Quantifying Dissipative Losses in Nanoparticle Films for Acoustic Gas Sensors

The integration of chemical sensing materials (i.e., receptors) with thin-film bulk acoustic resonators (FBARs) enables sensitive and selective gravimetric gas sensors. This approach offers devices with significantly smaller size, power, and cost compared to conventional resistive and optical-based approaches, making it a promising technology for Internet-of-Things sensing applications. One barrier to the application is the difficulty of achieving high quality receptor films that propagate short-wavelength acoustic waves (λ = 2.5 μm) without substantial energy loss. In this study, we quantify the dissipative energy losses in proxy receptor films made from SiO₂ nanoparticles that are deposited onto 3 GHz FBARs. Our findings demonstrate that decreasing the nanoparticle diameter from 100 nm to 20 nm decreases dissipative energy losses by a factor of ≈ 25 for drop-casting, and ≈ 3 for dip-coating deposition methods, respectively. Scanning electron microscopy reveals this reduction in energy losses is due to improved surface coverage for drop-casted films. For dip-coating, where surface coverage is more consistent across different nanoparticle diameters, we hypothesize that films made from smaller nanoparticles (possessing smaller ratio of nanoparticle diameter to acoustic wavelength) are closer to an "effective-medium" and are therefore less likely to scatter and/or attenuate acoustic waves. In practice, these findings can be applied as design guidelines to implement high-quality chemical sensing materials on high-frequency FBARs for gas sensing applications.