A chemically enhanced mechanism of surface-enhanced Raman scattering (SERS) was investigated using a series of metal-charge-transfer (CT) complex systems fabricated by a self-assembly method. The developed Ag/4-mercaptophenols (MPH)/ n -TiO 2 system presented layer number-dependent SERS spectra. By using the electron density values of the Ag 13 /MPH and Ag 13 /MPH/TiO 2 system calculated using the density functional theory (DFT) and by using these values in combination with the results of our previous investigations on the mechanism of the Ag/MPH/TiO 2 system, the absorption threshold of the CT complexes was clearly defined. The degree of CT was selected to study the layer number-dependent SERS spectra. Based on the layer number-dependent SERS data, it has been inferred that the degree of CT represents a resonance phenomenon. In addition, the CT resonance occurs at higher energy in the Ag/MPH/ n -TiO 2 system than in the monolayer TiO 2 system owing to the blue-shift of CT states with the continuous introduction of TiO 2 . Thus, we provide a good example of the use of a CT complex system to investigate the chemical mechanism of SERS.
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Ji et al. (2011) studied this question.