Crystalline superlattices from single-sized quantum dots

Despite the recent progress toward the synthesis of monodisperse semiconducting nanocrystals, it remains a challenge to prepare quantum dot structures with a precise number of atoms. Here, we report synthesis, crystal structure, and optical properties of a family of cadmium sulfide nanocrystal superlattices assembled through single-sized semiconducting clusters. Clusters of various sizes have been made. The largest cluster determined from single-crystal analysis has a total of 138 metal-chalcogen sites. It is the largest known single-sized II-VI quantum dot and is also the first one with more than 100 metal-chalcogen sites. X-ray powder diffraction (XRD) and optical absorption studies indicate the presence of even larger single-sized quantum dots (>200 metal-chalcogen sites). These clusters consist of cubic zinc blende-type core and hexagonal wurtzite-type corners and can exist in up to five isomeric forms that differ only in the position of the hexagonal-cubic interface.