The epitaxial growth of large-area single-domain graphene on hexagonal boron nitride by plasma-assisted deposition is now reported. New sets of Dirac points are produced as a result of a trigonal superlattice potential, while Dirac fermion physics near the original Dirac point remain unperturbed. This growth approach could enable band engineering in graphene through epitaxy on different substrates.

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Epitaxial growth of single-domain graphene on hexagonal boron nitride

The recent surge in graphene research has stimulated interest in the investigation of various 2-dimensional (2D) nanomaterials. Among these materials, the 2D boron nitride (BN) nanostructures are in a unique position. This is because they are the isoelectric analogs to graphene structures and share very similar structural characteristics and many physical properties except for the large band gap. The main forms of the 2D BN nanostructures include nanosheets (BNNSs), nanoribbons (BNNRs), and nanomeshes (BNNMs). BNNRs are essentially BNNSs with narrow widths in which the edge effects become significant; BNNMs are also variations of BNNSs, which are supported on certain metal substrates where strong interactions and the lattice mismatch between the substrate and the nanosheet result in periodic shallow regions on the nanosheet surface. Recently, the hybrids of 2D BN nanostructures with graphene, in the form of either in-plane hybrids or inter-plane heterolayers, have also drawn much attention. In particular, the BNNS–graphene heterolayer architectures are finding important electronic applications as BNNSs may serve as excellent dielectric substrates or separation layers for graphene electronic devices. In this article, we first discuss the structural basics, spectroscopic signatures, and physical properties of the 2D BN nanostructures. Then, various top-down and bottom-up preparation methodologies are reviewed in detail. Several sections are dedicated to the preparation of BNNRs, BNNMs, and BNNS–graphene hybrids, respectively. Following some more discussions on the applications of these unique materials, the article is concluded with a summary and perspectives of this exciting new field.

Advances in 2D boron nitride nanostructures: nanosheets, nanoribbons, nanomeshes, and hybrids with graphene

In recent years, two-dimensional atomic-level thickness crystal materials have attracted widespread interest such as graphene, hexagonal boron nitride (h-BN), silicene, germanium, black phosphorus (BP), transition metal sulfides and so on. These graphene-like two-dimensional (2D) materials have a lot of excellent characteristics such as high specific surface area and high Young's modulus, and many potential applications in diverse areas such as photo-electricity, catalysts, and transistors. In this review, we introduced the synthesis, structure, properties, and applications of graphene, h-BN, and their heterostructures, especially focused on their mechanical, optical, thermal, electric, and magnetic properties. Finally, we present the outlooks and perspectives for these types of excellent 2D materials and their potential applications.

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Graphene, hexagonal boron nitride, and their heterostructures: properties and applications

