Bulletin of the American Physical Society
2007 APS March Meeting
Volume 52, Number 1
Monday–Friday, March 5–9, 2007; Denver, Colorado
Session U18: Spectroscopy and Dynamics of Single Molecules and Nanoparticles |
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Sponsoring Units: DCP Chair: David Nesbitt, University of Colorado Room: Colorado Convention Center 103 |
Thursday, March 8, 2007 8:00AM - 8:12AM |
U18.00001: Single-Molecule Spectroscopic Investigations of RNA Structural Dynamics Julie L. Fiore, David J. Nesbitt To function properly, catalytic RNAs (ribozymes) fold into specific three-dimensional shapes stabilized by multiple tertiary interactions. However, only limited information is available on the contributions of individual tertiary contacts to RNA conformational dynamics. The Tetrahymena ribozymes's P4--P6 domain forms a hinged, ``candy-cane'' structure with parallel helices clamped by two motifs, the GAAA tetraloop-tetraloop receptor and adenosine (A)-rich bulge--P4 helix interactions. Previously, we characterized RNA folding due to a tetraloop-receptor interaction. In this study, we employ time-resolved single-molecule FRET methods to probe A-rich bulge induced structural dynamics. Specifically, fluorescently labeled RNA constructs excited by a pulsed 532 nm laser are detected in the confocal region of an inverted microscope, with each photon sorted by arrival time, color and polarization. We resolve the kinetic dependence of A-rich bulge-P4 helix docking/undocking on cationic environment (e.g. Na$^{+}$ and Mg$^{2+}$ concentration.) At saturating [Mg$^{2+}$], the docked structure appears only weakly stabilized, while only 50{\%} of the molecules exhibit efficient folding. [Preview Abstract] |
Thursday, March 8, 2007 8:12AM - 8:24AM |
U18.00002: Addressed Grids for Single-Nanoparticle Studies W.D. Tennyson, C.E. Allen, D.S. Hartnett, M.E. Curtis, A.R. Dedigama, D.J. Wasielewski, M.D. McCutchen, D.H. Dahanayaka, M.B. Johnson, L.A. Bumm We have developed a grid structure with a simple and robust address system to assist in locating and relocating individual substrate-supported nanoparticles. We demonstrate application of our addressed grids for facile characterization of the SAME nanoparticles in multiple instruments. Our grids can be prepared on a variety of substrates using lift-off photolithography. We will show addressed grids of Cr/Au on silicon, fused silica, and ITO coated glass as well as application to multiple measurements of the same nanoparticles by scanning electron microscopy, optical microscopy, atomic force microscopy, and single nanoparticle spectroscopy. [Preview Abstract] |
Thursday, March 8, 2007 8:24AM - 8:36AM |
U18.00003: Effect of Environment on Single-Photochromic-Molecule Satoshi Yokojima, Yasutaka Fujiu, Masanori Tachikawa, Jun-Wei Shen, Qi Gao, Paul Tchoupe, Takao Kobayashi, Akinori Murakami, Mitsuru Yoneyama, Katsuya Kanda, Shinichiro Nakamura, Toshikazu Ebisuzaki, Tuyoshi Fukaminato, Masahiro Irie Recent experimental results on the photochromic reactions of diarylethene derivatives at the single-molecule level by using a fluorescence technique [T.~Fukaminato, T.~Sasaki, T.~Kawai, N.~Tamai, and M.~Irie, J.~Am.~Chem.~Soc. {\bf 126} (2004) 14843; M.~Irie, T.~Fukaminato, T.~Sasaki, N.~Tamai, and T.~Kawai, Nature {\bf 420} (2002) 759.] is analyzed by the quantum chemical calculations and the molecular dynamics calculations. [Preview Abstract] |
Thursday, March 8, 2007 8:36AM - 8:48AM |
U18.00004: Single-molecule fluorescence quenching near small nanoparticles V. N. Pustovit, T. V. Shahbazyan We study theoretically radiative and nonradiative decay of a single molecule near small gold nanoparticle. The local field enhancement leads to an increased radiative decay rate while the energy transfer from molecule to optically-inactive electronic states in nanoparticle results in a decrease in fluorescence quantum efficiency for small molecule-nanoparticle distances. We performed a DFT-TDLDA calculation of both the enhancement and the quenching for small nanometer-sized gold nanoparticles. We found that in a close proximity to the surface, the non-radiative decay rate is dominated by generation of electron-hole pairs out of the Fermi sea resulting in a significantly lower quantum efficiency as compared to that obtained from electromagnetic calculations. For large distances, the efficiency is maximal for molecule polarized normal to the surface, whereas for small distances it is maximal for parallel orientation. [Preview Abstract] |
Thursday, March 8, 2007 8:48AM - 9:00AM |
