Bulletin of the American Physical Society
APS March Meeting 2011
Volume 56, Number 1
Monday–Friday, March 21–25, 2011; Dallas, Texas
Session Y39: Focus Session: Imaging and Interfaces in Energy Science |
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Sponsoring Units: DCP Chair: Annabella Selloni, Princeton University Room: A124/127 |
Friday, March 25, 2011 8:00AM - 8:36AM |
Y39.00001: Imaging Interfacial Structure and Reactivity with X-ray Reflectivity and Microscopy Invited Speaker: A fundamental understanding of interfacial reactions is best achieved with ability to observe the systems of interest directly, ideally with molecular-scale resolution and/or sensitivities. X-ray-based approaches offer broad opportunities for probing complex interfaces in environments (e.g., liquids) that are normally inaccessible. I will describe two complimentary approaches for imaging interfaces. The first, X-ray reflection interface microscopy (XRIM), uses the weak interface-reflected X-ray beam to image laterally heterogeneous interfacial structures and processes using a full-field imaging approach. This approach incorporates all of the sensitivities of X-ray reflectivity (XR, including sensitivity to interfacial topography, structure and composition) as potential contrast mechanisms. Recent applications of XRIM will be described, including the abilities to observe: elementary surface topography (i.e., 6.5 {\AA}-high steps) with $\sim $100 nm spatial resolution; interfacial reactivity; and liquid-solid interfaces, in-situ. A second, complementary, approach images the vertical distributions of element-specific sub-structures at an interface through the use of resonant dispersion at X-ray energies close to element's absorption edge (resonant anomalous X-ray reflectivity, RAXR). Recent applications of RAXR will be described including the ability to image element-specific distributions (i.e., ions near a charged liquid-solid interface) and its sensitivity for probing oxidation state specific structures at interfaces. The use of these techniques to observe charge transport at interfaces with respect to energy-related processes will be discussed. [Preview Abstract] |
Friday, March 25, 2011 8:36AM - 8:48AM |
Y39.00002: Small-Pore Molecular Sieves SAPO-34 with Chabazite Structure: Theoretical Study of Silicon Incorporation and Interrelated Catalytic Activity Hong Wang, James Lewis, Zhongmin Liu The catalytic conversion of methonal to olefin (MTO) has attracted attention both in industrial and academic fields. Strong evidence shows that small-pore molecular sieves with certain amount silicon incorporated (SAPO) present promising high catalytic activity in MTO conversion. Using DFT, we study the structural and electronic properties of chabazite SAPO-34. Although there are extensively experimental results show that silicon incorporation does not change the overall structure as the original AlPO structure, local structural changes are still created by silicon substitution, which probably accounted for the high catalytic activity. It is noted that the catalytic activity of SAPO-34 presents increasing trend along with the silicon incorporation amount increasing and maintain a flat peak even with more silicon incorporated. Hence, there is an optimal silicon incorporation amount which possibly yields the highest catalytic MTO conversion. [Preview Abstract] |
Friday, March 25, 2011 8:48AM - 9:24AM |
Y39.00003: Nanosecond Scanning Tunneling Microscopy: resolving spin dynamics at the atomic scale Invited Speaker: With the advent of nanoelectronics, functional electronic elements advance towards atomic dimensions and analysis techniques need to keep pace. Scanning tunneling microscopes (STM) have evolved into standard tools to measure the static electronic properties of nanostructures, molecules and atoms. Here we show how the STM can be used to access the equally important dynamical properties on time scales ranging from pico- to nanoseconds. We combine inelastic electron tunneling spectroscopy (IETS) with an all-electronic pump-probe measurement scheme and record the dynamical evolution of magnetic atoms on surfaces in the time domain [1]. We focus on the dynamics of electron spin relaxation in transition metal atoms placed onto a copper nitride decoupling layer on Cu(100). On this surface Fe atoms experience large magneto-crystalline anisotropy [2] that enables long spin lifetimes. At the same time the quantum mechanical nature of the discrete spin states allows for an additional path of spin relaxation: quantum tunneling of magnetization. We probe the dynamic behavior associated with this process and find that placing a Cu atom close to a Fe atom boosts the uniaxial anisotropy energy and creates a long-lived spin state with relaxation times in excess of 200 ns. The ability to probe individual nanostructures with atomic spatial and nanosecond temporal resolution opens a new avenue to explore spin dynamics and other dynamical phenomena on the intrinsic length scale of the underlying interactions. \\[4pt] [1] S. Loth, M. Etzkorn, C. P. Lutz, D. M. Eigler, A. J. Heinrich, Science 329 1628 (2010). \\[0pt] [2] C.F. Hirjibehedin, et al., Science 317, 1199 (2007). [Preview Abstract] |
Friday, March 25, 2011 9:24AM - 9:36AM |
