Scientists at the University of Illinois at Urbana-Champaign (UI) and the University of Pennsylvania (Penn) have created a new class of nanomaterial with tuneable electronic properties. Filling the cores of single-wall nanotubes created this latest class of nanomaterials, dubbed peapods by the researchers.
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 samples were produced using molecular self-assembly techniques by David E. Luzzi and his group at Penn, who were the first to synthesise these nanostructures.
The new findings are said to 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. ‘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 nanostructures, Yazdani and UI graduate student Daniel J. Hornbaker used a low-temperature scanning tunnelling 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 (cage-like molecules of 60 carbon atoms bound in a ball) 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 tunnelling microscope to manipulate the encapsulated molecules. With this 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 be used to define on-tube electronic devices.