Controllable Fabrication of Graphene and Related Two-Dimensional

Article Cite This: Acc. Chem. Res. 2018, 51, 2839−2847

pubs.acs.org/accounts

Controllable Fabrication of Graphene and Related Two-Dimensional Materials on Liquid Metals via Chemical Vapor Deposition Mengqi Zeng† 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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S Supporting Information *

CONSPECTUS: Due to the confinement of the charge, spin, and heat transport in the plane, graphene and related two-dimensional (2D) materials have been demonstrated to own many unique and excellent properties and witnessed many breakthroughs in physics. They show great application potential in many fields, especially for electronics and optoelectronics. However, a bottleneck to widespread applications is precise and reliable fabrication, in which the control of the layer number and domain assembly is the most basic and important since they directly determine the qualities and properties of 2D materials. The chemical vapor deposition (CVD) strategy was regarded as the frontrunner to achieve this target, and the design of the catalytic substrate is of great significance since it has the most direct influence on the catalysis and mass transfer, which can be the most essential elemental steps. In recent years, as compared to traditional solid metal catalysts, the emergence of liquid metal catalysts has brought a brand-new perspective and contributes to a huge change and optimization in the fabrication of 2D materials. On one hand, strictly self-limited growth behavior is discovered and is robust to the variation of the growth parameters. The atoms in the liquid metal tend to move intensely and arrange in an amorphous and isotropic way. The liquid surface is smooth and isotropic, and the vacancies in the fluidic liquid phase enable the embedding of heteroatoms. The phase transition from liquid to solid will facilitate the unique control of the mass-transfer path, which can trigger new growth mechanisms. On the other hand, the excellent rheological properties of liquid metals allow us to explore selfassembly of the 2D materials grown on the surface, which can activate new applications based on the derived collective properties, such as the integrated devices. Indeed, liquid metals show many unique behaviors in the catalytic growth and assembly of 2D materials. Thus, this Account aims to highlight the controllable fabrication of graphene and related 2D materials on liquid metals. By utilizing the phase transition of liquid metals, the segregation of precursors in the bulk can be controlled, leading to self-limited growth. By utilizing the fluidity of the liquid metals, 2D material crystals can achieve self-assembly on their surface, including oriented stitching, ordered assembly, and heterostacking, which enables the creation of new multilevel or hybrid structures, leading to property and function extension and even the emergence of new physics. Finally, the unique liquid characteristic of liquid metals can also offer us new ideas about the transfer process. By utilizing the shear transformation of liquid metals, the direct sliding transfer of 2D materials onto arbitrary substrates can be realized. The research concerning the self-limited growth, self-assembly, and sliding transfer of 2D materials on liquid metals is just raising the curtain on the behavioral study of 2D materials on liquid metals. We believe these primary technology developments revealed by liquid metals will establish a solid foundation for both fundamental research and practical application of 2D materials.

1. INTRODUCTION Two-dimensional (2D) materials represented by graphene are a kind of materials with thickness comparable to one or several atoms. The unique confinement structures endow them with novel physical and chemical properties.1 2D materials are expected to play important roles in future electronics, optoelectronics, and innovative ultrathin and flexible devices.2,3 However, whether this emerging material family can live up to our expectations depends on the establishment of controllable fabrication techniques.4 To date, chemical vapor deposition (CVD) has been regarded as the most cost-effective and promising method for controllably growing large-area and high-quality 2D materials. 4,5 Although the fabrication © 2018 American Chemical Society

techniques for graphene and related 2D materials have made much progress, how to preserve their various properties and device performances in the high-yield process is still an enormous challenge, including the fabrication of large-area high-quality films with precise and uniform layer number, the integration and assembly at wafer size, and the reliable property extension by constructing new multilevel or heterostacked structures. In a CVD process, the design of catalytic substrates will be very critical.6 By affecting the elemental steps, they determine Received: June 21, 2018 Published: September 17, 2018 2839

