Surface‐Enhanced Raman Scattering from a Single Nanoparticle–Plane Junction*

Abstract

Since the first observation of single-molecule level surface-enhanced Raman scattering (SERS) from randomly aggregated noble metal nanoparticles, much effort has been made to understand the electromagnetic and chemical mechanisms behind the observed enhancement, and to fabricate structures that exhibit strong and reproducible SERS activities. It is widely accepted that the locally enhanced electromagnetic (EM) field formed between the noble metal nanoparticles is mostly responsible for the strong SERS signals observed. However, the SERS-active junctions (hotspots) in self-assembled nanoparticle aggregates occur at unpredictable positions, and the SERS intensities vary by 5–6 orders magnitude from one hotspot to another within the same sample, which renders the quantitative prediction of the SERS activities challenging. The extremely irregular characters of the SERS signals appear to originate from the irregular geometries of the randomly formed junctions. Therefore, many “designed” SERS structures such as hexagonally close-packed gold nanospheres, rafts of silver nanowires, lithographically designed junctions, film-on-nanospheres structures (FON), and nanoparticle-plane assemblies, have recently been tested for consistent and reproducible SERS activities. However, the junction structures that exhibit site-reproducible (junctionto-junction invariant) SERS activities at the individual junction levels have yet to be reported. Herein we report that self-assembled nanoparticle–molecular monolayer–plane junctions show such reproducibility at the single junction level. Theoretical models predict that the nanosphere–plane junction would produce a strong local field and SERS signal via the coupling between the localized surface plasmon (LSP) of the nanosphere and the surface plasmon polariton (SPP) formed on the conducting surface. A few experimental reports have demonstrated that such junctions produce ensemble-averaged Raman enhancement of 10–10. However, the SERS activities and their microscopic mechanisms of the individual nanoparticle–plane junctions have not been experimentally examined in detail. Through the confocal SERS microscopy measurements on individual nanoparticle–plane junctions, we find that almost every nanoparticle–plane junction generates remarkably reproducible SERS signals. In addition, we observe that such junctions show strong preference towards the excitation and the SERS polarization directions parallel to the nanoparticle–plane junction axis, which strongly supports the LSP–SPP coupling model of the SERS from the nanoparticle–plane junction. The experiment was carried out with an epi-confocal Raman microscope combined with a tapping-mode atomic force microscope (AFM, Figure 1a, see also the Experimental Section), which enables the simultaneous measurements of the spatially