Bulletin of the American Physical Society
APS March Meeting 2015
Volume 60, Number 1
Monday–Friday, March 2–6, 2015; San Antonio, Texas
Session F8: Oxide Films and Surfaces |
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Sponsoring Units: DCMP Chair: Daniel Dougherty, North Carolina State University Room: 006C |
Tuesday, March 3, 2015 8:00AM - 8:12AM |
F8.00001: Re-visiting the O/Cu(111) system - When metastable surface oxides could become an issue! Aloysius Soon, Norina A. Richter, Chang-Eun Kim, Catherine Stampfl Surface oxidation processes are crucial for the functionality of Cu-based catalytic systems used for many industrially important chemical reactions. Based on recent findings in XPS and LEED experiments, we assess the stability and population of the experimentally proposed ``8"-structure, a new surface oxide phase, on the Cu(111) surface. Using density-functional theory calculations in combination with {\it ab initio} atomistic thermodynamics and Boltzmann statistical mechanics, we find that the proposed oxide superstructure is indeed metastable and that the population of the ``8"-structure is competitive with the known ``29" and ``44" oxide film structures on Cu(111). We show that the configuration of O and Cu atoms in the first and second layers of the ``8"-structure closely resembles the arrangement of atoms in the first two layers of Cu$_2$O(110), where the atoms in the ``8"-structure are more constricted. Cu$_2$O(110) has been suggested in the literature as the most active low index facet for reactions such as water splitting under light illumination. If the ``8"-structure were to form during a catalytic process, it is therefore likely to be one of the reactive phases. [Preview Abstract] |
Tuesday, March 3, 2015 8:12AM - 8:24AM |
F8.00002: Oxygen disorder, a way to accommodate large tensile strains in oxide thin films Y.Y. Zhang, R. Mishra, T.J. Pennycook, A.Y. Borisevich, S.J. Pennycook, S.T. Pantelides Strain induced by lattice mismatch of epitaxial layers is typically accommodated by misfit dislocations. In transition-metal oxides, other strain-relaxation modes have been observed (oxygen vacancy, octahedral tilts, etc.). Here we use density functional calculations to compare the total energies of different structures and to check for negative-frequency phonon modes, which is an good indicator of instability, and explore the stability of several oxide thin films. We find that when a ZrO2 thin film is sandwiched between SrTiO3 layers (7{\%} biaxial tensile strain), disorder in the oxygen sublattice lowers the energy by 1.4 eV/ZrO2 formula and leads to a stable configuration without negative-frequency phonon modes. Oxygen disorder can also accommodate a 6{\%} biaxial tensile strain in rutile TiO2 thin film. In contrast, we find that if a (LaFeO3)2/(SrFeO3) superlattice is grown on a substrate that imposes an overall biaxial tensile strain, there is a competition between O sublattice disorder, formation and ordering of O vacancies, and octahedral tilts. The mechanism for strain compensation varies with the extent of the strain. We conclude that oxygen-sublattice disorder is one of many ways that tensile strain can be accommodated in transition-metal oxide films. [Preview Abstract] |
Tuesday, March 3, 2015 8:24AM - 8:36AM |
F8.00003: Thermodynamic stability and band alignment at SrTiO3/GaAs(001) interface Joelson Cott, Ravi Droopad, Byounghak Lee The successful epitaxial growth of SrTiO3 on Si showed that it is possible to integrate the functional oxides with semiconductors incorporating unique multifunctional properties of oxides into various existing semiconductor technologies. While SrTiO3 has been also deposited on GaAs without amorphous interfacial layers, the exact interface structure has been controversial; On one hand, scanning Transmission Electron Microscopy (STEM) shows As atoms at the interface. X-ray photoelectron spectroscopy (XPS) measurements, on the other hand, do not show peaks associated with As-O bonding, indicating that the GaAs/STO interface does not involve an As/oxide layer, contradicting to STEM observations. Using ab initio calculations, we determine the interfacial structure of a SrTiO3 film on a GaAs substrate to help clarify the apparently discrepancy between the XPS and the STEM measurements. The calculations reveal that, under the condition that allows atomically abrupt interfaces, the energetically most stable interface is As/Sr/TiO2 structure, in accordance with both XPS and STEM measurements. We discuss the band offsets and the possibility of in-gap states of various interface structures. [Preview Abstract] |
Tuesday, March 3, 2015 8:36AM - 8:48AM |
