Bulletin of the American Physical Society
2008 APS March Meeting
Volume 53, Number 2
Monday–Friday, March 10–14, 2008; New Orleans, Louisiana
Session J21: Focus Session: Clusters, Cluster Assemblies, Nanoscale Materials IV |
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Sponsoring Units: DCP Chair: Meichun Qian, Virginia Commonwealth University Room: Morial Convention Center 213 |
Tuesday, March 11, 2008 11:15AM - 11:51AM |
J21.00001: Quantum Mechanics and Electrodynamics Studies of the Optical Properties of Metal Clusters/Nanoparticles Invited Speaker: This talk will describe the use of electrodynamics and quantum mechanics methods to describe the optical properties of silver and gold nanoparticles and other nanostructures. This work has been done in collaboration with several experimental colleagues, including Chad Mirkin, Rick Van Duyne and Teri Odom. Our recent work has focused on the optical properties of metal nanoparticles that are coated with molecules that are detected either through their influence plasmon resonance excitation, or via surface enhanced Raman spectroscopy (SERS). Electrodynamics calculations using either the DDA or FDTD methods provide a quantitative tool for characterizing far field properties, and at a more primitive level estimates of SERS intensities. Quantum mechanics, as developed using time dependent density functional theory, is restricted to small metal clusters, but the same methods of far field spectroscopy and SERS can still be studied. [Preview Abstract] |
Tuesday, March 11, 2008 11:51AM - 12:03PM |
J21.00002: Low Temperature Static Dipole Polarizability of Free Sodium Clusters with from 2 to 250 atoms Anthony Liang, John Bowlan, Xiao- Shan Xu, Shuange-Ye Yin, Walt A. de Heer The electric dipole polarizabilities of all sodium clusters Na$_{n}$ were measured from the atom up to n =250 using the molecular beam deflection method. Clusters were formed in cryogenic laser vaporization source operating at a temperature of 20 K. This complete sequence of high-resolution polarizablities measurements greatly enhances previous measurements. Electronic shell effects are observed as well as several features that are not readily understood in the shell model. The asymptotic limit of the measurements appears not to converge to the bulk sodium polarizability value. The data are compared with theoretical predictions. [Preview Abstract] |
Tuesday, March 11, 2008 12:03PM - 12:15PM |
J21.00003: Novel Properties of Diamondoid Molecules William Clay, Zhi Liu The recent isolation of a number of diamond-like hydrocarbons molecules (diamondoids) has sparked renewed interest in these unusual molecular systems. Several unique properties of these molecules are investigated. Diamondoid monolayers have been found to profoundly alter the electron emission tail of metal substrates in recent photoemission experiments, producing a sharp, nearly monochromatic peak. It is postulated that the cause of this effect is negative electron affinity combined with a strong electron-phonon interaction. New data and simulation results are presented to support this theory. Additionally, photoluminescence spectra for a number of diamondoid crystals are presented, taken with a 229 nm laser. To our knowledge, this is the first observation of UV photoluminescence in a saturated hydrocarbon molecule. Possible explanations for this phenomenon are discussed. [Preview Abstract] |
Tuesday, March 11, 2008 12:15PM - 12:27PM |
J21.00004: Room temperature stability of mass selected Ag clusters on C$_{60}$ functionalized surfaces Stefanie Duffe, Lukas Patryarcha, Torsten Richter, Benedikt Sieben, Heinz H\"{o}vel, Chunrong Yin, Bernd von Issendorff, Michael Moseler Mass selected clusters from Ag$^+_{55}$ to Ag$^+_{561\pm5}$ were soft landed on HOPG and Au(111) functionalized with 1 and 2 monolayers (ML) of C$_{60}$ molecules [1]. Depositions at 165 \,K gave extremely narrow cluster height distributions in STM images measured at 77\,K. Using C$_{60}$/HOPG or 2\,ML C$_{60} $/Au(111) the cluster heights are stable for more than 12\,h at room temperature (RT). For 1\,ML C$_{60}$/Au(111) the cluster height decreases and finally all clusters disappear at RT. Molecular dynamics simulations reveal a process by which the clusters decay atom by atom through 1\,ML C$_{60}$/Au(111) at RT. A sharp maximum at 1.7\,nm cluster height forms during the cluster decay, indicating that there exists some metastable 'supported magic number'. \par \noindent [1] S. Duffe et al., Eur. Phys. J. D (2007), published online [Preview Abstract] |
Tuesday, March 11, 2008 12:27PM - 12:39PM |
