Bulletin of the American Physical Society
2009 APS March Meeting
Volume 54, Number 1
Monday–Friday, March 16–20, 2009; Pittsburgh, Pennsylvania
Session H38: Focus Session: Theory of Electron Transport Through Molecules I |
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Sponsoring Units: DCP Chair: Matthias Ernzerhof, UniversitÃ© de MontrÃ©al Room: 410 |
Tuesday, March 17, 2009 8:00AM - 8:36AM |
H38.00001: The Role of Symmetry in Molecular Electronic Conduction Invited Speaker: Jeffrey Reimers The Greens Function Density-Functional Tight-Binding (gDFTB) method is applied to determine the role that molecular symmetry in single-molecule conductivity.~ Both coherent elastic electron transport and inelastic electron-tunnelling spectroscopy (IETS) are considered.~ Symmetry becomes manifest in various ways: through the molecular point-group symmetry of the conducting molecule (D$_{2h}$ for chemisorbed benzenedithiol between two gold electrodes), through the conductance point-group symmetry displayed by the gDFTB equations (this embodies junction asymmetry and may be very low and nominally non-existent), and through an approximate molecular-conductance point group (C$_{2v}$ for chemisorbed benzenedithiol).~ Indeed, the conductivities for a range of relevant problems are well approximated using the restriction of molecular-conductance point-group.~ This allows the complex transmission curves calculated by many research groups to be dramatically simplified and partitioned into symmetry-depicted channels.~ Means are introduced that isolate a very small number of component channels describing different aspects of single-molecule conductivity: input junction channels, through-molecule channels, and output-junction channels. For elastic transport, all through-molecule channels are totally symmetric and hence a rigorous selection rule appears that transport is allowed involving only input-junction and output-junction channels of the same symmetry.~ However, for IETS, the through-molecule channels have the symmetry of the scattering molecular vibration and hence the input-junction and output-junction channel symmetries may vary.~ In general, just one channel is expected to dominate the junctions, leading to the IETS propensity rule that totally symmetric transitions are the most intense ones. Simple physical pictures are presented showing the input, vibrational scattering, and output channels for IETS, leading to predictions of how this effect can be controlled chemically. [Preview Abstract] |
Tuesday, March 17, 2009 8:36AM - 9:12AM |
H38.00002: Molecular and Nano Scale Device-conductance: steady state and dynamical analysis Invited Speaker: Barry D. Dunietz A computational approach is used and developed to study electron transport through molecular and nano scale devices. New models and methods are employed to describe the dynamics of electron transport under the influence of time dependent (TD) perturbations. Quantum interferences affecting the TD conductance are analyzed for transient aspects, effects of present bound states and transport under the effect of coherent excitations. I will also discuss our modeling of several recent high-profile experimental studies achieving molecular scale (steady state) conductance which provides intriguing insight at the molecular structural level on the functionality of the conducting devices. The studies involve metal recognition properties of short peptides or fabricated molecular sockets based on surface confined terpyridine ligands. If time permits I will describe the required structural features for a gating field to tune the conductance of a molecular conjugated system. [Preview Abstract] |
Tuesday, March 17, 2009 9:12AM - 9:24AM |
H38.00003: The spectroscopic dynamics of electron transport through molecular junctions Alexander Prociuk, Barry Dunietz A non-equilibrium Green's-Function (NEGF) model based on time dependent perturbation theory is developed to compute the spectroscopic dynamics of electron transport through molecular junctions under the influence of weak time dependent classical fields. In this model, we use the two time variable nature of the Kadanoff-Baym equations of motion to formulate a mixed time-frequency representation for the electronic density. The resulting highly informative time dependent Wigner distributions are used to shed light on the features of dynamical observables, such as electron current, dipole moment and population. We analyze laser induced coherence and population transfer effects for both Markovian and non-Markovian electrode models. If time permits, the analysis of transient conductance with respect to the system's fundamental parameters will be discussed. [Preview Abstract] |