Y39.00004: Tracking Oxygen Vacancies in Thin Film SOFC Cathodes Donovan Leonard, Amit Kumar, Stephen Jesse, Sergei Kalinin, Yang Shao-Horn, Ethan Crumlin, Eva Mutoro, Michael Biegalski, Hans Christen, Stephen Pennycook, Albina Borisevich Oxygen vacancies have been proposed to control the rate of the oxygen reduction reaction and ionic transport in complex oxides used as solid oxide fuel cell (SOFC) cathodes [1,2]. In this study oxygen vacancies were tracked, both dynamically and statically, with the combined use of scanned probe microscopy (SPM) and scanning transmission electron microscopy (STEM). Epitaxial films of La$_{0.8}$Sr$_{0.2}$CoO$_{3}$ (LSC$_{113})_{ }$and LSC$_{113}$/LaSrCoO$_{4 }$(LSC$_{214})$ on a GDC/YSZ substrate were studied, where the latter showed increased electrocatalytic activity at moderate temperature. At atomic resolution, high angle annular dark field STEM micrographs revealed vacancy ordering in LSC$_{113}$ as evidenced by lattice parameter modulation and EELS studies. The evolution of oxygen vacancy concentration and ordering with applied bias and the effects of bias cycling on the SOFC cathode performance will be discussed. [Preview Abstract] |
Friday, March 25, 2011 9:36AM - 9:48AM |
Y39.00005: First principles study of GaN(10\underline{1}0)/Water interface Jue Wang, Maria Victoria Fernandez-Serra, Xiao Shen GaN/ZnO alloy semiconductors have been shown to be promising materials to serve as photo-anode in photocatalytical fuel cells. In recent study by Shen et al\footnote{X. Shen, Y.A. Small, J. Wang, P.B. Allen, M.V. Fernandez-Serra, M.S. Hybertsen and J.T. Muckerman \textit{J. Phys. Chem. C }\textbf{114(32)}, 13695 (2010)}, the non polar GaN(10\underline{1}0) surface has been studied with atomistic modeling and a sequence of intermediate steps for the water oxidation process at the interface are proposed. Here we present a first principles molecular dynamics study of the GaN(10\underline{1}0)/Water interface. We found dissociation events happen within 1ps and we show a detailed analysis of the changes in structure and dynamics of water molecules interacting with a dissociating wet surface. The complex hydrogen bond network near the surface is also analyzed in detail, including a throughout study of the proton diffusion processes. We perform a detailed analysis of the dynamics of the hole localization. The link between water surface dissociation and quantum efficiency will be discussed. [Preview Abstract] |
Friday, March 25, 2011 9:48AM - 10:24AM |
Y39.00006: Ions at interfaces and their spectroscopic consequences Invited Speaker: The affinity of relatively small ions for air-water interfaces challenges our basic understanding of the basic driving forces for solvation. Here I will show that this adsorption is a general phenomenon for ions in polar solvents. Its physical origin lies in a precarious and unexpected balance of strong nonlinear contributions. The statistics of solvent electric fields suggests a key role for interfacial fluctuations. I will also present an intuitive perspective on surface-specific vibrational spectroscopy, and discuss observable signatures for ion adsorption at aqueous interfaces. [Preview Abstract] |
Friday, March 25, 2011 10:24AM - 10:36AM |
Y39.00007: First-Principles Studies of Functionalized Si(111) in Air and in Water Yan Li, Giulia Galli We have investigated structural, electronic and vibrational properties of hydrogen and methyl-terminated Si(111) surfaces both in air and in contact with water, by combining density functional theory and many-body perturbation theory within the GW approximations. The computed surface dipole moments for both H-Si(111) and CH3-Si(111) surfaces were found to be consistent with measured electron affinities (EAs), and can be explain by simple electronegative trends. While GW self-energy corrections greatly improve the absolute values of EAs, the EA difference of the two surfaces remains overestimated by about 0.3 eV. The variations in CH3 frequencies, e.g. the umbrella mode and CH stretching mode, for the surface in air and water are also well reproduced by our calculations. The influence exerted by the adsorption of water molecules on the hydrophobic H-Si(111) and CH3-(111) surfaces, in particular, on the EAs and the surface vibrational frequencies will be discussed and compared with recent experiments.\\[4pt] [1] A. Aliano, Y. Li, G. Cicero and G. Galli, J. Phys. Chem. C 114, 11898 (2010).\\[0pt] [2] Y. Li and G. Galli, Phys. Rev. B 82, 045321 (2010). [Preview Abstract] |
Friday, March 25, 2011 10:36AM - 10:48AM |
Y39.00008: Interaction between surfaces with ionizable sites Stephen Barr, Athanassios Panagiotopoulos A key factor controlling the interaction between surfaces in aqueous solutions is the surface charge density. Surfaces typically become charged though a titration process where surface groups can become ionized based on their dissociation constant and the pH of the solution. In this work we use a Monte Carlo method to treat this process explicity in a system with two planar surfaces in a salt solution. We find that the surface charge density changes as the surfaces come close to contact due to interactions between the ionizable groups on each surface. In addition, we observe an attraction between the surfaces above a threshold surface charge, in good agreement with previous theoretical predictions based on uniformly charged surfaces. However, close to contact we find the force is significantly different than the uniformly charged case. We also explore the role of salt concentration and the density of the ionizable sites. [Preview Abstract] |
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