Bulletin of the American Physical Society
APS March Meeting 2011
Volume 56, Number 1
Monday–Friday, March 21–25, 2011; Dallas, Texas
Session D41: LeRoy Apker Prize Session: Clusters and Nanoscale Systems |
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Sponsoring Units: DCP Chair: Jim Lisy, University of Illinois at Urbana-Champaign Room: A115/117 |
Monday, March 21, 2011 2:30PM - 3:06PM |
D41.00001: LeRoy Apker Award Talk: Self-Assembly of DNA-Functionalized Nanoparticles Invited Speaker: Nanoparticles tethered with DNA strands can self-assemble into highly organized structures through the bonding of complementary nucleobases. These are promising building blocks for the bottom-up nanotechnology, and computational tools are useful to probe the behaviors of such complex materials. In this talk I will summarize my work on the phase behavior of nanoparticles tethered with a small number of DNA strands, and on the development of theories for the clustering and self-assembly kinetics of a specific case. Due to a separation of repulsion and attraction length scales, these nanoparticles exhibit an interesting hierarchy of phases made up of multiple interpenetrating structures. [Preview Abstract] |
Monday, March 21, 2011 3:06PM - 3:18PM |
D41.00002: DNA-driven assembly of phospholipid bilayer nanodiscs Nienke Geerts, Paul A. Beales, T. Kyle Vanderlick Phospholipid nanodiscs are a rare form of stable lipid self-assembly. The discs are formed by allowing lipids to self-assemble in the presence of membrane scaffold proteins (MSP). Each disc contains two MSP, wrapping around the edge of a leaflet of the bilayer. Although nanodiscs have become an important and versatile tool among model membrane systems to functionally reconstitute membrane proteins, they are yet to be utilized as building blocks in material science. However their highly monodisperse nanoscale structure make them ideal for this purpose. Here we report the first superstructures of nanodiscs self-assembled via membrane anchored single stranded DNA. The discs assemble into columnar stacks with high aspect ratio. The MSPs provide another powerful feature, as the His-tags of the protein can be used to attach the discs to colloids or other molecules of interest. This has strong potential for assembly of nanomaterials with greater degrees of complexity. [Preview Abstract] |
Monday, March 21, 2011 3:18PM - 3:30PM |
D41.00003: Orthogonal DNA-colloid Clusters Jesse W. Collins, Vinothan N. Manoharan We experimentally investigate the self-assembly of colloids labelled with different DNA strands into small clusters. We coat 1 micron diameter spheres with 65 base DNA strands having highly specific ``sticky ends.'' Particles with different surface-bound DNA sequences represent different particle ``types.'' We tune the short-ranged, pairwise interactions between some types to be attractive and interactions between other types to be purely repulsive; in this sense, the interactions are orthogonal. The magnitude of attraction (and repulsion) is constant across various types. We control the number and types of colloids at the single particle level, and distinguish the type of each particle from the types of their binding partners within each cluster. In an example experiment, 2 particles of each of 3 different types explore a volume less than 100 picoliters and assemble into equilibrium configurations. We characterize the structures with a microscope and compare observed averages with statistical mechanical predictions. [Preview Abstract] |
Monday, March 21, 2011 3:30PM - 3:42PM |
D41.00004: Size Characterization of Surfactant and Polymer Coated Gold Nanorods Christopher Grabowski, Paul Luchette, Peter Palffy-Muhoray Polarization-dependent dynamic light scattering was conducted on gold nanorods (Au NRs) coated with CTAB (hexadecyltrimethylammonium bromide) in water and coated with 50k MW PS (polystyrene) in toluene. The autocorrelation function of the scattered light intensity was determined for a series of scattering angles under VH and VV scattering geometries. The data were fit to a model of rotational and translational diffusivities. From this fit, we estimate the effective length (L) and diameter of the coated nanorods in solution. Au NRs coated with 50k PS show greatly reduced rotational diffusion compared to CTAB-coated NRs. Since the rotational diffusion coefficient scales as 1/L$^{3}$, this implies significant extension of the grafted PS chains in toluene. We investigate this phenomenon for PS grafted onto Au nanoparticles and nanorods of varying aspect ratio to determine the impact of surface curvature on polymer layer thickness. [Preview Abstract] |
