Surfaces and Epitaxial Films of Corundum-Structured Mixed Metal Oxides.

Author

Alan Richard Kramer, University of South FloridaFollow

Graduation Year

2019

Document Type

Dissertation

Degree

Ph.D.

Degree Name

Doctor of Philosophy (Ph.D.)

Degree Granting Department

Physics

Major Professor

Matthias Batzill, Ph.D.

Committee Member

Xiaomei Jiang, Ph.D.

Committee Member

Hariharan Srikanth, Ph.D.

Committee Member

Sarath Witanachchi, Ph.D.

Keywords

Transition Metal Oxides, XPS, XPD, RHEED, LEED

Abstract

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 t_2g and e_g– 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 a_1g 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 1^st 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). USF Tampa Graduate Theses and Dissertations.
https://digitalcommons.usf.edu/etd/7418