Edge Onset Research Articles

Synthesis of MAPbBr3‐iYi (Y=I, Cl and i=0, 1, 2, 3) thin film perovskites

AbstractMethylammonium lead halide perovskites with different halides (iodide, bromide and chloride) have been synthetized from methylamine, lead nitrate and the corresponding hydroX acid (X = I, Br, Cl) precursors. Subsequently MAPbBr3‐iYi (Y = I, Cl; i=0, 1, 2, 3) perovskites were deposited as thin films onto FTO substrates by spin coating or dipping. Thin film perovskites were then characterized by X‐Ray Diffraction, elemental analysis and optical spectrometry. Crystallites' sizes are between 100‐600 nm depending on the synthesis temperature. All synthetized MAPbX3‐iYi perovskites crystallized in the same cubic phase irrespective of the X and Y components and a unique phase is observed. Elemental analysis shows that in all cases the atomic components meet the expected stoichiometric formulae. The bandgap of thin film MAPbX3‐iYi perovskites were inferred from transmittance and reflectance spectral measurements. It is found that the onset of the absorption edge for thin film MAPbX3 perovskites is about 1.66, 2.55 and 3.37 eV for X= I, Br, Cl, respectively and it reaches intermediate values for mixed MAPbX3‐iYi perovskites. (© 2015 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Identification of the iron oxidation state and coordination geometry in iron oxide- and zeolite-based catalysts using pre-edge XAS analysis.

Analysis of the oxidation state and coordination geometry using pre-edge analysis is attractive for heterogeneous catalysis and materials science, especially for in situ and time-resolved studies or highly diluted systems. In the present study, focus is laid on iron-based catalysts. First a systematic investigation of the pre-edge region of the Fe K-edge using staurolite, FePO4, FeO and α-Fe2O3 as reference compounds for tetrahedral Fe(2+), tetrahedral Fe(3+), octahedral Fe(2+) and octahedral Fe(3+), respectively, is reported. In particular, high-resolution and conventional X-ray absorption spectra are compared, considering that in heterogeneous catalysis and material science a compromise between high-quality spectroscopic data acquisition and simultaneous analysis of functional properties is required. Results, which were obtained from reference spectra acquired with different resolution and quality, demonstrate that this analysis is also applicable to conventionally recorded pre-edge data. For this purpose, subtraction of the edge onset is preferentially carried out using an arctangent and a first-degree polynomial, independent of the resolution and quality of the data. For both standard and high-resolution data, multiplet analysis of pre-edge features has limitations due to weak transitions that cannot be identified. On the other hand, an arbitrary empirical peak fitting assists the analysis in that non-local transitions can be isolated. The analysis of the oxidation state and coordination geometry of the Fe sites using a variogram-based method is shown to be effective for standard-resolution data and leads to the same results as for high-resolution spectra. This method, validated by analysing spectra of reference compounds and their well defined mixtures, is finally applied to track structural changes in a 1% Fe/Al2O3 and a 0.5% Fe/BEA zeolite catalyst during reduction in 5% H2/He. The results, hardly accessible by other techniques, show that Fe(3+) is transformed into Fe(2+), while the local Fe-O coordination number of 4-5 is maintained, suggesting that the reduction involves a rearrangement of the oxygen neighbours rather than their removal. In conclusion, the variogram-based analysis of Fe K-edge spectra proves to be very useful in catalysis research.

Low-Temperature Growth of Inverted Hexagonal ZnS/CdS Quantum Dots: Functional and Luminescence Properties

A novel low-temperature wet chemical method is proposed for direct growth of type-I inverted hexagonal ZnS/CdS quantum dots (QD). 2-Mercaptoethanol (2-ME) was used as a capping agent for confinement by passivation, and also helped to prevent agglomeration of the QD. The band gap calculated from optical absorption spectra was 2.63 eV for the smallest core/shell QD. Absorption edge onset and results from transmission electron microscopy revealed formation of inverted core/shell QD. X-ray diffraction studies revealed the ZnS/CdS had a stable hexagonal crystal structure at low temperature. The average diameter of the core/shell QD was 4.2 nm. Tunable luminescence with substantial tunability was revealed by study of the photoluminescence of the inverted ZnS/CdS quantum dots. Surface passivation of ZnS/CdS QD by 2-ME was confirmed by Fourier-transform infrared spectroscopy.

