Dec 8, 2008
Paddle CNT resonators could make good mass sensors
By showing that paddle-shaped devices made from carbon nanotubes resonate in two very different modes when varying gate voltages are applied, researchers at the Delft University of Technology say that the nanotubes act as springs that detect their own motion.
Carbon nanotubes (CNTs) could be ideal for making mass sensors for use in nanoelectromechanical systems (NEMS). Thanks to the small radii of the nanotubes, the properties of these resonators significantly change when the devices deflect, or twist, by a very small amount (for example, when a mass is placed on the tubes). The change in electronic properties of CNTs that occurs when such a strain is applied can be detected and used for sensing applications.
Benoit Witkamp and co-workers have employed suspended CNTs as torsional and flexural springs in a paddle-shaped device to detect mechanical vibrations. The researchers found that measured resonances also tune with the DC voltage on a nearby gate electrode in the device.
The team noticed two types of specific gate-tuning behaviour in the resonators: resonances that tune to higher frequencies when the gate voltage is increased and resonances that tune to lower frequencies when the gate voltage is increased. According to the team, the first type of resonances come from the fact that the paddle moves back and forth towards the gate electrode and the second because the paddle island rotates around the nanotube axis. This distinct tuning behaviour is a new way to study how sensitive the different vibration modes are to applied masses or forces.
Compared with conventional top-down paddle resonators made from silicon, nanotube springs have smaller torsional spring constants, which means that they are easily twisted. This could make them perfect for mass-sensing applications, says Witkamp. Moreover, nanotubes can be used as self-detecting elements, while silicon-fabricated paddle resonators cannot. "This means you do not need a large optical set-up or SEM to detect vibrations," he told nanotechweb.org.
The researchers reported their work in Applied Physics Letters.
About the author
Belle Dumé is contributing editor at nanotechweb.org