Controllable Synthesis of Submillimeter Single-Crystal Monolayer Graphene Domains on Copper Foils by Suppressing Nucleation

Wafer scale growth of single crystal two-dimensional van der Waals materials

Two-dimensional (2D) van der Waals (vdW) materials, including graphene, hexagonal boron nitride (hBN), and metal dichalcogenides (MCs), form the basis of modern electronics and optoelectronics due to their unique electronic structure, chemical activity, and mechanical strength. Despite many proof-of-concept demonstrations so far, to fully realize their large-scale practical applications, especially in devices, wafer-scale single crystal atomically thin highly uniform films are indispensable. In this minireview, we present an overview on the strategies and highlight recent significant advances toward the synthesis of wafer-scale single crystal graphene, hBN, and MC 2D thin films. Currently, there are five distinct routes to synthesize wafer-scale single crystal 2D vdW thin films: (i) nucleation-controlled growth by suppressing the nucleation density, (ii) unidirectional alignment of multiple epitaxial nuclei and their seamless coalescence, (iii) self-collimation of randomly oriented grains on a molten metal, (iv) surface diffusion and epitaxial self-planarization and (v) seed-mediated 2D vertical epitaxy. Finally, the challenges that need to be addressed in future studies have also been described.

Grain Size Engineering of CVD-Grown Large-Area Graphene Films

Graphene, a single atomic layer of graphitic carbon, has attracted much attention because of its outstanding properties hold great promise for a wide range of technological applications. Large-area graphene films (GFs) grown by chemical vapor deposition (CVD) are highly desirable for both investigating their intrinsic properties and realizing their practical applications. However, the presence of grain boundaries (GBs) has significant impacts on their properties and related applications. According to the different grain sizes, GFs can be divided into polycrystalline, single-crystal, and nanocrystalline films. In the past decade, considerable progress has been made in engineering the grain sizes of GFs by modifying the CVD processes or developing some new growth approaches. The key strategies involve controlling the nucleation density, growth rate, and grain orientation. This review aims to provide a comprehensive description of grain size engineering research of GFs. The main strategies and underlying growth mechanisms of CVD-grown large-area GFs with nanocrystalline, polycrystalline, and single-crystal structures are summarized, in which the advantages and limitations are highlighted. In addition, the scaling law of physical properties in electricity, mechanics, and thermology as a function of grain sizes are briefly discussed. Finally, the perspectives for challenges and future development in this area are also presented.

Increasing coverage of mono-layer graphene grown on hexagonal boron nitride

Graphene sitting on hexagonal boron nitride (h-BN) always exhibits excellent electrical properties. And the properties of graphene onh-BN are often dominated by its domain size and boundaries. Chemical vapor deposition (CVD) is a promising approach to achieve large size graphene crystal. However, the CVD growth of graphene onh-BN still faces challenges in increasing coverage of monolayer graphene because of a weak control on nucleation and vertical growth. Here, an auxiliary source strategy is adapted to increase the nucleation density of graphene onh-BN and synthesis continuous graphene films. It is found that both silicon carbide and organic polymer e.g. methyl methacrylate can assist the nucleation of graphene, and then increases the coverage of graphene onh-BN. By optimizing the growth temperature, vertical accumulation of graphitic materials can be greatly suppressed. This work provides an effective approach for preparing continuous graphene film onh-BN, and may bring a new sight for the growth of high quality graphene.

Production of Large-Area Nucleus-Free Single-Crystal Graphene-Mesh Metamaterials with Zigzag Edges

In addition to conventional monolayer or bilayer graphene films, graphene-mesh metamaterials have attracted considerable research attention within the scientific community owing to their unique physical and optical properties. Currently, most graphene-mesh metamaterials are fabricated using common lithography techniques on exfoliated graphene flakes, which require the deposition and removal of chemicals during fabrication. This process may introduce contamination or doping, thereby limiting their production size and application in nanodevices. Herein, the controlled production of wafer-scale high-quality single-crystal nucleus-free graphene-mesh metamaterial films with zigzag edges is demonstrated. The ¹³ C-isotopic labeling graphene-growth approach, large-area Raman mapping techniques, and a uniquely designed high-voltage localized-space air-ionization etching method are utilized to directly remove the graphene nuclei. Subsequently, a hydrogen-assisted anisotropic etching process is employed for transforming irregular edges into zigzag edges within the hexagonal-shaped holes, producing a large-scale single-crystal high-quality graphene-mesh metamaterial film on a Cu(111) substrate. The carrier mobilities of the fabricated field-effect transistors on the as-produced films are measured. The findings of this study enable the large-scale production of high-quality low-dimensional graphene-mesh metamaterials and provide insights for the application of integrated circuits based on graphene and other 2D metamaterials.

