Quality factor (Q) equals the ratio of the resonant frequency to the range of frequencies over which resonance takes place. A radio receiver with high-Q circuitry, for instance, is more selective in separating one station from another. Other researchers have attained high Qs in nanostring resonators cooled to within a few degrees of absolute zero, but not at room temperature.
Such devices are typically made by a process called electron-beam lithography. Here, a tightly focused electron beam cuts a pattern into a chemical film covering a wafer of silicon or a similar substance. Acid then etches away the silicon in places resist has been removed.
Cornell uses an alternative method called electrospinning. It forces a liquid polymer through a row of openings a few nanometers in diameter. The resulting fibers are made to flow smoothly onto a moving silicon wafer, creating a series of parallel lines that act as a chemical resist. The process is faster and much cheaper than electron-beam lithography. "Given the substrate, I can make a nanobeam resonator in under an hour," says Cornell graduate student, Scott Verbridge.
The silicon-nitride nanostrings are "stretched" by controlling the temperature, pressure, and other factors as the film is deposited. For example, a 200-nm-wide, 105-nm-thick, and 60-mm-long string has a resonant frequency of 4.5 MHz and a Q of 207,000. Besides having a high quality factor, the stressed silicon-nitride nanostrings are mechanically robust, making them practical for consumer devices.
The group has used the nanostrings to detect masses as small as a single bacterium or virus. Coating the device with antibodies makes molecules of interest adhere to it. This weighs down the nanostring, lowering its resonant frequency. High-Q nanostrings are highly sensitive to small changes in mass and exhibit large frequency shifts.
What is being dubbed as the "world's most expensive radio" — built at Cornell from about $200,000 in lab equipment — mixes the vibration of a nanoscale resonator with the offtheair signal from local radio station WICB and reads the output with a laser. Nanostring resonators are about the size of a human hair and consume a fraction of the power of the quartz crystals ordinarily used in radios. Radio transmitters equipped with the devices could be made small enough to implant in the body and report on medical conditions. Cell phones could as well shrink to wristwatch size or smaller, researchers say.