Humidity-dependent characteristics of methyl-red thin film-based Ag/methyl-red/Ag surface-type cell

This paper describes the experimental results for humidity-dependent resistive and capacitive response of methyl-red thin films in a Ag/methyl-red/Ag surface-type cell. A 300-nm-thick film of methyl-red was deposited from 10 wt% solution in benzene by a spin coater at an angular speed of 2000 revolutions per minute (RPM) on a glass substrate with preliminary deposited metal electrodes to form the Ag/methyl-red/Ag surface-type cell. The length and width of the gap between the electrodes were 50 μm and 15 mm, respectively. The resistance of the film reduced from 37 to 17 MΩ with an elevation of relative humidity level over the whole humidity range. It was also observed that under the effect of humidity, the capacitance of the methyl-red thin film increased by 12 times. The capacitive/resistive sensor has a quasi-linear function with relative humidity in the range of 30–95% and has a small hysteresis. The response and recovery time of the sensor was about 10 s for the capacitive sensor. The humidity-dependent resistive and capacitive properties of this sensor make it promising for use in a humidity meter.