The performance and reliability of many electronic devices is frequently governed by the stability and durability of the device?s active component materials. For example, atomic level structural defects, or interfaces between different materials often cause decreases in device performance, or total failure at a macro level. At the same time, however, some future applications will actually rely on the presence of these atomic level defects and interfaces to improve the device?s performance. These defects and interfaces are unavoidable, but by understanding how to control and manipulate their influence on a material?s properties, we can improve the functionality of the material on the macro level. Accordingly, a fundamental understanding of how atomic-scale defects or interfaces influence a material?s macroscopic behavior is essential to continuing the development of reliable, next generation devices. Cobalt oxides are one class of ceramic materials that has recently attracted scientific attention as an inexpensive, non-toxic, and highly stabile group of compounds with properties such as superconductivity, thermo-electric behavior, or magneto-resistive behavior. These properties make cobalt oxides attractive for use in a wide range of new device applications. For example, the magnetic properties of cobalt oxides have the potential to revolutionize magnetic storage in high capacity solid-state computer hard-drives, while their thermo-electric properties may lead to the development of coatings that will allow heat from automotive tailpipes or furnace exhaust pipes to be converted into electricity. At present, however, cobalt oxides are neither reliable nor efficient enough for viable macroscopic applications, because there is a lack a fundamental understanding of what is happening with the material on the atomic-level. This CAREER grant utilizes atomic-resolution scanning transmission electron microscopy combined with electron energy-loss spectroscopy (EELS) to investigate the fundamental mechanisms governing the magnetic and electrical properties of cobalt-oxide ceramics and the effects of defects and interfaces on these properties. The educational aspect of this proposal involves training of undergraduate and graduate students, in particular underrepresented minorities, in state-of-the-art materials characterization. Summer programs for undergraduate students will be developed, and the participation of undergraduate students in research projects will be fostered further through the PI?s Journal of Undergraduate Research at the University of Illinois at Chicago.
objective of this CAREER proposal is to examine how charge, orbital and spin-interactions influence the magnetic and electronic structures of mixed-valence cobalt-oxide ceramics. Two systems will be examined: LaCoO3 and Ca3Co4O9. These systems were chosen for their intriguing, potentially widely useful properties, and for their status as model structures for other strongly correlated cobalt-oxide ceramics. LaCoO3 exhibits magneto-transport properties as well as two spin-state transitions that result in magnetic phase transitions without any structural transition, while the misfit-layered Ca3Co4O9 shows an exceptionally high thermo-power. Atomic-resolution Z-contrast imaging in a scanning transmission electron microscope (STEM) with (EELS) and in-situ heating/cooling experiments (10 < T < 1000 K) is being used to study the local atomic and electronic structures of these materials. The PI?s laboratory setup includes an aberration-corrected STEM, which will allow for sub-Ã… spatial-resolution and sub-eV energy-resolution, as well as a conventional TEM/STEM for atomic-resolution insitu heating and cooling experiments. The insitu STEM analysis will be supported by MBE thinfilm synthesis, macroscopic magnetization, transport measurements, and surface characterization, as well as first principles modeling. This combination of experimental and theoretical techniques enables the fundamental structure-properties relationship of charge, orbital and spin-interactions in mixed-valence cobalt-oxide ceramics to be unraveled, which could advance the field of electro-ceramics and lead to the discovery of emergent phenomena that can be used as innovative devices and sensors. An important feature of this program is the integration of research and education through the training of undergraduate and graduate students in cutting-edge transmission electron microscopy and theoretical materials science.
Over the last 5 years, the PI has focused on studying complex oxide interfaces using aberration-corrected scanning transmission electron microscopy. Intellectual Merit: Using a combination of atomic-resolution HAADF and ABF imaging, the PI has studied the effects of interfacial strain and doping on the ferromagnetic ordering transition in La1-xSrxCoO3 thin film and single crystal materials. The PI reported that the Co3+-ion spin state can be directly measured using EELS, and that the ferromagnetic ordering transition in epitaxially strained LaCoO3 thin films is the result of the stabilization of intermediate spin-state Co3+-ions. Electron-circular dichroism measurements were conducted to show that the strained films are uniformly ferromagnetic. In addition, the PI has established a collaboration with Prof. A. Gupta at the University of Alabama to growth single crystal thin films of Ca3Co4O9 on various substrates, including LaAlO3, Al2O3 and LSAT. It was demonstrated that the lattice mismatch between Ca3Co4O9 and the various substrates does not induce any strain in the films, but rather results in the formation of an interfacial buffer layer. As a result, a method was developed to grow ultra-thin films on SrTiO3 which has shown the highest Seebeck coefficient of any pristine Ca3Co4O9 material. The PI has shown that this increase in the Seebeck coefficient is related to the carefully controlled presence of stacking faults which increase the Co spin state and thus the thermoelectric transport properties. Dopants, such as Ti and Bi have not shown to affect the Seebeck coefficient. Furthermore, the PI has collaborated with Prof. S. Ogut at the University of Illinois - Chicago to establish first-principles calculation for the incommensurate Ca3Co4O9 system. As the result of this collaboration, the PI has demonstrated that the Co-ion spin state is responsible for the high Seebeck coefficient in Ca3Co4O9 and showed, from first principles, that the thermal conductivity is highly anisotropic in Ca3Co4O9. In addition to these projects, the PI has also studied the atomic and electronic structures of ultrathin SrTiO3 films on GaAs, the domain structure of ferroelectric BaTiO3 on GaAs and the dynamical control of orbital occupations via a ferroelectric-induced polar state in metallic manganite. Atomic-resolution annular bright field imaging enables the PI to characterize the position of light elements, such as oxygen, to determine the octahedral distortions and quantify the ferroelectric polarization down to a few picometers. This unprecedented resolution is afforded by the new aberration-corrected JEOL ARM200CF that was purchased using another NSF MRI-R2 grant (DMR-0959470). Broader Impacts: This grant has supported 1-2 graduate and 2-3 undergraduate students every year. More specifically, Yuan Zhao, Qiao Qiao and Ahmet Gulec were supported as graduate students over the last 4 years. In September 2013, Qiao Qiao defended her PhD thesis, and began a post-doctoral position at Vanderbilt University with S. Pantelides and S.J. Pennycook. Ahmet Gulec spent the summer of 2013 in the group of P. Schattschneider at the Technische Universitat in Vienna, where he learned about atomic-resolution circular dichroism measurements. Ahmet will defend his thesis in Sept. 2014. He has already accepted a post-doc position in the group of Prof. L.D. Marks and Northwestern University. Yuan Zhao was partially supported by this grant and he graduated in 2011. He now works for Johnson Matthey as an electron microscopists. Overall, 10 papers have been published as the result of this CAREER award, one preliminary patent has been filed and 16 oral presentations at national and international conferences have been given. The PI has also founded and maintained the Journal of Undergraduate Research as part of this grant. As the editor, the PI has published one issue per year over the last four year, each containing 10-14 papers by undergraduate student who have performed research at UIC.
