Green Persistent Luminescence Research Articles

(Invited) Investigation of Therml Ionization Quenching and Design of Persistent Luminescnece in Ce3+-Doped Garnet Phosphors

Ce3+-doped garnet materials have attracted a great deal of attention in the w-LED application because of their various optical properties. However, the mechanism of their thermal quenching has not clearly been elucidated yet. In this study, the two possibilities of thermally activated ionization and crossover were investigated as cause of quenching process of 5d luminescence in Ce3+-doped Y3Al5-xGaxO12. Based on the photoconductivity and thermoluminescence measurements, the quenching is found to be caused by thermal ionization. The thermal ionization quenching is a disadvantage for w-LED application, but for persistent phosphors, it is a required process. Using the thermal ionization process by blue light excitation, the blue light chargeable persistent phosphors were successfully developed. For the synthesis of polycrystalline ceramics of Y3Al5-xGaxO12(YAGG):Ce3+(0.5 %), Y3Al5-xGaxO12:Ce3+(0.5 %) -Cr3+(0.05% ), the Y2O3, Al2O3, Ga2O3, CeO2 and Cr2O3were used as starting materials. The mixed powder was pressed into a pellet and sintered at 1400~1600°C. The optoelectronic, photoluminescence and persistent luminescence properties in the obtained samples were investigated. From the PL spectra measurement of Ce3+-singly doped YAGG, the PL intensity decreases with increasing Ga content, x, and no photoluminescence was observed in the x=5 sample. In our photoconductivity measurement, the intense photocurrent excitation bands attributed to the transitions from the ground 4f level to the 5d1 and 5d2, which are the lowest and the 2nd lowest 5d levels, were observed at 300K in the x=3 and 5 samples doped with Ce3+. This result shows that the luminescence quenching is caused by thermal ionization process in the x=3 and 5 samples1). However, the photoconductivity by blue excitation was not observed in the x=0 sample (YAG:Ce3+) at 300K. For checking thermal ionization process of YAG:Ce3+ at high temperatures, thermoluminescence(TL) excitation spectra were measured. In the TL excitation (TLE) spectrum at room temperature there is no Ce3+:5d1 band (the lowest excited 5d level). However, in the TLE spectrum at 573 K, which corresponds to the onset temperature of luminescence quenching, a TLE band due to the Ce3+:5d1 excitation was observed at around 450 nm. This is the evidence that the luminescence quenching of YAG:Ce3+ at high temperatures is caused by the thermal ionization2). Using the thermal ionization process, we tried to prepare blue light chargeable persistent phosphors. In the Ce3+-singly doped YAGG phosphors, there is no appropriate electron traps for persistent phosphors at ambient temperature. We discovered that Cr3+ ions act efficiently as electron traps in the garnet materials. The Y3Al2Ga3O12:Ce3+–Cr3+ showed Ce3+:5d–4f green persistent luminescence (λem = 505 nm) for several hours after blue-light excitation3). The TL glow peak temperature of Y3Al2Ga3O12:Ce3+–Cr3+ is located at around 300K. By increasing the Ga content of Y3Al5-xGaxO12:Ce3+-Cr3+, the TL glow peak temperature are changed from 400 K to 150 K for glow curves measured at 10 K min-1. The corresponding trap depths vary from 0.41 eV to 1.2 eV4). A trap depth distribution with a width that changes with Ga content was observed in Y3Al5-xGaxO12:Ce3+–Cr3+. The width is minimum for x = 0, x = 3, and x = 5 which was explained by the statistics in the distribution of the Ga ions on the sites around the Cr defect. From simulating the distribution we conclude that Ga first start to occupy the tetrahedral sites and above x = 3 also the octahedral sites are being occupied. 1) J. Ueda, S. Tanabe, and T. Nakanishi, J. Appl. Phys., 110,053102 (2011). 2) J. Ueda, P. Dorenbos, A. J. J. Bos, A. Meijerink, and S. Tanabe, J. Phys. Chem. C, 119,25003-25008 (2015). 3) J. Ueda, K. Kuroishi, and S. Tanabe, Appl. Phys. Lett., 104,033519 (2014). 4) J. Ueda, P. Dorenbos, A. J. J. Bos, K. Kuroishi, and S. Tanabe, J. Mater. Chem. C, 3, 5642-5651 (2015).

