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Kim, Young-Min,Pennycook, Stephen J.,Borisevich, Albina Y. Elsevier 2017 Ultramicroscopy Vol.181 No.-
<P><B>Abstract</B></P> <P>Octahedral tilt behavior is increasingly recognized as an important contributing factor to the physical behavior of perovskite oxide materials and especially their interfaces, necessitating the development of high-resolution methods of tilt mapping. There are currently two major approaches for quantitative imaging of tilts in scanning transmission electron microscopy (STEM), bright field (BF) and annular bright field (ABF). In this paper, we show that BF STEM can be reliably used for measurements of oxygen octahedral tilts. While optimal conditions for BF imaging are more restricted with respect to sample thickness and defocus, we find that BF imaging with an aberration-corrected microscope with the accelerating voltage of 300kV gives us the most accurate quantitative measurement of the oxygen column positions. Using the tilted perovskite structure of BiFeO<SUB>3</SUB> (BFO) as our test sample, we simulate BF and ABF images in a wide range of conditions, identifying the optimal imaging conditions for each mode. We show that unlike ABF imaging, BF imaging remains directly quantitatively interpretable for a wide range of the specimen mistilt, suggesting that it should be preferable to the ABF STEM imaging for quantitative structure determination.</P> <P><B>Highlights</B></P> <P> <UL> <LI> An aberration-corrected BF STEM imaging at an accelerating voltage of 300kV provides a wide range of defocus-thickness imaging parameters and specimen misorientation for precisely locating oxygen positions in a tilted perovskite structure. </LI> <LI> BF STEM imaging for the measurements of oxygen octahedral tilts has quantitatively compared with ABF STEM imaging by systematic image simulations. </LI> <LI> BF STEM imaging working at 300kV shows superior accuracy for the measurements of the octahedral tilt angles as compared with ABF STEM imaging. </LI> </UL> </P>
Point Defect Configurations of Supersaturated Au Atoms Inside Si Nanowires
Oh, Sang Ho,Benthem, Klaus van,Molina, Sergio I.,Borisevich, Albina Y.,Luo, Weidong,Werner, Peter,Zakharov, Nikolai D.,Kumar, Dhananjay,Pantelides, Sokrates T.,Pennycook, Stephen J. American Chemical Society 2008 NANO LETTERS Vol.8 No.4
Kim, Ki Sung,Kim, Young-Min,Mun, Hyeona,Kim, Jisoo,Park, Jucheol,Borisevich, Albina Y.,Lee, Kyu Hyoung,Kim, Sung Wng Wiley (John WileySons) 2017 Advanced Materials Vol.29 No.36
<P>Structural defects often dominate the electronic- and thermal-transport properties of thermoelectric (TE) materials and are thus a central ingredient for improving their performance. However, understanding the relationship between TE performance and the disordered atomic defects that are generally inherent in nanostructured alloys remains a challenge. Herein, the use of scanning transmission electron microscopy to visualize atomic defects directly is described and disordered atomic-scale defects are demonstrated to be responsible for the enhancement of TE performance in nanostructured Ti1-xHfxNiSn1-ySby half-Heusler alloys. The disordered defects at all atomic sites induce a local composition fluctuation, effectively scattering phonons and improving the power factor. It is observed that the Ni interstitial and Ti,Hf/Sn antisite defects are collectively formed, leading to significant atomic disorder that causes the additional reduction of lattice thermal conductivity. The Ti1-xHfxNiSn1-ySby alloys containing inherent atomic-scale defect disorders are produced in one hour by a newly developed process of temperature-regulated rapid solidification followed by sintering. The collective atomic-scale defect disorder improves the zT to 1.09 +/- 0.12 at 800 K for the Ti0.5Hf0.5NiSn0.98Sb0.02 alloy. These results provide a promising avenue for improving the TE performance of state-of-the-art materials.</P>
Mishra, Rohan,Kim, Young-Min,Salafranca, Juan,Kim, Seong Keun,Chang, Seo Hyoung,Bhattacharya, Anand,Fong, Dillon D.,Pennycook, Stephen J.,Pantelides, Sokrates T.,Borisevich, Albina Y. American Chemical Society 2014 NANO LETTERS Vol.14 No.5
<P>Complex oxides displaying ferroelectric and/or multiferroic behavior are of high fundamental and applied interest. In this work, we show that it is possible to achieve polar order in a superlattice made up of two nonpolar oxides by means of oxygen vacancy ordering. Using scanning transmission electron microscopy imaging, we show the polar displacement of magnetic Fe ions in a superlattice of (LaFeO<SUB>3</SUB>)<SUB>2</SUB>/(SrFeO<SUB>3</SUB>) grown on a SrTiO<SUB>3</SUB> substrate. Using density functional theory calculations, we systematically study the effect of epitaxial strain, octahedral rotations, and surface terminations in the superlattice and find them to have a negligible effect on the antipolar displacements of the Fe ions lying in between SrO and LaO layers of the superlattice (i.e., within La<SUB>0.5</SUB>Sr<SUB>0.5</SUB>FeO<SUB>3</SUB> unit cells). The introduction of oxygen vacancies, on the other hand, triggers a polar displacement of the Fe ions. We confirm this important result using electron energy loss spectroscopy, which shows partial oxygen vacancy ordering in the region where polar displacements are observed and an absence of vacancy ordering outside of that area.</P><P><B>Graphic Abstract</B> <IMG SRC='http://pubs.acs.org/appl/literatum/publisher/achs/journals/content/nalefd/2014/nalefd.2014.14.issue-5/nl500601d/production/images/medium/nl-2014-00601d_0006.gif'></P><P><A href='http://pubs.acs.org/doi/suppl/10.1021/nl500601d'>ACS Electronic Supporting Info</A></P>
Kim, Young-Min,He, Jun,Biegalski, Michael D.,Ambaye, Hailemariam,Lauter, Valeria,Christen, Hans M.,Pantelides, Sokrates T.,Pennycook, Stephen J.,Kalinin, Sergei V.,Borisevich, Albina Y. Nature Publishing Group, a division of Macmillan P 2012 NATURE MATERIALS Vol.11 No.10
Oxygen vacancy distributions and dynamics directly control the operation of solid-oxide fuel cells and are intrinsically coupled with magnetic, electronic and transport properties of oxides. For understanding the atomistic mechanisms involved during operation of the cell it is highly desirable to know the distribution of vacancies on the unit-cell scale. Here, we develop an approach for direct mapping of oxygen vacancy concentrations based on local lattice parameter measurements by scanning transmission electron microscopy. The concept of chemical expansivity is demonstrated to be applicable on the subunit-cell level: local stoichiometry variations produce local lattice expansion that can be quantified. This approach was successfully applied to lanthanum strontium cobaltite thin films epitaxially grown on substrates of different symmetry, where polarized neutron reflectometry revealed a strong difference in magnetic properties. The different vacancy content found in the two films suggests the change in oxygen chemical potential as a source of distinct magnetic properties, opening pathways for structural tuning of the vacancy concentrations and their gradients.
