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Featuring work from the research group of Professor Jin Zhong Zhang, University of California, US, in collaboration with Hongmei Wang in Jiaxing University and Pengfei Fang in Wuhan University, China.

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Synthesis, properties, and optoelectronic applications of two-dimensional MoS2 and MoS2-based heterostructures The Zhang lab focus on synthesis, characterization, and applications of functional optical and electronic nanomaterials.

See Hongmei Wang, Jin Zhong Zhang et al., Chem. Soc. Rev., 2018, 47, 6101.

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Synthesis, properties, and optoelectronic applications of two-dimensional MoS2 and MoS2-based heterostructures Hongmei Wang,†*a Chunhe Li,†c Pengfei Fang, Jin Zhong Zhang *b

c

Zulei Zhang

a

and

As a two-dimensional (2D) material, molybdenum disulfide (MoS2) exhibits unique electronic and optical properties useful for a variety of optoelectronic applications including light harvesting. In this article, we review recent progress in the synthesis, properties and applications of MoS2 and related heterostructures. Heterostructured materials are developed to add more functionality or flexibility compared to single component materials. Our focus is on their novel properties and functionalities as well as emerging Received 20th April 2018

applications, especially in the areas of light energy harvesting or conversion. We highlight the correlation

DOI: 10.1039/c8cs00314a

between structural properties and other properties including electronic, optical, and dynamic. Whenever appropriate, we also try to provide fundamental insight gained from experimental as well as theoretical

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studies. Finally, we discuss some current challenges and opportunities in technological applications of MoS2.

1. Introduction Molybdenum disulfide (MoS2) is a typical member of a large class of layered transition metal dichalcogenides (TMDs) with the general formula MX2, where M refers to a transition metal a

College of Biological, Chemical Sciences and Engineering, Jiaxing University, Jiaxing 314001, China. E-mail: [email protected] b Department of Chemistry and Biochemistry, University of California, Santa Cruz, California 95064, USA. E-mail: [email protected] c Department of Physics and Hubei Nuclear Solid Physics Key Laboratory, Wuhan University, Wuhan 430072, China † These authors contributed equally to this work.

Hongmei Wang

Hongmei Wang received her BSc degree in 2001 and MSc degree in 2004 from China University of Geosciences (Wuhan), China. In 2007, she received her PhD degree from Wuhan University, China. Then, she had her visiting scholar experience from Wuhan University and University of California, Santa Cruz. Currently, she is a professor at Jiaxing University. Her research interests focus on semiconductorbased nanomaterials for energy conversion and storage, and photocatalysis.

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and X refers to a chalcogen (S, Se, or Te).1–4 Similar to graphene,5 single layers of the sandwich structure S–Mo–S can exist.6,7 Multiple layers with weak interlayer interactions can stack up to form bulk solids, similar to graphite composed of graphene layers. In contrast to graphene that has no bandgap between its valence and conduction bands,8–10 single-layered TMDs possess a bandgap and exhibit interesting electronic and optical properties that are ideal for optoelectronic and electronic applications.11–17 In particular, among the various TMD materials that exhibit stable 2D crystalline structures, such as tungsten diselenide (WSe2),18–22 molybdenum diselenide (MoSe2),23–29 tungsten

Chunhe Li

Chunhe Li received her BSc degree in physics from Zhoukou Normal University, Zhoukou, China in 2014. She is currently a PhD student in Prof. Fang’s group at Wuhan University, Wuhan, China. Her current research interests include design, synthesis, characterization and properties of transition metal dichalcogenide nanomaterials for energy and environmental applications, including photocatalytic hydrogen production, photocatalytic reforming of biomass, and photoelectrochemical cells.

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disulfide (WS2),30–34 and molybdenum disulfide (MoS2),30,35–38 MoS2 has received considerable attention due to its unique optical,39,40 electronic,41,42 and other characteristics, including its size-dependent bandgap.36,43,44 The Mo and S atoms in MoS2 have strong ionic bonding, and the different layers of MoS2 interact via van der Waals (vdW) forces.45 Bulk TMD crystals are composed of vertically stacked multilayers, separated by B6.5 Å for each layer.2,46 TMD layered-materials can be semiconducting or metallic, and have been explored widely in various forms including bulk,47 few-layers,48–51 nanotubes,52,53 or nanoparticles,54–56 with applications including catalysis and low-friction coatings. The recent new excitement in MoS2 was sparked by the possibility of generating and characterizing single layered or few layered structures with unique and interesting electronic and optical properties that are unlike their corresponding bulk

Pengfei Fang received his BSc degree in Chemistry in 1994 and his PhD in polymer chemistry and physics in 1999 from Wuhan University, Wuhan, China. His recent research interests focus on nanoscale functional materials and their utilization in various energy and environmental applications.

Pengfei Fang

Zulei Zhang received his BSc degree in Chemical Engineering from Jiaxing University, Jiaxing, China, in 2007 and his MSc degree in Applied Chemistry from Jiangsu University, Zhenjiang, China in 2009. In 2017, he received his PhD degree in Materials Science and Technology from East China University of Science and Technology, Shanghai, China. His recent research interests focus on synthesis, characterization, Zulei Zhang and application of functional polymers including imprinted polymers and advanced adsorbents, particularly in the areas of selective recognition and separation of environmental pollutants.

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phases. Bulk MoS2 has a direct bandgap of 1.8 eV. Based on the atomic stacking order, MoS2 can exist in four crystal phases including 3R (3-layer unit cell, rhombohedral, ABCABC stacking), 2H (2-layer crystal cell, hexagonal, ABAB stacking), 1T (1-layer crystal cell, trigonal, AA stacking), and 1H (1-layer unit cell, trigonal, AA stacking).50,57 Due to the large band-edge excitation of the metal centered d–d transitions, their unique electronic properties have potential applications in emerging technologies including solar cells,58–63 detectors,64–69 and sensors.70–74 In this review, we first discuss the latest progress in the synthetic methods for MoS2 synthesis, including bottom-up and topdown techniques. Then, we summarize the important properties of the materials, including exciton and charge carrier dynamics. In addition, we highlight the optoelectronic applications of the materials including photocatalytic hydrogen production, photoelectrochemical conversion, solar cells, photodiodes and phototransistors. Finally, we give our perspective on the current challenges and opportunities in the research and development of MoS2 layers. Due to the very large number of publications in this fast growing field, the review is not meant to provide a comprehensive coverage of all relevant work, but rather to illustrate some of the most interesting developments in the field, with a focus on light-harvesting applications. The structure of this review is summarized in the following scheme (Fig. 1).

2. Material synthesis Different synthesis methods can generate different 2D MoS2 materials with varied properties and applications. Therefore, it is important to first discuss the exfoliation and synthesis of

Jin Z. Zhang received his BSc degree in Chemistry from Fudan University, Shanghai, China, in 1983 and his PhD in physical chemistry from the University of Washington, Seattle, in 1989. He was a postdoctoral research fellow at the University of California Berkeley from 1989 to 1992. In 1992, he joined the faculty at UC Santa Cruz, where he is currently a full professor of chemistry and biochemistry. Jin Zhong Zhang Zhang’s recent research interests focus on design, synthesis, characterization, and exploration of applications of advanced materials including semiconductor, metal, and metal oxide nanomaterials, particularly in the areas of solar energy conversion, solid state lighting, sensing, and biomedical detection/therapy. He has authored 310 publications and three books. Zhang has been serving as a senior editor for JPC(L) published by ACS since 2004. He is a Fellow of AAAS, APS, and ACS. He is the recipient of the 2014 Richard A. Glenn Award of the ACS Energy and Fuel Division.

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Fig. 2 (a) Optical micrographs showing examples of monolayer MoS2 flakes obtained by anodic bonding; (b) atomic force microscopy (AFM) topography image of a MoS2 one-, two-, and three-layer flakes. Reproduced with permission from ref. 83, Copyright 2012, Institute of Physics Science.

Fig. 1

A summary of the structure of the review.

MoS2 layers. Two general synthesis methodologies for MoS2 layers have been established: (1) top-down methods, including mechanical exfoliation and chemical exfoliation,75,76 and (2) bottom-up methods, such as chemical vapor deposition77,78 and chemical synthesis.5,79,80 2.1.

