Graduation Year

2024

Document Type

Dissertation

Degree

Ph.D.

Degree Name

Doctor of Philosophy (Ph.D.)

Degree Granting Department

Physics

Major Professor

Hariharan Srikanth, Ph.D.

Co-Major Professor

Manh-Huong Phan, Ph.D.

Committee Member

Sarath Witanachchi, Ph.D.

Committee Member

Dario Arena, Ph.D.

Committee Member

Raja Das, Ph.D.

Keywords

Iron Oxide, Nanomagnetism, Nanoparticles, Phase-tunable

Abstract

Iron oxide nanoparticles (IONPs) hold immense potential across diverse fields, from spintronics, magnetic hyperthermia to drug delivery, biodetection, and magnetic resonance imaging. Unlocking these applications hinges on our ability to tailor the magnetic properties of IONPs, and this dissertation presents novel and powerful approaches for enhancing magnetic and hyperthermic responses of multiphase iron oxide nanostructures. This is achieved by manipulating their phase volume fraction, size, and shape with a focus on the magnetic tunability of nanostructures via strategic control over structural and environmental parameters. The aforementioned has been achieved by the following three key approaches: (i) phase-tunability of iron oxide nanocubes with varying sizes; (ii) phase engineering of iron oxide nanorods and nanocubes via controlled annealing; and (iii) precise bulk and crystallite size control within superparticles.

Our initial approach explored the tunability of magnetic properties and iron oxide phases within nanocubes as their size changed from sub-10 nm to 43 nm. Curiously, smaller nanocubes of sizes 10, 15, and 24 nm displayed the presence of three phases Fe3O4, α-Fe2O3, and FeO, and as the nanocubes became larger transitioned to the more stable Fe3O4 and α-Fe2O3 phases. The presence of the exchange bias effect, which typically arises due to the interfacial coupling of a ferromagnetic and an antiferromagnetic phase closely followed the presence of the FeO phase hinting at the interfacial coupling with ferrimagnetic Fe3O4, with the α-Fe2O3 phase abstaining from actively taking part in the exchange bias effect. Further, the stability of the FeO phase in different sizes of nanocubes was evaluated with the affirmed mechanism of the exchange bias effect.

Next, we investigated the critical role of annealing in shaping the phase coexistence and magnetism of iron oxide nanostructures, encompassing both nanorods and nanocubes. For better clarity and precision, magnetic measurements were carried out on high aspect ratio iron oxide nanorods (35 nm length, 7 nm width) for phase identification, as traditional characterization techniques like x-ray diffractometry tend to oversee the subtle variations of magnetic phases, especially with similar crystalline structures. Through meticulously controlled annealing at various temperatures and durations, phase transformations within the nanorods precisely trailed. Notably, the as-prepared nanorods, presumed to be solely Fe3O4, revealed the presence of α-Fe2O3 via cautious magnetic analysis, evidenced by the Morin transition. This investigation further elucidated how the more stable α-Fe2O3 phase progressively dominates with increasing anneal temperature and duration, this was further scrutinized in evaluating how a single parameter (anneal temperature or duration) influences the coexistence and evolution of these phases.

To further manipulate iron oxide phases, we explored the effects of annealing 30 nm as-prepared nanocubes containing Fe3O4 and α-Fe2O3 at 200 0C for 2 hours in various gas environments (ultra high purity O2, N2, He, and Ar). X-ray diffractometry and magnetic measurements revealed significant post-treatment modifications in nanocubes, specifically, the stark variations in saturation magnetization and coercivity across various gases prompted us to assess their magnetic hyperthermia efficiency. We explicitly focused on the two key factors affecting the efficacy; saturation magnetization and coercivity to determine which would play a major role in the response of the nanocubes in the presence of 400, 600, and 800 Oe AC fields in both water and agar mediums. Interestingly, the samples annealed with O2 exhibiting a higher coercivity led to a twofold increase in the specific absorption rate (1000 W/g) compared to the as-prepared nanocubes. This finding underscores the potential of controlled annealing in tailored environments to optimize the magnetic and structural properties.

Finally, we investigated the structural and magnetic properties of polycrystalline iron oxide superparticles, reaching particle sizes up to 400 nm, with each particle comprising nanocrystals, each with sizes in the 10-15 nm range, meticulously designed to retain the superparamagnetic behavior at room temperature. Large structures possessing superparamagnetic features of smaller nanostructures possess a compelling advantage enhanced controllability and reduced risk when navigating highly sensitive biological environments like the blood-brain barrier (BBB). With the inherent advantages, the structures were evaluated for their hyperthermia efficacy yielding highly promising specific absorption rates exceeding 250 W/g at a remarkably low 0.5 mg/ml. Furthermore, the structures were evaluated for the potential for polycrystalline size tunability.

In conclusion, the presented approaches underscore the paramount importance of nanostructure tunability in optimizing structural and magnetic characteristics for diverse applications. The strategical manipulation of phase composition, size, and shape can tailor iron oxide nanostructures to exhibit superior magnetic features and stability leading to enhanced performance in a variety of fields from biomedical applications to spintronics.

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