Modeling the phase behavior of nanoparticle superlattices with a molecular theory

MARIO TAGLIAZUCCHI

Conferencia; Kavli Conference Structure Design and Emerging Phenomena in Nanoparticle Assemblies: What?s Next?; 2023

Kavli Institute

3D arrays of nanoparticles (NPs), known as nanoparticle superlattices, exhibit unique phase behaviors due to their nanoscale nature and the interactions between their capping ligands. In this talk, I will present our theoretical approach to predict the phase behavior of NP superlattices using a statistical thermodynamic tool, known as molecular theory. The molecular theory explicitly considers the presence and degrees of freedom of the capping ligands on the surface of the NPs and of the solvent in which the nanoparticles are dissolved. We applied the theory to study the transition from body-centered-cubic (BCC) to face-centered-cubic (FCC) structures and the existence of the body-centered-tetragonal (BCT) intermediate phase for NPs coated by alkyl-chain ligands. The theory predicts that decreasing the solvent content during evaporation-driven crystallization leads to a FCCBCC transition, in good agreement with experiments in the literature. I?ll discuss the effects of the properties of the nanoparticles (ligand length and surface coverage, and nanoparticle radius) on this transition and compare our predictions with observations in the literature. Then, the presence of BCT as an intermediate phase in the FCC-BCC transition will be discussed. The BCT phase was recurrently observed in experiments for nanoparticle superlattices, but it is very rare in colloidal crystals of micrometer-sized particles. To explain this observation, the molecular theory was used to independently test the two mechanisms of BCT stabilization proposed in the literature: i) the influence of the substrate on which the superlattices are deposited, and ii) non-spherical NP shapes. I?ll show that the influence of the substrate can weakly stabilize the BCT phase, although it is unlikely to explain the experimentally observed abundancy of this structure. On the other hand, truncated octahedron NPs strongly stabilize BCT. In this case, the stabilization results from a symmetry breakdown of the ligand-mediated nanoparticle-nanoparticle interactions. Notably, this effect does not require different crystal facets to have different ligand properties, as previously thought.