Exploring Two-Dimensional Materials toward the Next-Generation

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Cite This: Chem. Rev. 2018, 118, 6236−6296

Exploring Two-Dimensional Materials toward the Next-Generation Circuits: From Monomer Design to Assembly Control Mengqi Zeng,† Yao Xiao,‡ Jinxin Liu,† Kena Yang,† and Lei Fu*,†,‡ †

College of Chemistry and Molecular Sciences, Wuhan University, Wuhan 430072, China The Institute for Advanced Studies (IAS), Wuhan University, Wuhan 430072, China

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‡

ABSTRACT: Two-dimensional (2D) materials have attracted tremendous research interest since the breakthrough of graphene. Their unique optical, electronic, and mechanical properties hold great potential for harnessing them as key components in novel applications for electronics and optoelectronics. Their atomic thickness and exposed huge surface even make them highly designable and manipulable, leading to the extensive application potentials. What’s more, after acquiring the qualification for being the candidate for next-generation devices, the assembly of 2D materials monomers into mass or ordered structure is also of great importance, which will determine their ultimate industrialization. By designing the monomers and regulating their assembling behavior, the exploration of 2D materials toward the next-generation circuits can be spectacularly achieved. In this review, we will first overview the emerging 2D materials and then offer a clear guideline of varied physical and chemical strategies for tuning their properties. Furthermore, assembly strategies of 2D materials will also be included. Finally, challenges and outlooks in this promising field are featured on the basis of its current progress.

CONTENTS 1. Introduction 2. Emerging 2D Materials for Next-Generation Electronics 2.1. Classification 2.2. Elemental 2D Materials 2.2.1. IV A Group 2.2.2. V A Group 2.2.3. III A Group 2.2.4. Transition Group 2.3. h-BN 2.4. Transition Metal Dichalcogenides (TMDs) 2.5. Transition Metal Carbides (TMCs) 2.6. Transition Metal Oxides (TMOs) 2.7. Main Group Metal Composites 2.7.1. III A Group 2.7.2. IV A Group 2.7.3. V A Group 2.8. Others 3. Designing Strategies for the Building Blocks 3.1. Dimension Controlling 3.1.1. Along the z Direction 3.1.2. Along the xy Direction 3.2. Composition Tuning 3.2.1. Atomic Doping 3.2.2. Alloying 3.2.3. Atomic Vacancy 3.3. External Field Tuning 3.3.1. Electric Field 3.3.2. Lighting 3.4. Structure Tuning © 2018 American Chemical Society

3.4.1. Phase Transition 3.4.2. Edge Structure Reconstruction 3.4.3. Crystal Lattice Deformation 4. Assembly Behaviors toward the Circuits 4.1. Inter-Monomers Assembly 4.1.1. Vertical Assembly 4.1.2. Lateral Assembly 4.2. Oriented Assembly 4.2.1. Rolling Assembly 4.2.2. Edge-to-Edge Coupling 4.3. Mass-Ordered Assembly 4.3.1. Patterning 4.3.2. Locating Strategy 4.3.3. Self-Assembly 5. Circuits Based On 2D Materials 6. Concluding Remarks and Outlook Author Information Corresponding Author ORCID Notes Biographies Acknowledgments References

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Special Issue: 2D Materials Chemistry Received: October 23, 2017 Published: January 30, 2018 6236

DOI: 10.1021/acs.chemrev.7b00633 Chem. Rev. 2018, 118, 6236−6296

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1. INTRODUCTION Since the exfoliation of graphene in 2004, two-dimensional (2D) materials have been receiving great attention because of qualitative changes in their physical and chemical properties due to quantum size effect, which is related to their nanosized thickness.1,2 The transports of charge carriers, heat, and photon will be strongly confined in the 2D plane, leading to remarkable changes in the electronic and optical properties of the 2D materials.3,4 The family of 2D materials encompasses a wide selection of compositions including most elements of the periodic table.5 This derives into a rich variety of electronic properties, including metals, semimetals, insulators, and semiconductors with direct and indirect bandgaps ranging from ultraviolet to infrared throughout the visible range.6−8 Even more, the 2D geometry owns excellent compatibility with current thin film manufacture techniques in the semiconductor industry, which can facilitate the 2D materials to be integrated with traditional electronic materials, such as Si, and to be placed on various substrates. Therefore, they have the potential to play a significant role in the future nanoelectronics, optoelectronics, and the assembly of novel ultrathin and flexible devices.6−18 2D Atomic crystal integration circuits, including memory, logic gate, amplifier, oscillator, mixer, switch, and modulator have been demonstrated. Being just one or several atoms thick, 2D materials immediately appear as the most suitable candidate to create a new generation of electronic devices. Functional integration circuits from 2D materials can indeed provide intensive for resolving technological and fundamental challenges in the electronic industry. Progress in 2D material-based nanoelectronics requires the reliable fabrication and controllable assembly of large-scale and high-quality 2D materials and the full exploitation of their unique properties in the 2D limit (Figure 1). The explosive popularity of 2D materials does not only rely on their intrinsic properties but also highly depends on the tunability. Due to the high anisotropy and “all-surface” crystal structure, the properties of 2D materials can be effectively tuned in a wide regime through diverse approaches, including reducing dimensions, composition tuning, external field tuning, and structural tuning.19 For example, the band structures of 2D materials are significantly changed as their thickness is thinned down from bulk to single-layer limit. Besides this, through the intercalation, some 2D materials can achieve a semiconductorto-metal conversion. Modern technologies and applications require a wide range of excellent performances, which can be hardly realized in a single material. Therefore, 2D materials provide a great platform of tuning their properties toward desired functions, further attracting a great deal of attention and opening up opportunities for a wide range of applications. In addition, being just one or several atoms thick, highly disparate 2D layered materials can be assembled together to create a wide range of heterostructures or macro-ordered structures without the constraints of lattice matching and processing compatibility. The creation of intermonomer heterostructure, including van der Waals vertical heterostructure and lateral heterostructure with atomically sharp interfaces and highly distinct electronic functions, offers a new material platform and a rich playground for probing the generation, confinement, and transport of charges, excitons, photons, and phonons at the limit of atomic thickness and promoting potential applications.18 The structure assembly aiming at individual 2D material monomers also deserves our attention

since it helps further deeply reveal the intrinsic property of the material, such as the in-plane anisotropy, the interlayer interaction, and so on. The oriented assembly derived structure deformation will lead to the property extension of the 2D materials and promote new applications. Moreover, challenges facing the scaling and the integrating of microelectronic, microphotonic, and microelectromechanical systems suggest that highly efficient assembly technologies are needed to produce mass nanoscale devices in an ordered structure.20,21 There are a number of important reports and summaries focused on the attractive properties of 2D materials already.3,19,22,23 However, the systematical review of the monomer regulation and the assembly of 2D material has not been presented yet. It is of great significance to understand how 2D materials can really live up to the expectation for being qualified as the key materials for the next-generation circuits. In this review, we will first overview the emerging 2D materials and then offer a clear guideline of the different strategies for tuning their properties. Furthermore, the assembly strategies of the 2D materials will also be included. By the precise design and the control of the monomer property and the assembly behavior, 2D atomic crystals are paving new ways for the nextgeneration nanoelectronics.

2. EMERGING 2D MATERIALS FOR NEXT-GENERATION ELECTRONICS Discovery of graphene and its astonishing properties has given birth to the new class of 2D materials. Beyond graphene, various 2D materials have also attracted considerable attention due to their various and unique physical and chemical properties. Origin of these properties is ascribed to the dimensionality effect and modulation in their band structure. In this part, we will introduce the emerging 2D materials from two aspects. First, we will offer an outline for cluing the 2D materials according to their structures. Then, various kinds of 2D materials classified by element composition will be exhibited in detail. 2.1. Classification

At first, 2D materials represented by graphene were known as layered materials with strong in-plane chemical bonds and weak coupling between the layers. These layered structures provide the opportunity for the bulk counterparts to be cleaved into individual freestanding atomic layers. Here, we call them 2D layered materials. Besides that, functional materials with superior and diversified characteristics were reported recently. They own crystal structures of chemical bonding in three dimensions rather than the two dimensions of layered materials. Here, we call them 2D nonlayered materials. Layered van der Waals materials are the front-runners and can be exfoliated into mono- and few-layered nanosheets. They exhibit strong in-plane covalent or ionic bonding and weak interlayer van der Waal or hydrogen bonding, which can facilitate the exfoliation of bulk parent crystals into nanosheets either by mechanical exfoliation or liquid exfoliation.8,24 Except the first identified graphene, the majority of transition metal dichalcogenides (TMDs) are also typical van der Waals materials, especially MoS2, WS2, MoSe2, and WSe2. So far, over 40 kinds of TMDs have been declared in the 2D layered form.25 Hexagonal boron nitride (h-BN),26 SiC,27 vanadium oxide,28 and Sb2Te329 are some other member of this class. In addition, there exists another important class of 2D materials that have been gaining popularity: layered ionic materials, 6237


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Figure 1. Exploring 2D materials toward the next-generation circuits: from monomer design to assembly control.

potential to be applied in field effect transistors (FETs), sensors, catalysis, supercapacitors, batteries, and thermoelectricity devices.44 2.2.1. IV A Group. Since graphene has been exfoliated in 2004 (Figure 2a), this honeycomb single layer 2D crystal has received more and more attention in recent years.1 A series of methods were developed to prepare graphene, including mechanical exfoliation, liquid-phase exfoliation, SiC epitaxy, CVD growth, and molecular assembly. Graphene has exhibited many excellent properties, especially an extremely high intrinsic mobility of 200000 cm2 V−1 s−1 at room temperature.45 Stampfer et al. reported the mobility of 350000 cm2 V−1 s−1 of graphene at low temperature, as presented in Figure 2b, which has reached up to the highest record so far.46 Figure 2c exhibited the schematic setup for measuring the thermal conductivity of graphene, where a trench was used to suspend it. A laser beam was focused on the center of the suspended graphene and then the heat flowed radially from the center of the graphene to the support edge. The Raman G peak was measured as a function of the excitation power (Figure 2d), and the thermal conductivity value of graphene was derived as about 5000 W m−1 K−1.47 Circular well array with the diameter of 1.5 mm or 1 mm and the depth of 500 nm patterned on a SiO2/Si substrate was used to suspend the graphene and characterize the Young’s modulus (Figure 2e). Figure 2f shows the statistical histogram of the elastic stiffness of the suspended graphene and the measured Young’s modulus are about 1.0 TPa.48 Besides, graphene also exhibits large theoretical specific surface area (2630 m2 g−1), high optical transmittance (∼97.7%), and good electrical conductivity.49 Due to the excellent properties, graphene shows great application potentials in many fields, such as electronics, optoelectronics, energy storage, and conversion. However, there still exhibits a huge challenge for graphene that hinders its ultimate application due to its zero-bandgap characteristic. Another kind of 2D allotropes of carbon breaks this impasse. Newly developed graphyne and its derivatives, which are consisted of sp- and sp2-hybridized carbon atoms, own a certain bandgap.50−52 For graphyne and its derivatives, acetylenic bonds serve as the structure units, which are inserted to modify graphene with the hexagonal symmetry remained. When inserting one acetylenic bond between two carbon atoms in different ways, graphene could be transformed into α-, β-, and γ-graphyne, as shown in Figure 3a.51 When inserting two acetylenic bonds, graphdiyne forms with the structure shown in Figure 3b.52 Variable lengths of acetylene chains result in a large

which consist of a charged polyhedral layer sandwiched between hydroxide or halide layers by electrostatic forces and can be easily exfoliated by ion intercalation or ion exchange. Perovskite type oxides are typical examples of layered materials exfoliated from the ion-exchange method.30−35 In recent years, 2D nonlayered materials have also witnessed the increasing wave of research. To date, more than 50 atomically thin nonlayered nanomaterials have been demonstrated, which mainly include metal oxides, some metal chalcogenides, and dichalcogenides. The study of the 2D nonlayered materials has not only enriched the 2D crystal library but also offered some novel electronic properties. They own the crystal structures of chemical bonding in three dimensions, which hamper them to be easily exfoliated to atomic-thickness. However, their property evolution with the thickness even should deserve more attention. There are numerous dangling bonds on their surfaces, enabling their surface highly chemically active and enhancing their capability of catalysis, sensing, and carriers transfer. For examples, III−V group semiconductors are typical nonlayered materials and have both high carrier mobility and direct bandgaps. Although many methods applied for fabricating 2D layered materials cannot work for these materials and the synthesis of them is still faced with many problems, it still would be a necessary attempt to see what new insight would be brought when it reaches the 2D limit. In addition, the emerging surface-assisted nonlayered materials without three-dimensional (3D) counterparts are artificially synthesized on a substrate via chemical vapor deposition (CVD) and epitaxial growth or assembly. Silicene, an analogue of graphene, is a perfect example of surface-assisted nonlayered solids. Low buckled hexagonal structure of Si atoms has been deposited on Ag(111),36 Ir(111)37 monocrystalline substrate, and a thin film of ZrB2 on silicon wafer.38 Unlike graphene, the instability of silicene at the ambient condition has limited its potential in electronic and optoelectronic applications. In addition, emerging 2D metal nanomaterials also demonstrated the existence of the 2D crystal without the 3D parent, such as Co,39 Fe,40,41 or Au42,43 layer with atomic thickness. 2.2. Elemental 2D Materials

Elemental 2D materials, including III−V A group and transition group materials, have aroused tremendous research interest. Compared with their bulk counterparts, they exhibit abnormal electronic, photonic, magnetic, and catalytic properties owing to their unique 2D structure. These materials have great 6238


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family of graph-n-ynes, named as graphyne, graphdiyne, graphtriyne, graphtetrayne, and so on, as shown in Figure 3c.53 Graphdiyne, one of the graphyne derivatives, could be traditionally synthesized via cross-coupling or Glaser-Hay coupling reaction by using hexaethynylbenzene (HEB) or tetraethynylethene (TEE) as precursors.54−58 Generally, graphdiyne could be obtained as nanowalls (NWs)57,58 and films55,56 or coating layers on TiO254 via such coupling reactions. Recently, graphdiyne has been fabricated on Au(111) in an ultrahigh-vacuum (UHV) chamber of the scanning tunneling microscopy (STM) by using molecule evaporator to deposit precursor molecules.59 Nishihara et al. demonstrated that multilayer and few-layer graphdiyne could be obtained via a liquid/liquid or a gas/liquid interfacial reaction.60 Their properties have been theoretically predicted. As semiconductors, graphyne allotropes have a natural bandgap (∼1.2 eV for graphynes and ∼0.46 eV for graphdiynes) and the direct bandgap of them indicates the application potential in photoelectronic devices.61,62 There still exist special family members with Dirac points and cones, for instance, α-graphyne with a single Dirac cone and 6,6,12-graphyne with two different Dirac cones.62,63 Differences of band structures mainly come from the different atomic structures of graphynes.64 Graphynefamily materials, as one kind of novel 2D materials, have excellent application prospect for electronic devices and photovoltaic devices.54,56,62 Solar cells with highly efficient electron transport have been achieved by Li et al. via doping phenyl-C61-butyric acid methyl ester (PCBM) with graphdiyne, as shown in Figure 3 (panels d−f).65 In spite of high carrier mobilities and suitable bandgaps of graphyne-family materials, the difficulties in synthesis of large-area and high-quality films or single crystals seriously restrict the development of graphyne-based applications. Therefore, new strategies are in urgent need to achieve such novel materials of high quality. Except carbon, other elementary substances of group IV A also have 2D structure, such as silicene, germanene, and stanene (tinene). But unlike graphene, the atoms of silicene and germanene are bonded with each other via sp3 hybrid orbitals, which are more stable than sp2 hybrid orbitals. They do not form a van der Waals layered structure in their bulk phase. Although they do not exist as freestanding sheets, the layered structures can still be successfully obtained. Here, several methods have been developed to fabricate silicene. Wet chemical method is one of the most effective approaches and

Figure 2. Properties of graphene. (a) Photograph of a large multilayer graphene flake with the thickness of ∼3 nm on an oxidized Si wafer. Reproduced with permission from ref 1. Copyright 2004 the American Association for the Advancement of Science (AAAS). (b) Conductance σ as the function of charge carrier density n. The black curve and the blue curve were tested at 300 and 1.6 K, respectively. The green curve corresponded with the data measured on another sample with carrier mobility of μ = 350000 cm2 V−1 s−1. Reproduced with permission from ref 46. Copyright 2015 AAAS. (c) Schematic of the experimental setup showing the excitation laser light focused on a graphene layer suspended across a trench. (d) Shift in G peak position versus change in total dissipated power. Reproduced from ref 47. Copyright 2008 American Chemical Society. (e) Schematic of nanoindentation on suspended graphene membrane. (f) Histogram of elastic stiffness. Reproduced with permission from ref 48. Copyright 2008 AAAS.

Figure 3. Structures and applications of graphyne family materials. Structures of (a) α-, β-, and γ- graphyne and (b) graphdiyne. Part (a) is reproduced with permission from ref 51. Copyright 2012 American Physical Society. Part (b) is reproduced from ref 52. Copyright 2011 American Chemical Society. (c) Classification of extended graph-n-yne by acetylenic carbon chain length. Reproduced with permission from ref 53. Copyright 2012 Royal Society of Chemistry. (d) Solar cell device construction based on graphdiyne. (e) Stability and (f) current−voltage (I−V) characteristic curves of the solar cell. The inset in (f) shows the external quantum efficiency of the solar cell. Reproduced from ref 65. Copyright 2015 American Chemical Society. 6239


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magneto-optical transport properties,130 thickness-, strain-, and orientation-induced band structure,131,132 mechanical flexibility,133 thermal properties134 (such as thermoelectric efficiency135 and anisotropic in-plane thermal conductivity136), anisotropic excitons,137 and electrical conductance.138 More specially, blue phase139,140 and the conversion from a normal insulator to a topological insulator (TI) via electric field141 have also been discovered based on BP. Arsenene is described as a single-atomic layer of gray arsenic with a rhombohedral structure. Arsenene has been prepared via the plasma-assisted process on the InAs substrate, and the thickness is 14 nm.142 The theoretical calculation predicted that the arsenene owned many unique properties, including highly anisotropic thermal conductivity143 and topological phase transitions to TI via the suitable strain modulation.144 Antimonene could also be defined as the monolayer gray antimony, which is the most stable phase among all its allotropes. It is a semiconductor with a theoretical bandgap up to 2.28 eV. The preparation methods of antimonene mainly include mechanical exfoliation,145 liquid exfoliation,146 MBE,147 and CVD.148 Some theoretical calculations show that antimonene has some unique properties, such as spin−orbit coupling effect,149 thermal conductivity effected by size and edge roughness,150 electronic properties influenced by Stone− Wales defects,151 and ultraviolet light absorption.152 A small strain applied to a monolayer antimonene will lead to the transformation from the indirect bandgap to the direct one, which could further promote its application prospect in optoelectronics.153,154 Bismuthene was first prepared on a silicon substrate with atomically smooth surface in 2005.155 Subsequently, wetchemical synthesis has also been reported.156 The theory calculation predicted that the bismuthene owned many unique characteristics. Bismuthene is stable against long wavelength vibrations and thermal excitations at high temperature. When this 3D crystal is thinned down to free-standing single-layer flake, the structure of bismuthene is compressed, which leads to the transformation from semimetal to semiconductor. Whereas, the topological characters of the bismuthene do not depend on its layer number.157,158 2.2.3. III A Group. For III A group, borophene is the exclusive elemental 2D material. Boustani et al., for the first time, put forward the quasi-planar boron structures via the theoretical calculation.159 After that, the preparation methods of borophene have been reported, including vacuum deposition160,161 and two-zone CVD.162 More theoretical calculations indicated that the borophene owned many unique properties, such as high work function,163 magnetic properties,164 and ultrahigh hydrogen storage capacity.165 Besides, the 2D borophene has the application potential in power generation, electricity transmittance, energy storage, and electric catalytic field.166,167 2.2.4. Transition Group. Since being discovered, 2D transition metal elemental materials have received significant attention. Until now, 2D transition metal metals, including Fe, Co, Ni, Cu, Ru, Rh, Pd, Ag, Pt, and Au, have been reported successively. The synthetic methods of 2D transition metal elemental materials have also developed a lot. Wu et al. reported the synthesis of 2D iron membrane. They stacked the iron and aluminum sheets and then folded and calendered them repeatedly for 20 times. After that, they selectively etched aluminum layers and thus obtained ultrathin Fe membrane.40 Solvent thermal method is often used in the 2D metal

refers to the exfoliation of CaSi2, which is consisted of commutative Ca layer and interconnected Si6 rings.66 To weaken the interaction between the layers, the researchers used HCl solution and Mg.67,68 In addition to this, the most commonly used method is the vacuum deposition on different substrates, including Ag(111),36 Ag(110),69 Au(110),70 Ir(111),37 MoS2,71 ZrB2,38 and h-MoSi2,72 with the homebuilt low-temperature STM. Some physical properties of silicene were predicted by theoretical calculations of structural and electronic properties,73 including quantum spin Hall effect,74,75 ferromagnetism,76,77 tunable thermal conductivity by Germanium doping,78 and half-metallic properties.79 Germanene can be synthesized by the similar preparation methods for fabricating silicene, such as CaGe2-based chemical exfoliation80 and vacuum deposition.81−83 Germanene has many unique properties, such as structural stability,84 halfmetallic behavior,85 strong excitonic resonances,86,87 large carrier mobility,88 Dirac features,89,90 photon properties in the ground state,91,92 negative thermal expansion,93 spin-polarized electronic transport,94 controllable magnetic properties,95 many body effects,96 infrared adsorption,97,98 and high thermoelectricity properties in room temperature.99 The electronic structures of silicence and germanene are quite similar to that of graphene and their charge carriers are also massless Fermions. A vertical electric field is able to open a band gap in single-layer buckled silicence and germanene.100 Besides, the modification of haloid elements or hydrogen would generate new properties such as the quantum spin Hall effect.101,102 Stanene (tinene) is composed of a biatomic layer of Sn(111), in which two triangular sublattices stack together, forming a buckled honeycomb lattice. The preparation of stanene was achieved by molecular beam epitaxy (MBE) on a Bi2Te3(111) substrate.103 Theoretical calculation showed that stanene has many unique properties, such as tension-induced mechanical properties,104 low thermal conductivity, and diffusive nature of thermal transport,105,106 special electronic properties (Dirac cone with zero-gap at the Fermi level and a Fermi velocity of vF = 0.97 × 106 m s−1),107 topological to trivial insulating phase transition,108 giant magnetoresistance (MR),109 superconductivity,110 quantum spin Hall effect,111 and so on. 2.2.2. V A Group. V A group elemental 2D materials include black phosphorus (BP), arsenene, antimonene, and bismuthene. The first fabrication of BP could be traced back to 1914.112,113 Each phosphorus atom is covalently bonded with three adjacent phosphorus atoms to form a puckered honeycomb structure in a single layer.114 The electronic bandgap of BP would increase from 0.3 to 1.5 eV with the thickness down to monolayer. In addition, it was demonstrated that the experimental (at room temperature) carrier mobility of BP is up to 1000 cm2 V−1 s−1 in few-layer quasi 2D phosphorene.115 Thus, BP has great application potentials in electronics and photonics.116 Mechanical exfoliation has been the most widely used method to obtain BP,117−119 beyond which liquid exfoliation120−123 and CVD 124 were also employed. In the meantime, the characters of BP highly depend on its thickness, such as photoluminescence (PL) and Raman spectra.119,120 BP was demonstrated to own the potential to be applied in photoelectronics,117 covering photodetectors, solar cells,125 and photosensitizers for singlet oxygen generation.126 Combining experiments with theoretical simulations, many properties of BP have been studied, such as electronic structure, which depends on the stacking order and the lattice strain,127−129 anisotropic magnetic behavior,124 6240


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Figure 4. Synthesis and characterization of the 2D transition metals. (a) Schematic formation process of the partially oxidized and pure Co 4-atomthick layer, respectively. (b) The high-angle annular dark-field scanning transmission electron microscopy (HAADF−STEM) image of Co 4-atomthick layer. Reproduced with permission from ref 39. Copyright 2016 Nature Publishing Group. (c) Schematic illustration of the formation process of Au square sheet. (d) Transmission electron microscopy (TEM) image of ∼2.4 nm-thick AuSSs on a GO surface. Inset: crystallographic models for a typical AuSS with its basal plane along the [110]h zone axis, showing ABAB stacking along the [001]h direction. (e) High-resolution TEM (HRTEM) image of a small region of a typical AuSS oriented normal to [110]h. Reproduced with permission from ref 168. Copyright 2011 Nature Publishing Group. (f) Schematic illustration of the ligand-induced phase change of Au nanoribbon, and HRTEM images of 4H and fcc Au NRB, respectively. Reproduced with permission from ref 42. Copyright 2015 Nature Publishing Group. (g) Low-voltage spherical aberration-corrected TEM (LVACTEM) micrograph of a monatomic Fe layer with the square unit cell. The inset below highlights the interatomic spacing of the square unit cell, and the inset above shows atomic structure of a suspended mono/atomic Fe layer in a graphene pore. Reproduced with permission from ref 41. Copyright 2014 AAAS.

containing rotational mismatched layers interconnected by a shared center.169 Besides, 2D metal materials also have some unique optical absorption and photothermal properties,170 and they are promising to be applied in the electrocatalysis area.39 Generally, the emerging elemental 2D materials are semiconductors, which imply their great potential applications in electronics and photonics. Although some progress has been achieved in this field, there still remain many challenges. One of the greatest difficulties for the synthesis of elemental 2D nanomaterials is to achieve large-scale yield with high quality and high purity. In addition, the synthesis of elemental nanosheets with designed structural features is demanded for specific applications, since the thickness, size, crystal phase, and defects play an important role in determining their physical, chemical, and electronic properties.

preparation. For example, Xie et al. synthesized 4-atom-thick Co layers by using n-butylamine for controlling the growth and assembly direction of cobalt and dimethylformamide for gradually reducing cobalt ions (Figure 4, panels a and b).39 Zhang et al. in situ synthesized dispersible hexagonal closepacked (hcp) Au square sheets (AuSSs) on graphene oxide sheets.168 The AuSSs were synthesized by heating a solution containing GO sheets, 4-mM HAuCl4 and 140-mM 1-amino-9octadecene (CH3(CH2)7CHCH(CH2)8NH2) in a solution of hexane and ethanol (V/V = 23:2) at 55 °C for 16 h (Figure 4c). The as-obtained AuSSs show an edge length of 200−500 nm and a thickness of ∼2.4 nm (∼16 Au atomic layers) (Figure 4d). The hcp structure is indicated in the inset of Figure 4 (panels d and e). Whereafter, they synthesized 4H hexagonal Au nanoribbon (new metastable phase of Au), which can be transformed to a face-centered cubic (fcc) structure via the ligand exchange under ambient conditions, as shown in Figure 4f. They further demonstrated that 4H hexagonal phase of Ag, Pd, and Pt can be readily stabilized through direct epitaxial growth on the 4H Au nanoribbon surface.42 In addition to this, they also reported that the AuSSs could be transformed from hcp to fcc structures via surface ligand exchange. They obtained 3 nm-thick {100}f-oriented fcc Au rather than {111}f basal planes at low energy.43 Different with the synthetic methods mentioned above, Rümmeli et al. reported the first freestanding single-atom-thick iron membranes. They found that these 2D Fe nanomembranes were suspended in graphene pores (Figure 4g). The as-formed square lattice shows the lattice constant of 2.65 Å at the room temperature (The inset in the bottom left of Figure 4g). The discovery broke the cognition that the nature of metallic bonding in the 3D structures of metals prohibits them from existing as a monatomic layer.41 Some researchers also found that the 2D metal had specific stacking structure. Yang et al., demonstrated an anisotropic, Hanoi Tower-like assembly of Pd nanosheets,

