By threading a magnetic field through a carbon nanotube, scientists have switched the molecule between metallic and semiconducting states, a phenomenon predicted by physicists some years ago, but never before clearly seen in individual molecules.
Researchers from the University of Illinois at Urbana-Champaign have presented experimental evidence that a nanotube’s electronic structure can be altered in response to a magnetic field. The research team consisted of physics professors Alexey Bezryadin and Paul Goldbart, postdoctoral research associate Smitha Vishveshwara and graduate students Ulas Coskun and Tzu-Chieh Wei.
Carbon nanotubes are built of sheets of graphite (a hexagonal lattice of carbon atoms) rolled into long cylinders. The manner in which the sheets are rolled and seamed determines whether the tubes are metallic or semiconducting.
“Unfortunately, we can’t undo the seam and rejoin it when we want to change the electronic properties of the nanotube,” Goldbart said. “However, we found that we can tune these materials not by restructuring the molecules themselves but by moving their energy levels with a strong magnetic field.”
Unlike other single molecules, multiwall carbon nanotubes are said to have the ideal size and shape for studying the Aharonov-Bohm effect. “The larger diameter nanotubes (about 30 nanometers) allow us to apply a magnetic field strong enough to significantly modify the energy spectrum and convert the nanotube’s electronic properties,” Bezryadin said.
“The Aharonov-Bohm effect goes to the heart of quantum mechanics, and is one of the most striking manifestations of the wave nature of matter,” Goldbart said. “As an electron moves, the wave actually takes multiple paths, including ones that encircle the nanotube and the magnetic flux threading it. Depending upon the strength of the magnetic field, the properties of the molecule will change from metallic to semiconducting, and back again.”
To probe the electronic energy spectrum and its dependence on a magnetic field, the researchers constructed a single-electron transistor by placing a multiwall carbon nanotube across a narrow trench (about 100 nanometers wide) etched in the surface of a silicon wafer. By measuring the conduction properties of their quantum dot device in various magnetic fields, the researchers were able to observe the modulation of the nanotube energy spectrum and the associated interconversion of semiconducting and metallic states.
Electrons in a nanotube can only occupy certain energy levels, and the tube’s conductance depends on how many of them there are at low energies.
“A semiconductor has a gap in the energy spectrum,” Bezryadin said. “Since it has no low-lying energy levels, the conductance is very small. In contrast, low-lying levels make the system metallic, as in our nanotube when no magnetic field is present. Passing a magnetic field through the nanotube changes the energies of electrons and opens up a gap, converting the nanotube into a semiconductor. Higher fields reverse the effect.”
In addition to its electronic properties, a nanotube’s mechanical and chemical properties also depend upon whether the tube is metallic or semiconducting, the researchers point out in their paper. These properties might also be controlled by a magnetic field.