Near-Field Properties Of Plasmon Coupled Individual Nanocavity

With the development of in situ and high spatial resolution near-field enhancement techniques and nanomaterials in recent years,especially the rapid development of surface enhanced Raman scattering(SERS)technology,near-field characterization has brought rapid changes.The accurate investigation of the transverse and longitudinal near-field distributions under small gaps and the enhancement limits under ultra-small gaps at high spatial resolution is still a key direction in the study of near-field properties.For the near-field distribution properties,it is still challenging to make accurate quantitative experimental measurements of the intensity distribution of the near-field generated by plasma excitation at the nanoscale.Due to the difficulties of molecular orientation and uncertainty of molecular position in the hot spot,it is difficult to accurately quantify the real near-field distribution;for the near-field limit properties,achieving higher SERS field strength at smaller nano-gaps has been the goal of the field.However,when the gap distance reaches the angstrom scale,quantum mechanical effects such as nonlocal effects or electron tunneling occur,leading to the phenomenon that the near-field enhancement decreases instead.In response to the above difficulties,the following four studies are carried out in this thesis.(1)Designed and constructed the Au nanoparticle-on-mirror(Au NPoM)system with the self-assembled monolayer of 4-mercaptobenzonitrile(SAM MBN)as a spacer layer.The transverse position-dependent Stark displacements of v(C=C)and v(C≡N)within a single nanocavity were measured by SERS in combination with density functional theory(DFT)simulations to accurately reveal the inhomogeneity of the transverse distribution of the near-field and quantify the transverse near-field intensity to the transverse near-field intensity was quantified to～1.9×109 V/m and compared with the predicted values by the finite element method(FEM),and the results were in high agreement.This work adds to the understanding of the plasma field excited by the surface of nanoparticles and assists other measurement methods to reveal the true nearfield distribution at higher resolution.(2)Designed and constructed the Au NPoM system with intercalated monolayer WS2 as SERS probes and 4 layers MoS2 as reference layers to quantify the SERS intensity of different layers of WS2 to form a unique "nanoscale" with a spatial resolution of～7 (?) to quantitatively probe the longitudinal near-field at high dielectric constants.A series of theoretical derivations,numerical calculations,and spectroscopic measurements were carried out to characterize the longitudinal near-field within a single nanocavity,which was inhomogeneously distributed with large gradients of 8.37 and 5.62.The SERS enhancement factor(EF)on the horizontal component was also analyzed,and the SERS EF showed a large decaying trend in the nanocavity,and the horizontal component distribution of the longitudinal near field was further precisely resolved.This work improves the understanding of the fundamentals of equivalent excimer,and provides guidance for ultra-high spatial resolution Raman imaging and monomolecular reaction manipulation.(3)Designed and constructed the Au NPoM system with graphene as the spacer layer.Combined with layer-dependent scattering spectroscopy experiments and FEM calculations,it was confirmed that the Au NPoM/monolayer graphene system does undergo electron tunneling.The maximum SERS EFs of different layers of graphene as spacers were obtained by AFM-SERS same-area imaging,and it was found that the quantum corrected model(QCM)fitted the experimental data well.When a single layer of graphene was in the gap,the electron tunneling effect greatly reduced the near-field enhancement in the gap.The limit of near-field enhancement within the nanocavity in the limit of quantum effects was probed by the SERS intensity of single-layer graphene.This work provides important guidance for further understanding of quantum mechanical effects in plasma enhancement,such as quantum plasmas and nanogap photodynamics.(4)Designed and constructed the Au NPoM system with h-BN as the spacer layer to explore the experimental and theoretical models of the layer number-dependent scattering spectra and to confirm the shielding effect of monolayer h-BN on electron tunneling effect inside the nanocavity.The SERS EF showed a monotonic increase as the number of layers decreases,and the correlation with the prediction of classical electromagnetic model(CEM)and QCM was further investigated,so that the SERS intensity of monolayer h-BN could be investigated by the SERS intensity of h-BN in a classical framework.The final near-field enhancement limit was probed in a classical framework.This work achieves active control of the electronic tunneling effect and detects the final near-field enhancement limit.This NPoM system is expected to improve the emission intensity of a single photon source,which is crucial for quantum optical devices.