Synthesis, Surface Functionalization, and Strain-Engineering of Luminescent Inorganic Nanostructures

Abstract

Quantum dots (QDs) are a class of luminescent materials with nanoscale dimensions and unique optical and photoluminescent properties. Due to the effects of quantum confinement, size-dependent changes to the electronic structure cause the QD band gap to increase in energy as the nanocrystal diameter decreases, resulting in size-tunable absorbance and photoluminescence characteristics. Their tunability and ultra-small size make QDs advantageous for a wide array of applications, including energy conversion, photodetection, biological imaging, and display technologies. Notably, nanocrystals with decreased toxicity and more widely accessible synthetic protocols are two areas that could lead to increases in the availability and continued commercial implementation of quantum dots. This dissertation describes investigations related to the impact of surface functionalization on nanocrystal toxicity, explores strain-engineering strategies in spherical quantum well (SQW) heterostructures, and presents alternate nanocrystal synthesis pathways that may lead toward greater accessibility. First, cadmium selenide / cadmium sulfide (CdSe/CdS) core-shell QDs are investigated as a model system to understand the impact that the surface functionalization has on toxicity, stability, and cellular uptake in biologically relevant fluids and mouse models. This study demonstrates that functionalizing the QDs with polyethylene glycol decreases toxicity, improves stability, and increases cellular uptake. Furthermore, in addition to surface chemistry, the QD concentration and type of biological model utilized play a large role in the observed QD behavior, which must be taken into consideration when choosing a model system to evaluate QDs for biological applications. In the next chapter, the sonochemical synthesis of CdSe QDs and magic-sized clusters (MSCs) is examined. The synthesis utilizes ultrasound to generate nanocrystals under atmospheric conditions without the need of a Schlenk line, specialized glassware, or heating equipment. In addition, when nanocrystal precursors are dissolved in a single-phase system and sonicated, ultra-small, white-light-emitting CdSe QDs were generated; however, when nanocrystal precursor were compartmentalized in emulsion droplets in a two-phase ethylene glycol system, atomically precise MSCs were produced. Moreover, since the nucleation and growth processes are entirely dependent on active sonication, this synthesis is highly controllable and can be turned on and off rapidly. The third chapter focuses on the generation of ZnS nanocrystals using a one-pot solvothermal synthesis with improved scalability relative to commonly used hot-injection syntheses. By utilizing a highly reactive sulfur precursor in combination with coordinating solvents that are liquid at room temperature, we demonstrate a highly reproducible ZnS nanocrystal synthesis with streamlined purification that consistently generates a high yield of monodisperse ZnS nanocrystals. The nanocrystal diameters can also be adjusted via the reaction temperature, allowing for straightforward tunability of the ZnS nanocrystal diameter and bandgap. Finally, the growth of zinc selenide/indium phosphide/zinc sulfide (ZnSe/InP/ZnS), ZnS/InP/ZnS, and ZnSe/InP/ZnSe core/shell/shell spherical quantum well (SQW) architectures were explored. This strain-engineering strategy has been utilized previously to generate high-quantum-yield cadmium chalcogenide SQWs with narrow emission linewidths and is a promising candidate for application in other semiconductor systems with lower toxicity. Herein, we investigate the impact that precursors and surface chemistry have on the successful epitaxial growth of InP onto zinc chalcogenide cores, while demonstrating the importance that symmetry, band alignment, and lattice match have on the photoluminescent characteristics of the resulting SQWs.