DOI: 10.1021/acs.accounts.8b00293 Acc. Chem. Res. 2018, 51, 2839−2847

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Accounts of Chemical Research

control over the layer number and the domain distribution. Smoothening (such as thermal annealing, electropolishing, chemical polishing, and oxygen induced surface reconstruction) the polycrystalline metal substrates becomes an endeavor to obtain homogeneous monolayer graphene.14 However, these processes are tedious and the as-produced graphene is still unsatisfactory. It is urgent to establish a more effective strategy. Single crystal substrates have become the preferred solution and attracted great attention.15 Regretfully, their exorbitant price seriously limits mass production. As for metal alloys, although they show apparent advantages over the layer number control, the as-grown graphene indeed consists of small domains, leading to sharp decline of the properties.11 In addition, for the growth of related 2D materials, such as hexagonal boron nitride (h-BN) and transition metal dichalcogenides (TMDs), they follow the surface adsorption growth mode. The as-obtained materials are usually nonuniform multilayer films or single crystals due to the uncontrollable adsorption of extra precursor. The liquid metals have witnessed landmark success for the controllable fabrication of 2D materials. When in a melted state, liquid metals possess an amorphous and isotropic atomic structure, which can effectively eliminate the negative effects derived from the grain boundaries. In addition, due to relatively free atom thermal movements in the liquid phase, the range of the atoms or electrons is extended, which ensures self-assembly of the 2D materials on the surface of the liquid metals. We can expect that the unique catalysis and masstransfer processes on liquid metals will trigger new growth and assembly behavior of 2D materials.

the decomposition efficiency of the precursors, the deposition morphology, and the final quality of 2D materials.7 Traditional growth of 2D materials is based on a stiff solid substrate. A large number of crystalline defects are randomly distributed on the surface, and the atom arrangement in the bulk is anisotropic with certain periodicity, which makes the nucleation of 2D materials very stochastic with high density and mass transfer efficiency relatively low. The resulting materials usually exhibit inhomogeneous layer number and indeed consist of small domains and massive grain boundaries, the properties of which exhibit huge disparity with theoretical expectations. In addition, since the subsequent growth of 2D materials after nucleation will continue at the fixed initial nucleation sites on the rigid solid substrate, the self-assembly of grains grown on solid metals is nearly impossible. In recent years, our group proposed the liquid metal CVD (LMCVD) strategy for the controllable fabrication of 2D materials. The change in the physical state of the metal catalyst brings about a qualitative change in the growth and assembly behavior of 2D materials. By utilizing the phase transition of liquid metals, self-limited growth of 2D materials can be triggered, leading to the obtainment of a strictly uniform monolayer. By use of the rheological properties of liquid metals, the artful self-assembly of the 2D material crystals can be effectively achieved, including oriented stitching, ordered assembly, and heterostack assembly. In addition, the particular shear transformation of liquid metals can also allow the new idea of the ultraclean sliding transfer process. This Account aims at summarizing our recent works concerning the unique self-limited growth and self-assembly of 2D materials on liquid metals. These primary and significant developments will become the solid foundation for future industrial applications of 2D materials.

3. SELF-LIMITED GROWTH For the growth of 2D materials, layer number control is the most basic requirement. Taking graphene as an example, its characteristics are highly dependent on the layer number.16 Monolayer graphene is gapless, while the band gap of bilayer graphene can be continuously tuned under an electric field. However, CVD growth of 2D materials with precise layer number is a huge challenge due to the uncontrollability of bulk segregation and surface adsorption of extra precursor species or atoms (Figure 1), which is nearly inevitable for the solid metal catalytic system. According to the growth mode of 2D materials, the self-limited growth can be generally achieved by two strategies. For the precipitation growth mode, selflimited growth can result from the substrate trapping of the extra dissolved precursors in the substrate bulk. For the surface adsorption growth mode, self-limited growth can be driven by

2. METAL CATALYSTS FOR CVD GROWTH OF 2D MATERIALS The metal catalyst plays a significant role in the growth of 2D materials. Taking graphene as an example, it goes through four most important elementary steps in the CVD process: (1) the precursors are adsorbed and decomposed on the surface of the metal catalyst; (2) the carbon species diffuse on the surface or dissolve into the bulk of the metal catalyst; (3) the dissolved carbon atoms are segregated from the bulk toward the surface; (4) graphene nucleates and grows on the surface. The metal catalysts can facilitate the feedstock decomposition since they will absorb and activate the reacting medium by forming intermediate compounds. In additon, the lattice mismatch and the carbon solubility of the substrate also have a significant effect on graphene growth.8,9 For example, compared to Ni with a high carbon solubility, Cu with an extremely low carbon solubility performs better for synthesizing uniform graphene. The design of alloy catalysts, such as Cu−Ni10 and Ni−Mo,11 is a very effective method of tuning the CVD elementary processes toward controllable growth of high-quality uniform graphene. In addition to the chemical composition, the surface structure, morphology, and physical state of the catalysts also deserve our attention.12 For the widely utilized solid metal substrates, there is a large amount of structural defects on the surface. In the CVD process for growing graphene, the carbon atoms tend to adsorb on the defects to launch the nucleation or the growth, and the dissolved ones in the metal bulk also preferentially precipitate at these sites,13 leading to poor