2.3. h-BN

h-BN, known as “white graphene”, is an excellent insulator with an extremely large direct bandgap of 5.97 eV.171,172 Boron and nitrogen atoms bond with each other by covalent bonds in each h-BN layer, which is shown in Figure 5a.173 The h-BN could be obtained by exfoliation,174,175 CVD process10,172,176−180 and cosegregation,181 among which CVD tends to be the main strategy due to the good controllability of crystal quality and size. Generally, the CVD growth of h-BN could be achieved with the assistance of metal catalysts containing the pure solid metals (Cu,10,177,178 Pt,179 Ni,176 etc.), solid metal alloys (such as Cu−Ni alloy182), and liquid metals (such as liquid Cu172) or liquid metal alloy (such as liquid Ni-Ga alloys180). It is notable that Jiang et al. obtained the largest h-BN grain of 7500 μm2 on Cu−Ni alloy and Fu et al. obtained self-aligned singlecrystalline h-BN arrays on liquid Cu for the first time, which could be important for integrated electronics.172,182 Recently, the mechanism of CVD growth of h-BN, especially for the nucleation and the formation of h-BN on metal catalysts, 6241


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compared with those on SiO2, which could improve the device performances.184 h-BN could be applied for gate dielectric layers (Figure 5, panels b and c),174,180,185 tunnel barrier layers,186,187 oxidation-resistant coating layers,188−190 and ultraviolet emission devices.191,192 However, the controllable fabrication of large-area and high-quality h-BN on other nonmetal substrates, which is significant for electronics, still needs to be achieved. 2.4. Transition Metal Dichalcogenides (TMDs)

TMDs have gained worldwide attention in recent years and are being heavily researched for use in photoelectronic devices because they show a wide range of electronic, optical, mechanical, chemical, and thermal properties.14 Some TMDs present nonlayered structures such as zinc blende or wurtzite, while layered TMDs are commonly restricted to metals in groups IV−VI and X. Depending on the coordination and oxidation state of the metal atoms, 2D layered TMDs can be semiconductors (MoS2, WS2), semimetals (WTe2, TiSe2), true metals (NbS2, VSe2), and superconductors (NbSe2, TaS2).7 TMDs (such as MoS2, WS2, and WSe2) consist of a sandwiched structure of a transition metal layer between two chalcogen layers (Figure 6a).11 Depending on the atomic stacking configurations, MX2 can form two kinds of typical crystal structures: a trigonal prismatic (2H) phase and an octahedral (1T) phase (Figure 6b).193 Taking MoS2 as an example, in 2HMoS2, each Mo atom is prismatically coordinated to six surrounding S atoms, forming a thermodynamically stable phase. In 1T-MoS2, six S atoms form a distorted octahedron around one Mo atom, which is a metastable phase. Interestingly, phase transition can occur among each phase via intralayer atomic gliding. 2H-MoS2 can be converted into 1T-MoS2 by intercalating Li or K.194,195

Figure 5. Structures and properties of h-BN. (a) Schematic illustrations of the structure of h-BN. Reproduced from ref 173. Copyright 2017 American Chemical Society. (b) Current−voltage curve of h-BN layer to show its excellent insulating nature. Reproduced from ref 180. Copyright 2016 American Chemical Society. (c) Resistivity−gate voltage (Vg) curves of monolayer graphene on h-BN measured at different temperatures. The inset shows the corresponding conductivity−carrier density curve. Reproduced with permission from ref 174. Copyright 2010 Nature Publishing Group.

attracted much attention. Edgar et al. simulated the nucleation and the formation of h-BN on Ni via first-principle based reactive molecular dynamics.183 It could help with the understanding of the role of temperature and the substrates used during h-BN growth. The h-BN layer could be an ultraflat substrate with lower electron−hole charge fluctuations as

Figure 6. Structures and properties of 2D TMDs. (a) Three-dimensional (3D) model of the MoS2 crystal structure. Reproduced with permission from ref 11. Copyright 2011 Nature Publishing Group. (b) Unit cell structures of 2H-MX2 and 1T-MX2. Reproduced from ref 193. Copyright 2015 American Chemical Society. (c) The dependency of the properties of TMDs on the number of layers. Reproduced from ref 199. Copyright 2015 American Chemical Society. (d) A schematic view of a monolayer MoS2 photodetector and time-resolved photoresponse of a monolayer MoS2 photodetector. Reproduced with permission from ref 202. Copyright 2013 Nature Publishing Group. (e) A schematic illustration of a split-gated WSe2 p−n diode and the scanning photocurrent map of a WSe2 p−n diode showing pronounced photocurrent generation localized to the junction. The orange dashed lines outline the source and drain contacts, whereas the blue dashed lines outline the back gates. Reproduced with permission from ref 203. Copyright 2014 Nature Publishing Group. (f) Atomic force microscopy (AFM) image of two devices fabricated on a 3.5 nm 2H-TaS2 flake. The full color scale of the topograph corresponds to a height of 100 nm. Below the image is the line profile of the flake taken at the location of the white dotted line. (g) I−V characteristics as a function of temperature for a bulk-like 14.9 nm device and the resistance (zero bias numerical derivative) versus temperature for the 14.9 nm device. (h) I−V characteristics as a function of temperature for a 5.8 nm device and the resistance versus temperature for the 5.8 nm device. Reproduced with permission from ref 206. Copyright 2016 Nature Publishing Group. 6242


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Figure 7. 2D α-Mo2C single crystal and its superconductivity property. (a) Typical structure of TMCs, fcc crystal structure, and hcp arrangement. Reproduced from ref 219. Copyright 2005 American Chemical Society. (b) Typical optical image of ultrathin α-Mo2C crystals. (c) Atomic-level HAADF-STEM images of the α-Mo2C sheet. (d and e) 2D Superconducting characteristics of ultrathin α-Mo2C crystals. (f) Berezinskii−Kosterlitz− Thouless (BKT) transition of 2D ultrathin α-Mo2C crystals. (g) Thickness dependence of superconductivity of 2D ultrathin α-Mo2C crystals. Reproduced with permission from ref 231. Copyright 2015 Nature Publishing Group.

including the photovoltaic effect and light-emitting properties in these atomically thin materials.203−205 Besides the semiconductive TMDs, recently discovered superconductive ones caught the most attention. Coronado et al. reported the 2D superconductivity in atomically thin 2HTaS2 layers (Figure 6, panels f−h).206 They found that the superconductive transition temperature (Tc) of this material was strongly enhanced from the bulk value as the thickness was decreased, which provided evidence of an unusual effect of the reduction of dimensionality on the properties of a superconducting 2D crystal and unveiled another aspect of the exotic manifestation of superconductivity in atomically thin TMDs.

Due to quantum confinement, interlayer coupling and surface effects, monolayer and few-layer TMDs exhibit unique exciting properties which are absent in their bulk counterparts. For example, bulk semiconducting trigonal prismatic TMDs possess an indirect bandgap, but when thinned down to monolayer thickness, they exhibit direct electronic and optical bandgaps,196 which leads to an enhanced PL intensity.197 More importantly, valley polarization has also been observed in monolayers of MoS2, an effect that is crucial for engineering valley-based electronic and optoelectronic devices.198 Importantly, the number of layers in TMDs remarkably affects their properties (Figure 6c).199 Heinz et al. isolated thin MoS2 layers (from 1 to 6) by the adhesive tape method and found that monolayers exhibit PL around 1.84 eV, which is related to the smallest direct transition at the K point in the Brillouin zone; for bilayers and thicker MoS2 structures the PL is quenched.196 In addition, the second-order nonlinear optical responses strongly depend on the number of layers. The centrosymmetry of bulk MoS2 crystals prohibits second-order nonlinear optical processes. However, for a monolayer or few-layer MoS2 film with an odd number of layers, the inversion center is removed, resulting in a strong second-order nonlinear optical response.200 Other semiconducting TMDs (e.g., WS2, WSe2, and MoSe2) with the same trigonal prismatic structure as MoS2 also exhibit direct bandgaps as single molecular layers and indirect gaps as bi- or multilayers.6 With an intrinsic bandgap typically in the range of 1−2 eV, 2D TMDs can overcome the key shortcomings of graphene for electronic applications, the lack of bandgap and low on/off current ratio and current saturation characteristics. On the basis of exfoliated MoS2 flakes as the semiconductor channels, FETs with an on/off ratio as large as 8 orders of magnitude have been achieved.11,201 The unique tunable electronic structure of 2D TMDs can also open up many other exciting opportunities, including highly sensitive photodetectors (Figure 6d).202 Yao et al. demonstrated the ambipolar behavior of the TMDs, which can be explored for the creation of gate-tunable atomically thin lateral p−n diodes by using locally patterned gate electrodes to electrostatically create a spatial modulation of the doping profile (Figure 6e).203 It can lead to many exciting functions

2.5. Transition Metal Carbides (TMCs)

Transition metal carbides (TMCs) are produced by incorporating carbon atoms into the interstitial sites of their parent metals, which typically include all 3d elements and 4d/5d elements of group III−VI B early transition metals. In general, TMCs of early transition metals possess unique physical and chemical properties.207−212 For example, TMC compounds combine the physical properties of three different classes of materials: covalent solids, ionic crystals, and transition metals. As a result, TMC compounds often demonstrate the extreme hardness of covalent solids, the high melting temperature of ionic crystals, and the excellent electric and thermal conductivity of transition metals. The unique combination of the desirable physical properties has led to the commercial applications of TMCs as cutting tools and hard-coating materials.209 In addition, TMCs are characterized by many unique and intriguing catalytic properties and electrochemical energy storage potential.213 What’s more, many members of this family are metallic and are demonstrated to own superconductivity, such as Mo2C, W2C, WC, TaC, NbC, and so on.207,211,214−216 In general, TMCs exhibit wide application potentials in electronics, catalyzation, and energy storage and should be a key kind of material in the future. It is indicated that in the early transition metals, the formulas MX and M2X are prevalent, in the later transition metals the stoichiometry shifts to M3X.211 The groups IV and V TMCs generally form monocarbides (TiC, ZrC, HfC, VC, NbC, and 6243


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Figure 8. Heterostructures constructed by TMCs and graphene. (a) The mechanism of the formation of 2D i-WC−G on a Ga−W substrate. (b) Typical optical microscope (OM) image of i-WC−G heterostructure. (c) Raman spectra sampled at the regions marked in (b). Reproduced with permission from ref 233. Copyright 2017 Elsevier Ltd. (d) The schematic showing the growth of vertical Mo2C−G heterostructure. (e) Typical OM image of vertical Mo2C−G heterostructure. (f) Raman spectra sampled at the regions marked in (e). Reproduced with permission from ref 234. Copyright 2017 John Wiley & Sons, Inc.

synthesized 2D α-Mo2C by CVD method from methane on a substrate of Cu and Mo foil.231 The as-synthesized samples have perfect crystalline quality with little observable defects, making them structurally robust and chemically stable, as seen in Figure 7 (panels b and c). The crystals are a few nanometers thick, over 100 μm in size, and very stable under ambient conditions. They show 2D characteristics of superconducting transitions that are consistent with Berezinskii−Kosterlitz− Thouless behavior and show strong anisotropy with magnetic field orientation; moreover, the superconductivity is also strongly dependent on the crystal thickness, as seen in Figure 7 (panels d−g). Such a CVD strategy even can be extended to the fabrication of other high-quality 2D TMC crystals, such as ultrathin WC and TaC crystals. They also investigated the domain structure of α-Mo2C and the influence of domain boundaries on their properties. The microstructure of chemical vapor deposited high-quality 2D αMo2C superconducting crystals of different regular shapes including triangles, rectangles, hexagons, octagons, nonagons, and dodecagons was studied.232 Except for rectangular and octagonal crystals, the C atom sublattices are composed of three or six domains with rotational-symmetry and well-defined line-shaped domain boundaries because of the presence of three equivalent off-center directions of interstitial carbon atoms in Mo octahedra. There is very small lattice shear strain across the domain boundary. In contrast to the single sharp transition observed in single-domain crystals, transport studies across domain boundaries show a broad resistive superconducting transition with two distinct transition processes due to the formation of localized phase slip events within the boundaries, indicating a significant influence of the boundary on 2D superconductivity. These findings provide new understandings on not only the microstructure of 2D TMCs, but also the intrinsic influence of domain boundaries on 2D superconductivity. The constructions of TMCs and graphene have also been reported by Fu et al. A liquid metal solvent based cosegregation (LMSCS) strategy was proposed to effectively promote the simultaneous segregation of W and C atoms. An in-plane WC− graphene heterostructure (i-WC−G) was fabricated in one step, as seen in Figure 8 (panels a−c).233 In addition to the inplane heterostructure, the vertical heterostructure composed by TMCs and graphene was constructed by Loh et al.234 They developed the one-step synthesis of 2D Mo2C-on-graphene film by molten Cu-catalyzed CVD, as seen in Figure 8 (panels d−f). The underlying graphene can serve as a diffusion barrier to the phase-segregated Mo, leading to the thin-layer Mo2C

TaC) with a fcc crystal structure.217 Similar to the NaCl structure, these carbides are characterized by two interpenetrating metal and carbon fcc lattices. In comparison, group VI metals (Cr, Mo, W) do not always produce stable monocarbides. The most commonly studied group VI TMCs include Mo2C, W2C, and WC. In general, the structures of group VI TMCs are much more complex than their groups IV and V counterparts. For example, the α-Mo2C phase has an orthorhombic crystal structure218 with the (100) surface corresponding to the closest-packed surface that can be terminated by either Mo or C atoms, as seen in Figure 7a.219 The arrangement of Mo is only slightly distorted from a hcp arrangement, the crystal structure of α-Mo2C can often be loosely described as a hcp structure with carbon atoms occupying the octahedral interstitial sites. Correspondingly, the closest-packed surface of α-Mo2C is also reported as hcp(0001). Early research about TMCs was primarily focused on polycrystalline carbide thin films or nanoparticles (NPs) or bulk single crystals by magnetron sputtering, solid-state reactions,220 and ball milling.221 It is important to point out that the surface structure of TMCs is often different from those of the bulk materials. Undoubtedly, the two dimensionalization of such materials will be of significant interest, especially for the electronic devices. Up to now, among the TMCs with various structures, the most actively studied ones are 2D-layered carbides. As the most common strategy, they can be prepared by an exfoliation method. Besides, functionalized TMCs layered structure named “MXenes” was reported.222,223 MXenes can be generated by selectively etching a certain element from Mn+1AXn phases.222−226 In Mn+1AXn phases, M represents an early transition metal, A is related to a main group (mostly group III A and IV A) element, X is C, and n = 1, 2, or 3.227 The procedure for synthesizing MXene carbide involves exfoliation coupled with etching.228,229 During this process, the relatively strong metallic bonds between M and A that form the corresponding Mn+1AXn phases are replaced by weaker hydrogen bonds (A layers and replacement by OH, O, or F) at room temperature using aqueous HF as the etching agent.222,230 Various carbides with layered structures such as Nb2C-, Ti2C-, Ti3C2-, V2C-, and Nb2C-based materials can be successfully obtained.226 However, they are not intrinsic 2D TMCs. Currently, it still remains a challenge to synthesize highcrystalline 2D TMCs, thus their physicochemical properties are limited by defects and surface terminal groups due to lack of large and high-quality crystals. In this regard, Ren et al. 6244


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metal (such as Ge, Sn and Pb) composites, and V A group metal (such as Sb and Bi) composites. Due to the distinct structures resulting from the different binding ways between main group metals and nonmetal elements, the corresponding chalcogenides, nitrides, and phosphides would be introduced one by one. 2.7.1. III A Group. III A group metal composites mainly contain chalcogenides, nitrides, phosphides, and arsenides of Ga and In. Their chalcogenides have been widely studied for their semiconducting properties. Due to the valence of group III A metals, M2X3 (M = Ga, In and X = S, Se, Te) should be the general chemical formula. However, for different constructions of atoms, MX could be another chemical formula of the group III A chalcogenides. α-In2Se3, a typical example of group III A chalcogenides in the formula of M2X3, shows a layered hexagonal structure (Figure 9a) and has been

growth. Both of these two heterostructures exhibit excellent interfacial charge transfer and show excellent elctrocatalytic potentials. Besides this, for the remaining transition metals (e. g., group VII B and I B), the formations of carbides are also emerging. Liu et al. revealed a novel transition from graphene to metal carbide on Re(0001) (reverse to that on VIII metals) for the first time.235 Matsuda et al. synthesized monolayer Cu2Si by directly evaporating atomic Si on Cu(111) in an ultrahigh vacuum system.236 They also reported the discovery of 2D Dirac nodal line Fermions in monolayer Cu2Si based on combined theoretical calculations and angle-resolved photoemission spectroscopy measurements, which provides the opportunities to realize high-speed low-dissipation devices. 2D TMCs have been interesting materials owing to their advantageous physical properties, such as excellent conductivity and thermal stability. However, so far, the fabrication of highquality 2D TMCs has been much less developed than that of the graphene and TMDs. Considering the multiphase of them, the large-scale controllable synthesis of 2D TMCs, especially with preferred structure and crystal plane, is urgent to be developed. 2.6. Transition Metal Oxides (TMOs)

Transition metal oxides (TMOs), which once served as the precursors for the synthesis of TMDs, have also been studied recently due to their special structures and properties. The O2− ions in TMOs are strongly polarizable, which could lead to the electrostatic screening and exceptional local surface and interfacial properties, such as Mott insulators.237 Layered TMOs, such as MoO3,238−242 WO3,243 Ti1−δO2,244 TaO3,245 and V2O5,246,247 can be obtained via exfoliation. Thin-layered TMOs are air- or water-stable due to the oxygen-terminated basal surface. 2D TMOs could be obtained by exfoliation, layerby-layer oxide epitaxy, MBE, pulsed laser deposition (PLD), atomic layer deposition (ALD), and CVD. Unlike TMDs and TMCs, TMOs exhibit great potentials in the future electronic applications, such as high κ dielectrics (such as HfO2,248 TiO2,249 and Ta2O5248), high mobility channels,250 and resistive switching components (such as memristors).251,252 As the most important dielectric, HfO2 films have been obtained through a room-temperature oxidation in a Ga liquid metal reaction environment very recently,253 showing atomic-level thickness and exhibiting excellent quality of dielectric. Moreover, the obtainment of 2D free-standing TiO2 has also been reported via a solvent-engineering route, which could absorb visible and near-infrared radiation (NIR) light and show good photocatalytic performances.254 However, the synthesis of 2D TMO highly crystalline films or single crystals still needs to be improved, mainly in the uniformity and crystal size.

Figure 9. Structural illustrations and electrical properties of 2D chalcogenides and nitrides of III A group metals. (a) Structure of αIn2Se3. Reproduced from ref 256. Copyright 2015 American Chemical Society. (b) Photograph and (c) optical image of In2Se3-based photodetector array. (d) Photoresponse of multiple In2Se3 photodetector channels at source−drain voltage (Vds) of 0.1 V and light power density of 4 mW cm−2. Reproduced with permission from ref 258. Copyright 2015 Nature Publishing Group. Schematic illustrations of (e) GaSe layers and (f) GaSe-based photodetector. (g) Photocurrent−Vds curves of photodetectors illuminated with different wavelengths. Reproduced from ref 261. Copyright 2012 American Chemical Society. (h) HAADF-STEM cross-sectional image of 2D GaN on SiC(0001) encapsulated by graphene. (i) I−V curve of graphene/2D GaN/SiC, collected with conductive AFM (the inset). Reproduced with permission from ref 271. Copyright 2016 Nature Publishing Group.

2.7. Main Group Metal Composites

synthesized via van der Waals epitaxy, physical vapor deposition (PVD), and CVD process.255−257 As a semiconductor, α-In2Se3 could be widely applied in photodetectors (Figure 9, panels b− d) and strain sensors.255−258 The difficulty of synthesizing these semiconductors, such as M2X3, is to control the target phase specifically due to the coexistence of complex phases.256,259 As for MX, gallium or indium monochalcogenides are also layered hexagonal 2D materials, which can be recognized as a double Ga or In layer intercalated in two layers of chalcogen to form the structure as X−M−M−X, as shown in Figure 9e.260−262 Group III A monochalcogenides could be obtained via exfoliation,261,263,264 physical vapor transport (PVT)265 and CVD process.266−268 These materials have excellent prospect in

Except for 2D composites based on transition metal, recent studies have reported a great deal of 2D main group metal composites and exhibited their potential applications in electronics and optoelectronics. For the main group metal composites, the electronic density of states (DOS) are different from those of transition metal composites since their valence electrons in s or p orbitals are nonlocalized. Thus, 2D main group metal composites could have different energy bands and exhibit unique physicochemical properties. Here, to facilitate the introduction, 2D main group metal composites are classified according to the groups of the metal, including III A group metal (such as Ga and In) composites, IV A group 6245


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the electronics263 and photodetector applications (Figure 9, panels f and g)261 due to their special band structures.260,269 Except for hexagonal structures, group III A monochalcogenides also have monoclinic structures and other phases which may have anisotropic semiconducting properties and different band structures.265,270 Nevertheless, the properties and new synthesizing strategies of group III chalcogenides still need further exploration. The 2D nitrides, phosphides, and arsenides of III A group metals have also been successfully synthesized, such as GaN, InP, GaP, and GaAs. It should be noted that their corresponding 3D counterparts were regarded as III−V group semiconductors due to their big bandgap, high breakdown electric field, and high thermal conductivity. Their property transformation in the 2D limit will be worthy of great attention. Taking GaN as an example, the planar structure exhibits an indirect Eg of 4.12 eV, while the buckled structure maintains a direct Eg of 5.28 eV. However, traditional exfoliation strategy is not suitable for the fabrication of the thin layers of these nonlayered structure, indicating the necessity of developing new synthesis methods. Robinson et al. developed the synthesis of 2D GaN via a migration-enhanced encapsulated growth technique utilizing epitaxial graphene.271 As shown in Figure 9h, GaN bilayer was clearly observed by cross-section STEM− HAADF image. The I−V characteristic curve also demonstrates its potential as a Schottky barrier layer (Figure 9i). In addition, Kim et al. reported the remote homoepitaxy technique to enable any type of semiconductor film (such as GaAs, InP, and GaP) to be copied from underlying substrates through 2D materials and the resultant epilayer to be rapidly released and transferred to a substrate of interest.272 The difficulties of the growth of 2D III A group nitrides, phosphides, and arsenides lie in the precise layer number control and the high crystallinity. Therefore, the applications of 2D nitrides, phosphides, and arsenides of III A group metals still face great challenges. 2.7.2. IV A Group. For IV A group metals, their chalcogenides have been focused on recently, containing monochalcogenides and dichalcogenides, which are distinctly different from each other in structures. Group IV A monochalcogenides (MX, where M = Ge, Sn, Pb and X = S, Se, Te) are isoelectronic with BP,260 which have similar structures with BP (Figure 10a).273 They could exhibit anisotropic electrical properties (Figure 10, panels b and c), electrical pulsing-induced inverting polar domains and other special thermoelectric and electromagnetic properties.273−276 Moreover, Hu et al. reported the anisotropic performances of polarized measurement for 2D GeSe, which has the capacity with the integrated short-wave NIR optical applications for polarization detection.277 That is, the anisotropy of monochalcogenides of IV A group metals could be the key point for relative applications. As for the dichalcogenides, the structures are similar to 1T phase TMDs.278 SnS2 and SnSe2, typical examples of group IV dichalcogenides, have already been obtained via CVD279,280 and vapor mass transport (VMT)278 process, and they could be widely applied in the photoelectronics.278−280 Moreover, Ozkan et al. reported the transition between SnS and SnS2, which could be achieved by controlling the temperature.281 It provides with the possibility of the transformation between monochalcogenides and dichalcogenides. Group IV A chalcogenides may have a good prospect in the electronics and optoelectronics, which still need further research. Additionally, 2D IV−V A group materials may exist, such as GeAs2, calculated to be a 2D semiconductor with

Figure 10. Structural illustrations and electrical properties of monochalcogenides of IV A group metals. (a) Different views of the structure of SnS. Temperature-dependent (b) carrier mobility and (c) conductance of SnS-based devices with the back gate grounded along zigzag (ZZ) or armchair (AC) directions. Reproduced from ref 273. Copyright 2017 American Chemical Society.

ultralow thermal conductivity and high thermoelectric efficiency.282 The experimental evidence of 2D IV−V A group materials (layered GeAs2 or even nonlayered Si3N4) still remain to be discovered, whether they could be obtained or not. 2.7.3. V A Group. V A group metal (Sb and Bi) chalcogenides are popular materials with special electrical transport properties, which are recognized as 2D TIs. TI, a material with special topological order,283 behaves as an insulator in its interior but permits electrons to move only along the surface due to the existence of conducting states on the surface of the material. The surface states of TIs are symmetrically protected by symmetry.284−288 Compounds with a general formula of A2B3 (where A = Sb and Bi and B = S, Se, Te) could be typical examples of layered TIs. As shown in Figure 11a, a monolayer A2B3 is constructed by five atom layers of B−A−B−A−B via ionic bonds, but A and B atoms could be different in the same layer like Bi2TeSe2 and Bi2Te2Se, which results in the variety of A2B3 compounds.289,290 2D A2B3 TIs could be obtained via various methods, containing hydrothermal growth,290 solvothermal growth,289,291,292 exfoliation, 293 vapor−solid growth, 29,294 vapor−liquid−solid growth,295 CVD synthesis,296 van der Waals epitaxy,258,297,298 and MBE growth.299 Even so, monolayer A2B3 TIs could be extremely difficult to achieve, which would have distinct charge transport behaviors from those of few-layer 2D A2B3 TIs. 2D A2B3 TIs have potentials in the application of flexible transparent electrodes,298,300 FETs,291,301 efficient photodetectors,258 strong nonlinear terahertz response devices,302 and high-temperature superconductors.303 Some 2D TIs could exhibit highly tunable chemical potential, pronounced weak antilocalization effect,29 emergent surface superconductivity (Figure 11b),304 and other surface electron transport or optical effects.12,305−308 Various properties of 2D A2B3 TIs are mainly attributed to the difference between their surface and interior. Moreover, the intercalation of 2D A2B3 TIs is also studied recently to obtain a variety of new ordered and disordered structures [including the super lattices and the charge density waves (CDWs)].309 Still, synthesis strategies and potential applications of such a large family of A2B3 TIs remain exploration for their excellent properties.310 6246


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s-triazine structure.173,313 The layered g-C3N4 could be obtained via both bottom-up and top-down methods.314,315 The bottom-up synthesis refers to condensation reactions of different precursors,313,316 and top-down strategies mainly refer to liquid exfoliation317−319 and thermal oxidation320 of bulk gC3N4. Due to the abundant active sites, g-C3N4 could be used in photocatalytic reactions,314,317,319−322 oxidation reactions,323 oxygen reduction reactions (ORR),324 and bioimaging.318 Although g-C3N4 has been demonstrated to be one of the most important catalysts for chemical reactions,316 the research concerned about its application in electronics and other devices is still limited to theoretical study325 since it is still a great challenge to obtain its monolayer single crystals. Novel strategies for synthesizing 2D g-C3N4 are urgently needed to be developed. Other 2D materials, such as transition metal polychalcogenides (e.g., ZrTe5,326 TiS3327), metal phosphorus trichalcogenides328 (e.g., Ti2PTe3329), PbI2,330 Ta2NiSe5,331,332 and βCu2S,333 have also been obtained and applied in many fields. On the basis of PbI2, 2D perovskites can be obtained through conversion reaction, which present great potential for applying in the electronics and optoelectronics.334,335 There are also some other 2D materials whose structures and properties have been predicted by theory, such as 2D phosphorus carbide,336 MoN2,337 and FeB2.338 For such a large 2D material family, its members and their novel properties remain to be discovered, and they would offer an excellent platform for fundamental research.

