Advanced Materials

Cover image for Advanced Materials

January, 2000

Volume 12, Issue 1

Pages 9–74

    1. New Trends in Colloidal Liquid Crystals Based on Mineral Moieties (pages 9–20)

      J.-C. P. Gabriel and P. Davidson

      Article first published online: 29 DEC 1999 | DOI: 10.1002/(SICI)1521-4095(200001)12:1<9::AID-ADMA9>3.0.CO;2-6

      Lyotropic mineral liquid crystals—materials that are generally electron rich, thermally stable, and often inexpensive—are reviewed. Molecular wires, ribbons, and disks as well as anisotropic crystallites are examined and guidelines useful in the search for new mineral liquid-crystalline suspensions are given. The Figure shows a centimeter-size single domain of nematic clay gel observed between crossed polarizers.

    2. Characterization of Supported Nanoporous Carbon Membranes (pages 21–25)

      M. B. Shiflett, J. F. Pedrick, S. R. McLean, S. Subramoney and H. C. Foley

      Article first published online: 29 DEC 1999 | DOI: 10.1002/(SICI)1521-4095(200001)12:1<21::AID-ADMA21>3.0.CO;2-P

      A new method for the fabrication of supported nanoporous carbon membranes, involving ultrasonic deposition of the polymer precursor, is presented here. The method has been used to produce mechanically robust, continuous films of nanoporous carbon on tubular, macroporous stainless steel supports using polyfurfuryl alcohol as the polymer precursor. The films, which were characterized using microscopy and spectrometry, show unprecedented molecular sieving selectivities for very small molecules, such as oxygen, nitrogen, helium, and hydrogen.

    3. Supermolecular Periodic Structures in Monolayers (pages 25–30)

      L. F. Chi, S. Jacobi, B. Anczykowski, M. Overs, H.-J. Schäfer and H. Fuchs

      Article first published online: 29 DEC 1999 | DOI: 10.1002/(SICI)1521-4095(200001)12:1<25::AID-ADMA25>3.0.CO;2-1

      Supermolecular periodic structures (such as that shown in the Figure) have been obtained by preparing Langmuir–Blodgett films of amphiphilic molecules with two vicinal hydroxyl groups attached to the hydrophobic chain. The periodicity is adjustable, and the ordered region can be extended to over one hundred micrometers. The undulations are suggested to be due to structural changes in defect arrays upon compression.

    4. Template-Assisted Growth of Hexagonal Poly- or Single-Crystalline Quasi-2D Palladium Nanoparticles (pages 31–33)

      J. Walter

      Article first published online: 29 DEC 1999 | DOI: 10.1002/(SICI)1521-4095(200001)12:1<31::AID-ADMA31>3.0.CO;2-N

      Reducing the dimensionality can dramatically improve the physical properties of some compounds. Quasi two-dimensional palladium nanoparticles have been obtained using a graphite lattice as a template. The hexagonal symmetry of the carbon host lattice was found to impose a hexagonal symmetry on the Pd atoms (see Figure; the black dots are the Pd atoms), producing a commensurate superstructure of the host.

    5. Frontier Orbital Model of Semiconductor Surface Passivation: Dicarboxylic Acids on n- and p-GaAs (pages 33–37)

      R. Cohen, L. Kronik, A. Vilan, A. Shanzer and D. Cahen

      Article first published online: 29 DEC 1999 | DOI: 10.1002/(SICI)1521-4095(200001)12:1<33::AID-ADMA33>3.0.CO;2-B

      Semiconductor surface states can affect the performance of many electronic devices, because of the significant role they have in electron transport across device interfaces. These authors use a series of dicarboxylic acids on GaAs surfaces to show that the effect is due to a HOMO–LUMO type of interaction between the frontier orbitals of the molecules and the semiconductor surface states.

    6. Formation of High-Magnesian Calcites via an Amorphous Precursor Phase: Possible Biological Implications (pages 38–42)

      S. Raz, S. Weiner and L. Addadi

      Article first published online: 29 DEC 1999 | DOI: 10.1002/(SICI)1521-4095(200001)12:1<38::AID-ADMA38>3.0.CO;2-I

      The skeletons of many organisms are composed of high-magnesian calcite(10–40 mol.-% Mg), which is not stable under equilibrium conditions. Here, high magnesian calcite particles (>20 mol.-%) are produced at ambient temperature in the presence and absence of additives. Most importantly these particles are shown to initiate from an amorphous precursor phase. The Figure shows the typical dumb-bell morphology of magnesian calcite spherulites.

    7. Soluble PPVs with Enhanced Performance—A Mechanistic Approach (pages 42–48)

      H. Becker, H. Spreitzer, W. Kreuder, E. Kluge, H. Schenk, I. Parker and Y. Cao

      Article first published online: 29 DEC 1999 | DOI: 10.1002/(SICI)1521-4095(200001)12:1<42::AID-ADMA42>3.0.CO;2-F

      Defects in the main chain of phenyl-substituted poly(p-phenylene vinylene)s (Ph-PPVs) can have a profound influence on the operational lifetime of LEDs, as demonstrated here. The formation of these tolane-bisbenzyl defects (shown in the Figure) is rationalized by examining the mechanism of polymerization, and monomers are designed that yield polymers with fewer defects and hence enhanced performance (see also cover).

    8. New Spiropyrans Showing Crystalline-State Photochromism (pages 48–50)

      S. Bénard and P. Yu

      Article first published online: 29 DEC 1999 | DOI: 10.1002/(SICI)1521-4095(200001)12:1<48::AID-ADMA48>3.0.CO;2-G

      Photochromic molecules are of great potential importance for optoelectronic devices. This communication presents the preparation of two new spiropyrans that display the first reported crystalline-state photochromism. UV irradiation of the pale yellow form produces the highly colored red form, both in the solid state and in solution. Equilibrium probably proceeds via the open and closed forms shown in the Figure for one of the spiropyrans.

