analogues, MoS2 Author for correspondence: C. N. R. Rao e-mail: [email protected]

The discovery of graphene marks a major event in the physics and chemistry of materials. The amazing properties of this two-dimensional (2D) material have prompted research on other 2D layered materials, of which layered transition metal dichalcogenides (TMDCs) are important members. Single-layer and few-layer TMDCs have been synthesized and characterized. They possess a wide range of properties many of which have not been known hitherto. A typical example of such materials is MoS2 . In this article, we briefly present various aspects of layered analogues of graphene as exemplified by TMDCs. The discussion includes not only synthesis and characterization, but also various properties and phenomena exhibited by the TMDCs. This article is part of the themed issue ‘Fullerenes: past, present and future, celebrating the 30th anniversary of Buckminster Fullerene’.

1. Introduction The discovery of fullerenes by Kroto et al. [1] initiated a new season of vital interest in low-dimensional materials as well as nanoscience. Carbon nanotubes [2] which are extended fullerenes gave a further thrust to these areas. The more recent discovery of the novel properties of graphene [3,4], the mother of all graphitic carbons, has particularly enthused the scientific community to explore two-dimensional (2D) materials. Fullerenes and nanotubes of layered inorganic materials such as MoS2 have been investigated extensively [5–7]. There is now a surge in research on layered 2D inorganic materials such as transition metal dichalcogenides (TMDCs). The

2016 The Author(s) Published by the Royal Society. All rights reserved.

Owing to anisotropic bonding, individual layers of TMDCs can be obtained by several physical and chemical methods, the choice of synthesis method usually varying with the relevant application. Important physical methods to synthesize single- or few-layer TMDCs comprise micromechanical exfoliation (scotch-tape technique), liquid-phase exfoliation, laser-thinning and sputtering [4,18]. Mechanical exfoliation provides electronic grade monolayers suitable for high-performance devices and for studies on condensed matter phenomena [11,12,19,20], but is limited to short-scale production. Liquid-phase exfoliation via ultrasonication in a solvent medium provides a route to large-scale production of large-area single- and few layers of a number of layered materials for applications in catalysis and electrochemical storage. Liquidphase exfoliation has been employed to successfully synthesize mono- and few layers of several materials such as MoS2 , MoSe2 , MoTe2 , WS2 , TaSe2 , NbSe2 and NiTe2 , h-BN, Bi2 Te3 [21]. Sonication techniques rely on the solvent (or surfactant) to overcome the cohesive energy between the neighbouring layers of the solute and, therefore, as a crude criterion, the surface energies of the dispersing solvent and the exfoliated material should match [21,22]. More specifically, the dispersion of a solute material in a given solvent depends on the minimization of exfoliation energy through optimizing the balance of solute–solute, solute–solvent and solvent– solvent binding energies. The Hansen solubility parameters are related to dispersion (δD ), hydrogen (δH ) and polar bonding (δP ) contributions to the cohesive energy density. The energy of exfoliation is represented by enthalpy of mixing and the latter is given by Hmix = ϕ(1 − ϕ)[(δD,A − δD,B )2 + (δP,A − δP,B )2 + (δH,A − δH,B )2 )], where A, B and ϕ denote solute, solvent and volume fraction of solute, respectively [21]. Hence for minimization of Hmix , all three Hansen solubility parameters of solvent should match those of solute. After screening a large number of solvents, dimethylformadimide (DMF), isopropyl alcohol (IPA) and N-methyl-2-pyrrolidone (NMP) have been shown to be most suitable solvents for dispersing a variety of TMDCs, owing to combination of suitable surface energies and Hansen solubility parameters [21]. Figure 1a,c shows photographs of dispersions of MoS2 , and WS2 in NMP, prepared by liquid-phase exfoliation and the corresponding transmission electron microscope (TEM) images which show transparent ultrathin layers with lateral dimensions spanning hundreds of nanometres. High-resolution TEM (HRTEM) images of single layers obtained by this route show no deviation from the 2H-structure (discussed below) of the bulk counterparts (figure 1c,d). A mixed-solvent strategy was used by

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2. Synthesis and characterization

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rsta.royalsocietypublishing.org Phil. Trans. R. Soc. A 374: 20150318

2D inorganic analogues of graphene constitute a new class of electronic materials and offer exciting avenues for fundamental and applied research in a wide variety of fields such as spintronics and valleytronics, electronics, optoelectronics, photovoltaics, catalysis, sensing and energy storage [8–11]. Of the TMDCs, MoS2 , in particular, has attracted greater attention due to its unique electronic, optical, magnetic and mechanical attributes, with a variety of potential applications [12]. Isolated atomic planes of these 2D materials can be reassembled into designer heterostructures bound by van der Waals interactions, with precise control over the sequence. These van der Waals heterostructures are remarkably complex and exhibit unusual properties and new phenomena [13–16]. Blends of graphene and 2D inorganic layered materials are also found to exhibit novel properties due to synergy [17]. Layered TMDCs with the generic formula MX2 (M = transition metal, X = chalcogen) are van der Waals solids with strong intralayer bonding and weak interlayer bonding. An individual layer of TMDCs constitutes three covalently bonded atomic planes forming a X–M–X sandwich. Quantum confinement of charge carriers and changes in the interlayer coupling and symmetry elements lead to dramatic changes in the electronic structure and thereby the properties of single- and few-layer TMDCs relative to their bulk counterparts [9]. The present article provides a perspective of the recent advances in the area of 2D TMDCs. The aspects covered include synthesis and characterization, electronic structure and properties, Raman spectra, magnetic properties, field-effect transistors, catalysis, Li and Na ion batteries, supercapacitors, valleytronics and trions, giant magnetoresistance (MR) and superconductivity.

(b)

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Figure 1. TEM images of nanosheets obtained by liquid-phase exfoliation. Low-resolution TEM images of (a) MoS2 and (b) WS2 flakes. Insets show photographs of dispersions of MoS2 and WS2 in NMP; high-resolution TEM images of (c) MoS2 and (d) WS2 single layers. The insets show corresponding Fast Fourier transforms (FFT) of the images. Adapted with permission from Coleman et al. [21]. Zhou et al. [23] wherein low-boiling solvent mixtures like ethanol–water in appropriate ratios are used to obtain MoS2 and WS2 monolayers in good yields. The optimum ratios of ethanol to water is determined by the Hansen solubility parameter. A variety of layered crystals including TMDCs, transition metal oxides and BN have been successfully exfoliated in aqueous medium by ultrasonication with sodium cholate as a surfactant to prevent re-aggregation of the dispersed nanosheets. Yao et al. [24] combined low-energy ball milling with ultrasonication for exfoliation of various inorganic layered materials into nanosheets. Ultrasonication has an undesired effect of breaking the nanosheets and reducing their lateral dimensions. However, the proportion of the moderately large (greater than 1 µm) and thin nanosheets can be optimized with a careful choice of the starting mass, sonication time and centrifugation conditions. But the yield of monolayers through liquid-phase exfoliation is not very high. Free-standing nanosheets of VS2 [25] and VSe2 [26] are obtained by refluxing the bulk materials in solvents like formamide. Intercalation of solvent molecules in between the layers causes swelling of bulk VS2 and VSe2 , and causes exfoliation to yield dispersions of nanosheets. Laser-thinning of multilayered MoS2 is another reliable approach to obtain single-layer MoS2 with defined shape and size in addition to retaining the semiconducting properties of the pristine material [27]. Irradiating dispersions of bulk TMDCs (MoS2 , WS2 , MoSe2 and WSe2 ) in DMF by a KrF excimer laser can also produce single- and few-layer flakes [28]. Turning to chemical methods, one of the most effective routes to the mass production of fully exfoliated TMDC nanosheets is the lithium intercalation–exfoliation procedure. It involves soaking of the bulk material in hexane solutions of n-butyl lithium (typically 3–4 days at 100◦ C or one to two weeks at room temperature, for MoS2 ) followed by exfoliation in ultrasonicated water during which profuse evolution of H2 gas occurs [29]. The yield of single-layer TMDCs