Monday, March 21, 2011 3:42PM - 3:54PM |
D41.00005: A ligand phase transition on nanorods and its effect on their surface forces Asaph Widmer-Cooper, Phillip Geissler Synthesizing nanometer-scale objects with controlled optical and electronic properties is now a relatively straightforward task, however organizing such objects into extended structures that could revolutionize technology remains a challenge, especially for anisotropic particles. Nanorods behave like liquid crystals in solution and can assemble into structures with the rods oriented perpendicular with respect to a substrate upon drying, assemblies that could potentially be used to print nanostructured solar cells and photoelectrochemical devices. Achieving complete control of this process, however, requires detailed understanding of the rod-rod and rod-surface interactions. Like most nanoparticles, CdS nanorods are passivated with ligands to stabilize them from random aggregation in solution. Using molecular dynamics simulations with explicit ligands and solvent we investigate the structure of phosphonic acid ligands on CdS nanorods as a function of temperature and show that they can undergo an ordering transition close to room temperature. We calculate the potential of mean force between the rods and show that this changes the rod-rod interaction from purely repulsive to attractive. This should have a significant effect on their self-assembly behavior. [Preview Abstract] |
Monday, March 21, 2011 3:54PM - 4:06PM |
D41.00006: Chemical Preparation and Characterization of Elemental Cu, Ni, and Cu/Ni Core/Shell Nanoparticles Laura Higgins, Michael Lattanzi, Brian Kelly, Gerald Poirier, Karl Unruh Elemental Ni, Cu, and Cu core/Ni shell nanoparticles have been prepared in a polyol-type process. The elemental nanoparticles were prepared by dissolving NiCl$_{2}\cdot $6H$_{2}$O and/or CuCl$_{2}\cdot $2H$_{2}$O in ethylene glycol (EG) and heating the solution to reflux prior to the addition of NaOH. The resulting precipitate was then extracted, dried, and characterized by scanning electron microscopy (SEM) with elemental analysis, x-ray diffraction (XRD), and in the case of the elemental Ni nanoparticles, vibrating sample magnetometry (VSM) measurements. The best fit lattice parameters obtained from the elemental Ni and Cu nanoparticles were 0.35289(28) and 0.36171(23) nm, respectively, in good agreement with the corresponding bulk values. On the other hand, the measured saturation magnetization of about 49 emu/g was somewhat smaller than the bulk Ni value. In the case of the Cu/Ni nanoparticles, the best fit lattice parameters for the Ni and Cu components of the core/shell structure were 0.35299(26) and 0.36101(10) nm, indicating the formation of an essentially pure Ni shell and a slight amount of Ni incorporation in the Cu core. [Preview Abstract] |
Monday, March 21, 2011 4:06PM - 4:18PM |
D41.00007: Solid-State Homogenization Reactions in Cu Core/Ni shell Nanoparticles Michael Lattanzi, Laura Higgins, Brian Kelly, Gerald Poirier, Karl Unruh Air stable Cu core/Ni shell nanoparticles have been prepared in a polyol-type process by heating an ethylene glycol (EG) solution containing CuCl$_{2}\cdot $2H$_{2}$O and NiCl$_{2}\cdot $6H$_{2}$O to its boiling temperature, adding an appropriate amount of NaOH, and allowing the reaction to proceed at reflux for 30 minutes prior to cooling. The as-prepared nanoparticles were characterized by scanning electron microscopy (SEM) with elemental mapping, x-ray diffraction (XRD), and vibrating sample magnetometry (VSM) measurements. Chemical composition maps