Stress sensor: This scanning-electron-microscope image shows a stress-triggered transistor in cross section. The zinc oxide nanowire, 25 nanometers in diameter, is embedded in a polymer (black area), leaving the top region free to bend.
ACS/Nano Letters
A novel nanoscale sensor responds to mechanical stresses.
Nanoscale sensors have many potential applications, from detecting disease molecules in blood to sensing sound within an artificial ear. But nanosensors typically have to be integrated with bulky power sources and integrated circuits. Now researchers at Georgia Tech have demonstrated a nanoscale sensor that doesn't need these other parts.
The new sensors consist of freestanding nanowires made of zinc oxide. When placed under stress, the nanowires generate an electrical potential, functioning as transistors.
Zhong Lin Wang, professor of materials science at Georgia Tech, has previously used piezoelectric nanowires to make nanogenerators that can harvest biomechanical energy, which he hopes will eventually be used to power portable electronics. Now Wang's group is taking advantage of the semiconducting properties of zinc oxide nanowires--the electrical potential generated when the new nanowires are bent, allowing them to act as transistors.
The Georgia Tech researchers used a vertical zinc oxide wire 25 nanometers in diameter to make a field-effect transistor. The nanowire is partially embedded in a substrate and connected at the root to gold electrodes that act as the source and the drain. When the wire is bent, the mechanical stress concentrates at the root, and charges build up. This creates an electrical potential that acts as a gate voltage, allowing electrical current to flow from source to drain, turning the device on. Wang's group has tested various triggers, including using a nanoscale probe to nudge the wire, and blowing gas over it.
Wang's group is "unique in using nanostructures to make something like this," says Liwei Lin, codirector of the University of California, Berkeley Sensor and Actuator Center. Nanowire sensors could be used for high-end sensing devices such as fingerprint scanners, Lin suggests.
Previous nanowire sensors have been tethered at both ends, limiting their range of motion. Wang says that the freestanding nanowires resemble the sensing hairs of the ear. If grouped into arrays of different lengths, each responsive to a different frequency of sound, the nanowires could potentially lead to battery-free hearing aids, he says.
The next step is to make arrays of the devices. "This is challenging because you have to make the electrical contact reliable, but we will be able to do that," says Wang.
Zhong Lin Wang thinks piezoelectric nanowires could power implantable medical devices and serve as tiny sensors.
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