In situ observation of nanoparticle ordering at the air-water-substrate boundary in colloidal solutions using x-ray nanobeams

The nanoscale structuring during evaporation of a droplet consisting of an aqueous colloidal solution of 2nm gold nanoparticles in water on a silicon substrate is followed in real time. The authors investigated the transfer of lateral order and vertical layering as a function of time at the three-phase contact line air-solution substrate combining a nanometer-sized x-ray beam with a grazing incidence geometry. A pronounced retardation of vertical ordering is observed with respect to lateral ordering. While individual layers are deposited during evaporation of the solvent, the growth parallel to the substrate shows a strongly nondiffusive behavior.