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Graphene

 

Graphene (born graphene) – a layer of carbon just one atom thick, connected by means of sp ² bonds in a two-dimensional hexagonal lattice. It can be represented as a single plane of graphite, separated from the bulk crystal. It is estimated that graphene has a high mechanical stiffness and good thermal conductivity (~ 1 TPa and ~ 5 × 103 W · m-1 · K-1, respectively).

The high carrier mobility at room temperature, making it a promising material for use in various applications, in particular, as the basis of nanoelectronics and possible replacement of silicon in integrated circuits.

Community of currently available methods for producing graphene is based on mechanical cleavage or flaking layers of graphite. It delivers the highest quality samples with high carrier mobility. This method does not involve the use of large-scale production, as this manual procedure. Another known method – the method of thermal decomposition of silicon carbide substrate is much closer to industrial production. Since graphene was first obtained in 2004 alone, it is still not well understood and is attracting increasing interest. Graphene is not merely a piece of other allotropic modifications of carbon: graphite, diamond – due to the nature of the energy spectrum of the carriers it shows specific, in contrast to the other two-dimensional systems, the electrical properties.

Graphene is a two-dimensional crystal consisting of a single layer of carbon atoms assembled in a hexagonal lattice. His theoretical research began long before the actual receipt of specimens, because of graphene can collect three-dimensional crystal of graphite. Graphene is the basis for the theory of the crystal. Graphite is a semimetal, and as was shown in 1947 P. Vollesom, in the band structure of graphene is also no band gap, and at the points of contact between the valence band and the conduction band energy spectrum of electrons and holes as a linear functions of the wave vector. This kind of spectrum has massless photons and ultrarelativistic particles and neutrinos. It is therefore said that the effective mass of electrons and holes in graphene near the contact zone is zero. But it is worth noting that, despite the similarity of the photons and massless carriers in graphene, there are several important differences that make the carriers in graphene are unique in their physical nature, namely, electrons and holes are fermions, and they are charged. Currently analogues for these massless charged fermions of the known elementary particles do not.

Despite these specific features, the experimental confirmation of these findings is not received before 2005, because it was not possible to create graphene. In addition, even before it was proved theoretically that ideal two free film can not be obtained due to the instability with respect to folding or twisting. Thermal fluctuations lead to the melting of two-dimensional crystal at any finite temperature.
Interest in graphene appeared again after the discovery of carbon nanotubes, as all the original theory was based on a simple model of the nanotube as the sweep of the cylinder. Therefore, the theory for graphene attached to the nanotubes well established.

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Attempts to obtain graphene attached to another material, began with experiments using a pencil, and continued using the atomic force microscope for the mechanical removal of layers of graphite, but do not succeed. The use of graphite embedded (intercalated graphite) in the interlayer space of foreign atoms (used to increase the distance between adjacent layers and their cleavage) also did not lead to the result.
In 2004, the Russian and British scientists published a paper in the journal Science, which reported obtaining graphene on oxidized silicon substrate. Thus, the stabilization of the two-dimensional film was achieved thanks to the connection with a thin layer of SiO2 insulator similar to the thin films grown by MBE. Was first measured conductivity, the Shubnikov – de Haas effect, Hall effect for samples consisting of carbon films with atomic thickness.

Exfoliation method is fairly simple and flexible because it allows you to work with all of the layered crystal, that is, those materials which appear to be weak (in comparison with the forces in the plane) linked layers of two-dimensional crystals. In a subsequent study, the authors have shown that it can be used for other two-dimensional crystals: BN, MoS2, NbSe2, Bi2Sr2CaCu2Ox.