Chemical Enhancement Effects in SERS Spectra: A Quantum Chemical Study of Pyridine Interacting with Copper, Silver, Gold and Platinum Metals

Abstract

Using density functional theory, we have studied the interactions between pyridine (Py) and coinage metals (silver, copper, and gold) as well as the transition metal platinum. We present here a detailed analysis of the influence of chemical enhancement effects on the SERS signals. We analyze the differential Raman scattering cross sections of the a1 vibrational modes related to Py. The results show that the relative Raman intensities of SERS spectra depend strongly on the binding interaction between Py and the SERS active centers, the electronic property of metal materials, and the incident wavelengths. When the bonding between Py and a SERS site is very weak, analogous to physical adsorption, the Raman spectra of the adsorbed Py are similar to that of free Py. For Py interacting strongly with copper, gold, and platinum clusters, we find that the Raman intensities of the v1, v6a, v9a, and v8a modes of Py are enhanced, whereas the intensity of the v12 mode decreases. To check the enhancement effect of the charge-transfer mechanism, we calculate the preresonance Raman spectra. For Py interacting with silver, copper, and gold clusters, the low-lying charge transfer states are formed from the valence shell s orbital of metal to the unoccupied π*-type orbitals (b1 and a2) of the Py ring; whereas for Py adsorbed on platinum clusters, the low-lying charge transfer states are formed due to the transitions from some d orbitals of the metal clusters to the two unoccupied π*-type orbitals of Py. The results show that the Raman spectral property depends strongly on the property of these excited states and the electronic structures of the metal materials.