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Etude par diffusion Raman de nanoparticules métalliques en matrice diélectrique amorphe

Hervé Portales
Abstract : The aim of this thesis is notably focused on the vibrational dynamics of metal nanoparticles embedded in an insulating matrix through a Raman scattering study. The coupling between the particle vibrations and the surface plasmon excitation results in an enhancement of the Raman scattering and so, leads to a very intense signal in despite of the low metal volume fraction in the studied samples. Various methods have been used to elaborate these samples resulting in different structural characteristics: matrix embedded nanoparticles are produced either by laser vaporization and deposition on a substrate by the LECBD technique, or by a rf-cosputtering technique, or by a fusion and heat treatment technique. Low-frequency Raman spectra exhibit a band corresponding to the scattering by the quadrupolar vibrations. For silver nanoparticles, the variation of the vibrational frequency with the particle size is in agreement with the one calculated in the frame of the elastic sphere model. When the excitation wavelength is varied, a shift of the Raman band is observed, due to a shape selection of the particles in the Raman process. Measurements of the depolarization ratio in two different configurations of the incident and scattered light polarizations confirm this hypothesis. Comparing as-deposited and annealed samples, a spatial coherence effect on the Raman scattering has been pointed out experimentaly and interpreted theoretically. Several new bands observed in the low-frequency Raman spectra of thermally treated samples have been ascribed to the Raman scattering by the nanoparticle radial modes, in very good agreement with results of time-resolved experiments. Various kinds of bimetallic systems have also been studied, consisting either of an alloy (gold-silver nanoparticles), or of a core-shell structure when the metals are segregated, like it seems to be the case for nickel-silver and cobalt-silver nanoparticles. High-frequency Raman scattering experiments have been performed on annealed samples of silver nanoparticles embedded in a silica matrix. A very broad band, with a maximum around 1000 1/cm, is observed and shifts when the particle size or the excitation wavelength are varied, similarly to the low-frequency band. The origin of this scattering is attributed to excitations of electron-hole pairs.
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Submitted on : Tuesday, December 17, 2002 - 8:34:03 PM
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Hervé Portales. Etude par diffusion Raman de nanoparticules métalliques en matrice diélectrique amorphe. Physique [physics]. Université Claude Bernard - Lyon I, 2001. Français. ⟨tel-00002169⟩

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