Mechanical exfoliation

The mechanical exfoliation method stemmed from the simple ‘‘Scotch tape method’’ commonly used to obtain layered graphene.81 In this process, bulk phase MoS2 is attached to an adhesive tape, repeatedly exfoliated, followed by attaching to a substrate.39,41,82 This results in ultrathin layered MoS2 transferred onto the substrate.46 The method can produce high quality monolayers. However, it is not appropriate for large-scale production because of its low production yield and limited ability to control flake thickness, shape, and size. In 2012, Gacem’s group reported a general technique for generating high quality 2D materials (as shown in Fig. 2), namely anodic bonding.83 Based on this technique, high quality sheets, in most instances much larger than those acquired using ‘‘Scotch tape’’ microcleavage, can be both obtained with high yield and readily transferred to other lacey supports. Besides, these sheets are pure and have good optical properties compared with those samples on glass support so that they are easy to identify. This method can in principle be used to generate nanosheets for any 2D materials. 2.2.

Liquid exfoliation

Liquid exfoliation (LE) is another technique for generating large quantities of single- and few-layered MoS2 nanoflakes, including an ion intercalation method and solvent exfoliation.84–86 In 2011, Zhang et al. proposed a rapid and easily controllable approach to exfoliate semiconducting layers.87 With an

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electrochemical lithiation discharge course, bulk phase MoS2 can be intercalated with lithium.88 With succeeding ultrasonication, a high yield of monolayer MoS2 was obtained and a large proportion of the ultimate products is metallic 1T MoS2.89–91 Solvent exfoliation was first proposed in 2011, which can achieve mainly 2H MoS2 from exfoliating dispersive bulk MoS2 slabs in organic solvents.92 O’Neill et al. improved this approach through optimizing the sonication time, eventually achieving a higher layer content of approximately 40 mg mL 1 and substantially larger layer size.93 LE can produce a suspension of single-layers, which can be used for composite construction. However, the sonication process can produce defects in layered construction and reduce the layer size down to thousands of nanometers, restricting their device applications. One challenge with liquid-generated MoS2 flakes is their subsequent separation, which is dependent on the layer size and thickness. For example, one study demonstrated thickness sorting of MoS2 into monolayers with strong photoluminescence (PL) using a block copolymer dispersant in aqueous solution.94 Another common approach to separation is centrifugation, which usually follows the generation of nanoflakes from sonication of MoS2 powders in solution.93 While these methods will narrow down the size and thickness distribution, it is challenging to achieve single size or single layer in general.95 2.3.

Chemical vapor deposition

Chemical vapor deposition (CVD) has been utilized to grow MoS2 thin layers on insulating supports, for instance, SiO2 or sapphire.35,96,97 Nevertheless, compared to graphene, it is more challenging to acquire crystallographic MoS2 via controlling the number of layers by CVD. At present, to generate MoS2 layers, precursor reagents such as Mo,35 (NH4)2MoS4,38 or MoO3,98 are firstly deposited on supports as thin layers prior to the sulfurization or thermal decomposition of the precursors at elevated working temperature to control the structure of MoS2. According to reports, atomically thin layers of MoS2 can be synthesized via a two-step thermal evaporation–exfoliation method, which has the advantage of only requiring modest conditions.99 Notably, the approach can grow large-sized MoS2 layers; however, the acquired layers are polycrystalline with small crystallite dimensions due to the non-crystalline precursors employed, but it is difficult to control the number of layers. To address such issues, a technique to grow high crystallinity MoS2 layers has been demonstrated by

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and high crystallinity 2D nanomaterials.103,104 There are many reactive sites and few defects in the nanosheets, which have been widely used in lithium battery anodes, electrocatalysis, photocatalysis, and supercapacitors.44,105–108 The major challenge lies in the limited utilization and high selectivity of suitable metal and chalcogen sources, as well as in mastering how to slow down vertical growth while simultaneously accelerating lateral growth. 2.5.

High temperature annealing method

High temperature annealing has been used to make large-sized MoS2 thin sheets with excellent electrical properties. A typical synthesis involves thermolysis of ammonium thiomolybdate under conditions of sulfur existence. Chang et al. described a new approach to precisely regulate the different phases of MoS2 at various calcination temperatures using lithium molten salts.109 After successive hydrolysis of the lithium salts, the precursors can be quickly exfoliated into 2H- or 1T-phased monolayer MoS2, depending on the annealing temperature of the precursors. The thermolysis reaction is given as follows: Fig. 3 (a) Schematics for the synthesis and cleavage of MoS2. MoO2 microplates were synthesized by reduction of MoO3 and then used as template to grow MoS2 by layer-by-layer surface sulfurization. The obtained MoS2 coating was separated from MoO2 and transferred to another substrate with the top layer facing up for further characterizations. 1L, 2L, and nL indicate single layer, bilayer, and n-layer, respectively; (b and c) optical and AFM (insets) images of as-made MoS2/MoO2 plates grown by annealing in sulfur vapor for 3 h and the transferred MoS2 flakes, respectively. Reproduced with permission from ref. 77, Copyright 2013, American Chemical Society.

controlling the number of layers using MoO2 microcrystallites as templates.77 A specific growth process of MoS2 layers is shown in Fig. 3(a). In brief, MoO3 powder is firstly evaporated and reduced by sulfur vapor to generate MoO2, which then nucleates on SiO2/Si substrates and grows into rhomboidal microsheets in a CVD furnace. The MoO2 microsheets are then annealed in sulfur vapor. In the process of annealing, varied numbers of MoS2 layers are obtained by sulfurizing the surface of MoO2 microsheets, according to the annealing duration. Then, a polymethyl methacrylate (PMMA) thin layer is coated on the surface of MoS2/MoO2 microsheets to separate the MoS2 thin flakes and the MoO2 layer, which can then be transferred onto other supports utilizing PMMA-mediated nano-transfer printing. Fig. 3(b) and (c) show photographs of the as-prepared MoS2/MoO2 layers produced by reducing MoO3 and then annealing, the stripped MoS2 layers, and the MoO2 layers retained on the supports, demonstrating successful synthesis of MoS2 layers from MoO2 precursors.

(NH4)2MoS4 + 2LiOH - MoS2 + Li2S + Sm + 2NH3m + 2H2Om (1) The reaction products, S, NH3 and H2O gas, can be volatile at high annealing temperature, where the MoS2 crystallite with Li2S can be extracted. In general, hydrolysis of Li2S into LiOH and H2S is exothermic and easy to take place, therefore, the MoS2 can be readily obtained after putting the reaction products into water. Fig. 4(a) and (b) show a photograph of the products calcined at different temperatures. The 2H-phase MoS2 can be easily exfoliated to a yellow-green suspension, while the 3R-phase MoS2 cannot be exfoliated. However, when calcined at 1000 1C, a black-brown suspension was obtained and attributed to 1T-phase MoS2. Fig. 4(f) reveals two major Raman peaks assigned to the in-plane (E12g) and out-of-plane (A1g) modes of MoS2 for all samples. The MoS2-1000 and MoS2-1100 samples show the other three weak peaks in the lower wave number region, which are labeled by shaded rectangles with J1, J2 and J3, respectively. These peaks correspond to vibration modes characteristic of the 1T-phase MoS2.91,110 Most of the exfoliated MoS2 layers were measured to be within 300 nm  2 mm in size and about 0.7 nm in thickness (Fig. 4(c) and (d)), approaching the theoretical value of monolayer MoS2 (0.68 nm). The microarea electron diffraction pattern in Fig. 4(e) clearly exhibits the 2a  2a structure with extra dark spots, indicating the representative 1T-phase MoS2. The properties of the 2H-MoS2 and 1T-MoS2 single-layers turn out to be quite different, with the former semiconductive with a direct bandgap of around 1.9 eV whereas the latter metallic. 2.6.

2.4.

Hydrothermal or solvothermal method

Hydrothermal or solvothermal methods usually use a low toxic organic sulfur source, such as thiourea100,101 and thioacetamide,102 to react with a molybdenum salt, e.g. (NH4)6Mo7O244H2O,100 in a Teflon-lined stainless steel autoclave at various temperatures. One advantage of this method is that it can achieve high-surface-area

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Microdomain reaction method

To obtain high crystallinity MoS2 nanosheets, mechanically activated MoO3 and S micro-particles were used to generate a microdomain that can be used to consequently grow nanolayers of MoS2.111 As shown in Fig. 5, this approach controls the growth of crystals that then result in the generation of nanoparticles by a solid-state route. The precursor MoO3 and S particles are

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Fig. 4 (a) Photograph of 50 mg of products calcined at different temperature after reacting with water. The exfoliated suspensions of 2H-MoS2 show the dark yellow-green color, while the suspensions of 1T-MoS2 show the dark brown color. 3R-type MoS2 cannot be exfoliated after reacting with water; (b) schematic illustrations of hydrolysis exfoliation of 2H and 1T MoS2 at different calcination temperatures; (c) AFM image of an individual exfoliated 1T-MoS2 monolayer. The height profile along the dashed line is overlaid on the image; (d) TEM image of the 1T-phase MoS2 monolayers; (e) SAED pattern of the MoS2 sheets in (d); (f) Raman spectrum of exfoliated MoS2 and their corresponding schematic illustration of 2H and 1T phase molecular structures. Reproduced with permission from ref. 109, Copyright 2016, John Wiley and Sons.