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Figure 2. SAED pattern from monolayer MoS2 with (a) 2H and (b) 1T structures, (c) schematic of 2H and 1T structures viewed down the c-axis; HRTEM images of monolayer MoS2 with (d) 2H and (e) 1T structures. The respective left insets in (d) and (e) are the magnified HRTEM images showing coordination around Mo and S atoms (indicated by red and yellow circles, respectively). The respective right insets in (d) and (e) show the intensity line scans along Mo and S atoms; (f ) HRTEM image of the heterojunction of 1T- and 2H-MoS2 phases obtained by Li-intercalation and exfoliation route. The insets show respective SAED patterns. The white dashed line represents the border between the 1T to the 2H phases. (a–e) Adapted with permission from Maitra et al. [35] and (f ) adapted with permission from Eda et al. [36]. is nearly 100%. Single layers of MoS2 [30], MoSe2 [31], WS2 [32], TiS2 [33], TaS2 [34] have been obtained through this method. An important aspect of this route is the transformation of the thermodynamically stable semiconducting 2H phase of Mo(S,Se)2 and W(S,Se)2 into metastable metallic 1T phases [9]. Both 2H and 1T phases are composed of X–M–X (M = metal, X = chalcogen) sandwich structures. The letters, H and T stand for hexagonal and trigonal, respectively, while the digit indicates the number of X–M–X stacks along the c-axis in a single unit cell. Figure 2c shows the schematic representation of 2H- and 1T-MoS2 viewed down the crystallographic c-axis. 2H-MoS2 has trigonal prismatic coordination of Mo and S atoms (figure 2c). The S atoms of upper plane lie directly over S atoms in the lower plane giving rise to AbA BaB type of stacking sequence (the upper and lower case alphabets denote X and M, respectively). In the 1T phase, the stacking sequence is of AbC AbC type with the S atoms in the upper and lower planes being off-set from each other by 30◦ such that the Mo atoms lie in the octahedral voids of the S-layers (figure 2c). Consistently, the electron diffraction pattern of 2H-polytpe shows a hexagonal diffraction pattern of spots (figure 2a), whereas that of 1T phase shows extra spots at 30◦ angular spacing in between the hexagonal spots of the 2H structure (figure 2b). Figure 2d,e shows the HRTEM images of 2H- and 1T monolayers, respectively. It can be seen from magnified HRTEM images (viewed down the c-axis) that for 2H monolayer, each Mo is surrounded by three S atoms (figure 2d, left inset), whereas six S atoms surround a Mo atom in the 1T-structure (figure 2e, left inset) [35]. The metallic 1T phase is induced through charge transfer from Li to the TMDCs which causes a local atomic rearrangement from the 2H to the 1T phase. The 1T phase of nanosheets obtained via Li-intercalation into TMDCs is metastable even after exfoliation through passivation of residual negative charge by water bilayer [9,36,37]. Chhowalla and co-workers [9,36,37] have identified the 2H and 1T phases of MoS2 by carrying out high-resolution scanning transmission electron microscope (STEM) imaging and shown that the samples prepared by Li-intercalation and exfoliation exist as coherent heterostructures of the 1T and 2H phases (figure 2f ). Because 1T- and 2H-MoS2 are metallic and semiconducting, respectively, their coexistence represents novel heterojunctions with possible application in molecular electronic devices. High-angle annular dark field (HAADF) imaging in an aberration-corrected STEM has revealed a high concentration of the zigzag-like superlattices identified as strained metallic 1T phase in the exfoliated WS2 nanosheets. The

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Figure 3. AFM images of Na-exfoliated single-layer (a) MoS2 and (b) WS2 ; TEM images of Na-exfoliated single-layer (c) MoS2 and (d) WS2 . The upper and lower insets show the corresponding SAED and aberration-corrected HRTEM images, respectively; (e) illustration of TMI intercalation process; (f –h) low-magnification and (i–k) magnified side-view TEM images of starting multilayer WSe2 flakes (f ,i), WSe2 intercalated with ethoxide in bilayer arrangement (g,j) and single-layer WSe2 nanosheets (h,k). (a–d) Adapted with permission from Zheng et al. [38] and (e–k) adapted with permission from Jeong et al. [39]. enhanced electrocatalytic hydrogen evolution reaction (HER) activity of WS2 nanosheets has been ascribed to the presence of the tensile strain induced by zigzag-like distortion in the 1T phase [32]. Interestingly, while Mo(S,Se)2 and W(S,Se)2 undergo the transition from the 2H phase to the 1T phase upon Li-intercalation, TaS2 undergoes the reverse transition, i.e. from the 1T phase to the 2H phase [33]. Despite nearly 100% yield of single-layer TMDCs, the Li-intercalation and exfoliation method suffers from drawbacks such as long lithiation time, difficulty in controlling the degree of lithium insertion and submicrometre flake-size. Zheng et al. [38] have reported a two-step expansion and intercalation method to produce large (up to 400 µm2 ) single-layer MoS2 flakes. Firstly, bulk MoS2 is reacted with hydrazine (N2 H4 ) under hydrothermal conditions (at 130◦ C for 48 h) where a part of N2 H4 rearranges in a redox fashion to N2 H+ 5 upon intercalation. The latter being thermally unstable, decomposes to N2 , NH3 and H2 at high temperatures, thereby expanding the MoS2 sheets by greater than 100 times compared with the original volume. Secondly, the expanded MoS2 flakes are intercalated by alkali naphthalenide followed by exfoliation in ultrasonicated water operated at low power to avoid fragmentation of the sheets. Figure 3a–d shows AFM and TEM images of as-exfoliated MoS2 and WS2 monolayers. Sodium naphthalenide has been shown to be most effective for obtaining large-area flakes. High yields of monolayer TiS2 , TaS2 and NbS2 , as well as few-layer TiSe2 , NbSe2 and MoSe2 have been prepared by this route. Jeong et al. [39] reported a tandem molecular intercalation (TMI) strategy (figure 3e) where a short initiator alkylamine intercalate expands the interlayer gap of TMDCs for an efficient intercalation of another long primary alkylamine. Owing to the difference in chain length, a bilayer arrangement with empty spaces between intercalates is adopted to reduce van der Waals force between them and spontaneous exfoliation occurs finally to yield single-layer TMDCs. The TMI process is advantageous in that it is a simple one-step process at room temperature without the need for harsh exfoliation processes involving ultrasonication or H2 generation. While a relatively weaker Lewis base such as alklyamine is effective for group IV and V TMDCs, a stronger Lewis base such as alkoxide is required for group VI TMDCs. The TEM images in figure 3f –k shows the step-wise TMI process during exfoliation of multilayer WSe2 into single layers. By employing appropriate intercalates, single-layer nanostructures of group IV (TiS2 , ZrS2 ), group V (NbS2 ) and VI (MoS2 , WSe2 ) have been successfully prepared.

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(f) Mo S


monolayer 1.0 nm

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Zeng et al. [40] report synthesis of nanosheets of various metal chalcogenides such as MoS2 , WS2 , TiS2 , TaS2 and ZrS2 by electrochemical Li-intercalation and subsequent exfoliation in water. Li-intercalation is carried out by controlled galvanostatic discharge at 0.05 mA current density in a standard battery test system with the layered bulk material as the cathode and the Li-foil as the anode. During the electrochemical discharge, the Li-foil anode is oxidized, giving out electrons that flow towards cathode. Simultaneously, Li-ions migrate through the electrolyte (LiPF6 ) from anode to cathode where they intercalate in between the layers, and get reduced to Li-metal by the incoming electrons. Subsequently, the lithiated material is ultrasonicated in water or ethanol during which, intercalated Li-metal reacts with water or ethanol to form Li(OH) and H2 gas. The latter expand the interlayer distance and cause exfoliation to give dispersions mainly containing single layers. Unlike the case of solution-phase Li-intercalation, the authors did not observe any phase-transition from 2H form of bulk to 1T form of monolayers upon electrochemical Li-intercalation of MoS2 [40]. Insufficient insertion of Li leads to ineffective exfoliation while excess Li-insertion leads to the decomposition of the crystal. Zeng et al. [41] have optimized the cut-off voltage and discharge current needed for the optimum Li-intercalation necessary for preparation of few-layer BN, NbSe2 , WSe2 , Sb2 Se3 and Bi2 Te3 . Another widely employed chemical method for fabricating large, highly uniform and electronic grade TMDC monolayers is chemical vapour deposition (CVD). CVD of TMDCs generally employs four strategies (i) sulfurization or selenization of pre-deposited thin films of metal or metal oxides, (ii) vapour-phase reaction of chalcogen and metal-based precursors, (iii) thermolysis of single-source precursors, and (iv) vapour-phase transport of TMDC powders. A pre-deposited thin film of Mo on SiO2 substrate can be converted to a MoS2 thin film by annealing in the presence of elemental sulfur [42]. However, both single- and few-layer MoS2 coexist on the substrates due to the challenge of forming a uniform metal thin film. On the other hand, single-layer MoS2 is obtained if S instead of Mo is preloaded on a Cu surface such that the sulfur source is available only to generate the first MoS2 monolayer [43]. Wafer-scale uniform MoS2 thin films with controlled thickness have been grown by layer-by-layer sulfurization of a MoO3 thin layer. A MoO3 thin film with the desired thickness is first grown on sapphire substrates by thermal evaporation and then reduced to MoO2 or other Mo forms in a H2 /Ar environment at 500◦ C. Subsequent annealing in a sulfur-rich environment at 1000◦ C leads to the formation of a wafer-scale MoS2 thin layer which can be transferred to arbitrary substrates for electronic device fabrication (figure 4a–c) [44,48]. Large-area WS2 sheets (approx. 1 cm2 ) with controllable thickness have been synthesized using a similar strategy by sulfurization of thermally evaporated WOx thin films [49]. Continuous films of single- and few-layer MoS2 have been prepared by the vapour-phase reaction of MoO3 and S powders as precursors on SiO2 /Si substrates coated with graphene oxide and perylene-3,4,9,10-tetracarboxylate of potassium. The organic coating promotes the growth of MoS2 layers by wetting of the growth surface and lowering the free energy for nucleation [50]. Ternary MoS2x Se2(1−x) nanosheets with tuneable band edge emission have been obtained by Li et al. [45] by a similar vapour-phase reaction of MoO3 with sulfur and selenium powders (figure 4d and e). Few-layer MoS2 has been grown on a graphitic surface by the thermal reduction of amorphous MoS3 in the presence of reduced graphite oxide at 1000◦ C under high-vacuum conditions [51]. However, achieving a complete coverage of the substrate was not possible by this method. Three-layer MoS2 sheets can be grown on a variety of insulating substrates by first dip-coating in ammonium thiomolybdate ((NH4 )2 MoS4 ) solution followed by annealing under Ar/H2 flow at 500◦ C and subsequently at 1000◦ C (figure 4h) [47]. The chemical reaction involved is (NH4 )2 MoS4 + H2 → 2NH3 + 2H2 S + MoS2 . Wu et al. [46] have reported a vapour– solid (versus) growth method for synthesizing MoS2 monolayer on insulating substrates such as Si wafer and sapphire, by physical vapour transport of MoS2 powder at 900◦ C (hot zone) under a low pressure of 20 Torr (figure 4f ,g). The substrates were placed in a cold zone at 650◦ C [46]. Feng et al. [52] have also reported the synthesis of uniformly distributed monolayer MoS2(1−x) Se2x alloys on SiO2 /Si substrates through the direct evaporation of MoSe2 and MoS2 powders at a high temperature of approximately 950◦ C followed by downstream alloying at approximately

