Date of Award

2018

Document Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

First Advisor

David M. Jonas

Second Advisor

Arthur J. Nozik

Third Advisor

Niels H. Damrauer

Fourth Advisor

Gordana Dukovic

Fifth Advisor

Daniel S. Dessau

Abstract

Quantum confinement in semiconductor nanocrystals opens doors to a variety of applications such as light displays, biomedical probes, and lighting harvesting. Lead chalcogenide quantum dots (QDs) are particularly interesting for light harvesting because their bandgap can be tuned to the shortwave infrared region, which is required for high-efficiency next-generation photovoltaics. Even with the best current synthesis, it is likely that every quantum dot in an ensemble is different from the others. Because of this static inhomogeneity, interesting optical properties of quantum dots are hidden under broad featureless peaks in linear absorption spectra but can be revealed in two-dimensional spectra.

In two-dimensional Fourier transform (2DFT) spectroscopy, the added frequency dimension allows intuitive separation of the static bandgap inhomogeneity from the dynamical absorption lineshape. Here, 2D peakshape analysis on 2D spectra collected at long relaxation times is demonstrated to extract the static bandgap inhomogeneity of lead sulfide (PbS) quantum dot samples. Two PbS QD ensembles with different ligand coverage are analyzed to test whether surface ligands affect the electronic properties of QDs. This peakshape analysis improves upon the previous peakshape analysis developed by our group and allows measurement of two-photon transition strengths. Peakshape analysis of the 2D spectra requires high precision 2D spectrometer calibration. To this end, an interlaced Fourier transform technique that doubles the Nyquist frequency and calibrates the spectrograph for precise 2D peakshape analysis is presented.

Finally, the measurement of the standard chemical potential for the creation of an exciton is demonstrated on PbS Quantum dots. This standard chemical potential defines the maximum energy that can be harvested upon absorption of a photon. The standard chemical potential connects the homogeneous absorption and stimulated emission cross- section through generalized Einstein relations that apply only to homogeneous ensembles. Because 2DFT spectroscopy separates dynamical lineshapes from static inhomogeneous broadening, peakshape analysis determines the dynamical absorption and stimulated emission lineshape with their relative strengths. This allows determination of the standard chemical potential for creation of an exciton in PbS QDs. Possible systematic errors from pulse spectra, Förster resonance energy transfer, and the self-absorption effect are also discussed. This 2D spectroscopy method for determining the standard chemical potential may be valuable for characterizing a variety of photovoltaic materials.

Available for download on Thursday, January 27, 2022

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