Persistent Luminescence Mechanism Research Articles

Low temperature photoluminescence and deformation luminescence of microparticles SrAl2O4:(Eu2+, Dy3+) in a matrix of photopolymer

The paper deals with low-temperature photoluminescence and deformation luminescence (mechanoluminescence) of a composite material based on fine disperse powder of phosphor SrAl 2 O 4:( Eu 2+, Dy 3+) and photopolymerizing resin that is transparent in the visible region. New information about the energy levels of impurities and defects was obtained. It has been found that at low temperatures (T = 15 ÷ 200 K) the photoluminescence spectrum of SrAl 2 O 4:( Eu 2+, Dy 3+) presents two partially overlapping wide bands with the maxima at λ1 max = 446 nm and λ2 max = 517 nm. The strong interaction of energy states of the bands results in temperature quenching of the short-wave band of luminescence (λ1 max = 446 nm) and its complete attenuation at T ≥ 200 K. The results were used to describe the mechanism of persistent luminescence and mechanoluminescence of SrAl 2 O 4:( Eu 2+, Dy 3+).

Photoluminescence and persistent luminescence in Bi3+-doped Zn2GeO4 phosphors

Bi3+-doped Zn2GeO4 were prepared by the high temperature solid-state reaction method. The phase purity and crystallinity of Zn2GeO4:Bi3+ samples were characterized by X-ray diffraction (XRD). The Photoluminescence and persistent properties of Zn2GeO4:Bi3+ phosphors were investigated through the excitation spectra, the emission spectra, the persistent luminescence, the persistent decay curves and thermoluminescence spectra. Excitation into the host absorption, Zn2GeO4:Bi3+ gives a blue broadband emission; a bluish-green emission of Zn2GeO4:Bi3+ is obtained for the direct excitation of Bi3+ under 300nm excitation. Zn2GeO4:Bi3+ shows a weak persistent luminescence after irradiation by 254nm UV light for 3min. The photoluminescence and persistent luminescence mechanism of Zn2GeO4:Bi3+ were discussed in detail.

Synthesis and Persistent Luminescence Mechanism of a Novel Orange Emitting Persistent Phosphor Sr5(BO3)3Cl:Eu2+

A series of Eu2+‐doped Sr5(BO3)3Cl phosphors were prepared successfully using a conventional solid‐state reaction method. The luminescent properties were studied systematically by utilizing photoluminescence spectra, decay curves, persistent luminescence spectra, and thermoluminescence glow curves. Energy transfer from host to emission center Eu2+ was affirmed. The orange persistent luminescence emission was observed for the first time. The optimal doping concentration of Eu2+ for persistent luminescence was experimentally to be approximately 0.5% and the orange persistent luminescence duration can persist about 15 min. On the basis of the experimental results, a model was constructed and the relevant mechanism of persistent luminescence was illustrated in detail. Two different ways of trapping the charge carriers were also discussed.

Origin of the visible light induced persistent luminescence of Cr3+-doped zinc gallate

ZnGa2O4:Cr3+ (ZGO:Cr) is a very bright persistent phosphor able to emit a near infrared light for hours following a UV (band to band excitation) or visible (Cr3 excitation) illumination. As such it serves as an outstanding biomarker for in vivo imaging. Persistent luminescence, due to trapping of electrons/holes at point defects, is studied here on a series of ZGO:Cr spinel compounds where the introduction of defects is controlled by varying the Zn/(Ga+Cr) nominal ratio during synthesis. Simulation of Electron Paramagnetic Resonance spectra revealed up to six types of Cr3+ ions with different neighboring defects and correlated to four emission lines in low temperature photoluminescence spectroscopy. Of particular importance, three EPR signals were attributed to Cr3+ with a pair of neighboring ZnGa′ and GaZn0° antisite defects. They were identified to the emission line N2 that plays a key role in the persistent luminescence mechanism for both storage of visible excitation and persistent luminescence emission. A model is proposed whereby the local electric field at Cr3+ created by the two neighboring antisite defects triggers the electron–hole separation and trapping upon excitation of Cr3+. The process is equivalent to a photoinduced electron transfer from a donor (here ZnGa′) to an acceptor (here GaZn0°) observed in some molecular systems.

