ARE CARBON NANOTUBES? Carbon nanotubes are tubular forms of carbon
that can be envisaged as graphitic sheets rolled into cylindrical
form. These nanotubes have diameters in the range of few nanometers
and their lengths are up to several micrometers. Each nanotube
is a single molecule made up of a hexagonal network of covalently
bonded carbon atoms.
nanotubes are of two types: single-walled and multi-walled. A
single-walled carbon nanotube (SWNT) consists of a single grapheme
cylinder, whereas a multi-walled carbon nanotube (MWNT) comprises
several concentric grapheme cylinders. A schematic representation
of a SWNT structure is shown in the figure below.
covalent bonding, unique one-dimensional structure and nanometer
size, together impart unusual properties to the nanotubes. These
properties include exceptionally high tensile strength, high resilience,
electronic properties ranging from metallic to semi-conducting,
the ability to sustain high current densities and high thermal
conductivity. Thus carbon nanotubes could be used as fillers in
super-strong composite materials, as wires and components in nano-electronic
devices, as tips of scanning probe microscopes and in flat panel
displays and gas sensors.
reported the preparation of MWNTs by the arc-discharge of graphite
electrodes in 1991. In 1993, Ijima and Ichihashi at NEC  and
Bethune et al. at IBM  independently reported the preparation
of SWNTs. Today, MWNTs are prepared in large quantities by the
chemical vapor deposition process. SWNTs can be prepared in reasonably
high yields by three techniques: arc-discharge of Ni-Y catalyzed
graphite electrodes, laser ablation of Ni-Co catalyzed graphite
targets and vapor phase paralysis of CO and Fe(CO)5 (HiPCO
process). Carbon nanotube samples are always contaminated with
impurities including amorphous carbon, residual metal catalyst
and graphitic nanoparticles. Thus the purification and chemical
processing of carbon nanotubes remains as a key step in any application.