Bulletin of the American Physical Society
APS March Meeting 2013
Volume 58, Number 1
Monday–Friday, March 18–22, 2013; Baltimore, Maryland
Session R43: Electron Transfer, Charge Transfer and Transport |
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Sponsoring Units: DCP Chair: Guy Cohen, Columbia University Room: Hilton Baltimore Holiday Ballroom 2 |
Wednesday, March 20, 2013 2:30PM - 2:42PM |
R43.00001: Numerically Exact Long Time Magnetization Dynamics Near the Nonequilibrium Kondo Regime Guy Cohen, Emanuel Gull, David Reichman, Andrew Millis, Eran Rabani The dynamical and steady-state spin response of the nonequilibrium Anderson impurity model to magnetic fields, bias voltages, and temperature is investigated by a numerically exact method which allows access to unprecedentedly long times. The method is based on using real, continuous time bold Monte Carlo techniques---quantum Monte Carlo sampling of diagrammatic corrections to a partial re-summation---in order to compute the kernel of a memory function, which is then used to determine the reduced density matrix. The method owes its effectiveness to the fact that the memory kernel is dominated by relatively short-time properties even when the system's dynamics are long-ranged. We make predictions regarding the non-monotonic temperature dependence of the system at high bias voltage and the oscillatory quench dynamics at high magnetic fields. We also discuss extensions of the method to the computation of transport properties and correlation functions, and its suitability as an impurity solver free from the need for analytical continuation in the context of dynamical mean field theory. [Preview Abstract] |
Wednesday, March 20, 2013 2:42PM - 2:54PM |
R43.00002: Transport and dynamics in multisite subsystems Malay Bandyopadhyay, Manas Kulkarni, Dvira Segal We consider a chain of quantum dots coupled to finite-size reservoirs (prepared out-of-equilibrium) in which each dot is susceptible to decoherence effects or inelastic scattering processes. We observe a ballistic to diffusive crossover in the electronic current. We further investigate the manifestation of this ballistic-diffusive crossover on the dynamics and electron reorganization in the fermionic reservoirs. We find regimes which can be described in a classical framework and regimes whose description is rooted in quantum statistics. Our work can be generalized to understand other multi-site systems and their feedback on the bath degrees of freedom. [Preview Abstract] |
Wednesday, March 20, 2013 2:54PM - 3:06PM |
R43.00003: Possibilities for Observation of Quantum Transport in (RE)Ba$_{2}$Cu$_{3}$O$_{\mathrm{7-y}}$ Perovskites Paul Grant For y $\approx $ 0, the crystal structure of the ``1-2-3'' family of rare earth copper oxide perovskites displays several curious ``porosity'' features. For example, along the b-axis direction of the region usually termed the ``CuO chains,'' one observes a dramatically wide ``channel'' bounded within a Ba-Cu-O tube. Similar channels can be found in both the b- and a-axes directions contained within RE, Cu and O ion groupings. The cross-sectional area of these channels is roughly that of a single-wall carbon nanotube, suggesting the former may manifest Buettiker-Landauer quantum conductance similar to that observed in the latter. Moreover, by employing various ratios of Pr/Y for the RE component of the host system, the bulk electrical properties of the surrounding host can be tailored from completely insulating to metallic. We test our conjecture predicting ballistic transport down these channels using density functional theory and report our initial findings here along with the likely consequences of paramagnetic spin scattering. We also discuss possible experimental embodiments which could lead to nano-controllable gate structures. [Preview Abstract] |
Wednesday, March 20, 2013 3:06PM - 3:18PM |
R43.00004: The Information Content of Conductance Histogram Peaks: Transport Mechanisms, Level Alignments, and Coupling Strengths Matthew Reuter, Patrick Williams We develop a theory for describing single channel peaks in conductance histograms by applying probability theory to electron transport. This produces analytical forms for fitting experimental conductance data, where the fitting parameters have physical significance. Depending on the transport mechanism (resonant vs.\ non-resonant tunneling), the peak's line shape contains information on the level alignment of the channel and the channel-electrode coupling(s). Ultimately, this work provides tools for extracting additional information from experimental data, helping us better understand electron transport processes. [Preview Abstract] |
Wednesday, March 20, 2013 3:18PM - 3:30PM |
