Assembly and self-organization of silver nanocrystal superlattices: Ordered "soft spheres"

Hydrophobic dodecanethiol-capped silver nanocrystals ranging from 50 to 80 Angstrom in diameter were synthesized using arrested growth methods. Size-selective precipitation was employed to isolate nanocrystals homogeneous in size and shape which exhibited close-packed structural order after drying on a carbon or mica substrate. Elemental analysis, H-1 NMR and FTIR spectroscopies were used to characterize the compositional features of the adsorbed thiolate ligands on nanocrystals suspended in solution and condensed in nanocrystal films. Small-angle X-ray scattering (SAXS) and transmission electron microscopy (TEM) were used to probe the structure of both individual nanocrystals and superlattices, and to estimate the energetic interactions between these sterically stabilized particles. In terms of these measurements, the effects of the capping ligand coverage, particle faceting and shape, and interparticle attractions on superlattice formation are elucidated. In particular, the concept of nanocrystals as "soft spheres" which order with an effective hard sphere diameter is introduced. In addition, the ways in which nanocrystals deviate from such an ideal model an discussed.