Narrow bandwidth red electroluminescence from solution-processed lanthanide-doped polymer thin films

Narrow bandwidth red electroluminescence from OLED devices fabricated using a simple solution-based approach is demonstrated. A spin-casting method is employed to fabricate organic light emitting diode (OLED) devices comprising a poly(N-vinylcarbazole) (PVK) host matrix doped with a europium beta-diketonate complex, Eu(dbM)(3)(Phen) (dibenzoylmethanate, dbm; 1,10-phenanthroline, Phen) on glass/ indium tin oxide (ITO)/3,4-polyethylene-dioxythiophene-polystyrene sulfonate (PEDOT) substrates. Saturated red europium ion emission, based on the (5)Do ->F-7(2) transition, is centered at a wavelength of 612 nm with a full width at half maximum of 3.5 rim. A maximum external quantum efficiency of 6.3 x 10(-2) cd/A (3.1 X 10(-2)%) and a maximum luminance of 130 cd/M-2 at 400 mA/cm(2) and 25 V is measured for ITO/PEDOT/PVK:Eu(dbM)3(Phen)/Ca/Al devices. This measured output luminance is comparable to that of devices fabricated using more sophisticated small molecule evaporation techniques. (c) 2005 Elsevier B.V All rights reserved.

Selectivity control in hydrogenation reactions by nanoconfinement of polymetallic nanoparticles in mesoporous thin films

A one-pot sol-gel synthesis method has been developed for the incorporation of metal nanoparticles into mesoporous oxide thin films deposited on various plane substrates by spin-coating and on the inner surface of fused silica capillaries by dip-coating. The size, the metal loading and the stoichiometry of the metal nanoparticles could be precisely controlled by following this methodology. In the first step, polymer stabilized Pt50Sn50 and Pt90Sn10 nanoparticles were obtained by a solvent-reduction method. Then, the nanoparticles were added to a metal oxide precursor sol, which was destabilized by solvent evaporation. After calcination, the obtained materials were tested in the hydrogenation of citral in both batch and continuous modes. The highest selectivity of 30% towards the unsaturated alcohols was obtained over supported Pt90Sn10 nanoparticles with a preferential formation of the cis-isomer (nerol) due to a unique confinement of the bimetallic nanoparticles in the mesoporous framework. The selectivity towards the unsaturated alcohols was further improved to 56% over the PtRu5Sn nanoparticles supported by impregnation onto mesoporous silica films. (C) 2009 Elsevier B.V. All rights reserved.

A comprehensive aerosol spray method for the rapid photocatalytic grid area analysis of semiconductor photocatalyst thin films

Indicator inks, previously shown to be capable of rapidly assessing photocatalytic activity via a novel photo-reductive mechanism, were simply applied via an aerosol spray onto commercially available pieces of Activ (TM) self-cleaning glass. Ink layers could be applied with high evenness of spread, with as little deviation as 5% upon UV-visible spectroscopic assessment of 25 equally distributed positions over a 10 cm x 10 cm glass cut. The inks were comprised of either a resazurin (Rz) or dichloroindophenol (DCIP) redox dye with a glycerol sacrificial electron donor in an aqueous hydroxyethyl cellulose (HEC) polymer media. The photo-reduction reaction under UVA light of a single spot was monitored by UV-vis spectroscopy and digital images attained from a flat-bed scanner in tandem for both inks. The photo-reduction of Rz ink underwent a two-step kinetic process, whereby the blue redox dye was initially reduced to a pink intermediate resorufin (Rf) and subsequently reduced to a bleached form of the dye. In contrast, a simple one-step kinetic process was observed for the reduction of the light blue redox dye DCIP to its bleached intermediates. Changes in red-green-blue colour extracted from digital images of the inks were inversely proportional to the changes seen at corresponding wavelengths via UV-visible absorption spectroscopy and wholly indicative of the reaction kinetics. The photocatalytic activity areas of cuts of Activ (TM) glass, 10 cm x 10 cm in size, were assessed using both Rz and DCIP indicator inks evenly sprayed over the films: firstly using UVA lamp light to activate the underlying Activ (TM) film (1.75 mW cm(-2)) and secondly under solar conditions (2.06 +/- 0.14 mW cm(-2)). The photo-reduction reactions were monitored solely by flat-bed digital scanning. Red-green-blue values of a generated 14 x 14 grid (196 positions) that covered the entire area of each film image were extracted using a Custom-built program entitled RGB Extractor(C). A homogenous degradation over the 196 positions analysed for both Rz (Red colour deviation = 19% UVA, 8% Solar: Green colour deviation = 17% UVA, 12% Solar) and DCIP (Red colour deviation = 22% UVA, 16% Solar) inks was seen in both UVA and solar experiments, demonstrating the consistency of the self-cleaning titania layer on Activ (TM). The method presented provides a good solution for the high-throughput photocatalytic screening of a number of homogenous photocatalytically active materials simultaneously or numerous positions on a single film; both useful in assessing the homogeneity of a film or determining the best combination of reaction components to produce the optimum performance photocatalytic film. (C) 2010 Elsevier B.V. All rights reserved.

