Bulletin of the American Physical Society
APS March Meeting 2018
Volume 63, Number 1
Monday–Friday, March 5–9, 2018; Los Angeles, California
Session S17: Oxide Surfaces, Films, Defects, and Chemistry |
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Sponsoring Units: DCMP Chair: Kristen Burson, Hamilton College Room: LACC 306A |
Thursday, March 8, 2018 11:15AM - 11:27AM |
S17.00001: Solid State Chemistry with DFT+U Meta Functionals Johannes Voss Supplementing density functional Hamiltonians with Hubbard-U on-site interaction terms is the computationally most efficient approach to remedy self-interaction errors present in standard density functional theory (DFT) calculations of the electronic structure of e.g. transitional metal oxides. Reactants and products typically require different values of the interaction strength U, rendering total energy differences meaningless. While an empirical correction scheme has been developed for bulk transition metal oxide sites [Jain et al., Phys. Rev. B 84, 045115 (2011)] based on experimental reference enthalpies, the presence of a surface or bulk defects requiring adjusted values of U again limits the predictive power of DFT+U for reaction energies. |
Thursday, March 8, 2018 11:27AM - 11:39AM |
S17.00002: Volatile 2D electron gas in ultra thin BaTiO3 films Peter Lutz, Simon Moser, Vedran Jovic, Young Jun Chang, Roland Koch, Søren Ulstrup, Ji Seop Oh, Luca Moreschini, Sara Fatale, Marco Grioni, Chris Jozwiak, Aaron Bostwick, Eli Rotenberg, Hendrik Bentmann, Friedrich Reinert We investigate the metallic surface state in ultra-thin films of c-axis oriented BaTiO3 by angle resolved photoemission spectroscopy (ARPES). We find Fermi surface contours derived from the Ti 3d t2g conduction band, with broad spectral features due to enhanced quasi-particle scattering. Oxygen vacancies created in the X-ray beam spot allow for reversible in situ doping control. Up to carrier densities as high as 2 × 1021 cm−3, they retain a charge state of 2+, preventing the formation of localized defect states and charge carrier trapping. Sub-surface oxygen vacancy migration at T > 285 K quenches the surface state. Our findings challenge the implementation of non-volatile room temperature applications based on non-stoichiometric BaTiO3. |
Thursday, March 8, 2018 11:39AM - 11:51AM |
S17.00003: High-resolution X-ray imaging of conductive filaments in VO2 nano-gaps Anatoly Shabalin, Javier Del Valle Granda, Martin Holt, Ivan Schuller, Oleg Shpyrko Electrically driven metal-insulator transition in vanadium dioxide (VO2) is of interest in novel memory devices, neural computation, and high-speed electronics. Resistive switching behavior of such devices is driven by the formation of Joule heating conductive nano-filaments between the contacts in the switched state and highly depends on the confinement and morphology of the nano-gap. We applied X-ray nano-diffraction to image the structure of 800x300 nm gap made of 100 nm VO2 film in operando conditions. High-resolution images of the several identical gaps in the switched state show tiny filaments of the metallic phase connecting the corners of the terminals. Interestingly, these filaments always appear at the same location which evidences a direct connection with the film morphology. |
Thursday, March 8, 2018 11:51AM - 12:03PM |
S17.00004: Large polaron evolution with temperature and carrier density in anatase TiO2 Bixing Yan, Dongyang Wan, Xiao Chi, Changjian Li, Mallikarjuna Motapothula, Sonu Hooda, Ping Yang, Zhen Huang, Shengwei Zeng, Akash Gadekar, Stephen Pennycook, Andrivo Rusydi, Ariando Ariando, Jens Martin, Thirumalai Venkatesan Electronic and magneto transport properties of reduced anatase TiO2 epitaxial thin films are analyzed as a function of temperature and carrier concentration. Semiconductor and metallic behaviors are observed at low and high temperature respectively, in agreement with previous studies. Unexpectedly, the mobility was found to increase with increasing carrier concentration, which rarely happens in a metallic system. Through our analysis, the increased mobility was attributed to a screening effect on the e-ph coupling. We also found that the magnetoresistance (MR) could be decomposed in to a linear and a quadratic component, separately characterizing the transport and trap behavior of carriers. Since the different polaron states have varying dependence on the temperature and carrier concentration, a phase diagram could be depicted by the analysis of the electronic and magneto transport data. |
