Breathing Colloidal Monolayers

Reversible polymorphic transitions are common in molecular crystals and have been utilized to design reconfigurable materials. However, translating this concept to colloidal superlattices, where microscale particles are the building blocks and distinct interactions are at play, remains a challenge. Herein, we report on 2D colloidal superlattices (or monolayers) that can anisotropically expand and contract, or "breathe", reversibly transforming between polymorphs with open and closed mesopores. The superlattices are assembled from polyhedral particles with a truncated hexagonal bipyramidal shape and through the combined use of depletion and AC electric field-induced interactions. Facet overlapping and dipolar alignment can be leveraged in the particle assembly, with their interplay enabling the formation of diverse rhombic superlattices with tunable axial angles and porosity. The assembly features various structural reconfiguration kinetics that promote crystal annealing. The fast and reversible switch between two porous superlattices manifests the stimuli-responsive breathing behavior. Our work offers a strategy toward responsive biomimetic colloidal materials.