Zinc-oxide-based nanostructures could help power wearable electonics

New research helps pave the way toward highly energy-efficient zinc-oxide-based micro energy harvesting devices with many applications in portable communications, healthcare, environmental monitoring, and more.

Article  |  Fall 2015

A group of researchers from the Korea Advanced Institute of Science and Technology (KAIST) have developed a new energy-efficient energy-harvesting device by exploring the potential of ZnO nanostructures to convert mechanical energy into electrical energy in order to power micro devices.

This research was reported in Applied Physics Letters (APL), published by the American Institute of Physics (AIP). The AIP also released a news article entitled “Zinc Oxide Materials Tapped for Tiny Energy Harvesting Devices” on January 13, 2015.

The research group reported in Applied Physics Letters that piezoelectric zinc oxide (ZnO) nanotechnology could be used to more effectively harvest mechanical energy and convert it into electrical energy, and the primary aim of the research was to exploit the piezoelectric capability of ZnO-based nanostructures to serve as nanogenerators for portable communications, wearable self-powering devices, healthcare monitoring systems, and so on (http://scitation.aip.org/content/aip/journal/apl/106/2/10.1063/1.4904270).

They discovered that inserting aluminum nitride (AlN) insulating layers into ZnO-based energy harvesting devices could lead to a significant improvement in the performance of nanogenerators (https://www.aip.org/publishing/journal-highlights/zinc-oxide-materials-tapped-tiny-energy-harvesting-devices).

“ZnO-based nanostructures appear to be highly suitable and promising as active functional elements of nanogenerators, mainly due to their numerous virtues including transparency, lead-free biocompatibility, nanostructural formability, chemical stability, and coupled piezoelectric and semiconductor properties,” said Giwan Yoon, a professor in the Department of Electrical Engineering at KAIST.

Device performance was an important feature that the researchers made considerable effort to improve in the ZnO-based nanogenerators. To achieve even higher device performance, the researchers designed and fabricated conceptually new nanostructured nanogenerators, not only by exploiting piezoelectric ZnO nanorods or nanowires arrays, but also by stacking ZnO layers with ultrathin AlN insulating layers. As a result, they developed entirely new insulating material configurations consisting of ZnO and AlN and their nanostructures, which distinguishes this research from other approaches previously reported.

Flexibility was another feature that the researchers tried to exploit in the ZnO-based nanogenerators. To this end, the nanogenerators were fabricated by stacking ZnO and AlN nanostructures sandwiched between two electrodes on flexible substrates.

“When flexible devices can be easily mechanically deformed by various external exertions or vibrations, strained ZnO nanorods or nanowires have a tendency to generate polarized charges that can, in turn, generate piezoelectric fields,” Yoon said. “This allows charges to accumulate on electrodes, which triggers an external current to start flowing, leading to electronic signals. Either we may use the electrical output signals directly or store them in energy storage devices,” explained Yoon. To ensure that charges can accumulate on the electrodes, research has focused on how to apply insulating materials so that a sufficiently large potential barrier, where necessary, could be induced without any adverse impact on the device operation.

“We discovered that inserting AlN insulating layers into ZnO-based energy harvesting devices led to a significant improvement of their performance, regardless of the layer thickness and/or layer positions in the devices,” said Eunju Lee, a postdoctoral researcher in Yoon’s group. “Also, the output voltage performance and polarity seem to depend on the relative position and thickness of the stacked ZnO and AlN layers, but this needs to be further studied.” Thus, the researchers seem to believe that this work will help them further study and refine devices to reach their optimum configurations and dimensions.

Yoon added, “This research is particularly useful for self-powered electronic systems, requiring both ubiquity and sustainability, such as portable communications, wearable devices, healthcare monitoring systems, environmental monitoring devices, and so forth.”

American Institute of Physics (AIP) (https://www.aip.org/publishing/journal-highlights/zinc-oxide-materials-tapped-tiny-energy-harvesting-devices)

CREDIT: Giwan Yoon/Korea Advanced Institute of Science and Technology