peer journals

Which forces do control the supracrystal nucleation in organic media ?

Here, two mechanisms of fcc Au supracrystal (assembly of Au nanocrystals) growth are proposed. The sizes of the Au nanocrystals and the solvent in which they are dispersed are major parameters that determine the final morphology of nanocrystal assemblies; fi lms by layer-by-layer growth (heterogeneous growth), characterized by their plastic deformation, or well-defined shapes grown in solution (homogeneous growth). Experiments supported by simulations demonstrate that supracrystal nucleation is mainly driven by solvent-mediated interactions and not solely by the van der Waals attraction between nanocrystal cores, as widely assumed in the literature.

Source : Which forces do control the supracrystal nucleation in organic media ? N.Goubet, J.Richardi, P.A. Albouy and M.P.Pileni. Adv.Funct.Mater., 2011,21, 2693-2704.

peer journals

Supracrystals of N‑Heterocyclic Carbene-Coated Au Nanocrystals.

Controlling the generation of organized 3D assemblies of individual nanocrystals, called supracrystals, as well as their properties, is an important challenge for the design of new materials in which the coating agent plays a major role. We present herein a new generation of structured fcc Au supracrystals made of N -heterocyclic carbene (NHC)-coated Au nanocrystals. The 3D assemblies were achieved byusing benzimidazole-derived NHCs tailored with long alkyl chains at diff erent positions. The average size of thenanocrystal precursors (4, 5, or 6 nm) and their ability to self-assemble were found to be dependent on the length, orientation, and number of alkyl chains on the NHC. Thick and large supracrystal domains were obtained from 5 nm Au nanocrystals coated with NHCs substituted by C14 alkyl chains on the nitrogen atoms. Here, the geometry of both the Ccarbene− Au and N− Calkyl  bonds induces a specifi c orientation of the alkyl chains, diff erent from that of alkylthiols, resulting in Au surface covering by the chains. However, the edge-to-edge distances in the supracrystals suggest that the supracrystals are stabilized by interdigitation of neighboring nanocrystals alkyl chains, whose terminal part must point outward with the appropriate geometry.

Source : Supracrystals of N‑Heterocyclic Carbene-Coated Au Nanocrystals. X.Ling, S. Roland and M.P. Pileni Chem. Mater.,2015, 27, 414−423.

peer journals

Impact of the metallic crystalline structure on the properties of nanocrystals and their mesoscopic assemblies.

The spontaneous assembly of uniform-sized globular entities into ordered arrays is a universal phenomenon observed for objects with diameters spanning a broad range of length scales. These extend from the atomic scale (10−8  cm), through molecular and macromolecular scales with proteins, synthetic low polymers, and colloidal crystals (∼ 10−6  cm), to the wavelength of visible light (∼ 10−5  cm). The associated concepts of sphere packing have had an infl uence in diverse fi elds ranging from pure geometrical analysis to architectural models or ideals. Self-assembly of atoms, supramolecules, or nanocrystals into ordered functional superstructures is a universal process and prevalent topic in science. About fi ve billion years ago in the early solar system, highly uniform magnetite particles of a few  hundred nanometers in size were assembled in 3D arrays. Thirty million years ago, silicate particles with submicrometer size were self-organized in the form of opal. Opal is colorless when composed of disordered silicate microparticles whereas it shows specifi c refl ectivity when particles order in arrays. Nowadays, nanocrystals, characterized by a narrow size distribution and coated with alkyl chains to maintain their integrity, self-assemble to form crystallographic orders called supracrystals. Nanocrystals and supracrystals are arrangements of highly ordered atoms and nanocrystals, respectively. The morphologies of nanocrystals, supracrystals, and minerals are similar at various scales from nanometer to millimeter scale. Such suprastructures, which enable the design of novel materials, are expected to become one of the main driving forces in material research for the 21st century. Nanocrystals vibrate coherently in a supracrystal as atoms in a nanocrystal. Longitudinal acoustic phonons are detected in supracrystals as with atomic crystals, where longitudinal acoustic phonons propagate through coherent movements of atoms of the lattice out of their equilibrium positions. These vibrational properties show a full analogy with atomic crystals: In supracrystals, atoms are replaced by (uncompressible) nanocrystals and atomic bonds by coating agents (carbon chains), which act like mechanical springs holding together the nanocrystals. Electronic properties of very thick (more than a few micrometers) supracrystals reveal homogeneous conductance with the fi ngerprint of the isolated nanocrystal. Triangular single crystals formed by heat-induced (50 °C) coalescence of thin supracrystals deposited on a substrate as epitaxial growth of metal particles on a substrate with specifi c orientation produced by ultrahigh vacuum (UHV). Here we demonstrate here that marked changes can occur in the chemical and physical properties of nanocrystals diff ering by their nanocrystallinity, that is, their crystalline structure. Furthermore, the properties (mechanical, growth processes) of supracrystals also change with the nanocrystallinity of the nanoparticles used as building blocks.

Source : Impact of the metallic crystalline structure on the properties of nanocrystals and their mesoscopic assemblies. MPPileni. Acc. Chem. Res., 2017, 50, 1946–1955