| Atlanta
(February 10, 2005) — Nanotubes are ubiquitous in the
world of science. Although several methods for making
them exist, little is known about how these techniques
physically produce the hollow fibers of carbon molecules
known as nanotubes, that is until now. A multinational
team of scientists has discovered that multi-walled
carbon nanotubes made by the pure carbon arc method
are, in fact, carbon crystals that form inside drops
of glass-coated liquid carbon. The research appears
in the 11 February 2005, issue of the journal Science,
published by the AAAS, the science society, the world’s
largest general scientific organization. See http://www.sciencemag.org,
and also http://www.aaas.org.
One way to make nanotubes involves
using a carbon arc to heat graphite to about 5,000
C. An electrical current is passed through the graphite
in a chamber filled with helium gas. The result is
a sooty deposit on one of the electrodes that contains
columns filled with nanotubes.
“We were doing research on
the electrical transport properties of carbon nanotubes
when we noticed that the nanotubes had these little
beads that looked like liquid drops on them, said
lead author Walt A. de Heer, physics professor at
the Georgia Institute of Technology.
Much like archeologists studying
artifacts to decipher what happened in centuries past,
the research team began with the photos of the liquid-like
beads coating the nanotube fibers and worked their
way back to try to find out how they got there.
“Just by looking at them we
realized that this has something to do with liquid,”
said de Heer. “So we asked the question, if the beads
were once liquid carbon and the nanotubes they are
attached to are also carbon, why didn’t the liquid
carbon dissolve the nanotube? The answer is that the
liquid must have been a glass at a lower temperature
than the nanotube.”
It is well known that glass
is made by rapidly cooling a liquid. The fast rate
of cooling doesn’t allow the molecules time to align
themselves in the orderly arrangement of crystals
and they remain in the disordered grouping of the
liquid.
The research team saw that
the beads had the disordered grouping characteristic
of glass, while the nanotubes they surrounded had
an orderly crystalline pattern. This lead them to
conclude that the carbon arc must have melted the
graphite into drops of liquid carbon, which had cooled
at a much faster rate on the outside, giving it a
glassy appearance.
Since the nanotubes in the
interior had a crystalline structure, the team reasoned
that the liquid carbon on the inside of the drops
had cooled so slowly it became a supercooled liquid,
which is a liquid below the temperature which normally
turns it into a solid. As the temperature of any supercooled
liquid drops to a certain critical temperature, it
begins to crystallize. Which in this case, researchers
reasoned, resulted in the orderly molecular structure
of the nanotubes.
As the nanotubes continue to
crystalize they lengthen - poking through the glass
layer - causing the glass to bead on the tubes much
like water beads on pine needles. This final portrait
of the beads on nanotube fibers is the photo that
began the research team’s initial questions.
“Before we began this work,
we had spent a lot of time investigating these fibers,
because they had special significance for our work.
Most people don’t look at the fibers. They open them
up to get the nanotubes inside, but the balls are
on the surface.” said de Heer. “It took us having
to see them several times - actually they were quite
annoying - but then we realized that they may have
some significance by themselves We hope our results
will open up the whole question of nanotube formation
again.”
The research team consists
of de Heer and Zhimin Song from the Georgia Institute
of Technology, Daniel Ugarte from Universidade Estadual
de Campinas and Laboratorio Nacional de Luz Sincrotron
(Brazil), Jefferson Bettini also from Laboratorio
Nacional de Luz Sincrotron, Philippe Poncharal from
Universite Montpellier (France), Claire Berger from
the Georgia Institute of Technology and the Laboratoire
d'Études des Propriétés Électroniques
des Solides (France) and Joseph Ghezo from the University
of Illinois at Urbana-Champaign.
Media Contact: David Terraso, 404-385-2966, david.terraso@icpa.gatech.edu
Technical Contact: Walt de
Heer, 404 894-5215, deheer@electra.physics.gatech.edu
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