Bulletin of the American Physical Society
2006 APS March Meeting
Monday–Friday, March 13–17, 2006; Baltimore, MD
Session V11: Focus Session: Aerosols, Clusters, Droplets: Physics and Chemistry of Nanoobjects V: Metal Clusters II |
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Sponsoring Units: DCP Chair: Michael Duncan, University of Georgia Room: Baltimore Convention Center 303 |
Thursday, March 16, 2006 11:15AM - 11:51AM |
V11.00001: Structure Determination of Noble Metal Clusters by Trapped Ion Electron Diffraction Invited Speaker: The structures of noble metal cluster ions have been studied by the recently developed technique of trapped ion electron diffraction (TIED)$^{1}$. In brief, cluster ions are generated by a magnetron sputter source and injected into a cooled (95~K) quadrupole ion trap. After mass selection and thermalization, the trapped ions are irradiated with a 40~keV electron beam. The resulting diffraction pattern is integrated with a CCD detector. The assignment of the structural motif is done via a comparison of the experimental and simulated scattering function, calculated from density functional theory structure calculations. \newline The structures of mass selected silver cluster cations Ag$_{19}^{+}$, Ag$_{38}^{+}$, Ag$_{55}^{+}$, Ag$_{59}^{+}$, Ag$_{75}^{+}$ and Ag$_{79}^{+}$ have been investigated$^{2}$. The resulting experimental data are best described by structures based on the icosahedral motif, while closed packed structures could be ruled out. Additionally, we present a comparison of the structures of Cu$_{20}^{+/-}$, Ag$_{20}^{+/-}$ and Au$_{20}^{+/-}$. Our findings show unambiguously that the structure of Au$_{20}^{-}$ is predominantly given by a tetrahedron in agreement with the results of L.S. Wang et al.$^{3}$ In contrast, structures of Ag$_{20}^{-}$ and Cu$_{20}^{-}$ based on the icosahedral motif agree best with the experimental data. Small structural differences between the charge states are observed. The possibilities and limitations of the TIED method are discussed. \newline (1) M. Maier-Borst, D. B. Cameron, M. Rokni, and J. H. Parks, Physical Review A \textbf{59} (5), R3162 (1999); S.~Kr\"{u}ckeberg, D. Schooss, M. Maier-Borst, and J. H. Parks, Physical Review Letters \textbf{85} (21), 4494 (2000). \newline (2) D. Schooss, M.N. Blom, B. v. Issendorff, J. H. Parks, and M.M. Kappes, Nano Letters \textbf{5} (10), 1972 (2005). \newline (3) J. Li, X. Li, H. J. Zhai, and L. S. Wang, Science \textbf{299}, 864 (2003) [Preview Abstract] |
Thursday, March 16, 2006 11:51AM - 12:03PM |
V11.00002: Electric deflection studies of rhodium clusters Mark Knickelbein, Martin Beyer The electric susceptibilities of rhodium clusters (Rh$_{n}$, n=5-32) have been studied via a DC molecular beam deflection technique. It is observed that all clusters are high-field seekers, indicating that the induced dipole moments are larger than any small permanent dipole moments that may be present. The per atom polarizabilities determined from the beam deflections exceed the classical value. Rh$_{7}$ and Rh$_{10}$ exhibit per- atom polarizabilities that are both anomalously large and temperature dependent. Peak broadening is most pronounced for Rh$_{7}$, indicating the presence of a small permanent dipole moment. It is proposed that the anomalous polarizabilities exhibited by Rh$_{7}$ and Rh$_{10}$ are a consequence of their being dynamic Jahn-Teller molecules. [Preview Abstract] |
Thursday, March 16, 2006 12:03PM - 12:15PM |
V11.00003: On the Possibility of Multiple Valence Superatoms Shiv Khanna, J. Ulises Reveles, A. Welford Castleman Jr., Dennis Bergeron, Patrick Roach It was recently demonstrated that an Al$_{13}$ cluster behaves like a halogen atom while an Al$_{14}^{ }$ cluster has properties analogous to an alkaline earth atom in gas phase clusters containing aluminum and iodine atoms. These observations, together with findings that Al$_{13}^{-}$ is inert like a rare gas atom, have reinforced the idea that chosen clusters can exhibit chemical behaviors reminiscent of atoms in the periodic table, offering the exciting prospect of a new dimension of the periodic table formed by cluster elements, called the superatoms. As the properties of the clusters can be controlled by size and composition, the superatoms offer the potential to create unique compounds with tailored properties. In this paper, we provide evidence of a new class of superatoms that exhibit multiple valences, like some of the atoms in the periodic table, and hence have the potential to form stable compounds when combined with diverse atoms. [Preview Abstract] |
