Tuning the Photoluminescence Properties of Indium Phosphide Quantum Dots Through Atomistic Surface Chemistry

Abstract

Colloidal semiconducting nanocrystals, also known as quantum dots, are a leading class of materials for optoelectronic devices that is shaping the technologies of the future. Quantum dots are 2-20 nm in diameter and get their name from the quantum confinement effect, where their size dictates their absorbance and emission wavelengths. The high surface to volume ratio of quantum dots requires that the surface chemistry be carefully controlled to access the desired optoelectronic properties. The solution processability, together with tunable size, shape, and chemical environment of quantum dots opens vast possibilities for surface chemistry and opportunities for prescriptive design through deeper understanding of structure-function relationships. This dissertation focuses on using atomistic surface chemistry of indium phosphide quantum dots (InP QDs) to improve their photoluminescence properties, including brightness, linewidth, and stability. Chapter 1 connects the surface chemistry of colloidal quantum dots with their luminescence properties. By following the historical arc that began with shell growth and led to an atomistic description of surface-derived charge trapping, the role of surface chemistry in luminescence properties is presented with an outlook toward emerging concepts such as surface dipoles and vibronic coupling. Chapter 2 discusses tuning of the InP core quantum dot stoichiometry and explores the impacts on the photoluminescence properties of an InP/ZnSe core/shell system. The stoichiometry of both anions (P, As, S, Se) and cations (In, Zn) was controlled at the InP/ZnSe core/shell interface and correlated with the resultant steady-state and time-resolved optical properties. Cluster-model density functional theory supported our experimental findings with the implication of the electronic defect states arising from the interfacial anions. Chapter 3 presents a novel, colloidal synthesis of metal oxide shells on InP QDs and their use as an interface within emissive core/shell heterostructures. Computational modeling was used to explore the optoelectronic properties of a set of metal oxide shelled InP QDs. Characterization techniques such as 1H NMR, X-ray emission spectroscopy (XES), X-ray diffraction (XRD), extended X-ray absorption fine structure spectroscopy (EXAFS) showed evidence of bulk and local structures of the metal oxide shells, and their impact on the final emission properties of InP/ZnSe core/shell QDs was examined.