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Structure of Graphene

wallpapers News 2021-05-14
Graphene is a planar film composed of carbon atoms with sp2 hybrid orbitals in a hexagonal honeycombed lattice. It is a two-dimensional material with only one atom layer thickness. As shown in Figure 1.1, the primal cells of graphene are defined by lattice vectors A1 and A2. Each primal cell contains two atoms, located on the lattice of A and B, respectively. The three electrons in the outer layer of C atom are hybridized by sp2 to form a strong σ bond (blue). The Angle between two adjacent bonds is 120°. The fourth electron is common, forming a weak π bond (purple). The carbon-carbon bond length of graphene is about 0.142nm, and there are three σ bonds in each lattice. The p orbitals of all carbon atoms are perpendicular to the sp2 hybrid plane and form a delocalized π bond in a side-by-side manner, which runs through the whole graphene.
Properties of graphene
1 Mechanical properties
In the two-dimensional plane of graphene, each carbon atom is connected to the three adjacent carbon atoms by a sigma bond. The Angle between the adjacent two bonds is 120°, and the bond length is about 0.142nm. These C-C bonds make graphene have good structural rigidity. Graphene is the strongest material known in the world with an intrinsic (breaking) strength of 130GPa, more than 100 times the Young's (tensile) modulus of 1100GPa of steel. Such thin film materials with high strength and light weight are expected to be used in many fields such as aerospace.
2 Electrical characteristics
There are three σ bonds in each lattice of graphene, and the p orbitals of all carbon atoms are perpendicular to the sp2 hybrid plane and form a delocalized π bond that runs through the whole graphene in a side-by-side manner. The unique structure of graphene enables it to have room temperature half-integer quantum Hall effect, bipolar electric field effect, superconductivity, high carrier rate and other excellent electrical properties. The carrier rate can reach 1.5×10-1. 6m/s at room temperature. Is 1/300 of the speed of light) to cross the forbidden band into the higher energy empty band, empty band after the existence of electrons become a conduction band, valence band after the absence of an electron to form a positive charge vacancy, become a hole. The electrons in the conduction band and the holes in the valence band are called electron-hole pairs, then the electrons and holes can move freely and become free carriers. Under the action of external electric field, they generate directional motion and form macroscopic current, which are electron conduction and hole conduction respectively.
The presence of two carbon atoms per unit lattice of graphene results in two equivalent cone-shaped intersections (K and K ') in each Brillouin region, near which the energy is linearly related to the wave vector. Thus, the effective mass of both the electrons and the holes in graphene is zero. All the electrons and the holes are called Dirac fermions. The intersection point is the Dirac point, near which the energy is zero, and the band gap (band gap) of ancient graphene is zero. Graphene's unique carrier properties and the mass free Dirac fermion property allow it to observe Hall effects and abnormal half-integer quantum Hall effects at room temperature (when a current passes through a conductor perpendicular to the external magnetic field, a potential difference appears at the two ends of the conductor perpendicular to the direction of the magnetic field and the current). It shows its unique carrier characteristics and excellent electrical properties.
The measured carrier mobility of graphene at room temperature reaches 15000cm2/V·s(electron density 100e4h-5Ω1 0.5π2≈1-eεhc=0α0137, e is the charge of photon, C is the speed of light αα8~104 cm2/Vs), which makes graphene have a very broad prospect in field effect transistor field. However, due to the fact that graphene is a zero-band-gap structure, it is impossible to achieve the switch state of the device, so the switch ratio is very low, which to some extent impedes the application of graphene.
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