Ligand Interactions Determine Orientational Order in Self-Assembled Nanocrystal Superlattices

The self-assembly of colloidal nanocrystals into ordered structures is a straightforward and scalable method of creating functional nanomaterials. Predicting and controlling the structure that form during self-assembly, however, requires detailed knowledge of the interactions between nanocrystals in solution. Recent work has revealed that small semiconductor nanocrystals with polyhedral shapes can be assembled into a range of different superlattices that do not only differ by their lattice symmetry but also by the degree of orientational alignment between nanocrystals. Here, we show with coarse-grained modeling and molecular simulations that interactions between organic ligands on nanoparticle surfaces are crucial in determining these self-assembly outcomes. Our simulations reproduce well the experimental results obtained with 5-nm PbS nanocrystals and show that modest changes in solvent quality and surface density of ligands can lead to superlattices with different lattice symmetry and degree of orientational order.