Formation and Healing of Vacancies in Graphene Chemical Vapor Deposition (CVD) Growth

Toward the Production of Super Graphene

The quality requirements of graphene depend on the applications. Some have a high tolerance for graphene quality and even require some defects, while others require graphene as perfect as possible to achieve good performance. So far, synthesis of large-area graphene films by chemical vapor deposition of carbon precursors on metal substrates, especially on Cu, remains the main way to produce high-quality graphene, which has been significantly developed in the past 15 years. However, although many prototypes are demonstrated, their performance is still more or less far from the theoretical property limit of graphene. This review focuses on how to make super graphene, namely graphene with a perfect structure and free of contaminations. More specially, this study focuses on graphene synthesis on Cu substrates. Typical defects in graphene are first discussed together with the formation mechanisms and how they are characterized normally, followed with a brief review of graphene properties and the effects of defects. Then, the synthesis progress of super graphene from the aspects of substrate, grain size, wrinkles, contamination, adlayers, and point defects are reviewed. Graphene transfer is briefly discussed as well. Finally, the challenges to make super graphene are discussed and a strategy is proposed.

Surface activation of Hastalex by vacuum argon plasma for cytocompatibility enhancement

Here, we present surface analysis and biocompatibility evaluation of novel composite material based on graphene oxide traded as Hastalex. First, the surface morphology and elemental analysis of the pristine material were examined by atomic force and scanning electron microscopies, and by energy-dispersive and X-ray photoelectron spectroscopies, respectively. The Hastalex surface was then modified by plasma (3 and 8 W with exposure times up to 240 s), the impact of which on the material surface wettability and morphology was further evaluated. In addition, the material aging was studied at room and elevated temperatures. Significant changes in surface roughness, morphology, and area were detected at the nanometer scale after plasma exposure. An increase in oxygen content due to the plasma exposure was observed both for 3 and 8 W. The plasma treatment had an outstanding effect on the cytocompatibility of Hastalex foil treated at both input powers of 3 and 8 W. The cell number of human MRC-5 fibroblasts on Hastalex foils exposed to plasma increased significantly compared to pristine Hastalex and even to tissue culture polystyrene. The plasma exposure also affected the fibroblasts' cell growth and shape.

Energy Application of Graphene Based Membrane: Hydrogen Separation

Hydrogen gas (H2 ) is a viable energy carrier that has the potential to replace the traditional fossil fuels and contribute to achieving zero net emissions, making it an attractive option for a hydrogen-based society. However, current H2 purification technologies are often limited by high energy consumption, and as a result, there is a growing demand for alternative techniques that offer higher H2 purity and energy efficiency. Membrane separation has emerged as a promising approach for obtaining high-purity H2 gas with low energy consumption. Nevertheless, despite years of development, commercial polymeric membranes have limited performance, prompting researchers to explore alternative materials. In this context, carbon-based membranes, specifically graphene-based nanomaterials, have gained significant attention as potential membrane materials due to their unique properties. In this review, we provide a comprehensive overview of carbon-based membranes for H2 gas separation, fabrication of the membrane, and its characterization, including their advantages and limitations. We also explore the current technological challenges and suggest insights into future research directions, highlighting potential ways to improve graphene-based membranes performance for H2 separations.

Gas Separations using Nanoporous Atomically Thin Membranes: Recent Theoretical, Simulation, and Experimental Advances

Porous graphene and other atomically thin 2D materials are regarded as highly promising membrane materials for high-performance gas separations due to their atomic thickness, large-scale synthesizability, excellent mechanical strength, and chemical stability. When these atomically thin materials contain a high areal density of gas-sieving nanoscale pores, they can exhibit both high gas permeances and high selectivities, which is beneficial for reducing the cost of gas-separation processes. Here, recent modeling and experimental advances in nanoporous atomically thin membranes for gas separations is discussed. The major challenges involved, including controlling pore size distributions, scaling up the membrane area, and matching theory with experimental results, are also highlighted. Finally, important future directions are proposed for real gas-separation applications of nanoporous atomically thin membranes.

