Optical,magneto-optical and mössbauer spectroscopy on Co3+ substituted cobalt ferrite Co2+Fe2−xCo3+xO4(0 ⩽ x ⩽ 2)

The magneto-optical spectra of Co_1+xFe_2?xO₄ show with increasing Co³⁺ content an increasing intensity of the 4A₂ ? 4T₁(F) and 4A₂ ? 4T₁(P) transition of Co²⁺ at 0.8 and 2.0 eV. A decrease in the Co²⁺-Fe³⁺ charge transfer transitions on octahedral sites is found. In the optical spectra a strong increase in optical absorption is found with dominant transitions at 0.8, 1.6 and 2.6 eV due to Co³⁺ crystal field transitions on octahedral sites and a Co²⁺-Co³⁺ charge transfer. Conversion Electron Mössbauer Spectroscopy has been used to determine the cation distribution in the surface layer of the samples. The results indicate a shift of Co²⁺ from octahedral to tetrahedral sites when Co³⁺ is substituted in CoFe₂O₄. This results in enhanced optical absorption, enhanced magneto-optical effects and a lower Curie temperature.     

Optical band-gap of Zn3As2

Absorption measurements of single Zn₃As₂ crystals were made at temperatures 5, 80 and 300 K. Free-carrier absorption is interpreted in the simple classical model. Interband absorption shows contributions from Urbach-like excitations. The direct optical gap has been estimated as 0.99 eV at 300 K, 1.09 eV at 80 K and 1.11 eV at 5 K. The linear dependence of band-gap on temperature was found in the range 80–300 K with dE_g/dT = ? 4.55 × 10^?4eVK^?1.     

Absorption spectra and optical gap energies of WxMo1−xO3

Optical absorption spectra of single crystals and powder specimens of W_xMo_1-xO₃ reveal fundamental absorption edges with gap energies of 2.77 eV (WO₃) and 3.05 eV (MoO₃). A continuous shift of the absorption edge and a simultanous appearance of absorption tails was found for ternary oxides. A theoretical model is proposed, based on the Anderson-localisation model with potential fluctuations due to tungsten-molybdenum disorder.     

Optical absorption due to transitions between bands and local levels in CdCr2Se4

The theory of optical absorption due to transitions between a valence band and a hydrogen-like local level associated with a conduction band is modified to permit an arbitrary power-law dependence of energy on the magnitude of the wave-vector of carriers in the valence band. The observed absorption for photon energies below 1.6 eV in the ferromagnetic semiconductor CdCr₂Se₄ is discussed in terms of a combination of two types of terms. The first type of absorption is due to transitions to a local level from a band with two branches, in each of which there is an energy region with a width of 0.28 eV or more beginning 0.10–0.16 eV from the band edge, in which the energy measured from some origin near but not necessarily equal to the band-edge is approximately proportional to (wave-vector)^{($\text{1}\text{3}$)}. The second type of absorption has a dependence on photon energy ?ω of the form (?ω ? E₃)², where E₃ is a threshold energy probably connected with indirect transitions between bands as suggested by Sakai, Sugano and Okabe. After constraints on parameters appearing in the theory are imposed by use of results of these authors and of Shepherd, it is found that curves of Harbeke and Lehmann on optical absorption in CdCr₂Se₄ at 4.2, 78, 130 and 298 K in the photon-energy range 1.14–1.42 eV can be fitted to a mean accuracy of 3%, using an average of 3.75 adjustable parameters for each curve. The strength of the indirect band-to-band absorption does not have the temperature dependence expected for phonon-assisted indirect band-to-band transitions, but can be described by a term independent of temperature plus another term proportional to the square of the deviation of the magnetization from saturation. The fitting of the absorption curves requires that the ratio of the widths of the two branches of the bands varies from about 1.6 at low temperatures to 1.35 at 298 K and that the total width of the bands involved is less than 1 eV.     

Investigation of the optoelectronic properties of columbite ZnNb2O6 crystal

A high-quality ZnNb₂O₆ single-crystal grown by optical floating zone method has been used as a research prototype to analyze the optoelectronic parameters by measuring the absorption coefficient and transmittance spectra along the b-axis from 200 nm to 1000 nm at room temperature. The optical interband transitions of ZnNb₂O₆ have been determined as a direct transition with a band gap of 3.84 eV. The refractive index, extinction coefficient, and real and imaginary parts of the complex dielectric constants as functions of the wavelength for ZnNb₂O₆ crystal are obtained from the measured absorption coefficients and transmittance spectra. In the Urbach tail of 3.16–3.60 eV, the validity of the Cauchy–Sellmeier equation has also been evaluated. Using the single effective oscillator model, the oscillator energy E_o is found to be 4.77 eV. The dispersion energy E_d is 26.88 eV and ZnNb₂O₆ crystal takes an ionic value.     

