Theoretical study on contribution of charge transfer effect to surface-enhanced Raman scattering spectra of pyridine adsorbed on Agn (n = 2–8) clusters

We investigate surface-enhanced Raman scattering (SERS) spectra of pyridine–Agn (n = 2–8) complexes by density functional theory (DFT) and time-dependent DFT (TDDFT) methods. In simulated normal Raman scattering (NRS) spectra, profiles of pyridine–Agn (n = 2–8) complexes are analogical with that of isolated pyridine. Nevertheless, calculated pre-SERS spectra are strongly dependent on electronic transition states of new complexes. Wavelengths at 335 nm, 394.8 nm, 316.9 nm and 342.6 nm, which are nearly resonant with pure charge transfer excitation states, are adopted as incident light when simulating pre-SERS spectra for pyridine–Agn (n = 2–8) complexes, respectively. We obtain enhancement factors from 103 to 105 in pre-SERS spectra compared with corresponding NRS spectra. The obvious increase in Raman intensities mainly result from charge transfer resonance Raman enhancement. A charge difference densities (CDDs) methodology is adopted in describing chemical enhancement mechanism. This methodology aims at visualizing charge transfer from Agn (n = 2–8) clusters to pyridine on resonant electronic transition, which is one of the most direct evidences for chemical enhancement mechanism.

Graphical abstractWe investigate the surface-enhanced Raman scattering spectra of pyridine–Agn (n = 2–8) complexes by density functional theory and time-dependent DFT methods. Wavelength at 335 nm, 394.8 nm, 316.6 nm and 346.2 nm, which are nearly resonant with the pure charge transfer excitation states, are adopted as incident light when simulating the pre-SERS spectra for pyridine–Agn (n = 2–8) complexes, respectively. We obtain the enhancement factors about 103 to 105 in pre-SERS spectra. The obvious increase in Raman intensities mainly result from charge transfer resonance Raman enhancement.Figure optionsDownload full-size imageDownload as PowerPoint slideHighlights► The enhancement factors are from 103 to 105 in pre-SERS spectra. ► The obvious enhancements result from charge transfer resonance Raman enhancement. ► Charge difference densities methodology is used in describing chemical enhancement mechanism.