Thursday, March 16, 2006 12:15PM - 12:27PM |
V11.00004: Geometrical and electronic properties of sodium, copper, and silver clusters: A comparative study Mingli Yang, Koblar A. Jackson, Julius Jellinek The structures of sodium, copper, and silver clusters with up to 20 atoms are investigated using density functional theory computations. Utilizing an extensive unbiased search procedure, we first find a large number of low-energy isomers of Cu clusters. The structures of these isomers are then reoptimized for Na and Ag clusters. We find a strong propensity to form similar conformations for clusters of these three elements. In most cases their energetically preferred forms have the same packing. As the cluster size increases, it changes from planar (less than 7 atoms) to layered (7 to 16 atoms) and eventually to compact (17 to 20 atoms). The similarities and differences between the clusters of the three elements will also be characterized in terms of their electronic properties such as ionization potential, HOMO-LUMO gap, dipole moment, and polarizability. [Preview Abstract] |
Thursday, March 16, 2006 12:27PM - 12:39PM |
V11.00005: Self-Similar Size Distribution of Atom Clusters in Cooling Vapors Yong W. Kim, Hedok Lee, Paul Belony, Jr. Aluminum atom clusters of nanometer dimensions grow out of dense vapors. We have both measured and modeled their formation process by first creating dense atomic vapors as remnants of a 3-D laser-produced plasma plume from a solid aluminum target. By real-time diagnosis, we have fully characterized the plume into more than 140,000 vapor cells. [Kim, Lee, \textit{Rev. Sci. Instr. }\textbf{75}, 3953 (2004)] Clustering of atoms in each cell has been followed during the cooling of the vapors by numerical simulation of atom-atom and atom-cluster collisions. The distribution of clusters by size evolves as the vapor cools, and is sensitively dependent on the initial atom density in each plasma cell. When the population and cluster size are rescaled as fractions of their respective maximal values, the distribution functions all collapse into a single functional form regardless of the time in the evolution or the initial value of the vapor density. The maximal population decreases while the maximal size grows over time. The cluster size distribution for the plasma plume is found by summing the distributions from all plasma cells. Independently, the clusters are captured onto electron microscope grids and size analyzed by means of transmission electron microscopy. The computed result is in excellent agreement with the measured histogram of clusters by size. The agreement is absolute, indicating the Brownian motion nature of cluster-cluster interaction. [Preview Abstract] |
Thursday, March 16, 2006 12:39PM - 1:15PM |
V11.00006: On Metal to Insulator Transitions in Bivalent Metal Clusters Invited Speaker: We discuss the issue of metal to insulator transitions in bivalent metal clusters, in view of our new photoelectron spectroscopy (PES) studies on Zn$_{n}^{-}$ clusters in the size range of n=3-117. We show that zinc clusters exhibit a distinct transition in their electronic structure characteristics as a function of their size. At small sizes up to $n$=18 the clusters follow the Bloch-Wilson picture of the development of a metal from closed-shell atoms, exhibiting a gradual decrease of the gap between the fully occupied $s $band and the empty $p $band. For large sizes (\textit{n$\ge $}32 ) the band overlap allows the valence electrons to fully delocalize. This leads to an almost perfect free-electron density of states, as is demonstrated by discussing the spectra in the light of standard free-electron models and by comparison to the results obtained on sodium clusters. These results will be compared with the PES of Hg$_{n}^{-}$ and Mg$_{n}^{-}$ clusters. [Preview Abstract] |