Tuesday, March 17, 2009 9:24AM - 9:36AM |
H38.00004: Model \textit{ab initio} studies of solvation and excess charge localization on conjugated carbon chains Michael Mayo, Yuri Gartstein Using long C$_N$H$_2$ conjugated carbon chains with the polyynic structure as prototypical examples of one-dimensional (1D) semiconductors, we discuss self-localization of excess charge carriers in the presence of the interaction with a surrounding polar solvent. The solvation mechanism of self-trapping is different from the self-localization due to coupling with bond-length modulations of the underlying atomic lattice well-known in conjugated polymers. Model \textit{ab initio} computations are carried out and compared that employ various methods such as hybrid density functionals and Hartree-Fock within the framework of the polarizable continuum model. We demonstrate the possibility of the formation of large 1D electron- and hole-polarons entirely due to solvation, but even larger degrees of charge localization occur when accompanied by atomic displacements. Also discussed are doubly-charged bipolaron states and topological kink-solitons that may be formed in these systems. For a brief report, see M.~L.~Mayo and Yu.~N.~Gartstein, Phys. Rev. B 78, 073402 (2008). [Preview Abstract] |
Tuesday, March 17, 2009 9:36AM - 9:48AM |
H38.00005: Measuring single electron charging energy in self-assembled single nanoparticle devices: Coulomb blockade threshold vs. Arrhenius energy Al-Amin Dhirani, Amir Zabet-Khosousi Single-nanoparticle (NP) devices formed by self-assembling NPs onto alkanedithiol-functionalized break junctions exhibit Coulomb blockade (CB) conductance suppressions at low temperatures. We have studied temperature dependence of conductance inside the CB region and find \textit{multiple }activation energies (\textit{Ea}): A small \textit{Ea }at low temperatures, and a larger \textit{Ea }at high temperatures. The small \textit{Ea }is independent of NP size and is attributed to an energy state located at the metal--molecule contact. The larger \textit{Ea }scales with NP size and is attributed to single electron charging energy of the NPs. Importantly, we observe a significant ($\sim $5--100 fold) discrepancy between values of charging energies obtained from CB voltage thresholds and \textit{Ea}. To account for the discrepancy, we propose a model in which electrons are temporarily localized at the energy states near the metal--molecule interface and lose energy. The proposed model is supported by ultraviolet photoelectron spectroscopy of alkanedithiol monolayers on gold which indicates a presence of energy states close to the Fermi level of gold likely arising from gold--thiolate bonds. A suitably modified Orthodox theory successfully describes our measurements. [Preview Abstract] |
Tuesday, March 17, 2009 9:48AM - 10:00AM |
H38.00006: Molecular transport in the language of many-body states Michael Galperin Recent advancements in experimental techniques at nanoscale caused a surge in research of transport through molecular junctions. Nonlinearity of current-voltage characteristic at resonance makes this regime particularly important for potential molecular based memory, switchers and logic devices. One of important differences of molecular junctions (compared e.g. to semiconductor QDs) is sensitivity of electronic and vibrational structure of the junction to oxidation/reduction of the molecule. This implies necessity of treating the transport at resonance in the language of molecular states rather than single particle orbitals. The latter are the choice of majority of available ab initio approaches. We consider two possible schemes capable of incorporating isolated molecule (many-body) states as a basis for transport calculations. The schemes utilize Hubbard operators for description of single electron transitions between many-body states and go beyond previously proposed scattering theory and standard quantum master equation approaches. [Preview Abstract] |
Tuesday, March 17, 2009 10:00AM - 10:12AM |
H38.00007: WKB modeling of single molecular transport and Molecular Nanometrology Vladimir Burtman, Andrei V. Pakoulev Wentzel--Kramers--Brillouin (WKB) approach to model transport mechanism in molecular nanostructures is discusses in content of molecular nanometrology. Two WKB models, direct tunneling (Simmons model) and field emission tunneling (Fowler-Nordhaim tunneling), could be used to model conductivity in single molecular structure at low and elevated biased. Potentially, Simmons model could extract two molecular barriers, one for electrons and one for holes from conductivity spectra. Following this assumption electrical and optical gap-probed molecular nanometrology (GMN) could be developed. The main GMN principle is the small difference between the values of the HOMO-LUMO energy gap detected by electrical and optical measurements. We will compare experimentally derived electrical and optical probed gap and energy offsets between E$_{F}$ and nearest molecular orbital to discus applicability and feasibility of this approach. [Preview Abstract] |
