Showing papers in "Advanced Functional Materials in 2004"

Nanoscale Morphology of Conjugated Polymer/Fullerene-Based Bulk- Heterojunction Solar Cells†

Abstract: The relation between the nanoscale morphology and associated device properties in conjugated polymer/fullerene bulk-heterojunction “plastic solar cells” is investigated. We perform complementary measurements on solid-state blends of poly[2-methoxy-5-(3,7-dimethyloctyloxy)]-1,4-phenylenevinylene (MDMO-PPV) and the soluble fullerene C60 derivative 1-(3-methoxycarbonyl) propyl-1-phenyl [6,6]C61 (PCBM), spin-cast from either toluene or chlorobenzene solutions. The characterization of the nanomorphology is carried out via scanning electron microscopy (SEM) and atomic force microscopy (AFM), while solar-cell devices were characterized by means of current–voltage (I–V) and spectral photocurrent measurements. In addition, the morphology is manipulated via annealing, to increase the extent of phase separation in the thin-film blends and to identify the distribution of materials. Photoluminescence measurements confirm the demixing of the materials under thermal treatment. Furthermore the photoluminescence of PCBM clusters with sizes of up to a few hundred nanometers indicates a photocurrent loss in films of the coarser phase-separated blends cast from toluene. For toluene-cast films the scale of phase separation depends strongly on the ratio of MDMO-PPV to PCBM, as well as on the total concentration of the casting solution. Finally we observe small beads of 20–30 nm diameter, attributed to MDMO-PPV, in blend films cast from both toluene and chlorobenzene.

Fundamentals of Mesostructuring Through Evaporation-Induced Self-Assembly†

Abstract: This article gives an overall view of the mechanisms involved in the mesostructuring that takes place during the formation of surfactant-templated inorganic materials by evaporation. Since such a method of preparation is well suited to fabricating thin films by dip coating, spin coating, casting, or spraying, it is of paramount interest to draw a general description of the processes occurring during the formation of self-assembled hybrid organic/inorganic materials, taking into account all critical parameters. The following study is based on very recent works on the meso-organization of thin silica films using tetraethylorthosilicate (TEOS) as the inorganic source and cetyltrimethylammonium bromide (CTAB) as the structuring agent, but we will show that the method can also be extended to other systems based on non-silica oxides and block copolymer surfactants. We demonstrate that the organization depends mainly on the chemical composition of the film when it reaches the modulable steady state (MSS), where the inorganic framework is still flexible and the composition is stable after reaching an equilibrium in the diffusion of volatile species. This MSS state is generally attained seconds after the drying line, and the film's composition depends on various parameters: the relative vapor pressures in the environment, the evaporation conditions, and the chemical conditions in the initial solution. Diagrams of textures, in which the stabilized structures are controlled by local minima, are proposed to explain the complex phenomena associated with mesostructuring induced by evaporation.

Effect of Molecular Weight and Annealing of Poly(3-hexylthiophene)s on the Performance of Organic Field-Effect Transistors

Abstract: The optical, structural, and electrical properties of thin layers made from poly(3-hexylthiophene) (P3HT) samples of different molecular weights are presented. As reported in a previous paper by Kline et al., Adv. Mater 2003, 15, 1519, the mobilities of these layers are a strong function of the molecular weight, with the largest mobility found for the largest molecular weight. Atomic force microscopy studies reveal a complex polycrystalline morphology which changes considerably upon annealing. X-ray studies show the occurrence of a layered phase for all P3HT fractions, especially after annealing at 1.50 degreesC . However, there is no clear correlation between the differences in the transport properties and the data from structural investigations. In order to reveal the processes limiting the mobility in these layers, the transistor properties were investigated as a function of temperature. The mobility decreases continuously with increasing temperatures; with the same trend pronounced thermochromic effects of the P3HT films occur. Apparently, the polymer chains adopt a more twisted, disordered conformation at higher temperatures, leading to interchain transport barriers. We conclude that the backbone conformation of the majority of the bulk material rather than the crystallinity of the layer is the most crucial parameter controlling the charge transport in these P3HT layers. This interpretation is supported by the significant blue-shift of the solid-state absorption spectra with decreasing molecular weight, which is indicative of a larger distortion of the P3HT backbone in the low-molecular weight P3HT layers