F8.00004: Study of electronic structure and magnetism at the relaxed SrTiO$_3$/LaMO$_3$ interface Soham Ghosh, Efstratios Manousakis We present a density functional theory study of the nature of bands in z-terminated metal oxides. It is shown that the bandstructure of pure SrTiO$_3$ near the fermi surface is modified by the presence of surfaces, besides being sensitive to ionic relaxations and thickness of the material. We also study the cases where layers of LaMO$_3$ (with M = Ti, Al) have been added to create an interface. We examine doping of the SrTiO$_3$ surface bands by the added layers as a possible reason for the presence and localization of the induced electron gas and we study the character of these bands. [Preview Abstract] |
Tuesday, March 3, 2015 8:48AM - 9:00AM |
F8.00005: Lattice-mismatch Strain Effects in Electron-Doped Calcium Manganese Oxide Thin Films Cacie Hart, Grace Yong, Zoey Warecki, Adeel Chaudhry, Prakash Sharma, Anthony Johnson, David Schaefer, Rajeswari Kolagani Electron-doped Calcium Manganese Oxide (CaMnO$_{3-\delta}$) thin films are of interest for use as photocatalysts and fuel cell electrodes in renewable energy applications. Oxygen stoichiometry of the films is a key parameter for the functionality in these applications. Currently, we are investigating the properties of (CaMnO$_{3-\delta}$) films grown by pulsed laser deposition. The thin films are epitaxially grown on LaAlO$_{3}$ and SrTiO$_{3}$ substrates. Both of these substrates have larger in-plane lattice parameters than CaMnO$_{3-\delta}$, which leads to bi-axial tensile strain in the thin films. We have characterized the thickness dependence of structural, electrical, and morphological properties of these films using high resolution x-ray diffraction, temperature dependent electrical resistivity measurements, and atomic force microscopy. The thickness dependence is characteristically different from what has been preciously observed in thin films of hole-doped manganites. Our results suggest that coupling between tensile strain and oxygen deficiency affect the electrical and structural properties of the material. [Preview Abstract] |
Tuesday, March 3, 2015 9:00AM - 9:12AM |
F8.00006: Epitaxial growth of in-plane-dimerized, single phase NbO$_2$ thin films for metal-insulator transition applications Agham Posadas, Tobias Hadamek, Andy O'Hara, Alexander Demkov NbO$_2$ is a exhibits a metal-insulator transition that may have potential applications in electronic devices. The strong conductivity change in NbO$_2$ occurs along the dimerization direction and for devices utilizing NbO$_2$ as a channel material (in-plane transport) such as transistors, one would like the dimerization direction to be in the plane of the film. The electrical properties of Nb oxides are strongly dependent on the oxidation state of Nb. It is therefore critical to be able to control the oxidation state of Nb during growth. Here, we describe the epitaxial growth of in-plane-dimerized NbO$_2$ using molecular beam epitaxy on a variety of substrates (STO, LSAT, MgO, BTO and GaN), growth temperatures, and oxygen-to-niobium flux ratios. We show that the particular substrate used significantly affects the bulk and surface crystallinity, as well as the degree of oxidation. We also show the evolution of the valence and core level photoemission spectra of Nb oxides as a function of oxygen-to-niobium flux ratio and point out the optimum growth conditions to achieve phase-pure, epitaxial NbO$_2$ films. [Preview Abstract] |
Tuesday, March 3, 2015 9:12AM - 9:24AM |
F8.00007: Band-gap engineering at a semiconductor - crystalline oxide interface Kamyar Ahmadi-Majlan, Mohammadreza Jahangir-Moghadam, Xuan Shen, Timothy Droubay, Mark Bowden, Matthew Chrysler, Dong Su, Scott A. Chambers, Joseph H. Ngai Abstract: The epitaxial growth of crystalline oxides on semiconductors provides a pathway to introduce new functionalities to semiconductor devices. Key to electrically coupling crystalline oxides with semiconductors to realize functional behavior is controlling the manner in which their bands align at interfaces. Here we apply principles of band gap engineering traditionally used at heterojunctions between conventional semiconductors to control the band offset between a single crystalline oxide and a semiconductor. Reactive molecular beam epitaxy is used to realize atomically abrupt and structurally coherent interfaces between SrZrxTi1$-$xO3 and Ge, in which the band-gap of the former is enhanced with Zr content x. We present structural and electrical characterization of SrZrxTi1$-$xO3-Ge heterojunctions for x $=$ 0.2 to 0.75 and demonstrate the band offset can be tuned from type-II to type-I, with the latter being verified using photoemission measurements. The type-I band offset provides a platform to integrate the dielectric, ferroelectric and ferromagnetic functionalities of oxides with semiconducting devices. [Preview Abstract] |