Tunable Nonlinear Optical Property and Photocatalytic Activity on Luminescent Chiral Lanthanide Chains

AbstractFive chiral (P21) isostructural lanthanide coordination complexes Ln(pic)3 [pic=picolinic acid, Ln=La (1), Gd (2), Nd (3), Sm (4) and Eu (5)] have been hydrothermally generated, featuring one dimensional right‐handed 21 helical chains constructed from LnN3O6 of square antiprism geometry and N,O‐chelated pic− groups, in which 3, 4 and 5 have been reported, but we explore undiscovered properties of their own systematically. In special, 1 undergoes an abnormal transformation of symmetry as the temperature cooling to 100 K. TG‐DSC studies demonstrated that all isologues have good thermal stability with the decomposition temperature up to 400°C. Then, UV‐Vis‐NIR transmission spectroscopy measurements indicated that they possess a short‐wavelength absorption edge onset at 377 nm, corresponding to the optical band gap of 3.29 eV. Meanwhile, powder second harmonic generation (SHG) measurements revealed that the SHG intensity of 1, 2, 4 and 5 may be about 1.2, 2.0, 0.9 and 0.75 times that of KH2PO4, respectively. Interestingly, 1 and 3 exhibit efficient photocatalytic degradation for MB (Methylene Blue) upon UV‐light irradiation. Moreover, both 1 and 2 show strong blue luminescence.

Detecting non-bridging oxygens: non-resonant inelastic X-ray scattering in crystalline lithium borates.

Probing the local environment of low-Z elements, such as oxygen, is of great interest for understanding the atomic-scale behavior in materials, but it requires experimental techniques allowing it to work with versatile sample environments. In this paper, the local environment of lithium borate crystals is investigated using non-resonant inelastic X-ray scattering (NRIXS) at energy losses corresponding to the oxygen K-edge. Large variations of the spectral features are observed close to the edge onset in the 535-540 eV energy range when varying the Li2O content. Calculations allow identification of contributions associated with bridging oxygen (BO) and non-bridging oxygen (NBO) atoms. The main result resides in the observed core-level shift of about 1.7 eV in the spectral signatures of the BO and NBO. The clear signature at 535 eV in the O K-edge NRXIS spectrum is thus an original way to probe the presence of NBOs in borates, with the great advantage of making possible the use of complex environments such as a high-pressure cell or high-temperature device for in situ measurements.

EELS modelling of graphitisation

The impact of graphitisation-type processes on the carbon K-edge ELNES is explored for model systems using the CASTEP density functional theory code. For c lattice direction expansion, contraction of spectral peaks occurs between 20 and 27 eV above the edge onset, for a/b lattice dimension expansion spectral peaks are heavily compressed in terms of energy separation and are shifted towards the edge-onset, consistent with a 1/a2 relationship. For a nanotube model system, it is shown that for higher curvature, an additional feature was observed in the spectrum ~5 eV, arguably consistent with 'fullerene'-type character.

Explorations of New Second-Order Nonlinear Optical Materials in the Ternary Rubidium Iodate System: Noncentrosymmetric β-RbIO3(HIO3)2 and Centrosymmetric Rb3(IO3)3(I2O5)(HIO3)4(H2O)

Two new rubidium iodates, namely, β-RbIO3(HIO3)2 (1, P1) and Rb3(IO3)3(I2O5)(HIO3)4(H2O) (2, P21/c), have been synthesized by hydrothermal reaction and their structures determined by single-crystal X-ray diffraction. Compound 1 exhibits IO3(-) anions and neutral HIO3 molecules which are interconnected by Rb(+) cations into three-dimensional structure. Compound 2 features a two-dimensional layered structure formed by IO3(-) anions and neutral HIO3 and dimeric I2O5 molecules interconnected by Rb(+) cations. Large bulk crystal of 1 with dimensions of several millimeters has been grown. UV-vis-NIR transmission spectroscopy measurements on a slab of a polished crystal of 1 indicated that the crystal possesses a short-wavelength absorption edge onset at 305 nm. Powder second-harmonic generation (SHG) measurements on sieved crystals revealed that 1 is a type I phase-matchable material with an SHG response about 1.5 times that of KH2PO4. The Vickers hardness of crystal of 1 has been measured to be 110 HV, and the laser-induced damage threshold has been confirmed to be 18.26 J/cm(2) with a laser wavelength of 1064 nm and a pulse duration of 10 ns. Moreover, thermal stabilities and vibrational spectra for both 1 and 2 have also been studied.

