High dispersion and fluorescence of anthracene doped in polyphenylsiloxane films

Phenylsilsesquioxane-diphenylsiloxane glass thick films doped with anthracene were prepared from homogeneous coating sols obtained from two different approaches. One approach consisted in incorporating the dye during the glass preparation (which implies the incorporation of the dye in an aqueous media). The doped-glass was further dissolved in the solvents mixture composed of cyclohexane and acetone. The other approach which is non-aqueous consisted simply in dissolving directly the preformed non-doped polyphenylsiloxane glass in the dye solutions. The stability of the organic-inorganic hybrid glass matrix in most organic solvents makes possible the incorporation of the dye without problems of miscibility and dispersion in the hybrid matrix. The coating was performed at room temperature using spin-coating technique prior to further heat-treatment. Crack-free and homogeneous films of high optical quality were obtained. The optical properties of the doped films based on their absorption and emission spectra were discussed owing the incorporation route of the dye. The results showed that the non-aqueous approach used to incorporate the dye minimizes the dye aggregation. This property associated to the preparation route permits to obtain optically active hybrid films loaded with high concentrations of anthracene (in the order of 10⁻² M) which enhance the fluorescence of the doped films. The hybrid doped-films obtained represent therefore a tremendous potential for applications in the field of optics and photonics including the development of new nonlinear optical materials. Bouzid Menaa and Masahide Takahashi both the author contributed equally.     

Room-temperature electrochemical reduction of epitaxial magnetite films to epitaxial iron films

The electrochemical reduction of oxides to metals has been studied for decades. Earlier work produced polycrystalline bulk metals. Here, we report that pre-electrodeposited epitaxial face-centered cubic magnetite thin films can be electrochemically reduced to epitaxial body-centered cubic iron thin films in aqueous solution on single-crystalline gold substrates at room temperature. This technique opens new possibilities to produce special epitaxial metal/metal oxide heterojunctions and a wide range of epitaxial metallic alloy films from the corresponding mixed metal oxides.     

Intermolecular band dispersion in quasi-one-dimensional adenine assemblies

Highly-ordered, hydrated adenine multilayer films grown on the surface of highly-oriented pyrolytic graphite, HOPG(0001), display extended electronic states, affording anisotropic band-like charge transport along the π-π stacking direction.     

Directional dispersion of excitons in anthracene,probed by electron spectroscopy

Anthracene single crystal foils have been investigated by electron energy loss in the range from 3 eV to 25 eV for various directions of the crystal excitation wave vector k. Experimental evidence for axial dispersion is obtained. Calculations in which a dielectric tensor is used in the oriented gas model explain the anisotropy in the energy loss data and allow the assignment of the losses in this whole energy range to ¹A_u and ¹B_u crystal excitations originating from molecular ¹A_1g → ¹B_1u and → ¹B_1u excitations.     

Photophysical properties of 9,10-disubstituted anthracene derivatives in solution and films

We have carried out absorption, time-resolved fluorescence, and fluorescence quantum yield measurements of four new soluble anthracene derivatives. They show natural radiative lifetimes in the range of 2.5-4.4 ns, which is 5-10 times shorter than those reported for unsubstituted anthracene. The 9,10-bis(phenylethynyl)anthracene (BPEA) derivatives show the largest fluorescence transition dipoles, which is attributed to extended π-conjugation between anthracene and phenyls through acetylene linkages. Spin-cast films of the BPEA derivatives show strong fluorescence quenching by weakly emitting low energy excitations, which is attributed to excimer-like traps. Quenching is significantly reduced when bulky dendrons are attached so that they give maximum coverage of the emitting chromophore and prevent their aggregation. The results show that anthracene derivatives can be developed into efficient solution-processable fluorescent emitters for the blue and green spectral regions.     

Contribution of axial dispersion to band spreading in perfusion chromatography

A new interpretation of the band spreading data in perfusion chromatography is proposed by investigating the relative importance of axial dispersion in perfusive beds. Elution chromatography of proteins (bovine serum albumin and lysozyme) under non-retained conditions on two kinds of reversed-phase perfusive supports (POROS R1/H and POROS R2/H), which have different pore structures, were carried out to obtain the axial dispersion data. The Knox equation and some empirical correlations for dispersion coefficients in porous media were applied to correlate the experimental data. The influences of particle properties, solute molecular sizes and flow velocity on the dispersion coefficient were elucidated. Axial dispersion was recognised to be the main contributor to peak broadening in perfusion chromatography. The dependence of the height equivalent to a theoretical plate on flow-rate was found to be the result of the velocity dependence of the axial dispersion. The dispersion coefficient in a perfusive column can be well represented both by a power-law relationship and a correlation derived based on stochastic theory. Pursuant to these, it was found that pore size distribution of the perfusive particles and solute molecular size are important parameters, which influenced the dispersion results significantly.     

