Implementation of diffractive optical element in four-wave mixing scheme for ex situ characterization of hydride vapor phase epitaxy-grown GaN layers

A holographic beam splitter has been integrated into a picosecond four-wave mixing (FWM) scheme. This modification significantly simplified the procedure of dynamic grating recording, thus making the FWM technique an easy-to-use tool for the holographic characterization of wide band gap materials. The novel FWM scheme was applied for characterization of hydride vapor phase epitaxy-grown undoped GaN layers of different thickness. It allowed the determination of carrier lifetime, diffusion coefficient, and carrier diffusion length by optical means, as well as the study of carrier recombination peculiarities with respect to dislocation and excess carrier density.     

A comparative Raman spectroelectrochemical study of selected polyaniline derivatives in a pH-neutral solution

Selected conducting polymers (polyaniline and poly-N-methylaniline), and copolymers of aniline with o-, m-, and p-phenylenediamines, as well as with metanilic acid, have been electropolymerized at a gold electrode, and studied with Raman spectroscopy (532 nm excitation) in a pH-neutral solution. Characteristic Raman features have been obtained and analysed for (co)polymers within electrode potential range of ?0.4 to 0.2 V vs. Ag/AgCl. Changes in Raman spectra depending on electrode potential have been analysed and assigned to different redox forms of (co)polymers. It has been shown that both oxidised and reduced forms of (co)polymers can exist, depending on electrode potential, even in a pH-neutral solution, where most of the polymers studied show no clearly defined electrochemical redox processes.     

Synthesis and properties of new derivatives of poly[9‐(2,3‐epoxypropyl)carbazole]

BACKGROUND: Carbazole derivatives are well known to exhibit interesting electro‐ and photo‐active properties due to their hole‐transporting ability, strong absorption in the ultraviolet spectral region and blue‐light emission. One of the most widely studied materials among carbazole‐containing oligomers is poly[9‐(2,3‐epoxypropyl)carbazole] (PEPK). The main field of application of this oligomer is electrophotographic microfilming. It is also used for the manufacture of multicolour slides and in the photothermoplastic recording of information. Unfortunately, due to its high ionization potential, which reaches 5.86 eV, the possibilities of application of this compound in optoelectronic devices are rather limited. RESULTS: PEPK‐based charge transporting oligomers, incorporating hydrazone moieties, are reported. The oligomers were prepared by chemical modification of PEPK. The materials obtained were examined using various techniques including differential scanning calorimetry and ultraviolet, infrared and NMR spectroscopy. Electron photoemission spectra of layers of the synthesized oligomers showed ionization potentials (I_p) in the range 5.4–5.5 eV. CONCLUSION: The synthesized oligomers possess a larger π‐conjugated system and show ionization potentials of ca 5.4 eV. Therefore, they are more suitable for use in optoelectronic devices with quicker photoresponse than unmodified PEPK. Copyright © 2008 Society of Chemical Industry     

Novel hydrazone based polymers as hole transporting materials

The novel family of hole transporting polymers containing hydrazone moieties is reported. The polymers were prepared in polyaddition reaction of hydrazone-containing diepoxydes with 4,4′-thiobisbenzenethiol in the presence of catalyst triethylamine (TEA). Obtained polymers were found to constitute novel polymeric hole transporting materials (TM) characterized by differential scanning calorimetry and time of flight method. The highest hole drift mobility in the newly synthesized polymers exceed 10⁻⁴ cm² V⁻¹ s⁻¹ at an electric field of 10⁶ V cm⁻¹ was observed in the TM with triphenylamine moieties.     

A survey of energy drink consumption patterns among college students

Background  

Energy drink consumption has continued to gain in popularity since the 1997 debut of Red Bull, the current leader in the energy drink market. Although energy drinks are targeted to young adult consumers, there has been little research regarding energy drink consumption patterns among college students in the United States. The purpose of this study was to determine energy drink consumption patterns among college students, prevalence and frequency of energy drink use for six situations, namely for insufficient sleep, to increase energy (in general), while studying, driving long periods of time, drinking with alcohol while partying, and to treat a hangover, and prevalence of adverse side effects and energy drink use dose effects among college energy drink users.     

