Nanocrystals surprise with superlattice formation

While using small-angle X-ray scattering to study palladium nanocrystal formation, SLAC National Accelerator Laboratory postdoc Liheng Wu and Stanford University graduate student Joshua J. Willis texted their adviser, Stanford’s Matteo Cargnello: “Something weird is happening.” The nanocrystals had suddenly and unexpectedly formed a three-dimensional superlattice—a material akin to a typical inorganic crystal but with nanocrystals serving as the “atoms.” Such superlattices have potential applications in magnetics, electronics, and catalysis, and they are typically prepared slowly through solvent evaporation at low temperatures. In the research led by Cargnello and SLAC’s Christopher J. Tassone, Wu and Willis saw superlattices form in just seconds at 230 °C (Nature 2017, DOI: 10.1038/nature23308). They were able to tune the superlattice structure by changing the synthesis conditions and the structure of a carboxylic acid surfactant. The approach also worked to create superlattices of lead telluride and iron nanocrystals. The researchers suggest that the nanocrystals grow individually until reaching a critical diameter—about 5 nm for palladium, 11 nm for iron. At that size, attractive forces between the nanocrystals cause them to come together, but the surfactant prevents aggregation, resulting in a superlattice.