Bulletin of the American Physical Society
2007 APS March Meeting
Volume 52, Number 1
Monday–Friday, March 5–9, 2007; Denver, Colorado
Session L18: Metal Clusters |
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Sponsoring Units: DCP Chair: A. Welford Castleman, Pennsylvania State University Room: Colorado Convention Center 103 |
Tuesday, March 6, 2007 2:30PM - 2:42PM |
L18.00001: ABSTRACT WITHDRAWN |
Tuesday, March 6, 2007 2:42PM - 2:54PM |
L18.00002: ABSTRACT WITHDRAWN |
Tuesday, March 6, 2007 2:54PM - 3:06PM |
L18.00003: Size-Selected Cluster Based Catalysts: Physical and Chemical Properties Studied by GISAXS, .Mass Spectrometry and UV-VIS Spectroscopy Stefan Vajda, Gregory Ballentine, Alexandre Bouhelier, Joseph Calo, Jeffrey Elam, Byeongdu Lee, Chieh-Tsung Lo, Christopher Marshall, Stephanie Mucherie, Michael Pellin, Soenke Seifert, Gary Wiederrecht, Randall Winans Properties of highly stable cluster-based model nanocatalysts are studied. Examples on size-selected clusters Au$_{n}$ (n=7-10), Ag$_{n}$ (n=15-19), Pt$_{n}$ (n=8-10) and 1-3 nm particles supported on thin oxide film coated flat and mesoporous supports address cluster stability under realistic reaction conditions, selective stabilization of particles and their reactivity. In situ GISAXS allows for correlation of catalyst performance with its size and shape of the catalyst. Pt-cluster based catalyst supported on mesoporous membranes were tested in a commercial tester in oxidative dehydrogenation of propane and exhibited excellent propane conversion and superb selectivity towards propene production at moderate temperatures, 400-550 $^{o}$C with and without SnO promoter. Au and Ag catalysts were tested in ethylene and propylene oxidation, showing an onset of the reactivity between 160-200 $^{o}$C. [Preview Abstract] |
Tuesday, March 6, 2007 3:06PM - 3:18PM |
L18.00004: Computational studies of small neutral vanadium oxide clusters and their reactions with sulfur dioxide Elena Jakubikova, Sheng-Gui He, Yan Xie, Yoshiyuki Matsuda, Elliot Bernstein Vanadium oxide is a catalytic system that plays an important role in the conversion of SO$_{2}$ to SO$_{3}$. Density functional theory at the BPW91/LANL2DZ level is employed to obtain structures of VO$_{y}$ (y=1,{\ldots},5), V$_{2}$O$_{y}$ (y=2,{\ldots},7), V$_{3}$O$_{y}$ (y=4,{\ldots},9), V$_{4}$O$_{y}$ (y=7,{\ldots},12) and their complexes with SO$_{2}$. BPW91/LANL2DZ is insufficient to describe properly relative V-O and S-O bond strengths of vanadium and sulfur oxides. Calibration of theoretical results with experimental data is necessary to compute enthalpies of reactions between V$_{x}$O$_{y}$ and SO$_{2}$. Theoretical results indicate SO$_{2}$ to SO conversion occurs for oxygen-deficient clusters and SO$_{2}$ to SO$_{3}$ conversion occurs for oxygen-rich clusters. Subsequent experimental studies confirm the presence of SO in the molecular beam as well as the presence of V$_{x}$O$_{y}$ complexes with SO$_{2}$. Some possible mechanisms for SO$_{3}$ formation and catalyst regeneration for solids are also suggested. [Preview Abstract] |
Tuesday, March 6, 2007 3:18PM - 3:30PM |
L18.00005: Unbiased studies on structural and electronic properties of gold clusters with up to 58 atoms Yi Dong, Michael Springborg, Ingolf Warnke Isolated neutral Au$_N$ clusters are studied using a parameterized density-functional tight-binding method combined with genetic algorithms for $N$ from 2 up to 58. Various descriptors are used in analysing the results, including stability, shape, and similarity functions, as well as radial distances of the atoms and the orbital energies, all as functions of $N$. Based on a harmonic approximation, also the heat capacity of the Au clusters are studied as a function of temperature. [Preview Abstract] |
