Spectroscopic studies and control of charge carrier dynamics in semiconductor nanocrystals Restricted; Files Only
He, Sheng (Summer 2024)
Abstract
Semiconductor nanocrystals (NCs) are promising in applications of photovoltaics, photocatalysis, light emitting, and quantum information, enabled by the quantum confinement on the charge carriers that can be fine-tuned by multifunctional synthesis in the nanometer scale. Fundamental understanding of the carrier dynamics in the NCs is crucial to their applications. However, the spectroscopic property of the charge carries in many semiconductor NCs are still not fully understood. In this dissertation, the spectroscopy signals of charge carriers in CdSe quantum dots (QDs) and CsPbBr3 NCs are studied using transient absorption (TA) and selective charge transfer. Novel approaches to control the charge carrier dynamics are explored, such as introducing crystal defects or dopants and introducing electrochemical charging.
The historically missing spectral signal of the valance band (VB) hole in prototypical CdSe QDs was revealed by removing the ultrafast hole trapping and the conduction band (CB) electron. The hole contribution to the exciton signal is only 22 % due to the hole population in low energy dark exciton states. In strongly confined perovskite NCs, the excited carrier activates the forbidden inter-band 1S-to-1P transitions due to polaron formation, which breaks the 1S and 1P wavefunction symmetry.
Introducing crystal defects and dopants are demonstrated to dramatically alter the charge transfer dynamics. Crystal defects in CsPbBr3 nanoplatelets form shallow electron traps (~ 40 meV) and cause slow electron trapping (~ 500 ps), which, however, slows down the electron transfer to surface oxidants by one order of magnitude. Cu-doped InP QDs show localized hole wavefunction with significant energy decrease of ~ 0.4 eV, which changes the hole transfer kinetics to various acceptors.
The study of triplet energy transfer (TET) from CdSe QDs to acetylene-bridged anthracenes demonstrated the potential of tuning the acceptors' wavefunction extension to control the energy transfer from QDs to distant acceptors. TET from QDs is also demonstrated to be applicable to sensitizing lanthanide ions.
Finally, colloidal QDs under electrochemical charging were studied using TA spectroscopy. Slow relaxation of higher energy electrons is revealed in charged QDs. It is expected that electrochemical charging of colloidal NCs is promising for studying charge transfer in multicarrier states.
Table of Contents
Chapter 1. Introduction 1
1.1 Electronic structures and optical transitions in semiconductor quantum dots 1
1.1.1 Quantum confinement and effective mass approximation 1
1.1.2 Exciton fine structures in CdSe QDs 4
1.1.3 Polarization dependence of band edge exciton transitions 8
1.2 Material and photophysical properties of metal halide perovskite 10
1.2.1 Material composite and crystal structures in metal halide perovskite 10
1.2.2 Electronic structure and exciton fine structure in lead halide perovskite nanocrystals. 13
1.2.3 Polaron formation in metal halide perovskite. 16
1.3 Charge transfer from quantum dots to molecular acceptors 18
1.3.1 Conventional Marcus electron transfer theory 18
1.3.2 Auger-assisted electron transfer from QD to molecules 22
1.4 References 24
Chapter 2. Experimental Methods 31
2.1 Sample preparation 31
2.1.1 Materials 31
2.1.2 Synthesis of CdSe quantum dots (QDs) and CdSe/CdS core/shell QDs 32
2.1.3 Preparation of QD-MV2+ complexes. 34
2.1.4 Synthesis of CsPbBr3 nanocrystals 34
2.1.5 Preparation of CsPbBr3 NC-acceptor complexes 36
2.1.6 Synthesis and surface passivation of CsPbBr3 nanoplatelets 36
