Direct observation of defect levels in InN by soft X-ray absorption spectroscopy

We have used synchrotron-based near-edge X-ray absorption fine structure (NEXAFS) spectroscopy to study the electronic structure of nitrogen-related defects in InN(0001). Several defect levels within the band gap or the conduction band of InN were clearly resolved in NEXAFS spectra around the nitrogen K-edge. We attribute the level observed at 0.3 eV below the conduction band minimum (CBM) to interstitial nitrogen, the level at 1.7 eV above the CBM to antisite nitrogen, and a sharp resonance at 3.2 eV above the CBM to molecular nitrogen, in full agreement with theoretical simulations.     

Direct electrodeposition of PbTe thin films on n-type silicon

High quality lead telluride thin films were directly deposited onto n-type silicon (1 0 0) substrates by electrodeposition at room temperature. The deposition mechanism was studied using cyclic voltammetry. The films were characterized by scanning electron microscopy, energy dispersive X-ray, X-ray diffraction, and Fourier transform infrared spectroscopy. The results indicated that the deposited PbTe films exhibited a polycrystalline rock salt structure and good optical properties with a direct band gap of 0.31 eV.     

Research on optical band gap of the novel GeSe2–In2Se3–KI chalcohalide glasses

The glass-forming region of the GeSe₂–In₂Se₃–KI system was reported firstly. The dependence of physical, thermal and optical properties on compositions as formula of (1 ? x)(0.8GeSe₂–0.2In₂Se₃)–xKI (x = 0, 0.1, 0.2, 0.3) chalcohalide glasses was investigated. The allowed direct transition and indirect transition, and Urbach energy of samples were calculated according to the classical Tauc equation. The results show that the glass system has good thermal stability and that there is an obvious blue-shift at the visible absorbing cutting-off edge. When the dissolved amount of KI increased from 0 to 30 mol%, the direct optical band gap and the indirect optical band gap were in the range from 1.617 to 1.893 eV and 1.573 to 1.857 eV. With the decrease of the molar refraction the refractive index decreases, optical band gap and metallization criterion increase. The relationship between energy band gap and metallization criterion was analyzed and the optical properties of chalcohalide glasses were summarized.     

Preparation and photocatalytic activity of visible light-active sulfur and nitrogen co-doped SrTiO3

Nitrogen and sulfur co-doped SrTiO₃ was prepared by high energy grinding of the mixture of SrTiO₃ and thiourea. A new band gap in visible light region (522 nm) corresponding to 2.37 eV could be formed by the co-doping. The photocatalytic activity for nitrogen monoxide oxidation of SrTiO₃ in visible light region especially in the long wavelength range (λ > 510 nm) could be improved greatly. Under the irradiation of light with wavelength larger than 510 nm, the photocatalytic activity of nitrogen and sulfur co-doped SrTiO₃ was 10.9 times greater than that of pure SrTiO₃. The high visible light photocatalytic activity of this substance may be due to the formation of a new band gap that enables to absorb visible light effectively.     

Investigation of supported nickel catalysts by X-ray absorption spectrometry and X-ray diffraction using synchrotron radiation

In a previous paper a study of the supported nickel catalysts based on extended X-ray absorption fine structure spectroscopy was presented for analysis of the first coordination shell. The present study evidenced a strong deformation of the local structure of the metal due to its interaction with oxide support. The average particle size, microstrains and probability of faults, the particle size distribution function of supported Ni catalysts were determined by X-ray diffraction method. The method is based on Fourier analysis of experimental X-ray line profile (1 1 1), (2 0 0) and (2 2 0). The global structure is obtained with a fitting method based on the generalized Fermi function facilities for approximation. A chemisorption model was elaborated by correlation of the local and global structure connected with the specific surface areas. The results obtained on supported Ni catalysts which are used in H/D isotopic exchange reactions are reported. Both types of measurements were performed on the Beijing synchrotron radiation facilities.     

