Bulletin of the American Physical Society
APS March Meeting 2015
Volume 60, Number 1
Monday–Friday, March 2–6, 2015; San Antonio, Texas
Session D8: Complex Films & Surfaces: Oxides, Alloys, and Semiconductors |
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Sponsoring Units: DCMP Room: 006C |
Monday, March 2, 2015 2:30PM - 2:42PM |
D8.00001: Investigation of Thin Layered Cobalt Oxide Nano-Islands on Gold Michal Bajdich, Alex S. Walton, Jakob Fester, Mohammad A. Arman, Jacek Osiecki, Jan Knudsen, Aleksandra Vojvodic, Jeppe V. Lauritsen Layered cobalt oxides have been shown to be highly active catalysts for the oxygen evolution reaction (OER), but the synergistic effect of contact with gold is yet to be fully understood. The synthesis of three distinct types of thin-layered cobalt oxide nano-islands supported on a single crystal gold (111) substrate is confirmed by combination of STM and XAS methods. In this work, we present DFT+U theoretical investigation of above nano-islands using several previously known structural models. Our calculations confirm stability of two low-oxygen pressure phases: (a) rock-salt Co-O bilayer and (b) wurtzite Co-O quadlayer and single high-oxygen pressure phase: (c) O-Co-O trilayer. The optimized geometries agree with STM structures and calculated oxidation states confirm the conversion from Co2+ to Co3+ found experimentally in XAS. The O-Co-O trilayer islands have the structure of a single layer of CoOOH proposed to be the true active phase for OER catalyst. For that reason, the effect of water on the Pourbaix stabilities of basal planes and edge sites is fully investigated. Lastly, we also present the corresponding OER theoretical overpotentials. [Preview Abstract] |
Monday, March 2, 2015 2:42PM - 2:54PM |
D8.00002: Effects of hydroxylated $\gamma $-Al$_{2}$O$_{3}$ support and H adsorbate on the Geometry and Electronic Structure of Pt Nanoparticles Ghazal Shafai, Sampyo Hong, Talat S. Rahman We have studied the effects of hydroxylated $\gamma $-Al$_{2}$O$_{3}$(110) support and H adsorbate on the geometry and electronic structures of Pt$_{x}$ (n$=$22,44) nanoparticles (NP) using DFT. We find that Pt$_{22}$ interacts more strongly with a less hydrated support, while Pt$_{44}$ more with a hydroxylated one. We also find a structural transition of the Pt$_{22}$ (and not Pt$_{44})$ from a biplanar to a 3D-like shape as a function of hydroxilation. H induces a much larger shift in the unoccupied d-band center than does the support. Also, these shifts are well correlated with metal-support interaction. The increased hydroxylation on $\gamma $-Al$_{2}$O$_{3}$(110) causes weaker metal-support interaction. As a result, the d-band width of a Pt NP decreases causing the center of the unoccupied d band to shift to lower energy (red shift). In the light of these results, we will discuss the features of XANES spectra obtained for $\gamma $-Al$_{2}$O$_{3}$(110) supported Pt nanoparticles [1]. \\[4pt] [1] Behafarid et al, Phys. Chem. Chem. Phys. 14, 11766 (2012). [Preview Abstract] |
Monday, March 2, 2015 2:54PM - 3:06PM |
D8.00003: A new model for $\sqrt {31} \times \sqrt {31} R9^{\circ}$ reconstruction of $\alpha $-Al2O3 (001) Hawoong Hong, Aaron Gray, Tai-C. Chiang Research in oxides surface suffers reproducibility problems on its structure. However, $\sqrt {31} \times \sqrt {31} R9^{\circ}$ reconstructed $\alpha $-Al2O3 (001) surface has been generated repeatedly by many different groups. This sapphire surface structure was known to be quite inert even to air exposure. The detailed structures have been studies with LEED, x-ray diffraction and AFM. The recent experimental studies conclude that the few topmost layers are composed of aluminum atoms and have metallic properties [1, 2]. Then a question arises why this surface seems to be inert to air exposure. Metallic aluminum is prone to oxidation according to theoretical investigation. Here we propose a new model for the $\sqrt {31} \times \sqrt {31} R9^{\circ}$ reconstruction involving oxidized top layers. For the direct structural information in the surface-normal direction, specular and non-specular crystal truncation rods were measured with x-ray diffraction. Only successful, if not perfect, fitting occurs with a dense top layer, which is reminiscent of oxidized aluminum (111) surface [3]. \\[4pt] [1] G. Renaud, B. Villette, I. Vilfan, and A. Bourret, Phys. Rev. Lett. \textbf{73}, 1825 (1994).\\[0pt] [2] J. V. Lauritsen, M. C. R. Jensen, K. Venkataramani, B. Hinnemann, S. Helveg, B. S. Clausen, and F. Besenbacher, Phys. Rev. Lett \textbf{103}, 076103 (2009).\\[0pt] [3] A. Kiejna, and B. I. Lundqvist, Phys. Rev. B63, 085405 (2001). [Preview Abstract] |
