Formation of cobalt sulfide hollow nanocrystals through a mechanism similar to the Kirkendall Effect has been investigated in detail. It was found that performing the reaction at >120oC leads to fast formation of a single void ins ide each shell, whereas at room temperature multiple voids are formed within each shell, which can be attributed to strongly temperature-dependent diffusivities for vacancies. The void formation process is dominated by outward diffusion of cobalt cations; still, significant inward transport of sulfur anions can be inferred to occur as the final voids are smaller in diameter than the original cobalt nanocrystals. Comparison of volume distributions for initial and final nanostructures indicates excess apparent volume in shells implying significant porosity and/or a defective structure. Indirect evidence for shells to fracture during growth at lower temperatures was observed in shell size statistics and TEM of as-grown shells. An idealized model of the diffusional process imposes two minimal requirements on material parameters for shell growth to be obtainable within a specific synthetic system.

Lattice mismatch strains are widely known to control nanoscale pattern formation in heteroepitaxy, but such effects have not been exploited in colloidal nanocrystal growth. We demonstrate a colloidal route to synthesizing CdS-Ag2S nanorod superlattices through partial cation exchange. Strain induces the spontaneous formation of periodic structures. Ab initio calculations of the interfacial energy and modeling of strain energies show that these forces drive the self-organization. The nanorod superlattices exhibit high stability against ripening and phase mixing. These materials are tunable near-infrared emitters with potential applications as nanometer-scale optoelectronic devices.