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Carbon nanotube encapsulated dye molecules probe surfaceenhanced Raman scattering

Heeg, Sebastian (Photonics Laboratory, ETH Zürich, Zürich, CHE); Müller, Niclas (Department of Physics, Freie Universität Berlin, Berlin, GER); Hübner, Uwe (Leibniz-Institut für Photonische Technologien, Jena, GER); Gaufrès, Etienne (Regroupement québécois sur les matériaux de pointe and Département de chimie, Université de Montréal , Montréal, CAN); Kusch, Partryk (Department of Physics, Freie Universität Berlin, Berlin, GER); Tang, Nathalie (Regroupement québécois sur les matériaux de pointe and Département de chimie, Université de Montréal , Montréal, CAN); Martel, Richard (Regroupement québécois sur les matériaux de pointe and Département de chimie, Université de Montréal , Montréal, CAN); Reich, Stephanie (Department of Physics, Freie Universität Berlin, Berlin, GER); Vijayaraghavan, Aravind (School of Materials, The University of Manchester, Manchester, GBR)

Surface-enhanced Raman scattering (SERS) is the giant increase in the Raman signal of molecules by surface plasmons of metal nanostructures. Even after 40 years of SERS, quantifying enhancement and refining our understanding of the underlying mechanism is challenging: the exact location of a molecule in a SERS hotspot, the orientation of its transition dipole and chemical interactions with the metal remain impossible to control experimentally. Here we overcome these limitations by probing SERS with aligned-sexithiophene (6T) molecules encapsulated inside carbon nanotubes. The tubes (i) carry the 6T into SERS hotspots by directed dielectrophoretic deposition, (ii) render them chemically inert and (iii) simultaneously define and reveal their location and orientation. We access SERS enhancement with unprecedented accuracy, both experimentally and by simulations according to the electromagnetic theory (EM) of SERS. The experimental enhancement exceeds the calculated value by two orders of magnitude and – in the absence of chemical enhancement - calls for treating SERS beyond the EM approach, e.g. within perturbation theory, where the plasmon forms an integral part of the Raman process

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