Chemical Reaction Technique Research Articles

Photoluminescence spectra of Zn1−xCdxAl2Se4-4xS4x single crystals

We investigated the photoluminescence as well as the crystal structure and optical energy gaps of the Zn1-xCdxAl2Se4-4xS4x solid solution system based on the Al-related compounds of ZnAl2Se4, ZnAl2S4, CdAl2Se4, and CdAl2S4. The single crystals of the system with 0.0 ≤ x ≤ 1.0 were grown by the chemical transport reaction technique. The Zn1-xCdxAl2Se4-4xS4x crystallizes in a defect chalcopyrite structure for a whole composition and has an optical energy gap ranging from 3.525 to 3.577 eV at 13 K. The photoluminescence spectra at 13 K showed a strong emission band in the blue spectral region and a weak broad emission band in the visible region due to donor–acceptor pair recombination. The composition and temperature dependence of these bands were examined in the investigated regions. The simple energy band scheme for the radiative mechanisms of the Zn1-xCdxAl2Se4-4xS4x is proposed on the basis of our experimental results along with photo-induced current transient spectroscopy measurements.

Cluster isolated chemical reaction (CICR) spectroscopy: Ba atoms and Ba(CH4)n complexes on large neon clusters

The cluster isolated chemical reaction (CICR) technique was applied to neon clusters (Ne≈7000) on which barium atoms and methane molecules were deposited. Clusters carrying barium only were studied first. Qualitatively, the present results on neon clusters are in line with our previous results on argon clusters. In particular, surface location of barium was observed. The central part of the present work concerns neon clusters carrying both one barium atom and one to ten methane molecules. Several types of spectroscopy were performed in the region of the resonance transition (6s2)1S→(6s6p)1P of barium (excitation spectrum of the total fluorescence, emission spectrum, action spectrum for forming (6s6p)3P), and experiments where the number of methane molecules per cluster, which was strictly controlled, was varied systematically. The corresponding results were interpreted on the ground of a model, which transposes both chemical thermodynamics of equilibria and reaction kinetics to CICR experiments. Such an approach has a strong relationship, although it is more simple, with the thermodynamical approach to reactions in micellar solutions. The present thermodynamical model helped us to determine the origin of the action spectrum for forming Ba(6s6p)3P in clusters carrying both one barium atom and an average of 2.5 methane molecules. This action spectrum was assigned to direct excitation of the Ba(CH4) and Ba(CH4)2 complexes. The present thermodynamical model was also applied to our former results on argon clusters. This allowed us to derive a consistent picture of the association reaction of barium with methane and of the quenching of electronic excitation of barium by methane in both environments.

Room Temperature Single Electron Tunneling in Nanoparticle-STM Tip Assemblies

A novel approach to constructing double barrier tunneling junctions has been proposed for observing single electron tunneling(SET) phenomena at room temperature. PbS nanoparticles were directly synthesized on gold STM tip by self-assembly and in-situ chemical reaction techniques in this method. The SAM/PbS modified tip was used for STM-based current-voltage(I-V) measurements. Highly oriented pyrolytic graphite(HOPG) was used as substrate for electron tunneling. Clear Coulomb blockade and Coulomb staircase with equidistant voltage of ∼; 110 mV were observed at room temperature with this novel assembly system. In addition. I-V curves also show an asymmetric shape at positive and negative voltages, suggesting a non-zero value of the fraction charge Q0 existing on the Coulomb island.

Photoluminescence properties of MgxCd1−xSe mixed single crystals

The photoluminescence spectra of MgxCd1−xSe (0.00⩽x⩽0.46) mixed single crystals grown by the chemical transport reaction technique were investigated. We observed an emission band (I2) due to excitons bound to a neutral donor, an emission band (DA) due to donor-acceptor pair recombination with LO-phonon replicas and a self-activated emission band. These emission bands were shifted to higher energy with increasing x. For the crystals with x=0.46, the donor-acceptor emission band was observed at 2.661 eV in the blue region.

