Abstract
A series of Gd2-x-yYbxEr(Ho)yGeMoO8 sheelites comprising Gd2GeMoO8 matrix doped by Yb3+-Er3+ and Yb3+-Ho3+in various concentrations (GGMO:Yb,Er and GGMO:Yb,Ho) have been prepared by solid state and sol-gel (Pechini) methods. Besides the confirmation of monophasic state (powder XRD) of the samples prepared at different temperatures, their morphology (scanning electron microscopy, SEM) as well as luminescence under infrared (IR) laser excitation (974 nm) with 1.0 and 2.5 W/cm2 pumping power density of GGMOs have been systematically investigated. All synthesized samples possess Scheelite structure (tetragonal syngony, sp. gr. I41/a). Under 974 nm excitation, GGMO:Yb,Er phosphors exhibit upconversion (UC) emission peaks at 525, 545 and 655 nm originating from 2H11/2, 4S3/2 - 4I15/2 and 4F9/2 - 4I15/2 Er3+ transitions with a significant predominance of green band. Virtually monochromatic bright green emission was observed for GGMO:Yb,Er with Yb:Er 8:3 ratio, synthesized by solid-state method at 1250 °C. In the UC luminescence spectra of GGMO: Yb, Ho the intensity of the Ho3+ red band (5F5 → 5I8 transition, λ = 665 nm) exceeds that of green band (5S2, 5F4 → 5I8, λ = 545 nm), but visually, the color of the luminescence was perceived as yellow.Among GGMO:Yb,Er and GGMO:Yb,Ho samples, those with Yb:Er and Yb:Ho ratios of 9:4 were revealed as more promising. The application of Pechini method affords nanosized GGMO:Yb,Er and GGMO:Yb,Ho UC phosphors already after annealing at 1000 °C, but phosphors annealed at 1200 °C were more efficient. Their morphology is irregularly shaped and strongly aggregated roughly-spherical grains with average size of 150–200 nm. Due to high thermal and chemical stability, these phosphors may be applied in various fields requiring IR detection.
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