| Under external excitation,charge carriers in metallic nanostructures can undergo col-lective oscillations and generate localized surface plasmons（LSP）.LSP can significantly en-hance the local electric field in the nanocavity as well as the optical response of the molecules confined in the cavity.Such an effect has been widely applied in high-precision spectroscopic methods such as surface/tip-enhanced Raman scattering and surface-enhanced fluorescence.The plasmonic response of metal nanocavity can be affected by many factors,including the dielectric properties of the metal,the morphology of the nanostructures,and the incident wavelength.Thus,it is difficult to fully understand the near-field plasmon response and its impact on the optical properties of molecular systems from experiments.Therefore,it is of great value to analyze the plasmonic properties of the metal nanocavity from a theoreti-cal perspective and explore the physical mechanism of its impact on the optical properties of molecular systems.This thesis is devoted to studying the influence of geometric mor-phology,material properties,and other factors of the metal nanocavity on their plasmonic response,as well as analyzing the influence of two-dimensional dielectric layers on the near-field properties of nanocavity.On this basis,the influence of localized plasmonic field on the fluorescence and Raman-scattering properties of molecular systems confined in the metallic nanocavity is discussed.A multiscale method that combines the finite element method and first-principles calculation is developed for the simulation of high-resolution TERS images of single molecular systems under the influence of a localized plasmonic field.The specific research contents are as follows:Firstly,the generation mechanism of localized plasmonic fields in two commonly used nanostructures with atomic-scale tips,namely nanoparticle-on-mirror（NPo M）structure and tip-substrate（TS）structure,with the same scale was investigated.The electric field en-hancement effect and the degree of electric field localization in the two nanostructures were analyzed,and the influence of the quantum tunneling effect on the localized electric field in the case of small spacing was explored.Through theoretical calculations of the local electric field in the tips,it was found that the NPo M structure can provide a horizontal localized elec-tric field distribution with a full-width at half-maximum of 0.9 nm at a distance of 0.5 nm above the substrate,which is half that of the TS structure under the same conditions.How-ever,its electric field enhancement effect is relatively weak,and it is easier to reduce the fluorescence quantum yield of the molecular system inside the cavity.In contrast,although the TS structure has a relatively low degree of plasmon localization,it can lead to a larger electric field enhancement.Under excitation at a wavelength of 590 nm,the localized electric field enhancement factor can reach 5300.This work clarifies the respective advantages of the NPo M and TS nanostructures,providing a theoretical basis for their subsequent applications.Secondly,the influence of a two-dimensional MoS2dielectric layer on the near-field properties of metal nanostructures composed of silver nanoparticle and silver substrate was systematically investigated.The computational results show that the introduction of a MoS2dielectric layer helps to enhance the electric field enhancement effect in the nanocavity.At the same silver nanoparticle-silver substrate spacing,the presence of a single layer of MoS2can increase the electric field enhancement factor from approximately 5×104to nearly9×104.In addition,based on the calculation of the fixed silver nanoparticle-MoS2top surface spacing,it was found that the increase in the number of MoS2layers can lead to a decrease in the electric field enhancement factor of plasmonic resonances in the nanocavity from 1.8×105to 0.1×105.On this basis,it was found that by changing the position and radius of the nanoparticle,the field enhancement effect caused by plasmonic resonances in the nanocavity can be tuned between 0.5×105and 3.2×105,and the full width at half maximum of the electric field distribution can be tuned between 4.5 nm and 7 nm.This work shows that the presence of a dielectric layer of two-dimensional materials can provide a new approach to the regulation of plasmonic properties in metal nanocavity.Finally,based on Gaussian function fitting technique,the multiscale simulation of the TERS imaging process of single-molecule systems in metal nanocavity was realized by com-bining the finite element method with first principle calculation and local field spectroscopy theory.Based on this method,the theoretical calculation of the TERS image of the single pentacene molecule system in the nanocavity was carried out,and the influence of the lo-cal field distribution on its imaging behavior was analyzed in detail.The calculation results showed that under non-resonant conditions,the TERS image of the molecular system had a good correlation with the vibrational mode.On this basis,by comparing the TERS image obtained using the finite element method to the ideal model field,it was found that the in-homogeneity of the local field in the direction perpendicular to the molecular surface had a significant impact on the imaging process of out-of-plane vibrations.The reason for this phenomenon is that the asymmetry of the local field in the perpendicular direction affects the coupling process between out-of-plane vibrations and electronic transitions,thus signif-icantly changing its image distribution.The Raman imaging corresponding to the in-plane vibrational mode of the molecule mainly depends on the degree of localization of the local field in the horizontal direction,while it is insensitive to the asymmetry of the local field in the perpendicular direction to the molecular surface.This work not only provides a re-liable computational scheme for analyzing the Raman imaging behavior of single-molecule systems under real experimental conditions but also provides an effective tool for further analyzing fundamental physical and chemical processes such as intra-molecular electronic-vibrational coupling. |