Cs2GeB4O9: a New Second-Order Nonlinear-Optical Crystal

A new alkali-metal borogermanate with noncentrosymmetric structure, namely, Cs2GeB4O9, has been discovered, and a large crystal with dimensions of 20 × 16 × 8 mm(3) has been grown by a high-temperature top-seeded solution method using Cs2O-B2O3 as a flux. The compound crystallizes in the tetragonal space group I4 with a = b = 6.8063(2) Å, c = 9.9523(7) Å, V = 461.05(4) Å(3), and Z = 2. It features a three-dimensional anionic open framework based on GeO4 tetrahedra and B4O9 clusters that are interconnected via corner-sharing, forming one-dimensional channels of nine-/ten-membered rings along the a and b axes, which are occupied by Cs(+) cations. Cs2GeB4O9 exhibits a very high thermal stability with a melting point of 849 °C, and it possesses a short-wavelength absorption edge onset at 198 nm determined by UV-vis transmission spectroscopy measurements on a slab of polished crystal. Powder second-harmonic generation (SHG) measurement on sieved crystals reveals that Cs2GeB4O9 is a type I phase-matchable material with a strong SHG response of about 2.8 × KH2PO4. The preliminary investigation indicates that Cs2GeB4O9 is a new promising second-order nonlinear-optical crystalline material.

Hierarchical microarchitectures of AgGa1−xInxS2: Long chain alcohol assisted synthesis, band gap tailoring and photocatalytic activities of hydrogen generation

An enhanced visible-light-driven photocatalyst, AgGa1−xInxS2, was prepared by a facile long chain alcohol assisted hydrothermal route. Phase structures, morphologies, optical properties, energy level and specific surface areas of the products were characterized by X-ray diffraction, scanning electron microscope, UV–vis spectra and BET analysis. The obtained products show unique hierarchical microarchitectures which were fabricated by micro-sphere and nano-sheets. The introducing of long chain alcohol (pentanol and heptanol) was investigated to be crucial for the forming of hierarchical microarchitectures. On the other hand, the band gap energy of AgGa1−xInxS2 (x = 0–1) estimated from the onset of absorption edge was found to be reduced from 2.71 eV (x = 0) to 1.93 eV (x = 1) by indium substitution. The photocatalytic activity for hydrogen evolution was investigated under visible light irradiation (λ > 400 nm). The rate of hydrogen evolution reached up to 813 μmol/h when the amount of In (x) was 0.1 and the roles of long chain alcohol and Ga/In were also carefully investigated.

Systematic structural and chemical characterization of the transition layer at the interface of NO-annealed 4H-SiC/SiO2 metal-oxide-semiconductor field-effect transistors

We present a systematic characterization of the transition layer at the 4H-SiC/SiO2 interface as a function of nitric oxide (NO) post-annealing time, using high-resolution transmission electron microscopy for structural characterization and spatially resolved electron energy-loss spectroscopy for chemical analysis. We propose a systematic method for determining transition layer width by measuring the monotonic chemical shift of the Si-L2,3 edge across the interface, and compare its efficacy to traditional measures from the literature, revealing the proposed method to be most reliable. A gradual shift in the Si-L2,3 edge onset energy suggests mixed Si-C/Si-O bonding in the transition layer. We confirm an inverse relationship between NO-anneal time and transition layer width, which correlates with improved channel mobility, enhanced N density at the interface, and decreased interface trap density. No excess C was noted in the interfacial region.