Direct Chemical Vapor Deposition Synthesis of Porous Single-Layer Graphene Membranes with High Gas Permeances and Selectivities

Single-layer graphene containing molecular-sized in-plane pores is regarded as a promising membrane material for high-performance gas separations due to its atomic thickness and low gas transport resistance. However, typical etching-based pore generation methods cannot decouple pore nucleation and pore growth, resulting in a trade-off between high areal pore density and high selectivity. In contrast, intrinsic pores in graphene formed during chemical vapor deposition are not created by etching. Therefore, intrinsically porous graphene can exhibit high pore density while maintaining its gas selectivity. In this work, the density of intrinsic graphene pores is systematically controlled for the first time, while appropriate pore sizes for gas sieving are precisely maintained. As a result, single-layer graphene membranes with the highest H₂ /CH₄ separation performances recorded to date (H₂ permeance > 4000 GPU and H₂ /CH₄ selectivity > 2000) are fabricated by manipulating growth temperature, precursor concentration, and non-covalent decoration of the graphene surface. Moreover, it is identified that nanoscale molecular fouling of the graphene surface during gas separation where graphene pores are partially blocked by hydrocarbon contaminants under experimental conditions, controls both selectivity and temperature dependent permeance. Overall, the direct synthesis of porous single-layer graphene exploits its tremendous potential as high-performance gas-sieving membranes.

Theoretical Study of Chemical Vapor Deposition Synthesis of Graphene and Beyond: Challenges and Perspectives

Two-dimensional (2D) materials have attracted great attention in recent years because of their unique dimensionality and related properties. Chemical vapor deposition (CVD), a crucial technique for thin-film epitaxial growth, has become the most promising method of synthesizing 2D materials. Different from traditional thin-film growth, where strong chemical bonds are involved in both thin films and substrates, the interaction in 2D materials and substrates involves the van der Waals force and is highly anisotropic, and therefore, traditional thin-film growth theories cannot be applied to 2D material CVD synthesis. During the last 15 years, extensive theoretical studies were devoted to the CVD synthesis of 2D materials. This Perspective attempts to present a theoretical framework for 2D material CVD synthesis as well as the challenges and opportunities in exploring CVD mechanisms. We hope that this Perspective can provide an in-depth understanding of 2D material CVD synthesis and can further stimulate 2D material synthesis.

A comparative density functional theory study of oxygen doping versus adsorption on graphene to tune its band gap

Graphene, having a perfect two-dimensional crystal structure, has many excellent features such as a high specific surface area, and extraordinary electrical, thermal and mechanical properties. However, its usage in electronic devices is possible only if band gap of desired value is induced in this gapless semi-metal. Therefore, first principle calculations have been carried out to investigate the role of oxygen (O) doping versus adsorption and, the impurity concentration and coverage to induce band gap in graphene employing PBE at GGA level. The band gap is induced owing to production of vacancies, dissociative adsorption of oxygen, subsituational doping and pre-dissociated oxygen adsorption. It is interesting to note that band gap is introduced by both the processes of doping and adsorption of O. The oxygen doping leads to induction of two energy gaps, smaller in value above and larger below the Fermi level; while adsorption irrespective of adsorption configuration produces single direct gap. Increase both in concentration and coverage leads to enhance band gap value maximum being 1.85 eV in case of hexagonal doping of 12.5% concentration with an exception in adsorption case. The results allow us to conclude that adsorption is as useful as doping to tune the band gap in graphene enabling its applications in designing high performance electronic devices.

Vacancies in graphene: an application of adiabatic quantum optimization

Quantum annealers have grown in complexity to the point that quantum computations involving a few thousand qubits are now possible. In this paper, with the intentions to show the feasibility of quantum annealing to tackle problems of physical relevance, we used a simple model, compatible with the capability of current quantum annealers, to study the relative stability of graphene vacancy defects. By mapping the crucial interactions that dominate carbon-vacancy interchange onto a quadratic unconstrained binary optimization problem, our approach exploits the ground state as well as the excited states found by the quantum annealer to extract all the possible arrangements of multiple defects on the graphene sheet together with their relative formation energies. This approach reproduces known results and provides a stepping stone towards applications of quantum annealing to problems of physical-chemical interest.

Graphene-based carbocatalysts for carbon-carbon bond formation

Organic transformations are usually catalyzed by metal-based catalysts. In contrast, metal-free catalysts have attracted considerable attention from the viewpoint of sustainability and safety. Among the studies in metal-free catalysis, graphene-based materials have been introduced in the reactions that are usually catalyzed by transition metal catalysts. This review covers the literature (up to the beginning of April 2020) on the use of graphene and its derivatives as carbocatalysts for C-C bond-forming reactions, which are one of the fundamental reactions in organic syntheses. Besides, mechanistic studies are included for the rational understanding of the catalysis. Graphene has significant potential in the field of metal-free catalysis because of the fine-tunable potential of the structure, high stability and durability, and no metal contamination, making it a next-generation candidate material in catalysis.

