Bulletin of the American Physical Society
2005 APS March Meeting
Monday–Friday, March 21–25, 2005; Los Angeles, CA
Session V35: Molecular Electronics III |
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Sponsoring Units: DCP DMP FIAP Chair: Duncan Stewart, Hewlett Packard Room: LACC 511 B |
Thursday, March 24, 2005 11:15AM - 11:51AM |
V35.00001: F-V/SMS: A New Technique for Studying the Structure and Dynamics of Single Molecules and Nanoparticles Invited Speaker: We recently introduced a new technique for semiconductor nanoparticle research that involves simultaneous single molecule spectroscopy (SMS) \textit{and} controllable oxidation/reduction in an electronic device. The new technique (denoted by fluorescence voltage F-V/SMS) is analogous to current vs. voltage ($I-V)$ measurements for devices and electrochemical cells. F-V/SMS data are reported for single molecules (nanoparticles) of the conjugated polymer MEH-PPV as a function of bias voltage on the device and bias sweep rate to obtain information on both the energetics and kinetics of the charge transfer (oxidation/reduction) process in situ on the nanoscale. The extensive F-V/SMS data presented in the talk reveal that the dynamics for oxidation/reduction of MEH-PPV nanoparticles are controlled by various factors including filling of deep hole traps in the charge transporting layer of the device, the oxidation/reduction chemical ``state'' of the MEH-PPV molecule, and the molecular scale heterogeneity of the device. \newline \newline 1.``Charge Injection and Photooxidation of Single Conjugated Polymer Molecules,'' J. Am. Chem. Soc., So-Jung Park, Andre J. Gesquiere, Ji Yu, and Paul F. Barbara, J. Am. Chem. Soc., 127, 4116 (2004). [Preview Abstract] |
Thursday, March 24, 2005 11:51AM - 12:03PM |
V35.00002: Electron-transfer diabatic free energy surfaces from first-principles molecular dynamics P. H.-L. Sit, Matteo Cococcioni, Nicola Marzari Electron transfer in aqueous solution is a fundamental process in physical chemistry, and since the introduction of Marcus theory it has been extensively studied using classical force-fields molecular dynamics. However, classical potentials contain fitted parameters, and have obvious limitations in describing structural effects related to hybridization and electronic polarization. In this work, we investigate electron transfer fully from first-principles, using the case of ferrous-ferric ions solvated in water as a paradigmatic example. The structure and dynamics of the aqua ions at ambient conditions are studied via Car-Parrinello molecular dynamics. The diabatic free energy surfaces in the limit of two ions infinitely apart are calculated with umbrella sampling, obtaining parabolic free energy curves, in agreement with Marcus theory. The extension of our approach to ions at finite distances is also discussed, and our solution to the specific challenges that this problem entails, ranging from self-interaction effects to localization constraints. [Preview Abstract] |
Thursday, March 24, 2005 12:03PM - 12:15PM |
V35.00003: Electron dynamics in anharmonic bath Yuri Dahnovsky, Jaremy Creechley Tunneling transition probability for a particle interacting with an \textit{anharmonic} bath is found in a time-dependent Hartree approximation. The theory presented is a direct generalization of an oscillator medium to an \textit{arbitrary anharmonic environment}. The general expression for the transition probability is presented in terms of medium \textit{Keldysh} functions that are assumed to be known. Furthermore, the transition probability is calculated in the noninteracting-blip approximation (NIBA) where the rate constant does not exhibit activation dependence at high temperatures. The parameters similar to the reorganization energy, E$_{r}$, and the reaction heat, $\varepsilon $, are expressed in terms of the correlation matrix for a solvent and internal modes in both quantum and classical regimes. It is shown that E$_{r}$ and $\varepsilon $ are temperature dependent. Such a behavior is demonstrated in the case of a particular electron transfer reaction in nonpolar solvents experimentally studied by Zimmt \textit{et al}. [Preview Abstract] |
Thursday, March 24, 2005 12:15PM - 12:27PM |
