A growing number of studies have demonstrated the synthesis and assembly of different types of anisotropic particles into a quasi-2D structures. However, our understanding of the factors controlling the properties of such structures is incomplete, e.g., while the assembly on hexagonal or square lattice 2D arrays is well established, much less is known about how a system can be designed to switch between these two ubiquitous lattice symmetries. We use both Monte Carlo (MC) and Molecular dynamics (MD) simulations to probe this question using different strategies to control particle n-fold symmetry and to enact monolayer confinement.<br/>First, we show that a mixture of hard disk and squares of appropriate sizes can assemble into a various substitutionally disordered 2D crystals and mesophases, which largely parallel those formed by the pure systems. Near the critical point for the hexatic-tetratic phase transition, we unveil a peculiar state where patches of tens of particles form, having either hexagonal or square symmetry which tile the 2D plane with a “mosaic” pattern.<br/>Second, we demonstrate how to control via slit confinement, the “flipping state” of a monolayer of hard particles such that different states correspond to different packing symmetries. This is enabled by particles shaped as short cylinders so that they can adopt one of two main orientational states: flipped and unflipped, whose “footprint” changes in area and n-fold symmetry. This system can be loosely seen as a colloidal realization of an Ising model. By varying the slit gap, we tune the restriction of the entropic degrees of freedom of these particles, thus controlling the fraction of particles that can access either orientation.<br/>Finally, we probe the mechanism of interfacial assembly of ligand-coated nanoparticles as the ligands are gradually removed and epitaxial bonding ensues. Such a system mimics experiments aimed to form quasi-2D nanoparticle superlattices via interfacial assembly. Our work reveals that the transition from a hexagonal preassembled structure (for particles fully coated) to the square superlattice (for particles with bare {100} facets) can occur through different mechanisms, depending on nanoparticle shape, and mediated by the interactions between ligands across particles. For truncated octahedral shapes, we unveil a novel serpentine-to-blob transition of the ligand clusters, which guides the martensitic assembly of particle rods into a square superlattice.

Tilting the balance between hexagonal and square quasi-2D crystalline colloidal assemblies by tuning entropic and energetic interactions

ACS Spring 2022

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