Tiny springs

Researchers at the Georgia Institute of Technology have developed a new class of nanometre-scale structures that spontaneously form helical shapes from long ribbon-like single crystals of zinc oxide.

Researchers at the Georgia Institute of Technology have developed a new class of nanometre-scale structures that spontaneously form helical shapes from long ribbon-like single crystals of zinc oxide (ZnO).

Just 10 to 60 nanometres wide and 5-20 nanometres thick – but up to several millimetres long – the new structures, dubbed nanosprings, have piezoelectric and electrostatic polarisation properties that could make them useful in small-scale sensing and micro-system applications.

The piezoelectric properties of the new structures could make them useful in detecting and measuring very small fluid flows, tiny strain/stress forces, high-frequency acoustical waves and even air flows that would otherwise be imperceptible. When deflected by the flow of air or fluids, the nanosprings would produce small but measurable electrical voltages.

‘They could be used to measure pressure in a bio-fluid or in other biomedical sensing applications,’ said Zhong L. Wang, director of Georgia Tech’s Center for Nanoscience and Nanotechnology. ‘You could use them to measure nano- or pico-newton forces.’

The piezoelectric properties could also make the structures useful as actuators in micro-systems and nanosystems, where applying a voltage would induce strains. ‘In micromechanical systems, these structures could provide the coupling between an electrical signal and a mechanical motion,’ Wang noted.

The new structures’ unusual electrostatic polarisation, with positively and negatively charged surfaces across the thickness of the nanoribbon, could be used to attract specific molecules, potentially allowing the nanosprings to be used as biosensors to detect single molecules or cells.

‘The polarised surfaces will attract different molecules with different charges, which would permit selectivity,’ Wang said. ‘The nanosprings have the promise of being able to do single-molecule detection because they are so small.’

Ultimately, he hopes the new structures could prove useful in biomedical monitoring applications, their small size allowing development of systems small enough to be implanted in the body.

‘We would like to use these materials for in-situ, real-time, non-destructive monitoring within the body,’ Wang said.

The new structures could give scientists a way to study the piezoelectric effect and polarisation-induced ferroelectricity at the nanoscale. The helical structure could also provide a new way to study nanometre-scale electromechanical coupling.

Wang and collaborator Xiang Yang Kong fabricate the structures using a solid-vapour process, evaporating high-purity zinc oxide powder in a horizontal tube furnace at temperatures of approximately 1,350 degrees C under vacuum. After an argon gas flow is introduced into the furnace, the nanostructures form on a cooler alumina substrate.

To induce spontaneous polarisation in the structures – one edge positively charged and the other negatively charged – the researchers use a proprietary process and precise control over production conditions. Wang believes the polarisation is what causes the nanosprings to spontaneously form helical structures.