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Figure 4. (a) Schematic showing the synthesis and cleavage of MoS2 ; (b,c) the optical images of as-synthesized MoS2 /MoO2 plates grown by annealing in sulfur vapour for 3 h and the transferred MoS2 flakes, respectively. Insets show the corresponding AFM images; (d,e) SEM image of the ternary MoS2x Se2(1−x) nanosheets obtained by vapour-phase reaction of MoO3 with sulfur and selenium powders and PL spectrum of the complete composition MoS2x Se2(1−x) nanosheets, respectively; (f ) optical microscope image of MoS2 crystallites grown on SiO2 /Si by physical vapour transport of MoS2 powders and (g) AFM image of a typical MoS2 monolayer; (h) schematic illustration of the two-step thermolysis of (NH4 )2 MoS4 for the synthesis of MoS2 thin layers and their subsequent transfer on to insulating substrates. (a–c) Adapted with permission from Wang et al. [44], (d,e) adapted with permission from Li et al. [45], (f ,g) adapted with permission from Wu et al. [46] and (h) adapted with permission from Liu et al. [47].

650◦ C. In all the above methods, careful optimization of reaction conditions at several stages in the process is highly desirable to ensure uniformity of the films especially over large area, and defect-free growth for achieving high-quality films. Other chemical strategies include thermal decomposition of precursors and hydrothermal synthesis. Heating a mixture of molybdic acid or tungstic acid and excess thiourea or selenourea (1 : 48 ratio) at 773 K for 3 h yields few-layer samples of MoS2 or MoSe2 and WS2 or WSe2 on a large scale [30,53]. Figure 5a shows the X-ray diffraction patterns of few-layer samples where the (002) reflections are suppressed due to ultrathin nature of few-layer nanosheets. Figure 5b shows a representative HRTEM image of a single-layer MoSe2 synthesized by the above thermal decomposition route. Few-layer MoS2 is obtained by the reaction of MoO3 and KSCN in aqueous solvent at 453 K and few-layer MoSe2 , by the reaction of molybdic acid and selenium metal in aqueous NaBH4 solution at 453 K [30,53]. With the same precursors, few-layer MoS2 and MoSe2 were obtained under microwave conditions by using ethylene glycol as the solvent [28]. Free-standing nanosheets of MoS2 and WS2 can be synthesized by the decomposition of singlesource precursors, such as (NH4 )2 MoS4 and (NH4 )2 WS4 , respectively, in oleylamine at 360◦ C [56]. Mahler et al. [54] have recently reported a colloidal synthesis of 2H-WS2 and distorted 1T-WS2 monolayers in oleylamine using WCl6 and CS2 as W- and S-precursors, respectively.

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Figure 5. (a) X-ray diffraction patterns of bulk and few-layer Mo(S,Se)2 and W(S,Se)2 synthesized by direct thermal decomposition of precursors and (b) representative Fourier filtered HRTEM image of single-layer MoSe2 prepared by above thermal decomposition route; (c) colloidal nanosheets of 1T- and 2H-WS2 synthesized solvothermally; low-magnification TEM image (d), HRTEM image along with FFT pattern as inset (e), SAED pattern (f ) and AFM image (g) of single-layer TiS2 nanosheets obtained through DCCI method. (a,b) Adapted with permission from Rao et al. [11], (c) adapted with permission from Mahler et al. [54] and (d–g) adapted with permission from Yoo et al. [55].

2H-WS2 forms instead of 1T-WS2 when hexamethyldisilazane (HMDS) is added to oleylamine [54]. Figure 5c shows the low- and high-resolution TEM images of 1T- and 2H-WS2 monolayers formed in the absence and presence of HMDS, respectively. Yoo et al. [55] have recently developed a solution-based protocol called ‘diluted chalcogen continuous influx’ (DCCI) for the synthesis of single-layer nanosheets of group IV metal disulfides. The continuous influx of dilute H2 S gas throughout the growth period is a key to obtaining large nanosheets through the exclusive lateral growth processes. 1-Dodecanethiol was used as the sulfur precursor for the slow in situ formation of H2 S which reacts with the metal chlorides (MCl4 , M = Ti, Zr, Hf) to form large single-layer nanosheets of MS2 [55]. Figure 5d shows TEM and AFM images of single-layer TiS2 nanosheets obtained through above DCCI method.

3. Electronic structure and properties Bulk 2H-MoS2 is centro-symmetric (P63/mmc) and is ans indirect-band gap semiconductor. The calculated electronic band structures show that bulk 2H-MoS2 has an indirect-band gap (approx. 1.2 eV) with the valence band maximum (VBM) and conduction band minimum (CBM) at the

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Figure 6. (a) Reflection and (b) photoluminescence spectra of ultrathin MoS2 deposited on quartz substrate. The observed peaks at 670 nm and 627 nm correspond to the A1 and B1 direct excitonic transitions at the K-point of Brillouin zone. The inset in (a) shows the electronic structure of bulk MoS2 with an indirect-band gap of approximately 1 eV and higher energy A1 and B1 direct excitonic transitions at the K-point, indicated by red and blue arrows, respectively; (c) electronic band structure of bulk, bilayer and monolayer MoS2 . The black arrows indicate the lowest energy transitions. Indirect-band gap (along Γ − K) increases with decreasing thickness while direct-band gap at K-point hardly changes; (d) photoluminescence spectra normalized by Raman intensity for MoS2 layers with different thickness, exhibiting a dramatic increase of luminescence efficiency in MoS2 monolayer. Adapted with permission from Splendiani et al. [57].

Γ point and midpoint along Γ –K path, respectively, in the reciprocal space (or k-space). The Γ point lies at the centre, whereas K points are at the six corners of the hexagonal Brillouin zone in the reciprocal space (figure 14a). Besides, a higher energy direct gap (approx. 1.8 eV) exists between CBM and VBM at the K point (figure 6a, inset) [57–60]. The indirect-band gap (along Γ –K) increases with decreasing number of layers, and exceeds the direct-band gap in the limit of monolayer (figure 6c) [57,59,60]. Thus, monolayer MoS2 becomes a direct-band gap semiconductor. The direct-band gap (at the K point) hardly changes with layer thickness because the conduction band states at the K point are primarily composed of strongly localized d-orbitals on Mo atoms, which have minimal interlayer coupling because Mo atoms are sandwiched between two S-planes (S–M–S) in the unit cell. On the other hand, states associated with the indirect-band gap (along Γ –K) originate from a linear combination of d-orbitals on Mo atoms and anti-bonding pz -orbitals on S atoms and hence have strong interlayer coupling and exhibit strong energy dependence on layer thickness [57,59,60]. Confinement of carriers owing to suppressed interlayer hopping in the ultrathin regime results in an increasing indirect-band gap as number of layers decrease. This indirect-to-direct gap crossover from bulk to monolayer is also evident from a sudden increase in photoconductivity at approximately 1.8 eV in the monolayer MoS2 as a consequence of the latter being a direct-band gap semiconductor [60]. Monolayer MoS2 (space group, P-6 m2) has time-reversal symmetry [E ↑ (k) = E ↓ (−k)] but lacks inversion symmetry

