“This is the first report on the growth of freestanding, single-crystal, complete nanorings, demonstrating the possibility of synthesizing extreme structures and offering a new nanostructure that was not previously thought possible,” said team leader Zhong Lin Wang. “The growth mechanism is a spontaneous self-coiling process – the ‘slinky’ growth mechanism – which is fundamentally a new crystal growth process. It sets the foundation for understanding the formation of polar-surface-induced nanostructures.”

Wang and colleagues made the nanorings by a solid-vapour technique, from powders of zinc oxide, indium oxide and lithium carbonate in a horizontal tube furnace. Heating the materials to 1400° C in argon caused material to deposit on a silicon substrate. Around 20 to 40% of this material was made up of zinc-oxide nanorings with diameters of 1 to 4 microns and shells around 10 to 30 nanometres thick.

“Nanorings are made up of fine nanobelts that are rolled up as coils layer-by-layer, with as many as a hundred loops,” said Wang.

Zinc oxide has a wurtzite crystal structure, as do materials such as gallium nitride, aluminium nitride, indium nitride and zinc sulphide. Wang reckons that the results received from zinc oxide should impact the growth of nanostructures for the entire wurtzite family.

“The [zinc oxide] structure can be used for fabricating piezoelectric-based fluid pumps and switches for biotechnology,” said Wang. “The near-term application will be in situ, real-time monitoring of blood pressure and blood flow rate.” The nanorings could also measure stress at the scale of a single cell. For applications outside biotechnology, the scientists now plan to integrate the piezoelectric nanorings into micro-electromechanical and nano-electromechanical systems, as well as using them to investigate fundamental physical phenomena.