Showing papers in "Advanced Functional Materials in 2003"

Effects of Postproduction Treatment on Plastic Solar Cells

Abstract: Efficiencies of organic solar cells based on an interpenetrating network of a conjugated polymer and a fullerene as donor and acceptor materials still need to be improved for commercial use. We have developed a postproduction treatment that improves the performance of solar cells based on poly(3-hexylthiophene) (P3HT) and [6,6]-phenyl C61-butyric acid methyl ester (PCBM) by means of a tempering cycle at elevated temperatures in which an external voltage is simultaneously applied, resulting in a significant increase of the short-circuit current. Using this postproduction treatment, an enhancement of the short-circuit current density, Isc, to 8.5 mA cm–2 under illumination with white light at an illumination intensity of 800 W m–2 and an increase in external quantum efficiency (IPCE, incident photon to collected electron efficiency) to 70 % are demonstrated.

Novel Nanostructures of Functional Oxides Synthesized by Thermal Evaporation

Abstract: Functional oxides are the fundamentals of smart devices. This article reviews novel nanostructures of functional oxides, including nanobelts, nanowires, nanosheets, and nanodiskettes, that have been synthesized in the authors’ laboratory. Among the group of ZnO, SnO2, In2O3, Ga2O3, CdO, and PbO2, which belong to different crystallographic systems and structures, a generic nanobelt structure has been synthesized. The nanobelts are single crystalline and dislocation-free, and their surfaces are atomically flat. The oxides are semiconductors, and have been used for fabrication of nanodevices such as field-effect transistors and gas sensors. Taking SnO2 and SnO as examples, other types of novel nanostructures are illustrated. Their growth, phase transformation, and stability are discussed. The nanobelts and related nanostructures are a unique group that is likely to have important applications in electronic, optical, sensor, and optoelectronic nanodevices.

Inorganic Luminescent Materials: 100 Years of Research and Application

Abstract: Up until approximately 80 years ago, only black-body radiation (including natural sources) was available to illuminate our environment. To realise state-of-the-art lamps, TV sets, monitors, and medical scanners, took an enormous scientific and technical effort. Inorganic luminescent materials are key components, which were, are, and will be prerequisite to the functionality and success of many lighting and display systems. In this Highlight, a hundred years of inorganic luminescent material research are reviewed.

Low-k Insulators as the Choice of Dielectrics in Organic Field-Effect Transistors

Abstract: In this paper, we present a new effect influencing the operation of organic field-effect transistors resulting from the choice of gate insulator material. In a series of studies it was found that the interaction between the insulator and the semiconductor materials plays an important role in carrier transport. The insulator is not only capable of affecting the morphology of the semiconductor layer, but can also change the density of states by local polarization effects. Carrier localization is enhanced by insulators with large permittivities, due to the random dipole field present at the interface. We have investigated this effect on a number of disordered organic semiconductor materials, and show here that significant benefits are achievable by the use of low-k dielectrics as opposed to the existing trend of increasing the permittivity for low operational voltage. We also discuss fundamental differences in the case of field-effect transistors with band-like semiconductors.

Electron transport in a methanofullerene

Abstract: The current-voltage characteristics of methanofullerene [6,6]-phenyl C-61-butyric acid methyl ester (PCBM)-based devices are investigated as a function of temperature. The occurrence of space-charge limited current enables a direct determination of the electron mobility. At room temperature, an electron mobility Of mu(e) = 2 x 10(-7) m(2) V-1 s(-1) has been obtained. This electron mobility is more than three orders of magnitude larger than the hole mobility of donor-type conjugated polymer poly(2-methoxy-5-(3',7'-dimethyloctyloxy)-p-phenylene vinylene) (OC1C10-PPV). As a result, the dark current in PCBM/OC1C10-PPV based devices is completely dominated by electrons. The observed field and temperature-dependence of the electron mobility 1 of PCBM can be described with a Gaussian disorder model. This provides information about the energetic disorder and average transport-site separation in PCBM.

