Solvent-Mediated Crystallization of Nanocrystal 3D Assemblies of Silver Nanocrystals: Unexpected Superlattice Ripening

Solvent ligand interactions in colloidal nanocrystals are of significant importance as they can be used to modulate the way they pack into superlattices. Here, we demonstrate that the crystal structures of the nanocrystal superlattices made of 2.2 nm Ag nanocrystals can be controlled by using different carrier solvents. Specifically, the superlattice structures are tuned from body-centered cubic (bcc) to facecentered cubic (fcc) when varying solvents from hexane to tetrachloroethylene (TCE). Furthermore, by simultaneously annealing these two samples at different temperatures, bcc structures originating from hexane solutions are dominated by a simple coalescence mechanism, while the fcc structure stemming from TCE solutions undergoes an Ostwald ripening process that can produce a variety of binary nanocrystal superlattices such as NaCl, AlB2, NaZn13, and MgZn2, the formation of those structures being well explained by a pure entropy driven process. This is believed to be due to variations in the ligand coverage ratio of the nanocrystals in different solvents that are changing the superlattice structures' stability. Those findings provide insights into the solvent-mediated nanocrystal superlattices and the Ostwald ripening process in nanocrystal superlattices.