Bulletin of the American Physical Society
2006 APS March Meeting
Monday–Friday, March 13–17, 2006; Baltimore, MD
Session H36: Focus Session: Single Molecule Conductance |
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Sponsoring Units: DMP Chair: Doug Natelson, Rice University Room: Baltimore Convention Center 339 |
Tuesday, March 14, 2006 11:15AM - 11:51AM |
H36.00001: Probing molecular electronics with mechanical break junctions Invited Speaker: We will report on experiments with single molecule junctions, performed with the mechanically controlled break-junction technique. A review is given on the capabilities of the technique and the results obtained so far. The importance of the molecular structure, the local environment, the contacts, and of the electronic polarizability will be elucidated. As a particular example, we will present an experiment with a molecule that was designed to form a single-molecule diode when contacted from two sides. The concept is closely related to Aviram-Ratner's Gedankenexperiment. Indeed, the IVs show a pronounced asymmetry, whereas a blind experiment with symmetric molecules resulted in symmetric IVs. A closer analysis of the data, involving theoretical models, suggests that the bias-dependent charge reconfiguration of the electronic structure is responsible for the diode-like characteristics. [Preview Abstract] |
Tuesday, March 14, 2006 11:51AM - 12:03PM |
H36.00002: Pulling gold nanowires with a hydrogen clamp Szabolcs Csonka, Andras Halbritter, George Mihaly Gold nanojunctions were found to be perfect candidates for studying the quantum nature of the conductance. It was shown that monoatomic gold contacts have a single conductance channel with perfect transmission. During the elongation of a single-atom gold contact stable atomic chain can be formed [2]. We have performed an experimentally study of the interaction of gold nanowires with hydrogen molecules by the Mechanically Controllable Break Junction technique [1]. Our results show, that in hydrogen environment the conductance of the chain is strongly reduced compared to the perfect transmission of pure Au chains. The comparison of the experiments with recent theoretical prediction for the hydrogen welding of Au nanowires [3] implies that a hydrogen molecule can even be incorporated in the gold nanocontact, and this hydrogen clamp is strong enough to pull a chain of gold atoms. [1] Csonka et al., to be published in Phys. Rev. B, cond-mat/0502421 (2005). [2] Agrait et al., Phys. Rep. \textbf{377}, 81 (2003). [3] Barnett, Nano Letters \textbf{4}, 1845 (2004). [Preview Abstract] |
Tuesday, March 14, 2006 12:03PM - 12:15PM |
H36.00003: Gold point-contact measurements of molecular junctions Latha Venkataraman, Jennifer Klare, Iris Tam, Colin Nuckolls, Mark Hybertsen, Michael Steigerwald The conductance of molecular junctions, formed by breaking gold point-contacts dressed with various thiol-functionalized organic molecules, is measured at 293 K and at 30 K. In the presence of molecules, individual conductance traces measured as a function of increasing gold electrode displacement show clear steps below the quantum conductance steps of the gold contact. These steps are distributed over a wide range of molecule-dependent conductance values. Histograms constructed from all conductance traces therefore do not show clear peaks either at room and low temperatures. Filtering of the data sets by an objective automated procedure does only marginally improve the visibility of such features. We conclude that the geometrical junction to junction variations dominate the conductance measurements. [Preview Abstract] |
Tuesday, March 14, 2006 12:15PM - 12:27PM |
H36.00004: Theoretical Exploration of the Impact of Link Chemistry on Single Molecule Junction Conductance Mark S. Hybertsen, Michael Steigewald, Latha Venkataraman, Jennifer E. Klare, Iris W. Tam, Colin Nuckolls Measurement of molecular conductance by repeatedly breaking an Au point contact in an environment of molecules allows for the study of a large number of fresh junctions and presents a statistical picture of the junction conductance. However, there is no direct control or knowledge of the atomic scale structure in each individual junction. It is important to distinguish effects on the conductance related to the structure of the metal-molecule link from those that are intrinsic to the backbone of the molecule in the junction. To that end, we examine the energetics and frontier electronic states of various link groups such as thiolate and isonitrile coupled to candidate Au structures using DFT based calculations. We discuss systematic trends in comparison with experiments. [Preview Abstract] |
Tuesday, March 14, 2006 12:27PM - 12:39PM |
