Bulletin of the American Physical Society
2006 APS March Meeting
Monday–Friday, March 13–17, 2006; Baltimore, MD
Session W28: Focus Session: Ordered Optoelectronic Organics |
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Sponsoring Units: DPOLY DMP Chair: Howard Katz, Johns Hopkins University Room: Baltimore Convention Center 325 |
Thursday, March 16, 2006 2:30PM - 2:42PM |
W28.00001: The phase diagram of the organic charge transfer salts (TMTTF)$_2$X F. Zhang, W. Yu, B. Alavi, A. Baur, C. A. Merlic, S. E. Brown We report on NMR spectroscopy and relaxation measurements probing the temperature/pressure phase diagram of the quasi-one-dimensional organic conductor (TMTTF)$_2$SbF$_6$. This material undergoes a charge ordering (CO) phase transition at $T=156K$, and the ground state is antiferromagnetic (AF) at ambient pressure. Our experiments show that the AF ground state is supressed sharply by applying hydrostatic pressure and a new phase appears at higher pressure. At intermediate pressures, magnetic ordering is suppressed to temperatures below the minimum measured so far, $T=2K$. The nature of the high pressure phase is not yet established; there are features consistent with both spin-Peierls (SP), and antiferromagnetic ordering. We discuss the possible role of counterion disorder in producing an inhomogenous system consistent with our observations, and the implications for a general phase diagram for the TMTTF/TMTSF family of organic conductors. [Preview Abstract] |
Thursday, March 16, 2006 2:42PM - 2:54PM |
W28.00002: Intermolecular bonding in conjugated polymers: The effect on aggregate morphology. Jeremy Schmit, Alex Levine Intermolecular interactions play an important role in the mechanical and optical properties of conjugated polymer films and solutions. However, the nature of such inter-polymer interactions is poorly understood. We present a calculation showing that in the tight-binding approximation, conjugated polymers that approach each other on the angstrom length scale will form weak, covalent-like bonds. These bonds then drive the formation of aggregate structures. The morphology of the aggregates formed by the polymers depends on the net binding strength of the interaction. At low bond strength, the polymers form loose aggregates, while strong bonds lead to the formation of tight bundles with an effective persistence length one order of magnitude greater than the free polymer. [Preview Abstract] |
Thursday, March 16, 2006 2:54PM - 3:06PM |
W28.00003: Effects of polymer side chains on the self-assembling of conjugated polymer in thin film Yunfei Jiang, Yiqing Wang, Uvw H. F. Bunz, Dvora Perahia Conjugated polymers are inherently semi-conducting and optically active materials, with immense potential applications in organic electro-optical devices. The chemical structure of the polymer including the rigidity of the backbone and the nature of substituents affect their association as well as their electro-optical response. The following work reports the effects of different side chains on the structure and fluorescence of highly conjugated polymer, poly(para phenyleneethynylene) (PPE). When substituted by long polylactide side chains they self-assemble into wires with fingerprint-like arrangement, casting from chloroform solutions on oxidized silicon wafer. With increasing content of poor solvent, the dimension of the structures increased and then crystallized area appeared, as showed in AFM studies. The introducing of the long flexible polymer side chains has significantly reduced the stacking between rigid backbones. This in tern results in a frequency shift in their fluoresces response, indication changes in the electronic levels. Direct measurements of the electronic levels using ATM are currently in progress. [Preview Abstract] |
Thursday, March 16, 2006 3:06PM - 3:18PM |
W28.00004: Chain morphologies in semi-crystalline polyfluorene: evidence from Raman scattering S. Guha, C. Volz, M. Arif Organic semiconductors, such as short-chain oligomers and long- chain polymers, are now a core constituent in numerous organic and organic-inorganic hybrid technologies. Polyfluorenes (PF) have emerged as attractive alternatives to other polymers due to their strong blue emission and high charge carrier mobilities. Almost all PF derivatives utilize side-chain substitution that improves solution processing and confers new functionality. There are many ambiguities regarding structure- property relationships in various side-chain substituted PFs. Di-octyl substituted PF (PF8) is known for its mesomorphic behavior, multitude of crystalline phases, and several conformational isomers that depend on the torsional angle between monomer units. In particular, the $\beta$ phase that represents a more planar backbone conformation dominates the optical emission although it appears as a minority constituent. We present Raman scattering studies of PF8 as a function of thermal cycling, which establishes a connection between the conformational isomers and the side and main chain morphology. Density-functional calculations of the vibrational spectra of single chain oligomers in conjunction with the experimental results demonstrate the incompatibility of the $\beta$ phase with the overall $\alpha$ crystalline phase in PF8. [Preview Abstract] |
Thursday, March 16, 2006 3:18PM - 3:30PM |
