Applications of evanescent microwave probes in gas and chemical sensors

Gas sensing using local probes, such as atomic force and scanning tunneling microscopes (STM), enables accurate measurement and detection of very small quantities of gas molecules and chemicals. Here we report a unique application of the evanescent microwave probes (EMP) in detecting hydrogen. The EMP imaging has previously been applied to mapping resistivity and other non-uniformities in a variety of materials including metals, insulators, semiconductors (both organic and inorganic), composites, and biological specimens. The EMP detects the microwave resistivity of the sample and it has an exponential sensitivity to distance and thickness variations. The EMP was used to detect deflections in a Pd-coated cantilever and to quantify the amount of stress and the resistivity change in the Pd film as a function of hydrogen concentration. The stress was in the range of 5.26–8.59×107 Pa for H2 concentrations of 0.5–1.4% at room temperature, which is about three times larger than that found in the bulk Pd for the same range of H2 concentrations. The Pd filmʹs resistivity changed by 13.5% at 3.0% H2 concentration and 18% change in the EMP signal was observed. The EMP with an appropriate frequency can also be used to resonantly detect various physi-absorbed molecules at the surface of an appropriate material as well. We discuss these possibilities along with some specific experimental data.