R43.00005: Collective Plasmon-Molecule Excitations in Nanojunctions: Quantum Consideration Alexander White, Boris Fainberg, Michael Galperin We use a pseudoparticle nonequilibrium Green function approach to study the coupling between plasmons and molecular excitons in nonequilibrium molecular junctions. This method is shown to be especially convenient for the calculation of the plasmon absorption spectrum of hybrid plasmon-exciton systems where coherent electron and energy transfer processes and strong system interactions play an important role. The formalism treats the intramolecular interactions and plasmon-exciton coupling exactly, going beyond the standard tool of molecular electronics - the nonequilibrium Green function. We demonstrate the sensitivity of the molecule-plasmon Fano resonance to junction bias, Coulomb repulsion, and intramolecular exciton coupling. We also compare our prediction for non-linear optical effects to previous mean-field equilibrium studies of isolated hybrid plasmon-exciton systems, and demonstrate the advantage of our approach. This study opens a way to deal with strongly interacting plasmon-exciton systems in nonequilibrium molecular devices. [Preview Abstract] |
Wednesday, March 20, 2013 3:30PM - 3:42PM |
R43.00006: Magnetic Field Control of Current through Molecular Ring Junctions Dhurba Rai, Michael Galperin, Oded Hod, Abraham Nitzan We study circular currents driven by voltage bias in molecular wires with ring structures [J. Phys. Chem. C 114, 20583 (2010)]. We revisit magnetic field effects on molecular ring structures presenting conditions under which magnetic field control of molecular ring conduction is realizable. [Phys. Rev. B 85, 155440 (2012)]. We find these conditions to be (a) weak molecule-lead coupling, implying relatively distinct conduction resonances, (b) asymmetric junction structure (e.g., meta or ortho connected benzene ring rather than a para connection), and (c) minimal dephasing (implying low temperature) so as to maintain coherence between multiple pathways of conduction. When these conditions are satisfied, considerable sensitivity to the applied magnetic field normal to the molecular ring plane is found. Although sensitivity to magnetic field is suppressed by dephasing, quantitative estimates indicate that magnetic field control can be observed in suitably constructed molecular ring junctions. We demonstrate control of the spin-flip inelastic electron tunnelling spectroscopy (IETS) signal and discuss spin polarization of total and circular currents in a benzene ring junction with spin impurity [Phys. Rev. B 86, 045420 (2012)]. [Preview Abstract] |
Wednesday, March 20, 2013 3:42PM - 3:54PM |
R43.00007: Interaction effects in electric transport through self-assembled molecular monolayers Martin Leijnse I will discuss a theoretical study of electric transport in molecular electronic devices based on self-assembled molecular monolayers (or other devices involving a large number of mesoscopic conductors contacted in parallel). In contrast to macroscopic conductors, Coulomb interactions between charge carriers being transported through neighboring molecules within the monolayer are large. I show that such inter-molecular Coulomb interactions not only lowers the conductance level, but lead to a correlated current and give rise to distinct signatures in the current-voltage characteristics. If some molecules fail to bond strongly to both electrodes, interactions can even give rise to negative differential resistance. Knowledge of the effects of Coulomb interactions between different conductors is important not only for the functionality of nanoelectronic devices, but also to isolate the genuine single-device properties, for example when trying to interpret a transport experiment using a molecular monolayer device in terms of single-molecule properties. Reference: Martin Leijnse, arXiv:1210.2843 [Preview Abstract] |
Wednesday, March 20, 2013 3:54PM - 4:06PM |
R43.00008: Zero-bias anomaly in thiol-bound molecular junction on Ag(111) Kees Flipse, Erwin Rossen, Jorge Cerda Single molecule transistors are widely regarded as the successor of current silicon-based technology. To investigate the electronic properties of single molecules, they must be connected to the macroscopic world via electrodes. The most used linker group to connect the molecule to the metal leads is a thiol group. One feature that is often observed in these systems is a significant reduction (10-20{\%}) in the conductance in a narrow region around the Fermi-level. While most authors choose to ignore this feature, it is in general attributed to excitations of the metal-sulphur mode and phonon interactions in the metal leads. We will discuss the origin of this zero-bias anomaly (ZBA) by presenting ab-initio calculation results in a Scanning Tunnelling Microscopy (STM) geometry for thiophenol molecules adsorbed on Ag(111), indicating the important role of the inelastic contributions of low energy vibrational modes in charge transport. [Preview Abstract] |
