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Epitaxial Growth of Two-Dimensional Layered Transition-Metal Dichalcogenides: Growth Mechanism, Controllability, and Scalability Henan Li,†,§ Ying Li,‡,§ Areej Aljarb,# Yumeng Shi,*,‡ and Lain-Jong Li*,# †

College of Electronic Science and Technology, Shenzhen University, Shenzhen 518060, China SZU-NUS Collaborative Innovation Center for Optoelectronic Science & Technology, International Collaborative Laboratory of 2D Materials for Optoelectronics Science and Technology, College of Optoelectronic Engineering, Shenzhen University, Shenzhen 518060, China # Physical Sciences and Engineering Division, King Abdullah University of Science and Technology, Thuwal 23955-6900, Saudi Arabia Downloaded via DURHAM UNIV on July 16, 2018 at 21:33:46 (UTC). See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles.

‡

ABSTRACT: Recently there have been many research breakthroughs in twodimensional (2D) materials including graphene, boron nitride (h-BN), black phosphors (BPs), and transition-metal dichalcogenides (TMDCs). The unique electrical, optical, and thermal properties in 2D materials are associated with their strictly defined low dimensionalities. These materials provide a wide range of basic building blocks for nextgeneration electronics. The chemical vapor deposition (CVD) technique has shown great promise to generate high-quality TMDC layers with scalable size, controllable thickness, and excellent electronic properties suitable for both technological applications and fundamental sciences. The capability to precisely engineer 2D materials by chemical approaches has also given rise to fascinating new physics, which could lead to exciting new applications. In this Review, we introduce the latest development of TMDC synthesis by CVD approaches and provide further insight for the controllable and reliable synthesis of atomically thin TMDCs. Understanding of the vapor-phase growth mechanism of 2D TMDCs could benefit the formation of complicated heterostructures and novel artificial 2D lattices.

CONTENTS 1. Introduction 2. Introduction of Synthetic Routes for TMDCs Layers 3. Growth Mechanism of TMDCs 3.1. van der Waals Epitaxial Growth Modes for TMDCs 3.2. Nucleation Sites Formation: Promoter-Induced versus Self-Seeding 3.3. Growth Temperature: Kinetic and Thermodynamic Control 3.4. Reactants Ratio between Transition Metal and Chalcogen 3.5. Reducing versus Sulphurization/Selenization Conditions 3.6. Growth Substrates for van der Waals Epitaxial Growth 4. Extending Monolayer Growth Mechanism to TMDCs Heterostructures 4.1. Vertical Stacked 2D Heterostructures 4.2. Lateral Stitched 2D Heterostructure 5. New 2D Candidates Extending the Group VIB Transition-Metal Dichalcogenides Catalog 6. Summary and Outlook Author Information Corresponding Authors ORCID Author Contributions Notes © 2017 American Chemical Society

Biographies Acknowledgments References

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1. INTRODUCTION The development of nanoscale structures and materials with novel properties has great potential to advance our knowledge and revolutionize many aspects of our lives.1−3 For examples, shrinking the size of modern electronic and optoelectronic devices down to nanoscale has been used to expand human capabilities and make human life more efficient, convenient, and comfortable. In most electronic devices, the integrated circuits are the key principal components. The development of integrated circuits has enormous impact on our lives, and the performance of electronic devices greatly relies on the continuous downscaling in size and thickness, especially the advances in novel nanoscale materials and structures.4 There are concerns on whether the current silicon-based microelectronics technologies can meet the requirements and overcome the ultimate limits when the transistors are scaled down to physical dimensions near the regime of 10 nm or beyond, because technical complexities and fabrication costs will substantially increase.5 Recent developments have identi-

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Special Issue: 2D Materials Chemistry Received: April 19, 2017 Published: July 6, 2017 6134

DOI: 10.1021/acs.chemrev.7b00212 Chem. Rev. 2018, 118, 6134−6150

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Figure 1. Commonly used CVD method for TMDCs monolayers. (a) Schematic illustration of the MoS2 growth using sulfur and MoO3 as precursors. (b) Phase diagram and possible reaction routes for MoS2 growth. (c) Schematic illustration of the gas-phase reaction and surface epitaxy of MoS2. (b, c) Adapted with permission from ref 49. Copyright 2015 The Royal Society of Chemistry. (d) Scheme showing a MOCVD reactor for MoS2 and WS2 growth. (e) Procedures of multilayer device fabrication. (f) SEM image of the two-layer devices; inset shows the magnified SEM image indicating the first- and second-layer devices. (g) IV curves of the TMDC layers, respectively, from the first and second deposition. (d−g) Adapted with permission from ref 58. Copyright 2015 Macmillan Publishers Limited.

fied several promising two-dimensional layered (2DL) materials6 such as graphene,7,8 transition-metal dichalcogenides (TMDCs),9 black phosphors (BPs),10−13 and boron nitride (hBN).14,15 In these 2DL materials, the electrons and holes are vertically confined, giving rise to numerous exotic physics phenomena, particularly at the monolayer limit;16,17 therefore, new electronics based on 2DL materials have been named as “monolayer electronics”. Due to the atomically thin layered structure, 2DL materials are likely to have the greatest impact on geometric scaling, and monolayer electronics are becoming the extreme scenarios for dimension downscaling in modern electronics aiming at high operating speed, lightweight, flexible, and low-power consumption.18,19 The emerging 2D monolayer materials possess a wide range of electronic properties.20,21 For example, h-BN is a wide-band-gap material that can function as

a gate dielectric or deep ultraviolet emitter;14,22 graphene performs as an excellent conductor23 with a high carrier mobility; and semiconducting TMDCs can serve as high on− off ratio semiconductors offering great potential for many high quantum efficiency optical/optoelectronic applications.9 Thus, most of the critical components such as resistors, memories,11 capacitors,24,25 diodes,23 and transistors7,9,10,26,27 that are formed by silicon in modern electronics/optoelectronics can be redesigned and fabricated based on 2DL materials.21 Furthermore, the great ability to tune the band gap, band offset, carrier density, carrier polarity, and switching characteristics in 2D materials provides unparalleled control over device properties and possibly new physical phenomena. Devices based on atomically thin monolayers are the extreme scenario 6135


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for the future lightweight, low-power consumption, and wearable electronics.1,21,27,28 Being an inorganic graphite analogue,29 each TMDC layer has a sandwich structure formed by hexagonally packed transition-metal atoms located between two layers of chalcogen atoms. Those TMDC members share the same material structure30 with a generalized formula of MX2, where M is a transition metal and X is a chalcogen representing a diverse and largely untapped source of 2D systems. Same as graphite, it is the weak van der Waals force that holds each sandwich TMDC layer together, which thus allows the bulk crystal to be exfoliated along the 2D surface.31 Besides, because these materials are a covalently bonded monolayer, they possess high carrier mobility, good flexibility (bendability),32 and transparency and are promising for flexible, lightweight, low-power consumption optoelectronic applications.1 In addition, their unique electronic structures and interesting valley-spin relations made them attractive for fundamental physics in TMDC materials.33

