Bulletin of the American Physical Society
2009 APS March Meeting
Volume 54, Number 1
Monday–Friday, March 16–20, 2009; Pittsburgh, Pennsylvania
Session L38: Focus Session: Theory of Electron Transport Through Molecules III |
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Sponsoring Units: DCP Chair: Kieron Burke, University of California, Irvine Room: 410 |
Tuesday, March 17, 2009 2:30PM - 3:06PM |
L38.00001: Statistical mechanics of non-equilibrium steady state systems Invited Speaker: One of the important classes of non-equilibrium systems is the systems, which are maintained in non-equilibrium steady state by the contact with several external macroscopic reservoirs. These systems are ubiquitous and their theoretical description has been a challenging fundamental scientific problem for many years. They are also of significant practical interest for various nanotechnological and biological applications, such as quantum contacts, molecular motors, nanowires, and molecular junctions. There is no unique theoretical approach to wide variety of non-equilibrium steady state systems. General theoretical description of non-equilibrium steady states has not been developed yet and many fundamental theoretical questions are yet to be answered. For example, how to include many-particle correlation effects into theoretical treatment, is there exist a general variational principle for non-equilibrium steady state, do we enforce by the choice of a particular theoretical treatment a specific non-equilibrium steady state which is not the same as the real system would establish under the same boundary conditions, do we have a unique steady state in a system of non-equilibrium interacting particles for given boundary conditions? In my talk, I will review these questions and their relevance to electron transport through molecules. I will also give account of our recent computational and theoretical work on non-equilibrium quantum transport through molecular nanostructures. [Preview Abstract] |
Tuesday, March 17, 2009 3:06PM - 3:42PM |
L38.00002: Vibronic effects in single molecule conductance Invited Speaker: Recent experimental advances have allowed to study the conductance properties of single-molecule junctions and revealed a wealth of intriguing transport phenomena. An important aspect that distinguishes nanoscale molecular conductors from mesoscopic devices is the influence of the nuclear degrees of freedom of the molecular bridge. Due to the small size of molecules, the charging of the molecular bridge is often accompanied by significant changes of the nuclear geometry that indicate strong coupling between electronic and nuclear (in particular vibrational) degrees of freedom. In this contribution, the effect of electron-vibrational (vibronic) coupling on the transport properties of single molecule junctions is studied. The study is based on a combination of first-principles electronic structure calculations to characterize the system and different transport methods including inelastic scattering theory, master equations and nonequilibrium Green's function theory. The basic mechanisms of vibrationally coupled electron transport are analyzed for a generic model of a molecular junction as well as benzenealkanethiolates between gold electrodes. The results show that vibronic coupling can have a significant effect on the conductance of molecular junctions. It manifests itself in pronounced structures in the current-voltage characteristics. Moreover, the current-induced excitation of vibrational modes mays result in a significant deviation of the vibrational degrees of freedom from their equilibrium distribution. [Preview Abstract] |
Tuesday, March 17, 2009 3:42PM - 3:54PM |
L38.00003: Controlling Current Flow Through Molecules With Electric Fields Emanating From Nearby Molecules: Theory and Experiment G. Kirczenow, P. G. Piva, R. A. Wolkow We show that electrical conduction through molecules can be strongly modulated by electric fields of nearby polar molecules. We study 1D organic heterostructures consisting of contiguous lines of CF$_3$- and OCH$_3$-styrene molecules on H-terminated Si(100). For suitable alignment of the OCH$_3$ groups in the molecular chain, their combined electric fields are shown by density functional calculations to give rise to potential profiles along the OCH$_3$-styrene chain that result in strongly enhanced conduction through molecules near the CF$_3$-styrene/OCH$_3$-styrene heterojunction for moderately low negative substrate bias, as is observed by STM. Under similar bias, dipoles associated with CF$_3$ groups are found in both theory and experiment to depress transport in the underlying Si. Under positive substrate bias, simulations suggest that the structural and electrostatic properties of CF$_3$-styrene molecules may lead to more sharply localized conduction enhancement near the heterojunction. Thus choice of substituents, their attachment site on the host styrene molecules on Si and the orientations of the molecular dipoles and multipoles provide a means of differentially tuning transport on the molecular scale. [Preview Abstract] |
Tuesday, March 17, 2009 3:54PM - 4:06PM |
L38.00004: Effects of Dephasing on DNA Sequencing via Transverse Electronic Transport Matt Krems, Yuriy Pershin, Michael Zwolak, Massimiliano Di Ventra We study theoretically the effects of dephasing on DNA sequencing in a nanopore via transverse electronic transport. To do this, we couple classical molecular dynamics simulations with transport calculations using scattering theory. Previous studies, which did not include dephasing, have shown that by measuring the transverse current of a particular base multiple times, one can get distributions of currents for each base that are distinguishable. We introduce a dephasing parameter into transport calculations to simulate the effects of the ions and other fluctuations. These effects lower the overall magnitude of the current, but have little effect on the current distributions themselves. The results of this work further implicate that distinguishing DNA bases via transverse electronic transport has potential as a sequencing tool. [Preview Abstract] |
Tuesday, March 17, 2009 4:06PM - 4:18PM |
