Bulletin of the American Physical Society
APS March Meeting 2010
Volume 55, Number 2
Monday–Friday, March 15–19, 2010; Portland, Oregon
Session B32: Structure and Morphology: Oxide Surfaces |
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Sponsoring Units: DMP DCMP Chair: Patrick Rinke, Fritz-Haber-Institut der Max-Planck-Gesellschaft Room: E142 |
Monday, March 15, 2010 11:15AM - 11:27AM |
B32.00001: Growth and Atomistic Mechanisms of Snowflake-like ZnO Islands on curved substrates Guo Li, Chen Li, Chengmin Shen, Chao Hui, Jifa Tian, Shixuan Du, Hong-Jun Gao, Zhenyu Zhang Various snowflake-like patterns are obtained during heretoepitaxial growth of ZnO on Zn-dominant spheres via chemical vapor deposition (CVD). Highly regular and symmetric dendritic patterns of varying compactness can be obtained at different cooling rates. Furthermore, a reduction in the relative concentration of oxygen supply leads to smooth compact islands with sharp or flat tops. The underlying atomistic formation mechanisms of these patterns are investigated using kinetic Monte Carlo simulations. We reproduce the dendritic patterns in the simulations and also propose a plausible mechanism for the growth of the compact patterns with different tops. [Preview Abstract] |
Monday, March 15, 2010 11:27AM - 11:39AM |
B32.00002: Temperature stabilised surface reconstructions at polar ZnO$(0001)$ Mira Todorova, Markus Valtiner, Guido Grundmeier, J\"org Neugebauer Combining diffraction experiments with density-functional theory calculations and thermodynamic considerations we study the atomic structure of the polar ZnO$(0001)$ surfaces. We show that a large number of very different reconstructions with similar stoichiometry are energetically almost degenerate, thus surface vibrational entropy contributions significantly affect their stabilisation. The large impact of the vibrational entropy on the surface structure gives rise to a strong dependence of surface phase diagrams on temperature and enables us to consistently describe and explain the experimentally observed surface structures on polar ZnO$(0001)$ surfaces.\\ M. Valtiner, M. Todorova, G. Grundmeier, and J. Neugebauer, Phys. Rev. Lett. 103, 065502 (2009). [Preview Abstract] |
Monday, March 15, 2010 11:39AM - 11:51AM |
B32.00003: Scanning tunneling spectroscopy on ZnO(0001) surfaces : evidence for an inhomogeneous electronic structure B. Hackens, M.S. Rodrigues, S. Faniel, P.O. Mouthuy, S. Melinte, J. Dumont, R. Sporken We performed low temperature (77 K) scanning tunneling microscopy (STM) and spectroscopy (STS) on the polar Zn-terminated ZnO(0001) surface [1]. STM and STS data show that the surface electronic structure strongly depends on the local morphology : we observe a narrow bandgap and surface states in the flat regions, and, in the defective surface regions, a wide bandgap without surface states. We also image atomically-resolved $(\sqrt{3} \times \sqrt{3})R30^{\circ}$ reconstructions in small defect-free areas.\\[4pt] [1] J. Dumont et al., Appl. Phys. Lett. 95, 132102 (2009). [Preview Abstract] |
Monday, March 15, 2010 11:51AM - 12:03PM |
B32.00004: Formation of islands of substitutional Li defects on the MgO (001) surface Sergey V. Levchenko, Matthias Scheffler We use density functional theory (DFT) calculations to evaluate electronic structure and formation energies of the substitutional Li defects in the (001) surface, subsurface, and bulk MgO. We analyze the sensitivity of the results to the approximations in the exchange-correlation functional by comparing semi-local (GGA PBE) and non-local (PBE0 and B3LYP) approximations. Calculated formation energies indicate that the Li defects prefer to form islands with molar concentration 50\% at the top layer, but not in the deeper layers. The formation energy of an isolated substitutional Li defect increases with the depth of the layer in which the defect is formed. We perform an {\em ab initio} atomistic thermodynamics~[1] analysis to determine the average island size under realistic conditions. The analysis shows that, despite the loss in the configurational entropy due to the island formation, the islands containing 10-20 Li defects are present on the surface at relevant temperatures (900- 1000~K). Within the islands, the oxygen vacancy formation energy is greatly decreased due to an electron transfer from the O vacancy state to the lower energy states near the top of the valence band, associated with the Li defects.---[1] K. Reuter and M. Scheffler, Phys. Rev. B \textbf{65}, 035406 (2001); C. M. Weinert and M. Scheffler, Mat. Sci. Forum \textbf{10-12}, 25 (1986); M. Scheffler and J. Dabrowski, Phil. Mag. A \textbf{58}, 107 (1988) [Preview Abstract] |