Tuesday, March 17, 2009 10:12AM - 10:24AM |
H38.00008: Exact real-time dynamics of electron transport in mesoscopic systems Xiao Zheng, Jinshuang Jin, YiJing Yan We present a formally exact and numerically tractable quantum dissipation theory for time-dependent quantum transport in mesoscopic systems. It is formulated in terms of hierarchically coupled equations of motion, which govern the non-Markovian dynamics of an arbitrary fermionic system interacting with grand canonical electron reservoirs, in the presence of arbitrary time-dependent applied voltages [1-2]. We also present numerical results on the real-time dynamics of open quantum dot systems. The linear response admittance is mapped to classical equivalent circuits; while the nonlinear response dynamics is associated with dot-state transitions, such as the dynamic Coulomb blockade effect involved in interacting quantum dots [3-4]. Real-time Kondo phenomena are also demonstrated, with the cotunneling induced Kondo transition distinguished in the transient response current. This work highlights the significance and versatility of quantum dissipation theory for transient dynamics calculations. [1] J. S. Jin, S. Welack, J. Y. Luo, X. Q. Li, P. Cui, R. X. Xu, and Y. J. Yan, J. Chem. Phys. \textbf{126}, 134113 (2007). [2] J. S. Jin, X. Zheng, and Y. J. Yan, J. Chem. Phys. \textbf{128}, 234703 (2008). [3] X. Zheng, J. S. Jin, and Y. J. Yan, J. Chem. Phys. \textbf{129}, 184112 (2008). [4] X. Zheng, J. S. Jin, and Y. J. Yan, New J. Phys. \textbf{10}, 093016 (2008). [Preview Abstract] |
Tuesday, March 17, 2009 10:24AM - 10:36AM |
H38.00009: Theoretical study of electron transport through $\pi $-stacked ethylbenzene lines bonded to a Si surface Manuel Smeu, Robert Wolkow, Hong Guo Recently, experimental techniques were developed for lines of $\pi $-stacked ethylbenzene molecules to self-assemble on an H-terminated Si (100) surface in the laboratory of one of the authors. In this work, we use density functional theory (DFT) combined with the nonequilibrium Green's function formalism (NEGF) to model electron transport through these ethylbenzene lines to determine if they could be used as molecular wires. In our calculations, the molecules are bonded to an H-terminated Si (100) surface and are bridging two Al leads. The transmission spectrum and its associated scattering states are determined by the NEGF-DFT technique. The presence of the Si substrate is found to play an important role for conduction: there is a dominant transmission peak near the Fermi level which is contributed by the Si substrate and not the $\pi $-stacked molecular line. The low-bias resistance is found to increase exponentially with the length of the molecular line, indicating a tunneling behavior in conduction. [Preview Abstract] |
Tuesday, March 17, 2009 10:36AM - 10:48AM |
H38.00010: Stark Spectroscopy of Conjugated Oligomers and Polymers Important for Organic Devices Alberto Moscatelli, David C. Coppock, Linda A. Peteanu Fluorescent conjugated polymers have attracted a great deal of attention among scientists and engineers for their potential use in opto-electronic devices. One of the points that remain to be fully understood, however, is the undesirable sensitivity of their charge transport efficiency and emission characteristics on variations of the polymer structure and morphology. Using Stark spectroscopy it is possible to measure directly two important photophysical molecular parameters: (i) the change in the dipole moment, which is related to the degree of charge transfer associated with an optical transition; and (ii) the change in polarizability, which is related to the extent of the electronic delocalization. Poly(phenylenevinylene) (PPV), poly(dialkylfluorene) (PDAF) and ladder-type polyphenylenes (LPPP), as well as related oligomers, have been tested using this approach. Comparison of the results from single chains and from aggregates reveal how intermolecular interactions impact charge transfer and electronic delocalization in these technologically-important systems. [Preview Abstract] |
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