Room-Temperature CH4 Detection Using PLD-Deposited ZnO/CuO and ZnO/In2O3 Bilayer SAW Sensors

This study investigates ZnO-based surface acoustic wave (SAW) sensors for the detection of methane (CH4) at room temperature (RT), focusing on the influence of film morphology and oxide heterostructures developed by pulsed laser deposition (PLD). Bilayer thin films—specifically ZnO/CuO and ZnO/In2O3—were deposited onto quartz substrates at 400 °C under oxygen pressures of 100 and 700 mTorr to evaluate how deposition conditions affect sensor performance. Structural and morphological characterization was performed using X-ray diffraction (XRD), scanning electron microscopy (SEM), and energy-dispersive X-ray spectroscopy (EDS). ZnO thin films deposited at 700 mTorr exhibited a more open, granular morphology compared to those deposited at 100 mTorr, a feature likely to facilitate gas access to the sensor’s active surface. The addition of CuO and In2O3 top layers further influenced the sensor response, presumably through surface and interface effects. Among the configurations studied, sensor S6 (ZnO/CuO, with both layers deposited at 700 mTorr) demonstrated the highest sensitivity—0.18 Hz/ppm—within the experimentally investigated CH4 concentration range (0.4–2%), with an estimated limit of detection (LOD) of 167 ppm. Short-term measurements performed on sensors S4 and S6 at a 2% CH4 concentration indicated reversible responses over two consecutive exposure and recovery cycles, with a return to the initial baseline value following the removal of the gas. These results suggest that ZnO-based double layers, fabricated via PLD, are promising candidates for further investigation as CH4-sensitive layers at RT.