【作者】 刘莎莎；

【导师】 韩克利； 陈茂笃；

【作者基本信息】 大连理工大学 ， 物理化学， 2009， 博士

【摘要】 20世纪70年代,人们在电化学拉曼散射实验中发现一种新颖的现象:当分子吸附或靠近某种特定金属表面时,分子的拉曼散射信号被极大地增强。这种现象后来被称为表面增强拉曼光谱散射（SERS）,它可将拉曼信号强度提高10⁶数量级。由于SERS现象使常规拉曼光谱（NRS）的拉曼信号得到显著增强,因此SERS已经成为一种重要的激光光谱技术。SERS光谱可以克服NRS光谱拉曼散射截面很低的缺点,并且成为一种有效的单分子检测工具。20世纪90年代,由于计算机科学和计算化学的发展,拉曼光谱的计算工作已经成为一个十分重要研究领域,对分子拉曼光谱的计算,为其谱峰指认提供了一种强有力的工具。在本论文中,采用量子化学方法研究了三种分子-金属系统的NRS和SERS光谱。基于密度泛函（DFT）和含时密度泛函（TDDFT）的计算结果,理论分析了三种体系的结合性质,吸收光谱,并对分子的拉曼谱峰进行指认。着重讨论了三种体系中表面增强拉曼光谱的化学增强机理,其中包括基态化学增强,电荷转移共振拉曼增强。同时,我们采用一种被称为电荷差异密度（CDDs）的理论方法来描述化学增强机理。这方法可以形象的展示金属团簇和吸附分子间在共振电子跃迁过程中的电荷转移,而分子间的电荷转移正是化学增强机理的直接证据之一。1.1977年,Albrecht等人和Van Duyne等人分别发现当吡啶分子吸附到粗糙银表面时其拉曼信号得到明显的提高,这种现象就是表面增强拉曼散射。我们采用DFT和TDDFT方法对吡啶-银团簇pyridine-Ag_n（n=2-8,20）和pyridine-Ag_4X（X=L（长轴）,S（短轴）,V（顶点））复合物的拉曼光谱进行研究。在NRS散射中,pyridine-Ag_n（n=2-8,20）和pyridine-Ag_4X（X=L,S,V）复合物的拉曼光谱与吡啶分子的拉曼光谱线型基本一致。通过量化计算和CDDs理论方法的分析,我们发现复合物中产生的电荷转移跃迁对预共振拉曼（pre-RRS）光谱的强度有着直接的影响,因此我们采用与纯电荷转移激发态共振的波长作为入射光计算pyridine-Ag_n（n=2-8）和pyridine-Ag_4X（X=L,S,V）复合物的预共振拉曼光谱。与NRS光谱相比pre-RRS光谱中获得的增强因子大约为10⁴到10⁵。拉曼信号的明显增强主要来源于电荷转移共振增强机理。计算结果显示,SERS光谱的强度取决于吸附位、团簇的尺寸以及复合体的构型。2.利用表面增强拉曼散射（SERS）效应,罗丹明6G（R6G）可用于高灵敏度标记检测。在可见光区,R6G阳离子染料有很强的光吸收同时伴随着剧烈的荧光效应。然而,强烈的荧光效应将妨碍R6G分子拉曼光谱的观测。有大量的实验和理论方面的研究揭示了R6G分子的几何结构,电子结构和光学性能。可是大多数前人的工作仅提供了SERS或者SERRS的增强机理的部分解释,而且化学增强机理还十分容易与电磁增强机理（EM）混淆。我们采用CDDs理论方法研究了吸附在银团簇上R6G分子的化学增强,并给出了电荷转移的直接证据。对于R6G分子在入射光为514.5 nm时产生的SERRS中,振动模v151和v154的增强分别来自分子间微弱的电荷转移（从Ag₂团簇到R6G分子）和强烈的分子内电荷转移（类似R6G的共振拉曼光谱）。当入射能量光接近金属-R6G复合物的纯电荷转移激发态1571.4 nm时,R6G的SERRS增强来自纯的分子间电荷转移共振增强。同时,为了与光吸收过程做对比,我们采用跃迁密度和CDDs方法研究了R6G的荧光效应。3.4-氨基苯硫酚（4-aminothiophenol,PATP）分子是一个带有π-共轭苯环的双官能团分子。在苯环的两端分别连接着一个电子供体（-NH₂）和一个电子受体（-SH）。当PATP分子吸附在Au表面时,-SH官能团容易劈裂形成Au-S键,同时-NH₂官能团通过静电力与Ag团簇作用。我们采用DFT和TDDFT方法研究金属-分子复合体和金属-分子-金属三明治结构的拉曼散射。两种复合体（Ag₂-PATP和PATP-Au₂）和两种三明治结构的（Ag₂-PATP-Au₂和Au₂-PATP-Ag₂）的NRS光谱线型是一致的,但是拉曼强度却存在明显不同。这是由于两种三明治结构拥有较大的静极化率,静极化率将直接影响NRS光谱的强度,Ag₂-PATP-Au₂和Au₂-PATP-Ag₂复合物的NRS强度比Ag₂-PATP和PATP-Au₂复合物的强度高约10²倍。我们计算了入射光为1064 nm的预共振拉曼散射（RRS）光谱,该入射光的能量距离银团簇或金团簇的表面等离子共振（SPR）带很远。在这种情况下,Ag₂-PATP-Au₂和Au₂-PATP-Ag₂结构比Ag₂-PATP和PATP-Au₂复合物获得更高的拉曼强度特别是b₂振动模。这种增强来自于PAPT分子和金属团簇之间的电荷转移激发态与入射光的共振,即电荷转移共振增强。通过CDDs理论方法,我们直观的描述了Ag₂-PATP-Au₂和Au₂-PATP-Ag₂结构中的分子间电荷转移现象,即Herberg-Teller效应的直接证据,从而给出了三明治结构pre-RRS光谱的化学增强机理。更多还原

【Abstract】 In 1970s, a novel phenomena had been presented that the Raman scattering could be enormously enhanced by molecular adsorbed on or near the rough surface of noble metal. This phenomenon was known as surface-enhanced Raman scattering （SERS） spectrum which enhanced the Raman intensity by 6 orders of magnitude. The great advancements in SERS spectra made it evolve as a significant laser spectroscopic characterization technique which overcame the defect of low scattering cross section in Normal Raman Scattering （NRS） spectrum and grew to an available tool in single-molecular