Radioluminescence in ZnO

ZnO powders were obtained by proteic sol–gel process using zinc sulfate as the precursor. They have an energy gap of 3.3 eV and display a radioluminescence signal centered in 550 nm, which corresponds to 2.6 eV. This signal has appeared due to defects in the gap (energy sublevels) caused by the distortions in its crystalline structure, which result from the loss of sulfur at high temperature. These powders were characterized by radioluminescence, XRD, SEM, EDS and DTA/TG.     

Structural and optical properties of sprayed Zn1−xMnxO films

In recent years the dilute magnetic semiconductors have received much attention due to the complementary properties of semiconductor and ferromagnetic behaviour. Zn_1−xMn_xO thin films have been synthesized by chemical spray pyrolysis at a substrate temperature of 400 °C with different manganese compositions that vary in the range, 0.0 ≤ x ≤ 0.25, on Corning 7059 glass substrates. The X-ray diffraction studies revealed that all the films were strongly oriented along the (002) orientation corresponding to the hexagonal wurtzite structure. The crystalline quality of the layers was found to decrease with the increase of x, however, no structural changes were observed over the ‘Mn’ composition range investigated. The optical absorption studies revealed that the energy band gap of the films followed the Vegard's law. The optical band gap of the films prepared at x = 0.15 was found to be ∼3.35 eV. The photoluminescence characteristics of Zn_1−xMn_xO films showed an emission peak at around 390 nm with a broad band about 530 nm. The details of these results were reported and discussed.     

Visible light active hydrogen modified (HM)-n-TiO2 thin films for photoelectrochemical splitting of water

Visible light active hydrogen modified n-type titanium oxide (HM-n-TiO₂) thin films were synthesized by thermal oxidation of Ti metal sheet (Alfa Co. 0.25 mm thick) in an electric oven followed by incorporation of hydrogen electrochemically under cathodic polarization at ?1.6 V vs Pt. The photoresponse of the HM-n-TiO₂ was evaluated by measuring the rate of water splitting reaction to hydrogen and oxygen in terms of photocurrent density, J_p. The optimized electric oven-made n-TiO₂ and HM-n-TiO₂ photoelectrodes showed photocurrent densities of 0.2 mA cm^?2 and 1.60 mA cm^?2, respectively, at a measured potential of ?0.4 V vs Pt at illumination intensity of 100 mW cm^?2 from a 150 W xenon lamp. This indicated an eightfold increase in photocurrent density for HM-n-TiO₂ compared to oven-made n-TiO₂ at the same measured electrode potential. The band-gap energy of HM-n-TiO₂ was found to be 2.7 eV compared to 2.82 eV for electric oven-made n-TiO₂ and a mid-gap band at 1.67 eV above the valence band was also observed. The HM-n-TiO₂ thin film photoelectrodes were characterized using photocurrent density under monochromatic light illumination and UV–Vis spectral measurements.     

Ab initio and XPS studies of pyrite (1 0 0) surface states

Ab initio quantum chemical modelling (GGA, CASTEP and B3LYP, CRYSTAL03) is used to predict differences in electronic structure between the (1 0 0) surface and bulk of pyrite. Experimental X-ray photoelectron spectroscopic (XPS) data for the S 2p core lines show the presence of two types of S surface states: surface S²⁻ monomers at a S 2p_3/2 binding energy (BE) of 161.2 eV, and (S–S)²⁻ surface dimer states at a S 2p_3/2 BE of 162.0 eV, compared to the S 2p_3/2 BE of bulk pyrite at 162.7 eV. The Fe 2p surface XPS displays several multiplets (implying high spin configuration) at higher BE than the bulk Fe 2p signal, which can be ascribed to surface state contributions. The quantum chemical simulation predicts an S 2p core level shift of 0.69 eV between the S bulk and S surface dimers, in good agreement with the 0.6 eV found in XPS measurements. A Mulliken population analysis confirms the conjectured charge distribution on the surface, which leads to the two different S surface states, as well as the surface high spin configuration responsible for the high BE Fe multiplets. Evidence for surface Fe²⁺ and Fe³⁺ surface states can be seen in the Fe projected valence band density of states, confirming the interpretation of the photoemission spectra.     

