peer journals

2D superlattices and 3D supracrystals of metal nanocrystals : a new scientific adventure.

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

peer journals

Hierarchy in Au Nanocrystal Ordering in Supracrystal: II. The Control of Inter Nanocrystal Distances.

Au nanocrystals coated with thiol derivatives differing by the length of their alkyl chains are used to build 3D superlattices called supracrystals. In this study, we used two sets of Au nanocrystals diff ering by their sizes and size distributions. The average sizes are 5 nm (Au5) and 7 nm (Au7). From one experiment to the other, the size distribution slightly changes. For Au5  nanocrystals, it evolves from 6 to 8% and for Au7  nanocrystals, it varies from 5 to 6%. The Au nanocrystals (Au5  and Au7) are fi rst dispersed in toluene and produce fcc supracrystals by solvent evaporation. Here, by small-angle grazing X-ray diff raction, we observe a control in the average interparticle distance within the supracrystals. When the supracrystals are grown at zero toluene vapor pressure, the interparticle distances increase linearly with the alkyl chain length of the nanocrystals’ coating agent regardless of their diameters. Furthermore, the dry supracrystals can swell and the interparticle distance within the superstructure be increased by subjecting the material to toluene vapor pressure after initial growth. This swelling process is reversible, and retraction occurs when the toluene vapor pressure drops. This indicates a strong ability of the dried supracrystals to trap toluene molecules. On increasing the toluene vapor pressure during the solvent evaporation process, the slope of the linear dependency of the interparticle distances to the alkyl chain length is markedlydecreased and the interparticle distance reaches a quasi-plateau. This is explained by the infl uence of depletion forces created by the presence of thiol-containing molecules physisorbed on the coating molecules on the internal structure of these supracrystals. Recently, we demonstrated that, by using the same nanocrystals (Au5  and Au7), a hierarchy in the supracrystal growth process takes place from heterogeneous nucleation with the formation of a layer-by-layer fi lm to homogeneous nucleation in solution with the formation of shaped supracrystals. Here it is shown that the interparticle distance is independent of the supracrystal growth mechanisms.

Source : Hierarchy in Au Nanocrystal Ordering in Supracrystal: II. The Control of Inter Nanocrystal Distances. Y.F.Wan, N.Goubet, P.A. Albouy and M.P.Pileni Langmuir,2013, 29, 13576-13581.

peer journals

Hierarchy in Au Nanocrystal Ordering in Supracrystal: 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 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).