Defect to R3+ energy transfer: colour tuning of persistent luminescence in CdSiO3

Luminescence from trivalent rare earth (R3+: La3+–Lu3+, excluding Pm3+) ions was studied in the CdSiO3 host. The positions of the R2+/3+ energy levels in the band structure of CdSiO3 suggest that the doping of CdSiO3 with R2+ ions is difficult if not impossible. Red, pink, blue, green and close to white persistent luminescence colours were obtained by doping with Pr3+, Sm3+, Gd3+, Tb3+ and Dy3+, respectively. The efficiency of the defect to R3+ energy transfer determines if persistent luminescence arises from the 4f–4f, defect or a combination of these two emissions. In contrast to what is observed for Pr3+ and Tb3+, the defect to R3+ energy transfer did not give efficient persistent luminescence for Sm3+ and Dy3+, probably due to high energy losses and/or back transfer from the rare earth to defects. In line with the experimental observations, the in situ synchrotron radiation XANES spectra indicated the presence of only the trivalent Pr3+ and Tb3+ species thus excluding the direct R3+ → RIV oxidation during the charging process of persistent luminescence. Finally, based on the band gap energy, R2+/3+ energy level positions, trap energies, and other optical and structural properties, the mechanism of persistent luminescence was developed for Pr3+ doped CdSiO3. For practical applications, the CdSiO3:R3+ system offers an excellent possibility for colour tuning of persistent luminescence by changing only the R3+ dopant instead of altering the host as is the case with the Eu2+ doped materials. Eventually, this will avoid the waste of both intellectual and financial resources.

Revealing trap depth distributions in persistent phosphors

Persistent luminescence or afterglow is caused by a gradual release of charge carriers from trapping centers. The energy needed to release these charge carriers is determined by the trap depths. Knowledge of these trap depths is therefore crucial in the understanding of the persistent luminescence mechanism. Unfortunately, the trap depths in persistent phosphors are often difficult to evaluate in an accurate and reliable way. The existing analysis methods are mostly based on single experiments, or they ignore the possibility of a continuous distribution of trap depths. We present a procedure to accurately probe the activation energies, even in the presence of a continuous distribution of energy levels. By performing a series of thermoluminescence experiments with varying excitation duration and at varying excitation temperature, and employing the initial rise analysis method, the depth and shape of such a distribution can be estimated. As an example, we investigated the trap system in the violet persistent phosphor CaAl2O4:Eu,Nd, and show that it consists of a Gaussian-shaped distribution of trap depths. The maximal density of traps lies in the region around 0.9 eV, but the distribution extends to 0.7 eV on the shallow side and 1.2 eV on the deep side. The described procedure can be used to obtain a clear view of the trap system in other persistent phosphors as well. This can lead to a better understanding of the nature of these trapping centers, and the role they play in the persistent luminescent mechanism.

Persistent luminescence behavior of materials doped with Eu^2+ and Tb^3+

In this work, the persistent luminescence mechanisms of Tb3+ (in CdSiO3) and Eu2+ (in BaAl2O4) based on solid experimental data are compared. The photoluminescence spectroscopy shows the different nature of the inter- and intraconfigurational transitions for Eu2+ and Tb3+, respectively. The electron is the charge carrier in both mechanisms, implying the presence of electron acceptor defects. The preliminary structural analysis shows a free space in CdSiO3 able to accommodate interstitial oxide ions needed by charge compensation during the initial preparation. The subsequent annealing removes this oxide leaving behind an electron trap. Despite the low band gap energy for CdSiO3, determined with synchrotron radiation UV-VUV excitation spectroscopy of Tb3+, the persistent luminescence from Tb3+ is observed only with UV irradiation. The need of high excitation energy is due to the position of 7F6 level deep below the bottom of the conduction band, as determined with the 4f8→4f75d1 and the ligand-to-metal charge-transfer transitions. Finally, the persistent luminescence mechanisms are constructed and, despite the differences, the mechanisms for Tb3+ and Eu2+ proved to be rather similar. This similarity confirms the solidity of the interpretation of experimental data for the Eu2+ doped persistent luminescence materials and encourages the use of similar models for other persistent luminescence materials.

