Luminescence, cathodoluminescence and Ce3+ → Eu2+ energy transfer and emission enhancement in the Sr5(PO4)3Cl:Ce3+,Eu2+ phosphor

Abstract

Ultraviolet and vacuum ultraviolet spectra are reported for Ce3+ and Eu2+ singly doped into the Sr-5(PO4)(3)Cl lattice, synthesized by a solid-state process. The maxima of the emission intensities are at 346 nm (Ce3+) and 442 nm (Eu2+), with lifetimes of 25 ns and 494 ns, respectively. The Ce3+ excitation spectra exhibit five bands assigned to vibronic structures of the 4f(1) -> 5d(1) electronic transition, in addition to the host absorption at 166 nm at 295 K. The 5d centroid shift is similar to that in SrCl2:Ce3+. The Eu2+ excitation spectrum is mainly comprised of two 4f(7) -> 4f(6)5d broad bands between 200 nm and 450 nm, peaking at 343 nm and 275 nm. Both of the singly doped systems exhibit concentration quenching, with energy transfer rates being in the low (mu s)(-1) range for concentrations up to 4 at.% of total cations. The energy transfer rates are linearly and quadratically related to dopant ion concentrations of Ce3+ and Eu2+, respectively. The energy transfer between Ce3+ and Eu2+ in the co-doped Sr-5(PO4)(3)Cl lattice has been studied by the analysis of intensity and decay measurements of both Ce3+ and Eu2+ emissions. The electric dipole-electric dipole transfer has an efficiency of 91% in Sr4.8-xCe0.01EuxNa0.01(PO4)(3)Cl, with a critical distance of 21 angstrom. Although energy transfer between Ce3+ and Eu2+ upon excitation into the overlapping Ce3+/Eu2+ absorption band is definitely demonstrated by the dramatic shortening of Ce3+ lifetime, the emission of Eu2+ is also enhanced at low Ce3+ concentrations using exclusive excitation into the Eu2+ absorption band. The quantum yields (QY) of the co-doped system approach those of BaMgAl10O17:Eu2+ (BAM) for the excitation wavelengths of 317 and 365 nm, but are inferior for 254 nm excitation. The addition of 1 at.% Ce3+ to Sr4.98Eu0.02(PO4)(3)Cl increases the QY by similar to 20% of the original value. The cathodoluminesce of the co-doped phosphor is comparable to that of BAM, with CIE coordinates of the emission (0.274, 0.237), so that the application as a field emission display phosphor is proposed.