Morphology and optical properties of p-type porous GaAs(1 0 0) layers made by electrochemical etching

Porous GaAs layers were formed by electrochemical etching of p-type GaAs(1 0 0) substrates in HF solution. A surface characterization has been performed on p-type GaAs samples using X-ray photoelectron spectroscopy (XPS) technique in order to get information about the chemical composition, particularly on the surface contamination. According to the XPS spectra, the oxide layer on as-received porous GaAs substrates contains As₂O₃, As₂O₅ and Ga₂O₃. Large amount of oxygen is present at the surface before the surface cleaning.Compared to untreated GaAs surface, room temperature photoluminescence (PL) investigations of the porous layers reveal the presence of two PL bands: a PL peak at ∼871 nm and a “visible” PL peak at ∼650-680 nm. Both peak wavelengths and intensities varied from sample to sample depending on the treatment that the samples have undergone. The short PL wavelength at 650-680 nm of the porous layers is attributed to quantum confinement effects in GaAs nano-crystallites. The surface morphology of porous GaAs has been studied using atomic force microscopy (AFM). Nano-sized crystallites were observed on the porous GaAs surface. An estimation of the mean size of the GaAs nano-crystals obtained from effective mass theory and based on PL data was close to the lowest value obtained from the AFM results.     

On the zero mode of the Poisson gauge theory

The fundamentals of the Diff(S²) and the SDiff(S²) gauge theories are developed. It is shown that the adjoint representation of SU (∞) is described by a divergentless two-dimensional vector field defined on the sphere. The SU (∞) Yang-Mills gauge action obtained earlier by Floratos et al. is reviewed. The problem of the zero modes is solved without need of any constraint. The fundamental representations of SU (∞) and the gauge-matter couplings are discussed.     

New exotic neutron-deficient nuclei nearN=82

Residual activities produced via³⁵Cl or³²S beams in thin¹¹²Sn and¹⁰⁶Cd targets have been studied by γ-ray and X-ray spectroscopy techniques behind a He-jet system. Mass identification of the products has been performed by coupling the He-jet to the medium current ion source of an isotope separator. Neutron-deficient nuclei with 137≦A≦144 have been identified. Decay schemes of several new isotopes as¹³⁸Eu (12s),¹⁴¹Gd (22s),¹⁴³Tb (12s and <17s),¹⁴⁴Dy (9s) and possibly¹⁴⁰Gd (11s) have been derived. New informations have also been provided for¹³⁷Sm (45s),¹³⁹Eu (24s) and¹⁴⁴Tb (5s). Data have been presented through systematics. The shapes of these nuclei and their trends versusN andZ have been analysed.     

Low-frequency Raman spectra of sub- and supercritical CO2: qualitative analysis of the diffusion coefficient behavior

We report the results of the low-frequency Raman experiments on CO(2) which were carried out in a wide density range, along the liquid-gas coexistence curve in a temperature range of 293-303 K, and on the critical isochore of 94.4 cm(3) mol(-1) in a temperature range of 304-315 K. In our approach, the qualitative behavior of the diffusion coefficient D is predicted, assuming the following: first, that the low-frequency Raman spectra can be interpreted in terms of the translation rotation motions; second, that the random force could be replaced by the total force to calculate the friction coefficient; and finally, that the Einstein frequency is associated with the position of the maximum of the low-frequency Raman spectrum. The results show that the diffusion coefficient increases along the coexistence curve, and its values are almost constant on the critical isochore. The predicted values reproduce qualitatively those obtained by other techniques. The values of D were also calculated by molecular-dynamics simulation and they qualitatively reproduce the behavior of D.     

An efficient and selective tosylation of alcohols with p-toluenesulfonic acid

Silica chloride has been found to be an efficient catalyst for facile tosylation of alcohols directly with p-toluenesulfonic acid in methylene chloride under reflux. The process is associated with selective tosylation of secondary alcohols over primary alcohols.     

High Brønsted beta nuc values in SNAr displacement. An indicator of the SET pathway?