Tuesday, March 6, 2007 3:30PM - 3:42PM |
L18.00006: Quantized Ferromagnetism in Free Cobalt and Iron Clusters Xiaoshan Xu, Shuangye Yin, Ramiro Moro, Anthony Liang, John Bowlan, Walt A. de Heer The magnetic moments $\mu$$_{N}$ for cobalt clusters \textit{Co}$_{N}$ (20$\le N\le $200) measured in a cryogenic molecular beam are found to be quantized both in the ground state: $\mu _{N}\sim $2$N$ยต$_{B}$ and in the metastable excited state: $\mu _{N}^{\ast }\sim N\mu _{B}$ in contrast with the bulk where it is fractional: $\mu _{N=\infty }$=1.7$N\mu _{B}$. For $N$=30, the ionization potentials of the excited state is about 0.1 eV lower than of the ground state while this difference diminishes with increasing size, which implies that the two states become degenerate at large sizes. The evolution from localized moments in small clusters to itinerant moments in the bulk appears to be related to the closing of this energy gap which results in a fluctuating ground state. These effects can be understood in terms of the Falicov-Kimball model. Two states are also observed in iron clusters, with $\mu _{N}\sim $3$N\mu _{B}$ for \textit{Fe}$_{N}$, and $\mu _{N}^{\ast }$ $\sim N\mu _{B}$ for \textit{Fe}$_{N}^{\ast }$ (20$\le N\le $150). [Preview Abstract] |
Tuesday, March 6, 2007 3:42PM - 3:54PM |
L18.00007: On the Stability of Sodium-Tin Zintl Ions in Gas phase experiments A. Reber, J.W. Garbus, S.N. Khanna, W.J. Zheng, O.C. Thomas, J.M. Nilles, K.H. Bowen A synergistic effort combining negative ion photoelectron spectroscopy of Na$_{n}$Sn$_{m}^{-}$ clusters along with the first principles electronic structure studies has been used to demonstrate that Zintl ions found in solutions also exist as stable species in free clusters. The theoretical investigations are carried out within a gradient corrected density functional approach. Our studies on Na$_{n}$Sn$_{4}^{-}$ clusters where n=0-4 and NaSn$_{m}^{-}$ clusters where m=4-7 show that Na$_{3}$Sn$_{4}^{-}$ is a very stable cluster marked by a distorted tetrahedral tin core and can be regarded as (Na+)$_{4}$(Sn$_{4})^{-4}$ gas phase analogue of the Na:Sn tetrahedral Zintl phase. In addition, the NaSn$_{5}^{-}$ cluster is shown to be the most abundant species in the mass spectrum in the NaSn$_{m}^{-}$ series and its stability can be reconciled with Sn$_{5}^{2-}$ Zintl ions. The existence of stable Zintl ions in the gas phase can provide an alternate approach to look for possible Zintl phases. [Preview Abstract] |
Tuesday, March 6, 2007 3:54PM - 4:06PM |
L18.00008: On the stability and vibrational properties of super-polyiodides Naiche Jones, J. Ulises Reveles, Shiv Khanna We had earlier shown that a new class of polyhalides can be formed by combining the Al$_{13}$ super-halogen with the conventional halogen, I. Experimental reactivity studies demonstrate that the new super-polyhalides, Al$_{13}$I$_{x}^{-}$, exhibit pronounced stability for even numbers of I atoms. Theoretical investigations probing the geometry and the electronic structure reveal that the enhanced stability is associated with pairs of I atoms occupying the on-top sites around the Al$_{13}^{-}$ core. We had also demonstrated another series, Al$_{14}$I$_{x}^{-}$, that exhibits stability for odd numbers of I atoms. In this work we have examined the vibrational properties of the new super-polyiodides using gradient corrected density functionals. It is shown that the low frequency modes involve motion of the central Al and that the geometrical progressions with high iodine coverage can be understood in terms of these vibrations. [Preview Abstract] |
Tuesday, March 6, 2007 4:06PM - 4:18PM |
