理论研究Ag/MPH/TiO2体系的SERS光谱化学增强机理

摘要

A chemically enhanced mechanism of surface-enhanced Raman scattering (SERS)spectroscopy was investigated using density functional theory (DFT).We studied the Raman spectra of the Ag13/MPH and Ag13/MPH/TiO2 systems under 514.5 nm excitation.We found that the intensities of the non-totally symmetric vibration modes were selectively enhanced after TiO2 was introduced into the Ag13/MPH system.By analyzing the ground state and excited states of the charge transfer (CT) complex we found that the system gave a photoinduced CT state from Ag to the MPH-TiO2 complex when the excitation wavelength exceeded the optical absorption threshold (635 nm) of the MPH-TiO2 complex.The selective enhancement of the b2 modes in the SERS spectra probably originates from the Herzberg-Teller mechanism through the coupling of the corresponding modes with the CT transition.Our theoretical results support the experimental results and also define the adsorption threshold of the CT complex clearly while providing an intelligible physical explanation for the laser wavelength-dependent SERS phenomenon.%采用密度泛函理论研究了Ag/对巯基苯酚(MPH)TiO2体系的表面增强拉曼散射(SERS)光谱化学增强机理.分别研究了Ag13/MPH和Ag13/MPH/TiO2复合物在514.5 nm激发波长下的拉曼光谱,发现由于TiO2的引入,发生了非完全对称振动模式峰选择性增强的现象.通过对电荷转移复合物基态和激发态的指认,发现当激发波长大于MPH-TiO2电荷转移复合物的光学吸收阈值(635 nm)时,该体系内将发生从Ag到MPH-TiO2部分的光诱导电荷转移现象.SERS光谱中b2模式的选择性增强,来源于相应振动模式与电荷转移跃迁的耦合(Herzberg-Teller机制).我们的理论结果不仅支持了实验现象,并且明确界定了电荷转移复合物,对于该体系存在的光依赖SERS现象提供了一个清晰的理论阐述.