Plasmon-sampled surface-enhanced Raman excitation spectroscopy

This work presents the first systematic study of the surface-enhanced Raman-scattering (SERS) properties of nanosphere lithography (NSL) derived Ag nanoparticles. Furthermore, it demonstrates the necessity of correlating nanoparticle structure and localized surface plasmon resonance (LSPR) spectroscopic data in order to effectively implement SERS on nanofabricated surfaces that have narrow (∼100 nm) LSPR line widths. Using nanoparticle substrates that are structurally well characterized by atomic force microscopy, the relationship between the LSPR extinction maximum (λ_max) and the SERS enhancement factor (EF) is explored in detail using the innovative approach of plasmon-sampled surface-enhanced Raman excitation spectroscopy (PS-SERES). PS-SERES studies were performed as a function of excitation wavelength, molecular adsorbate, vibrational band, and molecule-localized resonance or nonresonance excitation. In each case, high S/N ratio spectra are achieved for samples with an LSPR λ_max within a ∼120-nm window that encompasses both the excitation wavelength and the scattered wavelength. These results unambiguously demonstrate a systematic approach to the optimization of SER spectra on nanoparticle substrates with large interparticle spacings and consequently, weak or no electromagnetic coupling. In fact, this work demonstrates the largest SERS, EF > 1 × 10⁸, and SERRS, EF > 7 × 10⁹, enhancement factors measured to date on nanostructured substrates. The observation that EF_SERRS/EF_SERS ∼ 40 and E_RRS/EF_pre-RRS ∼ 40 in the Fe(bpy)₃²⁺system illustrates that this adsorbate, with its molecule-localized electronic transition, does not damp the nanoparticle-localized LSPR and that the SERS and RRS effects are strictly multiplicative.