Theoretical Investigation of the Chemical Effect on SERS Enhancement by Multiconfigurational Calculations

• Grant number: 24/17972-0
• Support Opportunities: Scholarships in Brazil - Scientific Initiation
• Start date: December 01, 2024
• End date: November 30, 2025
• Field of knowledge: Physical Sciences and Mathematics - Chemistry - Physical-Chemistry
• Principal Investigator: Rômulo Augusto Ando
• Grantee: Pedro Meireles Freire
• Host Institution: Instituto de Química (IQ). Universidade de São Paulo (USP). São Paulo , SP, Brazil
• Associated research grant: 22/11983-4 - Spectroscopy signal enhancement: nanomaterials, theory, and computer simulation, AP.TEM

Abstract

The enhancement of surface-enhanced Raman scattering (SERS) is generally attributed to contributions from two distinct mechanisms, the electromagnetic (EM) and the chemical effect (CE). The EM contribution, the main factor responsible for the enhancement, is well known and described, since it depends essentially on the plasmonic properties of the nanoparticle. Although the CE contribution is smaller for the absolute enhancement of the Raman signal, it is responsible for the observed spectral profile, especially when there is a significant interaction of the molecular species with the metal surface. In this sense, the CE contribution is more complicated to describe, since it depends on how the electronic and vibrational states of the molecule in question are affected by adsorption on the metal surface. This project aims to theoretically analyze the contribution of CE to the enhancement observed in SERS spectra, particularly to describe the charge transfer between the metal surface and the analyte. For this purpose, computational chemistry methods based on Density Functional Theory (DFT) and multiconfigurational methods (CASSCF, CASPT2 and/or NEVPT2) will be employed. The theoretical study will be based on experimental SERS spectra of simple aromatic compounds, focusing on para-halogenated benzonitriles adsorbed on silver surfaces, in which there is evidence of the formation of radical species. This fundamental research aims to contribute to a better understanding of the chemical effect on SERS spectra and thus obtain a theoretical basis for understanding the distinct spectral profiles of the technique in comparison to conventional Raman spectra.