Carbon Nanotube Resonator Weighs Individual Atoms

Weighing small masses of matter can be rather tricky in some situations, since such processes usually take place in high-resolution mass spectrometers that can easily destroy the sample while measuring it. However, resonators can now provide a much better solution, allowing highly accurate measuring processes to take place without endangering the sample. University of California scientists have recently created such a device sensitive enough to measure individual atoms with the help of carbon nanotubes.

A resonator is a material that has a strong natural oscillation at certain frequencies and is generally used to enhance the acoustic properties of musical instruments. The research team realized that by harnessing the properties of such materials, they could create a measuring device that calculates the mass of a tiny sample as it hits the surface of the resonator. When this happens, the oscillating frequency of the device is altered, providing physicists with a means to calculate the weight of the sample.

Similar devices use relatively dense material as resonators, like quartz, but in the case of weighing atoms, the force resulted during the collision is so small that it barely alters the oscillation frequency of the material. Thus, in order to make the idea work, lower density resonating materials had to be found.

Carbon nanotubes appeared to be perfect for the job, because they are hallow and have a mass at least four times smaller than that of typical resonators, meaning that their reaction to the force applied by an atom falling on its surface is several times stronger. In order to build the new resonator, Kenneth Jensen along with Kwanpyo Kim and Alex Zettl of the University of California first created a tiny springboard by using 250 nanometer long multi-walled carbon nanotubes, each attached to one end only.

The springboard was placed in an ultra-high-vacuum and forced to resonate with the help of radio signals. Then gold atoms were dropped on its surface, to see whether their mass would have any effect on the vibration frequency. The data resulted during the experiment showed that the new resonator was able to measure the force applied by 51 gold atoms, although later calculations revealed that it could in fact detect a mass variation as small as 0.13 zeptograms, just enough to detect individual gold atoms.

The problem is that calculations required to estimate the mass of a sample greatly depend on the place hit by the particle as it falls towards the springboard and that carbon nanotubes bend further as the falling mass approaches the other end.

"The analysis would be much simpler if all the atoms landed on just one part of the nanotube. But we did not force this to happen. Knowing that all gold atoms have the same mass, and assuming that they arrive randomly in time and land randomly along the nanotube lets us do the statistical analysis," says Zettl. "This device is more than a novelty," he said while adding that the new resonator is an exciting breakthrough in the field of nanoelectromechanical systems.