Bulletin of the American Physical Society
2008 APS March Meeting
Volume 53, Number 2
Monday–Friday, March 10–14, 2008; New Orleans, Louisiana
Session D28: Focus Session: Transport in Nanostructures III: Single Molecules |
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Sponsoring Units: DMP Chair: Ward Plummer, University of Tennessee Room: Morial Convention Center 220 |
Monday, March 10, 2008 2:30PM - 3:06PM |
D28.00001: Understanding the Conductance of Single-Molecule Junctions from First Principles Invited Speaker: Discovering the anatomy of single-molecule junctions, in order to exploit their transport behavior, poses fundamental challenges to nanoscience. First-principles calculations based on density-functional theory (DFT) can, together with experiment, provide detailed atomic-scale insights into the transport properties, and their relation to junction structure and electronic properties. Here, a DFT scattering state approach [1] is used to explore the single-molecule conductance of two prototypical junctions as a function of junction geometry, in the context of recent experiments. First, the computed conductance of 15 distinct benzene-diamine-Au junctions is compared to a large robust experimental data set [2]. The amine-gold bonding is shown to be highly selective, but flexible, resulting in a conductance that is insensitive to other details of the junction structure. The range of computed conductance corresponds well to the narrow distribution in experiment, although the average calculated conductance is approximately 7 times larger. This discrepancy is attributed to the absence of many-electron corrections in the DFT molecular orbital energies; a simple physically-motivated estimate for the self-energy corrections results in a conductance that is much closer to experiment [3]. Second, similar first-principles techniques are applied to a range of bipyridine-Au junctions. The extent to which Au-pyridine link bonding is affected by the constraints of forming bipyridine-Au junctions is investigated. In some contrast to the amine case, the computed conductance shows a strong sensitivity to the tilt of the bipyridine rings relative to the Au surfaces. Experiments probing the conductance of bipyridine-Au junctions are discussed in the context of these findings. [1] H. J. Choi et al, Phys Rev B, 76, 155420 (2007) [2] L. Venkataraman et al, Nano Lett 6, 458 (2006) [3] S. Y. Quek et al, Nano Lett. 7, 3477 (2007) [Preview Abstract] |
Monday, March 10, 2008 3:06PM - 3:18PM |
D28.00002: Single Molecule Conductance and Contact Chemistry Latha Venkataraman, Young Park, Adam Whalley, Masha Kamenetska, Michael Steigerwald, Colin Nuckolls, Mark Hybertsen Our previous experiments probing the conductance of single molecule circuits with amine-gold linkages have demonstrated the relationship between the electrical characteristics and the intrinsic molecular properties such as their length, conformation, gap between the highest occupied and lowest unoccupied molecular orbitals and the alignment of these levels to the metal Fermi level. Here we study different chemical linker groups expected to form donor-acceptor bonds to gold. We measure transport through single molecule junctions by repeatedly forming and breaking Au point contacts with a modified STM in a solution of the molecules terminated by Amine, Dimethyl Phosphine and Methyl Sulfide linker groups. The clear molecular signatures allow us to demonstrate a systematic dependence the link group. [Preview Abstract] |
Monday, March 10, 2008 3:18PM - 3:30PM |
D28.00003: Conductance Trends in Single Molecule Junctions Formed Using Donor-Acceptor Links: Theoretical Analysis Max B. Koentopp, Latha Venkataraman, Michael L. Steigerwald, Mark S. Hybertsen The conductance of single molecule junctions using amine-gold links has been understood based on formation of a donor- acceptor bond involving the N lone pair and the s-orbital on an undercoordinated Au site on the electrode. Experiments probing junctions formed with alkanes terminated by dimethyl phosphines and methyl sulfides also show an unambiguous conductance signature. The structure and bonding in these junctions is analyzed using density functional theory based calculations. Like the amine link, the dimethyl phosphine and methyl sulfide bond to an under-coordinated Au site through a donor-acceptor motif. While the bond energy for the amine and methyl sulfide links are similar (0.6 eV), the dimethyl phosphine is significantly stronger (1.2 eV). Trends in measured junction conductance (amine $<$ sulfide $<$ phosphine) are analyzed in terms of available electronic channels. [Preview Abstract] |
Monday, March 10, 2008 3:30PM - 3:42PM |
