Bulletin of the American Physical Society
2005 APS March Meeting
Monday–Friday, March 21–25, 2005; Los Angeles, CA
Session W30: Organic Thin Films |
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Sponsoring Units: DPOLY Chair: Yueh Lin Loo, University of Texas Room: LACC 505 |
Thursday, March 24, 2005 2:30PM - 2:42PM |
W30.00001: Molecular orientation in self-assembled azo-polymer thin films studied by second-harmonic generation Paulo B. Miranda, Fabio J. S. Lopes, Cleber R. Mendonca, Sergio C. Zilio Ultrathin films of polymers containing azo chromophores are important for many photonic applications. We have used optical second-harmonic generation (SHG) to study the molecular orientation of azo-polymer films prepared by Layer-by-Layer self-assembly onto a glass substrate. The SHG signal is proportional to the second-order nonlinear susceptibility of the film, which in turn depends on the orientational distribution of the azo chromophores in the film. Analyzing the SHG signal as a function of the input and output light polarizations, a few parameters of this orientational distribution can be deduced. The results indicate that there is a preferential orientation of the azo chromophores in the film, leading to a significant optical nonlinearity. The films were found to be anisotropic and inhomogeneous on the surface plane due to the self-assembly procedure used for their fabrication. Such detailed structural characterization is important for understanding and optimizing the self-assembly technique to yield samples with the desired microscopic arrangement. [Preview Abstract] |
Thursday, March 24, 2005 2:42PM - 2:54PM |
W30.00002: Polyelectrolytes with Azobenzene for Self-Assembled Smart Materials Christopher Barrett, Kevin Yager, Oleh Tanchak Electrostatic self-assembly of polyelectrolytes containing photoswitchable azo chromophores into multilayers from aqueous solutions is a promising method for the fabrication of thin films containing desired ``smart'' functionalities and/or sensing material. Using various reflection methods (Ellipsometry, SPR, Neutron) and an atomic force microscope (AFM) for force measurements, we show here how the structural, mechanical, and optical properties can depend on the ionization fraction of the assembling polymers. By controlling such film properties as layer thickness, density, kinetics of assembly, adhesion, and elastic response, we are able to custom-design polymer films with a desired architecture and mechanical response. Applications demonstrated will include a light-induced mechanical response for actuation as artificial muscles, and mass transport of the polymers on a Si surface for nanopatterning and holographic information storage. [Preview Abstract] |
Thursday, March 24, 2005 2:54PM - 3:06PM |
W30.00003: Beyond Force Measurements: Molecular Diffusion in Confined Fluida Jeff Turner, Ashis Mukhopadhyay, Sung Chul Bae, Sangmin Jeon, Steve Granick The thermal energy-driven rotation of an ultra-dilute concentration of a fluorescent probe was measured in a series of confined fluids. Crossed cylindrical walls of atomic smoothness were used to confine these samples. The systems of interest included a model lubricant, OMCTS, and a strongly associative and hydrogen bonding system, 1,2-propanediol. These molecules, when confined, showed very different probe rotation. In the highly associative system a slight increase in rotational diffusion coefficient is observed, while rotation slows slightly in the oily system (similar to our group's translational diffusion measurements). In both cases, the rotational mobility still shows the signature of a liquid system, in contrast to previous force studies suggesting ``confinement-induced solidification.'' [Preview Abstract] |
Thursday, March 24, 2005 3:06PM - 3:18PM |
W30.00004: Exciton absorption in thin PTCDA and PTCDA/Alq3 multilayers V.R. Gangilenka, Ajith DeSilva, H.P. Wagner We investigate the absorption of Frenkel excitons in crystalline PTCDA thin films and PTCDA/Alq3 multilayers in the temperature range from 10 to 300 K by optical transmission spectroscopy. The organic nanostructures are fabricated under high vacuum using organic molecular beam deposition. The measurements provide information about microscopic interactions between molecules, the coupling of Frenkel excitons with various vibronic states and possible strain fields within different organic films. Of particular interest are absorption changes in the PTCDA/Alq3 multilayers. Compared to pure PTCDA films the 0-0 vibronic transition in multilayer structures is shifted to lower energy by 10 meV and the whole absorption spectrum is shifted to higher energy by 20 meV. The observed energy shifts are mainly attributed to strain fields present in multilayered films. Other reasons might be dielectric effects and/or exciton confinement within the nanometer thick PTCDA layers. [Preview Abstract] |