Monday, March 2, 2015 3:06PM - 3:18PM |
D8.00004: Preparation of atomically flat TiO$_{2}$(001) surfaces Yang Wang, Hanno H. Weitering, Paul C. Snijders Transition metal oxides with the rutile structure (MO$_{2}$, M$=$ e.g. Ti, V, or Nb) have highly directional partially occupied t$_{\mathrm{2g}}$ orbitals. Some of these orbitals form quasi-1D electronic bands along the rutile c-axis, and Peierls-like ordering phenomena have been observed in VO$_{2}$ and NbO$_{2}$. Tailoring the electronic properties of these materials \textit{via} quantum confinement requires epitaxial growth on suitable substrates such as low index TiO$_{2}$ surfaces. Because of the high surface energy of rutile TiO$_{2}$(001), the standard approach of sputtering and annealing usually introduces faceting. Here we demonstrate a facile method to create atomically flat, non-faceted TiO$_{2}$(001) surfaces. Using scanning tunneling microscopy we observe terraces with a width of approximately 150 nm. Step heights of approximately 0.3 nm are observed, consistent with the $c$ lattice parameter of rutile TiO$_{2}$. Low energy electron diffraction patterns reveal sharp diffraction spots with an in-plane lattice constant of 0.358 nm which is consistent with a (1x1) ordering of the (001) plane. These TiO$_{2}$(001) single crystal surfaces can serve as an ideal substrate for further growth of rutile heterostructures. [Preview Abstract] |
Monday, March 2, 2015 3:18PM - 3:30PM |
D8.00005: Physical Character and Morphology of Platinum Nanocrystals on Strontium Titanate Joshua Gild, Michael Pierce, Vladimir Komanicky, Andi Barbour, Hoydoo You The physical characteristics of platinum nanocrystals on single crystal strontium titanate, $SrTiO_{3}$, can effect the chemical properties of this important model catalyst. The morphology, epitaxy, distribution, and size of the Pt nano-crystals can all be controlled through different growth and processing mechanisms. Nanometer scale platinum thin films are deposited on strontium titanate at ambient temperatures then annealed at range of temperatures and in various oxidizing environments. The process of how these conditions influence the formation of uniformly epitaxial platinum crystals on the sample surface has been investigated using basic materials characterization techniques. Single crystal x-ray diffraction is the primary tool for these experiments, coupled with atomic force microscopy for morphology and x-ray and electron spectroscopy to determine chemical bonding between the particles and gases introduced into the system. These substrate supported nanoparticle samples will then be utilized in experiments to test their catalytic activity compared to an amorphous platinum film. [Preview Abstract] |
Monday, March 2, 2015 3:30PM - 3:42PM |
D8.00006: Imaging of Pt Nanocrystals on SrTiO$_3$ Substrate: Coherent X-ray Diffraction and Scanning Microscopy Studies Travis Douglas, Joshua Gild, Michael Pierce, Vladimir Komanicky, Andi Barbour, Hoydoo You I\textunderscore maging of nano-scale structures, particularly those in real-world environments, presents a significant challenge. X-ray Coherent Diffractive Imaging (CDI) provides one avenue of accessing the structural information of a nano-scaled sample in a harsh environment.~ However, while this problem has been solved for Au and Pb nano-crystals in clean, vacuum environments, much work remains before it can be rapidly employed in other systems. Our efforts center on determining the real-space structure of Pt nanocrystals grown on SrTiO3 substrates using a combination of CDI and atomic force scanning microscopy (AFM). X-ray speckle patterns have been obtained using coherent diffraction of these crystals which can be transformed back to real space coordinates to calculate the crystal structure using CDI algorithms. Microscopy provides complementary information allowing us to simulate the speckled diffraction patterns from real-space images of the actual particles. This dual approach of using both real and reciprocal space information to solve the structures should lead to a practical set of algorithms and procedures whereupon the samples can be imaged quickly in the environments and conditions of interest. [Preview Abstract] |
