Formation of bilayer bernal graphene: layer-by-layer epitaxy via chemical vapor deposition

Excimer-ultraviolet-lamp-assisted selective etching of single-layer graphene and its application in edge-contact devices

Since the discovery of graphene and its remarkable properties, researchers have actively explored advanced graphene-patterning technologies. While the etching process is pivotal in shaping graphene channels, existing etching techniques have limitations such as low speed, high cost, residue contamination, and rough edges. Therefore, the development of facile and efficient etching methods is necessary. This study entailed the development of a novel technique for patterning graphene through dry etching, utilizing selective photochemical reactions precisely targeted at single-layer graphene (SLG) surfaces. This process is facilitated by an excimer ultraviolet lamp emitting light at a wavelength of 172 nm. The effectiveness of this technique in selectively removing SLG over large areas, leaving the few-layer graphene intact and clean, was confirmed by various spectroscopic analyses. Furthermore, we explored the application of this technique to device fabrication, revealing its potential to enhance the electrical properties of SLG-based devices. One-dimensional (1D) edge contacts fabricated using this method not only exhibited enhanced electrical transport characteristics compared to two-dimensional contact devices but also demonstrated enhanced efficiency in fabricating conventional 1D-contacted devices. This study addresses the demand for advanced technologies suitable for next-generation graphene devices, providing a promising and versatile graphene-patterning approach with broad applicability and high efficiency.

Hybrid van der Waals Epitaxy

The successful growth of non-van der Waals (vdW) group-III nitride epilayers on vdW substrates not only opens an unprecedented opportunity to obtain high-quality semiconductor thinfilm but also raises a strong debate for its growth mechanism. Here, combining multiscale computational approaches and experimental characterization, we propose that the growth of a nitride epilayer on a vdW substrate, e.g., AlN on graphene, may belong to a previously unknown model, named hybrid vdW epitaxy (HVE). Atomic-scale simulations demonstrate that a unique interfacial hybrid-vdW interaction can be created between AlN and graphene, and, consequently, a first-principles-based continuum growth model is developed to capture the unusual features of HVE. Surprisingly, it is revealed that the in-plane and out-of-plane growth are strongly correlated in HVE, which is absent in existing growth models. The concept of HVE is confirmed by our experimental measurements, presenting a new growth mechanism beyond the current category of material growth.

Toward Fully Multiferroic van der Waals SpinFETs: Basic Design and Quantum Calculations

Manipulating spin transport enhances the functionality of electronic devices, allowing them to surpass physical constraints related to speed and power. For this reason, the use of van der Waals multiferroics at the interface of heterostructures offers promising prospects for developing high-performance devices, enabling the electrical control of spin information. Our work focuses primarily on a mechanism for multiferroicity in two-dimensional van der Waals materials that stems from an interplay between antiferromagnetism and the breaking of inversion symmetry in certain bilayers. We provide evidence for spin-electrical couplings that include manipulating van der Waals multiferroic edges via external voltages and the subsequent control of spin transport including for fully multiferroic spin field-effect transistors.

Understanding epitaxial growth of two-dimensional materials and their homostructures

The exceptional physical properties of two-dimensional (2D) van der Waals (vdW) materials have been extensively researched, driving advances in material synthesis. Epitaxial growth, a prominent synthesis strategy, enables the production of large-area, high-quality 2D films compatible with advanced integrated circuits. Typical 2D single crystals, such as graphene, transition metal dichalcogenides and hexagonal boron nitride, have been epitaxially grown at a wafer scale. A systematic summary is required to offer strategic guidance for the epitaxy of emerging 2D materials. Here we focus on the epitaxy methodologies for 2D vdW materials in two directions: the growth of in-plane single-crystal monolayers and the fabrication of out-of-plane homostructures. We first discuss nucleation control of a single domain and orientation control over multiple domains to achieve large-scale single-crystal monolayers. We analyse the defect levels and measures of crystalline quality of typical 2D vdW materials with various epitaxial growth techniques. We then outline technical routes for the growth of homogeneous multilayers and twisted homostructures. We further summarize the current strategies to guide future efforts in optimizing on-demand fabrication of 2D vdW materials, as well as subsequent device manufacturing for their industrial applications.

