The growth and characterization of GaN grown on an Al2O3 coated (001)Si substrate by metalorganic vapor phase epitaxy

Single crystal GaN films have been grown on to an Al2O3 coated (001)Si substrate in a horizontal-type low-pressure MOVPE system. A thin Al2O3 layer is an intermediate layer for the growth of single crystal GaN on to Si although it is only an oriented polycrystal him as shown by reflection high electron diffraction. Moreover, the oxide was not yet converted to a fully single crystal film, even at the stage of high temperature for the GaN overlayer as studied by transmission electron microscopy. Double crystal X-ray diffraction showed that the linewidth of (0002) peak of the X-ray rocking curve of the 1.3 mu m sample was 54 arcmin and the films had heavy mosaic structures. A near band edge peaking at 3.4 eV at room temperature was observed by photoluminescence spectroscopy. (C) 1998 Elsevier Science B.V. All rights reserved.

The growth and characterization of GaN grown on a gamma-Al2O3/(001) Si substrate by metalorganic vapor phase epitaxy

Wurtzite single crystal GaN films have been grown onto a gamma-Al2O3/Si(001) substrate in a horizontal-type low pressure MOVPE system. A thin gamma-Al2O3 layer is an intermediate layer for the growth of single crystal GaN on Si although it is only an oriented polycrystal film as shown by reflection high electron diffraction. Moreover, the oxide is not yet converted to a fully single crystal film, even at the stage of high temperature for the GaN layer as studied by transmission electron microscopy. Double crystal x-ray linewidth of (0002) peak of the 1.3 mu m sample is 54 arcmin and the films have heavy mosaic structures. A near band edge peaking at 3.4 eV at room temperature is observed by photoluminescence spectroscopy. Raman scattering does not detect any cubic phase coexistence.

Initiation and development of water film by seepage

When water seeps upwards through a saturated soil layer, the soil layer may become instability and water films occur and develop. Water film serves as a natural sliding surface because of its very small friction. Accordingly, debris flow may happen. To investigate this phenomenon, a pseudo-three-phase media is presented first. Then discontinuity method is used to analyze the expansion velocity of water film. Finally, perturbation method is used to analyze the case that a water flow is forced to seep upwards through the soil layer while the movement of the skeleton may be neglected relative to that of water. The theoretical evolutions of pore pressure gradient, effective stress, water velocity, the porosity and the eroded fine grains are obtained. It can be seen clearly that with the erosion and re-deposited of fine grains, permeability at some positions in the soil layer becomes smaller and smaller and, the pore pressure gradient becomes bigger and bigger, while the effective stress becomes smaller and smaller. When the effective stress equals zero, e.f. liquefaction, the water film occurs. It is shown also that once a water film occurs, it will be expanded in a speed of (U)(t)/(1 - E >).

Synthesis and gas transport property of polyimide from 2,2 '-disubstituted biphenyltetracarboxylic dianhydrides (BPDA)

A series of dianhydride monomers, 2,2'-disubstituted-4,4',5,5'-biphenyltetracarboxylic dianhydride (substituents = phenoxy, p-methylphenoxy, p-tert-butylphenoxy, nitro, and methoxy) were synthesized by the nitration of an N-methyl protected 3,3',4,4'-biphenyttetracarboxylic dianhydride (BPDA) and subsequent aromatic nucleophilic substitutions with aroxides (NaOAr) or methoxide. These dianhydrides were polymerized with various aromatic diamines in refluxing m-cresol containing isoquinoline to afford a series of aromatic polyintides. The effects of varying 2,2'-substituents of the dianhydride (BPDA) moiety on the properties of polyimides were investigated. It was found that polyimides from the dianhydrides containing phenoxy, p-methylphenoxy, and p-tert-butylphenoxy side groups possessed excellent solubility and film forming capability whereas polyimides from 2,2'-dinitro-BPDA and 2,2'-dimethoxy-BPDA were less soluble in organic solvent. The soluble polymers formed flexible, tough and transparent films. The films had a tensile strength, elongation at break, and Young's modulus in the ranges 102-168 MPa, 8-21%, 2.02-2.38 GPa, respectively. The polymer gas permeability coefficients (P) and ideal selectivities for N-2, O-2, CO2 and CH4 were determined for the -OAr substituted polyimides. The oxygen permeability coefficient (P-O2) and permselectivity of oxygen to nitrogen (PO2/N-2) of the films were in the ranges 3.4-11.3 barrer and 3.8-4.6, respectively.

