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Now showing items 1 - 11 of 11

  • Origin of piezoelectricity in monolayer halogenated graphane piezoelectrics

    Hye Jung Kim   Mohammad Noor-A-Alam   Jong Yeog Son   Young-Han Shin  

    Graphical abstract Highlights • (C 2 HF) n for a good fatigue-free and long-retention-time piezoelectric material. • Density functional study of a two-dimensional single-layer fluorinated graphane. • The four conformations are stable with band gaps as high as 6 eV. • Their piezoelectricity is comparable to bulk polyvinylidene difluoride (PVDF). • The effect of bulkier elements such as chlorine. Abstract Periodic patterning with adatoms or defect is one of the methods for opening the band gap of graphene. In particular, under certain configurations controlled by the order of hydrogen and halogen atoms attached on graphene, inversion symmetry of graphene can be broken to give piezoelectricity as well as pyroelectricity. Using first-principles calculations, we examine the structural stability and electronic properties of four polar conformations of halogenated graphane (C 2 HX) n to understand the origin of piezoelectricity in this two-dimensional system. The formation energies and piezoelectric coefficients manifest that the four conformations of (C 2 HF) n are energetically stable with considerable piezoelectricity. We find that the electronic contribution of the proper piezoelectricity in (C 2 HF) n is mainly related to the change of the electron distribution around F atoms. By substituting flourine with chlorine, we confirm that the piezoelectricity enhances at the expense of stability degradation.
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  • Ferroelectric switching response of P(VDF-TrFE) nanodots with and without nanomolds

    Yoonho Ahn   Young-Han Shin   Jong Yeog Son  

    We investigated ferroelectric switching response of poly(vinylidene fluoride-ran-trifluoroethylene) (P(VDF-TrFE)) nanodots with and without nanomolds using piezoelectric force microscopy. Nanomolds with diameter of about 20 and 40 nm were formed on Nb-doped SrTiO3 substrate by nanointaglio process using an atomic force microscopy tip. The P(VDF-TrFE) nanodots were fabricated with and without nanomolds on the surface of the Nb-doped SrTiO3 substrate, which was conducted by precisely controllable dip-pen nanolithography. These P(VDF-TrFE) nanodots exhibited good ferroelectric properties with asymmetric behavior in ferroelectric switching response. In particular, the P(VDF-TrFE) nanodots with nanomolds exhibited larger asymmetric switching characteristic compared to the P(VDF-TrFE) nanodots without nanomolds, which came from the difference of contact area between the P(VDF-TrFE) nanodots and Nb-doped SrTiO3 substrate (bottom electrode).
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  • Order–disorder character of PbTiO3

    Young-Han Shin   Jong-Yeog Son   Byeong-Joo Lee   Ilya Grinberg   Andrew M Rappe  

    We report the displacive and order–disorder characters ofPbTiO3 analyzed using a classical atomic model which was developed under the inverse relationbetween the bond length and the bond valence. The ferroelectric phase changes to theparaelectric phase around 700 K, and the histograms of Ti–O and Pb–O bond lengths havethree peaks which become a single peak above the phase transition temperature. Theorder–disorder character of this material was clearly shown by analyzing the iondisplacement magnitudes.
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  • Prediction of dielectric dispersion for lead based perovskites and study of local dielectric response in 0.75Pb(Mg 1/3Nb 2/3)O 3-0.25PbtiO 3

    Grinberg, I.   Takenaka, H.   Young-han Shin   Rappe, A.M.  

    Relaxors exhibit unique dielectric response properties such as diffuse phase transitions and strong dispersion of the dielectric constant. The origin of these properties is still not fully understood. Here, we review our work on using first-principles-based methods to elucidate the connections between composition, local structure and dynamics in Pb-based relaxors.
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  • Development of a bond-valence molecular-dynamics model for complex oxides

    Young-Han Shin   Valentino R. Cooper   Ilya Grinberg  

    A simple ten-parameter interatomic potential model is described that is capable of accurately reproducing the static and dynamical properties of complex oxides. The accuracy of this model stems from the crystal-chemical bond-valence theory of ionic and covalent bonding. The development of a specific variant of this model for ferroelectric PbTiO3 sPTd is discussed in detail; and comparison of the model is made with density functional theory computations and with experimental data. Bond-valence molecular dynamics sBVMDd simulations for PT show a ferroelectric transition at 575 K. The BVMD model correctly reproduces the mixed order-disorder and displacive phase transition character; the magnitudes of cation displacements in the ferroelectric and paraelectric phases; and the energy of 180° domain walls. The success of this simple and physically motivated model makes the simulation of extended defects tractable in PT and other complex oxides.
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  • Collective coherent control: synchronization of polarization in ferroelectric PbTiO3 by shaped THz fields.

