Organics

• Optical properties of hybrid polymers as barrier materials
  Authors: D. Georgiou, A. Laskarakis, S. Logothetidis, S. Amberg-Scwhab, U. Weber, M. Schmidt, K. Noller
  APPLIED SURFACE SCIENCE 255 8023-8029 (2009)
  Abstract: The development of high barrier films for the encapsulation of organic electronics devices onto flexible polymeric substrates is attracting a considerable scientific interest, since it is important to protect the organic semiconductor layers of these devices from corrosion due to atmospheric gas molecule permeation. The barrier layers for encapsulation consist of a sequence of inorganic and hybrid polymer thin films that are deposited onto flexible polymeric substrates, such as polyethylene terephthalate (PET). In addition to their barrier response, these multilayer systems should also exhibit high transparency and good adhesion between the hybrid polymer and inorganic layers. The knowledge of their optical properties and the correlation of the optical response with their structure and the final barrier response are of major importance since it will contribute towards the optimization of their functionality. In this work, the optical properties of hybrid polymers deposited onto silicon oxide inorganic thin films that were grown onto flexible polymeric substrates, have been investigated by the use of spectroscopic ellipsometry in a wide spectral region from the infrared to the visible-ultra violet. As it has been found, the increase of the solid content in the hybrid polymers is associated with a reduction in the refractive index values. This behavior can be correlated to a lower density of the hybrid polymer, and furthermore to a poor barrier response, due to the less cohesive inorganic-organic bonding network. Finally, from the investigation of the optical response of the hybrid polymers in the IR spectral region has revealed information on their bonding structure that has been discussed together with their barrier response.
  
  
• Preparation of thin phthalocyanine layers and their structural and absorption properties
  Authors: S. Kment, P. Kluson, M. Drobek, R. Kuzel, I. Gregora, K. Kohout, Z. Hubicka
  THIN SOLID FILMS 517 5274-5279 (2009)
  Abstract: Phthalocyanines (Pcs) may act as efficient absorbents of photons in the visible region. These structures upon contact with photons from well-defined regions, specifically between 600 and 700 nm, may produce an excited triple state. Its extinction is accompanied by the formation of highly active singlet oxygen species. Its utilization ranges from sensors to selective medical diagnosis tools. In this work we report on preparation of Al and Zn phthalocyanines thin layers of a constant thickness via a method of vacuum sublimation. The produced films were smooth, homogeneous, resistant and well transparent. The films were examined by means of AFM. SEM, profilometry, ellipsometry, Raman spectroscopy and UV-VIS. Special attention was paid to the crystallinity changes with temperature observable by XRD. Light absorption properties of the Pcs films were compared with those recorded in solution (sulphonated Pcs dissolved in water).
  Email: kment@fzu.cz
  
  
• Impact of fluorinated vinylene units on supramolecular organization and optical properties of poly(p-phenylenedifluorovinylene) thin films as a class of blue band gap conjugated polymers
  Authors: Losurdo M, Giangregorio MM, Capezzuto P, et al.
  Source: POLYMER 49 (19) 4133-4140 (2008)
  DOI: 10.1016/j.polymer.2008.07.024
  Abstract: This study is an investigation on the interplay between supramolecular organization and optical properties of thin films of conjugated polymers with fluorinated vinylene units such as poly[2-(2-ethylhexyloxy)-5-methoxyl-1,4-phenylenedifluorovinylene (MEH-PPDFV) and poly(2-methoxy-5-propyloxysulfonatephenylenedifluorovinylene) (MPS-PPDFV), which are both PPV polymers with fluorinated double bonds with alkoxy chains in the 2 and 5 positions. MEH-PPDFV is the fluorinated version of the widely investigated MEH-PPV, and MPS-PPDFV is characterized by the presence of ionic alkoxy side chains. This interplay is elucidated exploiting atomic force microscopy, spectroscopic ellipsometry, and photoluminescence to obtain complementary information. It is demonstrated that the presence of F-atoms in the vinylene units of the MEH-PPDFV yields a blue optical band gap with the maximum of the fundamental HOMO-LUMO transition and of the room temperature photoluminescence at 3.74 eV (331 nm) and at 2.71 eV (458 nm), respectively. The blue-absorption and emission in the thin films are ascribed to the fact that fluorine atoms on the vinylene units prevent pi-stacking of polymeric chains. Furthermore, the dependence of morphology, anisotropy in optical properties and photoluminescence properties of films on deposition methodology is also discussed. MEH-PPDFV also emits homogeneous blue-greenish electroluminescence at 2.46 eV (504 nm). (C) 2008 Elsevier Ltd. All rights reserved.
  Email: maria.losurdo@ba.imip.cnr.it
  