Cluster isolated chemical reactions: Reactivity of Ba atoms and small Ba clusters with SF6 and CO2 molecules on large argon clusters

The cluster isolated chemical reaction technique was used to investigate the reactivity of the Ba/CO2 and Ba/SF6 systems in the environment of Ar≈6000 clusters. The method was extended to document several aspects of the reactivity. Notably, mass spectrometry gives insight into the full reactivity of the system deposited on the clusters. Laser induced fluorescence (LIF) and chemiluminescence are also used as detection tools. Unexpectedly, we found that a single barium atom neither reacts with CO2 nor with SF6 at the cluster temperature (32 K). In contrast, the LIF results suggest the formation of a weakly bound covalent Ba…CO2 complex. Finally, Ba2 and larger barium aggregates react with CO2, and Ba3 and larger aggregates react with SF6. The chemiluminescent products are Ba2O in the first case, and BaF in the second. These observations are rationalized on the ground of the harpoon model.

Growth and characterization of Mg xCd 1 − xSe single crystals

Single crystals of Mg x Cd 1 − x Se over the range 0 ⩽ x ⩽ 0.46 were grown by the modified chemical transport reaction technique. The structural and optical properties of the Mg x Cd 1 − x Se single crystals were investigated at room temperature from X-ray powder diffraction and optical absorption measurements. It has been found that the Mg x Cd 1 − x Se has a wurtzite structure over the investigated composition range. The lattice constants of a and c decrease linearly with increasing a composition x. The band gap increases linearly with increasing a composition x, which is represented by a linear relationship E g( x) = 1.742 + 1.813 x.

Cluster Isolated Chemical Reactions: Evidence for the Formation of Ba2O in Oxidation Reactions of Barium Dimers

The present paper reports on a new experimental technique, the cluster isolated chemical reaction technique. A single pair of reactants (or a controlled number of reactants) is isolated on a large atomic or molecular cluster. The reactants are free to migrate and to collide with each other on the cluster surface. The collision between two reactants results in a chemical reaction and yields observable signals, the cluster playing the role of a reaction medium of finite size. The CICR technique is used here to study the reaction Ba2 + N2O on (N2)8000 clusters and the reaction Ba2 + CO2 on (Ar)5500 clusters. Both these reactions are chemiluminescent and yield rather unstructured emission spectra. The power of the CICR method is exemplified here since it allows unambiguous determination of the emitter as Ba2O in both reactions. This allows us to identify, then, the yet unassigned chemiluminescence spectrum observed by West et al. (J. Chem. Phys. 1977, 66, 2139) in flames of Ba/CO2 and Ba/CO mixtures.

Determination of the chemical mechanism of neurotransmitter receptor-mediated reactions by rapid chemical kinetic techniques.

