Interfacial Electron Transfer Dynamics from Single Molecules and Quantum Dots Studied by Single-Molecule Fluorescence Spectroscopy Open Access

Jin, Shengye (2010)

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Interfacial Electron Transfer Dynamics from Single Molecules and Quantum Dots Studied by Single-Molecule Fluorescence Spectroscopy

By Shengye Jin

Interfacial electron transfer (ET) dynamics from single organic molecules and quantum dots (QDs) to semiconductor nanocrystalline thin films or molecular electron acceptors have been studied by using single-molecule fluorescence spectroscopy.

The photoinduced interfacial ET dynamics in sulforhodamine B (SRhB)-aminosilane-TiO2/SnO2 nanoparticle (donor-bridge-acceptor) complexes have been studied on the single molecule level. The presence of the silane bridge enabled the complete sampling of these molecules under single molecule conditions. Shorter fluorescence lifetimes for molecules on TiO2 and SnO2 compared to on ZrO2 were observed and attributed to ET from SRhB to TiO2 and SnO2. The single molecule fluorescence lifetimes fluctuated with time and varied among single donor-bridge-acceptor complexes, suggesting that both static and dynamic ET rate distributions contribute to the heterogeneity of ET in this system.

The ET dynamics from single CdSe core/multi-shell QDs to adsorbed Fluorescein (F27) molecules and to TiO2 nanoparticles have also been studied by single particle spectroscopy. The QDs in both systems showed intermittent ET dynamics modulated by their blinking activities. The excited state lifetime of QDs in the on state reflected the ET rate, whereas in the off state, QD excitons decayed by fast non-radiative Auger relaxation. Furthermore, interfacial ET provided an additional pathway for generating off states, leading to the intermittent ET dynamics. The ET dynamics from single QDs to TiO2 rutile and single crystals were also studied. The ET rates for QDs on rutile (110) and (001) were found to differ, probably due to the different surface structures of these two single crystals.

The exciton quenching dynamics of QDs on ITO were also studied and compared with results for QDs on In2O3 and glass. Single QDs on ITO showed suppressed blinking activities and reduced fluorescence lifetimes, which was attributed to negative charging of the QDs. In these negatively charged QDs, the off states were suppressed due to the effective removal of the valence band holes, and their fluorescence lifetimes were shortened because of Auger relaxation processes involving the additional electrons.

Table of Contents

Table of Contents
Chapter 1. Introduction...1

1.1 Single-Molecule Fluorescence Spectroscopy and Interfacial Electron Transfer...1

1.1.1 Single-Molecule Fluorescence Spectroscopy...2
1.1.2 Using SMFS to study interfacial ET...6

1.2 Interfacial Electron Transfer from Single Molecules...4
1.3 Interfacial Electron Transfer from Single QDs...8

1.3.1 Introduction...8
1.3.2 ET from QD to molecular adsorbates...11
1.3.3 ET from QD to semiconductor oxides...12
1.3.4 Fluorescence and ET dynamics of QDs on conducting materials...13

1.4 Summary...14

Chapter 2. Experimental Section...28

2.1 Preparation of Semiconductor Colloid and Nanocrystalline Thin Films...28

2.1.1 Preparation of SnO2 Colloid and Thin Films...28
2.1.2 Preparation of TiO2 Colloid and Thin Films...30
2.1.3 Preparation of ZnO Colloid and Thin Films...31
2.1.4 Preparation of ZrO2 Colloid and Thin Films...31
2.1.5 Preparation of In2O3 Colloid and Thin Films...32

2.2 Fabrication of Electron Donor-Bridge-Acceptor Complex...32

2.2.1 Synthesis of Silane Conjugated Sulforhodamine B...32
2.2.2 Fabrication of Electron Donor-Bridge-Acceptor Complex...34

2.3 Fabrication of QD ET complexes...35

2.3.1 QD-F27 Complexes...35
2.3.2 QD-TiO2 Nanoparticle Complexes...36
2.3.3 QD-In2O3 and ITO Complexes...37

2.4 Single-Molecule Fluorescence Spectroscopy...37

2.4.1 Time-Correlated Single Photon Counting (TCSPC) Technique...37
2.4.2 Single-Molecule Detection...40


Chapter 3. Single Molecule Interfacial Electron Transfer Dynamics in Donor-Bridge-Nanoparticle Acceptor complexes...44

3.1 Introduction...44
3.2 Results and Discussion...48

3.2.1 Ensemble-Averaged Electron Transfer Dynamics...48
3.2.2 Wide Field Imaging of Single Molecules...54
3.2.3 Single-Molecule Dynamics on ZrO2...57
3.2.4 Single-Molecule Dynamics on SnO2 and TiO2...62
3.2.5 Distributions of ET Rates on SnO2 and TiO2...67
3.2.6 Computational Modeling of Interfacial ET...69

3.3 Summary...78

Chapter 4. Interfacial Electron Transfer Dynamics from Single QDs to Adsorbed F27 Molecules...88

4.1 Introduction...88
4.2 Results and Discussion...90

4.2.1 Ensemble-Averaged ET Dynamics...90
4.2.2 Single QD ET Dynamics...99
4.2.3 Correlation between Single QD Emission and Fluorescence Intensity...109
4.2.4 Comparison of Ensemble and Single QDs ET dynamics...112
4.2.5 Single QD Blinking and Interfacial ET Model...115

4.3 Summary...117

Chapter 5. Interfacial Electron Transfer Dynamics from Single Quantum Dots to TiO2 nanoparticles and rutile single crystals...124

5.1 Interfacial Electron Transfer from Single QDs to TiO2 Nanoparticles...124

5.1.1 Introduction...124
5.1.2 Results and Discussion...125 Single QD Dynamics...126 Comparison of Ensemble and Single QDs ET Dynamics...134 ET Rate Distribution on TiO2...140 Single QDs ET Model on TiO2...142

5.1.3 Summary...143

5.2 Electron Transfer Dynamics of Single Quantum Dots on The (110 and 001) Surfaces of Rutile TiO2 Single Crystals...144

5.2.1 Introduction...144
5.2.2 Results and Discussion...146
5.2.3 Summary and Future Work...153


Chapter 6. Exciton Quenching and Suppressed Blinking Dynamics of Single Quantum Dots on Tin Doped Indium Oxidize...161

6.1 Introduction...161
6.2 Results...163

6.2.1 Suppressed Blinking Dynamics...165
6.2.2 Exciton Quenching Dynamics...168

6.3 Discussion...173

6.3.1 Charging of QDs on ITO...173
6.3.2 Fluorescence and Lifetime Dynamics in Charged QDs...176

6.4 Summary...179
Appendix 1...184
Appendix 2...187

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