2.1.7 Preparation of CsPbBr3 NPLA(E)-Acceptor complexes 37
2.1.8 Synthesis of InP@ZnSe QDs and Cu doping 37
2.1.9 Preparation of InP@ZnSe QD-acceptor complexes 38
2.1.10 Synthesis of 9-anthracene carboxylic acid 39
2.1.11 Synthesis of 9-carboxylic acid acetylene anthracene 39
2.1.12 Synthesis of 9-carboxylic acid di-acetylene anthracene 40
2.1.13 Synthesis of CdS QDs 42
2.1.14 Preparation of 1-NCA capped QDs and the CdS-1-NCA-Eu upconversion system 42
2.2 Static state spectroscopy measurements 43
2.3 Transient absorption (TA) measurements 43
2.3.1 Femtosecond TA setup 43
2.3.2 Nanosecond TA setup 44
2.4 Photoluminescence lifetime measurements 44
2.5 Transmission electron microscopy measurements 45
2.6 Photoluminescence quantum yield measurements 45
2.7 Spectroelectrochemistry (SEC) setup and sample preparation. 46
2.8 Reference 48
Chapter 3: Contributions of Exciton Fine Structure and Hole Trapping on the Hole State Filling Effect in the Transient Absorption Spectra of CdSe Quantum Dots 49
3.1 Introduction 49
3.2 Results and discussions 51
3.2.1 Sample characterization and transient absorption studies 51
3.2.2 Transient absorption spectra fitting 62
3.2.3 Exciton fine structure and hole contribution 67
3.3 Conclusion 70
Appendix 3.1 Estimation of the QD band edge positions 71
Appendix 3.2 Kinetics fitting of the 1S and 2S XB signals of the HQD sample 72
Appendix 3.3 Kinetics fitting of the CdS XB signal, the MV+• radical signal, and the CS state signal of the HQD-MV2+ sample 74
Appendix 3.4 Fitting model and parameters for the absorption spectra of the HQD and HQD-MV2+ samples 75
Appendix 3.5 Fitting procedures and parameters for the TA spectra of the QD-MV2+ samples 78
Appendix 3.6 Discussion of the transition-counting model and the hole contribution calculated from kinetics fitting 82
3.4 References 84
Chapter 4: Excited State Transitions and Hot Electron Transfer in Quantum Confined CsPbBr3 Perovskite Nanocrystals 89
4.1 Introduction 89
4.2 Results and discussions 92
4.2.1 Excited state transitions in strongly confined CsPbBr3 NCs 92
4.2.2 Activation of high energy biexciton absorption by carrier state filling 102
4.2.3 Hot exciton relaxation and hot electron transfer 107
4.3 Conclusion 115
Appendix 4.1 TA spectral fitting method and results 116
A4.1.1 Fitting of the ground state absorption spectrum 116
A4.1.2 Fitting of the circularly polarized TA spectrum 118
A4.1.3 Fitting of the linearly polarized TA spectrum 121
Appendix 4.2 Hot exciton relaxation and electron transfer kinetics fitting 123
Appendix 4.3 Size dependent hot exciton relaxation and electron transfer kinetics 125
Appendix 4.4 Estimation of the electron transfer driving force and reorganization energy 129
Appendix 4.5 Calculation of the electron population at NC surfaces 132
Appendix 4.6 Comparison to conventional Marcus electron transfer theory 134
Appendix 4.7 Simulation of the activation of forbidden S-to-P transitions 135
4.4 Reference 137
Chapter 5 Electron Trapping Prolongs the Lifetime of Charge Separated States in 2D Perovskite Nanoplatelet-Hole Acceptor Complexes 144
5.1 Introduction 144
5.2 Results and discussions 146
5.2.1 Surface passivation effect on charge carrier dynamics 146
5.2.2 Selective electron and hole transfer 154
5.2.3 Discussion on electron tapping 162
5.3 Conclusion 167
Appendix 5.1 Fitting of the electron transfer kinetics 168
Appendix 5.2 Calculation of the average number of PTZ on each NPLA and NPLE 169
Appendix 5.3 Fitting the XB decay kinetics in pure NPLA and NPLE 171
Appendix 5.4 Fitting of the hole transfer kinetics 172
Appendix 5.5 Electron trapping-detrapping model 175
5.4 References 181
