Nanoparticle superlattices (NPSLs) are a unique class of nanostructured materials comprised of periodic two or three-dimensional arrays of nanoparticles. Multiple methods have been developed to assemble NPSLs, and these crystallization processes have often been guided by chemical intuition derived from the formation of traditional atomic crystals. Chemical concepts like bond valency or packing factors can be directly analogized to structural features and phenomena observed in NPSLs, suggesting that more complex behaviors like microstructure formation or phase transitions could also be induced. Given that the properties of NPSLs are often governed by their crystal symmetry and lattice parameters, the ability to induce a phase transition to reversibly tune the crystal structure has the potential to lead to dynamic materials with switchable properties for a variety of applications, including optoelectronics, plasmonics, mechanics, and catalysis. As such, there has been significant interest in engineering NPSLs that are able to exhibit reversible polymorphic phase transitions.<br/><br/>Here, we report the assembly of nanocomposite tectons (NCTs), polymer grafted nanoparticles that have supramolecular recognition groups at the chain ends, into unary NPSLs that are capable of exhibiting a martensitic phase transition between a face-centered cubic (FCC) and a body-centered cubic (BCC) structure when the solvent quality is changed. The phase transition is rapid, reversible, and appears to proceed through an order-to-order crystal transition. We show the impact that NCT composition has on the phase transition behavior and discuss the potential mechanisms behind it. Importantly, we demonstrate that the phase transition can be kinetically arrested, allowing both polymorphs to be obtained in the solid form and examined. The BCC phase was observed to maintain the same crystal habit as the parent FCC phase but exhibited significant twinning, analogous to those observed in martensitic transformations. Given the importance of phase transitions and twinning in atomic materials, the ability to replicate them in NPSLs represents a step forward in not only understanding their formation, but also in engineering complex materials that can utilize them to augment their properties.

Reversible martensitic phase transitions in 3D nanoparticle superlattices

ACS Spring 2022

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