Figure 11. Structures and electrical properties of V A group metal chalcogenides. (a) Structural illustrations of Bi2TexSe3−x with different x. Reproduced from ref 289. Copyright 2015 American Chemical Society. (b) Temperature-dependent resistivity of a thin Sb2Te3 crystal. The critical superconducting transition temperature (Tc) is 9 K. Inset shows Hall contact configuration used in the measurement. Reproduced with permission from ref 304. Copyright 2015 Nature Publishing Group. (c) Crystal structure of Bi2O2Se. (d) Temperaturedependent Hall mobility (μHall) and carrier density (n) in a Bi2O2Se nanoplate. Inset shows the optical image of a Hall-bar device. Reproduced with permission from ref 311. Copyright 2017 Nature Publishing Group.

When the chalcogen atoms of 2D Bi2B3 TIs (B = S, Se, and Te) are partly substituted by oxygen atoms, a novel series of materials could be fabricated. Peng et al. for the first time reported the synthesis of Bi2O2Se on fluorophlogopite mica through a CVD process.311 As shown in Figure 11c, the [Bi2O2]n2n+ layer was sandwiched by [Se]n2n− layers.312 The asobtained Bi2O2Se single crystal exhibited high electron mobility (Figure 11d), thickness-dependent bandgaps and excellent stability in the air, which make it an excellent candidate in highspeed and low-power electronics.311

3. DESIGNING STRATEGIES FOR THE BUILDING BLOCKS Although the springing of 2D materials has been continuously refreshing and enriching their physical properties, we still need to be aware that a material with fixed properties may not meet the demands for the versatile applications. Due to the unique crystal structures, the physical and chemical properties of 2D materials can be effectively tuned through different strategies, leading to the achievement of the design of the 2D material building blocks for the specific electronic devices. In consideration of the quantum confinement effect, the dimension controlling becomes an indispensable research area in the 2D materials’ synthesization. Meanwhile, chemical composition tunings, such as doping, alloying, atomic vacancy, and physical field tunings via electric field and lighting, are classic tuning approaches in the materials’ properties engineering. Moreover, structure tunings, such as phase transition, edge structure reconstruction and crystal structure deformation, could help with regulating properties of 2D materials.

2.8. Others

Except for elemental 2D materials, h-BN, TMDs, TMCs, and main group metal composites, there are still many other 2D materials having been synthesized recently due to their diversified structures and properties. Graphitic carbon nitride (g-C3N4) can be recognized as the graphite framework with the C atoms partially substituted by N atoms. The g-C3N4 layer could be formed by sp2-hybridized C and N atoms. As shown in Figure 12 (panels a and b), there are two different structures, referring to s-triazine structure and tri-

3.1. Dimension Controlling

On the basis of quantum confinement effect, 2D crystals present obviously distinct characters compared with the bulk even few-layer ones owing to their atomic-thickness structure.9 Further reducing the dimension of them, such as synthesis of nanoribbons and quantum dots, is also an effective approach to design the single building block. In addition, fabrication of large-scale domains of single 2D crystals is also necessary to promote their further practical applications. 3.1.1. Along the z Direction. For a large number of 2D materials, their electron structures vary from different thicknesses due to van der Waals interlayer coupling. Here, controllable fabrication of 2D materials with expected layer

Figure 12. Structural illustrations of (a) s-triazine-constructed and (b) tri-s-triazine-constructed g-C3N4. Reproduced from ref 173. Copyright 2017 American Chemical Society. 6247


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formation of metal carbides and suppressed upward precipitation of the carbons, respectively. Here, Mo component could trap all of the dissolved excess carbon species in the form of molybdenum carbides, which are chemically stable throughout the high-temperature growing process, as well as Ni component was employed as high-efficiency catalyst and the growth of graphene becomes governed by a self-limited surface catalytic process. This strategy allowed the synthesis of strictly singlelayer graphene with 100% surface coverage and an extremely high tolerance to variations in experimental conditions (such as growth time, temperature, and cooling rate), which can be extended to any alloy that contains an active catalyst component. Fu et al. developed a facile way to fabricate strictly monolayer graphene with 100% surface coverage using highly carbonsoluble iron-group metals, which are characterized in high catalytic efficiency and low cost.346 A peculiar carbon barrier, a chemically stable antiperovskite layer, was established in this strategy by introducing gallium to react with iron-group metal and then to seal the passageway of carbon segregation during cooling reasonably. The schematic illustration of this method was presented in Figure 13b and the typical OM image was displayed in Figure 13c. The simplicity and scalability of utilizing iron-group metals with high catalytic ability and low cost may greatly facilitate future graphene research and industrial applications. In addition to this, Fu et al. also offered a simple method to grow uniform monolayer graphene via utilizing liquid metal catalysts.347,348 Here, the underlying liquid bulk can act as a container to buffer the excess carbon supply, thus making the carbon precipitation process blocked by the frozen metal lattices. Therefore, the self-limited growing behavior with simplicity, scalability, and a large growth window was achieved on this quasi-atomic smooth liquid metal surface. The use of liquid metals provides an attractive solution to obtain uniform graphene. 3.1.1.2. Growth of Bilayer Graphene. Beyond monolayer graphene without bandgap, bilayer graphene has an electric field induced bandgap of up to 250 meV.349 The unique abilities of bandgap tuning and electrical properties make bilayer graphene a promising candidate in the electronic device construction,350 which also promotes the growth technique of it. However, the precipitation of carbon atoms and uncontrollable segregation of them during cooling make the carbonsoluble catalyst (such as Ni and Co) hard to grow uniform bilayer graphene. Most works are devoted to obtaining uniform bilayer crystals via overcoming the self-limited restriction. Zhong et al. first reported the synthesis of wafer-scale bilayer graphene films on Cu via CVD method.351 The key parameter for this growing process is the slow cooling process, which is different from the reported self-limited growing process.340,342 Liu et al. offered a scalable strategy to produce high quality bilayer Bernal graphene via a layer-by-layer epitaxy CVD process utilizing Cu.352 The self-limited effect is broken through the introduction of a second growing process. Here, fresh Cu foil placed upstream was introduced as the catalyst, and uniform monolayer graphene on the Cu foil was placed downstream as the substrate. Carbon radicals were then transported downstream and deposited onto the existing monolayer graphene film during the growth process. In this way, the coverage of Bernal bilayer regions could reach 67% before further optimization. Duan et al. reported an approach to produce larger-area high-quality AB-stacked bilayer graphene on Cu.353 A high H2/CH4 ratio was introduced in this low-

number is the fundamental issue to construct the available circuit. 3.1.1.1. Growth of Monolayer Graphene. After being exfoliated from graphite,1 great efforts have been devoted to the controllable growth of uniform monolayer graphene, the first fabricated and widely studied 2D materials because of its unique properties,2 via CVD method, which has been suggested as the most controllable and promising approach to grow 2D materials.339 Ruoff et al. proposed the first self-limited growth behavior of monolayer graphene.340 Here, Cu foil with low carbon solubility was employed as the catalyst to grow monolayer graphene, where the peculiarity of Cu appears to help make this growth process self-limiting. It is proved that graphene growth on Cu is self-limited and monolayer graphene structure can be yielded no matter the growth time or if the thickness of the Cu foil is varied. According to these observations, it can be concluded that graphene may be grown by a surface-catalyzed process rather than a precipitation process, as reported by others for Ni.341 Notably, two distinguishing growing processes were then proved to be assigned to Cu and Ni, respectively, via carbon isotope labeling strategy.342 However, bilayer and even multilayer graphene can still be obtained on Cu by using a lower cooling rate and an elevated growth pressure or increased concentrations of carbon sources.343,344 The dissolution of carbon atoms into bulk metals and their subsequent nonequilibrium precipitation are the crucial steps that govern the uniformity of CVD graphene. A suitable catalytic metal system was therefore designed to control these processes. A facile control of graphene layer thickness could be achieved via utilizing a Ni−Mo binary alloy by Liu et al.345 The schematic drawing of the as-designed binary alloy for CVD growth of monolayer graphene by suppressing carbon precipitation was displayed in Figure 13a. The numbers from 1 to 4 represent the elementary steps in the Ni−Mo CVD process, including surface catalytic decomposition of the carbon source and reconstruction of the carbons, downward dissolution of the carbon atoms into the bulk metal, the

Figure 13. Controllable growth of monolayer and bilayer graphene. (a) Schematic illustration of a designed binary alloy for CVD growth of single-layer graphene by suppressing carbon precipitation. Reproduced with permission from ref 345. Copyright 2011 Nature Publishing Group. (b) Schematic illustration of the monolayer graphene growth process via introducing a compact antiperovskite layer as the carbon barrier and (c) the as-grown monolayer graphene. Reproduced from ref 346. Copyright 2015 American Chemical Society. (d) Schematics of the bilayer growing dynamic processes on the exterior surfaces of the Cu pocket and (e) the as-grown bilayer graphene domain. Reproduced with permission from ref 354. Copyright 2016 Nature Publishing Group. 6248


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pressure CVD process, which enabled to break the self-limited growing behavior and continue the growth of bilayer graphene. Bilayer graphene with a high AB stacking ratio (up to 90%) and high coverage (up to 99%) could be obtained. However, the synthesis of large-scale single crystals is necessary in the practical applications for eliminating the unfavorable effect of the grain boundary on the transport properties. Therefore, an oxygen-assisted strategy was developed to obtain 0.5 mm size, Bernal-stacked bilayer graphene single crystals by Hone et al.354 An oxygen-rich Cu (OR-Cu) pocket was used to carry out the bilayer graphene growth process. The low diffusivity of CH4 on the interior surface, which was limited by the crimped edge of the pocket, suppressed the nucleation and growth rate of graphene and large bilayer graphene domains were then achieved after a long-time growing process (Figure 13d). It is said that the growth of the first layer is followed by the surfacemediated mechanism and the second layer is produced by the diffused carbon atoms from the interior, as illustrated in Figure 13e. 3.1.1.3. Growth of Other 2D Materials with Uniform Layer Number. Known from semimetal graphene, monolayer TMDs become principal building blocks in various optoelectronic devices on account of its direct bandgap.196,355 Thus, the controllable growth of atomic TMDs crystals in a self-limited way is the first priority to enable the practical applications. Cao et al. utilized the low pressure CVD system to realize the growth of uniform MoS2 films.356 The layer numbers of the films were controlled by the partial pressure of the MoCl5 precursors. Liu et al. realized the growth of uniform mono-, bi-, and trilayer MoS2 films by utilizing a plasma-treated substrate.357 The plasma surface treatment could lead to the Si−(O or OH)4 bonding, thus providing a higher surface reactivity and a better lattice matching for hexagonal MoS2 growth. The layer numbers of the films were therefore controlled by the treatment time of the substrates. Similarly, Zhai et al. reported the growth of monolayer WS2 films via using a pregrowth substrate silanization treatment.358 First, the surface of the Si/SiO2 substrate was decorated with 3aminopropyltriethoxysilane and a thin film of tungstic acid (WO3·xH2O) solution was then spin coated on the substrate as the precursor. The silanization reaction of the substrate was introduced to effectively improving the dispersion of WO3 precursors. Therefore, large-area, uniform and continuous WS2 film was achieved. However, the growth of monolayer TMDs films with waferscale size as well as spatial homogeneity and high electrical performance remains a great challenge. Park et al. achieved the preparation of high-mobility 4-in. monolayer MoS2 films, which were grown directly on insulating Si/SiO2 substrates, with excellent spatial homogeneity over the entire films.359 Here, they used the metal−organic CVD (MOCVD) technique and the concentration of each reactant was precisely controlled during the entire growth time (Figure 14a). In addition, they exhibited the batch-fabricated MoS2 FET circuits on a 4-in. Si/ SiO2 wafer (Figure 14b), which indicated the possibility of immediate batch fabrication of TMD-based integrated circuitry devices on a technologically relevant multi-inch wafer-scale. However, the grain boundaries of the films would strongly decline the properties of the TMDs crystals and it would be the stumbling block of TMDs in their further applications. Fu et al., therefore, reported the Cu-assisted self-limited growth strategy to fabricate WSe2 single crystals with atomic thickness.360 As Figure 14c illustrated, Cu atoms were introduced in the CVD

Figure 14. Controllable growth of monolayer TMDs. (a) The diagram of the MOCVD growth setup and (b) the photograph of the synthesized and patterned monolayer MoS2 film on a 4-in. Si/SiO2 wafer. Reproduced with permission from ref 359. Copyright 2015 Nature Publishing Group. (c) Schematic illustration of the Cu-assisted self-limited growth process and (d) the OM image of the as-prepared strictly monolayer WSe2 crystals. Reproduced with permission from ref 360. Copyright 2016 John Wiley & Sons, Inc.

growth process to occupy active sites positioned at the WSe2 surface, which was demonstrated by DFT calculations. Thus, the W source can only adsorb on the edge of monolayer WSe2, which could achieve the size extension growth instead of layer accumulation. Monolayer WSe2 flakes with large-area distribution were therefore presented (Figure 14d), which demonstrated the self-limited growing behavior. However, there are multiple factors which complicate the growing kinetics of TMDs and contribute to the inevitable formation of the multilayers. The core issue lies in that the overflowing precursors, which makes the mass transport process the ratelimiting step and leads to the formation of thermodynamically stable multilayers, is hardly to be suppressed. Aiming at the blank, Fu et al. also proposed a substrate-trapping strategy (STS) to achieve the growth of monolayer TMDs crystals over the whole substrate surface.361 Here, the key factor was employing a designed substrate, which was liquid soda-lime glass, to trap the overflowing Mo source. That is, a kinetically controlled growing process was driven and strictly monolayer MoS2 crystals with almost 100% ratio was easily achieved. Reasonable excellent tolerance to variations in growing parameters of STS was also proved and, in addition, the universality of this strategy was also demonstrated by the realization of strictly monolayer WS2 crystals. As a high-quality dielectric material with remarkable chemical stability and oxidation resistance, h-BN has become a promising building block in numerous applications, especially being an ideal substrate for 2D electronics. Therefore, improving synthetic methods for producing high-quality uniform h-BN film is of great importance to exploit its prospective applications. Lee et al. reported the growth of large-area monolayer h-BN on Pt foil.362 The self-limited growing behavior was thought to be induced by the catalytical decomposition process of the borazine source on the Pt surface, where the associating precipitation was forbidden. Else, a more efficient catalytic effect of Pt(001) surface was encouraged, compared to the Pt(111) surface in this work. In addition, Kalantar-Zadeh et al. realized the growth of large area 2D GaS with uniform unit cell thickness.363 The highly wettable oxide layer of the Ga metal with uniform thickness, which was obtained in a self-limited oxidation reaction, was utilized as precursors to be sulfurized. Meanwhile, selective 6249


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Figure 15. Growth of large-scale graphene single crystal. (a) The picture of the Cu pocket and the schematic of the CVD growth system and (b) SEM image of the as-grown large-scale graphene domain. Reproduced from ref 376. Copyright 2011 American Chemical Society. (c) DFT-calculated energies of different configurations of H attachment and the schematic of graphene edge growth on Cu with the assistance of O. (d) SEM images of smaller graphene domains. Reproduced with permission from ref 379. Copyright 2013 the American Association for the Advancement of Science. (e) Schematic illustration of catalytic growth of single-crystalline monolayer graphene from unidirectional aligned multiple seeds on the hydrogenterminated germanium and the photograph of graphene grown on the wafer. (f) The SEM image of the graphene seeds at the early stage of growth. Reproduced with permission from ref 382. Copyright 2014 the American Association for the Advancement of Science. (g) Schematic drawing of the localized feeding growth process and (h) the optical picture of the 1.5-in. graphene single crystal grown on Cu85Ni15 alloy. Reproduced with permission from ref 381. Copyright 2016 Nature Publishing Group.

patterning of the 2D GaS could also be achieved via controlling the self-limited oxide skin. 3.1.1.4. Layer Thinning of 2D Materials with Precise Layer Numbers. Except CVD growth approaches, layer thinning methods are another way to precisely control the layer number of the as-acquired 2D materials. Tour et al. reported the layerby-layer removal of graphene via sputter-coating with Zn and then dissolving it.364 It is believed that the sputtering process damaged the top layer of graphene, and the acid treatment could then remove the sputtered Zn as well as the damaged carbon layer. Therefore, this method can also be regarded as a lithography approach to pattern graphene via using predesigned Zn patterns. In addition, the authors found that the reported strategy could be applied to graphene oxide, chemically converted graphene, and CVD graphene. Lee et al. reported the laser thinning approach for achieving monolayer graphene.365 It is said that the top graphene layers on a Si/ SiO2 substrate could be etched completely by scanning laser irradiation, whereas the bottom monolayer graphene could remain unetched. In this process, the Si substrate played a crucial role as a heat sink, thus preventing the undesirable damage for the bottom monolayer graphene. Similarly, Steele et al. reported the laser thinning method to obtain monolayer MoS2.366 Here, single layers with arbitrary shapes and sizes down to 200 nm could be achieved. Except laser, ion beam367,368 and plasma369−371 can also be used to thin the few-layered 2D materials. Kim et al. reported the controlled layer-by-layer removal of MoS2 via utilizing ion beam.367 The proposed process was a circulation of Cl-radical adsorption and Ar-ion-beam desorption, whereas the etching was not observed only after Cl-radical adsorption or low-energy Ar-ion-beam desorption. Ni et al. reported the layer thinning of MoS2 via Ar plasma.370 The authors indicated that this reliable approach is easily scaled up, thus making it possible to generate heterostructures or patterns on the nanometer scale. Furthermore, Zhang et al. reported the layer thinning of MoS2 by simply thermal annealing in air.372 The thinning of MoS2 may result from the formation of MoO3 after the annealing, therefore, the thinned MoS2 layer performed p-type characteristics in fabricated FET. In addition, Cheung et al. reported the chemical layer thinning of WSe2 by vapor XeF2.373

It is found that the thickness and the surface roughness of WSe2 is varied with the vapor XeF2 exposure time and the exposure pressure. Thus, the monolayer and bilayer WSe2 could be obtained from thick-layered flakes by selecting appropriate vapor XeF2 exposure times. However, all the layer thinning methods will destroy the original crystal structure and lead to undesired characteristic changing. Generally, the layer number controlling of graphene can be effectively realized via modifying the catalytic substrate since the graphene formation is highly related to the dissolution and precipitation of the carbon atoms in the bulk of the catalysts. While for the other 2D materials, the selection of the precursor is usually regarded as the most effective strategy due to that the growth mainly occurs as the surface reaction. In addition, the layer thinning method provides a postadjusting approach to obtain monolayer flakes from thick-layered materials, but the unavoidable damage of crystal structure may limit their further application. 3.1.2. Along the xy Direction. Synthesis of 2D materials with large-scale single-crystal domains is in line with the construction of practical applications. Meanwhile, synthesis of nanostructured 2D materials is also an effective approach to tune the properties. 3.1.2.1. Growth of Large-Scale Crystals. 3.1.2.1.1. Growth of Large-Scale Graphene Single Crystal. In consideration of the grain boundaries induced retardation of carrier transport,374 it is desirable to grow 2D single crystals with large domains to eliminate those drawbacks. A series of research have been reported to achieve the goal via optimizing CVD growing process. The suppression of nucleation is the most facile approach to achieve the large-scale single crystals of graphene.375 In addition, merging smaller crystals with the same lattice orientation into a large one is also an optional method to meet the requirement. Rouff et al. first yielded graphene single crystals with large-scale size (about 0.5 mm) via utilizing a Cu-foil enclosure, which was displayed in Figure 15a.376 Nucleation sites with a much lower density and crystals with larger domain were presented inside of the enclosure (Figure 15b), whereas uniform graphene films grown on the outside exhibited the reported self-limited behavior.340 The anticipant low density of nucleation sites inside was regarded as 6250


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Figure 16. Growth of other 2D crystals with large domain. (a) Schematic drawing of the CVD process for the synthesis of MoSe2 crystals on molten glass, where the isotropic liquid surface can effectively suppress the nucleation sites. (b) OM image of the as-grown MoSe2 large domains. Reproduced from ref 388. Copyright 2017 American Chemical Society. (c) Schematic drawing of the CVD process for the synthesis of large singlecrystalline h-BN grains on the Cu−Ni alloy foils with low nucleation density and (d) the SEM image of the as-grown h-BN large domains. Reproduced with permission from ref 182. Copyright 2015 Nature Publishing Group.

the single-crystalline surface enables the growth of graphene seeds with the same orientation, which can merge into one grain-boundary-free crystal. The basic idea of the as-proposed monolayer single-crystalline graphene growing method was schematically illustrated in the upside of Figure 15e. The graphene seeds with well-defined orientation at the early stage were presented in Figure 15f. In addition, continual growth could also be realized after a recycling process of the hydrogenterminated Ge(110) substrate, which can be attributed to the weak interaction between the graphene single crystal and the underlying surface, thus enabling a facile nondestructive etchfree transfer process. Up to now, growth of inch-scale graphene single crystals has been realized via several strategies, such as localized feeding technique381 and hydrogen-terminated Ge(110) single-crystal substrate.382 However, those costly approaches still hinder up the yield of wafer-scale graphene single crystals, and a facile and efficient method is still required to promote the industrialized application of graphene. 3.1.2.1.2. Growth of Large-Scale Single Crystal of Other 2D Materials. As other promising 2D building blocks in the electronics industries, monolayer TMDs are also highly required in the growth of large-scale domains.383 Similar to graphene, the suppression of the nucleation is also very important for the large-scale single crystal growth of other 2D materials. Zhang et al. reported the successful growth of large single-crystal MoS2 via an O2 assisted CVD approach at sapphire substrate.384 First, the anisotropic etching effect of O2 toward MoS2385 contributes to suppressing the nucleation density via etching off the unsteady nucleus as well as chemical oxidation sites. In addition to the etching ability, O2 could also help to prevent the poisoning of the MoO3 source resulting from sulfurization and to enable the ongoing growth of large domains via guaranteeing the continuous evaporation of Mo source. However, compared to the fixed etching rate, the continuous decline of the growing rate caused by the enlargement of the domain size, limited the maximum area of MoS2 crystals. Therefore, controllable growth of larger size crystals was still required for 2D MoS2 and other TMDs.386,387 Loh et al. reported the fast growth of millimeter-sized monolayer MoSe2 crystals via utilizing a molten glass as growth substrate.388 The schematic drawing of the CVD process for the synthesis of MoSe2 crystals on molten glass was presented in Figure 16a, and the OM image of the as-grown MoSe2 large domains was displayed in Figure 16b. The isotropic surface of the molten glass could greatly suppress the density of

the result of a lower partial pressure of methane and an equalized Cu vapor environment during the growth process. Otherwise, Zhang et al. reported a nucleation density controlling method via optimizing the surface of Cu foils.375 A long-time pregrowth annealing stage was introduced before the growth procedure to suppress the impurities or defects on the substrate surface, which may act as active heteronucleation sites,377 and submillimeter-sized graphene crystals were therefore realized. Gu et al. also proposed a facile method to obtain millimeter-sized graphene domains via optimizing the Cu foils in a resolidified way.378 The smooth surface of the resolidified Cu foils resulted in a low nucleation density of graphene domains. Furthermore, Ruoff et al. developed an oxygenassisted CVD strategy to fabricate larger single-crystalline graphene on Cu.379 Here, centimeter-scale graphene domains could be achieved on OR-Cu. On the one hand, the oxygen could passivate the Cu surface to substantially suppress the active nucleation sites. On the other hand, it can shift the growth kinetics from edge-attachment-limited step to diffusionlimited step and accelerate the crystal growing rate on account of the C−O attachment at the domain edges, which was demonstrated by the DFT calculations (Figure 15c) and domain shape change (Figure 15d). In addition, this group also reported a Cu-vapor-suppress method to fabricate large-size single-crystal graphene via using Cu tube or stacked Cu foils.380 However, those approaches were still hard to further reduce the nucleation sites and realize the wafer-scale graphene domains. Xie et al. reported the single nucleus-induced inch-sized singlecrystal graphene growing method.381 Here, an optimized Cu− Ni alloy was selected as a growth substrate, which activated the isothermal segregation and then greatly accelerated the growing rate. In addition, a localized feeding strategy was adopted to form the single nucleus. The schematic illustration of the growing process was displayed in Figure 15g, and an optical image in Figure 15h showed the 1.5-in. graphene domain grown on the substrate. In addition to the nucleation control method, utilizing a particular substrate can also enable the fabrication of wafer-scale graphene crystals. Whang et al. reported the growth of monolayer graphene single crystals on a hydrogen-terminated single-crystalline Ge(110) surface.382 First of all, Ge possesses a high catalytic activity to catalytically decompose the carbon source and form graphitic carbon on the surface. In addition, the extremely low solubility of carbon in Ge enables the self-limited growth of strictly monolayer graphene. Especially the well-defined atomic arrangement of 6251


