The roles of different point defects in persistent luminescence of SrAl2O4：Eu,Dy phosphors were investigated. The research results showed that Dyer plays an important role in the persistent luminescence of SrA1EO4：E...The roles of different point defects in persistent luminescence of SrAl2O4：Eu,Dy phosphors were investigated. The research results showed that Dyer plays an important role in the persistent luminescence of SrA1EO4：Eu, Dy phosphors. It can serve as the electron trap of suitable depth for persistent luminescence. V~ does not serve as the electron trap of suitable depth, but its existence can increase the depth of electron traps. There is interaction between the Dy^3＋（ DySr ） and the Eu^2＋（Eu^x Sr ）, and only if the distance between the Dy^3＋（DySr） and the Eu^2＋ （Eu^x Sr） is close enough, the Dyer can work as an effective electron trap. The point defect of V＂ Sr can be hole trap, but the change of its density in crystal matrix does not arouse the obvious change of persistent luminescence.展开更多
The study on the effects of SrO/Al 2O 3 molar ratio on the crystalline phases and photoluminescence characteristics of strontium aluminate phosphors co-activated with Eu 2+ and Dy 3+ were conducted by X-ray po...The study on the effects of SrO/Al 2O 3 molar ratio on the crystalline phases and photoluminescence characteristics of strontium aluminate phosphors co-activated with Eu 2+ and Dy 3+ were conducted by X-ray powder diffractometry, fluorescence spectrometer and photometer. The strontium aluminate luminescent materials with different SrO/Al 2O 3 molar ratio emit the visible lights with different color tone after removal of excitation. The peak wavelengths of the emission spectra drift in the direction of short wave, the quantity of Sr 4Al 14O 25 crystalline phase increases and the afterglow time lengthens with the SrO/Al 2O 3 reduction. The results show that when the SrO/Al 2O 3 molar ratio is near 1, the photoluminescence materials have high luminescent intensity, and when it is near 0.75, they have long afterglow time. However, when SrO/Al 2O 3 molar ratio is more than 1, the luminescent materials appear strong alkaline in water solution; when SrO/Al 2O 3 molar ratio is much less than 0.75, the samples need a higher temperature to be sintered.展开更多
Strontium aluminate long persistence phos phors are synthesized by combustion method. By control- ling the raw material ratio （Sr/Al）, the effects of phase composition on subsequent spectroscopic properties of phosp...Strontium aluminate long persistence phos phors are synthesized by combustion method. By control- ling the raw material ratio （Sr/Al）, the effects of phase composition on subsequent spectroscopic properties of phosphors are studied. Results show that the phase com-position changes from strontium-rich phase to aluminum- rich phase with the decrease of Sr/AI： when the rate of Al/Sr changes from 3：1 to 1：1, the main crystal phase of samples is Sr3Al206, and it exhibits the characteristic fluorescence of Eu^3＋ in the lattice of Sr3Al206; when the rate of Al/Sr is between 1：2 and 2：7, phase composition is the mixture of SrAl204 and SrAl4OT, and it emits the characteristic fluorescence of Eu^2＋ in SrAl204 but not in SrAl4OT; when Al/Sr decreases to 1：4 or even 1：12, the main crystal phase of samples transform into SrAl12019, and the characteristic emission peak is about 470 nm, which corresponds to the characteristic emission of Eu2＋ in SrAl12019. At the end of the article, the influence laws of two different synthesis methods on phase composition of samples between high-temperature solid method and combustion method are compared. Compared with the high-temperature solid method, the rule of influence is similar, but the mole ratio of Al/Sr in products is always higher than the initial ratio of the raw material, and com-pounds like Sr4Al14025 are not obtained by combustion method.展开更多