  
• Polarimetric studies of polyethylene terephtalate flexible substrates
  Authors: Stchakovsky M, Garcia-Caurel E, Warenghem M
  EUROPEAN PHYSICAL JOURNAL-APPLIED PHYSICS 44 (3) 229-233 (2008)
  DOI: 10.1051/epjap:2008156
  Abstract: Polymer sheets are currently used worldwide in a wide range of applications. The manufacturing process of these sheets involves extruding machines that stretch the material in both lateral and longitudinal directions with respect to the machine direction, thus inducing birefringence. In most cases, the film obtained is optically biaxial. Polarimetric spectroscopy (Ellipsometry and Mueller Matrix) combined with conoscopic observation are the methods of choice to study these properties. In this work we report an analysis of commercially available polyethylene terephtalate (PET) films used as substrate for food packaging as well as for embedded electronic devices such as solar cells or flexible displays. Initial observation of these films through polarizing microscope in conoscopic mode reveals first the trace of optical axis plane with respect to the film surface and second, whether the optical axis is acute or not. This preliminary study allows optimal sample positioning for further polarimetric studies. The measurements and modelling are done in both reflection and transmission mode on several spectroscopic polarimetric setups from UV to NIR. The models give as a main result, the dielectric tensor of the film as well as its orientation with respect to the laboratory reference frame.
  Email: michel.stchakovsky@jobinyvon.fr
  
  
• Vacuum ultraviolet ellipsometry investigation of ultrathin organic films and their heterostructures
  Authors: J. Sindu, S. Rudra, D. Lehmann, M. Friedrich, D.R.T. Zahn
  APPLIED SURFACE SCIENCE 255 (3) 694-697 (2008)
  Abstract: Thin layers and multilayer structures of the DNA bases guanine and cytosine on ZnO substrates were prepared by organic molecular beam deposition under ultra-high vacuum conditions and measured in situ by means of vacuum ultraviolet spectroscopic ellipsometry at the synchrotron source BESSY. Using the dielectric function of the individual layers the optical response of the G/C and C/G heterostructures was modeled. Deviations between simulated and experimental data were mainly attributed to the ordering of cytosine over guanine in the G/C and C/G structures and to the influence of the substrate.
  Email: sindu.louis@physik.tu-chemnitz.de
  
  
• Oligo(vinylidene fluoride) Langmuir-Blodgett films studied by spectroscopic ellipsometry and the density functional theory
  Authors: Korlacki R, Johnson JT, Kim J, Ducharme S, Thompson DW, Fridkin VM, Ge ZX, Takacs JM
  JOURNAL OF CHEMICAL PHYSICS 129 (6) 064704 (2008)
  DOI: 10.1063/1.2965819
  Abstract: Thin films of amphiphilic vinylidene fluoride oligomers prepared by Langmuir-Blodgett deposition on silicone substrates were investigated by comparing experimental and theoretical mid-infrared (IR) spectra. The experimental spectra were obtained using infrared spectroscopic ellipsometry. Theoretical spectra were calculated using density functional theory. Excellent correspondence of major IR bands in both data sets shows that the molecular backbone is oriented with the long axis normal to the substrate plane. This is in contrast to poly(vinylidene fluoride) LB films, in which the polymer chains are parallel to the substrate.
  
