Biomedical Engineering Reference
In-Depth Information
layers. CNTs are related to graphite because they are broadly sp
2
hybridized. However, graphene layers in CNTs are rolled up to form
seamless hollow tubes; an individual atomic carbon layer forms a single-
walled carbon nanotube (SWCNT), while multiple layers rolled up
concentrically constitute a multi-walled carbon nanotube (MWCNT). A
structure containing pentagons or half fullerenes closes the tube ends. The
interlayer spacing of MWCNTs is close to that of graphite layers (3.35A
˚
)
(Terrones, 2003). The diameter of the innermost tube can be as small as
0.4 nm and up to a large fraction of the outer diameter, which possibly can
reach more than 100 nm (Meyyappan, 2005). SWCNTs are synthesized in
bundles or ropes of up to roughly 100 tubes arranged in a hexagonal array.
SWCNTs are either metallic or semiconducting depending on the chirality
and diameter of nanotubes; armchair nanotubes are metallic, while chiral
and zigzag nanotubes can be metallic or semiconducting (Poole and Owens,
2003). Generally, a ratio of 1:2 is found between metallic and semiconduct-
ing SWCNTs.
7.2.2 Production of carbon nanotubes
CNTs are commonly produced by three methods:
.
electric arc discharge (EAD)
.
laser ablation or evaporation (LAE)
.
chemical vapor deposition (CVD).
An even simpler classification (Shaffer and Sandler, 2007) is into high-
temperature (EAD and LAE) and low-temperature (CVD) synthesis
techniques. The chosen synthesis route and specific parameters control the
quality and yield of CNTs produced. Techniques using high temperature
produce highly crystalline CNTs but are costly and provide low yield due to
the presence of undesirable carbonaceous by-products including fullerenes
and graphitic nanoparticles. Low-temperature synthesis techniques produce
cheap CNTs in large quantities but these CNTs are structurally defective
and often coated with amorphous carbon.
Electric arc discharge
Electric arc discharge was the technique used by Iijima (1991), when CNTs
were characterized for the first time. Typically, a current of 50-100 A, at a
voltage of 20-40 V, is passed through a pair of graphite electrodes of
diameter 6-12mm, separated by a distance of 1-4mm in an inert
atmosphere, usually helium. Carbon vaporizes away from the anode and
arranges itself into MWCNTs, which deposit on the cathode as a soft and
dark black fibrous material (Wilson et al., 2002). Introducing transition
