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Plasmonic Au nanoparticles (AuNPs) embedded into a TiO2 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 degrees 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-TiO2 film, resulting in a negligible LSPR response. The in-vacuum thermal annealing at 400 degrees 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 TiO2 matrix, whereas the larger NPs are located at the surface. Annealing at 600 degrees 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 authors thank the METSA network (FR3507 CNRS) for the financial support for this work, the Clym consortium for granting access to the Jeol 2010F and TITAN 80-300 keV ETEM. S.K. thanks the French Ministry of Higher Education and Research for the financial support during his PhD which was also supported by the LABEX iMUST (ANR-10-LABX-0064) of Universite de Lyon, within the program "Investissements d'Avenir" (ANR-11-IDEX-0007) operated by the French National Research Agency (ANR). This research was partly supported by the 3Dclean ANR-15-CE09-0009 project granted by the ANR. This research was also sponsored by the Portuguese Foundation for Science and Technology (FCT) in the framework of the Strategic Funding UID/FIS/04650/2013 and FCT Project 9471-RIDTI, co-funded by FEDER and with reference PTDC/FIS-NAN/1154/2014. J.B. acknowledges FCT for his Postdoc Grant (SFRH/BPD/117010/2016) and M.S.R. acknowledges FCT for his PhD Grant (SFRH/BD/118684/2016)., info:eu-repo/semantics/publishedVersion