Relating the morphology of poly(p-phenylene vinylene)/methanofullerene blends to solar-cell performance

Abstract: The performance of bulk-heterojunction solar cells based on a phase-separated mixture of donor and acceptor materials is known to be critically dependent on the morphology of the active layer. Here we use a combination of techniques to resolve the morphology of spin cast films of poly(p-phenylene vinylene)/methanofullerene blends in three dimensions on a nanometer scale and relate the results to the performance of the corresponding solar cells. Atomic force microscopy (AFM), transmission electron microscopy (TEM), and depth profiling using dynamic time-of-flight secondary ion mass spectrometry (TOF-SIMS) clearly show that for the two materials used in this study, 1-(3-methoxycarbonyl)propyl-1-phenyl-[6,6]-methanofullerene (PCBM) and poly[2-methoxy-5-(3',7'-dimethyloctyloxy)-1,4-phenylene vinylene] (MDMO-PPV), phase separation is not observed up to 50 wt.-% PCBM. Nanoscale phase separation throughout the film sets in for concentrations of more than 67 wt.-% PCBM, to give domains of rather pure PCBM in a homogenous matrix of 50:50 wt.-% MDMO-PPV/PCBM. Electrical characterization, under illumination and in the dark, of the corresponding photovoltaic devices revealed a strong increase of power conversion efficiency when the phase-separated network develops, with a sharp increase of the photocurrent and fill factor between 50 and 67 wt.-% PCBM. As the phase separation sets in, enhanced electron transport and a reduction of bimolecular charge recombination provide the conditions for improved performance. The results are interpreted in terms of a model that proposes a hierarchical build up of two cooperative interpenetrating networks at different length scales.

High Mechanical Strength Double‐Network Hydrogel with Bacterial Cellulose

Abstract: Double-network (DN) hydrogels with high mechanical strength have been synthesized using the natural polymers bacterial cellulose (BC) and gelatin. As-prepared BC contains 90 % water that can easily be squeezed out, with no more recovery in its swelling property. Gelatin gel is brittle and is easily broken into fragments under a modest compression. In contrast, the fracture strength and elastic modulus of a BC–gelatin DN gel under compressive stress are on the order of megapascals, which are several orders of magnitude higher than those of gelatin gel, and almost equivalent to those of articular cartilage. A similar enhancement in the mechanical strength was also observed for the combination of BC with polysaccharides, such as sodium alginate, gellan gum, and ι-carrageenan.

High Performance Nanotube‐Reinforced Plastics: Understanding the Mechanism of Strength Increase

Abstract: Polymer–multiwalled carbon nanotube composite films were fabricated using two types of polymer matrices, namely poly(vinyl alcohol), (PVA) and chlorinated polypropylene. In the first case, the PVA was observed to form a crystalline coating around the nanotubes, maximising interfacial stress transfer. In the second case the interface was engineered by covalently attaching chlorinated polypropylene chains to the nanotubes, again resulting in large stress transfer. Increases in Young's modulus, tensile strength, and toughness of × 3.7, × 4.3, and × 1.7, respectively were observed for the PVA-based materials at less than 1 wt.-% nanotubes. Similarily for the polypropylene-based composites, increases in Young's modulus, tensile strength and toughness of × 3.1, × 3.9, and × 4.4, respectively, were observed at equivalent nanotube loading levels. In addition, a model to describe composite strength was derived. This model shows that the tensile strength increases in proportion to the thickness of the interface region. This suggests that composite strength can be optimized by maximising the thickness of the crystalline coating or the thickness of the interfacial volume partially occupied by functional groups.