Photoluminescence and long persistent luminescence properties of a novel green emitting phosphor Sr3TaAl3Si2O14:Tb3+

A series of green emitting persistent phosphors Sr3TaAl3Si2O14:Tb3+ were synthesized via a conventional solid-state reaction method. These phosphors show a green persistent luminescence dominated at ~542 nm at room temperature after irradiated by 254 nm ultraviolet light for minutes. X-ray diffraction, scanning micrograph images, photoluminescence spectra, decay curves, afterglow spectra and thermal luminescence were performed to investigate the physical properties of the persistent phosphors. The influence of Tb3+ concentrations on the photoluminescence and long persistent luminescence properties was studied. The optimal concentration of Tb3+ ions for the best afterglow characteristic was experimentally to be 0.5 mol%. A feasible model was proposed on the basis of experimental results to discuss mechanism of long afterglow in Sr3TaAl3Si2O14:Tb3+ in detail.

Combustion synthesis of SrAl4O7:Eu2+, Dy3+, Gd3+ long lasting phosphor

Long afterglow strontium aluminates SrAl4O7:Eu2+,Dy3+,Gd3+ phosphor was prepared by combustion method. The X-ray diffraction (XRD), Scanning electron microscope (SEM) and photoluminescence (PL) spectrum as well as the luminescence decay were used to characterize the long lasting phosphor. The emission spectrum of the phosphor shows broadened band at 495 nm assigned to the 4f65d→4f7(8S7/2) transition of Eu2+. The present phosphor shows green persistent luminescence after removal of the UV light.

A novel green long-lasting phosphorescence phosphor Ca14Mg2(SiO4)8:Eu2+, Dy3+

A novel silicate long-lasting phosphorescence phosphor Ca14Mg2(SiO4)8:Eu2+, Dy3+ was prepared by high-temperature solid state reaction. Its properties were investigated systematically by powder X-ray diffraction patterns, Rietveld refinements, photoluminescence spectra, long-lasting phosphorescence spectra, phosphorescence decay curves, and thermoluminescence spectra. Under the excitation of 400nm, Ca14Mg2(SiO4)8:Eu2+, Dy3+ phosphor exhibits green light with the peak at 505nm, which is attributed to the 4f65d1–4f7 electronic transition of Eu2+ ions. It is also found that Ca14Mg2(SiO4)8:Eu2+, Dy3+ phosphor can emit green persistent phosphorescence after the irradiation of ultraviolet or visible light is stopped. And for the optimal sample, the green persistent luminescence can last for about half an hour. The co-doping of Dy3+ ions greatly enhances the intensity and duration of the persistent luminescence in Ca14Mg2(SiO4)8:Eu2+, Dy3+ phosphor. However, the peak position of the long-lasting phosphorescence spectrum is red shift compared to that of the photoluminescence spectrum, and the possible reason was analyzed. Meanwhile, the long-lasting phosphorescence mechanism of this phosphor was also discussed.

Promising long-lasting phosphor material: a novel metal–organic framework showing intriguing luminescent performance

A distinct way to target long-lasting phosphors (LLPs) is disclosed. This new material is a metal-organic framework featuring a 1D zig-zag chain and 3D hydrogen bonded PtS net with three-fold interpenetration. It exhibits persistent luminescence lasting about 1 s which can be traced by the naked eye. The green persistent luminescence is exclusively due to emission from multiple triplet excited states. In this way, LLP can be easily achieved by using a simple hydrothermal synthesis without any codopant that in nature is responsible for well-known inorganic LLPs.

Photoluminescence and persistent luminescence properties of non-doped and Ti4+-doped Mg2SnO4 phosphors

Mg2SnO4 exhibits green photoluminescence and persistent luminescence, which originate from oxygen vacancies. When Ti4+ ions were doped, an interesting Mg2SnO4:Ti4+ phosphor with bluish white photoluminescence under ultraviolet irradiation and with green persistent luminescence was first obtained. Our investigation reveals that two emission centres exist in Mg2SnO4:Ti4+. The centres responsible for the green emission are considered to be the F centres (oxygen vacancies) and the blue centres are the TiO6 complex. Trap clusters in the band gap with different depths, such as [Sn••Mg−O″i], [Sn••Mg−V•O], [Sn••Mg−V×O] and Mg″Sn, correspond to the components at 85 °C, 146 °C and 213 °C of the thermoluminescence curve.

