Nanoelectromechanical Disk Resonators as Highly Sensitive Mass Sensors

This letter presents electro-thermally actuated nanomechanical disk resonators and characterization of their performance as highly sensitive mass sensors both in air and liquid. Operating in-plane rotational mode, the disk resonator surfaces slide in parallel to the liquid interface, hence minimizing the consequent motion of surrounding liquid and the resulting viscous damping. Disk resonators with diameters ranging from 2 to
$20~\mu \text{m}$
with thermal actuator beams as narrow as 35 nm were fabricated via electron beam lithography. Quality factors (
${Q}$
) as high as 3740 in air and 183 in liquid have been achieved for such resonators with resonance frequencies in the 35-MHz range. Resonator mass sensitivities were characterized in air by the formation of a self-assembled monolayer of hexamethyldisilazane on the surfaces. Frequency shift as high as 318 Hz/ag was measured using a 2-
$\mu \text{m}$
diameter disk resonator, resonating at 221 MHz with the thickness of 25 nm. In liquid, when a 20-
$\mu \text{m}$
disk resonator was exposed to a 10-mM solution of mercaptohexanol (MCH) in ethyl alcohol (ethanol), its resonance frequency was shifted as MCH molecules gradually adhered to the resonators gold-coated surface in real time. A frequency shift of 20 kHz because of 1.4-pg MCH mass was obtained for such experiment.