Bulletin of the American Physical Society
2006 APS March Meeting
Monday–Friday, March 13–17, 2006; Baltimore, MD
Session H28: Focus Session: Energetics and Transport in Conjugated Organics |
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Sponsoring Units: DPOLY DMP Chair: David Gundlach, National Institute of Standards and Technology Room: Baltimore Convention Center 325 |
Tuesday, March 14, 2006 11:15AM - 11:51AM |
H28.00001: Building-up supramolecular self-assemblies on surfaces: toward organic nanoelectronics Invited Speaker: Self-assembly of functional organic molecules on atomically flat surfaces opens new perspectives towards ``molecular nanoelectronics,'' a realistic strategy in the miniaturization of electronic devices. One simple way to investigate 2D supramolecular self-assemblies at the nanoscale is to generate them at a liquid-solid interface and observe them in situ by means of scanning tunneling microscopy (STM). A variety of 2D architectures have been recently obtained with functional molecules (liquid crystals, molecular wires, graphite-like molecules, etc) by controlling the subtle interplay between molecule-molecule and molecule-substrate interactions. In this presentation, we first illustrate this approach with various families of conjugated compounds such as alkoxy-triphenylenes, star-shaped oligothiophenes, triazatrinaphthylenes, etc. One step further, we demonstrate the possibility to work in the vertical dimension and pile-up two or three layers of similar or different molecules on top of each other in homo- or heteroepitaxy. Even further, we use very long n-alkanes physisorbed on a solid substrate (graphite, gold) as tailored templates for the growth of complex self-assembled bi- and trilayers of large conjugated discal molecules possessing electrical and/or optical properties. Finally, we show that STM allows for the first time to image the surface of a bulk single crystal organic semiconductor down to molecular resolution and to record its local I/V curves in the dark and under photoexcitation. [Preview Abstract] |
Tuesday, March 14, 2006 11:51AM - 12:03PM |
H28.00002: Narrow Band Tails in Organic Semiconductor Crystals C. Krellner, S. Haas, D. J. Gundlach, B. Batlogg In order to study the electronic states in the bulk of high quality organic semiconductor crystals, we have applied the technique of temperature-dependent SCLC measurements. The result is a quantitative density of states curve extending from typically 0.5 eV to (0.05--0.1) eV above the valence band mobility edge. We have studied dozens of crystals of pentacene, rubrene and related compounds, and find in the purest samples a very low DOS in the range of $10^{15}$cm$^{-3}$eV$^{-1}$ and a characteristic exponential distribution parameter of $\sim$180 meV. Close to the mobility edge, a rapidly rising DOS is found with a narrow typical energy of $\sim$10 meV. Extrapolated to the mobility edge, these curves meet the DOS derived from electronic structure calculations. The band tails are significantly narrower than in amorphous Si:H, or derived for pentacene in previous measurements, where surface or interface effects might play a significant role. [Preview Abstract] |
Tuesday, March 14, 2006 12:03PM - 12:15PM |
H28.00003: H\"uckle $\pi$-electron theory of self-assembled metalloporphyrin films W. Schwalm, J. Moreno, A. Brandt Porphyrin-based thin films including transition metal ions can be formed by self-assembly (Y. Ni and Q. Huo, J. of Porphyrins and Phthalocyanines {\bf 9}, 275 (2005)). They have significant magnetic and optical properties. Assuming that the structures are planar and two-dimensionally periodic, the $\pi$-electron theory, though highly approximate, may give some rough insight into their electronic properties. We model the two-dimensional array of metalloporphyrin molecules using a double-exchange Hamiltonian, that couples the local moments on the metal ions with the metal-like extended $\pi$-orbitals. Within a parametrized Local Combination of Atomic Orbitals (LCAO) approximation the electronic wave-function is treated in a minimal basis of states odd in reflection through the plane of the molecule. Local Density of States and other simple properties are computed by Green function methods. The support surface interaction breaks the reflection symmetry and contributes self energy. This effect is included through a simple model approximation. [Preview Abstract] |
Tuesday, March 14, 2006 12:15PM - 12:27PM |
H28.00004: Triplet exciton formation and decay in polyfluorene light emitting diode H.H. Liao, H.F. Meng, S.F. Horng, J.T. Shy, K Chen, C.S. Hsu We study the triplet excitons in poly (9,9-dioctylfluorene-2,7-diyl) (PFO) light emitting diode using infrared induced absorption. The infrared absorption is exclusively due to the triplet excitons and there is no spectral overlap with any other species. A strong suppression of the triplet exciton density relative to the singlet by voltage is observed. Through an unique independent measurement on the triplet exciton lifetime it is shown that the suppression solely comes from triplet exciton quenching by current injection. The triplet-to-singlet exciton formation ratio is independent of voltage as well as temperature, implying a spin-independent exciton formation. [Preview Abstract] |
Tuesday, March 14, 2006 12:27PM - 12:39PM |
H28.00005: Estimating the electronic conductivity of size-expanded DNA: a complex bandstructure study Miguel Fuentes-Cabrera, Jack C. Wells, Oscar Huertas, F. Javier Luque, Hao Wang, James P. Lewis, Modesto Orozco, Otto F. Sankey xDNA is a new class of synthetic nucleic acid with one of the bases larger than the natural congeners. The larger bases, called x-bases, can be viewed as a synthesis of benzene and a natural base. We recently have found that the x-bases have HOMO- LUMO gaps smaller than their natural congeners, and that size- expanded duplexes have stronger $\pi$-$\pi$ stacking interactions than B-DNA duplexes. These findings suggest that xDNA is likely to have a smaller band gap than B-DNA, which could make xDNA a candidate for molecular wire applications. Here we use the complex band-structure method to estimate the conductive properties of polyxG.polyC and polyxG.polyxC. These results are compared to those we obtained previously on polyG.polyC. In this manner, we systematically probe how changes in the $\pi$-$\pi$ stacking interactions affect the conductivity of DNA-like molecules. [Preview Abstract] |