Trends in the Change in Graphene Conductivity upon Gas Adsorption: The Relevance of Orbital Distortion

The experimental ability to alter graphene (G) conductivity by adsorption of a single gas molecule is promoting the development of ultra-high-sensitivity gas detectors and could ultimately provide a novel playground for future nanoelectronics devices. At present, the underpinning effect is broadly attributed to a variation of G carrier concentration, caused by an adsorption-induced Fermi-level shift. By means of first-principle Kubo-Greenwood calculations, here we demonstrate that adsorbate-induced orbital distortion could also lead to small but finite G conductivity changes, even in the absence of Fermi-level shifts. This mechanism enables a sound physical interpretation of the observed variable sensitivity of G devices to different chemical moieties, and it can be strongly enhanced by using a suitable Ni substrate, thereby opening new pathways for the optimal design of operational nanoscale detectors.

Single-Crystalline Monolayer Graphene Wafer on Dielectric Substrate of SiON without Metal Catalysts

Many scientific and engineering efforts have been made to realize graphene electronics by fully utilizing intrinsic properties of ideal graphene for last decades. The most technical huddles come from the absence of wafer-scale graphene with a single-crystallinity on dielectric substrates. Here, we report an epitaxial growth of single-crystalline monolayer graphene directly on a single-crystalline dielectric SiON-SiC(0001) with a full coverage via epitaxial chemical vapor deposition (CVD) without metal catalyst. The dielectric surface of SiON provides atomically flat and chemically inert interface by passivation of dangling bonds, which keeps intrinsic properties of graphene. Atomic structures with a clean interface, full coverage of single-crystalline monolayer, and the epitaxy of graphene on SiON were confirmed macroscopically by mapping low energy electron diffraction (LEED) and Raman spectroscopy, and atomically by scanning tunneling microscopy (STM). Both of measured and calculated local density of states (LDOS) exhibit a symmetric and sharp Dirac cone with a Dirac point located at a Fermi level. Our method provides a route to utilize a single-crystalline dielectric substrate for ideal graphene growth for future applications.

Doping of epitaxial graphene by direct incorporation of nickel adatoms

Direct incorporation of Ni adatoms during graphene growth on Ni(111) is evidenced by scanning tunneling microscopy. The structure and energetics of the observed defects is thoroughly characterized at the atomic level on the basis of density functional theory calculations. Our results show the feasibility of a simple scalable method, which could be potentially used for the realization of macroscopic practical devices, to dope graphene with a transition metal. The method exploits the kinetics of the growth process for the incorporation of Ni adatoms in the graphene network.

Defective Graphene on the Transition-Metal Surface: Formation of Efficient Bifunctional Catalysts for Oxygen Evolution/Reduction Reactions in Alkaline Media

Supported single-atom catalysts (SACs) have attracted enormous attention because of their high selectivity, activity, and efficiency, compared to conventional nanoparticles and metal bulk catalysts. However, all of these unique merits rely on the stability of the SAC, as reported by many investigators. To avoid aggregation of single-metal atoms and maintain the high performance of the SAC, various substrates have been tried to support them, particularly on graphene nanosheets. A spontaneous interface phenomenon between graphene and the Co (and Ni) substrate discovered in this work is that the holes in the graphene layer can stimulate metal atoms to pop up from a metal substrate and fill the double vacancy in graphene (DV-G) and stabilize on the graphene surface. The unique structure of the lifted metal atom is expected to be useful for the bifunctional SAC for electrocatalytic oxygen evolution reactions (OERs) and oxygen reduction reactions (ORRs). Our first-principles calculations indicate that the DV-G on the Co(0001) surface can serve as an excellent bifunctional OER/ORR catalyst in alkaline media with extremely low overpotentials of 0.39 V for OER and only 0.36 V for ORR processes, which are even lower than those for previously reported bifunctional catalysts. We believe that the catalytic activity stems from the interface coupling effect between the DV-G and metal substrate, as well as the charge redistribution in the graphitic sheet.

Atomic-Scale Structural Modification of 2D Materials

2D materials have attracted much attention since the discovery of graphene in 2004. Due to their unique electrical, optical, and magnetic properties, they have potential for various applications such as electronics and optoelectronics. Owing to thermal motion and lattice growth kinetics, different atomic-scale structures (ASSs) can originate from natural or intentional regulation of 2D material atomic configurations. The transformations of ASSs can result in the variation of the charge density, electronic density of state and lattice symmetry so that the property tuning of 2D materials can be achieved and the functional devices can be constructed. Here, several kinds of ASSs of 2D materials are introduced, including grain boundaries, atomic defects, edge structures, and stacking arrangements. The design strategies of these structures are also summarized, especially for atomic defects and edge structures. Moreover, toward multifunctional integration of applications, the modulation of electrical, optical, and magnetic properties based on atomic-scale structural modification are presented. Finally, challenges and outlooks are featured in the aspects of controllable structure design and accurate property tuning for 2D materials with ASSs. This work may promote research on the atomic-scale structural modification of 2D materials toward functional applications.

