Low Cost Environmental Sensors Using Zinc Oxide Nanowires and Nanostructures
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Low Cost Environmental Sensors Using Zinc Oxide Nanowires and Nanostructures Nima Mohseni Kiasari1, Saeid Soltanian1, Bobak Gholamkhass1, and Peyman Servati1 1
Department of Electrical and Computer Engineering, University of British Columbia 2332 Main Mall, Vancouver BC V6T 1Z4, Canada. ABSTRACT Zinc oxide (ZnO) nanowires (NW) are grown on both silicon and sapphire substrates using conventional chemical vapor deposition (CVD) system. As-grown nanostructures are characterized by scanning electron microscope (SEM), X-ray diffraction (XRD) as well as energy dispersive spectroscopy (EDS) and the results confirm high-quality c-axis growth of single-crystalline zinc oxide nanowires. Nanowire are dispersed in solvent and then placed between micro-patterned gold electrodes fabricated on silicon wafers using low cost and scalable dielectrophoresis (DEP) process for fabrication of oxygen and humidity sensors. These sensors are characterized in a vacuum chamber connected to a semiconductor analyzer. Current-voltage characteristics of each device are systematically investigated under different hydrostatic pressure of various gaseous environments such as nitrogen, argon, dry and humid air. It is observed that the electrical conductivity of the nanowires is significantly dependent on the number of oxygen and water molecules adsorbed to the surface of the metal oxide nanowire. These results are critical for development of low cost metal oxide sensors for high performance ubiquitous environmental sensors of oxygen and humidity. INTRODUCTION Zinc oxide (ZnO) nanowires (NWs) are promising semiconductor materials in view of their distinct optical, electro-mechanical, physical and chemical properties. Examples of these applications include flexible electronic skin, piezoelectric power generation, transparent electrode, solar cells, and chemical sensing [1, 2, 3, 4, 5]. In particular, the high surface area and the presence of oxygen vacancies[6] on the surface makes ZnO a potential candidate for sensing of oxygen and water species in gaseous and aqueous environments. In addition to oxygen vacancies, the variety of morphological structures (e.g. nanorods, nanobelts, and NWs[7]) as well as the diversity of synthesis methods such as hydrothermal method[8], chemical vapor deposition (CVD)[9], and laser ablation[10], and the economical price of the materials make ZnO a suitable candidate for low cost, fast and accurate sensing of oxygen and humidity which are critical for many applications including hydrogen fuel cells and water quality monitoring. For instance, in hydrogen fuel cells the ratio between hydrogen and oxygen as well as the precise management of water at the cathode are critical to keep the cell working efficiently[11]. In other words, to achieve the optimum impact of oxygen ratio and relative humidity on the fuel cell operation the first step would be in-situ, fast and accurate sensing of these parameters inside a cell during operation which is potentially feasible with embedded nanometer-sized and accurate oxygen and humidity
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