Exploring the Indium Phosphide Landscape: Precursor and Solvent Effects on Nanocrystal Size, Morphology, and Crystallization Temperature

Abstract

Understanding the synthesis of colloidal quantum dots (QDs) allows for precise control over desired properties such as emission wavelength, morphology, phase, or monodispersity. InP is a crucial QD material due to its direct bandgap tunable across the visible and near-infrared wavelengths, strong absorbance coefficient, and lack of restricted heavy metals. Important developments in colloidal InP synthesis include size control through secondary injections, high photoluminescence quantum yield through surface passivation, and discovery of atomically-precise clusters. However, challenges remain including the lack of mechanisms for precursor reactivity-based size control, minimal routes to anisotropic morphologies, and high temperatures required for crystallization. The studies presented herein address these challenges by taking inspiration from how nature controls ionic strength for facile crystallization, nucleation with organized cation-binding moieties, and crystallization with confined spaces. Chapter 1 introduces colloidal nanocrystal nucleation and growth, and details how biomineralization can inspire solutions for outstanding InP synthetic challenges. Chapter 2 explores InP QD synthesis in polar solvent environments. Polar solvents are hypothesized to lower the crystallization temperature of InP by imparting ionicity or stabilizing charged reaction species. High-throughput experimentation identifies key factors for low temperature growth as increased percentage of polar solvent, carboxylic acid choice, and inclusion of polar additives. Guided by these insights, high-quality, crystalline InP QDs were synthesized at 60 °C using a toluene-dimethylformamide mixture, and exhibited a photoluminescence quantum yield of 25% after HF treatment. Chapter 3 demonstrates indium precursor reactivity-based size control for InP QDs using aminophosphine chemistry. Increasing the equivalents of metal-chelating aminopolycarboxylic acid EDTA ([CH2N(CH2CO2H)2]2) was found to decrease the final diameter of InP QDs from 4.5 to 2.3 nm by lowering the initial InP growth rate. This size trend was rationalized by invoking a continuous nucleation model in which suppressed initial growth rates are attributed to a competitive decrease in reactivity caused by indium-EDTA complexation and a lower effective concentration. Chapter 4 presents the synthesis of zinc blende InP nanowires made using indium tris(trifluoroacetate) and tris(diethylaminophosphine). Transmission electron microscopy and nuclear magnetic resonance spectroscopy identified a solution-liquid-solid growth mechanism using in situ-formed indium metal nanoparticles with the aminophosphine as the reducing agent.