Engineering Plasmonic Nanopillar Arrays for Surface-enhanced Raman Spectroscopy

Publication: ResearchPh.D. thesis – Annual report year: 2016

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This Ph.D. thesis presents (i) an in-depth understanding of the localized surface plasmon resonances (LSPRs) in the nanopillar arrays (NPs) for surface-enhanced Raman spectroscopy (SERS), and (ii) systematic ways of optimizing the fabrication process of NPs to improve their SERS efficiencies. This Ph.D. project is part of the NAPLAS - NAnoPLAsmonic Sensors project, funded by The Danish Council for Independent Research. LSPRs in silver capped silicon NPs are studied using numerical simulations and dark-field scattering microscopy. Simulations show that a standalone NP supports two LSPR modes, i.e., the particle mode and the cavity mode. The particle mode can be hybridized via leaning of pillars. The LSPR wavelength of the cavity mode is dominant only by the diameter of the Si pillar. The presence of a substrate dramatically changes the intensities of these two LSPR modes, by introducing constructive and destructive interference patterns with the excitation fields. Experimental scattering spectra can be interpreted using theoretical simulations. The processes, which affect the SERS efficiencies of the silver NPs, are systematically evaluated. Short exposures to the O2-plasma and the use of 1-3 nm Cr adhesion layers are advantageous for reducing the SERS background signals. Influence of the NP height and silver deposition thickness on SERS intensities is also investigated. Using an optimized recipe, the measured SERS enhancement factor (EF) reaches 108, andthe SERS signal intensity exhibits a standard deviation of ~14% (660 data points) across a 5 x 5 mm2 surface area. Lastly, a further improved process shows that high-density NPs exhibit unrivalled macroscale SERS uniformities (RSD: ∼2.5% in mm scale, ∼7% in inch scale) and SERS reproducibilities (RSD: ∼1.5% across three wafers), while at the same time displaying a very large average SERS EF of >108. From a practical point of view, the developed SERS substrates are particularity interesting, since they are easy to handle and store and the fabrication is scalable, facilitating a wide and simple use of SERS in sensing applications.
Original languageEnglish
PublisherDTU Nanotech
Number of pages152
StatePublished - 2016
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