Bulletin of the American Physical Society
APS March Meeting 2016
Volume 61, Number 2
Monday–Friday, March 14–18, 2016; Baltimore, Maryland
Session X47: Thin Insulating Adlayers and Films |
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Sponsoring Units: DCMP Room: 312 |
Friday, March 18, 2016 8:00AM - 8:12AM |
X47.00001: Oxidic copper on the Au(111) surface:A theoretical surface science approach Taehun Lee, Yonghyuk Lee, Kisung Kang, Aloysius Soon Recently, via reactive Cu deposition in an oxygen ambience, high quality gold-supported cuprous oxide (Cu$_2$O) ultrathin nanofilms [1] have been prepared as a model system to further such catalytic studies. Nonetheless, an accurate atomic picture of these ultrathin Cu$_2$O nanofilms, which largely depends on its immediate oxygen environment, is currently lacking. In this work, we perform density-functional theory (DFT) calculations using the Vienna ab initio Simulation Package in combination with \textit{ab initio} atomistic thermodynamics [2] to investigate stability of Cu$_2$O thin films on Au(111) as a function of oxygen chemical potential. Our results indeed show that some of the surface structures suggested in Ref. [1] are energetically more stable than the traditional copper oxide thin film structures on copper substrate, and elucidated the electronic structure of these ultrathin copper oxide films on gold, in comparison with available experimental data. [1] H. Str\"{a}er et al., J. Phys. Chem. C 119, 5975 (2015); [2] A. Soon et al., Phys. Rev. B 73, 165424 (2006) [Preview Abstract] |
Friday, March 18, 2016 8:12AM - 8:24AM |
X47.00002: Interaction of Molecular Oxygen with Au (001) Surface Michael Pierce, Andrew Loheac, Andi Barbour, Vladimir Komanicky, Anthony Ruffino, Hoydoo You Kinetics of dioxygen - Au (001) surface interaction has been studied at high temperature and near atmospheric pressures with in situ x-ray scattering measurements. We find that the hexagonal reconstruction (hex) of Au (001) surface lifts to (1\texttimes 1) in the presence of dioxygen. The measured lifting rate constant vs. temperature shows a `volcano'-type behavior indicating that oxygen adsorption limits at high temperature while activation barrier of the lifting limits at low temperature. The pressure--temperature (PT) phase diagram obtained in our study shows three regions: hex at low P and T, (1\texttimes 1) at high P and T, and coexistence of the hex and the (1\texttimes 1) at the intermediate P and T. [Preview Abstract] |
Friday, March 18, 2016 8:24AM - 8:36AM |
X47.00003: Stabilization of the O p2x2 phase on Cu(001) sheltered by wrinkled BN over-layer Yong-Sung Kim, Chuanxu Ma, An-Ping Li, Mina Yoon The 2$\surd $3x$\surd $3R45\textdegree phase of oxygen (O) on the Cu(001) surface has been observed in scanning tunneling microscopy (STM) measurements. Although the p2x2 phase of O on the Cu(001) surface has been proposed theoretically to be the most stable in O-lean conditions, it has not been observed in experiments for a long time. Recently, the O p2x2 phase has been found in STM on the Cu(001) surface with an overlying BN monolayer. In this theoretical study, we investigate what the role of BN over-layer is to stabilize the O p2x2 phase on the Cu(001) surface. The BN over-layer is lattice-matched with the Cu(001) surface and the BN mono-layer sheet is periodically wrinkled along the BN arm-chair direction and along the [100] or [010] direction on the Cu(001) surface. The interlayer space between the Cu(001) surface and the bulge of the wrinkled BN sheet is found to play as a preferential shelter for O to be adsorbed, and the boundary of the BN inner wall along the [010] or [100] direction makes the p2x2 phase more favorable against the 45\textdegree -tilted 2$\surd $3x$\surd $3R45\textdegree phase of O on the Cu(001) surface. [Preview Abstract] |
Friday, March 18, 2016 8:36AM - 8:48AM |
