Mechanisms in Surface Enhanced Raman Scattering

<p>This thesis focusses on a number of topics in surface enhanced Raman scattering (SERS). The aim of the undertaken research was to deepen the general understanding of the SERS effect and, thereby, to clarify some of the disputed issues, among them: What is the origin of the enhancement? What is the physical or chemical effect of 'salt activation' in SERS systems? Can we observe single-molecules using SERS? Can we determine the absorbate's orientation on the surface? In part one (chapters 1-3), as a general introduction, I start with a short overview of the Raman effect and its relation to other molecular spectroscopic effects (such as fluorescence, Rayleigh scattering, etc... ). Following these basic remarks, the surface enhancementmechanisms underlying SERS are explained (as a largely electromagnetic field enhancement) and are investigated theoretically on the canonicalmodel of a nanoscopic dimer of silver spheres. The second part (chapter 4) reports on the experimental investigation (electron microscopy, in-situ Raman measurements) of a typical real SERS system: Lee & Meisel silver colloids. An emphasis is put on the self-limiting aggregation kinetics which is observed in such systems after salt addition. This is also investigated and rationalised by means of Monte-Carlo simulations which are footed on empiric theoretical considerations for the interaction potential. Part three (chapter 5) contains a discussion of the early attempts on singlemolecule SERS and points out the shortcomings of the previously used ultra-lowconcentration approach. In response, an improved andmore rigorous approach is presented: Bi-Analyte SERS. Examplary applications of the technique are discussed. Within these experiments the capability of the technique to prove/disprove (with statistical soundness) single-molecule sensitivity in any SERS system is demonstrated, and single-molecule enhancement factors are derived. The last part (chapter 6) presents computational studies based on densityfunctional theory and its use in the context of Raman spectroscopy and SERS. Of particular interest here were the Raman tensors, their visual representation appropriate in the SERS case, their relation to the relative intensities of Raman peaks, and their modification when the photon energy approaches the electronic resonance of the molecule. Last, but not least, a conclusion chapter is presented, where I highlight what has contributed by the thesis to the general understanding of the SERS effect.</p>