UC Irvine

Quantum dots (QDs) are semiconductor nanocrystals with optical and electronic properties that can be manipulated through subtle changes to their size, shape, and composition. This remarkable tunability—coupled with their ease of manufacturing and low cost—makes them promising building blocks for emerging and next-generation semiconductor, optoelectronics, and energy conversion devices. QDs are already proving their utility and commercial viability in modern television displays, which use QDs to expand their color gamut beyond that of traditional technologies. However, many impactful applications (e.g., solar cells, cameras, machine vision, quantum information systems, illumination systems) of QD thin films are precluded by their poor charge transport properties. This thesis is broadly focused on improving charge transport in QD thin films. Towards achieving this goal, we focused on the development and characterization of an epitaxially-fused QD superlattice (epi-SL), which is a periodic assembly of QDs in which the constituent QDs are crystallographically aligned and epitaxially connected to form a porous single-crystal. Epi-SLs are exciting because of their predicted ability to combine the electronic and optical tunability of individual QDs with the efficient charge transport of bulk semiconductors through the emergence of collective, delocalized electronic states. This thesis details the conversion of colloidal QDs into electronic devices composed of individual epi-SL grains, establishing foundational insight into key aspects of epi-SL synthesis, chemistry, structure, and electronic behavior. The first chapter provides a broad introduction to QDs and QD-based devices. In the second chapter, multi-modal structural analysis reveals how colloidal PbSe QDs assemble into a 3-D epi-SL. The structural metamorphosis occurs with the help of a critical intermediate parent superlattice (SL) phase, which converts into the epi-SL through an impressive choreography of QD translation and rotation. Chemical analysis of parent and epi-SL phases reveals the underlying chemical processes that initiate the SL phase transformation. In the third chapter, these insights are leveraged to make epi-SLs using UV light (rather than injection of a chemical) to trigger the SL phase transformation. The final chapter of this work contains a demonstration of, for the first time, single-grain epi-SL field-effect transistors. Individual epi-SL grains are deterministically integrated into optoelectronic devices of arbitrary geometry and architecture (although here we focus on field-effect transistors) by way of a novel nanofabrication process. Single-grain epi-SL transistors show hole mobilities approaching 10 cm2/Vs, which represents a ~10x improvement in charge carrier mobility over previously reported PbX QD solids. Variable-temperature transistor measurements reveal several interesting electronic phenomena that warrant future study.