To measure the effectiveness of the new nanowires, the researchers used a piezoelectric device–one that converts electrical signals into mechanical vibrations–to shake the nanowires at different frequencies. They then used a scanning electron microscope (SEM) to observe the wire’s vibration and calculate its resonance frequency and Q factor. The Q values ranged from 2,700 to 60,000–up to 10 times higher than measured for previous experimental nanoscale resonators.
The widely varying values are a result of limitations in the SEM measurement technique, Bertness says. Indeed, the Q values changed with different measurements even on the same wire. Bertness says that this is because the intense electron beam causes carbon molecules in the air to deposit on the nanowire, damping its vibrations.
Hong Tang, an electrical-engineering professor at Yale University, who is also working on nanoscale resonators, is skeptical about the researchers’ results. He says that combining a piezoelectric shaking with SEM detection artificially raises the Q value. Because SEM uses a tightly focused electron beam, he says, if the nanowire vibrates more than the beam spot size, the measurement of the wire’s displacement is not accurate. Tang’s guess is that the actual Q factors are probably lower than the reported values, although they are still likely to be higher than those reported for silicon-based nanowires, which have been around 1,000. He says that the researchers would have to use other measurement methods to verify their nanowires’ Q factors.
Bertness acknowledges the need for better measurements, adding that the nano resonator is far from practical right now. To be used in a cell-phone receiver, the nanowire will have to be driven by an electrical signal, not a mechanical shaking. Because gallium nitride is piezoelectric, the researchers believe that this should be possible, she says, and they are now trying to prove that theory.