L18.00009: New Assemblies Combining Super-halogens and Super-alkalis Shiv Khanna, Arthur Reber, A. Welford Castleman Jr. An Al$_{13}$ cluster has been shown to exhibit behaviors reminiscent of halogen atoms with an electron affinity comparable to a Cl atom while molecular units like K$_{3}$O, called superalkalis, are known to have low ionization potentials. We have carried out first principles electronic structure calculations to examine the stability and the electronic properties of compound clusters formed by combining super halogens with superalkalis. An Al$_{13}$K$_{3}$O unit is shown to be a strongly bound ionic molecule that can be assembled into stable cluster superatom assemblies of composition (Al$_{13}$K$_{3}$O)$_{n}$. It will be shown that the individual clusters maintain their identity during the growth. The nature of the super-assemblies and their electronic properties will be highlighted. [Preview Abstract] |
Tuesday, March 6, 2007 4:18PM - 4:30PM |
L18.00010: Nano-Assemblies using Designer Clusters A.C. Reber, M. Qian, S.N. Khanna, A.W. Castleman Jr., A. Sen, K.M. Davis, S.J. Peppernick, A. Ugrinov, M.D. Merritt It is shown that a new procedure that combines studies on clusters in gas phase, theoretical investigation of the stability patterns, and the directed assembly in solutions can enable synthesis of nano-assemblies where the building blocks are designer clusters identified in gas phase. As a demonstration of its viability, we first examine As$_{7}$K$_{3}$ as a potential building block through gas phase molecular beam experiments starting from a dispersed mixture of bulk arsenic and potassium. Combining the experimental results with first principles electronic structure calculations, we identify As$_{7}$K$_{3}$ species as a uniquely stable Zintl entity that could affect self-assembly. Through directed assembly, we report success in synthesizing and characterizing a lattice of analogous super-cluster assembled material crystallized from the liquid phase. Electronic structure calculations on the nanoassembled material indicate that it is a wide band gap semiconductor. [Preview Abstract] |
Tuesday, March 6, 2007 4:30PM - 4:42PM |
L18.00011: Optical and magnetic excitations in small transition-metal clusters using TDDFT Micael Oliveira, Fernando Nogueira, Angel Rubio Magnetic properties of transition-metal clusters have been the subject of intensive study in the last decades both theoretical and experimentaly. In particular, the importance of noncollinear effects and spin-orbit coupling in those systems has recently gained great interest. In this work we use time dependent density functional theory (TDDFT) to study optical and magnetic excitations of small transition-metal clusters. In particular, we investigate the role of non-collinear magnetism and spin-orbit coupling in such phenomena. We present some results concerning the linear response calculations and show how noncollinear effects and spin-orbit coupling modify the optical and/or magnetic spectra (including the natural magnetic dichroism spectroscopy). We will discuss further line of research that we are conducting to understand the size dependence magnetic response of this clusters for potential technological applications. The calculations are done within a real-time real-space TDDFT framework using the Octopus code. [Preview Abstract] |
Tuesday, March 6, 2007 4:42PM - 4:54PM |
