Facile preparation of Silicon/ZnO thin film heterostructures and ultrasensitive toxic gas sensing at room temperature: Substrate dependence on specificity.

Abstract Two types of silicon-Zinc oxide (ZnO) heterostructures were prepared simply by depositing (drop casting) chemically prepared ZnO nanoparticles onto single crystalline (p-type) silicon substrates (Si) as well as electrochemically prepared p-type porous silicon (PS). ZnO nanoparticles and PS/ZnO structures were characterized structurally by various techniques. By depositing in-plane gold contacts on the heterostructures, gas sensors were fabricated and characterized electrochemically by dc and ac impedance measurements. The PS/ZnO sensors showed specific response at room temperature for NO 2 with increase in current and no significant response for other reducing and oxidizing gases. The sensor is sensitive to 200 ppb NO 2 at 25 °C with 35% change in current and 50 s response time. Temperature dependent studies of sensor in the range of 25–100 °C have shown maximum sensitivity at 40 °C (50% change for 200 ppb) with decreasing sensitivity thereafter (23% change at 60 °C), indicating the suitability of the sensor till 60 °C. Alternatively Si/ZnO heterostructures showed maximum response with NO 2 , along with lesser specific responses for SO 2 and NH 3. Detailed multifrequency impedance studies with temperature suggested the role of space charge layers at various interfaces in the charge transport properties of PS/ZnO and Si/ZnO heterostructures resulting in their specific gas sensing properties. Graphical abstract Image 1 Highlights • Nanostructured porous p-silicon and silicon/ZnO sensors were prepared by simple method. • Porous silicon/ZnO sensors showed highly specific response with NO 2 at room temperature. • Gas sensing of both sensors depends mainly on the space charge change at various interfaces. • Multitudes of space charge layers at interfaces of porous silicon/ZnO increases specificity. [ABSTRACT FROM AUTHOR]

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