Interactions of single and few organic molecules with SERS hot spots investigated by orientational imaging and super-resolution optical imaging
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Dynamics between organic molecules and surface enhanced Raman scattering (SERS) hot spots are extracted from far-field optical images by two experimental methods presented in this thesis: orientational imaging and super-resolution optical imaging. We introduce SERS orientational imaging as an all-optical technique able to determine the three-dimensional orientations of SERS-active Ag nanoparticle dimers. This is accomplished by observing lobe positions in SERS emission patterns formed by the directional polarization of SERS emission along the longitudinal axis of the dimer. We further extend this technique to discriminate nanoparticle dimers from higher order aggregates by observing the wavelength-dependence of SERS emission patterns, which are unchanged in nanoparticle dimers, but show differences in higher order aggregates involving two or more nanoparticle junctions. Dynamic fluctuations in the SERS emission pattern lobes are observed in aggregates labeled with low dye concentrations, as molecules diffuse into regions of higher electromagnetic enhancement in multiple nanoparticle junctions. In order to investigate these dynamic interactions between single organic molecules and nanoparticle hot spots we present the first super-resolution optical images of single-molecule SERS (SM-SERS), introducing super-resolution imaging as a powerful new tool for SM-SERS studies. Mapping the dynamic movement of SM-SERS centroid positions with +/- 5 nm resolution reveals the position-dependent SERS intensity as the centroid samples different positions in space. We have proposed that the diffusion of the SERS centroid is due to diffusion of a single molecule on the surface of the nanoparticle, which leads to changes in coupling between the scattering dipole and the optical near field of the nanoparticle. Finally, we combine an isotope-edited bi-analyte SERS spectral approach with super-resolution optical imaging and atomic force microscopy (AFM) structural analysis for a more complete picture of molecular dynamics in SERS hot spots. We demonstrate the ability to observe multiple molecule dynamics in a single hot spot and show that in addition to the single-molecule regime, a "few" molecule regime is able to report on position-dependent SERS intensities in a hot spot. Furthermore, we are able to identify multiple local hot spots in single nanoparticle aggregates.