Bulletin of the American Physical Society
APS March Meeting 2010
Volume 55, Number 2
Monday–Friday, March 15–19, 2010; Portland, Oregon
Session Y25: Nanoparticles |
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Sponsoring Units: DCP Chair: Christopher Sorensen, Kansas State University Room: D135 |
Friday, March 19, 2010 8:00AM - 8:12AM |
Y25.00001: Phase Equilibria in Nanoparticle Solutions Christopher Sorensen, Hao Yan, Brandon Lohman, Amit Chakrabarti Quasi-monodisperse nanoparticle suspensions can act as solutions. We have studied the thermally reversible precipitation/dissolution of ca. 5nm gold nanoparticles (NP) in various solvents. The NPs have a narrow size distribution. They are ligated with alkyl thiols: C8SH, C10SH and C12SH. The solubility depends on both the solvent and the ligand. Solvents used were the series of normal alkanes from hexane to hexadecane, simple aromatics and mixtures of these with 2- butanone. Concentration versus temperature yields enthalpies of dissolution which can be compared to superlattice cohesive energies. [Preview Abstract] |
Friday, March 19, 2010 8:12AM - 8:24AM |
Y25.00002: Detection of Nucleic Acids with Graphene Nanopores: Ab Initio Characterization of a Novel Sequencing Device Tammie Nelson, Bo Zhang, Oleg Prezhdo We report an ab initio study of the interaction of two nucleobases, cytosine and adenine, with a novel graphene nanopore device for detecting the base sequence of a single-stranded nucleic acid (ssDNA or RNA). The nucleobases were inserted into a pore in a graphene nanoribbon, and the electrical current and conductance spectra were calculated as functions of voltage applied across the nanoribbon. The conductance spectra and charge densities were analyzed in the presence of each nucleobase in the graphene nanopore. The results indicate that, due to significant differences in the conductance spectra, the proposed device has adequate sensitivity to discriminate between different nucleotides. Moreover, we show that the nucleotide conductance spectra is not affected by its orientation inside the graphene nanopore. The proposed technique may be extremely useful for real applications in developing ultrafast, low cost DNA sequencing methods. [Preview Abstract] |
Friday, March 19, 2010 8:24AM - 8:36AM |
Y25.00003: Puzzles and promises in finding nanostructure from pair distribution function data Corey Musolff, Christopher Farrow, Phillip Duxbury, Simon Billinge, Pavol Juhas The pair distribution function (PDF) method provides detailed information about nanostructure and efficient software is available for refining PDF data to starting structures provided by the user. Starting structures may be developed using intuition, homologous structures from the literature, or by using ab-initio theoretical structures. However, in studies of nanoparticles good starting structures are hard to identify, particularly in systems where complex surface distortions occur. Moreover, refinement from random structures typically fails, or converges to unphysical structures. We quantify this problem through a systematic study of PDF refinement using starting structures that are randomly perturbed variants of the correct structures and identify the threshold perturbations beyond which refinement fails. To alleviate the difficulties in guessing or searching over starting structures it would be ideal if good starting structures could be found directly from PDF data. We outline our progress in this direction. [Preview Abstract] |
Friday, March 19, 2010 8:36AM - 8:48AM |
Y25.00004: Vibrational modes of metal core/dielectric shell nanoparticles Arman Kirakosyan, Tigran Shahbazyan We study theoretically the spectrum of radial vibrational modes in composite spherical nanoparticles with metal core and dielectric shell in an environment. We calculate frequency and damping rate of the fundamental (breathing) mode and find that they are significantly higher than those for solid metal particles of the same overall size. For excited modes, we find that both frequency and damping rate exhibit pronounced oscillations as a function of aspect ratio and the frequency of higher modes is nearly independent of the environment, while the damping rate shows strong sensitivity to outside medium. [Preview Abstract] |