Thursday, March 24, 2005 3:18PM - 3:30PM |
W30.00005: Strain Modified Exciton Emision in Organic Multilayers Ajith DeSilva, H.P. Wagner, T.U. Kampen We study the exciton emission in 3,4,9,10-perylene tetracarboxylic dianhydride (PTCDA) films and in PTCDA/ aluminum-tris- hydroxyqinoline (Alq$_{3})$ multilayers by analyzing the photoluminescence (PL) spectrum at temperatures between 10 and 300 K. In multilayers we observe a strain modified charge transfer exciton (CTE) emission while the indirect Frenkel exciton transition is not strongly affected. The PL intensity enhancement and the red shift of the CTE emission in attributed to tilted PTCDA crystallites within the multilayer structure due to compressive strain fields that increase the CTE trapping probability and binding energy. This assignment is supported by x-ray diffraction that reveals an increase of out of plane disorder of PTCDA molecules in the multilayer structure. Furthermore uniaxial strain dependent PL measurements on pure PTCDA films show similar strain modifications of the CTE emission as observed in multilayer structures. [Preview Abstract] |
Thursday, March 24, 2005 3:30PM - 3:42PM |
W30.00006: Enhancement of Raman signal for nanoconfined samples Malgorzata Graca, Sang Chul Bae, Steve Granick Previously, studies on confined fluids in parallel confinement geometries were limited to perturbation/relaxation force measurements. Recent advances in our research group have allowed the probing of these systems by spectroscopic means. One major hurdle in these experiments is the fact that a vibrational spectrum is very hard to collect from a confined fluid, due to very thin (few molecules) sample. To this end we have developed a novel experimental platform by which introduce a smooth surface with the ability to enhance a Raman signal - without perturbing the film structure. The goal of this work is to enhance the signal from the confined fluid to the point where Raman signals can be collected from a film only a few nm thick in a short period of time with sufficient signal as compared to background. [Preview Abstract] |
Thursday, March 24, 2005 3:42PM - 3:54PM |
W30.00007: The self-assembly of alkyl-trichlorosilanes on model surfaces of biphenylthiols Svetlana Stoycheva, Joerg Fick, Steffen Franzka, Nils Hartmann, Alexander Kornviakov, Avi Ulman, Michael Himmelhaus, Michael Grunze Despite of its technological relevance, the self assembly of alkyl-trichlorosilanes onto oxidized metal or semiconductor surfaces is still not fully understood. Phenomena, such as island formation and polymerization, hamper the formation of densely packed and well-ordered self assembled monolayers (SAM). To elucidate the influence of the distribution of OH groups on the surface, we have used 4-mercaptobiphenyls (MBP) to prepare stable, molecularly engineered surfaces and used them as substrates for the assembly of alkyl-trichlorosilanes. The monolayers of MBP as well as the bilayer assemblies were characterized by contact angle, ellipsometry, IR spectroscopy, X-ray photoelectron spectroscopy, sum frequency generation, and AFM. In combination with the results of ab initio calculations, a detailed picture of the film formation of the silane overlayer could be achieved. [Preview Abstract] |
Thursday, March 24, 2005 3:54PM - 4:06PM |
W30.00008: Influence of lateral packing density and tailgroup hydrophilicity on the protein resistance of oligoether-terminated alkanethiols studied by IR-vis sum frequency generation (SFG) Joerg Fick, Rongyao Wang, Sascha Herrwerth, Wolfgang Eck, Michael Himmelhaus, Michael Grunze A recent study indicates that the protein resistance of self-assembled monolayers (SAM) consisting of oligoether-terminated alkanethiols is determined by different factors such as internal and terminal hydrophilicity and lateral packing density. In this contribution we analyze the influence of lateral packing density and hydrophilicity on the protein resistance using in-situ broadband SFG to track conformational changes before, during and after water contact. We observe that the protein resistance is not dependent on the substrate, but is a pure density effect. SFG results are supplemented by ellipsometry and IR measurements examining the SAMs in the different stages of film treatment. Additionally, a mechanism of degradation of OEG-SAMs in aqueous solution has been investigated and clarified. [Preview Abstract] |
Thursday, March 24, 2005 4:06PM - 4:18PM |
