Bulletin of the American Physical Society
APS March Meeting 2010
Volume 55, Number 2
Monday–Friday, March 15–19, 2010; Portland, Oregon
Session X32: Quantum Size Effects and Interfaces |
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Sponsoring Units: DMP Chair: Wenguang Zhu, University of Tennessee Room: E142 |
Thursday, March 18, 2010 2:30PM - 2:42PM |
X32.00001: Influence of quantum well states on homo-epitaxy of Pb on ultra-thin Pb films Jisun Kim, Alexander Khajetoorians, Chih-Kang Shih Pb on the Si(111) surface is a well-investigated system, partly because of its unique electronic properties. Due to the nearly half-integer matching between the Fermi wavelength and the inter-layer spacing, many properties of Pb films, including thermodynamic stability, exhibit bi-layer quantum oscillations. Here we study epitaxial growth of Pb overlayers on such films. We start with flat-top Pb mesas on Si(111) exhibiting a range of thicknesses including both quantum stable and unstable thicknesses. We then investigate the behavior, nucleation, and growth of 2D Pb nanoislands on such quantum films. Issues of kinetics are investigated by controlling the annealing temperatures. We examine these results in the context of recent investigations of adsorbates on Pb quantum films. [Preview Abstract] |
Thursday, March 18, 2010 2:42PM - 2:54PM |
X32.00002: ABSTRACT WITHDRAWN |
Thursday, March 18, 2010 2:54PM - 3:06PM |
X32.00003: Stability of the wetting layer in the Pb/Si(111)7x7 quantum-size-effect system M.W. Gramlich, S.T. Hayden, P.F. Miceli, C. Kim, M.C. Tringides, E.H. Conrad Studies of Pb/Si(111) have revealed novel physical phenomena, such as Pb nano-islands with quantized height selection (quantum size effects, or QSE) as well as non-classical coarsening behavior with anomalously fast relaxation rates [C.A. Jeffrey et al., PRL \textbf{96,} 106105 (2006)]. It is, therefore, of interest to study the dense Pb wetting layer which exists in between the islands and which apparently enables the fast transport of Pb adatoms required for coarsening. This talk will report recent in situ x-ray scattering studies of the stability and structural transitions of the wetting layer without the presence of the nano-islands. Depending on the temperature, it is observed that the Pb8x8 wetting layer on Si(111)7x7 either becomes more ordered or it decays to the alpha phase. For coverage above a monolayer, large 3D crystallites are found to coexist with the alpha phase. The crystallite diffraction peaks appear in the same reciprocal lattice position as for the previously reported domain-wall satellite peaks, which were not observed in our study. Experiments were performed at the MUCAT beamline at the Advanced Photon Source (supported by US-DOE). Research funding is supported by NSF DMR-0706278. [Preview Abstract] |
Thursday, March 18, 2010 3:06PM - 3:18PM |
X32.00004: Utilizing Electronic Coherence to Probe a Deeply Embedded Quantum Well in Bimetallic Pb/Ag Films on Si(111) Matthew Brinkley, Yang Liu, Nathan Speer, Thomas Miller, Tai-Chang Chiang We report an experiment in which we utilize electronic coherence to probe a deeply embedded thin film as a quantum well. An atomically uniform Ag film prepared on Si(111) was covered by Pb films up to 70 {\AA} thick, and the resulting electronic structure was examined by angle-resolved photoemission spectroscopy. Despite a photoemission escape depth of just a few {\AA}ngstr\"{o}ms and an incommensurate Pb/Ag interface, the data reveal a striking Fabry-P\'{e}rot-like structure characteristic of an Ag etalon buried deeply under the Pb overlayers. Our simulations clearly illustrate the manifest coherence of the electronic structures, permitting the characterization of the embedded Ag quantum well. [Preview Abstract] |
Thursday, March 18, 2010 3:18PM - 3:30PM |
