Steven Noyce and Faculty Mentor: Robert Davis, Physics
Many chemical sensing methods rely on the binding mechanism of the analyte to create a measurable response, making it difficult to create new sensors quickly, but resonant sensors require only that an analyte be bound and rely on the resulting change in mass to obtain a measurement. Solid resonant microcantilevers, or small vibrating fixed-free beams, are a type of resonant sensor that have shown extremely high sensitivities in vacuum environments. The sensitivity of these cantilevers, however, decreases greatly in fluid environments such as air or water due to fluid damping. We propose that porous microcantilever sensors offers both a ten thousand fold increase in surface area available for analyte adsorption as well as a higher quality factor in the presence of fluid damping, allowing for high sensitivities to be obtained in gas and liquid environments. We also demonstrate fabrication, characterization, and testing of these porous microcantilever devices.
Cantilever devices were fabricated by synthesizing patterned carbon nanotubes and subsequently coating them with carbon as one of several possible infiltration materials. Carbon nanotube synthesis was accomplished by patterning a 4 nm iron film onto an alumina coated silicon wafer and exposing this substrate to a flow of hydrogen and ethylene gas at 750 C. Afterward, carbon was coated onto the nanotubes by flowing hydrogen and ethylene gas over the sample at 900 C.
A new chemical flow chamber was built with pressure control from 700 to 60 Torr. This chamber contains a machined clamp that holds a piezoelectric bimorph and the fabricated cantilever device. The piezo is used to drive the cantilever into resonance while the deflection of a laser reflected from the cantilever tip is measured by a photodiode as an indicator of cantilever movement.
Various pressure and chemical environments were maintained within the chamber while the frequency response of cantilever devices was measured.
The damping ratio of the beams (related to the quality factor), is found to follow a linear trend with respect to pressure, with the damping ratio at atmospheric pressure being a factor of two or more larger than the damping ratios at small pressures.
Resonant shifts due to cantilever exposure to acetone, ethanol, and water vapors are observed.
The radius of carbon coated carbon nanotubes is found to vary almost linearly from 13 nm at 1 minute of infiltration to 21 nm at 6 minutes of infiltration.
The relative sizes of the damping ratios observed in atmospheric pressure and in vacuum show that fluid damping is contributing at least half of the total damping the cantilevers are experiencing when the beams are in air. One worry is that internal damping in the beam may be limiting for cantilevers that are porous, but the data collected here clearly show that fluid damping is dominant in the environments of interest.
Detecting acetone and ethanol vapors suggests that these beams may be capable of detecting many other volatile organic molecules that currently have high demand for sensors to be developed.
A model of cantilever sensor behavior is also developed and used to explore optimal beam parameters.
These porous resonant microcantilever sensors are shown to be capable of accurate sensing in fluid environments. These devices have been characterized, increasing understanding of the fabrication process and allowing future work in this regime to be effective.