Fig. 5 Schematic illustration of the synthesis procedure and structural model of MoS2 NSs. Reproduced with permission from ref. 111, Copyright 2013, American Chemical Society.

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well ball-milled to obtain nanoscale powders with intimate contact, establishing isolated microdomains for sulfurization during the annealing course. This represents an efficient route to produce MoS2 with a high density of active sites and large surface area.

3. Material properties 3.1.

Structural properties

Bulk phase MoS2 has an indirect bandgap and its color is black due to absorption of all visible light. When MoS2 is transformed from bulk into monolayer, its bandgap can be increased from 1.3 eV to 1.8 eV, which is due to quantum confinement effects. For example, as seen from Fig. 6(a), the color of the MoS2 samples, made from commercial MoS2 powders in N-methyl-2pyrrolidone (NMP) by sonication and centrifugation, changes from black to light-yellow with decreasing number of layers and size, implying the widening of the bandgap.112 Fig. 6(b)–(h) show the morphology of obtained MoS2 with different centrifugation speeds. Since gradient centrifugation can influence the number of layers and grain scale of flakes, the MoS2 layers

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become thinner and smaller from Sample 1 (S1) to Sample 5 (S5) with increasing centrifugation speed. Fig. 6(b) shows the SEM image of commercial MoS2 with the typical layered construction and micrometer thickness. Fig. 6(c) shows the SEM image of the sediments centrifuged at 1000 rpm, revealing a layer thickness of B300 nm and size of around 2–5 mm. The sediments from centrifugation at 2000 rpm are primarily few-layer flakes of size 0.5–1.0 mm and thickness B 10.5 nm (Fig. 6(d)). Centrifugation at 3000 rpm resulted in layers with a thickness of B4.2 nm and size of B200 nm (Fig. 6(e)). With centrifugation at 4000 rpm, uniform MoS2 single-layers are obtained with a diameter of B100–200 nm, as shown in Fig. 6(f). The dynamic force mode (DFM) picture shows the single layer with an apparent thickness of 0.8 nm. The difference from the theoretical thickness of 0.65 nm is attributed to surface corrugation from distortions. The TEM picture of S5 indicates the existence of a thin MoS2 film (Fig. 6(g)). The high-resolution picture (Fig. 6(h)) exhibits the hexagonal structure generated by Mo and S atoms with a distance of 0.23 nm, which displays the graphene-like construction, and the Fourier transform (FFT) image in Fig. 6(h) shows the hexagonal symmetry of the atomic configuration as well. As shown in Fig. 6(i), all samples exist in

Fig. 6 (a) Photograph of different MoS2 samples, from S1 to S5, 0.5 mg mL 1 in NMP solution; morphology of different MoS2 samples with different layer numbers; (b) SEM image of commercial MoS2 (S1); (c) SEM image of Sample 2 (S2); (d) SEM image of Sample 3 (S3); (e) SEM image of Sample 4 (S4); (f) DFM image of Sample 5 (S5); (g) TEM image of SL MoS2 (S5); and (h) its corresponding HRTEM image, the inset shows the FFT pattern of MoS2 nanosheet plane; (i) XRD patterns of different MoS2 samples, from the bulk one to single layer. Reproduced with permission from ref. 112, Copyright 2015, John Wiley and Sons.

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Fig. 7 Morphology comparison between two layers is about 6.5 (f) diffraction pattern taken from Copyright 2012, John Wiley and

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of CVD-MoS2 (a–d) and LE-MoS2 (e–g): (a) edge area of the atomic MoS2 layer; (b) two layers of MoS2. The distance Å; (c) diffraction patterns of the atomic layers; (d) typical edges of CVD-MoS2; (e) BF-TEM image of a LE-MoS2 flake; a region in (e) showing a single-crystal MoS2; (g) typical edges of LE-MoS2. Reproduced with permission from ref. 35, Sons.

the (002) crystal plane along the c-axis. And the intensity of the (002) crystal plane decreases from S1 to S5, indicating that the development of MoS2 along the c-axis was prohibited. Calculating from the (002) crystal plane using the Scherrer equation (D = Kl/b cos y), it is easy to get the mean c-stacking thickness, that is 69.4, 39.7, 11.2, and 3.7 nm for S1–S4, respectively. Thus, the number of layers of S1–S4 is estimated to be 112, 64, 18, and 6, respectively. For S5, the (002) crystal plane could hardly be detected, implying the possible presence of single-layer MoS2, and the XRD results are mostly in agreement with the analysis of DFM images in Fig. 6(f). In 2012, Zhan et al. fabricated large-sized MoS2 layers on SiO2 supports by an operable CVD method.35 The as-prepared MoS2 can either be easily used for device manufacturing or be readily exfoliated from SiO2 and then transferred to other substrates. For comparison, liquid exfoliated MoS2 (LE-MoS2) was prepared.92 HRTEM and X-ray diffraction (XRD) patterns are shown in Fig. 7. Fig. 7(a)–(c) shows that the CVD-MoS2 is layer-like and the interlayer distance is about 6.5 Å. Fig. 7(e) and (f) show the TEM images and diffraction pattern of LE-MoS2, which indicate that the size of LE-MoS2 is smaller than that of CVD-MoS2. The edges in CVD-MoS2 and LE-MoS2 were also examined, as shown in Fig. 7(d) and (g). The results reveal four(4L) and three-layered (3L) MoS2 at a length of about 10 nm, 2L MoS2 at a length of about 20 nm in CVD-MoS2, and 4L MoS2 at a length of about 90 nm in LE-MoS2.

3.2.

calculate optical spectra using the Bethe–Salpeter equation (BSE) approach.118,119 Electronic structure computations can offer significant insight into the change in band structure with tuning of the layer numbers from the bulk phase to single-layer. As shown in Fig. 8, bulk MoS2 has an indirect bandgap of 1.29 eV and a valence-band maximum (VBM) that is highly dependent on the interlayer interactions.120–123 In the situation of a single layer, the VBM at the G point shifts below the K point, and the bandgap turns direct.124 Experimentally, two electronic transitions (A and B) are measured at B1.83–1.90 and 1.98–2.06 eV,39,124 arising from transitions near the K point. The A/B transition energies are relatively independent of the layer number.122 The computed splitting of the VBM states at

Electronic structures from first-principles calculations

Electronic structure calculations are important in investigating the physical properties of single-layer MoS2. Particularly, firstprinciples calculations according to density functional theory (DFT) have been widely employed,6,113,114 often using exchange correlation functions involving the generalized gradient approximation (GGA)115 and the local density approximation (LDA).116 Recently, first-principles calculations based on many-body perturbation theory117 have been used to predict the quasiparticle band structures in conjunction with GW approximation and to

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Fig. 8 Band structure of bulk MoS2 and the dominant transition matrix elements for direct transitions. Each transition corresponds to a pair of valence and conduction band states, which are then marked relying on the energy and strength of the transition. The radius of the circle (or width of the line) represents the magnitude of the matrix element,126 and the color represents the respective transition energy. Reproduced with permission from ref. 125, Copyright 2012, American Physical Society.

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the K point due to spin–orbit coupling is in agreement with the energy difference between the A and B peaks.125 The calculations also show that the transitions at the K point are dominant, as indicated by the circles in Fig. 8.

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3.3.

Optical properties

The optical properties of MoS2 are usually studied using Ultravioletvisible (UV-vis) spectroscopy, PL, and Raman spectroscopy.

Fig. 9 Absorption spectrum of the MoS2 dispersions. The dashed line is the calculated scattering background. Inset: absorption without the scattering. Reproduced with permission from ref. 127, Copyright 2013, American Chemical Society.

