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- ItemImpact of the precursor chemistry and process conditions on the cell-to-cell variability in 1T-1R based HfO2 RRAM devices(London : Nature Publishing Group, 2018) Grossi, A.; Perez, E.; Zambelli, C.; Olivo, P.; Miranda, E.; Roelofs, R.; Woodruff, J.; Raisanen, P.; Li, W.; Givens, M.; Costina, I.; Schubert, M.A.; Wenger, C.The Resistive RAM (RRAM) technology is currently in a level of maturity that calls for its integration into CMOS compatible memory arrays. This CMOS integration requires a perfect understanding of the cells performance and reliability in relation to the deposition processes used for their manufacturing. In this paper, the impact of the precursor chemistries and process conditions on the performance of HfO2 based memristive cells is studied. An extensive characterization of HfO2 based 1T1R cells, a comparison of the cell-to-cell variability, and reliability study is performed. The cells’ behaviors during forming, set, and reset operations are monitored in order to relate their features to conductive filament properties and process-induced variability of the switching parameters. The modeling of the high resistance state (HRS) is performed by applying the Quantum-Point Contact model to assess the link between the deposition condition and the precursor chemistry with the resulting physical cells characteristics.
- ItemFemtosecond X-ray diffraction from nanolayered oxides(Amsterdam : Elsevier, 2010) Von Korff Schmising, C.; Harpoeth, A.; Zhavoronkov, N.; Woerner, M.; Elsaesser, T.; Bargheer, M.; Schmidbauer, M.; Vrejoiu, I.; Hesse, D.; Alexe, M.Femtosecond X-ray scattering offers the opportunity to investigate reversible lattice dynamics with unprecedented accuracy. We show in a prototype experiment how strain propagation modifies the functionality of a ferroelectric system on its intrinsic time scale.
- ItemUltrafast structural changes in SrTiO3 due to a superconducting phase transition in a YBa2Cu3O7 top layer(College Park, MD : Institute of Physics Publishing, 2010) Lübcke, A.; Zamponi, F.; Loetzsch, R.; Kämpfer, T.; Uschmann, I.; Große, V.; Schmidl, F.; Köttig, T.; Thürk, M.; Schwoerer, H.; Förster, E.; Seidel, P.; Sauerbrey, R.We investigate the structural response of SrTiO3 when Cooper pairs are broken in an epitaxially grown YBa2Cu3O 7 top layer due to both heating and optical excitation. The crystal structure is investigated by static, temperaturedependent and time-resolved x-ray diffraction. In the static case, a large strain field in SrTiO3 is formed in the proximity of the onset of the superconducting phase in the top layer, suggesting a relationship between both effects. For the time-dependent studies, we likewise find a large fraction of the probed volume of the SrTiO3 substrate strained if the top layer is superconducting. Upon optical breaking of Cooper pairs, the observed width of the rocking curve is reduced and its position is slightly shifted towards smaller angles. The dynamical theory of x-ray diffraction is used to model the measured rocking curves. We find that the thickness of the strained layer is reduced by about 200 nm on a sub-ps to ps timescale, but the strain value at the interface between SrTiO3 and YBa2Cu3O7 remains unaffected. © IOP Publishing Ltd and Deutsche Physikalische Gesellschaft.
- ItemSingle molecule magnet with an unpaired electron trapped between two lanthanide ions inside a fullerene(London : Nature Publishing Group, 2017) Liu, F.; Krylov, D.S.; Spree, L.; Avdoshenko, S.M.; Samoylova, N.A.; Rosenkranz, M.; Kostanyan, A.; Greber, T.; Wolter, A.U.B.; Büchner, B.; Popov, A.A.Increasing the temperature at which molecules behave as single-molecule magnets is a serious challenge in molecular magnetism. One of the ways to address this problem is to create the molecules with strongly coupled lanthanide ions. In this work, endohedral metallofullerenes Y 2 @C 80 and Dy 2 @C 80 are obtained in the form of air-stable benzyl monoadducts. Both feature an unpaired electron trapped between metal ions, thus forming a single-electron metal-metal bond. Giant exchange interactions between lanthanide ions and the unpaired electron result in single-molecule magnetism of Dy 2 @C 80 (CH 2 Ph) with a record-high 100 s blocking temperature of 18 K. All magnetic moments in Dy 2 @C 80 (CH 2 Ph) are parallel and couple ferromagnetically to form a single spin unit of 21 μ B with a dysprosium-electron exchange constant of 32 cm -1. The barrier of the magnetization reversal of 613 K is assigned to the state in which the spin of one Dy centre is flipped.
