08. How to Tune Nanostrings
(l. to r.) Graduate students Leon Bellan and Scott Verbridge and professors Harold Craighead and Jeevak Parpia
Harold G. Craighead, Applied and Engineering Physics, Jeevak M. Parpia, Physics, and graduate student Scott Verbridge have created nanoscale resonators—tiny vibrating strings—with the highest quality factor (Q) to date obtainable for devices this small. The researchers made the nanostrings from silicon nitrate under stress, using a fast, low-cost fabrication technique that allows for inexpensive testing of a wide variety of materials. The longest string was 200 nanometers (nm) wide, 105 nm thick, and 60 microns long, with a resonant frequency of 4.5 megaHertz and a quality factor of 207,000—the highest Q ever achieved at room temperature. The stressed silicon nitrate strings are very mechanically robust which makes them practical for consumer products. Previously, the researchers used vibrating strings and cantilevers to detect masses as small as a single bacterium or virus. Resonant frequency depends on the mass of a vibrating object. If a nanoscale vibrator is coated with antibodies that cause a virus or other molecule to adhere to it, the change in mass causes a measurable change in frequency. In a high Q nanostring, a small change in mass will produce a much more noticeable shift. This new research gets closer to the “laboratory on a chip.” In addition to their application in devices that detect and identify biological molecules, the nanostrings can be used as very precisely tuned oscillators in radio-frequency circuits, replacing bulky quartz crystals.