© 2020 The Author(s). We present an overview of the main techniques for production and processing of graphene and related materials (GRMs), as well as the key characterization procedures. We adopt a 'hands-on' approach, providing practical details and procedures as derived from literature as well as from the authors' experience, in order to enable the reader to reproduce the results. Section I is devoted to 'bottom up' approaches, whereby individual constituents are pieced together into more complex structures. We consider graphene nanoribbons (GNRs) produced either by solution processing or by on-surface synthesis in ultra high vacuum (UHV), as well carbon nanomembranes (CNM). Production of a variety of GNRs with tailored band gaps and edge shapes is now possible. CNMs can be tuned in terms of porosity, crystallinity and electronic behaviour. Section II covers 'top down' techniques. These rely on breaking down of a layered precursor, in the graphene case usually natural crystals like graphite or artificially synthesized materials, such as highly oriented pyrolythic graphite, monolayers or few layers (FL) flakes. The main focus of this section is on various exfoliation techniques in a liquid media, either intercalation or liquid phase exfoliation (LPE). The choice of precursor, exfoliation method, medium as well as the control of parameters such as time or temperature are crucial. A definite choice of parameters and conditions yields a particular material with specific properties that makes it more suitable for a targeted application. We cover protocols for the graphitic precursors to graphene oxide (GO). This is an important material for a range of applications in biomedicine, energy storage, nanocomposites, etc. Hummers' and modified Hummers' methods are used to make GO that subsequently can be reduced to obtain reduced graphene oxide (RGO) with a variety of strategies. GO flakes are also employed to prepare three-dimensional (3d) low density structures, such as sponges, foams, hydro- or aerogels. The assembly of flakes into 3d structures can provide improved mechanical properties. Aerogels with a highly open structure, with interconnected hierarchical pores, can enhance the accessibility to the whole surface area, as relevant for a number of applications, such as energy storage. The main recipes to yield graphite intercalation compounds (GICs) are also discussed. GICs are suitable precursors for covalent functionalization of graphene, but can also be used for the synthesis of uncharged graphene in solution. Degradation of the molecules intercalated in GICs can be triggered by high temperature treatment or microwave irradiation, creating a gas pressure surge in graphite and exfoliation. Electrochemical exfoliation by applying a voltage in an electrolyte to a graphite electrode can be tuned by varying precursors, electrolytes and potential. Graphite electrodes can be either negatively or positively intercalated to obtain GICs that are subsequently exfoliated. We also discuss the materials that can be amenable to exfoliation, by employing a theoretical data-mining approach. The exfoliation of LMs usually results in a heterogeneous dispersion of flakes with different lateral size and thickness. This is a critical bottleneck for applications, and hinders the full exploitation of GRMs produced by solution processing. The establishment of procedures to control the morphological properties of exfoliated GRMs, which also need to be industrially scalable, is one of the key needs. Section III deals with the processing of flakes. (Ultra)centrifugation techniques have thus far been the most investigated to sort GRMs following ultrasonication, shear mixing, ball milling, microfluidization, and wet-jet milling. It allows sorting by size and thickness. Inks formulated from GRM dispersions can be printed using a number of processes, from inkjet to screen printing. Each technique has specific rheological requirements, as well as geometrical constraints. The solvent choice is critical, not only for the GRM stability, but also in terms of optimizing printing on different substrates, such as glass, Si, plastic, paper, etc, all with different surface energies. Chemical modifications of such substrates is also a key step. Sections IV-VII are devoted to the growth of GRMs on various substrates and their processing after growth to place them on the surface of choice for specific applications. The substrate for graphene growth is a key determinant of the nature and quality of the resultant film. The lattice mismatch between graphene and substrate influences the resulting crystallinity. Growth on insulators, such as SiO2, typically results in films with small crystallites, whereas growth on the close-packed surfaces of metals yields highly crystalline films. Section IV outlines the growth of graphene on SiC substrates. This satisfies the requirements for electronic applications, with well-defined graphene-substrate interface, low trapped impurities and no need for transfer. It also allows graphene structures and devices to be measured directly on the growth substrate. The flatness of the substrate results in graphene with minimal strain and ripples on large areas, allowing spectroscopies and surface science to be performed. We also discuss the surface engineering by intercalation of the resulting graphene, its integration with Si-wafers and the production of nanostructures with the desired shape, with no need for patterning. Section V deals with chemical vapour deposition (CVD) onto various transition metals and on insulators. Growth on Ni results in graphitized polycrystalline films. While the thickness of these films can be optimized by controlling the deposition parameters, such as the type of hydrocarbon precursor and temperature, it is difficult to attain single layer graphene (SLG) across large areas, owing to the simultaneous nucleation/growth and solution/precipitation mechanisms. The differing characteristics of polycrystalline Ni films facilitate the growth of graphitic layers at different rates, resulting in regions with differing numbers of graphitic layers. High-quality films can be grown on Cu. Cu is available in a variety of shapes and forms, such as foils, bulks, foams, thin films on other materials and powders, making it attractive for industrial production of large area graphene films. The push to use CVD graphene in applications has also triggered a research line for the direct growth on insulators. The quality of the resulting films is lower than possible to date on metals, but enough, in terms of transmittance and resistivity, for many applications as described in section V. Transfer technologies are the focus of section VI. CVD synthesis of graphene on metals and bottom up molecular approaches require SLG to be transferred to the final target substrates. To have technological impact, the advances in production of high-quality large-area CVD graphene must be commensurate with those on transfer and placement on the final substrates. This is a prerequisite for most applications, such as touch panels, anticorrosion coatings, transparent electrodes and gas sensors etc. New strategies have improved the transferred graphene quality, making CVD graphene a feasible option for CMOS foundries. Methods based on complete etching of the metal substrate in suitable etchants, typically iron chloride, ammonium persulfate, or hydrogen chloride although reliable, are time- and resourceconsuming, with damage to graphene and production of metal and etchant residues. Electrochemical delamination in a low-concentration aqueous solution is an alternative. In this case metallic substrates can be reused. Dry transfer is less detrimental for the SLG quality, enabling a deterministic transfer. There is a large range of layered materials (LMs) beyond graphite. Only few of them have been already exfoliated and fully characterized. Section VII deals with the growth of some of these materials. Amongst them, h-BN, transition metal tri- and di-chalcogenides are of paramount importance. The growth of h-BN is at present considered essential for the development of graphene in (opto) electronic applications, as h-BN is ideal as capping layer or substrate. The interesting optical and electronic properties of TMDs also require the development of scalable methods for their production. Large scale growth using chemical/physical vapour deposition or thermal assisted conversion has been thus far limited to a small set, such as h-BN or some TMDs. Heterostructures could also be directly grown.

