Bulletin of the American Physical Society
2005 APS March Meeting
Monday–Friday, March 21–25, 2005; Los Angeles, CA
Session S31: Organic Electronics Materials Characterization |
Hide Abstracts |
Sponsoring Units: DPOLY FIAP Chair: Graciela Blanchet, DuPont Room: LACC 503 |
Wednesday, March 23, 2005 2:30PM - 2:42PM |
S31.00001: Optical characterization of single crystals of the organic semiconductor rubrene J.R. Weinberg-Wolf, L.E. McNeil, Shubin Liu, Christian Kloc 5,6,11,12-tetraphenyl tetracene (rubrene) is an organic semiconductor with a reported mobility of up to 20 cm\^{}2/Vs and a near 100{\%} photoluminescence yield. It is receiving much attention for its possible uses in electronic devices. A detailed analysis of the Raman spectrum of rubrene single crystals will be discussed in light of isolated-molecule Raman simulations. The spectra will also be compared to measurements and simulations of single crystal tetracene Raman spectra, as the tetracene molecule is the backbone of the rubrene molecule. The effects of temperature on the Raman spectrum of the crystalline rubrene will also be presented. One important conclusion from the vibrational studies is that there is very small intermolecular coupling between rubrene molecules in the solid state. [Preview Abstract] |
Wednesday, March 23, 2005 2:42PM - 2:54PM |
S31.00002: Crystal Structures and Band Structures of Acene Chalcogenides: Their Application for OFET. A. Ugawa, T. Kunikiyo, Y. Ohta, M. Murakami, J. Kasahara We have systematically studied acene chalcogenides as active channel materials for Organic Field Effect Transistor (OFET). The molecules have a common structural feature which is of great advantage for carrier channel: chalcogen atoms are located at outside of molecule so that larger orbitals of chalcogen atoms would intensify intermolecular interactions not only for the molecular stacking direction but also for the inter-stacking directions. We then expect that the conduction channels would become more isotropic and the effective mass of the carriers would become lighter compared to the case of aromatic hydrocarbons. The materials we have surveyed are as follows: Hexathiopentacene (HTP), Tetrathiotetracene (TTT), Hexathioanthracene (HTA), Tetrathioanthracene (TTA), Benzo[1,2-c;3,4-c';5,6-c'']tris[1,2]dithiole-1,4,7-trithione (abbreviated as C$_{9}$S$_{9}$, which is its chemical formula), and some selenium analogues. The first-principle band structure calculations based on the crystal structures determined by x-ray analysis reveal that the materials have an anisotropic 2-D HOMO band with an effective mass of 1.1$m_{e}$ for TSeA, 1.3$m_{e}$ for HTA, 1.4$m_{e}$ for TTT, respectively. HTP is proved to have a 1-D HOMO band with $m$*=2.5$m_{e}$ in spite of 2-D structural feature. It is interesting that C$_{9}$S$_{9}$ has an isotropic 3-D HOMO band with $m$*=0.68$m_{e}$, properties which are suitable for OFET channel. [Preview Abstract] |
Wednesday, March 23, 2005 2:54PM - 3:06PM |
S31.00003: Microscopic evidence for spatially inhomogeneous charge trapping in pentacene. Erik Muller, John Marohn Using high sensitivity electric force microscopy we are investigating the electronic properties at the semiconductor-dielectric interface in pentacene thin film devices. It is believed that the conduction takes place within the first few monolayers of the organic and is adversely affected by the presence of charge traps. We find that charge traps in polycrystalline pentacene are distributed inhomogeneously but do not appear to be associated with grain boundaries as is generally supposed. We will also report on ongoing studies of thin (1-3 monolayers) devices, where the relationship between the topography and the location of the charge traps is more easily interpreted. [Preview Abstract] |
Wednesday, March 23, 2005 3:06PM - 3:18PM |
S31.00004: Optical properties of pentacene clusters and ultra-thin films (*) Rui He, Nancy G. Tassi, Graciela B. Blanchet, Aron Pinczuk Photoluminescene spectra of pentacene clusters and films of few monolayer in thickness reveal two fundamental excitations that are assigned to the Davydov doublets of the lowest singlet exciton. While the energy splittings of the doublets have minor dependence on cluster thickness, their bandwidths become narrower as the pentacene clusters grow larger and into continuous ultra-thin films. The marked similarity of these Davydov doublets to those in optical absorption spectra of thicker pentacene films and crystals suggests a similarity in molecular arrangements. Luminescence of self-trapped excitons is quenched in the few monolayer clusters and ultra-thin films. Asymmetric 0-0 and 1-0 resonance Raman scattering excitation profiles are observed in these samples. (*) Supported primarily by the Nanoscale Science and Engineering Initiative of the NSF under NSF Award Number CHE- 0117752 and by the NYSTAR, and by a research grant of the W. M. Keck Foundation. We thank I. Dujovne and C. F. Hirjibehedin for their helpful discussions. [Preview Abstract] |
