Energy transfer dynamics and photoluminescence properties of sol-gel synthesized dense-packed Ca3−(x+y)TbxSmyMgSi2O8 phosphor

Abstract

The Ca3−(x+y)TbxSmyMgSi2O8 (x = 0–0.15, y = 0–0.04 mol), dense packed merwinite type crystalline phosphors were studied for color tunability via Tb+3 → Sm+3 energy transfer. All the phosphor samples were prepared by acid catalyzed sol-gel reaction route followed by high temperature annealing. The structural and optical characteristics of the phosphors were ascertained from X-ray diffraction, scanning electron microscopy, Fourier transforms infrared and photoluminescence spectroscopy. The synthesized phosphors were devoid of any detectable impurities or phase separation and belonged to phase pure monoclinic crystal system in the P121/a1 space group. Tb+3 doped samples demonstrated intense green emission from 5D4 → 7FS (S = 6, 5) upon f-d (243 nm) and f-f excitation (378 nm). Sm+3 doped samples did not exhibit charge transfer excitation and however, strong orange-red emission was realized due to the 4G5∕2 → 6HS emission upon 404 nm (f-f) excitation. The onset of concentration quenching occurred at 0.07 for Tb+3 and 0.02 mol for Sm+3 corresponding to the Tb+3-Tb+3 and Sm+3-Sm+3 atomic distance of 11.4 and 17.4 Å respectively. Both Tb+3 and Sm+3 exhibited biexponential decay with τavg of 2.23 (Tb+3) and 2.16 ms (Sm+3), which is explained on the basis of their two different oxygen coordination environment in the lattice. The occurrence of Tb+3 → Sm+3 energy transfer (ET) was established from the analysis of excitation and emission spectra, and luminescence decay characteristics of Tb+3 and Tb+3-Sm+3 doped phosphors. Tb+3 → Sm+3 ET mechanism was further illustrated from the energy level diagram, and emission decay curves. The Tb+3 → Sm+3 ET efficiency and ET probability were observed to be maximum for the Tb0.07Sm0.02 codoped phosphor and determined to be 52.9% and 0.504 × 10−3·s−1, respectively. The Dexter hypothesis explicated the dominant energy transfer mechanism to be Tb-Sm dipole-dipole interaction. The Tb+3 → Sm+3 ET lead to the emission colour tunability with discrete CIE coordinates.