Intermolecular and molecule-substrate interactions in plasmonic nanostructures
Résumé
Surface-enhanced Raman spectroscopy (SERS) is a powerful method to probe the changes that occur in a molecule in contact with a metal. However, the molecules interacting with a plasmonic substrate make a complicated picture of the vibrational spectra. Understanding the different contributions requires a multi-dimensional approach involving the spectroscopic investigation under various experimental parameters such as plasmonic nanostructure material, density and kind of hotspots, substrate temperature, laser energy, and molecular layer density (from bulk crystals to ultrathin 1 and 2 nm thick layers). In this contribution, we report our findings deduced from a systematic investigation of all these parameters. The new insights deduced from this analysis include the equivalency between Cu and Au nanostructures. For these two materials, we found that it is not the chemistry what makes the largest changes but the density and geometry of hotspots and electric field amplification. The chemical contribution is evidenced in Ag and bimetallic nanostructures under resonance and non-resonance excitations. The temperature-dependent investigations indicate that the temperature coefficients and minima in intensity vs. T curves observed for some molecule-plasmonic nanostructures can be correlated to perturbations in the electronic structure of the molecule. These results show that controlling internal and external parameters in plasmon-enhanced spectroscopy allows deducing novel insights on the chemical and electromagnetic effects in these hybrid organic/inorganic systems.