Exciton photoluminescence, photoconductivity and absorption in GaSe0.9Te0.1 alloy crystals

The photoluminescence, photoconductivity and absorption in GaSe_0.9Te_0.1 alloy crystals have been investigated as a function of temperature and external electric field. It has been observed that the exciton peaks shift to lower energy in GaSe_0.9Te_0.1 alloy crystals compared to GaSe crystals. The long wavelength tails of interband photoluminescence, photoconductivity and absorption spectra are determined by the free exciton states and show an Urbach-Martienssen-type dependence to the photon energy. The maxima of the extrinsic photoluminescence and photoconductivity spectra were found to be determined by the acceptor centers with an energy of E_A=E_V+0.19 eV formed by the polytypism and defects complexes that include Se and Te anions.     

Photoconductivity spectra of HgCr2Se4

The photoconductivity spectra of ferromagnetic semiconductor HgCr₂Se₄ single crystals in the intrinsic transition region were measured at a number of fixed temperatures covering T_c = 106 K. The photoconductivity edge, 0.88 eV, at room temperature shifted to 0.35 eV at 52 K.The temperature dependence of the photoconductivity edge is in good correspondence with that of the absorption edge, and is explained qualitatively well with the calculated spin correlation functions in spinels.From the theoretical calculations by Kambara et al., the lowest transition associated with the photoconduction in HgCr₂Se₄ was interpreted to be the electronic transition from the highest d? to the lowest A [dγ-p] states for up-spin at X point. The many doby effects should be taken into account.     

Photoluminescence of polycrystalline CuIn0.5Ga0.5Te2 thin films grown by flash evaporation

Polycrystalline CuIn_0.5Ga_0.5Te₂ films were deposited by flash evaporation from ingot prepared by reacting, in stoichiometric proportions, high purity Cu, In, Ga and Te elements in vacuum sealed quartz. The as-obtained films were characterized by X – ray diffraction (XRD), transmission electron microscopy (TEM) combined with energy dispersive spectroscopy (EDS). XRD and TEM results showed that the layer has a chalcopyrite-type structure, predominantly oriented along (112) planes, with lattice parameters a?=?0.61?nm and c?=?1.22?nm. The optical properties in the near - infrared and visible range 600–2400?nm have been studied. The analysis of absorption coefficient yielded an energy gap value of 1.27?eV. Photoluminescence analysis of as-grown sample shows two main emission peaks located at 0.87 and 1.19?eV at 4?K.     

Mg substitutions in ZnO-Al2O3 thin films and its effect on the optical absorption spectra of the nanocomposite

ZnO-Al₂O₃ nanocomposite thin films were prepared by sol-gel technique. The room temperature synthesis was mainly based on the successful peptization of boehmite (AlO(OH)) and Al(OH)₃ compounds, so as to use it as matrix to confine ZnO nanoparticles. The relative molar concentrations of xZnO to (1 − x) Al₂O₃ were varied as x = 0.1, 0.2 and 0.5. The optical absorption spectra of the thin films showed intense UV absorption peaks with long tails of variable absorption in the visible region of the spectra. The ZnO-Al₂O₃ nanocomposites thin films were doped with MgO by varying its molar concentrations as y = 0.05, 0.75, 0.1, 0.125, 0.15 and 0.2 with respect to the ZnO present in the composite. The MgO doped thin films showed suppression of the intense absorption peaks that was previously attained for undoped samples. The disappearance of the absorption peaks was analyzed in terms of the crystalline features and lattice defects in the nanocomposite system. The bulk absorption edge, which is reportedly found at 3.37 eV, was shifted to 5.44 eV (for y = 0.05), 5.63 eV (for y = 0.075) and maximum to 5.77 eV (for y = 0.1). In contrast, beyond the concentration, y = 0.1 the absorption edges were moved to 5.67 eV (for y = 0.125), 5.61 eV (for y = 0.15) and to 5.49 eV (for y = 0.2). This trend was explained in terms of the Burstein-Moss shift of the absorption edges.     

