Carbon nanotubes (CNTs) are a family of long, cylindrical, hollow molecules of pure carbon with a structure that can have a length-to-diameter ratio greater than 1,000,000. The properties of CNTs make them potentially useful in different applications in nanotechnology, electronics, optics, medicine. Some nanotubes are the strongest, stiffest fibers known, comparable in stiffness to diamond. Their thermal conductivity is arguably equal or greater than diamond, the best thermal conductor. Other species are among the best electrical conductors, better than even copper or the best-performing semiconducting material at room temperature.
Carbon nanotubes have been accidentally produced and observed under a
variety of conditions several times as far back as 1952, when
L.V.Radushkevich and V.M.Lukyanovich published clear images of 50
nanometer diameter carbon tubes in the Soviet Journal of Physical
Chemistry. It is likely that CNTs were produced before this date, but
they couldn’t be observed without Transmission Electron Microscope or
Scanned Probe Microscopes (STM and AFM). Several pages were published by
Japan (Oberlin, Endo, and Koyama, 1976) and USA (Abrahamson, 1979)
scientists and showed hollow carbon fibres with nanometer-scale
diameters and one or several walls of graphene.
Nanotube research accelerated greatly following the independent
discoveries by Bethune at IBM and Iijima at NEC of single-walled carbon
nanotubes (1991) and methods to specifically produce them by adding
transition-metal catalysts to the carbon in an arc discharge. The arc
discharge technique was well-known to produce the famed Buckminster
fullerene on a preparative scale, and these results appeared to extend
the run of accidental discoveries relating to fullerenes.
An ideal single-wall carbon nanotube (SWCNT) can be represented as a
strip of a graphitic plane wrapped in the form of a cylinder. This can
be done in several different directions and different diameters, so a
pair of integer indexes (n,m) is used to uniquely identify a type of
nanotube and reflects its geometrical structure. The numbers (n,m)
determine the radius of nanotube, its chiral angle and all other
properties. There are theoretically an infinite number of carbon
nanotube species with indices (n,m). However, in practice the diameter
of the nanotubes is limited by physical constraints (e.g. (1,1) is
impossible), so only several hundred species are created.
Nanotubes where the two indices are equal (n=m) are called armchair
nanotubes (Figure 1), and those in which index m is zero (m=0) are
referred to as zigzag nanotubes (Figure 2). The armchair family of
nanotubes are inherently metallic and zigzag family of tubes are
inherently semiconducting. These two families are most easy to
illustrate and calculate. All other types are referred to as chiral
nanotubes (Figure 3).
Because there are hundreds of species of nanotubes, it's important to
note that properties can vary greatly among them. Additionally, although
single-walled carbon nanotubes (SWCNTs) are the simplest, least-messy
case, many methods of production result in the concentric
nanotubes-within-nanotube case known as multi-walled carbon nanotubes
(MWCNTs). Many methods produce predominantly MWCNTs, which are less
desirable for many applications because they combine different types of
nanotubes. For example, a mixture of metallic and semiconducting tubes
is neither an ideal semiconductor nor an ideal metal. The mixture of
species with differing properties is a major obstacle to the application
of carbon nanotubes and efforts to selectively synthesize, destroy or
separate certain species or sub-types are ongoing.
Scientifically, their attractive properties, combined with their simple
structure and small diameters makes them ideal one-dimensional test
subjects for theories and hypotheses of electrical and thermal
conduction at the quantum scale. As such, they enabled many experiments
which would otherwise have been far more challenging or impossible, and
they became an area of intense research in condensed matter physics and
materials science. While understanding the origin of their properties
requires familiarity with concepts of quantum physics, the calculations
are simple enough to serve as a good introductory example to that field.
Technologically, carbon nanotubes are a good example of emerging
bottom-up fabrication in contrast to traditional top-down fabrication.
Developing techniques to define smaller and smaller transitors
(currently 45nm) onto ever-larger silicon crystal wafers has been the
focus of microelectronics fabrication development for decades. In
contrast, creating carbon nanotubes for electronics is more akin to
chemistry or baking : The appropriate selection of ingredients
(substrate, catalyst particles and gases) and temperature results in
self-assembled arrays of well-aligned, mostly semiconducting carbon
nanotubes with diameters well under 2nm (Liu, Nano Letters, 1/20/09).
With further development, carbon nanotubes could potentially be
integrated into electronics as heat dissipators, electrical vias,
transistors, diodes, capacitors or could be formed into brand-new kinds
As mechanically strong and electrically conducting fibers, nanotubes can
be mixed into polymers to increase their strength or conductivity. The
main obstacle in the use of carbon nanotubes as fiber reinforcements is
their smooth and inert surface.