Kinetics of Graphene and 2D Materials Growth

During the last 10 years, remarkable achievements on the chemical vapor deposition (CVD) growth of 2D materials have been made, but the understanding of the underlying mechanisms is still relatively limited. Here, the current progress on the understanding of the growth kinetics of 2D materials, especially for their CVD synthesis, is reviewed. In order to present a complete picture of 2D materials' growth kinetics, the following factors are discussed: i) two types of growth modes, namely attachment-limited growth and diffusion-limited growth; ii) the etching of 2D materials, which offers an additional degree of freedom for growth control; iii) a number of experimental factors in graphene CVD synthesis, such as structure of the substrate, pressure of hydrogen or oxygen, temperature, etc., which are found to have profound effects on the growth kinetics; iv) double-layer and few-layer 2D materials' growth, which has distinct features different from the growth of single-layer 2D materials; and v) the growth of polycrystalline 2D materials by the coalescence of a few single crystalline domains. Finally, the current challenges and opportunities in future 2D materials' synthesis are summarized.

Facile Fabrication of Large-Area Atomically Thin Membranes by Direct Synthesis of Graphene with Nanoscale Porosity

Direct synthesis of graphene with well-defined nanoscale pores over large areas can transform the fabrication of nanoporous atomically thin membranes (NATMs) and greatly enhance their potential for practical applications. However, scalable bottom-up synthesis of continuous sheets of nanoporous graphene that maintain integrity over large areas has not been demonstrated. Here, it is shown that a simple reduction in temperature during chemical vapor deposition (CVD) on Cu induces in-situ formation of nanoscale defects (≤2-3 nm) in the graphene lattice, enabling direct and scalable synthesis of nanoporous monolayer graphene. By solution-casting of hierarchically porous polyether sulfone supports on the as-grown nanoporous CVD graphene, large-area (>5 cm² ) NATMs for dialysis applications are demonstrated. The synthesized NATMs show size-selective diffusive transport and effective separation of small molecules and salts from a model protein, with ≈2-100× increase in permeance along with selectivity better than or comparable to state-of-the-art commercially available polymeric dialysis membranes. The membranes constitute the largest fully functional NATMs fabricated via bottom-up nanopore formation, and can be easily scaled up to larger sizes permitted by CVD synthesis. The results highlight synergistic benefits in blending traditional membrane casting with bottom-up pore creation during graphene CVD for advancing NATMs toward practical applications.

Cobalt-assisted recrystallization and alignment of pure and doped graphene

Recrystallization of bulk materials is a well-known phenomenon, which is widely used in commercial manufacturing. However, for low-dimensional materials like graphene, this process still remains an unresolved puzzle. Thus, the understanding of the underlying mechanisms and the required conditions for recrystallization in low dimensions is essential for the elaboration of routes towards the inexpensive and reliable production of high-quality nanomaterials. Here, we unveil the details of the efficient recrystallization of one-atom-thick pure and boron-doped polycrystalline graphene layers on a Co(0001) surface. By applying photoemission and electron diffraction, we show how more than 90% of the initially misoriented graphene grains can be reconstructed into a well-oriented and single-crystalline layer. The obtained recrystallized graphene/Co interface exhibits high structural quality with a pronounced sublattice asymmetry, which is important for achieving an unbalanced sublattice doping of graphene. By exploring the kinetics of recrystallization for native and B-doped graphene on Co, we were able to estimate the activation energy and propose a mechanism of this process.

