Optische und strukturelle Eigenschaften von Edelmetallnanopartikeln
Noble-metal nanoparticles exhibit rich optical behavior, such as resonant light scattering and absorption and non-linear signal enhancement. This makes them attractive for a multitude of physical, chemical, and biophysical applications. For instance, recent biomedical experiments demonstrate the suitability of noble-metal nanoparticles for selective photothermal apoptosis by heat transport by laser irradiation. The applications of nanoparticles largely exploit that plasmons, i. e. collective oscillations of the conduction electrons, can be optically excited in these nanoparticles. In optical spectroscopy, these are seen as pronounced resonances. In the first part of this work, model calculations are employed to elucidate how radiation damping in noble-metal nanoparticles, i. e. the transformation of plasmons into photons, depends on particle size, particle shape, and on electromagnetic coupling between individual particles. Exact electrodynamic calculations are carried out for individual spheroidal particles and for pairs of spherical particles. These calculations for spheroidal particles demonstrate for the first time that radiative plasmon decay is determined by both the particle volume and the particle shape. Model calculations for pairs of large spherical particles reveal that the electromagnetic fields radiated by the particles mediate electromagnetic coupling at interparticle distances in the micrometer range. This coupling can lead to immense modulations of the plasmonic linewidth. The question whether this coupling is sufficiently strong to mediate extended, propagating, plasmon modes in nanoparticle arrays is addressed next. Detailed analysis reveals that this is not the case; instead, for the particle spacings regarded here, a non-resonant, purely diffractive coupling is observed, which is identified by steplike signatures in reflection spectra of the particle arrays.In the second part of this work, structural and optical properties of noble-metal nanoparticles are examined. It is shown that laser-irradiation of silver nanoparticles embedded in titania induces structural and optical changes in the nanoparticles. Consequently, the sample color changes to that of the illuminating light. Optical spectroscopy and x-ray diffractrometry show that the amount of metallic, crystalline, silver is reduced in the particles under irradiation. Based on these experimental findings a model of this recently discovered photochromic effect is presented. In another project, the cooling dynamics of laser-excited gold nanoparticles are investigated by solving the heat transfer equation. These theoretical results are compared to time-resolved x-ray scattering experiments. The thermal conductivity of the nanometer-sized interface between the particle and the surrounding water phase is determined by combining theory and experiment. Furthermore, the threshold temperature for the nucleation of nanoscale water vapor bubbles on a picosecond timescale is determined.
info:eu-repo/classification/ddc/530, Physik, Plasmon, Oberflächenplasmonresonanz, Nanopartikel, Photochrome Verbindung, Edelmetallnanopartikel, surface plasmon resonance, nanoparticle, noble-metal nanoparticle, photochromism
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