Tuesday, March 3, 2015 9:24AM - 9:36AM |
F8.00008: Effect of post-deposition annealing on the structure and physical properties of strained epitaxial Ni$_{\mathrm{1-x}}$Ti$_{\mathrm{1-y}}$O$_{3}$ thin films Tamas Varga, Timothy Droubay, Libor Kovarik, Scott Chambers Ferroelectrically induced weak ferromagnetism had been predicted in compounds MTiO$_{3}$ (M$=$Fe,Mn,Ni) with the LiNbO$_{3}$-type structure. In order to stabilize this metastable structure by oxide heteroepitaxy, we attempted to grow epitaxial NiTiO$_{3}$ films on Al$_{2}$O$_{3}$, and Fe$_{2}$O$_{3}$/Al$_{2}$O$_{3}$ substrates by pulsed laser deposition. Given the structural imperfections of the as-deposited films arising from the large lattice mismatch, which resulted in weak ferroic ordering, we investigated the effect of post-synthesis annealing on the films' properties. Our structural data from x-ray diffraction and electron microscopy suggest that the crystalline quality of the Ni$_{\mathrm{1-x}}$Ti$_{\mathrm{1-y}}$O$_{3}$ films was greatly improved by annealing the films at 1000 $^{\circ}$C for 8 hours. Our physical property characterization indicates increased ferromagnetism in the films. The specific changes in film structure and magnetic as well as polar properties will be discussed. These results suggest that the ferroic domain properties of the films can be favorably altered by post-synthesis heat treatment. [Preview Abstract] |
Tuesday, March 3, 2015 9:36AM - 9:48AM |
F8.00009: Epitaxial Growth of Ca$_{2}$IrO$_{4}$ Single-Crystal Thin-Films Maryam Souri, John H. Gruenewald, Jasminka Terzic, Gang Cao, Joseph W. Brill, Sung S. Ambrose Seo Complex oxides containing 5$d$ transition metals including iridates have attracted substantial attention due to their potential to create novel electronic and magnetic states that originate from strong spin--orbit coupling and the electron-correlation of 5$d$ electrons. However, the progress of experimental research on the 5$d$ transition-metal oxides is hindered by the limited number of available materials. To further understand the layered iridates ($A_{2}$IrO$_{4}$, $A$: alkaline-earth elements) featuring the J$_{\mathrm{eff}}=$1/2 Mott state, we have synthesized epitaxial thin-films of Ca$_{2}$IrO$_{4}$. The single crystal Ruddlesden-Popper (R-P) phase of Ca$_{\mathrm{n+1}}$Ir$_{\mathrm{n}}$O$_{\mathrm{3n+1}}$ (n$=$1) is thermodynamically unstable; hence, we have used epitaxial-stabilization strategies to grow metastable thin-films of Ca$_{2}$IrO$_{4}$. The R-P phase of Ca$_{2}$IrO$_{4}$ is synthesized on yttrium aluminum oxide and lanthanum aluminum oxide substrates by pulsed laser deposition. We have studied the electronic structure of these films by transport and optical spectroscopic measurements. The dc-resistivity shows that these Ca$_{2}$IrO$_{4}$ thin-films are insulating with activation energy of about 100 meV. The optical spectroscopy shows that the optical gap energy is about 0.5 eV. We will discuss the electronic structure of Ca$_{2}$IrO$_{4}$ by comparing with Sr$_{2}$IrO$_{4}$ and Ba$_{2}$IrO$_{4}$. [Preview Abstract] |
Tuesday, March 3, 2015 9:48AM - 10:00AM |
F8.00010: Structure-Property relationship for H covered Fe3O4(001) Fangyang Liu, Orhan Kizilkaya, Phillip Sprunger, Richard Kurtz, Rongying Jin, Jiandi Zhang, Ward Plummer Magnetite (Fe3O4), the oldest permanent magnet, is still being studied, due to the fascinating surface properties. Clean B layer terminated Fe3O4(001) surface exhibits a ($\surd $2 $\times$ $\surd $2)R45 reconstruction, which as reported by LEED experiments can be removed by hydrogen adsorption at RT. However, the mechanism of this surface structural change is unknown. Combining HREELS, LEIS, ARXPS, UPS and XANES, we discovered a very unusual adsorption mechanism. Hydrogen appears to be bonded to the surface iron atoms not oxygen as expected. We observe H-Fe vibration mode with HREELS while no OH mode is present. Furthermore LEIS experiments confirmed H is on the iron atoms site. We will discuss the adsorption mechanism and electronic structure change with information provided by the core level photoemission techniques. [Preview Abstract] |
Tuesday, March 3, 2015 10:00AM - 10:12AM |
