We report the first real-time observation of the collective motion of nanocrystals self-assembled in a 3D supracrystal. The long-range ordering in the fcc cobalt supra-crystals studied here is ideal to launch coherent vibrations of the nanocrystals when suddenly heated by femtosecond laser pulses. The interdigitated aliphatic chains linking the particles act as mechanical nanosprings, which can be resonantly excited by ultrashort laser perturbations. This macroscopic supramolecular motion finds its origin at the microscopic level where, similarly, the cobalt atoms in each nanocrystal have their own dynamics, including their electron and lattice relaxation energies.
Source : Vibration Dynamics of Supra-Crystals of cobalt nanocrystals studied with femtosecond laser pulses. I.Lisiecki, V. Halté, C. Petit, M-P Pileni and J-Y Bigot Adv. Mat., 2008, 20, 1-4.