We use broadband picosecond acoustics to detect longitudinal acoustic phonons with few-gigahertz frequency in three-dimensional supracrystals(with face-centered cubic lattice) of 7 nm cobalt nanocrystal spheres. In full analogy with atomic crystals, where longitudinal acoustic phonons propagate with the speed of sound through coherent movements of atoms of the lattice out of their equilibrium positions, in these supracrystals atoms are replaced by (uncompressible) nanocrystals and atomic bonds by coating agents (carbon chains) that act like mechanical springs holding together thenanocrystals. By repeating the measurements at diff erent laser angles of incidence it was possible to accurately determine both the index of refraction of the supracrystal (n = 1.26 ± 0.03) and the room-temperature longitudinal speed of sound (vs = 1235 ± 12 m/s), which is quite low due to the heavyweight of the spheres (with respect to atoms in a crystal) and the soft carbon chains (with respect to atomic bonds). Interestingly, the speed of sound inside the supracrystal was found to dramaticallyincrease by decreasing the sample temperature due to a change in the stiff ness of the dodecanoic acid chains which coat the Co nanocrystals.
Source : Coherent Longitudinal Acoustic Phonons in Three-Dimensional Supracrystals of Cobalt Nanocrystals I. Lisiecki, D. Polli, C. Yan, G. Soavi, E. Duval, G. Cerullo and M.P. PileniNano Letters , 2013, 13, 4914-4919.