Rapid chemical reaction techniques play an important role in unraveling the mechanism of reactions mediated by soluble proteins, including enzymes involved in the regulation of intracellular processes and the biosynthesis of proteins and nucleic acids. Regulatory proteins change conformation rapidly and must, therefore, be studied in the microsecond-to-millisecond time region. Similar techniques for investigating reactions mediated by membrane-bound neurotransmitter receptors were not available, and the mechanisms of the reactions are poorly understood. These proteins must be studied in a membrane-bound form in cells or vesicles, over a wide range of reactant concentrations, and in the microsecond-to-millisecond time region. Four rapid-mixing techniques for making kinetic measurements in the microsecond and millisecond time regions have now been developed for investigating neurotransmitter receptors in the membranes of neurons and muscle cells, thus extending chemical kinetic approaches to membrane-bound proteins and intercellular processes. Neurotransmitter receptors regulate transmission of signals between neurons (approximately 10(12) in the human nervous system), thereby allowing perception of stimuli, integration and storage of information, and reaction to the environment. Six structurally related neurotransmitter receptors, and many isoforms, have been identified by use of recombinant DNA technology. Modern electrophysiological techniques show that these different proteins, upon binding a specific chemical signal (neurotransmitter), transiently open transmembrane channels, which are characterized by their ion selectivity, conductance, and lifetime. To be able to account for the receptor-mediated voltage changes that trigger signal transmission between cells, we still need to know the concentration of open receptor-channels. This concentration changes with time and is affected by the concentration of neurotransmitter. Rapid reaction techniques are particularly suitable for determining the relationship between neurotransmitter concentration and the time-dependent concentration of the open receptor-channels. The four rapid reaction techniques adapted or developed for studying receptor mechanisms are quench- and stopped-flow, adapted for use with vesicles, and cell-flow and laser-pulse photolysis for use with single cells. The approach was initiated when it was found that the neurotransmitter receptors desensitize (become transiently inactive) faster, by almost two orders of magnitude, than was believed. Before fast reaction techniques were used, the chemical properties of only desensitized forms were investigated, although this was not recognized. So far, the chemical mechanism(s) of the excitatory (cation-specific) acetylcholine receptor in membrane vesicles, electroplax cells, and single clonal cells, and the inhibitory (anion-specific) gamma-aminobutyric acid (GABA) receptor in primary cerebral cortical cells have been investigated with the new techniques.(ABSTRACT TRUNCATED AT 400 WORDS)

On the mechanism of the gamma-aminobutyric acid receptor in the mammalian (mouse) cerebral cortex. Chemical kinetic investigations with a 10-ms time resolution adapted to measurements of neuronal receptor function in single cells.

The gamma-aminobutyric acidA (GABA) receptor belongs to a superfamily of proteins involved in chemical reactions that regulate signal transmission between cells of the nervous system and is the target of some of the agents most frequently used in medicine to control disorders of the central nervous system. In contrast to the nicotinic acetylcholine receptor, which initiates signal transmission and is the best characterized member of the superfamily, the GABA receptor forms anion-specific transmembrane channels and inhibits signal transmission. The chemical kinetic experiments described here, in which fast chemical reaction techniques were used, indicate that both receptor proteins may operate by the same mechanism. Also described is the use of a chemical kinetic technique with a 10-ms time resolution that we have developed for making measurements with single cells isolated from specific areas of the nervous system, in this case the cerebral cortex of embryonic mice. A flow device was used to equilibrate receptors on the cell surface with GABA, and the concentration of open transmembrane channels in the cells was then measured by recording the whole-cell currents at pH 7.2, 21-23 degrees C, and a transmembrane voltage of -70 mV. Two different receptor forms, A alpha and A beta, were detected in cerebral cortical cells. Although the ratio of A alpha to A beta varied from cell to cell, on average 35% and 65% of the receptor-controlled current was associated with receptor forms A alpha and A beta, respectively. At saturating concentrations of GABA, the rate coefficients of desensitization, alpha and beta, associated with these two forms have maximal values of 4.4 and 0.7 s-1, respectively. The constants of a mechanism that accounts for the open transmembrane channels of both receptor forms were evaluated over a 50-fold range of GABA concentration. The dissociation constant of the site controlling channel opening was 40 microM for A alpha and 320 microM for A beta. The channel-opening equilibrium constant, phi-1, was 3.5 for A alpha and 20 for A beta. The evaluated constants allow one to calculate Po, the conditional probability that at a given concentration of GABA the receptor-channel is open. Po could also be determined in the presence of 100 microM GABA by an independent method in which different assumptions are made in the interpretation of the experimental results, the single-channel current-recording technique. The value of Po obtained (0.56) was in good agreement with the Po value (0.61) calculated for receptor form A alpha from chemical kinetic measurements at 100 microM GABA.(ABSTRACT TRUNCATED AT 400 WORDS)

Electrical properties of SmGa 2S 4 single crystal

SmGa 2S 4 single crystals have been obtained by chemical transport reaction technique and are crystallized into the orthorhombic system with lattice parameters a = 10.28; b = 10.42; c =6.16 A ̊ and space group Fddd. SmGa 2S 4 compound melts congruently at 1740 K. The investigation results of current-voltage characteristics and conductance temperature dependence are given. It is shown that in In-SmGa 2S 4-In structures current passage peak in the CVC linear region is associated with the Frenkel-Poole thermal field injection. The trap occurence depth, ionization centre concentration, potential well shape, and charge carrier free length are determined.

