In a paper published in Nano Letters this summer, Wang demonstrated a nanogenerator capable producing 11 milliwatts per cubic centimeter—enough to light up an LED. Wang notes that a pacemaker requires 5 milliwatts to run, an iPod 80 milliwatts. “We’re almost there,” he says.
The devices made by the Georgia Tech group are “getting into the realm where the power output is reasonable,” says Michael McAlpine, professor of mechanical engineering at Princeton University and a 2010 TR35 awardee. “Getting impressive power outputs is a matter of scaling up,” he adds.
Both Wang and McAlpine are looking to more efficient materials for making nanogenerators. Both have recently demonstrated making nanowires from PZT, a crystalline material that is standard in commercial piezoelectric devices. PZT, a compound that contains lead, zirconium, and titanium, is the most efficient piezoelectric material known, but making it into nanowires has been tricky because there are no good catalysts for growing PZT nanowires.
Wang and McAlpine have found different solutions to this problem. Wang treats his starting solution at high temperature and pressure, which does away with the need for an efficient catalyst. McAlpine grows a flat film of PZT, and then uses a mask to pattern nanowires through chemical etching. Energy harvesters made from PZT nanowires aren’t as efficient as the zinc-oxide ones yet, but McAlpine says this is because he and Wang have only just begun to work with them.