What's Happening Inside the Nanogenerator
A nanogenerator consists of an integrated circuit, with components made from silicone and a piezoelectric ceramic, etched onto a flexible surface, called a substrate. While the strength and other properties of the substrate are important in engineering the nanogenerator, the magic happens in the circuitry. On the surface, using the naked eye, we can see a series of lines and boxes that appear as a flat, two-dimensional image. However, a microscopic look beneath the outer layers of the flexible chip reveals a completely different three-dimensional picture.
The electricity is generated in the piezoelectric material. As mentioned before, Wang's team has used ZnO to develop nanowires. Each nanowire measures between 100 and 300 nanometers in diameter (the width of the wire). Each nanowire's length is about 100 microns; one micron = 100,000 nanometers. To put this in perspective, note that the length of the wire (not the width) is about the same as the width of two human hairs.
In their nanogenerators, Wang's team attaches an array of nanowires to the substrate and places a silicone electrode at the other end of the wires. The electrode has a zigzag pattern on its surface. When a small physical pressure is applied to the nanogenerator, each nanowire flexes and generates an electrical charge. The electrode captures that charge and carries it through the rest of the nanogenerator circuit. The entire nanogenerator might have several electrodes capturing power from millions of nanowires [sources: Greenemeier, Science Daily].
In their own research groups, McAlpine and Purohit have taken a different approach to nanogenerators, using PZT to create nanoribbons. Each nanoribbon is about 10 micron wide and 250 to 500 nm thick. They first form the nanoribbons on a magnesium oxide surface and then remove them using phosphoric acid. Then, they fix the nanoribbons to a pre-stretched silicone rubber surface that, when relaxed, causes the nanoribbons to buckle without breaking. When the nanoribbons are bent, their movement generates electricity without breaking them away from the surface [source: Berger].
Building on the basic premise of forming flexible wires from piezoelectric material, researchers have studied ways to get more power out of each generator. For example, Wang's lab has improved both the nanowires and the circuitry in each successive design. Wang reports that over the last decade, he's seen output improved to over a billion times better than when he started.
So far, you've seen how the nanogenerator works from the inside. Now, let's examine where you might find nanogenerators at work within the next few years.