Multisite-occupation properties, spectral characteristics and energy-transfer behaviors of Ca2ZrSi4O12:Ce3+,Tb3+ phosphor for NUV-based LEDs

Abstract

In this paper, the Ce3+- and Tb3+-activated Ca2ZrSi4O12 (CZSO) sample series were prepared by employing a solid-solid synthesis route at high temperatures. The phase composition and crystal structure of as-prepared phosphors were studied by x-ray diffraction (XRD). The ionic lattice occupation, photoluminescence properties, energy-transfer behaviors, decay time, and thermal stability were investigated systematically. With excitation of 322 nm, CZSO:0.01Ce3+ phosphor can produce an asymmetrical wide emission spectrum peaking at 382 nm, in which two types of emission centers (marked as Ce I and Ce Ⅱ) originated from Ce3+ occupying 9- and 8-coordinated Ca2+ sites have been identified and discussed. The Tb3+-activated CZSO phosphor can emit several narrow bands ranging from 350 to 650 nm, corresponding to the 5Di → 7Fj (i = 3, 4; j = 3, 4, 5, 6) transitions of Tb3+. With the strategy of Ce3+ → Tb3+energy transfer (ET), intense blue-green light has been obtained in CZSO:Ce3+, Tb3+ samples. The ET mechanism was confirmed as dipole-dipole interaction using the critical distance equation suggested by Dexter and Readeld. The internal quantum efficiency (IQE) and external quantum efficiency (EQE) of phosphor with optimum doping concentrations (CZSO:0.01Ce3+,0.045Tb3+) were calculated to be 59.2% and 32.7%, respectively. Furthermore, the ET process was verified to be dominated by Ce Ⅱ→Tb3+ through the analysis of spectra-shifting and lattice occupancy. The calculated chromaticity coordinates suggested by CIE can be adjusted from the blue position to the greenish-yellow region according to the variation of Tb3+ concentration. The experimental data indicate that the CZSO:Ce3+, Tb3+ phosphors have potential value for NUV-based LEDs.