Wednesday, March 20, 2013 4:06PM - 4:18PM |
R43.00009: Bi-Stable States in Highly Conductive Pyrazine Molacular Junction G.P. Brivio, C. Motta, S. Kanenko, M. Kiguchi Bi-stable molecular junctions are recently deserving attention for their potential in molecular electronics applications. In the present work, we investigate the bi-stable conductance of highly conductive single-molecule pyrazine/Pt junctions. Break-junction measurements show two distinct conductance states of 1.0 G$_{\mathrm{0}}$ and 0.3 G$_{\mathrm{0}}$, G$_{\mathrm{0}}$ being the quantum of conductance. First-principles calculations reveal that the two states could be assigned to different geometrical configurations of pyrazine exhibiting larger and lower coupling with the electrodes, respectively. Inelastic tunneling electron spectroscopy measurements and theoretical analysis of the system vibrations are able to further characterize the configuration dependent conductance of such junctions. We demonstrate that the controlled torsion of the molecule is capable to switch between the two conducting regimes. This process triggered only by mechanical manipulation of the junction is reversible. [Preview Abstract] |
Wednesday, March 20, 2013 4:18PM - 4:30PM |
R43.00010: Switching of Current in a Molecular Junction Kamal Dhungana, Subhasish Mandal, Ranjit Pati Achieving atomic level control at the metal-molecule interface in a single molecule conductance measurement is a difficult task that hinders the progress in molecular electronics. The lack of atomic level structural information of the interface makes the theoretical interpretation of experimental data much harder. Herein, we create an ensemble of device structure by varying metal-molecule binding sites, the orientation of the molecule at the interface, interface distance, and conformational change within the molecule to study junction dependent conductance behavior in Ruthenium-Bis(terpyridine) molecular wire, which has been fabricated and characterized. An orbital dependent DFT in conjunction with a parameter free, single particle Green's function approach is used to study the I-V characteristics. Our calculation for the weakly-coupled ONTOP junction geometry yields a relatively small (OFF state) current value below a threshold voltage $(V_{th})$. The current value is found to increase at ~ $V_{th}$ and remains flat (ON state) after the threshold value. A similar non-linear I-V curve with a current switching behavior has been reported experimentally. [Preview Abstract] |
Wednesday, March 20, 2013 4:30PM - 4:42PM |
R43.00011: Effect of electron transfer on direct vs. indirect contact of CdSe quantum dots on TiO$_{2}$ nanoparticles Tess R. Senty, Oshadha Ranasingha, Scott K. Cushing, Congjun Wang, James P. Lewis, Christopher Matranga, Alan D. Bristow CdSe quantum dots (QD) attached to TiO$_{2}$ semiconductors (SC) have been extensively studied over the last decade. They have shown promising results for uses as energy materials including capture of light in solar cells [1] and photocatalytic reduction of CO$_{2}$ [2]. The length of linker molecules between the QD and SC has been shown to decrease the electron transfer (ET) rate exponentially for an increasing linker length [3]. Studies also indicate that this exponential decrease breaks down for direct contact [4] although the exact mechanism is not fully understood. Through visible and NIR transient absorption spectroscopy we directly probe the electron and hole dynamics of CdSe QDs on TiO$_{2}$ nanoparticles comparing intimate contact with mercaptopropionic acid linked QDs. We find that with this direct contact, the ET rate decreases, deviating from previous results. We investigate the mechanisms for this deviation including the effect of oxidation on the QD surface.\\[4pt] [1] Robel, I. et al, JACS, \textbf{128}, 2385 (2006)\\[0pt] [2] Wang, C. et al, J. Phys. Chem. Lett. \textbf{1}, 48 (2010)\\[0pt] [3] Dibbell, R. S. and Watson, D. F., J. Phys. Chem. C, \textbf{113}, 3139 (2009)\\[0pt] [4] Pernik, D. R. et al, J. Phys. Chem. C, \textbf{115}, 13511 (2012) [Preview Abstract] |
Wednesday, March 20, 2013 4:42PM - 4:54PM |
R43.00012: Surface plasmonic responses in semiconductor and metal nanostructures investigated by ultrafast electron diffraction Kiseok Chang, Tzong-Ru T. Han, Fei Yuan, Chong-Yu Ruan Incorporating metallic nanostrcutures in the molecular sensing, nanoelectronics, and catalysis devices has often yielded significantly enhanced efficiency, despite many open questions remain. Using ultrafast electron diffraction as a sensitive contact free voltammetry probe, we find the photoinduced voltages across the heterojunctions consisting of nanostructures and semiconductor or metal surfaces can be highly enhanced when the surface plasmon excitation is used as the ponderomotive drive to induce photocurrent. By using the effective circuit model, and aided by the time domain finite difference method, we are able to describe the observed timescales and spectral responses in the context of dielectric coupling, interfacial charge transfer, and strong proximity-field induced at the interfaces between the nanostrcutures, the substrate, and the surrounding medium, which help understand different origins of the surface plasmon enhancement effect. [Preview Abstract] |
