Onset Of Absorption Edge Research Articles

Electron beam induced changes in optical properties of glassy As35S65 chalcogenide thin films studied by imaging ellipsometry

Spectroscopic imaging ellipsometry was used to study 50 μm × 50 μm squares recorded into glassy As35S65 thin film by electron beam with different exposure doses. Optical constants (refractive index n and extinction coefficient k) of electron exposed areas were determined from imaging ellipsometric data recorded over a spectral range 390–800 nm. Complex dielectric permittivity of each of electron exposed square (glassy As35S65 with different electron exposure dose) was parametrized by a Tauc-Lorentz oscillator. Gradient optical model with the refractive index changing linearly within the layer thickness was used.Evaluation of parameters of Tauc-Lorentz oscillator revealed decrease of energy bandgap, increase of the refractive index in whole recorded spectra (more pronounced at the top of the film) and increase of extinction coefficient in the part of spectra where the material absorbs with the increasing electron exposure dose used for exposure of the squares.The decrease in local disorder with increasing electron exposure dose is quantified from the short wavelength absorption edge onset using a Mott-Davis model. The dependence of the parameters of Wemple-DiDomenico model for the transparent part of the optical constants spectra on electron exposure dose is shown.Thin film with recorded image was subsequently etched in amine based solution in aprotic solvent. While native film was fully etched off the remaining height of the exposed areas is increasing with the increasing electron exposure dose used.Knowledge of the changes of optical properties and etching rates induced by electron beam of different exposure doses can be used for fabricating of optical elements by high resolution electron beam lithography.

Spectroscopic ellipsometry characterization of spin-coated Ge25S75 chalcogenide thin films

Abstract Spectroscopic ellipsometry study on spin-coated non-toxic Ge25S75 thin films annealed at different temperatures were conducted. Multi sample analysis with two sets of samples spin-coated onto soda-lime glass and onto silicon wafers was utilized. Optical constants (refractive index n and extinction coefficient k) of these films were determined from ellipsometric data recorded over a wide spectral range (0.05–6 eV). Different parametrization of Ge25S75 complex dielectric permittivity which consists of a Tauc-Lorentz or Cody-Lorentz oscillator describing the short wavelength absorption edge, a Lorentz or Gauss oscillators describing phonon absorption or optically active absorption of alkyl ammonium germanium salts in the middle infrared part of spectra is discussed. Using a Mott-Davis model, the decrease in local disorder with increasing annealing temperature is quantified from the short wavelength absorption edge onset. Using the Wemple-DiDomenico single oscillator model for the transparent part of the optical constants spectra, a decrease in the centroid distance of the valence and conduction bands with increasing annealing temperature is shown and increase in intensity of the inter-band optical transition due to annealing temperature occurs. Intensity of absorption near 3000 cm−1 could be used as alternative method to evaluation of quality of prepared films.

Spectroscopic ellipsometry characterization of ZnO:Sn thin films with various Sn composition deposited by remote-plasma reactive sputtering

ZnO:Sn thin films were deposited onto thermally oxidized silicon substrates using a remote plasma reactive sputtering. Their optical constants (refractive index n and extinction coefficient k) were determined from ellipsometric data recorded over a wide spectral range (0.05–6eV). Parametrization of ZnO:Sn complex dielectric permittivity consists of a parameterized semiconductor oscillator function describing the short wavelength absorption edge, a Drude oscillator describing free carrier absorption in near-infrared part of spectra and a Lorentz oscillator describing the long wavelength absorption edge and intra-band absorption in the ultra-violet part of the spectra. Using a Mott-Davis model, the increase in local disorder with increasing Sn doping is quantified from the short wavelength absorption edge onset. Using the Wemple-DiDomenico single oscillator model for the transparent part of the optical constants spectra, an increase in the centroid distance of the valence and conduction bands with increasing Sn doping is shown and only slight increase in intensity of the inter-band optical transition due to Sn doping occurs. The Drude model applied in the near-infrared part of the spectra revealed the free carrier concentration and mobility of ZnO:Sn. Results show that the range of transparency of prepared ZnO:Sn layers is not dramatically affected by Sn doping whereas electrical conductivity could be controlled by Sn doping. Refractive index in the transparent part is comparable with amorphous Indium Gallium Zinc Oxide allowing utilization of prepared ZnO:Sn layers as an indium-free alternative.

