Graphene is a honeycomb planar film composed of sp2 hybridization of carbon atoms. It is separated from the graphite material and is a two-dimensional crystal with a thickness of one atom. Graphene narrowly refers to a single layer of graphite with a thickness of 0.335 nm. In practice, however, a graphite structure within 10 layers may also be referred to as graphene, and a layer of 10 or more is referred to as a graphite film. The perfect graphene has an ideal two-dimensional crystal structure consisting of a hexagonal lattice with a theoretical specific surface area of up to 2.6×102 m2/g. In addition, the regular hexagonal lattice structure gives graphene excellent electrical conductivity, and the electron mobility at room temperature is as high as 1.5×104 cm 2 /(V.s). The special structure, outstanding thermal conductivity and mechanical properties of graphene have aroused great interest in the scientific community and become a hotspot in materials science research.
Structure of graphene
Graphene refers to a thick single-layer graphite layer with only one atomic scale, which is composed of a honeycomb crystal structure in which sp2 hybridized carbon atoms are closely arranged. The carbon-carbon bond length in graphene is about 0.142 nm. There are three sigma bonds in each crystal lattice, and the connection is very strong, so that a stable hexagon structure is formed. The π bond perpendicular to the crystal plane plays a large role in the process of graphene conduction. Graphene is the basic building block of graphite, carbon nanotubes and fullerenes. It can be regarded as an infinite aromatic molecule. In a single-layer graphene, each carbon atom is rectified by a sp2 hybridization bond with a surrounding carbon atom. Each hexagonal unit is actually similar to a benzene ring, and the carbon atoms contribute an unbonded electron. The thickness of the single-layer graphene is only 0.35 nm, which is about one-hundred thousandth of the diameter of the hair. The structure of graphene is very stable, and the connection between carbon atoms is flexible. When subjected to external forces, the carbon atom will be bent and deformed, so that the carbon atoms do not have to be rearranged to adapt to external forces, thereby ensuring their structural stability.
Properties of graphene
Graphene is an ultra-light material with an areal density of 0.77 mg/m2. Its main properties are: First, it has superior electrical conductivity. Graphene has a higher electron mobility than carbon nanotubes or silicon crystals. Its electron mobility is 100 times that of silicon and can reach 15 000 cm 2 /( V·s ) at room temperature. And its resistivity is much lower than aluminum, copper and silver, is only about 10 to 6 Ω·cm. Second, it has super thermal conductivity. The thermal conductivity of graphene is as high as 5300 W/m•K and it is better than that of carbon nanotubes, which is 10 times that of metals such as copper and aluminum. Third, it has superior mechanical properties. The hardness of graphene exceeds that of diamond, and the breaking strength reaches 100 times that of steel. Fourth, it has super light transmission. The light absorption of graphene is very small, and the light transmittance is as high as 97.7%. Fifth, it has a strong specific surface area. The specific surface area of graphene is 1130 m2 per gram higher than that of ordinary activated carbon, reaching 2630 m 2 /g.
