Matchsticks, Zig-Zags, S-shapes, and Bedsprings

Liquid crystals, that curious phase of matter between solid and liquid, play tricks with light: they change its direction of vibration as it traverses them. Some liquid crystals split light into left and right-handed rays, whose vibrations describe opposing spirals as they pass through a crystal's layers. "One ray rotates clockwise, the other counterclockwise, at different velocities," explains Akhlesh Lakhtakia. "Or both rays may describe clockwise spirals, or counterclockwise, but still move at different velocities." One ray may bounce off the material, while another passes through it unscathed.

Liquid crystals, however, are not very stable: their properties change with temperature—as witness the mood ring, or the more recent LCD-studded t-shirts, both of which respond not to emotional fluctuations but to body heat.

What if you could design a stable material with the same light-altering properties?

That's the question Akhlesh Lakhtakia asked himself a few years ago.

Lakhtakia, Penn State associate professor of engineering science and mechanics, considers himself a theoretical electromagneticist. It's his business, in other words, to ponder how light moves through materials. "Especially exotic materials."

His study of liquid crystals taught him that "a material's microstructure gravely influences wave propagation through it—that shape controls properties."

This principle Lakhtakia was used to hearing from Russell Messier, who works across the hall. Messier, professor of engineering science and mechanics, is an expert on columnar thin films, another kind of in-between material, which possesses some of the qualities of bulk materials, some of the properties of molecules.

Columnar thin films are grown by painstakingly depositing gaseous atoms of metal, ceramic or other material onto a chosen substrate, or base material. If conditions are right, tiny clusters of particles can be directed to settle on top of one another, "grown" as tiny columns. Messier was one of the pioneers in explaining how such structure is achieved.

In recent years, Messier and other researchers have demonstrated that the shaping of a thin film can be extremely fine-tuned: they have successfully changed the direction of a column's growth within a space of three nanometers or less. (That's three millionths of a millimeter!)

If we can influence a thin film's structure at such a scale, Lakhtakia reasoned, why can't we engineer columns in the shapes we want—as helices, say, replicating the spiraling of liquid crystals? If we could, presumably we'd have a stable material that exhibited liquid crystal properties.

He asked Messier, who said he thought the concept was sound. But Messier's lab was not set up to make such a material. "It would have required a major redesign of his equipment," says Lakhtakia.

Messier, however, knew of two researchers at the University of Alberta, Michael Brett and Kevin Robbie, who had previously published a paper depicting some two-dimensional zig-zag columns they had grown. Lakhtakia called these colleagues in Canada, who agreed to give the idea a try.

Lakhtakia picks up the story: "One day in March of last year I turned on my Mac and found the SEMs waiting. They had made the material, and had e-mailed me the micrographs."

The pictures show thick forests of spirals made of magnesium fluoride rising from a flat substrate. "Bedsprings," Lakhtakia calls them.

Now Lakhtakia, Messier, and their colleagues envision a whole class of such materials, which they call "sculptured thin films." Already, they have drawn up four basic shapes as building blocks: matchsticks, zigzags, and S-shapes, as well as bedsprings.

"If we can engineer at the scale of three nanometers," Lahtakia says, "we can build these shapes. They can be conformed to allow certain wavelengths of light to pass through, and to polarize or to focus light. And unlike liquid crystals, they should be able to withstand wide temperature changes and high pressures, because they will have no internal stresses to make them brittle."

These materials will be useful, he says, in many devices, from optical sensors to broad-range thermometers to ultra-thin photographic lenses—even for biomedical applications. Sculptured thin films, Lakhtakia suggests, could be fashioned into tiny sieves for trapping viral particles, which run to about 50 nanometers in diameter—too small for your ordinary microfilter.

"These films are highly porous," he explains. "They're up to 80 percent air. We could vary the shape in such a way that a virus passing through fits the microstructure like a key fits a lock.

"A directed microstructure," Lakhtakia concludes, "is something we can exploit in many ways. This is true nano-engineering."

Akhlesh Lakhtakia, Ph.D., is associate professor of engineering science and mechanics in the College of Engineering, 224C Hammond Building, University Park, PA 16801; 814-863-4319. Russell F. Messier, Ph.D., is professor of engineering science and mechanics. The results reported above appeared in the Nov/Dec 1995 issue of the Journal of Vacuum Science and Technology A.

Last Updated September 01, 1996