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Roughening of a stepped GaN grain boundary with increasing driving force for migration
Bo Lee, Sung,Jo Yoo, Seung,van Aken, Peter A. Editions de Physique 2017 Europhysics letters Vol.120 No.1
<P>High-resolution transmission electron microscopy is applied to characterize the structure of a GaN grain boundary, which is subjected to the driving force for migration arising from the surface energy anisotropy during annealing at 500 degrees C in a transmission electron microscope. At a low driving force for migration, the grain boundary consists of atomically flat terrace regions interrupted by steps. As the driving force increases, grain-boundary steps become more frequent and undergo meandering, which is a typical attribute of kinetic roughening transition. Copyright (C) EPLA, 2017</P>
Formation of Pt–Zn Alloy Nanoparticles by Electron-Beam Irradiation of Wurtzite ZnO in the TEM
Lee, Sung Bo,Park, Jucheol,van Aken, Peter A. SPRINGER SCIENCE + BUSINESS MEDIA 2016 NANOSCALE RESEARCH LETTERS Vol.11 No.1
<P>As is well documented, platinum nanoparticles, promising for catalysts for fuel cells, exhibit better catalytic activities, when alloyed with Zn. Pre-existing syntheses of Pt–Zn alloy catalysts are composed of a number of complex steps. In this study, we have demonstrated that nanoparticles of Pt–Zn alloys are simply generated by electron-beam irradiation in a transmission electron microscope of a wurtzite ZnO single-crystal specimen. The initial ZnO specimen is considered to have been contaminated by Pt during specimen preparation by focused ion beam milling. The formation of the nanoparticle is explained within the framework of ionization damage (radiolysis) by electron-beam irradiation and accompanying electrostatic charging.</P>
Lee, S.B.,Lee, J.H.,Cho, Y.H.,Kim, D.Y.,Sigle, W.,Phillipp, F.,van Aken, P.A. Elsevier Science 2008 ACTA MATERIALIA Vol.56 No.18
Dependence of the electrical properties on grain-boundary plane orientation is examined by a combination of high-resolution transmission electron microscopy, impedance spectroscopy, and electron energy-loss spectrometry using two kinds of SrTiO<SUB>3</SUB> Σ5 ([100]/36.8<SUP>o</SUP>) bicrystalline grain boundaries: symmetric (310) (18.4<SUP>o</SUP>/18.4<SUP>o</SUP>) and asymmetric (8.4<SUP>o</SUP>/28.4<SUP>o</SUP>). While the symmetric grain boundary is observed to be straight with the symmetric (310)//(310) plane orientation, the asymmetric grain boundary is faceted into symmetric (310)//(310) and (210)//(210), and asymmetric (100)//(430). Grain-boundary impedance is observed only in the asymmetric grain boundary, and the electron energy-loss spectrometry quantification indicates that the asymmetric (100)//(430) facets are more oxygen-deficient than the symmetric ones. The results suggest that the asymmetric (100)//(430) facets are the most resistive among the three different facets.