L18.00012: Methanol Formation from Carbon Monoxide and Hydrogen on Neutral Nb$_{8}$ Clusters in the Gas Phase Yan Xie, Sheng-Gui He, Elliot Bernstein Reactions of neutral V$_{n}$, Nb$_{n}$, and Ta$_{n}$ metal clusters ($n \le $ 11) with (CO + H$_{2})$/He mixed gases and CH$_{3}$OH/He in a flow tube reactor (P $\sim $ 14 Torr) are studied by time of flight mass spectroscopy. Metal clusters are generated by 532 nm laser ablation and reactants and products are ionized by low fluence ($\sim $200 $\mu $J/cm$^{2})$ 193 nm excimer laser light. Nb$_{n}$ clusters exhibit strong size dependent reactivity in reactions both with CO + H$_{2}$ and CH$_{3}$OH compared with V$_{n}$ and Ta$_{n }$ clusters. A remarkably strong mass peak Nb$_{8}$COH$_{4}$ is observed in the reaction of Nb$_{n}$ clusters with the mixed gases CO + H$_{2}$ at various concentration of H$_{2}$. This suggests a stable, low energy CH$_{3}$OH structure may form on an Nb$_{8}$ cluster. Methanol formation is not found on other Nb$_{n}$ ($n \ne $ 8), V$_{n}$, and Ta$_{n}$ clusters. In reactions of CH$_{3}$OH with metal clusters $M_{n }(M $= V, Nb, Ta, $n$ = 3-11), molecularly adsorbed products ($M_{n}$CH$_{3}$OH) are only observed on Nb$_{8}$ and Nb$_{10}$, whereas dehydrogenated products ($M_{n}$CO) are observed for all other clusters. This observation supports the suggestion that CH$_{3}$OH can be formed on Nb$_{8}$ in the reaction of Nb$_{n}$ with CO + H$_{2}$. Reaction mechanisms are discussed based on the experimental results in this work and those in the literature. Theoretical calculations are carried out to confirm our experimental results and suggested reaction mechanisms. [Preview Abstract] |
Tuesday, March 6, 2007 4:54PM - 5:06PM |
L18.00013: Potential for Strong Pairing and High Transition Temperatures in Metallic Nanoclusters Amber Post, Louis Bloomfield Studies of atomic clusters containing tens or hundreds of atoms have gained much interest in recent decades because of their potential to bridge the gap between isolated atoms and bulk systems. Notable results include the observation of a shell structure$^{1}$ similar to that found in electronic shells of single atoms. Theoretical calculations$^{2}$ show that certain levels within this shell structure allow for strong Cooper pairing. These calculations also show that the particular shell levels, which are realistically attainable, have high density of states in the HOS and LUS levels and could show substantially higher values of the superconducting transition temperature $T_{C}$ than are observed in the bulk material. At temperatures near $T_{C}$, the onset of strong pairing can be experimentally observed by an increase in the minimum excitation energy of the particular cluster. Our group will first look for this energy increase in Al clusters at around 90K, the predicted $T_{C}$ for Al clusters of interest. Here we present a progress report on Al and describe future work. \newline \newline $^{1}$ W. Knight, K. Clemenger, W. de Heer, W. Saunders, M. Chou, and M. Cohen, Phys. Rev. Lett. {\bf 52}, 2141 (1984). \newline $^{2}$V. Z.Kresin and Y. N. Ovchinnikov, Phys. Rev. B {\bf 74}, 024514 (2006). [Preview Abstract] |
Tuesday, March 6, 2007 5:06PM - 5:18PM |
L18.00014: Electron pairing in pure Niobium and Niobium alloy clusters Anthony Liang, Xiaoshan Xu, Shuangye Yin, John Bowlan, Walt de Heer Electrons in pure niobium and in niobium alloy clusters are ferroelectric at low temperatures. The ferroelectric effect is enhanced for niobium clusters doped with non-magnetic metals and reduced when doped with magnetic atoms. The effect is enhanced (reduced) for clusters with an even (odd) total number of valence electrons. For specific alloy clusters the ferroelectric state persists up to room temperature. Ferroelectricity in these clusters and superconductivity in the corresponding bulk appear to be related, with similar transitions temperatures and similar responses to specific impurities. The spontaneous polarization of a ground state involving a Cooper pair explains the observations. [Preview Abstract] |
Tuesday, March 6, 2007 5:18PM - 5:30PM |
L18.00015: ABSTRACT WITHDRAWN |
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