Hierarchy in Au nanocrystal ordering in supracrystals: III. Competition between van der Waals and dynamic processes.

Au nanocrystals coated with thiol derivatives of varying chain sizes ranging from C12 to C16 were produced; two different size nanocrystals have been synthesized (5 and 7 nm in diameter) for each coating agent. All of those specimens are characterized by a low size distribution (below 7%). Those Au nanocrystals were used as building blocks to grow larger self-assembled crystalline structures or supracrystals. These crystalline growths were carried out by slow and controlled solvent evaporation at different temperatures and under non-null partial solvent vapor pressure (Pt). We show that the order within the supracrystals is temperature-dependent when they are made of hexadecanethiol-coated gold nanocrystals, regardless of the size of the nanocrystals. The interparticle distances within the various supracrystals that were produced were determined by small-angle X-ray diffraction (SAXRD). We demonstrate that the interparticle distance is controlled not only by the presence of physisorbed thiol residues, as previously reported, but also, at higher temperatures, by the dynamics of the organic chains and the van der Waals forces involved between the metallic cores of the nanocrystals forming the structure.