Carbon nanotubes, rolled-up cylinders of graphite so small that 50 000 could fit side-by-side across the width of a human hair, are of strong interest for future information processing systems and an ideal tool for exploring wave properties of electronic systems in restricted geometries, where quantum phenomena become especially prominent. A group led by Professors Alexey Bezryadin and Paul Goldbart at the University of Illinois at Urbana-Champaign has recently demonstrated that a multiwall carbon nanotube (MWNT) can be switched between metallic and semiconducting states by threading a strong magnetic field through the tube, a phenomenon predicted by theorists some years ago but never before clearly seen in individual molecules.
In a paper published in the May 21 issue of Science (Ulas C. Coskun, Tzu-Chieh Wei, Smitha Vishveshwara, Paul M. Goldbart, and Alexey Bezryadin, "h/e Magnetic Flux Modulation of the Energy Gap in Nanotube Quantum Dots," Science 304, 1132–1134 [2004]), the researchers report that a strong magnetic field alters a nanotube's electronic structure. The research team included Department of Physics experimentalists Bezryadin and graduate student Ulas Coskun and theorists Goldbart, postdoctoral research associate Smitha Vishveshwara, and graduate student Tzu-Chieh Wei.
Nanotubes have previously been shown to be either metallic or semiconducting, depending upon how they have been rolled and seamed when fabricated. "Unfortunately, we can't undo the seam and rejoin it when we want to change the nanotube's electronic properties," according to Goldbart. "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."
Carbon nanotubes are also particularly well suited to study the Aharonov-Bohm effect—the quantum mechanical phenomenon by which a charged particle is affected by electromagnetic fields in regions from which the particle is excluded. "By using a larger-diameter (about 30 nm) MWNT, we are able to achieve a higher magnetic flux value (more than one flux quantum) through the tube, which significantly modifies the energy spectrum and converts the nanotube's electronic properties," Bezryadin said.
The dependence of the single-electron energy levels (vertical axis) on the magnetic flux through the tube (horizontal axis), computed using a simple model, is shown below left. Note the opening and closing of diamond-shaped gaps as the applied magnetic field, and correspondingly the magnetic flux through the tube, is varied. The figure below right is a map of the actual measured electrical conductance as a function of magnetic field on the x axis and bias potential on the y axis, showing the opening and closing of the energy gap as the nanotube switches from metallic to semiconducting.
"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, Bezryadin and Coskun constructed a single-electron tunneling (SET) transistor by placing a MWNT across a narrow trench (about 100 nm wide) etched in the surface of a silicon wafer. By measuring the conduction properties of the SET in various magnetic fields, the researchers were able to observe the modulation of the nanotube energy spectrum and the associated switching between semiconducting and metallic states.
Electrons in a nanotube can occupy only 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. Yet higher ones are expected to cause it again, and so on."
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.
Ulas Coskun (left) and Tzu-Chieh Wei (right)
This work was supported by the National Science Foundation and the U.S. Department of Energy. The conclusions presented are those of the authors and not necessarily those of the funding agencies.