Chemical Vapor Deposition Growth and Applications of Two

Review Cite This: Chem. Rev. XXXX, XXX, XXX−XXX

pubs.acs.org/CR

Chemical Vapor Deposition Growth and Applications of TwoDimensional Materials and Their Heterostructures Zhengyang Cai,† Bilu Liu,*,† Xiaolong Zou,† and Hui-Ming Cheng*,†,‡,§ †

Shenzhen Geim Graphene Center (SGC), Tsinghua−Berkeley Shenzhen Institute (TBSI), Tsinghua University, Shenzhen, Guangdong 518055, People’s Republic of China ‡ Shenyang National Laboratory for Materials Sciences, Institute of Metal Research, Chinese Academy of Sciences, Shenyang, Liaoning 110016, People’s Republic of China § Center of Excellence in Environmental Studies (CEES), King Abdulaziz University, Jeddah 21589, Saudi Arabia ABSTRACT: Two-dimensional (2D) materials have attracted increasing research interest because of the abundant choice of materials with diverse and tunable electronic, optical, and chemical properties. Moreover, 2D material based heterostructures combining several individual 2D materials provide unique platforms to create an almost infinite number of materials and show exotic physical phenomena as well as new properties and applications. To achieve these high expectations, methods for the scalable preparation of 2D materials and 2D heterostructures of high quality and low cost must be developed. Chemical vapor deposition (CVD) is a powerful method which may meet the above requirements, and has been extensively used to grow 2D materials and their heterostructures in recent years, despite several challenges remaining. In this review of the challenges in the CVD growth of 2D materials, we highlight recent advances in the controlled growth of single crystal 2D materials, with an emphasis on semiconducting transition metal dichalcogenides. We provide insight into the growth mechanisms of single crystal 2D domains and the key technologies used to realize wafer-scale growth of continuous and homogeneous 2D films which are important for practical applications. Meanwhile, strategies to design and grow various kinds of 2D material based heterostructures are thoroughly discussed. The applications of CVD-grown 2D materials and their heterostructures in electronics, optoelectronics, sensors, flexible devices, and electrocatalysis are also discussed. Finally, we suggest solutions to these challenges and ideas concerning future developments in this emerging field.

CONTENTS 1. Introduction 2. Overview of the Key Parameters in the CVD Growth of 2D Materials 2.1. Precursor 2.2. Temperature 2.3. Substrate 2.4. Pressure 2.5. Other Parameters 3. CVD Growth of Single Crystal 2D Materials 3.1. Theory of 2D Materials Growth by CVD 3.2. Controlled Growth of Single Crystal 2D Materials 3.2.1. Grain Size 3.2.2. Layer 3.2.3. Orientation 3.2.4. Morphology 3.2.5. Phase 3.2.6. Doping 3.2.7. Quality and Defects 4. CVD Growth of Wafer-Scale Continuous 2D Material Films 4.1. Motivation for and Challenges of Growing Wafer-Scale Continuous and Homogeneous 2D Material Films © XXXX American Chemical Society

4.2. Growth of Continuous 2D Films on Rigid Substrates 4.2.1. MOCVD Method 4.2.2. Conventional CVD Method 4.2.3. Thin Film Deposition Method 4.3. Growth of Continuous 2D Films on Flexible Substrates 4.3.1. Direct Growth on Flexible Substrates 4.3.2. Postgrowth Transfer 2D Materials onto Flexible Substrates 5. CVD Growth of 2D Material Based Heterostructures 5.1. Vertical 2D Heterostructures 5.1.1. Metal/Semiconductor 5.1.2. Semiconductor/Semiconductor 5.1.3. Semiconductor/Insulator 5.1.4. Metal/Insulator 5.2. Lateral 2D Heterostructures 5.2.1. Metal/Insulator 5.2.2. Semiconductor/Semiconductor

B C D D D E E E E F F G H J K K L M

M M O O P P Q Q R R S U U V V V

Special Issue: 2D Materials Chemistry Received: September 5, 2017

M A

DOI: 10.1021/acs.chemrev.7b00536 Chem. Rev. XXXX, XXX, XXX−XXX

Chemical Reviews 5.2.3. Metal/Semiconductor 5.3. Other Dimension/2D (xD/2D) Heterostructures 5.3.1. 1D/2D 5.3.2. 0D/2D 6. Applications of CVD-Grown 2D Materials and Their Heterostructures 6.1. Electronics 6.2. Optoelectronics 6.2.1. LEDs 6.2.2. Photodetectors 6.2.3. Solar Cells 6.3. Sensors 6.4. Flexible and Wearable Devices 6.5. Hydrogen Evolution Reactions 7. Concluding Remarks and Outlook 7.1. Improvement and Innovation of CVD Growth Systems 7.2. Salt-Assisted Growth of TMDCs: A General and Efficient Route 7.3. High Quality 2D Materials 7.4. Large Single Crystal and Fast Growth Methods: A Long Way to Go 7.5. Exploration of Novel 2D Materials with Exotic Properties 7.6. Low Temperature Growth of 2D Materials for Flexible Device Applications 7.7. Heterostructures: Variations and Easy Growth 7.8. High-Performance Scalable Electronics and Optoelectronics Author Information Corresponding Authors ORCID Notes Biographies Acknowledgments References

Review

example, 1D and 0D materials are usually prepared by bottom-up synthesis approaches, while 2D materials can be prepared by either bottom-up synthesis or the top-down exfoliation of bulk layered materials. In addition, layer−layer interactions in 2D materials can result in unusual material properties and provide excellent platforms to study fundamental physics. As a representative 2D material, graphene, a monolayer of graphite, exhibits extraordinarily high charge carrier mobility, along with many other excellent properties including high thermal conductivity, high mechanical strength, large surface area and broadband optical absorption, etc.1,2 Intriguing physical phenomena such as the quantum hall effect and a Berry phase have been reported in graphene.3 These features make graphene a “superstar” in condensed matter physics and materials sciences and have promoted extensive research on 2D materials analogous to graphene, including hexagonal boron nitride (hBN), transition metal dichalcogenides (TMDCs), black phosphene, silicene, germanene, etc., forming a large family of 2D materials.4,5 Importantly, the 2D materials family exhibits a wide spectrum of electronic properties covering metals, semimetals, semiconductors with various energy band gaps, and also insulators.6,7 Besides their wide range of electronic properties, 2D materials also have high surface areas, dangling bonds, and surface state free nature, and may exhibit strong spin−orbit coupling and quantum spin Hall effects, making them attractive for studying new physics and exploring devices using new concepts.8,9 Moreover, when different 2D materials stack or merge, vertical or lateral heterostructures with an atomically sharp interface can be obtained, providing perfect platforms to investigate layer−layer interactions at the atomic scale as well as devices with new functions.10−12 Taken together, these unique features of 2D materials allow them to have potential in many fields including electronics, optoelectronics, sensors, flexible and wearable devices, catalysis, and many others. The properties of 2D materials are closely related to their structures, including size, number of layers, morphologies, orientations, phases, doping, defects, grain boundaries, etc.13 For instance, (i) the number of layers, the phases, and any doping of 2D materials each have a crucial influence on the electronic structures, and further determine the performance of 2D material based electronic and optoelectronic devices.14,15 (ii) High quality 2D materials are often defect-free or contain few grain boundaries, resulting in a high charge mobility and a low interface scattering and excellent electronic performance. (iii) Large area continuous 2D crystals with uniform properties are of great importance for devices with reproducible performance. For practical applications of 2D materials, especially in electronics and optoelectronics, high quality and large area materials with controlled properties are prerequisites. This fact calls for a deep understanding of the material growth mechanism for preparing 2D materials with controlled quality, structure, and other properties. Generally, mechanical exfoliation, liquid exfoliation, and vapor phase growth are commonly adopted strategies for 2D material preparation. For example, the Scotch-tape-based mechanical exfoliation method led to the discovery of graphene,16 and it is still an important method for preparing virtually any 2D material. It can produce the highest quality 2D materials, making it an ideal tool for fundamental physics studies.17 However, it is obvious that such a method is not suitable for the large-scale production of 2D materials. In addition, it is difficult to control the number of layers and their size, orientation, and phase using this technique since this method highly relies on the experience of researchers.

W W W X Y Y Z Z AA AB AB AC AC AE AE AF AF AF AF AG AG AG AG AG AG AG AG AH AH

1. INTRODUCTION Advanced materials play important roles in our daily lives. It is well-known that human civilization can be classified by the key material used at the time, e.g., Stone Age and Bronze Age. Historically, people have spent thousands of years to improve the properties of materials by playing with their chemical compositions, structures, processing recipes, etc. Since the 1980s, dimensionality has entered the horizons of condensed matter physics and materials science research and triggered rapid developments in low-dimensional (LD) materials. In comparison to traditional bulk materials which are commonly called three-dimensional (3D) materials, LD materials refer to materials that have a nanoscale size in at least one dimension. LD materials exhibit unique properties originating from the quantum confinement effect of their nanoscale size, which provides researchers the ability to precisely tune the material’s properties at the atomic level. In general, LD materials can be divided into three categories: two-dimensional (2D), onedimensional (1D), and zero-dimensional (0D). Of the above three types of LD materials, 2D materials are rather unique because they have bulk counterparts, i.e., layered materials. In contrast, neither 0D nor 1D materials have such bulk counterparts. This feature leads to several noticeable differences between 2D materials and 1D or 0D materials. For B

DOI: 10.1021/acs.chemrev.7b00536 Chem. Rev. XXXX, XXX, XXX−XXX

Chemical Reviews

Review

Figure 1. Overview of the CVD growth of 2D materials, from single crystals to continuous films, and further to 2D material based heterostructures, as well as their device applications. Reprinted from ref 24. Copyright 2015 American Chemical Society. Reprinted with permission from ref 25. Copyright 2015, Rights Managed by Nature Publishing Group. Reprinted with permission from ref 26. Copyright 2015, Rights Managed by Nature Publishing Group. Reprinted from ref 27. Copyright 2014 American Chemical Society.

In the past few years, researchers have made great achievements in the CVD growth of 2D materials and the exploration of their use.23 Here, we present a comprehensive review of recent progress in the controllable growth of 2D materials and their heterostructures, as well as the applications of these advanced materials in various areas (Figure 1). We focus on both the fundamental nucleation and growth mechanism of 2D materials, as well as technologies developed on how to grow large single crystal 2D material domains, wafer-scale 2D material films, and the recently emerging 2D material based heterostructures. We also list several key challenges and opportunities in this field related to both the growth and applications of 2D materials, and suggest how these problems might be solved to further push developments in this important field.

On the other hand, a liquid phase exfoliation method is suitable for the low-cost production of 2D materials with scale-up capability. However, the size and quality of solution-prepared 2D materials are of concern for some applications.18 In this regard, the vapor phase based direct growth approaches, especially chemical vapor deposition (CVD), provide a scalable and controllable way to grow high quality and large area 2D materials with a reasonable cost. These advantages make CVD very important for fundamental research and exploring the applications of 2D materials.19,20 Several correlations have been discovered between the parameters used in CVD and 2D materials produced. On one hand, CVD is a process in which gaseous materials react in the vapor phase or on the surface of substrates, and thus form solid products that are deposited on the substrates. The number of layers, their size, morphology and orientation, and the introduction of any dopants or defects can be controlled by playing with the growth parameters such as temperature, chamber pressure, carrier gas flow rate, relative amounts of source materials, and source−substrate distance. Recently, modifications of 2D material growth methods have been widely used to fabricate their heterostructures, making the 2D family more versatile than uniphase 2D materials. On the other hand, theoretical and technological advances in the growth of 2D materials can improve the development of the CVD method itself. Compared with traditional CVD approaches to grow silica, tungsten, and diamond, some modified CVD methods have been developed to grow large area and high quality 2D materials.21 For example, Wu et al. developed a local feed method by supplying the source gas to a single point with a small area in the CVD furnace to create only one nucleus, which later grew into an inch size single crystal graphene domain.22 In this regard, the development of CVD methods and 2D material growth may together promote the rapid development of research into 2D materials and accelerate their wide use with the availability of large-scale and high quality CVD-grown samples.

2. OVERVIEW OF THE KEY PARAMETERS IN THE CVD GROWTH OF 2D MATERIALS In general, 2D materials can be prepared by two strategies, i.e., top-down-based exfoliation methods and bottom-up-based growth methods. The former includes mechanical and liquid phase exfoliation, while the latter includes CVD, physical vapor deposition (PVD), vapor phase transport (VPT), molecular assembly, atomic layer deposition, etc. Note that, in this review, we use the term “CVD” as a general term to cover several vapor transport based methods including CVD, PVD, VPT, and other closely related techniques. Such CVD-based vapor phase growth methods are powerful in preparing high quality 2D materials, because they can grow materials with a high quality and scale-up capability. Hence, it is important to highlight several key CVD parameters in growing 2D materials. The properties of 2D materials are largely dependent on their size, morphology, phase, any interfaces present, etc. These features can be controlled by rational design and careful tuning of the CVD growth processes. Therefore, it is important to understand the general mechanisms of CVD growth, i.e., how C

DOI: 10.1021/acs.chemrev.7b00536 Chem. Rev. XXXX, XXX, XXX−XXX

Chemical Reviews

Review

2.2. Temperature

parameters such as the precursor, substrate, pressure, and temperature affect mass and heat transport, interface reactions, and consequently the growth of the materials (Figure 2).

Generally speaking, the temperature in a CVD system may affect the flow of the carrier gas, the chemical reactions of precursors in the gas phase, and the deposition rate of products on the substrate. These features suggest that temperature can determine the composition and uniformity of the products. High quality products can usually be obtained at relatively high temperatures but only at the price of high energy consumption and limited suitable substrates. In a more important aspect, temperature may result in fast diffusion of species in the gas flow on the order of 105 cm/s, which is much higher than the flux velocity in a typical CVD process. This would result in a concentration gradient, especially with respect to mass transport near the solid surface in the CVD system. Temperature plays an even more important role in TMDC growth than in graphene growth in the following two aspects. First, as mentioned above, the temperature in the source zone is important because a slight temperature change may lead to a large change in the saturation pressure of gasified solid precursors, which in turn will significantly affect the growth of TMDCs. Second, the temperature in the reaction zone can affect the mass transport of species and their reaction at the vapor− solid interface, providing an effective approach to adjusting the reaction rate. The temperature in the source zone is used to gasify solid precursors such as sulfur and selenium in TMDC growth. In this case, the higher the temperature, the higher the gas concentration, and this kind of sufficient supply of precursors can make the growth controlled by the chemical reaction rate. On the other hand, a relatively low temperature would result in a mass transport limited growth mechanism. Furthermore, with the consumption of solid precursors, their concentrations in the gas phase would decrease gradually, leading to a gradient of precursor feed in TMDC growth thus making CVD growth difficult to control. For nucleation at the vapor−solid interface, generally speaking, a high temperature leads to a thermodynamic process while a low temperature leads to a kinetic process. In recent experiments, multilayer MoSe2 nucleation, that is when the upper triangles grow from the same nucleation site at the center of the bottom triangle, was found to occur at temperatures higher than 800 °C, suggesting that temperature acts as an important factor in controlling the number of layers of 2D materials.31 It is therefore obvious that, for growth at this interface, a high temperature would accelerate the growth rate. The importance of growth temperature on the layer size, number of layers, and shape of as-grown WSe2 flakes has been systematically studied,24 showing the importance of temperature on the controlled growth of TMDCs.

Figure 2. Schematic showing how various parameters including precursor, temperature, pressure, and substrate affect the thermodynamics and kinetics in the CVD growth of 2D materials.

2.1. Precursor

Precursors serve as reactants in the CVD process. Briefly, three reactions, including thermal decomposition reactions, chemical synthesis reactions, and chemical transport reactions, are generally involved in the conversion of precursors into the desired products. In a commercial CVD setup to grow silicon, vapor sources along with a carrier gas (SiH4 or SiH2Cl2, H2, and Ar) are directly introduced into the reaction system, and the use of these gaseous precursors allow precise control of the number of molecules involved (e.g., 1 sccm at atmospheric pressure equals 7.4 × 10−7 mol/s; here sccm stands for standard cubic centimeters per minute) by changing the flow rate and partial pressure of each precursor. In addition, a large amount of hydrogen gas in the reactor can terminate any broken or dangling bonds and result in high quality samples. However, high purity precursors are often needed to avoid detrimental contaminants and unwanted side reactions. Basically, this is also true for graphene deposition, where gaseous sources such as CH4 and H2 are commonly used as precursors. This type of gaseous precursor is convenient for the CVD process due to the easy and accurate control of the gas flow rate over a wide range, allowing one to precisely control the structure, morphology, and size of the graphene. In addition, by introducing gaseous NH3 or PH3, the graphene can be doped with N or P atoms.28,29 In comparison, solid sources are currently most commonly used for the growth of TMDCs. In these processes, transition metal oxides (e.g., MoO3, WO3) or chlorides (MoCl5) or metal foils often serve as metal (molybdenum or tungsten) sources while S or Se powders act as the source of sulfur or selenium. These solid precursors require very accurate temperature control of the source zone since the vapor pressure of solid materials is very sensitive to temperature.30 Therefore, the controllability of TMDC growth is a real challenge and the technology for it is much less mature than that for graphene growth. The use of all gaseous precursors such as Mo(CO)6, CS(CH3)2, and H2S has been shown to be able to improve the uniformity of as-grown TMDC films.25

2.3. Substrate

“Substrate” refers to where the material is deposited in the CVD process, but more than that, the substrate provides other functions. Catalytic active nickel and copper substrates are usually used as substrates as well as catalysts for multilayer and monolayer graphene growth, due to their different carbon solubilities and catalytic abilities. In contrast, inert Si/SiO2,24 mica,32 and polyimide33 are commonly used substrates for 2D TMDC growth, while the use of metals such as gold or tungsten foils for TMDC growth has also been reported.34−36 Specifically, the microstructure and lattice structure of the substrate can significantly affect the growth of 2D materials. For the simple fabrication of electronic and optoelectronic devices, TMDC materials are commonly grown on Si/SiO2 substrates without adding any catalysts. On this kind of SiO2/Si substrate, certain D

DOI: 10.1021/acs.chemrev.7b00536 Chem. Rev. XXXX, XXX, XXX−XXX

Chemical Reviews

Review

temperature of thousands of kelvin. Such high temperatures can induce unusual processes that would not happen at regular temperatures, such as the dissociation of precursor molecules. Moreover, interactions between the surface and energetic ions lead to an increase in the density of the deposited film, and help remove contaminants and improve the quality of the films. It should be noted that the inductively coupled plasma CVD (ICPCVD) method permits film deposition at an even lower temperature than normal PECVD. These methods have great potential in achieving high quality 2D films at moderate temperatures, which is important for growth on polymeric or glass substrates. Different from the conventional growth of graphene on metal foils (e.g., Cu, Ni) at temperatures around 1000 °C,51,52 Sun et al. reported the growth of graphene on glass without any catalyst at low temperatures in the range 400−600 °C by the PECVD method.53 The structure and electrical properties of the as-grown graphene have been shown to be well-tailored during PECVD. In addition, this method also works for TMDCs. For example, Kim et al. used the PECVD approach and realized the growth of MoS2 films at temperatures from 150 to 200 °C using a Mo thin film and H2S gas as precursors.54 Notably, ICP-CVD provides a powerful approach to grow TMDC materials with novel structures. For example, Lu et al. used ICP-CVD to strip the top-layer S atoms from an as-grown MoS2 monolayer and obtained intermediate MoSH, and followed this with thermal selenization to grow Janus MoSSe monolayer TMDC materials.55 These results demonstrate that advanced CVD systems can help to create new materials and make 2D materials grown in an easier way.

substances such as perylene-3,4,9,10-tetracarboxylic acid tetrapotassium salt (PTAS), perylene-3,4,9,10-tetracarboxylic dianhydride (PTCDA), and reduced graphene oxide (rGO) were used to promote the nucleation and growth of TMDCs.37 In addition to these seeding promoters, additives such as alkali metal halides (NaCl, KI, KCl, and NaBr), sodium cholate, and NaOH have been increasingly frequently used to promote the growth of TMDCs. Such additives, or growth promoters, can react with high melting point precursor (such as WO3 and MoO3), and form volatile intermediates.38−43 Via adding such growth promoters, the domain sizes of TMDCs usually increase, and the growth windows become broader. However, the functions of such growth promoters are not very clear, and more efforts are needed to gain better understanding about the mechanism of these growth promoters. However, a very different situation occurred for sapphire substrates. Due to the specific lattice orientation of c-plane sapphire and the atomically smooth sapphire terraces on the surface, TMDC growth on this kind of substrate showed a specific orientation preference, related to the crystal symmetry and the surface terraces of the substrates.44,45 For metal foil substrates, the crystallography of gold can be used to determine the nucleation and domain size of TMDCs by selecting different facets. This preferential nucleation and growth on specific facets originates from the facet-dependent binding energy between TMDCs and substrates.46 In addition, the orientation of the substrate is important for morphology control of the as-grown sample. For example, a MoS2 continuous film with a domain size as large as 20 μm was grown by using a growth substrate oriented in a vertical position in the chamber rather than a horizontally oriented growth substrate; in this case, the uniformity of the precursor feedstock was significantly improved.47

3. CVD GROWTH OF SINGLE CRYSTAL 2D MATERIALS Recent studies have shown that CVD is very powerful in synthesizing 2D materials. For practical applications of CVDgrown 2D materials, it is crucial to improve the reproducibility of the growth process, which needs a deep understanding of the general growth mechanism. Hence, the link between the theory of 2D material growth and the CVD process needs to be studied and understood. Single crystal materials provide a valuable platform to investigate fundamental nucleation and growth mechanisms, as well as develop approaches to controlling the properties of the materials. In this section, based on an understanding of the growth mechanisms of 2D materials, theory and practical approaches on how to control several key features of the single crystals, including their size, number of layers, orientation, morphology, and phase, and any dopants and defects they contain are discussed.

2.4. Pressure

Pressure in a CVD chamber can vary over a wide range from a few atmospheres to several millitorr or even lower, and this has a tremendous influence on the gas flow behavior. At a low pressure, the volume flow and the velocity of gas are much increased for the same molar flow, while the precursor concentration decreases based on the ideal gas equation PV = nRT. As a result, the low concentration and high velocity of the mass feed of the precursor can make the reaction more controllable. Hence, for the growth of a wafer-scale, continuous TMDC film, a low pressure CVD approach is usually used.48,49 In addition, the partial pressure of a specific precursor can affect the layer-by-layer mechanism for TMDC growth. In the growth of MoS2, the partial pressure of Mo(CO)6 plays a vital role. At a low pressure, the nucleation of the second layer can happen only at the grain boundaries, whereas at a high pressure nucleation occurs randomly on the top of the first layer and leads to a mixture of monolayer and multilayer products.

3.1. Theory of 2D Materials Growth by CVD

In early research, it was reported that a typical growth process of 2D TMDCs consists of evaporation and reduction of metal oxide solid precursors to suboxides, which further react with reducer vapors (such as sulfur or selenium) to form TMDCs on a substrate. Notably, a metastable nanoparticle reaction intermediate (MOxXy) was suggested to serve as both metal source and nucleation site for the subsequent formation of layered TMDCs.56,57 In the CVD growth of 2D materials, various parameters including temperature, gas flow rate, pressure, type of substrate, etc. have effects on the nucleation and growth of 2D materials. So far, several nucleation and growth models for 2D materials have been reported, including layer-by-layer (LBL), layer-over-layer (LOL), screw-dislocation-driven (SDD),58 and dendritic growth models.59 Here, degrees of supersaturation of

2.5. Other Parameters

In addition to thermal CVD where heat is a major means of breaking chemical bonds in the precursors so as to promote following reactions, there are many other methods of achieving this purpose, for example, a plasma, light, or a laser. Among these modified CVD methods, plasma-enhanced CVD (PECVD) is a powerful tool to enable many film deposition processes that are impossible or very difficult to achieve by solely adjusting temperatures and other parameters in a typical thermal CVD reaction.50 A plasma is any gas where atoms or molecules are ionized into negatively charged electrons, positively charged ions, and neutral species. The electrons are so light and have a high E

DOI: 10.1021/acs.chemrev.7b00536 Chem. Rev. XXXX, XXX, XXX−XXX

Chemical Reviews

Review

carrier scattering at the grain boundaries, while such scattering does not exist in single crystal samples due to a lack of grain boundaries.66 In another aspect, large area 2D materials are preferred for fabricating devices based on current silicon-based semiconducting technology. In addition, compared to small single crystal samples or large polycrystalline samples, large area single crystals can provide more active area for photodetectors, which gives them a high sensitivity and short response time. As a result, large single crystal 2D materials are preferred for many applications including high-performance electronics, optoelectronics, flexible and wearable devices, thermal management applications, etc. Hence, it is of vital importance to design effective methods to grow large single crystal 2D TMDCs. Currently, 2D MoS2,67,68 WS2,69 WSe2,70 and MoTe271 crystals with large grain sizes have been grown. In the past few years, researchers have devoted much effort toward this goal and the sizes of 2D materials have been increased from submicrometers to several micrometers, and at present even to millimeter or centimeter levels.72−74 To achieve the growth of large single crystals, a reasonable strategy is to decrease the nucleation density. Based on this consideration, Chen et al. have reported simple CVD growth of monolayer MoSe2 single crystals with a size as large as ∼2.5 mm on molten glass, as shown in Figure 4a,b. The molten glass at high temperature forms a very smooth surface, resulting in few nucleation sites for growing large MoSe2 crystals. A decent fieldeffect mobility of 95 cm2 V−1 s−1 and an on/off current ratio of 107 for MoSe2 field-effect transistor (FET) were obtained and were attributed to the high quality large crystals.75 In principle, the use of molten glass or, more generally, other liquid or quasiliquid substrates may help in the growth of a variety of 2D materials besides MoSe2. This method certainly needs more exploration and development to look for other molten substrates. In addition, additives in the CVD process may have similar effects in decreasing the nucleation density. By introducing a small amount of oxygen into the CVD chamber to suppress the nucleation density by etching away unstable nuclei, large triangular monolayer MoS2 domains with sizes up to 350 μm were grown on c-plane sapphire.76 These results exemplify the positive effects of decreasing the nucleation density to achieve large size TMDCs. Another important aspect of increasing crystal size is maintaining the reaction activity for the material growth over long time. Decreasing the energy barrier for the reaction should effectively achieve this goal. A typical example of this idea was reported by Gao et al.26 According to a self-limited catalytic surface growth mechanism, large area monolayer WS2 single crystals were grown on Au foils with sizes up to the millimeter level. The importance of the Au foil lies in that S2 can dissociate into two S atoms on Au surface, which results in a lower energy barrier for the sulfurization of the WO3 precursor. As a result of this lower energy barrier, high-quality single crystal WS2 domains have been grown, as shown in Figure 4c,d. In a similar way, large single crystal MoS2 domains with an edge length of 80 μm were grown on Au foils.77 In addition, WS2 single crystals with sizes as large as 135 μm were grown on c-plane sapphire by maintaining the activity of the lateral growth process a over long time.78 Up to now, the sizes of CVD-grown TMDC single crystals are still far smaller than those of CVD-grown single crystal graphene. Understanding the grain boundary formation kinetics,79 decreasing the nucleation density, increasing the growth rate,80 and maintaining long periods of growth deserve extensive further

the reactants play a role in determining the growth modes of the materials. LBL growth is usually observed in 2D graphene and TMDC growth processes. Because nucleation of new layers has to overcome high energy barriers, high supersaturation conditions are usually necessary to initiate the nucleation and enable a reasonable crystal growth rate for LBL growth.60 For the growth of 2D materials in a CVD system, good control of the precursor concentration may lead to precise control of the number of layers and well-stacked vertical heterostructures. The SDD growth model was previously reported in the solvent synthesis of onedimensional oxides, hydroxides, and nitrides.61 Recent research indicates that 2D materials grown by CVD under low supersaturation conditions also obey a SDD model.62,63 It is shown that a new layer is initiated at the center of the first layer, and grows faster than the bottom one. When the upper layer catches up with the bottom layer, its growth will stop. This growth model results in a pyramid-like morphology with steps and helices inside, which exhibits a unique thicknessindependent vertical conductance as shown in a recent study.64 This growth model provides a potential way to control the number of layers or even stacking rotation in 2D materials. 3.2. Controlled Growth of Single Crystal 2D Materials

Based on the above theory of the nucleation and growth of 2D materials in CVD processes, in this section we will discuss the relationship between various properties of 2D single crystals and the CVD growth parameters as well as how to control their properties by changing these parameters. Specifically, we divide the controlled growth of 2D materials into seven aspects, i.e., size, number of layers, orientation, morphology, phase, dopants, and defects and quality, as illustrated in Figure 3. 3.2.1. Grain Size. The first observation of grain boundaries in CVD-grown MoS2 was reported by Najmaei et al.65 These grain boundaries may worsen the electronic, mechanical, and thermal properties of 2D materials. For instance, charge carrier mobility in polycrystalline materials would be seriously impaired due to

Figure 3. Schematic showing several key features that need control during CVD growth of 2D single crystals. F

DOI: 10.1021/acs.chemrev.7b00536 Chem. Rev. XXXX, XXX, XXX−XXX

Chemical Reviews

Review

Figure 4. Growing large single crystal 2D materials. (a) Optical image of MoSe2 crystals grown on molten glass. (b) LEED pattern of MoSe2 crystal. (c) Optical image of millimeter-size triangular monolayer single crystal WS2 domains grown on Au foil. (d) Superimposed image of 12 SAED patterns taken from different parts of the same WS2 domain, confirming it is a single crystal. (a, b) Reprinted from ref 75. Copyright 2017 American Chemical Society. (c, d) Reprinted with permission from ref 26. Copyright 2015, Rights Managed by Nature Publishing Group.

