Infrared Optical Characterization of Thermally Oxidized CuInSe2 Single Crystals

Infrared reflectivity spectra of thermally oxidized CuInSe₂ single crystals are measured at room temperature in the wavenumber range from 180 to 4000 cm⁻¹. A Kramers-Kronig analysis of the spectra reveals seven vibrational modes with frequencies which agree with mode frequencies in In₂O₃. No vibrational modes due to Cu–O and Se–O bonds could be observed. The results obtained are compared with previous studies of oxidized CuInSe₂ crystals.     

XPS Analysis of Bridgman-grown CuInTe2 and of its Native Oxide

X-ray excited photoelectron spectroscopy is used to analyse the near-surface properties of CuInTe₂ single crystals grown by the vertical Bridgman method. It is found that keeping the crystals at room temperature in air for relatively short times is sufficient for the formation of an oxide layer which consists mainly of TeO₂. No detectable amounts of copper or indium oxides could be observed.     

Photoacoustic Spectroscopy of Defect States in Etched and Air-annealed CuInSe2 Single Crystals

Photoacoustic spectra of cleaved, polished and etched, and air-annealed n-type CuInSe₂ single crystals are measured at different frequencies between 30 and 312 Hz. The spectra related to the bulk of the crystals exhibit five structures due to defects that are also present in p-type crystals. Polishing and etching as well as subsequent air annealing at 100, 200 and 300 °C reveal rather complex changes of the defect equilibrium in the near-surface region of the crystals which include both relative concentration changes of existing defects and creation of new defects. The results for polished and etched crystals correspond to trends expected from etching induced local modifications of the composition and structure as revealed by electron spectroscopies and ion channeling. Air annealing is found to affect all existing defects and to create up to five new defects which cannot be explained in terms of the related point defect model proposed by CAHEN and NOUFI.     

Optical Properties of CuInSe2 Thin Films Prepared by R.F. Sputtering

The optical absorption of r.f. sputtered CuInSe₂ thin films was studied in the photon energy range from 1 to 3 eV. The gap energy and the spin-orbit splitting are found to be (1.01 ± 0.01) eV and (0.24 ± 0.02) eV, respectively. From the photon energy dependence of the absorption coefficient it is concluded that the heavy and light hole bands are parabolic whilst the split-off band contains terms linear in the wavevector. The optical transition probability for valence band- to- conduction band transitions is estimated to be (10.8 ± 1.0) eV which yields an admixture of copper d states to the valence band of (30 ± 8) %.     

Splines and Linear Control Theory

In this work, the relationship between splines and the linear control theory has been analyzed. We show that spline functions can be constructed naturally from the control theory. By establishing a framework based on control theory, we provide a simple and systematic way to construct splines. We have constructed the traditional spline functions including polynomial splines and the classical exponential spline. We have also discovered some new spline functions such as the combination of polynomial, exponential and trigonometric splines. The method proposed in this paper is easy to implement. Some numerical experiments are performed to investigate properties of different spline approximations.     

Zinc and copper complexes of prodigiosin: implications for copper-mediated double-strand DNA cleavage

[reaction: see text] Zinc(II) and copper(II) complexes of prodigiosin (1) have been characterized. All N-atoms of 1 bind Cu(II) to generate 5: the complex exhibits regiospecific oxidation of the C-pyrrole. In contrast, coordination by Zn(II) to 1 produces Zn(1)(2) (8), a 4-coordinate tetrahedral complex. The influence of these binding geometries on Cu-mediated double-strand (ds) DNA cleavage by 1 is discussed.     

Why isn't 'standard' heme good enough for c-type and d1-type cytochromes?

This perspective seeks to discuss why biology often modifies the fundamental iron-protoporphyrin IX moiety that is the very versatile cofactor of many heme proteins. A very common modification is the attachment of this cofactor via covalent bonds to two (or rarely one) sulfur atoms of cysteine residue side chains. This modification results in c-type cytochromes, which have diverse structures and functions. The covalent bonds are made in different ways depending on the cell type. There is little understanding of the reasons for this complexity in assembly routes but proposals for the rationale behind the covalent modification are presented. In contrast to the widespread c-type cytochromes, the d1 heme is restricted to a single enzyme, the cytochrome cd1 nitrite reductase that catalyses the one-electron reduction of nitrite to nitric oxide. This is an extensively derivatised heme; a comparison is drawn with another type of respiratory nitrite reductase in which the active site is a c-type heme, but the product ammonia.     

On-line sample clean-up and chromatography coupled with electrospray ionization mass spectrometry to characterize the primary sequence and disulfide bond content of recombinant calcium binding proteins

We have used on-line sample clean-up, concentration, and chromatography with electrospray ionization mass spectrometry (ESI-MS), to characterize and determine the presence of disulfide bonds in recombinant full-length rat brain calbindin D28K and two deletion mutants of the protein, one lacking EF-hand 2 (calbindin delta 2) and the other lacking EF-hands 2 and 6 (calbindin delta 2,6). The molecular weights of the expressed proteins dissolved in biological buffers were determined with high accuracy using a low-flow, pressurized chamber infusion system, that allows on-line protein clean-up by removing buffers/salts incompatible with ESI-MS. The molecular weight determinations showed that the amino-terminal methionine residues had been cleaved during the expression and isolation of the recombinant proteins. Approximately 85-90% of the protein sequences were confirmed by on-line HPLC-ESI-MS analysis of peptides generated by a lysyl endoproteinase C digestion. Comparisons of ESI-MS spectra of native and reduced calbindin D28K and delta 2 show that the full length- and delta 2 mutant-protein contain one disulfide bond. Molecular mass determinations of calbindin delta 2,6 showed that this protein contains a highly active cysteine residue that covalently binds a mercaptoethanol group, or forms a homodimer via a disulfide bond. The results show surprising differences amongst the deletion mutants of calbindin D28K with respect to the formation of disulfide bonds. These differences are not readily detected by other techniques and show that ESI-MS is a powerful, rapid method by which to detect disulfide linkages for intact proteins.     

Polymerization of vinyl stearate multilayers

A study was made of the solid-state polymerization of built-up monomolecular layers of vinyl stearate. Polymerization was initiated by exposing the samples to ⁶⁰Co γ-radiation. The reactions were followed by multiple internal reflection infrared spectroscopy. The structure of the multilayer monomer and polymer was studied by x-ray diffraction. Surface properties of the multilayers were studied by contact-angle measurements. Results showed that, under the proper conditions, monomer multilayers of vinyl stearate could be polymerized to virtually 100% completion. Reaction was favored by higher irradiation temperatures, larger doses of radiation, and by higher reaction temperatures. The x-ray diffraction photographs indicated that both the monomer and polymer multilayers possessed some single crystal character. Contact-angle measurements indicated that the molecules in both the monomer and polymer multilayers were closely packed.