Graphene

Graphene is a two-dimensional allotropic form of carbon. The atoms, combined into a hexagonal crystal lattice, form a layer with a thickness of one atom (Fig. 8). Graphene was opened in 2004 by A. Geim and K. Novoselov. For the discovery of graphene Geim and Novoselov in 2010 received the Nobel Prize in Physics.

Fig. 8. The structure of graphene

The graphene has aroused an enormous interest, because it has in a single material a series of very remarkable properties:

Graphene has very high strength. Its sheet with area of one square meter and a thickness of one atom can hold an object weighing 4 kilograms. This is really surprising, if you think that the graphene sheet would not weigh more than 1 mg!

Graphene is a material with very high conductivity of electricity and heat, which makes it ideal for use in various electronic devices, especially if we recall its flexibility and full optical transparency.

Chemistry of graphene is poorly studied. The reactivity of graphene is determined by the presence in it of an extended polyaromatic π-system and terminal coordinatively unsaturated C atoms. The latter are usually associated with -OH or (more rarely) -COOH groups whose properties differ little from phenols and aromatic carboxylic acids. The easily polarized π-graphene system is equally active both with respect to electrophilic and nucleophilic reagents (Fig. 9).

Fig. 9. The scheme of interaction of graphene with nucleophile and electrophilic compounds.

An example of the active π-π interaction of graphene is fixed in the formation of supramolecular complexes with porphyrin derivatives. It is believed that the graphene flakes are negatively charged, therefore, in the formation of the supramolecular ensemble, along with the π-π interaction, there is an electrostatic interaction with a flat positively charged porphyrin cycle.

At the same time, it is considered that bromine and iodine in pairs are only sorbed on the surface of graphene without forming a bond with the C atoms. Nevertheless, fluoridation is possible to occur. Then hydrogen atoms are substituted by fluorine atoms. With this reaction is possible to obtain a new material with new properties for application in the electronic industry.

As a polyaromatic system, graphene also adds other active reagents with the formation of covalent bonds. The addition of hydrogen locally disrupts the π-system of graphene. The process is reversible: holding the graphane at 450 °C for 24 hours leads to dehydrogenation of graphane and the restoration of the π-system graphene authors succeeded in obtaining "pure" graphene by annealing graphane at a temperature of 800 ºC. Based on these experiments, it is believed that graphene can be used as a material for hydrogen storage. (Graphane has a monolayer structure like that of graphene, with the difference that the carbon atoms, in addition to being linked to each other, are also to hydrogen atoms located on both sides of the layer). Unlike graphene, graphane does not conduct electric current. The bonds with the hydrogen 'tie' the electrons responsible for the good electrical conductivity of the graphane, turning it into an insulator. However, graphane maintains the good mechanical properties of its predecessor: very good mechanical strength, high density and flexibility).

Graphene is highly reactive material and its chemistry is not sufficiently studied. It is believed that by chemical modification it will be possible to change the electrophysical characteristics of graphene - to convert from a conductor to a semiconductor and to change the width of the forbidden band of the latter. One of the problems with the graphene monolayer is the "absence of a forbidden band". It is a very good conductor but, unlike other materials, it has no banned band, which is the one that allows to interrupt the entire flow of current.

One of the most promising ways of obtaining materials based on graphene is to create a uniform dispersion of graphene in organic solvents, which can be further used to produce macroscopic materials based on graphene. Like any nanoobject, graphene is characterized by high surface energy.