Epitaxial and graphoepitaxial growth of materials on highly orientated PTFE substrates

The mechanism by which friction-deposited, highly orientated poly(tetrafluoroethylene) (PTFE) films promote orientated growth of materials was investigated. For this purpose, transmission electron microscopy was used to determine the orientation of polyethylene and 1,4-bis-2-(5-phenyloxazolyl)benzene (POPOP) crystals grown from the vapour phase onto the single crystal like PTFE films. Electron diffraction patterns revealed that the polyethylene crystals adopted an orientation that minimized the lattice mismatch at the interface between this material and the PTFE substrate. On the other hand, the POPOP crystals aligned in a fibre pattern, implying that orientated growth occurred because of the grating-shaped surface topography of the PTFE films. Evidently, the latter films were capable of promoting orientated growth of materials by a graphoepitaxial mechanism or conventional epitaxy, depending on the material used.     

Heat sensitive property of sputtered titanium oxide thin films for uncooled IR detector application

Titanium oxide (TiO_x) thin films were deposited on thermally grown Si₃N₄-film-coated Si substrates by dc reactive magnetron sputtering. The crystallographic structure, microscopic morphology, electrical resistivity, temperature coefficient of resistance (TCR) and activation energy of the films were proven as functions of the oxygen partial pressure (pO₂) and substrate temperature (T _s ). The mixed valence TiO_x thin films deposited with temperature from 30 to 250 °C in different pO₂ present various electrical properties even though all the samples are amorphous. The TCR and activation energy of the films vary with different pO₂ and T _s . It has been demonstrated that the sputtered TiO_x films are promising heat sensitive materials for uncooled Infrared detector application.     

Epitaxial crystallization of selenium on linear polyphenyls

Selenium has been crystallized in the presence of linear polyphenyls, namelyp-ter andp-quaterphenyl, crystals by reheating from the glassy state to temperatures in the range 80 to 180°C. Oriented growth of selenium is observed. The epitaxial relationship is based on lattice matchings parallel and normal to the selenium chains. Similarities with epitaxial growth of polyethylene on the same organic substrates are discussed.     

Transient characteristics of organic light-emitting diodes with efficient energy transfer in emitting material

We have investigated the combination effect of host–guest materials in an emitting layer on the transient property of an organic light-emitting diode (OLED). We found that an efficient energy transfer owing to the large overlap between the photoluminescence spectrum of host material and the absorption spectrum of guest material was an important factor to improve the response speed of the OLED. As a result, the rise time of optical response was mainly affected by the combination of host–guest materials, and it increased using the optimal guest material, 1,4-bis[2-[4-N,N-di(p-tolyl)amino]phenyl]vinyl]benzene (DSB). A maximum −3 dB cutoff frequency of 15.8 MHz was achieved for an OLED with DSB as a guest material.     

Mechanical properties and structure of glassy and semicrystalline random copolymers of poly(ethylene terephthalate) and poly(ethylene naphthalene-2,6-dicarboxilate)

The microhardness,H, of random copolymers of poly(ethylene terephthalate) (PET) and poly(ethylene naphthalene-2,6-dicarboxilate) (PEN) was determined over a wide range of compositions. It is shown that microhardness of the materials is strongly affected by the composition. The mechanical property,H, of the quenched amorphous copolyester films is discussed in terms of a simple model given by the additivity values of the single componentsH _a ^PET andH _a ^PEN . In materials containing up to 30% PEN, crystals of PET are found after annealing at temperatures 10 °C below their melting points. In materials containing 80% PEN, after annealing at about 20 °C below the melting point, crystals of PEN are formed. The observed deviation ofH for the crystallized films from the additive behaviour of the single components can be quantitatively related to two factors: the changes occurring in the crystallinity value and in the thickness of PET and PEN crystals.     

Flexible Luminescent Hydrogen-bonded Organic Framework for the Separation of Benzene and Cyclohexane

A nonplanar phenothiazine derivative with three cyano moieties (PTTCN) is designed and synthesized to achieve functional crystals for absorptive separation of benzene and cyclohexane. PTTCN can crystallize into two kinds of crystals with different fluorescence colors in different solvent systems. The molecules in two crystals are in different stereo isomeric forms of nitrogen, quasi axial (ax), and quasi equatorial (eq). The crystals with blue fluorescence in ax form may selectively adsorb benzene by a single-crystal-to-single-crystal (SCSC) transformation, but separated benzene from a benzene/cyclohexane equimolar mixture with a low purity of 79.6%. Interestingly, PTTCN molecules with eq form and benzene co-assembled to construct a hydrogen-bonded framework (X-HOF-4) with S-type solvent channels and yellow-green fluorescence, and can release benzene to form nonporous guest-free crystal under heating. Such nonporous crystals strongly favor aromatic benzene over cyclohexane and may selectively reabsorb benzene from benzene/cyclohexane equimolar mixture to recover original framework, and the purity of benzene can reach ≈96.5% after release from framework. Moreover, reversible transformation between the nonporous crystals and the guest-containing crystals allows the material to be reused.     