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.

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.

A mechanism for thickness-controllable single crystalline 2D materials growth

Recent efforts in growing two-dimensional (2D) multilayers have enabled the synthesis of single crystalline 2D multilayers in a wafer scale through the seamless stitching of multiple epitaxial 2D islands. Unlike previously observed wedding-cake or inverted-wedding-cake structures, these multilayer islands have the same size and shape in each layer with aligned edges. In this study, we investigated the underlying growth mechanisms of synchronic 2D multilayers growth and have showed that a heterogenous layer on a crystalline substrate is critical for maintaining the synchronic growth of 2D multilayers. During growth, the heterogenous layer passivates the edges of multilayer 2D island and thus prevents the coalescence of these active edges, while the high interfacial energy between the heterogenous surface layer and the substrate stabilizes the synchronic structure. Based on this model, we have successfully explained the previously observed synchronic growth of graphene and hexagonal boron nitride multilayers (Nat Nanotech 2020, 15: 861; Nature 2022, 606: 88). The deep understanding on the mechanism paves a way towards the synthesis of wafer-scale single-crystal 2D multilayers with a uniform thickness.

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.

Stack bonding in polyaromatic hydrocarbons

Parallel displacement of π-stacked component molecules enhances the efficiency of organic semiconductors by maximizing interpenetration of the π-densities. Dimers of symmetric polyaromatic hydrocarbons coronene, hexabenzo[bc,de,gh,kl,no,qr]coronene, circumcoronene, kekulene, and circumcircumcoronene are examined using density functional theory from the stack bonding perspective which considers π-stacking interactions in terms of contributions of monomer π-orbital overlap to the character of dimer orbitals. Energetically favored parallel displaced and/or twisted dimer conformations are consistent with patterns of mixing of the monomer molecular orbitals (MOs) that maximize interpenetration of the π densities. The multiple minima found along parallel displacement (PD) coordinates coincide with the formation of dimer MOs formally antibonding between the monomers at the sandwich conformation to bonding at the PD minima. Minima identified with favorable stack bonding are consistent with polymorphs found in large polyaromatic hydrocarbons.

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.

Stepwise rapid electrolytic synthesis of graphene oxide for efficient adsorption of organic pollutants

Graphene oxide (GO) has been widely used in energy devices, biomedicine, environmental protection, composite materials and other fields. Hummers' method is currently one of the most powerful strategies for the preparation of GO. However, many deficiencies, including severe environmental pollution, operation safety issues and low oxidation efficiency are major obstacles for the large-scale green synthesis of GO. Here, we report a stepwise electrochemical method for the fast preparation of GO using spontaneous persulfate intercalation followed by anodic electrolytic oxidation. Such a step-by-step process not only avoids uneven intercalation and insufficient oxidation in traditional one-pot methods, but also largely shortens the overall duration by two orders of magnitude. In particular, the oxygen content of the obtained GO is as high as 33.7 at%, almost double that from Hummers' method (17.4 at%). The abundant surface functional groups render this GO an excellent adsorption platform for methylene blue with an adsorption capacity of 358 mg g^-1, 1.8-fold higher than conventional GO.

Helical Trilayer Nanographenes with Tunable Interlayer Overlaps

The synthesis of well-defined nanocarbon multilayers, beyond the bilayer structure, is still a challenging goal. Herein, two trilayer nanographenes were synthesized by covalently linking nanographene layers through helicene bridges. The structural characterization of the trilayer nanographenes revealed a compact trilayer-stacked architecture. The introduction of a furan ring into the helicene linker modulates the interlayer overlap and π-conjugation of the trilayer nanographenes, enabling the tuning of the interlayer coupling, as demonstrated by optical, electrochemical, and theoretical analyses. Both synthesized trilayer nanographenes are rigid chiral nanocarbons and show a chirality transfer from the helicene moiety to the stacked nanographene layers. These helical trilayer nanographenes reported here represent the covalently linked multilayer nanographenes rather than bilayer ones, showing the tunable multilayer stacking structure.