Monday, March 2, 2015 3:42PM - 3:54PM |
D8.00007: Probing the role of Ga in amorphous conducting oxides through local structure studies Stephanie Moffitt, Qimin Zhu, Qing Ma, Donald Buchholz, Robert Chang, Thomas Mason, Tobin Marks, Michael Bedzyk The study of amorphous (a-) conducting oxides is an emerging field. The lack of grain boundaries, smooth surfaces, and low temperature deposition position these materials as ideal candidates for large area applications and flexible electronics. Most impressively, these materials maintain high electron mobility in the amorphous state. These benefits have led the recent commercialization of a-IGZO (Ga and Zn doped indium oxide) as a replacement for a-Si as the channel layer of thin film transistors in display technology. Despite this success, fundamental understanding of structure-property relationships is still lacking and must be improved to guide further development of amorphous conducting oxides. X-ray absorption spectroscopy (XAS) is one of the few tools that can be used to probe the structure of amorphous materials. Amorphous indium oxide doped with Ga (a-IGO) is a model system to help develop the role of dopants in amorphous oxides. An in depth XAS study was carried out to determine inter-atomic distances, coordination numbers, and structural disorder parameters as a function of Ga doping level. The correlation between XAS-derived structural features and the dopant-dependent evolution of both electrical properties and thermal stability of a-IGO will be discussed. [Preview Abstract] |
Monday, March 2, 2015 3:54PM - 4:06PM |
D8.00008: Optical constants and transient absorption of solution-deposited RuO2 thin films Jeffrey Owrutsky, Ryan Compton, James Long, Christopher Chrevin, Konrad Bussmann, Adam Dunkelberger, Bryan Spann, Irina Palin, Debra Rolison, Paul Cunningham, Joseph Melinger, Paul Desario, Dan Weidinger, Edwin Heilweil Optical and electrical conductivity properties are determined for the promising, broadband transparent conductor material, solution-deposited RuO$_{2}$ nanostructured films. The 10-30 nm thick films or nanoskins are less conductive but more optically transmissive than polycrystalline, sputtered RuO$_{2}$ films which are inherently metallic. The optical constants (0.6 to 4.5 eV) determined by ellipsometry show that $\epsilon_{1}$ is positive for the nanoskins in the spectral region investigated so they are not plasmonic. Transient picosecond absorption with visible (400 nm) pump and various probe wavelengths (visible and THz) are performed for nanoskins calcined to different temperatures. When heated to 200$^{\circ}$C the absorption increases in the visible and THz. After heating to 300$^{\circ}$C, the films become more polycrystalline and there is evidence for the appearance of a new absorption. Deceased absorptions or bleaches are observed in the THz and for longer visible wavelengths ($>$ 750 nm). The absorption is ascribed to a damped plasmon band of the crystalline nanoparticles formed in the film upon heating. [Preview Abstract] |
Monday, March 2, 2015 4:06PM - 4:18PM |
