Enhancement of green light emission from the LiSrVO4:Tb3+ doped vanadate phosphor by charge compensation via co-doping of the alkali metal ions (Li+/Na+/K+)

Terbium (Tb3+) doped and alkali metal ions (Li+/Na+/K+) co-doped lithium strontium vanadate phosphors were synthesized by the conventional solid state reaction technique. The concentration of the dopant ions was varied in order to identify the optimum value (4 mol %). The prepared samples were characterized for their phase formation by powder XRD and their morphological characteristics were analyzed by SEM, where the latter confirmed that the particles were agglomerated with irregular shape. The elements present in the prepared samples were confirmed by the EDAX spectrum. The DR spectrum (200-700 nm) of the pure and the Tb3+ ions doped samples showed a wide band gap of 3.69 and 3.64 eV respectively. The photoluminescence spectrum of the Tb3+ ions doped vanadate phosphor, under NUV excitation (317 nm), showed its characteristics peaks at 487 nm (D-5(4) -> F-7(6)), 544 nm (D-5(4)-> F-7(5)), 586 nm (D-5(4) -> F-7(4)) and 620 nm (D-5(4) -> F-7(3)). On adding the alkali metal ions (co-doping), the luminescence intensity increased up to similar to 2.2, 5.9, 3.7 times for the Li+, Na+ and K+ ions respectively, compared with that of the singly doped phosphor. Moreover, the thermal quenching results showed that, the representative phosphors were relatively very sensitive to temperature changes, as the temperature increased from 30 degrees C to 210 degrees C. Under `317 nm excitation and 544 nm emission', the optimum phosphor showed the lifetime value in the microsecond range. In addition, the temperature dependent lifetime measurements were carried out to find the sensitivities of the prepared phosphor. The value of the relative sensitivities was calculated to be 0.44-1.07% (degrees C 1) in the 30. C-170 degrees C temperature range. The optimal phosphor's CIE (Commission Internationale de l'Elcairage) co-ordinates were found to be x = 0.2718; y = 0.6214, which were in the greenish region. The correlated color temperature showed a cool greenish emission with high color purity. The above results reveal that, the green emitting LiSrVO4:Tb3+ phosphor can be a suitable candidate for fabricating phosphor converted white light emitting diode.