| Self-assembled hetero-epitaxial films-with fruitful configurations benefitted by the advances made in synthesis technologies-have drawn considerable spotlight due to enthralling physics and their potential applications for next-generation devices. Recently, a high interface-to-volume ratio vertical nanostructure has exhibited abundant structural parameters, such as the component species, topological morphology, orientation relationship, hetero-interface bonding and the complex strain distribution, etc. The manipulations of structural parameters manipulation may lead to novel functionalities of nanocomposite material and thus promote the designing of novel flexible electronic devices. In this thesis, different hetero-epitaxial thin film systems synthesized from perovskite oxides and functional oxides such as ferrimagnetic spinels, optoelectronic semiconductors, etc, were investigated systematically by using transmission electron microscopy and our work paved diversified and efficient routes to manipulate structural parameters and improve the performance of these nanocomposites.First, we demonstrated the conversion relationships of different perovskite crystal structures by using theoretical inference, software simulation and transmission electron microscopy experiments. The orientational transformation matrices were obtained though theoretical derivations on the orientation conversion among cubic, orthorhombic and rhombohedral perovskite structures, which is confirmed by the results of simulation and high resolution transmission electron microscopy. This work provides a versatile way to discussion the distorted perovskite structures with the pseudo-cubic structure consistently.The CoFe2O4-BiFeO3 nanocomposite thin films, as a model system of perovskite-spinel heterostructure, were demonstrated novel nanostructure configurations with new orientation relationships and heterointerface structures grown on the (001)c-oriented substrates with different crystal structures and lattice parameters. Three different substrates was selected for the films grown. The results show that two phases spontaneously separated and heteroepitaxially grew on the different substrates exhibited a high degree of crystal Unity. CoFe2O4 nanopillars with the shapes of nano-cuboid, nano-plate, triangular prism, were embedded in the BiFeO3 matrix and went throughout the whole films in all of the cases. Unlike the unitary cube-on-cube orientation relationship reported widely, the crystal orientation of CoFe2O4 nanopillars was tuned among [001], [011] and [111] respectively, while the BiFeO3 matrix kept [001]c matching the substrates. Moreover, resemble results were observed in NiFe2O4-BiFeO3 heterostructure on (001)pc SrRuO3 buffered layer with nano-plate patterns, confirmed the front results. The different hetero-interface structures between the two component phases and also the substrate at an atomic scale have been investigated systematically by high resolution transmission electron microscopy. We demonstrated that structural continuity, surface energy anisotropy, and strain state at the heterointerfaces would affect the growth of these heterostructures with new orientation relation. The elastic energy at the heterointerface and surface energy of the nanopillars should be the determined factors in our present work. We may provide an effective way to explore more possible combinations of heterostructures and suggest the methods for the functionality regulation in the complex oxides films by controlling their orientations or interfaces.We report a new type of heteroepitaxial mesocrystal-embedded nanocomposites, (NiFe204)0.33:(La0.67Ca0.33Mn03)0.67, in which tiny NiFe2O4 nanocrystals aggregate into ordered octahedral mesocrystal arrays with{111} facets together with a concomitant structural phase transition of the La0.67Ca0.33MnO3 matrix upon post-annealing process. Based on combined magnetic and x-ray absorption spectroscopic measurements, significant enhancement in magnetic properties at the room temperature is observed due to the structure evolution induced magnetic transition of NiFe2O4 and the consequent magnetic coupling at the heterointerface mediating. This work demonstrates an elegant route to manipulate the intriguing physical properties of material systems by integrating desired functionalities of constituents via synthesis of a self-assembled mesocrystal embedded nanocomposite system.Heteroepitaxial ZnO and SrRuO3 have been successfully grown on SrTiO3 (111) substrates, forming a self-assembled nanostructure. The SrRuO3 nanopillars were homogeneously embedded in the ZnO matrix. A spontaneous orientation-tuning of the SrRuO3 pillars was observed, with the growth direction changing from [111]SRO to [011]SRO as the film thickness increases. The growth behavior of the SrRuO3 nanopillars can be attributed to a misfit strain transition from the biaxial strain controlled by the SrTiO3 substrate to the vertical strain controlled by the ZnO matrix. The combination of [011]-SrRuO3 and [0001]-ZnO gives a favorable matching in the nanocomposite films, resulting in a higher mobility of charge carriers and obvious photoconductivity. This vertically integrated configuration and regulation on the crystallographic orientations are expected to be employed in designing new functional nanocomposite systems for the applications in electronic devices.Self-assembled LaNiO3-NiO nanocomposite thin films were investigated to explore more interesting heterointerface structures in this vertical aligned nanostructure. A special growth behavior of the nanocomposites was demonsted with the orientation relationship:[011]LNOdomain-[001]NO-[001]LNOmatrix, which could result in abundant enthralling heterointerfaces. The plentiful combination forms with diverse heterointerfaces in the heterostructures may offer a desirable platform to manipulate and improve the intrinsic physical properties of each component phase in nanocomposite materials. |
PDF Full Text Request