High-yield chemical vapor deposition growth of high-quality large-area AB-stacked bilayer graphene

Probing the Critical Role of Interfaces for Superior Performance in PbS Quantum Dot/Graphene Nanohybrid Broadband Photodetectors

Colloidal quantum dots/graphene (QD/Gr) nanohybrids have been studied intensively for photodetection in a broadband spectrum including ultraviolet, visible, near-infrared, and shortwave infrared (UV-vis-NIR-SWIR). Since the optoelectronic process in the QD/Gr nanohybrid relies on the photogenerated charge carrier transfer from QDs to graphene, understanding the role of the QD-QD and QD-Gr interfaces is imperative to the QD/Gr nanohybrid photodetection. Herein, a systematic study is carried out to probe the effect of these interfaces on the noise, photoresponse, and specific detectivity in the UV-vis-NIR-SWIR spectrum. Interestingly, the photoresponse has been found to be negligible without a 3-mercaptopropionic acid (MPA) ligand exchange, moderate with a single ligand exchange after all QD layers are deposited on graphene, and maximum if it is performed after each QD layer deposition up to five layers of total QD thickness of 260-280 nm. Furthermore, exposure of graphene to C-band UV (UVC) for a short period of 4-5 min before QD deposition leads to improved photoresponse via removal of polar molecules at the QD/Gr interface. With the combination of the MPA ligand exchange and UVC exposure, optimal optoelectronic properties can be obtained on the PbS QD/Gr nanohybrids with high specific detectivity up to 2.6 × 1011, 1.5 × 1011, 5 × 1010, and 1.9 × 109 Jones at 400, 550, 1000, and 1700 nm, respectively, making the nanohybrids promising for broadband photodetection.

Precise synthesis of graphene by chemical vapor deposition

Graphene, a typical representative of the family of two-dimensional (2D) materials, possesses a series of phenomenal physical properties. To date, numerous inspiring discoveries have been made on its structures, properties, characterization, synthesis, transfer and applications. The real practical applications of this magic material indeed require large-scale synthesis and precise control over its structures, such as size, crystallinity, layer number, stacking order, edge type and contamination levels. Nonetheless, studies on the precise synthesis of graphene are far from satisfactory currently. Our review aims to deal with the precise synthesis of four critical graphene structures, including single-crystal graphene (SCG), AB-stacked bilayer graphene (AB-BLG), etched graphene and clean graphene. Meanwhile, existing problems and future directions in the precise synthesis of graphene are also briefly discussed.

Stacking Order Engineering of Two-Dimensional Materials and Device Applications

Stacking orders in 2D van der Waals (vdW) materials dictate the relative sliding (lateral displacement) and twisting (rotation) between atomically thin layers. By altering the stacking order, many new ferroic, strongly correlated and topological orderings emerge with exotic electrical, optical and magnetic properties. Thanks to the weak vdW interlayer bonding, such highly flexible and energy-efficient stacking order engineering has transformed the design of quantum properties in 2D vdW materials, unleashing the potential for miniaturized high-performance device applications in electronics, spintronics, photonics, and surface chemistry. This Review provides a comprehensive overview of stacking order engineering in 2D vdW materials and their device applications, ranging from the typical fabrication and characterization methods to the novel physical properties and the emergent slidetronics and twistronics device prototyping. The main emphasis is on the critical role of stacking orders affecting the interlayer charge transfer, orbital coupling and flat band formation for the design of innovative materials with on-demand quantum properties and surface potentials. By demonstrating a correlation between the stacking configurations and device functionality, we highlight their implications for next-generation electronic, photonic and chemical energy conversion devices. We conclude with our perspective of this exciting field including challenges and opportunities for future stacking order engineering research.

Sequential Bayesian-optimized graphene synthesis by direct solar-thermal chemical vapor deposition

This work reports the use of a high-flux solar simulator that mimics the solar spectrum and a cold-wall CVD reactor to demonstrate the feasibility of utilizing a renewable energy resource in synthesizing graphene under various conditions. A parametric study of process parameters was conducted using a probabilistic approach. Gaussian process regression serves as a surrogate to establish a prior for Bayesian optimization, and an information acquisition function is employed to identify conditions that yield high-quality products. Backscattered electron images and Raman mapping were used to assess the effects of growth conditions on graphene characteristic sizes, film quality, and uniformity. We report the synthesis of high-quality single-layer graphene (SLG) and AB-stacked bilayer graphene films in a one-step, short-time process with [Formula: see text] ratios of 0.21 and 0.14, respectively. Electron diffraction analysis shows peak intensities that resemble SLG and AB-bilayer graphene with up to 5 and 20 [Formula: see text]m grain sizes, respectively. The optical transmissivities of SLG and AB-bilayer graphene fall between 0.959-0.977 and 0.929-0.953, whereas the sheet resistances measured by a 4-point probe with 1 mm spacing are 15.5 ± 4.6 and 3.4 ± 1.5 k[Formula: see text]/sq, respectively. Further scale-up of the optimized graphene growth area was achieved by flattening the insolation profile, leading to spatial uniformity up to 13 mm in radius. Direct solar capture for CVD synthesis enable a practical and sustainable option for synthesizing graphene films applicable for photonic and electronic applications.

Controllable Synthesis and Growth Mechanism of Interlayer-Coupled Multilayer Graphene

The potential applications of multilayer graphene in many fields, such as superconductivity and thermal conductivity, continue to emerge. However, there are still many problems in the growth mechanism of multilayer graphene. In this paper, a simple control strategy for the preparation of interlayer-coupled multilayer graphene on a liquid Cu substrate was developed. By adjusting the flow rate of a carrier gas in the CVD system, the effect for finely controlling the carbon source supply was achieved. Therefore, the carbon could diffuse from the edge of the single-layer graphene to underneath the layer of graphene and then interlayer-coupled multilayer graphene with different shapes were prepared. Through a variety of characterization methods, it was determined that the stacked mode of interlayer-coupled multilayer graphene conformed to AB-stacking structure. The small multilayer graphene domains stacked under single-layer graphene was first found, and the growth process and growth mechanism of interlayer-coupled multilayer graphene with winged and umbrella shapes were studied, respectively. This study reveals the growth mechanism of multilayer graphene grown by using a carbon source through edge diffusion, paving the way for the controllable preparation of multilayer graphene on a liquid Cu surface.