Van der Waals Epitaxy of c-Oriented Wurtzite AlGaN on Polycrystalline Mo Substrates for Enhanced Heat Dissipation

The epitaxy of III-nitrides on metallic substrates is competitive due to the advantages of vertical carrier injection, enhanced heat dissipation, and flexible application in various III-nitride-based devices. However, the serious lattice mismatch, atom diffusion, and interface reaction under the rigorous growth conditions have caused enormous obstacles. Based on the thermal and chemical stability of the graphene layer, we propose the van der Waals epitaxy of c-oriented wurtzite AlGaN on the polycrystalline Mo substrate by high-temperature metal-organic chemical vapor deposition. The insertion of a graphene layer interrupts the chaotic epitaxial relationship between the polycrystalline metal and epilayers, resulting in the single-crystalline orientation along the wurtzite (0002) plane and residual stress release in AlGaN because of the weak van der Waals interaction. We also demonstrate that the epitaxy of AlGaN on Mo metal possesses enhanced heat dissipation ability, in which the epilayer temperature is controlled at only 28.7 °C by the heating of a ∼54 °C hot plate. The heat dissipation enhancement for the present epitaxial structures provides a desirable strategy for the fabrication of efficient ultraviolet devices with excellent stability and lifetime.

Multigraphene Prepared by One-Pot Pyrolysis of Diatomite/Polypropylene Composites

Multigraphene was prepared via a one-pot pyrolysis method using polypropylene (PP) as the carbon source and diatomite (DM) as the catalyst. The obtained graphene had 4–6 layers and a D/G intensity ratio of 0.70 and a 2D/G intensity ratio of 0.72, indicating a high degree of graphitization. When the pyrolysis temperature was higher than 850 °C under argon, the graphene yield was greatly dependent on the DM content. The highest graphene yield of 25.86% was obtained by pyrolysis of PP with 30 wt.% DM at the temperature of 1000 °C. A catalytic effect of DM and infusible cross-linking structure formation were proposed to explain the possible mechanism of graphene growth during the pyrolysis of the DM/PP composites.

Epitaxy of 2D Materials toward Single Crystals

Two-dimensional (2D) materials exhibit unique electronic, optical, magnetic, mechanical, and thermal properties due to their special crystal structure and thus have promising potential in many fields, such as in electronics and optoelectronics. To realize their real applications, especially in integrated devices, the growth of large-size single crystal is a prerequisite. Up to now, the most feasible way to achieve 2D single crystal growth is the epitaxy: growth of 2D materials of one or more specific orientations with single-crystal substrate. Only when the 2D domains have the same orientation, they can stitch together seamlessly and single-crystal 2D films can be obtained. In this view, four different epitaxy modes of 2D materials on various substrates are presented, including van der Waals epitaxy, edge epitaxy, step-guided epitaxy, and in-plane epitaxy focusing on the growth of graphene, hexagonal boron nitride (h-BN), and transition metal dichalcogenide (TMDC). The lattice symmetry relation and the interaction between 2D materials and the substrate are the key factors determining the epitaxy behaviors and thus are systematically discussed. Finally, the opportunities and challenges about the epitaxy of 2D single crystals in the future are summarized.

In Situ Investigation of the Motion Behavior of Graphene on Liquid Copper

The in situ investigation of the dynamic growth process and novel assembly phenomena of graphene on liquid copper (Cu) is of great significance to deeply understand the special behavior of graphene and self-assembly mechanism. Here, the direct observation of the graphene growth and motion behavior on liquid Cu via in situ imaging is reported. Evidence of graphene movement on liquid Cu is offered and it is demonstrated that the translation and rotation behaviors of graphene are affected by the surface condition of liquid Cu. The self-assembly process of graphene array is also revealed by capturing the dynamic changes of graphene in real-time. Further analysis highlights the importance of surface energy of liquid Cu and the interaction between graphene building blocks during the self-assembling process. The growth parameters are also investigated to flexibly control the assembly configuration of graphene arrays. This work provides an insight into the mechanism of graphene motion and assembly behavior that can be used to guide the controllable manipulation of 2D materials and on-demand fabrication assembly structures with desired properties.

Controllable Synthesis of Wafer-Scale Graphene Films: Challenges, Status, and Perspectives

The availability of high-quality, large-scale, and single-crystal wafer-scale graphene films is fundamental for key device applications in the field of electronics, optics, and sensors. Synthesis determines the future: unleashing the full potentials of such emerging materials relies heavily upon their tailored synthesis in a scalable fashion, which is by no means an easy task to date. This review covers the state-of-the-art progress in the synthesis of wafer-scale graphene films by virtue of chemical vapor deposition (CVD), with a focus on main challenges and present status. Particularly, prevailing synthetic strategies are highlighted on a basis of the discussion in the reaction kinetics and gas-phase dynamics during CVD process. Perspectives with respect to key opportunities and promising research directions are proposed to guide the future development of wafer-scale graphene films.

The Mechanism of Graphene Vapor-Solid Growth on Insulating Substrates

Wafer-scale single-crystal graphene film directly grown on insulating substrates via the chemical vapor deposition (CVD) method is desired for building high-performance graphene-based devices. In comparison with the well-studied mechanism of graphene growth on transition metal substrates, the lack of understanding on the mechanism of graphene growth on insulating surfaces greatly hinders the progress. Here, by using first-principles calculation, we systematically explored the absorption of various carbon species CH_x (x = 0, 1, 2, 3, 4) on three typical insulating substrates [h-BN, sapphire, and quartz] and reveal that graphene growth on an insulating surface is dominated by the reaction of active carbon species with the hydrogen-passivated graphene edges and thus is less sensitive to the type of the substrate. The dominating gas phase precursor, CH₃, plays two key roles in graphene CVD growth on an insulating substrate: (i) to feed the graphene growth and (ii) to remove excessive hydrogen atoms from the edge of graphene. The threshold reaction barriers for the growth of graphene armchair (AC) and zigzag (ZZ) edges were calculated as 3.00 and 1.94 eV, respectively; thus the ZZ edge grows faster than the AC one. Our theory successfully explained why the circumference of a graphene island grown on insulating substrates is generally dominated by AC edges, which is a long-standing puzzle of graphene growth. In addition, the very slow graphene growth rate on an insulating substrate is calculated and agrees well with existing experimental observations. The comprehensive insights on the graphene growth on insulating surfaces at the atomic scale provide guidance on the experimental design for high-quality graphene growth on insulating substrates.

