Below-band-gap waveguiding behaviors of a weakly index-guided GaAs/AlGaAs quantum well laser

We report the reduced waveguiding efficiency for the signals around 1560 nm as the injection current of an GaAs/AlGaAs multiple quantum well laser diode (lasing wavelength at 840 nm) with a ridge-loading waveguide configuration increased. This reduction trend stopped when the injection current reached the threshold condition of the laser diode. The decreased waveguide transmission and the more expanded mode profile indicated the variation of the effective refractive index gradient in the lateral dimension with injection current. This variation was due to the refractive index decrease with increasing carrier density even below band gap. A slab waveguide model was used to simulate the lateral mode profile variation with injection current. The refractive index differences between the guiding layer and claddings in the slab waveguide model provided estimates of refractive index contrasts of the laser diode at a concerned wavelength under various injection conditions.     

Chemistry of thienopyridines. VII. Mechanistic studies on the synthesis of thieno[2,3-c] pyridine by thermolytic cyclization

A study was made of the formation of thieno[2,3-c]pyridine (1) from hydrogen sulfide and 4-vinylpyridine in a flow system at 630°. 2-(4-Pyridyl)ethanethiol and bis-2-(4-pyridyl)ethyl sulfide were found to be likely intermediates. Based on these studies, there was devised a two-step practical preparative method for I (optimum overall yield 58%) which consists of preliminary conversion of 4-vinylpyridine to benzyl 2-(4-pyridyl)ethyl sulfide and subsequent thermolysis (at 605°) of this substance.     

Nucleation theory: Fisher's droplet picture and microscopic surface tension

Fisher's droplet picture is used to define a “microscopic” surface tension for small droplets. The nucleation rate is then calculated. Excellent agreement between experiments and our calculations has been found for H₂O, CH₃OH, C₂H₅OH, NH₃, C₆H₆, CHCl₃ and CCl₃F.     

Computational structural determination and energy landscape analysis of the hepatic carcinogen 2-(acetylamino)fluorene

 2-(Acetylamino)fluorene (AAF), a potent mutagen and a prototypical example of the mutagenic aromatic amines, forms covalent adducts to DNA after metabolic activation in the liver. A benchmark study of AAF is presented using a number of the most widely used molecular mechanics and semiempirical computational methods and models. The results are compared to higher-level quantum calculations and to experimentally obtained crystal structures. Hydrogen bonding between AAF molecules in the crystal phase complicates the direct comparison of gas-phase theoretical calculations with experiment, so Hartree–Fock (HF) and Becke–Perdew (BP) density functional theory (DFT) calculations are used as benchmarks for the semiempirical and molecular mechanics results. Systematic conformer searches and dihedral energy landscapes were carried out for AAF using the SYBYL and MMFF94 molecular mechanics force fields; the AM1, PM3 and MNDO semiempirical quantum mechanics methods; HF using the 3-21G*and 6-31G* basis sets; and DFT using the nonlocal BP functional and double numerical polarization basis sets. MMFF94, AM1, HF and DFT calculations all predict the same planar structures, whereas SYBYL, MNDO and PM3 all predict various nonplanar geometries. The AM1 energy landscape is in substantial agreement with HF and DFT predictions; MMFF94 is qualitatively similar to HF and DFT; and the MNDO, PM3 and SYBYL results are qualitatively different from the HF and DFT results and from each other. These results are attributed to deficiencies in MNDO, PM3 and SYBYL. The MNDO, PM3 and SYBYL models may be unreliable for compounds in which an amide group is immediately adjacent to an aromatic ring. Received: 26 May 2002 / Accepted: 12 December 2002 / Published online: 14 February 2003     

X-Ray Structure and Proton NMR Study of a Hexacoordinated Lithium Complex

The structure of the lithium complex with1,3,5-tris[oxymethylene(N,N-dicyclohexyl)carboxyamido]cyclohexanehas been determined by the X-ray method.The compound is triclinic, space group P¯1,a = 15.623(3), b = 19.279(4),c = 19.295(4)Å = 102.32(3), = 92.45(3), = 105.67(3)0, V = 5436(2)Å3, Z = 4. Itscomposition is represented by the formulaC48H82N3O6LiI 0.5H2O. The lithium cationis encapsulated in a polar pseudo-cavity of six oxygen atoms of the ligandmolecule and displays a distorted trigonal prism coordination. The conformationof the ligand in the solid state complex has been compared with the conformationof the complex in solution determined by 1H-NMR measurements.Supplementary data relevant to this publication have been deposited with the British Library, No. SUP 82224 (21 pages).     