Reinforcing Epoxy Polymer Composites Through Covalent Integration of Functionalized Nanotubes

Abstract: Strong interfacial bonding and homogenous dispersion have been found to be necessary conditions to take full advantage of the extraordinary properties of nanotubes for reinforcement of composites. We have developed a fully integrated nanotube composite material through the use of functionalized single-walled carbon nanotubes (SWNTs). The functionalization was performed via the reaction of terminal diamines with alkylcarboxyl groups attached to the SWNTs in the course of a dicarboxylic acid acyl peroxide treatment. Nanotube-reinforced epoxy polymer composites were prepared by dissolving the functionalized SWNTs in organic solvent followed by mixing with epoxy resin and curing agent. In this hybrid material system, nanotubes are covalently integrated into the epoxy matrix and become part of the crosslinked structure rather than just a separate component. Results demonstrated dramatic enhancement in the mechanical properties of an epoxy polymer material, for example, 30–70 % increase in ultimate strength and modulus with the addition of only small quantities (1–4 wt.-%) of functionalized SWNTs. The nanotube-reinforced epoxy composites also exhibited an increased strain to failure, which suggests higher toughness.

Photoluminescence and Electron Paramagnetic Resonance of ZnO Tetrapod Structures

Abstract: ZnO tetrapod nanostructures have been prepared by the evaporation of Zn in air (no flow), dry and humid argon flow, and dry and humid nitrogen flow. Their properties have been investigated using scanning electron microscopy (SEM), X-ray diffraction (XRD), photoluminescence (PL) and photoluminescence excitation (PLE) spectroscopies (at different temperatures), and electron paramagnetic resonance (EPR) spectroscopy at –160 °C and room temperature. It is found that the fabrication conditions significantly influence the EPR and PL spectra obtained. While a g = 1.96 EPR signal is present in some of the samples, green PL emission can be observed from all the samples. Therefore, the green emission in our samples does not originate from the commonly assumed transition between a singly charged oxygen vacancy and a photoexcited hole [K. Vanheusden, C. H. Seager, W. L. Warren, D. R. Tallant, J. A. Voigt, Appl. Phys. Lett. 1996, 68, 403]. However, the green emission can be suppressed by coating the nanostructures with a surfactant for all fabrication conditions, which indicates that this emission originates from surface defects.

Semiconducting and Piezoelectric Oxide Nanostructures Induced by Polar Surfaces

Abstract: Zinc oxide, an important semiconducting and piezoelectric material, has three key characteristics. First, it is a semiconductor, with a direct bandgap of 3.37 eV and a large excitation binding energy (60 meV), and exhibits near-UV emission and transparent conductivity. Secondly, due to its non-centrosymmetric symmetry, it is piezoelectric, which is a key phenomenon in building electro-mechanical coupled sensors and transducers. Finally, ZnO is bio-safe and bio-compatible, and can be used for biomedical applications without coating. With these unique advantages, ZnO is one of the most important nanomaterials for integration with microsystems and biotechnology. Structurally, due to the three types of fastest growth directions— , , and —as well as the ±(0001) polar surfaces, a diverse group of ZnO nanostructures have been grown in our laboratory. These include nanocombs, nanosaws, nanosprings, nanorings, nanobows, and nanopropellers. This article reviews our recent progress in the synthesis and characterization of polar-surface-induced ZnO nanostructures, their growth mechanisms, and possible applications as sensors, transducers, and resonators. It is suggested that ZnO could be the next most important nanomaterial after carbon nanotubes.

Electric-Field-Directed Growth of Gold Nanorods in Aqueous Surfactant Solutions

Abstract: The factors affecting the nucleation and growth of gold nanorods, (Jana et al., Adv. Mater.2001, 13, 1389) have been investigated. It is shown that the size and aspect ratio can be controlled through the use of different sized seed particles. The length of the rods can be tuned from 25–170 nm, while the width remains almost constant at 22–25 nm. The formation of rods requires the presence of the cationic surfactant cetyltrimethylammonium bromide (CTAB). Lower temperature favors rod formation, although this reduces CTAB solubility. The addition of chloride ions or the use of dodecyltrimethylammonium bromide (DTAB) leads to shorter-aspect rods. AuIII and AuI are shown to be quantitatively bound to the CTAB micelles. We propose an electrochemical mechanism for rod formation, whereby the flux of AuI bound to cationic micelles to the seed surface is maximized at points of highest curvature, where the electrical double layer gradient is highest. Initial numerical solutions to the electric potential and field around an ellipsoid in a 1:1 electrolyte are provided, which indicate that the field at the particle tip scales linearly with the aspect ratio. Mean free passage times for ions are found to be shortest at the tips. The results provide a general explanation for the formation of non-equilibrium crystal habits and a mechanism for controlling crystal growth.