High pressure Raman spectroscopic study of phase transformation in TaO2F

High pressure Raman spectroscopic measurements on nearly zero thermal expansion material TaO₂F are carried out up to 19 GPa. Earlier report of high pressure X-ray diffraction studies shows two phase transitions, one at 0.7 and the other at 4 GPa with rhombohedral (R-3c) structure above 4 GPa, but the structure between 0.7 GPa and 4 GPa remained unclear. In high pressure Raman measurements, a reversible, cubic to rhombohedral phase transformation onsets around 0.8 GPa and gets completed at 4.4 GPa with all four predicted normal modes corresponding to R-3c phase and retaining the structure up to 19 GPa. A mixture of cubic and rhombohedral phases is observed between 0.8 and 4.4 GPa. Optically silent modes in the ambient cubic structure exhibit strong, broad Raman bands due to anionic (O/F) disorder in TaO₂F altering the local symmetry and allowing for first order Raman scattering. On compression, these disorder induced first order Raman bands gradually decrease in intensity and disappear around 4.4 GPa due to inhibition of local distortion caused by anions, and the modes corresponding to the rhombohedral phase appear. This is a clear evidence of disorder-free rhombohedral single phase exists above 4.4 GPa in agreement with the reported HPXRD results. Temperature dependent Raman measurements reveal that the intensities of Raman bands remain almost unchanged with rise in temperature indicating static disorder in TaO₂F. Disorder-induced first order Raman modes at 176, 212, 381 and 485 cm⁻¹ soften with increase in pressure whereas the other modes show low positive Gruneisen parameter. The thermal expansion coefficient calculated using these Gruneisen parameters (−2.91 ppm K⁻¹) is in fair agreement with the reported values (−1 to +1 ppm K⁻¹). On the other hand, all four modes of disorder-free rhombohedral phase show the usual hardening behavior with increase in pressure contributing to positive thermal expansion.     

A study on solution deposited CuSCN thin films: Structural,electrochemical, optical properties

A cost-effective successive ionic layer adsorption and reaction (SILAR) method was used to deposit copper (I) thiocyanate (CuSCN) thin films on glass and steel substrates for this study. The deposited thin films were characterized for their structural, morphological, optical and electrochemical properties using X-ray diffraction (XRD), scanning electron microscopy (SEM), UV–visible spectroscopy and VersaSTAT potentiostat. A direct band gap of 3.88 eV and 3.6 eV with film thickness of 0.7 μm and 0.9 μm was obtained at 20 and 30 deposition cycles respectively. The band gap, microstrain, dislocation density and crystal size were observed to be thickness dependent. The specific capacitance of the CuSCN thin film electrode at 20 mV/s was 760 F g⁻¹ for deposition 20 cycles and 729 F g⁻¹ for deposition 30 cycles.     

Investigation of structural,optical and luminescent properties of sprayed N-doped zinc oxide thin films

N-doped ZnO (NZO) thin films are synthesized via spray pyrolysis technique in aqueous medium treating zinc acetate and N,N-dimethylformamide as precursors. Influence of N doping on structural, optical and luminescence properties have been investigated. Films are nanocrystalline having hexagonal crystal structure. Raman analysis depicts an existence of NZnO structure in NZO thin film. XPS spectrum of N 1s shows the 400 eV peak terminally bonded, well screened molecular nitrogen (γ-N₂). Lowest direct band gap of 3.17 eV has been observed for 10 at% NZO thin film. The UV, blue, and green deep-level emissions in photoluminescence of NZO films are due to Zn interstitials and O vacancies.     

