Portland, Ore. 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.
Others have etched silicon MEMS rods to create guitar-stringlike mechanical oscillators, but those were at lower-kHz frequencies, and were not electrically tunable, like McEuen's group's design.
In the McEuen oscillator, an ac voltage is applied to the silicon substrate to start the string oscillating. A dc voltage was able to change the tension of the "guitar string," thereby precisely tuning the frequency of its oscillation, which ranged from 3 to 200 millions of cycles per second.
"We discovered that a voltage applied to the substrate electrostatically attracted the nanotube downward, enabling us to precisely tune the tension in the string by changing the voltage," said McEuen.
McEuen performed the work with Toms A. Arias, Cornell associate professor of physics; Vera Sazonova, a Cornell graduate student in physics; David Roundy, a postdoctoral associate; and Yuval Yaish, a visiting scientist in the Laboratory of Atomic and Solid State Physics (LASSP) at Cornell.
An electrically tunable nanoscale mechanical oscillator has the potential for applications in the same areas today in which engineers are conducting experiments using radio frequency MEMS devices such as wireless sensor nets. It could also be configured as a mechanical RF receiver in the spirit of the original "cat's whisker" mechanical RF receivers that predate electronics. As the world's smallest radio frequency detector, such a NEMS device could be tuned into a particular frequency just by changing the substrate voltage.
The device could also be used as a sensor. Because of its size a nanometer is the width of about four atoms any molecules sticking to the nanotube would change the nanotube's frequency of oscillation by an amount proportional to its weight, making it possible to weigh individual molecules or even atoms.
"We are not trying to make a single-molecule sensor for detecting anthrax or anything. . . . But we are going to do some work for NASA to try to weigh different kinds of individual atoms or molecules.
"Right now," said McEuen, "by our calculations, we are right on the edge of being able to measure the weight of a single molecule. So what we want to do next is to cool the nanotube down and see if it gets more accurate when we get it very, very cold. If it does, then we may be able to study the effect of individual molecules, atoms, even electrons," he said.
The group is now retrofitting its laboratory to cool its test setup to cryogenic temperatures. By combining the unknown effects of quantum confinement and electrically tunable mechanical vibration, McEuen predicts that new discoveries are inevitable.
The group (www.lassp.cornell.edu/assp_data/mceuen/homepage/pubs.html) was funded by the National Science Foundation, the Microelectronics Advanced Research Corp. Focus Center on Materials, Structures & Devices and the Semiconductor Research Corp. The devices were fabricated at the NSF-funded Cornell Nanoscale Facility.