Stability of E′ centers induced by 4.7 eV laser radiation in SiO2

The kinetics of E′ centers ( Si) induced by 4.7 eV pulsed laser irradiation in dry fused silica was investigated by in situ optical absorption spectroscopy. The stability of the defects, conditioned by reaction with mobile hydrogen of radiolytic origin, is discussed and compared to results of similar experiments performed on wet fused silica. A portion of E′ centers and hydrogen are most likely generated by laser-induced breaking of Si–H precursors, while an additional fraction of the paramagnetic centers arise from another formation mechanism. Both typologies of E′ centers participate in the reaction with H₂ leading to the post-irradiation decay of the defects. This annealing process is slowed down on decreasing temperature and is frozen at T = 200 K, consistently with the diffusion properties of H₂ in silica.     

Rare earth centers properties and electron trapping in SnO2 thin films produced by sol–gel route

Some very relevant optical, electrical, and structural properties of SnO₂ doped with rare-earth ions Er³⁺ and Eu³⁺ are presented. Films are produced by the sol–gel-dip coating process, and may be described as a combination of nanoscopic dimension crystallites (about 3–10 nm) with their respective intergrain potential barriers. The Er³⁺ and Eu³⁺ ions are expected to act as acceptors in SnO_2, which is a natural n-type conductor, inducing a high degree of charge compensation. Electron trapping and emission spectra data are presented and are rather distinct, depending on the location of the rare-earth impurity. This behavior allows the identification of two distinct centers: located either in the SnO₂ lattice or segregated at the particles surface. Based on a model for thermally activated cross-section defects, the difference between the capture energy of the photo-excited electron and the intergrain potential barrier is evaluated, leading to distinct values for high and low symmetry sites. A higher distortion in the lattice of undoped SnO₂ and SnO₂:Eu (1 at.%) was evaluated from Rietveld refinements of X-ray diffraction data. This was confirmed by Raman spectra, which are associated with the particles size and disorder. By comparing the samples with the same doping concentration, it was found that this disorder is higher in Eu-doped SnO₂ than in Er-doped SnO₂, which is in agreement with a higher energy for the lattice relaxation in the trapping process by Eu³⁺ centers.     

Synthesis and characterization of amorphous aluminosilicates prepared by sol–gel to encapsulate organic dyes

We synthesized via the sol–gel process three amorphous aluminosilicates having the stoichiometry of zeolite types A, X, and Y, respectively. The aluminosilicate, so-called SX, having the chemical composition of faujasite X, proved to be amorphous in the range 298 K up to 1273 K. At the higher temperature the sample begins to crystallize as nepheline. The anionic organic dye 1,2-dihydroxyanthraquinone (Alizarine, AL) was successfully encapsulated in the aluminosilicate matrix SX. Still, 7-α-D-glucopyranosil-9,10-dihydro-3,5,6,8-tetrahydroxy-7-methyl-9, 10 dioxanthracene carboxylic acid (carminic acid, CA) could only be deposited on the matrix surface. The adsorbents and the dye impregnated matrices were characterized using several techniques: Fourier Transform Infrared (FTIR) and Diffuse Reflectance Spectroscopies used to determine the hydroxylation as well as the adsorbed species, Small Angle X-ray Scattering (SAXS) to know the shape of the heterogeneities and the fractal dimension of each sample, nitrogen physisorption to measure the specific surface area and X-ray Diffraction (XRD) to identify the crystalline compounds present in the samples. Therefore, with these complementary techniques, the structure and the morphology of the samples were obtained.     

Crystallization of sputtered amorphous silicon induced by silver–copper alloy with high crystalline volumeratio

Silver–copper alloy-induced crystallization of sputtered a-Si has been studied. In this alloy, Cu acts as a catalyst to accelerate the crystallization, while Ag acts as a new kind of buffer layer, different from Al₂O₃ and Si₃N₄, to obtain well-crystallized poly-Si films with short annealing time and free of post-treatment for ohmic contact. When the Cu content is limited to below 30%, Ag can effectively slow down the diffusion of Cu into Si and decrease the Cu–silicide nuclei density to improve the crystalline volume ratio from 80% to over 90%. A 1:4 ratio of Cu to Ag yields the best result. The crystalline volume ratio and Hall mobility reach nearly 100% and 29.4 cm²/V s, respectively. This high quality poly-Si film demonstrates a promising application in solarcells.     

Junction capacitance study of an oxygen impurity defect exhibiting configuration relaxation in amorphous silicon–germanium alloys deposited by hot-wire CVD

We report the observation of light induced electron capture in oxygen contaminated (～5 × 10²⁰ cm^?3) hydrogenated amorphous silicon–germanium alloys grown by hot-wire chemical vapor deposition (HWCVD). By examining the time evolution of dark capacitance after 1.2 eV photoexcitation, we are able to estimate the free energy barrier (?0.8 eV) for the release of electrons into the conduction band. Such a large thermal barrier, for a defect whose optical threshold is centered (～1.35 eV) so close to the band-gap (1.5 eV), indicates significant configurational relaxation once the oxygen impurity state is occupied with photoexcited electrons.     

