title: Quantitative Plasmon Mode and Surface-Enhanced Raman Scattering Analyses of Strongly Coupled Plasmonic Nanotrimers with Diverse Geometries.
abstract: Here, we quantitatively monitored and analyzed the spectral redistributions of the coupled plasmonic modes of trimeric Au nanostructures with two ∼1 nm interparticle gaps and single-dye-labeled DNA in each gap as a function of varying trimer symmetries. Our precise Mie scattering measurement with the laser-scanning-assisted dark-field microscopy allows for individual visualization of the orientations of the radiation fields of the coupled plasmon modes of the trimers and analyzing the magnitude and direction of the surface-enhanced Raman scattering (SERS) signals from the individual plasmonic trimers. We found that the geometric transition from acute-angled trimer to linear trimer induces the red shift of the longitudinally polarized mode and the blue shift of the axially polarized mode. The finite element method (FEM) calculation results show the distinct "on" and "off" of the plasmonic modes at the two gaps of the trimer. Importantly, the single-molecule-level systematic correlation studies among the near-field, far-field, and surface-enhanced Raman scattering reveal that the SERS signals from the trimers are determined by the largely excited coupled plasmon between the two competing plasmon modes, longitudinal and axial modes. Further, the FEM calculation revealed that even 0.5 nm or smaller discrepancy in the sizes of two gaps of the linear trimer led to >10-fold difference in the SERS signal. Granted that two gap sizes are not likely to be completely the same in actual experiments, one of two gaps plays a more significant role in generating the SERS signal. Overall, this work provides the knowledge and handles for the understanding and systematic control of the magnitude and polarization direction of the both plasmonic response and SERS signal from trimeric nanostructures and sets up the platform for the optical properties and the applications of plasmonically coupled trimers and higher multimeric nanostructures.
authors.affiliation: †Laboratory for Advanced Molecular Probing (LAMP), Research Center for Convergence Nanobiotechnology, Korea Research Institute of Chemical Technology (KRICT), Daejeon 305-600, South Korea.
authors.forename: Haemi
authors.initials: H
authors.lastname: Lee
authors.affiliation: ‡Department of Chemistry, Seoul National University, Seoul 151-747, South Korea.
authors.forename: Gyeong-Hwan
authors.initials: GH
authors.lastname: Kim
authors.affiliation: ‡Department of Chemistry, Seoul National University, Seoul 151-747, South Korea.
authors.forename: Jung-Hoon
authors.initials: JH
authors.lastname: Lee
authors.affiliation: †Laboratory for Advanced Molecular Probing (LAMP), Research Center for Convergence Nanobiotechnology, Korea Research Institute of Chemical Technology (KRICT), Daejeon 305-600, South Korea.
authors.forename: Nam Hoon
authors.initials: NH
authors.lastname: Kim
authors.affiliation: ‡Department of Chemistry, Seoul National University, Seoul 151-747, South Korea.
authors.forename: Jwa-Min
authors.initials: JM
authors.lastname: Nam
authors.affiliation: †Laboratory for Advanced Molecular Probing (LAMP), Research Center for Convergence Nanobiotechnology, Korea Research Institute of Chemical Technology (KRICT), Daejeon 305-600, South Korea.|§School of Chemical Engineering, Sungkyunkwan University (SKKU), Suwon 440-746, South Korea.
authors.forename: Yung Doug
authors.initials: YD
authors.lastname: Suh
keywords: laser-scanning-assisted dark-field microscopy; localized surface plasmon; nanogap-enhanced Raman scattering (NERS); single-molecule SERS; single-nanoparticle spectroscopy