Fluorescence Quenching in single CdSe/CdS Dot-In-Rod Nanostructures using Modified Atomic Force Microscopy tips Open Access

Abstract

The main objective of this thesis is to study the quenching of fluorescence in single CdSe/CdS dot-in-rod (DIR) nanostructures via electron transfer (ET) using modified AFM tips. AFM tips where either modified with TiO2 nanoparticles (NP), or commercially available Pt AFM tips were used for this study. Electron Transfer from DIR to TiO2 or Pt was shown, by measuring the quenching in photoluminescence (PL) intensities and lifetimes of single CdSe/CdS DIRs. It was seen that the fluorescence quenching is uniform along the length of the DIR, suggesting the ET process from DIR to TiO2 NP or Pt is the rate-determining step and dominates over ionization, diffusion, and recombination rates. Cross-section dependent study showed that quenching occurs only within the tip radius in contact with the DIR and is dependent on the tip material and geometry. Vertical tip-DIR separation study showed that the quenching can occur only when the tip is in close contact with the DIR. Control experiments with regular Si AFM tips showed that quenching can occur only when an electron acceptor like TiO2 or Pt is attached to the AFM tip, suggesting charge transfer to be occurring.

Table of Contents

Chapter 1 Introduction. 1

1.1 Theory of Fluorescence. 1

1.2 Fluorescence Lifetime and Quantum Yield. 3

1.3 Semiconductor Nanoparticles. 4

1.4 From bulk CdSe to quantum confined nanocrystals. 5

1.5 Relaxation process in Semiconductor Nanocrystals: Non-radiative and radiative transitions. 7

1.5.1 Non-radiative relaxations. 7

1.5.2 Radiative relaxation or Fluorescence. 9

1.6 Core/Shell Nanocrystals. 11

1.7 CdSe/CdS dot-in-rods. 13

1.8 CdSe/CdS DIRs coupled with metal nanocrystals. 15

1.9 Fluorescence Quenching. 16

Chapter 2 Experimental Setup and Sample Preparation. 20

2.1 Synthesis of CdSe/CdS DIRs. 20

2.2 Synthesis of TiO2 Nanoparticles. 20

2.3 Atomic Force Microscopy (AFM) and Tip Functionalization (single TiO2 NP attachment to an AFM Tip). 21

2.4 Single Molecule Fluorescence Spectroscopy. 22

2.5 Time-Correlated Single Photon Counting (TCSPC) Technique. 24

2.6 Fluorescence Intensity and Lifetime Measurements. 25

2.7 Fluorescence Intermittency or Blinking. 26

2.8 Using Single Molecule Fluorescence to study interfacial ET. 29

Chapter 3 Results and Discussion. 32

Summary and Future Outlook. 41

References. 42

About this Master's Thesis

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School	Laney Graduate School
Department	Chemistry
Degree	MS
Submission	Master's Thesis
Language	English
Research Field	Chemistry, Physical
Keyword	Single particle fluorescence AFM Fluorescence Quenching
Committee Chair / Thesis Advisor	Lian, Tim, Emory University
Committee Members	Widicus Weaver, Susanna, Emory University Evangelista, Francesco , Emory University

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