Dielectric properties of silver nanoparticles coated with silica shells of different thicknesses

Core/shell nanoparticles having metallic silver nanoparticle cores of similar to 38 nm in diameter and silica shells of different thicknesses ranging from similar to 3.6-20 nm were prepared. For the silica coating, a slightly modified Stober method was used which allowed preparing grams of core/shell nanoparticles for the first time. The particles were characterized by UV-vis spectroscopy, dynamic light scattering, scanning electron microscopy, transmission electron microscopy, and energy-dispersive X-ray scattering. Their dielectric properties were measured as pellets in parallel-plate capacitors. It was found that the permittivity is much influenced by the silica shell thickness with an increase in permittivity for thinner shells. A shell thickness of 20 +/- 2 nm allowed fabrication of capacitors which have similar characteristics to those of silica, thus, there is no influence of the metal core on the dielectric properties anymore. However, by decreasing the silica shell to 17 +/- 2, 8 +/- 1.5, and 6.6 +/- 1.5 nm the permittivity at high frequencies is increasing from 10, 34, to 41, respectively. The insulator to metal transition was observed for a silica shell thickness of 3.6 +/- 1 nm. Functionalization of the silica surface with a hydrophobic coating removes surface adsorbed water as observed by the flat dielectric permittivity over a large frequency domain

Photoinduced hole-transfer in semiconducting polymer/low-bandgap cyanine dye blends: evidence for unit charge separation quantum yield

Power-conversion efficiencies of organic heterojunction solar cells can be increased by using semiconducting donor-acceptor materials with complementary absorption spectra extending to the near-infrared region. Here, we used continuous wave fluorescence and absorption, as well as nanosecond transient absorption spectroscopy to study the initial charge transfer step for blends of a donor poly(p-phenylenevinylene) derivative and low-band gap cyanine dyes serving as electron acceptors. Electron transfer is the dominant relaxation process after photoexcitation of the donor. Hole transfer after cyanine photoexcitation occurs with an efficiency close to unity up to dye concentrations of similar to 30 wt%. Cyanines present an efficient self-quenching mechanism of their fluorescence, and for higher dye loadings in the blend, or pure cyanine films, this process effectively reduces the hole transfer. Comparison between dye emission in an inert polystyrene matrix and the donor matrix allowed us to separate the influence of self-quenching and charge transfer mechanisms. Favorable photovoltaic bilayer performance, including high open-circuit voltages of similar to 1 V confirmed the results from optical experiments. The characteristics of solar cells using different dyes also highlighted the need for balanced adjustment of the energy levels and their offsets at the heterojunction when using low-bandgap materials, and accentuated important effects of interface interactions and solid-state packing on charge generation and transport

Interface Dipoles for Tuning Energy Level Alignment in Organic Thin Film Devices

One of the key features of organic optoelectronic and electronic devices resides in the multilayer architecture of the device stack. The performance of the latter strongly depends on the interface quality between organic layers or at the electrode heterojunction. Apart from interface thermodynamics governing adhesion and wetting, the electronic energy levels of the organic semiconductor are affected by the interface properties in a drastic way. This mini review gives a short overview on the possibilities to adjust frontier orbital energy levels using oriented electrical dipoles at the interfaces

H-aggregation and correlated absorption and emission of a merocyanine dye in solution, at the surface and in the solid state. A link between crystal structure and photophysical properties

The formation of H-aggregates as a function of solution, substrate and ambient variables is considered for the merocyanine dye 3-acetyl-5-12-(3-ethyl-2-benzothiazolydene) rhodanine. Colloidal semiconductor particles are shown to be a powerful tool to control the size of the aggregates. In water the blue shifted absorption band has been assigned to a dimer. Its spectrum has been isolated and the thermodynamical variables derived for the dissociation reaction are: ΔrG0=21.16 kJ/mol, ΔrH0=32.36 kJ/mol and ΔrS0=37.24 J/mol K. Exciton band absorption maxima for aggregates in solution and at the water-TiO2 and water-Al2O3 interface, respectively, have been correlated to the aggregation geometry using the extended dipole model in conjunction with crystallographic data. Microcrystals showing a hypsochromical shift in the absorption band have been produced within the pores of nanocrystalline semiconductor films. The calculation shows that these aggregates are needle shaped and are composed of about 2250 monomer units. A broad emission band appears when the organic molecules assemble in a head to tail stacking geometry which could be attributed to excimer fluorescence. It is not quenched by charge injection into TiO2 and indicates the existence of dislocations within the merocyanine stacks

Synthesis, Characterization, and Dielectric Properties of Phthalocyanines with Ester and Carboxylic Acid Functionalities

