Formation and Stepwise Self-Assembly of Cadmium Chalcogenide Nanocrystals to Colloidal Supra-Quantum Dots and the Superlattices

Nearly monodisperse colloidal superstructures of cadmium chalcogenide quantum dots (QDs) are reported. The superstructures, which we named as supra quantum dot (SQD), are typically composed of hundreds of a-few-nanometer-sized QDs three-dimensionally (3D) assembled by oriented attachment. The synthesis route for SQD is quite universal and can be extended to CdS, CdSe, CdTe, and CdSeTe alloy. The size of SQD can be tuned from tens of nanometers to over a hundred nanometers. In the case of CdSe SQD, zinc-blende seeds (primary QDs) act as the building block for the formation of the 3D assembled structures, SQDs, with discrete intermediates nanostructures. Primary seeds, 4 nm tetrahedral shaped QDs, assembled into a large tetrahedron of 20 nm. The 20 nm tetrahedrons, in turn, self-assembled into a larger tetrahedron of 40 nm. The discrete-in-size and sequential assemblies were followed by conventional growth from the remaining precursors and ripening within the particles to result in spheroidal SQDs. SQDs allow surface ligand exchange without losing the structural integrity. Size selective precipitation of SQDs can provide monodisperse SQDs that can assemble into ordered superlattices. The size and composition tunability of SQDs and their capability to form superlattices can provide a new solution-processable building block for superstructure with programmable physical and chemical properties