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The long afterglow phosphor, CaWO4: Eu3+, is synthesized and the intensity and duration of its afterglow can be enhanced by the Ti4+ and Mg2+ incorporation. The x-ray diffraction patterns depict pure tetragonal CaWO4 of all samples. The emission spectra show the Eu3+ emission and the charge transfer (CT) emission of WO42
. The intensity of CT increases with the Mg2+ incorporation. The excitation spectra monitoring 616 nm exhibit the strongest CT band with Ti4+ incorporation. These results indicate that Mg2+ enhances the efficiency of CT emission of WO42
while the Ti4+ enhances the energy transfer rate from CT to Eu3+. Since the thermoluminescence (TL) curves do not imply a new trap, the enhancement of the afterglow results from the coreinforcement of CT efficiency and energy transfer rate.

I. INTRODUCTION

Phosphors have a broad application in our daily life, e.g., cathode ray tubes, television, fluorescent lamps, detectors, and light-emitting diodes.1,2 The luminescent lifetime of phosphors applied in above fields is usually short, while some phosphors possess a long lifetime, which is called long afterglow. It refers to a luminescence phenomenon that persists for a long time after excitation. Materials with long afterglow can store the absorbed energy from external irradiation and release the energy usually as visible light gradually. Due to this energy storage characteristic, visible light can be obtained from long afterglow materials in dark environment without electricity. It more or less assists to energy saving and sustainable development. Phosphors with long afterglow are commercially applied in emergency indications, road signs, displays, decoration, etc.3,4 More recently, the application of long afterglow phosphors (LAPs) was extended to medical diagnostics.5,6 The employment of LAPs substantially enhanced the signal-to-noise ratio because the autofluorescence of tissues was avoided during the detection. Hence, LAPs possess a considerable commercial prospect. By now, tricolor LAPs have been achieved, i.e., Sr2MgSi2O7: Eu2+, Dy3+ (blue, ;470 nm),7,8 SrAl2O4: Eu2+, Dy3+ (green, ;510 nm),9,10 and Y2O2S: Eu3+, Ti4+, Mg2+ (red, 625 nm).11,12 Among these, the afterglow properties of blue and green phosphors are much superior to the red one (the afterglow duration of Sr2MgSi2O7: Eu2+, Dy3+ and SrAl2O4: Eu2+, Dy3+ lasts more than 10 h a)

Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/jmr.2012.16 J. Mater. Res., Vol. 27, No. 6, Mar 28, 2012

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while the one of Y2O2S: Eu3+, Ti4+, Mg2+ maintains several minutes, which is far away from commercial application). To enhance the red long afterglow, some new materials are developed. Ca2Si5N8: Eu2+13,14 and Ca2SnO4: Sm3+4,15 have been reported to exhibit red long afterglows. However, calcining temperatures for synthesis of these materials are high (.1300 °C), and it is not conducive to industrial manufacture. In 2004, Liu et al.16 observed the long afterglow of Eu3+ in CaWO4 matrix. The aft