Wha-Tzong Whang

Low dielectric polyimide/poly(silsesquioxane)-like nanocomposite material

A new type of low dielectric polyimide/poly(silsesquioxane)-like (PI/PSSQ-like) hybrid nanocomposite material is successfully prepared from the polyimide (ODA‐ODPA) precursor containing phenyltrialkoxysilane (PTS) at two chain ends and monoaryltrialkoxysilane with a self-catalyzed sol‐gel process. We employ p-aminophenyltrimethoxysilane (APTS) to provide bonding between the PTS and ODPA‐ODA phase. It is shown by transmission electron microscopy (TEM) and scanning electron microscopy (SEM) that the PSSQ-like domain sizes with uniform size are fairly well separated in the hybrid films. The silica domain sizes of 5000-PIS and 5000-PIS‐50-PTS films are in the range of 30‐100 nm, of 5000-PIS‐100-PTS and 10000-PIS‐100-PTS in the range of 80‐200 and 300‐600 nm, respectively. The dielectric constant can be 2.79 for 5000-PIS‐140-PTS with fairly good mechanical properties. The PI/PSSQ-like hybrid films have higher onset decomposition temperature and char yield in thermogravimetric analysis (TGA) and higher Tg in differential scanning calorimetry (DSC) than the pure PI. Moreover, the PI/PSSQ-like hybrid films have excellent transparency even under high PTS content. In the series of X-PIS hybrid films, the coefficient of thermal expansion (CTE) below Tg increases with the PI block chain length, but in the series of X-PIS‐y-PTS films, it slightly increases with the PTS content. However, above Tg the CTE of X-PIS and X-PIS‐24-PTS is much lower than that of the pure PI. The dielectric constant and water absorption of X-PIS‐y-PTS films decrease with the PTS content because of the higher free volume and hydrophobicity. q 2001 Elsevier Science Ltd. All rights reserved.

Effect of surface stabilization of nanoparticles on luminescent characteristics in ZnO/poly(hydroxyethyl methacrylate) nanohybrid films

Highly transparent and stable luminescent ZnO/poly(hydroxyethyl methacrylate) nanocomposites have been synthesized via a nanoparticle surface modified method. 3-(Trimethoxysilyl)propyl methacrylate (TPM) was used as the stabilizing agent in a simple, mild sol–gel route to prepare TPM-modified ZnO nanoparticles. The existence of TPM on the nanoparticle surface effectively promotes the stability of colloidal ZnO nanoparticles and the compatibility between the inorganic nanoparticles and the organic matrix in the solid nanohybrid. The resulting ZnO/PHEMA nanocomposites with TPM-modified nanoparticles have better dispersibility and controllable luminescent properties. The characteristics of TPM-modified and unmodified ZnO nanoparticles have been studied by ultraviolet-visible (UV-vis) absorption spectroscopy, powder X-ray diffraction (XRD), transmission electron microscopy (TEM), Fourier transform infrared (FTIR) and 1H NMR spectroscopy.

High-conductivity poly(3,4-ethylenedioxythiophene):poly(styrene sulfonate) film for use in ITO-free polymer solar cells

In this study, we devised a simple method to enhance the conductivity of poly(3,4-ethylenedioxythiophene)-poly(styrene-sulfonate) (PEDOT:PSS) films through spin-coating with various surface-modified compounds, and then applied this technique to the preparation of ITO-free polymer solar cells (PSCs). The electrical conductivity of PEDOT:PSS films can be increased by more than two order of magnitudes merely by spin-coating a compound containing one or more polar groups—such as ethanol, methoxyethanol, 1,2-dimethoxyethane, and ethylene glycol—onto the films. In this paper, we discuss the phenomena occurring through conductivities, morphologies, and chemical properties of the modified PEDOT-PSS films as determined using Raman spectroscopy, a four-point probe, scanning electron microscopy (SEM), atomic force microscopy (AFM), transmission electron microscopy (TEM), and X-ray photoelectron spectroscopy (XPS). The schematic 3D morphological model of directly solvent-modified PEDOT:PSS films is presumed for ITO-free devices. The desirable conductivity enhancements of these materials make them attractive candidates for use as anode materials in ITO-free PSCs.

Physical and mechanical properties of polyimide/titania hybrid films

Abstract Titania-containing polyimide (PI) hybrid films with homogeneous and flexible properties have been fabricated via sol–gel process. Various contents of titania are added to three different PI systems by using acetylacetone to reduce the reaction rate and to prohibit gelation. The thermal, electrical and mechanical properties of the hybrid films will be measured and compared to pure PI. Results indicate that the metal-containing PI possesses lower thermal expansion and resistivity than pure PI. Incorporation of small amount of titania enhances mechanical properties of the hybrid films at both low and elevated temperature. The relationship between the effect of titania concentration, structure and properties will also be discussed.