Effect of Temperature and Illumination on the Electrical Characteristics of Polymer-Fullerene Bulk-Heterojunction Solar Cells

Abstract: The current-voltage characteristics of ITO/PEDOT:PSS/OC1C10-PPV:PCBM/Al solar cells were measured in the temperature range 125-320 K under variable illumination, between 0.03 and 100 mW cm(-2) (white light), with the aim of determining the efficiency-limiting mechanism(s) in these devices, and the temperature and/or illumination range(s) in which these devices demonstrate optimal performance. (ITO: indium tin oxide; PEDOT:PSS: poly(styrene sulfonate)-doped poly(ethylene dioxythiophene); OC1C10-PPV: poly[2-methoxy-5-(3,7-dimethyl octyloxy)-1,4-phenylene vinylene]; PCBM: phenyl-C-61 butyric acid methyl ester.) The short-circuit current density and the fill factor grow monotonically with temperature until 320 K. This is indicative of a thermally activated transport of photogenerated charge carriers, influenced by recombination with shallow traps. A gradual increase of the open-circuit voltage to 0.91 V was observed upon cooling the devices down to 125 K. This fits the picture in which the open-circuit voltage is not limited by the work-function difference of electrode materials used. The overall effect of temperature on solar-cell parameters results in a positive temperature coefficient of the power conversion efficiency, which is 1.9% at T = 320 K and 100 mW cm(-2) (2.5% at 0.7 mW cm(-2)). The almost-linear variation of the short-circuit current density with light intensity confirms that the internal recombination losses are predominantly of monomolecular type under short-circuit conditions. We present evidence that the efficiency of this type of solar cell is limited by a light-dependent shunt resistance. Furthermore, the electronic transport properties of the absorber materials, e.g., low effective charge-carrier mobility with a strong temperature dependence, limit the photogenerated current due to a high series resistance, therefore the active layer thickness must be kept low, which results in low absorption for this particular composite absorber.

Efficient isolation and solubilization of pristine single-walled nanotubes in bile salt micelles

Abstract: We have investigated a wide variety of surfactants for their efficiency in dissolving isolated single-walled carbon nanotubes (SWNTs) in water. In doing so, we have completely avoided the harsh chemical or mechanical conditions, such as acid or ultrasonic treatments, that are known to damage SWNTs. Bile salts in particular are found to be exceptionally effective in dissolving individual tubes, as evidenced by highly resolved optical absorption spectra, bright bandgap fluorescence, and the unprecedented resolution (∼ 2.5 cm - 1 ) of the radial breathing modes in Raman spectra. This is attributed to the formation of very regular and stable micelles around the nanotubes providing an unusually homogeneous environment. Quantitative information concerning the degree of solubilization is obtained from absorption spectroscopy.

Hole transport in poly(phenylene vinylene)/methanofullerene bulk-heterojunction solar cells

Abstract: A fundamental limitation of the photocurrent of solar cells based on a blend of poly(2-methoxy-5-(3′,7′-dimethyloctyloxy)-p-phenylene vinylene) (MDMO-PPV) and [6,6]-phenyl C61-butyric acid methyl ester (PCBM) is caused by the mobility of the slowest charge-carrier species, the holes in the MDMO-PPV. In order to allow the experimentally observed photocurrents electrostatically, a hole mobility of at least 10–8 m2 V–1 s–1 is required, which exceeds the observed hole mobility in pristine MDMO-PPV by more than two orders of magnitude. However, from space-charge-limited conduction, admittance spectroscopy, and transient electroluminescence measurements, we found a hole mobility of 2 × 10–8 m2 V–1 s–1 for the MDMO-PPV phase in the blend at room temperature. Consequently, the charge-carrier transport in a MDMO-PPV:PCBM-based solar cell is much more balanced than previously assumed, which is a necessary requirement for the reported high fill factors of above 50 %.

