Piano nanowires

Dutch researchers have made what they call the world's smallest piano wire. In fact, these wires are made of carbon nanotubes measuring approximately 1 micrometer long and approximately 2 nanometers in diameter. After applying alternating current of various frequencies to these nanotubes, they started to vibrate like real piano wires.

Dutch researchers have made what they call the world's smallest piano wire. In fact, these wires are made of carbon nanotubes measuring approximately 1 micrometer long and approximately 2 nanometers in diameter. After attaching these nanotubes to electrodes and applying alternating current of various frequencies, the nanotubes started to vibrate like real piano wires. These nano-electromechanical systems could soon be used as mass sensors to detect viruses or in GSM-related applications.

These nanowires have been created by researchers from the Kavli Institute of Nanoscience at Delft University of Technology and from the Foundation for Fundamental Research on Matter (FOM). Here is a short description of their work with the carbon nanotubes.

The tubes were attached to electrodes and initially placed above a layer of silicon oxide. This layer of silicon oxide was then partially etched away with acid, which caused the tubes to detach and hang.
A layer of silicon is contained beneath the silicon oxide. A strong and frequently variable alternating current is applied to this layer, which causes the hanging nanotubes to vibrate. The suspended tube is alternately attracted and repelled. The largest measured deviation for one tube was 8 nanometers.

The researchers, led by Herre van der Zant, are working in the Molecular Electronics and Devices group and describe what they do about Nano-electromechanical systems (NEMS).

We focus on the exploration of NEM-physics and the development of NEM-devices that can be used as extremely sensitive sensors for force and mass detection down to the single molecule level, as high-frequency resonators up to the GHz range, or as ultra-fast, low-power switches. Both a top-down and bottom-up approach is followed.
In the bottom-up approach suspended structures of single-walled carbon nanotubes and of (semiconducting) nanowires are fabricated. In particular, (new) mechanisms for detection of displacements and eigenfrequencies are studied with the goal to reveal the physical processes (e.g. damping, thermal effects, momentum noise) that limit the sensitivity of the devices.

Below is a scanning electron microscope (SEM) image of such carbon nanotubes. (Credit: Delft University of Technology)

Carbon piano nanowire

The latest research work has been published online by Nano Letters under the name "Bending-Mode Vibration of a Suspended Nanotube Resonator" (November 22, 2006). Here is a link to the abstract.

We have used a suspended carbon nanotube as a frequency mixer to detect its own mechanical motion. A single gate-dependent resonance is observed, which we attribute to the fundamental bending mode vibration of the suspended carbon nanotubes. A continuum model is used to fit the gate dependence of the resonance frequency, from which we obtain values for the fundamental frequency, the residual and gate-induced tension in the nanotube. This analysis shows that the nanotubes in our devices have no slack and that, by applying a gate voltage, the nanotube can be tuned from a regime without strain to a regime where it behaves as a vibrating string under tension.

As you can deduct from the various elements mentioned above, these vibrating nanowires will not be useful for music. So what will be they used for?

Van der Zant identifies one possibility as a hypersensitive mass sensor. "The nanotubes are extremely lightweight. If you suspend something from the tube that is also extremely lightweight, like a virus, then the change in mass is rendered by a different vibration pattern. From this, you can determine the size of the extra mass and deduce if it involves the virus concerned." The vibrating tubes may also be of interest for GSM-related applications (which now use resonators that vibrate in the GHz-field.)

So you'll not be able to buy a nanopiano to your kid for the holidays...

Sources: Delft University of Technology news release; and various websites

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