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2000

bulk

Raman spectroscopy has been a powerful analytical tool for determining thickness and stacking of 2D TMDC layered materials to study their mechanical and thermal properties and to directly probe and monitor the charge-doping. Group theory predicts 2H-MoS2 to exhibit four first-order Raman modes, E22g , E1g , E12g and A1g with frequencies at 32, 286, 383 and 408 cm−1 , respectively [11,72]. E12g and A1g are the only intense modes which correspond to the in-plane (intralayer) and out-of-plane (interlayer) vibrations, respectively (the inset of figure 7b) [73]. Upon reducing the number of layers, the A1g mode softens and the E12g mode stiffens (figure 7a and b). Within the model of coupled harmonic oscillators, both the A1g and E12g modes are expected to stiffen with increasing number of layers as the interlayer van der Waals interactions increase the effective restoring forces acting on the atoms [76]. While the shift in A1g mode is consistent with the model, the anomalous shift in the E12g mode was ascribed to the dielectric screening of long-range Coulomb interactions which increase with increasing number of layers [73,76]. For 2H-MoSe2 , the A1g vibration is the most intense and occurs at a lower frequency than the E12g vibration [77]. The E12g mode stiffens and the A1g mode softens as the number of layers is reduced [77,78]. In bulk 2H-WSe2 , the E12g and A1g modes degenerate, giving rise to only a single Raman band. When the dimensionality is decreased, this peak splits into two, with the E12g band at a frequency lower than that of the A1g band and with greater intensity [79] Raman bands of mechanically exfoliated monolayer TMDCs show significant temperature dependence. The temperature coefficients (α) of the A1g and E12g modes of MoS2 were found to be

−0.0123 cm−1 K−1 and −0.0132 cm−1 K−1 , respectively [80–82]. The value of α for the A1g mode in monolayer MoSe2 is much smaller (−0.0045 × 10−2 cm−1 K−1 ) than that of monolayer MoS2 due to difference in the strain-phonon modes as revealed by first-principles calculations [77]. When exposed to shockwaves, few-layer MoS2 and other TMDCs undergo morphological changes with

.........................................................

4. Raman spectroscopy

10


[E ↑ (k) = E ↑ (−k)] due to which the spin–orbit coupling (SOC) lifts the spin degeneracy of otherwise twofold degenerate valence bands at the K point into two bands with spin-up and spin-down character (figure 14a) [61,62]. Thus two direct excitonic transitions, namely A1 and B1, are allowed at the K point (figure 6a, inset). Consistently, the experimental reflectance and photoluminescence (PL) spectra of monolayer MoS2 reveal two peaks at 670 nm and 627 nm corresponding to A1 and B1 direct excitonic transitions, respectively (figure 6a and b) [57,63,64]. As a consequence of indirect to direct-band gap transition, PL is significantly enhanced in monolayers of MoS2 relative to its multilayered counterparts (figure 6d). The quantum efficiency of PL in a monolayer MoS2 is about 104 times that of the bulk material [57,60] and the enhanced PL emission from MoS2 monolayer has been attributed to the significant lowering of the intraband relaxation rate [57,60]. On the other hand, PL emission is lost in the metastable metallic 1T-MoS2 monolayers generated by Li-intercalation and the excitonic features emerge only upon annealing up to 300◦ C when the 2H phase is restored [37]. Besides the number of layers, the band gap in group VI TMDCs can be tuned by application of electric field or a mechanical strain. For instance, the indirect-band gap of bilayer TMDCs can be driven to zero at an electric field of 2–3 Vnm−1 applied perpendicular to the layers, allowing for larger band gap tuneability than that in graphene [65]. Under strain, the band gap of mono- and bi-layer MoS2 decreases and the material undergoes an insulator-to-metal transition [66–69]. Chemical stimuli can also modulate the band gap. MoS2 interacts with electron-donating tetrathiafulvalene (TTF), which forms a radical cation by donating an electron. However, MoS2 being p-type, does not interact with electron-accepting tetracyanoethylene (TCE). First-principles calculations show a large reduction in the band gap of MoS2 upon interaction with TTF molecules [70]. Electron doping can be induced by gate voltage in a field-effect transistor (FET) [71], though hole-doping is not possible by this means in contrast with graphene which can be doped by both electrons and holes.

0 x 2 –20 360

layer 1

402

S Mo

layer 2

384

E12g E12g

382

(e)


J1 J2

1T MoS2 E1g

J3 E12g

2H-MoS2

150

200

250

300

A1g

E12g

350

Raman shift (cm–1)

A1g

400

450

Raman intensity (arb. units)

(d)

20

A1g 18

0 1 2 3 4 5 6 thickness (L)

380 400 420 Raman shift (cm–1)

22

Eg-like

100

11

ReS2

Ag-like

bulk monolayer

2nd order

200 300 Raman shift (cm–1)

400

bulk

A¢1

A¢¢1

( f ) 218

138

217

136

3L

216

4L 5L 6L 10 L bulk 100 120 140 160 180 200 220 240 Raman shift (cm–1)

peak position (cm–1)

20

24

404

Eg

134

215

132

214 213

130

212 169 168 167 166 165 164 163 162 161 160

128 122 120 118 116 114 112 110 108

0

5

10 15 no. layers

20

25

Figure 7. (a) Raman spectra of bulk MoS2 and thin layers (nL) of MoS2 ; (b) frequencies of in-pane E12g and out-of-plane A1g Raman bands (left vertical axis) and their difference (right vertical axis) as a function of layer thickness. Inset shows the atomic displacements associated with both the modes; (c) Raman spectrum obtained from bulk (blue) and monolayer (red) ReS2 ; (d) Raman spectra of Li-exfoliated monolayer 1T- and 2H-MoS2 ; (e) Raman spectra of Td-WTe2 flakes of different thicknesses and (f ) variation of peak frequencies as a function of number of layers. (a,b) Adapted with permission from Lee et al. [73], (c) adapted with permission from Tongay et al. [74], (d) adapted with permission from Gupta et al. [31] and (e, f ) adapted with permission from Jana et al. [75]. (Online version in colour.)

reduction in interlayer (002) separation leading to softening of the A1g and E12g bands [83]. Raman spectra of 1T-MoS2 and 1T-MoSe2 prepared by the Li-intercalation and exfoliation method are different from those of respective 2H phases due to change in the symmetry [31]. Figure 7d compares the Raman spectra of 1T-MoS2 with that of the 2H-MoS2 . Besides E12g and A1g modes which are blue-shifted with respect to 2H-MoS2 , new bands of J1, J2 and J3 appear in the Raman spectrum of 1T-MoS2 . √ WTe2 occurs in the orthorhombic distorted 1T structure (or Td phase) with a 3 × 1 superstructure of dimerized zigzag W–W chains running along crystallographic a-axis (figure 15b). First-principles calculations reveal that electronic band structures for bulk and monolayer of Td-WTe2 are rather similar, both being semimetallic in nature [75]. This is in contrast with other TMDCs which exhibit a strong dependence of the electronic structure on the number of layers. There are 33 possible zone-centre Raman active modes for Td-WTe2 , of which five distinct Raman bands are observed around 112, 118, 134, 165 and 212 cm−1 in bulk TdWTe2 (figure 7e). The intense bands at 165 cm−1 and 212 cm−1 are assigned to the A1 and the A1 modes, respectively, based on symmetry analysis and the Raman tensor [75]. In contrast to the 2Hpolytypes of group VI TMDCs, the low crystal symmetry of Td-WTe2 leads to mixing of in-plane (along the a–b plane of the unit cell) and out-of-plane (along the c-axis of the unit cell) components of atomic displacements. The A1 mode involves out-of-plane (z-direction) displacements of Te atoms and in-plane displacements of W atoms while their motion is vice versa in the A1 mode. From the layer-dependent Raman spectroscopy, it was found that the intense A1 band is most sensitive to the number of layers, exhibiting an up-shift of about 4 cm−1 in a 3-layer flake relative to bulk Td-WTe2 , whereas the intense A1 band does not change with the number of layers though both belong to the same A1 symmetry (figure 7f ). In the A1 mode of vibration, the Te atoms of the same plane vibrate in-phase while their motion is out-of-phase in the mode of vibration. Thus, the A1 mode is plausibly more localized to a layer of WTe2 and shows weaker or no dependence on the number of layers [75].

.........................................................

6L 5L 4L 3L 2L 1L bulk

40

26

Raman signal (arb. units)

60

(c)

A1g

406



A1g

E12g

frequency difference (cm–1)

(b) 408

80

peak frequency (cm–1)

(a)

