Poking at a nanotube with a pointy rod can lead to intriguing
nano-science.
Nanotubes, tiny carbon pipes, may someday play a pivotal role in
bringing nanoscale technology into everyday use. Now
researchers have discovered that prodding these tubes with a
pointy tip can alter their ability to carry an electric current.
The results, the first to demonstrate how mechanical deformations
can affect a molecular wire's electrical properties, were published
in the June 15 issue of the journal Nature by Hongjie D,ai
Stanford assistant professor of chemistry, and graduate student
Thomas Tobler in collaboration with University of Kentucky
theoretical physicists.
The discovery could be used for making tiny electromechanical
devices, such as transducers that convert mechanical movements
into electrical signals. Other applications include creating
high-frequency telephone lines to carry voice and data and
making on/off switches for nanoscale computer chips.
Dai and his colleagues studied carbon nanotubes that are less than
one-millionth the diameter of a human hair and just millionths of
an inch long. Each tiny structure resembles a rolled-up graphitic
sheet of carbon atoms arranged in a honeycomb pattern.
To prod the nanotube, the researchers used the sharp tip of an
atomic force microscope (AFM). A real-space microscopy
technique, the AFM makes images of surface topography by
dragging a pointy tip over a structure's bumps and folds. The tip
reads the shape like a blind person reads Braille. The textures are
then translated into a visual image.
To conduct experiments on a single nanotube, Dai's group used a
technique he perfected with AFM co-inventor Calvin Quate ,
professor of electrical engineering at Stanford, and graduate
student Jing Kong. They placed an array of finely powdered
metal nanoparticles on a silicon dioxide substrate, and then fed a
carbon-containing gas (methane) over the substrate heated to a
high temperature. The carbon infused into the metal particles,
which acted as catalysts that converted carbon atoms into
honeycomb-lattice nanotubes.
The researchers used the technique to grow a single nanotube
across a silicon dioxide trench. They then attached an electrode to
each end of the tube. They used the AFM tip to push the wire
down into the trench, while measuring the wire's electrical
conductance.
The group was initially surprised to observe that the flow of
electricity dropped sharply as the nanotube bent. When the AFM
tip was removed, the tube straightened and the flow of electricity
returned to normal. Previous theoretical studies predicted no
significant change in the conductance of nanotubes due to
mechanical deformation.
Dai hypothesized that a dent that formed near the AFM tip could
be responsible for strongly affecting the electrical flow. To make
sense of the results, Dai enlisted the help of theoretician Shi-yu
Wu at the University of Kentucky. Wu and his colleagues used
computer simulations to show that the AFM tip dented one wall
toward the other, as when a garden hose gets kinked and the flow
of water is stopped.
As one side of the tube is pushed closer to the other, carbon atoms
form bonds across the inside of the tube. Normally, each carbon
atom binds to three other carbons, leaving one electron free for
use in conducting electricity. But when the walls of the tube come
close together, each carbon binds to four rather than three
carbons. The resulting decrease in the number of free electrons
causes the electrical conductance to slow.
"The AFM tip squashes the tube, causing each atom to bond with
more atoms," said Dai. "This causes the tube to turn from an
electrical conductor into an insulating structure similar to that
found in diamonds." Remarkably, the dent disappears once the
perturbing tip is removed. This high mechanical reversibility
allows the full recovery of the nanotube's electrical property, Dai
said.
"Dai's work is a very exciting experimental demonstration of
what our theoretical work predicted," said Kyeongjae Cho,
Stanford assistant professor of mechanical engineering, "namely
that local nanotube deformation is a way to develop different
functional components of nanotube transistors."
Editor's Note: The original news release can be found at
http://www.stanford.edu/dept/news/pr/00/nanowires.html
Note: This story has been adapted from a news release issued by
Stanford University for journalists and other members of the public. If you
wish to quote from any part of this story, please credit Stanford University
as the original source. You may also wish to include the following link in
any citation:
http://www.sciencedaily.com/releases/2000/07/000717072439.htm
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