Electrochemical Li+ Insertion/Extraction Reactions at LiPON/Epitaxial Graphene Interfaces

Redox reactions of the Li+ insertion/extraction from one to two interlayers of graphene (Gr) on area-defined single-crystalline SiC substrates are investigated using lithium phosphorus oxynitride glass (LiPON) as the solid-state electrolyte. Unlike an organic liquid electrolyte, this glassy electrolyte does not induce a reduction current and excludes the desolvation reaction of Li+. Gr electrodes with less than two Gr layers show a single reduction peak and one or two oxidation peaks below +0.21 V (vs Li+/Li), differing distinctly from those of graphite and multilayer Gr, which display multiple peaks (multiple stage transitions). However, this finding aligns with the conventional understanding that graphite stage structure transitions proceed with stepwise increases or decreases in the number of Gr layers between adjacent Li-inserted interlayers. Cyclic voltammetry measurements indicate the presence of surface capacity due to Li+ adsorption/desorption at the LiPON/Gr interface. Moreover, Li+ insertion and extraction induce different charge transfer resistances at the level of a single interlayer. These sensitive measurements are achieved using high-quality epitaxial Gr and LiPON electrolyte, which prevent the formation of a solid electrolyte interphase and the desolvation reaction of Li+. Similar measurements using bilayer Gr produced by chemical vapor deposition coupled with a Gr transfer method and an ethylene carbonate/dimethyl carbonate liquid electrolyte are not reliable. Thus, the proposed method is effective for electrochemical measurement of Gr electrodes with a controlled number of layers.

Recent Understanding in the Chemical Vapor Deposition of Multilayer Graphene: Controlling Uniformity, Thickness, and Stacking Configuration

Multilayer graphene has attracted significant attention because its physical properties can be tuned by stacking its layers in a particular configuration. To apply the intriguing properties of multilayer graphene in various optoelectronic or spintronic devices, it is essential to develop a synthetic method that enables the control of the stacking configuration. This review article presents the recent progress in the synthesis of multilayer graphene by chemical vapor deposition (CVD). First, we discuss the CVD of multilayer graphene, utilizing the precipitation or segregation of carbon atoms from metal catalysts with high carbon solubility. Subsequently, we present novel CVD approaches to yield uniform and thickness-controlled multilayer graphene, which goes beyond the conventional precipitation or segregation methods. Finally, we introduce the latest studies on the control of stacking configurations in bilayer graphene during CVD processes.

A Wrinkling and Etching-Assisted Regrowth Strategy for Large-Area Bilayer Graphene Preparation on Cu

Bilayer graphene is a contender of interest for functional electronic applications because of its variable band gap due to interlayer interactions. Graphene growth on Cu is self-limiting, thus despite the fact that chemical vapor deposition (CVD) has made substantial strides in the production of monolayer and single-crystal graphene on Cu substrates, the direct synthesizing of high-quality, large-area bilayer graphene remains an enormous challenge. In order to tackle this issue, we present a simple technique using typical CVD graphene growth followed by a repetitive wrinkling-etching-regrowth procedure. The key element of our approach is the rapid cooling process that causes high-density wrinkles to form in the monolayer area rather than the bilayer area. Next, wrinkled sites are selectively etched with hydrogen, exposing a significant portion of the active Cu surface, and leaving the remaining bilayer areas, which enhance the nucleation and growth of the second graphene layer. A fully covered graphene with 78 ± 2.8% bilayer coverage and a bilayer transmittance of 95.6% at room temperature can be achieved by modifying the process settings. Bilayer graphene samples are examined using optical microscopy (OM), scanning electron microscopy (SEM), Raman spectroscopy, and an atomic force microscope (AFM) during this process. The outcomes of our research are beneficial in clarifying the growth processes and future commercial applications of bilayer graphene.

Fast synthesis of large-area bilayer graphene film on Cu

Bilayer graphene (BLG) is intriguing for its unique properties and potential applications in electronics, photonics, and mechanics. However, the chemical vapor deposition synthesis of large-area high-quality bilayer graphene on Cu is suffering from a low growth rate and limited bilayer coverage. Herein, we demonstrate the fast synthesis of meter-sized bilayer graphene film on commercial polycrystalline Cu foils by introducing trace CO₂ during high-temperature growth. Continuous bilayer graphene with a high ratio of AB-stacking structure can be obtained within 20 min, which exhibits enhanced mechanical strength, uniform transmittance, and low sheet resistance in large area. Moreover, 96 and 100% AB-stacking structures were achieved in bilayer graphene grown on single-crystal Cu(111) foil and ultraflat single-crystal Cu(111)/sapphire substrates, respectively. The AB-stacking bilayer graphene exhibits tunable bandgap and performs well in photodetection. This work provides important insights into the growth mechanism and the mass production of large-area high-quality BLG on Cu.

Sensitive label-free detection of the biomarker phosphorylated tau-217 protein in Alzheimer's disease using a graphene-based solution-gated field effect transistor

Alzheimer's disease (AD) is generally diagnosed using advanced imaging, but recent research suggests early screening using biomarkers in peripheral blood is feasible; among them, plasma tau proteins phosphorylated at threonine 231, threonine 181, and threonine 217 (p-tau217) are potential targets. A recent study indicates that the p-tau217 protein is the most efficacious biomarker. However, a clinical study found a pg/ml threshold for AD screening beyond standard detection methods. A biosensor with high sensitivity and specificity p-tau217 detection has not yet been reported. In this study, we developed a label-free solution-gated field effect transistor (SGFET)-based biosensor featuring a graphene oxide/graphene (GO/G) layered composite. The top layer of bilayer graphene grown using chemical vapor deposition was functionalized with oxidative groups serving as active sites for forming covalent bonds with the biorecognition element (antibodies); the bottom G could act as a transducer to respond to the attachment of the target analytes onto the top GO conjugated with the biorecognition element via π-π interactions between the GO and G layers. With this unique atomically layered G composite, we obtained a good linear electrical response in the Dirac point shift to p-tau217 protein concentrations in the range of 10 fg/ml to 100 pg/ml. The biosensor exhibited a high sensitivity of 18.6 mV/decade with a high linearity of 0.991 in phosphate-buffered saline (PBS); in human serum albumin, it showed approximately 90% of the sensitivity (16.7 mV/decade) in PBS, demonstrating high specificity. High stability of the biosensor was also displayed in this study.