J21.00005: Modification on the melting of aluminum nanoclusters by a copper atom: heat capacities of CuAl$_{n-1}^{-}$ nanoalloys Baopeng Cao, Colleen M. Neal, Anne M. Starace, Martin F. Jarrold The melting of alloyclusters is currently of great interest and emerging as an important research area. In this talk, we report the synthesis and melting transition of CuAl$_{n-1}^{-}$ nanoalloy clusters (n = 49 -- 62). Heat capacities and melting behaviors have been determined for CuAl$_{n-1}^{-}$ nanoalloy clusters using a novel collision induced dissociation method and are compared with those of pure aluminum cluster Al$_{n}^{+}$. All these nanoalloys present a first order melting transition at temperatures well-below the melting temperature of the bulk aluminum and the eutectic temperature of their bulk alloys. No eutectic characteristic is detected for these nanoalloyclusters. Upon substitution of Al with a single copper atom, the melting of pure aluminum clusters has been altered considerably. Size and charge effects of the doping atom on the melting of host nanoclusters are discussed. [Preview Abstract] |
Tuesday, March 11, 2008 12:39PM - 12:51PM |
J21.00006: Forces between Functionalized Silica Nanoparticles J. Matthew D. Lane, Ahmed E. Ismail, Michael Chandross, Gary S. Grest Polymer-coated nanoparticles have a wide variety of applications, including drug delivery, adhesives, coatings, and magnetics. Although, the complexity of these nanoparticles precludes atomistic simulations of large numbers of nanoparticles in solution, it is possible to study the interaction between pairs of nanoparticles in an explicit solvent using molecular dynamics. From these simulations, we can compute the potential of mean force (PMF) between nanoparticles, which can be used in coarse-grained simulations at larger length and time scales. In particular, we present results for PMFs between polymer-grafted silica nanoparticles as a function of chain length, core size, and approach velocity. We report results for explicit-atom models of poly(ethylene oxide)-coated nanoparticles in water and alkylsilane-coated nanoparticles in decane. [Preview Abstract] |
Tuesday, March 11, 2008 12:51PM - 1:03PM |
J21.00007: Photo-fragmentation of the closo-carboranes Jing Liu, Danqin Feng, P.A. Dowben, A.P. Hitchcock, A.L.D. Kilcoyne, T. Tyliszczak, J.D. Bozek, E. R\"uhl Single and multi-cation ionic fragmentation of three isomeric carborane cage compounds [closo-1,2-orthocarborane, closo-1,7-metacarborane, closo-1,12-paracarborane (C$_{2}$B$_{10}$H$_{12})$] following B 1s and C 1s excitation were studied by time of flight mass analysis. The energetic cycles were constructed to gain some insights into some of decomposition processes. CH$^{+}$ or BH$_{2}^{+}$ fragment dominates the single ion fragmentation of the closo-carboranes. Double ion fragmentation yields and charge separation mass spectra of all three isomers are generally quite similar in that H$^{+}$ and BH$_{2}^{+}$/CH$^{+}$ ion pairs, BH$_{2}^{+}$/CH$^{+}$ and Y$_{11}^{+ }$ion pairs (where Y = (BH) or (CH)), Y$_{3}^{+}$ and Y$_{9}^{+ }$ion pairs (where Y = (BH) or (CH)) yields dominate. The H$^{+}$ and BH$_{2}^{+}$/CH$^{+}$ ion pairs dominate at the B 1s threshold for ortho- and metacarborane, while Y$_{1}^{+}$ and Y$_{11}^{+ }$ion pairs (where Y = (BH) or (CH)) dominates the multi-photofragment ion yield of paracarborane at the B1s threshold. [Preview Abstract] |
Tuesday, March 11, 2008 1:03PM - 1:15PM |
J21.00008: Coupled quantum dot / quantum shell systems: optical gain, ultrafast charge transport, and single particle blinking. Patanjali Kambhampati, D.M. Sagar, Eva Dias, Samuel Sewall, Ryan Cooney, Amy Grimes, Douglas English The CdSe/ZnS/CdSe core/barrier/shell nanostructure forms an electronically coupled quantum system that is a spherical analog to the quantum well superlattice. The core's brightness is enhanced via light harvesting by the shell. This material offers an opportunity to study charge transport in spherical nanoscale materials. Here, we present new results on the femtosecond dynamics of radial charge transport in these materials. With a combination of excitonic state selectivity and femtosecond time resolution, we monitor the ultrafast relaxation dynamics of either the core or the shell, having optically excited either phase. The femtosecond experiments reveal strong optical gain as well as evidence of spatially separated biexcitons, and coupling between phases. Finally, we present single dot data on the two-color blinking kinetics of these coupled quantum dot quantum shell systems. [Preview Abstract] |
Tuesday, March 11, 2008 1:15PM - 1:27PM |
