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The experimental setup has been described previously in detail 23, 24. In what follows, we will recall the important steps. The silver nanoparticles are produced by the laser vaporization technique, where the beam of a Nd: YAG pulsed laser is focused on a rod of the metal to be studied. The atomic plasma obtained is thermalised by the helium gas and expands in a vacuum through a nozzle. In this thermalization operation, the gas used is either helium at high pressure delivered by a pulsed valve, or a continuous flow of helium at low pressure (a few tens of mbar). To control the size of the free nanoparticles and thus obtain samples of different size distributions, it acts on the pressure conditions. The neutral nanoparticles obtained then pass through a skimmer in a high-vacuum chamber where they can be photoionized and analyzed by means of a time-of-flight reflector mass spectrometer. The silver clusters produced by the source at room temperature are probably spherical because the size-resolved mass distribution reveals the same series of magic numbers, as predicted by the Jellium model. Finally, the nanoparticles and the dielectric matrix (alumina) are simultaneously deposited on a substrate in a vacuum chamber (a few mbar). We can check the volume fraction of the metal using quartz balances, which provide the two deposit rates. The thickness of the film depends on the type of study to be performed; it varies from 15 nm for TEM observations up to 200 nm for optical absorption analysis. The volume fraction of the metal nanoparticles was maintained between 3% and 10% in order to avoid the correlation effects between the optical response of these nanoparticles and that of the bulk material. The RBS and EDX techniques confirmed these values. TEM micrographs of our samples were also made. They show quasi-spherical clusters distributed randomly in the alumina matrix. The size distributions give average values of 2.0 and 3.7 nm in diameter (or an average number of 300 to 3000 atoms per nanoparticle). X-ray diffraction analysis shows that the crystalline structure of the nanoparticles is of the FCC type. Small angle X-ray scattering measurements (GISAXS) confirm the average size and sphericity of nanoparticles as well as the absence of any spatial correlation.

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