Monday, March 15, 2010 12:03PM - 12:15PM |
B32.00005: Combined Theoretical and Experimental Model of an Oxide-Supported Heterogeneous Catalyst: WO$_{x}$/$\alpha$-Fe$_{2}$O$_{3}$(0001) Martin McBriarty, Zhenxing Feng, Michael Bedzyk, Donald Ellis Spin-polarized density functional theory (DFT) calculations were combined with surface-sensitive experimental techniques to evaluate models of monolayers and sub-monolayers of catalytic WO$_{x}$ on the (0001) surface of $\alpha$-Fe$_{2}$O$_{3}$ (hematite). Relaxed structures for various surface configurations were calculated, taking into account the surface lattice position and oxygen coordination of W as well as the presence of hydroxyl groups or adsorbed water. These structures were compared to surface atomic density maps generated by synchrotron x-ray standing wave (XSW) imaging under reducing and oxidizing conditions to deduce the most plausible atomic configurations. Theoretical ionicity of W atoms increased with oxygen coordination; as expected, the formal charges (W$^{5+}$ and W$^{6+}$) inferred from x-ray photoelectron spectroscopy (XPS) were not found. Using charge density maps and local densities of states, surface W-O and W-W interactions were studied and compared to Fe-O interactions in the bulk. [Preview Abstract] |
Monday, March 15, 2010 12:15PM - 12:27PM |
B32.00006: Polarization effect on the growth of Pt thin films on PbTiO$_3$ (100) surfaces Miguel Angel Mendez Polanco, Ilya Grinberg, Wissam Al-Saidi, Andrew Rappe Results of Density Functional Theory calculations performed on Pt thin films grown on PbTiO$_3$ (100) Ferroelectric (FE) ideal surface terminations are discussed. We analyze the effect of the ferroelectric polarization on the metal-oxide interface and how this in turn influences the growth of Pt thin films on PbTiO$_3$. Our results suggest a preferential stacking ordering for the Pt layers that depends on the polarization of the surface: The \textbf{\textit{hollow}}-top-hollow... arrangement is preferred over the \textbf{\textit{top}}-hollow-top... registry for the positive (P$^+$) polarization surface; however, this predilection is reversed for the negative (P$^-$) polarization surface. The effect of strain in the adsorption of Pt layers will also be discussed. This polarization-dependent preference for the adsorption of Pt layers onto FE surfaces could be of interest in understanding the mechanism of polarization fatigue. [Preview Abstract] |
Monday, March 15, 2010 12:27PM - 12:39PM |
B32.00007: Structure evolution of the polar SrTiO$_{3}$(110) surface upon Sr/Ti concentration Zhiming Wang, Kehui Wu, Qinlin Guo, Jiandong Guo In the artificial oxide heterostructures, the atom arrangement at the interface may be determinant for the property [1]. To study the lattice and electronic structures of SrTiO$_{3}$ surface at the atomic scale is important since it has been extensively used as a template for the growth of functional oxide materials. We report the investigation on the structure of the polar SrTiO$_{3}$ (110) surface treated by Ar ion sputtering followed by annealing in ultra high vacuum. Tow types of termination are obtained and their relative ratio is tuned by varying the sputtering dose or the Sr/Ti metal adsorption. The tuning mechanism is revealed as a chemical content driven process that is also responsible for the charge compensation on the polar surface. Further changing the relative concentration of Sr and Ti induces the formation of different surface reconstructions. \\[4pt] [1] A. Ohtomo and H. Y. Hwang, Nature 427, 423 (2004). [Preview Abstract] |
Monday, March 15, 2010 12:39PM - 12:51PM |
