Patterning and Structural Engineering of Dimensionally Constrained Functional Oxide Nanostructures

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The current trend of ceramic nanotechnology has motivated an ever-increasing need to achieve exquisite control over size, shape, and spatial confinement for functional oxide architectures, in an equivalent manner demonstrated for semiconductors. However, the unique nature of ceramics has posed major challenges for most traditional nanofabrication technologies, putting the development of innovative oxide nanopatterning schemes under the spotlight. Dimensional and spatial confinement of functional oxides has also raised extensive intellectual interests since it carries a profound bearing upon their microstructure variation and leads to often superior performances. This further underlines the need for exploring the "materials science and engineering" of nano-constrained oxides, i.e., to fabricate nanopatterns with precise geometrical control at various dimensionalities, and to tailor their microstructural and functional characteristics. This dissertation presents one strategy to achieve such objectives. We have developed a versatile nanofabrication approach, termed variable pressure-soft-electron beam lithography (VP-soft-eBL) that successfully resolves the generic challenges in patterning oxides and enables high resolution fabrication of diverse materials on a multitude of substrates. A strategy based on VP-soft-eBL was derived for microstructural and morphological control on the nanostructures, particularly that of ferroelectrics and ferrimagnets. The effect of pattern aspect ratio on the microstructure evolution has been investigated for CoFe2O4 and BaTiO3 nanodiscs on single crystal substrates with appropriate lattice matching. Following this strategy, high quality epitaxial patterns can be readily achieved from amorphous form during annealing. VP-soft-eBL portfolio was then expanded significantly towards multi-dimensional patterning capability to facilitate systematic study on the confinement phenomena. We investigated the beam skirt effect on electron energy deposition profile in VP-eBL and demonstrated successful fabrication of architectures spaning 0-dimensional dots, 1-dimensional lines, 2-dimensional high density grids, and 3-dimensional heterostructures. Embedded in these efforts were varied site-specific characterizations on the nanostructures. Our initial study using x-ray microdiffraction indicated localized deformation field induced in the substrate at vicinity of epitaxial pattern edges. Our approach of building and tailoring nano-constrained functional oxides is highly flexible, serving for a wide variety of materials and diverse types of architectures. The protocol reported in this dissertation lays the groundwork for in-depth exploration of the rich phenomena related to spatial/dimensional confinement. Further refinement of our strategy may provide an effective tool to harness the technological opportunities of diverse functional materials and pave the way for innovative applications which require nanopatterned architectures.

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  • 09/06/2018
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