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Abstract
This thesis investigates the interfacial chemistry and lattice structure via transmission electron microscopy (TEM) of (001) epitaxial nickel oxide (NiO) nanostructures grown on strontium titanate (SrTiO3) and magnesium aluminate (MgAl2O4) substrates. Here the ‘interface’ exclusively refers to the interface between the nanostructure and the underlying substrate. A wide range of TEM techniques such as bright field imaging, electron diffraction, high annular angle dark field (HAADF), energy dispersive x-ray spectroscopy (EDS) and electron energy loss spectroscopy (EELS), etc. are used to characterize the nanostructures.
The NiO-SrTiO3 component of this thesis finds the nanostructures have two distinct sets of bounding facets, belonging to the {111} and {113} family of planes. Atomic level investigations reveal that the interface is wavy and incoherent. A terrace is observed between the base of the nanostructure and the substrate in addition to the deformation of the overall interface. High angle annular dark field (HAADF) scanning transmission microscopy (STEM) imaging shows a dark layer at the interface, thus indicating that the interface is composed of relatively lighter elements such as titanium and oxygen. Electron energy loss spectroscopy (EELS) shows the elements from the substrate diffuse across the interface and reach as far as 2nm into the nanostructure side. Oxygen vacancies were found at the interface. This finding gives evidence to a previously postulated model [1] that the oxygen vacancies act as minority carriers and trigger the resistive switching properties of nickel oxide.
The study of the NiO-MgAl2O4 system shows nanostructures are much smaller than those grown on SrTiO3. The facets also differ for these nanostructures; showing only {111} and {001} planes. The interface of these nanostructures is found to be also incoherent despite a lower mismatch strain between NiO and MgAl2O4 (~3.3%) compared to the former system (~6.8%). Within resolution, energy dispersive x-ray spectroscopy (EDS) analysis finds no chemical mixing between the substrate and nanostructure.
[1] Sullaphen, J., et al., Interface mediated resistive switching in epitaxial NiO nanostructures. Applied Physics Letters, 2012. 100(20): p. 203115-203115-5.