Date of Completion

4-1-2020

Embargo Period

3-27-2021

Keywords

Quantum dots, semiconductor nanocrystals, tetrapods, blinking, nanoparticle mercury interaction

Major Advisor

Dr. Jing Zhao

Associate Advisor

Dr. Yao Lin

Associate Advisor

Dr. Jie He

Field of Study

Chemistry

Degree

Doctor of Philosophy

Open Access

Open Access

Abstract

The study in this dissertation mainly focused on semiconductor nanocrystals ranging from controllable synthesis of II-VI quantum dots in both organic phase and aqueous phase, to their optical property and potential environmental impact. Besides, our study of fluorescent nanomaterials was extended from semiconductor nanocrystals to another system, carbon dots. In the first project, an unconventional method to induce anisotropic growth of CdS arms on CdSe/CdS seeds was developed to form nanotetrapods. Our control experiments suggested the key role of CdS shell thickness of CdSe/CdS seeds and concentration of precursor for CdS growth in the growth of nanotetrapods. Our results show that the branching came from a zinc blende phase formation at the slow growing end of c-axis of wurtzite CdSe/CdS seeds due to the enrichment of stacking fault. The second work focused on the study of blinking properties of CdSe/CdS core/shell QDs with various morphology, surface capping ligands, and shell growth chemistry. Outstanding blinking suppression has been observed with hexagonal pyramid and bipyramid QDs, especially the bipyramid QDs. Besides, there are more off-state events observed with QDs coated with hydrophilic ligands, while the quantum yield was slightly decreased after ligand exchange reaction. Additionally, QDs prepared with the newly developed shell growth chemistry have been observed with lower on time fraction and an effect of core size on the blinking properties. The third work focused on the study on how Hg interacts with nanoparticles using aqueous CdSe nanoparticles capped with l-cysteine as a model system. The fluorescent nature of CdSe nanoparticles allows us to study the interaction of Hg ion with CdSe nanoparticles via photoluminescence spectra and photoluminescence decay. Results of ICP-MS measurements of CdSe NPs and the separated solution obtained after addition of Hg ions excluded the cation exchange of Cd2+ by Hg2+ except at very high Hg to Cd ratio. XPS further suggested that the Hg was bound to the amine and carboxylate group of l-cysteine capping agent on the nanoparticle surface. The fourth project is an experimental study of fluorescent carbon nanodots on their photoluminescence response to external electric field. Fluorescent carbon dots were synthesized using procedures modified from oil phase synthesis of QDs with citric acid as the reactants. The carbon nanodots showed remarkable excitation dependent emission properties. After encapsulated in artificial lipid bilayers, the carbon dots showed competitive sensitivity of voltage compared to the most used commercial membrane potential sensing dyes, which indicates great potential of carbon dots as alternative voltage sensing probes.

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