Bulletin of the American Physical Society
APS March Meeting 2010
Volume 55, Number 2
Monday–Friday, March 15–19, 2010; Portland, Oregon
Session Z28: Clusters |
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Sponsoring Units: DCP Chair: Rich Loomis, Washington University in St. Louis Room: C124 |
Friday, March 19, 2010 11:15AM - 11:27AM |
Z28.00001: Why Boron clusters are Planar? Kiran Boggavarapu, Anil Kandalam The origin of the unusual stability of planar and quasi-planar B12 and B13$^{+}$ clusters is explored. Our results demonstrate that in B$_{12}$ and B$_{13}^{+}$ clusters a 6$_{\pi }$-6$_{\sigma -delo}$-6$_{\sigma -3ring}$ trifurcation leads to the triple aromaticity, which is unique to these clusters. Most importantly, the H-L gaps of these clusters are strongly dependent on the strength of the interaction between the inner- and the outer-rings, which make up these clusters. Furthermore, the similarities and the differences between B$_{12}$ and other stable boron species, B$_{10}$ and B$_{14}$ clusters are also discussed. The implication of the current analysis is discussed with respect to Carbon, Silicon and Aluminum clusters. [Preview Abstract] |
Friday, March 19, 2010 11:27AM - 11:39AM |
Z28.00002: Adsorption of CO and O2 molecules on supported small Au clusters Martin Amft, Natalia Skorodumova We investigate the catalytic activity of metal-oxide/metal supported tiny gold clusters towards the carbon monoxide oxidation by means of density functional theory calculations. Our focus lies on cluster-size effects, the influence of different support materials and co-adsorption of other molecules, e.g. water. In agreement with experimental data we could explain, why Au ad-atoms and dimers on MgO do not show any catalytic activity towards CO oxidation and why a Langmuir-Hinshelwood reaction mechanism via co-adsorption is possible for the trimer and tetramer. Furthermore we thoroughly studied the influence of spin-orbit coupling, a hitherto widely neglected effect in these systems, on the adsorption of gold clusters on the surface and of small molecules on the cluster/surface system. Last but not least clusters consisting of an odd number of gold atoms carry a spin moment from one unpaired 6s electron. We studied its coupling to the moments of a magnetic metal beneath a thin supporting metal-oxide layer. [Preview Abstract] |
Friday, March 19, 2010 11:39AM - 11:51AM |
Z28.00003: Hydrogen production from methanol on transition metals: A study of thermodynamics and kinetics on subnanometer clusters Faisal Mehmood, Jeffery P. Greeley, Peter Zapol, Larry A. Curtiss The mechanistic studies of Pd-based catalysts and its interaction with methanol have attracted huge attention because of the possibility of using methanol as an on-board source of hydrogen for fuel cells. Stabilizing subnanometer metal clusters is a challenging process that has exhibited novel catalytic properties for various industrially important reactions such as production of hydrogen from hydrogen-rich molecules. One such reaction is methanol decomposition that was modeled by applying DFT methods on metal clusters. The thermodynamics and kinetics of three decomposition routes involving C-O, C-H and O-H scission were investigated; activation energy barriers were determined with the nudged elastic band method on Pd clusters with a comparison to Co and Cu clusters. A detailed analysis of the PES for methanol decomposition shows C-O activation to be the least favorable step on all three metal clusters. However we find activation to be $\sim$0.30 eV smaller on Co cluster. In addition, estimated thermodynamical data for a large number of transition metals has been generated from linear correlations constructed from the binding energies of Pd, Cu and Co to broaden our understanding of the role such metal clusters can play as catalyst for such reactions. [Preview Abstract] |
Friday, March 19, 2010 11:51AM - 12:03PM |