Etching behavior of CdTe in aqueous H<Subscript>2</Subscript>O<Subscript>2</Subscript>-HI-C<Subscript>6</Subscript>H<Subscript>8</Subscript>O<Subscript>7</Subscript> solutions

This paper examines the dissolution behavior of the (111)A, (111)B, (110), and (100) surfaces of CdTe single crystals in aqueous H₂O₂-HI-C₆H₈O₇ (citric acid) solutions. We have determined the dissolution rate of the crystals as a function of temperature and solution concentration, located the composition regions of polishing and selective etchants, and studied the microstructure and roughness of surfaces polished with optimized etchants. The etching behavior of CdTe is shown to depend on its crystallographic orientation.     

Kinetics of crystallization of trans-1, 4-polyisoprene crystallized in thin films

The lamellar thickness and crystal growth rates of both high melting form and low melting form trans-1,4-polyisoprene crystals growing from the same melt in thin films, have been determined by transmission electron microscopy. Values of the fold surface energy ( _e) have been determined and compare well with values for solution grown and bulk melt grown crystals. It is suggested that the crystals have a similar form in all three cases. The crystal growth rate data can be described by equations derived from secondary nucleation theory and the product of the surface energies ( _{e s}) is calculated. The value of the product is compared with recalculated values determined by using previously published optical growth rate data.     

Optically Rewritable Transparent Liquid Crystal Displays Enabled by Light‐Driven Chiral Fluorescent Molecular Switches

Functional soft materials exhibiting distinct functionalities in response to a specific stimulus are highly desirable towards the fabrication of advanced devices with superior dynamic performances. Herein, two novel light‐driven chiral fluorescent molecular switches have been designed and synthesized that are able to exhibit unprecedented reversible Z/E photoisomerization behavior along with tunable fluorescence intensity in both isotropic and anisotropic media. Cholesteric liquid crystals fabricated using these new fluorescent molecular switches as chiral dopants exhibit reversible reflection color tuning spanning the visible and infrared region of the spectrum. Transparent display devices have been fabricated using both low chirality and high chirality cholesteric films that operate either exclusively in fluorescent mode or in both fluorescent and reflection mode, respectively. The dual mode display device employing short pitch cholesteric film is able to function on demand under all ambient light conditions including daylight and darkness with fast response and high resolution. Moreover, the proof‐of‐concept for a “remote‐writing board” using cholesteric films containing one of the light‐driven chiral fluorescent molecular switches with ease of fabrication and operation is disclosed herein. Such optically rewritable transparent display devices enabled by light‐driven chiral fluorescent molecular switches pave a new way for developing novel display technology under different lighting conditions.     

A mesoporous silica modified conjugated polymer film: Preparation and detection nitroaromatics in aqueous phase

The linear conjugated polymer of polyfluorene/poly(p-phenylenevinylenes) (PFO/PPV) was synthesized and selected as the matrix film. The incorporation of a small fraction of inorganic mesoporous silicas into the polymer matrix resulted in a significant increase in overall detection efficiency of nitroaromatics. The structure of the obtained mixed films was characterized by ¹H-NMR, IR, and XRD. The optical and fluorescence properties of the mixed films were demonstrated by ultraviolet and visible spectroscopy (UV-Vis) and photoluminescence (PL) spectra in ethanol/water solution. Compared with the pure conjugated polymer film, the mixed films had high fluorescence quenching sensitivity toward nitroaromatic compounds, especially 2,4,6-trinitrophenol (TNP), in the aqueous phase. The fluorescent emission quenching is quantitative and can be analyzed on the basis of the Stern-Volmer model relation with the quenching process. The results revealed that such mixed matrix films of conjugated polymer/mesoporous silicas can be a promising strategy of designing chemosensory materials for detecting nitroaromatic compounds.     

