Nanoscale device measures up

May 1, 2007
Measuring miniscule vibratory movements on scanning tunneling microscopes (STMs) may soon be accomplished with a new nanoscale apparatus developed by

Measuring miniscule vibratory movements on scanning tunneling microscopes (STMs) may soon be accomplished with a new nanoscale apparatus developed by JILA, a joint venture of the National Institute of Standards and Technology (NIST) and the University of Colorado at Boulder. In the device, 40 million vibrations/sec are measured by hopping electrons on a tiny gold beam. The technology offers the potential for a 500-fold increase in speed of STMs and may allow scientists to watch atoms vibrate in high definition and real time.

The new device measures the wiggling of the beam, or, more precisely, the space between it and an electrically conducting point just a single atom wide, based on the speed of electrons tunneling across the gap. The work is the first use of an atomic point contact to sense a nanomechanical device oscillating at its resonant frequency, where it naturally vibrates like a tuning fork.

The technique is not as precise as more complex and much colder methods of measuring very fast motions of ultra-small devices, but it minimizes unwanted electronic noise and measures the random shaking of the beam caused by back-action or recoil. This level of sensitivity is possible because the atomic point contact acts as an amplifier for these otherwise imperceptible factors, and the gold beam is tiny and floppy enough — just 100 nm thick, and 5.6 µm long by 220 nm wide — to respond to single electrons.

The new method involves bringing the sharp point within one nanometer of the gold beam. A current is applied through the point across the gap, until an increase in resistance indicates that electrons are tunneling across the gap. Gap size is monitored based on variations in the current. The beam's undulations were measured with tens to hundreds of times greater precision than a typical STM result. That's because the oscillations were measured using microwave electronics, which are much faster than the audio frequency technology traditionally used with STMs.

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