Due to their smallness, nanomechanical resonators can sense masses down to a few atoms in vacuum. They typically transduce added mass into a change in resonance frequency of an oscillating cantilever. Such a description of the sensor response, however, is deceptively simple. In reality, the sensor response is determined not only by the mass, but also by the rigidity of adsorbed material. This complicates an accurate interpretation of the sensor response, jeopardising its applicability for sensing thin metal films for, e.g., micromachined electronic circuits, or for detecting molecular markers in biomedical sensing and diagnostics.
At the London Centre for Nanotechnology, researchers in the groups of Bart Hoogenboom and Rachel McKendry have now developed a method for disentangling these mechanical and mass effects on nanomechanical resonators. They measured the resonance frequency and the width of the resonance, and combined this information to accurately determine both the mass and the rigidity of material adsorbed on the resonators.
Using ultra-small cantilevers developed in collaboration with IBM, they measured the sensor response to nanometre-thick metal adsorbates, and determined the transition from small-island adsorbates to a homogeneous coverage of the cantilevers, as an illustrative application.
The work was published in Applied Physics Letters (R. R. Grüter et al., Appl. Phys. Lett. 96, 023113 (2010))
Journal link Applied Physics Letters
Figure 1: Ultra-small cantilever of 200 nm thickness.
Figure 2: Surface of 1 square micrometre covered with metal islands.