Why and how do the shapes of surface-enhanced Raman scattering spectra change? Recent progress from mechanistic studies

The present review is focused on understanding the enhancement mechanisms in surface-enhanced Raman scattering (SERS) from the point of view of the various spectral changes, which have brought considerable confusion in this research field and have obstructed practical SERS applications. The electromagnetic mechanism provides us with the quantitative explanation of unexpected SERS spectral changes, such as larger Raman intensities for anti-Stokes peaks than for Stokes peaks and overtone peak intensities comparable with fundamental ones. Other mechanisms classified as chemical effects, i.e. the resonance Raman and the charge transfer mechanisms, are also introduced to explain the spectral changes in SERS. These mechanisms may explain the emergence of non-totally symmetric vibrational modes in SERS spectra, which is usually forbidden in Raman spectra. Copyright (c) 2016 John Wiley & Sons, Ltd.