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For instance, Wang et al. measured the UV-vis spectra of high crystallization mono- and few-layered MoS2 flakes obtained by employing liquid-phase exfoliation, as shown in Fig. 9.127 The four characteristic peaks, in the 600–700 nm and 400–450 nm regions and marked A, B, C, and D in the inset, agree well with the characteristic features of 2H polytyped MoS2 nanosheets. For the sake of clarity, the background induced by Mie scattering, represented by the dashed line in Fig. 9 and calculated using scattering p l 2.3592, is subtracted from the spectrum for the inset of Fig. 9. The doublet in the 600–700 nm region results from interband excitonic transitions at the K point of MoS2 nanosheets. The isolation between A and B is attributed to the spin–orbit dissociation of transitions at the K point. The characteristic peaks in the 400–450 nm region (i.e. C and D) originate from transitions involving higher lying excited states or bands. Mouri et al. systematically studied few-layered MoS2 and the effect of molecular doping using PL and photoconductivity spectroscopy.128 Fig. 10 shows a photograph of ultrasonically exfoliated single layer (1L), 2L, and 3L MoS2 and their corresponding Raman and PL results. The frequency differences (D) between A1g and E12g, which changed with the layer number of MoS2, are observed to be 19.2 cm 1, 22.3 cm 1, and 24.4 cm 1 for 1L, 2L, and 3L MoS2, respectively. Jeon et al. obtained largearea, high-quality IL, 2L, and 3L MoS2 layers using the CVD approach.96 Fig. 10(c) shows the UV-vis absorbance spectra of the 1L, 2L, and 3L MoS2 layers. There are two evidently different absorption signals at 655 nm (1.89 eV) and 610 nm (2.03 eV), which are the A and B exciton transitions.39,124 The energy difference between the A and B exciton transitions was 0.14 eV, which is in accordance with the calculated result of the desired

Fig. 10 (a) Optical pictures of mechanically exfoliated 1L, 2L, and 3L MoS2 on SiO2/Si substrates. (b) Raman spectra of 1L, 2L, and 3L MoS2; (c) UV-vis absorbance spectra of 1L, 2L, and 3L MoS2. (d) PL spectra of 1L, 2L, and 3L MoS2. Reproduced with permission from (a and b) ref. 128, Copyright 2013, American Chemical Society; (c and d) ref. 96, Copyright 2015, The Royal Society of Chemistry.

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MoS2 (0.148 eV).129 Fig. 10(d) displays the PL spectra of the 1L, 2L, and 3L MoS2 layers. A distinctly strong PL intensity was detected for the 1L MoS2 layers at about 665 nm,39 and the PL intensity is inversely proportional to the layer number. The evolution of the optical properties and electronic structure of MoS2 was comprehensively analyzed by Mak et al. employing optical absorption spectroscopy, PL, and photocurrent by altering the layer number.124 The PL spectrum indicates a direct bandgap of B1.9 eV for 1L MoS2, and an indirect bandgap of B1.6 eV for 2L MoS2. The 1L MoS2 is strongly photoluminescent, with a quantum yield about 103 times that of bulk MoS2. There was no photocurrent below the direct bandgap for 1L MoS2, while a sudden increase in photoconductivity was observed just near the direct bandgap, again proving that 1L MoS2 has a direct bandgap, while bilayer and few-layer MoS2 have an indirect bandgap. Interestingly, the optical properties of MoS2 can be manipulated by electrochemical ion interactions. For example, the PL of 2D MoS2 nanoflakes can be modulated by electrochemically introducing Li+, Na+ or K+ ions in between the MoS2 layers.130 This is potentially useful for bio-optical sensors or optical modulators or switches. Another recent study has demonstrated that an acoustic wave can be used to modulate the trion and exciton behavior and thereby

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optical properties of piezoelectrical 2D MoS2.131 Another intriguing aspect of optical properties of MoS2 is related to its plasmonic resonances when it is highly doped. For instance, electrochemical interactions of Li+ with 2D MoS2 nanoflakes were found to induce plasmon resonance absorption in the visible and UV regions.132 3.4.

Electronic properties

Bulk MoS2 consists of vertically stacked, weakly interacting S–Mo–S layers (Fig. 11(a)),46 which can be used to generate monolayers by micromechanical cleavage and liquid exfoliation. For instance, a single layer, 6.5 Å thick, can be produced with scotch tape or lithium-based intercalation.41,133,134 As shown in Fig. 11(b), the unit cell consists of a hexagonal lattice,135 with a Mo–S bond of 2.42 Å and optimized lattice constant of 1L MoS2 as 3.18 Å.134 As shown in Fig. 12, MoS2 can exist in four poly types 1T, 1H, 2H and 3R.136 While the 1H phase is the most stable, the 1T and 3R phases are meta-stable. The 1T phase has the Mo atoms coordinated octahedrally with the S and Mo atoms, while the 1H phase has the Mo atoms coordinated octahedrally with the S atoms and sandwiched between S–Mo–S layers. Both 2H and 3R phases have trigonal prismatic coordination around the Mo atoms,

Fig. 11 (a) Three dimensional representation of the structure of MoS2: single layer, 6.5 Å thick, can be extracted using scotch tape-based micromechanical cleavage; (b) optimized structures of the MoS2 monolayer with four adsorption sites: (1) hollow site, (2) top site of the S atom, (3) Mo–S bridge site, and (4) top site of the Mo atom. Reproduced with permission from: (a) ref. 46, Copyright 2011, Nature Publishing Group; (b) ref. 135, Copyright 2013, International Journal of Electro. Chem. Sci.

Fig. 12 Schematic drawing of common poly-types for MoS2. The yellow dots represent Mo atoms and the blue dots represent S atoms. The green rectangles show a basic unit, while the purple rectangles represent a unit cell. Reproduced with permission from: ref. 136, Copyright 2002, Elsevier Science B.V.

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Fig. 13 Unit cell schematics of (a) 2H-MoS2 and (b)1T-MoS2; filling of the non-bonding d-orbitals for a typical d2 TMD along with the band structure and the representative position of the Fermi level for the (c) 2H phase and (d) 1T phase. Reproduced with permission from (a and b) ref. 50, Copyright 2015, American Chemical Society; (c and d) ref. 1, Copyright 2013, Nature Publishing Group.

with the difference that the 2H phase has two S–Mo–S units per cell while the 3R phase has three S–Mo–S units per elemental cell. Generally, the crystal type or metal coordination strongly influences the electronic properties of MoS2. The 1H and 3R MoS2 have rare applications in light-harvesting due to their intrinsic properties themselves. For 2H-MoS2, every Mo atom is prismatically packed by six ambient S atoms, constituting a thermodynamically stable structure (Fig. 13(a)), while for the metallic 1T-MoS2 phase, six S atoms coordinate to form a twisted octahedron near one Mo atom (Fig. 13(b)).50 The stable semiconducting 2H-MoS2 phase can be easily changed into the metallic 1T-structure by phase engineering.137 Interestingly, 2H-MoS2 can also be reversely transformed into 1T-MoS2 using intercalating Li or K.138,139 XPS was used to identify the 1T and 2H phases. For high-quality 1T-MoS2, the Mo 3d exhibits signals at B228.1 and B231.1 eV, whose binding energy is slightly lower than those of pure 2H-MoS2 (B229.5 and B232.0 eV).140 By analyzing the Mo4+ 3d5/2 and Mo4+ 3d3/2 signals, one can quantify the relative amount of the 1T and 2H crystal phases. Based on the ligand field theory, the reason that 2H-MoS2 is a semiconductor is the fully filled dz2, vacant dxy and dx2 y2 orbitals (Fig. 13(c)).1 1T-MoS2 is metallic due to its half-filled t2g band (dxy, dxz, dyz) (Fig. 13(d)). Based on their excellent electronic properties, atomically thin layers of MoS2 has been studied in field effect transistors (FETs)141 and light harvesting devices.142 FETs based on bulk and few-layer MoS2 as well as monolayer MoS2 have been demonstrated.46 Charge transport characteristics of bulk TMD materials have been widely explored since the early report by Fivaz and Mooser.143 Nevertheless, specific electrical properties of 2D materials were hindered because of their small size. The electronic properties will be discussed from the perspectives of charge carrier transport and mobility.

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3.4.1. Charge transport. Charge carrier transport in 2D MoS2 is highly dependent on the carrier density or the Fermi energy EF, as shown in Fig. 14. The dependence of resistivity is shown in Fig. 14(a).144 Fig. 14(b) shows the insulating condition with EF located in the bandgap and devoid of mobile carriers.17 Positive gate bias moves EF to the conduction band edge (Fig. 14(c)). A further increase in gate bias results in the EF moved above the mobility edge (Fig. 14(d)). The transition from the insulating to the conducting regime is gradual, often involving localized states or band edge states (Fig. 14(e)). 3.4.2. Carrier mobility. The electron mobility of 1L MoS2 (Fig. 15(a)) varies significantly (1–1000 cm2 V 1 s 1),17 much higher at low temperature than room temperature due to the difference in phonon scattering. Between samples from mechanical exfoliation and CVD-growth, the mobility is very similar, indicating similar quality between the samples. As shown in Fig. 15(b), the mobility at room temperature increases with increasing layer number, attributed to the existence of charged impurities on the support that affects thinner samples more.145 Bao et al. reported that a PMMA-coated support shows enhanced carrier mobilities due to long-range disorder, implying that the unfavorable effects due to the SiO2 support can be relieved by PMMA.146 3.5. Exciton and charge carrier dynamics in monolayer and few-layer MoS2 The dynamics of excitons (photoinduced electron–hole pair with Coulombic attraction) and photogenerated charge carriers (free or trapped electrons and holes) directly affect the electronic and optical properties as well as the type of optoelectronic device of semiconductors in bulk or nanostructured form, including TMDs.147–150 Dynamics studies can often provide new insight into the fundamental photophysical mechanisms, complementing

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Fig. 14 Charge transport regimes. (a) Temperature dependent resistivity of ionically gated few-layer MoS2 devices. The temperature dependence evolves significantly from insulating at low carrier densities (top curves) to metallic and superconducting at high carrier densities (bottom curves); (b–d) schematic energy diagram representing different charge transport regimes: (b) insulating; (c) conducting by thermal activation and by hopping; and (d) band transport. (e) Density of states diagram showing the disorderderived band tail states. Reproduced with permission from (a) ref. 144, Copyright 2012, AAAS; (b–e) ref. 17, Copyright 2015, The Royal Society of Chemistry.