Growth and in situ characterization of 2D materials by chemical vapour deposition on liquid metal catalysts: a review

2D materials (2DMs) have now been established as unique and attractive alternatives to replace current technological materials in a number of applications. Chemical vapour deposition (CVD), is undoubtedly the most renowned technique for thin film synthesis and meets all requirements for automated large-scale production of 2DMs. Currently most CVD methods employ solid metal catalysts (SMCat) for the growth of 2DMs however their use has been found to induce structural defects such as wrinkles, fissures, and grain boundaries among others. On the other hand, liquid metal catalysts (LMCat), constitute a possible alternative for the production of defect-free 2DMs albeit with a small temperature penalty. This review is a comprehensive report of past attempts to employ LMCat for the production of 2DMs with emphasis on graphene growth. Special attention is paid to the underlying mechanisms that govern crystal growth and/or grain consolidation and film coverage. Finally, the advent of online metrology which is particularly effective for monitoring the chemical processes under LMCat conditions is also reviewed and certain directions for future development are drawn.

Atomically Thin Hexagonal Boron Nitride and Its Heterostructures

Atomically thin hexagonal boron nitride (h-BN) is an emerging star of 2D materials. It is taken as an optimal substrate for other 2D-material-based devices owing to its atomical flatness, absence of dangling bonds, and excellent stability. Specifically, h-BN is found to be a natural hyperbolic material in the mid-infrared range, as well as a piezoelectric material. All the unique properties are beneficial for novel applications in optoelectronics and electronics. Currently, most of these applications are merely based on exfoliated h-BN flakes at their proof-of-concept stages. Chemical vapor deposition (CVD) is considered as the most promising approach for producing large-scale, high-quality, atomically thin h-BN films and heterostructures. Herein, CVD synthesis of atomically thin h-BN is the focus. Also, the growth kinetics are systematically investigated to point out general strategies for controllable and scalable preparation of single-crystal h-BN film. Meanwhile, epitaxial growth of 2D materials onto h-BN and at its edge to construct heterostructures is summarized, emphasizing that the specific orientation of constituent parts in heterostructures can introduce novel properties. Finally, recent applications of atomically thin h-BN and its heterostructures in optoelectronics and electronics are summarized.

Hydrogen-Assisted Fast Growth of Large Graphene Grains by Recrystallization of Nanograins

Chemical vapor deposition has been highlighted as a promising tool for facile graphene growth in a large area. However, grain boundaries impose detrimental effects on the mechanical strength or electrical mobility of graphene. Here, we demonstrate that high-pressure hydrogen treatment in the preannealing step plays a key role in fast and large grain growth and leads to the successful synthesis of large grain graphene in 10 s. Large single grains with a maximum size of ∼160 μm grow by recrystallization of nanograins, but ∼1% areal coverage of nanograins remains with 28-30° misorientation angles. Our findings will provide insights into mass production of high-quality graphene.

Growth and Grain Boundaries in 2D Materials

Grain boundaries (GBs) are a kind of lattice imperfection widely existing in two-dimensional materials, playing a critical role in materials' properties and device performance. Related key issues in this area have drawn much attention and are still under intense investigation. These issues include the characterization of GBs at different length scales, the dynamic formation of GBs during the synthesis, the manipulation of the configuration and density of GBs for specific material functionality, and the understanding of structure-property relationships and device applications. This review will provide a general introduction of progress in this field. Several techniques for characterizing GBs, such as direct imaging by high-resolution transmission electron microscopy, visualization techniques of GBs by optical microscopy, plasmon propagation, or second harmonic generation, are presented. To understand the dynamic formation process of GBs during the growth, a general geometric approach and theoretical consideration are reviewed. Moreover, strategies controlling the density of GBs for GB-free materials or materials with tunable GB patterns are summarized, and the effects of GBs on materials' properties are discussed. Finally, challenges and outlook are provided.

Designed Growth of Large-Size 2D Single Crystals

In the "post-Moore's Law" era, new materials are highly expected to bring next revolutionary technologies in electronics and optoelectronics, wherein 2D materials are considered as very promising candidates beyond bulk materials due to their superiorities of atomic thickness, excellent properties, full components, and the compatibility with the processing technologies of traditional complementary metal-oxide semiconductors, enabling great potential in fabrication of logic, storage, optoelectronic, and photonic 2D devices with better performances than state-of-the-art ones. Toward the massive applications of highly integrated 2D devices, large-size 2D single crystals are a prerequisite for the ultimate quality of materials and extreme uniformity of properties. However, at present, it is still very challenging to grow all 2D single crystals into the wafer scale. Therefore, a systematic understanding for controlled growth of various 2D single crystals needs to be further established. Here, four key aspects are reviewed, i.e., nucleation control, growth promotion, surface engineering, and phase control, which are expected to be controllable at different periods during the growth. In addition, the perspectives on designed growth and potential applications are discussed for showing the bright future of these advanced material systems of 2D single crystals.

