Kimika Teorikoa Seminar: Addressing chemical effects in Surface-Enhanced Raman Scattering

Molecular Raman spectroscopy is a spectroscopy technique that uses inelastic light scattering (Raman scattering) to identify molecular vibrational fingerprints, and thus to shed light on chemical structures and their environment. Compared to other vibrational spectroscopic techniques, Raman signals are weak for typical organic molecules, and the amplification of the Raman signal is required to improve the sensitivity and specificity of this spectroscopy technique. The Raman signal can be amplified by locating a molecule near a metallic particle. This is the basic principle of Surface-Enhanced Raman Scattering (SERS) [1]. The amplification of the Raman signal in SERS is typically ascribed to two mechanisms: (i) the chemical (CHEM) mechanism, which results from the chemical interaction between the metallic surface and the molecule, and (ii) the electromagnetic (EM) mechanism due to the extremely localised electromagnetic fields induced near the metallic particle by collective oscillations of the electron density (plasmon resonances). To compute the EM contribution to the total SERS enhancement, the classical Maxwell’s equations are solved for the plasmonic response of the metallic particle. The CHEM contribution is typically calculated within the Density Functional Theory (DFT) framework and ignores the role of the EM mechanism, although the latter can also influence the obtained results [2]. In this contribution, we present a theoretical procedure to evaluate the contribution of the CHEM and EM mechanisms to the total SERS enhancement [3]. This approach uses DFT and Time-Dependent DFT to isolate bare chemical effects from EM contributions. We apply this approach to study the chemical enhancement of biphenyl-4,4’-dithiol (BPDT) embedded between two gold clusters. We show that for small cluster sizes, the total SERS enhancement is mainly driven by the CHEM mechanism, but the EM contribution grows with cluster size. This approach can be applied to rationalize SERS enhancements in challenging SERS configurations that involve larger metallic nanoparticles.

[1] E. C. Le Ru, and P. G. Etchegoin., Principles of Surface- Enhanced Raman Spectroscopy and related plasmonic effect (Elsevier Science and Technology, United Kingdom, 2011).
[2]. S. M. Morton, D. W. Silverstein, et al. Chem. Rev. 2011, 111, 3692.
[3] R. A. Boto, R. Esteban, et al. J. Phys. Chem. C. 2024,128, 18293.