Image: Physical Review Letters

Hollow nanotubes made of pure carbon possess remarkable strength and potentially unique electrical properties, making them an interesting starting material for new sorts of devices. To that end, researchers have investigated how the tiny tubes--measuring as little as one nanometer in diameter--can be filled with other compounds to tweak their behavior. In 1998 David Luzzi and colleagues at the University of Pennsylvania created "nanoscopic peapods," which were in essence nanotubes stuffed with chains of carbon spheres called buckyballs. Now Sumio Iijima of Meijo University and NEC Research Corporation, who discovered carbon nanotubes in 1991, Hisanori Shinohara of Nagoya University and colleagues have done the same thing using a different sort of pea. Instead of the standard buckyball sphere of 60 carbon atoms, C₆₀, the Japanese scientists filled nanotubes with C₈₂ buckyballs containing gadolinium atoms. Their work appears in the December 18th issue of Physical Review Letters.

Because adding metal atoms such as gadolinium to buckyballs--turning them into so-called metallofullerenes--changes their electronic structure, the team hoped that adding metallofullerenes to nanotubes might have a similar effect. To assemble their pea pod, they first formed the metallofullerenes (Gd@C₈₂) and single-walled carbon nanotubes (SWNTs), opening their ends in dry air at 420 degrees Celsius. The Gd@C₈₂, once heated to a vapor, readily entered the opened tubes, and lined up inside the tubes at roughly 1 nanometer intervals (see image). Next they probed the new structure, (Gd @C₈₂)_n@SWNT, with electrons. What they discovered was that the electrical resistance of the filled SWNTs varied more dramatically with respect to temperature than did empty SWNTs. The scientists were unable to cofirm anything more conclusive, but the work is significant in that it is the first to show that stuffing can change SWNTs' electrical properties.