B32.00008: The structure of the (001) surface of Sr$_{3}$(Ru$_{1-x}$Mn$_{x })_{2}$O$_{7}$ Von Braun Nascimento, Biao Hu, Rongying Jin, E. Ward Plummer It has become quite clear that small structural distortions at the surface of complex materials like the transition-metal-oxides can have profound effects on the physical properties at the surface. We have investigated the surface structure of Sr$_{3}$(Ru$_{1-x}$Mn$_{x })_{2}$O$_{7 }$as a function of Mn substitution for Ru. Analysis of Sr$_{3}$Ru$_{2}$O$_{7}$ indicates an unexpected tilt (4\r{ }) as well as a larger rotation for the top RuO$_{6}$ octahedron (12\r{ }). Preliminary results indicate the tilted structural phase seems to be progressively suppressed by the substitution of Ru for Mn in the doped system. Electron diffraction patterns for a Mn doping level of only 10{\%}, already indicate a very small tilt angle at the surface ($\sim $1\r{ }). The question is what effect this structural change will have on the magnetic ordering in the bilayered manganite. Extensive studies on the single-layered (Sr,Ca)2RuO$_{4}$ system reveal that rotation and tilt of RuO$_{6}$ octahedra plays a critical role in their physical properties, both in bulk and at the surface. [Preview Abstract] |
Monday, March 15, 2010 12:51PM - 1:03PM |
B32.00009: Submonolayer C$_{60}$ films on ultrathin SiO$_2$ Michelle Groce, Brad Conrad, William Cullen, Ellen Williams The nucleation and growth of C$_{60}$ thin films on ultrathin SiO$_2$ are studied using room temperature scanning tunneling microscopy. Organic electronic devices are typically grown on a relatively thick SiO$_2$ substrate, which limits the techniques which may be used to characterize them. The ultrathin oxide layer grown on Si(111)-(7x7) presents a chemically equivalent interface, yet is thin enough to allow characterization by STM which achieves the highest possible spatial resolution. It has been reported that C$_{60}$ films initially follow the Volmer-Weber growth mode on insulating substrates, but there has previously been little investigation of their nucleation in the submonolayer regime. We report the morphological characteristics of films from 0.02 to 1 monolayer in thickness, varying the physical vapor deposition flux rate and the substrate temperature during deposition. [Preview Abstract] |
Monday, March 15, 2010 1:03PM - 1:15PM |
B32.00010: ABSTRACT WITHDRAWN |
Monday, March 15, 2010 1:15PM - 1:27PM |
B32.00011: Manganese doped Scandium Oxide as a Cathode for Fuel Cells Bridger Anderson, Paul Rugheimer, Yves Idzerda Current fuel cells contain a porous mixed ionic and electronic conductive cathode such as LSCF (Lanthanum Strontium Cobalt Ferrite). The porosity of LSCF is necessary to transport oxygen to the cathode electrolyte interface. The potential advantage of using an alternate cathode material such as Manganese doped Scandium Oxide (MSO) is the possibility of oxygen transport through a solid cathode, eliminating the additional complications introduced by cathode pores. We are investigating structural properties of rare earth and transition metal doped Scandium Oxide for application as a cathode layer in fuel cells. Here we report the results of X-ray Absorption Spectroscopy and X-ray Diffraction measurements done on pure and rare earth or transition metal doped ScO3. [Preview Abstract] |
Monday, March 15, 2010 1:27PM - 1:39PM |
B32.00012: Degradation Mechanisms in Oxygen Ion Conducting Materials Alexandre Lussier, Martin Finsterbusch, Yves U. Idzerda We have developed a testing apparatus to characterize degradation mechanisms in oxygen ion conducting materials, with an emphasis on Solid Oxide Fuel Cell (SOFC) materials. While chemical potentials drive currents in SOFCs, we utilize a simple electrical potential to drive oxygen ionic currents through materials and interfaces. We can additionally adjust the temperature and gaseous environment of our experiment, enabling us to identify and characterize degradation mechanisms and their causes. Early performance results confirm multiple SOFC cathode degradation mechanisms driven by both high temperatures and ion currents. In particular, cation inter-diffusion is prevalent at interfaces such as those between La(0.6)Sr(0.4)Co(0.2)Fe(0.8)O(3) and Ga-doped CeO(2) resulting in an interfacial structure which is increasingly resistant to subsequent oxygen ion flow. By isolating and understanding various degradation mechanisms we can more effectively address those mechanisms to improve long term ion conducting material performance. [Preview Abstract] |
