Date of Graduation

12-2021

Document Type

Thesis

Degree Name

Master of Science in Microelectronics-Photonics (MS)

Degree Level

Graduate

Department

Microelectronics-Photonics

Advisor

Colin D. Heyes

Committee Member

Robert H. Coridan

Second Committee Member

Jingyi Chen

Third Committee Member

Fisher Yu

Fourth Committee Member

Matt Leftwich

Keywords

fluorescence, nanocrystals, nanomaterials, nanoparticles, quantum dots, semiconductor

Abstract

Since the early 2000s heavy-metal-free quantum dots (QDs) such as CuInS2/ZnS have attempted to replace CdSe, their heavy-metal-containing counterparts. CuInS2/ZnS is synthesized in a two-step process that involves the fabrication of CuInS2 (CIS) nanocrystals (NCs) followed by the addition of zinc precursors. Instead of the usual core/shell architecture often exhibited by binary QDs, coating CIS QDs results in alloyed and/or partially alloyed cation-exchange (CATEX) QDs. The effect that zinc has on the properties of CIS NCs was studied by incorporating zinc during the first step of the synthesis. Different In:Cu:Zn ratios were employed in this study, maintaining a constant 4:1 In:Cu molar ratio. When a 4:1:4 In:Cu:Zn molar ratio was employed, the photoluminescence quantum yield (PL QY) increased from 12% (for the Zn-free CIS QDs) to 28%, and the fluorescence lifetime increased from 191 ns to 293 ns. The polydispersity of these NCs was also reduced, shrinking the full-width-at-half-maximum (FWHM) by 40 nm, while only increasing the average particle diameter from 3.3 nm to 3.9 nm. The superior optical performance of Zn-containing QD cores was, nevertheless, not observed after NCs underwent 24 hours of cation exchange. Zn-free QD cores reached PL QY of up to 75% upon cation exchange, while Zn-containing cores showed PL QY as low as 35%. After six hours of reaction zinc overtook indium and copper, making up above 97% of the metallic elements present in all CATEX QDs. The presence of zinc prior cation exchange did not influence the fluorescence lifetime, with 300 ns being measured for nearly all CATEX QDs, but had a detrimental effect on particle growth. Quantum Dot cores that lacked zinc showed an average particle size of over seven nanometers, a significant increase when compared to the minimal growth of over four nanometers experienced by those with highest zinc content.

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