In 1991, Sumio Iijima noticed novel structures in the byproduct soot created during fullerene
production by electric arc-discharge.1 Investigation revealed the tubular nature of these
interesting structures, now known as carbon nanotubes, which have likely existed for years,
but remained undiscovered up to this point. A carbon nanotube may be described as a
graphene sheet rolled up to form a tube, with half-fullerene caps, for closed tubes. The caps
are fairly reactive due to curvature strain, and are readily destroyed during processing,
yielding open tubes.2 Nanotubes are formed in a range of diameters and may be singlewalled
or multi-walled, depending on the reaction conditions. The single-walled variety exist
as a bundle made up of strands, like a rope, in which each strand is a single-walled nanotube
(SWNT); each bundle contains typically 10 – 50 strands and is 5 – 20 nm in diameter.3 The
diameter of SWNTs is on the order of a nanometer (~ 1.4 nm), and the intertube spacing
within the bundle has been determined to be ~ 0.3 nm, at closest approach.3 Multi-walled
nanotubes (MWNT) are comprised of several graphene sheets rolled up, resulting in
concentric tubes with successively larger radii. Multi-walled nanotubes also aggregate, due to
significant van der Waals forces.