A chemiresistive biosensor for detection of cancer biomarker in biological fluids using CVD-grown bilayer graphene

A chemiresistive biosensor is described for simple and selective detection of miRNA-21. We developed chemical vapor deposition (CVD) and low-damage plasma treatment (LDPT)-treated bilayer graphene composite of graphene oxide/graphene (GO/GR) for the determination of a reliable biomarker. We have successfully overcome the self-limiting growth mechanism by using CVD method to grow more than one layer of graphene on copper foil. In addition, LDPT can be used to form GO/GR structures for chemiresistive biosensor applications. Due to the direct formation of BLGR (bilayer graphene), the coupling between graphene layers is theoretically superior to that of stacked BLGR, which is also confirmed by the blue shift of the characteristic peak of graphene in Raman spectroscopy. The shift is about double compared with that of stacked BLGR. Based on the results, the limit of detection for the target miRNA-21 was calculated to be 5.20 fM and detection rage is calculated as 100 fM to 10 nM, which is obviously better performance. Compared with previous work, this chemiresistive biosensor has good selectivity, and stability towards detection of miRNA-21. The ability to detect miRNA-21 in different biological fluids was almost identical to that in pH 7.4 phosphate-buffered saline (PBS). Thus, the proposed bilayer GO/GR of modified chemiresistive biosensor may potentially be applied to detect cancer cells in clinical examinations.

Direct growth of hBN/Graphene heterostructure via surface deposition and segregation for independent thickness regulation

Hexagonal boron nitride/graphene (hBN/G) vertical heterostructures have attracted extensive attention, owing to the unusual physical properties for basic research and electronic device applications. Here we report a facile deposition-segregation technique to synthesize hBN/G heterostructures on recyclable platinum (Pt) foil via low pressure chemical vapor deposition. The growth mechanism of the vertical hBN/G is demonstrated to be the surface deposition of hBN on top of the graphene segregated from the Pt foil with pre-dissolved carbon. The thickness of hBN and graphene can be controlled separately from sub-monolayer to multilayer through the fine control of the growth parameters. Further investigations by Raman, scanning Kelvin probe microscopy and transmission electron microscope show that the hBN/G inclines to form a heterostructure with strong interlayer coupling and with interlayer twist angle smaller than 1.5°. This deposition-segregation approach paves a new pathway for large-scale production of hBN/G heterostructures and could be applied to synthesize of other van der Waals heterostructures.

Phase transition from a nonmagnetic to a ferromagnetic state in a twisted bilayer graphene nanoflake: the role of electronic pressure on the magic-twist

The electronic properties of a bilayer graphene nanoflake (BLGNF) depend sensitively upon the strength of the inter-layer electronic coupling (IEC) energy. Upon tuning the IEC via changing the twist angle between the layer, a ferromagnetic gap state emerges in a BLGNF due to spontaneous symmetry breaking at the magic-twist. Herein, using first-principles density functional theory, we demonstrate the magic twist angle (θ_M) in a bilayer graphene nanoflake at which the transition from a nonmagnetic to a ferromagnetic phase occurs can be tuned by exerting uniaxial electronic pressure (P_e). Electronic pressure, which provides another route to control the IEC, is simulated by varying the interlayer spacing in the nanoflake. Our result shows a P_e of 0.125 GPa corresponding to interlayer spacing (h) of 3.58 Å yielding a magic twist angle of ∼1° and a negative value of P_e (-0.042 GPa) at h = 3.38 Å producing a θ_M of ∼2.4°. The higher value of θ_M at a negative P_e (smaller h) is attributed to an increase in the energy gap due to strong inter-layer electronic coupling energy in the nanoflake under uniaxial compression. This finding in the nanoflake agrees with the experimental observation in two-dimensional bilayer graphene (M. Yankowitz, S. Chen, H. Polshyn, Y. Zhang, K. Watanabe, T. Taniguchi, D. Graf, A. F. Young and C. R. Dean, Science, 2019, 363, 1059-1064) that indicated an increase in the magic angle value for the phase transition upon application of hydrostatic pressure.

Combinatorial Cu-Ni Alloy Thin-Film Catalysts for Layer Number Control in Chemical Vapor-Deposited Graphene

We synthesized a combinatorial library of CuxNi1−x alloy thin films via co-sputtering from Cu and Ni targets to catalyze graphene chemical vapor deposition. The alloy morphology, composition, and microstructure were characterized via scanning electron microscopy (SEM), energy dispersive x-ray spectroscopy (EDS), and X-ray diffraction (XRD), respectively. Subsequently, the CuxNi1−x alloy thin films were used to grow graphene in a CH4-Ar-H2 ambient at atmospheric pressure. The underlying rationale is to adjust the CuxNi1−x composition to control the graphene. Energy dispersive x-ray spectroscopy (EDS) analysis revealed that a continuous gradient of CuxNi1−x (25 at. % < x < 83 at.%) was initially achieved across the 100 mm diameter substrate (~0.9%/mm composition gradient). The XRD spectra confirmed a solid solution was realized and the face-centered cubic lattice parameter varied from ~3.52 to 3.58 A˙, consistent with the measured composition gradient, assuming Vegard’s law. Optical microscopy and Raman analysis of the graphene layers suggest single layer growth occurs with x > 69 at.%, bilayer growth dominates from 48 at.% < x < 69 at.%, and multilayer (≥3) growth occurs for x < 48 at.%, where x is the Cu concentration. Finally, a large area of bi-layer graphene was grown via a CuxNi1−x catalyst with optimized catalyst composition and growth temperature.