Persistent luminescence mechanisms: human imagination at work

The present status and future progress of the mechanisms of persistent luminescence are critically treated with the present knowledge. The advantages to be achieved by a further need as well as the pitfalls of the excessive use of imagination are shown. As usual, in the beginning of the present era of persistent luminescence since the mid 1990s, the imagination played a more important role than the sparse solid experimental data and the chemical common sense and knowledge was largely ignored. Since some five years, the mechanistic studies seem to have reached the maturity and – perhaps deceivingly – it seems that there are only details to be solved. However, the development of red emitting nanocrystalline materials poses a challenge also to the more fundamental studies and interpretation. The questions still luring in the darkness include the problems how the increased surface area affects the defect structure and how the “persistent energy transfer” really works. There is still some light to be thrown onto these matters starting with agreeing on the terminology: the term phosphorescence should be abandoned altogether. The long lifetime of persistent luminescence is due to trapping of excitation energy, not to the forbidden nature of the luminescent transition. However, the technically well-suited term “afterglow” should be retained for harmful, short persistent luminescence.

Persistent luminescence fading in Sr_2MgSi_2O_7:Eu^2+,R^3+ materials: a thermoluminescence study

The fading of persistent luminescence in Sr2MgSi2O7:Eu2+,R3+ (R: Y, La-Nd, Sm-Lu) was studied combining thermoluminescence (TL) and room temperature (persistent) luminescence measurements to gain more information on the mechanism of persistent luminescence. The TL glow curves showed the main trap signal at ca. 80 °C, corresponding to 0.6 eV as the trap depth, with every R co-dopant. The TL measurements carried out with different irradiation times revealed the general order nature of the TL bands. The results obtained from the deconvolutions of the glow curves allowed the prediction of the fading of persistent luminescence with good accuracy, though only when using the Becquerel decay law.

Thermal Release Behavior of Carriers in Persistent Luminescence Material CaAl2O4:Eu2+,Nd3+

To reveal the detailed migration processes of carriers thermally released from defects in CaAl2O4:Eu2+,Nd3+, their thermoluminescence curves recorded at different fading time were investigated. The results indicated that their trapping levels and kinetic orders were varying with fading time. As the persistent luminescence properties of CaAl2O4:Eu2+,Nd3+ are closely related with the migration manners of carriers, the variety of kinetic order indicated that the mechanism(s) of persistent luminescence is(are) more complicated than we previous proposed. Furthermore, the details of carrier thermal release processes and carrier transfer processes are illustrated with the aid of variety of parameters derived from TL curves.

Luminescence and x-ray absorption measurements of persistent SrAl2O4:Eu,Dy powders: Evidence for valence state changes

The development of new efficient afterglow phosphors is currently hampered by a limited understanding of the persistent luminescence mechanism. Radioluminescence (RL) and x-ray absorption measurements on the persistent phosphor SrAl${}_{2}$O${}_{4}$:Eu,Dy were combined to reveal possible valence state changes for the rare earth (co)dopants. Traps in the phosphor material are quickly filled when exposing thermally emptied SrAl${}_{2}$O${}_{4}$:Eu,Dy powder to x rays. On the same time scale a partial oxidation of Eu${}^{2+}$ to Eu${}^{3+}$ is observed by x-ray absorption near-edge spectroscopy (XANES), while for the trivalent dysprosium the valence state remains unchanged. The impact of these observations on the recently proposed models for persistent luminescence is discussed.

Long-lasting near-infrared persistent luminescence from β-Ga 2O 3:Cr 3+ nanowire assemblies

Near-infrared (NIR) persistent luminescent β-Ga 2O 3:Cr 3+ nanowire assemblies were synthesized by a hydrothermal process followed by calcination. The phosphor exhibits more than 4 h afterglow in the wavelength range of 650–850 nm after ceasing the ultraviolet light (280–360 nm) irradiation. The trap structure and persistent luminescence mechanism were revealed by thermoluminescence measurement. The β-Ga 2O 3:Cr 3+ nanowire assemblies may find applications as identification taggants in security and optical probes in bio-imaging.