The polycrystalline Eu^2＋ and Dy ^3＋ co-doped strontium aluminates SrAl2O4： Eu^2＋, Dy^3＋ with different compositions were prepared by solid state reactions. The UV-excited photoluminescence, persistent luminescen...The polycrystalline Eu^2＋ and Dy ^3＋ co-doped strontium aluminates SrAl2O4： Eu^2＋, Dy^3＋ with different compositions were prepared by solid state reactions. The UV-excited photoluminescence, persistent luminescence and thermo-luminescence were studied and compared. Results show that the doped Eu^2＋ ion in SrAl2O4： Eu^2＋, Dy^3＋ phosphors works as not only the UV-excited luminescent center but also the persistent luminescent center. The doped Dy^3＋ ion can hardly yield any luminescence under UV-excitation, but effectively enhance the persistent luminescence and thermo-luminescence of SrAl2O4： Eu^2＋. Dy^3＋ co-doping can help form electron traps with appropriate depth due to its suitable electro-negativity, and increase the density and depth of electron traps. Based on above observations, a persistent luminescence mechanism, electron transfer model, is proposed and illustrated.展开更多
The electronic structure of the strontium aluminate （SrAl2O4：Eu^2＋） materials was studied with a combined experimental and theo- retical approach. The UV-VUV synchrotron radiation was applied in the experimental s...The electronic structure of the strontium aluminate （SrAl2O4：Eu^2＋） materials was studied with a combined experimental and theo- retical approach. The UV-VUV synchrotron radiation was applied in the experimental study while the electronic structure of the non-optimized and optimized crystal structure were investigated theoretically by using the density functional theory. The structure of the valence and conduction bands as well as the band gap energy of the material together with the position of the Eu2＋ 4f7 85712 ground state were calculated. The calculated band gap energy （6.4 eV） agreed well with the experimental value of 6.6 eV. The valence band consisted mainly of oxygen states whereas the bottom of the conduction band of strontium states. In agreement with the experimental results, the calculated 4f7 8S7r2 ground state of Eu2＋ lies in the energy gap of the host. The position of the 4f7 ground state depended on the Coulomb repulsion strength. The position of the 4f7 ground state with respect to the valence and conduction bands was discussed using theoretical and experimental evidence available.展开更多
Eu 2+, Dy 3+ co-doped nanocrystalline strontium aluminate phosphor powders with brightness and long afterglow were synthesized by the sol-gel method at 1200 ℃ for 2 h. The samples were characterized by X-ray diffract...Eu 2+, Dy 3+ co-doped nanocrystalline strontium aluminate phosphor powders with brightness and long afterglow were synthesized by the sol-gel method at 1200 ℃ for 2 h. The samples were characterized by X-ray diffraction (XRD) and scanning electron microscopy (SEM). The results of XRD show that the single crystalline phase is α-SrAl_2O_4. According to the observation of SEM, the particles of the samples are needle-like. Compared with the samples synthesized by solid state reaction, the grain size of the sol-gel method is to nanometer grade. A clear blue shift occurs in the excitation and emission spectra. The blue shift in nanocrystalline SrAl_2O_4∶Eu, Dy phosphor can be attributed to the quantum-size-effect of the luminescent particles.展开更多
A series of novel lanthanum strontium aluminate phosphors, Ce3＋-doped LaSr2AlO（5–0.5x）Fx（LSAF） phosphors were success-fully synthesized using the high-temperature solid state synthesis procedure under reducing a...A series of novel lanthanum strontium aluminate phosphors, Ce3＋-doped LaSr2AlO（5–0.5x）Fx（LSAF） phosphors were success-fully synthesized using the high-temperature solid state synthesis procedure under reducing atmosphere （H2/N2=1/4）. The X-ray diffrac-tion pattern revealed that LSAF sample was a pure LaSr2AlO5 phase with a sintering temperature of 1250 °C. With the increasing amount of SrF2, the particle size of powders increased and the shape of particles changed more regularly. Being an orange/red emitting phosphors for orange and white light emitting diodes, LSAF：Ce3＋ could be effectively excited by blue lights with typical 4f→5d transitions of Ce3＋ ions. PL and PLE spectra showed inhomogeneous enlargement effects with the increase of F/O ratios, which suggested that electronega-tivity effect was promoted when F/O ratio increased. This was further confirmed by a red shift of PL spectra peak when F/O was in-creased. It can be concluded that the LSAF：Ce phosphors have the potential to meet the development of white light-emitting diodes.展开更多
