Photo-induced Charge Carrier Dynamics in Artificial Atoms, Molecules, and Solids Studied by Transient Absorption Spectroscopy Open Access

Yang, Ye (2013)

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Quantum confined semiconductor nanoparticles are referred to as artificial atoms due to atomic like energy levels. In analogy to molecules or solids, artificial molecules or solids can be constructed by covalent bonding or by close packing artificial atoms. Recently, these artificial atoms, molecules, and solids have attracted intensive interest in solar-to-electricity and solar-to-fuel applications. To further improve the solar energy conversion efficiency, the fundamental understanding of the dynamics of the photo-induced charge carriers in these artificial systems is required.

In this dissertation, we first investigated the multiple exciton generation (MEG) and dissociation in a model system consisted of PbS quantum dots and electron acceptors. We demonstrated that the presence of electron acceptors did not affect the MEG efficiency of QDs and all generated excitons could be dissociated by electron transfer to the acceptor, achieving MEG and multiple exciton dissociation efficiencies of 112%. We also demonstrated that these efficiencies were not affected by the charging of QDs.

We also studied the strong electronic coupling and hot electron transfer between PbS QDs and TiO2 nanocrystalline films. In this study, we reported that, due to the strong electronic coupling between 1S electron state and TiO2 conduction band, the 1S electron transfer time from PbS QDs to TiO2 films was estimated as 6.4 ± 0.4 fs. We further demonstrated that the 1P electron of the PbS QDs could inject to the TiO2 films coated by aluminum oxide layers prior to its relaxation to 1S level. The 1P electron injection yield is determined to be 18±4 %.

Finally, we studied the carrier transport and interfacial electron transfer in CdSe heterostructured tetrapod and CdSe QD solids. We find that the electrons in the tetrapod can quickly migrate from the branch to the core and the rate of this electron localization process is not affected by the presence of another electron in the core, suggesting an electron and hole coupled transport in one dimensional confinement. We also showed that photogenerated electrons in quantum dot solid electrodes could be transported to the surface to reduce methyl viologen with 100% quantum efficiency.

Table of Contents

Chapter 1. Introduction. 1
1.1. Ultrafast multiple exciton dissociation and hot electron extraction from PbS QDs. 2
1.2. Charge transport and interfacial transfer in CdSe tetrapods. 5
1.3. Charge transport and interfacial transfer in CdSe solids. 7
1.4. Summary. 8
Reference. 8
Chapter 2. Experimental Methods. 18
2.1. Sample synthesis. 18
2.1.1. Synthesis of PbS quantum dots and preparation of PbS/methylene blue complex. 18
2.1.2. Synthesis CdSe tetrapod nanocrystals. 19
2.1.3. Synthesis of CdSe Quantum dots and preparation of CdSe QD solids. 19
2.1.4. Synthesis of TiO2 nanoparticles and preparation of TiO2 nanoporous films. 20
2.2. Pump-probe transient absorption setup. 20
2.2.1. Femtosecond visible-NIR transient absorption setup. 20
2.2.2. Femtosecond mid-IR transient absorption setup. 22
2.2.3. Nanosecond visible transient absorption setup. 23
2.3. Three-pulse visible transient absorption setup. 23
2.4. Time resolved fluorescence setup. 24
Reference. 24
Chapter 3. Electron and Hole Dynamics in PbS Quantum Dot-Methylene Blue Complexes. 25
3.1. Introduction. 25
3.2. Results. 26
3.2.1. Energy levels and absorption spectra of PbS QD and PbS-MB complex. 27
3.2.2. Charge separation and recombination measured by visible-NIR TA. 28
3.2.3. Charge separation and recombination measured by mid-IR TA. 31
3.3. Discussions. 33
3.4. Conclusion. 35
Reference. 35
Chapter 4. Multiple Exciton Generation and Dissociation in PbS Quantum Dot-Electron Acceptor Complexes 38
4.1. Introduction. 38
4.2. Results and discussions. 40
4.2.1. Steady-state and Transient Absorption Spectra of PbS QDs. 41
4.2.2. Multiple Exciton Dynamics in PbS QDs. 41
4.2.3. Single Exciton Dissociation in PbS-MB+. 50
4.2.4. MEG and MED in PbS-MB+ complexes. 51
4.2.5. Effect of the charged PbS-MB+ complexes. 56
4.3. Conclusion. 56
Reference. 57
Appendix 1. 61
Chapter 5. Strong Electronic Coupling and Ultrafast Electron Transfer between PbS Quantum Dots and TiO2 Nanocrystalline. 68
5.1. Introduction. 68
5.2. Results and discussions. 71
5.2.1. Exciton band broadening in absorption spectra. 71
5.2.2. Visible-NIR transient absorption measurements. 74
5.2.3. Mid-IR transient absorption measurements. 76
5.2.4. Strong electronic coupling and adiabatic electron transfer. 79
5.3 Conclusion. 84
References. 85
Chapter 6. Efficient Room Temperature Hot Electron Transfer from PbS QDs to Nanocrystalline Oxide Films 90
6.1.Introduction. 90
6.2. Results and discussions. 92
6.2.1. Hot exciton induced TA in free PbS QDs. 92
6.2.2. 1S electron injection in PbS QDs/ Al2O3/TiO2. 96
6.2.3. 1P electron injection in PbS QDs/ Al2O3/TiO2. 97
6.2.4. Hot electron injection from high energy level (higher than 1P) of PbS to TiO2 100
6.3. Conclusion. 103
Reference. 103
Appendix 1. 107
Chapter 7. Coupled Electron and Hole Transport in CdSe Tetrapod Nanocrystals. 109
7.1. Introduction. 109
7.2 Results and discussions. 112
7.2.1. Band alignment and electronic transitions in CdSe TPs. 112
7.2.2. Three-pulse and two-pulse TA measurements. 113
7.2.3. Association of electron and hole transport. 118
7.3. Conclusion. 119
Reference. 120
Appendix 1. 123
Appendix 2. 124
Appendix 3. 126
Chapter 8. Bulk Transport and Interfacial Transfer Dynamics of Photogenerated Carriers in CdSe Quantum Dot Solid Electrodes. 127
8.1. Introduction. 127
8.2. Results and discussions. 130
8.2.1. Absorption spectrum, exciton band broadening and solid thickness. 130
8.2.2. TA measurements of QD solid and QD solid in MV2+ solution. 131
8.2.3. Electron transfer to MV2+: Hot electron transfer model vs. 1S electron transfer model. 137
8.2.3. Electron and hole transport and interfacial transfer rate in Solid/MV2+. 139
8.2.3. QD solid thickness dependent electron transport rate. 141
8.3. Conclusion. 145
Reference. 145
Appendix 1 149

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