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.

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.

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.

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.

Indium induced band gap tailoring in AgGa1−xInxS2 chalcopyrite structure for visible light photocatalysis

Indium was substituted at gallium site in chalcopyrite AgGaS(2) structure by using a simple solid solution method. The spectroscopic analysis using extended x-ray absorption fine structure and x-ray photoelectron spectroscopy confirmed the indium substitution in AgGaS(2) lattice. The band gap energy of AgGa(1-x)In(x)S(2) (x=0-1) estimated from the onset of absorption edge was found to be reduced from 2.67 eV (x=0) to 1.9 eV (x=1) by indium substitution. The theoretical and experimental studies showed that the indium s orbitals in AgGa(1-x)In(x)S(2) tailored the band gap energy, thereby modified the photocatalytic activity of the AgGa(1-x)In(x)S(2).

Analysis of SiOxCyHz polymeric materials by x-ray absorption spectroscopy: Anomalous behavior of the resonant SiKLL Auger spectra

Silicone-like SiOxCyHz polymeric thin films with different C∕O atomic ratios have been studied by x-ray absorption spectroscopy and resonant Auger electron spectroscopy. The energy of the onset of the SiK absorption edge varies with the film composition from the value observed in SiC (i.e.,∼1840eV) to that in SiO2 (i.e.,∼1844eV). In the samples with a low C∕O atomic ratio (i.e.,∼1),the main band of the SiK spectra depicts two well defined peaks that can be associated with the Si-C and Si-O bonds present in the samples. Their relative intensity can be used to assess the concentration of these two bond types. Resonant SiKLL Auger spectra of the polymeric thin film have been measured using photon energies across the SiK absorption edge. A first Auger peak appearing at a high kinetic energy was recorded for photon energies around the onset of the absorption edge and the first peak/shoulder of the SiK spectrum associated with the Si-C bonds. This resonant SiKLL Auger signal depicts the typical behavior of a resonant Auger process, i.e., its intensity increases when increasing the photon energy, passes through a maximum and then decreases to disappear. In parallel with the evolution of this resonant spectrum, a “normal” SiKLL Auger spectrum with a lower kinetic energy appeared for photon energies around the middle of the absorption curve. This spectrum is the only one prevailing at photon energies well above the absorption edge. An outstanding behavior of the SiKLL resonant Auger peaks in the polymers with a high C∕O atomic ratio (i.e., ⩾4) is that they present a negative dispersion with photon energy, i.e., their kinetic energy decreases as the photon energy increases. This behavior is opposite to that generally found for resonant Auger peaks in most systems and particularly for SiO2. Although the origin of such behavior is still unclear, the study of the SiK absorption edge and the corresponding SiKLL resonant Auger spectra results helpful for the structural and electronic characterization of SiOxCyHz polymers.

Spectral Properties of AOT-Protected CdS Nanoparticles: Quantum Yield Enhancement by Photolysis

Absorption and steady-state emission spectra and time-resolved luminescence decays have been recorded for CdS nanoparticles prepared using the reverse-micelle method. Spectra were collected in hexane solution, with the surfactant AOT as stabilizer. The emission spectra of these particles are characteristic of trap-state emissions, being broad and red-shifted from the absorption-edge onset. Quantum yields for CdS particles prepared with w0's of 3−7 (w0 = [H2O]/[AOT]) are low (∼1−4%) and are not size dependent. CdS nanoparticles with w0 = 4 were irradiated at the maximum of the first absorption peak. Irradiation resulted in a red shift and overall decrease in intensity of the absorption spectra, a red shift and increase in intensity of the emission spectra, an increase in the apparent lifetimes, and an increase in the quantum efficiency. Final quantum yields range from ∼9 to 13%.