GEORGIA TECH (US)—A new class of electronic logic device generates a current-switching electric field by applying mechanical strain to zinc oxide nanowires.

The strain could be as simple as pushing a button, or be created by the flow of a liquid, stretching of muscles, or the movement of a robotic component.

The devices, which include transistors and diodes, could be used in nanometer-scale robotics and very small electromechanical systems.

“When we apply a strain to a nanowire placed across two metal electrodes, we create a field, which is strong enough to serve as the gating voltage,” says Zhong Lin Wang, a professor at the Georgia Institute of Technology.

“This type of device would allow mechanical action to be interfaced with electronics, and could be the basis for a new form of logic device that uses the piezoelectric potential in place of a gate voltage.”

In traditional field-effect transistors, an electrical field switches—or “gates”—the flow of electrical current through a semiconductor. Instead of using an electrical signal, the new logic devices create the switching field by mechanically deforming zinc oxide nanowires.

The deformation creates strain in the nanowires, generating an electric field through the piezoelectric effect—which creates electrical charge in certain crystalline materials when they are subjected to mechanical strain.

Wang, who has published a series of articles on the devices in such journals as Nano Letters, Advanced Materials, and Applied Physics Letters, calls this new class of nanometer-scale device “piezotronics” because they use piezoelectric potential to tune and gate the charge transport process in semiconductors.


Researchers at Georgia Tech study zinc oxide devices on flexible substrate. (Credit: Gary Meek)

The devices rely on the unique properties of zinc oxide nanostructures, which are both semiconducting and piezoelectric.

The transistors and diodes add to the family of nanodevices developed by Wang and his research team, and could be combined into systems in which all components are based on the same zinc oxide material.

The researchers have previously announced development of nanometer-scale generators that produce a voltage by converting mechanical motion from the environment, and nanowire sensors for measuring pH and detecting ultraviolet light.

“The family of devices we have developed can be joined together to create self-powered, autonomous and intelligent nanoscale systems,” Wang says. “We can create complex systems totally based on zinc oxide nanowires that have memory, processing, and sensing capabilities powered by electrical energy scavenged from the environment.”

A strain-gated transistor is made of a single zinc oxide nanowire with its two ends—the source and drain electrodes—fixed to a polymer substrate by metal contacts. Flexing the devices reverses their polarity as the strain changes from compressive to tensile on opposite sides.

The devices operate at low frequencies—the kind created by human interaction and the ambient environment.

“These transistors could provide new processing and memory capabilities in very small and portable devices,” says Wang.

The Georgia Tech group has also learned to control conductivity in zinc oxide nanodevices using laser emissions that take advantage of the unique photo-excitation properties of the material.

When ultraviolet light from a laser strikes a metal contact attached to a zinc oxide structure, it creates electron-hole pairs which change the height of the Schottky barrier at the zinc oxide-metal contact.

These conductivity-changing characteristics of the laser emissions can be used in tandem with alterations in mechanical strain to provide more precise control over the conducting capabilities of a device.

“The laser improves the conductivity of the structure,” Wang notes. “The laser effect is in contrast to the piezoelectric effect. The laser effect reduces the barrier height, while the piezoelectric effect increases the barrier height.”

Wang has called these new devices fabricated by coupling piezoelectric, photon excitation, and semiconductor properties “piezo-phototronic” devices.

The research has been supported by the National Science Foundation, the Defense Advanced Research Projects Agency, and the U.S. Department of Energy.

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