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Plasmonic enhancement of SERS measured on molecules in carbon nanotubes


We isolated and measured the plasmonic contribution to surface-enhanced Raman scattering (SERS) and find it much stronger than expected. Organic dyes encapsulated in single-walled carbon nanotubes are ideal probes for quantifying plasmonic enhancement in a Raman experiment. The molecules are chemically protected through the nanotube wall and spatially isolated from the metal, which prevents enhancement by chemical means and through surface roughness. The tubes carry molecules into SERS hotspots, thereby defining molecular position and making it accessible for structural characterization with atomic-force and electron microscopy. We measured a SERS enhancement factor of 106 on α-sexithiophene (6T) molecules in the gap of a plasmonic nanodimer. This is two orders of magnitude stronger than predicted by the electromagnetic enhancement theory (104). We discuss various phenomena that may explain the discrepancy (including hybridization, static and dynamic charge transfer, surface roughness, uncertainties in molecular position and orientation), but found all of them lacking in enhancement for our probe system. We conclude that plasmonic enhancement in SERS is, in fact, much stronger than currently anticipated. We outline novel approaches for treating SERS quantum mechanically as a higher-order Raman process where the plasmon forms an integral part of the Raman process. The plasmonic enhancement obtained from this treatment very well describes our experimental results. Our findings have important consequences on the understanding of SERS as well as for designing and optimizing plasmonic substrates.

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Publication details

The article was received on 10 Apr 2017, accepted on 23 May 2017 and first published on 23 Jun 2017

Article type: Paper
DOI: 10.1039/C7FD00127D
Citation: Faraday Discuss., 2017, Accepted Manuscript
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    Plasmonic enhancement of SERS measured on molecules in carbon nanotubes

    N. S. Müller, S. Heeg, P. Kusch, E. Gaufres , N. Tang, U. Hübner, R. Martel, A. Vijayaraghavan and S. Reich, Faraday Discuss., 2017, Accepted Manuscript , DOI: 10.1039/C7FD00127D

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