Time-Resolved Mass Sensing of a Molecular Adsorbate Nonuniformly Distributed Along a Nanomechnical String

We show that the particular distribution of mass deposited on the surface of a nanomechanical resonator can be estimated by tracking the evolution of the device's resonance frequencies during the process of desorption. The technique, which relies on analytical models we have developed for the multimodal response of the system, enables mass sensing at much higher levels of accuracy than is typically achieved with a single frequency-shift measurement and no rigorous knowledge of the mass profile. We report on a series of demonstration experiments, in which the explosive molecule 1,3,5-trinitroperhydro-1,3,5-triazine (RDX) is vapor deposited along the length of a silicon nitride nanostring to create a dense, random covering of RDX crystallites on the surface. In some cases, the deposition is biased to produce distributions with a slight excess or deficit of mass at the string midpoint. The added mass is then allowed to sublimate away under vacuum conditions, with the device returning to its original state over about 4 h (and the resonance frequencies, measured via optical interferometry, relaxing back to their pre-mass-deposition values). Our claim is that the detailed time trace of observed frequency shifts is rich in information---not only about the quantity of RDX initially deposited but also about its spatial arrangement along the nanostring. The data also reveal that sublimation in this case follows a nontrivial rate law, consistent with mass loss occurring at the exposed surface area of the RDX crystallites.