Size and oxygen passivation induced reversal of photoconducting behaviour in CdS nanorods

The effect of oxygen adsorption and desorption on the photoconducting gain, spectral dependence of quantum efficiency and optical switching was studied in CdS nanorods with diameters of 20, 50 and 100 nm. These were found to have an increasing degree of crystallinity and consequently a decreasing overall density of defects leading to better stoichiometry being maintained. These properties, along with the complete depletion of electrons from the nanorod volume and oxygen induced passivation of defects, resulted in: (i) a large difference in photoconducting gain, (ii) reversal of the photoconducting behaviour on annealing in oxygen and a vacuum, and (iii) onset of an absorption edge in the spectral dependence of quantum efficiency on oxygen annealing in 20 nm diameter nanorods in comparison to the normal photoconducting behaviour expected from an n-type semiconductor observed in 50 and 100 nm diameter nanorods. Single CdS nanorod devices show stable I–V characteristics in dark and light conditions under a wide temperature range and the effect of oxygen and vacuum annealing can be clearly observed. The oxygen induced defect passivation observed in this study is important for the application of compound semiconductor nanorods in optoelectronic devices.

Photoconducting nanocrystalline lead sulphide thin films obtained by chemical bath deposition

A chemical bath deposition method of preparing photoconducting nanocrystalline lead sulphide (PbS) thin films at room temperature (RT) is described. The aqueous bath of lead acetate, thiourea, and ammonium hydroxide produce films of about 100 nm thicknesses in 45 minutes. X-ray diffraction (XRD) studies show that these films are nanocrystalline cubic PbS with 10 nm crystallite size. Atomic Force Microscope (AFM) and Scanning Electron Microscope (SEM) revealed that the films consist of spherical grains of sizes 100 to 200 nm. The transmission spectra of the films show onset of absorption edge around 850 nm and the bandgap is around 1.65 eV. The films are p-type with dark conductivity of 2.5×10−3 S/cm and mobility of 0.07 cm2/V⋅s. The photosensitivity is 6–7 for an illumination of 80 mW/cm2 from a halogen lamp (50 W, 12 V). Transient photoconductivity measurements reveal short and long life times of minority carriers. Thermoelectric and photothermoelectric studies show that photoconductivity in these films is mainly due to photogenerated majority carriers.

Textured Fluorine‐Doped Tin Dioxide Films formed by Chemical Vapour Deposition

The use of an aerosol delivery system enabled fluorine-doped tin dioxide films to be formed from monobutyltin trichloride methanolic solutions at 350-550 °C with enhanced functional properties compared with commercial standards. It was noted that small aerosol droplets (0.3 μm) gave films with better figures of merit than larger aerosol droplets (45 μm) or use of a similar precursor set using atmospheric pressure chemical vapour deposition (CVD) conditions. Control over the surface texturing and physical properties of the thin films were investigated by variation in the deposition temperature and dopant concentration. Optimum deposition conditions for low-emissivity coatings were found to be at a substrate temperature of about 450 °C with a dopant concentration of 1.6 atm% (30 mol% F:Sn in solution), which resulted in films with a low visible light haze value (1.74%), a high charge-carrier mobility (25 cm(2) V s(-1)) and a high charge-carrier density (5.7×10(20) cm(-3)) resulting in a high transmittance across the visible (≈80%), a high reflectance in the IR (80% at 2500 nm) and plasma-edge onset at 1400 nm. Optimum deposition conditions for coatings with applications as top electrodes in thin film photovoltaics were found to be a substrate temperature of about 500 °C with a dopant concentration of 2.2 atm% (30 mol% F:Sn in solution), which resulted in films with a low sheet resistance (3 Ω sq(-1)), high charge-carrier density (6.4×10(20) cm(-3)), a plasma edge onset of 1440 nm and the films also showed pyramidal surface texturing on the micrometer scale which corresponded to a high visible light haze value (8%) for light scattering and trapping within thin film photovoltaic devices.

