Nanocrystal gels facilitate an ambitious feat for multiscale bulk materials: retaining the distinctive properties of individual nanocrystals and controllably harnessing their collective and structure-dependent response to assembly. Inducing the irreversible aggregation of colloidal dispersions through ligand removal and the formation of chemical bonds between surface-bound ligands was pivotal for the emergence of nanocrystal gels, but these conventional methods limit the development of more diverse, adaptable, and functional systems. While reversible physical interactions offer such opportunity, they have been primarily explored for microparticle self-assembly. Therefore, translating, characterizing, and validating these underlying principles at the nanoscale remains challenging. This talk will discuss a general and tunable approach, based on nonspecific and purely entropic depletion attractions, to control gelation for three types of metal oxide nanocrystals. We observe that gel formation, induced by adding polyethylene glycol depletants, is influenced by the nanocrystal size and shape but not the composition, which is consistent with theoretical phase behavior predictions. In addition, we demonstrate that nanocrystal gels formed through this approach are reversible and exhibit infrared optical properties responsive to the structure of the assembly. These findings motivate further exploring depletion interactions in nanocrystal assemblies in combination with thermodynamic predictions to design and engineer responsive and reconfigurable functional gels.

Colloidal metal oxide nanocrystal gels via depletion attractions

ACS Spring 2022

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