Computational Model for Electrochemical Surface-Enhanced Raman Scattering: Key Role of the Surface Charges and Synergy between Electromagnetic and Charge-Transfer Enhancement Mechanisms

Abstract

We present a computational model for electrochemical surface-enhanced Raman scattering (EC-SERS). The surface excess of charge induced by the electrode potential (V el) was introduced by applying an external electric field to a set of clusters [Ag n ] q with (n, q) of (19, ±1) or (20, 0) on which a molecule adsorbs. Using DFT/TD-DFT calculations, these metal–molecule complexes were classified by the adsorbate partial charge, and the main V el-dependent properties were simultaneously studied with the aid of vibronic resonance Raman computations, namely, changes on the vibrational wavenumbers, relative intensities, and enhancement factors (EFs) for all SERS mechanisms: chemical or nonresonant, and resonance Raman with bright states of the adsorbate, charge-transfer (CT) states, and plasmon-like excitations on the metal cluster. We selected two molecules to test our model, pyridine, for which V el has a remarkable effect, and 9,10-bis­((E)-2-(pyridin-4-yl)­vinyl)­anthracene, which is almost insensitive to the applied bias. The results nicely reproduced most of the experimental observations, while the limitations of our approach were critically evaluated. We detected that accounting explicitly for the surface charges is key for EC-SERS models and that the highest calculated EFs, up to 107 to 108, are obtained by interstate coupling of bright local excitations of the metal cluster and CT states. These results highlight the importance of nonadiabatic effects in SERS and the capabilities of EC-SERS as a technique with potential to study excited-state coupling by tuning the CT and plasmon-like states by manipulating V el.

Linear Vibronic Coupling Approach for Surface-Enhanced Raman Scattering: Quantifying the Charge-Transfer Enhancement Mechanism

“…On the other hand, as shown in Figure S56, the calculated intensity of the normal Raman spectrum of isolated Py, expressed as the differential cross section, is in the order of 10 −31 cm 2 /sr, in agreement with previous values reported in the literature. 8,21,41 Because the intensities shown in Figure 5b are in the order of 10 −26 to 10 −25 cm 2 /sr, the enhancement factor is of about 10 S48. The maximum intensity is found for the field value triggering the fastest and largest population transfer (E = 10 −4 a.u.)…”

“…This model has been previously adopted in the literature and provides a good balance between adclusters and cost for the necessary electronic structure computations. 21,35,41 The nitrogen of Py is bonded to a single silver atom of the respective clusters, and we use the label [Ag N TPy]+E ⃗ to specify the system under investigation, where N denotes the number of atoms and the shape of the metallic moiety, T the conformation of the metal−molecule surface complex, and E ⃗ the external electric field along the Z Cartesian axis of module E (Figure 1); therefore, E ⃗ can be either E•ẑor −E•ẑwhen the field is applied parallel or antiparallel to the Z axis. This axis is aligned with the N−Ag bond in all cases, and E ⃗ mimics the effects of applied electrode potential V el .…”

“…Such possibilities have been proposed in the literature to explain the intensification of mode 9a. 21,62 Resonance with either PL states of charged adsorption sites or Rydberg CT states is also associated to a small reinforcement of mode 8a. 21,62 Because such states are not included in the neutral, i.e., not charged, simple models we considered here, it can be expected that the behavior at more negative potentials is not fully reproduced in our calculations.…”

Effects of Au6 and Au20 Adsorption Sites of Cyromazine–Au Complexes by Raman Spectroscopy and Density Functional Theory

“…Among them, the charge-transfer (CT) enhancement is driven by photons from the metal surface near the Fermi level to the lowest unoccupied molecular orbital (LUMO) under chemisorption. From the highest occupied molecular orbital (HOMO) of the molecule to the LUMO level on the metal surface, the Raman polarization of the adsorbent increases, resulting in the observed amplified SERS signal. , …”

“…From the highest occupied molecular orbital (HOMO) of the molecule to the LUMO level on the metal surface, the Raman polarization of the adsorbent increases, resulting in the observed amplified SERS signal. 9,10 Using the Raman and SERS techniques to study cyromazine degradation is usually accompanied by solving the problem with density functional theory (DFT), which is an approximate quantum mechanical method based on the Kohn−Sham electron density response described by specific DFT functional forms used to determine the electronic structure of atoms, molecules, and materials. The DFT method is known to provide a good compromise between computational cost and accuracy compared with the more time-consuming post-Hartree−Fock ab initio method, in particular for vibrational spectra of macromolecules, where analytic gradients are available.…”