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Material structure, properties, and dynamics through scanning transmission electron microscopy
Pennycook, Stephen J.,Li, Changjian,Li, Mengsha,Tang, Chunhua,Okunishi, Eiji,Varela, Maria,Kim, Young-Min,Jang, Jae Hyuck Springer (Biomed Central Ltd.) 2018 Journal of analytical science & technology Vol.9 No.1
Principled Polycentrism and Resourceful Speakers
Alastair Pennycook 아시아영어교육학회 2014 The Journal of Asia TEFL Vol.11 No.4
A central goal of language education is the development of resourceful speakers, people who have both good access to a range of linguistic resources and are good at shifting between styles, discourses, registers and genres. Communication becomes possible not because we adhere to global or even regional norms, but because language users are able to bring their communication into alignment with each other. Drawing on a series of studies of both online and face-to-face interaction in different cities in Asia, this paper suggests that to understand communication in contexts of diversity, we need to focus less on a supposed shared code and more on the interactions among language resources, activities and space. This in turn suggests that in order to pursue intelligibility in multilingual contexts we need a model of principled polycentrism, not the polycentrism of a World Englishes focus, with its established norms of regional varieties of English, nor the reduced communicative domain of the English as a lingua franca framework, but a more fluid yet principled approach to the diversity of contemporary contexts of communication.
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>
Yin, Y. W.,Burton, J. D.,Kim, Y-M.,Borisevich, A. Y.,Pennycook, S. J.,Yang, S. M.,Noh, T. W.,Gruverman, A.,Li, X. G.,Tsymbal, E. Y.,Li, Qi Nature Publishing Group 2013 NATURE MATERIALS Vol.12 No.5
The range of recently discovered phenomena in complex oxide heterostructures, made possible owing to advances in fabrication techniques, promise new functionalities and device concepts. One issue that has received attention is the bistable electrical modulation of conductivity in ferroelectric tunnel junctions (FTJs) in response to a ferroelectric polarization of the tunnelling barrier, a phenomenon known as the tunnelling electroresistance (TER) effect. Ferroelectric tunnel junctions with ferromagnetic electrodes allow ferroelectric control of the tunnelling spin polarization through the magnetoelectric coupling at the ferromagnet/ferroelectric interface. Here we demonstrate a significant enhancement of TER due to a ferroelectrically induced phase transition at a magnetic complex oxide interface. Ferroelectric tunnel junctions consisting of BaTiO<SUB>3</SUB> tunnelling barriers and La<SUB>0.7</SUB>Sr<SUB>0.3</SUB>MnO<SUB>3</SUB> electrodes exhibit a TER enhanced by up to ~ 10,000% by a nanometre-thick La<SUB>0.5</SUB>Ca<SUB>0.5</SUB>MnO<SUB>3</SUB> interlayer inserted at one of the interfaces. The observed phenomenon originates from the metal-to-insulator phase transition in La<SUB>0.5</SUB>Ca<SUB>0.5</SUB>MnO<SUB>3</SUB>, driven by the modulation of carrier density through ferroelectric polarization switching. Electrical, ferroelectric and magnetoresistive measurements combined with first-principles calculations provide evidence for a magnetoelectric origin of the enhanced TER, and indicate the presence of defect-mediated conduction in the FTJs. The effect is robust and may serve as a viable route for electronic and spintronic applications.
Fabrication and growth mechanism of ultra‑crystalline C60 on silicon substrate in vacuum
Hongyu Wang,Ashutosh Rath,Shu Hearn Yu,Meng Seng Tan,Stephen J. Pennycook,Daniel H. C. Chua 한국탄소학회 2021 Carbon Letters Vol.31 No.2
We report a simple procedure to fabricate single crystals 3D C60 having an FCC structure on silicon substrates using a vapour–solid set-up in vacuum conditions. The morphology of the deposited film can be tuned by controlling the temperature and position of the substrate. The as-fabricated samples are extensively characterised by transmission electron microscopy, scanning electron microscope, X-ray powder diffraction, Raman spectroscopy, X-ray photoelectron spectroscopy and nanoindentation, which allow us to shed light on the recrystallization process of the C60. In addition, the growth mechanism of the formation of crystalline 3D structure of the C60 film is discussed in detail. Based on the newly gained knowledge of mechanism and its unique properties, fullerene has shown huge potential as a solid lubricant on various kinds of substrates.
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.
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>
Multiferroic tunnel junctions and ferroelectric control of magnetic state at interface (invited)
Yin, Y. W.,Raju, M.,Hu, W. J.,Burton, J. D.,Kim, Y.-M.,Borisevich, A. Y.,Pennycook, S. J.,Yang, S. M.,Noh, T. W.,Gruverman, A.,Li, X. G.,Zhang, Z. D.,Tsymbal, E. Y.,Li, Qi American Institute of Physics 2015 Journal of Applied Physics Vol.117 No.17
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