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Gebhardt, Julian,Kim, Youngkuk,Rappe, Andrew M. American Chemical Society 2017 JOURNAL OF PHYSICAL CHEMISTRY C - Vol.121 No.12
<P>Layered perovskites open a plethora of possibilities for tuning organometallic halide perovskite (OMHP) properties via the incorporation of larger organic cations. Promising applications of this increased structural freedom include enhanced chemical stability and tunable exciton binding. Owing to the larger cation, crystal and electronic structures vary with layer stacking, having layered bulk and a monolayer as limiting cases. Using ab initio calculations, here we study the atomic and electronic structures of such a layered material, (C6H5C2H4NH3)(2)PbI4, which has recently attracted attention as a promising solar cell material. The reduction of layer thickness between the bulk and a monolayer is investigated and compared to that of the benchmark OMHP, (CH3NH3)PbI3, showing that the bulkier C6H5C2H4NH3 cations largely preserve the two-dimensional nature of the electronic structure in the layered bulk OMHP.</P>
Bond-Valence Model of Ferroelectric PbTiO₃
Young-Han Shin,Byeong-Joo Lee,Andrew M. RAPPE 한국물리학회 2008 THE JOURNAL OF THE KOREAN PHYSICAL SOCIETY Vol.52 No.4
A classical potential parameterized for the reproduction of density functional calculations is used to describe the behavior of complex ferroelectric PbTiO₃. A scoring function is defined in terms of the energy differences and the forces of the reference structures that are generated from \emph{ab-initio} molecular dynamics simulations with various strained lattice vectors. The elastic properties of ferroelectric PbTiO₃, as well as the phase transition temperature, have been improved by the addition of the strained reference structures. A classical potential parameterized for the reproduction of density functional calculations is used to describe the behavior of complex ferroelectric PbTiO₃. A scoring function is defined in terms of the energy differences and the forces of the reference structures that are generated from \emph{ab-initio} molecular dynamics simulations with various strained lattice vectors. The elastic properties of ferroelectric PbTiO₃, as well as the phase transition temperature, have been improved by the addition of the strained reference structures.
Wallpaper fermions and the nonsymmorphic Dirac insulator
Wieder, Benjamin J.,Bradlyn, Barry,Wang, Zhijun,Cano, Jennifer,Kim, Youngkuk,Kim, Hyeong-Seok D.,Rappe, Andrew M.,Kane, C. L.,Bernevig, B. Andrei American Association for the Advancement of Scienc 2018 Science Vol.361 No.6399
<P>Materials whose gapless surface states are protected by crystal symmetries include mirror topological crystalline insulators and nonsymmorphic hourglass insulators. There exists only a very limited set of possible surface crystal symmetries, captured by the 17 'wallpaper groups.' Here we show that a consideration of symmetry-allowed band degeneracies in the wallpaper groups can be used to understand previously described topological crystalline insulators and to predict phenomenologically distinct examples. In particular, the two wallpaper groups with multiple glide lines, pgg and p4g, allow for a topological insulating phase whose surface spectrum consists of only a single, fourfolddegenerate, true Dirac fermion, representing an exception to a symmetry-enhanced fermion-doubling theorem. We theoretically predict the presence of this phase in Sr2Pb3 in space group 127 (P4/mbm).</P>
Crystalline Bilayer Graphene with Preferential Stacking from Ni-Cu Gradient Alloy
Gao, Zhaoli,Zhang, Qicheng,Naylor, Carl H.,Kim, Youngkuk,Abidi, Irfan Haider,Ping, Jinglei,Ducos, Pedro,Zauberman, Jonathan,Zhao, Meng-Qiang,Rappe, Andrew M.,Luo, Zhengtang,Ren, Li,Johnson, Alan T. Ch American Chemical Society 2018 ACS NANO Vol.12 No.3
