Mechanical Properties of Ag Nanoparticle Thin Films Synthesized by Supersonic Cluster Beam Deposition

The morphological and mechanical properties of nanoparticles-based ultrathin Ag films, synthesized by supersonic cluster beam deposition over a sapphire substrate, are unveiled exploiting ultrafast optoacoustic, atomic,force microscopy, Xray photoelectron spectroscopies, and X-ray diffraction techniques. The films, with thicknesses in the 10-50 nm range, have a porous structure composed of metallic Ag nanoparticles with a crystalline structure and average diameter of 6 nm. The films acoustic modes are in the hypersonic frequency range, the thinner films frequencies exceeding 100 GHz. The acoustic spectra are well accounted for modeling the nanoparticles film as an effective continuous medium. The modes quality factors show the existence of acoustically quasi-dark and bright states. The film effective density and effective elastic stiffness constants are respectively 0.8 and 0.5 that of bulk Ag. The present results are relevant in view of applications for optoacoustic transducers in the hypersonic frequency range, for optical coatings technology and for the production of mechanically stable bactericidal coatings.