The Visualization Study For The Charge-Transfer Of Chemical Enhancement In SERS

Surface-enhanced Raman scattering (SERS) mainly includes two enhanced mechanisms, namely electromagnetic enhancement mechanism and chemical enhancement mechanism. For the chemical enhancement, its main form is charge-transfer model. Many researchers discussed the charge-transfer model from different viewpoints, trying to find a complete chemical enhancement theory. In this thesis, we adopted three-dimensional real-space analysis by a more intuitive way, from density functional theory (DFT), time-dependent density functional theory (TDDFT) method and charge difference density (CDD) and other quantum chemical calculations and achieved visualization of charge-transfer process for molecule-metal system. The researchers also have used this method, but still lacked application and experiments on different molecules. In this thesis, this method was applied to pyridazine^nd thiourea adsorbed on silver electrode surface. We constructed proper theoretical model to study these systems, and distinguished electromagnetic (EM) and charge-transfer (CT) enhancement mechanism of SERS in different excitation region.In the experiments, we obtained SERS spectra of pyridazine adsorbed on the silver electrode, excited by785nm and532nm laser; ethylene thiourea and thiourea adsorbed on the silver electrode, excited by514.5nm laser, through applying the different potentials of the electrode to adjust Fermi level to charge-transfer resonance level. Experimental results show that, in the SERS of pyridazine adsorbed on the silver electrode excited by532nm laser, the intensity of Raman peaks is reversed when the applied electrode voltage is at-0.6V (vs saturated KCI/AgCI electrode) and in the SERS of ethylene thiourea adsorbed on the silver electrode excited by514.5nm laser, the relative intensity of Raman peaks changed significantly when the applied electrode voltage is at-0.9V (vs saturated KCI/AgCI electrode). These phenomena were consistent with the theoretical prediction. The SERS of thiourea adsorbed on the silver electrode not only was consistent with theoretical simulations, but also appeared in an unexpected "additional" Raman peak attributed to the Darling-Dennison resonances. Darling-Dennison resonance indicates that two degenerate Raman modes interact with each other and their degenerate first-order overtone modes obtain energy and appear in Raman spectra. Our study showed that the ratio of intensity of the Darling-Dennison resonance is up to0.24of its fundamental Raman intensity, when the applied electrode voltage is at-0.4V versus the saturated calomel electrode. This phenomenon was also observed on the Ag island film surface at ambient condition. These observations demonstrated strong evidence for Darling-Dennison resonance band in SERS. The implications of these observations are also discussed.Finally, the thesis also studied the charge transfer of donor-acceptor in the three-component organic solar cells by visualization method and explored this research.