Process Development for Scalable Templated Synthesis of Compund Semiconductor Nanocrystals

Semiconductor nanocrystals, or quantum dots (QDs), are interesting nanomaterials whose size-dependent, tunable optical and electronic properties make them ideal for applications in biological sensing and imaging, light-emitting devices, displays, and solar cells. The commercial exploitation of these materials requires the development of synthesis techniques that are scalable, economical, and environmentally friendly, while enabling precise control of the size, shape and size distribution of the nanocrystals. The most common synthesis technique for these nanocrystals employs small batch reactors in which nanocrystals grow as a function of time following a rapid injection of organometallic precursors into a hot coordinating solvent. The limitations of this process for large-scale commercial exploitation stem from the incomplete mixing of the precursors in large batches that can lead to non-uniform nucleation and broad particle size distributions. Limitations also include the high cost, flammability, and toxicity of the organometallic precursors and its operator-intensive nature. Templated synthesis techniques for nanocrystals have distinct advantages over other methods, including more precise control of particle size, shape, and size distribution and easier scalability for commercial applications. This thesis presents the templated synthesis of semiconducting nanocrystals in stable microemulsions and liquid crystals, formed by the self-assembly of an amphiphilic block copolymer in the presence of a polar and non-polar solvent. The work of this thesis investigates microemulsion templates for the scalable synthesis of semiconductor nanocrystals including: materials composition and particle size control, continuous production of nanocrystals, improvement of optical properties, and alternative non-toxic reactants. The nanocrystals were formed by reacting a group-II salt dissolved in the dispersed phase of the template with a group-VI hydride gas inside the nanodomains. The versatility of nanomaterials and precision of size control of this synthesis method were demonstrated by adjusting the metal salt composition and concentration. The scalability of this technique was displayed by developing a counter-current flow, packed-bed reactor for continuous synthesis of nanocrystals in templating microemulsions. Limitations of the optical properties of nanoparticles synthesized with microemulsion template were addressed by post-processing techniques including extraction and functionalization of the nanocrystals, annealing, and overcoating the quantum dots with an inorganic shell to optimize fluorescence emission and quantum yield. This post-process annealing allowed for the investigation of Mn-dopant incorporation and expulsion from the ZnSe nanocrystal. To eliminate the toxic and flammable group-VI hydride gases, a microwave-assisted templated synthesis route was developed. This employed bursts of microwaves to selectively heat the aqueous, dispersed droplets of water-in-oil microemulsions that contain the water-soluble precursors of the group-II and VI elements, thus leading to nucleation and formation of a single nanocrystal inside each nanodomain