The discovery of defect-induced room temperature ferromagnetism in otherwise non-magnetic graphene has aroused significant attention and led to investigations on layer-dependent magnetism in TMDCs. MoS2 bears striking similarities with graphene such as ferromagnetic ordering along zigzag edges and novel exchange bias phenomenon which makes the former attractive for spintronics applications. Panich et al. [84] reported electron paramagnetic resonance (EPR) investigations on inorganic fullerene-like MoS2 nanoparticles which indicated the presence of a large density of dangling bonds carrying unpaired electrons. The unsaturated metal centres with partially filled d-orbitals render MoS2 and WS2 clusters (Mo6 S12 and W6 S12 ) magnetic [85]. Zhang et al. [86] report that MoS2 exhibits room-temperature ferromagnetism owing to a high density of prismatic edges containing unsaturated Mo and S atoms. Lithiated MoS2 prepared by soaking the bulk MoS2 in n-butyl lithium also exhibits room-temperature ferromagnetism [87]. Upon exposure to a 2 MeV proton beam, MoS2 exhibits room-temperature magnetic ordering and the temperature dependence of magnetization displays ferrimagnetic behaviour with a Curie temperature of 895 K. Proton irradiation can induce formation of isolated vacancies, vacancy clusters, edge states and reconstructions of the lattice which may give rise to room-temperature magnetism [88]. Graphene-like MoS2 is ferromagnetic at room temperature with the fieldcooled (FC) and the zero-field-cooled (ZFC) magnetization diverging from about 300 K [28,89]. Such divergence of the ZFC and FC curves is found in magnetically frustrated systems like spin-glasses where the ferromagnetic and antiferromagnetic domains are randomly distributed. Figure 8a shows temperature-dependent magnetization of few-layer MoS2 . Few-layer MoS2 is ferromagnetic at room temperature, as seen from the hysteresis in magnetization versus applied field in the inset of figure 8a. The coexistence of both ferromagnetic and antiferromagnetic interactions in few-layer MoS2 was evident from the exchange bias behaviour in few-layer MoS2 samples at different applied fields (figure 8b). A negative exchange bias of 15.5 Oe observed at 1T field decreases with the decreasing applied field and reverses to −15.5 Oe at −1T [28]. The existence of ferromagnetism in MoS2 is ascribed partly to the presence of zigzag edges in the magnetic ground state [90]. First-principles calculations show that the MoS2 nanoribbon with armchair-terminated edges is semiconducting with a non-magnetic (spin-unpolarized) ground state, while that with zigzag-terminated edges is metallic with a magnetic (spin-polarized state) ground state [91]. Magnetic force microscopy of few-layer MoS2 nanosheets have further revealed that the nanosheets become non-magnetic beyond a certain thickness [92].

6. Field-effect transistors The relative ease of fabricating complex device structures renders 2D materials attractive for nanoelectronics. Graphene has been widely studied because of its rich physics and very high mobilities. Graphene-based FETs with a charge carrier mobility as high as 106 cm2 V−1 s−1 have been fabricated previously [93]. However, pristine graphene is a zero-band gap material and a finite band gap is required for several applications including transistors. On the other hand, monolayer MoS2 with a large intrinsic direct-band gap (approx. 1.8 eV) is a potential candidate

.........................................................

5. Magnetic properties

12


ReS2 occurs in a distorted 1T structure with dimerized zigzag Re–Re chains. Bulk ReS2 behaves as electronically and vibrationally decoupled monolayers stacked together [74]. Bulk and monolayer ReS2 have nearly identical band structures both being direct-band gap semiconductors (e.g. approx. 1.5 eV). Consequently, in contrast with other TMDCs, the Raman spectrum shows no dependence on the number of layers (figure 7c) and the optical absorption is insensitive to interlayer distance modulated by hydrostatic pressure (dEgap /dP ∼ 0.02 eV per GPa) [74]. Firstprinciples calculations ascribe the decoupling to Peierls distortion of the 1T structure of ReS2 , which prevents ordered stacking and minimizes the interlayer overlap of wave functions. Weak intralayer polarization due to small charge differences between neighbouring Re and S planes is another factor that leads to very weak van der Waals interlayer interaction [74].

(a)

(b)

2.4

13

M (emu g–1)

300 K

–0.004 –4

1.2

0.6

M (×10–3 emu g–1)

FC

0.01 0

–2

0 2 H (kOe)

4

0

–0.01

ZFC 0

50

100

150 200 T (K)

250

300

50

150 H (kOe)

250

Figure 8. (a) Temperature-dependent magnetization (FC and ZFC) of few-layer MoS2 under 100-Oe applied field. The inset shows hysteresis in few-layer MoS2 at 300 K; (b) field-dependent magnetization in few-layer MoS2 at 2 K, showing the dependence of exchange bias on applied field. Adapted with permission from Matte et al. [28]. (a)

(b) HfO2

gates

Vds

Vtg Ids

Au

0.65 nm

SiO2

Cr/Au HfO2

30 nm

Au

SiO2 monolayer MoS2

270 nm

Si leads

Vbg

(c) Vds = 10 mV

–1.0

200 –40 –20 0 20 40 Vds (mV)

m = 217 cm2 V–1 s–1

100 50 0 –10

–5 0 5 back gate voltage Vbg (V)

Vds = 500 mV

Vbg = 0 V

100 mV 10 mV

current Ids (A)

current Ids (nA)

0

250

Ids (mA)

1.0

300

150

(d) 10–4 10–5 10–6 10–7 10–8 10–9 10–10 10–11 10–12 10–13 10–14 10

200 Ids (nA)

350

50 nm

S = 74 mV dec–1

–4

Vds = 10 mV

100

Vbg

0 –4

0 Vtg

–2 0 2 top gate voltage Vtg (V)

4

4

Figure 9. (a) An optical image of a field-effect transistor (FET) fabricated on a single layer of MoS2 shown in the inset; (b) schematic of a cross-sectional view of the FET device; (c) room-temperature transfer characteristics of the FET at an applied bias voltage, Vds of 10 mV. Inset shows the Ids –Vds curve acquired for different Vbg values (0, 1 and 5 V); (d) Ids –Vtg curve recorded for a bias voltage ranging from 10 to 500 mV. bg, back gate; ds, drain and source; tg, top gate; μ, mobility and S, subthreshold swing with unit mV dec−1 , where a dec (decade) represents a 10-fold increase in the drain current, Id . Adapted with permission from Radisavljevic et al. [94]. and can complement graphene in applications requiring ultrathin transparent semiconductors, such as optoelectronic and energy harvesting applications [8,94]. An n-type FET based on singlelayer MoS2 with HfO2 as the top-gate dielectric has been demonstrated to exhibit high current on/off ratios exceeding 108 and FET mobilities as high as about 200 cm2 V−1 s−1 (figure 9), being

.........................................................

1.8

0.1 T 0.5 T 1T –1 T


M (×10–3 emu g–1)

0.004

14 .........................................................


comparable with thin silicon films and graphene nanoribbons [94]. Late et al. [95] have observed hysteresis in back-gated FETs fabricated out of mechanically exfoliated single-layer MoS2 , which increases with increasing relative humidity and under illumination. The hysteresis is ascribed to the trapping states induced by surface-absorbed water molecules and the photosensitivity of MoS2 devices. After passivating the device with a 30 nm thick Si3 N4 layer, the conductivity of the transistor increases by greater than 100 times, and the hysteresis is almost suppressed. Further, the mobility also increases, plausibly due to the improved contact and suppressed Coulomb scattering [95]. Hao et al. [96] studied the effect of chemisorbed oxygen and water on conductance values of back-gated MoS2 transistors. Transistors with high-k Al2 O3 as the top-gate dielectric exhibit high electron mobility of about 517 cm2 V−1 s−1 and the current on/off ratio, greater than 108 [97]. Min et al. [98] report that the single-layered MoS2 FET, top-gated with high-k Al2 O3 , exhibited greater mobility of approximately 170 cm2 V−1 s−1 compared with the double- and triple-layered MoS2 FETs which showed only approximately 25 and approximately 15 cm2 V−1 s−1 , respectively. The degradation of performance with MoS2 thickness was ascribed to the increasing dielectric constant with thickness [98]. Flexible, thin-film MoS2 transistors with a current on/off ratio of 105 were fabricated using ion gel gate dielectrics [99]. Chakraborty et al. [71] prepared top-gated single-layer MoS2 transistors with a current on/off ratio of about 105 and a mobility of 50 cm2 V−1 s−1 and also carried out Raman studies on these transistors. Although MoS2 is p-type, transistors made of 15 nm thick sheets are ambipolar and the measured Hall mobility of holes is greater (86 cm2 V−1 s−1 ) than that of electrons (44 cm2 V−1 s−1 ) suggesting that p-type operation is more favoured [100]. Lee et al. [101] have demonstrated that a FET based on MoS2 prepared by liquid-phase exfoliation shows n-type characteristics with low current on/off ratios of 3–4 and a low mobility of 0.117 cm2 V−1 s−1 . Temperaturedependent electrical transport measurements revealed variable range hopping transport at lower temperatures which can be explained by the Coulomb potential of randomly distributed charges at the MoS2 –SiO2 interface [102]. Integrated small-signal analogue amplifiers were fabricated by connecting in series two top-gated single-layer MoS2 transistors [103]. A FET fabricated by sandwiching few-layer MoS2 as the semiconducting channel between single-layer graphene and a metal thin film exhibited current on/off ratio greater than 103 , and a high current density up to 5000 A cm−2 [104]. Transistors made of mechanically exfoliated MoS2 nanosheets with one- to four-layer thicknesses have been used to detect up to 2 ppm levels of NO with 80% decrease in the channel conductance [105]. He et al. [106] have fabricated flexible transistors with MoS2 as the thin-film channel and reduced graphene oxide (RGO) as the electrode material for sensing NO2 . When Ptdecorated MoS2 was used as the channel, the sensitivity increased by about three times, with a detection limit of 2 ppb [106]. Devices made of electrochemically reduced MoS2 thin films showed good conductivity and rapid electron transfer and were used for the selective detection of glucose and dopamine [107]. Transistors made of few-layer MoS2 are shown to sense humidity and gases such as NH3 and NO2 (figure 10) [108]. The sensing is based on the change in deviceresistivity in the presence of these gases, which occurs due to charge transfer to MoS2 at different applied fields. Phototransistors have been fabricated of single- and bilayer MoS2 to detect green light and of triple-layer MoS2 to detect red light [109]. MoS2 with thickness ranging from 10 to 60 nm was used to detect light in UV-to-near-IR radiation [109]. FETs fabricated using fewlayer WS2 prepared by sonication in IPA exhibit ambipolar behaviour, with a high current on/off ratio of approximately 105 and good photosensitivity to visible light [110]. Back-gated FET of ultrathin MoSe2 obtained by mechanical exfoliation showed n-type characteristics with a mobility of 50 cm2 V−1 s−1 and current on/off ratio exceeding 106 [111]. The temperature dependence of FET mobilities in these devices suggests a strong, phonon-dominated scattering. FETs of single-layer WSe2 with chemically doped source/drain contacts and high-κ gate dielectric exhibited ambipolar characteristics, with large hole mobilities in the order of approximately 250 cm2 V−1 s−1 and high current on/off ratios greater than 106 at room temperature [112,113].