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Figure 17. Fabrication of GNRs. (a) Schematic drawing of the unzipping CNT process and (b) AFM images of the synthesized GNRs. Reproduced with permission from ref 402. Copyright 2010 Nature Publishing Group. (c) Schematic illustration of the process for tailoring of the SiC crystal for selective GNR growth and (d) the cross-sectional HRTEM images of the GNRs templated growth on SiC. Reproduced with permission from ref 413. Copyright 2010 Nature Publishing Group. (e) Reaction scheme of the synthesis process of GNRs from precursors and a STM image of a typical GNR and (f) HRSTM image of the synthesized GNRs. Reproduced with permission from ref 409. Copyright 2010 Nature Publishing Group. (g) Reaction scheme of the synthesis process of the 7−13 GNR heterojunction and (h) HRSTM image of a 7−13 GNR heterojunction. Reproduced with permission from ref 410. Copyright 2015 Nature Publishing Group.

as patterning or etching graphene films, unzipping CNTs, and bottom-up approaches, such as template growth and chemical synthesis. Generally, the former one, which can be attributed to the physical strategy is relatively universal. Patterning graphene films is the most effective method to fabricate graphene nanostructures with controlled width and shape.395−397 Kim et al. first realized the fabrication of graphene nanoribbons (GNRs) via electron beam lithography (EBL) technique, which utilized an e-beam resist as patterned masks.395 GNRs with varying widths running parallel could be obtained successfully. In addition, Duan et al. reported the fabrication of graphene nanomesh via utilizing porous polystyrene film as patterned mask.396 Strano et al. developed a deoxyribonucleic acid (DNA) lithography method, where DNA molecules with self-assembly ability were utilized as the patterned masks, to obtain graphene nanostructures with designed width and orientation.397 However, an etching mask is required to perform this strategy, and it indicates a complicated pre-etch process and a possibility of residuals as well as a limited GNR width.395 Dai et al. first reported the production of sub-10 nm GNRs, via sonochemical cutting, where the as-derived bandgap could be observed at room-temperature electrical operation.398 Here, ultrahot gas bubbles were involved in the sonication process to break the graphene sheets (GSs) into GNRs with various widths. This approach was proved as an effective and simple chemical route to fabricate graphene nanostructures with sub-10 nm width. However, the uncontrollability nature of sonochemical approach restricted its further evolution. Biro et al. developed a STM-based lithography technology to engineer the intrinsic graphene into GNRs with atomic precision.399 The etching of graphene by an STM tip was performed under a constant higher bias potential, which could cut the GNRs into a desired geometry following the moving of tip. The atomic STM tip and an appropriate applied velocity ensure the nanometer-scale controllability of this lithography process. The presented high-resolution STM (HRSTM) images

nucleation sites in the growing process, thus promoting the growth of large-size crystals. Furthermore, the large migration rates of the surface of the molten glass greatly improve the growth rates of large crystalline domains, thus leading to the fast growth of millimeter-sized TMD crystals. As an ideal dielectric layer for 2D material-based devices, hBN has attracted significant attention. Therefore, it is of great importance to synthesize large single-crystalline h-BN grains to avoid the drawbacks brought from the grain boundaries and defects. Teo et al. reported the growth of large single-crystalline h-BN on the electropolished Cu.389 The nucleation density of h-BN could be significantly reduced as well as the domain sizes could increase, where the size of the largest crystal could reach to 35 μm2. Xie et al. reported the synthesis of large singlecrystalline h-BN grains on the Cu−Ni alloy foils, as exhibited in Figure 16c.182 It was observed that the nucleation density of hBN could be dramatically decreased by introducing Ni to the Cu substrate, thus obtaining single-crystalline h-BN grains with a size of 7500 μm2 (Figure 16d). Liu et al. reported the waterassisted fast growth of large-sized h-BN single-crystalline domain with a size of 330 mm.390 Zhang et al. achieved the epitaxial growth of millimeter-sized h-BN single-crystalline domains on Ni(111).391 It is thought that the formation of large-sized domains mainly results from the reduced grain boundaries and the improved crystallinity of Ni film. Meanwhile, reasonable well-aligned orientation of the h-BN domains was exhibited, and it provides a possibility to grow further larger h-BN grains. 3.1.2.2. Fabrication of 2D Materials Nanoribbons. To optimize the band structure of gapless 2D graphene, numerous efforts have been devoted to introducing a bandgap.392,393 Constructing of nanostructured graphene is a quite effective approach to access a bandgap, which could possess the possibility of reaching the full potential of graphene.394 The commonly used fabrication methods could be classified into two major categories, named as the top-down approaches, such 6252


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of the GNR indicated the atomically flat edges as well as the intrinsic graphene lattice structure. Obviously, the expensive instrument and high-precision control make it difficult to perform the as-proposed technique in the practical applications. Facile methods were also reported to prepare GNRs via “unzipping” carbon nanotubes (CNTs), which can be regarded as the reverse process of fabricating a tubular structure created by rolling and zipping single-layer or few-layer graphene narrow strips. Tour et al. proposed a solution-based oxidative approach to transform CNTs into GNRs.400 The multiwall CNTs (MWCNTs) were first oxidated to break the C−C bonds in the concentrated sulfuric acid (H2SO4) with potassium permanganate (KMnO4). Then the CNTs were unzipped into GNRs and mass production of them was demonstrated. Meanwhile, Dai et al. reported a dry etching based unzipping method.401 The MWCNTs were first deposited onto a Si substrate and then spin-coated by poly(methyl methacrylate) (PMMA). After being peeled off from the substrate, the PMMA−MWCNTs film was then exposed to Ar plasma in vacuum. Then, the MWCNTs could be etched partially and unzipped into expected GNRs by selecting befitting etch time and MWCNTs’ size. However, defects at edges were unavoidable during the oxidation reaction and the etching process. A simple two-step fabrication process of GNRs to avoid obvious doping and defects was then reported by this group.402 A mild gas-phase oxidation process was first performed in pristine MWCNTs to achieve artificial defects at the original defect sites, which could avoid the oxidization at the pristine sidewalls of them (Figure 17a). GNRs were obtained by a sonication-based unzipping approach, where the sonochemistry could enlarge the introduced defect pits and then unpack the CNTs (Figure 17b). Nevertheless, defects and doping could inevitably be introduced into the high-quality CNTs, and the unzipped GNRs were therefore doped. Wild patterning, sonochemical cutting, and unzipping methods were mild approaches, and the edge of the asobtained GNRs could not be precisely controlled, thus making the properties of them heterogeneous. Hence, anisotropic etching methods based on chemical reagents, such as hydrogen and metal NPs, were proposed, which could achieve GNRs with clean and orientation-controllable edges. The reason lies in the thermodynamic stability difference between C−C bond on the AC and ZZ face, which makes the etching tend to move along the ZZ edge of graphene. A hydrogen anisotropic etching approach based on graphene grown on Cu foil was reported by Zhou et al.403 The Cu substrate was regarded as the catalyzer in the etching reaction, and it can be interpreted as the inverse process of graphene growth or catalytic decomposition of CH4. Besides, Zhang et al. achieved the etching of graphene on insulated SiO2/Si substrate by introducing defect sites and utilizing hydrogen-plasma etching.404 In addition, anisotropic etching strategies could also be used to reduce the width and optimize the edge roughness of patterned GNRs. Dai et al. reported the fabrication of GNRs via a chemical narrowing method.405 The EBL patterned samples were exposed to an atmosphere mixed by O2, Ar, and H2/NH3 in a vacuum furnace, which made the widths of GNRs to be decreased to 5−10 nm. Metal NPs such as Co and Fe could also lead to the catalytic hydrogenation of graphene. Thong et al. reported the Coassisted etching approach of graphene to obtain nanostructures.406 Several Co NPs were deposited at the edge of the EBL-patterned graphene strips and graphene nanostructures

with smooth edges were then obtained with the movement of Co NPs. Above-mentioned GNRs fabrication approaches based on crystallized sp2 C substances such as graphene, CNTs, and graphite were classified into the “top-down” methods, where these postoptimizing strategies were usually difficult to realize the mass production of high-quality GNRs with controlled width, distribution, and orientation. Direct synthesis approaches via combining small molecules chemically have a great potential to achieve it, which are named as “bottom-up” methods. Chemical synthesis method based on aromatic compounds is an efficient way to atomically precisely fabricate GNRs with a well-defined edge structure and sub-1 nm width. Sinitskii et al. reported an approach to yield gram quantities of GNRs with high aspect ratio and atomically smooth AC edges via solution-mediated synthesis.407 This process was realized via a Scholl reaction where the polymerization of presynthesized molecular precursors was based on a Ni0-mediated Yamamoto coupling and a cyclodehydrogenation. Mullen et al. designed a modified structure of GNRs that installed with dodecyl chains to enhance the dispersibility of them.408 Ruffieux et al. reported the surface-assisted synthesis of GNRs to avoid the problems that were induced by the solubility of oligophenylene precursors and GNRs with strong intermolecular π−π stacking.409 In this process, thermal activation was utilized to sublimate the biradical species (monomers) onto the Au(111) surface, and their halogen substituents were then removed to form the linear polymer chains via chemical functionality pattern (Figure 17e). The targeted GNRs were then formed via a surface-assisted cyclodehydrogenation and the typical STM image of them was presented in Figure 17f. It is worth noting that this chemical synthesis method is also an efficient approach to construct the in-plane heterojunctions of GNRs, enabling to control the electronic properties for the GNRs-based applications. Here, GNR heterojunction means the interfaces between nanoribbons with unequal bandgaps. Crommie et al. demonstrated the synthesis of 7−13 GNR heterojunctions via a surface-assisted synthesis method (Figure 17, panels g and h), where 7 and 13 indicated the number of rows of carbon atoms across the GNRs in this width-modulated AC GNRs (AGNRs) heterostructures.410 In addition, Fasel et al. also achieved the synthesis of a chemical component-modulated GNR heterostructures, where the partial structure part of them was replaced by the nitrogen-substituted equivalents.411 Sakaguchi et al. proposed a “conformation-controlled surface catalysis” methodology to achieve efficient production of organized acene-type GNRs.412 This reaction was driven by the conformation adaptation of the precursor on the surface, which was analogous to enzymatic catalysis. Here, a precursor with a “Z-bar linkage” was designed, where the exhibited geometric flexibility allowed it to adopt an optimized conformation on Au(111). Thus, acene-type GNRs could be achieved via this conformationally controlled chemical reactions. In addition, bottom-up template growth methods were also proposed to controllably prepare GNRs with scalable length at the desired position. Templates with a nanoscale width are employed to grow graphene with a limited width. de Heer et al. reported the direct growth of GNRs utilizing SiC templates.413 The operational substrates were first etched by fluorine-based reaction ion etching (RIE) to achieve a nanometer-scale step and then were heated at a high temperature in low vacuum to relax the nanoscale-width template. After the fabrication of SiC templates, the growth of graphene growth was performed via an 6253


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Figure 18. Fabrication of 2D materials quantum dots. (a) Schematic illustration of the synthesis process of GQDs and (b) TEM images of the GQDs with regular size and shape distribution. Reproduced with permission from ref 420. Copyright 2013 Nature Publishing Group. (c) Schematic drawing of the growth process of GQDs via using C60 and (d) STM images of the GQDs with triangular and hexagonal morphology. Reproduced with permission from ref 422. Copyright 2011 Nature Publishing Group. (e) Schematic illustration of the synthesis process of TMD nanodots from layered bulk TMDs and (f) TEM images of the synthesized TMD nanodots. Reproduced with permission from ref 423. Copyright 2015 John Wiley & Sons, Inc.

realize the preparation of GQDs.420 The root cause lies in that the crystalline carbon in the coal structure was easier to be oxidatively displaced compared to that in the pure sp2-carbon structures (Figure 18a). The synthesized GQDs with regular size and shape distribution was exhibited in Figure 18b. A bottom-up chemical synthesis approach was also proposed to prepare GQDs with monodispersed structures by Li et al.421 Graphene moieties that formatted GQDs were synthesized from the polyphenylene dendritic precursors, where an oxidative condensation process was performed through a stepwise organic synthesis approach. However, the size and shape of GQDs, which could largely decide the PL emission, were still needed to be well-controlled. Therefore, a welldefined GQDs synthesis method was developed by Loh et al. via unzipping the C60 molecules.422 Here, Ru was utilized as the catalyst to open the cage of C60, and the shape of as-prepared GQDs could be tailored to a defined shape via optimizing the annealing temperature (Figure 18c). Meanwhile, the size of these GQDs could also be controlled by using organic precursors as template (Figure 18d). The synthesis of TMDs nanodots was also reported by Zhang et al.423 A combination of sonication and grinding technique was utilized in preparing a series of TMDs nanodots (such as MoS2, WS2, WSe2, and MoSe2) from their bulk crystals (Figure 18e). The synthesized TMD nanodots were exhibited in Figure 18f.

epitaxial method, as exhibited in Figure 17c. Thus, the GNRs were obtained on these etching-induced facets (Figure 17d). Bao et al. reported an approach to fabricate GNRs on DNA template, where metal salts were infused to act as catalysts.414 Besides, this group also proposed a growth method of GNR by using an electrospun polymer template.415 High-quality GNRs with controlled width were therefore yielded on these random or aligned palladium-incorporated nanofiber templates. Liu et al. developed a facile way to fabricate GNR arrays via taking advantage of the wrinkles on the Cu substrate.416 Graphene films with well-defined parallel wrinkles were fabricated on a Cu foil with slip lines and then transferred to the SiO2/Si substrate. Plasma etching was then performed to leave behind the graphene wrinkles with sub-10 nm width from the films, where the wrinkles were performed as a nanometer-sized etching mask. GNR arrays with massive productivity and high scalability were therefore achieved. 3.1.2.3. Fabrication of 2D Materials Quantum Dots. Quantum dot is another available nanostructured material that has drawn great attention in biology and medicine toward biolabeling and bioimaging applications.417 Benefiting from the electronic, thermal, and mechanical properties of 2D materials, quantum dots with strong quantum confinement and edge effects become the promising building blocks in practical applications. Solution-based exfoliation method is a common way to fabricate quantum dots of graphene and TMDs. In addition, the direct synthesis via unzipping the C60 molecules is also an alternative method to obtain graphene quantum dots (GQDs). Loh et al. reported a facile synthesis method to fabricate fluorescent carbon NPs.418 Here, ionic liquid was utilized in the electrochemical exfoliation of graphite, which was induced by a complex interplay of the anionic intercalation of it and the anodic oxidation from water. Wu et al. developed a hydrothermal method to fabricate intrinsic GQDs via cutting preoxidized graphene nanosheets.419 Here, epoxy groups were introduced on the surface of the graphene lattice to serve as the cleavage sites, and GQDs were finally produced. Tour et al. reported a fabrication approach with coal, which was easy to

3.2. Composition Tuning

The composition tuning strategies for 2D materials can be divided into three aspects, atomic doping, alloying, and atomic vacancy. Heteroatom substitution and molecule adsorption are two approaches to realize the atomic doping of 2D materials with great universality. The band structure of doped materials could be modulated by controlling dopant type and doping level, leading the adjustment of their electrical properties. The alloying strategy can only be applied to two kinds of 2D layered materials with small lattice mismatch. For the atomic vacancy tuning strategy, the vacancy defects can lead to the significant change of the properties of 2D materials and can be introduced by chemical treatments or physical processes, such as thermal 6254


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Figure 19. Atomic doping of 2D materials. (a) Schematic structure of NG and (b) Ids−Vg output characteristics of the FET devices based on NG and PG. Reproduced with permission from ref 425. Copyright 2011 John Wiley & Sons, Inc. (c) PL spectra of monolayer MoS2 treated with different oxygen plasma irradiations and the change of the PL intensities with the irradiation times. Reproduced from ref 430. Copyright 2014 American Chemical Society. (d) Schematic drawing of the surface doping of MoS2 by BV and (f) Ids−Vg output characteristics of the MoS2 FET devices before (green) doping, immediately after doping (purple), and after 1 day in air (blue). The inset shows the OM image of the device. Part (d and f) is reproduced from ref 436. Copyright 2014 American Chemical Society. (e) Transfer characteristics (Ids−Vg) of the graphene device before and after Cl plasma treatment. Reproduced from ref 433. Copyright 2011 American Chemical Society.

homojunctions based on vertically stacked doped and undoped MoS2 flakes. Robinson et al. reported the Mn-doping of monolayer MoS2.428 Dimanganese decacarbonyl (Mn2(CO)10) powder was introduced as Mn precursors into the MoS2 CVD process. The authors found that the doping efficiency of Mn atoms is highly dependent on the growth substrate. The heteroatoms can be incorporated into monolayer MoS2 flakes on an inert substrate without dangling bonds, such as graphene, whereas it is inefficient to realize Mn doping of 2D MoS2 on the substrates with reactive surface terminations, such as SiO2 and sapphire. This work indicated that the substrate surface chemistry may play an important role in the doping chemistry of 2D materials. Duesberg et al. reported the Re doping of 2D MoS2 flakes by substitutionally replacing a Mo atom in pristine MoS2 by a Re atom.429 Here, the doped films were exfoliated from bulk ones, and n-type conduction was presented in the electrical Hall measurements based on the Re-doped MoS2 flakes. The above doping approaches via heteroatom substitution involve a massive destruction of the intrinsic structure. In addition, another method that bonding or adsorption of a certain substance on the surface of the crystals possesses much more controllability. The oxygen bonding of TMD crystals was researched by Ni et al.430 Thermal annealing was performed to monolayer MoS2 in a low pressure, which led to the adsorption of the O atoms on the defect sites of MoS2 with strong bonding effect. Hence, heavy p-doping of MoS2 was introduced, which achieved a high PL quantum efficiency. The root cause lies in that p-doping in MoS2 could convert the trion into the exciton, and it made the radiative recombination process dominated by the exciton. The PL intensity was enhanced along with the increase of the oxygen plasma irradiation intensity, as exhibited in Figure 19c. Koratkar et al. reported the water adsorption approach of graphene surface to open a bandgap of it.431 An environmental chamber was introduced to control the surface adsorption quantity of water via tuning the humidity level of the environment. And a tunable bandgap (up to ∼0.206 eV) was therefore opened in the water-adsorbed graphene. The sensitivity of graphene to environmental conditions makes it possible to explore the environment-induced performance of

annealing, electron beam irradiation, or H2 plasma treatment, which is also a very universal strategy for 2D materials. 3.2.1. Atomic Doping. Atomic doping is a property tuning strategy referring to conscious introduction of heteroatoms into the intrinsic crystals, aimed at changing the intrinsic characteristics through those heteroatoms. The band structure of doped materials semiconductors could be modulated by controlling dopant type and doping level, leading to the adjustment of their electrical properties. Graphene has been considered as a promising material in the next-generation electronics on account of its unique properties. Therefore, it is of great technological importance to modulate its electrical properties, and doping is considered to be an appropriate approach to achieve it. Liu et al. reported the CVD growth of N-doped graphene (NG) via introducing NH3 during the growth process.424 The presented electrical properties of the NG indicate an n-type semiconductor characteristic, which confirmed the electrical properties tuning ability of doping toward graphene. A facile approach to fabricate NG based on the segregation phenomenon was reported by Liu et al.425 Here, a trace amount of nitrogen and carbon that dissolved in metals was utilized to grow NG. The schematic structure of NG was presented in Figure 19a and the Ids−Vg output characteristics of the FET devices based on NG and pristine graphene (PG) were exhibited in Figure 19b. This approach offers a convenient strategy to control the concentration and the location of the dopants, which can also be used to grow heterostructures. Graphene p−n junctions that formed by single-crystalline intrinsic graphene and NG were also achieved via a wellcontrolled doping technique,426 which was named the Mosaic graphene. Here, different gas precursors were introduced during the nucleation and the extending stages. The samples displayed good carrier transport properties in both the p and n region on account of their single-crystal nature. Wu et al. reported the electrical performance of Nb-doped MoS 2 flakes,427 where the Mo atoms were substituted by Nb atoms. The characterized MoS2 flake was exfoliated from the bulk Nbdoped single crystal, and the stable p-type conduction was observed in the FET based on it. Meanwhile, gate-voltagemediated current rectification was also presented in the p−n 6255


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Figure 20. 2D Materials alloying. (a) Schematic structure of the atomic model of the h-BNC film, which exhibited hybridized h-BN and graphene domains and (b) Ids−Vds output characteristics of the FET devices based on the as-grown BNC with different percentages in carbon. Reproduced with permission from ref 10. Copyright 2010 Nature Publishing Group. (c) HAADF−STEM image of MoS1.6Se0.4 monolayer in false color and (d) the PL spectra of MoS2(1−x)Se2x monolayers with different x. Reproduced with permission from ref 446. Copyright 2014 John Wiley & Sons, Inc. (e) Schematic structure of the layered AsP and (f) Raman spectra of AsP with different chemical compositions. Reproduced with permission from ref 452. Copyright 2015 John Wiley & Sons, Inc. (g) Schematic structure of the vertically composition-controlled Mo1−xWxSy and (h) the Raman and PL spectra for monolayer Mo1−xWxS2 alloy. Reproduced with permission from ref 451. Copyright 2015 Nature Publishing Group.

and MoS2 flakes with selective K-doping at the metal contacts. Suenaga et al. investigated the properties of Re- and Au-doped MoS2.440 They found that the adsorbed Re-dopant atoms changed their positions on the monolayer MoS2 surface in the TEM characterization, whereas the position of substitutional dopants at the Mo sites remain unchanged. Meanwhile, the adsorbed Au atoms were also easy to migrate under the electron beam. Furthermore, local enhancement of chemical reactivity was observed in the metal atom doping sites, and the author indicated that they could provide catalytically active sites. The doping strategies can also be applied in the 2D materials beyond graphene and TMDs. Chen et al. reported the surface doping of few-layer BP to modulate its electrical conduction characteristics.441 Cesium carbonate (Cs2CO3) and molybdenum trioxide (MoO3) were used to perform the surface functionalization of BP. The electron mobility of Cs2CO3doped BP can be significantly enhanced, whereas the doping of MoO3 was indicated as a giant hole-doping effect. In addition, a largely enhanced photodetection behavior was also presented in the devices based on the doped BP crystals. Yu et al. reported the enhanced stability and electrical performance of BP via adsorption of metal ions.442 Ag+ ions were spontaneously adsorbed on the BP surface via cation−π interactions, thus passivating the lone-pair electrons of BP and stabilizing it in air. Meanwhile, the FET based on Ag+-modified BP exhibited greatly enhanced hole mobility. In addition, the as-reported strategy can also be extended to other metal ions such as Fe3+, Mg2+, and Hg2+. Cui et al. reported the properties of Sb-doped Bi2Se3 TI nanoribbons.443 The large concentration induced by bulk residual carriers of the TI crystals is regarded as a major challenge to reveal its properties. Therefore, the authors suppressed the bulk electron concentration of Bi2Se3 TI nanoribbons via doping by Sb, thus realizing the surface-statedominant transport. It is reported that an extremely low 2D carrier concentration of 2 × 1011 cm−2 is achieved. 3.2.2. Alloying. If doping was assigned to the quantitative variation, alloying should be assigned to the qualitative change. Alloying, which can be regarded as a mixture of different materials (especially semiconductors here), is an efficient method to adjust their electronic structure and lattice parameters.5 Ajayan et al. reported the growth of uniform hBNC film, which was a mixture of graphene and h-BN, via a

graphene-based devices. Johnston et al. reported the photoconductivity of graphene-based terahertz-frequency devices modulated by surface adsorption of environmental gases.432 The photoconductivity spectra of graphene devices exposed to different atmospheric gases indicated that the high-frequency electrical response of graphene flakes could be dramatically altered by surface gas adsorption. Dai et al. reported a noninvasive doping method for graphene via controllable Cl plasma.433 Compared with H and F plasmas, a milder reaction between Cl plasma and graphene made the chlorination wellcontrolled. The slower reaction kinetics may be induced by the weaker binding energy between Cl atoms and graphene. Distinct transport characteristic curves (Ids−Vg) of the graphene device before and after Cl plasma treatment were presented (Figure 19e). Bao et al. reported the n-type doping of graphene via a solution-processed method, such as spin coating and inkjet printing at graphene surface.434 In this process, 2-(2-methoxyphenyl)-1,3-dimethyl-2,3-dihydro-1Hbenzoimidazole (o-MeO−DMBI) was utilized as the strong n-type dopant of graphene. The surface adsorption doping of MoS2 was reported by Matsuda et al. by using p-type dopants containing 2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane (F4TCNQ) and 7,7,8,8-tetracyanoquinodimethane (TCNQ).435 This solution-based approach was performed by a drop-cast method and therefore resulted in a PL enhancement. Javey et al. proposed a n-type doping method of MoS2, serving benzyl viologen (BV) as the air-stable electron donor organic compounds.436 These dopant molecules could be reversibly removed by immersion in toluene and that made it possible to control the carrier density of MoS2 sheets via selective removal of surface dopant adsorptions. Figure 19d displayed the schematic illustration of the device based on doped MoS2, and the electronic transport properties were presented in Figure 19f, which demonstrated the air-stable ndoping of MoS2. In addition, Ye et al. reported the Cl2 molecular doping of TMDs437 and p-doping of WSe2 through NO2 was proposed by Javey et al.438 Except the inorganic small molecules, metal atoms are also mostly used substances in the molecule adsorption doping process of 2D materials. Javey et al. reported the K doping of few-layered TMDs.439 N-Type conduction of doped MoS2 and WSe2 flakes were obtained due to the small electron affinity of K. Furthermore, low contact resistances was exhibited in the top-gated FETs based on WSe2 6256


