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

Abstract

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. † Part of the "Martin Moskovits Festschrift".

Temperature dependence of a1 and b2 type modes in the surface enhanced Raman from 4-Aminobenzenethiol

“…Topography images of the nanocavity are shown in Fig 1a. The AFM image shows the presence of arrays of nanocavities, the size of these cavities was measured (see Fig 1b) to be 120 nm in height and 480 nm in width. A Raman spectrum recorded using a 532 nm excitation wavelength is shown in Fig 1c. The Raman spectrum shows several peaks which are characteristic of modes observed previously from surface enhanced Raman spectra of 4-ABT 18 . A transmission spectrum of the substrate is shown in Fig 1d. The transmission spectrum of the nanocavity substrate (see Fig 1d) shows that the optimum plasmonic enhancement induced by the cavity is likely to occur between c.a.…”

“…These intense Raman peaks are assigned to a combination of a and b type vibrational symmetries and are in agreement in both their frequency position and relative intensities with literature Raman studies for 4-ABT on silver 16 . The Raman spectra is assigned to arise from surface enhanced Raman scattering ie SERS [16][17][18] .…”

“…In order to investigate the SERS activity as a function of temperature a monolayer of 4-Aminobenzenethiol (4-ABT) was prepared on a SSA substrate. SERS spectrum from 4-ABT possess a combination of totally symmetric (a 1 ) vibrations and non-totally symmetric (b 2 ) vibrations 18 . Studies have shown that the b 2 vibration arise from intensity borrowing from an intense * molecular transition ( 1 A 1 -1 B 2 ) at 300 nm as a consequence 4-ABT has is used as a model adsorbate for probing the chemical enhancement mechanism in SERS.…”

Disentangling the Peak and Background Signals in Surface-Enhanced Raman Scattering

“…The CE is generally far less than the EM enhancement, but the CE is strongly system dependent, varying with applied potential and wavelength, and in certain charge-transfer systems it can dominate over the EM enhancement. 8 In experimental SERS spectra, therefore, it is difficult to precisely distinguish between the EM and CE nature of the enhancement and most current work focuses on the larger EM enhancement.…”

Comparative Characteristics of Gold-Gold and Gold-Silver Nanogaps Probed by Raman Scattering Spectroscopy of 1,4-Phenylenediisocyanide

“…Considering the potential and excitation wavelength dependence, the bands have been assumed in the past 15 years to belong to b 2 symmetry vibrations of 4-ABT, arising from the metal-to-adsorbate or the vice versa charge transfer transition. [17][18][19] Very recently, however, there is a debate that the bands would be due to the A g modes of dimercaptoazobenzene supposedly produced from 4-ABT via a catalytic coupling reaction on metal substrates.…”