Bulletin of the American Physical Society
2006 APS March Meeting
Monday–Friday, March 13–17, 2006; Baltimore, MD
Session P30: Focus Session: Organic Interfaces |
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Sponsoring Units: DPOLY DMP Chair: Lynn Loo, University of Texas, Austin Room: Baltimore Convention Center 327 |
Wednesday, March 15, 2006 11:15AM - 11:51AM |
P30.00001: Properties of Organic Molecules at Metal Surfaces Invited Speaker: The adsorption and self–assembly of organic molecules at surfaces has recently been investigated extensively, both because of the fundamental interest and for prospective applications in nanoelectronics and nanophotonics [1, 2]. Molecule–molecule and molecule–substrate interactions can be tuned by the appropriate choice of substrate material and symmetry. Upon molecular adsorption, surfaces typically do not behave as static templates, but often rearrange dramatically to accommodate different molecular species [3, 4]. This presentation reviews recent experimental work using Scanning Tunneling Microscopy, which is providing new insight into fundamental properties such as molecular diffusion [5, 6] and self–assembly via surface templating [7] and hydrogen bonding driven by co-adsorption [8]. \newline \newline [1] F. Rosei et al., \textit{Prog. Surf. Science} \textbf{71}, 95 (2003). \newline [2] F. Rosei, \textit{J. Phys. Condens. Matter} \textbf{16}, S1373 (2004). \newline [3] F. Rosei et al., \textit{Science} \textbf{296}, 328 (2002). \newline [4] R. Otero, F. Rosei, et al., \textit{Nanoletters} \textbf{4}, 75 (2004). \newline [5] M. Schunack, T.R. Linderoth, F. Rosei, et al., \textit{Phys. Rev. Lett.} \textbf{88}, 156102 (2002). \newline [6] J.A. Miwa, S. Weigelt, H. Gersen, F. Besenbacher, F. Rosei, T.R. Linderoth, submitted. \newline [7] R. Otero, Y. Naitoh, F. Rosei et al., \textit{Angew. Chem.} \textbf{43}, 4092 (2004). \newline [8] K.G. Nath, O. Ivasenko, J.A. Miwa, H. Dang, J.D. Wuest, A. Nanci, D.F. Perepichka, F. Rosei, submitted. [Preview Abstract] |
Wednesday, March 15, 2006 11:51AM - 12:03PM |
P30.00002: Direct measurements of contact resistances in asymmetric pentacene thin-film transistors with polyaniline and gold electrodes Kwang Seok Lee, Timothy Smith, Joung Eun Yoo, Keith Stevenson, Yueh-Lin (Lynn) Loo We have fabricated asymmetric pentacene thin-film transistors with one gold electrode and one polyaniline (PANI) electrode connected to the same pentacene channel. Surface potential measurements reveal large potential drops at the gold/pentacene contact, but not at the PANI/pentacene contact during operation. We observe, however, some potential drop along the PANI electrode outside the scan window due to the bulk resistance of PANI. To minimize the potential drop across the PANI electrode, we have fabricated asymmetric devices with one exposed gold electrode, and one PANI-coated gold electrode. The surface potential profiles of these asymmetric devices with the PANI-coated gold electrode as the source electrode and the exposed gold electrode as the drain electrode, and vice versa, reveal no additional potential drop across the PANI-coated gold electrode. Quantification of the contact resistance indicates that the PANI/pentacene contact is significantly less resistive than the gold/pentacene contact whether the PANI-coated gold electrode is used as the source, or the drain electrode. Additionally, charge injection from the exposed gold electrode and the PANI-coated electrode appears to be more difficult than charge extraction from these electrodes. [Preview Abstract] |
Wednesday, March 15, 2006 12:03PM - 12:15PM |
P30.00003: Nm-resolution studies of Au/molecular-film/GaAs junctions using ballistic electron emission microscopy (BEEM) C. Marginean, C. Tivarus, J.P. Pelz, Hossam Haick, David Cahen BEEM was used to image and quantify lateral homogeneity and energy band alignments at molecule/electrode interfaces in Au/dC-X/GaAs structures, where dC-X are dicarboxylic ligands with X= H, OCH$_{3}$, CF$_{3}$, CN, or CH$_{3}$ [1]. Transport through such junctions was proposed to be dominated by ``pinholes'' in the dC-X film, with the Au/GaAs Schottky barrier height (SBH) at pinholes modified by the surrounding molecular film dipole [1]. BEEM images of dC-CH$_{3}$ with V$_{tip }< \quad \sim $1.38 eV indeed revealed isolated 20-40 nm sized ``pinholes'' with measured local SBHs ranging from 0.90 -- 1.0 eV, consistent with the model [1]. However, between the pinholes we also observed a new conduction channel for V$_{tip }> \quad \sim $1.38 eV, possibly due to transport through the LUMO of the dC-CH$_{3}$ film itself. BEEM measurements for the other --X groups also showed non-uniform, film-dependent SBH, but for those films the pinholes were too dense or the films too transparent to resolve isolated pinholes. All dC-X films were stable under the BEEM hot-electron flux. \newline \newline [1] H. Haick \textit{et al}., Adv. Mater. \textbf{16}, 2145 (2004). [Preview Abstract] |
