Theoretical Research On The Surface Enhanced Raman Scattering And Fluorescence Of Several Typical Nanostructures

The huge electromagnetic?EM?field arising from the localized surface plasmon reso-nance?LSPR?of metal nanoparticles can dramatically increase the Raman and fluorescence activity of nearby molecules,which results in plasmon-enhanced phenomenons commonly known as surface-enhanced Raman scattering?SERS?and surface-enhanced fluorescence?SEF?.Thanks to the ultrahigh sensitivity and stability,these two techniques have become valuable tools for many research fileds such as surface science,applied optics,materials science,environmental science and biomedical science.At present,plasmonic nanosctru-cutures including conventional pure metal nanoparticles?NPs?and the emerging core-shell nanoparticles and tip-substrate configuration can all achieved ultrahigh detection limit and spatial resolutions.However,there are still some key problems that need to be solved.Exam-ples of such include the formation mechanisms of SERS and SEF,the rule for the selection of the optimal material and configuration as well as the often observed discrepencies between experimental and theoreical results.In view of the above problems,we apply high precision theoretical analysis in the thesis to study the SERS,SEF and the tip enhanced Raman/flu-orescence?TERS/TEF?effects of molecular systems confined in several typical plasmonic nanosctutures.The obtained results were applied to improve our understanding of these ef-fects which could promot the development and further application of the relevent techniques.The main contents of the thesis are as follows:Firstly,the basic principles,development history and application fields of SERS and SEF effects were briefly summarized.Especailly,the experimental and theoretical research status of the coventional metal nanoparticels dimer,the core-shell nanoparticles and tip-substrate nanosctutures were introduced.Then,the theoretical methods used in the thesis were described in detail.Starting from the classical EM field theory,the commonly used methods for the calculation of the EM enhancement factor in surface enhancement theory were introduced.Meanwhile,the theoretical description of the electronic transitions in free molecular systems were described.The computation method for the transition dipole mo-ment,escpecially the method for the calcualtion of the Franck-Condon terms were discussed in detail.A density-matrix method which could be applied to treat both of the SERS and SEF effects of molecules confined in plasmonic nanosctutures was also introduced.Such a method laid the fundation for the subsequent calculations.Secondly,the density-matrix method was applied to calculate the surface-enhanced res-onant Raman scattering?SERRS?and SEF spectra of a general model molecule confined in metallic dimers consisting of Ag,Au and hybrid AuAg NPs.The EM enhancement to the incident laser field and the enhancement of the metallic dimer on the radiative and non-radiative decay rates of the molecule were both computed with the generalized Mie theory.The nonlocal dielectric response effect,which was mainly caused by creation of electron-hole pairs,was also account for with the d-parameter methods described by Johansson et al.The influence of the nonlocal dielectric response effect,EM enhancement and quantum yield on the SERRS and SEF spectra were investigated in detail.The results show that the nonlocal dielectric response is very sensitive to the gap distance of the NPs dimers,and it undergoes much faster decay with the increase of the separation than the radiative and energy transfer rates.The Raman and fuorescence peaks as simulated with the nonlocal dielectric response are relative weaker than that without the nonlocal effect for smaller NP separations because the extra decay rates of the nonlocal effect could reduce both the population of the excited state and the interband coherence between the ground and excited states.Meanwhile,the nonlocal effect is more prominent on the SEF process than the SERRS process.Moreover,the SERRS mainly related to EM enhancement and the SEF depended on the competition between EM enhancement and quantum yield,both of which could be controlled by tun-ing the radius and separation of the metallic dimers.The optimal configuration of the NPs as predicted were obtained for the three types of structures by the theoretical simulations,respectively.These results could offer valuable information for the design of metallic sub-strates for surface enhanced Raman and fluorescence measurements.Then,the SERS and SEF effect of a general model molecule placed in the middle of silica coated Ag or Au nanoparticle dimers?Ag@SiO₂or Au@SiO₂?were computed for better understanding of the enhancement mechanism.The EM enhancement and fluores-cence quantum yield of the dimers were simulated by using three-dimensional finite element method?3D-FEM?,and then the Raman and fluorescence enhancement factors were ob-tained.By comparing the EM enhancement and quantum yield obtained for dimers with and without SiO₂shell,we show that the SiO₂shell can efficiently transmit the strong LSPR from the metal core by effectively reducing the gap distance,which lead to the better Ra-man and fluorescence enhancement.Meanwhile,the size of the metal core and the thickness of the shell can obviously influence the SERS and SEF effects.Moreover,the calculated maximum Raman and fluorescence enhancement with excitation wavelength of 500 nm can reach up to?10⁹and?10⁴which shows that such core-shelled substrates were capable for ultrasensitive detections.Finally,we theoretically studied the EM enhancement to the Raman and fluorescence signals of a molecule placed in the center of a nanocavity formed by a metallic tip and sub-strate that mimics a tip-enhanced Raman scattering?TERS?setup using 3D-FEM method calculations.The influence of tip size and tip-molecule distance on the EM enhancements to the incident field as well as the radiative and non-radiative decay rates of the molecule were systematically calculated and analyzed.The simulation results show that the maximum EM enhancement to the incident light as provided by the LSPR in the nanocavity can reach?285for the configuration considered in the present work.Meanwhile,it was found that,at the classical limit considered in the present work,decreasing the apex radius or the tip-molecule distance can both reduce the spatial distribution?as characterized by the full width at half maximum?of the local field in a linear fashion.The fluorescence quantum yield becomes lower due to the competition between the radiative and non-radiative decay rates,especial-ly the contribution of the nonlocal dielectric effect.However,it was found that the strong EM enhancement to the excitation is the dominating factor for the tip enhanced fluorescence?TEF?effect and stronger fluorescence enhancement has been found when increasing the apex radius or reducing the tip-molecule distance when excited at 532 nm.