World's Smallest Car Built From a Single Molecule

HOUSTON, Texas, October 21, 2005 (ENS) - The smallest car ever constructed - a single molecule "nanocar" just one 80 thousandth the size of a human hair - has been built by scientists at Rice University.

The nanocar has a chassis, axles and four wheels that roll. The unique car is described in a research paper that is available online and due to appear in an upcoming issue of the journal "Nano Letters."

Other research groups have created nanoscale objects that are shaped like automobiles, but study co-author Kevin 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, provided the measurements and experimental evidence that verified the rolling movement.

Seen through a scanning tunneling microscope, the nanocar zips across a surface of gold. (Photo by T. Sasaki courtesy Rice U.)

The wheels are buckyballs, spheres of pure carbon containing 60 atoms apiece. The entire car measures just three to four nanometers across, making it slightly wider than a strand of DNA. A human hair, by comparison, is about 80,000 nanometers in diameter.

"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 scanning tunneling microscopy (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.

Kelly and Tour and their 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.

Dr. James Tour, a synthetic organic chemist, is the Chao Professor of Chemistry, professor of mechanical engineering and materials science and professor of computer science at Rice University. (Photo courtesy Rice U.)

"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," Tour said. "They're helping us learn the ground rules."

Synthesis of the nanocars 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, Tour said. 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 trial and error.

Tour and his team constructed an oligo (phenylene ethynylene) chassis and axle which were then mounted onto four fullerene wheels.

Palladium catalysts were used to create the reactions needed to synthesize the axle and chassis together.

Funded by the Welch Foundation, Zyvex Corporation and the National Science Foundation, the research was conducted as a proof-of-concept for directional control of nanoscale transporters. Once perfected, the transports would be able to ferry atoms and molecules in non-living fabrication environments.

The Rice team has already followed up the nanocar work by designing a light-driven nanocar and a nanotruck that is capable of carrying a payload.