Bulletin of the American Physical Society
2008 APS March Meeting
Volume 53, Number 2
Monday–Friday, March 10–14, 2008; New Orleans, Louisiana
Session L21: Focus Session: Clusters, Cluster Assemblies, Nanoscale Materials V |
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Sponsoring Units: DCP Chair: Penee Clayborne, Virginia Commonwealth University Room: Morial Convention Center 213 |
Tuesday, March 11, 2008 2:30PM - 2:42PM |
L21.00001: Ab Initio Simulations of Nano-Diamond Surface Reconstruction William Mattson, Radhakrishnan Balu We have simulated with in the Density Functional Theory (DFT) spherically cut nano-diamonds from bulk diamond at ambient conditions. The 2.6 nanometer diameter sphere is then allowed to relax at 250K and surface reconstruction is observed. Four hemispherical fullerene like regions form on the surface and while the interior maintains the diamond structure, it undergoes compression equivalent to over 30GPa in the bulk. Results of dynamic shearing will be presented. [Preview Abstract] |
Tuesday, March 11, 2008 2:42PM - 2:54PM |
L21.00002: TiO$_{2}$ nanostructures prepared by ferrocene/cobalt catalyst agents M.E. Gomez, J.C. Caicedo, G. Zambrano, A-M. Lazar, D. Chaumont, Y. Lacroute, M. Sacilotti We present the growth and characterization of TiO$_{2}$ nanocrystals. Nanostructured growth is obtained in a low-pressure CVD system by using an organometallic precursor Ti(OC$_{3}$H$_{7})_{4}$ as both the Ti and O source catalyzed by both ferrocene (an organometallic precursor) and cobalt metallic clusters prepared by the microwave-assisted polyol method. Two kinds of TiO$_{2}$ structures were obtained in the cobalt clusters: a) pine-tree like (with short-leaf structure) and b) long-leaf structures as large as a few micrometers in size and both under 10-nm in thickness. Long-leaf TiO$_{2}$ structures were grown at cobalt grain boundaries. For the growth conditions utilized, the TiO$_{2}$ structures are composed of both anatase and rutile crystallographic phases. [Preview Abstract] |
Tuesday, March 11, 2008 2:54PM - 3:06PM |
L21.00003: Metal cluster anions produced by attachment of slow electrons: Evaporative cooling, cluster energetics, and restructuring of the abundance spectra Vitaly Kresin, Roman Rabinovitch, Chunlei Xia Metal clusters are able to attach low-energy electrons with very large cross sections by capturing them in a strong long-range polarization potential. But little information has been available about the last stage of the collision process: what happens to the energy deposited by the captured electron, and are the cluster size distributions modified? We have carried out measurements of the mass spectra of negative sodium cluster ions born in the electron-cluster interaction region. Importantly, the arrangement allowed us to monitor the parent and the daughter cluster beams simultaneously. It is found that the electron affinity energy is quickly thermalized and is sufficient to cause rapid evaporative cooling. As a result, the magic numbers shift from Na$_{n}$ to $\mbox{Na}_{\mbox{n-1}}^{\mbox{-}} $, and a host of other significant changes in the abundance structure are observed, compared to the parent cluster beam. These are well reproduced by a detailed analysis based on the evaporation cascade model, and yield new information about cluster binding energies. [Preview Abstract] |
Tuesday, March 11, 2008 3:06PM - 3:18PM |
L21.00004: Anion Photoelectron Spectroscopy and Density Functional Investigation of Diniobium-Carbon Clusters P.A. Clayborne, K.L. Knappenberger, Jr., J.U. Reveles, M.A. Sobhy, C.E. Jones, Jr., U.U. Gupta, I. Iordanov, J. Sofo, A.W. Castleman, Jr., S.N. Khanna Experimental photoelectron and computational results show diniobium-carbon (Nb$_{2}$C$_{n})$ clusters to coexist in multiple structural isomers: three-dimensional geometries, planar rings and linear chains. Three-dimensional clusters having up to five carbons are formed preferentially with Nb-Nb bonding, whereas only Nb-C bonding is observed experimentally at six carbons. Clusters consisting of an odd number of atoms are also observed with linear geometries. The larger binary clusters (n $\ge$ 7) display properties similar to pure carbon clusters. We provide evidence for niobium substitution of carbon atoms. [Preview Abstract] |
Tuesday, March 11, 2008 3:18PM - 3:30PM |
L21.00005: Towards Artificial Molecules: Metallodielectric Clusters Dina Aronzon, Vinothan Manoharan, Jonathan Fan Recent advances in the synthesis of metallic colloids have allowed for an explosion in research into their optical properties. It is now possible to synthesize solid metallic colloids, core-shell colloids that mix different metals, and core shell colloids of metallodielectrics. In this talk I propose a mechanism for further exploring the optical properties of such materials by producing clusters of metallodielectric colloids and studying the relationship between the structure and composition of a colloidal cluster and its optical response. To this end, we synthesized a number of solutions, each of clusters consisting of a different average number of colloids. By doing this, we hope to study the effects of different dielectrics and near neighbor interactions on the plasmonic resonances of the metallic shells in the colloid. In the future, we hope to produce and study high yield, pure samples, both in solution and as photonic crystals. All of these options provide new ways of producing specific optical resonances that can be used in sensors, spectroscopy, optical triggers, and many other applications. [Preview Abstract] |
Tuesday, March 11, 2008 3:30PM - 3:42PM |