Changes in functional behavior of 93 % Pb(Mg1/3Nb2/3)O-3-7 % PbTiO3 thin films induced by ac electric fields

The dielectric properties of Au/[93%Pb(Mg1/3Nb2/3)O-3-7%PbTiO3] (PMN-PT)/(La0.5Sr0.5)CoO3/MgO thin-film capacitor heterostructures, made using pulsed laser deposition, have been investigated, with particular emphasis on the changes in response associated with increasing the magnitude of the ac measuring field. It was found that increasing the ac field caused a change in the frequency spectrum of relaxators, increasing the speed of response of "slow" relaxators, with an associated decrease in the freezing temperature (T-f) of the relaxor system; in addition, other characteristic parameters relating to polar relaxation (activation energy E-a and attempt frequency 1/tau(0)), described by fitting of the dielectric response to a Vogel-Fulcher expression, were found to change continuously as ac field levels were increased.

Phase transitions in epitaxial Ba0.5Sr0.5TiO3 thin films

Ba0.5Sr0.5TiO3 (BST) thin-film capacitor structures with various thicknesses, (50-1200 nm) and different strain conditions (on lanthanum strontium cobalt oxide La0.5Sr0.5CoO3 and strontium ruthenate SrRuO3 buffer layers) were made using pulsed laser deposition, and characterized by x-ray diffraction. The out-of-plane lattice parameter was followed as a function of temperature within the 100-300 K temperature interval. The phase sequence (cubic-tetragonal-orthorhombic-rhombohedral) known to exist in the bulk analog is shown to be strongly affected by both the stress conditions imposed by the buffer layer and the thickness of the BST film itself. Thus, no phase transition was found for the in-plane compressed BST films. On the stress-free BST films, on the contrary, more phase transitions were observed. It appeared that the complexity of structural phase transitions increased as the film thickness in this system was reduced.

The effect of flexoelectricity on the dielectric properties of inhomogeneously strained ferroelectric thin films

Recent experimental measurements of large flexoelectric coefficients in ferroelectric ceramics suggest that strain gradients can affect the polarization and permittivity behaviour of inhomogeneously strained ferroelectrics. Here we present a phenomenological model of the effect of flexoelectricity on the dielectric constant, polarization, Curie temperature (T-C), temperature of maximum dielectric constant (T-m) and temperature of the onset of reversible polarization (T-ferro) for ferroelectric thin films subject to substrate-induced epitaxial strains that are allowed to relax with thickness, and the qualitative and quantitative predictions of the model are compared with experimental results for (Ba0.5Sr0.5)TiO3 thin films on SrRuO3 electrodes. It is shown that flexoelectricity can play an important role in decreasing the maximum dielectric constant of ferroelectric thin films under inhomogeneous in-plane strain, regardless of the sign of the strain gradient.

Studies of switching kinetics in ferroelectric thin films

Experimental studies are reported concerning the importance of interfacial capacitance (including electrode screening, space-charge layers, and/or chemically discrete dead layers). on domain switching behaviour in thin films of ferroelectric lead zirconate-titanate (PZT), strontium bismuth tantalate (SBT), and barium strontium titanate (BST). Emphasis is placed upon studies at applied field values very near the coercive field E, asymmetry in fatigue for positive and negative polarity coercive fields, and in the case of BST, of the coexistence of ferroelectric and paraelectric phases Studies of dielectric loss show important correlations between tan 6 and fatigue (polarization decrease) as a function of bipolar switching cycles N. This is a priori not obvious, since the former is a linear response and the latter, a nonlinear response. Modelling of enlarged interfacial,space-charge layers in PZT films and chemically distinct dead (paraelectric) layers in BST films shows contradictory tendencies of coercive-voltage changes with the growth of passive layers.

Evidence for two-phase regions in BST thin films from capacitance-voltage data

The Curie-Weiss plots of reciprocal dielectric constant versus temperature, in Ba0.5Sr0.5TiO3 films grown onto SrRuO3 lower electrodes by pulsed-laser deposition, show two minima below film thicknesses of 280 nm. This double minima implies possible mixed phases in the thin films. A graphical plot of capacitance for decreasing dc voltage versus that of increasing dc voltage shows a well-defined triangular shape for both Pb(Zr0.4Ti0.6)O-3 and SrBi2Ta2O9 thin films. However, for a 175-nm-thick Ba0.5Sr0.5TiO3 thin film, the plot shows an overlapping of two triangles, suggesting mixed phases. This graphical method appears to be effective in detecting structural subtleties in ferroelectric capacitors.