Study On Preparation And Mechanism Of Rare Earth Ions Doped Conductive Phosphor12CaO·7Al₂O₃for Field Emission Displays

Recently, field emission displays (FEDs) have been considered as one of the mostpromising technologies in flat panel displays because they provide thin panels, self-emission,a distortion-free image, wide viewing angle, low weight, and quick response, as well as lowpower consumption. Phosphors for FEDs are required to have high emission efficiency, goodchemical stability, and effective conductivity with high beam current density. FED phosphorsneed to have suitable electrical conductivity because insulating phosphors do not transferelectrons. Although many efficient sulfide-based compounds with suitable electricalconductivity have been explored as possible low voltage phosphors, the volatility of sulfur hasprohibited their use in the FEDs. Oxide-based phosphors are more stable and environmentallyfriendly compared to sulfides. However, most oxide-based phosphors have low electricalconductivity, which results in charge build-up on the phosphor and decreases the efficiency ofthe phosphor. Therefore, it is an challenge to find novel phosphors that suitable for FEDs.12CaO7Al2O3(abbreviated as C12A7:O2or C12A7) is found to have unique physicaland chemical properties based on its special crystal nanocage structure. These free O2-ionscan be replaced easily by other anions like F-, Cl-, OH-, O-, H-and even by electrons. C12A7can be converted from an insulator into a semiconductor by introducing electrons. On theother hand，the encaged anions in C12A7can induce a considerable deformation of its lattice.The different the deformation induced, the different local crystal field created, which makethis compound very attractive as functional material with highly variable properties.Therefore, it is significative to investigate the relation between luminescence properties ofrare earth ions and the crystal field by tuning the encaged anions in C12A7.Based on the above, we prepared novel white-light-emitting conductive phosphor12CaO7Al2O3:Ce3+，Dy3+and green phosphor12CaO7Al2O3-CaCeAl3O7:Ce3+, Tb3+. Theeffects of energy transfer, electrical conductivity and multiphase strategy onphotoluminescence and cathodoluminescence properties have been investigated in detail. Thestudied conclusions came following:A novel white-light-emitting conductive phosphor,12CaO7Al2O3:Ce3+, Dy3+wasprepared by the solid-state reaction in H2atmosphere followed by irradiation with ultravioletlight. Upon excitation at362nm or low-voltage (5kV) electron beam, the phosphor showsexcellent white-light emission ((CIE coordinate of (0.324,0.323)) that combines the blue andyellow emissions at476and576nm, assigned to the4F9/2â†'6H15/2and4F9/2â†'6H13/2transitions of Dy3+, respectively. The luminescence of Dy3+is greatly enhanced by effective energytransfer from Ce3+to Dy3+. The results suggest that the charge compensation due to thesubstitution of trivalent Ce3+and Dy3+ions for divalent Ca2+ions might induce more free O2-ions inC12A7:Ce3+, Dy3+, which are be beneficial for more encaged H-formation during theannealing process.A multiphase strategy is proposed and successfully applied to make the insulating greenphosphor CaCeAl3O7:Tb3+conductive in the form of12CaO7Al2O3–CaCeAl3O7:Ce3+,Tb3+.The phosphor shows bright green-light emission with a short lifetime (2.51ms) underlow-voltage electron beam excitation (3kV). The green photo-and cathodoluminescencefrom5D4â†'7FJ(J=6,5,4,3) transitions of Tb3+are significantly enhanced in comparisonwith pure C12A7:Tb3+. It was confirmed that this enhancement is the consequence of the jointeffects of energy transfer from Ce3+to Tb3+and broadening of the absorption spectrum of Ce3+due to the existence of multiple phases. In particular, under800V electron beam excitation,cathodoluminescence is improved by the modified electrical conductivity of the phosphor.Pure Ca12Al14O32Cl2:Ce3+and Ca12Al10.6Si3.4O32Cl5.4:Ce3+were prepared by thesolid-state reaction in H2atmosphere. The main emission peak of Ce3+-activatedCa12Al14O32Cl2exhibits greatly red shift in comparison with that of Ce3+-activated C12A7(from430nm to503nm). The excitation spectrum monitored at503nm consists of two broadabsorption bands at292, and330nm, which ascribes to the4f–5d transitions of Ce3+. Theemission color of Ce3+is tuned from blue to yellow by controlling anions in the nanocages.Sr2+has no effect on the luminescence of Ce3+, but is good for the stability of Cl-.Ca12Al10.6Si3.4O32Cl5.4: Ce3+emits two emission centers, which peaks at around415and490nm, respectively. The emission band at415nm are probably due to the Ce3+substituted forsome Ca2+coordinated by six O2-and one Cl-ions, the emission band at490nm due to theCe3+substituted some Ca2+coordinated by six O2-and one Cl-ions.