Controllable growth of two-dimensional materials on noble metal substrates

Chemical vapor deposition (CVD) is a promising approach for the controllable synthesis of two-dimensional (2D) materials. Many studies have demonstrated that the morphology and structure of 2D materials are highly dependent on growth substrates. Hence, the choice of growth substrates is essential to achieve the precise control of CVD growth. Noble metal substrates have attracted enormous interest owing to the high catalytic activity and rich surface morphology for 2D material growth. In this review, we introduce recent progress in noble metals as substrates for the controllable growth of 2D materials. The underlying growth mechanism and substrate designs of noble metals based on their unique features are thoroughly discussed. In the end, we outline the advantages and challenges of using noble metal substrates and prospect the possible approaches to extend the uses of noble metal substrates for 2D material growth and enhance the structural controllability of the grown materials.

Atomically Thin Materials for Next-Generation Rechargeable Batteries

Atomically thin materials (ATMs) with thicknesses in the atomic scale (typically <5 nm) offer inherent advantages of large specific surface areas, proper crystal lattice distortion, abundant surface dangling bonds, and strong in-plane chemical bonds, making them ideal 2D platforms to construct high-performance electrode materials for rechargeable metal-ion batteries, metal-sulfur batteries, and metal-air batteries. This work reviews the synthesis and electronic property tuning of state-of-the-art ATMs, including graphene and graphene derivatives (GE/GO/rGO), graphitic carbon nitride (g-C₃N₄), phosphorene, covalent organic frameworks (COFs), layered transition metal dichalcogenides (TMDs), transition metal carbides, carbonitrides, and nitrides (MXenes), transition metal oxides (TMOs), and metal-organic frameworks (MOFs) for constructing next-generation high-energy-density and high-power-density rechargeable batteries to meet the needs of the rapid developments in portable electronics, electric vehicles, and smart electricity grids. We also present our viewpoints on future challenges and opportunities of constructing efficient ATMs for next-generation rechargeable batteries.

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.

Growth of wrinkle-free and ultra-flat Bi-layer graphene on sapphire substrate using Cu sacrificial layer

The transfer process of chemical vapor deposition graphene film leads to unavoidable crack, wrinkles, doping, and contamination, which limits its function to establish stable and high-performance devices. It raises a growing interest to fabricate high-quality graphene on the target substrate directly. Here, bi-layer graphene (BLG) film can be grown on sapphire substrate by a Cu sacrificial layer using atmospheric-pressure chemical vapor deposition. The as-obtained BLG at the interface between sapphire and Cu layer is free of wrinkles, and the corresponding surface roughness Ra is as low as 0.66 nm. The square resistance of the graphene is 898.1 ohm sq^-1, which is the lowest among the records of graphene film directly grown on nonmetal substrates.

Multistep Fractionation of Coal and Application for Graphene Synthesis

Despite its complex structure, coal has shown to be a promising precursor for graphene synthesis by chemical vapor deposition (CVD). However, the presence of heteroatoms and aliphatic chains in coal can lead to defects in the graphene lattice, preventing the formation of pristine graphene layers. Therefore, the goal of this study was to formulate a multistep coal fractionation scheme to extract and characterize the most aromatic fractions and explore their potential as graphene precursors. The scheme consisted of direct coal liquefaction under different conditions, Soxhlet extraction with heptane then toluene, and preparative liquid chromatography on silica gel using heptol solutions with different heptane/toluene ratios. The fractions obtained by this process were analyzed by proton nuclear magnetic resonance, thermogravimetric and elemental analyses, and automated SAR-AD (saturates, aromatics, resins-asphaltene determinator) separations. This characterization allowed the identification of two aromatic fractions with and without heteroatoms, which were subsequently used for graphene synthesis by CVD on nickel and copper foils. Raman spectrometry revealed that both fractions primarily formed defect-free multilayered graphene with approximately 11 layers on nickel due to the high solubility of carbon and the defect-healing effect of nickel. On the other hand, these fractions generated amorphous carbon on copper due to the high solubility of hydrogen in copper, which competed with carbon. Molecules in the more aromatic heteroatom-free fraction still contained alkyl pendant substituents and did not share the same planarity and symmetry to form defect-free graphene on copper. Thus, the quality of graphene was governed by the substrate on nickel and by the precursor quality on copper. When deposited directly on lacey carbon-coated copper grids of a transmission electron microscope, the heteroatom-free fraction gave rise to much larger graphene domains. The presence of heteroatoms promoted the formation of small self-assembled agglomerates of amorphous carbon.

Strategies, Status, and Challenges in Wafer Scale Single Crystalline Two-Dimensional Materials Synthesis

The successful exfoliation of graphene has given a tremendous boost to research on various two-dimensional (2D) materials in the last 15 years. Different from traditional thin films, a 2D material is composed of one to a few atomic layers. While atoms within a layer are chemically bonded, interactions between layers are generally weak van der Waals (vdW) interactions. Due to their particular dimensionality, 2D materials exhibit special electronic, magnetic, mechanical, and thermal properties, not found in their 3D counterparts, and therefore they have great potential in various applications, such as 2D materials-based devices. To fully realize their large-scale practical applications, especially in devices, wafer scale single crystalline (WSSC) 2D materials are indispensable. In this review, we present a detailed overview on strategies toward the synthesis of WSSC 2D materials while highlighting the recent progress on WSSC graphene, hexagonal boron nitride (hBN), and transition metal dichalcogenide (TMDC) synthesis. The challenges that need to be addressed in future studies have also been described. In general, there have been two distinct routes to synthesize WSSC 2D materials: (i) allowing only one nucleus on a wafer scale substrate to be formed and developed into a large single crystal and (ii) seamlessly stitching a large number of unidirectionally aligned 2D islands on a wafer scale substrate, which is generally single crystalline. Currently, the synthesis of WSSC graphene has been realized by both routes, and WSSC hBN and MoS₂ have been synthesized by route (ii). On the other hand, the growth of other WSSC 2D materials and WSSC multilayer 2D materials still remains a big challenge. In the last section, we wrap up this review by summarizing the future challenges and opportunities in the synthesis of various WSSC 2D materials.

