Adatom Hypothesis as a Predominant Mechanism of Surface Enhanced Raman Scattering: A Review of Experimental Argumentation

Abstract

A contemporary review of research concerning the mechanism of surface enhanced Raman scattering (SERS) is presented and it is stated that this problem is yet to be solved. The necessity to yet once again analyze some of the familiar experimental observations and the deep-rooted beliefs and revise quantitative evaluations of SERS enhancement via major mechanisms (electromagnetic and chemical) with allowance made for most important experimental characteristics of SERS is substantiated. The characteristics are enumerated and the role they play in the above evaluations is sketched. Attention is focused mainly on electrochemical aspects of SERS problem, which to a large extent define the content and salient features of SERS and provide basic experimental justification for the so-called adatom hypothesis.

Surface enhanced Raman spectroscopy of organic molecules deposited on gold sputtered substrates

“…In any case, the fact that the SERS signal could be detected with 4-aminothiophenol and not with cysteamine indicates that the nature of the bond between gold and molecules is not responsible for such an effect. The origin of the chemical enhancement has not been clear up to now but it seems that a charge transfer mechanism plays an essential role [7,8]. In 1999 Kneipp et al proposed the following four-step excitation mechanism [1]:…”

“…However, the EM effect is not the only mechanism responsible for the enhancement observed in SERS: the interaction between molecules and metallic nanoparticles is also implied in a part of this enhancement. This feature is not clearly understood yet but it seems that a charge transfer is necessary to achieve a strong enhancement [7,8]. This effect is known as chemical enhancement.…”

On the chemical bonding effects in the Raman response: Benzenethiol adsorbed on silver clusters

“…The importance of the so-called chemical enhancement 1,16,17 has been debated since the early years after the discovery of SERS. The contributions of the electromagnetic coupling and the chemical binding are not clearly separable in the experiments, 17 because the first process is essential for SERS detection. This adds to the ambiguity in the interpretation of SERS experiments.…”

Metal−Molecule Schottky Junction Effects in Surface Enhanced Raman Scattering

“…The huge enhancement factor (10 14 −10 15 ) as seen in many single-molecule experiments are normally attributed to the low concentration level of the analyte in SERS experiments and hence, mainly due to the difference in the number of molecules showing scattering in SERS as compared to bulk Raman . However, the contribution from electromagnetic coupling and chemical binding are clearly inseparable in experiments due to the fact that the SERS detection itself depends on the electromagnetic enhancement process . Thus the kind of charge transfer (CT) contribution in those high enhancement experiments (10 14 −10 15 ) is still unknown.…”