Lithium inserted ZnSnN2 thin films for solar absorber: n to p-type conversion

ZnSnN₂ is a non-toxic and earth-abundant photoabsorber material for flexible photovoltaic devices because of its excellent optoelectronic behavior. However, theoretical studies show that the alkaline-earth metallic (Li, Na, K, Rb, Cs, and Fr) dopants in ZnSnN₂, particularly lithium (Li), display shallow-acceptor behavior and improve the performance of ZnSnN₂ semiconductors. Orthorhombic phase structure with (002) preferred orientation was observed for Li-doped films and the lattice parameters agree well with reported standards. Secondary ion mass spectroscopy (SIMS) analysis revealed the incorporation of Li in Li:ZnSnN₂ films. XPS, the density of states, and Born effective charge analysis revealed the chemical bonding states of Li–ZnSnN₂. In contrast to the pristine n-type ZnSnN₂, Li:ZnSnN₂ thin films showed conductivity with p-type hole concentrations varying between 1.14 × 10²⁰–9.47 × 10¹⁹ cm^?3 and the highest mobility of 20.03 cm²V^?1s^?1. Therefore, we obtained p-type conductivity by substituting an organolithium reagent (C?H?Li) on the Zn site, which highlights that Li:ZnSnN₂ can be effectively used as the photoanode layer for next-generation thin-film solar cell devices.     

Validated reversed-phase column high-performance liquid chromatographic method for separation and quantification of polyphenolics and furocoumarins in herbal drugs

A rapid column high-performance liquid chromatographic-photodiode array method has been developed for the separation and identification of secondary metabolites, especially different types of phenols and furocoumarins, in a 35 min chromatographic run. The method has been optimized and validated for selectivity, precision, recovery, and robustness with the aim of application for standardization of selected herbal drugs. Almost all of the tested compounds had linearity of >98%, with relative standard deviation <10% in terms of variation of retention time. Interday and intraday variability was <5%. The developed method has been successfully applied in identification and quantification of phenols and furocoumarins present in different plants, viz., Artemisia pallens (whole plant), Hibiscus rosa-sinensis DC (flower), Heracleum candicans DC (fruit), and Ficus carica Linn (bark). The results indicate that the method is rapid, accurate, and robust for the analysis of different types of phenols and furocoumarins and, hence, can be successfully used in the quality control and standardization of plant extracts and herbal drugs.     

Contrasting effects of temperature on the rheology of normal and reverse wormlike micelles

Wormlike micelles are flexible polymerlike chains formed by the self-assembly of amphiphilic molecules either in water ("normal" worms) or in oil ("reverse" worms). Normal and reverse worms have both been studied extensively and have generally been found to exhibit analogous rheological properties (e.g., Maxwell fluidlike behavior). Here, we report a hitherto unexplored difference between these two classes of micelles pertaining to the effect of temperature on their rheological properties. For normal worms, the plateau modulus remains constant as the sample is heated while the relaxation time exponentially decreases. For reverse worms, however, both the plateau modulus and the relaxation time decrease exponentially upon heating. Consequently, the zero-shear viscosity of reverse worms decreases more rapidly with temperature than for normal worms. To explain these differences, we propose that increasing the temperature weakens the driving force for micellization in reverse worms whereas it only accelerates the dynamics of surfactant exchange in normal worms.     

Photoisomerization of trans‐2‐[4′‐(Dimethylamino)styryl]benzothiazole

Photoreaction of trans‐2‐[4′‐(dimethylamino)styryl]benzothiazole (t‐DMASBT) under direct irradiation has been investigated in dioxane, chloroform, methanol and glycerol to understand the mechanism of photoisomerization. Contrary to an earlier report, isomerization takes place in all these solvents including glycerol. The results show that restriction on photoisomerization leads to the increase in fluorescence quantum yield in glycerol. The results are consistent with the theoretically simulated potential energy surface reported earlier using time‐dependent density functional theory (TDDFT) calculations. DFT calculations on cis isomers under isolated condition have suggested that cis‐B conformer is more stable than cis‐A conformer due to hydrogen‐bonding interaction. In the ground state, cis‐DMASBT is predominantly present as cis‐B. The fluorescence spectra of the irradiated t‐DMASBT suggested that photoisomerization follows not the adiabatic path as proposed by Saha et al., but the nonadiabatic path.     

