Jan. 4, 2002 PHILADELPHIA – First came fullerenes, those cage-like molecules of 60 carbon atoms bound in a ball. Then came long, thin soda straws of carbon atoms called nanotubes. Now there are fullerenes nested within nanotubes, like so many peas in a pod.
This week, scientists at the University of Illinois at Urbana-Champaign and the University of Pennsylvania report in the journal Science on their recent discovery that these nanoscopic peapods – the latest class of nanomaterials created by filling the cores of single-wall nanotubes – have tunable electronic properties. For shrinking circuits, nanotubes are the silicon of nanoelectronics, and the new findings could have far-reaching implications for the fabrication of single-molecule-based devices, such as diodes, transistors and memory elements.
The peapod samples were produced using molecular self-assembly techniques by David E. Luzzi and his group at Penn, who were the first to synthesize these complex nanostructures.
"When we first created peapods, it provided the first glimpse of a toolbox of nanomaterials that could provide the same excellent mechanical strength and thermal conductivity of nanotubes but would have other tunable properties – optical, electrical or catalytic – to provide the diverse functionality needed for integrated and complex nanodevices," said Luzzi, professor of materials science and engineering at Penn and a co-author of this week’s Science paper. "This work confirms that these materials are not peas in a pod but actually peapods, a completely new material."
The new findings point to the future design of other hybrid nanoscale structures that could be tailored for a particular electronic function. Much like the dopant added to silicon, which turns beach sand into today’s computer chips, the encapsulated molecules could make nanotubes more attractive as the material of choice for future nanocircuits.
"Our measurements show that encapsulation of molecules can dramatically modify the electronic properties of single-wall nanotubes," said Ali Yazdani, a professor of physics at UI and senior author of a paper to appear in Science, as part of the Science Express web site, on Jan. 3. "We also show that an ordered array of encapsulated molecules can be used to engineer electron motion inside nanotubes in a predictable way."
To explore the properties of these novel nanostructures, Yazdani and UI graduate student Daniel J. Hornbaker used a low-temperature scanning tunneling microscope that they built. With their high resolution microscope, the researchers were able to image the physical structure of individual peapods and to map the motion of electrons inside them.
By examining the images of individual peapods, the UI researchers found that the encapsulated fullerenes modify the electronic properties of the nanotube without affecting its atomic structure.
"In contrast to unfilled nanotubes, peapods exhibit additional electronic features that are strongly dependent on the location along the tube," Yazdani said. "By mapping electron waves of different energies inside these nanoscale structures, we can begin to unravel the complex interaction in these systems and better understand their electronic properties."
To further demonstrate the importance of the C-60 molecules in determining the electronic properties of the peapods, the researchers used the scanning tunneling microscope to manipulate the encapsulated molecules. With this unique experimental technique, they were able to compare the measurements performed on the same section of nanotube with and without the encapsulated molecules.
How the measured electronic properties of the peapod differed in the two cases provides insight into what could become design rules for hybrid structures having a specific type of electronic functionality. Because the local electronic properties of single-wall nanotubes can be selectively modified by the encapsulation of a single molecule, for example, the technique might one day be used to define on-tube electronic devices.
Penn physics Professor Eugene J. Mele modeled the experimental findings.
"The encapsulated balls have a much stronger effect on the electronic structure of the tube than we had expected," said Mele, also a co-author of the Science paper. "Fortunately, we were saved by the high geometrical symmetry of these structures. That allowed us to develop a good model and in the end the physics turned out to be very intuitive and pretty."
The researchers speculate that the lessons learned in unraveling the properties of this complex nanostructure also may apply elsewhere.
"As the drive toward miniaturization of electronic devices continues, concepts such as symmetry of electronic states may be useful in controlling the electronic properties of individual nanostructures and for coupling them together," Yazdani said.
Luzzi, Yazdani, Hornbaker and Mele were joined on the Science paper by Alan T. "Charlie" Johnson, associate professor of physics and astronomy at Penn; former Penn researcher Brian W. Smith; and Se-Jong Kahng and S. Misra at UI. The work was funded by the National Science Foundation.
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