Intro

For several decades, studying materials microstructure was quite challenging as majority of the research works were theoretical. More importantly, providing direct evidence of the atomic scale evolution of material property was nearly impossible due to the limited analysis tools and techniques. However, with the invention of the new analysis and interpretational techniques, elucidating the correlation between material nanostructure and device characteristics has become crucial for the development of novel materials. Our research focuses on the application of aberration-corrected STEM with emphasis on the unprecedented atomic scale imaging techniques aimed at unraveling and understanding the various material property-dependent device performance. In-depth investigation of the various phenomena such as atomic-scale visualization of antisite defects such as those observed in LiFePO4 are tackled.

Intro

Publications

Our Major Publications

"Direct Imaging of Reconstructed Atoms on TiO2 (110) Surfaces", Science, 322, pp. 570-573 (2008)

Determining the atomic structures of oxide surfaces is critical for understanding their physical and chemical properties but also challenging because the breaking of atomic bonds in the formation of the surface termination can involve complex reconstructions. We used advanced transmission electron microscopy to directly observe the atomic structure of reduced titania (TiO2) (110) surfaces from directions parallel to the surface. In our direct atomic-resolution images, reconstructed titanium atoms at the top surface layer are clearly imaged and are found to occupy the interstitial sites of the TiO2 structure. Combining observations from two orthogonal directions, the three-dimensional positioning of the Ti interstitials is identified at atomic dimensions and allows a resolution of two previous models that differ in their oxygen stoichiometries. [pdf]

"Atomic-Scale Visualization of Antisite Defects in LiFePO4", Physical Review Letters, 100, 125502 (2008)

We visualize the antisite exchange defects in LiFePO4 crystals with an ordered olivine structure by using annular dark-field scanning transmission electron microscopy (STEM). A recognizable bright contrast is observed in some of the Li columns of STEM images in a sample annealed at a lower temperature, which directly demonstrates the disordered occupations by Fe atoms. Furthermore, such exchange defects appear to be locally aggregated rather than homogeneously dispersed in the lattice, although their overall concentration is fairly low. The present study emphasizes the significance of atomic-level observations for the defect distribution that cannot be predicted by macroscopic analytical methods. [pdf]

"Direct Determination of Dopant Site Selectivity in Ordered Perovskite CaCu3Ti4O12 Polycrystals by Aberration-Corrected STEM", Advanced Materials, 21, pp. 885-889 (2009)

A direct determination of dopant site selectivity in ordered perovskite CaCu3Ti4O12 polycrystals was reported. Scanning Transmission Electron Microscopy (STEM) with a spherical aberration corrector and Electron Energy Loss Spectroscopy (EELS) analysis were utilized for direct visualization and chemical identification of each atomic column. Image simulation was carried out before the observation of CaCu3Ti4O12 doped with 5% La in STEM, to predict the variations in the intensity of each atomic column by doping. Considering CaCu3Ti4O12 as a complex perovskite oxide showed that doped aliovalent La cations are substituted for Ca, even though 75% of A sites are composed of Cu. The results reveals that La cations are preferentially substituted for Ca sites in CaCu3Ti4O12 indicating an ordered behavior of occupations. [pdf]

"Distinct configurations of antisite defects in ordered metal phosphates: Comparison between LiMnPO4 and LiFePO4", Physical Review Letters, 108, 195501 (2012)

By using a combination of aberration-corrected high-angle annular dark-field scanning transmission electron microscopy, ab initio density-functional theory calculations, and neutron powder diffraction techniques, We have found completely different configurations of the antisite exchange defects in LiMnPO4 and LiFePO4 , with a random distribution of the exchange pairs without aggregation in the former and with zigzag-type clustering behavior preferred in the latter. Recalling the compositional analogy and identical crystal structure of the two metal phosphates, such unexpectedly distinct arrangement of the same type of point defects is a notable structural aspect. [pdf]

"Assessment of Strain-Generated Oxygen Vacancies Using SrTiO3 Bicrystals", Nano Letters, 15, pp. 4129-4134 (2015)

