Tracking adsorbed atoms on a nanoscale mass sensor

An electromechanical system capable of sensing the mass of a single molecule or a few atoms typically consists of a nanometer-thick beam whose resonant frequency measurably shifts in response to the loaded mass. Naturally, such devices are sensitive to the adsorption and desorption of individual analytes or their diffusion along the beam’s surface; both processes, shown in the schematic, cause the resonator’s frequency to fluctuate. Now, Michael Roukes and colleagues at Caltech’s Kavli Nanoscience Institute have determined the contribution of those processes to frequency noise for a nanoscale resonator vibrating at 190.5 MHz. The experiment, which was conducted in a temperature-controlled vacuum cryostat, relied on a nozzle that delivered a steady stream of xenon atoms to a silicon carbide resonator, shown in the inset. As the resonator was cooled, the average number of adsorbed xenon atoms increased; subsequently, so did the magnitude of the frequency shift. The researchers measured the fluctuation of surface adsorbates and found that diffusion is the favored route for atoms leaving the resonator; diffusion thus contributes the most to frequency noise in excess of the resonator’s inherent thermal fluctuations. The combination of experimental data and analytical models also revealed previously unknown power-law dependence in the system’s noise spectrum, a finding that could be used to probe the sensitivity limits of wide classes of nanoscale frequency-shift sensors. (Y. T. Yang et al., Nano Lett., in press, doi:10.1021/nl2003158 .)--Jermey N. A. Matthews