W30.00009: Low Energy Electron Diffraction and Photoemission Study of Dodecanethiol on Pt(111) and Pt(100) T.M. Sweeney, P.S. Robbert, J.W. Hobson, S.M. Huston, C.A. Ventrice, Jr., H. Geisler The geometric and electronic structure of self-assembled monolayers of dodecanethiol grown on Pt(100) and Pt(111) has been studied by low energy electron diffraction and synchrotron-based ultra-violet photoelectron spectroscopy. Ordered overlayer structures are observed during the initial stages of thiol growth on both surfaces, and the photoemission spectra show a very slow uptake of thiol (s $<$ 10 $^{- 5}$), which provides evidence for dissociation of the thiol molecules at room temperature. The electronic structure of the thiol films grown from solution on Pt (four peaks observed at 6.5, 10, 14, and 20 eV) are similar to the structure observed previously on Au(111) and Ag(111). However, the photoemission spectra of the thiol films grown by vapor deposition at $\sim$180 K are dramatically different than those observed previously on Au(111) and Ag(111) at low temperature. Relative shifts in the binding energy of some of the molecular orbitals by as much as 6.5 eV on Pt again points to a dissociative adsorption geometry. [Preview Abstract] |
Thursday, March 24, 2005 4:18PM - 4:30PM |
W30.00010: Molecular Simulation of Oligomeric Nanofilms Confined Between Iron and Iron Oxide Surfaces David Rigby, Rajesh Khare The past several years have witnessed significant advances in areas of molecular simulation of relevance to the study of confined oligomeric nanofilms. Two noteworthy advances include developments in classical force fields, which promise to make accurate predictions of physical properties, such as equation of state behavior under a wide range of conditions of temperature and pressure, accessible via simulation, and developments in procedures for modeling the behavior of films with nanoscopic dimensions under shear and at equilibrium . In this work, we report on recent calculations in which we have combined these two developments to perform realistic simulations of oligomers confined between iron and iron oxide (as FeO) surfaces. We begin by presenting the results of validation data for the bulk substances of interest -- including alkanes, alcohols and bulk metal oxide, before comparing the structure and shear behavior of films consisting either of pure alkanes, or hydroxyl terminated chains. Finally, we examine the effect of enhancing the substrate interaction by use of molecules with additional hydroxyl groups. [Preview Abstract] |
Thursday, March 24, 2005 4:30PM - 4:42PM |
W30.00011: Structural characterization of iron phthalocyanine thin films by X-ray diffractometry Casey W. Miller, A. Sharoni, G. Liu, C. N. Colesniuc, B. Fruhberger, Ivan K. Schuller The crystal structure of iron phthalocyanine (FePc) thin films was investigated by X-ray diffractometry (XRD) as a function of deposition temperature and \textit{in situ} annealing. FePc was deposited on A-plane sapphire in an organic molecular beam epitaxy (OMBE) system equipped with a low temperature effusion cell. The pressure of the OMBE was better than 5x10$^{-9}$ during deposition. The substrates were held at constant temperatures ranging from ambient to 300 $^{o}$C during deposition. For each substrate temperature, post-deposition \textit{in situ} annealing at the same temperature was performed for 1, 2, 4, and 8 hours. XRD analysis shows that films were grown to $\sim $10 monolayers thick with 1-2 monolayers of roughness. XRD also reveals the emergence of higher order peaks as deposition temperature increases. Supporting AFM results show improvement toward the deposition of pinhole-free FePc films. OMBE grown films are compared to others deposited using a simple thermal evaporator with a deposition pressure of $\sim $10$^{-7}$ Torr. [Preview Abstract] |
Thursday, March 24, 2005 4:42PM - 4:54PM |
W30.00012: Electronic structures of self-assembled monolayer of molecules of symmetric disulfides of benzoic acid Y. -H. Tang, M. -H. Tsai Electronic structures of self-assembled monolayer (SAM) of molecules of symmetric disulfides of benzoic acid have been studied using the first-principles calculation method. The functional elements considered are H, Br and F. It is found that the highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) are composed mainly of S orbital at the end and C-H bonding orbitals in the middle benzene-ring of the molecule, respectively. The separation between HOMO and LUMO energy levels (bands) are found to be less than 1.0eV. It is also found that when the distance between adjacent molecules are small enough, SAM becomes semimetallic due to lateral couplings among molecules. The dipole moments per molecule of these SAM's with Br and F functional elements are found to decrease substantially when the lateral distance between molecules is reduced. The electronic structure of SAM or a single organic molecule used for molecular electronics is often treated as unaltered under an applied electric field in the calculation of electric current using the Green function method. However, an applied field is found to cause significant redistribution of charges in the molecule. [Preview Abstract] |
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