Electrochemical sensors based on conducting polymer—polypyrrole

Conducting polymers can be exploited as an excellent tool for the preparation of nanocomposites with nano-scaled biomolecules. Polypyrrole (Ppy) is one of the most extensively used conducting polymers in design of bioanalytical sensors. In this review article significant attention is paid to immobilization of biologically active molecules within Ppy during electrochemical deposition of this polymer. Such unique properties of this polymer as prevention of some undesirable electrochemical interactions and facilitation of electron transfer from some redox enzymes are discussed. Recent advances in application of polypyrrole in immunosensors and DNA sensors are presented. Some new electrochemical target DNA and target protein detection methods based on changes of semiconducting properties of electrochemically generated Ppy doped by affinity agents are introduced. Recent progress and problems in development of molecularly imprinted polypyrrole are considered.     

Closely packed hexagonal conical microlens array fabricated by direct laser photopolymerization

We apply femtosecond laser direct writing in photopolymers for manufacturing of conical microlenses and closely packed arrays thereof. We demonstrate the fabrication of high optical quality axicons of 15?μm in radius, having 150°, 160°, and 170° cone angles. Their optical properties and performance are modeled using the finite-difference time-domain method and compared with experimentally measured data. Additionally, optimization of the laser direct writing parameters regarding these types of micro-objects is presented. Possible applications of closely packed arrays of axicon microlenses are discussed, having potential attractivity in the fields of modern microscopy, light-based material processing, particle manipulation in microfluidic, and optofluidic applications.     

Optical evaluation of carrier lifetime and diffusion length in synthetic diamonds

The key electronic parameters of high-pressure-high-temperature and chemical-vapor-deposition grown diamonds have been determined at interband (hν = 5.82 eV) or below bandgap (hν = 4.68 eV) photoexcitation, using a picosecond transient grating (TG) technique. TG kinetics directly provided the values of ambipolar diffusion coefficient (6–10 cm²/s) and carrier lifetime (in a range from 0.17 to 2.8 ns) for crystals grown under different conditions. The carrier diffusion length was found to vary from 0.5 μm in CVD layers to 1.6 μm for IIa type HPHT diamond crystal. The carrier lifetimes correlated well with the nitrogen-related defect density in both types of diamonds.     

A study of electropolymerization of N,N-dimethylaniline

Electrochemical oxidation and polymerization of N,N-dimethylaniline at a gold electrode have been studied in an acidic solution. Both potential cycling and electrolysis at a controlled potential yield a thin film of a polymer that shows electrochemical activity in acidic and pH-neutral solutions. Raman spectra of sulfate- and polystyrenesulfonate-doped polymer have been obtained and analyzed. Two possibilities for electropolymerization have been considered – a simple polymerization yielding a “true” poly(N,N-dimethylaniline), and polymerization accompanied by a partial demethylation yielding poly(N-methylaniline). Based on the results obtained, the first one has been considered as most likely.     

Synthesis of new hole-transporting molecular glass with pendant carbazolyl moieties

The synthesis and properties of amorphous hole-transporting 4,4′-bis[11-carbazol-9-yl)-6,8,10-tri(carbazol-9-methyl)-3-hydroxy-5,7,9-trioxa-1-thia]thiobisbenzene with two end and six pendant carbazolyl moieties, separated from the main chain by CH₂ spacers, is reported. It was prepared by a multistep synthesis route from 1,6-di(carbazol-9-yl)-5-(carbazol-9-methyl)-4-oxa-2-hexanol glycidyl ether. Glass transition temperatures of intermediate and final products established by differential scanning calorimetry are reported. The hole drift mobility, measured by the time of flight technique, in this well defined molecular glass depends little on the strength of the electric field and exceeds 10⁻⁵ cm² V⁻¹ s⁻¹. The ionization potentials measured by electron photoemission method, light absorption and fluorescence spectra are closed to those reported for the other organic photoconductors containing electronically isolated carbazole moieties.