Lithium-ion capacitors with 2D Nb<sub>2</sub>CT<sub>x</sub> (MXene) – carbon nanotube electrodes
Byeon, Ayeong,Glushenkov, Alexey M.,Anasori, Babak,Urbankowski, Patrick,Li, Jingwen,Byles, Bryan W.,Blake, Brian,Van Aken, Katherine L.,Kota, Sankalp,Pomerantseva, Ekaterina,Lee, Jae W.,Chen, Ying,Gog Elsevier 2016 Journal of Power Sources Vol.326 No.-
<P><B>Abstract</B></P> <P>There is a growing interest to hybrid energy storage devices, such as lithium-ion capacitors, in which battery-type electrodes are combined with capacitor-type ones. It is anticipated that the energy density (either gravimetric or volumetric) of lithium-ion capacitors is improved if pseudocapacitive or fast insertion materials are used instead of conventional activated carbon (AC) in the capacitor-type electrode. MXenes, a new family of two-dimensional transition metal carbides, demonstrate metallic conductivity and fast charge-discharge behavior that make them suitable for this application. In this study, we move beyond single electrodes, half-cell studies and demonstrate three types of hybrid cells using Nb<SUB>2</SUB>CT<SUB>x</SUB>–carbon nanotube (CNT) films. It is shown that lithiated graphite/Nb<SUB>2</SUB>CT<SUB>x</SUB>-CNT, Nb<SUB>2</SUB>CT<SUB>x</SUB>-CNT/LiFePO<SUB>4</SUB> and lithiated Nb<SUB>2</SUB>CT<SUB>x</SUB>-CNT/Nb<SUB>2</SUB>CT<SUB>x</SUB>-CNT cells are all able to operate within 3 V voltage windows and deliver capacities of 43, 24 and 36 mAh/g (per total weight of two electrodes), respectively. Moreover, the polarity of the electrodes can be reversed in the symmetric Nb<SUB>2</SUB>CT<SUB>x</SUB>-CNT cells from providing a positive potential between 0 and 3 V to a negative one from −3 to 0 V. It is shown that the volumetric energy density (50–70 Wh/L) of our first-generation devices with MXene electrodes exceeds that of a lithium titanate/AC capacitor.</P> <P><B>Highlights</B></P> <P> <UL> <LI> 3 types of lithium-ion capacitors using Nb<SUB>2</SUB>CT<SUB>x</SUB>-CNT as one electrode were tested. </LI> <LI> The highest volumetric energy density of 50–70 Wh/L was achieved. </LI> <LI> Energy density of symmetric cell exceeds that of lithium titanate/activated carbon. </LI> <LI> The lithiated graphite/Nb<SUB>2</SUB>CT<SUB>x</SUB>-CNT shows the highest gravimetric performance. </LI> </UL> </P> <P><B>Graphical abstract</B></P> <P>[DISPLAY OMISSION]</P>
Tailoring the electronic properties of Ca<sub>2</sub>RuO<sub>4</sub> via epitaxial strain
Dietl, C.,Sinha, S. K.,Christiani, G.,Khaydukov, Y.,Keller, T.,Putzky, D.,Ibrahimkutty, S.,Wochner, P.,Logvenov, G.,van Aken, P. A.,Kim, B. J.,Keimer, B. American Institute of Physics 2018 APPLIED PHYSICS LETTERS Vol.112 No.3
Strain-induced indium clustering in non-polar <i>a</i>-plane InGaN quantum wells
Lee, Ja Kyung,Park, Bumsu,Song, Kyung,Jung, Woo Young,Tyutyunnikov, Dmitry,Yang, Tiannan,Koch, Christoph T.,Park, Chan Gyung,van Aken, Peter A.,Kim, Young-Min,Kim, Jong Kyu,Bang, Junhyeok,Chen, Long-Q Elsevier 2018 Acta materialia Vol.145 No.-
<P><B>Abstract</B></P> <P>In conventional light-emitting diodes the epitaxial strain and related piezoelectric polarization arising along the polar [0001] growth direction of the InGaN/GaN quantum wells (QWs) induce internal fields which adversely affect the radiative recombination of electron-hole pairs therein. Growing the quantum wells along a nonpolar orientation can, in principle, avoid this problem but seems to face with another problem associated with indium clustering. In this study, we present experimental evidence that supports the inhomogeneous distribution of indium in non-polar <I>a</I>-plane InGaN QWs by using dark-field inline electron holography as well as atom probe tomography measurements and discuss the possible origin by density functional theory calculation. A model non-polar <I>a</I>-plane QW structure with 10 nm-thick In<SUB>0.1</SUB>Ga<SUB>0.9</SUB>N double QWs was investigated and compared with the polar <I>c</I>-plane QWs with the same QW structure. Unlike the random distribution in the polar QWs, the indium atoms in the non-polar QW exhibit inhomogeneous distribution and show a tendency of periodic clustering. We suggest the dipole interaction energy and the strain energy associated with indium substitution could have a substantial influence on the local composition of strained InGaN QWs and, particularly, triggers In clustering in the non-polar <I>a</I>-plane QW structure. Accompanying phase field modeling rationalizes that In clustering can also modify the in-plane polarization through piezoelectric effects, preventing the electrostatic potential from diverging along the in-plane polar direction.</P> <P><B>Graphical abstract</B></P> <P>[DISPLAY OMISSION]</P>