Fully reversible homogeneous and heterogeneous Li storage in RuO2 with high capacity

Abstract: In this paper, we report that Li can be stored in RuO2 with an unusually high coulombic efficiency. The process involves three electrochemical steps: i) formation of a Ru/Li2O nanocomposite, ii) formation of a Li-containing surface film, and iii) interfac

Sulfur Composite Cathode Materials for Rechargeable Lithium Batteries

Abstract: Conductive polymer/sulfur composite materials were prepared by heating the mixture of polyacrylonitrile (PAN) and sublimed sulfur. During the heating process, PAN was dehydrogenated by sulfur, forming a conductive main chain similar to polyacetylene. At the same time, the high-polarity functional group –CN cyclized at the melt state, forming a thermally stable heterocyclic compound in which sulfur was embedded. The nanodispersed composites showed excellent electrochemical properties. Tested as cathode material in a non-aqueous lithium cell based on poly(vinylidene fluoride) (PVDF) gel electrolyte at room temperature, the composite exhibited a specific capacity up to 850 mA h g–1 in the initial cycle. Its specific capacity remained above 600 mA h g–1 after 50 cycles, about five times that of LiCoO2, and recovered partly after replacement of the anode with a fresh lithium sheet. The utilization of the electrochemically active sulfur was about 90 % assuming a complete reaction to the product, Li2S.

Controlling the morphology of nanocrystal-polymer composites for solar cells

Abstract: We have shown recently that the use of high-aspect-ratio inorganic nanorods in conjunction with conjugated polymers is a route to obtaining efficient solar cells processed from solution. Here, we demonstrate that the use of binary solvent mixtures in which one of the components is a ligand for the nanocrystals is effective in controlling the dispersion of nanocrystals in a polymer. By varying the concentration of the solvent mixture, phase separation between the nanocrystal and polymer could be tuned from micrometer scale to nanometer scale. In addition, we can achieve nanocrystal surfaces that are free of surfactant through the use of weak binding ligands that can be removed through heating. When combined, the control of film morphology together with surfactant removal result in nanorod–polymer blend photovoltaic cells with a high external quantum efficiency of 59 % under 0.1 mW cm–2 illumination at 450 nm.

Template-Assisted Self-Assembly of Spherical Colloids into Complex and Controllable Structures†

Abstract: Colloidal aggregates with well-controlled sizes, shapes, and structures have been fabricated by dewetting aqueous dispersions of monodispersed spherical colloids across surfaces patterned with two-dimensional arrays of relief structures (or templates). The capability and feasibility of this approach have been demonstrated with the organization of polymer latex or silica beads into homo-aggregates, including circular rings; polygonal and polyhedral clusters; and linear, zigzag, and spiral chains. It was also possible to generate hetero-aggregates in the configuration of HF and H2O molecules that contained spherical colloids of different sizes, compositions, densities, functions, or a combination of these features. These uniform, well-defined aggregates of spherical colloids are ideal model systems to investigate the aerodynamic, hydrodynamic, and optical properties of colloidal particles characterized by non-spherical shapes and/or complex topologies. They can also serve as a new class of building blocks to generate hierarchically self-assembled structures that are expected to exhibit interesting features valuable to areas ranging from condensed matter physics to photonics.

General Synthesis of Single-Crystal Tungstate Nanorods/Nanowires: A Facile, Low-Temperature Solution Approach

Abstract: The general large-scale synthesis of a family of single-crystalline transition metal tungstate nanorods/nanowires is easily realized by a hydrothermal crystallization technique under mild conditions using cheap and simple inorganic salts as precursors. Uniform tungstate nanorods/nanowires such as MWO4 (M = Zn, Mn, Fe), Bi2WO6, Ag2WO4, and Ag2W2O7 with diameters of 20–40 nm, lengths of up to micrometers, and controlled aspect ratios can be readily obtained by hydrothermal transformation and recrystallization of amorphous particulates. This novel and efficient pathway toward various kinds of related low-dimensional tungstate nanocrystals under mild conditions could open new opportunities for further investigating the novel properties of tungstate materials.

Infiltrating Semiconducting Polymers into Self-Assembled Mesoporous Titania Films for Photovoltaic Applications

Abstract: Interpenetrating networks of organic and inorganic semiconductors are attractive for photovoltaic cells because electron transfer between the two semiconductors splits excitons. In this paper we show that films of titania with a uniform distribution of pore sizes can be made using a block copolymer as a structure-directing agent, and that 33 % of the total volume of the film can be filled with a semiconducting polymer.