Interfacial engineering of plasmonic nanoparticle metasurfaces

SignificanceMolecules interacting with metallic nanostructures can show tunable exciton-plasmon coupling, ranging from weak to strong. One factor that influences the interactions is the spatial organization of the molecules relative to the localized plasmon-enhanced electromagnetic fields. In this work, we show that the arrangement of aromatic dye molecules can be tuned within plasmonic hotspots by interfacial engineering of nanoparticle surfaces. By controlling the local chemical and physical interactions, we could modulate lasing thresholds. Surface-functionalized plasmonic metasurfaces open prospects for programmable light-matter interactions at the nanoscale.

Epitaxial growth of structure-tunable ZnO/ZnS core/shell nanowire arrays using HfO2 as the buffer layer

Synthesis of high-quality ZnO/ZnS heterostructures with tunable phase and controlled structures is in high demand due to their adjustable band gap and efficient electron-hole pair separation. In this report, for the first time, remote heteroepitaxy of single-crystalline ZnO/ZnS core/shell nanowire arrays has been realized using amorphous HfO₂ as the buffer layer. Zinc blende or wurtzite ZnS epilayer can be efficiently fabricated under the same thermal deposition condition by adjusting the buffer layer thickness, even among the same batch of products, respectively. Structural characterization reveals "(01-10)ZnO_wz//(2-20)ZnS_ZB, [0001]ZnO_WZ//[001]ZnS_ZB" and "(01-10)ZnO_WZ//(01-10)ZnS_WZ, [0002]ZnO_WZ//[0002]ZnS_WZ" epitaxial relationships between the core and the shell, respectively. The cathodoluminescence measurement demonstrates that the tuning of the optical properties can be accomplished by preparing a heterostructure with HfO₂, in which a strong green emission increases at the expense of the quenching of UV emission. In addition, the core/shell heterostructure based Schottky diode exhibits an asymmetrical rectifying behavior and an outstanding photo-electronic switching-effect. We believe that the aforementioned results could provide fundamental insights for epitaxial growth of structure-tunable ZnO/ZnS heterostructures on the nanoscale. Furthermore, this promising route buffered by the high-k material can broaden the options for fabricating heterojunctions and promote their application in photoelectric nanodevices.

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.

2D Materials for All-Solid-State Lithium Batteries

Although one of the most mature battery technologies, lithium-ion batteries still have many aspects that have not reached the desired requirements, such as energy density, current density, safety, environmental compatibility, and price. To solve these problems, all-solid-state lithium batteries (ASSLB) based on lithium metal anodes with high energy density and safety have been proposed and become a research hotpot in recent years. Due to the advanced electrochemical properties of 2D materials (2DM), they have been applied to mitigate some of the current problems of ASSLBs, such as high interface impedance and low electrolyte ionic conductivity. In this work, the background and fabrication method of 2DMs are reviewed initially. The improvement strategies of 2DMs are categorized based on their application in the three main components of ASSLBs: The anode, cathode, and electrolyte. Finally, to elucidate the mechanisms of 2DMs in ASSLBs, the role of in situ characterization, synchrotron X-ray techniques, and other advanced characterization are discussed.

Precise CO2 Reduction for Bilayer Graphene

It is of great significance to explore unique and diverse chemical pathways to convert CO2 into high-value-added products. Bilayer graphene (BLG), with a tunable twist angle and band structure, holds tremendous promise in both fundamental physics and next-generation high-performance devices. However, the π-conjugation and precise two-atom thickness are hindering the selective pathway, through an uncontrolled CO2 reduction and perplexing growth mechanism. Here, we developed a chemical vapor deposition method to catalytically convert CO2 into a high-quality BLG single crystal with a room temperature mobility of 2346 cm2 V-1 s-1. In a finely controlled growth window, the CO2 molecule works as both the carbon source and the oxygen etchant, helping to precisely define the BLG nucleus and set a record growth rate of 300 μm h-1.

