White light emitting nanocrystalline Y1−xGdxPO4:Dy3+ and improved PLQY on Gd3+ co-doping

Abstract

White Light emitting phosphors are in the realm of scientific evolution owing to their several superior photophysical features and maximum commercial reach. Here in this work we have carried out doping of Dy3+ in Y1-xGdxPO4 wherein we have moved from YPO4 to GdPO4 to harness the potential of dysprosium ion in achieving white light emission and correlating the structural evolution in these phosphates with luminescence properties. Y1-xGdxPO4:Dy3+ phosphor has been synthesized using co-precipitation method and characterized using x-ray diffraction (XRD), field emission scanning electron microscopy (FESEM), time resolved photoluminescence (TRPL) and positron annihilation lifetime spectroscopy (PALS). Based on emission spectroscopy it was deciphered that Dy3+ lacks inversion symmetry in all the samples with asymmetry ratio of ∼1.2 in YPO4 and 2.75 in GdPO4. The same is correlated structurally wherein YO8 is in dodecahedron with D2d symmetry while GdO8 is highly distorted. The photoluminescence quantum yield (PLQY) increases monotonically with Gd3+ insertion and GdPO4:Dy3+ showing the highest PLQY endowed by enhanced asymmetry around Dy3+ making the f-f transitions more relaxed and allowable. The surface defects as revealed by positrons also decreased with increase in Gd content and that also aids the increase in emission intensities. Moreover PLQY value was higher with 274 nm compared to excitation with f-f transitions (∼351 nm) due to Gd3+ to Dy3+ energy transfer. The phosphors have similar color coordinate value and correlated color temperature of (∼0.33, ∼0.36), ∼5400 K, respectively, which are close to the bluish cool-white emission. Therefore, the GdPO4:Dy3+ phosphor with longer excited state lifetime, moderate PLQY, effective CIE coordinates close to white and high CCT can be explored for generation of cool white light emission excitable by UV/NUV LED for solid-state lighting and optoelectronic devices.