J21.00009: Sampling of stable and metastable cluster structures by a first-principles Monte Carlo approach Ralf Gehrke, Karsten Reuter Size-selected nano-scale atomic clusters are now systematically becoming accessible in experiment, but characterizing their ground-state and metastable isomer ensemble averages from first principles requires a global and local exploration of vast configuration spaces. We here explore a first-principles Monte Carlo scheme to efficiently sample the minima of the corresponding total energy landscapes. The energetics is obtained at the density-functional theory level, using an all-electron local orbital based first principles code,$^1$ which allows to switch seamlessly from minimal size effective tight-binding like to meV-level chemically accurate basis sets within a single fundamental framework. The sampling strategies rely on basin hopping, using different schemes to create new trial structures. We demonstrate the reliability and performance of the approach for Cu and Si clusters, discussing in particular the scaling behaviour with the system size.\newline $^1$M. Scheffler and V. Blum; R. Gehrke, F. Hanke, P. Havu, V. Havu, X. Ren, K. Reuter, P. Rinke, A. Sanfilippo, A. Tkatchenko, The FHI - Ab Initio Molecular Simulations (aims) project, www.fhi-berlin.mpg.de/aims [Preview Abstract] |
Tuesday, March 11, 2008 1:27PM - 1:39PM |
J21.00010: The Effects of pH and Acid Type on Porous Alumina Formation Adam Friedman, Derrick Brittain, Latika Menon Porous aluminum oxide prepared by anodization has an enormous variety of uses in nanomanufacturing, as it can be used as scaffolding to grow nanowires and tubes of exacting size specifications. However, there is no complete physical model for its growth. Three models in particular have been suggested in the past. We experimentally study the stability of porous alumina formation and the effects of changing anodization voltage, acid pH, and acid type. Using this information, we show that the models err in their primary assumptions, we pinpoint the location of these errors, and we suggest a method to correct them. [Preview Abstract] |
Tuesday, March 11, 2008 1:39PM - 1:51PM |
J21.00011: Probing the Coulomb Barrier Towards Ionic Fragmentation Shaun Ard, Nasrin Mirsaleh-Kohan, Robert Compton Quintessential to understanding the stability of multiply charged anions (MCAs) is the characterization of the so-called ``Coulomb Barrier'' (CB) toward the loss of an excess electron and/or dissociation into charged fragments. The CB arises due to the superposition of the long-range Coulomb repulsion of the excess electron (or anion), and the short-range attractive polarization binding energy of the anion (or fragment). The CB adds to the stability of MCAs, often rendering thermodynamically unstable species to be metastable toward autodetachment or dissociation. The magnitude and shape of the CB is expected to depend heavily on the decay pathway. Whereas dissociation into charged fragments is often the lowest energy pathway for many MCAs, the vast majority of previous research has focused on electron loss. In this work, collision-induced dissociation (CID) is employed to study fragmentation of disulfonic dianions of increasing ``length''. Energy threshold for the production of SO$_{3}^{-}$ plus its conjugate anion are used to estimate the magnitude of the Coulomb barrier to dissociation. These measured thresholds are compared with \textit{ab initio }calculations of the dissociation energy. The relationship between these magnitudes and the distance between the excess charges will then be discussed. [Preview Abstract] |
Tuesday, March 11, 2008 1:51PM - 2:03PM |
J21.00012: Vibrational modes in metal core-shell nanoparticles A.S. Kirakosyan, T.V. Shahbazyan We study the spectrum of lowest vibrational modes of bimetallic nanoparticles in dielectric surrounding. For solid spherical particles, the mode's period and decay time are determined by the ratio of particle radius to longitudinal sound velocities in metal and outside medium, respectively. In contrast, in bimetallic nanoparticles, the dependence of both period and damping time on aspect ratio is highly non-monotonic. In particular, for Au/Ag core-shell nanoparticles, in a wide range of aspect ratios, the fundamental mode period is lower than in both Au and Ag solid particles of the same overall size, while the damping time exhibits a minimum at aspect ratios around 0.5. The unique acoustical signature of complex nanostructures allows unambiguous determination of their composition from ultrafast pump-probe and Raman spectroscopy measurements. [Preview Abstract] |
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