Atomistic Simulations of Graphene Growth: From Kinetics to Mechanism

Epitaxial growth is a promising strategy to produce high-quality graphene samples. At the same time, this method has great flexibility for industrial scale-up. To optimize growth protocols, it is essential to understand the underlying growth mechanisms. This is, however, very challenging, as the growth process is complicated and involves many elementary steps. Experimentally, atomic-scale in situ characterization methods are generally not feasible at the high temperature of graphene growth. Therefore, kinetics is the main experimental information to study growth mechanisms. Theoretically, first-principles calculations routinely provide atomic structures and energetics but have a stringent limit on the accessible spatial and time scales. Such gap between experiment and theory can be bridged by atomistic simulations using first-principles atomic details as input and providing the overall growth kinetics, which can be directly compared with experiment, as output. Typically, system-specific approximations should be applied to make such simulations computationally feasible. By feeding kinetic Monte Carlo (kMC) simulations with first-principles parameters, we can directly simulate the graphene growth process and thus understand the growth mechanisms. Our simulations suggest that the carbon dimer is the dominant feeding species in the epitaxial growth of graphene on both Cu(111) and Cu(100) surfaces, which enables us to understand why the reaction is diffusion limited on Cu(111) but attachment limited on Cu(100). When hydrogen is explicitly considered in the simulation, the central role hydrogen plays in graphene growth is revealed, which solves the long-standing puzzle into why H₂ should be fed in the chemical vapor deposition of graphene. The simulation results can be directly compared with the experimental kinetic data, if available. Our kMC simulations reproduce the experimentally observed quintic-like behavior of graphene growth on Ir(111). By checking the simulation results, we find that such nonlinearity is caused by lattice mismatch, and the induced growth front inhomogeneity can be universally used to predict growth behaviors in other heteroepitaxial systems. Notably, although experimental kinetics usually gives useful insight into atomic mechanisms, it can sometimes be misleading. Such pitfalls can be avoided via atomistic simulations, as demonstrated in our study of the graphene etching process. Growth protocols can be designed theoretically with computational kinetic and mechanistic information. By contrasting the different activation energies involved in an atom-exchange-based carbon penetration process for monolayer and bilayer graphene, we propose a three-step strategy to grow high-quality bilayer graphene. Based on first-principles parameters, a kinetic pathway toward the high-density, ordered N doping of epitaxial graphene on Cu(111) using a C₅NCl₅ precursor is also identified. These studies demonstrate that atomistic simulations can unambiguously produce or reproduce the kinetic information on graphene growth, which is pivotal to understanding the growth mechanism and designing better growth protocols. A similar strategy can be used in growth mechanism studies of other two-dimensional atomic crystals.

Self-cleaning by harnessing wrinkles in two-dimensional layered crystals

Two-dimensional (2D) layered crystals are prone to bending and folding owing to their ultra-low bending stiffness. Folds are traditionally viewed as defects that degrade the material performance. Here, we demonstrate that folds and cohesive forces in 2D layered crystals like graphene and MoS₂ can be exploited to collect and clean up interlayer impurities, wherein multiple separated impurities agglomerate into a single, large cluster. We combine classical molecular dynamics simulations and an analytical model to elucidate the competing roles of membrane bending and impurity-membrane cohesive energies in the self-cleaning process. Our findings shed light on the mechanisms by which the forces that are present in 2D layered crystals can positively impact, through the possibility of intrinsic cleaning and defect engineering, the synthesis of van der Waals homo- and heterostructures with improved reliability and functionalities.

The transition metal surface dependent methane decomposition in graphene chemical vapor deposition growth

By using density-functional theory (DFT) calculations, the dissociation of CH₄ on various metal surfaces, including Ni, Cu, Ru, Pd, Pt, Ir, Co, Au, and Rh, is systematically explored. For all the explored face-centered cubic (fcc) metal substrates, the (100) surface is found to be more active than the (111) surface, which explains the higher activity of the (100) surface in graphene chemical vapor deposition (CVD) growth. The catalytic activity order of these metals is found to be Ni ≈ Rh ≈ Co ≈ Ru > Pd ≈ Pt ≈ Ir > Cu > Au, which explained the catalyst type dependent growth behavior of graphene. It was found that the main dissociation product of CH₄ on Ni, Pd, Pt, Ir, Rh, Co, and Ru substrates is a carbon monomer and a very high rate of dissociation is expected, but a low rate of dissociation and the dissociation products of CH_i (i = 1, 2, 3) are expected on Cu and Au surfaces, which explained the diffusion-limited growth of graphene on Cu and Au surfaces and attachment limited growth on other active metal surfaces. Furthermore, our study shows that the dissociation of CH₄ on all these metal substrates follows the well-known Brønsted-Evans-Polanyi (BEP) principles, or the reaction barrier is roughly linear to the reaction energy.

Formation mechanism of overlapping grain boundaries in graphene chemical vapor deposition growth

The formation mechanisms of two different types of grain boundaries (GBs), the weakly bound overlapping GB and the covalent bound GB, during graphene domain coalescence are revealed by both theoretical modeling and experimental observations.