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static property measurements. For example, Cha et al. probed the 1s intraexcitonic transient dynamics in 1L MoS2 by employing timeresolved mid-IR spectroscopy.151 Inspired by a theoretical prediction that the A (first) and B (second) excitons are located very close in energy (the 0.32 and 0.3 eV) due to 1s–2p transitions, a monolayer MoS2 suspension was studied using an ultrafast mid-IR probe pulse (0.23–0.37 eV probe) following excitation with a tunable ultrafast laser pulse in the UV-vis region. As shown in Fig. 16, the transient mid-IR results indicate that there are two 1s–3p transitions for the A and B excitons and 1s–2p between 1s (A exciton) and 2p (B exciton). The transient IR spectrum is very broad, indicating that the transient absorption may be from the 1s to the quasi-continuum states following the pump pulse photoexcitation. Time- and angle-resolved photoemission spectroscopy (TR-ARPES) has been used to study the free charge carrier dynamics of 1L MoS2 on a metallic substrate, as shown in Fig. 17.152 The results reveal a direct quasiparticle bandgap of 1.95 eV with ultrafast (50 fs) extraction of excited free carriers from MoS2 by the metal substrate. Fig. 17(a) shows schematically the photogeneration of excited free carriers around the K% point, while Fig. 17(b) and (c) show the dynamics as a function of time and band edge energy, with the red contrast interpreted as excited electrons, blue corresponding to holes, and band edges and Fermi level represented by dashed horizontal lines. Similarly, fs transient absorption (TA) spectroscopy and microscopy have been utilized to probe the exciton dynamics in single layer and few-layer MoS2.153 Fig. 18 shows a summary of the key relaxation processes. The A exciton dynamics is found to be very different from that of thick crystals. The intraband relaxation rate was enhanced significantly in the monolayer compared with the thick crystals, which is attributed to defect assisted scattering. The results also suggest fast trapping of excitons by surface trap states. In order to determine the dependence of charge carrier lifetime on the layer thickness, ultrafast nondegenerate optical pump–probe experiments were conducted on few-layer MoS2.154,155 The results reveal two distinct time scales: a fast one of B2 ps and a slow one of B100 ps, as shown in Fig. 19.154 The results suggest that

Fig. 15 Carrier mobility. Carrier mobilities reported for (a) monolayer and (b) few-layer MoS2. Reproduced with permission from ref. 17, Copyright 2015, The Royal Society of Chemistry.

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Fig. 16 Ultrafast intraexcitonic and band-to-band spectroscopy in monolayer MoS2. (a) Schematic illustration of the ultrafast intraexcitonic (green) and conventional band-to-band (red) interband spectroscopy. The 3.1 eV optical pump (blue arrow) creates e –h+ pairs from the ground to quasi-continuum states or C-band. The mid-IR pulse (green) measures the 1s to np transitions, while the white-light continuum pulse measures the ground-to-1s transition; (b) transient dynamics of DT/T0 (DT/T0 is the change in optical transmission expressed in percentage) measured at two representative energies of 1.86 eV (red) and 0.35 eV (green). The positive sign of DT/T0 is observed for the 1.86 eV probe, while the 0.35 eV probe exhibits a negative sign. A clear temporal delay (B0.2 ps) between the two rising transients was observed. Reproduced with permission from ref. 151, Copyright 2016, Nature Publishing Group.

Fig. 17 (a) Electrons (black spheres) excited by a pulse tuned across the visible to infrared range are photoemitted from epitaxial single layer (SL) MoS2 on Au(111) using a 25 eV probe pulse and detected along the high  K% in the hexagonal BZ at the bandgap energy; symmetry direction G– snapshots of excited free carriers around the K% point; (b and c) intensity difference with the corresponding spectrum obtained before optical excitation (t o 0) at the time delays marked on the time line. Red contrast can be interpreted as excited electrons, while blue corresponds to excited holes. The positions of the band edges and Fermi level are exhibited by dashed horizontal lines with the energy differences given in eV. Reproduced with permission from ref. 152, Copyright 2015, American Chemical Society.

defect-assisted recombination is dominant with electrons and holes captured by defects via Auger processes. The lifetime was found to increase with layer thickness,155 and the inverse carrier lifetime scales with carrier density in surface layers, and linearly with the photoexcited carrier density. The exact nature of the defects is still unclear. Sulfur vacancies in MoS2 were identified to be a possible candidate in a recent theoretical study156 due to their low formation energies.157 3.6. Improved interfacial electron transfer in MoS2-based heterostructures Improved charge carrier separation in MoS2 is important to many electronic and photoelectronic device applications including

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solar cells and photoelectrochemical cells. One strategy is to form heterojunctions between MoS2 and other semiconductors. For example, van der Waals heterostructures (vdWHs) have recently been found as a new type of heterostructure, where quantum coupling between stacked atomically thin 2D layers, such as MX2, results in interesting new properties. vdWHs in composite materials are specifically attractive for optoelectronic and photovoltaic (PV) applications because 2D MX2 monolayers can absorb strongly a broad spectrum of light from near-infrared to visible. Meanwhile, theoretical studies have demonstrated that many stacked MX2 heterostructures form type II heterojunctions that can promote electron–hole separation. 3.6.1. MoS2/WS2 heterostructure. In type II heterojunctions, the conduction band minimum (CBM) and VBM exist in the two materials forming the junction, and this facilitates separation of photogenerated electrons and holes. For van der Waals (vdW)coupled MoS2/WS2 heterostructures, one question of interest is the effect of electron–electron interactions or excitonic interactions on charge transfer. For instance, the band alignment of a MoS2/ WS2 heterostructure is theoretically predicted to form a type II heterojunction.14,158–160 Optical excitation of the MoS2 A-exciton can lead to layer-separated electron (e ) and hole (h+) carriers (Fig. 20). Using fs pump–probe spectroscopy, the hole transfer from the MoS2 layer to the WS2 layer was found to take place within 50 fs.161 Employing energy-state-resolved ultrafast visible/ infrared microspectroscopy, a charge transfer intermediate state was identified during the transition from an intraexciton to an interlayer exciton at the WS2/MoS2 interface.162 3.6.2. MoS2/MoSe2 heterostructure. One issue with vdWHs is their low photovoltaic conversion efficiency due to ineffective electron–hole pair separation into free charge carriers. Coulombic electron–hole attraction in MX2 often dominates over the charge transfer driving force, resulting in inefficient charge separation at MX2 heterojunctions.163,164 To address this issue, computational studies suggested that quantum coherence and donor–acceptor

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Fig. 18 Schematic summary of relaxation processes in a monolayer and a thick MoS2 crystal. Reproduced with permission from ref. 153, Copyright 2013, American Chemical Society.

Fig. 19 (a) Schematic diagram of pump–probe studies of monolayer single layer MoS2; (b) the measured differential transmission DT/T of the probe pulse is plotted as a function of the probe delay with respect to the pump pulse. The pump fluence is B16 mJ cm 2 and T = 300 K; (c) a zoomed-in plot of the data in (b) shows three different temporal regions; (d) illustration of the ultrafast carrier dynamics in MoS2 in the three temporal regions: (I) After photoexcitation, the carriers thermalize and cool and form a correlated electron plasma. (II) Most of the holes, followed by the electrons, are captured by the fast defects within 1–2 ps. A small fraction of the photogenerated holes is also captured by the slow defects. (III) After all the photoexcited holes have been captured and electrons have filled the fast traps completely, the remaining photoexcited electrons are captured by the slow defects on a 60–70 ps time scale and the slow transient lasts for more than 100 ps. Reproduced with permission from ref. 154, Copyright 2015, American Chemical Society.

