Acoustic Vibrations of Core-Shell Nanospheres: Probing the Mechanical Contact at the Metal-Dielectric Interface

The acoustic vibrations of metal-dielectric core-shell nanoparticles (Ag@SiO2 and Au@SiO2) are investigated using ultrafast pump-probe spectroscopy. The experimental results are in good agreement with calculations of the period of the breathing vibration of the core-shell particles. The transition from one overtone of the breathing mode to the next one as the size of the silica shell increases is observed for slightly thinner SiO2 shells than previously reported. Our results confirm that this optical technique permits the determination of the quality of the mechanical contact at the metal dielectric interface. This is of major importance for many applications, in particular because it modifies significantly the thermal conductivity at the nanoscale. Moreover, we observed that for two different synthesis routes, poor contact core-shell samples could sneak in among good sample series based on the same metallic core particles. This confirms the suitability of this all optical and noninvasive technique to obtain information hardly reachable by other means such as electron microscopy or absorption spectroscopy.