THE UNIVERSITY OF CHICAGO PROBING QUANTUM DYNAMICS AND SPECTRAL BROADENING IN SEMICONDUCTOR NANOCRYSTALS WITH NONLINEAR SPECTROSCOPIES A DISSERTATION SUBMITTED TO THE FACULTY OF THE DIVISION OF THE PHYSICAL SCIENCES IN CANDIDACY FOR THE DEGREE OF DOCTOR OF PHILOSOPHY DEPARTMENT OF CHEMISTRY BY NICHOLAS EDWARD WILLIAMS CHICAGO, ILLINOIS JUNE 2019 Copyright c 2019 by Nicholas Edward Williams All Rights Reserved In loving memory of my father, Edward T. Williams TABLE OF CONTENTS LIST OF FIGURES . vii ACKNOWLEDGMENTS . ix ABSTRACT . xi 1 INTRODUCTION . 1 1.1 Semiconductor Nanocrystals . 1 1.2 Improving Semiconductor Nanocrystals by Understanding Electronic Struc- ture and Quantum Dynamics . 2 1.3 Spectroscopy . 3 1.4 Nanomaterial Systems . 6 1.4.1 InP Quantum Dots . 6 1.4.2 Perovskite Quantum Dots . 7 1.4.3 CdSe Nanoplatelets . 9 1.5 Outline of Dissertation . 10 REFERENCES . 12 2 BACKGROUND . 22 2.1 Electronic Structure of Semiconductors . 22 2.1.1 Band Structure Introduction . 22 2.1.2 Semiconductor Carrier Dynamics . 24 2.1.3 Confined Semiconductors . 27 2.1.4 Colloidal Nanocrystals . 29 2.2 2D Electronic Spectroscopy . 30 2.2.1 Theory of 2DES . 31 2.2.2 Data Acquisition . 36 2.2.3 Data Processing . 40 REFERENCES . 43 3 THE ORIGIN OF BROAD EMISSION SPECTRA OF INP QUANTUM DOTS: CONTRIBUTIONS FROM STRUCTURAL AND ELECTRONIC DISORDER . 48 3.1 Abstract . 48 3.2 Introduction . 49 3.3 Results and Discussion . 52 3.3.1 Synthesis and Surface Passivation of InP Quantum Dots for Improved Photoluminescence . 52 3.3.2 Photoluminescence Excitation (PLE) Spectroscopy . 54 3.3.3 Transient Absorption and Streak Camera Studies of the Picosecond Excited Carrier Dynamics . 58 iv 3.3.4 Femtosecond Excited Carrier Dynamics Accessed by Two-Dimensional Electronic Spectroscopy . 64 3.3.5 Structural Characterization by Raman and EXAFS . 69 3.3.6 Exciton-Phonon Coupling Probed by 2DES Quantum-Beating Maps . 78 3.3.7 Computational Insights into Electronic Structure and Impurity Levels of InP Quantum Dots . 81 3.4 Conclusions . 83 3.5 Acknowledgements . 86 REFERENCES . 88 4 SCALABLE LIGAND-MEDIATED TRANSPORT SYNTHESIS OF ORGANIC- INORGANIC HYBRID PEROVSKITE NANOCRYSTALS WITH RESOLVED ELECTRONIC STRUCTURE AND ULTRAFAST DYNAMICS . 96 4.1 Abstract . 96 4.2 Introduction . 97 4.3 Materials and Methods . 99 4.3.1 Materials . 99 4.3.2 Preparation of CH3NH3Br . 99 4.3.3 Preparation of CH3NH3I . 100 4.3.4 Synthesis of MAPbBr3 Nanocrystals . 100 4.3.5 Synthesis of MAPb(Br/I)3 Nanocrystals . 100 4.3.6 Synthesis of MAPbI3 Nanocrystals . 102 4.3.7 Powder X-ray Diffraction . 102 4.3.8 Photoluminescence Measurements . 102 4.3.9 Transmission Electron Microscopy . 102 4.3.10 Transient Absorption Spectroscopy . 103 4.3.11 Two-Dimensional Electronic Spectroscopy . 103 4.4 Results and Discussion . 104 4.5 Conclusions . 113 4.6 Acknowledgements . 113 REFERENCES . 115 5 TWO-DIMENSIONAL ELECTRONIC SPECTROSCOPY OF CDSE NANOPLATELETS THIN FILMS . 121 5.1 Introduction . 121 5.2 Methods . 122 5.2.1 Synthesis of 6 ML CdSe Nanoplatelets . 122 5.2.2 Two-Dimensional Electronic Spectroscopy . 123 5.3 Results and Discussion . 124 5.4 Conclusion . 131 REFERENCES . 132 v 6 PROPOSED DIRECTIONS FOR FUTURE RESEARCH . 137 6.1 Further studies of InP Quantum Dots . 