Atomic-scale defects strongly influence the electrical and optical properties of materials, and their impact can be more pronounced in localized dimensions. Here, we directly demonstrate that strain triggers the formation of oxygen vacancies in complex oxides by examining the tilt boundary of SrTiO3 bicrystals. Through transmission electron microscopy and electron energy loss spectroscopy, we identify strains along the tilt boundary and oxygen vacancies in the strain-imposed regions between dislocation cores. First-principles calculations support that strains, irrespective of their type or sign, lower the formation energy of oxygen vacancies, thereby enhancing vacancy formation. Finally, current-voltage measurements confirm that such oxygen vacancies at the strained boundary result in a decrease of the nonlinearity of the I-V curve as well as the resistivity. Our results strongly indicate that oxygen vacancies are preferentially formed and are segregated at the regions where strains accumulate, such as heterogeneous interfaces and grain boundaries. [pdf]

"Emergence of room-temperature ferroelectricity at reduced dimensions", Science, 349, pp. 1314-1317 (2015)

The enhancement of the functional properties of materials at reduced dimensions is crucial for continuous advancements in nanoelectronic applications. Here, we report that the scale reduction leads to the emergence of an important functional property, ferroelectricity, challenging the long-standing notion that ferroelectricity is inevitably suppressed at the scale of a few nanometers. A combination of theoretical calculations, electrical measurements, and structural analyses provides evidence of room-temperature ferroelectricity in strain-free epitaxial nanometer-thick films of otherwise nonferroelectric strontium titanate (SrTiO3). We show that electrically induced alignment of naturally existing polar nanoregions is responsible for the appearance of a stable net ferroelectric polarization in these films. This finding can be useful for the development of low-dimensional material systems with enhanced functional properties relevant to emerging nanoelectronic devices. [pdf]

"Enhancement of the anisotropic photocurrent in ferroelectric oxides by strain gradients", Nature Nanotechnology, 10, pp. 972-979 (2015)

The phase separation of multiple competing structural/ferroelectric phases has attracted particular attention owing to its excellent electromechanical properties. Little is known, however, about the strain-gradient-induced electronic phenomena at the interface of competing structural phases. Here, we investigate the polymorphic phase interface of bismuth ferrites using spatially resolved photocurrent measurements, present the observation of a large enhancement of the anisotropic interfacial photocurrent by two orders of magnitude, and discuss the possible mechanism on the basis of the flexoelectric effect. Nanoscale characterizations of the photosensitive area through position-sensitive angle-resolved piezoresponse force microscopy and electron holography techniques, in conjunction with phase field simulation, reveal that regularly ordered dipole-charged domain walls emerge. These findings offer practical implications for complex oxide optoelectronics. [pdf]

"Reversible phase modulation and hydrogen storage in multivalent VO2 epitaxial thin films", Nature Materials, 15, pp. 1113-1119 (2016)

Hydrogen, the smallest and the lightest atomic element, is reversibly incorporated into interstitial sites in vanadium dioxide (VO2), a correlated oxide with a 3d 1 electronic configuration, and induces electronic phase modulation. It is widely reported that low hydrogen concentrations stabilize the metallic phase, but the understanding of hydrogen in the high doping regime is limited. Here, we demonstrate that as many as two hydrogen atoms can be incorporated into each VO2 unit cell, and that hydrogen is reversibly absorbed into, and released from, VO2 without destroying its lattice framework. This hydrogenation process allows us to elucidate electronic phase modulation of vanadium oxyhydride, demonstrating two-step insulator (VO2)-metal (HxVO2)-insulator (HVO2) phase modulation during inter-integer d-band filling. Our finding suggests the possibility of reversible and dynamic control of topotactic phase modulation in VO2 and opens up the potential application in proton-based Mottronics and novel hydrogen storage. [pdf]

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Members

Si-Young Choi

Professor

Si-Young Choi Associate Professor Department of Materials Science and Engineering (MSE) Pohang University of Science and Technology (POSTECH) 77 Cheongam-Ro, Nam-Gu, Pohang, Gyeongbuk, 37673, Korea Tel. +82-54-279-2161, Fax. +82-54-279-2399, M.P. +82-10-8625-1158 E-mail: youngchoi@postech.ac.kr, siyoung.choi@gmail.com Education 1993.03 – 1999.02 B.S. inFind More

James P. Buban

Research Professor

Gi-Yeop Kim

Post-Doctor

Odongo Francis Ngome Okello

Ph.D. Candidate

Jinhyuk Jang

M.S. Candidate

Collaborator