Crystallization of semiconductor nanorods into perfectly faceted hexagonal superstructures

Super crystallization of ligand-capped nanocrystals into defined periodic solids from solution is the definitive demonstration of their self-organizing properties. To date, this has been mainly limited to spherical nanocrystals where organization emulates atom or molecule packing in regular crystals with the most thermodynamically stable arrangement being eventually preferred. Here, the crystallization of wurtzite CdS nanorods into micrometer-sized CdS superstructures with regular hexagonal symmetry is demonstrated by fine-tuning the nanorod dispersibility over time. It is shown that the supercrystals have a long nucleation stage to form monolayer hexagons followed by a relatively faster growth stage both occurring rod by rod (in-plane) and layer by layer (out of plane). The perfectly symmetrical hexagon shape of the final structure is mapped from the wurtzite crystal structure of each individual nanorod where they pack in side by side and end to end arrangements. These well-defined superstructures are highly attractive for applications that collectively exploit electronic or optical properties that are synthetically tunable through the size and shape of each nanorod building block. Complete crystallization of CdS nanorods into micrometer sized perfectly faceted hexagonal superstructures is reported. The concept of crystallization for highly anisotropic material (nanorods) and the directing effect of the wurtzite structure of nanocrystal in the final shape of the superstructure are perfectly demonstrated in this system.