Experimental Identification of Chemical Effects in Surface Enhanced Raman Scattering of 4-Aminothiophenol

We report on the experimental identification of Raman modes that are enhanced through the chemical effect in surface enhanced Raman spectroscopy of 4-aminothiophenol (also known as p-mercaptoaniline) adsorbed on gold substrate. Introduction of a thin spacer layer between the metal and the sample can prevent any possible chemical bonding between metal atoms and sample molecules, hence such a sample shows only those Raman modes that are enhanced through the electromagnetic effect. Alternatively, a significant increase in the chemical effect could be observed in the presence of halide ions as compared to their absence. This result provides another way to experimentally identify those Raman modes that undergo chemical enhancement. In addition, apart from the electromagnetic-based resonance in SERS, chemical enhancement also shows a resonance with varying wavelength of the excitation light, which provides yet another way to experimentally identify chemically enhanced Raman modes in SERS. Some new chemically enhanced modes could be observed when the sample molecules were sandwiched between gold substrate and a gold nanotip.