Thermally Stable Two‐Dimensional Hexagonal Mesoporous Nanocrystalline Anatase, Meso‐nc‐TiO2: Bulk and Crack‐Free Thin Film Morphologies

Abstract: Herein a novel synthetic route is described for the production of thermally stable, structurally well-defined two-dimensional (2D) hexagonal mesoporous nanocrystalline anatase (meso-nc-TiO2), with a large pore diameter, narrow pore-size distribution, high surface area, and robust inorganic walls comprised of nanocrystalline anatase The synthetic approach involves the evaporation-induced co-assembly of a non-ionic amphiphilic triblock-copolymer template and titanium tetraethoxide, but with a pivotal change in the main solvent of the system, where the commonly used ethanol is replaced with 1-butanol This seemingly minor modification in solvent type from ethanol to 1-butanol turns out to be the key synthetic strategy for achieving a robust, structurally well-ordered meso-nc-TiO2 material in the form of either thick or thin films The beneficial “solvent” effect originates from the higher hydrophobicity of 1-butanol than ethanol, enhancing microphase separation and templating, lower critical micelle concentration of the template in 1-butanol, and the ability to increase the relative concentration of the inorganic precursor to template in the co-assembly synthesis Moreover, thin films with dimensions of several centimeters that are devoid of cracks down to the length scale of the mesostructure itself, having high porosity, well-defined mesostructural features, and semi-crystalline pore walls were straightforwardly and reproducibly obtained as a result of the physicochemical property advantages of 1-butanol over ethanol within our synthesis scheme

High-Contrast Electrochromism and Controllable Dissolution of Assembled Prussian Blue/Polymer Nanocomposites†

Abstract: To maintain the momentum and impact of the field, assembled materials systems must increasingly incorporate broad functionality to meet real-world applications. Here we describe nanocomposite films of specially synthesized inorganic Prussian blue (PB) nanoparticles and linear poly(ethylene imine) (LPEI) that possess the unusual functional combination of high-performance electrochromism for displays and controllable dissolution for drug delivery. Fabrication using layer-by-layer (LBL) assembly was followed by spectroelectrochemical characterization, allowing a full composition determination rarely achieved for LBL films. The electrochromic performance of thick LPEI/PB nanocomposites most relevant to applications surpassed that of inorganic PB films with competitive switching speed and superior contrast. Oxidation beyond the primary electrochromic transition removes nanoparticle ionization and can controllably dissolve the films. Because PB is non-toxic we suggest this mechanism for controlled in-vivo drug delivery. The performance and multifunctional quality of these nanocomposites promise a strong impact on flexible displays, electrochromic windows, and even biomedical devices.

Color Tuning in Benzo[1,2,5]thiadiazole‐Based Small Molecules by Amino Conjugation/Deconjugation: Bright Red‐Light‐Emitting Diodes

Abstract: Bipolar compounds (referred to in general as btza) containing a benzo[1,2,5]thiadiazole core and peripheral diarylamines and/or 4-tert-butylphenyl moieties have been synthesized via palladium-catalyzed cross-coupling reactions of 4,7-dibromobenzo[1,2,5]thiadiazole with appropriate stannyl compounds. These compounds are fluorescent and the emission color ranges from green to red. The fluorescence of the compounds originates from a charge-transfer process and exhibits solvatochromism. These red-light-emitting materials are amorphous and devices of different configurations were fabricated: I) ITO/btza/TPBI/Mg:Ag; II) ITO/btza/Alq3/Mg:Ag; III) ITO/btza/Mg:Ag (where ITO = indium tin oxide, TPBI = 1,3,5-tris(N-phenylbezimidazol-2-yl)benzene, and Alq3 = tris(8-hydroxyquinoline)aluminum). The performance of some of the red-light-emitting devices appears to be very promising.

