Preparation and application of graphene

Graphene (Graphene) as a typical representative of carbon nanomaterials, with its excellent crystal structure and electrical performance and has attracted widespread attention and great interest of scientists. In this paper, on the one hand, main preparation methods of graphene and introduces the principle, on the other hand, in view of the graphene in the field of nanoscale electronic devices, and many other widely used to make an overview. Low cost mass preparation of graphene materials is of great significance to the research and application of graphene.

Carbon nanomaterials is the research hot spot of popular in the field of new materials, including carbon nanotubes (CNT Carbon Nanotube) and Graphene (Graphene), as well as fullerenes (Fullerene) is a typical representative of carbon nanomaterials. Because they have strong and unique optical, electrical and mechanical properties, thus has broad application prospect. In the three typical carbon nanomaterials, two-dimensional graphene is a one dimensional carbon nanotubes and zero dimension of the basic unit of the fullerene (figure 1), has the extremely good crystal structure and electrical performance.

Graphene (Graphene) since 2004 was Geim and other reports, a professor at the university of Manchester, England, with its peculiar performance has attracted wide attention and interest of scientists. Single-layer graphene exist in two-dimensional crystal structure, the thickness of only 0.334 nm, it is the basic unit of the other dimensions of the carbon material, it can be wrapped to form a zero dimensional hungry fullerenes, roll up form one dimensional carbon nanotubes, graphite layer upon layer to form a three-dimensional. Graphene is a kind of no energy gap of the semiconductor, has more than 100 times the silicon carrier mobility (2 x 105 cm2 / V), at room temperature with micron grade free path and greater coherence length, so the graphene is ideal material of nanometer circuit. Graphene has good thermal conductivity of 3000 w/(m, K), high strength (110 GPA) and large specific surface area (2630 m2 / g).These excellent performance make graphene in nanoscale electronic devices, gas sensors, energy storage and composite materials, and other fields have a bright application prospect.

1, the method of preparation of graphene

At present, the main preparation methods of graphene are mechanical method, the thermal decomposition of graphite oxide reduction, growth of SiC method, chemical deposition method, extension method, etc.

1.1, micro mechanical stripping method

In 2004, Geim first has the micro mechanical stripping method, such as high success from directional thermal pyrolysis graphite (highlyoriented pyrolytic graphite) dissection and observation to the single-layer graphene. Geim team preparation of monolayer graphene one of the biggest width can be up to 10 microns. The main method is to use oxygen plasma beam in high oriented pyrolytic graphite (HOPG) surface etching out 20 mu m ~ 2 mm wide, deep groove face 5 microns, and suppress them in with photoresist SiO2 / Si substrate, after roasting, repeatedly with transparent tape stripping out redundant graphite flake, residual graphite flake on Si wafer soaked in acetone, and in a lot of water and propanol in ultrasonic cleaning, remove most of the thicker lamella after get the thickness less than 10 nm layer, the thin slice layer relies mainly on the van der Waals force or capillary force and SiO2, integrated in atomic force microscope finally selected only a few single atomic layer thick graphene layers, this method can get the width of micron size of graphene films, but not easy to get independent pieces of graphene single atomic layer thickness, the yield is low, so is not suitable for large-scale production and application.

Then he will Meyer microcomputer stripping method, Si wafer containing monolayer graphene placed in a after etching of metal rack, the Si wafer with acid corrosion, the successful preparation of impending monolayer graphene supported by metal stents, its morphology by tem observation. They found they monolayer graphene is not a flat plane, but the plane has a certain height (5 ~ 10 nm) fold, fold monolayer graphene surface was significantly more than double the graphene, and with the increase of graphene layers fold degree is more and more small, this may be due to single layer graphene to reduce its surface energy, from two-dimensional to three-dimensional morphology transformation, thus can be speculated that fold on the surface of the graphene may be a necessary condition for the existence of two-dimensional graphene and graphene on the surface of the fold on its performance needs further exploration. Micro mechanical stripping method with high quality graphene can be prepared, but the shortcomings of low production rate and high cost, did not meet the requirements of industrialization and mass production, at present only as a small-scale laboratory preparation.

1.2, chemical vapor deposition method

Chemical vapor deposition method (Chemical vapored position) is the most widely used a method of mass industrial preparation of semiconductor thin film materials. CVD method is refers to the material in the gas phase reaction conditions are issued by biochemical reaction, generate solids deposition on the surface of the solid substrate that heating, which made solid material process. Its production process is very perfect, researchers have become a way of preparation of graphene.

Chemical vapor deposition (CVD) method provides a method of effective control of graphene can be different from the preparation of CNTs, the graphene prepared by CVD method need not granular catalyst, it is base on the plane (such as a metal film, metal single crystal, etc.) at high temperature decomposable precursor (such as methane, ethylene) atmosphere, make through high temperature annealing carbon deposition in the basal surface graphene, finally by chemical etching method to remove the metal base after obtains the independent pieces of graphene. By selecting the base type, growth temperature, the precursor flow parameters such as the growth of the graphene regulation (such as growth rate, thickness, size, etc.), this method has been successfully prepared area of single layer or multilayer graphene square centimeter level, its biggest advantage is that can be prepared with larger area of the graphene films.

