Liheng Wu1 2 Matteo Cargnello1 Christopher Tassone2

1, Stanford University, Stanford, California, United States
2, SLAC National Accelerator Laboratory, Menlo Park, California, United States

Developing nanocrystals with precisely controlled size, shape, and structure is of great importance for understanding their properties. Although significant developments have been achieved in colloidal synthesis in the past two decades, it remains challenging to synthesize nanocrystals in a predictive way due to a lack of mechanistic understanding of their formation. Here we use synchrotron-based X-ray scattering to probe the formation of nanocrystals in situ under typical colloidal synthetic conditions.

By coupling the small angle X-ray scattering with the wide angle X-ray scattering, we were able to quantitatively analyze the nucleation and growth kinetics of nanocrystals and understand the evolution of their atomic crystalline structures. Using PtSn as a model system, we observed a unique formation mechanism of PtSn nanocrystals in a one-pot synthesis through sequential diffusion of the Sn component inside pre-formed Pt nanocrystals. The in situ characterization provides a general strategy for the synthesis of bimetallic nanocrystals. Moreover, the in situ X-ray scattering enabled the unprecedented discovery of crystallization of various nanocrystals (i.e., metallic, and semiconducting) into three-dimensional superlattices in solution, even at very high temperatures (up to 300 oC), providing new insights on interparticle interactions during colloidal synthesis. These studies demonstrate new opportunities offered by in situ X-ray scattering techniques in understanding not only the formation mechanisms of nanocrystals, but also the interparticle interactions for creating diverse nanocrystal superlattices.