Thursday, March 16, 2006 1:15PM - 1:27PM |
V11.00007: Quantized Ferromagnetic Moments of Free Cobalt Clusters Xiaoshan Xu, Shuangye Yin, Anthony Liang, Walter de Heer The magnetic moments $\mu $(N) of free Co{\_}N(20$<$=N$<$=200) clusters has been measured in a cryogenic molecular beam. Besides the ground states~ with~ $\mu $(N)/N$\sim $ 2 $\mu $B, an electronic state has been found in all clusters studied. The magnetic moments $\mu $*(N)/N of this state is approximately 1/2 $\mu $(N)/N. Ionization potential of Co clusters at each of the two states are determined from photo-ionization efficiency measurements. The ionization potentials for the excited states are systematically lower than that for ground states by about 100meV for small clusters, and merge for larger clusters(N$>$100). This suggests that molecular magnetism of small clusters evolves to itinerant (band) ferromagnetism in the bulk ($\mu $(N)/N$\sim $1.7 $\mu $B) when the energy gap between these two states vanishes. [Preview Abstract] |
Thursday, March 16, 2006 1:27PM - 1:39PM |
V11.00008: Magnetism in Gold Nano-clusters R.J. Magyar, V. Mujica, C. Gonzalez, M. Marquez Intrinsic magnetism in thiolated and unthiolated gold nano-clusters is explored from a first-principles perspective. The computed electronic structures elucidate the relevant mechanism whereby these nano-clusters develop a permanent magnetic moment. Polarized ground states are favorable due to surface paramagnetism, and bare clusters are expected to be magnetic for Au38 and larger. How the thiolates effect the magnetic moment is also considered. [Preview Abstract] |
Thursday, March 16, 2006 1:39PM - 1:51PM |
V11.00009: Anomalous magnetic enhancement by doping Mn in Co clusters Shuangye Yin, Ramiro Moro, Xiaoshan Xu, Walter de Heer Magnetic moments of Co$_N$Mn$_M$ and Co$_N$V$_M$ clusters (with Mn and V concentration less than 30\%) are measured using Stern-Gerlach deflection method in molecular beams. Their average moments per atom as a function of impurity concentrations are studied and compared with bulk. Both Co$_N$Mn$_M$ and Co$_N$V$_M$ clusters have higher average moments than their bulk counterpart due to reduced sizes. The average moments for Co$_N$Mn$_M$ clusters are found to increase with Mn concentration, opposite to that of CoMn bulk. The enhancement effect is found to be independent of cluster size and composition. Meanwhile, Co$_N$V$_M$ clusters show reduction of average moments with V doping, consistent with what are expected in CoV bulk. The results are discussed within the virtual bound states model. [Preview Abstract] |
Thursday, March 16, 2006 1:51PM - 2:03PM |
V11.00010: Analysis of the response of atomic clusters to static electric fields in terms of position-dependent polarizabilities Koblar A. Jackson, Mingli Yang, Julius Jellinek To explore in detail the response of atomic clusters to external electric fields, we have developed a method to compute position-dependent polarizabilities (PDP's). The essence of the method is to partition the overall cluster dipole into local, atom-centered contributions. The local moments are naturally decomposed further into charge-transfer and dipole components. This decomposition furnishes added insight into the response behavior of the clusters. By tracking the changes in the local moments with an external field, we arrive at the PDP's. In this talk we will present the details of the method and will compare and contrast different approaches to computing the local moments. We will also discuss results for Na$_{n}$, Si$_{n}$ and Ar$_{n}$ as a function of cluster size. These results show strong qualitative similarities in the response of Na$_{n}$ and Si$_{n}$ clusters, including clear evidence for metallic screening of the cluster interiors. [Preview Abstract] |
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