<P>We developed a high-yield synthesis of highly crystalline bilayer graphene (BLG) with two preferential stacking modes using a Ni-Cu gradient alloy growth substrate. Previously reported approaches for BLG growth include flat growth substrates of Cu or Ni-Cu <I>uniform</I> alloys and “copper pocket” structures. Use of flat substrates has the advantage of being scalable, but the growth mechanism is either “surface limited” (for Cu) or carbon precipitation (for uniform Ni-Cu), which results in multicrystalline BLG grains. For copper pockets, growth proceeds through a carbon back-diffusion mechanism, which leads to the formation of highly crystalline BLG, but scaling of the copper pocket structure is expected to be difficult. Here we demonstrate a Ni-Cu gradient alloy that combines the advantages of these earlier methods: the substrate is flat, so easy to scale, while growth proceeds by a carbon back-diffusion mechanism leading to high-yield growth of BLG with high crystallinity. The BLG layer stacking was almost exclusively Bernal or twisted with an angle of 30°, consistent with first-principles calculations we conducted. Furthermore, we demonstrated scalable production of transistor arrays based crystalline Bernal-stacked BLG with a band gap that was tunable at room temperature.</P> [FIG OMISSION]</BR>
Martirez, John Mark P.,Kim, Seungchul,Morales, Erie H.,Diroll, Benjamin T.,Cargnello, Matteo,Gordon, Thomas R.,Murray, Christopher B.,Bonnell, Dawn A.,Rappe, Andrew M. American Chemical Society 2015 JOURNAL OF THE AMERICAN CHEMICAL SOCIETY - Vol.137 No.8
<P>In addition to composition, the structure of a catalyst is another fundamental determinant of its catalytic reactivity. Recently, anomalous Ti oxide-rich surface phases of ternary oxides have been stabilized as nonstoichiometric epitaxial overlayers. These structures give rise to different modes of oxygen binding, which may lead to different oxidative chemistry. Through density functional theory investigations and electrochemical measurements, we predict and subsequently show that such a TiO<SUB>2</SUB> double-layer surface reconstruction enhances the oxygen evolving activity of the perovskite-type oxide SrTiO<SUB>3</SUB>. Our theoretical work suggests that the improved activity of the restructured TiO<SUB>2</SUB>(001) surface toward oxygen formation stems from (i) having two Ti sites with distinct oxidation activity and (ii) being able to form a strong O–O moiety (which reduces overbonding at Ti sites), which is a direct consequence of (iii) having a labile lattice O that is able to directly participate in the reaction. Here, we demonstrate the improvement of the catalytic performance of a well-known and well-studied oxide catalyst through more modern methods of materials processing, predicted through first-principles theoretical modeling.</P><P><B>Graphic Abstract</B> <IMG SRC='http://pubs.acs.org/appl/literatum/publisher/achs/journals/content/jacsat/2015/jacsat.2015.137.issue-8/ja511332y/production/images/medium/ja-2014-11332y_0008.gif'></P><P><A href='http://pubs.acs.org/doi/suppl/10.1021/ja511332y'>ACS Electronic Supporting Info</A></P>
Displacement field estimation for a two-dimensional structure using fiber Bragg grating sensors
Rapp, Stephan,Kang, Lae-Hyong,Han, Jae-Hung,Mueller, Uwe C,Baier, Horst Institute of Physics Publishing 2009 Smart materials & structures Vol.18 No.2
<P>The structure shape itself is of great interest for many aerospace applications. For example, the stability of the surface shape of large, high precision or space reflectors is essential for the communication performance. The knowledge of static and dynamic displacements of these structures would provide the possibility to enhance their performance by appropriate countermeasures. During operation, however, the direct measurement of displacements of the whole structure is often difficult. This study investigates whole displacement field estimation using strain measurement and a displacement–strain-transformation approach. The use of fiber Bragg gratings (FBGs) as strain sensors for this application offers the possible implementation of an integrated sensor network including many measurement points within only a few optical fibers. This paper discusses many issues related to the displacement field estimation of a dynamically excited plate using a transformation matrix based on a modal approach. In order to reduce systematic displacement estimation errors due to aliasing, a parametric study was performed and the sensor locations were optimized. Experimental validation was also conducted using a cantilever plate equipped with 16 FBG sensors in an optimized configuration. The estimated displacements showed good agreements with those measured directly from laser displacement sensors. </P>