Near-ultraviolet excited down-conversion Sm3+-doped Ba5Zn4Gd8O21 reddish-orange emitting nano-diametric rods for white LEDs

Abstract Down-conversion Ba5Zn4Gd8O21:Sm3+ reddish-orange nanocrystalline phosphors with high quantum efficiency were well-synthesized by solution combustion route. The structural and morphological characterizations were systematically done via X-ray diffractometry (XRD) and transmission electron microscopy (TEM). Refined atomic positions as well as unit cell parameters for optimum composition were calculated using refinement technique. The results revealed that the substitution of Gd3+ ions by Sm3+ ions in Ba5Zn4Gd7.76Sm0.24O21 nanophosphor did not induce any major structural change and confirmed its tetragonal framework with I4/m (87) space group. TEM studies indicated the rod-shaped morphology with uniform samarium ion distribution of combustion derived nanophosphor ranging from 40 to 90 nm. The energy band-gap values for Ba5Zn4Gd8O21 (5.1 eV) and Ba5Zn4Gd7.76Sm0.24O21 (4.88 eV) were estimated from diffuse reflectance (DR) measurements. Further, the photoluminescence studies upon 410 nm excitation yielded the reddish-orange luminescence (at 610 nm) attributed to 4G5/2→6H7/2 transition of Sm3+ ions. The luminescence intensity of samarium-activated phosphor reached the maximum at the Sm3+ content of 3 mol%, for which the critical separation (RSm-RSm) was ≈ 16.5 A. In addition, the energy relaxation process in Ba5Zn4Gd8O21:Sm3+ was clearly observed, which was confirmed via electric dipole-quadrupole interaction pathway. Besides, the radiative lifetime (1.19 ms), non-radiative relaxation rate (544.31 s−1) and most importantly, the quantum efficiency of 4G5/2 electronic state (60%) were also analyzed in detail. The obtained CIE chromaticity coordinates favour the applications of samarium activated Ba5Zn4Gd8O21 phosphor in near-ultraviolet (NUV) excited tricolor phosphor based white light-emitting diodes (WLEDs).