D8.00009: Effect of Ions and pH on the H-bond Network at the Quartz(101)-Water Interface Mark DelloStritto, James Kubicki, Jorge Sofo Reactions in aqueous systems are common, and yet can be difficult to study, as the structure and dynamics of the H-bond network can dominate the rates of reactions by determining the access reactants have to each other. This is thought to be the case at the quartz-water interface, where the addition of ions to solution or an increase in pH can increase the dissolution rate by an order of magnitude without any change in the activation energy. This suggests that the dissolution reaction is unchanged and the ions modify the structure of water at the interface such that the reaction occurs at a greater frequency. To investigate this effect, we performed ab-initio molecular dynamics (AIMD) simulations of the quartz(101)-water interface in several different ionic solutions under different pH and temperature conditions. We found that both anions and cations in a near-surface configuration tend to increase the order of interfacial water, although at neutral pH this competes with a decrease in the H-bond lifetime induced by the presence of an anion-cation pair. We also found that neither the H or O atoms of H$_{2}$O or OH$^{-}$ have greater access to the surface with the introduction of ions, but there is a greater incidence of intrasurface H-bonding in all systems studied. This suggests that intrasurface proton transfer could be an important component of the dissolution reaction, while nucleophilic attack of Si by H$_{2}$O is unlikely to occur at the interface studied. [Preview Abstract] |
Monday, March 2, 2015 4:18PM - 4:30PM |
D8.00010: N vacancy, self-interstitial diffusion, and Frenkel-pair formation/dissociation in TiN studied by \textit{ab-initio} and classical molecular dynamics Davide G. Sangiovanni, Bj\"{o}rn Alling, Lars Hultman, Igor A. Abrikosov We use \textit{ab-initio} and classical molecular dynamics (AIMD, CMD) to simulate diffusion of N vacancy and N self-interstitial point-defects in B1 TiN. The physical properties of TiN, important material system for thin film and coatings applications, are largely dictated by concentration and mobility of point defects. We determine N dilute-point-defect diffusion pathways, activation energies, attempt frequencies, and diffusion coefficients as a function of temperature. In addition, MD simulations reveal an unanticipated atomistic process, which controls the spontaneous formation of N-self-interstitial/N-vacancy pairs (Frenkel pairs) in defect-free TiN. This entails that a N lattice atom leaves its bulk position and bonds to a neighboring N lattice atom. In most cases, Frenkel-pair N$^{\mathrm{I}}$ and N$^{\mathrm{V}}$ recombine within a fraction of ns; 50{\%} of these processes result in the exchange of two nitrogen lattice atoms. Occasionally, however, Frenkel-pair N-interstitial atoms permanently escape from the anion vacancy site, thus producing unpaired N$^{\mathrm{I}}$ and N$^{\mathrm{V}}$ point defects. [Preview Abstract] |
Monday, March 2, 2015 4:30PM - 4:42PM |
D8.00011: Structural analysis of ferromagnetic Ni-Mn-Sn thin films fabricated by co-sputter deposition Sema Guvenc, Mehmet Yumak, A. Quintana Nedelcos, J. L. Sanchez Llamazares, Carlos Garcia Garcia Ferromagnetic Ni-Mn-X (Sn, In) alloys are predicted as potential candidates for energy efficient Magnetocaloric Effect (MCE) technologies. The MCE is the basis of magnetic refrigeration and it leads to a groundbreaking progress on conventional refrigeration methods. In our research, Ni-Mn-Sn thin films were fabricated by co-sputter deposition method. The Mn losses due to the high Mn vapor pressure produce a deviation from the desired Ni$_{\mathrm{50}}$Mn$_{\mathrm{37}}$Sn$_{\mathrm{13}}$ composition, which are partially compensated by increasing power of the Mn target gun. A systematic study of thin film co-sputter fabrication was divided into three stages; i) a Ni-Mn-Sn target was evaporated at different temperatures in order to study the grain size temperature dependence, ii) the nominal chemical composition (Ni$_{\mathrm{50}}$Mn$_{\mathrm{37}}$Sn$_{\mathrm{13}})$ was reached by controlling the power value applied to the Mn target, iii) the dependence of the phase transformation temperature with the grain size of the thin film alloys was studied. Grain size can be controlled by modifying the substrate temperature (T$_{\mathrm{s}})$. [Preview Abstract] |
Monday, March 2, 2015 4:42PM - 4:54PM |