W28.00005: Structural Changes in Phthalocyanine Thin Films from Analyte Vapor Exposure Thomas Gredig, Ge Liu, Corneliu N. Colesniuc, Forest I. Bohrer, Andrew C. Kummel, Ivan K. Schuller Organic phthalocyanine thin films were fabricated with varied thicknesses and varied grain size structure. These films act as gas sensors as detected via conduction measurements due to analyte vapors. We have observed structural changes with high-resolution x-ray diffraction due to exposure to different analyte vapors. In thin films with small grains ($\sim $30nm diameter), low-angle irreversible changes and high-angle reversible variations were observed. We associate the irreversible behavior with current drift observed in transport measurements. In contrast, reversible variations of the first Bragg peak of the phthalocyanine are compared to the sensing changes in conductivity from exposure to analyte vapors. [Preview Abstract] |
Thursday, March 16, 2006 3:30PM - 3:42PM |
W28.00006: Electronic Structure of Potassium-doped Magnesium Phthalocyanine measured using Soft X-ray Spectroscopies. Yufeng Zhang, Shancai Wang, Leyla Colerkerol, Timothy Learmonth, Lukasz Plukinski, Kevin Smith, James Downes, Anne Matsuura We report a synchrotron radiation-excited resonant soft x-ray emission spectroscopy (XES) study of the electronic structure of magnesium phthalocyanine (MgPc) doped with potassium. XES measures directly the element specific partial density of states (PDOS) in solids. The electronic structure near the Fermi level in organic systems can be accurately measured by using this non-ionizing spectroscopy. The MgPc films were grown \textit{in-situ} by using a custom designed ultra-high vacuum organic molecular beam deposition system, and transferred under vacuum to the spectrometer system. As with our earlier study of Cu-Pc and vanadium oxide phthalocyanine (VO-Pc), the K-doped MgPc films were discovered to be highly susceptible to synchrotron radiation beam damage. We successfully circumvented this effect by continuous translation of the films during measurement. We find that the measured C $2p$ PDOS for K-doped MgPc differs from that of pure MgPc, and will discuss the possible origins of these results. Supported in part by ACS Petroleum Research Fund, and by the NSF. Experiments were performed at the NSLS. [Preview Abstract] |
Thursday, March 16, 2006 3:42PM - 3:54PM |
W28.00007: Polaron transport in triphenylene-based discotic liquid crystals Volodimyr Duzhko, Alexander N. Semyonov, Robert J. Twieg, Kenneth D. Singer We report on the investigation of charge carrier transport in the columnar phase of the triphenylene-based discotic liquid crystal, hexapentyloxytriphenylene, by the time-of-flight technique. The hole mobility was found to be temperature and electric field dependent with a maximum value of 2$\times $10$^{-3}$ cm$^{2}$/Vs. Its temperature dependence is described by a T$^{-n}$ power law, with an electric field dependent n varying from 2.5 to 4.5 corresponding to electric field values from 5$\times $10$^{4}$ V/cm to 5$\times $10$^{3}$ V/cm, respectively. The drift velocity of charge carriers is a linear function of the electric field for small fields below 5$\times $10$^{4 }$V/cm and tends to saturation at strong fields. These results are interpreted in the framework of correlated polaron motion as described by the non-adiabatic low-temperature limit of Holstein's small polaron theory. The applicability of the Holstein polaron model to triphenylene-based materials showing thermally activated or temperature-independent carrier mobilities will be discussed. [Preview Abstract] |
Thursday, March 16, 2006 3:54PM - 4:06PM |
W28.00008: Atomic Force Microscope-Based Surface Potential and Surface Photovoltage Studies of Porphyrin Nanorod Thin Films Todd Holden, Walter Smith, A.D. Schwab, J.C. de Paula We have performed atomic force microscope-based surface potential and surface photovoltage measurements on porphyrin grown as nanorods about 5 nm in diameter and 1 micron long. These nanorods have been shown to have peculiar photoconducting properties in that the photoconductivity grows under light illumination for up to 1 hour. In addition, when a current is flowed through the nanorods, they become ``trained.'' That is after the light is turned off and the ends of the nanorod are short circuited together, a small current will flow opposite to the direction of the original photocurrent. The material exhibits nanoscale potential fluctuations as well as selective surface potential sensitivity to light. These offer some insight to the material's novel properties. [Preview Abstract] |
Thursday, March 16, 2006 4:06PM - 4:18PM |
W28.00009: Holographic liquid crystal photonic materials stabilized with monoacrylate LC monomer. Augustine Urbas, Eric Beckel, Vincent Tondiglia, Lalgudi Natarajan, Timothy Bunning Active structured optical materials such as dynamically tunable photonic crystals have potential technological applications in imaging and communications. Structured liquid crystal materials are especially promising for their observed high performance and wide range of dynamic response. In addition, they provide multiple routes of response, including electro-optic and photo-optic. Reverse mode holographic polymer dispersed liquid crystals (HPDLCs) are typically fabricated by multiple laser beam exposure of an LC photo-sensitive syrup containing diacrylate liquid crystal (LC) monomer. The function of the diacrylate monomer is to form the basis of a highly cross-linked network which stabilizes regions with high polymer content. The use of monoacrylate functional liquid crystals in this application has the potential of tuning the stabilizing effect of the polymer network by changing the crosslink density. This affects the switching time and the contrast between stabilized and unstabilized regions of the patterned structure. These parameters are particularly important to photo-switchable patterned photonic structures containing azobenzene derivatives. The effects on electro-optic and photo-optic properties of reverse mode HPDLCs containg of different monoacrlyate functional liquid crystal fractions in the stabilizing patterned polymer network will be presented. [Preview Abstract] |
