Strong magnetic field converts nanotube from metal to semiconductor and back

May 20, 2004

CHAMPAIGN, Ill. -- 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.

In the May 21 issue of the journal Science, researchers from the University of Illinois at Urbana-Champaign present 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 remarkable molecules 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 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.
-end-
The National Science Foundation and the U.S. Department of Energy funded the work.

University of Illinois at Urbana-Champaign

Related Magnetic Field Articles from Brightsurf:

Investigating optical activity under an external magnetic field
A new study published in EPJ B by Chengping Yin, Guangdong Provincial Key Laboratory of Quantum Engineering and Quantum Materials, South China, aims to derive an analytical model of optical activity in black phosphorous under an external magnetic field.

Magnetic field and hydrogels could be used to grow new cartilage
Instead of using synthetic materials, Penn Medicine study shows magnets could be used to arrange cells to grow new tissues

Magnetic field with the edge!
This study overturns a dominant six-decade old notion that the giant magnetic field in a high intensity laser produced plasma evolves from the nanometre scale.

Global magnetic field of the solar corona measured for the first time
An international team led by Professor Tian Hui from Peking University has recently measured the global magnetic field of the solar corona for the first time.

Magnetic field of a spiral galaxy
A new image from the VLA dramatically reveals the extended magnetic field of a spiral galaxy seen edge-on from Earth.

How does Earth sustain its magnetic field?
Life as we know it could not exist without Earth's magnetic field and its ability to deflect dangerous ionizing particles.

Scholes finds novel magnetic field effect in diamagnetic molecules
The Princeton University Department of Chemistry publishes research this week proving that an applied magnetic field will interact with the electronic structure of weakly magnetic, or diamagnetic, molecules to induce a magnetic-field effect that, to their knowledge, has never before been documented.

Origins of Earth's magnetic field remain a mystery
The existence of a magnetic field beyond 3.5 billion years ago is still up for debate.

New research provides evidence of strong early magnetic field around Earth
New research from the University of Rochester provides evidence that the magnetic field that first formed around Earth was even stronger than scientists previously believed.

Massive photons in an artificial magnetic field
An international research collaboration from Poland, the UK and Russia has created a two-dimensional system -- a thin optical cavity filled with liquid crystal -- in which they trapped photons.

Read More: Magnetic Field News and Magnetic Field Current Events
Brightsurf.com is a participant in the Amazon Services LLC Associates Program, an affiliate advertising program designed to provide a means for sites to earn advertising fees by advertising and linking to Amazon.com.