Nanotube Oscillator Weighs A Single Atom

By Roland Piquepaille

Recent developments in nanoelectromechanical systems (NEMS) have typically used vibrating silicon rods so small that they oscillate at radio frequencies. But now, Cornell University researchers have replaced the silicon rod by a carbon nanotube. This new electromechanical oscillator might be capable of weighing a single atom. The oscillator consists "of a carbon nanotube from one to four nanometers in diameter and about one-and-a-half micrometers long, suspended between two electrodes above a conducting silicon plate." Such an oscillator, tunable from 3 to 200 MHz, could be used in future cell phones, which have to change frequently their operating frequencies. The only problem is that the current production of carbon nanotubes is too small for such a huge market. Read more...

Here are some details about this carbon nanotube oscillator.

The Cornell device consists of a carbon nanotube from one to four nanometers in diameter and about one-and-a-half micrometers 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 just as tightening or loosening a guitar string changes its pitch. 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 (millions of cycles per second).

Here is a scanning electron microscope (SEM) photo of the device. You can see the carbon nanotube suspended over a trench (typically 1.2-1.5 µm wide, 500 nm deep) between two metal (Au/Cr) electrodes. Below is a cross-section of the device. (Credit: Cornell University)

What can we expect from such a device?

Such a tunable 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," said Paul McEuen, Cornell professor of physics. "We're pushing the ultimate limit, maybe weighing individual atoms."

For more information, you can visit the McEuen Lab Homepage.

The research work has been published by Nature in its September 16, 2004 issue under the name "A tunable carbon nanotube electromechanical oscillator." Here are two links to the abstract and to a full preview of the paper.

Sources: Cornell University news release, September 15, 2004; and various web pages