The role of gas-phase dynamics in interfacial phenomena during few-layer graphene growth through atmospheric pressure chemical vapour deposition

The complicated chemical vapour deposition (CVD) is currently the most viable method of producing graphene. Most studies have extensively focused on chemical aspects either through experiments or computational studies. However, gas-phase dynamics in CVD reportedly plays an important role in improving graphene quality. Given that mass transport is the rate-limiting step for graphene deposition in atmospheric-pressure CVD (APCVD), the interfacial phenomena at the gas-solid interface (i.e., the boundary layer) are a crucial controlling factor. Accordingly, only by understanding and controlling the boundary-layer thickness can uniform full-coverage graphene deposition be achieved. In this study, a simplified computational fluid dynamics analysis of APCVD was performed to investigate gas-phase dynamics during deposition. Boundary-layer thickness was also estimated through the development of a customised homogeneous gas model. Interfacial phenomena, particularly the boundary layer and mass transport within it, were studied. The effects of Reynolds number on these factors were explored and compared with experimentally obtained results of the characterised graphene deposit. We then discussed and elucidated the important relation of fluid dynamics to graphene growth through APCVD.

A Strategy To Prepare High-Quality Monocrystalline Graphene: Inducing Graphene Growth with Seeding Chemical Vapor Deposition and Its Mechanism

High-quality monocrystalline graphene has gained considerable attention in fundamental physics, materials science, and nanoelectronics. However, the performance of the graphene obtained by chemical synthesis methods is currently significantly restricted by the crystal quality. Herein, a seeding chemical vapor deposition (SCVD) method is designed to cultivate high-quality monocrystalline graphene on a Cu(111) substrate with hexagonal boron nitride (h-BN) as the seed crystal. Combining the experimental and theoretical research, the nucleation behavior of the growth-induced graphene on the h-BN seed crystal is investigated, and the induced growth mechanism on the Cu(111) substrate is studied. The results show that the h-BN seed crystal can dramatically reduce the adsorption energy of active carbon atoms and the energy barrier for C-C aggregation at the BN/Cu(111) step, thus promoting graphene growth around the h-BN seed. Large monocrystalline graphene domains are obtained by the proposed SCVD method. Further study shows that the growth-induced graphene has good crystal quality and could maintain high structural integrity. This new strategy can be applied for growing high-quality graphene and other two-dimensional materials.

Controlled Growth of Single-Crystal Graphene Films

Grain boundaries produced during material synthesis affect both the intrinsic properties of materials and their potential for high-end applications. This effect is commonly observed in graphene film grown using chemical vapor deposition and therefore caused intense interest in controlled growth of grain-boundary-free graphene single crystals in the past ten years. The main methods for enlarging graphene domain size and reducing graphene grain boundary density are classified into single-seed and multiseed approaches, wherein reduction of nucleation density and alignment of nucleation orientation are respectively realized in the nucleation stage. On this basis, detailed synthesis strategies, corresponding mechanisms, and key parameters in the representative methods of these two approaches are separately reviewed, with the aim of providing comprehensive knowledge and a snapshot of the latest status of controlled growth of single-crystal graphene films. Finally, perspectives on opportunities and challenges in synthesizing large-area single-crystal graphene films are discussed.

Graphene at Fifteen

Graphene is a two-dimensional nanomaterial composed of 1-10 layers of carbon atoms in a honeycomb lattice. It has been 15 years since the first isolation of few-layer graphene from graphite by the Scotch Tape method. Worldwide research efforts on graphene have been rewarded with enormous breakthroughs in fundamental science and innovative applications. To achieve an influential impact on society, graphene must be manufactured at large scales, be superior to existing products, and be safe to use. In this Perspective, we highlight relevant issues in the quest for commercialization of graphene-containing products. We showcase achievements in improving graphene synthesis while also discussing concerns regarding graphene standardization and graphene's impact on the environment and human health.

Precise Vapor-Phase Synthesis of Two-Dimensional Atomic Single Crystals

Two-dimensional atomic single crystals (2DASCs) have drawn immense attention because of their potential for fundamental research and new technologies. Novel properties of 2DASCs are closely related to their atomic structures, and effective modulation of the structures allows for exploring various practical applications. Precise vapor-phase synthesis of 2DASCs with tunable thickness, selectable phase, and controllable chemical composition can be realized to adjust their band structures and electronic properties. This review highlights the latest advances in the precise vapor-phase synthesis of 2DASCs. We thoroughly elaborate on strategies toward the accurate control of layer number, phase, chemical composition of layered 2DASCs, and thickness of non-layered 2DASCs. Finally, we suggest forward-looking solutions to the challenges and directions of future developments in this emerging field.