X47.00004: Multiscale Investigations of the Early Stage Oxidation on Cu Surfaces Qing Zhu, Penghao Xiao, Xin Lian, Shen-Che Yang, Grame Henkelman, Wissam Saidi, Judith Yang Previous \textit{in situ} TEM experiments have shown that the oxidation of the three low index Cu surfaces (100), (110) and (111) exhibit different oxide nucleation rates, and the resulting oxides have 3-dimensional (3D) island shapes or 2D rafts under different conditions. In order to better understand these results, we have investigated the early stages of Cu oxidation using a multiscale computational approach that employs density functional theory (DFT), reactive force field (ReaxFF), and kinetic Mote Carlo (KMC). With DFT calculation, we have compared O$_{\mathrm{2}}$ dissociation barriers on Cu (100), (110) and (111) surfaces at high oxygen coverage to evaluate the kinetic barrier of sublayer oxidization. We found that O$_{\mathrm{2}}$ dissociation barriers on Cu(111) surface are all lower than those on (110) and (100) surfaces. This trend agrees with experimental observations that (111) surface is easier to oxidize. These DFT calculated energy barriers are then incorporated into KMC simulations. The large scale ReaxFF molecular dynamics and KMC simulations detail the oxidation dynamics of the different Cu surfaces, and show the formation of various oxide morphologies that are consistent with experimental observations. [Preview Abstract] |
Friday, March 18, 2016 8:48AM - 9:00AM |
X47.00005: Inducing electric polarization in ultrathin insulating layers Jose Martinez-Castro, Marten Piantek, Mats Persson, David Serrate, Cyrus F. Hirjibehedin Studies of ultrathin polar oxide films have attracted the interest of researchers for a long time due to their different properties compared to bulk materials. However they present several challenges such as the difficulty in the stabilization of the polar surfaces and the limited success in tailoring their properties. Moreover, recently developed Van der Waals materials have shown that the stacking of 2D-layers trigger new collective states thanks to the interaction between layers. Similarly, interface phenomena emerge in polar oxides, like induced ferroelectricity. This represents a promising way for the creation of new materials with customized properties that differ from those of the isolated layers. Here we present a new approach for the fabrication and study of atomically thin insulating films. We show that the properties of insulating polar layers of sodium chloride (NaCl) can be engineered when they are placed on top of a charge modulated template of copper nitride (Cu$_2$N). STM studies carried out in ultra-high vacuum and at low temperatures over NaCl/Cu2N/Cu(001) show that we are able to build up and stabilize interfaces of polar surface at the limit of one atomic layer showing new properties not present before at the atomic scale. [Preview Abstract] |
Friday, March 18, 2016 9:00AM - 9:12AM |
X47.00006: Role of oxygen diffusion at Ni/Cr$_2$O$_3$ interface in intergranular oxidation of Ni-Cr alloy Bharat Medasani, Maria Sushko, Daniel Schreiber, Kevin Rosso, Stephen Bruemmer Certain Ni-Cr alloys used in nuclear systems experience intergranular oxidation and stress corrosion cracking when exposed to high-temperature water leading to their degradation and unexpected failure. To develop a mechanistic understanding of grain boundary oxidation processes, we proposed a mesoscale metal alloy oxidation model that combines quantum Density Functional Theory (DFT) with mesoscopic Poisson-Nernst-Planck/classical DFT. This framework encompasses the chemical specificity of elementary diffusion processes and mesoscale reactive dynamics, and allows modeling oxidation processes on experimentally relevant length scales from first principles. As a proof of concept, a preliminary model was previously employed that limited oxygen diffusion pathways to those through the oxide phase and did not allow oxygen diffusion in the alloy or across oxide/alloy interfaces. In this work, we expand the model to include oxygen diffusion pathways along Ni/Cr$_2$O$_3$ interfaces and demonstrate the increasing importance of such pathways for intergranular oxidation of Ni-Cr alloys with high Cr content. [Preview Abstract] |