Polyol-Mediated Synthesis of Nanoscale Functional Materials†

Abstract: Nanoscale functional materials such as luminescent materials (ZnS:Ag+, Cl–; LaPO4:Ce3+,Tb3+; Y2O3:Eu3+), color pigments (CoAl2O4; Cr2O3; ZnCo2O4; (Ti0.85Ni0.05Nb0.10)O2; α-Fe2O3; Cu(Fe,Cr)O4; TiO2), transparent conducting oxides (ZnO:In3+), and catalytically active oxides (CeO2; Mn3O4; V2O5) are prepared with the polyol method. All these materials are yielded as crystalline, spherical, and almost monodisperse particles, 30–200 nm in size. Characterization is carried out based on scanning electron microscopy (SEM), X-ray powder diffraction (XRD), optical spectroscopy, and conductance measurements. The preparation via the polyol method is singled out due to its broad and easy applicability. The resulting material properties are similar to or better than nanoscale materials prepared by other measures. Some materials and their properties, e.g., ZnS:Ag+,Cl– as a phosphor, ZnCo2O4 and Cu(Fe,Cr)O4 as pigments, and ZnO:In3+ as transparent conductive oxide, are presented for the first time at the nanoscale.

Self‐Assembly of Polyaniline—From Nanotubes to Hollow Microspheres

Abstract: By simply changing the molar ratio of the dopant to monomer, the morphology of salicylic acid (SA)-doped polyaniline (PANI) can be changed from one-dimensional nanotubes (∼ 109–150 nm in diameter) to three-dimensional hollow microspheres (∼ 1.5–3.1 μm in diameter) via a self-assembly process. Freeze–fracture electron microscopy (FFEM) proved that hollow spherical micelles composed of SA/aniline act as templates in the formation of either nanotubes or hollow spheres. FTIR and X-ray diffraction measurements suggest that the hydrogen bond of the –OH group of SA with the amine group of PANI might be a driving force for self-assembling hollow microspheres, while the hydrogen bond through hydrogen and oxygen of the adjacent SA doped on the polymer chains results in short-range order of the counter-ions along the polymer chain in the nanotubes.

Characterization and Field‐Emission Properties of Needle‐like Zinc Oxide Nanowires Grown Vertically on Conductive Zinc Oxide Films

Abstract: Needle-like ZnO nanowires with high density are grown uniformly and vertically over an entire Ga-doped conductive ZnO film at 550 °C. The nanowires are grown preferentially in the c-axis direction. The X-ray diffraction (XRD) θ-scan curve shows a full width at half maximum (FWHM) value of 2°. This indicates that the c-axes of the nanorods are along the normal direction of the substrate surface. The investigation using high-resolution transmission electron microscopy (HRTEM) confirmed that each nanowire is a single crystal. A room-temperature photoluminescence (PL) spectrum of the wires consists of a strong and sharp UV emission band at 380 nm and a weak and broad green–yellow band. It reveals a low concentration of oxygen vacancies in the ZnO nanowires and their high optical quality. Field electron emission from the wires was also investigated. The turn-on field for the ZnO nanowires was found to be about 18 V μm–1 at a current density of 0.01 μA cm–2. The emission current density from the ZnO nanowires reached 0.1 mA cm–2 at a bias field of 24 V μm–1.

Dendrite-Assisted Growth of Silicon Nanowires in Electroless Metal Deposition

Abstract: This article concerns the detailed investigations on the silver dendrite-assisted growth of single-crystalline silicon nanowires, and their possible self-assembling nanoelectrochemistry growth mechanism. The growth of silicon nanowires was carried out through an electroless metal deposition process in a conventional autoclave containing aqueous HF and AgNO3 solution near room temperature. In order to explore the mechanism and prove the centrality of silver dendrites in the growth of silicon nanowires, other etching solution systems with different metal species were also investigated in this work. The morphology of etched silicon substrates strongly depends upon the composition of the etching solution, especially the metal species. Our experimental results prove that the simultaneous formation of silver dendrites is a guarantee of the preservation of free-standing nanoscale electrolytic cells on the silicon substrate, and also assists in the final formation of silicon nanowire arrays on the substrate surface.