Fig. 20 Band alignment and structure of MoS2/WS2 heterostructures. (a) Schematic of the theoretically predicted band alignment of a MoS2/WS2 heterostructure, which forms a type II heterojunction. Optical excitation of the MoS2 A-exciton will lead to layer-separated electron (e ) and hole (h+) carriers; (b) illustration of a MoS2/WS2 heterostructure with a MoS2 monolayer lying on top of a WS2 monolayer. Electrons and holes created by light are shown to separate into different layers. Reproduced with permission from ref. 161, Copyright 2014, Nature Pulishing Group.

delocalization can promote charge transfer at a MoS2/MoSe2 interface.165 Both electron and hole transfers are on the subpicosecond time scale, consistent with experimental

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results,153,166,167 which are facilitated mainly by the out-ofplane Mo–S modes.168–170 The slow relaxation of the ‘‘hot’’ hole suggests long-distance band-like transport.167 The electron–hole

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recombination is slow at the interface, leading to better charge separation. 3.6.3. MoS2/WSe2 heterostructure. Theoretical studies suggest that stacked MoS2 and WSe2 single layers form a type II p–n junction with a built-in electric field across the interface that can enhance electron–hole separation and transfer.171–173 Exciton dissociation and charge transfer dynamics in MoS2/WSe2 vdWHs has been investigated using a spatially-resolved micro pump–probe and time-resolved PL system.174 Photogenerated electrons in WSe2 are transferred to MoS2 within 470 fs resulting in fast separation of electrons and holes with long lifetimes. Likewise, ultrafast charge carrier dynamics in 1L and 3L of the TMDs MoS2 and WSe2 has been studied using a combination of time-resolved PL and terahertz spectroscopy.10 Photoconductivity was measured within 350 fs. Fast quenching of PL and THz conductivity was attributed to rapid surface trapping of charge carriers. 3.6.4. GaTe/MoS2 heterostructure. GaTe is a p-type vdW semiconductor with a high charge density175 and a direct bandgap (1.7 eV for multilayer). Few-layer GaTe has been used in phototransistors that show high photoresponsivity (104 A W 1) and a rapid response time (6 ms).176 Also, a p-GaTe/n-MoS2 vdW p–n junction has been demonstrated,177 as shown in Fig. 21(a). As shown in Fig. 21(b), there is a correlation between the normalized Iforward and Vg, which depends on temperature. The Ids–Vds characteristics are shown in Fig. 21(c). Fig. 21(d) shows Ids as a function of time with Vds = 1 V at 80 K (blue line) and 300 K (red line).

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3.6.5. PbS/MoS2 heterostructure. PbS has broad absorption in the UV to mid-IR region, desirable for many optical applications.178,179 An infrared photodetector has been demonstrated by creating layered PbS nanosheets/MoS2 heterostructures, as shown in Fig. 22(a).180 DFT calculations show that PbS can have intimate contact with MoS2, providing an effective transport pathway for charge carriers. PbS/MoS2 heterostructure arrays can be fabricated using electron beam lithography, as shown in Fig. 22(b). A gate-tunable response is found for the photogain (G) and detectivity (D*), as shown in Fig. 22(c), and the overall high detectivity is attributed to strong responsivity and low dark current (Fig. 22(d)). 3.7.

MoS2-organic heterojunctions

Hybrid organic–inorganic heterostructures are attractive for optoelectronic applications. 2D inorganic and organic semiconductors without dangling bonds at their surfaces can interact via vdW forces and be used to create ideal interfaces between them. For example, MoS2 has been used in heterojunctions with few-layer molecular crystals,181,182 small-molecule thin films,183,184 and conjugated polymers.185 The charge transfer dynamics between an organic polymer:fullerene blend and MoS2 has been studied, as shown in Fig. 23(a).186 In addition, plasmonic metasurfaces have been used to improve light absorption and charge generation in a MoS2–(P3HT:PCBM) heterojunction (Fig. 23(b) and (c)). Similarly, a hybrid system based on p-conjugated organic aluminum(III) (Alq3) and single-layer MoS2 has shown evidence of charge transfer between Alq3 and MoS2.187

Fig. 21 (a) Schematic of a GaTe/MoS2 vdWH p–n junction. GaTe was exfoliated on top of the MoS2 flake, and 8/60 nm Cr/Au was used as the electrode; (b) the normalized drain current (Vds = 1 V) at various temperatures as a function of the gate voltage; (c) current–voltage characteristics at various gate voltages, while the temperature was 80 K; (d) the time-dependent photoresponse characteristics at 300 K (red line) and 80 K (blue line), respectively; Vds = 1 V and Vg = 0 V. A 473 nm laser with Popt = 12.77 mW used as the light source. Reproduced with permission from ref. 177, Copyright 2015, American Chemical Society.

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Fig. 22 (a) Schematic illustration of PbS nanoplates–MoS2 heterostructures; (b) schematic illustration of PbS nanoplates–MoS2 heterostructure arrays; (c) gate-tunable behavior of photogain (G) and detectivity with P = 0.15 mW cm 2; (d) ratio of photocurrent on–off switching is as high as 102 at Vgs = 20 V and P = 124 mW cm 2. Reproduced with permission from ref. 180, Copyright 2016, American Chemical Society.

Fig. 23 (a) Schematic of the hybrid P3HT:PCBM/MoS2 heterojunction on a plasmonic metasurface; (b) nanosecond-scale transient pump–probe absorptance measurements from P3HT:PCBM and the hybrid P3HT: PCBM/MoS2 active layers with and without the plasmonic metasurfaces; (c) picosecond-scale transient pump–probe reflection measurements from P3HT:PCBM and the hybrid P3HT:PCBM/MoS2 active layers with and without the plasmonic metasurfaces. Reproduced with permission from ref. 186, Copyright 2016, American Chemical Society.

In another study, a zinc phthalocyanine (ZnPc)–MoS2 interface was studied to understand interfacial charge transfer (Fig. 24). A photogenerated electron in ZnPc is transferred to MoS2 in 80 fs, forming a charge transfer exciton.188 Back electron transfer is slower (B1–100 ps), leading to the formation of a triplet exciton in ZnPc. The overall fast singlet–triplet conversion is attributed to the large singlet–triplet splitting in ZnPc and strong spin–orbit coupling in MoS2. Spin-selective triplet exciton formation may be useful in controlling the electron spin in organic-TMD heterostructures.

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Fig. 24 (a) Schematic of the charge transfer (CT) dynamics between ZnPc and MoS2; (b) energy level diagram at the MoS2–ZnPc interface. Electronic processes including (i) optical excitation; (ii) CT; and (iii) back electron transfer (BET) are indicated in the figure. Reproduced with permission from ref. 188, Copyright 2017, American Chemical Society.

Similarly, molecular doping of monolayer MoS2 was used as a means to modulate its electronic properties for potential applications.189 DFT computations were carried out to study charge transfer and electrostatic potential modulation with pentacene molecules adsorbed on the surface of a MoS2 monolayer. The interfacial charge transfer is insignificant between pentacene and 2H-MoS2 while substantial in the pentacene/ 1T-MoS2 complex, attributed to better alignment of energy levels near the Fermi level for the latter case. Doping with pentacene resulted in major structural changes of the substrate and large adsorption energy stabilizing the 1T-MoS2 monolayer.

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The results suggest the possibility of developing organic/inorganic complexes by molecular doping through charge transfer and interface engineering.

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3.8.

Doping of MoS2

Doping is a simple and powerful technique to alter the electronic, optical, and magnetic properties of solids, especially for

Fig. 25 (a) The Mn is shown to be incorporated at the MoS2 domain boundary and substitutionally; (b) two-terminal conductance versus back gate voltage measurements indicate that Mn-doping leads to an increase in the density of states in the bandgap of MoS2 and thus lower saturation conductance. Reproduced with permission from ref. 190, Copyright 2015, American Chemical Society.

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semiconductors. Different doping strategies have been demonstrated for MoS2, including intercalation doping, adatom doping, and substitutional doping. 3.8.1. Intercalation doping. Mn atoms can be introduced as a dopant into monolayer MoS2 via in situ vapor phase deposition (Fig. 25(a)).190 Gate-dependent conductance G (Vbg) with Mn-doped MoS2 saturates more abruptly at smaller Vbg (Fig. 25(b)). The results indicate slow movement of the Fermi level, possible due to a high density of localized states within the bandgap.191 3.8.2. Adatom doping. Using Z-contrast annular dark field (ADF) imaging, Re and Au dopants in a host MoS2 lattice have been observed, along with correlation between their migration behaviors and energetics.192 Fig. 26(a)–(c) show a set of sequential ADF images revealing the migration of Re atoms in a MoS2 single layer. Au atom migration is illustrated in the ADF images in Fig. 26(d)–(f). The atomic structure evolution shows the interaction of the dopant atoms with impurity atoms on the MoS2 sheet, and the migration behavior is different between Re and Au atoms. 3.8.3. Substitutional doping. Nb-Doped MoS2 has been grown by chemical vapor transport (CVT) (Fig. 27(a)).193 A vdW p–n junction fabricated with MoS2:Nb (60 nm) and undoped MoS2 (4 nm) (inset of Fig. 27(b)) shows good rectification characteristics (Fig. 27(b)).