V35.00004: Phosphine Adsorption and Dissociation on the Si(001) Surface: An Ab Initio Survey of Structures Oliver Warschkow, Hugh F. Wilson, Nigel A. Marks, David R. McKenzie, Steven R. Schofield, Neil J. Curson, Michelle Y. Simmons, Phil V. Smith, Marian W. Radny The continued down-scaling of electronic components to the atomic scale leads to a number of novel devices including quantum cellular automata, single electon transistors and quantum computers. The fabrication of such devices invariably requires scanning probe microscopies and an atomic-level understanding of doping mechanisms and the intermediate species involved. In the case of phosphorus, it is well established that PH$_3$ adsorbs dissociatively on the Si(001) surface, much controversy surrounds the numerous intermediate species observed in STM experiments. We present an extensive density functional theory (DFT) survey of possible PH$_3$ dissociation products and assign three prominent STM features as PH$_2$, PH and P species, respectively. All three structures are fully consistent with STM data and collectively outline a mechanism for the complete PH$_3$ dissociation on the Si(001) surface. [Preview Abstract] |
Thursday, March 24, 2005 12:27PM - 12:39PM |
V35.00005: Molecular Dynamics Simulation of Alkanethiol Monolayer with Azobenzene Molecule on the Au(111) Surface Ping Jiang, Adrian Roitberg, Jeffrey Krause, Hai-Ping Cheng Azobenzene-based molecules have been investigated widely in various applications, such as optomechanical devices and switching elements for microelectronics. A heterogeneous system consisting of an alkanethoil (dodecanethiol) monolayer with azobenzene (trans and cis) molecules on a Au(111) surface has been studied using classical molecular dynamics (MD) to control and predict the properties of this system. The temperature dependence of the tilt angle and diffusion properties of the molecule on the surface have been analyzed in detail compared to the corresponding homogeneous system. Finally, we discuss the implications of our results for the interpretation of recent experiments. [Preview Abstract] |
Thursday, March 24, 2005 12:39PM - 12:51PM |
V35.00006: Structural and Excited States Properties of Semiconductor Nanoparticle-Organic Molecule Heterocomposites: Theoretical Modelling D. S. Kilin$^*$, E. I. Zenkevich, O.V. Prezhdo$^*$, C. von Borczyskowski Long fluorescence lifetime, slow electronic relaxation, and discretization of states of the wurzite CdSe semiconductor quantum dots are calculated by the ab initio methods. The electronic structure is affected by the differece in the core/surface atoms: The inner 4-coordinated atoms maintain the crystalline structure, surface 2- and 3- coordinated atoms link to each other in vacuum [Phys. Rev. Lett. {\bf 92} 217401 (2004)] or to the TOPO/thiol ligand groups in a solvent. According to the simulations, ligand groups are preferably attached to the 2-coordinated surface atoms that increases the VB-CB gap compare to the vacuum case. In contrast, a trial attachment of ligand groups to 3-coordinated atoms creates unphysical unoccupied states on the top of the valence band so that system gains metallic properties. The information on ligand-friendly sites is used to model the anchoring of the pyridyl-substituted tetrapyrrolic organic molecules to semiconductor nanoparticles that triggers additional energy-transfer relaxation pathways of the semiconductor optical excitations as compared to the experimental results on CdSe/ZnS-porphyrin nanocomposites [E. Zenkevich et al, J. Phys. Chem. B (in print)]. [Preview Abstract] |
Thursday, March 24, 2005 12:51PM - 1:03PM |
V35.00007: Size control of thiol-stabilized gold nanoparticles I. Pister, L. Soussan, S. Nemzer, T. Harris, A.I. Frenkel, Y. Sun, M.H. Rafailovich The goal of this research was to investigate the possibility to control the size of the thiol-stabilized gold nanoparticles by a gold/thiol ratio. Several samples of gold nanoparticles stabilized by dodecanethiol chains were prepared by using two different (``one phase'' and ``two phase'') methods. For each method, the only difference between the seven samples analyzed was the gold/thiol ratio. The samples were analyzed by Extended X-Ray Absorption Fine Structure (EXAFS) and Transmission Electron Microscopy (TEM). The results demonstrate that as the gold-thiol ratio decreases, the average size of the particle decreases as well. Surprisingly, we obtained that at the values of the gold/thiol ratio less than 1:2, the cluster size stabilizes. The smallest clusters were obtained by EXAFS analysis to be cuboctahedral in shape where Au atoms occupy close packed structure positions. The average size of the clusters was ca. 11 {\AA}, corresponding to a 55 atom regular cuboctahedron. Due to a finite distribution of sizes obtained by TEM, we conclude that the significant amount of clusters were the 13 atom clusters, i.e., the smallest possible regular polyhedral clusters. This result explains why the further decrease of the Au/thiol ratio (below 1:2) does not change the average cluster size. Another factor contributing to the stabilization of the nanoparticle size at small Au/thiol ratio is the steric repulsion of thiol chains. [Preview Abstract] |
