Bulletin of the American Physical Society
2005 APS March Meeting
Monday–Friday, March 21–25, 2005; Los Angeles, CA
Session W35: Molecular Electronics IV |
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Sponsoring Units: DCP DMP FIAP Chair: Douglas Natelson, Rice University Room: LACC 511B |
Thursday, March 24, 2005 2:30PM - 3:06PM |
W35.00001: Invited Speaker: |
Thursday, March 24, 2005 3:06PM - 3:42PM |
W35.00002: Current and Charge Control in Molecular Transistors Invited Speaker: In a simple, conceptual model for a molecular transistor\footnote{ P.M. Solomon and C.R. Kagan, ``Understanding the Molecular Transistors,'' in \textit{Future Trends In Microelectronics-The Nano Millenium}, Luryi, Zaslavsky, Xu (Eds.), Wiley Interscience, New York, 2004.} current and charge control mechanisms analogous to that of a conventional ballistic field effect transistor were assumed, where the molecular levels are translated with respect to source and drain Fermi levels by the self-consistent potential in the center of the molecule resulting from the gate, source and drain voltages and the internal `space' charge. It was also shown that a single resonant level is inadequate for achieving a large on-off ratio concomitant with high performance, leading to the concept of using molecules to design an electronic filter. The assumptions of this model were tested by the present authors using a model `biphenyl' molecular transistor, and self-consistently solving Schr\"{o}dinger's and Poisson's equations within the density-functional formalism\footnote{ N. D. Lang, Phys. Rev. B \textbf{2001}, 64, 235121.}. While some gross features of the simple model were preserved, other types of behavior were completely unexpected, such as the observation of non-monotonic potential progression with a monotonically increasing gate voltage. In contrast to the simple model, polarization of levels well below the Fermi level dominated the charge control. This new, complex and interesting behavior will be discussed along with its bearing on the molecular transistor as a logic element. [Preview Abstract] |
Thursday, March 24, 2005 3:42PM - 4:18PM |
W35.00003: Tools for Studying Electron and Spin Transport in Single Molecules Invited Speaker: Experiments in the field of single-molecule electronics are challenging in part because it can be very difficult to control and characterize the device structure. Molecules contacted by metal electrodes cannot easily be imaged by microscopy techniques. Moreover, if one attempts to characterize the device structure simply by measuring a current-voltage curve, it is easy to mistake nonlinear transport across a bare tunnel junction or a metallic short for a molecular signal. I will discuss the development of a set of experimental test structures that enable the properties of a molecular device to be tuned controllably in-situ, so that the transport mechanisms can be studied more systematically and compared with theoretical predictions. My collaborators and I are developing the means to use several different types of such experimental "knobs" in coordination: electrostatic gating to shift the energy levels in a molecule, mechanical motion to adjust the molecular configuration or the molecule-electrode coupling strength, illumination with light to promote electrons to excited states or to make and break chemical bonds, and the use of ferromagnetic electrodes to study spin-polarized transport. Our work so far has provided new insights into Kondo physics, the coupling between a molecule's electronic and mechanical degrees of freedom, and spin transport through a molecule between magnetic electrodes. \newline \newline Collaborators: Radek Bialczak, Alex Champagne, Luke Donev, Jonas Goldsmith, Jacob Grose, Janice Guikema, Jiwoong Park, Josh Parks, Abhay Pasupathy, Jason Petta, Sara Slater, Burak Ulgut, Alexander Soldatov, H\'{e}ctor Abru\~{n}a, and Paul McEuen. [Preview Abstract] |
Thursday, March 24, 2005 4:18PM - 4:30PM |