Figure 5. Controlling the number of layers in 2D materials during CVD growth. (a−c) Optical images of the CVD growth of (a) 1−2 layer, (b) 1−3 layer, and (c) 1−4 layer MoSe2. (d) Atomic configuration, (e) optical image, and (f) HAADF-STEM image of AB stacked bilayer MoS2. (g) Atomic configuration, (h) optical image, and (i) HAADF-STEM image of AA′ stacked bilayer MoS2. (a−c) Reprinted with permission from ref 31. Copyright 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim. (d−i) Reprinted from ref 87. Copyright 2015 American Chemical Society.

investigation to obtain centimeter or even inch size wafer high quality 2D TMDC single crystals. 3.2.2. Layer. One of the fascinating advantages of 2D materials is that their properties are tunable by controlling the number of layers. For the majority of TMDC materials, the band gap decreases with an increase in the number of layers and this

tendency is much more obvious when a few layers are involved. Obviously, materials with a tunable number of layers are desirable for many applications including electronics, optoelectronics, catalysis, and energy applications.81−84 More interestingly, when the layers of TMDCs such as MoS2, MoSe2, WS2, and WSe2 decrease down to monolayer, there is a transition from G

DOI: 10.1021/acs.chemrev.7b00536 Chem. Rev. XXXX, XXX, XXX−XXX

Chemical Reviews

Review

Figure 6. Controlling the grain orientations of TMDCs during growth. (a) SEM image of aligned WSe2 flakes grown at 950 °C on c-plane sapphire. (b) AFM image showing initial WSe2 nuclei aligned at sapphire step edges. (c) Atomic models illustrating aligned nucleation of WSe2 on c-plane sapphire. (d) Schematic showing top view of relative lattice orientations of monolayer MoS2 and c-plane sapphire. (e) Optical image of monolayer MoS2 grains grown on atomically smooth sapphire; inset, RHEED pattern. (f) Orientation histogram based on the area shown in image e. (a−c) Reprinted from ref 45. Copyright 2015 American Chemical Society. (d−f) Reprinted from ref 92. Copyright 2015 American Chemical Society.

indirect to direct band gaps. This characteristic is responsible for their potential applications in optoelectronic devices such as light-emitting diodes (LEDs), laser diodes, and photovoltaic (PV) devices. Apart from the importance of precise control of the number of layers for device applications, it is also significant in using layer-by-layer stacking and designing multifunctional van der Waals heterostructures, which provide versatility in many new properties and novel devices. All these opportunities stem from the number of layers, and it is therefore important to develop effective means to precisely control this parameter. To control the number of layers in TMDCs, some effective approaches have been developed, such as using oxygen-plasmatreated substrates for one to three layer MoS2 growth.85 In addition, by controlling the growth temperatures in a one-step CVD reaction, MoSe2 crystals with a number of layers from one to four have been grown.31 Specifically, growth at 750, 825, and 900 °C leads to the formation of monolayer, bilayer, and three to four layer MoSe2, respectively. As shown in Figure 5a−c, the uniform and atomically smooth surface of MoSe2 crystals with a well-defined number of layers can be clearly distinguished. In a similar manner, with an increase of growth temperature, the number of layers in WSe2 increased with the same tendency.24 By using different 2D materials such as SnS2, TaS2, and graphene as substrates, MoS2 crystals with the number of layers tuned from one to six have been grown by using volatile MoCl5 as precursor in a typical low-pressure CVD (LPCVD) system, at temperatures lower than 500 °C.86 Uniquely for these structures, due to the band structures dependent on the number of layers, the boundary area between adjacent layers can act as semiconductor junctions for further advanced device applications. On the basis of the precise control of the number of layers, the stacking of the layers may determine the properties of the materials and consequently their device applications. Specifically, 3R phase TMDC materials with ABC type stacking and noncentrosymmetric property facilitate their unique applications in spintronics and valleytronics. Considering the theoretical predictions of the existence of two different stacking orders for

TMDC materials, i.e., AB order with a 0° twist angle between adjacent layers and AA′ order with a 60° twist angle, Xia et al. have grown these two stacked types of MoS2 bilayer crystals by CVD.87 The different stacking orders are clearly observed by optical images (Figure 5e,h) and high angle annular dark field scanning transmission electron microscope (HAADF-STEM) images as shown in Figure 5f,i. Very recently, Shearer et al. showed that stacking in WSe2 was controlled by a screw dislocation driven growth model. The stacking arrangements in centrosymmetric or noncentrosymmetric styles are strongly correlated to the number and rotation of screw dislocation spirals (triangular, hexagonal, or mixed). Importantly, these screw dislocation spirals can be adjusted by changing the ratio of precursor Se and WO3 powders in the CVD reactions.88 These results confirm that the number of layers and the rotation angle of TMDC layers can be tuned by changing the CVD reaction conditions. It is noted that a clear relationship between the number of layers and rotation angles of 2D materials and their CVD growth parameters are still not well understood. Instead, most currently established methods are based more on experience and may or may not work for other materials or CVD systems. This needs further study in order to obtain a deep understanding of the mechanism controlling the number of layers in 2D materials. For example, it has been demonstrated that the concentrations of precursors have an important influence on the nucleation locations of 2D materials, either at the edges or on the surface of existing layers. This feature would help in controlling the layer formation during the growth of TMDCs. 3.2.3. Orientation. Controlling grain orientation to obtain alignment of 2D materials is another important factor in understanding their nucleation and growth mechanism. On the one hand, the relative angle between them and the underlying substrates is a clue to a nucleation model and it provides a novel way to investigate their nucleation. On the other hand, misoriented lattice growth may lead to the formation of grain boundaries and defects during the merging of adjacent 2D H

DOI: 10.1021/acs.chemrev.7b00536 Chem. Rev. XXXX, XXX, XXX−XXX

Chemical Reviews

Review

Figure 7. Morphology control of TMDCs. (a) Schematic shows the spatial sectioning of the growth substrate with various MoO3 vapor concentrations as a function of distance between Mo source and substrate (upper) and corresponding morphologies of the MoS2 grown (lower). (b) Schematic of the relationships between the Mo:S atom ratios and the domain shapes of MoS2. (a, b) Reprinted from ref 100. Copyright 2014 American Chemical Society.

been reported to grow aligned WSe2 on a c-plane sapphire substrate.45 In this approach, the nucleation of WSe2 prefers to occur along the periodic atomic step/terrace structure on the cplane sapphire surface, resulting in a specific orientation and consequently the aligned growth of WSe2 crystals where one edge of a WSe2 domain is parallel to a step/terrace direction of the sapphire substrate, as confirmed by scanning electron microscopy (SEM; Figure 6a) and atomic force microscopy (AFM; Figure 6b). Another strategy to control the orientation of 2D crystals is to use an epitaxial relationship between the substrate and the material. For example, according to the epitaxial growth of MoS2 on a c-plane sapphire substrate, a limited number of lattice orientations with lattice rotation angles θ between the crystallographic axes of monolayer MoS2 and cplane sapphire including 0 and 60° as majority orientations and 30 and 90° as minority orientations are clearly demonstrated in Figure 6d−f.92 This good control over orientations can be ascribed to the match of the sapphire lattice with MoS2 lattice. In addition to the sapphire substrate, highly oriented pyrolytic graphite (HOPG) is another substrate which can help in forming apparently ordered orientations of MoS2 domains during growth.93 At this point, it seems that the substrate lattice

domains, which would have negative effects on their electrical, thermal, and mechanical properties. In contrast, it has been reported in graphene growth that when parallel hexagonal graphene domains (domains with a 60° rotation) merge, they form meter-size single crystal graphene.89,90 It is reasonable that parallel triangular TMDC domains (domains with a 120° rotation) would merge into a single crystal TMDC in a similar way. Hence it is very meaningful to control the orientation and grow well-aligned TMDC domains in order to prepare waferscale single crystalline TMDCs for the fabrication of large-scale device arrays with few grain boundaries, which would improve the uniformity and performance of devices significantly. Recently, researchers have demonstrated that the substrate plays a key role in the lattice-orientation control of TMDCs during their growth. The strong van der Waals interactions and the degree of lattice matching between the substrate and the grown material determines the orientation preference of the grown samples. c-plane (0010) facet sapphire is often used as the substrate for nanomaterial growth because of its high quality single crystal nature and lattice symmetry considerations.91 Inspired by the principles involved in the aligned growth of carbon nanotubes, a new step-edge-guided growth method has I

DOI: 10.1021/acs.chemrev.7b00536 Chem. Rev. XXXX, XXX, XXX−XXX

Chemical Reviews

Review

Figure 8. Phase control of TMDCs. (a) Sequential annular dark-field STEM images showing phase transformation of MoS2 at 600 °C. (b) Corresponding model of atom movements during the 2H−1T phase transition. (c) Schematic of the synthesis of a MoTe2 thin film by two-zone CVD. (d) Temperature dependent phase evolution of MoTe2 thin films synthesized at different N2 flow rates. (a, b) Reprinted with permission from ref 109. Copyright 2014, Rights Managed by Nature Publishing Group. (c, d) Reprinted from ref 119. Copyright 2017 American Chemical Society.

evolution reaction (HER) is closely related to the density of edge sites, suggesting that the morphology can have an important influence on the catalytic performance.97 From another perspective, because of the threefold lattice symmetry of TMDCs, triangular crystals are commonly obtained in CVD growth. TMDCs with different morphologies, including dendritic, truncated triangles, three-pointed stars, hexagons, and six-pointed stars, have also been grown. The morphologies of as-grown materials are determined by the barrier for edge diffusion of atoms. When the edge diffusion barrier is high, attached atoms can hardly find the most favorable binding sites, leading to the formation of irregular morphologies such as dendritic.98 Theoretical calculations have shown that the morphologies and edge structures of MoS2 can be tuned by changing concentrations of Mo and S species, and these influence the magnetic and electronic properties of MoS2.99 The shape and edge structure are largely dependent on the reaction conditions, such as precursor concentration ratio, substrates used, gas flow rate, etc., providing an opportunity to study the growth kinetics of TMDCs in-depth. According to the basic principles of the theory of crystal growth, the shape of a crystal is governed by the growth rate of different crystal facets. In this respect, Wang et al. have systematically studied the relationship between the mass ratio of the precursors, and their temperature, and flow rate on the morphology evolution of MoS2. As shown in Figure 7a, the MoO3 vapor concentration is related to the distance between the Mo source and the substrate, providing an effective way of adjusting the ratio of Mo and S in the reaction. Consequently, the morphologies of MoS2 show a transformation from medium-size

structure, especially c-plane sapphire, is relatively efficient in its ability to adjust and control domain lattice orientations. Other strategies, such as controlling the gas flow direction and regulating the growth temperature, could be combined with the choice of a special substrate, to further improve the alignment and orientation of 2D materials. Compared with the control of the crystal size and number of layers, there are fewer reports about controlling the orientation of the 2D material grown. Considering the importance of the orientation in the basic growth mechanism, an understanding of how to grow large single crystal 2D materials by parallel domains merging requires more effort. For example, more substrates need to be explored, including those of single crystal, polycrystalline, or amorphous metals, and their oxides and nitrides with engineered surface properties and textures. Of these we need to pay special attention to single crystal metal substrates because Au has been shown to be able to grow WSe2 at an ultrafast rate.80 If this could be achieved with the same orientation of each domain, one could produce fast growth of large single crystal 2D materials over single crystal metal substrates, which would make a big contribution to the field. The gas flow direction and dynamics in the CVD chamber have been shown to influence the alignment of carbon nanotubes,94−96 but whether the use of such parameters can be used to control the alignment of 2D materials is unknown and needs further investigation. 3.2.4. Morphology. Morphology control is important to understanding the relationships between the structure and performance of 2D materials in catalysis, sensors, energy applications, etc. For example, it is argued that the electrocatalytic activity of TMDC materials toward the hydrogen J

DOI: 10.1021/acs.chemrev.7b00536 Chem. Rev. XXXX, XXX, XXX−XXX

Chemical Reviews

Review

Recent research demonstrates that an efficient way to obtain metastable phase TMDCs relies on post-treatment of the samples by alkali metal intercalation, 110 electron beam irradiation,111 plasma treatment,112 mechanical force,113 etc. Lithium-containing salts, such as n-butyl lithium, are widely used in the phase engineering of TMDCs.114 Alternatively, by using the lithium-containing molten salts LiOH and (NH4)2MoS4 as precursors, Chang et al. have grown 1T phase MoS2 through the intercalation of lithium ions into MoS2 layers when it was calcined at a high temperature.115 Although the Li-intercalated MoS2 1T phase is metastable, recent work by Tan et al. has demonstrated that hydrogenating the intercalated lithium to form lithium hydride enhances its stability in air.116 Although metastable phases of TMDCs grown directly by a CVD reaction have rarely been reported, some important studies have been made. Previously, bulk MoS2 with the metastable 1T phase was grown in a CVD system by the sulfurization of potassium molybdate. It was shown that the potassium counterions in the as-grown material promote the formation and stabilization of the MoS2 1T phase.117 In addition, a metastable 1T WS2 monolayer has been prepared by colloidal synthesis in nonaqueous solvents.118 The 2H phase or the 1T phase materials are synthesized by simply controlling the reactivity of the tungsten precursors and the reaction times. Very recently, Yang et al. reported a continuous phase transformation among phases by adjusting the tellurium vapor mass-transfer (carrier gas flow rate) and tellurization reaction coefficient (reaction temperature), as illustrated in Figure 8c.119 The relationships between the four phases and conditions are clearly depicted in Figure 8d, showing that a high gas flow rate and a low temperature are preferrable for growing 1T′ MoTe2. In another study, single crystal monolayer 1T′ MoTe2 has been grown by CVD using ammonium heptamolybdate and tellurium as precursors and sodium cholate as a promoter in the reactions.120 Intriguingly, single- and few-layer samples of all three phases of MoTe2 (2H, 1T, and 1T′) have been selectively grown by changing the growth and quench temperatures in the same CVD process.121 These studies indicate that introducing functional ions or changing the reaction conditions are effective approaches for phase control, and it is therefore reasonable to believe that the controlled growth of high quality and metastable phase TMDC crystals, especially S and Se based, by CVD methods may be possible in the near future. 3.2.6. Doping. As a supplement to the limited intrinsic properties of specific 2D materials, elemental doping provides another approach for tuning the properties. Specifically, elemental doping has been proposed for modulating the electronic structures of TMDC materials as well as changing their physical and chemical properties. Considering the similarities in the atomic structures of various TMDCs, there is great potential for making ternary TMDCs with metal dopants (e.g., MoxW1−xS2) or chalcogen dopants (e.g., MoS2xSe2−2x). However, because the metal atoms are stacked between adjacent chalcogen atoms, it may be more challenging to dope metals than to dope chalcogen atoms in the CVD growth of TMDCs. Equally importantly, control of the atomic concentration is still difficult and deserves further study. Nevertheless, CVD growth provides a promising solution to the control of the concentration and position of dopant atoms in TMDCs. A common strategy for preparing ternary TMDC materials is to introduce a dopant precursor into a CVD system during material growth. In this way, MoS2xSe2(1−x) materials were grown by simultaneously putting S and Se precursors into the CVD

truncated triangles to large triangles, and then back to mediumsize truncated triangles followed by decreasing sizes of hexagons, and finally very small triangles. The reason for these changes is ascribed to the differences of the growth rates of different crystal facets of MoS2 (Figure 7b). Specifically, the growth rate of S-zz terminations and Mo-zz terminations are proportional to their relative concentrations. Here S-zz termination refers to a zigzag S edge while Mo-zz refers to a zigzag Mo edge. At high Mo precursor concentrations, the S-zz termination will be bonded to Mo atoms to obtain Mo-zz terminated MoS2 triangles.100 This work provides an effective way to control the morphologies of TMDCs during CVD growth. When changing the relative concentrations of Mo and S, the edge features, such as chalcogenterminated edges or metal-terminated edges, can be modified. Based on this report, a generalized TMDC growth theory considering both thermodynamic and kinetic mechanisms has been proposed.101 It has also been reported that temperature has an effect on the shape evolution of WSe2, from triangles and truncated triangles to hexagons as the temperature is increased during CVD growth.24 The growth of dendritic shape monolayer MoS2 has been reported by Zhang et al. that shows a high catalytic activity in HER.102 These strategies to control the morphology may also work for other 2D materials such as graphene and Mo2C in terms of tuning their shapes and edge structures.103,104 Although several typical morphologies and edges have been grown and can be controlled well, some novel structures with complicated shapes such as the symmetric twinned nanoislands of TMDCs,105 which was predicted in theory recently and may exhibit intriguing physics properties, have not been grown yet and further investigations are needed. 3.2.5. Phase. TMDCs have a wide variety of phases, and phase control is another very important aspect of their growth. In MX2 which has an X−M−X arrangement with three layers, where the chalcogen atoms are vertically aligned along the z-axis (ABA stacking), a trigonal prismatic phase, also called the 2H phase, is formed with a trigonal prismatic coordination of the metal atoms. Differently, when the adjacent chalcogen layers shift toward each other (stacking by an ABC model), an octahedral phase is formed, also called the 1T phase, with an octahedral coordination of the metal atoms. Typically, a distorted octahedral phase (1T′ phase) is often observed, especially for group 7 transition metals.106 Different phases have distinctly different properties; for example, 2H phase TMDCs are semiconductors while 1T phase ones are metallic. It is reported that the semiconductor−metal transition can be correlated to the stacking sequence of the 1T phase,107 and this feature could provide a simple way to alter the properties without adding or removing any atoms. Based on this example, it is of great importance to directly control the phase produced in the growth process in order to make it easier to use the material in many areas.108 Generally, stable phase TMDCs are easy to grow in a typical CVD system, while the direct growth of metastable phases is challenging for thermodynamic reasons. Previously, the structural transformation of monolayer MoS2 (2H to 1T) at the atomic level has been observed by in situ STEM studies.109 This shows the step-by-step progress (Figure 8a) and the atom movements (Figure 8b) in MoS2 during such a 2H to 1T phase transformation. It was noted that an intermediate phase (α phase) with a band-like structure is necessary to promote this phase transformation and an electron beam helps the growth of small 1T phase MoS2 domains. K

DOI: 10.1021/acs.chemrev.7b00536 Chem. Rev. XXXX, XXX, XXX−XXX

Chemical Reviews

Review

Figure 9. Doping 2D TMDCs. (a) Different phases depending on concentration x in WSe2(1−x)Te2x. (b) Raman spectra of monolayer 2H WSe2(1−x)Te2x (x = 0−0.6) alloys. (c) Raman spectra of monolayer 1T WSe2(1−x)Te2x (x = 0.6−1) alloys. (d) Optical image and AFM image of as-grown triangular WxMo1−xS2 flake (left); Raman intensity map and EDX W and Mo elemental maps for as-grown WxMo1−xS2 flake (right). (e) Normalized roomtemperature PL spectra of a single layer MoS2 film after sputtering and DS insertion cycles (left) and DFT-based band gap of MoS2(1−x)Se2x between the limits of pure MoS2 and pure MoSe2. (a−c) Reprinted with permission from ref 125. Copyright 2017 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim. (d) Reprinted with permission from ref 128. Copyright 2017 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim. (e) Reprinted from ref 131. Copyright 2014 American Chemical Society.

cm−1 and energy dispersive X-ray (EDX) maps of elemental Mo and W are not uniform across the whole crystal, indicating spatial segregation of the Mo and W. In a further experiment, using a mixture of MoO 3 and WO 3 as precursor, monolayer Mo(1−x)WxS2 was obtained in a one-step CVD process.129 When annealing mixed MoS2/WO3 powders in sulfur vapor, triangular MoxW1−xS2 monolayers with gradual concentration profiles of W and Mo have been grown.130 It is reasonable to believe that if the Mo and W atoms are uniformly distributed, performance and reproducibility of a device made from the material would be improved. Post-treatment of as-grown samples to obtain ternary TMDCs is another easy and efficient way to dope them. Ma et al. have grown monolayer MoS2 by a conventional CVD method followed by the removal of certain sulfur atoms by exposure to an Ar plasma. Diselenodiphenyl (DS) was then used to insert selenium atoms into the sulfur vacancies to obtain MoS2(1−x)Se2x crystals.131 The photoluminescence (PL) spectra in Figure 9e show evidence for the successful doping of selenium atoms into the MoS2 crystals, and the composition-based band gap change is consistent with theoretical predictions. Combining this method with patterning, it is possible to produce doping at desired locations, overcoming the limitations in a one-step CVD growth process and showing potential for device applications. 3.2.7. Quality and Defects. Quality and defect control is another important aspect of TMDC growth. On the one hand, high quality TMDCs are highly desired for high performance electronics and optoelectronics,132 although, unfortunately, defects and impurities are inevitably generated during their CVD growth in current growth methods, and these charged impurities, defects, and traps may seriously degrade charge carrier mobility.133 On the other hand, a controlled type and density of defects in TMDCs has effects on the 2D structure itself

chamber for the reaction. 122,123 Few-layer MoS2x Se 2(1−x) materials with about 2 wt % Se doping throughout the whole crystals were grown in a similar way.124 However, this does not work for less reactive chalcogens such as Te. To tackle this problem, Yu et al. reported the chemical vapor transport (CVT) growth of bulk single crystal ternary WSe2(1−x)Te2x followed by Scotch-tape-based mechanical exfoliation to prepare monolayer WSe2(1−x)Te2x (0 < x < 1). As shown in Figure 9a, the phase can be precisely tuned as a function of Te concentration, where x = 0−0.4 for the 2H structure, x = 0.5 and 0.6 for 2H and 1Td mixed structures, and x = 0.7−1.0 for the 1Td structure. In Figure 9b,c, the doping concentrations of monolayer WSe2(1−x)Te2x were measured by the peak shift and sharpness of the two characteristic E2g and A1g peaks in their Raman spectra.125 In another respect, monolayer MoS(1−x)Se2x crystals with a large composition tunnability (x = 0.41−1) and controllable edge orientations have been obtained in a PVD method.126 Although the above results show good control of the TMDC phases, the direct CVD growth of monolayer WSe2(1−x)Te2x crystals is still to come. To obtain metal-doped ternary TMDCs, Wang et al. reported the growth of a Mo1−xWxS2 monolayer by an LPCVD method. Here a highly volatile precursor, WCl6, was selected as the W source, MoO3 was used as the Mo source, and the growth temperature was determined by the reaction of MoO3 with S, which can be as low as 700 °C. The Mo/W ratio can be tailored by tuning the concentrations of the MoO3 and WCl6 supplies, and the chemical composition of the as-grown Mo1−xWxS2 monolayer is homogeneous.127 In a different way, Liu et al. grew monolayer triangular WxMo1−xS2 crystals with a size of 20 μm using atmospheric pressure CVD.128 The MoO3 and WO3 precursor powders were located separately in the chamber. As shown in Figure 9d, Raman intensity maps at a frequency of 408 L

DOI: 10.1021/acs.chemrev.7b00536 Chem. Rev. XXXX, XXX, XXX−XXX

Chemical Reviews

Review

4. CVD GROWTH OF WAFER-SCALE CONTINUOUS 2D MATERIAL FILMS In section 3, we talked about the controlled growth of TMDCs with emphasis on the properties of the individual domains, including their grain size, number of layers, orientation, morphology, phase, doping, and defects. In this section, we will focus on how to grow large, continuous, and homogeneous films of 2D materials, because the preparation of such wafer-scale uniform samples is a prerequisite for their practical use. We focus on two key questions in this regard. The first is how to develop methods for growing wafer-scale uniform 2D films by CVD, and the second is how to achieve low temperature growth of 2D continuous films on flexible and transparent substrates, which are usually unable to withstand high-temperature processing.

and may be desirable for applications in catalysis, energy storage and conversion, sensors, etc.134,135 For example, theoretical calculations show that a line of S vacancies in MoS2 can lead to a band structure transition from semiconductor to metal,136 and ndoping high conductivity WSe2 was achieved by creating Se vacancies through H2/He plasma treatment.137 Therefore, the control of defect type and density is an important problem that must be solved. To realize this, defect formation mechanisms in TMDCs, especially vacancies which are the major defect, need to be carefully studied. In addition, defect regulation methods and principles during CVD growth need to be understood and efficient approaches to the recovery of defects found. Chalcogen vacancies are the typical defects in TMDCs because they have the lowest formation energy. Previously, the formation mechanisms of S vacancies using electron irradiation have been predicted by density functional theory (DFT) calculations138 and the relationship between the number of S vacancies and the fieldeffect mobility and photoluminescence of MoS2 have been studied.139,140 For native chalcogen vacancies in few-layer MoS2, S vacancies on the surface can act as electron donors and induce localized states in the band gap, which play a key role in the charge transport by nearest-neighbor hopping or by variablerange hopping.141 Understanding the influence of defects may help tune the charge mobility for achieving high performance devices. In addition, metal vacancies in TMDCs can influence the macroscopic material properties. The metal vacancies with deeptrap states have a more serious effect on the electron mobility and photoluminescence than that of chalcogen vacancies with shallow-trap states. Jeong et al. have grown hexagonal WS2 monolayer by a CVD method using ammonium metatungstate hydrate as liquid W precursor and sodium cholate hydrate as a promoter in a convention CVD system.42 The S vacancy domains and W vacancy domains, in triangular shape, alternatively coexisted within the hexagonal WS2 layer. This unique defect arrangement was well-characterized by PL intensity and STEM images. The metal vacancies are formed because of the tungsten concentration variation at certain conditions. To recover these defects, post-treatment is another effective way. Yu et al. have developed a thiol chemistry method to repair sulfur vacancies in MoS2 by (3-mercaptopropyl)trimethoxysilane treatment. After repair, the MoS2 devices show enhanced electron mobility because of the reduction of charged impurities and short-range scattering.142 For selenium-based TMDCs, Han et al. used hydrohalic acid to reduce the Se vacancies and suppress the trap states, and increased the photoluminescence intensity by a factor of 30.143 For the recovery of these chalcogen vacancies, the recombination kinetics of exctions before and after repair by organic molecules are also investigated by transient optical characterization.144 Different from the above methods, Lu et al. used oxygen to fill the chalcogen vacancies by a focused laser beam treatment in ambient conditions.145 These results demonstrate that proper chemical treatment is efficient in removing the point defects and degrade the traps in TMDCs. Moreover, the in situ introduction of additives to the CVD reaction systems may serve as another simple way to repair defects in TMDCs during growth. As shown by Zhou et al., molecular sieves were used to trap the precursors of the byproduct during MoTe2 growth.146 This approach can be developed further; for example, some additives may fill vacancies in the crystal lattices of TMDCs to obtain a crystal without vacancies.