D28.00004: Molecular Conductance of oligophenylene-vinylene in Metallic Break Junctions Patrick Wheeler, Meng Lu, David Corley, James Tour, Doug Natelson Break junctions between a metallic tip and a metallic substrate have proven to be extremely useful tools for characterizing single-molecule electrical conductance. Conductance measurements while repeatedly breaking and reforming junctions are conducive to rapid statistical characterization. We will present preliminary results of room temperature break junction conductance measurements on amine-terminated oligophenylene-vinylene (OPV) oligomers. Recent low temperature measurements of OPV oligomers in the electromigrated gap configuration imply a large renormalization downward of the HOMO-LUMO gap. Since the HOMO-LUMO gap is correlated with the conductance and the tunneling coefficient, beta, break junction measurements should provide clarification about the HOMO-LUMO gap in these molecules. [Preview Abstract] |
Monday, March 10, 2008 3:42PM - 3:54PM |
D28.00005: Probing mechanisms of electrical conduction in single organic molecules Lyudmyla Adamska, Ivan Oleynik, Mortko Kozhushner The electrical conduction of relatively long (1-2 nm) single organic molecules occurs via resonant tunneling of charge carriers, electrons and/or holes, through the energy levels of negative molecular ion (electrons) and/or positive molecular ion (holes). The position of these resonant energy levels with respect to the Fermi levels of the anode and cathode determines the relative contributions of electron and hole conduction to the resonant current. These resonant levels depend on the applied bias, and are also influenced by several physical factors such as the polarization of the molecule, image potential and metal/molecule interfaces that are difficult to control under conditions of real experiment. In this presentation we suggest a method of \textit{unambiguous experimental} determination of specific type of the conduction mechanism (electron or hole conduction) which is based on the idea of utilizing experimental techniques of nanocalorimetry. [Preview Abstract] |
Monday, March 10, 2008 3:54PM - 4:06PM |
D28.00006: Single Molecule Junctions: Conductance, Formation and Persistence Statistics. Maria Kamenetska, Michael Frei, Mark Hybertsen, Latha Venkataraman We measure the conductance of single molecules attached to gold electrodes by repeatedly forming and breaking Au point contacts with a modified STM in a solution of molecules. Conductance traces measured while pulling the point-contacts reveal steps due to the formation of single molecule junctions which can be elongated without a significant change in junction conductance. To better understand the mechanical stability of these single molecule junctions, we analyze data sets of 20000 or more individual conductance traces for a series of diamine molecules, measuring the distance over which junctions can persist. We find that the distance that a junction can be pulled is affected by the metal-molecule binding energy. In addition, we see an unambiguous relationship between geometry and stability, where both the length of the molecule as well as the atomic configuration of the contact electrode affect the distance over which a junction can persist. [Preview Abstract] |
Monday, March 10, 2008 4:06PM - 4:18PM |
D28.00007: Tunneling Transport through Long Molecular Chains Emil Prodan, Roberto Car The Riemann structure of the bands and other properties of the evanescent Bloch functions have been used to derived an asymptotic expression for the tunneling conductance through long molecular chains. Our results give the contact conductance in terms of an overlap integral of three well defined and physically relevant quantities. In particular, this formula shows how the conducting states of the leads couple to the evanescent Bloch functions of the insulating chain. The theory is applied to amine-linked alkyl and aromatic chains and the results are compared with the experiment. Using these applications, we discuss the key aspects and advantages of the theory. Extensions to spin dependent transport will be also discussed. [Preview Abstract] |
Monday, March 10, 2008 4:18PM - 4:30PM |
D28.00008: Transport properties of molecular wires from ab initio calculations Kenji Hirose, Nobuhiko Kobayashi Understanding of electron transport through nanostructures becomes important with the advancement of fabrication process to construct atomic-scale devices. Due to the drastic change of transport properties by contact conditions to electrodes in local electric fields, first-principles calculation approaches are indispensable to understand and characterize the transport properties of nanometer-scale molecular devices. Here we focus on the transport properties of molecular wires bridged between metallic electrodes, especially on the effects of contacts to electrodes and on the dependence of the length of molecular wires on transport properties. We use an ab initio calculation method based on the scattering waves, which are obtained by the recursion-transfer-matrix (RTM) method, combined with non-equilibrium Green's function (NEGF) method. We find that conductance shows exponential behaviors as a function of the length of molecular wires due to tunneling process determined by the HOMO-LUMO energy gap. From the voltage drop behaviors inside the molecular wires, we show that the contact resistances are well separated for the long molecular wires. We will present detailed data of electronic states at contacts to metallic electrodes under strong electronic fields and will discuss the polarization, screening effect, and potential barrier formation at contacts on the transport properties of molecular wires, comparing them with those of metallic atomic wires. [Preview Abstract] |
