Physical Properties and Dynamics of Excitons in Two-Dimensional Semiconductor Nanoplatelets for Lasing and Solar-to-Fuel Conversion Público
Li, Qiuyang (Spring 2019)
Published
Abstract
2D semiconductor nanoplatelets (NPLs) exhibit many unique properties, such as uniform quantum confinement, large exciton binding energy, and giant oscillator strength transition effect, long Auger lifetime, and high emission quantum yield, and have been intensively investigated for lasing and solar-to-fuel conversion applications. The optoelectronic and photocatalytic performances of 2D NPLs depend on the physical properties of band-edge excitons, including the exciton size, exciton in-plane transport, multi-exciton annihilation, and exciton dissociation. In this dissertation, we focus on the physical properties and dynamics of 2D excitons and how these properties affect the optical gain (gain mechanism, gain threshold) and light-driven H2 generation performance in cadmium chalcogenide (CdX, X=Se, S, Te) and CsPbBr3 Perovskite NPLs.
We first discuss the 2D exciton in-plane transport using CdSe/CdS type I and CdSe/CdTe type II core/crown (CC) NPLs as model systems, respectively. Both hot and cold excitons transport diffusively in 2D NPLs with a diffusion constant close to their bulk counterparts. This is followed by an investigation of multi-exciton Auger recombination of NPLs, emphasizing the difference of Auger mechanisms in 2D NPLs, 0D QDs, and 1D NRs, and the deviation from the “universal volume scaling” law. We established a new Auger recombination model for 2D NPLs that accounts for both a lateral area dependent exciton collision frequency and thickness dependent Auger probability per collision. The Auger recombination rate depends on the product of the exciton collision frequency and Auger probability, which provides a general scheme for Auger recombination in 1D and 2D nanocrystals.
Next, we discuss optical gain (OG) mechanisms and models of CdSe NPLs and show that OG thresholds are independent on the NPL lateral size but increase with the optical density of NPLs at the excitation wavelength. We also show that the extension of exciton coherent area at low temperature (<4K) and the resulting giant oscillator strength transition effect reduces the OG threshold by ~4-fold compared to the value at room temperature (298K), providing a new strategy for the rational design of NC-based low-threshold lasing materials. We also discuss exciton gain mechanism in CdSe/CdTe type II CC NPLs, where the OG is achieved in a tri-exciton state with two charge-transfer excitons at the core/crown interface and one CdTe exciton localized in CdTe crown.
Finally, we summarize our study of exciton dissociation in 2D NPLs and show that the light-to-H2 conversion quantum efficiency can be significantly improved using 2D CdS NPLs. Detailed transient absorption study and modeling indicate that the improvement comes from slowing down charge recombination by the 2D morphology. We also show efficient exciton dissociation in CsPbBr3 perovskite 2D NPLs by selectively transferring electron and hole to different acceptors and an over 100-fold longer-lived charge-separated state in CsPbBr3 NPL-hole acceptor complexes compared to that in QD-hole acceptor complexes. These results indicate great potentials of 2D NPLs as in absorbers in solar-to-fuel conversion.
Table of Contents
Chapter 1. Introduction. 1
1.1. Backgrounds of Semiconductor 2D Nanoplatelets. 1
1.2. Physical Properties of 2D Excitons of Nanoplatelets for Optoelectrical and Photocatalytic Applications 4
1.2.1. COM coherent delocalization of band-edge excitons. 5
1.2.2. Exciton in-plane transport 6
1.2.3. Multi-exciton annihilation. 8
1.2.4. Exciton dissociation. 10
1.3. Summary. 11
Chapter 2. Experimental Methods. 14
2.1. Sample Preparation. 14
2.1.1. Synthesis of cadmium chalcogenide nanoplatelets. 14
2.1.2. Synthesis of core/crown nanoplatelet heterostructures. 16
2.1.3. Synthesis of CdS nanorods. 17
2.1.4. Synthesis of nanocrystal-Pt heterostructures. 17
2.1.5. Synthesis of CsPbBr3 perovskite nanoplatelets. 19
2.1.6. Sample preparation for low temperature measurement 20
2.1.7. Procedures of ligand exchange. 20
2.2. Spectroscopy Setup. 21
2.2.1. Femtosecond transient absorption spectroscopy setup. 21
2.2.2. Nanosecond transient absorption spectroscopy setup. 22
2.2.3. Static-photoluminescence measurement setup. 22
2.2.4. Time-resolved photoluminescence measurement setup. 22
2.2.5. Amplified spontaneous emission measurement setup. 23
