Advanced Materials

Cover image for Advanced Materials

August, 1999

Volume 11, Issue 11

Pages 895–961

    1. Polarized Luminescence from Oriented Molecular Materials (pages 895–905)

      Martin Grell and Donal D. C. Bradley

      Article first published online: 5 AUG 1999 | DOI: 10.1002/(SICI)1521-4095(199908)11:11<895::AID-ADMA895>3.0.CO;2-Y

      Organic electroluminescent devices that emit polarized light will be useful as backlights in conventional liquid-crystal displays. The necessary high polarization ratios require, in turn, highly aligned emitters. The various methods of orientation are discussed, together with the materials to which they are applicable. The Figure shows poly(phenylene vinylene), which is used as a luminescent guest molecule in an aligned host matrix.

    2. A Full-Color Transparent Metal-Free Stacked Organic Light Emitting Device with Simplified Pixel Biasing (pages 907–910)

      Gautam Parthasarathy, Gong Gu and Stephen R. Forrest

      Article first published online: 5 AUG 1999 | DOI: 10.1002/(SICI)1521-4095(199908)11:11<907::AID-ADMA907>3.0.CO;2-L

      The integration of organic light-emitting diodes (OLEDs) into full-color flat-panel displays is a current research objective. Metal-free stacked OLEDs (MF-SOLEDs) are the latest development: a red, a blue, and a green subpixel, each of which is transparent to the light emitted by itself and the other subpixels, are stacked on top of each other. A MF-SOLED is described in which an insulating layer is interposed between the middle and top highly transparent subpixels, allowing the three subpixels to be referenced to a common ground and eliminating the differential drive schemes required in previous SOLEDs.

    3. Combinatorial Color Generation with Mixtures of Dithienyl Photochromes (pages 910–913)

      Alvaro Fernandez-Acebes and Jean-Marie Lehn

      Article first published online: 5 AUG 1999 | DOI: 10.1002/(SICI)1521-4095(199908)11:11<910::AID-ADMA910>3.0.CO;2-8

      The open–closed interconversion of four photochromes, including their photochemical interconversion kinetics and their use in multicolor systems based on multicomponent mixtures, is presented. The Figure shows a picture prepared with three dithienyl photochromes in closed form on a thin-layer chromatography plate after exposure to UV light for 2 min (left) and subsequently to daylight for 12 h (right).

    4. Enhancement of the Magnetic Ordering Temperature and Air Stability of a Mixed Valent Vanadium Hexacyanochromate(III) Magnet to 99 °C (372 K) (pages 914–918)

      Øyvind Hatlevik, Wayne E. Buschmann, Jie Zhang, Jamie L. Manson and Joel S. Miller

      Article first published online: 5 AUG 1999 | DOI: 10.1002/(SICI)1521-4095(199908)11:11<914::AID-ADMA914>3.0.CO;2-T

      Two ambient-temperature molecule-based magnets (Curie temperatures Tc of ∼350 K and 372 K) are presented, which are the result of research to identify factors in synthesis that control the magnetic properties of compounds. The goal is to increase Tc. The magnetic properties of the two hexacyanochromate magnets are described (see also the cover of this issue) and the reasons for the differences between them and earlier hexacyanochromate magnets (Tc ∼ 315 K) are discussed. High Curie temperature materials may find applications in magnetic shielding and memory storage devices, for example.

    5. Controlled Nanometer-Scale Surface Roughening and Its Effect on the Ordering and Stability of Liquid-Crystalline Polymer Films (pages 918–923)

      Maarten W. J. van der Wielen, Martien A. Cohen Stuart and Gerard J. Fleer

      Article first published online: 5 AUG 1999 | DOI: 10.1002/(SICI)1521-4095(199908)11:11<918::AID-ADMA918>3.0.CO;2-D

      Control over surface texture in thin film production is necessary for preparing smooth films (e.g., for optical surfaces), but also where some roughening of the surface is required (e.g., patterning for liquid-crystalline display devices). These authors have found a way to control nanoscale roughening by adsorbing negatively charged colloidal silica particles onto a positively charged polyelectrolyte preadsorbed onto a silicon wafer. Using this method, model surfaces have been prepared which are then substrates for liquid-crystalline polymers. The properties of these films (surface ordering and film stability) are compared with their smooth counterparts.

