“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,” Zhong Lin (ZL) Wang told nanotechweb.org. “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, starting with 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 nm 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.

Last year, Wang and his team reported the formation of zinc-oxide nanobelts by a similar process. The surfaces of the nanobelts contained large (0001) facets, with a positive charge on one side of the belt due to the termination of zinc ions on that surface and a negative charge on the opposite side of the belt because of the termination of oxygen ions.

In their most recent study, the scientists believed that the nanobelts spontaneously coiled up into nanorings in order to neutralize the polarization across their surfaces, minimizing the electrostatic energy. The heating process then caused the individual coils of the nanobelts to fuse together to form a single crystal. The resulting nanorings contained between five and one hundred loops. The presence of indium and lithium ions helps the growth process by inducing a planar defect in the zinc-oxide nanobelt, which makes the energetic conditions for growth more favourable.

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. And for applications outside biotech, the scientists plan to integrate the piezoelectric nanorings into micro-electromechanical and nano-electromechanical systems, as well as using them to investigate fundamental physical phenomena such as the Aharonov-Bohm oscillations in exciton luminescence.

The researchers reported their work in Science.