| Surface-enhanced Raman scattering(SERS)is an unique spectroscopic technique based on plasmonic enhancement effect of Raman signal when the molecule was placed near nanostructures with plasmonic characteristics such as rough metal surface.SERS has many excellent properties such as high sensitivity,spatial resolution,portability and repeatability which made it suitable for applications in molecular imaging,surface science,material chemistry,environmental science and biomedicine.Many theoretical and experimental studies have been carried out to study the SERS properties of different molecules and nano-substrates with different materials and configurations.However,the influence of the quantum tunneling effect on SERS,which is expected to become important when the characteristic size of nanoparticles reach few nanometer or subnanometer level,remains less explored.This thesis is devoted to study such an effect in several typical nanoparticles with subnanometer structure by using theoretical calculations.The main contents of the thesis are as follows:Firstly,the research background,purpose and significance of the works in the thesis were introduced.The history and basic principle of Raman scattering were them described.The shortcomings of traditional Raman spectroscopy and the advantages of SERS technology were pointed out.The generation mechanism of SERS and its applications were also briefly discussed.Then,we presented the theoretical model and calculation method used in this thesis.The working principle of the quantum correction model and its implementation in finite element method simulations was introduced.The density matrix method that was used to generate the molecular SERS spectrum was also presented.Secondly,the SERS properties of a model molecule confined in spherical metallic dimers constructed by two types of commonly used noble metals,Au and Ag,were systematically simulated.Based on finite element method calculations,the local field enhancement in the gap of the metal nanoparticles under the quantum effect was obtained by using the quantum correction model.The SERS spectra of the model molecules under the quantum tunneling effect were then calculated by using the density matrix method.Simulation results show that the local field enhancement and SERS spectrum are very sensitive to the size of the gap between the nanoparticles.As the separation of the nanoparticles decreases,the local field enhancement first increases and then begins to be significantly suppressed.This is because the tunneling effect neutralizes the positive and negative induced charges in the nanocavity.At the same time,the reduced local field enhancement in the range of small gap distances also led to a significant decrease in SERS intensity of the model molecule.In addition,the results also show that both the local field enhancement and the enhanced molecular decay rates has to be considered at the same time to better understand the SERS characteristics of molecules in the subnanometer cavity.Thirdly,we studied the local field enhancement and molecular Raman enhancement of bow-tie Ag NPs dimers by taking into account the quantum tunneling effect.The finite element method simulation with the quantum correction model were also applied here to calculate the local field enhancement of the bow-tie nanoparticles.The simple electromagnetic enhancement mechanism was used to deduce the Raman enhancement factor of molecules placed in the center of the dimer.The results show that the quantum tunneling effect can significantly affect the local field enhancement and molecular Raman enhancement of the bow-tie silver nanoparticles when the separation is less than0.38 nm,and the local field enhancement and molecular Raman enhancement of the bow-tie silver nanoparticles are consistent with the classical electromagnetic calculation results in the large separation range.The results show that the effect of quantum tunneling is significantly reduced in the short distance range. |
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