An Interdigital Capacitive Humidity Sensor With Layered Black Phosphorus Flakes as a Sensing Material

Black phosphorus (BP) atomic layers have received significant attention for chemical sensing applications due to its exceptional electrical, mechanical, and surface properties. However, they are known to undergo chemical degradation in ambient conditions. In this paper, an interdigital capacitive humidity sensor with layered BP flakes as a sensing material is reported. The BP flakes were prepared from crystal powders by the mechanical exfoliation and then deposited on the interdigital electrode (IDE) structures by a spin-coating method. The deposited BP flakes were characterized by the atomic force microscope (AFM) and Raman spectroscopy, which demonstrated the uniformity of highly crystalline BP flakes. The interdigital capacitive humidity sensors were measured under different relative humidity (RH) levels. The results obtained show that the capacitance increases gradually as the RH levels with the sensitivity of about 0.2pF/%RH at the low RH levels (<; 45%RH) while the capacitance increases rapidly as the RH levels with the high sensitivity of about 10pF%RH. The maximum hysteresis was measured to be about 3%RH. Stability measurement showed that the capacitance of the sensors increased in the first 15 hours and then tended to be stable. Good repeatability was observed in the measurement, suggesting that degradation was prevented. Oxide layers were formed on the surface instead and the inside layers of BP kept intact. The estimated response and recovery time of the capacitive sensors was about 17s and 4.7s, respectively. The equivalent RC networks model for the capacitive sensors is proposed to explain the mechanisms. The fast response and excellent sensitivity of the capacitive sensors at the high RH levels showed potential applications in high RH monitoring areas.