- ItemMixed dysprosium-lanthanide nitride clusterfullerenes DyM2N@C80-: I h and Dy2MN@C80- i h (M = Gd, Er, Tm, and Lu): Synthesis, molecular structure, and quantum motion of the endohedral nitrogen atom(Cambridge : RSC Publ., 2019) Schlesier, C.; Liu, F.; Dubrovin, V.; Spree, L.; Büchner, B.; Avdoshenko, S.M.; Popov, A.A.Systematic exploration of the synthesis of mixed-metal Dy-M nitride clusterfullerenes (NCFs, M = Gd, Er, Tm, Lu) is performed, and the impact of the second metal on the relative yield is evaluated. We demonstrate that the ionic radius of the metal appears to be the main factor allowing explanation of the relative yields in Dy-M mixed-metal systems with M = Sc, Lu, Er, and Gd. At the same time, Dy-Tm NCFs show anomalously low yields, which is not consistent with the relatively small ionic radius of Tm3+ but can be explained by the high third ionization potential of Tm. Complete separation of Dy-Gd and Dy-Er, as well as partial separation of Dy-Lu M3N@C80 nitride clusterfullerenes, is accomplished by recycling HPLC. The molecular structures of DyGd2N@C80 and DyEr2N@C80 are analyzed by means of single-crystal X-ray diffraction. A remarkable ordering of mixed-metal nitride clusters is found despite similar size and electronic properties of the metals. Possible pyramidalization of the nitride clusters in these and other nitride clusterfullerenes is critically analyzed with the help of DFT calculations and reconstruction of the nitrogen inversion barrier in M3N@C80 molecules is performed. Although a double-well potential with a pyramidal cluster structure is found to be common for most of them, the small size of the inversion barrier often leads to an apparent planar structure of the cluster. This situation is found for those M3N@C80 molecules in which the energy of the lowest vibrational level exceeds that of the inversion barrier, including Dy3N@C80 and DyEr2N@C80. The genuine pyramidal structure can be observed by X-ray diffraction only when the lowest vibrational level is below the inversion barrier, such as those found in Gd3N@C80 and DyGd2N@C80. The quantum nature of molecular vibrations becomes especially apparent when the size of the inversion barrier is comparable to the energy of the lowest vibrational levels.
- ItemGiant thermal expansion and α-precipitation pathways in Ti-Alloys(London : Nature Publishing Group, 2017) Bönisch, M.; Panigrahi, A.; Stoica, M.; Calin, M.; Ahrens, E.; Zehetbauer, M.; Skrotzki, W.; Eckert, J.Ti-Alloys represent the principal structural materials in both aerospace development and metallic biomaterials. Key to optimizing their mechanical and functional behaviour is in-depth know-how of their phases and the complex interplay of diffusive vs. displacive phase transformations to permit the tailoring of intricate microstructures across a wide spectrum of configurations. Here, we report on structural changes and phase transformations of Ti-Nb alloys during heating by in situ synchrotron diffraction. These materials exhibit anisotropic thermal expansion yielding some of the largest linear expansion coefficients (+ 163.9×10-6 to-95.1×10-6 °C-1) ever reported. Moreover, we describe two pathways leading to the precipitation of the α-phase mediated by diffusion-based orthorhombic structures, α″lean and α″iso. Via coupling the lattice parameters to composition both phases evolve into α through rejection of Nb. These findings have the potential to promote new microstructural design approaches for Ti-Nb alloys and β-stabilized Ti-Alloys in general.
- ItemThe Contrasting Character of Early and Late Transition Metal Fluorides as Hydrogen Bond Acceptors(Washington, DC : ACS Publications, 2015) Smith, Dan A.; Beweries, Torsten; Blasius, Clemens; Jasim, Naseralla; Nazir, Ruqia; Nazir, Sadia; Robertson, Craig C.; Whitwood, Adrian C.; Hunter, Christopher A.; Brammer, Lee; Perutz, Robin N.The association constants and enthalpies for the binding of hydrogen bond donors to group 10 transition metal complexes featuring a single fluoride ligand (trans-[Ni(F)(2-C5NF4)(PR3)2], R = Et 1a, Cy 1b, trans-[Pd(F)(4-C5NF4)(PCy3)2] 2, trans-[Pt(F){2-C5NF2H(CF3)}(PCy3)2] 3 and of group 4 difluorides (Cp2MF2, M = Ti 4a, Zr 5a, Hf 6a; Cp*2MF2, M = Ti 4b, Zr 5b, Hf 6b) are reported. These measurements allow placement of these fluoride ligands on the scales of organic H-bond acceptor strength. The H-bond acceptor capability β (Hunter scale) for the group 10 metal fluorides is far greater (1a 12.1, 1b 9.7, 2 11.6, 3 11.0) than that for group 4 metal fluorides (4a 5.8, 5a 4.7, 6a 4.7, 4b 6.9, 5b 5.6, 6b 5.4), demonstrating that the group 10 fluorides are comparable to the strongest organic H-bond acceptors, such as Me3NO, whereas group 4 fluorides fall in the same range as N-bases aniline through pyridine. Additionally, the measurement of the binding enthalpy of 4-fluorophenol to 1a in carbon tetrachloride (−23.5 ± 0.3 kJ mol–1) interlocks our study with Laurence’s scale of H-bond basicity of organic molecules. The much greater polarity of group 10 metal fluorides than that of the group 4 metal fluorides is consistent with the importance of pπ–dπ bonding in the latter. The polarity of the group 10 metal fluorides indicates their potential as building blocks for hydrogen-bonded assemblies. The synthesis of trans-[Ni(F){2-C5NF3(NH2)}(PEt3)2], which exhibits an extended chain structure assembled by hydrogen bonds between the amine and metal-fluoride groups, confirms this hypothesis.