X32.00005: Quantum Size Effects in the Properties of Non-Magnetic Americium-I (0001) Surface: A Hybrid DFT Study Raymond Atta-Fynn, Asok Ray Hybrid density functional theory (HDFT) and a periodic slab model (up to nine layers thick) have been used to study the surface properties and electronic structure of non-magnetic Americium-I (0001) surface. The work function and surface energy of the semi-infinite surface is predicted to 3.45eV and 1.09 J/m$^{2}$ respectively. The surface properties, namely the surface energy, work function, and slab incremental energy exhibit no variations after five layers. A five layer slab is thus predicted to accurately model the adsorbate-induced changes in the surface properties of Americium-I (0001). The electronic structure is in excellent agreement with recent photoemission spectroscopy data. A layer-by-layer examination 5$f$ electron localization using the thickest slab clearly indicates that the 5$f$ electrons are localized on each layer and the nature of the localization is independent of the local geometry indicating that there is no variation in the 5$f$ electron localization at the surface and in the bulk region. [Preview Abstract] |
Thursday, March 18, 2010 3:30PM - 3:42PM |
X32.00006: Atomic and Electronic Structure of SrTiO$_{3}$/GaAs Hetero-Interfaces Qiao Qiao, Weronika Walkosz, Serdar Ogut, Robert Klie Metal-oxide semiconductor interfaces have received much attention in recent years due to their potential applications in metal-oxide-semiconductor field-effect transistors. In this study we examine the atomic and electronic structures of epitaxial SrTiO$_{3}$[100] thin films on GaAs[001] using atomic-resolution Z-contrast imaging and electron energy loss spectroscopy in combination with first principles calculations to develop a fundamental understanding of the interfacial structure-property relationships. We will demonstrate that it is energetically favorable for SrO layer of SrTiO$_{3}$ to be in direct contact with the terminating As layer of GaAs, as opposed to the TiO$_{2}$ layer. However, the model with the simplest 1x1 surface unit cell is not in agreement with the photoemission data.$^{2}$ First principles total energy calculations will be used to consider how different GaAs(001) surface reconstructions are modified in the presence of SrTiO$_{3}$ to find the low-energy semiconducting interface. $^{2}$Y. Liang et al., \textit{Appl. Phys. Lett.,} \textbf{86} (8), 082905 (2005). [Preview Abstract] |
Thursday, March 18, 2010 3:42PM - 3:54PM |
X32.00007: Intermixtures at LaAlO$_3$/SrTiO$_3$ interfaces Hanghui Chen, Alexie Kolpak, Sohrab Ismail-Beigi The intriguing transport properties observed at the LaAlO$_3$/SrTiO$_3$ $n$-type interface have precipitated numerous studies in the past few years. However, it remains uncertain whether the interface obtained experimentally is atomically sharp, and if not, what role the disorder plays in the unique behavior of this system. We use first principles density functional theory to find the energetics of cation intermixing, specifically La-Sr and Al-Ti, at the LaAlO$_3$/SrTiO$_3$ $n$-type interface. We find that an ideal interface with no intermixing is not thermodynamically stable. Ti-Al intermixing reduces the total energy while Sr-La intermixing increases the total energy. We explain the energetics and this asymmetry in terms of a simple electrostatic model, which is able to accurately describe the DFT results. We also discuss how intermixing affects the polar field in the LaAlO$_3$, the ``polar catastrophe'', and the critical thickness of LaAlO$_3$ needed to induce a metal-insulator transition. [Preview Abstract] |
Thursday, March 18, 2010 3:54PM - 4:06PM |
X32.00008: A generalized diffraction approach to predict the coupling of interface structure and function with lattice displacements F. J. Walker, J. W. Reiner, A. M. Kolpak, Y. Segal, D. Kumah, Z. Zhang, S. Ismail-Beigi, C. H. Ahn Abrupt changes in symmetry, bonding and structure strongly affect functional behavior at oxide-semiconductor interfaces. In this talk, we consider specific examples of crystalline BaO and SrTiO$_{3}$ deposited on Si. Charge transfer at the interface between the oxide and semiconductor is found using synchrotron x-ray diffraction. This charge movement couples to optical modes in the BaO and SrTiO$_{3}$ layers. This effect extends deep into the films because the polarization of the optical mode is not effectively screened in the insulating BaO and SrTiO$_{3}$ layers. Differences in atomic displacements between the BaO and SrTiO$_{3}$ thin films result in a polarization that alternates sign in the case of BaO, as opposed to a uniform polarization in SrTiO$_{3}$. These differences impact on the electronic properties of the system, including how the semiconductor and oxide bands align. [Preview Abstract] |
Thursday, March 18, 2010 4:06PM - 4:18PM |
