Theoretical Study On Contribution Of Chemical Enhancement To Surface-enhanced Raman Scattering Spectra

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 aÏ€-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.