Graduate Thesis Or Dissertation
 

Understanding Charge and Mass Transfer Phenomena in Functional Oxide Heterostructures

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  • Oxides are a versatile class of materials with a wide range of electronic, optical, and magnetic properties. This makes them ideal for use in a multitude of functional devices for oxide electronics, electrochemical conversion, and storage. The overall performance of these devices is governed by atomic scale charge and mass transfer processes which necessitate fundamental level studies. Epitaxial thin films free from impurities and additives are ideally suited as model systems for mechanistic studies owing to the well-defined atomic interfaces, controlled orientation, and tunable defects. This PhD thesis is focused on using epitaxial oxide heterostructures as model systems to study intrinsic charge and mass transfer processes in selected material systems. We utilize a number of advanced experimental techniques such as in situ atomic force microscopy (AFM) and surface X-ray diffraction (SXRD) along with ab initio modeling using density functional theory (DFT) to establish critical structure-property relationships. In one of the studies, we used in situ electrochemical AFM and other supplementary characterization techniques to monitor the structural and morphological evolution of epitaxial LiCoO2 (LCO) films on SrTiO3 (STO) during charging and overcharging. We revealed that atomical scale defects in LCO such as antiphase boundaries (APBs) act as viable Li+ diffusion pathways during delithiation but are more susceptible to irreversible phase transformations under over-delithiation conditions. Moreover, LCO showed a higher degree of charge transfer for the same loading when the Li-containing planes were directly exposed on the surface rather than being parallel to it. Next, we presented a growth mechanism for Li2WO4, an electrolyte in solid-state Li-ion batteries (LIBs) which formed due to the reaction of out-diffusing Li+ in LCO/STO with WO3 species, by using high-resolution X-ray phi scans, AFM, scanning transmission electron microscopy (STEM) and crystallographic modeling. Our mechanism proposed that twin boundary defects in LCO directed along the STO[100/010] direction acted as preferred nucleation sites for the growth of Li2WO4 islands. Finally, we also investigated the reasons for the lower-than-expected 2-dimensional electron gas (2DEG) carrier density at the NdTiO3/SrTiO3 (NTO/STO) heterojunction using SXRD and coherent Bragg rod analysis (COBRA) along with DFT. We provided an atomic scale mechanism which suggested that Nd vacancies (VNd), stabilized by O and O2 species on the film surface during growth deplete the conduction band (CB) electrons causing a drop in carrier density. Through our comprehensive studies, we show that epitaxial model oxide heterostructures along with an appropriate combination of experimental and theoretical characterization allow for deepening our understanding on atomic scale charge and mass transfer processes in functional materials.
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  • This work was financially supported by the OSU-PNNL Distinguished Graduate Fellowship, Oregon State University and National Science Foundation (NSF) (CBET-1949870, CBET-2016192) and the U.S. Department of Energy (DOE), Office of Science (SC), Office of Basic Energy Sciences (BES), Early Career Research Program under Award No. 68278. X-ray photoelectron spectroscopy (XPS) and XRD studies were supported by U.S. DOE BES Materials Science and Engineering Division under Award No. 10122. In situ AFM monitoring work was supported by U. S. DOE under Award KC020105-FWP12152. A portion of the work was performed at the W. R. Wiley Environmental Molecular Sciences Laboratory, a DOE User Facility sponsored by the Office of Biological and Environmental Research. PNNL is a multi-program national laboratory operated for DOE by Battelle. The hybrid-MBE growth of NTO/STO was done at University of Minnesota supported by U.S. DOE through DE-SC002021. The SXRD and X-ray phi scans were carried out at beamlines 33-ID, and 33-BM of the Advanced Photon Source, respectively, a DOE SC User Facility, operated for the DOE SC by Argonne National Laboratory under Contract No. DE-AC02-06CH11357. This work also used resources of the National Energy Research Scientific Computing Center; a DOE SC User Facility supported by the SC of the U.S. DOE under Contract No. DE-AC02-05CH11231 using NERSC award BES-ERCAP0021800.
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  • Pending Publication
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  • 2022-06-10 to 2023-07-11

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