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