4.1. Motivation for and Challenges of Growing Wafer-Scale Continuous and Homogeneous 2D Material Films

Wafer-scale, continuous films of 2D materials that are compatible with the current silicon-based microfabrication processes are greatly needed for applications in electronic and optoelectronics. By patterning the large area 2D films into arrays, batches of devices could be fabricated on a single substrate technologically relevant scale. Ideally, the as-prepared films should be homogeneous over the entire substrate to guarantee structural and electrical uniformity of all the devices in these arrays. Although alternative approaches such using solution-processed 2D materials in spray coating or in an inkjet printer are capable of fabricating FETs covering large area substrates, the performance of these devices is relatively poor (with a charge carrier mobility of around 0.1 cm2 V−1 s−1 and an on/off current ratio of 600).147 Therefore, direct CVD growth of continuous and high quality 2D films over large areas is urgently needed for them to be used in electronic devices. The key challenges in growing high quality wafer-scale film are as follows. (i) The first challenge is the growth of a continuous film with large grain. Small grains are necessarily accompanied by a high density of grain boundaries, increasing the scattering of carriers and consequently degrading the electronic properties of the films. (ii) The present uniformity of as-grown films is not great. Growth at different areas may vary and layer distribution may be random, both of which hinder the scalable fabrication of device arrays. (iii) Transfer of 2D films from growth substrates to target ones is usually tedious and difficult to scale up. The transfer process is unavoidable for specific applications, while the damage and contamination introduced during transfer is more serious for large area films than for an individual flake. (iv) The mass production of continuous films remains a major challenge. The slow growth speed and strict growth condition requirements still limit the scalable production of large area 2D materials. In section 4.2, we discuss the progress made in solving these challenges in recent years. 4.2. Growth of Continuous 2D Films on Rigid Substrates

To overcome the above challenges, researchers have devoted extensive efforts to growing high quality continuous films by modified CVD methods, such as MOCVD, LPCVD, ICP-CVD, the predeposition of metal or metal oxide films,148 and thermal decomposition of precursors such as (NH4)2MoS4, etc. The main achievements in the growth of wafer-scale continuous films are given in Table 1, and are discussed in sections 4.2.1, 4.2.2, and 4.2.3. 4.2.1. MOCVD Method. Metal−organic precursors are very attractive for the controlled growth of large area TMDC materials because they can be fed into the CVD chamber in M

DOI: 10.1021/acs.chemrev.7b00536 Chem. Rev. XXXX, XXX, XXX−XXX

N

wafer scale with layers adjusted

wafer scale, trilayer MoS2 film, crystal domains larger than 160 nm

MoS2/various substrates

MoS2/Al2O3

MoS2/SiO2/Si

PLD

ALD

dip coating

wafer scale continuous film (1 cm × 9 cm); bilayer, trilayer, few layers size up to substrate scale, bilayer

MoS2/SiO2/Si

MoS2/c-plane sapphire

thermal deposition

magnetron sputtering

2-in. wafer scale; trilayer

monolayer (few layer), WSe2/SiO2/ Si

vapor transport

wafer scale film; 3.1 nm thickness

1 cm2

MoSe2

vapor reaction method

1T′ MoTe2/SiO2/Si

35 mg of MoO3; 600 mg of sulfur; 750−800 °C; 130 sccm Ar; 0.67 Torr

wafer scale; grain size 20−1000 nm 2 × 2 cm2; grain size 0.3−0.7 mm

monolayer MoS2

vapor reaction method

electron-beam deposition

0.01 sccm Mo(CO)6/W (CO)6; 0.4 sccm (C2H5)2S; 5 sccm H2; 150 sccm Ar; 7.5 Torr, 550 °C; 26 h

4-in. wafer scale; grain size ∼1 μm

monolayer MoS2, WS2/silicon

MOCVD

PLD targets: MoS2/S ratio: 1:1, particle size: S(20 μm), MoS2 (43 μm, 2 μm); deposition T: 700 °C Mo(NMe2)4 and H2S in THF, ALD at 60 °C; sulfurization in sulfur vapor at 1000 °C for 5 h 1.25 wt % (NH4)2MoS4 in DMF, pull rate of 0.5 mm/s, sulfurization at 500 and 1000 °C, 1 Torr, Ar/H2 = 4:1

MoO3 film deposition method; Ar/H2 (4:1), 1 Torr, 500 °C, 1 h; sulfur, 1000 °C, 30 min, 70 sccm Ar, 600 Torr 1 nm Mo/MoO3 film by electron beam evaporation; Te with molecular sieves, 3 sccm Ar + 4 sccm H2, 700 °C, 1 h; atmospheric pressure MoO3: RF magnetron sputtering at 300 °C; sulfurization at 650 °C for 1 h, 100 sccm Ar, 2 × 10−2 Torr

300 mg of Se; 20 mg of MoO3; Si/SiO2; 750 °C; 20 min; H2/Ar (15%) 75 sccm WSe2 powder, 1060 °C for source T, 765 °C for substrate T, 100 sccm, 40 min

growth parameters

size of films

materials/substrates

methods

on/off current ratio: 1.6 × 105, carrier mobility: 6 cm2 V−1 s−1

162

160

159

electrical resistivity: 1.5 × 104 Ω cm−1

none

158

154

Thin Film Deposition Method on/off current ratio: 105; electron mobility 0.8 cm2 V−1 s−1

mobility of ∼21 cm2 V−1 s−1 (bilayer) and ∼25 cm2 V−1 s−1 (trilayer), on/off ratios in the range of ∼107 (bilayer) and 104−105 (trilayer)

153

p-type behavior, hole carrier mobility of 100 cm2 V−1 s−1 (monolayer), 350 cm2 V−1 s−1 (few layer)

157

152

42 cm2 V−1 s−1; on/off ratio: 106

sheet resistance of the film is 1120 Ω sq−1, sheet resistance of Hall bar devices is 1900−2028 Ω sq−1

67

Conventional CVD Method 7 cm2 V−1 s−1

ref 25

device performance Metal−Organic CVD Method mobility: 30 cm2 V−1 s−1 at room temp, and 114 cm2 V−1 s−1 at 90 K

Table 1. Summary of Growth of Continuous Films of TMDCs by Various Methods

Chemical Reviews Review

DOI: 10.1021/acs.chemrev.7b00536 Chem. Rev. XXXX, XXX, XXX−XXX

Review

164

precise amounts that can be controlled to be constant over a long time. Using this method, WSe2 monolayer and few-layer films with a domain size of 8 μm have been grown by MOCVD with W(CO)6 and (CH3)2Se as precursors.149 In a recent breakthrough, Kang et al. used gas-phase Mo(CO)6, W(CO)6, and (C2H5)2S as precursors and H2/Ar as carrier gas, and grew continuous 4-in. wafer-scale, homogeneous MoS2 and WS2 films (Figure 10a,b).25 The growth was found to follow a layer-by-layer growth mode. From the MoS2 film, a wafer-scale batch of 8100 high performance MoS2 FETs with a global back gate was fabricated by photolithography (Figure 10c). More practically, two stacked MoS2 FETs were fabricated at vertical levels on silicon wafer with the external deposition of 500 nm SiO2 dielectric layers in between. The transfer curves from both types of FETs show back gate dependent conductance changes, indicating that their semiconducting behavior is as expected in MoS2 (Figure 10d). All the above processes demonstrate the good compatibility of wafer-scale 2D films with conventional fabrication procedures, which can be further developed into three-dimensional structures for scalable device fabrication. Because of the easy control of reaction parameters and the constant concentration of each precursor, MOCVD using vaporphase precursors has great potential for the production of high quality and uniform TMDC films. 4.2.2. Conventional CVD Method. Normally, a combination of MoO3 (WO3) and S (Se) powders, or WSe2 (MoS2) powders, serves as the precursors for CVD growth of TMDCs. Single crystals with defined domain sizes are commonly grown in such setups. By increasing the growth time, TMDC continuous films can be grown.150 Zhang et al. reported the use of LPCVD for the scalable growth of polycrystalline monolayer MoS2 films on Si/SiO2 substrate, using MoO3 and S as precursors.67 The film is composed of individual domains with sizes ranging from 20 nm to 1 μm that have coalesced and was used to fabricate FETs with a back gate. They show a relatively decent mobility of 7 cm2 V−1 s−1 and an on/off current ratio of 106. In another work, by selective growth at the gap between two stacked substrates, a continuous monolayer MoS2 film as large as a few square centimeters was obtained.151 In addition, MoS2, MoSe2, and WS2 films have been grown in a similar way. Gong et al. used MoO3 and Se as precursors and achieved wafer-scale growth of continuous MoSe2 films with a controlled number of layers and few defects, by tuning the flow rate, growth temperature, and H2 content during CVD growth. The wafer-scale films led to a relative high mobility of 42 cm2 V−1 s−1 for back gate MoSe2 FETs.152 In addition, large area (1 cm2) atomically thin continuous WSe2 films have been grown using WSe2 powder as precursor in a simple growth process.153 For these kinds of growth systems, the continuous films are often polycrystalline and are formed by the merging of numerous individual domains. 4.2.3. Thin Film Deposition Method. Another general strategy to grow wafer-scale continuous 2D films is the sulfurization (or selenization) or decomposition of predeposited films (Mo or W foil, MoO3 or WO3 or their salts). There are many ways to carry out the predeposition process, such as thermal deposition, electron beam deposition, atomic layer deposition, magnetron sputtering, and spin-coating method. For example, to grow wafer-scale MoS2 thin films, a MoO3 precursor with the desired thickness was thermally deposited on the (0001) surface of a sapphire substrate. Then, a postannealing process at 500 °C in a hydrogen atmosphere and sulfurization at 1000 °C in sulfur vapor were used to achieve the growth of MoS2 over a 1 × 1 cm2 substrate.154 In another study, Kong et al.

1.25 wt % (NH4)2MoS4 in ethylene glycol; 280 °C 30 min, 450 °C 30 min; H2/N2 100 sccm; 1.8 Torr wafer scale; grain size: N/A MoS2/polyimide thermal decomposition

size of films materials/substrates methods

Table 1. continued

growth parameters

Flexible and Transparent Substrate high photocurrent durability when bending for 105 cycles

device performance

ref

Chemical Reviews

O

DOI: 10.1021/acs.chemrev.7b00536 Chem. Rev. XXXX, XXX, XXX−XXX

Chemical Reviews

Review

Figure 10. MOCVD growth of wafer-scale continuous 2D material films. (a) Diagram of MOCVD growth setup. Precursors were introduced using individual mass flow controllers. (b) Photographs of monolayer MoS2 andWS2 films grown on 4-in. fused silica substrates. The left halves show the bare silica substrate for comparison. (c) Batch-fabricated 8100 MoS2 FET devices on 4-in. Si/SiO2 wafer. Top inset: enlarged image of one square containing 100 devices. Middle and bottom insets: corresponding color maps of δ at gate biases VB = +50 V and −50 V. (d) Diagram of fabrication of MoS2 device/ SiO2 stack by alternating MOCVD growth. SEM image of MoS2 FET arrays on first and second layers. ISD−VSD curves measured from two neighboring devices on first and second layers of MoS2 FETs. Reprinted with permission from ref 25. Copyright 2015, Rights Managed by Nature Publishing Group.

film on its surface. Then, a two-step annealing process was used to decompose and convert the (NH4 ) 2MoS 4 into MoS 2 continuous films.162 A similar strategy with a (NH4)2MoS4 film deposited by spin coating was later reported with atomically thin and wafer-scale MoS2 layers.163 For these predeposition methods, the size, the number of layers, and the quality of the films heavily depend on the deposition condition.

reported that large areas of vertically aligned MoS2 or MoSe2 layers were grown from an electron beam deposited Mo film followed by conventional sulfurization or selenization in a typical CVD system.155 The thickness of the electron beam deposited Mo films can be tuned, resulting in a good control of the number of layers in as-grown MoS2 or MoSe2. By using this method, wafer-scale MoS2 films156 and 2H as well as 1T′ phase MoTe2 continuous films157 have been obtained. Radio frequency magnetron sputtering was also used to deposit an MoO3 film followed by sulfurization at 650 °C to prepare MoS2.158 The thickness of the MoS2 film can be tuned in atomic scale by the sputtering time, and the fabricated MoS2 FETs show a high charge carrier mobility and a high on/off current ratio, which are dependent on the MoS2 thickness. Pulsed laser deposition (PLD) is another effective approach to deposit thin films for 2D material growth. In a recent study, a large area MoS2 film with a controlled number of layers ranging from 1 to 10 was deposited by controlling the target composition, MoS2, and sulfur particle size in the target, and other PLD deposition parameters. It is noted that no additional sulfurization step was required.159 Atomic layer deposition (ALD) is a very powerful method to grow atomically thin coatings on different substrates. Very recently, MoS2 films were synthesized at a temperature as low as 60 °C in an ALD system by using volatile Mo(NMe2)4 and H2S as precursors. The growth rate of MoS2 was 0.12 nm/cycle, which makes the number of layers and thickness control of MoS2 very precise.160 It is reasonable to argue that this Mo(NMe2)4 precursor can also be used in a MOCVD system for the scalable preparation of 2D TMDCs. Also, wafer-scale WS2 nanosheets, whose number of layers could be systematically controlled, were synthesized by the sulfurization of an ALD WO3 film.161 Besides the above-mentioned thin film deposition techniques, dip coating or spin coating is a relatively simple way to prepare uniform thin films of precursors on a substrate. For example, (NH4)2MoS4 was dissolved in dimethylformamide (DMF) to obtain a 1.25 wt % solution and a silicon wafer was immersed into the solution and slowly pulled out to form a thin (NH4)2MoS4

4.3. Growth of Continuous 2D Films on Flexible Substrates

Flexible electronic and optoelectronic devices, wearable sensors, and other portable devices have great potential in next generation roll-up displays, electronic paper, electronic skin, etc. Flexible electronics is an area where 2D materials may surpass silicon due to their good mechanical properties. For flexible devices, a variety of polymers are commonly used as substrates, such as polyester (PET), poly(ethylene napthalate) (PEN), poly(ether sulfone) (PES), and polyimide (PI). To fabricate device arrays on the flexible substrates, a continuous film is a prerequisite. Generally, direct growth and postgrowth transfer are two widely used methods to prepare continuous films on flexible substrates. 4.3.1. Direct Growth on Flexible Substrates. Flexible substrates are usually polymeric-based and have a low processing temperature, high surface roughness, and low thermal expansion coefficient, making them not preferred substrates for conventional high temperature CVD growth. However, researchers have developed effective ways to solving these problems. Modification of the substrate can play a key role in lowering the growth temperature for 2D materials. Using alloy substrates with low melting points and the formation of a liquid surface at reaction temperatures can promote the growth of TMDC continuous films. For example, Zhou et al. used PI-supported gallium or a eutectic indium−gallium alloy as the substrate, and synthesized large area high quality GaSe or GaxIn1−xSe2 films at a growth temperature as low as 100 °C.165 In this reaction, the alloys serve as both precursors and substrate with a liquid surface. In principle, if the correct Mo- or W-containing eutectic could be found, it would be easy to achieve the direct growth of TMDC films on flexible substrates. Also, Ling et al. showed that loading P

DOI: 10.1021/acs.chemrev.7b00536 Chem. Rev. XXXX, XXX, XXX−XXX

Chemical Reviews

Review

Figure 11. Direct growth of continuous 2D material films on flexible polymeric substrates. (a) Schematic of temperature−time profile for two-step thermal decomposition process for synthesis of large-scale, continuous MoS2 layers and thermogravimetric analysis (TGA) of (NH4)2MoS4. (b) Photograph of homogeneous MoS2 films synthesized directly on PI substrate. (c) Photocurrents of MoS2-based visible-light photodetectors recorded at P = 12.5 mW cm−2 and V = 20 V before and after bending. Reprinted with permission from ref 164. Copyright 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.

growth substrate/MoS2/glue/PET stacked substrate, the flexible MoS2/glue/PET layer could be easily separated from the growth substrate, and MoS2 films with size up to 2 cm × 10 cm were successfully transferred onto PET without any obvious deterioration of their properties. These high efficiency transfer methods will accelerate the applications of 2D materials in flexible devices. Currently, such transfer methods are usually manual and the development of automatic or semiautomatic methods is important to increase the yield and efficiency of the process.

specific organic molecules onto the substrates to lower the CVD reaction barrier and achieve low temperature growth is a potential method to achieve the direct growth of TMDCs on flexible polymeric substrates, that the use of PTAS as a promoter can decrease the temperature for MoS2 growth to 650 °C.166 Nevertheless, there are only a few reports on this technique, and suitable organic molecules deserve further exploration. Recently, the thermal decomposition of molybdenum salt, such as (NH4)2MoS4, has appeared to be an efficient way of obtaining MoS2 continuous films at relatively low temperatures. For example, Lim et al. reported a modified two-step thermal decomposition process for preparing wafer-scale, homogeneous MoS2 films at low temperatures on a plastic PI substrate.164 In Figure 11a, intermediate MoS3 was first formed at 280 °C, followed by decomposition at 450 °C to grow MoS2 continuous films. A flexible MoS2-based visible light photodetector was then fabricated on the PI substrate and showed good stability of the photocurrent after bending to a radius of 5 mm for 105 cycles (Figure 11b,c). This strategy provided an easy way to directly grow 2D films on a flexible substrate. However, this decomposition temperature is still too high for many flexible substrates. Some volatile additives may be added to the (NH4)2MoS4 solution to lower the decomposition temperature to achieve MoS2 growth at even lower temperatures. 4.3.2. Postgrowth Transfer 2D Materials onto Flexible Substrates. The direct growth of 2D films on flexible substrates is challenging, and from a more practical aspect, a highly efficient transfer method is necessary to obtain 2D continuous films on flexible substrates. Previously, different from a conventional polymer (PMMA) assisted etching method, Gao et al. developed a face-to-face transfer method and achieved the wafer-scale transfer of graphene films.167 The nascent gas bubbles and capillary bridges between the films and the substrate promote the detachment of as-grown graphene. For TMDC films grown on metal substrates such as Au foil, this face-to-face method should work as well. Recently, Phan et al. reported a direct one-step method for transferring MoS2 films grown on sapphire or SiO2/ Si substrates to a flexible and transparent PET substrate by using epoxy glue as a mediator without any etching process.168 For the

5. CVD GROWTH OF 2D MATERIAL BASED HETEROSTRUCTURES Along with extensive research and the rapid development of 2D materials consisting of graphene, h-BN, TMDCs, phosphorene, etc., 2D material based heterostructures have recently emerged as a new research frontier for condensed matter physics and materials science. From structure and materials points of view, heterostructures provide additional ways to design and prepare novel materials with more than one building block. From a physics perspective, heterostructures provide interesting platforms to exploit new physics as well as investigate the coupling between elementary particles in different building blocks.169 These 2D material based heterostructures exhibit new physics related to electron−electron coupling and electron−phonon coupling, originating from the layer−layer interactions.170 Furthermore, such heterostructures may deliver improved device performance compared with the separate building blocks.169,171 The energy band alignment and charge carrier mobility in these heterostructures can be tuned by selecting the components in the heterostructures, to satisfy different application requirements. Hence, by precisely designing the components used to build the heterostructure and sequences in which they are arranged, one could open up countless opportunities to create new materials and systems aimed at high-performance device applications.172 Generally speaking, there are two types of 2D material based heterostructures. The first is vertical heterostructures, where different 2D materials are stacked layer by layer vertically with no strong interaction between them, while the second is lateral Q

DOI: 10.1021/acs.chemrev.7b00536 Chem. Rev. XXXX, XXX, XXX−XXX

Chemical Reviews

R

180

179

xD/2D heterostructures

Bi2S3/MoS2

Bi2S3 and MoO3 mixed powder, 0.5−1 g of S, 20 sccm N2, 650 °C, 6 min

graphene: Cu foil, APCVD, 1050 °C, Ar/H2 (930/60 sccm) CH4 20 sccm hydrogen etch graphene BN: NH3−BH3, 120 °C, 10−30 min graphene/BN

178 MoS2/MoSe2

177

step 1: 0.6 g of WO3 260 °C (Se), Ar/H2 (90/6 sccm) 20 Torr, 925 °C, 15 min; step 2: 0.6 g of MoO3 190 °C (Se), Ar (70 sccm) 40 Torr, 755 °C, 15 min 0.7 g of MoO3, 0.4 g of S, 0.6 g of Se, 750 °C for sulfurization, 700 °C with 5 sccm H2 for selenization, 15 min MoS2/WSe2

176 174 Ni−Ga/Mo foil substrate, NH3−BH3 (110−130 °C), Ar/H2 75 sccm, 30 min, 1000 °C; 4 sccm H2S precursor, 680 °C, 25 min, 10 sccm Ar one-step CVD: 10 mg of W + 100 mg of Te, 25 mg of MoO3, 500 mg of S; Si/SiO2 (285 nm); 100 sccm Ar; 650 °C, 15 min; atmospheric MoS2/h-BN MoS2/WS2 lateral heterostructures

graphene: Au foil, AP, CH4 1.5 sccm, H2 30 sccm, Ar 200 sccm, 970 °C MoS2: MoO3(530 °C), S (102 °C), substrate (680 °C), Ar/H2 (50/5 sccm) MoS2/ graphene

175

triangular MoS2 and WS2; type II band alignment monolayer graphene film at bottom; monolayer layer MoS2 at top; weak n-doping level with whole sizes up to 200 μm2 WS2−MoS2 interface roughness is four unit cells with a width of 15 nm junction depletion width is ∼320 nm, type II band alignment triangular geometry thickness of 0.8 nm interface transition in scale of ∼40 nm zigzag oriented boundaries; sharp interface boundary with width of 0.5 nm monolayer MoS2, Bi2S3: diameter 0.12 μm, length 7.3 μm one-step CVD: 10 mg of W + 100 mg of Te, 25 mg of MoO3, 500 mg of S; Si/SiO2 (285 nm); 100 sccm Ar; 850 °C, 15 min; atmospheric MoS2/WS2 vertical heterostructures

characterization materials

Table 2. Summary of Representative CVD Grown 2D Heterostructures

Due to the existence of abundant 2D materials with different electronic properties and weak requirements of a lattice match for stacking them together, there are unlimited combinations in 2D vertical heterostructures. Currently, the CVD-based method can grow four main types of 2D vertical heterostructures, including (i) metal/semiconductor junctions consisting of graphene/s-TMDCs, m-TMDC/s-TMDCs, graphene/phosphorus, etc.; (ii) semiconductor/semiconductor junctions consisting of different s-TMDCs or s-TMDCs/phosphorus; (iii) semiconductor/insulator junctions consisting of s-TMDCs/ h-BN; and (iv) metal/insulator junctions consisting of graphene/h-BN, where s-TMDCs and m-TMDCs refer to semiconducting and metallic TMDCs, respectively. In addition, complicated heterostructures which are made of more than two stacked materials are also reviewed in this section. 5.1.1. Metal/Semiconductor. Graphene/TMDC-based metal/semiconductor vertical heterostructures are very important because they take full advantage of both graphene with its high transparency, high conductivity, and tunable work function, and TMDCs with a wide choice of compositions and band gaps. As a result, these heterostructures demonstrate interesting properties in electronic and optoelectronic devices.181−183 For example, high performance electronic devices,184 a photoresponsive memory,185 biological sensors,186 and flexible transparent devices183 have been fabricated by stacking the mechanical exfoliated graphene and TMDC monolayers into heterostructures that clearly point out the advantages of these materials in electronic and optoelectronic applications. Considering the fact that Scotch-tape-based exfoliation and aligned

type

5.1. Vertical 2D Heterostructures

methods

ref

heterostructures, where different 2D materials are seamlessly joined to each other, usually by covalent bonds, in the same plane. For vertical 2D heterostructures, the interlayer interaction is mainly by van der Waals forces (they are sometimes called van der Waals heterostructures), and there is no or little requirement for lattice matching between the different layers. Because of this, several methods have been developed to fabricate such vertical heterostructures, including (i) Scotch-tape-based mechanical exfoliation and multiple step aligned transfer,173 (ii) sequential deposition of 2D material solutions, (iii) one-step or multistep CVD growth, and (iv) some combination of these methods (e.g., exfoliation a 2D material followed by CVD to grow another 2D material on top). In contrast, lateral 2D heterostructures usually require atomic level sharpness at the junctions between the materials on each side. These strict requirements make the bottom-up approach, especially CVD growth, one of the most suitable choices that can be used to grow lateral 2D heterostructures. In the following sections, recent progress and achievements on the preparation of both vertical and lateral 2D heterostructures are presented. Moreover, heterostructures consisting of 2D materials and LD materials (e.g., 1D nanowires and nanotubes, 0D quantum dots (QDs)), which we call xD/2D heterostructures, have recently attracted increasing attention, are also highlighted. Although reports of CVD-grown heterostructures are not as plentiful as those concerned with Scotch-tapebased exfoliation and aligned transfer ones, the high yield and comparable high quality make CVD-grown heterostructures promising for practical applications. Hence, this section mainly focuses on heterostructures where CVD growth is involved in their preparation and Table 2 is a summary of the typical examples of these three kinds of heterostructures, which will be discussed in detail in section 5.1, section 5.2, and section 5.3.

174

Review

DOI: 10.1021/acs.chemrev.7b00536 Chem. Rev. XXXX, XXX, XXX−XXX

Chemical Reviews

Review

Figure 12. CVD growth of graphene/TMDC based metal/semiconductor and TMDC/TMDC based semiconductor/semiconductor vertical heterostructures. (a) SEM image of as-grown MoS2/graphene vertical heterostructures on Au foil. (b) Large area STM image (1.11 V, 0.20 nA, 78 K; 44.0 × 44.0 nm) of MoS2/graphene. (inset) Corresponding SEM image. (c) Atomic resolution STM image (0.81 V, 0.23 nA, 78 K; 3.2 × 3.2 nm) of MoS2/graphene. (inset) Corresponding FFT pattern. (d) Schematic of the three types of heterostructures. (e) Schematics showing monolayer MoSe2 used for WSe2 growth and stack 1 and stack 2 WSe2/MoSe2 heterostructures with corresponding optical images. (a−c) Reprinted with permission from ref 175. Copyright 2015 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim. (e) Reprinted from ref 195. Copyright 2015 American Chemical Society.

different TMDCs can form three types of heterostructures (Figure 12d). Type I heterostructures are called straddling heterostructures because the valence band maximum and conduction band minimum of one material fall within the band gap of the other. Type II heterostructures, staggered heterostructures, are where the valence band maximum occurs in one material while the conduction band minimum is in the other. In this instance the electrons and holes are confined in two separate materials, which may result in excellent performance in optoelectronic device applications. Type III heterostructures, broken-gap heterostructures, are where the conduction band minimum in one material is lower than the valence band maximum in another material, indicating that the valence band and conduction band overlap at the interface. Compared with single layer TMDC materials, these 2D material based heterostructures with different band alignments provide additional freedom and unique properties in electronic and optoelectronic devices.189,190 The most straightforward way to prepare layer-by-layer heterostructures is stacking the as-grown individual TMDCs by traditional transfer methods. For example, monolayer MoS2/ WSe2 heterostructures were stacked by transferring flakes of one material onto a flake of the other using a PMMA assisted method.191,192 A similar method was used to assemble MoS2/

transfer methods are time-consuming and are only suitable for individual devices, the development of CVD-based methods to prepare heterostructures is very important. In an early study, Shi et al. used a two-step CVD method to grow graphene/MoS2 vertical heterostructures.187 In this work, they first grew multilayer graphene by CVD and later used it as the substrate for (NH4)2MoS4 deposition and decomposition to prepare graphene/MoS2 heterostructures. In another study, a CVD method was used to first grow a complete monolayer of graphene on an Au foil, followed by the growth of monolayer MoS2 on top of graphene. An SEM image of the graphene/MoS2 heterostructures is shown in Figure 12a. The high quality of the materials was confirmed by atomic resolution scanning tunneling microscopy (STM) imaging (Figure 12b,c). We note that, although the sizes of graphene and MoS2 domains are large, the overlapped area between them, i.e., the heterostructures, is small.175 Similarly, vertical heterostructures made of graphene/ MoS2 nanoribbons were grown by two individual CVD processes, and showed a high photoresponsivity to visible light.188 It should be noted that all current heterostructures have only been formed in small areas and the wafer-scale CVD growth of vertical heterostructures has not been reported. 5.1.2. Semiconductor/Semiconductor. Due to their wide range of band gaps and electronic affinities, the stacking of S

DOI: 10.1021/acs.chemrev.7b00536 Chem. Rev. XXXX, XXX, XXX−XXX

Chemical Reviews

Review

Figure 13. CVD growth of TMDC/h-BN based semiconductor/insulator vertical heterostructures. (a) Schematic showing preparation of TMDC/hBN heterostructures. (b) SEM image of directly grown single crystal MoS2 on h-BN. (inset) MoS2 crystal with a grain size up to 200 μm2. (c) TEM characterization of MoS2/h-BN heterostructures. (inset) SAED patterns corresponding to heterostructures. (d) Photoluminescence performance of directly grown and transferred TMDC/h-BN heterostructures. Reprinted from ref 176. Copyright 2016 American Chemical Society.

considerable interest. Recently, CdI2 nanoplate/TMDC (i.e., MoS2, WS2, WSe2) vertical heterostructures were prepared by an all-CVD approach. The single CdI2 crystals show hexagonal and triangular morphologies with sizes of 2−10 μm and a thickness of 5−220 nm.198 This new heterostructure design will help to expand the family of 2D material based heterostructures and enable new device functionalities. Black phosphorus (bP) has a high theoretical charge carrier mobility of up to ∼10 000 cm2 V−1 s−1 and a direct band gap of about 0.3 eV in the bulk form. In addition, bP FETs typically exhibit ambipolar behavior with the p-branch stronger than the n-branch. These features make bP a good complement to most TMDC materials which are n-type semiconductors and have band gaps in the visible light range. Hence, the assembly of bP with TMDCs can produce van der Waals p−n heterostructures as building blocks for novel devices, as well as extending the optical spectral range from the visible region to the mid-infrared region for optoelectronic applications.199 So far, TMDC/bP vertical heterostructures can only be made by the stacking of components exfoliated using Scotch tape.200−202 To achieve the scalable production of the heterostructures, some CVD-based methods have recently been proposed. For example, Deng et al. reported a high performance and gate-tunable p−n diode based on n-type monolayer MoS2/p-type bP. The heterostructures were made by first putting Scotch-tape-exfoliated bP onto a silicon wafer, followed by the CVD growth of MoS2 on top of the bP.203 This approach was also used to fabricate other TMDC/bP heterostructures, such as MoSe2/bP, etc.204 In addition, recent work has shown that alloying of bP with arsenic produces b-AsP with band gaps reaching the long-wavelength infrared regime (0.15 eV).205 Therefore, fabrication of b-AsP/TMDCs heterostructures is another interesting research direction to pursue. As already mentioned, a transfer method is unavoidable in fabricating TMDC/bP heterostructures because of the difficulty of growing bP thin films by CVD. It is obvious that a CVD approach to grow monolayer or few-layer bP is urgently needed in order to achieve the all-CVD growth of large area and high quality TMDC/bP vertical heterostructures. The development of such a CVD method will significantly accelerate the science and applications of bP.

