Gel Assemblies of Colloidal Nanocrystals

Colloidal nanocrystals have distinctive optical properties that are broadly tunable by varying size, shape, and composition. When nanocrystals are assembled into ordered superlattices or disordered gel networks, the resulting materials exhibit properties inherited from their nanocrystal building blocks that also depend on their structural organization at the mesoscale. I will describe our efforts to make reconfigurable, dynamic assemblies of nanocrystals by making controlled, reversible bonds between them. Low volume fraction gels are assembled from indium tin oxide nanocrystals, which exhibit tunable plasmonic absorption of infrared light. Gelation is induced by combining the nanocrystals with low molecular weight polyethylene oxide (PEO). At moderate PEO concentrations, the polymer can act as a cross-linking agent, connecting adjacent nanocrystals in the gel. At high PEO concentrations, depletion forces can induce physical bonds between nanocrystals. Gels can be caused to flow again by changing the relative concentration of nanocrystals and PEO, suggesting reversible bonding between nanocrystals. Additional strategies for reversible bonding between assembling nanocrystals are under development to introduce chemical specificity and chemo-responsivity. These materials offer an outlook toward structurally and optically responsive mesoscale assemblies.