3D Macroscale Metal Chalcogenide Architectures from the Assembly of Nanoscale Objects—The Intersection of Form and Function

The oxidative assembly of metal chalcogenide nanoparticles into three-dimensional gel architectures has been shown to be an effective strategy to enable facile electronic and charge transport between particles (matter network interfaces), and interactions between the complex particle network and the surroundings (matter-pore network interfaces) for applications including catalysis, sensing, and sorption. The ability to design functional architectures is predicated on a deep understanding of how the native chemical, structural and morphological features of nanoparticle components dictate the complex structural and functional properties of the resultant gel networks. Previously, we have shown that the kinetics of oxidative assembly depend on a range of parameters including nanocrystal size, concentration, surface ligand characteristics, oxidant concentration, the redox thermodynamics of the chalcogenides, and the solubility of metal ions. Notably, the crystal structure has a profound influence on the kinetics of oxidative assembly and the characteristics of the resultant gel network, with hexagonal polymorphs (such as wurtzite CdSe) undergoing rapid assembly whereas cubic polymorphs (e.g., zinc blende CdSe) are quite slow. We hypothesize that the structure-based kinetic and morphological differences arise from differences in the facet energies and/or polarity. To test this premise, we are investigating the assembly of anisotropic CdSe nanoparticles with well-defined facets and probing the kinetics and resultant gel morphologies as a function of facet type and prevalence within the structure. Our preliminary studies show that wurtzite CdSe nanorods with polar facets on the tips assemble preferentially into open structures by tip-to-tip joins, and at a significantly larger rate than for CdSe nanorice, which have no polar facets (and also form compact structures). However, our understanding of the driving forces for assembly and the interfacial chemistry between the nanocrystals during the growth of the network is limited, as are the effects on the functional behavior of the resultant network. In this presentation, using a combination of in situ liquid-phase and static electron microscopies, time-dependent dynamic light scattering, and graph theoretical analyses, we will describe how the native features of the nanoparticle components, including facet energy and polarity, dictate the structure of the resultant networks, and discuss implications for design of functional materials.