Surface plasmon excitation and surface-enhanced raman scattering using two-dimensionally close-packed gold nanoparticles

We investigated plasmon excitation and surface-enhanced Raman scattering (SERS) of crystal violet dye adsorbed on two-dimensionally (2D) close-packed gold nanostructures. With reference to the absorption maximum of colloidal nanoparticles, the 2D nanostructures showed red-shifted maxima at ~560 and 740 nm. Interestingly, we found that the surface plasmon resonance (SPR) and SERS signals were preferentially enhanced near the edge of the 2D nanostructures. Both the SPR and SERS images suggest good correlation between the SPR-mediated localized electromagnetic field and the localized optical field, particularly near the edge of the 2D nanostructures. We observed broad SPR bands near the edge and red-shifted and intense bands near the center of the 2D nanostructures. Also, the overall SERS enhancement on the 2D nanostructures was comparable to that reported on engineered gold nanostructures. The integral SERS intensity near the edge was higher than that measured near the center. We carried out finite-different time-domain simulations and demonstrated that the experimental data are consistent with the simulated ones. Here, several proximal localization sites appeared for nanoparticles with their out-of-plane polarization parallel to the interparticle axis. Thus, the experimental results and the simulation predicted the existence of multipolar plasmon excitation and a narrow passage for energy percolation, which are often obscured in ensemble SERS measurements.