Combustion Synthesis and Study of Double Charge Transfer in Highly Efficient Cool White-emitting Dy3+ Activated Vanadate-based Nanophosphor for Advanced Solid-state Lighting

A series of Ca9Gd(VO4)(7): Dy3+ (x = 0.01-0.20) nanophosphor crystals emitting a cool white light were synthesized by solution combustion methodology. The X-ray diffraction patterns were analyzed and processed using Rietveld refinement. The fabricated nanophosphor was found to crystallize in a trigonal crystal lattice with space group R3c(161). The morphological behavior of the prepared nanophosphor was investigated using transmission electron microscopy (TEM) and scanning electron microscopy (SEM). The photoluminescence properties of the nanophosphor correspond to cool white emission upon near-ultraviolet (NUV) excitation at 327 nm due to F-4(9/2) & RARR; H-6(15/2) (bluish) and F-4(9/2) & RARR; H-6(13/2) (yellowish) radiative relaxations at 487 nm and 576 nm respectively. Also, there is a strong occurrence of double charge transfer from O2- ions to Dy3+ and V5+ ions with the latter being stronger due to the high positive charge of V5+ ions. Color coordinates (x = 0.2878, y = 0.3259) are consistent with white emission. Auzel's model was implemented to examine the non-radiative relaxation (113.5 ms(-1)), radiative lifetime (1.4856 ms), and quantum efficiency (83.13%) values. The crystalline and optical behavior of the synthesized cool white emitting nanophosphor facilitates its use in near-UV-based WLEDs and other advanced solid-state lighting.