Adding layer to a piezoelectric nanostructure increases output voltage

Left: A conventional VING comprising a top electrode, the active ZnO element, a bottom electrode, and a substrate. Right: an insulating layer added between the active element and the bottom electrode. (Credit: adapted from Eunju Lee et al./Applied Physics Letters)

Sometimes very simple modifications of nanoscale structure can have large practical implications. Last month we noted the unexpected discovery of piezoelectricity in a molecular monolayer. The research noted today achieved a large increase in voltage output of a nanostructure several hundred nanometers thick (a vertically integrated nanogenerator, or VING) through the insertion of an insulating layer. From Kurzweil Accelerating Intelligence News “Energy-harvesting discovery generates 200 times higher voltage to power wearables, other portable devices“:

Korea Advanced Institute of Science and Technology (KAIST) researchers have discovered how to radically improve conversion of ambient energy (such as body movement) to electrical energy for powering wearable and portable devices.

As has been noted on KurzweilAI, energy-harvesting devices can convert ambient mechanical energy sources — including body movement, sound, and other forms of vibration — into electricity. The energy-harvesting devices or “nanogenerators” typically use piezoelectric materials such as zinc oxide* (ZnO) to convert mechanical energy to electricity. Uses of such devices include wearables and devices for portable communication, healthcare monitoring, environmental monitoring; and for medical implants.

The researchers explored ways to improve “vertically integrated nanogenerator” energy-harvesting chips based on ZnO. They inserted an aluminum-nitride insulating layer into a conventional energy-harvesting chip based on ZnO and found that the added layer increased the output voltage a whopping 140 to 200 times (from 7 millivolts to 1 volt, in one configuration). This increase was the result of the high dielectric constant (increasing the electric field) and large Young’s modulus (stiffness). …

The findings are reported in an open-access paper in the journal Applied Physics Letters: “Characteristics of piezoelectric ZnO/AlN-stacked flexible nanogenerators for energy harvesting applications“. Additional news coverage is provided by the AIP Publishing news staff “Zinc Oxide Materials Tapped for Tiny Energy Harvesting Devices“:

… “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,” explained 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,” noted Yoon.

The key concept behind the group’s work? Flexible ZnO-based micro energy harvesting devices, aka “nanogenerators,” can essentially be comprised of piezoelectric ZnO nanorod or nanowire arrays sandwiched between two electrodes formed on the flexible substrates. In brief, the working mechanisms involved can be explained as a transient flow of electrons driven by the piezoelectric potential. …

Following up on reports from others that the use of insulating materials can provide a large potential barrier, the AIP news article continues:

The KAIST researchers proposed, for the first time, new piezoelectric ZnO/aluminum nitride (AlN) stacked layers for use in nanogenerators.

“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,” said 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.”

Compared to a VING without an aluminum nitride insulating layer, the insertion of either a 30-nm-thick or a 150-nm-thick AlN layer atop, below, or in the middle of the active element led to a large increase in output voltage. Although the eventual achievement of atomically precise manufacturing will provide the ultimate accomplishments of nanotechnology, the energy field is a particularly good example of an application area where substantial rewards await at several stops in the journey through nanoscale technology toward atomic precision.
—James Lewis, PhD

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