Friday, September 17, 2004

"NANOTECH: Tuned radio frequency oscillator built from nanotubes"
Researchers at Cornell University have created the world's smallest mechanical oscillator that is capable of being tuned electrically. The nanoelectromechanical system (NEMS), which might be a forerunner of sensors that can detect individual atoms, stretches a 1-nanometer-diameter nanotube across a 1,500-nm-wide trench. The system creates a guitar-stringlike device that could also be used as a mechanical RF oscillator or as a clock reference in future nanoscale chips. "Very simply, what we have here is a smaller version of a MEMS [microelectromechanical systems] RF oscillator, but using carbon nanotubes and being electrically tunable," said Paul McEuen, a Cornell physics professor. "All of its applications are years away from being practical, but it is an interesting new direction for researchers plumbing the nanoscale." To construct the nanoscale oscillator, which the researchers tuned as high as 200 MHz in the lab, the team first grew an oxide on a standard single-crystal silicon wafer. Next they grew nanotubes with 1- to 4-nm diameters and laid them on the oxide's surface, then etched a micronwide trench under the nanotubes' middle so that they were suspended. "We use an atomic-force microscope to locate the nanotube. Then we use lithography to define a trench with photoresist. Then we just etch out about a 1-micron-wide section underneath the nanotube. The middle of the nanotube was suspended over the trench with nothing more than van der Waals forces holding it there," said McEuen. Since the width of the trench � about 1.2 to 1.5 microns � is more than 1,000x wider than the width of the nanotube, the arrangement is similar to a stretched guitar string.