Fullerene Nanotubes: C1,000,000 and Beyond
Some unusual new molecules—long, hollow fibers with tantalizing electronic and mechanical properties—have joined diamonds and graphite in the carbon family
Nanotubes owe their discovery to Sumio Iijima of NEC Corporation.
Iijima's samples were created by a direct-current arc discharge
between carbon electrodes immersed in a noble gas. A similar
apparatus had been used by Wolfgang Kratschmer of the Max Planck
Institute for Nuclear Physics and Donald Huffman of the University
of Arizona to mass-produce fullerenes.
The electric arc was used by Roger Bacon in the early 1960s to make
"thick" carbon whiskers, and one can speculate that the
nanotube discovery was a matter of looking more closely at the
smallest products hidden in the soot. Iijima himself suggests that
nanotubes may have been formed in those old experiments, but Bacon
lacked the high-power microscope required to see them. Although
various fullerenes can be produced by different ways of vaporizing
carbon, followed by condensation in tiny clusters, the presence of
an electric field in the arc discharge seems to promote the growth
of the long tubules. Indeed, the nanotubes form only where the
current flows, on the larger negative electrode. The typical rate of
deposit is about a millimeter per minute at a current and voltage in
the range of 100 amperes and 20 volts respectively, which maintains
a high temperature of 2,000-3,000 degrees Celsius.
A year later, quite by chance, Thomas Ebbesen and P. M. Ajayan at
NEC found a way to produce nanotubes in higher yields and make them
available for studies by different techniques. Subsequently they
found a way to purify them. An addition of a small amount of
transition-metal powder (cobalt, nickel or iron) favors the growth
of so-called single-walled nanotubes, a fact independently noticed
by Donald Bethune at IBM Corporation and Iijima.
Metal clearly serves as a catalyst, preventing the growing tubular
structure from wrapping around and closing into a smaller fullerene
cage. The presence of a catalyst also allows one to lower the
temperature. Without such cooling the arc is just too hot a place:
Nanotubes coalesce and merge like the foam bubbles in a glass of
beer. Condensation of a laser-vaporized carbon-catalyst mixture at a
lower temperature (1,200 degrees Celsius in an oven-heated quartz
tube) allowed a recent breakthrough at Rice University.
Single-walled nanotubes now can be produced in yield proportions of
more than 70 percent. Moreover, these nanotubes self-organize into
bundles--ropes more than one-tenth of a millimeter long that look
very promising for engineering applications. It is in this context,
connecting dreams with the strides of progress, that we discuss the
properties of nanotubes.