A graphical evaluation of characteristic polynomials of huckel trees

Characteristic polynomials of acyclic carbon chains (Huckel trees) are treated in a systematic way. Formulas of coefficients (a_k) of the polynomial are obtained in terms of connectivities that were introduced for dealing with moments in a previous paper. Based on the meaning of a_k, a graph-theoretical analysis is given such that a_k can be expressed as a linear combination of binomial factors specified by a set of graphs containing ½k edges. The numerical relationship is disclosed between each binomial factor and its specified graph. This stimulates the proposal of a novel approach for evaluating ak by simply collecting the graph set of defnite edges. The approach is equally applicable for the evaluation of matching polynomials of cyclic systems and extendable to the investigation of general trees.     

High‐efficiency polymer solar cells based on phenylenevinylene copolymer with BF2‐azopyrrole complex and CN‐PC70BM with solvent additive

Power conversion efficiency (PCE) of phenylenevinylene‐based copolymer with BF₂ azopyrrole complex (PB)/modified PC₇₀BM, that is, CN‐PC₇₀BM bulk heterojunction solar cells improves from 2.16 to 4.90% using a processing additive and drying condition. The results demonstrate that a processing additive and drying condition provides an effective means to control both the surface roughness and finer interpenetrating networks to enhance the exciton dissociation into free charge carriers, charge transportation, and collection. Taking into the account of simple device fabrication process without thermal annealing, the PCE of the polymer solar cell can further improved by chloronapthalene (CN) additive under the fast drying condition. The average carrier lifetimes extracted from the impedance spectra and found to correlate with measured PCEs. At short circuit conditions and illumination, the average charge carrier lifetime was found vary from 16.8 to 32 μs with power conversion efficiencies ranging from 3.0 to 4.9%. © 2012 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys, 2012     

Unusual Inherent Electrochemistry of Graphene Oxides Prepared Using Permanganate Oxidants

Graphene and graphene oxides are materials of significant interest in electrochemical devices such as supercapacitors, batteries, fuel cells, and sensors. Graphene oxides and reduced graphenes are typically prepared by oxidizing graphite in strong mineral acid mixtures with chlorate (Staudenmaier, Hofmann) or permanganate (Hummers, Tour) oxidants. Herein, we reveal that graphene oxides pose inherent electrochemistry, that is, they can be oxidized or reduced at relatively mild potentials (within the range ±1 V) that are lower than typical battery potentials. This inherent electrochemistry of graphene differs dramatically from that of the used oxidants. Graphene oxides prepared using chlorate exhibit chemically irreversible reductions, whereas graphene oxides prepared through permanganate‐based methods exhibit very unusual inherent chemically reversible electrochemistry of oxygen‐containing groups. Insight into the electrochemical behaviour was obtained through cyclic voltammetry, chronoamperometry, and X‐ray photoelectron spectroscopy experiments. Our findings are of extreme importance for the electrochemistry community as they reveal that electrode materials undergo cyclic changes in charge/discharge cycles, which has strong implications for energy‐storage and sensing devices.     

Three-dimensional constitutive equations for Ni–Mn–Ga single crystals

This paper presents a formulation of isothermal three-dimensional (3D) quasi-static magneto-mechanical constitutive equations and 3D magnetisation constitutive equations for tetragonal martensite Ni–Mn–Ga FSMA single crystals (c/a<1)$(c/a<1)$ with both ends restrained from twin-boundary motion. The formulated 3D constitutive equations model the 3D quasi-static magnetic fields as well as the coupling between uniaxial strains and stresses, and shear strains and stresses. The constitutive equations are compared with experimental results available in the literature and are found to correlate well with the experimental results, including magnetic field reversals. Both sets of 3D constitutive equations require only macroscopic parameters that are readily obtainable from magnetisation and mechanical stress–strain curves.