Separation of Electromagnetic and Chemical Contributions to Surface-Enhanced Raman Spectra on Nanoengineered Plasmonic Substrates

TL;DR

This paper investigates how electromagnetic and chemical effects influence surface-enhanced Raman spectra on nanostructured gold, proposing methods to distinguish their individual contributions for better spectral analysis.

Contribution

It introduces a computational approach to separate electromagnetic and chemical effects in SERS spectra, emphasizing the role of electronic structure changes due to chemical binding.

Findings

Chemical binding alters molecular electronic structure significantly.

Electromagnetic enhancement varies with frequency, affecting spectral features.

Chemical effects are primarily electronic rather than orientation-based.

Abstract

Raman signals from molecules adsorbed on a noble metal surface are enhanced by many orders of magnitude due to the plasmon resonances of the substrate. Additionally, the enhanced spectra are modified compared to the spectra of neat molecules: many vibrational frequencies are shifted and relative intensities undergo significant changes upon attachment to the metal. With the goal of devising an effective scheme for separating the electromagnetic and chemical effects, we explore the origin of the Raman spectra modification of benzenethiol adsorbed on nanostructured gold surfaces. The spectral modifications are attributed to the frequency dependence of the electromagnetic enhancement and to the effect of chemical binding. The latter contribution can be reproduced computationally using molecule-metal cluster models. We present evidence that the effect of chemical binding is mostly due to changes in the electronic structure of the molecule rather than to the fixed orientation of molecules relative to the substrate.