Chapter 6 Doping of Colloidal Nanocrystals for Optimizing Interfacial Charge Transfer: a Double-Edged Sword 189
6.1 Introduction 189
6.2 Results and discussions 191
6.2.1 Sample preparation and characterization 191
6.2.2 Exciton dynamics of doped QDs 194
6.2.3 Hole transfer from QDs to acceptors 198
6.2.4 Electron transfer to anthraquinone-2,3-dicarboxylic acid and charge recombination 199
6.2.5 Electron transfer to methyl viologen dichloride and charge recombination 206
6.2.6 Comparison of charge transfer rate constants with and without Cu doping 211
6.2.7 Discussion on the mechanism of Cu doping impact on charge transfer 215
6.3 Conclusion 217
Appendix 6.1 TA spectra of the pure QD component and the charge separated states before and after blue shift 218
Appendix 6.2 Reference spectrum of AQ- and AQ-H+ 221
Appendix 6.3: TA kinetics fitting in QD(Cu)-AQ samples 223
Appendix 6.4 Calculations of energy levels and surface charge densities 227
A6.4.1 Calculations of reorganization energies 227
A6.4.2 Calculations of electron and hole energy levels and surface charge densities 229
A6.4.3 Driving force of charge recombination in InP@ZnSe-AQ. 233
A6.4.4 Factors contributing to the charge transfer rate constants 234
6.4 References 236
Chapter 7 Oligoyne Bridges Enable Strong Through-Bond Coupling and Efficient Triplet Transfer from CdSe QD Trap Excitons for Photon Upconversion 242
7.1 Introduction 242
7.2 Results and discussions 244
7.2.1 Steady state photon upconversion using acetylene functionalized anthracenes 244
7.2.2 Triplet energy transfer kinetics 258
7.2.3 Triplet energy transfer mechanisms through the acetylene bridges 265
7.3 Conclusion 274
Appendix 7.1 Spectral analysis of TA data 275
A7.1.1 Double difference spectrum to extract the CAA triplet kinetics at early times (ps) 275
A7.1.2 Linear fit to extract the CAA triplet kinetics at later times (ns) 277
A7.1.3 Double difference spectrum to extract the 9ACA triplet kinetics at early time (ps) 279
A7.1.4 Linear fit to extract the 9ACA triplet kinetics at later time (ns) 280
A7.1.5 Discussion on the CDAA TA signal 282
Appendix 7.2 Calculation of QD exciton radial distribution function 283
Appendix 7.3 Calculation of T1 state spin density at the anchoring group in 9ACA, CAA, and CDAA 286
Appendix 7.4 DFT Calculation of the T1 state energy and reorganization energy in 9ACA, CAA, and CDAA 287
7.4 References 288
Chapter 8 Triplet Energy Transfer from Quantum Dots Increases Ln(III) Photoluminescence, Enabling Excitation at Visible Wavelengths 294
8.1 Introduction 294
8.2 Results and discussions 297
8.2.1 Steady state photon upconversion 297
8.2.2 Triplet energy transfer mechanism and kinetics 305
8.3 Conclusions 316
8.4 References 317
Chapter 9 Ultrafast Spectroelectrochemistry of Colloidal CdSe/CdS core/shell Quantum Dots under Cathodic Potentials 322
9.1 Introduction 322
9.2 Results and discussions 324
9.2.1 Effect of adding oxygen scavengers (trioctylphosphine, TOP) on SEC results 325
9.2.2 Measure the conduction band edge and trap states energies 328
9.2.3 Transient absorption measurements of QDs under cathodic potentials 332
9.2.4 electrochemistry-assisted photocharging and the impact on TA signals 338
9.2.5 1S kinetics of CdSe/CdS QDs under cathodic potentials 344
9.2.6 1P kinetics of CdSe/CdS QDs under cathodic potentials 349
9.2.7 Fitting the 1S and 1P kinetics at cathodic potentials 356
9.2.8 TA with direct 1P3/2 exciton excitation 358
9.2.9 QD charging state in TA-SEC measurements 360
9.2.10 TA spectra in QDs with maximum charging 363
9.2.11 Compare colloidal QD SEC with film QD SEC 365
9.3 Conclusion 368
9.4 References 369
Chapter 10 Conclusion and Outlook 374
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