Pillars of the thin-film community.

For a computer chip to have a chance of working properly, the thin layers of materials that make up the chip must be perfectly crystalline and flat. Now, however, researchers are making thin films with unusual microstructures that could have a host of other applications, such as sensors, magnetic recording media, or flat-panel displays. Two recent studies describe ways to create films with a microstructure reminiscent of trees growing in a forest.

At the University of Toronto, a team of scientists has succeeded in planting a forest of carbon nanotubes on an aluminum foundation. The technique not only allows the nanotubes to grow in a regular arrangement but is also "a way of mass-producing nanotubes of the diameter that you want," says study coauthor Jimmy Xu, soon to be at Brown University in Providence, R.I.

Xu and his colleagues etch an array of tiny holes into a thin piece of aluminum. Inside each hole, they deposit a cobalt catalyst. When the template is exposed to an acetylene vapor at 650 [degrees] C, carbon nanotubes, about 47 nanometers in diameter, grow out of the holes. They can reach heights of up to 100 micrometers, Xu says.

Researchers commonly make nanotubes by coursing electricity through a rod of solid carbon, which then throws off soot containing carbon molecules of many shapes and sizes. Bundles of nanotubes produced this way must be chemically separated, sorted by size, and manipulated. "That's a pain," Xu notes. Nanotubes produced with the Toronto group's technique, by contrast, are already pure, uniform in size, and untangled. He and his colleagues report their finding in the July 19 APPLIED PHYSICS LETTERS.

Since carbon nanotubes can emit electrons from their tips, scientists are tapping them for flat-panel television and computer screens (SN: 9/16/95, p. 183). Today's flat-panel technology uses crystalline semiconductors that can't be bent easily. Xu says that his nanotube film can be deposited on curved surfaces to make flexible displays.

Michael J. Brett of the University of Alberta in Edmonton and his colleagues take another route to growing films thickly populated with tiny pillars. Their trick is to rotate the foundation while bombarding it with atoms at a very shallow angle. Once a pillar begins to grow at some location, it shadows an area around it, preventing atoms from sticking. Depending on how fast they rotate the foundation, the researchers can create posts that are straight and smooth, twisty like a spring, or something in between. They describe their technique in the July JOURNAL OF MATERIALS RESEARCH.

Scientists developing better magnetic storage media are "considering having individual pillars as recording bits. That might allow greater density," Brett says. "The standard way is to make a film, then etch away the parts you don't want, leaving pillars. Maybe we have a simple process to make the pillars in just one step."