Doctor of Philosophy (Ph.D.)
Degree Granting Department
Matthias Batzill, Ph.D.
Xiaomei Jiang, Ph.D.
Hariharan Srikanth, Ph.D.
Sarath Witanachchi, Ph.D.
Transition Metal Oxides, XPS, XPD, RHEED, LEED
Throughout the last half century of materials science, significant motivations came from, and still do, the industrial applications of these materials. Whether it is electronic, thermal, tribological or chemical in nature, the study of metals, semiconductors and insulators eventually reveals that the surface plays a significant part in the properties of these materials. Understanding metal terminations reveals often that an oxide is the stable state of the metallic surface in an ambient atmosphere and the ability to predict and control these oxides has led to significant strides forward in not just the metallic bulk but the oxide as well.
Here we add to the understanding of the class of materials known as transition metal oxides by focusing on the structural and chemical nature of their surfaces. Vanadia, chromia and a new mixed metal oxide, VTiO3, all of which form the corundum structure and have physical properties that need further study. Specifically, Cr2O3 has been at the center of much debate over how oxygen chemical potential influences surface terminations and top layer relaxation. Chromia is a wide band gap (~3.4eV) insulator with substantial ligand field interaction and measurements of the 3d states reveal these states split to t2g and eg– consistent with the distorted octahedral.
V2O3 is known to be a Mott insulator and paramagnetic, properties that can be modified through dopants, stoichiometry and strain. In this work, solid solutions of V2O3 and Ti2O3 are studied. VTiO3, has been synthesized in a corundum – like structure by epitaxial growth on an isostructural α-Al2O3 substrate.
Section I offers a review of corundum like transition metal oxides and their surface properties and motivations of continued research. In section II we describe in detail, the critical components of PLD thin film growth and in the next section a review of the pertinent characterization techniques utilized in the process. Finally, the results are presented of the study of two transition metal oxide structures namely:
1) Novel VTiO3 in a corundum structure has been grown via Pulsed Laser Deposition – Molecular Beam Epitaxy on a single crystal Al2O3(0001) substrate. The sapphire substrate with modest lattice mismatch was utilized in an effort to compel heteroepitaxial growth of the VTiO3 film. Confirmation of the films structure & chemical state were performed by X-Ray diffraction, Transmission Electron Microscopy (HR), X-Ray Photo-electron Diffraction, Ultra-Violet Photo-Electron Diffraction and Reflection High Energy Electron expected that the metal ions exist in a 3+ charge state. While XPS clearly points to a V3+ charge state and this suggests that Ti should as well, however there is also a strong Ti4+ component present. EELS spectra support the existence of a mixed state Ti3+ & 4+. Broadening of the valance band edge as revealed by UPS spectra indicate that the 3d orbitals are occupied and that the a1g molecular states are occupied. The conflict in diffraction data supporting corundum and PES/EELS data suggesting a mixed state implies that additional final state effects are present and/or an oxygen rich structure.
2) Additionally, corundum like Chromium(III) Oxide is formed on a Cr(110) surface and characterized with X-Ray Photoelectron Diffraction, Low Energy Electron Diffraction and XPS for the purpose of characterizing surface termination and terminating layer relaxation. Comparison of the XPD diffraction data with known and previously discussed terminations reveal the as grown film does not conform. Consequently, we propose a new, stoichiometric termination with oxygen termination and 1st layer chromium interstitials. Atop this structure was grown an ultra-thin film of V2O3 by vanadium e-beam evaporation in background oxygen. This final structure supports the previously proposed vanadyl structured surface
Scholar Commons Citation
Kramer, Alan Richard, "Surfaces and Epitaxial Films of Corundum-Structured Mixed Metal Oxides." (2017). Graduate Theses and Dissertations.