Fabrication of colloidal arrays by self-assembly of sub-100 nm silica particles

Self-assembly of sub-100 nm spherical silica nanoparticles into ordered, tightly packed three- and two-dimensional arrays was studied. Self-assembly by vertical evaporation was investigated for particles made by two methods: an optimized Stِber method recently reported from this laboratory and a modified reverse micelle method. Ordered, close-packed, two- and three-dimensional structures were formed with spherical nanoparticles made by the optimized Stِber method. Fast Fourier transforms of top-view scanning electron microscopy images document close-packed hexagonal packing for three-dimensional arrays consisting of particles as small as 50 nm. Ranges over which evaporation temperature and suspension particle concentration can be altered as strategies for improving packing quality of 50 nm particles have been defined. Self-assembly behavior that is distinct from that of larger particles (>200 nm) is observed for these sub-100 nm particles in that the ranges over which these variables can be altered to affect array packing quality are much smaller than for larger particles. In contrast, for particles made by the reverse micelle method, only structures with poor packing quality were obtained despite the fact that such particles are typically more uniform than those made by the Stِber method. These results provide clear evidence that, in addition to particle uniformity, other particle properties deriving from fabrication method play important roles in directing self-assembly of sub-100 nm particles. Finally, a rapid self-assembly method based on horizontal evaporation was used to produce close-packed three-dimensional structures of these sub-100 nm particles spanning several millimeters. Although these arrays are not as ordered as those made by vertical evaporation, the strategy reported herein allows tightly packed, crack-free arrays up to microns in thickness to be fabricated. A mechanism for self-assembly by this process is proposed.