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ORCID

http://orcid.org/0000-0002-1734-4390

Date of Award

Summer 8-15-2019

Author's School

Graduate School of Arts and Sciences

Author's Department

Chemistry

Degree Name

Doctor of Philosophy (PhD)

Degree Type

Dissertation

Abstract

Colloidal semiconducting nanocrystals hold significant potential for third generation photovoltaics as solution processable materials that can surpass the Shockley-Queisser limit through multiexciton generation. In pursuit of this goal, the synthesis and optical characterization of CdSe/PbS core/shell quantum dots is reported. The spectroscopic behavior of these particles demonstrates their potential for use in optoelectronic devices, taking advantage of wave function engineering of the electron and hole. The rock salt PbS shell grows on all sides of the underlying zinc blende CdSe quantum dot, creating a core/shell structure. With increasing shell thickness, the band edge absorption and photoluminescence transitions decrease in energy as a result of reduced quantum confinement. At the same time, the strength of the first absorption transition decreases by over an order of magnitude relative to the higher energy transitions due to reduced electron—hole wavefunction overlap in the core/shell quantum dots. This leads to a tunable energetic shift of up to 550 meV between the onset of strong absorption and photoluminescence. These proposed changes in wave function overlap are further corroborated by effective mass approximation wave function calculations, which indicate a transition between quasi-type-I and quasi-type-II behavior. These results demonstrate the prospects for this system as luminescent solar concentrators with a tunable emission energy with prospects for multiexciton generation in third generation photovoltaics. The same synthetic scheme can be used to synthesize CdSe/PbS core/shell nanoplatelets, allowing for applications requiring directional charge transport. The PbS shell growth results in a change of morphology of the nanoplatelets from rectangular to oval shaped with increasing thickness. The distinct heavy hole, light hole, and split off hole transitions in CdSe nanoplatelets make this system a useful tool for further probing the effects of the changing wave functions on the spectroscopy and dynamics. The changes to the spectral properties of the nanoplatelets were rationalized using a model of the differing band structures of CdSe and PbS. The band gap of CdSe occurs at the Γ point, where the gap between the conduction and valence bands of PbS is large, while the band gap of PbS is at the L point, where the gap between the conduction and valence bands of CdSe is large. This leads to differing absorption transition energies and strengths with increasing PbS shell thickness. The charge carrier wave function overlap is further studied by measuring the photoluminescence lifetimes, which increase with increasing shell thickness, suggesting the wave function overlap between the electron and hole at the band gap decreases. The shape changes induced during PbS shell deposition on CdSe nanoplatelets is also investigated by treating the nanoplatelets with a variety of chemical precursors used in the PbS shell synthesis. The lead precursor, lead oleate, is found to etch the CdSe nanoplatelets. Transmission electron microscopy images show that both the size and shape of the nanoplatelets change with etching, but the morphology of the product nanocrystals varies depending on the lead oleate solvent. Etching in the presence of non-coordinating solvents causes the formation of nanodots separate from the CdSe nanoplatelets, while etching in the presence of a coordinating solvent, octylamine, causes the formation of nanodots attached to the CdSe nanoplatelets. Absorption spectroscopy of the nanoplatelets with varying etching times shows a modest shift of the absorption features to higher energy. With increased etching, the nanoplatelet width decreases, causing the quantum confinement to increase, and resulting in the observed spectral shift. Elemental analysis of the product nanocrystals indicates that the newly formed nanodots contain lead. Together, these results suggest that the etching begins from the smallest {110} edges of the nanoplatelets, which also explains the observed shape changes to the nanoplatelets during the CdSe/PbS core/shell nanoplatelet synthesis.

Language

English (en)

Chair and Committee

Richard A. Loomis William Buhro

Committee Members

Bryce F. Sadtler, Elijah Thimsen, John R. Bleeke,

Available for download on Wednesday, November 04, 2020

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