Sweating the small stuff Home

Nanotech Fine-Tuning
By Mark K. Anderson

The versatility of carbon nanotubes, those sheets of graphite rolled into long tubes mere nanometers in diameter, has long been trumpeted. But until recently no one knew the nanotube was like a trombone.

According to a team of physicists from the United States and South Korea, nanotubes can be tuned with the movement of molecules rolling around inside -- like a trombone changes its pitch with the up and down motion of its slide. These adjustable electric properties offer a new kind of tunable circuit component, one that will join regular, unfilled nanotubes as the great multi-purpose device of the nanometer-sized world.

"If we're going to use nanotubes for any application, we're hoping that they'll one day replace not only wires but also be a sort of template for molecular scale electronics," said Ali Yazdani of the University of Illinois.

Yazdani is one of eight co-authors on the paper that first studied the electronic properties of these stuffed nanotubes. The paper will appear in an upcoming issue of Science and now appears on the journal's Science Express website.

"In a transistor, you want to modulate the electronic properties to control or gate the flow of electrons through it," Yazdani said. "That's the basic idea behind an electronic device."

Yazdani's device is called a nanotube "peapod" -- whose "peas" are typically the spherical C60 molecule, also known as buckminsterfullerene or buckyballs. By encapsulating C60 within a nanotube, Yazdani's team found that the electronic properties of the system varied from semiconductor to conductor to insulator, depending on the peas' positions.

They note that, if the peas are spaced periodically, quantum wave resonances of the electrons traveling through the system can also be tuned. This could, in turn, open the door to using the peapod as a medium for quantum computations.

"People talk about using quantum dots for quantum computations," Yazdani said. "A C60 molecule is a small dot ... and understanding these (peapod) structures may give us a clue how to engineer these quantum dot-like states."

Yazdani, et al, further speculate that such tunable-nanotube effects may extend to internal molecules other than C60.

"It's an intriguing structure," said Cees Dekker of the Delft University of Technology in the Netherlands. "Up till now there were only images, and Yazdani's study is the first physical study of the properties. In that sense, it's important and interesting.

"But if you ask me what's the great prospect of these peapods -- are they going to replace nanotubes? I would say no."

Dekker, whose nanotube circuit-making has also recently made headlines, said that he is still reserving judgment on the nanotube peapod's usefulness until he sees more.

Yazdani cited the 1999 computer simulations of an NEC-Michigan State team who studied a closed peapod system with one pea. They discovered that this pea could in fact switch between one end of the pod and the other -- think "0" and "1" -- in only 10 trillionths of a second (10 Pico seconds).

This beats the fastest computer memory chips on the market today by several orders of magnitude.

"These things can be manipulated," Yazdani said. "This is going to make nanotubes even more attractive."

Still, the plain old nanotube has already demonstrated its usefulness in computer circuits, said Calvin Quate of Stanford, thanks in no small part to Dekker's work (PDF).

"Moving these peas around in a peapod is futuristic," he said. "That's a possibility for the future. But Dekker's work is here and now."

Yazdani noted that he doesn't see peapods as any substitute for the power of the unadorned nanotube. He just argues that the introduction of peas to this pod adds a powerful element that could increase the nanotube's functionality in unanticipated ways.

"Nanotubes are starting to be used everywhere now -- in molecular scale device making and logic circuits," he said. "This is a new twist, and it could be a very important twist, because you can then start tuning the nanotube's properties with encapsulation."