metallic are metal nanotubes?
Technology Research News
As researchers examine carbon nanotubes
more closely, they're finding that the tiny rolls of graphite are more
complicated than they may have first appeared.
Researchers from Harvard have examined metallic nanotubes and found that
a nanotube's environment affects whether it is truly metallic or not.
The finding throws at least a small complication into plans to eventually
use the nanotubes as ready-made wires for nanoscale electronics.
Nanotubes come in two basic types: metal, which allow electrical current
to flow freely, and semiconducting,
which contain energy gaps that slow current. Electrons and their charge
opposites, positive holes, inhabit certain orbits around an atom; bandgaps
are the spaces between orbits where charges cannot flow.
Nanotubes can be smaller than one nanometer in diameter and are made of
one-molecule-thick sheets of graphite that roll up to form the tubes.
A combination of the angle of the roll and the diameter of the tube determines
its electrical properties.
There are two basic types of metal nanotubes: armchair and zigzag. The
difference is in the direction the graphite sheet is rolled. The sheet's
hexagonal rows of carbon molecules run along the nanotube axis in armchair
nanotubes and around it's circumference in Zigzag nanotubes.
The researchers found that zigzag nanotubes contain energy gaps that stop
current from flowing freely. "They're truly semiconductors," said Charles
Lieber, a chemistry professor at Harvard University.
The way armchair nanotubes conduct is a little more complicated. When
bundled with other nanotubes, they contain pseudo energy gaps that lone
armchair nanotubes do not. "Thus... to obtain good electrical interconnects,
[or] wires, we will need to use armchair carbon nanotubes, with the tubes
isolated -- not in bundles. Only these will be true metals," he said.
The pseudo gaps are actually caused by the molecular interactions among
two or more nanotubes, which destroy the rotational symmetry that gives
lone armchair nanotubes metallic properties, said Lieber. Rotational symmetry
simply means the tube looks the same when rotated a certain amount. For
instance a wheel with six spokes will look the same after it is rotated
The researchers do not know precisely how much the pseudo energy gaps
in bundled armchair nanotubes will slow current. "We do not yet know how
dramatic an effect the pseudogap will have on the conducting properties.
However, all theory would suggest that this type of gap will degrade their
conducting properties," said Lieber.
The research shows that controlling the environment around the nanotube
is important, said Lieber. "It's not so simple as you just take something
that should be a metal and throw it on a surface," he said.
In light of the research, it may be useful to find a way to isolate armchair
nanotubes. This is tricky, because nanotubes like to stick together, Lieber
said. "You don't get single sheets of graphite floating around -- they
have a very strong attraction for one another," he said.
One way to both sort nanotubes into the correct types and keep conducting
armchair nanotubes apart is to grow them in place. "You could probably
grow isolated tubes, but I just don't know how to at this time," said
Lieber. "We know now what needs to be done to get the electronic properties.
It's not physically impossible, there's just some hard work that's going
to have to be done."
These gaps could also be used in a positive way, Lieber added. Both the
true energy gaps and the pseudo gaps make the nanotubes conductivity sensitive,
meaning the conductivity changes when chemicals bind to the tubes. Because
of this, these types of nanotubes could be used as nanoscale chemical
or biological sensors, he said.
The researchers are now more closely examining the armchair nanotubes'
pseudo gap, including how it forms and how environment can be used to
control this type of gap, said Lieber. They are looking to better understand
the role of defects, or misalignments in determining nanotube electronic
properties, he said.
It's nice work, said Dan Ralph, an associate professor of physics at Cornell
University. "It turns out that the gap is pretty small," however, which
means the tubes may still be useful as wires, at least for room temperature
applications, Ralph said. "There may be small differences in conductivity
costs, but by tens of percent rather than big orders of magnitude," he
The gap may loom larger for low temperature applications because colder
electrons have less thermal energy. It also may turn out to matter more
after larger problems like resistance between a contact and a nanotube,
which is often comparable to the resistance of the whole tube, have been
solved, he said.
Researchers generally agree that it will be more than five or even 10
years before nanotubes can be used practically in circuitry.
Lieber's research colleagues were Min Ouyang, Jin-Lin Huang and Chin Li
Chang of Harvard University. They published the research in the April
27, 2001 issue of the journal Science. The research was funded by the
National Science Foundation (NSF).
Timeline: 5-10 years
TRN Categories: Nanotechnology
Story Type: News
Related Elements: Technical paper, "Energy Gaps in 'Metallic'
Single Walled Carbon Nanotubes," Science, April 27, 2001.
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