Bulletin of the American Physical Society
2009 APS March Meeting
Volume 54, Number 1
Monday–Friday, March 16–20, 2009; Pittsburgh, Pennsylvania
Session Y37: Mesoscopic Systems, Clusters, and Nanoscale Systems I |
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Sponsoring Units: DCP Chair: Julius Jellinek, Argonne National Laboratory Room: 409 |
Friday, March 20, 2009 8:00AM - 8:12AM |
Y37.00001: Single-Molecule Interfacial Electron Transfer Dynamics at Dye-Sensitized TiO$_{2}$ Nanoparticles H. Peter Lu, Yuanmin Wang, Yufan He Interfacial electron transfer dynamics is important for environmental and catalytic reactions. Extensive ensemble-averaged studies have indicated inhomogeneous and complex dynamics of interfacial ET reaction. To characterize the inhomogeniety and the complex mechanism, we have applied single-molecule spectroscopy and correlated AFM/STM imaging to study the Interfacial ET dynamics of dye molecules adsorbed at the surface of TiO$_{2}$ nanoparticles. The interfacial ET activity of individual dye molecules showed fluctuations and intermittency at time scale of milliseconds to seconds. The fluctuation dynamics were found to be inhomogeneous from molecule to molecule and from time to time, showing significant static and dynamic disorders in the dynamics. Furthermore, we have applied site-specific AFM-Raman spectroscopy on analyzing ET associated mode-specific vibrational reorganization energy barriers. Our experiments revealed site-to-site variations in the vibrational reorganization energy barriers in the interfacial ET systems. Our recent experiments on single-molecule metal-to-ligand electron transfer (3) and single-molecule STM manipulation will also be discussed. [Preview Abstract] |
Friday, March 20, 2009 8:12AM - 8:24AM |
Y37.00002: Phase Diagram of a Model of Nanoparticles in Electrolyte Solutions Xiaofei Li, Steven Lettieri, Nathaniel Wentzel, James Gunton We obtain accurate fluid-fluid coexistence curves for a recent simple model of interacting nanoparticles that includes the effects of ion-dispersion forces. It has been proposed that these ion dispersion forces provide at least a partial explanation for the Hofmeister effect [Phys. Rev. Lett., 87:168103, 2001]. We study a model of aluminum oxide nanoparticle [Colloids and Surfaces A, 319:98-102, 2008] for three different electrolyte solutions with added salt type being sodium chloride, sodium iodide and a non-polarizable salt. We observe that the fluid-fluid coexistence curves depend substantially on the identity of added salt; this provides an efficient way of tuning the phase behavior of nanoparticles. The methods we employ include finite-size scaling (FSS), multicanonical histogram reweighting and Gibbs ensemble methods. We show that, as expected, all three cases belong to the universality class. The scaling fields and critical point parameters are obtained in the thermodynamic limit of infinite system size by extrapolation of our FSS results. [Preview Abstract] |
Friday, March 20, 2009 8:24AM - 8:36AM |
Y37.00003: Prediction of Zeolite Framework Types by a Machine Learning Approach Shujiang Yang, Mohammed Lach-hab, Iosif Vaisman, Estela Blaisten-Barojas Zeolites are microporous crystalline materials with highly regular framework structures consisting of molecular-sized pores and channels. Characteristic framework types of zeolites are traditionally determined by the combined information of coordination sequences and vertex symbols. Here we present a machine learning model for classifying zeolite crystals according to their framework types. An eighteen-dimensional feature vector is defined including topological descriptors and physical/chemical properties of zeolite crystals [Microporous and Mesoporous Materials 117, 339 (2009)]. Trained with crystallographic data of known zeolites, the new model can predict the framework types of unknown zeolite crystals with up to 98 \% accuracy. Compared with conventional methods, the machine learning model is more robust handling crystal disorder and/or crystal defects in a more effective manner. This model can be adapted for classifying and clustering other crystalline species. [Preview Abstract] |
Friday, March 20, 2009 8:36AM - 8:48AM |
Y37.00004: Site-specific polarizabilities as predictors of optimal binding sites of H$_{2}$O on Na$_{n}$ clusters Li Ma, Koblar Jackson, Julius Jellinek We have used density functional theory (DFT), in the generalized gradient approximation to study the adsorption of water molecules on sodium clusters, Na$_{n}$, for n = 7, 12, 18, and 25. These clusters span a range of sizes and surface topographies. In each case, we conduct an extensive search to identify the optimal binding site of the ad-molecule on the cluster. We analyze the results within the framework of cluster polarizabilities, making use of a new methodology for partitioning the cluster polarizability into atomic components (Jackson et al., J. Chem. Phys. \textbf{129}, 144309 (2008)). We show that the most favorable adsorption sites are at surface atoms that have the largest atomic polarizabilities. We will compare and contrast these results with corresponding findings for the adsorption of O$_{2}$ and NH$_{3}$ molecules on Na$_{n}$ clusters. [Preview Abstract] |
