A group led by Alex Zettl, a condensed matter physicist at the University of California, Berkeley, US, used electron-beam lithography and reactive ion etching to create a trench 300 nanometres across with electrodes on either side and a chargeable plate at the bottom.
The researchers then coaxed carbon nanotubes to grow across the gap. Driven by high-frequency signals, the tiny bridges resonate at 1.3 billion cycles per second, or 1.3 gigahertz.
“Other groups have made resonators that were very small, and have gotten into the one gigahertz regime, but only at very low temperatures and pressures,” says Zettl. “So getting these very high frequencies under room temperature and atmospheric pressure is a real breakthrough.”
the team measured the nanotubes' oscillations in a novel way. Instead of mixing the signal from the nanotubes' vibrations into a test current alternating at a similar frequency, they used a signal at twice the resonant frequency. The result was a much sharper measurement.
it is that high sensitivity that lets them detect the how a test mass placed on the string causes it to vibrate more slowly. The device can detect masses of just 10-18 grams. “This is where you can measure molecules,” says Zettl. “Biological molecules, threat molecules – the sort of things airport screeners are interested in – have masses in this range. We’d like to push it to where you can measure individual atoms, but in everyday life the most interesting things are molecules.”