Role of doping-induced photochemical and microstructural properties in the photocatalytic activity of InVO4 for splitting of water

We report in this paper on microstructural, optical and photocatalytic properties of single-phase indium orthovanadates, as a function of doping at lattice sites. The UV–visible spectra of these samples exhibited intense UV-region bands at 250 and 350 nm, besides broad absorption band in visible region (350–700 nm). The wavelength at absorption edge and the intensity of visible absorption showed considerable increase on doping of an impurity, particularly at V or O lattice sites. Also, the samples gave rise to blue-green photoluminescence emission, with overriding bands at ca. 420, 450, 460 and 485 nm, on excitation at 240–420 nm wavelengths. The intensity of these fluorescence bands varied with excitation wavelength and impurity content of a sample. In deviation with several earlier studies, only oxygen and no hydrogen were produced during photocatalytic splitting of water, in the experiments conducted under visible light (>395 nm) and at a pH of ~6.5. The O2 yield depended on the dispersed metal co-catalyst, impurity content and the addition of methanol as sacrificial reagent. On the other hand, small quantities of hydrogen and no oxygen were evolved on UV-irradiation of pure water using metal/InVO4. These results are ascribed to flat band potentials and the doping-induced inter-band donor and acceptor charge trapping states of InVO4, the presence of which is revealed by XRD, luminescence and XPS studies. Our study also confirms that the onset of absorption edge may not necessarily correspond to band-to-band energy gap of a semiconducting material. This accounts for some anomalous band gap energies reported earlier for InVO4.

Transmittance and optical constants of erbium films in the 325−1580 eV spectral range

The optical constants of erbium (Er) films were obtained in the 3.25-1580 eV range from transmittance measurements performed at room temperature. Thin films of Er were deposited by evaporation in ultra high vacuum conditions and their transmittance was measured in situ. Substrates consisted of a thin C film supported on a grid. Transmittance measurements were used to obtain the extinction coefficient k of the Er films. The refractive index n of Er was calculated using the Kramers-Krönig analysis. k data were extrapolated both on the high- and low-energy parts of the spectrum by using experimental data and calculated k values available in the literature. Er, similar to other lanthanides, has a low-absorption band below the O(2,3) edge onset; the smallest absorption was measured at ~22.5 eV. Therefore, Er is a promising material for filters and multilayer coatings in the energy range below the O(2,3) edge, in which materials typically have an absorption stronger than at other energies. Good consistency of the data resulted from the application of f and inertial sum rules.

The speed of context integration in the visual cortex

The observation of figure-ground selectivity in neurons of the visual cortex shows that these neurons can be influenced by the image context far beyond the classical receptive field. To clarify the nature of the context integration mechanism, we studied the latencies of neural edge signals, comparing the emergence of context-dependent definition of border ownership with the onset of local edge definition (contrast polarity; stereoscopic depth order). Single-neuron activity was recorded in areas V1 and V2 of Macaca mulatta under behaviorally induced fixation. Whereas local edge definition emerged immediately (<13 ms) after the edge onset response, the context-dependent signal was delayed by about 30 ms. To see if the context influence was mediated by horizontal fibers within cortex, we measured the latencies of border ownership signals for two conditions in which the relevant context information was located at different distances from the receptive field and compared the latency difference with the difference predicted from horizontal signal propagation. The prediction was based on the increase in cortical distance, computed from the mapping of the test stimuli in the cortex, and the known conduction velocities of horizontal fibers. The measured latencies increased with cortical distance, but much less than predicted by the horizontal propagation hypothesis. Probability calculations showed that an explanation of the context influence by horizontal signal propagation alone is highly unlikely, whereas mechanisms involving back projections from other extrastriate areas are plausible.

Transmittance and optical constants of Ho films in the 3–1340 eV spectral range

The optical constants n and k of holmium (Ho) films were obtained in the 3–1340−eV range from transmittance measurements performed at room temperature. Thin films of Ho with various thicknesses were deposited by evaporation in ultra high vacuum conditions and their transmittance was measured in situ. Ho films were deposited onto thin C-film substrates supported on high transmittance grids. Transmittance measurements were used to obtain the extinction coefficient k of Ho films. The refractive index n of Ho was calculated with Kramers−Krönig analysis; in order to do this, k data were extrapolated both on the high and on the low energy parts of the spectrum by using experimental and calculated k values available in the literature. Ho, similar to other lanthanides, has a low-absorption band below the O2,3 edge onset; the lowest absorption was measured at ∼22 eV. Therefore, Ho is a promising material for filters and multilayer coatings in the energy range below the O2,3 edge in which most materials have a large absorption. Good consistency of the data resulted from the application of f and inertial sum rules.