Z28.00004: Intrinsic nano-transformation of Al$_{55}$ clusters below the melting temperature Joongoo Kang, Su-Huai Wei, Yong-Hyun Kim A recent series of experiments [1] have shown diverse melting behaviors in size-selected Al nanoclusters (Al$_{n})$. In particular, Al$_{55}$ is a magic cluster that serves as a boundary for abrupt change of melting points when $n$ is around 55. Here, resulting from first-principles molecular dynamics simulations of Al$_{55}$ clusters, we reveal a new dynamic melting state that has both solid and liquid characteristics. In thermal fluctuations near the melting point, the low-energy tetrahedral Al$_{55}$ survives through rapid, collective surface transformations --- such as parity conversions and intervened row hopping --- without losing its structural orders. The emergence of the collective motions is due to efficient thermal excitation of soft phonon modes at nanoscale. A series of spontaneous surface reconfigurations result in a mixture or effective flow of surface atoms as is random color shuffling of a Rubik's cube. This novel ``lattice-liquid'' state will provide useful insights into understanding stability and functionality of nano systems near or below melting temperatures. [1] G. A. Breaux et al., Phys. Rev. Lett. \textbf{94}, 173401 (2005). [Preview Abstract] |
Friday, March 19, 2010 12:03PM - 12:15PM |
Z28.00005: Theoretical Studies of the Stability and Electronic Properties of Pd$_{n}$, and Pd$_{n}$O$_{2}$ (1$\le $n$\le $13) Clusters Debesh R. Roy, J. Ulises Reveles, Shiv N. Khanna, Andreas M. Koster First principles electronic structure studies on the ground state geometry, electronic structure and magnetic moment of Pd$_{n}$(1$\le $n$\le $13) clusters have been carried out using a gradient corrected density functional approach. The clusters are found to be magnetic with a moment per atom that varies with cluster size. In particular, Pd$_{13}$ is shown to have a two layers structure that can be looked upon as a fragment of the bulk and has a spin magnetic moment of 6 Bohr magnetons. The calculated magnetic moments are compared with available data from Stern Gerlach experiments. We also study the effect of adding an O$_{2}$ molecule on the electronic and magnetic properties by carrying out corresponding studies on Pd$_{n}$O$_{2}$ (1$\le $n$\le $13) clusters. Our findings on the strength of binding of oxygen will be compared with recent experiments on the oxidation of palladium clusters by oxygen. [Preview Abstract] |
Friday, March 19, 2010 12:15PM - 12:27PM |
Z28.00006: A density functional theory investigation of the 3d, 4d, and 5d metals 13-atom clusters Paulo Piquini, Mauricio Piotrowski, Juarez Da Silva In this work, we report a first-principles study based on density functional theory calculations of the atomic structure, binding energies, coordination numbers, average bond lengths, magnetic properties, and vibrational frequencies of 3d, 4d, and 5d metal clusters (30 elements) containing 13 atoms, M13. A set of lower energy local minimum structures were obtained by combining high-temperature first-principles molecular dynamics simulations with geometric optimizations at zero temperature for different spin configurations. The ground state structures for the M13 clusters show very clear features: (i) Compact icosahedral-like forms at the beginning of each metal series, (ii) more opened structures such as hexagonal bilayer-like and double cubic layer in the middle of each metal series, and (iii) structures with increasing average coordination number, e.g., icosahedron (Hg), for elements with valence d-states having more than half-occupation, at the end of each series. The magnetic exchange interactions play an important role for particular systems such as Fe, Cr, and Mn. Most of the properties can be explained by the occupation of the bonding and anti-bonding states [Preview Abstract] |
Friday, March 19, 2010 12:27PM - 12:39PM |
Z28.00007: The Origin of ``Magic-Number'' Stability and Chiral Selectivity for Serine Clusters in Gas Phase Anthony Costa, R. Graham Cooks Serine ``magic-number'' clusters have attracted substantial experimental and theoretical interest since their discovery. They have been implicated in one possible mechanism leading to the origin of homochirality, as certain clusters exhibit remarkable chiral selectivity. We aim to develop a ``structural landscape'' for these clusters over a range of relevant cluster sizes, enantiomeric compositions, and ionizing charge states using theoretical tools of statistical and quantum mechanics. In this work, we search for low-lying stationary points and global minima of the potential energy landscape via a combined annealing, replica exchange and basin-hopping molecular dynamics approach in a modified AMBER forcefield. These structures are used as inputs for further DFT-based optimization and energy decomposition analysis. It is shown that the behavior and stability of these systems is due to major structural rearrangements as a function of size and charge. Further, the experimentally observed chiral selectivity may be understood in part by the unique network of hydrogen bonds facilitated by the serine hydroxyl side chain. The influence of a further kinetic mechanism is not ruled out by the current results and is discussed. [Preview Abstract] |
