5.7 GHz Ultrasensitive Shear Horizontal-Surface Acoustic Wave Humidity Sensor Based on LiNbO₃/SiO₂/SiC Heterostructures with a Sensitive Layer of Polyethyleneimine-SiO₂ Nanocomposites

Humidity sensing and water molecule monitoring have becomehotresearch topics attributed to their potential applications in monitoringbreathing/physiological conditions of humans, air conditioning ingreenhouses, and soil moisture in agriculture. However, there is ahuge challenge for highly sensitive and precision humidity detectionwith wireless and fast responsive capabilities. In this work, a hybrid/synergisticstrategy was proposed using a LiNbO3/SiO2/SiCheterostructure to generate shear-horizontal (SH) surface acousticwaves (SAWs) and using a nanocomposite of polyethylenimine-silicondioxide nanoparticles (PEI-SiO2 NPs) to form a sensitivelayer, thus achieving an ultrahigh sensitivity of SAW humidity sensors.Ultrahigh frequencies (1 & SIM;15 GHz) of SAW devices were obtainedon a high-velocity heterostructure of LiNbO3/SiO2/SiC. Among the multimodal wave modes, we selected SH waves for humiditysensing and achieved a high mass-sensitivity of 5383 MHz & BULL; mm(2) & BULL; & mu;g(-1). With the PEI-SiO2 NP composite as the sensitive layer, an ultrahigh sensitivityof 2.02 MHz/% RH was obtained, which is two orders of magnitude higherthan those of the conventional SAW humidity sensors (& SIM;202.5MHz frequency) within a humidity range of 20-80% RH.