WS2 vertical heterostructures. For both vertical and planar devices, they exhibit novel rectifying and bipolar behavior, a high on/off current ratio, and a high mobility.193 It is shown that this stacking method is able to fabricate TMDC/TMDC heterostructures with relatively good properties. However, contamination caused by trapped impurities or residue at the interface and mechanical damage during the transfer process may seriously modify the intrinsic electronic properties of each individual material and consequently the properties of the heterostructure. In addition, it is very difficult to achieve the large-scale production of TMDC/TMDC heterostructures using such an aligned transfer method. Intriguingly, a one-step CVD growth approach has been demonstrated as being able to grow this kind of heterostructure, as demonstrated by Gong et al. for the growth of MoS2/WS2 in 2014.174 Furthermore, sulfurization of deposited Mo and W stacks, which served as seed layers, was used to grow MoS2/WS2 vertical heterostructures.194 However, the one-step method has limitations in the spatial and size control of the heterostructures. Gong et al. discovered a new two-step CVD method for growing WSe 2 /MoSe 2 heterostructures, where MoSe2 was grown in the first step, followed by epitaxial growth of WSe2 along the edges and on top of the MoSe2.195 The sizes of both the MoSe2 and WSe2 could be easily adjusted, providing a powerful way to control the size of heterostructures, which reached a size of 169 μm. As shown in Figure 12d, heterostructures with bilayer WSe2 and bilayer WSe2/MoSe2 at the edges (stack 1) and vertically stacked WSe2/ MoSe2 heterostructures with an additional WSe2 monolayer and bilayer at the edges (stack 2) are clearly observed. However, the edges of these large vertical heterostructures are not clean, and more effort must be devoted to passivating the edge sites and activating the top surface of the bottom layers. For periodic patterns of MoS2/WS2 vertical heterostructures, WO3 and MoO3 were predeposited separately, followed by a thermal reduction sulfurization process.196 More interestingly, a two-step temperature mediated strategy was used to grow A−A stacked MoS2/ WS2 or A−B stacked WS2/MoS2 vertical heterostructures without cross contamination of Mo and W atoms.197 In addition to the TMDC/TMDC heterostructures, other metal chalcogenide materials (i.e., GaS, InSe, SnS2) and metal halide materials (i.e., CdI2, PbI2) have been the focus of T

DOI: 10.1021/acs.chemrev.7b00536 Chem. Rev. XXXX, XXX, XXX−XXX

Chemical Reviews

Review

Figure 14. CVD growth of graphene/h-BN lateral 2D heterostructures. (a) Illustration of fabrication procedure for in-plane graphene/h-BN heterostructures. (b) STEM bright field imaging of graphene/h-BN interface, highlighted by the dashed line. (c) Illustration of epitaxial growth of h-BN onto graphene edges. Black, blue, red, and small gray spheres represent C, N, B, and H atoms, respectively. (d) Full coverage of etched holes in a graphene island by BN. (e) Atomic-resolution STM image of graphene/h-BN boundary. A honeycomb lattice and zigzag boundaries are overlaid on the image. (a, b) Reprinted with permission from ref 219. Copyright 2013, Rights Managed by Nature Publishing Group. (c−e) Reproduced with permission from ref 179. Copyright 2014 American Association for the Advancement of Science.

5.1.3. Semiconductor/Insulator. It is known that the surfaces of h-BN are atomically flat, chemically inert, free of dangling bonds, and surface state free. In addition, the phonon energy in h-BN is very small, making it a substrate with very weak electron−phonon interactions. Together, these features make hBN an ideal substrate or protective layer for 2D channel materials to achieve high electronic device performance. For example, an extremely high Hall mobility of ∼34 000 cm2 V−1 s−1 was reported for a six-layer MoS2 FET sandwiched between two hBN monolayers.206 CVD is seen as a promising way to fabricate high quality TMDC/h-BN vertical heterostructures. A general approach is to combine CVD and Scotch-tape exfoliation. For instance, Yan et al. developed an efficient and scalable CVD method to directly grow single- and few-layer MoS2 flakes on hBN that had been exfoliated using Scotch tape.207 The as-grown single layer MoS2/h-BN heterostructures have a relatively stable electrical environment with less electron scattering, and show potential for use in high quality MoS2-based devices. Despite the convenience of this combinational method, it is very difficult to produce large millimeter or centimeter scale TMDC/h-BN vertical heterostructures. Recently, an interesting all-CVD method to grow MoS2/h-BN vertical heterostructure was reported by Wang et al.208 First, a large area h-BN film was grown on a copper substrate by CVD followed by transfer onto a SiO2/Si substrate. Second, individual MoS2 domains were grown directly on the whole h-BN surface with MoO3 and S powders as precursors. A transfer process was still necessary in this strategy because the nickel or copper substrate can react with sulfur. To solve this, a nickel−gallium alloy with an excellent sulfide resistance and a high catalytic activity for h-BN growth was used by Fu et al. As exhibited in Figure 13a, this is an all-CVD process without any transfer steps. Using Mo foil and sulfur powders as precursors, single crystal, high quality MoS2 domains as large as 200 μm2 were grown on the h-BN surface (Figure 13b,c). Furthermore, the superior properties (low level of charged impurities at the interface) of the direct CVD-grown MoS2/hBN heterostructure were verified by PL measurements (Figure 13d).176 Another recent study showed that h-BN/MoS2 vertical heterostructures could be grown on a Au foil using a two-step CVD method which also did not need a transfer process.34 These

results indicate that the growth of high quality and large area TMDC/h-BN heterostructures can be achieved by an all-CVD approach. Such direct CVD-grown h-BN/MoS2 heterostructures tend to eliminate contamination introduced in the Scotch-tapebased transfer approaches, and increase interlayer interactions at the interface, which may benefit the electronic applications of TMDCs. Furthermore, these strategies may be used to grow some metastable 2D materials, where the use of h-BN substrates may help to stabilize them and further benefit their device applications. Except for h-BN, mica has been extensively used as a substrate for 2D material growth and TMDC/mica vertical heterostructures have been reported.209−211 Because of the atomic flatness, surface inertness, and hexagonal in-plane lattice of mica, it is suitable for the epitaxial growth of materials with the same lattice symmetry, such as TMDCs.32 Notably, due to the flexibility of a mica substrate, this kind of vertical heterostructure has been used as a flexible and transparent optoelectronic device with a photoresponse of about 30 mA/W.210 5.1.4. Metal/Insulator. Graphene/h-BN heterostructures are typical 2D metal/2D insulator vertical heterostructures that have been extensively studied, because they provide platforms for novel physics research, including Hofstadter’s butterfly effect, as well as achieving high-performance graphene electronics.212,213 Generally, Scotch tape exfoliation, CVD growth, and cosegregation growth are used for the preparation of graphene/h-BN vertical heterostructures.214,215 The CVD method is the prevailing one because it has the advantages of growing large area samples with good thickness uniformity. The commonly used two-step process for the growth of graphene/h-BN vertical heterostructures, i.e., growing graphene on top of h-BN layers,216 suffers from the inert catalytic activity of h-BN substrate and a low decomposition rate of carbon precursors. To solve this issue, Li et al., have developed an inspiring nickelocene precursor facilitated route to realize fast growth of graphene/h-BN vertical heterostructures.217 In this method, h-BN was first grown on a Cu substrate using ammonia borane as precursors, followed by graphene deposition through nickelocene decomposition. In this process, the decomposed carbon rings serve as the carbon source, while the nickel atoms serve as a gaseous catalyst to increase the U

DOI: 10.1021/acs.chemrev.7b00536 Chem. Rev. XXXX, XXX, XXX−XXX

Chemical Reviews

Review

Figure 15. CVD growth of semiconductor/semiconductor and semiconductor/metal lateral 2D heterostructures. (a) Schematic of synthesis process for s-MoS2/s-WS2 lateral 2D heterostructures. (b) Atomic-resolution Z-contrast STEM images of in-plane interface betweenWS2 and MoS2 domains. The orange and pink dashed lines depict the atomic planes along the armchair and zigzag directions, respectively. (c) Combined Raman intensity mapping at 351 (yellow) and 381 cm−1 (purple). (d) Combined PL intensity mapping at 630 (orange) and 680 nm (green). (e) TEM image and corresponding FFT patterns of WSe2/WO3−x lateral heterostructures, where “H” stands for a hexagonal WSe2 domain and “C” stands for a cubic WO3−x domain. (f) Schematic of structural evolution during heating of WSe2 in air. (a−d) Reprinted with permission from ref 174. Copyright 2014, Rights Managed by Nature Publishing Group. (e, f) Reprinted from ref 220. Copyright 2016 American Chemical Society.

grown materials are made of randomly distributed domains of hBN and graphene. Furthermore, to precisely control the shapes and sizes of these two components in the heterostructures, the authors used a two-step CVD method.219 As shown in Figure 14a, h-BN was first grown on copper or nickel foils, and was then selectively etched away over an exposed region by argon ion plasma, followed by the growth of graphene at the etched area. This method is more controllable, and a large area crystal domain and a highly desired arrangement of heterostructures were obtained. The high quality nature of the heterostructures was evidenced by the sharp atomic level boundary (Figure 14b). Similarly, a two-step CVD approach but with a inverse feed sequence, i.e., first growing graphene followed by h-BN growth, was developed by Liu et al. to grow graphene/h-BN heterostructures.179 In Figure 14c, after the growth of monolayer graphene, H2 was introduced to in situ etch part of the graphene to obtain fresh zigzag edges, which immediately act as nucleation sites for subsequent orientated h-BN growth. As a result, an atomic level abrupt interface was formed in the graphene/h-BN lateral heterostructures (Figure 14d,e). It is clear that, after this great effort, high quality graphene/h-BN lateral heterostructures with sharp interfaces can now be synthesized by different methods. 5.2.2. Semiconductor/Semiconductor. Direct CVD growth has been shown to be very powerful in growing sTMDC/s-TMDC lateral heterostructures.218 For example, MoS2/WS2 lateral heterostructures were prepared by a simple one-step CVD method with sulfur, molybdenum trioxide, and

growth rate of graphene. The authors have achieved an 8−10 times faster growth rate compared to that without the nickel catalyst, and large-crystal domains up to 20 μm. To date, there are few reports on the one-step CVD growth of graphene/h-BN vertical heterostructures, mainly because it is difficult to separate the graphene deposition and the h-BN deposition, resulting in the formation of mixed domains of these two components. Learning from a recent study that used reversed flows during the temperature change stage in a sequential CVD process,218 it is possible to grow graphene/hBN vertical heterostructures by a one-step CVD method using a similar reverse flow method. 5.2. Lateral 2D Heterostructures

Lateral 2D heterostructures refer to heterostructures where each component is stitched together in the same plane and it is obvious that there must be strict lattice match requirements for the components on each side of the interface. Currently, researchers have grown lateral heterostructures that were metal/ insulator (e.g., graphene/h-BN),179,219 semiconducting/semiconducting (e.g., s-TMDC/s-TMDC),177,191 and semiconducting/metallic (e.g., s-TMDC/m-TMDC).220 5.2.1. Metal/Insulator. Considering their similar lattice structures and lattice constants (1.7% difference), graphene and h-BN have the potential to form lateral heterostructures. With great effort, graphene/h-BN lateral heterostructures have been realized by different approaches. The first synthesis of this kind of heterostructure reported was by introducing extra ammonia borane into the typical graphene growth process.221 The asV

DOI: 10.1021/acs.chemrev.7b00536 Chem. Rev. XXXX, XXX, XXX−XXX

Chemical Reviews

Review

tungsten as precursors (Figure 15a−d).174 An atomically sharp interface between MoS2 and WS2 was clearly observed with both atomic planes (armchair and zigzag) sharing the same crystal orientation. This high quality lateral heterostructure is a triangular core−shell shape with MoS2 inside and WS2 outside, as revealed by Raman and PL mapping results. The atomic level p−n junctions formed between the MoS2 and WS2 deliver a strong PL enhancement and photovoltaic effect. This one-step CVD method not only works for the growth of MoSe2/WSe2 lateral heterostructures where the two components have similar lattice constants, but also works well for 1H MoS2/1T′ MoTe2 lateral heterostructures where the two components have relatively large lattice mismatch.222,223 In addition, a two-step CVD method was developed to grow large WSe2/MoSe2 heterostructures, which shows the ability to control both the shape and size of each TMDC component in the heterostructure.195 MoS2/WS2 lateral heterostructures were also grown by a two-step CVD method using a MoS2 layer as a seed.224 Similarly, lateral heterostructures of MoS2/MoSe2,178 WS2/ WSe2,178 and MoS2/MoS2(1−x)Se2x225 have been synthesized. These heterostructures have either the same metal elements or the same chalcogen elements. In a further step, Li et al. use a twostep epitaxial method to grow WSe2/MoS2 lateral heterostructures with good controllability, overcoming the large lattice mismatch between these two crystals.177 The above results demonstrate that, among the various s-TMDCs, semiconducting/semiconducting lateral heterostructures have been successfully grown. 5.2.3. Metal/Semiconductor. As mentioned in section 3, TMDCs can be either metallic or semiconducting, depending on the phases. For example, 2H phase MoS2 and 2H phase WSe2 are semiconductors while their 1T phase counterparts are metals. Taking advantage of this, a general approach to prepare metal/ semiconducting TMDC lateral heterostructures lies in the treatment of part of an s-TMDC layer to transform it into mTMDC. For example, it has been reported that treating CVDgrown 2H phase WSe2 or MoS2 by n-butyl lithium converted them both into the 1T phase.226 The FETs made of 2H/1T lateral heterostructures of WSe2 with metallic phase as contacts and a semiconductor phase as channel materials exhibit an excellent FET performance with a mobility up to 66 cm2 V−1 s−1 and on/off current ratios of 107.227 In another interesting study, a one-step CVD approach was used to grow WSe2−NbSe2 lateral semiconductor−metal heterostructures with a WxNb1−xSe2 interface region, which can lower the Schottky barrier height of WSe2 FETs.228 Moreover, even for the same composition, fewlayer 2H/1T′ MoTe2 lateral homostructures were grown by introducing the proper concentration of Te vapor into predeposited Mo nanoislands, while a high Te concentration led to 2H phase and a low one led to the 1T′ phase.229 This lateral homostructure shows abrupt interfaces with a potential difference of about 100 eV between the 2H and 1T′ phases. In addition to the above-mentioned lateral heterostructures where the 2D components at the interfaces have very similar lattice constants, there are also several studies on the preparation of lateral heterostructures which are made of two materials with very different lattice structures and lattice constants.230−232 For example, lateral heterostructures made of monolayer MoS2 and graphene were grown by Zhao et al. The single graphene was first patterned to form a channel by using an oxygen plasma, and the graphene edges with dangling bonds promoted MoS2 nucleation and finally grew graphene/MoS2/graphene heterostructures.230 It is known that monolayer MoS2 has three atomic layers consists

of S−Mo−S, while monolayer graphene has only one carbon layer. It will be very interesting to study the interface and binding between MoS2 and graphene in such heterostructures.233 Recently, electron microscopy was used to study the formation mechanism of a lateral interface between two components with large lattice mismatch, such as ZnO/graphene and SiC/ graphene. Moreover, it is revealed that graphene can serve as a template to grow these novel heterostructures as well as to stabilize the unstable atomically thin oxides and carbides. These works significantly extend the range of possible 2D material based lateral heterostructures.234,235 In addition, we have developed a simple approach for the preparation of WSe2/ WO3−x in-plane heterostructures by simply heating a CVDgrown monolayer or few-layer WSe2 in air. As shown in Figure 15e,f, highly conductive WO3−x with a cubic lattice and domain sizes in the range of a few to a few tens of nanometers were formed within the hexagonal WSe2 matrix, after heating in air.220 The formation of such WSe2/WO3−x in-plane heterostructures was found to be able to significantly improve the mobility and on/off current ratios of WSe2 FETs. 5.3. Other Dimension/2D (xD/2D) Heterostructures

Beyond the 2D/2D heterostructures discussed above, combining 2D materials with other LD materials such as 1D nanowires and nanotubes, and 0D QDs, atoms, and small molecules (e.g., fullerenes, organic molecules) can further expand the family of 2D material based heterostructures. In these, the 2D material is just like a piece of paper and one can put whatever interesting materials on top of it in order to achieve certain functionalities. We term such heterostructures xD/2D heterostructures. It has been shown recently that putting 0D or 1D materials on top of 2D crystals results in unique electronic and optical properties and improved device performance in electronics, optoelectronics, energy conversion, etc.236−238 5.3.1. 1D/2D. One-dimensional materials such as carbon nanotube and silicon nanowire have tunable morphology, aspect ratio, doping, and conductivity, as well as the potential to be aligned into arrays.239 When combined with 2D materials, a large family of 1D/2D heterostructures and composite materials can be designed and fabricated, which are interesting for electronic, optoelectronic, and energy related applications. In addition, different crystal structures of 1D and 2D materials make the 1D/ 2D interface different from 2D/2D ones, and they provide interesting platforms to study some fundamental science such as charge transfer at 1D/2D interfaces. Artificial stacking of a 1D nanowire and a 2D material is a common approach to fabricating 1D/2D heterostructures. There is no strict requirement of lattice matching for the components, and therefore, numerous heterostructures have been fabricated.180,240−242 However, the currently used solution process unavoidably introduces contaminants during the preparation. As an improvement, researchers have developed the growth of 1D nanowires such as InxGa1−xAs on graphene by MOCVD or molecular beam epitaxy (MBE) methods.243 In a recent study using a reverse process, 2D MoS2 nanosheets were grown on 1D MgFe2O4 nanowires to form 1D/2D heterostructures.244 In a recent paper, Li et al. reported a simple one-step CVD method to grow 1D Bi 2 S 3 nanowires and 2D MoS 2 heterostructures.180 Mixed powders of MoO3 and Bi2O3 were used as the respective precursors of MoS2 and Bi2S3, and this was followed by sulfurization at 650 °C. This method produced 1D Bi2S3 nanowires with a length of 7.3 μm and diameter of 120 nm on top of 2D monolayer MoS2 flakes (Figure 16a). Although the W

DOI: 10.1021/acs.chemrev.7b00536 Chem. Rev. XXXX, XXX, XXX−XXX

Chemical Reviews

Review

Figure 16. CVD growth of xD/2D vertical heterostructures. (a) Growth and morphologies of Bi2S3/MoS2 heterostructures. (b) Schematic and SEM image of SWCNT/MoS2/SWCNT vertical point heterostructures. (c) AFM image of the 2D-WS2 crystal after spin coating of CdSe/ZnS 0D-QDs and a representation of hybrid indirect exciton formation upon photoexcitation. (a) Reprinted from ref 180. Copyright 2016 American Chemical Society. (b) Reprinted with permission from ref 237. Copyright 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim. (c) Reprinted from ref 236. Copyright 2016 American Chemical Society.

onto CVD-grown horizontally aligned SWCNTs, followed by the transfer of another SWCNT layer aligned perpendicular to the first layer, on top of MoS2. A schematic and SEM image of the device are shown in Figure 16b. The fabricated FET exhibits a high on/off current ratio of 105−106 at room temperature, and shows its superiority in making nanoscale 2D material based devices for logic circuits.237 Notably, FETs with ultrashort gate electrodes have been fabricated using this 1D/2D heterostructure design.238 These noble-metal-free electrode devices are promising for scalable applications in nanodevices.249 As a complement to the 1D nanowire/2D TMDCs and 1D SWCNT/2D TMDC heterostructures, 1D TMDC nanoribbon/ 2D TMDCs demonstrate great potential in unique applications that differ from the 2D TMDC/2D TMDC heterostructures, because of the metallicity at inversion domain boundaries in the 1D structure.250 However, this novel structure has seldom been reported. Further attention should be paid to novel properties and applications, and more effort needs to be devoted to the CVD growth of 1D TMDC nanoribbon/2D TMDC layered heterostructures. 5.3.2. 0D/2D. QDs are typical 0D materials, which have a large variety, a broad size tunability, and a tunability of band gaps. When QDs are in contact with 2D materials, the interface may exhibit interesting properties such as nonradiative energy transfer, charge transfer, and surface plasmonic effects, making such 0D/2D heterostructures increasingly attractive for catalysis and optoelectronic applications.251 In general, the QDs are prepared by solution-based methods, followed by dispersion on the surface of 2D materials. Using this strategy, Boulesbaa et al. reported 2D WS2 monolayer/CdSe− ZnS core−shell QD heterostructures, which show ultrafast charge transfer, where the WS2 monolayer was grown using CVD and the CdSe−ZnS QDs were spin-coated onto the WS2. In Figure 16c, the height of the WS2 is 0.7 nm and the height of the CdSe−ZnS QDs is about 8 nm with a uniform coverage on the

nanowires were located in random angles with respect to the lattice of 2D MoS2, it is noted that a relatively high proportion of nanowires tended to be either parallel or perpendicular to the edge directions of MoS2, resulting from the low formation energy barrier in such configurations. Despite this progress, a wellaligned distribution of Bi2S3 on TMDCs needs to be grown and studied in-depth in order to fabricate device arrays. In a further consideration, although m-TMDC nanowire/s-TMDC heterostructures have been prepared by steering a focused electronic beam,245 a simpler way of growing TMDC-based 1D/2D heterostructures by a CVD method would be promising for applications and needs to be further explored. Besides nanowires, carbon nanotubes (CNTs) are another important 1D material. It is noted that single-wall CNTs (SWCNTs) can be either semiconducting or metallic, depending on the way the graphene sheet is wrapped in the tube, especially the chirality.246 SWCNTs will different chiralities have different band gaps and band positions, which in combination with many different TMDCs would result in a multitude of combinations of energy alignments at the interface between a 1D SWCNT and 2D TMDCs. Considering the facts that researchers have made significant progress in the chirality-controlled synthesis of SWCNTs and can now produce high purity single chirality SWCNT species,246,247 it is reasonable to anticipate that 1D SWCNT/2D TMDC heterostructures have much science to be explored and may show promising applications. For example, recently, by using an SWCNT as a p-type semiconductor and monolayer MoS2 as an n-type semiconductor, gate-tunable mixed p−n heterostructures were formed.248 In a further step, Zhang et al. reported the construction of SWCNT/MoS2/SWCNT vertical heterostructures, where the MoS2 monolayer was sandwiched between two cross-stacked metallic SWCNTs and the contact areas were as low as 1−2 nm. The device was stacked in the following sequence. Scotch-tape-exfoliated MoS2 layer was first transferred X

DOI: 10.1021/acs.chemrev.7b00536 Chem. Rev. XXXX, XXX, XXX−XXX

Chemical Reviews

Review

Figure 17. Electronic devices for 2D materials and heterostructures. (a) Schematic of 1D/2D-FET with MoS2 channel and SWCNT gate. (b) Falsecolored SEM image of the device. (c) Cross-sectional TEM image of representative sample showing SWCNT gate, ZrO2 gate dielectric, and bilayer MoS2 channel. (d) ID−VGS characteristics of bilayer MoS2 channel SWCNT gated FET at VBS = 5 V and VDS = 50 mV and 1 V (left) and ID−VGS characteristics at VDS = 1 V and varying VBS (right). (e) Schematic showing cross-sectional view of an ATLAS-TFET with ultrathin bilayer MoS2 as the channel and p-type Ge as the source. (f) Cross-sectional TEM image of bilayer MoS2 on Ge. Scale bar, 2 nm. (g) Drain current as a function of gate voltage for drain voltages of 0.1, 0.5, and 1 V. (h) SS as a function of drain current for an ATLAS-TFET (green triangles) as well as a conventional MOSFET (blue squares) at VDS = 0.5 V. (a−d) Reproduced with permission from ref 238. Copyright 2016 American Association for the Advancement of Science. (e−h) Reprinted with permission from ref 275. Copyright 2015, Rights Managed by Nature Publishing Group.

6.1), optoelectronics (section 6.2), sensors (section 6.3), flexible devices (section 6.4), and catalysis (section 6.5) are discussed.

WS2 surface. These heterostructures form indirect excitons by hole and electron transfer through the interface under the illumination of laser light.236 Similarly, PbS colloidal QD/2D WSe2 heterostructures have been developed and used as high sensitivity and broadband photodetectors.252 QD/TMDC heterostructures of CdS/MoS2 were also formed to produce a high performance photocatalyst for the hydrogen evolution reaction,253 and this potential heterostructure need also be grown by CVD in the future. To achieve more accurate deposition by additives, a CVD method combined with thermal evaporation was used to synthesize PbS/MoS2 heterostructures, where the edges of nonlayered PbS nanoplates were in contact with the edges of MoS2.254 Very recently, C60 molecules were deposited onto graphene using a low temperature effusion cell at 156 °C,255 and the heterostructures showed good electronic properties in terms of an on/off current ratio above 3 × 103.256 Based on these results, we believe this approach can be an effective way to obtain C60/TMDC heterostructures for new types of electronic devices.

6.1. Electronics

The FET is the most fundamental and important component in electronics. In a typical FET device, a channel material is connected to contact pads called source and drain electrodes on both sides, as well as to a top gate electrode with a dielectric layer between the channel material and the gate electrode. The current flow in the channel is associated with the source−drain voltage (VDS) and can be modulated by the electric field through the applied gate voltage (VG). For high performance FETs, (i) the charge carrier mobility and on-state current (Ion) should be high enough to realize a fast response, and (ii) the on/off current ratio of the FET should be large and the leakage current at the off state (Ioff) should be as low as possible, in order to achieve effective switching and low power consumption during transistor operation. The well-known Moore’s law predicts that the numbers of transistors per unit area will double every 18 months, indicating that each transistor will become smaller and smaller. One obvious problem for small transistors is the so-called short channel effect; i.e., the effect of source−drain voltage on the channel material would impair the ability of gate voltage to tune the channel current flow, resulting in a bad switching performance. Another problem is the heat dissipation, where the accumulated heat may seriously degrade device performance, cause technical failure, and reduce the lifetime. In addition, future devices require electronics to be not only small in size and fast in operation speed, but also to have multiple functionalities such as

6. APPLICATIONS OF CVD-GROWN 2D MATERIALS AND THEIR HETEROSTRUCTURES The rapid developments in the CVD technique have shown the ability to grow large-area high quality 2D materials with versatile structures and properties, and the growth of 2D material based vertical and lateral heterostructures. The availability of such material systems provides a foundation for the exploration of their uses in various fields. In this section, the uses of CVD-grown 2D materials and 2D heterostructures for electronics (section Y

DOI: 10.1021/acs.chemrev.7b00536 Chem. Rev. XXXX, XXX, XXX−XXX

Chemical Reviews

Review

being flexible, wearable, power efficient, or even self-powered. Taken together, these exacting requirements stand as significant challenges for current silicon and III−V semiconductors, and provide exciting opportunities for new semiconducting materials, especially LD materials such as 2D graphene, TMDCs, bP, etc. Taking 2D TMDCs as an example, they are atomically flat with excellent mechanical flexibility, and free from dangling bonds and surface states. In addition, the availability of a large number of TMDCs with diverse electronic properties and different band gaps have made them being used to fabricated TMDC-based FETs with a high on/off current ratio and a low off current,257 as well as high mobility.258 In the following paragraphs, we highlight the latest strategies to optimize the FET performance of 2D transistors, mainly based on the improvement of CVD-grown TMDCs. High mobility is very important for many electronic applications including FETs and radio frequency devices. Recently, charge scattering mechanisms259,260 in TMDCs have been extensively studied to achieve a high charge carrier mobility in FETs. For example, the first monolayer MoS2-based FET with high a mobility of 200 cm2 V−1 s−1 and a high on/off current ratio of 108 at room temperature was reported in 2011,261 where the researchers used Scotch-tape-exfoliated MoS2. CVD-grown TMDCs with high quality and large area have recently been used to fabricate MoS2, MoSe2, WS2, and WSe2 FETs with superior performance.27,262 Notably, Yu et al. used a “gate first” process to fabricate FETs with a high mobility using CVD-grown MoS2 as the channel material, and then to fabricate various logic gates and circuits for large area high performance electronics.263 In addition to the above typical TMDC materials, newly emerging 2D materials, such as PtSe2, have been grown by CVT and exhibit a high room temperature mobility of 210 cm2 V−1 s−1 and good stability in a back gate FET.264 To date, high mobility individual 2D-material-based FETs have been realized using an ionic liquid as the gate dielectric, decreasing the number of charge traps, or making good electrode/channel material contacts, etc.142,265−268 Another important research direction is to decrease the device size of 2D material based FETs. To achieve this goal, a MoS2based FET with a narrow SWCNT (diameter ∼ 1 nm) as gate electrode has been designed and fabricated by Desai et al. (Figure 17a−c).238 In this structure, the aligned SWCNTs were grown by CVD, followed by ALD deposition of a ZrO2 gate dielectric. Later, Scotch-tape-exfoliated MoS2 channel material was put on top of the ZrO2. The ID−VGS characteristics in Figure 17d at VBS = 5 V and VDS = 50 mV and 1 V demonstrate the ability of the ∼1 nm SWCNT gate to deplete the MoS2 channel and turn off the device. The FET exhibited excellent subthreshold characteristics with a subthreshold swing (SS) of ∼65 mV/decade (dec) and a high on/off current ratio of ∼106. Moreover, with the increase of the VBS values, VGS had to decrease in order to deplete the MoS2 channel, which in turn made the threshold voltage (Vth) more negative. Above VBS = 1 V, the SS and ION did not improve any further, and the extension regions were strongly inverted. Thus, the 1D/2D FET operated as a short-channel device. In achieving an ultrashort channel length, Xu et al. reported a more universal technological technique to make sub-10-nm gaps with sharp edges and steep sidewalls, exhibiting no obvious short channel effects.269 This monolayer MoS2 FET shows a high on/off current ratio of 106 and an SS of 140 mV/dec. With a decrease of both the gate length and the channel length, the heat dissipation from electronics has emerged as a crucial problem that deserves more attention.