(a)

(b)

15

dry air or N2 mass flow controller

gass chamber

gass outlet

Keithley semiconductor analyser

20 mm

(c) 20

two-layer five-layer

0

S (%)

–20 –40

gas on

–60 –80 –100 500

1000

1500 2000 time (s)

2500

(d) 900 800 700 600 500 400 300 200 100 0

two-layer five-layer

gas on

0

500

1000 1500 time (s)

2000

2500

Figure 10. (a) SEM image of transistor device fabricated of a two-layer MoS2 flake. Inset shows the optical microscope image of two-layer MoS2 ; (b) schematic of experimental set-up used for NH3 and NO2 gas-sensing; (c,d) comparative cyclic sensing performances of two- and five-layer MoS2 flakes with (c) NH3 and (d) NO2 gases (100, 200, 500 and 1000 ppm). Adapted with permission from Late et al. [108].

7. Catalysis MoS2 is a well-known catalyst for the hydrodesulfurization reaction of sulfur-containing hydrocarbons [114–116]. Few-layer MoS2 decorated with Co and Ni nanoparticles has been shown to be a highly efficient catalyst for the hydrodesulfurization of thiophene to butane with a conversion-efficiency of approximately 98% at approximately 375◦ C [117]. Ni-Fe/MoS2 composites have exhibited good catalytic activity for hydrazine oxidation [118]. Hinnemann et al. [119] have shown that MoS2 nanoparticles are good catalysts for the HER with an over potential as low as 0.1–0.2 V. Theoretical studies have revealed that the edge sites [120] and vacancies [121] in MoS2 are catalytically active sites for HER. Enhanced photocatalytic and electrochemical HER activities have been achieved by increasing the surface area of or the density of active edge sites in the MoS2 catalyst [122–126]. With the MoS2 /graphene composite, electrocatalytic HER activity was further enhanced [127] due to a better electrical coupling to the electrode facilitated by graphene [128]. Photocatalytic HER has been carried out with MoS2 as a co-catalyst in the presence of light-absorbing semiconductors such as CdS [129,130], CdSe [131] and TiO2 [132], and dyes such as [Ru(bpy)3 ] and eosin [133]. Incorporation of a small weight per cent of graphene in the composites enhances the activity as graphene can act as an electron sink and promote better charge separation [134]. The charge separation is further improved by forming a p–n junction with p-type MoS2 with n-type N-doped graphene. N-doped graphene composites with MoS2 showed enhanced photocatalytic [35] and electrocatalytic HER activity [135] compared with that of MoS2 /graphene composites. The 1T-MoS2 (or 1T-MoSe2 ) is metallic with t2g (dyx , dzx , dzy ) orbitals of Mo being partly filled with two electrons. An extra electron leads to the stable half-filled configuration of t2g orbitals (figure 11a). Therefore, 1T-MoS2 (or 1T-MoSe2 ) can easily accept an electron from a dye like eosin and catalyse the HER (figure 11b). 1T-MoS2 prepared by Li intercalation and exfoliation indeed

.........................................................

NH3 or NO2 mass flow controller


MoS2 FET

mixer

20 mm

EY–

EY1* EY

e–

dx2 –y2, dz2 dxz, dyz, dyx

e–

H2O/H+

energy (eV)

h –1 sg

100 50

1

0 0

200

5

gh–

1T MoSe2

300

150

10

mo les

EY3*

400

200

time (h)

75 m

(b)

TEOA TEOA+

H2 evolved (mmoles g–1)

dxz, dyz, dyx

1T-MoS2

100 1T-MoSe2

H2

0 0

4

8 time (h)

12

Figure 11. (a) The crystal field induced electronic configuration of (i) 2H-MoSe2 , (ii) Li-intercalated MoSe2 and (iii) 1T-MoSe2 ; (b) schematic showing the plausible mechanism of hydrogen evolution reaction. EY, eosin; EY1 , singlet-state EY; EY3 , tripletstate EY; TEOA, triethanolamine. (c) Time course of hydrogen evolution with 1T-MoSe2 and that of 1T-MoS2 (inset). Adapted with permission from Gupta et al. [31]. (Online version in colour.) showed 600 times higher activity than the few-layer 2H-MoS2 (inset in figure 11c) [35]. 1T-MoSe2 shows even higher HER activity (approx. 62 ± 5 mmol g−1 h−1 ) than that of 1T-MoS2 (figure 11c) [31]. The lower work function of MoSe2 , and the weaker binding of hydrogen to Se than to S at the edge sites of MoS2 are considered to be responsible for the higher activity of 1T-MoSe2 . 1T-WS2 nanosheets prepared by Li-intercalation and exfoliation show high activity for electrocatalytic HER [32,136] which is ascribed to domains of the strained 1T phase. Density functional theory calculations reveal that tensile strain increases the density of states near the Fermi level and facilitates hydrogen binding [32]. 1T-MoS2 also shows better electrocatalytic HER activity than 2H-MoS2 ; oxidation of MoS2 edges revealed that, in addition to the edges, the basal plane of 1T-MoS2 nanosheets is also active for HER [137].

8. Lithium and sodium ion batteries Layered TMDCs (MX2 ; M = Mo, W, Ti, V and X = S, Se) are good host materials for batteries, allowing for rapid intercalation and de-intercalation of guest ions such as lithium or sodium ions between the host layers. The first lithium-ion battery using MoS2 as the anode material was demonstrated by Haering et al. [138]. Chemically exfoliated and restacked MoS2 with an enlarged c-lattice parameter and good surface area exhibits a high charge capacity of 800 mAh−1 g−1 in the first cycle which is retained till 750 mAh−1 g−1 after 20 cycles at a current density of 50 mAg−1 [139]. Electrodes prepared from hydrothermally synthesized MoS2 nanoflakes and nanoplates exhibited capacities of as high as 1000 mAh−1 g−1 [140] and 1062 mAh−1 g−1 [141], respectively. WS2 nanosheets and nanoflakes also show good reversible capacity [142–144]. Julien et al. [145] have shown that 0.6 mol of Li ions intercalates per mole of WS2 . Mesoporous WS2 , prepared using SBA-15 as the template, has a high surface area and exhibits a storage capacity of 805 mAh−1 g−1 at a current density of 100 mA g−1 [146]. Owing to its high surface area and chemical stability, graphene has been used as a commercial anode material [147,148]. Composites of TMDCs and graphene have been investigated extensively for Li-ion storage. MoS2 /graphene composites exhibited a high reversible capacity of approximately 1290 mAh−1 g−1 maintained up to 50 cycles at a current density of 100 mA g−1 . A high specific capacity of 1040 mAh−1 g−1 was retained even at a higher current density of 1000 mAg−1 [149]. MoS2 /graphene composites prepared by different routes show high reversible capacity with good capacity retention [150– 153]. As shown in figure 12a–c, replacing graphene with N-doped graphene in MoS2 /graphene composites further improves the capacity and retention [154]. Increasing vacancies and defects in N-doped graphene appears to promote more Li-ion insertion leading to the observed

.........................................................