Suitable Surface Oxygen Concentration on Copper Contributes to the Growth of Large Graphene Single Crystals

In this Letter, we found that the growth of graphene on Cu oxide foil is significantly affected by the concentration of oxygen. The grain size of graphene grown on a Cu substrate with a relatively high oxygen concentration is much smaller than that on the substrate with lower oxygen concentration. By controlling the oxidation of the Cu substrate at a proper degree, we can obtain millimeter scale graphene single crystals at a growth temperature of 1050 °C. On the basis of our experimental observations, the dual role of oxygen in the CVD growth of graphene was revealed: (i) Oxygen on a Cu surface can contribute to the decomposition of hydrocarbon feedstock and decrease the graphene growth barrier, resulting in an increased growth rate and a larger grain size of graphene; (ii) excess oxygen in the Cu substrate leads to etching of the graphene edge. Our research provides insights to obtain large-area and single-crystalline graphene by choosing a proper Cu oxide substrate.

High-Mobility, Wet-Transferred Graphene Grown by Chemical Vapor Deposition

We report high room-temperature mobility in single-layer graphene grown by chemical vapor deposition (CVD) after wet transfer on SiO₂ and hexagonal boron nitride (hBN) encapsulation. By removing contaminations, trapped at the interfaces between single-crystal graphene and hBN, we achieve mobilities up to ∼70000 cm² V^-1 s^-1 at room temperature and ∼120 000 cm² V^-1 s^-1 at 9K. These are more than twice those of previous wet-transferred graphene and comparable to samples prepared by dry transfer. We also investigate the combined approach of thermal annealing and encapsulation in polycrystalline graphene, achieving room-temperature mobilities of ∼30 000 cm² V^-1 s^-1. These results show that, with appropriate encapsulation and cleaning, room-temperature mobilities well above 10 000 cm² V^-1 s^-1 can be obtained in samples grown by CVD and transferred using a conventional, easily scalable PMMA-based wet approach.

Graphene on Group-IV Elementary Semiconductors: The Direct Growth Approach and Its Applications

Since the first development of large-area graphene synthesis by the chemical vapor deposition (CVD) method in 2009, CVD-graphene has been considered to be a key material in the future electronics, energy, and display industries, which require transparent, flexible, and stretchable characteristics. Although many graphene-based prototype applications have been demonstrated, several important issues must be addressed in order for them to be compatible with current complementary metal-oxide-semiconductor (CMOS)-based manufacturing processes. In particular, metal contamination and mechanical damage, caused by the metal catalyst for graphene growth, are known to cause severe and irreversible deterioration in the performance of devices. The most effective way to solve the problems is to grow the graphene directly on the semiconductor substrate. Herein, recent advances in the direct growth of graphene on group-IV semiconductors are reviewed, focusing mainly on the growth mechanism and initial growth behavior when graphene is synthesized on Si and Ge. Furthermore, recent progress in the device applications of graphene with Si and Ge are presented. Finally, perspectives for future research in graphene with a semiconductor are discussed.

On the Dynamics of Intrinsic Carbon in Copper during the Annealing Phase of Chemical Vapor Deposition Growth of Graphene

In chemical vapor deposition (CVD) growth of graphene, intrinsic carbon in copper has been shown to play a role, especially during the nucleation phase. Here, we report experimental results on depletion of carbon from the bulk of a Cu foil to its surface at different hydrogen pressures, which explain new aspects of the interplay between hydrogen and intrinsic carbon prior to growth. We observed that rising H₂ pressure boosts carbon depletion to the surface, but at the same time, at elevated H₂ pressures, the graphitic film formed on the Cu surface is etched away at a faster rate. This effect led us to practice annealing of copper under high hydrogen pressure as an approach to decrease the total content of carbon in the copper foil and consequently reducing the nucleation density of graphene flakes. These results enhance our understanding about the role of H₂ in the CVD process and explain some of the inconsistencies among the earlier reports.

Growth of U-Shaped Graphene Domains on Copper Foil by Chemical Vapor Deposition

U-shaped graphene domains have been prepared on a copper substrate by chemical vapor deposition (CVD), which can be precisely tuned for the shape of graphene domains by optimizing the growth parameters. The U-shaped graphene is characterized by using scanning electron microscopy (SEM), atomic force microscopy (AFM), transmission electron microscopy (TEM), and Raman. These show that the U-shaped graphene has a smooth edge, which is beneficial to the seamless stitching of adjacent graphene domains. We also studied the morphology evolution of graphene by varying the flow rate of hydrogen. These findings are more conducive to the study of morphology evolution, nucleation, and growth of graphene domains on the copper substrate.

Restoring self-limited growth of single-layer graphene on copper foil via backside coating

The growth of single-layer graphene (SLG) by chemical vapor deposition (CVD) on copper surfaces is very popular because of the self-limiting effect that, in principle, prevents the growth of few-layer graphene (FLG). However, the reproducibility of the CVD growth of homogeneous SLG remains a major challenge, especially if one wants to avoid heavy surface treatments, monocrystalline substrates and expensive equipment to control the atmosphere inside the growth system. We demonstrate here that backside tungsten coating of copper foils allows for the exclusive growth of SLG with full coverage by atmospheric pressure CVD implemented in a vacuum-free furnace. We show that the absence of FLG patches is related to the suppression of carbon diffusion through copper. In the perspective of large-scale production of graphene, this approach constitutes a significant improvement to the traditional CVD growth process since (1) a tight control of the hydrocarbon flow is no longer required to avoid FLG formation and, consequently, (2) the growth duration necessary to reach full coverage can be drastically shortened.

