Carbon nanotube or CNT is not a new term in the present scenario actually it is the allotrope of carbon sharing a cylindrical nanostructure. The length-to-diameter of nanotubes lies in between 132,000,000:1 and have very fascinating properties to be used in nanotechnology, optics, material science, electronics and other fields of science. Due to their extraordinary thermal conductivity, mechanical and electrical properties carbon nanotubes are used as additives for various structural materials for example, in baseball bats, car parts and golf clubs nanotubes form a very tiny fraction of the material. Nanotubes are members of fullerene family which also includes the buckyballs and the ends of these nanotubes may be capped with the hemisphere of buckyballs. Their name has been derived from their long, hollow structure with walls formed by one-atom thick sheets of carbon known as graphene. These sheets are then rolled at specific and dicrete angle and the combination of rolling angle and radius decides the properties of these nanotubes. Nanotubes are either single-walled nanotubes (SWNTs) or multi-walled nanotubes (MWNTs). The particles of nanotubes are held together by van der Waals forces. Applied quantum chemistry specially the orbital hybridization best describes chemical bonding in them. Chemical bonds are chiefly composed of sp2 bonds similar to those occurring in graphite and are stronger than the sp3 bonds found in diamond and alkanes and so are responsible for great strength of these structures.
Historical Background
In 1952, L.V. Radushkevich and L.M. Lukyanovich published clear images of 50 nm tubes made up of carbon in the Soviet Journal of Physical Chemistry but the article failed to arouse interest among the western scientists because it was published in Russian language and access was not open due to cold war. The invention of the transmission electron microscope (TEM) made the visualization of these structures possible. A paper published by Oberlin, Endo and Koyama in 1976 indicated about hollow carbon fibers with nanometer scale diameter by using vapour growth technique. In 1979, John Abrahamson presented evidence of carbon nanotubes in the 14th Biennial Conference on Carbon of Pennsylvania State University.
The whole credit to the current interest in the carbon nanotube goes to the discovery of the buckminsterfullerene C60 and other allied fullerenes in 1985. The discovery that carbon can form other stable structures apart from graphite and diamond compelled the researchers to find new forms of carbon and the result came out in the form of C60 that can be made available in all laboratories in simple arc evaporation apparatus. Sumio Lijima, a Japanese scientist discovered the fullerene related carbon nanaotube by using the simple arc evaporation apparatus in 1991. The tubes were made up of two layers with a diameter ranging from 3-30 nm and closed at both the ends. In 1993 single layered carbon nanotubes were discovered with a diameter of 1-2 nm and can be curved but they failed to create much interest among the researchers as they were structurally imperfect so the researchers are now working to improve the catalytic properties of these nanotubes.
Single walled nanotubes (SWNTs)
Most single walled nanotubes share a diameter close to 1nm with a length million times longer and the structure can be imagined by wrapping one atom thick layer of graphite called graphene into a seamless cylinder. The way by which graphene is wrapped in represented by a pair of indices (n, m) and the integers n and m represent the unit vectors along the two directions in the honey comb crystal lattice of graphene. If m=0 then nanotubes are called as zigzag nanotubes and if n=m then they are called armchair otherwise they are chiral. The SWNTs are very important variety of nanotubes because their properties changes with change in the n and m values and are widely used in the development of the first intermolecular field effect transistors. The price of these nanotubes has declined in the present era.
Multi waled nanotubes (MWNTs)
They consist of multiple rolled layers of graphene are there are two layers that can better define the structure of these nanotubes. The Russian Doll model says that the layers of graphite are arranged in concentric cylinders for example a single walled nanotube within a single walled nanotube. The Parchment model says that a single sheet of graphite is rolled around itself resembling a rolled newspaper. The interlayer distance in these nanotubes is 3.4. The Russian Doll model is generally considered while studying the structure of MWNTs. Double-walled nanotubes (DWNTs) is a special type of nanotube with morphology and properties similar to MWNTs with highly improved resistance against the chemicals.
Torus
A nanotorus is a carbon nanotube bent in the form of a torus and bears many unique properties like magnetic moment 1000 times more. Thermal stability and magnetic moment depends on the radius of the torus as well as the radius of the tube.
