Self-assembled supracrystals and hetero-structures made from colloidal nanocrystals

Editorial article to the themed collection: "Colloidal Self- Assembled Supracrystals and Heterostructures

How the Level of Ordering of 2D Nanocrystal Superlattices Is Controlled by Their Deposition Mode

Here, we show that droplet deposition of 5 nm Ag nanocrystals induces the formation of two bidimensionnal arrays differing by their level of long-range ordering. This is valid for the alkanethiol/solvent combinations using decanethiol or dodecanethiol as the coating agent and hexane, octane, decane, or dodecane as the solvent to disperse the nanocrystals. The spacing between the nanocrystals does not diverge from one network to the other for a given combination but depends conversely on this combination. This is demonstrated by employing an intrinsic property of highly ordered Ag nanocrystals subjected to oxygen plasma and also classical methods usually used for this purpose

Coherent longitudinal acoustic phonons in three-dimensional supracrystals of cobalt nanocrystals

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 (vs= 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. © 2013 American Chemical Society