Effects of Li+ Codoping on the Optical Properties of SrAl2O4 Long Afterglow Ceramic Phosphors

Rare-earths codoped long afterglow strontium aluminate phosphors with high brightness were synthesized via a facile combustion synthesis method using urea as a fuel. The resulted phosphor particles were analyzed by using X-ray diffraction and field emission scanning electron microscope, whereas their optical properties were monitored by photoluminescence spectroscopy. The prepared SrAl2O4:Eu2+, Dy3+, Li+ samples showed a broad green-yellowish emission, peaking at 512？nm when excited by 348？nm. Compared to traditional SrAl2O4:Eu2+, Dy3+ phosphor, the initial luminescence brightness of SrAl2O4:Eu2+, Dy3+, codoped with Li+ improved from 1.89？cd/m2 to 2.71？cd/m2 and the afterglow decay time was prolonged from 103 to 121？min. The possible mechanism of SrAl2O4:Eu2+, Dy3+, Li+ phosphorescence enhancement has been discussed. 1. Introduction Long afterglow phosphor is a kind of energy-storing material, which can absorb the light photons, store the energy, and then release the energy as visible light. Alkaline aluminates based phosphor materials generally generate more defect-related traps when they are doped with rare-earths RE. The luminescence emission of these RE-doped alkaline aluminates is strongly dependent on the host lattice and can occur from the ultraviolet to the red region. For example, CaAl4O7:Eu2+, Dy3+ phosphors exhibited a broad blue emission band with the main peak at 445？nm [1]. The emission colour of MgAl4O7:Eu2+, Dy3+ nanoparticles shifts from bluish (700°C) to reddish (900°C) with increasing calcination temperature [2]. Recently, strontium aluminates activated with rare-earths have attracted a lot of attention since they exhibit excellent properties such as long afterglow time, high quantum efficiency, good phase stability, and bright emitting color [3], when compared with known sulfide long afterglow phosphors. Since the lifetime greatly exceeds that of any other material, this offers an unexpectedly large field of applications for the use of these aluminates [4, 5]. Although many studies have examined the optical properties of RE-doped alkaline aluminates, only a few have investigated the codoping of three or more different ions in the same host material. It is also known that optical properties of phosphorescent materials are strongly influenced by the incorporation of auxiliary activators. Among them, smaller cationic radius of Li+ ions are favored for their movement and site occupation in the host matrix and these advantages make them attractive for tailoring the crystal field of host lattice [6]. There are many reports on significant