Hydrogen detection by single and bilayer sensor structures in Surface Acoustic Wave system*

Presented here are the results concerning an investigations of sensor properties of a single and bilayer sensor structures in a Surface Acoustic Wave systems. Among the single structure is: vacuum deposited 20 nm palladium thin layer. Whereas the bilayer structures consists of two layers produced in two different vapour deposition processes. The first one is a phthalocyanine thin layer, whereas the second is a 20 nm thin palladium (Pd) film. The following bilayer structures are investigated: Pd+CoPc, Pd+PbPc and Pd+H$_{2}$Pc from the point of view their sensor properties towards hydrogen and other such toxic gases like: NO$_{2}$, SO$_{2}$ in synthetic dry air. These structures were formed in one of the dual delay line systems on a LiNbO$_{3 }$ Y-cut Z-propagation substrate, while the other serves as a reference, permitting an easy detection of the arising differential frequency $\Delta$f. This frequency, depending on the operating frequency modes, is in the range of 50 kHz to 400 kHz, whereas the oscillator frequencies are in the range of 43,6 MHz. The wavelength is 80 $\mu$m. Investigations have been performed in a two different temperatures $\sim $22 $^{\circ}$C and ~50 $^{\circ}$C