The formation of grain boundaries (GBs) in graphene films is both fundamentally interesting and practically important for many applications. A GB in graphene is known as a linear defect and is formed during the coalescence of two single crystalline graphene domains. The covalent binding between domains is broadly known as the mechanism of GB formation during graphene chemical vapor deposition (CVD) growth. Here, we demonstrate another GB formation mechanism, where two graphene domains are connected by weak van der Waals interactions between overlapping graphene layers. The formation mechanism of the overlapping GBs (OLGBs) is systematically explored theoretically and the proposed conditions for forming OLGBs are validated by experimental observations. This discovery leads to a deep understanding of the mechanism of graphene CVD growth and reveals potential means for graphene quality control in CVD synthesis.

An ab initio study of the nickel-catalyzed transformation of amorphous carbon into graphene in rapid thermal processing

Ab initio molecular dynamics (AIMD) simulations are employed to investigate the chemical mechanism underlying the Ni-catalyzed transformation of amorphous carbon (a-C) into graphene in the rapid thermal processing (RTP) experiment to directly grow graphene on various dielectric surfaces via the evaporation of surplus Ni and C at 1100 °C (below the melting point of bulk Ni). It is found that the a-C-to-graphene transformation entails the metal-induced crystallization and layer exchange mechanism, rather than the conventional dissolution/precipitation mechanism typically involved in Ni-catalyzed chemical vapor deposition (CVD) growth of graphene. The multi-layer graphene can be tuned by changing the relative thicknesses of deposited a-C and Ni thin films. Our AIMD simulations suggest that the easy evaporation of surplus Ni with excess C is likely attributed to the formation of a viscous-liquid-like Ni-C solution within the temperature range of 900-1800 K and to the faster diffusion of C atoms than that of Ni atoms above 600 K. Even at room temperature, sp(3)-C atoms in a-C are quickly converted to sp(2)-C atoms in the course of the simulation, and the graphitic C formation can occur at low temperature. When the temperature is as high as 1200 K, the grown graphitic structures reversely dissolve into Ni. Because the rate of temperature increase is considerably faster in the AIMD simulations than in realistic experiments, defects in the grown graphitic structures are kinetically trapped. In this kinetic growth stage, the carbon structures grown from sp(3)-carbon or from sp(2)-carbon exhibit marked differences.

Large scale atomistic simulation of single-layer graphene growth on Ni(111) surface: molecular dynamics simulation based on a new generation of carbon-metal potential

To explore the mechanism of graphene chemical vapor deposition (CVD) growth on a catalyst surface, a molecular dynamics (MD) simulation of carbon atom self-assembly on a Ni(111) surface based on a well-designed empirical reactive bond order potential was performed. We simulated single layer graphene with recorded size (up to 300 atoms per super-cell) and reasonably good quality by MD trajectories up to 15 ns. Detailed processes of graphene CVD growth, such as carbon atom dissolution and precipitation, formation of carbon chains of various lengths, polygons and small graphene domains were observed during the initial process of the MD simulation. The atomistic processes of typical defect healing, such as the transformation from a pentagon into a hexagon and from a pentagon-heptagon pair (5|7) to two adjacent hexagons (6|6), were revealed as well. The study also showed that higher temperature and longer annealing time are essential to form high quality graphene layers, which is in agreement with experimental reports and previous theoretical results.

Direct Synthesis of Unilamellar MgAl-LDH Nanosheets and Stacking in Aqueous Solution

Two-dimensional (2D) materials, such as graphene, inorganic oxides, and hydroxides, are one of the most extensively studied classes of materials due to their unilamellar crystallites or nanosheet structures. In this study, instead of using the universal exfoliation method of the bulky crystal precursor, 2D crystals/nanosheets of MgAl-layered double hydroxides (LDHs) were synthesized in formamide. We propose that the obtained crystals are unilamellar according to the XRD, TEM, and AFM observations. The HRTEM and fast Fourier transform images confirm that the crystal structures are the same as those of the exfoliated MgAl-LDH nanosheets. The directly synthesized sheets can stack into a 3D crystal structure, which is the same as that of typical LDHs except for the disordered orientation of the a-/b- crystal axis of each sheet. This result provides not only a novel approach to the preparation of 2D crystals but also insight into the formation mechanism of LDHs.

Coexistence of metallic and insulating-like states in graphene

Since graphene has been taken as the potential host material for next-generation electric devices, coexistence of high carrier mobility and an energy gap has the determining role in its real applications. However, in conventional methods of band-gap engineering, the energy gap and carrier mobility in graphene are seemed to be the two terminals of a seesaw, which limit its rapid development in electronic devices. Here we demonstrated the realization of insulating-like state in graphene without breaking Dirac cone. Using first-principles calculations, we found that ferroelectric substrate not only well reserves the Dirac fermions, but also induces pseudo-gap states in graphene. Calculated transport results clearly revealed that electrons cannot move along the ferroelectric direction. Thus, our work established a new concept of opening an energy gap in graphene without reducing the high mobility of carriers, which is a step towards manufacturing graphene-based devices.

