Date of Award

Spring 1-1-2014

Document Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Department

Chemistry & Biochemistry

First Advisor

David Jonas

Second Advisor

Gordana Dukovic

Third Advisor

Veronica Vaida

Fourth Advisor

Robert Parson

Fifth Advisor

Henry Kapteyn

Abstract

While quantum dots (QDs) potentially offer routes to increase the efficiency of solar energy harvesting, fundamental photophysical processes, such as carrier cooling, are still not fully understood. Ultrafast degenerate pump-probe spectroscopy is used to investigate carrier dynamics in both isolated colloidal quantum dots, uncoupled quantum dot arrays, and most importantly, coupled quantum dot arrays that may be suitable for device applications. Pumping and probing at the same wavelength overcomes frequency-dependent absorption cross-section issues that may complicate results. Additionally, utilizing novel sample refreshing techniques minimizes photodegradation and photocharging.

The time-dependent behavior of PbS nanocrystal arrays and colloidal solutions is measured at the band gap with sub-100fs time resolution and at higher photon energies with sub-40fs resolution. Different ligand treatments are employed to exchange bulky long-chain organics for shorter chains with different functional groups. Ligand exchange is performed on films as well as a colloidal solution.

Investigations of 2nm diameter PbS quantum dots at the band gap and at 1.4 times the band gap show the biexciton lifetime in an uncoupled array is the same as in colloidal solution. This is in contrast to previous reports of different decay times for various sizes of PbSe QD solutions and uncoupled arrays. Degenerate studies of these same 2nm diameter PbS QDs as coupled arrays at the band gap indicate substantial excited state absorption that increases with increasing pump fluence.

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