The Transient Phase of Amorphous Calcium Carbonate in Sea Urchin Larval Spicules: The Involvement of Proteins and Magnesium Ions in Its Formation and Stabilization

Abstract: Amorphous calcium carbonate (ACC) is a precursor phase of calcite in the formation of the sea urchin larval spicule. The goal of this research is to study the formation and stabilization mode of this transient phase. We first characterized the mineralogy of the spicules from the sea urchin Strongylocentrotus purpuratus. We then examined the role of the macromolecules extracted from the spicules at different growth stages in the formation of transient ACC in vitro.The biogenic amorphous transient phase is shown to be both structurally and compositionally different from the known stable ACC phases. It does not contain bound water, and is thus the first dehydrated ACC phase to be detected. The macromolecules that were extracted at early stages of spicule growth, when the amorphous content of the biogenic mineral is high, induced the formation of transient ACC in vitro in the presence of magnesium ions. In contrast, the macromolecules extracted at a later stage, when the spicules are completely crystalline, induced the formation of single crystals of low magnesian calcite. We therefore deduce that the macromolecules from the sea urchin larval spicules together with magnesium ions, mediate the transient formation of ACC as a precursor to calcite. These observations may well provide novel ideas for improved materials synthesis.

Stabilized Blue Emission from Polyfluorene‐Based Light‐Emitting Diodes: Elimination of Fluorenone Defects

Abstract: Polyfluorene (PF)-based light-emitting diodes (LEDs) typically exhibit device degradation under operation with the emergence of a strong low-energy emission band (at ∼ 2.2–2.4 eV). This longer wavelength band converts the desired blue emission to blue–green or even yellow. We have studied both the photoluminescence (PL) and electroluminescence (EL) of PFs with different molecular structures and found that the low-energy emission band originates from fluorenone defects which are introduced by photo-oxidization, thermal oxidation, or during device fabrication. X-ray photo-emission spectroscopy (XPS) results show that the oxidation of PF is strongly catalyzed by the presence of calcium. The fluorenone defects generate a stronger contribution to the EL than to the PL. By utilization of a novel electron-transporting material as a buffer layer between the emissive PF and the Ca/Ag (Ba/Ag) cathode, the blue EL emission from the PF was stabilized.

Electrophosphorescence from a Polymer Guest–Host System with an Iridium Complex as Guest: Förster Energy Transfer and Charge Trapping†

Abstract: We report high-efficiency green electrophosphorescent light-emitting diodes obtained by using tris[9,9-dihexyl-2-(phenyl-4′-(-pyridin-2″-yl))fluorene]iridium(III) (Ir(DPPF)3) as the guest, and a blend of poly(vinylcarbazole) (PVK) with 2-tert-butylphenyl-5-biphenyl-1,3,4-oxadiazol (PBD) as the host. The electrophosphorescent emission is characteristic of Ir(DPPF)3, with its maximum at 550 nm. An external quantum efficiency of 8 % photons per electron and luminous efficiency of 29 cd A–1, with maximum brightness of 3500 cd m–2, were achieved at 1 wt.-% concentration of Ir(DPPF)3. The devices exhibited no emission from PVK or PBD, even at the lowest concentration of Ir(DPPF)3 (0.1 wt.-%). The results indicate that Forster energy transfer plays a minor role in achieving high efficiencies in these devices. Direct charge trapping appears to be the main operating mechanism.

Laser-Ablation Growth and Optical Properties of Wide and Long Single-Crystal SnO2 Ribbons

Abstract: Wide and long ribbons of single-crystalline SnO2 have been achieved via laser ablation of a SnO2 target. Transmission electron microscopy (TEM) shows the as-grown SnO2 ribbons are structurally perfect and uniform, with widths of 300-500 nm, thicknesses of 30-50 nm (width-to-thickness ratio of ∼ 10), and lengths ranging from several hundreds of micrometers to the order of millimeters. X-ray diffraction (XRD) pattern and energy-dispersive X-ray spectroscopy (EDS) spectral analysis indicate that the ribbons have the phase structure and chemical composition of the rutile form of SnO2. Selected-area electron diffraction (SAED) patterns and high-resolution TEM images reveal that the ribbons are single crystals and grow along the [100] crystal direction. Photoluminescence measurements show that the synthesized SnO2 ribbons have one strong emission band at ∼ 605 nm and a red-shift of ∼ 30 nm, as compared to standard SnO2 powder, which may be attributed to crystal defects and residual strains accommodated during the growth of the ribbons.