detection. From the nineties of the 20th century, advance in computer science and computational chemistry made Raman spectrum simulation become a well-established and important research area. And Raman spectrum simulation became a powerful tool for band assignment. In the current thesis, quantum chemistry was preformed to investigate three molecule-metal system Raman spectra and SERS spectra. Theoretical analysis of binding properties, band assignment, adsorption spectra was been carried out based on Density functional theory （DFT） and Time-dependent DFT （TDDFT） methods. We focused on chemical enhancement mechanism of three system, including ground state chemical enhancement and charge transfer resonant enhancement. In addition, a theoretical methodology called Charge Difference Densities （CDDs） is adopted in describing chemical enhancement mechanism. This methodology aims at visualizing charge transfer between metal clusters and target molecule on resonant electronic transition, which is one of the most direct evidences for chemical enhancement mechanism. 1. The Raman intensity of pyridine was strongly enhanced when the molecular adsorbed on rough silver surface which was found by Albrecht et al. and Van Duyne et al. in 1977, respectively. This phenomenon was commonly said to be SERS spectra. We investigated the Raman Scattering spectra of pyridine-Ag_n （n = 2-8, 20） and pyridine-Ag_4X （X = L, S, V） complexes by DFT and TDDFT methods. In normal Raman scattering （NRS） spectra, profiles of pyridine-Ag_n （n = 2-8, 20） and pyridine-Ag_4X （X = L, S, V） complexes were analogical with that of single pyridine. Through quantum chemistry computation and CDDs results, we found the pre-Resonance Raman Scattering （RRS） spectra were strongly dependent on the electronic transition state of new complexes. Wavelengths were nearly resonant with the pure charge transfer excitation states, which were adopted as incident light when simulating the pre-RRS spectra for pyridine-Ag_n （n = 2-8） pyridine-Ag_4X （X = L, S, V） complexes, respectively. We obtained the enhancement factors about 10⁴ to 10⁵ in pre-RRS spectra compared with the corresponding NRS spectra. The obvious increase in Raman intensities mainly resulted from charge transfer resonance Raman enhancement. The calculated results showed that the SERS spectra were strongly dependent on adsorption site, cluster size and the configuration of new complexes.2. Owing to the significant role in surface-enhanced Raman spectroscopy, Rhodamine 6G （R6G） has been used in high sensitive detection. In visible light R6G cationic dye has a strong adsorption accompanying with a severe fluorescence yield. However, the strong fluorescence yield of R6G may prevent observation of the Raman spectrum. Numerous experimental and theoretical investigations have