    6. CdSe Nanocrystal Rods/Poly(3-hexylthiophene) Composite Photovoltaic Devices (pages 923–927)

      Wendy U. Huynh, Xiaogang Peng and A. Paul Alivisatos

      Article first published online: 5 AUG 1999 | DOI: 10.1002/(SICI)1521-4095(199908)11:11<923::AID-ADMA923>3.0.CO;2-T

      Photovoltaic devices remain an important aim for thin films of conjugated polymers. Here is reported the construction of devices with improved photovoltaic performance, which is achieved by blending elongated CdSe nanocrystals (see Figure) with regioregular poly(3-hexylthiophene). Improved transport arising from denser aggregation between the elongated particles is a probable source of the enhanced energy conversion.

    7. Monolayers of Functionalized Oligothiophenes on Graphite—STM Investigation of the Influence of Intermolecular Interactions on the Epitaxy (pages 927–931)

      Ralf Stecher, Bruno Gompf, Jürgen S. R. Münter and Franz Effenberger

      Article first published online: 5 AUG 1999 | DOI: 10.1002/(SICI)1521-4095(199908)11:11<927::AID-ADMA927>3.0.CO;2-D

      Intermolecular interactions caused by substituents, van der Waals interactions, hydrogen bonds, etc. need to be understood if the directed formation of supramolecular structures is to succeed. A study of the influence of intermolecular and substrate–molecule interactions on the epitaxy of monolayers of terminally mono- and disubstituted 3,3-dipentyl-α-quinquethiophenes such as that shown in the Figure is presented.

    8. Dependence of Elastic Properties on Morphology in Single-Wall Carbon Nanotubes (pages 931–934)

      Oleg Lourie, H. Daniel Wagner, Yaogang Zhang and Sumio Iijima

      Article first published online: 5 AUG 1999 | DOI: 10.1002/(SICI)1521-4095(199908)11:11<931::AID-ADMA931>3.0.CO;2-X

      Small-diameter nanotubes are predicted to exhibit quantum size effects involving an interdependence between the diameter of the nanotubes and their electronic and magnetic properties. These authors ask whether this dependence on diameter might be extended to the mechanical properties of single-wall carbon nanotubes (SWNTs). Results were obtained by inducing a compressive deformation in SWNTs embedded in a polymer matrix by cooling the specimen to 81 K and monitoring the deformation via the Raman D* band frequency shift of the embedded nanotubes.

    9. Microtribology and Direct Force Measurement of WS2 Nested Fullerene-Like Nanostructures (pages 934–937)

      Yuval Golan, Carlos Drummond, Moshe Homyonfer, Yishai Feldman, Reshef Tenne and Jacob Israelachvili

      Article first published online: 5 AUG 1999 | DOI: 10.1002/(SICI)1521-4095(199908)11:11<934::AID-ADMA934>3.0.CO;2-L

      Inorganic fullerene-like nested nanostructures (IFs) such as the WS2 IFs shown in the Figure deposited on an a-SiOx-coated TEM grid are confirmed to confer tribological advantages when added to a lubricant fluid (tetradecane) between two shearing mica surfaces. It is shown that the lower friction is associated with friction-induced material transfer of WS2 from the IFs to the mica surfaces.

    10. Fabrication and Characterization of Carbon Nanotube/Poly(vinyl alcohol) Composites (pages 937–941)

      Milo S. P. Shaffer and Alan H. Windle

      Article first published online: 5 AUG 1999 | DOI: 10.1002/(SICI)1521-4095(199908)11:11<937::AID-ADMA937>3.0.CO;2-9

      Are nanotubes ideally suited to a straightforward reinforcing role? Results reported here indicate that that may not be the case, but they could find application as a polymer modifier. A successful route is described for the fabrication of large composite films containing carbon nanotubes based on the formation of a stable colloidal intermediate, a route that should be broadly applicable to a range of nanotube materials and polymers. The resulting thermo-mechanical and electrical properties are discussed. While the stiffness of the composites at room temperature is rather low they show promise at high temperatures.

    11. One-Step Solid-State Reactions at Ambient Temperatures—A Novel Approach to Nanocrystal Synthesis (pages 941–942)

      Xiang R. Ye, Dian Z. Jia, Jian Q. Yu, Xin Q. Xin and Ziling Xue

      Article first published online: 5 AUG 1999 | DOI: 10.1002/(SICI)1521-4095(199908)11:11<941::AID-ADMA941>3.0.CO;2-T

      High yields of nanocrystals at ambient temperaturehave been attained for several inorganic salts such as ZnS and CuO. The authors report one-step, solid-state reactions between easily obtained starting materials such as CuCl2·2H2O and NaOH to give nanoparticles with narrow size distribution. The process is carried out in air and requires no complex apparatus, reagents, or techniques and thus it shows potential for mass production of nanocrystals. All of the reactions investigated involve hydrated salts, and the role of the water in the reaction mechanism is discussed.

