How to Tune the Au Internanocrystal Distance in Two-Dimensional Self-Ordered Superlattices

Hydrated and anhydrous Au nanocrystals with the same size dispersion and coating agent are synthesized from two procedures. Intriguing, the interparticle distance in two-dimensional (2D) self-ordered Au nanocrystals can be tuned by the amount of water molecules added to anhydrous Au nanocrystal solution. This behavior could be attributed to adsorption of water molecules either to the Au nanocrystal surface or on the dodecanethiol alkyl chains inducing their shrinkage. Taking into account various syntheses performed in hydrated media and the influence of water molecules on the appearance of attractive interactions between nanocrystals (measured by others), we claim that the presence of water molecules induces a competition between their adsorption at the metallic hydrophilic surface of Au nanocrystals and the chemisorption of the dodecanethiol. These adsorbed water molecules lead to additional attractive adhesion forces, which decrease the internanocrystal distance, whereas the strong thiol−Au bonding maintains the 2D long-range ordering of the nanocrystals.