  
• Comprehensive investigation of absolute optical properties of organic materials
  Authors: Choy WCH, Fong HH
  JOURNAL OF PHYSICS D-APPLIED PHYSICS 41(15) 155109 (2008)
  DOI: 10.1088/0022-372741/15/155109
  Abstract: The dispersive absorption coefficient and refractive index are the key optical functions in optimizing the performance of organic optoelectronic devices. The optical functions of organic materials have been generally determined and reported by, for example, using ellipsometry and photoluminescence excitation. However, these methods cannot provide much physical understanding of the optical functions. In addition, conclusive studies on the origins of the optical functions of the organic materials are limited. In this paper, we first determine the absolute optical functions by using ellipsometry against different oscillator models. We can then find out the better oscillator model to be used with ellipsometry together for determining the experimental optical functions. The next step is to determine and explain the origins of the experimentally determined optical functions by investigating the molecular orbitals and electronic transitions of organic molecules through time-dependent density functional theory. This comprehensive study is conducted on two popular materials of tris(8-hydroxyquinoline) aluminium (Alq(3)) and 4, 4' bis[N-(1-napthyl)-N-phenyl-amino]-biphenyl (NPD).
  
  
• Development of methods to determine the infrared-optical properties of polymer films
  Authors: Oreski G, Tscharnuter D, Wallner GM
  Conference Information: 17th European Symposium on Polymer Spectroscopy (ESOPS 17), SEP 09-12, 2007 Seggauberg, AUSTRIA
  MACROMOLECULAR SYMPOSIA 265 124-133 (2008)
  Abstract: In this paper, two methods for the determination of infrared optical properties of thick polymer films, based on FTIR spectroscopy, were implemented and used. Complex index of refraction data were generated for various ethylene copolymer films. Transmittance and reflectance spectra were measured in the mid infrared range using a gold-coated loo mm-diameter integrating sphere. For the investigated films n and k values ranging from 1.3 to 1.6 and from 10(-4) to 0.25 were determined, respectively. Regarding n, a good agreement was obtained for both methods, the transmittance/reflectance procedure (T/R method) used for transparent and semi-transparent regions, and Single Substractive Kramers-Kronig (SSKK) algorithm applied for non-transparent regions. The highest k values were determined for the CH, stretching vibration. The k values are dependent on comonomer content and film thickness. The combination of both methods allows for an accurate determination of n and k in the entire IR region relevant for solar application.
  
  
• Flexible organic electronic devices: Materials, process and applications
  Author: Logothetidis S
  MATERIALS SCIENCE AND ENGINEERING B-ADVANCED FUNCTIONAL SOLID-STATE MATERIALS 152(1-3) 96-104 (2008)
  DOI: 10.1016/j.mseb.2008.06.009
  Abstract: The research for the development of flexible organic electronic devices (FEDs) is rapidly increasing worldwide, since FEDs will change radically several aspects of everyday life. Although there has been considerable progress in the area of flexible inorganic devices (a-Si or solution processed Si), there are numerous advances in the organic (semiconducting, conducting and insulating), inorganic and hybrid (organic-inorganic) materials that exhibit customized properties and stability, and in the synthesis and preparation methods, which are characterized by a significant amount of multidisciplinary efforts. Furthermore, the development and encapsulation of organic electronic devices onto flexible polymeric substrates by large-scale and low-cost roll-to-roll production processes will allow their market implementation in numerous application areas, including displays, lighting, photovoltaics, radio-frequency identification circuitry and chemical sensors, as well as to a newgeneration of modern exotic applications. In this work, we report on some of the latest advances in the fields of polymeric substrates, hybrid barrier layers, inorganic and organic materials to be used as novel active and functional thin films and nanomaterials as well as for the encapsulation of the materials components for the production of FEDs (flexible organic light-emitting diodes, and organic photovoltaics). Moreover, we will emphasize on the real-time optical monitoring and characterization of the growing films onto the flexible polymeric substrates by spectroscopic ellipsometry methods. Finally, the potentiality for the in-line characterization processes for the development of organic electronics materials will be emphasized, since it will also establish the framework for the achievement of the future scientific and technological breakthroughs.
  Email: logot@auth.gr
  