Friday, March 19, 2010 12:39PM - 12:51PM |
Z28.00008: Effect of Charge and Composition on the Structural Fluxionality and Stability of Nine Atom Tin-Bismuth Zintl Analogues Penee Clayborne, Ujjwal Gupta, Arthur Reber, Joshua Melko, A.W. Castleman, Shiv Khanna Synergistic studies of bismuth doped tin clusters combining photoelectron spectra with first principles theoretical investigations establish that highly charged Zintl ions, observed in the condensed phase, can be stabilized as isolated gas phase clusters through atomic substitution that preserves the overall electron count but reduces the net charge. Mass spectrometry studies reveal that Sn$_{8}$Bi$^{-}$, Sn$_{7}$Bi$_{2}^{-}$, and Sn$_{6}$Bi$_{3}^{-}$ exhibit higher abundances than neighboring species, and photoelectron spectroscopy show that all of these heteroatomic gas phase species have high adiabatic electron detachment energies. Sn$_{6}$Bi$_{3}^{-}$ is found to be a particularly stable cluster, having a large highest occupied molecular orbital (HOMO)-lowest unoccupied molecular orbital (LUMO) gap. Theoretical calculations demonstrate that Sn$_{8}$Bi$^{-}$, Sn$_{7}$Bi$_{2}^{-}$, and Sn$_{6}$Bi$_{3}^{-}$ are deltahedral clusters and isoelectronic to the Zintl polyatomic clusters Sn$_{9}^{2-}$, Sn$_{9}^{3-}$ and Sn$_{9}^{4-}$, respectively. However, the fluxionality reported for tin-Zintl clusters is suppressed by substituting Sn atoms with Bi atoms in Sn$_{8}$Bi$^{-}$ and Sn$_{6}$Bi$_{3}^{-}$. The similarities between bismuth doped deltahedral tin clusters and deltahedral Zintl polyanions, suggest these gas phase Zintl clusters (GPZC) may find use for building blocks of cluster assembled materials. [Preview Abstract] |
Friday, March 19, 2010 12:51PM - 1:03PM |
Z28.00009: Activity and Selectivity of Size-Selected Sub-nm to Nanometer Size Silver Clusters in the Selective Oxidation of Propylene Stefan Vajda, Sungsik Lee, Kristian Sell, Yu Lei, Ingo Barke, Armin Kleibert, Arantxa Fraile-Rodriguez, Viola von Oeynhausen, Karl-Heinz Meiwes-Broer, Randall Meyer, Byeongdu Lee, Soenke Seifert, Randall Winans, Jeffrey Elam, Michael Pellin, Suzanne Giorgio, Claude Henry, Detre Teschner, Robert Schloegl The activity and selectivity of Ag nanoparticles in propene oxidation will be discussed and compared with the performance of Ag3 clusters. The experimental studies are based on 1) chemically uniform support fabrication, 2) size-selected cluster deposition, 3) electron microscopy of nanoclusters, and 4) in situ synchrotron X-ray characterization of clusters under working conditions, combined with mass spectroscopy analysis of reaction products. [Preview Abstract] |
Friday, March 19, 2010 1:03PM - 1:15PM |
Z28.00010: On the Oxidation of Palladium clusters supported on alumina/NiAl(110) Roberto Robles, Shiv N. Khanna Palladium nanostructures supported on alumina substrates have been widely studied as catalysts for several combustion processes. Following recent experimental results [1], we have performed electronic structure density functional calculations in order to study the geometry, stability and electronic properties of small Pd clusters supported on alumina/NiAl(110). Using an accurate model of such surface, the properties of selected cluster sizes have been determined, as well as the effect of the addition of oxygen. A carefully analysis of the results as well as a comparison with the experimental data will be presented. \\[4pt] [1] T.Wu et al, Surface Science 603, 2764 (2009). [Preview Abstract] |
Friday, March 19, 2010 1:15PM - 1:27PM |
Z28.00011: Not so loosely bound: temperature dependent vibrational fingerprints of Au$_N$Kr$_M$ clusters Luca M. Ghiringhelli, Philipp Gruene, Gerard Meijer, Andr\'{e} Fielicke, Matthias Scheffler In order to interpret the vibrational spectra of \textit{neutral} Au$_N$Kr$_M$ clusters, as measured in a Multiple Photon Dissociation Far-IR experiment (P. Gruene \emph{et al.}, Science \textbf{321}, 674 (2008)), we calculated their finite temperature vibrational spectra, by means of all electron density functional theory, including the Tkatchenko-Scheffler van der Waals correction. We surprisingly find that Kr forms weak chemical bonds (binding energy around 0.2 eV per Kr atom) with 1- to 3-fold coordinated Au atoms belonging to small Au$_N$ clusters ($N \leq 4$). Such Au$_N$Kr$_M$ clusters have a vibrational spectrum which is different from the related Au$_N$ clusters. For bigger Au$_N$ clusters, Kr physisorbs to the clusters, forming a complex whose vibrational spectrum is practically identical to the spectrum of the bare cluster. Anharmonicities affect the spectrum by changing (with respect to the harmonic spectrum) the relative intensity of the peaks and by showing new peaks, due to interactions among eigenmodes. [Preview Abstract] |
