The university's nanocars, which will be described in an upcoming issue of the journal Nano Letters, are actually complex molecules that behave like vehicles. They roll on wheels and can be steered with electrical fields or by other means.
There is a nanocar that drives across a thin gold film, as well as a nanotruck that can carry a payload. A future nanocar will be propelled by a photon-powered engine, while another upcoming vehicle, a nanotrain of sorts, will consist of several molecular boxcars.
The cars, which took eight years to develop, measure only a few nanometers long and are thinner than human DNA. (A nanometer is a billionth of a meter. A human hair is about 80,000 nanometers wide.)
"We even have a Mini Cooper. It's two by two nanometers," said James Tour, who is a professor of chemistry, mechanical engineering, materials science and computer science at Rice. "We have a six-wheeled version, too."
The idea behind the research is to create molecules that will act as tools in the chemical reactions that will be employed to build microprocessors or other components in the future.
Transistors and interconnects in chips are currently etched into place by equipment that, collectively, costs chip factory owners billions of dollars. Ideally, circuits in the future will assemble themselves through the principles of chemistry--in the same manner, for example, that cold water will turn into ice crystals under the right conditions.
Self-assembly, though, works best in simple reactions, in which only heat and a few different types of molecules are needed. Building complex structures requires helper molecules.
Roughly speaking, the nanocars would act like enzymes for industrial applications, he said. Enzymes in living beings help break down molecules and transport the by-products to the appropriate molecule for the next step in processing. Hemoglobin, for example, transports oxygen. Tour does not envision these molecules being used in medicines or for biological applications.
"When you eat food in the morning and then it is part of your ear the next day, how did that happen?" Tour said. "Self-assembly alone is not sufficient."
Although the cars contain several atoms each that move in different directions, they actually consist of one single molecule. The nanocars typically contain four wheels attached to two axles. Each wheel is a, a spherical molecule made up of 60 carbon atoms. The axles are made of carbon as well. Although the bond between the wheels and the axles is strong, the wheels rotate like regular wheels on the axle.
"You can't pull it apart, but it does spin," he said.
The axles also pivot with respect to the chassis, thereby permitting navigation and steering.
Although the nano road is currently paved with gold, other materials will likely suffice. Gold is used because it makes very flat surfaces at the atomic level and doesn't form an oxide layer that can interfere with driving. At room temperature, strong electrical bonds hold the buckyball wheels tightly against the gold, but heating the gold to about 200 degrees Celsius frees them to roll.
The tip of a scanning tunneling microscope is applied to the film. This creates an electrical field that draws the car toward the tip, Tour believes.
Tour and his team have also conducted studies to ensure that the cars are rolling rather than just sliding across the surface. In these tests, the tip is placed on the side of a car. Images show that, instead of sliding sideways, the car takes a turn first and then heads toward the tip.
"If it were spinning, you would expect it to slide from the front as easily as the side," he said. A three-wheeled nanocar, which should slide easier than the four-wheeled versions, also did a turn before heading to the tip.
Tour hopes to be able to share experimental results about the motorized nanocar in about a year. In this car, photons, the smallest unit of light, turn a paddle wheel-like structure, which can propel the car. Experiments conducted so far show that photons will turn the paddle wheel.