Date of Graduation

12-2016

Document Type

Dissertation

Degree Name

Doctor of Philosophy in Chemistry (PhD)

Degree Level

Graduate

Department

Chemistry & Biochemistry

Advisor

Colin D. Heyes

Committee Member

Neil T. Allison

Second Committee Member

Nan Zheng

Third Committee Member

Jingyi Chen

Fourth Committee Member

Feng Wang

Keywords

Pure sciences; Blinking; Exciton decay; Fluorescence spectroscopy; Grey state; Nanocrystals (quantam dots); Quantam yield

Abstract

Colloidal semiconductor nanocrystals (quantum dots, QDs) have received much attention in recent years due to their uniquely size-tunable properties leading to a number of promising applications. Some of their most popular applications include their use as fluorescent probes in biology, as electro-optical components and in photovoltaic devices. CdSe-based QDs are particularly important because of their ease of synthesis, high photoluminescence quantum yields (PL QYs) across the whole visible spectrum and their photostabilty. Shelling of core QDs is usually carried out to improve their optical properties, minimize outer environmental effects on their properties, and avoid toxic element exposure to the environment. However, choosing the shell composition is not trivial, since the band-edge energy offset, interfacial lattice mismatch, shell thickness and chemical stability all play roles in influencing the optical properties. Interfacial lattice strain can be alleviated by either forming multi-shells or gradient-alloyed shells, but this comes at the expense of reducing charge carrier confinement. However, a comprehensive model to decide which shell configuration is best is not yet available. In this dissertation, a systematic comprehensive study of CdSe-based core/multi-shells and core/gradient-alloyed-shells is carried out in terms of their PL QYs, various blinking states and multiple radiative and non-radiative exciton decay rates. The experimental results for the ensemble and single particle optical properties for the different core-multishell QDs proves that the ensemble quantum yield is not a good indicator for single QD blinking. The exciton decay pathways in terms of radiative and non-radiative decay for different core-multishell architectures are shown to be strongly influenced by the lattice strain and band edge confinement. These studies were then extended to the study of multiple fluorescence intensity levels in single QDs as a function of the various shells using a range of time-resolved fluorescence spectroscopies. From this data, a mechanistic model showing various physical transitions was proposed. Through a systematic, quantitative study, this dissertation highlights the factors of both lattice strain and band edge confinement potential in controlling exciton decay that is needed to design and synthesize QDs to reach their full potential in a range of future applications.

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