Thursday, March 8, 2018 12:03PM - 12:15PM |
S17.00005: Interface physics of TiO2 anatase and rutile growth on c-cut sapphire substrates using PLD technique Alexandra Gordienko, Anthony Kaye Titanium dioxide (hereafter, titania or TiO2) is a well-studied material, but despite of its 100 year history we still don't know everything about this material. For the last few decades it used to be thought that only one of tetragonal phases of titania - rutile - can be grown on c-cut sapphire substrates. As a part of this study we grew both anatase and rutile titania on c-cut sapphire by manipulating pulsed laser deposition (PLD) growth parameters, such as substrate temperature, background gas pressure, laser pulse repetition rate and laser pulse energy. All films were analyzed using X-ray diffraction technique. The resulting films show pure anatase and pure rutile structures. |
Thursday, March 8, 2018 12:15PM - 12:27PM |
S17.00006: The Observation of Bipolarons on the Anatase TiO2(001) surface Xuefeng Cui, Xiaochuan Ma, Shihui Dong, Bing Wang The polaron is a fermionic quasi-particle formed by a conduction electron repelling the negative ions and attracting the positive ions in an ionic crystal or a polar semiconductor. Among the polarizable examples of metal oxides, anatase TiO2 and SrTiO3 have been mostly studied. Here we epitaxially grow anatase TiO2(001) thin films on the SrTiO3(001) substrate with pulsed laser deposition method. The anatase TiO2(001) surface terminal is demonstrated to be the (1×4) reconstruction by scanning tunneling microscopy and LEED pattern. Subsequent in-situ angle-resolved photoemission measurements demonstrate that the two-dimensional electron gas, polarons and bipolarons can be available on this surface at different charge carrier densities. The strong electron-phonon coupling is found on this surface. Our findings reveal the 2D nature of electron gas and polarons on this surface and provide the essential role of the phonon-related bipolarons in understanding the electronic structure of anatase TiO2(001) surface. |
Thursday, March 8, 2018 12:27PM - 12:39PM |
S17.00007: Ab Initio Thermodynamics Of Surface Oxide Structures Under Controlled Growth Conditions Taehun Lee, Yonghyuk Lee, Simone Piccinin, Aloysius Soon Having a robust and predictive ab initio thermodynamic model to examine and describe the interplay of the oxygen gas and evaporated metal atoms on another metal substrate may prove to be very helpful in understanding the surface phase diagrams of these oxygen/metal systems. In this work, we examine the O/Cu/Au(111) system and provide a refined atomistic thermodynamic model which takes different definitions of the chemical potential of the less abundant metal, Cu into account. We argue that the latter highly depends on the various surface structures (overlayers and alloys) that forms on the metal substrate under growth conditions. We demonstrate that our improved thermodynamic model rationalizes new experimentally observed oxide structures and may pave a systematic way to predict new surface structures of reduced stoichiometries, which would otherwise be missed by the common practice of taking only the bulk limits. |
Thursday, March 8, 2018 12:39PM - 12:51PM |
S17.00008: Systematic characterization of metal-supported ultrathin copper oxide layers from first-principles calculations Taehun Lee, Yonghyuk Lee, Simone Piccinin, Aloysius Soon The nanocatalyst system composed of thin copper oxide layers on different metal supports (e.g. gold) is gaining a lot of attention due to its exceptional performance in photochemical and oxidation reactions. The atomic configuration of the metal support is known to assist in the tuning of the overall electronic structure of the nanocatalyst system which, in turn, determines its catalytic properties. However, before turning our attention to the explicit physical structure of these metallic supports, we find that a systematic and thorough characterization of all supported Cu surface oxides is currently lacking and far from complete. Here, we survey and study the various possible ultrathin oxide structures of Cu using first-principles density-functional theory (DFT) calculations. Namely, we report the DFT-derived surface core-level shifts (CLS) and simulated scanning tunneling microscopy (STM) images (where the orbitals of the STM tip are explicitly considered) for O/Cu(111), ``29'', ``44'', ``8'', and other defect structures, and provide an atomistic picture to explain and interpret the structural ambiguities in recent CLS and STM experiments. |
