Synthesis and characterization of down−conversion nanophosphors
Tshabalala, Kamohelo George
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Luminescent zinc aluminate (ZnAl2O4) nanoparticles, crystalline–low quartz and amorphous silica powders were incorporated with Ce3+ and Tb3+ ions. These powders were successfully synthesized by the solution combustion and sol-gel routes. Phase analysis, particle sizes and morphology of the ZnAl2O4 nanoparticles were determined with X-ray diffraction (XRD), high resolution transmission electron microscopy (HRTEM) and scanning electron microscopy (SEM). Similarly, both low-quartz and amorphous phases of silica were determined the same way. The photoluminescence (PL) data were collected at room temperature using a variable UV Xenon lamp mounted into the F7000 Fluorescence and Cary Eclipse fluorescence spectrophotometers. The cathodoluminescence (CL) data were collected at room temperature using Ocean Optics CL spectrometer attached to the vaccum chamber of the Physical Electronics PHI 549 Auger electron spectrometer. The surface characterization was carried out using Auger electron spectroscopy (AES) and X-ray photoelectron spectroscopy (XPS). The average crystallite sizes for zinc aluminate powder phosphors reduced in the hydrogen atmosphere were ranging from 20 nm to 50 nm. The results from XRD and HRTEM showed that ZnAl2O4:Ce3+, Tb3+ powder phosphors were crystalline and the lattice spacing estimated form SAED was 0.24 nm, corresponding to the (311) lattice of ZnAl2O4. The PL intensity of the green line emission from Tb3+ at 544 nm (5D4→7F5 transition) increased as a result of Ce3+ co-doping. The fact that the increase was simultaneous with the decrease in blue emission from Ce3+ (5d → 4f transition) suggests that excitation energy was transferred from Ce3+ to Tb3+. The AES and CL data were collected simultaneously when the powders were irradiated with a beam of electrons ( for 10 hours) in a vacuum chamber maintained at 1 × 10−7 TorrO2 atmosphere. The AES elemental composition data for the degraded powder phosphors gave all the main elements in the ZnAl2O4:Ce3+, Tb3+, namely; Zn, Al, O and adventitious C. The ratio of Zn APPH to that of oxygen was almost stable during the electron beam irradiation. The Al/O ratio increased from 0 – 300 C.cm−2 and then stabilized while the adventitious C peak decreased drastically from 0 – 600 C.cm−2 before stabilizing. The simultaneous increase of the CL intensity with the removal of C between 0 – 600 C.cm−2 suggests that the presence of C on the surface inhibited light emission from the surface. The decrease in the C/O APPH ratio was due to removal of C from the surface due to the presence of Al2O3 investigated using electron stimulated surface chemical reactions (ESSCRs) model. The CL intensity then decreased slightly after 600 C.cm−2 electron dose and then remained stable. According to ESSCR model, electron beam irradiation may dissociate the O-O (from O2 introduced in the vacuum chamber) and Zn-Al-O bonds resulting in highly reactive O2−, Zn2+, and Al3+. The XPS data collected from the sample of ZnAl2O4:Ce3+, Tb3+ proved that there was structural readjustment from inversion to normal spinel as a result of annealing in reduced H2 atmosphere. In a low quartz and amorphous silica samples, efficient energy transfer from Ce3+ to Tb3+ ions was observed when the powder phosphors were excited at the wavelength of 322 nm. The transfer rate was shown to be more efficient for samples reduced in a mixture of N2 and H2 compared to those annealed in air. Thus, the maximum energy transfer was observed from the sample co-doped with SiO2: 2 mol%Ce3+, 4 mol%Tb3+. The excitation of 322 nm is ascribed to direct excitation of Ce3+ ions from 4f → 5d transition of Ce. The improved down-converted emission indicates that our materials can be used as wavelength shifting layer in Si photovoltaic cells to improve their power conversion efficiency.