of the particles revealed a well-defined core/shell structure consisting of a Cu core about 100-150 nm in diameter surrounded by a Ni shell about 30-40 nm in thickness. XRD measurements indicated that while the Cu core contained a small amount of incorporated Ni, the shell was essentially pure Ni. The solid-state transformation from the as-prepared core/shell structure to an essentially homogeneous Cu-Ni alloy was studied by high temperature VSM and XRD measurements as a function of annealing temperature and time. These measurements reveal that the core/shell structure remains largely intact to temperatures above 400 \r{ }C and that complete homogenization occurs at temperatures above about 600 \r{ }C. [Preview Abstract] |
Monday, March 21, 2011 4:18PM - 4:30PM |
D41.00008: The Discovery of a New Class of Magnetic Superhalogens P. Jena, M.M. Wu, H. Wang, Y. Ko, Q. Wang, Q. Sun, B. Kiran, A. Kandalam, K. Bowen We report the discovery of a new class of magnetic superhalogens and their unusually stable molecular anions. These are formed when a hot plume of manganese atoms is cooled through collisions with an inert gas in the presence of chlorine atoms. The anions, with a composition of (Mn$_{x}$Cl$_{2x+1})^{-}$ (x = 1, 2, 3. . . ), appear as prominent (magic) peaks in mass spectra. Using calculations based on density functional theory and experiments utilizing anion photoelectron spectroscopy, we traced the origin of their unusual stability to the half-filled d-shell of the Mn atoms in anionic clusters and the large electron affinities of their neutral counterparts. The calculated and measured electron affinities are almost twice as high as that of the chlorine atom. However, unlike conventional superhalogens which are non-magnetic and consist of a single metal atom at the core surrounded by halogen atoms, the superhalogens discovered here are magnetic and have (MnCl$_{2})_{x}$ moiety as a core to which a chlorine atom is attached. In addition, our calculations show that Mn atoms carry large magnetic moments and Mn$_{x}$Cl$_{2x+1}$ superhalogen moieties can serve as building blocks of a new category of salts with magnetic properties. [Preview Abstract] |
Monday, March 21, 2011 4:30PM - 4:42PM |
D41.00009: Dielectron Attachment and Hydrogen Evolution Reaction in Water Clusters Robert Barnett, Rina Giniger, Ori Cheshnovsky, Uzi Landman Binding of excess electrons to nano-size water droplets, with a focus on the hitherto largely unexplored properties of doubly-charged clusters, were investigated experimentally using mass spectrometry and theoretically with large-scale first-principles quantum simulations. Doubly-charged clusters were measured in the range of 83 $\le $ n $\le $ 123, with (H$_{2}$O)$_{n}^{-2 }$ clusters found for 83 $\le $ n $<$ 105, and mass-shifted peaks corresponding to (H$_{2}$O)$_{n-2}$(OH$^{-})_{2}$ detected for n $\ge $ 105. Simulations revealed surface and internal dielectron, e$^{-}_{2}$, localization modes and elucidated the mechanism of the reaction (H$_{2}$O)$_{n}^{-2}\to $ (H$_{2}$O)$_{n-2 }$(OH$^{-})_{2}$ + H$_{2}$ (for n $\ge $ 105), which was found to occur via concerted approach of a pair of protons belonging to two water molecules located in the first shell of the dielectron internal hydration cavity, culminating in hydrogen formation 2H$^{+}$ + e$^{-}_{2 }\to $ H$_{2}$. Instability of the dielectron internal localization impedes the reaction for smaller ( n $<$ 105) clusters. [Preview Abstract] |
Monday, March 21, 2011 4:42PM - 4:54PM |
D41.00010: Investigations into aggregate growth dynamics \textit{via in situ} structural quantification of flame synthesized silica nanoparticle aggregates Durgesh Rai, Gregory Beaucage, Jan Ilavsky, Hendrik Kammler, Sotiris Pratsinis Ramified aggregates are formed in many dynamic processes such as in flames. The structures are disordered and present a challenge to quantification. The topological quantification of such nanostructured materials is important to understand their growth processes. Small-angle X-ray scattering (SAXS) is widely used to characterize such nanoparticle aggregates. Recently, we have developed a method for the quantification of topology in aggregated material using SAXS. This methodology will be used to describe topologies from \textit{in-situ} SAXS studies on flame synthesized silica aggregates on millisecond time scales. This is an important step to facilitate understanding of the growth dynamics and the structural rearrangements that occur during flame synthesis. [Preview Abstract] |