Hetero-site nucleation for growing twisted bilayer graphene with a wide range of twist angles

Twisted bilayer graphene (tBLG) has recently attracted growing interest due to its unique twist-angle-dependent electronic properties. The preparation of high-quality large-area bilayer graphene with rich rotation angles would be important for the investigation of angle-dependent physics and applications, which, however, is still challenging. Here, we demonstrate a chemical vapor deposition (CVD) approach for growing high-quality tBLG using a hetero-site nucleation strategy, which enables the nucleation of the second layer at a different site from that of the first layer. The fraction of tBLGs in bilayer graphene domains with twist angles ranging from 0° to 30° was found to be improved to 88%, which is significantly higher than those reported previously. The hetero-site nucleation behavior was carefully investigated using an isotope-labeling technique. Furthermore, the clear Moiré patterns and ultrahigh room-temperature carrier mobility of 68,000 cm² V^-1 s^-1 confirmed the high crystalline quality of our tBLG. Our study opens an avenue for the controllable growth of tBLGs for both fundamental research and practical applications.

Fabrication Strategies of Twisted Bilayer Graphenes and Their Unique Properties

Twisted bilayer graphene (tBLG) exhibits a host of innovative physical phenomena owing to the formation of moiré superlattice. Especially, the discovery of superconducting behavior has generated new interest in graphene. The growing studies of tBLG mainly focus on its physical properties, while the fabrication of high-quality tBLG is a prerequisite for achieving the desired properties due to the great dependence on the twist angle and the interfacial contact. Here, the cutting-edge preparation strategies and challenges of tBLG fabrication are reviewed. The advantages and disadvantages of chemical vapor deposition, epitaxial growth on silicon carbide, stacking monolayer graphene, and folding monolayer graphene methods for the fabrication of tBLG are analyzed in detail, providing a reference for further development of preparation methods. Moreover, the characterization methods of twist angle for the tBLG are presented. Then, the unique physicochemical properties and corresponding applications of tBLG, containing correlated insulating and superconducting states, ferromagnetic state, soliton, enhanced optical absorption, tunable bandgap, and lithium intercalation and diffusion, are described. Finally, the opportunities and challenges for fabricating high-quality and large-area tBLG are discussed, unique physical properties are displayed, and new applications inferred from its angle-dependent features are explored, thereby impelling the commercialization of tBLG from laboratory to market.

Polycrystalline Few-Layer Graphene as a Durable Anticorrosion Film for Copper

Corrosion of metals in atmospheric environments is a worldwide problem in industry and daily life. Traditional anticorrosion methods including sacrificial anodes or protective coatings have performance limitations. Here, we report atomically thin, polycrystalline few-layer graphene (FLG) grown by chemical vapor deposition as a long-term protective coating film for copper (Cu). A six-year old, FLG-protected Cu is visually shiny and detailed material characterizations capture no sign of oxidation. The success of the durable anticorrosion film depends on the misalignment of grain boundaries between adjacent graphene layers. Theoretical calculations further found that corrosive molecules always encounter extremely high energy barrier when diffusing through the FLG layers. Therefore, the FLG is able to prevent the corrosive molecules from reaching the underlying Cu surface. This work highlights the interesting structures of polycrystalline FLG and sheds insight into the atomically thin coatings for various applications.

Copper-based etalon filter using antioxidant graphene layer

Copper is a low-cost material compared to silver and gold, having high reflectivity in the near infrared spectral range as well as good electrical and thermal conductivity. Its properties make it a good candidate for metal-based low-cost multilayer thin-film devices and optical components. However, its high reflectance in the devices is reduced because copper is easily oxidized. Here, we suggest a copper-based Fabry-Perot optical filter consisting of a thin dielectric layer stacked between two copper films, which can realize low-cost production compared to a conventional silver-based etalon filter. The reduced performance due to the inherent oxidation of the copper surface can be overcome by passivating the copper films with monolayer graphene. The anti-oxidation of copper film is investigated by optical microscopy, x-ray photoelectron spectroscopy, and transmission measurement in UV-vi spectral ranges. Our results show that the graphene coating can be expanded for various metal-based optical devices in terms of anti-corrosion.

Layer-controlled single-crystalline graphene film with stacking order via Cu-Si alloy formation

Multilayer graphene and its stacking order provide both fundamentally intriguing properties and technological engineering applications. Several approaches to control the stacking order have been demonstrated, but a method of precisely controlling the number of layers with desired stacking sequences is still lacking. Here, we propose an approach for controlling the layer thickness and crystallographic stacking sequence of multilayer graphene films at the wafer scale via Cu-Si alloy formation using direct chemical vapour deposition. C atoms are introduced by tuning the ultra-low-limit CH₄ concentration to form a SiC layer, reaching one to four graphene layers at the wafer scale after Si sublimation. The crystallographic structure of single-crystalline or uniformly oriented bilayer (AB), trilayer (ABA) and tetralayer (ABCA) graphene are determined via nano-angle-resolved photoemission spectroscopy, which agrees with theoretical calculations, Raman spectroscopy and transport measurements. The present study takes a step towards the layer-controlled growth of graphite and other two-dimensional materials.

3D Graphene Materials: From Understanding to Design and Synthesis Control

Carbon materials, with their diverse allotropes, have played significant roles in our daily life and the development of material science. Following 0D C₆₀ and 1D carbon nanotube, 2D graphene materials, with their distinctively fascinating properties, have been receiving tremendous attention since 2004. To fulfill the efficient utilization of 2D graphene sheets in applications such as energy storage and conversion, electrochemical catalysis, and environmental remediation, 3D structures constructed by graphene sheets have been attempted over the past decade, giving birth to a new generation of graphene materials called 3D graphene materials. This review starts with the definition, classifications, brief history, and basic synthesis chemistries of 3D graphene materials. Then a critical discussion on the design considerations of 3D graphene materials for diverse applications is provided. Subsequently, after emphasizing the importance of normalized property characterization for the 3D structures, approaches for 3D graphene material synthesis from three major types of carbon sources (GO, hydrocarbons and inorganic carbon compounds) based on GO chemistry, hydrocarbon chemistry, and new alkali-metal chemistry, respectively, are comprehensively reviewed with a focus on their synthesis mechanisms, controllable aspects, and scalability. At last, current challenges and future perspectives for the development of 3D graphene materials are addressed.

