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Abstract

We deposited epitaxial thin films of Morphotropic Phase Boundary (MPB) Pb0.65Ba0.35Nb2O6 (PBN:65) on MgO substrates using pulsed laser deposition. Afterwards, a novel transmission optical experiment was developed to measure the electric field-induced bending angle of the thin film sample using a divergent incident light. From which the electric field-induced strain was obtained, and it was used to calculate the electrostrictive constant of the PBN thin film. The result is 0.000875 μm2/V2, and it is consistent with what we measured in the reflection experiment

Abstract

A new approach for determination of refractive index dispersion n(λ) (the real part of the complex refractive index) and thickness d of thin films of negligible absorption and weak dispersion is proposed. The calculation procedure is based on determination of the phase thickness of the film in the spectral region of measured transmittance data. All points of measured spectra are included in the calculations. Barium titanate thin films are investigated in the spectral region 0.38–0.78 μm and their n(λ) and d are calculated. The approach is validated using Swanepoel’s method and it is found to be applicable for relatively thin films when measured transmittance spectra have one minimum and one maximum only.

Abstract

In our study, the 1% mol Eu2+ doped Li2CaSiO4: B3+ phosphors were prepared by the combustion method as fluorescent material for ultraviolet, light-emitting diodes (UV-LEDs) used as a light source. The properties of Li2 (Ca0.99, Eu0.01) SiO4: B3+ phosphors with urea concentration, doping boric acid and a series of initiating combustion temperature were investigated. The crystallization and particle sizes of Li2 (Ca0.99, Eu0.01) SiO4: B3+ has been investigated by using powder X-ray diffraction (XRD) and transmission electron microscopy (TEM). Luminescence measurements showed that the phosphors can be efficiently excited by UV to the visible region, and exhibited bluish green light with a peak of 480 nm. The results showed that the boric acid was effective in improving the luminescence intensity of Li2 (Ca0.99, Eu0.01) SiO4: B3+ and the optimum molar ratio of boric acid to calcium nitrate was about 0.06. The optimized phosphors Li2 (Ca0.99, Eu0.01) SiO4: B0.063+ showed 180% improved emission intensity compared with that of the Li2 (Ca0.99, Eu0.01) SiO4 phosphors under ultraviolet (λex =287 nm) excitation.

Abstract

A blue-green emitting phosphor (Ba1.95, Eu0.05)ZnSi2O7: Bx3+ was prepared by combustion synthesis and an efficient blue-green emission under near-ultraviolet was observed. The luminescence, crystallinity and particle sizes were investigated by using luminescence spectrometry, X-ray diffractometry (XRD) and scanning electron microscopy (SEM). The emission spectrum shows a single band centered at 503 nm, which corresponds to the 4f 65d 1 →4f 7 transition of Eu2+. The excitation spectrum is a broad band extending from 260 to 465 nm, which matches the emission of ultraviolet light-emitting diodes. The optical absorption spectra of the (Ba1.95, Eu0.05)ZnSi2O7: B0.063+ exhibited band-gap energies of 3.9 eV. The results showed that boric acid was effective in improving the luminescence intensity of (Ba1.95, Eu0.05)ZnSi2O7, and the optimum molar ratio of boric acid to zinc nitrate was about 0.06. The phosphor (Ba1.95, Eu0.05)ZnSi2O7: B0.063+ synthesized by combustion method showed 1.5 times improved emission intensity compared with that of the Ba1.95ZnSi2O7: Eu0.052+ phosphor under λex = 353 nm.

Abstract

In this study, the phosphors (Sr1−x , Znx)0.9(Al2−y , By)O4 doped 10 mol % Eu2+, were prepared by combustion method as the fluorescent material for white light emitting diodes (WLEDs), performing as a light source. The luminescent properties were investigated by changing the combustion temperature, the boron concentration, and the ratio of Sr to Zn. The luminescence, crystallinity and particle morphology were investigated by using a luminescence spectrometer, X-ray diffractometer (XRD) and transmission electron microscopy (TEM), respectively. The highest intensity of Sr0.9(Al2−y , By)O4: Eu0.12+ phosphor was achieved when the combustion temperature was 600° and the concentration of B3+ was 8 mol % of the aluminate. A new blue emission was observed when the high Zn concentration (x ⩾ 0.8), and this blue emission disappeared with the Zn concentration became lower than 0.8. The combustion method synthesized phosphor (Sr0.6, Zn0.4)0.9(Al1.92, B0.08)O4: Eu0.12+ showed 3.3 times improved emission intensity compared with that of the Sr0.9(Al1.92, B0.08)O4:Eu0.12+ phosphor under λex = 390 nm.

