(74e) Engineering Entropic Self-Assembly of Faceted Nanoparticles

Authors: 
Sharma, A. K., Cornell University
Thapar, V., University of Chicago
Escobedo, F. A., Cornell University
Faceted particles are known to exhibit complex ordered phases arising from purely entropic interactions. In this work, we use Monte Carlo simulations to understand how such entropic effects can not only help the thermodynamics, but also the kinetics of self-assembly of faceted nanoparticles. It has been conjectured that configurations where the facets of a pair of neighboring particles are aligned are entropically favorable. We anticipate that this local tendency can influence the kinetics of a global transition – such as a disorder-to-order transition to a crystal phase. We develop a novel metric to capture the notion of facet alignment among neighboring particles. Coupled with particle orientational correlations, our facet alignment measure reveals that various members of truncated cube family have varying levels of incompatibility between orientational order (present in the crystal) and facet alignment (which occur in the liquid). Such incompatibility manifests in an increased free energy barrier for the solid-phase nucleation. This is because if locally preferred configurations do not contribute to the nucleation pathway then they constitute ‘false starts’ or ‘dead ends’. Similarly, we observe that if the local tendency to align faces is cooperative with the global tendency to form orientationally ordered phases, then those phases are easier to self-assemble. Finally, we also discuss some of our recent results on the isotropic-to-solid phase transition for an extreme member of the family, hard cubes, where the lack of corner truncation leads to a high level of facet alignment and a non-trivial transition mechanism. Thus, we provide an important design consideration with respect to the shape of colloidal nanoparticles: the degree of facet alignment in the disordered phase structure correlates with the ease of self-assembly.