title: Coherent longitudinal acoustic phonons in three-dimensional supracrystals of cobalt nanocrystals.
abstract: 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 the nanocrystals. By repeating the measurements at different 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 (v(s)= 1235 ± 12 m/s), which is quite low due to the heavy weight 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 dramatically increase by decreasing the sample temperature due to a change in the stiffness of the dodecanoic acid chains which coat the Co nanocrystals.
authors.affiliation: Université Pierre et Marie Curie, UMR 7070, LM2N, 4 place Jussieu, 75005 Paris, France.
authors.forename: Isabelle
authors.initials: I
authors.lastname: Lisiecki
authors.forename: Dario
authors.initials: D
authors.lastname: Polli
authors.forename: Cong
authors.initials: C
authors.lastname: Yan
authors.forename: Giancarlo
authors.initials: G
authors.lastname: Soavi
authors.forename: Eugène
authors.initials: E
authors.lastname: Duval
authors.forename: Giulio
authors.initials: G
authors.lastname: Cerullo
authors.forename: Marie-Paule
authors.initials: MP
authors.lastname: Pileni
mesh_terms: D000162:Acoustics; D003035:Cobalt / Q000737:chemistry*; D007850:Lauric Acids / Q000737:chemistry; D053768:Metal Nanoparticles / Q000737:chemistry*; D060739:Phonons; D013696:Temperature
chemical_list: D007850:Lauric Acids; C030358:lauric acid; D003035:Cobalt