Resistive switches based on piezoelectric nanowires allow electrical signals to be produced from mechanical actions

Jul 26, 2011 by John Toon
Regents professor Zhong Lin Wang holds an array of piezoelectrically modulated resistive memory (PRM) cells on which metal electrodes have been patterned using lithography. Credit: Gary Meek

( -- Taking advantage of the unique properties of zinc oxide nanowires, researchers have demonstrated a new type of piezoelectric resistive switching device in which the write-read access of memory cells is controlled by electromechanical modulation. Operating on flexible substrates, arrays of these devices could provide a new way to interface the mechanical actions of the biological world to conventional electronic circuitry.

The piezoelectrically modulated resistive memory (PRM) devices take advantage of the fact that the of piezoelectric such as (ZnO) can be controlled through the application of strain from a mechanical action. The change in resistance can be detected electronically, providing a simple way to obtain an from a mechanical action.

“We can provide the interface between biology and electronics,” said Zhong Lin Wang, Regents professor in the School of Materials Science and Engineering at the Georgia Institute of Technology. “This technology, which is based on , allows communication between a mechanical action in the biological world and conventional devices in the electronic world.”

The research was reported online June 22 in the journal Nano Letters. The work was sponsored by the Defense Advanced Research Projects Agency (DARPA), the National Science Foundation (NSF), the U.S. Air Force and the U.S. Department of Energy.

In conventional transistors, the flow of current between a source and a drain is controlled by a gate voltage applied to the device. That gate voltage determines whether the device is on or off.

The piezotronic memory devices developed by Wang and graduate student Wenzhuo Wu take advantage of the fact that piezoelectric materials like zinc oxide produce a charge potential when they are mechanically deformed or otherwise put under strain. These PRM devices use the piezoelectric charge created by the deformation to control the current flowing through the zinc oxide that are at the heart of the devices – the basic principle of piezotronics. The charge creates polarity in the nanowires – and increases the electrical resistance much like gate voltage in a conventional transistor.

“We are replacing the application of an external voltage with the production of an internal voltage,” Wang explained. “Because zinc oxide is both piezoelectric and semiconducting, when you strain the material with a mechanical action, you create a piezopotential. This piezopotential tunes the charge transport across the interface – instead of controlling channel width as in conventional field effect transistors.”

An array of piezoelectrically modulated resistive memory (PRM) cells is shown being studied in an optical microscope. Credit: Gary Meek

The mechanical strain could come from mechanical activities as diverse as signing a name with a pen, the motion of an actuator on a nanorobot, or biological activities of the human body such as a heart beating.

“We control the charge flow across the interface using strain,” Wang explained. “If you have no strain, the charge flows normally. But if you apply a strain, the resulting voltage builds a barrier that controls the flow.”

The piezotronic switching affects current flowing in just one direction, depending whether the strain is tensile or compressive. That means the memory stored in the piezotronic devices has both a sign and a magnitude. The information in this memory can be read, processed and stored through conventional electronic means.

Taking advantage of large-scale fabrication techniques for zinc oxide nanowire arrays, the Georgia Tech researchers have built non-volatile resistive switching memories for use as a storage medium. They have shown that these piezotronic devices can be written, that information can be read from them, and that they can be erased for re-use. About 20 of the arrays have been built so far for testing.

The zinc oxide nanowires, which are about 500 nanometers in diameter and about 50 microns long, are produced with a physical vapor deposition process that uses a high-temperature furnace. The resulting structures are then treated with oxygen plasma to reduce the number of crystalline defects – which helps to control their conductivity. The arrays are then transferred to a flexible substrate.

“The switching voltage is tunable, depending on the number of oxygen vacancies in the structure,” Wang said. “The more defects you quench away with the oxygen plasma, the larger the voltage that will be required to drive current flow.”

The piezotronic operate at low frequencies, which are appropriate for the kind of biologically-generated signals they will record, Wang said.

Image shows an array of piezoelectrically modulated resistive memory (PRM) cells on which metal electrodes have been patterned using lithography. Credit: Gary Meek

These piezotronic memory elements provide another component needed for fabricating complete self-powered nanoelectromechanical systems (NEMS) on a single chip. Wang’s research team has already demonstrated other key elements such as nanogenerators, sensors and wireless transmitters.

“We are taking another step toward the goal of self-powered complete systems,” Wang said. “The challenges now are to make them small enough to be integrated onto a single chip. We believe these systems will solve important problems in people’s lives.”

Wang believes this new memory will become increasingly important as devices become more closely connected to individual human activities. The ability to build these devices on means they can be used in the body – and with other electronic devices now being built on materials that are not traditional silicon.

“As computers and other electronic devices become more personalized and human-like, we will need to develop new types of signals, interfacing mechanical actions to electronics,” he said. “Piezoelectric materials provide the most sensitive way to translate these gentle mechanical actions into electronic signals that can be used by electronic devices.”

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not rated yet Jul 26, 2011
It sounds more like an array than a memory, it's not clear how, once something is written to it by an external force, the strain is retained? Is the deformation retained and then reformed by an external voltage on reading?
1 / 5 (1) Jul 26, 2011
The Japanese will invent a sex toy that charges itself when used.
not rated yet Aug 04, 2011
HUX -- The strain is not retained, it changes continually with the external forces. It does however allow continuous reading of the nature of that strain with high sensitivity and resolution. These are an electronic sense of touch.

The self powered systems mentioned in the article rely on different advances made by the same institute. But these arrays by themselves are very impressive.

I could see them being used in structural integrity testing and monitoring. Think of being in a car accident and your car knows where and how bad all of the damage is, or a pacemaker that only fires when needed. How about a prosthesis covered in them? with the current state of electrical stimulation of nerves, it could even translate signals into sensations.