Organic electroluminescent derivatives containing dibenzothiophene and diarylamine segments

A new series of 2,8-disubstituted dibenzothiophenes have been successfully synthesized via palladium-catalyzed C–N or C–C bond formation using 2,8-dibromodibenzothiophene and diarylamines as starting materials. These new dibenzothiophene derivatives are amorphous with a glass transition temperature ranging from 86 to 190 °C. Furthermore, they are fluorescent in the blue to bluish green region. Two types of light-emitting diodes (LED) were constructed from these compounds, (I) ITO/Cpd/TPBI/LiF/Al and (II) ITO/Cpd/Alq3/LiF/Al, where TPBI and Alq3 are 1,3,5-tris(N-phenylbenzimidazol-2-yl)benzene and tris(8-hydroxyquinolinato)aluminium, respectively. In type I devices, the compounds function as the hole-transporting and emitting material. In type II devices, emission from Alq3 is observed. Several type I devices emit pure blue light, and most of the devices II have very promising performance. The relation between the energy levels of the materials and the performance of the light-emitting diodes is discussed.

Observation of enhanced cell adhesion and transfection efficiency on superhydrophobic surfaces

It was found that cells attached preferentially on the roughened area of patterned superhydrophobic surfaces allowing the formation of cell microarrays with the advantages of improved cell adhesion, natural separation of colonies and enhanced transfection efficiency.

Deposition of Carbon Nanowall Flowers on Two-Dimensional Sheet for Electrochemical Capacitor Application

We have used bias-assisted microwave plasma chemical vapor deposition to synthesize (i) two-dimensional carbon nanowalls [(2D CNWs), monolayer] and (ii) three-dimensional carbon nanowalls [(3D CNWs), double-layer] on carbon cloth in a CH 4 /CO 2 system. The specific surface area of the 3D CNW electrode (1580 m 2 /g) was considerably larger than that of the 2D CNW electrode (364 m 2 /g). This higher surface area and more porous structure of the 3D CNW electrode resulted in its capacitance reaching as high as ca. 198 F/g at a scan rate of 10 mV/s. Electrochemical measurements suggested that this 3D electrode is a promising material for application in electrical storage devices.

Characteristics of an electropolymerized PPV and its light-emitting diode

Abstract Poly(phenylene vinylene) (PPV) has been prepared successfully from p -xylylenebis(triphenylphosphonium bromide) in acetonitrile solution via an electroreduction polymerization. p -Xylylenebis(triphenylphosphonium bromide) also acted as an electrolyte in the solution. The resultant electropolymerized PPV film was transparent and dense, and had considerable adhesion to an indium tin oxide (ITO) electrode. The electropolymerized PPV showed a blue shift both in the ultraviolet-visible absorption spectrum and photoluminescence, compared with the PPV prepared from a p -xylylenebis(tetrahydrothiophenium chloride) precursor route, indicating that it has a shorter π-conjugated chain length. The infra-red absorption spectra showed a stable PC bond on the electropolymerized PPV chain. The Al/electropolymerized PPV/ITO light-emitting diode emitted green light with an emission maximum at 530 nm. Annealing the electroluminescence device in a high vacuum oven at 160°C for 3 h significantly enhanced the electroluminescence performance as a result of improvement of the interfacial contact between PPV and the Al metal electrode.

A study of the relationship between the electroluminescence characteristics and compositions of PPV-PVA-based polymers

Abstract A series of poly( p -phenylene vinylene) (PPV)-poly(vinyl alcohol) (PVA)-based polymers were prepared from the PPV precursor and PVA aqueous solution mixtures. The luminescence characteristics of the PPV-PVA-based polymers were dependent on their composition and the following treating conditions. The emitted light of the PPV-PVA-based light-emitting diodes (LEDs) shifted from yellow-green (550 nm) to blue (485 nm) as the PVA component increased. The infra-red (i.r.) absorption spectra showed that some part of the PPV precursor could react with PVA to form COC linkages and interrupted the conjugation of PPV polymer chains. This resulted in decrease of the conjugated chain length, as evidenced in ultraviolet-visible u.v.-vis. absorption spectra. The relative photoluminescence (PL) intensity increases with decreasing the conjugated chain length. The relative electroluminescence (EL) decreases with the increase of the PVA component in the polymers. However, both PL and EL intensity normalized with PPV molar ratio of PPV-PVA-based polymers were higher than that of pure PPV. The relative low threshold voltage at 4–6 V was found in these PPV-PVA-based polymer LEDs with Al (negative) and indium-tin oxide ITO (positive) as the two electrodes.

Effect of ZnMA on Optical and Holographic Characteristics of Doped PQ/PMMA Photopolymer

We report on the fabrication and experimental characteristics of doping Zinc methylacrylate (ZnMA) and 9,10-phenanthrenequinone (PQ) into a poly(methyl methacrylate) (PMMA) photopolymer. The effects of a ZnMA dopant on the optical and holographic characteristics of ZnMA/PQ/PMMA have been examined. From the optical and holographic measurements of different samples with different composition ratios, we found that the time to reach the maximum diffraction efficiency has been reduced from 102 to 59 (reduced by 1.7 times) and the diffraction efficiency has been raised up from 0.57 to 1.62% (increased by 2.9 times). We also found that holographic gratings can be recorded in pure PQ-doped PMMA, but cannot be recorded in pure ZnMA-doped PMMA. The higher the concentration of PQ in codoped PMMA, the higher the diffraction efficiency. In contrast, the higher ZnMA concentration in the codoped PMMA, the shorter the response time. It is proposed that PQ serves as a photosensitive reactant and plays a major role in the formation of phase gratings. ZnMA serves as a sufficient catalyst to accelerate the reaction of PQ with a polymer or ZnMA. By adding ZnMA with a proper concentration into a PQ/PMMA photopolymer, we succeeded in improving the holographic characteristic of the photopolymer.