Computer chip technology and scientific breakthroughs have marched in step for decades. But the ability to cram ever more circuitry onto silicon chips now faces fundamental limits. In recent years, scientists have been going for the ultimate shrinkage: turning single molecules and small chemical groups into transistors and other standard components of computer chips. This year, researchers wired up their first molecular-scale circuits, a feat that Science selects as the Breakthrough of 2001 and that may herald a new generation of molecular electronics.
Today's state-of-the-art computer chips pack some 40 million transistors onto a slab of silicon no bigger than a postage stamp. But as impressive as this sounds, the transistors on these chips are still about 60,000 times bigger than molecules. By the late 1990s a spate of studies had shown that individual molecules could conduct electricity like wires or semiconductors, the building blocks of modern microprocessors. Turning individual molecules into devices was not far behind, and by the end of 2000 researchers had amassed a grab bag of molecular electronic devices but no demonstrations of wiring them together.
2001 brought a world of difference, when five labs succeeded in hooking up these devices into more complex circuits that could carry out rudimentary computing operations:
- In January, a team led by Charles Lieber, a chemist at Harvard University, reported arranging nanowires into a simple configuration that resembled the lines in a ticktacktoe board that was electronically active. The tiny arrangement wasn't a circuit yet, but it was the first step, showing that separate nanowires could communicate with one another.
- In April, James Heath and his colleagues at the University of California, Los Angeles, reported at the American Chemical Society meeting that they'd made semiconducting crossbars. Heath's team placed molecules called rotaxanes, which function as molecular transistors, at each junction. By controlling the input voltages to each arm of the crossbar, the scientists showed that they could make working 16-bit memory circuits.
- In the 26 August online edition of Nano Letters, a team led by Phaedon Avouris of IBM reported making a circuit out of a single semiconducting carbon nanotube. The team coaxed the device to work like a simple circuit called an inverter, another of the basic building blocks for more complex circuitry. Crucially, the IBM circuit also demonstrated another advantage: "gain," the ability to turn a weak electrical input into a stronger output, which is a necessary feature for sending signals through multiple devices.
- A pair of papers in the 9 November issue of Science reported circuits with even stronger gain. The first, by Cees Dekker and his colleagues at Delft University of Technology in the Netherlands, also relied on carbon nanotubes. By carefully controlling the formation of metal gate electrodes, Dekker's group created transistors with an output signal 10 times stronger than the input. Lieber and colleagues at Harvard also got in on the act, constructing circuits with their semiconducting nanowires, in this case made from silicon and gallium nitride.
- Finally, in a report published online by Science on 8 November, a group led by physicist Jan Hendrik Schön of Lucent Technologies' Bell Laboratories in Murray Hill, New Jersey, reported similar success in crafting circuits from transistors made from organic molecules that chemically assemble themselves between pairs of gold electrodes.
Researchers now face the truly formidable task of taking the technology from demonstrations of rudimentary circuits to highly complex integrated circuitry that can improve upon silicon's speed, reliability, and low cost. Reaching that level of complexity will undoubtedly require a revolution in chip fabrication. But as chip designers race ever closer to the limits of silicon, pressure to extend this year's breakthroughs in molecular electronics will only intensify.