Capacitive Mode Vapor Sensing Phenomenon in ZnO Homojunction: An Insight Through Space Charge Model and Electrical Equivalent Circuit

Conventionally employed resistive mode, vis-à-vis relatively non-conventional capacitive mode measurement technique for ZnO homojunction based alcohol vapor sensor is reported in this paper. ZnO homojunction comprising of sol-gel grown p-ZnO (sodium doped) nanoparticles and electrochemically derived undoped n-ZnO nanotubes was fabricated and after structural (FESEM) and electrical (I-V and C-V) characterizations, the sensing study (in both resistive and in capacitive mode) was carried out at room temperature (~25 °C). Compared to the DC resistance change based measurement, three distinctive features were noticed in capacitive mode sensing; (i) For methanol, the fabricated p-n homojunction device showed higher (~2415%) selectivity window (response magnitude difference with the nearest interfering species) in the capacitive mode, (ii) Capacitive measurement offered a dramatic improvement in the response magnitude (RM ~3021%) compared to its resistive counterpart (RM ~121%), (iii) Transient characteristics were sluggish in capacitive mode due to diffusion limited interactions. An innovative approach was adopted to understand the underlying sensing mechanism through correlation of an ambient dependent (in inert, in air and in vapor environment) space charge model of the homojunction device with the corresponding quantitative equivalent circuit.