Enhancement of red phosphorescence properties of Pr3+ activated calcium titanate

Luminescence is the phenomenon of light emission as a result of absorbed energy from an excitation source. Luminescence consists of two stages as excitation and emission. At the first stage, as a result of energy absorption, the molecule is excited to a higher energy state from the ground state. The excited molecule is unstable and wants to return to ground state. In the second stage, part of absorbed energy is emitted as light during return of the molecule to the ground state. Luminescent materials are essential materials in imaging, lighting and display applications. They have been well known for a long time from traditional lighting and display applications like fluorescent lamps, cathode ray tubes, TV screens and also X-rays screens to new application such as white LEDs, plasma display panels and detectors for X-ray imaging. In addition to being classified according to types of excitation such as photoluminescence, cathodoluminescence, chemiluminescence, bioluminescence, thermoluminescence and triboluminescence; luminescence can be separated two subgroups as fluorescence and phosphorescence. Fluorescence and phosphorescence are separated from each other in terms of afterglow times and excitation states. Fluorescence refers to the instantaneous emission of light within the transition from the single excited states to the ground state. Phosphorescence is the light emission that occurs during the transition from the triplet excited state to ground state. Transition from triplet excited state to ground state is forbidden and the emission rate is slow. Light emission can be detected even after excitation source has been removed. Phosphorescence systems consist of a host lattice and dopant element which serve as luminescence center. As the main matrix, many different structures such as aluminate, silicate, sulfur, phosphate, gallate, germanate and titanate are being studied. Even if dopants are referred as luminescence center, the host lattice has a significant effect on the luminescence properties. Rare earth elements and some transition metals are generally used as additive elements. A suitable host lattice and additive element must be selected to be able to form a phosphorescence system. Persistent phosphors are optical functional materials. When they are excited, the resulting light emission remains visible for an appreciable time (from milliseconds to seconds and hours) after the excitation source has removed. There are two types of active centers in persistent phosphors; emitters and traps. While the emitters provide light emission, the traps store the absorbed energy for a certain period of time and provide long lasting phosphorescence. The afterglow time of the light emission depends on the type of trap, its depth and intensity. Recently, many different researches are being carried out in order to improve phosphorescent materials that emit red, green and blue primary colors. Significant improvements were obtained in phosphorescent materials which emit light in green blue emission range after discovering of CaAl2O4: Eu2+, Nd3+ (blue) and SrAl2O4: Eu2+, Dy3+ (green). However, the phosphorescent materials that emit red light still cannot meet desirable luminescence intensity and afterglow time properties. The research on luminescent materials is mainly driven by on the lack of efficient red phosphorescent materials. Recently, rare-earth doped titanate phosphors which are novel luminescent materials with advanced physical and chemical properties, have attracted significant attention owing to their promising photoluminescent properties and potential applications in white light emitting diodes (LEDs) due to very close to the chromaticity coordinates of the ideal red. Calcium titanate (CaTiO3) is preferred as a host material thanks to exhibited good chemical and thermal stability as well as its better mechanical resistance. This study includes a promising approach for the improvement of a red light emitting CaTiO3: Pr3+ phosphor. In this study, Pr3+ doped CaTiO3 material with strong red emission at 612 nm wavelength through 1D23H4 transition (f-f) of praseodymium electrons was prepared by two different methods; Pechini and solid state reaction. As first the step of study, samples were prepared by the Pechini method in CaTiO3: x% Pr3+ (x = 0.10, 0.15, 0.20, 0.25, 0.50, 0.75, 1) formulation and were sintered at 1500 °C for 6 hours to investigate the effect of increasing praseodymium concentration on emission intensity and afterglow time. In order to determine the proper sintering temperature, the samples with CaTiO3: 0.2% Pr3+ formulation were prepared by Pechini method and sintered at four different temperatures 1000, 1200, 1400 and 1500 ° C. The samples were prepared as CaTiO3: 0.2% Pr3+ composition by the solid state reaction and the Pechini methods and sintered at 1500 ° C for 6 hours to examine the effect of the production method on the phosphorescence properties. Eu, La, Lu, Gd, Y, Sc and Nb elements were added to the structure as a co-dopant in order to improve the emission intensity and afterglow time. The samples which were prepared by the Pechini method according to the composition of Ca0.99TiO3: 0.2% Pr3+, 1% A3+ (Y, Gd, Sc, La, Lu, Eu, Nb) were sintered at 1500 °C for 6 hours. The samples were characterized by using Differential Thermal Analysis (DTA) technique to determine reaction and phase transformation temperatures, X-Ray Diffraction (XRD) method for phase analysis, Scanning Electron Microscope (SEM) analysis for investigate microstructure, and Spectrophotometer analysis for determination of emission intensity and afterglow time. As a result of the characterization studies; the optimum praseodymium concentration was determined as 0.2% at which the maximum emission intensity and afterglow time were obtained. While the increase in emission intensity was proportional to the increasing temperature, a decrease in the emission intensity was observed at the sample sintered at 1200 °C. It was foreseen that this decline was caused by the phase transformation that occurred around 1200 °C. Maximum emission intensity and afterglow time were achieved in samples sintered at 1500 °C. When the production methods were compared; although there is no difference between the two production methods in the emission wavelength, it was observed that the sample produced by the solid state reaction method has higher emission intensity according to the sample produced by the Pechini method. However, it was observed that the sample produced with the solid state method showed almost no afterglow time, while the sample produced with the Pechini method showed persistent luminescence. As a result of the spectrophotometer analysis; it was found that all co-dopants (Y, Gd, Sc, La, Lu, Eu, Nb) significantly improved both the emission intensity and the afterglow time when compared with only praseodymium doped reference sample (CaTiO3: 0.2% Pr3+). The maximum emission intensity was obtained with Ca0.99TiO3: 0.2% Pr3+, 1% Y3+ sample and it was found that yttrium co-doped calcium titanate has four times higher emission intensity than the reference sample. The maximum afterglow time was also obtained with yttrium co-doped calcium titanate sample; and it was observed that it has three times higher afterglow time when compared again with the reference sample.