Novel hydrogen sensors using evanescent microwave probes

Gas sensing using local probes, such as atomic force and scanning tunneling microscopes, enables accurate measurement and detection of very small quan tities of gas molecules and chemicals. Here, we report a unique application of the evanescent microwave probes (EMP) in detecting hydrogen. The EMP is extensively used to map resistivity and other nonuniformities in a variety of materials including metals, insulators, semiconductors (both organic an d inorganic), composites, and biological specimens. The EMP detects the mic rowave resistivity of the sample and it has an exponential sensitivity to d istance and thickness variations. Here, the EMP is used to detect deflectio ns in a Pd-coated cantilever and to quantify the amount of stress and the r esistivity change in the Pd film as a function of hydrogen concentration. T he stress was in the range of 5.26-8.59x10(7) Pa for H-2 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 H-2 concentrations. The Pd film' s resistivity changed by 13.5% at 3.0% H-2 concentration and it resulted in an 18% change in the EMP signal. 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. (C) 1999 American Institute of Physics. [S0034-6748(99)04809-1].