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

Abstract

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 a few papers in the literature reporting 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 a hexagonal cross-section demonstrate self-organization, which is very similar to spherical colloids. Likewise, one would also expect that trigonal nanoparticles behave similarly. Unfortunately, it is very hard to obtain monodisperse semiconductor colloids with a trigonal shape, because this requires a symmetry break during morphogenesis of the nanocrystal. While such a symmetry break is known in the literature for structures attached to a solid substrate, herein, colloidal synthesis of trigonal ZnO nanorods is successfully demonstrated, and the mechanism is elucidated via experimental and theoretical methods. 2D-superlattices formed by such particles with trigonal cross-section are compared to hexagonal analogues. It is found that there are distinct differences, which result in important differences in properties such as the formation of voids and also in optical properties.