Epitaxial Growth of Oriented Metalloporphyrin Network Thin Film for Improved Selectivity of Volatile Organic Compounds

This study reports an oriented and homogenous cobalt‐metalloporphyrin network (PIZA‐1) thin film prepared by liquid phase epitaxial (LPE) method. The thickness of the obtained thin films can be well controlled, and their photocurrent properties can also be tuned by LPE cycles or the introduction of conductive guest molecules (tetracyanoquinodimethane and C₆₀) into the PIZA‐1 pores. The study of quartz crystal microbalance adsorption confirms that the PIZA‐1 thin film with [110]‐orientation presents much higher selectivity of benzene over toluene and p‐xylene than that of the PIZA‐1 powder with mixed orientations. These results reveal that the selective adsorption of volatile organic compounds highly depends on the growth orientations of porphyrin‐based metal‐organic framework thin films. Furthermore, the work will provide a new perspective for developing important semiconductive sensing materials with improved selectivity of guest compounds.     

Structure and mechanical properties of polyethylene-fullerene composites

The microhardness of films of fullerene-polyethylene composites prepared by gelation from semidilute solution, using ultrahigh molecular weight polyethylene (PE) (6×10⁶), has been determined. The composite materials were characterized by optical microscopy and X-ray diffraction techniques. The microhardness of the films is shown to increase notably with the concentration of fullerene particles within the films. In addition, a substantial hardening of the composites is obtained after annealing the materials at high temperatures (T _a=130 °C) and long annealing times (t _a=10⁵s). The hardening of the composites with annealing temperature has been identified with the thickening of the PE crystalline lamellae. Comparison of X-ray scattering data and the microhardness values upon annealing leads to the conclusion of phase separation of C₆₀ molecules from the polyethylene crystals within the material. The temperature dependence is discussed in terms of the independent contribution of the PE matrix of the C₆₀ aggregates to the hardness value.     

MOLECULAR AND CRYSTAL STRUCTURE OF THE ADDUCTS OF C60F18 WITH AROMATIC HYDROCARBONS

ABSTRACT

The solubility of C₆₀F₁₈ in aromatic hydrocarbons, benzene, toluene, and xylenes (0.48–1.23?mg/mL), and decomposition enthalpies for the 1?:?1 and 1?:?2 complexes (31–70?kJ/mole) have been determined. The C₆₀F₁₈ molecule has near perfect C_3v symmetry, and the x-ray single-crystal structures of the C₆₀F₁₈?L complexes (L=hexamethylbenzene, o-, m-, p-xylene, and bromobenzene) are compared in terms of 12 types of C?C and four types of C–F bonds. Analysis of the packing modes in the crystals shows an influence of the size and polarity of the aromatic hydrocarbon molecule.     

Electrodeposited Thin Film ZnTe Semiconductors for Photovoltaic Applications

The electrodeposition of zinc telluride thin films on tin conductive oxide substrates in aqueous solution containing TeO₂ and ZnCl₂ was studied. The electrodeposition mechanism was investigated by cyclic-photovoltammetry. The appropriate potential region where formation of stoichiometric ZnTe semiconductor occurs, was found to be close to _-0.75 V vs. SCE. Annealing of the as-deposited films was carried out at 400^°C to obtain a crystalline phase. The crystallographic structure and film morphology were studied by XRD and SEM analyses, respectively. ZnTe films have the hexagonal structure of wurtzite and they are characterized by good homogeneity. A direct energy gap of 2.25 eV was determined by NIR-VIS-UV spectroscopy, in close agreement with the energy gap of ZnTe single crystals. A fractal dimension of 2.4 was determined by AFM analysis of ZnTe films. The mechanism of the thin film growth has been interpreted in terms of diffusion limited aggregation model.     

Binding energies of Si 2<Emphasis Type="Italic">p</Emphasis> and Co 3<Emphasis Type="Italic">p</Emphasis> electrons in cobalt silicides

The binding energies of Si 2p and Co 3p core-shell electrons in four stable cobalt silicides (Co₃Si, Co₂Si, CoSi, and CoSi₂) have been determined by high-resolution photoelectron spectroscopy using synchrotron radiation. The silicides were formed by solid-state epitaxy under identical conditions on Si(100), Si(110), and Si(111) faces of silicon single crystals.     