H36.00005: Charge Storage Based Hysteretic Negative-Differential-Resistance in Metal-Molecule-Metal Junctions Richard Kiehl, John Le, Panglijen Candra, Rebecca Hoye, Thomas Hoye Experimental results on the electrical characteristics of Hg-alkanethiol//arenethiol-Au molecular junctions are used to develop a physical model for the hysteretic negative differential resistance observed for these, and possibly other, metal-molecule-metal junctions. The dependence of the current-voltage characteristic on sweep direction and rate are examined together with the voltage dependence of the junction's ac conductance. Based on several specific electronic properties, it is concluded that the observed behavior is caused by a slow charge storage process. The implications of this model on potential electronic applications are discussed. [Preview Abstract] |
Tuesday, March 14, 2006 12:39PM - 12:51PM |
H36.00006: Kondo resonances and anomalous gate dependence of electronic conduction in single-molecule transistors D. Natelson, L.H. Yu, Z.K. Keane, J.W. Ciszek, L. Cheng, J.M. Tour, T. Baruah, M.R. Pederson We report Kondo resonances in the conduction of single-molecule transistors based on transition metal coordination complexes. We find Kondo temperatures in excess of 50~K, comparable to those in purely metallic systems. The observed gate dependence of the Kondo temperature is inconsistent with observations in semiconductor quantum dots and a simple single-dot-level model. We discuss possible explanations of this effect, in light of electronic structure calculations. [Preview Abstract] |
Tuesday, March 14, 2006 12:51PM - 1:03PM |
H36.00007: Self-aligned lithography and in-situ assembly of chemically responsive single-molecule transistors Jinyao Tang, jennifer Klare, Yiliang Wang, Etienne De Poortere, Colin Nuckolls, Shalom Wind We report the fabrication and assembly of single-molecule transistors comprising ultrathin metal electrodes separated by a nanoscale gap, which is bridged by a single molecule or a small number of molecules. The electrodes sit upon a conductive substrate, which serves as a gate, separated by a thin gate dielectric, and the gap is defined by a completely self-aligned process involving the lateral oxidation of a sacrificial thin film of Al. Devices with gaps ranging from $\sim $ 2 - 10 nm are fabricated with yields approaching 80{\%}. Highly conjugated bis-oxazole molecules are assembled within the gaps in a sequential fashion, relying upon individually designed end-group chemistry to control the attachment of molecular units to the metal electrodes and the modular assembly of the bis-oxazole units, respectively. In addition, metal ion complexes are used to reversibly attach and detach terpyridyl molecular units from one another. Fully assembled devices display distinctive electrical response, which is strongly modulated by the molecular assembly and attachment. [Preview Abstract] |
Tuesday, March 14, 2006 1:03PM - 1:15PM |
H36.00008: Prevalence of Coulomb blockade in conjugated and non-conjugated molecules on gold versus palladium electro-migrated junctions Artur Erbe, LaRue Dunkleberger, Kirk Baldwin, Robert Willett, Anat de Picciotto, Jennifer Klare, Kenji Sugo, Colin Nuckolls The conduction properties of electro-migration gap junctions made with Au or Pd and with various organic molecules incorporated in the gaps are studied to expose the full range of possible transport processes, and their prevalence. Primarily comparisons are made between molecules with an electron delocalized vs. electron-localized backbone, between molecules with one vs. two thiol end groups, and between molecules with and without large side chains on the Au junctions. Within these junctions Coulomb blockade can be observed in all molecular species tested, including bare junctions and those coated by molecules with no electron-accepting properties, but at significantly lower prevalence than molecules with delocalized electron backbones. Importantly, Coulomb blockade with high charging energy values is seen almost exclusively on junctions with molecules possessing the delocalized electrons. An overall lower prevalence of Coulomb blockade is observed in the Pd junctions, but with the same relative molecular dependence. These results indicate the scope of variation in transport possible for molecules on electro-migrated junctions of various metals. [Preview Abstract] |
Tuesday, March 14, 2006 1:15PM - 1:27PM |
