Characterization and Control of the Structure and Properties of Low-Density, Linker-Mediated Nanocrystal Gels

Owing to their open, porous structure and high surface area, aerogels have found application in a variety of areas, including insulation, water purification, and catalysis. The use of nanocrystals to form aerogels or other low density gels is a particularly promising strategy to form novel materials: the high surface area and porosity allow the material to retain any unique properties (enhanced catalytic activity, for example) that are often inherent to nanocrystals, while creating a material with macroscopic dimensions. One strategy to form such gels is to mediate attractions between the nanocrystals via a linker species, e.g., a cation that is coordinated by negatively charged ligands. A variety of low-density gels have been formed experimentally from nanocrystals using this strategy, but how to control the physical properties of these gels is not well understood in general. Using a combination of theoretical calculations and Monte Carlo simulations, we investigate gelation in a system where the attractions are provided by an intermediate linker species, focusing on elucidating the effect of varying (1) linker concentration, and (2) the effective interactions between nanocrystals. We connect the underlying phase behavior of the linker-mediated nanocrystalline system to the resulting properties of the gel, and we compare to other systems known to form low-density gels.