Se Composition Research Articles

Electrical properties of semi-insulating CdTe 0:9Se 0:1:Cl crystal and its surface preparation

CdTe 0.9Se 0.1:Cl crystals doped with chlorine at 5×10 17 cm −3 level were grown by the vertical Bridgman method. The composition of Se throughout the ingot was nearly constant at x=0.110±0.016. The electrical resistivity of CdTeSe:Cl was 4.5×10 9 Ω cm. Chemical etchants were employed to obtain stoichiometric and flat surfaces for electrode deposition, and the effects of the etchants on CdTeSe surfaces were analyzed by photoluminescence (PL) and AFM with different bromine concentrations and etching times. The mobility-lifetime products of electron and hole in CdTeSe:Cl crystals were of the order of ∼10 −2 cm 2/V and its values are greater than those for CdTe crystals. The energy resolution of a CdTeSe:Cl detector was tested using a 241Am radioactive source.

Activity of selenium modified ruthenium-electrodes and determination of the real surface area

The determination of the surface area of Pt and Ru electrocatalyst surfaces by oxidation of adsorbed CO and by oxidation of a Cu upd layer are compared. The amount of adsorbed CO was determined mass-spectrometrically from the ionic current for CO2 formation during an oxidative potential sweep. On Ru, the Faradaic charge is too large (by approx. 55%) due to Faradaic effects (oxygen adsorption). For massive Ru electrodes a Cu upd charge of 520 μC cm−2 is found after normalization to the area determined by CO oxidation. Using this value, both methods yield identical surface areas for nanoparticulate Ru catalysts. On Ru surfaces (both massive and nanoparticulate) completely covered by Se the amount of Cu upd charge decreases to one fourth of the value observed for pure Ru. Since CO is only adsorbed on free Ru sites and not on Se covered sites, the oxidation charge for the latter can be used to determine the number of free Ru sites, whereas the decrease of the Cu upd charge on Se modified surfaces can be used to calculate the area which is modified by Se. This method, previously tested on the model electrodes, was extended to Ru nanoparticle and Ru/Se electrodes. Using this surface determination it is possible to draw conclusions about the active surface area and the Se composition of the outer shell of Ru/Se nanoparticles.

Superconductors in confined geometries

ABSTRACTThe synthesis of nanoscale superconductors with controlled geometries is extremely challenging. In this paper we present results on synthesis and characterization of one-dimensional (1D) NbSe2 superconducting nanowires/nanoribbons. Our synthesis approach includes the synthesis of 1D NbSe3 nanostructure precursors followed by nondestructive and controlled adjustment of the Se composition to formulate NbSe2. The morphology, composition and crystallinity of the synthesized 1D NbSe2 nanostructures were analyzed with scanning electron microscopy, x-ray diffraction and transmission electron microscopy. Transport measurements were carried out to explore the electronic properties of these confined superconducting nanostructures.

A solid-state hydroxide ion selective electrode for the measurement of high pH values

A solid-state ion selective electrode, which is sensitive to hydroxide ion concentration has been prepared. This electrode with a 10% Cu 2 S and 90% Ag 2 Se composition exhibited an analytically useful potential change from 10 −6 to 10 −1 M hydroxide ion, pH 8–13. The slope of linear portion is about 58 mV per decade change in hydroxide concentration. There was no interference of K + , Na + , Ca 2+ , Zn 2+ and Mg 2+ cations and Cl − , NO 3 − and SO 4 2− anions. The response time was 30 s and the lifetime was about 2 years.

CdSe quantum dots grown on ZnSe and Zn0.97Be0.03Se by molecular-beam epitaxy: Optical studies

We report detailed studies of the photoluminescence (PL) properties of CdSe quantum dots (QDs) grown on ZnSe and Zn0.97Be0.03Se by molecular-beam epitaxy. We performed steady-state and time-resolved PL measurements and observed that nonradiative processes dominate at room temperature (RT) in the CdSe/Zn0.97Be0.03Se QD structures while these nonradiative processes do not dominate in the CdSe/ZnSe QD structures up to RT. We developed a method to estimate the capped CdSe/Zn0.97Be0.03Se QD size and composition, based on PL and photoluminescence excitation as well as Raman scattering spectroscopy measurements. Assuming spherical QDs, we estimate the QD radii to be in the range of 2.5 nm to 4.0 nm with the Cd concentration in the range of 47%–54%. This size is smaller than the exciton Bohr radius, suggesting a nearly zero-dimensional character. We also performed contactless electroreflectance measurements on the CdSe/ZnSe QDs, and observed the transitions due to the QDs and the wetting layer. In this case, assuming lens-shaped QDs, we extracted the QD radius to be around 9.52 nm, the height about 3.24 nm. This size is larger than the exciton Bohr radius, indicating a quasi-two-dimensional character. Studies based on Raman scattering spectroscopy suggest that the Cd concentration is about 44% for this structure. The temperature dependences of the PL lifetimes are consistent with the results of the QD size and Cd concentration estimated by these two methods.

