March 1999

Will the Real Nanotech Please Stand Up?

Continued from page 2

By David Rotman

For now, these materials are mostly made by traditional methods of chemical synthesis, but Siegel says the availability of tools for atomic imaging has begun to enable scientists to make selective nano-structures. Siegel points, for example, to the development of nanocrystalline materials used in the giant magnetoresistance (GMR) devices that have in the past few years dramatically accelerated the pace of improvement in information storage. GMR technology relies on multiple layers of thin films, some only a few atoms thick; the precise layering of these thin films at the molecular level is responsible for the high sensitivity of the device. Siegel argues that "the huge impact of nanotech will come in nanoelectronics." The nanocrystals used in GMR, he suggests, are "only the tip of that iceberg."

For those making micrometer-sized devices (now common in advanced electronics and optics), the collision with the nanoscale is rapidly approaching. The expanding field of MEMs (micro-electromechanical machines), which is developing tiny machines to act as everything from microphones to miniature rockets, is also bumping up against the nanoworld and routinely making working parts as small as a few hundred nanometers.

For purists, however, you need to think smaller-much smaller-before you enter the real nanoworld. For these chemists and physicists, it is below about 50 nanometers where the fun begins. In this new arena, forces such as gravity that govern the everyday world rapidly lose their familiar meanings. "Physical intuition fails miserably in the nanoworld. You have to throw away your preconceived notions," says Reed. "You see all kinds of unusual effects." For one thing, electrons can go places that, according to classical physics, they can't be. In some cases, says Reed, "It's like throwing a tennis ball at a garage door and having the ball pop out the other side."

This is also where today's silicon-based electronics begin to fail. On the nanoscale, conventional transistors leak electrons like sieves, and the "dopant" atoms inserted into silicon to control its properties behave like huge, awkward boulders. Yet if the nanoscale poses sharp obstacles to conventional electronic technologies, it also opens up remarkable new possibilities that may leave today's electronics looking like the Model T.
If electronic devices could be reduced to the size of individual molecules, then the game would be entirely altered. Molecular electronics was proposed in the 1970s by Mark Ratner, who is now at Northwestern University, and Ari Aviram of IBM. For years it remained a tantalizing idea far beyond the abilities of experimentalists. But during the past couple of years, leading-edge researchers have begun making actual wires and components out of single molecules. And now they have begun to make crude devices that actually work.

At Yale, Reed and his coworkers have, for one, made a diode out of several individual organic molecules. The simple diode, which is several nanometers long, is far from being a practical device, says Reed. But, he adds, it's a first, encouraging step to making transistors and logic devices at that scale.