Nano-Switches Could Yield Even Smaller Gadgets

Ben Harder
for National Geographic News
August 16, 2005
Gadgets seem to get smaller every day, but the silicon-based technology used in most of today's electronics can only get so much smaller.

Now researchers have cooked up tiny Y-shaped tubes of carbon that act like electrical switches. The new tubes could someday become the foundation for ultraminiature electronics.

Conventional carbon nanotubes are hollow, unbranched cylinders of rolled up carbon atoms. These nanotubes, first made in the early 1990s, are more than a thousand times thinner than human hairs and are good electrical conductors.

"Electrons can stream through these nanotubes without any problems," said Apparao M. Rao, a physicist at Clemson University in South Carolina and coauthor of the study. Because the tubes are good at carrying a current, he said, "they're ideal for nano-electronics."

But conductive materials aren't enough to make an electronic gadget work. Any device needs built-in switches to turn currents on and off.

The new carbon nanotubes are shaped like y's and have small metal particles embedded at their junctions. The structure allows researchers to control the flow of electricity through the different branches.

The researchers' findings appear in the September issue of the journal Nature Materials.

Rewriting Moore's Law

Today's commercial electronics use silicon switches called transistors—often millions of them—to control electrical flow.

For decades the number of silicon transistors that engineers have been able to squeeze onto each inch (3 centimeters) of circuitry has roughly doubled every 18 months.

This clockwork-like increase in computing power is called Moore's Law. The law is named after Gordon E. Moore, co-founder of computer chip maker Intel, who made the observation in 1965.

But the law also implies a limit to manufacturers' abilities to scale down electronics. Reducing the bulkiness of silicon transistors has become a challenge. At some point, engineers won't be able to squeeze any more transistors on a chip.

Researchers hope that it might eventually be possible to replace standard silicon transistors with much smaller versions fashioned from carbon nanotubes.

Rao and his colleagues at Clemson University figured out how to make the Y-shaped nanotubes by adding metal particles during the formation process. When a particle is trapped in a half-formed nanotube, the tube branches.

Prabhakar Bandaru, a materials scientist at the University of California, San Diego, heard the news and started thinking: The new tubes had three prongs—just like a traditional transistor.

"I was curious whether there could be some transistor-like properties" in the nanotubes, Bandaru said.

Bandaru and his colleagues borrowed some nanotubes from Rao and performed a series of experiments. They noticed that applying a current to one prong of the Y-shaped nanotube affected a separate current that was already running between the other two prongs.

Regardless of which prong they used to apply current, they could manipulate the current in the other prongs as if they were opening and closing a gate.

"We created a completely new kind of transistor," Bandaru said. "There's no silicon at all."

From Lab Bench to Laptop

"The gate modulates the current between the source and the drain," commented Jia Chen, an IBM research scientist in Yorktown Heights, New York. "That's the basic element of a transistor."

But consumers shouldn't expect to find carbon transistors in their electronics any time soon. Chen and Bandaru agree that commercial applications are still years away.

To make a working chip, engineers would need to assemble millions of evenly spaced, predictably shaped nanotube transistors.

"We don't know yet how to assemble these in a regular fashion," Bandaru said. Even making tidy arrays of standard cylindrical nanotubes has been a challenge. "There have been a lot of attempts, but nothing that is economically feasible."

The new Y-shaped nanotubes also don't perform nearly as well as current silicon-based transistors. According to Chen, the gate in the carbon nanotube couldn't completely halt the flow of electrons as it would in a true transistor. "There was leakage," she said.

As a material for making transistors, carbon "is probably not going to catch silicon in [the next] 10 to 20 years," Chen predicted. "At this stage nothing really competes."

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