Radical-recombination luminescence,ion-luminescence and photoluminescence of CaGa2S4:Eu

CaGa₂S₄ based luminophors have recently attracted significant attention due to their high efficiency. Very little is known now about the surface and near surface luminescence of these luminophors. The ion-luminescence of CaGa₂S₄:Eu excited by low energy hydrogen ions (up to 3 keV) and the radical-recombination luminescence excited by neutral hydrogen atoms of thermal energies have been studied. The latter is due to chemical energy released at the surface during heterogeneous recombination of atoms into molecules (∼4 eV per recombination event). The radical-recombination luminescence is likely the most surface kind of luminescence, while the ion-luminescence is the luminescence of near-surface layers. In this regard the photoluminescence is the bulk one of with which the characteristics of the surface luminescence might be compared.     

Luminescence of CaGa<Subscript>2</Subscript>Se<Subscript>4</Subscript>:Eu crystals

We have studied photoluminescence and thermoluminescence (PL and TL) in CaGa₂Se₄:Eu crystals in the temperature range 77–400 K. We have established that broadband photoluminescence with maximum at 571 nm is due to intracenter transitions 4f⁶ 5d–4f⁷ (⁸S_7/2) of the Eu²⁺ ions. From the temperature dependence of the intensity (log I–10³/T), we determined the activation energy (E _a = 0.04 eV) for thermal quenching of photoluminescence. From the thermoluminescence spectra, we determined the trap depths: 0.31, 0.44, 0.53, 0.59 eV. The lifetime of the excited state 4f6 5d of the Eu²⁺ ions in the CaGa₂Se₄ crystal found from the luminescence decay kinetics is 3.8 μsec. Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 76, No. 1, pp. 112–116, January–February, 2009.     

Chemical spray pyrolysis of β-In2S3 thin films deposited at different temperatures

In₂S₃ thin films were deposited onto indium tin oxide-coated glass substrates by chemical spray pyrolysis while keeping the substrates at different temperatures. The structures of the sprayed In₂S₃ thin films were characterized by X-ray diffraction (XFD). The quality of the thin films was determined by Raman spectroscopy. Scanning electron microscopy (SEM) and atomic force microscopy were used to explore the surface morphology and topography of the thin films, respectively. The optical band gap was determined based on optical transmission measurements. The indium sulfide phase exhibited a preferential orientation in the (0, 0, 12) crystallographic direction according to the XRD analysis. The phonon vibration modes determined by Raman spectroscopy also confirmed the presence of the In₂S₃ phase in our samples. According to SEM, the surface morphologies of the films were free of defects. The optical band gap energy varied from 2.82 eV to 2.95 eV.     

Study on the defect of sulfur substituting for phosphorus in potassium dihydrogen phosphate

The electronic structure and geometric distribution of neutral and charge states of the point defect sulfur substituting for phosphorus (S_p) in potassium dihydrogen phosphate (KH₂PO₄) have been investigated using a first-principles method. The energy gap is lowered to about 2.2 eV and 6.5 eV from the Density Function Theory (DFT) value of 7.3 eV by a -2 charge and neutral state, respectively. However, no defect states appear in the energy gap for the +1, -1, and +2 charge states of the S_p defect. The results show that the defects S_p with neutral and -2 charge states might contribute to the transient optical absorption under the high energy laser irradiation.     

Absorption Spectra of Single Crystals of EuGa2S4 and EuGa2S4:Co

The optical properties of EuGa₂S₄ and EuGa₂S₄:Co single crystals in a range of temperatures from 77 to 300 K are investigated. The single crystals are obtained by the Bridgman method and are characterized by tetragonal syngony. The behavior of the optical transitions in the photon energy range 1.70–2.45 eV and the temperature range 77–300 K is determined. It is established that in the energy range 1.77–1.90 eV absorption is associated with transitions of the Co²⁺ ion, while in the range 2.20–2.40 eV, with indirect allowed optical transitions.     

Electronic and optical properties of defect chalcopyrite HgAl2Se4

The structural, electronic and optical properties of HgAl₂Se₄ are investigated using the full potential linear augmented plane wave method based on density functional theory. The calculated structural parameters using LDA are in excellent agreement with the available experimental result. The obtained energy band gap (2.24 eV) using EV-GGA approximation is in excellent agreement with experimental data (2.20 eV). Variation in the energy band gap as a function of the unit cell lattice parameter has been studied. The optical properties show a considerable anisotropy, which makes this compound very useful for various linear–nonlinear optical devices.     