Structure and photoluminescence properties of Ce 3+-doped novel silicon-oxynitride Ba 4− zMzSi 8O 20−3 xN 2 x ( M=Mg, Ca, Sr)

Structural and photoluminescence properties of undoped and Ce 3+-doped novel silicon-oxynitride phosphors of Ba 4− z M z Si 8O 20−3 x N 2 x ( M=Mg, Sr, Ca) are reported. Single-phase solid solutions of Ba 4− z M z Si 8O 20−3 x N 2 x oxynitride were synthesized by partial substitutions of 3O 2−→2N 3− and Ba→ M ( M=Mg, Ca, Sr) in orthorhombic Ba 2Si 4O 10. The influences of the type of alkaline earth ions of M, the Ce 3+ concentration on the photoluminescence properties and thermal quenching behaviors of Ba 3 MSi 8O 20−3 x N 2 x ( M=Mg, Ca, Sr, x=0.5) were investigated. Under excitation at about 330 nm, Ba 3 MSi 8O 20−3 x N 2 x :Ce 3+ ( x=0.5) exhibits efficient blue emission centered at 400–450 nm in the range of 350–650 nm owing to the 5 d→4 f transition of Ce 3+. The emission band of Ce 3+ shifts to long wavelength by increasing the ionic size of M due to the modification of the crystal field, as well as the Ce 3+ concentrations due to the Stokes shift and energy transfer or reabsorption of Ce 3+ ions. Among the silicon-oxynitride phosphors of Ba 3 MSi 8O 18.5N:Ce 3+, M=Sr 0.6Ca 0.4 possesses the best thermal stability probably related to its high onset of the absorption edge of Ce 3+.

Optical absorption at its onset in sputter deposited hafnia–titania nanolaminates

The onset of the fundamental optical absorption edge in sputter deposited HfO2–TiO2 nanolaminate films grown on unheated substrates was investigated. Three bilayer architectures were examined, representing overall film chemistry from 0.51 to 0.72 atom fraction Hf. The goal was to determine the absorption coefficient, α(E), versus incident photon energy, E, and to model this dependence in terms of the absorption behavior of specific functional units within the nanolaminate. Persistence and amalgamation models were applied, representing the extremes of segregated cation and mixed cation structures, respectively, and both were found to be unsatisfactory. Consideration of physiochemical data for the nanolaminates led to the development of a modified persistence model for absorption. α(E) was decomposed into contributions from (I) broad (9 nm-thick) interfacial regions that were chiefly o-HfTiO4, and (II) regions remote from interfaces that contained material based on a highly defective m-HfO2 lattice. The absorption edge at its onset in all nanolaminates was determined by short-range atomic order characteristic of o-HfTiO4. An indirect band gap of EG=3.25±0.02 eV was determined for this compound.

Transmittance and optical constants of Lu films in the 3–1800 eV spectral range

The optical constants n and k of lutetium (Lu) films were obtained in the 3–1800 eV range from transmittance measurements performed at room temperature. These are the first experimental optical constant data of Lu in the whole range. Thin films of Lu with various thicknesses were deposited by evaporation in ultrahigh vacuum conditions and their transmittance was measured in situ. Lu films were deposited onto grids coated with a thin, C support film. Transmittance measurements were used to obtain the extinction coefficient k of Lu films. The refractive index n of Lu was calculated with Kramers–Krönig analysis. k data were extrapolated both on the high and on the low-energy sides by using experimental and calculated k values available in the literature. Lu, similar to other lanthanides, has a low-absorption band below the O2,3 edge onset; the lowest absorption was measured at ∼25.1 eV. Therefore, Lu is a promising material for filters and multilayer coatings in the energy range below the O2,3 edge in which most materials have a large absorption. Good consistency of the data was obtained through f and inertial sum rules.