A Simple and Effective Route for the Synthesis of Crystalline Silver Nanorods and Nanowires

Abstract: A simple and effective approach to the aqueous-phase synthesis of crystalline silver nanorods and nanowires is demonstrated, using which their diameters and aspect ratios can be effectively controlled. The synthesis involves a template-less and non-seed process to high-quality nanoparticles, which is low-cost and proceeds at moderate temperatures. The nanorods and nanowires were synthesized by the reduction of silver nitrate with tri-sodium citrate in the presence of sodium dodecylsulfonate. The concentration of tri-sodium citrate plays a critical role while sodium dodecylsulfonate, as a capping agent, only plays an assistant role in controlling the diameters and aspect ratios of the products. High-resolution transmission electron microscopy (HRTEM) and selected-area electron diffraction (SAED) investigations show that the silver nanocrystals are generated with a twinned crystalline structure. We also put forward a primary experimental model to shed light on their growth mechanisms.

Enhanced Dielectric and Electromechanical Responses in High Dielectric Constant All-Polymer Percolative Composites

Abstract: A type of all-polymer percolative composite is introduced which exhibits a very high dielectric constant (> 7000). The experimental results also show that the dielectric behavior of this new class of percolative composites follows the predictions of the percolation theory and the analysis of conductive percolation phenomena. The very high dielectric constant of the all-polymer composites, which are also very flexible and possesses an elastic modulus close to that of the insulation polymer matrix, makes it possible to induce a high electromechanical response under a very reduced electric field (a strain of 2.65 % with an elastic energy density of 0.18 J cm–3 can be achieved under a field of 16 MV m–1). Data analysis also suggests that within the composites, the non-uniform local field distribution as well as interface effects can significantly enhance the strain responses. Furthermore, the experimental data as well as the data analysis indicate that conduction loss in the composites will not affect the strain hysteresis.

Well‐Aligned ZnO Nanowire Arrays Fabricated on Silicon Substrates

Abstract: Arrays of well-aligned single-crystal zinc oxide (ZnO) nanowires of uniform diameter and length have been synthesized on a (100) silicon substrate via a simple horizontal double-tube system using chemical vapor transport and condensation method. X-ray diffraction and transmission electron microscopy (TEM) characterizations showed that the as-grown nanowires had the single-crystal hexagonal wurtzite structure with detectable defects and a growth direction. Raman spectra revealed phonon confinement effect when compared with those of ZnO bulk powder, nanoribbons, and nanoparticles. Photoluminescence exhibited strong ultraviolet emission at 3.29 eV under 355 nm excitation and green emission at 2.21 eV under 514.5 nm excitation. No catalyst particles were found at the tip of the nanowires, suggesting that the growth mechanism followed a self-catalyzed and saturated vapor–liquid–solid (VLS) model. Self-alignment of nanowires was attributed to the local balance and steady state of vapor flow at the substrate. The growth technique would be of particular interest for direct integration in the current silicon-technology-based optoelectronic devices.

Towards a Transparent, Highly Conductive Poly(3,4-ethylenedioxythiophene)

Abstract: A detailed investigation of the processing parameters influencing the oxidative polymerization of 3,4-ethylenedioxythiophene (EDOT) and a methanol-substituted derivative (EDOT-CH2OH) was performed with the goal of maximizing the conductivity of the polymer. We show that the conductivity can be significantly enhanced by varying the monomer, oxidant (iron(III) p-toluenesulfonate (Fe(OTs)3)), weak base (imidazole (Im)), solvent (various alcohols), and solution concentrations. The effect of each variable on the final materials properties is investigated, and the parameters have been optimized to achieve conductivities as high as 900 S cm−1. Surface resistance below 150 Ω/□ for 80-90 nm thick films with visible-spectrum transparency exceeding 80 % is achieved. The combination of these properties makes the films highly suitable for numerous device applications.

Synthesis and Properties of a Water‐Soluble Single‐Walled Carbon Nanotube–Poly(m‐aminobenzene sulfonic acid) Graft Copolymer

Abstract: Poly(m-aminobenzene sulfonic acid) (PABS), was covalently bonded to single-walled carbon nanotubes (SWNTs) to form a water-soluble nanotube–polymer compound (SWNT–PABS). The conductivity of the SWNT–PABS graft copolymer was about 5.6 × 10–3 S cm–1, which is much higher than that of neat PABS (5.4 × 10–7 S cm–1). The mid-IR spectrum confirmed the formation of an amide bond between the SWNTs and PABS. The 1H NMR spectrum of SWNT–PABS showed the absence of free PABS, while the UV/VIS/NIR spectrum of SWNT–PABS showed the presence of the interband transitions of the semiconducting SWNTs and an absorption at 17 750 cm–1 due to the PABS addend.