138 6.2 Electronic Structure and Femtosecond Dynamics of Cd-Free Quantum Dots . 140 REFERENCES . 144 7 CONCLUSION . 150 APPENDICES . 153 A PHASE CORRECTIONS IN 2D ELECTRONIC SPECTROSCOPY . 154 A.1 Spectral Drift between 2DES Experiment and Pump Probe . 157 A.2 Spectral drift and beam pointing changes during a 2DES experiment . 159 REFERENCES . 162 B RELATION BETWEEN THE BULK AND THE NANOSCALE . 163 REFERENCES . 166 vi LIST OF FIGURES 1.1 InP/ZnS Quanum Dots emit light via shallow hole trap states . 6 1.2 Methyl-Ammonium Lead Halide Perovskite Quantum Dots probed by Two- Dimensional Electronic Spectroscopy . 8 1.3 6 ML CdSe Nanoplatelets probed by Two-Dimensional Electronic Spectroscopy 9 2.1 Description of band structure with a chain of molecular orbitals . 23 2.2 Band Structure of bulk InP . 25 2.3 Recombination possibilities in semiconductor materials . 27 2.4 Examples of common Feynman diagrams in third-order spectroscopies . 33 2.5 Mock 2D Spectrum . 35 2.6 Layout of the Redfield Lab Laser Setup . 38 2.7 Layout of the Redfield Lab 2D Spectrometer and Pulse Sequence . 39 2.8 Example of 2D Spectroscopy Analysis . 41 3.1 Absorption, SAXS, and TEM measurements of CdTe and InP nanocrystals . 50 3.2 Small angle x-ray scattering for size distribution analysis . 51 3.3 Emission red shift of InP quantum dots . 53 3.4 Absorption, PL, and PLE spectra of CdTe and InP QDs . 55 3.5 Comparison of ensemble absorption spectrum and PLE spectrum line widths . 57 3.6 Transient absorption spectra of as-synthesized InP QDs . 59 3.7 Transient absorption studies show that electron hole trapping rates are negligible for all samples . 61 3.8 A comparison of bleach recovery and PL decay at the hundreds of nanoseconds timescale . 63 3.9 Real-valued 2D spectra of as-synthesized and core-shell InP quantum dots . 66 3.10 Large Stokes shifts are correlated with lattice disorder . 70 3.11 Luminescent InP QDs have larger apparent Stokes shifts compared to as- synthesized InP and CdSe QDs . 73 3.12 Raman spectroscopy and EXAFS data for InP QDs under various conditions and capping ligands . 76 3.13 Comparison of quantum beating modes between different points of the 2D spectrum 79 3.14 Quantum-beating maps of InP/ZnS and as-synthesized InP QDs . 80 3.15 Density of states traces and their respective electronic structure maps . 82 3.16 Pictorial Description of the Emissive Trap Model in InP/ZnS Quantum Dots . 84 4.1 Illustrative reaction mechanism of the ligand-mediated transport strategy . 99 4.2 EDX spectra of MAPb(Br/I)3 nanocrystals. 101 4.3 Comparison between reaction mechanisms of the solid-extraction strategy versus other strategies . 105 4.4 Characterization of the as-synthesized organic-inorganic perovskite NCs . 107 4.5 TEM characterization on MAPbI3 NCs . 108 4.6 Nanosecond TA spectroscopy on MAPbI3 NCs . 109 4.7 TCSPC measurement of MAPbI3 nanocrystals . 110 4.8 2DES spectra of MAPbI3 NCs with different sizes . 111 vii 4.9 Femtosecond 2D electronic spectroscopy on MAPbI3 NCs (2.6 nm) . 112 5.1 Linear Spectra and Pictorial band structure of CdSe Nanoplatelets . 125 5.2 2D spectra of thin films of 6 ML CdSe Nanoplatelets at room temperature and various pump powers . 126 5.3 Waiting time traces of points in the room temperature 2D spectra of 6.