Fundamental optical properties of α-RuCl3

The fundamental optical properties in the paramagnetic phase of α-RuCl₃ are studied at different temperatures in the photon energy interval 0.03 to 10 eV. Infrared reflectivity spectra show a transverse optical frequency at 0.038 eV (32 μm) for an E_u mode (E⊥c, in plane atomic displacements). The absorption spectra in the energy range 0.2 to 1 eV reveal three bands (0.29, 0.51, 0.71 eV) attributed to d-d electronic transitions. Reflectance and thermo-reflectance measurements indicate the onset of the charge-transfer transitions at 1.1 eV and show structure at 1.85, 2.55, 3.05, 4.5 eV. The marked reflectivity peak at 5.2 eV is probably related to p(Cl) → s(Ru) band-to-band transitions.     

Optical properties of amorphous CuInSe2

Optical absorption spectra of polycrystalline and amorphous CuInSe₂ thin films were measured at room temperature in the photon energy range from 0.8 to 2.1 eV. In amorphous CuInSe₂ the absorption coefficient follows the relation α(hv) = A(hv?E₀)/hv characteristic of optical transitions between extended states in both the valence and conduction band. The optical gap of E₀ = 1.38 ± 0.01 eV is larger than the fundamental gap energy of E_g = 1.01 ± 0.01 eV in crystalline CuInSe₂. A comparison of the results for CuInSe₂ with those for ZnSe is given.     

Formation of Ge35In8S57 amorphous films for optical applications

Thin films of In-doped Ge-S in the form of Ge₃₅In₈S₅₇ with different film thickness were deposited using an evaporation method. The X-ray diffraction studies demonstrate that the as-prepared films are amorphous in nature for these films. Some optical constants were calculated at a thickness of 150, 300, 450 and 900?nm and annealing temperature of 373, 413, 437 and 513?K. Our optical observations show that the mechanism of the optical transition obeys the indirect transition. It was found that the energy gap, E_g, decreases from 2.44 to 2.20?eV with expanding the thickness of the film from 150 to 900?nm. On the other hand, it was found that E_g increases with annealing temperature from 373 to 513?K. The increment in the band gap can be attributed to the gradual annealing out of the unsaturated bonds delivering a decreasing the density of localized states in the band structure. Using the single oscillator model, the dispersion of the refractive index is described. The dispersion constants of these films were calculated with different both thickness and annealing temperatures. Additionally, both of nonlinear susceptibility, χ⁽³⁾ and nonlinear refractive index, n₂ were calculated.     

Electrical conduction and 1ƒ noise in Li-DOPED MnO

Electrical resistivity, thermoelectric power and current noise were measured on Li-doped MnO single crystals in the temperature range from 300 to 1000 K. Below 700 K the crystals are p-type and the activation energy of the resistivity is 0.75 eV. Around 700 K the activation energy changes from 0.75 to 1.25 eV owing to a change from p- to n-type conduction. The depth of the Li acceptor is found to be 0.65 eV. From resistivity and thermoelectric power data it is concluded that the bandgap in first approximation can be written as E_s(T) = E_o ? γT between 750 and 1000 K, with E_o = 1.9 eV and γ = 6 × 10^?4 eV/K. The current noise spectra show $\text{1}\text{?}$ noise. The magnitude of the $\text{1}\text{?}$ noise is strongly temperature dependent. From the noise data it is deduced that E_o = 2.2 eV and γ = 10^?3 eV/K in the temperature range 430–700 K.     

Physical properties and lithium intercalates of CrPS4

Chromium thio-phosphate, CrPS₄, has a layered structure based on hexagonally close packed sulfur layers. Magnetic susceptibility measurements indicated an antiferromagnetic ordering perpendicular to the C axis and the presence of trivalent chromium in the paramagnetic region. Optical transmission spectra showed the fundamental absorption edge to occur at 2.4 eV. Using n-butyl lithium techniques, it was found that CrPS₄ can readily accomodate lithium. This insertion is not allowed by any parameter change in the host structure nor reduction of the Cr³⁺ magnetic moment. Lithium NMR measurements gave a rather high (0.31 eV) energy of activation for lithium movement and use of CrPS₄ as a cathode against a lithium anode showed a sharp drop in potential.     

A shock-tube measurement of the SiO(E1Σ+ − X1Σ+) transition moment

The sum of the squares of the electronic transition moments, Σ|R_e|², for the E¹Σ⁺ ?X¹Σ⁺ band system of SiO has been determined from absorption measurements conducted in the reflected-shock region of a shock tube. The test gas was produced by shock-heating a mixture of SiCl₄, N₂O and Ar, and the spectra were recorded photographically in the 150–230 nm wavelength range. The values of the Σ|R_e|² were determined by comparing the measured absorption spectra with those produced by a line-be-line synthetic spectrum calculation. The value of the Σ|R_e|² so deduced at an r-centroid value of 3.0 Bohr was 0.86±0.10 atomic units.     