What are the active carbon species during graphene chemical vapor deposition growth?

The dissociation of carbon feedstock is a crucial step for understanding the mechanism of graphene chemical vapor deposition (CVD) growth. Using first-principles calculations, we performed a comprehensive theoretical study for the population of various active carbon species, including carbon monomers and various radicals, CHi (i = 1, 2, 3, 4), on four representative transition-metal surfaces, Cu(111), Ni(111), Ir(111) and Rh(111), under different experimental conditions. On the Cu surface, which is less active, the population of CH and C monomers at the subsurface is found to be very high and thus they are the most important precursors for graphene CVD growth. On the Ni surface, which is more active than Cu, C monomers at the subsurface dominate graphene CVD growth under most experimental conditions. In contrast, on the active Ir and Rh surfaces, C monomers on the surfaces are found to be very stable and thus are the main precursors for graphene growth. This study shows that the mechanism of graphene CVD growth depends on the activity of catalyst surfaces and the detailed graphene growth process at the atomic level can be controlled by varying the temperature or partial pressure of hydrogen.

Insights into carbon nanotube and graphene formation mechanisms from molecular simulations: a review

The discovery of carbon nanotubes (CNTs) and graphene over the last two decades has heralded a new era in physics, chemistry and nanotechnology. During this time, intense efforts have been made towards understanding the atomic-scale mechanisms by which these remarkable nanostructures grow. Molecular simulations have made significant contributions in this regard; indeed, they are responsible for many of the key discoveries and advancements towards this goal. Here we review molecular simulations of CNT and graphene growth, and in doing so we highlight the many invaluable insights gained from molecular simulations into these complex nanoscale self-assembly processes. This review highlights an often-overlooked aspect of CNT and graphene formation-that the two processes, although seldom discussed in the same terms, are in fact remarkably similar. Both can be viewed as a 0D → 1D → 2D transformation, which converts carbon atoms (0D) to polyyne chains (1D) to a complete sp(2)-carbon network (2D). The difference in the final structure (CNT or graphene) is determined only by the curvature of the catalyst and the strength of the carbon-metal interaction. We conclude our review by summarizing the present shortcomings of CNT/graphene growth simulations, and future challenges to this important area.

Repeated growth-etching-regrowth for large-area defect-free single-crystal graphene by chemical vapor deposition

Reducing nucleation density and healing structural defects are two challenges for fabricating large-area high-quality single-crystal graphene, which is essential for its electronic and optoelectronic applications. We have developed a method involving chemical vapor deposition (CVD) growth followed by repeated etching-regrowth, to solve both problems at once. Using this method, we can obtain single-crystal graphene domains with a size much larger than that allowed by the nucleation density in the initial growth and efficiently heal structural defects similar to graphitization but at a much lower temperature, both of which are impossible to realize by conventional CVD. Using this method with Pt as a growth substrate, we have grown ∼3 mm defect-free single-crystal graphene domains with a carrier mobility up to 13,000 cm2 V(-1) s(-1) under ambient conditions.

Mechanisms of graphene growth on metal surfaces: theoretical perspectives

Graphene is an important material with unique electronic properties. Aiming to obtain high quality samples at a large scale, graphene growth on metal surfaces has been widely studied. An important topic in these studies is the atomic scale growth mechanism, which is the precondition for a rational optimization of growth conditions. Theoretical studies have provided useful insights for understanding graphene growth mechanisms, which are reviewed in this article. On the mostly used Cu substrate, graphene growth is found to be more complicated than a simple adsorption-dehydrogenation-growth model. Growth on Ni surface is precipitation dominated. On surfaces with a large lattice mismatch to graphene, epitaxial geometry determin a robust nonlinear growth behavior. Further progresses in understanding graphene growth mechanisms is expected with intense theoretical studies using advanced simulation techniques, which will make a guided design of growth protocols practical.