The emergence of 2D/2D heterostructures has produced a platform for the design of novel structure and high performance FETs. With stacked 2D materials as a conductive channel, decreased gate voltage and power consumption can be realized for electronics. Previously, MoS2/WSe2,270,271 WSe2/SnSe2,272 bp/MoS2,273 and MoS2/α-MoTe2274 based vertical heterostructures have been used for tunneling FETs using the Scotch-tape-exfoliation method. In a further step based on the CVD growth of MoS2, Sarkar et al. reported the fabrication of an atomically thin and layered semiconducting-channel tunnel FET device as shown in Figure 17e,f.275 In this structure, the substrate consisted of a p-doped Ge wafer with parts of it being a 300 nm SiO2 dielectric layer, bilayer MoS2 synthesized by CVD was then transferred onto this engineered substrate, and the Ge contacts the source electrode while MoS2 contacts the drain electrode making a unique tunneling structure. A solid polymer electrolyte composed of poly(ethylene oxide) and lithium perchlorate was used as a high quality gate dielectric material. For this band-toband tunneling device, the transfer characteristics (Figure 17g) for different VDS’s shows that the atomically thin and layered semiconducting-channel tunnel FET (ATLAS-TFET) can overcome the fundamental limitations on SS (60 mV/dec at room temperature) in conventional MOSFETs, and SS values below 60 mV/dec were obtained over about 4 decades of current. As shown in Figure 17h, the lowest SS for the ATLAS-TFET can be as low as 3.9 mV/dec, and excellent average SS values of 5.5, 12.8, 22, and 31.1 mV/dec were obtained over 1, 2, 3, and 4 decades of current, respectively. The design of these tunneling FETs largely increases the gate capability to control the channel mobility. To facilitate the fabrication process without using a bulk p-type three-dimensional Ge substrate, the 2D/2D heterostructures such as MoS2/WSe2 need to be grown by CVD for the production of tunneling FETs. 6.2. Optoelectronics

An optoelectronic device features a mutual charge-to-photon conversion process. For a light-emitting device, photons are generated due to the recombination of electron−hole pairs, which are injected from a cathode and an anode, respectively. The light emission process is named “electroluminescence” (EL). For a photovoltaic device, free charges are generated due to the dissociation of excitons, which are excited by the incident photons. Two typical applications of photovoltaic cells include solar cells and photodetectors. Generally, a semiconductor with a direct band gap has high efficiency of exciton formation or recombination. As a result, an optoelectronic device with an active semiconductor of a direct band gap shows higher device efficiency in comparison with that of an indirect band gap. For 2D materials, most direct band gap semiconductors are TMDC monolayers. Consequently, 2D optoelectronic devices select TMDC monolayer as the active layer. It is important to note that several TMDC multilayers can also have a direct band gap due to the introduction of tensile strain.276 Based on the above considerations, TMDC-based LEDs, photodiodes, and solar cells will be briefly introduced in sections 6.2.1, 6.2.2, and 6.2.3. 6.2.1. LEDs. An LED structure typically has a p−n junction. Under a given bias, electrons and holes can be injected from electrodes into n-type and p-type semiconductors, respectively. In the depletion area of a p−n junction, the electrons and holes form excitons or electron−hole pairs, which undergo recombination and emit photons. The EL spectra for an LED can cover the ultraviolet, visible, or infrared range by selecting active semiconductors of proper band gaps. Commonly, a highZ

DOI: 10.1021/acs.chemrev.7b00536 Chem. Rev. XXXX, XXX, XXX−XXX

Chemical Reviews

Review

Figure 18. Optoelectronic devices based on 2D materials and heterostructures. (a) Schematic view and I−V characteristic of p+-Si/i-WS2/n−-ITO heterostructured device. Inset shows images of LEDs and EL mapping. Current-dependent (b) EL spectra and (c) degree of EL circular polarization for full emission and each component emission of the LED device. (d) Schematic view of electron−hole pair generation under laser beam illumination and hole tunneling across the h-BN layer. (e) I−V characteristic of the photodetector under the dark and excitation of 405 nm laser light. (f) J−V characteristics, EQE spectra, and parameters of MoS2/p-Si heterostructured solar cells. (a−c) Reprinted from ref 303. Copyright 2014 American Chemical Society. (d, e) Reprinted from ref 287. Copyright 2016 American Chemical Society. (f) Reprinted from ref 297. Copyright 2016 American Chemical Society.

injection from electrodes to the emissive layer. Another important function of 2D LEDs is the single-photon emission, which has been observed at edges of WSe2 crystals at low temperature, featuring a 0.1 meV line width.283,288,289 In this case, it can be anticipated that the higher single-photon-emission efficiency can be achieved by precise control over edge structures and defects during the CVD growth process, and thus promote the development of future applications. 6.2.2. Photodetectors. For a photodetector, a built-in electric field is formed, and crosses the depletion region of the p− n junction. The electron−hole pairs, excited by the incident light, are dissociated into free electrons and holes by the built-in field. Under driving of the applied bias, the free charges transport across the active layer, followed by the collection of electrodes, which finally contribute to the photocurrent. In addition, the sensitivity to the wavelength of the incident light is crucial for the device selectivity. To achieve high performance, a thick depletion region is preferred to harvest more incident light for the generation of more electron−hole pairs, while p-type and n-type layers on two sides of the junction are kept thin enough to shorten the charge transport distance, ensuring the high responsivity.290 Several significant progresses, including the use of individual TMDC layers,291,292 2D/2D heterostructures,293 1D/2D heterostructures, and novel 0D/2D heterostructures,294 have been made to optimize both absorption and transport distance. Among different types of heterostructures, TMDC ones are promising for advanced photodetectors. Especially for type II band alignment, the dissociation efficiency of electron−hole pairs is very high due to the existence of a strong built-in electric field.295,296 As an example, the typical time constant of hole transfer has been observed to be 7300 and an H-index of 39. Xiaolong Zou received his bachelor’s degree in 2006 and Ph.D. in 2011, both in physics from Tsinghua University. After working as a research associate at Rice University, Houston, TX, USA, he joined Tsinghua− Berkeley Shenzhen Institute (TBSI), Tsinghua University, China, as an assistant professor in 2016. Dr. Zou’s current research focus concerns the theoretical description of the growth of 2D materials and their electronic, optical, and catalytic applications. Hui-Ming Cheng received his Ph.D. in materials science from the Institute of Metal Research, Chinese Academy of Sciences (IMR, CAS), in 1992. He has worked at AISTKyushu and Nagasaki University of Japan, and the Massachusetts Institute of Technology, USA. He is a member of CAS. He has been professor and founding director of the Advanced Carbon Research Division at Shenyang National Laboratory for Materials Science, IMR, CAS, since 2001, and also the founding director of the Low-Dimensional Materials and Devices Laboratory at Tsinghua−Berkeley Shenzhen Institute, Tsinghua University, China, since 2016. His research interests focus on the synthesis and applications of carbon nanotubes, graphene, other 2D materials, and highperformance bulk carbons, and on the development of new energy materials for batteries, electrochemical capacitors, and hydrogen production from water by photosplitting. He has published more than 550 scientific papers and obtained more than 100 patents.

ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China (Nos. 51722206 and 51521091), the Youth 1000-Talent Program of China, the Shenzhen Basic Research Project (Nos. JCYJ20170307140956657 and JCYJ20170407155608882), and the Development and Reform Commission of Shenzhen Municipality for the development of the “Low-Dimensional Materials and Devices” discipline. REFERENCES (1) Allen, M. J.; Tung, V. C.; Kaner, R. B. Honeycomb Carbon: A Review of Graphene. Chem. Rev. 2010, 110, 132−145. (2) Kim, K.; Choi, J.-Y.; Kim, T.; Cho, S.-H.; Chung, H.-J. A Role for Graphene in Silicon-Based Semiconductor Devices. Nature 2011, 479, 338−344. (3) Zhang, Y. B.; Tan, Y. W.; Stormer, H. L.; Kim, P. Experimental Observation of the Quantum Hall Effect and Berry’s Phase in Graphene. Nature 2005, 438, 201−204. (4) Tan, C. L.; Cao, X. H.; Wu, X. J.; He, Q. Y.; Yang, J.; Zhang, X.; Chen, J. Z.; Zhao, W.; Han, S. K.; Nam, G. H.; et al. Recent Advances in AH

DOI: 10.1021/acs.chemrev.7b00536 Chem. Rev. XXXX, XXX, XXX−XXX

Chemical Reviews

Review

Sapphire Via a Layer-over-Layer Growth Mode. ACS Nano 2015, 9, 8368−8375. (46) Shi, J. P.; Zhang, X. N.; Ma, D. L.; Zhu, J. B.; Zhang, Y.; Guo, Z. X.; Yao, Y.; Ji, Q. Q.; Song, X. J.; Zhang, Y. S.; et al. Substrate Facet Effect on the Growth of Monolayer MoS2 on Au Foils. ACS Nano 2015, 9, 4017− 4025. (47) Wang, S.; Pacios, M.; Bhaskaran, H.; Warner, J. H. Substrate Control for Large Area Continuous Films of Monolayer MoS2 by Atmospheric Pressure Chemical Vapor Deposition. Nanotechnology 2016, 27, 085604. (48) Yu, Y.; Li, C.; Liu, Y.; Su, L.; Zhang, Y.; Cao, L. Controlled Scalable Synthesis of Uniform, High-Quality Monolayer and Few-Layer MoS2 Films. Sci. Rep. 2013, 3, 1866. (49) Elías, A. L.; Perea-López, N.; Castro-Beltrán, A.; Berkdemir, A.; Lv, R.; Feng, S.; Long, A. D.; Hayashi, T.; Kim, Y. A.; Endo, M.; et al. Controlled Synthesis and Transfer of Large-Area WS2 Sheets: From Single Layer to Few Layers. ACS Nano 2013, 7, 5235−5242. (50) Wei, D.; Peng, L.; Li, M.; Mao, H.; Niu, T.; Han, C.; Chen, W.; Wee, A. T. S. Low Temperature Critical Growth of High Quality Nitrogen Doped Graphene on Dielectrics by Plasma-Enhanced Chemical Vapor Deposition. ACS Nano 2015, 9, 164−171. (51) Chen, Z.; Ren, W.; Gao, L.; Liu, B.; Pei, S.; Cheng, H.-M. ThreeDimensional Flexible and Conductive Interconnected Graphene Networks Grown by Chemical Vapour Deposition. Nat. Mater. 2011, 10, 424−428. (52) Gao, L.; Ren, W.; Zhao, J.; Ma, L.-P.; Chen, Z.; Cheng, H.-M. Efficient Growth of High-Quality Graphene Films on Cu Foils by Ambient Pressure Chemical Vapor Deposition. Appl. Phys. Lett. 2010, 97, 183109. (53) Sun, J.; Chen, Y.; Cai, X.; Ma, B.; Chen, Z.; Priydarshi, M. K.; Chen, K.; Gao, T.; Song, X.; Ji, Q.; et al. Direct Low-Temperature Synthesis of Graphene on Various Glasses by Plasma-Enhanced Chemical Vapor Deposition for Versatile, Cost-Effective Electrodes. Nano Res. 2015, 8, 3496−3504. (54) Kim, H.; Ahn, C.; Arabale, G.; Lee, C.; Kim, T. Synthesis of MoS2 Atomic Layers Using PECVD. ECS Trans. 2013, 58, 47−50. (55) Lu, A.-Y.; Zhu, H.; Xiao, J.; Chuu, C.-P.; Han, Y.; Chiu, M.-H.; Cheng, C.-C.; Yang, C.-W.; Wei, K.-H.; Yang, Y.; et al. Janus Monolayers of Transition Metal Dichalcogenides. Nat. Nanotechnol. 2017, 12, 744− 749. (56) Cain, J. D.; Shi, F.; Wu, J.; Dravid, V. P. Growth Mechanism of Transition Metal Dichalcogenide Monolayers: The Role of Self-Seeding Fullerene Nuclei. ACS Nano 2016, 10, 5440−5445. (57) Li, B.; Gong, Y. J.; Hu, Z. L.; Brunetto, G.; Yang, Y. C.; Ye, G. L.; Zhang, Z. H.; Lei, S. D.; Jin, Z. H.; Bianco, E.; et al. Solid-Vapor Reaction Growth of Transition-Metal Dichalcogenide Monolayers. Angew. Chem., Int. Ed. 2016, 55, 10656−10661. (58) Xia, J.; Zhu, D. D.; Wang, L.; Huang, B.; Huang, X.; Meng, X. M. Large-Scale Growth of Two-Dimensional SnS2 Crystals Driven by Screw Dislocations and Application to Photodetectors. Adv. Funct. Mater. 2015, 25, 4255−4261. (59) Zhang, Y.; Ji, Q. Q.; Wen, J. X.; Li, J.; Li, C.; Shi, J. P.; Zhou, X. B.; Shi, K. B.; Chen, H. J.; Li, Y. C.; et al. Monolayer MoS2 Dendrites on a Symmetry-Disparate SrTiO3 (001) Substrate: Formation Mechanism and Interface Interaction. Adv. Funct. Mater. 2016, 26, 3299−3305. (60) Jin, S.; Bierman, M. J.; Morin, S. A. A New Twist on Nanowire Formation: Screw-Dislocation-Driven Growth of Nanowires and Nanotubes. J. Phys. Chem. Lett. 2010, 1, 1472−1480. (61) Meng, F.; Morin, S. A.; Forticaux, A.; Jin, S. Screw Dislocation Driven Growth of Nanomaterials. Acc. Chem. Res. 2013, 46, 1616−1626. (62) Chen, L.; Liu, B.; Abbas, A. N.; Ma, Y.; Fang, X.; Liu, Y.; Zhou, C. Screw-Dislocation-Driven Growth of Two-Dimensional Few-Layer and Pyramid-Like WSe2 by Sulfur-Assisted Chemical Vapor Deposition. ACS Nano 2014, 8, 11543−11551. (63) Wu, J. J.; Hu, Z. L.; Jin, Z. H.; Lei, S. D.; Guo, H.; Chatterjee, K.; Zhang, J.; Yang, Y. C.; Li, B.; Liu, Y.; et al. Spiral Growth of SnSe2 Crystals by Chemical Vapor Deposition. Adv. Mater. Interfaces 2016, 3, 1600383.

(27) Roy, T.; Tosun, M.; Kang, J. S.; Sachid, A. B.; Desai, S. B.; Hettick, M.; Hu, C. C.; Javey, A. Field-Effect Transistors Built from All TwoDimensional Material Components. ACS Nano 2014, 8, 6259−6264. (28) Wei, D. C.; Liu, Y. Q.; Wang, Y.; Zhang, H. L.; Huang, L. P.; Yu, G. Synthesis of N-Doped Graphene by Chemical Vapor Deposition and Its Electrical Properties. Nano Lett. 2009, 9, 1752−1758. (29) Lv, R.; Li, Q.; Botello-Méndez, A. R.; Hayashi, T.; Wang, B.; Berkdemir, A.; Hao, Q.; Elías, A. L.; Cruz-Silva, R.; Gutiérrez, H. R.; et al. Nitrogen-Doped Graphene: Beyond Single Substitution and Enhanced Molecular Sensing. Sci. Rep. 2012, 2, 586. (30) Haynes, W. M. CRC Handbook of Chemistry and Physics; CRC Press: 2014. (31) He, Y. M.; Sobhani, A.; Lei, S. D.; Zhang, Z. H.; Gong, Y. J.; Jin, Z. H.; Zhou, W.; Yang, Y. C.; Zhang, Y.; Wang, X. F.; et al. Layer Engineering of 2D Semiconductor Junctions. Adv. Mater. 2016, 28, 5126−5132. (32) Ji, Q.; Zhang, Y.; Gao, T.; Zhang, Y.; Ma, D.; Liu, M.; Chen, Y.; Qiao, X.; Tan, P.-H.; Kan, M.; et al. Epitaxial Monolayer MoS2 on Mica with Novel Photoluminescence. Nano Lett. 2013, 13, 3870−3877. (33) Gong, Y. J.; Li, B.; Ye, G. L.; Yang, S. Z.; Zou, X. L.; Lei, S. D.; Jin, Z. H.; Bianco, E.; Vinod, S.; Yakobson, B. I.; et al. Direct Growth of MoS2 Single Crystals on Polyimide Substrates. 2D Mater. 2017, 4, 021028. (34) Zhang, Z. P.; Ji, X. J.; Shi, J. P.; Zhou, X. B.; Zhang, S.; Hou, Y.; Qi, Y.; Fang, Q. Y.; Ji, Q. Q.; Zhang, Y.; et al. Direct Chemical Vapor Deposition Growth and Band-Gap Characterization of MoS2/h-BN Van Der Waals Heterostructures on Au Foils. ACS Nano 2017, 11, 4328− 4336. (35) Zhou, X. B.; Shi, J. P.; Qi, Y.; Liu, M. X.; Ma, D. L.; Zhang, Y.; Ji, J. G.; Zhang, Z. P.; Li, C.; Liu, Z. F.; et al. Periodic Modulation of the Doping Level in Striped MoS2 Superstructures. ACS Nano 2016, 10, 3461−3468. (36) Sanne, A.; Ghosh, R.; Rai, A.; Movva, H. C. P.; Sharma, A.; Rao, R.; Mathew, L.; Banerjee, S. K. Top-Gated Chemical Vapor Deposited MoS2 Field-Effect Transistors on Si3N4 Substrates. Appl. Phys. Lett. 2015, 106, 062101. (37) Lee, Y.-H.; Yu, L.; Wang, H.; Fang, W.; Ling, X.; Shi, Y.; Lin, C.-T.; Huang, J.-K.; Chang, M.-T.; Chang, C.-S.; et al. Synthesis and Transfer of Single-Layer Transition Metal Disulfides on Diverse Surfaces. Nano Lett. 2013, 13, 1852−1857. (38) Li, S.; Wang, S.; Tang, D.-M.; Zhao, W.; Xu, H.; Chu, L.; Bando, Y.; Golberg, D.; Eda, G. Halide-Assisted Atmospheric Pressure Growth of Large WSe2 and WS2 Monolayer Crystals. Applied Materials Today 2015, 1, 60−66. (39) Wang, Z.; Xie, Y.; Wang, H.; Wu, R.; Nan, T.; Zhan, Y.; Sun, J.; Jiang, T.; Zhao, Y.; Lei, Y.; et al. NaCl-Assisted One-Step Growth of MoS2−WS2 in-Plane Heterostructures. Nanotechnology 2017, 28, 325602. (40) Kim, H.; Ovchinnikov, D.; Deiana, D.; Unuchek, D.; Kis, A. Suppressing Nucleation in Metal-Organic Chemical Vapor Deposition of MoS2 Monolayers by Alkali Metal Halides. Nano Lett. 2017, 17, 5056−5063. (41) Wang, H.; Huang, X.; Lin, J.; Cui, J.; Chen, Y.; Zhu, C.; Liu, F.; Zeng, Q.; Zhou, J.; Yu, P.; et al. High-Quality Monolayer Superconductor NbSe2 Grown by Chemical Vapour Deposition. Nat. Commun. 2017, 8, 394. (42) Jeong, H. Y.; Jin, Y.; Yun, S. J.; Zhao, J.; Baik, J.; Keum, D. H.; Lee, H. S.; Lee, Y. H. Heterogeneous Defect Domains in Single-Crystalline Hexagonal WS2. Adv. Mater. 2017, 29, 1605043. (43) Yun, S. J.; Han, G. H.; Kim, H.; Duong, D. L.; Shin, B. G.; Zhao, J.; Vu, Q. A.; Lee, J.; Lee, S. M.; Lee, Y. H. Telluriding Monolayer MoS2 and WS2 Via Alkali Metal Scooter. Nat. Commun. 2017, 8, 2163. (44) Zhang, Y.; Zhang, Y. F.; Ji, Q. Q.; Ju, J.; Yuan, H. T.; Shi, J. P.; Gao, T.; Ma, D. L.; Liu, M. X.; Chen, Y. B.; et al. Controlled Growth of HighQuality Monolayer WS2 Layers on Sapphire and Imaging Its Grain Boundary. ACS Nano 2013, 7, 8963−8971. (45) Chen, L.; Liu, B. L.; Ge, M. Y.; Ma, Y. Q.; Abbas, A. N.; Zhou, C. W. Step-Edge-Guided Nucleation and Growth of Aligned WSe2 on AI

DOI: 10.1021/acs.chemrev.7b00536 Chem. Rev. XXXX, XXX, XXX−XXX

Chemical Reviews

Review

(83) Zhou, X.; Gan, L.; Tian, W.; Zhang, Q.; Jin, S.; Li, H.; Bando, Y.; Golberg, D.; Zhai, T. Ultrathin SnSe2 Flakes Grown by Chemical Vapor Deposition for High-Performance Photodetectors. Adv. Mater. 2015, 27, 8035−8041. (84) Babu, G.; Masurkar, N.; Al Salem, H.; Arava, L. M. R. Transition Metal Dichalcogenide Atomic Layers for Lithium Polysulfides Electrocatalysis. J. Am. Chem. Soc. 2017, 139, 171−178. (85) Jeon, J.; Jang, S. K.; Jeon, S. M.; Yoo, G.; Jang, Y. H.; Park, J.-H.; Lee, S. Layer-Controlled Cvd Growth of Large-Area Two-Dimensional MoS2 Films. Nanoscale 2015, 7, 1688−1695. (86) Samad, L.; Bladow, S. M.; Ding, Q.; Zhuo, J. Q.; Jacobberger, R. M.; Arnold, M. S.; Jin, S. Layer-Controlled Chemical Vapor Deposition Growth of MoS2 Vertical Heterostructures Via Van Der Waals Epitaxy. ACS Nano 2016, 10, 7039−7046. (87) Xia, M.; Yin, K.; Capellini, G.; Niu, G.; Gong, Y. J.; Zhou, W.; Ajayan, P. M.; Xie, Y. H.; Li, B. Spectroscopic Signatures of AA′ and AB Stacking of Chemical Vapor Deposited Bilayer MoS2. ACS Nano 2015, 9, 12246−12254. (88) Shearer, M. J.; Samad, L.; Zhang, Y.; Zhao, Y.; Puretzky, A.; Eliceiri, K. W.; Wright, J. C.; Hamers, R. J.; Jin, S. Complex and Noncentrosymmetric Stacking of Layered Metal Dichalcogenide Materials Created by Screw Dislocations. J. Am. Chem. Soc. 2017, 139, 3496−3504. (89) Lee, J.-H.; Lee, E. K.; Joo, W.-J.; Jang, Y.; Kim, B.-S.; Lim, J. Y.; Choi, S.-H.; Ahn, S. J.; Ahn, J. R.; Park, M.-H.; et al. Wafer-Scale Growth of Single-Crystal Monolayer Graphene on Reusable HydrogenTerminated Germanium. Science 2014, 344, 286−289. (90) Xu, X.; Zhang, Z.; Dong, J.; Yi, D.; Niu, J.; Wu, M.; Lin, L.; Yin, R.; Li, M.; Zhou, J.; et al. Ultrafast Epitaxial Growth of Metre-Sized SingleCrystal Graphene on Industrial Cu Foil. Sci. Bull. 2017, 62, 1074−1080. (91) Aljarb, A.; Cao, Z.; Tang, H.-L.; Huang, J.-K.; Li, M.; Hu, W.; Cavallo, L.; Li, L.-J. Substrate Lattice-Guided Seed Formation Controls the Orientation of 2D Transition-Metal Dichalcogenides. ACS Nano 2017, 11, 9215−9222. (92) Dumcenco, D.; Ovchinnikov, D.; Marinov, K.; Lazic, P.; Gibertini, M.; Marzari, N.; Sanchez, O. L.; Kung, Y.-C.; Krasnozhon, D.; Chen, M.W.; et al. Large-Area Epitaxial Monolayer MoS2. ACS Nano 2015, 9, 4611−4620. (93) Lu, C.; Butler, C. J.; Huang, J.-K.; Hsing, C.-R.; Yang, H.-H.; Chu, Y.-H.; Luo, C.-H.; Sun, Y.-C.; Hsu, S.-H.; Yang, K.-H. O.; et al. Graphite Edge Controlled Registration of Monolayer MoS2 Crystal Orientation. Appl. Phys. Lett. 2015, 106, 181904. (94) Zhang, Q.; Zhao, M. Q.; Tang, D. M.; Li, F.; Huang, J. Q.; Liu, B.; Zhu, W. C.; Zhang, Y. H.; Wei, F. Carbon-Nanotube-Array Double Helices. Angew. Chem., Int. Ed. 2010, 49, 3642−3645. (95) Liu, B.; Wang, C.; Liu, J.; Che, Y.; Zhou, C. Aligned Carbon Nanotubes: From Controlled Synthesis to Electronic Applications. Nanoscale 2013, 5, 9483−9502. (96) Liu, B.; Ren, W.; Liu, C.; Sun, C.-H.; Gao, L.; Li, S.; Jiang, C.; Cheng, H.-M. Growth Velocity and Direct Length-Sorted Growth of Short Single-Walled Carbon Nanotubes by a Metal-Catalyst-Free Chemical Vapor Deposition Process. ACS Nano 2009, 3, 3421−3430. (97) Ji, Q. Q.; Zhang, Y.; Shi, J. P.; Sun, J. Y.; Zhang, Y. F.; Liu, Z. F. Morphological Engineering of CVD-Grown Transition Metal Dichalcogenides for Efficient Electrochemical Hydrogen Evolution. Adv. Mater. 2016, 28, 6207−6212. (98) Einax, M.; Dieterich, W.; Maass, P. Colloquium: Cluster Growth on Surfaces: Densities, Size Distributions, and Morphologies. Rev. Mod. Phys. 2013, 85, 921−939. (99) Cao, D.; Shen, T.; Liang, P.; Chen, X. S.; Shu, H. B. Role of Chemical Potential in Flake Shape and Edge Properties of Monolayer MoS2. J. Phys. Chem. C 2015, 119, 4294−4301. (100) Wang, S. S.; Rong, Y. M.; Fan, Y.; Pacios, M.; Bhaskaran, H.; He, K.; Warner, J. H. Shape Evolution of Monolayer MoS2 Crystals Grown by Chemical Vapor Deposition. Chem. Mater. 2014, 26, 6371−6379. (101) Govind Rajan, A.; Warner, J. H.; Blankschtein, D.; Strano, M. S. Generalized Mechanistic Model for the Chemical Vapor Deposition of 2D Transition Metal Dichalcogenide Monolayers. ACS Nano 2016, 10, 4330−4344.

