Date of Award

5-12-2024

Degree Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Department

Chemistry

Advisor(s)

Weiwei Zheng

Subject Categories

Chemistry | Physical Sciences and Mathematics

Abstract

The study of transition metal ion-doped semiconductor nanocrystals (NCs) has attracted increasing attention due to their unique optical, electronic, and magnetic properties which sparked considerable interest towards a wide range of applications in material science, renewable energy and biological systems. Besides the size- and shape-dependent behavior, doped nanomaterials exhibit new and/or enhanced optical properties, which are sensitive to the dopant location or dopant distribution throughout the solid crystal lattice. Our work primarily focuses on understanding and elucidating the mechanism of dopant ion migration inside the NCs during post-synthetic treatments, such as shell passivation and high-temperature annealing. Mn(II) dopants migration behavior in Mn(II) doped CdS-based core/shell quantum dots (QDs) and Mn(II) doped ZnSe-based core/shell nanowires were established in the previous work of the Zheng group. This dissertation will expand the study of dopant migration behavior in semiconductor QDs, specifically, to elucidate the effect of an inserted alloyed layer, with a small cationic size mismatch of dopant ions, for directional dopant migration and diffusion. Our research reveals that the intentionally inserted alloyed layer, with a small cationic size mismatch with Mn(II) dopants, could serve as an atomic “trap” to facilitate the directional dopants migration pathways, including both outward and inward migration in core and shell doped core/multi-shell QDs. The detailed mechanistic study reveals that dopants migration behavior between different tetrahedral sites inside a II-VI group semiconductor lattice are sensitive to the migration temperature and microenvironment within the NCs. Furthermore, a larger Cd(II) substitutional doping site (92 pm) with larger local lattice distortion is critical for efficient Mn dopant (80 pm) trapping and migration. Density functional theory (DFT) calculation reveals a higher energy barrier for a Mn(II) dopant hopping from the smaller Zn substitutional tetrahedral site (74 pm) as compared to a larger Cd substitutional tetrahedral site in the migration model. The ratio of relative rate constant (kZnS/kCdS) indicates that the rate of Mn migration is about three-orders of magnitude larger in the CdS lattice compared to ZnS lattice for the temperature range investigated in this study (180 – 230 ℃). Furthermore, we also illustrated the progress and challenges of the Mn(II) dopants migration behavior between the perovskite NCs and the II-VI chalcogenides such as CdS or ZnS NCs. The large difference between the ionic nature of perovskite and the covalent nature of chalcogenides, along with their coordination structure (corner-sharing octahedral for perovskite, and core-sharing tetrahedral for metal chalcogenides), as well as the disparate growth kinetics (rapid for perovskite and slow for chalcogenides), pose substantial challenges for the synthesis of heterostructures. Although the optical results indicate that substantial further research is required, these initial steps are promising and paving the way for further exploration of dopant behavior in perovskite based heterostructures.

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