Hydrophilic Gold Supracrystals Differing by the Nanoparticle Crystalline Structure

Very few studies concern water-soluble nanocrystals self-assembled in crystalline 3D superlattices called supracrystals. Furthermore, the control of the crystalline structure of nanocrystals known as nanocrystallinity has not been yet achieved with water-soluble nanocrystals. Here we produce, selectively, 5 nm Au single-domain (SD) and polycrystalline (POLY) water-soluble nanocrystals. These nanocrystals self-assembled in face-centered-cubic (fcc) supracrystals. The supracrystal stiffness evolves with the nanocrystallinity, the nanocrystal surface charge, as well as the steric effect of the coating agent. The optical properties of SD and POLY nanoparticles and those of the related supracrystals are also presented. In addition, a nanocrystallinity segregation event was observed upon drying-assisted self-assembly of aqueous stoichiometric mixtures of SD and POLY NCs, as in the case of their hydrophobic counterparts

Mechanical Properties of Au Supracrystals Tuned by Flexible Ligand Interactions

Here mechanical properties of face cubic centered colloidal crystals obtained out of equilibrium by solvent evaporation of coated Au nanocrystals suspension, called supracrystals, are reported as a function ligand chain length (<i>n</i>) and interparticle edge-to-edge distance within the supracrystals (δ_pp) for two nanocrystal sizes (<i>d</i>). Young’s modulus (<i>E</i>*) and hardness (<i>H</i>) are independent of δ_pp and of the supracrystal morphology. Both <i>E</i>* and <i>H</i> are in the range of few tenths of a MPa to a few GPa. Tuning of δ_pp by 50% is achieved by controlling the solvent vapor pressure (<i>P</i>_t) during the evaporation process. For any nanocrystal size, at <i>P</i>_t = 0, <i>E</i>* and <i>H</i> values markedly increase with increasing <i>n</i> from 12 to 14. At <i>P</i>_t = 39% and 75%, such dependency disappears. This trend differs from classical nanocomposite materials and is attributed to a change in the conformation of flexible ligands with <i>n</i> and to free thiol-containing molecules trapped in the supracrystal lattices

Unexpected Electronic Properties of Micrometer-Thick Supracrystals of Au Nanocrystals

We investigated the electronic properties of highly ordered three-dimensional colloidal crystals of gold nanocrystals (7 ± 0.4 nm), called supracrystals. Two kinds of Au supracrystals with typical thicknesses of 300 nm and 5 μm, respectively, are probed for the first time with scanning tunneling microscopy/spectroscopy at 5 K revealing similar power law behavior and showing homogeneous conductance with the fingerprint of isolated nanocrystal. Potential applications evading the size-related risks of nanocrystals could be then considered

Simultaneous Growths of Gold Colloidal Crystals

Natural systems give the route to design periodic arrangements with mesoscopic architecture using individual nanocrystals as building blocks forming colloidal crystals or <i>supracrystals</i>. The collective properties of such supracrystals are one of the main driving forces in materials research for the 21st century with potential applications in electronics or biomedical environments. Here we describe two simultaneous supracrystal growth processes from gold nanocrystal suspension, taking place in solution and at the air–liquid interface. Furthermore, the growth processes involve the crystallinity selection of nanocrystals and induce marked changes in the supracrystal mechanical properties