Sol-gel synthesis of and luminescent properties of Pr³⁺ in different host matrices
Mbule, Pontsho Sylvia
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Luminescent ZrO2:Pr3+ , SiO2:Pr3+, ZnO:Pr3+ and ZnS:Pr3+ nanophosphors were synthesized by a sol-gel method, dried, ground and annealed in air at 600oC (SiO2:Pr3+, ZrO2:Pr3+, ZnO:Pr3+ and ZnS:Pr3+) or 280oC (ZrO2:Pr3+). The chemical composition of the powder phosphors was analyzed by energy dispersive x-ray spectrometer (EDS). The structure and particle sizes were determined with x-ray diffraction (XRD) and particle morphology was analyzed using scanning electron microscopy (SEM) and transmission electron microscopy (TEM). SiO2:Pr3+ was amorphous even after annealing at 600oC. ZrO2:Pr3+ annealed at 280oC showed an amorphous structure but the material crystallized when the annealing temperature was increased to 600oC. The particle sizes estimated from the XRD peaks were ∼2±0.2 nm (dried ZnS and ZnO) and ∼8±0.1 nm (ZrO2:Pr3+ annealed at 600oC). Particle sizes increased to ∼17-20±0.2 nm in diameter for annealed ZnS:Pr3+ and ZnO:Pr3+. The UV-Vis spectrophotometer was used to determine the absorption properties of the nanophosphors and their band absorption showed a blue shift compared to their bulk counterparts. Powder phosphors were also irradiated with 325 nm (He-Cd) laser to study photoluminescence (PL) properties. PL spectra were obtained for both undoped and Pr3+ -doped nanophosphors. A broad emission band was observed at 498 nm with a shoulder at 416 nm from SiO2:Pr3+ annealed at 600oC. ZrO2:Pr3+ annealed at 280oC showed two emission bands in the visible range at 459 nm and 554 nm. A broad green emission band at 567 nm and a shoulder at 607 nm were observed for dried ZnO:Pr3+ nanophosphor and the shoulder at 607 nm was enhanced significantly when the Pr3+ concentration was increased. Annealed ZnO:Pr3+ (280oC) nanophosphor showed a green emission band centered around 533 nm and a shoulder at 624 nm. Dried ZnS:Pr3+ nanophosphor showed a blue emission centered at 445 nm and the PL intensity increased with an increase of Pr3+ ions concentration. All these emissions were coming from the host matrices and not from the Pr3+ ion when the powders were excited by 325 nm (3 eV) photons. SiO2 and SiO2:Pr3+ powder phosphors were subjected to prolonged 2 keV electron beam irradiation in an ultra high vacuum (UHV) chamber at a base pressure of 1x10-9 torr. The surface reactions and degradation of cathodoluminescence intensity were monitored using Auger electron spectroscopy (AES) and cathodoluminescence (CL) spectroscopy respectively. CL emission of SiO2 showed a maximum emission peak at 451 nm and a shoulder at 478 nm and SiO2:Pr3+ showed a multiple peak emissions located at 510 nm, 614 nm, 730 nm, 780 nm and 970 nm which are attributed to the transitions in the Pr3+ ions. The SiO2:Pr3+ CL intensity decreased with time as a result of continuous exposure to 2 keV electrons. The Auger peak-to-peak height as a function of energy spectrum showed that there were changes on the surface chemistry of the powders as a result of prolonged irradiation by 2 keV electrons. It is most likely that non-luminescent layers were formed on the surface and they contributed to the CL intensity degradation. A high concentration of volatile gas species, which might have contributed to the CL degradation, was detected with a residual gas analyzer (RGA). Cathodoluminescence was not measured for ZnO:Pr3+,ZnS:Pr3+ and ZrO2:Pr3+ due to charging of the powder phosphors and ZrO2:Pr3+ did not emit light under high energy electron exposure (2 keV).