Thickness effect of sputtered ZnO seed layer on the fabrication of ZnO nanorods on flexible polyimide films and their et
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Thickness effect of sputtered ZnO seed layer on the fabrication of ZnO nanorods on flexible polyimide films and their ethanol gas sensing properties Hosang Ahn, Seon-Bae Kim and Dong-Joo Kim Materials Research and Education Center, Auburn University, Auburn AL 36849 USA
ABSTRACT Controlled ZnO nanostructures were grown on a flexible substrate for the future development of smart sensing tags. Thermolysis-assisted chemical solution deposition was used to grow ZnO nanorods at 85°C from 0.01mol of Zinc nitrate hexahydrate and HMT (Hexamethyltetramine) solution. To promote and modulate the ZnO nanorods, R.F. sputtered ZnO seed layers were deposited on polyimide substrates at various film thicknesses in the range of 8 to 160 nm. The optimum processing conditions to fabricate ZnO nanostructures have been investigated to examine the growth behaviors and to correlate the process parameters with the morphological characteristics. When the ethanol gas sensitivities were measured at different thickness of ZnO seed layers before growing ZnO nanorods, the highest sensitivity was obtained at 40 nm thick ZnO film at 300oC where the film thickness is similar to the Debye length. When ZnO nanorods were grown on such a ZnO seed layer, the sensitivities were more heavily influenced by the ZnO nanostructures rather than the thickness of the seed layer probably due to the dominant proportion of carrier density involved with the gas absorption. INTRODUCTION Semiconducting ZnO having large band gap energy (3.37ev) have been researched as a promising material in several applications, such as high power LED, solar cell, and gas sensors [1]. To maximize its electronic and optical properties, nanostructured ZnO was studied by several technologies, such as MOCVD, sol-gel spin coating, sputtering, and electrochemical deposition [2]. In recent device development, technologies, efficient, accessible at low working temperature at economic cost, is highly demanded. Among above methods, electrochemical deposition was satisfied to be applicable to the devices in large scale at low cost, especially for the device integrated on flexible substrate. In the growth of ZnO nanostructure by electrochemical approach, a seed layer was reported to be necessary in selectively growing nanostructure and vertical alignment [3]. The influence of seed layer, i.e. ZnO thin film on the property of nanostructured ZnO, such as field emission, photoconductivity and transmittance were researched by several groups [4]. However, rare studies were carried out to investigate the relationship between seed layer effect and nanostructured ZnO and its gas sensing property. In this study, 8 to 160nm thick ZnO thin films were deposited by R.F. sputtering to prepare ZnO nanorods. FE-SEM and XRD characterizations were performed to investigate the influence of seed layer on the volumetric change of ZnO nanorods and its gas sensing property. To clearly understand the effects of seed layer on nanostructured ZnO growth and its sensitivity, two separated groups were studied, one group was
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