Inching closer to molecular electronics.

Inching closer to molecular electronics

When it comes to electronics, thinking small is thinking big. Cramming ever more components onto ever smaller chip areas has shrunk super-expensive, room-filling computers into affordable and even more capable gizmos that you can carry with one hand if you can fork over $1,000 or so with the other.

As the miniaturization trend continues, a diverse community of small-thinking scientists has set its sights on the tiniest imaginable electronic landscape, where components measure millionths of a centimeter or about the size of small protein molecules. That would translate to components at least 1,000 times smaller than today's -- a prospect that some view as theoretically sound and others doom as wishful thinking.

In 1988, Ari Aviram of the IBM Thomas J. Watson Research Center in Yorktown Heights, N.Y., proposed a theoretical class of molecules, which his calculations indicated should behave as diminutive memory, logic and amplification components--that is, as a set of molecular computing devices.

Excited by the chemical challenge of this proposal, James M. Tour of the University of South Carolina in Columbia began efforts to make real versions of Aviram's mind-confined molecules.

Now he has something to show for his efforts. In the July 4 JOURNAL OF THE AMERICAN CHEMICAL SOCIETY, Tour and graduate students Ruilian Wu and Jeffry S. Schumm report making batches of two molecules with structures close to the ones Aviram proposed.

Aviram's hypothetical structures consist of two polymeric chains, each about a millionth of a centimeter long, linked to each other at a 90[degrees] angle via a twisted chemical bridge that would control the passage of electrons from one chain to the other. In an actual electronic component, the electrons would flow in and out through four electrodes, one at each end of the two chains. An additional pair of electrodes flanking the bridge would serve as a switch, opening or closing the bridge to electron traffic.

So far, the South Carolina chemists have made two types of twisted bridges and attached as many as three molecular chain links -- either pentagonal thiophenes or hexagonal phenylenes -- that extend from the bridge like outstretched arms. They still need to add three or more links to each arm to get chains that fit Aviram's specifications, Tours notes.

"The chemistry is lovely, a real tour de force," says Joel S. Miller, a solid-state chemist at Du Pont's Experimental Station in Wilmington, Del. "Tour has made a tremendous stride toward these molecules." But Miller questions an important assumption underlying Aviram's vision of molecular electronics -- namely, that single molecules can behave like electronic devices.

Inherent quantum fluctuations add a random factor to the location of electrons within individual molecules, and that would make the solo molecules unreliable components at best, Miller says. Reliability could be won by averaging the collective behavior of tens of thousands of such molecules, he adds, but such an ensemble would take as much space as the electronic components on today's chips.

Aviram and Tour acknowledge that molecular electronics research is a high-risk venture riddled with obstacles, but they say that experimental deeds, rather than theoretical words, will determine the venture's success or failure. Miller says he agrees that the time for experimental tests has nearly arrived. As Aviram puts it, "Let the molecules speak for themselves."