MoO3 − x + (7 − x)/2S → MoS2 + (3 − x)/2SO2

where the transition-metal suboxides are likely formed during the reaction. As shown in Figure 1c, the reaction intermediates diffuse to the substrate surface and further react with sulfur vapors to grow MoS2 layers. Thus, the evaporation temperature of S and MoO3 is critical to control the partial pressure of the gaseous species in order to govern subsequent adsorption and surface-bound reaction on the substrates. Besides, the excess in sulfur supply could suppress the volatilization of MoO3, which may lead to a low yield in TMDC growth. In addition, insufficient chalcogen supply may not allow S to replace the oxygen in metal oxides completely, which is not preferred for TMDC formation. Therefore, the CVD of TMDCs via the sulfurization/selenization transition-metal oxides requires a well-controlled sulphurization/selenization rate especially for monolayer TMDC production. To ensure a consistent precursor supply and improve the scalability and controllability, metal organic chemical vapor deposition (MOCVD) was then adopted, where organic compounds containing Mo or W were used as precursors in the synthesis.58,59 As shown in Figure 1d, a MOCVD method for monolayer MoS2 and WS2 with wafer-scale homogeneity has been developed by Park’s group.58 In MOCVD, precursors with a high equilibrium vapor pressure are required; thus, molybdenum hexacarbonyl (Mo(CO)6) and diethyl sulfide ((C2H5)2S) are used instead. According to their report, layerby-layer growth mode can be achieved by a low partial pressure of metal vapor (Figure 1e−g). This provides better control over metal and chalcogen precursor delivery during the synthesis, and hence this work shows significant promise for wafer-size growth of TMDCs. Remarkably, after the first TMDC layer growth, SiO2 can be redeposited for the production of multistacked monolayer MoS2 films as well as electronic devices fabricated at different vertical levels. Not alone, Eichfeld et al. have separately demonstrated a scalable synthesis of largearea, monolayer, and few-layer WSe2 via cold wall MOCVD using W(CO)6 and (CH3)2Se as the precursor.59 In general, MOCVD is more capable for large-scale synthesis due to the easier control of precursor delivery. However, carbon-based contaminations from the organic compounds could be introduced during the reaction, which may be an issue for electronic applications. The physical vapor deposition (PVD) methods such as magnetron sputtering and pulsed laser deposition techniques60 provide an alternative route to grow various TMDCs. For example, direct sputtering of transition-metal dichalcogenide targets to produce continuous few-layer TMDCs has been reported with superior area scalability and layer-by-layer controllability.61−63 Huang et al. have demonstrated that large-area bilayer to few-layer MoS2 growth can be achieved by magnetron sputtering MoS2 target followed by postdeposition annealing process.61 Meanwhile, Tao et al. have demonstrated that MoS2 film can be grown at high temperatures (>700 °C) by Mo metal sputtering in a vaporized sulfur ambient.64

2. INTRODUCTION OF SYNTHETIC ROUTES FOR TMDCS LAYERS One major and essential research field of TMDCs is the reliable synthesis of atomically thin TMDCs with good layer controllability and large-area uniformity,34,35 for practical applications in electronic and optical devices. A variety of synthetic techniques have been developed to synthesize TMDCs layer such as chemical vapor deposition (CVD),36,37 thermolysis,38−40 and physical vapor transport41,42 and layer-bylayer conversion.43,44 Among which, the vapor-deposition methods are capable of achieving better layer number controllability. To date, various monolayers and few-layer TMDC nanosheets like MoS2,36 WS2,45,46 WSe2,37 and MoSe2,47 with scalable sizes, controllable thicknesses, and excellent electronic properties, have been synthesized by the CVD process. It has been shown that the intrinsic optical and electrical properties of monolayer TMDCs can be defined by their stoichiometry via substitutional doping.42,48,49 Besides, wafer-scale deposition and “roll-toroll”50 production of monolayer to few-layer TMDC films have also been realized.51 Substantial research studies on extending the methodology enable the formation of monolayer alloys and TMDC stacked structures or super lattices.52,53 The advances in synthesizing monolayer and few-layer TMDC films enable the fast development in a broad range of applications such as field effect/tunneling transistors, chemical sensors, light-emitting diodes, light detectors, photovoltaic applications, and energy-storage devices.54−57 Since the report on vapor-phase growth of single-crystalline MoS2 monolayer, there has been a growing body of research on the growth of various TMDC structures. The vapor-phase reaction, suitable for wafer-scale fabrication, provides highquality TMDC films that are useful for electronic devices and have become the most widely used method for synthesizing TMDCs monolayers and heterostructures. In the pioneer experimental report for monolayer TMDC CVD synthesis, sulfur and MoO3 powders were chosen as the growth precursors (Figure 1a).36 Suggested by the Mo−O−S ternary phase diagram (Figure 1b),35 it is proposed that the gas-phase MoO3 precursors undergo a two-step reaction during the CVD growth, MoO3 + x /2S → MoO3 − x + x /2SO2

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3. GROWTH MECHANISM OF TMDCS Suggested by both CVD and MOCVD experimental works, the quality of TMDCs can be determined by many factors including but not limited to the growth temperature, pressure, precursor ratio and flow rate, and choice of substrates. Depending on the growth reactors and growth condition

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Figure 2. Nucleation in TMDC growth promoted by substrate surface-seeding and self-seeding process. (a) Schematics for the experimental setup and the growth process on diverse surfaces. The chemical structure of PTAS is shown on the right. (b) Pictures showing the MoS2 flakes synthesized on SiO2/Si substrates using three typical organic promoters. The triangular shape of MoS2 crystallites reflects their 3-fold symmetry. Insets display the corresponding molecular structures of the promoters. (a, b) Reproduced with permission from ref 68. Copyright 2014 American Chemical Society. (c−e) SEM images for the MoS2 synthesized with various patterns, where a higher density of nucleation is frequently observed on the crosssectional surfaces and at substrate edges. Adapted with permission from ref 69. Copyright 2013 Macmillan Publishers Limited. (d) Induced nucleation of MoS2 domains are started from the artificially made circular edges. (f) Optical image of the large-grain MoS2 grown by CVD method using ultraclean growth substrates and fresh growth precursors. (g) MoS2 triangle can grow up to ∼123 μm in lateral length. Adapted with permission from ref 70. Copyright 2013 Macmillan Publishers Limited.

monolayer is completed. Beyond the critical layer number (1 for monolayer TMDCs), the growth continues through the nucleation and coalescence of TMDC nanoparticles or fewlayer islands, leading to few-layer epitaxy and multilayer growth. Hence, a large-scale and high-uniformity TMDC layer can be obtained with Stranski−Krastanov growth mode. The Frank Van der Merwe is an “island growth” mode, where the TMDCs layers formed in isolated islands with different layer thicknesses and then stitched to form a complete thin film. It can produce TMDCs with abundant structural edges, which is desirable in catalytic applications.