Solid-diffusion-facilitated cleaning of copper foil improves the quality of CVD graphene

The quality of CVD-grown graphene is limited by the parallel nucleation of grains from surface impurities which leads to increased grain boundary densities. Currently employed cleaning methods cannot completely remove surface impurities since impurity diffusion from the bulk to the surface occurs during growth. We here introduce a new method to remove impurities not only on the surface but also from the bulk. By employing a solid cap during annealing that acts as a sink for impurities and leads to an enhancement of copper purity throughout the catalyst thickness. The high efficiency of the solid-diffusion-based transport pathway results in a drastic decrease in the surface particle concentration in a relatively short time, as evident in AFM and SIMS characterization of copper foils. Graphene grown on those substrates displays enhanced grain sizes and room-temperature, large-area carrier mobilities in excess of 5000 cm²/Vs which emphasizes the suitability of our approach for future graphene applications.

Influence of cooling-induced edge morphology evolution during chemical vapor deposition on H2 etching of graphene domains

In this paper, we studied the influence of edge morphology evolution during the chemical vapor deposition cooling process on H₂ etching of graphene domains. Hexagonal graphene domains were synthesized on a Cu substrate and etched with H₂ at atmospheric pressure. After etching, two kinds of graphene edge morphologies were observed, which were closely associated with the cooling process. A visible curvature was observed at the graphene edges via an atomic force microscope, indicating that the graphene edges sank into the Cu surface during the cooling process, which protected the graphene edges from etching. This work demonstrates the changes in graphene edges during cooling and sheds light on the etching mechanism of graphene edges on a Cu substrate.

The entire morphological variation of CVD graphene during cooling and etching.

Graphene Nucleation Preference at CuO Defects Rather Than Cu2O on Cu(111): A Combination of DFT Calculation and Experiment

It is well-known that reducing the nucleation density is an effective way to enhance the growth quality of graphene. In this work, we explore the mechanism of graphene nucleation and growth around CuO defects on a Cu(111) substrate by using density functional theory combined with the nudged elastic band method. The defect formation mechanism at the initial nucleation stage is also studied. Our calculation results of the C adsorption energy and the reaction barrier of C-C dimer formation illustrate that the initial nucleation of graphene could be promoted by artificially introducing CuO defects on a Cu(111) surface and the nucleation on the clean Cu(111) substrate could thus be suppressed. These conclusions have been verified by graphene growth experiments using a chemical vapor deposition method. Further studies showed that graphene grown around CuO "seed crystals" could maintain its structural integrity without significantly producing defective carbon rings. This work provides a fundamental understanding and theoretical guidance for the controllable preparation of large-dimension and high-quality graphene by artificially introducing CuO seeds.

Role of the Cu substrate in the growth of ultra-flat crack-free highly-crystalline single-layer graphene

Producing ultra-flat crack-free single-layer high-quality graphene over large areas has remained the key challenge to fully exploit graphene's potential into next-generation technological applications. In this regard, we show that epitaxial Cu(111) film represents the most promising catalyst for the chemical vapor deposition (CVD) of graphene with superior planarity and physical integrity. We first compare the most widely used Cu catalysts (foils, polycrystalline films and epitaxial films) in order to benchmark the roughness of the Cu surface which serves as a template for graphene growth. We then discuss the correlation between the formation of cracks and wrinkles in as-grown graphene and the surface morphology of these various Cu catalysts. In particular, Cu grain boundary grooves, inherently present in polycrystalline substrates, are found to contribute to the formation of cracks. Finally, we focused on tuning the CVD protocol in order to successfully grow highly crystalline graphene made of millimeter-size domains on every type of catalyst while mitigating Cu surface roughening. Putting into context the challenges and opportunities associated with the most widely used Cu catalysts provides valuable guidelines for high-throughput manufacturing of graphene suitable for emerging industrial applications.

Selectively Patterned Regrowth of Bilayer Graphene for Self-Integrated Electronics by Sequential Chemical Vapor Deposition

There is a critical demand for the highly qualified synthesis of graphene with precisely controlled thickness over a large coverage area. Selective growth can be considered as one method of preparing a vertically stacked graphene, but it usually requires elaborately alloyed substrates for chemical vapor deposition (CVD). Here, we report on a newly developed synthesis strategy for a selectively patterned grown graphene sheet in a spatially defined multithickness scale, exhibiting single- and bilayer graphene produced by a conventional CVD process. In particular, a sequential CVD growth technique on a single Cu substrate was used to produce highly ordered and alternatively patterned single- and bilayer graphene, maintaining its continuous configuration in a simplified and scalable manner. Our regrowth process did not require multiple transfer procedures or an alloying catalytic substrate to satisfy the properties of graphene associated with the needs for various applications. We also investigated the most valid mechanisms for our regrowth CVD process, which suggests that it is useful for the cost-effective synthetic approach into a built-in heterostructured single- and bilayer graphene. Finally, we demonstrated the possible accesses of transparent flexible electrodes and monolithically self-integrated all-graphene-based thin-film transistors to fully utilize regrown graphene.