Vacancy inter-layer migration in multi-layered graphene

The migration of vacancies between graphene layers and carbon nanotube walls has been observed in experiments, in which it is well known that the migration of vacancies between adjacent layers is prohibited by a very large energy barrier (∼7.0 eV). This contradiction has been a major puzzle for a number of years. In the present study, by using density functional tight-binding molecular dynamic simulations and first principle calculations, we have found that interaction between vacancies or vacancy holes in neighbouring graphene layers can greatly reduce the barrier, to ∼3 eV or less, and this expedites the migration process. In addition, all the vacancies in a multi-layered graphene gather to form a single hole in one layer. Our study has revealed a new mechanism for healing the defect in graphene materials and successfully explains the experimental puzzle. Our results have important applications in the engineering of graphene materials.

Ultrasmooth metallic foils for growth of high quality graphene by chemical vapor deposition

Synthesis of graphene by chemical vapor deposition is a promising route for manufacturing large-scale high-quality graphene for electronic applications. The quality of the employed substrates plays a crucial role, since the surface roughness and defects alter the graphene growth and cause difficulties in the subsequent graphene transfer. Here, we report on ultrasmooth high-purity copper foils prepared by sputter deposition of Cu thin film on a SiO2/Si template, and the subsequent peeling off of the metallic layer from the template. The surface displays a low level of oxidation and contamination, and the roughness of the foil surface is generally defined by the template, and was below 0.6 nm even on a large scale. The roughness and grain size increase occurred during both the annealing of the foils, and catalytic growth of graphene from methane (≈1000 °C), but on the large scale still remained far below the roughness typical for commercial foils. The micro-Raman spectroscopy and transport measurements proved the high quality of graphene grown on such foils, and the room temperature mobility of the graphene grown on the template stripped foil was three times higher compared to that of one grown on the commercial copper foil. The presented high-quality copper foils are expected to provide large-area substrates for the production of graphene suitable for electronic applications.

The edge termination controlled kinetics in graphene chemical vapor deposition growth

The kinetics of graphene CVD growth is dominated by the type of edge passivation.

Signatures in vibrational and UV-visible absorption spectra for identifying cyclic hydrocarbons by graphene fragments

To promote possible applications of graphene in molecular identification based on stacking effects, in particular in recognizing aromatic amino acids and even sequencing nucleobases in life sciences, we comprehensively study the interaction between graphene segments and different cyclic organic hydrocarbons including benzene (C6H6), cyclohexane (C6H12), benzyne (C6H4), cyclohexene (C6H10), 1,3-cyclohexadiene (C6H8(1)) and 1,4-cyclohexadiene (C6H8(2)), using the density-functional tight-binding (DFTB) method. Interestingly, we find obviously different characteristics in Raman vibrational and ultraviolet visible absorption spectra of the small molecules adsorbed on the graphene sheet. Specifically, we find that both spectra involve clearly different characteristic peaks, belonging to the different small molecules upon adsorption, with the ones of ionized molecules being more substantial. Further analysis shows that the adsorptions are almost all due to the presence of dispersion energy in neutral cases and involve charge transfer from the graphene to the small molecules. In contrast, the main binding force in the ionic adsorption systems is the electronic interaction. The results present clear signatures that can be used to recognize different kinds of aromatic hydrocarbon rings on graphene sheets. We expect that our findings will be helpful for designing molecular recognition devices using graphene.

Strains induced by point defects in graphene on a metal

Strains strongly affect the properties of low-dimensional materials, such as graphene. By combining in situ, in operando, reflection high-energy electron diffraction experiments with first-principles calculations, we show that large strains, above 2%, are present in graphene during its growth by chemical vapor deposition on Ir(111) and when it is subjected to oxygen etching and ion bombardment. Our results unravel the microscopic relationship between point defects and strains in epitaxial graphene and suggest new avenues for graphene nanostructuring and engineering its properties through introduction of defects and intercalation of atoms and molecules between graphene and its metal substrate.

Epitaxial graphene growth and shape dynamics on copper: phase-field modeling and experiments

The epitaxial growth of graphene on copper foils is a complex process, influenced by thermodynamic, kinetic, and growth parameters, often leading to diverse island shapes including dendrites, squares, stars, hexagons, butterflies, and lobes. Here, we introduce a phase-field model that provides a unified description of these diverse growth morphologies and compare the model results with new experiments. Our model explicitly accounts for the anisotropies in the energies of growing graphene edges, kinetics of attachment of carbon at the edges, and the crystallinity of the underlying copper substrate (through anisotropy in surface diffusion). We show that anisotropic diffusion has a very important, counterintuitive role in the determination of the shape of islands, and we present a "phase diagram" of growth shapes as a function of growth rate for different copper facets. Our results are shown to be in excellent agreement with growth shapes observed for high symmetry facets such as (111) and (001) as well as for high-index surfaces such as (221) and (310).