Graduate School of Science, Technology, and Innovation, Kobe University, Kobe 657-8501, Japan.
Department of Chemistry, Faculty of Science, Hokkaido University, Sapporo 060-0810, Japan.
J Chem Phys. 2019 Sep 7;151(9):094102. doi: 10.1063/1.5111944.
The cause for the huge enhancement factors of surface-enhanced Raman spectroscopy (SERS) by the addition of small silver nanoclusters is theoretically investigated by focusing on the difference between resonance Raman activity and surface plasmon effects. First, the resonance and off-resonance Raman spectra are calculated using the incident light wavenumbers of the low-lying charge transfer excitations for the surface (S) and vertex (V) complexes of the pyridine molecule attaching to three small silver nanoclusters: Ag, Ag, and Ag. As a result, it is found that the incident radiation dramatically increases the resonance Raman activities with the enhancement factors up to 10. This indicates that the resonance Raman effects are dominant in the enhancement factors of SERS, at least when to use small silver clusters. It is also found that the resonance Raman spectra significantly depend on the adsorption sites given in S or V complexes, and on the inclusion or exclusion of the long-range correction for density functional theory, irrespective of the size of the silver clusters. The electromagnetic field enhancement effects called "surface plasmon effects" are also examined for the Ag cluster to confirm this conclusion. Consequently, the enhancement in the electric field is roughly evaluated as less than one for the static polarizability of this small cluster. It is, therefore, concluded that the resonance Raman activity effect is dominant in the huge SERS enhancement factors for, at least, small silver nanoclusters.
通过聚焦于共振拉曼活性与表面等离子体效应之间的差异，从理论上研究了在添加少量银纳米簇后表面增强拉曼光谱（SERS）的巨大增强因子的原因。首先，使用吸附在三个小银纳米簇：Ag、Ag 和 Ag 上的吡啶分子的表面（S）和顶点（V）复合物的低能电荷转移激发的入射光波数计算共振和非共振拉曼光谱。结果发现，入射辐射极大地增加了共振拉曼活性，增强因子高达 10。这表明在 SERS 的增强因子中，共振拉曼效应占主导地位，至少在使用小银簇时是这样。还发现，共振拉曼光谱显著依赖于 S 或 V 复合物中的吸附位，以及密度泛函理论的远程校正的包含或排除，而与银簇的大小无关。还检查了所谓的“表面等离子体效应”的电磁场增强效应，以证实这一结论。因此，对于这个小簇的静态极化率，电场的增强大致评估为小于一。因此，结论是，对于至少小的银纳米簇，共振拉曼活性效应是巨大 SERS 增强因子的主导因素。