n-channel, ambipolar, and p-channel organic heterojunction transistors fabricated with various film morphologies

The relationship between the performance characteristics of organic field-effect transistors (OFETs) with 2,5-bis(4-biphenylyl)-bithiophene/copper hexadecafluorophthalocyanine (BP2T/F16CuPc) heterojunctions and the thickness of the BP2T bottom layer is investigated. Three operating modes (n-channel, ambipolar, and p-channel) are obtained by varying the thickness of the organic semiconductor layer. The changes in operating mode are attributable to the morphology of the film and the hetero-junction effect, which also leads to an evolution of the field-effect mobility with increasing film thickness. In BP2T/F16CuPc heterojunctions the mobile charge carriers accumulate at both sides of the heterojunction interface, with an accumulation layer thickness of ca. 10 nm. High field-effect mobility values can be achieved in continuous and flat films that exhibit the heterojunction effect.

Composite film of carbon nanotubes and chitosan for preparation of amperometric hydrogen peroxide biosensor

A new amperometric biosensor for hydrogen peroxide was developed based on cross-linking horseradish peroxidase (HRP) by glutaraldehyde with multiwall carbon nanotubes/chitosan (MWNTs/chitosan) composite film coated on a glassy carbon electrode. MWNTs were firstly dissolved in a chitosan solution. Then the morphology of MWNTs/chitosan composite film was characterized by field-emission scanning electron microscopy. The results showed that MWNTs were well soluble in chitosan and robust films could be formed on the surface. HRP was cross-linked by glutaraldehyde with MWNTs/chitosan film to prepare a hydrogen peroxide biosensor. The enzyme electrode exhibited excellent electrocatalytic activity and rapid response for H2O2 in the absence of a mediator. The linear range of detection towards H2O2 (applied potential: -0.2 V) was from 1.67 x 10(-5) to 7.40 x 10(-4) M with correction coefficient of 0.998. The biosensor had good repeatability and stability for the determination of H2O2. There were no interferences from ascorbic acid, glucose, citrate acid and lactic acid.

Polymer concentration, shear and stretch field effects on the surface morphology evolution during the spin-coating

Polymer concentration and shear and stretch field effects on the surface morphology evolution of three different kinds of polymers (polystyrene (PS), polybutadiene (PB) and polystyrene-b-polybutadiene-b-polystyrene (SBS)) during the spin-coating were investigated by means of atomic force microscopy (AFM). For PS and SBS, continuous film, net-like structure and particle structure were observed at different concentrations. For PB, net-like structures were not observed and continuous films and radial array of droplets emerged. Moreover, we compared surface morphology transitions on different substrate locations from the center to the edge. For PS, net-like structure, broken net-like structure and irregular array of particles were observed. For SBS, net-like structure, periodically orientated string-like structure and broken-line structure appeared. But for PB, flower-like holes in the continuous film, distorted stream-like structure and irregular distributions of droplets emerged. These different transitions of surface morphologies were discussed in terms of individual material property.

