Using correlated super-resolution optical and structural studies to investigate surface-enhanced Raman scattering on silver & gold nanoparticles
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Surface-enhanced Raman scattering (SERS) from organic molecules is used to study how geometry affects plasmonic enhancement in noble metal nanoparticles using super-resolution analysis techniques coupled with scanning electron microscope (SEM) imaging. We demonstrate that super-resolution analysis can track emission from Raman-active molecules on nanoaggregate surfaces with resolution typically better than 5 nm, a significant improvement over the diffraction limit. This optical analysis technique is used to identify active junction regions, known as hot spots, in correlated SEM structural images of aggregated particles based on shape, size, and angular position of SERS emission. This correlated analysis is further extended to study the mechanism of silver nanoparticle luminescence and its positional relationship to SERS emission. These studies support the hypothesis that silver luminescence is enhanced via a different mechanism than SERS. Discrete dipole approximation calculations agree with experimental results, indicating that SERS emission is a product of local plasmonic enhancement in specific junction regions between nanoparticles whereas silver luminescence is dependent upon nanoaggregate geometry and the collective plasmon modes within a structure. These theoretical calculations of luminescence centroid location can assist with SERS-active junction region assignment in higher order aggregates (e.g. trimers, tetramers, etc.) Finally, correlated optical and structural studies are used to develop understanding of site-specific electrochemical potentials on silver and gold nanoparticle aggregates using a redox active Raman reporter molecule. We demonstrate that there is a weak correlation between emission centroid location and bulk sample potential. Silver colloid studies demonstrate variability in modulation behavior and are highly unpredictable, whereas gold colloids with the redox molecule covalently tethered to their surface via a gold-thiol bond show more reproducible modulation. Furthermore, these tethered studies demonstrate geometrical agreement between site-specific emission at negative potentials and junction regions in nanoaggregate SEM images indicating that plasmonic enhancement may affect local nanoparticle surface potentials.