Hyung Bin Bae

Frenkel-Defect-Mediated Chemical Ordering Transition in a Li–Mn–Ni Spinel Oxide†

Using spinel-type Li(Mn(1.5)Ni(0.5) )O4 with two different cations, Mn and Ni, in the oxygen octahedra as a model system, we show that a cation ordering transition takes place through the formation of Frenkel-type point defects. A series of experimental results based on atomic-scale observations and in situ powder diffractions along with ab initio calculations consistently support such defect-mediated transition behavior. In addition to providing a precise suggestion of the intermediate transient states and the resulting kinetic pathway during the transition between two phases, our findings emphasize the significant role of point defects in ordering transformation of complex oxides.

Formation of Two-Dimensional Homologous Faults and Oxygen Electrocatalytic Activities in a Perovskite Nickelate

Atomic-scale direct probing of active sites and subsequent elucidation of the structure-activity relationship are important issues involving oxide-based electrocatalysts to achieve better electrochemical conversion efficiency. By generating Ruddlesden-Popper (RP) two-dimensional homologous faults via simple control of the cation nonstoichiometry in LaNiO3 thin films, we demonstrate that strong tetragonal distortion of [NiO6] octahedra is induced by more than 20% elongation of Ni-O bonds in the faults. In addition to direct visualization of the elongation by scanning transmission electron microscopy, we identify that the distorted [NiO6] octahedra in the faults show considerably higher electrocatalytic activities than other surface sites during the electrochemical oxygen evolution reaction. This unequivocal evidence of the octahedral distortion and its impact on electrocatalysis in LaNiO3 suggests that the formation of RP-type faults can provide an efficient way to control the octahedral geometry and thereby remarkably enhance the oxygen catalytic performance of perovskite oxides.

Probing dopant segregation in distinct cation sites at perovskite oxide polycrystal interfaces

Although theoretical studies and experimental investigations have demonstrated the presence of space-charge-induced dopant segregation, most work has been confined largely to the crystal-free surface and some special grain boundaries, and to the best of our knowledge there has been no systematic comparison to understand how the segregation varies at different types of interfaces in polycrystals. Here, through atomic-column resolved scanning transmission electron microscopy in real polycrystalline samples, we directly elucidate the space-charge segregation features at five distinct types of interfaces in an ABO3 perovskite oxide doped with A- and B-site donors. A series of observations reveals that both the interfacial atomic structure and the subsequent segregation behaviour are invariant regardless of the interface type. The findings in this study thus suggest that the electrostatic potential variation by the interface excess charge and compensating space charge provides a crucial contribution to determining not only the distribution of dopants but also the interfacial structure in oxides.Space-charges in polycrystalline materials can drive segregation of dopants, however an in-depth understanding of this process is still missing. Here, the authors show that in polycrystalline perovskites the space-charge segregation and interfacial structure are nearly identical irrespective of the interface type.

Subsurface Space-Charge Dopant Segregation to Compensate Surface Excess Charge in a Perovskite Oxide

Since the first prediction by Frenkel, many follow-up studies have been carried out to show the presence of subsurface space-charge layers having the opposite sign to that of the excess charge at the surface, producing overall neutrality in ionic crystals. However, no precise experimental evidence demonstrating how the aliovalent solutes segregate in the space-charge region beneath the surface has been provided over the past several decades. By utilizing atomic-scale imaging and chemical probing in a perovskite oxide, the origin of the surface excess charge at the topmost surface and the position of segregated dopants in the space-charge region is precisely determined. The impact of the space-charge contribution to the dopant distribution near the surface in oxide crystals is explored.

Defect-engineered Si1−xGex alloy under electron beam irradiation for thermoelectrics

We report the development of a defect-engineered thermoelectric material using Si1−xGex alloys grown on a c-plane sapphire substrate via electron beam (E-beam) irradiation. This paper outlines the idea of growing the Si1−xGex film at relatively high temperatures to obtain good crystalline properties, then controlling the amount of twins or dislocations through ex situ electron-beam irradiation. The current work suggests that structure reconstruction by bond rearrangement through E-beam irradiation may be used for tailoring thermoelectric properties.

High-Electron-Mobility SiGe on Sapphire Substrate for Fast Chipsets

High-quality strain-relaxed SiGe films with a low twin defect density, high electron mobility, and smooth surface are critical for device fabrication to achieve designed performance. The mobilities of SiGe can be a few times higher than those of silicon due to the content of high carrier mobilities of germanium (p-type Si: 430 cm2/V·s, p-type Ge: 2200 cm2/V·s, n-type Si: 1300 cm2/V·s, and n-type Ge: 3000 cm2/V·s at 1016 per cm3 doping density). Therefore, radio frequency devices which are made with rhombohedral SiGe on -plane sapphire can potentially run a few times faster than RF devices on SOS wafers. NASA Langley has successfully grown highly ordered single crystal rhombohedral epitaxy using an atomic alignment of the direction of cubic SiGe on top of the direction of the sapphire basal plane. Several samples of rhombohedrally grown SiGe on -plane sapphire show high percentage of a single crystalline over 95% to 99.5%. The electron mobilities of the tested samples are between those of single crystals Si and Ge. The measured electron mobility of 95% single crystal SiGe was 1538 cm2/V·s which is between 350 cm2/V·s (Si) and 1550 cm2/V·s (Ge) at 6 × 1017/cm3 doping concentration.

Manipulation of Nanoscale Intergranular Phases for High Proton Conduction and Decomposition Tolerance in BaCeO3 Polycrystals

In many ion-conducting polycrystalline oxides, grain boundaries are generally accepted as rate-limiting obstacles to rapid ionic diffusion, often resulting in overall sluggish transport. Consequently, based on a precise understanding of the structural and compositional features at grain boundaries, systematic control of the polycrystalline microstructure is a key factor to achieve better ionic conduction performance. In this study, we clarify that a nanometer-thick amorphous phase at most grain boundaries in proton-conducting BaCeO3 polycrystals is responsible for substantial retardation of proton migration and moreover is very reactive with water and carbon dioxide gas. By a combination of atomic-scale chemical analysis and physical imaging, we demonstrate that highly densified BaCeO3 polycrystals free of a grain-boundary amorphous phase can be easily fabricated by a conventional ceramic process and show sufficiently high proton conductivity together with significantly improved chemical stability. These findings emphasize the value of direct identification of intergranular phases and subsequent manipulation of their distribution in ion-conducting oxide polycrystals.