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Preparation and Luminescent Properties of ZnGa_2O_4 Doped with Mn~(2+)/Eu~(3+) by the Citric-gel Method

YU Min, LIN Jun, ZHOU Yong hui (Key Laboratory of Rare Earth Chemistry and Physics, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, Changchun 130022, China)  
The development of flat panel display devices such as field emission displays (FEDs) and vacuum fluorescent displays (VFDs) requires highly efficient cathodoluminescent materials. While many efficient sulfide based phosphors have been explored as possible low voltage phosphors, the volatility of sulfur has prohibited their use in FEDs or VFDs. Sulfide base phosphors often degrade under high energy electron bombardment due to dissociation of the cation sulfur bonds. Oxide phosphors offer potential advantage because of their superior stability under electron bombardment and excellent luminescent properties. ZnGa 2O 4 is an attractive oxide phosphor for low voltage cathodoluminscence applications in VFDs and FEDs. ZnGa 2O 4 is a binary compound oxide consisting of ZnO and Ga 2O 3 with the formula AB 2O 4 and has a cubic structure that can be viewed as a combination of rock salt and zinc blend structures, a normal spinel structure [space group is Fd3m (O 7 h)]. The optical bandgap of ZnGa 2O 4 is about 4 4eV. Like other wide bandgap semiconductors ZnGa 2O 4 exhibits a blue emission under excitation by both ultraviolet light and low voltage electrons. Activation with Mn 2+ or Eu 3+ ions, the emission shifts to green or red. The citrate gel method allows preparation of highly dispersed mixed oxide at low temperature. This method involves the formation of a mixed ions citrate that due to the three ligand nature of the citric acid, resulting a transparent three dimensional network upon drying (gel). The pyrolysis of the gel yields a homogeneous mixed oxides. The method was widely used to prepare multicomponent oxides, such as superconductors. In this paper, we report the synthesis of ZnGa 2O 4 and ZnGa 2O 4∶Mn 2+ /Eu 3+ by a citrate gel method. XRD, TG DTA, and luminescence excitation and emission spectra were used to characterize the resulting products. The results of XRD reveal that the powders begin to crystallize at 500℃ and pure ZnGa 2O 4 phase is obtained at 700℃, which agrees well with the results of TG DTA and FT IR. This temperature is much lower than that (1200℃) by solid state reaction method. In the crystalline ZnGa 2O 4, the Eu 3+ shows its characteristic red (613nm, 5D 0 → 7F 2 ) emission with a quenching concentration of 5mol%(of Ga 3+ ), and the Mn 2+ shows green emission (506nm, 4T 1 → 6A 1 ) with a quenching concentration of 0 1 mol%(of Zn 2+ ). The luminescence mechanism of ZnGa 2O 4∶Mn 2+ /Eu 3+ is presented. Activator sensitization and host to activator energy transfer are suggested.
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