Friday, March 18, 2016 9:12AM - 9:24AM |
X47.00007: Theoretical Investigation of Supported Utra-Thin Cobalt/Nickel/Iron/Manganese Oxides Michal Bajdich, Max García Melchor, Alexandra Vojvodic In the last decade, a number of experiments have shown that ultra-thin layers of transition metal oxides (TMOs) can be stabilized when interfaced with precious metal supports such as Au(111) and Pt(111) or Ir(100). Moreover, gold supported Co/Ni/Mn-based catalysts have been experimentally proven to exhibit higher oxygen evolution reaction (OER) activities than other metal supported oxide catalysts. However, the synergistic effect of contact with gold support is yet to be fully understood. In this talk, I will report on our recent investigation of thermodynamic stability and and high water reactivity of ultra-thin cobalt oxide nanoislands supported on Au(111). Furthermore, the stability trends, scaling of the metal-support interaction and charge transfer of several Mn/Fe/Co/Ni supported oxides on all FCC(111) metals will be analyzed. The type and role of different edge sites for the OER activity of these nanoislands will be discussed. [Preview Abstract] |
Friday, March 18, 2016 9:24AM - 9:36AM |
X47.00008: A DFT study of metastable \textit{h}-WO$_{3}$ surfaces Yonghyuk Lee, Taehun Lee, Woosun Jang, Aloysius Soon Polycrystalline WO$_{3}$ has gained considerable interest as an efficient oxide material for photoreactions [1], and its surface-dependent catalytic properties have been exploited by shape-control crystal engineering of this oxide for photochemistry reactions e.g. water-splitting. Recently, hexagonal single crystal WO$_{3}$ nanorods with dominant (0001) and (11$\bar{2}$0) facets were synthesized and these nanorods are found to be highly effective photoanode [2] However, the precise local atomic structures and surface orientations of this metastable \textit{h}-WO$_{3}$, which are important for understanding surface-dependent photoreactions, are not well studied. In this work, using first-principles density-functional theory (DFT), we consider the various orientations and terminations of \textit{h}-WO$_{3}$ surfaces and address the predicted nanomorphologies under corresponding experimental conditions based on the DFT-derived Gibbs-Wulff polyhedrons. We provide a microscopic perspective for its potential applications in photoreactions by studying the surface energetics and electronic structure. [1] Y. Ping and G. Galli, \textit{J. Phys. Chem. C} \textbf{118}, 6019 (2014); [2] P. M. Rao \textit{et al.}, \textit{Nano Lett.} \textbf{14}, 1099 (2014) [Preview Abstract] |
Friday, March 18, 2016 9:36AM - 9:48AM |
X47.00009: \textbf{Structural and electrical characterization of NbO}$_{\mathrm{\mathbf{2}}}$\textbf{ vertical devices grown on TiN coated SiO}$_{\mathrm{\mathbf{2}}}$\textbf{/Si substrate} Toyanath Joshi, Pavel Borisov, David Lederman Due to its relatively high MIT temperature (1081 K) and current-controlled negative differential resistance, NbO$_{\mathrm{2}}$ is a robust candidate for memory devices and electrical switching applications. In this work, we present in-depth analysis of NbO$_{\mathrm{2}}$ thin film vertical devices grown on TiN coated SiO$_{\mathrm{2}}$/Si substrates using pulsed laser deposition (PLD). Two of the films grown in 1 mTorr and 10 mTorr O$_{\mathrm{2}}$/Ar (\textasciitilde 7{\%} O$_{\mathrm{2}})$ mixed growth pressures were studied. The formation of NbO$_{\mathrm{2}}$ phase was confirmed by Grazing Incidence X-ray Diffractometry (GIXRD), X-ray Photoelectron Spectroscopy (XPS) and current vs. voltage measurements. A probe station tip (tip size \textasciitilde 2 $\mu $m) or conductive AFM tip was used as a top and TiN bottom layer was used as a bottom contact. Device conductivity showed film thickness and contact size dependence. Current pulse measurements, performed in response to applied triangular voltage pulses, showed a non-linear threshold switching behavior for voltage pulse durations of \textasciitilde 100 ns and above. Self-sustained current oscillations were analyzed in terms of defect density presented in the film. [Preview Abstract] |