    12. Preparation and Characterization of Nanostructured Titania Thick Films (pages 943–946)

      Maria Cristina Carotta, Matteo Ferroni, Vincenzo Guidi and Giuliano Martinelli

      Article first published online: 5 AUG 1999 | DOI: 10.1002/(SICI)1521-4095(199908)11:11<943::AID-ADMA943>3.0.CO;2-L

      A low-cost method of producing nanostructured titania thick films is required if nanostructured titania is to be used to its full potential in applications in areas ranging from optics via solar energy to gas sensing. It is demonstrated that screen printing forms the basis of such a method, and the effect of annealing on the nanophase and the role of doping to prevent coarse-grain growth at relatively high temperature are discussed. Finally, miniaturized devices with deposited titania layers are shown to act as sensors for CO, their performance depending on the size of the grains.

    13. Large-Area Patterning by Vacuum-Assisted Micromolding (pages 946–950)

      Noo Li Jeon, Insung S. Choi, Bing Xu and George M. Whitesides

      Article first published online: 5 AUG 1999 | DOI: 10.1002/(SICI)1521-4095(199908)11:11<946::AID-ADMA946>3.0.CO;2-9

      Rapid micropatterning of polymers on rigid and flexible substrates can be achieved by the method—vacuum-assisted micromolding in capillaries (MIMIC)—introduced here. The Figure shows an oblique-view SEM image of a single hexagonal cell of a pattern produced by vacuum-assisted MIMIC. The use of vacuum reduced the time taken to fill the pattern with UV-curable polyurethane from ∼30 min to ∼15 s.

    14. Magnetic Core–Shell Particles: Preparation of Magnetite Multilayers on Polymer Latex Microspheres (pages 950–953)

      Frank Caruso, Andrei S. Susha, Michael Giersig and Helmuth Möhwald

      Article first published online: 5 AUG 1999 | DOI: 10.1002/(SICI)1521-4095(199908)11:11<950::AID-ADMA950>3.0.CO;2-T

      Colloidal particles with magnetic properties are attractive due to their tunable anisotropic interactions. Here is reported a new class of polystyrene-core magnetite-shell particles produced by the sequential adsorption of nanoparticles and polyelectrolyte, a process that allows the shell thickness and composition to be controlled with nanometer precision. The Figure shows the alignment of the nanocomposite particles in the presence of a magnetic field.

    15. Surprising Volume Change in PPy(DBS): An Atomic Force Microscopy Study (pages 953–957)

      Elisabeth Smela and Nikolaj Gadegaard

      Article first published online: 5 AUG 1999 | DOI: 10.1002/(SICI)1521-4095(199908)11:11<953::AID-ADMA953>3.0.CO;2-H

      Conjugated polymers such as polypyrrole (PPy) have potential use as artificial muscles or in microsystems such as valves for microfluid handling. One of the most important parameters in such uses is the magnitude of volume change during associated redox processes; however, until now, estimates have varied greatly. Atomic force microscopy is reported here as allowing direct measurement of the in situ thickness change during oxidation and reduction of thin films of PPy doped with dodecylbenzenesulfonate.

    16. Cu(In,Ga)Se2 Solar Cells: Device Stability Based on Chemical Flexibility (pages 957–961)

      Jean-François Guillemoles, Uwe Rau, Leeor Kronik, Hans-Werner Schock and David Cahen

      Article first published online: 5 AUG 1999 | DOI: 10.1002/(SICI)1521-4095(199908)11:11<957::AID-ADMA957>3.0.CO;2-1

      Is “self-healing” the source of the stability of Cu(In, Ga)Se2-based solar modules? The proven remarkable stability and radiation hardness of Cu(In,Ga)Se2 (CIGS) solar cells stand in apparent contradiction to the fact that CIGS shows both short-range (metastable defect centers) and long-range (significant Cu migration) instabilities. The authors suggest that these instabilities may in fact be a prerequisite for CIGS's stability as they allow a degree of flexibility or “smartness” in accommodating externally imposed changes. Two self-healing cycles are proposed, in which copper species play a particularly important role.