AVS2001 Session OF+TF-ThM: Characterization of Organic Thin Films

Thursday, November 1, 2001 8:20 AM in Room 131

Thursday Morning

Start	Invited?	Item
8:20 AM		OF+TF-ThM-1 Characterization and Modification of ITO Surfaces: Use of Chemisorbed Probe Molecules as Indicators of Electron Transfer Rates and Their Use in Improving OLED Performance C. Carter, C.L. Donley, N.R. Armstrong (University of Arizona) XPS characterization of ITO surfaces, following various pretreatments, shows that the surface coverage of hydroxyl groups, and oxygen defects, can be varied significantly. Probe molecules, such as ferrocene dicaroboxylic acid (Fc(COOH)2) can be chemisorbed to these surfaces through hydrogen bonding interactions, at coverages up to 40% of a compact monolayer. The coverage and electron transfer rates of these chemisorbed molecules vary by up to 100% depending upon pretreatments used. Organic light emitting diodes, prepared by spin coating single polymer layers (PVK), doped with Alq3, and its tri-sulfonamide analog, Al(qs)3, show signficant lowering of onset potentials for electroluminescence for ITO films modified with the Fc(COOH)2 probe, following air plasma cleaning. These differences in onset potential are most significant for devices where electron injection is not the chief limitation to electroluminescence.
8:40 AM		OF+TF-ThM-2 XPS Characterization of Photo-Alignment of Liquid Crystals using Adsorbed Dichroic Materials L. Su, J.L. West (Kent State University); Y. Reznikov (Ukraine Academy of Science); K. Artyushkova, J.E. Fulghum (Kent State University) The alignment layer is one of the most important parts of a twisted nematic (TN) display. Many methods exist for producing alignment, and the quality of the alignment layer directly influences the performance of the display. The most popular alignment method used by the Liquid crystal display (LCD) industry is mechanical rubbing of a polymer film. However, there are many disadvantages associated with this method, such as the introduction of dust particles, formation of electrostatic charges and other defects on the rubbed surface. Photo-alignment of liquid crystals is emerging as one of the most promising substitutes for mechanical rubbing. Photo-alignment generates surface anisotropy through the interaction between the irradiated substrate and the light source. Photo-alignment using adsorbed dichroic materials provides an efficient and versatile way to produce effective alignment of liquid crystals. Successful photo-alignment results from properly matching the adsorbed and substrate materials. In this study, poly(vinyl) alcohol (PVA) and adsorbed dichroic materials, including the dye Brilliant Yellow, are utilized as the alignment layer. We will demonstrate the use of polarized UV-Vis spectroscopy and angle-resolved x-ray photoelectron spectroscopy (XPS) for studying the interaction of the polymer, dye and liquid crystal. Using ARXPS, it is possible to study the distribution of all three organic layers. Both techniques indicate that intermolecular interaction between the adsorbed dichroic molecules and the substrate polymer is critical to the outcome of photoalignment. This work has been supported in part by NSF ALCOM (DMR 89-20147), and the Air Force (DAGSI SN-AFIT-9903).
9:00 AM		OF+TF-ThM-3 An STM , XPS and RAIRS Study of Cobalt(II) Hexadecafluorophthalocyanine (CoF16Pc) and Its Co-adsorption with Nickel(II) Tetraphenylporphyrin (NiTPP)on Au (111) S.L Scudiero, D.E. Barlow, K.W. Hipps (Washington State University) Scanning tunneling microscopy (STM) images show that cobalt(II) hexadecafluorophthalocyanine (CoF16Pc) forms disordered structures when deposited on Au(111) under UHV conditions at 300K, while nickel(II) tetraphenylporphyrin (NiTPP) forms a tightly packed well organized structure under the same conditions. X-ray photoelectron (XPS) and reflection-absorption infrared spectroscopy (RAIRS) data obtained on thin films of the pure compounds confirm that the chemical composition of these films is the same as the starting bulk materials. When CoF16Pc and NiTPP are co-deposited with a monolayer coverage, the resulting surface structure is a well ordered 2D array having a 1:1 ratio and a nearly square unit cell as revealed by high resolution STM images. The composition of the mixture was confirmed by XPS. We believe that this new structure is due to the weak electrostatic interactions associated with the local partial charges and their images, along with differences in Van der Waals forces.