Hybrid Solar Cells Based on Nanoparticles of CuInS2 in Organic Matrices

Abstract: We report on solution-processed hybrid solar cells consisting of a nanocrystalline inorganic semiconductor, CuInS2, and organic materials. Synthesis of quantized CuInS2 nanoparticles was performed using a colloidal route, where the particle surface was shielded by an organic surfactant. First attempts were made to use nanocrystalline CuInS2 with fullerene derivatives to form flat-interface donor–acceptor heterojunction solar cells. We investigated also bulk heterojunctions by replacing the CuInS2 single layer by a blend of CuInS2 and p-type polymer (PEDOT:PSS; poly(3,4-ethylenedioxythiophene:poly(styrene sulfonic acid) in the same cell configuration. Bulk heterojunction solar cells show better photovoltaic response with external quantum efficiencies up to 20 %.

The Origin of Green Emission in Polyfluorene‐Based Conjugated Polymers: On‐Chain Defect Fluorescence

Abstract: The low emission band at 2.2–2.3 eV in polyfluorene-based conjugated materials is studied by various spectroscopic methods on defined copolymers of 9–9′-difarnesyl-fluorene with 9-fluorenone, which can be seen as a model compound for degraded polyfluorenes. Absorption, electroluminescence, and photoluminescence in the film (temperature-dependent) and solution (room temperature) reveal the optical properties of this low-energy emission band emerging in polyfluorene-type polymers upon degradation. All the experimental evidence presented yield direct evidence against excimer or aggregate formation as the primary source of the low-energy emission band. Instead keto defect sites can be shown to be responsible for the emissive defect.

Formation of Thick Porous Anodic Alumina Films and Nanowire Arrays on Silicon Wafers and Glass

Abstract: A method for the fabrication of thick films of porous anodic alumina on rigid substrates is described. The anodic alumina film was generated by the anodization of an aluminum film evaporated on the substrate. The morphology of the barrier layer between the porous film and the substrate was different from that of anodic films grown on aluminum substrates. The removal of the barrier layer and the electrochemical growth of nanowires within the ordered pores were accomplished without the need to remove the anodic film from the substrate. We fabricated porous anodic alumina samples over large areas (up to 70 cm2), and deposited in them nanowire arrays of various materials. Long nanowires were obtained with lengths of at least 9 μm and aspect ratios as high as 300. Due to their mechanical robustness and the built-in contact between the conducting substrate and the nanowires, the structures were useful for electrical transport measurements on the arrays. The method was also demonstrated on patterned and non-planar substrates, further expanding the range of applications of these porous alumina and nanowire assemblies.

Nanoparticle Arrays on Surfaces Fabricated Using Anodic Alumina Films as Templates

Abstract: High density nanoparticle arrays on surfaces have been created using a template-assisted approach. Templates were produced by evaporating aluminum onto substrates and subsequently anodizing the aluminum to produce nanoporous alumina films. The resulting templates have a narrow distribution of pore sizes tunable from ∼ 25 to ∼ 70 nm. To demonstrate the flexibility of this approach for producing nanoparticle arrays on various substrates, templates have been fabricated on silicon oxide, silicon, and gold surfaces. In all cases, a final chemical etching step yielded pores that extended completely through the template to the underlying substrate. Because the templates remain in intimate contact with the substrate throughout processing, they may be used with either vacuum-based or wet chemical deposition methods to direct the deposition of nanoparticles onto the underlying substrates. Here we have produced gold nanodot arrays using evaporation and gold nanorod arrays by electrodeposition. In each case, the diameter and height of the nanoparticles can be controlled using the confining dimensions of the templates, resulting in high density (∼ 1010 cm–2) arrays of nanoparticles over large areas (> 1 cm2).

