Ligand-Programmed Consecutive Symmetry Break(s) in Nanoparticle Based Materials Showing Emergent Phenomena: Transitioning from Six-Fold to Three-Fold Symmetry in Anisotropic ZnO Colloids

The central promise of nanoparticle-based materials is that cooperative properties may emerge, when individual quantum dots are positioned on a periodic lattice. Yet, there are only few papers in literature reporting about such effects. Nevertheless, it is clear that the symmetry of the superlattice is decisive for the desired emergent phenomena. An interesting question is, how the symmetry of the initial monodisperse nanoparticles affects the structure of the colloidal crystal during self-assembly processes. For instance, particles with hexagonal cross-section show self-organization which is very similar to spherical colloids. Like-wise one would also expect that trigonal nanoparticles behave similar. Unfortunately, it is very hard to obtain monodisperse semiconductur colloids with trigonal shape, because this requires a symmetry break during morphogenesis of the nanocrystal. While such a symmetry-break is known in literature for structures attached to a solid substrate, it is shown here, colloidal synthesis of trigonal ZnO nanorods is successful, and the mechanism is elucidated by experimental and theoretical methods. 2D-superlattices formed by such particles with trigonal cross-section were compared to hexagonal analogues. It was found, there are distinct differences, which result in important differences in properties such as the formation of voids and also in optical properties.