U18.00005: Atomic-Scale Coupling of Photons to Single-Molecule Junctions Shiwei Wu, Naoki Ogawa, Wilson Ho The study of optical phenomena at the atomic scale is expected to provide new understanding of molecules and their chemical dynamics. The combination of lasers with a low temperature scanning tunneling microscope (STM) has led to the observation of photo-induced resonant tunneling with sub-molecular spatial resolution for single molecules adsorbed on a solid surface [\textit{Science} \textbf{312}, 1362, (2006)]. Furthermore, the irradiation of femtosecond laser pulses into this single-molecule junction defined by STM results in non-linear coupling by two-photon excitation, rather than the single-photon coupling in the case of continuous wave (CW) lasers. These experiments lead to new opportunities by tapping into the unique properties of lasers and the STM. [Preview Abstract] |
Thursday, March 8, 2007 9:00AM - 9:12AM |
U18.00006: Energy Spectra of Individual Gold Monolayer Protected Clusters Measured by Single Electron Tunneling Force Microscopy Ning Zheng, Jon Johnson, Gangli Wang, Clayton Williams Monolayer Protected Clusters (MPCs) exhibit strong quantum confinement effects and size dependent electronic, optical and chemical properties. The energy levels of individual gold MPCs (Au$_{38}$ {\&} Au$_{140})$ have been directly measured by Single Electron Tunneling Force Microscopy at room temperature in UHV.$^{[1,2]}$ Single electrons, tunneling between a probe tip and individual gold MPCs are detected using a novel surface potential measurement technique.$^{[2]}$ Tunneling events to and from the MPCs are recorded as a function of the applied bias voltage. A clear electronic spectrum is obtained, showing a HOMO-LUMO gap for Au$_{38}$ but not for Au$_{140}$. For both MPCs the single electron charging energy is measured. Spectral differences from particle to particle are observed. The energy spectra obtained by this method are directly compared with existing electrochemical data,$^{[3]}$ showing good agreement. The methodology will be described and the measured electronic spectra for Au$_{38}$ and Au$_{140}$ will be presented and discussed. 1. E. Bussmann, D. J. Kim, and C.C. Williams, \textbf{\textit{Appl. Phys. Lett. 85, 2538 (2004)}} 2. E. Bussmann, N. Zheng, and C. C. Williams, \textbf{\textit{Nano Lett.; 2006; 6(11)}} 3. Lee, D el at, \textbf{\textit{J. Am. Chem. Soc. 2004, 126, 6193}} [Preview Abstract] |
Thursday, March 8, 2007 9:12AM - 9:24AM |
U18.00007: On the reactivity of neutral metal oxide clusters in the gas phase: Detection through 118 nm single photon ionization Sheng-Gui He, Yan Xie, Elliot Bernstein Single photon ionization by a vacuum ultra-violet (VUV, 118 nm) laser is successfully employed for the study of reactions of \textit{neutral} metal oxide clusters (Ti$_{m}$O$_{n}$, Fe$_{m}$O$_{n}$, Co$_{m}$O$_{n})$ with various simple molecules (CO, NO, SO$_{2}$, H$_{2}$O) in the gas phase. Neutral clusters are generated by reaction of laser ablation generated metal plasma with O$_{2}$ in a supersonic expansion. Clusters are reacted with reactant gases in a flow tube reactor. Detection of neutral clusters and products is through ionization with 118 nm laser radiation and time of flight mass spectroscopy. Rich neutral cluster chemistry is observed: (1) Ti$_{m}$O$_{2m}$ and Ti$_{m}$O$_{2m+1}$ absorb one or more H$_{2}$O molecules for $m \quad \ge $ 2 and $m \ge $ 1, respectively; (2) FeO$_{2}$, FeO$_{3}$, and possibly FeO are reactive with CO while Fe$_{2}$O$_{4}$ and Fe$_{2}$O$_{5}$ are less reactive; (3) Fe$_{2}$O$_{5}$ is reactive with NO and SO$_{2}$, but FeO$_{2}$ is much less reactive with them; and (4) small Co$_{m}$O$_{n}$ clusters ($m \quad \le $ 4 and $n \quad \le $ 6) are more reactive than large clusters (6 $\le \quad m \quad \le $ 12 and 8 $\le \quad n \le $ 17) with CO, and among these small clusters, Co$_{3}$O$_{4}$ is particularly reactive. A detailed quantum chemistry study of Fe$_{m}$O$_{n}$ reactions with CO is in progress. Preliminary calculations indicate that reactions of FeO and FeO$_{2}$ with CO to produce CO$_{2}$ are overall barrierless, in agreement with the experimental observations. [Preview Abstract] |
Thursday, March 8, 2007 9:24AM - 9:36AM |
U18.00008: ABSTRACT WITHDRAWN |
Thursday, March 8, 2007 9:36AM - 9:48AM |
U18.00009: ABSTRACT WITHDRAWN |
Thursday, March 8, 2007 9:48AM - 10:00AM |
