A Density Functional Theoretical Study on the Charge-Transfer Enhancement in Surface-Enhanced Raman Scattering

The chemical enhancement due to ground-state charge transfer (GSCT) and photon-driven charge transfer (PDCT) in surface-enhanced Raman scattering (SERS) has been investigated by density functional theory. Para-substituted thiophenol derivatives adsorbed on silver and gold surfaces are selected as model systems to evaluate the chemical enhancement factor. By changing the functional groups on thiophenol, we are allowed to modulate the chemical interactions between the thiophenol and the metal cluster in both ground state and charge transfer excited state. Both off-resonance and pre-resonance SERS spectra are simulated to calculate the chemical enhancement factors. The GSCT enhancement factor, EF_GSCT , shows a roughly linear relationship to (ω_TP /ω_M-TP )⁴ , where ω_TP denotes the HOMO-LUMO gap of free molecule, and ω_M-TP denotes the energy difference between the HOMO of the molecule and the LUMO of the metal. The PDCT enhancement factor, EF_PDCT , is governed by the energy difference between the incident light energy and the excitation energy to the CT excited state. EF_PDCT first increases and then decreases with the increase of incident light energy.