Thursday, March 8, 2018 12:51PM - 1:03PM |
S17.00009: Synthesis and electronic properties of Ruddlesden-Popper strontium iridate epitaxial thin films stabilized by kinetic control Xiaoran Liu, Yanwei Cao, Banabir Pal, Mikhail Kareev, Srimanta Middey, Yongseong Choi, Padraic Shafer, Daniel Haskel, Elke Arenholz, Jak Chakhalian We report on the selective fabrication of high-quality Sr2IrO4 and SrIrO3 epitaxial thin films from a single Sr2IrO4 target by pulsed laser deposition. We discover that within a relatively narrow range of substrate temperature, the oxygen partial pressure plays a critical role in the cation stoichiometric ratio of the films and triggers the stablization of different Ruddlesden-Popper (RP) phases. Resonnant X-ray abosprtion spectra taken at Ir L-edge and O K-edge demonstrate the presence of strong spin-orbit coupling, and reveal the electronic and orbital structures of both compounds. These results offer a new approach to the synthesis of ultra-thin films of RP series of iridates and can be extended to other compelx oxides with layered structure. |
Thursday, March 8, 2018 1:03PM - 1:15PM |
S17.00010: Exploring Interfacial Reconstructions by adapting the Minima Hopping Method Shane Patel, Maximilian Amsler, Christopher Wolverton While advances have been made in crystal structure prediction, there have been limited attempts at predicting interfacial structure. The Minima Hopping Method (MHM) is a structure prediction method that uses short molecular dynamics escape trials to explore a potential energy landscape, and a fingerprinting approach to avoid previously visited minima. The MHM has been successfully employed in studying molecular clusters, bulk solids, and surfaces. In this work, we extend the MHM to search for low-energy structures at solid-solid interfaces. We use as a test system non-stoichiometric grain boundaries in strontium titanate (SrTiO3), which have been studied twice previously in efforts to predict interfacial structure. For each non-stoichiometric grain boundary considered, our MHM approach is able to predict lower energy structures than previously reported. Our method is simple to generalize to other interfacial systems and may be used with empirical potentials as well as density functional theory to explore feature-rich interfacial potential energy landscapes. |
Thursday, March 8, 2018 1:15PM - 1:27PM |
S17.00011: Defect Chemistry and Size Effects in SrRuO3 Chiara Gattinoni, Nicola Spaldin The metal-insulator transition in strontium ruthenate (SrRuO3) is an unsolved issue of great relevance. |
Thursday, March 8, 2018 1:27PM - 1:39PM |
S17.00012: Oxygen Diode Formed in Nickelate Heterostructures by Chemical Potential Mismatch Er-Jia Guo, Yaohua Liu, Changhee Sohn, Ryan Desautels, Andreas Herklotz, Zhaoliang Liao, John Nichols, John Freeland, Michael Fitzsimmons, Ho Nyung Lee Deliberate control of oxygen vacancy formation and migration in perovskite oxide thin films is important for developing novel iontronics devices. In this talk, we will discuss our observation that the concentration of oxygen vacancies formed in LaNiO3 (LNO) during pulsed laser deposition is strongly affected by the chemical potential mismatch between the LNO film and its proximal layers. Increasing the oxygen vacancy concentration in LNO drives the metal-insulator transition and thereby significantly modifies the degree of orbital polarization. Changes in the nickel oxidization state and carrier concentration of the films have been confirmed by soft x-ray absorption spectroscopy and optical spectroscopy. Our discovery on the unidirectional-control the oxygen flow across the heterointerface, e.g. a so-called “oxygen diode”, owing to the chemical potential mismatch at interfaces in oxide heterostructures provides a new avenue to tune the physical and electrochemical properties of complex oxides useful for developing advanced information and energy materials and devices. |