Electronic states of lanthanide in the ternary thiogallate CaGa2S4

The electronic states of lanthanide (Ln) doped CaGa₂S₄ are investigated by the molecular orbital calculations for a spherical cluster of LnCa₈Ga₁₂S₂₄ using the FORTRAN program DVSCAT on the basis of the Discrete Variational method with Xα potentials (DV-Xα). In view of the SCF convergence, the Ln-doped lattice should contract to 85-90% of the mother crystal around the Ln atom for the lightweight lanthanides from Ce to Sm. On the other hand, the lattice contraction is very small for the heavyweight lanthanides, especially for Er, Tm and Yb in contrast to the generally known lanthanide contraction for Ln³⁺ ions. This is probably attributed to the effective charges of Ln atoms calculated here to be less than +1 for all lanthanides contrary to the chemically accepted value of +3. The energy level scheme of 4f and 5d related molecular orbitals is proposed for each Ln substituting Ca in CaGa₂S₄, showing that the optical processes relating to the 5d→4f transition must be complicated especially for the lightweight Ln-doped CaGa₂S₄.     

Optical characterization of CuIn5S8 crystals by ellipsometry measurements

Optical properties of CuIn₅S₈ crystals grown by Bridgman method were investigated by ellipsometry measurements. Spectral dependence of optical parameters; real and imaginary parts of the pseudodielectric function, pseudorefractive index, pseudoextinction coefficient, reflectivity and absorption coefficients were obtained from the analysis of ellipsometry experiments performed in the 1.2–6.2 eV spectral region. Analysis of spectral dependence of the absorption coefficient revealed the existence of direct band gap transitions with energy 1.53 eV. Wemple–DiDomenico and Spitzer–Fan models were used to find the oscillator energy, dispersion energy, zero-frequency refractive index and high-frequency dielectric constant values. Structural properties of the CuIn₅S₈ crystals were investigated using X-ray diffraction and energy dispersive spectroscopy analysis.     

Band gap tunable and improved microstructure characteristics of Cu2ZnSn(S1−x,Sex)4 thin films by annealing under atmosphere containing S and Se

Cu₂ZnSn(S_xS_1?x)₄ (CZTSSe) thin films were prepared by annealing a stacked precursor prepared on Mo coated glass substrates by the sputtering technique. The stacked precursor thin films were prepared from Cu, SnS₂, and ZnS targets at room temperature with stacking orders of Cu/SnS₂/ZnS. The stacked precursor thin films were annealed using a tubular two zone furnace system under a mixed N₂ (95%) + H₂S (5%) + Se vaporization atmosphere at 580 °C for 2 h. The effects of different Se vaporization temperature from 250 °C to 500 °C on the structural, morphological, chemical, and optical properties of the CZTSSe thin films were investigated. X-ray diffraction patterns, Raman spectroscopy, and X-ray photoelectron spectroscopy results showed that the annealed thin films had a single kesterite crystal structure without a secondary phase. The 2θ angle position for the peaks from the (112) plane in the annealed thin films decreased with increasing Se vaporization temperature. Energy dispersive X-ray results showed that the presence of Se in annealed thin films increased from 0 at% to 42.7 at% with increasing Se vaporization temperatures. UV–VIS spectroscopy results showed that the absorption coefficient of all the annealed thin films was over 10⁴ cm^?1 and that the optical band gap energy decreased from 1.5 eV to 1.05 eV with increasing Se vaporization temperature.     

Inclusion of polyaniline in electrodeposited bismuth sulphide thin films: Synthesis and characterization

Structural, electrical and optical properties of polyaniline (PAni) doped Bi₂S₃ composite thin films prepared by electrodeposition method are reported. X-ray diffraction pattern indicates its polycrystalline nature and crystallite size increases with increase in the concentration of PAni. FTIR studies reveal that the dopant PAni has affected the absorption phenomenon in the IR region of the Bi₂S₃ thin films. The optical band gap energy is found to be 1.91 eV for as-deposited Bi₂S₃ thin film and it decreases with increase in the concentration of PAni. The morphology of the doped films changes due to the addition of PAni. Electrical studies indicate that the conductivity increases with increase in the concentration of PAni. The conduction results from a hopping due to localized states in the temperature range 300–358 K. Above 358 K, the conduction process is explained by the traps at grain boundaries of partially depleted grains.     

