Photoinduced charge carrier generation in a poly(3-hexylthiophene) and methanofullerene bulk heterojunction investigated by time-resolved terahertz spectroscopy

We report on the ultrafast photoinduced charge separation processes in varying compositions of poly(3-hexylthiophene) (P3HT) blended with the electron acceptor [6,6]-phenyl-C61-butyric acid methyl ester (PCBM). Through the use of time-resolved terahertz spectroscopy, the time- and frequency-dependent complex photoconductivity is measured for samples with PCBM weight fractions (WPCBM) of 0, 0.2, 0.5, and 0.8. By analysis of the frequency-dependent complex conductivity, both the charge carrier yield and the average charge carrier mobility have been determined analytically and indicate a short (<0.2 nm) carrier mean free path and a suppressed long-range transport that is characteristic of carrier localization. Studies on pure films of P3HT demonstrate that charge carrier generation is an intrinsic feature of the polymer that occurs on the time scale of the excitation light, and this is attributed to the dissociation of bound polaron pairs that reside on adjacent polymer chains due to interchain charge transfer. Both interchain and interfacial charge transfer contribute to the measured photoconductivity from the blended samples; interfacial charge transfer increases as a function of increasing PCBM. The addition of PCBM to the polymer films surprisingly does not dramatically increase the production of charge carriers within the first 2 ps. However, charge carriers in the 0.2 and 0.5 blended films survive to much longer times than those in the P3HT and 0.8 films.     

Transition temperature of a magnetic semiconductor with angular momentum j

We employ dynamical mean-field theory to identify the materials properties that optimize T(c) for a generalized double-exchange model. We reach the surprising conclusion that T(c) achieves a maximum when the band angular momentum j equals 3/2 and when the masses in the m(j) = +/- 1/2 and +/-3/2 and subbands are equal. However, we also find that T(c) is significantly reduced as the ratio of the masses decreases from one. Consequently, the search for dilute-magnetic semiconductor materials with high T(c) should proceed on two fronts. In semiconductors with p bands, such as the currently studied Mn-doped Ge and GaAs semiconductors, T(c) may be optimized by tuning the band masses through strain engineering or artificial nanostructures. On the other hand, semiconductors with s or d bands with nearly equal effective masses might prove to have higher T(c)'s than p-band materials with disparate effective masses.     

New results in equal sums of like powers

This paper reports on new results for the equation

i.e., equal sums of like powers. Since the 1967 Lander, Parkin and Selfridge survey paper [4], few other numeric results have been published (see Elkies [6] and Ekl [3]). The present paper reports on several new smallest primitive solutions. Further, search limits have been extended in many cases, and tables of solutions are presented. Additionally, new solutions to the same class of problems in distinct integers have been discovered.

     

A soluble base for the copper-catalyzed imidazole N-arylations with aryl halides

[reaction: see text] CuI-catalyzed N-arylation of imidazoles with aryl bromides has been achieved in a near-homogeneous system that utilizes tetraethylammonium carbonate as base, 8-hydroxyquinoline as ligand, and H2O as cosolvent. Preliminary results with aryl chlorides are also reported.     

Effects of melt‐processing conditions on the quality of poly(ethylene terephthalate) montmorillonite clay nanocomposites

Organically modified montmorillonite was synthesized with a novel 1,2‐dimethyl‐3‐N‐alkyl imidazolium salt or a typical quaternary ammonium salt as a control. Poly(ethylene terephthalate) montmorillonite clay nanocomposites were compounded via melt‐blending in a corotating mini twin‐screw extruder operating at 285 °C. The nanocomposites were characterized with thermal analysis, X‐ray diffraction, and transmission electron microscopy to determine the extent of intercalation and/or exfoliation present in the system. Nanocomposites produced with N,N‐dimethyl‐N,N‐dioctadecylammonium treated montmorillonite (DMDODA‐MMT), which has a decomposition temperature of 250 °C, were black, brittle, and tarlike resulting from DMDODA degradation under the processing conditions. Nanocomposites compounded with 1,2‐dimethyl‐3‐N‐hexadecyl imidazolium treated MMT, which has a decomposition temperature of 350 °C, showed high levels of dispersion and delamination. © 2002 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 40: 2661–2666, 2002     

Base-triggered self-amplifying degradable polyurethanes with the ability to translate local stimulation to continuous long-range degradation

A new type of base-triggered self-amplifying degradable polyurethane is reported that degrades under mild conditions, with the release of increasing amounts of amine product leading to self-amplified degradation. The polymer incorporates a base-sensitive Fmoc-derivative into every repeating unit to enable highly sensitive amine amplified degradation. A sigmoidal degradation curve for the linear polymer was observed consistent with a self-amplifying degradation mechanism. An analogous cross-linked polyurethane gel was prepared and also found to undergo amplified breakdown. In this case, a trace amount of localized base initiates the degradation, which in turn propagates through the material in an amplified manner. The results demonstrate the potential utility of these new generation polyurethanes in enhanced disposability and as stimuli responsive materials.

A new type of base-triggered self-amplifying degradable polyurethane is reported that degrades under mild conditions, with the release of increasing amounts of amine product leading to self-amplified degradation.     

Atmospheric pressure ion trapping in a tandem FAIMS-FAIMS coupled to a TOFMS: Studies with electrospray generated gramicidin S ions

A tandem FAIMS–FAIMS system for ion trapping at room temperature and atmospheric pressure is described. The first FAIMS device consisted of a side-to-side configuration (sFAIMS) suitable for ion separation, whereas the second FAIMS device was appropriate for ion trapping (tFAIMS). Ions pre-selected by the sFAIMS entered the tFAIMS and were captured by virtual trapping fields at the hemispherical tip of the inner electrode. The use of the sFAIMS, with wider electrode diameters, and consequently better ion separation efficiency than the tFAIMS, lowered the number of background ions captured in the trapping region of tFAIMS, and thus reduced the space charge effects in the trap. This tandem device was coupled to a laboratory built time-of-flight mass spectrometer and was evaluated using the electrospray generated [M + 2H]²⁺ ion of gramicidin S. The half-time (t_1/2) of the exponential decay of the ion cloud in tFAIMS, determined by monitoring the residual intensity of ions extracted from the ion trapping region of tFAIMS after various delay times, was about 2 s.