X32.00009: The effect of epitaxial strain on interfaces between polar perovskites and SrTiO$_{3}$ Mark C. Monti, Carl J. Stolle, John T. Markert We have embarked on a systematic study of novel charge states at oxide interfaces. We have performed pulsed laser deposition (PLD) growth of epitaxial oxide thin films on single crystal oxide substrates. We plan to study the effects of epitaxial strain, of the termination layer, and of the metal oxide layers. We have successfully created TiO$_{2}$ terminated SrTiO$_{3}$ (STO) substrates and have grown epitaxial thin films of LaAlO$_{3}$ (LAO) on STO using a KrF pulsed excimer laser. Current work emphasizes the importance of understanding the effect of epitaxial strain on the metallic interface. We will study the effect of both lateral compression and stretching of the charged layered films on STO. We have made polycrystalline targets of LaGaO$_{3}$ and LaAlO$_{3}$ to study the effect of successively larger tensions as well as LaTiO$_{3}$ and KNbO$_{3}$ to examine the effect of compression on the films. Studying the effect of epitaxial strain and of the valence of the alternating layers of these films grown on STO will lead to a deeper understanding of charge doping and conduction at oxide interfaces. This work was supported by: Texas Advanced Research Program 003658-0126, The Robert A. Welch Foundation F-1191, and the National Science Foundation DMR-0605828. [Preview Abstract] |
Thursday, March 18, 2010 4:18PM - 4:30PM |
X32.00010: Direct Imaging and First Principles Studies of Si$_{3}$N$_{4}$/SiO$_{2}$ Interface Weronika Walkosz, Robert Klie, Serdar Ogut, Bilijana Mikijelj, Stephen Pennycook, Juan C. Idrobo It is well known that the composition of the integranular films (IGFs) in sintered polycrystalline silicon nitride (Si$_{3}$N$_{4})$ ceramics controls many of their physical and mechanical properties. A considerable effort has been made to characterize these films on the atomic scale using both experimental and theoretical methods. In this talk, we present results from a combined atomic-resolution Z-contrast and annular bright field imaging, electron energy-loss spectroscopy, as well as ab initio studies of the interface between $\beta $-Si$_{3}$N$_{4 }$(10-10) and SiO$_{2}$ intergranular film. Our results show that O replaces N at the interface between the two materials in agreement with our theoretical calculations and that N is present in the SiO$_{2}$ IGF. Moreover, they indicate the presence of atomic columns completing Si$_{3}$N$_{4}$ open rings, which have not been observed experimentally at the recently imaged Si$_{3}$N$_{4}$/rare-earth oxides interfaces, but have been predicted theoretically on bare Si$_{3}$N$_{4}$ surfaces. The structural and electronic variations at the Si$_{3}$N$_{4}$/SiO$_{2}$ interface will be discussed in detail, focusing in particular on bonding characteristics. [Preview Abstract] |
Thursday, March 18, 2010 4:30PM - 4:42PM |
X32.00011: Fisrt-principles calculations on metal-induced gap states at metal-semiconductor interfaces Y. Gohda, S. Tsuneyuki Metal-induced gap states (MIGS) are responsible for Fermi-level pinning for narrow-gap semiconductors such as Si and GaAs. First-principles calculations have demonstrated that MIGS are related to the tails of metal states penetrating into the semiconductor corresponding to Bloch states with wave vectors having an imaginary part. Thus, their existence is a consequence of intrinsic properties of the bulk semiconductor. In contrast, a removal of FLP has been reported experimentally at atomically controlled Al-Si(100) interfaces, suggesting that MIGS play a less dominant role in determining the interface properties. This inconsistency between experimental results and the accepted view of MIGS calls for a detailed theoretical investigation. Here, we report our recent progresses on MIGS at a few metal-semiconductor interfaces investigated by means of first-principles calculations. [Preview Abstract] |
Thursday, March 18, 2010 4:42PM - 4:54PM |
