High-temperature crystallization of nanocrystals into three-dimensional superlattices

A bottom-up process to achieve rapid growth of micrometre-sized three-dimensional nanocrystal superlattices during colloidal synthesis at high temperatures is revealed by in situ small-angle X-ray scattering; the process is applicable to several colloidal materials. Nanoparticles grown with their superlattices Nanoparticles can be ordered into superlattice structures that mimic the ordering of atoms in crystalline lattices. These superlattices could harness the unique properties of their nanocrystal components. Matteo Cargnello and colleagues report the synthesis of nanocrystals directly coupled with the rapid growth of superlattices of the nanocrystals at high temperatures, as observed by small-angle X-ray scattering. Subsequent electron microscopy revealed micrometre-sized three-dimensional superlattices composed of monodisperse nanocrystals. Tuning the composition of the ligands on the surface of the nanocrystals was instrumental in controlling superlattice formation. The authors saw that as the nanocrystals continued to grow within the lattice, the lattice also expanded. Three different materials systems all demonstrated superlattice formation in this way. Furthermore, both face-centred-cubic and hexagonal close-packed lattices were achieved by manipulating the reaction temperature. Crystallization of colloidal nanocrystals into superlattices represents a practical bottom-up process with which to create ordered metamaterials with emergent functionalities 1 , 2 , 3 . With precise control over the size, shape and composition of individual nanocrystals 4 , 5 , 6 , various single- and multi-component nanocrystal superlattices have been produced, the lattice structures and chemical compositions of which can be accurately engineered 7 , 8 , 9 . Nanocrystal superlattices are typically prepared by carefully controlling the assembly process through solvent evaporation or destabilization 2 , 10 , 11 , 12 , 13 , 14 , 15 or through DNA-guided crystallization 16 , 17 , 18 . Slow solvent evaporation or cooling of nanocrystal solutions (over hours or days) is the key element for successful crystallization processes 10 , 18 . Here we report the rapid growth (seconds) of micrometre-sized, face-centred-cubic, three-dimensional nanocrystal superlattices during colloidal synthesis at high temperatures (more than 230 degrees Celsius). Using in situ small-angle X-ray scattering, we observe continuous growth of individual nanocrystals within the lattices, which results in si