STM technique yields tiniest battery.

Electrochemists looking to apply their skills to the nascent field of nanotechnology have created an itsy-bitsy battery, 100 of which would fit into a single human red blood cell.

The record-small battery consists of pillars of copper and silver laid down on a graphite surface with a scanning tunneling microscope (STM), says Reginald M. Penner of the University of California, Irvine. Penner calculates that the battery generates one-fiftieth of a volt during its 45-minute lifespan. He and his colleagues describe how to make the battery in the Aug. 6 JOURNAL OF PHYSICAL CHEMISTRY.

Although other scientists have used STMs to move or deposit one kind of atom in lines or piles on a surface, the Irvine group is the first to succeed in placing different metal atoms close to each other, Penner told SCIENCE NEWS.

"That he was able to deposit different materials in a controlled fashion is significant," says Bruce Parkinson, an electrochemist at Colorado State University in Fort Collins. "There are new experiments that you can do now that you can deposit different materials."

To site a pillar, Penner and his colleagues first turn up the STM voltage, which digs a small pit in the very smooth graphite surface. During this step, the graphite and the STM tip are immersed in a dilute solution containing silver ions. At the pit, a few silver ions lose their positive charges and come out of solution. Then more silver naturally gathers at this site. The technique works at room temperature, Penner says.

Next, the researchers replace the silver solution with a copper sulfate solution and use voltage pulses from the STM to make pillars of copper near two silver piles. When they first did this experiment, they expected to have to switch the copper solution for a silver solution in order to discharge the battery. That's the way silver-copper batteries typically function.

"But this battery didn't work the way we expected it to," Penner recalls. Even before the researchers switched solutions, they noticed the copper pillars shrinking and the silver ones growing. These changes indicated that a voltage existed in the tiny battery.

In this case, "the copper wants to deposit on silver more than it wants to deposit on itself," Penner explains. On the basis of a macroscopic version of this battery, the researchers conclude that once a two-atom layer of copper coats the silver pillar, dissolution and deposition cease, and the battery dies. Overall, about 75,000 copper atoms dissolve.

Because of its size, the 500,000-atom battery will never generate very much current for very long, but the small size means that it does produce a large electric field, says Penner. He hopes to use the battery to study how proteins, such as muscle's actin, orient in this field.

The battery will also let researchers study corrosion -- another electrochemical reaction -- on a nanometer scale, he says.

Penner plans to try other combinations of metals, and he expects that cadmium-silver nanobatteries will discharge much faster and be much more powerful.