(64) Ly, T. H.; Zhao, J.; Kim, H.; Han, G. H.; Nam, H.; Lee, Y. H. Vertically Conductive MoS2 Spiral Pyramid. Adv. Mater. 2016, 28, 7723−7728. (65) Najmaei, S.; Liu, Z.; Zhou, W.; Zou, X.; Shi, G.; Lei, S.; Yakobson, B. I.; Idrobo, J.-C.; Ajayan, P. M.; Lou, J. Vapour Phase Growth and Grain Boundary Structure of Molybdenum Disulphide Atomic Layers. Nat. Mater. 2013, 12, 754−759. (66) Yan, Z.; Peng, Z. W.; Tour, J. M. Chemical Vapor Deposition of Graphene Single Crystals. Acc. Chem. Res. 2014, 47, 1327−1337. (67) Zhang, J.; Yu, H.; Chen, W.; Tian, X.; Liu, D.; Cheng, M.; Xie, G.; Yang, W.; Yang, R.; Bai, X.; et al. Scalable Growth of High-Quality Polycrystalline MoS2 Monolayers on SiO2 with Tunable Grain Sizes. ACS Nano 2014, 8, 6024−6030. (68) Tu, Z.; Li, G.; Ni, X.; Meng, L.; Bai, S.; Chen, X.; Lou, J.; Qin, Y. Synthesis of Large Monolayer Single Crystal MoS2 Nanosheets with Uniform Size through a Double-Tube Technology. Appl. Phys. Lett. 2016, 109, 223101. (69) Rong, Y. M.; Fan, Y.; Koh, A. L.; Robertson, A. W.; He, K.; Wang, S. S.; Tan, H. J.; Sinclair, R.; Warner, J. H. Controlling Sulphur Precursor Addition for Large Single Crystal Domains of WS2. Nanoscale 2014, 6, 12096−12103. (70) Chen, J.; Liu, B.; Liu, Y.; Tang, W.; Nai, C. T.; Li, L.; Zheng, J.; Gao, L.; Zheng, Y.; Shin, H. S.; et al. Chemical Vapor Deposition of Large-Sized Hexagonal WSe2 Crystals on Dielectric Substrates. Adv. Mater. 2015, 27, 6722−6727. (71) Zhou, J.; Liu, F.; Lin, J.; Huang, X.; Xia, J.; Zhang, B.; Zeng, Q.; Wang, H.; Zhu, C.; Niu, L.; et al. Large-Area and High-Quality 2D Transition Metal Telluride. Adv. Mater. 2017, 29, 1603471. (72) Zhan, L. J.; Wan, W.; Zhu, Z. W.; Xu, Y. X.; Shih, T. M.; Zhang, C. K.; Lin, W. Y.; Li, X. T.; Zhao, Z. J.; Ying, H.; et al. Centimeter-Scale Nearly Single-Crystal Monolayer MoS2 Via Self Limiting Vapor Deposition Epitaxy. J. Phys. Chem. C 2017, 121, 4703−4707. (73) Liu, Z. B.; Xu, C.; Kang, N.; Wang, L. B.; Jiang, Y. X.; Du, J.; Liu, Y.; Ma, X. L.; Cheng, H. M.; Ren, W. C. Unique Domain Structure of Two-Dimensional Alpha-Mo2C Superconducting Crystals. Nano Lett. 2016, 16, 4243−4250. (74) Kim, S. M.; Hsu, A.; Park, M. H.; Chae, S. H.; Yun, S. J.; Lee, J. S.; Cho, D.-H.; Fang, W.; Lee, C.; Palacios, T.; et al. Synthesis of Large-Area Multilayer Hexagonal Boron Nitride for High Material Performance. Nat. Commun. 2015, 6, 8662. (75) Chen, J.; Zhao, X.; Tan, S. J. R.; Xu, H.; Wu, B.; Liu, B.; Fu, D.; Fu, W.; Geng, D.; Liu, Y.; et al. Chemical Vapor Deposition of Large-Size Monolayer MoSe2 Crystals on Molten Glass. J. Am. Chem. Soc. 2017, 139, 1073−1076. (76) Chen, W.; Zhao, J.; Zhang, J.; Gu, L.; Yang, Z. Z.; Li, X. M.; Yu, H.; Zhu, X. T.; Yang, R.; Shi, D. X.; et al. Oxygen-Assisted Chemical Vapor Deposition Growth of Large Single-Crystal and High-Quality Monolayer MoS2. J. Am. Chem. Soc. 2015, 137, 15632−15635. (77) Shi, J. P.; Yang, Y.; Zhang, Y.; Ma, D. L.; Wei, W.; Ji, Q. Q.; Zhang, Y. S.; Song, X. J.; Gao, T.; Li, C.; et al. Monolayer MoS2 Growth on Au Foils and on-Site Domain Boundary Imaging. Adv. Funct. Mater. 2015, 25, 842−849. (78) Xu, Z. Q.; Zhang, Y. P.; Lin, S. H.; Zheng, C. X.; Zhong, Y. L.; Xia, X.; Li, Z. P.; Sophia, P. J.; Fuhrer, M. S.; Cheng, Y. B.; et al. Synthesis and Transfer of Large-Area Monolayer WS2 Crystals: Moving toward the Recyclable Use of Sapphire Substrates. ACS Nano 2015, 9, 6178−6187. (79) Cheng, J. X.; Jiang, T.; Ji, Q. Q.; Zhang, Y.; Li, Z. M.; Shan, Y. W.; Zhang, Y. F.; Gong, X. G.; Liu, W. T.; Wu, S. W. Kinetic Nature of Grain Boundary Formation in as-Grown MoS2 Monolayers. Adv. Mater. 2015, 27, 4069−4074. (80) Gao, Y.; Hong, Y. L.; Yin, L. C.; Wu, Z.; Yang, Z.; Chen, M. L.; Liu, Z.; Ma, T.; Sun, D. M.; Ni, Z.; et al. Ultrafast Growth of High-Quality Monolayer WSe2 on Au. Adv. Mater. 2017, 29, 1700990. (81) Lan, Y. W.; Torres, C. M.; Tsai, S. H.; Zhu, X. D.; Shi, Y. M.; Li, M. Y.; Li, L. J.; Yeh, W. K.; Wang, K. L. Atomic-Monolayer MoS2 Band-toBand Tunneling Field-Effect Transistor. Small 2016, 12, 5676−5683. (82) Yu, Y.; Huang, S.-Y.; Li, Y.; Steinmann, S. N.; Yang, W.; Cao, L. Layer-Dependent Electrocatalysis of MoS2 for Hydrogen Evolution. Nano Lett. 2014, 14, 553−558. AJ

DOI: 10.1021/acs.chemrev.7b00536 Chem. Rev. XXXX, XXX, XXX−XXX

Chemical Reviews

Review

Deposition Exhibits Weak Antilocalization Effect. Nano Lett. 2016, 16, 4297−4304. (121) Empante, T. A.; Zhou, Y.; Klee, V.; Nguyen, A. E.; Lu, I. H.; Valentin, M. D.; Naghibi Alvillar, S. A.; Preciado, E.; Berges, A. J.; Merida, C. S.; et al. Chemical Vapor Deposition Growth of Few Layer MoTe2 in the 2H, 1T′, and 1T Phases: Tunable Properties of MoTe2 Films. ACS Nano 2017, 11, 900−905. (122) Li, H. L.; Duan, X. D.; Wu, X. P.; Zhuang, X. J.; Zhou, H.; Zhang, Q. L.; Zhu, X. L.; Hu, W.; Ren, P. Y.; Guo, P. F.; et al. Growth of Alloy MoS2xSe2(1‑x) Nanosheets with Fully Tunable Chemical Compositions and Optical Properties. J. Am. Chem. Soc. 2014, 136, 3756−3759. (123) Gong, Y. J.; Liu, Z.; Lupini, A. R.; Shi, G.; Lin, J. H.; Najmaei, S.; Lin, Z.; Elias, A. L.; Berkdemir, A.; You, G.; et al. Band Gap Engineering and Layer-by-Layer Mapping of Selenium-Doped Molybdenum Disulfide. Nano Lett. 2014, 14, 442−449. (124) Umrao, S.; Jeon, J.; Jeon, S. M.; Choi, Y. J.; Lee, S. A Homogeneous Atomic Layer MoS2(1‑x)Se2x Alloy Prepared by LowPressure Chemical Vapor Deposition, and Its Properties. Nanoscale 2017, 9, 594−603. (125) Yu, P.; Lin, J. H.; Sun, L. F.; Le, Q. L.; Yu, X. C.; Gao, G. H.; Hsu, C. H.; Wu, D.; Chang, T. R.; Zeng, Q. S.; et al. Metal-Semiconductor Phase-Transition in WSe2(1‑x)Te2x Monolayer. Adv. Mater. 2017, 29, 1603991. (126) Feng, Q. L.; Mao, N. N.; Wu, J. X.; Xu, H.; Wang, C. M.; Zhang, J.; Xie, L. M. Growth of MoS2(1‑x)Se2x (x = 0.41−1.00) Monolayer Alloys with Controlled Morphology by Physical Vapor Deposition. ACS Nano 2015, 9, 7450−7455. (127) Wang, Z. Q.; Liu, P.; Ito, Y.; Ning, S. C.; Tan, Y. W.; Fujita, T.; Hirata, A.; Chen, M. W. Chemical Vapor Deposition of Monolayer Mo1‑xWxS2 Crystals with Tunable Band Gaps. Sci. Rep. 2016, 6, 21536. (128) Liu, X. K.; Wu, J.; Yu, W. J.; Chen, L.; Huang, Z. H.; Jiang, H.; He, J. Z.; Liu, Q.; Lu, Y. M.; Zhu, D. L.; et al. Monolayer WxMo1‑xS2 Grown by Atmospheric Pressure Chemical Vapor Deposition: Bandgap Engineering and Field Effect Transistors. Adv. Funct. Mater. 2017, 27, 1606469. (129) Zhang, W. T.; Li, X. D.; Jiang, T. T.; Song, J. L. Q.; Lin, Y.; Zhu, L. X.; Xu, X. L. CVD Synthesis of Mo(1‑x)WxS2 and MoS2(1‑x)Se2x Alloy Monolayers Aimed at Tuning the Bandgap of Molybdenum Disulfide. Nanoscale 2015, 7, 13554−13560. (130) Lin, Z.; Thee, M. T.; Elias, A. L.; Feng, S. M.; Zhou, C. J.; Fujisawa, K.; Perea-Lopez, N.; Carozo, V.; Terrones, H.; Terrones, M. Facile Synthesis of MoS2 and MoxW1‑xS2 Triangular Monolayers. APL Mater. 2014, 2, 092514. (131) Ma, Q.; Isarraraz, M.; Wang, C. S.; Preciado, E.; Klee, V.; Bobek, S.; Yamaguchi, K.; Li, E.; Odenthal, P. M.; Nguyen, A.; et al. Postgrowth Tuning of the Bandgap of Single-Layer Molybdenum Disulfide Films by Sulfur/Selenium Exchange. ACS Nano 2014, 8, 4672−4677. (132) Zou, X.; Yakobson, B. I. An Open Canvas-2D Materials with Defects, Disorder, and Functionality. Acc. Chem. Res. 2015, 48, 73−80. (133) Najmaei, S.; Yuan, J. T.; Zhang, J.; Ajayan, P.; Lou, J. Synthesis and Defect Investigation of Two-Dimensional Molybdenum Disulfide Atomic Layers. Acc. Chem. Res. 2015, 48, 31−40. (134) Jin, K.; Xie, L. M.; Tian, Y.; Liu, D. M. Au-Modified Monolayer MoS2 Sensor for DNA Detection. J. Phys. Chem. C 2016, 120, 11204− 11209. (135) Lin, Z.; Carvalho, B. R.; Kahn, E.; Lv, R. T.; Rao, R.; Terrones, H.; Pimenta, M. A.; Terrones, M. Defect Engineering of TwoDimensional Transition Metal Dichalcogenides. 2D Mater. 2016, 3, 022002. (136) Wang, S. S.; Lee, G. D.; Lee, S.; Yoon, E.; Warner, J. H. Detailed Atomic Reconstruction of Extended Line Defects in Monolayer MoS2. ACS Nano 2016, 10, 5419−5430. (137) Tosun, M.; Chan, L.; Amani, M.; Roy, T.; Ahn, G. H.; Taheri, P.; Carraro, C.; Ager, J. W.; Maboudian, R.; Javey, A. Air-Stable N-Doping of WSe2 by Anion Vacancy Formation with Mild Plasma Treatment. ACS Nano 2016, 10, 6853−6860. (138) Komsa, H. P.; Kotakoski, J.; Kurasch, S.; Lehtinen, O.; Kaiser, U.; Krasheninnikov, A. V. Two-Dimensional Transition Metal Dichalcoge-

(102) Zhang, Y.; Ji, Q.; Han, G.-F.; Ju, J.; Shi, J.; Ma, D.; Sun, J.; Zhang, Y.; Li, M.; Lang, X.-Y.; et al. Dendritic, Transferable, Strictly Monolayer MoS2 Flakes Synthesized on SrTiO3 Single Crystals for Efficient Electrocatalytic Applications. ACS Nano 2014, 8, 8617−8624. (103) Ma, T.; Ren, W. C.; Zhang, X. Y.; Liu, Z. B.; Gao, Y.; Yin, L. C.; Ma, X. L.; Ding, F.; Cheng, H. M. Edge-Controlled Growth and Kinetics of Single-Crystal Graphene Domains by Chemical Vapor Deposition. Proc. Natl. Acad. Sci. U. S. A. 2013, 110, 20386−20391. (104) Xu, C.; Wang, L. B.; Liu, Z. B.; Chen, L.; Guo, J. K.; Kang, N.; Ma, X. L.; Cheng, H. M.; Ren, W. C. Large-Area High-Quality 2D Ultrathin Mo2C Superconducting Crystals. Nat. Mater. 2015, 14, 1135− 1142. (105) Artyukhov, V. I.; Hu, Z. L.; Zhang, Z. H.; Yakobson, B. I. Topochemistry of Bowtie- and Star-Shaped Metal Dichalcogenide Nanoisland Formation. Nano Lett. 2016, 16, 3696−3702. (106) Voiry, D.; Mohite, A.; Chhowalla, M. Phase Engineering of Transition Metal Dichalcogenides. Chem. Soc. Rev. 2015, 44, 2702− 2712. (107) Wang, Z. Q.; Ning, S. C.; Fujita, T.; Hirata, A.; Chen, M. W. Unveiling Three-Dimensional Stacking Sequences of 1T Phase MoS2 Monolayers by Electron Diffraction. ACS Nano 2016, 10, 10308− 10316. (108) Acerce, M.; Voiry, D.; Chhowalla, M. Metallic 1t Phase MoS2 Nanosheets as Supercapacitor Electrode Materials. Nat. Nanotechnol. 2015, 10, 313−318. (109) Lin, Y. C.; Dumcenco, D. O.; Huang, Y. S.; Suenaga, K. Atomic Mechanism of the Semiconducting-to-Metallic Phase Transition in Single-Layered MoS2. Nat. Nanotechnol. 2014, 9, 391−396. (110) Huang, Q.; Li, X.; Sun, M.; Zhang, L.; Song, C.; Zhu, L.; Chen, P.; Xu, Z.; Wang, W.; Bai, X. The Mechanistic Insights into the 2H-1T Phase Transition of MoS2 Upon Alkali Metal Intercalation: From the Study of Dynamic Sodiation Processes of MoS2 Nanosheets. Adv. Mater. Interfaces 2017, 4, 1700171. (111) Kretschmer, S.; Komsa, H.-P.; Boggild, P.; Krasheninnikov, A. V. Structural Transformations in Two-Dimensional Transition-Metal Dichalcogenide MoS2 under Electron Beam: Insights from FirstPrinciples Calculations. J. Phys. Chem. Lett. 2017, 8, 3061−3067. (112) Zhu, J.; Wang, Z.; Yu, H.; Li, N.; Zhang, J.; Meng, J.; Liao, M.; Zhao, J.; Lu, X.; Du, L.; et al. Argon Plasma Induced Phase Transition in Monolayer MoS2. J. Am. Chem. Soc. 2017, 139, 10216−10219. (113) Song, S.; Keum, D. H.; Cho, S.; Perello, D.; Kim, Y.; Lee, Y. H. Room Temperature Semiconductor-Metal Transition of MoTe2 Thin Films Engineered by Strain. Nano Lett. 2016, 16, 188−193. (114) Kappera, R.; Voiry, D.; Yalcin, S. E.; Branch, B.; Gupta, G.; Mohite, A. D.; Chhowalla, M. Phase-Engineered Low-Resistance Contacts for Ultrathin MoS2 Transistors. Nat. Mater. 2014, 13, 1128− 1134. (115) Chang, K.; Hai, X.; Pang, H.; Zhang, H. B.; Shi, L.; Liu, G. G.; Liu, H. M.; Zhao, G. X.; Li, M.; Ye, J. H. Targeted Synthesis of 2H-and 1T Phase MoS2 Monolayers for Catalytic Hydrogen Evolution. Adv. Mater. 2016, 28, 10033−10041. (116) Tan, S. J. R.; Abdelwahab, I.; Ding, Z.; Zhao, X.; Yang, T.; Loke, G. Z. J.; Lin, H.; Verzhbitskiy, I.; Poh, S. M.; Xu, H.; et al. Chemical Stabilization of 1T′ Phase Transition Metal Dichalcogenides with Giant Optical Kerr Nonlinearity. J. Am. Chem. Soc. 2017, 139, 2504−2511. (117) Wypych, F.; Schollhorn, R. 1T-MoS2, A New Metallic Modification of Molybdenum Disulfide. J. Chem. Soc., Chem. Commun. 1992, 19, 1386−1388. (118) Mahler, B.; Hoepfner, V.; Liao, K.; Ozin, G. A. Colloidal Synthesis of 1T-WS2 and 2H-WS2 Nanosheets: Applications for Photocatalytic Hydrogen Evolution. J. Am. Chem. Soc. 2014, 136, 14121−14127. (119) Yang, L.; Zhang, W. F.; Li, J.; Cheng, S.; Xie, Z. J.; Chang, H. X. Tellurization Velocity-Dependent Metallic-Semiconducting Metallic Phase Evolution in Chemical Vapor Deposition Growth of Large Area, Few-Layer MoTe2. ACS Nano 2017, 11, 1964−1972. (120) Naylor, C. H.; Parkin, W. M.; Ping, J. L.; Gao, Z. L.; Zhou, Y. R.; Kim, Y.; Streller, F.; Carpick, R. W.; Rappe, A. M.; Drndic, M.; et al. Monolayer Single-Crystal 1T′-MoTe2 Grown by Chemical Vapor AK

DOI: 10.1021/acs.chemrev.7b00536 Chem. Rev. XXXX, XXX, XXX−XXX

Chemical Reviews

Review

nides under Electron Irradiation: Defect Production and Doping. Phys. Rev. Lett. 2012, 109, 035503. (139) Kim, I. S.; Sangwan, V. K.; Jariwala, D.; Wood, J. D.; Park, S.; Chen, K.-S.; Shi, F.; Ruiz-Zepeda, F.; Ponce, A.; Jose-Yacaman, M.; et al. Influence of Stoichiometry on the Optical and Electrical Properties of Chemical Vapor Deposition Derived MoS2. ACS Nano 2014, 8, 10551− 10558. (140) Amani, M.; Lien, D.-H.; Kiriya, D.; Xiao, J.; Azcatl, A.; Noh, J.; Madhvapathy, S. R.; Addou, R.; KC, S.; Dubey, M.; et al. Near-Unity Photoluminescence Quantum Yield in MoS2. Science 2015, 350, 1065− 1068. (141) Qiu, H.; Xu, T.; Wang, Z. L.; Ren, W.; Nan, H. Y.; Ni, Z. H.; Chen, Q.; Yuan, S. J.; Miao, F.; Song, F. Q.; et al. Hopping Transport through Defect-Induced Localized States in Molybdenum Disulphide. Nat. Commun. 2013, 4, 2642. (142) Yu, Z.; Pan, Y.; Shen, Y.; Wang, Z.; Ong, Z.-Y.; Xu, T.; Xin, R.; Pan, L.; Wang, B.; Sun, L.; et al. Towards Intrinsic Charge Transport in Monolayer Molybdenum Disulfide by Defect and Interface Engineering. Nat. Commun. 2014, 5, 5290. (143) Han, H. V.; Lu, A. Y.; Lu, L. S.; Huang, J. K.; Li, H.; Hsu, C. L.; Lin, Y. C.; Chiu, M. H.; Suenaga, K.; Chu, C. W.; et al. Photoluminescence Enhancement and Structure Repairing of Monolayer MoSe2 by Hydrohalic Acid Treatment. ACS Nano 2016, 10, 1454− 1461. (144) Amani, M.; Taheri, P.; Addou, R.; Ahn, G. H.; Kiriya, D.; Lien, D.-H.; Ager, J. W., III; Wallace, R. M.; Javey, A. Recombination Kinetics and Effects of Superacid Treatment in Sulfur- and Selenium-Based Transition Metal Dichalcogenides. Nano Lett. 2016, 16, 2786−2791. (145) Lu, J. P.; Carvalho, A.; Chan, X. K.; Liu, H. W.; Liu, B.; Tok, E. S.; Loh, K. P.; Castro Neto, A. H.; Sow, C. H. Atomic Healing of Defects in Transition Metal Dichalcogenides. Nano Lett. 2015, 15, 3524−3532. (146) Zhou, L.; Xu, K.; Zubair, A.; Zhang, X.; Ouyang, F. P.; Palacios, T.; Dresselhaus, M. S.; Li, Y. F.; Kong, J. Role of Molecular Sieves in the CVD Synthesis of Large-Area 2D MoTe2. Adv. Funct. Mater. 2017, 27, 1603491. (147) Kelly, A. G.; Hallam, T.; Backes, C.; Harvey, A.; Esmaeily, A. S.; Godwin, I.; Coelho, J.; Nicolosi, V.; Lauth, J.; Kulkarni, A.; et al. AllPrinted Thin-Film Transistors from Networks of Liquid-Exfoliated Nanosheets. Science 2017, 356, 69−73. (148) Zhan, Y.; Liu, Z.; Najmaei, S.; Ajayan, P. M.; Lou, J. Large-Area Vapor-Phase Growth and Characterization of MoS2 Atomic Layers on a SiO2 Substrate. Small 2012, 8, 966−971. (149) Eichfeld, S. M.; Hossain, L.; Lin, Y.-C.; Piasecki, A. F.; Kupp, B.; Birdwell, A. G.; Burke, R. A.; Lu, N.; Peng, X.; Li, J.; et al. Highly Scalable, Atomically Thin WSe2 Grown Via Metal-Organic Chemical Vapor Deposition. ACS Nano 2015, 9, 2080−2087. (150) Senthilkumar, V.; Tam, L. C.; Kim, Y. S.; Sim, Y.; Seong, M.-J.; Jang, J. I. Direct Vapor Phase Growth Process and Robust Photoluminescence Properties of Large Area MoS2 Layers. Nano Res. 2014, 7, 1759−1768. (151) Mohapatra, P. K.; Deb, S.; Singh, B. P.; Vasa, P.; Dhar, S. Strictly Monolayer Large Continuous MoS2 Films on Diverse Substrates and Their Luminescence Properties. Appl. Phys. Lett. 2016, 108, 042101. (152) Gong, Y. J.; Ye, G. L.; Lei, S. D.; Shi, G.; He, Y. M.; Lin, J. H.; Zhang, X.; Vajtai, R.; Pantelides, S. T.; Zhou, W.; et al. Synthesis of Millimeter-Scale Transition Metal Dichalcogenides Single Crystals. Adv. Funct. Mater. 2016, 26, 2009−2015. (153) Zhou, H. L.; Wang, C.; Shaw, J. C.; Cheng, R.; Chen, Y.; Huang, X. Q.; Liu, Y.; Weiss, N. O.; Lin, Z. Y.; Huang, Y.; et al. Large Area Growth and Electrical Properties of P-Type WSe2 Atomic Layers. Nano Lett. 2015, 15, 709−713. (154) Lin, Y. C.; Zhang, W. J.; Huang, J. K.; Liu, K. K.; Lee, Y. H.; Liang, C. T.; Chu, C. W.; Li, L. J. Wafer-Scale MoS2 Thin Layers Prepared by MoO3 Sulfurization. Nanoscale 2012, 4, 6637−6641. (155) Kong, D. S.; Wang, H. T.; Cha, J. J.; Pasta, M.; Koski, K. J.; Yao, J.; Cui, Y. Synthesis of MoS2 and MoSe2 Films with Vertically Aligned Layers. Nano Lett. 2013, 13, 1341−1347.

(156) Yu, F. F.; Liu, Q. W.; Gan, X.; Hu, M. X.; Zhang, T. Y.; Li, C.; Kang, F. Y.; Terrones, M.; Lv, R. T. Ultrasensitive Pressure Detection of Few-Layer MoS2. Adv. Mater. 2017, 29, 1603266. (157) Zhou, L.; Zubair, A.; Wang, Z.; Zhang, X.; Ouyang, F.; Xu, K.; Fang, W.; Ueno, K.; Li, J.; Palacios, T. S. Synthesis of High-Quality Large-Area Homogenous 1T′ MoTe2 from Chemical Vapor Deposition. Adv. Mater. 2016, 28, 9526−9531. (158) Hussain, S.; Shehzad, M. A.; Vikraman, D.; Khan, M. F.; Singh, J.; Choi, D. C.; Seo, Y.; Eom, J.; Lee, W. G.; Jung, J. Synthesis and Characterization of Large-Area and Continuous MoS2 Atomic Layers by Rf Magnetron Sputtering. Nanoscale 2016, 8, 4340−4347. (159) Serna, M. I.; Yoo, S. H.; Moreno, S.; Xi, Y.; Oviedo, J. P.; Choi, H.; Alshareef, H. N.; Kim, M. J.; Minary-Jolandan, M.; Quevedo-Lopez, M. A. Large-Area Deposition of MoS2 by Pulsed Laser Deposition with in Situ Thickness Control. ACS Nano 2016, 10, 6054−6061. (160) Jurca, T.; Moody, M. J.; Henning, A.; Emery, J. D.; Wang, B. H.; Tan, J. M.; Lohr, T. L.; Lauhon, L. J.; Marks, T. J. Low-Temperature Atomic Layer Deposition of MoS2 Films. Angew. Chem., Int. Ed. 2017, 56, 4991−4995. (161) Song, J. G.; Park, J.; Lee, W.; Choi, T.; Jung, H.; Lee, C. W.; Hwang, S. H.; Myoung, J. M.; Jung, J. H.; Kim, S. H.; et al. LayerControlled, Wafer-Scale, and Conformal Synthesis of Tungsten Disulfide Nanosheets Using Atomic Layer Deposition. ACS Nano 2013, 7, 11333−11340. (162) Liu, K. K.; Zhang, W. J.; Lee, Y. H.; Lin, Y. C.; Chang, M. T.; Su, C.; Chang, C. S.; Li, H.; Shi, Y. M.; Zhang, H.; et al. Growth of LargeArea and Highly Crystalline MoS2 Thin Layers on Insulating Substrates. Nano Lett. 2012, 12, 1538−1544. (163) George, A. S.; Mutlu, Z.; Ionescu, R.; Wu, R. J.; Jeong, J. S.; Bay, H. H.; Chai, Y.; Mkhoyan, K. A.; Ozkan, M.; Ozkan, C. S. Wafer Scale Synthesis and High Resolution Structural Characterization of Atomically Thin MoS2 Layers. Adv. Funct. Mater. 2014, 24, 7461−7466. (164) Lim, Y. R.; Song, W.; Han, J. K.; Lee, Y. B.; Kim, S. J.; Myung, S.; Lee, S. S.; An, K.-S.; Choi, C.-J.; Lim, J. Wafer-Scale, Homogeneous MoS2 Layers on Plastic Substrates for Flexible Visible-Light Photodetectors. Adv. Mater. 2016, 28, 5025−5030. (165) Zhou, Y. B.; Deng, B.; Zhou, Y.; Ren, X. B.; Yin, J. B.; Jin, C. H.; Liu, Z. F.; Peng, H. L. Low-Temperature Growth of Two-Dimensional Layered Chalcogenide Crystals on Liquid. Nano Lett. 2016, 16, 2103− 2107. (166) Ling, X.; Lee, Y.-H.; Lin, Y.; Fang, W.; Yu, L.; Dresselhaus, M. S.; Kong, J. Role of the Seeding Promoter in MoS2 Growth by Chemical Vapor Deposition. Nano Lett. 2014, 14, 464−472. (167) Gao, L.; Ni, G.-X.; Liu, Y.; Liu, B.; Castro Neto, A. H.; Loh, K. P. Face-to-Face Transfer of Wafer-Scale Graphene Films. Nature 2014, 505, 190−194. (168) Phan, H. D.; Kim, Y.; Lee, J.; Liu, R.; Choi, Y.; Cho, J. H.; Lee, C. Ultraclean and Direct Transfer of a Wafer-Scale MoS2 Thin Film onto a Plastic Substrate. Adv. Mater. 2017, 29, 1603928. (169) Novoselov, K. S.; Mishchenko, A.; Carvalho, A.; Castro Neto, A. H. 2D Materials and Van Der Waals Heterostructures. Science 2016, 353, aac9439. (170) Jin, C. H.; Kim, J.; Suh, J.; Shi, Z. W.; Chen, B.; Fan, X.; Kam, M.; Watanabe, K.; Taniguchi, T.; Tongay, S.; et al. Interlayer ElectronPhonon Coupling in WSe2/h-BN Heterostructures. Nat. Phys. 2017, 13, 127−131. (171) Withers, F.; Del Pozo-Zamudio, O.; Mishchenko, A.; Rooney, A. P.; Gholinia, A.; Watanabe, K.; Taniguchi, T.; Haigh, S. J.; Geim, A. K.; Tartakovskii, A. I.; et al. Light-Emitting Diodes by Band-Structure Engineering in Van Der Waals Heterostructures. Nat. Mater. 2015, 14, 301−306. (172) Liu, Y.; Weiss, N. O.; Duan, X. D.; Cheng, H. C.; Huang, Y.; Duan, X. F. Van Der Waals Heterostructures and Devices. Nat. Rev. Mater. 2016, 1, 16042. (173) Lee, C. H.; Lee, G. H.; van der Zande, A. M.; Chen, W. C.; Li, Y. L.; Han, M. Y.; Cui, X.; Arefe, G.; Nuckolls, C.; Heinz, T. F.; et al. Atomically Thin P-N Junctions with Van Der Waals Heterointerfaces. Nat. Nanotechnol. 2014, 9, 676−681. AL

