Surface-enhanced Raman Scattering From Nanostructures Studied By Near-field Spectroscopy And Buried Molecule Systems

The surface-enhanced Raman scattering (SERS) effect can prodigiously enhance the Raman scattering intensity of molecules (ions) adsorbed onto the surface of metal nano-structures. Therefore, it is broadly applied in the research fields of the interface sciences, such as electrochemistry, and analytical science. In order to fully develop surface-enhanced Raman spectroscopy and further broaden its application system, a comprehensive understanding of the SERS mechanisms is required.As a special optical effect at the nano-scale range, SERS effect is closely related with nano-science. As the syntheses of the laser, molecule and nano-structure, it involves profound rules for physics and chemistry. Nevertheless, the traditional studies on SERS are mostly focused on the macroscopic views and averaged information of SERS spectroscopy. Thus, the studies on the interaction between the nano-structure and molecules on the solid surface from the SERS signal at nano-scale and the external and idiographic analysis of the hypostasis of the SERS phenomenon are usually ignored. As a consequence, to obtain better understanding for SERS phenomenon and mechanisms, it is very necessary to develop new methods for probing at nano-scale level and new methods for the preparation of nano-structure samples. Furthermore, the new nano-structure systems in different styles should be induced in the investigation process. Based on these expects, this thesis mainly focuses on the following areas to study and obtained the following results:1) Jointing the techniques of cantilever-type scanning near-field optical microscopy (SNOM) and SERS, and the near-field SERS spectroscopy and imaging technique were developed and improved. The combination of SNOM and SERS would have many practical difficulties technically. For example, the near-field excitation signal is extremely weak, the critical environmental requirements of precision and stability for the collection of weak nano-scale spectra. Therefore we have optimized the experimental system and the working status of instrument.2) The near-field SERS spectra and images for the [Ru(Bpy)₃]²⁺ molecule adsorbed on silver nanoparticle substrate were achieved. Based on the results, some specific phenomena for near-field SERS were found. By observing the near-field SERS spectra collected at different location at the nano-scale, the different spectral characteristics which are caused by the local environmental differences of molecules have been found. On the substrate, the highly SERS active sites (hot spots) are much more than SEF hot spots, and the distribution of them is not entirely consistent. This phenomenon could be explained by the influences of several factors including the different enhancement efficiencies existed for SERS and SEF, the quenching effects for fluorescence by the metal substrate and metal-coated tip, and the presence of different numbers of probing molecules in SERS and SEF.3) It is of interest that the near-field and far-field SER spectra demonstrate several distinctively different spectral features. The peak wavenumber in the near-field spectra is not as stable as those of the far-field spectra. This could be mainly due to the different molecular adsorption states within the small but irregular nanoparticle junctions. When excited by 532 nm laser, the far-field SER spectrum represents a typical resonance Raman spectrum whereas the near-field SER spectral behaves like the far-field non-resonance Raman spectrum. We tried to figure out this abnormal phenomenon by the electric field gradient effect in the near-field, and the heterogeneous polarization of output laser from the tip aperture. It can be found that the main bands in near-field SERS spectrum are blue-shifted compared with the far-field SERS bands. This may due to the different excitation methods between near-field and far-field induced slight difference between the energy levels, thus leading to the small displacements. The observation of some extra bands including the IR active bands in the near-field SERS was accounted for also by the electric field gradient effect, and the existence of excited state bpy^- anion.4) The near-field spectroscopic and imaging studies of PATP and R6G molecules adsorbed on silver nanoparticles-coated substrates were preliminarily investigated. The relationship of the near-field SERS or SEF intensities of two molecules with the incident near-field light intensity can be obtained from the experimental results. After comparison of the near-field images which are derived from different vibrational modes, it can be found that the intensity changing trends are the same for these modes. When the probe molecules of R6G are separated from silver nanopartiles with protecting agent, the near-field SEF image of R6G can be obtained. It can be found that the SEF hot spots are distributed at the junction positions between particles where stronger electromagnetic fields were located. In addition, the effects of different scanning conditions on AFM imaging quality were preliminary discussed.5) The buried-SERS of the probe molecules buried beneath the outermost gold film was systematically investigated. The top gold films were roughed by different methods, therefore the BM-SERS (Buried Molecule-SERS) nano-structure system can be considered as a kind of novel SERS system. Firstly, after the thickness of the surface gold layers were changed, the results of the Pb-UPD, potential step method, cyclic voltammetry, molecular replacement, pH changing, temperature in-situ SERS, XPS etc. demonstrated that the probe molecules can be really buried by the metal atoms while vacuum sputtering process. Afterwards, the mechanism of the charge transfer induced with the SERS of the BM-SERS system can be investigated with the electrochemical in-situ SERS, theory calculations etc. methods.