Fig. 26 Sequential ADF images of single-layer Re-doped and Au-doped MoS2. (a) Three [email protected] are indicated by green arrows and one Re–S adatom is indicated by the open arrow; (b) at t = 16.7 s, the Re–S adatom moves to the Mo site as Re–Mo; (c) moves to another S site as Re–S again at t = 28.5 s; (d–f) Au adatoms on single- and bilayer regions (indicated by red open arrows) change their atomic positions in every frame. Reproduced with permission from ref. 192, Copyright 2014, John Wiley and Sons.

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Fig. 27 (a) Cross-section illustration of van der Waals (vdW) coupled MoS2:Nb layers where Nb dopants replace the Mo host atoms in a substitutional manner; (b) I–V characteristic at variable back-gate voltages measured across the vdW p–n junction assembled with MoS2:Nb (60 nm) and undoped MoS2 (4 nm). Inset is a false-color scanning electron microscopy image along with a scale bar of 10 mm. Reproduced with permission from ref. 193, Copyright 2014, American Chemical Society.

Structural and X-ray measurements determine that the Nb atoms are substitutionally incorporated into MoS2.

the 1T-MoS2 cocatalyst for H2 generation. The initial fast electron transfer from CdS to MoS2 reduces electron–hole recombination and enhances photocatalytic activity.

4. Applications

4.2.

4.1.

Photocatalytic hydrogen production with MoS2

MoS2 has received major attention as electrocatalysts for the hydrogen evolution reaction (HER).194–196 Among the four phases of MoS2 (1T, 1H, 2H and 3R), metallic 1T-MoS2 has attracted the most attention in applications involving HER. Studies have suggested that the catalytic activity is derived from active sites along the edges of the 2H phase of MoS2 flakes,6,197–201 while the majority of basal surfaces are inactive.202 In contrast, both the edges and basal planes of the 1T phase have high catalytic activity and effective charge transport, inhibiting electron–hole recombination.110,203 1T-MoS2 with both metallic conductivity204,205 and high density of active sites is desired for HER. For example, Liu et al. conducted in situ construction of a binary heterostructure including few-layered 1T-MoS2 nanosheets consistently and intimately loaded onto the surface of CdS nanorods (NRs), as shown in Fig. 28(a).206 The MoS2 nanosheets of about 2–5 layers are intimately adhered to the surfaces of the CdS NRs, which should facilitate charge separation and thereby improve photocatalytic activity. To directly confirm the constitution of the 1T phase MoS2 deposited on the surfaces of the CdS NRs, extended X-ray absorption fine structure (EXAFS) was used. Fig. 28(b) shows that the oscillating curves are strikingly distinct from those of 2H-MoS2 (commercial bulk phase), which indicates a significant change in the atomic arrangements. Ultrafast transient absorption spectroscopy (TA) data shown in Fig. 28(c) and (d) reveal efficient charge transfer from the photoexcited CdS NRs to the 1T-MoS2, leading to ultimately enhanced photocatalytic activity in water splitting. Fig. 28(e) displays the rate of H2 production on [email protected] catalysts with various deposition amounts of 1T-MoS2. Notably, the heterojunction with 0.2 wt% 1T-MoS2 achieves a maximum H2 evolution rate of nearly 17.479 mmol g 1 h 1, which is 39 times that of bare CdS NRs. Fig. 28(f) shows a proposed model for electron transfer involving

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Photoelectrochemical conversion (PEC) with MoS2

4.2.1. PEC hydrogen generation with MoS2. Solar-driven PEC water splitting is attractive for solar energy conversion into storable fuels.207–209 Early PEC water-splitting systems based on expensive semiconductors and catalysts were not stable.210 Photocathodes based on MoS2 for solar-driven HER have recently been reported.211 Compared to 2H-MoS2, 1T-MoS2 shows enhanced HER activity.212 Similarly, a 1T-MoS2/Si heterostructure was demonstrated for PEC H2 generation with good stability (Fig. 29).213 4.2.2. PEC sensors with MoS2. PEC measurement has been applied in bioanalytical chemistry as an analytical method.214,215 The mechanism of PEC sensing is based on the reaction between photogenerated charge carriers and the analyte.216,217 In a typical PEC cell for sensing, the photocurrent is measured and is sensitive to the presence of an analyte.218 Several PEC platforms have been used to detect small molecules. Usually, 2D layered MoS2 is beneficial for charge transport time and distance. To date, most PEC platforms involving MoS2 are based on graphene nanocomposites.219,220 Hun et al. designed a PEC sensor with a 1L nano MoS2 modified gold electrode to detect dopamine, which showed a linear and wide detection limit for dopamine under optimal conditions (Fig. 30(a) and (b)).221 Similarly, Wu et al. have demonstrated MoS2 nanosheets for high performance PEC glucose biosensing under visible light (Fig. 30(c)).222 4.3.

Solar cell applications of MoS2

2D materials exhibit some unique optoelectronic properties that are suitable for solar cells and other optoelectronics applications.147,223–229 Although TMDs usually have narrow bandgaps, MoS2 has a tunable bandgap from 1.29 to 1.90 eV, depending on the number of layers. For instance, sheetlike MoS2 has been used in solar cells in various capacities including a hole-transport layer (HTL),230–232 electron-transport

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Fig. 28 (a) The lapped edges of the 1T-MoS2 layers on the CdS NRs; (b) Mo K-edge oscillation curves k3w(k); (c) TA spectra of [email protected] (excitation wavelength l = 400 nm) showing the evolution of the dominant 1S excitonic bleach of [email protected] The signal (i.e., the absorbance changes) is given in mOD, for which OD is the optical density; (d) the relaxation kinetics of the CdS NRs and [email protected] 1S excitonic bleach; (e) the rate of H2 production on [email protected] photocatalysts deposited with various amounts of 1T-MoS2 under visible-light irradiation; (f) schematic illustration of the 1T-MoS2 cocatalyst strategy for accelerating. Reproduced with permission from ref. 206, Copyright 2016, John Wiley and Sons.

layer (ETL),233–235 interfacial layer,236 and protective layer.237 However, most of these applications involve MoS2 on the micrometer scale and did not take advantage of the unique optical properties of monolayer or few layer TMDs including MoS2. Recently, MoS2 sheets have been used as the electron acceptor in organic solar cells, achieving a large-sized type-II heterojunction by coupling MoS2 with the organic p-donor polymer PTB7 (Fig. 31(a)).238 There is significant PL quenching in the PTB7 and MoS2 heterojunction, with minimal PTB7 PL left (Fig. 31(b), inset). PL quenching of both materials involved indicates charge transfer and the construction of a type-II MoS2–PTB7 heterojunction. A schematic of the solar cell is shown in Fig. 31(c), along with the energy band diagrammatic sketch shown in Fig. 31(d). The PV performance of the PTB7–MoS2 heterojunction

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solar cell is shown in Fig. 31(e) and compared to that of PTB7-only and MoS2-only regular cells that produce no photovoltage, indicating the resistant behavior from a mono material, with no obvious Schottky barrier voltage between the substrate and MoS2. In contrast, the PTB7–MoS2 bilayer heterojunction shows a short-circuit current density ( Jsc) of 1.98 mA cm 2, open-circuit voltage (Voc) of 0.21 V, and a power conversion efficiency (PCE) of 0.1%. The Jsc is measured by integrating the external quantum efficiency (EQE) spectrum shown in Fig. 31(f). The EQE displays clear absorbance from the monolayer MoS2 and a small contribution from the PTB7 layer, as demonstrated by the EQE beyond the MoS2 absorption boundary. For practical applications, materials and device stability and longevity are very important. To improve the stability of MoS2,

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Fig. 29 PEC hydrogen evolution using 1T-MoS2 on p-Si. (A and B) Comparison of top-down and cross-sectional (insets) SEM images of (A) 2H- and (B) 1T-MoS2/Si; (C–E) illustration of a 1T-MoS2 catalyst on a p-Si semiconductor and the schematic band energy diagram of p-Si, 1T-MoS2, and a H+/H2 redox couple at 0 V versus RHE in the dark (D) before and (E) after equilibrium; (F) J–E curves of a CVD-grown 2H-MoS2/Si photocathode (CVD 2H), a CVD-grown 1T-MoS2/Si photocathode (CVD 1T), and a drop-cast 1T-MoS2/Si photocathode (Dropcast 1T) measured in 0.5 M H2SO4 under simulated one sun irradiation. The J–E curve of a bare Si photocathode is also shown for comparison. Reproduced with permission from ref. 213, Copyright 2014, American Chemical Society.

