Showing papers by "Kung-Hwa Wei published in 2006"

Enhanced Collective Electron Transport by CdSe Quantum Dots Confined in the Poly(4-vinylpyridine) Nanodomains of a Poly(styrene-b-4-vinylpyridine) Diblock Copolymer Thin Film†

Abstract: Semiconductor nanoparticle (NP) quantum dots (QDs) that have sizes close to their Bohr exciton radius (typically between 1 and 10 nm) display size-dependent band gaps and hence tunable optical properties. As a result, they exhibit a wide range of electrical and optical properties and can be used for various applications, such as light-emitting diodes, solar cells, lasers, and transistors. In these applications, composite materials consisting of nanoparticles and organic materials are often adopted. Thus, an understanding of the collective

Thiophenol-modified cds nanoparticles enhance the luminescence of benzoxyl dendron-substituted polyfluorene copolymers

Abstract: We have prepared highly luminescent dendron-substituted copolyfluorenes that incorporate surface-modified cadmium sulfide nanoparticles. Both the photoluminescence and electroluminescence efficiencies of the polymer nanocomposites are dramatically enhanced relative to the values of the pure polymer.

Synthesis and Characterization of Rod−Coil Polymers Based on Poly(ethylene oxide)s and Novel Luminescent Aromatic Cores

Abstract: A novel conjugated aromatic core containing direct-coupled fluorene, thiophene, and biphenyl groups via Suzuki coupling reaction was synthesized. The asymmetrical molecules contain two kinds of poly(ethylene oxide)s (PEO, Mn = 750 and 2000, n = 17 and 44) on one side of the rigid cores. Asymmetrical FOC8PEO17 and FOC16PEO17 contain flexible PEO chains (n = 17) displaying the smectic phases. However, FOC8PEO44 and FOC16PEO44 consisting of flexible PEO chains (n = 44) exhibit two kinds of columnar phases, Colh and Colr. Besides, alkoxy groups with different lengths (−OC8H17and −OC16H33) on both sides of the rigid cores were used as another flexible chain to form symmetrical molecules. Symmetrical FOC8 and FOC16 exhibit the nematic and smectic mesophases, respectively. Optical textures (POM) and XRD patterns have confirmed the structure of the mesophases and the molecular arrangements. The photophysical characteristics of all luminescent compounds were studied by photoluminescence and UV−vis absorption. In a...

Synergistic Effect of Compatibilizer in Organo-Modified Layered Silicate Reinforced Butadiene Rubber Nanocomposites

Abstract: We have prepared nanocomposites of intercalated and exfoliated organosilicates in butadiene rubber (BR) by using a two-stage melt blending process. We used X-ray diffraction and transmission electron microscopy to examine, respectively, the intergallery spacing of the organosilicates and their dispersion in the BR. Marked enhancements in the mechanical and thermal properties of BR occurred when it incorporated <10 parts of organosilicates and the loading ratio of the organosilicate to dicarboxylic acid-terminated butadiene oligomer was approximately three. In particular, the addition of 10 parts of organosilicate and 3 parts of compatibilizer in the BR led to a more than four-fold increase in the tensile strength, a 150% increase in modulus at 100% elongation (M100), and 232 and 410% enhancements in the tear strength and elongation at break, respectively, relative to those of neat BR. The degradation temperature for the BR nanocomposite containing only a 10-part loading of organosilicate was 51°C higher than that of neat BR; these increases reduced, however, to 9 –13°C upon the addition of the CTB compatibilizer. In addition, the relative water vapor permeabilities of the BR nanocomposites containing 10 parts of organosilicate— both in the presence and absence of the compatibilizer—reduced to 20% of that of the neat BR. POLYM. ENG. SCI., 46:80 – 88, 2006. © 2005 Society of Plastics Engineers

Synthesis and characterization of liquid-crystalline block copolymers with cyanoterphenyl moieties by atom transfer radical polymerization

Abstract: A series of new mesomorphic block copolymers composedofdifferentmacroinitiators, including poly(ethylene oxide), polystyrene, and poly(ethylene oxide)-b-polystyrene,and polymethacrylate with a pendent cyanoterphenyl group were synthesized through atom transfer radical polymerization. The number-average molecular weights of the three diblock copolymers, determined by gel permeation chromatography, were 10,254, 9,772, and 15,632 g mol−1, and their polydispersity indices were 1.17, 1.28, and 1.34. The mesomorphic and optical properties of all the block copolymers were investigated, and they possessed a smectic A phase with mesophasic ranges wider than 100 °C. Moreover, X-ray diffraction patterns provided evidence of the smectic A phase and the corresponding interdigitated packing of all the polymers. © 2006 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 44: 4593–4602, 2006

Effect of the orientation of CdSe nanorods on the electron mobility of CdSe/P4VP nanodomains self-assembled within a poly(styrene-b-4-vinylpyridine) diblock copolymer thin film.