Artificial Monovalent Metal Ion-Selective Fluidic Devices Based on Crown Ether@Metal-Organic Frameworks with Subnanochannels

Precise regulation of ion transport through nanoscale pores will profoundly impact diverse fields from separation to energy conversion but is still challenging to achieve in artificial ion channels. Herein, inspired by the exquisite ion selectivity of biological Na⁺ channels, we have successfully fabricated hierarchically grown metal-organic frameworks (MOFs) on an asymmetrical substrate assisted by atomically thin nanoporous graphene. Efficient separation of monovalent metal ions is realized by encapsulating 18-crown-6 into MOF crystals. The resulting 18-crown-6@ZIF-67/ZIF-8 device, with subnanochannels and specific K⁺ binding sites, shows an ultrahigh Li⁺ conductivity of 1.46 × 10^-2 S cm^-1 and selectivities of 9.56 and 6.43 for Li⁺/K⁺ and Na⁺/K⁺, respectively. The Li⁺ conductivity is around 1-2 orders of magnitude higher than reported values for the other MOF materials. It is the first time that MOFs with subnanochannels realize selective transport of Na⁺ (ionic diameter of 1.9 Å) over K⁺ (2.6 Å) based on subangstrom differences in their ionic diameter. Our work opens new avenues to develop crown ether@MOF platforms toward efficient ion transistors, fluidic logic devices, and biosensors.

In-situ interfacial compatibilization via edge-sulfurated few layer graphene during the formation of crosslinked graphene-rubber nanocomposites

Herein, we report various physico-chemical approaches to probe the nature of the interface between few layers graphene (FLG) and carboxylated nitrile rubber (XNBR) nanocomposites prepared through efficient blending of XNBR latex with an aqueous dispersion of FLG. The extent of physical interaction between FLG and XNBR was investigated using Lorentz-Park and Cunneen-Russell models. The chemical interface between FLG and sulfur crosslinked XNBR was studied using model reactions between sulfur and graphene in presence of zinc 2-mercaptobenzothiazole (ZMBT). We propose that an edge sulfurated FLG is formed, which could chemically bond with XNBR during the vulcanization process. Density Functional Theory (DFT) was employed to unravel the mechanistic insights, which support this hypothesis and suggest a kinetically favorable sulfuration of both XNBR and FLG. The formation of a chemical bond between edge-FLG and XNBR through the proposed intermediacy of sulfurated FLG leads to the observed improvement in mechanical properties of the nanocomposites.

Large-Scale Syntheses of 2D Materials: Flash Joule Heating and Other Methods

In the past 17 years, the larger-scale production of graphene and graphene family materials has proven difficult and costly, thus slowing wider-scale commercial applications. The quality of the graphene that is prepared on larger scales has often been poor, demonstrating a need for improved quality controls. Here, current industrial graphene synthetic and analytical methods, as well as recent academic advancements in larger-scale or sustainable synthesis of graphene, defined here as weights more than 200 mg or films larger than 200 cm² , are compiled and reviewed. There is a specific emphasis on recent research in the use of flash Joule heating as a rapid, efficient, and scalable method to produce graphene and other 2D nanomaterials. Reactor design, synthetic strategies, safety considerations, feedstock selection, Raman spectroscopy, and future outlooks for flash Joule heating syntheses are presented. To conclude, the remaining challenges and opportunities in the larger-scale synthesis of graphene and a perspective on the broader use of flash Joule heating for larger-scale 2D materials synthesis are discussed.

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.

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.

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.

Practical Route for the Low-Temperature Growth of Large-Area Bilayer Graphene on Polycrystalline Nickel by Cold-Wall Chemical Vapor Deposition

We report a practical chemical vapor deposition (CVD) route to produce bilayer graphene on a polycrystalline Ni film from liquid benzene (C6H6) source at a temperature as low as 400 °C in a vertical cold-wall reaction chamber. The low activation energy of C6H6 and the low solubility of carbon in Ni at such a low temperature play a key role in enabling the growth of large-area bilayer graphene in a controlled manner by a Ni surface-mediated reaction. All experiments performed using this method are reproducible with growth capabilities up to an 8 in. wafer-scale substrate. Raman spectra analysis, high-resolution transmission electron microscopy, and selective area electron diffraction studies confirm the growth of Bernal-stacked bilayer graphene with good uniformity over large areas. Electrical characterization studies indicate that the bilayer graphene behaves much like a semiconductor with predominant p-type doping. These findings provide important insights into the wafer-scale fabrication of low-temperature CVD bilayer graphene for next-generation nanoelectronics.