1.3, and the method of epitaxial growth

This method is generally by heating 6 h - SiC surface of single crystal, stripping Si (0001) atomic graphene is prepared. First 6 h - SiC single crystal surface oxidation or H2 etching pretreatment, under the ultra-high vacuum (1.33 x - 8-10 pa) get rid of the surface oxide heated to 1000 ℃, by auger electron spectroscopy (Augerelectronspectroscopy) determined to oxide has been completely removed, the sample and then heating to 1250 ~ 1450 ℃ and temperature 10 ~ 20 min, graphene was prepared by the slice thickness is mainly decided by the temperature of this step, this approach to the preparation of the 1 ~ 2 carbon atom thick layer of graphene, but as a result of SiC crystal surface structure is relatively complex, difficult to get the thickness of the large area, have the graphene. Berger respectively using the method such as the preparation of the monolayer and multilayer graphene and study its performance. Compared with the mechanical stripping method of graphene, epitaxial growth method of preparation of graphene showed higher carrier mobility and other features, but it cannot detect the quantum hall effect.

1.4, electrochemical methods

Liu was prepared by the method of electrochemical oxidation graphite rod such as graphene. They will both high purity graphite rod in the aqueous solution containing the ionic liquids in parallel, the control voltage in 10 to 20 v, 30 min after the anode graphite rod corrosion, cation radicals formed in the cathodic reduction of ionic liquids, and graphene in PI electronic union, form ionic liquids functionalization of graphene films, finally with anhydrous ethanol washing black deposits in electrolytic cell, dry 2 h can get graphene under 60 ℃.This method can step in the preparation of ionic liquid functionalization of graphene, but the preparation of graphene layer is greater than the single atomic layer thickness.

1.5, organic synthesis

Qian with definite structure which was prepared by organic synthesis such as graphene nanobelts and zero defect. They choose four bromine) imide (tetrabromo - perylenebisimides) as the monomer, the compounds in cuprous iodide and L - proline can occur under the activation of many molecules, the coupling reaction between the different scales and) imide, implements the graphene nano containing imide group with efficient chemical synthesis; They also by high performance liquid separation out of the two and three) imide isomer, combined with the theoretical calculation to further illustrate their structure.

2, the application of graphene

Graphene have excellent electron transport, optical coupling, electromagnetic, thermal and mechanical properties, so in nanoscale electronic devices, high-performance liquid crystal display materials, solar energy battery, field emission materials, gas sensors and energy storage, and other fields have a wide range of applications.

2.1, transparent electrode

Industry has been commercialized transparent membrane material is indium tin oxide (ITO), due to the content of indium element on the earth is limited, the price is expensive, especially the toxicity is very big, make its application is restricted. As new star of carbonaceous materials, graphene with low dimension and the penetration under the condition of low density can form conductive network is considered to be the characteristics of indium tin oxide of alternative materials, preparation of graphene to the advantages of simple process, low cost paving the way for its commercialization. Mullen team by dip coating deposited by thermal annealing reduction of graphene, thin film resistor of 900 Ω, light transmittance was 70%, the film was made into dye-sensitised solar cells positive, energy conversion efficiency of solar cell is 0.26%.In 2009, the team using acetylene reducing gas and carbon source, using high temperature reducing method preparation of the high conductivity (1425 s/cm) of graphene, as conductive glass replacement for graphene materials.

2.2, sensors,

Electrochemical biosensor technology is a combination of information technology and biotechnology, involving cross subjects such as chemistry, biology, physics, and electronics. Coming of graphene, the researchers found that graphene provides a two-dimensional electronic transmission environment and the fast heterogeneous electron transfer in edge part, which makes it become the ideal material of electrochemical biosensors. Chen, adopt the method of low temperature thermal annealing such as electrode materials, preparation of graphene as a sensor to detect low concentration NO2 at room temperature, the author thinks that if to improve the quality of graphene, will improve the sensitivity of sensors for gas detection. Graphene in sensor showed the potential of different from other materials, making it more and more medical attention, the graphene is also used to detection of dopamine on medicine, glucose, etc.

2.3, super capacitor

Super capacitor is a highly efficient storage and transmission of energy system, it has big power density, large capacity, long service life, economic advantages such as environmental protection, is widely used in various places of power supply. Graphene has high specific surface area and high conductivity, unlike porous carbon electrode materials depend on hole distribution, which makes it become the most promising electrode materials. Chen et al. Preparation of electrode materials for graphene supercapacitor power density of 10 kw/kg, the energy density of 28.5 Wh/kg, the maximum specific capacitance is 205 f/g, and after a 1200 - cycle recharge test still retain 90% of the specific capacitance, have longer cycle life. Graphene in super capacitor potential should receive more attention of researchers.

2.4, composite materials

Graphene unique physical, chemical and mechanical can offer the motive force for the development of composite materials, is expected to open up many new application fields, such as a new type of conductive polymer materials, multi-functional polymer composite materials and high strength of porous ceramic materials. Fan using graphene, such as high specific surface area and high electron mobility, the preparation of supported by graphene materials of polyaniline graphene composites, the compound has high specific capacitance (1046 f/g) is greater than that of pure polyaniline than capacitance 115 f/g. The addition of graphene improved the versatility of composite materials and composite materials processing performance and so on, for the application of composite material provides a wide field.

3, conclusion

Heald above knowable, graphene as a new type of two-dimensional carbon material, has excellent electron transport, optical coupling, electromagnetic, thermal and mechanical properties, so in nanoscale electronic devices, high-performance liquid crystal display materials, solar cells, gas sensors, field emission materials and energy storage, and other fields have broad application, and thus become a research hotspot at home and abroad. Low cost mass preparation of graphene materials is of great significance to the research and application of graphene.

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