used, TMDCs with various geometric shapes, sizes, and aspect ratios can be obtained. The vapor-phase synthesis techniques are notoriously difficult to extend across different reactors, and subtle changes in the growth conditions usually confound the reproducibility. A further understanding of the general growth mechanism is critical toward controllable synthesis of TMDCs materials. 3.1. van der Waals Epitaxial Growth Modes for TMDCs

Three primary growth modes are commonly known for thinfilm epitaxy: the Volmer−Weber, Frank Van der Merwe, and Stranski−Krastanov modes.30,65 The 2D TMDCs van der Waals epitaxial growth commonly follows the Stranski− Krastanov or Frank Van der Merwe growth modes. For Stranski−Krastanov growth mode, also known as “layer-plusisland growth”, monolayer TMDC domains initially gather and interconnect with each other until the full coverage of the

3.2. Nucleation Sites Formation: Promoter-Induced versus Self-Seeding

The sulfurization of metal oxides in vapor phases results in either single-crystalline TMDC flakes or continuous monolayer thin films formed from merging triangular single-crystal island 6137


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Figure 3. Atomic structure and chemical composition of the nucleation center in CVD-grown TMDCs. (a) Top panel shows the high-magnification HAADF image of the nucleation center, and the bottom panel is the low-magnification of the TMDC sheet. (b) TEM image of the nucleation center and the schematic model of the cross section of the nucleation center with the core−shell fullerene-like TMDC layers. (c) Schematic illustration of the proposed nucleation and growth dynamics. At stage 1 of the CVD growth process, partial reduction of the MoO3 leads to MoO3−x(S,Se)y molecular clusters deposited on the growth substrates. With the increase in growth temperature, a more complete reduction of the oxide precursor and full transformation of the metal oxides to Mo(S,Se)2 can be reached, resulting in monolayer Mo(SSe)2 growth. (d) Molybdenum−oxygen− chalcogen ternary phase diagram that explains the nucleation and growth route. (a−d) Reproduced with permission from ref 71. Copyright 2016 American Chemical Society.

domains on arbitrary substrates depending on the nucleation density. The pioneering work for growing large-area MoS2 atomic layers reported by Li’s group is based on the direct chemical vapor phase reaction of MoO3 and S powders.36,37,47 This method allows the growth of single-crystalline MoS2 flakes directly on arbitrary substrates depending on the control of nucleation density and growth process. Recently, Jeon et al. have reported that a high-quality, centimeter-scale continuous monolayer MoS2 film can be grown on noncrystallized substrate such as SiO2 by pretreatment on substrates.66 Substrates that are pretreated by oxygen plasma exhibit lower surface energy, and this would help to facilitate heterogeneous nucleation and MoS2 layer growth. Besides, the number of layers can able be controlled by changing plasma treatment time.66 To facilitate the TMDCs growth on noncrystallized substrates, substrate surface seeding with graphene-like species has also been explored (Figure 2a). It is anticipated that the presence of aromatic molecules provides a better wetting of the growth surface and lowers the free energy for the nucleation.67

It was then found that the lateral growth of monolayer MoS2 can be promoted by the substrate treatment using aromatic molecules, such as reduced graphene oxide (r-GO), perylene3,4,9,10-tetracarboxylic acid tetrapotassium salt (PTAS), and perylene-3,4,9,10-tetracarboxylic dianhydride (PTCDA).36 These graphene-like molecules act as the seeds for growing MoS2 thin layers. Therefore, various seeding promoters have been further studied to enhance MoS2 growth.68 As shown in Figure 2b, it was concluded that aromatic molecules, such as copper(II) 1,2,3,4,8,9,10,11,15,16,17,18,22,23,24,25-hexadecafluoro-29H,31H-phthalocyanine (F16CuPc), copper phthalocyanine (CuPc), and dibenzo{[f,f ′]-4,4′,7,7′-tetraphenyldiindeno [1,2,3-cd:1′,2′,3′-lm]perylene (DBP), can facilitate the growth, while the inorganic materials including aluminum oxide (Al2O3), hafnium oxide (HfO2), and SiO2 do not promote the growth of MoS2. Another strategy to control the growth of TMDCs is to pattern the substrates using conventional lithography processes. By patterning the substrates surface, the nucleation and growth of MoS2 layers can be controlled. As shown in Figure 2c−e, 6138


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lower temperature) than that controlled by the thermodynamics. One of the best illustrated examples for the kinetically controlled growth process is lateral heteroepitaxy growth, where its edge features can promote the formation of TMDCs without forming alloys at the interfaces.69 The thermodynamically controlled process, on the other hand, is typically used to produce high-quality crystals with stable atomic structures, such as bilayer or thicker TMDCs.53 So, these two growth-control strategies can be utilized to manipulate the growth process. Besides, the growth temperature can affect the crystallization and growth of TMDCs in several manners. Zhang et al. observed that the increase in WS2 growth temperature from 880 to 900 °C can dramatically increase the flake size from ∼15 to ∼50 μm.74 As shown in Figure 4a−f, Liu et al. have

Najmaei et al. found that the MoS2 triangular crystals nucleated at the step edges, and this edge-dominated catalytic process is due to a significant reduction in the nucleation energy barrier at step edges.69 Note that the seeding and patterning approaches that control the growth of monolayer TMDCs have attracted a lot of discussion. Different from the seeding method, van der Zande et al. reported that a clean substrate and the use of fresh precursor are critical for promoting ultralarge MoS2 singlecrystal growth on SiO2 surfaces.70 The samples prepared by this method could have a higher-quality monolayer MoS2, as no seeding molecules were used to promote the nucleation, and a larger growth size up to 120 μm in lateral, as illustrated in Figure 2f and g. By contrast to the seeding method, according to the report by van der Zande et al., the yield of monolayer MoS2 can be significantly decreased if contaminated substrates or degraded precursors are used. Besides, the knowledge of the nucleation and growth dynamics is still limited and largely based on speculation. Especially for CVD reaction, both metal oxide and chalcogenides are heated up to their vapor phases. Yet, the delivery rate of precursors could vary significantly due to their huge melting temperature difference, which further complicates the growth process of monolayer TMDCs. Two possible reaction pathways have been proposed for the van der Waals epitaxial growth of monolayer MoS2: (1) MoO3 in vapor phase undergoes a two-step reaction, where MoO3−x is likely the intermediate phase, and these suboxide compounds will diffuse onto the substrate and further react with sulfur vapors to grow MoS2 layers. (2) TMDCs are formed in vapor phase and are directly crystallized into larger domains on the surfaces. To further specify the reaction pathway and understand the role of the nucleation center or “seeds”, Cain et al. used aberration-corrected scanning transmission electron microscopy (STEM) along with other analytical techniques to reveal the atomic structure and chemical composition of the nucleation center in CVD-grown TMDCs.71 Interestingly, as shown in Figure 3a and b, the STEM images consistently show all the monolayer TMDCs flakes have a nucleation center formed by a core/shell structure with a size ranging from 10 to 30 nm. The nucleation center has a core/shell-like structure, in which a partially sulfurized or selenized molybdenum trioxide core was wrapped with a fullerene-like shell of TMDCs. On the basis of the direct observation and analysis of the nucleation center, a step-by-step nucleation and growth mechanism of CVD TMDCs has been proposed (Figure 3c and d). Initially, the evaporation of Mo−O molecular clusters condensed on the growth substrates forming MoO3−x(S,Se)y nanoparticles in a chalcogen-deficient atmosphere. With the increase of growth temperature, the transformation of TMDCs is completed, resulting in a core/shell structure that serves as the nucleation center for the monolayer to continue the growth. Therefore, the TMDCs monolayer growth essentially follows a “selfseeding” process.