Understanding the nature of vanadium species supported on activated carbon and its catalytic properties in the aerobic oxidation of aromatic alcohols

Vanadium oxide catalysts supported on activated carbon (V/AC) with V loadings ranging from 1 to 20 wt.% were prepared by a wet-impregnation method. Various physicochemical characterization techniques, including nitrogen physisorption, X-ray diffraction (XRD), Raman spectroscopy, X-ray absorption (XANES and EXAFS), X-ray photoelectron spectroscopy (XPS), and electron spin resonance (ESR), were employed to understand the nature of vanadium species on activated carbon. The results revealed that vanadium oxide mainly existed in a highly dispersed state for 10 wt.% or less vanadium loadings; a large amount of vanadium resulted in aggregated microcrystalline phase. Vanadium species on activated carbon surface showed a similar local coordination structure to that of NH₄VO₃ with a distorted tetrahedral symmetry at low vanadium loadings, whereas octahedral coordination was dominant at high vanadium loadings (>10 wt.%). All V/AC samples showed V⁵⁺ as the major oxidation state. Nevertheless, V⁴⁺ centered in a distorted tetrahedral symmetry could be detected at a vanadium loading greater than 4 wt.%. The catalytic activity for the benzyl alcohol oxidation largely depended on the dispersion, oxidation state, and local coordination of vanadium oxides on activated carbon. Highly dispersed vanadium (5+) species with a distorted tetrahedral coordination were postulated to account for the excellent catalytic performances of V/AC catalysts (TOF = 39.1 h^?1).     

Synthesis,crystal structure and luminescent properties of pyrovanadates A2CaV2O7 (A = Rb,Cs)

The novel vanadium oxides Rb₂CaV₂O₇ and Cs₂CaV₂O₇ have been prepared by solid-state reaction and their crystal structures determined and refined using X-ray, neutron powder and electron diffraction data. Rb₂CaV₂O₇ and Cs₂CaV₂O₇ are isostructural, crystallizing in space group P2₁/n with unit cell parameters: a = 13.8780(1), b = 5.96394(5), c = 10.3376(1) Å, β = 104.960(1)° and a = 14.0713(2), b = 6.0934(1), c = 10.5944(1) Å, β = 104.608(1)°, respectively. Their crystal structures can be described as a framework of CaO₆ octahedra and V₂O₇ pyrogroups with alkaline metals found in the tunnels formed. Photoluminescence (PL) and PL excitation spectra of the considered pyrovanadates have been studied in the vacuum ultraviolet (VUV) to visible light (Vis) range as well as their pulse cathode luminescence (PCL) spectra and the kinetic parameters of PCL. In the PL and the PCL spectra of both pyrovanadates recorded at T = 300 K a broad band with maxima at 2.2, 2.4 eV and two shoulders (bands) at 2.0 and 2.58 eV have been observed. At T = 10 K the band at 2.0 eV becomes the main band in the spectra. Two types of luminescence centers for each pyrovanadate, with very similar excitation bands at 3.75, 4.84, 6.2, 7.3 and 9.1 eV, have been found. The nature of the luminescence centers connected with the bands at 2.0, 2.2, 2.4 and 2.58 eV is discussed.     

Electron momentum density in ZnSe: Theory and experiment

We present experimental Compton profiles of ZnSe along [1 0 0] and [1 1 0] directions using our 740 GBq ¹³⁷Cs Compton spectrometer. We have also computed the momentum densities, energy bands, density of states (DOS) and band gaps using density functional theory (local density and generalized gradient approximations) and pseudopotential (PP) approach. The anisotropy in the momentum density is well reproduced by the density functional calculations. The energy bands and bond length are interpreted in terms of the anisotropies.     

Influence of redox behavior of copper ions on dielectric and spectroscopic properties of Li2O–MoO3–B2O3: CuO glass system

