By pushing carbon nanotubes close to their breaking point, researchers at
the University of Illinois
have demonstrated a remarkable increase in the current-carrying capacity of
the nanotubes, well beyond what was previously thought possible.
The researchers drove semiconducting carbon nanotubes into an avalanche process
that carries more electrons down more paths, similar to the way a multilane
highway carries more traffic than a one-lane road.
"Single-wall carbon nanotubes are already known to carry current densities
up to 100 times higher than the best metals like copper," said Eric Pop,
a professor of electrical and computer engineering at the U. of I. "We
now show that semiconducting nanotubes can carry nearly twice as much current
as previously thought."
As reported in the journal Physical Review Letters, the researchers found that
at high electric fields (10 volts per micron), energetic electrons and holes
can create additional electron-hole pairs, leading to an avalanche effect where
the free carriers multiply and the current rapidly increases until the nanotube
The sharp increase in current, Pop said, is due to the onset of avalanche impact
ionization, a phenomenon observed in certain semiconductor diodes and transistors
at high electric fields, but not previously seen in nanotubes.
While the maximum current carrying capacity for metallic nanotubes has been
measured at about 25 microamps, the maximum current carrying capacity for semiconducting
nanotubes is less established. Previous theoretical predictions suggested a
similar limit for single-band conduction in semiconducting nanotubes.
To study current behavior, Pop, graduate student Albert Liao and undergraduate
student Yang Zhao first grew single-wall carbon nanotubes by chemical vapor
deposition from a patterned iron catalyst. Palladium contacts were used for
measurement purposes. The researchers then pushed the nanotubes close to their
breaking point in an oxygen-free environment.
"We found that the current first plateaus near 25 microamps, and then
sharply increases at higher electric fields," said Pop, who also is affiliated
with the Beckman Institute and the Micro and Nanotechnology Laboratory at the
U. of I. "We performed repeated measurements, obtaining currents of up
to 40 microamps, nearly twice those of previous reports."
By inducing very high electric fields in the nanotubes, the researchers drove
some of the charge carriers into nearby subbands, as part of the avalanche process.
Instead of being in just one "lane," the electrons and holes could
occupy several available lanes, resulting in much greater current.
The avalanche process (which cannot be observed in metallic carbon nanotubes
because an energy gap is required for electron-hole multiplication) offers additional
functionality to semiconducting nanotubes, Pop said. "Our results suggest
that avalanche-driven devices with highly nonlinear turn-on characteristics
can be fashioned from semiconducting single wall nanotubes."
Funding was provided by the National Science Foundation and the National Institute
of Standards and Technology through the Nanoelectronics Research Initiative.