    9. Photoluminescence, Electroluminescence, Nonlinear Optical, and Humidity Sensitive Properties of Poly(p-diethynylbenzene) Prepared with a Nickel Acetylide Catalyst (pages 51–53)

      X. Zhan, M. Yang, Z. Lei, Y. Li, Y. Liu, G. Yu and D. Zhu

      Article first published online: 29 DEC 1999 | DOI: 10.1002/(SICI)1521-4095(200001)12:1<51::AID-ADMA51>3.0.CO;2-J

      The π-conjugated polymer poly(p-diethynylbenzene) (PDEB) has potential applications in electronics and optics; however, its poor solubility and processability have limited its usefulness. The new synthesis reported here makes use of a novel nickel catalyst, and gives rise to PDEB with good solubility and excellent thermal stability characteristics. The polymer exhibits fair photoluminescence and electroluminescence, a large third-order nonlinear optical susceptibility, and good humidity sensitivity, while doped PDEB shows semiconductor characteristics.

    10. Halides of BET-TTF: Novel Hydrated Molecular Metals (pages 54–58)

      E. Laukhina, E. Ribera, J. Vidal-Gancedo, S. Khasanov, L. Zorina, R. Shibaeva, E. Canadell, V. Laukhin, M. Honold, M.-S. Nam, J. Singleton, J. Veciana and C. Rovira

      Article first published online: 29 DEC 1999 | DOI: 10.1002/(SICI)1521-4095(200001)12:1<54::AID-ADMA54>3.0.CO;2-1

      A hint of superconducting transition has been observed for the first time in a cation radical salt derived from bisethylenethio-tetrathiafulvalene (BET-TTF), the salt (BET-TTF)2Br·3H2O. Here the synthesis, X-ray structure, and physical properties of two hydrated halides of BET-TTF that are isostructural and present stable metallic properties are described. The Figure is a perspective view of the crystal structure of (BET-TTF)2Br·3H2O, layers of BET-TTF radical cation alternating along the a-direction with polymeric anion networks.

    11. Efficient Energy Transfer from Blue to Red in Tetraphenylporphyrin-Doped Poly(9,9-dioctylfluorene) Light-Emitting Diodes (pages 58–62)

      T. Virgili, D. G. Lidzey and D. D. C. Bradley

      Article first published online: 29 DEC 1999 | DOI: 10.1002/(SICI)1521-4095(200001)12:1<58::AID-ADMA58>3.0.CO;2-E

      A saturated-red emitting system suitable for solution-processable LEDs (light-emitting diodes) is reported. In their approach the authors dope the blue-emitting conjugated polymer poly(9,9-dioctylfluorene) (see Figure) with the red dye tetraphenylporphyrin, which leads to an efficient single-step Förster transfer from blue to red. The fabrication and efficiency of LEDs based on this system are also described.

    12. Preparation of Mesoporous High-Surface-Area Activated Carbon (pages 62–65)

      Z. Hu, M. P. Srinivasan and Y. Ni

      Article first published online: 29 DEC 1999 | DOI: 10.1002/(SICI)1521-4095(200001)12:1<62::AID-ADMA62>3.0.CO;2-B

      A simple, cost effective method for producing carbons with high mesopore contentis presented. The effect of the ratio of activating agent (ZnCl2) to carbon precursor (coconut shell) on pore size is investigated as is the nitrogen adsorption isotherm of these carbonaceous materials at 77 K. The mesoporosity is substantiated by the absorption capacity for phenol, methylene blue, and erythrosine red. Mesoporous high-surface-area activated carbons, which up to now have proved difficult to prepare, have applications as catalytic, battery electrode, capacitor, gas storage, and biomedical engineering materials.

    13. Magnetic and Electronic Structure of the CMR Chalcospinel Fe0.5Cu0.5Cr2S4 (pages 65–69)

      O. Lang, C. Felser, R. Seshadri, F. Renz, J.-M. Kiat, J. Ensling, P. Gütlich and W. Tremel

      Article first published online: 29 DEC 1999 | DOI: 10.1002/(SICI)1521-4095(200001)12:1<65::AID-ADMA65>3.0.CO;2-U

      The search for magnetic analogues of superconducting compounds has lead to the discovery of compounds with giant magnetoresistance. For example, a colossal negative magnetoresistance of up to 20 % at 180 K has been measured for the chalcospinel Fe0.5Cu0.5Cr2S4. Here spin-polarized ab-initio band structure calculations are performed on the title compound in order to gain an understanding of its electronic and magnetic behavior, the structural information for these calculations being obtained from neutron diffraction data. The valency of iron is also investigated by temperature-dependent Mössbauer spectroscopy.

    14. Rapid Conversion of Poly(p-phenylenevinylene) Films at Low Temperatures (pages 69–74)

      A. Marletta, D. Gonçalves, O. N. Oliveira Jr., R. M. Faria and F. E. G. Guimarães

      Article first published online: 29 DEC 1999 | DOI: 10.1002/(SICI)1521-4095(200001)12:1<69::AID-ADMA69>3.0.CO;2-6

      Poly(p-phenylenevinylene) (PPV) films with good optical and electrical properties are desirable for optoelectronic applications. The standard synthesis involves thermal elimination from a non-conjugated precursor. Although conversion at low temperatures would be preferable in terms of improved quantum efficiency and ease of device fabrication, relatively high temperatures are needed for full PPV conversion. Here a route is presented in which rapid elimination takes place at low temperature to yield thermally stable PPV films with a high degree of conjugation and improved photoluminescence properties.