The structure, performance and potential application of graphene

The most critical issue in graphene research remains the synthesis of graphene. Although there are many methods of graphene preparation, preparation is still a major factor limiting graphene research and application.
synthesis and transfer
sopen phase method
according to carbon source phase and synthesis environment, graphene preparation method can be divided into solid phase method, liquid phase method and gas phase method (Figure 1) . Among them, solid phase method includes mechanical stripping method and SiC extension method. High-quality graphene can be obtained by mechanical stripping of highly directional pyration graphite (Figure 1 (a)), which is inefficient and costly.
1 Graphene preparation (a-c) solid phase method: (a) mechanical stripping method; (b) extension growth on SiC; (c) plasma etching open CNTs to obtain graphene nanobands; (d-f) solution method: (d) liquid phase stripping to obtain graphene oxide tablets; (e) by thermal AFM needle tip and laser reduction GO; (f) GNRs for different shapes assembled by monolith assembly; (g) CVD device schematic; (h) CVD growth mechanism: methane lysis to produce carbon; Ni substrate dissolves and delineates carbon (left), copper substrate adsorption carbon (right); follow-up growth of graphene
Graphene can be obtained by vacuum graphiteization extension growth on monocrystalline SiC (Figure 1 (b)). The resulting extended graphene is of high quality, layer number control, and can be prepared for large size graphene, but due to the high reaction temperature and the high cost of SiC material, SiC extended growth graphene is very expensive, and is slightly inferior to graphene obtained by mechanical peeling in terms of product quality and grain size.
liquid phase method
redox method is a common method of graphene materials in liquid phase legal system, which is low cost and high yield, but the product is defective. Graphene derivatives such as graphene oxide (graphene oxide, GO) are commonly used in liquid phase preparation. The liquid-phased GO solution can be completely dispersed in water to obtain a suspended solution of the almost independent GO layer (Figure 1 (d)). GO solutions can be deposited into film on a variety of surfaces, reducing the reduced graphene oxide (rGO) film. In addition to the use of reducing agents, GO heating in inert gases, catalyst-assisted lighting or high temperature action, electric reduction, etc. can also be reduced. The thermal needle tip, laser beam, and pulse microwave of the Atomic Force Microscope (AFM) enable fine local GO reduction (Figure 1 (e)). By heating the AFM probe for thermochemical nano-lithographic, nanoscale modeling of rGO can be obtained without causing the probe wear and sample breakage. The width of the rGO sample can be controlled at 12-20 m. Laser irradiation reduction can also achieve rGO modeling. Hot probe reduction and laser reduction GO have the advantages of reliability, cleanliness, fast and easy operation.
graphene nano-striped
is a two-dimensional zero-band gap semi-metallic material. In order to apply it to electronic devices, the first thing to do is to open the band gap so that it exhibits semiconductor characteristics. Due to the quantum limit and boundary effects, graphene nanobands (GNRs) have certain band gaps (Eg to 1/w alpha, where w is the width of the GNRs stripe and α is constant). When w is less than 10 nm, GNRs exhibit semiconductor characteristics with band gaps (Eg>0.3 eV). GNRs can be obtained by chemical ultrasound, e.g. by dispersing the expanding graphite ultrasound in organic solvents to obtain a stripped graphite suspension, which can be obtained by further centrifugation. The GNRs produced by this method are narrow, but the yield is low and the width is uncontrollable.
graphene film is a common method for obtaining wide GNRs, but the width and flatness of GNRs obtained by photolithing are limited. Carbon nanotubes (CNTs) can be seen as a barrel structure seamlessly curled by GNRs, with one idea being to cut out CNTs to prepare GNRs. The CNTs are partially embedded in the polymer, and then GNRs can be obtained by plasma etching (Figure 1 (c)). If the initial CNTs diameter is small and the hand is determined, GNRs with controlled width and edge types are available. The method can prepare a large number of shape rules, structure control, band gap adjustable GNRs. The stripping of GNRs can be achieved by heating a single layer of graphene on a silicon substrate for a long period of time at 150 degrees C. Thermal activity causes graphene to slide, tear and peel spontaneously, forming GNRs. Longitudinal cutting of multi-wall CNTs in a solution produces strip structures, which can then be obtained with chemical reduction and restored conductive properties. This method generates GNRs for jagged edges, but cannot obtain a specific structure.
