'Nanosprings' Show Promise for Self-Powered Devices
Self-powered nanosystems such as environmental and structural integrity sensors could be enabled by coiled semiconductor nanowires that absorb light. Computational simulations by researchers at Bilkent University show that twisting straight nanowires into springlike shapes can increase light absorption by up to 23 percent. The nanowires, meanwhile, take up 50 percent less area than if they were kept straight. In 2007, U.S. researchers introduced a single nanowire photosensor that produced up to 200 pW of electricity, enough to power nanoscale electronic circuits. More recently, a European team built an InP nanowire solar cell with almost 14 percent efficiency.
Researchers have calculated that twisting conventional nanowires into springs can boost their light absorption by up to 23 percent, while using up to 50 percent less area for the same length of wire. Courtesy of Mehmet Bayindir/Bilkent University. The efficiency is not enough to beat the best crystalline silicon solar cells on the market, but because nanowires can cover more area with less material, the nanowire solar cells could ultimately be cheaper. "There is huge potential in the area of nanoscale photosensors," said Mehmet Bayindir, director of the university's National Nanotechnology Research Center. "More efficient outputs might induce the emergence of a new generation of photosensor technology and eventual commercialization of these products." Bayindir and his colleague Tural Khudiyev, now a postdoctoral associate at MIT, found that adjusting the geometry of the typical nanowire may be one way to realize the desired efficiency enhancement. Nanowires are usually long, thin and straight. Their tiny dimensions mean they interact with light differently than ordinary materials. Certain wavelengths of light will match up in just the right way with the dimensions of the nanowire, causing the light to resonate inside the wire, a phenomenon called Mie resonance. "When the nanospring period matches with the Mie resonance points, a 'double resonance' condition occurs which boosts light harvesting efficiency," Khudiyev said. The group has already developed an easy way to produce nanosprings by first making long nanowire arrays, then heating them to a temperature at which the arrays can be twisted into the nanospring shape. The technique can be varied to control the diameter of the spring and the tightness of the curl.