Production and processing of graphene and related materials

Due to the outstanding physicochemical properties arising from its truly two-dimensional (2D) planar structure with a single-atom thickness, graphene exhibits great potential for use in sensors, catalysts, electrodes, and in biological applications, etc. With further developments in the theoretical understanding and assembly techniques, graphene should enable great changes both in scientific research and practical industrial applications. By the look of development, it is of fundamental and practical significance to translate the novel physical and chemical properties of individual graphene nanosheets into the macroscale by the assembly of graphene building blocks into macroscopic architectures with structural specialities and functional novelties. The combined features of a 2D planar structure and abundant functional groups of graphene oxide (GO) should provide great possibilities for the assembly of GO nanosheets into macroscopic architectures with different macroscaled shapes through various assembly techniques under different bonding interactions. Moreover, macroscopic graphene frameworks can be used as ideal scaffolds for the incorporation of functional materials to offset the shortage of pure graphene in the specific desired functionality. The advantages of light weight, supra-flexibility, large surface area, tough mechanical strength, and high electrical conductivity guarantee graphene-based architectures wide application fields. This critical review mainly addresses recent advances in the design and fabrication of graphene-based macroscopic assemblies and architectures and their potential applications. Herein, we first provide overviews of the functional macroscopic graphene materials from three aspects, i.e., 1D graphene fibers/ribbons, 2D graphene films/papers, 3D network-structured graphene monoliths, and their composite counterparts with either polymers or nano-objects. Then, we present the promising potential applications of graphene-based macroscopic assemblies in the fields of electronic and optoelectronic devices, sensors, electrochemical energy devices, and in water treatment. Last, the personal conclusions and perspectives for this intriguing field are given.

Graphene-based macroscopic assemblies and architectures: an emerging material system.

A method for growing a graphene nanoribbon on an insulating substrate having a cleavage plane with atomic level flatness is provided, and belongs to the field of low-dimensional materials and new materials. The method includes the following steps. Step 1: Cleave an insulating substrate to obtain a cleavage plane with atomic level flatness, and prepare a single atomic layer step. Step 2: Directly grow a graphene nanoribbon on the insulating substrate having regular single atomic steps. In the method, a characteristic that nucleation energy of graphene on the atomic step is different from that on the flat cleavage plane is used, and conditions, such as the temperature, intensity of pressure and supersaturation degree of activated carbon atoms, are adjusted, so that the graphene grows only along a step edge into a graphene nanoribbon of an adjustable size. The method is mainly applied to the field of new-type graphene optoelectronic devices.