Highly conductive single molecular junctions by direct binding of π-conjugated molecule to metal electrodes

We have investigated electrical conductance of the single C₆₀ and benzene molecules bridging between metal electrodes. The single C₆₀ and benzene molecular junctions were prepared in ultra high vacuum. The single molecular junctions showed the high conductance values (around 0.1–1 G₀: G₀ = 2e²/h), which were comparable to that of the metal atomic contact. The highly conductive single molecular junctions could be prepared by direct binding of the π-conjugated organic molecule to the metal electrodes without the use of anchoring groups. For comparison, the single 1,4-benzenediamine molecular junction was investigated in solution. The benzene molecule was bound to the Au electrodes via amine (anchoring group) for the single 1,4-benzenediamine molecular junction. The conductance of the single 1,4-benzenediamine molecular junction was 8 × 10⁻³G_0. It was suggested that the anchoring groups acted as resistive spacers between the molecule and metal.     

Potentiometric study of polyaniline film synthesized with various dopants and composite-dopant: A comparative study

The potentiometric study of polyaniline (PANI) film synthesized with dopants viz. polyvinyl sulfonic acid (PVS),p-toluene sulfonic acid (p TS), dodecyl benzene sulfonic acid (DBS) and composite-dopants viz. PVS-p TS and PVS-DBS, has been carried out. The synthesized PANI films were characterized by electrochemical technique, UV-visible spectroscopy, Fourier transform infrared spectroscopy (FTIR), scanning electron microscope (SEM) and conductivity measurement. It was found that the PANI doped with PVS gives good electrochemical properties, conductivity as well as surface morphology as compared to p TS and DBS, whereas in composite dopants the PANI doped with PVS-pTS gives good polymer matrix as compared to PVS-DBS.     

Thermal and fluorescent properties of optical brighteners and their whitening effect for pelletization of cycloolefin copolymers

The thermal and fluorescent properties of 4,4-bis(5-methyl-2-beazoxoazol)ethylene (Hostalux KS-N), 1,4-bis(benzoxazolyl-2-yl)naphthalene (Hostalux KCB), 2,5-bis-(5-tertbutylbenzoxazole-2-yl) thiophene (Uvitex OB), 2,2′-(4,4′-diphenolvinyl)dibenzoxazol (Uvitex OB-1), and 1,1′-biphenyl-4,4′-bis(2-(methoxyphenyl)ethenyl) (Uvitex 127) have been investigated. All of them exhibit good thermal stability (> 300 °C) and high fluorescent quantum yields (> 0.8). Furthermore, we also have successfully blended these optical brighteners into cycloolefin copolymers with the antioxidant (Irganox HP2921) to improve the discoloration after pelletization based on their whitening effect.     

Growth of CdS nanoparticles in Y- and Z-type Langmuir–Blodgett thin film using 1,3-bis-(p-iminobenzoic acid)indane

Cadmium sulphide (CdS) nanoparticles were formed in 1,3-bis-(p-iminobenzoic acid)indane by exposing Cd²⁺ doped Y- and Z-type multilayered Langmuir–Blodgett (LB) films to H₂S gas. The growth of CdS nanoparticles were monitored by UV–visible spectroscopy measurements. It was observed that CdS nanoparticles in both Y- and Z-type LB films cause a blue-shift in absorption spectra. The surface morphology of LB films were characterized with atomic force microscopy DC electrical measurements were carried out for these LB films grown in a metal/LB film/metal sandwich structures with and without CdS nanoparticles. By analyzing I–V curves and assuming Schottky conduction mechanism the barrier height was found to be as 1.25 and 1.17?eV for Y-type unexposed and exposed samples; 1.18 and 1.25?eV for Z-type unexposed and exposed samples, respectively.     

Study of the Superconducting Cs-doped WO<Subscript>3</Subscript> Crystal Surface by Electron Backscattered Diffraction

The crystallographic surface structure of thermal vapor grown Cs _X WO₃ 0.005≤x≤0.3 crystals was investigated locally by electron backscattered diffraction in the environmental scanning electron microscope. Monoclinic to hexagonal phase transformation was shown to take place upon Cs doping to nominal concentrations of x=0.005 and x=0.05, while monoclinic to trigonal phase transition was observed at a concentration of x=0.3. In particular, the 2D superconducting crystals, of x=0.005 nominal concentration, were of inhomogeneous crystallographic phase according to the local Cs doping. The superconducting Cs-doped regions of the hexagonal phase were shown to be epitaxially grown on the WO₃ monoclinic crystal surface, the (0001) of this phase being parallel to the (001) plane of the WO₃ crystal. Our results support previous observations in these 2D superconducting Cs _X WO₃ crystals.