Nanoscale generators can turn ambient mechanical energy--vibrations, fluid flow, and even biological movement--into a power source. Now researchers have combined a nanogenerator with a solar cell to create an integrated mechanical- and solar-energy-harvesting device. This hybrid generator is the first of its kind and might be used, for instance, to power airplane sensors by capturing sunlight as well as engine vibrations.

Nanogenerators typically use piezoelectric nanowires--hairlike zinc oxide structures that generate an electrical potential when mechanically stressed--to produce small amounts of power. The first such devices were made by Zhong Lin Wang, a professor at Georgia Tech and director of the institute's Center for Nanostructure Characterization. Wang hopes that nanogenerators will one day eliminate the need for batteries in implantable medical sensors, and will eventually generate enough power to charge up larger personal electronics.

Compared with solar cells, nanogenerators are still a relatively inefficient way of harvesting energy, says Wang, but "sometimes solar energy isn't available." So he collaborated with Xudong Wang, an assistant professor of materials science and engineering at the University of Wisconsin-Madison, to make the new hybrid device.

It combines two previously developed technologies, both of which rely on zinc oxide nanowires, in a layered silicon substrate. The top layer consists of a thin-film solar cell embedded with dye-coated zinc oxide nanowires. The large surface area of the nanowires boosts the device's light absorption, a design based on work by Peidong Yang, a professor of chemistry at the University of California, Berkeley. The bottom layer contains Wang's nanogenerator. On the underside of the silicon is a jagged array of polymer-coated zinc oxide nanowires in a toothlike arrangement. When the device is exposed to vibrations, these "teeth" scrape against an underlying array of vertically aligned zinc oxide nanowires, creating an electrical potential.

The solar cell and the nanogenerator are electrically connected by the silicon substrate itself, which acts as both the anode of the solar cell and the cathode of the nanogenerator. It is possible to string together large groups of solar cells and nanogenerators, but having them integrated in a single system takes up less space and is also more energy efficient. The prototype device can generate 0.6 volts of solar power and 10 millivolts of piezoelectric power. While the prototype device had only one nanogenerator, Wang expects to increase the power output by creating devices with multiple layers of nanogenerators. He says that a likely first application of these devices might be in sensor-laden military aircraft. The U.S. Air Force recently issued a call for research funding proposals related to hybrid energy-scavenging devices.

Charles Lieber, a professor of chemistry at Harvard University, says that Wang's device is "creative" and is, to his knowledge, the first hybrid nanoscale device capable of harvesting two types of energy. "That is particularly important, given that one is light active, while the other can work in the dark," says Lieber. He expects Wang's work to inspire other researchers to focus on hybrid nanogenerator devices, as well as on devices that combine nanogenerators with "complementary nano-enabled power storage."