Bulletin of the American Physical Society
2007 APS March Meeting
Volume 52, Number 1
Monday–Friday, March 5–9, 2007; Denver, Colorado
Session X26: Focus Session: Charge Transport in Nanostructures IV |
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Sponsoring Units: DCP Chair: Paul McEuen, Cornell University Room: Colorado Convention Center 205 |
Friday, March 9, 2007 8:00AM - 8:36AM |
X26.00001: Molecular Junction Transport - Some Vibronic Effects Invited Speaker: The behavior of molecular transport junctions in the coherent tunneling (Landauer-Imry) regime is rapidly becoming understood. But vibronic effects characterize molecules, and understanding how they act in such junctions is a significant issue. This talk will deal with the role of both weak and strong vibronic interactions in molecular junctions. The weak mixing appears in IETS spectra, and can be handled by perturbation theory in the coherent tunneling limit. It provides some quantitative comparisons between calculation and experiment, And can clarify pathways for transport. But strong vibronic interaction requires a more elaborate analysis, and changes the mechanisms for transport. Hysteresis and switching behaviors will be discussed. [Preview Abstract] |
Friday, March 9, 2007 8:36AM - 8:48AM |
X26.00002: Vibrationally induced two-level systems in single molecule junctions Robert Thijssen, Darko Djukic, Sander Otte, Rolf Bremmer, Jan van Ruitenbeek It is found that differential conductance spectra of small single molecules contacted by metal electrodes display positive or negative peaks. The positions in energy of these peaks correspond with the energies of local vibration modes of the molecule in the junction. A model of vibrationally induced two-level systems is made in order to explain the physics responsible for the observed features. A molecule in an atomic junction can be contacted in two geometrically different configurations, each of which results into a different junction conductance. These two energy minima are separated by a large energy barrier. Only by vibrationally exciting the molecule above the barrier, a transition between the configurations is possible. This results in a sudden jump in conductance and a peak in differential conductance. The vibrationally induced two-level switching is expected to be quite general, since we have observed dI/dV peaks in many different single molecule-metal junctions. It acts as an intrinsic amplification mechanism for local vibration mode features, even when large conductance fluctuations are present. Therefore it could be exploited as a new spectroscopic tool for identifying local vibration mode energies. [Preview Abstract] |
Friday, March 9, 2007 8:48AM - 9:00AM |
X26.00003: Electronic and Vibronic Spectroscopy of Molecular Junctions James Kushmerick Transition voltage spectroscopy and inelastic electron tunneling spectroscopy are used to explore charge transport in molecular junctions. Our recent work has shown that a mechanistic transition occurs from direct tunneling to field emission in molecular junctions. The magnitude of the voltage required to enact this transition is molecule-specific, and thus constitutes a form of spectroscopy. We demonstrate that the transition voltage of a conjugated molecule depends directly on the manner in which the conjugation path is extended. Furthermore using inelastic electron tunneling spectroscopy to measure the vibronic structure of non-equilibrium molecular transport, aided by a quantitative interpretation scheme based on non-equilibrium Greens function/density functional theory methods, we are able to characterize the actual pathways that the electrons traverse when moving through a molecule in a molecular transport junction. [Preview Abstract] |
Friday, March 9, 2007 9:00AM - 9:12AM |
X26.00004: Charge Modification of Vibrational Features in Inelastic Electron Tunneling Spectroscopy Lam Yu, James Kushmerick Inelastic electron tunneling (IET) spectroscopy of Au-decandithiol-Ni atoms-benzenethiol-Au, Au-decandithiol-Au colloids-benzenethiol-Au and Au-decandithiol-benzenethiol-Au junctions are investigated by cross-wire tunnel junctions. Both the IET spectroscopic features' intensities and line shapes are observed to be significantly modified by the presence of a metal-sandwich layer. We attribute the vibronic features modification to the interaction between the metallic electronic levels and molecularly coupled phonons in the molecular junctions. Our results provide experimental insights into understanding the origin of some of the differences observed in two previous molecular IET spectroscopy experiments by Kushmerick et al. (Nano lett. 2004, 4, 639) and Wang et al. (Nano lett. 2004, 4, 643). [Preview Abstract] |
Friday, March 9, 2007 9:12AM - 9:24AM |
X26.00005: Dynamic Molecular Nanostructures Assembled with Atomic Manipulation Brian K. Foster, Hari C. Manoharan Molecular nanostructures of CO were engineered on a Cu(111) surface by single molecule manipulation in a custom-built low-temperature scanning tunneling microscope (STM). The structures were designed to allow for well-defined motion of either a single molecule or linked sets of molecules on the surface, for the purpose of studying the system's dynamic translational behavior. Dynamics such as meta-stability, bi-stability and molecular vibration were observed; STM measurements were used to decipher dynamic behavior though coupling to electronic charge via elastic and inelastic tunneling. We also explore the coupling of vibrational modes of individual molecules (at THz frequencies) to the molecular translation rates (at kHz frequencies or below). The dynamic behavior of such structures offers the potential to control and transmit information across surfaces. [Preview Abstract] |
Friday, March 9, 2007 9:24AM - 9:36AM |
X26.00006: Classical Nuclear Motion in Quantum Transport Claudio Verdozzi, Gianluca Stefanucci, Carl-Olof Almbladh A quantum-classical scheme is presented to study nuclear motion in time-dependent quantum transport. The nuclei are treated in the Ehrenfest approximation. We illustrate the method in terms of model systems results. We show how electron-lattice interactions may induce dynamical Peierls distortions in short wires, and change their conducting behavior. We also show time-resolved results for current-induced molecular desorption and suggest that AC biases could provide a way to tailor electromigration. The results illustrate the importance of non-adiabatic effects for transient phenomena in nanodevices. [Preview Abstract] |