    Tingting Qi   Young-Han Shin   Ka-Lo Yeh   Keith A. Nelson   Andrew M Rappe  

    We show that properly shaped terahertz (THz) fields can be used to move ions in ferroelectric crystals from their positions in an initial domain orientation along well-defined collective microscopic paths into the positions they occupy in a new domain orientation. Collective coherent control will enable direct observation of fast highly nonlinear material responses and far-from-equilibrium structures that can be harnessed in electro-optic devices and nonvolatile computer memory.
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  • Defense Technical Information Center Compilation Part Notice Adp023824 Title: Accelerated Modeling and New Ferroelectric Materials for Naval Sonar Distribution: Approved for Public Release, Distribution Unlimited Accelerated Modeling and New Ferroelectric Materials for Naval Sonar

    Ilya Grinberg   Valentino R. Cooper   Thomas J. Baring   Sam B. Cable   Young-Han Shin   Andrew M Rappe  

    Most of these materials are complex systems with some degree of disorder; making them challenging to We have computationally designed new materials for study experimentally and theoretically. However; as it is use in Naval SOund NAvigation Ranging (SONAR). Our their complexity which gives them their favorable quantum-mechanical studies show that these lead-free; properties; highly accurate modeling which captures the non-toxic oxides will be high-performance piezoelectrics essential features of the disordered structure is necessary with promise for use in Naval SONAR and to explain the behavior of current materials and predict communications applications. To enable this research; favorable compositions for new materials. Recently; a we also present techniques for greatly accelerated combination of methodological improvements i and a rise modeling of oxide materials. We show that a simple in computer speed has made first-principles calculations a atomistic model accurately reproduces our quantumviable tool for understanding these comp!ex systems. In mechanical results yet is thousands of times faster. We particular; the density functional Itheory (DFT) also report successful porting and performance tuning of approach Ol "1 offers a combination of accuracy and our computer codes to the CRAY X1; resulting in a great computational speed that can reveal the microscopic speed-up over previous architectures. structure and interactions of complex systems[3 51. Rational design of new materials with improved properties requires simulations that are accurate and
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  • Studies of Perovskite Materials for High-Performance Storage Media, Piezoelectric, and Solar Energy Conversion Devices

    Tingting Qi   Ilya Grinberg   Joseph W Bennett   Young-Han Shin   Andrew M Rappe   Ka-Lo Yeh   Keith A. Nelson  

    Perovskite materials are crucial in a variety of important technological applications. Using quantum-mechanical simulations and accurate molecular dynamics models, we have computationally investigated ferroelectric materials for applications in computer memory, piezoelectrics and photovoltaics. By tuning the applied electric pulses' shaping and delay separation, we find the lower limit for tetragonal PbTiO$ 3$ polarization switching time period is on the scale of 10 pico seconds. Structural and electronic properties of a semiconductor non-oxide perovskite BaZrS$ {3}$ are investigated in detail. We also microscopically model relax or behavior demonstrated by the next generation piezoelectric devices, not only reproducing dielectric response but also revealing the significance of local crystal chemistry.
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  • Modeling of Materials for naval SONAR, Pollution Control, and Nonvolatile Memory Application

    Ilya Grinberg   A.M. Kolpak   Young-Han Shin   A. M. Rappe  

    Ferroelectric perovskite oxides exhibit unique physical properties and are used in variety of technological applications. Using quantum-mechanical simulations, we have computationally investigated recently synthesized materials for use in naval SONAR and have developed composition-property correlations in ferroelectric perovskites. We also use density functional theory (DFT) calculations to show that the use of ferroelectric oxide as support for metal catalysts can greatly affect the reactive properties of the metal surface and its interactions with the gas phase molecules. This can enable dynamic, real time control of catalytic performance. Additionally, accurate atomistic calculations are used to study dynamics of polarization switching in ferroelectric materials.
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  • Molecular dynamics study of dielectric response in a relaxor ferroelectric.

    Ilya Grinberg   Young-Han Shin   Andrew M Rappe  

    We use atomistic molecular dynamics simulations to study relaxor behavior in the 0.75 PbMg(1/3)Nb(2/3)O(3)-0.25 PbTiO(3) material. Even for a fairly small simulation size of 1000 atoms, the system exhibits frequency dispersion and deviation from the Curie-Weiss law typical of relaxor materials. Analysis of the time autocorrelation functions for individual atoms allows us to identify the Nb atoms with a high concentration of neighboring Ti atoms as the nucleation sites for the relaxor behavior. This is due to the higher coupling between the cation displacements induced by the presence of overbonded oxygen atoms.
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  • Thermoelectric and phonon transport properties of two-dimensional IV–VI compounds

    Aamir Shafique   Young-Han Shin  

    We explore the thermoelectric and phonon transport properties of two-dimensional monochalcogenides (SnSe; SnS; GeSe; and GeS) using density functional theory combined with Boltzmann transport theory. We studied the electronic structures; Seebeck coefficients; electrical conductivities; lattice thermal conductivities; and figures of merit of these two-dimensional materials; which showed that the thermoelectric performance of monolayer of these compounds is improved in comparison compared to their bulk phases. High figures of merit (ZT) are predicted for SnSe (ZT = 2.63; 2.46); SnS (ZT = 1.75; 1.88); GeSe (ZT = 1.99; 1.73); and GeS (ZT = 1.85; 1.29) at 700 K along armchair and zigzag directions; respectively. Phonon dispersion calculations confirm the dynamical stability of these compounds. The calculated lattice thermal conductivities are low while the electrical conductivities and Seebeck coefficients are high. Thus; the properties of the monolayers show high potential toward thermoelectric applications.
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