Friday, March 20, 2009 8:48AM - 9:00AM |
Y37.00005: Effect of adsorbates on the isomer stability of Ir$_4$ clusters Vladan Stevanovic, Zeljko Sljivancanin, Alfonso Baldereschi The relative stability of Ir$_4$ isomers, both in the gas phase and on MgO(100) substrate, is studied using density functional theory. The square Ir$_4$ is the most stable in both cases. The metastable tetrahedral isomer, which experimental data suggest as the most stable form of Ir$_4$ on MgO(100), is highly distorted by the strong Ir--O interactions. The relative stability of Ir$_4$ isomers is strongly altered by adsorption of a single C atom since the binding energy of the C adatom to tetrahedral and butterfly Ir$_4$ is much larger ($\sim$ 1.7 eV) than that to the square one, both in the gas-phase and on MgO(100). After carbon adsorption, the most stable structure of Ir$_4$ is the butterfly geometry for free clusters and the ``tetrahedral'' one for Ir$_4$/MgO(100). The C adatom binds in a bridge configuration in all cases and reduces the distortions produced by the MgO substrate. Energetics and equilibrium atomic geometries will also be discussed for the adsorption ({\it i}) one H or O atom and ({\it ii}) one CO molecule. [Preview Abstract] |
Friday, March 20, 2009 9:00AM - 9:12AM |
Y37.00006: Control and manipulation of Au nanocatalysis: effects of metal oxide support thickness and composition Bokwon Yoon, Chris Harding, Vahideh Habibpour, Sebastian Kunz, Adrian Nam-Su Farnbacher, Ueli Heiz, Uzi Landman Control and tunability of the catalytic oxidation of CO by gold clusters deposited on MgO surfaces grown on molybdenum, Mo(100), to various thicknesses, are explored through temperature programmed reaction measurements on mass selected 20-atom gold clusters and via first-principles density-functional theory calculations. Dependencies of the catalytic activities and microscopic reaction mechanisms on the thickness and stoichiometry of the MgO films, and on the dimensionalities and structures of the adsorbed gold clusters are demonstrated and elucidated. Langmuir-Hinshelwood mechanisms and reaction barriers corresponding to observed low and high temperature CO oxidation reactions are calculated and analyzed. Along with the oxidation reactions on stoichiometric ultra thin MgO films we also study reactions catalyzed by Au$_{20}$ nanoclusters adsorbed on relatively thick defect-poor MgO films supported on Mo, and on defect-rich thick MgO surfaces containing oxygen vacancy defects. [Preview Abstract] |
Friday, March 20, 2009 9:12AM - 9:24AM |
Y37.00007: Role of anharmonic contributions for the elasticity of ice Mira Todorova, Lars Ismer, J\"org Neugebauer Water, one of the simplest molecules in chemistry, forms a liquid and solid phase with features essential to live and environment. Many of these can be attributed to hydrogen bonding, but that does not mean that they are fully understood. Ice should be an easier material to understand, because its molecules are arranged on a regular lattice. Yet even the determination of such basic properties as the bulk modulus and the elastic constants proves to be a challenge. Using first principles calculations we investigate the bulk properties of hexagonal ice. Our initial density-functional theory calculations (GGA-PBE level) yield values, which are much too high when compared to experiment. Even though the consideration of thermal effects within the quasi-harmonic approximation leads to a qualitative agreement between measured and calculated quantities, such as the linear expansion coefficient, ice remains much too hard. The large overestimation of the ice' softness demonstrates the importance of anharmonic contributions, which will be shown to be crucial and lead to a dramatic reduction of the bulk modulus and the elastic constants. [Preview Abstract] |
Friday, March 20, 2009 9:24AM - 9:36AM |
Y37.00008: An Assessment of Hubbard U Corrections on Manganese Oxide Clusters Elise Y. Li, Davide Ceresoli, Nicola Marzari Conventional density-functional approaches often fail in offering an accurate description of the spin-resolved energetics in transition metal complexes, due to spurious self-interaction errors (SIE). Previous studies have shown that a self-consistent DFT + U approach [1] can accurately correct SIE in TM complexes, providing excellent agreement with high-level quantum chemistry calculations. In this work we report a systematic evaluation of DFT + U in a series of small manganese oxide clusters (MnO$_{x}$, x=1-4) and their anions, focusing on structural, electronic and magnetic properties. It is found that DFT + U succeeds in systems where the valence electrons are essentially localized on the TM ion, but progressively worsens when the coordination number increases and more covalency comes into play between the TM center and neighboring atoms, pointing to the importance of choosing the appropriate correlated manifold when Hubbard corrections are added. [1] H. J. Kulik, M. Cococcioni, D. A. Scherlis, and N. Marzari, Phys. Rev. Lett., 2006, 97, 103001 . [Preview Abstract] |
Friday, March 20, 2009 9:36AM - 9:48AM |
Y37.00009: ABSTRACT WITHDRAWN |
Friday, March 20, 2009 9:48AM - 10:00AM |