Electrodeposition-Assisted Rapid Preparation of Pt Nanocluster/3D Graphene Hybrid Nanozymes with Outstanding Multiple Oxidase-Like Activity for Distinguishing Colorimetric Determination of Dihydroxybenzene Isomers

Here, we demonstrate a facile bottom-up strategy to fabricate Pt nanoclusters (Pt NCs) grafted onto three-dimensional graphene foam (3D GF) assisted by cetyltrimethyl ammonium bromide (CTAB) using the electrodeposition method. The homogeneous grafting of Pt NC onto 3D GF is due to the formation of hemimicelles above some CTAB concentration. With the unique nanocluster structure and the high content of Pt⁰, the Pt NC/3D GF nanohybrid exhibits extremely high activity and shows higher reusability and stability. Apart from the intrinsic oxidase-like activity with 3,3',5,5'-tetramethylbenzidine (TMB) as the substrate, the Pt NC/3D GF nanohybrid can act simultaneously as an effective polyphenol oxidase (PPO) mimic, such as tyrosinase, catechol oxidase, and laccase. More importantly, utilizing intrinsic catechol oxidase-like activity and the oxidase-like activity with TMB as the substrate of the nanohybrid, distinguishing colorimetric determination of dihydroxybenzene isomers (catechol and hydroquinone) is performed. Distinguishing colorimetric analysis of dihydroxybenzene isomers was first developed using nanozymes. The present work provides a simple bottom-up approach for the reasonable fabrication of various nanostructured nanozymes with excellent performance using the electrodeposition method assisted with surfactants.

Large-area single-crystal AB-bilayer and ABA-trilayer graphene grown on a Cu/Ni(111) foil

High-quality AB-stacked bilayer or multilayer graphene larger than a centimetre has not been reported. Here, we report the fabrication and use of single-crystal Cu/Ni(111) alloy foils with controllable concentrations of Ni for the growth of large-area, high-quality AB-stacked bilayer and ABA-stacked trilayer graphene films by chemical vapour deposition. The stacking order, coverage and uniformity of the graphene films were evaluated by Raman spectroscopy and transmission electron microscopy including selected area electron diffraction and atomic resolution imaging. Electrical transport (carrier mobility and band-gap tunability) and thermal conductivity (the bilayer graphene has a thermal conductivity value of about 2,300 W m^-1 K^-1) measurements indicated the superior quality of the films. The tensile loading response of centimetre-scale bilayer graphene films supported by a 260-nm thick polycarbonate film was measured and the average values of the Young's modulus (478 GPa) and fracture strength (3.31 GPa) were obtained.

Molecular dynamics simulation on the formation and development of interlayer dislocations in bilayer graphene

Molecular dynamics simulations are used to study the formation and development of interlayer dislocations in bilayer graphene (BLG) subjected to uniaxial tension. Two different BLGs are employed for the simulation: armchair (AC-BLG) and zigzag (ZZ-BLG). The atomic-level strains are calculated and the parameter 'dislocation intensity' is introduced to identify the dislocations. The interlayer dislocation is found to start at the edge and propagate to the center. For AC-BLG, the dislocations arise successively with the increase of applied strain, and all dislocations have the same width. For ZZ-BLG, the first dislocation arises alone. After that, two dislocations with different widths appear together every time. The simulated dislocation widths are in good agreement with existing experimental results. Across every dislocation, there is a transition from AB stacking to AC stacking, or vice versa. When temperature is taken into account, the dislocation boundaries become indistinct and the formation of dislocations is postponed due to the existence of dispersive small slippages. Due to the disturbance of temperature, dislocations present reciprocating movement. These findings contribute to the understanding of interlayer dislocations in two-dimensional materials, and will enable the exploration of many more strain related fundamental science problems and application challenges.

Chemically induced transformation of chemical vapour deposition grown bilayer graphene into fluorinated single-layer diamond

Notwithstanding the numerous density functional studies on the chemically induced transformation of multilayer graphene into a diamond-like film carried out to date, a comprehensive convincing experimental proof of such a conversion is still lacking. We show that the fluorination of graphene sheets in Bernal (AB)-stacked bilayer graphene grown by chemical vapour deposition on a single-crystal CuNi(111) surface triggers the formation of interlayer carbon-carbon bonds, resulting in a fluorinated diamond monolayer ('F-diamane'). Induced by fluorine chemisorption, the phase transition from (AB)-stacked bilayer graphene to single-layer diamond was studied and verified by X-ray photoelectron, UV photoelectron, Raman, UV-Vis and electron energy loss spectroscopies, transmission electron microscopy and density functional theory calculations.

Dispersed graphene materials of biomedical interest and their toxicological consequences

Graphene is one-atom thick nanocarbon displaying a unique honeycomb structure and extensive conjugation. In addition to high surface area to mass ratio, it displays unique optical, thermal, electronic and mechanical properties. Atomic scale tunability of graphene has attracted immense research interest with a prospective utility in electronics, desalination, energy sectors, and beyond. Its intrinsic opto-thermal properties are appealing from the standpoint of multimodal drug delivery, imaging and biosensing applications. Hydrophobic basal plane of sheets can be efficiently loaded with aromatic molecules via non-specific forces. With intense biomedical interest, methods are evolving to produce defect-free and dispersion stable sheets. This review summarizes advancements in synthetic approaches and strategies of stabilizing graphene derivatives in aqueous medium. We have described the interaction of colloidal graphene with cellular and sub-cellular components, and subsequent physiological signaling. Finally, a systematic discussion is provided covering toxicological challenges and possible solutions on utilizing graphene formulations for high-end biomedical applications.

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.