Figure 1. Design principle for the self-limited growth of 2D materials via CVD process. 2840


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on non-metal surfaces by the filtration of the undesired kinetic reaction path. Commonly used non-metal substrates do not exhibit excellent catalytic reactivity for the growth of graphene, leading to poor crystallinity, quality, and layer number controllability. Our group proposed a Ga vapor assisted growth of high-quality monolayer graphene on quartz.23 Ga can effectively grasp the hydrocarbons24 and then efficiently catalyze their decomposition into carbon atoms or fragments. This eliminates the accumulation of undecomposed precursors and ensures that high-quality graphene follows self-limited growth behavior on the quartz substrates. In addition to graphene, the liquid metal vapor assisted growth strategy also can be extended to achieve the self-limited growth of other 2D materials. Our group prepared strictly single-layer WSe2 crystals by the Cu-assisted self-limited growth (CASLG) strategy.25 Generally, layer accumulation and size expansion coexist during the CVD growth process, which is the main cause for the formation of undesirable multilayer structure. The key to the CASLG strategy lies in designing a significantly energetically favorable competition, thus filtering the undesired reaction path. Cu vapor was introduced to occupy the center site of the six membered ring of the WSe2 surface, allowing only the size expansion process to occur, thus achieving monolayer WSe2 (Figure 3). The self-limited growth behavior

the filtering adsorption of the precursors on the as-formed crystals. Table S1 summarizes the self-limited growth of various 2D materials based on liquid metals. When liquid metals serve as the catalytic substrates, the graphene exhibits strictly self-limited growth behavior, which we can attribute to the precise control of the segregation process of carbon atoms from the liquid bulk toward the surface in the phase transition process. Our group successfully grew strictly single-layer graphene over a series of accessible liquid metals (liquid Ga, In, and Cu).17,18 The LMCVD strategy even shows high tolerance to the experimental parameter variations. The mechanism of the self-limited growth is vividly illustrated in Figure 2. Generally, the solid

Figure 2. Self-limited growth of graphene on liquid metals. Schematic illustration of the self-limited growth of monolayer graphene on liquid metals achieved by substrate trapping of the extra carbon atoms in the phase transition from liquid to bulk. Reproduced from ref 18. Copyright 2014 American Chemical Society.

Cu has a very low carbon solubility. However, when it is melted, the nonperiodic and fluctuant atom arrangement in the liquid bulk enables the embedding of carbon atoms. In the cooling process, the solidification starts from the surface and the atoms arrange in a periodic structure. Thus, the further segregation of carbon atoms in the liquid bulk will be prevented due to the barrier of the frozen surface. In addition, the solidified surface is derived from the homogeneous liquid one and thus can be quasi-atomically smooth without random defects, which facilitates the self-limited growth of high-quality graphene with uniform layer number. The design of the liquid alloy can further extend the advantage of liquid catalysts. The widely employed solid catalysts only can achieve growth of high-quality graphene employing CH4 as the precursor at 1000 °C at least, which undoubtedly limits future industrialization. Our group artificially combined liquid metals (such as Ga) and irongroup metals (such as Fe and Ni) that have extremely high catalytic reactivity and high carbon solubility to form liquid alloy catalysts.19 We achieved the synthesis of uniform singlelayer graphene at 600 °C. Beyond graphene, the controllable uniform growth of other 2D materials on liquid metals was also reported, including chalcogenide crystals (GaSe and GaxIn1−xSe)20 and transition metal carbides (Mo2C, WC, and TaC).21 Notably, subnanometer layers of HfO2, Al2O3, and Gd2O3 were thermodynamically created via the liquidmetal-based solvent reaction route at the room temperature, which fully utilizes the alloying with liquid metal elements.22 Apart from acting as catalytic substrates, liquid metals can enable remote catalysis for self-limited growth of 2D materials

