Novel Piezoelectric Logic Devices Use Zinc Oxide Nanowires

Researchers at the Georgia Institute of Technology have developed a new class of electronic logic devices in which current is switched by an electric field generated by the application of mechanical strain to zinc oxide nanowires. The devices, which include transistors and diodes, could be used in nanometer-scale robotics, nanoelectromechanical systems (NEMS), microelectromechanical systems (MEMS), and microfluidic devices. The mechanical action used to initiate the strain could be as simple as pushing a button, or it could be created by the flow of a liquid, the stretching of muscles, or the movement of a robotic component.

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.

"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," remarks Zhong Lin Wang, a professor in the Georgia Tech School of Materials Science and Engineering. "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."

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

Using strain-gated transistors fabricated on a flexible polymer substrate, the researchers have demonstrated basic logic operations-including NOR, XOR, and NAND gates and multiplexer/demultiplexer functions-by simply applying different types of strain to the zinc oxide nanowires. They have also created an inverter by placing strain-gated transistors on both sides of a flexible substrate.

"Using the strain-gated transistor as a building block, we can build complicated logic," Wang says. "This is the first time that a mechanical action has been used to create a logic operation."

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 using 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-and would not challenge traditional CMOS transistors for speed in conventional applications. The devices respond to very small mechanical forces, Wang notes.

More information on the research is available from the Georgia Institute of Technology.