F8.00011: Thin-film growth of the quasi-one-dimensional metal Li$_{0.9}$Mo$_6$O$_{17}$ Alexandra Cote, Saeed Moshfeghyeganeh, Joshua L. Cohn, John J. Neumeier Attempts to grow epitaxial thin films of Li$_{0.9}$Mo$_6$O$_{17}$ by pulsed-laser deposition will be discussed. Single crystals of this quasi-one-dimensional (q1D) metal exhibit\footnote{J. L. Cohn {\it et al.}, Phys. Rev. Lett., {\bf 112}, 186602 (2014).} highly anisotropic Seebeck coefficients with $\Delta S=S_c-S_b\approx 200\ \mu$~V/K near 450~K (the $b$ axis corresponds to the most conducting, q1D chain direction). Suitably oriented thin films could enable possible applications in energy detection using the transverse Seebeck effect. X-ray diffraction results will be presented for films grown from a polycrystalline target on several substrates under a narrow range of temperature and pressure conditions. [Preview Abstract] |
Tuesday, March 3, 2015 10:12AM - 10:24AM |
F8.00012: Superconducting single crystalline YBa2Cu3O7-$\delta $ on SrTiO3 buffered Si (100) Mohammadreza Jahangir Moghadam, Kamyar Ahmadi Majlan, Hao Zhang, Xuan Shen, Matthew Chrysler, Patrick Conlin, Ricky Hensley, Dong Su, John Wei, Joseph Ngai The growth of crystalline oxides on semiconductors enables new functionalities to be integrated with semiconducting technologies. Here, thin films of optimally-doped (001)-oriented YBa2Cu3O7-$\delta $ are epitaxially integrated on silicon (001) through growth on a SrTiO3 buffer. The former is grown using pulsed-laser deposition and the latter is grown on Si using oxide molecular beam epitaxy. The single crystal nature of the SrTiO3 buffer enables very high transition temperatures to be achieved. For a 30 nm thick SrTiO3 buffer, YBa2Cu3O7-$\delta $ films exhibiting a transition temperature of $\sim$ 95 K, and a narrow transition width (\textless 5 K) are achieved. The integration of single crystalline YBa2Cu3O7-$\delta $ on Si (001) paves the way for the potential exploration of cuprate materials in a variety of applications. [Preview Abstract] |
Tuesday, March 3, 2015 10:24AM - 10:36AM |
F8.00013: Optical and magnetic properties of Ca$_{3}$CoMnO$_{6}$ thin films Jitendra Saha, Gyaneshwar Sharma, Satyabrata Patnaik Ca$_{3}$CoMnO$_{6}$ is one of the initial one-dimensional Ising chain compounds that has shown large magnetoelectric coupling below its antiferromagnetic temperature (15 K). We report on the growth and characterization of Ca$_{3}$CoMnO$_{6}$ thin films deposited by pulse laser deposition. The films of thickness 220 nm are grown on 0001-oriented sapphire substrates at 750 $^{\circ}$C. The band gap ($\sim$ 1.73eV) derived from UV visible absorption spectroscopy and temperature dependent resistivity is consistent with one another. It is seen that the films can be grown at various oxygen pressures but the optimal deposition pressure is found to be 5x10$^{-2}$ mbar. The effect of oxygen pressure on the texture of the film and band gap indicates that the oxygen vacancies play a major role in the optical and electrical properties of the films. AFM measurements show a homogeneous growth of the films. Magnetization measurement shows that the transition temperature increased to 39 K, much above the bulk Neel temperature. The increase in magnetic transition is supposed to be due to stronger inter-chain interaction caused by tensile strain effected by lattice mismatch. [Preview Abstract] |
Tuesday, March 3, 2015 10:36AM - 10:48AM |
F8.00014: ABSTRACT WITHDRAWN |
Tuesday, March 3, 2015 10:48AM - 11:00AM |
F8.00015: Effects of ferroelectric polarization on surface phase diagram: an evolutionary algorithm study of the BaTiO$_{3}$(001) surface Pengcheng Chen, Yong Xu, Na Wang, Artem R. Oganov, Wenhui Duan We have constructed the surface phase diagram of the BaTiO$_{3}$(001) surface by employing an evolutionary algorithm for surface structure prediction, where the ferroelectric polarization is included as a degree of freedom. Among over 1000 candidate structures explored, a surface reconstruction of (2$\times$1)-TiO is discovered to be thermodynamically stable and have the \emph{p2mm} plane group symmetry as observed experimentally. We find that the influence of ferroelectric polarization on the surface free energy can be either negligibly small or sizably large (over 1 eV per ($2 \times 1$) supercell), depending strongly on the surface structure and resulting in a significant distinction of surface phase diagram with varying ferroelectric polarization. It is therefore feasible to control the surface stability by applying an external electric field. Our results may have important implications in understanding the surface reconstruction of ferroelectric materials and tuning surface properties. [Preview Abstract] |
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