Acoustic phonon propagation and elastic properties of cluster-assembled carbon films investigated by Brillouin light scattering

We demonstrate that elastic properties of cluster-assembled carbon films can be measured by surface Brillouin scattering. We have studied films with different nano- and mesostructures produced with different precursor cluster mass distributions. In particular, films showing very high and low surface roughness have been compared over a wide range of film thicknesses. We have found that the propagation of surface and bulk acoustic phonons is strongly affected by the film mesostructure. Absence of phonon propagation and localization effects have been observed in films assembled with large clusters due to surface roughness, whereas small clusters produced smooth films showing well defined Brillouin peaks. This occurrence, together with the determination of the material density by x-ray reflectivity, made the evaluation of the elastic properties of nanostructured carbon films possible. The values of the elastic constants, and in particular the anomalous (extremely low) value of the Poisson's ratio, suggest that cluster-assembled carbon films are a unique system among different inorganic carbon-based materials.