Exploring exciton and trap state recombination dynamics in cadmium sulfide quantum dots

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Woodall, D. (2015). Exploring exciton and trap state recombination dynamics in cadmium sulfide quantum dots. Unc Charlotte Electronic Theses And Dissertations.
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Abstract

Colloidal semiconducting quantum dots (QDs) are ideal candidates for optoelectronic devices such as solar cells and LEDs because of their high photoluminescence (PL) quantum yields and photostability. In order to improve device efficiencies, we must understand how QD surfaces affect charge dynamics within the QD. In this work we demonstrate that CdS is an excellent model system for studying QD carrier dynamics for two principle reasons: (i) they produce both exciton and surface-localized trap state emission and (ii) their surface chemistry is simplified by the presence of only one ligand species. First we, use temperature-dependent time-resolved photoluminescence (TRPL) to measure both exciton and trap recombination dynamics and find that an activated trapping process influences exciton dynamics while trap dynamics are dominated by non-radiative recombination. We propose a model to explain the exciton dynamics in which electron transfer from the exciton state to the deep emissive trap state occurs via a temperature-dependent multi-step process requiring multiple non-emissive trap state distributions. Second, we identify the nature of the trap state using excitation-dependent TRPL, and demonstrate for the first time that trap excited state lifetimes are up to four times longer when influenced by the exciton state compared to direct excitation. Finally, we attempt to control trap emission by altering the amount of native ligand, oleic acid on the QD surface. We present evidence that the ligand coverage on the QD surface and in solution affects not only trap emission but also exciton emission, and suggest that oleic acid plays a more complicated role in QD emission than previously expected. By developing CdS as a model system for trapping dynamics, we further our understanding of these dynamics and enable the development of new technologies that take advantage of the trap state benefits such as broad emission and long-lived charge separated states.

Details
Author

Woodall, Danielle

Title

Exploring exciton and trap state recombination dynamics in cadmium sulfide quantum dots

Physical Description

1 online resource (166 pages) : PDF

Date

2015

Degree Granting Institution

University of North Carolina at Charlotte

Abstract

Colloidal semiconducting quantum dots (QDs) are ideal candidates for optoelectronic devices such as solar cells and LEDs because of their high photoluminescence (PL) quantum yields and photostability. In order to improve device efficiencies, we must understand how QD surfaces affect charge dynamics within the QD. In this work we demonstrate that CdS is an excellent model system for studying QD carrier dynamics for two principle reasons: (i) they produce both exciton and surface-localized trap state emission and (ii) their surface chemistry is simplified by the presence of only one ligand species. First we, use temperature-dependent time-resolved photoluminescence (TRPL) to measure both exciton and trap recombination dynamics and find that an activated trapping process influences exciton dynamics while trap dynamics are dominated by non-radiative recombination. We propose a model to explain the exciton dynamics in which electron transfer from the exciton state to the deep emissive trap state occurs via a temperature-dependent multi-step process requiring multiple non-emissive trap state distributions. Second, we identify the nature of the trap state using excitation-dependent TRPL, and demonstrate for the first time that trap excited state lifetimes are up to four times longer when influenced by the exciton state compared to direct excitation. Finally, we attempt to control trap emission by altering the amount of native ligand, oleic acid on the QD surface. We present evidence that the ligand coverage on the QD surface and in solution affects not only trap emission but also exciton emission, and suggest that oleic acid plays a more complicated role in QD emission than previously expected. By developing CdS as a model system for trapping dynamics, we further our understanding of these dynamics and enable the development of new technologies that take advantage of the trap state benefits such as broad emission and long-lived charge separated states.

Genre

doctoral dissertations

Subjects--Topics

Nanoscience
Chemistry, Physical and theoretical

Degree

Ph.D.

Keywords

Cadmium Sulfide
Excitation-Dependent TRPL
Quantum Dot
Temperature-Dependent TRPL
Time-Resolved PL
Trap Emission

Subject Area

Nanoscale Science

Advisor(s)

Jones, Marcus

Committee Members

Fiddy, Michael
Schmedake, Thomas
Zhang, Yong
Ringwood, Amy

Degree Note

Thesis (Ph.D.)--University of North Carolina at Charlotte, 2015.

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This Item is protected by copyright and/or related rights. You are free to use this Item in any way that is permitted by the copyright and related rights legislation that applies to your use. For other uses you need to obtain permission from the rights-holder(s). For additional information, see http://rightsstatements.org/page/InC/1.0/.

Rights Holder Information

Copyright is held by the author unless otherwise indicated.

Identifier

Woodall_uncc_0694D_10744

Permalink

http://hdl.handle.net/20.500.13093/etd:1389