Investigation of Interfacial Phenomena in Nanoscale Functional Oxides via Transmission Electron Microscopy

Transition metal oxides are some of the most diverse and rich materials in terms of flexibility and versatility of properties and structures. As a result, they have been exploited in many advanced engineering applications, ranging from effective visible-light photocatalysis, high performance batteries, to computer memristors and super-capacitors, etc. The demand for higher density and faster performance has pushed the size of multifunctional devices into the nanoscale regime. Nanoscale functional materials are usually integrated on to a substrate to make a viable and robust device. This places significant emphasis on the quality of the interface between the active functional material (typically on top) and the underlying substrate, as the properties of the functional material are usually correlated with the substrate used. A slight change in lattice structure, chemical intermixing or even electronic structure can result in significant modifications in material properties and functionality. This coupling calls for an understanding of the mechanical properties of functional materials at small length scales.Currently, the continued development of transmission electron microscopy (TEM) techniques has reinforced its role as one of the most important characterization methods to understand the properties of functional nanomaterials. In this thesis, a combination of TEM imaging and spectroscopy techniques are applied in various of transition metal oxide interfaces, in order to understand the role of local structural and electro-chemical fluctuation in property tuning in the functional nanomaterials. Firstly local strain has been investigated at the interface between the CoFe2O4 nanopillars and the Bi5Ti3FeO15 matrix thin film, which further explains the enhanced polarization properties. In addition, a detailed TEM investigation is carried out at the interface of NiO nanocrystals epitaxially grown on SrTiO3 substrate. It indicates that the resistive switching properties of NiO-SrTiO3 are interface-mediated phenomena. This study is followed by a TEM investigation of encapsulation at metal-oxide/metal-oxide interface. 3D morphology of SrTiO3 substrate deformation is revealed, despite of 2D limitation of TEM imaging. In the last part of this thesis, a full-scale model is developed. Supported by detailed microscopy evidence, it is able to successfully rationalize and predict nanocrystal morphology and multiple growth orientations