Precursor of SrAl_2O_4∶Eu, Dy phosphor was prepared by co-precipitation method, in which dispersing agent was added to control particle size. Microwave radiation method was used to synthesize SrAl_2O_4∶Eu, Dy phosph...Precursor of SrAl_2O_4∶Eu, Dy phosphor was prepared by co-precipitation method, in which dispersing agent was added to control particle size. Microwave radiation method was used to synthesize SrAl_2O_4∶Eu, Dy phosphors. Experimental results show that the precursor was composed of crystalline ammonium dawsonite hydrate and strontium carbonate, and the adding of dispersing agent can effectively diminish precursor size. The precursor can be transformed to pure SrAl_2O_4 phase after being calcined for 40 min in microwave chamber. The phosphors possess good persistent luminescent properties, finer grain size ranged from 0.2 μm to 5 μm and better size distribution than that of solid-state reaction. The doping of B^(3+) will result in the increasing of crystallite size, but can effectively improve phosphors′ persistent luminescent properties.展开更多
Long lasting strontium aluminate phosphors were synthesized by means of coated precipitation in the experiment. SrAl 2O 4 monoclinic phase with long afterglow was synthesized at 1 100 ℃ with the aid of H 3BO 3, and t...Long lasting strontium aluminate phosphors were synthesized by means of coated precipitation in the experiment. SrAl 2O 4 monoclinic phase with long afterglow was synthesized at 1 100 ℃ with the aid of H 3BO 3, and the synthetic temperature decreased about 100 ℃—150 ℃ than in solid state reaction. The process of heat treatment was studied by TG-DTA. The phase compositions of powders were analyzed by XRD. The micrographs of the samples were investigated by SEM and the luminescent properties were researched by optical spectrum. The effects of H 3BO 3 on the phase composition and microstructure of the calcined powders were investigated. The results indicated that H 3BO 3 accelerated the formation of SrAl 2O 4 by consuming the intermediate phase. The addition of Dy 2O 3 stimulated the afterglow property due to the increase in the depth of the trap center in the materials.展开更多
Transparent inorganic pigment is the color-filler material for manufacturing spectrum-fingerprint anti-counterfeiting fiber, and has great effects on the emission spectral characteristics of the fiber. In order to exp...Transparent inorganic pigment is the color-filler material for manufacturing spectrum-fingerprint anti-counterfeiting fiber, and has great effects on the emission spectral characteristics of the fiber. In order to explain the change mechanism of the emission spectral character- istics of spectrum-fingerprint anti-counterfeiting fiber and to promote the development of the fiber, several kinds of spectrum-fingerprint fiber samples were prepared by using rare-earth strontium aluminate and polyethylene terephthalate （PET） as raw materials and by adding different kinds of transparent inorganic pigments respectively in this research. The effect of transparent inorganic pigment on the spectral characteristics of the fiber was analyzed in detail by means of scanning electron microscopy （SEM）, X-ray diffraction （XRD） and fluorescence spectrometer. The results showed that transparent inorganic pigment had no effect on the dispersion state and phase structure of rare-earth luminescent mate- rial in spectrum-fingerprint fiber and the microscopic morphology of the fiber. However, transparent inorganic pigment did affect the excitation and emission process of spectrum-fingerprint fiber so as to decrease its excitation and emission efficiency. The change of both the type and content of transparent inorganic pigment exerted great effects on the excitation and emission spectra of the fiber. Therefore, it was a feasible way to control the emission spectrum of spectrum-fingerprint fiber through changing the type and content of transparent inorganic pigment.展开更多