D8.00012: Surface nanocrystalline and hardening effects of Ti--Al--V alloy by electropulsing ultrasonic shock Xiaoxin Ye, Guoyi Tang The effect of electropulsing ultrasonic shock (EUS) on the surface hardening and microstructure of Ti6Al4V alloy was studied. It was found that electropulsing improved the microhardness dramatically both in the influential depth and maximum value, compared with the only ultrasonic-shocked sample. It's indicated that refined surface layer with nanocrystalline and improved microhardness were obtained on account of surface severe plastic deformation, dynamic recrystallization (DRX) and phase change, which was implemented at relative low temperature and high strain rate/capacity due to the coupling of the thermal and athermal effects of EUS. It's different from conventional experiments and theory. It's discussed that the positive contributions of EPT in the thermodynamics and kinetics of microstructure and properties change were attributed to the reduction of nucleation energy barrier and acceleration of atomic diffusion. Therefore, it's supposed that EUS is an energy-saving and high-efficiency method of surface treatment technique with the help of high-energy electropulses, which is promising in cost reduction of the surface engineering and energy management. [Preview Abstract] |
Monday, March 2, 2015 4:54PM - 5:06PM |
D8.00013: Shedding Light on the Emission Mechanisms of In$_{0.54}$Ga$_{0.46}$N Disks in GaN Nanowires Using C.W. Non-Linear Spectroscopy Cameron Nelson, Albert Liu, Saniya Deshpande, Shafat Jahangir, Pallab Bhattacharya, Duncan Steel Linear and non-linear spectroscopy has been performed on an ensemble of 3 nm thick In$_{0.54}$Ga$_{0.46}$N disks grown in self-assembled GaN nanowires ($\sim$ 30 nm diameter). PLE measurements nearly resonant with the PL show a mostly broad, featureless spectrum with a linear increase in absorption as a function of energy, similar to InGaN/GaN quantum wells. Unlike InGaN quantum wells, the centroid of the PL spectrum shows a negligible intensity-dependent shift in PL emission wavelength. Further, the non-linear optical spectrum is dominated by excitonic resonances with line widths $\sim$ 20-30 meV in the same region as the PLE data. Distinguishable peaks in the PL spectrum overlap with the non-linear resonances. Continuous -wave nearly degenerate pump-probe absorption measurements show no evidence of spectral hole burning within the resonances; however there is evidence of population pulsations in the 3$^{\mathrm{rd}}$ order signal. This data shows evidence consistent with regular excitonic saturation and two beam coupling similar to that expected in discrete (e.g. 2 or 3 level systems). The excitonic behavior is also consistent with anti-bunching seen in g2 measurements from single dots. [Preview Abstract] |
Monday, March 2, 2015 5:06PM - 5:18PM |
D8.00014: Chemical Reactions and Atomic Removal Dynamics during Gallium Nitride Chemical Mechanical Polishing Process: Quantum Chemical Molecular Dynamics Simulations Kentaro Kawaguchi, Yuji Higuchi, Nobuki Ozawa, Momoji Kubo The chemical mechanical polishing (CMP) is promising for efficient polishing of the GaN substrate, and it is essential for manufacturing of GaN devices. However, the detailed CMP mechanisms are unclear, and then the design of efficient and precise CMP process is difficult. We performed polishing simulations of a GaN substrate by a SiO$_2$ abrasive grain in a solution including OH radicals in order to reveal effects of OH radicals on the polishing process. The OH radicals in the solution are adsorbed on the GaN surface and occupy the hollow sites on the surface. Then, a surface-adsorbed O atom is generated by the chemical reaction between the surface-adsorbed OH species and a OH radical in the solution. In the friction interface between the GaN substrate and the abrasive grain, the surface-adsorbed O atom is mechanically pushed into the GaN substrate by the abrasive grain. This O atom intrusion induces the dissociation of Ga-N bonds of the GaN substrate. Moreover, volatile N$_2$ molecules and soluble Ga(OH)$_3$ molecules are generated due to the dissociation of Ga-N bonds. Then, we suggested that the GaN CMP process efficiently proceeds by the mechanically induced chemical reactions: a surface-adsorbed O atom is generated and pushed into the GaN bulk by the abrasive grain. [Preview Abstract] |
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