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CVD method.10 Cu was chosen as the substrate and methane and ammonia borane were both served as the precursors for graphene and h-BN, which made it possible to adjust the atomic ratio of C and B, N by controlling the growing parameters. Schematic structure of the atomic model of the hBNC film, which exhibited hybridized h-BN and graphene domains, was presented in Figure 20a. The electronic transport properties of the samples with different atomic ratios were also shown in Figure 20b. The resistivity of the h-BNC samples decreased toward the increase of its percentage of carbon. It confirmed that the electrical properties of this alloying sample were highly dependent to the atomic ratio of C and B, N. In addition, Chen et al. reported the bandgap tuning of BNC crystals via various the BN concentration.444 Compared to graphene, the alloying strategy is more suitable to the TMDs family, which possess similar crystal structure and various properties highly dependent on their chemical components. Xie et al. first reported the properties tuning of exfoliated atomically 2D Mo1−xWxS2 alloys.445 The bandgap modulating of the monolayer alloys by tuning the W composition percent was confirmed by both PL experiments and DFT simulations. This group also reported the direct growth of monolayer MoS2(1−x)Se2x semiconductor alloys.446 Here, MoS2 and MoSe2 powders were introduced as the reaction precursors and placed in separated heat zones, so that the evaporation temperature of the precursors could be well-controlled. An atomic-resolution HAADF−STEM was utilized to reveal the random arrangement of Se, S, and W atoms of these crystals (Figure 20c). PL spectra were performed to verify the bandgap tuning effect (Figure 20d), and the electrical measurement was conducted to confirm the semiconductive behavior of these alloys. Ajayan et al. also reported the growth of Se-doped MoS2 (MoS2(1−x)Se2x) via utilizing S, Se, and MoO3 powers as the precursors.447 In addition, Pan et al. reported the growth of it with fully tunable chemical compositions.448 Furthermore, Xiang et al. proposed the growth of monolayer WS2(1−x)Se2x alloys, which could enhance the hydrogen evolution reaction (HER) catalytic performance.449 Beside the chemical compositions varying among different alloy crystals, the atomic ratio tuning in a certain crystal can also be realized. Pan et al. reported in the growth of monolayer ternary MoS2(1−x)Se2x alloys that the composition was varied from center to the edge of the crystals.450 An improved CVD growing strategy was introduced that the heating temperature of Se powders was increased gradually followed by a decrease of maintaining temperature of S powders at the same time. Thus, the lateral composition-graded alloy crystals could be synthesized via a meticulously controlling of the vaporization of precursors. The PL mapping characterization of the crystal indicated a lateral composition variation, and it was further verified by the position-dependent PL spectrum of the crystals. Otherwise, a vertically composition varying alloy was synthesized by Kim et al.451 The synthesis process was realized via sulfurizing Mo1−xWxOy, which was obtained from supercycle ALD. The schematic structure of the as-synthesized vertically composition-controlled Mo1−xWxSy was presented in Figure 20g. The Raman and PL spectra of monolayer alloy crystals synthesized after one-cycle ALD was shown in Figure 20h, which confirmed the variation of the composition. In addition, the arsenic− phosphorus (AsxP1−x) alloy based on BP was also realized by Zhou et al.452 The red phosphorus and ultrapure gray arsenic were utilized as the reaction precursors to achieve the synthesis of AsxP1−x. The schematic structure was displayed in Figure

20e, and the Raman spectra of AsxP1−x with different chemical compositions was presented in Figure 20f. In addition, Li and Zhang et al. reported the growth of Janus monolayers of TMDs, where the structure of MoSSe was confirmed only when the top-layer S of MoS2 monolayer was fully replaced with Se atoms.453 3.2.3. Atomic Vacancy. Vacancy defects in 2D materials had been predicted to be the motivation of various novel properties by theoretical calculations,454−457 and it made vacancy-introduced approaches a promising property-tuning method. Wei et al. reported that ferromagnetic interactions were highly related to the exchange interactions between Mo4+ ions and sulfur vacancies in a MoS2 nanosheet.458 Here, the ferromagnetism of MoS2, which is an originally nonmagnetic material, was induced by a phase-incorporation strategy. The incorporated 1T-MoS2 phase was introduced to the original 2H-MoS2 nanosheet via a two-step hydrothermal process. The raw materials were first synthesized by maintaining Mo ions precursors and thiourea at a temperature of 200 °C. Then the as-obtained MoS2 nanosheets were utilized to generate sulfur vacancies, where the surrounding lattice of 2H phase could be transformed into 1T-MoS2, via subjecting them to centrifugation, ultrasonation, and autoclavation in ethanol solution (Figure 21a). Then electronic paramagnetic resonance (EPR)

Figure 21. Atomic vacancy of 2D materials. (a) Schematic illustration of the two-step hydrothermal synthesis process of the 1T@2H-MoS2 nanosheets and (b) EPR spectrum of the synthesized 1T@2H-MoS2 nanosheets. Reproduced from ref 458. Copyright 2015 American Chemical Society. (c) Schematic drawing of WSe2 flake with Se vacancies after the H2 plasma treatment and (d) transfer characteristics (Ids−Vg) of the WSe2 devices with different plasma treatment processes. Reproduced from ref 461. Copyright 2016 American Chemical Society.

spectrum was conducted to explore the magnetism properties of as-synthesized samples (Figure 21b), and it presented an enhanced ferromagnetism in the obtained 1T@2H-MoS2 nanosheets. Zhou et al. studied the formation of inversion domains induced by vacancies.459 Thermal annealing process of monolayer MoS2 was performed to generate chalcogenide vacancies, which induced large-scale inversion domains. Meanwhile, Se vacancies of MoSe2 monolayers were generated by electron beam irradiation, where Se vacancies induced the formation of inversion domains. The atomic structure of Sedeficient MoSe2 sheets, grown by MBE, was investigated by Lehtinen et al.460 A mirror-twin boundary structure was 6257


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Figure 22. Property modulation of 2D materials via using an electric field. (a) SEM image of a typical FET device based on graphene and (b) the field effect curves of the device. Reproduced with permission from ref 1. Copyright 2004 AAAS. (c) Schematic illustration of the gap opening of bilayer graphene at strong electrical gating and (d) gate-induced absorption spectra of bilayer graphene after applying different displacement fields. Reproduced with permission from ref 467. Copyright 2009 Nature Publishing Group. (e) Schematic structure of the graphene tunneling FET and the band structure, corresponding to the situation with no gate voltage applied, a finite gate voltage and zero bias, and finite gate voltage and bias, respectively. (f) Zero-bias conductivity dependence on gate voltage. Reproduced with permission from ref 186. Copyright 2012 AAAS. (g) Ids−Vg output characteristics of the FET devices based on monolayer MoS2. Inset displayed the Ids−Vds curves and the typical schematic view of the device. Reproduced with permission from ref 11. Copyright 2011 Nature Publishing Group. (h) Transfer characteristics of the FET devices based on BP and the inset displayed the OM image of the devices. Reproduced with permission from ref 118. Copyright 2014 Nature Publishing Group. (i) OM image of the WSe2 p−n junction devices and (j) the transfer characteristics of the devices. Reproduced with permission from ref 203. Copyright 2014 Nature Publishing Group.

mobility of graphene of 10000 cm2 V−1 s−1 at room temperature as well as the carrier concentrations of 1013 cm−1 were also exhibited. The root cause lied in that a tiny overlap existed between the valence and conductance bands of 2D graphene, which made it a semimetal substance. Later, Lukose et al. predicted the electric field induced Landau level contraction and eventual collapse.462 Pinczuk et al. reported the tuning of electron−phonon coupling in graphene via electric field effect,463 and Kim et al. proposed the study of graphene work function tuning by electric field effect.464 In addition, Wang et al. reported that the optical transition properties of graphene could also be modulated by electrical gating.465 More than 2% and 6% of the incident IR radiation could be absorbed by monolayer and bilayer graphene, respectively. However, the gapless nature of graphene hinders its practical applications, which made it of great significance to open the bandgap of graphene. Therefore, Rotenberg et al. proposed the electric field tuning method toward the bandgap of bilayer graphene.349 The Dirac crossing at the band structure of bilayer graphene could be adjusted via controlling carrier concentration of two graphene layers or varying the applied electric field. The root cause lies in the high sensitivity between the graphene band structure and its lattice symmetry. An energy gap can be achieved when the individual layers of bilayer graphene are rendered inequivalent. Angle-resolved photoemission spectroscopy (ARPES) was utilized to investigate the band structure of bilayer graphene, and the result was fully proved. Castro et al. reported the experimental evidence of the electric field modulated gap opening in bilayer graphene.350 A pronounced plateau at zero Hall conductivity was found only at the biased bilayer after applying a magnetic field, and the dependence of cyclotron mass toward carrier density also indicated the gap opening. In addition, Heinz et al. achieved the direct observation of the bandgap opening in electric-fieldapplied bilayer graphene via infrared radiation spectroscopy.466 It is noted that Wang et al. realized a wide-range tunable bandgap of bilayer graphene in the electric field, which was directly observed in the experiment.467 Dual-gate FETs based

observed in this crystal, and it was regarded as the energetically favorable structure in Se-deficient conditions. Javey et al. proposed the research of mild plasma induced Se vacancies in WSe2 flakes as well as the electronic transport properties based on it.461 Mild He or H2 plasma treatment was introduced to engineering Se vacancies in the WSe2 lattice. Schematic structure of Se-deficient WSe2 crystal structure after treated by plasma was displayed in Figure 21c. Meanwhile, a 200 meV shift of the Fermi level toward the conduction band was observed, which was rooted in the Se-vacancy induced electron doping. Therefore, this strategy was utilized to improve the conductivity at the contact regions of FETs, where the WSe2 region contacted the electrode was treated by H2 plasma, as presented in Figure 21d. 3.3. External Field Tuning

External field tuning approaches are nondestructive physical tuning methods for modifying the properties of 2D materials, which not only can achieve the continuous tuning but also is universal for all 2D materials. Electric field and light were universal physical means to tune the properties of 2D building blocks via modifying the carrier density and the kinetic energy of the electrons. 3.3.1. Electric Field. Applying electric field is an effective physical tuning method that can modulate the properties of semiconductor effectively and controllably. FET is an electric device that uses an electric field to control the electrical conductivity of its channel material (generally semiconductor). The applied voltage (electric field) can tune the carrier density of the semiconductor and hence influence the conductivity of it. Geim et al. first reported the electric field effect in graphene, which is the pathbreaking work in the field of 2D materials.1 Here, the first reported graphene flake was fabricated by a mechanical exfoliation strategy and a FET device was constructed on it (Figure 22a). The typical dependence curves of its resistivity, conductivity, and Hall coefficient on Vg (or applied electric field) were displayed in Figure 22b as well as the temperature-dependent curve of its carrier concentration. A strong ambipolar electric field effect can be observed, and the 6258


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Figure 23. Property modulation of 2D materials via using illumination. (a) The generated photocurrent as a function of the illumination power of the graphene based device, where the external responsivity is 1.5 mA W−1. Inset is the schematic of the illumination. Reproduced with permission from ref 469. Copyright 2010 Nature Publishing Group. (b) Ids−Vds output characteristic of the device based on monolayer MoS2 in the dark and under different illumination intensities. Inset is the schematic of the illumination. Reproduced with permission from ref 202. Copyright 2013 Nature Publishing Group. (c) Ids−Vds output characteristic of the device based on BP in the dark and under illumination with different powers. Inset is the schematic of the device. Reproduced with permission from ref 475. Copyright 2015 Nature Publishing Group.

physical field, which is another mostly used method to tune 2D materials’ properties. The energy could be transferred from a photon to an electron according to the photoelectric effect, and the transformation of the electrons’ kinetic energy as well as the electrical properties of the matter are therefore changed. Thus, lighting becomes a commonly adopted approach to modulate physical properties of the semiconductors, and one of the derived applications is a photodetector. Photodetector is a device that can control the electrical conductivity of its channel material via using illumination. Various materials, such as silicon and GaN, were introduced in those devices for improving their performance and expanding the wavelength range of operation. Here, Avouris and Xia et al. first reported the performance of the photodetector based on graphene.469 This device was constructed by metal−graphene−metal structure, which could break the mirror symmetry of the internal electric-field profile in graphene,470 thus allowing for efficient photodetection. Therefore, photodetector with the external photoresponsivity up to 6.1 mA W−1 (at a wavelength of 1.55 mm) could be realized based on this interdigitated structure. In addition, the construction possibility of the optoelectronics with a photoresponse in the 300−6000 nm range was also indicated in this work, on account of the unique band structure of graphene.471 In consideration of the radiative recombination, the direct bandgap could allow a high absorption coefficient and efficient electron−hole pair generation under photoexcitation. Monolayer MoS2 with quantum-confinement-induced direct bandgap is thought to be a promising material in optoelectronics. Zhang et al. first investigated the photodetector based on monolayer MoS2.472 It exhibited a better photoresponsivity (7.5 mA W−1) compared to the device based on graphene (Figure 23a). Later, Kis and Radenovic et al. studied ultrasensitive photodetectors based on monolayer MoS2 with outstanding performance.202 The devices exhibited a high photoresponse with an operational wavelength range from 400 to 680 nm, where a maximum external photoresponsivity of 880 A W−1 could be obtained at a wavelength of 561 nm (Figure 23b). The 100000-fold improvement of the photoresponsivity of the monolayer MoS2-based photodetectors compared to Zhang’s work472 is thought to be attributed by the improved mobility, as well as the optimized contact quality and positioning technique. In addition, Cui et al. reported the valley polarization of monolayer MoS2 via optical pumping.473 Valleytronics is a term that refers to the technology to control the valley degree of freedom of multivalley semiconductors, which present multiple valleys inside the first Brillouin zone.474 Monolayer MoS2 was considered as a promising material to control the number of electrons in the valleys and then made a valleytronic

on bilayer graphene were conducted to perform the experiments. In addition, IR absorption spectroscopy was utilized to directly observe the electric-field-induced bandgap with widely tuning range in bilayer graphene. The carrier doping concentration and energy bandgap, which can be regarded as the key parameters of a semiconductor, could be modulated independently via using the top and bottom gates. The schematic illustration of the electric-field-modulated electronic bands of bilayer graphene was displayed in Figure 22c, and the absorption spectra of the bilayer sample corresponding to various gate voltages were presented in Figure 22d, where the electric-field-induced bandgaps could be directly observed and tuned in a wide-range manner. In addition to bilayer graphene with extra electric field, FETs based on graphene, h-BN and MoS2 vertical heterostructures can also present satisfactory on/ off ratios.186 The schematic structure of the experimental device was presented in Figure 22 (panels e and f) and displayed the zero-bias conductivity dependence of Vg. Compared to semimetal graphene, FETs based on the 2D semiconductors such as TMDs and BP could achieve a more outstanding performance. Kis et al. investigated the FETs based on monolayer MoS2.11 The typical transfer characteristic of the as-constructed FET was displayed in Figure 22g as well as the schematic view of the device structure inside. A roomtemperature mobility of 200 cm2 V−1 s−1 and on/off ratio of 108 were presented, which made monolayer MoS2 a promising building block in electronic and optoelectronic devices. Xu et al. reported the modulating valley magnetic moment of bilayer MoS2 via applying an electric field.468 The root cause lies in the broken of crystal symmetry, via applying an extra electric field. Furthermore, this group also proposed the electrically tuning of monolayer WSe2 based excitonic light-emitting diodes.203 Here, an electrostatically induced monolayer WSe2 lateral p−n junctions was utilized as the channel materials to construct the optoelectronic device, where h-BN film was introduced as the dielectric layer as well as multiple metal gates were used (Figure 22i). The as-constructed device enables the injection of electrons effectively. Thus, the electroluminescence of the monolayer WSe2 lateral p−n junctions could be optimized greatly. The typical I−V curves were presented in Figure 22j, which indicated an excellent performance. In addition, Zhang et al. investigated the BP-based FET device.118 The typical curves of Hall coefficient (blue curve) and conductance (red curve) between gate voltage were displayed in Figure 22h as well as the OM image of the as-constructed device based on few-layer BP was presented inside. 3.3.2. Lighting. In consideration of the interaction between light and matter, lighting could be regarded as an external 6259


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Figure 24. Phase transition via physical methods for 2D materials. (a) Schematic illustration of the growth of 1T′-MoTe2 and the phase transition process from it to 2H-MoTe2. Reproduced from ref 478. Copyright 2015 American Chemical Society. (b) Thermoelectrical properties of 2H-MoTe2 and 1T′-MoTe2, evaluated by Seebeck coefficients of the two materials. (c) MR of 2H-MoTe2 and 1T′-MoTe2 in a magnetic field, which shows large change of resistance for 1T′-MoTe2 but little change for 2H- MoTe2 under a strong magnetic field. Reproduced with permission from ref 479. Copyright 2015 Nature Publishing Group. (d) Bandgap energies of 2H-MoS2 (of different layers) and 1T-MoS2 under hydrostatic pressure. (e) Pressure of phase transition from semiconducting states to metallic states for the MoS2 polytypes containing 2H phase of different thicknesses and 1T′ phase. Reproduced from ref 480. Copyright 2015 American Chemical Society. (f) Schematic illustration of laser-irradiation process. Reproduced with permission from ref 481. Copyright 2015 AAAS.

to achieve the phase transition, and alkali metal intercalation should be the most mature chemical strategy to achieve the phase transition. Both of these mentioned strategies are universal for almost all 2D materials. For the edge structure reconstruction strategy, the Joule heating and electron beam illumination strategy can be quite useful and universal since the stability of each edge configuration differs. In terms of the crystal lattice deformation tuning strategy, it has a significant influence on the properties of all layered 2D materials because the energy band characteristics around the Fermi level are very sensitive to the orbital coupling of neighboring atoms. 3.4.1. Phase Transition. According to the specific combinations of metal and chalcogen atoms, monolayer TMDs could exhibit trigonal prismatic (1H) phase and 1T phase. For 1H TMDs, there are two different stacking ways, including hexagonal symmetry (2H phase) and rhombohedral symmetry (3R phase).23 And for 1T phases, distortion could happen when the filling electrons of d orbitals are different, resulting in distorted 1T phase (1T′ phase). Changing atomic arrangement or symmetry of TMDs could lead to new properties. Here, methods of phase transition between 2H and 1T are focused on, as well as novel properties or phenomena resulting from the phase transition. To achieve the phase transition, energy barriers between different phases should be overcome. For instance, energies of 1T phase Moand W-dichalcogenides are generally higher than those of 2H phase compounds, except for WTe2.477 Through physical or chemical methods, additional energies could be provided to materials to overcome the energy barriers between different phases. 3.4.1.1. Physical Strategies. Physical methods contain introducing heat, pressure, and electromagnetic wave (laser or microwave), which could provide large energy to materials for the phase transition.

device due to that both the conduction and valence band edges of it have two energy-degenerate valleys at the corners of the first Brillouin zone. However, due to the zero-bandgap nature of graphene and low mobility of monolayer MoS2, photodetectors based on graphene suffer from very high dark current, and those based on monolayer MoS2 suffer from low photo current. Here, narrow but finite bandgap and promising mobility make BP an ideal building block in constructing photodetector applications. Therefore, Li et al. investigated the photodetectors based on layered BP.475 Here, very low dark current as well as high intrinsic responsivity, which is up to 135 mA W−1 for 11.5 nmthick devices and 657 mA W−1 for 100 nm-thick ones, were achieved in the BP photodetectors (Figure 23c). Meanwhile, the devices also exhibited a high response bandwidth exceeding 3 GHz. In addition, Cui and Hwang et al. reported the construction of polarization-sensitive photodetector based on BP vertical p−n junction.476 The in-plane optical anisotropy of BP lead to a strong intrinsic linear dichroism, which makes it a promising material to realize the polarization-sensitivity photoresponse. Furthermore, a perpendicular electric field was induced by gating to achieve the vertical p−n junction of BP, which could lead to the more spatial separation and faster recombination of photogenerated electrons and holes in the channel, and the performance of the device could be enhanced effectively. 3.4. Structure Tuning

The properties of 2D materials are highly dependent on their structures. We introduce three kinds of structure-tuning strategies in this part, including phase transition, edge reconstruction, and crystal lattice deformation. As for phase transition methods, thermal approaches and pressure controlling are widely used physical strategies because temperature or pressure increasing could help to provide with external energies 6260


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Figure 25. Phase transition via chemical methods and different properties of the devices based on different phases for 2D materials. (a) Energy change toward the ion relaxation steps from the initial supercell to the final one. Reproduced from ref 485. Copyright 2014 American Chemical Society. (b) Schematic illustrations of the phase transition process from 2H-MoS2 to 1T-MoS2 by using SC CO2. Reproduced from ref 487. Copyright 2016 American Chemical Society. (c) Schematic illustrations of two-step solvothermal synthesizing and coating process for 1T@2H-MoS2 nanosheets. The I−V characteristics of (d) the 2H-MoS2-polyvinylpyrrolidone (PVP)-based and (e) the 1T@2H-MoS2−PVP-based flexible memory device. The insets of (d) and (e) show the constructions of (d) 2H-MoS2-based and (e) 1T@2H-MoS2−PVP-based device, respectively. (f) Retention characteristics in the ON and OFF states of the 1T@2H-MoS2−PVP-based memory device at a reading voltage of 0.1 V. (g) Current curve toward reading cycles of the 1T@2H-MoS2−PVP-based memory device. Reproduced with permission from ref 488. Copyright 2016 John Wiley & Sons, Inc.

et al. used a diamond anvil cell (DAC) with a soft neon pressure medium to exert a uniform hydrostatic pressure to MoS2 for large volume strain change. They discovered that the pressure of phase transition decreased as the number of MoS2 layers increased, as shown in Figure 24d. As pressure increases, the bandgap of 2H-MoS2 (1 to 3 layers) rises up to a peak value and then decreases, while the bandgap of bulk 2H-MoS2 decreases rapidly. In Figure 24e, the pressure of the phase transition from semiconducting 2H phase to metallic 1T phase decreases as the number of 2H-MoS2 layers increases due to the increment of the interlayer interaction.480 However, the shortcoming is also obvious. The ultrahigh pressure produced by the DAC is too hard to achieve, and this is the biggest limitation of pressure controlling for phase engineering. Electromagnetic waves containing lasers and microwaves could also provide energy to achieve phase transition. Lee et al. obtained 2H-MoTe2 films with patterned areas transited to 1T′ phase via laser irradiation process. 2H-MoTe2 multilayer flakes were mechanically exfoliated onto the substrate and then were patterned into desired area by laser irradiating (Figure 24f). During the process, 2H-MoTe2 could be thinned and changed into 1T′-MoTe2, but they also mentioned that the reverse phase transition from 1T′ to 2H phase could not be observed via laser irradiation.481 Due to the controllability of the laser, laser irradiation is an efficient way to achieve a phase transition in patterned areas, but high energy produced by irradiation could lead to the damage of monolayer 2D materials. Microwave irradiation has also been reported to transform 1T-MoS2 to 2H-MoS2.482 Unfortunately, for the dispersibility of the microwave irradiation energy, the method is not appropriate for phase patterning and engineering for 2D materials on substrates. Recently, Zhang et al. suggestively achieved the phase transition of MoTe2 driven by electrostatic doping, for which 2H and 1T′ phases could be transformed from one to another

Thermal methods are one of the most convenient strategies because the phase transition process could be easily controlled by thermal pretreatment at different temperatures according to phase diagrams. Phase transition could result from the frequent vibration of atoms and the formation of new bonds at a high temperature. Lee et al. studied the phase transition between 2H- and 1T′-MoTe2 during a two-zone CVD synthesis. They used a slow tellurization strategy to obtain large 2H-MoTe2 films. As shown in Figure 24a, the growth illustration indicates that the 1T′-MoTe2 film is first obtained on the substrate and then it gradually transforms into 2H phase during slow tellurization. They also demonstrated that reverse phase transition from 2H to the 1T′ phase could happen due to further tellurization or rapid cooling.478 Kim et al. synthesized 2H- and 1T′- MoTe2 via flux method by using Mo and Te powders as the precursors. 2H-MoTe2 could be obtained via continuous slow cooling from 900 °C to room temperature, but 1T′-MoTe2 could be yielded just by quenching which referred to the fast cooling process. The authors compared the electrical properties of 2H- and 1T′-MoTe2. As shown in Figure 24b, 2HMoTe2 exhibits higher Seebeck coefficients (∼230 μV K−1) than 1T′-MoTe2 (∼30 μV K−1), which indicates that 2HMoTe2 has more excellent thermoelectrical properties than 1T′-MoTe2. However, 1T′-MoTe2 shows an extremely high MR of about 16000%, while 2H-MoTe2 shows little change of resistance under a strong magnetic field (Figure 24c).479 Though thermal methods are efficient and convenient, the uncontrollability of the transition process would seriously limit the application of the phase engineering for few-layered materials. Pressure controlling, which could directly affect the volume strain in the materials, is also an efficient way to achieve the phase transition. Actually, chemical bonds between atoms could change a lot at an extremely high pressure so that phase transition of almost all the materials could happen. Akinwande 6261


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I−V curves of the 2H-MoS2-based memory device show a typical nonvolatile memory effect during a sequence of voltage bias sweeping (Figure 25d). Whereas, as the 2H-MoS2 partly transformed into 1T phase, the I−V characteristics of the 1T@ 2H-MoS2-based memory device show an inerasable data storage (Figure 25e). The ON/OFF current ratio is about 200, and the ON and OFF states could stay with little fluctuation for at least 600 s, which shows a good retention characteristic of the resistance state (Figure 25f). Moreover, Figure 25g shows steady resistance switching properties over 104 cycles with small fluctuations. In conclusion, based on solvothermal synthesis, partly phase transited 2H-MoS2 could be applied to high-performance memory devices. Recently, Fu et al. first developed an iodine-mediated CVD process for the controllable direct synthesis of metastable TMDs, which inspires the researchers to fabricate metastable materials.489 It could be an important way for the direct synthesis of materials which could be obtained only by the phase transition process at present. To sum up, phase transition plays an important role in material-property modification and high-performance device construction. A much more efficient and specific method needs to be explored for further applications. 3.4.2. Edge Structure Reconstruction. The structure of edges in 2D materials has a great influence on its mesoscopic properties.490−496 Nanostructures, such as nanoribbons,395,497 quantum dots,498 and nanojunctions,499−501 are largely influenced by the edge configurations due to the high edge/ bulk ratio. Taking graphene as example, generally, the edge configuration could be classified into three types, AM, ZZ, and chiral. Although there exist abundant studies and structural characterizations carried out on the geometry of edge structures, being able to control the edge termination freely still remains a challenging but significant topic. Most top-down production techniques such as EBL and photolithography result in defective edges.395,400,401,502 Anisotropic etching with the assistance of metal NPs and hydrogen at elevated temperatures has also been demonstrated to produce predominately ZZ edges.503,504 Direct bottom-up synthesis strategy usually provides nanostructures with specific-type edge and faces the disadvantage of low yield. In fact, except the control of the edge configuration during the fabrication process,505 the edge reconstruction is quite an effective and universal way to tune the properties of 2D materials, especially for the corresponding nanostructures.506−509 Since that graphene is gapless and nanostructuralization is an effective way to open its bandgap, the research that focuses on the edge reconstruction of graphene is very hot. The GNR with ZZ edge (ZGNR) has distinct magnetic properties because of its spin polarization. Theoretical studies showed that a ZGNR can be turned into half-metallic by applying an in-plane electric field or by chemical modification.493,510−512 The GNR with AC edge (AGNR) exhibits metallic or semiconducting depending on the periodicity.513 Joule heating and electron beam irradiation are both demonstrated to be able to achieve the edge reconstruction.507,509,514,515 Dresselhaus et al. demonstrated the efficient shaping of graphitic nanoribbon edges into ZZ or AC edges via Joule heating inside a TEM-STM system.507 An individual nanoribbon sample is attached to the sample holder at one end and to the STM tip at the other end, both these two ends can serve as the two electrodes. When applying a voltage over the ribbon, current flows, leading to the improvement of the degree of crystallinity of the ribbon and the

reversibly. Ionic liquid covering on the MoTe2 monolayer and electrode could manipulate the electrons on the MoTe2 surface, which resulted in the phase transition between 2H and 1T′ phases. The observed hysteresis of Raman results and transformation of second-harmonic generation (SHG) could be evident showing the phase transition. Such a reversible phase transition driven by electrostatic doping could provide a platform for fundamental studies, and the large hysteresis may be potential for the application of memory devices.483 To sum up, introducing heat, pressure, and laser could be general methods to achieve the phase transition, but it is very hard to realize precise regulation. Microwaves and electrostatic doping may be efficient for the phase transition of specific materials. For the phase transition through physical processes, precision and efficiency remain to be improved. 3.4.1.2. Chemical Strategies. Apart from physical methods, chemical methods are usually controllable and lead to less damage of materials, which contain alkaki metal intercalation, molecule induction, and solvothermal reaction. Alkaki metal intercalation is one of the most common methods to achieve the phase transition, among which lithium ions are the most commonly used alkaki metal ions. Bai et al. reported lithium intercalation process in MoS2 nanosheets through in situ TEM observation. They explained the phase transition mechanism on the basis of the experimental results that polytype superlattices could be formed with a lithium ion in the interlayer S−S tetrahedron site, which led to the phase transition from pristine 2H-MoS2 to 1T-MoS2.484 Schwingenschlogl et al. detailedly simulated the phase transition process via DFT calculations. The theoretical calculation shows that the electron−phonon coupling in conduction band valleys results in the electron doping, which is the main reason for the phase transition. As shown in Figure 25a, the energy of the supercell decreases toward the ion relaxation, and 1T-MoS2 could be finally achieved after full ion relaxation.485 Moreover, sodium and potassium ions could also be applied in the phase transition by alkaki metal intercalation.23 However, lithiation process or other alkaki metal intercalation should be stopped at an appropriate time otherwise TMDs would be irreversibly transformed into transition metal and alkaki metal chalcogenides,485 which means that the poor controllability and impure residues of the lithiation process could be the restriction. Very recently, Loh et al. reported that 1T′-TMDs could be stabilized for about 3 months in air when the intercalated alkali metal ions were hydrogenated to form alkali metal hydrides.486 This discovery may help to achieve the phase transition and stabilization of TMDs in the future. Besides alkaki metal ions, small molecules could also induce the phase transition. Chen et al. demonstrated that a lateral 2HMoS2/1T-MoS2 heterostructure could be fabricated by partial phase engineering of 2H-MoS2 flakes with the assistance of supercritical CO2 (SC CO2). Because the adsorption of CO2 could stabilize 1T-MoS2, SC CO2 could be adsorbed on two sides of 2H-MoS2, inducing the phase transition from 2H phase to 1T phase, as shown in Figure 25b.487 The method could provide a new strategy for fabricating lateral heterostructures of TMDs with different phases. Xue et al. reported that solvothermal reaction could also be applied to achieve the phase transition.488 As shown in Figure 25c, 1T@2H-MoS2 nanosheets could be synthesized via a twostep solvothermal synthetic route. 2H-MoS2 was obtained through a hydrothermal process at 200 °C, and the phase transition happened in ethanol solution at 220 °C for 8 h. The 6262


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Figure 26. Edge reconstruction of graphene and BP. (a) Crystallization and edge formation in graphitic nanoribbons and edge motion under Joule heating inside the TEM. Reproduced with permission from ref 507. Copyright 2009 AAAS. (b) Straight graphene torn edge with AC edge configuration. (c) Straight graphene torn edge with ZZ edge configuration. (d) A time series of TEM images of a graphene torn edge under electron beam. Reproduced with permission from ref 509. Copyright 2013 Nature Publishing Group. (e) A series of structural transformation in the reconstruction of BP at the flat ZZ edge. Reproduced with permission from ref 516. Copyright 2016 Royal Society of Chemistry.

