On the mechanism responsible of Raman enhancement on carbon allotropes surfaces: the role of molecule-surface vibrational coupling in SERS

摘要

The ability of different carbon allotropes surfaces as potential substrates for enhanced Raman spectroscopy is analysed theoretically and the factors responsible of the Raman-enhancing mechanism deeply scrutinised. Our analysis is based on the partition of the Raman tensor into molecule and surface terms, which leads to three different contributions to the Raman activity ('molecule', 'surface' and 'intermolecular'). Both static and pre-resonance conditions are considered in our analysis of the Raman spectra of pyridine adsorbed on model planar and curved surfaces and the three contributions to the Raman activity obtained separately. At static conditions, there is a general decrease in the Raman activity of vibrational modes associated to the molecule, proportional to the strength of the molecule-surface interaction. This stems from a reduction of the polarizability of pyridine upon its adsorption on the carbon surface. Under pre-resonance conditions, the surface contributes significantly to the Raman activity of the pyridine vibrational modes, even if the electronic transition involves exclusively energy levels from the surface. This is because of small vibrational couplings between molecular and surface modes which are negligible in metallic surface-like silver. It suggests also the possibility of finding similar effects in metallic surface built from lighter atoms like silicon or aluminium. Copyright (c) 2015 John Wiley & Sons, Ltd.