Abstract: Block copolymers are versatile platform materials because they can self-assemble—if they have appropriate compositions and are subjected to suitable conditions—into various nanostructures having period thicknesses between 10 and 100 nm through microphase separation of incompatible blocks. Nanostructured block copolymers can be used as templates for selectively controlling the spatial position of semiconductor nanoparticles within one of the blocks. For example, the selective sequestrations of pre-synthesized CdS, CdSe, and TiO2 [6b] nanoparticles into one block of a diblock copolymer can be performed by ensuring the presence of strong interactions between that block and the surface ligands of the nanoparticles. Block copolymers can also be used as nanoreactors for the synthesis of nanomaterials. For example, quasi-regular arrays of Au clusters and TiO2 needles [6c] have been obtained through the selective sequestration of metal ions into one block of poly(styrene-bvinylpyridine). The unique optical and electrical properties of onedimensional (1D) semiconductor nanostructures, such as nanorods (NRs) and nanowires (NWs), can be exploited for use in a number of applications, including solar cells, photonic crystals, lasers, transistors, and sensors. 8,11a,b] To take full advantage of the material properties of these 1D nanostructures, they must be pre-aligned or ordered in some other way. Several physical and chemical strategies have been used for the alignment or ordering of arrays of 1D nanostructures. Examples for in-plane two-dimensional (2D) arrays include the unidirectional alignment of CdSe NRs, the assembly of BaCrO4 NR monolayers through the use of the Langmuir–Blodgett technique, and the alignment of a liquid-crystalline phase of CdSe NRs through surface deposition. For out-of-plane 2D arrays, some examples are the use of electron-beam (e-beam) lithography to obtain arrays of CdSe pillars and TiO2 nanoneedles through electrochemical deposition and solution crystal growth and the preparation of a periodic array of uniform ZnO NWs through vapor-phase transport and catalytic growth. Although there are many bottom–up growth techniques for preparing 1D nanostructures in the out-of-plane 2D arrays and many techniques for pre-synthesized NRs in the inplane 2D arrays, finding techniques for arranging pre-synthesized NRs in out-of-plane 2D arrays remains a great challenge. The powerful aligning force of an electric field can be used to manipulate the orientation of anisotropic materials by taking advantage of their different anisotropic dielectric constants. Examples of this approach include the alignment of ZnO NRs and Au NWs, the alignment of carbon nanotubes suspended in a columnar liquid-crystalline polymer melt, and the electrically induced sphere-to-cylinder transition and patterning in diblock copolymer films. In this present study, we applied an electric field to induce the orientation of CdSe NRs that had been self-assembled in the poly(4-vinylpyridine) (P4VP) nanodomains of a poly(styrene-b-4-vinylpyridine) (PS-b-P4VP) diblock copolymer thin film. We varied the number of CdSe nanorods incorporated in the P4VP nanodomains by controlling the strength of the polar interactions and the loading concentration. The electron mobilities and the electron-barrier heights of the CdSe/P4VP nanodomains incorporating out-of-plane and inplane CdSe NRs were monitored. Scheme 1 illustrates the process we used to prepare a monolayered (CdSe/P4VP)-b-PS thin film. Pyridine-modified CdSe NRs and the PS-b-P4VP block copolymer were dissolved and mixed in pyridine; the CdSe NRs were distributed selectively in the P4VP phase as a result of their preference for experiencing polar interactions. Subsequently, toluene, which is a good solvent for PS but a poor one for P4VP, was added to form a solution containing micelles having CdSe/P4VP cores and PS shells. Figure 1 displays TEM images of the P4VP domains of PS-b-P4VP samples containing different amounts of CdSe NRs. The dark regions represent the CdSe NRs (because of the high electron density of cadmium). The orientation of these CdSe NRs was largely in the plane direction of the PS-b-P4VP film. The interdomain distance of CdSe/P4VP domains was about 90 nm. In Figure 1a and b, where the loading concentrations of CdSe NRs were 65 and 48%, respectively, the average numbers of NRs incorporated within single P4VP nanodomains were approximately five and three, respectively. When the loading amount was 33%, some individually distinct CdSe nanorods were incorporated into the P4VP nanodomains (Figure 1c). [*] C. P. Li, H. C. Chang, Prof. K. H. Wei Department of Materials Science and Engineering National Chiao Tung University 1001 Ta Hsueh Road, Hsinch 30050 Taiwan (ROC) Fax: (+886)35-724727 E-mail: khwei@mail.nctu.edu.tw

Light-emitting diode of polymer nanocomposite with quantum dots

Abstract: The present invention provides a light-emitting diode made of a polymer nanocomposite doped with quantum dots to improve luminescence efficiency and to increase stability and electrical characteristics.

Enhanced electroluminescence of poly(2-methoxy-5-(2′-ethylhexyloxy)- 1,4-phenylene vinylene) films in the presence of TiO2 nanocrystals

Abstract: We demonstrated that in a nanocrystalline TiO2/poly(2-methoxy-5-(2 0 -ethylhexyloxy)-1,4-phenylene vinylene) (MEHPPV) composite, electroluminescence of this composite is enhanced via addition of TiO2 nanoneedles. The TiO2 nanoneedles enhance the partial crystallization of MEH-PPV around TiO2, which in turn causes a decrease in the hole barrier height and an increase in hole mobility. The IYV measurement was established on an indium tin oxide/MEHPPV:TiO2/Al device to identify the electrical properties of the composites.