Polyaniline–Silica Composite Conductive Capsules and Hollow Spheres†

Abstract: In this paper, we report on the preparation of monodisperse polyaniline (PANi)–silica composite capsules and hollow spheres on monodisperse core–gel-shell template particles. An extension of the previously reported inward growth method was used. The samples were self-stabilized without external additives. The core–gel-shell particles were prepared by the inward sulfonation of monodisperse polystyrene particles. The introduced sulfonic acid and sulfone groups are responsible for the gel properties. The gel-shell thickness and core size were synchronously controlled over the whole particle radius range. After aniline (ANi) monomer was preferentially absorbed in the sulfonated polystyrene shell, PANi was formed by polymerization. PANi was doped in situ with a sulfonic acid group to give the capsules a high conductivity. PANi hollow spheres were derived after the polystyrene cores were dissolved: their cavity size and shell thickness were synchronously controlled by using different core–gel-shell particles. The PANi–silica composite capsules and hollow spheres were therefore prepared by a sol–gel process using tetraethylorthosilicate in the conducting shell. The PANi shell became more robust while maintaining the same conductivity level. Morphological results indicate that the PANi and silica formed a bicontinuous network. Fourier-transform infrared (FTIR) spectra revealed that the hydrogen bonding in the PANi–gel shell was enhanced after the silica phase was incorporated, which could explain the high conductivity level after the silica phase was added. In a converse procedure, silica capsules and hollow spheres were prepared by a sol–gel process that incorporated tetraethylorthosilicate into the core–gel-shell templates, which was followed by the absorption and polymerization of aniline in the silica shell thus forming PANi–silica composite capsules and hollow spheres. The silica capsules and hollow spheres thereby became conductive.

Micellar Nanoreactors—Preparation and Characterization of Hexagonally Ordered Arrays of Metallic Nanodots†

Abstract: The preparation of hexagonally ordered metallic nanodots was studied in detail with emphasis on the chemical state of the resulting particles. To obtain these dots, in a first step micellar structures were formed from diblock copolymers in solution. The reverse micelles themselves are capable of ligating defined amounts of a metal salt within their cores, acting as nanoreactors. After transfer of the metal-loaded reverse micelles onto a substrate, the polymer was removed by means of different plasmas (oxygen and/or hydrogen), which also allow the metal salt to be reduced to the metallic state. In this way, ordered arrays of metallic nanodots can be prepared on various substrates. By adjusting the appropriate parameters, the separation and the size of the dots can be varied and controlled. To determine their purity, chemical state, and surface cleanliness—all of which are crucial for subsequent experiments since nanoscale structures are intrinsically surface dominated—in-situ X-ray photoelectron spectroscopy (XPS) and ex-situ transmission electron microscopy (TEM) were applied, also giving information on the formation of the nanodots.

Surface Functionalization of Thermoplastic Polymers for the Fabrication of Microfluidic Devices by Photoinitiated Grafting

Abstract: Photografting has been used for the surface modification of a wide range of commercial commodity polymers such as poly- styrene, poly(methyl methacrylate), poly(dimethyl siloxane), polycarbonate, Parylene C, polypropylene, cyclic olefin copolymer, and hydrogenated polystyrene that are useful substrate materials for the fabrication of microfluidic chips. Since the chain propagation is initiated after UV light-activated abstraction of a hydrogen atom from the surface of channels within the materials, their permeability for UV light was tested and polyolefins were found to be the best candidates. A number of monomers with a variety of functional groups such as perfluorinated, hydrophobic, hydrophilic, reactive, acidic, basic, and zwitterionic have been successfully grafted from the surface of selected substrates, and the grafting efficiency determined using X-ray photoemission spectroscopy. Layered surface structures were prepared by consecutive grafting of different monomers. Our approach also enables photolithographic patterning of surfaces and specific functionalization of confined areas within the microchannel.

