Elemental theory of a SAW gas sensor based on electrical conductivity changes in bi-layer nanostructures

An elemental theory for a surface acoustic wave (SAW) gas sensor is reported within this work. The sensor mechanism is generally based on changes in electrical conductivity of thin bi-layer sensor nanostructures under the influence of gas molecules. In turn, the interaction between the electrical surface potential (associated with SAW propagating on a piezoelectric substrate) and the mobile electric charges of bi-layer sensor nanostructures (characterized by different electrical conductivities σ1, σ2 and thicknesses h1, h2), cause an acoustoelectric perturbation of the SAW propagation. Both films are very thin in comparison to the SAW wavelength and the Debye screening length. This allows for a one-dimensional treatment of the problem. As a result the relative changes in SAW velocity and attenuation is highly dependent on the electrical surface conductivities ratio of the thin film bi-layer nanostructures (x = σs2/σs1). A proper fitting of the construction parameter x and the initial “work point” (σs1 or σs2 vs. voCs) for such structures allows for obtaining considerable sensitivities in SAW gas sensors.