Nanocrystals are able to self-assemble in hexagonal networks (2D) and in supracrystals (3D). Here it is shown that the interparticle distance is tuned by the presence of water molecules adsorbed at the nanocrystal interface and on the alkyl chains used as coating agents. By using an intrinsic property due to the nanocrystal ordering, a new, but destructive, method is proposed to detect defects on a large monolayer scale. The supracrystal growth mechanism changes with the nanocrystal size from a heterogeneous (layer-by-layer) to a homogeneous (growth in solution) process. Co supracrystals are highly stable after annealing at 350 !C with an improvement in the nanocrystal ordering, i.e., in the supracrystallinity. With Ag supracrystals it was possible, from the same batch of 5 nm Ag nanocrystals, to control the supracrystallinity with phase transitions of hcp to fcc and amorphous solids to hcp and bcc. Finally a tentative analogy between atoms and nanocrystals is proposed in the crystal growth process. These data open a new research area with a large potential for discovering new chemical and physical properties.
Source : 2D superlattices and 3D supracrystals of metal nanocrystals : a new scientific adventure. M.P.Pileni J. Mater. Chem., 2011, 21, 16748 – 16758