L21.00006: Quantum transport in molecular electronic devices described with complex source and sink potentials Francois Goyer, Ali Goker, Matthias Ernzerhof We present a non-Hermitian model Hamiltonian containing complex potentials [1,2] that is devised to study ballistic transport in molecular electronic devices (MEDs). The complex potentials replace semi-infinite contacts and act as source and sink of probability current density. This approach is rigorous in the sense that the exact wave function is recovered in the interior of the MED. We employ this technique to calculate the conductance through certain prototypical MEDs [3]. We also extend this method [4] such that we can go beyond the one- electron picture by constructing two-electron states explicitly. We present results for simple model system described by Hubbard-type Hamiltonians. The impact of electron correlation effects on the molecular conductance is discussed. [1] F. Goyer, M. Ernzerhof, M. Zhuang, JCP, 126, 144104 (2007). [2] M. Ernzerhof, JCP, to appear nov. 2007. [3] M. Ernzerhof, H. Bahmann, F. Goyer, M. Zhuang, P. Rocheleau, J. Chem. Theory Comput., 2, 1291 (2006); M. Ernzerhof, M. Zhuang, P. Rocheleau, JCP, 123, 134704 (2005). [4] A. Goker, F. Goyer, M. Ernzerhof, work in pogress. [Preview Abstract] |
Tuesday, March 11, 2008 3:42PM - 3:54PM |
L21.00007: Charge carrier solvation on a polymer chain revealed in \textit{ab initio} computations Michael Mayo, Yuri Gartstein When an excess charge carrier (electron or hole) is added to a semiconducting polymer chain in vacuum, it is well known that the carrier may self-trap into a polaronic state accompanied by a self-consistent localized bond alternation pattern. A different mechanism of self-localization is a solvation of the charge carrier expected to take place when the polymer chain is immersed in a polar medium (such as a common solvents) - in this case a self-consistent pattern of the orientational polarization is formed around a localized charge. The goal of our study is to identify this solvation effect within \textit{ab initio} computations. Using long carbon atom chains (both hydrogen terminated and rings) as model systems, we employ the hybrid B3LYP density functional within the DFT and the Polarizable Continuum Model to find the resulting electronic level structure and atomic charge densities. Our results clearly show trends towards excess charge self-localization due to the solvation. We suggest that this effect may be of importance for various semiconductor nanostructures in polar environments. [Preview Abstract] |
Tuesday, March 11, 2008 3:54PM - 4:06PM |
L21.00008: Nanotubes in polar environments: Solvated charge carriers and their dynamics Geoffrey Ussery, Yuri Gartstein Excess charge carriers on semiconducting nanotubes immersed in sluggish polar environments (such as common solvents) can undergo self-localization into polaronic states whose properties are profoundly different from the free band states. We explore such solvated states within the adiabatic continuum framework using a simplified picture of an electron or a hole confined to a cylindrical surface in the 3D polar medium. At the static level, the binding energy of a polaron is evaluated and found to be a sizable fraction ($\sim$ 0.3) of the corresponding Wannier- Mott exciton binding energy, which is expected to substantially decrease the thermal activation energy for the exciton dissociation. We discuss the diffusion and mobility of polarons caused by the dielectric fluctuations of the medium and applied electric fields, as well as the local dielectric relaxation modes in the vicinity of the polaron. We also discuss the electronic (optical) transitions between the localized electronic states within a self-consistent potential well due to the orientational polarization pattern. \newline [1] Yu.N. Gartstein, Phys. Lett. A 349, 377 (2006). \newline [2] Yu.N. Gartstein, T.D. Bustamante, S. Ortega Castillo, J. Phys.: Cond. Matter 17, 156210 (2007). [Preview Abstract] |
Tuesday, March 11, 2008 4:06PM - 4:18PM |
L21.00009: Fluorescent silver atom clusters in DNA hairpin loops Patrick O'Neill, Elisabeth Gwinn, Dirk Bouwmeester, Deborah Fygenson We synthesize fluorescent clusters of silver atoms on DNA hairpins, and systematically vary the loop region to probe the effects of DNA sequence and structure on the optical properties and chemical stability of the Ag clusters. We find that these novel fluorophores only form on the single stranded hairpin loop, have Stoke's shifts ranging from 60nm to 120nm and can be tuned to emit at wavelengths ranging from 525nm to 670nm. Furthermore, certain DNA geometries support strong excitation of visible fluorescence by 260-280nm light. [Preview Abstract] |
Tuesday, March 11, 2008 4:18PM - 4:30PM |
L21.00010: H$_2$O Nucleation Around Noble Metal Cations Patrizia Calaminici, Pavel Oropeza Alfaro, Martin Juarez Flores, Andreas K\"oster, Marcela Beltran, J. Ulises Reveles, Shiv N. Khanna First principle electronic structure calculations have been carried out to investigate the ground state geometry, electronic structure and binding energy of noble metal cations (H$_2$O)$_n^+$ clusters containing up to 10 H$_2$O molecules. The calculations are performed with the density functional theory code deMon2k [1]. Due to the very flat potential energy surface of these systems special care to the numerical stability of energy and gradient calculation must be taken.Comparison of the results obtained with Cu$^+$, Ag$^+$ and Au$^+$ will be shown. This investigation provides insight into the structural arrangement of the water molecules around these metals and a microscopic understanding of the observed incremental binding energy in the case of the gold cation based on collision induced dissociation experiments. \newline [1] A.M. K\"oster, P. Calaminici, M.E. Casida, R. Flores-Moreno, G. Geudtner, A. Goursot, T. Heine, A. Ipatov, F. Janetzko, J. Martin del Campo, S. Patchkovski, J.U. Reveles, A. Vela and D.R. Salahub, deMon2k, The deMon Developers, Cinvestav, 2006 [Preview Abstract] |
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