500 dxz, dyz, dyx

dz2

250


dx2 –y2, dyx

16

600

dx2 –y2, dz2

ole

(iii)

dx2 –y2, dz2

mm

(ii) dxz, dyz

30

(c) (i)

H2 evolved (mmoles g–1)

(a)

(a)

(b)

17

1800

1600

charge: discharge:

1600 MoS2/N-G

1400

MoS2/N-G

1400

1200

1200

1000

1000

MoS2/G

800

–1

A

600

–1

A

2

600

MoS2

400

g

m 00

800

m 00

5 –1

–1

g

A 0m

g

0

10

100 mA g–1

Ag 100 m

400 200

200 0

10

30 20 cycle number

(c)

40

10

20

30 40 cycle number

50

60

70

160 140

Rf

Re

120 –Zmg (W)

0

50

Rcf Zw

CPE1 CPE2

100 80

MoS2/N-G at different cycles: 5th 20th 40th

60 40 20 0 0

20

40

60 80 100 Zreal (W)

120

140

Figure 12. Comparison of electrochemical performance in half-cells fabricated of MoS2 , MoS2 /graphene (MoS2 /G) and MoS2 /Ndoped graphene (MoS2 /N-G). (a) Specific capacity versus cycle number for MoS2 , MoS2 /G and MoS2 /N-G at a current density of 100 mA h−1 g−1 ; (b) cycling behaviour of MoS2 /N-G electrode at various current densities; (c) Nyquist plots of the MoS2 , MoS2 /G and MoS2 /N-G electrodes obtained using an applied sine wave of 5.0 mV amplitude in the frequency range from 200 KHz to 0.01 Hz. The inset shows the equivalent circuit model of the studied system. Re , Rf and Rcf denote the internal resistance of the test battery, resistance due to SEI film and charge-transfer resistance, respectively. CPE1 and CPE2 represent the constant phase element of SEI film and the electrode/electrolyte interface, respectively. Zw denoted Warburg impedance corresponding to the lithium diffusion process. Adapted with permission from Chang et al. [154].

increase in capacity with an increasing number of cycles (figure 12a) [154]. WS2 /graphene composites also showed improved properties such as high specific capacity at high current densities and good capacity-retention compared with the bare WS2 [155]. Amorphous carboncoated FeS nanosheets synthesized via a solution-phase route exhibited a discharge capacity of approximately 623 mAh−1 g−1 at a current density of 0.1 Ag−1 with good cyclability even at higher discharge rates [156]. David et al. [157] have reported MoS2 /graphene composites as anode materials in Na-ion batteries which showed a stable capacity of 230 mAh−1 g−1 , with a capacityretention up to several cycles, even at high current densities of 200 mAg−1 . Graphene or carbon in the composites enhances their electrochemical performance and cyclability by absorbing the mechanical stress during volume changes accompanying the insertion and de-insertion cycles, by increasing the electrical conductivity and by preventing the diffusion of polysulfides into the electrolyte.

9. Supercapacitors Electrodes fabricated with MoS2 samples containing a high density of nanowalls exhibit an electrochemical double layer capacitance comparable with that of electrodes made of carbon

.........................................................

charge: discharge:


specific capacity (mAh g–1)

1800

(b)

2

MoS2 RGO MoS2-RGO (1 : 2)

current (mA)

–1

MoS2

300

RGO MoS2-RGO (1 : 2) MoS2-RGO (2 : 1) MoS2-RGO (1 : 3)

250 200 150 100 50 0

–0.2

0.6

0

(d )

0.6 MoS2 RGO MoS2-RGO (1 : 2) MoS2-RGO (2 : 1) MoS2-RGO (1 : 3)

0.4

0.2

0

20

60 40 scan rate (mV s–1)

80

100

300 MoS2

270 specific capacitance (F/g)

potential (V vs Ag/AgCl)

(c)

0.0 0.2 0.4 potential (V versus Ag/AgCl)

RGO MoS2-RGO (1 : 2) MoS2-RGO (2 : 1) MoS2-RGO (1 : 3)

240 210 180 150 120 90 60 30

–0.2 0

100

200 time (s)

300

400

0

1

3 4 2 current density (A g–1)

5

Figure 13. (a) Cyclic voltammograms of MoS2 , RGO and MoS2 /RGO nanocomposites at a scan rate of 100 mV s−1 . (b) Specific capacitance of MoS2 , RGO and MoS2 /RGO at different scan rates. (c) Galvanostatic charge–discharge curves of MoS2 , RGO and MoS2 /RGO at a current density of 1 A g−1 . (d) Specific capacitance of MoS2 , RGO and MoS2 /RGO nanocomposites at different current densities. Adapted with permission from Gopalakrishnan et al. [160]. nanotube arrays [158]. Edge-oriented MoS2 nanoporous films synthesized by electrochemical anodization of molybdenum metal in the presence of sulfur vapour showed a capacitance up to 12.5 mF cm−2 from cyclic voltammetry (CV) measurements at a scan rate of 50 mV s−1 and 14.5 mF cm−2 from galvanostatic charge-discharge measurements at a current density of 1 mA cm−2 . These films showed an increase in the capacitance from 2.2 to 10.5 mF cm−2 after 10 000 testing cycles at a current density of 10 mA cm−2 [159]. Supercapacitor performance of composites of MoS2 and RGO of different compositions has been studied [160]. MoS2 /RGO (1 : 2) composite exhibited best results as shown in figure 13. The quasi-rectangular CV curves for MoS2 /RGO electrodes resemble those of an ideal supercapacitor, showing a maximum capacitance of 416 F g−1 with MoS2 /RGO (1 : 2) at a scan rate of 5 mV s−1 (figure 13a). The current increases and the capacitance decreases with scan rate as shown in figure 13b. Figure 13c shows the charge-discharge curves obtained at a current density of 1 A g−1 . The discharge time of MoS2 /RGO (1 : 2) is significantly longer than the other composites, and the nearly symmetrical curve indicates the remarkable charge-storing ability of this composite. The specific capacitance of this composite is calculated to be 249 F g−1 at a current density of 0.3 A g−1 . Figure 13d shows the specific capacitance of electrodes made of MoS2 , RGO and MoS2 /RGO composites obtained at different current densities. Combination of high electronic conductivity of RGO and surface properties of both MoS2 and RGO gives rise to enhanced specific capacitance of MoS2 /RGO composites. The synergistic effect is calculated to be 118% in the case of MoS2 /RGO (1 : 2) composite. Composites of MoS2 and graphene synthesized by cysteine-assisted solutionphase synthesis exhibit a maximum specific capacitance of 243 F g−1 , which is significantly higher that of the individual components. The electrodes exhibit long-term cyclic stability, with

.........................................................

0

350


MoS2-RGO (2 : 1) MoS2-RGO (1 : 3)

1

18

450 400

specific capacitance (F/g)

(a)

(c)

(a)

circular luminescence (arb. units)

(b) 750

S Mo

–K

circular luminescence (arb. units)

K

800 K

left-handed circular excitation right-handed circular excitation

500

Ps + = 32 ± 2%

250 0 –250

Ps – = –32 ± 2%

600 6.5 Å

monolayer bilayer

1.75

2.00

400 monolayer 200

0 bilayer

–500 1.80

1.82 1.84 1.86 1.88 photon energy (eV)

1.90

1.80

1.85 photon energy (eV)

1.90

Figure 14. (a) Schematic of the electronic structure of MoS2 showing six valleys and opposite spin–orbit splitting (SOC) of valence band (VB) maxima at the high symmetry K(-K) points of hexagonal Brillouin zone in reciprocal space. Red and blue surfaces represent VB maxima split by SOC, each of which is associated with up- or down-spin. Green surfaces represent conduction band valleys; (b) polarization resolved luminescence spectra under circularly polarized excitation from a HeNe laser (1.96 eV) at 10 K; (c) circularly polarized components of luminescence spectra from MoS2 bilayers (green) and monolayers (blue) under circular excitation (1.96 eV) at 10 K. Upper left and right insets show, respectively, schematic of a MoS2 bilayer unit cell and the photoluminescence spectra of monolayer (blue) and bilayer (green) MoS2 . (a) Adapted with permission from Xiao et al. [164] and (b,c) adapted with permission from Zeng et al. [165].

only a 7.7% decrease in specific capacitance after 1000 cycles [161]. Composites of MoS2 and polyaniline (PANI) with different compositions exhibit excellent supercapacitor performance [162]. The MoS2 /PANI composites with compositions of 1 : 1 and 1 : 6 showed capacitance values of 417 and 567 F g−1 , respectively, with improved cyclic stability compared with PANI alone. Nanocomposites of MoS2 and polypyrrole prepared by in situ oxidative polymerization show a specific capacitance of about 700 F g−1 at a scan rate of 10 mV s−1 [159].

10. Valleytronics and trions While electronic and spintronics devices generally exploit the charge and spin of electrons, respectively, valleytronics relies on the fact that the conduction (valence) bands of some materials have two or more degenerate minima (maxima), separated in the momentum space [163]. To realize a valleytronic device, it is necessary to produce a valley-polarization by controlling the number of carriers in these valleys. Monolayer MoS2 is ideal for valleytronics with conduction and valence band edges having two degenerate valleys at the corners (K and K points) of the 2D hexagonal Brillouin zone (figure 14a). The direct-band gaps (approx. 1.9 eV) at these two valley points allow for valley-polarization by optical pumping [163–165]. Owing to the broken inversion symmetry, spin–orbit interactions split the valence bands by approximately 160 meV and the spin projection, Sz, is well defined along the c-axis of the crystal with the two bands being of spin down

.........................................................