Monday, March 21, 2011 4:54PM - 5:06PM |
D41.00011: Influence of Nanoparticles on Fragility and Collective Particle Motion in Polymer Glass-Formation Jack Douglas, Francis Starr We investigate the impact of nanoparticles (NP) on glass-formation in polymer melts by molecular dynamics simulation. The NP cause significant changes in both fragility and the average length of string-like cooperative motion, where the extent of the effect depends on the NP-polymer interaction and NP concentration. These dynamical changes can be interpreted via the Adam-Gibbs (AG) theory if we assume the strings represent the abstract cooperatively rearranging regions (CRR) of the AG model, whose basic assumptions are reviewed. Molecular additives are also effective at altering the fragility of glass-formation and extent of string-like collective motion so the modulation of fragility and cooperative motion with additives seems to be a general effect. We find that the fragility of glass formation is mainly controlled mainly by the differential change of L with respect to T near the glass transition rather than the actual size L of the collective motion. We also find a near proportionality between m and the glass transition temperature in our nanocomposite system, which greatly simplifies the T dependence of structural. The classical entropy theory of glass-formation is considered as a complementary tool to gain analytic insights into these additive effects on polymer glass formation. [Preview Abstract] |
Monday, March 21, 2011 5:06PM - 5:18PM |
D41.00012: Anion Photoelectron Spectroscopy and First-Principles Study of PbxIny Clusters S. Vincent Ong, Joshua Melko, Ujjwal Gupta, J. Ulises Reveles, Jonathan D'Emidio, Shiv Khanna, A.W. Castleman Anionic and neutral Pb$_{x}$In$_{y}$ clusters containing up to 5 Pb and up to 7 In atoms have been investigated using negative ion photodetachment spectroscopy along with first-principles electronic structure studies within a gradient corrected density functional approach. The stability and electronic properties of these clusters have been characterized through studies of the detachment energies, gaps in the electronic spectrum, variations in binding energy, and nature of the electronic states. Particularly stable clusters have been grouped into two families of stable species. PbIn$_{3}^{-}$, Pb$_{2}$In$_{2}$, and Pb$_{3}$In$_{2}$ exhibit enhanced stability compared to their neighbors and the stability is linked to the aromatic character identified in their molecular orbitals. On the other hand, PbIn$_{5}^{-}$ and Pb$_{2}$In$_{4}$ exhibit enhanced stability associated with filled electronic shells within a confined nearly free electron gas. [Preview Abstract] |
Monday, March 21, 2011 5:18PM - 5:30PM |
D41.00013: Structural Analysis of Bonding in Au-Ge Clusters Danielle McDermott, Kathie Newman The study of Gold-Germanium clusters is important in understanding systems such as gold catalyzed nanowire growth. Of particular concern is the bonding behavior between the two chemical elements, one tending to form metallic bonds, the other covalent. DFT calculations and Conjugate Gradient relaxations were performed on clusters ranging in size from 50 to 150 atoms using the SIESTA code to find the geometries of metastable states. Emphasis has been placed on developing accurate and dependable bases to be used to study nano-sized systems. The binding energy, coordination number, bond lengths and bond angles are studied as a function of the size and composition of Ge-Au clusters. We will discuss a nanoscale ``phase diagram'' for gold and germanium and will also discuss the topology of the bonding network. [Preview Abstract] |
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