Mobility enhanced photoactivity in sol-gel grown epitaxial anatase TiO2 films

Epitaxial anatase thin films were grown on single-crystal LaAlO3 substrates by a sol-gel process. The epitaxial relationship between TiO2 and LaAlO3 was found to be [100]TiO2||[100]LaAlO3 and (001)TiO2||(001)LaAlO3 based on X-ray diffraction and a high-resolution transmission electron microscopy. The epitaxial anatase films show significantly improved photocatalytic properties, compared with polycrystalline anatase film on fused silica substrate. The increase in the photocatalytic activity of epitaxial anatase films is explained by enhanced charge carrier mobility, which is traced to the decreased grain boundary density in the epitaxial anatase film.     

Electrodeposition of PbSe epitaxial films on (111) InP

Epitaxial growth of PbSe is addressed on (111) InP single crystals by electrodeposition from aqueous solutions containing Se(IV), Pb(II) and Cd(II). The structure of the deposits is studied by reflection high-energy electron diffraction and X-ray diffraction. It is shown that an epitaxial growth is only observed in the presence of cadmium in the electrolyte. The best epitaxy is obtained for an optimal 0.5 M cadmium concentration.     

Deposition and characterisation of epitaxial oxide thin films for SOFCs

This paper reviews the recent advances in the use of thin films, mostly epitaxial, for fundamental studies of materials for solid oxide fuel cell (SOFC) applications. These studies include the influence of film microstructure, crystal orientation and strain in oxide ionic conducting materials used as electrolytes, such as fluorites, and in mixed ionic and electronic conducting materials used as electrodes, typically oxides with perovskite or perovskite-related layered structures. The recent effort towards the enhancement of the electrochemical performance of SOFC materials through the deposition of artificial film heterostructures is also presented. These thin films have been engineered at a nanoscale level, such as the case of epitaxial multilayers or nanocomposite cermet materials. The recent progress in the implementation of thin films in SOFC devices is also reported.     

Selective reaction and chemical anisotropy in epitaxial bismuth layer-structured ferroelectric thin films

Bismuth layer-structured ferroelectric thin films consisting of a stacking of pseudoperovskite and bismuth oxide blocks along the c-axis were epitaxially grown on single-crystal substrates by metalorganic chemical vapor deposition (MOCVD), and a selective reaction of the bismuth oxide layer with HCl was demonstrated. Epitaxial films with contrasting crystal orientations were used for the acid treatment in order to probe chemical anisotropy. For a/b-axis-oriented SrBi₂Ta₂O₉, Bi₄Ti₃O₁₂, and SrBi₄Ti₄O₁₅ films with sequential stacking of the two vertical blocks, notable structural selectivity of the reaction was observed only for SrBi₂Ta₂O₉. For this film, the pseudoperovskite block remained and the bismuth oxide block was removed, while both blocks of Bi₄Ti₃O₁₂ and SrBi₄Ti₄O₁₅ dissolved into the acid. In addition to the bismuth, the other cations in the pseudoperovskite blocks, strontium and titanium, also decreased for Bi₄Ti₃O₁₂ and SrBi₄Ti₄O₁₅. The selective reaction observed for a/b-axis-oriented SrBi₂Ta₂O₉, however, was not observed for c-axis-oriented SrBi₂Ta₂O₉ films in which the pseudoperovskite and bismuth oxide blocks were stacked horizontally. The results clearly show that SrBi₂Ta₂O₉ has sub-nano-order structural selectivity and chemical anisotropy in the unit cell in the reaction with HCl.     

Langmuir-Blodgett films of 9-phenyl anthracene molecules incorporated into different matrices

This communication reports the surface pressure (pi) versus area per molecule (A) isotherm characteristics of the mixed films of 9-phenyl anthracene (PA) in stearic acid (SA) and polymethyl methacrylate (PMMA) matrices, at the air-water interface. The mixed Langmuir films at the air-water interface have been observed to be easily transferred onto solid substrates to form uniform Langmuir-Blodgett films. By changing various parameters, namely molefraction, surface pressure of lifting and number of layers, the mixed Langmuir-Blodgett (LB) films of various types have been fabricated successfully and their spectroscopic characteristics have been reported. From the isotherm characteristics and the area per molecule versus molefraction plot, it is evident that the PA molecules are successfully incorporated into mixed Langmuir-Blodgett films. UV-vis absorption spectroscopic study of the mixed LB films at various molefractions of PA in two different matrices reveal that formation of I-type aggregate in PMMA matrix whereas both I- and H-type aggregates are playing their dominant role in SA matrix. Moreover, fluorescence spectroscopic study reveals reabsorption effect. Molecular movement persists in the freshly prepared LB films, as is evident from the time dependent changes in both UV-vis absorption and fluorescence spectra of the mixed LB films in both matrices. From our observation it is evident that about 200 h is required to get the LB films in a stable condition. Dimers and higher order n-mers are formed at a higher surface pressure of 30 mNm(-1).     

