The discovery of carbon nanotubes
(CNTs) in 1991 has stimulated intensive research to characterize their
structure and to determine their physical properties both by direct measurement
and through predictive methods using modeling techniques. Many of their
fundamental and remarkable properties are now well-known, and their
exploitation in a wide range of applications forms a large part of the research
effort now in progress. For example, their electronic properties have already
had a significant impact on field emission applications in various electronic
devices, while their geometry offers a new and exciting means of precision-controlled
drug delivery. However, it is their extraordinary mechanical properties (exceptionally
high tensile strength and stiffness) that has aroused interest and promoted
research into the fabrication of nanotube composite materials. The credit for
discovering multi-walled carbon nanotubes is given to Sumio Iijima, who in 1991
reported on carbon microtubulii found as by-products in the Krätschmer
electronic arc discharge reactor used for fullerene synthesis. Multi-walled
carbon nanotubes can be distinguished clearly from single-walled and
double-walled carbon nanotubes based on their larger diameter and their Raman
spectrum. The distinction between MWCNTs and carbon nanofibers relies on the
well-defined structure of carbon nanotubes: any elongated carbonaceous object
with diameter below ∼500 nm that does not feature a carbon nanotube structure
is a carbon nanofiber (CNF). Multi-walled carbon nanotubes (MWCNTs) are
elongated hollow cylindrical nanoobjects made of sp2 carbon. Their diameter is
3–30 nm and they can grow several cm long, thus their aspect ratio can vary
between ten and ten million. The wall thickness of a MWCNT is fairly
constant along the axis, and therefore the inner channel is straight. This
channel is not directly accessible from the outside because the ends of perfect
MWCNTs are capped by half fullerene spheres; however, it can be accessed by
opening the nanotube using, e.g., oxidation, milling or ion beam treatment.