Friday, March 9, 2007 9:36AM - 9:48AM |
X26.00007: Inelastic effects in noise properties of molecular junctions Michael Galperin, Abraham Nitzan, Mark A. Ratner The effect of electron-phonon coupling on the current noise in a molecular junction is investigated within a simple model. The model comprises a one-level bridge representing a molecular level that connects between two free electron reservoirs and is coupled to a vibrational degree of freedom representing a molecular vibrational mode. The latter in turn is coupled to a phonon bath that represents the thermal environment. We focus on the zero frequency noise spectrum and study the changes in its behavior under weak and strong electron-phonon interactions. In the weak coupling regime we find that the noise amplitude can increase or decrease as a result of opening of an inelastic channel. In particular the relative Fano factor decreases with increasing off resonance distance and junction asymmetry. For resonant inelastic tunneling with strong electron-phonon coupling the differential noise spectrum can show phonon sidebands in addition to a central feature. A striking crossover of the central feature from double to single peak is found for increasing asymmetry in the molecule-leads coupling or increasing electron-phonon interaction. A possible use of noise data from scanning tunneling microscopy experiments for estimating the magnitude of the electron-phonon interaction on the bridge is proposed. [Preview Abstract] |
Friday, March 9, 2007 9:48AM - 10:00AM |
X26.00008: \textit{In-situ} Inelastic Electron Tunneling Spectroscopy of Oligoaniline Molecular Junctions Heayoung Yoon, Masato Maitani, Lintao Cai, David Allara, Theresa Mayer Recently, several studies have reported that self assembled monolayers of oligoaniline dimmers showed room temperature bistable switching behavior. In this talk, we will discuss the electrical and spectroscopic properties of thiol-substituted oligoaniline (OA) molecular junctions at the interface of lithographically-defined bottom metal nanowire contacts and metal nanowire top contacts. The junctions showed reproducible room temperature bistable switching with the threshold voltages of approximately $\pm $ 1.5 V and I-V(T) showed the dominant transport mechanism is coherent tunneling. Inelastic electron tunneling (IET) spectra in low and high current states were obtained at 5 K using a standard AC modulation technique to collect the second harmonic signal directly. The observed IET peaks in a plot of d2I/dV2 versus V were compared to infrared and Raman spectra for the OA self-assembled monolayers. The measurement confirms that the measured transport properties of molecular junctions are due to the intended molecule rather than process induced artifacts. In addition, the intensity change of vibrational modes of the benzene ring (185mV) and the quinon (197mV) of the OA in the low to the high current state suggest that the switching behavior is attributed to an inherent molecular feature of the OA molecules that form the junction. [Preview Abstract] |
Friday, March 9, 2007 10:00AM - 10:12AM |
X26.00009: Shot noise measurements on a single molecule Jan M. van Ruitenbeek, Oren Tal Fabrication of molecular junctions with diverse and controlled functionality requires a fundamental understanding of the relation between the structure and conductance properties of these junctions. We address this issue using simple organic molecules (e.g., hydrogen, carbon monoxide, and benzene) as a molecular bridge between two Pt electrodes formed by the mechanical break junction technique. Shot noise is used to reveal the number of conductance channels through the molecular junction, and their probabilities, while point contact spectroscopy yielded its characteristic vibration modes. This diverse information combined with theoretical calculations allows us to present a detailed picture of the relation between the conductance and the junction structure. [Preview Abstract] |
Friday, March 9, 2007 10:12AM - 10:24AM |
X26.00010: Quantum-state-resolved probing of molecular inner-sphere reorganization using a single-molecule transistor Nathalie de Leon, Wenjie Liang, Qian Gu, Hongkun Park The coupling of electron transport through a single molecule to various degrees of freedom, such as spin, charge, and vibrations, can be probed using a single molecule transistor. The addition or subtraction of electrons usually accompanies a change in molecular geometry, a phenomenon known as inner-sphere reorganization. We have studied the effects of inner-sphere reorganization on electron transport using two complexes, ferrocene and Fe(bpy)$_{3}^{n+}$ (n = 1, 2, 3), as model systems. The reported energies of vibrational excitations in Fe(bpy)$_{3}^{n+}${\-} are in agreement with existing IR and Raman spectroscopic data. [Preview Abstract] |
Friday, March 9, 2007 10:24AM - 10:36AM |
X26.00011: Quantum Channels and Conductance Oscillations in Metal/Molecule/Metal Switches Feng Miao, Douglas Ohlberg, R. Stanley Williams, C. N. Lau We investigate conductance switching in Pt/stearic acid monolayer/Ti devices by pressure-modulated conductance microscopy. For devices with conductance G$>>$G$_{Q}$ and G$<<$ G$_{Q}$, where G$_{Q}$ is the conductance quantum, localized pressure-induced conductance peaks are observed, indicating formation of nanoscale conductance pathways on the electrodes. For devices with G$\sim $ 1- 2 G$_{Q}$, in addition of conductance peaks, we also observed conductance dips and oscillations in response to localized pressure. These results suggest formation of quantum conductance channels in our devices, and can be satisfactorily modeled by considering interfering electron waves between two partially transmitting electrodes. Moreover, the force dependence of such conductance modulations is fully consistent with this model. Our findings underscore the possible use of these devices as atomic switches. [Preview Abstract] |
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