Tunable multicolor emission from Dy3+/Sm3+/Eu3+ activated lithium-aluminum-bismuth-zinc phosphate glasses for warm white LED applications

The spectroscopic characterization of lithium-aluminum-bismuth-zinc phosphate (LABZ) glasses singly doped with Sm3+ (LABZ-S) and co-/tri-doped with Dy3+ and Eu3+ (LABZ-DS, LABZ-SE, LABZ-DSE) is reported. The amorphous nature of a representative glass sample was confirmed by analysis of powder X-ray diffraction pattern. Absorption, photoluminescence and time decay measurements, and Judd-Ofelt (J-O) analysis, were combined with optical-edge (Tauc) and Urbach analyses. In LABZ-S glass, the oscillator strengths are well reproduced by J-O theory, and the intensity parameters follow the tendency Omega(4) > Omega(6) > Omega(2), indicating that the rigidity and viscosity dominate on the asymmetry and covalency around the Sm3+ ions. The total radiative rate from level (4)G(5/2) is A(tot) = 70.59 s(-1), yielding tau(rad) = 14.17 ms; comparison with the measured lifetime (3.52 ms) shows significant radiative relaxation. Peak stimulated-emission cross-sections for transitions (4)G(5/2) -> H-6(7/2,9/2) are of order 10(-22) cm(2); although smaller than those reported for phosphate/lead-fluoroborate systems. LABZ-S glass shows competitive optical-gain values (sigma(p) center dot tau(rad) = 27.8 and 23.1 & times; 10(-25 )cm(2) s for transitions (4)G(5/2) -> H-6(7/2) and (4)G(5/2) -> H-6(9/2), respectively). Direct and indirect band-gap energies (3.79 and 3.65 eV, respectively), together with a low Urbach energy (0.02 eV), confirm a transparent and low-disorder host. In the LABZ-SE glass, Sm -> Eu non-radiative energy transfer occurs via a dominant electric dipole-dipole interaction with an efficiency of 5.7%. In the LABZ-DS glass, Dy -> Sm non-radiative energy transfer is present through a dominant electric dipole-dipole interaction with an efficiency of 4.8%. The tri-doped LABZ-DSE glass enables excitation-dependent color tuning from orange to warm white (CCT similar to 3989 K), in which occurs non-radiative energy transfer Sm3+ to Eu3+ through a dominant electric dipole-dipole interaction as well a non-radiative energy transfer Dy3+ to Sm3+/Eu3+. These results position LABZ glasses activated by Dy3+/Sm3+/Eu3+ ions as promising multicolor emitters for NUV-activated WLEDs.