Variation of lattice dimensions in epitaxial SnS films on MgO(001)

Epitaxial and polycrystalline SnS films were prepared on MgO(001) and glass substrates using molecular beam epitaxy. The films were characterized by X-ray diffraction method. The orientations of epitaxial films were (010)[100]SnS||(001)[100]MgO or (010)[001]SnS||(001)[100]MgO. Lattice parameters of the polycrystalline film closely resembled those of bulk SnS at room temperature. However, the lattice parameters of epitaxial films varied widely and were very different from those of bulk SnS at room temperature. Considering the lattice dimensions and a/c ratio, the films roughly correspond to bulk SnS at elevated temperatures from 371 to 666 K. SEM images of the films showed needle- or circular-like SnS crystallites segregated from the epitaxial films. Respective energies of indirect band gaps of the films and refractive index of the polycrystalline film were estimated using results of optical transmission experiments.     

Optimization of electrochemical aspects for epitaxial depositing nanoscale ZnSe thin films

Studies of the electrochemical optimization of ZnSe thin film deposition on polycrystalline Au substrates using electrochemical atomic layer epitaxy are reported. Electrochemical aspects were characterized by means of cyclic voltammetry, differential pulse voltammetry, and coulometry. To study the growth mechanism of the underpotential deposition in the formation of ZnSe, the effects of Zn and Se deposition potentials and a Se-stripping potential were adjusted to optimize the deposition program. The deposit, grown using the optimized program, was proved to be a single-phase ZnSe compound with a strong (220)-preferred orientation by X-ray diffraction analysis, and scanning electronic microscopy observation shows the deposit consisted of nanoscale particles with an average size about 100 nm. The right 1:1 stoichiometric ratio of Zn to Se according to the coulometry suggests that ZnSe is formed.     

Magnetically driven growth of anthracene thin films by organic molecular beam deposition

The possible use of a static magnetic field during organic molecular beam deposition of thin molecular films for inducing some preferential growth is discussed and the magnetic properties of diamagnetic molecules and molecular crystals are recalled. Considering prototypical materials, namely anthracene molecules and potassium phthalate substrates, which interact and may give rise to polycrystalline films with specific orientations, we show that in the presence of a magnetic field the films display a macroscopic preferential orientation as a result of minimization of the magnetic energy contribution. A very good agreement between the results of optical spectroscopy, atomic force microscopy, and predictions made on the basis of the anisotropic magnetic susceptibility of anthracene is found.     

In situ growth of epitaxial cerium tungstate (100) thin films

The deposition of ceria on a preoxidized W(110) crystal at 870 K has been studied in situ by photoelectron spectroscopy and low-energy electron diffraction. Formation of an epitaxial layer of crystalline cerium tungstate Ce(6)WO(12)(100), with the metals in the Ce(3+) and W(6+) chemical states, has been observed. The interface between the tungsten substrate and the tungstate film consists of WO suboxide. At thicknesses above 0.89 nm, cerium dioxide grows on the surface of Ce(6)WO(12), favoured by the limited diffusion of tungsten from the substrate.     

Faster axial band dispersion in a monolithic silica column than in a particle-packed column

The contribution of molecular diffusion to peak broadening was studied in a reversed-phase HPLC system, consisting of a monolithic silica C18 column and methanol-water mobile phase. Study on the band broadening effect of holding a solute in a column or elution at very low linear velocity of mobile phase allowed facile determination of the contribution of the molecular diffusion term. Less obstruction against molecular diffusion, or the faster axial band dispersion in a monolithic silica column than in a particle-packed column, was found both in mobile phase and in stationary phase.     

Solidification-induced and shear-induced band texture in thermotropic liquid crystalline polymer films

Two phase diagrams of a six-ring double-swallow-tailed compound are presented where the mixing components involve electron-acceptor compounds. In the mixed phase region, nematic, smectic A and C, cubic and columnar phases are induced. In this way transitions between lamellar, cubic and columnar phases can be realized by variation of the con- centration. The mesophases occurring in these systems have been characterized by X-ray investigations.     

Fluorescence spectra and their temperature dependence of amorphous anthracene films doped with naphthacene

Fluorescence spectra and their temperature dependence of amorphous anthracene films doped with naphthacene were observed. An amorphous film was prepared by vacuum evaporation technique on a quartz plate cooled down to 30–40°K. The obtained results were discussed in relation to the structural change in the amorphous film.