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 specimensare characterized by a low size distribution (below 7%). ThoseAu nanocrystals were used as building blocks to grow larger self-assembled crystalline structures or supracrystals. Thesecrystalline growths were carried out by slow and controlledsolvent evaporation at different temperatures and under nonnull partial solvent vapor pressure (Pt). We show that theorder within the supracrystals is temperature-dependent whenthey 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 diff raction (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.
Source : Hierarchy in Au Nanocrystal Ordering in Supracrystal: III. Competition between Van der Waals and Dynamic Processes. N. Schaeffer, Y.Wan, M.P. Pileni. Langmuir ,2014, 30, 7177 (2014).