High-energy ion induced physical and surface modifications in antimony sulphide thin films

Sb₂S₃ thin films prepared by electrodeposition on indium tin oxide coated glass substrate were irradiated with 150 MeV Ni¹¹⁺ ions for various fluence in the range of 10¹¹–10¹³ ions/cm². The modifications in the structure, surface morphology and optical properties have been studied as a function of ion fluence. X-ray diffraction (XRD) analysis indicates a shift in the (2 4 0) peak position towards lower diffraction angle and a decrease in grain size with increase in ion fluence. Presence of microcracks due to irradiation induced grain splitting effect has been observed from the SEM micrograph at higher ion fluence. The optical absorbance spectrum revealed a shift in the fundamental absorption edge and the band gap energy increased from a value of 1.63 eV for as-deposited films to 1.80 eV for the films irradiated with 10¹³ ions/cm².     

Optical energy gaps of undoped and Co-doped ZnIn2Se4 single crystals

Single crystals of undoped and Co-doped ZnIn₂Se₄ were grown by the vertical Bridgman technique. The optical energy gaps of the single crystals were investigated in the temperature range of 10–300 K from the optical absorption measurements. The indirect optical energy gaps of the single crystals were found to be 1.624 eV for undoped ZnIn₂Se₄ and 1.277 eV for Co-doped one at 300 K. Also, the direct optical energy gaps were given by 1.774 and 1.413 eV for undoped ZnIn₂Se₄ and co-doped one, respectively. The temperature dependence of the optical energy gaps was well fitted by the Varshni equation.     

Electronic structure and optical properties of LaNiO3: First-principles calculations

Using a first-principles method, we investigate the electronic structure and optical properties of rhombohedral LaNiO₃. The total density of states shows that there is no band gap and bulk LaNiO₃ is metallic. There is a strong hybridization between Ni and O orbits near the Fermi level, suggesting that the metallic nature of LaNiO₃ mainly originates from Ni 3d states and La atoms have no noticeable contribution to this. The absorption coefficient of LaNiO₃ is one order of magnitude less than that of nickel in the lower energy region (0–5 eV), and the interband optical transitions are mainly derived from O 2p and Ni 3d states. In reflectivity spectrum of LaNiO₃, there are three main reflectance peaks located at 0 eV, 15.6 eV and 22.9 eV, respectively. In the visible–ultraviolet energy range, the reflectivity of LaNiO₃ remarkably decreases with the increasing photon energy and the value is always smaller than that of nickel in the region.     

Synthesis and photoluminescence properties of Bi2S3 nanowires via surfactant micelle-template inducing reaction

Single-crystalline Bi₂S₃ nanowires, with diameters in the range of 80-200 nm and lengths up to tens of micrometers, have been successfully synthesized through surfactant micelle-template inducing reaction at ambient-pressure and low-temperature. The synthetic route is simple, effective and can provide great opportunities for both fundamental and technological applications. The optical properties of the Bi₂S₃ nanowires with different diameters were firstly examined by means of photoluminescence spectroscopy at room temperature. The representative photoluminescence spectrum exhibits a great blue-shift from the band gap of 1.30 eV of bulk Bi₂S₃ to high energy of 1.44 eV, which indicated that these nanostructures showed quantum confinement effects.     

Feasibility of TFEL application of Ce-doped CaGa2S4 and SrGa2S4 films prepared by flash evaporation method

Ce-doped CaGa₂S₄ and SrGa₂S₄ thin films were prepared for the first time by the flash evaporation method. The films were characterized before and after annealing in H₂S(10%)+Ar gas stream by measuring photoluminescence and absorption spectra, X-ray diffraction and electron probe micro analyses. X-ray diffraction curves and absorption spectra before annealing show amorphous behaviour, whereas the annealing leads to a significant crystallization and improves the stoichiometry of the films. Based on the performance data obtained from dispersive type EL cells using CaGa₂S₄:Eu powder together with photoluminescence property comparison between CaGa₂S₄:Eu and CaGa₂S₄:Ce powders, the annealed films prepared by flash evaporation can be considered to become one of the candidates for TFEL flat panel devices.     

