Controlling the Optical Properties of a Conjugated Co-polymer through Variation of Backbone Isomerism and the Introduction of Carbon Nanotubes

The need to control the formation of weakly emitting species in polymers such as aggregates and excimers, which are normally detrimental to device performance, is illustrated for the example of the polymer poly(m-phenylenevinylene-co-2,5-dioctyloxy-p-phenylenevinylene), using the model compound, 2,5-dioctyloxy-p-distyrylbenzene as a comparison. Two different methods, namely a Homer-Emmons polycondensation in dimethylformamide (DMF) and a Wittig polycondensation in dry toluene, have been used during synthesis resulting in a polymer with a predominantly trans-vinylene backbone and a polymer with a predominantly cis-vinylene backbone, respectively. Photoluminescence and absorption spectroscopy indicate that the polymer forms aggregate species in solution with spectra that are distinctly red-shifted from those associated with the intra-chain exciton. Concentration dependent optical studies were used to probe the evolution of aggregation in solution for both polymers. The results indicate that inter-chain coupling in the predominantly cis-polymer is prominent at lower concentrations than in the case of the trans-counterpart. These results are supported by pico-second pump and probe transient absorption measurements where, in dilute solutions, the polymer in a cis-configuration exhibits highly complex excited state dynamics, whereas the polymer in a trans-configuration behaves similarly to the model compound. It is proposed therefore that the degree of backbone isomerism has a profound impact on the morphology of the polymeric solid and control over it is a route towards optimising the performance of the material in thin film form. Another method to inhibit inter-chain effects using multi walled carbon nanotubes (MWNT) as nano-spacers in the polymer solutions is proposed. By comparison to spectroscopic analysis, aggregation effects are shown to be reduced by the introduction of nanotubes. Electron microscopy and computer simulation suggest a well-defined interaction between the polymer backbone and the lattice of the nanotube

The Reelin Receptors Apoer2 and Vldlr Coordinate the Patterning of Purkinje Cell Topography in the Developing Mouse Cerebellum

The adult cerebellar cortex is comprised of reproducible arrays of transverse zones and parasagittal stripes of Purkinje cells. Adult stripes are created through the perinatal rostrocaudal dispersion of embryonic Purkinje cell clusters, triggered by signaling through the Reelin pathway. Reelin is secreted by neurons in the external granular layer and deep cerebellar nuclei and binds to two high affinity extracellular receptors on Purkinje cells-the Very low density lipoprotein receptor (Vldlr) and apolipoprotein E receptor 2 (Apoer2). In mice null for either Reelin or double null for Vldlr and Apoer2, Purkinje cell clusters fail to disperse. Here we report that animals null for either Vldlr or Apoer2 individually, exhibit specific and parasagittally-restricted Purkinje cell ectopias. For example, in mice lacking Apoer2 function immunostaining reveals ectopic Purkinje cells that are largely restricted to the zebrin II-immunonegative population of the anterior vermis. In contrast, mice null for Vldlr have a much larger population of ectopic Purkinje cells that includes members from both the zebrin II-immunonegative and -immunopositive phenotypes. HSP25 immunoreactivity reveals that in Vldlr null animals a large portion of zebrin II-immunopositive ectopic cells are probably destined to become stripes in the central zone (lobules VI–VII). A small population of ectopic zebrin II-immunonegative Purkinje cells is also observed in animals heterozygous for both receptors (Apoer2+/−: Vldlr+/−), but no ectopia is present in mice heterozygous for either receptor alone. These results indicate that Apoer2 and Vldlr coordinate the dispersal of distinct, but overlapping subsets of Purkinje cells in the developing cerebellum

Photochromisme transitoire du sexithiophène. Vers un modulateur spatial de lumière ultra-rapide

Les modulateurs spatiaux de lumière (MSL) sont les composants de base des dispositifs de traitement optique de l'information. Les matériaux optiques non-linéaires du troisième ordre sont susceptibles d’améliorer leurs performances en termes de densité d'éléments de résolution et de vitesse de commutation. Nous montrons içi que le photochromisme transitoire du sexithiophène (α-6T) présente des caractéristiques spectrales et dynamiques adaptées à un convertisseur incohérent-à-cohérent ultra-rapide, une classe particulière de MSL à commande optique

Near infrared emission in rubrene:fullerene heterojunction devices

Near infrared (NIR) emission was studied in rubrene:fullerene planar and bulk heterojunction bifunctional devices. The degree of mixing was controlled by changing the substrate temperature for planar heterojunctions and changing the ratio of the two materials for bulk heterojunctions. We found that there was no simple correlation between the presence of the NIR emission and the photovoltaic efficiency and the turn-on voltage of the devices. The origin of the NIR emission and the relationship between EL spectra, photovoltaic efficiency, and the degree of mixing of the donor and acceptor materials is discussed. © 2009 Elsevier B.V. All rights reserved.link_to_subscribed_fulltex

Second harmonic generation in zinc oxide nanorods

Second-order optical nonlinearities of zinc oxide (ZnO) nanorods grown on quartz substrate were determined by optical second harmonic generation (SHG) measurements at 1064 nm fundamental wavelength. The average length of the zinc oxide nanorods ranged from 50 nm to 700 nm. By employing the Maker fringes technique, we obtained the second-order nonlinear optical coefficients d 333 and d 311. Their magnitudes and ratio are compared with that of zinc oxide thin film fabricated by different techniques. We see variations of the second-order nonlinear optical coefficients with respect to the aspect ratio of the nanorods. This is attributed to local field effects.link_to_subscribed_fulltex

Nanocomposite hole injection layer for organic device applications

Nanocomposite layers consisting of poly(3,4,-ethylene dioxythiophene): polystyrene sulfonic acid (PEDOT:PSS) and inorganic nanoparticles (Ni, Cu, and NiO) were investigated as hole injection layers in organic light emitting diodes (OLEDs) consisting of N,N′-di(naphthalene-1-yl)-N,N′- diphenylbenzidine as a hole transporting and tris (8-hydroxyquinoline) aluminum as electron transporting and emitting layer. It was found that the addition of Cu and NiO nanoparticles resulted in inferior OLED performance for all nanoparticle concentrations, while for Ni nanoparticles, improved performance was obtained for optimized nanoparticle concentration compared to the devices with pure PEDOT:PSS. Reasons for the improved performance of devices with PEDOT:PSS:Ni layers are discussed. It appears that increase of the current driven into the diode is the reason for the improvement of OLED performances achieved by incorporating Ni-nanoparticles into the injection layer. © 2005 Elsevier B.V. All rights reserved.link_to_subscribed_fulltex