(107c) Molecular Simulation of Ligand-Capped Lead Selenide Nanocrystals

Nanocrystalline solids have become the subject of intense study due to their unique optical properties and their capacity to form self-assembled superlattices. These properties make them suitable for use in a variety of applications such as solar cells, light emitting devices, transistors, etc. The self-assembly of these nanoparticles is governed by interactions at the molecular level, and hence, understanding the nature of these interactions could be instrumental in achieving precise control over the self-assembly process. The nanocrystals are "capped" by organic molecules which are believed to drive the self-assembly and stabilize the final superlattice and which are responsible for most of the interparticle forces. Despite the importance of the ligand-ligand interactions, there is very little fundamental understanding of these hybrid systems.  In our work, we use atomically and molecularly explicit Molecular Dynamics simulations to understand the interactions between the ligands on the surface of the nanocrystals. Specifically, we observe the energetic interactions of experimentally sized nanocrystals capped with ligands in various lattices including face-centered cubic (fcc) and body-centered cubic (bcc) structures in collaboration with the Hanrath group at Cornell. We also undertake the study of key controlling parameters of self-assembly including nanocrystal diameter, ligand grafting density, ligand length, nanocrystal orientation and size of nanocrystal facets, etc.

These results form the first report of computer simulations of experimentally relevant sized ligand-capped nanocrystals (3-6 nm). All prior simulation work in the literature has focused on nanocrystal diameters of less than 3 nm. The importance of nanocrystal size was clearly uncovered in our studies, which showed that the high ratio of ligand length to crystal facet size for 3 nm particles facilitates the wrapping of ligands around the nanocrystal cores. This, in turn, produces an effectively spherical interaction among nanocrystals that is orientation-independent. For larger nanocrystals (4 and 6 nm), the lower ligand-to-diameter ratio allows many ligands to experience more face than edge interactions and tend to stick out from the facet rather than wrapping around the crystal. Our simulations have also shown that these interactions are dependent on the grafting density of ligands and on the density of ligands on various facets of the nanocrystals.

We are also investigating the orientational alignment of nanocrystals in superlattices and the effect of local asymmetries in ligand density on the selection of preferred morphologies. The alignment of nanocrystals in the superlattice is primarily due to the orientations of the capping ligands. And the anisotropy of ligand coverage is responsible for orientational alignment of the nanocrystals, which, in turn, drives superlattice symmetry.

The choice of solvents on ligand morphologies also plays a role; the solvent appears to be responsible for superlattice stability as well superlattice symmetry. The solvent may also be responsible for different ligand morphologies on the nanocrystal surface. This kind of study provides key insights into molecular-scale information about ligand interactions which can be leveraged into more coarse-grained mesoscale simulations of nucleation and growth phenomena which may to be used to predict and guide future experimental studies.