Palladium and Metal-Free Phthalocyanine Bilayer Structures for Hydrogen Detection in the SAW Sensor System Based on Interaction Speed

Bilayer structures with various thicknesses of metal-free phthalocyanine (H₂Pc) (~80, ~120, and ~160 nm) but the same thickness of palladium (Pd) (~20 nm), have been studied for hydrogen gas-sensing application at temperatures of ~30degC and ~50degC with a method based on interaction speed. The structures were fabricated in two different vacuum deposition processes (first the H₂Pc film and than the Pd) onto an LiNbO₃ Y- cut Z-propagating substrate for the surface acoustic wave (SAW) method and additionally (in these same technological processes) onto a glass substrate with a planar microelectrode array for the simultaneous monitoring of the structure planar resistance. A very good correlation has been observed between these two methods (frequency changes for the SAW method coincide with the decrease of the bilayer structure resistance), especially for higher hydrogen concentrations. Although simultaneous measurements were not always feasible (too great resistance in the samples for the structure with the thinnest H₂Pc), they can provide information about the acoustoelectric interactions between SAW and charge carriers in the bilayer structure. The interaction speed method is based on the great variance in interaction speeds at various hydrogen concentrations (from 2.5% to 4% in synthetic dry air), even though the amplitude signal reaches almost the same frequency level. For a particular chosen initial interaction time interval, a distinct interaction speed can be distinguished with great resolution (from 7.5 Hz/s for 2.5% H₂ in air to 29.1 Hz/s for 4% for the structure with 160-nm H₂Pc and 20-nm Pd). These initial interaction fragments are linear versus time for the investigated medium hydrogen concentrations in synthetic dry air. In the case of the investigated bilayer structures, the interaction speed is higher for the structure with the thinnest H₂Pc film (~80 nm)