DOI: 10.1002/cphc.200800563 Surface-Enhanced Raman Scattering from a Single Nanoparticle–Plane Junction* Won-Hwa Park, Sung-Hyun Ahn, and Zee Hwan Kim* [a] Since the first observation of single-molecule level surface-en- hanced Raman scattering (SERS) [1–3] from randomly aggregated noble metal nanoparticles, much effort has been made to un- derstand the electromagnetic and chemical mechanisms behind the observed enhancement, and to fabricate structures that exhibit strong and reproducible SERS activities. It is widely accepted [4, 5] that the locally enhanced electro- magnetic (EM) field formed between the noble metal nanopar- ticles is mostly responsible for the strong SERS signals ob- served. However, the SERS-active junctions (hotspots) in self-as- sembled nanoparticle aggregates occur at unpredictable posi- tions, and the SERS intensities vary by 5–6 orders magnitude from one hotspot to another within the same sample, [6, 7] which renders the quantitative prediction of the SERS activities challenging. The extremely irregular characters of the SERS sig- nals appear to originate [4] from the irregular geometries of the randomly formed junctions. Therefore, many “designed” SERS structures such as hexagonally close-packed gold nano- spheres, [8] rafts of silver nanowires, [9–11] lithographically de- signed junctions, [12–14] film-on-nanospheres structures (FON), [15] and nanoparticle-plane assemblies, [16–19] have recently been tested for consistent and reproducible SERS activities. However, the junction structures that exhibit site-reproducible (junction- to-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 [20–22] 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 po- lariton (SPP) formed on the conducting surface. A few experi- mental reports [16–18, 23–26] have demonstrated that such junctions produce ensemble-averaged Raman enhancement of 10 5 –10 7 . 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 in- dividual nanoparticle–plane junctions, we find that almost every nanoparticle–plane junction generates remarkably repro- ducible SERS signals. In addition, we observe that such junc- tions 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 mi- croscope (AFM, Figure 1 a, see also the Experimental Section), which enables the simultaneous measurements of the spatially Figure 1. a) Experimental setup and sample structure. The AFM tip scans above the sample and the illumination and detection are carried out in the epi-direction. b) SERS spectra obtained from the nanoparticle–benzenethiol (BT)–plane junction position (red line, position A in Figure 1 c), BT-plane (blue line, position B in Figure 1 c), and a BT-coated gold nanoparticle on glass (black line, position D in Supporting Information). Also shown are the spectral assignments based on Joo et al.. [22] c) AFM topography and d) con- focal SERS images of the nanoparticle–BT–plane sample obtained simultane- ously. The inset images in (c) and (d) are the magnified scans (1.3 mm  1.3 mm) of the squared areas. The scale bars represent 1 mm length scale. The position C is an aggregate of nanoparticles (two or three) on the surface that produce visibly stronger SERS signals than average. e) Represen- tative collection of SERS and AFM images of the nanoparticle–BT–plane system obtained from different regions of the same sample [the color scales are the same as the ones used in (c) and (d)]. [a] W.-H. Park, S.-H. Ahn, Prof. Z. H. Kim Department of Chemistry and Center for Electro- and Photo-Responsive Molecules Korea University, Anam-Dong, Seongbuk-Gu, Seoul 137-701 (Korea) Fax: (+ 82) 2-3290-3121 E-mail : zhkim@korea.ac.kr Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/cphc.200800563. ChemPhysChem 2008, 9, 2491 – 2494  2008 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim 2491