Sun Mengtao, Xu Hongxing
Beijing National Laboratory for Condensed Matter Physics and Laboratory of Soft Matter Physics, Institute of Physics, Chinese Academy of Sciences, P. O. Box 603-146, Beijing, 100190, PR China.
Chemphyschem. 2009 Feb 2;10(2):392-9. doi: 10.1002/cphc.200800596.
Direct visualization of photoinduced tunneling charge transfer (TCT) in an Au(5)/para-aminothiophenol (PATP)/Ag(6) junction in which Au and Ag clusters form the first and second layer, respectively, is provided by the charge difference density (see picture; green and red stand for holes and electrons, respectively). We theoretically investigate the mechanism of chemical enhancement of surface-enhanced resonance Raman scattering (SERRS) of para-aminothiophenol (PATP)/metal complexes and metal/PATP/metal junctions. The method of charge difference density is used to visualize intracluster excitation and charge transfer (CT) between PATP and metal during the process of resonant electronic transitions. It is found that the selective enhancement of the b(2) mode in SERRS spectra result not only from Albrecht's A term (the Frank-Condon term), but also from the Herzberg-Teller term (Albrecht's B mechanism) via resonant CT. For the metal/PATP/metal junctions, the calculated results reveal that the Raman spectrum is of SERRS nature and the nontotally symmetric b(2) mode is strongly enhanced at the incident wavelength of 1064 nm when Au and Ag nanoparticles are the first and second layer, respectively, and the dominant enhancement mechanism is the Herzberg-Teller term in chemical enhancement via tunneling charge transfer (intervalence electron transfer from the Ag cluster to the Au cluster). When the first and second layers were inverted (i.e. the Ag and Au nanoparticles are the first and second layers, respectively), the Raman spectrum at an incident wavelength of 1064 nm is due to normal Raman scattering, and the nontotally symmetric b(2) mode is not strongly enhanced. Our theoretical results not only support the experimental findings, but also provide a clear physical interpretation.
通过电荷差密度（见图；绿色和红色分别代表空穴和电子）实现了对金（5）/对氨基硫酚（PATP）/银（6）结中光诱导隧穿电荷转移（TCT）的直接可视化，其中金和银团簇分别构成第一层和第二层。我们从理论上研究了对氨基硫酚（PATP）/金属配合物和金属/PATP/金属结的表面增强共振拉曼散射（SERRS）的化学增强机制。电荷差密度方法用于可视化共振电子跃迁过程中PATP与金属之间的团簇内激发和电荷转移（CT）。研究发现，SERRS光谱中b(2)模式的选择性增强不仅源于阿尔布雷希特A项（弗兰克 - 康登项），还源于通过共振CT的赫兹伯格 - 泰勒项（阿尔布雷希特B机制）。对于金属/PATP/金属结，计算结果表明拉曼光谱具有SERRS性质，当金和银纳米颗粒分别为第一层和第二层时，在1064 nm入射波长下非完全对称的b(2)模式会强烈增强，并且主要增强机制是通过隧穿电荷转移（从银团簇到金团簇的价间电子转移）的化学增强中的赫兹伯格 - 泰勒项。当第一层和第二层颠倒时（即银和金纳米颗粒分别为第一层和第二层），1064 nm入射波长下的拉曼光谱是由于正常拉曼散射，非完全对称的b(2)模式没有强烈增强。我们的理论结果不仅支持了实验发现，还提供了清晰的物理解释。