Abstract

In the present work, a CdTe alloy doped with a relatively high concentration of chromium (1%), and a CdTe:Cr layer, have been studied. Absorption and reflectivity spectra were measured at room temperature. They indicate the presence of chromium in the divalent state, both in the alloy and in the layer.

Abstract

Current–voltage characteristics of indium-embedded indium oxide thin films (600–850 Å), with Ag electrodes approximately 1000 Å thick, prepared by reactive evaporation of pure metallic indium in partial air pressure have been studied for substrate temperatures between 50 and 125°C. The optical properties of these films have also been investigated as a function of metallic indium concentration and substrate temperature. IV characteristics of all the samples are non-ohmic, independent of metallic indium concentration. The conductivity of the films increases but the optical transmission decreases with increasing metallic indium concentration. Metallic indium concentration was found to be an important parameter affecting the film properties. Furthermore, two possible conduction mechanisms are proposed.

Abstract

We report the photobehaviour of a series of eight structurally related arylacetylene derivatives, in solution as well as in pristine and PC61BM blended thin-_lms. The formation of both H- and J-aggregates in the solid state have been demonstrated, and, interestingly, an energy transfer from H-aggregates or/and from residual "unstacked" molecules to J-aggregates has been found, the latter being the only emitting species. The fuorescence quenching by PC61BM at di_erent loadings has been studied in blend films, and it has been found particularly effcient in the case of a symmetrical peripheral substitution of the acetylene derivative core. Preliminary time-resolved measurements in emission (ns resolution) and in absorption (fs resolution) con_rmed the H⟶J energy transfer and underlined the presence of delayed fuorescence from Jaggregates, formed by energy transfer from the long-lived first excited singlet state of H-aggregates. In all cases, a homogeneous surface morphology of thin films

Abstract

We report results obtained from measurements of optical transmittance spectra carried out on a series of silicon thin films deposited by plasma-enhanced chemical vapour deposition (PECVD) from silane diluted with hydrogen. Hydrogen dilution of silane results in an inhomogeneous growth during which the material evolves from amorphous hydrogenated silicon (a-Si:H) to microcrystalline hydrogenated silicon (µc-Si:H). Spectral refractive indices and absorption coefficients were determined from transmittance spectra. The spectral absorption coefficients were used to determine the Tauc optical band gap energy, the B factor of the Tauc plots, E 04 (energy at which the absorption coefficient is equal to 104 cm−1), and the Urbach energy as a function of the hydrogen dilution. The results were correlated with microstructure, namely volume fractions of the amorphous and crystalline phase with voids, and with the grain size.

Abstract

This paper gives an overview about recent theoretical and experimental work on electronic and optical properties of spinel ferrites MFe2O4. These compounds have come into focus of research due to their possible application as photocatalyst material for photoelectrochemical water splitting. The theoretical background of state-of-the-art quantum-chemical approaches applied for predicting electronic and optical band gaps, absolute band positions, optical absorption spectra, dielectric functions and Raman spectra, is briefly reviewed. Recent applications of first-principles methods on magnetic and electronic properties of ferrites with M = Mg and the first row of subgroup elements Sc to Zn are presented, where it is shown that the fundamental band gap is strongly dependent on the spin state and the degree of inversion of the spinel structure. The observed variation of electronic properties may serve as an explanation for the large scattering of experimental results. The exchange of M and Fe cations has also a pronounced effect on the Raman spectra of ferrites, which is analyzed at atomic scale from first principles. Calculated optical absorption spectra of ferrites are compared to experimental spectra. The electronic nature of the first excitations and the role of oxygen vacancies are discussed. For the calculation of absolute band positions, which have a significant impact on the photoelectrochemical activity of the ferrites, models of the most stable ferrite surfaces are developed that take into account their polar nature and the interaction with the solvent. Theoretically predicted valence and conduction band edges are compared to results from electrochemical measurements. The role of cation exchange on the surface electronic structure is investigated both theoretically and experimentally.