Types of Carbon Nanotubes

TYPES OF CARBON NANOTUBES

The nano‒tubes possess atleast two layers, often many more, and ranged in outer diameter from about 3 nm to 30 nm. The bonding in carbon nanotubes is sp² hybridization, 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 (Fig. 3.17) in which a graphene sheet can be rolled into a tube, viz.,

(i) Arm chair structure

(ii) Zig‒zag structure and

(iii) Chiral structure.

Here the armchair structure and zig‒zag structure have a high degree of symmetry. The terms "armchair" and "zig‒zag” refer to the arrangement of hexagons around the circumference. The "chiral" term means that it can exist in two mirror‒related forms.

The internal arrangements of atoms in the structure of a nanotube is as shown in Fig. 3.18 The structure is specified by a vector, (n,m), which defines how the graphene sheet is rolled up. The carbon nanotube can be produced with various values of 'n' and 'm'.

For example, to produce a nanotube with the indices (7, 6), the sheet has to be rolled up in such a way that the atom labelled (0, 0) has to be superimposed on the one labelled (7, 6). It can be seen from the figure that m = 0 for all zig‒zag tubes, while n = m for all armchair tubes.