Monday, March 10, 2008 4:30PM - 4:42PM |
D28.00009: The molecular electronics of protein fragments David Cardamone, George Kirczenow Small fragments of polypeptide chains provide a uniquely scalable, customizable basis for nanoelectronic devices. Using a combination of \emph{ab initio} and semi-empirical techniques, we arrive at a quantitative understanding of the charge transport properties of these molecules. This allows us to investigate their chemical and physical properties, such as lead-molecule bonding geometry, lead-induced distortion of molecular structure (e.g., molecular stretching), and device properties. We explain the observed current rectification in these molecules and further predict negative differential resistance, opening the way to protein-based nanoelectronic devices. [Preview Abstract] |
Monday, March 10, 2008 4:42PM - 4:54PM |
D28.00010: Ab initio studies of electronic transport through amine-Au-linked junctions of photoactive molecules David A. Strubbe, Su Ying Quek, Latha Venkataraman, Hyoung Joon Choi, J.B. Neaton, Steven G. Louie Molecules linked to Au electrodes via amine groups have been shown to result in reproducible molecular conductance values for a wide range of single-molecule junctions [1,2]. Recent calculations have shown that these linkages result in a junction conductance relatively insensitive to atomic structure [3]. Here we exploit these well-defined linkages to study the effect of isomerization on conductance for the photoactive molecule 4,4'-diaminoazobenzene. We use a first-principles scattering-state method based on density-functional theory to explore structure and transport properties of the cis and trans isomers of the molecule, and we discuss implications for experiment. [1] L Venkataraman et al., Nature 442, 904-907 (2006); [2] L Venkataraman et al., Nano Lett. 6, 458-462 (2006); [3] SY Quek et al., Nano Lett. 7, 3477-3482 (2007). [Preview Abstract] |
Monday, March 10, 2008 4:54PM - 5:06PM |
D28.00011: Electronic spectrum and orbital filling in a single-molecule junction Edgar A. Osorio, Kevin O'Neill, Maarten Wegewijs, Nicolai Stuhr-Hansen, Jens Paaske, Thomas Bjornholm, Herre van der Zant We study single-electron tunneling in three-terminal devices in which a single molecule bridges the gap between source and drain electrode. The molecular devices are made by electromigration and at low temperatures excitations appear in the stability diagram. For the OPV-5 molecule more than fifteen different excitations are visible, of which twelve match RAMAN spectra and the remaining ones are due to vibratrions of the molecule attached to gold electrodes at energies below 10 meV. Similar to carbon nanotubes, the observation of a singlet-triplet transition allow us to determine the orbital filling and spin configuration of the molecule. [Preview Abstract] |
Monday, March 10, 2008 5:06PM - 5:18PM |
D28.00012: Gate effects on electronic transport in alkanedithiol single molecular junctions. Hyunwook Song, Takhee Lee, Youngsang Kim, Heejun Jeong We investigate the gate effects on the electronic transport properties in alkanedithiol single molecular junctions. Using electromigration-induced break junction technique, we fabricated an array type of electrode pairs with nanometer-sized separation on top of naturally oxidized aluminum gate electrodes. The alkanedithiol molecules were bridged between the nanometer-sized gap that was achieved by breaking gold nanowires fabricated using electron-beam lithography with a controlled passage of current. The electric potential applied to the aluminum gate electrode shifts the molecular energy levels relative to the Fermi energy in the metallic contacts. We will discuss the gate-bias dependent current-voltage characteristics and other observed transport properties of alkanedithiol single molecular junctions in the off-resonant tunneling transport regime. [Preview Abstract] |
Monday, March 10, 2008 5:18PM - 5:30PM |
D28.00013: Modeling of N@C$_{60}$ single-molecule transistors Carsten Timm, Jacob E. Grose, Wolfgang Harneit, Daniel C. Ralph We report on recent experimental and theoretical results for single-molecule transistors involving endohedral N@C$_{60}$ fullerene molecules. In this talk, we will focus on the theoretical modeling. The observed differential conductance shows strong evidence for the exchange interaction between electrons in the fullerene LUMO and the nitrogen p-electrons, favoring an antiferromagnetic interaction. In addition, soft vibrational modes are seen, which are attributed to oscillations of the molecule as a whole. We discuss a model Hamiltonian that reproduces the main features of the experimental conductance. [Preview Abstract] |
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