Photoacid cross-linkable polyfluorenes for optoelectronics applications

Polyfluorene copolymers are functionalised with oxetane side groups which make them cross-linkable upon exposure to an acid. Upon addition of a photoacid they become photo-cross-linkable if exposed to UV light. Optical and electrochemical properties of these copolymers, both in the soluble and cross-linked forms are compared. We further show that the use of this type of copolymer allows the fabrication of multilayer structures without restriction of the organic solvent used to deposit an upper layer. In particular, we fabricate multilayer LEDs with enhanced luminous efficiency using one of the copolymers. The possibility to photo-cross-link these materials is also used to prepare micrometer size patterns, using a transmission electron microscope grid as a shadow mask. Furthermore, the use of a scanning near-field optical microscope allows submicron patterns to be prepared. We propose that this approach, which can be extended to a wide range of conjugated polymers, can be useful for the fabrication of optical, electronic and optoelectronic structures.     

Photoconductivity and recombination in amorphous Se/CdSe multilayers

Transport and recombination in single layers of a-Se, CdSe as well as amorphous multilayers of Se/CdSe have been studied by measuring temperature dependencies of the photocurrent and the exponent in the intensity dependence of the photocurrent. It has been suggested that the transport: (i) in a-Se was controlled by traps situated at 0.25 eV above the top of the valence band or the localized states of the valence band tail; (ii) in CdSe was controlled by traps at about 0.12 eV below the conduction band. The recombination was considered to be dominated by deep centers in both Se and CdSe layers. Values of around 0.6 eV and 0.7 eV above the valence band have been determined for the position of the centers dominating recombination in CdSe at temperatures lower than 110 K and higher than 200 K respectively. The position of the recombination centers in a-Se has been determined at about 0.97 eV above the valence band. This coincides with the position of the three-fold coordinated Se atoms carrying a positive charge (C₃⁺ defects) determined by other authors.     

Thickness dependence of structural and ferroelectric properties of sol-gel Pb(Zr0.56Ti0.44)0.90(Mg1/3Nb2/3)0.10O3 films

Sol-gel Pb(Zr_0.56Ti_0.44)_0.90(Mg_1/3Nb_2/3)_0.10O₃ (PZT-PMN) films were prepared onto the Ti/Pt/Ti bottom electrode by multilayer spin coating. The film thickness ranged from 0.22 to 0.88 μm. The Pt top electrodes were deposited on the PZT-PMN films by DC sputtering. The structural and ferroelectric properties of PZT-PMN films were investigated as a function of film thickness by X-ray diffraction (XRD), scanning electron microscopy (SEM), cross-sectional transmission electron microscopy (XTEM), and by measuring the relative permittivity. The film retains the tetragonal perovskite structure with the [111] and [100] preferred orientations perpendicular to the film surface independent of film thickness. The [100] texture increases with increasing film thickness although the [111] texture is always predominant. The film consists of columnar grains. The average grain size is nearly independent of film thickness. The surface layer containing fine grains about 30 nm in diameter is induced on the top of the film. As the film thickness exceeds 0.44 μm, the number of the fine grains decreases remarkably. The crystalline interface layer about 10 nm thick is formed between the film and the bottom electrode. This interface layer is composed of Pt, Pb, Zr, Ti and O, while it is rich in Ti and deficient in Pb and O as compared with the inside of the film. The measured relative permittivity of the film increases with increasing film thickness, following the low permittivity interface model. On the basis of this model, the relative permittivity is estimated to be 3200 for the intrinsic PZT-PMN film, 750 for the surface layer and 50 for the interface layer.     

Preparation,characterization and thermal stability of bentonite modified with bis-imidazolium salts

Sodium bentonite was modified with several organic bis-imidazolium salts. Organoclays with water soluble surfactants were prepared by the traditional cation exchange reaction. The bis-imidazolium-bentonites were characterized by Fourier transform infrared spectroscopy (FTIR), powder X-ray diffraction analysis (PXRD) and thermogravimetric analysis (TGA). The effect of chemical composition and molecular weight of the salts on the thermal stability and basal spacing were evaluated. The bis-imidazolium-bentonites showed enhanced thermal stability (300–400 °C) and may be potentially useful materials for melt processing of polymer/layered silicates nanocomposites.     