–G

19


–K

–K

photoluminescence (arb. units)

K

In contrast to the regular layered counterparts, distorted layered ditellurides of Mo and W with zigzag metal–metal chains (figure 15a and b) are gaining considerable attention due to their unique properties such as giant MR and pressure-driven superconductivity. Recently, a giant, unidirectional (along W–W chains) positive MR of 452 700% at 4.5 K in a magnetic field of 14.7 T and a record 13 million per cent at 0.53 K in a field of 60 T (figure 15d), has been reported in single crystals of semimetallic orthorhombic Td-WTe2 (figure 15b) [169]. Even at very high applied magnetic fields, MR in Td-WTe2 does not saturate, which is attributed to a perfect balanced electron-hole resonance. Bulk monoclinic 1T -MoTe2 with Mo–Mo zigzag chains (figure 15a) is semimetallic and also exhibits giant MR of 16 000% in a magnetic field of 14 T at 1.8 K (figure 15c) [168]. Pan et al. [170] have reported pressure-driven dome-shaped superconductivity in Td-WTe2 . Superconductivity appears at a pressure of 2.5 GPa. The critical temperature (Tc ) reaches a maximum of 7 K at around 16.8 GPa and monotonically decreases with increasing pressure [170]. Kang et al. [171] also reported superconductivity in Td-WTe2 arising from a suppressed MR state under pressure. With increasing pressure, MR is gradually suppressed and turned off at a critical pressure of 10.5 GPa, where superconductivity begins to emerge [171]. Pressure-driven superconductivity has also been reported in single crystals of orthorhombic Td-MoTe2 (below 120 K), a low-temperature polymorph of 1T -MoTe2 . A maximum Tc of 8.2 K is observed at 11.7 GPa [172].

12. Other layered chalcogenides Besides group VI-metal dichalcogenides, there are other layered metal chalcogenides of considerable interest. Bi2 Se3 , Bi2 Te3 and Sb2 Te3 are exotic spin Hall quantum phases known as topological insulators (TIs) wherein gapless, metallic surface states exist within insulating bulk band gap. These Bi- and Sb-based TIs are also the state-of-the-art thermoelectric materials which can reversibly convert waste heat into electricity [173]. Bi2 Se3 (Bi2 Te3 or Sb2 Te3 ) is an anisotropic layered material containing quintuple layers (QLs) each of approximately 1 nm thickness and

.........................................................

11. Giant magnetoresistance and superconductivity in distorted transition metal dichalcogenides

20


(E↓) and spin up (E↑) in character [62,164,166]. This broken spin degeneracy together with timereversal symmetry leads to inherent coupling of the valley and the spin of the valence bands in monolayer MoS2 leading to the valley-dependent optical selection rule [163,165]. Consequently, inter-band transitions in the vicinity of the K or K valleys couple exclusively to left or right circularly polarized light, respectively. The circular component of the band edge luminescence is of the same polarization as that of the circularly polarized excitation (figure 14b) [165]. On the other hand, inversion symmetry is preserved in bilayer MoS2 , which in combination with time-reversal symmetry, restores the spin-degeneracy by splitting the fourfold degenerate valence bands into two spin-degenerate valence bands. As valley and spin are decoupled in bilayer MoS2 , the valley-dependent selection rule is not allowed and, therefore, negligible circular polarization is exhibited by bilayer MoS2 under the same conditions (figure 14c) [165]. Tightly bound negative trions, a quasi-particle composed of two electrons and a hole, have been spectroscopically identified in monolayer MoS2 FET [167]. These quasi-particles can be created optically with spin and valley polarized holes. These trions possess a large binding energy (approx. 20 meV), nearly an order of magnitude larger than that found in conventional quasi2D systems like quantum wells. This is due to the significantly enhanced Coulomb interactions in monolayer MoS2 , arising from reduced dielectric screening. The existence of tightly bound trions with dynamically controllable hole valley and spin in monolayer MoS2 gives rise to possibilities of novel many-body phenomena and may impact the development of new photonic and optoelectronic devices, such as photovoltaic cells or optical detectors.

(a)

(b)

21

2H

5 b a

4.3

7

c b

a

1T¢

(c)

(d) MR (%)

1.5

N = 23 8 000 12

13

1T¢-MoTe2

0.5

4

10 H (T)

15

0

2

70 90 300

0

2

4 6 m0H (T)

10

8

0

–2 0.06

5

50

2

2H-MoTe2 0

30

200

0

1.0 Dr (mW cm)

MR (% × 105)

MR (%)

400

T = 1.8 K

16 000

1

0.14 0.10 1/(m0H) (T–1)

2

3

4 5 6 m0H (T)

7

8

9

Figure 15. (a) Crystal structures of 2H- and 1T -MoTe2 and (b) Td-WTe2 ; (c) field–dependent magnetoresistance (MR) of 1T -MoTe2 and 2H-MoTe2 at T = 1.8 K. Inset shows the Shubnikov-de Haas (SdH) oscillations at T = 1.8 K for 1T -MoTe2 ; (d) field-dependent MR in Td-WTe2 at representative temperatures (numbers in kelvins). Upper inset shows the MR at higher temperatures and the lower inset shows the SdH oscillations at 4.5 K. (a,c) Adapted with permission Keum et al. [168], (b,d) adapted with permission from Ali et al. [169]. (Online version in colour.)

composed of five covalently bonded atomic planes [Se2 -Bi-Se1 -Bi-Se2 ]. These QLs are periodically stacked along the crystallographic c-axis by weak van der Waals interactions. Free-standing five-atom-thick Bi2 Se3 single layers can be synthesized via Li-exfoliation of the bulk material [174]. Ultrathin nanodiscs of Bi2 Se3 are obtained by heating Bi(NO3 )3 and Na2 SeO3 in ethylene glycol with a small amount of NH2 OH [175]. Few-layer Bi2 Se3 nanosheets have been prepared by green, ionothermal synthesis, using a mixture of bismuth acetate and selenourea in an ionic liquid solvent, 1-ethyl-3-methylimidazolium tetrafluoroborate ([EMIM][BF4 ]). Ionothermal decomposition of single-source precursor, Bi(Se-C6 H6 N)3 in the same solvent yields few-layer nanodiscs of Bi2 Se3 [176]. Mechanical exfoliation [177] chemical vapour transport [178] and vapour-phase deposition [179] have been employed for synthesis of few-layer Bi2 Te3 nanosheets. Sb2 Te3 nanosheets have been synthesized using SbCl3 , Na2 TeO3 and hydrazine hydrate in ethylene glycol under microwave heating [180]. Single-crystalline nanobelts and nanosheets of Sb2 Te3 have been synthesized by the reaction of SbCl3 and TeO2 with hydrazine hydrate as the reducing agent [181]. A pressure-induced insulator-metal transition has been found in rhombohedral BiI3 (near 1.5 GPa), whose high symmetry parent structure is similar to that of Bi2 Se3 TI. The calculated Born effective charges suggest the presence of metallic states in the structural vicinity of rhombohedral BiI3 [182].

13. Conclusion and future directions The previous sections must clearly bring out the fascinating properties of layered inorganic materials especially when they are made up of single- or few layers. The properties that we

.........................................................

3.48

W


2.8

Te

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Authors’ contributions. The article was written jointly by both the authors. Competing interests. We declare we have no competing interests. Funding. M.K.J. acknowledges CSIR, India for research fellowship.

22


described cover a wide range including optical properties, FETs, GMR, catalysis and energy devices. It is interesting that the discovery of single-layer MoS2 has given rise to new areas in physics, namely valleytronics and trions. We believe that these areas will be explored extensively in the coming years. Many new properties are yet to be explored, as evidenced from new discoveries coming out ever so frequently in the last year or two. A typical example is the unusually high magnetoresistance by WTe2 . We believe that those who are interested in physics and chemistry of materials can expect to have an exciting period of research in 2D inorganic materials in the years to come.

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Cytotoxicity of exfoliated transition-metal dichalcogenides (MoS2 , WS2 , and WSe2 ) is lower than that of graphene and its analogues.

Topological superconductivity in monolayer transition metal dichalcogenides.

Exciton Dynamics in Monolayer Transition Metal Dichalcogenides.

Comparative study of Raman spectroscopy in graphene and MoS2-type transition metal dichalcogenides.

Ultrasound exfoliation of inorganic analogues of graphene.

Electronic and thermoelectric properties of few-layer transition metal dichalcogenides.

Atomic healing of defects in transition metal dichalcogenides.

Three-fold rotational defects in two-dimensional transition metal dichalcogenides.

Exciton radiative lifetimes in two-dimensional transition metal dichalcogenides.

Physical and chemical tuning of two-dimensional transition metal dichalcogenides.

Protein Induces Layer-by-Layer Exfoliation of Transition Metal Dichalcogenides.

Photocarrier relaxation pathway in two-dimensional semiconducting transition metal dichalcogenides.

Chiral topological excitons in the monolayer transition metal dichalcogenides.

Self-Limiting Layer Synthesis of Transition Metal Dichalcogenides.

Covalent functionalization of monolayered transition metal dichalcogenides by phase engineering.

Continuously tunable electronic structure of transition metal dichalcogenides superlattices.

Observing grain boundaries in CVD-grown monolayer transition metal dichalcogenides.

Emerging device applications for semiconducting two-dimensional transition metal dichalcogenides.

Tailoring Nanoscale Friction in MX2 Transition Metal Dichalcogenides.

Unusual stacking variations in liquid-phase exfoliated transition metal dichalcogenides.

Toward low-power electronics: tunneling phenomena in transition metal dichalcogenides.

Two dimensional materials beyond MoS2: noble-transition-metal dichalcogenides.

Fabrication of ultralong hybrid microfibers from nanosheets of reduced graphene oxide and transition-metal dichalcogenides and their application as supercapacitors.

Chalcogen Dependant Interactions of Hairpin DNA with Transition Metal Dichalcogenides, MX2.