Fig. 30 (a and b) Photocurrent response of different concentrations of dopamine from 1.0  10 11 to 1.0  10 5 M; blank was performed in PBS (pH 7.0) without dopamine (n = 9); (c) schematic illustration for the reaction process of the MoS2–glucose oxidase (GOD) modified ITO electrode to glucose. Reproduced with permission from: (a and b) ref. 221, Copyright 2017, Elsevier B.V.; (c) ref. 222, Copyright 2016, Elsevier B.V.

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Fig. 31 (a) Schematic diagram of a MoS2–PTB7 film; (b) PL spectra for films of PTB7 alone, MoS2 alone, and a PTB7–MoS2 bilayer. The inset shows nearly compelte quenching of MoS2 PL and a small contribution of PTB7 PL; MoS2–PTB7 solar cells: (c) device cross-section structure; (d) energy levels of the cell depicted in (c); (e) current–voltage response of a device with PTB7 only, MoS2 only, and a PTB7–MoS2 bilayer; (f) EQE of the solar cell in (e), with the optical absorbance spectra of PTB7 and MoS2 for reference; (g) caculated IQE of the solar cell. Reproduced with permission from ref. 238, Copyright 2016, American Chemical Society.

Liu et al. employed a chemical exfoliation method to obtain MoS2 layers, where their surface was decorated with a CTAC.239 The MoS2 layers, with Li intercalation, were used in organic solar cells, in which the MoS2 layer was the HTL, a PFN layer deposited with a PTB7 and PC71BM mixture was the active layer and Al was the cathode (shown in Fig. 32(a)). Fig. 32(b)–(d) show the J–V curves of the solar cell made from MoS2 solution after devices were kept for 6, 9, and 100 days,

which suggested the long-term stability of organic solar cells. As compared to the poor stability of the solution of pure layered-MoS2, the CTAC decorated MoS2 solution exhibited long-term stability, and remained unchanged up to 100 days, which is mainly due to the electrostatic repulsion and the steric hindrance effect of CTAC molecules at the MoS2 surfaces. This high stability and functionality of MoS2 provide potential applications for the emerging layered-materials and

Fig. 32 (a) Chemical structures of solar cell architectures; J–V curves of organic solar cells built by using MoS2 solution after devices were stored for (b) 6 days, (c) 9 days, and (d) 100 days. To prepare MoS2 solution, CTAC aqueous solution was added to the MoS2 sheets dispersed in deionized water. Reproduced with permission from ref. 239, Copyright 2014, The Royal Society of Chemistry.

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operable optoelectronic applications with low-cost solutionprocessable procedures. For industrial PV application fields, a combination of high PCE, easy and cost-effective production, stability, and compatibility with required substrates needs to be considered. These requirements may be met with MoS2 hybrid heterostructures, from chemical functionalization to composite formation, so as to match energy levels with other desired material units. 4.4.

Application of MoS2 in photodiodes and phototransistors

Graphene and other similar 2D layered materials have gained considerable attention because of their extensive applications in future nanoelectronics.75,240 In spite of the predominant properties of graphene, the intrinsic difficulty resulting from the lack of a bandgap restricts its applications in photodiodes and phototransistors.241–243 2D MoS2, as an alternative layered material, exhibits luminescence and light-absorption properties that may overcome the drawbacks of graphene.124,244 With the wide bandgap and 2D morphology for easy integration, it is particularly attractive for low-power electronics applications, including photodiodes and phototransistors.141 For example, highly efficient photocurrent generation was demonstrated from vertical MoS2 devices fabricated using asymmetric metal contacts, with an external quantum efficiency of up to 7%, attributed to the large junction area.245 Similarly, an excellent photoresponse and photodiode-like behavior have been observed for vertical Si/monolayer MoS2 heterostructures.246 After depositing Au nanoparticles onto few-layer MoS2 phototransistors, an increase in the photocurrent was observed.247 For phototransistors, the top-gate with a MoS2 channel showed a very low mobility.244 For practical applications, high mobility and subthreshold swing are critical requirements, which can be met with the transparent and patterned top-gate approaches.46 As shown in Fig. 33(a) and (b), Lee et al. first proved that 1L and 2L MoS2, with corresponding bandgap energies of 1.8 and 1.65 eV, are well-suited for green light detection, and 3L MoS2 with an indirect bandgap of 1.35 eV can be used for red light detection.248 In particular, those with 1L and 2L MoS2 show good

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nanodevice properties including high mobilities and unsaturated output currents. The distinct features in the MoS2-based phototransistor lay the foundation for further development of other 1L semiconductors for multifunctional optoelectronic applications.

4.5.

Some challenges in applications of MoS2

Although MoS2 has demonstrated considerable potential for applications in a broad range of technologies, some challenges still remain for large scale practical applications. Firstly, uniform growth of high quality MoS2 in large area for device applications is still challenging. To achieve control over stoichiometry, purity, number of layers, size, and phases of 2D MoS2 has proven to be difficult. For example, solution synthesis, the most frequently utilized method, tends to produce a mixture of 2D flakes, and their separation is a major bottleneck and is strongly dependent on their thickness and size. Second, environmental stability of MoS2 is another major challenge. For instance, WS2 can be degraded by laser light in the presence of moisture,249 which could occur to MoS2. Therefore, environmental factors such as water and oxygen are issues of concern. Third, while vdWHs involving MoS2 have emerged as promising for various applications, the interfacial contact between the building layers needs to be better understood and further optimized.250 For example, in light-harvesting applications, the MoS2-based heterostructures often suffer from poor light harvesting, weak contact, and low carrier motilities, largely due to poor interfacial contact. While the pure metallic phase (1T) of MoS2 shows low resistance, its composites or heterostructures need improvement in charge transport.251 Chemical doping of 2D MoS2 is a useful method to improve charge transport and other properties. However, control of doping can also be challenging and requires further studies. Finally, an in-depth understanding and control of charge carrier properties, including dynamics and spin properties, are also areas that need further research both in the experimental and theoretical fronts.252 This will require more advanced investigations with high time and spatial resolution, including possibly X-ray based experimental techniques that can provide element-specific information.

Fig. 33 (a) The schematic energy band diagrams of ITO (gate)/Al2O3 (dielectric)/1L, 2L, 3L-layer MoS2 (n-channel) under light (Elight = hn) illustrate the photoelectric effects for the bandgap measurements; (b) schematic 3D model of a single-layer transistor with a hexagonal phase MoS2 nanosheet, 50 nm-thick Al2O3 dielectric, and ITO top-gate under monochromatic light. Reproduced with permission from ref. 248 Copyright 2012, American Chemical Society.

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5. Conclusion We have reviewed recent progress in fundamental studies as well as applications of ultrathin MoS2, with an emphasis on single- or few-layer MoS2 and their heterostructures. The last few years have witnessed a dramatic increase in research efforts both at the fundamental level and in technological applications of thin layered MoS2. Combining different 2D MoS2 into vdWHs is an emerging research area. The distinct advantages of optical transparency, direct bandgap and unique electronic properties make thin-layered MoS2 attractive for the next generation of technologies including low cost and low power electronics, flexible and transparent solar cells, and displays or wearable computing devices. Various heterostructured materials involving MoS2 have also been developed and studied, which add further flexibility and functionality to MoS2 and some show great promise for emerging applications in addition to interesting new properties. To continue at this pace of progress, several important issues still need to be addressed. First, new methods need to be developed to dope MoS2 and related materials for achieving ambipolar and p-type behavior. Second, for device applications, we need to understand and develop materials that can integrate or interface with MoS2. Third, the heterojunction interfaces need to be better understood fundamentally still, especially at the atomic level. In principle, there is also still room for improving the charge carrier mobility that is important for many applications. To address these issues will require multidisciplinary and interdisciplinary research and development involving both experimentalists and theoreticians. It can be anticipated that research and development involving MoS2 will continue to be an active area in the near and possibly longer future.

Conflicts of interest There are no conflicts to declare.

Acknowledgements This work was financially supported by the National Natural Science Foundation of China (51402126) and the Natural Science Foundation of Zhejiang Province (No. LQ13B010003). JZZ is grateful to support from NASA through MACES (NNX15AQ01A) and UCSC Committee on Research Special Research Grant.

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Chem. Soc. Rev., 2018, 47, 6101--6127 | 6127