New metal fillings for carbon tubules.

Like hollow pastry shells begging for a creamy filling, carbon nanotubes cry out to be filled.

The exquisitely small carbon straws, first reported in 1991, show all the promise of a tantalizing new material -- if only scientists could figure out how to stuff them with something useful.

Researchers have tried various methods to fill the tubules with metal. But most such efforts turn out only sparse amounts of stuffed nanotubes, making studies of bulk quantities difficult.

Two recent reports offer some new clues for filling the tiny tubes. In the Dec. 22/29 NATURE, H. Pascard, a materials scientist at the Ecole Polytechnique in Palaiseau, France, and his colleagues describe some general principles for inserting metals into carbon nanotubes.

Using an arc-discharge technique, which involves very high temperatures and electrical voltages, the group filled carbon straws with 15 different metals. They found, among other things, that the elements chromium and gadolinium formed nanowires most readily within the slender carbon casings.

Studying data from nanotube fillings, the scientists concluded that "the propensity for forming continuous 'nanowires' throughout the length of the tubes seems to be strongly correlated with the existence of an incomplete electronic shell" in the metal's charged state. In other words, metals most likely to accept electrons in vacant "holes" are the ones most likely to succeed as carbon nanotube fillings, they say.

In a related report in the Nov. 10 NATURE, chemists Malcolm L.H. Green and S.C. Tsang of the University of Oxford in England and their colleagues describe a "wet-chemical" technique that enables them to open and fill nanotubes with many metal oxides.

The boiled tubules in a bath of nitric acid and nickel nitrate, baked them in an oven, and then cooked them some more in superheated helium. Bypassing high temperatures and voltages, they managed to fill up to 70 percent of the carbon tubes with nickel, a formidable yield. Other experiments turned out tubules full of cobalt, iron, and uranium.

Speculating on possible uses for nanotubes filled in solution, Rodney S. Ruoff, a chemist at SRI International in Menlo Park, Calif., suggests thinking of the tiny cylinders as test tubes for studying chemical reactions.

The British team says that such metalfilled tubes "might find applications in catalysis, separation and storage technology, and in the development of materials with new magnetic and electrical properties."