Oxygen detection with zinc oxide nanoparticle structures

Semiconductor metal oxide (SMO) sensors have been utilized as oxygen sensors in industry and research for decades. Oxygen molecules adsorb to the SMO surface which leads to a measurable increase of resistivity [1]. Those sensors are valued for their high accuracy, but they operate at high temperatures [2]. Therefore, heating circuits inside the sensors are required, increasing size and power consumption of the sensors. This paper investigates the applicability of zinc oxide nanoparticle (ZnO) structures as low-cost oxygen detectors for measurements at room temperature. The main advantage of ZnO nanoparticle-based electronics is their low production cost since the nanoparticles can be deposited by cheap process techniques like spray-coating, spin-coating, inkjet-printing or the doctor blade process. Furthermore, they provide a high surface area to volume ratio which leads to higher sensitivity to oxygen. The most critical disadvantage is the high inhomogeneity of particle size and shape which causes nanoporous ZnO layers with low conductivity and nonuniform electrical characteristics. Therefore, gate structures were integrated into the sensors, so that the ZnO nanoparticle conductivity can be adjusted by applying a gate voltage. ZnO nanoparticle transistors with different electrode geometries and channel length were manufactured and analyzed. In different oxygen concentrations ZnO layer resistance, dependent on the applied gate voltage, was measured. Based on the results, a new layout for low-cost sensors without heating structures was developed. Since this work is part of a project, in which a low-cost water quality sensor is developed, the sensors are designed for oxygen concentration measurements in liquids as well