Small-angle x-ray-scattering study of silver-nanocrystal disorder-order phase transitions

A conceptually unique approach was developed to study the interparticle interactions between organized alkanethiol-capped silver nanocrystals. Dense nanocrystal fluids were formed by evaporating the solvent from a "size-polydisperse" (sigma greater than or similar to +/- 12%) nanocrystal dispersion on a substrate. The sample polydispersity prevented the disorder-order phase transition (i.e., superlattice formation) from occurring. Small-angle x-ray scattering was then used to measure the static structure factors S(q), of these disordered nanocrystal films as a function of the ratio [L]/R between the capping ligand chain length to the core nanocrystal radius. The pair-distribution and direct correlation functions were then calculated from Fourier transformations of S(q). This enabled the use of the hypernetted chain approximation to calculate the pair interparticle potential ct(r). The 6-12 Lennard-Jones potential provided reasonable fits to all experimentally determined values of u(r), indicating the predominance of relatively short-range repulsion between nanocrystals. Monodisperse dodecanethiol- and octanethiol-capped silver nanocrystals were then condensed into ordered arrays. Face-centered-cubic (fcc) packing was favored for [L]/R<0.60, and body-centered-cubic (bcc) packing was favored when [L]/R>0.60. Lower-symmetry body-centered-tetragonal packing was observed for octanethiol-capped silver nanocrystals with [L]/R>0.66. A simple model employing the experimentally determined values for u(r), predicts that the fcc-->bcc superlattice phase transition occurs when [L]/R greater than or similar to 0.65. [S0163-1829(99)13421-0].