Analysis of amphoteric surfactants by liquid chromatography with post-column detection : III. Salt-free-type imidazoline amphoteric surfactants

Salt-free-type imidazoline amphoteric surfactants were analysed by reversed-phase high-performance liquid chromatography with ultraviolet and/or post-column detection. The active ingredients of the surfactants were isolated and identified directly by field desorption mass spectrometry and some specific chemical reaction techniques. Use of the method has clarified the reaction pathways by which the compounds are transformed and demonstrated the specificity of the proposed post-column detection in commercial products analysis. By this method, information on the alkyl-chain distribution of the orginal fatty acids, the degree of substitution by electrophilic reactants and the reaction process and conditions under which the surfactants were prepared can be obtained.

Analysis of amphoteric surfactants by liquid chromatography with post-column detection : II. Imidazoline-type amphoteric surfactants derived from sodium chloroacetate

A convenient and direct high-performance liquid chromatographic (HPLC) determination of imidazoline-type amphoteric surfactants derived from sodium chloroacetate, using post-column detection, is described. The active ingredients of the surfactants were isolated by semi-preparative HPLC and identified directly by using both field desorption mass spectra and some specific chemical reaction techniques. The post-column detection system was based on a specific reaction with the amine structures of the surfactants, developed previously. The proposed method gave information on alkyl chain distribution of the original fatty acids, the degree of N-substitution by an electrophilic reactant, the reaction mechanism and conditions under which the surfactants were prepared. The method was applied successfully to toiletry products analysis.

Growth and physical properties of CdCr 2Se 4 defect single crystals

Results for Cd 1- x Me x Cr 2Se 4 (Me=In,Ag; 0⩽ x⩽0.1) single crystals grown by chemical transport reaction technique are presented. The influence of dopant as well as deviation from stoichiometry on electrical and magnetic characteristics have been investigated. The results are discussed taking into account the variable valence of chromium ions.

Croissance et caractérisation de couches ZnSe, ZnS et ZnS xSe 1- x épitaxiées sur flourine

Epitaxial films of ZnSe, ZnS and ZnS x Se 1- x , 3 to 12 αm thick and of 1 cm 2 area, have been grown by the vapour phase chemical reaction technique, on the cleavage faces of CaF 2. Microscopic and X-ray investigations have shown that these compounds crystallized in the cubic phase, in the 600–650°C range of temperature. Some indications are given about the photoluminescence spectra of the ZnSe/CaF 2 layers.

Préparation de couches minces orientées de CdS obtenues par voie chimique en phase vapeur

Epitaxial films of CdS, 15 to 25 μ thick and 1.5 cm 2 area, have been grown by a vapor phase chemical reaction technique on the cleavage faces of NaCl, KCl, KBr, CaF 2 and muscovite. The optical and photoelectric properties of the films on the two substrates are equal to those of the bulk materials.

Preparation and Properties of GaAs ‐ GaP , GaAs ‐ Ge , and GaP ‐ Ge Heterojunctions

Epitaxial films of and have been deposited on , , and Ge substrates by a vapor phase chemical reaction technique. The growth variables, such as source temperature, seed temperature, crystal orientation, surface preparation, and gas flow rate, have been investigated. The optimum conditions for the growth of epitaxial films and the electrical characteristics of these films are reported. junctions grown by a solution growth technique (TSM) are abrupt, whereas junctions grown by the vapor phase technique are graded. Detailed measurements of the I–V characteristics and capacitance of , , and abrupt heterojunctions are interpreted in terms of Anderson's model. Kinks in the I–V characteristic are explained by discontinuities in the valence and conduction bands.