Friday, March 19, 2010 8:48AM - 9:00AM |
Y25.00005: Capillary adhesion at nanometer scales Mark Robbins, Shengfeng Cheng Molecular dynamics simulations are used to study the capillary adhesion due to a liquid bridge between a spherical tip and a flat substrate. The atomic scale roughness of the tip, tip radius, contact angles on the two surfaces, and the volume of the liquid bridge are varied. The simulations consider a nonvolatile fluid with constant volume. The adhesive force on the tip is measured as a function of tip-surface separation. The force agrees with continuum predictions for separations down to 5 to 10nm. At even smaller separations, the adhesion oscillates due to liquid layering in the narrow gap between surfaces. This effect is most pronounced for large tip radii and smooth surfaces. There is also a decrease in the average adhesive force that can be as large as a factor of two. This shift is due to an anisotropy in the pressure stress tensor. The in-plane component is consistent with the Laplace pressure predicted by continuum theory, but the normal pressure that determines adhesion is much more positive. [Preview Abstract] |
Friday, March 19, 2010 9:00AM - 9:12AM |
Y25.00006: Computer Simulation Evidence for a Liquid-Liquid Phase Transition in Gallium: Bulk and Nanodroplets Alex Antonelli, Diego Jara, Mateus Michelon, Maurice de Koning Over the last decade, there has been an increasing interest in the first-order liquid-liquid phase transition (LLPT) between liquids of the same chemical composition. While LLPT has been speculated to occur in several liquids that exhibit anomalies in their thermodynamic properties, so far in only two cases it has been experimentally verified. This lack of evidence stems from difficult experimental conditions, since in many cases the LLPT is expected to occur in the metastable supercooled regime. Gallium is a very promising substance for the study of LLPT, since it has low melting point (303 K), displays anomalous behavior, and can be kept liquid about 100 K below its melting temperature. In this work, we report on molecular dynamics simulations of liquid Ga that provide theoretical evidence of a LLPT from a high density to a low density liquid in bulk [1] and nanodroplets. Our results indicate a reduction in the temperature and latent heat of the LLPT as the size of the system decreases. \noindent [1] D. A. C. Jara, M. F. MIchelon, A. Antonelli, and M. de Koning, Journal of Chemical Physics \textbf{130}, 221101 (2009). [Preview Abstract] |
Friday, March 19, 2010 9:12AM - 9:24AM |
Y25.00007: Electric field induced alignment of gold nanorods on surfaces Waqqar Ahmed, Stefan Kooij, Arend van Silfhout, Bene Poelsema We have studied the alignment of colloidal gold nanorods, deposited from solution onto well-defined substrates in the presence of an AC electric field generated by micrometer spaced electrodes. The field strengths employed in our experiments are sufficiently large to overcome the rotational and translation Brownian motion of nanorods. However, despite the large fields, we find that the degree of alignment is considerably smaller than what was previously reported for field-induced nanorod alignment in suspension. The limited alignment of nanorods is ascribed to the different experimental configuration, and the correspondingly larger density of nanorods. The mutual interactions of nanorods give rise to a disturbance of the local electric field and therewith their orientation. For sufficiently large field strengths, these interactions lead to the formation of nanorod chains that ultimately bridge the electrode gap. Furthermore, for small electrode spacing, the nanorods accumulate on the electrode surface, and the screening of their mutual interactions leads to considerably improved alignment. [Preview Abstract] |
Friday, March 19, 2010 9:24AM - 9:36AM |
