Date of Award

Summer 8-21-2020

Level of Access

Open-Access Thesis

Degree Name

Doctor of Philosophy (PhD)

Department

Physics

Advisor

Robert W. Meulenberg

Second Committee Member

Robert J. Lad

Third Committee Member

James P. McClymer

Additional Committee Members

Carl P. Tripp

Liping Yu

Abstract

Semiconductor nanocrystals (NC) are well known for their unique size tunable optical properties making them suitable candidates for devices such as light emitting diodes (LEDs), solar cells, and cellular labels. II-VI semiconductors in the bulk form behave diamagnetically, but can inherit paramagnetic (PM) or ferromagnetic (FM) properties at the nanoscale. Reports suggest that the emergence of weak PM or FM behavior in undoped NCs are attributed to the increased surface to volume ratio compared for NCs. Traditionally, these NCs only obtain magnetic properties after doping with certain transition metals, such as Co, Mn, or Fe. Many mechanisms have been proposed to determine the source of magnetism in undoped NCs, ranging from dangling bonds, surface vacancies, and ligand exchange interactions. This thesis focuses on the role of dangling bonds and atomic vacancies on the surface of colloidal CdSe and ZnO NCs via controlled ligand removal along with CdS and CdS/ZnS core/shell nanoplatelets doped with Mn. Through magnetic measurements we show that for CdSe and ZnO NCs, the surface ligand density can drastically affect the magnetization results through a liquid phase post processing technique. For CdSe NCs the exact source of magnetism is complex and can arise from the uncoordinated surface atoms as seen with varying total angular momentum, J, values. In general, modification of magnetism in ZnO NCs can be attributed to the formation of oxygen vacancies as seen from consistent J values. Lastly, CdS and CdS/ZnS NPLs inherently possess surface defects, such as Cd or Zn vacancies, which coupled with Mn dopants can promote strong spin coupling between the core and NC surface.

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