First there was the Mini. Then the Smart Car. Can automobiles get any smaller? Quite a bit smaller, according to scientists at Rice University (Houston). They recently produced the first nanoscale vehicle with an internal motor.
The nanocar measures just 3-by-4 nanometers, about the same width as a strand of DNA, but much shorter than DNA. About 20,000 nanocars could be parked, side-by-side, across the diameter of human hair.
The nanocar consists of a rigid chassis and four alkyne axles that spin freely and swivel independently of one another. Four buckyball wheels are made from spherical molecules of carbon, hydrogen and boron called p-carborane.
The motorized nanocar is powered by light. When light strikes the motor, it rotates in one direction, pushing the car along like a paddlewheel. Initial tests were carried out in a bath of toluene solvent. Follow-up tests are underway to determine whether the motorized car can be driven across a flat surface.
"We want to construct things from the bottom up, one molecule at a time, in much the same way that biological cells use enzymes to assemble proteins and other supermolecules," says James Tour, a professor of chemistry, mechanical engineering and materials science. "Everything that's produced through biology-from the tallest redwood to the largest whale-is built one molecule at a time. Nanocars and other synthetic transporters may prove to be a suitable alternative for bottom-up systems where biological methods aren't practical." In addition to the nanocar, Tour and his colleagues have created a nanotruck that's capable of carrying a payload.
Assembling the nanocar posed numerous challenges. In fact, the researchers spent almost 8 years perfecting the process. 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, but the buckyballs had a tendency to shut down the palladium reactions. "Finding the right method to attach the wheels involved a good bit of trial and error," says Tour.
Proving that the nanocar was rolling across a surface, not slipping and sliding, was another difficult challenge. Kevin Kelly, assistant professor of electrical and computer engineering, measured the movement of the nanocar across a gold surface. At room temperature, strong electrical bonds hold the buckyball wheels tightly against the gold, but heating to about 200 C frees them to roll.
To prove that the car was rolling rather than sliding, Kelly and his associates took scanning tunneling microscopy (STM) images every minute and watched the cars progress. Because the nanocar's axles are slightly longer than the wheelbase, they could determine the way the car was oriented and whether it moved perpendicular to the axles.
In addition, the researchers 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.