Carbon nanotubes, described as the reigning celebrity of the
advanced materials world, are all the rage. Recently researchers at
Rice University and Rensselaer Polytechnic Institute used them to make
the “blackest black” — the darkest known
material, reflecting only 0.045 percent of all light shined on it.
When Sandia's François Léonard explains the physics of carbon nanotubes to students, he uses chicken wire as a metaphor. Léonard has written a book on the subject to be published later this summer. (Photo by Randy Wong)
National Laboratories is also in on the carbon nanotube game,
with research led by physicist François Léonard.
Léonard has considerable experience in the subject, so much
that he wrote the book on it — literally. He’s the
author of a forthcoming work, Physics of Carbon Nanotube Devices, which
could become the definitive text on the topic.
Carbon nanotubes are long thin cylinders composed entirely of
carbon atoms. While their diameters are in the nanometer range (1-10),
they can be very long, up to centimeters in length. The carbon-carbon
bond is very strong, making carbon nanotubes very robust and resistant
to any kind of deformation. The properties of other single-element
materials are obvious — gold is a metal and silicon is a
semiconductor, for example. Carbon nanotubes, on the other hand, have a
sort of dual personality not found in other materials made from a
single element. They’re special because they can be either
metallic or semiconducting.
Léonard explains that this results from the actual
structure of a carbon nanotube; the way the atoms are arranged around
the tube determines its electronic properties. To explain this concept
to a group of undergraduates at the University of California, Berkeley,
he uses three rolls of chicken wire, each cut at a different angle. The
chicken wire represents the sheet of graphene from which the nanotube
is cut. The angle of that cut creates a different bond geometry along
the nanotube, which results in different properties.
Working in uncharted territory
Léonard’s experience with carbon
nanotubes began when the field was just emerging. While the discovery
of carbon nanotubes is credited to Japanese physicist Sumio Iijima in
1991, work on applications didn’t begin until the late 1990s.
Léonard was at IBM as a postdoc when researchers there built
the first transistor from carbon nanotubes.
As a theoretical physicist, Léonard was working in
uncharted territory. From the beginning, he worked on modeling
approaches to understand how carbon nanotubes might behave in certain
applications. He joined Sandia in 2000, where he has continued his
carbon nanotube research.
The semiconducting side of carbon nanotubes holds a lot of
promise for the development of new nanoelectronic devices. “A
carbon nanotube creates a transistor that is only one nanometer
wide,” says Léonard. “This makes it
possible, in principle, to achieve very high device densities compared
with the current state of the art.” The field emission
properties of carbon nanotubes are also exciting. Flat panel displays
are typically made from a high density of sharp tips, to which high
voltage is applied to extract electrons. These electrons strike and
activate the pixels in the screen. Carbon nanotubes can serve this
purpose because they are very sharp, long, and can sustain high fields
and high temperatures.
‘Layla’ on a nanotube receiver
Researchers have demonstrated the ability to assemble such
devices with a single carbon nanotube. At a recent conference, one
scientist played Eric Clapton’s “Layla”
on a carbon nanotube device acting as a radio receiver.
Another potential use is in chemical and biological sensors.
Carbon nanotubes, because of their small diameter, can serve as very
sensitive detectors, with the ability to detect a single molecule of a
target substance. DNA detection has also been demonstrated. Currently,
Léonard is leading a team to develop optical detection using
carbon nanotubes. The project is a partnership with Lockheed Martin.
Unique electronic properties
Semiconducting carbon nanotubes have many properties that make
them attractive for optical detection. They have unique electronic
properties that favor light absorption. In addition, the wavelength
over which light is absorbed can be controlled with nanotubes of
different diameters. Importantly, the device fabrication process could
be entirely compatible with fabrication processes used by the
semiconductor industry. In addition to carbon nanotubes,
Léonard is interested in electronic transport in other
nanostructures — carbon nanotubes as well as nanowires and
single molecules. The question, he says, is how does current pass
across nanostructures? How is transport of electrons different than in
Léonard’s book is expected to be out by
the end of August. See the publisher’s website here for details.