Sep 9, 2005
Zinc-oxide nanohelix gets in a twist
Researchers at the Georgia Institute of Technology, US, Peking University, China, and the National Center for Nanoscience and Technology of China (NCNST) have created nanohelices of zinc oxide for the first time. Zinc oxide is a piezoelectric semiconductor that's also biocompatible - until now its only nano-forms have been nanobelts, nanosprings and nanorings.
"A previously unknown zinc-oxide nanostructure that resembles the helical configuration of DNA could provide engineers with a new building block for creating nanometre-scale sensors, transducers, resonators and other devices that rely on electromechanical coupling," Zhong Lin Wang of Georgia Institute of Technology, US, Peking University, China, and NCNST told nanotechweb.org.
To make the nanohelices, Wang and colleagues used a vapour-solid growth process. They heated zinc-oxide powder, which has a wurtzite-crystal structure, to 1000 °C in a vacuum before introducing an argon carrier gas. Further heating at 1400 °C caused nanohelix structures to form on a polycrystalline aluminium oxide substrate.
"The key difference between growing nanohelices and the earlier types of nanobelt is that we control raising the temperature and when we introduce the carrier gas," said Wang. "With the earlier structures, we introduced the carrier-gas flow at the beginning. With these nanohelices, we only introduce the carrier gas when the temperature reaches a certain level. That allows formation to begin in a vacuum, which is the key to controlling the helix formation."
Heating zinc oxide in a vacuum creates structures with polar surfaces. The scientists believe that the introduction of the carrier gas caused the growth to minimize the amount of polar surface produced, leading to a superlattice structure with mismatches at the crystalline interfaces.
The superlattice consisted of alternating long bands of single crystal positioned with their c-axes perpendicular to one another. The bands were about 3.5 nm wide and each was offset by about 5°. "This twist angle induces the rigid rotation of the stripes, leading to the formation of the helical structure," said Wang. The nanohelices had conventional single-crystal nanobelt structures at both ends.
The typical yield of nanohelices was roughly 10%, while the ratio of right-handed to left-handed helices was about 50:50. The structures were up to 100 μm long and had diameters of 300-700 nm and widths of 100-500 nm. Unlike the single-crystal nanosprings that the researchers have produced previously, nanohelices are rigid and retain their shape even when cut apart.
"This is a brand-new structure that shows a new growth model for nanomaterials," said Wang. "But from the properties point of view, these are like the earlier nanobelts in having semiconducting and piezoelectric properties which make them good for electromechanical coupling."
For now the researchers are focused on building novel resonator, inductor and piezoelectric devices based on the nanohelix for use in sensors. After that they plan to make multifunctional devices by integrating nanohelix devices with those made from other materials.
The researchers reported their work in Science.
About the author
Liz Kalaugher is editor of nanotechweb.org.