Interlayer epitaxy of wafer-scale high-quality uniform AB-stacked bilayer graphene films on liquid Pt3Si/solid Pt

Large-area high-quality AB-stacked bilayer graphene films are highly desired for the applications in electronics, photonics and spintronics. However, the existing growth methods can only produce discontinuous bilayer graphene with variable stacking orders because of the non-uniform surface and strong potential field of the solid substrates used. Here we report the growth of wafer-scale continuous uniform AB-stacked bilayer graphene films on a liquid Pt₃Si/solid Pt substrate by chemical vapor deposition. The films show quality, mechanical and electrical properties comparable to the mechanically exfoliated samples. Growth mechanism studies show that the second layer is grown underneath the first layer by precipitation of carbon atoms from the solid Pt, and the small energy requirements for the movements of graphene nucleus on the liquid Pt₃Si enables the interlayer epitaxy to form energy-favorable AB stacking. This interlayer epitaxy also allows the growth of ABA-stacked trilayer graphene and is applicable to other liquid/solid substrates.

Specific stacking sequence of graphene can enable observation of unusual properties however it has been difficult to obtain this over wider areas. Here, the authors report wafer-scale growth of 100% AB-stacked bilayer graphene films by CVD on liquid Pt₃Si/solid Pt substrates showing high quality and improved mechanical properties comparable to the mechanically exfoliated flakes.

Optical properties of GaN nanowires grown on chemical vapor deposited-graphene

Optical properties of GaN nanowires (NWs) grown on chemical vapor deposited-graphene transferred on an amorphous support are reported. The growth temperature was optimized to achieve a high NW density with a perfect selectivity with respect to a SiO₂ surface. The growth temperature window was found to be rather narrow (815°C ± 5°C). Steady-state and time-resolved photoluminescence from GaN NWs grown on graphene was compared with the results for GaN NWs grown on conventional substrates within the same molecular beam epitaxy reactor showing a comparable optical quality for different substrates. Growth at temperatures above 820 °C led to a strong NW density reduction accompanied with a diameter narrowing. This morphology change leads to a spectral blueshift of the donor-bound exciton emission line due to either surface stress or dielectric confinement. Graphene multi-layered micro-domains were explored as a way to arrange GaN NWs in a hollow hexagonal pattern. The NWs grown on these domains show a luminescence spectral linewidth as low as 0.28 meV (close to the set-up resolution limit).

Growth of Large-Area High-Quality Graphene on Different Types of Copper Foil Preannealed under Positive Pressure H2 Ambience

Large-area high-quality graphene was synthesized on different types of copper foils preannealed under positive pressure H2 atmosphere between 1 and 2 standard atmospheres. The prepared graphene showed good electrical conductivity, transmittance, and uniformity. The sheet resistance values of the grown monolayer graphene film were all about 500 Ω/□, and the transmittance was as high as 97.24%. The carrier mobility of the monolayer graphene film was around 2000-3000 cm2/(V s). Furthermore, the monolayer coverage could be more than 95.00% controlled by adjusting the process parameters. The properties of the four layer-superposed graphene film nearly reached that of the commercialized indium tin oxide (ITO) glasses, which showed that the prepared graphene could be well applied to the transparent conductive electrode. The obtained graphene film has been used to construct Si/graphene solar cells without an antireflection film, which showed energy conversion efficiency among 4.99-5.62%.

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.

Toward Mass Production of CVD Graphene Films

Chemical vapor deposition (CVD) is considered to be an efficient method for fabricating large-area and high-quality graphene films due to its excellent controllability and scalability. Great efforts have been made to control the growth of graphene to achieve large domain sizes, uniform layers, fast growth, and low synthesis temperatures. Some attempts have been made by both the scientific community and startup companies to mass produce graphene films; however, there is a large difference in the quality of graphene synthesized on a laboratory scale and an industrial scale. Here, recent progress toward the mass production of CVD graphene films is summarized, including the manufacturing process, equipment, and critical process parameters. Moreover, the large-scale homogeneity of graphene films and fast characterization methods are also discussed, which are crucial for quality control in mass production.

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.

Continuous Growth of Highly Reproducible Single-Layer Graphene Deposition on Cu Foil by Indigenously Developed LPCVD Setup

Continuous growth of high-quality single-layer graphene (SLG) is highly desirable in several electronic and optoelectronic applications. To fulfill such requirements, we proposed a low-cost, highly reproducible high-quality SLG synthesized by indigenously developed low-pressure chemical vapor deposition (LPCVD) setup. The quality of SLG is examined by Raman spectroscopy, where we have probed the I _2D/I _G ratio for continuous 30 runs to assess the reproducibility and quality of single-layer using proposed indigenous LPCVD setup for device fabrication. The highest I _2D/I _G ratio of SLG (5.82) was found with full width at half maximum values of 2D peak and G peak of ∼30.10 cm^-1 and ∼20.86 cm^-1, respectively. Further, high-resolution transmission electron microscopy and X-ray photoelectron spectroscopy have been performed to study the quality of SLG. Thickness measurement of graphene with graphene grain size is calculated from atomic force microscopy studies, and the average grain size is found to be 1-3 μm. Moreover, I-V characteristics have also been investigated by the two-probe method to ensure the quality of SLG. The lowest resistance of the SLG (∼387 Ω) was found at room temperature. Thus, this new indigenously developed low-cost setup provides a novel alternative method to produce highly reproducible metrology-grade continuous SLG on Cu substrate for next-generation quantum devices.

Manufacturing Graphene-Encapsulated Copper Particles by Chemical Vapor Deposition in a Cold Wall Reactor

Functional fillers, such as Ag, are commonly employed for effectively improving the thermal or electrical conductivity in polymer composites. However, a disadvantage of such a strategy is that the cost and performance cannot be balanced simultaneously. Therefore, the drive to find a material with both a cost efficient fabrication process and excellent performance attracts intense research interest. In this work, inspired by the core-shell structure, we developed a facile manufacturing method to prepare graphene-encapsulated Cu nanoparticles (GCPs) through utilizing an improved chemical vapor deposition (CVD) system with a cold wall reactor. The obtained GCPs could retain their spherical shape and exhibited an outstanding thermal stability up to 179 °C. Owing to the superior thermal conductivity of graphene and excellent oxidation resistance of GCPs, the produced GCPs are practically used in a thermally conductive adhesive (TCA), which commonly consists of Ag as the functional filler. Measurement shows a substantial 74.6 % improvement by partial replacement of Ag with GCPs.

