Design and assembly of metal oxide nanocrystal gels via depletion attractions

Achieving and implementing macroscopic materials capable of displaying the unique properties inherent to inorganic nanocrystals requires bridging the nanoscale and the length scales of larger orders of magnitude in a systematic, controllable, and scalable way. Over the last 15 years, nanocrystal gels have been developed and investigated as potential materials to tackle this need. However, available gelation methods rely on chemical reactions and interactions specific to the stabilizing molecules on the nanocrystal surface, and are therefore not readily adaptable across broad types of materials. Specifically, most studies have focused on gelation methods for metal chalcogenides and noble metals, whereas progress on metal oxide nanocrystal gels has lagged behind. This dissertation investigates and demonstrates an alternative gelation method based on entropic depletion attractions that are not dependent on specific surface chemistries and have not been explored in nanoscale colloidal gels. In the first study, a proof of concept system is developed, where depletion attractions induce the gelation of tin-doped indium oxide nanocrystals in the presence of a polymer depletant and achieve a macroscopic material with optical properties reflective of both the microstructure and the nanostructured building blocks. The mechanism of gelation is assessed by comparing the observed phase behavior to theoretical predictions and the microstructure is characterized by small-angle X-ray scattering (SAXS). Next, the universal applicability of depletion attractions is demonstrated by varying the composition and shape of the building blocks while fixing size and nanocrystal volume fraction. The gelation of spherical nanocrystals occurs at the same depletant concentration and this phase transition threshold does not depend on the specific composition of the metal oxide nanocrystal. Consistent with theoretical phase boundary calculations, cubic nanocrystals form gels at a lower depletant concentration than spherical nanocrystals due to the ability to pack face-to-face and therefore increase the overlap excluded volume during assembly. Finally, a method to polymer-wrap tin-doped indium oxide nanocrystals in a controllable way while maintaining colloidal stability is investigated in an effort to tune the physicochemical properties of the metal oxide building blocks available for gelation.