Y25.00008: Selective Chemical Raman Enhancement for Organic Adsorbates at Metal Surfaces Alexey T. Zayak, Ying Hu, Hyuck Choo, Stefano Cabrini, P. James Schuck, Jeffrey B. Neaton It has long been observed that in surface enhanced Raman spectroscopy (SERS) relative mode intensities differ from gas- and solution-phase data, which obscures understanding of SERS in general. Using first-principles methods, we examine how chemisorption affects Raman scattering of molecules on metal surfaces relative to gas-phase, and provide a quantitative description of this effect. Calculated Raman spectra for benzene thiol bound at different sites on Au(111) show that chemical enhancement arises from the mode dependent electron-phonon coupling of the metal-molecule interface. Site-dependent enhancements are explained correlated to interfacial electronic structure. Comparison to experiments suggests affinity of benzene thiol for bridge sites on Au(111) surfaces. [Preview Abstract] |
Friday, March 19, 2010 9:36AM - 9:48AM |
Y25.00009: Self-Assembly of Ligated Gold Nanoparticles A. Chakrabarti, S.J. Khan, C.M. Sorensen We study assembly of ligated gold nanoparticles by both phenomenological modeling and computer simulations for various ligand chain lengths. First, we develop an effective nanoparticle-nanoparticle pair potential by treating the ligands as flexible polymer chains. Besides van der Waals interactions, we incorporate both free-energy of mixing and elastic contributions from compression of the ligands in our effective pair potentials. Next, we use the calculated pair potentials as input to Brownian Dynamics simulations for studying nucleation and growth of nanoparticle superlattice formation in three dimensions. [Preview Abstract] |
Friday, March 19, 2010 9:48AM - 10:00AM |
Y25.00010: Quantification of Aggregate Topology, the Minimum Dimension and Connectivity Durgesh Rai, Gregory Beaucage, Jan Ilavsky, Hendrik Kammler The properties (electrical conductivity, diffusion coefficient, spring constant) of nanostructured ceramic aggregates can be determined only if details of the structural topology are known. For example, the mechanical strength of an aggregate depends only on the shortest average path through the aggregate, called the minimum path. Most characterization methods fail to quantify the topology. Values of the minimum dimension, associated with the minimum path, and the spectral dimension, associated with energy distribution in an aggregate have been considered only in simulations and models. Recently we have developed a method using small-angle neutron and x-ray scattering for the quantification of the details of topology in aggregated materials (Beaucage 2004, Ramachandran 2008, 2009). In situ SAXS studies of flame aerosols containing nanostructured aggregates will be presented. Their topology as a function of growth time on the millisecond time scale will be described. Beaucage G, \textit{Phys. Rev. E} \textbf{70} 031401 (2004).; Ramachandran R, et al. \textit{Macromolecules} \textbf{41} 9802-9806 (2008).; Ramachandran R, et al. \textit{Macromolecules}, \textbf{42} 4746-4750 (2009). [Preview Abstract] |
Friday, March 19, 2010 10:00AM - 10:12AM |
Y25.00011: An XPS and STM/STS Study of Tetra(sulfonatophenyl)porphyrin Nanorods K.W. Hipps, U. Mazur, B. Friesen, B. Wiggins Tetra(sulfonatophenyl)porphyrin, TSPP, forms nanorods on Au(111) or HOPG. These nanorods were examined by XPS, STM, and STS. N1s XPS data demonstrate that the central four nitrogens of the porphyrin are completely protonated, generating a positive central core and a negative molecular periphery that plays a role in the rod formation process. STM studies in UHV also suggest the rods may be collapsed tubes. STS results show that while single rods are somewhat rectifying with primarily n-type conduction, bundles of rods show very strong rectification. This difference in bundle versus single rod conduction may be an artifact of the measurement method, with the single rod I-V curves containing both conduction and tunneling currents while thicker bundle data is dominated by direct conduction. Comparison of the STS of the nanorods with that of NiTPP molecules suggests that the lowest unoccupied molecular orbitals are those that participate in the conduction process. It is also interesting to note that the STS of the nanorods on gold is exactly the same as on HOPG, indicating that these rods have an effective Fermi energy that equilibrates with the substrate. [Preview Abstract] |
