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Promoteur de la thèse: Monsieur Michel Voué

Résumé de la thèse:

Plasmonic nanocomposites based on noble metals have numerous applications such as optical sensors, non-linear optical activity-based devices, spectrally selective coatings and sensors for bio-medical diagnostics. This class of new attractive materials, composed of a dielectric matrix and embedded metal nanoparticles (NPs), is a great topic of research due to a peculiar aspect of their optical properties: the Localized Surface Plasmon Resonance (LSPR). Well-known in its effect since e.g. the optical aspect of the Lycurgus cup (4th CE) or of the mosaics of Saint Sabina (5th CE), this LSPR appears when free electrons of the metal are excited by the electromagnetic field associated to the propagation of light. This collective oscillation depends on the geometry of NPs, on the properties of their environment and also their distribution inside the dielectric matrix.
The most popular way to synthesize these plasmonic nanocomposites is the « bottom up » approach, which has two separate steps: metal NPs are synthesized as colloidal solutions and dispersed in a dielectric matrix. In this thesis, we used a different approach based on a « one-pot synthesis » scheme to synthesize the plasmonic materials: an aqueous noble metal salt solution (AgNO3 for silver or HAuCl4 for gold) is directly mixed with a polymer solution, the poly-(vinyl) alcohol (PVA). The polymer directly plays the role of the reducing agent and acts as a stabilizer of the NPs, preventing them to aggregate. This synthesis approach has the advantages to be simpler, faster and lead to a concentration of NPs in the nanocomposite higher than other synthesis schemes. However, in the case of ‘soft’ matrices such as polymers, the reproducibility of the NPs distributions in size, shape … remains difficult to achieve and slight variations in the experimental conditions leads to different results evidenced by modifications of the plasmon resonance band parameters. Moreover, the detailed mechanism linking the optical properties of the nanocomposite to their structural parameters is today not fully understood although still being the subject of an increasing number of publications.
In this context, we have prepared nanocomposites embedding silver or gold nanoparticles. We first studied the impact of the thickness of the film on the localized surface plasmon resonance parameters. We have shown that this experimental parameter has a large impact on the size of the nanoparticles in situ synthesized in the polymer matrix: the red shift of the plasmon band lead us to conclude that thin nanocomposite films induce a larger size of the produced nanoparticles than in the thicker ones. The variation in the spatial distribution in thin (2D) and thick films (3D) is a possible explanation for the different behavior on the optical properties.
In a second time, we presented an experimental study of the optical properties of gold-doped nanocomposites using single wavelength imaging ellipsometry. We have found that, for a thin polymer film (thickness less than 500 nm) and at low gold-doping level (volume fraction of gold lower than 0.2%), the growth of the gold NPs inhomogeneously occurred within the film. Superimposition of ellipsometric measurements recorded during the annealing and theoretical curves allowed us to decompose the global growth dynamics in two parts: the first one, at the early stage of annealing, corresponding to a decrease of film thickness while the second one is attributed to an increase of the refractive index.
Finally, at this low doping level, the optical constant determined by the conventional spectroscopic ellipsometry spectra is only relevant for the polymer matrix. We have shown the advantage of using imaging spectroscopic ellipsometry by choosing areas on the image with a different intensity contrast. The ellipsometric spectra of each region of interest were different, especially between 400 nm and 600 nm, the spectral region corresponding to the localized surface plasmon resonance of gold nanoparticles (AuNPs). Using a theoretical model based on Effective Medium Approximations (EMAs), we have shown that each zone presents a point-to-point variation of the volume fraction of gold in the polymer. From the ellipsometric maps near the resonance wavelength, we were able to calculate two maps decorrelated from each other: one of the local thickness of the film and a second one of the local volume fraction of AuNPs. Beyond unravelling the role played by the film thickness in the growth of the noble metal nanoparticles, this last result highlights the strength of the spectroscopic imaging ellipsometry as a local optical analysis technique.