Thermoluminescence excitation spectroscopy: A versatile technique to study persistent luminescence phosphors

A versatile new facility to study photoionization processes in impurity doped compounds is presented. In this new facility monochromatic light is coupled to a thermoluminescence reader, enabling a fully automated recording of glow curves as a function of photon excitation wavelength. It provides detailed information on the mechanism of trap filling preceding persistent luminescence. The technique is first demonstrated with a study on Lu 2SiO 5:Ce 3+ and then applied to commercial modern day double lanthanide doped SrAl 2O 4:Eu 2+,Dy 3+, Sr 4Al 14O 25:Eu 2+,Dy 3+, CaAl 2O 4:Eu 2+,Nd 3+; and to the classical ZnS:Cu + persistent luminescence phosphors. The presented data provide new insight into the mechanism of persistent luminescence.

Optical energy storage properties of Sr2MgSi2O7:Eu2+,R3+ persistent luminescence materials

The details of the mechanism of persistent luminescence were probed by investigating the trap level structure of Sr2MgSi2O7:Eu2+,R3+ materials (R: Y, La-Lu, excluding Pm and Eu) with thermoluminescence (TL) measurements and Density Functional Theory (DFT) calculations. The TL results indicated that the shallowest traps for each Sr2MgSi2O7:Eu2+,R3+ material above room temperature were always ca. 0.7 eV corresponding to a strong TL maximum at ca. 90 °C. This main trap energy was only slightly modified by the different co-dopants, which, in contrast, had a significant effect on the depths of the deeper traps. The combined results of the trap level energies obtained from the experimental data and DFT calculations suggest that the main trap responsible for the persistent luminescence of the Sr2MgSi2O7:Eu2+,R3+ materials is created by charge compensation lattice defects, identified tentatively as oxygen vacancies, induced by the R3+ co-dopants.

Red persistent luminescent silicate nanoparticles

Red persistent luminescent diopside nanoparticles doped with Mn 2+ and codoped with RE 3+ (Eu 2+ , Dy 3+ ) have been obtained by sol-gel method. The influence of codoping rare earth ions on the persistent luminescence was studied by wavelength-resolved thermally stimulated luminescence (TSL) measurements from 30 to 650 K after X-ray irradiation. From these first results, a mechanism of persistent luminescence is proposed. In this mechanism Mn 2+ and Eu 2+ act as TSL recombination centers, Mn 3+ and Eu 3+ being stable hole centers, whereas Dy 3+ acts as a good electron trap giving rise to a TSL peak at high temperature. Finally, persistent luminescence was measured. Intensity and persistence of the red luminescence of CaMgSi 2 O 6 : Mn 2+ -Dy 3+ are better than those of CaMgSi 2 O 6 : Mn 2+ and CaMgSi 2 O 6 : Mn 2+ -Eu 2+ , which are in agreement with the results of TSL.

Persistent luminescence in rare earth ion-doped gadolinium oxysulfide phosphors

A series of rare-earth ion-doped gadolinium oxysulfide phosphors Gd 2O 2S:RE 3+, Ti, Mg (RE = Ce, Pr, Nd, Sm, Eu, Tb, Dy, Ho, Er, Tm, Yb) were synthesized by solid-state reaction. The excitation and photoluminescence spectra, afterglow spectra, afterglow decay curves and thermoluminescence spectra of the phosphors were examined. According to the afterglow spectra, gadolinium oxysulfide doped with rare-earth ions were classified into three groups. When rare earth ions such as Eu 3+, Sm 3+, Dy 3+, Ho 3+, Er 3+ and Tm 3+ were introduced into the Gd 2O 2S host, their characteristic emission as well as that from Gd 2O 2S:Ti, Mg were observed. In case of Yb 3+ and Nd 3+, only the broadband luminescence of Gd 2O 2S:Ti, Mg was obtained. Gadolinium oxysulfide doped with Pr 3+, Tb 3+ and Ce 3+ did not show afterglow emission. The calculated trap energy levels of the samples were compared. The role of Ti and Mg ions and a potential mechanism for persistent luminescence in the samples were discussed.