Figure 27. Crystal lattice deformation of 2D materials. (a) Schematic of a typical bending device used to apply strain to the materials. Reproduced from ref 524. Copyright 2013 American Chemical Society. (b) Raman spectra of the strained graphene. Reproduced with permission from ref 517. Copyright 2009 National Academy of Sciences. (c) DFT calculated electronic band structure of monolayer MoS2 under different biaxial strains. Reproduced with permission from ref 523. Copyright 2012 Nature Publishing Group. (d) PL spectra of the strained MoS2. Reproduced with permission from ref 524. Copyright 2013 American Chemical Society. (e) Schematic illustration of the operation process of the single-layer MoS2 piezoelectric device. (f) Direct-current electrical characterizations of single-layer and bilayer MoS2 devices under strains. Reproduced with permission from ref 527. Copyright 2014 Nature Publishing Group. (g) STM image of the highly strained graphene nanobubbles and the inset exhibited an enlarged image of a graphene nanobubble. (h) Experimental topographic line scan (red line) and experimentally determined magnetic fields profile (black line). Reproduced with permission from ref 534. Copyright 2010 AAAS. (i) Schematic of the top and side views of MoS2 with strained Svacancies. (j) HER characterized curves for the Au substrate, Pt electrode, unstrained MoS2 without S-vacancies, strained MoS2 without S-vacancies, unstrained MoS2 with S-vacancies, and strained MoS2 with S-vacancies. Reproduced with permission from ref 535. Copyright 2016 Nature Publishing Group.

decrease of the thickness of the sample, as seen in Figure 26a. From the edge terminations observed, it can be concluded that the majority of edges are either ZZ or AC, which is attributed to that the activation energy of atoms forming ZZ or AC edges is lower than for other edge configurations. The dynamics of the edge reconstruction are also shown. When applying the bias voltage, the AC edge above the ZZ edge starts to evaporate, resulting in the upward movement of the ZZ edge. Eventually, the AC edge is eliminated, and the lower ZZ edge joins with the upper ZZ edge and forms a stable ZZ-ZZ junction. Zettl et al. reported that the atomic structure of graphene edge could also be reconstructed by electron beam-initiated

mechanical rupture or tearing in high vacuum, oriented in either the AC or ZZ direction. Figure 26b shows the atomic resolution TEM images of a torn graphene edge nominally aligned with the AC lattice direction.509 Notably, the torn graphene edge is extremely clean, regular, and straight even at the atomic scale. Extended ZZ torn edges with atomically smooth, ideal edge structure are also observed, as seen in Figure 26c. It should be noticed that a pentagon-heptagon (5−7) reconstruction at the ZZ edge occuured, which has been previously investigated via TEM over a limited range. Under the electron illumination conditions, both AC and ZZ edges show dynamical effects, with the ZZ edge being much more 6263


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band shifts of the E12g (∼385.3 cm−1) Raman mode of monolayer MoS2 and very small shifts of A1g (∼402.4 cm−1) mode. The electronic properties of 2D crystals were also sensitive to the lattice deformation. Guinea and Geim et al. suggested that the bandgap of graphene could be significantly opened via strain engineering.520 Meanwhile, Neto et al. explored the influence of local strain on the electron structure of graphene.521 Satpathy et al. calculated the electronic structure of bilayer graphene under strain.522 The conclusion could be drawn that the bandgap tuning of 2D materials could be achieved rapidly and reversibly via applying strain. Moreover, the construction of optoelectronics with the ability to capture a broad range of the solar spectrum required an optoelectronic material with a spatially varying bandgap. Li et al. performed the theoretical study about the bandgap extension of a strain engineered monolayer MoS2.523 The strain-dependent theoretical electron structures of monolayer MoS2 were exhibited in Figure 27c, which indicated that both the calculated direct and indirect bandgaps of it were narrowed along with the increase of strain. In addition, the absorption spectra and the optical gap of the monolayer MoS2 can also be modified by the biaxial strain. Bolotin et al. then carried out the experiments to demonstrate strain-induced bandgap tuning of monolayer MoS2.524 They conducted the PL spectra of MoS2 crystal under uniaxial tensile mechanical strain in the range of 0−2.2% to investigate the band structure of it. As shown in Figure 27d, the red shift of A peaks in PL of monolayer MoS2, which was induced by a direct transition between the conduction band and the valence band, was confirmed. Furthermore, the PL intensity was decreased rapidly after enhancing the strain, and it may be caused by the anticipated strain-induced optical bandgap transformation (from direct to indirect). In addition, they also observed the split of the E12g Raman mode of monolayer MoS2. The strain engineering toward BP was reported by Pourfath et al.,525 which predicated a straininduced semiconductor-to-metal transition as well as a peculiar Dirac-shaped dispersion. Furthermore, the lattice deformation of 2D materials can also be used to achieve the piezoelectricity, which is the electric charge accumulation of the materials in response to applied mechanical strain. The root cause also lies the strain-induced lattice distortion. This characteristic of materials has great potential to be applied in the next-generation sensors, transducers, energy conversion, and electronics.526 Wang et al. first investigated the piezoelectricity of monolayer MoS2,527 which was predicted to be strongly piezoelectric, owing to the opposite orientation of adjacent atomic layers.528 The operation process of the single-layer MoS2 piezoelectric device was schematically illustrated in Figure 27e. A mechanical-toelectrical energy conversion efficiency of 5.08% was obtained in a monolayer flake that strained by 0.53% and a peak output of 15 mV and 20 pA was generated. They also indicated that the output decreases with increasing layer number, and the sign reversed after the strain direction was rotated by 90°. Therefore, the strain-induced piezoelectricity of MoS2 could also modulate the carrier transport properties of it, as exhibited in Figure 27f. Moreover, Zhang et al. observed the piezoelectricity of free-standing monolayer MoS2.529 Then, Zhang et al. reported the study of piezoelectric effect in CVD-grown triangular MoS2.530 The piezoelectric effect on biaxial-strained graphene was also reported by Ren et al.531 In addition to

active. Figure 26d shows a time series of TEM images of a relatively torn graphene edge corner. Under the influence of the electron beam, the ZZ edge frequently undergoes dramatic, extended, and fully reversible structural transitions between a 5−7 reconstructed edge and a 6−6 ZZ edge. The AC edge (shown with yellow dashed lines) remains relatively stable under the electron beam, consistent with previous theoretical calculations. Besides graphene, the edge reconstruction of other 2D materials was also studied, including h-BN, TMDs, and black phosphors. However, almost all the works focus on the theoretical research. Zhang et al. identified a novel nanotubelike structure for the highly stable ZZ edge of phosphorene.516 A pristine edge of BP with unsaturated dangling bonds is unstable, leading to the atomic reconstruction of the edge since forming triple bonds in the armrests can easily lower the edge energy. The transition pathway for the first pair of edge atoms is shown in Figure 26e. Initially, the two adjacent edge atoms roll up (from a1 to a2) to reach the intermediate state (a3). Then, by overcoming a small barrier (from a3 to a4), the edge state a5 is formed, of which the energy is much lower than that of the a1 state. Subsequently, the adjacent two edge atoms begin to roll up and repeat similar reconstruction behavior to the first pair. It should be noted that the subsequent flipping barrier may be monotonically decaying. Furthermore, their calculations show that the tube-like terminated edge has a bandgap of 1.23 eV with an obvious edge state close to the valence band. 3.4.3. Crystal Lattice Deformation. Crystal lattice deformation has a significant influence on the properties of layered 2D materials because the energy band characteristics around the Fermi level are very sensitive to the orbital coupling of neighboring atoms. Other than the visible deformation, small shift of atomic positions (∼a couple of Å) can lead to lattice deformation of 2D materials, thus affecting their surface properties. Generally, lattice deformation of the crystals can be achieved by introducing strain, including applying mechanical force, employing substrates with mis-matched lattice or different thermal expansion coefficients. Regarding that Raman scattering of the crystals is highly related to the vibrational, rotational, and other low-frequency modes of their structure, Raman spectrum is regarded as a powerful characterization technique to detect the strain in the 2D materials with lattice deformation. A typical strain applying process toward 2D materials was exhibited in Figure 27a. Hone et al. reported the systematic study of the Raman spectra of the strained monolayer graphene.517 They measured the Raman spectra of the monolayers, which were transferred onto flexible substrates, under uniaxial tensile strain. As shown in Figure 27b, significant red shifts along with the enhancement of stain were presented both in the G and 2D bands of graphene. Meanwhile, the strain-induced symmetry breaking makes the G band split into two distinct sub-bands. In addition, distinctive polarization dependence of Raman scattering from the two G sub-bands was also found in this work, reflecting the angle between the axis of the stress and the underlying graphene crystal axes. It indicated that the polarized Raman spectroscopy could be a purely optical characterization to determinate the crystallographic orientation of graphene. Later, Cheong et al. reported that the 2D band of graphene in Raman spectra could also be split into two peaks under homogeneous uniaxial strain.518 Ramanscattering measurements of monolayer MoS2 under uniaxial strain were reported by Young et al.519 They found observable 6264


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heterostructures have been reported to exhibit many exciting physical phenomena. Generally, we classified the intermonomers assembly as two catagories, the vertical assembly and the lateral assembly. And the assembled constructures are recognized as vertical or lateral heterostructures, respectively. Here, we will review the assembling strategies of the intermonomer heterostructures of 2D materials, including mechanical exfoliation combined with layer-by-layer transfer, CVD, epitaxy, or combinations of them (Figure 28). Among

piezoelectrics, strain modulated FETs and optoelectronics were also studied by Appenzeller et al.532 and Wang et al.533 Crommie et al. observed that magnetic field greater than 300 T could be induced by crystal deformation of graphene nanobubbles.534 This effect is unique to graphene because of its massless Dirac Fermion-like band structure and particular lattice symmetry (C3v). Here, the experimental measurements were conducted by STM of highly strained graphene nanobubbles (Figure 27g). Figure 27h exhibited the experimental topographic line scan (red line) and experimentally determined magnetic fields profile (black line), which indicated a relatively uniform pseudo-magnetic field of 300 to 400 T. Zheng et al. reported the strain engineering effect in HER.535 Schematic illustration of the top and side views of MoS2 with strained S-vacancies was presented in Figure 27i. The HER activity of monolayer 2H-MoS2 was further optimized by introducing S vacancies and strain, as exhibited in Figure 27j. Recently, Javey et al. realized the introduction of controlled strain engineering in the growth process of bilayer and monolayer WSe2 via utilizing the thermal coefficient of expansion mismatch between the substrate and semiconductor.536 Here, stable built-in strains of WSe2 crystals ranging from 1% tensile to 0.2% compressive at substrates with different thermal coefficient of expansion are achieved. Notably, brightening of the dark exciton in strained bilayer and monolayer WSe2 flakes is observed besides the modulation of the band structure and indirect-to-direct bandgap transition of them.

4. ASSEMBLY BEHAVIORS TOWARD THE CIRCUITS The discovery of graphene and other 2D materials together with recent advances in fabrication and property tuning technique has set the foundations for the application of this large material family. The family of 2D materials encompasses a wide selection of compositions including most elements of the periodic table. This derives into a rich variety of electronic properties, including metals, semimetals, insulators, and semiconductors with direct and indirect bandgaps ranging from ultraviolet to infrared throughout the visible range, even not to mention that there exist many strategies to achieve their further property tuning. Thus, they offer the potential to play a fundamental role in the future of nanoelectronics, optoelectronics, and the assembly of novel ultrathin and flexible devices. Besides this, the integration of the functional layer and the batch production of the electronic devices are urgently needed when considering the industrial applications. Or in other evocative ways, after finishing the design of the individual 2D material monomers, the assembly of them into the ordered or mass structures is also of great importance. Here, we will in detail introduce the controllable assembly methods of the 2D materials monomers, from the assembly of the intermonomers for the synergetic functionalization, to the oriented assembly of the monomers for the development of the collective properties, to the mass assembly toward the integrated devices.

Figure 28. Schematics of strategies of the intermonomers assembly.

them, exfoliation combined with transfer owns the most universality and almost can be used to assemble any 2D layered monomers in a stack form since the unconstraint of the lattice match. But it is regretful that such a strategy faces with a huge challenge in terms of the large-size fabrication of 2D material heterostructure. CVD and epitaxy could be applied to achieve both the vertical and lateral intermonomers assembly of 2D materials at a relatively large scale. However, for the CVD methods, it relies much on both the fabrication feasibility of the monomers and even the compatibility of the growth parameters of the different monomers. For the epitaxy, it is highly dependent on the selection of the substrate material with low lattice mismatch, which also limits its application. Existent intermonomers assembly strategies are often the efficient combination of these methods mentioned above. 4.1.1. Vertical Assembly. In general, a single layer of a 2Dlayered material consists of a single- or few-atom thick, covalently bonded lattice. These dangling-bond-free atomic sheets often exhibit extraordinary electronic and optical properties, in contrast to typical nanostructures that are plagued by dangling bonds and trap states at the surface. Additionally, with fully saturated chemical bonds on the surface, the interactions between neighboring layers of 2D materials are usually characterized as van der Waals forces. Without direct chemical bonding, van der Waals interactions allow the integration of highly disparate materials without the constraints of crystal lattice matching. This allows considerable freedom in integrating 2D materials with various nanoscale materials to create diverse van der Waals heterostructures with functions

4.1. Inter-Monomers Assembly

The extensive library of 2D materials with selectable properties opens up the possibility of the heterogeneous integration at the atomic scale, thus creating novel hybrid structures that exhibit totally new physics and enable unique functionality. When conducting the intermonomers assembly, the synergetic effects can be awaked. The distribution of the charge between the different monomers in the assembled structure will occur. Such 6265


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each layer of the MoS2/WSe2 heterostructure, the device could be successfully fabricated, as shown in Figure 29b. The individual MoS2 and WSe2 layers exhibit n- and p-type channel characteristics, respectively (the inset of Figure 29c). According to the threshold voltages of the two materials, the overlapping region could be regarded as a p−n junction. The gate-voltagetuned current rectification could be observed in the I−V curves of the junction at various Vg (Figure 29c), due to the modulation of the density of free electrons and holes via electrostatic doping.537 Recently, Park et al. fabricated a BP/ ReS2 heterostructrue via exfoliation and transferring process and then constructed a negative differential resistance device, which could be applied to multivalued logical circuits.538 As a commonly used method, it is convenient that a great many of 2D materials could be obtained via exfoliation and their vertical heterostructures can be further formed by a layer-by-layer transfer and stacking process. As long as the individual materials constituting heterostructures could be exfoliated, the method could be used for the synthesis of TMD/TMD,539−541 graphene/TMD,542 graphene/h-BN,543 BP/TMD,544 and other binary or ternary vertical heterostructures.545−547 However, due to the uncontrollability of size and thickness, exfoliation is not an ideal approach to fabricate 2D monomers. As the CVD growth process of graphene and TMDs has been greatly improved, researchers tend to adopt CVD method to partially replace exfoliation due to its excellent controllability in the size and thickness of the monomers. Lee et al. reported the construction of a two-terminal floating gate memory device. The exfoliated h-BN and MoS2 flakes were transferred onto a CVD graphene layer through a dry transfer approach.187 In such a strategy, different monomers (such as graphene, TMDs, BP, GaSe, and so on) are obtained by two different processes, exfoliation and CVD, but the heterostructure assembling is still based on the layer-by-layer transfer process.548−550 However, the problems such as impurities, cracks, or wrinkles resulting from the transfer process could be hardly avoided. Direct growth of the upper layers for constructing vertical heterostructures could effectively alleviate the problem, which is recognized as direct assembly. Kitaura et al. demonstrated that WS2 single crystals could be directly obtained via CVD on the exfoliated h-BN flakes, leading to the effective construction of WS2/h-BN vertical heterostructures.551 Bi2Se3/h-BN vertical heterostructure could be fabricated via van der Waals epitaxy growth of Bi2Se3 on exfoliated h-BN flakes.552 Through the direct growth of 2D monomers on the exfoliated ones, the impurities at the interlayer interface may be effectively suppressed. CVD synthesis techniques of high-quality films or single crystals of graphene, h-BN, TMDs, and TIs are becoming mature,10,172,176−180,296,346,360 which lay good foundation for their layer-by-layer vertical assembly. Ferrari et al. obtained monolayer MoS2 and graphene via CVD synthesis and stacked them on a polyethylene terephthalate (PET) substrate and then fabricated a flexible photodetector with the polymer electrolyte served as the gate dielectric, as shown in Figure 30 (panels a and b). The necessity of the CVD synthesis is to control the size and the thickness of MoS2 and graphene.553 The method could be also used for the construction of MoS2/h-BN, WS2/ MoS2, and Bi2Se3/graphene heterostructures.554−556 Park et al. reported the programmed vacuum stack (PVS) process, which is a novel strategy using a layer-by-layer transfer process to construct wafer-scale vertical heterostructures of CVD grown MoS2 and WS2 films. They achieved ultraclean interfaces,

that were not previously possible. We call such construction method as vertical stacks of 2D materials monomers. The vertically stacked structures of 2D materials monomers are recognized as vertical heterostructures, which could demonstrate various coupling effects and carrier transport behaviors of different van der Waals interfaces. For the combinations of 2D monomers with different energy bands, vertical heterostructures have greatly enriched the existing 2D material family. The construction of vertical heterostructures could be generally described as fabricating methods and assembling strategies. Monomer fabricating methods contain exfoliation, CVD, MBE, and van der Waals epitaxy, while assembling strategies mainly refer to layer-by-layer transfer process and direct growth assembly (directly growing the upper layers). Constructing strategies distinguish with each other mostly on how to combine monomer fabricating with heterostructure assembling. Here, we would review different constructing strategies and various electrical applications of vertical heterostructures. Exfoliation combined with layer-by-layer transfer is the most commonly used strategy in the fabrication of vertical heterostructures. The constructing process could be described as stacking exfoliated materials with the assistance of polymers. Figure 29a presents a schematic illustration of MoS2/WSe2 vertical heterostructure. The heterostructrues were fabricated on the SiO2/Si substrate using a colamination and mechanical transfer technique. After metal electrodes were deposited on

Figure 29. Exfoliation combined with layer-by-layer transfer or CVD process used for the fabrication of vertical heterostructures. (a) Atomic structure of MoS2/WSe2 vertical heterostructure. (b) Schematic illustration of the construction of MoS2/WSe2 vertical heterostructure based device. (c) I−V curves at various Vg measured across the heterostructure. The inset shows gate-dependent transport characteristics for individual monolayers of MoS2 (blue curve) and WSe2 (red curve). Reproduced with permission from ref 537. Copyright 2014 Nature Publishing Group. 6266


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difficulty troubling researchers. Such heterostructures could be applied to devices with excellent properties, including batchfabricated tunnel device arrays, band-engineered heterostructure tunnel diodes, and millimeter-scale ultrathin membranes and windows. The novel method is promising to be employed in heterostructure construction for the electronics and optical devices.557 As is mentioned above, in the polymer-assisted layer-by-layer assembly process, the residues, wrinkles, or damages in the materials introduced by the transfer process could severely affect the properties of the vertical stacks of 2D monomers. And the precise stacking method537 is hard to operate. Instead of the subsequent assembly via layer-by-layer transfer process, van der Waals epitaxy, MBE and all CVD synthesis could be employed to directly assemble 2D materials together during the monomer fabrication process, which are much more controllable. What’s more, little impure residues will be introduced at the interfaces. By using van der Waals epitaxy, heterostructures of expected size could be obtained without any impurities resulting from the transfer process. Robinson et al. reported the van der Waals epitaxy growth of MoS2 on epitaxy graphene. High-quality epitaxy graphene was obtained on diced SiC wafers via sublimation of Si from 6H-SiC(0001), and then MoS2 was grown on epitaxy graphene via vapor-phase reaction of MoO3−x and sulfur powder. Due to the residual strain in epitaxy graphene on SiC, the epitaxy of MoS2 could be impacted by improving the commensurability and the structural symmetry in MoS2/graphene heterostructures could be enhanced. In accordance with cross-sectional HRTEM, the epitaxy MoS2/ graphene heterostructure on SiC substrate could be clearly observed (Figure 30c). The MoS2/graphene heterostructurebased photosensor in Figure 30d exhibits an excellent photoresponsibility of 40 mA W−1 with a light power (Plight) of 40 μW (Figure 30e), and the photocurrent remains stable over 1000 s of continuous operation (Figure 30f).558 When graphene was grown on 4H-SiC through epitaxy or on Cu foil

Figure 30. CVD synthesis combined with layer-by-layer transfer or van der Waals epitaxy process for vertical heterostructure growth. (a) Schematic illustration and (b) picture of the construction of the flexible photodetector based on vertical MoS2/graphene heterostructure fabricated via CVD and layer-by-layer transfer. The inset in (b) shows the optical image of photodetectors with different channel length. Reproduced from ref 553. Copyright 2016 American Chemical Society. (c) Cross-sectional HRTEM image of MoS2/graphene heterostructure constructed by using CVD and van der Waals epitaxy, (d) Optical image of the photosensor based on vertical MoS2/ graphene heterostructure. (e) Ids−Vds curves of the photosensor when illuminated with a 488 nm laser at the power of 0 (referring to dark), 4, 20, and 40 μW, respectively. (f) Transient measurement of the photosensor illuminated with 488 nm laser at Vds = 3.0 V and Plight = 40 μW. Reproduced from ref 558. Copyright 2014 American Chemical Society.

ultralarge size, and controllable stacking structures of MoS2/ WS2 vertical heterostructures, which was once the largest

Figure 31. All CVD process for the synthesis of vertical heterostructures. (a) Schematic illustrations of two-step CVD process for the construction of MoS2/h-BN vertical heterostructure on a Ni−Ga alloy supported by Mo foil. Reproduced from ref 180. Copyright 2016 American Chemical Society. (b) Schematic illustration of the atomic structure and (c) optical image of WS2/MoS2 vertical heterostructure when the reacting temperature is ∼850 °C. (d) Ids−VG curves of a CVD-grown WS2/MoS2 bilayer, a mechanically transferred WS2/MoS2 bilayer, a MoS2 bilayer, and monolayer MoS2 demonstrating that the CVD-grown WS2/MoS2 bilayer has the best performance. Reproduced with permission from ref 566. Copyright 2014 Nature Publishing Group. (e) Schematic illustration of CVD twinned growth of ReS2/WS2 vertical heterostructures. Raman mapping images of the E2g peak intensity of (f) ReS2 and (g) WS2. Reproduced with permission from ref 568. Copyright 2016 Nature Publishing Group. 6267