Luminescence properties of RP1-xVxO4: A (R=Y, Gd, La; A=Sm3+Er3+ x=0, 0.5, 1) thin films prepared by pechini sol-gel process

Thin film phosphors with compositions of RP1-xVxO4: A (R = Y, Gd, La; A = Sm3+, Et3+; x = 0, 0.5, 1) have been prepared by a Pechini sol-gel process. X-Ray diffraction, atomic force microscopy (AFM), photoluminescence excitation and emission spectra were utilized to characterize the thin film phosphors. The results of XRD showed that a solid solution formed in YVxP1-xO4: A film series from x = 0 to x = 1 with zircon structure, which also held for GdVO4: A film. However, LaVO4: A film crystallized with a different structure, monazite. AFM study revealed that the phosphor films consisted of homogeneous particles ranging from 90 to 400 nm depending on the compositions. Upon short ultraviolet excitation, the films exhibit the characteristic Sm(3+ 4)G(5/2)-H-6(J) (J=5/2, 7/2, 9/2) emission in the red region and Er3+ H-2(11/2), S-4(3/2)-I-4(15/2) emission in the green region, respectively With the increase of x values in YVxP1-xO4: SM3+ (Er3+) films, the emission intensity Of SM3+ (Er3+) increases due to the increase of energy transfer probability from VO43- to Sm3+ (Er3+). Due to the structural effects, the Sm3+ (Er3+) shows similar spectral properties in YVO4 and GdVO4 films, which are much different from those in LaVO4 film.

Synthesis, characterization and fabrication on a glassy carbon electrode of a tetra-iron substituted sandwich-type pentadecatungstodiarsenate heteropolyanion

A novel sandwich-type compound, Na-12[Fe-4(H2O)(2)(As2W15O56)2].41H(2)O, has been synthesized. The compound was well-characterized by means of IR, UV-vis, W-183 NMR and elemental analyses. The compound crystallizes in the triclinic, P (1) over bar symmetry group. The structure of the compound is similar to that of Na-16[M-4(H2O)(2)(As2W15O56)(2)].nH(2)O (M = Cu, Zn, Co, Ni, Mn, Cd), and consists of an oxo-aqua tetranuclear iron core, [(Fe4O14)-O-III(H2O)(2)], sandwiched by two trivacant alpha-Wells-Dawson structural moieties, alpha-[As2W15O56]. Redoxelectrochemistry of the compound has been studied in buffer solutions at pH = 4.7 using polarography and cyclic voltammetry ( CV). The compound exhibited four one-electron couples associated with the Fe(III) center followed by three four-electron redox processes attributed to the tungsten-oxo framework. The compound-containing monolayer and multilayer films have been fabricated on a 4-aminobenzoic acid modified glassy carbon electrode surface by alternating deposition with a quaternized poly(4-vinylpyridine) partially complexed with [Os(bpy)(2)Cl](2+/-). CV, X-ray photoelectron spectroscopy (XPS), UV-vis spectroscopy and atomic force microscopy (AFM) have been used to characterize the multilayer films.

Cardo polyarylethersulfones and polyaryletherketones bearing alkyl substituents on the phenylene unit

A series of cardo polyaryletherketones and polyaryletersulfones containing alkyl substituents of a different kind, number and volume were synthesized from bis(4-nitrophenyl)ketone or bis(4-fluorophenyl)sulfone with various alkyl substituted phenolphthaleins by polycondensation using K2CO3 as catalyst. Their chemical and aggregation structures were confirmed by FT-IR, H-1-NMR and WAXD. The resulting polymers were soluble in a variety of common polar solvents and, transparent, colorless, and tough films could be easily cast from 1,1,2-trichluoroethane solution. Their tensile strength, elongation at break and tensile modulis were in the range of 70.5 similar to 97.1MPa, 4.49%similar to7.81%, and 1.69 similar to2.27GPa, respectively. The prepared polymers had reasonably high glass transition temperatures at 207 to 269 degreesC, and showed fairly good thermal stability with 5% thermal decomposition loss above 410 degreesC.

Stimulated emission in the film of polymer/dye blend

Optically pumped stimulated emission behavior in an organic film was demonstrated in this study. The gain material consists of a laser dye perylene doped into polystyrene (PS) matrix in an appropriate weight ratio. The sample was transversely pumped by the three harmonic output of a mode-locked Nd:YAG laser. The change of the emission spectra showed a clear threshold action and gain narrowing phenomenon when increasing the excitation intensity. Three emission peaks were observed below the excitation threshold, which are locate at 446, 475 and 506 nm, respectively. However, only the gain narrowing peak centered at 475 nm could be detected above the threshold. The spectra narrowing observed results from the amplified spontaneous emission (ASE) in the gain material. (C) 2000 Elsevier Science S.A. All rights reserved.

