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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.

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 (n) and interparticle edge-to-edge distance within the supracrystals (δpp) for two nanocrystal sizes (d). Young’s modulus (E*) and hardness (H) are independent of δpp and of the supracrystal morphology. Both E* and H are in the range of few tenths of a MPa to a few GPa. Tuning of δpp by 50% is achieved by controlling the solvent vapor pressure (Pt) during the evaporation process. For any nanocrystal size, at Pt = 0, E* and H values markedly increase with increasing n from 12 to 14. At Pt = 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 n and to free thiol-containing molecules trapped in the supracrystal lattices.