Cooling Dodecanethiol-Capped 2 nm Diameter Gold Nanocrystal Superlattices below Room Temperature Induces a Reversible Order-Disorder Structure Transition

We recently observed that a disordered assembly of octadecanethiol-capped gold (Au) nanocrystals can order when heated from room temperature to 60 degrees C [Yu, Y.; Jain, A.; Guillaussier, A.; Voggu, V. R; Truskett, T. M.; Smilgies, D.-M.; Korgel, B. A. Faraday Discuss. 2015, 181, 181-192]. This inverse melting structural transition was reversible and occurred near the melting-solidification temperature of the capping ligands. To determine the generality of this phenomenon, we studied by in situ grazing incidence small-angle X-ray scattering (GISAXS) the structure of assemblies of Au nanocrystals with shorter C-12 and C-5 alkanethiol capping ligands that form ordered superlattices at room temperature and have a ligand melting-solidification temperature below room temperature. Superlattices of dodecanethiol-capped Au nanocrystals disorder when cooled below 260 K, which is the melting-solidification temperature for dodecanethiol. Au nanocrystals capped with even shorter pentanethiol ligands that have a melting transition below 100 K (the lowest experimentally accessible temperature) do not undergo the disorder transition.