W35.00004: A 4.5 kbit molecular-electronic memory at 3x10$^{10}$ elements/cm$^{2}$ Erica DeIonno, Y. Luo, E. Johnston-Halperin, R.A. Beckman, J.E. Green, K. Beverly, J.R. Heath, S. Nygaarel, J.O. Jeppsen, B.W. Luarsen, J.F. Stoddart We present the fabrication of 4.5 kbit random access molecular-electronic memory devices. The devices are based on a two-dimensional crossbar architecture with the bottom electrode array fabricated by SNAP and consisting of 150 n-type Si nanowires at a pitch of 34 nm, while the top electrode array is metallic and consists of 30 wires at a pitch of 100 nm fabricated by e-beam lithography. The active layer consists of a monolayer of bi-stable [2]-rotaxane supramolecule prepared on a Langmuir-Blodgett trough and deposited between the top and bottom electrodes. As a result, each crossing point between the electrodes serves as an independently addressable molecular switch tunnel junction. A group of 64 randomly selected bits from each device was tested, revealing reliable point-addressability and multiple-cycle lifetimes for individual bits. [Preview Abstract] |
Thursday, March 24, 2005 4:30PM - 4:42PM |
W35.00005: Monte Carlo Simulation of Exciton Dynamics in Supramolecular Semiconductor Architectures Carlos Silva, Cl\'{e}ment Daniel, David Beljonne, Laura Herz, Freek Hoeben, Pascal Jonkheijm, Albertus Schenning, Bert Meijer Supramolecular chemistry is useful to construct molecular architectures with functional semiconductor properties. To explore the consequences of this approach in molecular electronics, we have carried out ultrafast measurements of exciton dynamics in supramolecular assemblies of an oligo-\emph{p}-phenyl\-ene\-vinyl\-ene derivative functionalized to form chiral stacks in dodecane solution in a thermotropically reversible manner. We apply a model of incoherent exciton hopping within a Monte Carlo scheme to extract microscopic physical quantities. The simulation first builds the chiral stacks with a Gaussian disorder of site energies and then simulates exciton hopping on the structure and exciton-exciton annihilation to reproduce ensemble-averaged experimental data. The exciton transfer rates are calculated beyond the point-dipole approximation using the so-called line-dipole approach in combination with the F\"{o}rster expression. The model of incoherent hopping successfully reproduces the data and we extract a high diffusion coefficient illustrating the polymeric properties of such supramolecular assemblies. The scope and limitations of the line-dipole approximation as well as the resonance energy transfer concept in this system are discussed. [Preview Abstract] |
Thursday, March 24, 2005 4:42PM - 4:54PM |
W35.00006: Internal structures in single molecule bipolar conduction in a double barrier tunnel junction Naoki Ogawa, Gareguin Mikaelian, Xiuwen Tu, Wilson Ho Electronic properties of molecules decoupled from metal leads are of current interest due to their extended lifetime of excitation and potential applications. Here we use scanning tunneling microscopy and spectroscopy to study charge transport through a single molecule in a double barrier tunnel junction composed of isolated copper phthalocyanine molecules adsorbed on an ultrathin Al$_{2}$O$_{3}$ film grown on a NiAl(110) surface. The differential conductance spectra show several types of features at positive and negative biases, exhibiting bipolar conduction. STM topographic images about these conductance features reveal that different parts of the same molecular orbital are imaged at different sample biases. In addition spatially resolved conductance microscopy shows rich and highly anisotropic conductance patterns in the single molecule. [Preview Abstract] |
Thursday, March 24, 2005 4:54PM - 5:06PM |
W35.00007: Inelastic electron transport: IETS, NDR, switching, and hysteresis Michael Galperin, Abraham Nitzan, Mark Ratner We study the effect of the mutual influence between the phonon and the electron subsystems using nonequilibrium Green function (NEGF) formalism at the level of self-consistent Born approximation. Regarding the inelastic spectrum, two types of inelastic contributions are discussed. Features associated with real and virtual energy transfer to phonons are usually observed in the second derivative of the current $I$ with respect to the voltage $V$. Signatures of resonant tunneling driven by an intermediate molecular ion appear as peaks in the first derivative $dI/dV$ and may show phonon sidebands. The dependence of the observed vibrationally induced lineshapes on the junction characteristics, and the linewidths associated with these features are also discussed. Polaron formation on a molecular wire as a possible mechanism for observed NDR, switching and/or hysteresis in the $I/V$ characteristic of molecular junctions is discussed within a simple mean-field model (self-consistent Hartree approximation). This mechanism differs from earlier proposed mechanisms of charging and conformational change. The polaron model captures the essential physics and provides qualitative correspondence with experimental data. The importance of active redox centers in the molecule is indicated. [Preview Abstract] |
