Formation and Molecular Sensing Property of Silver Nanoparticles from Sputtered Silver Oxide Layers
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Formation and Molecular Sensing Property of Silver Nanoparticles from Sputtered Silver Oxide Layers Makoto Fujimaki, Yasuhiko Iwanabe1, Koichi Awazu, and Junji Tominaga Center for Applied Near-Field Optics Research (CAN-FOR) National Institute of Advanced Industrial Science and Technology (AIST) Tsukuba Central 4, 1-1-1 Higashi, Tsukuba, 305-8562, Japan 1 Graduate School of Engineering, Tokyo Denki University 2-2 Kanda, Chiyoda, Tokyo, 101-8457, Japan ABSTRACT Surface-enhanced Raman scattering (SERS) efficiency of silver nanoparticles formed by laser irradiation or thermal annealing in sputtered silver oxide layers was examined. Silver nanoparticles formed by irradiation of He-Ne laser light (632.8 nm) to a sputtered silver oxide thin film thermally annealed at 300 °C show good SERS, while silver nanoparticles formed by thermal annealing at 600 °C scarcely show SERS. From these results, it is deduced that thermal annealing at a proper temperature results in formation of silver nuclei that can be precursors of silver nanoparticles with desirable sizes to induce the SERS, while thermal annealing at a higher temperature results in the formation of large silver particles that no longer cause the SERS. INTRODUCTION High-sensitivity molecular-detection sensors, which are able to detect a small quantity of specific molecules, have been anxious in medical, biotechnological, and life-science fields. Raman spectroscopy is one of the most powerful tools to specify molecules, molecular structures, molecular orientation, and interaction among molecules close to each other. However, Raman spectroscopy has a great disadvantage that the cross section of Raman scattering is extremely small, and it is quite difficult for a common Raman spectroscopy to detect a small amount of molecules. Therefore, the finding of SERS with a rough silver electrode attracted much attention [1-4]. At the beginning, the Raman enhancement factors were estimated to be 103 to 105. The latest reports, however, indicated that cross sections of SERS were 14 to 15 orders of magnitude greater than these of common Roman scattering [5-7]. The SERS is caused by nano structures of noble metals, such as rough surfaces, nanoparticles, and nano rods. Typical fabrication methods of nano structures of noble metals utilize crystal growth of noble metals in solutions [8,9]. Fabrication of noble-metal nanoparticles using colloidal solutions is also well known [10,11], and sizes and shapes of nanoparticles are controlled by mixing solutions or adjusting concentrations. However, these solutions are not suitable for the SERS measurement, since the solutions may cause chemical reactions with molecules to be detected. Accordingly, lots of efforts to adhere noble-metal colloidal nanoparticles or nano rods on substrates, such as the Langmuir-Blodgett method, have been made. However, these methods usually require long processing time and are not suitable for industrial use. It has been reported that silver nanoparticles formed from silver-oxide thin films caused effective S
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