About a year ago, Zhong Lin Wang of Georgia Tech in the US discovered that, when he disturbed zinc oxide nanowires, they gave off a tiny electrical current, a phenomenon called piezoelectricity. At the time, he had to use the tip of an atomic force microscope – a $250,000 instrument – to create about one-billionth of a watt of power.
Wang has improved his design at least a thousand-fold. Using gold nanoparticle as seeds, he grew a small forest of 1-micron-high zinc oxide wires on a conductive substrate 2 millimetres square.
Then he placed a saw-toothed electrode on top, which is designed to make contact with as many nanowires as possible. Finally, by rattling the whole thing with ultrasound, he found that he could generate a few microwatts of electricity. The movement of the top electrode disturbs the nanowires, providing a potential power source for anything that moves.
The generator suffers from a few key limitations, however. First, growing uniform nanowires is difficult – they are usually of slightly different height or diameter. As a result, in a generator containing many thousands of nanowires, only a few hundred or so successfully generate electricity when shaken, as they do not all make contact with the electrode.
So there is another tenfold or better improvement to be gained by getting all of the nanowires to make contact. Being able to pack the nanowires more efficiently could also increase the power generation.
Wang says: “I have full confidence that within three years we will have something that is useful commercially.”