Mass-Dependent Fractionation of Selenium and Chromium Isotopes in Low-Temperature Environments

Selenium (masses 74, 76, 77, 78, 80, and 82; Table 1⇓) and chromium (masses 50, 52, 53 54; Table 1⇓) are treated together in this chapter because of their geochemical similarities and similar isotope systematics. Both of these elements are important contaminants in surface and ground water. They are redox-active and their mobility and environmental impact depend strongly on valence state and redox transformations. Isotope ratio shifts occur primarily during oxyanion reduction reactions, and the isotope ratios should serve as indicators of those reactions. In addition to environmental applications, we expect that there will be geological applications for Se and Cr isotope measurements. The redox properties of Se and Cr make them promising candidates as recorders of marine chemistry and paleoredox conditions. View this table: Table 1. Compositions of natural Se and Cr and currently used double spikes (atom %). There are only about a dozen published studies on Se isotopes and only two on Cr isotopes. This chapter summarizes what has been learned thus far, and almost all of this work concentrates on aqueous reactions at earth surface temperatures. It also attempts to provide some geochemical background and reviews some relevant points from the sulfur isotope literature, which provides insight into the isotopic systematics of Se and Cr. ### Se geochemistry Se is chemically similar to sulfur, which is immediately above it in the periodic table. Its concentration in the earth’s crust is small, with most rocks containing less than 0.1 ppm Se except for shales, which span a wide range of concentration and average roughly 1ppm (Faure 1991). Coal is also relatively rich in Se, averaging 3 ppm (Cooper et al. 1974). Se can substitute extensively for S in pyrite (Coleman and Delevaux 1957). However, Se concentrations of many sulfide minerals are quite small, suggesting strong decoupling of Se from S in some systems. …

Epitaxy and structural characterization of ZnSeTe layers grown on InP substrates

ZnSeTe epilayers were grown on InP substrates with and without ZnCdSe buffer layers. Structural X-ray and Raman characterization results indicate a large range of Se composition in the epilayers. The introduction of a buffer layer and the control of the growth conditions improve the structure by limiting compositional variations close to the stoichiometric value where lattice matching conditions are met. Effects of plastic strain relaxation are thereby minimized.

Natural variations of Se isotopic composition determined by hydride generation multiple collector inductively coupled plasma mass spectrometry

Multiple-collector inductively coupled plasma mass spectrometry has been used for the precise measurement of the isotopic composition of Se in geological samples. Se is chemically purified before analysis by using cotton impregnated with thioglycollic acid. This preconcentration step is required for the removal of matrix-interfering elements for hydride generation, such as transitional metals, and also for the quantitative separation of other hydride-forming elements, such as Ge, Sb, and As. The analyte is introduced in the plasma torch with a continuous-flow hydride generation system. Instrumental mass fractionation is corrected with a “standard-sample bracketing” approach. By use of this new technique, the minimum Se required per analysis is lowered to 10 ng, which is one order of magnitude less than the amount needed for the N-TIMS technique. The estimated external precision calculated for the 82Se/ 76Se isotope ratio is 0.25‰ (2σ), and the data are reported as delta notation (‰) relative to our internal standard (MERCK elemental standard solution). Measurements of Se isotopes are presented for samples of standard solutions and geological reference materials, such as silicate rocks, soils, and sediments. The Se isotopic composition of selected terrestrial and extraterrestrial materials are also presented. An overall Se isotope variation of 8‰ has been observed, suggesting that Se isotopes fractionate readily and are extremely useful tracers of natural processes.

Composition control of CdSeTe layers grown by molecular beam epitaxy

CdSeTe layers were grown on GaAs(1 0 0) substrates by molecular beam epitaxy. The beam-intensity ratios of Se to Te atoms were changed to control the composition of the CdSeTe mixed crystals. The Te atoms were more readily incorporated into the epilayers than the Se atoms under the competition for incorporation of Se and Te atoms. The J VI/ J II beam-intensity ratio dependence of the Se composition was studied. The Se composition increased with decrease in J VI/ J II ratio. At a J VI/ J II ratio of 0.05 the beam-intensity ratio of Se to Se+Te coincided with the Se composition. This result indicates that the Se and Te atoms are incorporated without competition on the Cd stabilized surface. We considered the behavior of physisorbed atoms and the heat of formation of the compounds and discussed the situation of the growing surface of the CdSeTe epilayers, comparing it with the reported results of ZnSeTe epilayers.

