A previously unknown zinc-oxide nanostructure could provide a new building block for creating nanometer-scale sensors, transducers, resonators and other devices that rely on electromechanical coupling.
Based on a superlattice of alternating single-crystal “stripes” a few nanometers wide, the “nanohelix” structure is part of a family of nanobelts — tiny ribbon-like structures with semiconducting and piezoelectric properties — that were first reported in 2001.
The nanohelices, which get their helical shape from twisting forces created by a small mismatch between the stripes, are produced using a vapor-solid growth process at high temperature, says Zhong Lin Wang, a Regents professor in the School of Materials Science and Engineering, Georgia Institute of Technology.
The research was sponsored by the National Science Foundation, NASA Vehicle Systems Program, U.S. Department of Defense Research and Engineering, the Defense Advanced Research Projects Agency, and the Chinese Academy of Sciences.
The nanohelices reach lengths of up to 100 µm, with diameters from 300 to 700 nm, and widths from 100 to 500 nm. The nanohelices exist in both right and left-handed versions, with production split about 50-50 between the two directions.
Unlike earlier single-crystal nanosprings, which are elastic, the nanohelices are rigid and retain their shape even when cut apart.
They are formed using a process similar to that for fabricating other nanobelts: Heating zinc-oxide powder in a vacuum creates structures with polar surfaces. When carrier gas is introduced, the polar surfaces are minimized, forming a superlattice structure with mismatched crystalline interfaces. The nanohelices begin and end with conventional single-crystal nanobelt structures.
A nanohelix forms from a single-crystal stiff nanoribbon that is dominated by polar surfaces. Abruptly changing the nanoribbon's structure into stripes produces a uniform nanohelix.
The first dozen batches of nanohelices produced a yield of about 10%. Wang's research team has produced nearly 20 different zinc-oxide nanostructures, including nanobelts, aligned nanowires, nanotubes, nanopropellor arrays, nanobows, nanosprings, nanorings, nanobowls, and others.