  
• Optical properties of polymer nanocomposites
  Authors: Srivastava S, Haridas M, Basu JK
  BULLETIN OF MATERIALS SCIENCE 31 (3) 213-217 (2008)
  Abstract: Nanomaterials have emerged as an area of interest motivated by potential applications of these materials in light emitting diodes, solar cells, polarizers, light - stable colour filters, optical sensors, optical data communication and optical data storage. Nanomaterials are of particular interest as they combine the properties of two or more different materials with the possibility of possessing novel mechanical, electronic or chemical behaviour. Understanding and tuning such effects could lead to hybrid devices based on these nanocomposites with improved optical properties. We have prepared polymer nanocomposites of well-defined compositions and studied the optical properties of powders and their thin films. UV-vis absorption spectroscopy on nanocomposite powders and spectroscopic ellipsometry measurements on thin films was used to study the effect of interfacial morphology, interparticle spacing and finite size effects on optical properties of nanocomposites. Systematic shift in the imaginary part of the dielectric function can be seen with variation in size and fraction of the gold nanoparticle. The thickness of the film also plays a significant role in the tunability of the optical spectra.
  Email: basu@physics.iisc.emet.in
  
  
• Investigations of the effect of curing conditions on the structure and stability of amino-functionalized organic films on silicon substrates by Fourier transform infrared spectroscopy, ellipsometry, and fluorescence microscopy
  Authors: Kim J, Seidler P, Fill C, Wan LS
  SURFACE SCIENCE 602 (21) 3323-3330 (2008)
  DOI: 10.1016/j.susc.2008.09.001
  Abstract: The effect of curing on the structure and stability of amino-functionalized organic films on silicon substrates was investigated using Fourier transform infrared spectroscopy (FTIR), ellipsometry, and fluorescence microscopy. Amino-terminated organic films were prepared on silicon wafers by self-assembling 3-aminopropyltriethoxysilane (APTES) in toluene, cured at different conditions, and washed by sonication in water. For APTES films without curing/or cured at 25 degrees C, the thickness of APTES films decreased by as much as 65% after sonication in water. FTIR spectra indicate that a substantial amount of physisorbed and/or partially condensed APTES was removed and surface amino groups were oxidized to imines. For APTES films cured at 100 degrees C, ellipsometric measurements and FTIR spectra showed that the thickness was decreased by similar to 10% and that remaining loosely bound APTES had become condensed after curing. Further sonication in water caused no significant changes in the thickness and structure. Independent fluorescence measurements support ellipsometric results and FTIR studies. Our investigations suggest that freshly prepared APTES films are mechanically unstable in aqueous solutions and that proper curing process is required to form physically stable APTES films.
  Email: kimj@buffalostate.edu
  
  
• Application of transmission ellipsometry to the determination of CD spectra of porphyrin J-aggregates solid-state samples
  Authors: Arteaga O, El-Hachemi Z, Canillas A
  PHYSICA STATUS SOLIDI A-APPLICATIONS AND MATERIALS SCIENCE 205 (4) 797-801 (2008)
  DOI: 10.1002/pssa.200777802
  Abstract: Transmission-mode perpendicular incidence phase modulated ellipsometry was applied to study anisotropic thin films of porphyrin J-aggregates. A customized data acquisition system and measurement method was implemented to measure seven Mueller matrix elements, which allow us to determine the circular birefringence and dichroism as well as the linear birefringence and dichroism of solid-state films. The examples shown illustrate the performance of the method for obtaining the true circular dichroism and consequently for detecting structural differences in chiral aggregates.
  Email: oarteaga@ub.edu