Figure 3. Self-limited growth of uniform WSe2 on insulated substrates assisted by liquid metal vapor. (a) Self-limited growth of monolayer WSe2 achieved by filtering the adsorption of W atoms on the WSe2 surface that is occupied by the Cu vapor. (b) Optical microscope (OM) image of the uniform monolayer WSe2. (c) Unlimited growth of nonuniform multilayer WSe2 resulted from the uncontrollable adsorption of W on the WSe2 surface. (d) OM image of the nonuniform multilayer WSe2. Reproduced with permission from ref 25. Copyright 2016 Wiley.

of 2D materials on liquid metals can also be extended to other liquid substrates. Based on liquid glass, the growth of graphene and a series of monolayer transition metal dichalcogenides (TMDs) was achieved.26−29 The smooth isotropic surface, fast diffusion rate, and chemical inertness of the liquid insulated substrates make them suitable to grow various 2D materials from precursors with corrosivity. However, their inert characteristic will also lead to poor catalytic ability.

4. SELF-ASSEMBLY Generally, 2D materials obtained on solid metals by CVD are polycrystalline films or randomly distributed single crystals. In order to fully explore their potential applications, multilevel assembled or heterostacked structures should be constructed to extend their properties. The excellent rheological features of liquid metals provide a good platform to develop innovative 2841


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Figure 4. Isotropic growth of graphene and smooth oriented stitching of the grains on liquid Cu. (a) Scheme of the IGGs grown on the liquid Cu. (b) Typical scanning tunneling microscopy (STM) image showing the edge structure of an individual IGG. (c) Atom-resolution STM image showing the precise edge structure and the simulative atom configuration. (d) Raman mapping of the peak intensity of the 2D band. (e) Schematic illustration of the stitching process of IGGs. Reproduced from ref 30. Copyright 2016 American Chemical Society.

liquid Cu. Therefore, for the IGGs with low rotational steric hindrance and highly active mixed edge structure, only low energy is needed to drive their rotation in the stitching process on the liquid Cu substrate with excellent rheological properties. The OM image and Raman mapping shown in Figure 4d confirm the smooth oriented stitching since no defects can be detected across the intergrain region. Figure 4e illustrated the self-adjustment of the orientation of the IGG toward the smooth oriented stitching with its neighboring grains, in which the carbon atoms catalytically decomposed by the Cu substrate will fill the atom vacancies in the stitching site. Hexagonal boron nitride (h-BN) single crystal grown on liquid Cu in certain growth conditions can also be isotropically round with alternately B-terminated and N-terminated edge configuration.31 However, TMD single crystals with more complex atom configurations grown on liquid metals or insulated substrates still exhibit regular shapes,27,32 which results from the significant energy difference of the different edge terminations, the ratio of the chalcogen and transition metal atoms, and the intrinsic sandwich crystalline structure of the TMDs.33

self-assembly of the 2D materials grown on the surface. How to control the morphology of the domains and their stitching or assembly will be the key issue in 2D materials-based scientific research and technological applications. The diffusion of the precursor atoms on the liquid surface, their adsorption and attachment on the as-formed islands, and the interaction of the islands are worthy of attention. Furthermore, the heterostacking of the different 2D material crystals can immensely extend their properties. We should fully understand the van der Waals interaction of two kinds of 2D material components. 4.1. Oriented Stitching

The inability to control the shape or orientation of graphene single crystals grown on polycrystalline solid metal catalysts will have a negative influence on the commensurate oriented stitching of the grains due to the discrepant atom arrangements of the neighboring grains at the edge in various directions. The incommensurate dislocated stitching of the graphene grains will lead to the formation of grain boundaries and decay of the as-spliced film’s quality. Our group attempted to design the growth of isotropic graphene grains (IGGs) on the isotropic liquid surface to achieve their commensurate oriented stitching (Figure 4a).30 A carbon source at extremely low concentration is introduced, and the resulting slow growth rate will allow the full diffusion of carbon atoms on the liquid surface until they reach the energetically favorable sites. Unlike the solid metal with a specific crystal orientation, the liquid metal with an amorphous surface will make the assembly of the carbon atoms isotropic instead of following a dominant direction. Therefore, the as-grown graphene grains are isotropically round and the rotational steric hindrance is much lower than that of the graphene grains with an angular shape. Figure 4b shows that the edge of the IGG seems like a nanozipper at the nanometer level, which consists of a number of sawteeth in a certain periodicity. Figure 4c shows its more precise edge structure, in which the carbon atoms are randomly arranged in both the zigzag and armchair configurations. Such a mixed edge structure is highly active, allowing the rotation of IGGs on

