Emission studies of InGaN layers and LEDs grown by plasma-assisted MBE

We prepared InGaN layers on GaN/sapphire substrates using rf-MBE. Photoluminescence (PL) from these layers, grown at different temperatures T_S, shows that there is a strong tendency of GaN to form a separate phase as T_S is increased from 600°C to 650°C. Concomitant with the phase separation, the PL from the InGaN phase broadens, which indicates that indium composition in this phase becomes increasingly non-uniform. Indium compositions measured by Rutherford backscattering (RBS) are consistent with these results. We also observed an increase in PL intensity for InGaN layers grown at higher temperatures. In this paper, we also report on preparing a top-contact InGaN/GaN light emitting diode. The device was operated at 447 nm and had the emission line width of 37 nm with no observable impurity related features. The turn-on voltage was 3.0 V. The output power was 20 μW at 60 mA drive current.     

Synthesis of aryl- and heteroaryl-substituted 3-benzyloxyisothiazoles via Suzuki and Negishi cross-coupling reactions

Introduction of aryl and heteroaryl substituents into the 5-position of 3-benzyloxyisothiazole (1) using palladium-catalyzed Suzuki and Negishi cross-coupling reactions was investigated. Attempts to generate synthetically viable nucleophilic species from 1 for Suzuki- or Negishi-type cross-couplings were unsuccessful. However, using 3-benzyloxy-5-iodoisothiazole 2 as an intermediate, a range of aromatic and heteroaromatic substituents were successfully introduced under Suzuki or Negishi cross-coupling conditions in good to excellent yields.     

Synthesis,characterization, and structural investigations of 1‐amino‐3‐substituted‐1,2,3‐triazolium salts,and a new route to 1‐substituted‐1,2,3‐triazoles

Quarternary salts based upon 3‐alkyl substituted 1‐amino‐1,2,3‐triazolium cations (alkyl = methyl, ethyl, nypropyl, 2‐propenyl, and n‐butyl) have been synthesized and characterized by vibrational spectra, multinuclear NMR, elemental analysis, and DSC studies. Subsequent diazotization of these salts results in the exclusive formation of 1‐alkyl‐1,2,3‐triazoles. Single crystal X‐ray studies were carried out for 1‐amino‐3‐methyl‐1,2,3‐triazolium iodide, 1‐amino‐3‐ethyl‐1,2,3‐triazolium bromide, 1‐amino‐3‐n‐propyl‐1,2,3‐triazolium bromide, and 1‐amino‐3‐n‐butyl‐1,2,3‐triazolium bromide as well as the starting heterocycle, 1‐amino‐1,2,3‐triazole, and all of the structures are discussed.     

A multivariate characterization of crystal transformations of cellulose

A spectroscopic study of cellulose transformation processes, such as alkali treatment and annealing, showed that, in combination with multivariate data analysis techniques, a detailed understanding of the crystalline transformation processes could be reached.¹³C cross-polarization magic-angle spinning (CPMAS) NMR and near-infrared (NIR) spectroscopy of cotton linters and softwood pulps analysed during the processing revealed information, after data reduction using principal components data analysis, that could be connected to structural changes of the cellulose polymorphs. The data showed that alkali treatment of cotton linters led to a cellulose conversion from cellulose I to II, while annealing, both for linters and pulps, yielded a transformation from I to I.     

{4(S)‐(Piperidin‐1‐ylcarbonyl)‐4‐[3‐(trifluoro­methyl)phenylsulfon­amido]butyl}guanidinium chloride: a model of a graftable thrombin inhibitor

In the title compound, C₁₈H₂₇F₃N₅O₃S⁺·Cl⁻, the guanidine group forms N—H⋯Cl hydrogen bonds, with four N⋯Cl distances in the range 3.164 (3)–3.337 (4) Å. In the crystal packing, the cations are further linked by N—H⋯O and C—H⋯O inter­actions. The structure is compared with that of argatroban complexed with thrombin and is the subject of docking studies in the active site of thrombin.     

Magnetic quantum dots: synthesis, spectroscopy, and magnetism of Co2+ - and Ni2+-doped ZnO nanocrystals

We report a method for the preparation of colloidal ZnO-diluted magnetic semiconductor quantum dots (DMS-QDs) by alkaline-activated hydrolysis and condensation of zinc acetate solutions in dimethyl sulfoxide (DMSO). Mechanistic studies reveal that Co(2+) and Ni(2+) dopants inhibit nucleation and growth of ZnO nanocrystals. In particular, dopants are quantitatively excluded from the critical nuclei but are incorporated nearly isotropically during subsequent growth of the nanocrystals. The smaller nanocrystal diameters that result upon doping are explained by the Gibbs-Thompson relationship between lattice strain and crystal solubility. We describe methods for cleaning the nanocrystal surfaces of exposed dopants and for redispersion of the final DMS-QDs. Homogeneous substitutional doping is verified by high-resolution low-temperature electronic absorption and magnetic circular dichroism (MCD) spectroscopies. A "giant Zeeman effect" is observed in the band gap transition of Co(2+):ZnO DMS-QDs. MCD and Zeeman spectroscopies are used to quantify the magnitude of the p-d exchange interaction (N(0)beta) that gives rise to this effect. N(0)beta values of -2.3 +/- 0.3 eV (-18 500 cm(-1)) for Co(2+):ZnO and -4.5 +/- 0.6 eV (-36 300 cm(-1)) for Ni(2+):ZnO have been determined. Ligand-to-metal charge-transfer transitions are observed in the MCD spectra of both Co(2+):ZnO and Ni(2+):ZnO DMS-QDs and are analyzed in the context of an optical electronegativity model. The importance of these charge-transfer states in determining N(0)beta is discussed. Ferromagnetism with T(C) > 350 K is observed in aggregated nanocrystals of Co(2+):ZnO that unambiguously demonstrates the existence of intrinsic high-T(C) ferromagnetism in this class of DMSs.     

Convergence analysis for column-action methods in image reconstruction

Column-oriented versions of algebraic iterative methods are interesting alternatives to their row-version counterparts: they converge to a least squares solution, and they provide a basis for saving computational work by skipping small updates. In this paper we consider the case of noise-free data. We present a convergence analysis of the column algorithms, we discuss two techniques (loping and flagging) for reducing the work, and we establish some convergence results for methods that utilize these techniques. The performance of the algorithms is illustrated with numerical examples from computed tomography.     

Large-Scale Synthesis of a Lewis b Tetrasaccharide Derivative,its Acrylamide Copolymer,and Related DI- and Trisaccharides for Use in Adhesion Inhibition Studies with Helicobacter Pylori.

ABSTRACT

The 2-aminoethyl glycoside of O-α-L-fucopyranosyl-(1→2)-O-β-D-galactopyranosyl-(1→3)-[O-α-L-fucopyranosyl-(1→4)]-2-acetamido-2-deoxy-β-D-glucopyranose (Lewis B tetrasaccharide) was synthesized on a large scale and acryloylated with acryloyl chloride. The obtained oligosaccharide 2-acrylamidoethyl glycoside was then copolymerized with acrylamide to form a water-soluble, high molecular weight polymer, suitable for use in adhesion inhibition studies with Helicobacter pylori. Also synthesized were the corresponding derivatives of O-α-L-fucopyranosyl-(1→2)-O-β-D-galactopyranosyl-(1→3)-2-acetamido-2-deoxy-β-D-glucopyranose and O-β-L-fucopyranosyl-(1→2)-β-D-galactopyranose.