Ultrafast Detection of ppb-Level NH₃ Gas at Room Temperature Using CuO Nanoparticles Decorated AlN-Based Surface Acoustic Wave Sensor

Rational design of heterostructure (HS)-based surface acoustic wave (SAW) smart gas sensors for efficient and accurate subppm level ammonia (NH₃) detection at room temperature (RT) is of great significance in environmental protection and human safety. This study introduced a novel HS composed of an AlN-based SAW resonator and CuO nanoparticles (NPs) as a chemical interface for NH₃ detection at RT (∼26 °C). The structural, morphological, and chemical compositions were detailly investigated, which demonstrates that the CuO/AlN HS was successfully formed via interfacial modulation. The CuO/AlN HS SAW sensor exhibited a significant positive frequency shift of 52.60 kHz in response to 100 ppm of NH₃, which is 4.8 times higher than that of the as-grown AlN SAW sensor. Additionally, the CuO/AlN HS SAW sensor exhibited ultrafast response/recovery times of 5/25 s, a remarkably low limit of detection (LOD) of 24 ppb, and excellent long-term stability and selectivity. These results are attributed to the high porosity and defect sites of CuO NPs, which enhanced charge transfer at the heterointerface, as well as decreased mass loading and conductivity effects. The CuO/AlN HS SAW sensor also demonstrated distinct frequency responses to 100 ppm of NH₃, under varying relative humidity (RH): a positive shift at low RH (5%-10%) due to increased conductivity, and a negative shift at high RH (20%-80%) due to enhanced mass loading. These NH₃ gas sensing characteristics of the CuO/AlN HS SAW sensor were validated through X-ray photoelectron spectroscopy band diagram analysis and resistive-type gas sensing measurements. These findings highlight the potential of the integrating metal oxide with nitride semiconductors for advanced SAW-based gas sensing technology in environmental and industrial applications.