Zinc oxide materials show promise as piezoelectric nanogenerators
The KAIST team discovered that inserting aluminum nitride insulating layers into ZnO-based energy harvesting devices led to an improvement of the devices’ performance which will help to make wearable electronics that are flexible, sustainable and powered by ambient renewable energy.
The researchers have reported their findings in the journal Applied Physics Letters.
“Mechanical energy exists everywhere, all the time, and in a variety of forms – including movement, sound and vibration. The conversion from mechanical energy to electrical energy is a reliable approach to obtain electricity for powering the sustainable, wireless and flexible devices – free of environmental limitations,” suggested Giwan Yoon, a professor in the Department of Electrical Engineering at KAIST.
Piezoelectric materials such as ZnO, as well as several others, have the ability to convert mechanical energy to electrical energy, and vice versa. “ZnO nanostructures are particularly suitable as nanogenerator functional elements, thanks to their numerous virtues including transparency, lead-free biocompatibility, nanostructural formability, chemical stability, and coupled piezoelectric and semiconductor properties,” explained Yoon.
The key concept behind the group’s work? Flexible ZnO-based micro energy harvesting devices, or ‘nanogenerators’ can be comprised of piezoelectric ZnO nanorod or nanowire arrays sandwiched between two electrodes formed on the flexible substrates. The working mechanisms involved can be explained as a transient flow of electrons driven by the piezoelectric potential.
“When flexible devices can be easily mechanically deformed by various external excitations, strained ZnO nanorods or nanowires tend to generate polarized charges, which, in turn, generate piezoelectronic fields,” said Yoon. “This allows charges to accumulate on electrodes and it generates an external current flow, which leads to electronic signals. Either we can use the electrical output signals directly or store them in energy storage devices.”
Other researchers have reported that the use of insulating materials can help provide a large potential barrier. “This makes it critically important that insulating materials are carefully selected and designed – taking both the material properties and the device operation mechanism into consideration,” said Eunju Lee, a postdoctoral researcher in Yoon’s group.
To date, however, there have been few efforts made to develop new insulating materials and assess their applicability to nanogenerator devices or determine their effects on the device output performance.
The KAIST researchers proposed, for the first time, new piezoelectric ZnO/aluminum nitride (AlN) stacked layers for use in nanogenerators.
Stacked flexible nanogenerators: The illustration shows stacked flexible nanogenerators (left), and a cross-sectional transmission electron microscopy image of the ZnO/AlN-stacked structure. The scale bar on the right represents 200 nm. Credit: Giwan Yoon/Korea Advanced Institute of Science and Technology
“We discovered that inserting AlN insulating layers into ZnO-based harvesting devices led to a significant improvement of their performance – regardless of the layer thickness and/or layer position in the devices,” explained Lee. “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 explored further.”
The group’s findings are expected to provide an effective approach for realizing highly energy-efficient ZnO-based micro energy harvesting devices. “This is particularly useful for self-powered electronic systems that require both ubiquity and sustainability – portable communication devices, healthcare monitoring devices, environmental monitoring devices and implantable medical devices,” pointed out Yoon.
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