Can a Procedure for the Growth of Single-layer Graphene on Copper be used in Different Chemical Vapor Deposition Reactors?

In the last decade, catalytic chemical vapor deposition (CVD) has been intensively explored for the growth of single-layer graphene (SLG). Despite the scattering of guidelines and procedures, variables such as the surface texture/chemistry of catalyst metal foils, carbon feedstock, and growth process parameters have been well-scrutinized. Still, questions remain on how best to standardize the growth procedure. The possible correlation of procedures between different CVD setups is an example. Here, two thermal CVD reactors were explored to grow graphene on Cu foil. The design of these setups was entirely distinct, one being a "showerhead" cold-wall type, whereas the other represented the popular "tubular" hot-wall type. Upon standardizing the Cu foil surface, it was possible to develop a procedure for cm² -scale SLG growth that differed only by the carrier gas flow rate used in the two reactors.

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.

Micro-photoluminescence of Carbon Dots Deposited on Twisted Double-Layer Graphene Grown by Chemical Vapor Deposition

Carbon-based nanomaterials, such as carbon dots (CDs) and graphene (Gr), feature outstanding optical and electronic properties. Hence, their integration in optoelectronic and photonic devices is easier thanks to their low dimensionality and offers the possibility to reach high-quality performances. In this context, the combination of CDs and Gr into new nanocomposite materials CDs/Gr can further improve their optoelectronic properties and eventually create new ones, paving the way for the development of advanced carbon nanotechnology. In this work, we have thoroughly investigated the structural and emission properties of CDs deposited on single-layer and bilayer graphene lying on a SiO₂/Si substrate. A systematic Raman analysis points out that bilayer (BL) graphene grown by chemical vapor deposition does not always respect the Bernal (AB) stacking, but it is rather a mixture of twisted bilayer (t-BL) featuring domains with different twist angles. Moreover, in-depth micro-photoluminescence measurements, combined with atomic force microscopy (AFM) morphological analysis, show that CD emission efficiency is strongly depleted by the presence of graphene and in particular is dependent on the number of layers as well as on the twist angle of BL graphene. Finally, we propose a model which explains these results on the basis of photoinduced charge-transfer processes, taking into account the energy levels of the hybrid nanosystem formed by coupling CDs with t-BL/SiO₂.

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.

Sheet Resistance Analysis of Interface-Engineered Multilayer Graphene: Mobility Versus Sheet Carrier Concentration

Both interlayer-undoped and interlayer-doped multilayer graphenes were prepared by the multiple transfers of graphene layers with multiple Cu etching (either dopant-free or doped during etching) and transfer, and the effect of interface properties on the electrical properties of multilayer graphene was investigated by varying the number of layers from 1 to 12. In both the cases, the sheet resistance decreased with increasing number of layers from 700 to 104 Ω/sq for the interlayer-undoped graphene and from 280 to 25 Ω/sq for the interlayer-doped graphene. Further, Hall measurements revealed that the origins of the sheet resistance reduction in the two cases are different. In the interlayer-undoped graphene, the sheet resistance decreased because of the increase in mobility with the addition of inner layers, which has a low carrier density and a high carrier mobility. On the other hand, it decreased because of the increase in sheet carrier density in the interlayer-doped multilayer graphene. The mobility and carrier density variations in both the cases were confirmed by fitting with the model of Hall effect in the heterojunction. In addition, we found that surface property modification by the doping of the top layer and the formation of double-layer graphene with different partial coverages allow the separate control of carrier density and mobility. Our study provides an effective approach for controlling the properties of multilayer graphene for electronic applications.

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.

