Applications of nanotubes

Unusual electrical properties of nanotubes will make them one of the main materials of nanoelectronics. On their basis prototypes of new elements for computers are made. These elements provide a reduction of devices in comparison with silicon ones by several orders of magnitude. And nanotubes are given an unquestionably leading position among prospective candidates for the place of silicon.

Another use of nanotubes in nanoelectronics is the creation of semiconductor heterostructures, i.e. structures such as "metal / semiconductor" or the junction of two different semiconductors (nanotransistors). To do this, it is necessary to create a structural defect in the nanotube growth process (namely, to replace one of the carbon hexagons with a pentagon) by simply breaking it in the middle in a special way. Then one part of the nanotube will have metallic properties, and the other will have the properties of semiconductors.

Several applications of nanotubes in the computer industry have been developed. Under the influence of the voltage applied to one of the ends of the nanotube, the other end begins to emit electrons that fall on the phosphorescent screen and cause illumination. The resultant grain of the image will be fantastically small: of the order of a micron.

Due to their superior electrical properties carbon nanomaterials such as one-dimensional (1D) carbon nanotubes and two-dimensional (2D) graphene are evolving as significant tools which provide a faster and more power-efficient electronics. In addition, high surface to volume ratio, robust and durable mechanical properties have established them a highly sensitive and lower energy consuming building block for nano sensors. All the carbon nanotubes have an ability to emit electrons in the presence of an electric field. This property has been utilized in the field emission devices like television, computer screens and other instruments which are based on cathode-ray emission. Nanotechnology is facilitating the production of brighter, clearer and more efficient displays for computers and television.

The chemical and physical state of the environment may alter the electrical conductivity of carbon nanotubes and thus carbon nanotubes have been developed as chemical and physical sensors. The efficiency of carbon nanotubes to detect any changes in chemical composition of the environment is three times higher in magnitude than conventional solid state sensors. The other advantage of these electronic chips is that they are smaller in size and can be used at room temperature.

Another example is the use of a nanotube as a needle for a scanning microscope. Usually such a point is an acute sharpened tungsten needle, but according to atomic standards such sharpening is still quite rough. Nanotube is an ideal needle with a diameter of the order of several atoms. Applying a certain voltage, you can pick up the atoms and whole molecules that are on the substrate directly under the needle, and carry them from place to place.

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 800ºC. 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.