Investigation of the persistent luminescence of LiBaPO 4 :Eu 2+
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We investigated the persistent luminescence in europium-doped LiBaPO4. The persistent phosphors were synthesized via solid-state reaction method under mild reducing atmosphere. Its properties were investigated by x-ray diffraction, diffuse reflectance, photoluminescence, persistent luminescence, and thermoluminescence spectra. Under UV irradiation, broad-band persistent luminescence peaked at ;470 nm was observed in the phosphors at room temperature. The effects of Eu21 concentration on the persistent luminescence of LiBaPO4:Eu21 were discussed. An energy level scheme was constructed to convey reasonable trapping and detrapping processes in the material.
I. INTRODUCTION
Persistent luminescence, also known as long afterglow, is a special case of thermally stimulated luminescence. The energy stored in the material, absorbed from sunlight or lamps, is gradually released as visible light by the thermal energy available at ambience.1–5 Persistent phosphors were widely used in areas, such as safety indication, emergency lighting, road signs, billboards, graphic arts, and interior decoration.6,7 New applications for persistent phosphors have been extended to medical field in recent years, such as cancer photodynamic therapy,8 cancer diagnoses, and in vivo imaging.9–11 The most known efficient persistent phosphors are Eu21-activated alkaline earth aluminates, represented by SrAl2O4:Eu21,Dy31 (green)12 and Sr2MgSi2O7:Eu21, Dy31 (blue).13 Other rare-earth ions are usually codoped to enhance the performance of these materials. In particular, Dy31 and Nd31 are often selected for this motive. The best phosphors (SrAl2O4:Eu21,Dy31) can continue emitting green light even in excess of 24 h in the dark. It could be extended to 15 days if infrared persistent phosphor Zn3Ga2Ge2O10:Cr31 is included,14 though the costly raw materials may hinder its commercialization. Recently, Hölsä et al.2,15 have developed plausible mechanisms for Eu 21- or Tb 31-activated persistent phosphors. They depicted figures to elucidate the trapping and detrapping processes of charge carriers. Nevertheless, there are many issues that still need consideration, such as, the nature of traps, the role of the codoping lanthanide ions, the concentration quenching of persistent luminescence, the energy levels of dopants
II. EXPERIMENTAL A. Synthesis
LiBaPO4:Eu21 powders were synthesized using a high temperature solid state reaction method. Stoichiometric amounts of starting materials Li2CO3 (98.0%, Kemiou Chemical Reagent Co., Ltd., Tianjin, China), BaCO3 (99.0%, Kemiou Chemical Reagent Co. Ltd., Tianjin, China),
a)
Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/jmr.2014.1 J. Mater. Res., Vol. 29, No. 4, Feb 28, 2014
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with respect to host conduction/valence band (CB/VB), the point defects resulting from doping, etc. The structure of LiBaPO4 was reported to be crystallized in a hexagonal crystal system by Boutinaud et al.,16,17 which is isomorphic with the b-SiO2 tridymite. In the crystal lattice, ever
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