|Posted: July 3, 2006
Carbon nanotube building blocks open up possibilities for advanced electronics
(Nanowerk News) 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
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
One dimer, two dimer…
After they understood how to add one dimer, the researchers began to add dimers
"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
"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."
- 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.
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.