Thursday, March 8, 2018 1:39PM - 1:51PM |
S17.00013: Interfacial mechanical coupling at the interface of Pb0.2Zr0.8TiO3/LaNiO3/SrTiO3 heterostructures Claudia Lau, Cristina Visani, Stephen Albright, Zhan Zhang, Ankit Disa, Divine Kumah, Charles Ahn, Fred Walker The coupling of ferroelectric polarization to the conducting oxide LaNiO3 results in large changes of the conductivity at an epitaxial ferroelectric-LaNiO3 interface. Systematic measurements of the conductivity with varying LaNiO3 conducting channel thickness show that the effect is confined to within a few atomic layers of the interface. We correlate changes in conductivity with structural changes in the ferroelectric and conducting oxide as the ferroelectric polarization is switched through in operando synchrotron diffraction measurements. We observe changes in the tetragonality and displacement of the transition metal cations and oxygen anions in both Pb0.2Zr0.8TiO3 and LaNiO3. We also characterize rotations of oxygen octahedra in LaNiO3 via measurement of half order Bragg peaks. These rotations exhibit a hysteresis loop as the applied voltage is swept through the coercive field of the ferroelectric and are correlated with the conductivity of the channel, with larger rotations leading to smaller conductivity. The observed changes in conductivity are larger than that expected from band theory alone, implying a large role of electronic correlations. By using the ferroelectric to effect changes in the structure, we explore the correlated transport behavior of LaNiO3 in new ways. |
Thursday, March 8, 2018 1:51PM - 2:03PM |
S17.00014: Internal strain distribution of PbZr0.2Ti0.8O3/ZnO heterostructure Ruohanyang Leng, Juan Wang, Pavel Salev, Alexei Grigoriev The piezoelectric response of epitaxial ZnO films and PbZr0.2Ti0.8O3/ZnO heterostructure were studied by time-resolved x-ray diffraction (TRXRD). Epitaxial thin film samples were prepared by RF magnetron sputtering. ZnO was grown epitaxially on n-GaN/ Al2O3 (0001)-oriented substrates, and PbZr0.2Ti0.8O3 thin film was deposited on top of ZnO. TRXRD revealed piezoelectric response in ZnO and PZT layers under an applied voltage. Piezoelectric properties were quantitatively analyzed by fitting the Laue oscillations of ZnO and PZT Bragg peaks. A non-uniform strain distribution model had to be used in order to explain experimental results. The results of this study improve the understanding of internal polarization dynamics and polarization coupling in semiconductor/ferroelectric heterostructures, and they provide new opportunities for controlling the electronic properties of semiconductors by switchable electric polarization of ferroelectric materials. |
Thursday, March 8, 2018 2:03PM - 2:15PM |
S17.00015: Band offsets at the interfaces in the PbZr0.2Ti0.8O3/ZnO/GaN ferroelectric-semiconductor heterostructure studied by X-ray photoelectron spectroscopy Juan Wang, Xiaxia Liao, Roberto Félix, Thomas Kunze, Regan G. Wilks, Marcus Bär, Alexei Grigoriev Knowledge of the band offsets at the interfaces in the PbZr0.2Ti0.8O3/ZnO/GaN heterostructure are necessary for the insight-driven design and development of corresponding ferroelectric-semiconductor based electronic devices. We have directly measured the valence band offset (VBO) at the ZnO/GaN and PbZr0.2Ti0.8O3/ZnO heterointerfaces using X-ray photoelectron spectroscopy. For this, ZnO and PbZr0.2Ti0.8O3 (PZT) films of different thicknesses were deposited on GaN/(0001) Al2O3 and ZnO/GaN/(0001) Al2O3, respectively, by RF sputtering to be able to monitor core level positions upon interface formation in order to access changes in band bending at the interface. The derived interface-induced band bending and the positions of the valence band maxima (VBM) were used to derive the VBO. The gained information provides detailed insights into the underlying mechanisms explaining the electrical properties of ferroelectric-semiconductor heterostructures opening a knowledge-based route to explore new approaches to engineer functional nanomaterials. |
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