Friday, March 19, 2010 1:27PM - 1:39PM |
Z28.00012: Signature of the Superatom to Superhalogen Behavior of Au$_{n}$(BO$_{2})_{m}$ clusters Anil Kandalam, Matthias Gotz, Mary Willis, Gerd Gantefor, Puru Jena We report the discovery of a new class of clusters consisting of Au$_{n}$(BO$_{2})_{m}$ which formed during the oxygenation of gold clusters when boron nitride was used as insulation in the pulsed arc cluster ion source (PACIS). Using DFT based calculations, we trace the origin of these species to be due to the unusual stability of the BO$_{2}$ moiety as well as shed light on their formation process. PES measurements and the corresponding DFT calculations further reveal some rather remarkable properties of Au$_{n}$(BO$_{2})_{m}$ clusters such as large HOMO-LUMO gaps in the range of 3.00 eV -- 3.95 eV and electron affinities substantially larger than that of F, the most electronegative element in the periodic table. In addition, some of the most predominant features of the electronic structure of the bare Au clusters, namely odd-even alternation in the electron affinity, are preserved in the Au$_{n}$(BO$_{2})$ species. The synergy between theory and experiment illustrates that Au$_{n}$(BO$_{2})_{m}$ clusters, behave as superatoms and superhalogens, opening the door for the synthesis of a new class of cluster-assembled materials. [Preview Abstract] |
Friday, March 19, 2010 1:39PM - 1:51PM |
Z28.00013: Ab Initio Study of Atomic and Molecular Adsorption to Pt Clusters on Graphenes Gyubong Kim, Seung-Hoon Jhi In recent years, Pt nanostructures have received a great deal of attention because of their exceptional chemical/electrochemical reaction properties with high catalytic efficiency. Particularly, a number of studies have focused on their versatile catalytic properties depending on their sizes, shapes, substrate and/or alloy compositions {\it etc}. Motivated by those studies, we investigated the adsorption of several important adsorbates (H, O, and CO) to Pt$_x-$graphene complexes (x = 1 or 13) with the use of {\it ab initio} density functional methods. Our calculations of Pt adsorption on various graphene defects demonstrate that Pt {\it d} band profiles remarkably vary depending on the type of graphene defects and the corresponding adsorption strength of the adsorbates is also substantially affected. Also for Pt$_ {13}-$graphene complexes, the overall adsorption strength of H, O, and CO is remarkably changed depending on graphene defects, which indicates the variation of their catalytic behavior. The role of graphene defects for the interaction between Pt and the adsorbates will be presented. [Preview Abstract] |
Friday, March 19, 2010 1:51PM - 2:03PM |
Z28.00014: Magnetic Superatoms J. Ulises Reveles, Penee A. Clayborne, Arthur C. Reber, Shiv N. Khanna, Kalpataru Pradhan, Prasenjit Sen, Mark R. Pederson The electronic states in metal clusters are grouped in shells much in the same way as in atoms. Filling of the electronic shells leads to stable species called magic numbers. This has led to the proposition that selected stable metal clusters can mimic chemical properties of atoms in the periodic table and can be classified as superatoms. So far the work on superatoms has focused on non-magnetic species. Here, we propose an extension of the superatom concept to magnetic species by invoking systems that have both localized and delocalized electronic states. Here, the localized electrons stabilize spin magnetic moments while filled nearly free electron shells lead to stable superatoms. We demonstrate it for an isolated VCs$_{8}$ and a ligated MnAu$_{24}$(SH)$_{18}$ motifs that are shown to be magnetic superatoms. The magnetic superatoms assemblies offer prospect of tunable molecular electronic devices, as the coupling can be altered by applying fields. [Preview Abstract] |
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