Optical Gas Sensor Using Cobalt Oxide Thin Film Prepared by Pulsed Laser Ablation
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Optical Gas Sensor Using Cobalt Oxide Thin Film Prepared by Pulsed Laser Ablation
Hyun-Jeong Nam, Takeshi Sasaki, and Naoto Koshizaki Nanoarchitectonics Research Center (NARC), National Institute of Advanced Industrial Science and Technology (AIST), Central 5, 1-1-1 Higashi, Tsukuba, Ibaraki 305-8565, Japan ABSTRACT We investigated the optical gas sensing properties of cobalt oxide thin films prepared by pulsed laser deposition (PLD) at various Ar pressures from 0.07 to 133 Pa. Morphology of films was changed by Ar pressure. Sensitivity was estimated as transmittance change of film between dry air and 200 ppm of CO gas ambient at 350°C depended on the film morphology. The cobalt oxide films prepared at Ar pressures from 13.3 to 133 Pa showed high sensitivity. Especially, the film prepared at 13.3 Pa was suitable for optical CO sensor with a high sensitivity and mechanical stability.
INTRODUCTION Gas sensors based on optical absorption changes in thin films are attracting interest for detection of various gases such as CO, NO and H2 [1-4]. Optical gas sensors have several advantages over conventional electronic gas sensors, including the potential for higher sensitivity, reduced signal noise, and compatibility with combustible gases [2]. Recently, our group reported that an optical gas sensor using a cobalt oxide films prepared by pulsed laser deposition (PLD) exhibited remarkably high sensitivity [5]. Although morphology and crystallite size of film have been reported to have a significant effect on the sensitivity of conventional electronic gas sensor [6], factors affecting the sensitivity of this optical gas sensor have not been sufficiently investigated. PLD is an excellent method to prepare thin films with controllable thickness, composition, and microstructures [7-9]. Ambient gas pressure is one of the important parameters controlling the morphology of films prepared by PLD [10-12]. In this study, we prepared cobalt oxide composites by PLD at different pressures of Ar, and investigated the relationship between morphology and sensitivity of optical gas sensors.
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EXPERIMENTAL DETAILS Preparation and characterization of films First we evacuated the deposition chamber to a base pressure of ca. 2.66×10 -4 Pa and then filled it with argon (Ar) gas at pressures varying from 0.07 to 133 Pa during film deposition. The beam of a Nd:YAG laser (Continuum, Precision 8000) (355 nm, 10 Hz, 70 mJ/pulse) was focused onto the surface of a cobalt monoxide (CoO) single crystal target, rotating at 30 rpm. The cobalt oxide films were deposited on quartz glass substrates, which were positioned in an off-axis geometry and were rotating at 30 rpm. Silicon wafers were also used as substrates for morphology observation using a field emission scanning electron microscopy (FE-SEM, Hitachi S-4800). PLD was carried out for 800 s at room temperature. Crystallinity comparison and phase identification were performed by X-ray diffraction (XRD, Rigaku RAD-C) using Cu Kα radiation. Films morphology was characterize
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