DOI: 10.1021/acs.chemrev.7b00536 Chem. Rev. XXXX, XXX, XXX−XXX

Chemical Reviews

Review

(174) Gong, Y.; Lin, J.; Wang, X.; Shi, G.; Lei, S.; Lin, Z.; Zou, X.; Ye, G.; Vajtai, R.; Yakobson, B. I. Vertical and in-Plane Heterostructures from WS2/MoS2 Monolayers. Nat. Mater. 2014, 13, 1135−1142. (175) Shi, J. P.; Liu, M. X.; Wen, J. X.; Ren, X. B.; Zhou, X. B.; Ji, Q. Q.; Ma, D. L.; Zhang, Y.; Jin, C. H.; Chen, H. J.; et al. All Chemical Vapor Deposition Synthesis and Intrinsic Bandgap Observation of MoS2/ Graphene Heterostructures. Adv. Mater. 2015, 27, 7086−7092. (176) Fu, L.; Sun, Y. Y.; Wu, N.; Mendes, R. G.; Chen, L. F.; Xu, Z.; Zhang, T.; Rummeli, M. H.; Rellinghaus, B.; Pohl, D.; et al. Direct Growth of MoS2/h-BN Heterostructures Via a Sulfide-Resistant Alloy. ACS Nano 2016, 10, 2063−2070. (177) Li, M. Y.; Shi, Y. M.; Cheng, C. C.; Lu, L. S.; Lin, Y. C.; Tang, H. L.; Tsai, M. L.; Chu, C. W.; Wei, K. H.; He, J. H.; et al. Epitaxial Growth of a Monolayer WSe2-MoS2 Lateral P-N Junction with an Atomically Sharp Interface. Science 2015, 349, 524−528. (178) Duan, X. D.; Wang, C.; Shaw, J. C.; Cheng, R.; Chen, Y.; Li, H. L.; Wu, X. P.; Tang, Y.; Zhang, Q. L.; Pan, A. L.; et al. Lateral Epitaxial Growth of Two-Dimensional Layered Semiconductor Heterojunctions. Nat. Nanotechnol. 2014, 9, 1024−1030. (179) Liu, L.; Park, J.; Siegel, D. A.; McCarty, K. F.; Clark, K. W.; Deng, W.; Basile, L.; Idrobo, J. C.; Li, A. P.; Gu, G. Heteroepitaxial Growth of Two-Dimensional Hexagonal Boron Nitride Templated by Graphene Edges. Science 2014, 343, 163−167. (180) Li, Y. T.; Huang, L.; Li, B.; Wang, X. T.; Zhou, Z. Q.; Li, J. B.; Wei, Z. M. Co-Nucleus 1D/2D Heterostructures with Bi2S3 Nanowire and MoS2 Monolayer: One-Step Growth and Defect-Induced Formation Mechanism. ACS Nano 2016, 10, 8938−8946. (181) Li, Y.; Qin, J. K.; Xu, C. Y.; Cao, J.; Sun, Z. Y.; Ma, L. P.; Hu, P. A.; Ren, W. C.; Zhen, L. Electric Field Tunable Interlayer Relaxation Process and Interlayer Coupling in WSe2/Graphene Heterostructures. Adv. Funct. Mater. 2016, 26, 4319−4328. (182) Yu, W. J.; Liu, Y.; Zhou, H.; Yin, A.; Li, Z.; Huang, Y.; Duan, X. Highly Efficient Gate-Tunable Photocurrent Generation in Vertical Heterostructures of Layered Materials. Nat. Nanotechnol. 2013, 8, 952− 958. (183) Georgiou, T.; Jalil, R.; Belle, B. D.; Britnell, L.; Gorbachev, R. V.; Morozov, S. V.; Kim, Y.-J.; Gholinia, A.; Haigh, S. J.; Makarovsky, O.; et al. Vertical Field-Effect Transistor Based on Graphene-WS 2 Heterostructures for Flexible and Transparent Electronics. Nat. Nanotechnol. 2013, 8, 100−103. (184) Yu, L. L.; Lee, Y. H.; Ling, X.; Santos, E. J. G.; Shin, Y. C.; Lin, Y. X.; Dubey, M.; Kaxiras, E.; Kong, J.; Wang, H.; et al. Graphene/MoS2 Hybrid Technology for Large-Scale Two-Dimensional Electronics. Nano Lett. 2014, 14, 3055−3063. (185) Roy, K.; Padmanabhan, M.; Goswami, S.; Sai, T. P.; Ramalingam, G.; Raghavan, S.; Ghosh, A. Graphene-MoS2 Hybrid Structures for Multifunctional Photoresponsive Memory Devices. Nat. Nanotechnol. 2013, 8, 826−830. (186) Loan, P. T. K.; Zhang, W.; Lin, C.-T.; Wei, K.-H.; Li, L.-J.; Chen, C.-H. Graphene/MoS2 Heterostructures for Ultrasensitive Detection of DNA Hybridisation. Adv. Mater. 2014, 26, 4838−4844. (187) Shi, Y. M.; Zhou, W.; Lu, A. Y.; Fang, W. J.; Lee, Y. H.; Hsu, A. L.; Kim, S. M.; Kim, K. K.; Yang, H. Y.; Li, L. J.; et al. Van Der Waals Epitaxy of MoS2 Layers Using Graphene as Growth Templates. Nano Lett. 2012, 12, 2784−2791. (188) Yunus, R. M.; Endo, H.; Tsuji, M.; Ago, H. Vertical Heterostructures of MoS2 and Graphene Nanoribbons Grown by Two-Step Chemical Vapor Deposition for High-Gain Photodetectors. Phys. Chem. Chem. Phys. 2015, 17, 25210−25215. (189) Hong, X. P.; Kim, J.; Shi, S. F.; Zhang, Y.; Jin, C. H.; Sun, Y. H.; Tongay, S.; Wu, J. Q.; Zhang, Y. F.; Wang, F. Ultrafast Charge Transfer in Atomically Thin MoS2/WS2 Heterostructures. Nat. Nanotechnol. 2014, 9, 682−686. (190) Wang, F.; Yin, L.; Wang, Z. X.; Xu, K.; Wang, F. M.; Shifa, T. A.; Huang, Y.; Jiang, C.; He, J. Configuration-Dependent Electrically Tunable Van Der Waals Heterostructures Based on MoTe2/MoS2. Adv. Funct. Mater. 2016, 26, 5499−5506. (191) Chiu, M. H.; Zhang, C. D.; Shiu, H. W.; Chuu, C. P.; Chen, C. H.; Chang, C. Y. S.; Chen, C. H.; Chou, M. Y.; Shih, C. K.; Li, L. J.

Determination of Band Alignment in the Single-Layer MoS2/WSe2 Heterojunction. Nat. Commun. 2015, 6, 7666. (192) Furchi, M. M.; Pospischil, A.; Libisch, F.; Burgdorfer, J.; Mueller, T. Photovoltaic Effect in an Electrically Tunable Van Der Waals Heterojunction. Nano Lett. 2014, 14, 4785−4791. (193) Huo, N. J.; Kang, J.; Wei, Z. M.; Li, S. S.; Li, J. B.; Wei, S. H. Novel and Enhanced Optoelectronic Performances of Multilayer MoS2WS2 Heterostructure Transistors. Adv. Funct. Mater. 2014, 24, 7025− 7031. (194) Woods, J. M.; Jung, Y.; Xie, Y. J.; Liu, W.; Liu, Y. H.; Wang, H. H.; Cha, J. J. One-Step Synthesis of MoS2/WS2 Layered Heterostructures and Catalytic Activity of Defective Transition Metal Dichalcogenide Films. ACS Nano 2016, 10, 2004−2009. (195) Gong, Y. J.; Lei, S. D.; Ye, G. L.; Li, B.; He, Y. M.; Keyshar, K.; Zhang, X.; Wang, Q. Z.; Lou, J.; Liu, Z.; et al. Two-Step Growth of TwoDimensional WSe2/MoSe2 Heterostructures. Nano Lett. 2015, 15, 6135−6141. (196) Xue, Y.; Zhang, Y.; Liu, Y.; Liu, H.; Song, J.; Sophia, J.; Liu, J.; Xu, Z.; Xu, Q.; Wang, Z.; et al. Scalable Production of a Few-Layer MoS2/ WS2 Vertical Heterojunction Array and Its Application for Photodetectors. ACS Nano 2016, 10, 573−580. (197) Shi, J.; Tong, R.; Zhou, X.; Gong, Y.; Zhang, Z.; Ji, Q.; Zhang, Y.; Fang, Q.; Gu, L.; Wang, X.; et al. Temperature-Mediated Selective Growth of MoS2/WS2 and WS2/MoS2 Vertical Stacks on Au Foils for Direct Photocatalytic Applications. Adv. Mater. 2016, 28, 10664−10672. (198) Ai, R.; Guan, X.; Li, J.; Yao, K.; Chen, P.; Zhang, Z.; Duan, X.; Duan, X. Growth of Single-Crystalline Cadmium Iodide Nanoplates, CdI2/MoS2 (WS2, WSe2) Van Der Waals Heterostructures, and Patterned Arrays. ACS Nano 2017, 11, 3413−3419. (199) Ye, L.; Li, H.; Chen, Z. F.; Xu, J. B. Near-Infrared Photodetector Based on MoS2/Black Phosphorus Heterojunction. ACS Photonics 2016, 3, 692−699. (200) Chen, P.; Zhang, T. T.; Zhang, J.; Xiang, J.; Yu, H.; Wu, S.; Lu, X.; Wang, G.; Wen, F.; Liu, Z.; et al. Gate Tunable WSe2-BP Van Der Waals Heterojunction Devices. Nanoscale 2016, 8, 3254−3258. (201) Li, D.; Wang, B.; Chen, M.; Zhou, J.; Zhang, Z. Gate-Controlled BP-WSe2 Heterojunction Diode for Logic Rectifiers and Logic Optoelectronics. Small 2017, 13, 1603726. (202) Ye, L.; Wang, P.; Luo, W.; Gong, F.; Liao, L.; Liu, T.; Tong, L.; Zang, J.; Xu, J.; Hu, W. Highly Polarization Sensitive Infrared Photodetector Based on Black Phosphorus-on-WSe2 Photogate Vertical Heterostructure. Nano Energy 2017, 37, 53−60. (203) Deng, Y. X.; Luo, Z.; Conrad, N. J.; Liu, H.; Gong, Y. J.; Najmaei, S.; Ajayan, P. M.; Lou, J.; Xu, X. F.; Ye, P. D. Black PhosphorusMonolayer MoS2 Van Der Waals Heterojunction P-N Diode. ACS Nano 2014, 8, 8292−8299. (204) Feng, Z. H.; Chen, B. Y.; Qian, S. B.; Xu, L. Y.; Feng, L. F.; Yu, Y. Y.; Zhang, R.; Chen, J. C.; Li, Q. Q.; Li, Q. N.; et al. Chemical Sensing by Band Modulation of a Black Phosphorus/Molybdenum Diselenide Van Der Waals Hetero-Structure. 2D Mater. 2016, 3, 035021. (205) Liu, B.; Koepf, M.; Abbas, A. N.; Wang, X.; Guo, Q.; Jia, Y.; Xia, F.; Weihrich, R.; Bachhuber, F.; Pielnhofer, F.; et al. Black ArsenicPhosphorus: Layered Anisotropic Infrared Semiconductors with Highly Tunable Compositions and Properties. Adv. Mater. 2015, 27, 4423− 4429. (206) Cui, X.; Lee, G.-H.; Kim, Y. D.; Arefe, G.; Huang, P. Y.; Lee, C.H.; Chenet, D. A.; Zhang, X.; Wang, L.; Ye, F.; et al. Multi-Terminal Transport Measurements of MoS 2 Using a Van Der Waals Heterostructure Device Platform. Nat. Nanotechnol. 2015, 10, 534−540. (207) Yan, A. M.; Velasco, J.; Kahn, S.; Watanabe, K.; Taniguchi, T.; Wang, F.; Crommie, M. F.; Zettl, A. Direct Growth of Single- and FewLayer MoS2 on h-BN with Preferred Relative Rotation Angles. Nano Lett. 2015, 15, 6324−6331. (208) Wang, S.; Wang, X.; Warner, J. H. All Chemical Vapor Deposition Growth of MoS2 : h-BN Vertical Van Der Waals Heterostructures. ACS Nano 2015, 9, 5246−5254. (209) Cui, F.; Li, X.; Feng, Q.; Yin, J.; Zhou, L.; Liu, D.; Liu, K.; He, X.; Liang, X.; Liu, S.; et al. Epitaxial Growth of Large-Area and Highly AM

DOI: 10.1021/acs.chemrev.7b00536 Chem. Rev. XXXX, XXX, XXX−XXX

Chemical Reviews

Review

Crystalline Anisotropic ReSe2 Atomic Layer. Nano Res. 2017, 10, 2732− 2742. (210) Zhou, Y.; Nie, Y.; Liu, Y.; Yan, K.; Hong, J.; Jin, C.; Zhou, Y.; Yin, J.; Liu, Z.; Peng, H. Epitaxy and Photoresponse of Two-Dimensional Gase Crystals on Flexible Transparent Mica Sheets. ACS Nano 2014, 8, 1485−1490. (211) Liu, Y.; Tang, M.; Meng, M.; Wang, M.; Wu, J.; Yin, J.; Zhou, Y.; Guo, Y.; Tan, C.; Dang, W.; et al. Epitaxial Growth of Ternary Topological Insulator Bi2Te2Se 2D Crystals on Mica. Small 2017, 13, 1603572. (212) Dean, C. R.; Wang, L.; Maher, P.; Forsythe, C.; Ghahari, F.; Gao, Y.; Katoch, J.; Ishigami, M.; Moon, P.; Koshino, M.; et al. Hofstadter’s Butterfly and the Fractal Quantum Hall Effect in Moire Superlattices. Nature 2013, 497, 598−602. (213) Dean, C. R.; Young, A. F.; Meric, I.; Lee, C.; Wang, L.; Sorgenfrei, S.; Watanabe, K.; Taniguchi, T.; Kim, P.; Shepard, K. L.; et al. Boron Nitride Substrates for High-Quality Graphene Electronics. Nat. Nanotechnol. 2010, 5, 722−726. (214) Zhang, C.; Zhao, S.; Jin, C.; Koh, A. L.; Zhou, Y.; Xu, W.; Li, Q.; Xiong, Q.; Peng, H.; Liu, Z. Direct Growth of Large-Area Graphene and Boron Nitride Heterostructures by a Co-Segregation Method. Nat. Commun. 2015, 6, 6519. (215) Song, X.; Sun, J.; Qi, Y.; Gao, T.; Zhang, Y.; Liu, Z. Graphene/hBN Heterostructures: Recent Advances in Controllable Preparation and Functional Applications. Adv. Energy Mater. 2016, 6, 1600541. (216) Gao, T.; Song, X.; Du, H.; Nie, Y.; Chen, Y.; Ji, Q.; Sun, J.; Yang, Y.; Zhang, Y.; Liu, Z. Temperature-Triggered Chemical Switching Growth of in-Plane and Vertically Stacked Graphene-Boron Nitride Heterostructures. Nat. Commun. 2015, 6, 6835. (217) Li, Q.; Zhao, Z.; Yan, B.; Song, X.; Zhang, Z.; Li, J.; Wu, X.; Bian, Z.; Zou, X.; Zhang, Y.; et al. Nickelocene-Precursor-Facilitated Fast Growth of Graphene/h-BN Vertical Heterostructures and Its Applications in Oleds. Adv. Mater. 2017, 29, 1701325. (218) Zhang, Z.; Chen, P.; Duan, X.; Zang, K.; Luo, J.; Duan, X. Robust Epitaxial Growth of Two-Dimensional Heterostructures, Multiheterostructures, and Superlattices. Science 2017, 357, 788−792. (219) Liu, Z.; Ma, L. L.; Shi, G.; Zhou, W.; Gong, Y. J.; Lei, S. D.; Yang, X. B.; Zhang, J. N.; Yu, J. J.; Hackenberg, K. P.; et al. In-Plane Heterostructures of Graphene and Hexagonal Boron Nitride with Controlled Domain Sizes. Nat. Nanotechnol. 2013, 8, 119−124. (220) Liu, B.; Ma, Y.; Zhang, A.; Chen, L.; Abbas, A. N.; Liu, Y.; Shen, C.; Wan, H.; Zhou, C. High-Performance WSe2 Field-Effect Transistors Via Controlled Formation of in-Plane Heterojunctions. ACS Nano 2016, 10, 5153−5160. (221) Ci, L.; Song, L.; Jin, C. H.; Jariwala, D.; Wu, D. X.; Li, Y. J.; Srivastava, A.; Wang, Z. F.; Storr, K.; Balicas, L.; et al. Atomic Layers of Hybridized Boron Nitride and Graphene Domains. Nat. Mater. 2010, 9, 430−435. (222) Huang, C. M.; Wu, S. F.; Sanchez, A. M.; Peters, J. J. P.; Beanland, R.; Ross, J. S.; Rivera, P.; Yao, W.; Cobden, D. H.; Xu, X. D. Lateral Heterojunctions within Monolayer MoSe2-WSe2 Semiconductors. Nat. Mater. 2014, 13, 1096−1101. (223) Naylor, C. H.; Parkin, W. M.; Gao, Z.; Berry, J.; Zhou, S.; Zhang, Q.; McClimon, J. B.; Tan, L. Z.; Kehayias, C. E.; Zhao, M. Q.; et al. Synthesis and Physical Properties of Phase-Engineered Transition Metal Dichalcogenide Monolayer Heterostructures. ACS Nano 2017, 11, 8619−8627. (224) Yoo, Y. D.; Degregorio, Z. P.; Johns, J. E. Seed Crystal Homogeneity Controls Lateral and Vertical Heteroepitaxy of Monolayer MoS2 and WS2. J. Am. Chem. Soc. 2015, 137, 14281−14287. (225) Li, H.; Wu, X.; Liu, H.; Zheng, B.; Zhang, Q.; Zhu, X.; Wei, Z.; Zhuang, X.; Zhou, H.; Tang, W.; et al. Composition-Modulated TwoDimensional Semiconductor Lateral Heterostructures Via LayerSelected Atomic Substitution. ACS Nano 2017, 11, 961−967. (226) Kappera, R.; Voiry, D.; Yalcin, S. E.; Jen, W.; Acerce, M.; Torrel, S.; Branch, B.; Lei, S.; Chen, W.; Najmaei, S.; et al. Metallic 1T Phase Source/Drain Electrodes for Field Effect Transistors from Chemical Vapor Deposited MoS2. APL Mater. 2014, 2, 092516.

(227) Ma, Y.; Liu, B.; Zhang, A.; Chen, L.; Fathi, M.; Shen, C.; Abbas, A. N.; Ge, M.; Mecklenburg, M.; Zhou, C. Reversible Semiconductingto-Metallic Phase Transition in Chemical Vapor Deposition Grown Monolayer WSe2 and Applications for Devices. ACS Nano 2015, 9, 7383−7391. (228) Kim, A. R.; Kim, Y.; Nam, J.; Chung, H. S.; Kim, D. J.; Kwon, J. D.; Park, S. W.; Park, J.; Choi, S. Y.; Lee, B. H.; et al. Alloyed 2D MetalSemiconductor Atomic Layer Junctions. Nano Lett. 2016, 16, 1890− 1895. (229) Yoo, Y.; DeGregorio, Z. P.; Su, Y.; Koester, S. J.; Johns, J. E. InPlane 2H-1T′ MoTe2 Homojunctions Synthesized by Flux-Controlled Phase Engineering. Adv. Mater. 2017, 29, 1605461. (230) Zhao, M.; Ye, Y.; Han, Y.; Xia, Y.; Zhu, H.; Wang, S.; Wang, Y.; Muller, D. A.; Zhang, X. Large-Scale Chemical Assembly of Atomically Thin Transistors and Circuits. Nat. Nanotechnol. 2016, 11, 954−959. (231) Zheng, C.; Zhang, Q.; Weber, B.; Ilatikhameneh, H.; Chen, F.; Sahasrabudhe, H.; Rahman, R.; Li, S.; Chen, Z.; Hellerstedt, J.; et al. Direct Observation of 2D Electrostatics and Ohmic Contacts in Template-Grown Graphene/WS2 Heterostructures. ACS Nano 2017, 11, 2785−2793. (232) Chen, W.; Yang, Y.; Zhang, Z. Y.; Kaxiras, E. Properties of InPlane Graphene/MoS2 Heterojunctions. 2D Mater. 2017, 4, 045001. (233) Ling, X.; Lin, Y.; Ma, Q.; Wang, Z.; Song, Y.; Yu, L.; Huang, S.; Fang, W.; Zhang, X.; Hsu, A. L.; et al. Parallel Stitching of 2D Materials. Adv. Mater. 2016, 28, 2322−2329. (234) Hong, H. K.; Jo, J.; Hwang, D.; Lee, J.; Kim, N. Y.; Son, S.; Kim, J. H.; Jin, M. J.; Jun, Y. C.; Erni, R.; et al. Atomic Scale Study on Growth and Heteroepitaxy of ZnO Monolayer on Graphene. Nano Lett. 2017, 17, 120−127. (235) Susi, T.; Skakalova, V.; Mittelberger, A.; Kotrusz, P.; Hulman, M.; Pennycook, T. J.; Mangler, C.; Kotakoski, J.; Meyer, J. C. Computational Insights and the Observation of SiC Nanograin Assembly: Towards 2D Silicon Carbide. Sci. Rep. 2017, 7, 4399. (236) Boulesbaa, A.; Wang, K.; Mahjouri-Samani, M.; Tian, M.; Puretzky, A. A.; Ivanov, I.; Rouleau, C. M.; Xiao, K.; Sumpter, B. G.; Geohegan, D. B. Ultrafast Charge Transfer and Hybrid Exciton Formation in 2D/0D Heterostructures. J. Am. Chem. Soc. 2016, 138, 14713−14719. (237) Zhang, J.; Wei, Y.; Yao, F. R.; Li, D. Q.; Ma, H.; Lei, P.; Fang, H. H.; Xiao, X. Y.; Lu, Z. X.; Yang, J. H.; et al. SWCNT-MoS2-SWCNT Vertical Point Heterostructures. Adv. Mater. 2017, 29, 1604469. (238) Desai, S. B.; Madhvapathy, S. R.; Sachid, A. B.; Llinas, J. P.; Wang, Q.; Ahn, G. H.; Pitner, G.; Kim, M. J.; Bokor, J.; Hu, C.; et al. MoS2 Transistors with 1-Nanometer Gate Lengths. Science 2016, 354, 99−102. (239) Mai, L.; Tian, X.; Xu, X.; Chang, L.; Xu, L. Nanowire Electrodes for Electrochemical Energy Storage Devices. Chem. Rev. 2014, 114, 11828−11862. (240) Xu, B.; He, P. L.; Liu, H. L.; Wang, P. P.; Zhou, G.; Wang, X. A 1D/2D Helical CdS/ZnIn2S4 Nano-Heterostructure. Angew. Chem., Int. Ed. 2014, 53, 2339−2343. (241) Lou, Z.; Xue, C. In Situ Growth of WO3−x Nanowires on g-C3N4 Nanosheets: 1D/2D Heterostructures with Enhanced Photocatalytic Activity. CrystEngComm 2016, 18, 8406−8410. (242) Kiriya, D.; Zhou, Y.; Nelson, C.; Hettick, M.; Madhvapathy, S. R.; Chen, K.; Zhao, P.; Tosun, M.; Minor, A. M.; Chrzan, D. C.; et al. Oriented Growth of Gold Nanowires on MoS2. Adv. Funct. Mater. 2015, 25, 6257−6264. (243) Tchoe, Y.; Jo, J.; Kim, M.; Yi, G.-C. Catalyst-Free Growth of InAs/InxGa1−xAs Coaxial Nanorod Heterostructures on Graphene Layers Using Molecular Beam Epitaxy. NPG Asia Mater. 2015, 7, e206. (244) Fan, W.; Li, M.; Bai, H.; Xu, D.; Chen, C.; Li, C.; Ge, Y.; Shi, W. Fabrication of MgFe2O4/MoS2 Heterostructure Nanowires for Photoelectrochemical Catalysis. Langmuir 2016, 32, 1629−1636. (245) Lin, J.; Cretu, O.; Zhou, W.; Suenaga, K.; Prasai, D.; Bolotin, K. I.; Cuong, N. T.; Otani, M.; Okada, S.; Lupini, A. R.; et al. Flexible Metallic Nanowires with Self-Adaptive Contacts to Semiconducting Transition-Metal Dichalcogenide Monolayers. Nat. Nanotechnol. 2014, 9, 436−442. AN

DOI: 10.1021/acs.chemrev.7b00536 Chem. Rev. XXXX, XXX, XXX−XXX

Chemical Reviews

Review

(246) Liu, B.; Wu, F.; Gui, H.; Zheng, M.; Zhou, C. ChiralityControlled Synthesis and Applications of Single-Wall Carbon Nanotubes. ACS Nano 2017, 11, 31−53. (247) Liu, B.; Liu, J.; Tu, X.; Zhang, J.; Zheng, M.; Zhou, C. ChiralityDependent Vapor-Phase Epitaxial Growth and Termination of SingleWall Carbon Nanotubes. Nano Lett. 2013, 13, 4416−4421. (248) Jariwala, D.; Sangwan, V. K.; Wu, C. C.; Prabhumirashi, P. L.; Geier, M. L.; Marks, T. J.; Lauhon, L. J.; Hersam, M. C. Gate-Tunable Carbon Nanotube-MoS2 Heterojunction P-N Diode. Proc. Natl. Acad. Sci. U. S. A. 2013, 110, 18076−18080. (249) Chen, Y.; Shen, Z.; Xu, Z.; Hu, Y.; Xu, H.; Wang, S.; Guo, X.; Zhang, Y.; Peng, L.; Ding, F.; et al. Helicity-Dependent Single-Walled Carbon Nanotube Alignment on Graphite for Helical Angle and Handedness Recognition. Nat. Commun. 2013, 4, 2205. (250) Cheng, F.; Xu, H.; Xu, W.; Zhou, P.; Martin, J.; Loh, K. P. Controlled Growth of 1D MoSe2 Nanoribbons with Spatially Modulated Edge States. Nano Lett. 2017, 17, 1116−1120. (251) Raja, A.; Montoya Castillo, A.; Zultak, J.; Zhang, X. X.; Ye, Z.; Roquelet, C.; Chenet, D. A.; van der Zande, A. M.; Huang, P.; Jockusch, S.; et al. Energy Transfer from Quantum Dots to Graphene and MoS2: The Role of Absorption and Screening in Two-Dimensional Materials. Nano Lett. 2016, 16, 2328−2333. (252) Hu, C.; Dong, D. D.; Yang, X. K.; Qiao, K. K.; Yang, D.; Deng, H.; Yuan, S. J.; Khan, J.; Lan, Y.; Song, H. S.; et al. Synergistic Effect of Hybrid PbS Quantum Dots/2D-WSe2 Toward High Performance and Broadband Phototransistors. Adv. Funct. Mater. 2017, 27, 1603605. (253) Zhang, J.; Zhu, Z.; Feng, X. Construction of Two-Dimensional MoS2/CdS P-N Nanohybrids for Highly Efficient Photocatalytic Hydrogen Evolution. Chem. - Eur. J. 2014, 20, 10632−10635. (254) Wen, Y.; Yin, L.; He, P.; Wang, Z. X.; Zhang, X. K.; Wang, Q. S.; Shifa, T. A.; Xu, K.; Wang, F. M.; Zhan, X. Y.; et al. Integrated HighPerformance Infrared Phototransistor Arrays Composed of Nonlayered PbS-MoS2 Heterostructures with Edge Contacts. Nano Lett. 2016, 16, 6437−6444. (255) Ojeda-Aristizabal, C.; Santos, E. J. G.; Onishi, S.; Yan, A.; Rasool, H. I.; Kahn, S.; Lv, Y.; Latzke, D. W.; Velasco, J., Jr.; Crommie, M. F.; et al. Molecular Arrangement and Charge Transfer in C60/Graphene Heterostructures. ACS Nano 2017, 11, 4686−4693. (256) Kim, K.; Lee, T. H.; Santos, E. J.; Jo, P. S.; Salleo, A.; Nishi, Y.; Bao, Z. Structural and Electrical Investigation of C60−Graphene Vertical Heterostructures. ACS Nano 2015, 9, 5922−5928. (257) Ovchinnikov, D.; Allain, A.; Huang, Y.-S.; Dumcenco, D.; Kis, A. Electrical Transport Properties of Single-Layer WS2. ACS Nano 2014, 8, 8174−8181. (258) Lembke, D.; Bertolazzi, S.; Kis, A. Single-Layer MoS2 Electronics. Acc. Chem. Res. 2015, 48, 100−110. (259) Yu, Z.; Ong, Z.-Y.; Li, S.; Xu, J.-B.; Zhang, G.; Zhang, Y.-W.; Shi, Y.; Wang, X. Analyzing the Carrier Mobility in Transition-Metal Dichalcogenide MoS2 Field-Effect Transistors. Adv. Funct. Mater. 2017, 27, 1604093. (260) Radisavljevic, B.; Kis, A. Mobility Engineering and a MetalInsulator Transition in Monolayer MoS2. Nat. Mater. 2013, 12, 815− 820. (261) Radisavljevic, B.; Radenovic, A.; Brivio, J.; Giacometti, i. V.; Kis, A. Single-Layer MoS2 Transistors. Nat. Nanotechnol. 2011, 6, 147−150. (262) Huang, J.-K.; Pu, J.; Hsu, C.-L.; Chiu, M.-H.; Juang, Z.-Y.; Chang, Y.-H.; Chang, W.-H.; Iwasa, Y.; Takenobu, T.; Li, L.-J. LargeArea Synthesis of Highly Crystalline WSe2 Monolayers and Device Applications. ACS Nano 2014, 8, 923−930. (263) Yu, L.; El-Damak, D.; Radhakrishna, U.; Ling, X.; Zubair, A.; Lin, Y.; Zhang, Y.; Chuang, M. H.; Lee, Y. H.; Antoniadis, D.; et al. Design, Modeling, and Fabrication of Chemical Vapor Deposition Grown MoS2 Circuits with E-Mode FETs for Large-Area Electronics. Nano Lett. 2016, 16, 6349−6356. (264) Zhao, Y. D.; Qiao, J. S.; Yu, Z. H.; Yu, P.; Xu, K.; Lau, S. P.; Zhou, W.; Liu, Z.; Wang, X. R.; Ji, W.; et al. High-Electron-Mobility and AirStable 2D Layered PtSe2 FETs. Adv. Mater. 2017, 29, 1604230. (265) Chuang, H.-J.; Tan, X.; Ghimire, N. J.; Perera, M. M.; Chamlagain, B.; Cheng, M. M.-C.; Yan, J.; Mandrus, D.; Tomanek,

