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Abstract
This PhD thesis investigates the role of structural and chemical defects located at interfaces of thin-film functional materials using transmission electron microscopy (TEM) techniques. The outcomes obtained demonstrate how advanced electron microscopy can be an extremely powerful method to provide insight into the interfacial mechanisms that govern the structural, chemical and functional properties of semiconductor and multiferroic materials systems.
The thesis first investigates the chemical and structural properties in one of the most well-known ferromagnetic materials, lanthanum strontium manganite (La0.67Sr0.33MnO3 (LSMO)) thin films, epitaxially fabricated on (001) strontium titanate (SrTiO3 (STO)) substrates. This system is chosen as a model epitaxial oxide thin film to develop the required skills for advanced TEM techniques. It employs comprehensive high-resolution transmission electron microscopy (HRTEM) and electron diffraction methods to unravel the structure-property correlations in the LSMO/STO system. The LSMO is epitaxially grown on STO with no structural defects. The LSMO/STO interface is chemically sharp and no diffusion is revealed at the interface.
The thesis then provides a complete structure-photoelectrochemical property correlation study in non-oxide ZnS/GaP (ZG) multilayered films. It investigates nano-scale origins of the observed photoelectrochemical (PEC) properties under visible light in the aforementioned system. The investigations primarily use TEM techniques accompanied by relevant theory to reveal the significance of the interfacial structure for PEC performance. It shows that the multilayers carry a large volume fraction of defects at the interfaces, associated with an inhomogeneous residual strain at the aforementioned defect cores. These defects themselves act as excellent channels for diffusion. Indeed for each ZG interface, a ~5 nm interdiffused region with an effective chemical composition of a ZnS-GaP solid solution is observed. This solid solution is expected to have better PEC performance compared to each individual parent constituent, based on prior density functional theory (DFT) predictions. This mechanism is shown to be extendable through the whole film by creating ZG multilayers with intentionally designed ultra-short periods.
Finally, the thesis demonstrates a structure-chemical study in a more complex case of multiferroic solid solution of BiTi0.1Fe0.8Mg0.1O3 (BTFMO) and CaTiO3 (CTO) film on LSMO/STO substrate. The impact of changes in structure, composition and bonding that occur in only a few atoms either around an interfacial defect site or in the films on the overall properties of the material needs to be understood on a local level. It reveals that the BTFMO-CTO film is grown epitaxially on the substrate. The crystal structure of the film undergoes a phase transition to a tetragonal phase, caused by the tensile strain in an out of plane direction. The energy-dispersive X-ray spectroscopy (EDS) confirms the homogeneous distribution of elements through the film. This TEM study is successfully conducted as a part of planning to investigate the origin of the strong polarization in BTFMO-CTO thin film on LSMO/STO substrate.