Plasmonic Au nanostructures for surface-enhanced raman spectroscopy
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Raman spectroscopy is a spectroscopic technique that provides rich structural information for identifying chemical species but finds limited applications owing to its low sensitivity. Surface-enhanced Raman spectroscopy (SERS) is capable of solving the issue of sensitivity by enormously amplifying the Raman signal through localized surface plasmon resonance (LSPR) that is induced by so-called plasmonic nanostructures. Since the inception of SERS in 1970s, significant efforts have been put in developing SERS-active substrates with high quality in terms of sensitivity, reliability, reproducibility, scalability, throughput, and cost. At present, however, SERS substrates with sufficiently high quality for both research activities and real-world applications have not stood out yet. In this dissertation, four types of plasmonic Au nanostructures will be reviewed with respects to fabrication, characterization, optimization, and evaluation for SERS applications. Firstly, faceted ZnO/Au nanonecklace arrays epitaxially grown on r-plane sapphire substrates by chemical vapor deposition and sputtering will be introduced. Secondly, Au nanoisland arrays prepared by repeated sputtering and post-deposition annealing will be presented. Thirdly, nanoporous Si/Au composites resulting from metal-assisted wet etching and sputtering will be reported. Lastly, we will present a novel plasma nanocoating technique that overcoats SiO2/Au SERS-active nanostructures with an ultra thin polymer layer, followed by the demonstration of benefits brought by such plasma nanocoating. The properties and growth mechanisms of above mentioned plasmonic Au nanostructures were investigated with scanning electron microscopy (SEM), atomic force microscopy (AFM), transmission electron microscopy (TEM), ellipsometry and contact angle analyzer. By correlating the enhancement of Raman signal with the experimental parameters, recipes for optimized plasmonic nanostructures were established. Furthermore, the applicability of these plasmonic Au nanostructures for SERS purposes was demonstrated by successfully detecting various chemical species at trace level. At the end of the dissertation, a brief summary on these four plasmonic Au nanostructures will be reviewed against the standards of high quality SERS substrates and corresponding recommendations will be proposed to further improve the SERS performance.