One-dimensional Nanostructures of Hafnium Oxides: Fabrication with and without Ti/Fe Doping, and Magnetic Properties
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Since the discovery of giant magnetoresistance (GMR) in the late 1980s, which marked the emergence of spintronic technology, the integration of conventional semiconductor-based electronics with spintronics has always been of great interest due to important technological advantages. Dilute magnetic semiconductors (DMSs) have been developed as a viable solution for realizing this idea. The recent generation of DMSs are dilute magnetic semiconducting oxides (DMSOs) that promise real-life device applications due to their high Curie temperatures well above room temperature. Among all DMSOs, HfO2 has received increased attention because of its technologically important properties such as high dielectric constant (κ~25), wide bandgap (~5.7 eV), high refractive index (n=2.9), and excellent thermal and chemical stabilities. More importantly, HfO2 is the most CMOS-technology compatible metal oxide, and its ferromagnetic properties therefore promise easy integration of CMOS technology with spintronics. As the structural defects, particularly oxygen vacancies, are believed to play an important role in inducing ferromagnetism in DMSOs, nanostructures of HfO2, especially one-dimensional (1D) nanostructures with high specific surface areas, are expected to exhibit enhanced ferromagnetic properties. Moreover, single-crystalline 1D nanostructures with their high crystal quality provide an excellent material system for studying the correlation between oxygen vacancy defects and ferromagnetism due to the minimal effects of other structural defects. However, the synthesis of single-crystalline HfO2 1D nanostructures have hitherto been unsuccessful because of the extremely low vapor pressures and high melting points of Hf (2233 ℃) and HfO2 (2800 ℃), and also of the suppression of vapor-liquid-solid (VLS) growth arising from pulsed laser ablation of oxide targets. In the present work, we have synthesized undoped and doped HfO2 nanostructures with different morphologies to investigate their novel magnetic properties. By precisely controlling the growth parameters in a catalyst-assisted pulsed laser deposition (PLD) system and by using different growth templates including chemically oxidized Si (Ox-Si), Ox-Si predeposited with gold nanoislands (GNI/Ox-Si) and with tin alloyed gold nanoislands (Sn-GNI/Ox-Si), we have been able to grow HfO2 nano square pyramids and nano triangular pyramids on Ox-Si and GNI/Ox-Si, and nano-tetrahedrons, undoped 1D nanostructures [nanowires (NWs), nanospikes, nano-columns], Ti-doped and Fe-doped nanospikes, and Fe-doped distorted nanocubes on Sn-GNI/Ox-Si. The 1D nanostructures synthesized in this work are the first single-crystalline 1D HfO2 nanostructures ever reported. In the first phase of the present work, we characterize the morphology, crystal structure and growth direction of the undoped HfO2 nanostructures using scanning electron microscopy, glancing incidence X-ray diffraction, and high resolution transmission electron microscopy, along with structural simulation by calculated atomic models. Our investigations reveal that Sn plays a crucial role in promoting the VLS growth of 1D nanostructures by alloying with GNIs to form Sn-GNI to increase both the VLS growth nucleation and growth rates. Furthermore, X-ray photoelectron spectroscopy shows that HfO2 NWs are more oxygen-deficient than the HfO2 nano square pyramids. Their room temperature ferromagnetic behavior compared to the weak paramagnetic behavior of HfO2 nano square pyramids therefore confirms a strong correlation between high temperature ferromagnetism and the amount of oxygen vacancies in the lattice. Employing a modified bound magnetic polaron-hybridized band ferromagnetism model, we explain the role of oxygen vacancies in inducing high temperature ferromagnetism in HfO2 NWs. In an attempt to fabricate HfO2 nanostructures with higher magnetic saturation, Ti-doped and Fe-doped HfO2 nanostructures are also synthesized in this work. Morphological investigations reveal that Ti doping has a minor effect on the VLS growth mechanism producing Hf1-xTixO2 1D nanostructures (nanospikes) at different Ti atomic concentrations (x=0.01, 0.10, 0.25 and 0.50). On the other hand, doping with a magnetic material such as Fe is found to restrict VLS growth significantly, yielding nanospikes for 1 at. % Fe doping (Hf0.99Fe0.01O2 nanospikes), but distorted nanocubes with stacked crystal flakes for 5 at. % (Hf0.95Fe0.05O2), 10 at. % (Hf0.90Fe0.10O2) and 20 at. % (Hf0.80Fe0.20O2) Fe doping. Moreover, Ti doping up to 10 at. % is found to slightly increase the magnetic saturations of the HfO2 nanostructures, while further doping (25 at. % and 50 at. %) reduces the magnetic saturation back to the same order of magnitude as that of the undoped HfO2 NWs, which is attributed to the presence of the HfTiO4 phase with a possible higher oxygen vacancy formation energy. In contrast, Fe doping has significantly increased magnetic saturations (e.g., up to two orders of magnitude higher than that of undoped HfO2 nanostructures for 10 at. % doping) through F-center exchange interaction in the lattice. Further doping (20 at. %), however, is found to produce extrinsic properties due to possible atomic-scale magnetic ion clustering. The present work offers valuable insights to the synthesis of other oxide nanostructures, especially 1D nanostructures of complex oxides. Indeed, the use of appropriate metal-alloy catalysts could provide the key to the PLD growth of other hitherto unobtainable 1D nanostructures of other metal oxides. Their magnetization measurements could further advance our understanding of defect-induced ferromagnetism in DMSO materials. The single-crystalline HfO2 nanostructures introduced here could be an important stepping stone toward the integration of CMOS and spintronics technologies, given their high Curie temperatures and high material compatibility.
Cite this version of the work
Mahdi Beedel (2022). One-dimensional Nanostructures of Hafnium Oxides: Fabrication with and without Ti/Fe Doping, and Magnetic Properties. UWSpace. http://hdl.handle.net/10012/17852