ARGONNE,
Ill. (June 30, 2006) — A new method to systematically
modify the structure of single-walled carbon nanotubes
could expand their electronic properties and open
the path to nano-electronics.
Carbon cylinders a few billionths of a meter in
diameter and a few microns long, these nanotubes
are one of the strongest structures known and have
unique electrical and thermal properties.
This promising method to add defects to carbon nanotube
walls was developed by researchers at the U.S. Department
of Energy's Argonne National Laboratory, who are
interested in improving the materials for thermoelectric
power generation, the use of heat differences to
generate electricity. Thermoelectric conversion is
the principle behind thermocouples, thermal
diodes and solid-state refrigerators .
"If you change the electronic structure," said Argonne
chemist Larry Curtiss, "by adding defects in an ordered
way, theoretically you can make more efficient thermoelectric
materials. So we could produce electricity more efficiently
from solar, nuclear or any thermal power generation." Curtiss
is group leader of the Molecular
Materials Group in Argonne's Materials
Science Division .
One dimer at a time
Creating defects by adding molecules to nanotubes
is challenging because of their extremely small size.
And researchers are seeking a controlled, reproducible
method. So the Argonne team, which includes Curtiss,
Michael Sternberg, Peter Zapol, Dieter Gruen, Gary
Kedziora, Paul Redfern and David Horner, used computer
simulation tools to learn how to add a single carbon
dimer – a molecule of two bonded carbons – to a single-walled
carbon nanotube.
The single-walled nanotubes – believed to be the
best candidates for next step of miniaturizing modern
electronics – resemble a long tube of chain-link
fence made of hexagons. The Argonne team simulated
a variety of approaches to attach the carbon dimer
to the nanotube. They found the easiest and strongest
method is by horizontally inserting a carbon dimer
into two hexagonal bonds, creating two adjacent pentagons
and heptagons (seven-sided structures) in the chain
link.
One dimer, two dimer…
After they understood how to add one dimer, the
researchers began to add dimers in patterns.
"The interesting thing was going into the multiple
patterns," Curtiss said. "We started building up
patterns using the dimers like building blocks and
adding them to the tubes." The researchers found
a number of interesting modifications:
- The "bumpy" tube has carbon dimers added symmetrically
around the circumference of the tube to create
a stable bulge.
- The "zipper" tube has dimers added horizontally
along the axial direction to every other hexagon,
creating alternating single octagons and pairs
of pentagons.
- The "multiple zipper" tube has six axial "zippers" spaced
by hexagon rows around a tube.
"The structures we simulated," said physicist Zapol, "have
new and unexpected features. They modify the electronic
properties in the nanotubes, and that will be useful
in future electronic applications."
Guided by the simulations, Argonne materials scientists,
led by Gruen, with expertise in carbon nanomaterials
are creating materials for testing.
"But we think that some of these structures exist
already," said Curtiss. Zapol's literature review
revealed that some researchers have found these structures,
but they did not know what they were.
The zipper structure particularly appeals to Argonne
researchers because the atomic spacings in the openings
are just the right size to bond nanotubes to Ultrananocrystalline™ diamond
and combine the properties of both. Ultrananocrystalline
diamond is a novel form of nanocarbon developed by
Argonne that has many of the properties of diamond – the
hardest known material on earth – and can be deposited
on a variety of surfaces. Unlike diamond, its properties
can be optimized depending on the application.
Researchers plan to use the carbon nanotubes as
a scaffolding to attach other molecules and study
their functions. They will also connect the tubes
into arrays and study the effects.
This research was funded by DOE's Office
of Science , Office of Basic
Energy Sciences ' Division of Materials
Sciences and Engineering . — Evelyn Brown
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