From: Brian D Williams (talon57@well.com)
Date: Thu Sep 10 1998 - 08:48:32 MDT
4. ON THE FUTURE OF CARBON NANOTUBE CHEMISTRY
Fullerenes are large molecules composed entirely of carbon, with
the chemical formula C(sub n), where n is any even number from 32
to over 100. They apparently have the structure of a hollow
spheroidal cage with a surface network of carbon atoms connected
in hexagonal and pentagonal rings. Carbon nanotubes are similar
to fullerenes, except their shape is tubular. They were first
discovered by Sumio Iijima (NEC Laboratories, JP) in 1991, they
come in both multi-walled and single-walled versions, and they
have diameters of the order of 10 to 30 nanometers.
.... ... Robert F. Service (*Science*, US) reviews the reasons for
current excitement concerning nanotubes and recent research in
the field, and makes the following points: 1) Carbon nanotubes
are stronger than steel; lightweight; able to withstand repeated
bending, buckling, and twisting; can conduct electricity as well
as copper, or semiconduct like silicon; and they transport heat
better than any other known material. 2) The possible important
applications of carbon nanotubes include superstrong cables,
wires for microscale electronic devices, charge storage devices
in batteries, and microscale electron guns for flat-screen
television. 3) The key to the potential applications of carbon
nanotubes lies in the unique structure of carbon nanotubes and in
the possible defects in that structure that confer special
properties. The structure itself depends on the unique properties
of carbon. Under intense pressure, carbon atoms form bonds with 4
neighboring atoms to produce diamond. Under special conditions,
however, sheet-like carbon structures involving 3-bonded carbon
can be formed (graphite), and with still further specialization
of conditions, these sheets organize into spherical (fullerene)
or tubular (nanotube) arrays. One critical aspect of the 3-bonded
carbon array is that the nature of the bonding produces a cloud
of unpaired electrons floating above and below the sheet, and
these electrons are mobile enough to make the material a good
electrical conductor. The importance of defects in this context
is that specifically designed defects in the carbon arrays can
alter the physical properties of these arrays, including the
electrical properties -- all in a controlled manner. It has
recently been reported, for example, that single-walled carbon
nanotubes can function not only as conductors but also as
semiconductors, depending on the conditions, which is of
considerable significance for the possible use of carbon
nanotubes as semiconductor switches in computer devices. It has
also been possible to form hybrid carbon nanotubes such that one
end of the tube behaves as a metallic conductor while the other
end of the tube behaves as a semiconductor, and such tubes have
the potential to act as molecular diodes, devices that allow
electric current to flow in one direction, from a semiconductor
to a metal but not in reverse. 4) Perhaps of greatest interest is
the recent demonstration by Heer et al (Georgia Institute of
Technology, US), confirming theoretical predictions, that carbon
nanotubes can carry current at room temperature with essentially
no resistance. The mechanism for this involves so-called
"ballistic transport", which refers to the passage of electrons
through a semiconductor whose length is less than the mean free
path of electrons in the system, so that most of the electrons
pass through the semiconductor without scattering. 5) At the
present time, one critical aspect of carbon nanotube research is
that before the mentioned potential applications can be achieved,
the technology of carbon nanotube production must be improved so
that carbon nanotubes are available in bulk quantities for
materials research and development. At the present time, single-
walled carbon nanotubes are commercially available for
approximately US$200 per gram. This price needs to be severely
reduced by technological advances before these new structures can
be fully developed for practical use.
QY: Robert F. Service <science_editors@aaas.org>
(Science 14 Aug 98 281:940) (Science-Week 11 Sep 98)
-------------------
Brian
Member,Extropy Institute
Current reading: "Blue Mars" by Kim Stanley Robinson
"Ultimate Spanish" Basic/Intermediate Living Language/Random House
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