The persistent phosphor SrAl_(2)O_(4):Eu^(2+),Dy^(3+)is the subject of numerous investigations.One often neglected aspect is that in this phosphor,as well as in Sr_(4)Al_(14)O_(25):Eu^(2+),Dy^(3+),there are two differ...The persistent phosphor SrAl_(2)O_(4):Eu^(2+),Dy^(3+)is the subject of numerous investigations.One often neglected aspect is that in this phosphor,as well as in Sr_(4)Al_(14)O_(25):Eu^(2+),Dy^(3+),there are two different Sr^(2)+sites which can be occupied by the dopant Eu^(2+)ions,We first introduce a general scheme of possible energy transfers in these persistent phosphor materials including explicitly both europium ions,This scheme is used as a generic starting point to study experimentally specific pathways.We illustrate this application with the study of the effect of excitation wavelength(444 and 382 nm)on the afterglow of differently doped SrAl_(2)O_(4):Eu^(2+),Dy^(3+)samples,as well as on the emission decay curves.With the same excitation intensity under 444 nm excitation,the resulting afterglow intensity is stronger than under near UV excitation.At 382 nm,Eu^(2+)ions on both Sr^(2)+s ites in SrAl_(2)O_(4)are excited,but at room temperature the blue emission is quenched,leading to a loss of photons.The observed effects can further be associated with the ratio of Eu^(2+)ions and trap states which are modulated by the concentrations of Eu^(2+)and Dy^(3+)in SrAl_(2)O_(4),as well as by temperature,Increasing the nominal Dy^(3+)content from 0.1 mol%to 0.5 mol%with respect to Sr results in the doubling of the integrated afterglow intensity and confirms thus that Dy^(3+)ions are indeed involved in the trapping process,The concentration of trap states is much lower than the concentration of Eu^(2+)ions,as even with low excitation densities,a plateau of integrated afterglow intensity(corresponding to the total number of accessible traps)is reached.We postulate that an important fraction of excited Eu^(2+)ions can potentially transfer their energy to trap states.Once that all traps are filled or in a dynamical filling-depletion process under illumination(with thermal and/or optical depletion processes),for the remaining Eu^(2+)a"normal"steady-state emission is observed.The luminescence decay curves at 520 nm measured at 77 K show a mono-exponential decay with a common lifetime of about 1140 ns for all 5 samples under 437 nm excitation,while under 375 nm excitation,a feed process originating from the energy transfer between Eu^(2+)ions is demonstrated.Under 375 nm excitation,the non-exponential decay observed at 440 nm can be quantitatively associated to a Forster energy transfer process with R_(0)=1.58(8)nm.For the overall understanding of the afterglow processes,it appears that one has to consider the individual contributions of all active ions on different lattice sites.展开更多
基金This study is financially supported by the Jiangxi University of Finance and Economy (Project No. 0421205) Jiangxi Provincial Department of Education (Project No. (2007) 260)
文摘The roles of different point defects in persistent luminescence of SrAl2O4：Eu,Dy phosphors were investigated. The research results showed that Dyer plays an important role in the persistent luminescence of SrA1EO4：Eu, Dy phosphors. It can serve as the electron trap of suitable depth for persistent luminescence. V~ does not serve as the electron trap of suitable depth, but its existence can increase the depth of electron traps. There is interaction between the Dy^3＋（ DySr ） and the Eu^2＋（Eu^x Sr ）, and only if the distance between the Dy^3＋（DySr） and the Eu^2＋ （Eu^x Sr） is close enough, the Dyer can work as an effective electron trap. The point defect of V＂ Sr can be hole trap, but the change of its density in crystal matrix does not arouse the obvious change of persistent luminescence.
文摘The study on the effects of SrO/Al 2O 3 molar ratio on the crystalline phases and photoluminescence characteristics of strontium aluminate phosphors co-activated with Eu 2+ and Dy 3+ were conducted by X-ray powder diffractometry, fluorescence spectrometer and photometer. The strontium aluminate luminescent materials with different SrO/Al 2O 3 molar ratio emit the visible lights with different color tone after removal of excitation. The peak wavelengths of the emission spectra drift in the direction of short wave, the quantity of Sr 4Al 14O 25 crystalline phase increases and the afterglow time lengthens with the SrO/Al 2O 3 reduction. The results show that when the SrO/Al 2O 3 molar ratio is near 1, the photoluminescence materials have high luminescent intensity, and when it is near 0.75, they have long afterglow time. However, when SrO/Al 2O 3 molar ratio is more than 1, the luminescent materials appear strong alkaline in water solution; when SrO/Al 2O 3 molar ratio is much less than 0.75, the samples need a higher temperature to be sintered.
