(34b) Molecular Simulations of Ligand-Capped 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 these ligand-ligand interactions, there is very little fundamental understanding of their role in structure-direction. We have used atomically and molecularly explicit Molecular Dynamics simulations to create this understanding. Specifically, we study the links between processing conditions and the resultant symmetry of the superlattice, whether face-centered cubic (fcc) or body-centered cubic (bcc) structures, in a close collaboration with the Hanrath group.

These results form the first report of computer simulations of experimentally relevant sized ligand-capped nanocrystals (3-6 nm) in contrast to prior simulation work in the literature (sub-3 nm). Our results for the free energies of the system provide definitive proof of a clear dependence of preferred superlattice symmetry on the ratio of ligand length to nanocrystal size. Importantly, we have also uncovered the two key molecular mechanisms that are at play to direct the eventual preference. These simulations have also shown that nanoparticle interactions are critically dependent on parameters such as ligand grafting density and ligand coverage on specific facets of the nanocrystals, which ultimately help govern the self-assembly process.

In our most recent work, we are investigating the effect of shape and size of nanocrystals on the electronic energy states, which are critically important to their performance as photovoltaic solar cells. Our preliminary results confirm experimentalists’ suspicions that surface ligands play a role in modifying the electronic properties. We are investigating the properties of excitons by modeling excited states using Time Dependent Density Functional Theory (TD-DFT). We are also studying the electronic properties of nanocrystals coupled through organic linker molecules that will help inform the feasibility of this approach for experimentalists.

The impact of this kind of study is the provision of key insights into molecular-scale information about the relative roles of surface-bound ligand and nanoparticle cores that are impossible to determine experimentally. In addition, this insight can be leveraged in the future into more coarse-grained mesoscale simulations of the whole self-assembly process, namely nucleation and growth phenomena, which may then to be used to guide future experimental studies.