Oxygen-Promoted Chemical Vapor Deposition of Graphene on Copper: A Combined Modeling and Experimental Study

Mass production of large, high-quality single-crystalline graphene is dependent on a complex coupling of factors including substrate material, temperature, pressure, gas flow, and the concentration of carbon and hydrogen species. Recent studies have shown that the oxidation of the substrate surface such as Cu before the introduction of the C precursor, methane, results in a significant increase in the growth rate of graphene while the number of nuclei on the surface of the Cu substrate decreases. We report on a phase-field model, where we include the effects of oxygen on the number of nuclei, the energetics at the growth front, and the graphene island morphology on Cu. Our calculations reproduce the experimental observations, thus validating the proposed model. Finally, and more importantly, we present growth rate from our model as a function of O concentration and precursor flux to guide the efficient growth of large single-crystal graphene of high quality.

Bridging the Gap between Reality and Ideal in Chemical Vapor Deposition Growth of Graphene

Graphene, in its ideal form, is a two-dimensional (2D) material consisting of a single layer of carbon atoms arranged in a hexagonal lattice. The richness in morphological, physical, mechanical, and optical properties of ideal graphene has stimulated enormous scientific and industrial interest, since its first exfoliation in 2004. In turn, the production of graphene in a reliable, controllable, and scalable manner has become significantly important to bring us closer to practical applications of graphene. To this end, chemical vapor deposition (CVD) offers tantalizing opportunities for the synthesis of large-area, uniform, and high-quality graphene films. However, quite different from the ideal 2D structure of graphene, in reality, the currently available CVD-grown graphene films are still suffering from intrinsic defective grain boundaries, surface contaminations, and wrinkles, together with low growth rate and the requirement of inevitable transfer. Clearly, a gap still exits between the reality of CVD-derived graphene, especially in industrial production, and ideal graphene with outstanding properties. This Review will emphasize the recent advances and strategies in CVD production of graphene for settling these issues to bridge the giant gap. We begin with brief background information about the synthesis of nanoscale carbon allotropes, followed by the discussion of fundamental growth mechanism and kinetics of CVD growth of graphene. We then discuss the strategies for perfecting the quality of CVD-derived graphene with regard to domain size, cleanness, flatness, growth rate, scalability, and direct growth of graphene on functional substrate. Finally, a perspective on future development in the research relevant to scalable growth of high-quality graphene is presented.

Effects of carbon-based impurities on graphene growth

In this paper, we studied the growth of graphene on an untreated Cu substrate and further studied the effect of carbon-based impurities on the nucleation of graphene in different growth environments. It is found that the impurities on the surface of the Cu substrate easily lead to damage of the graphene, and the impurities do not always promote nucleation as previously reported, but inhibit nucleation in a high etching environment. Finally, based on experimental results, a model of nucleation and growth of graphene around impurities is presented.

How Low Nucleation Density of Graphene on CuNi Alloy is Achieved

CuNi alloy foils are demonstrated to be one of the best substrates for synthesizing large area single-crystalline graphene because a very fast growth rate and low nucleation density can be simultaneously achieved. The fast growth rate is understood to be due the abundance of carbon precursor supply, as a result of the high catalytic activity of Ni atoms. However, a theoretical understanding of the low nucleation density remains controversial because it is known that a high carbon precursor concentration on the surface normally leads to a high nucleation density. Here, the graphene nucleation on the CuNi alloy surfaces is systematically explored and it is revealed that: i) carbon atom dissolution into the CuNi alloy passivates the alloy surface, thereby drastically increasing the graphene nucleation barrier; ii) carbon atom diffusion on the CuNi alloy surface is greatly suppressed by the inhomogeneous atomic structure of the surface; and iii) a prominent increase in the rate of carbon diffusion into the bulk occurs when the Ni composition is higher than the percolation threshold. This study reveals the key mechanism for graphene nucleation on CuNi alloy surfaces and provides a guideline for the catalyst design for the synthesis of graphene and other 2D materials.

Highly sensitive MoS2 photodetectors with graphene contacts

Two-dimensional materials such as graphene and transition metal dichalcogenides (TMDs) are ideal candidates to create ultra-thin electronics suitable for flexible substrates. Although optoelectronic devices based on TMDs have demonstrated remarkable performance, scalability is still a significant issue. Most devices are created using techniques that are not suitable for mass production, such as mechanical exfoliation of monolayer flakes and patterning by electron-beam lithography. Here we show that large-area MoS₂ grown by chemical vapor deposition and patterned by photolithography yields highly sensitive photodetectors, with record shot-noise-limited detectivities of 8.7 × 10¹⁴ Jones in ambient condition and even higher when sealed with a protective layer. These detectivity values are higher than the highest values reported for photodetectors based on exfoliated MoS₂. We study MoS₂ devices with gold electrodes and graphene electrodes. The devices with graphene electrodes have a tunable band alignment and are especially attractive for scalable ultra-thin flexible optoelectronics.