9:20 AM		OF+TF-ThM-4 Solution-Assisted Tribological Modification of Surfaces Using an Atomic Force Microscope F. Stevens, R. Leach, J.T. Dickinson (Washington State University) The response of thin polymer films to combined stress and solvent is important for applications such as protective barriers (e.g., various wrappings), in controlled drug release from polymer hemispheres, resists for lithography, and nanometer scale surface modification. Over a wide range of normal forces, when a polymer is scanned by SFM in contact mode in a solvent, material is not worn away, but rather the polymer surface expands forming a nanometer scale "bump" at and surrounding the scanned location. Furthermore, for sub-micron scan areas one often observes a series of parallel ridges (moguls) perpendicular to the fast scan direction. Previous reports of the formation of raised material have nearly all been in air, required long times or very high forces to form; little evidence has been presented for the mechanism of formation. We have engaged in a detailed study of protrusion formation and raised ridges using poly(methyl methacrylate) in alcohol based solvents. In addition to scanning in air, we have scanned the polymer surface in four solvents with dramatic differences in response. We have also observed the effects of varying contact force, and the effect of using cantilevers with different force constants. We present evidence that both plastic deformation and tip induced swelling play major roles in the observed polymer surface modification by SFM. The stresses applied by the tip generate tensile forces around the tip that likely increase the quantity of solvent that can enter the surface. We show that adding Rhodamine 6G dye to the solvent provides us with evidence that indeed solvent is going into the polymer. Using fluorescence microscopy we can monitor the uptake of dye as a function of scanning and solvent parameters. This study also shows the possibility of introducing small quantities of a chemical into the polymer surface in a highly localized (nanometer scale) fashion.
9:40 AM		OF+TF-ThM-5 Viscoelastic Properties of Thin Liquid Crystal Films I. Zori@aa c@, P. Borchard, T. Carlsson, B. Kasemo (Chalmers University of Technology, Sweden) Viscoelastic properties of thin liquid crystal (LC) films are strongly affected by the changes in orientational and/or translational order in the system. These changes may be induced via a temperature variation or by a presence of the two phase interface (e.g. a free surface or a LC-solid substrate interface). In this contribution we report the viscoelastic properties of thin (500-7000Å) 5CB films, spin coated on the Au electrode of the quartz crystal microbalance (QCM), with one free surface. The system (LC film) is exposed to a periodic shear force (at 5 MHz respectively 15MHz) and the changes in the QCM oscillator frequency, Δf, and dissipation factor, ΔD, are measured as a function of temperature while the system undergoes a series of phase transitions (smectic-nematic-isotropic). Both Δf and ΔD show unusual temperature dependence in the vicinity of the nematic-isotropic phase transition. Two approaches are used to deduce the temperature dependent viscoelastic coefficients from the measured frequency and dissipation factor changes. In both approaches the QCM is treated as a harmonic oscillator and the Navier Stokes equation is used to calculate the velocity profile in the viscous overlayer caused by the periodic shear. Once the velocity profile in the film is known, a frictional force causing a change in the oscillator frequency and dissipation, may be calculated. In the first case the overlayer is treated as a homogenous isotropic thin liquid film (Voight model) while in the second case a proper anisotropic structure of the LC film is taken into account in the hydrodynamic continuum approach (Leslie-Ericksen theory). We compare our results to generalized viscosities obtained using different methods.
10:00 AM		OF+TF-ThM-6 Structure of Ultrathin Pentacene Films S. Lukas, G. Witte, C. Wöll (Ruhr-Universität Bochum, Germany) The structure of ultrathin films (1-3ML) of pentacene grown by evaporation under UHV conditions on various copper surfaces has been studied by LEED, STM and NEXAFS. The enhanced binding energy of acene molecules at step edges which was measured systematically by TDS can be utilized to prepare ordered monolayer films on vicinal copper surfaces with close packed terraces while only disordered films grow on the flat Cu(111) surface.¹ Highly ordered mono- and multilayer films were also obtained for the Cu(110) surface and their corresponding molecular orientation was determined by NEXAFS as a function of the film thickness. For comparison additional films were grown also on gold and sapphire surfaces. In addition to the geometrical structure the electronical properties of these pentacene films were investigated by UPS. ¹ S. Lukas et al., J. Chem. Phys. 114 (2001) .