Friday, March 19, 2010 10:12AM - 10:24AM |
Y25.00012: Self-Doped Titanium Oxide Nanonail Growth on Silicon by Solid-State-Reaction Jaewu Choi, Chang-Mook Lee Titanium oxide nanonails are directly grown on silicon with a thermal oxide by the solid-state-reaction of the metallic multilayer on a silicon substrate with a thick thermal oxide at a relatively low growth temperature, 800 $^{\circ}$C, made possible in the presence of a carbon precursor. The pattern of the multilayer growth pad on silicon promoted the nanonail growth. The nanonails are spontaneously doped by the constituent elements of the multilayer and carbon of the hydrocarbon. The doped titanium oxide nanonails show cathode luminescence in a visible and near-infrared spectrum range. The vertical location of the catalyst layer in the multilayer growth pad directly controlled the head formation of the titanium oxide nanonails. [Preview Abstract] |
Friday, March 19, 2010 10:24AM - 10:36AM |
Y25.00013: Synthesis and solid-state transformations of Cu core/Ni shell nanoparticles Karl Unruh, Michael Lattanzi, Laura Higgins, Stephen Johnson, Thomas Ekiert Air stable Cu core/Ni shell nanoparticles have been prepared in a two step polyol-type process by adding CuCl$_{2}\cdot $2H$_{2}$O and NaOH to an ethylene glycol solution at 160 \r{ }C followed by the subsequent addition of NiCl$_{2}\cdot $6H$_{2}$O and NaOH at a solution temperature of 180 \r{ }C. Allowing the low temperature step of the reaction to proceed to completion ensured that the high temperature step resulted in the formation of elemental Ni rather than a Cu-Ni alloy as verified from the near bulk values of the measured Cu and Ni lattice parameters. The solid-state transformation from the as-prepared core/shell structure to an essentially homogeneous Cu-Ni alloy has been studied by differential scanning calorimetry, x-ray diffraction, and vibrating sample magnetometry measurements. These measurements reveal that the core/shell structure remains largely intact to temperatures above 400 \r{ }C during an annealing profile consisting of a 20 \r{ }C/min temperature ramp followed by a rapid temperature quench. [Preview Abstract] |
Friday, March 19, 2010 10:36AM - 10:48AM |
Y25.00014: Water-Gas --Shift Reaction on Gold Nanoparticles Supported on Iron Oxide Surfaces: A Scanning Tunneling Microscopy/Spectroscopy Study Kwang Taeg Rim, Daejin Eom, Li Liu, Joan Raitano, Siu-Wai Chan, Maria Flytzani-Stepanopoulous, George Flynn We present a Scanning Tunneling Microscopy (STM)/Scanning Tunneling Spectroscopy (STS) study of the Water-Gas-Shift reaction on a model catalyst system consisting of supported gold nanoparticles on a reduced Fe$_{3}$O$_{4}$(111) surface in ultrahigh vacuum. Gold forms two electrically distinct nanoparticles on an iron oxide surface upon annealing multilayer Au/Fe$_{3}$O$_{4}$(111) at 500 $^{\circ}$C for 15 minutes. STS measurements show that large gold nanoparticles ($\sim $ 8 nm) exhibit a metallic electronic structure while single gold adatoms bonded to the oxygen sites of the Fe$_{3}$O$_{4}$(111) surface are likely positively charged. This Au/Fe$_{3}$O$_{4}$(111) system is dosed with CO and H$_{2}$O at various temperatures. The site specific adsorption and the interaction of CO with the Au nanoparticles will be presented and discussed along with the observation of the site specific dissociative adsorption of H$_{2}$O to elucidate the catalytic properties of Au nanoparticles on a reducible iron oxide surface. [Preview Abstract] |
Friday, March 19, 2010 10:48AM - 11:00AM |
Y25.00015: Nonequilibrium Transport in a biased Quantum Dot Sung Chao, Guillaume Palacios, Andres Jerez, Carlos Bolech, Pankaj Mehta, Natan Andrei We derive the transport properties of a quantum dot subject to a source-drain bias at zero temperature and magnetic field. Using the Scattering Bethe Anstaz, a generalization of the traditional Thermodynamic Bethe Ansatz to open systems out of equilibrium, to obtain dot occupation and current as a function of voltage numerically. [Preview Abstract] |
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