Synthesis of a new kind of polyamide and polyester including aromatic spirodilactone

A new kind of monomers including aromatic spirodilactone-5, 5'-carboxy-7,7'-dioxo-2,2'-spirobi(benzo-[c]tetrahydrofuran) is synthesized from m-xylene and paraformaldehyde. It is converted to a series of polyamides and polyesters by means of low-temperature solution polycondensation and interfacial polycondensation. NMR and IR spectra, solubility, mechanical and thermal properties of all these polymers are investigated. The polymers have high glass transition temperatures and good thermal oxidative properties. All polyamides have high viscosity and good solubility in strong polar organic solvents such as DMSO, DMAc, DMF and NMP. All polyamides can be cast into transparent, flexible and tough films possessing good tensile properties.

Synthesis and investigation of monoesters of polymaleic acids with azo-group

A series of amphiphilic polymers, monoesters of polymaleic acid containing ate-side groups (PMAN-A(n)Me) was synthesized by the reaction of alcohol(A,Me) with polymaleic anhydride (PMAN) for Langmuir-Blodgett assembly. The effects of the length of flexible spacer in the side chain on the pi-A isotherms and LB films were investigated. The UV-Vis and infrared linear dichroism spectra showed that the ate-side groups were highly oriented in PMAN-A(6)Me LB film deposited on silica or CaF2 substrate.

Novel polyaryletherketones containing various pendant groups .1. Synthesis and characterization

Novel high glass transition temperature polyaryletherketones, containing pendant amido, alkyl, and carboxyl groups with reduced viscosity above 0.54 dL/g, were synthesized via solution nucleophilic polycondensation reaction of phenolphthalin, 2',2 ''-diisopropyl-5',5 ''-dimethylphenolphthalin, and 3,3'-bis(4-hydroxyphenyl)isobenzopyrrolidone with bis(4-nitrophenyl)ketone in the presence of potassium carbonate. By ion exchange with Na+ and K+, four ionomers were also prepared. A new monomer simultaneously containing carboxyl and algyl substituents was synthesized by reduction reaction of 2',2 ''-diisopropyl-5',5 ''-dimethyl-phenolphthalein. The resulting polymers were soluble in a few polar aprotic solvents; transparent, colorless, and tough films could easily be cast from DMF or DMSO solution. The mechanical properties of the films were excellent; and their tensile strength, elongation at break, and tensile moduli were in the range of 67.1-97.1 MPa, 7.8-165%, and 1.47-2.27 GPa, respectively. The prepared polymers showed fairly good thermal stability and resonably high glass transition temperatures above 210 degrees C. (C) 1997 John Wiley & Sons, Inc.

ELECTROCHEMICAL STUDY OF ISOPOLYOXOMETALLATES AND HETEROPOLYOXOMETALLATES FILM MODIFIED MICROELECTRODES .1. PRETREATMENT AND MODIFICATION OF THE MO80-26(4-) MODIFIED CARBON-FIBER MICROELECTRODE

In this paper the preparation of isopoly- and heteropolyoxometallates (IPA and HPA) thin film modified carbon fiber (CF) microelectrodes and the factor that influences the modification of IPA and HPA films are described. IPA and HPA film modified CF microelectrodes can all be prepared by cyclic potential scan and simple dip coating. The modified electrodes prepared are very stable and reversible in acidic solution with monolayer characteristics. The electrochemical pretreatment of CF microelectrodes plays an important role in the modification of IPA and HPA film. The absorption of IPA and HPA film on electrode surfaces has been discussed on the basis of surface conditions of the CF microelectrode and the structure of IPA and HPA.