By Peg Brickley
Small Times Correspondent

Feb. 27, 2002 — South Korean scientists are hailing another advance in tailoring carbon nanotubes, the long thin structures that many are looking to as the building block for the next generation of nanoscale devices.

Researchers at Seoul National University in South Korea packed nanotubes with tiny spherical molecules called fullerenes to create regions of varying semiconducting properties within each tube.

The result is a hollow structure containing the equivalent of a series of tiny transistors


This illustration shows the atomic structure
of a single-wall carbon nanotube “peapod” filled
with fullerenes. Superimposed on the image are
electron waves.

Image courtesy of D. Hornbaker and A. Yazdani
far smaller than any now in existence, according to a letter to appear in this week’s issue of Nature magazine.

The breakthrough could lead to an expanded universe of applications for devices like diodes, memory elements and logic circuits based on single molecules.

The advance is similar to a breakthrough made by a U.S. group earlier this month that made what it called “nanoscopic peapods.” Those results were printed in the journal Science earlier this month.

Excitement over the possibility of making molecular-level transistors had researchers on both sides of the Pacific working to figure out how to manipulate and measure the conducting properties of the tubes.

Young Kuk, speaking for the team from Seoul, said the day when carbon nanotubes will replace silicon-based transistors to kick off a new era in miniaturization is still years away. But the peas-in-a-pod structure, where the fullerenes are the peas and the carbon nanotube is the pod, is a significant step toward that future.

The advance demonstrates a way to fine-tune the carbon straws, opening up the field of potential uses, Kuk said.

Making dot devices in two or three dimensions is “expensive and difficult,” the Korean scientist said. But inserting fullerenes into the single-walled carbon nanotubes produces multiple one-dimensional quantum dots, raising the possibility of creating light-emitting devices and integrated devices with high density.

Called single wall nanotubes, or SWNT for short, the structures were promising in theory, but difficult to assess until recently. Researchers at the University of Pennsylvania in Philadelphia, who claim to be the first to use molecular self-assembly techniques to synthesize the peapods, worked with physicists at the University of Illinois in Urbana to assess the effect of the encapsulated balls on the electronic structure of the tube.

Illinois physicists used a low temperature scanning tunneling microscope to evaluate the conducting properties of the fullerene-packed carbon nanotubes, mapping the movement of electrons in the hybrid structure.

The conclusion? “Our paper showed that the encapsulation of molecules inside the nanotube’s hollow core is a viable way to control or manipulate their electronic properties,” said Ali Yazdani, a professor of physics at the University of Illinois and senior author of the paper published in Science.

He added that he would be eager to read what the Koreans had devised in their variation on the peas-in-a-pod composite structure, which used larger fullerenes than the American version.

Illinois and Pennsylvania researchers used spheres made of 60 carbon atoms, while the Korean “peas” were made up of 82 carbon atoms and one of gadolinium.

“Their structure, as I understand it, puts quite a bit of strain on the nanotube cage, kind of like a snake swallowing a mouse,” the Illinois physicist said. In contrast, the smaller American molecule fits into the nanotube in a way that allows it to slide around without modifying the structure.

Kuk said his group had begun the patent process in Korea and elsewhere, with an eye toward the day when silicon-based scaled down technology hits a “technical and economic brick wall sometime around 2010.”

Related News
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