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.
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.
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.
we can't undo the seam and rejoin it when we want
to change the electronic properties of the nanotube,"
"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
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,"
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."
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.
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.
semiconductor has a gap in the energy spectrum,"
"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."
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.
National Science Foundation and the U.S. Department
of Energy funded the work.