Carbon nanotubes, long thin cylinders of carbon, were discovered in 1991 by Iijima. Carbon nanotubes (CNTs) are allotropes of carbon which are members of the fullerene structural family, which also includes the spherical buckyballs. These are large macromolecules which are unique for there size, shape and remarkable physical properties.
The nature of the bonding of a nanotube is described by applied quantum chemistry, specifically, orbital hybridization. The chemical bonding of nanotubes is composed entirely of sp2 bonds, similar to those of graphite. This bonding structure, which is stronger than the sp3 bonds found in diamond, provides the molecules with their unique strength. Nanotubes naturally align themselves into “ropes” held together by Van der Waals forces. Under high pressure, nanotubes can merge together, trading some sp² bonds for sp³ bonds, giving the possibility of producing strong, unlimited-length wires through high-pressure nanotube linking.
HISTORY: The discovery that carbon could form stable, ordered structures other than graphite and diamond stimulated researchers worldwide to search for other new forms of carbon. The search was given new impetus when it was shown in 1990 that carbon-60(buckminister fullerence) could be produced in a simple arc-evaporation apparatus readily available in all laboratories. It was using such a evaporator that the Japanese scientist Sumio Iijima discovered fullerence-related carbon nanotubes in 1991. The tubes contained atleast two layers, often many more, and ranged in outer diameter from about 3nm to 30nm.
In 1993, a new class of carbon nanotubes was discovered, with just a single layer. These single-walled nanotubes are generally narrower than the multiwalled tubes, with diameters typically in the range 1-2 nm, and tend to be curved rather than straight. It was soon established that these new fibers had a range of exceptional properties, and this sparked off an explosion of research in to carbon nanotubes. It is important to note, however, that nano scale tubes of carbon produced catalytically, had been known for many years before Iijimas discovery. The main reason why these early tubes did not excite wide interest is that they were structurally rarther imperfect, so
did not have particularly interesting properties. Recent reseach has focused on improving the quality of catalytically-produced nanotubes.
STRUCTURE: The bonding in carbon nanotubes is sp², with each atom joined to three neighbours, as in graphite. The tubes can therefore be considered as rolled-up graphene sheets (graphene is an individual graphite layer). There are three distinct ways in which a graphene sheet can be rolled into a tube, as shown below.
The first two of these, known as armchair‚¬ (fig.(b)) and zig-zag‚¬ (fig.(c)) have a high degree of symmetry. The terms “armchair” and “zig-zag” refer to the arrangement of hexagons around the circumference. The third class of tube, which in practice is the most common, is known as chiral, meaning that it can exist in two mirror-related forms. An example of a chiral nanotube is as shown in fig.(d).
TYPES OF CARBON NANOTUBES
a) SINGLE-WALLED CNTs
Most single-walled nanotubes(SWNT) have a diameter close to 1nm, with a tube length that can be many thousands of times longer.
b) MULTI-WALLED CNTs:
c) FULLERITE:
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