10:40 AM		OF+TF-ThM-8 Photoelectron Spectroscopy of Exciton Dynamics and Interactions in Organic Thin Films: C₆₀ and Photopolymerized C₆₀ J.P. Long, S.J. Chase, M.N. Kabler (Naval Research Laboratory) Of fundamental importance to the operation of many organic electronic devices are the controlling nonequilbrium populations of singlet (S₁) and triplet (T₁) excitons and charged carriers. For example, optoelectronic device efficiencies may be depressed if dark T₁ excitons compete with fluorescent S₁ states or if annihilation interactions limit excited state population densities. Because pump-probe photoelectron spectroscopy is uniquely capable, in principle, of resolving in energy and time the electrons of all nonequilibrium species, it promises to provide useful insights into the densities, lifetimes, and mutual interactions of the various nonequilibrium populations. Using both synchrotron and laser-harmonic photoemission sources, we have applied this technique to study the dynamics of excitons pumped by visible laser radiation in the model systems of C₆₀ and photopolymerized C₆₀ films prepared in ultrahigh vacuum and studied in situ. We report the unambiguous identification of the transient photoelectron spectra of both S₁ and T₁ excitons. Studies on time scales from 100 ps to 10 µs and of exciton concentrations from below 10¹⁸ cm^-3 to more than 10¹⁹ cm^-3 reveal a rich dynamics. For exciton densities above ~10¹⁹ cm^-3, excited-state interactions lead to a surprising excess of T₁ excitons at the expense of S₁ excitons for times much less than the intersystem crossing time of ~2.5 ns. Such excess T₁ generation suggests that interactions among excited species may interfere with the operation of potential devices that would operate at high nonequilibrium densities. A rate-equation model that couples interacting S₁, T₁, and carrier populations reproduces the complex dynamics, including non-Markovian decays, and provides evidence for the annihilation of excitons by charged carriers.
11:00 AM		OF+TF-ThM-9 Optical Properties of Highly Ordered Ultrathin Epitaxial Films of PTCDA on Au(111) R. Nitsche, S. Mannsfeld, H. Proehl, T. Fritz (TU Dresden, Institut fuer Angewandte Photophysik, Germany) Highly ordered organic thin films on a gold single crystal have been prepared by means of organic molecular beam epitaxy and investigated by optical absorption spectroscopy. The organic dye molecule perylene-3,4,9,10- tetracarboxylic-3,4,9,10-dianhydride (PTCDA) has been deposited on Au(111) with submonolayer to multilayer coverage. All films were structurally characterized by combining Scanning Tunneling Microscopy (STM) with Low Energy Electron Diffraction (LEED), clearly indicating epitaxial growth in the point-on-line mode. In order to determine the optical constants of those films, Differential Reflection Spectroscopy (DRS) has been applied. Subsequently, the optical constants have been calculated based on a model dispersion formula, thereby deconvoluting the absorption index k into several molecular transitions. The thickness dependence of the parameters (position, width, strength) of those transitions is discussed. Contrary to a simple discussion of the peak position of the entire spectrum, which would indicate an energetic shift, our thorough analysis reveals that the dominant effect consists of a change in the relative contributions of the several transitions rather than an energetic shift. The results are compared to polycrystalline PTCDA layers of comparable thickness on polycrystalline gold films, exhibiting a larger inhomogeneous broadening.
11:20 AM		OF+TF-ThM-10 Importance of Structural Order for the Low Surface Energy of Perfluoroalkyl Substituted Polymethacrylates J. Luning (Stanford University); D.Y. Yoon (Seoul National University, Korea); J. Stohr (Stanford University) Perfluoroalkyl substituted PolyMethacrylate (PFPM) polymers are today widely used as low surface-energy coatings playing an essential role in microelectronics, anti-fogging, and anti-fouling applications, and even have promising medical applications. It is generally believed that the anti-wetting properties -solely determined by the structures present in the surface region- of these and related polymers arise from the segregation of CF₃ groups to the surface. However, proof of a direct correlation between surface structure and surface energy, and the importance of order in the underlying bulk, is still lacking as most experimental techniques do not have the required surface sensitivity. For preferentially oriented polymers the asymmetry in chain orientation translates to an asymmetry in the electron charge density as the electron orbitals are oriented along the molecular bonds. Such a charge asymmetry can give rise to a dependence of the Near Edge X-ray Absorption Fine Structure (NEXAFS) on the orientation of the electric field vector of the linearly polarized x-rays relative to the sample. Consequently, such a linear dichroism can be used to study orientation phenomena quantitatively. The required surface sensitivity is obtained by recording simultaneously the more bulk sensitive total (TEY) and the more surface sensitive Auger (AEY) electron yield. Our studies of three PFPM's with different bulk order phases reveal a greater order at the surface than in the bulk, and the surface order parameter is found to correlate with the surface energy. Most importantly, temperature dependent NEXAFS measurements covering several bulk phase transitions show that the achievable surface order, and hence the surface properties, is ultimately limited by the bulk order (phase).