4.2. Ordered Assembly

The assembly of 2D single crystals with unique physicochemical properties into an ordered structure with tunable periodicity is highly desirable. However, the research on selfassembly of high-quality 2D material single crystals is still a blank field. With regard to CVD growth of 2D materials on traditional solid substrates, self-assembly is a huge challenge. Precursors tend to absorb on the defect sites of the solid substrate, leading to nonuniform nucleation distribution. The subsequent growth will continue at the fixed initial nucleation sites on such a rigid substrate. Promoting relatively uniform nucleation and offering a rheological assembly platform will be the key to conquer the challenge. The employment of liquid metal substrates with smooth surfaces and excellent fluidity will be an artful strategy. 2842


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Figure 5. Self-assembly of graphene single crystals into a 2DSOS. (a) Schematic diagram showing the self-assembly mechanism of GSOS. (b) Scanning electron microscope (SEM) image of GSOS. (c) Statistics of the deviations of the size and distance of GSOS. (d) Percentile curve of the distribution of the rotation angle. The inset shows the histograms of the rotation angle. Reproduced from ref 34. Copyright 2016 American Chemical Society.

Figure 6. Self-symmetrical etching growth of h-BN-G CSAs. (a) Schematic illustration showing the self-symmetrical etching growth of h-BN-G CSAs. (b) OM image of large-area CSA on the Cu substrate. (c) Raman mapping to identify the composition of the core−shell structure, including the intensity mapping of the 2D peak (graphene) and the E2g peak (h-BN). (d) Statistics of the deviation of the distributions of the inner and outer diameters, included angles and center distances, to show the periodicity of the CSAs. Reproduced from ref 36. Copyright 2017 American Chemical Society.

size and the distance or even the orientation, they all exhibit excellent uniformity. However, if one wants to achieve selfassembly of 2D single crystals on a large scale, a more robust self-assembly system urgently needs to be developed. Our group reported the self-assembly of metal oxide nanoparticles in liquid metal triggered by electrical charge and thus achieved precise nucleation control of graphene in both space and time, yielding a graphene single crystal array at a very large scale.35 Besides this, our group also observed the self-assembly behavior of other 2D materials on liquid metals, such as hexagonal boron nitride (h-BN).31 We first prepared a selfaligned single crystal array (SASCA) of h-BN on the surface of

Our group successfully achieved the self-assembly of graphene single crystals into arrays with uniform size and orientation, which is the first identified 2D superordered structure (2DSOS).34 We employ thermally decomposed poly(methyl methacrylate) (PMMA) as the carbon source for providing the nucleation seeds and simultaneously introduce airflow to trigger the self-assembly. In the subsequent growth process, the mutual electrostatic repulsive force possessed by the graphene crystals in the hydrogen atmosphere gradually increases, contributing to the self-assembly of the graphene superordered structure (GSOS), as seen in Figure 5a. Outstanding periodicity is fully shown in Figure 5b,c,d. No matter the 2843


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Figure 7. Twinned heterostack assembly of 100% overlapped ReS2/WS2 vertical heterostructures on liquid Au and their characterizations. (a) Schematic for the twinned assembly of ReS2/WS2 heterostructures and the typical OM and Raman mapping results. (b) Low-magnification TEM image of the ReS2/WS2 twinned vertical heterostructures. (c) High-resolution TEM image showing the resulting Moiré pattern. (d) Fast Fourier transform of the heterostructures in panel c. (e) Tentative orientation model of rotating the upper ReS2 by an angle of 5.6° with respect to the ground WS2. Reproduced with permission from ref 32. Copyright 2016 Nature Publishing Group.