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c). The as-obtained vertical heterostructure-based device could show a much higher mobility than those devices based on individual MoS2 layers and WS2/MoS2 heterostructures stacked via transfer process (Figure 31d).566 This clearly indicated the cleaner heterointerface in the WS2/MoS2 heterostructure fabricated by one-step CVD growth than that fabricated via a layer-by-layer transfer method. Such a one-step CVD growth strategy could open up a new territory for efficiently synthesizing vertical heterostructures with clean interfaces, which is significant for the applications of the as-obtained heterostructures. When a monomer is directly fabricated via a CVD process on the metal catalytic substrates, the catalyst would be deactivated due to that its surface is fully covered by the first layer monomers. Thus, decomposition and nucleation rates of the gas precursor are greatly reduced, which could lead to the depression of formation rate of the upper layers for vertical heterostructures. To avoid this problem, it is an effective way to use solid-state dissolved precursors in the bulk of metal as precursors and follow an underneath growth mode, which is regarded as CVD cosegregation. Liu et al. reported the controllable synthesis of graphene/h-BN heterostructure by using C-doped Ni/(B, N) source/Ni sandwiched substrate, in which the C-doped Ni served as C source for graphene synthesis and (B, N) source was used for h-BN growth. The sandwiched substrate was placed in the vacuum annealing furnace and was heated to desired temperatures after the furnace was evacuated to a low pressure. During the annealing, dissolved C atoms in the C-doped Ni top layer first segregated to the surface, nucleated, and formed the graphene layer on the top of the metal surface. Then B and N atoms diffused to the surface and started to nucleate and form a h-BN layer, directly producing vertically stacked graphene/h-BN heterostructures.567 It is possible that mass production of wafer-scale vertical heterostructure could be achieved via CVD cosegregation. However, such a CVD cosegregation strategy could not be universally applied to the synthesis of other vertical heterostructures due to the limited selection of the segregative element in the substrate bulk. Therefore, it is necessary to develop more universal methods for efficiently fabricating 2D material heterostructures. Due to the random nucleation process and uncontrollable growth rates of ordinary CVD process, it is extremely difficult to precisely control the stacking area and location of vertical heterostructures, thus laying the foundation for the subsequent controllable device fabrication. Recently, Fu et al. reported the twinned growth of 2D material.568 They designed a “reaction vessel”−metal alloy of Re, W, and Au and achieved the fully stacked ReS2/WS2 vertical heterostructures, as shown in Figure 31e. The Raman mapping images in Figure 31 (panels f and g) showed the complete stacking of ReS2 and WS2. Due to the lower adsorption energy for Re atoms on WS2(001) than that on Au(111), only vertical heterostructures in the form of ReS2 on WS2 could be grown on Au(111). The strategy could also allow the vertical twinned assembly between other TMDs, such as MoS2/WS2. It could provide vertical TMD heterostructures with controllable stacking location, 100% overlap percentage, and ultraclean interfaces via the twinned growth, which could help present excellent optical and electrical properties. Due to the van der Waals interfaces and various combinations of energy bands, multilayer vertical heterostructures with over two kinds of materials involved in could be excellent platforms for the explorations of physical

via the CVD process, the epitaxy MoS2 could show rotationally commensurate growing behavior or different orientations on polycrystalline graphene.559,560 TMDs could also be obtained on h-BN via van der Waals epitaxy.561 Although van der Waals epitaxy has been known as an effective method to fabricate heterostructures with clean heterointerfaces, it is still faced with a huge limitation due to its high demands on the lattice match of the 2D material monomers. For the 2D monomers that are extremely difficult to obtain through conventional methods (such as CVD and exfoliation), MBE could be a good choice, which refers to an epitaxy growth process by supplying precursors via molecule beams. Gao et al. reported a superconductor−TI−normal metal layered heterostructure, referring to the HfTe3/HfTe5/Hf heterostructure. The synthesis process included two steps: the direct reaction between Te and Hf for growing HfTe5 on Hf(0001) substrate and epitaxy growth of HfTe3 on HfTe5 layer.562 MBE could be an effective way for the construction of vertical heterostructures with ultraclean interfaces, but the strict requirements of UHV, super smooth surface of the substrates, and expensive instruments limit its further application. As a common approach in the fabrication of 2D materials, CVD growth methods have already been adopted to directly construct vertical heterostructures, including two-step CVD growth and one-step CVD growth. Two-step CVD growth is a commonly used strategy in the synthesis of vertical heterostructures consisting of two or more different materials. It was reported by Liu et al. that graphene/MoS2 vertical heterostructures could be obtained by two-step CVD growth on Au foil. First, graphene was grown on Au substrate with CH4 serving as C source. Then MoS2 was directly synthesized on graphene by using MoO3 and S as the precursors.563 WSe2/ MoSe2 heterostructure could also be obtained via a two-step CVD growth process at a specific reacting temperature.564 Therefore, vertical heterostructures with clean interlayer interface could be obtained, and their excellent electrical and optoelectrical properties could be fully exhibited. By using hBN as an excellent dielectric substrate to avoid dangling bonds and charge trapping at the interface, it can enable 2D materials such as TMDs to show their intrinsic physical properties. However, the stacking of TMDs/h-BN heterostructures was initially achieved through a sequential mechanical transfer process, in which the expected performance was not wellexhibited due to the interfacial contamination and poor interlayer contact. On the basis of the synthesis of graphene on liquid metal alloy,346,565 Fu et al. reported direct two-step CVD growth of MoS2/h-BN vertical heterostructure on a sulfide-resistant alloy substrate. As shown in Figure 31a, the hBN was obtained directly via CVD synthesis on a Ni−Ga alloy supported by Mo foil, and then MoS2 was synthesized on h-BN with H2S gas and Mo foil served as the precursors. MoS2/h-BN vertical heterostructure with clean interface could be successfully obtained without an intermediate transfer process. The authors demonstrated that the direct two-step CVD synthesis based on a sulfide-resistant liquid metal alloy substrate could be versatile for the growth of other 2D material heterostructures.180 Due to the compatible growth process of some TMDs, it is possible to achieve TMDs vertical heterostructures via one-step CVD growth. Ajayan et al. demonstrated a one-step CVD method for the synthesis of WS2/MoS2 heterostructures. Vertical WS2/MoS2 heterostructures could be obtained when the reacting temperature was ∼850 °C (Figure 31, panels b and 6268


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Figure 32. Synthesis of lateral heterostructures via two-step growth. (a) Schematic illustration of the sequential growth of the monolayer WSe2/ MoS2 lateral heterostructure. (b) Optical image of the monolayer WSe2/MoS2 lateral heterostructure. Reproduced with permission from ref 576. Copyright 2015 AAAS. (c) Schematic illustration of lateral epitaxy growth of WS2/WSe2 and MoS2/MoSe2 lateral heterostructures. (d) Gate-tunable Ids−Vds curves of a p−n diode based on WSe2/WS2 lateral heterostructure. The VG varies from 80 to 20 V in steps of 10 V. The inset shows the optical image of the lateral heterostructure-based p−n diode device, in which the scale bar is 2 μm. (e) The Vout−Vin curve and the voltage gain at different Vin of the CMOS inverter obtained by integrating a p-type WSe2 and n-type WS2 FET. The inset shows the optical image and the circuit diagram of the WSe2/WS2 CMOS inverter, in which the scale bar is 2 μm. Reproduced with permission from ref 577. Copyright 2014 Nature Publishing Group.

due to their low lattice mismatches (e.g., 1.7% for graphene and h-BN).575 Remarkably, these artificial lateral heterostructures possess atomically clean and sharp interfaces, which are crucial for the high-performance applications. In contrast to the vertical heterostructures, the interfaces in lateral ones are reduced from 2D to 1D, finally becoming an “interlining”. Herein, atoms located at interlinings are connected by covalent bonds, further ensuring the epitaxial quality and stability and thus boosting the optical, electrical, and topological performances of the heterostructures. Lateral assembly could be achieved through two-step growth, one-step CVD growth, and catalytic conversion. Two-step growth means that two different materials assembling lateral heterostructures by two separated growing processes. Li et al. obtained a monolayer WSe2/MoS2 lateral heterostructure via such a two-step epitaxy growth.576 As shown in Figure 32a, WSe2 started to grow from the WSe2 seeds and expanded via van der Waals epitaxy on sapphire (925 °C), and then the MoS2 epitaxy growth from the edge of WSe2 was performed at a lower temperature of 755 °C in a separated furnace. In Figure 32b, the optical image shows the lateral WSe2/MoS2 heterostructure with the domain of WSe2 and MoS2 clearly distinguished by the optical contrast. Such a twostep epitaxy growth offers a controllable method to obtain lateral heterostructures with atomically sharp interfaces. Duan et al. also adopted the two-step growth in the synthesis of WS2/ WSe2 and MoS2/MoSe2 lateral heterostructures.577 As shown in Figure 32c, the triangular domain of WS2 (or MoS2) was obtained by the first CVD process and then switched to the vapor source of WSe2 (or MoSe2) in the middle of the growth process, thus the lateral heterostructure could be formed via the heteroepitaxy growth at the peripheral active growth front. On the basis of the WS2/WSe2 lateral heterostructure, the p-n diode could be naturally formed (the inset of Figure 32d) due to the typical p-type and n-type characteristics in WSe2 and WS2, respectively. Obvious current rectification effect could be observed in source-drain current−source-drain voltage (Ids− Vds) curves, in which the current was able to pass through the diode device only when positive voltage bias was added to the p-type WSe2. Moreover, the WS2/WSe2 lateral heterostructure

properties based on the as-fabricated devices. However, due to the incompatibility of the growth parameters of each 2D material in the heterostructures, they are generally fabricated by exfoliation and layer-by-layer transfer. On the basis of the multilayer sandwiched vertical heterostructures, Novoselov and Geim et al. have reported many novel physical phenomena, such as quantum state tuning of Dirac electrons,569 strong lightmatter interactions,570 high-temperature superfluidity,571 and twist-controlled resonant tunneling,572 as well as devices covering field-effect tunneling transistors,186 light-emitting tunneling transistors,573 and light-emitting diodes.574 It could be expected that the complex logic and integrated circuits (ICs) based on 2D material heterostructures would shed light on the future electronics. Thus, a convenient fabrication method for multilayer heterostructure should be established. To sum up, constructing strategies of vertical heterostructures have been optimized from two aspects: the monomer fabrication and the monomer assembling. The mechanical exfoliation method can yield the highest-quality 2D material and even achieve the restack with random orientation, which is the most suitable for fundamental studies but is highly limited for the practical applications due to the poor yield. CVD and epitaxy methods showed the great potential for manufacturing vertical heterostructures with large sizes due to their excellent controllability both in the fabrication and vertical stacking assembly of 2D monomer. To suppress the impurities at the heterointerfaces, direct intermonomer assembly integrated in the growth of the monomer tends to replace the polymerassisted layer-by-layer transfer process, which is significant for the performance improvement for the as-constructed electronics and optoelectronics. 4.1.2. Lateral Assembly. The vertical heterostructure, which is commonly achieved by vertically stacking the 2D materials monomers layer-by-layer, has been systematically introduced above. Except for the vertical stacking, the 2D materials monomers could also be laterally jointed together to form lateral heterostructures. In the interface of lateral heterostructure, atoms are connected by covalent bonds. So far, graphene/h-BN and selective TMDs have been stitched together seamlessly in-plane to form lateral heterostructures 6269


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Figure 33. Synthesis of lateral heterostructures via one-step growth or conversion reaction. (a) Schematic illustration of WS2/MoS2 lateral heterostructure obtained by controlling the reacting temperature. (b and c) Optical images and (d) SEM image of WS2/MoS2 lateral heterostructure with clear interfaces. Reproduced with permission from ref 566. Copyright 2014 Nature Publishing Group. (e) Schematic illustrations of the synthesis process of graphene/h-BN lateral heterostructure via the conversion reaction from h-BN to graphene by using a Pt-patterned SiO2/Si substrate. (f) Optical image of graphene/h-BN lateral heterostructure on SiO2/Si substrate. Reproduced from ref 586. Copyright 2015 American Chemical Society.

inhomogeneity of the lateral heterostructures. Therefore, precise reaction conditions should be controlled for the synthesis of lateral heterostructures via the two-step growth. One-step growth has also been used for the synthesis of lateral heterostructures by controlling the growth temperature. As mentioned above, Ajayan et al. obtained vertical WS2/MoS2 heterostructures at ∼850 °C by using the one-step CVD method. When the reacting temperature was set to 650 °C, lateral WS2/MoS2 heterostructures could also be obtained.566 As shown in Figure 33a, the active sites of the reaction changes to the edge of MoS2 at 650 °C, so that WS2 could grow along the active edges but not nucleate on the surface of MoS2. The OM and SEM images (Figure 33, panels b−d) could show the lateral heterostructures with clear interfaces. Such a one-step growth could also be employed to the synthesis of graphene/hBN. Johnson et al. demonstrated that graphene/h-BN heterostructures could be obtained on Cu foil through a continuous CVD process. Graphene was first synthesized at a high temperature and then h-BN could be formed along the straight edges of graphene where the furnace was cooled down to a lower temperature. The mechanism of the continuous growth was studied by DFT calculation, which exactly presented that a cooling-down process to different temperatures during the continuous growth process could lead to different constructions of the interfaces of graphene/h-BN lateral heterostructures.584 Bi2Te3/Bi2Se3 lateral heterostructure could also be fabricated through a one-step growth.585 Compared to two-step growth, one-step growth could efficiently simplify the synthesis process without reducing the quality of lateral heterostructures, but the reacting temperature is hard to control to prevent the residues and defects, which may restrict its applications. Therefore, detailed mechanism of the one-step growth process remains for further studies, and the difficulties of controlling the reaction conditions are in urgent need of being overcome. Other methods were also used for the construction of lateral heterostructures. Shin et al. reported that they succeeded in fabricating graphene/h-BN lateral heterostructures via the conversion reaction.586 As shown in Figure 33e, CVD-grown h-BN film was transferred to a SiO2/Si substrate patterned by Pt film, and then graphene could be obtained at the patterned area in the CVD furnace with carrier gases containing CH4. After the reaction, the graphene/h-BN lateral heterostructure film was transferred to SiO2/Si substrate and the optical image

was ultrathin so that the gate-tunable electrical transport properties could be achieved as shown in Figure 32d, and the Ids increased toward the increase of positive Vg, which indicated that the n-type WS2 partly limits the charge transport in the diode device. Other functional devices based on the WS2/WSe2 lateral heterostructures could also be fabricated, such as complementary metal oxide semiconductor (CMOS) inverters. As shown in Figure 32e, a CMOS inverter using a logic NOT gate could be constructed by integrating a p- and an n- channel transistor (referring to WSe2 and WS2, respectively) in series across the lateral heterostructure interface, which shows a voltage gain as large as 24 when the input voltage (Vin) is increased to ∼1.5 V, and the output voltage (Vout) is quickly switched from a high level of 8 V to nearly 0 V. Similarly, MoS2/WS2 lateral heterostructures could be obtained by the two-step epitaxy growth.578,579 It predicted that lateral heterostructures based on not only 2D layered semiconductors but also other 2D superconducting, magnetic and topologically insulating materials could be obtained via such a two-step epitaxy growth approach.577 Recently, Duan et al. achieved general synthesis of 2D lateral heterostructures (such as WS2− WSe2 and WS2−MoSe2), multiheterostructures (such as WS2− WSe2−MoS2 and WS2−MoSe2−WSe2), and superlattices (such as WS2−WSe2−WS2−WSe2−WS2) by adopting a reverse flow to the sequential growth process mentioned above. The interfaces of lateral heterostructures were sharp, and the different components were precisely controlled. On the basis of WS2−WSe2 lateral heterostructures, excellent rectification properties could be achieved. The general strategy for 2D lateral heterostructure growth could effectively promote the development of synthesis and applications of 2D lateral superlattices.580 Besides, graphene/h-BN lateral heterostructure could also be obtained by the two-step growth process. The general strategy could be described as growing graphene (or hBN) and growing another material after etching the former one by RIE or Ar ions.581,582 Liu et al. demonstrated that a complete h-BN layer on Cu was obtained at 500 °C via CVD synthesis, and it could be partly etched at 900 °C so that graphene islands could subsequently grow on the exposed Cu surface, which could lead to the formation of graphene/h-BN lateral heterostructure.583 That is, two-step growth could be a commonly used method to obtain lateral heterostructures, but etching and vertically nucleating may happen during the second growth process, which could result in the defects and 6270


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exfoliated from natural flake graphite with the assistance of microwave sparks in liquid nitrogen and study the electronic properties of the resulting CNSs.615 As shown in Figure 34a,

shown in Figure 33f indicated that the as-obtained lateral heterostructure could be recognized as the alternative graphene/h-BN film on the substrate. The mechanism could be described as the reaction of the h-BN film and CH4 on the Pt films due to the catalysis of Pt for decomposing h-BN and producing graphene films. Such a catalytic conversion reaction could also help with the fabrication of devices based on graphene/h-BN lateral heterostructures and understanding the growth mechanism of graphene and h-BN. Moreover, such a conversion method could be a promising approach for fabricating other patterned lateral heterostructures and electronic arrays. In conclusion, 2D materials could be laterally assembled via two-step growth, one-step growth, and conversion reaction. Only when the lattice mismatch is low enough, the 2D materials could form lateral heterostructures. This is the main reason why the number of 2D lateral heterostructures is less than that of 2D vertical heterostructures. As a matter of fact, lateral heterostructures could be employed to investigate the electrical properties and transport behaviors at the interfaces of them for understanding the interaction of 2D materials. Therefore, it is necessary to develop more new and efficient methods to construct 2D lateral structures.

Figure 34. Rolling up of CNS with microwave spark assistance in liquid nitrogen. (a) Schematic illustration of the exfoliation process for the fabrication of a CNS. (b) High-resolution characterization of CNS and its intermediate state. Reproduced with permission from ref 615. Copyright 2011 John Wiley & Sons, Inc.

when irradiated by microwaves, the graphene flakes (GFs) were heated to high temperature, resulting in an expansion in volume. At the same time, the GSs coated on the surface of the GFs were cooled by the liquid nitrogen, resulting in their shrinkage. The resulting surface strain drives the rolling up of a GS to produce a CNS, which is the energetically favorable state. Furthermore, the violent release of heat by the GFs resulted in a rapid boiling of the liquid nitrogen on the surface, which also contributed to the rolling up of the GSs. Figure 34b shows a typical transmission electron microscopy (TEM) image of a CNS exfoliated from a GF. The maximum mobilities of holes and electrons of the as-obtained CNS were estimated to be 3117 cm2 V−1 s−1 and 4595 cm2 V−1 s−1, respectively, which were larger than the mobility of pristine GS (∼100 to 2000 cm2 V−1 s−1) produced by CVD. The pure and high-quality CNSs obtained by this process will boost the rapid development of CNSs in terms of both fundamental research and applications. 4.2.1.2. Surface Dissymmetry-Induced Self-Rolling. Zhang et al. reported the rolling up a monolayer MoS2 sheet from the edges or the grain boundaries of MoS2 sheet assisted by a weak argon plasma treatment.616 The plasma bombardment could selectively remove the top layer of sulfur atoms partially, causing a tensile stress within the MoS2 basal plane and enabling the scroll formation, as shown in Figure 35. The approach offers several advantages. The fabrication of MoS2

4.2. Oriented Assembly

Intermonomer assembly, that is the construction of vertical or lateral heterostructure, can significantly tune the band structure and Fermi level of each 2D material monomer due to the strong interfacial coupling, while the structure assembly aiming at an individual 2D material deserves our attention since it helps further deeply reveal the intrinsic property of this material, such as the in-plane anisotropy and the interlayer interaction and so on. The high anisotropy of 2D materials in xy and z direction makes the oriented assembly possible and interesting. The oriented assembly strategy requires the small building blocks to exhibit specific structural evolution or to connect with each other in a confined 2D space by sharing a common crystallographic face. The oriented assembly derived structure deformation will lead to the property extension of the 2D materials and promote new applications. In this section, we will introduce the monomer structure assembly from two aspects, in-plane rolling and out-of-plane edge-to-edge coupling. 4.2.1. Rolling Assembly. Rolling up a thin solid film into nanoscale scrolls is an interesting topic in the field of nanoscience and nanotechnology. Clearly the geometry of a nanoscroll is different from its thin film matrix and would have a significant impact on its properties as well.587−596 So far, various thin films of metals,597 semiconductors,598,599 and insulators600−602 have been explored for nanoscroll rolling-up. Recent advances in the discovery of a wide variety of 2D materials, including graphene,1,2,603,604 h-BN,177 silicene,38,605 and MoS2,196,606 just to mention a few, allow one to explore the scrolling-up of them in an atomic-scale thickness limit. As predicted by theory, these nanoscrolls may have superior electronic and electromechanical properties,607−611 thus being useful in FETs.612 4.2.1.1. Microwave Irradiation-Assisted Self-Rolling. Microwave irradiation is a powerful tool for heating. Graphite displays strong microwave absorption ability because of its low resistance, with a dramatic temperature increase accompanied by luminous sparks.613,614 Zheng et al. explored a new, simple strategy to produce high-quality carbon nanoscrolls (CNSs)

Figure 35. Formation of MoS2 induced by surface dissymmetry. (a) The schematic diagram of the process of MoS2 nanoscroll formation induced by surface dissymmetry due to the S vacancy on one side. (b) AFM images showing the forming process of MoS2 nanoscroll. (c) TEM image of MoS2 nanoscroll to show its morphology. Reproduced with permission from ref 616. Copyright 2016 John Wiley & Sons, Inc. 6271


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which is developed the most sufficiently in the field of array fabrication, as an example. Fabrication of GNR arrays with narrower width is a bottleneck for quite a long time. The traditional photolithography failed to achieve higher precision due to the optical printing precision. Zhou reported patterning of ultranarrow aligned GNR arrays down to 5 nm width using helium ion beam lithography (HIBL) for the first time (Figure 37a).620 The as-fabricated GNRs arrays owned narrow, uniform, and adjustable widths with high precision and reproducibility.

nanoscrolls is fulfilled by Ar-plasma treatment, and this approach is one step. Such a fabrication method is solventsfree and the Ar-plasma treatment is in the gas phase. In addition, the yielding of nanoscrolls is very high. In their experiments, they used centimeter-scale continuous MoS2 films on substrates as the raw material and the formation of nanoscrolls are everywhere across the entire substrate. This convenient, solvents-free, and high-yielding approach for nanoscroll fabrication is also suitable for the fabrication of other 2D TMDs. 4.2.2. Edge-to-Edge Coupling. Due to the strong interlayer coupling of the 2D nanosheets, they tended to follow a face-to-face stacking rather than an edge-to-edge geometry.617 Undoubtedly, face-to-face stacking materials with poor surface area will significantly decrease the active functional area. Fu et al. demonstrated, for the first time, the abnormal edgeto-edge self-assembly process of a 2D nanomaterial, namely ReS2.618 The extremely weak vdW coupling and large anisotropy of ReS2 allow one to realize an oriented selfassembly (OSA) process spanning the nanometer to micrometer range. Also, ReS2 nanoflakes with the same size and shape tend to self-assemble in an edge-to-edge manner and eventually curl into micrometer-long nanoscrolls. Figure 36

Figure 37. Patterning strategy for the mass assembly of graphene structure. (a) HIBL for fabricating ultranarrow aligned graphene nanoribbon arrays. Reproduced from ref 620. Copyright 2014 American Chemical Society. (b) Substrate-controlled and metalassisted etching of graphene with controllable etching direction for fabricating aligned GNR arrays. Reproduced with permission from ref 623. Copyright 2013 John Wiley & Sons, Inc.

One of the most promising methods to produce GNR arrays is the catalytic etching of graphene (for example, using Ni or Fe) with relatively controllable etching direction.502,621,622 Ago et al. demonstrated the large-scale production of dense arrays of highly aligned GNRs by a controllable etching of graphene, in which the substrate offered a certain direction and metal NPs served as etching active reactants, as illustrated in Figure 37b.623 It is worth noting that the Ni NPs were deposited on the graphene film by radio frequency (RF) magnetron sputtering. The etching direction was precisely parallel to the [11̅01̅] direction of sapphire, which was analogous to the CVD growth of horizontally aligned CNTs due to the anisotropic van der Waals interaction between the substrate and the SWCNT. They produced GNRs with densities up to 25 nanoribbons μm−1 and widths of ∼19 nm, and the device on/off ratios were as high as 5000 at room temperature. 4.3.2. Locating Strategy. Nucleation is the first stage of the crystal growth, and the nucleation potential energy is the barrier that must be overcome during the crystal growth process. Before reaching the nucleation barrier, the clusters will aggregate and decompose repeatedly, leading a very slow growth process.624 Generally, the surficial electrons can significantly saturate the edges of the clusters and the activity of the platform is significantly lower than that of the defects such as atomistic steps and point defects on a metal surface, which makes them act as the dominant factors governing the site and density of heterogeneous nucleation.625−628 However, the distribution of the defects on a metal surface is usually random. Therefore, providing the controllable preponderant nucleation site, we can achieve the locating site of the 2D materials on the substrate.629−632 Johnson et al. reported the use of lithographically patterned islands of MoO3 or ammonium heptamolybdate (AHM) as the seeds for the growth of crystalline MoS2 monolayers at predefined locations on oxidized silicon substrates, as seen in Figure 38a.632 The process begins with patterning of an array of

Figure 36. Edge-to-edge OSA of ReS2 nanoflakes. (a) Schematic for the process of OSA of ReS2 nanoscrolls. (b) TEM characterization of the ReS2 NSs. Reproduced from ref 618. Copyright 2016 American Chemical Society.

demonstrates the formation process of ReS2 nanosrolls (NSs). They grew the ReS2 nanowalls (NWs) on the surface of a 3D graphene foam (3DGF). An electrochemical lithium intercalation process triggers the ReS2 NWs to exfoliate into nanoflakes. Due to the competition between oriented attaching and self-rolling processes, they can tune which process dominates and regulates the aspect ratio of the resulting ReS2 NSs. This method can be expanded to the edge-to-edge OSA of other 2D nanomaterials. 4.3. Mass-Ordered Assembly

When facing the future industrialization, the successful construction of an ordered structure of 2D monomers facing the integrated electronics is of great significance. Generally, the mass-ordered assembly could be classified into three categories, up-down patterning and bottom-up prelocating and selfassembly. 4.3.1. Patterning. Nowadays, 2D materials-based electronics produced or adjusted by various methods were demonstrated to show excellent transport properties. There is still a great deal of challenges in scalability and manufacturability. That means, various 2D materials monomers should be orderly integrated into assemblies. Processes are needed to achieve dense, aligned functional component arrays in order to allow a high current to pass through the channel materials for practical device applications.619 Here we still take graphene, 6272


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Figure 38. Patterned growth of patterned 2D materials and their heterostructure using seed locating strategy. (a) Schematic of the growth process for MoS2 using patterned molybdenum sources. (b) Optical image of an array of flakes of monolayer MoS2 grown by CVD. Reproduced with permission from ref 632. Copyright 2015 Nature Publishing Group. (c) Schematic diagrams of the growth of patterned G/h-BN domains and monolayers using predeposited PMMA seeds on Cu foils as nucleation sites. (d) SEM image of patterned graphene domains grown from the PMMA seeds on the predeposited monolayer h-BN. Reproduced from ref 631. Copyright 2016 American Chemical Society. (e) Schematic illustration of the patterned growth of regular arrays of perovskite microplate crystals. (f) OM image of PbI2 seed arrays after flow seeding process and the further growth in saturated PbI2 solutions. Reproduced with permission from ref 335. Copyright 2015 AAAS.

cm2 V−1 s−1 at room temperature, which is comparable with that of CVD graphene devices on exfoliated h-BN by transfer. Large-scale transparent electronic devices were also fabricated on the patterned G/h- BN samples after transfer onto quartz glass to demonstrate its great potential application in highperformance optoelectronics. Besides the locating strategy for 2D-layered material single crystal array, Duan et al. reported the first-patterned growth of regular arrays of perovskite crystals, as seen in Figure 38e.335 The substrates were first functionalized with a self-assembled monolayer of (octadecyl)trichlorosilane (OTS) to produce a hydrophobic surface. Then they employ a photolithography process and oxygen plasma treatment to create periodic arrays of hydrophilic areas on the hydrophobic surface. Subsequently, when an aqueous seeding solution flows over the surface of the substrate, the PbI2 seeds will only form in hydrophilic regions, that is in the patterned regions on the substrate. The seeded substrate was then immersed in saturated PbI2 aqueous solution to allow seeded growth of PbI2 microplates. The typical growth result is shown in Figure 38f. They further showed that the seeded growth strategy could be employed for the selective growth of perovskite crystals on prepatterned gold electrodes, thus creating photodetector arrays that are independently addressable and discrete transistors. 4.3.3. Self-Assembly. As a common phenomenon in nature, self-assembly is a process in which disordered building blocks spontaneously organize with each other to form an ordered structure by applied external field or interactions. Therefore, it is suggested that self-assembly can be employed as an effective tool for nanofabrication. Directed assembly of 2D layered materials holds great promise for large-scale electronic

square windows, typically 5−10 mm on a side, by e-beam or optical lithography. Next, molybdenum-containing seed material is deposited into the windows, either by thermal evaporation of MoO3 or by spin-casting of a saturated solution of AHM in water. The resist and unwanted molybdenum source are removed in a standard lift-off step. Before the growth, the molybdenum-containing seed particles were aggregated and the substrate was also pretreated to promote the monolayer growth. The typical growth result of the MoS2 arrays was shown in Figure 38b. The patterned growth approach to monolayer MoS2 has the advantage of enabling direct fabrication of multiple devices. As the MoS2 flakes were grown at known, prepatterned locations, it would be straightforward to align an electrode pattern to the MoS2 flakes, with no need for a separate etch step. Device mobilities were between 8.2 and 11.4 cm2V−1 s−1 and the on/off ratios were larger than 106, which was comparable to earlier best reports for exfoliated MoS2 monolayers. Except the pure 2D materials, the located growth of 2D material heterostructures has also been achieved successfully. Liu et al. realized the effective control over the nucleation densities and locations of graphene domains on the predeposited h-BN monolayers with the aid of the well-defined PMMA seeds, leading to the formation of patterned G/h-BN arrays, as seen in Figure 38c.631 The seed-assisted growth approach was expected to facilitate uniformity of graphene domain size, location, and the crystal quality, providing a promising route to a large-scale integration of high-performance G/h-BN based electronics, as seen in Figure 38d. Electrical measurements were then performed on G/h-BNbased FETs. The carrier mobility was calculated to be ∼11000 6273


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Figure 39. Self-assembly strategy for obtaining ordered 2D single crystals or nanostructure. (a) Schematic of the electric-field-assisted assembly system for WS2 single crystal. (b) Step-by-step magnification SEM images of WS2 flakes to show the assembly process. Reproduced from ref 635. Copyright 2016 American Chemical Society. (c) Schematic of experimental setup for applying magnetic field and magnetic field induced the graphene flake orientation control of a diluted suspension. (d) Dynamics of a multilayer graphene flake in water under a magnetic field. Reproduced with permission from ref 642. Copyright 2017 John Wiley & Sons, Inc.