基金financially supported by the National Natural Science Foundation of China(No.NSFC50962001)the Scientific Research Foundation of Beifang Ethnic University for Nationalities(No.2010Y042)
文摘Strontium aluminate long persistence phos phors are synthesized by combustion method. By control- ling the raw material ratio （Sr/Al）, the effects of phase composition on subsequent spectroscopic properties of phosphors are studied. Results show that the phase com-position changes from strontium-rich phase to aluminum- rich phase with the decrease of Sr/AI： when the rate of Al/Sr changes from 3：1 to 1：1, the main crystal phase of samples is Sr3Al206, and it exhibits the characteristic fluorescence of Eu^3＋ in the lattice of Sr3Al206; when the rate of Al/Sr is between 1：2 and 2：7, phase composition is the mixture of SrAl204 and SrAl4OT, and it emits the characteristic fluorescence of Eu^2＋ in SrAl204 but not in SrAl4OT; when Al/Sr decreases to 1：4 or even 1：12, the main crystal phase of samples transform into SrAl12019, and the characteristic emission peak is about 470 nm, which corresponds to the characteristic emission of Eu2＋ in SrAl12019. At the end of the article, the influence laws of two different synthesis methods on phase composition of samples between high-temperature solid method and combustion method are compared. Compared with the high-temperature solid method, the rule of influence is similar, but the mole ratio of Al/Sr in products is always higher than the initial ratio of the raw material, and com-pounds like Sr4Al14025 are not obtained by combustion method.
基金Jiangxi University of Finance and Economy and Jiangxi Provin-cial Department of Education (No.[2007]260)
文摘The polycrystalline Eu^2＋ and Dy ^3＋ co-doped strontium aluminates SrAl2O4： Eu^2＋, Dy^3＋ with different compositions were prepared by solid state reactions. The UV-excited photoluminescence, persistent luminescence and thermo-luminescence were studied and compared. Results show that the doped Eu^2＋ ion in SrAl2O4： Eu^2＋, Dy^3＋ phosphors works as not only the UV-excited luminescent center but also the persistent luminescent center. The doped Dy^3＋ ion can hardly yield any luminescence under UV-excitation, but effectively enhance the persistent luminescence and thermo-luminescence of SrAl2O4： Eu^2＋. Dy^3＋ co-doping can help form electron traps with appropriate depth due to its suitable electro-negativity, and increase the density and depth of electron traps. Based on above observations, a persistent luminescence mechanism, electron transfer model, is proposed and illustrated.
基金supported by the Jenny and Antti Wihuri Foundation and the Academy of Finland Project (117057/2006)supported by the European Community-Research Infrastructure Action under the FP6 Structuring the European Research Area Programme (RII3-CT-2004-506008 (IA-SFS))supported by research mobility agreements (112816/2006/JH and 116142/2006/JH, 123976/2007/TL) between the Academy of Finland and the Academy of Sciences of the Czech Republic
文摘The electronic structure of the strontium aluminate （SrAl2O4：Eu^2＋） materials was studied with a combined experimental and theo- retical approach. The UV-VUV synchrotron radiation was applied in the experimental study while the electronic structure of the non-optimized and optimized crystal structure were investigated theoretically by using the density functional theory. The structure of the valence and conduction bands as well as the band gap energy of the material together with the position of the Eu2＋ 4f7 85712 ground state were calculated. The calculated band gap energy （6.4 eV） agreed well with the experimental value of 6.6 eV. The valence band consisted mainly of oxygen states whereas the bottom of the conduction band of strontium states. In agreement with the experimental results, the calculated 4f7 8S7r2 ground state of Eu2＋ lies in the energy gap of the host. The position of the 4f7 ground state depended on the Coulomb repulsion strength. The position of the 4f7 ground state with respect to the valence and conduction bands was discussed using theoretical and experimental evidence available.
