Abstract
An electronic nose (e-nose), an array of different individual chemiresistive sensors, has been proposed to mimic olfaction system of mammals. The e-nose provides a unique pattern for each gas species, which enables one to distinguish gases and their concentrations. Therefore, it demands as much reliable data as possible to improve gas selectivity. Increasing the number of constituting sensors allows to collect massive data, thus gives more chances to reduce errors, but this suffers from the complex and high-cost fabrication. More economical solution is collecting highly-distinguishable data from each sensor, which can produce more reliable results with reduced errors. In this presentation, we propose a new concept of collecting two-dimensional response data by making a single device show ambipolar sensing behaviors, thus, both p-type (the resistance increases in oxidative gases) and n-type (the resistance decreases in oxidative gases) sensing behaviors. The proposed concept utilizes the response direction (sign of the gas response) as well as the response amplitude, differentiated from other methods that produces monotonous data from each sensor by reading only the response amplitude.Firstly, we have demonstrated the ambipolar gas sensing behaviors in diode-type device; p-type NiO/n-type SnO2 diode, and NiO/Ti/Au Schottky diode. The ambipolar behavior is attributed to the predominant modulation of heterojunction barrier. Our unique device architecture has a single well-defined heterojunction formed by vertically aligned nanostructures and electrodes in a top-bottom configuration, facilitating the heterojunction barrier modulation. We also show the ambipolar sensing behavior in an organic thin-film transistor (OTFT)-based gas sensor that can operate in both n-channel and p-channel modes depending on the gate bias. Indeed the ambipolar OTFT-based gas sensor produces two-dimensional data depending on the gate biases. Our result reveal that the proposed ambipolar sensing concept allows the collection of two-dimensional data from a single device in a reliable manner, which can significantly contribute to enhance gas selectivity of e-nose.