U18.00010: Structures and energetics of hydrocarbon molecules in the full hydrogen chemical potential range Yongxin Yao, Tzu-Liang Chan, Cai-Zhuang Wang, Kai-Ming Ho Hydrocarbons, especially polycyclic aromatic hydrocarbons (PAH), attract much interest as candidates for the unidentified infrared bands (UIRs), UV extinction curve and diffuse interstellar bands in the interstellar medium. While many experiments and quantum chemical calculations of the infrared spectra on human-selected hydrocarbons have been done, it remains an open question. A key reason may be the complicated phase space of hydrocarbons originated from the enormous bonding ability of carbon. Here we present a series of unbiased global search for hydrocarbon molecules (CnHm, 1$<$n$<$50) in the full hydrogen chemical potential range. The search is based on Genetic Algorithm (GA) combined with Brenner's hydrocarbon empirical potential and our new tight-binding potential. The structures and energies from GA are checked at both DFT and MP2 level. Such a mini database is reported, which is supposed to give more helpful information on hydrocarbon-related researches. [Preview Abstract] |
Thursday, March 8, 2007 10:00AM - 10:12AM |
U18.00011: Structural evolution of Au nanoclusters: From planar to cage to tubular motifs Bokwon Yoon, Xiaopeng Xing, Uzi Landman, Joel H. Parks The evolution of structural motifs of gold cluster anions, Au$_{n}^{-}$, in the size range $n=11-24$ has been determined through a comparison of electron diffraction data with density functional calculations. The results provide clear evidence for a transformation from planar to three-dimensional structures in the range $n=12-14$, the development of cage structures for $n=16$ and 17, the appearance of a tetrahedral structure at $n=20$, and the emergence of a highly symmetric tubular structure for $n=24$. [Preview Abstract] |
Thursday, March 8, 2007 10:12AM - 10:24AM |
U18.00012: Bulk Quantities of Noble Gas Nanoclusters with Five-Fold Symmetry Stabilized in Superfulid Helium V. Kiryukhin, E.P. Bernard, V.V. Khmelenko, R.E. Boltnev, D.M. Lee, N.V. Krainyukova Bulk quantities (volume$\sim$cm$^3$, atomic density$\sim$10$^ {19}$-10$^{20}$ cm$^{-3}$) of noble gas nanoclusters (size$\sim$5-6 nm) were produced in superfluid helium by injection technique. X-ray diffraction measurements show that the samples consist of weakly interacting nanoclusters with five-fold symmetry axes, such as icosahedra and decahedra. These results open new opportunities for fundamental research of nanoclusters of noble gases and other materials in well-controlled environments using experimental techniques requiring bulk samples. [Preview Abstract] |
Thursday, March 8, 2007 10:24AM - 10:36AM |
U18.00013: Support-induced Catalytic Activity of Gold Nanocluster Chun Zhang, Bokwon Yoon, Uzi Landman The catalytic activity of gold nanoclusters supported on metal-oxide surfaces is a topic of active research efforts. Recently, a dimensionality crossover of gold clusters, adsorbed on a metal-supported thin film of MgO(100), has been predicted$^{1}$. We present here a first- principles study of the catalytic activity of a planer Au$_{20}$ cluster on two- layer MgO(100) film supported by a Mo surface. Both Langmuir-Hinshelwood (LH) and Eley--Rideal (ER) mechanisms of CO oxidation are investigated. The barrier of the LH mechanism is found to be 0.15 eV. For the ER mechanism, the barrier depends on the direction of approach of the CO molecule to the preadsorbed oxygen molecule, varying between a vanishing barrier height and 0.2 eV. Charge transfer from the Mo surface to the cluster supported on the thin MgO(100) film plays a key role in the catalyzed CO oxidation process. [Preview Abstract] |
Thursday, March 8, 2007 10:36AM - 10:48AM |
U18.00014: The Electronic Structure of Diamondoids: When does a Molecule Become a Nanoparticle? Trevor Willey, Christoph Bostedt, T. Moller, J. E. Dahl, S. G. Liu, R. M. K. Carlson, T. van Buuren, R. W. Meulenberg, E. Nelson, L. J. Terminello Methane and diamond are common materials with well-known, but extremely different properties. Fundamental, compelling questions then arise: with sp$^{3}$ bound carbon molecules/clusters, at what sizes do diamond-like properties emerge? Pure, defect-free, perfectly hydrogen-terminated diamondoids, from $\sim $0.5 nm to $\sim $1 nm, bridge the gap between molecules and nanoparticles. Furthermore, experiments in the gas phase eliminate particle-particle interaction and ensure measurement of pure, pristine, and undamaged molecules. X-ray absorption probes the unoccupied electronic states; the carbon K-edge reveals rich electronic structure in the series methane, cyclohexane, adamantane, diamantane, through to hexamantane. Diamondoids show the emergence of a diamond-like band structure. Using x-ray absorption and soft x-ray emission, we will compare our experimental LUMO/conduction band and HOMO/valence band positions with predicted changes in HOMO-LUMO gap from several calculations. [Preview Abstract] |
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