文摘Eu 2+, Dy 3+ co-doped nanocrystalline strontium aluminate phosphor powders with brightness and long afterglow were synthesized by the sol-gel method at 1200 ℃ for 2 h. The samples were characterized by X-ray diffraction (XRD) and scanning electron microscopy (SEM). The results of XRD show that the single crystalline phase is α-SrAl_2O_4. According to the observation of SEM, the particles of the samples are needle-like. Compared with the samples synthesized by solid state reaction, the grain size of the sol-gel method is to nanometer grade. A clear blue shift occurs in the excitation and emission spectra. The blue shift in nanocrystalline SrAl_2O_4∶Eu, Dy phosphor can be attributed to the quantum-size-effect of the luminescent particles.
基金supported by Jiangsu Innovation Program for Graduate Education(CXZZ12_0147)A Project Funded by the Priority Academic Program Development of Jiangsu Higher Education Institutions
文摘A series of novel lanthanum strontium aluminate phosphors, Ce3＋-doped LaSr2AlO（5–0.5x）Fx（LSAF） phosphors were success-fully synthesized using the high-temperature solid state synthesis procedure under reducing atmosphere （H2/N2=1/4）. The X-ray diffrac-tion pattern revealed that LSAF sample was a pure LaSr2AlO5 phase with a sintering temperature of 1250 °C. With the increasing amount of SrF2, the particle size of powders increased and the shape of particles changed more regularly. Being an orange/red emitting phosphors for orange and white light emitting diodes, LSAF：Ce3＋ could be effectively excited by blue lights with typical 4f→5d transitions of Ce3＋ ions. PL and PLE spectra showed inhomogeneous enlargement effects with the increase of F/O ratios, which suggested that electronega-tivity effect was promoted when F/O ratio increased. This was further confirmed by a red shift of PL spectra peak when F/O was in-creased. It can be concluded that the LSAF：Ce phosphors have the potential to meet the development of white light-emitting diodes.
文摘Precursor of SrAl_2O_4∶Eu, Dy phosphor was prepared by co-precipitation method, in which dispersing agent was added to control particle size. Microwave radiation method was used to synthesize SrAl_2O_4∶Eu, Dy phosphors. Experimental results show that the precursor was composed of crystalline ammonium dawsonite hydrate and strontium carbonate, and the adding of dispersing agent can effectively diminish precursor size. The precursor can be transformed to pure SrAl_2O_4 phase after being calcined for 40 min in microwave chamber. The phosphors possess good persistent luminescent properties, finer grain size ranged from 0.2 μm to 5 μm and better size distribution than that of solid-state reaction. The doping of B^(3+) will result in the increasing of crystallite size, but can effectively improve phosphors′ persistent luminescent properties.
文摘Long lasting strontium aluminate phosphors were synthesized by means of coated precipitation in the experiment. SrAl 2O 4 monoclinic phase with long afterglow was synthesized at 1 100 ℃ with the aid of H 3BO 3, and the synthetic temperature decreased about 100 ℃—150 ℃ than in solid state reaction. The process of heat treatment was studied by TG-DTA. The phase compositions of powders were analyzed by XRD. The micrographs of the samples were investigated by SEM and the luminescent properties were researched by optical spectrum. The effects of H 3BO 3 on the phase composition and microstructure of the calcined powders were investigated. The results indicated that H 3BO 3 accelerated the formation of SrAl 2O 4 by consuming the intermediate phase. The addition of Dy 2O 3 stimulated the afterglow property due to the increase in the depth of the trap center in the materials.
