Using a carbon nanotube, Cornell University researchers have produced a tiny electromechanical oscillator that might be capable of weighing a single atom. The device, perhaps the smallest of its kind ever produced, can be tuned across a wide range of radio frequencies, and one day might replace bulky power-hungry elements in electronic circuits.
Recent research in nanoelectromechanical systems (NEMS) has focused on vibrating silicon rods so small that they oscillate at radio frequencies. By replacing the silicon rod with a carbon nanotube, a cylinder of carbon atoms arranged in a hexagonal pattern, the Cornell researchers have created an oscillator that is even smaller and very durable. Besides serving as a radio frequency circuit element, the new device has applications in mass sensing and basic research.
Paul McEuen, Cornell professor of physics, Vera Sazonova, Cornell graduate student in physics and Yuval Yaish, a visiting scientist in the Laboratory of Atomic and Solid State Physics (LASSP) at Cornell, report on the device in the latest issue of the journal Nature.
The Cornell device consists of a carbon nanotube from one to four nanometres in diameter and about one-and-a-half micrometres long, suspended between two electrodes above a conducting silicon plate. The tube is not stretched tight, but hangs like a chain between two posts in a shallow curve called a catenary.
The tube itself is a conductor, and when a voltage is applied between the tube and the underlying plate, electrostatic force attracts the tube to the plate. An alternating voltage sets up vibration as the tube is alternately attracted and repelled. A static voltage applied at the same time increases the tension on the tube, changing its frequency of vibration. The entire assembly of tube and plate behaves as a transistor, so the tube’s motion can be read out by measuring the current flow. Experimenting with various sizes and lengths of tubes, the researchers have made oscillators that tune over a range from 3 to 200 megaHertz.
Such a tuneable oscillator could be used as a detector in a radio-frequency device such as a cellular phone, which must constantly change its operating frequency to avoid conflicts with other phones.
Like their larger cousins, nanotube oscillators also could be used for mass sensing. Since the frequency of vibration is a function of the mass of the vibrating string, adding a very small mass can change the frequency. Silicon rod oscillators have been used to weigh bacteria and viruses.
“This is so much smaller that mass sensitivity should be that much higher,” McEuen said. “We’re pushing the ultimate limit, maybe weighing individual atoms.”