Thursday, March 24, 2005 1:03PM - 1:15PM |
V35.00008: Electronic structure and bonding of Au on SiO$_2$ cluster: A nano bullet for tumor Qiang Sun, Qian Wang, Puru Jena It is shown for the first time that gold atoms bind to silicon atoms with dangling bonds and serve as seeds for the growth of Au islands. The large electron affinity of gold causes significant change in the electronic structure of silica resulting in a substantial reduction in the HOMO-LUMO and the optical gap, thus allowing it to absorb near infrared radiation. This suggests that a small cluster can have similar functionality in the treatment of cancer as the large size nano-shell, but for a different mechanism. The advantage of having a small cluster with similar functionality as the large particle is that it can easily penetrate the crowded environments such as the biological milieu of cells and live tissues for effective drug delivery. [Preview Abstract] |
Thursday, March 24, 2005 1:15PM - 1:27PM |
V35.00009: One- and two-dimensional Ne adsorbed on carbon nanotube bundles Subramanian Ramachandran, Oscar Vilches, Tate Wilson, Damien Ramunno-Johnson We report initial results from an experiment that measures the heat capacity of Ne adsorbed on HiPco$^{TM}$ purified single- wall close-ended carbon nanotube bundles. The calorimeter is a thin wall copper cell containing 413 mg of bundles. We have obtained data for two coverages of about 0.05 and 0.36 monolayers in the temperature range from 1.9K to 18K. The first coverage is at a density within the one-line of atoms on either heterogeneous interstitial sites or outside of the bundle grooves, while the second coverage is in the expected three- line of Ne atoms formed on the outside grooves phase. We don't find any anomalies in the heat capacity in these 1d phases in the range measured. The experiment is continuing into densities of films adsorbed on the outside surface of the bundles, where we expect to see remnants of the two-dimensional triple point melting of monolayer Ne/graphite or Ne/graphitized carbon blacks. [Preview Abstract] |
Thursday, March 24, 2005 1:27PM - 1:39PM |
V35.00010: Control of Harmonic Genration in SuperLattices-Coherence and Relaxation Andre D. Bandrauk, Kyril Pronin We consider a d-dimentional conductor (a superlattice- Phys Rev B 69,195308(2004)) within the independent-electron one-band model taking into account relaxation effects. Its nonperturbative response to time-periodic electric fields is studied analytically for the dynamic and kinetic harmonic generation spectra. It is shown that in the dynamic (short time) regime field induced localization does not influence the high order harmonic spectra whereas in the kinetic (long time) regime all harmonic amplitudes can be suppressed by localization. The harmonic spectra reveal a plateau as in molecular systems (Phys Rev A 49,3943(1994)) and these plateaus are characterized by the parameter eEa/hv, where E is the maximum field amplitude of frequency v and a is the periodicity of the lattice. We show that in the case of a periodic rectangular electric field a new effect of single-mode response occurs:for specific values of the field E and frequency v, all harmonic amplitudes vanish except for a single one. Coherent control of such spectra will be illustrated. [Preview Abstract] |
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V35.00011: Electron Propagator Calculations on Molecular Wires Yuri Dahnovsky, Vyacheslav Zakrzhewski, Alexei Kletsov, Vince Ortiz Several molecular wires are studied by an \textit{electron propagator }method. This \textit{ab initio} method does not include any adjustable parameter or an additional potential introduced into a Hamiltonian. Such an approach is based on an accurate calculation of infinite series of diagrams rigorously describing the electron correlation in a bridge molecule. In this work we compute nonequilibrium Keldysh functions in order to find the dependence of current on applied voltage for particular molecular wires. Quantum chemical calculations are performed for several molecular bridges. The extended molecule approach is adopted. Results are compared with available experimental data and other quantum chemical approaches, in particular those based on density functional theory. [Preview Abstract] |
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