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Rice scientists build world's first single-molecule car
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'Nanocar' with buckyball wheels paves way for other molecular machines

Rice University scientists have constructed the world's smallest car -- a single molecule "nanocar" that contains a chassis, axles and four buckyball wheels.

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.