Cytotoxicity of Formulated Graphene and Its Natural Rubber Nanocomposite Thin Film in Human Vaginal Epithelial Cells: An Influence of Noncovalent Interaction

Graphene family materials (GFMs) are extensively explored for various biomedical applications due to their unique physical properties. The prime challenge is to establish a conclusive safety profile of these nanomaterials and their respective products or devices. Formulating GFMs with appropriate ingredients (e.g., surfactant/compatibilizer) will help to disperse them homogeneously (i.e., within the polymer matrix in the case of polymer–graphene nanocomposites) and aid in good interfacial interaction to achieve the desired properties. However, no cytotoxicity report is available on the effects of the additives on graphene and its incorporated materials. Here, we report in vitro cytotoxicity of formulated FLG (FLG-C), i.e., a mixture of FLG, melamine, and sodium poly(naphthalene sulfonate) (SPS), along with natural rubber (NR) latex and FLG-C-included NR latex nanocomposite (FLG-C-NR) thin films on human vaginal epithelial (HVE) cells. FLG-C shows reduced cellular proliferation (∼55%) only at a longer exposure time (72 h) even at a low concentration (50 μg/mL). It also displays significant down- and upregulation in mitochondrial membrane potential (MMP) and reactive oxygen species (ROS), respectively, whereas no changes are observed in lactate dehydrogenase (LDH), propidium iodide (PI), uptake, and cell cycle analysis at 48 h. In vitro experiments on NR latex and FLG-C-NR latex thin films demonstrate that the incorporation of FLG-C does not compromise the biocompatibility of the NR latex. Further substantiation from the in vivo experiments on the thin films recommends that FLG-C could be suitable to prepare a range of biocompatible rubber latex nanocomposites-based products, viz., next-generation condoms (male and female), surgical gloves, catheters, vaginal rings, bladder–rectum spacer balloon, etc.

Edge-State-Induced Stacking of Zigzag Graphene Nanoribbons

We have investigated the structural stabilities and electronic properties for AA and the Bernal-stacked AB bilayer zigzag graphene nanoribbons (ZZGNRs) using first-principles calculations within density functional theory. The AB-stacked ZZGNR exhibits the spin-polarized state, while the AA-stacked ZZGNR has the nonmagnetic ground state, being more energetically stable than the AB-stacked one. For the AA-stacked ZZGNR, the interaction between the so-called edge states rather than the van der Waals (vdW) interaction plays an important role: the occupied up-spin and the unoccupied down-spin states at one end of ZZGNR interact with each other, and vice versa at the other end, forming the non-spin-polarized bonding and antibonding states at the zigzag edge. Thus, the structural stability for the AA-stacked ZZGNR is dominated by the trade-off between the edge-edge interaction and the vdW interaction of the basal plane of GNRs.

Molecular bilayer graphene

Bilayer graphene consists of two stacked graphene layers bound together by van der Waals interaction. As the molecular analog of bilayer graphene, molecular bilayer graphene (MBLG) can offer useful insights into the structural and functional properties of bilayer graphene. However, synthesis of MBLG, which requires discrete assembly of two graphene fragments, has proved to be challenging. Here, we show the synthesis and characterization of two structurally well-defined MBLGs, both consisting of two π-π stacked nanographene sheets. We find they have excellent stability against variation of concentration, temperature and solvents. The MBLGs show sharp absorption and emission peaks, and further time-resolved spectroscopic studies reveal drastically different lifetimes for the bright and dark Davydov states in these MBLGs.

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

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

Edge Segregated Polymorphism in 2D Molybdenum Carbide

Molybdenum carbide (Mo₂ C), a class of unterminated MXene, is endowed with rich polymorph chemistry, but the growth conditions of the various polymorphs are not understood. Other than the most commonly observed T-phase Mo₂ C, little is known about other phases. Here, Mo₂ C crystals are successfully grown consisting of mixed polymorphs and polytypes via a diffusion-mediated mechanism, using liquid copper as the diffusion barrier between the elemental precursors of Mo and C. By controlling the thickness of the copper diffusion barrier layer, the crystal growth can be controlled between a highly uniform AA-stacked T-phase Mo₂ C and a "wedding cake" like Mo₂ C crystal with spatially delineated zone in which the Bernal-stacked Mo₂ C predominate. The atomic structures, as well as the transformations between distinct stackings, are simulated and analyzed using density functional theory (DFT)-based calculations. Bernal-stacked Mo₂ C has a d band closer to the Fermi energy, leading to a promising performance in catalysis as verified in hydrogen evolution reaction (HER).