gas phase method
graphene is a prerequisite for the application of electronic devices is to obtain high-quality, large area graphene, both liquid phase method and mechanical peeling method is difficult to obtain. However, large areas of single-, double-layer or multi-layer graphene films can be obtained through chemical vapor deposition (CVD). A typical CVD unit is shown in Figure 1 (g). Because gaseous carbon sources such as methane limit the types of available carbon sources, some inexpensive solid carbon sources (e.g. sucrose and polymethyl acrylates (PMMA)) are used to grow graphene, based on copper or nickel, with a reaction temperature of 800-1000 degrees C to obtain a controlled thickness of graphene, and can be controlled at the same time. The flexible selection of raw materials for CVD is an effective way to obtain high-quality graphene over a large area. However, the CVD growth process usually takes several hours and is less efficient, and the growth process and subsequent transfer process introduce defects in graphene. The growth temperature of 1000 degrees C leads to high energy consumption of graphene growth, and the metal substrate etching needs to be removed during the transfer process, which is difficult to reuse and waste. Combined with the above reasons, the cost of CVD to grow graphene is higher than that of liquid phase.
the growth mechanism of CVD preparation graphene (Figure 1 (h)) is closely related to the substrate, and the growth mechanism of graphene on the nickel substrate and the copper substrate is different. For the nickel substrate, due to the high temperature ni carbon solubility is large, in the high temperature area carbon source under Ni's catalytic action decomposed into active carbon atoms, solidified in Ni, cooling at an appropriate cooling speed in the process of carbon in Ni's solubility decreased, carbon atoms, the formation of graphene on the ni substrate surface. Carbon atoms are almost insoluble in Cu, and carbon sources are catalyticly cracked into carbon atoms by copper, which are deposited directly on the copper surface and crystallized to produce graphene. A single layer of single-crystal graphene with no folds can be grown on a hydrogen-attached niobium substrate, and the substrate can be reused. The parameters that can be regulated during the CVD process include C/H ratio, substrate mass, temperature and pressure, which can change the quality and thickness of graphene. For graphene growth on the surface of Cu, oxygen can passivate copper and inhibit graphene-shaped nuclei, and oxygen has dehydrogenation, graphene edge binding hydrogen atoms in aerobic conditions easy to remove, exposed graphene edge carbon atoms make the newly cracked carbon source and graphene edge binding, thereby promoting graphene growth.
transfer
the complete transfer of graphene on any substrate is the key technology to realize the practical application of graphene in electronics and other fields. For CVD-grown graphene, transfer is usually achieved by polymethylsiloxane (PDMS) transfer and floating transfer. Ni, Cu substrate with FeCl3, Fe (NO3) 3, (NH4) 2S2O8 solution etching removal. PDMS is used to protect graphene films, especially for graphene wafers that do not have continuous filming, and PDMS protection can be effectively transferred. The target substrate SiO2/Si is first treated with N2 plasma to form a "bubble source", and when the Cu is etched, N2 forms a capillary bridge between the graphene and SiO2/Si substrates, thus ensuring that the graphene film remains attached to the SiO2/Si substrate. This direct face-to-face transfer method reduces defects in the transfer process and is very suitable in semiconductor production lines. SiC extended growth of graphene can be transferred with metal adhesion, graphene on different metals binding force is different, you can choose the appropriate binding force of the two metals to achieve selective stripping. This dry transfer reduces the consumption of SiC tablets and controls the number of layers of transferred graphene. Similarly, there is a method of patterned fossil ene film, which sputters Zn onto multiple layers of graphene with a specific pattern and removes a layer of graphene during HCl cleaning of Zn, thus achieving the patterning of graphene.
Structure and Shape
Graphene is a two-dimensional material consisting of single-atom cellular carbon atoms, carbon atoms are sp2 hybridization, carbon atom p orbit of the remaining electrons together constitute a large π key. The keys made up of carbon σ sp2 bond make graphene structurally stable and flexible. Theoretical studies have shown that thermal disturbances can destroy long-range, orderly two-dimensional lattic lattic, so the structure of graphene has long been considered impossible to actually exist. Following the discovery of graphene in 2004, microstructure indications indicate that graphene has elastic folds (Figure 2 (a)), which resist thermal disturbances by regulating bond length, thus ensuring the stable presence of macro 2D graphene. For double-layer graphene (BLG), the buckling at the wrong place completely eliminates the residual pull stress and pressure stress of the two layers. When a flat surface at the atomic level, such as mica, is used as the base for graphene, its initial ripples are strongly inhibited by the interaction forces of the interface.
figure 2 The structure and shape of graphene (a) the ups and downs of graphene; (b) the jagged edge GNR and the armchair edge GNR; (c) the atomic resolution ADF-STEM photo of the graphene crystal boundary; and (d) stone There are four defect types of ink: adsorption atoms, two empty positions (V1 and V2), 5-8-5 yuan ring reconstruction
graphene edges are jagged and armchair edges (Figure 2 (b)). The jagged graphene nanobands are metallic, while the armchair-type graphene nanobands can be neither metallic nor semiconductor. The reconstruction status of carbon atoms indicates that the jagged edge is more stable during long-term irradiation by the spratly correction transmission mirror and analog comparative analysis of SLG edges. The nitrogen atomic energy loss spectrum obtains a large amount of chemical information about carbon atoms at the edge of graphene.
