“Piezoelectric and ferroelectric-based smart materials are just as important as semiconductor nanostructures, because they are the transducers and actuators for nanoscale machines and devices,” researcher Zhong Lin Wang told nanotechweb.org. “We have reported, for the very first time, the successful synthesis of structurally controlled piezoelectric and ferroelectric ZnO nanobelts 10-60 nm wide and 5-20 nm thick. The most exciting result is the formation of single crystalline ZnO helical nanosprings due to spontaneous polarization.”

To grow the nanobelts, Wang and colleague Xiang Yang Kong used a solid-vapour process. They heated ZnO powder in a horizontal tube furnace, depositing ZnO nanostructures onto an alumina substrate. Most of the nanostructures were nanobelts up to several hundreds of microns long, but the scientists also saw nanobelts in a ring shape, and helical structures consisting of coiled-up nanobelts. These helices, or nanosprings, had radii of about 500-800 nm.

For the greatest piezoelectric effect, it’s desirable to have ZnO nanostructures with a large area of polarized (0001) zinc- and oxygen-terminated surfaces. However, the (0001) surface has a high surface energy and so its growth is energetically unfavourable. By carefully controlling the experimental conditions, the Georgia team succeeded in producing structures with surfaces dominated by (0001) facets. In fact, electron diffraction studies indicated that more than 90% of the nanobelts had top flat surfaces consisting of polar ±(0001) facets.

According to the scientists, the nanobelts and nanosprings are an ideal system for understanding piezoelectricity and polarization-induced ferroelectricity at the nanoscale. They could also have applications as sensors, nanoinductors, transducers, actuators, and tunable functional components for microelectromechanical systems (MEMS) and nanoelectromechanical systems (NEMS). In addition, the different polar surfaces - zinc- and oxygen-terminated - could find a use as selective catalysts, and the helical nanosprings’ tunable pitch distance could help to separate DNA double-helix chains and tailor DNA structures via electromechanical coupling.

“My future research will focus on two directions: application and integration of piezoelectric nanobelt materials with microsystems, and applications of nanobelts in biomedical science,” added Wang. “We will work on the emergence of the nanobelt structure for improving the performance of MEMS and NEMS. We’d also like to use these materials for in-situ, real-time, non-destructive and remote monitoring and detection of cancer cells.”

The researchers reported their work in Nano Letters.