Thursday, March 24, 2005 5:06PM - 5:18PM |
W35.00008: Self-assembly of Fibrous Proteins for Molecular Electronics Jianhua Gu, Debin Li, David Lederman, Aaron Timperman Biomolecules can exhibit self-assembly, which would remove the need to individually pattern them into structures, and greatly aid the mass production of nanostructues.\footnote{T.~Scheible, R.~Parthasathy, G.~Sawick, X.~Lin, H.~Jaeger, and S.~Lindquist, PNAS 100, 4527 (2003)} Fibrous proteins tend to have relatively simple, regular linear structures, making them ideal candidates for the formation of nanowires. We have synthesized tropomyosin fibers for molecular electronics. Nanowire fibers with lengths ranging from 500nm-2000nm and 15nm-150nm in diameter have been fabricated. The length and diameter can be controlled with the ion (Na$^{+}$ or Mg$^{+}$) concentration in the protein solution. These deposition processes have been characterized with AFM and SEM. These wires can be deposited on to gold and silicon substrates using a self-assembly technique. These wires can be bridged between two-point or four-point gold nanoelectrtrodes. Electric conducting properties with these wires or wire-templates will be discussed. \\ \\ Work supported by the WVNano Initiative. [Preview Abstract] |
Thursday, March 24, 2005 5:18PM - 5:30PM |
W35.00009: Control of band alignment through coupling chemistry in prototypical molecular wire systems S.W. Robey, C.D. Zangmeister, R.D. Van Zee The performance of conjugated molecular systems in electronic applications, either for organic light emitting diodes (OLED's) and field effect transistors, or in more speculative applications proposed for molecular electronics, depends critically on coupling at the molecule-electrode interface. Interactions at this interface determine the alignment of the contact Fermi level with the transport levels in the molecular system and control charge injection into the molecular $\pi$ levels. We have used one-and two-photon photoemission to examine the influence of coupling chemistry on Fermi level alignment and electronic structure in the prototypical “molecular wire”, 4,4'-(ethynylphenyl)-1- benzenethiol on Au. These studies reveal a rigid shift of the Fermi level relative to the valence and C (1s) levels upon substitution of the isocyanide coupling for thiol without significant modifications to the overall spectral shape. Absorption measurements reveal no change in optical band gap. These results indicate that substitution of the isocyanide linking chemistry for thiol shifts E$_{f}$ away from the highest occupied level in the molecule by about 0.5 eV, with little modification of the extended $\pi$ electronic structure. The interaction at the Au-thiol-OPE interface will be compared and contrasted with the Au-isocyanide-OPE interface in terms of bonding and charge transfer effects and contact made to RAIRS and transport data for related systems. [Preview Abstract] |
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W35.00010: Coulomb blockade and current-induced switching in single molecular devices Ivan Oleynik, Mortko Kozhushner, Vladimir Posvyanskii Electron transport properties of single organic molecules exhibit intriguing and unusual features due to reduced dimensionality of the single molecular structures and their unique electronic properties. We will discuss the particular aspect of electron transport that is characterized by dominance of many-electron effects. The interplay between Coulomb and exchange interactions in the course of electron transfer through the molecule results in modification of the energy spectrum of the tunneling electron. We predict dynamical evolution of the energy spectrum as applied bias is increased which results in current-induced switching in single molecular devices. [Preview Abstract] |
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