- ItemTailor-made nanostructures bridging chaos and order for highly efficient white organic light-emitting diodes(London : Nature Publishing Group, 2019) Li, Y.; Kovačič, M.; Westphalen, J.; Oswald, S.; Ma, Z.; Hänisch, C.; Will, P.-A.; Jiang, L.; Junghaehnel, M.; Scholz, R.; Lenk, S.; Reineke, S.Organic light-emitting diodes (OLEDs) suffer from notorious light trapping, resulting in only moderate external quantum efficiencies. Here, we report a facile, scalable, lithography-free method to generate controllable nanostructures with directional randomness and dimensional order, significantly boosting the efficiency of white OLEDs. Mechanical deformations form on the surface of poly(dimethylsiloxane) in response to compressive stress release, initialized by reactive ions etching with periodicity and depth distribution ranging from dozens of nanometers to micrometers. We demonstrate the possibility of independently tuning the average depth and the dominant periodicity. Integrating these nanostructures into a two-unit tandem white organic light-emitting diode, a maximum external quantum efficiency of 76.3% and a luminous efficacy of 95.7 lm W−1 are achieved with extracted substrate modes. The enhancement factor of 1.53 ± 0.12 at 10,000 cd m−2 is obtained. An optical model is built by considering the dipole orientation, emitting wavelength, and the dipole position on the sinusoidal nanotexture.
- ItemDimethyl carbonate synthesis from carbon dioxide using ceria–zirconia catalysts prepared using a templating method: characterization, parametric optimization and chemical equilibrium modeling(London : RSC Publishing, 2016) Kumar, Praveen; With, Patrick; Srivastava, Vimal Chandra; Shukla, Kartikeya; Gläser, Roger; Mishra, Indra ManiIn this paper, a series of CexZr1−xO2 solid solution spheres were synthesized by exo- and endo-templating methods and tested for dimethyl carbonate (DMC) synthesis using direct conversion of CO2. The synthesized catalysts were characterized by X-ray diffraction (XRD), N2-physisorption, scanning electron microscopy (SEM), and CO2/NH3-temperature-programmed desorption (TPD). Formation of CexZr1−xO2 solid solutions with tetragonal and cubic crystal structures depending on cerium/zirconium compositions was confirmed by XRD analysis. The specific surface area of the mixed oxide decreased and the average pore diameter increased with an increase in the ceria content, with the exception of the mixed oxides with x = 0.4–0.5 i.e. Ce0.4Zr0.6O2 and Ce0.5Zr0.5O2. The basic and acidic site density of the synthesized catalysts was in the order: ZrO2 < CeO2 < Ce0.5Zr0.5O2, and the basic and acidic site density per unit area followed the same order. The best Ce0.5Zr0.5O2 catalyst was further used for the optimization of reaction conditions such as reaction time, reaction temperature, catalyst dose and reusability for DMC synthesis. Furthermore, study of chemical equilibrium modeling was done using the Peng–Robinson–Stryjek–Vera equation of state (PRSV-EoS) along with the van der Waals one-fluid reaction condition so as to calculate change of Gibbs free energy (ΔG°) and heat of reaction (ΔH°).
- ItemPulsed laser deposition of thick BaHfO3-doped YBa 2Cu307-δ films on highly alloyed textured Ni-W tapes(Bristol : Institute of Physics Publishing, 2014) Sieger, M.; Hänisch, J.; Iida, K.; Gaitzsch, U.; Rodig, C.; Schultz, L.; Holzapfel, B.; Hühne, R.YBa2Cu3O7-δ (YBCO) films with a thickness of up to 3 μm containing nano-sized BaHfO3 (BHO) have been grown on Y2O3/Y-stabilized ZrO2/CeO 2 buffered Ni-9at% W tapes by pulsed laser deposition (PLD). Structural characterization by means of X-ray diffraction confirmed that the YBCO layer grew epitaxial. A superconducting transition temperature T c of about 89 K with a transition width of 1 K was determined, decreasing with increasing BHO content. Critical current density in self-field and at 0.3 T increased with increasing dopant level.