The persistent luminescence and up conversion photostimulated luminescence properties of nondoped Mg2SnO4 material

The nondoped Mg2SnO4 material with inverse spinel structure was synthesized by solid state reaction. This phosphor showed a broad green emission band covering 470–550 nm under 291 nm excitation, which was due to the recombination of F centers with holes. Stimulated by 980 nm infrared laser, the green photostimulated luminescence was first observed in a nondoped oxide. After ultraviolet irradiation, the green persistent luminescence of Mg2SnO4 could be seen in darkness for about 5 h. The decay curves revealed that the long persistent luminescence was governed by tunneling mechanism and it proved the presence of different trap clusters in Mg2SnO4. These trap clusters (such as [SnMg••–Oi″], [SnMg••–2e′], and [SnMg••–e″]) induced the trap levels with different depths in band gap and corresponded to the three components (at 110, 168, and 213 °C) of the thermoluminescence glow curve of Mg2SnO4. These trap levels with different depths were proved to be not independent. It revealed that the shallow traps (110 °C) and part of the deep traps (168 and 213 °C) were involved in the persistent luminescence. Meanwhile, all the shallow and deep traps were responsible for the photostimulated luminescence. Accordingly, the photoluminescence, persistent, and photostimulated luminescence mechanisms of the nondoped Mg2SnO4 material were first proposed.

Spectroscopic Properties of Persistent Luminescence Phosphors: Lu2O3:Tb3+,M2+ (M = Ca, Sr, Ba)

A new family of persistent luminescence phosphors, Lu2O3:Tb3+,M2+ (M = Ca, Sr, Ba), was found and the materials’ most important spectroscopic properties related to the persistent luminescence phenomenon were reported and discussed. The persistent luminescence seen by the naked eye in the dark lasted for at least 10 h. The green persistent luminescence is exclusively due to emission from the Tb3+ ion. The thermoluminescence glow curves change with the codopant. For Ca2+ codoping, the main TL band is located around 100 °C. When Sr2+ is added, a similar TL band is accompanied with another structure around 240 °C, and for Ba2+ codoped material, the latter band becomes the most prominent feature in the TL glow curve. Analysis of the various findings leads to the conclusion that the persistent luminescence is governed by a temperature-assisted tunneling mechanism and the point defects get spatially linked during the materials processing into [TbLu×-VO••-2MLuˈ] cluster entities.

Luminescent nanoparticles of MgAl 2O 4:Eu, Dy prepared by citrate sol–gel method

MgAl 2O 4:Eu, Dy nanoparticles were prepared by citrate sol–gel method and thermally treated at 600, 700, 800 and 900 °C. The trivalent europium ion is partially reduced to the divalent state at 700 and 800 °C. Infrared spectra of the phosphors showed bands around 700 and 520 cm −1 corresponding to the AlO 6 groups. X-ray diffraction patterns present sharp reflections of samples heated from 700 to 900 °C indicating the MgAl 2O 4 spinel phase. Grain size in the range 20–30 nm were observed by measurement of transmission electron microscopy (TEM). The emission spectra of the phosphors show a broadened band at 480 nm assigned to the 4f 65d → 4f 7 ( 8S 7/2) transition of Eu 2+ ion overlapped to the 4F 9/2 → 6H 15/2 transition of the Dy 3+ ion. Besides, the 4F 9/2 → 6H 13/2 transition (579 nm) of Dy 3+ ion is overlapped with the 5D 0 → 7F 0 (578 nm) and 5D 0 → 7F 1 (595 nm) transitions from the Eu 3+ ion. Excitation spectra of the sample heated at 900 °C monitoring the excitation at 615 nm of 5D 0 → 7F 2 transition of Eu 3+ ion exhibit a broad band assigned to the O → Eu 3+ ligand-to-metal charge-transfer states (LMCT) around 280 nm. The samples present green persistent luminescence after exposure to UV radiation. The chromaticity coordinates were obtained from the luminescence emission spectrum.