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We apply soft x-ray emission spectroscopy (XES) to measure the electronic structure of crystalline silicon supersaturated with sulfur (up to 0.7 at. %), a candidate intermediate-band solar cell material. Si L2,3 emission features are observed above the conventional Si valence band maximum, with intensity scaling linearly with S concentration. The lineshape of the S-induced features change across the insulator-to-metal transition, indicating a significant modification of the local electronic structure concurrent with the change in macroscopic electronic behavior. The relationship between the Si L2,3 XES spectral features and the anomalously high sub-band gap infrared absorption is discussed.
In view of the complexity of thin-film solar cells, which are comprised of a multitude of layers, interfaces, surfaces, elements, impurities, etc., it is crucial to characterize and understand the
chemical and electronic structure of these components. Because of the high complexity of the Cu2ZnSn(S,Se)4 compound semiconductor absorber material alone, this is particularly true for kesterite-based devices. Hence, this paper reviews our recent progress in the characterization of Cu2ZnSnS4 (CZTS) thin films. It is demonstrated that a combination of different soft x-ray spectroscopies is an extraordinarily powerful method for illuminating the chemical and electronic material characteristics from many different perspectives, ultimately resulting in a comprehensive picture of these properties. The focus of the article will be on secondary impurity phases, electronic structure, native oxidation, and the CZTS surface composition.
Capacitive approach to determine phospholipase A2 activity towards artificial and natural substrates
(1998)
A capacitive approach has been employed to develop a novel method to determine phospholipase activity. The sensing electrodes have a structure like Au/S(CH₂)₁₇CH₃/substrate/electrolyte. Hydrolysis of the substrate, mediated by phospholipase A₂, leads to the formation of water-soluble products from the insoluble substrate. This results in desorption of these products into aqueous phase and corresponding increase of the electrode capacitance. The requirement of high water solubility of the reaction products can be achieved in two ways. In the first, short-chain phospholipids are used as the substrate, in which case, water-soluble products are formed and no additional reagents are required to promote desorption of these products. The sensors prepared by this strategy provide sensitive qualitative detection of phospholipases. The second way is based on the use of a water-soluble acceptor (for example, β-cyclodextrin) to solubilize the products of hydrolysis. It allows semiquantitative detection of phospholipase activity toward long-chain natural substrates. The reaction kinetics for this case was found to be monoexponential and linearly dependent on the phospholipase concentration. The detection limit of this method, as tested with phospholipase A₂ from bee venom and soy bean lecithin as the substrate, is ∼0.5 ng/mL (500 μunits/mL).
Nonspecific adsorption of surfactants on polycrystalline gold electrodes, covered by a chemically adsorbed monomolecular layer of octadecanethiol, was studied by means of capacitive measurements. Homologous series of fatty acids (from decane to tetradecane acids) and dodecanepyridinium chloride were used as surfactants. The behavior of the electrode was completely different in the first and subsequent cycles of adsorption/desorption. In the first adsorption cycle, an initial capacitance jump at low concentrations (<10 μM) occurs, while later the curves show the usual saturation behavior. The first surfactant layer could not be completely desorbed; only the electrode capacitance shown after the jump was reached. Subsequent cycles do not reveal any capacitance jumps and were completely reversible. Adsorption curves in the series of monoalkyl acids are shifted according to the Traube rule. A comparison with the surfactant adsorption at the air/water interface has been done.
The preparation of the capacitive biosensor for monitoring the activity of enzymes (lipases, phospholipases, etc.) hydrolysing waterinsoluble substrates into water-soluble products is described. The sensor is based on a sandwich-like structure: Au/S(CH₂)₁₇CH₁/substrate/electrolyte. Hydrolysis of the substrate leads to the formation of water-soluble products and desorption of these compounds from the electrode. When the product formation is the rate-limiting step of this process, the desorption rate is determined by the enzyme activity. The desorption can be easily monitored as an increase of the electrode capacitance. By following this principle it was possible to develop a sensor monitoring phospholipase A₂ activity even at the 50 pg ml-1 level.
Laminar flame speeds of dimethyl ether and dimethoxymethane at pressures from 1 to 5 bar and initial temperatures from 298 to 373 K were determined experimentally using a constant volume spherical vessel and a heat flux burner setup. This study is the first to report dimethoxymethane laminar flame speeds at a pressure higher than 1 bar. Using these experimental data along with data available in the literature, a new kinetic model for the prediction of the oxidation behavior of dimethyl ether and dimethoxymethane in freely propagating and burner stabilized premixed flames, in shock tubes, rapid compression machines, flow reactors, and a jet-stirred reactor has been developed. The experimental results from the present work and literature are interpreted with the help of the derived kinetic model. This newly developed reaction mechanism considers the redox chemistry of NOx to accommodate the influence of the oxygen level on the onset of fuel conversion and interconversion of NO and NO2. The current model suggests that an increased O2 level promotes the HO2 production, which in turn leads to the formation of OH radicals, which promotes the combustion of the fuel/air mixture under lean conditions. The increase of OH radical concentrations is mainly via the NO/NO2 interconversion reaction channel, NO+HO2=NO2+OH, NO2+H=NO+OH, CH3OCH3+NO2=CH3OCH2+HONO, followed by the thermal decomposition of HONO. This work extends the kinetic database and helps to improve the understanding of dimethyl ether and dimethoxymethane combustion behavior. The kinetic model presented in this work can serve as a base model for hydrocarbons and oxygenated fuels higher than C2.