The Impact of Nanostructuring on the Thermal Conductivity of Thermoelectric CoSb3

Abstract: The high concentration of grain boundaries provided by nanostructuring is expected to lower the thermal conductivity of thermoelectric materials, which favors an increase in their thermoelectric fi ...

Efficient Load Transfer to Polymer-Grafted Multiwalled Carbon Nanotubes in Polymer Composites†

Abstract: Multiwalled carbon nanotubes (MWNTs) grafted with poly(methyl methacrylate) (PMMA) were synthesized by emulsion reactions and used as a reinforcement for commercial PMMA. Both scanning electron microscopy (SEM) and transmission electron microscopy (TEM) revealed that the applied tensile load on the composites was transferred to the PMMA-grafted MWNTs, leading to a strain failure of the MWNTs rather than an adhesive failure between the MWNTs and the matrix. Dynamic mechanical analysis (DMA) data showed that the storage modulus at 20 °C of the PMMA composite containing 20 wt.-% of the PMMA-grafted MWNTs was significantly enhanced by ∼ 29 GPa (or by ∼ 1100 %) as compared with commercial PMMA.

Siliceous ZSM-5 Membranes by Secondary Growth of b-Oriented Seed Layers

Abstract: A modified seeded growth procedure, which allowed us to prepare the first functional b-oriented siliceous ZSM-5 membrane (Z. Lai et al., Science, 2003, 300, 456), is described and discussed in detail. The procedure involves growing a b-oriented ZSM-5 seed monolayer into a dense b-oriented membrane. Preservation of the orientation is achieved by enhancing the growth rate of siliceous ZSM-5 crystals along the b-axis using trimer-TPAOH (bis-N,N-(tripropylammoniumhexamethylene) di-N,N-propylammonium trihydroxide) as the structure-directing agent (SDA). Under similar synthesis conditions but using monomer-TPAOH (tetrapropylammonium hydroxide) as the SDA for the secondary growth of the b-oriented seed layer, two other films with mixed grain orientations, namely b&a- or b&a&h0h/c-oriented ZSM-5 membranes were obtained depending on temperature of the secondary growth. Membrane microstructures were characterized by X-ray diffraction, pole figure analysis, scanning electron microscopy, fluorescent confocal optical microscopy, electron probe microanalysis, and X-ray photoelectron spectroscopy. The separation performance of the three types of membranes was tested by using a mixture of xylene isomers. Significant improvement in both permeance and separation factor is achieved by the b-oriented siliceous ZSM-5 membrane compared to the other two types of membranes described here or other reported ZSM-5 membranes in the literature. The permeation results indicate that the membrane performance depends strongly on membrane microstructure.

The effect of hydrogen bonding on self-assembled polyaniline nanostructures

Abstract: Polyaniline (PANT) nanotubes with an outer diameter of 165-240 nm and an inner diameter of 10-70 nm were prepared by a self-assembly process in the presence of six different carboxylic acids-propionic acid (PA), lactic acid (LA), succinic acid (SA), malic acid (MA), tartaric acid (TA), and citric acid (CA)-as the dopants. These nanotubes aggregated to form nanotube dendrites when the carboxylic acids contain an OH group. Moreover, the number of OH and COOH groups of the carboxylic acids affected the size, aggregated dendrite morphology, and thermal and electrical properties of the nanotubes. It was proposed that the micelle formed by the carboxylic acids acts as a template in the formation of the nanotubes, while the hydrogen bonds between the polymer chain of PANI and the OH group of the carboxylic acids supply a driving force to form the aggregated nanotube dendrites.

