Short-Ranged Attractive Forces Enable the Self-Assembly of Binary Nanocrystal Superlattices

The self-assembly of nanocrystals into binary superlattices enables the targeted integration of orthogonal physical properties, like photoluminescence and magnetism, into a single superstructure, unlocking a vast design space for multifunctional materials. Yet, the formation of binary nanocrystal superlattices remains poorly understood, restricting the use of simulation to predict structure and properties of the final superlattices. Here, we use <i>in situ </i>scattering experiments to unravel the time-dependent self-assembly of nanocrystals into 3D binary superlattices, and molecular dynamics simulations to obtain interparticle interactions consistent with experimental observations. We show definitively that short-ranged, attractive interparticle forces are necessary to obtain the binary crystalline phases observed in experiment. The short-ranged attraction stabilizes these crystalline phases relative to fluid phases, dramatically enhancing their formation kinetics over the purely repulsive interactions of the hard-sphere model. In these conditions, the formation of binary nanocrystal superlattices proceeds through homogeneous nucleation in the absence of intermediate ordered structures. These results establish a robust correspondence between experiment and theory, paving the way towards <i>a</i> <i>priori </i>prediction of binary nanocrystal superlattices.