A New "Twist" On Nanostructures
These so-called nanohelices have both piezoelectric and semiconducting properties. They get their shape from twisting forces caused by a small mismatch between alternating single-crystal "stripes" just 3.5 nm wide that are offset by about 5°. The nanohelices are roughly 100 m long, about 300 to 700 nm in diameter, and from 100 to 500 nm wide. They come in both right and left-handed versions, with production split nearly 50-50 between the two directions.
In the lab, a process similar to that used for nanobelts and nanosprings makes nanohelices. Zinc-oxide powder goes inside an alumina tube, which itself goes in a horizontal high-temperature tube furnace. The furnace heats the material under vacuum to about 1,000°C, at which point an argon carrier gas is introduced. Heating continues until the furnace reaches about 1,400°C. The nanohelix structures form on a polycrystalline aluminum-oxide substrate. Unlike earlier single-crystal nanosprings which are elastic, nanohelices are rigid and retain their shape even when cut apart. A slight change in the fab process accounts for the different structures.
"With the earlier structures we let in the carrier gas at the beginning," explains Zhong Lin Wang, a professor of materials science and engineering at Georgia Tech. "For nanohelices, we start carrier-gas flow when the furnace reaches a certain temperature. That allows helix formation to begin in a vacuum, which is key to controlling helix shape."
The first dozen batches of nanohelices had a yield of about 10%, though Wang believes that can improve over time. So far the team has made about 20 different zinc-oxide nanostructures, including nanobelts, aligned nanowires, nanotubes, nanopropellor arrays, nanobows, nanosprings, nanorings, nanobowls, and others. Funding for the research comes from the National Science Foundation, NASA Vehicle Systems Program, U.S. Dept. of Defense Research and Engineering (DDR&E), Darpa, and the Chinese Academy of Sciences.