2.3. Light-Driven H2 generation. 24
2.4. Electrochemical method for the estimation of Nanocrystal band-edge position. 25
Chapter 3. Size-Independent Exciton Localization Efficiency in CdSe/CdS Type-I Core/Crown Nanoplatelet Heterostructures. 29
3.1. Introduction. 29
3.2. Results and Discussion. 31
3.2.1. Sample Preparation and Characterizations. 31
3.2.2. Type I Band alignment in CdSe/CdS core/crown heterostructure. 33
3.2.3. Size independent crown-to-core exciton localization (XL) efficiency. 37
3.2.4. Size dependent crown-to-core exciton localization dynamics. 40
3.2.5. Mechanisms of exciton transport and trapping. 45
3.3. Conclusion. 50
Appendix 3.1. 52
Appendix 3.2. 54
Appendix 3.3. 55
Appendix 3.4. 59
Appendix 3.5. 60
Appendix 3.6. 63
Chapter 4. Efficient Diffusive Transport of Hot and Cold Excitons in CdSe/CdS Type-II Core/Crown Nanoplatelet Heterostructures. 72
4.1. Introduction. 72
4.2. Results and Discussion. 75
4.2.1. Sample characterization. 75
4.2.2. Unity CT exciton formation efficiency. 77
4.2.3. Size dependent exciton in-plane transport. 78
4.2.4. 2D exciton diffusion model. 82
4.2.5. Hot exciton transport. 85
4.3. Conclusion. 90
Appendix 4.1. 92
Appendix 4.2. 94
Appendix 4.3. 95
Appendix 4.4. 96
Appendix 4.5. 101
Chapter 5. Area- and Thickness-Dependent Biexciton Auger Recombination in CdSe Nanoplatelets 112
5.1. Introduction. 112
5.2. Results and Discussion. 113
5.2.1. Sample characterization. 113
5.2.2. Transient absorption spectroscopy on colloidal CdSe NPLs. 115
5.2.3. Lateral size and thickness dependent Auger recombination. 117
5.3. Conclusion. 124
Appendix 5.1. 126
Appendix 5.2. 128
Appendix 5.3. 129
Appendix 5.4. 132
Appendix 5.5. 135
Chapter 6. A Model for Optical Gain in CdSe Nanoplatelets. 137
6.1. Introduction. 137
6.2. Results and Discussion. 139
6.2.1. Sample characterization. 139
6.2.2. Lateral area independent optical gain threshold. 140
6.2.3. Optical density dependent OG threshold. 143
6.2.4. Amplified spontaneous emission measurements. 144
6.2.5. Model of optical gain threshold. 146
6.3. Conclusions. 151
Appendix 6.1. 152
Appendix 6.2. 153
Appendix 6.3. 154
Chapter 7. Reducing Optical Gain Threshold in Two-Dimensional CdSe Nanoplatelets by Giant Oscillator Strength Transition Effect 160
7.1. Introduction. 160
7.2. Results and Discussion. 163
7.2.1. Sample characterization. 163
7.2.2. Temperature-dependent OG threshold by GOST effect 164
7.2.3. Measurement of exciton coherent area. 167
7.2.4. Temperature-dependent SX. 171
7.2.5. Temperature-dependence of gain threshold in 0D, 1D, 2D, and bulk materials. 174
7.3. Conclusion. 175
Appendix 7.1. 176
Appendix 7.2. 178
Appendix 7.3. 179
Appendix 7.4. 181
Appendix 7.5. 189
Appendix 7.6. 199
Chapter 8. Low Threshold Multi-Exciton Optical Gain in CdSe/CdTe Core/Crown Type-II Nanoplatelets Heterostructures. 201
8.1. Introduction. 201
8.2. Results and Discussion. 202
8.2.1. Sample characterization. 202
8.2.2. Charge transfer exciton state. 205
8.2.3. Long-lived excitons in the CdTe crown. 208
8.2.4. Higher energy excitons in the CdSe core. 212
8.2.5. Spatial distribution and energetics of exciton states. 215
8.2.6. Pump wavelength dependent optical gain threshold. 216
8.3. Conclusion. 218
Appendix 8.1. 220
Appendix 8.2. 221
Appendix 8.3. 225
Appendix 8.4. 232
Chapter 9. Two-Dimensional Morphology Enhances Light-Driven H2 Generation Efficiency in CdS Nanoplatelet-Pt Heterostructures. 236
9.1. Introduction. 236
9.2. Results and Discussion. 238
9.2.1. Sample characterization. 238
9.2.2. Enhanced H2 generation internal quantum efficiency. 240
9.2.3. Ultrafast charge separation. 243
9.2.4. Ultrafast and pH dependent hole removal 247
9.2.5. pH and morphology dependent charge recombination in NPL-MV2+ complexes. 249
9.2.6. A Model for pH dependent H2 generation IQE.. 252
9.2.7. A Model for Enhanced H2 IQE in 2D NPL-Pt Heterostructures. 253
9.3. Conclusion. 256
Appendix 9.1. 258
Appendix 9.2. 263
Appendix 9.3. 266
Appendix 9.4. 271
Appendix 9.5. 283
Appendix 9.6. 285
Chapter 10. Efficient Charge Separation in Two-Dimensional CsPbBr3 Perovskite Nanoplatelets 291
10.1. Introduction. 291
10.2. Results and Discussion. 292
10.2.1. Sample characterization. 292
10.2.2. Spectroscopy of CsPbBr3 NPLs. 294
10.2.3. Charge separation in CsPbBr3 NPLs. 296
10.3. Conclusion. 301
Appendix 10.1. 302
Appendix 10.2. 304
Appendix 10.3. 307
Appendix 10.4. 311
Chapter 11. Conclusion and Outlooks. 314
References 318
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