Energy transfer dynamics and time resolved photoluminescence in BaWO4:Eu3+ nanophosphors synthesized by mechanical activation

Abstract

This study reports the synthesis of nanocrystalline undoped and europium doped BaWO₄:Eu³⁺ nanophosphors by a mechanical activation process using high energy ball mill (HEBM) technique. The prepared nanophosphors were characterized systematically by Rietveld analysis of the X-ray powder diffraction (XRD) data, Raman spectroscopy, energy dispersive spectroscopy (EDS), scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The synthesized nanophosphors were found to possess spherical morphology and small particle sizes ∼25 nm. To explore this material for application in luminescence, detailed time resolved photoluminescence studies were carried out. It was observed that it is a rare breed of phosphor material that exhibits exceptional luminescence properties such as the capability to give high emission output till 5.0 mol% without undergoing quenching and unique property to get excited by mid ultraviolet (MUV) light, near ultraviolet (NUV) light, as well as by visible light such as blue and green. It was also observed that energy transfer primarily takes place from tungstate group of host to Eu³⁺. In fact energy transfer from the host to Eu³⁺ becomes more and more efficient with the increasing concentration of Eu³⁺ ions. Two lifetime values are observed for all europium ion (Eu³⁺) concentrations, one of the order of 465–650 μs (T_S, short lifetime) and the other of the order of 1050–1320 ms (T_L, longer lifetime). This reflects the fact that Eu³⁺ ions occupy two sites; regular symmetric Ba²⁺ site (S₁) and asymmetric site (S₂), which will be closer to charge compensating defects. Calculation of Judd–Ofelt parameter illustrated that at all europium concentrations, the local surrounding around it lacked inversion symmetry (asymmetric environment), and the exact point group symmetry around europium ion was found to be C_6v. The red purity (intense ⁵D₀–⁷F₂) and high quantum efficiency (78%) of BaWO₄:Eu³⁺ projects it as a new red phosphor for application in white light emitting diodes.