Figure 40. Self-assembly of graphene single crystals into the 2DSOS. (a) SEM image of GSOS. (b) Statistical distribution about the deviation of size and distance of GSOS. (c) Percentile curve and (inset) histograms of the rotation angle of GSOS. (d) Schematic of the self-assembly mechanism of the graphene single crystals in order to form GSOS. Reproduced from ref 643. Copyright 2016 American Chemical Society.

of graphene flakes, as seen in Figure 39 (panels c and d).642 Multilayer graphene flakes with an average thickness of 2.4 nm were synthesized via intercalation and exfoliation of graphite. When the surface of a diamagnetic graphene flake is oriented perpendicular to an external magnetic field, the induced magnetic field is in a direction opposite to the external field, leading to repulsive magnetic force and increased total interaction energy. In order to minimize the magnetic potential energy, the flake will rotate away from the field until its surface is in parallel with the external field. To better understand the dynamics of flakes under magnetic field, the orientation of a multilayer graphene flake in real time was monitored. It can be seen that the flake is initially laid flat at the bottom of the beaker. After the application of a 24 mT vertical magnetic field, the flake began to turn and finally stood up vertically on one of its edges, in alignment with the magnetic field. By combining macroscopic alignment with anisotropic optical property of graphene, novel device applications of graphene in magnetic field sensing and magnetic field controlled displays were also demonstrated. Besides the assembly after the synthesis of 2D material single crystals, direct growth of 2D material single crystal superordered structures can be a huger challenge. Fu et al. succeeded for the first time in synthesizing a 2D superordered structure (2DSOS), as seen in Figure 40a.643 They preset condensed

and optoelectronic applications.633,634 Mayer et al. demonstrated controlled placement of solution-suspended monolayer WS2 sheets on a substrate using electric-field-assisted assembly. Micrometer-sized triangular WS2 monolayers are selectively positioned on a lithographically defined interdigitated guiding electrode structure using the dielectrophoretic force induced on the sheets in a nonuniform field, as shown in Figure 39 (panels a and b).635 Triangular sheets with sizes comparable to the interelectrode gap assemble with an observed preferential orientation where one side of the triangle spans across the electrode gap. Semianalytical calculations were employed to confirm that the orientation of the sheets relative to the guiding electrode is consistent with the lowest energy configuration. Nearly all sheets assemble without observable physical deformation, and postassembly PL and Raman spectroscopy characterization of the monolayers reveal that they retain their as-grown crystalline quality. Such field-assisted assembly process may be used for large-scale bottom-up integration of 2D monolayer material for nanodevice applications. Macroscopically ordered nanomaterials have many benefits in device applications.636−638 When 2D material flakes, take graphene as an example, are assembled in the same planar direction, they show excellent thermal, optical, electrical, and electromagnetic shielding properties.639−641 Bao et al., for the first time, demonstrated the magnetic response and alignment 6274


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Figure 41. Controllable fabrication of h-BN-G CSA. (a) Schematic for the formation process of h-BN-G CSA. (b) Large-area optical image of the CSA with high uniformity. (c) Statistical distributions of the inner, outer diameters, included angles and center distances. (d) Raman mapping of the intensity of 2D peak (attributed to graphene) and the intensity of E2g peak (attributed to h-BN) of a core−shell circular unit. Reproduced from ref 644. Copyright 2017 American Chemical Society.

PMMA seeds on a fluidic liquid Cu surface. With an appropriate airflow with uniform speed and direction for disturbance, the original distribution of the seeds will be broken and the seeds tend to arrange in an energetically favorable way on the rheological surface, in which the spacing between the neighboring seeds will be nearly the same so as to balance evenly the whole energy on the liquid surface. Then CH4 is introduced, the seeds continue to grow. As the growth progresses, the mutual electrostatic force between the adjacent crystals continually increases, thus ultimately resulting in the formation of graphene superordered structure (GSOS). The asobtained 2DSOS exhibits tunable periodicity in the crystal space and outstanding uniformity in size and distance, as seen in Figure 40b. The most fascinating feature of the presented approach is its precise control of the crystallographic orientation, as seen in Figure 40c. The root cause lies in the electrostatic interaction of the as-grown graphene single crystals on the liquid metal catalyst. The edge of this structure has an anisotropic electrostatic potential distribution owning to the directivity of the static electric field, each individual graphene single crystal tends to adjust its own orientation to match that with the neighboring ones. When it comes to a group of seedprelocated graphene single crystals, each graphene single crystal in the array will try to rotate to match itself due to the anisotropic static electric field, as exhibited in Figure 40d. The rheological property of the underlying amorphous liquid substrate makes a self-adjusting rotation possible. Moreover, the intrinsic property of each building block is preserved. The electrical performance of the GSOS was examined by constructing back-gated FET array. The derived mobility is 3882 ± 896 cm2 V−1 s−1, which reflects the high quality of the GSOS and is comparable with that of the unassembled graphene single crystals. In addition, the GSOS was employed as electrode pairs for connecting MoS2 flakes, the as-derived carrier mobility based on the MoS2 single crystal is as high as 441 cm2 V−1 s−1, thus demonstrating its potential for building nanoscale device arrays. Fu et al. also demonstrated the controllable construction of self-aligned single-crystals array (SASCA) on the liquid Cu substrate.172 The formation of the circular h-BN single-crystals on the isotropic liquid Cu surface is the key since they can be easily closely packed to each other with a certain accordant spatial orientation. The as-obtained SASCA can be expected to serve as gate dielectrics for highly integrated and individually switching FETs.

In addition to the self-assembly of the high-quality onecomponent 2D single crystals, Fu et al. succeeded, for the first time, in synthesizing 2D h-BN−G core−shell arrays (CSA), which possess extremely high uniformity in shapes, sizes, and distributions.644 Each of the core−shell units is composed of G ring-shaped shell internally filled with h-BN circular core. The schematic for the formation of h-BN−G CSA was shown in Figure 41a. By using liquid metal as the catalyst, large-area uniform graphene films were obtained, which is a crucial premise for the further large-area self-assembly of h-BN. The growth of h-BN was then carried out on the solidified Cu with graphene covering on it, in which the formation of core−shell -ordered structure involved the adsorption and self-symmetrical arrangement of h-BN building blocks (such as borazine) on graphene platform and subsequent in situ etching and substitution of graphene by h-BN building blocks. Finally, the growth of h-BN led to both cores and shells formation, thus the h-BN−G CSA was successfully synthesized. The morphology and uniformity of the as-grown h-BN−G CAS were further evaluated by the statistics, as shown in Figure 41 (panels b and c). They also employed Raman mapping for further characterizing the distribution of h-BN and graphene in the CSA, thus demonstrating the h-BN-G core−shell structure, as seen in Figure 41d. Such a unique self-assembly strategy, which possesses considerable controllability and efficiency, is going to open new territory for the precise and large-scale synthesis of more 2D-ordered arrays with complex and functional material structures and will facilitate their application in 2D integrated systems and devices.

5. CIRCUITS BASED ON 2D MATERIALS 2D Materials could be widely applied to electronics due to their unique advantages in carrier mobility or switching performance. Such electronics could finally be used in the construction of logic gates or complex electrical elements, which are recognized as electronic circuits. An electronic circuit is composed of individual electronic components (such as resistors, transistors, capacitors, inductors, and diodes) connected by conductive wires and trace. Electronic circuits could also be known as ICs, and they could be further connected to form multifunctional and highly integrated devices. As one of the most important kinds of components in ICs, FETs with different channels could serve as different logic circuits. Here, n-channel metaloxide-semiconductor (NMOS) and p-channel MOS (PMOS) 6275


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Figure 42. Electronic circuits such as inverters and logic gates based on 2D materials. (a) Schematic illustrations of the electronic circuit based on monolayer MoS2. Vout as a function of Vin of (b) a MoS2-based inverter and (c) a MoS2-based NOR gate circuit, and schematic drawing of electronic circuits and truth tables are shown in the insets of (b) and (c). Reproduced from ref 645. Copyright 2011 American Chemical Society. (d) Schematic illustration of the vertical stacked inverter. (e) Characteristics of the vertical stacked inverter, and the relative voltage gain is about 1.7. Reproduced with permission from ref 647. Copyright 2013 Nature Publishing Group. (f) Schematic illustration of MoS2 gigahertz signal inverter. (g) Input (a square wave of 200 MHz with amplitude of 100 mV and −1.3 V dc gate bias) and output signals of the few layer MoS2 inverter, and the output signal is shifted in phase by 180° with a relative voltage gain of 2 over the input one. (h) Schematic drawing of MoS2-based RF amplifier circuit. Sinusoidal input signals of (i) 100 MHz and (j) 1 GHz coupled with −0.5 dc gate bias, and the output signals show gains of (i) 2 and (j) 1.07, respectively. (k) The propagation delay detected by applying a square wave to the input electrode and measuring the output signal via an oscilloscope. Reproduced with permission from ref 649. Copyright 2014 Nature Publishing Group.

effect.646 On the basis of graphene and MoS2, Duan et al. put forward a vertical stacked FET (VFET) which could satisfy two important requirements of high-performance logic applications−high on−off current ratio and high current density. As shown in Figure 42d, the graphene-MoS2−Ti VFET is connected with graphene-Bi2Sr2Co2O8 (BSCO, p-channel)graphene VFET to form a CMOS transistor, which could be used as an inverter due to its NOT logic operation (Figure 42e).647 They also used graphene-indium gallium zinc oxide (IGZO)-Ti vertical thin film transistors (VTFTs) to form logic circuits.648 For short channel FETs based on MoS2, Duan et al. fabricated a dual-channel MoS2 FET which could work as an inverter, as shown in Figure 42f. At a direct current bias of 5 V, an input signal of 200 MHz square wave could be inverted, and the inverted output signal with a relative voltage gain of 2 could be obtained from the output electrode at the same operating frequency without any noticeable delay (Figure 42g). Such a MoS2 inverter could be used in a RF amplifier, and the integrated structure of it is shown in Figure 42h. With the input of sinusoidal waves of 100 MHz and 1 GHz, sinusoidal waves with larger amplitude−relative voltage gains of 2 and 1.07 could be observed in the output electrode, as shown in Figure 42 (panesl i and j, respectively). The propagation delay of the amplifier is detected by applying a square wave on the input electrode and measuring the output signal through an oscilloscope. As shown in Figure 42k, the rise times of input and output signals are of 210 and 580 ps, respectively, which indicates the propagation delay is of 370 ps of a MoS2 RF amplifier.649 These constructions could also be applied in flexible electronic circuits.647−649 In addition, ReS2 or WSe2 transistors connected in series, as well as CMOS transistors based on WSe2/MoS2 or BP/MoS2, could also be applied in current inverters.650−652 Except for semiconductors, 2D materials with other properties, such as TaS2 with a CDW phase, could be utilized in other electronic circuits, for example, oscillators.653 More and more simplified 2D material-based

FETs could be used as different switches in an electronic circuit. For a NMOS transistor, when the input Vg is low, the switch will be turned off, and when the input Vg is high, the switch will be turned on. The OFF and ON states of a PMOS transistor are opposite to those of a NMOS transistor. Therefore, such FETs could be connected with each other to form logic gates (AND, OR, NOT, NAND, and NOR gates) and finally to construct logic circuits. For instance, a NMOS transistor could be a NOT gate, and when two NMOS transistors are put in parallel and in series, a NOR gate and a NAND gate could be constructed, respectively. Three NMOS transistors complicatedly connected with each other could also form AND and OR gates. Moreover, a CMOS transistor constructed by in series connection of a PMOS and a NMOS transistor could work as a NOT gate. When FETs connected with resistors and other components, the Vout directly depends on the Vin. After defining low and high voltage as 0 and 1, respectively, truth tables could be easily calculated according to the logic circuits so that various logical calculation and operation could be achieved. 2D Material-based electronics are therefore very important elements in electronic circuits. Recently, researchers have been taking more and more interest in 2D material-based electronic circuits. The first step of the fabrication of complicated ICs is to construct 2D material-based logic gates containing two or three components. Kis et al. constructed an IC based on a monolayer MoS2, as shown in Figure 42a. With different wiring, the circuit could work as a NOT or a NOR gate logic circuit (Figure 42, panels b and c). As shown in Figure 42b, the electronic circuit is equal to two NMOS transistors (MoS2 as an n-channel) in series, acting as a NOT gate, which could be used in an integrated MoS2 inverter. When the circuit is constructed by two MoS2 transistors connected in parallel and a resistor of 1 MΩ is served as a load, the circuit could show a NOR gate operation, as shown in Figure 42c.645 The MoS2-based ICs take good advantage of monolayer MoS2 to reduce the short-channel 6276


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Figure 43. Complicated ICs based on 2D materials. (a) Optical image and schematic drawing of the 5-stage ring oscillator based on MoS2. Reproduced from ref 654. Copyright 2012 American Chemical Society. (b) Layout and optical image of the MoS2-based test chip using the proposed flow. Reproduced with permission from ref 655. Copyright 2016 American Chemical Society. (c) Optical image and schematic drawings of a MoS2based microprocessor. Reproduced with permission from ref 656. Copyright 2017 Nature Publishing Group. Schematic illustrations of (d) impression of the CVD-graphene transfer process on a single die containing an image sensor read-out circuit and (e) detailed construction of the image sensor array. (f) NIR and SWIR light photograph of an apple and a pear, and (g) VIS, NIR, and SWIR light photograph of a box of apples, illuminated by an incandescent light source (1000 W, 3200 K). Reproduced with permission from ref 658. Copyright 2017 Nature Publishing Group.

qualified as the key materials for the next-generation electronics. Being just one or several atoms thick, highly disparate 2D materials can be assembled together to create a wide range of heterostructures or ordered structures in macro scale without the constraints of lattice matching and processing compatibility. The novel properties in the 2D assemblies with diverse layering structures promote the emergence of new design of electronic devices, including tunneling transistors, barristors, and flexible electronics, as well as optoelectronic devices, referring to photodetectors, photovoltaics, and lightemitting devices with unprecedented characteristics or unique functionalities. The 2D materials can be tailored or assembled to achieve specific properties, thus opening the new territory for next-generation electronics. In this review, we summarized the recent progress from the design of the 2D material monomer to their assemblies at various scales. In accordance with the layer structure and chemical composition, the classical or emerging 2D materials were exhibited from the structure and property as well as the potential application in the future electronics. What should be noted is that the properties of the 2D materials building blocks can be highly designed. Different synthetic methods can be used to prepare 2D materials with varying structural features, including dimensions in each direction, doping, alloying, and vacancy. In addition, external field tuning techniques are also very effective for remodeling the properties of the 2D materials due to their atomic thickness. Electric field and light are both demonstrated to be powerful tools in modifying the band structure and Fermi level of the 2D materials. What’s more, the structure tuning also deserves our attention since it plays significant roles in determining the electric properties of the 2D materials, including the phase transition, edge structure reconstruction, and crystal lattice deformation. In spite of the monomer design, the assembly of the 2D materials toward circuits is another core concern of this review. We summarized the intermonomer assembly behavior, including the construction of vertical and lateral heterostructures. In spite of this, the specific oriented assemblies were also included, which will greatly enrich the property of 2D materials. Furthermore, mass-ordered assembly strategies were concluded as well, which laid the solid foundation for the future practical applications. At last, an overview of the circuits based on 2D materials was

electronic circuits containing several electronics have been fabricated, which could promote the development of ICs. However, the practical application of ICs based on 2D materials is still very difficult to achieve. For such high integration ICs, MoS2 and graphene have been involved as the channels. On the basis of MoS2, 5-stage ring oscillators, test chips, and microprocessors have been achieved by complex circuit design, as shown in Figure 43 (panels a−c).654−656 As for graphene, it could also be used in oscillators.657 More recently, CVD graphene has been applied in hybrid graphenePbS colloidal quantum dots (CQDs)-based broadband image sensor array by integrating graphene-CMOS circuits (Figure 43, panels d and e). The imaging could cover NIR, short-wave infrared radiation (SWIR) and visible (VIS) lights, as shown in Figure 43 (panels f and g).658 Though ICs based on MoS2 and graphene exhibit an excellent application prospect, complicated ICs, such as very large scale integration (VLSI)659 and chips, based on other 2D materials are still gaps in the synthesis method and device construction. To sum up, electronic circuits based on 2D materials have been studied since the very recent years. Lower dielectric constants of 2D materials than those of conventional semiconductors, combined with ultimate thinness, could result in extremely short scale lengths and significant reduction of short-channel effect. Furthermore, the heavy electron effective mass in 2D materials could also reduce the source-drain tunneling in ultrashort MOS FETs. Another advantage is that flexible electronic circuits could be easily achieved by 2D material-based electronics.660 However, there are still many difficulties in the construction of large-scale homogeneous 2D materials for complicated ICs. The contacts are also problems, referring to the contact resistances between the source or the drain terminals and the 2D material channels.660 New constructions of 2D material-based ICs still need to be developed, as well as novel detecting strategies, such as ptychographic X-ray computed tomography (PXCT), which could directly but nondestructively detect the inside structures of electronic circuits.661

6. CONCLUDING REMARKS AND OUTLOOK Interest in 2D materials is growing exponentially across the scientific community owing to their fascinating electrical, optical, and thermal properties, which facilitate them to be 6277


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2D material component makes it extremely difficult. Furthermore, the macro mass assembly of 2D material crystals into designed ordered structure calls for urgent development, which is highly demanded for the ICs. The competition of the 2D materials with the conventional semiconductors is enthralling. However, issues related to the large-area growth, reliable and repeatable synthesis techniques, and low contact resistance are still the key challenges, considering the commercially viable products. In addition, the device fabrication process is still quite complicated and inefficient for practical applications due to the complexity and unreliability of 2D materials, especially for their heterostructures. Therefore, developing a new and reliable process on a large scale will be the ultimate challenge for successfully integrating the 2D materials based devices into the mainstream electronics. There is still another area of concern. In the shortterm, it is unlikely that 2D materials will replace Si CMOS technology in some areas. However, significant progress is being made in transparent displays and flexible electronics in the near future. The availability of an increasingly broad library of 2D materials with variable electronic properties and the unique ability to be thinned and restacked into functional and complex assembled structures enables a new dimension for materials engineering and device design. However, despite the extraordinary potential and considerable progress so far, the actual realization of 2D materials’ applications in the practical technologies should not be ignored. A complete understanding of the assembled structures of 2D materials monomers to unlock their potential is yet to be fully developed. Nonetheless, the continuous developments of the elaborate monomer design and assembly control of 2D materials offer unprecedented opportunities for new applications that represent the future of advanced next-generation circuits.

provided to exhibit the development progress toward the nextgeneration electronics. In order to achieve progress in the fundamental research and their practical applications, the exploration in the design of the 2D materials monomers and their assemblies are faced with more and more challenges. First, untraditional 2D layered materials or even nonlayered materials are on the rise. Traditional layered 2D materials, represented by graphene, consist of a covalently bonded lattice and are weakly bonded to neighboring layers via van der Waals interactions, which make them feasible to be isolated. Nowadays, some layered materials bonded by strong interlayer interaction, such as ionic bonding, or even nonlayered materials were also successfully thinned to atomic thickness by some specific designs. New physical properties will be generated and innovative applications can be brought forth. Undoubtedly, developing non van der Waals layered or nonlayered materials will enrich the 2D crystal library. However, on the other hand, nonlayered structures determine that their corresponding ultrathin structure with atomic thickness is difficult to achieve. As the top-down exfoliation method is ineffectual for the preparation of ultrathin nonlayered nanomaterials, the existent preparation methods almost focus on the wet-chemical synthesis and topochemical transformation, in which the relatively poor quality of the crystals hinders their application in the nanoelectronics. Therefore, the preparation of ultrathin high-quality 2D nonlayered crystal is one of the challenges in this field. Second, from the point view of material synthesis and property tuning, the current production or property-tuning yield, quality, and synthesis rate of 2D materials are still far from the criteria that are demanded for industrialization and commercialization. On the one hand, the very limited selection of the growth substrate, the uncontrolled, and complex thermodynamic and dynamic processes make the reliable and steady production of 2D materials a harsh task. On the other hand, existent property tuning strategies, including chemical and physical ones, also face problems in tuning uniformity and efficiency. In addition, another challenge in this field lies in the development of effective characterization techniques to explore the growth mechanism of ultrathin 2D nanomaterials and the property tuning principles. Excitingly, some in situ characterization techniques, such as the in situ TEM, in situ X-ray diffraction (XRD), in situ Raman spectroscopy, and in situ X-ray photoelectron spectroscopy (XPS) have been explored to identify the construction of the 2D materials and figure out the relationship between the structure and the property. However, limited to the spatial and time resolution, the precise mechanism identification of the growth and property tuning of 2D materials still remains unclear. Third, the precise and large-scale assembly of the 2D materials has rarely been explored, especially on the premise of the crystal quality. The creation of van der Waals heterostructures with atomically sharp interfaces and highly distinct electronic functions offers a new material platform and a rich playground for probing the generation, confinement, and transport of charges, excitons, photons, and phonons at the limit of atomic thickness and promotes the related potential applications. However, the majority of the existing research relies on the layer-by-layer exfoliating and stacking with the assistance of polymers and inevitably will suffer from the uncontrollability of size and thickness and even the contamination. Direct synthesis of the 2D material heterostructures is of great importance, although the incompatibility of the growth parameter of each individual

AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected]. ORCID

Lei Fu: 0000-0003-1356-4422 Notes

The authors declare no competing financial interest. Biographies Mengqi Zeng received her B.S. from Wuhan University in 2013 and continued her studies as a Ph.D. candidate under the supervision of Prof. Lei Fu at College of Chemistry and Molecular Sciences at Wuhan University. Her current research interests include the controllable growth, assembly, and transfer of two-dimensional materials on liquid metal catalysts. Yao Xiao received his B.S. in chemistry from Wuhan University in 2016 and continued for a Ph.D. candidate under the supervision of Prof. Lei Fu at the Institute of Advanced Studies at Wuhan University. His current scientific interests are focused on the special growing phenomena and electronics of two-dimensional materials and heterostructures. Jinxin Liu received his bachelor’s degree in chemistry (2015) and M.S. degree in physical chemistry (2017) from Wuhan University. Currently, he worked as a Ph.D. candidate in the research group of Prof. Lei Fu at the Laboratory of Advanced Nanomaterials of Wuhan 6278


Chemical Reviews

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University. His current scientific interests are focused on the controllable growth of two-dimensional materials and heterostructures. Kena Yang received her B.S. from Ocean University of China in 2016 and continued her studies as a master’s degree candidate under the supervision of Prof. Lei Fu at College of Chemistry and Molecular Sciences at Wuhan University. Her current research interest is the controllable growth of two-dimensional material. Prof. Lei Fu received his B.S. degree in chemistry from Wuhan University in 2001. He obtained his Ph.D. degree from the Institute of Chemistry, Chinese Academy of Sciences in 2006. After obtaining his Ph.D., he worked as a Director’s Postdoctoral Fellow at the Los Alamos National Laboratory, Los Alamos, NM (2006−2007). Thereafter, he became an Associate Professor at Peking University. In 2012, he joined Wuhan University as a Full Professor. His current interests of research relate to the controlled growth and novel property exploration of 2D atomic layer thin crystals.

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