Y37.00010: Advances in a Joint Density-Functional Theory for Electronic Systems in Contact with Liquid Water: A New Form of Density Functional for Water Johannes Lischner, Tomas Arias We present a framework for studying complex electronic systems, such as biological molecules or electrochemical interfaces, that are dissolved in liquid water. The key ingredient, that renders calculations possible, is the usage of an approximate, yet accurate ``classical'' density-functional theory of water, while the electronic system is described by traditional Kohn-Sham theory. The electronic system (e.g. biosolute) is then coupled to the aqueous environment via molecular pseudopotentials. Here, we present a new form of ``classical'' density functional for water which is the first to properly account for the structure of the molecule in an exact way. We accomplish this by writing the free energy in terms of three effective potentials, one for the oxygen and one for each hydrogen atom, in which fictitious non-interacting water molecules move. In this talk, we will show that the resulting functional correctly reproduces the following properties of water: the linear and {\em nonlinear} dielectric response, the site-site correlation functions, the surface tension, the bulk modulus of the liquid and its variation with pressure, the density of the liquid and the vapor phases, and their coexistance. [Preview Abstract] |
Friday, March 20, 2009 10:00AM - 10:12AM |
Y37.00011: The AM05 density functional applied to the water molecule, dimer, and bulk liquid Ann E. Mattsson, Thomas R. Mattsson We show that the AM05 exchange-correlation density functional (Armiento and Mattsson, Phys.~Rev.~B~{\bf 72}, 085108 (2005)) yields a H$_2$O dimer binding energy of 4.9 kcal/mol. The result is thus within 0.15 kcal/mol of CCSD(T) level theory ($5.02 \pm 0.05$ kcal/mol). We compare the AM05 results with those of five other functionals: LDA, PBE, PBEsol, RPBE, and BLYP. For liquid water, AM05 yields an O-O pair correlation function that is more structured than the ones of PBE and BLYP, which, in turn, are more structured than the one of RPBE. However, LDA and PBEsol yields more structured water than AM05. We confirm that accuracy in the water dimer binding energy is not a strong indicator for the fidelity of the resulting structure of liquid water. We will also report on the performance of AM05 for other systems and discuss the sub-system functional scheme used in the construction of AM05. Sandia is a multiprogram laboratory operated by Sandia Corporation, a Lockheed Martin Company, for the United States Department of Energy's National Nuclear Security Administration under contract DE-AC04-94AL85000. [Preview Abstract] |
Friday, March 20, 2009 10:12AM - 10:24AM |
Y37.00012: Heat capacities of aluminum clusters Anne Starace, Baopeng Cao, Oscar Judd, Martin Jarrold Clusters of certain elements are known to undergo phase transitions from solid-like to liquid-like states. Aluminum clusters have emerged as a model system for metal cluster phase transitions [1]. We report here the measurement of heat capacities of cationic clusters containing 84 to 127 Al atoms using a multi-collision induced dissociation mass spectrometry method [2]. We find two major changes in the heat capacities with increasing cluster size: (1) the fluctuations in the temperature of the phase transition vary more smoothly and (2) the peaks in heat capacity become sharper. Furthermore, we have found a range of cluster sizes (115-117 atoms) that contain two distinct peaks, separated by baseline, in their heat capacities. The origin of the extra peaks in the heat capacity, which is suspected to be due either to a pre-melting transition or to a solid-to-solid transition prior to the melting transition, will be further investigated by means of annealing experiments. The current work extends prior work on singly charged Al cluster cations having 16-83 atoms [2, 3]. [1] Breaux, G. A.; Neal, C. M.; Cao, B.; Jarrold, M. F. Physical Review Letters 2005, 94. [2] Neal, C. M.; Starace, A. K.; Jarrold, M. F. Journal of the American Society for Mass Spectrometry 2007, 18, 74-81. [3] Neal, C. M.; Starace, A. K.; Jarrold, M. F. Physical Review B 2007, 76. [4] This work is supported by NSF. [Preview Abstract] |
Friday, March 20, 2009 10:24AM - 10:36AM |
Y37.00013: From Atomic Clusters towards Nano-Materials with Controlled properties S.N. Khanna, M.C. Qian, A.C. Reber, J.U. Reveles, R. Robles, P.A. Clayborne, S.V. Ong, K. Casalenuovo, A.W. Castleman Jr., A. Sen, P.W. Weiss, H. Saavedra, A. Ugrinov, N. Chaki One pathway towards the synthesis of nanomaterials with controllable properties is to assemble solids using chosen clusters as the building blocks. The talk will outline a new protocol that enables synthesis of nanomaterials from clusters and highlight how the character of the cluster emerges in the assembled material. Through studies on assemblies involving polyvalent anions As$_{7}^{-3}$ and As$_{11}^{-3}$ and alkali based cations, we will show how the studies can provide novel ways of controlling the bandgap through energy level of the countercation and the degree of charge transfer. The theoretical predictions will be compared with experimental findings. [Preview Abstract] |
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