Nanohelix Structure Provides New Building Block For Nanoscale Piezoelectric Devices

Basedon a superlattice composed of alternating single-crystal "stripes" justa few nanometers wide, the "nanohelix" structure is part of a family ofnanobelts -- tiny ribbon-like structures with semiconducting andpiezoelectric properties -- that were first reported in 2001.

Thenanohelices, which get their shape from twisting forces created by asmall mismatch between the stripes, are produced using a vapor-solidgrowth process at high temperature. Information about the growth andanalysis of the new structures will be reported in the September 9issue of the journal Science.

The research was sponsored by theNational Science Foundation, NASA Vehicle Systems Program, U.S.Department of Defense Research and Engineering (DDR&E), the DefenseAdvanced Research Projects Agency (DARPA), and the Chinese Academy ofSciences.

"This structure provides a new building block fornanodevices," said Zhong Lin Wang, a Regents professor in the School ofMaterials Science and Engineering at the Georgia Institute ofTechnology. "From them we can make resonators, place molecules on theirsurfaces to create frequency shifts -- and because they arepiezoelectric, make electromechanical couplings. This adds a newstructure to the toolbox of nanomaterials."

With theirsuperlattices composed of many near-parallel single-crystal stripeseach about 3.5 nanometers wide and offset about five degrees, thenanohelices are very different from the nanosprings and nanorings ofzinc oxide reported by the same research group in Science in 2004.Nanosprings are composed of a single crystal whose shape is governed bybalancing the electrostatic forces created by opposite electricalcharges on their edges with the elastic deformation energy of theentire structure.

The nanohelices reach lengths of up to 100microns, with diameters from 300 to 700 nanometers and widths from 100to 500 nanometers. The nanohelices exist in both right- and left-handedversions, with production split approximately 50-50 between the twodirections.

"This is a brand new structure which shows a newgrowth model for nanomaterials," Wang said. "But from the propertiespoint of view, these are like the earlier nanobelts in havingsemiconducting and piezoelectric properties which makes them good forelectromechanical coupling."

However, unlike the earliersingle-crystal nanosprings which are elastic, the nanohelices are rigidand retain their shape even when cut apart.

"When we first sawthese structures, we were amazed by their perfection," said Wang, whois also director of Georgia Tech's Center for Nanoscience andNanotechnology. "Once you form a nanohelix, it is perfectly uniform."

Thenanohelices are formed using a simple process similar to the one usedfor fabricating other nanobelts. However, changing the growthconditions leads to entirely different structures.

Zinc oxide(ZnO) powder is positioned inside an alumina tube in a horizontalhigh-temperature tube furnace. Under vacuum, the material is heated toapproximately 1,000 degrees Celsius, at which point an argon carriergas is introduced. Heating continues until the furnace reachesapproximately 1,400 degrees. The nanohelix structures form on apolycrystalline aluminum oxide (Al2O3) substrate in the furnace.

"Thekey difference between growing nanohelices and the earlier types ofnanobelt is that we control raising the temperature and when weintroduce the carrier gas," explained Wang. "With the earlierstructures, we introduced the carrier gas flow at the beginning. Withthese nanohelices, we only introduce the carrier gas when thetemperature reaches a certain level. That allows formation to begin ina vacuum, which is the key to controlling the helix formation."

Heatingthe zinc oxide powder in a vacuum leads to formation of structures withpolar surfaces. When the carrier gas is introduced, the growth changesto minimize the polar surfaces, creating the superlattice structurewith mismatches at the crystalline interfaces. The nanohelices beginand end with conventional single-crystal nanobelt structures. "By thetime the carrier gas is introduced, the crystal orientation is fixed,but the structures must continue to grow," Wang explained. "Introducingthe carrier gas initiates a transition to the superlattice structure."

Formationof a nanohelix is initiated from a single-crystal stiff nanoribbon thatis dominated by polar surfaces. An abrupt structural transformation ofthe single-crystal nanoribbon into stripes of thesuperlattice-structured nanobelt leads to the formation of a uniformnanohelix due to rigid structural alteration, Wang said. Thesuperlattice nanobelt is a periodic, coherent, epitaxial and parallelassembly of two alternating stripes of zinc oxide crystals orientedwith their c-axes perpendicular to one another. Growth of the nanohelixis terminated by transforming the partially polar-surface-dominatednanobelt into a non-polar-surface-dominated single-crystal nanobelt.

"Thedata suggest that reducing the polar surfaces could be the drivingforce behind the formation of the superlattice structure, and the rigidstructural rotation and twist caused by the superlattice results in theinitiation and formation of the nanohelix," Wang explained.

Thefirst dozen batches of nanohelices produced a yield of only about 10percent, but Wang believes that can be improved over time. Thus far,Wang's research team has produced nearly 20 different zinc oxidenanostructures, including nanobelts, aligned nanowires, nanotubes,nanopropellor arrays, nanobows, nanosprings, nanorings, nanobowls andothers. And there may yet be other structures discovered.

"Younever know what other structures might be out there that could be addedto this toolbox," he said. "From the richness of this configuration andthe complete properties, this is a unique material that could becomethe new material for nanotechnology following carbon nanotubes."

Awideband semiconductor, zinc oxide also has interesting piezoelectricand optical properties, can produce ultraviolet laser emissions andshows electroluminescence at room temperature. Those properties make itpotentially useful in many applications.

"You can use it forspintronics, biomedical applications and many things you can make withsilicon technology," Wang said. "Zinc oxide is much cheaper and easierto work with than gallium nitride."

Other collaborators on thiswork included Pu Xian Gao, Yong Ding, Wenjie Mai, William Hughes, andChangshi Lao, all in Georgia Tech's School of Materials Science andEngineering.