Carbon nanotubes (CNT)

CARBON NANOTUBES (CNT)

A group of nanostructures with large potential applications carbon nanotubes. The hexagonal lattice of carbon is simply graphite. A single layer of graphite is called graphene. (Fig. 5.22)

The carbon nanotube (CNT) consists of a graphene layer which is rolled up into a cylindrical shape as shown in fig. 5.22.

When the graphene layer is rolled, the structure is tube like and it is a single molecule. Each single molecule nanotube is made up of a hexagonal network of covalently bonded carbon atoms.

In some cases, the hexagons are arranged in a spiral form. The layer appears like a rolled-up chicken wire (net having a large hexagonal mesh) with carbon atoms at the apexes of the hexagon as shown in fig. 5.23.

The carbon nanotubes are hollow cylinders of extremely thin diameter, 10,000 times smaller than a human hair.

Structures of CNT

The CNTs have many structures on the basis of their length, type of spiral and number of layers. Their electrical properties, depend on their structure and they act as both a metal or a semiconductor.

There are a variety of structures of carbon nanotubes with different properties.

Types of CNT structures

Three types of nanotube structures are considered by rolling a graphite sheet with different orientations about the axis.

They are

(i) Armchair structure

(ii) Zig-zag structure

(iii) Chiral structure

Armchair structure

When the axis of the tube parallel to C-C bonds of the carbon hexagons, the structure shown in fig. 5.24 (a) is obtained. It is referred as "armchair" structure.

Zig-zag and Chiral structure

The tubes sketched in figs. 5.24 (b) and 5.24 (c), referred as zig-zag and chiral structure. They are formed by rolling graphene sheet such that the axis of the tube is not parallel to C-C bonds.

Zig-zag structure consists of tube axis perpendicular to C-C bonds.

In chiral structure, C-C bond is inclined towards the axis of the tube.

Generally, nanotubes are closed at both ends with half of fullerene structure.

Classification of CNT

Based on the number of layers, the carbon nanotubes are classified as

(i) Single-walled (SWNTs)

(ii) Multi-walled (MWNTs).

In multi-walled nanotubes, more than one CNTs are coaxially arranged.