Optical properties of metallic nanoparticles and metallic nanocomposite materials

摘要

The optical behaviour of both gold and silver nanoparticles has been studied in both experimental and theoretical aspects. In the theoretical part of our thesis, to contribute to a better understanding of particles of various shapes and configurations, the versatile Discrete Dipole Approximation (DDA) has been employed to simulate in depth the absorption spectra of single isolated oriented nanoparticles of different symmetry (nanocube, nanobar and nanoellipsoid). The effect of the plasmonic coupling and the size of spherical particles assemblies on their optical response have also been addressed. It was shown that the plasmonic coupling in the interacting particles in close proximity configuration disturbs the homogenous distribution of surface charges and results in splitting the plasmonic band into two bands. The excitations of two different bands (longitudinal and transverse bands) have been also observed in the absorption spectra of many fold symmetry particles. The diversity of the polarization factors along different symmetry axis was established as the main key for observing several bands. Thus the importance of particle shape and the different interesting possibilities offered by this single factor has been well demonstrated in the DDA calculations performed while our treatment of ensembles of nanospheres showed in detail the effect of interacting particles on the overall optical properties of actual samples. ududIn the experimental part of this thesis, a first part is devoted to the study of the influence of dielectric host material on the optical properties of gold nanoparticles. For this purpose, gold, -poly(methyl methacrylate) (PMMA) and -gelatin nanocomposite materials have been prepared by an in-situ method. Two reduction methods (photochemical and chemical) were used to reduce the gold salt in the presence of the polymer matrix. Firstly, annealed and non annealed samples were prepared by different photochemical methods (UV-, thermal-, and MW-irradiation). Gold-poly(methyl methacrylate) nanocomposites were prepared by irradiating spin-coated films containing the polymer and the gold precursor dissolved in acetone. The reduction of gold ions resulted in the formation of gold that nucleated and grew within the polymer films. It was shown that, depending on the energy source, gold nanoparticles with different shapes could be formed. The nanocomposites prepared through the photochemical methods, showed a low sensitivity toward the environment. However, by annealing the samples at temperatures well above the glass transition temperature of the polymer, the response to dielectric environment appeared to be enhanced significantly. The increased sensitivity of the annealed sample (increase the surface particle density) was accounted for by the increased mobility of both polymer chains and gold nanoparticles in the rubbery state of the material and the presence of the monomer. The results showed that, by using adequate post-synthesis heat treatments, gold-polymer nanocomposites could be used as plasmonic sensing platforms.ududSecondly, gold–gelatin bionanocomposite films were prepared by the reduction of gold ions by sodium borohydride in an aqueous solution. It was shown that both the solution and the films on glass substrates contained entrapped hydrogen micro- and nanobubbles with diameters in the range of 200 nm–3 μm. The composite films having micro- and nanobubble inclusions have been found to be very stable. The optical properties of gold nanoparticles in the presence of gelatin and hydrogen nanobubbles were measured and simulated by using the discrete dipole approximation (DDA) method. The calculated localized surface plasmon resonance band was found in agreement with the experimental band position only when the presence of hydrogen bubbles around the gold nanoparticles was taken into account. The different morphological features engendered by the presence of the bubbles in the film (gelatin receptacles for the nanoparticles, gelatin hemispheres raised by the bubbles under the surface, cavities on the surface of the film, etc) are potential candidates for many applications. ud