Mixes make tiniest transistors

Eric Smalley, Technology Research News

Think of chemistry and you usually picture drugs, plastics and household cleaners, not the future of computing. But work is steadily progressing toward a day when it will be possible to whip up a batch of molecular computer circuits.

Two research teams have fashioned individual molecules into transistors, the electrical-switch building blocks of computer circuits. The Cornell and Harvard University teams hooked single molecules to electrodes, ran electricity through the tiny transistors and measured the results.

The electrical properties of individual molecules have been measured before, but the Cornell and Harvard molecules act as transistors, not simply wires; the researchers controlled the amount of electricity the molecules allowed through by changing the strength of a surrounding electrical field.

In both teams' demonstrations, the transistor molecules spanned a gap between a pair of gold electrodes. "[We] synthesized molecules [that] act like a transistor, and then we inserted the molecules individually into a circuit and demonstrated that the transistor worked," said Dan Ralph, an associate professor of physics at Cornell.

One of the gold electrodes is a source electrode, which channels electrons into a transistor, and the other is a drain electrode, which channels electrons out of it. The electric field controls the flow of electrons through the molecule in order to turn the transistor on and off.

Millions of transistors wired together can form computer circuits because the output of one transistor can switch another transistor on. The complicated patterns of transistors switching on and off form the logic of computer processors.

The researchers' molecules are between one and three nanometers long, or about 30 to 100 times shorter than the transistors in today's computer chips.

The Cornell molecule has a single cobalt atom at its center and all of the electrons flow through the atom. "We can regulate electronic flow at the scale of a single atom," said Ralph. The Harvard molecule consists of a pair of atoms of the metal vanadium.

In a similar experiment last year, researchers at Bell Labs showed a single molecule acting as a transistor in a one-molecule layer containing a mix of insulating and active molecules that was so dilute only a single active molecule was likely to be positioned between a pair of electrodes. Bell Labs is reevaluating these results in the wake of scientific misconduct allegations, however.

The chemical process used to make molecular electronics is easier and potentially cheaper to carry out than today's semiconductor manufacturing process, which uses light and chemicals to etch lines into silicon wafers. The field of molecular electronics is still in its infancy, however, said Hongkun Park, an assistant professor of chemistry at Harvard.

Researchers have to overcome three principal challenges before they can produce practical molecular transistors. The first is achieving gain, which is the ability to put a small signal into a device and have it amplified to get a big signal out, said Ralph. Gain allows electrical signals to pass through many transistors without dying out.

The second challenge is boosting the speed of the devices, he said. "Our molecular transistors are much slower than silicon transistors."

And the third challenge is connecting molecular transistors together into computer circuits. "It will take a lot of imagination to discover ways to reproducibly connect several molecules in the right way to make useful technologies," said Ralph.

Practical applications for molecular electronics are possible in 10 to 20 years, but "that might be too optimistic," said Ralph. Applications are at least five years away, said Park.

Ralph's research colleagues were Jiwoong Park, Abhay Pasupathy, Jonas Goldsmith, Connie Chang, Yuval Yaish, Jason Petta, Marie Rinkoski, James Sethna, Héctor Abruña and Paul McEuen of Cornell University. They published the research in the June 13, 2002 issue of the journal Nature. The research was funded by the National Science Foundation (NSF), the Department of Energy, the Department of Education and the Packard Foundation.

Park's research colleagues were Wenjie Liang and Marc Bockrath of Harvard University and Matthew Shores and Jeffrey Long of the University of California at Berkeley. They published the research in the June 13, 2002 issue of the journal Nature. The research was funded by the National Science Foundation (NSF), the Defense Advanced Research Projects Agency (DARPA) and the Packard Foundation.

Timeline:   >5 years, 10-20 years
Funding:   Government, Private
TRN Categories:   Biological, Chemical, DNA and Molecular Computing; Nanotechnology
Story Type:   News
Related Elements:  Technical papers, "Coulomb blockade and the Kondo effect in single-atom transistors," Nature, June 13, 2002 and "Kondo resonance in a single-molecular transistor," Nature, June 13, 2002


June 26/July 3, 2002

Page One

PCs augment reality

Stamps bang out tiny silicon lines

Bent wires make cheap circuits

Mixes make tiniest transistors

Plastic computer memory advances


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