Friday, March 18, 2016 9:48AM - 10:00AM |
X47.00010: Multiferroic fluoride BaCoF$_{\mathrm{4}}$ Thin Films Grown Via Molecular Beam Epitaxy Pavel Borisov, Trent Johnson, Camilo García-Castro, Amit KC, Dustin Schrecongost, Cheng Cen, Aldo Romero, David Lederman Multiferroic materials exhibit exciting physics related to the simultaneous presence of multiple long-range orders, in many cases consisting of antiferromagnetic (AF) and ferroelectric (FE) orderings. In order to provide a new, promising route for fluoride-based multiferroic material engineering, we grew multiferroic fluoride BaCoF$_{\mathrm{4}}$ in thin film form on Al$_{\mathrm{2}}$O$_{\mathrm{3}}$ (0001) substrates by molecular beam epitaxy. The films grow with the orthorhombic b-axis out-of-plane and with three in-plane structural twin domains along the polar c-axis directions. The FE ordering in thin films was verified by FE remanent hysteresis loops measurements at T $=$ 14 K and by room temperature piezoresponse force microscopy (PFM). An AF behavior was found below Neel temperature T$_{\mathrm{N}} \quad \sim $ 80 K, which is in agreement with the bulk properties. At lower temperatures two additional magnetic phase transitions at 19 K and 41 K were found. First-principles calculations demonstrated that the growth strain applied to the bulk BaCoF$_{\mathrm{4}}$ indeed favors two canted spin orders, along the b- and a-axes, respectively, in addition to the main AF spin order along the c-axis. [Preview Abstract] |
Friday, March 18, 2016 10:00AM - 10:12AM |
X47.00011: ABSTRACT WITHDRAWN |
Friday, March 18, 2016 10:12AM - 10:24AM |
X47.00012: The effect of Si impurities on the effective work function at TiN/tetragonal-HfO$_{\mathrm{2}}$ interface. Geun-Myeong Kim, Young Jun Oh, Kee Joo Chang The TiN/monoclinc-HfO$_{\mathrm{2}}$ (TiN/m-HfO$_{\mathrm{2}})$ interface structure is widely used in high-k/metal gate stacks of metal-oxide-semiconductor field-effect transistors. As the device size is continuously reduced, high-k dielectric materials are required to reduce the gate leakage current. The tetragonal HfO$_{\mathrm{2}}$ (t-HfO$_{\mathrm{2}})$ is beneficial in high-k/metal gate stacks because its dielectric constant is much higher than that of m-HfO$_{\mathrm{2}}$. It is known that Si doping can reduces the crystalline temperature of t-HfO$_{\mathrm{2}}$. However, there is a lack of studies for the effect of Si impurities at TiN/t-HfO$_{\mathrm{2}}$ interface. Here we perform first-principles density functional calculations to investigate the effect of Si impurities on the work function at TiN/t-HfO$_{\mathrm{2}}$ interface. It is energetically favorable for Si atoms to substitutes for interface N atoms. The change of interface bonds by the Si atoms enhances the effective work function(EWF). On the other hand, when the Si atoms replace the N atoms in bulk region, the EWF is almost unchanged. Our results indicate that the Si impurities incorporated in the interface region mostly affect the work function at TiN/t-HfO$_{\mathrm{2}}$ interface. [Preview Abstract] |
Friday, March 18, 2016 10:24AM - 10:36AM |
