Vibration dynamics of supra-crystals of Cobalt nano-crystals studied with femtosecond laser pulses

Using femtosecond laser pulses, we have observed in real time the coherent motion of nanocrystals self-assembled in a three-dimensional (3D) supra-crystal. The self-organisation of nanosystems is of great interest since it leads to the elaboration of mesoscopic materials which have specific physical properties differing both from those of the bulk and of the nano-material. The nanostructures that we have studied are 3D long-range ordered or amorphous assemblies of cobalt nanoparticles elaborated by soft chemistry. It consists in a micellar solution of cobalt nanoparticles passivated by aliphatic chains and evaporated on an HOPG substrate. Depending on the evaporation conditions, it results in amorphous assemblies or 3D supra-crystals with perfectly organised pavements of 10 mum².