Nanotuned to perfection

Researchers from Delft University of Technology and FOM Foundation claim to have successfully made and 'tuned' the world's smallest piano wire.

The wires are made of carbon nanotubes that measure approximately two nanometres in diameter. The researchers have published an article on the subject this week in the scientific journal Nano Letters.

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 was applied to this layer, which caused the hanging nanotubes to vibrate. The suspended tube was alternately attracted and repelled. The largest measured deviation for one tube was eight nanometres. The distance of the nanotubes to the layer of silicon influenced the electrical capacity to the layer of silicon. The movement of the nanowires was derived from these changes in capacity.

When the frequency of the applied current approached the level of the suspended tube's eigenfrequency, it began to vibrate more powerfully. By varying the strength and frequency of the applied current, the research group led by Professor Herre van der Zant succeeded in transposing the wire from a freely suspended state, to a state in which it is taut and vibrates. Van der Zant said: ‘As such it is like tightening a piano wire or guitar string. You can, as it were, tune the wire.’

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researchers claim to have developed a model that can predict the vibrations of the nanotubes. The vibrating nanotubes can be used for a host of specific applications. Van der Zant identified one possibility as a hypersensitive mass sensor. ‘The nanotubes are extremely lightweight,’ said Van der Zant. ‘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.’