Catalytic CVD synthesis of boron nitride and carbon nanomaterials - synergies between experiment and theory

Low-dimensional carbon and boron nitride nanomaterials - hexagonal boron nitride, graphene, boron nitride nanotubes and carbon nanotubes - remain at the forefront of advanced materials research. Catalytic chemical vapour deposition has become an invaluable technique for reliably and cost-effectively synthesising these materials. In this review, we will emphasise how a synergy between experimental and theoretical methods has enhanced the understanding and optimisation of this synthetic technique. This review examines recent advances in the application of CVD to synthesising boron nitride and carbon nanomaterials and highlights where, in many cases, molecular simulations and quantum chemistry have provided key insights complementary to experimental investigation. This synergy is particularly prominent in the field of carbon nanotube and graphene CVD synthesis, and we propose here it will be the key to future advances in optimisation of CVD synthesis of boron nitride nanomaterials, boron nitride - carbon composite materials, and other nanomaterials generally.

Low-Temperature and Rapid Growth of Large Single-Crystalline Graphene with Ethane

Future applications of graphene rely highly on the production of large-area high-quality graphene, especially large single-crystalline graphene, due to the reduction of defects caused by grain boundaries. However, current large single-crystalline graphene growing methodologies are suffering from low growth rate and as a result, industrial graphene production is always confronted by high energy consumption, which is primarily caused by high growth temperature and long growth time. Herein, a new growth condition achieved via ethane being the carbon feedstock to achieve low-temperature yet rapid growth of large single-crystalline graphene is reported. Ethane condition gives a growth rate about four times faster than methane, achieving about 420 µm min^-1 for the growth of sub-centimeter graphene single crystals at temperature about 1000 °C. In addition, the temperature threshold to obtain graphene using ethane can be reduced to 750 °C, lower than the general growth temperature threshold (about 1000 °C) with methane on copper foil. Meanwhile ethane always keeps higher graphene growth rate than methane under the same growth temperature. This study demonstrates that ethane is indeed a potential carbon source for efficient growth of large single-crystalline graphene, thus paves the way for graphene in high-end electronical and optoelectronical applications.

Switching isotropic and anisotropic graphene growth in a solid source CVD system

Controlling the flow rate of carrier gases tunes the mode of growth from anisotropic to isotropic along with enlarging the crystal size.

Suppression of Graphene Nucleation by Turning Off Hydrogen Supply Just before Atmospheric Pressure Chemical Vapor Deposition Growth

To exploit the extraordinary property of graphene in practical electrical and optical devices, it is necessary to produce large-sized, single-crystal graphene. Atmospheric pressure chemical vapor deposition (APCVD) on polycrystalline Cu surface is a promising scalable route of graphene synthesis but the unavoidable multiple nucleation limits their reachable domain size. Here, we report that effective suppression of nucleation was achieved by only turning off hydrogen supply before introduction of the carbon source for graphene growth. The density of graphene decreased from 72.0 to 2.2 domains/cm2 by turning off hydrogen for 15 min. X-ray photoelectron spectroscopy and Raman spectroscopy studies show that the Cu surface was covered with 3–4 nm thick highly crystalline Cu2O, which would be caused by oxidation by residual oxidative gasses in the chamber during the turning off period. It was also revealed that elevating the temperature in Ar followed by annealing in H2/Ar before turning off hydrogen led to the enlargement of the Cu domain, resulting in the further suppression of nucleation. By optimizing such growth parameters in the CVD process, a single-crystal graphene with ~2.6 mm in diameter was successfully obtained.

Water at the Interface Between Defective Graphene and Cu or Pt (111) Surfaces

The presence of defects in the graphenic layers deposited on metal surfaces modifies the nature of the interaction. Unsaturated carbon atoms, due to vacancies in the lattice, form strong organometallic bonds with surface metal atoms that highly enhance the binding energy between the two materials. We investigate by means of a wide set of dispersion-corrected density functional theory calculations how such strong chemical bonds affect both the electronic properties of these hybrid interfaces and the chemical reactivity with water, which is commonly present in the working conditions. We compare different metal substrates (Cu vs Pt) that present a different type of interaction with graphene and with defective graphene. This comparative analysis allows us to unravel the controlling factors of water reactivity, the role played by the carbon vacancies and by the confinement or "graphene cover effect". Water is capable of breaking the C-Cu bond by dissociating at the undercoordinated carbon atom of the vacancy, restoring the weak van der Waals type of interaction between the two materials that allows for an easy detachment of graphene from the metal, but the same is not true in the case of Pt, where C-Pt bonds are much stronger. These conclusions can be used to rationalize water reactivity at other defective graphene/metal interfaces.

Low-energy transmission electron diffraction and imaging of large-area graphene

Two-dimensional (2D) materials have attracted interest because of their excellent properties and potential applications. A key step in realizing industrial applications is to synthesize wafer-scale single-crystal samples. Until now, single-crystal samples, such as graphene domains up to the centimeter scale, have been synthesized. However, a new challenge is to efficiently characterize large-area samples. Currently, the crystalline characterization of these samples still relies on selected-area electron diffraction (SAED) or low-energy electron diffraction (LEED), which is more suitable for characterizing very small local regions. This paper presents a highly efficient characterization technique that adopts a low-energy electrostatically focused electron gun and a super-aligned carbon nanotube (SACNT) film sample support. It allows rapid crystalline characterization of large-area graphene through a single photograph of a transmission-diffracted image at a large beam size. Additionally, the low-energy electron beam enables the observation of a unique diffraction pattern of adsorbates on the suspended graphene at room temperature. This work presents a simple and convenient method for characterizing the macroscopic structures of 2D materials, and the instrument we constructed allows the study of the weak interaction with 2D materials.