Mechanical and tribological properties of CrN/Ag and CrSiN/Ag nanoscale multilayers

This paper presents a concept of combining hard CrN (or CrSiN) and soft Ag lubricating materials in a nanoscale multilayer structure. The CrN/Ag and CrSiN/Ag multilayers are synthesized by reactive direct current (DC) magnetron sputtering techniques. The thickness of the Ag nanolayer in the multilayers is fixed to 4 nm while that of CrN and CrSiN nanolayers vary from 4 to 20 nm. The nitride layer effects on the mechanical and tribological properties of the multilayers have been investigated. Different mechanisms of the adhesion failure have been observed on the multilayer surfaces, depending on the nitride nanolayer types and their thicknesses. The CrSiN/Ag films exhibit poor adhesion whereas CrN/Ag multilayers demonstrate very good adhesion to AISI M2 steel substrates. The study of friction behaviors of the CrN/Ag multilayers against 100Cr6 steel balls reveals that the multilayers have low friction coefficients in comparison with that of a CrN single layer. The decrease of friction coefficients is related to the presence of the solid lubricant (Ag) nanolayer in the coatings.     

Electrochemical removal of NOx with porous cell stacks

In this study porous cell stacks were investigated for their ability to remove NO_x electrochemically. The cell stacks were made from laminated tapes of porous electrolyte Ce_0.9Gd_0.1O_1.95 and composite electrodes of La_1−xSr_xMnO₃ (x = 0.15, and 0.5) and ceria doped with Gd or Pr. The cell stacks were infiltrated with nano-particles of pure ceria, Ce_0.9Gd_0.1O_1.95 and Ce_0.8Pr_0.2O_2−δ after sintering. A gas stream containing NO were sent through the cell stack. When the cell stacks were polarised with 0.75 V per cell then it was possible to remove some of the NO_x in the temperature interval of 250-400 °C. The cell stacks infiltrated with ceria showed the highest activity, while the ones infiltrated with Ce_0.9Gd_0.1O_1.95 had the highest selectivity towards NO compared to O₂. When the cell stack was polarised with 1.5 V for each cell it was possible to remove up to 35% of NO present.     

High-frequency ferromagnetic properties of multilayered FeCoHfO/AlOx films

For mobile communication, the ferromagnetic resonance frequency of magnetic films must be over 3 GHz. A suitable anisotropic field and high resistivity for high frequency applications were obtained by inserting insulator (AlO_x) layers into ferromagnetic layers (FeCoHfO). With this optimum configuration of three layers structure [FeCoHfO (400 nm)/AlO_x (10 nm)]₃, high frequency characteristics (permeability ∼ 100 at 100 MHz and ferromagnetic resonance frequency over 3 GHz) and high resistivity (ρ ∼ 1088) μΩ cm were achieved.     

Near infrared quazi-omnidirectional reflector in chalcogenide glasses

The quazi-omnidirectional reflector was designed as a planar quarter wave stack consisting of the alternating amorphous chalcogenide Ge₂₅S₇₅ and Sb₄₀Se₆₀ films. Photonic bandgap calculation of the intended reflector predicted 240 nm omnidirectional and 450 nm normal incidence first-order bandgaps centred near 1.55 μm for appropriate values of the index of refraction and thickness of the films. The TEM and HR-TEM images of the prepared 7.5 pairs reflector verified good periodicity, smooth interface and amorphous structure of the chalcogenide films deposited by thermal and flash evaporation, respectively. The optical reflectivity measurements revealed 98.8% normal incidence stopband of the reflector at 1.55 μm. We also report the ellipsometry study of the prepared reflector. The TEM and ellipsometry studies confirmed the thickness variation of prepared individual layers to be ±7 and ±9 nm, respectively, compared to theoretical predictions.