Band Gap Control in Bilayer Graphene by Co-Doping with B-N Pairs

The electronic band structure of bilayer graphene is studied systematically in the presence of substitutional B and/or N doping, using density functional theory with van der Waals correction. We show that introduction of B-N pairs into bilayer graphene can be used to create a substantial band gap, stable against thermal fluctuations at room temperature, but otherwise leaves the electronic band structure in the vicinity of the Fermi energy largely unaffected. Introduction of B-N pairs into B and/or N doped bilayer graphene likewise hardly modifies the band dispersions. In semiconducting systems (same amount of B and N dopants), however, the size of the band gap is effectively tuned in the presence of B-N pairs.

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.

Large-Area High-Quality AB-Stacked Bilayer Graphene on h-BN/Pt Foil by Chemical Vapor Deposition

Large-area, high-quality bilayer graphene (BLG) has attracted great interest because of its immense potential for many viable applications. However, its growth is still greatly limited owing to its small size and low carrier mobility. In this article, we report the successful growth of large-area, high-quality AB-stacked BLG on hexagonal boron nitride (h-BN)/Pt foil by chemical vapor deposition (CVD). Optical microscopy and scanning electron microscopy observations reveal the formation of uniform and continuous BLG films with sizes of up to 500 μm, which are 4-5 times larger than those reported elsewhere for CVD-grown BLG films. A large carrier mobility of up to 9000 cm² V^-1 s^-1 is observed for the BLG films grown on h-BN/Pt foils under ambient conditions. We also propose a plausible growth mechanism of BLG growth on h-BN/Pt foils. Our findings will contribute for the better understanding of the fundamental BLG physics and the development of BLG-based devices.

Intergrain Diffusion of Carbon Radical for Wafer-Scale, Direct Growth of Graphene on Silicon-Based Dielectrics

Graphene intrinsically hosts charge-carriers with ultrahigh mobility and possesses a high quantum capacitance, which are attractive attributes for nanoelectronic applications requiring graphene-on-substrate base architecture. Most of the current techniques for graphene production rely on the growth on metal catalyst surfaces, followed by a contamination-prone transfer process to put graphene on a desired dielectric substrate. Therefore, a direct graphene deposition process on dielectric surfaces is crucial to avoid polymer-adsorption-related contamination from the transfer process. Here, we present a chemical-diffusion mechanism of a process for transfer-free growth of graphene on silicon-based gate-dielectric substrates via low-pressure chemical vapor deposition. The process relies on the diffusion of catalytically produced carbon radicals through polycrystalline copper (Cu) grain boundaries and their crystallization at the interface of Cu and underneath silicon-based gate-dielectric substrates. The graphene produced exhibits low-defect multilayer domains ( L_a ∼ 140 nm) with turbostratic orientations as revealed by selected area electron diffraction. Further, graphene growth between Cu and the substrate was 2-fold faster on SiO₂/Si(111) substrate than on SiO₂/Si(100). The process parameters such as growth temperature and gas compositions (hydrogen (H₂)/methane (CH₄) flow rate ratio) play critical roles in the formation of high-quality graphene films. The low-temperature back-gating charge transport measurements of the interfacial graphene show density-independent mobility for holes and electrons. Consequently, the analysis of electronic transport at various temperatures reveals a dominant Coulombic scattering, a thermal activation energy (2.0 ± 0.2 meV), and two-dimensional hopping conduction in the graphene field-effect transistor. A band overlapping energy of 2.3 ± 0.4 meV is estimated by employing the simple two-band model.

Electrical property of macroscopic graphene composite fibers prepared by chemical vapor deposition

Graphene fibers are promising candidates in portable and wearable electronics due to their tiny volume, flexibility and wearability. Here, we successfully synthesized macroscopic graphene composite fibers via a two-step process, i.e. first electrospinning and then chemical vapor deposition (CVD). Briefly, the well-dispersed PAN nanofibers were sprayed onto the copper surface in an electrified thin liquid jet by electrospinning. Subsequently, CVD growth process induced the formation of graphene films using a PAN-solid source of carbon and a copper catalyst. Finally, crumpled and macroscopic graphene composite fibers were obtained from carbon nanofiber/graphene composite webs by self-assembly process in the deionized water. Temperature-dependent conduct behavior reveals that electron transport of the graphene composite fibers belongs to hopping mechanism and the typical electrical conductivity reaches 4.59 × 10³ S m^-1. These results demonstrated that the graphene composite fibers are promising for the next-generation flexible and wearable electronics.

Unraveling the Water Impermeability Discrepancy in CVD-Grown Graphene

Graphene has recently attracted particular interest as a flexible barrier film preventing permeation of gases and moistures. However, it has been proved to be exceptionally challenging to develop large-scale graphene films with little oxygen and moisture permeation suitable for industrial uses, mainly due to the presence of nanometer-sized defects of obscure origins. Here, the origins of water permeable routes on graphene-coated Cu foils are investigated by observing the micrometer-sized rusts in the underlying Cu substrates, and a site-selective passivation method of the nanometer-sized routes is devised. It is revealed that nanometer-sized holes or cracks are primarily concentrated on graphene wrinkles rather than on other structural imperfections, resulting in severe degradation of its water impermeability. They are found to be predominantly induced by the delamination of graphene bound to Cu as a release of thermal stress during the cooling stage after graphene growth, especially at the intersection of the Cu step edges and wrinkles owing to their higher adhesion energy. Furthermore, the investigated routes are site-selectively passivated by an electron-beam-induced amorphous carbon layer, thus a substantial improvement in water impermeability is achieved. This approach is likely to be extended for offering novel barrier properties in flexible films based on graphene and on other atomic crystals.