Bandgap Engineering of ZnO Using Se

We have grown the compound semiconductor ZnO 1-x Se x by MBE. A decrease in bandgap energy with increasing Se composition x was observed and a large bowing parameter of 12.7 ± 1.6 eV was measured. This indicates the possibility of bandgap bowing in the ZnO 1-x Se x compound system and may open up new bandgap control techniques that allow bandgap changes to be made with small lattice mismatch.

DC electrochemical deposition of CdSe nanorods array using porous anodic aluminum oxide template

CdSe nanorods array were prepared using direct current potentiostatic deposition in porous anodic aluminum oxide template from aqueous CdSO 4 and H 2SeO 2 at room temperature (298 K). Electron microscope results show that these CdSe nanorods have a uniform length and diameter. X-ray energy dispersion analysis indicates that the atomic composition of Cd and Se is very close to a 1:1 stoichiometry. X-ray diffraction and electron diffraction pattern investigations demonstrate that CdSe nanorods are a uniform cubic CdSe crystal.

Electrochemical Preparation of CdSe Nanowire Arrays

We have successfully prepared CdSe nanowire arrays by direct-current electrodeposition in porous anodic aluminum oxide templates from a dimethyl sulfoxide solution containing CdCl2 and elemental Se. Electron microscopy results show that the length, diameter, and direction of growth of the nanowires are quite uniform. X-ray energy dispersion analysis indicates that the atomic composition of Cd and Se is very close to a 1:1 stoichiometry. Furthermore, X-ray diffraction and high-resolution electron microscope investigations demonstrate that the CdSe nanowires are a uniform hexagonal CdSe crystal.

Photoresponse studies of ZnSSe visible–blind ultraviolet detectors: A comparison to ZnSTe detectors

This work focuses on the investigation of the difference between the photoresponse of ZnS, ZnSSe, and that of ZnSTe Schottky-barrier photodiodes, with a particular aim to reveal the underlying causes of the gradual turn-on characteristic of low-Te-containing ZnSTe Schottky barrier photodiodes. To form the bottom electrode layer for the newly developed ZnSSe diode, n-type doping of ZnSSe by incorporating Al flux during molecular beam epitaxial growth was studied. Excellent-to-good dopant activation is achieved for Se composition up to 50%. The measured photoresponse of the diodes clearly indicates that the Te isoelectronic trapping effect is responsible for the gradual turn-on characteristic of low-Te-containing ZnSTe Schottky-barrier photodiodes. The results also reveal that the ZnSSe diode, having a much better visible rejection power, is a more suitable choice for high-performance visible–blind ultraviolet detection applications.

Crystal structure and band-edge transitions of ReS 2− xSe x layered compounds

Single crystals of the ReS 2− x Se x solid solution series have been grown by the chemical vapor transport method using Br 2 as a transport agent. From analyzing the X-ray patterns, the series crystals are determined to be single-phase and crystallized in the triclinic layered structure. The lattice parameters of the compound series are evaluated and discussed. The optical absorption edge was measured on basal-plane at room temperature. The results reveal that ReS 2− x Se x are indirect semiconductors and their energy gaps are determined. The band gap energy varies smoothly with the Se composition x, indicating that the nature of band edges are similar for the end members ReS 2 and ReSe 2, and the compounds of intermediate compositions.

Piezoreflectance study of band-edge excitons ofReS2−xSexsingle crystals

The temperature dependence of the spectral features in the vicinity of the direct band edge of ${\mathrm{ReS}}_{2\ensuremath{-}x}{\mathrm{Se}}_{x}$ single crystals is measured over a temperature range of 25--300 K using piezoreflectance (PzR). From a detailed line-shape fit of the PzR spectra, the temperature dependence of the energies and broadening parameters of the band-edge excitons are determined accurately. The excitonic transition energies at different temperature vary smoothly with the Se composition x, indicating that the natures of the direct band edges of ${\mathrm{ReS}}_{2\ensuremath{-}x}{\mathrm{Se}}_{x}$ are similar. The parameters that describe the temperature variation of the energies and broadening function of the excitonic transitions are evaluated and discussed.

Ionic Conduction and Electronic Properties in Liquid In–Se System

Electronic and ionic conductivities and thermoelectric power of liquid In 1- x –Se x system have been measured as a function of concentration and temperature. The electronic conductivity in the system shows a deep minimum near the Se composition x =0.58 and the ionic conductivity has a broad maximum at this composition, where the thermoelectric power shows a large negative maximum. The electronic conductivity and thermoelectric power were analyzed in terms of the Fermi-Dirac integrals. It is found that the shape of the density of state, N ( E ), changes gradually with concentration.