Influence of vacuum annealing on the structural and photoelectrochemical properties of nanocrystalline MoBi2S5 thin films

In the present paper we report structural, optical, morphological and electrical properties of thin films of MoBi₂S₅ prepared by facile self organized arrested precipitation technique (APT) from aqueous alkaline bath. X-ray diffraction study on thin films suggests orthorhombic and rhombohedral mixed phase structure. The samples are further annealed under vacuum at 373 and 473 K. The EDS pattern shows minor loss of sulphur upto 473 K. The optical absorption in visible region shows direct allowed transition with band gap variation over 1.2–1.1 eV. Post-heat treated samples exhibit n-type electrical conductivity. SEM images show uniform distribution of spherical grains with diameter ∼200 nm for as-synthesized MoBi₂S₅ thin film. The grain size increases with annealing temperature and morphology becomes more compact due to crystallization of thin film. The surface roughness deduced from AFM, was in the range of 1.29–1.92 nm. The MoBi₂S₅ thin films are employed for the fabrication of photoelectrochemical solar cells as all the samples exhibit strong absorption in visible to near IR region. Due to vacuum annealing it gives a significant enhancement of power conversion efficiency (η) upto 0.14% as compared to as-synthesized MoBi₂S₅ thin film.     

Effect of Nb doping on morphology,crystal structure,optical band gap energy of TiO2 thin films

Nb–TiO₂ nanofibers and thin films were prepared using a sol–gel derived electrospinning and spin coating, respectively, by varying the Nb/Ti molar ratios from 0 to 0.59 to investigate the effect of Nb doping on morphology, crystal structure, and optical band gap energy of Nb–TiO₂. XRD results indicated that Nb–TiO₂ is composed of anatase and rutile phases as a function of Nb/Ti molar ratio. As the Nb/Ti molar ratio rose, the anatase to rutile phase transformation and the reduction in crystallite size occurred. The band gap energy of Nb–TiO₂ was changed from 3.25 eV to 2.87 eV when the anatase phase was transformed to rutile phase with increasing the Nb doping. Experimental results indicated that the Nb doping was mainly attributed to the morphology, the crystal structure, the optical band gap energy of Nb–TiO₂, and the photocatalytic degradation of methylene blue.     

Effect of Cd and Pb impurities on the optical properties of?fresh?evaporated amorphous (As2Se3)90Ge10 thin films

Transmission spectra (400–1500 nm) of thermally evaporated amorphous [(As₂Se₃)₉₀Ge₁₀]₉₅M₅ thin films have been analyzed to study the effect of impurities (M = Cd and Pb) on their optical properties. The refractive index increases with addition of metal impurities. The dispersion of refractive index has been studied using Wemple–DiDomenico single oscillator model. The optical gap has been estimated using Tauc’s extrapolation and was found to decrease with the addition of metal impurities from 1.46 to 1.36 eV (Cd) and 1.41 eV (Pb) with an uncertainty of ±0.01 eV. The change in optical properties with metal impurities has been explained on the basis of density, polarizability and bond energy of the system.     

Comparative first-principle analysis of un-doped and V3+-doped α-ZnAl2S4 spinel

The experimental and theoretical studies of the optical properties of pure α-ZnAl₂S₄ and α-ZnAl₂S₄:V³⁺ crystals were carried out. The ab initio and crystal field calculations of the structural and optical properties of α-ZnAl₂S₄:V³⁺ were compared with the corresponding experimental data. It was shown that the lowest vanadium 3d states are located at about 1.36 eV above the valence band's top. The complete energy level scheme of the α-ZnAl₂S₄:V³⁺ system, which includes the host's electronic band structure and impurity ion's energy levels, was suggested on the basis of the performed calculations.     

Optoelectronic properties of Cu1−xPtxFeO2 (0 ≤ x ≤ 0.05) delafossite for p-type transparent conducting oxide

The samples of Cu_1−xPt_xFeO₂ (0 ≤ x ≤ 0.05) delafossite have been synthesized by solid-state reaction method to investigate their optical and electrical properties. The properties of electrical resistivity and Seebeck coefficient were measured in the high temperature ranging from 300 to 960 K, and the Hall effect and the optical properties were measured at room temperature. The obtained results of Seebeck showed the samples are p-type conductor. The optical properties at room temperature exhibited the samples are transparent visible light material with optical direct gap 3.45 eV. The low electrical resistivity, hole mobility and carrier density at room temperature displayed value ranging from 0.29 to 0.08 Ω cm, 1.8 to 8.6 cm²/V s and 1.56 × 10¹⁸ to 4.04 × 10¹⁹ cm⁻³, respectively. The temperature range for transparent visible light is below 820 K because the direct energy gap contains value above 3.1 eV. Consequently, the Cu_1−xPt_xFeO₂ delafossite enhance performance for materials of p-type transparent conducting oxide (TCO) with low electrical resistivity.