Figure 4. Growth temperature and duration effects on the sizes, layer numbers, and shapes of TMDCs. (a) AFM topographic images showing the WSe2 domain shapes and sizes at the growth temperature of (a) 900 °C, (b) 950 °C, (c, d) 1025 °C, and (e, f) 1050 °C. As the growth temperature increases, triangular WSe2 flakes gradually change to nontriangular shapes. The truncated few-layer triangles and hexagons with curve edges started to appear when the growth temperature was raised above 1025 °C. Reproduced with permission from ref 75. Copyright 2015 American Chemical Society.

demonstrated how the growth temperature and duration affect the sizes, layer numbers, and shapes of WSe2 flakes.75 According to their report, the flake sizes and layer numbers of WSe2 increase with the increasing growth temperature. In addition to the usually seen WSe2 triangular domains, truncated triangles and hexagons with different edge features have also been observed from the samples grown at a higher temperature. It was found that, under a suitable monolayer growthtemperature condition, the long growth duration led to the growth of larger WSe2 domains while retaining their monolayer property and triangle shape. It can be reasoned that a higher growth temperature facilitates the surface diffusion of WO3 and WO3−x and the reaction of WO3−x with sulfur/selenium can also be promoted. Meanwhile, a higher growth temperature contributes to the increase in desorption and evaporation of the monolayers; therefore, the nucleation and deposition is restrained, resulting in decreased nucleation density. It is noteworthy that the sulfurization/selenization reaction in TMDCs growth typically requires a high temperature (for example, >600 °C), which consumes a lot of energy. Gong et al. have reported that tellurium-assisted synthesis of MoS2 and WS2 monolayers can be achieved at a significantly lower temperature, ∼500 °C.76 Surprisingly, the as-grown monolayers

3.3. Growth Temperature: Kinetic and Thermodynamic Control

The growth behavior could also change between kinetically controlled and thermodynamically controlled at various temperatures. The kinetically controlled process prefers metastable structure formation such as vertical aligned TMDCs layers grown perpendicularly to the growth substrates.72,73 Usually it requires a lower activation energy (or 6139


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Figure 5. CVD synthesized TMDCs monolayers with various shapes: triangle, truncated triangle, three-point star, six-point star, and hexagon. (a) Schematic illustration of the MoS2 CVD growth system. The optical microscope images show the spatial sectioning of the MoS2 with various sizes and shapes. Reproduced with permission from ref 77. Copyright 2014 American Chemical Society. (b) Schematic illustration of the relationship between the Mo and S atomic ratio in precursors and the resulting domain shapes. The ball models on the left show two types of MoS2 edge termination structures. The ball-and-stick models in the central part display the top-view microstructure of the monolayer MoS2 crystal in different shapes. The schematic diagrams on the right demonstrate the domain shape evolution procedure determined by the growing rates of two types of termination, S-zz and Mo-zz. Reproduced with permission from ref 78. Copyright 2016 American Chemical Society.

gradient of transition-metal oxide precursor concentration during the CVD process (see Figure 5a),77 and the shape of TMDCs is highly dependent on the local changes in the transition metal/chalcogenide ratio of precursors, as illustrated in Figure 5b. As TMDC crystals have two types of competing crystal facets, Mo-zigzag (zz) and S-zz, the ratio between Mo and S atoms on the growth surface influences the energetic stability of Mo-zz and S-zz terminations, thus leading to a different growth rate and resulting in different domain shapes. Rajan et al. developed a stochastic model utilizing a sitedependent activation energy barrier based on the intrinsic TMDCs bond energies and a series of Evans−Polanyi relations.78 The stochastic model has reached good agreement with the shape and size evolution of TMDCs as reported by Wang and co-workers. Furthermore, a unified analytical theory

show good optical characteristics and electrical performance, which are comparable to the samples prepared by CVD at higher temperatures without Te. The introduction of Te makes the reaction conditions milder, because Te has a lower melting point at ∼450 °C, and a small fraction of W can dissolve into Te. Therefore, WS2 crystals will then precipitate from Te agglomerates and grow on SiO2 substrates when a sulfur-rich reaction environment is achieved during the growth. 3.4. Reactants Ratio between Transition Metal and Chalcogen

Monolayer TMDCs with different shapes have been synthesized by CVD, such as triangles, truncated triangles, three-point stars, six-point stars, and hexagons. To understand the shape evolution in TMDCs, Wang et al. intentionally created a sharp 6140


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Figure 6. Producing monolayer TMDCs on flexible and conductive substrates. (a) Schemes showing the CVD growth process and the morphology of MoSe2 monolayers grown on molten glass substrates. (b) Photo of the MoSe2 crystals grown on molten glass substrates with millimeter-sized monolayer MoSe2 flakes, which can be observed by naked eyes. (c) Optical image of a large-size and single-domain WS2 transferred to a SiO2/Si substrate. (d, e, f, g) Photos of various monolayer and stacked bilayer 2D materials transferred onto flexible PET substrates. Adapted with permission from ref 51. Copyright 2015 Macmillan Publishers Limited.

while under a more reductive condition (H2S/H2 = 5:15), hexagonal morphology starts being observed. The CVD technique commonly produces the TMDC layers that conformally formed on the substrate surfaces to minimize the surface energy. Interestingly, it was observed that H2S/H2 injection at a high temperature (700 °C) produces vertically aligned multilayer MoS2 domains with a lateral size of 10 μm. This observation is similar to the previous reports that smooth and uniform edge-terminated TMDCs films can be produced under a rapid sulfurization/selenization process,72,73,83 where the chemical conversion rate of transition-metal species is much faster than the diffusion rate of sulfur/selenide gas into the film. With the anisotropic structure, the diffusion of chalcogens through van der Waals gaps is much faster than diffusion across the layers. The TMDC layers naturally orient perpendicular to the film, exposing van der Waals gaps for fast reaction results in the TMDC vertical structures with abundant TMDC edge sites. It is notable that the vertically grown TMDCs are metastable with high chemical activity in many catalytic reactions.72,73,83

is derived for the growth of truncated triangles, equilateral triangles, and regular hexagons. 3.5. Reducing versus Sulphurization/Selenization Conditions

As it is known that H2 is a more reductive reagent than sulfur, it may directly promote the reduction of WO3 or the formation of H2S to reduce WO3. Zhang et al. have demonstrated that largearea and high-quality WS2 layers can be achieved by using a carrier gas mixture of H2 and Ar because H2 is able to generate a WO3−x-rich environment for WS2 growth, favoring the growth of triangular WS2 flakes.74 The growth with H2 gas is considered to be mediated by a kinetic effect, leading to the thermodynamically stable geometry of triangular-shaped WS2. Besides, highly reactive sulfuric precursor such as H2S has also been used for wafer-scale growth of MoS2 film.79−81 Dumcenco et al. have reported the growth of MoS2 on sapphire substrates using a chemical reaction between MoO3 and H2S/ H2 gas mixture because H2S will lead to a more favorable chemical reaction:

3.6. Growth Substrates for van der Waals Epitaxial Growth

MoO3 + 2H 2S + H 2 → MoS2 + 3H 2O

The growth of TMDC monolayers is very sensitive to the substrate surface properties. The pioneer CVD process for monolayer TMDC synthesis is based on insulating and rigid substrates such as Si wafer,36 mica,65 and sapphire37,47,84 because of their atomically flat and chemically inert features. It has been demonstrated that, even though the van der Waals interaction is relatively weak, monolayer MoS2 with a high degree of control over lattice orientation can be achieved by epitaxial CVD growth on c-plane sapphire.84 Recently, centimeter-scale continuous monolayer MoS2 film was obtained via merging single-crystal domains with the same lattice

H2S/H2 gas mixture in the range of 5:15−15:5 sccm is introduced at different temperatures from 600 to 700 °C. The injection temperature of the H2S/H2 gas and the ratio of H2S/ H2 are critical factors in determining the size of MoS2 islands and switching the growth direction of domains.82 Early introduction of H2S/H2 gas mixture at lower temperature (600 °C) will favor the horizontal growth. When the growth condition is performed in S-rich conditions, for example, H2S/ H2 = 15:5, the triangular shape is the equilibrium form of MoS2, 6141


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Figure 7. Usage of molten substrate in CVD growth process. (a) Schematic illustration of the roll-to-roll production of large-area flexible monolayer WS2 films. The WS2 monolayer was first synthesized on Au foil and transferred from Au foil to the target flexible substrates using bubbling/etching method. (b) Optical image of millimeter-sized triangular monolayer WS2 on Au foil. (c, d) Optical images of MoSe2 monolayers grown on molten glass substrates. Adapted with permission from ref 90. Copyright 2017 American Chemical Society.

to-roll production of large-area flexible film production with uniform monolayer, double-layer WS2, and WS2/graphene heterostructures.51 Such roll-to-roll technique could enable the realization of fabricating flexible TMDC-based film transistor arrays. However, the lack of low-cost chemically inert metal substrates for TMDC growth could limit the practical applications of this technique. Besides the solid dielectric/metallic substrates widely used for TMDCs synthesis, the use of molten substrates in CVD growth process can effectively promote the quality of 2D crystals. As aforementioned, the nucleation of TMDCs occurs at impurities or defect sites. The molten surface can be quasiatomic smooth and homogeneous; thus, “liquid-state” molten substrates will reduce the nucleation density, leading to the growth of large-size TMDC crystals. As shown in Figure 7, millimeter-size TMDC monolayers with triangular morphology have been achieved by using a molten glass substrate.90 The glass substrate melts at >750 °C and the molten glass has a nonperiodic and time-dependent ionic structure that induces the fluctuation of the interatomic distance, wakening the interaction between adatoms and growth substrates, which will then lower the migration barrier energy. The much lower migration coefficient on molten glass leads to a higher growth rate and a lower nucleation density, resulting in millimeter-sized TMDCs growth.90 At 1050 °C, the nucleation density can be controlled as below 20 nuclei cm−2, enabling the growth of triangular monolayer MoSe2 crystals with a lateral size up to ∼2.5 mm and with a high carrier mobility up to ∼95 cm2/(Vs).

orientation.84 In addition, dendritic monolayer MoS2 samples have been achieved through a facile CVD method on a symmetry-disparate SrTiO3 (STO) (001) substrate.85 It has been proposed that the monolayer MoS2 on STO (001) follows a diffusion-limited aggregation mechanism, where the prominent diffusion anisotropy of monomer precursors on STO (001) and the adlayer−substrate symmetry disparity lead to the dendritic growth of monolayer MoS2.85 Shi et al. have reported that a 2D surface like graphene surface can significantly promote the growth of TMDCs, due to the van der Waals epitaxy.39 Although the lattice spacing for MoS2 is 28% larger than that for graphene, the crystal orientation can be incoherent due to the large strains between layers. However, the weak van der Waals interaction between the vertically stacked layers allows all the remaining strain to get accommodated in the van der Waals (vdW) gap. Besides graphene, fluorophlogopite mica (KMg3AlSi3O10F2) has also been used as growth substrates for TMDC synthesis because of its atomically flat and chemically inert surface with a hexagonally arranged in-plane lattice. In addition, a recent work reported by Ji et al. has demonstrated that centimeterscale uniform monolayer MoS2 can be synthesized on mica through the low-pressure CVD (LPCVD) process.65 Nowadays there is an increasing demand for producing monolayer TMDCs on flexible and conductive substrates. For example, the edges of MoS2 and WS2 present an ideal hydrogen-binding energy,45 which makes them promising to replace the Pt-based electrocatalysts for hydrogen generation.83 Thus, synthesis of TMDCs with desired electrochemical properties on conductive substrates has attracted a lot of attention. Nonetheless, introducing flexible metal foil substrates enables the “roll-to-roll”50 techniques for monolayer TMDC production, enabling the batch and continuous production as displayed in Figure 6. Au foils have been proven to be excellent growth substrates for monolayer TMDCs.45,51,86,87 The limited molybdenum (tungsten) and sulfur (selenium) solubilities in gold foil allow a self-limited growth of monolayer TMDCs. It has been demonstrated that the crystallography of Au foil substrates can significantly affect the growth of monolayer TMDCs and the large single-domain size of MoS2 can be achieved by carefully tuning the growth conditions.88,89 As shown in Figure 6b−g, Gao et al. have demonstrated the roll-

4. EXTENDING MONOLAYER GROWTH MECHANISM TO TMDCS HETEROSTRUCTURES Chemical vapor deposition (CVD) has resulted in great advances in the growth of 2D monolayer materials such as graphene, h-BN, and TMDCs. Another very attractive aspect is the heterostructural 2D systems in which two or more 2D materials with different electrical properties combined “face to face” or “edge to edge”.54 Heterostructural 2D materials can be fabricated physically, where 2D crystal thin layers with desired thickness are first isolated and then proceed to a mechanical transfer process to restack and assemble into 2D heterostructure.91 Because 2D materials are sensitive to their environment, the interface of 2D 6142