Wednesday, March 23, 2005 3:18PM - 3:30PM |
S31.00005: Raman spectroscopic studies of polyfluorenes upon thermal cycling M. Arif, S. Guha, B. Tanto, M.J. Winokur Polyfluorenes (PFs) have emerged as attractive alternatives to other pi-conjugated polymers for organic optoelectronics due to their strong blue emission, high charge mobility and excellent chemical and thermal stability. Almost all PFs utilize side- chain substituents that improve solution processing as well as confer new functionality. Thermal cycling of PFs results in distinct backbone and side-chain conformations that lead to improved optical and electronic properties. In this work we present detailed Raman scattering studies of a branched (PF2/6) and non-branched (PF8) PF as a function of temperature to investigate phase transitions and molecular ordering. The low frequency Raman peaks between 100-1000 cm$^{-1}$ are extremely sensitive to the local chain conformation, side chain moiety, and are strongly impacted by thermal cycling. Our results are further compared with X-ray diffraction and gas phase molecular modeling calculations. [Preview Abstract] |
Wednesday, March 23, 2005 3:30PM - 3:42PM |
S31.00006: Intermolecular bonding in conjugated polymers Jeremy D. Schmit, Alex J. Levine Soluble conjugated polymers may enable the development of fast sensitive biosensors. However, the tendency of these molecules to aggregate even at low concentrations has a profound effect on the fluorescence signal that these sensors rely on. We propose that the aggregation of doped conjugated polymers occurs due to the formation of weak interpolymeric bonds resulting from intermolecular electron tunneling at crossing points of the chains. Although these bonds are essentially covalent in character, they are significantly weaker ($\sim 2 k_B T$) due to poor the intermolecular overlap of the electron wavefunctions as well as the delocalization of the pi-electrons along the polymer backbone. We show that the aggregates resulting from these bonds form either loosely bound braids or tight bundles of parallel chains depending on the strength of the electrostatic repulsion between the polymers. Surprisingly, we find that undoped polymers are unable to form parallel bundles. We also explore the interaction of SSH solitons on the chains with these intermolecular binding sites and demonstrate a roughly a four-fold enhancement of the binding strength when each chain has a soliton at the binding site. [Preview Abstract] |
Wednesday, March 23, 2005 3:42PM - 3:54PM |
S31.00007: Resonant Soft X-Ray Emission (SXE) and Resonant Inelastic X-Ray Scattering (RIXS) study of the Electronic Structure of Thin Film Vanadium Oxide Phthalocyanine (VO-Pc). Kevin E. Smith, Yufeng Zhang, Lukasz Plucinski, Shancai Wang, Sarah Bernardis, Timothy Learmonth, James Downes We report a synchrotron radiation-excited resonant soft x-ray emission (SXE) spectroscopy study of the electronic structure of thin films of the organic semiconductor vanadium oxide phthalocyanine (VO-Pc). SXE measures directly the element-resolved partial density of states (PDOS) in materials. The VO-Pc films were grown \textit{in-situ} on Si(100) substrates at beamline X1B at the National Synchrotron Light Source. We present measurement of the V, O, N, and C PDOS in VO-Pc, as well as the observation of dipole forbidden V $3d$ -- V $3d$ excitations and O $2p$ - V $3d$ charge transfer excitations. The relationship of these excitations to those observed in bulk vanadium oxide crystals will be explored, and our data will be compared to our earlier study of Cu-Pc ( J.E. Downes, C. McGuinness, P.-A. Glans, T. Learmonth, D. Fu, P. Sheridan, and K.E. Smith, Chem. Phys. Lett. 390, 203 (2004)) [Preview Abstract] |
Wednesday, March 23, 2005 3:54PM - 4:06PM |
S31.00008: Two-dimensional dispersion of image electrons on C$_{60}$ thin films on Au(111) and Cu(111) Daniel Quinn, Gregory Dutton, Chad Lindstrom, Xiaoyang Zhu Two-photon photoemission (2PPE) has been used for many years to investigate occupied and unoccupied electronic states in clean and adsorbate-covered metal substrates. In this report, femtosecond 2PPE is employed to investigate charge transfer across a metal/organic-semiconductor interface and electronic structure in the thin film overlayer. Monolayer films of C$_ {60}$ have been grown using organic molecular beam deposition in ultrahigh vacuum on Au(111) and Cu(111) substrates. Such films represent a model system consisting of a metal/organic- semiconductor interface. Due to slightly different interatomic spacing in the two substrates, the epitaxial C$_{60}$ films grow as C$_{60}(4\times 4)$/Cu(111) or C$_{60}(2\sqrt{3}\times 2 \sqrt{3})$R30$^\circ$/Au(111). These distinct overlayers have previously been established by low energy electron diffraction and scanning tunneling microscopy experiments. By studying angle-resolved 2PPE, dispersion of image electrons in the conduction band along the surface plane can be measured directly. The fact that the C$_{60}$ overlayer is rotated by 30 $^\circ$ in the Au(111) case with respect to the Cu(111) case leads to distinct dispersion characteristics which correspond to different cuts in the two-dimensional band structure of the C$_{60}$ thin film. Application of an \emph{s}-band tight binding model leads to a reasonable quantitative fit. [Preview Abstract] |