D.; Zhou, Z. High Mobility WSe2 P- and N-Type Field-Effect Transistors Contacted by Highly Doped Graphene for Low-Resistance Contacts. Nano Lett. 2014, 14, 3594−3601. (266) Fang, H.; Chuang, S.; Chang, T. C.; Takei, K.; Takahashi, T.; Javey, A. High-Performance Single Layered WSe2 P-FETs with Chemically Doped Contacts. Nano Lett. 2012, 12, 3788−3792. (267) Lin, M. W.; Liu, L. Z.; Lan, Q.; Tan, X. B.; Dhindsa, K. S.; Zeng, P.; Naik, V. M.; Cheng, M. M. C.; Zhou, Z. X. Mobility Enhancement and Highly Efficient Gating of Monolayer MoS2 Transistors with Polymer Electrolyte. J. Phys. D: Appl. Phys. 2012, 45, 345102. (268) Amit, I.; Octon, T. J.; Townsend, N. J.; Reale, F.; Wright, C. D.; Mattevi, C.; Craciun, M. F.; Russo, S. Role of Charge Traps in the Performance of Atomically Thin Transistors. Adv. Mater. 2017, 29, 1605598. (269) Xu, K.; Chen, D.; Yang, F.; Wang, Z.; Yin, L.; Wang, F.; Cheng, R.; Liu, K.; Xiong, J.; Liu, Q.; et al. Sub-10 Nm Nanopattern Architecture for 2D Material Field-Effect Transistors. Nano Lett. 2017, 17, 1065− 1070. (270) Roy, T.; Tosun, M.; Cao, X.; Fang, H.; Lien, D.-H.; Zhao, P.; Chen, Y.-Z.; Chueh, Y.-L.; Guo, J.; Javey, A. Dual-Gated MoS2/WSe2 Van Der Waals Tunnel Diodes and Transistors. ACS Nano 2015, 9, 2071−2079. (271) Nourbakhsh, A.; Zubair, A.; Dresselhaus, M. S.; Palacios, T. S. Transport Properties of a MoS2/WSe2 Heterojunction Transistor and Its Potential for Application. Nano Lett. 2016, 16, 1359−1366. (272) Roy, T.; Tosun, M.; Hettick, M.; Ahn, G. H.; Hu, C.; Javey, A. 2D-2D Tunneling Field-Effect Transistors Using WSe2/SnSe2 Heterostructures. Appl. Phys. Lett. 2016, 108, 083111. (273) Xu, J.; Jia, J. Y.; Lai, S.; Ju, J.; Lee, S. Tunneling Field Effect Transistor Integrated with Black Phosphorus-MoS2 Junction and Ion Gel Dielectric. Appl. Phys. Lett. 2017, 110, 033103. (274) Pezeshki, A.; Shokouh, S. H. H.; Nazari, T.; Oh, K.; Im, S. Electric and Photovoltaic Behavior of a Few-Layer Alpha-MoTe2/MoS2 Dichalcogenide Heterojunction. Adv. Mater. 2016, 28, 3216−3222. (275) Sarkar, D.; Xie, X.; Liu, W.; Cao, W.; Kang, J.; Gong, Y.; Kraemer, S.; Ajayan, P. M.; Banerjee, K. A Subthermionic Tunnel Field-Effect Transistor with an Atomically Thin Channel. Nature 2015, 526, 91−95. (276) Desai, S. B.; Seol, G.; Kang, J. S.; Fang, H.; Battaglia, C.; Kapadia, R.; Ager, J. W.; Guo, J.; Javey, A. Strain-Induced Indirect to Direct Bandgap Transition in Multilayer WSe2. Nano Lett. 2014, 14, 4592− 4597. (277) Jo, S.; Ubrig, N.; Berger, H.; Kuzmenko, A. B.; Morpurgo, A. F. Mono- and Bilayer WS2 Light-Emitting Transistors. Nano Lett. 2014, 14, 2019−2025. (278) Ross, J. S.; Klement, P.; Jones, A. M.; Ghimire, N. J.; Yan, J.; Mandrus, D. G.; Taniguchi, T.; Watanabe, K.; Kitamura, K.; Yao, W.; et al. Electrically Tunable Excitonic Light-Emitting Diodes Based on Monolayer WSe2 P-N Junctions. Nat. Nanotechnol. 2014, 9, 268−272. (279) Liu, C.-H.; Clark, G.; Fryett, T.; Wu, S.; Zheng, J.; Hatami, F.; Xu, X.; Majumdar, A. Nano-Cavity Integrated Van Der Waals Heterostructure Light-Emitting Tunneling Diode. Nano Lett. 2017, 17, 200−205. (280) Withers, F.; Del Pozo-Zamudio, O.; Schwarz, S.; Dufferwiel, S.; Walker, P. M.; Godde, T.; Rooney, A. P.; Gholinia, A.; Woods, C. R.; Blake, P.; et al. WSe2 Light-Emitting Tunneling Transistors with Enhanced Brightness at Room Temperature. Nano Lett. 2015, 15, 8223−8228. (281) Zhang, Y. J.; Oka, T.; Suzuki, R.; Ye, J. T.; Iwasa, Y. Electrically Switchable Chiral Light-Emitting Transistor. Science 2014, 344, 725− 728. (282) Sanchez, O. L.; Ovchinnikov, D.; Misra, S.; Allain, A.; Kis, A. Valley Polarization by Spin Injection in a Light-Emitting Van Der Waals Heterojunction. Nano Lett. 2016, 16, 5792−5797. (283) Koperski, M.; Nogajewski, K.; Arora, A.; Cherkez, V.; Mallet, P.; Veuillen, J. Y.; Marcus, J.; Kossacki, P.; Potemski, M. Single Photon Emitters in Exfoliated WSe2 Structures. Nat. Nanotechnol. 2015, 10, 503−506. (284) Clark, G.; Schaibley, J. R.; Ross, J.; Taniguchi, T.; Watanabe, K.; Hendrickson, J. R.; Mou, S.; Yao, W.; Xu, X. Single Defect LightAO

DOI: 10.1021/acs.chemrev.7b00536 Chem. Rev. XXXX, XXX, XXX−XXX

Chemical Reviews

Review

Emitting Diode in a Van Der Waals Heterostructure. Nano Lett. 2016, 16, 3944−3948. (285) Nikam, R. D.; Sonawane, P. A.; Sankar, R.; Chen, Y. T. Epitaxial Growth of Vertically Stacked P-MoS2/N-MoS2 Heterostructures by Chemical Vapor Deposition for Light Emitting Devices. Nano Energy 2017, 32, 454−462. (286) Cheng, R.; Li, D. H.; Zhou, H. L.; Wang, C.; Yin, A. X.; Jiang, S.; Liu, Y.; Chen, Y.; Huang, Y.; Duan, X. F. Electroluminescence and Photocurrent Generation from Atomically Sharp WSe2/MoS2 Heterojunction P-N Diodes. Nano Lett. 2014, 14, 5590−5597. (287) Yang, W.; Shang, J.; Wang, J.; Shen, X.; Cao, B.; Peimyoo, N.; Zou, C.; Chen, Y.; Wang, Y.; Cong, C.; et al. Electrically Tunable ValleyLight Emitting Diode (vLED) Based on CVD-Grown Monolayer WS2. Nano Lett. 2016, 16, 1560−1567. (288) Palacios-Berraquero, C.; Barbone, M.; Kara, D. M.; Chen, X.; Goykhman, I.; Yoon, D.; Ott, A. K.; Beitner, J.; Watanabe, K.; Taniguchi, T.; et al. Atomically Thin Quantum Light-Emitting Diodes. Nat. Commun. 2016, 7, 12978. (289) He, Y.-M.; Clark, G.; Schaibley, J. R.; He, Y.; Chen, M.-C.; Wei, Y.-J.; Ding, X.; Zhang, Q.; Yao, W.; Xu, X.; et al. Single Quantum Emitters in Monolayer Semiconductors. Nat. Nanotechnol. 2015, 10, 497−502. (290) Kufer, D.; Konstantatos, G. Photo-FETs: Phototransistors Enabled by 2D and 0D Nanomaterials. ACS Photonics 2016, 3, 2197− 2210. (291) Lopez-Sanchez, O.; Lembke, D.; Kayci, M.; Radenovic, A.; Kis, A. Ultrasensitive Photodetectors Based on Monolayer MoS2. Nat. Nanotechnol. 2013, 8, 497−501. (292) Chang, Y.-H.; Zhang, W.; Zhu, Y.; Han, Y.; Pu, J.; Chang, J.-K.; Hsu, W.-T.; Huang, J.-K.; Hsu, C.-L.; Chiu, M.-H.; et al. Monolayer MoSe2 Grown by Chemical Vapor Deposition for Fast Photodetection. ACS Nano 2014, 8, 8582−8590. (293) Rosner, M.; Steinke, C.; Lorke, M.; Gies, C.; Jahnke, F.; Wehling, T. O. Two-Dimensional Heterojunctions from Nonlocal Manipulations of the Interactions. Nano Lett. 2016, 16, 2322−2327. (294) Huo, N.; Gupta, S.; Konstantatos, G. MoS2−HgTe Quantum Dot Hybrid Photodetectors Beyond 2 μm. Adv. Mater. 2017, 29, 1606576. (295) Che, W.; Cheng, W. R.; Yao, T.; Tang, F. M.; Liu, W.; Su, H.; Huang, Y. Y.; Liu, Q. H.; Liu, J. K.; Hu, F. C.; et al. Fast Photoelectron Transfer in (Cring)-C3N4 Plane Heterostructural Nanosheets for Overall Water Splitting. J. Am. Chem. Soc. 2017, 139, 3021−3026. (296) Massicotte, M.; Schmidt, P.; Vialla, F.; Schaedler, K. G.; Reserbat-Plantey, A.; Watanabe, K.; Taniguchi, T.; Tielrooij, K. J.; Koppens, F. H. L. Picosecond Photoresponse in Van Der Waals Heterostructures. Nat. Nanotechnol. 2016, 11, 42−46. (297) Vu, Q. A.; Lee, J. H.; Nguyen, V. L.; Shin, Y. S.; Lim, S. C.; Lee, K.; Heo, J.; Park, S.; Kim, K.; Lee, Y. H.; et al. Tuning Carrier Tunneling in Van Der Waals Heterostructures for Ultrahigh Detectivity. Nano Lett. 2017, 17, 453−459. (298) Floery, N.; Jain, A.; Bharadwaj, P.; Parzefall, M.; Taniguchi, T.; Watanabe, K.; Novotny, L. A WSe2/MoSe2 Heterostructure Photovoltaic Device. Appl. Phys. Lett. 2015, 107, 123106. (299) Memaran, S.; Pradhan, N. R.; Lu, Z.; Rhodes, D.; Ludwig, J.; Zhou, Q.; Ogunsolu, O.; Ajayan, P. M.; Smirnov, D.; FernandezDominguez, A. I.; et al. Pronounced Photovoltaic Response from Multilayered Transition-Metal Dichalcogenides PN-Junctions. Nano Lett. 2015, 15, 7532−7538. (300) Wi, S.; Chen, M.; Li, D.; Nam, H.; Meyhofer, E.; Liang, X. Photovoltaic Response in Pristine WSe2 Layers Modulated by MetalInduced Surface-Charge-Transfer Doping. Appl. Phys. Lett. 2015, 107, 062102. (301) Wang, F.; Wang, Z.; Xu, K.; Wang, F.; Wang, Q.; Huang, Y.; Yin, L.; He, J. Tunable Gate-MoS2 Van Der Waals P-N Junctions with Novel Optoelectronic Performance. Nano Lett. 2015, 15, 7558−7566. (302) Bernardi, M.; Palummo, M.; Grossman, J. C. Extraordinary Sunlight Absorption and One Nanometer Thick Photovoltaics Using Two-Dimensional Monolayer Materials. Nano Lett. 2013, 13, 3664− 3670.

(303) Tsai, M. L.; Su, S. H.; Chang, J. K.; Tsai, D. S.; Chen, C. H.; Wu, C. I.; Li, L. J.; Chen, L. J.; He, J. H. Monolayer MoS2 Heterojunction Solar Cells. ACS Nano 2014, 8, 8317−8322. (304) Long, M.; Liu, E.; Wang, P.; Gao, A.; Xia, H.; Luo, W.; Wang, B.; Zeng, J.; Fu, Y.; Xu, K.; et al. Broadband Photovoltaic Detectors Based on an Atomically Thin Heterostructure. Nano Lett. 2016, 16, 2254− 2259. (305) Wong, J.; Jariwala, D.; Tagliabue, G.; Tat, K.; Davoyan, A. R.; Sherrott, M. C.; Atwater, H. A. High Photovoltaic Quantum Efficiency in Ultrathin Van Der Waals Heterostructures. ACS Nano 2017, 11, 7230−7240. (306) Furchi, M. M.; Zechmeister, A. A.; Hoeller, F.; Wachter, S.; Pospischil, A.; Mueller, T. Photovoltaics in Van Der Waals Heterostructures. IEEE J. Sel. Top. Quantum Electron. 2017, 23, 106− 116. (307) Gan, L.-Y.; Zhang, Q.; Cheng, Y.; Schwingenschloegl, U. Photovoltaic Heterojunctions of Fullerenes with MoS2 and WS2 Monolayers. J. Phys. Chem. Lett. 2014, 5, 1445−1449. (308) Li, B. L.; Wang, J. P.; Zou, H. L.; Garaj, S.; Lim, C. T.; Xie, J. P.; Li, N. B.; Leong, D. T. Low-Dimensional Transition Metal Dichalcogenide Nanostructures Based Sensors. Adv. Funct. Mater. 2016, 26, 7034−7056. (309) Borini, S.; White, R.; Wei, D.; Astley, M.; Haque, S.; Spigone, E.; Harris, N.; Kivioja, J.; Ryhanen, T. Ultrafast Graphene Oxide Humidity Sensors. ACS Nano 2013, 7, 11166−11173. (310) Heerema, S. J.; Dekker, C. Graphene Nanodevices for DNA Sequencing. Nat. Nanotechnol. 2016, 11, 127−136. (311) Abbas, A. N.; Liu, B.; Chen, L.; Ma, Y.; Cong, S.; Aroonyadet, N.; Koepf, M.; Nilges, T.; Zhou, C. Black Phosphorus Gas Sensors. ACS Nano 2015, 9, 5618−5624. (312) Late, D. J.; Doneux, T.; Bougouma, M. Single-Layer MoSe2 Based NH3 Gas Sensor. Appl. Phys. Lett. 2014, 105, 233103. (313) Jiang, S.; Cheng, R.; Ng, R.; Huang, Y.; Duan, X. Highly Sensitive Detection of Mercury(II) Ions with Few-Layer Molybdenum Disulfide. Nano Res. 2015, 8, 257−262. (314) Parvin, N.; Jin, Q.; Wei, Y.; Yu, R.; Zheng, B.; Huang, L.; Zhang, Y.; Wang, L.; Zhang, H.; Gao, M.; et al. Few-Layer Graphdiyne Nanosheets Applied for Multiplexed Real-Time DNA Detection. Adv. Mater. 2017, 29, 1606755. (315) Zhu, C.; Zeng, Z.; Li, H.; Li, F.; Fan, C.; Zhang, H. Single-Layer MoS2-Based Nanoprobes for Homogeneous Detection of Biomolecules. J. Am. Chem. Soc. 2013, 135, 5998−6001. (316) Park, M.; Park, Y. J.; Chen, X.; Park, Y. K.; Kim, M. S.; Ahn, J. H. MoS2-Based Tactile Sensor for Electronic Skin Applications. Adv. Mater. 2016, 28, 2556−2562. (317) Schedin, F.; Geim, A. K.; Morozov, S. V.; Hill, E. W.; Blake, P.; Katsnelson, M. I.; Novoselov, K. S. Detection of Individual Gas Molecules Adsorbed on Graphene. Nat. Mater. 2007, 6, 652−655. (318) Liu, B.; Chen, L.; Liu, G.; Abbas, A. N.; Fathi, M.; Zhou, C. HighPerformance Chemical Sensing Using Schottky-Contacted Chemical Vapor Deposition Grown Mono Layer MoS2 Transistors. ACS Nano 2014, 8, 5304−5314. (319) Kalantar-zadeh, K.; Ou, J. Z. Biosensors Based on TwoDimensional MoS2. ACS Sensors 2016, 1, 5−16. (320) Naylor, C. H.; Kybert, N. J.; Schneier, C.; Xi, J.; Romero, G.; Saven, J. G.; Liu, R.; Johnson, A. T. Scalable Production of Molybdenum Disulfide Based Biosensors. ACS Nano 2016, 10, 6173−6179. (321) Akinwande, D.; Petrone, N.; Hone, J. Two-Dimensional Flexible Nanoelectronics. Nat. Commun. 2014, 5, 5678. (322) Sun, D. M.; Liu, C.; Ren, W. C.; Cheng, H. M. All-Carbon ThinFilm Transistors as a Step Towards Flexible and Transparent Electronics. Adv. Electron. Mater. 2016, 2, 1600229. (323) Fuhrer, M. S.; Hone, J. Measurement of Mobility in Dual-Gated MoS2 Transistors. Nat. Nanotechnol. 2013, 8, 146−147. (324) Das, S.; Gulotty, R.; Sumant, A. V.; Roelofs, A. All TwoDimensional, Flexible, Transparent, and Thinnest Thin Film Transistor. Nano Lett. 2014, 14, 2861−2866. (325) Zhao, J.; Chen, W.; Meng, J. L.; Yu, H.; Liao, M. Z.; Zhu, J. Q.; Yang, R.; Shi, D. X.; Zhang, G. Y. Integrated Flexible and High-Quality AP

DOI: 10.1021/acs.chemrev.7b00536 Chem. Rev. XXXX, XXX, XXX−XXX

Chemical Reviews

Review

Thin Film Transistors Based on Monolayer MoS2. Adv. Electron. Mater. 2016, 2, 1500379. (326) De Fazio, D.; Goykhman, I.; Yoon, D.; Bruna, M.; Eiden, A.; Milana, S.; Sassi, U.; Barbone, M.; Dumcenco, D.; Marinov, K.; et al. High Responsivity, Large-Area Graphene/MoS2 Flexible Photodetectors. ACS Nano 2016, 10, 8252−8262. (327) Shen, T.; Penumatcha, A. V.; Appenzeller, J. Strain Engineering for Transition Metal Dichalcogenides Based Field Effect Transistors. ACS Nano 2016, 10, 4712−4718. (328) He, Y.; Yang, Y.; Zhang, Z.; Gong, Y.; Zhou, W.; Hu, Z.; Ye, G.; Zhang, X.; Bianco, E.; Lei, S.; et al. Strain-Induced Electronic Structure Changes in Stacked Van Der Waals Heterostructures. Nano Lett. 2016, 16, 3314−3320. (329) Deng, D. H.; Novoselov, K. S.; Fu, Q.; Zheng, N. F.; Tian, Z. Q.; Bao, X. H. Catalysis with Two-Dimensional Materials and Their Heterostructures. Nat. Nanotechnol. 2016, 11, 218−230. (330) Chia, X.; Eng, A. Y. S.; Ambrosi, A.; Tan, S. M.; Pumera, M. Electrochemistry of Nanostructured Layered Transition-Metal Dichalcogenides. Chem. Rev. 2015, 115, 11941−11966. (331) An, Y.; Fan, X.; Luo, Z.; Lau, W.-M. Nano-Polygons of Monolayer MS2−Best Morphology and Size for Her Catalysis. Nano Lett. 2017, 17, 368−376. (332) Tang, Q.; Jiang, D. E. Mechanism of Hydrogen Evolution Reaction on 1T-MoS2 from First Principles. ACS Catal. 2016, 6, 4953− 4961. (333) Li, G.; Zhang, D.; Qiao, Q.; Yu, Y.; Peterson, D.; Zafar, A.; Kumar, R.; Curtarolo, S.; Hunte, F.; Shannon, S.; et al. All the Catalytic Active Sites of MoS2 for Hydrogen Evolution. J. Am. Chem. Soc. 2016, 138, 16632−16638. (334) Xie, J.; Zhang, H.; Li, S.; Wang, R.; Sun, X.; Zhou, M.; Zhou, J.; Lou, X. W. D.; Xie, Y. Defect-Rich MoS2 Ultrathin Nanosheets with Additional Active Edge Sites for Enhanced Electrocatalytic Hydrogen Evolution. Adv. Mater. 2013, 25, 5807−5813. (335) Ye, G. L.; Gong, Y. J.; Lin, J. H.; Li, B.; He, Y. M.; Pantelides, S. T.; Zhou, W.; Vajtai, R.; Ajayan, P. M. Defects Engineered Monolayer MoS2 for Improved Hydrogen Evolution Reaction. Nano Lett. 2016, 16, 1097−1103. (336) Wang, J.; Yan, M.; Zhao, K.; Liao, X.; Wang, P.; Pan, X.; Yang, W.; Mai, L. Field Effect Enhanced Hydrogen Evolution Reaction of MoS2 Nanosheets. Adv. Mater. 2017, 29, 1604464. (337) Yu, X. Y.; Feng, Y.; Jeon, Y.; Guan, B.; Lou, X. W.; Paik, U. Formation of Ni−Co−MoS2 Nanoboxes with Enhanced Electrocatalytic Activity for Hydrogen Evolution. Adv. Mater. 2016, 28, 9006−9011. (338) Li, S.; Wang, S. S.; Salamone, M. M.; Robertson, A. W.; Nayak, S.; Kim, H.; Tsang, S. C. E.; Pasta, M.; Warner, J. H. Edge-Enriched 2D MoS2 Thin Films Grown by Chemical Vapor Deposition for Enhanced Catalytic Performance. ACS Catal. 2017, 7, 877−886. (339) Tsai, C.; Li, H.; Park, S.; Park, J.; Han, H. S.; Norskov, J. K.; Zheng, X. L.; Abild-Pedersen, F. Electrochemical Generation of Sulfur Vacancies in the Basal Plane of MoS2 for Hydrogen Evolution. Nat. Commun. 2017, 8, 15113. (340) Li, H.; Tsai, C.; Koh, A. L.; Cai, L.; Contryman, A. W.; Fragapane, A. H.; Zhao, J.; Han, H. S.; Manoharan, H. C.; AbildPedersen, F.; et al. Activating and Optimizing MoS2 Basal Planes for Hydrogen Evolution through the Formation of Strained Sulphur Vacancies. Nat. Mater. 2016, 15, 48−53. (341) Cheng, C. C.; Lu, A. Y.; Tseng, C. C.; Yang, X. L.; Hedhili, M. N.; Chen, M. C.; Wei, K. H.; Li, L. J. Activating Basal-Plane Catalytic Activity of Two-Dimensional MoS2 Monolayer with Remote Hydrogen Plasma. Nano Energy 2016, 30, 846−852. (342) Li, H.; Du, M.; Mleczko, M. J.; Koh, A. L.; Nishi, Y.; Pop, E.; Bard, A. J.; Zheng, X. Kinetic Study of Hydrogen Evolution Reaction over Strained MoS2 with Sulfur Vacancies Using Scanning Electrochemical Microscopy. J. Am. Chem. Soc. 2016, 138, 5123−5129. (343) Yuan, J. T.; Wu, J. J.; Hardy, W. J.; Loya, P.; Lou, M.; Yang, Y. C.; Najmaei, S.; Jiang, M. L.; Qin, F.; Keyshar, K.; et al. Facile Synthesis of Single Crystal Vanadium Disulfide Nanosheets by Chemical Vapor

Deposition for Efficient Hydrogen Evolution Reaction. Adv. Mater. 2015, 27, 5605−5609. (344) Shi, J.; Ma, D.; Han, G.-F.; Zhang, Y.; Ji, Q.; Gao, T.; Sun, J.; Song, X.; Li, C.; Zhang, Y.; et al. Controllable Growth and Transfer of Monolayer MoS2 on Au Foils and Its Potential Application in Hydrogen Evolution Reaction. ACS Nano 2014, 8, 10196−10204. (345) Xiang, Q.; Yu, J.; Jaroniec, M. Synergetic Effect of MoS2 and Graphene as Cocatalysts for Enhanced Photocatalytic H2 Production Activity of TiO2 Nanoparticles. J. Am. Chem. Soc. 2012, 134, 6575−6578. (346) Tang, C.; Zhong, L.; Zhang, B.; Wang, H.-F.; Zhang, Q. 3D Mesoporous Van Der Waals Heterostructures for Trifunctional Energy Electrocatalysis. Adv. Mater. 2017, 1705110. (347) Lei, Y.; Pakhira, S.; Fujisawa, K.; Wang, X.; Iyiola, O. O.; Perea López, N.; Laura Elías, A.; Pulickal Rajukumar, L.; Zhou, C.; Kabius, B.; et al. Low-Temperature Synthesis of Heterostructures of Transition Metal Dichalcogenide Alloys (WxMo1−xS2) and Graphene with Superior Catalytic Performance for Hydrogen Evolution. ACS Nano 2017, 11, 5103−5112. (348) Wu, J.; Yuan, H.; Meng, M.; Chen, C.; Sun, Y.; Chen, Z.; Dang, W.; Tan, C.; Liu, Y.; Yin, J.; et al. High Electron Mobility and Quantum Oscillations in Non-Encapsulated Ultrathin Semiconducting Bi2O2Se. Nat. Nanotechnol. 2017, 12, 530−535. (349) Gong, Y. J.; Lin, Z.; Ye, G. L.; Shi, G.; Feng, S. M.; Lei, Y.; Elias, A. L.; Perea-Lopez, N.; Vajtai, R.; Terrones, H.; et al. Tellurium-Assisted Low-Temperature Synthesis of MoS2 and WS2 Monolayers. ACS Nano 2015, 9, 11658−11666. (350) Velusamy, D. B.; Haque, M. A.; Parida, M. R.; Zhang, F.; Wu, T.; Mohammed, O. F.; Alshareef, H. N. 2D Organic-Inorganic Hybrid Thin Films for Flexible UV-Visible Photodetectors. Adv. Funct. Mater. 2017, 27, 1605554.

AQ

DOI: 10.1021/acs.chemrev.7b00536 Chem. Rev. XXXX, XXX, XXX−XXX