Papers by Keyword: Optical Absorption

Abstract: Herein, the lithium yttrium fluoride (LiYF4) compound doped with rare-earth Nd3+ ions are synthesized by solid-state reaction methods. The results of XRD, optical absorption, and emission measurements confirm that the doped powder sample was successfully synthesized. The optical properties of Nd-doped LiYF₄ powder samples were investigated through a comparison of ab initio and experimental methods. We predicted the crystal and electronic structures using ab initio total energy calculations within the framework of density functional theory (DFT). Our results for the optical properties, including the dielectric function, absorption coefficient, and refractive index of the Nd-doped LiYF₄ compound, were calculated and compared using the independent particle approximation (IPA), time-dependent density functional perturbation theory (TDDFpT), and random phase approximation (RPA).

Abstract: The thermochromic properties (color change with temperature) of n type doped SiC wafers of different polytypes (3C, 4H and 6H) have been investigated up to 500°C under air. It was found that 3C-SiC color passes from bright yellow at room temperature to deep orangeat 500°C leading to a color contrast (ΔE) as high as 64. The hexagonal polytypes undergo also a color change upon heating but far less pronounced, with ΔE values <20. All these semiconductors undergo band gap shrinkage upon heating which effect largely participated to the observed color change. This effect is very sensitive for 3C polytypesince its bandgap is already in the visible energy range at room temperature. The thermochromicity of 3C-SiC was found to be reversible thanks to its thermal stability and its resistance towards oxidation.

Abstract: Doping of perovskite solar cells is a regularly used approach to adjust and modify the structures and properties of organic-inorganic hybrid perovskite such as CH3NH3PbI3 material, and subsequently increase the conversion efficiency. In this work, optical absorption is calculated for copper-doped-perovskite material based on the density functional theory analysis for tetragonal crystal structures. We investigated the effect of doping CH3NH3PbI3 with copper, as an optical alternative to Pb atom in central tetragonal structure. As a conclusion, some enhancements of the optical properties by the replacement of Cu by the Pb atom are observed, such as optical absorption at certain visible spectrum regions, along with more intensive field mapping. This optical enhancement can lead to a better improvement of perovskite solar cell according to metallic-doing substitutional defect concept.

Abstract: The intermediate band semiconductor of AgGa_1-xCr_xS₂ is investigated by the first principles calculations and further confirmed by the experimental results. The band structures of pure and Cr-doped crystals were calculated and it is shown that the crystal with a direct energy band gap of about 0.95 eV for AgGaS₂. Because of Cr dopant, a metallic intermediate band (IB) is successfully formed in the host. From the partial density of states (PDOS) of Cr-doped AgGaS₂, the IB mainly comes from the hybridization of the Cr-3d and S-3p states. Based on the theoretical predications, the Cr-doped AgGaS₂ is synthesized by the high-temperature solid state reaction. Two extra absorption responses are detected in the absorption spectra. The optical absorption coefficients are enhanced in the visible radiation range due to the formation of metallic and isolated IB. Therefore, Cr-doped AgGaS₂ with an intermediate band is suggested as a potential material to enhance the efficiency of solar cells.

Abstract: Optical absorption and photoluminescence (PL) spectra were measured on defect-stannite-type semiconductor ZnGa₂Se₄ at temperatures T from 11 to 300 K. The square of the absorption coefficient spectra showed distinct two absorption edges, which were E_0A,B and E_0C,D transitions at Γ point in the Brillouin zone. The temperature dependence of the direct-gap energies, E_0A,B and E_0C,D, of ZnGa₂Se₄ were determined and fit using the analytical four-parameter expression developed for the explanation of the band-gap shrinkage effect in semiconductors. The PL emissions at near band-edge and at higher energy than band-edge were also observed at T ≤ 150 K.

Abstract: This work reported the influence of seaweed (Sargassum crassifolium) extract on the absorption of magnetic iron oxide nanoparticles (MIONPs) via green synthesis route. The seaweed extract acted as reducing and capping agent. Ultraviolet-visible (UV-Vis) spectra revealed the presence of sharp peaks at around 366 nm to 371 nm which corresponds to the absorption spectra of MIONPs. The absorption peaks corresponding to MIONPs was significantly affected by the varying amount of seaweed extract. On the other hand, Fourier Transform-Infrared (FTIR) spectra revealed the presence of the vibration mode around 532 cm^-1 to 551 cm^-1 which corresponds to Fe-O bands suggesting the successful formation of MIONPs. Thermogravimetric (TGA) spectra showed that the produced MIONPs stabilizes at 400 °C. The synthesized MIONPs are spherical in shape with an estimated average diameter of 20 nm as revealed by the Field Emission Scanning Electron Microscopy (FESEM). Finally, the produced MIONPs exhibit magnetic property via attraction with an external magnet. This straight forward green synthesis of MIONPs can be a good route for possible industrial and environmental application.