Absorption spectra near the absorption edge of the system HgxZn1−xCr2Se4

Optical absorption spectra near the absorption edge of the system Hg_xZn_1?xCr₂Se₄ have been measured for various compositions in the temperature range between room and liquid helium temperatures. A correlation between the optical properties and magnetic structures becomes obvious from the measurements of the energy at the absorption edge and the change in the magnetic property with composition variation, and by measuring the optical absorption spectra under the applied magnetic field.     

Structural,electronic and optical properties of Ca3Bi2: First-principles investigation

In this work by applying first principles calculations structural, electronic and optical properties of Ca₃Bi₂ compound in hexagonal and cubic phases are studied within the framework of the density functional theory using the full potential linearized augmented plane wave (FP-LAPW) approach. According to our study band gap for Ca₃Bi₂ in hexagonal phase are 0.47, 0.96 and 1?eV within the PBE-GGA, EV-GGA and mBJ-GGA, respectively. The corresponding values for cubic phase are 1.24, 2.08 and 2.14?eV, respectively. The effects of hydrostatic pressure on the behavior of the electronic properties such as band gap, valence bandwidths and anti-symmetry gap are investigated. It is found that the hydrostatic pressure increases the band widths of all bands below the Fermi energy while it decreases the band gap and the anti-symmetry gap. In our calculations, the dielectric tensor is derived within the random phase approximation (RPA). The first absorption peak in imaginary part of dielectric function for both phases is located in the energy range 2.0–2.5?eV which are beneficial to practical applications in optoelectronic devices in the visible spectral range. For instance, hexagonal phase of Ca₃Bi₂ with a band gap around 1?eV can be applied for photovoltaic application and cubic phase with a band gap of 2?eV can be used for water splitting application. Moreover, we found the optical spectra of hexagonal phase are anisotropic along E||x and E||z.     

IR absorption in p-type pb0.97Sn0.03Se

IR absorption in p-type melt grown Pb_0.97Sn_0.03Se crystals is reported. The results for the room temperature absorption coefficient (α) in the wavelength region 2–15 μm are analysed. The indirect absorption edge is found to be at 0.26 eV for this ternary alloy. In the longwavelength region α is found to be proportional to λ², in agreement with the classical free carrier absorption expression. The conductivity effective mass of holes is found to be 0.067 m₀ at 300°K.     

Polarization-modulated magnetoreflection in magnetic semiconductors: CdCr2S4 and EuSe

The polarization-modulated (PM) magnetoreflection technique has been used to study magnetic ordering effects in CdCr₂S₄ and EuSe. The energy gap for direct band-band transitions in CdCr₂S₄ has been measured to be E_G = 2.3 eV, and the exchange splitting of the valence band to be about 0.03 eV. Good agreement with thin film absorption measurements is obtained in the temperature dependence of spectral structure, observed at energies less than E_G, associated with crystal field and charge transfer transitions. No strongly blue-shifting peak is observed with magnetic order. PM magnetoreflection spectra of the E₁ peak of EuSe show a direct manifestation of the spin alignment from the ferrimagnetic to ferromagnetic state of this crystal at 2 K in external fields up to 16 kOe. An attempt is made to explain resonant Raman scattering in EuSe (observed by other workers) in terms of the field-induced shift of one of the polarized E₁ reflectivity components into coincidence with the 5145 Å argon-ion laser line.     

Electrical and optical absorption studies of Cu7GeS5I fast-ion conductor

Electrical conductivity and fundamental absorption spectra of monocrystalline Cu₇GeS₅I were measured in the temperature ranges 95-370 and 77-373 K, respectively. A rather high electrical conductivity (σ_t=6.98×10⁻³Ω⁻¹ cm⁻¹ at 300 K) and low activation energy (ΔE_a=0.183 eV) was found. The influence of different types of disordering on the Urbach absorption edge and electron-phonon interaction parameters were calculated, discussed and compared with the same parameters in Cu₇GeS₅I, Cu₆PX₅I (X=S,Se) and Ag₇GeX₅I (X=S,Se) compounds. We have concluded that the P→Ge and Cu→Ag cation substitution results in an increase of the electrical conductivity and a decrease of the activation energy. Besides, P→Ge substitution, results in complete smearing and disappearance of the exciton absorption bands and in blue shift of the Urbach absorption edge, an increase of the edge energy width and an electron-phonon-interaction enhancement.