X32.00012: Modification of interfacial electronic structure as a function of organic overlayer stereochemistry N.M. Santagata, K.M. Andrews, A. Calzolari, M. Buongiorno Nardelli, T. Pearl Hydrogen bonding interactions involving molecular chiral centers control the organizational structure at the tartaric acid/Ag(111) interface. Specifically, for enantiopure tartaric acid films, a single molecule basis adsorbs with the molecular axis orientated parallel to the surface plane. Conversely, a paired basis unit composed of opposite enantiomers adsorbs with the combined molecular axis perpendicular to the surface plane for racemic tartaric acid films. Here, the unique electronic structure of each interface will be discussed. Scanning tunneling spectroscopy results give quantitative shifts of the Ag(111) Shockley-type surface state (67 meV below the Fermi level) of 881 meV and 55 meV for enantiopure and racemic films, respectively. Density functional theory computations have also been carried out in support of the experimental results. The respective energetic shifts will be discussed in terms of a modified surface potential, film polarizability, and work function changes. [Preview Abstract] |
Thursday, March 18, 2010 4:54PM - 5:06PM |
X32.00013: Cohesive Strength of Metal/Ceramic Interfaces Fe/$\it M$[C,N] and the Role of Misfit Dislocations and Interface Roughness Oleg Y. Kontsevoi, A.J. Freeman, G.B. Olson The dispersed inclusions of carbides and nitrides were shown to provide superior fracture toughness and increased strength for steels. To obtain a fundamental understanding of their effect, we investigated the theoretical cohesive strength of Fe/$\it M$[C,N] ($\it M$=Ti,V,Nb,Mo) interfaces using the first principles calculations with the full-potential linearized augmented plane wave (FLAPW) method. For the coherent $\langle 100 \rangle$\{001\}Fe//$\langle 110 \rangle$\{001\}$\it M$C interfaces, the highest theoretical strength of 3.78 J/m$^2$ was obtained for the Fe/VC interface, and it decreases from V-Ti-Nb-Mo. The strength of nitride interfaces is 10-15\% lower than for carbides. Within the Peierls-Nabarro model with {\it ab initio} generalized stacking fault energetics, we estimated that the misfit dislocations which may form due to lattice misfit decrease the interface strength by 20\%. The strength of $\langle 110 \rangle$\{110\}Fe//$\langle 010 \rangle$\{100\}TiC semicoherent interfaces that contain primary misfit dislocations was found to be 30\% lower. The effect of interface steps and roughness on strength is also investigated. We further identify the origins of strong interfacial bonding based on electronic structure and charge densities analyzes. [Preview Abstract] |
Thursday, March 18, 2010 5:06PM - 5:18PM |
X32.00014: Role of Pt nanoparticles in photocatalytic activities of CdS surfaces: Atomic and Electronic structure of Pt/CdS interface Sefa Dag, Lin-Wang Wang CdS photocatalyst is one of the important material for solar cell applications and it has a profound interest in academical and industrial area. By using first-principles Density Functional theory method, we analyzed the physical characteristics behind the the outstanding catalytic activity of the Pt/CdS heterostructures. We found strong bonding interaction in the interface region between Pt and CdS and this interaction promotes new surface states occur in the band gap. Our calculations also reveal that interface dipole at the Pt/CdS interface was created by this bonding interaction and differ with respect to number of Pt layers. [Preview Abstract] |
Thursday, March 18, 2010 5:18PM - 5:30PM |
X32.00015: Structural and Electronic Properties of Interface between Metal and HAT-CN Molecule Young-Kyun Kwon, Ji Hoon Kim, Yongsup Park Using {\em ab initio} density functional theory, we study the equilibrium structures and electronic properties of interface between metal surface and organic molecule, called 1,4,5,8,9,11- hexaazatriphenylene-hexacarbonitrile (HAT-CN), which is known for a strong electron acceptor and used as an efficient hole injection layer for organic light emitting diodes. We consider Ca (111) and Cu (111) surfaces, on which HAT-CN is placed. The geometrical and electronic properties of the interfaces are significantly different when HAT-CN is on one metal surface than when it is on the other. We find that the nitrogen atoms at the edge of HAT-CN have stronger interaction on metal surfaces than other atoms in the molecule. Due to the resultant charge transfer between the HAT-CN molecule and the metal surface, the surface dipole is formed at the interface. We discuss the dependence of the work function on the surface dipole formation and compare with the experimental results. [Preview Abstract] |
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