liquid Cu. In addition, our group also succeeded in preparing a 2D heterostructure array, that is, 2D h-BN−G core−shell arrays (CSAs).36 Figure 6a clearly exhibits the formation process. First, we conducted the growth of a high-quality and uniform graphene film based on liquid Cu, which is a crucial prerequisite for the subsequent self-assembly. The precursor, such as borazine, can adsorb and arrange on the graphene film in a self-symmetrical way, which was then followed by the in situ etching of graphene and substitution of h-BN. Thus, h-BN core and graphene shell distributed in a periodic structure can be formed, that is, the 2D h-BN−G CSA (Figure 6b). The composition and structure of the core−shell component could be confirmed by the Raman mapping, as seen in Figure 6c. The excellent periodicity of the 2D h-BN−G CSA was further exhibited by the statistics (Figure 6d). The as-presented selfsymmetrical etching growth strategy suggests new ways to think about the synthesis of 2D ordered arrays of heterostructures with high controllability and efficiency, which opens access to new applications in 2D integrated systems and devices. So far, self-assembly of TMD single crystals on a liquid metal or glass substrate has not been reported. For one thing, many liquid metals (such as liquid Cu, Ga) are easily corroded by the precursors of the chalcogenide, which limits the preparation of TMDs with various components. For another thing, the anisotropic regular shape (triangle) of many TMDs single crystals hampers their self-symmetrical arrangement on the liquid surface.31 Design of anticorrosion liquid metal catalysts with appropriate viscosity and the precise morphology of the TMDs components might facilitate the achievement of their self-assembly.

The as-obtained heterostructures are also faced with the disadvantage of weak interlayer interaction and interfacial contamination. The development of a direct and one-step synthesis strategy for 2D heterostructures can be a promising and important target. Our group designed a sulfide-resistant liquid substrate (Ni− Ga alloy) for fabricating TMD/h-BN heterostructures.38 A MoS2 single crystal with an area up to 200 μm2 was directly grown on the h-BN film. The direct band gap of the MoS2 on h-BN grown on Ni−Ga alloy is 1.85 eV. This value is in consistence with that for freestanding exfoliated equivalents. Although many researchers, including us, have achieved the one-step CVD growth of stacked 2D materials domains. However, it is still extremely difficult to achieve precise control of the overlap for each component due to diverse nucleation sites and growth rates. Very intriguingly, our group, demonstrated the twinned assembly of two kinds of 2D materials, that is, ReS2/WS2.32 Liquid Au was chosen as the liquid solvent to dissolve the Re and W, which lowers the barrier energies for this special twinned assembly process. For one thing, the adsorption energy of W atoms over Au(111) is much stronger than that of Re atoms over Au(111). For another thing, the adsorption energy of Re atoms on WS2(001) is also high, which leads to the adsorption, nucleation and twinned assembly of ReS2 on the as-formed WS2; 100% overlap for each of the stacked TMD structures is achieved, as seen in Figure 7a. High-resolution transmission electron microscopy (TEM) characterizations further revealed the crystalline structure and the controllable stack orientation, as seen in Figure 7b−e. A clear Moiré pattern can be observed. The simplicity of the process may be expanded to construct other vertically stacked 2D material domains. In addition, a vertical heterostack of Mo2C and graphene was reported on liquid Cu.39 The liquid metal catalysts can also be applied to assemble lateral heterostructures between different 2D material domains. Our group achieved the simultaneous segregation of W and C atoms in the liquid Ga solvent and fabricated an inplane WC−graphene heterostructure.40

4.3. Heterostacking

Various 2D materials with rich properties allow the possibility of heterostacking of different 2D material domains, thus constructing novel heterostructures with new physical properties and enabling the mining of unique functionality. To allow full range of their properties, we should first achieve the assembly of 2D heterostructures in a designable and controllable manner. Generally, the construction of vertical heterostructures refers to multiple mechanical exfoliations with layerby-layer transfer, which is hard to achieve with high yield.37

5. TRANSFER Compared to traditional solid catalysts, liquid counterparts exhibit significant advantages in layer number control and 2844


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Figure 8. Sliding transfer of graphene on liquid metals and its characterization. (a) Illustration of the sliding transfer process. (b) Illustration and photograph of the sliding transfer installation. (c) OM image of sliding transferred graphene on a SiO2/Si substrate. (d) High resolution XPS spectrum of Ga 2p. (e) AFM image of sliding transferred graphene on SiO2/Si. (f) TEM image of large-area sliding transferred graphene resting on lacey carbon at low magnification. (g) High-resolution atomic image of the sliding transferred graphene. Reproduced with permission from ref 41. Copyright 2016 Wiley.