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Figure 8. Diverse parallel stitched and vertical stacked 2D heterostructures. (a, b) TEM images showing the parallel stitched graphene−MoS2 heterojunction. (c−e) Images demonstrating the parallel stitched graphene−MoS2 heterojunction in a large scale with arbitrary patterns. (c) Typical optical images of the graphene−MoS2 periodic array. (d) Optical images before and after MoS2 grown on patterned graphene pattern with the “MIT” logo. (e) Optical image of a MoS2-filled MIT mascot “Tim the beaver”. (a−e) Adapted with permission from ref 98. Copyright 2016 WileyVCH Verlag GmbH & Co. KGaA, Weinheim. (f) Schematic illustration of van der Waals epitaxy forming WS2/MoS2 bilayer at 850 °C. The right image shows the optical microscopy of WS2/MoS2 bilayer. (g) Schematic atomic structure of in-plane WS2−MoS2 monolayer formed at 650 °C. The optical microscope image on the right displays the WS2−MoS2 heterostructure on SiO2/Si substrates. (f, g) Adapted with permission from ref 53. Copyright 2014 Macmillan Publishers Limited. (h) Two-step growth of in-plane WSe2−MoS2 lateral heterojunction with atomically sharp interfaces. (i) STEM images and molecular diagram showing the heterojunction with a sharp interface. (h, i) Adapted with permission from ref 104. Copyright 2015 American Association for the Advancement of Science.

materials is critical for their electrical performance.92 Nevertheless, vertical stacking of TMDCs homo- and heterostructures with precise control of twist angles could create new materials with a tailored interlayer coupling depending on the stacking orientation.93,94 Thus, by manipulating the lattice structure or layer stacking manner or alternating the sequence of each individual layer, it is possible to manipulate the properties of the heterostructure and create atomically thin van der Waals materials with unique and unexplored physical properties. However, the physical exfoliation and stacking

method can only produce vertically stacked 2D layers that are not scalable for large-scale device fabrication due to the absence of the 2D materials with controllable thickness, size, and uniformity. To realize the various practical applications, controllable synthetic approaches for 2D heterostructures are essential. The fabrication of 2D semiconducting heterostructures requires advanced synthetic technologies to well define the interfaces. There are still several technical issues that need to be solved, including layer number controllability, single-crystal size tuning, 6143


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TMDC heterojunction, one key component for constructing monolayer p−n rectifying diodes, light-emitting diodes, photovoltaic devices, and bipolar junction transistors, has received interest for applications such as integrated circuit,100 optoelectronics,101 and energy generation.83,102 Recently, the MoS2− MoSe2,52 WS2−WSe252 and WS2−MoS2,53 and MoSe2−WSe299 lateral heterostructures, with interesting optical and electrical properties, have been obtained by one-pot synthetic processes. Duan and co-workers have demonstrated the growth of MoS2/ MoSe2 and WS2/WSe2 lateral heterostructures by changing of the chalcogen reactants during growth.52 Besides, seamless high-quality in-plane MoSe2 andWSe2 monolayer heterojunctions have also been reported by Huang’s group.99 However, the atomically resolved tunnelling electron microscope (TEM) image reveals that the substitution of one transition metal by another occurs across the interface forming TMDCs alloys. It is very challenging to obtain well-defined and atomically sharp interface between two TMDC regions. Because all precursors coexist in vapor phases during the growth, TMDC alloys are thermodynamically preferred at the interface regions at elevated temperatures.83,103 As aforementioned, Gong and co-workers have reported a one-step growth strategy for highquality in-plane interconnected heterostructures of WS2/MoS2 by controlling the growth temperature, displayed in Figure 8f and g.53 Vapor growth at low temperature (∼650 °C) leads to lateral epitaxy of WS2 on MoS2 edges, where the lateral epitaxy of WS2 on the MoS2 edge occurs preferentially along the zigzag direction. The seamless and atomically sharp in-plane heterostructures generate intrinsic p−n junctions, and yet such processes only allow the growth of heterostructures with either different metals or chalcogens. Li and co-workers have developed a two-step CVD approach for epitaxial growth of WSe2/MoS2 lateral junction as shown in Figure 8h and i,104 where WSe2 is first synthesized through van der Waals epitaxy on substrate followed by the edge epitaxy of MoS2 along the W growth front. This two-step CVD approach is able to avoid the alloy formation and obtain atomically sharp interface at the WSe2−MoS2 interface. The 2D lateral WSe2−MoS2 heterojunction was synthesized on c-plane sapphire substrates by sequential chemical vapor deposition of WSe2 and MoS2. The crucial point to grow heterostructure without alloy formation is to control the relative vapor amount of MoO3 and S during the second step of MoS2 growth. The excess in Mo precursors may enhance the MoS2 vertical growth, whereas the excess in S vapor promotes the formation of undesired WS2 at the interface.104 Therefore, single-crystalline triangular WSe2 monolayer requiring a higher growth temperature (925 °C) was first prepared, and then the MoS2 was grown at a lower temperature (650 °C) in a separate furnace to avoid the alloy reaction as observed in one-pot synthesis.104 The TMDC−TMDC lateral junction formed among the four TMDCs, WSe2, MoSe2, WS2, and MoS2, is always a type-II semiconductor junction,105,106 and a p−n junction can be easily formed after Fermi energy alignment. With all the 2D building blocks and knowledge of lateral junction formation, it is crucial to be able to grow the junction in a controllable way. Areaselective growth is the key step to develop. However, the lateral junctions can only be randomly formed by CVD method on the growth substrates, which hinders the TMDC-based large-scale integrated circuit fabrication. Therefore, area-selective growth and photolithographic processes shall be used to well define a junction on substrate. Recently, Li and co-workers demonstrated a photoresist-free method to pattern as-grown TMDC

location-selective growth, and layer-by-layer formation. Here we present a comprehensive review of the structural characteristics and synthetic routes of 2D stacking and their functionalities. 4.1. Vertical Stacked 2D Heterostructures

The epitaxial process may start from the basal plane or the lateral edge depending on the lattice, edge morphology of the 2D layer materials, and growth regime (thermodynamically or kinetically controlled). Vertically stacked TMDCs have a large interface area, which can significantly promote the photon carrier interaction and thus have immense potential applications in solar cell, light-emitting diodes, and photodetectors. Ionescu et al. have demonstrated highly ordered MoS2/WS2 bilayer domains by sulfurization of drop-casted MoO3 and WO3 nanobelts in sequence.95 Besides, Gong et al. also have reported the synthesis of WS2 and MoS2 heterostructures,53 where the mixture of molybdenum trioxide (MoO3) and tungsten was used as the metal sources and tellurium powders were added to accelerate the melting of tungsten powder during the growth. TMDC layers with different stacking manners can be achieved by controlling the reaction temperature, where in-plane lateral heterojunctions are formed at ∼650 °C, and vertically stacked bilayers dominate when the synthesis is carried out at ∼850 °C. Gong and co-workers further report the vapor-phase growth of 2D TMDC heterostructures with vertical stacking using a twostep approach, where the MoSe2 layers are synthesized first and are followed by an epitaxial growth of WSe2 on top.44 4.2. Lateral Stitched 2D Heterostructure

