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Credit: Y. Shira/Rice
University
Credit: T. Sasaki/Rice
University
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"The synthesis and testing of nanocars and
other molecular machines is providing critical insight in our
investigations of bottom-up molecular manufacturing," said one of the
two lead researchers, James M. Tour, the Chao Professor of Chemistry,
professor of mechanical engineering and materials science and
professor of computer science. "We'd eventually like to move objects
and do work in a controlled fashion on the molecular scale, and these
vehicles are great test beds for that. They're helping us learn the
ground rules."
The nanocar consists of a chassis and axles made of
well-defined organic groups with pivoting suspension and freely
rotating axles. The wheels are buckyballs, spheres of pure carbon
containing 60 atoms apiece. The entire car measures just 3-4
nanometers across, making it slightly wider than a strand of DNA. A
human hair, by comparison, is about 80,000 nanometers in diameter.
Other research groups have created nanoscale
objects that are shaped like automobiles, but study co-author Kevin F.
Kelly, assistant professor of electrical and computer engineering,
said Rice's vehicle is the first that actually functions like a car,
rolling on four wheels in a direction perpendicular to its axles.
Kelly and his group, experts in scanning tunneling
microscopy (STM), provided the measurements and experimental evidence
that verified the rolling movement.
"It's fairly easy to build nanoscale objects that
slide around on a surface," Kelly said. "Proving that we were rolling
– not slipping and sliding – was one of the most difficult parts of
this project."
To do that, Kelly and graduate student Andrew
Osgood measured the movement of the nanocars across a gold surface. At
room temperature, strong electrical bonds hold the buckyball wheels
tightly against the gold, but heating to about 200 degrees Celsius
frees them to roll. To prove that the cars were rolling rather than
sliding, Kelly and Osgood took STM images every minute and watched the
cars progress. Because nanocars' axles are slightly longer than the
wheelbase – the distance between axles – they could determine the way
the cars were oriented and whether they moved perpendicular to the
axles.
In addition, Kelly's team found a way to grab the
cars with an STM probe tip and pull them. Tests showed it was easier
to drag the cars in the direction of wheel rotation than it was to
pull them sideways.
Synthesis of the nanocars also produced major
challenges. Tour's research group spent almost eight years perfecting
the techniques used to make them. Much of the delay involved finding a
way to attach the buckyball wheels without destroying the rest of the
car. Palladium was used as a catalyst in the formation of the axle and
chassis, and buckyballs had a tendency to shut down the palladium
reactions, so finding the right method to attach the wheels involved a
good bit of trial and error.
The Rice team has already followed up the nanocar
work by designing a light-driven nanocar and a nanotruck that's
capable of carrying a payload. |
