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Piezoelectric wires: The mechanical stress produced by bending a zinc oxide nanowire creates an electrical potential across the wire. This drives current through a circuit. The conversion of mechanical energy to electrical energy is called the piezoelectric effect. It's harnessed in the devices on the next page, which might be made from the nanowires.
Credit: Bryan Christie Design
Last November, Wang embedded zinc oxide nanowires in a layer of polymer; the resulting sheets put out 50 millivolts when flexed. This is a major step forward in powering tiny sensors.
And Wang hopes that these generators could eventually be woven into fabric; the rustling of a shirt could generate enough power to charge the batteries of devices like iPods. For now, the nanogenerator's output is too low for that. "We need to get to 200 millivolts or more," says Wang. He'll get there by layering the wires, he says, though it might take five to ten more years of careful engineering.
Meanwhile, Wang has demonstrated the first components for a new class of nanoscale sensors. Nanopiezotronics, as he calls this technology, exploit the fact that zinc oxide nanowires not only exhibit the piezoelectric effect but are semiconductors. The first property lets them act as mechanical sensors, because they produce an electrical response to mechanical stress. The second means that they can be used to make the basic components of integrated circuits, including transistors and diodes. Unlike traditional electronic components, nanopiezotronics don't need an external source of electricity. They generate their own when exposed to the same kinds of mechanical stresses that power nanogenerators.