Monday, March 15, 2010 1:39PM - 1:51PM |
B32.00013: Modified Cr valence in symmetric oxygen ion conducting half cells with ion flow Martin Finsterbusch, Alexandre Lussier, Ezana Negusse, Yves Idzerda The degradation mechanisms in ion conducting materials, including solid oxide fuel cells (SOFC), are still of high interest in current energy research, especially with regards to material and interface stability, fuel impurities and impurities originating from sealing or interconnect materials. A common practice is the use of symmetric half-cells (e.g. cathode/electrolyte/cathode) to determine interface stability via cross section Energy Dispersive x-ray Spectroscopy line scans and overpotential magnitude and degradation via AC-impedance spectroscopy. Using these electrically driven half-cells, we have developed a new method to directly measure degradation due to oxygen ion flow through ion conducting materials and their associated interface structures. By using X-ray absorption spectroscopy of cells before and after oxygen ion flow (800\r{ }C for 100 hours), we determined that the valence state of Cr that migrated from a metallic interconnect into the porous cathode changes in valence from +3 (Cr$_{2}$O$_{3})$ to +6 (CrO$_{3})$, depending on the direction of the oxygen ion flow. This observation is strong evidence of the influence of the oxygen ion flux on the degradation mechanisms of ion conducting materials. [Preview Abstract] |
Monday, March 15, 2010 1:51PM - 2:03PM |
B32.00014: Resistance Switching in Epitaxial Pr$_{0.7}$(Sr$_{1-x}$Ca$_{x}$)$_{0.3}$MnO$_{3}$ Films T. Venkatesan, W. Lv, C. Xin, A. Roy Barman, S. Dhar, Y. Chen, D. Lian, G. Xiong Epitaxial Pr$_{0.7}$(Sr$_{1-x}$Ca$_{x})_{0.3}$MnO$_{3}$ (PSCMO) thin films exhibit hysteretic resistance switching (RS) behavior at room temperature that demonstrate resistance changes from high resistance state (HRS) to low resistance state under an applied voltage and back to HRS in zero field. Dramatic changes of RS direction from anticlockwise to clockwise loops of $I-V$ data were observed in the same films when measurements were performed at temperatures lower than that of the metal-insulator transitions of the samples. Observations of RS behaviors obtained in the epitaxial PSCMO thin films provide clear evidence that besides thermally activated vacancy diffusion leading to conducting filaments, another field mediated process should be considered for understanding the driving mechanism of RS phenomenon in oxides. [Preview Abstract] |
Monday, March 15, 2010 2:03PM - 2:15PM |
B32.00015: Partial dissociation of Water on Fe$_3$O$_4$ (001): Adsorbate Induced Charge and Orbital Order Narasimham Mulakaluri, Rossitza Pentcheva, Maria Wieland, Wolfgang Moritz, Matthias Scheffler Besides potential applications in spintronics, magnetite plays an important role in many environmental redox reactions. Using density-functional theory together with an on-site Coulomb repulsion term (GGA+$U$), we investigate the interaction of water and its influence on the electronic properties and surface reconstruction of Fe$_3$O$_4$(001). The surface phase diagram, compiled within the framework of {\sl ab initio} atomistic thermodynamics, reveals dissociative adsorption especially at surface defect sites. At higher water vapour pressures a partial dissociation sets in where a chain of hydrogen bonded H$_2$O and OH groups forms. The mixed adsorption mode and a suppression of the $(\sqrt{2}\times \sqrt{2})R45^{\circ}$ reconstruction is confirmed by a quantitative low energy electron diffraction analysis. The defects and adsorbates induce a unique charge and orbital order on the Fe$_3$O$_4$(001) surface[1]. \begin{enumerate} \item N.Mulakaluri {\sl et al.}, Phys. Rev. Lett \textbf{103}, 176102 (2009). \end{enumerate} [Preview Abstract] |
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