Size-Specific ZrO₂ and HfₓZr₁₋ₓO₂ Nanoclusters and Their Magnetic and Nanodevice Applications
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Nanoclusters, generally defined as assemblies of a discrete number of atoms or molecules in the size range of 1-10 nm, have attracted a lot of attention due to their unique properties that bridge the gap between isolated atoms or molecules and bulk materials. Nanoclusters possess distinct electronic, optical, magnetic, and catalytic characteristics, which are often profoundly different from those observed in bulk materials or even larger nanoparticles. These notable differences arise from quantum confinement effects, high surface-to-volume ratios, and the specific atomic arrangements in these nanoscale clusters that often include the presence of defects. The understanding and manipulation of these properties are pivotal for harnessing nanoclusters in diverse fields, ranging from nanoelectronics to catalysis, biomedicine, and advanced material design. The oxides of group IV-B transition metals, notably ZrO2 and HfO2, have attracted significant research interest due to their high dielectric constants, wide bandgaps, pronounced refractive indices, and superior thermal stability. Additionally, oxygen vacancy defects within these oxides, particularly in the nanocrystalline forms, contribute to the manifestation of intriguing phenomena. The present work delves deeply into the synthesis methodologies, precise characterization techniques, and the multifaceted application potential of the ZrO2 and HfxZr1-xO2 (x≤1) nanoclusters. Through this exploration, we aim to elucidate the intricate relationship between the defects in the physical structure of the nanocluster, composition, and their resulting macroscopic behaviors, with a special focus on the ferromagnetism, providing insights that could pave the way for future innovations in nanotechnology. Monosized ZrO2 nanoclusters (NCs) are deposited over a large area by using gas- phase condensation followed by in-situ size selection by a quadrupole mass filter. These size-specific NCs exhibit sub-oxide photoemission features at binding energies that are dependent on the cluster size (from 3 to 9 nm), which are attributed to different oxygen vacancy defect states. These dopant- free ZrO2 NCs also show strong size-dependent ferromagnetism, which provides distinct advantages in solubility and homogeneity of magnetism when compared to traditional dilute magnetic semiconductors. A defect-band hybridization-induced magnetic polaron model is proposed to explain the origin of this size-dependent ferromagnetism. This work demonstrates a new protocol of magnetization manipulation by nanocluster size control and promises potential applications based on these defect-rich size-selected NCs. Using two metal targets in the gas-phase condensation technique, we synthesize, for the first time, size-specific hybrid HfxZr1-xO2 (x ≤ 1) NCs that can be precisely tuned from 5 nm to 14 nm in size while adjusting the Zr and Hf composition. The crystallinity of the hybrid NCs is found to vary with the NC size obtained under specific deposition conditions, from amorphous for small NCs < 6 nm, to single crystalline for 6-10 nm NCs, to core-shell for NCs with higher Hf content and to polycrystalline for larger NCs > 10 nm with high Zr content. For the single-crystalline HfxZr1-xO2 NCs, we observe, for the first time for NCs, the special orthorhombic (Pca21) structure found only in the HfZrO2 film prepared under extreme conditions. Surprisingly, the measured bandgaps of these NCs are found to increase with the cluster size, in contrast to expected increasing band gap with decreasing NC size. The XPS spectra clearly show that the Zr 3d components can be attributed to oxygen vacancy defects and substitution of Hf for Zr in the lattice. A new model involving Hf induced polaron is proposed to describe the physical and electronic structures of these novel bimetallic hybrid oxide NCs. This work establishes a general formation protocol for other hybrid semiconductor NCs, while the HfxZr1-xO2 (x<1) NCs with novel phase and polarization could provide promising electrical properties for the next generation non-volatile memory device applications. To understand the behavior of the electrons within the defects of these NCs and explore their electronic properties, we fabricate nano-electrodes, including nano interdigital electrodes and nanogaps. As one of the most crucial procedures in the electronic device fabrication, patterning is studied by comparing the results obtained by maskless optical lithography, electron beam lithography, and ion beam lithography with a gas field He ion source and with a SiAu liquid metal alloy ion source. Helium ion beam lithography is found to offer the most refined feature resolution, while the Si ion beam lithography demonstrates its fastest patterning speed in creating nanofeatures, particularly by taking advantage of its unique direct-write capability. Using ion beam lithography, a nano-IDE with a 43-nm gap is created with direct writing of Au++ ion beam on a Pt film. This technology also enables precise nanogap device production with sharp edges that are crucial for tunneling and electron hopping studies. The spacing of these nanogaps can be fine-tuned through ion beam exposure. We also fabricate a single-nanocluster device, for the first time, by integrating HfxZr1- xO2 NCs into these nanogap devices. Such nano-electrodes serve as platforms for measuring the electronic properties of NCs, with promising potential for other nano and quantum device applications.
Cite this version of the work
Xiaoyi Guan (2024). Size-Specific ZrO₂ and HfₓZr₁₋ₓO₂ Nanoclusters and Their Magnetic and Nanodevice Applications. UWSpace. http://hdl.handle.net/10012/20266