Microwave Resonators Embedded With Carbon Nanotubes for Real-Time Gas Sensing

This study proposed the application of microwave resonators embedded with carbon nanotube (CNT) sensing films fabricated using inkjet printing technology as gas sensors. The density and uniformity of CNTs dominated resistance, which was related to inkjet printing droplet spacing (DS), layer numbers, and electrode patterns. Although the high-density resistive-type CNT (DS = 20
$\mu \text{m}$
and 20 layers) sensor with the regular electrode (RE) pattern had a lower response than the low-density CNT (DS = 30
$\mu \text{m}$
) sensor, its response presented a narrow repeatability distribution. The response of the high-density CNT sensor with interdigital electrode (IDE) pattern can be improved and was even higher than that of the low-density CNT sensor with the RE pattern. Based on the results of resistive-type sensors, the frequency responses of the CNT films with DS = 20 and
$30 ~\mu \text{m}$
, 20 layers, and the IDE pattern embedded into transmission-type resonators were studied. During NH3 absorption, the insertion and return losses of the resonator embedded with the sensing film with DS = 20
$\mu \text{m}$
and 20 layers increased, whereas those of the resonator embedded with the sensing film with DS = 30
$\mu \text{m}$
and 20 layers decreased. The resonator frequency of both films increased because the CNT resistance increased during NH3 absorption. These results indicated that the frequency response of the microwave sensors was related to the resistance of CNT films. A transmission-type microwave resonator can provide multidimensional frequency responses and high repeatability and has the potential for integration with the IoT and RFID for wireless gas-sensing applications.