Synthetic and dynamic control in strongly correlated transition metal oxides
Marks, Samuel David
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Binary and mixed oxides incorporating transition metal cations host a broad range of scientifically compelling and technologically significant optical, electronic, and magnetic properties. Transition metal oxides (TMOs) are being explored for applications in energy storage, optoelectronics, sensors, and magnetic storage among many other potential uses. The diverse range of physical properties within this class of materials arises from the large flexibility in chemical compositions and crystal structures. These compositions and structures can be generated during synthesis or tailored after synthesis with external stimuli. In this thesis, I develop strategies for reaching structural and chemical states in transition metal oxides with technologically important optical and electronic properties. I demonstrate a new synthesis strategy for single-crystal SrVO3 films – a transparent conducting oxide with potential applications in display and photovoltaic technologies – using solid-phase epitaxy. By this technique, epitaxial layers of SrVO3 are crystallized from amorphous precursor films. The electrical conductivity and visible light transmission in these epilayers are comparable with SrVO3 formed through other epitaxial synthesis methods. This synthesis route employs thin film deposition and crystallization techniques that are scalable to m2 surface areas. Scalability is a crucial step for commercial applications of transparent conducting oxide layers. Crystal growth from amorphous precursor films is a recent development for transition metal oxides that have traditionally been synthesized using vapor-phase epitaxy. As a result, fundamental insight into the amorphous-to-crystalline transformation and defect formation processes in solid-phase epitaxy for transition metal oxides is comparatively rare. In situ synchrotron x-ray characterization is a powerful experimental approach for gathering mechanistic insight for crystal growth processes. I have designed new instrumentation for synchrotron x-ray studies of the amorphous layer deposition, crystallization, and defect formation processes inherent to solid-phase epitaxy. This instrumentation combines a vacuum sample deposition and crystallization environment with x-ray focusing optics for in situ x-ray microbeam diffraction, reflectivity, and scattering studies. Design features and key capabilities are demonstrated through a series of results from experiments performed during the commissioning of the instrument at the Advanced Photon Source. In a separate ex-situ study, I examine the crystal structure of micrometer-scale regions of SrTiO3 crystallized from nanoscale seeds using lateral solid-phase crystallization. Using a high-energy synchrotron x-ray beam focused to 200 nm, I reveal a continuous rotation in the lattice planes in the laterally crystallized regions. A rotation of nearly 25° per micrometer of lateral crystallization is measured for several SrTiO3 crystals independent of the crystallographic orientation of the growth front. The uniform lattice rotation rate suggests a single defect formation process that is characteristic of lateral crystal growth through an amorphous precursor layer. These findings support a hypothesis that the lattice rotation is driven by dislocations that form in response to mechanical stresses arising from the density difference across the crystal-amorphous interface. ¬Controlling the oxygen environment is crucial to forming specific structural phases during synthesis. Similarly, modifications to oxygen stoichiometry can be used to modify the physical properties in epitaxial thin films of multivalent transition metal oxides. In this project, I use x-ray nanobeam diffraction and scanning near-field optical microscopy to simultaneously probe the structural and optoelectronic features of oxygen-deficient epitaxial monoclinic vanadium dioxide thin-films. In this study, an electrically conductive phase is patterned in insulating vanadium dioxide using intense electric fields delivered from an atomic force microscope probe. Electrical conductivity arises from oxygen vacancies created in the presence of the electric field that modify the electronic band structure. The stability and relaxation of the electrically conducting state are governed by the oxygen vacancy dynamics that can be manipulated with hard x-ray irradiation. This study demonstrates a way to manipulate nanoscale structural and electronic states in vanadium dioxide with local electric fields and focused hard x-rays, bringing new insights into the stability of the oxygen-deficient conductive phase of vanadium dioxide.
Materials Science and Engineering; Electronic Materials; In Situ X-ray Scattering