F.B. Dejene

Effects of catalyst/zinc mole fraction on ZnAl2O4:0.01% Cr3+ nanocrystals synthesized using sol–gel process

A report on the sol–gel preparation of nanosized ZnAl2O4:0.01% Cr3+ at a relatively low temperature (~80 °C) is presented. The catalyst/Zn mole fraction in the solution was varied from 0–3 during the synthesis. The x-ray diffraction (XRD) data show that the annealed samples were pure cubic crystalline structures at the lower catalyst/Zn mole fractions. However, minor diffraction peaks associated with ZnO were detected at the higher catalyst/Zn mole fractions. An increase in the mole fraction led to transformation of morphology from spherical particles to rod-like-structures. Time-of-flight secondary ion mass spectrometry (TOF-SIMS) analysis and elemental mapping revealed the presence of Zn, Al, O and Cr. In addition, the catalytic mole fraction influenced the band-gap of the ZnAl2O4 host. Photoluminescence (PL) spectra indicated that the undoped and 0.01% Cr3+-doped powders exhibit a bluish-violet emission at slightly different peak positions, suggesting that the emission could either be originating from the host or the Cr3+ dopant ion. The emission from the host was attributed to oxygen vacancies (Vo*), while emissions from the Cr3+ were attributed to the transitions from different energy levels in the Cr3+ ion. It was also noted that the PL emission intensity was dependent on the catalyst/Zn mole fraction. The colour chromaticity showed that the emission colour of the ZnAl2O4:0.01% Cr3+ phosphor could be tuned by varying the catalyst/Zn mole fraction.

Influence of acidic pH on the formulation of TiO2 nanocrystalline powders with enhanced photoluminescence property

Titanium dioxide (TiO2) nanoparticles were prepared by the sol-gel method at different pH values (3.2–6.8) with a hydrochloric acid (HCl) solution. Raw samples were calcined at 500 °C for 2 h. The effects of pH on the structural, morphological and optical properties of TiO2 nanoparticles were investigated. At pH 4.4–6.8, only the anatase phase of TiO2 was observed. Under strong acidic condition at pH 3.2 rutile, brookite and anatase co-exist, but rutile is the predominant phase. The strain value increased and the crystallite size decreased as the HCl content increased. The increased crystallite sizes in the range 21–24 nm and enhanced blue emission intensity around 432 nm was obtained for the sample at pH 5.0. Experimental results showed that TiO2 nanoparticles synthesized at pH 5.0 exhibited the best luminescence property with pure anatase phase.

Effect of Eu3+ Ion Concentration on Phase Transition, Site Symmetry and Quantum Efficiency of ZrO2 Nanocrystal Rods

This work report the influence of Eu3+ ion concentration on the photophysical properties of zirconia nanocrystal rods including its intrinsic quantum efficiency (IQE). A simple chemical route was employed in the synthesis procedure. X-ray diffraction results show mixed phases of monoclinic and tetragonal structures. Phase transition occurred at low (1 mol%) and high (7 and 8 mol%) Eu3+ concentrations. There are three forms of excitations for this phosphor; band edge excitation at 216 nm, charge transfer state transition at 247 and 263 nm, and direct excitation at 362, 395 and 535 nm. Photoluminescence emission for all the doped samples at room temperature appeared to be entirely from intraconfigurational Eu3+ emissions and depends both on the site symmetry as well as the Eu3+ concentration. Low temperature measurements indicate the presence of defect bands associated with oxygen vacancies. Defects were also shown to be temperature dependent. The Eu3+ ions were distributed in both phases especially at high ion concentrations (7 and 8 mol% Eu3+). Two multipolar processes where found to be responsible for the luminescence quenching process in the mixed-structure; the dipole-dipole and the dipole-quadrupole transitions. The intensity parameters (Ω2, Ω4), asymmetry ratio, R0 and the average decay lifetime of the nanocrystals show dependence on concentration and excitation wavelength. High IQE values were obtained at 1, 7 and 8 mol% Eu3+ where the monoclinic phase is dominant. The CIE coordinates values are comparable to existing red phosphors and in combination with high average IQE of 55% makes this phosphor a good candidate for red emitting phosphor application.

Defect Related Green–Red Luminescence of Sb-Doped ZnO Nanorods Grown by Vapor-Phase Oxidation Method

Vertically aligned undoped and Sb-doped ZnO (ZnSbO) nanorods were grown by the vapor-phase oxidation process on sapphire substrates at 700 °C using Fe as the catalyst. The effect of Sb doping on surface morphology, structural and optical properties of ZnO were studied. The grown undoped and Sb-doped ZnO nanorods have a hexagonal wurtzite structure with preferential c-axis orientation. The as-grown nanorods morphology has shown length of 2-5 μm and a diameter of 100-250 nm. An elongated nanorods and high aspect ratio was observed in Sb-doped ZnO. The undoped ZnO nanorods showed a strong and sharp ultraviolet (UV) emission centered at ~381 nm, whereas Sb-doped ZnO nanorods exhibited two defect related emissions centered at ~530 nm (green) and ~630 nm (red) with a suppressed UV emission. The dominance of the red emission in the nanorods indicates that it exhibits a much higher defect caused by the Sb doping.