Understanding the Origin of the 535 nm Emission Band in Oxidized Poly(9,9‐dioctylfluorene): The Essential Role of Inter‐Chain/Inter‐Segment Interactions

Abstract: We present a careful study of the effects of photo-oxidation on the emissive properties of poly(9,9-dioctylfluorene) (PFO) that addresses important issues raised by a recent flurry of publications concerning the degradation of blue light-emitting, fluorene-based homo- and copolymers. The photoluminescence (PL) spectra of thin PFO films oxidized at room temperature comprise two major components, namely a vibronically structured blue band and a green, structureless component, referred to hereafter as the ‘g-band’. These are common features in a wide range of poly(fluorene)s (PFs) and whilst the former is uniformly accepted to be the result of intra-chain, fluorene-based, singlet-exciton emission, the origin of the ‘g-band’ is subject to increasing debate. Our studies, described in detail below, support the proposed formation of oxidation-induced fluorenone defects that quench intra-chain, singlet-exciton emission and activate the g-band emission. However, whilst these fluorenone defects are concluded to be necessary for the g-band emission to be observed, they are considered not to be, alone, sufficient. We show that inter-chain/inter-segment interactions are required for the appearance of the g-band in the PL spectra of PFO and propose that the g-band is attributable to emission from fluorenone-based excimers rather than from localized fluorenone π–π* transitions as recently suggested.

The Singlet–Triplet Exchange Energy in Conjugated Polymers†

Abstract: Electron–electron interactions in organic semiconductors split the lowest singlet and triplet states by the exchange energy, ΔEST. Measurement of singlet and triplet emission spectra in a large number of conjugated polymers yield an almost constant ΔEST value close to 0.7 eV. This is in contrast to the situation in molecules, where the exchange energy is found to depend on molecular size and to vary over a wide range. Quantum-chemical calculations are performed to address the origin of the constant exchange energy in phenylene-based conjugated polymers. The electron–hole separation in the lowest singlet and triplet excited states is found to be independent of the π-conjugated backbone, and saturates for chains longer than a few repeating units, resulting in a constant exchange energy. In shorter conjugated oligomers, confinement of the excitations destabilizes the singlet with respect to the triplet through exchange interactions and leads to a larger and size-dependent singlet–triplet energy separation.

Spatially Selective Nucleation of Metal Clusters on the Tobacco Mosaic Virus

Abstract: Tobacco mosaic virus (TMV) is a very stable nanotube complex of a helical RNA and 2130 coat proteins. The special shape makes it an interesting nano-object, especially as a template for chemical reactions. Here we use TMV as a chemically functionalized template for binding metal ions. Different chemical groups of the coat protein can be used as ligands or to electrostatically bind metal ions. Following this activation step, chemical reduction and electroless plating produces metal clusters of several nanometers in diameter. The clusters are attached to the virion without destroying its structure. Gold clusters generated from an ascorbic acid bath bind to the exterior surface as well as to the central channel of the hollow tube. Very high selectivity is reached by tuning PdII and PtII activations with phosphate: When TMV is first activated with PdII, and thereafter metallized with a nickel–phosphinate bath, 3 nm nickel clusters grow in the central channel; when TMV from phosphate-buffered suspensions is employed, larger nickel clusters grow on the exterior surface. Phosphate buffers have to be avoided when 3 nm nickel and cobalt wires of several 100 nm in length are synthesized from borane-based baths inside the TMV channel. The results are discussed with respect to the inorganic complex chemistry of precursor molecules and the distribution of binding sites in TMV.

Peptide Templates for Nanoparticle Synthesis Derived from Polymerase Chain Reaction-Driven Phage Display†

Abstract: Phage peptide display libraries are commonly used to select peptides that bind to inorganic surfaces (metals, metal oxides, and semiconductors). These binding peptides can serve as templates to control the nucleation and growth of inorganic nanoparticles in vitro. In this report, we describe the identification of a unique set of sequences that bind to silver and cobalt nanoparticles from a phage peptide display library using a polymerase chain reaction (PCR)-driven method. The amino acid sequences obtained by the PCR method are a distinct set of sequences that would otherwise be missed using the regular panning method. Peptides identified by the method described here are also shown to function as templates for the synthesis of silver and cobalt platinum nanoparticles.