Nanoparticle superlattices (NPSLs) are 3D arrays of nanoparticles stabilized by weak interactions, which can be thought as the nanoscopic counterparts of atomic crystalline solids. However, NPSLs exhibit phase behaviors that are unique to their nanoscale nature and that arise from the interactions between the capping ligands on the surfaces of the nanoparticles. In this talk, I will introduce our efforts to model the phase behavior of NPSLs using a statistical thermodynamic tool known as molecular theory. I will present theoretical predictions and their comparison with experiments in the literature for the effects of the capping ligands and the crystallization solvent on the phase behavior of NPSLs. The decrease in solvent content during evaporation-driven crystallization leads to a transition from the FCC (face centered cubic) phase to the BCC (body centered cubic) phase, which our theory explains as a competition between the mixing entropy of the solvent and the entropy and internal energy of the ligands. The presence of the BTC (body centered tetragonal) intermediate in the BCC-FCC transition, which was recurrently observed in experiments, is theoretically predicted in the case of non-spherical particles, but not for spherical ones. For truncated octahedron nanoparticles, the occurrence of the BCT phase depends (among other factors) of the amount of solvent in the lattice and the shape of the nanoparticles (see figure). The stabilization of the BCT phase results from a symmetry breakdown of the ligand-mediated nanoparticle-nanoparticle interactions for non-spherical nanoparticles. Notably, this effect does not require different crystal facets to have different ligand properties, as previously thought.

Modeling the phase behavior of nanoparticle superlattices

ACS Spring Meeting 2021

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