Imaging out-of-plane polarized emission patterns on gap mode SERS substrates: from high molecular coverage to the single molecule regime

Abstract

Gap mode surface-enhanced Raman scattering (SERS) substrates are created when a single nanoparticle is deposited on a thin metal film, creating a region of significant electromagnetic field enhancement in the gap between the nanoparticle and the film due to excitation of a vertically-oriented, out-of-plane dipole plasmon mode, e.g. the gap plasmon. When molecules are located in the gap and couple to the gap plasmon mode, the resulting emission is polarized perpendicular to the thin film, generating SERS emission patterns that have a characteristic donut shape. We analyze these SERS emission patterns using a dipole emission model and extract out-of-plane and in-plane emission angles associated with the gap plasmon mode. Fluctuations in both of these angles reveal dynamic heterogeneity due to molecular motion within the hot spot that changes as a function of molecular coverage. We also reveal static heterogeneity associated with structural defects in the thin film component of the gap mode substrates, indicating that even nanometer-scale surface roughness can impact the quality of gap mode emission.