Compared to the vertically stacked TMDCs, another very attractive structure is a lateral heterostructure in which the junction is atomically sharp and the active region can be as narrow as a few strings of atoms at the junction areas. This structure offers much easier band-offset tuning because materials are spatially separated. However, the lateral heterostructure is not achievable by the layer-stacking techniques. Several attempts of using vapor-phase reaction of transition-metal oxides and chalcogenides have been done to grow lateral heterostructures.53,96,97 Until today, the controlled growth of 2D heterostructure is still mostly at the exploration stage. Recent works have shown that the use of graphene−MoS2 heterostructures, as the contacts to MoS2 channels, can lead to a much lower contact resistance than conventional metal contacts. A strong photocurrent was observed from the heterojunction. As shown in Figure 8a−e, Lin and co-workers have reported a recent development of graphene−MoS2 lateral heterojunctions with nanometer overlapped region.98 This lateral heterojunctions was obtained by first transfer of the CVD graphene on SiO2 /Si and then patterning with lithography, followed by MoS2 synthesis using CVD methods. High-resolution transmission microscope (TEM) imaging in Figure 8b reveals that, instead of the two materials forming an in-plane junction directly, they present as an overlapped junction a few nanometers up to 20 nm in width. The overlapped junction releases the requirement to match the lattices at the interface. It has been shown that such onedimensional (1D) “edge contact” is very advantageous for electrically contacting graphene and other 2D materials to achieve high-performance devices. Similarly, the heterojunction formed by two different TMDCs monolayers can also be produced via lateral heteroepitaxy using CVD method.55,99 The spatially connected 6144


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TiS2 is promising for energy-storage application owing to highly reversible and high-rate charge-storage characteristics displayed in the 2D morphology.34 Basically, transitions from indirect to direct band gap in 2DTMDC materials associated with the decrease in number of layers is a unique feature of TMDCs. However, some new members in the 2D-TMDC family lack indirect to direct band gap transitions but exhibit in-plane anisotropic properties such as ReS2,120 ReSe2,121 and WTe2122 with tunable optical and electrical properties by local strain engineering, which also opens new routes for novel electrical and optical applications. Moreover, WTe2 with a high current density and low thermal conductivity is desirable for applications such as phase change memories.123 Furthermore, strong spin orbit coupling of MoTe2 and WTe2, as demonstrated theoretically, renders applications in spinotronics.47 In addition, the theoretical calculation shows that the phonon-limited mobilities of MoTe2 and HfSe2 are ∼2500 and 3500 cm2/(V·s) at room temperature, respectively.124 Several new 2D-TMDCs have exhibited remarkable performance in field-effect transistors and photodetectors. For example, 1T-phase 2D MoTe2 used in a topological field-effect transistor with fast on/off switching can be realized by using the topological phase transition.125 Besides the electronic properties, MoTe2 extends the spectral range from the visible to the near-infrared.126 Also, field-effect transistors based on a few-layered SnSe2 have shown high drive currents,127 as well as high on/off ratios.128 Unlike most 2D TMDCs, the PtS2 exhibits a strong interlayer interaction that narrows its band gap, where the band gap largely changes with the number of layers and a relatively high mobility can be achieved.111 On the other hand, ultrathin SnSe2,129 ReSe2,130 HfS2,131 and In2Se3132 photodetectors have demonstrated high photoresponsivity and relatively fast response to light. For catalysis applications, layered platinum dichalcogenides PtS2, PtSe2, and PtTe2 are promising owing to their excellent electrocatalytic activity.133 All these Pt-based dichalcogenides manifest inherent electrochemical activities, and the electrocatalytic properties are strongly associated with their electronic structures. For example, the hydrogen evolution reaction (HER) catalytic activity of the Pt chalcogenides changes with the chalcogens following the order PtTe2 > PtSe2 > PtS2.133 Promising thermoelectric properties and multiple thermoelectric transport effects are shown in monolayer ZrSe2 and HfSe2 .134 In wearable applications, a 2D In2 Se3 -based electronic-skin strain sensor array was fabricated and displayed high spatial resolution in strain detection.135 To explore the properties of the new candidates and possible applications, high-quality 2D layered materials are required. So far, there have been plenty of methods for the synthesis of 2D layered materials. Among the new candidates, MoTe2,136,137 HfSe2,138 PbI2,137 ReSe2,130 In2Se3,135 and SnSe2139 were successfully synthesized by CVD methods. The epitaxial growth of high-quality single-crystalline monolayer PtSe2 has been demonstrated by direct selenization of a Pt (111) substrate.140 High-quality PtSe2 crystals have also been successfully grown by the chemical vapor transport (CVT) method.141 Recently, Hu’s group has successfully synthesized 2D In2Se3 nanosheets on flexible mica by van der Waals epitaxial (vdW) CVD method using In2O3 and Se powder as precursors at temperature 660 °C with a mixture of H2 and Ar as a carrier gas.135 Moreover, Shi’s group has reported the growth of PbI2 by heating PbI2 powders to 250−300 °C under a pressure of 120 Torr and a constant flow of Ar.137 In Zhai’s group, ultrathin hexagonal

monolayers, which offers better controllability for TMDC heterostructure synthesis. The controlled patterning of TMDC monolayer was realized by a focused ion beam (FIB) method, where the exposed edges serve as the growth seeds enabling the CVD growth of a second TMDC material to form desired lateral heterostructures with arbitrary layouts. Devices fabricated based on the synthesized TMDCs heterostructure show excellent stability and obvious rectification behavior.107

5. NEW 2D CANDIDATES EXTENDING THE GROUP VIB TRANSITION-METAL DICHALCOGENIDES CATALOG Besides the extensively investigated group VIB TMDCs members such as MoS2, WS2, MoSe2, and WSe2, increasing efforts have been devoted to other TMDCs candidates with their own interesting physical and chemical properties that enable promising applications in electronics,108 optoelectronic devices,108,109 and sensors.110 New types of 2D materials such as indirect band gap semiconductor PtS2,111 metallic 3R phase NbS2,112 and 1T′ phase MoTe258,113 are emerging. The synthesis of these TMDC layers is targeted at expanding the band gap coverage in the 2D family, improving the electrode contact properties, and pursuing the ultrahigh electron/hole mobility and new physical properties in electronic devices. Since the discovery of charge-density-wave (CDW) in TMDCs, the layered TMDCs have become model materials for investigating the mechanism of CDW.114 For example, TaS2 crystals consist of a S−Ta−S sandwich structure with covalent bonding among S and Ta inside sandwiches and weak van der Waals bonding between the layers. The coordination of Ta atoms can be either octahedral or trigonal prismatic, leading to the stacking with 1T or 2H coordination polytypes. The 1TTaS2 polytype exhibits a wide range of CDW phases.114 The 2H-MX2 TMDCs, where M = Ti, Ta, or Nb and X = S or Se, exhibit many similarities with the high-temperature superconductors. Not alone, many typical TMDCs also show the coexistence and competition between superconductivity and CDWs. For example, titanium diselenide (TiSe2) is a CDW material, and it can become a superconductor with a transition temperature at ∼4 K under a hydrostatic pressure115 and copper doping.116 Contrary to other TMDCs, NbSe2 has a relatively high superconducting transition temperature (∼7.15 K).117 Neutron diffraction studies on 2H-NbSe2 show that it undergoes a phase transition to an incommensurate, triangular charge-density-wave phase at a temperature ∼33 K. Experiments show that the incommensurate CDW remains at least to 1.3 K.118 On the other hand, 2H-NbSe2 also becomes a prototypical anisotropic type-II superconductor below a temperature of ∼7.2 K. Thus, superconductivity and the CDW ordered states coexist in 2H-NbSe2 below 7.2 K. The coexistence of the density wave and superconducting phases and the nature of their interplay has been a subject of much recent interest, especially in the context of competing order in strongly correlated systems.118 Besides, the phase transitions between many-body states are studied by tuning the material through the CDW and superconductivity phases using the electric-field effect, where the electronic state modulation is fundamentally important to the emergence of 2D superconductivity.104 On the other hand, the electronic structure of titanium disulfide (TiS2) can be tuned by strain or alloying to bring back the CDW instability because this phase is incipient in this material as concluded from the electronic band structure calculations in the bulk and monolayer TiS2.119 Additionally, 6145


