Surface Engineering of Colloidal Group IV Nanocrystals for Optoelectronics

Abstract

Colloidal nanocrystals (NCs), often synonymous with"quantum dots," represent a burgeoning class of next-generation optoelectronic materials. The promise of NCs is twofold: (i) Their optical properties are tunable and offer unique opportunities for enhanced energy conversion due to quantum confinement effects. (ii) The NCs can be processed into thin films using cost-efficient roll-to-roll printing techniques for large-scale integration into devices. Taken together, these two attributes enable a new platform for optoelectronic technology where energy-efficient devices can be produced at low costs. There is an array of research efforts to produce NC-based optoelectronic devices such as photovoltaic cells, light emitting devices, and photodetectors. Much of the recent progress in this direction hinges on the ability to manipulate the NC surface. Conventional solution synthesis yields NCs with ligands bound to metal surface atoms through a labile acid-base complex. The electrically-insulating native ligands are thus routinely exchanged to produce conductive NC arrays for devices integration. Just as surface manipulation has launched metal-based NCs to the forefront of optoelectronic technology, it is the inability to do so with the covalent surface of group IV (germanium and silicon) NCs that has greatly hindered progress. The motivation of this research is to bridge the gap between group IV and metal-based NCs in order to establish an abundant, non-toxic alternative to NCs that contain toxic lead or cadmium. The bridge is built by developing new Si NC surface chemistries, understanding how they interact with molecules, and applying chemical and physical models to uncover the mechanism of NC colloidal stability. The research begins by developing nonthermal plasma synthesis of Si NCs from a new precursor, silicon tetrachloride. This work builds on previous studies on chlorine-terminated germanium NCs synthesized from germanium tetrachloride, which were observed to form stable colloids without covalent ligand attachment. Synthesis from silicon tetrachloride offers the same flexibility for tuning size and crystallinity as typical silane synthesis but yields a new chlorinated surface chemistry. Si-Cl surface groups of the NCs are shown to be crucial for achieving the same colloidal stability observed in Ge NCs. It was determined spectroscopically the polarized Si-Cl surface bond renders the surface Si atoms Lewis acidic and capable of hypervalent interactions with Lewis basic molecules. The NCs were thus dispersible in select Lewis basic solvents. Interestingly, these interactions are also shown to be responsible for a reversible "surface doping" effect, which was also explored spectroscopically and by electrical characterization of a thin film device. The notion of a Lewis acidic surface gave rise to the development of a more robust Si NC surface chemistry. In this work, plasma synthesis that includes diborane is applied. The resulting Si NC surface is then terminated by a classic Lewis acid, boron, which is demonstrated to be an even more versatile chemistry than the Si-Cl surface. These NCs are also used as a model system for uncovering the mechanism of colloidal stability due to these surface interactions with solvent molecules. It is found that conventional theory cannot account for the stability observed, and a simple alternative model is developed. In light of this model, we are able to demonstrate stable Si NC colloids in media that spans hexane to water. The thesis concludes with a peripheral effort on Ge NCs, a material lacking in maturity even to Si NCs. In this work, the NC surface is modified to enhance the optical properties of the material as opposed to the ability to process the NCs into films from solution. Size-tunable band gap emission is demonstrated for the first time in gas-phase synthesized Ge NCs by applying Grignard chemistry to the Ge-Cl surface groups. The emission is narrower than any previous report, and emission near the bulk band gap of Ge is attained for the first time.