Wednesday, March 20, 2013 4:54PM - 5:06PM |
R43.00013: Frontier Orbital Energy Change of Poly(3-Hexylthiophene) Oligomers: Effect of Large Amplitude Torsional Motion Ram Bhatta, Mesfin Tsige, David Perry Poly(3-hexylthiophene) (P3HT) and [6,6]-phenyl C61-butyric acid methylester (PCBM) based solar cells remain the most promising organic photovoltaics so far. Despite their promise for organic solar cells, practical application is hindered by low efficiency, associated with poor electron transport from P3HT to PCBM. For isolated P3HT oligomers, we investigate the torsional dependence of electronic properties by performing DFT calculations and extrapolate to the long chain limit. The fully relaxed isolated P3HT oligomers are non-planar with a 47 degree twist angle between each pair of rings and are lower in energy by 0.03 eV per monomer unit than the fully planar oligomers. The non-planarity lowers highest occupied molecular orbital (HOMO) energy by 1 eV and rises lowest unoccupied molecular orbital (LUMO) energy by 0.6 eV compared to the respective orbital energies in a planar P3HT. The shifts in HOMO and LUMO energies increase the band gap from 1.9 eV in planar P3HT up to 3.3 eV when all backbone angles are non-planar and point to a reduced electrical conductivity. The larger band gap in non-planar P3HT accounts for the observed blue shift in the visible P3HT absorption band in P3HT/PCBM mixtures. [Preview Abstract] |
Wednesday, March 20, 2013 5:06PM - 5:18PM |
R43.00014: Effect of geometrical orientation on the charge transfer energetics of supramolecular (tetraphenyl)-porphyrin/fullerens dyads Marco Olguin, Rajendra Zope, Tunna Baruah We present our study of several low lying charge-transfer (CT) excitation energies for a widely used donor-acceptor system composed of a porphyrin-fullerene pair. The dyad systems consist of C$_{60}$ and C$_{70}$ acceptor systems coupled to tetraphenyl-porphyrin (TPP) and tetraphenyl-(zinc)porphyrin (ZnTPP) donor systems in a co-facial orientation. We find that replacing C$_{60}$ by C$_{70}$ in a given dyad may increase the lowest charge transfer excitation energy by about 0.27 eV, whereas varying the donor in these complexes had marginal effect on the lowest charge transfer excitation energy. Additionally, we examined the effect of geometrical orientation on the CT energy by calculating several CT excited state energies for an end-on orientation of the porphyrin-fullerene dyads. The CT excitation energies are larger for the end-on orientation in comparison to the co-facial orientation by 0.6 eV - 0.75 eV. The difference is attributed to a reduced exciton binding energy in going from the co-facial to the end-on orientation. [Preview Abstract] |
Wednesday, March 20, 2013 5:18PM - 5:30PM |
R43.00015: Electron Transfer Mechanism in Gold Surface Modified with Self-Assembly Monolayers from First Principles Filipe C. D. A. Lima, Rodrigo M. Iost, Frank N. Crespilho, Mar\'Ilia J. Caldas, Arrigo Calzolari, Helena M. Petrilli We report the investigation of electron tunneling mechanism of peptide ferrocenyl-glycylcystamine self-assembled monolayers (SAMs) onto Au (111) electrode surfaces. Recent experimental investigations showed that electron transfer in peptides can occur across long distances by separating the donor from the acceptor. This mechanism can be further fostered by the presence of electron donor terminations of Fc terminal units on SAMs but the charge transfer mechanism is still not clear. We study the interaction of the peptide ferrocenyl-glycylcystamine on the Au (111) from first principles calculations to evaluate the electron transfer mechanism. For this purpose, we used the Kohn Sham (KS) scheme for the Density Functional Theory (DFT) as implemented in the Quantum-ESPRESSO suit of codes, using Vandebilt ultrasoft pseudopotentials and GGA-PBE exchange correlation functional to evaluate the ground-state atomic and electronic structure of the system. The analysis of KS orbital at the Fermi Energy showed high electronic density localized in Fc molecules and the observation of a minor contribution from the solvent and counter ion. Based on the results, we infer evidences of electron tunneling mechanism from the molecule to the Au(111). [Preview Abstract] |
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