Method for preparing graphene nanoribbon on insulating substrate

The invention provides a preparation method of a graphene flash memory. According to the method, a single-layer or multi-layer continuous graphene thin film replaces a polysilicon gate or nitric oxide to store electric charges, the storage capacity of the electric charges is improved within a limited physical space, and as the graphene is relatively thin, the influence of capacity coupling in a device is eliminated while the longitudinal dimension of the device is diminished, so that the problem of crosstalk of adjacent storage units in work is effectively avoided. The graphene flash memory is simple in process and convenient in operation, and the data of the graphene flash memory are rapidly written, erased and read at the low power consumption.

Preparation method of graphene flash memory

The invention discloses a method for preparing a graphene device on a flexible substrate. The method for preparing the graphene device on the flexible substrate at least includes: 1) providing the flexible substrate, attaching the flexible substrate to a hard substrate, and forming graphene conducting channels in the flexible substrate; 2) carrying out electron beam lithography patterning to form a source electrode pattern and a drain electrode pattern, and depositing metal and peeling to form a metal source electrode at one end of each graphene conducting channel and form a metal drain electrode at the other end; 3) using a low-temperature deposition process to deposit at the surface of the structure obtained through the step 2) to form a gate medium layer; 4) etching the gate medium layer to expose the metal source electrode and metal drain electrode at two ends of the graphene conducting channel; 5) carrying out electron beam lithography patterning to form a gate electrode pattern, and depositing metal and peeling to form a gate electrode on the gate medium layer between graphene conducting channels; 6) forming a contact electrode; 7) separating the flexible substrate from the hard substrate.

Method for preparing graphene device on flexible substrate

The invention provides a preparation method of a graphene-boron nitride heterojunction. The method includes the steps of: S1. adhering graphene to a heat release adhesive tape; S2. attaching the heat release adhesive tape side adhered with the graphene to boron nitride; and S3. conducting heating to make the heat release adhesive tape lose viscosity, and then peeling off the heat release adhesive tape to obtain the graphene-boron nitride heterojunction. The method provided by the invention utilizes continuous transfer of the heat release adhesive tape to obtain grapheme/boron nitride with dry and clean upper and lower surfaces. The transfer method has simple conditions, is low in cost, has good repeatability, is environment friendly, and successfully avoids metal ions and chemical groups introduced by corrosion of metal growth substrate in a wet transfer process, also avoids damage of graphene and boron nitride surface appearance, and further avoids the possibility of incomplete removal of residual adhesive. The high quality graphene-boron nitride heterojunction lays a good foundation for later stage preparation of graphene electronic devices.

Preparation method of graphene-boron nitride heterojunction

The invention provides a method for transferring graphene. The method comprises the following steps: providing graphene on a first substrate; covering a buffer layer on the graphene; adhering a heat release adhesive tape on the buffer layer; soaking the first substrate in a corrosive solution to remove the first substrate under the graphene; covering the graphene without the first substrate on a target substrate; heating to make the heat release adhesive belt become non-sticky, removing the heat release adhesive belt; and soaking the buffer layer in a dissolving solution to remove the buffer layer on the graphene to obtain graphene on a target substrate to finish the transferring of graphene. According to the method, a buffer layer is used to assist the transferring of graphene, the required steps are few, the process is fast and convenient, the upper and lower surfaces of graphene are clean, no substance is adhered on the upper and lower surfaces of the graphene, the wrinkles in the microscopic level are reduced, thus the effective usable area of graphene is enlarged, and the graphene can be more suitable for being used to prepare electronic devices.

Method for transferring graphene

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