Jang, Jae Hyuck,Kim, Young-Min,He, Qian,Mishra, Rohan,Qiao, Liang,Biegalski, Michael D.,Lupini, Andrew R.,Pantelides, Sokrates T.,Pennycook, Stephen J.,Kalinin, Sergei V.,Borisevich, Albina Y. American Chemical Society 2017 ACS NANO Vol.11 No.7
<P>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.</P>
Chang, Hye Jung,Kalinin, Sergei V.,Morozovska, Anna N.,Huijben, Mark,Chu, Ying‐,Hao,Yu, Pu,Ramesh, Ramamoorthy,Eliseev, Evgeny A.,Svechnikov, George S.,Pennycook, Stephen J.,Borisevich, Albina Y WILEY‐VCH Verlag 2011 Advanced Materials Vol.23 No.21
<P><B>Direct atomic displacement mapping at ferroelectric interfaces</B> by aberration corrected scanning transmission electron microscopy(STEM) (a‐STEM image, b‐corresponding displacement profile) is combined with Landau‐Ginsburg‐Devonshire theory to obtain the complete interface electrostatics in real space, including separate estimates for the polarization and intrinsic interface charge contributions. </P>
Kim, Young-Min,Morozovska, Anna,Eliseev, Eugene,Oxley, Mark P.,Mishra, Rohan,Selbach, Sverre M.,Grande, Tor,Pantelides, S. T.,Kalinin, Sergei V.,Borisevich, Albina Y. Nature Publishing Group, a division of Macmillan P 2014 NATURE MATERIALS Vol.13 No.11
The development of interface-based magnetoelectric devices necessitates an understanding of polarization-mediated electronic phenomena and atomistic polarization screening mechanisms. In this work, the LSMO/BFO interface is studied on a single unit-cell level through a combination of direct order parameter mapping by scanning transmission electron microscopy and electron energy-loss spectroscopy. We demonstrate an unexpected ~5% lattice expansion for regions with negative polarization charge, with a concurrent anomalous decrease of the Mn valence and change in oxygen K-edge intensity. We interpret this behaviour as direct evidence for screening by oxygen vacancies. The vacancies are predominantly accumulated at the second atomic layer of BFO, reflecting the difference of ionic conductivity between the components. This vacancy exclusion from the interface leads to the formation of a tail-to-tail domain wall. At the same time, purely electronic screening is realized for positive polarization charge, with insignificant changes in lattice and electronic properties. These results underline the non-trivial role of electrochemical phenomena in determining the functional properties of oxide interfaces. Furthermore, these behaviours suggest that vacancy dynamics and exclusion play major roles in determining interface functionality in oxide multilayers, providing clear implications for novel functionalities in potential electronic devices.
Mishra, Rohan,Kim, Young-Min,He, Qian,Huang, Xing,Kim, Seong Keun,Susner, Michael A.,Bhattacharya, Anand,Fong, Dillon D.,Pantelides, Sokrates T.,Borisevich, Albina Y. American Physical Society 2016 Physical Review B Vol.94 No.4
<P>The surfaces of transition-metal oxides with the perovskite structure are fertile grounds for the discovery of novel electronic and magnetic phenomena. In this article, we combine scanning transmission electron microscopy (STEM) with density functional theory (DFT) calculations to obtain the electronic and magnetic properties of the (001) surface of a (LaFeO3)(8)/(SrFeO3)(1) superlattice film capped with four layers of LaFeO3. Simultaneously acquired STEM images and electron-energy-loss spectra reveal the surface structure and a reduction in the oxidation state of iron from Fe3+ in the bulk to Fe2+ at the surface, extending over several atomic layers, which signals the presence of oxygen vacancies. The DFT calculations confirm the reduction in terms of oxygen vacancies and further demonstrate the stabilization of an exotic phase in which the surface layer is half metallic and ferromagnetic, while the bulk remains antiferromagnetic and insulating. Based on the calculations, we predict that the surface magnetism and conductivity can be controlled by tuning the partial pressure of oxygen.</P>