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Author ORCID Identifier
Open Access Dissertation
Doctor of Philosophy (PhD)
Year Degree Awarded
Month Degree Awarded
Kevin R. Kittilstved
Inorganic Chemistry | Materials Chemistry | Physical Chemistry
The intentional incorporation of impurities or dopants in semiconductors is fundamental to manipulate the properties that render them useful for spintronics, photocatalysis, and optoelectronics. One long-standing challenge in integrating the doped semiconductors in various applications is the design of materials with controlled individual dopant properties such as dopants speciation, valence state, and spin dynamics. Despite several elegant studies to circumvent these material challenges, the quest for new materials with tunable dopant properties to address the theoretical and experimental understanding continues. In this work, we combine synthetic chemistry and various spectroscopies to study a class of materials possessing both substitutional magnetic dopants, surface defects, and tunable charge carrier densities.
This thesis builds off a substantial body of work to understand the defect chemistry and dopant-carrier interactions in colloidal SrTiO3, BaTiO3, and TiO2 nanocrystals (NCs). At the initial stages of this work, the formation of paramagnetic surface defects during hydrothermal synthesis of colloidal SrTiO3 and BaTiO3 NCs is investigated. These defects are identified as surface adsorbed superoxide radicals formed by reducing molecular oxygen during synthesis. The critical roles of lactate ions as reducing agents, hydrazine as an oxygen scavenger, and choice of precursor are thoroughly explored. In a continuation of this work, the critical role of precursor and slow hydrolysis in dopant incorporation are discussed that has enabled us to prepare sub-10 nm colloidal NCs of BaTiO3and TiO2 doped with an array of aliovalent magnetic dopants at substitutional Ti4+ sites. In a post-synthetic modification, the effect of excess electrons added by the photochemical method on dopant properties is explored. Using various spectroscopies, we presented a reversible control over Fe valence states between Fe3+ and Fe2+. This observation exists to at least in SrTiO3, BaTiO3, and TiO2 NCs and contrasts with the acceleration of Cr3+ spin relaxation time in SrTiO3 NCs found previously. This work demonstrates that dopant-carrier interactions are unique, which depends on the relative position of redox-level of dopants in the band structure and their Zeeman energies under applied magnetic field. This work helped us build the electronic structure of Ti(IV)-based oxide NCs where Fe3+/2+ redox-level located within the bandgap acts as trap center for photoexcited carriers while Cr3+/2+ is situated above the conduction band. The ability to refine and control these magnetic properties by simply using photons contributes to the fundamental understanding of carriers' interactions in oxide semiconductors and provides insights into the pathways for charge and spin-based applications for energy storage and quantum processing.
Abdullah, Muhammad, "Manipulating the Aliovalent Magnetic Dopants in Ti(IV)-based Oxide Nanocrystals" (2021). Doctoral Dissertations. 2248.
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