Resistive and Capacitive Measurement of Nano-Structured Gas Sensors

Abstract

With the advent of industrial renaissance and world population exploration, atmospheric pollution is being elevated beyond the predicted roadmap. Development of effective and inexpensive systems for detection as well as selective quantification of environmentally hazardous species (i.e. NO2, NO, N2O, H2S, CO, NH3, CH4, CO2, volatile organic compounds etc.), for industrial and domestic air quality monitoring, are the timely demand. Presently, the most reliable gas measurement techniques are optical spectroscopy, infra-red spectroscopy and gas chromatography/spectroscopy; which are precise but non-portable, expertize is needed to operate these systems and are expensive ones also. As a cost effective alternative, solid-state gas sensors (with nanostructured material(s) as the sensing layer) have widely been researched for environmental gas detection offering promisingly high sensitivity with easy portability. However, solid-state gas sensors often suffer from the limitations like, high operating temperature, low carrier mobility and poor selectivity. To mitigate these glitches, various types of gas sensors have been reported by tuning the properties of the sensing materials and/or by employing different transduction/measurement strategies. The major transduction/measurement types include resistive type (includes planar, metal insulator metal, junction, field effect transistor based device structure), capacitive type, surface acoustic wave (SAW) type, quartz crystal microbalance (QCM) type and electrochemical type. Among these techniques, resistive and capacitive type sensors have already been proved to be the potential candidate due to simple electronic interface, ease of use/portability and low maintenance cost. However, for the last three or four decades, most widely investigated/employed transducing technique is the resistive mode/conductometric sensing measurement, unfortunately analysis of which does not provide any information regarding the device parasitic capacitance, and hence fails to correlate the transient response of the device because the equivalent circuit of the device cannot be derived in a quantitative manner (only partial and qualitative explanation is possible). Thus, without proper understanding (quantitative) of underlying sensing mechanism/physics, no efficient sensor device can be fabricated with a predefined functionality. While several review/book chapters have so far been published on theory, synthesis and influence of different nanostructures for gas sensing applications, no work has so far been published by critically discussing the prospects and the constraints of resistive and capacitive type transduction/measurement techniques.