X47.00013: Absolute surface energy calculations of Wurtzite (0001)/(000-1): a study of ZnO and GaN Jingzhao Zhang, Yiou Zhang, Kinfai Tse, Bei Deng, Hu Xu, Junyi Zhu The accurate absolute surface energies of (0001)/(000-1) surfaces of wurtzite structures are crucial in determining the thin film growth mode of important energy materials. However, the surface energies still remain to be solved due to the intrinsic difficulty of calculating dangling bond energy of asymmetrically bonded surface atoms. We used a pseudo-hydrogen passivation method to estimate the dangling bond energy and calculate the polar surfaces of ZnO and GaN. The calculations were based on the pseudo chemical potentials obtained from a set of tetrahedral clusters or simple pseudo-molecules, using density functional theory approaches, for both GGA and HSE. And the surface energies of (0001)/(000-1) surfaces of wurtzite ZnO and GaN we obtained showed relatively high self-consistencies. A wedge structure calculation with a new bottom surface passivation scheme of group I and group VII elements was also proposed and performed to show converged absolute surface energy of wurtzite ZnO polar surfaces. [Preview Abstract] |
Friday, March 18, 2016 10:36AM - 10:48AM |
X47.00014: The Effect of Gate -Bias Stress and Light illumination on the performance of ZnO Thin-Film Field Effect Transistors Prakash Gajurel, Michael Aldridge, Yuri Glinka, Pavel Borisov, Kevin Daly, David Lederman We have investigated the stability of ZnO thin film field effect transistors (TFFETs) grown on Si/SiO2 under the application of positive gate bias stress and light illumination at room temperature. A gate voltage applied over a few seconds in ZnO TFFET devices is known to induce a positive shift in the threshold voltage as a consequence of charge trapping at or near the conducting channel / insulator interface. This bias stress remains unchanged even if a negative gate voltage stress is applied. A negative shift of the transfer curve for stressed devices was achieved while exposing the transistor to light in the presence of a small source voltage. The negative shift in threshold voltage depended on the photon energy and exposure time. Our experimental results indicate that the traps responsible for the stress are approximately 2.1 eV below the bottom of the ZnO conduction band with an energy distribution width of 3.40 eV. Stressed devices recovered their original characteristics with the photon energy of UV light (365 nm, 3.6 mW/cm2) at room temperature within 1818s. This approach could be used to reset stressed TFFETs using light sources. [Preview Abstract] |
Friday, March 18, 2016 10:48AM - 11:00AM |
X47.00015: ABSTRACT WITHDRAWN |
Friday, March 18, 2016 11:00AM - 11:12AM |
X47.00016: Sand effects on thermal barrier coatings for gas turbine engines. Michael Walock, Blake Barnett, Anindya Ghoshal, Muthuvel Murugan, Jeffrey Swab, Marc Pepi, David Hopkins, George Gazonas, Kevin Kerner Accumulation and infiltration of molten/ semi-molten sand and subsequent formation of calcia-magnesia-alumina-silicate (CMAS) deposits in gas turbine engines continues to be a significant problem for aviation assets. This complex problem is compounded by the large variations in the composition, size, and topology of natural sands, gas generator turbine temperatures, thermal barrier coating properties, and the incoming particulate's momentum. In order to simplify the materials testing process, significant time and resources have been spent in the development of synthetic sand mixtures. However, there is debate whether these mixtures accurately mimic the damage observed in field-returned engines. With this study, we provide a direct comparison of CMAS deposits from both natural and synthetic sands. Using spray deposition techniques, 7{\%} yttria-stabilized zirconia coatings are deposited onto bond-coated, Ni-superalloy discs. Each sample is coated with a sand slurry, either natural or synthetic, and exposed to a high temperature flame for 1 hour. Test samples are characterized before and after flame exposure. In addition, the test samples will be compared to field-returned equipment. [Preview Abstract] |
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