Wednesday, March 15, 2006 12:15PM - 12:27PM |
P30.00004: A Possible Mechanism For Photoinduced Effects In Molecule-Based Magnets Serkan Erdin, Michel Van Veenendaal A mechanism based on charge transfer processes between ligand and metal, for photoinduced effects on magnetic order that are observed in manganese-tetracyanoethylene molecule-based magnet is proposed. In order to support the mechanism, Monte Carlo calculations for a double exchange model with antiferromagnetic interaction between nearest neighbor manganese core spins, J$_{AF}$ on two dimensional metal-ligand lattice are performed. Depending on strength of J$_{AF }$ and the number of electrons in the system, total average magnetization and average angles of core spins are calculated. [Preview Abstract] |
Wednesday, March 15, 2006 12:27PM - 12:39PM |
P30.00005: The electronic structure and polymerization of a self-assembled monolayer Danqin Feng, David Wisbey, Yian Tai, Yaroslav Losovyj, Michael Zharnikov, Peter Dowben Irradiation-induced modifications of electronic structure in the monomolecular insulator [1,1';4',1''-terphenyl]-4,4''-dimethanethiol (TPDMT) films have been investigated by photoemission and inverse photoemission. A dominant effect is cross-linking of the TPDMT film, which result in a quasipolymeric material with a smaller gap between the highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) but improved the dielectric properties. The cross-linking processes are also accompanied by changes in molecular orientation. The photoemission intensities of organic molecular layers generally obey the Debye-Waller temperature dependence but not always. With the example of a monomolecular film formed from [1,1';4',1''-terphenyl]-4,4''-dimethanethiol, we show that pronounced deviations from Debye-Waller temperature behavior are possible and are likely caused by temperature dependent changes in molecular orientation. [Preview Abstract] |
Wednesday, March 15, 2006 12:39PM - 12:51PM |
P30.00006: A first-principles study of $\pi$-stacking in charged oligothiophenes in the presence of counterions Nicholas Miller, Damian Scherlis, Nicola Marzari We investigate the structural and electronic properties of charged oligothiophenes and of their $\pi$-stacking interactions with extensive density-functional theory (PBE, B3LYP) and post-Hartree-Fock (MP2) calculations. We pay particular attention to the role of counterions and of the solvation medium in tuning the balance between electrostatic repulsion and chemical bonding, including explicitly hexafluorophosphate counterions, and exploring the role of polarizability and surface tension for different solvents. Our calculations show that Coulomb's repulsion of the charged oligothiophenes is rapidly screened by solvation, counterions, or both, leading to stable $\pi$-dimerized systems (or higher stackings) where binding is driven by $\pi$-bond hybridization. Furthermore, we studied the charge-transfer properties of the counterion-oligomer system, as well as site preferences for counterions, highlighting the relevance of a proper treatment of correlations and self-interaction in describing the electronic-structure of these systems. [Preview Abstract] |
Wednesday, March 15, 2006 12:51PM - 1:03PM |
P30.00007: Orientation of Fluorophenols on Si(111) Fan Zheng, J.L. McChesney, Xiaosong Liu, F.J. Himpsel Oriented adsorption of switchable organic molecules at surfaces is an important prerequisite for single molecular electronics [1, 2]. As model systems we select polar fluorophenols with tailored dipole moments and investigate their adsorption on the Si(111)7$\times $7 surface by near edge x-ray absorption fine structure spectroscopy (NEXAFS). A strong polarization dependence of the $\pi $* transitions is observed in fluorinated phenols, while phenol itself is isotropic. A quantitative model is developed to convert polarization-dependent NEXAFS data into orientational information. The model includes three angular degrees of freedom, two of them fixed and the other with a Gaussian distribution. Such a situation is encountered in a variety of self-assembled monolayers (SAMs) with tailored end groups [3]. [1] T. A. Jung, R. R. Schlittler, J. K. Gimzewski, Nature \textbf{386}, 696, (1997) [2] A. J. Mayne, M. Lastapis, G. Baffou, L. Soukiassian, G. Comtet, L. Hellener and G. Dujardin, Phys. Rev. B \textbf{69}, 045409 (2004) [3] Y.Y. Luk, N. L. Abbott, J. N. Crain and F. J. Himpsel, J. Chem. Phys. \textbf{120,} 10792 (2004) [Preview Abstract] |
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