Enhancement Mechanisms and Theory of SERS

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<div>This chapter starts with a brief introduction to basic Raman Spectroscopy,</div><div>its strengths and bottlenecks leading to ineffective utilization of the technique in its</div><div>conventional form. Further, the basic principle of Surface Enhanced Raman</div><div>Spectroscopy (SERS) has been discussed along with its development as a vibrational</div><div>spectroscopic tool of analytical importance. The milestones related to the development</div><div>of new methods in SERS have also been covered. The major focus of the chapter is to</div><div>describe the enhancement mechanisms responsible for magnificent enhancement</div><div>obtained in SERS as compared to normal Raman Spectroscopy. Out of various</div><div>mechanisms, the electromagnetic (EM) mechanism has been considered to play the</div><div>most significant role in enhancement. Localized Surface Plasmon Resonance (LSPR) is</div><div>chiefly responsible for the whole EM enhancement leading to E4 enhancement and</div><div>therefore, it has been discussed in detail. Other mechanisms like chemical and</div><div>electronic enhancement mechanisms have also been discussed extensively. The</div><div>dependence of SERS on various factors like substrate, excitation wavelength, size and</div><div>shape of nanoparticles, etc., has been explained with emphasis on the reported data</div><div>followed by analysis. Two photon excited SERS has been presented as a special class</div><div>of SERS. Towards the end, the application of SERS to achieve the ultimate limit of</div><div>detection by probing single molecules has been emphasised. In this context, Hot spots,</div><div>the heterogeneous nanoregions causing extremely large enhancements and their various</div><div>generations have been presented along with preliminary theoretical and experimental</div><div>results.</div>