Electronic Soup: Molecules in acid broth act as circuit parts

Single molecules may someday replace the transistors and other already tiny components in today’s microchips. Yet many of the molecules that chemists have found to have promising electronic traits don’t behave as expected in circuits.

GOLDEN HANDCUFFS. Suspended in a sulfuric acid solution between an ultrasharp gold probe (top cylinder) and a gold electrode (bottom spike), a single molecular strand (chain of rings) transforms into a precisely adjusted electronic amplifier. Its conductivity varies with the voltages produced by the sources in the setup. Water molecules are shown surrounding the chain. Lindsay

Now, Stuart Lindsay of Arizona State University in Tempe and his colleagues have fabricated single-molecule switches that the scientists say are more predictable. The key, Lindsay says, is to immerse molecules in solutions rich in electrically charged particles, or ions. That liquid, he says, provides a means for controlling the electronic states of the individual molecules.

“You have a much better-controlled environment,” comments Mark A. Ratner of Northwestern University in Evanston, Ill.

Commercial chip makers might be disturbed at needing “sulfuric acid sloshing around on their chips,” Lindsay acknowledges. However, he maintains that malleable solids could also provide suitable ionic environments “that folks at IBM would be happy with.”

Even if the headaches of an ionic setting for chip components prove overwhelming, a similar strategy might work in energy-production applications. For example, solar cells and hydrogen generators could rely on arrays of trillions of electronically active molecules. “Squishy” ionic environments could be particularly useful in efforts to put molecules derived from plants and bacteria into such devices, Lindsay says.

“Charge transfer in nature takes place in the presence of a solution with ions,” he notes. “It’s very different than silicon.”

He described his research team’s work on Feb. 18 in Washington, D.C., at the annual meeting of the American Association for the Advancement of Science.

In recent experiments, Lindsay and his colleagues at Arizona State and Columbia University turned to oligoaniline molecules, which link together as polyaniline. This conducting polymer is increasingly used in flexible circuits and displays, lighting, and conductive films. “It’s the most widely used organo-electronic material in the world,” Lindsay says (SN: 5/17/03, p. 312: Available to subscribers at Plastic Electric).

The experimental setup that Lindsay and his colleagues developed enabled the team to adjust the electronic states hordes of oligoaniline molecules and simultaneously control the states of single molecules, Lindsay says.

This dual approach took some chemistry: The team had to synthesize strands of oligoaniline, modify both ends of each strand to include a chemical group that sticks to gold, coat a gold electrode with a forest of the molecules, and finally immerse the molecule-bedecked electrode in sulfuric acid.

To study the electronic behavior of individual molecular strands within that forest, the researchers used an extremely fine gold probe to briefly catch an individual oligoaniline strand and monitor its voltage and current. Simultaneously, the researchers adjusted the voltage on the electrode to simultaneously flip all the oligoaniline molecules between electrically conducting and insulating states.

Thanks to the acid bath, the researchers say, they could induce individual molecules to act as electronic switches and amplifiers whose performance could be controlled by precise adjustment of electric fields in the solution. Such components could prove useful as molecule-scale computer components, Lindsay suggests (SN: 9/20/03, p. 182: Available to subscribers at Molecular Memory: Carbon-nanotube device stores data in molecules).

The team found that in a nonionic solution, the molecules performed only as resistors. For circuits to do much more than dissipate energy, they require sophisticated components, such as amplifiers and transistors, Lindsay explains.

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