ultrafast transfer method without any carrier. By integration in the existing floating flat glass technique based on liquid metal, the controllable fabrication and transfer of large-area and highquality 2D materials are promising to add to future industrial fabrication.42 We believe liquid metals can stimulate the controllable fabrication of 2D materials much more than what have been reported. To date, the tentative exploration of the LMCVD strategy has mainly been conducted with graphene. There still exists a research gap in the unique growth behavior of other 2D materials on the liquids, which really deserves huge exploration. Nevertheless, liquid metals are faced with some problems in terms of the catalytic growth of 2D materials. Traditionally employed transition metal catalysts have high melting points. For example, Cu has a melting point of 1083 °C, while Ni has a melting point as high as 1453 °C, which greatly hinders their conversion to liquid. The nanocrystallization of these metals can facilitate the lowering of their melting point.44 In addition, the theoretical understanding of liquid metals is still in its infancy. The amorphous surface of liquid metals without specific crystalline structure makes it hard to directly apply existing methods for determining the influence of the liquid surface structure on the elementary steps in the CVD process. Notably, compared to solid metal catalysts, more abundant reactive or mass-transfer interfaces in liquid systems, including gas−solid interface, liquid−solid interface, and gas−liquid interface, make the mechanism more complex. The understanding of some special growth behaviors, such as the growth of large-size crystals and unique stacked microstructures, are still hidden in the mist.45,46 Providing the combination between systematic theoretical guidance and experimental verification is achieved, the controllable growth and self-assembly of 2D materials on liquid metals will be further developed. It can be predicted that liquid metals will see a new push in the research of 2D materials.

domain control. In subsequent device fabrication, the unique physical state of liquid metal offers new clues for the transfer of 2D materials. Our group developed a novel sliding transfer strategy.41 Figure 8a,b exhibits the sliding transfer installation. The original liquid metal substrate with graphene on the surface was aligned with the target substrate. Upon sliding the original liquid metal substrate, the graphene on its surface will be adhered and transferred onto the target substrate. The total transfer process only takes a few seconds. A typical OM image of the as-transferred graphene film on the SiO2/Si substrate is shown in Figure 8c. The X-ray photoelectron spectrum (XPS) reveals that the graphene is metal-free (Figure 8d). Atomic force microscopy (AFM) and low magnification TEM images reveal the completeness, cleanliness, and flatness of the transferred graphene film, in which there are nearly no wrinkles, cracks, or impurities in the interface between the graphene and the substrate (Figure 8e,f). Figure 8g further demonstrated the high crystallinity of the transferred graphene. The sliding transfer strategy has good industrial application prospects since it can be effectively integrated in the existing floating flat glass technique, which also works based on the ultrasmooth surface of the liquid metal and has been one of the most mainstream techniques for preparing large-size glass.42 It can be expected that the proposed sliding transfer strategy should be versatile and highly efficient to transfer the 2D materials on the liquid metal onto various target substrates and will promote future applications of 2D materials.

6. CONCLUSION AND OUTLOOK Liquid metals exhibit many wonderful properties, such as the isotropic surface, bulk with abundant vacancies, an excellent rheology, which make the 2D materials on them exhibit very unique growth and assembly behavior. Up to now, liquid metals have been demonstrated in the self-limited catalytic growth of large-area, strictly single-layer, and high-quality graphene.6 Apart from the layer number control, they exhibit more impressive ability in the self-assembly of the 2D material crystals, including oriented stitching, ordered assembly, and twinned heterostacking. So far, the self-assembly of 2D material single crystals, such as graphene and h-BN, has been demonstrated only on liquid metals with rheological surface.12,34,36,43 Liquid metals even brought us a brand-new

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The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.accounts.8b00293. 2845


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Summary of liquid metals for the growth of various 2D materials (PDF)

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 is an Associate Professor at Wuhan University. She focuses on the controllable growth of two-dimensional materials. Lei Fu is a Professor at Wuhan University. He leads a research group focused on the controlled growth and novel property exploration of two-dimensional materials.

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ACKNOWLEDGMENTS The research was supported by the National Natural Science Foundation of China (Grants 21673161 and 21473124) and the Sino-German Center for Research Promotion (Grant 1400).

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