Spatiotemporal solitons in inhomogeneous nonlinear media

We investigate the possibility of forming spatiotemporal solitons (optical bullets) in inhomogeneous, dispersive nonlinear media using a graded-index Kerr medium as an example. We use a variational approach to solve the multidimensional, inhomogeneous, nonlinear Schrödinger equation and show that spatiotemporal solitons can be stabilized under certain conditions. We verify their existence by means of a full numerical analysis and show that such solitons should be observable experimentally.     

Self-assembly of surfactant vesicles that transform into viscoelastic wormlike micelles upon heating

Unilamellar vesicles are observed to form in aqueous solutions of the cationic surfactant, cetyl trimethylammonium bromide (CTAB), when 5-methyl salicylic acid (5mS) is added at slightly larger than equimolar concentrations. When these vesicles are heated above a critical temperature, they transform into long, flexible wormlike micelles. In this process, the solutions switch from low-viscosity, Newtonian fluids to viscoelastic, shear-thinning fluids having much larger zero-shear viscosities (e.g., 1000-fold higher). The onset temperature for this transition increases with the concentration of 5mS at a fixed CTAB content. Small-angle neutron scattering (SANS) measurements show that the phase transition from vesicles to micelles is a continuous one, with the vesicles and micelles coexisting over a narrow range of temperatures. The tunable vesicle-to-micelle transition and the concomitant viscosity increase upon heating may have utility in a range of areas, including microfluidics, controlled release, and tertiary oil recovery.     

Enhanced diastereoselectivity via confinement: diastereoselective photoisomerization of 2,3-diphenyl-1-benzoylcyclopropane derivatives within zeolites

Photochemistry of optically pure trans-2,3-diphenyl-1-benzoylcyclopropane has been examined in isotropic solution and within zeolites. Results suggest that it isomerizes by cleavage of either the C1-C2 or C1-C3 bond. From the perspective of chiral induction, photoisomerization of cis-2,3-diphenyl-1-benzoylcyclopropane derivatives with chiral auxiliaries placed at the meta and para positions of the benzoyl group have been examined both in isotropic solution and within zeolites. Whereas in isotropic solution the chiral auxiliaries placed at the meta position exhibit very little influence during the conversion of triplet cis-2,3-diphenyl-1-benzoylcyclopropane derivatives, they have significant influence within zeolites. For example, alpha-methyl benzylamine placed at the meta position of the benzoyl group (via an amide bond) yields the trans isomer with a diastereoselectivity (de) of 71% within NaY zeolite, whereas in solution no de is obtained. The chiral induction process within zeolites depends on the nature of the alkali ion and on the presence of water. Results suggest that the chiral auxiliary is able to control the bond being cleaved (C1-C2 vs. C1-C3 bond) within a zeolite, but it is unable to do so in an isotropic solution.     

Parabolic Higgs bundles and Teichmüller spaces for punctured surfaces

In this paper we study the relation between parabolic Higgs vector bundles and irreducible representations of the fundamental group of punctured Riemann surfaces established by Simpson. We generalize a result of Hitchin, identifying those parabolic Higgs bundles that correspond to Fuchsian representations. We also study the Higgs bundles that give representations whose image is contained, after conjugation, in SL(). We compute the real dimension of one of the components of this space of representations, which in the absence of punctures is the generalized Teichmüller space introduced by Hitchin, and which in the case of is the usual Teichmüller space of the punctured surface.

     

Probing the temperature profile of energy production in the sun

The particle kinetic energies of thermonuclear pp fusion in the Sun (Gamow energy) produce small changes in the energies of pp solar neutrinos relative to those due only to exothermal energetics. This effect may be observable via the unique tools of the LENS solar neutrino detector. The temperature profile of energy production in the Sun may thus be directly probed for the first time.