L38.00005: Single-Electron Transistors made by chemical patterning of silicon dioxide substrates and selective deposition of gold nanoparticles Ulas Coskun, Henok Mebrahtu, Thom LaBean, Gleb Finkelstein We describe a method to pattern SiO$_2$ surfaces with colloidal gold nanoparticles by e-beam lithography and selective nanoparticle deposition. The method allows us to deposit nanoparticles in different shapes, including long continuous lines just one nanoparticle wide. We contact the pre-positioned nanoparticles with metal leads to form Single Electron Transistors. The Coulomb blockade pattern surprisingly does not show the parasitic ``offset charges'' at low temperatures, indicating relatively little surface contamination. [Preview Abstract] |
Tuesday, March 17, 2009 4:18PM - 4:30PM |
L38.00006: Contact Transparency of Nanotube-Molecule-Nanotube Junctions S. H. Ke, H. U. Baranger, Weitao Yang The transparency of contacts between conjugated molecules and metallic single-walled carbon nanotubes is investigated using a single-particle Green's function method which combines a Landauer approach with \textit{ab initio }density functional theory. We find that the overall conjugation required for good contact transparency is broken by connecting through a six-member ring on the tube. Full conjugation achieved by an all-carbon contact through a five-member ring leads to near perfect contact transparency for different conjugated molecular bridges. [Phys. Rev. Lett. \textbf{99, }146802 (2007)] [Preview Abstract] |
Tuesday, March 17, 2009 4:30PM - 4:42PM |
L38.00007: Quantum many-body effects on the electric and thermoelectric response of molecular heterojunctions Justin Bergfield, Charles Stafford A semi-empirical $\pi$-electron Hamiltonian (extended Hubbard model) is used to model the electronic degrees of freedom most relevant for transport in a heterojunction consisting of a conjugated organic molecule coupled to two (or more) metallic electrodes. With an appropriate choice of parameters, the {\em complete spectrum of electronic excitations} of the molecule up to 8--10eV can be accurately described,$^1$ which is essential to accurately model transport far from equilibrium. The electric and thermoelectric response of the junction is calculated within a many-body theory of transport based on nonequilibrium Green's functions. For benzenedithiol-Au junctions, the parameters characterizing the lead-molecule coupling (tunneling width and chemical potential offset) are determined by comparison to linear-response measurements of conductance and thermopower. The nonlinear transport can then be predicted: the differential conductance as a function of gate and bias voltages exhibits clear signatures of charge quantization and resonant tunneling through excited states, with an irregular ``molecular diamond'' structure analogous to the regular Coulomb diamonds observed in quantum dot transport experiments. Several other small conjugated organic molecules are also investigated. $^1$C.\ W.\ M.\ Castelton and W.\ Barford, J.\ Chem.\ Phys.\ {\bf 117}, 3570 (2002). [Preview Abstract] |
Tuesday, March 17, 2009 4:42PM - 4:54PM |
L38.00008: Current induced local heating and heat transport in single molecular bridge junction Yoshihiro Asai Current induced local heating will be discussed theoretically. Both electric conductance and heat conductance of electronic and phonon origins are calculated in a microscopic way, including inelastic scattering effects due to electron-phonon couplings. [1] Based on the self-consistent solution for an alkanethiol molecule bridging gold electrodes, we found that the effective temperature T$_{eff}$ due to the local heating is largely reduced by the heat conductance, which releases the Joule heat out of the molecule. All these calculations are made in a fully microscopic way without introduction of the phenomenological phonon diffusion effect used in literature. Theoretical voltage dependence of T$_{eff}$ agrees nicely with an experiment. [2] Ref.) [1]Y. Asai, Phys. Rev. B78, 045434 (2008). [2] Z.Huang et al, Nano Letters, 6, 1240 (2006). [Preview Abstract] |
Tuesday, March 17, 2009 4:54PM - 5:06PM |
L38.00009: Fabrication and characterization of vertically aligned and interconnected nickel oxide Nanowalls Latha Kumari, Wenzhi Li, Charles H. Vannoy, Roger M. Leblanc , Dezhi Wang Vertically aligned and well interconnected NiO nanowalls were fabricated on Ni foil by a two step hydrothermal route. The as-prepared nickel hydroxide was converted to NiO by further heat treatment. The NiO nanowalls are typically 15 nm thick and around 1-1.5 $\mu $m wide. The NiO nanowalls have cubic crystal structure with their growth plane along the [111] direction. The NiO nanowalls show an optical band gap of about 3.8 eV and exhibit broad photoluminescence emission band centered at around 390 nm. The present synthesis technique supports the growth of well aligned 2D nanostructures with large surface area for possible applications in nanoscale devices. [Preview Abstract] |
Tuesday, March 17, 2009 5:06PM - 5:18PM |
L38.00010: Conductance switching in organic monolayers Luis Agapito, Sabri Alkis, Jeffrey Krause, Hai-Ping Cheng Self-assembled monolayers of some organic molecules, such as the bipyridyl-dinitro (BPDN) [1], present conductance switching (toggling between ON and OFF states). The switching happens upon crossing fixed threshold values in the applied bias voltage and the device can ``remember'' its previous state; thus, they have potential value as electronic memory devices. We use density-functional theory to elucidate the atomistic origins of this phenomenon. Extensive geometry relaxations revealed two adsorption states; namely, an atop and a hollow adsorption geometry. The electronic structure of both adsorption states were further recalculated using localized basis functions and the electrical currents through these devices were estimated within the Landauer approximation. The atop state shows a higher current than the hollow state, which matches the ON and OFF conductance states observed experimentally. We attribute the conductance switching to fluctuations in the adsorption geometry of the monolayers. Ab initio calculations can help us to understand the atomistic causes of the memory effect, which is essential for having a systematic approach to theory-guided molecular synthesis. Supported by DOE grant DE-FG02-02ER45995. [1] A. S. Blum, et al., Nature Materials 4, 167 (2005). [Preview Abstract] |
Tuesday, March 17, 2009 5:18PM - 5:30PM |
L38.00011: ABSTRACT WITHDRAWN |
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