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Abstract Colloidal nanocrystal gels offer tunable optical properties governed by both the nature of the building blocks and their spatial arrangement. When assembled via reversible molecular linkers, their phase behavior and structure are primarily dictated by bond strength and lability. However, precise control over these interactions remains a significant synthetic challenge and is often system‐specific. Here, we present a simple, broadly tunable linking strategy that modulates nanocrystal phase behavior and assembly structure by leveraging competitive metal–ligand equilibria. We achieve programmable control over gelation temperature and network structure by tuning competitive metal–terpyridine and metal–halide equilibria in terpyridine‐functionalized tin‐doped indium oxide (ITO) nanocrystals, governed by metal and halide identity, concentration, and temperature, enabling wide‐range infrared optical modulation. Combined kinetic Monte Carlo and optical simulations reveal that weaker, more labile links facilitate particle crawling, leading to denser gel structures with enhanced plasmon coupling. This strategy eliminates the need for complex ligand or linker design and establishes competitive coordination chemistry as a versatile platform for engineering dynamic, stimuli‐responsive colloidal assemblies. 

AbstractThis article is based on the MRS Medal presentation given by Delia J. Milliron at the 2023 MRS Fall Meeting & Exhibit in Boston, Mass. Milliron is cited “for the development of optically tunable metal oxide nanomaterials for applications such as energy-saving electrochromic windows.”Doped metal oxide nanocrystals (NCs) provide a highly tunable platform for localized surface plasmon resonance (LSPR) in the near- to mid-IR. This tunability can be achieved synthetically, through the size, shape, and composition of the NCs, or post-synthetically through reversible redox reactions, enabling a host of emerging applications. While the broad strokes of this tunability have been understood for a decade, over the last few years, there has been tremendous progress in understanding the relationships between the electronic structure, defect chemistry, and synthetic and post-synthetic tunability of metal oxide NCs. This article aims to provide an up-to-date picture of the optical tunability of metal oxide NC LSPR, in particular focusing on recent insights into how the NC electronic structure plays a role in LSPR tunability. Graphical abstract 

Controlling the structure and function of colloidal gels requires a detailed understanding of how the various components govern network formation and aging. In particular, molecular additives like salts are widely used to tune interparticle interactions, yet their influence on gelation pathways in complex systems such as colloidal nanocrystal gels remains inadequately understood. Here, we investigate how noncoordinating salts modulate the evolution of gels formed using chemically linked tin-doped indium oxide nanocrystals. Through combined structural, dynamic, and kinetic analyses, we demonstrate that increasing salt concentration accelerates gelation. When rescaled by salt-dependent characteristic times, the evolution collapses onto universal trajectories, revealing a time-salt superposition principle. The universality extends across length scales, suggesting a consistent salt-dependent mechanism that controls both local structuring and macroscopic network formation. This observed salt modulation of structure and dynamics provides a predictive basis for controlling the kinetics of nonequilibrium nanocrystal gel assembly, enhancing the rational design of functional nanomaterials with tunable properties.