Nanotubes switch back and forth
21 May 2004
Two groups of physicists have shown that carbon nanotubes respond to
magnetic fields in ways that are not seen in other materials. Junichiro Kono
and colleagues at Rice University and Florida State University and Alexey
Bezryadin and co-workers at the University of Illinois at Urbana-Champaign
have discovered that semiconducting nanotubes can be made metallic, and vice
versa, by applying a magnetic field. In addition to their fundamental
importance, the results could have practical applications (Science 304 1129
Carbon nanotubes are essentially rolled up sheets of graphite, just
nanometres in diameter, that can be metallic or semiconducting depending on
the direction in which the sheet has been rolled up. Kono and co-workers
performed optical absorption and emission spectroscopy on solutions of
semiconducting single wall nanotubes placed in strong magnetic fields of 45
Tesla. They found that the band gap between the conduction and valence bands
in the nanotubes became smaller as the strength of the magnetic field was
"This phenomenon is unique among known materials," Kono told PhysicsWeb.
"Ordinary semiconductors show the opposite behaviour." The team believes
that the band gap could disappear completely in higher fields, which would
cause the semiconducting nanotubes to become metallic.
Meanwhile, Bezryadin and colleagues found that the band gap in a
multi-walled metallic nanotube -- which was initially zero -- gradually
widened as a magnetic field was applied, turning it into a semiconductor.
Moreover, as the applied field was increased further, the band gap dropped
back to zero and the nanotube became a metal again.
Although these effects have never been observed in nanotubes before, they
agree with theoretical predictions. Both experiments also highlight the
importance of a subtle quantum effect known as the Aharonov-Bohm effect.
Although this effect has been observed in many systems before, including
nanotubes, this is the first time that it has been shown to have an effect
on the band structure of a solid.
"The discovery could lead to novel magneto-optical or magneto-electrical
switching devices by magnetically controlling the metallicity of nanotubes,"
Kono told PhysicsWeb. "It could also lead to novel experiments on
"Our work demonstrates that hollow molecules can change the energies of
their orbitals in response to the magnetic flux threaded through the
molecule," said Bezryadin. "This observation may have interdisciplinary
importance, since electronic orbitals not only determine the energy of the
molecule but also its chemical, mechanical and other properties. It might
therefore be possible to control these properties by a magnetic field."
Kono's group now plans to study the effects of even stronger magnetic fields
on nanotubes, while Bezryadin and co-workers will repeat their experiment at
ultracold temperatures to obtain an even clearer picture of how the electron
energy levels in the nanotubes respond to magnetic fields.
Bubba Do Wah Ditty
"One year ago today, the time for excuse-making has come to an end."