DFT study on the influence of electric field on surface‐enhanced Raman scattering from pyridine–metal complex

Abstract

The influence of a static external electric field on surface-enhanced Raman scattering is investigated by calculating the Raman spectra and excited state properties of pyridine-Au 20 complex with the density functional theory and time-dependent density functional theory method. The external electric field with orientation parallel (positive) or antiparallel (negative) to the permanent dipole moment is respectively applied on the complex. This field slightly changes the equilibrium geometry and polarizabilities, which results in shifted vibration frequencies and selectively enhanced Raman intensities. The changes of charge transfer (CT) excited states in response to the electric field are visualized by employing the charge difference densities. Further, the energy of charge transfer transition is tuned by electric field to be resonant or not with the incident light, leading to the Raman intensities are enhanced or not enhanced. At the same time, the intensities of vibration modes are sensitive to the orientation of the field. The positive electric field enhances the totally symmetric ring breathing mode (~1009 cm À1 ) but suppresses the trigonal ring breathing mode (~1051 cm À1 ). On the contrary, the mode at 1051 cm À1 is more enhanced than the mode at 1009 cm À1 when the negative electric field is applied on the complex. The Raman spectra could be modulated by tuning the strength and direction of the electric field.

The SERS effect in coordination chemistry

“…The higher polarizability of the Au 20 -enol system, in relation to the free molecule, would also facilitate the occurrence of a non-resonant chemical mechanism (CHEM). However, in spite of the several theoretical attempts to explain the interactions between the ligand and the metallic clusters [188,[206][207][208]226,227], their contribution to the transition polarizability moment remains yet poorly understood.…”

“…However, this mechanism is not yet completely understood, representing a current theoretical [181,201,[206][207][208][209][210][211][212] and experimental challenge [198,199,213]. It requires the understanding of the nature of the complexes formed at the nanoparticle-molecule interface, including their geometry, bond strengths and electronic transitions [214].…”

“…For instance, F. Dufour et al[225] performed a systematic analysis of the adsorption of single ligands with different binding strengths and charge transfer properties on the different conformers of the neutral and trication Au 11 cluster. The influence of the cluster charge, solvent, nature and number of the ligands (thiol, thiolate, thiyl radical, phosphine, chloride, and chlorine) on the cluster electronic structure has been evaluated, with special attention to the structural motifs present in the metallic core of the different conformers.Chen et al[207] investigated a pyridine-Au 20 complex and showed that the HOMO-LUMO gap can change according to the intensity of the applied electromagnetic field, leading to the selective enhancement of some normal modes and shifting the charge transfer transitions. This phenomenon was attributed to the non-resonant chemical mechanism, considering that when the complex is immersed in an electric field the electronic charges are redistributed, changing its polarizability.Schatz, Jensen and Zhao[226] showed by TD-DFT that the coordination of pyridine on an adatom of the Ag 20 metallic cluster is stronger than for the coordination on the cluster surface.…”

Electric Field-Induced Chemical Surface-Enhanced Raman Spectroscopy Enhancement from Aligned Peptide Nanotube–Graphene Oxide Templates for Universal Trace Detection of Biomolecules

“…This resulting in changes of the polarizability and dipole momentum of the probe molecule until the system reaches equilibrium resulting in an increased Raman scattering cross section resulting in an increased SERS signal intensity. [35][36][37] Additionally, FF-PNTs are well known to have both piezo- 22,23 and pyroelectric 19 properties, these effects might be activated by applying the electric field which might induce vibrational and heating of the tubes (Fig. S11 and S12), as discussed above.…”

Visualized method of chemical enhancement mechanism on SERS and TERS

“…Besides, Aikens and Schatz theoretically investigated the absorption and SERS spectra of Py interacting with Au 20 cluster. Herein, we adopt the S‐complex in the work reported by Aikens and Schatz to model Py adsorbed on metal surface to study the electric field effect on SERS. The external electric field is applied along the metal–molecule Au–N bond and perpendicular to the metal surface (Fig.…”