Influence of metal nanoparticles on the light emission properties of rare-earth ions

Dissertation, RWTH Aachen University, 2019; Aachen 1 Online-Ressource (iv, 199 Seiten) : Illustrationen, Diagramme (2019). = Dissertation, RWTH Aachen University, 2019
The optical properties of metal nanoparticles (MNPs) can differ significantly from those of bulk material. Light can excite a collective oscillation of conduction electrons inside an MNP, thus creating a particle plasmon polariton in the MNP. At the resonance frequencies of the particle plasmon polaritons, the MNP absorbs and scatters light strongly, and the local field near the surface of the MNP changes more drastically as compared to other frequencies. When an oscillating electric or magnetic dipole emitter is placed in the vicinity of the nanoparticle, which acts as an optical nanoantenna, the radiative and non-radiative decay rates of the emitter can be modified due to the coupling between the particle plasmon of the MNP and the dipole emitter. The main focus of this thesis work is to study and understand the modification of the excitation and emission rates of the dipole emitters due to the changes in the local fields in the emitter’s surroundings introduced by metal nanoparticles with various shapes and sizes. The particle plasmon polariton properties of a metal nanoparticle can be utilized for the luminescence rate enhancement of the trivalent rare-earth ions, which can be incorporated into Si solar cells for the enhancement of their overall light-to-current conversion efficiency. At the beginning of this thesis work, the spatial consequences of various metal nanoparticles on radiative (γR) and non-radiative (γNR) decay rates of electric dipole (ED) and magnetic dipole (MD) emitters are investigated. For example, in the extreme nearfield region (d ≤10 nm) of a silver nanosphere, γNR of the ED and MD is observed to vary with 1/d3 and 1/d, respectively, where d is the distance between the dipole emitter and the surface of the silver nanosphere. Numerical results presented in this thesis show that the electric dipole transitions are quenched more strongly than the magnetic dipole transitions at extremely short distances from a metal nanoparticle surface. Furthermore, it is learned from the presented simulation results that when the dipole emitters can couple to a metallic nanoparticle with both electric and magnetic resonances, the electric dipole transitions are still quenched more than the magnetic dipole transitions at extremely close distance to the nanoparticle surface. In the next step, the effects of various MNPs on the photoluminescence emission rate of Sm3+ ions are studied by investigating the effects of the metal nanoparticles with various shapes and sizes on both the excitation and emission decay rates of the Sm3+ ion. The presented calculation results using the finite-element method show that it is more efficient to use nanoparticles made of aluminum than a noble metal for the implementation of rare-earth ions like Sm3+ (doped in a photoluminescence layer) in devices such as photovoltaic solar cells. The physical dimensions such as shape and size of the aluminum nanoparticle can be further modified to obtain an even more suiting aluminum nanoparticle for an experimentally feasible sample configuration with Sm3+ ions. Finally, the influence of gold nanoparticles of various shapes and sizes on the upconversion luminescence emission rate of Er3+ ions are presented and discussed. The presented results in this thesis work are the outcome of various numerical methods. The overall upconversion luminescence enhancement due to a large gold nanosphere (diameters = 300 nm) is compared quantitatively with the results from literature for smaller gold nanospheres (diameters: 100 nm, 140nm and 200 nm). A maximum overall enhancement of around 5% in the volume averaged upconversion luminescence is observed for the largest gold nanosphere with diameter 300 nm. There are almost no enhancements in the volume averaged upconversion luminescence observed for the smaller gold nanospheres discussed in this thesis work. In comparison to the maximum of only 5% in the volume averaged upconversion luminescence enhancement due to the large gold nanosphere, a maximum of around 33% is observed due to the gold nanodisk with length 300nm and height 25 nm. From the presented analysis, it is observed that a metal nanoparticle with large scattering efficiency at both the excitation and emission frequencies of the Er3+ion seems to be the most suitable for the enhancement of the upconversion luminescence rate. Furthermore, it is also observed that it is even more important for a metal nanoparticle to have a large scattering efficiency at the absorption frequency than at the emission frequency of the Er3+ ion in order to achieve a large upconversion luminescence rate enhancement.
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