Tuesday, March 14, 2006 12:39PM - 12:51PM |
H28.00006: Excitonic effects in molecular crystals built up by small organic molecules Peter Puschnig, Claudia Ambrosch-Draxl, Kerstin Hummer, Stephan Sagmeister The excitonic effects of biphenyl and 2,2'-bithiophene are investigated within an ab-initio framework. For this purpose the Bethe-Salpeter equation for the two-particle Greens function is solved. Therefrom the imaginary part of the dielectric function is derived, which includes the electron-hole interaction in the absorption process. It turns out that these organic molecular crystals, which are built by small molecules, give rise to sizeable exciton binding energies, which are between 0.7 and 0.8 eV. To study the influence of the intermolecular interaction, the exciton binding energy of crystalline biphenyl is calculated as a function of pressure. The decrease of both, the band gap and the exciton binding energy, results in a slight red-shift of the lowest optically active singlet exciton. [Preview Abstract] |
Tuesday, March 14, 2006 12:51PM - 1:03PM |
H28.00007: Doping Induced Energy Level Shift in Organic Semiconductors Huanjun Ding, Kate Green, Yongli Gao Using photoemission and inverse photoemission spectroscopy (UPS, XPS and IPES), we investigated the electronic structure of alkali metal (Cs and Na) doped copper phthalocyanine (CuPc) and tris(8-hydroxyquinoline) aluminum (Alq) films. We found that doping induces energy level shift, which can be seen as in two different stages. The first stage is predominantly due to the Fermi level moving in the energy gap as a result of the doping of electrons from the alkaline metal to the organic, and the second stage is characterized by the significant modification of organic energy levels such as the introduction of a new gap state and new core level components. In addition, we observed that the energy level shift in the first stage depended in a semi-logarithmic fashion on the doping concentration, whose slope could not be explained by the conventional model used in inorganic semiconductors. Furthermore, we also observed a reversal shift by depositing Au on alkali metal doped Alq film. The initial Au deposition quenches the Alq gap state, as well as the new component in N 1s core level, caused by the alkali metal doping. Further Au depositions shift gradually the energy levels opposite to that induced by doping. The results indicate that the gap state and energy level positions can be decoupled in the organic semiconductors, and that it's possible to fine tune the electronic structure by selective doping in the interface region. [Preview Abstract] |
Tuesday, March 14, 2006 1:03PM - 1:15PM |
H28.00008: Optical Studies of Excited States in Polyfluorene Minghong Tong, Chuanxiang Sheng, Z.Valy Vardeny Polyfluorene is an attractive $\pi $-conjugated polymer for display applications owing to efficient blue emission and relatively large hole mobility with trap free transport. The understanding of its photophysics is important for improving its applications. We have applied a variety of ps transient and cw optical techniques to study the optical properties of poly(9,9-dioctyl) fluorene (PFO). These spectroscopies include absorption, photoluminescence, electroabsorption (EA), two-photon absorption (TPA), and ps transient photoinduced absorption. Using these techniques we probed odd- and even-parity excited states that are complementary to each other. We fitted the EA and TPA spectra by calculating the imaginary part of the third order optical susceptibility using a summation over states model, which include the effects of strongly coupled vibrations and distribution of conjugation lengths. [Preview Abstract] |
Tuesday, March 14, 2006 1:15PM - 1:27PM |
H28.00009: On the Charge transport regime of crystalline organic semiconductors: diffusion limited by thermal off-diagonal electronic disorder Alessandro Troisi In organic crystalline semiconductor molecular components are held together by very weak interactions and the transfer integrals between neighboring molecular orbitals are extremely sensitive to small nuclear displacements. We used a mixed quantum chemical and molecular dynamic methodology to assess the effect of thermal structural fluctuations on the modulation of the transfer integrals between close molecules. We have found that the fluctuations of the transfer integrals are of the same order of magnitude of their average value for pentacene and anthracene. This condition makes the band description inadequate because a dynamic localization takes place and the translational symmetry is completely broken for the electronic states. We also present a simple one-dimensional semiclassical model that incorporates the effects of dynamical localization and allows the numerical computation of the charge mobility for ordered organic semiconductors. These results explain several contrasting experimental observations pointing sometimes to a delocalized ``band-like'' transport and sometimes to the existence of strongly localized charge carriers. [Preview Abstract] |
Tuesday, March 14, 2006 1:27PM - 1:39PM |
H28.00010: Charge Transport Simulations for Amorphous Organic Thin Film Devices Conor Madigan, Vladimir Bulovic We employ Monte Carlo simulations to analyze charge transport through amorphous organic thin films using the well-established ``Miller-Abrahams'' hopping model within a disordered manifold of molecular energies. Whereas most existing studies calculate non-dispersive, equilibrium mobilities at low carrier concentrations, in this work we instead calculate current densities for film thicknesses (i.e. 10 to 100 nm) and applied voltages (i.e. 0.1 to 100 V) typical of common thin film devices (e.g. organic light emitting devices and photovoltaics). We find that at these thicknesses one can not assume the non-dispersive, equilibrium condition. Also, since conduction typically occurs at high carrier concentrations and is space charge limited, we must employ simulations which treat both conditions. We compare our results with those predicted by existing analytic theories, and find that the analytic theories are often highly inaccurate, mainly due to faulty use of Fermi statistics and improper calculation of backward hopping. [Preview Abstract] |
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