Monomeric phthalocyanine pentylester derivatives containing Cu and Zn were synthesized via the cyclotetramerization of 4,5-dipentylphthalonitrile. Conversion of the pentylester derivatives by hydrolysis yielded the octacarboxylic acid phthalocyanine derivatives in high purity, without any contamination by oligomers. The dielectric properties of pressed pellets were investigated as a function of frequency. When exposing the phthalocyanine powders to water vapor, a high increase in the dielectric constant was found for the carboxylic acid derivatives, whereas the esters showed no effect. For Cu containing octacarboxylic acid phthalocyanine derivatives, the low-frequency dielectric constant of the pellets was raised from 12 to > 1 x 10(5) when going from a water content of 6.4 to 14.7 wt %, respectively. The concomitant sharp rise in conductivity was attributed to the protons released from the acid groups after water uptake. The present work clearly demonstrates that water uptake and not oxygen is the major factor for achieving surprisingly high dielectric constants in carboxylic acid phthalocyanine derivatives

Self-Repairable, High Permittivity Dielectric Elastomers with Large Actuation Strains at Low Electric Fields

A one-step process for the synthesis of elastomers with high permittivity, excellent mechanical properties and increased electromechanical sensitivity is presented. It starts from a high molecular weight polymethylvinylsiloxane, P1, whose vinyl groups serve two functions: the introduction of polar nitrile moieties by reacting P1 with 3-mercaptopropionitrile (1) and the introduction of cross-links to fine tune mechanical properties by reacting P1 with 2,2'-(ethylene-dioxy) diethanethiol (2). This twofold chemical modification furnished a material, C2, with a powerful combination of properties: permittivity of up to 10.1 at 10(4) Hz, elastic modulus Y-10% = 154 kPa, and strain at break of 260%. Actuators made of C2 show lateral actuation strains of 20.5% at an electric field as low as 10.8 mu m(-1). Additionally, such actuators can self-repair after a breakdown, which is essential for an improved device lifetime and an attractive reliability. The actuators can be operated repeatedly and reversibly at voltages below the first breakdown. Due to the low actuation voltage and the large actuation strain applications of this material in commercial products might become reality

The optimisation of the laser-induced forward transfer process for fabrication of polyfluorene-based organic light-emitting diode pixels

Laser-induced forward transfer (LIFT) has already been used to fabricate various types of organic light-emitting diodes (OLEDs), and the process itself has been optimised and refined considerably since OLED pixels were first demonstrated. In particular, a dynamic release layer (DRL) of triazene polymer has been used, the environmental pressure has been reduced down to a medium vacuum, and the donor receiver gap has been controlled with the use of spacers. Insight into the LIFT process's effect upon OLED pixel performance is presented here, obtained through optimisation of three-colour polyfluorene-based OLEDs. A marked dependence of the pixel morphology quality on the cathode metal is observed, and the laser transfer fluence dependence is also analysed. The pixel device performances are compared to conventionally fabricated devices, and cathode effects have been looked at in detail. The silver cathode pixels show more heterogeneous pixel morphologies, and a correspondingly poorer efficiency characteristics. The aluminium cathode pixels have greater green electroluminescent emission than both the silver cathode pixels and the conventionally fabricated aluminium devices, and the green emission has a fluence dependence for silver cathode pixels. (C) 2012 Elsevier B.V. All rights reserved

Synthesis of poly(ethylene-co-butylene)-block-poly(ethylene oxide) surfactant and its use in the synthesis of polyhydroxyethyl methacrylate nanoparticles containing azo-dye

A synthetic path to poly(ethylene-co-butylene)-block-poly(ethylene oxide), a substitute for the "KLE" surfactant was developed. For this purpose, mono hydroxyl end-functionalized poly(ethylene-co-butylene) (similar to 3800 g mol(-1)) was modified with a triple bond at its hydroxyl group, while two samples of mono hydroxyl end-functionalized poly(ethylene oxide) (similar to 2000 g mol(-1) and 5000 g mol(-1)) were modified with an azide group. The two blocks were bound together by click chemistry and the products characterized by 1H NMR spectroscopy, EA and GPC. The ability of these new block copolymers to stabilize nanodroplets in inverse miniemulsion polymerization of 2-hydroxyethyl methacrylate (HEMA) was investigated. It was found that the molecular weight of the poly(ethylene oxide) block strongly influences the stability of the miniemulsions; only the copolymer with the short oligoethylene block gave satisfactory results. The possibility of encapsulating Disperse Red 1 (DR1) dye in polyhydroxyethyl methacrylate (PHEMA) was also investigated and the miniemulsions were optimized such as to prepare particles with a maximum DR1 loading of approximately 21 wt%. To avoid dye agglomeration and phase separation during polymerization, DR1 was equipped with a polymerizable group. The resulting particles were characterized by DLS, SEM, TGA, UV-vis and DSC. These particles are of importance as fillers in polydimethylsiloxane composites that are used as electrostrictive materials