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."
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