Preparation and Characterization of Blue-Luminescent Tris(8-hydroxyquinoline)-aluminum (Alq3)

Abstract: Using differential scanning calorimetry (DSC) measurements in combination with structural and optical characterization we have investigated the formation conditions of different phases of tris(8-hydroxyquinoline)aluminum (Alq3). We have identified the δ-phase as a high-temperature phase of Alq3 being composed of the facial stereoisomer, and report an efficient method to obtain blue luminescent Alq3 by a simple annealing process. This allows the preparation of large amounts of pure δ-Alq3 by choosing appropriate annealing conditions, which is necessary for further characterization of this blue-luminescent phase, and offers the possibility of fabricating blue organic light-emitting devices (OLEDs) from this material.

Stabilization of semiconducting polymers with silsesquioxane

Abstract: Polyhedral oligomeric silsesquioxanes (POSS) anchored to poly(2-methoxy-5-(2-ethylhexyloxy)-1.4-phenylenevinylene) (MEH-PPV) (MEH-PPV–POSS), and to poly(9,9-dihexylfluorenyl-2,7-diyl) (PFO) (PFO–POSS) were synthesized. Compared with the corresponding parent polymers, MEH-PPV and PFO, MEH-PPV–POSS and PFO–POSS have better thermal stability. MEH-PPV–POSS and MEH-PPV have identical absorption and photoluminescent (PL) spectra, both in solution and as thin films. They also have identical electroluminescent (EL) spectra. Devices made from MEH-PPV–POSS exhibit higher brightness (1320 cd m–2 at 3.5 V) and higher external quantum efficiency (ηext = 2.2 % photons per electron) compared to MEH-PPV (230 cd m–2 at 3.5 V and ηext = 1.5 % ph el–1). Compared with PFO in the same device configuration, PFO–POSS has improved blue EL emission and higher ηext.

Preparation of Metallic Films on Elastomeric Stamps and Their Application for Contact Processing and Contact Printing

Abstract: The formation of permanent or reversible metallic patterns on a substrate has applications in microfabrication and analytical techniques. Here, we investigate how to metallize an elastomeric stamp, either for processing of a substrate mediated by the proximity between the metal on the stamp and an active layer on the substrate, or for contact printing of the metal from a stamp to a substrate. The stamps were made from poly(dimethylsiloxane) (PDMS) and were modified before metallizing them with Au by adding to or removing from their bulk mobile silicone residues, by oxidizing their surface with an O2-plasma, by surface-fluorination via silanization, or by priming them with a Ti layer. The interplay between the adhesion of the different layers defines two categories of application: contact processing and contact printing. Contact processing corresponds to keeping the metal on the stamp after contacting a substrate; it is reversible and nondestructive, and useful to define transient electrical contacts or quench fluorescence on a surface, for example. Contact printing occurs when the metal on the stamp adheres to the printed surface. Contact printing can transfer a metal, layers of metals, or an oxide onto a substrate with submicrometer lateral resolution. The transfer can be total or localized to the regions of contact, depending on the morphology of the metal on the stamp and/or the surface chemistry of the substrate.

Star‐Shaped Oligothiophenes for Solution‐Processible Organic Field‐Effect Transistors

Abstract: We have designed and successfully synthesized star-shaped oligothiophenes, which could be used as semiconducting materials for solution-processible organic field-effect transistors (FETs). By systematically changing the chemical structure of the star-shaped oligothiophenes we obtained the structural requirements needed for making working FETs from them. UV-vis fluorescence measurements showed that a molecule of the star-shaped compounds under consideration is not a fully conjugated molecule, but it has three independently conjugated oligothienyl-phenylene blocks. A possible scheme of molecular packing of the star-shaped oligothiophenes in a lamellar structure was proposed and confirmed by atomic force microscopy (AFM) and X-ray diffraction (XRD) measurements. Although the star-shaped semiconductors show a somewhat lower mobility than their linear analogs, they possess better solubility and film-forming properties, leading to improved spin-coating processing. The best FETs were made by spin-coating 1,3,5-tris(5″-decyl-2,2′:5′,2″-terthien-5-yl)benzene from a chloroform solution, which resulted in a mobility of 2 × 10–4 cm2 V –1s–1, a 102 on/off ratio at gate voltages of 0 V and –20 V, and a threshold voltage close to 0 V.