In 1974 two researchers, Mark Ratner of Northwestern University and Ari Aviram of IBM, had a wild vision. They imagined a day when computers would use individual molecules to perform digital calculations, yielding machines of breathtaking capability. That day is near: Two groups of researchers have just developed the first practical logic circuits whose components have been shrunk to the size of molecules.
No matter how they're constructed, computer circuits work by guiding electricity through a sequence of on-off amplifiers, or transistors. Charles Lieber, a chemist at Harvard, downsized the transistor, using silicon or gallium nitride wires just 10 atoms thick. He and his collaborators created a grooved template that forms little tunnels when placed on a flat surface. Then they forced a liquid containing the silicon wires to run through those tunnels. "Like logs in a river, the wires line up in the flow," Lieber says. Finally, the researchers rotated the template and repeated the process with gallium nitride wires, creating a crosshatch pattern in which each intersection functions as a transistor. The completed circuits have successfully performed simple addition problems.
Cees Dekker of Delft University of Technology in the Netherlands and his colleagues had previously built similarly small transistors from cylindrical carbon molecules, called carbon nanotubes. Now they've spread thousands of nanotubes on a chip, three of which they attached to aluminum contacts and joined together with gold bridges to create a simple test circuit. It may take a decade or two before either approach leads to a practical molecular computer, but Dekker now can imagine how it will happen: "We have single molecules combined to make a logic circuit. That was the holy grail with this field for 25 years."
Carbon nanotube transistors with gold contacts—the tiny future of computers?Photograph courtesy of Tu Delft/Gripp