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ReSe2 flakes are successfully synthesized using CVD method on SiO2/Si substrates. Precursor materials ReO3 and Se were used at temperature 625 °C under the constant flow of N2 and H2.130 High-quality single-crystalline SnSe2 nanosheets on SiO2/Si substrates are synthesized for the first time by He’s group, where a mixture of Se and SnSe powder was heated at 750 °C with Ar as a carrier gas.139 Dresselhaus’ group successfully synthesized two phases of MoTe2 (pure 2H and pure 1T′MoTe2) on SiO2/Si using MoO3137 or Mo136 as precursors, and they found that the phase of MoTe2 grown from Mo precursors is controlled by selecting different Mo precursors.136,137 In Li’s group, HfS2 nanosheets have been successfully synthesized by CVD using hafnium tetrachloride (HfCl4) as precursor instead of HfO2 because of the much lower sublimation temperature of HfCl4 (317 °C) and sulfur powder. The growth has been done in a mixture of Ar and H2 at a temperature of 950 °C for the SiO2/Si substrate and 160 °C for the sources.138

ORCID

Lain-Jong Li: 0000-0002-4059-7783 Author Contributions §

H.L. and Y.L. contributed equally to this Review

Notes

The authors declare no competing financial interest. Biographies Dr. Henan Li received her Bachelor’s degree (BE) in Materials Science and Engineering from Nanyang Technological University in 2008. She was then awarded Nanyang President’s Graduate Scholarship to pursue her doctoral studies at the same university, and she obtained her Ph.D. degree in 2014. Her research interests include the synthesis and detailed characterization of single crystals, nanostructures, and 2D materials by means of various diffraction techniques (X-ray, synchrotron, and neutron), SEM, and TEM. Dr. Ying Li is an Associate Professor in the International Collaborative Laboratory of 2D Materials for Optoelectronics Science and Technology of Shenzhen University. She received her Ph.D. in optical engineering from Fudan University, China, in 2010. Her current research interest focuses on the nonlinear optics and all-optical device including the broadband optical nonlinearities of 2D materials and their applications in microwave photonics. She has authored more than 50 publications in international journals and more than 10 patents.

6. SUMMARY AND OUTLOOK In the past few years, there have been many breakthroughs in monolayer 2D nanomaterials. The latest developments of CVD techniques have provided insights for the controllable and reliable synthesis of atomically thin 2D layers. However, it is demanding but still challenging to realize fast synthesis of TMDCs with large domain sizes. Growth conditions such as total pressure, temperature, partial pressure of reactants, and evaporation rate should be optimized to maintain a low nucleation density but high growth rate. On the basis of the growth mechanism, a two-step growth process can be proposed as a promising strategy, where the process can be performed at a high growth temperature with lower reactants flow rate (or partial pressure) to achieve low nucleation density, followed by the increase in flow rate to facilitate the lateral growth from the domain edges. Alternative to the conventional hot wall CVD methods, other bottom-up synthesis techniques such as coldwall CVD, magnetron sputtering, and pulsed laser deposition are also emerging.60−63 With better control over reactants delivery during synthesis and good industrial compatibility, these emerging techniques show significant promise for a scalable synthesis of TMDCs with high uniformity and reliability. Meanwhile, semiconducting monolayer TMDC materials with tunable doping levels and suitable band structure make them complementary to other 2D materials such as metallic graphene and insulating boron nitride. The capability to precisely engineer 2D materials by chemical approaches has also given rise to fascinating new physics leading to exciting new applications. To realize the practical applications, efforts have been made to tackle issues related to the synthesis and device fabrication. Considering numerous possibilities in terms of designing and fabricating different 2D layered compounds, as well as integrating them in heterolattices vertically and/or laterally to achieve tailored properties for specific electronic applications, there is still a strong demand to deepen the understanding of the growth mechanisms of TMDCs and further develop new techniques for the 2D electronic devices.

Miss Areej Aljarb received her B.Sc. degree from Physics department at King Abdulaziz University (KAAU) in 2009 and received her M.Sc. from the same department in 2012. She worked as a lecturer from 2009 to 2014 in KAAU. In 2016, she joined 2D Materials Research lab in the Material Science and Engineering program at King Abdulla University for Science and Technology (KAUST) as a Ph.D. student. Her research interests relate to the controlled growth and exploration of fundamental phenomena of two-dimensional atomic layer thin materials. Dr. Yumeng Shi obtained his Ph.D. in Nanyang Technological University, Singapore, in 2011. He worked in Nanyang Technological University, Singapore (2010−2011), Massachusetts Institute of Technology, U.S.A. (2011−2012), and Singapore University of Technology and Design, Singapore (2012−2014), as a postdoctoral fellow. Since 2014, he became a research scientist in Physical Sciences and Engineering Division at King Abdullah University of Science and Technology, Saudi Arabia. He started his professorship at Shenzhen University in 2016, with support from the National Thousand-YoungTalents Program of China. His research interests include synthesis, characterizations, and energy applications of 2-dimensional materials. Dr. Lain-Jong (Lance) Li received his B.Sc. at National Taiwan University as a Dr. Yuan T. Lee Fellow and obtained his M.Sc. at the same department. After 5 years of R&D at the TSMC, he obtained his Ph.D. in condensed matter physics from Oxford University in 2006 as a Swire Scholarship Fellow. He was an assistant professor in MSE at Nanyang Tech. Univ. Singapore (2006−2009). Since 2010, he has become an Associate Research Fellow at Academia Sinica Taiwan. He started his Associate Professorship at KAUST, in 2014. His research interests include chemical vapor deposition and characterizations of 2dimensional materials and their energy applications.

AUTHOR INFORMATION

ACKNOWLEDGMENTS This work was supported by National Natural Science Foundation of China (Grant no. 51602200), Educational Commission of Guangdong Province (Grant no.

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. 6146


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2016KZDXM008), Natural Science Foundation of SZU (Grant no. 2017011), and King Abdullah University of Science and Technology, Saudi Arabia. This work was partially supported by the Science and Technology Planning Project of Guangdong Province (Grant no. 2016B050501005), the Educational Commission of Guangdong Province (Grant no. 2016KCXTD006), and Shenzhen Peacock Plan (Grant no. KQTD2016053112042971).

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