graphene crystal boundary (GBs) is formed when adjacent crystals are in different orientations. The crystal boundary of graphene is produced during the initial growth phase by combining the island-shaped crystals. A spherically corrected ring darkfield scanning transmission electron microscope (ADF-STEM) with atomic resolution can observe that graphene wafers are stitched together by a crystal boundary made up of a pentogram-seven ring pair (Figure 2 (c)). The hydroxyl produced by light decomposition water tends to bind to the graphene crystal boundary, while oxygen is oxidized on the Cu substrate through the crystal boundary through the graphene film, where the oxidation is located in the graphene crystal boundary. Because the structure of the crystal boundary is different from that of the wafer, the crystal boundary changes the performance of graphene. The defect density and strength of polycrystalline graphene are affected by the angle of the crystal boundary. Graphene with large angle crystal boundary has higher strength than small angle sample, and the strength of large angle polycrystalline graphene is similar to that of initial graphene. The results of nano-indentation of atomic force microscope show that the combination of good crystal boundaries does not affect the electrometic properties of graphene. When the edges of graphene wafers overlap instead of co-priced connections, the crystal boundary strength is poor, but the conductivity is better than that of a good co-priced combination. In contrast to overlapping crystal boundaries, when there is a small gap between wafers, their conductivity decreases significantly.
performance
electrical properties
graphene carbon atom sp2 hybridization constitutes σ bonds, carbon atomic p orbit of the remaining electrons constitute a large π bonds. In 1 graphene monotone, 3 σ-state electrons form a lower price state, while the off-domain π and π state form the highest occupying state and the lowest non-occupying guide belt. Graphene is a zero-band gap semi-metallic material with tapered distribution of guide and price bands at Dirac Point (Figure 3 (a)). Since electrons are distributed linearly at Dirac point, the effective mass here is m=0. Taking into account the interaction between quasiparticles, the Dirac spectrum is reconstructed and reconstructed to include multiple intersections: the intersection between pure charge bands, the intersection between pure plasma bands, and the circular intersection between charge and plasma bands. The speed of graphene carriers is independent of quantum energy, so Landau can vary in magnitude. Electrons in graphene are restricted by two-dimensional films and the abnormal quantum Hall effect (QHE) can be observed (Figure 3 (b).)
Figure 3 The electrical properties of graphene (a) have a quantum Landau energy-grade graphene electronic structure with tapered guide and price bands, intersected at Dirac Point; Sub-Hall effect; (c) the energy belt structure of armchair type (left) and jagged type (right) GNRs; (d) the electronic structure of single-layer graphene (SLG) and symmetrical double-layer graphene (BLG). Perturbation, adsorption doping and strong gate pressure alter the band gap of graphene μ (e) electron migration rate μ and conductivity σ change with the concentration of the carrier fluid n, (f) resistance distribution map of electronic motion in a single layer of graphene
graphene edge determines its electrical and magnetic properties. The jagged GNRs show the characteristics of zero band gap semi-metal and provide a platform for the study of spin electronics. Armchair-type GNRs are narrow-band-gap semiconductors (Figure 3 (c)). Adjusting the edges of graphene to obtain a specific crystal orientation can improve magnetic order. Narrow-saw GNRs (5 nm) are anti-ferromagnetic semiconductors, and wide-jagged GNRs (>8 nm) are ferromagnetic semimetallic.
optical properties
graphene's transmission (T) and reflectivity (R) are calculated by formulas T≡ (1 plus 2 G/c)-2 and R≡0.25 to 2 alpha 2T, where G=e2/4ħ is The high frequency conductivity of Diracfemizi in graphene (ħ sh/2 s, h is the Pronk constant, e is the electron charge, c is the speed of light, α is the speed of light, α is e2/ħc≈1/137 is the structural constant that describes coupling between light and relative electrons). The effective structural constant of graphene α 0.14. Graphene has a very small reflectivity of R<0.1%, SLG is only a single atom thick, and its absorbentity can reach (1-T) ≈≈2.3%. For each additional layer of graphene, the film's transmission is reduced by 2.3% and is not affected by the wavelength of the incoming light. Regulating the gate pressure can change the light transmission of graphene. Adjusting the drive voltage adjusts the Fermi energy stage of graphene, based on which a wide-band optical modulator based on graphene's adjustable response is constructed. Because photons are neutral, the light field is difficult to control, but regulating the concentration of carriers in graphene regulates the light field.
thermal properties
carbon atoms in graphene are highly binding and heat can be lost less during transmission, so graphene