Directly observing individual adsorbed atoms on a material surface is no easy task. However, in recent years, researchers have developed ultrasensitive resonant nanoelectromechanical systems (NEMS) with high "quality factors" that allow them to detect the presence of several, or even individual, adsorbed noble gas atoms. This technique can thus be used to probe how surface adsorbates behave on very small surfaces just nanometres across.

In the new work, Michael Roukes' team at Caltech employed a NEMS resonator made of a 100 nm thick silicon carbide film vibrating at 190 MHz to study how xenon atoms moved on the device surface. The researchers did this by tracking how the frequency of the resonator fluctuates in real time using a high-precision frequency-locking and tracking circuit. When xenon atoms are adsorbed onto the surface of the resonator, the frequency at which it vibrates changes. This change can be monitored and the mass of the particles adsorbed then calculated.

Statistical noise
By measuring the mass fluctuations (also known as statistical noise), the team found that the atoms diffused along the 1D device. "We also observed interesting frequency noise," team member Philip Feng (now at Case Western Reserve University) told nanotechweb.org. "Through careful analysis of two possible noise processes, associated with adsorption-desorption and surface diffusion respectively, we identified surface diffusion as the dominant mechanism."

More interestingly still, the noise process due to surface diffusion in such a low-dimensional system was found to exhibit new power laws in the frequency noise spectra, never before reported for classical oscillators, he adds. "Now the NEMS devices are sensitive enough to reveal such weak power laws that were invisible for conventional much larger oscillators."

Understanding how adsorbates behave on the surfaces of nanoscale devices is important for both fundamental studies and technology applications. For example, it could help scientists think about how to engineer or even control such adsorbates by programming the physical conditions on a surface.

The researchers now hope to do just this by controlling diffusion of certain adsorbates on structures on which they can engineer specific diffusion pathways. "We are also interested in probing related processes in even smaller systems," revealed Feng.

The work was published in Nano Letters.