基金Project supported by the Key (Key grant) Project of Chinese Ministry of Education (309016)National High-Tech R&D Program of China (863 Program) (2012AA030313)
文摘Transparent inorganic pigment is the color-filler material for manufacturing spectrum-fingerprint anti-counterfeiting fiber, and has great effects on the emission spectral characteristics of the fiber. In order to explain the change mechanism of the emission spectral character- istics of spectrum-fingerprint anti-counterfeiting fiber and to promote the development of the fiber, several kinds of spectrum-fingerprint fiber samples were prepared by using rare-earth strontium aluminate and polyethylene terephthalate （PET） as raw materials and by adding different kinds of transparent inorganic pigments respectively in this research. The effect of transparent inorganic pigment on the spectral characteristics of the fiber was analyzed in detail by means of scanning electron microscopy （SEM）, X-ray diffraction （XRD） and fluorescence spectrometer. The results showed that transparent inorganic pigment had no effect on the dispersion state and phase structure of rare-earth luminescent mate- rial in spectrum-fingerprint fiber and the microscopic morphology of the fiber. However, transparent inorganic pigment did affect the excitation and emission process of spectrum-fingerprint fiber so as to decrease its excitation and emission efficiency. The change of both the type and content of transparent inorganic pigment exerted great effects on the excitation and emission spectra of the fiber. Therefore, it was a feasible way to control the emission spectrum of spectrum-fingerprint fiber through changing the type and content of transparent inorganic pigment.
基金Project supported by the Swiss National Science Foundation(200020_182494,200021_169033)Innosuisse(25902.1 PFNM-NM,15217.1PFIW-IW)。
文摘The persistent phosphor SrAl_(2)O_(4):Eu^(2+),Dy^(3+)is the subject of numerous investigations.One often neglected aspect is that in this phosphor,as well as in Sr_(4)Al_(14)O_(25):Eu^(2+),Dy^(3+),there are two different Sr^(2)+sites which can be occupied by the dopant Eu^(2+)ions,We first introduce a general scheme of possible energy transfers in these persistent phosphor materials including explicitly both europium ions,This scheme is used as a generic starting point to study experimentally specific pathways.We illustrate this application with the study of the effect of excitation wavelength(444 and 382 nm)on the afterglow of differently doped SrAl_(2)O_(4):Eu^(2+),Dy^(3+)samples,as well as on the emission decay curves.With the same excitation intensity under 444 nm excitation,the resulting afterglow intensity is stronger than under near UV excitation.At 382 nm,Eu^(2+)ions on both Sr^(2)+s ites in SrAl_(2)O_(4)are excited,but at room temperature the blue emission is quenched,leading to a loss of photons.The observed effects can further be associated with the ratio of Eu^(2+)ions and trap states which are modulated by the concentrations of Eu^(2+)and Dy^(3+)in SrAl_(2)O_(4),as well as by temperature,Increasing the nominal Dy^(3+)content from 0.1 mol%to 0.5 mol%with respect to Sr results in the doubling of the integrated afterglow intensity and confirms thus that Dy^(3+)ions are indeed involved in the trapping process,The concentration of trap states is much lower than the concentration of Eu^(2+)ions,as even with low excitation densities,a plateau of integrated afterglow intensity(corresponding to the total number of accessible traps)is reached.We postulate that an important fraction of excited Eu^(2+)ions can potentially transfer their energy to trap states.Once that all traps are filled or in a dynamical filling-depletion process under illumination(with thermal and/or optical depletion processes),for the remaining Eu^(2+)a"normal"steady-state emission is observed.The luminescence decay curves at 520 nm measured at 77 K show a mono-exponential decay with a common lifetime of about 1140 ns for all 5 samples under 437 nm excitation,while under 375 nm excitation,a feed process originating from the energy transfer between Eu^(2+)ions is demonstrated.Under 375 nm excitation,the non-exponential decay observed at 440 nm can be quantitatively associated to a Forster energy transfer process with R_(0)=1.58(8)nm.For the overall understanding of the afterglow processes,it appears that one has to consider the individual contributions of all active ions on different lattice sites.
