Highly ordered superlattices from polydisperse Ag nanoparticles: a comparative study of fractionation and self-correction.

We have examined two different routes to construct highly ordered two- or three-dimensional (2D or 3D) superlattice structures from hydrophilic polydisperse mercaptosuccinic acid (MSA)-modified Ag nanoparticles of the average size of 2.5 nm. First, polydisperse particles were fractionized by the polyacrylamide gel electrophoresis (PAGE) method. Due to the size-dependent migration under the electric field, the particles were isolated into a series of gel bands and each band contained particles with significantly narrow size distribution. Subsequent to phase transfer into chloroform by cationic surfactant, long-range 2D superlattices were simultaneously formed on the substrate upon evaporation of chloroform. Second, 3D superlattices were directly grown at an air-water interface from the polydisperse bulk dispersion by diffusion of HCl vapor without any pretreatment for the size narrowing. The influence of diffusion rate of HCl was also studied. The achievement of 3D superlattices via this route was ascribed as a long-time self-correction process. Furthermore, it was revealed that the superlattice structures obtained by the above two procedures exhibited distinct features though the starting material was the same MSA-Ag nanoparticles. The surface distance of core between component particles, the orientation of particles inside the superlattice, and the process of superlattice formation were comprehensively studied. We confirmed that each growth process depended on a corresponding self-assembly mechanism.