Significant theoretical work has been devoted to understanding why colloidal nanocrystals (NCs) form the diverse set of structures they do. This research has focused on the ratio of ligand shell to core size, core faceting, ligand coverage, and processing conditions as controllable parameters. Here, we demonstrate the central role unbound/free ligands play in tuning NC superlattice (SL) structure.  We perform a series of controlled additions of unbound oleic acid ligands to solutions of identically sized, oleate-capped PbS NCs with differing ligand coverages and track the structures formed upon deposition via spin-coating with grazing-incidence small-angle X-ray scattering (GISAXS). The unbound ligand swells the bound ligand-shell and tunes the SL continuously with increasing addition of ligand from a body-centered cubic (BCC) phase, through a series of body-centered tetragonal (BCT) phases, to the face-centered cubic (FCC) symmetry. We establish limitations on the range of tunability based on Flory-Rehner theory of gel-swelling. Molecular dynamics simulations of single NCs confirm the unbound ligand readily swells the bound ligand-shell over all exposed NC facets, even without explicit rebinding to the NC surface. Furthermore, we explain the disruption of SL orientation relative to the substrate observed for high volume fraction samples by considering the early-time dynamics of spin-coating viscous solutions. This study establishes the ability to rationally tune SL structure through the addition of unbound ligand and highlights the importance of fully characterizing the bound and unbound ligand populations in a batch of NCs to successfully predict SL structure. This opens the door to new functionality of NC assemblies through the incorporation of free ligand.

Importance of unbound ligand in determining nanocrystal superlattice structure and orientation

Macromolecular Chemistry: The Second Century

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ACS Spring 2020 National Meeting & Expo

The official archive and repository for all ACS Meetings & Expositions abstracts and select posters or presentations.