Wednesday, March 23, 2005 4:06PM - 4:18PM |
S31.00009: DMRG study of pi-conjugated polymers with additional pi-conjugation in the transverse direction Yongguo Yan, Sumit Mazumdar The excited state ordering in trans-polyacetylene is not conducive to light emission: the 2A$_g$ occurs below theoptical 1B$_u$ in this system, and the excited 1B$_u$ rapidly decays to the 2A$_g$, radiative transition from which to the ground state is forbidden. It has been suggested that systems which have the same backbone pi-conjugation as linear polyenes, but which also have transverse pi-conjugation with finite extent, will simultaneously have small optical gap and excited state ordering that allows light emission. To verify this prediction, we have carried out DMRG calculations within a Hubbard Hamiltonian for a hypothetical simplified polymeric structure. The critical Hubbard U at which the 2Ag occurs below the 1Bu is larger in the subtituted polymer than in the unsubstituted linear polyene of the same length. Our results introduce the possibility of synthesizing conjugated polymers that will emit in the IR. [Preview Abstract] |
Wednesday, March 23, 2005 4:18PM - 4:30PM |
S31.00010: Optical Coherent Control of Lattice Deformations in Organic Semiconductors M. V. Katkov, C. Piermarocchi We investigate theoretically a semiconducting polymer chain under the effect of an intense off-resonant laser field. The coherent polarization induced by the field couples to the lattice and causes local deformations. Due to the off-resonant nature of the excitation, the deformations are reversible and controllable by the intensity and frequency of the laser. We derive and solve numerically a nonlinear equation describing the distribution of the optical polarization in the chain. Localized solutions exhibit characteristic saturation features. We analyze the light-induced force acting on the lattice in the case of polydiacetylene. [Preview Abstract] |
Wednesday, March 23, 2005 4:30PM - 4:42PM |
S31.00011: Density functional theory and Molecular Dynamics Studies on Energetics and Kinetics for Electro-Active Polymers: PVDF and P(VDF-TrFE) Haibin Su, A. Strachan, William Goddard III We use first principles methods to study static and dynamical mechanical properties of the ferroelectric polymer Poly(vinylidene fluoride) (PVDF) and its copolymer with trifluoroethylene (TrFE). We find that the energy barrier necessary to nucleate a kink (gauche pairs separated by trans bonds) in an all-T crystal is much lower (14.9 kcal/mol) in P(VDF-TrFE) copolymer than in PVDF (24.8 kcal/mol). This correlates with the observation that the polar phase of the copolymer exhibits a solid-solid a transition to a non-polar phase under heating while PVDF directly melts. We also studied the mobility of an interface between a polar and non-polar phases under uniaxial stress; we find a lower threshold stress and a higher mobility in the copolymer as compared with PVDF. Finally, considering plastic deformation under applied shear, we find that the chains for P(VDF-TrFE) have a very low resistance to sliding, particularly along the chain direction. [Preview Abstract] |
Wednesday, March 23, 2005 4:42PM - 4:54PM |
S31.00012: Force Field Parameterization and Property Calculation of Aminofluorene-Based Chromophores David Rigby, Rajiv Berry In recent years, a variety of aminofluorene-based chromophores have been synthesized and studied experimentally for potential use in applications ranging from high density storage and imaging to detection of chemical and biological agents. In view of the large number of compounds of this type, it is desirable to be able to perform property prediction on interesting molecules as efficiently and accurately as possible -- a need which may be realized by use of molecular modeling techniques in combination with increasingly accurate force fields. Currently the most significant obstacle to accurate property prediction is the lack of accurate force field parameters for the chromophore functional groups of interest. Accordingly, we have performed ab-initio based valence parameter development combined with a classical approach to nonbond parameter refinement (as employed for the COMPASS force field) for relevant model compounds. The resulting force field has then been applied to investigate prediction of properties, including glass transition temperatures, for several molecules of interest. [Preview Abstract] |
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