Abstract: In the present work, we studied the structural and optical properties of zinc-titania tellurite glass (75-x)TeO₂-20ZnO-5TiO₂-xEr₂O₃ doped with erbium ion Er³⁺ (where mol % : x= 0 (host), 0.5, 1, 1.5). The reported glasses were synthesized using tradition melt quench method. The X-ray diffraction and thermal profile were utilized to study the amorphous nature of the mentioned glass. Optical absorption UV-Visible-NIR spectra were measured for optical energy gap and Er³⁺ absorption bands analysis. Both visible and near infrared luminescence were performed at room temperature using 980 nm laser diode excitation where the optical transition mechanism of the Er³⁺ in the glass is finally proposed. Broad luminescence in the present glass suggests its potential for broadband optical application.

Abstract: Lanthanum and silver co-doped zinc borotellurite glasses having chemical composition of [{[(TeO₂)_0.7(B₂O₃)_0.3]_0.7(ZnO)_0.3}_0.96 (La NPs)_0.04]_1-x(Ag₂O)_x with 0.02 ≤ x ≤ 0.10 molar fraction were fabricated via melt-quenching technique. X-ray diffraction (XRD) analysis confirmed the amorphous nature of the samples through the presence of broad hump instead of sharp peaks. Optical properties of the samples were determined through ultraviolet-visible spectroscopy (UV-Vis) in the range of 220 to 800 nm. Fundamental absorption edge observed in the optical absorption spectra shifts to longer wavelength as the concentration of silver in the glass system increases. Decreasing trend for indirect energy band gap and increasing trend of Urbach energy values with the increment of Ag content suggest the contribution of Ag to the formation of nonbridging oxygen. Enhanced optical properties of the prepared glass hints the possible application of the glass material as optical fiber in photonic field.

Abstract: Metal nanoparticles have attracted more and more attention in the last years due to their unique chemical and physical properties which are very different from the metal bulk material. In particular, when the size of nanoparticles decreases below two nm, nanoparticles can be described as nanoclusters (NCs), and they present peculiar optical properties. The excited electrons in addition to specific absorption bands show also a bright luminescence related to the quantum size effect which produce discrete energy levels. Optical properties (absorption and fluorescence) of these NCs are widely used in many different applications in science and engineering, such as chemical sensors, fluorescent probes for bio imaging or in environmental issues. In the present study, we report on the synthesis of silver nanoclusters (AgNCs) in aqueous phase using silver nitrate as precursor salt and L-Glutathione (GSH) as stabilizer. AgNCs were characterized using absorption and fluorescence spectroscopy, and transmission electron microscopy (TEM). The strong absorption and luminescence shown by these NCs are very promising for a possible exploitation both as label for bioimaging and for optical sensors for heavy metal ions.

Abstract: Absorption spectra of different composition of tellurite glasses were analyzed in order to estimate their lasing properties. Composition of the investigated tellurite glasses are: 55TeO₂-(43-x)ZnO-2Bi₂O₃-xEr₂O₃ (x = 0.5, 1.0, 1.5, 2.0, 2.5, 3.0 mol%). Glasses absorption spectra and density were measured at room temperature using UV-VIS-NIR Spectrophotometer and pycnometer, respectively; while their refractive index were measured by implying Brewster’s angle method. Lasing performance of glasses were then analyzed using Judd-Ofelt theory in order to obtain line strength (S), Judd-Ofelt parameters (Ω_t, t = 1, 2, 3), and radiative life-times (τ). It is seen that incorporating Er³⁺ ions of up to 2.5 mol% resulted in decreasing the radiative lifetime of electrons sitting at ⁴I_11/2. From Judd-Ofelt parameters analysis it can be concluded that these tellurite glasses tend to have the covalence nature of Er-O bond and thus represents the asymmetry around the Er³⁺ ions site.