Jae Hyuck Jang

Dimensionality Controlled Octahedral Symmetry-Mismatch and Functionalities in Epitaxial LaCoO3/SrTiO3 Heterostructures

Epitaxial strain provides a powerful approach to manipulate physical properties of materials through rigid compression or extension of their chemical bonds via lattice-mismatch. Although symmetry-mismatch can lead to new physics by stabilizing novel interfacial structures, challenges in obtaining atomic-level structural information as well as lack of a suitable approach to separate it from the parasitical lattice-mismatch have limited the development of this field. Here, we present unambiguous experimental evidence that the symmetry-mismatch can be strongly controlled by dimensionality and significantly impact the collective electronic and magnetic functionalities in ultrathin perovskite LaCoO3/SrTiO3 heterojunctions. State-of-art diffraction and microscopy reveal that symmetry breaking dramatically modifies the interfacial structure of CoO6 octahedral building-blocks, resulting in expanded octahedron volume, reduced covalent screening, and stronger electron correlations. Such phenomena fundamentally alter the electronic and magnetic behaviors of LaCoO3 thin-films. We conclude that for epitaxial systems, correlation strength can be tuned by changing orbital hybridization, thus affecting the Coulomb repulsion, U, instead of by changing the band structure as the common paradigm in bulks. These results clarify the origin of magnetic ordering for epitaxial LaCoO3 and provide a route to manipulate electron correlation and magnetic functionality by orbital engineering at oxide heterojunctions.

Water-mediated electrochemical nano-writing on thin ceria films

Bias dependent mechanisms of irreversible cathodic and anodic processes on a pure CeO2 film are studied using modified atomic force microscopy (AFM). For a moderate positive bias applied to the AFM tip an irreversible electrochemical reduction reaction is found, associated with significant local volume expansion. By changing the experimental conditions we are able to deduce the possible role of water in this process. Simultaneous detection of tip height and current allows the onset of conductivity and the electrochemical charge transfer process to be separated, further elucidating the reaction mechanism. The standard anodic/cathodic behavior is recovered in the high bias regime, where a sizable transport current flows between the tip and the film. These studies give insight into the mechanisms of the tip-induced electrochemical reactions as mediated by electronic currents, and into the role of water in these processes, as well as providing a different approach for electrochemical nano-writing.

Local probing of electrochemically induced negative differential resistance in TiO2 memristive materials

The early stages of electroforming in TiO(2) were explored using a combination of electrochemical strain microscopy and local I-V curve measurements. Negative differential resistance and corresponding surface deformation were observed below the electroforming voltages. Electrochemical strain microscopy allowed probing of the changes in local electrochemical activity during the pre-forming and forming stages. The associated structural changes were visualized by transmission electron microscopy. The results allowed an understanding of the electrochemical processes in the early stages of electroforming, and provide a comprehensive approach for exploring irreversible and partially reversible bias-induced transformations in solids.

In Situ Observation of Oxygen Vacancy Dynamics and Ordering in the Epitaxial LaCoO3 System

Vacancy dynamics and ordering underpin the electrochemical functionality of complex oxides and strongly couple to their physical properties. In the field of the epitaxial thin films, where connection between chemistry and film properties can be most clearly revealed, the effects related to oxygen vacancies are attracting increasing attention. In this article, we report a direct, real-time, atomic level observation of the formation of oxygen vacancies in the epitaxial LaCoO3 thin films and heterostructures under the influence of the electron beam utilizing scanning transmission electron microscopy (STEM). In the case of LaCoO3/SrTiO3 superlattice, the formation of the oxygen vacancies is shown to produce quantifiable changes in the interatomic distances, as well as qualitative changes in the symmetry of the Co sites manifested as off-center displacements. The onset of these changes was observed in both the [100]pc and [110]pc orientations in real time. Additionally, annular bright field images directly show the formation of oxygen vacancy channels along [110]pc direction. In the case of 15 u.c. LaCoO3 thin film, we observe the sequence of events during beam-induced formation of oxygen vacancy ordered phases and find them consistent with similar processes in the bulk. Moreover, we record the dynamics of the nucleation, growth, and defect interaction at the atomic scale as these transformations happen. These results demonstrate that we can track dynamic oxygen vacancy behavior with STEM, generating atomic-level quantitative information on phase transformation and oxygen diffusion.

Studying Dynamics of Oxygen Vacancy Ordering in Epitaxial LaCoO 3 / SrTiO 3 Superlattice with Real-Time Observation

Oxygen vacancies in perovskite oxides are of great interest both from fundamental viewpoint due to their effect on electronic and magnetic properties of oxides – and from applications viewpoint due to their importance for solid oxide fuel cells. In the latter case, understanding the dynamic behavior of vacancies is crucial. Oxygen vacancy (Vo) ordering can substantially affect local structure and ionic conductivity of the oxide matrix. Recently, Kim et al. have reported that local oxygen content and thus vacancy ordering can be quantified using lattice expansion via atomic position mapping in high angle annular dark field (HAADF) images with unit cell resolution in the La0.5Sr0.5CoO3- system.[1] The composition studied in [1] is remarkably beam resistant, however, in other related compounds such as LaCoO3 (LCO) exposure to the electron beam can lead to the emergence of vacancy ordering in previously disordered matrix. Adapting Kim et al. approach to a dynamic setting, we explore Vo behavior in the PLD grown LaCoO3/SrTiO3 (LCO/STO) superlattices and LCO films by HAADF and annular bright field (ABF) STEM.

Uncovering Structure-Properties Relations in Fuel Cells and Catalysts with Quantitative Aberration-Corrected STEM and EELS

Advances in materials science of fuel cells and catalysts promise global benefits: increased efficiency in power generation, decreased harmful emissions, and reduced energy use. Keys to fundamental understanding of the behavior of these materials, however, often lie on nanoor even atomic scales: functionality of solid oxide fuel cells is ultimately controlled by the static and dynamic behavior of oxygen vacancies; catalyst properties are governed by configuration and chemical composition of active centers. It is therefore crucial to be able to study relevant aspects of material behavior with atomic resolution. Recent advances in aberration-corrected electron microscopy and spectroscopy provide a wealth of structural and chemical information: local structure can be characterized with picometer precision [1], chemical identity of a single atom can be determined [2], and concentration of oxygen vacancies can be measured for every unit cell [3]. Scanning transmission electron microscopy (STEM) setting is especially useful since multiple types of signals can be collected simultaneously and correlated. These methods can be applied in both in situ and ex situ configurations.