Surface Plasmon Effect And Optical Properties Of Gold Nanostructures

A comprehensive study on the surface plasmons（SPs）and the optical properties of gold（Au）nanostructure is important,which can not only broaden the understanding of the interaction between light and matter,but also contribute to their applications for enhancing solar cell efficiency,fluorescence enhancement,three-dimensional display and so on.The SPs arise from the collective oscillation of the free electrons driven by the electromagnetic field of the incident light.Two important characters of the SPs are the near-field intensity enhancement and the localization,which can manipulate the electromagnetic waves at nanoscale.This thesis focuses on the preparation and optical study of chiral Au nanostructures and Au-fluorophore composite structures.In this wok,the chiral effect and fluorescent effect of the Au nanostructure are systematically studied.Optical properties of chiral Au nanostructures are calculated by simulation and the effect of the localized surface plasmon（LSP）on the fluorescence properties is studied using simple composite structures.The main research contents of this thesis are listed as follows:1.The optical properties including the circular dichroism（CD）of the double“L”structure and the double-spiral structure were carried out by the finite element method with the commercial software COMSOL.Different electric fields and extinction coefficients of the different structures were obtained under the left-handed circular polarized（LCP）light and the right-handed circular polarized（RCP）light.The chiral effect could be analyzed by using the plasmonic hybridization.The single“L”structure and the single-spiral structure didn’t have chiral effects.The double“L”structure had a distinct chiral effect of the distance less than 300 nm（the distance of the double-spiral structure was less than 400 nm）.This was because the local and near-field enhanced SPs of the Au nanostructures.The maximum CD values of the double“L”structure were almost at the same wavelength for the distance 175–300nm,while the wavelength increased with the decrease of the distance from 175 to 100nm.The chiral effect of the double“L”structure for the angle 75°was similar with the structure of the angle 90°,while it was opposite with the structure of the angle105°.In the range above 900 nm,the two kinds of double-spiral structures,the structure 1 with an angle difference of 0°and the structure 2 with an angle difference of 180°,were like linear polarizers.The double-spiral structure had an evidently chiral effect of the long wavelength,in which the structure 1 had 4 CD peaks while the structure 2 only had 3 CD peaks.For the double-spiral structure,the CD spectra had obvious blue shift with the increase of the arm width.2.Au nanoclusters（NCs）with an average size of 1.4 nm were prepared.The temperature-dependent photoluminescence（PL）spectra of the LA-PEG-COOH protected Au NCs in the temperature range of 60 K to 290 K were investigated.At a temperature of 290 K,the main emission peak energy of the Au NCs was 1.478 eV,and as the temperature decreased,the main emission peak was significantly blue-shifted.The variation of the band gap with temperature and the phonon energy of 47 meV could be analyzed using the O’Donnell-Chen model including electron-phonon interaction.The results of absorption spectra and CD spectra could explain the electron-phonon interaction and the energy level distribution.3.We provided a simple chemical method to prepare the composite structure of the InP/ZnS quantum dots（QDs）and the Au nanorods.The effects of the LSP on the fluorescence properties were studied.At temperatures below 130 K,the emissions of the bright-dark double exciton state,the upper-exciton state and the defect state were observed in both samples.The upper-exciton state was observed in static fluorescence experiment for the first time.In the composite structure,the emission intensities of the defect state confirmed that the energy transfer was mainly from the LSP resonance state.In addition,when the temperature rose from 11 K to 300 K,the main emission peak energy of the pure QDs was reduced by 26 meV,and the main emission peak energy of the composite structure was reduced by 54 meV.That was to say,the coupling effect between the InP/ZnS QDs and the LSP made the main emission peak energy of the composite structure vary with temperature by 28 meV larger than that of the pure QDs.4.A mixture sample of the Yb³⁺,Tb³⁺,Eu³⁺triply-doped YVO₄ nanomaterials and the Au nanoparticles（NPs）was synthesized.The effects of the LSP on the fluorescence properties of the rare earth materials were studied.The light emitting process of the sample involved a two-photon process.The LSP could increase the number of photons absorbed during the emitting process.The results showed that a series of up-conversion（UC）emission peaks of the Eu³⁺ion and the Tb³⁺ion were observed in samples.With the coupling,the emission peak of the Eu³⁺ion at about2.095 eV of the ⁵D₀→⁷F₁ transition decreased significantly,while the emission peak of the Tb³⁺ion at about 2.113 eV of the ⁵D₄→⁷F₄ transition increased.One possible reason was that the LSP had diverse effects on the magnetic dipole transition and the electric dipole transition.