Mass spectrometry quantification of hydrogen produced under illumination of p-CuInSe 2 and modified surfaces

Hydrogen gas obtained from illuminated p-CuInSe 2 (CIS) on 0.5 M sulfuric acid solution was collected in a glass balloon using argon as carrier gas, and quantified with a mass spectrometer. p-CuInSe 2 thin films were electrodeposited on SnO 2-coated glass plates, according to references already reported. Electrochemical deposition of Se and chemical modifications on the semiconducting surfaces with adsorbed ruthenium were also tested for the hydrogen evolution reaction. The E g and E fb of the modified p-CuInSe 2 were determined. Atomic absorption spectrometry was used to quantify the chemical compositions of the semiconductors before and after the illumination. The modified semiconductors had a composition of CIS Se 24 and CIS/Se 6/(RuO 4) 0.12, under illumination at 0.8 V in 0.5 M H 2SO 4 for 4 min, the volumes of evolved hydrogen were 1 cm 3 and 2.4 cm 3, respectively.

Site Occupation Preferences in CdMnTeSe Quaternary Alloys. EXAFS Data Analysis

Site occupation preferences in zinc-blende Cd 1-x Mn x Te 1-y Se y (0.0 ≤ x ≤ 0.15; 0.0 ≤ y ≤ 0.20) quaternary alloys have been determined by the Extended X-ray Absorption Fine Structure (EXAFS) analysis. The experimental data have been collected with the use of synchrotron radiation for Se- and Mn-K edges. For both K edges, the preferences of manganese distribution around selenium have been observed and analyzed. The Mn-Se with Cd-Se, and Mn-Se with Mn-Te distances have been found as linearly dependent on concentration of Se and Mn. The average coordination numbers of Mn around Se versus Mn composition was approximated by the quadratic behavior instead of the linear dependence expected for the virtual crystal approximation, in which a full random distribution of elements in the sublattices is assumed. The obtained results support the preferential occurring of Mn and Se ions in the considered compounds. A structural and magnetic interaction reasons of the preferences were discussed.

Effects of diet and chemical form of selenium on selenium metabolism in sheep.

The effects of diet composition and chemical form of Se on intestinal flow, absorption, and retention of Se were determined in sheep by the balance technique and by disappearance of Se from sites along the gastrointestinal tract with reference to dual-phase digesta markers. Six sheep with ruminal and duodenal cannulas were used in a crossover design with a split-plot arrangement of the Se isotope treatments. Sheep were fed a forage (alfalfa hay)-based (.37 mg Se/kg) or concentrate (barley)-based (.27 mg Se/kg) diet at 90% of ad libitum intake. Selenium stable isotopes (enriched [77Se]yeast, enriched [82Se]selenite) and fluid (Co-EDTA) and particulate (Cr-mordanted fiber) markers were administered simultaneously into the rumen four times daily for 7 d, and total collections of feces and urine were made every 24 h for these and the following 7 d. A larger proportion (51 to 61%) of the Se tracers flowing to the duodenum was associated with the particulate fraction, mainly as bacteria-associated Se, than with the fluid fraction. The [82Se]selenite was more available (P < .05) for absorption and retention than [77Se]yeast, indicating that inorganic chemical forms of Se are as available to the ruminant as organic forms of Se commonly found in feedstuffs. Selenium absorption and retention were greater (P < .05) in sheep receiving the concentrate-based diet than in sheep receiving the forage-based diet. Thus, the availability of Se from inorganic and organic sources in sheep seems to be influenced by diet composition.

Absorption and Raman spectra of Se 8-ring clusters in zeolite 5A

Samples with different weight ratio of Se to zeolite 5A (Se composition) have been prepared by loading Se into the cages of zeolite 5A and the measurements of the absorption and Raman spectra have been carried out for the prepared samples. The measured absorption edges of the samples are close and blue shifted to the value for monoclinic Se containing Se 8-ring, suggesting the formation of Se 8-ring clusters in the cages. The continuous and broadening features of the absorption spectra are interpreted by the strong electron-phonon coupling in Se 8-ring clusters. The sample with high Se composition has a red shift of the absorption band edge relative to the samples with less Se composition. It is tentatively attributed to the reason that with different Se composition, single Se 8-ring clusters and double Se 8-ring clusters are formed in the cages of zeolite 5A. A single broad band at about 262 cm −1 is observed in the Raman spectra, that gives the further support of the formation of Se 8-ring clusters.