H36.00009: Control of topography, stress and diffusion at molecule-metal interface Nikolai Zhitenev, Donald Tennant, Raymond Cirelli, Weirong Jiang, Eric Garfunkel, Artur Erbe, Zhenan Bao The electronic properties of molecular devices that are just a few atomic layers thick are determined not solely by the properties of the molecules but are equally dependent on dopants, defects and electronic states at the interfaces. We study the phenomena affecting the conductance of molecular devices by systematically varying the growth conditions at the metal-molecule interface. Transport properties of metal-molecule-metal junctions containing monolayer of conjugated and saturated molecules with characteristic dimensions in the range of 30-300 nm are correlated with microscopic topography, stress and chemical bonding at metal-molecule interfaces. Small shadow masks defined within a stack of Si/SiO$_{2}$/SiN$_{x}$/SiO$_{2}$ layers are used to obtain features below the usual lithographic limits. We demonstrate that the defects/interfaces can be rationally controlled and that their properties are often more important than the electronic properties of molecules in determining the device conductance. The density of defects caused by metal penetration into monolayers is significantly reduced yielding $>$95{\%} of non-shorted devices. Our statistically significant dataset allows us to conclude that the conductivity of organic molecules $\sim $1.5 nm long is at least 4 orders of magnitude lower than is commonly believed. [Preview Abstract] |
Tuesday, March 14, 2006 1:27PM - 1:39PM |
H36.00010: Electronic Transport through Organic Monolayer Devices Duncan Stewart, Jason Blackstock, Carrie Donley, Zhiyong Li, Douglas Ohlberg, R. Stanley Williams, Sehun Kim, Regina Ragan We report experimental studies of electronic transport through molecular monolayers. Particular emphasis is placed on combining detailed chemical, physical and electronic characterization in a single test structure, and to whatever degree possible, fabricating well-defined interfaces that enable quantitative chemical and physical analysis. To this end, we describe physical characterization of ultra-flat template-stripped Au and Pt metal electrodes including UHV-STM imaging and incorporation into a new stencil-based nanopore structure. In-situ XPS and IR spectroscopy are used to characterize the organic monolayers and the buried inorganic/organic interfaces. Using this well-characterized device structure, we present detailed I-V characterization including temperature dependence and IETS spectroscopy of several alkane self-assembled and Langmuir-Blodgett monolayers, correlating both electrical switching behavior and IETS spectral responses to the measured chemical and physical structure of the device. [Preview Abstract] |
Tuesday, March 14, 2006 1:39PM - 1:51PM |
H36.00011: Structural Characterization of a Molecular Junction by X-ray Reflectivity Julian Baumert, Michael Lefenfeld, Eli Sloutskin, Peter Pershan, Moshe Deutsch, Collin Nuckolls, Ben Ocko In the field of molecular electronics, the nature and pathways of charge transfer through molecules is among the most intensely studied open questions. Experimental studies, employing both single molecules and self-assembled monolayers attached to the electrodes, have demonstrated that the electronic characteristic of these junctions is difficult to reproduce. We report x-ray reflectivity studies of the structure of organic mono- and bi-layers self-assembled between two conducting electrodes: silicon and mercury. At high molecular coverage, the Angstrom resolution high-energy synchrotron x-ray measurements reveal densely-packed layers of roughly interface-normal molecules. The interface normal structure is stable and relatively insensitive to electric fields when a voltage is applied across the junction. Furthermore, our x-ray studies reveal that variation of the molecular coverage of the electrodes influences the structure and quality of the molecular junctions. [Preview Abstract] |
Tuesday, March 14, 2006 1:51PM - 2:03PM |
H36.00012: Charge addition effects in phenylene ethynylene oligomers: the effect of -NO$_{2}$ substitution S. W. Robey, C. D. Zangmeister, R. D. van Zee Interest in aromatic systems based on phenylene ethynylene oligomers (OPE) has been spurred by reports of interesting negative differential resistance/hysteretic behavior in metal-molecule-metal junctions. Theoretical work has invoked the importance of charge addition effects on conformation and electronic structure and polaronic effects to provide potential explanations for this behavior. We have investigated the influence of charge addition, via ``doping'' with K, in pristine versus nitro-substituted OPE using photoelectron spectroscopies. Sequential addition of K in unsubstituted OPE produces a gradual shift to higher binding energy, reminiscent of ``rigid band'' behavior, with relative shifts of $\pi $ levels suggesting some distortion/conformation change. For a nitro-substituted oligomer, a markedly different behavior is observed with K addition. After an initial shift, the electronic levels are pinned for subsequent K addition before a final shift brings the spectrum to near coincidence with that for the pristine oligomer. Potential explanations for this behavior are provided based on model calculations of the effects of charge addition on the molecular electronic structure. We have also investigated differences in the reorganization energy that accompanies charge addition/removal in these systems by a combination of photoelectron and optical spectroscopies and theoretical calculation. [Preview Abstract] |
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