Thursday, March 16, 2006 4:18PM - 4:30PM |
W28.00010: Electrooptic Properties of Holographic Polymer-Stabilized Cholesteric Liquid Crystals Eric Beckel, Melissa Ingram, Lalgudi Natarajan, Vincent Tondiglia, Richard Sutherland, Timothy Bunning Cholesteric liquid crystals (CLCs) have attracted significant attention for uses in photonic and electrooptic devices, such as photonic crystals, light switches, and display applications. These materials selectively reflect circularly polarized light due to the existence of a macroscopic helical structure. Application of an electric field reorients the LC from a planar geometry to a homeotropic alignment, eliminating the reflection notch. This LC reorientation to the clear state is rapid with the application of an electric field. After the electric field is removed, the return to the cholesteric orientation is compounded by the long lifetime of a highly scattering focal conic state. To avoid this undesired prolonged focal conic lifetime, a small concentration of monomer can be polymerized, which acts as a memory for the rapid return into the cholesteric state. In this research, we examine the effect of holographically patterning the polymer stabilization. Reflection grating patterning is studied and varying bilayer spacings are examined. This research shows the possibility of minimal notch broadening, with no apparent chirp in the reflection notch. Additionally, we examine the possibility of incorporating a narrow notch Bragg reflection at a differing wavelength than the CLC broad reflection notch, within the same device. [Preview Abstract] |
Thursday, March 16, 2006 4:30PM - 4:42PM |
W28.00011: High Refractive Index Poly(thiophene) for Organic 3-D Photonic Crystals with a Complete Photonic Band Gap Matthew J. Graham, Shi Jin, Frank W. Harris, Stephen Z.D. Cheng Photonic crystals (PC) with a complete 3-D photonic band gap (PBG) require materials with sufficient refractive index ($n)$ contrast to be in specific 3-D periodic structures on the length scale of light. Currently, only inorganics have an adequate $n$ to open a complete 3-D PBG. Poly(thiophene) (PT), a sulfur containing conjugated polymer, is predicted to have a sufficient $n$, but this has not been realized. By optimizing the electropolymerization of PT including reaction rates, temperatures, additives, and reactant concentrations, high quality PT films with an adequately high $n$ can be synthesized. Using a density differential colloidal crystallization technique, which allows the crystallization process to approach thermodynamic equilibrium, high quality templates were produced. A nano-mechanical annealing technique was developed to enable the further perfection of the entropy driven structures. The next step is to combine these to fabricate an organic 3-D PC with a complete PBG. [Preview Abstract] |
Thursday, March 16, 2006 4:42PM - 4:54PM |
W28.00012: Thermopower of Pentacene Thin-film Transistors Daniel Lenski, Alexandra Curtin, M.S. Fuhrer The mechanism of conduction in organic thin-film transistors remains poorly understood, though it is generally thought that conduction occurs via hopping in a disordered band tail. Thermopower in principle can give additional information about the density of states D(E), and the dependence of the mobility on D(E); such information can be used to discriminate between various conduction mechanisms. We report for the first time measurements of the thermopower of pentacene thin films on SiO$_{2}$/Si in a field-effect transistor configuration as a function of charge carrier density and temperature. This work was supported by the Laboratory for Physical Sciences. [Preview Abstract] |
Thursday, March 16, 2006 4:54PM - 5:06PM |
W28.00013: In-Situ Measurements of Organic Electronic Devices Fabricated via Transfer Printing on Flexible Substrates Andrew Tunnell, Daniel R. Hines, Vince W. Ballarotto, Mihaela Breban, Ellen D. Williams Transfer printing was used to fabricate high quality organic thin-film transistors (TFT) on flexible substrates.~ The model system of a pentacene (Pn) TFT with 600 nm thick poly(methyl methacrylate) dielectric layer and gold electrodes on a polyethylene terephthalate substrate has shown a mobility (adjusted for contact resistance) of 0.237 cm$^{2}$/Vs, on/off ratio of 10$^{5}$ and threshold voltage of -7 V.~ To optimize the transfer printing parameters of the Pn semiconductor layer, mobility and contact resistance were studied as a function of printing temperature and pressure.~ The best TFT devices resulted from printing at 120 $^{o}$C and 600 psi.~ A detailed study of the effect of transfer printing on the device properties was performed via in-situ measurements of drain current (ID) as a function of both drain (VD) and gate (VG) voltages.~ Details of the in-situ measurements while transfer printing the Pn layer will be presented and discussed. *Work supported by the Laboratory for Physical Sciences, College Park, MD and ARDA. [Preview Abstract] |
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