Nanobud
Nanobuds are newly created materials made by joining two allotropes of carbon namely carbon nanotubes and fullerenes. In this material the fullerene like buds are covalently bonded to the outer sidewalls of the underlying nanotube. This new material shares the properties of both fullerenes and carbon nanotubes. They are supposed to be good field emitters.
Graphenated carbon nanotubes
They are relatively newly developed hybrid materials combining graphitic foliates grown along the sidewalls of a multiwalled nanotube. Stoner and co-workers have reported that these hybrid materials have enhanced supercapacitor ability.
Peapod
Carbon peapod is a new hybrid material composed of network of fullerene trapped inside a carbon nanotube. It possesses interesting magnetic, heating and irradiating properties.
Cup-stacked carbon nanotubes
They differ from other quasi 1D carbon materials that behave as quasi metallic conductors of electrons. The semiconducting behaviour of these structures is due to the presence of stacking microstructure of graphene layers.
Extreme carbon nanotubes
The longest carbon nanotube was reported in 2009 measuring 18.5 cm grown on Si substrates by chemical vapour deposition method and represent electrically uniform arrays of single walled carbon nanotubes. Cycloparaphenylene was the shortest carbon nantube reported in 2009. The thinnest carbon nanotube is the armchair with a diameter of 3.
Properties
1. Strength
Carbon nanotubes have the strongest tensile strength and elastic modulus among all the materials yet discovered. The tensile strength is due to the presence of sp2 hybridization among the individual carbon atoms. The tensile strength of multi-walled tube was reported to be 63 gigapascals (GPa) in 2000. Further studies carried out in 2008 have found that the shell of these tubes is of 100 gigapascals strength which is in good agreement with the quantum models. Since these tubes have a low density their strength is high. If excessive tensile strain is given of these tubes they undergo plastic deformation which means that they are permanently altered. Although the strength of individual tubes is very high but weak shear interactions between the adjacent shells and tubes result in weakening of the strength of the multi-walled tubes. They are also not strong when compressed. Due to their hollow structure and high aspect ratio they show buckling when kept under torsional or bending stress.
2. Hardness
Standard single-walled nanotubes can tolerate a pressure of about 24GPa without being deformed and can undergo transformation to superhard phase nanotubes. Maximum pressure tolerated under current experimental techniques is 55 GPa. But these superhard nanotubes can collapse at pressures higher than 55 GPa. The bulk modulus of these nanotubes is 462-546 GPa much higher than that of diamond.
3. Kinetic Properties
Multi-walled nanotubes are concentric multiple nanotubes folded within each other and gifted with striking teleoscopic property where the inner tube may slide without friction within its outer shell therefore, creating an rotational bearing. This is perhaps the first true examples of molecular nanotechnology useful in making machines. This property has already been utilized in making world’s smallest rotational motor.
4. Electrical Properties
The symmetry and unique electronic structure of graphene is responsible for providing the carbon naotubes their astonishing electrical properties. Intrinsic superconductivity has been observed in nanotubes but it is a controversial issue in the present context.
5. Wave absorption
The most recently worked properties of the multi-walled carbon nanotubes is their efficiency to show microwave absorption and is the current area of research by the researchers for radar absorbing materials (RAM) so as to provide better strength to the aircraft and military vehicles. The research is under progress where researchers are trying to fill the MWNTs with metals like iron, nickel or cobalt to increase the effectiveness of these tubes for microwave regime and the results have shown improvement in maximum absorption and bandwidth of adequate absorption.
6. Thermal Properties
All the nanotubes are generally believed to be good thermal conductors exhibiting the property of ballistic conduction.
Defects
Crystallographic defect affects the material property of any material and defect is due to the presence of atomic vacancies and such defects can reduce the tensile strength of the material to about 85%. The Strong Wales Defect creates a pentagon and heptagon by the rearrangement of bonds. The tensile strength of the carbon nanotubes is dependent of the weakest segment. Crystallographic defect also affects the electrical properties of the tubes by lowering the conductivity. Crystallograhic defect also affects the thermal conductivity of the tubes resulting in the phonon scattering that reduces the mean free path.
Applications
Nanotubes are widely used in making tips of the atomic force microscopic probes. They are also used in tissue engineering acting as a scaffold for bone growth. Their potential strength helps them to be used as a filling material for increasing the tensile strength of other nanotubes. Their mechanical property helps them to be used in making clothes, sports jackets and space elevators. They are also used in making electrical circuits, cables ad wires.