been carried out to reveal the structural, electronic and optical properties of R6G. Those previous works mentioned above have provided some understandings of the enhancement of SERS or SERRS but it is easy to confuse chemical enhancement with EM in SERS or SERRS. The problem of the chemical enhancement of R6G absorbed on silver cluster has been theoretically investigated by CDDs to show direct evidence of charge transfer. For SERRS of R6G excited at 514.5 nm, the enhancements of v151 and v154 are resulted from weak intermolecular （from Ag₂ cluster to R6G） CT and the strong intramolecular charge transfer （similar to that of RRS of R6G）, respectively. The possibility of the SERRS of R6G contributed from pure intermolecular CT is also discussed, when the incident light is close to the new metal-R6G charge transfer excited state at 1571.4 ran. Meanwhile, compared with the absorption process, the fluorescence yield of R6G is investigated by transition densities and CDDs.3. 4-aminothiophenol （PATP） is a typical bifunctional molecule with aπ-conjugated benzene ring linked by an electron-donor （-NH₂） and an electron-accepter （-SH） group in each side. When the PATP molecule absorbed on Au surface, the -SH group easily splits to form Au-S bond and amino group attaches to silver NPs through the electrostatic force. DFT and TDDFT methods have been performed to investigate the Raman Scattering spectra of metal-molecule complex and metal-molecule-metal junction architectures interconnected with PATP molecule. Profiles of calculated NRS spectra for two complexes （Ag₂-PATP and PATP-Au₂） and two junctions （Ag₂-PATP-Au₂ and Au₂-PATP-Ag₂） are similar to each other, but with obviously different Raman intensities. Due to two junctions possess lager static polarizabilities, which directly influence the ground state chemical enhancement in NRS spectra, the calculated normal Raman intensities of them are stronger than those of two complexes by the factor of 10². We calculate pre-Resonance Raman Scattering （RRS） spectra with incident light at 1064 nm, which is far away from the Surface Plasma Resonance （SPR） bands of silver or gold nanoparticles. Ag₂-PATP-Au₂ and Au₂-PATP-Ag₂ junctions obtain higher Raman intensities than those of Ag₂-PATP and PATP-Au₂ complexes especially for b₂ modes. It is mainly caused by charge transfer between metal gap and PAPT molecule which results in the occurrence of charge transfer resonance enhancement. Calculated pre-RRS spectra are strongly dependent on the electronic transition state produced by new structures. CDDs method has been used to visually describe chemical enhancement mechanism of Ag₂-PATP-Au₂ and Au₂-PATP-Ag₂ junctions in pre-RRS spectrum. This methodology aims at visualizing intermolecular CT which is the direct evidence of Herberg-Teller mechanism.更多还原