Temperature dependence of the generation and decay of E′ centers induced in silica by 4.7 eV laser radiation

We report a study of the generation of silicon dangling bonds (E′ centers) induced in fused silica by 4.7 eV laser irradiation in the 10 < T < 475 K temperature range, carried out by in situ optical absorption spectroscopy. The generation of the defects, occurring by transformation of pre-existing precursors, results to be a thermally activated process, quenched below 150 K and with a 0.044 eV activation energy. At T > 200 K the induced defects undergo a post-irradiation decay due to their reaction with mobile H₂. The interplay between generation and annealing gives rise to a bell-shaped temperature dependence of the concentration of induced E′ centers, peaking at 250 K.     

Preparation and characterization of ZnO particles embedded in organic–inorganic planar waveguide by sol–gel route

Hybrid organic–inorganic waveguides based on ZnO-(3-glycidoxypropil)trimethoxisilane (GPTS) have been fabricated by sol–gel route. A transparent sol of ZnO was added to the GPTS host and the resulting sol was deposited on silica substrates by spin coating technique. Waveguides with different molar composition (100?x)GPTS?xZnO (x = 10, 20, 30) were investigated by different diagnostic techniques. Morphological measurements were carried out by means of an AFM apparatus, and a roughness of few nanometers was estimated for all the waveguides. Optical properties such as refractive index, thickness, number of propagating modes and attenuation coefficient were measured at 632.8, 543.5, 1319 and 1542 nm by the prism coupling technique as a function of the ZnO content. Photoluminescence measurements, upon excitation at 325 nm, showed a large luminescence band in the region between 350 and 600 nm with a main peak centered at about 380 nm, due to the presence of ZnO nanoparticles.     

Concentration growth and thermal stability of γ-ray induced germanium lone pair center in Ge-doped sol–gel a-SiO2

We report an experimental study of the concentration growth by γ-ray irradiation of germanium lone pair center (GLPC) in 10⁴ part per million molar Ge-doped sol–gel silica. The data show that γ-ray induced GLPC concentration increases linearly up to ～5 MGy and then it seems to reach a limit value. In addition to the dose dependence, we have studied the thermal stability of the radiation induced GLPC in ambient atmosphere up to 415 °C. We found that the concentration of this latter GLPC starts to decrease at ～300 °C, at variance to native GLPC, suggesting that the annealing is related to irradiation products. After the thermal treatments the photoluminescence (PL) activity of the γ induced GLPC shows some spectroscopic differences with respect to that related to the native. These differences have been investigated both as a function of excitation energy and by recording the photoluminescence time decay at low temperature. The obtained results are interpreted on the basis of thermally induced changes in the environment of the defect.     

The effect of zirconia content on properties of Al2O3–ZrO2 (Y2O3) composite nanopowders synthesized by aqueous sol–gel method

Al₂O₃–ZrO₂ (Y₂O₃) nanopowders containing 5, 10 and 15 wt% ZrO₂ were synthesized by aqueous sol–gel method using aluminum sec-butoxide and zirconium butoxide as precursors. BET analysis shows that, increasing the zirconia content results in a decrease in surface area, 152, 125 and 121 m²/g, and an increase in pore size, 5.63, 9.79 and 11.05 nm for 5, 10 and 15 wt% ZrO₂, respectively. Furthermore, a shift toward higher temperatures is observed for transition of transitional aluminas to stable α-alumina phase through increasing the zirconia content. SEM micrograph of calcined nanopowders revealed nanosize spherical particles in the range of 15–75 nm.     

The role of surfactant in synthesis of magnetic nanocrystalline powder of NiFe2O4 by sol–gel auto-combustion method

In this work, a new sol–gel auto-combustion method has been performed to synthesize single phase nickel ferrite nanocrystalline powders by using n-cetyltrimethylammonium bromide, as a cationic surfactant. The gels were prepared from ferric and nickel nitrates and citric acid. Ammonia was used as pH adjusting agent as well. The effects of the surfactant on the after combustion calcination process and the reduction of the resulting powder crystallite size which affects the magnetic properties of the material were investigated by XRD and DTA/TGA techniques. The results showed that the ignition of the gels in air have a self-propagating behavior. Addition of surfactant to the starting solution affected the crystallite size of the synthesized powders and their phase constitution. The crystallite size of nickel ferrite powder in sample with surfactant was obtained 31.2 nm. The other important result of this study was production of single phase nickel ferrite directly after combustion while without surfactant the nickel ferrite single phase was obtained only after a post-calcination process at 1000 °C.