Nucleophilic substitutions of 4-chloro-7-nitrobenzofurazan (NBD-Cl) and 3-methyl-1-(4-nitrobenzofurazanyl)-imidazolium ions (NBD-Im+) with a series of 4-X-substituted anilines have been kinetically investigated in 70-30 (v/v) and 20-80 (v/v) H2O-Me2SO mixtures. The rate-limiting step in these reactions is nucleophilic addition with formation of Meisenheimer-type sigma-adducts followed by fast expulsion of the leaving group (Cl- or Im). The reactions are characterized by a notable sensitivity to basicity of the aniline nucleophiles, with Hammett rho values of -2.68 and -3.82 in 30% and 80% Me2SO, respectively, for NBD-Cl and even more negative values, -3.43 and -5.27, respectively, for NBD-Im+. This is consistent with significant development of positive charge at the nitrogen atom of the zwitterionic sigma-adduct. Unexpectedly, the Br?nsted-type plots reveal abnormally high beta nuc values, ca. 1.0 and 1.3-1.4, respectively. Satisfactory correlations between the rates of the reactions and the oxidation potentials of the respective anilines support a SET mechanism for this process, i.e. initial (fast) electron-transfer from the aniline donor to the nitrobenzofurazan acceptor moiety and subsequent (slow) coupling of the resulting cation and anion radicals within the solvent cage with formation of the sigma-adduct. An alternative possible explanation of the high beta nuc values being related to the strong--I effect exerted by the negatively charged 4-nitrobenzofurazanyl structure, which would induce a greater positive charge at the developing anilinium nitrogen atom in the sigma-adduct-like transition state as compared with the situation in the reference protonation equilibria of anilines, is considered less probable. It is thus proposed that obtention of abnormal beta nuc values may be an indicator of electron-transfer in nucleophilic aromatic substitution and highlights the transition from the polar (SNAr) to the single electron-transfer (SET) mechanism.     

A linear tetranuclear Ni(II) acyl hydrazone Schiff base complex: preparation,crystal structure and catalytic application

A novel linear tetranuclear Ni(II) complex Ni₄L₂ {[Ni₄L₂(H₂O)₃CH₃OH]·CH₃OH·2CH₃CN·2H₂O} was constructed by employing a well-tailored compartmental ligand H₄L = (2E,N'E)-N'-(1-(3-((E)-2-hydroxy-3-methoxybenzylideneamino)-2-hydroxyphenyl)ethylidene)-2 -(hydroxyimino)propanehydrazideas with equal equivalent NiCl₂·6H₂O and NaOH via solvothermal reaction. The complex was well characterized by X-ray crystallography, elemental analysis, infrared emission spectra, UV–visible absorption spectra and thermogravimetry. The crystal structure shows that four nickel ions coordinate with two ligands to form a unique linear structure of Ni₄L₂. Besides, Ni₄L₂ act as a catalyst showed good catalytic activity for polymerization of methyl methacrylate (MMA) to obtain PMMA under mild condition.

Graphical abstract

     

Cyclopentadienyl tricarbonyl complexes of Tc for the in vivo imaging of the serotonin 5-HT1A receptor in the brain

Technetium and rhenium tricarbonyl complexes with derivatized cyclopentadienyl ligands were prepared starting from pertechnetate and an appropriate ferrocene ligand. Furthermore, the complexes (M(CO)₃cp-COOC₅H₉N-R, M = Tc, Re; R = Me, isopropyl) could be obtained starting from the precursor complexes [^99mTc(CO)₃(H₂O)₃]⁺ and [Re(CO)₃Br₃]²⁻. Their chemical identity was confirmed by chromatographic methods and electron spray mass spectrometry. The biodistribution of the ^99mTc complexes (cytectrene I and cytectrene II) in Wistar rats was studied. Both compounds show high uptake in the brain and fast blood clearance. The pattern of regional distribution in the brain demonstrated in autoradiographic studies indicates binding to the 5-HT_1A and α₁ adrenergic receptors.     

A novel procedure for conversion of epoxides to α-hydroxyphosphonates with a trialkylphosphite mediated by LiClO4

A new method has been developed for the direct conversion of epoxides to α-hydroxyphosphonates by the reaction of a trialkylphosphite with the epoxide in 5 M lithium perchlorate in diethyl ether (LPDE). The reaction is highly regioselective and efficient with excellent yields under mild and neutral conditions.     

Center-like subsets in rings with derivations or endomorphisms

Our purpose in this paper is to show that certain subsets, defined by commutativity conditions involving derivations or endomorphisms, coincide with the center in prime rings. Moreover, we provide examples to show that the assumed restrictions cannot be relaxed.