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Plasmonic Au nanoparticles (AuNPs) embedded into a TiO 2 dielectric matrix were analyzed by combining two-dimensional and three-dimensional electron microscopy techniques. The preparation method was reactive magnetron sputtering, followed by thermal annealing treatments at 400 and 600 °C. The goal was to assess the nanostructural characteristics and correlate them with the optical properties of the AuNPs, particularly the localized surface plasmon resonance (LSPR) behavior. High-angle annular dark field-scanning transmission electron microscopy results showed the presence of small-sized AuNPs (quantum size regime) in the as-deposited Au-TiO 2 film, resulting in a negligible LSPR response. The in-vacuum thermal annealing at 400 °C induced the formation of intermediate-sized nanoparticles (NPs), in the range of 10-40 nm, which led to the appearance of a well-defined LSPR band, positioned at 636 nm. Electron tomography revealed that most of the NPs are small-sized and are embedded into the TiO 2 matrix, whereas the larger NPs are located at the surface. Annealing at 600 °C promotes a bimodal size distribution with intermediate-sized NPs embedded in the matrix and big-sized NPs, up to 100 nm, appearing at the surface. The latter are responsible for a broadening and a redshift, to 645 nm, in the LSPR band because of increase of scattering-to-absorption ratio. Beyond differentiating and quantifying the surface and embedded NPs, electron tomography also provided the identification of "hot-spots". The presence of NPs at the surface, individual or in dimers, permits adsorption sites for LSPR sensing and for surface-enhanced spectroscopies, such as surface-enhanced Raman scattering.

The electron tomography technique applied in a quantitative way allowed us to characterize a heterogeneous catalyst made of Pd nanoparticles deposited on a δ-Al(2)O(3) lamellar support. In the first step, high resolution tomographic experiments carried out on several typical areas of support have confirmed the hypothesis of formation of δ-Al(2)O(3) proposed in the literature by the coalescence of lateral facets of the γ-Al(2)O(3) precursor. A bimodal porosity was also observed in the arrangement of δ-Al(2)O(3) platelets. In the second step, the Pd nanoparticles were found preferentially anchored on the lateral facets of δ-Al(2)O(3) platelets or on the defects situated on their basal planes. From a general point of view, we have demonstrated once again that the electron tomography technique implemented with nanometre resolution provides unique insight into the structure, morphology and spatial arrangement of components in a complex 3D nanostructure., (This journal is © The Royal Society of Chemistry 2012)