HOUSTON -- Rice University scientists have developed the first method for sorting
semiconducting carbon nanotubes based on their size, a long-awaited development
that could form the basis of a nanotube purification system capable of producing
the necessary feedstocks for nano-circuits, therapeutic agents, next-generation
power cables and more.
Nanotubes, tiny cylinders of carbon no wider than
a strand of DNA, possess a tantalizing array of properties
coveted by materials scientists. Nanotubes are stronger
than steel, but weigh one sixth as much. Some varieties
are excellent semiconductors, while others are metals
that conduct electricity as well as copper.
But there are dozens of varieties of nanotubes,
each slightly different in size and atomic structure
and each with very different properties. For many
applications, engineers need to use just one type
of nanotube, but that's not possible today because
all production methods turn out a mishmash of types.
New research due to appear in an upcoming issue
of the Journal of the American Chemical Society describes
a new method that uses electric fields to sort nanotubes
by size.
"People have developed sorting methods based on
both chemical and electrical properties, but ours
is the first that's capable of sorting semiconducting
nanotubes based upon their dielectric constant, which
is determined by their diameter," said corresponding
author, Howard Schmidt, executive director of Rice's
Carbon Nanotechnology Laboratory (CNL).
To sort nanotubes, the CNL team built a system that
capitalizes on the fact that each type of nanotube
has a unique dielectric constant – a term that refers
to a material's ability to store electrostatic energy.
CNL scientists created an electrified chamber and
pumped a solution of dissolved nanotubes through
it. The chamber traps metallic nanotubes and causes
semiconducting varieties to float at different levels
in the chamber. The smaller the diameter of the nanotube,
the larger the dielectric constant and the lower
in the system the tubes float. By varying the speed
of flow through the system – with upper-level currents
traveling faster than lower-level currents – the
scientists were able to collect samples that had
three times more small tubes than large and vice
versa.
The experimental work was primarily performed by
research scientist Haiqing Peng and first-year graduate
student Noe Alvarez. Co-authors on the paper include
research scientist Carter Kittrell and distinguished
faculty fellow Robert Hauge. The research was supported
by NASA, the Department of Energy, the Army Research
Laboratory and the Air Force Office of Scientific
Research.
Contact: Jade Boyd
jadeboyd@rice.edu
713-348-6778
Rice University
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