Synchrotron radiation investigations of the Sr 2MgSi 2O 7:Eu 2+,R 3+ persistent luminescence materials

The electronic and defect energy level structure of polycrystalline Sr 2MgSi 2O 7:Eu 2+,R 3+ persistent luminescence materials were studied with thermoluminescence and different synchrotron radiation spectroscopies (UV-VUV emission and excitation, X-ray absorption near-edge spectroscopy (XANES) and extended X-ray absorption fine structure (EXAFS)). Special attention was paid on the effect of the R 3+ co-dopants on the persistent luminescence properties of the materials. Theoretical calculations using the density functional theory (DFT) were carried out simultaneously with the experimental work. The experimental band gap energy ( E g) value of ca. 7.1 eV agreed very well with the DFT value of 6.7 eV. The variation of the E g value was attempted to relate with the trap structure as well as with the different properties of the R 3+ co-dopants. The trap level energy distribution depended strongly on the R 3+ co-dopant except for the shallowest trap energy above the room temperature remaining practically the same, however. The different processes in the mechanism of persistent luminescence from Sr 2MgSi 2O 7:Eu 2+,R 3+ were assembled and their contributions discussed.

Synchrotron radiation spectroscopy of rare earth doped persistent luminescence materials

The electronic structure of the polycrystalline CaAl 2O 4:Eu 2+,Ce 3+ persistent luminescence materials were studied with X-ray absorption (XANES) and UV–VUV emission and excitation spectroscopy by using synchrotron radiation. Theoretical calculations using the density functional theory (DFT) were carried out simultaneously with the experimental work. The experimental band gap energy ( E g) value of 6.7 eV agrees very well with the DFT value of 6.4 eV. From the 4f 7→4f 65d 1 excitation bands of Eu 2+, the positions of the 4f 7 ground as well as the 4f 65d 1 excited levels were established. The excitonic fine structure which could act as trap levels close to the bottom of the conduction band could not be observed, however. The different processes contributing to the mechanism of persistent luminescence from CaAl 2O 4:Eu 2+ were constructed and discussed.

UV–VUV spectroscopy of rare earth doped persistent luminescence materials

The electronic and defect energy level structure of polycrystalline SrAl 2O 4:Eu 2+,R 3+ persistent luminescence materials were studied with thermoluminescence and UV–VUV synchrotron radiation emission and excitation spectroscopy. Theoretical calculations using the density functional theory (DFT) were carried out simultaneously with the experimental work. The experimental band gap energy ( E g) value of 6.6 eV agrees very well with the DFT value of 6.4 eV. The 4f 7 → 4f 65d 1 excitation bands of Eu 2+ were found rather similar irrespective of the R 3+ co-dopant. The trap level energy distribution depended strongly on the R 3+ co-dopant except for the shallowest trap energy above the room temperature remaining the same, however. The different processes in the mechanism of persistent luminescence from SrAl 2O 4:Eu 2+,R 3+ was constructed and discussed.

X-ray absorption study of rare earth ions in Sr 2MgSi 2O 7:Eu 2+,R 3+ persistent luminescence materials

The valence of the europium dopant and selected rare earth co-dopants (Ce 3+, Dy 3+, and Yb 3+) in the Sr 2MgSi 2O 7:Eu 2+,R 3+ persistent luminescence materials were studied by room temperature XANES measurements. The results indicated the co-existence of both divalent and trivalent europium in all the studied materials. The relative amount of Eu 3+ was observed to increase upon increasing exposure to X-rays, as expected by the persistent luminescence mechanism. This suggests a simultaneous filling of oxygen vacancies initially created by the reducing preparation conditions. For the Dy and Yb co-dopants, only trivalent species were observed. On the other hand, traces of tetravalent cerium were present in the Eu,Ce co-doped materials.