Strong Magnetic Field Converts Nanotube from Metal to Semiconductor and Back


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 theoretical calculations for the changes in energy level, E/EM with variations in the applied magnetic field, Phi_0     conductance map of the actual measured electrical properties as a function of magnetic field and bias potential, 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        Tzu-Chieh Wei
Ulas Coskun (left) and Tzu-Chieh Wei (right)

Recent News

  • In the Media

The goal of the experiment, Fermilab Muon g-2, is to better understand the properties of muons, which are essentially heavier versions of electrons, and use them to probe the limitations of the Standard Model of particle physics. Specifically, physicists want to know about the muons’ “magnetic moment”—that is, how much do they rotate on their axes in a powerful magnetic field— as they race around the magnet? 

  • Accolades

Physical Review B is celebrating its 50th anniversary in 2020. The journal emerged out of its revered parent, The Physical Review, in response to the explosive growth of specialized physics content. It has excelled in front-edge coverage of condensed matter and materials physics research. As part of the celebration, in 2020 the editors are presenting a Milestone collection of papers that have made lasting contributions to condensed matter physics. Selection of papers of such importance is not an easy task. It is inevitable that some very important work will not be featured because of the abundance of gems in the treasure trove of the largest journal for physics. The Milestones will be highlighted on the journal website and in social media throughout the year.

  • Accolades

Illinois Physics Professor James Eckstein has been selected for the American Physical Society’s 2021 James C. McGroddy Prize for New Materials. This prize recognizes outstanding achievement in the science and application of new materials.

Eckstein shares the prize with two colleagues—Brookhaven National Laboratory Senior Scientist Ivan Bozovic and Cornell University Industrial Chemistry Professor Darrell G. Schlom—with whom he worked at Varian, Inc., in Palo Alto, CA, in the 1990s. There they developed atomic-layer-by-layer molecular beam epitaxy (MBE) as an effective method of growing artificially structured oxide materials in which each atomic-oxide layer can be individually specified.

The citation reads, “For pioneering the atomic-layer-by-layer synthesis of new metastable complex oxide materials, and the discovery of resulting novel phenomena.”

  • Research Funding

The University of Illinois Writing Across Engineering & Science (WAES) program has been awarded $599,999 by the National Science Foundation (NSF) for the research project Advancing Adaptation of Writing Pedagogies for Undergraduate STEM Education Through Transdisciplinary Action Research. This research program, which ultimately aims to incorporate effective technical writing as a core skill taught in STEM courses across the university, is funded through NSF’s Improving Undergraduate STEM Education Program: Education and Human Resources.