Effects of site asymmetry and valley mixing on Hofstadter-type spectra of bilayer graphene in a square-scatter array potential

Under a magnetic field perpendicular to an monolayer graphene, the existence of a two-dimensional periodic scatter array can not only mix Landau levels of the same valley for displaying split electron-hole Hofstadter-type energy spectra, but also couple two sets of Landau subbands from different valleys in a bilayer graphene. Such a valley mixing effect with a strong scattering strength has been found observable and studied thoroughly in this paper by using a Bloch-wave expansion approach and a projected [Formula: see text] effective Hamiltonian including interlayer effective mass, interlayer coupling and asymmetrical on-site energies due to a vertically-applied electric field. For bilayer graphene, we find two important characteristics, i.e. mixing and interference of intervalley scatterings in the presence of a scatter array, as well as a perpendicular-field induced site-energy asymmetry which deforms severely or even destroys completely the Hofstadter-type band structures due to the dependence of Bloch-wave expansion coefficients on the applied electric field.

Direct Growth of Graphene on Insulator Using Liquid Precursor Via an Intermediate Nanostructured State Carbon Nanotube

Synthesis of high-quality graphene layers on insulating substrates is highly desirable for future graphene-based high-speed electronics. Besides the use of gaseous hydrocarbon sources, solid and liquid hydrocarbon sources have recently shown great promises for high-quality graphene growth. Here, I report chemical vapor deposition growth of mono- to few-layer graphene directly on SiO₂ substrate using ethanol as liquid hydrocarbon feedstock. The growth process of graphene has been systematically investigated as a function of annealing temperature as well as different seed layers. Interestingly, it was found that the carbon atoms produced by thermal decomposition of ethanol form sp² carbon network on SiO₂ surface thereby forming nanographene flakes via an intermediate carbon-based nanostructured state carbon nanotube. This work might pave the way to an understanding for economical and catalyst-free graphene growth compatible with current silicon-processing techniques, and it can be applied on a variety of insulating surfaces including quartz, sapphire, and fused silica.

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.

Reprogrammable Graphene-based Metasurface Mirror with Adaptive Focal Point for THz Imaging

Recent emergence of metasurfaces has enabled the development of ultra-thin flat optical components through different wavefront shaping techniques at various wavelengths. However, due to the non-adaptive nature of conventional metasurfaces, the focal point of the resulting optics needs to be fixed at the design stage, thus severely limiting its reconfigurability and applicability. In this paper, we aim to overcome such constraint by presenting a flat reflective component that can be reprogrammed to focus terahertz waves at a desired point in the near-field region. To this end, we first propose a graphene-based unit cell with phase reconfigurability, and then employ the coding metasurface approach to draw the phase profile required to set the focus on the target point. Our results show that the proposed component can operate close to the diffraction limit with high focusing range and low focusing error. We also demonstrate that, through appropriate automation, the reprogrammability of the metamirror could be leveraged to develop compact terahertz scanning and imaging systems, as well as novel reconfigurable components for terahertz wireless communications.

A light emitter based on practicable and mass-producible polycrystalline graphene patterned directly on silicon substrates from a solid-state carbon source

We developed a procedure for direct patterning of graphene with arbitrary position, size, and shape on silicon substrates from a solid-state carbon source without dry etching processing. Our light emitting graphene devices perform on a par with those based on high crystallinity graphene obtained via mechanical exfoliation or chemical vapor deposition.

We developed a procedure for direct patterning graphene with arbitrary position, size, and shape on Si from a solid-state carbon source without dry etching. Our light emitting devices perform on a par with those based on high crystallinity graphene.

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

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