Li₂O–MoO₃–B₂O₃ glasses mixed with different concentrations of CuO (ranging from 0 to 1.2 mol%) were prepared. The samples were characterized by X-ray diffraction, scanning electron microscopy and differential scanning calorimetry. Optical absorption, luminescence, ESR, IR and dielectric properties (viz., dielectric constant ?′, loss tan δ and a.c. conductivity σ_ac, over a wide range of frequency and temperature) of these glass materials have been investigated. The results of differential scanning calorimetric studies suggest that the glass forming ability is higher for the glasses containing CuO beyond 0.6 mol%. The analysis of results of the dielectric properties has revealed that the glasses possess high insulating strength when the concentration of CuO is >0.6 mol%. The variation of a.c. conductivity with the concentration of CuO passes through a maximum at 0.6 mol%. In the high-temperature region, the a.c. conduction seems to be connected with the mixed conduction viz., electronic conduction and ionic conduction. The optical absorption spectra of these glasses exhibited bands due to Cu⁺ ions in the UV region in addition to the conventional band due to Cu²⁺ ions in the visible region. The ESR spectral studies have indicated that there is a gradual adoption of Cu²⁺ ions from ionic environment to covalent environment as the concentration of CuO increases beyond 0.6 mol% in the glass matrix. The luminescence spectra excited at 271 nm have exhibited an intense yellow emission band centered at about 550 nm and a relatively broad blue emission band at about 450 nm; these bands have been attributed to the ³D₁ → ¹S₀ transition of isolated Cu⁺ ions and ³D₁ → ¹S₀ transition of (Cu⁺)₂ pairs, respectively. The quantitative analysis of the results of all these studies has indicated that as the concentration of CuO is increased beyond 0.6 mol% in the glass matrix, a part of Cu²⁺ ions have been reduced to Cu⁺ ions that have influenced the physical properties of these glasses to a substantial extent.     

Alternate hot and cold electrodes

Alternate hot and cold electrodes, actively heated and chilled, were facilely fabricated for the first time. It consists of two symmetrically placed semiconductor thermoelectric coolers and an embedded interlayer with a Pt-PtRh thermocouple. The tip of Pt-PtRh thermocouple can be used as both an electrochemical electrode and a temperature-sensing device simultaneously. Theoretical simulation shows that a local steady-state supercooled water within a radius of ca. 12.6 mm around the electrode can be obtained. In this paper, the activity of horseradish peroxidase is evidenced at supercooled temperature (− 9 °C) in 0.1 M PBS.     

Synthesis,structure and properties of Ag2PdO2

The first silver palladium oxide, Ag₂PdO₂, was synthesised from a co-precipitated oxide precursor by annealing at 423–823 K, applying an oxygen pressure of 73 MPa. The crystal structure has been determined from X-ray and neutron powder diffraction data. The new compound crystallises in space group Immm. The lattice constants as determined from X-ray powder diffraction are a=4.55523(5) Å, b=3.00803(3) Å and c=9.8977(1) Å. The crystal structure constitutes a new structure type showing some features in common with the Li₂CuO₂-type. Palladium is found in a nearly square planar arrangement while silver has an almost linear co-ordination. The overall structure can be considered as a rocksalt defect structure. Ag₂PdO₂ is diamagnetic and semiconducting. The band gap, estimated from conductivity measurements in the temperature range of 240–300 K, is 0.18(2) eV.     

Visible LED light photoelectrochemical sensor for detection of L-Dopa based on oxygen reduction on TiO2 sensitized with iron phthalocyanine

The present work describes the development of a new strategy to photoelectrochemical detection of L-Dopa at low potential based on oxygen reduction on TiO₂ sensitized with iron phthalocyanine (FePc/TiO₂). The FePc/TiO₂ composite shows a photocurrent 10-fold higher than that of pure TiO₂ nanoparticles and it was 4-fold higher than that of FePc exploiting visible light. The band gaps of pure TiO₂ nanoparticles, FePc and FePc/TiO₂, calculated according to the Kubelka–Munk equation, were 3.22 eV, 3.11 eV and 2.82 eV, respectively. The FePc/TiO₂ composite showed a low charge transfer resistance in comparison to the photoelectrode modified with FePc or TiO₂. Under optimized conditions, the photoelectrochemical sensor shows a linear response range from 20 up to 190 μmol L^− 1 with a sensitivity of 31.8 μA L mmol^− 1 and limit of detection of 1.5 μmol L^− 1 for the detection of L-Dopa.     

Small copper nanoclusters: X-ray absorption analysis

X-ray absorption near edge structure (XANES) spectra at the cooper L_2,3-edge of small cooper nanoclusters has been studied. Theoretical interpretation of XANES spectra measured for small copper nanoclusters have been performed. Theoretical analysis is based on a self-consistent full multiple scattering theory of X-ray absorption as well as on non-muffin-tin finite difference method (FDM). This approach allows choosing the best model for local geometry of small cooper nanoclusters.