CARBON nanotubes can be switched from acting like a semiconductor to acting like a metal and back again simply by using a magnetic field. This fulfils a prediction based on one of the most fundamental phenomena in quantum physics that says electrons can “sense” a magnetic field even when they are apparently shielded from it.
Some carbon nanotubes are metals and others are semiconductors. Metals are good conductors because their electrons can easily jump from the highest energy levels they normally occupy to the conduction band. In semiconductors the energy needed to jump this “band gap” is greater, so the material can conduct only when the electrons are boosted into the conduction band with the right amount of external energy.
Now, two teams have used different methods to show that a magnetic field can alter the band gap in carbon nanotubes, switching metallic properties to semiconducting ones or vice versa. A team led by Junichiro Kono of Rice University in Houston, Texas, determined that the band gap in nanotubes with a diameter of 1 nanometre shrank as the magnetic field was increased, though not enough to turn them into metals (Science, vol 304, p 1129). To do that would have required magnetic fields far stronger than currently achievable in their lab, says Kono.
Advertisement
But another approach would be to increase the diameter of the nanotubes, and that is exactly what researchers at the University of Illinois at Urbana-Champaign have done. They threaded a magnetic field through the core of a nanotube about 30 nanometres in diameter. As the field reached 6 tesla, the nanotube started conducting current just as a metal would. Increasing the magnetic field further turned the nanotube back into a semiconductor (Science, vol 304, p 1132).
The transformation is predicted by a phenomenon called the Aharonov-Bohm effect, put forward in 1959 by physicist David Bohm and his student Yakir Aharonov, which says that electrons will be affected by a magnetic field even if the field is inside a metal tube that should completely shield the electrons from its influence.
To understand how the field influences the nanotubes, think of an electron flowing through the wall of a carbon nanotube as taking all possible paths from one end of the nanotube to the other. “The different trajectories the electron can take interfere with each other,” says Paul Goldbart a member of the University of Illinois team. Threading a magnetic field through the nanotube changes how the multiple paths of the electron interfere, which in turn can lower or raise the band gap in the carbon molecules.
Both teams say their research is complementary. “It’s very good that the same effect has been confirmed with different methods,” says Alexey Bezryadin of the Illinois team. “It makes certain that our understanding of the physics of these molecules is correct.”
